Silyl Hydride Complexes Research Articles

Breaking Conventional Site Selectivity in C-H Bond Activation: Selective sp3 versus sp2 Silylation by a Pincer-Based Pocket.

A deeply ingrained assumption in the conventional understanding and practice of organometallic chemistry is that an unactivated aliphatic C(sp3)-H bond is less reactive than an aromatic C(sp2)-H bond within the same molecule given that they are at positions unbiasedly accessible for activation. Herein, we demonstrate that a pincer-ligated iridium complex catalyzes intramolecular dehydrogenative silylation of the unactivated δ-C(sp3)-H (δ to the Si atom) with exclusive site selectivity over typically more reactive ortho δ-C(sp2)-H bonds. A variety of tertiary hydrosilanes undergo δ-C(sp3)-H silylation to form 5-membered silolanes, including chiral silolanes, which can undergo further oxidation to produce enantiopure β-aryl-substituted 1,4-diols. Combined computational and experimental studies reveal that the silylation occurs via the Si-H addition to a 14-electron Ir(I) fragment to give an Ir(III) silyl hydride complex, which then activates the C(sp3)-H bond to form a 7-coordinate, 18-electron Ir(V) dihydride silyl intermediate, followed by sequential reductive elimination of H2 and silolane. The unprecedented site selectivity is governed by the distortion energy difference between the rate-determining δ-C(sp3)-H and δ-C(sp2)-H activation, although the activation at sp2 sites is much more favorable than sp3 sites by the interaction energy.

Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C-H Borylation of Arenes and Alkanes.

Indenyl (Ind) rhodium N‐heterocyclic carbene (NHC) complexes [Rh(η 5‐Ind)(NHC)(L)] were synthesised for 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIPr) with L=C2H4 (1), CO (2 a) and cyclooctene (COE; 3), for 1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C2H4)2] did not give the desired SIPr complex, thus demonstrating the “indenyl effect” in the synthesis of 1. Oxidative addition of HSi(OEt)3 to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)3}(H)(SIPr)] (6) with loss of COE. Tethered‐fluorenyl NHC rhodium complexes [Rh{(η 5‐C13H8)C2H4N(C)C2H x NR}(L)] (x=4, R=Dipp, L=C2H4: 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5–31 %) in comparison to good yields for the monodentate complexes (49–79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B2pin2 (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl‐tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B2pin2 at 120 and 140 °C, respectively, was monitored by 11B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp3 C−H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C2H4 and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained.

An H-Substituted Rhodium Silylene.

Divergent reactivity of organometallic rhodium(I) complexes, which led to the isolation of neutral rhodium silylenes, is described. Addition of PhRSiH2 (R=H, Ph) to the rhodium cyclooctene complex (iPr NNN)Rh(COE) (1-COE; iPr NNN=2,5-[iPr2 P=N(4-iPrC6 H4 )]2 N(C6 H2 )- , COE=cyclooctene) resulted in the oxidative addition of an Si-H bond, providing rhodium(III) silyl hydride complexes (iPr NNN)Rh(H)SiHRPh (R=H, 2-SiH2 Ph; Ph, 2-SiHPh2 ). When the carbonyl complex (iPr NNN)Rh(CO) (1-CO) was treated with hydrosilanes, base-stabilized rhodium(I) silylenes κ2 -N,N-(iPr NNN)(CO)Rh=SiRPh (R=H, 3-SiHPh; Ph, 3-SiPh2 ) were isolated and characterized using multinuclear NMR spectroscopy and X-ray crystallography. Both silylene species feature short Rh-Si bonds [2.262(1) Å, 3-SiHPh; 2.2702(7) Å, 3-SiPh2 ] that agree well with the DFT-computed structures. The overall reaction led to a change in the iPr NNN ligand bonding mode (κ3 →κ2 ) and loss of H2 from PhSiRH2 , as corroborated by deuterium labelling experiments.

