Mark R. Crimmin's research while affiliated with Imperial College London and other places

Publications (99)

In the presence of a catalytic quantity of a palladium bis(phosphine) complex, a reagent containing a Mg-Mg bond effects the C-H functionalisation of benzene. The resulting 100% atom efficient transformation generates an unprecedented aryl magnesium hydride.
The reaction of [Ru(H)2(N2)2(PCy3)2] (1) with β-diketiminate stabilized hydrides of Al, Zn, and Mg generates a series of new heterobimetallic complexes with either H2 or N2 ligated to the ruthenium center. Changing the main-group fragment of the M·Ru-N2 (M = Al, Zn, Mg) complexes can subtly alter the degree of binding, and therefore activation, of the diatomic ligand, as evidenced by the νN≡N absorptions in the infrared data. Experimental and computational data rationalize this tunable binding; decreasing the electronegativity of the main group in the order Al > Zn > Mg infers greater ionic character of these M·Ru-N2 complexes, and this in turn results in greater destabilization of the frontier molecular orbitals of ruthenium and therefore greater Ru(4d) → π*(N2) back-donation.
We report the first catalytic methods for the transformation of C–H bonds of unactivated arenes into C–Al bonds. The catalytic reactions occur at 25 °C (benzene, toluene and xylenes) with palladium loadings as low as 0.1 mol%. Remarkably, the C–H activation of toluene and xylenes proceeds with ortho- and meta-selectivity. This selectivity is highly unusual and complementary to both Friedel-Crafts and the majority of C–H borylation methods. Through a detailed mechanistic analysis (Eyring analysis, KIE, DFT, QTAIM) we show that unusual Pd–Al intermetallic complexes are on the catalytic cycle and that the selectivity is determined by weak attractive dispersion forces in the transition state for C–H bond breaking.
The preparation and structural characterisation of three new heterobimetallic hydride complexes containing 3-centre,2-electron W–H–Cu bonds is reported. These complexes have been characterised by single crystal X-ray crystallography and multinuclear NMR...
A series of industrially relevant fluoroalkenes react with a monomeric Al(I) complex. These reactions break either strong sp2 or sp3 C‐F bonds and result in the formation of a diverse array of organoaluminium compounds. Mechanistic studies show that two mechanisms are likely in operation: Oxidative addition of the C‐F bond to Al(I) occurs with retention of alkene stereochemistry, while stepwise formation and decomposition of a metallocyclopropane intermediate occurs with inversion of alkene stereochemistry. As part of this mechanistic analysis, we have isolated the first aluminium metallocyclopropane complex from oxidative addition of an alkene to Al(I). Remarkably this reaction is reversible and reductive elimination of the alkene occurs at higher temperature reforming Al(I). Furthermore, we show that, in selected cases, the organoaluminium products are susceptible toward beta‐fluoride elimination yielding a double C‐F activation pathway.
The reaction of a series of M·Zr heterobimetallic hydride complexes with dienes and alkynes has been investigated (M = Al, Zn, and Mg). Reaction of M·Zr with 1,5-cyclooctadiene led to diene isomerization to 1,3-cyclooctadiene, but for M = Zn also result in an on-metal diene-to-alkyne isomerization. The resulting cyclooctyne fragment is trapped between Zr and Zn metals in a heterobimetallic species that does not form for M = Mg or Al. The scope of diene isomerization and alkyne trapping has been explored leading to the isolation of three new heterobimetallic slipped metallocyclopropene complexes. The mechanism of diene-to-alkyne isomerization was investigated through kinetics. While the reaction is first-order in Zn·Zr at high diene concentration and proceeds with ΔH‡ = +33.6 ± 0.7 kcal mol–1, ΔS‡ = +23.2 ± 1.7 cal mol–1 K–1, and ΔG⧧298 K = +26.7 ± 1.2 kcal mol–1, the rate is dependent on the nature of the diene. The positive activation entropy is suggestive of involvement of a dissociative step. On the basis of DFT calculations, a heterobimetallic rebound mechanism for diene-to-alkyne isomerization has been proposed. This mechanism explains the origin of heterobimetallic control over selectivity: Mg---Zr complexes are too strongly bound to generate reactive fragments, while Al---Zr complexes are too weakly bound to compensate for the contrathermodynamic isomerization process. Zn---Zr complexes have favorable energetics for both dissociation and trapping steps.
A series of industrially relevant fluoroalkenes react with a monomeric Al(I) complex. These reactions break either strong sp2 or sp3 C‐F bonds and result in the formation of a diverse array of organoaluminium compounds. Mechanistic studies show that two mechanisms are likely in operation: Oxidative addition of the C‐F bond to Al(I) occurs with retention of alkene stereochemistry, while stepwise formation and decomposition of a metallocyclopropane intermediate occurs with inversion of alkene stereochemistry. As part of this mechanistic analysis, we have isolated the first aluminium metallocyclopropane complex from oxidative addition of an alkene to Al(I). Remarkably this reaction is reversible and reductive elimination of the alkene occurs at higher temperature reforming Al(I). Furthermore, we show that, in selected cases, the organoaluminium products are susceptible toward beta‐fluoride elimination yielding a double C‐F activation pathway.
Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to Mg-Mg bonds has been determined as a concerted SNAr-like pathway in which one Mg centre acts as a nucleophile and the other an electrophile. The experimentally determined Gibbs activation energy for the addition of C6F6 to a Mg-Mg bond of a molecular complex, ΔG‡298 K(experiment) = 21.3 kcal mol-1 is modelled by DFT with the ωB97X functional, ΔG‡298 K(DFT) = 25.7 kcal mol-1. The transition state for C-F activation involves a polarisation of the Mg-Mg bond and significant negative charge localisation on the fluoroarene moiety. This transition state is augmented by stabilising closed-shell Mg···F ortho interactions that, in combination with the known trends in C-F and C-M bond strengths in fluoroarenes, provide an explanation for the experimentally determined preference for C-F bond activation to occur at sites flanked by ortho-fluorine atoms. The effect of modification of both the ligand coordination sphere and the nature and polarity of the M-M bond (M = Mg, Zn, Al) on C-F activation has been investigated. A series of highly novel β-diketiminate stabilised complexes containing Zn-Mg, Zn-Zn-Zn, Zn-Al and Mg-Al bonds has been prepared, including the first crystallographic characterisation of a Mg-Al bond. Reactions of these new M-M containing complexes with perfluoroarenes were conducted and modelled by DFT. C-F bond activation is dictated by the steric accessibility, and not the polarity, of the M-M bond. The more open coordination complexes lead to enhanced Mg···F ortho interactions which in turn lower the energy of the transition states for C-F bond activation.
A convergent, nine-step (LLS), enantioselective synthesis of α-cyclopiazonic acid and related natural products is reported. The route features (a) an enantioselective aziridination of an imine with a chiral sulfur ylide; (b) a bioinspired (3+2)-cycloaddition of the aziridine onto an alkene; and (c) installation of the acetyltetramic acid by an unprecedented tandem carbonylative lactamization/N-O cleavage of a bromoisoxazole.
A convergent, nine-step (LLS), enantioselective synthesis of α-cyclopiazonic acid and related natural products is reported. The route features (a) an enantioselective aziridination of an imine with a chiral sulfur ylide; (b) a bioinspired (3+2)-cycloaddition of the aziridine onto an alkene; and (c) installation of the acetyltetramic acid by an unprecedented tandem carbonylative lactamization/N-O cleavage of a bromoisoxazole.
