A dynamic nuclear magnetic resonance study of the effects of methyl group substitution of the hexahapto-arene on the intramolecular rotations of (η6-arene)dicarbonyl(triphenylphosphine)-chromium(0) and -molybdenum(0) complexes

Abstract

The variable-temperature 75.49 MHz ¹³ C-{¹H} n.m.r. spectra of (η⁶-arene)-dicarbonyl(triphenylphosphine)metal(0) complexes (arene = C₆H_{6 –n}Me_n, n= 0–6, metal = Cr; arene = 1,3,5-trimethylbenzene, metal = Mo) have been observed down to 145 K. The steric requirements for intramolecular rotations for these complexes are visualised as two cones with a common apex at the centre of the metal atom. Methyl substitution of the benzene ring can lead to restricted rotation of the triphenylphosphine ligand about the metal–phosphorus bond, with ΔG₂₀₀^‡ca. 36.6 kJ mol^–1. However, if two adjacent positions on the benzene ring are unsubstituted then rapid interconversion of the phenyl-group environments of the PPh₃ligand occurs even at the lowest experimentally accessible temperature. Slowed rotation about the metal–arene bond is not observed in any of the complexes.