THE NATURE OF REDUCED AND EXCITED-STATES OF PI-ELECTRON-DEFICIENT COMPLEXES BETWEEN RE(CO)3HAL AND DIIMINE LIGANDS

Neutral dinuclear complexes (μ-L) [Re(CO)3Hal]2 (Hal = Cl, Br) with the bridging bis-chelate ligands 2,2′-bipyrimidine, 2,5- bis(2-pyridyl)pyrazine, 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine, and 2,2′-azopyridine were synthesized. Coordination of two Re(CO)3Hal fragments causes a very pronounced lowering of the ligand π* LUMOs, the dinuclear 2,2′-azopyridine complexes being reduced most easily at about +0.45 V vs SCE. The resulting anion-radical complexes were characterized by ESR and, in part, by their visible absorption and carbonyl vibrational spectra. ESR data rule out a “Re(O)” or “Re(O)/Re(I)” formulation for the singly reduced monomers and dimers and show the spectral effect of halide loss for the labile mononuclear complex (bpy·−)Re(CO)3Br. The well-separated long wavelength absorption bands from MLCT transitions of the most π-electron-deficient neutral complexes display a distinct structure. A comparison is made between the electronic structures of α-diimine complexes with W(CO)4, Re(CO)3Hal, and [Ru(bpy)2]2+ fragments. © 1990, American Chemical Society. All rights reserved.