Observation of buttressing effect and hindered rotation about C(sp2)-C(sp2) single bond in styrenes coordinated to a ruthenium cation, Cp(diphosphine)Ru+

Article abstract of DOI:10.1139/v96-220

Complexes of (η ⁵-cyclopentadienyl)(bis(di-p-tolylphosphino)ethane)ruthenium(II) cation with some styrenes containing meta or para substituents were prepared and their NMR spectra examined in detail. Variable-temperature NMR studies on the unsubstituted and para-substituted styrene analogues demonstrated occurrence of a restricted rotation about the C(sp²)-C(sp²) single bond of the styrenes where one of the ortho hydrogens of the styrene phenyl group receives a very large diamagnetic shielding effect by one of the phosphine tolyl groups. Similar studies on the meta-substituted styrene complexes showed existence of two unequally populated conformational isomers arising from the similar restricted rotation where the meta substituent in the dominant isomer was placed further away from the C=C group. The origin of such conformational isomerism was deduced to be the buttressing effect of the meta substituent transmitted via the ortho-hydrogen atom.