[show abstract][hide abstract] ABSTRACT: Reaction of (PNP)Li with TaF5 produces pentagonal-bipyramidal (PNP)TaF4 (2). Alkylation of 2 with MeMgBr allows for the isolation of (PNP)TaMe4 (3). (PNP)TaMe4 (3) evolves thermally and/or photochemically into a bis(methylidene) complex (PNP)Ta(CH2)2 (4). The identity of the latter has been established by X-ray structural, NMR spectroscopic, and DFT computational studies. It does not appear that 4 possesses agostic interactions in solution.
[show abstract][hide abstract] ABSTRACT: Operationally unsaturated (i.e., 16/18-electron) (PNPR)Re(H)4, where PNPR is N(SiMe2CH2PR2)2, is reactive at 22 degrees C with cyclic olefins. The first observed products are generally (PNPR)Re(H)2(cycloalkylidene), with hydrogenated olefin as the product of hydrogen abstraction from the tetrahydride. The tetrahydride complex with R = tBu generally fails to react (too bulky), that with R = cyclohexyl suffers a (controllable) tendency to abstraction of 3H from one ring, forming an eta3-cyclohexenyl compound, and that with R = iPr generally gives the richest bimolecular reactivity. The cyclic monoolefins studied show distinct reactivity, C6 giving first the carbene and then coordinated cyclohexadiene, C5 giving carbene, then diene, and then eta5-C5H5, C8 giving carbene and then eta2-cyclooctyne, and C12 giving an eta3-allyl. Norbornene gives a pi-complex of the norbornene in thermal equilibrium with its carbene isomer; at 90 degrees C, hydrocarbon ligand Calpha-Cbeta bond cleavage occurs to give, for the first time, a carbyne complex from an internal olefin. Two compounds synthesized here have the formal composition "(PNPR)Re + olefin", and each of these is capable of dehydrogenating the methyl group of a variety of alkanes at 110 degrees C to form (PNP)ReH triple bond (CR).
Journal of the American Chemical Society 06/2007; 129(18):6003-16. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: A series of titanium complexes containing a terminal neopentylidene functionality have been prepared by a one electron oxidatively induced α-hydrogen abstraction from the corresponding bis-neopentyl precursor (Nacnac)Ti(CH2tBu)2 (Nacnac- = [Ar]NC(CH3)CHC(CH3)N[Ar], Ar = 2,6-(CHMe2)2C6H3), among them (Nacnac)TiCHtBu(OTf) and (Nacnac)TiCHtBu(I). It was determined that bulky alkyl groups bound to titanium as well as a bulky coordinating anion from the oxidant are needed to promote α-hydrogen abstraction. Complex (Nacnac)TiCHtBu(OTf) serves as a template for other four-coordinate titanium neopentylidene complexes such as (Nacnac)TiCHtBu(X) (X- = Cl, Br, and BH4). Complexes (Nacnac)TiCHtBu(X) undergo cross-metathesis reactivity with the imine functionality of the Nacnac- ligand forming the imido complexes (HtBuCC(Me)CHC(Me)N[Ar])TiNAr(X) (X- = OTf, Cl, Br, I, BH4). In addition, C−H activation of two tertiary carbons also takes place to afford the titanacycles Ti[2,6-(CMe2)(CHMe2)C6H3]NC(Me)CHC(Me)N[2,6-(CMe2)(CHMe2)C6H3](X) (X- = OTf, Cl, Br and η2-BH4). Kinetic studies in C6D6 reveal the formation of (HtBuCC(Me)CHC(Me)N[Ar])TiNAr(I) from (Nacnac)TiCHtBu(I) to be independent of solvent (C6D6, Et2O−d10, THF-d8) and the reaction to be first order in titanium (k = 8.06 × 10-4 s-1 at 57 °C, with activation parameters ΔH = 21.3(2) kcal/mol, ΔS = −8(3) cal/mol K). Compound (Nacnac)TiCHtBu(OTf) reacts with various substrates to afford products in which the alkylidene functionality has been significantly transformed. When the alkylidene derivatives (NacnactBu)TiCHtBu(X) (X- = OTf, I; NacnactBu- = [Ar]NC(tBu)CHC(tBu)N[Ar]) were prepared, the intramolecular cross-metathesis transformation observed with (Nacnac)TiCHtBu(X) was inhibited completely.
