The fluxional behaviour of hexaphenylbenzene chromium tricarbonyl: A variable-temperature 13C NMR spectroscopic study

The 125 MHz 13C NMR spectrum of (C6Ph6)Cr(CO)3 reveals that the metal is π-bonded to a peripheral ring. At −80°C this chromium-bonded phenyl exhibits slowed rotation on the NMR time-scale and is effectively orthogonal to the central ring. The fluxional process is shown to involve rotation of the Cr(CO)3-complexed phenyl ring; the activation energy barrier is ≈ 12.2 kcal mol−1, a value much lower than those previously reported for non-complexed hexa-aryl benzenes bearing ortho or meta methyl or methoxy substituents.