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Tripodal carbene and aryloxide ligands for small-molecule activation at electron-rich uranium and transition metal centers
Abstract
Chelating tripodal are ligands ideal candidates to support highly reactive metal complexes, as their anchors and pendant arms are highly tunable.Recently, such ligands were developed that employ N-heterocyclic carbene arms, and either a nitrogen (TIMEN) or carbon (TIME) anchor.The carbon anchored ligand has been shown to coordinate to a range of metals in the transition series, forming polynuclear species.Using the nitrogen anchor instead allows the isolation of a mononuclear cobalt complex, [(TIMENAr)Co]Cl, with the ligand coordinated in a k3 fashion.This cobalt complex is highly reactive towards both dioxygen and organic azides.A tripodal ligand derivative with tunable aryloxide arms and a triazacyclononane anchor has been used to support low- and high-valent uranium complexes as well.Using an aryloxide arm substituted with a tert-butyl group creates an open uranium(III) center in the complex [((t-BuArO)3tacn)U], which activates hydrocarbons such as methylcyclohexane and splits carbon dioxide, forming a uranium m-oxo dinuclear complex, [((t-BuArO)3tacn)U]{µ-O}, and producing carbon monoxide as a byproduct.Using the bulky adamantyl groups on the aryloxide arms makes a complex with a highly protected uranium center, [((AdArO)3tacn)U], featuring a small cylindrical cavity forcoordination and reduction of carbon dioxide to form a uranium(IV) carbon dioxide complex with radical residing on the reduced CO2 ligand.The complexes described herein are distinctive as they represent a set of isostructural complexes possessing a range of metals, oxidation states, and differing electronic and magnetic behaviours.
... Tripodal examples, which exhibit approximate C 3 symmetry in solution, have proved particularly effective in controlling the coordination sphere of uranium by encapsulating the metal ion in a single well-defined "steric pocket" along the C 3 axis. Such motifs have supported unusual uranium oxidation states and bonding regimes, and coordinatively unsaturated U(III) centers that support rich small molecule activation chemistry [3][4][5][6][7]. Over the last 25 years, the tripodal ligands that have proved most popular for generating landmark uranium complexes include the anchored tris-aryloxides {N(CH 2 OAr) 3 } (Ar = substituted aryl), {tacn(CH 2 OAr) 3 } (tacn = 1,3,7-triazacyclononane) and {Mes(CH 2 OAr) 3 } (Mes = C 6 H 2 Me 3 -2,4,6); and the tris-amides {N(CH 2 CH 2 NSiR 3 ) 3 } (SiR 3 = SiMe 3 , Si t BuMe 2, Si i Pr 3 ) [3][4][5][6][7]. ...
... Such motifs have supported unusual uranium oxidation states and bonding regimes, and coordinatively unsaturated U(III) centers that support rich small molecule activation chemistry [3][4][5][6][7]. Over the last 25 years, the tripodal ligands that have proved most popular for generating landmark uranium complexes include the anchored tris-aryloxides {N(CH 2 OAr) 3 } (Ar = substituted aryl), {tacn(CH 2 OAr) 3 } (tacn = 1,3,7-triazacyclononane) and {Mes(CH 2 OAr) 3 } (Mes = C 6 H 2 Me 3 -2,4,6); and the tris-amides {N(CH 2 CH 2 NSiR 3 ) 3 } (SiR 3 = SiMe 3 , Si t BuMe 2, Si i Pr 3 ) [3][4][5][6][7]. Several related macrocyclic ligand systems have more recently been applied in f-element chemistry [8][9][10]. ...
... For example, the reaction of CO 2 with [U{tacn(CH 2 Ar tBu,tBu O) 3 [14], whereas the more sterically demanding [U{tacn (CH 2 Ar Ad,Ad O) 3 }] (Ar Ad,Ad O = 3-adamantyl-5-tert-butyl-2-oxybenzyl) reacts with CO 2 to yield the remarkable terminal CO 2 U(IV) complex [U{tacn(Ar Ad,Ad O) 3 }(CO 2 )] [15]. Those examples showcase how the outcomes of small molecule activation reactions for tripodal U(III) complexes are dependent upon the size of the axial reactivity pocket [3][4][5][6][7]. ...
Tripodal multidentate ligands have become increasingly popular in f-element chemistry for stabilizing unusual bonding motifs and supporting small molecule activation processes. The steric and electronic effects of ligand donor atom substituents have proved crucial in both of these applications. In this study we functionalized the previously reported tris-anilide ligand {tacn(SiMe2NPh)3} (tacn = 1,3,7-triazacyclononane) to incorporate substituted aromatic rings, with the aim of modifying f-element complex solubility and ligand steric effects. We report the synthesis of two proligands, {tacn(SiMe2NHAr)3} (Ar = C6H3Me2-3,5 or C6H4Me-4), and their respective group 1 transfer agents—{tacn(SiMe2NKAr)3}, M(III) complexes [M{tacn(SiMe2NAr)3}] for M = La and U, and U(IV) complexes [M{tacn(SiMe2NAr)3}(Cl)]. These compounds were characterized by multinuclear NMR and FTIR spectroscopy and elemental analysis. The paramagnetic uranium complexes were also characterized by solid state magnetic measurements and UV/Vis/NIR spectroscopy. U(III) complexes were additionally studied by EPR spectroscopy. The solid state structures of all f-block complexes were authenticated by single-crystal X-ray diffraction (XRD), together with a minor byproduct [U{tacn(SiMe2NC6H4Me-4)3}(I)]. Comparisons of the characterization data of our f-element complexes with similar literature examples containing the {tacn(SiMe2NPh)3} ligand set showed minor changes in physicochemical properties resulting from the different aromatic ring substitution patterns we investigated.
... Compared to CASSCF, the significant decrease in the MS-CASPT2 computed χT at high temperature is due to the increased energy gap between the ground and excited KD (see Table S7). To capture the metal−ligand covalency, we next incorporated two ligand-based occupied π-orbitals in the active space, that is, CAS (7,9), and computed all the quartets and doublets ( Figure 5). These calculations show that the ground state wave function is composed of the (π Lig ) 4 5f 3 configuration with a weight of 78% and the (π Lig ) 3 5f 4 configuration with a weight of 7% followed by other smaller terms. ...
... The incorporation of dynamic correlation adds corrections to the low-lying energy spectrum and g values. The steepness of the magnetic susceptibility curves computed with CASSCF- (7,9) and CASPT2 (7,9) is in reasonably good agreement with experiment ( Figure 4). Most importantly, the incorporation of LMCT transitions in the active space predicts the correct electronic structure of complex 2 along with a reasonably good estimate of the low-lying energy spectrum and magnetic properties. ...
... The incorporation of dynamic correlation adds corrections to the low-lying energy spectrum and g values. The steepness of the magnetic susceptibility curves computed with CASSCF- (7,9) and CASPT2 (7,9) is in reasonably good agreement with experiment ( Figure 4). Most importantly, the incorporation of LMCT transitions in the active space predicts the correct electronic structure of complex 2 along with a reasonably good estimate of the low-lying energy spectrum and magnetic properties. ...
... [178] In particular, these ligands were capable of stabilizing reactive intermediate species and low to high metal oxidation states. [179,180] In the chelating tris-carbene ligand family, Biffis, Santini, et al. [181] developed the first tripodal tris(imidazolium)methane ligand and the related trinuclear Ag I -NHC complex in 2008. The field of poly-NHC ligands was recently and excellently reviewed by Meyer, [178,180] Peris, [124] and Biffis. ...
... [179,180] In the chelating tris-carbene ligand family, Biffis, Santini, et al. [181] developed the first tripodal tris(imidazolium)methane ligand and the related trinuclear Ag I -NHC complex in 2008. The field of poly-NHC ligands was recently and excellently reviewed by Meyer, [178,180] Peris, [124] and Biffis. [125] In addition, the coordination and organometallic chemistry of "pincer"-type polydentate ligands incorporating one or more NHC units and other conventional donors was recently reviewed by Peris and Crabtree, [126] Danopoulos, [140] Braunstein, [182] and Chaplin. ...
Within the research field of antitumor metal-based agents alternative to platinum drugs, gold(I/III) coordination complexes have always been in the forefront due mainly to the familiarity of medicinal chemists with gold compounds, whose application in medicine goes back in the ancient times, and to the rich chemistry shown by this metal. In the last decade, N-heterocyclic carbene ligands (NHC), a class of ligands that largely resembles the chemical properties of phosphines, became of interest for gold(I) medicinal applications, and since then, the research on NHC-gold(I/III) coordination complexes as potential antiproliferative agents boosted dramatically. Different classes of gold(I/III)-NHC complexes often showed an outstanding in vitro antiproliferative activity, however up to now very few in vivo data have been reported to corroborate the in vitro results. This review summarizes all achievements in the field of gold (I/III) complexes comprising NHC ligands proposed as potential antiproliferative agents in the period 2004-2016, and critically analyses biological data (mainly IC50 values) in relation to the chemical structures of Au compounds. The state of art of the in vivo studies so far described is also reported.
... [178] In particular, these ligands were capable of stabilizing reactive intermediate species and low to high metal oxidation states. [179,180] In the chelating tris-carbene ligand family, Biffis, Santini, et al. [181] developed the first tripodal tris(imidazolium)methane ligand and the related trinuclear Ag I -NHC complex in 2008. The field of poly-NHC ligands was recently and excellently reviewed by Meyer, [178,180] Peris, [124] and Biffis. ...
... [179,180] In the chelating tris-carbene ligand family, Biffis, Santini, et al. [181] developed the first tripodal tris(imidazolium)methane ligand and the related trinuclear Ag I -NHC complex in 2008. The field of poly-NHC ligands was recently and excellently reviewed by Meyer, [178,180] Peris, [124] and Biffis. [125] In addition, the coordination and organometallic chemistry of " pincer " -type polydentate ligands incorporating one or more NHC units and other conventional donors was recently reviewed by Peris and Crabtree, [126] Dano- poulos, [140] Braunstein, [182] and Chaplin. ...
The success enjoyed in the last 50 years by use of poly(pyrazolyl)borates as ligands has inspired the development of a great number of scorpionates containing other donor elements. These include polytopic bis- and tris(NHC)borate ligands, featuring N-heterocyclic carbene (NHC) moieties that generally coordinate as bidentate κ2C or tridentate κ3C, respectively. In spite of structural similarities between poly(pyrazolyl)borates and poly(NHC)borates, their methods of synthesis and their reactivity are different and reflect the variations in topology, flexibility, and donor properties. The structural and electronic properties of poly(NHC)borate ligands are compared and correlated with their coordination chemistry, particularly towards transition metals. The advances in the chemistry of scorpionate-type bis- and tris(NHC)borate ligands are reviewed by highlighting and comparing their structural properties and their ability to stabilize low to high metal oxidation states in transition-metal complexes.
