Investigations of the effect of electronic distribution and magnetic anisotropy on the proton magnetic resonance spectra of substituted benzenechromium tricarbonyl complexes

Abstract

The 1H n.m.r. spectra of a series of methylbenzenes and the corresponding methylbenzenechromium tricarbonyl complexes have been recorded and analysed. The upfield shift of the ring proton resonances on complex formation is considered in terms of withdrawal of π-electron density from the ring, the magnetic anisotropy of the chromium tricarbonyl moiety, and quenching of the ring current and it is concluded that all three effects are important. The change of meta-substituent parameters on complex formation (for the methyl and other groups) is discussed in terms of preferred conformers and ascribed to differential magnetic anisotropy effects. The increase of one-bond ring 13C–H coupling constants on complex formation is shown to arise largely from increase in electron density at the ring carbon nucleus. The decrease of coupling constants between ring protons on complex formation is discussed in terms of reduced π-electron density on the complexed arene ring.