METHANE ACTIVATION BY TRIS(IMIDO) COMPLEXES - THE EFFECT OF METAL, LIGAND AND D-ORBITAL OCCUPATION

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 = Mo, W), [M(=NH)(3)](-) (M = Ta), and [M(=NH)(3)](+) (M = Tc, Re), and for d(2) systems [M(=NH)(3)](-) (M = Tc) and M(=NH)(3) (M = Os). The 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 pi-loading in the product. Methane elimination barriers (Delta H(elim)double dagger) to form tris(imido) complexes are much higher than Delta H(elim)double dagger to form bis(imido) complexes and mono(imido) complexes. Activation barriers for [2(sigma) + 2(pi)] CH activation (Delta H(act)double dagger) of methane by d(2) imidos are considerably larger (approximate to 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 Delta H-add = -8.3, -15.6, and -26.3 kcal mol(-1), respectively. For this isoelectronic series more negative Delta H-add correlate with larger Delta H(act)double dagger. 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.