... [178] In particular, these ligands were capable of stabilizing reactive intermediate species and low to high metal oxidation states. [179,180] In the chelating tris-carbene ligand family, Biffis, Santini, et al. [181] developed the first tripodal tris(imidazolium)methane ligand and the related trinuclear Ag I -NHC complex in 2008. The field of poly-NHC ligands was recently and excellently reviewed by Meyer, [178,180] Peris, [124] and Biffis. ...
... [179,180] In the chelating tris-carbene ligand family, Biffis, Santini, et al. [181] developed the first tripodal tris(imidazolium)methane ligand and the related trinuclear Ag I -NHC complex in 2008. The field of poly-NHC ligands was recently and excellently reviewed by Meyer, [178,180] Peris, [124] and Biffis. [125] In addition, the coordination and organometallic chemistry of " pincer " -type polydentate ligands incorporating one or more NHC units and other conventional donors was recently reviewed by Peris and Crabtree, [126] Dano- poulos, [140] Braunstein, [182] and Chaplin. ...
The enhanced application of N-heterocyclic carbene (NHC) complexes has over the last few years also triggered a steadily increasing number of studies in the fields of medicinal chemistry, which take advantage of the fascinating chemical properties of these complexes. In fact it has been demonstrated that metal-NHC complexes can be used to develop highly efficient metal based drugs with possible applications in the treatment of cancer or infectious diseases. In particular they represent new anticancer agents, normally with low toxicity profiles, and provide a range of versatile structures for targeted biological applications. Most of these complexes have shown higher cytotoxicity than cisplatin. In the present review the medicinal applications of copper(I) and silver(I) metal NHC complexes are summarized. Specifically, azolium precursors and related Cu(I)- and Ag(I)-NHC complexes of functionalized and non-functionalized imidazole-, triazole- thiazole and benzimidazole-based NHC systems employed in anticancer applications are reviewed. The possibility of using novel azolium precursors and copper(I) and silver(I) acceptors should offer interesting perspectives for the rapid and selective construction of libraries of high molecular diversity and complexity, thus providing a basis for the design and development of metal-based drugs associated with unprecedented biological targets. An overview of the most relevant chemical features is presented and copper(I) and silver(I) NHCs complexes used as anticancer drugs are given together with the description of structure-activity relationships as far as possible.
... These distances are consistent with those for uranium-(IV)−aryloxide interactions. 30 Accordingly, the U−N bond distance has also decreased (2.371(4) Å), but it is still in the Inorg. Chem. ...
... XXXX, XXX, XXX−XXX range for a U(IV)-amide. 26,30,31 The intraligand distances corroborate a higher degree of reduction as compared to 1, with C−O distances (1.350(6) and 1.355(6) Å) that are elongated by ∼0.1 Å, in agreement with single bond character that is expected for the iminosemiquinone resonance structure shown in Scheme 1. The corresponding C−N bonds are very similar to those in 1, showing that they are less sensitive to ligand oxidation state changes. ...
Uranium derivatives of a redox-active, dioxophenoxazine ligand, (DOPO(q))2UO2, (DOPO(sq))UI2(THF)2, (DOPO(cat))UI(THF)2, and Cp*U(DOPO(cat))(THF)2 (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate), have been synthesized from U(VI) and U(III) starting materials. Full characterization of these species show uranium complexes bearing ligands in three different oxidation states. The electronic structures of these complexes have been explored using (1)H NMR and electronic absorption spectroscopies, and where possible, X-ray crystallography and SQUID magnetometry.
Terminal uranium oxido, sulfido, and selenido metallocenes were synthesized, and their reactivity was comprehensively studied. Heating of an equimolar mixture of [η5-1,2,4-(Me3Si)3C5H2]2UMe2 (2) and [η5-1,2,4-(Me3Si)3C5H2]2U(NH-p-tolyl)2 (3) in the presence of 4-dimethylaminopyridine (dmap) in refluxing toluene forms [η5-1,2,4-(Me3Si)3C5H2]2U═N(p-tolyl)(dmap) (4), which is a useful precursor for the preparation of the terminal uranium oxido, sulfido, and selenido metallocenes [η5-1,2,4-(Me3Si)3C5H2]2U═E(dmap) (E = O (5), S (6), Se (7)) employing a cycloaddition-elimination methodology with Ph2C═E (E = O, S) or (p-MeOPh)2CSe, respectively. Metallocenes 5-7 are inert toward alkynes, but they act as nucleophiles in the presence of alkylsilyl halides. The oxido and sulfido metallocenes 5 and 6 undergo [2 + 2] cycloadditions with isothiocyanate PhNCS or CS2, while the selenido derivative 7 does not. The experimental studies are complemented by density functional theory (DFT) computations.
The uranium metallacyclocumulene, [η5-1,3-(Me3Si)2C5H3]2U(η4-C4Ph2) (3) was isolated from the reaction mixture containing [η5-1,3-(Me3Si)2C5H3]2UCl2 (1), potassium graphite (KC8) and 1,4-diphenylbutadiyne (PhCC-CCPh) in good yield. The reactivity of 3 towards various small organic molecules was evaluated. For example, while complex 3 shows no reactivity towards alkynes and 2,2'-bipyridine, it may deliver the [η5-1,3-(Me3Si)2C5H3]2U(II) fragment in the presence of Ph2E2 (E = S, Se) and Ph3CN3, or react as a nucleophile in the presence of carbodiimides, isothiocyanates, aldehydes, ketones, and pyridine derivatives, forming five-, seven- or nine-membered heterometallacycles. On the contrary, addition of Ph2CS to 3 induces CS bond cleavage yielding the dithiolate complex [η5-1,3-(Me3Si)2C5H3]2U[S2(C12H5Ph5)] (14). In contrast, the closely related, but sterically more encumbered uranium metallacyclocumulene [η5-1,2,4-(Me3Si)3C5H2]2U(η4-C4Ph2) (4) features a more limited reactivity which is restricted to mono- and double insertions with small unsaturated organic molecules such as isothiocyanates, ketones and nitriles.
This paper describes the synthesis and reactivity of [η5-1,3-(Me3Si)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3) (6) which is accessible from a ligand exchange reaction between [η5-1,3-(Me3Si)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPPh3) (2) and Me3PO at ambient temperature. Phosphinidene 6 exhibits no reactivity towards internal alkynes, but readily reacts with various hetero-unsaturated molecules such as isothiocyanates, aldehydes, nitriles, isonitriles, and organic azides, forming uranium sulfido, oxido, imido, and uranaheterocyclic compounds. Nevertheless, with the bidentate ortho-dicyanobenzene o-C6H4(CN)2 the zwitterionic species [η5-1,3-(Me3Si)2C5H3]2U[NHC(N){C6H4CP(2,4,6-iPr3C6H2)CH2PMe2O}] (13) is isolated in good yield. Moreover, 6 converts with Ph2S2 to the uranium(iii) phenylthiolate compound [η5-1,3-(Me3Si)2C5H3]2USPh(OPMe3) (7) in good isolated yield. Furthermore, the influence of the Lewis base on the reactivity of the uranium phosphinidene metallocenes has also been evaluated.
The Lewis base supported terminal uranium phosphinidene metallocene [η5-1,3-(Me3C)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3) (2) could be isolated from a salt metathesis reaction in toluene at ambient temperature between [η5-1,3-(Me3C)2C5H3]2U(Cl)Me (1) and 2,4,6-iPr3C6H2PHK in the presence of Me3PO, and its structure and reactivity were probed in detail. No reaction of 2 with internal alkynes was observed, but it reacts in the presence of various heterounsaturated molecules such as CS2, isothiocyanates, aldehydes, imines, diazenes, carbodiimides, nitriles, isonitriles, diazoalkane, and organic azides, forming carbodithioates, sulfidos, oxidos, metallaheterocycles, and imido complexes, in good yields. Moreover, on reaction with the diazoalkane derivative Me3SiCHN2 the pseudophosphinimido uranium(iii) complex [η5-1,3-(Me3C)2C5H3]2U(N[double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3) (20) can be isolated in good yield.
The synthesis, electronic structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Addition of diphenylacetylene (PhC≡CPh) to the uranium phosphinidene metallocene [η5-1,2,4-(Me3C)3C5H2]2U=P-2,4,6- t Bu3C6H2 (1) yields the stable uranium metallacyclopropene, [η5-1,2,4-(Me3C)3C5H2]2U[η2-C2Ph2] (2). Based on density functional theory (DFT) results the 5f orbital contributions to the bonding within the metallacyclopropene U-(η2-C=C) moiety increases significantly compared to the related Th(IV) compound [η5-1,2,4-(Me3C)3C5H2]2Th[η2-C2Ph2], which also results in more covalent bonds between the [η5-1,2,4-(Me3C)3C5H2]2U2+ and [η2-C2Ph2]2- fragments. Therefore, although the thorium and uranium complexes are structurally closely related, different reaction patterns are observed. For example, 2 reacts as a masked synthon for the low-valent uranium(II) metallocene [η5-1,2,4-(Me3C)3C5H2]2U(II) when reacted with Ph2E2 (E = S, Se), alkynes and a variety of hetero-unsaturated molecules such as imines, ketazine, bipy, nitriles, organic azides, and azo derivatives. In contrast, five-membered metallaheterocycles are accessible when 2 is treated with isothiocyanate, aldehydes, and ketones.
The first stable base‐free terminal uranium phosphinidene metallocene is presented. Its structure and reactivity have been studied in detail and compared to that of the corresponding thorium derivative. Salt metathesis reaction of the methyl iodide uranium metallocene Cp′′′ 2 U(I)Me ( 2 , Cp′′′ = η 5 ‐1,2,4‐(Me 3 C) 3 C 5 H 2 ) with Mes*PHK (Mes* = 2,4,6‐(Me 3 C) 3 C 6 H 2 ) in THF yields the base‐free terminal uranium phosphinidene metallocene, Cp′′′ 2 U=PMes* ( 3 ). In addition, density functional theory (DFT) studies suggest substantial 5f orbital contributions to the bonding within the uranium phosphinidene [U]=PAr moiety, which result in a more covalent bonding between the [Cp′′′ 2 U] 2+ and [Mes*P] 2‐ fragments than that for the related thorium derivative. This difference in bonding causes different reactivity patterns. Therefore, the uranium derivative 3 may act as a Cp′′′ 2 U(II) synthon releasing the phosphinidene moiety (Mes*P:) when treated with alkynes or a variety of hetero‐unsaturated molecules such as imines, thiazoles, ketazines, bipy, organic azides, diazene derivatives, ketones, and carbodiimides.
