Hydridosilylamido complexes of Ta and Mo isolobal with Berry's zirconocenes: syntheses, β-Si-H agostic interactions, catalytic hydrosilylation, and insight into mechanism.

The syntheses of novel Group 5 and Group 6 hydrosilylamido complexes of the type R(ArN[double bond, length as m-dash])M{N((t)Bu)SiMe2-H}X (M = Ta, R = Cp; M = Mo, R = ArN; X = Cl, H, OBn, Me) are described. The various substituents in the X position seem to play the key role in determining the extent of β-agostic interaction with the Si-H bond. The Mo agostic hydrido complex (ArN[double bond, length as m-dash])2Mo{η(3)-N((t)Bu)SiMe2-H}H is a pre-catalyst for the hydrosilylation of carbonyls.

Multiple coupling of silanes with imido complexes of Mo.

The bis(imido) complexes ((t)BuN[double bond, length as m-dash])2Mo(PMe3)(L) (L = PMe3, C2H4) react with up to three equivalents of silane PhSiH3 to give the imido-bridged disilyl silyl Mo(vi) complex ((t)BuN){μ-(t)BuN(SiHPh)2}Mo(H)(SiH2Ph)(PMe3)2 (3) studied by NMR, IR and X-ray diffraction. NMR data supported by DFT calculations show that complex 3 is an unusual example of a silyl hydride of Mo(VI), without significant SiH interaction. Mechanistic NMR studies revealed that silane addition proceeds in a stepwise manner via a series of Si-H∙∙∙M agostic and silanimine complexes whose structures were further elucidated by DFT calculations.

Group 5 hydride and borohydride complexes supported by cyclopentadienyl-imido ligand sets.

Reactions of Cp*Ta(NAr)Cl2 and CpM(NAr)Cl2 (M = Nb, Ta; Ar = 2,6-C6H3(i)Pr2) with NaBH4 in the presence of an excess of PMe3 provided facile access to the corresponding dihydride derivatives Cp(R)M(NAr)H2(PMe3) (Cp(R) = Cp, Cp*). Reaction of Cp*Nb(NAr)Cl2 with NaBH4 in the absence of phosphine gave the Nb(+5) borohydride-hydride complex Cp*Nb(NAr)H(η(2)-BH4). When the corresponding reactions for CpM(NAr)Cl2 (M = Nb, Ta) were carried out in the absence of an excess of phosphine, dimeric M(IV) products [CpM(μ-NAr)(η(2)-BH4)]2 containing M-M single bonds were formed.