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-C₆H ₃Me₂) 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 N₂, 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 dⁿ 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^IIITa^III dimer, [Ar(R)N]₃-Re-CO-Ta[N(R)Ar]₃, is thermodynamically best suited for cleaving CO.