A three-membered thorium metallaheterocycle [η ⁵ -1,3-(Me 3 C) 2 C 5 H 3 ] 2 Th[η ² -P 2 (2,4,6- ⁱ Pr 3 C 6 H 2 ) 2 ] (4) is readily prepared besides H 2 from [η ⁵ -1,3-(Me 3 C) 2 C 5 H 3 ] 2 Th(PH-2,4,6- ⁱ Pr 3 C 6 H 2 ) 2 (3) upon heating in toluene solution. Density functional theory (DFT) studies were performed to elucidate the 5f orbital contribution to the bonding within Th-(η ² -P-P) revealing more covalent bonds between the [η ⁵ -1,3-(Me 3 C) 2 C 5 H 3 ] 2 Th ²⁺ and [η ² -P 2 (2,4,6- ⁱ Pr 3 C 6 H 2 ) 2 ] ²⁻ fragments than those in the related thorium metallacyclopropene. Consequently, distinctively different reactivity patterns emerge, e.g., while 4 reacts with pyridine derivatives such as 4-dimethyaminopyridnie (DMAP) and forms the DMAP adduct [η ⁵ -1,3-(Me 3 C) 2 C 5 H 3 ] 2 Th[η ² -P 2 (2,4,6- ⁱ Pr 3 C 6 H 2 ) 2 ](DMAP) (5), it may also act as a [η ⁵ -1,3-(Me 3 C) 2 C 5 H 3 ] 2 Th(ii) synthon when reacted with bipy, Ph 2 S 2 or Ph 2 Se 2 . Nevertheless, no reaction of complex 4 with alkynes is observed, but it reacts as a nucleophile towards nitriles and aldehydes resulting in five- or seven-membered metallaheterocycles, respectively. DFT computations provide some additional insights into the experimental observations.
The stable base-free terminal phosphinidene thorium metallocene, [η5-1,2,4-(Me3C)3C5H2]2Th[double bond, length as m-dash]P-2,4,6-tBu3C6H2 (2), can be isolated from the reaction of the thorium dichloride complex [η5-1,2,4-(Me3C)3C5H2]2ThCl2 (1) with 2 equiv. of 2,4,6-(Me3C)3C6H2PHK in THF. The reactivity of 2 in the activation of various small organic molecules such as diselenides, phosphines, imines, ketones, phosphine oxides, thiazole, imidazole derivatives and amines was explored. For example, when complex 2 is treated with Ph2Se2, the phosphinidene is replaced, yielding diselenido compound [η5-1,2,4-(Me3C)3C5H2]2Th(SePh)2 (3). Moreover, E-H (E = P, N, C) bond activation occurs on exposure of 2 to 2,4,6-iPr3C6H2PH2, PhPH2, (p-tolyl)2C[double bond, length as m-dash]NH, 1-indanone, cyclohexanone, Me3PO, thiazole, 1-methylimidazole and p-toluidine, resulting in the phosphido complex [η5-1,2,4-(Me3C)3C5H2][η5,κ-C-1,2-(Me3C)2-4-(CH2CMe2)C5H2]Th(PH-2,4,6-iPr3C6H2) (4), the metallaheterocycle [η5-1,2,4-(Me3C)3C5H2]2Th(η2-P2Ph2) (5), the iminato phosphido complex [η5-1,2,4-(Me3C)3C5H2]2Th(PH-2,4,6-tBu3C6H2)[N[double bond, length as m-dash]C(p-tolyl)2] (6), the phosphido enolyl compound [η5-1,2,4-(Me3C)3C5H2]2Th(PH-2,4,6-tBu3C6H2)(κ-O-1-OC9H7) (7), the enolyl complex [η5-1,2,4-(Me3C)3C5H2][η5,κ-C-1,2-(Me3C)2-4-(CH2CMe2)C5H2]Th(κ-O-1-OC6H9) (8), the alkyl complex [η5-1,2,4-(Me3C)3C5H2][η5,κ-C-1,2-(Me3C)2-4-(CH2CMe2)C5H2]Th(κ-O,C-OPMe2CH2) (9), the phosphido thiazolyl complex [η5-1,2,4-(Me3C)3C5H2]2Th(PH-2,4,6-tBu3C6H2)(C3H2NS) (10), the bis-imidazolyl complex [η5-1,2,4-(Me3C)3C5H2]2Th[2-(1-MeC3H2N2)]2 (11), and the imido complex [η5-1,2,4-(Me3C)3C5H2]2Th[double bond, length as m-dash]N(p-tolyl) (12), respectively. Several spectroscopic techniques were employed for the characterisation of the new complexes 3-11, and in addition the solid-state molecular structures of compounds 3-6, 8-9 and 11 were further confirmed by X-ray diffraction analyses.
The salt metathesis reaction of the thorium methyl chloride complex [η⁵-1,3-(Me3C)2C5H3]2Th(Cl)Me (3) with 2,4,6-(Me3C)3C6H2PHK in benzene furnishes an alkali-metal halide-bridged phosphinidiide actinide metallocene, {[η⁵-1,3-(Me3C)2C5H3]2Th(P-2,4,6-tBu3C6H2)(ClK)}2 (4), whose structure and reactivity was investigated in detail. On the basis of density functional theory (DFT) studies, the 5f orbitals in the model complex [η⁵-1,3-(Me3C)2C5H3]2Th(P-2,4,6-tBu3C6H2) (4′) contribute significantly to the bonding of the phosphinidene ThP(2,4,6-tBu3C6H2) moiety. Furthermore, compared to the related thorium imido complex, the bonds between the [η⁵-1,3-(Me3C)3C5H2]2Th²⁺ and [P-2,4,6-tBu3C6H2]²⁻ fragments are more covalent. The reactivity of compound 4 toward alkynes and a variety of heterounsaturated molecules such as nitriles, isonitriles, carbodiimides, imines, isothiocyanates, aldehydes, ketones, thiazoles, quinolines, organic azides, pyridines, and imidazoles, forming metallacycles, phospholes, imidos, metallaheterocycles, sulfidos, oxidos, pinacolates, pseudophosphinimidos, and phosphidos, was comprehensively studied. Moreover, complex 4 reacts with elemental selenium and PhSSPh, yielding selenido and sulfido compounds, respectively. DFT computations were performed to complement these experimental investigations and to provide further insights.
The synthesis, structure, and reactivity of a base-free terminal actinide phosphinidene metallocene have been comprehensively studied. The salt metathesis reaction of the thorium methyl iodide complex Cp‴2Th(I)Me (2; Cp‴ = η5-1,2,4-(Me3C)3C5H2) with Mes*PHK (Mes* = 2,4,6-(Me3C)3C6H2) in THF furnishes the first stable base-free terminal phosphinidene actinide metallocene, Cp‴2Th═PMes* (3). Density functional theory (DFT) shows that the bonds between the Cp‴2Th2+ and [PMes*]2- fragments are more covalent than those in the related thorium imido complex. While the phosphinidene complex 3 shows no reactivity toward alkynes, it reacts with a variety of heterounsaturated molecules such as CS2, isothiocyanate, nitriles, isonitriles, and organic azides, forming carbodithioates, imido complexes, metallaaziridines, and azido compounds. These experimental observations are complemented by DFT computations.
All-carbon metallacycles of the d-transition metals have received widespread attention over the past three decades because of their exceptional intrinsic reactivity. However, in recent years, significant progress has also been made in the synthesis and characterization of the actinide metallacyclopropenes, metallacyclopentadienes, and metallacyclocumulenes (metallacyclopentatrienes). Such actinide metallacycles are of interest because of their unique structural properties, their potential application in novel group transfer reactions and catalysis, as well as their ability to engage the 5f orbitals in metal-ligand bonding. This short review summarizes the latest developments in this area focusing on all-carbon actinide metallacycles, i.e., metallacyclopropenes, metallacyclopentadienes, and metallacyclocumulenes (metallacyclopentatrienes).
The isolation of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3], formally containing LnII, for all lanthanides (excluding Pm) was surprising given that +2 oxidation states are typically regarded as inaccessible for most 4f-elements. Herein, X-ray absorption near-edge spectroscopy (XANES), ground-state density functional theory (DFT), and transition dipole moment calculations are used to investigate the possibility that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds represented molecular LnII complexes. Results from the ground-state DFT calculations were supported by additional calculations that utilized complete-active-space multi-configuration approach with second-order perturbation theoretical correction (CASPT2). Through comparisons with standards, Ln(C5H4SiMe3)31- (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f6 5d0 (SmII), 4f13 5d0 (TmII), 4f14 5d0 (YbII), 4f14 5d1 (LuII), and 4d1 (YII) electronic configurations. Additionally, our results suggest that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain LnII ions, but with 4f
n
5d1 configurations (not 4f n+1 5d0). In these 4f
n
5d1 complexes, the C3h-symmetric ligand environment provides a highly shielded 5d-orbital of a' symmetry that made the 4f
n
5d1 electronic configurations lower in energy than the more typical 4f n+1 5d0 configuration.
The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [TismPrⁱBenz], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [TismPrⁱBenz]Li, [TismPrⁱBenz]MgMe, [TismPrⁱBenz]Cu and [TismPrⁱBenz]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ³-coordination, namely [κ³-TismPrⁱBenz]ZnMe, has also been obtained. Furthermore, the [TismPrⁱBenz] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [TismPrⁱBenz]Cu to [κ⁴-C4-TismPrⁱBenz*]Cu. The transannular M–C bond lengths in the atrane compounds are 0.19–0.32 Å longer than the sum of the respective covalent radii, which is consistent with a bonding description that features a formally zwitterionic component. Interestingly, computational studies demonstrate that the Cu–Catrane interactions in [TismPrⁱBenz]Cu and [κ⁴-C4-TismPrⁱBenz*]Cu are characterized by an “inverted ligand field”, in which the occupied antibonding orbitals are localized more on carbon than on copper.
In der Zeit seit 1990 entwickelte sich die Forschung auf anorganisch-chemischem Gebiet in einem zuvor nicht gekannten Maße, so dass es schwierig ist, allen Themen gerecht zu werden. In der Festkörperchemie galt das Interesse insbesondere den Hochtemperatur- und Ionenleitern, den Zintl-Phasen und den in ihnen vorgebildeten intermetallischen Clustern, der Syntheseplanung mit Hilfe der Computerchemie, der Herstellung von Metal-Organic-Frameworks (MOFs) sowie den sensorischen und katalytischen Eigenschaften von Nanopartikeln. Große Beachtung fanden weiterhin Arbeiten über ringförmige, wasserlösliche Polyoxymetallate mit bis zu 700 Gerüstatomen, die als „Nanoschwämme“ wirken können. Höhepunkte in der Nichtmetallchemie waren die Isolierung neuartiger Edelgasverbindungen, die Synthese von stabilen Molekülen mit B=B-Doppel und B≡B-Dreifachbindungen, die Isolierung von Aluminium- und Siliciumverbindungen in niederen Oxidationsstufen sowie die Darstellung aurierter Derivate der nicht-isolierbaren Teilchen , und , was den Begriff „Aurophilie“ kreierte. Erstmals gelang auch die Synthese von Metallkomplexen mit M≡Si-, M≡Ge- und M≡Sn-Dreifachbindungen und einer Carbonylverbindung des Bors.
