Formation and reactivity of 1,3-cyclohexadiene complexes of niobium and tantalum containing aryloxide ligation: Selectivity differences in the hydrogenation of 1,3-cyclohexadiene

Article abstract of DOI:10.1016/0277-5387(95)00252-N

The sodium amalgam reduction (2 Na per M) of hydrocarbon solutions of [M(OAr)₃Cl₂] (M = Nb, Ta) or [Nb(OAr)₂Cl₃]₂ (OAr = 2,6-di-isopropylphenoxide) in the presence of either 1,3-or 1,4-cyclohexadiene yielded the complexes [M(OAr)₃(β⁴-C₆H₈)] (1a: M = Nb; 1b: M = Ta) and [Nb(OAr)₂Cl(η⁴-C₆H₈)], 2, respectively. The solid state structures of 1a, isomorphous 1b and 2 showed the 1,3-cyclohexadiene strongly bound to the metal. The ligand is not symmetrically bound to the metal in 1 but in 2 there is a crystallographic mirror plane. The solution NMR spectra of 1a and 1b show only one set of aryloxide ligand signals and only four proton and three carbon resonances for the C₆H₈ group. The hydrolysis of 1 or 2 yielded 2,6-di-isopropylphenol and one equivalent of cyclohexane (¹H NMR). The niobium compounds 1a and 2 will catalyse the disproportionation and hydrogenation of 1,3-cyclohexadiene with differing selectivity.