A series of heterobimetallic complexes containing 3-center,2-electron Au-H-Cu bonds have been prepared from addition of a parent gold hydride to a bent d10 copper(I) fragment. These highly unusual heterobimetallic complexes represent a missing link in the widely investigated series of neutral and cationic coinage metal hydride complexes containing Cu-H-Cu and M-H-M+ moieties (M = Cu, Ag). The well-defined heterobimetallic hydride complexes act as pre-catalysts for the conversion of CO2 to HCO2Bpin using HBpin as a reductant. The selectivity of the heterobimetallic complexes for the catalytic production of a formate equivalent surpasses that of the parent monomeric group 11 complexes
A series of heterobimetallic complexes containing 3-center,2-electron Au-H-Cu bonds have been prepared from addition of a parent gold hydride to a bent d10 copper(I) fragment. These highly unusual heterobimetallic complexes represent a missing link in the widely investigated series of neutral and cationic coinage metal hydride complexes containing Cu-H-Cu and M-H-M+ moieties (M = Cu, Ag). The well-defined heterobimetallic hydride complexes act as pre-catalysts for the conversion of CO2 to HCO2Bpin using HBpin as a reductant. The selectivity of the heterobimetallic complexes for the catalytic production of a formate equivalent surpasses that of the parent monomeric group 11 complexes
The isolable ruthenium(II) bis(dinitrogen) complex [Ru(H)2(N2)2(PCy3)2] (1) reacts with aryl ethers (Ar–OR, R = Me and Ar) containing a ketone directing group to effect sp²C–O bond activation at temperatures below 40 °C. DFT studies support a low-energy Ru(II)/Ru(IV) pathway for C–O bond activation: oxidative addition of the C–O bond to Ru(II) occurs in an asynchronous manner with Ru–C bond formation preceding C–O bond breaking. Alternative pathways based on a Ru(0)/Ru(II) couple are competitive but less accessible due to the high energy of the Ru(0) precursors. Both experimentally and by DFT calculations, sp²C–H bond activation is shown to be more facile than sp²C–O bond activation. The kinetic preference for C–H bond activation over C–O activation is attributed to unfavorable approach of the C–O bond toward the metal in the selectivity determining step of the reaction pathway.
Through serendipitous discovery, a palladium bis(phosphine) complex was identified as a catalyst for the selective transformation of sp2C-F and sp2C-H bonds of fluoroarenes and heteroarenes to sp2C-Al bonds (19 examples, 1 mol% Pd loading). The carbon-fluorine bond functionalization reaction is highly selective for the formation of organoaluminum products in preference to hydrodefluorination products (selectivity = 4.4:1 to 27:1). Evidence is presented for a two step mechanism in which hydrodefluorination is followed by sp2C-H alumination.
Through serendipitous discovery, a palladium bis(phosphine) complex was identified as a catalyst for the selective transformation of sp2C-F and sp2C-H bonds of fluoroarenes and heteroarenes to sp2C-Al bonds (19 examples, 1 mol% Pd loading). The carbon-fluorine bond functionalization reaction is highly selective for the formation of organoaluminum products in preference to hydrodefluorination products (selectivity = 4.4:1 to 27:1). Evidence is presented for a two step mechanism in which hydrodefluorination is followed by sp2C-H alumination.
The first bis(sigma-zincane) complexes, heterotrimetallic species [M(CO)4(2-HZnBDI)2], have been prepared (BDI = 2-{2,6-i-Pr2C6H3NCMe}2CH). For M = Cr a single stereoisomer is observed in solution and the solid-state. For M = Mo and W, cis and trans isomers where found to reversibly interconvert at 297 K. Despite the huge steric demands of the ligand on zinc, in all cases the cis isomer was found to be the most thermodynamically stable. The activation parameters for the isomerization when M = Mo are H‡ = 20.8 kcal mol-1, S‡ = -12.8 cal K-1 mol-1. In combination with DFT calculations, the negative activation entropy suggests an intramolecular rotation mechanism for isomerization.
A series of copper(I) complexes bearing electron-deficient β-diketiminate ligands have been prepared. The study includes [{{ArNC(CR3)}2CH}Cu(η(2)-toluene)n] (Ar = Mes, R = F, n = 0.5, [12·tol]; Ar = C6F5, R = Me, n = 1, [2·tol]; Ar = 2,6-Cl2C6H3, R = H, n = 0.5, [32·tol]). Reactions of [1-3n·tol] with boranes, alanes, a zinc hydride, a magnesium hydride, and a calcium hydride generate the corresponding σ complexes ([1-3·B], [3·B'], [3·Al], [3·Al'], [1-3·Zn], [1·Mg], and [1·Ca]). These species all form reversibly, being in equilibrium with the arene solvates in solution. With the exception of the calcium complex, the complexes have all been characterized by single-crystal X-ray diffraction studies. In solution, the σ-hydride of the aluminum, zinc, magnesium, and calcium derivatives resonates between -0.12 and -1.77 ppm (C6D6 or toluene-d8, 193-298 K). For the σ-borane complexes, the hydrides are observed as a single resonance between 2 and 3.5 ppm (C6D6, 298 K) and bridging and terminal hydrides rapidly exchange on the NMR time scale even at 193 K. Quantification of the solution dynamics by van't Hoff analysis yields expectedly small values of ΔH° and negative values of ΔS° consistent with weak binding and a reversible process that does not involve aggregation of the copper species. The donor-acceptor complexes can be rationalized in terms of the Dewar-Chatt-Duncanson model. Density functional theory calculations show that the donation of σ-M-H (or E-H) electrons into the 4s-based orbital (LUMO or LUMO+1) of the copper fragment is accompanied by weak back-donation from a dxz-based orbital (HOMO or HOMO-1) into the σ*-M-H (or E-H) orbital.
Fluorobenzenes, in particular fluorobenzene (FB) and 1,2-difluorobenzene (1,2-DiFB), are increasingly becoming recognised as versatile solvents for conducting organometallic chemistry and transition-metal-based catalysis. The presence of fluorine substituents reduces the ability to donate π-electron density from the arene and consequently fluorobenzenes generally bind weakly to metal centres, allowing them to be used as essentially non-coordinating solvents or as readily displaced ligands. In this context, examples of well-defined complexes of fluorobenzenes are discussed, including trends in binding strength with increasing fluorination and different substitution patterns. Compared to more highly fluorinated benzenes, FB and 1,2-DiFB typically demonstrate greater chemical inertness, however, C-H and C-F bond activation reactions can be induced using appropriately reactive transition metal complexes. Such reactions are surveyed, including catalytic examples, not only to provide perspective for the use of FB and 1,2-DiFB as innocent solvent media, but also to highlight opportunities for their exploitation in contemporary organic synthesis.
The reaction of a series of dinucleating bis(β-diketiminate) pro-ligands with mesitylcopper in the presence and absence of mono and diphosphines has allowed the isolation of a new series of dicopper(i) complexes. Inclusion of trans-1,2-cyclohexyl (1), 2,6-pyridyl (2), and 2,2'-oxydiaryl (3) spacers between the β-diketiminate units has been studied. The isolation of three new copper(i) phosphine complexes [1·Cu2(PPh3)2], [2·Cu2(PPh3)2] and [3·Cu2(PPh3)2] is reported. While these compounds display large CuCu separations of 5.4-7.9 Å in the solid state, solution data are consistent with a large degree of conformational freedom. Modification of the monophosphine to a diphosphine, DPPE, allowed the isolation of the novel 11-membered bimetallic macrocycle [2·Cu2(DPPE)] containing both a binucleating nitrogen based ligand and a chelating diphosphine. While acetonitrile adducts of this series could also be generated in situ, under forcing conditions reaction of the 2,6-pyridyl bridged ligand with mesityl copper led to the formation [2·Cu2]2. This latter complex is a dimer of dicopper(i) units in which the bis(β-diketiminate) ligand now binds four copper(i) centers through not only the expected κ(2)-N,N'-chelation but also κ(1)- and η(2)-binding of the central pyridine through orthogonal Cu-N and Cu-arene interactions. Reversible coordination of alkenes, pyridine and quinoline to the copper cluster was identified allowing the isolation and structural characterisation of a further series of dinuclear complexes [2·Cu2(pyridine)2], [2·Cu2(cyclopentene)2] and [2·Cu2(norbornene)2]. Solution studies allow quantification of the reversible binding event through a van't Hoff analysis. Both solution and the solid state data suggest a weak anagostic interaction exists in the latter two alkene complexes of copper(i). The new complexes have been characterized by X-ray diffraction, multinuclear NMR spectroscopy and CHN analysis.
