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′-Me2-2,2′-bpy or N,N′-bis-isopropyl-1,4-diazabutadiene) in the solid state and solution: an X-ray and NOESY

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)](PF6)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.