An der 1743 gegründeten Markgräflichen Universität Erlangen wurde die anorganische Chemie erst 1952 eine eigenständige Disziplin. Der zuvor als Extraordinarius tätige Alwin Meuwsen erhielt 1958 die „Amtsbezeichnung, Rechte und Pflichten eines o. Professors“. 1962 wurde ein zweiter Lehrstuhl geschaffen und auf diesen Helmut Behrens berufen. Gemeinsam mit dem 1964 auf Meuwsen folgenden Klaus Brodersen brachte er in den nächsten drei Jahrzehnten die anorganische Chemie sehr gut voran und konnte auch den Lehrkörper bedeutend erweitern. In dieser Zeit waren die Koordinationschemie und die Festkörperchemie die dominierenden Forschungsgebiete. Die Nachfolger von Behrens und Brodersen, Dieter Sellmann und Rudi van Eldik, wandten sich vorrangig der bioanorganischen Chemie zu und versuchten, die Aktivierung kleiner Moleküle, wie z. B. O2, N2, NO und CO2, durch Übergangskomplexe zu verstehen. Neben synthetischen Arbeiten spielten dabei auch reaktionsmechanistische Untersuchungen eine große Rolle. Die Forschungsschwerpunkte der jetzt aktiven Anorganiker sind darauf gerichtet, Brücken zwischen der klassischen Koordinationschemie und der supramolekularen, bioanorganischen und Organometallchemie zu schlagen und nicht nur Verbindungen der Übergangsmetalle, sondern auch solche des Urans und der Erdalkalimetalle einzubeziehen.
New tetradentate Schiff base chloro complexes of U(IV) and Th(IV) are prepared by salt metathesis approaches. These complexes undergo clean salt metathesis with NaN3 to generate stable pseudo-trans azide compounds. The uranium dichloro complex also undergoes 2 e⁻ oxidation with excess NaNO2 to generate the uranyl derivative. All classes of complexes are characterized by single crystal X-ray diffraction and NMR spectroscopy.
Addition of potassium graphite (KC8) to a solution of (η⁵-C5Me5)2UCl2 (1) and 2,2′-bipyridine (bipy) gives the uranium bipyridyl metallocene (η⁵-C5Me5)2U(bipy) (2) in good yield. In complex 2 a bipy radical anion is coordinated to a U(III) atom, and it is therefore an ideal starting material for small-molecule activation: e.g., it serves as a synthetic equivalent for the (η⁵-C5Me5)2UII fragment on treatment with conjugated alkynes and a variety of heterounsaturated molecules such as imines, carbodiimide, organic azides, hydrazine, and azo derivatives. Alternatively, it may also react with aldehydes, ketones, nitriles, and α,β-unsaturated reagents such as p-ClPhCHO, (CH2)5CO, PhCN, and methyl methacrylate (MMA), forming the C–C bond coupling products (η⁵-C5Me5)2U[(bipy)(p-ClPhCHO)] (10), (η⁵-C5Me5)2U[(bipy){(CH2)5CO}] (11), (η⁵-C5Me5)2U[(bipy)(PhCN)] (12), (η⁵-C5Me5)2U[(bipy){CH2═C(Me)CO(OMe)] (13a), and [(η⁵-C5Me5)2U{OC(OMe)═C(Me)CH2–3-bipy}]2 (13b), respectively, in quantitative conversion. Furthermore, addition of CuI to complex 2 induces a single-electron-transfer process to form the uranium(III) iodide complex (η⁵-C5Me5)2U(I)(bipy) (14).
The uranium metallacyclocumulene, [η(5)-1,3-(Me3C)2C5H3]2U(η(4)-C4Ph2) (2) was isolated by the reduction of [η(5)-1,3-(Me3C)2C5H3]2UCl2 (1) with potassium graphite (KC8) in the presence of 1,4-diphenylbutadiyne (PhC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]CPh) in good yield. Furthermore it was fully characterized including the determination of its molecular structure; and the reactivity of 2 towards various small unsaturated organic molecules was explored. For example, while complex 2 shows no reactivity with alkynes and 2,2'-bipyridine (bipy), it reacts as a nucleophile when exposed to carbodiimides, diazabutadienes, isothiocyanates, ketones, and pyridine derivatives, leading to five-, seven- or nine-membered heterometallacycles. In contrast, treatment of complex 2 with CS2 results in C[double bond, length as m-dash]S bond cleavage and forms the binuclear complex [η(5)-1,3-(Me3C)2C5H3]2U[μ-η(4):η(3)-PhC[double bond, length as m-dash]C[double bond, length as m-dash]C(S)C(Ph)[double bond, length as m-dash]CS]U[η(5)-1,3-(Me3C)2C5H3]2 (10). Density functional theory (DFT) studies complement the experimental study.
The synthesis, electronic structure, and reactivity of a uranium metallacyclocumulene were studied. Reduction of [(η⁵-C5Me5)2UCl2] (1) with potassium graphite (KC8) in the presence of 1,4-bis(trimethylsilyl)butadiyne (Me3SiC≡C–C≡CSiMe3) forms the uranium metallacyclocumulene [(η⁵-C5Me5)2U{η⁴-C4(SiMe3)2}] (2) in good yield. Magnetic susceptibility data confirm that 2 behaves as a U(IV) complex, and density functional theory (DFT) studies indicate a substantial 5f orbital contribution to the bonding of the metallacyclopentatriene U(η⁴-C═C═C═C) moiety, leading to more covalent bonds between the [(η⁵-C5Me5)2U]²⁺ and [η⁴-C4(SiMe3)2]2– fragments than those found in the related Th(IV) compound. Consequently, very different reactivity patterns emerge; e.g., 2 can act as a synthetic equivalent for the (η⁵-C5Me5)2U(II) fragment when reacted with conjugated species such as butadiyne, bipy, and diazabutadiene derivatives. Alternatively, the [(η⁴-Me3SiC═C═C═CSiMe3)]2– moiety in 2 may react as a nucleophile when exposed to a variety of simple heterounsaturated molecules such as aldehydes, ketones, nitriles, isothiocyanates, carbodiimides, pyridines, and organic azides. DFT studies are included to complement the experimental observations.
The first crystallographically characterizable complex of Sc2+, [Sc(NR2)3]− (R=SiMe3), has been obtained by LnA3/M reactions (Ln=rare earth metal; A=anionic ligand; M=alkali metal) involving reduction of Sc(NR2)3 with K in the presence of 2.2.2-cryptand (crypt) and 18-crown-6 (18-c-6) and with Cs in the presence of crypt. Dark maroon [K(crypt)]+, [K(18-c-6)]+, and [Cs(crypt)]+ salts of the [Sc(NR2)3]− anion are formed, respectively. The formation of this oxidation state of Sc is also indicated by the eight-line EPR spectra arising from the I=7/2 45Sc nucleus. The Sc(NR2)3 reduction differs from Ln(NR2)3 reactions (Ln=Y and lanthanides) in that it occurs under N2 without formation of isolable reduced dinitrogen species. [K(18-c-6)][Sc(NR2)3] reacts with CO2 to produce an oxalate complex, {K2(18-c-6)3}{[(R2N)3Sc]2(μ-C2O4-κ1O:κ1O′′)}, and a CO2− radical anion complex, [(R2N)3Sc(μ-OCO-κ1O:κ1O′)K(18-c-6)]n.
The first crystallographically characterizable complex of Sc(2+) , [Sc(NR2 )3 ](-) (R=SiMe3 ), has been obtained by LnA3 /M reactions (Ln=rare earth metal; A=anionic ligand; M=alkali metal) involving reduction of Sc(NR2 )3 with K in the presence of 2.2.2-cryptand (crypt) and 18-crown-6 (18-c-6) and with Cs in the presence of crypt. Dark maroon [K(crypt)](+) , [K(18-c-6)](+) , and [Cs(crypt)](+) salts of the [Sc(NR2 )3 ](-) anion are formed, respectively. The formation of this oxidation state of Sc is also indicated by the eight-line EPR spectra arising from the I=7/2 (45) Sc nucleus. The Sc(NR2 )3 reduction differs from Ln(NR2 )3 reactions (Ln=Y and lanthanides) in that it occurs under N2 without formation of isolable reduced dinitrogen species. [K(18-c-6)][Sc(NR2 )3 ] reacts with CO2 to produce an oxalate complex, {K2 (18-c-6)3 }{[(R2 N)3 Sc]2 (μ-C2 O4 -κ(1) O:κ(1) O'')}, and a CO2(-) radical anion complex, [(R2 N)3 Sc(μ-OCO-κ(1) O:κ(1) O')K(18-c-6)]n .
The uranium metallacyclopropene (η(5)-C5Me5)2U[η(2)-C2(SiMe3)2] (1) reacts with various small unsaturated organic molecules. For example, replacement of bis(trimethylsilyl)acetylene occurs when complex 1 is exposed to alkynes, conjugated alkenes, nitriles and quinones. Reaction of 1 with internal phenyl(alkyl)acetylene PhC[triple bond, length as m-dash]CMe selectively yields the Cs symmetric uranium metallacyclopentadiene (η(5)-C5Me5)2U[η(2)-C(Ph)[double bond, length as m-dash]C(Me)-C(Ph)[double bond, length as m-dash]C(Me)] (6) after the loss of bis(trimethylsilyl)acetylene, while treatment of 1 with phenyl(silyl)acetylenes (PhC[triple bond, length as m-dash]CR, R = SiHMe2, SiMe3) gives the corresponding C2v symmetric isomers (η(5)-C5Me5)2U[η(2)-C(R)[double bond, length as m-dash]C(Ph)-C(Ph)[double bond, length as m-dash]C(R)] (R = SiHMe2 (7), SiMe3 (8)). Furthermore, while no deprotonation occurs between complex 1 and pyridine derivatives, cyclohexanone can be inserted into the uranium metallacyclopropene moiety of 1 to yield the five-membered, heterocyclic complex (η(5)-C5Me5)2U[OC(CH2)5(C2(SiMe3)2)] (14) in quantitative conversion. Density functional theory (DFT) studies have been performed to complement the experimental studies.