N‐Heterocyclic Carbene Iron Silyl Hydride Complexes

AbstractReaction of N‐heterocyclic carbene iron(0) tetracarbonyl complexes with hydrosilanes (triphenylsilane, methyldiphenylsilane and diphenylsilane) under UV irradiation (350 nm) afforded the octahedral N‐heterocyclic carbene iron(II) silyl hydride complexes in yields up to 95 %, resulting from the oxidative addition of the hydrosilane. Eight iron silyl hydrides were obtained and characterized by NMR and X‐ray diffraction analysis. Noticeably, the geometry of the iron‐hydride complexes and the bent structures of the carbonyl ligands were rationalized by DFT calculations.

Unusual Structure, Fluxionality, and Reaction Mechanism of Carbonyl Hydrosilylation by Silyl Hydride Complex [(ArN)Mo(H)(SiH2Ph)(PMe3)3

The reactions of bis(borohydride) complexes [(RN=)Mo(BH4)2(PMe3)2] (4: R = 2,6-Me2C6H3; 5: R = 2,6-iPr2C6H3) with hydrosilanes afford new silyl hydride derivatives [(RN=)Mo(H)(SiR'3)(PMe3)3] (3: R = Ar, R'3 = H2Ph; 8: R = Ar', R'3 = H2Ph; 9: R = Ar, R'3 = (OEt)3; 10: R = Ar, R'3 = HMePh). These compounds can also be conveniently prepared by reacting [(RN=)Mo(H)(Cl)(PMe3)3] with one equivalent of LiBH4 in the presence of a silane. Complex 3 undergoes intramolecular and intermolecular phosphine exchange, as well as exchange between the silyl ligand and the free silane. Kinetic and DFT studies show that the intermolecular phosphine exchange occurs through the predissociation of a PMe3 group, which, surprisingly, is facilitated by the silane. The intramolecular exchange proceeds through a new non-Bailar-twist pathway. The silyl/silane exchange proceeds through an unusual Mo(VI) intermediate, [(ArN=)Mo(H)2(SiH2Ph)2(PMe3)2] (19). Complex 3 was found to be the catalyst of a variety of hydrosilylation reactions of carbonyl compounds (aldehydes and ketones) and nitriles, as well as of silane alcoholysis. Stoichiometric mechanistic studies of the hydrosilylation of acetone, supported by DFT calculations, suggest the operation of an unexpected mechanism, in that the silyl ligand of compound 3 plays an unusual role as a spectator ligand. The addition of acetone to compound 3 leads to the formation of [trans-(ArN)Mo(OiPr)(SiH2Ph)(PMe3)2] (18). This latter species does not undergo the elimination of a Si-O group (which corresponds to the conventional Ojima's mechanism of hydrosilylation). Rather, complex 18 undergoes unusual reversible β-CH activation of the isopropoxy ligand. In the hydrosilylation of benzaldehyde, the reaction proceeds through the formation of a new intermediate bis(benzaldehyde) adduct, [(ArN=)Mo(η(2)-PhC(O)H)2(PMe3)], which reacts further with hydrosilane through a η(1)-silane complex, as studied by DFT calculations.

An Unexpected Mechanism of Hydrosilylation by a Silyl Hydride Complex of Molybdenum

Carbonyl hydrosilylation catalyzed by (ArN)Mo(H)(SiH(2)Ph)(PMe(3))(3) (3) is unusual in that it does not involve the expected Si-O elimination from intermediate (ArN)Mo(SiH(2)Ph)(O(i)Pr)(PMe(3))(2) (7). Instead, 7 reversibly transfers β-CH hydrogen from the alkoxide ligand to metal.

Synthesis and reactivity of rhodium and iridium alkene, alkyl and silyl complexes supported by a phenyl-substituted PNP pincer ligand

