Metal π complexes of benzene derivatives: Part 56. (η6-Fluoranthene)(tricarbonyl)chromium: isomerism and haptotropic equilibration

Reaction of fluoranthene with Cr(CO)3Py3/BF3·OEt2 at 25 °C affords a mixture of two isomeric complexes: traces of tricarbonyl(1-5,15-η6-fluoranthene)chromium (3) (coordination to benzene) and, as the major product, tricarbonyl(1-4,15,16-η6-fluoranthene)chromium (2) (coordination to naphthalene). The ratio 3:2 is less than 0.05 according to 1H-NMR of crude product before crystallization. Complex 2 is thermodynamically less stable than 3: at 100 °C in decane or C6D6 for 8 h or at 90 °C in C6F6 for 100 h 2 rearranges irreversibly to 3 via an inter-ring haptotropic shift of the Cr(CO)3 group from the naphthalene moiety to the benzene part of the fluoranthene ligand. NMR evidence for a degenerate reversible haptotropic shift within the naphthalene moiety is absent. The isomers 2 and 3 have been characterized by X-ray structural analysis.