Reduction of (η5-C5Me5)2ThCl2 (1) with potassium graphite (KC8) in the presence of 2,2′-bipyridine forms the thorium bipy metallocene (η5-C5Me5)2Th(bipy) (2) in good yield. Complex 2 was fully characterized and reacts with various small molecules. For example, 2 serves as a source for the (η5-C5Me5)2Th(II) fragment when exposed to conjugated alkynes, elemental sulfur and their organic derivatives, diazabutadiene, carbodiimide, CS2, isothiocyanate, and organic azides. Furthermore, treatment of 2 with ketone Ph2CO, thio-ketone Ph2CS, imine PhCH═NPh, and nitrile PhCN results in C–C bond coupling products (η5-C5Me5)2Th[(bipy)(Ph2CO)] (10), (η5-C5Me5)2Th[(bipy)(Ph2CS)] (11), (η5-C5Me5)2Th[(bipy)(PhCHNPh)] (12), and (η5-C5Me5)2Th[(bipy)(PhCN)] (13), respectively, in quantitative conversion.
The synthesis, structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Reduction of (η(5)-C5Me5)2UCl2 (1) with potassium graphite (KC8) in the presence of bis(trimethylsilyl)acetylene (Me3SiC≡CSiMe3) allows the first stable uranium metallacyclopropene (η(5)-C5Me5)2U[η(2)-C2(SiMe3)2] (2) to be isolated. Magnetic susceptibility data confirm that 2 is a U(IV) complex, and density functional theory (DFT) studies indicate substantial 5f orbital contributions to the bonding of the metallacyclopropene U-(η(2)-C═C) moiety, leading to more covalent bonds between the (η(5)-C5Me5)2U(2+) and [η(2)-C2(SiMe3)2](2-) fragments than those in the related Th(IV) compound. Consequently, very different reactivity patterns emerge, e.g., 2 can act as a source for the (η(5)-C5Me5)2U(II) fragment when reacted with alkynes and a variety of heterounsaturated molecules such as imines, bipy, carbodiimide, organic azides, hydrazine, and azo derivatives.
Reduction of (η5-C5Me5)2ThCl2 (1) with potassium graphite (KC8) in the presence of 1,4-diphenylbutadiyne (PhC≡CC≡CPh) yields the first actinide metallacyclocumulene, the thorium metallacyclopentatriene (η5-C5Me5)2Th(η4-C4Ph2) (2). The structure and reactivity of 2 were investigated in detail; structural parameters and density functional theory (DFT) studies confirm the presence of a metallacyclopentatriene unit in 2. Furthermore, DFT computations also indicate a notable contribution of the 5f orbitals to the bonding of the metallacyclopentatriene Th-(η4-C = C = C = C) moiety. While complex 2 shows no reactivity toward alkynes, it reacts with a variety of heterounsaturated molecules such as isothiocyanates, carbodiimides, aldehydes, ketones, nitriles, pyridines, and diazoalkane derivatives. DFT studies complement the experimental observations and provide additional insights. Furthermore, in comparison to group 4 metals, the thorium metallacyclopentatriene 2 exhibits distinctively different reactivity patterns.
A family of Ag complexes of the type [{BnN(CH(2)CH(2)(R)Im)(2)}Ag]center dot[AgX2] (X = I, (R)Im = 1-methylimidazole (3a); X = Cl, (R)Im = 1-tert-butylimidazole (3b), 1-benzylimidazole (3c) or 1-methylbenzimidazole (3d)) or [{BnN(CH(2)CH(2)CH(2)(R)Im)(2)}Ag]center dot[AgCl2]((R)Im = 1-methylimidazole (4a), 1-tert-butylimidazole (4b), 1-benzylimidazole (4c) or 1-methylbenzimidazole (4d)) with a flexible unsaturated linker connecting the two NHC ligands has been prepared. These silver complexes undergo facile transmetalliation with [Ir(COD)Cl](2) to generate the bimetallic species (COD)ClIr{BnN(CH(2)CH(2)(R)Im)(2)}IrCl(COD) ((R)Im = 1-methylimidazole (5a), 1-tert-butylimidazole (5b), 1-benzylimidazole (5c) or 1-methylbenzimidazole (5d)) or (COD)ClIr{BnN(CH2 CH(2)CH(2)(R)Im)(2)}IrCl(COD) ((R)Im = 1-methylimidazole (6a), 1-tert-butylimidazole (6b), 1-benzylimidazole (6c) or 1-methylbenzimidazole (6d)) with a bridging bidentate NHC ligand. A similar reaction can be performed using [Rh(COD)Cl](2) to generate the analogous Rh species (COD) ClRh {BnN(CH(2)CH(2)CH(2)(R)Im)(2)}RhCl (COD) ((R)Im = 1-methylimidazole (7a), 1-benzylimidazole (7c) or 1-methylbenzimidazole (7d)). Complexes 5d, 6d and 7d were characterized by X-ray crystallography. All of our new Rh and Ir species possess axial chirality and are prepared as a mixture of isomers. However, crystals of 5d, 6d and 7d contain only one diastereomer. Dissolution of the diastereometrically pure crystals results in epimerization and the formation of an equilibrium mixture. Our new Ir complexes are active catalysts for both olefin and transfer hydrogenation and we present a comparison of their catalytic activity based on the linker length of the ligand.
This chapter reviews the reaction chemistry of uranium complexes with small molecules of industrial and biological importance. Specifically, carbon monoxide (CO), nitrogen monoxide (NO), dinitrogen (N2), dioxygen (O2), carbon dioxide (CO2), nitrous oxide (N2O), dihydrogen (H2), saturated hydrocarbons, unsaturated hydrocarbons (alkenes, alkynes, and arenes), and water (H2O) are covered. This chapter is limited to molecular systems, and, where appropriate, comparisons with lanthanide or transition metal systems will be made, but are by no means exhaustive. Small-molecule activation studies with uranium do allow for the correlation of molecular-electronic structure/reactivity relationships. These fundamental studies may provide the design criteria for valuable chemical processes and for the development of nuclear waste remediation technologies.
Uncatalysed 1,3-dipolar cycloaddition reactions between two phosphaalkynes, P[triple bond, length as m-dash]CR (R = Bu(t) or Me), and a series of di-, tri- and poly-azido precursor compounds have given very high yields of a range of triazaphosphole substituted systems. These comprise the 1,1'-bis(triazaphosphole)ferrocenes, [Fe{C5H4(N3PCR)}2], the tris(triazaphosphole)cyclohexane, cis-1,3,5-C6H9(N3PCBu(t))3, and the poly(allyltriazaphosphole)s, {C3H5(N3PCR)}∞. Electrochemical studies on the 1,1'-bis(triazaphosphole)ferrocenes reveal the compounds to undergo reversible 1-electron oxidation processes, at significantly more positive potentials than ferrocene itself. Attempts to chemically oxidise one 1,1'-bis(triazaphosphole)ferrocene with a silver salt, Ag[Al{OC(CF3)3}4] were not successful, and led to the formation of a silver coordination complex, [{Fe[μ-C5H4(N3PCBu(t))]2(μ-Ag)}2][Al{OC(CF3)3}4]2, thereby demonstrating the potential the reported triazaphosphole substituted systems possess as novel ligands in coordination chemistry.
The tris-palladium tripodal N-heterocyclic carbene (NHC) complexes (timtebtBu){Pd(ICy)I2}3 (4a), (timtebtBu){Pd(PPh3)I2}3 (5a), and (timtebdipp){Pd(PPh3)I2}3 (5b) (timtebtBu = 1,3,5-{tris(tert-butylimidazol-2-ylidene)methyl}-2,4,6-triethylbenzene, 3a; ICy = 1,3-dicyclohexylimidazol-2-ylidene; timtebdipp = 1,3,5-{tris({2,6-diisopropylphenyl}imidazol-2-ylidene)methyl}-2,4,6-triethylbenzene, 3b) were prepared by reaction of Pd(II) precursors with either the free carbenes or imidazolium salts. Treatment of [Cu(NCMe)4]X (X = PF6, BF4) with 3a or 3b produced the tris-copper(I) bridged complexes [(timtebR)Cu3(μ3-O)]X (R = tBu, X = PF6, 6a; R = dipp, X = BF4, 6b). Complexes 4a, 5a, and 6a were structurally characterized. The palladium complexes were tested as catalysts for Suzuki−Miyaura and Sonogashira coupling reactions and the copper species also employed for the Sonogashira reaction, as well as for the Ullmann-type arylation of imidazoles and phenols.
High yielding "click" reactions between phosphaalkynes and organo-triazides have afforded examples of tris(triazaphosphole)s, the utility of which as a new class of tripodal P(3)-ligand has been demonstrated with the preparation of an unusual diplatinum complex.
A comparative examination of the electronic interactions across a series of trimetallic actinide and mixed lanthanide-actinide and lanthanum-actinide complexes is presented. Using reduced, radical terpyridyl ligands as conduits in a bridging framework to promote intramolecular metal-metal communication, studies containing structural, electrochemical, and X-ray absorption spectroscopy are reported for (C(5)Me(5))(2)An[-N horizontal lineC(Bn)(tpy-M{C(5)Me(4)R}(2))](2) (where An = Th(IV), U(IV); Bn = CH(2)C(6)H(5); M = La(III), Sm(III), Yb(III), U(III); R = H, Me, Et) to reveal effects dependent on the identities of the metal ions and R-groups. The electrochemical results show differences in redox energetics at the peripheral "M" site between complexes and significant wave splitting of the metal- and ligand-based processes indicating substantial electronic interactions between multiple redox sites across the actinide-containing bridge. Most striking is the appearance of strong electronic coupling for the trimetallic Yb(III)-U(IV)-Yb(III), Sm(III)-U(IV)-Sm(III), and La(III)-U(IV)-La(III) complexes, [8](-), [9b](-), and [10b](-), respectively, whose calculated comproportionation constant K(c) is slightly larger than that reported for the benchmark Creutz-Taube ion. X-ray absorption studies for monometallic metallocene complexes of U(III), U(IV), and U(V) reveal small but detectable energy differences in the "white-line" feature of the uranium L(III)-edges consistent with these variations in nominal oxidation state. The sum of these data provides evidence of 5f/6d-orbital participation in bonding and electronic delocalization in these multimetallic f-element complexes. An improved, high-yielding synthesis of 4'-cyano-2,2':6',2''-terpyridine is also reported.
