Correlation of 31 P nuclear magnetic resonance chemical shifts in aryl phosphinates with Hammett substituent constants: Inductive versus resonance interactions and relevance to p π – d π bonding

Substituent-induced chemical shifts and coupling constants in the 31 P, 13 C, and 1 H nuclear magnetic resonance spectra of meta- and para-substituted phenyl dimethylphosphinates (1), methylphenylphosphinates (2), and diphenylphosphinates (3) have been determined in CDCl 3 solvent. For all three series, a correlation of δ 31 P with Hammett–Taft σ 0 (or σ) constants is preferred over σ − on the basis of the correlation coefficient and standard deviations of the slope and intercept values. Electron-withdrawing substituents induce downfield shifts in δ 31 P, in contrast to the inverse trends observed for structurally related series of oxyphosphorus acids and their derivatives. It is proposed that electron-withdrawing substituents act to deplete the electron density on the aryl oxygen, thereby weakening a p π –d π bonding interaction between the aryl oxygen and phosphorus. The resultants loss of d-orbital density on phosphorus causes a downfield shift in δ 31 P in each of the phosphinate series. Phenyl substituents attached directly to phosphorus in series 2 and 3 increase the phosphoryl p π –d π back-bonding interactions, either through inductive or resonance effects, which leads to shielding of the phosphorus atom, overriding the anticipated downfield shift through inductive electron withdrawal of the phenyl substituents in series 2 and 3, relative to the methyls in series 1. Trends in Hammett ρ values for the plots of δ 31 P and δ 13 C versus σ 0 and differences in the shielding of 13 C and 1 H nuclei of the methyl attached to phosphorus in series 1 and 2 suggest that the phenyl groups may interact in π bonding with the phosphorus atom through a resonance interaction.