Molecular structures of electron-transfer active complexes [Re(XQ +)(CO) 3(NN)] 2+ (XQ += N-Me-4,4 ′-bipyridinium or N-Ph-4,4 ′-bipyridinium; NN=bpy, 4,4 ′-Me 2-2,2 ′-bpy or N, N′-bis-isopropyl-1,4-diazabutadiene) in the solid state and solution: an X-ray and NOESY NMR study

Abstract

Crystal and molecular structures of a series of complexes [Re(XQ +)(CO) 3(NN)] 2+ (XQ= N-methyl-4,4 ′-bipyridinium (MQ +) and N-phenyl-4,4 ′-bipyridinium (PQ +) and NN=bpy, 4,4 ′-dimethyl-bpy (dmb) or N, N ′-bis-isopropyl-1,4-diazabutadiene ( iPr-DAB)) in the solid state have been determined by X-ray diffraction. Aromatic rings within the XQ + ligand were found to be highly staggered. The dihedral angles between the pyridine and pyridinium rings were found in the range 39°–45° for MQ + and 28°–46° for PQ +. The exceptionally low dihedral angle of 8° in [Re(MQ +)(CO) 3(dmb)] 2+ is due to crystal-packing effects. The pyridinium and phenyl rings of the PQ + ligand are even more staggered, with dihedral angles in the range 40°–55°. The pyridine ring of the XQ + ligand is oriented relative to the equatorial ligands in such a way that it bisects the angles between the equatorial Re–N and Re–C bonds in all complexes, except for [Re(PQ +)(CO) 3( iPr-DAB)](PF 6) 2, where it bisects the DAB ligand. Structures of the complexes [Re(XQ +)(CO) 3(NN)] 2+ (NN=bpy, dmb) were also studied in solution using NOESY NMR. It was found that the orientation of the XQ + ligand relative to the equatorial ligands is the same as in the solid state. The XQ + ligands become even more staggered on going to the solution where pyridine–pyridinium dihedral angles range from 42° to 45°. A value of ∼69° was found for the pyridinium-phenyl dihedral angle in PQ + complexes. The structural data obtained are related to electron-transfer activity of XQ + complexes. It follows that any ground- or excited-state electron transfer reactions or optical charge transfer excitation have to be coupled with a major reorganization of the XQ + ligand, namely twisting of its aromatic rings and shortening of the interring C–C bond. This conclusion has important implications for estimates of Marcus inner reorganization energy and electron transfer dynamics.