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 (η 6 -cpp)ML n to (η 5 -cpp)ML n, where cppH is 4 H -cyclopenta[ def ]phenanthrene and ML n = Fe(C 5 H 5 ) 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 (η 3 -cpp)ML n species, stabilized by the presence of a naphthalene-type 10π aromatic system. In an attempt to generate (η 3 -cpp)Fe(CO)(C 5 H 5 ), the corresponding (η 1 -cpp)Fe(CO) 2 (C 5 H 5 ) complex was prepared and allowed to decompose; the major products were [(C 5 H 5 )Fe(CO) 2 ] 2 and the cpp trimer, C 15 H 8 (C 15 H 9 ) 2, 20, that was shown to adopt a rigid geometry with C 2 symmetry. Treatment of (η 5 -cpp)Mn(CO) 3 with triethylphosphine yields (η 1 -cpp)Mn(CO) 3 (PEt 3 ) 2, 23, which exhibits hindered rotation about the C(4)−Mn bond with a barrier of 16.5 kcal mol - 1 . The molecule [(η 6 -cppH)Fe(C 5 H 5 )]PF 6, 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.