Intermolecular-directed reactivity in solid media. Radiogenic formation of phosphorus-centered radicals in chiral diphosphine disulfides studied by ESR

Abstract

AWct Singlecrystal, powder, and frozen-matrix ESR experiments have been performed to study the radiogenic electron-capture properties of several diastereoisomeric and asymmetric diphosphine disulfides (R,RzP(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@-Tol)P(S)P(S)Me@-Tol), and Ph(PhCHz)P(S)P(S)Ph(CH2Ph) does not lead to the formation of a three-electron bond PAP u* 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 PLP u* 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 PAP u* configurations in the first. X irradiation of diphosphine disulfides in a frozen THF matrix almost invariably results in a single radical product, being the PAP u* 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. In recent years the radiochemistry of organophosphorus compounds has received considerable attention. The phosphoruscentered radicals, formed in the radiation process, are of special interest because of their diverse structural and dynamical prop erties and their possible relevance to radiation damage in biochemical systems.’” It has been shown from a number of single-crystal ESR studies that the precise nature of the configuration of a phosphoranyl radical depends on the type of ligands surrounding the central phosphorus nucleus. Only recently have effects other than the nature of the substituents been envisaged as being essential to the formation of radiogenic phosphoruscentered radicals. In this respect, aspects of conformation and configuration of the precursor molecules and the involvement of the precursor environment in the electron-capture process have been studied.4d Substituted diphosphine disulfides (R,R,P(S)P(S)R,R,) can serve as good probes to study phosphoranyl radical formation because of their intrinsic ability to adopt various radical configurations from a single molecule (Figure l).’ X irradiation of a diphosphine disulfide almost invariably results in the formation of an electron-capture radical in which the unpaired electron is accommodated in the antibonding u* orbital of the inter-phosphorus bond. As a consequence, the electron is symmetrically distributed over the two phosphorus nuclei. Complementary to this symmetrical PAP u* configuration, several radical structures with an asymmetrical spin density distribution have been established. One of the asymmetric structures frequently observed is a radical in which the odd electron is located in the vacant equatorial position of a trigonal-bipyramidal configuration (TBP-e). Another asymmetric structure has been identified as a phosphoranyl radical in which the unpaired electron resides in an antibonding u* orbital of the phosphorussulfur bond (PS U’).