Investigating the formation of metal nitride complexes employing a tetradentate bis-carbene bis-phenolate ligand

The synthesis of MnV and CrV nitride complexes of a pro-radical tetradentate bis-phenol bis-N-heterocyclic carbene ligand H2LC2O2 were investigated. Employing either azide photolysis of the MnIII precursor complex MnLC2O2(N3) or a nitride exchange reaction between MnLC2O2(Br) and the nitride exchange reagent Mnsalen(N) failed to provide a useful route to the target nitride MnLC2O2(N). Experimental results support initial formation of the target nitride MnLC2O2(N), however, the nitride rapidly inserts into a Mn-C_NHC bond. A second insertion reaction results in the isolation of the doubly inserted ligand product [H2LC2O2(N)]+ in good yield. In contrast, the Cr analogue CrLC2O2(N) was readily prepared and characterized by a number of experimental methods, including X-ray crystallography. Theoretical calculations predict a lower transition state energy for nitride insertion into the M-CNHC bond for Mn in comparison to Cr, and in addition the N-inserted product is stabilized for Mn while destabilized for Cr. Natural bond order (NBO) analysis predicts that the major bonding interaction (π MN to σ* M-CNHC) indicates nucleophilic attack of the nitride on the carbene. Finally, one-electron oxidation of CrLC2O2(N) affords a relatively stable cation that is characterized by experimental and theoretical analysis to be a metal-oxidized d0 CrVI species.