THE REMARKABLE PHOTOCHEMISTRY OF FAC-XMN(CO)3(ALPHA-DIIMINE) (X = HALIDE) - FORMATION OF MN2(CO)6(ALPHA-DIIMINE)2 VIA THE MER ISOMER AND PHOTOCATALYTIC SUBSTITUTION OF X- IN THE PRESENCE OF PR3

This article describes the remarkable photochemistry of complexes fac-XMn(CO)3(alpha-diimine) (X = Cl, Br, I) upon irradiation into their lowest-energy CT band. The primary photoprocess is loss of CO, as was established by flash photolysis and low-temperature IR spectroscopy. The CO-loss product reacted thermally and photochemically to give mer-Xmn(CO)3(alpha-diimine), which in turn transformed thermally into the fac isomer at temperatures above 213 K. Irradiation of mer-Xmn(CO)3(alpha-diimine) resulted in the formation of the 16-electron radical complexes Mn+(CO)3(alpha-diimine.-), which were identified at room temperature by ESR spectroscopy and which dimerized to give Mn2(CO)6(alpha-diimine)2 for alpha-diimine = bpy. At low temperatures these radical complexes formed rather stable adducts Mn+(CO)3(alpha-diimine.-)(PR3) with PR3 (R = nBu, Ome). At room temperature these adducts initiated the catalytic substitution of X- by PR3 in the starting complex fac-Xmn(CO)3(alpha-diimine). The primary photoprocess of fac-Xmn(CO)3(alpha-diimine) (loss of CO) most likely occurs from the lowest LF state. Contrary to this, irradiation of mer-Xmn(CO)3(alpha-diimine) gives rise to homolysis of the Mn-X bond. This remarkable difference in photochemical behavior of the fac and mer isomers is discussed.