Mixed phosphito–phosphonato rhodium(I) complexes

Abstract

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= cycloocta-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 ³¹P-{¹H} NMR spectra indicate for 1 and 2 the existence either of a P–OH to PO 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·Et₂O. 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 [Rh₂L₂(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 also 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].