The photochemistry of M(CO) 4(η 4-norbornadiene) complexes of group 6 transition metals (M = Cr, Mo, W) in low-temperature matrices

Abstract

Photolysis of M(CO) 4(η 4-norbornadiene) group 6 metal complexes ( 1) in low-temperature matrices involves both loss of CO and cleavage of metal—olefin bonds to an extent depending on the metal and on the excitation wavelength. In inert matrices mer-M(CO) 3(η 4-NBD) ( 2, M = Cr), fac-M(CO) 3(η 4-NBD) ( 3, M = Cr, Mo, W), and trans-vacant M(CO) 4(η 2-NBD) ( 7, M = Mo) fragments are observed as primary photoproducts and characterized by means of IR and UV-visible spectroscopy. Secondary wavelength-dependent photoreactions of these fragments include partial regeneration of the starting material and reversible 2 ⇄ 3 photoisomerization (M = Cr). Experiments in 13CO-doped matrices and with 13CO-labelled starting material complement the characterization of 1, 2, and 3 (M = Cr). Remarkably, the cis-vacant M(CO) 4(η 2-NBD) fragment 9 could not be detected. For all three metals photolysis of 1 in the presence of excess carbon monoxide results in stepwise displacement of the NBD ligand, presumably via the initial formation of 7, yielding M(CO) 5(η 2-NBD) ( 4) and M(CO) 6. The fragments 2, 3, and 7 can take up N 2 from the matrix environment to form mer-M(CO) 3(η 4-NBD)(N 2) ( 5, M = Cr), fac-M(CO) 3(η 4-NBD)(N 2) ( 6, M = Cr, Mo, W), and trans-M(CO) 4(η 2-NBD)(N 2) ( 8, M = Mo) as the major products. The relevance of these results with regard to the mechanism of the photocatalytic hydrogenation of norbornadiene is discussed.