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

Imido silyl complexes of molybdenum: synthesis, structure, and unusual H2/silane exchange

Reaction of complex (ArN)Mo(H)(Cl)(PMe3)3 (1) with BPh3 and silane H3SiPh affords a new silyl complex (ArN)Mo(SiH2Ph)(Cl)(PMe3)2 (3) that catalyses facile H–H/Si–H exchange. EXSY experiments suggest that this exchange does not involve the classical pathways such as oxidative addition of hydrogen or silane to the complex or σ-bond metathesis. It is suggested that the exchange may occur in an FLP cage comprised by the complex and the silane.

Synthesis of (Arylmido)niobium(V) Complexes Containing Ketimide, Phenoxide Ligands, and Some Reactions with Phenols and Alcohols

Reactions of Nb(NAr)(N=C t Bu2)3 (3a, Ar = 2,6-Me2C6H3) with 1.0, 2.0, or 3.0 equiv of Ar'OH (Ar' = 2,6- i Pr2C6H3) afforded Nb(NAr)(N=C t Bu2)2(OAr'), Nb(NAr)(N=C t Bu2)(OAr')2, or Nb(NAr)(OAr')3, respectively (at 25 °C), whereas the reaction with 2.0 equiv of 2,6- t Bu2C6H3OH afforded Nb(NAr)(N=C t Bu2)2(O-2,6- t Bu2C6H3) upon heating (70 °C) without the formation of bis(phenoxide) and the reaction of 3a with 2.0 equiv of 2,4,6-Me3C6H2OH afforded Nb(NAr)(N=C t Bu2)(O-2,4,6-Me3C6H2)2(HN=C t Bu2). Similar reactions of 3a with 1.0 equiv of (CF3)3COH or 2.0 equiv of (CF3)2CHOH afforded Nb(NAr)(N=C t Bu2)2[OC(CF3)3](HN=C t Bu2) or Nb(NAr)(N=C t Bu2)[OCH(CF3)2]2(HN=C t Bu2), respectively. On the basis of their structural analyses and the reaction chemistry, it was suggested that these reactions proceeded via coordination of phenol (alcohol) to Nb and the subsequent proton (hydrogen) transfer to the ketimide (N=C t Bu2) ligand. The reaction of Nb(NAr)(N=C t Bu2)2(OAr') with 1.0 equiv of 2,4,6-Me3C6H2OH gave the disproportionation products Nb(NAr)(N=C t Bu2)(OAr')2 and Nb(NAr)(N=C t Bu2)(O-2,4,6-Me3C6H2)2(HN=C t Bu2) with 1:1 ratio, clearly indicating the presence of the above mechanism and the fast equilibrium (between the ketimide and the phenoxide). The reaction of 3a with 1.0 or 2.0 equiv of C6F5OH afforded Nb(N=C t Bu2)2(OC6F5)3(HN=C t Bu2) as the sole isolated product, which was formed from once generated Nb(NAr)(N=C t Bu2)2(OC6F5)(HN=C t Bu2) by treating with C6F5OH.

Triple bonds of niobium with silicon, germaniun and tin: the tetrylidyne complexes [(κ3-tmps)(CO)2Nb[triple bond, length as m-dash]E-R] (E = Si, Ge, Sn; tmps = MeSi(CH2PMe2)3; R = aryl)

A systematic, efficient approach to first complexes containing a triple bond between niobium and the elements silicon, germanium or tin is reported. The approach involves a metathetical exchange of the niobium-centered nucleophile (NMe4)[Nb(CO)4(κ2-tmps)] (1) (tmps = MeSi(CH2PMe2)3) with a suitable organotetrel(ii)halide. Compound 1 was obtained from (NMe4)[Nb(CO)6] and the triphosphane tmps by photodecarbonylation. Reaction of 1 with the disilene E-Tbb(Br)Si[double bond, length as m-dash]Si(Br)Tbb in the presence of 4-dimethylaminopyridine afforded selectively the red-brown silylidyne complex [(κ3-tmps)(CO)2Nb[triple bond, length as m-dash]Si-Tbb] (2-Si, Tbb = 4-tert-butyl-2,6-bis(bis(trimethylsilyl)methyl)phenyl). Similarly, treatment of 1 with E(ArMes)Cl (E = Ge, Sn; ArMes = 2,6-mesitylphenyl) afforded after elimination of (NMe4)Cl and two CO ligands the deep magenta colored germylidyne complex [(κ3-tmps)(CO)2Nb[triple bond, length as m-dash]Ge-ArMes] (3-Ge), and the deep violet, light-sensitive stannylidyne complex [(κ3-tmps)(CO)2Nb[triple bond, length as m-dash]Sn-ArMes] (3-Sn), respectively. Formation of 3-Sn proceeds via the niobiastannylene [(κ3-tmps)(CO)3Nb-SnArMes] (4-Sn), which was detected by IR and NMR spectroscopy. The niobium tetrylidyne complexes 2-Si, 3-Ge and 3-Sn were fully characterized and their solid-state structures determined by single-crystal X-ray diffraction studies. All complexes feature an almost linear tetrel coordination and the shortest Nb-E bond lengths (d(Nb-Si) = 232.7(2) pm; d(Nb-Ge) = 235.79(4) pm; d(Nb-Sn) = 253.3(1) pm) reported to date. Reaction of 3-Ge with a large excess of H2O afforded upon cleavage of the Nb-Ge triple bond the hydridogermanediol Ge(ArMes)H(OH)2. Photodecarbonylation of [CpNb(CO)4] (Cp = η5-C5H5) in the presence of Ge(ArMes)Cl afforded the red-orange chlorogermylidene complex [Cp(CO)3Nb[double bond, length as m-dash]Ge(ArMes)Cl] (5-Ge). The molecular structure of 5-Ge features an upright conformation of the germylidene ligand, a trigonal-planar coordinated Ge atom, and a Nb-Ge double bond length of 251.78(6) pm, which lies in-between the Nb-Ge triple bond length of 3-Ge (235.79(4) pm) and a Nb-Ge single bond length (267.3 pm). Cyclic voltammetric studies of 2-Si, 3-Ge, and 3-Sn reveal several electron-transfer steps. One-electron oxidation and reduction of the germylidyne complex of 3-Ge in THF are electrochemically reversible suggesting that both the radical cation and radical anion of 3-Ge are accessible species in solution.

