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

Abstract

The synthesis of Mn^V and Cr^V nitride complexes of a pro-radical tetradentate bis-phenol bis-N-heterocyclic carbene ligand H₂L^C2O2 was investigated. Employing either azide photolysis of the Mn^III precursor complex MnL^C2O2(N₃) or a nitride exchange reaction between MnL^C2O2(Br) and the nitride exchange reagent Mnsalen(N) failed to provide a useful route to the target nitride MnL^C2O2(N). Experimental results support initial formation of the target nitride MnL^C2O2(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 [H₂L^C2O2(N)]⁺ in good yield. In contrast, the Cr analogue CrL^C2O2(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–C_NHC 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 → σ* M–C_NHC) promotes nucleophilic attack of the nitride on the carbene as the major reaction pathway. Finally, one-electron oxidation of CrL^C2O2(N) affords a relatively stable cation that is characterized by experimental and theoretical analysis to be a metal-oxidized d⁰ Cr^VI species.