Addition of 2 equiv. of CuI to a [Cu]4 multimetallic complex results in cluster formation leading to the isolation of a rare bicapped tetrahedral [Cu6I2] cluster that is stabilised by two conformationally constrained polynucleating ligands.
The preparation and applications of heterobimetallic complexes continue to occupy researchers in the fields of organometallic, main group, and coordination chemistry. This interest stems from the promise these complexes hold as precursors to materials, reagents in synthesis and as new catalysis. Here we survey and organise the state-of-the-art understanding of the TM-H-M linkage (M = Mg, Zn, Al, Ga). We discuss the structure and bonding in these complexes, their known reactivity, and their largely unrealised potential in catalysis.
Synthetic approaches to produce reactive chemical building blocks from fluorinated molecules by the functionalization of carbon–fluorine bonds with main group reagents are reviewed. The reaction types can be categorized as: (i) the formal 1,2-addition of C–F bonds across Si–Si, B–B, or Mg–Mg bonds; (ii) the oxidative addition of C–F bonds to Si(II), Ge(II), and Al(I) centres; and (iii) the dehydrogenative coupling of C–F bonds with Al–H or B–H bonds. Many of the advances have emerged between 2015–2016 and are largely focused upon aromatic substrates that contain sp² C–F bonds. 1 Introduction 2 C–F Borylation of Aromatic sp² C–F Bonds 2.1 Rhodium Catalysis 2.2 Nickel Catalysis 3 C–F Alumination of sp² C–F Aromatic and sp³ C–F Aliphatic Bonds 4 C–F Silylation and Germylation of Aromatic sp² C–F Bonds 5 C–F Magnesiation of Aromatic sp² C–F Bonds 6 C–F Silylation and Borylation of Alkenes 7 Conclusions and Future Directions
Through a dramatic advance in the coordination chemistry of the zinc–hydride bond, we describe the trajectory for the approach of this bond to transition metals. The dynamic reaction coordinate was interrogated through analysis of a series of solid state structures and is one in which the TM-H-Zn angle becomes increasingly acute as the TM–Zn distance decreases. Parallels may be drawn with the oxidative addition of boron–hydrogen and silicon–hydrogen bonds to transition metal centers.
Through a dramatic advance in the coordination chemistry of the zinc–hydride bond, we describe the trajectory for the approach of this bond to transition metals. The dynamic reaction coordinate was interrogated through analysis of a series of solid state structures and is one in which the TM-H-Zn angle becomes increasingly acute as the TM–Zn distance decreases. Parallels may be drawn with the oxidative addition of boron–hydrogen and silicon–hydrogen bonds to transition metal centers.
Addition of the carbon-fluorine bond of a series of perfluorinated and polyfluorinated arenes across the Mg-Mg bond of a simple coordination complex proceeds rapidly in solution. The reaction results in the formation of a new carbon-magnesium bond and a new fluorine-magnesium bond and is analogous to Grignard formation in homogeneous solution.
Reaction of a zinc/zirconium heterobimetallic complex with 1,5-cyclooctadiene (1,5-COD) results in slow isomerization to 1,3-cyclooctadiene (1,3-COD), along with the formation of a new complex that includes a cyclooctyne ligand bridging two metal centers. While analogous magnesium/zirconium and aluminum/zirconium heterobimetallic complexes are competent for the catalytic isomerization of 1,5-COD to 1,3-COD, only in the case of the zinc species is the cyclooctyne adduct observed.
A series of Cu-, Fe- and Fe/Cu-containing zeolite (ZSM-5, beta, Y) catalysts were prepared to investigate the effect of zeolite’s physicochemical properties on the total oxygenates production and MeOH selectivity from the partial methane oxidation using H2O2 as oxidizing agent. The NH3-TPD studies have shown that the zeolite type and Si/Al molar ratio are correlated with the acid sites strength and concentration. The latter surface property was proved to have a strong influence on the oxygenate productivity. In particular, a significant increase of the methanol production was observed when lowering Si/Al ratio in the ZSM-5, Fe/ZSM-5, Cu/ZSM-5 and Cu–Fe/ZSM-5 catalysts. This can be explained by the increased amount of Brønsted acid sites capable of accommodating the active catalyst (Fe species). The Fe-only ZSM-5 catalysts exhibited the highest catalytic activity (total oxygenated products) with HCOOH being the major product, whereas the presence of only Cu was found to suppress the production of MeOOH and HCOOH. On contrary, the deposition of both Fe3+ and Cu2+ results to a switch in selectivity and the target product, MeOH, was observed in ~80 % selectivity. In the case of Cu-only ZSM-5 catalysts, a similar activity to methanol was observed regardless the copper source and synthesis method. The activity/selectivity findings of the present study confirm and complement the conclusions of the previous work by Hammond et al. (ACS Catal 3:689, 2013; ACS Catal 3:1835, 2013; Angew Chem Int Ed 51:5129, 2012; Chem Eur J 18:15735, 2012) over the well-studied Cu–Fe–zeolite system, providing also complete material balance based on both gas and liquid reaction products. Graphical Abstract
Addition of fluoroarenes, fluoroalkanes or benzofuran to [{(2,6-(i)Pr2C6H3NCMe)2CH}Al] results in facile oxidative addition of either a C-F or C-O bond to the Al(i) centre.
The scope of the catalytic dehydrocoupling of primary and secondary amines with phenylsilanes has been investigated using [Y{N(SiMe3)2}3] and a four-coordinate analogue bearing a cyclometalated phosphonium methylide ligand. Inclusion of the phosphorus-based ligand on yttrium results in increased substrate scope in comparison to the tris(amide) analogue. While reversible C-H bond activation of the cyclometalated ligand was observed in stoichiometric experiments, D-labeling experiments and DFT calculations suggest that reversible ligand activation is not involved in silazane formation under catalytic conditions. We suggest that the extended reaction scope with the four-coordinate yttrium phosphonium methylide complex relative to the three-coordinate yttrium (tris)amide complex is a result of differences in the ease of amine inhibition of catalysis.
We report the addition of M–H bonds (M = Al, Zn, Mg) to a Rh(III) intermediate generated from the reductive elimination of triethylsilane from [Cp*Rh(H)2(SiEt3)2]. A series of new heterobimetallic complexes possessing Rh–M bonds have been isolated and characterised by a number of spectroscopic (1H, 29Si, 13C, 103Rh NMR, infrared, and X-ray diffraction) and computational techniques (NBO and QTAIM analysis). Experimental and computational data are consistent with cleavage of the M–H bond upon addition to rhodium with formation of new Rh–M and Rh–H bonds. Upon photolysis the Al analogue of this series undergoes a further elimination reaction producing triethylsilane and a highly unusual Rh2Al2H4 containing cluster proposed to contain an Al(I) bridging ligand.
A series of multimetallic copper(II) complexes have been re-investigated for methane oxidation with H2O2. The preparation and properties of trinuclear copper(II) complexes of the form [Cu3(triazole)n(OH2)12-n] (n = 8,10) are reported. While these complexes are trimeric in the solid-state, 1H NMR studies suggest that facile ligand dissociation occurs in solution. The oxidation of cyclohexane with H2O2 catalyzed by [Cu3(triazole)n(OH2)12-n] (n = 8, 10) is compared against a literature known oxo-centered tetrameric cluster (Angew. Chem., Int. Ed. 2005, 44, 4345) and these catalysts display moderate activities. The series have also been investigated in methane oxidation at 30 bar and 40 oC. Analytical techniques including a solvent suppression 1H NMR method have been applied to quantify the liquid- and gas-phase products. The multi-metallic copper(II) complexes and copper(II) nitrate control samples produce only methanol and CO2. While TONs for methanol production range from 1.4-4.6 in all cases approximately 50 times the amount of CO2 is produced relative to methanol. We conclude that selectivity is a determining factor in methane oxidation under these conditions and should be considered in future studies.
