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

Abstract

Abstract We used extended Hückel calculations, variable‐temperature, homonuclear long‐range shift‐correlated 2‐D 1 H NMR and dynamic NOE measurements to investigate rotational phenomena of substituent phenyl rings on (η 5 ‐cyclopentadienyl)(η 4 tetraphenylcyclobutadiene)cobalt (1). Two closely related compounds, (η 5 ‐cyclopentadienyl)(η 4 ‐1,3‐diphenylcyclobutadiene)cobalt (A) and (η 5 ‐cyclopentadienyl)(η 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° for I and 0° for A according to extended Hückel calculations. In 1, ortho protons of phenyl rings belong to one set of multiplet in 1 H NMR; meta and para protons belong to the other. It was supported by a long‐range coupling 2‐D 1 H NMR and NOE experiments. A sharp line due to phenyl rings was observed in the low‐temperature 1 H NMR spectrum of A, which indicates that the five protons are magnetically equivalent at that temperature.