Computational and spectroscopic investigations of radical anions of triosmium benzoheterocycle complexes

The electrochemical behavior of benzoheterocycle triosmium clusters of the general formula [Os3(CO)10-1(mum-eta 2-(L-H)(mu-H)] (1 = 0, 1; m = 2 or 3; L = benzoheterocycle) as well as the clusters derived from these by further modification with EPh3 (E = Si, Sn, P) and CH2N2 (1--36) have been studied. The ligands in these clusters bind to the metal core in a variety of different bonding modes. In general, the electron deficient clusters with mu3-eta2-bonding mode showed reduction potentials at less negative values than their electron precise precursors. The observed electrochemical potentials correlate with n to sigma* electronic transitions (excluding 3 and 6) but not with the other absorptions for the two series of complexes and their corresponding free ligands. The complexes all exhibit electrochemically quasi-reversible or irreversible reduction processes except in the case of benzoheterocycle clusters containing phenanthridine, 5; 5,6-benzoquinoline, 6; quinoxaline and substituted quinoxaline (7', 19, 20, 21, 25, 26, 31, 32, 34 and 36) and the quinoline-4-carboxaldehyde derivatives (13, 14, 15 and 24), which showed one-electron reversible reductions and resulted in stable radical anions after one-electron electrochemical and/or chemical reduction. Spectroscopic investigations of the radical anions were performed in an attempt to confirm the stability of these radical anions and to further elucidate their electronic and molecular structures and to determine the locations of spin densities. The radical anions of 5, 6, 7', 13, 14 and 15 were studied by infrared spectroelectrochemistry, NMR, EPR and UV-Vis spectroscopies. Very interesting phenomenon were found in the case of partially reduced radical anions of 5, 6 and 13, which showed selective line broadening in their 1H and 13C NMR resonances, shifting of the IR frequencies to lower frequencies and the disappearance of low energy UV-Vis band. The fully reduced radical anion of 14 showed a well defined ESR signal. Density functional theory calculations were performed in order to understand their HOMO-LUMO energy gaps, natural charge populations and spin density distributions. Reasonable correlations were found between spectroscopy and computational data.