Photochemical studies of Co2(CO)6(acetylene) complexes and their phosphine derivatives in frozen Nujol matrices

Photolysis of Co2(CO)6(alkyne), where alkyne=H2C2 (I), (C2H5)2C2 (II) (C6H5)2C2 (III) and C6H5C2H (IV) in frozen Nujol at ca. 90 K results in loss of CO to give Co2(CO)5(alkyne) in which CO-loss appears to be from the axial position. Complete conversion to a second isomer, presumed to have a vacancy in an equatorial position, or to starting material is observed for all compounds upon annealing the matrices to ca. 140 K. The second isomer is also formed by Co2(CO)5[(C6H5)2C2] upon photolysis at λ<280 nm. Photolysis of Co2(CO)6As2 (V), gave photochemical and annealing products whose spectra were analogous to those of the alkyne compounds. Photolysis of phosphine substituted derivatives such as axial-Co2(CO)5(PR3)(C2R′2), where R=Bu; R′=H (VI), R=Ph; R′=H (VII), R=OPh, R′=H (VIII), R=Ph; R′=Ph (M) yields two isomeric CO-loss products. Annealing samples of V–IX to ca. 140 K results in simple reaction reversal. Solution photolysis of IX with PPh3 gives Co2(CO)4(PPh3)2(Ph2C2). Photolysis of Co2(CO)4(PPh3)2(H2C2) (X) yields two products. Annealing the sample reforms the starting material and also generates a new species. Photolysis of Co2(CO)4(Ph2P)2CH2][(C2H5)2C2] (XI) forms a single product that reforms XI upon annealing.