Investigating CN– Cleavage by Three-coordinate M[N(R)Ar]₃ Complexes

Abstract

Three-coordinate Mo[N(tBu)Ar]3 binds cyanide to form the intermediate [Ar(tBu)N]3Mo–CN–Mo[N(tBu)Ar]3 but, unlike its N2 analogue which spontaneously cleaves dinitrogen, the C–N bond remains intact. DFT calculations on the model [NH2]3Mo/CN– system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol–1, accounting for why C–N bond cleavage is not observed experimentally. The situation is improved for the [H2N]3W/CN– system where the intermediate and products are closer in energy but not enough for CN– cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H2N]3Re+/CN–/W[NH2]3 and [H2N]3Re+/CN–/Ta[NH2]3– systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H2N]3Re+/CN–/W[NH2]3 system are more favourable than those for the [H2N]3W/CN– system, the final C–N cleavage step is still endothermic by 32 kJ mol–1 when symmetry constraints are relaxed. The resistance of these systems to C–N cleavage was examined by a bond decomposition analysis of [H2N]M–L1L2–M[NH2]3 intermediates for L1L2 = N2, CO and CN– which showed that backbonding from the metal into the L1L2 π* orbitals is significantly less for CN– than for N2 or CO due to the negative charge on CN– which results in a large energy gap between the metal dπ and the π* orbitals of CN–. This, combined with the very strong M–CN– σ interaction which stabilises the CN– intermediate, makes C–N bond cleavage in these systems unfavourable even though the CN triple bond is not as strong as the bond in N2 or CO.