A series of bis(σ-B–H) complexes of copper(I) have been prepared by displacement of arene solvent from a β-diketiminate copper(I) complex by four-coordinate boranes, H3B–L (L = NMe3, lutidine). In the presence of the same copper arene complex, the secondary amine-borane H3B–NMe2H undergoes dehydrogenation. We provide evidence for formation of a heterogenous catalyst from decomposition of the solution species.
[Cp*RhCl(μ-Cl)]2 is reported as a highly efficient and selective precatalyst for the hydrodefluorination of perfluoroarenes using a hydrocarbon-soluble aluminum dihydride as the terminal reductant. Reactions are directed to cleave a C–F bond adjacent to an existing C–H bond with high regioselectivity (98.5–99%). A heterobimetallic complex containing an extremely rare Al–H–Rh functional group has been isolated and shown to be catalytically competent.
We report [Y{N(SiMe3)2}3] as a precatalyst for the dehydrocoupling of sterically demanding amines with β-diketiminate stabilised aluminium dihydrides. While simple fluorinated anilines readily undergo Al-H/N-H dehydrocoupling under thermal conditions, catalytic methods are required to achieve reasonable rates of reaction for ortho-substituted anilines or hindered aliphatic amines.
This chapter provides details of the recent progress in heavier Group 2-catalyzed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX2, where L is a monoanionic ligand and X is a reactive σ-bonded substituent and M = Mg, Ca, Sr, and Ba. The intra- and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation, hydroboration, hydrogenation, and hydroacetylation) of alkenes, alkynes, dienes, carbodiimides, isocyanates, pyridines, quinolines, and ketones is discussed, along with the dimerization of aldehydes, the trimerization of isocyanates, and the dehydrogenation of amine-boranes and the dehydrogenative coupling of amines with silanes. While studies in this field have focused largely on biocompatible and inexpensive catalysts of calcium and the heavier elements, the field has renewed interest in the chemistry of organomagnesium complexes. Graphical Abstract
We report the synthesis and isolation of three new σ-complexes of Cu(I) in which E–H (E = Al, Zn) σ-bonds are coordinated to copper. The addition of the main group hydride to a toluene-solvated Cu(I) complex results in reversible ligand exchange, and the Cu(I) σ-complexes have been crystallized. Experimental and computational data provide a wealth of evidence for weak binding of the E–H bond to Cu(I), which can be ascribed to σ-donation from the E–H bond into the 4s orbital of copper and back-donation from copper into the E–H σ* orbital.
The homoleptic alkaline earth hexamethyldisilazides, [M{N(SiMe3)2}2(THF)2] (M = Mg ; Ca ; Sr ), have been shown to act as efficient precatalysts for the hydroacetylenation of organic carbodiimides with alkyl- and arylacetylenes. Catalytic activity was observed to increase with the size of the group 2 metal centre employed and to be strongly influenced by the steric properties of the carbodiimide substrate. The intermediate dimeric calcium and strontium bis(amidinate) complexes, [{PhC[triple bond, length as m-dash]CC(N(i)Pr)2}2M]2 (M = Ca , Sr ), have been isolated and crystallographically characterised. Kinetic studies using the strontium precursor, , provided a reaction rate law independent of [acetylene] but proportional to [carbodiimide](2) and inversely proportional to the concentration of the amidine product in solution.
Over the past few decades, researchers have made substantial progress in the development of transition metal complexes that activate and functionalize C–H bonds. For the most part, chemists have focused on aliphatic and aromatic C–H bonds and have put less effort into complexes that activate and functionalize vinylic C–H bonds. Our groups have recently developed a novel method to functionalize vinylic C–H bonds that takes advantage of the unique ligand-based reactivity of a rare class of metal dinitrosyl complexes. In this Account, we compare and discuss the chemistry of cobalt and ruthenium dinitrosyl complexes, emphasizing alkene binding, C–H functionalization, and catalysis.
The chemoselective C–O bond functionalization of benzofuran with an aluminum dihydride may be catalyzed by zirconocene dichlorides. The reaction proceeds with the formal addition of a C–O bond to, and elimination of dihydrogen from, aluminum. The product of C–O bond alumination reacts with benzaldehyde via insertion of the carbonyl into the newly formed Al–C bond.
The synthesis of a diverse series of hydride complexes of aluminium coordinated by N,N'-chelating ligands is reported. Reaction of [{2,6-(i)Pr2C6H3}NC(Me)CHC(Me)N(H)CH2CH2NMe2] with either LiAlH4 or Me3N·AlH3 allows isolation of the corresponding five-coordinate aluminium dihydride [κ(3)-{(2,6-(i)Pr2C6H3)NC(Me)CHC(Me)NCH2CH2NMe2}AlH2] (). The latter complex demonstrates trigonal bipyramidal geometry in the solid-state. Correlation of solid and n-hexane solution infrared spectroscopy data reveals that this coordination is retained in solution. To evaluate the observed coordination geometry, the dissociation of the pendant ligand of was investigated by DFT methods conducted with the M06-2X functional and a hybrid 6,31G+(d,p)/Lanl2DZ basis-set. Reaction of Me3N·AlH3 with both N,N'-bis(di-iso-propylphenyl)ethylenediamine and N,N'-bis(mesityl)ethylenediamine gave [{κ(2)-(ArNCH2)2}AlH(NMe3)] (Ar = Mes, ; Ar = 2,6-di-iso-propylphenyl, ) in moderate yields. Removal of NMe3 from by heating under dynamic vacuum allowed the isolation of cis-[AlH{μ-N(Ar)CH2CH2N(Ar)}] (Ar = 2,6-di-iso-propylphenyl, cis-) as a single diastereomer following crystallization. DFT studies in combination with infrared and NMR spectroscopy and single crystal X-ray diffraction data provide a weight of evidence consistent with the robust dimeric structure of cis- remaining intact in solution. An unusual reaction in which the aluminium dihydride, [κ(2)-{(2,6-Me2C6H3NHCH2)2CH}AlH2], promotes the P-C bond cleavage of Ph3PCH2 is also reported.
A series of beta-diketiminate-supported magnesium and calcium acetylide complexes have been synthesized by a-bond metathesis of magnesium n-butyl or magnesium and calcium amido precursors and a range of terminal acetylenes. The dimeric complexes have been characterized by NMR spectroscopy and X-ray diffraction analysis. The homoleptic bis(amido) and dialkyl complexes [M{X-(SiMe3)(2)}(2)(THF)(2)] (M = Ca, Sr; X = N, CH) have been assessed for the atom-efficient, catalytic head-to-head dimerization of donor-functionalized terminal alkynes into butatrienes and aryl-/silyl-substituted terminal acetylenes into 1,3-enynes. Deuterium labeling studies of the catalytic reactions are suggested to imply that triene formation requires concerted proton delivery and rearrangement via an adjacent methylene group at a bimetallic alkaline-earth species.
The phenyl-substituted β-diketiminate ligand precursor (Ph)LH, [(Dipp)NC(Ph)CHC(Ph)NH(Dipp)] (Dipp = 2,6-di-isopropylphenyl) and its lithium and beryllium halide derivatives [(Ph)LLi(OEt2)], [(Ph)LBeCl] and [(Ph)LBeI] have been synthesised and characterised by NMR and X-ray structural analysis. The iodoberyllium complex [(Ph)LBeI] reacts with THF in a well-defined ring-opening insertion reaction to form the 4-iodo-n-butoxide complex [(Ph)LBeO(CH2)4I].
Four-coordinate diketiminate-stabilized aluminum dihydrides are found to be effective terminal reductants in the zirconocene-catalyzed dehydrofluorination of fluoroarenes (yields determined by NMR).
We report the [Y{N(SiMe3)2}3] catalysed dehydrocoupling of triphenylphosphonium methylide with phenylsilane to form the silylated ylide Ph3PCHSiH2Ph. Attempts to catalyse this reaction with the related group 2 hexamethyldisilazide base [Ca{N(SiMe3)2}2] led to the catalytic formation of the phosphine Ph2PCHSiH2Ph along with Ph2PMe in low selectivity, while group 1 bases [M{N(SiMe3)2}] (M = Li, Na, K) proved ineffective for both transformations. The stoichiometric reactions of Ph3PCH2 with [M{N(SiMe3)2}n] have been investigated and allowed the isolation and characterisation of a cyclometallated phosphonium methylide complex of yttrium.