Catalytic dinitrogen reduction with the Schrock complex is still hampered by low turn-over numbers that are likely to result from a degradation of the chelate ligand. In this work, we investigate modifications of the original HIPTN(3)N ligand applied by Schrock and co-workers in catalytic reduction of dinitrogen with density functional methods. We focus on ligands that are substituted in the para position of the central phenyl ring of the terphenyl moieties and on a ligand where the bridging nitrogen is exchanged by phosphorus. In addition, results for tris(pyrrolyl-alpha-methyl)amine, tris(pyrrolyl-alpha-ethyl)amine, and tris[2-(3-xylyl-imidazol-2-ylidene)ethyl]amine are reported. For this study, we take into account the full ligands without approximating them by model systems. Reaction energies for the various derivatives of HIPTN(3)N are found to be similar to those of the unchanged parent system. However, the most promising results for catalysis are obtained for the [{tris[2-(3-xylyl-imidazol-2-ylidene)ethyl]amine}Mo](N(2)) complex. Feasibility of the exchange of NH(3) by N(2) is found to be the pivotal question whether a complex can become a potential catalyst or not. A structure-reactivity relationship is derived which allows for the convenient estimation of the reaction energy for the NH(3)/N(2) exchange reaction solely from the wavenumber of the N[triple bond]N stretching vibration. This relationship may guide experiments as soon as a dinitrogen Mo complex is formed.
The trivalent uranium metallocenes (MeC5H4)3U·THF and (Me3SiC5H4)3U react with CS2 to form the binuclear U(IV) complexes [(RC5H4)3U]2[μ-η 1,η2-CS2] (1, R = Me; 2, R = SiMe3). Crystals of 1 are monoclinic, P21/n, with a = 14.127 (4) Å, b = 14.182 (4) Å, c = 8.123 (2) Å, and β= 92.36 (3)° at 23°C; for Z = 2 the calculated density is 2.097 g/cm3. The structure was refined by full-matrix least-squares to a conventional R factor of 0.025, for 1402 data with F2 > 2σ(F2). The central carbon atom of the complex is disordered across a center of symmetry. The geometry about the CS2 ligand (U-S = 2.973 (3) Å and U-C = 2.53 (2) Å), as well as NMR and susceptibility data, is consistent with two full one-electron transfers into the CS2. There is no measurable magnetic interaction between the paramagnetic ions to 5 K.
The reaction of 1,1,1-tris(chlormethyl)ethane H3CC(CH2Cl)3 with Ar2PH in DMSO as the solvent using KOH/H2O as the base gives good yields of tripod ligands H3CC(CH2PAr2)3, 2. Using Ph2PH as the phosphine component, it is shown that the chloride substituents of H3CC(CH2Cl)3 are exchanged in sequence by the formation of H3CC(CH2Cl)2(CH2PPh2), 1a, and H3CC(CH2Cl)(CH2PPh2)2, 1b, respectively. The product composition is almost exclusively determined by the applied stoichiometry. The tripod ligands 2 are obtained with Ar = phenyl, 2a, 3-tolyl, 2b, 4-tolyl, 2c, 4-tert-butylphenyl, 2d, 1-naphthyl, 2f, and Ar2P dibenzophospholyl, 2e, as Ar2P groups. Their capability of facial coordination in molybdenum and iron complexes is demonstrated by the characterization of compounds of the type tripod-Mo(CO)3, 3, and [tripod-Fe(NCCH3)3](BF4)2, 4. Compounds 1-4 are characterized by the usual spectroscopic and analytical methods as well as by X-ray analysis on selected examples.
The syntheses of [RhH3(triphos)] (8, triphos = CH3C(CH2PPh2)3) and the related compound [RhH3(triphos-I)] (triphos-I = EtC(CH2PPh2)3) are described, and the X-ray crystal structure of the latter is reported. The crystals are monoclinic space group P21/a, a 15.985(6), b 19.683(7), c 11.900(4) Å, β 103.73(6)°, Z=4. The structure was solved by the heavy atom method and refined by full-matrix least-squares to the conventional R factor value of 0.057 for 2649 observed reflections. The metal atom is octahedrally coordinated by three phosphorus atoms and by three hydrogen atoms, each trans to one phosphorus.Complex 8 has been shown to react with CO to give [RhH(CO)(triphos)] (2). Compound 2 reacts with CH2=CHCO2Me to give only the branched insertion product [Rh(CH(CH3)CO2Me)(CO)(triphos)]. The complex [RhCl(CO)(triphos)] has been re-investigated and was obtained in a single isomeric form which has been assigned a five-coordinate structure. The five-coordinate compounds (RhX(CO)(triphos)] (X = Br and I) are also described.
[Ni(CO2)(PCy3)2],0·75(C7H8), where Cy = cyclohexyl, can be made either by treating [Ni(PCy3)3] or [{Ni(PCy3)2}2N2] with CO2 in toluene, or by direct reduction of [NiBr2(PCy3)2] with sodium sand under CO2; the complex is planar, the CO2 ligand possesses bent geometry and is co-ordinated through the carbon atom and one of the oxygen atoms.
Zinc, as a constituent of more than 300 enzymes, plays
essential roles in biological systems. The active sites of these enzymes
feature a zinc center attached to the protein backbone by three or four
amino acid residues, the nature of which influences the specific function
of the enzyme. In order to understand why different zinc enzymes utilize
different amino acid residues at the active site, it is necessary to
understand how, and why, the chemistry of zinc is modulated by its
coordination environment. Answers to these questions are being provided by
a study of synthetic analogues of zinc enzymes, i.e. small
molecules that resemble the enzyme active sites. This article provides an
account of those studies that have specifically used tripodal ligands to
mimic the active site protein residues in an effort to ascertain the
bioinorganic chemistry of zinc.
The reaction of [NiL4](L = PEt3 or PBun3) with CO2 in toluene affords complexes of formula [Ni(CO2)L2], via the [Ni(CO2)L3] species. The reaction of [Ni(CO2){P(C6H11)3}2]·0.75C6H5Me with O2 to give (peroxocarbonato)-bis( tricyclohexylphosphine)nickel(II) is also reported.
The trivalent uranium metallocenes (MeCâHâ)âU x THF and (MeâSiCâHâ)âU react with CSâ to form the binuclear U(IV) complexes ((RCâHâ)âU)â(..mu..-eta¹,eta²-CSâ) (1, R = Me; 2, R = SiMeâ). Crystals of 1 are monoclinic, P2â/n, with a = 14.127 (4) A, b = 14.182 (4) A, c = 8.123 (2) A, and ..beta.. = 92.36 (3)° at 23°C; for Z = 2 the calculated density is 2.097 g/cm³. The structure was refined by full-matrix least-squares to a conventional R factor of 0.025, for 1402 data with F² > 2 sigma (F²). The central carbon atom of the complex is disordered across a center of symmetry. The geometry about the CSâ ligand (U-S = 2.973 (3) A and U-C = 2.53 (2) A), as well as NMR and susceptibility data, is consistent with two full one-electron transfers into the CSâ. There is no measurable magnetic interaction between the paramagnetic ions to 5 K.
This review deals with rhodium and iridium complexes of the hydridotris(pyrazolyl)borate (Tp′) ligands. In addition to outlining the synthesis of precursor compounds, an overview of the coordination modes of the Tp′ ligands is given. Recent developments in the chemistry of some important families of compounds (carbonyls, isonitriles, classical and non-classical polyhydrides) are discussed. Particular attention is given to CH activation reactions with these compounds. Over 100 references are covered, of which approximately half stem from the last 3 years.
The X-ray crystal structure (−110°C); space group P3 1c, a = b = 16.370(2) Å, c = 8.302(1) Å, α = β = 90°, γ = 120°, V = 1926.7(1) Å3, Z = 2; shows that the geometry of U[N(SiMe3)2]3·13 cyclohexane is pyramidal, consistent with deductions made on the basis of infrared spectroscopy. The UN distance is 2.320(4) Å and the NUN angle is 116.24(7)°. The solid state magnetic susceptibility from 35–280 K gives μeff of 3.354(4) withe θ = −13 K and 3.385(4) B.M. with θ = −11 K at 5 and 40 kGauss, respectively. The effective magnetic moments are similar to that found for UF3 and other trivalent uranium metallocenes, consistent with a 5f3 electron configuration.
The bulkiness and multidenticity of the ligands stabilize the unusual trigonal-monopyramidal coordination geometry of the title complexes, which were prepared by "reductive" (Ti, V), "direct" (Cr, Fe), and "oxidative" (Mn) routes. The V complex was characterized by X-ray crystallography (structure shown here).
Migratory insertion of an anionic group onto coordinate carbon monoxide or an isocyanide is an important mechanistic postulate in organoactinide chemistry. In contrast to transition metals, where carbon monoxide complexes abound, only three examples of carbon monoxide coordination to uranium have been observed in matrix isolation studies at cyrogenic temperatures. These studies showed that U(CO)â can exist below ca. 20 K and that nu/sub CO/ of 1961 cmâ»Â¹ is similar to that found for W(CO)â, nu/sub CO/ is 1987 cmâ»Â¹ under similar conditions. The nu/sub CO/ is lowered substantially from gaseous CO (nu = 2145 cmâ»Â¹) which implies that uranium metal is a ..pi..-donor, though the bonds are either kinetically labile, thermodynamically weak, or both. In UFâ(CO) the nu/sub CO/ of 2182 cmâ»Â¹ at 20 K/sup 2c/ shows that the tetravalent compound does not engage in ..pi..-back-bonding to CO. In another study, UOâ has been shown to absorb CO at temperatures below 20 K; the CO stretching frequency was not measured. In this paper, evidence is given for (MeâSiCâHâ)âUCO, the first molecular actinide complex of carbon monoxide in solution and solid phase. 8 references.
Quantum chemical calculations at the MP2 level of theory using relativistic ECPs with large valence basis sets for the metals are reported for the complexes of CuCl, AgCl, and AuCl with the N-heterocyclic carbene imidazol-2-ylidene 1 and the related silylene 2 and germylene 3. The metal−ligand bond dissociation energies are predicted at CCSD(T). The metal−carbene bonds are very strong. The strongest bond is predicted for 1-AuCl, which has a bond strength De = 82.8 kcal/mol. Even the silylene and germylene complexes have substantial bond energies between 37.4 and 64.1 kcal/mol for 2 and between 29.9 and 49.4 kcal/mol for 3. The trend of the bond energies for the metal fragments is AuCl > CuCl > AgCl, and for the ligands it is 1 > 2 > 3. The metal−ligand bonds have a strong ionic character which comes from the Coulomb attraction between the positively charged metal atom and the σ-electron pair of the donor atom. The covalent part of the bonding shows little π-back-bonding from the metal to the ligand. The aromaticity of the N-heterocyclic ligands is slightly enhanced in the metal complexes.
