Generation of catalysts by photolysis of transition metal complexes

The concepts of photocatalysis and photoassistance are defined and recent experimental results involving transition metal carbonyls as photocatalysts and photoassistance agents are described. Photolysis of Fe(CO)5is shown to lead to olefin hydrogenation under mild conditions (H21 atm, 25°), and d8 → d6 oxidative addition to photogenerated coordinatively unsaturated iron carbonyl intermediates is shown to be a possible pathway both for hydrogenation and olefin isomerization. Olefin isomerization, hydrogenation, and hydrosilation via photolysis of Cr(CO)6, Mo(CO)6. and W(CO)6are also described, and d6 → d4oxidative additions to photogenerated coordinatively unsaturated intermediates are invoked to account for the olefin activations. Photoisomerization of the coordinated styrylpyridines in M(CO)n(styrylpyridine)6-n (n = 5, 4; M = W, Mo, Cr) and XRe(CO)3(traws-4-styrylpyridine)2(X = Cl. Br) is discussed in terms of model photoassistance situations. Metal—metal bond cleavage is shown to dominate the photochemistry of the compounds M2(CO)10(M = Mn, Re), Mn2(CO)9PPh3, Mn2(CO)8(PPh3)8, and [h5-C5H5M(CO)3]2(M = Mo, W). The resulting mononuclear metal fragments react efficiently with other photogenerated metal fragments, I2or alkyl halides.