Optical resolution, configurational stability, and coordination chemistry of the P-chiral heterocyclic diphosphine 1,1 '-diphenyl-3,3 ',4,4 '-tetramethyl-2,2 '-diphosphole-3,3 '-diene

The optical resolution of the C-2-symmetrical racemic ligand (R-P*S-C*)-1,1 '-diphenyl-3,3',4,4'-tetramethyl-2,2'-diphosphole-3,3'-diene (+/-)-(1) has been achieved efficiently via fractional crystallization of a pair of diast ereomeric palladium(II) complex cations containing the appropriate form of the chelating diphosphine ligand and orthometalated (S)-(1-(dimethylamino)e thyl)naphthalene. The absolute stereochemistries of both diastereomeric com plexes have been confirmed by single-crystal X-ray crystallography. The lib eration of the optically active diphosphine ligand from the corresponding r esolved complexes involved two steps: the removal of the naphthylamine liga nd with concentrated hydrochloric acid followed by the decomposition of the intermediate [Pd(1)Cl-2] species with aqueous potassium cyanide. Thus, (RP SC)-(-)-(1) and (SPRC)-(+)-(1) were obtained in high yields with [alpha](D) = +/-124.3 degrees (C6H6). These resolved ligands are configurationally st able, as their optical purities remained unchanged upon heating at 140 degr ees C for 48 h in benzene and in propanol. They are able to form the optica lly active digold complexes [Au-2(mu-1)Cl-2], despite the fact that the two rigid heterocyclic rings are linked with a carbon-carbon bond of restricte d rotation.