The noncarbonylative photochemistry of group 6 Fischer carbene complexes

The different photochemical reaction mechanisms of group 6 Fischer carbene complexes have been studied by a joint experimental and density functional theory (DFT) computational study. This study unifies the photochemcal behavior of group 6 metal carbene complexes (M = Cr or W), which may occur by three different photoreactions, namely photocarbonylation, stepwise 1,2-dyotropic rearrangement, and alpha-fragmentation. The metal atom and the substituent on the nitrogen atom attached to the carbene carbon atom influence the topography of the potential energy surface, which leads to two different activated species, diradicals or metallacyclopropanones. Complexes having a hydrogen substituent at the nitrogen atom experience a stepwise dyotropic migration with no significant differences between the chromium- and tungsten-carbene complexes. The excitation of complexes having substituents other than hydrogen at the nitrogen atom leads to two different T1 species, and hence to the formation of mixtures of products arising from alpha-fragmentation (noncarbonylation) and amino esters (photocarbonylation). The two triplet states are isoenergetic in the Cr derivatives, but the T1 species that leads to alpha-fragmentation is strongly favored for the W derivatives. Finally, both the chromium and tungsten-carbene complexes are photoreactive in spite of the longstanding claim that tungsten-carbene complexes were photochemically inert. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).