A two-metal job: Four-coordinate aluminum dihydrides such as 1 are reported as terminal reductants for the selective title reaction. The heterobimetallic complex 2 has been isolated and shown to be catalytically competent.
The title complex (I) is an exceedingly rare example of a hydrocarbon-soluble, thermally stable, metal-coordinated betaine which allows to study the Wittig olefination from an isolated intermediate.
We report the synthesis of an yttrium phosphonium methylide complex and its reaction with benzophenone to form an excedingly rare hydrocarbon soluble metal-coordinated betaine. While this reaction models the C-C σ-bond formation step of the Wittig reaction under salt-conditions, addition of Ph(3)P=O to the betaine complex results in formation of 1,1-diphenylethene.
Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}(9) species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me(2)-pyrazolyl)borate). The metric parameters about the metal center and the pre-edge region of the Co K-edge X-ray absorption spectrum were reproduced by density functional theory (DFT), providing a qualitative description of the Co-NO bonding interaction as a Co(II) (S(Co) = 3/2) metal center, antiferromagnetically coupled to a triplet NO(-) anion (S(NO) = 1), an interpretation of the electronic structure that was validated by ab initio multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant g-anisotropy in the S = ½ ground state, but the linear-response DFT performed poorly at calculating the g-values. Instead, CASSCF/MRCI computational studies in conjunction with quasi-degenerate perturbation theory with respect to spin-orbit coupling were required for obtaining accurate modeling of the molecular g-tensor. The computational portion of this work was extended to the diamagnetic Ni analogue of the Co complex, Tp*Ni(NO), which was found to consist of a Ni(II) (S(Ni) = 1) metal center antiferromagnetically coupled to an S(NO) = 1 NO(-). The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO(0) formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and ab initio computational methods for their ability to predict various spectroscopic and molecular features.
Alkene gezähmt: Die Synthese des Nitrosylkomplexes [RuCl2(NO)2(THF)] wurde durch die Reaktion von [{(Cymol)RuCl2}2] mit NO in THF erreicht. In Gegenwart eines chelatisierenden L2-Liganden bindet [RuCl2(NO)2(THF)] Alkene an die Nitrosyl-Stickstoffatome (siehe Schema).
The magnesium methyl and the calcium and strontium silylamide β-diketiminate derivatives [{ArNC(Me)CHC(Me)NAr}MX(THF)n] (M = Mg, X = CH3, n = 0; M = Ca, X = N(SiMe3)2, n = 0 or 1; M = Sr, X = N(SiMe3)2, n = 1, Ar = 2,6-diisopropylphenyl), the silylamides [M{N(SiMe3)2}2]2 and [M{N(SiMe3)2}2(THF)2] (M = Mg, Ca, Sr, Ba), and the alkyl species [M{CH(SiMe3)2}2(THF)2] (M = Mg, Ca, Sr, Ba) have been studied as precatalysts for the hydroamination/cyclization of aminoalkenes. Hydroamination afforded a series of five- and six-membered pyrrolidine and piperidine derivatives in near quantitative yields with all precatalysts, apart from the barium species, which were apparently limited to a maximum of two turnovers with even the favorable 1-amino-2,2-diphenyl-4-pentene substrate. Significant formation of hexahydroazepines was observed only with the magnesium amide species, while the group 2 dialkyl derivatives decomposed at high reaction temperatures and proved to be more limited in scope. In general, the calcium precatalysts proved to be more reactive than their strontium analogues, which, in turn, proved to be far more reactive than the magnesium species. Among the β-diketiminate derivatives, the greater coordinative unsaturation of the THF-free calcium derivative provided increased activity for the cyclization of primary aminoalkene substrates in comparison to its THF-solvated counterpart. In contrast, the unsolvated bis(amides) displayed lower activity with these substrates than their THF-solvated analogues. Use of silylamide precatalysts provides potentially reversible entry into the catalytic manifold. The position of the equilibrium is perturbed by both the nature of the substrate and the identity of the group 2 element, highlighted by the reaction of [{ArNC(Me)CHC(Me)NAr}Sr{N(SiMe3)2(THF)] with 2-methoxyethylamine, which results in equilibration between the starting silylamide, an isolable amine adduct, and the product of amine/silylamide transamination. Performing the same reaction with the calcium precatalyst provided the analogue of the latter product as the only observable species. A kinetic study of the cyclization of (1-allylcyclohexyl)methanamine with each of the calcium and strontium silylamide precatalysts provided an apparent first-order dependence in [catalyst], while determination of the activation barriers and Eyring analyses provided quantitative evidence that the group 2 catalysts reported herein provide activities at least commensurate with previously reported lanthanide-based catalyses. In the particular cases of systems based upon calcium and strontium, an enhanced catalytic performance is proposed to arise from a tangible entropic advantage resulting from the reduced charge density of the larger divalent alkaline earth cations and consequentially less constrained rate-determining alkene insertion transition states. The rate of cyclization was also found to decrease with increasing substrate concentration. This latter observation, along with the observation of large kinetic isotope effects (>4), proposed to be a result of a beneficial and concerted proton transfer step associated with rate-determining alkene insertion, are reasoned to be consistent with Michaelis−Menten-type kinetics.
A series of N(1),N(1),N(3)-tri-substituted benzamidrazones of the general formula [PhC(NHR)=NNMe(2)] (R = Me, n-Pr, i-Pr, n-Bu, Bn, Ph; 1a-f) was synthesized via condensation of 1,1-dimethylhydrazine with the corresponding imidoyl chloride, [PhC(Cl)=NR]. Multinuclear NMR data, and zero-point energy DFT calculations conducted with the B3LYP functional and 6-31G+(d,p) basis set, suggest that these compounds exist as a single tautomer in solution; possessing a weak intramolecular hydrogen bond and a structure dominated by the localised resonance structure ArC(NHR)=N-NMe(2). An X-ray crystallographic study upon PhC(NHPh)=NNMe(2) (1f) demonstrated that this compound adopts an identical tautomer in the solid state. Reactions of [PhC(NHMe)=NNMe(2)] (1a) with [LMCl(2)](2) (M = Ru, L = cymene; M = Rh, Ir, L = Cp*) results in the stoichiometric formation of products of the formula [LM{PhC(=NMe)NHNMe(2)}Cl](+)Cl(-) (2a-c) in which the amidrazone chelates the metal in a κ(2)-N(1),N(3)-coordination mode. Formation of this five-membered chelate occurs with a concomitant tautomerisation of the amidrazone ligand to an alternative tautomer, i.e. [PhC(=NMe)NHNMe(2)], the latter tautomer is expected to be readily energetically accessible based upon the aforementioned DFT calculations. This series of salts may be deprotonated with lithium hexamethyldisilazide to form the corresponding charge neutral complexes [LM{PhC(NMe)=NNMe(2)}] (3a-c). In contrast, the reaction of N(1),N(1),N(3)-tri-substituted benzamidrazones with [(cymene)RuCl(2)](2) in the presence of NaOAc yielded a mixture of cyclometallation (C-H activation) and amidrazone chelation/deprotonation (N-H activation) products. Reaction of 1a yielded an inseparable mixture of products, whilst the reaction of 1c resulted in formation of the cyclometallated product [LM{C(6)H(5)C(=N(i)Pr)NHNMe(2)}] (L = cymene, M = Ru; 4a) in a modest 62% yield. This latter complex could be isolated as a crystalline orange solid, full characterisation including single crystal X-ray diffraction demonstrated that the amidrazone coordinates in a κ(2)-N(2),C-coordination mode.
The asymmetric C-H functionalization of norbornene and norbornadiene with five-, six-, and seven-membered cyclic enones mediated by the reactive intermediate [{η(5)-((t)BuMe(2)Si)C(5)H(4)}Co(NO)(2)] is reported. A novel base mixture derived from enantiopure ammonium salts and NaHMDS was used as a source of chirality, and this enantioselective desymmetrization of C(s) alkenes has been applied to the asymmetric synthesis of C(2)- and C(1)-symmetric diene ligands in high regioselectivity (3.7-20:1 anti/syn), near perfect diastereoselectivity (>99:1 dr), and high enantioselectivity (90-96% ee).