NHC carbene supported half-sandwich hydridosilyl complexes of ruthenium: the impact of supporting ligands on Si⋯H interligand interactions

Electron donating NHC carbene ligand IPr exerts stronger RuH⋯Si interactions in complexes Cp(IPr)RuH₂(SiR₃) than in related ⁱPr₃P complexes Cp(ⁱPr₃P)RuH₂(SiR₃).

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.

Non-Innocent Behaviour of Imido Ligands in the Reactions of Silanes with Half-Sandwich Imido Complexes of Nb and V: A Silane/Imido Coupling Route to Compounds with Nonclassical SiH Interactions

Reactions of imido complexes [M(Cp)(=NR')(PR''3)2] (M=V, Nb) with silanes afford a plethora of products, depending on the nature of the metal, substitution at silicon and nitrogen and the steric properties of the phosphine. The main products are [M(Cp)(=NR')(PR3)(H)(SiRnCl3-n)] (M=V, Nb; R'=2,6-diisopropylphenyl (Ar), 2,6-dimethylphenyl (Ar')), [Nb(Cp)(=NR')(PR''3)(H)(SiPhR2)] (R2=MeH, H2), [Nb(Cp)(==NR')(PR''3)(Cl)(SiHRnCl2-n)] and [Nb(Cp)(eta 3-N(R)SiR2--H...)(PR''3)(Cl)]. Complexes with the smaller Ar' substituent at nitrogen react faster, as do more acidic silanes. Bulkier groups at silicon and phosphorus slow down the reaction substantially. Kinetic NMR experiments supported by DFT calculations reveal an associative mechanism going via an intermediate N-silane adduct [Nb(Cp){=N(-->SiHClR2)R'}(PR''3)2] bearing a penta-coordinate silicon centre, which then rearranges into the final products through a Si--H or Si--Cl bond activation process. DFT calculations show that this imido-silane adduct is additionally stabilized by a Si--HM agostic interaction. Si--H activation is kinetically preferred even when Si--Cl activation affords thermodynamically more stable products. The niobium complexes [NbCp(=NAr)(PMe3)(H)(SiR2Cl)] (R=Ph, Cl) are classical according to X-ray studies, but DFT calculations suggest the presence of interligand hypervalent interactions (IHI) in the model complex [Nb(Cp) (==NMe)(PMe3)(H)(SiMe2Cl)]. The extent of Si--H activation in the beta-Si--HM agostic complexes [Cp{eta 3-N(R')SiR2--H}M(PR''3)(Cl)] (R''=PMe3, PMe2Ph) primarily depends on the identity of the ligand trans to the Si--H bond. A trans phosphine leads to a stronger Si--H bond, manifested by a larger J(Si--H) coupling constant. The Si--H activation diminishes slightly when a less basic phosphine is employed, consistent with decreased back-donation from the metal.

Synthesis and structural studies of germyl and germyl/stannyl substituted tungstenocenes

