PENTACYANO(PYRIDINE)CHROMATE(III) - SYNTHESIS, CHARACTERIZATION, AND PHOTOCHEMISTRY

The Cr(CN)(5)(py)(2-) anion (py pyridine) has been prepared by acid-pr omoted methanolysis of Cr(CN)(6)(3-) followed by reaction with pyridin e, isolated as the potassium salt, and characterized by absorption spe ctra (lambda(max): 403 and 256 nm in H2O; 411 nm in Me2SO) and phospho rescence, observed in Me2SO (lambda(max), 774 nm; tau = 56 mu s at 20 degrees C) but not in H2O. In acid aqueous solution the complex decomp oses stepwise to Cr(H2O)(5)(py)(3+); by contrast, the thermal reaction in Me2SO leads to Cr(CN)(5)(Me2SO)(2-) with first-order kinetics (k(2 5) = 9.8 x 10(-7) s(-1), Delta H-double dagger = 138 +/- 8 kJ mol(-1)) . Ligand-field (LF) band irradiation results in substitution of py and CN-. The quantum yields, measured by ligand analysis, spectrophotomet ry, and HPLC, are as follows: Phi(py) = 0.08, Phi(CN) = 0.01 in H2O (p H 7.2, phosphate buffer) and Phi(py) = 0.04, Phi(CN) = 0.002 in Me2SO. The preference for py release obeys the prediction of the Vanquickenb orne-Ceulemans, additive angular overlap model (AOM) A notable feature of this complex is that both types of ligands are pi accepters, and t he pi effect of py on bond labializing is evidenced by comparison with the photolysis of Cr(CN)(5)(NH3)(2-). Irradiation of the intense UV a bsorption due to overlap of charge-transfer (CT) and pi --> pi, py lo calized transitions causes the increase of both quantum yields, sugges ting the involvement of higher-energy states besides the LF ones. Co(s ep)3+ (sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane = sepul chrate) quenches the phosphorescence (k(q) = 1.6 x 10(9) M-1 s(-1)) bu t has no effect on the photoreaction efficiencies: the photochemistry is thus inferred to originate entirely from the lowest quartet excited state(s) in competition with intersystem crossing. The marked solvent effects on the absorption spectrum, on the emission behavior, on the thermal reactivity, on the photolysis quantum yields, and, in particul ar, on the Phi(py)/Phi(CN) ratio, are discussed in terms of the pronen ess of the cyanide ligand to either protonation or hydrogen bonding an d of solvent orientation toward anionic complexes.