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

Abstract

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 wi...