THE METAL-MEDIATED CONVERSION OF OCTACHLOROCYCLOHEPTATRIENE INTO DODECACHLOROHEPTAFULVALENE - A SYNTHETIC, STRUCTURAL, AND EHMO STUDY

Octachlorocycloheptatriene, C7Cl8, 3, reacts with a variety of organom etallic precursors, e.g., Fe-2(CO)(9), Co-2(CO)(8), [(C5H5)Fe(CO)(2)]( -), to give dodecachlorohepafulvalene, C14C12, 12, in yields ranging f rom 54 to 68%. There is no evidence to support the intermediacy of car bene complexes of the type C7Cl6=ML(n), and it is suggested that the r eaction proceeds by the dechlorination of C7Cl8 to give hexachlorotrop ylidene, which dimerizes to the observed product. Extended Huckel mole cular orbital calculations are used to show that the chlorinated carbe ne, C7Cl6, should be more stable than the corresponding parent system, C7H6. The EHMO calculations also reveal that planar heptafulvalenes a re electronically disfavored and should exist preferentially in the S- anti conformation, as is the case for C14H12. The molecule C14C12, 12, is shown by X-ray crystallography to be formed only as the syn confor mer, which isomerizes to the anti structure only at high temperature. The conformations of the two seven-membered rings in 12 closely resemb le that found in octachlorocycloheptatrie itself. C7Cl8, 3, crystalliz es in the orthorhombic space group Pnma with n = 7.130(1) Angstrom, b = 13.329(3) Angstrom, c = 12.595(3) Angstrom, and V = 1198.7(4) Angstr om(3) for Z = 4, 3 adopts a severely bent boat structure in which the planes C(1)-C(7)-C(6) and C(2)-C(3)-C(4)-C(5) make angles of 51.8 degr ees and 32.4 degrees, respectively, with the C(1)-C(2)-C(5)-C(6) plane .