Photodissociation of CO from [Ru3(μ3-O)(μ-OOCCH3)6(CO)L2] in acetonitrile, where L = pyridine, 4-cyanopyridine and methanol

Photodissociation of CO from oxo-centered trinuclear ruthenium clusters [Ru-3(mu(3)-O)(mu-OOCCH3)(6)(CO)L-2] (L=pyridine (py): 1; 4-cyanopyridine (cpy): 2; methanol: 3) dissolved in organic solvents has been examined. Upon photolysis (greater than or equal to290 nm, a 450-W Xe lamp), an absorption peak at 585 nm observed for 1 in CH3CN decreases its intensity and a new absorption band appears and grows at 896 nm. This spectral change, presenting isosbestic points, corresponds to photosubstitution of CO in 1 to form [Ru-3(mu(3)-O)(mu-OOCCH3)(6)(CH3CN)(py)(2)] 4. Photoexcitation of carbonyl complexes 2 and 3 in CH3CN also affords the corresponding CH3CN-coordinated complexes [Ru-3(mu(3)-O)(mu-OOCCH3)(6)(CH3CN)(cpy)(2)] 6 and [Ru-3(mu(3)-O)(mu-OOCCH3)(6)(CH3CN)(3)] 7, respectively. The photosubstitution reactions (excitation wavelength, greater than or equal to290 nm) are well described by the first-order kinetics: k=7.3x10(-4) s(-1) for 1, 4.9x10(-4) s(-1) for 2 and 5.1x10(-4) s(-1) for 3 (298 K). In the presence of a 100-fold excess of py, photolysis of 1 yields a tris(py) complex [Ru-3(mu(3)-O)(mu-OOCCH3)(6)(py)(3)] 5 via photochemical loss of CO followed by coordination of py. The overall reaction (photochemical and thermal) is also confirmed by H-1 NMR spectroscopy. The dissociative character of the photosubstitution is supported by negligible effects of the concentration of the entering pyridine molecule, the nature of solvents and the type of terminal monodentate ligands (other than CO) attached to the cluster. Quantum yield measurements with varied excitation wavelengths have shown that absorption bands located in the UV region (<400 nm) play a principal role in photosubstitution, whereas an absorption band in the visible region (centered at similar to 580 nm), ascribed to an "intracluster" charge transfer, is not at all responsible for photosubstitution.