A series of homoleptic guanidinate-type complexes of the heavier alkaline earth (Ae) metals calcium and strontium have been prepared. The six-coordinate compounds [Ae{(i)PrNC(NPh(2))CN(i)Pr}(2)(THF)(2)] (Ae = Ca and Sr) were synthesised through reactions of the appropriate THF-solvated hexamethyldisilazide [Ae{N(SiMe(3))(2)}(2)(THF)(2)] with two molar equivalents of diphenylamine and 1,3-di-iso-propylcarbodiimide. Both compounds were shown to crystallise with a cisoid arrangement of the two THF molecules at the metal centres. In contrast, the (Me(3)Si)(2)N-substituted calcium guanidinate, [Ca{CyNC{N(SiMe(3))(2)}CNCy}(2)(THF)(2)], contains two coordinated THF molecules with a trans disposition. Further reactions of the free amidine [{(2-FC(6)H(4))N}C(NH(i)Pr)(2)] with either [Ca{N(SiMe(3))(2)}(2)(THF)(2)] or its dimeric unsolvated analogue provided monomeric or dimeric derivatives respectively in which the ligands had tautomerised to an anisobidentate form. A further reaction of the phosphaguanidine [CyN=C(PPh(2))N(H)Cy] with [Sr{N(SiMe(3))(2)}(2)(THF)(2)] provided the first example of a phosphaguanidinate complex of this heavier alkaline earth metal. This compound has also been characterised in the solid state and shown to exist with a transoid configuration of the coordinated THF molecules.
Reaction of the β-diketiminato calcium amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(OEt2)] (Ar = 2,6-di-iso-propylphenyl) with triethylaluminium yields a novel calcium aluminate complex, [κ3-{ArNC(Me)C(AlEt3)HC(Me)NAr}Ca{Et4Al}], in which coordination at calcium is provided by not only bridging interactions with the ethyl groups of the aluminate anion but also a tripodal κ3-N,N,C-coordinated ligand derived from further reaction of the ‘spectator’ β-diketiminate ligand with triethylaluminium. In the solid state, this compound exists as a 1 ∶ 1 mixture of coordination isomers in which the Et4Al anion binds to calciumvia either μ2- or μ3-coordination modes. Although persistent, and rapidly interconverting, at room temperature in solution, variable temperature NMR studies suggest that these two isomers undergo dissociative loss of triethylaluminium from the diketiminate ligand at low temperature.
Well defined dialkyls of the heavier alkaline earth elements, [M{CH(SiMe(3))(2)}(2)(THF)(n)] [M = Ca, n = 2; M = Sr, n = 3; M = Ba, n = 3] react to form either the heteroleptic beta-diketiminato alkyl when M = Ca or unusual 'C-H activation' products in which a methyl group of the ligand is deprotonated when M = Sr or Ba.
A density functional theory assessment of the use of the group 2 elements Mg, Ca, Sr, and Ba for the intermolecular hydroamination of ethene indicated that the efficiency of the catalysis is dependent upon both the polarity and the deformability of the electron density within the metal-substituent bonds of key intermediates and transition states. The validity of this analysis was supplemented by a preliminary study of the use of group 2 amides for the intermolecular hydroamination of vinyl arenes. Although strontium was found to provide the highest catalytic activity, in line with the expectation provided by the theoretical study, a preliminary kinetic analysis demonstrated that this is possibly a consequence of the increased radius and accessibility of this cation rather than a reflection of a reduced barrier for rate-determining alkene insertion.
The beta-diketiminate-stabilized calcium amide complex [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe(3))(2)}(THF)] (Ar = 2,6-diisopropylphenyl) and magnesium methyl complex [{ArNC(Me)CHC(Me)NAr}Mg(Me)(THF)] are reported as efficient precatalysts for hydroamination/cyclization of aminoalkenes. The reactions proceeded under mild conditions, allowing the synthesis of five-, six-, and seven-membered heterocyclic compounds. Qualitative assessment of these reactions revealed that the ease of catalytic turnover increases (i) for smaller ring sizes (5 > 6 > 7), (ii) substrates that benefit from favorable Thorpe-Ingold effects, and (iii) substrates that do not possess additional substitution on the alkene entity. Prochiral substrates may undergo diastereoselective hydroamination/cyclization depending upon the position of the existing stereocenter. Furthermore, a number of minor byproducts of these reactions, arising from competitive alkene isomerization reactions, were identified. A series of stoichiometric reactions between the precatalysts and primary amines provided an important model for catalyst initiation and suggested that these reactions are facile at room temperature, with the reaction of the calcium precatalyst with benzylamine proceeding with DeltaG(o)(298 K) = -2.7 kcal mol(-1). Both external amine/amide exchange and coordinated amine/amide exchange were observed in model complexes, and the data suggest that these processes occur via low-activation-energy pathways. As a result of the formation of potentially reactive byproducts such as hexamethyldisilazane, calcium-catalyst initiation is reversible, whereas for the magnesium precatalyst, this process is nonreversible. Further stoichiometric reactions of the two precatalysts with 1-amino-2,2-diphenyl-4-pentene demonstrated that the alkene insertion step proceeds via a highly reactive transient alkylmetal intermediate that readily reacts with N-H sigma bonds under catalytically relevant conditions. The results of deuterium-labeling studies are consistent with the formation of a single transient alkyl complex for both the magnesium and calcium precatalysts. Kinetic analysis of the nonreversible magnesium system revealed that the reaction rate depends directly upon catalyst concentration and inversely upon substrate concentration, suggesting that substrate-inhibited alkene insertion is rate-determining.
Heteroleptic calcium amides effect the catalytic dimerisation of the terminal alkyne CH(3)OCH(2)C[triple bond, length as m-dash]CH to the hexatriene (CH(3)OCH(2))CH[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]CH(CH(2)OCH(3)) under mild conditions; the reaction is proposed to occur via a dimeric calcium acetylide intermediate and to be promoted by polarisation of the electron density within the alkynide C[triple bond, length as m-dash]C bonds.
In a study relevant to group 2-mediated hydroamination catalysis, the reaction of the beta-diketiminato magnesium alkyl complex [{ArNC(Me)CHC(Me)NAr}Mg((n/s)Bu)] (Ar = 2,6-(i)Pr(2)C(6)H(3)) with benzylamine, 2-methoxyethylamine, pyrrolidine, and 2-methyl-4,4-diphenylpyrrolidine has been shown to yield the corresponding magnesium amide complexes [{ArNC(Me)CHC(Me)NAr}Mg(NR(1)R(2))] (R(1) = H, R(2) = CH(2)Ph, CH(2)CH(2)OMe; R(1) = R(2) = -(CH(2))(4)-, -CH(Me)CH(2)CPh(2)CH(2)-) within the first point of analysis (30 min) at room temperature in near quantitative yield as monitored by (1)H NMR spectroscopy. Reactions proceeded non-reversibly, and the products have been characterized in both solution and the solid state. While single crystal X-ray diffraction analysis demonstrated that the magnesium amides are dimeric in the solid state, with aggregation occurring via mu(2)-coordinated amide ligands, NMR studies suggest that for more sterically crowded amide ligands discreet monomeric species exist in solution. In contrast, the calcium complex [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe(3))(2)}(THF)] reacted reversibly with benzylamine at room temperature to form an equilibrium mixture of a calcium benzylamide and bis(trimethylsilyl)amide. A series of Pulsed-Gradient Spin-Echo NMR studies upon beta-diketiminato calcium amides were consistent with the formation of a dimer in solution. A van't Hoff analysis performed on this mixture allowed DeltaH degrees = -51.3 kJ mol(-1) and DeltaS degrees = -134 J mol(-1) of the protonolysis/dimerization reaction to be derived and the Gibbs' free energy to be calculated as DeltaG degrees (298 K) = -11.4 kJ mol(-1).