New rhodium and iridium complexes supported by the phenyl-substituted PNP pincer ligand PNPPhH (HN(2-PPh2-4-Me-C6H3)2) (1) were synthesized. The reaction of 2 equiv. of 1 with [(COD)IrCl]2 afforded the coordination complex [(PNPPhH)Ir(COD)]Cl (2) featuring hydrogen bonding between the N–H group and the chloride anion, as characterized by NMR spectroscopy and X-ray crystallography. Reaction of 1 with [(COE)2IrCl]2 or [(COE)2RhCl]2 in benzene provided a mixture of complexes including (PNPPhH)MHCl2 (M = Ir (4), M = Rh (7)) and (PNP)M(COE) (M = Ir (5), M = Rh (8)). Alkene complexes of the type (PNPPh)M(L) (M = Ir, L = COD (3) and COE (5); M = Rh, L = COE (8) and L = ethylene (9)) were synthesized by reaction of (PNPPh)Li with the appropriate alkene chloride complexes. Reactions of silanes with 5, 8 or 9 produced silyl hydride complexes (PNPPh)MH(SiR3) (M = Ir, R = Ph (16) and R = Et (17); M = Rh, R = Ph (18), Et (19) and Ph2Cl (20)) via Si–H oxidative addition. The JSiH coupling constants for rhodium complexes 18, 19 and 20 were determined to be ca. 35 Hz, while iridium complexes 16 and 17 exhibited coupling constants less than 10 Hz. X-Ray crystal structures of 16 and 18 reveal isostructural complexes featuring a trigonal bipyramidal geometry about iridium with a mer binding of the PNPPh ligand. A hydride ligand, located from the Fourier map for 18, has a short contact of 1.83(3) Å with the silicon atom. Oxidative addition of iodomethane to 5 and 8 afforded (PNPPh)M(Me)(I)(THF) (M = Rh (14), M = Ir (12)), respectively. Arene C–H activation upon thermolysis of 12 in benzene produced (PNPPh)M(Ph)(I)(THF). Iridium silyl iodide complexes (PNPPh)IrI(SiR3) (SiR3 = SiPh3 (21), SiH2Mes (22) and SiH2Xyl (23)) resulted from addition of organosilanes to 12, via elimination of CH4.

Synthesis and Structure of PNP-Supported Iridium Silyl and Silylene Complexes: Catalytic Hydrosilation of Alkenes

Oxidative addition of bulky primary, secondary, and tertiary silanes to PNP (PNP = [N(2-P(i)Pr(2)-4-Me-C(6)H(3))(2)](-)) iridium complexes (PNP)IrH(2) and (PNP)Ir(COE) (11) afforded iridium silyl hydride complexes (PNP)Ir(H)(SiRR'R'') (3-8). Addition of 2 equiv of PhSiH(3) or (3,5-Me(2)C(6)H(3))SiH(3) to (PNP)IrH(2) or 11 yielded disilyl complexes (PNP)Ir(SiH(2)R)(2) (R = Ph (9), 3,5-Me(2)C(6)H(3) (10)). Hydride abstraction from (PNP)Ir(H)(SiH(2)R) (R = Trip (5), Dmp (6)) by [Ph(3)C][B(C(6)F(5))(4)] afforded iridium silylene complexes [(PNP)(H)Ir=SiR(H)][B(C(6)F(5))(4)] (R = Trip (12), Dmp (13)) exhibiting downfield (29)Si NMR resonances (234 ppm (12), 226 ppm (13)) and downfield (1)H NMR resonances for the Si-H group (10.76 ppm (12), 9.76 ppm (13)). Thermally stable disubstituted silylene complexes [(PNP)(H)Ir=SiPh(2)][A] (A = (-)B(C(6)F(5))(4) (14), (-)CB(11)H(6)Br(6) (16)) were isolated via hydride abstraction from (PNP)Ir(H)(SiHPh(2)). The X-ray structure of 16 confirmed sp(2) hybridization at silicon and revealed a short Ir-Si bond of 2.210(2) A. Catalytic hydrosilation of alkenes by hydrogen-substituted silylene complexes [(PNP)(H)Ir=SiMes(H)][B(C(6)F(5))(4)] (1) and 14 exhibited anti-Markovnikov regioselectivity with an array of alkene substrates. Addition of H(3)SiMes to complex 1 afforded [(PNP)(SiH(Mes)(Hex))IrH(SiH(2)Mes)][B(C(6)F(5))(4)] (19), featuring a beta-agostic interaction demonstrated by a J(SiH) of 102 Hz for the N-SiH hydrogen. Similarly, addition of H(2)SiPh(2) to 16 afforded the structurally characterized Ir(V) disilyl complex [(PNP)(SiPh(2))Ir(SiPh(2)H)(H)(2)][CB(11)H(6)Br(6)] (20). Complex 20 was found to be catalytically active for the hydrosilation of alkenes, which is consistent with its intermediacy in the catalytic cycle.