One-pot reactions of [WCp2(H)2] and [WCp2(H)(SnMe3)] with "BuLi followed by an equivalent of GeMe2Cl2 afford mono(germyl) substituted tungstenocenes [WCp2(H)(GeMe2Cl)] (1) and [WCp2(SnMe3)(GeMe2Cl)] (7). Reactions of the products with tin halides in the presence NEt3 afford the mixed complexes [WCp2(SnR2X)(GeMe2Y)] (X, Y = Cl, Br, R = Me, Et), which were further converted to moderately stable compounds [WCp2(SnR2H)(GeMe2H)] (R = Me (10), Et (11)). A number of asymmetric mono(halo)-substituted germyl/stannyl tungstenocenes [WCp2(SnMe2X)(GeMe2H)] (X = Cl (12), Br (13)) and [WCp2(SnEt2Y)(GeMe2H)] (Y = Br (14), I (15)) were prepared by selective halogenation of the Sn-H bond in 10 and 11. X-Ray studies of [WCp2(H)(GeMe2Cl)] (1), [WCp2(SnEt2Br)(GeMe2Cl)] (4), and [WCp2(SnEt2Br)(GeMe2H)] (14) established classical structures of these compounds. X-Ray study of complex [WCp2(SnMeCl)(GeMe2Cl)] (3) revealed the presence of interligand Ge-Cl...Sn-Cl interactions in a highly Ge/Sn disordered structure. Analyses of molecular parameters of 1, 4, and 14 suggest the presence of a negative hyperconjugation between metal lone pairs and the sigma*-orbital of the E-X bond, which is stronger in bromo substituted complexes in comparison with chloro substituted ones.

Syntheses and Structures of Asymmetric Bis(silyl) Niobocene Hydrides

This paper deals with the preparation and structural investigation of asymmetric bis(silyl) niobocene hydrides, Cp2Nb(SiHMe2)(H)(SiXMe2) (2; X = F (a), Cl (b), Br (c), I (d)) and Cp2Nb(SiXMe2)(H)(SiYMe2) (X,Y= F-I; X not equal Y). Complexes 2a-d were prepared by selective electrophilic activation of the Si-H bond in Cp2Nb(SiHMe2)2(H). The Cp2Nb(SiXMe2)(H)(SiYMe2) complexes were prepared by electrophilic activation of the Si-H bond in 2a-d and, in some cases, by electrophilic exchange of the X halides in Cp2Nb(SiXMe2)2(H) (1) for other halides, Y. The structures of complexes 2b and 2c have been studied by X-ray and neutron diffraction (ND). The ND results unequivocally established that the hydride ligand in 2c is shifted toward the SiBrMe2 ligand and that in 2b is positioned symmetrically between two nonequivalent silyl groups, with the H...SiClMe2 distance being shorter because of the shorter Nb-SiClMe2 bond length. Analysis of the X-ray structures of complexes 2a-d and complexes Cp2Nb(SiXMe2)(H)(SiYMe2) shows that the largest structural distortions are observed for the silyl groups substituted by heavy halogen atoms. These trends are rationalized in terms of stronger interligand hypervalent interactions (IHI) Nb-H...Si-X for heavy atoms X from Group 7.

Synthesis and Reactivity of Tris(imido)rhenium Complexes Containing Rhenium−Main Group Element Bonds. Silicon−Carbon Bond Activations of PhSiH3 by Silyl Complexes

The synthesis and reactivity of a series of complexes of the (DippN=)(3)Re (Dipp = 2,6-(i)Pr(2)C(6)H(3)) fragment are reported. The anionic, Re(V) complex (THF)(2)Li(micro,micro-NDipp)(2)Re(=NDipp) (1), prepared by the reaction of (DippN=)(3)ReCl with (THF)(3)LiSi(SiMe(3))(3) or (t)BuLi (2 equiv) in the presence of THF (4 equiv), served as an important starting material for the synthesis of rhenium-element-bonded complexes. For example, treatment of 1 with ClSiR(3) gave the corresponding silyl complexes (DippN=)(3)ReSiR(3) (SiR(3) = SiMe(3) (2a), SiHPh(2) (2b), SiH(2)Ph (2c)). Complexes 2a-c are thought to exist in equilibrium between the Re(VII) (DippN=)(3)ReSiR(3) and Re(V) (DippN=)(2)ReN(SiR(3))Dipp isomers. Complexes 2a,b reacted with PhSiH(3) to give reaction mixtures that included 2c, Ph(2)SiH(2), SiH(4), and C(6)H(6). The silane and organic products arise from Si-C bond formation and cleavage. Treatment of 2a with CO gave (DippN=)(2)Re[N(SiMe(3))Dipp](CO) (3), which appears to result from trapping of the reactive Re(V) isomer of 2a by CO. Complex 1 reacted with the main group halides MeI, Ph(3)GeCl, Me(3)SnCl, Ph(2)PCl, and PhSeCl to give the corresponding rhenium complexes (DippN=)(3)ReER(n) (ER(n)() = Me (4), GePh(3) (5), SnMe(3) (6), PPh(2) (7), SePh (8)) in high yields. X-ray diffraction data for 5 indicate that the germyl ligand is bonded to rhenium, but positional disorder of the phenyl and Dipp groups prevented refinement of accurate metric parameters.

Unexpected features of stretched Si–H⋯Mo β-agostic interactions

Coupling of silanes with the imido group of (Ar'N)2Mo(PMe3)3 gives either the silanimine dimer (ArN-SiHCl)2 or Si-H agostic silylamido complexes which do not exhibit the commonly expected correlation between the nature of the substituents on silicon, the degree of Si-H addition and the value of the Si-H coupling constant.