Haptotropic Shifts in (C5H5)Fe and Mn(CO)3 Complexes of 4H-Cyclopenta[def]phenanthrene (cppH): X-ray Crystal Structure and NMR Fluxionality of (η1-cpp)Mn(CO)3(PEt3)2

The favored pathways for the haptotropic shifts from (η6-cpp)ML n to (η5-cpp)ML n , where cppH is 4H-cyclopenta[def]phenanthrene and ML n = Fe(C5H5) or Mn(CO)3, have been investigated by means of extended Hückel molecular orbital calculations, and energy hypersurfaces for these processes have been obtained. These data suggest the intermediacy of an exocyclic (η3-cpp)ML n species, stabilized by the presence of a naphthalene-type 10π aromatic system. In an attempt to generate (η3-cpp)Fe(CO)(C5H5), the corresponding (η1-cpp)Fe(CO)2(C5H5) complex was prepared and allowed to decompose; the major products were [(C5H5)Fe(CO)2]2 and the cpp trimer, C15H8(C15H9)2, 20, that was shown to adopt a rigid geometry with C 2 symmetry. Treatment of (η5-cpp)Mn(CO)3 with triethylphosphine yields (η1-cpp)Mn(CO)3(PEt3)2, 23, which exhibits hindered rotation about the C(4)−Mn bond with a barrier of 16.5 kcal mol-1. The molecule [(η6-cppH)Fe(C5H5)]PF6, 2b, and also the manganese complex 23 have been characterized by X-ray crystallography. The relevance of these (and other literature data) to the mechanisms of haptotropic shifts are discussed.