Photochemistry of Surface-Confined Organometallics. Photochemical Release of a Surface-Confined Cobalt Carbonyl Catalyst

Article abstract of DOI:10.1021/ja00414a024

Inorganic oxide surfaces, SiO2 and Al2O3, bearing -OH functionality have been functionalized with -Co(CO)4 by first treating the solids with (EtO)3SiH, Me2ClSiH, or Cl3SiH to introduce <*>SiH functionality followed by reaction with Co2(CO)8.Derivatized surfaces have been characterized by infrared spectroscopy and compared to solution analogues to confirm the presence of <*>SiH and <*>SiCo(CO)4 groups on the surface.The surface-confined <*>SiCo(CO)4 undergoes photoreactions that begin with loss of CO subsequent to optical excitation in the near-ultraviolet.The photochemistry closely parallels the behavior of solution R3SiCo(CO)4 analogues; CO can be photosubstituted by P(OPh)3 and the surface-confined <*>SiCo(CO)3(P(OPh)3) species is detectable by infrared spectroscopy.Irradiation of the oxide powders bearing <*>SiCo(CO)4 suspended in Et3SiH solutions results in the release of the -Co(CO)4 into solution at Et3SiCo(CO)4 concurrent with the regeneration of surface <*>SiH functionality.Irradiation of the powders (<*>SiCo(CO)4) in the presence of 1-pentene yields Co4(CO)12 in solution and surface <*>Si(pentenyl) groups.In the presence of Et3SiH/1-pentene photoactivated catalysis by the derivatized powders (<*>SiCo(CO)4) occurs to give isomerization of the alkene, hydrosilation to Et3Si(n-pentyl), and small ammounts of pentane.Reaction under H2 improves the relative yield of pentane, while H2/CO mixtures yield no hydroformylation products and lower the observed rate due to CO competing for coordinatively unsaturated species.The surface <*>SiH groups also add to 1-pentene to give powders bearing <*>Si(n-pentyl) functianality.The use of powders functionalized with <*>SiCo(CO)3(P(OPh)3) also gives catalysis upon photoexcitation, but the product distribution differs significantly and includes at least two isomers of Et3Si(pentenyl).In all cases the bulk of the catalysis appears to result from Co-carbonyl fragments photoreleased from the oxide support.The initial rate of catalysis appears to depend on the initial rate at which the fragments are released into homogeneous solution.The heterogeneous photocatalysts thus give the same product distribution as appropriate homogeneous precursors, but the oxide supported <*>SiCo(CO)4 is more easily isolated and handled and more durable than R3SiCo(CO)4.