Methane Activation by Group VB Bis(imido) Complexes

Abstract

An effective core potential, parallel supercomputing study of methane activation by group VB bis(imido) complexes, M(=NH)[sub 2](NH[sub 2]) (M = V, Nb and Ta), is presented in order to probe the efficacy of [pi]-loading (i.e., repeated coordination of strong [pi]-bonding ligands such as imides to a metal) as a strategy for designing more potent methane activators. Several important conclusions were reached. An activation pathway involving C-H addition across the metal-amido bond (i.e., [sigma]-bond metathesis) is disfavored relative to a pathway involving C-H addition across the metal-imido bond (i.e., [2[sub [sigma]] + 2[sub [pi]]] addition). The M(=NH)[sub 2](NH[sub 2]) complexes possess substantially more pyramidal coordination geometries than group IVB imides, M(NH[sub 2])[sub 2](=NH). Comparisons of metal-imido bond lengths for d[sup 0] imido and bis(imido) complexes are consistent with weakening of the metal-imido linkage, an important component of the [pi]-loading strategy. Calculated methane elimination barriers ([Delta]H[sub elim]) also show reasonable agreement with experiment and reproduce trends as a function of the metal. Methane activation by M(=NH)[sub 2](NH[sub 2]) (M = V, Nb, Ta) is approximately 10 kcal mol[sup [minus]1] more exothermic, versus separated reactants, than the analogous reaction for M(NH[sub 2])[sub 2](=NH) (M = Ti, Zr, Hf), a result consistent with themore » [pi]-loading hypothesis. 33 refs., 2 figs., 7 tabs.« less