Synthesis, reduction chemistry, and spectroscopic and computational studies of isomeric quinolinecarboxaldehyde triosmium clusters

The quinoline derivatives Os-3(CO)(n)(mu(m)-eta(2)-L-H)(mu-H) (L = quinoline-4-carboxaldehyde, n 10, m = 2, 4; n = 9, N-C(8) bound, m = 3, 5; n = 10, N-C(2) bound, m = 2, 6; L = quinoline-3-carboxaldehyde, n = 9, N-C(8) bound, m = 3, 7) are obtained by the reaction of Os-3(CO)(10)(CH3CN)(2) with the quinoline-3-carboxaldehyde or -4-carboxaldehyde ligand followed by photochemical decarbonylation in the case of 5 and 7. The quinoline-4-carboxaldehyde clusters and the free ligand all show reversible 1e(-) reductions, but the electron-deficient 5 shows two separate reversible 1e(-) reductions, the first at much less negative potential than for the previously investigated Os-3(Co)(9)(mu(3)-eta(2) -L-H)(mu-H) (L = phenanthridine, 1; L = 5,6-benzoquinoline, 2), while the free ligand is reduced at a more negative potential than any of the clusters. The spectroscopic properties of the clusters upon electrochemical and chemical reduction have been investigated, and in the case of 4 selective line broadening of its H-1 NMR resonances is observed. Infrared spectrophotoelectrochemistry reveals that the CO stretching modes are shifted to lower frequencies, as expected. The pattern of unpaired spin densities in the reduced clusters predicted from the DFT calculations correlates with the observed selective line broadening, and EPR measurements confirm the formation of the radical anions at g values similar to those observed for pure organic radical anions.