Alkene Hydrosilation by a Cationic Hydrogen-Substituted Iridium Silylene Complex

A cationic hydrogen-substituted iridium silylene complex [(PNP)(H)Ir Si(Mes)H][B(C6F5)4] (2) was synthesized via hydride abstraction from the corresponding neutral iridium silyl hydride complex. DFT calculations for 2 indicate that the cationic charge is localized at the silicon center and depict a LUMO with predominant silicon p-orbital character. Notably, complex 2 reacts rapidly with unhindered alkenes at ambient temperatures to afford disubstituted silylene complexes via Si-C bond formation. Complex 2 is also the catalyst for alkene hydrosilation of primary silanes with a high degree of anti-Markovnikov selectivity.

Unexpected effect of the ring on the extent of Si⋯H interligand interactions in half-sandwich silyl hydrides of ruthenium

This paper reports preparation of new silyl hydride complexes of ruthenium supported by the Cp/PR(3) ligand set. It is shown that the easiest and most general route to these complexes is provided by the thermal reaction of [RuCp(PR(3))(H)(3)] with hydrosilanes. Complexes [RuCp(PR(3))(H)(2)(SiMe(2)Cl)] exhibit Interligand Hypervalent Interactions (IHI) between the hydride and silyl ligands. Comparison of the X-ray structures of complexes [RuCp(PPr(i)(3))(H)(2)(SiMe(2)Cl)], [RuCp(PPhPr(i)(2))(H)(2)(SiMe(2)Cl)], and [RuCp(PPh(3))(H)(2)(SiMe(2)Cl)] shows that the IHI weakens with the decreasing electron-releasing ability of the phosphine. Comparison of the X-ray structure of [RuCp(PPh(3))(H)(2)(SiMe(2)Cl)] with the structure of the previously reported complex [RuCp*(PPh(3))(H)(2)(SiMe(2)Cl)] reveals that the lack of IHI in the latter compound is due to unfavourable steric interactions between the bulkier Cp* ring and the SiMe(2)Cl group.

Rhodium Silyl Hydrides in Oxidation State +5: Classical or Nonclassical?

Four families of silyl hydride complexes of rhodium in the formal oxidation state +5 were investigated by means of DFT calculations, supplemented by the calculation of Mayer bond indices and Si−H coupling constants. In each case some degree of interligand Si−H interaction has been found. In the compounds CpRh(SiMe3)2H2 (1), CpRh(SiMe3)3H (2), CpRh(SiMe3)2(SiEt3)H (3), and [Cp(Me3P)Rh(SiMe3)2H]+ (4) the hydride(s) interact(s) with both silyl ligands. Relaxed potential energy scans indicate that the potential energy surface is extremely flat. It takes only 1 kcal·mol-1 to compress the Si−H bond from 2.3 A to 2.0 A in 1, and 2 kcal·mol-1 to compress the Si−H distance from 1.990 A to 1.70 A in complex 2. ONIOM calculations of the compound CpRh(SiMe3)3H and optimization of the lowest energy conformers of complex CpRh(SiMe3)2(SiEt3)H show that their geometries are largely determined by steric effects. Increasing steric hindrance promotes Si−H interactions because they result in longer Rh−Si bond lengths, leadin...

Low temperature in situ UV irradiation of [(η5-C5H5)Co(C2H4)2] in the NMR probe: formation of Co(iii) silyl hydride complexes

Low temperature in situ UV irradiation of [(eta(5)-C(5)H(5))Co(C(2)H(4))(2)] in the presence of silanes enables the characterisation of unstable fluxional Co(III) silyl hydride complexes [(eta(5)-C(5)H(5))Co(SiR(3))(H)(C(2)H(4))] (SiR(3) = SiEt(3), SiMe(3) or SiHEt(2)) by NMR spectroscopy; the reaction of [Co(eta(5)-C(5)H(5))(C(2)H(4))(2)] with HSiR(3) proceeds thermally to reach an equilibrium when SiR(3) = Si(OMe)(3) or SiClMePh.

