INTERMOLECULAR-DIRECTED REACTIVITY IN SOLID MEDIA - RADIOGENIC FORMATION OF PHOSPHORUS-CENTERED RADICALS IN CHIRAL DIPHOSPHINE DISULFIDES STUDIED BY ESR

Single-crystal, powder, and frozen-matrix ESR experiments have been performed to study the radiogenic electron-capture properties of several diastereoisomeric and asymmetric diphosphine disulfides (R1R2P(S)P(S)R3R4). The principal values of the hyperfine couplings of several phosphorus-centered radical configurations are determined and related to the spin density distribution. Attention is focused on the strong differences in radiogenic properties, observed between the meso and racemic forms of phenyl- and tolyl-substituted diphosphine disulfides. The most striking result is that X irradiation of the crystalline meso compounds MePhP(S)P(S)MePh, Me(p-Tol)P(S)P(S)Me(p-Tol), and Ph(PhCH2)P(S)P(S)Ph(CH2Ph) does not lead to the formation of a three-electron bond P∸P σ⋆ radical but invariably results in configurations in which the unpaired electron is primarily localized on one half of the molecule. X irradiation of the corresponding racemic forms, on the other hand, gives rise to P∸P σ⋆ configurations. The observed discrimination between symmetric and asymmetric configurations is explained in terms of intermolecular steric interactions affecting the geometry relaxation of the precursor molecule after initial electron addition. For a quantitative assessment, the change in van der Waals energy resulting from elongation of the P-P bond of the molecules in their respective crystal lattices was calculated with X-ray crystallographic data. The calculations reveal significantly stronger steric interactions for the aromatic meso compounds than for their racemic forms, in agreement with the absence of P∸P σ⋆ configurations in the first. X irradiation of diphosphine disulfides in a frozen THF matrix almost invariably results in a single radical product, being the P∸P σ⋆ configuration, and differences between meso and racemic isomers disappear. This is a consequence of the fact that in a randomly oriented solid matrix the molecular packing is less tight than in a molecular crystal, making more space available to the precursor molecule. It is concluded that in case stabilization of the initial electron adduct via P-P bond length elongation is unfavorable because of steric interactions, other relaxation pathways become accessible, resulting in alternative phosphoranyl radical configurations. © 1990, American Chemical Society. All rights reserved.