Structures and rearrangement mechanisms for some bicyclo[6.1.0]nona-2,4,6-triene complexes of chromium, molybdenum, and tungsten

Article abstract of DOI:10.1021/om00148a020

The structures of (bicyclo[6.1.0]nona-2,4,6-triene)tricarbonylmolybdenum and (endo-9-bromobicyclo-[3.1.0]nona-2,4,6-triene)tricarbonylmolybdenum have been investigated by X-ray crystallography. The first crystallized in the orthorhombic space group Pnma: a = 14.185, b = 10.417, c = 7.230 ?; Z = 4; 1143 reflections were measured, of which 756 were considered observed; the final structure had R = 0.115 and R_w = 0.137. The second complex crystallized in monoclinic space group P2₁/m: a = 8.740, b = 10.038, c = 13.742 ?; β = 85.9°; Z = 4; 1736 reflections were measured, of which 1494 were considered observed; R = 0.068 and R_w = 0.077. Both are shown to have geometries in which the cyclopropane ring is syn to the metal. In the 9-bromo complex only two of the three C=C bond are coordinated to the metal; the third coordination site is occupied by the halogen. The mechanism of thermal rearrangement of (bicyclo[6.1.0]nona-2,4,6-triene)tricarbonylmolybdenum to (bicyclo[4.2.1]nona-2,4,7-triene)tricarbonylmolybdenum has been investigated by deuterium-labeling and kinetic studies. A new, degenerate rearrangement of the starting complex has been discovered in the course of this investigation. Similar processes are shown to occur for the corresponding chromium and tungsten complexes. It is concluded that both types of rearrangement are sigmatropic processes in which the metal does not directly participate in cleavage of the C-C bond. The difference in thermal chemistry of the complexed and uncomplexed hydrocarbon is proposed to be due to selective inhibition of certain reaction pathways by the metal.