Mono- and di-nuclear rhodium complexes of meso- and dl-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane. Stereochemical control of reactivity and complexation geometry

Thermolysis of meso- and dl-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (200°C, 10 min) followed by fractional crystallisation from ethanol/dichloromethane gives two sharp-melting diastereomers. The higher melting compound, herein shown to be the meso-isomer, reacts with 1,5-cyclooctadiene-2,4-pentanedionatorhodium and HBF₄ to give the dinuclear rhodium complex (3). This underwent hydrogenation slowly in methanol solution with deposition of rhodium metal and formation of a mononuclear complex (5) with four coordinated phosphorus nuclei, also obtained by independent synthesis. This proved to be highly susceptible to oxidation, forming a dioxygen complex (6) with P(1) and P(3) mutually trans. The lower melting dl-isomer likewise formed a dinuclear rhodium complex (4) on reaction with 1,5-cyclooctadiene-2,4-pentanedionatorhodium and HBF₄. This reacted more rapidly than complex 3 with hydrogen forming a mononuclear dihydride (7) and metallic rhodium. In the presence of cyclohexene, a tetracoordinate phosphinerhodium complex (9) was formed. This reacted with oxygen to give dioxygen complex (10), although here P(1) and P(4) are mutually trans, and with carbon monoxide to give a five-coordinate monocarbonyl (11).The corresponding dirhodium bis-cyclooctadiene complex of 1,1,4,8,11,11-hexaphenyl-1,4,8,11-tetraphosphaundecane (13) (a single diastereomer of unknown stereochemistry), reacted with hydrogen in methanol to form a dinuclear solvate without reductive degradation.