Rotational Phenomena on Substituent Phenyl Rings of (η5‐Cyclopentadienyl)(η4‐Tetraphenylcyclobutadiene)Cobalt

We used extended Huckel calculations, variable-temperature, homonuclear long-range shift-correlated 2-D H-1 NMR and dynamic NOE measurements to investigate rotational phenomena of substituent phenyl rings on (eta(5)-cyclopentadienyl)(eta(4)-tetraphenylcyclobutadiene)cobalt (1). Two closely related compounds, (eta(5)-cyclopentadienyl)(eta(4)-1,3-diphenylcyclobutadiene)cobalt (A) and (eta(5)-cyclopentadienyl)(eta(4)-1,2-diphenylcyclobutadiene)cobalt (A'), were prepared. Energy minima appeared at conformations of which the dihedral angles between phenyl and cyclobutadiene rings are about 30 degrees for 1 and 0 degrees for A according to extended Huckel calculations. In 1, ortho protons of phenyl rings belong to one set of multiplet in H-1 NMR; meta and para protons belong to the other. It was supported by a long-range coupling 2-D H-1 NMR and NOE experiments. A sharp line due to phenyl rings was observed in the low-temperature H-1 NMR spectrum of A, which indicates that the five protons are magnetically equivalent at that temperature.