STEREOCHEMISTRY AND STABILITY OF FREE AND COORDINATED SECONDARY PHOSPHINES - CRYSTAL AND MOLECULAR-STRUCTURE OF [S-[(R(ASTERISK),R(ASTERISK)),(R(ASTERISK))]]-(+)(589)-[PTCL(1,2-C6H4(PMEPH)(2))(PHMEPH)]PF6-CENTER-DOT-CH2CL2

(S,S)-[PtCl2{1,2-C6H4(PMePh)(2)}], (S,S)-1, reacts with 1 equiv of silver nitrate in acetone to give the corresponding chloro-bridged diplatinum complex, which, when treated with (+/-)-methylphenylphosphine in the presence of trifluoromethanesulfonic acid, affords, following metathesis of intermediate salts with ammonium hexafluorophosphate, the complex [PtCl{1,2-C6H4(PMePh)(2)}(PHMePh)]PF6, 2, as a diastereomeric pair, epimeric at the secondary phosphine-P stereocenter with (S,S),(S)/(S,S),(R) = 2/1 in dichloromethane-d(2). Recrystallization of this mixture from dichloromethane-diethyl ether gives colorless prisms of (S,S),(S)-[PtCl{1,2-C6H4(PMePh)(2)}(PHMePh)]PF6.CH2Cl2, (S,S),(S)-2.CH2Cl2, having [alpha](589) +56 (c 1, CH2Cl2). The crystal structure of the configurationally homogeneous secondary phosphine complex has been determined. Crystal data for C28H31Cl3F6P4Pt: triclinic, P1, a=8.816(1) Angstrom, b=9.122(1) Angstrom, c=10.556(1) Angstrom, alpha=93.50(1)degrees beta=97.81(1)degrees, delta=94.76(1)degrees Z = 1, and R=0.024. Pure (S,S),(S)-2.CH2Cl2 is stable in dichloromethane solution, but epimerization at the secondary phosphine-P stereocenter occurs rapidly in the presence of traces of chloride or water to give an equilibrium mixture of the two epimers with (S,S),(S)/(S,S),(R) = 2/1 at 23 degrees C. All attempts at displacing resolved (+/-)-PHMePh from (S,S),(S)-2.CH2Cl2 were unsuccessful. Indeed, simple secondary phosphines of this type, although possessing a considerable barrier to unimolecular inversion, viz. E(inv) > 97.5 +/- 0.5 kJ mol(-1), undergo facile intermolecular proton exchange in the presence of traces of acids or cationic secondary phosphine complexes, for example, (S,S),(S)-2.CH2Cl2. Nevertheless, isotopic exchange experiments between (+/-)-PHMePh and (+/-)-PDEtPh revealed negligible proton exchange between the phosphines in highly purified acetonitrile containing sodium acetylacetonate as a proton scavenger, thus indicating that the optical resolution of a simple secondary phosphine of the type (+/-)-PHR(1)R(2) may be feasible under mildly basic conditions.