Despite the routine employment of Grignard reagents and Hauser bases as stoichiometric carbanion reagents in organic and inorganic synthesis, a defined reaction chemistry encompassing the heavier elements of Group II (M = Ca, Sr and Ba) has, until recently, remained unreported. This article provides details of the recent progress in heavier Group II catalysed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX₂; where L is a mono-anionic ligand and X is a reactive σ-bounded substituent. The intra- and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation and hydrogenation) of alkenes, alkynes, dienes, carbodiimides, isocyanates and ketones is discussed.
The heteroleptic calcium amides [{ArNC(Me)CHC(Me)NAr}Ca(NR(2))(THF)] (Ar=2,6-di-iso-propylphenyl, R=SiMe(3), Ph) and the homoleptic heavier alkaline earth amides, [M{N(SiMe(3))(2)}(2)] (M=Ca, Sr and Ba) are reported as pre-catalysts for the hydroamination of isocyanates.
The β-diketiminate-stabilized calcium amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] (1) reacts with terminal acetylenes in hydrocarbon solvents to yield the corresponding calcium acetylide complexes [{ArNC(Me)CHCN(Me)Ar}Ca{CCR1}]2 (R1 = n-Bu, t-Bu, Ph, 4-MeC6H4, ferrocenyl, Ar = 2,6-di-isopropylphenyl, 2a−e). Although in all instances solid and solution state data were consistent with the reaction products existing as dimeric species with aggregation occurring via three-center−two-electron bridging acetylide units, a further reaction of 1 with HCCSi(iPr)3 demonstrated that both monomeric solvated [{ArNC(Me)CHC(Me)NAr}Ca{CCSi(iPr)3}(THF)2] (3b) or dimeric acetylide [{ArNC(Me)CHC(Me)NAr}Ca{CCSi(iPr)3}]2 (3a) species could be isolated from the reaction depending upon the exact conditions of the crystallization of the reaction product from solution. Further solution studies demonstrated the presence of a monomer−dimer equilibrium in solution. A van’t Hoff analysis allowed ΔG°(298 K) for the dimerization reaction to be calculated as +27.0 kJ mol−1. The reaction of these hydrocarbon-soluble kinetically stabilized calcium acetylides with 1,3-dialkylcarbodiimides gave the corresponding heteroleptic calcium C-propargyl amidinate complexes [{ArNC(Me)CHCN(Me)Ar}Ca{(R2N)2CCCR1}(THF)n] (R1 = 4-MeC6H4, n = 0, 4a; 4-MeC6H4, n = 1, 4a·THF; R2 = iPr; R1 = Si(iPr3), R2 = Cy, n = 1, 4b·THF) via insertion of the carbodiimide into the calcium−carbon σ-bond. The latter complexes have been characterized in both solution and the solid state including single-crystal X-ray analysis of 4a·THF. Extension of this reactivity to catalytic systems has allowed the application of amide 1 (5 mol %) to the catalytic hydroacetylenation of 1,3-di-isopropylcarbodiimide with phenylacetylene, yielding the corresponding propargyl amidine in 59% yield following crystallization from hexane solution.
The straightforward syntheses of dialkyl calcium (1), strontium (2) and barium (3) species were studied. The X-ray analyses of 1-3 are notable for the identification of typical structural parameters within complexes of the type. The bis-THF-solvate of V. Compound 1 was synthesized straightforwardly and on a preparative scale by using Lappert's previously described salt metathesis reaction in THF. All the reactions and manipulations under rigorous exclusion of water and oxygen, by using either a double manifold vacuum line or an Ar-filled drybox operating at less than 0.1ppm O2. All reagents and solvents were purified using standard procedures. The results showed that complex 1 reacts with two equivalents of 1,3-dialkyl carbodiimides to give the anticipated amidinate insertion product in fashion analogous to the reported explorations of calcium amide chemistry.
A series of heteroleptic beta-diketiminate-stabilised calcium amides of the form [{ArNC(Me)CHC(Me)NAr}Ca{NR1R2}(THF)] (Ar = 2,6-diisopropylphenyl; R1 = H, R2 = Ar; R1 = H, R2 = CH2CH2OMe; R1 = R2 = Ph) react with 1,3-dialkylcarbodiimides, R3NCNR3 (R3 = Cy, iPr), to yield the corresponding insertion products [{ArNC(Me)CHC(Me)NAr}Ca{(R3N)2CNR1R2}(THF)] at room temperature in hydrocarbon solutions. These latter compounds contain both beta-diketiminate and guanidinate ligands bound to calcium. Solid-state data are consistent with the guanidinate ligands adopting a number of binding modes including 2 through 3 coordination, with varying degrees of delocalisation of the non-bound guanidinate nitrogen lone-pair across the -framework of the ligand. DFT computational studies have been conducted to address these variations in coordination behaviour.
The heteroleptic calcium amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] (Ar = 2,6-diisopropylphenyl) and the homoleptic heavier alkaline earth amides, [M{N(SiMe3)2}2(THF)2] (M = Ca, Sr and Ba) are reported as competent pre-catalysts for the hydroamination of 1,3-carbodiimides. Whilst the reaction scope is currently limited to reactions of aromatic amines with 1,3-dialkylcarbodiimides, in most cases preparations in hydrocarbon solvents proceed rapidly at room temperature with catalyst loadings as low as 0.2 mol-% and the guanidine reaction products crystallize directly from the reaction mixture. Initial studies are consistent with the intermediacy of heavier group-2 guanidinate complexes.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
A series of triazenide complexes of the heavier alkaline earths, Ca, Sr and Ba, have been synthesized by either protonolysis or salt metathesis routes. Although complexes of the form [{Ar 2N 3}M{N(SiMe 3) 2}(THF) n ] (M = Ca, n = 2; M = Sr, n = 3; Ar = 2,6-diisopropylphenyl) and [{Ar 2N 3}Ca(I)(THF) 2] 2 could be isolated and characterized by X-ray crystallography, solution studies revealed the propensity of these species to undergo Schlenk-like redistribution with the formation of [{Ar 2N 3} 2M(THF) n ] (M = Ca, n = 1; M = Sr, n = 2). The latter compounds have been synthesized independently. In the case of the large barium dication, attempts to synthesize the heaviest analogue of the series, [{Ar 2N 3} 2Ba(THF) n ], failed and led instead to the isolation of the potassium barate complex [K{Ar 2N 3}Ba{N(SiMe 3) 2} 2(THF) 4]. Single crystal X-ray diffraction studies demonstrated that, although in all the aforementioned cases the triazenide ligand binds to the electrophilic group 2 metal centers via symmetrical kappa (2)- N, N-chelates, in the latter compound an unprecedented bridging mode is observed in which the triazenide ligand coordinates through both terminal and internal nitrogen centers. A series of density-functional theory computational experiments have been undertaken to assist in our understanding of this phenomenon. In further experiments, the calcium and strontium amide derivatives [{Ar 2N 3}M{N(SiMe 3) 2}(THF) n ] (M = Ca, n = 2; M = Sr, n = 3) proved to be catalytically active for the intramolecular hydroamination of 1-amino-2,2-diphenylpent-4-ene to form 2-methyl-4,4-diphenylpyrrolidine, with the calcium species demonstrating a higher turnover number than the strontium analogue ( 2a, TOF = 500 h (-1); 2b, TOF = 75 h (-1)). In these instances, because of ambiguities in the structural charcterization of the precatalyst in solution, such quantification holds little value and detailed catalytic studies have not been conducted.
