Pentacyano(pyridine)chromate(III):  Synthesis, Characterization, and Photochemistry

The Cr(CN)₅(py)^2- anion (py = pyridine) has been prepared by acid-promoted methanolysis of Cr(CN)₆^3- followed by reaction with pyridine, isolated as the potassium salt, and characterized by absorption spectra (λ_max:  403 and 256 nm in H₂O; 411 nm in Me₂SO) and phosphorescence, observed in Me₂SO (λ_max, 774 nm; τ = 56 μs at 20 °C) but not in H₂O. In acid aqueous solution the complex decomposes 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 (k₂₅ = 9.8 × 10^-7 s^-1, ΔH^⧧ = 138 ± 8 kJ mol^-1). Ligand-field (LF) band irradiation results in substitution of py and CN^-. The quantum yields, measured by ligand analysis, spectrophotometry, and HPLC, are as follows:  Φ_py = 0.08, Φ_CN = 0.01 in H₂O (pH 7.2, phosphate buffer) and Φ_py = 0.04, Φ_CN = 0.002 in Me₂SO. The preference for py release obeys the prediction of the Vanquickenborne−Ceulemans, additive angular overlap model (AOM). A notable feature of this complex is that both types of ligands are π acceptors, and the π effect of py on bond labilization is evidenced by comparison with the photolysis of Cr(CN)₅(NH₃)^2-. Irradiation of the intense UV absorption due to overlap of charge-transfer (CT) and π → π*, py localized transitions causes the increase of both quantum yields, suggesting the involvement of higher-energy states besides the LF ones. Co(sep)³⁺ (sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane = sepulchrate) quenches the phosphorescence (k_q = 1.6 × 10⁹ M^-1 s^-1) but 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 particular, on the Φ_py/Φ_CN ratio, are discussed in terms of the proneness of the cyanide ligand to either protonation or hydrogen bonding and of solvent orientation toward anionic complexes.