The yttrium hydride {[DADMB]YH(THF)}2 (2; DADMB = 2,2‘-bis(tert-butyldimethylsilylamido)-6,6‘-dimethylbiphenyl), conveniently generated in situ from [DADMB]YMe(THF)2 (1), is an active olefin hydrosilylation catalyst. This system represents the first use of a d0 metal complex with non-Cp ligands in the catalytic hydrosilylation of olefins. Studies of the reactivity and regio- and enantioselectivity for various olefins and silanes are presented. High preference for terminal addition in the case of aliphatic olefins is observed, while aromatic olefins exhibit preference for 2,1-addition. Both primary and secondary silanes have been employed. Preliminary studies of the enantioselectivity of the chiral catalyst show that 90% ee can be achieved in the hydrosilylation of norbornene with PhSiH3. Kinetic studies support a mechanism consistent with the generally accepted one for hydrosilylation catalyzed by early transition metal species, involving rapid olefin insertion into a Y−H bond followed by a Si−C bond-forming σ-bond metathesis of the resulting yttrium alkyl with silane.

The synthesis, structural characterization, and reactivity of tantalum complexes with chelating imido−amido ligands are reported. The highly bent imido Cp* = η5-C5Me5), with a TaN−C bond angle of 116.3(4)°, was synthesized from Cp*TaCl4 and the lithiated bis(silylamino)biphenyl (C6H3Me)2(NLiSiMe3)2 (3). Compound 4 undergoes reactions with electrophiles at the nucleophilic imido nitrogen atom. The methyl reacts with xylyl isonitrile to give an insertion product, 6, which was structurally characterized. Addition of MeI to 5 gives a cationic diamide tantalum the ionic structure of which was confirmed by X-ray crystallography. Reactions of 4 and 5 with unhindered silanes result in addition of the silane Si−H bond across the TaN double bond. Addition of PhSiH3 to 4 and 5 gave the respectively. The crystal structure of 9 was determined. Compounds 8 and 9 are unstable and decompose via elimination of HSiMe3. In the presence of CH2Cl2 and PhSiH3, 4 was slowly converted to another hydrido A mechanism for this transformation, involving a sequence of silane addition/elimination reactions, is proposed. X-ray structural characterization of 12 revealed the presence of a nonclassical bonding interaction between the hydride ligand and a neighboring silyl group, leading to a short H−Si contact of 1.86(4) Å and a distorted pentagonal bipyramidal geometry at silicon. Reactions of PhSiH3 and (CH2)3SiH2 (silacyclobutane) with 5 follow second-order kinetics, and an inverse deuterium isotope effect of k H/k D = 0.78(1) for the addition of PhSiH3 to 5 was observed. The elimination of HSiMe3 from 9 was found to follow a first-order rate law with approach to equilibrium (K H = 0.025(2) mol/L) and exhibit an inverse isotope effect of k H/k D = 0.85(2). A study of the temperature dependence of the first-order rate constant for HSiMe3 elimination from 9 provided the activation parameters ΔH ⧧= 25.5(3) kcal/mol and ΔS ⧧ = −0.3(1.0) cal/(mol·K). These findings are interpreted in terms of a mechanism involving slow, rate-determining formation of pentacoordinate silicon intermediates, coupled with a fast hydride shift between Ta and Si.

The yttrium bis(silylamido)biphenyl complex [DADMB]YCl(THF)2 (2; DADMB = 2,2‘-bis((tert-butyldimethylsilyl)amido)-6,6‘-dimethylbiphenyl) was prepared from Li2[DADMB]·2THF and YCl3(THF)3, and its structure was determined by X-ray crystallography. Compound 2 reacts with MeLi and (Me3Si)2CHLi to give the corresponding alkyl derivatives [DADMB]YMe(THF)2 (3) and [DADMB]Y[CH(SiMe3)2](THF)(OEt2) (5), respectively. In the presence of silicone grease, the reaction of 2 with MeLi produces the yttrium trimethylsiloxide complex [DADMB]Y(OSiMe3)(THF)2 (4), which was also crystallographically characterized. Both 3 and 5 react with phenylsilane or H2 to give the insoluble, dimeric yttrium hydride {[DADMB]Y(μ-H)(THF)}2·C6H6 (6). The alkyl derivatives 3 and 5 exhibit only limited olefin polymerization activity; however, the hydride 6 reacts rapidly with ethylene or 1-hexene to give a single insertion product. The resulting yttrium ethyl complex 7 was structurally characterized. Compound 6 reacts with pyridine to produce a mixture of the isomeric 1,2- and 1,4-insertion products 9 and 10, [DADMB]Y(NC5H6)(pyr)2. Compound 9 converts quantitatively to 10 upon heating.