[Ag(Et2-Bimy)2][AgBr2] (1; Et2-Bimy = diethylbenzimidazol-2-ylidene) was obtained readily from the reaction of [Et2-BimyH]Br with Ag2O. Compound 2, [Ag(Et2-Bimy)2]PF6, was prepared by the reaction of [Et2-Bimy]PF6 with Ag2O under basic phase transfer catalysis conditions. Both compounds 1 and 2 are good carbene transfer agents. Thus, Pd(Et2-Bimy)2Cl2, Au(Et2-Bimy)Br, and [Au(Et2-Bimy)2]PF6 were obtained in high yields using 1 and 2 as carbene sources. The byproduct AgBr or AgCl can be reused to generate 1 under basic phase transfer catalysis conditions. The crystal structure of compound 1 revealed that linear [Ag(Et2-Bimy)2]+ and [AgBr2]- groups were associated through a short AgI−AgI contact (2.956 Å). FAB/mass spectrometry and molar conductivity measurements indicate the existence of ligand-unsupported AgI−AgI interactions in the gas phase and acetonitrile solution. 13C NMR studies suggest that the two Bimy ligands in 1 are fluxional in solution.
The nature of the chemical bond in mixed carbene−halogen complexes (1)TMX (X = F−I) and bis(carbene) complexes (1)2TM+ of group 11 metals (TM = Cu, Ag, Au) with imidazol-1-ylidene (1) as ligand has been investigated at the BP86 level of theory using an energy decomposition analysis (EDA). The metal−carbene bonds are mainly held together by classical electrostatic attraction, which contributes >65% of the binding interactions. The metal−carbene bonds are very strong. In the bis(carbene) complexes, the N-heterocyclic carbene ligand 1 is bonded even more strongly than in the mixed carbene−halogen complexes. In the bis(carbene) complexes, orbital interactions are slightly more important than in the mixed carbene−halogen complexes but the covalent contribution is always <35% of the total attractive interaction. The orbital interaction part of the bonding has only 20% π back-bonding. The calculated data are not very different from previous EDA results for the Fischer carbene complex (CO)5W−C(OH)2. The EDA results suggest that R2C←MLn π back-donation in complexes with N-heterocyclic carbenes is not substantially smaller than in classical Fischer carbene complexes bearing two π donor groups R.
Density functional calculations rationalize the bonding in [Rh2(μ-O2CR)4L] complexes with strong and very weak axial donor−acceptor ligands L such as “Arduengo” carbenes and aromatic hydrocarbons.
A reaction of [Ru(bpy)2(CO)2](PF6)2 with 2 equiv of Bu4NOH in H2O/EtOH (1:1 v/v) affords an eta(1)-CO2 complex, [Ru(bpy)2(CO)(COO)].3H2O. An addition of an aqueous HCl solution to a MeOH solution of [Ru(bpy)2-(CO)(COO)].3H2O quantitatively regenerates [Ru(bpy)2-(CO)2]2+.
The syntheses and characterizations of homoleptic bis(carbene) adducts of Ag(I) and Cu(I) are described. These carbene adducts are available directly from the reaction of the stable nucleophilic carbene 1,3-dimesitylimidazol-2-ylidene and the corresponding metal triflate. NMR data are consistent with the bis(carbene)metal structures and suggest a level of delocalization in the imidazole ring that is intermediate between those of the free carbene and imidazolium ions. The X-ray crystal structure of the silver-carbene adduct is reported: [C42H48N4Ag]CF3SO3, monoclinic, space group P21/c (No. 14), a = 1167.5(1) pm, b = 1472.9(2) pm, c = 2479.8(2) pm, β = 95.31(1)°; T = -70°C, Z = 4. The silver is essentially linearly coordinated with the imidazole rings twisted 39.7° relative to one another. The solid-state structure of the silver-carbene adduct is compared with those of related gold-carbene adducts.
The tetradentate "tripod-like" ligand tris(2-diphenylphosphinoethyl)amine (NP3), which has donor atoms that tend to give both low- and high-spin five-coordinate complexes, forms complexes with nickel(II) and cobalt(II) salts of the general formula [M(NP3)X]Y (where X = Cl, Br, I, NCS; Y = X or BPh4). All of the complexes are five-coordinate, probably with a trigonal-bipyramidal structure. The nickel complexes are diamagnetic. The cobalt ones are high spin when X is Cl or Br and low spin when X is NCS. When the set of donor atoms is NP3I, a high-spin complex is obtained when Y is BPh4 and a low-spin complex is formed when Y is I. The set NP3I appears to represent the crossover point between high- and low-spin species. In fact, both these high- and low-spin species are present together in solution in inert solvents.
The synthesis of homolytic uranium (III) alkoxide complexes was undertaken because it was felt that these complexes would be valuable starting materials for further investigations of nonaqueous uranium(III) chemistry and for uranium alkoxide cluster syntheses via compropoetionation reactions with higher oxidation state uranium alkoxides and oxo-alkoxides. The successful synthesis of two uranium(III) aryloxide complexes are described herein. Both compounds were found to be ether- and hydrocarbon-soluble and very air-sensitive. The results of the ¹H NMR spectra of the complexes are reported. 17 references, 2 figures.
We wish to report that the duodecamethyl metallocene (C[sub 5]Me[sub 4]H)[sub 3]U (2) provides an electronic and steric environment that enables the synthesis and structural characterization by X-ray methods of the first isolable carbonyl complex of an actinide element, (C[sub 5]Me[sub 4]H)U(CO). Results of our study has demonstrated that, under appropriate conditions and with a suitable choice of coligands, uranium carbonyl complexes can be isolated. Our results question the current view of actinide carbonyls as unstable or unisolable entities. Preliminary results indicate that the electron-rich metal center in 2 may give rise to a correspondingly rich and diverse chemistry, an area which we are now exploring. 15 refs., 1 fig.
The monomeric terminal imido complexes Cp*IrNR (Cp* = eta-5-C5Me5; 1a, R = t-Bu; 1b, R = SiMe2t-Bu; 1c, R = 2,6-Me2C6H3; 1d, R = 2,6-i-Pr2C6H3) were prepared from [Cp*IrCl2]2 (2) and 4 equiv of the corresponding lithium amide LiNHR in THF. In addition, the complexes Cp*Ir(RNH2)Cl2 (3a, R = t-Bu; 3d, R = 2,6-i-Pr2C6H3) were made from an amine and [Cp*IrCl2]2 (2) and dehydrochlorinated with KN(SiMe3)2 to provide an alternate route to 1a,d. Efficient exchange occurred between 1a and arylamines, leading to 1c,d and tert-butylamine. tert-Butylimido complex 1a, a weak nucleophile, reacted with MeI to form [Cp*IrI2]2 and Me3Nt-Bu+I-. Coupling of the imido ligand in 1a with CNt-Bu and CO gave Cp*Ir(t-BuNCNt-Bu)(CNt-Bu) (4) and Cp*Ir(t-BuNCO)(CO) (5a), respectively. Cp*IrPPh3(t-BuNCO) (5b) was formed from 1a, PPh3, and CO. The bridging imido complex Cp*IrNt-Bu(dppePt) (6, dppe = 1,2-bis(diphenylphosphino)ethane) was prepared from 1a and dppePt(C2H4). Complex 1a and CO2 gave the cycloadduct Cp*Ir(Nt-BuOCO) (7a), which added PPh3 to form Cp*IrPPh3(Nt-BuOCO) (7b). Two equivalents of dimethylacetylenedicarboxylate reacted with 1a to yield the pyrrole complex Cp(Ir(eta-4-RCCRNt-BuRCCR) (8, R = CO2Me). Maleic anhydride was added to 1a to give Cp*Ir[Nt-BuC(O)CH=CHCO2] (9a), which reacted with CO to yield Cp*Ir(CO)[Nt-BuC(O)CH=CHCO2] (9b). Compounds 1a-d, 7a, and 8 were structurally characterized by X-ray diffraction; imido complexes 1a-d have short Ir-N distances and nearly linear Ir-N-C(Si) angles, consistent with the presence of a metal-nitrogen triple bond.
The potentially tetradentate tripod ligand tris(2-diphenylphosphinoethyl)phosphine, pp3, reacts with cobalt(II) salts in the presence of sodium borohydride to give low-spin five-coordinate cobalt(I) complexes [CoX(pp3)], where X = halide, thiocyanate, and hydride. With nickel(II) salts five-coordinate hydrido complexes of the general formula [NiH(pp3)]Y (Y = iodide, tetrafluoroborate, tetraphenylborate) are formed. The cobalt-hydrido complex [CoH(pp3)]·1/2(CH3)2CO has been characterized by an X-ray structure analysis (trigonal with hexagonal dimensions a = 13.573 (3) Å, c = 36.404 (8) Å, Z = 6, space group R3) and has been found to have a trigonal-bipyramidal geometry with C3v symmetry. On the basis mainly of the electronic spectra the same geometry must be attributed to all of the cobalt(I) complexes.
The trivalent uranium metallocene (MeC5H4)3U·THF reacts with COS, SPPh3, SePPh3, or TeP(n-Bu)3 to form the bridging chalcogenide complexes [(MeC5H4)3U]2E, where E is S, Se, or Te. Crystals of [(MeC5H4)3U]2S are monoclinic, P21/c, with a = 19.740 (6) Å, b = 8.302 (3) Å, c = 21.602 (4) Å, and β= 97.28 (3)° at 23°C; for Z = 4 the calculated density is 1.920 g/cm3. The structure was refined by full-matrix least squares to a conventional R factor of 0.053 by using 2061 data with F2 > 2σ(F2). The average U-C distance is 2.77 ± 0.06 Å, the U-S distance is 2.60 (1) Å, and the U-S-U angle is 164.9 (5)°. Triphenyl-phosphine oxide does not behave in a similar fashion; the trivalent uranium coordination complex (MeC5H4)3UOPPh3 is instead isolated. Crystals of (MeC5H4)3UOPPh3 are monoclinic, P21/n, with a = 16.268 (3) Å, b = 17.948 (3) Å, c = 10.900 (2) Å, and β = 105.01 (2)° at 23°C; for Z = 4 the calculated density is 1.628 g/cm3. The structure was refined by full-matrix least squares to a conventional R factor of 0.028 [2427 data, F2 > 2σ(F2)]. The average U-C distance is 2.82 ± 0.04 Å, the U-O distance is 2.389 (6) Å, and the U-O-P angle is 162.8 (4)°.
The complexes BpCu (1) and Tp*Cu (2) catalyse the transformation of styrene into styrene oxide using Oxone as the oxidising agent; the use of silica gel-supported 1 or 2 as heterogeneous catalysts affords similar results, using water as the sole solvent.
Recently (1) the rate data for the generalized nucleophilic displacement reaction were reviewed.
Rarity value is something that main group complexes of N-heterocyclic carbenes still have. With a triscarbene chelating carbon analogue of Trofimenko's tris(pyrazolyl)borate the solvent free, dinuclear lithium complex 1 was successfully synthesized, in which the lithium atoms are only bound to carbene carbon atoms.
