Achieving C-N bond cleavage in dinuclear metal cyanide complexes

Cleavage of cyanide is more difficult to achieve compared to dinitrogen and carbon monoxide, even though these species contain triple bonds of greater strength. In this work, we have used computational methods to investigate thermodynamic and mechanistic aspects of the C-N bond cleavage process in [L3M-CN-M'L-3] systems consisting of a central cyanide unit bound in an end-on fashion to two terminal metal tris-amide complexes. In these systems, [M] is a d(3) transition metal from the 3d, 4d, 5d, or 6d series and groups 4 through 7, and [L] is either [NH2], [NMe2], [(NPrPh)-Pr-i], or [(NBuAr)-Bu-t]. A comparison of various models for the experimentally relevant [L3Mo-CN-MoL3] system has shown that while the C-N cleavage step appears to be an energetically favourable process, a large barrier exists for the dissociation of [L3Mo-CN-MoL3]((-)) into [L3Mo-C]((-)) and [N-MoL3], which possibly explains why C-N bond scission is not observed experimentally. The general structural, bonding, and thermochemical trends across the transition metal series investigated, indicate that the systems exhibiting the greatest degree of C-N activation, and most favourable energetics for C-N cleavage, also possess the most favourable electronic properties, namely, a close match between the relevant pi-like orbitals on the metal-based and cyanide fragments. The negative charge on the cyanide fragment leads to significant destabilization of the pi* level which needs to be populated through back-donation from the metal centres in order for C-N bond scission to be achieved. Therefore, metal-based systems with high-lying d(pi) orbitals are best suited to C-N cleavage. In terms of chemical periodicity, these systems can be identified as the heavier members within a group and the earlier members within a period. As a consequence, Mo complexes are not well suited to cleaving the C-N bond, whereas the Ta analogues are the most favourable systems and should, in principle, be capable of cleaving cyanide under relatively mild conditions. An important conclusion from this work is that a successful strategy for achieving cleavage of multiply-bonded, and relatively unreactive, molecular fragments, may simply lie in tuning the electronic structures and orbital interactions by judicious choice of metal sites and ligand groups.