Mixed phosphito-phosphonato rhodium(I) complexes

O,O'-3,3'-Di-tert-butyl-5,5'-dimethoxy-1,1'-biphenyl-2,2'-diyl phosphonate (HL) I was prepared by hydrolysis of (3,3'-di-tert-butyl-5,5'-dimethoxy-1,1'-biphenyl-2,2'-diyl)phosphorus chloride. Its tautomeric equilibrium with the corresponding phosphite form was completely shifted toward the phosphonate form. The reaction of I with the solvato complex [Rh(C8H12)(thf)(2)]ClO4 (C8H12 = cydoocta-1,5-diene, thf = tetrahydrofuran), in thf, in the molar ratio 2:1, afforded the rhodium(I) complex [Rh(HL)L(C8H12)] containing I co-ordinated both as a phosphonate and phosphite ligand. Carbon monoxide replaces the C8H12 ligand of 1 to afford the corresponding dicarbonyl species 2. The latter was better obtained by treating [Rh(acac)(CO)(2)] (acac = acetylacetonate) with I, in benzene solution, in the molar ratio 1:2. The P-31-{H-1} NMR spectra indicate for 1 and 2 the existence either of a P-OH to P=O proton-exchange process faster than the NMR time-scale or of a symmetrical O...H...O bridge. The existence of a symmetrical O...H...O bridge was confirmed by treating 2 with BF3 . Et(2)O. The acidic character of the hydrogen of the O...H...O framework was also confirmed treating 2 with NaOH. The reaction of the resulting anionic species with the solvato species [Rh(C8H12)(thf)(2)]ClO4 afforded a binuclear dirhodium(I) zwitterionic compound [Rh(2)L(2)(C8H12)(2)] containing bridging phosphonate ligands both bonded to a rhodium(I) centre through the phosphorus atoms and to the other one through the oxygen atoms. Attempts to crystallize 1 from a dilute diethyl ether-dichloromethane (4:1) solution abo afforded the binuclear complex in low yield. Its crystal structure has been determined by single-crystal X-ray diffraction. Under catalytic hydroformylation conditions, the complexes lose the phosphorus-containing moieties to give the catalyst [RhH(CO)(4)].