Unique {H(SiR3)2}, (H2SiR3), H(HSiR3), and (H2)SiR3Ligand Sets Supported by the {Fe(Cp)(L)} Platform (L=CO, PR3)

This work deals with the type and incidence of nonclassical Si--H and H--H interactions in a family of silylhydride complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(X)] (X=SiMe(n)Cl(3-n), H, Me, n=0-3) and [Fe(Cp)(Me(3)P)(SiMe(n)Cl(3-n))(2)H] (n=0-3). DFT calculations complemented by atom-in-molecule analysis and calculations of NMR hydrogen-silicon coupling constants revealed a surprising diversity of nonclassical Si--H and H--H interligand interactions. The compounds [Fe(Cp)(L)(SiMe(n)Cl(3-n))(2)H] (L=CO, PMe(3); n=0-3) exhibit an unusual distortion from the ideal piano-stool geometry in that the silyl ligands are strongly shifted toward the hydride and there is a strong trend towards flattening of the {FeSi(2)H} fragment. Such a distortion leads to short Si--H contacts (range 2.030-2.075 A) and large Mayer bond orders. A novel feature of these extended Si--H interactions is that they are rather insensitive towards the substitution at the silicon atom and the orientation of the silyl ligand relatively the Fe--H bond. NMR spectroscopy and bonding features of the related complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(Me)] (n=0-3) allow for their rationalization as usual eta(2)-Si--H silane sigma-complexes. The series of "dihydride" complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(2)] (n=0-3) is different from the previous two families in that the type of interligand interactions strongly depends on the substitution on silicon. They can be classified either as usual dihydrogen complexes, for example, [Fe(Cp)(OC)(SiMe(2)Cl)(eta(2)-H(2))], or as compounds with nonclassical H--Si interactions, for example, [Fe(Cp)(OC)(H)(2)(SiMe(3))] (16). These nonclassical interligand interactions are characterized by increased negative J(H,Si) (e.g. -27.5 Hz) and increased J(H,H) (e.g. 67.7 Hz).

Comparisons of Photoinduced Oxidative Addition of B−H, B−B, and Si−H Bonds at Rhodium(η5-cyclopentadienyl)phosphine Centers

Ultraviolet irradiation of [Rh(η5-C5H5)(PMe3)(C2H4)] (1a), [Rh(η5-C5H5)(PPh3)(C2H4)] (1b), and [Rh(η5-C5H4CF3)(PMe3)(C2H4)] (1c) (collectively abbreviated as [Rh(Cp‘)(PR3)(C2H4)]) in the presence of HBpin (pinacolate = pin = 1,2-O2C2Me4) results in elimination of C2H4 and B−H oxidative addition, leading to the formation of boryl hydride complexes [Rh(Cp‘)(Bpin)(H)(PR3)]. Complete conversion is achieved in liquid HBpin or by photolysis in hexane at −10 °C. Similarly, photolysis of 1a−c in the presence of B2pin2 in hexane at −10 °C leads to B−B oxidative addition products, [Rh(Cp‘)(Bpin)2(PR3)]. Irradiation at room temperature leads to formation of [Rh(Cp‘)(PR3)2] in addition to the desired products. The rhodium boryl products were characterized by multinuclear NMR spectroscopy and, in the case of [Rh(η5-C5H5)(Bpin)(H)(PPh3)], by X-ray crystallography. The structure reveals a Rh−B distance of 2.0196(15) Å. The H···B separation of 2.09(2) Å together with the bond angles at the metal suggest some residual H···B interaction. Photolysis of 1a−c in the presence of tertiary and secondary silanes (HSiEt3, HSiiPr3, HSi(OMe)3, HSiMe2Et, HSiMeEt2, and H2SiEt2) results in rhodium silyl hydride complexes [Rh(Cp‘)(SiR‘2R‘ ‘)(H)(PR3)]. The structure of [Rh(η5-C5H5)(SiiPr3)(H)(PMe3)] was determined by single-crystal X-ray diffraction, yielding a Rh−Si bond length of 2.3617(3) Å and a Rh−H bond length of 1.508(17) Å. The H···Si distance of 2.278(17) Å and the very unequal H−Rh−P and H−Rh−Si angles suggest some residual H···Si interaction. Competition reactions were performed with 1b dissolved in hexane in the presence of HBpin and B2pin2 simultaneously. 31P NMR measurements, made after brief irradiation, showed a slight preference for B−B oxidative addition over B−H oxidative addition. Similar experiments with three-way competition among HBpin, HSiMe2Et, and HC6F5, analyzed by 1H NMR spectroscopy, showed negligible selectivity among H−B, H−C, and H−Si oxidative addition. Molecular structures were also determined by single-crystal X-ray diffraction for 1b, 1c, [Rh(η5-C5H5)(PPh3)2], and [Rh(η5-C5H5)Cl2(PPh3)].

