Organometallic conformational equilibria : XVIII. Preferred orientations and rotational barriers of π-olefins in cyclopentadienyl and indenyl complexes of iron and ruthenium

Abstract

The synthesis and spectral analysis of a series of some η 5-cyclopentadienyl-and η 5-indenyl-ironolefin complexes have allowed the elucidation of the orientational preferences and dynamic properties of the olefin ligand. The olefins rotate rapidly about the metalolefin bond with the barrier to rotation on the order of 8 kcal as determined by the observation of signal broadening in several of the ethylene complexes. Dissociation of the olefin and rotation about the carbon double bond are excluded as possible mechanisms on the basis of spectral evidence. The mode of rotation is consistent with the behavior of the olefin ligands in η 5-C 5H 5Rh(C 2H 4) 2 first noted by R. Cramer [J. Amer. Chem. Soc., 86 (1964) 217]. Thetermodynamically preferred orientations were defermined for each of the olefin complexes. Chemical shift differences resulting from the substitution of an indenyl ligand for a cyclopentadienyl ligand allow determination of the preferred orientations in the ethylene and propylene complexes. For ethylene, the orientation in which the CC bond is parallel to the plane of the cyclopentadienyl ring is preferred. Methyl substitution of the olefin produces a deviation of preferred orientation. A dihedral angle of about 10° is estimated for the propylene complex.