[show abstract][hide abstract] ABSTRACT: A Ru center ligated by a pincer bis(o-phosphinoaryl)amine (PNP) ligand in (PNP)RuH(H2) is sufficiently electron rich to break C−O and C−C bonds, resulting in the ultimate decarbonylation of hydrocarbonate and acetone, respectively, to give (PNP)RuH(CO). The decarbonylation of acetone is accompanied by hydrogenolysis of the C−C bonds to produce methane.
[show abstract][hide abstract] ABSTRACT: Synthesis of (PNP(R))ReOCl(2) (PNP(R) = (R(2)PCH(2)SiMe(2))(2)N, R = (i)()Pr, Cy, and (t)()Bu) from (Me(2)S)(2)ReOCl(3) and (PNP(R))MgCl is described. Magnesium and H(2) convert (PNP(R))ReOCl(2) first to (PNP(R))ReO(H)(2) and then to (PNP(R))Re(H)(4), the last being an operationally unsaturated species which can bind PMe(3) or p-toluidine. Acyclic alkenes react with (PNP(R))Re(H)(4) at 22 degrees C to give first (PNP(R))Re(H)(2)(olefin) and then (PNP(R))ReH(carbyne), in equilibrium with its eta(2)-olefin adduct. Re can also migrate to the terminal carbon of internal olefins to form a carbyne complex. Allylic C-SiMe(3) or C-NH(2) bonds are not broken, but OEt, OPh, and F vinyl substituents (X) are ultimately cleaved from carbon to give the ReC-CH(3) complex and liberate HX. DFT calculations, together with detection of intermediates for certain olefins, help to define a mechanism for these conversions.
Journal of the American Chemical Society 06/2004; 126(20):6363-78. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Both (PNP)Re(H)(4) and (PNP)ReH(cyclooctyne) (PNP(i)(Pr) = ((i)Pr(2)PCH(2)SiMe(2))(2)N) react with alkylpyridines NC(5)H(4)R to give first (PNP)ReH(2)(eta(2)-pyridyl) and cyclooctene and then, when not sterically blocked, (PNP)Re(eta(2)-pyridyl)(2) and cyclooctane. The latter are shown by NMR, X-ray diffraction, and DFT calculations to have several energetically competitive isomeric structures and pyridyl N donation in preference to PNP amide pi-donation. DFT studies support NMR solution evidence that the most stable bis pyridyl structure is one that is doubly eta(2)- with the pyridyl N donating to the metal center. When both ortho positions carry methyl substituents, cyclooctane and the carbyne complex (PNP)ReH(tbd1;C-pyridyl) are produced. Excess 2-vinyl pyridine reacts with (PNP)Re(H)(4) preferentially at the vinyl group, to give 2-ethyl pyridine and the sigma-vinyl complex (PNP)ReH[eta(2)-CH=CH(2-py)]. The DFT and X-ray structures show, by various comparisons, the ability of the PNP amide nitrogen to pi-donate to an otherwise unsaturated d(4) Re(III) center, showing short Re-N distances consistent with the presence of pi-donation.
Journal of the American Chemical Society 03/2004; 126(7):2105-13. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Presented on the occasion of the 70th birthday of Prof J.J. Ziółkowski, in recognition of the great vision he has brought to the Institute of Chemistry in Wrocław. Abstract The reaction of NO with highly unsaturated, triplet spin state (PNP)Ru II Cl ("PNP" = (t Bu 2 PCH 2 SiMe 2) 2 N) in benzene at 20 • C is reported. The reaction proceeds through three major intermediate species, ultimately forming [(PNP)Ru(NO) 2 + ][Ru(NO)(OH)(NO 2) 2 Cl 2 − ], whose structure is determined by X-ray diffraction. The implications of PNP ligand loss, NO 2 − production, and partial oxidation of ruthenium to Ru(III) (the anion above) are discussed, together with the observed oxygen transfer which represents NO disproportionation. The two NO ligands in the cation are chemically inequivalent (one bent, NO − , and one linear, NO +), features which are studied by density functional theory (DFT) geometry optimization. Two isomers of (PNP)Ru(NO) 2 Cl, as well as (PNP)Ru(NO)Cl are evaluated as possible reaction intermediates by DFT geometry optimization.
Journal of Molecular Catalysis A Chemical 01/2004; 224:51-59. · 3.19 Impact Factor
[show abstract][hide abstract] ABSTRACT: The reaction of alkynes RCCH (R = H, Ph) with (PNP)RuCl, where PNP is (tBu2PCH2SiMe2)2N, occurs rapidly below 23 °C to give first an η2-alkyne adduct and then a final product with a vinylidene group, CCHR, inserted into the N−Ru bond. Characterization included X-ray diffraction (R = Ph) and DFT calculations to probe mechanistic aspects of the reaction.
