Synthesis and structural investigation of mono- and dimetallic N-heterocyclic carbene complexes of group VII transition metals

Abstract

N-heterocyclic carbene complexes of group VII transition metals (Mn and Re) were synthesized and the electronic and structural aspects theoretically investigated. The synthesis yielded carbene complexes of type [Mx(CO)yL(NHC)] where M represents Mn or Re, x = 1, y = 4 and L = Cl, Br, I or M = Mn, x = 2, y = 9, L = CO. The synthesis of the mono- and dimetallic NHC complexes involved the substitution of labile carbonyl ligands for symmetrical and asymmetrical 1,3-disubstituted NHC ligands. Both aliphatic and aromatic N-substituent groups were incorporated into the heteroatomic, aromatic backbone of the imidazolylidene ligands. Coordination of the carbene ligand to the metal centre generated different results when comparing manganese NHC complexes to the rhenium NHC analogues. Analysis of the results indicated that only monometallic, halogenated NHC complex of Re could be synthesized and isolated, whereas the coordination of the imidazolium salts to Mn2(CO)10 produced axially substituted dimetallic carbene complexes. Spectroscopic (IR, NMR (1H and 13C) and structural (X-ray)) properties are reported and compared to those of other N-heterocyclic carbene derivatives. The structural features, coordination patterns and reactivity centres were examined to provide insight into the electronic and bonding environments of the metal-metal bond and the metal-NHC moiety.