Synthesis, characterization, and reactivity of manganese(II) complexes containing N-heterocyclic carbene ligands

A series of divalent manganese complexes containing N-heterocyclic carbene (NHC) ligands were synthesized and characterized in solution and the solid state. Chlorido complexes of Mn(II) containing the NHC ligands 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) were prepared in straightforward fashion by direct carbene addition to MnCl2(thf)1.5. The chlorido complexes were found to be ineffective precatalysts in Kumada-type coupling reactions of simple aryl halides. Cyclic voltammetry studies of one of the chloride complexes, [MnCl2(IMes)2], demonstrate no electrochemically accessible reduction events in tetrahydrofuran, possibly accounting for the lack of catalytic activity. Chlorido complexes containing both IMes and IPr ligands serve as precursors to alkyl and aryl compounds of Mn(II). Characterization of the alkyl and aryl complexes is reported, and comparisons are made between these complexes and similar compounds in the chemical literature.