Platinum Bis(tricyclohexylphosphine) Silyl Hydride Complexes

A series of platinum metal silyl hydride complexes, cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ = SiPh2H, SiEt2H, SiPh3, SiEt3, SiMe2(OSiHMe2), Si(OSiMe3)2Me, SiMe2(CH2CHCH2), SiMe2Et, SiMe2[OCH2C(Me)CH2], Si(OMe)2(CH2CHCH2), SiPh2(OSiPh2H)), have been prepared in solution by reaction of Pt(PCy3)2 with the appropriate silane, HSiR2R‘. The complex cis-Pt(PCy3)2(H)(HSiPh2) (1-cis) has been characterized by X-ray crystallography at −100 °C. The platinum center exhibits a distorted-square-planar geometry with angles P(1)−Pt−P(2) = 113.55(3)°, P(1)−Pt−Si = 146.83(3)°, and P(2)−Pt−Si = 99.37(3)°. The reaction of Pt(PCy3)2 with chlorinated hydrosilanes at −78 °C yields the analogous complexes cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ = SiMe2Cl, SiMeCl2, SiCl3), which isomerize to their trans isomers on warming to room temperature. The complex 1-cis and several analogues convert to the trans isomers photochemically at room temperature. Ready silane exchange is demonstrated by the reaction of HSiPh3 with cis-Pt(PCy3)2(D)(SiPh3) and b...

Nonclassical titanocene silyl hydrides.

The titanocene silyl hydride complexes [Ti(Cp)2(PMe3)(H)(SiR3)] [SiR3=SiMePhCl (6), SiPh2Cl (7), SiMeCl2 (8), SiCl3 (9)] were prepared by HSiR3 addition to [Ti(Cp)2(PMe3)2] and were studied by NMR and IR spectroscopy, X-ray diffraction (for 6, 8, and 9), and DFT calculations. Spectroscopic and structural data established that these complexes exhibit nonclassical Ti-H-Si-Cl interligand hypervalent interactions. In particular, the observation of silicon-hydride coupling constants J(Si,H) in 6-9 in the range 22-40 Hz, the signs of which we found to be negative for 8 and 9, is conclusive evidence of the presence of a direct Si-H bond. The analogous reaction of [Ti(Cp)2(PMe3)2] with HSi(OEt)3 does not afford the expected classical silyl hydride complex [Ti(Cp)2(PMe3)(H)[Si(OEt)3]], and instead NMR-silent titanium (apparently TiIII) complex(es) and the silane redistribution product Si(OEt)4 are formed. The structural data and DFT calculations for the compounds [Ti(Cp)2(PMe3)(H)(SiR3)] show that the strength of interligand hypervalent interactions in the chlorosilyl complexes decreases as the number of chloro groups on silicon increases. However, in the absence of an Si-bound electron-withdrawing group trans to the Si-H moiety, a silane sigma complex is formed, characterized by a long Ti-Si bond of 2.658 A and short Si-H contact of 1.840 A in the model complex [Ti(Cp)2(PMe3)(H)(SiMe3)]. Both the silane sigma complexes and silyl hydride complexes with interligand hypervalent interactions exhibit bond paths between the silicon and hydride atoms in Atoms in Molecules (AIM) studies. To date a classical titanocene phosphane silyl hydride complex without any Si-H interaction has not been observed, and therefore titanocene silyl hydrides are, depending on the nature of the R groups on Si, either silane sigma complexes or compounds with an interligand hypervalent interaction.

Tantalizing chemistry of the half-sandwich silylhydride complexes of niobium: identification of likely intermediates on the way to agostic complexes.