A series of N-heterocyclic carbene (NHC) adducts of the heavier group 2 bis(trimethylsilyl)amides, of the general formulas [(L1)M{N(SiMe3)2}2], [(L2)Ca{N(SiMe3)2}2], and [(L1)Ca{N(SiMe3)2}Cl] (L1 = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and L2 = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, M = Ca, Sr, and Ba), have been synthesized by either the addition of the group 2 amide to the appropriate imidazolium salt or direct addition of the carbene to the solvent-free, homoleptic, metal amide. X-ray diffraction studies of the reaction products revealed the formation of monomeric three-coordinate alkaline earth metal species in which the NHC binds via conventional σ-donation of the lone pair to the electrophilic metal center. Although 1H, 13C, and NOESY NMR experiments suggest this coordination is retained in solution, reactions with protic substrates such as 2-methoxyethylamine, diphenylamine, and di-p-tolylphosphine demonstrate the lability of the NHC under catalytically-relevant conditions. Furthermore, a series of reactions of [L1Ca{N(SiMe3)2}2] with Lewis bases suggest that the strength of the interaction between the metal and neutral ligand decreases across the series Ph3PO > NHC THF > PPh3. In the case of triphenylphosphine oxide the structure of the reaction product, [(Ph3PO)2Ca{N(SiMe3)2}2], was confirmed by independent synthesis from addition of Ph3PO to [Ca{N(SiMe3)2}2].
Protonolysis of the beta-diketiminate calcium bis(trimethylsilyl)amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] with benzylamine is reversible and forms a quantifiable equilibrium mixture.
Amides of the heavier group 2 elements Ca, Sr, and Ba are effective precatalysts for the atom-efficient addition of phosphine P−H bonds to carbodiimides. A number of intermediates within the catalytic cycle have been identified by in situ NMR methods and by stoichiometric synthesis.
A number of heavier alkaline-earth (Ae) diphenylphosphides have been prepared by protonolysis of the corresponding bis(trimethylsilyl)amides and completely characterized in the solid state and in solution. Through careful control of the reaction and workup conditions, preparations performed with tetrahydrofuran (THF) as the only donor solvent crystallize as octahedral tetrakis-solvated THF adducts [Ae(PPh2)2(THF)4] when Ae=Ca and Sr. Both structures are similar and contain the two diphenylphosphide substituents in a trans disposition. In contrast, the analogous reaction performed upon the barium amide yielded a one-dimensional polymeric complex containing both five- and six-coordinated Ba centers in which the polymer is propagated via bridging diphenylphosphide units. Further strictly mononuclear derivatives of the Ca and Ba diphenylphosphides may be straightforwardly prepared by treatment of the THF adducts with 18-crown-6. Although the THF-solvated species are evidently labile to loss of solute, solution (diffusion-ordered spectroscopy NMR, THF-d8) studies indicate that, under catalytically relevant conditions, a similar mononuclear constitution is most likely retained.
Fluoral arrangement: The calcium β‐diketiminate complex 1 (see scheme) not only exhibits an unexpected and unprecedented binding mode of a CF3 group, from which one fluorine atom coordinates to the Ca center, but also undergoes CF bond cleavage through the formation of a heteroleptic calcium fluoride complex.
The reaction of a β-diketiminate-stabilized calcium diphenylamide with the dialkylborane 9-BBN allows the synthesis of a β-diketiminate-stabilized calcium borohydride along with an amidoborane reaction coproduct via a likely σ-bond metathesis mechanism.
The β-diketiminato complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(SiMe3)2}(THF)] effects intermolecular hydrophosphination of a range of alkenes and alkynes. In behavior reminiscent of lanthanocene(III) catalysis, a more electrophilic alkene is polymerized to phosphine-terminated macromolecules.
Homoleptic heavier alkaline earth amides, M{N(SiMe3)2}2(THF)2 (M = Ca, Sr, and Ba) are reported as precatalysts for the dimerization of aldehydes to the analogous carboxylic esters (Tischenko reaction). [reaction: see text].
A series of heteroleptic calcium η5-C5R5 cyclopentadienides supported by an N-Dipp (Dipp=2,6-iPr2C6H3)-substituted β-diketiminate ligand have been synthesised by selective protonolysis of the readily available reagent [HC{(C(Me)N(Dipp))}2Ca{N(SiMe3)2}(THF)] with tetramethylcyclopentadiene, fluorene, indene or cyclopentadiene. No reaction was observed with pentamethylcyclopentadiene, presumably for steric reasons. The tetramethylcyclopentadienyl, fluorenyl and indenyl compounds were characterised by variable temperature 1H NMR and X-ray crystallography. Each complex was found to exist as a mononuclear species both in solution and in the solid state and to be highly sterically crowded, as evidenced by the variable temperature NMR studies. DFT (B3LYP/LANL2DZ) calculations on the model complexes [CaH(C5Me4H)], [CaH(C13H9)] and [CaH(C9H7)] indicate that the precise structures of such heteroleptic compounds are a result of both stereoelectronic and steric influences. Attempts to isolate the unsubstituted cyclopentadienyl were unsuccessful, but resulted in the crystallographic analysis of the dimeric calcium siloxide [HC{(C(Me)N(Dipp))}2Ca(μ-OSiMe3)]2.
The calcium-catalyzed intramolecular hydroamination of alkenes and alkynes is reported. The beta-diketiminato complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}-Ca{N(SiMe3)2}(THF)] affects catalytic cyclization of a range of aminoalkenes and aminoalkynes with activities that are broadly commensurate to those of established rare earth catalysts.
The “selective” protonolysis of the β-diketiminato calcium derivative [Ca{(NDippCMe)2CH}{N(SiMe3)2}(THF)] (Dipp = C6H3iPr2-2,6) with a selection of terminal alkynes has produced a series of dimeric acetylides [Ca{(NDippCMe)2CH}{μ-CCR}]2, which have been characterized in solution and in the solid state. The asymmetry of the Ca−C−Ca‘ bridges is subtly dependent upon the extent of “side-on” (π-type) interaction between the acetylide unit and the Ca‘ centers.
The potential of the heteroleptic heavier alkaline-earth hexamethyldisilazides [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ae{N(SiMe3)2}(THF)] (Ae = Ca, Sr, Ba) as kinetically-stable reagents for further protolytic reaction chemistry has been assessed. Only the previously reported calcium complex was found to be stable to solution dismutation and dynamic ligand exchange. The barium complex was isolated in sufficient purity to enable characterisation by an X-ray analysis. Reactions of the kinetically robust calcium complex with cyclohexylamine and tert-butylamine resulted in displacement of THF and formation of solvated structures, which could be characterised by 1H NMR spectroscopy. Attempts to isolate these solvated complexes were unsuccessful due to redistribution to the homoleptic complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}2Ca]. In contrast, the more acidic amine [H2NCH2CH2OMe] was cleanly deprotonated resulting in the isolation of the first well defined primary amido derivative of a heavier alkaline-earth element, [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{NHCH2CH2OMe}]2, which retains its dimeric constitution in solution and is stable to further intermolecular ligand exchange. Reactions of [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(SiMe3)2}(THF)] with a variety of ortho-disubstituted anilines also resulted in immediate protonation of the hexamethyldisilazide ligand. Only the most sterically demanding 2,6-diisopropylphenyl anilide derivative possessed sufficient kinetic stability to allow isolation of the heteroleptic complex. The crystal structure of [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(H)-2,6-iPrC6H3}(THF)] was shown to exist as a mononuclear, pseudo-five-coordinate complex in which the coordinative unsaturation of the calcium centre is relieved by a Ca⋯H–C agostic-type interaction to one of the ortho isopropyl groups of the anilide ligand. This interaction is not maintained in solution however and the complex slowly redistributes to the homoleptic β-diketiminato species and ill-defined polymeric calcium anilido products.
'Selective' protonolysis of the beta-diketiminato calcium derivative [Ca[(NDippCMe)(2)CH][N(SiMe(3))(2)](THF)] Dipp = C(6)H(3)(i)Pr(2)-2,6) with H(2)N(CH(2))(2)OCH(3) produced the dimeric species [Ca[(NDippCMe)(2)CH][mu-NH(CH(2))(2)OMe]](2), which has been fully characterised in solution and in the solid state.

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Affiliations

Imperial College London
Department
  • Department of Chemistry
University of Bristol
Department
  • School of Chemistry
University of Bath
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  • Department of Chemistry
University of California, Berkeley
Department
  • Department of Chemistry
University College London
Department
  • Department of Chemistry

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