RHENIUM(I) CARBONYL DIOXOLENE COMPLEXES - ELECTROCHEMICAL AND SPECTROELECTROCHEMICAL (RESONANCE RAMAN, UV-VIS, IR) STUDY OF [RE(CO)3L(DIOX)]Z AND [RE(CO)2(PPH3)2(DIOX)]Z (L = CO, PPH3, P-DPPE, THF, PH3PO, ME2CO, PY, Z = -1, 0, +1) REDOX SERIES

The fac-[ReI(CO)3L(DBSQ)] (L = CO, THF, PPh3, Ph3PO, py, Me2CO, monodentate P-dppe) and [ReI(CO)2(PPh3)2(DBSQ)] complexes containing the 3,5-di-tert-butyl-1,2-benzosemiquinone radical anionic ligand (DBSQ) are electrochemically reduced and oxidized without a change in coordination number or stereochemistry to corresponding complexes of the catecholate (DBCat) and quinone (DBQ) ligand, respectively. The spectroelectrochemical study allowed the characterization of [ReI(CO)3(PPh3)-(DBCat)]-and [ReI(CO)2(PPh3)2(DBCat)]- by IR and UV-vis absorption spectroscopies. The [ReI(CO)3L(DBCat)]- (L = CO, THF) complexes were generated chemically by the reaction between Re(CO)5- and DBQ in dry THF or by the reduction of [Re(CO)3(THF)(DBSQ)] with CP2Co and were characterized by IR spectroscopy. The IR spectra of the oxidized [ReI(CO)3-(PPh3)(DBQ)]+ and [ReI(CO)2(PPh3)2(DBQ)]+ species were obtained spectroelectrochemically. The dicarbonyl complex [ReI(CO)2(PPh3)2(DBQ)]+ exhibits a very strong absorption band at 575 nm (epsilon = 18 000 M-1 cm-1). A Raman peak corresponding to the Re-O stretching vibration at 568 cm-1 is very strongly intensity-enhanced by resonance with this electronic transition. The IR, UV-vis, and resonance Raman spectra of the radical [ReI(CO)2(PPh3)2(DBSQ)] complex are reported for the first time. A comparison of the substitution effects on redox potentials and spectra and, especially, the study of the UV-vis, resonance Raman, and IR spectra (average C=O stretching force constants) as a function of the oxidation state of the dioxolene ligand along the [ReI(CO)3L(DBDiox)]z and [ReI(CO)2(PPh3)2(DBDiox)]z (z = -1, 0, +1; DBDiox = DBCat, DBSQ, DBQ, respectively) redox series allowed the characterization of the bonding properties of the DBDiox ligand in each of the three oxidation states. The DBCat ligand behaves as a strong sigma-donor, and its Re complexes are unique examples of 6-coordinated carbonyls of a d6-metal with a dianionic ligand in their coordination spheres. They exhibit LMCT transitions of relatively low intensity. The DBSQ radical anionic ligand is a weak pi-acceptor. Spectra of its complexes show a typical MLCT transition. The DBQ ligand is a strong pi-acceptor which forms a strongly stabilizing delocalized pi-bond with the Re atom. The extent of this delocalization is very flexible and compensates for the substitution effects in the rest of the coordination sphere. The intense visible absorption is assigned to a transition between orbitals which are pi-bonding and pi-antibonding with respect to the Re-O bonds. [ReI(CO)2(PPh3)2(DBQ)]+ is one of the very few stable quinone complexes known.