Tetracyanide-Bridged Divanadium Complexes:  Redox Switching between Strong Antiferromagnetic and Strong Ferromagnetic Coupling

Reaction of [(Me3tacn)V(CF3SO3)3] (Me3tacn = N,N ‘,N ‘ ‘-trimethyl-1,4,7-triazacyclononane) with LiCN in DMF results in oligomerization of cyanide to form [(Me3tacn)2V2(CN)4(μ-C4N4)]. The structure of this binuclear complex features a planar tetracyanide unit bridging two VIV centers via imido type linkages. The conjugated pathway provided by the bridging ligand leads to strong antiferromagnetic coupling (J = −112 cm-1) and an S = 0 ground state. Reduction of the complex with cobaltocene generates the Class III mixed-valence anion [(Me3tacn)2V2(CN)4(μ-C4N4)] -, wherein resonance exchange induces strong ferromagnetic coupling to give a well-isolated S = 3/2 ground state.