Reactions of the compound [NbCp(ArN)(PMe3)2] with chlorosilanes HSiRR'Cl give a series of silyl complexes [NbCp(ArN)(PMe3)(H)-(SiRR'Cl)] and [NbCp(ArN)(PMe3)(Cl)(SiRR'H)] which are likely intermediates to the agostic complexes [NbCp(ArN(RR'2Si-H...))-(PMe3)(Cl)].

Structural and Bonding Characteristics in Transition Metal—Silane Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Neutral and Anionic Silyl Hydride Derivatives of the Tantalum Imido Fragment Cp*(DippN)Ta (Cp* = η5-C5Me5; Dipp = 2,6-iPr2C6H3). Reactive σ-Bonds and Intramolecular C−H Bond Activations Involving the Silyl Ligands

The syntheses and reactivities of alkyl, silyl, and hydride complexes of the type Cp*(DippN)TaRR‘ (Cp* = η5-C5Me5; Dipp = 2,6-iPr2C6H3; R = silyl, alkyl; R‘ = alkyl, hydride) are described. The compounds Cp*(DippN)Ta(SiR3)Cl (SiR3 = Si(SiMe3)3 (1), SiPh3 (2), SiHMes2 (3); Mes = 2,4,6-Me3C6H2), prepared from Cp*(DippN)TaCl2 and (THF)nLiSiR3, react with H2 to yield the dimeric hydride [Cp*(DippN)TaCl(μ-H)]2 (4). The reaction of 4 with (THF)3LiSi(SiMe3)3 (2 equiv) gave the 16-electron silyl hydride complex Cp*(DippN)Ta[Si(SiMe3)3]H (6), which is Lewis acidic and reversibly binds halide ions to afford anionic Ta(V) complexes of the type A+{Cp*(DippN)Ta(X)[Si(SiMe3)3]H}- (A = Li(THF)3, X = Cl (5); A = NBu4, X = Br (7) and I (8)). For complexes 5, 7, and 8, NMR and X-ray data suggest the presence of weak three-center interactions involving Ta, Si, and H. Silyl hydride 6 reacts with xylyl isocyanide (xylyl = 2,6-dimethylphenyl) and acetonitrile to give the corresponding η2-silaimine and azomethine insertion products, respectively, while treatment with acetone yields the stable isopropoxide product arising from insertion into the Ta−H bond. When complex 6 is treated with ethylene or diphenylacetylene, elimination of HSi(SiMe3)3 occurs, along with formation of a five-membered tantalacycle resulting from coupling of the unsaturated substrate at Ta. Hydrogenolysis of 6 yields HSi(SiMe3)3 and a dimeric dihydride [Cp*(DippN)TaH(μ-H)]2 (14). At room temperature, 6 rearranges (t1/2 = 8.5 h) to an unusual alkyl hydride species resulting from C−H bond activation, Cp*(DippN)Ta[CH2Si(SiMe3)2SiMe2H]H (15). This complex exhibits a γ-agostic Si−H interaction with the metal center. An attempt to prepare Cp*(DippN)Ta(SiHMes2)H produced the alkyl hydride product Cp*(DippN)Ta[η2-CH2(2-SiH2Mes-3,5-Me2C6H2)]H (20), which apparently results from decomposition of the expected silyl hydride. The alkyl hydride complex [Cp*(DippN)TaMe(μ-H)]2 (25) was prepared by the hydrogenolysis of Cp*(DippN)Ta[Si(SiMe3)3]Me (23), whereas Cp*(DippN)Ta(CH2CMe3)H (26) was obtained by treatment of 4 with NpMgCl. Complex 26 possesses an α-agostic C−H interaction, and the corresponding deuteride 26-d slowly scrambles deuterium into the methylene positions.

Structural and bonding characteristics in transition metal-silane complexes.

Transition metal-silane complexes containing metal-eta 2-H-Si coordination display different structural and bonding characteristics in comparison to other sigma-complexes, such as dihydrogen and alkane (agostic) complexes. The different characteristics can be related to the strong sigma*-accepting properties of the eta 2-silane ligand(s) because of the weaker H-Si sigma bond. Various examples of metal-silane complexes have been reviewed and their structural stabilities have been systematically discussed. Silyl-hydride complexes having substantial silyl-hydrido interactions have also been emphasized.