[show abstract][hide abstract] ABSTRACT: (PNPtBu)Re(H)4, where PNPtBu is (tBu2PCH2SiMe2)2N, reacts at 23 °C with RCCH (R = tBu, SiMe3, Ph) to give first H2 and mirror-symmetric (PNPtBu)ReH3(CCR), then H2 and C2ν symmetric (PNPtBu)Re(CCR)2. The diacetylide compounds show temperature-independent paramagnetism and 13C and 31P chemical shifts far beyond their normal values for other (PNPtBu)ReXn compounds. Single-crystal X-ray diffraction shows very similar structures for the cases R = Ph and R = SiMe3, each having an approximately C2v geometry with equivalent acetylides with C−Re−C approximately 108°. No hydride or H2 ligands are detected in final difference Fourier maps. DFT(B3PW91) calculations give minimum energy geometries of these species, of their products upon adding H2, and of mechanistically significant analogues [(H2PCH2SiH2)2N]ReHnR‘mH2-m, with n = 0, 2, m = 1, 2, and R‘ = H or Ph. These calculated geometries, when compared to those from X-ray diffraction, indicate that the isolated compounds have no hydride or H2 ligands and are thus (PNP)ReIII(CCR)2, making them more unsaturated than the reagent (PNP)ReV(H)4 by two electrons. Triplet state geometries of (PNP)ReXY are calculated and analyzed, as are their frontier orbitals.
[show abstract][hide abstract] ABSTRACT: The X-ray diffraction structures of the olefin complexes [CpFe(CO)2(H2CCHDo)]PF6 (Do=OEt and NMe2) have been determined to further evaluate the previous report that the distance from Fe to the olefin carbon substituted by Do
(referred to as Cβ) is long or even nonbonding. These Fe–C distances are determined here to be long [2.402(10)
Å for Do=OEt] or nonbonding [2.823(11)
Å for Do=NMe2]. DFT optimization of the geometries of these, together with CpFe(CO)2−n(PH3)n(H2CCHDo])+ for n=1 and 2, show (a) agreement with experiment for n=0, (b) a progression of Fe–Cβ distances to shorter values with increasing n for Do=OEt, (c) persistence of the Fe–Cβ distance at a nonbonding value for all n when Do=NMe2 and (d) the shortest Fe–Cβ distances for the weakest π-donor substituent, Do=F. These results are rationalized in terms of increased localization of nucleophilicity on the olefin Cα as the π-donor ability of Do strengthens. Therefore, not all olefins will show η2-binding.
New Journal of Chemistry 11/2003; 27(12):1769-1774. · 2.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Treatment of the terminal titanium–phosphinidene (Nacnac)TiPMes*(CH2tBu)
= 2,4,6-tBu3C6H2) with CNtBu and N2CPh2, in pentane at −35 °C, affords η2-(N,C)-phosphaazaallene and phosphinylimide complexes, respectively.
[show abstract][hide abstract] ABSTRACT: The molecule (PNPR)Re(H)4 (PNPR = (R2PCH2SiMe2)2N, R = iPr or cyclohexyl) reacts at 20 degrees C with 2 mol of cyclohexene to form equimolar cyclohexane and (PNPR)Re(H)2[=C(CH2)5]. This product is characterized by 1H, 13C, and 31P NMR and by X-ray diffraction as having one CH2 hydrogen (from a carbon located beta to Re) donating to the metal ("agostic CH"). This interaction occurs in preference to PNPR amide nitrogen pi-donation. DFT calculations confirm this agostic interaction, and show that the (PNPR)Re(H)2 fragment indeed reverses the greater stability of free olefin vs free carbene.
Journal of the American Chemical Society 09/2003; 125(32):9604-5. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The ligand (tBu2PCH2SiMe2)2N1- (PNP) in [PNP]RuCl leads to an intermediate spin ground state, S = 1, which has been characterized by NMR and X-ray diffraction as having a planar structure. This spin state is attributed in part to N --> Ru pi donation. DFT calculations confirm that the singlet state lies higher in energy and is nonplanar. The molecule is converted to a diamagnetic product by addition of 2 mol of PhCN. The half-filled orbitals of the S = 1 state are suggested to be the reason agostic interactions do not compensate for the 14-valence electron count.
