Optimizing small molecule activation and cleavage in three-coordinate M[N(R)Ar]3 complexes.

The sterically hindered, three-coordinate metal systems M[N(R)Ar]3 (R = tBu, iPr; Ar = 3,5-C6H3Me2) are known to bind and activate a number of fundamental diatomic molecules via a [Ar(R)N]3M-L-L-M[N(R)Ar]3 dimer intermediate. To predict which metals are most suitable for activating and cleaving small molecules such as N(2), NO, CO, and CN(-), the M-L bond energies in the L-M(NH2)3 (L = O, N, C) model complexes were calculated for a wide range of metals, oxidation states, and dn (n = 2-6) configurations. The strongest M-O, M-N, and M-C bonds occurred for the d2, d3, and d4 metals, respectively, and for these d(n) configurations, the M-C and M-O bonds were calculated to be stronger than the M-N bonds. For isoelectronic metals, the bond strengths were found to increase both down a group and to the left of a period. Both the calculated N-N bond lengths and activation barriers for N2 bond cleavage in the (H2N)3M-N-N-M(NH2)3 intermediate dimers were shown to follow the trends in the M-N bond energies. The three-coordinate complexes of Ta(II), W(III), and Nb(II) are predicted to deliver more favorable N2 cleavage reactions than the experimentally known Mo(III) system and the Re(III)Ta(III) dimer, [Ar(R)N]3Re-CO-Ta[N(R)Ar]3, is thermodynamically best suited for cleaving CO.