A New Way to Scorpionate Niobium Complexes: Terminal Alkyne, Imido, and Oxo Complexes and the Rearrangement of α-Agostic Ethyl Complexes

This paper describes an efficient, straightforward synthesis of new hydrotris(pyrazolyl)borate (Tp‘) niobium terminal alkyne complexes and some reactions of these complexes.
Reduction of the niobium(IV) complexes Tp‘NbCl3 (Tp‘ = TpMe2 (1), TpMe2,4Cl (1-Cl)) with
zinc in the presence of a terminal alkyne HC⋮CR gives good yields (ca. 75%) of the new
alkyne complexes Tp‘NbCl2(HC⋮CR) (2-R) (R = H, Ph, C6H4-4-Me, CH2Ph, CF3) and 2-Cl-Ph. The strategy is significant since the previously reported synthesis of Tp‘NbCl2(RC⋮CR‘) was possible only with internal alkynes. NMR data indicate that the four-electron donor
alkyne sits in the molecular mirror plane. Restricted alkyne rotation is observed by NMR
in solution. The more abundant rotamer has the large substituent located distal to the Tp‘
ligand, thereby alleviating steric interactions. Only this rotamer is observed in the crystal
structures of 2-R (R = Ph, C6H4-4-Me, CH2Ph). In the crystal structure of 2-CF3, however,
three independent molecules are observed with both alkyne orientations. The one that has
the CF3 group proximal to TpMe2 exhibits H···F bridges between the alkyne CF3 group and
the methyl group at the 3-position of the TpMe2 ligand. Oxo and imidoniobium complexes
are also available via the same synthetic procedure. α-C−H agostic ethyl complexes TpMe2NbCl(μ-H-CHCH3)(HC⋮CR) (4-R) (R = Ph, C6H4-4-Me, CH2Ph) have been synthesized. Their
thermolyses have been studied with the goal of observing the Nb−CH2CH3/H−C⋮CR
exchange leading to the putative TpMe2NbCl(H)(CH3CH2C⋮CR). This reaction mainly gives
decomposition products, although spectrocopic data suggest that it occurs. This reaction is
significant in the context of comparing kinetic and thermodynamic data for M−H versus
M−C stability and reactivity.