Journal of the American Chemical Society 08/2003; 125(28):8426-7. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The chemistry of the ligand (R2PCH2SiMe2)(2)N- (R = cyclohexyl and Bu-t), "PNP-R", on ruthenium is developed, including RuH(PNP-Cy)(PPh3) and (HPNP-R)RuH3Cl. The latter contains a protonated nitrogen (i.e., amine as a donor to Ru) and one H-2 ligand (X-ray structure for R = Bu-t). This compound can be dehydrohalogenated to give (PNP-Cy)RuH3, which undergoes H/D exchange of D-2 into its cyclohexyl rings, and is itself dehydrogenated by excess H2C=CHR to give [Cy2PCH2SiMe2NSiMe2CH2PCy(C6H8)] Ru, which contains a triply dehydrogenated cyclohexyl ring p allyl bonded to Ru. ( PNP-Cy) RuH3 reacts with dihydrofurans to give the heteroatom-stabilized carbene complex (PNP-Cy)RuH[=CO(CH2)(3)].
New Journal of Chemistry 01/2003; 27(2):263-273. · 2.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Reaction of [RuHClL2](2) (L = (PPr3)-Pr-i) with the C/C unsaturated cyclic carbene:C(NMeCH)(2) produces the 16-electron square-pyramidal RuHCl[CNMeCH=CHNMe]L-2 by a chloride bridge-splitting reaction. Double H2C(sp(3)) dehydrogenation of cyclic H2C(NMeCH2)(2) is successful for producing the C/C saturated carbene (boundto Ru); the two hydrogens removed are found as RuHCl(H-2)L-2. In this case, the free carbene is unstable with respect to dimerization to the olefin. The C-13 chemical shifts of the carbene carbons of these two complexes, the Ru/C distance, the N-C(carbene) distance, and a variety of reaction energies (from DFT calculations) and calculated atomic charges are generally consistent with these two carbenes, aromatic and non-aromatic, both binding similarly, and with little back donation from this electron-rich center. The C-13 chemical shifts are perhaps the most sensitive parameter. Collectively, these results suggest that, if the C/C unsaturated and the C/C saturated Arduengo carbenes differ in their binding to this electron-rich metal center, the difference is at or below detection limits.
New Journal of Chemistry 01/2003; 27(10):1446-1450. · 2.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The reaction of (R(2)PCH(2)SiMe(2))(2)NM (PNP(R)M; R = Cy; M = Li, Na, MgHal, Ag) with L(2)ReOX(3) [L(2) = (Ph(3)P)(2) or (Ph(3)PO)(Me(2)S); X = Cl, Br] gives (PNP(Cy))ReOX(2) as two isomers, mer,trans and mer,cis. These compounds undergo a double Si migration from N to O at 90 degrees C to form (POP(Cy))ReNX(2) as a mixture of mer,trans and fac,cis isomers. Additional thermolysis effects migration of CH(3) from Si to Re, along with compensating migration of halide from Re to Si. DFT calculations on various structural isomers support the greater thermodynamic stability of the POP/ReN isomer vs PNP/ReO and highlight the influence of the template effect on the reactivities of these species.
[show abstract][hide abstract] ABSTRACT: The reaction of (R2PCH2SiMe2)2NM (PNPRM; R = Cy; M = Li, Na, MgHal, Ag) with L2ReOX3 [L2 = (Ph3P)2 or (Ph3PO)(Me2S); X = Cl, Br] gives (PNPCy)ReOX2 as two isomers, mer,trans and mer,cis. These compounds undergo a double Si migration from N to O at 90 °C to form (POPCy)ReNX2 as a mixture of mer,trans and fac,cis isomers. Additional thermolysis effects migration of CH3 from Si to Re, along with compensating migration of halide from Re to Si. DFT calculations on various structural isomers support the greater thermodynamic stability of the POP/ReN isomer vs PNP/ReO and highlight the influence of the template effect on the reactivities of these species.
[show abstract][hide abstract] ABSTRACT: DFT calculations on a [N(SiH2CH2PH2)2]MH3 model (M = Ru and Os) of experimentally known compounds show that the three H have different bonding modes for Ru (H + H2) and for Os (three independent H). Calculation shows that the Ru(H)(H2) compound adds H2 to give an RuH(H2)2 substructure; the Os(H)2 species adds H2 to give the Os(H)3(H2) substructure. The impact of these intramolecular redox processes on bond lengths is discussed, as are attempts to provide an improved computational model of P(alkyl) n without the introduction of additional alkyl atoms.