A combined experimental and DFT/TDDFT investigation of structural, electronic, and pH-induced tuning of photophysical and redox properties of osmium(II) mixed-chelates derived from imidazole-4,5-dicarboxylic acid and 2,2'-bipyridine.

Experimental results coupled with computational studies were utilized to investigate the structural and electronic properties of mixed-ligand monometallic osmium(II) complexes of composition [(bipy)(2)Os(H(2)Imdc)](+) (1(+)), the N-H deprotonated form [(bipy)(2)Os(HImdc)] (1), and the COOH deprotonated form [(bipy)(2)Os(Imdc)](-) (1(-)), where H(3)Imdc = imidazole-4,5-dicarboxylic acid and bipy = 2,2'-bipyridine. The X-ray crystal structures of [(bipy)(2)Os(H(2)Imdc)](+) (1(+)) and [(bipy)(2)Os(HImdc)] (1) have been determined, which showed that compound 1(+) crystallizes in a monoclinic form with the space group P2(1)/c, while 1 is obtained in a triclinic form with the space group P1[combining macron]. The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the available X-ray data. The influence of pH on the photophysical and redox properties of the complexes has been thoroughly investigated. As compared to protonated complex (1(+)), which undergoes reversible oxidation at 0.50 V (vs. Ag/AgCl) in acetonitrile, the redox potential of the fully deprotonated complex (1(-)) is shifted to a much lower value, 0.16 V. The proton-coupled redox activity of 1(+) has been studied over the pH range 2-12 in an acetonitrile-water (3 : 2) medium. From the pH versus E(1/2) profile, the equilibrium constants of the complex species in the protonated/deprotonated forms and the metal ion in +2/+3 oxidation states have been determined. Using these values the bond dissociation free energies for the imidazole N-H and COOH bonds have also been estimated. The pK(a) values for 1(+) in the +2 state have also been determined spectrophotometrically. Substantial red shifts in the MLCT bands and the large shift in the E(1/2) value to a less positive potential that occur on deprotonation are energetically correlated. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies provide insight into the nature of the ground and excited states, with resulting detailed assignments of the orbitals involved in the absorption and emission transitions.