Methane activation by tris(imido) complexes: the effect of metal, ligand and d orbital occupation

Abstract

An effective core potential (ECP) and molecular mechanics (MM) study of methane activation by tris(imido) complexes is presented. The effects of metal, ligand, and d-orbital occupation on the potential energy surface are determined for d 0 systems M(NH) 3 (M W), [M(NH) 3] − (M  Ta), and [M(NH) 3] + (M  Tc, Re), and for d 2 systems. The [M(NH) 3] − (M  Tc) and M(NH) 3 (M  Os). d 2 (20 electron) tris(imido) complexes are planar, while d 0 (18 electron) complexes are pyramidal, except for [Ta(=NH) 3] −, which is planar. Methane activation is more exothermic for neutral, tris(imido) complexes than related bis(imido) and mono(imido) complexes. For a series of tris(imido) complexes the reaction enthalpy is more sensitive to d orbital occupation than metal or overall charge on the complex. The enthalpic barrier to methane elimination (microscopic reverse of methane activation) is strongly dependent on the extent of π-loading in the product. Methane elimination barriers ( ΔH elim ‡ ) to form tris(imido) complexes are much higher than ΔH elim ‡ to form bis(imido) complexes and mono(imido) complexes. Activation barriers for [2 σ + 2 π] CH activation ( ΔH act ‡ ) of methane by d 2 imidos are considerably larger (≈ 50 kcal mol −1) than activation by d 0 analogues, owing primarily to overcoming a repulsive methane-complex interaction. The methane-complex interaction increases upon going from anion to cation, [Ta(=NH) 3 −] < W(=NH) 3 < [Re(=NH) 3] +, with ΔH add = −8.3, −15.6, and −26.3 kcal mol −1, respectively. For this isoelectronic series more negative ΔH add correlate with larger ΔH act ‡. A combination of ECP and MM calculations suggests that an alkane adduct of [Tc(NAr) 3] + (and its Re analogue) is a plausible experimental target.