Silyl Hydrides of Tantalum Supported by Cyclopentadienyl-imido Ligand Sets: Syntheses, X-ray, NMR, and DFT Studies

Reactions of the imido complex Cp(ArN)Ta(PMe3)2 (1, Ar = 2,6-diisopropylphenyl) with silanes afford the silyl hydrides Cp(ArN)Ta(PMe3)(H)(SiRnCl3−n) (2b−e) and Cp(ArN)Ta(PMe3)(H)(SiPhMeH) (2a) as the first kinetic products. However, the hydride compounds Cp(ArN)Ta(PMe3)(H)(SiRnCl3−n) are metastable and, first, rearrange in the presence of phosphine to the chlorides Cp(ArN)Ta(PMe3)(Cl)(SiHRnCl2−n) (5) and then decompose to Cp(ArN)Ta(PMe3)(Cl)(H) (4) and eventually to Cp(ArN)Ta(PMe3)Cl2 (3). Complexes with a smaller Ar′ substituent at nitrogen (Ar′ = 2,6-dimethylphenyl) react faster, as do more Lewis acidic silanes. The occurrence of interligand hypervalent interactions in the tantalum complexes Cp(ArN)Ta(PMe3)(H)(SiRnCl3−n) has been revealed by X-ray structure analysis, DFT calculations, and the experimental determination of the sign of the coupling constant J(Si−H). The J(Si−H) was found to be negative for Cp(ArN)Ta(PMe3)(H)(SiMenCl3−n) (J(Si−H) = −40 Hz for n = 1; J(Si−H) = −50 Hz for n = 0), indicative of the presence of Si−H bonding, but positive for Cp(ArN)Ta(PMe3)(H)(SiMeHPh) (J(Si−H) = +14 Hz), suggesting the absence of direct Si−H interactions. A DFT study of the mechanism of silane coupling with the model imido complex Cp(MeN)Ta(PMe3)2 established the feasibility of the direct addition of silanes HSiMenCl3−n (n = 1−3) to the imido group to give the adduct Cp(MeN{→SiR3-H})Ta(PMe3)2, as previously found in the related niobium chemistry.