Coordination ambivalence of the electroactive bis-chelate ligand bis(1-methyl-2-imidazolyl)glyoxal (BIG) in mononuclear and dinuclear complexes with Re(CO)3Cl

Abstract

Bis(1-methyl-2-imidazolyl)glyoxal (BIG) is reversibly reduced to an EPR, IR and UV-VIS spectroelectrochemically detectable α-diketyl radical anion. As a ligand BIG can coordinate through the carbonyl oxygen centres O and O′ and through the imine nitrogen atoms N and N′ of the imidazolyl rings. For fac-(BIG)Re(CO)₃Cl metal coordination occurs via both imine nitrogen centres, leading to a non-planar seven-membered chelate ring with a free α-dicarbonyl moiety as evident from X-ray structural analysis. For dinuclear (μ-BIG)[fac-Re(CO)₃Cl]₂ three essentially different structural coordination alternatives A–C are possible in a bis-chelate situation: Separate N,N′- and O,O′-coordination with one seven-membered and one five-membered chelate ring (A), N,O′- and N′,O-coordination with two edge-sharing six-membered chelate rings (B), or N,O- and N′,O′-coordination with two five-membered chelate rings and free rotation around the C(O)–C′(O′) single bond. Spectroscopic, electrochemical and spectroelectrochemical results obtained for the mono- and di-nuclear complexes suggest the π-conjugated structure alternative B for (μ-BIG)[fac-Re(CO)₃Cl]₂.