Methyltriarylphosphonium Arenesulfonates. Preparation and Characterization by 1H, 31P, and 17O FTNMR, Cyclic Voltammetry, and Molecular Orbital Calculations

Article abstract of DOI:10.1021/jo00272a023

Two series of methyltriarylphosphonium arenesulfonates (1a-i, 2a-g) were prepared in which either the anion or the cation is systematically varied.The frontier molecular orbitals of these salts were evaluated by a combination of spectroscopic, electrochemical, and theoretical techniques.For the cation variations, irreversible reductions are observed by cyclic voltammetry, which correlate well with substituent electronegativities. 1H NMR (for the P-methyl group) and 31P NMR (for the phosphorus itself) data also correlate well with substituent constants.For the anion variations, irreversible oxidations are observed which correlate better with ? than with ?⁰, implying that the aromatic ?-system is involved in the oxidation. 17O NMR chemical shifts for the sulfonate group, on the other hand, yield a better correlation with ?⁰, as would be expected for a purely inductive effect.Semiempirical molecular orbital calculations at the AM1 level were also performed on both series of ions.The calculations are consistent with the expectation that, for a given phosphonium sulfonate salt, the LUMO is on the cation and the HOMO is on the anion.Both HOMO and LUMO energies exhibit very good linear correlations with substituent constants.While the LUMO energies also correlate well with the cathodic peak potentials, the HOMO energies give a substantially poorer correlation with the anodic peak potentials.However, by using the energies calculated for the highest occupied ?-orbitals (HOPO's) for the anions, a much improved correlation is observed.This is also consistent with the removal of an electron from the aromatic ?-system rather than the charge-bearing sulfonate group.