Treatment of the tripod compounds tripodCoCl2 (1), and tripodCoCl (2), [tripod = CH3C(CH2PPh2)3] in THF solution under argon atmosphere with strong reducing agents such as KC8 leads to the generation of reactive species. While the dinitrogen compound tripodCo(N2)Cotripod (3), which is formed under N2 atmosphere, is rather unreactive, the species formed under argon atmosphere undergo selective reactions with compounds containing P−H or Sn−H functions. With PhPH2 as the reagent, the diphosphene compound tripodCo(η2-PhP=PPh) (8), is formed in a yield (44%) similar to that achieved in the preparation of 8 from 2 and PhPHNa (60%). With Ph3SnH as the reagent, tripodCoSnPh3 (9), is obtained (yield 61%), while reaction with Bu3SnH produces tripodCo(H)2SnBu3 (10a, 50%). The CoI species 9 undergoes oxidative addition of dihydrogen to produce the CoIII compound tripodCo(H)2SnPh3 (10b), which is an analogue of 10a. The properties of these new compounds are characterized by the usual analytical techniques, including NMR spectroscopy, cyclic voltammetry, and X-ray analysis.
IntroductionComplexes of 2-Monohaptopyrazoles (Rpz*)Pyrazole Ligands with Additional Coordination SitesGeminal Poly(I-Pyrazolyl) Compounds RnE(pz*)mComplexes Involving E(pz*)nE BridgesCompounds RnE(pz*)Complexes Containing NO M–pz* BondLigands and MiscellanyConcluding Remarks
A stable multidentate carbene ligand containing three imidazol-2-ylidenes was synthesized and characterized. The spectroscopic data suggest that there is no intramolecular interaction between the neighboring carbene sites.
The physical organic chemistry and spectroscopic features of the ligand phenoxyl are reviewed. Some well-characterized metalloproteins known to contain tyrosyl radicals are briefly introduced and then the coordination chemistry of uncoordinated and coordinated phenoxyls is systematically described. The chapter concludes with a description of the reactivity of coordinated phenoxyls toward some organic substrates.
The new tris-carbene ligand [TIMEt-Bu] has been synthesized and fully characterized. Reaction of the free carbene with copper(I) salt provided an unprecedented dinuclear Cu(I)-Cu(I) complex, in which the cuprous ion is coordinated in a trigonal planar ligand environment of three different carbon ligators. Interestingly, one of the three chelating carbon atoms can be formed only via C-H activation of the unsatd. imidazole backbone of the carbene chelator. Accordingly, the title complex is described as a bis-carbenealkenyl copper(I) complex. The crystal structure of the prepd. complexes were detd. [on SciFinder (R)]
Deacetoxycephalosporin C synthase (DAOCS) is an iron(II) and 2-oxoglutarate-dependent oxygenase that catalyzes the conversion of penicillin N to deacetoxycephalosporin C, the committed step in the biosynthesis of cephalosporin antibiotics. The crystal structure of DAOCS revealed that the C terminus of one molecule is inserted into the active site of its neighbor in a cyclical fashion within a trimeric unit. This arrangement has hindered the generation of crystalline enzyme-substrate complexes. Therefore, we constructed a series of DAOCS mutants with modified C termini. Oxidation of 2-oxoglutarate was significantly uncoupled from oxidation of the penicillin substrate in certain truncated mutants. The extent of uncoupling varied with the number of residues deleted and the penicillin substrate used. Crystal structures were determined for the DeltaR306 mutant complexed with iron(II) and 2-oxoglutarate (to 2.10 A) and the DeltaR306A mutant complexed with iron(II), succinate and unhydrated carbon dioxide (to 1.96 A). The latter may mimic a product complex, and supports proposals for a metal-bound CO(2) intermediate during catalysis.
The possibility of recycling COâ from industrial emissions and of removing some of this greenhouse gas, an environmental pollutant, is receiving increased attention. Also, the possibility of using COâ as the starting material for the synthesis of fine chemicals provides an attractive alternative to compounds presently derived from petroleum. Efforts to convert COâ to useful chemicals will inevitably center on transition metal catalysts. Furthermore, efforts to enhance the yield of hydrogen in water gas shift reactions are also centered on carbon dioxide interactions with transition metal catalysts. For all these reasons, a broader understanding of the organometallic chemistry of COâ is being sought. In this article, only those compounds that can be clearly identified as having carbon dioxide bound to metal centers through carbon will be considered. Thus the discussions will not include metal formate complexes whose chemistry has been reviewed recently. Also, the discussions will not include metallocarboxylic acids or metallocarboxylate esters except where these compounds have been used as reagents for the synthesis of COâ complexes or result from reactions of these compounds. 111 refs.
We have synthesized a triamidoamine ligand ([(RNCH2CH2)3N]3-) in which R is 3,5-(2,4,6-i-Pr3C6H2)2C6H3 (HexaIsoPropylTerphenyl or HIPT). The reaction between MoCl4(THF)2 and H3[HIPTN3N] in THF followed by 3.1 equiv of LiN(SiMe3)2 led to formation of orange [HIPTN3N]MoCl. Reduction of [HIPTN3N]MoCl with magnesium in THF under dinitrogen led to formation of salts that contain the {[HIPTN3N]Mo(N2)}- ion. The {[HIPTN3N]Mo(N2)}- ion can be oxidized by zinc chloride to give [HIPTN3N]Mo(N2) or protonated to give [HIPTN3N]Mo-N=N-H. Other relevant compounds that have been prepared include {[HIPTN3N]Mo-N=NH2}+, [HIPTN3N]MoN, {[HIPTN3N]Mo=NH}+, and {[HIPTN3N]Mo(NH3)}+. (The anion is usually {B(3,5-(CF3)2C6H3)4}- = {BAr'4}-.) Reduction of [HIPTN3N]Mo(N2) with CoCp2 in the presence of {2,6-lutidinium}BAr'4 in benzene leads to formation of ammonia and {[HIPTN3N]Mo(NH3)}+. Preliminary X-ray studies suggest that the HIPT substituent creates a deep, three-fold symmetric cavity that protects a variety of dinitrogen reduction products against bimolecular decomposition reactions, while at the same time the metal is left relatively open toward reactions near the equatorial amido ligands.
We show that imidazolium salts do not always give normal or even aromatic carbenes on metalation, and the chemistry of these ligands can be much more complicated than previously thought. N,N'-disubstituted imidazolium salts of type [(2-py)(CH(2))(n)(C(3)H(3)N(2))R]BF(4) react with IrH(5)(PPh(3))(2) to give N,C-chelated products (n = 0, 1; 2-py = 2-pyridyl; C(3)H(3)N(2) = imidazolium; R = mesityl, n-butyl, i-propyl, methyl). Depending on the circumstances, three types of kinetic products can be formed: in one, the imidazole metalation site is the normal C2 as expected; in another, the metalation occurs at the abnormal C4 site; and in the third, C4 metalation is accompanied by hydrogenation of the imidazolium ring. The bonding mode is confirmed by structural studies, and spectroscopic criteria can also distinguish the cases. Initial hydrogen transfer can take place from the metal to the carbene to give the imidazolium ring hydrogenation product, as shown by isotope labeling; this hydrogen transfer proves reversible on reflux when the abnormal aromatic carbene is obtained as final product. Care may therefore be needed in the future in verifying the structure(s) formed in cases where a catalyst is generated in situ from imidazolium salt and metal precursor.
2-Pyridylmethylimidazolium salts and IrH5(PPh3)2 give an [(N-C)IrH2(PPh3)2]+ species with the imidazole ring bound in the 'wrong way': at C-5, not at the expected C-2.
We have prepared a series of divalent cobalt(II) complexes supported by the [PhBP(3)] ligand ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)) to probe certain structural and electronic phenomena that arise from this strong field, anionic tris(phosphine) donor ligand. The solid-state structure of the complex [PhBP(3)]CoI (1), accompanied by SQUID, EPR, and optical data, indicates that it is a pseudotetrahedral cobalt(II) species with a doublet ground state-the first of its type. To our knowledge, all previous examples of 4-coordinate cobalt(II) complexes with doublet ground states have adopted square planar structure types. Complex 1 provided a useful precursor to the corresponding bromide and chloride complexes, ([PhBP(3)]Co(mu-Br))(2), (2), and ([PhBP(3)]Co(mu-Cl))(2), (3). These complexes were similarly characterized and shown to be dimeric in the solid-state. In solution, however, the monomeric low spin form of 2 and 3 dominates at 25 degrees C. There is spectroscopic evidence for a temperature-dependent monomer/dimer equilibrium in solution for complex 3. Furthermore, the dimers 2 and 3 did not display appreciable antiferromagnetic coupling that is typical of halide and oxo-bridged copper(II) and cobalt(II) dimers. Rather, the EPR and SQUID data for solid samples of 2 and 3 suggest that they have triplet ground states. Complexes 1, 2, and 3 are extremely oxygen sensitive. Thus, stoichiometric oxidation of 1 by dioxygen produced the 4-coordinate, high spin complex [PhB(CH(2)P(O)Ph(2))(2)(CH(2)PPh(2))]CoI, (4), in which the [PhBP(3)] ligand had undergone a 4-electron oxidation. Reaction of 1 with TlOAr (Ar = 2,6-Me(2)Ph) afforded an example of a 4-coordinate, high spin complex, [PhBP(3)]Co(O-2,6-Me(2)Ph) (5), with an intact [PhBP(3)] ligand. The latter two complexes were spectroscopically and structurally characterized for comparison to complexes 1, 2, and 3. Our data for these complexes collectively suggest that the [PhBP(3)] ligand provides an unusually strong ligand-field to these divalent cobalt complexes that is chemically distinct from typical tris(phosphine) donor ligand sets, and distinct from tridentate borato ligands that have been previously studied. Coupling this strong ligand-field with a pronounced axial distortion away from tetrahedral symmetry, a geometric consequence that is enforced by the [PhBP(3)] ligand, provides access to monomeric [PhBP(3)]CoX complexes with doublet rather than quartet ground states.
The complexes TpxCu (Tpx = homoscorpionate) catalyse the insertion of diazo compounds into nitrogen-hydrogen bonds of amines and amides, under very mild conditions, with quantitative yields being obtained with equimolar ratios of reactants.
Reaction of 1,3-dimesitylimidazol-2-ylidene and trichloro-oxo-vanadium(V) yields an air stable 1:1 adduct, which demonstrates the utility of N-heterocyclic carbenes to stabilize metal complexes in high oxidation states. The molecular structure of this compound reveals that the chloride ligands cis to the carbene are oriented toward the Ccarbene atom. Density functional theory calculations show that a bonding interaction occurs between lone pairs of these chlorides and the formally unoccupied p-orbital of the carbene. Previous studies indicated that this orbital was not involved in the bonding of N-heterocyclic carbenes to transition metals. The observed interaction therefore represents a new bonding mode for these widely used ligands.