Synthesis and crystal structure of a dinuclear rhodium complex. Catalytic activity of mono- and di-nuclear rhodium phosphite complexes in hydroformylation

A new bidentate phosphite, {bis[2-(diphenoxyphosphinoxy)-1-naphthyl]methyl}benzene L(1) and a tetradentate phosphite, 1,4-bis{bis[2-(diphenoxyphosphinoxy)-1-naphthyl]methyl}benzene L(2) were prepared in a facile two-step procedure involving condensation of 2-naphthol with respectively benzaldehyde or terephthalaldehyde, followed by treatment with chlorodiphenoxyphosphine. The corresponding monuclear rhodium(I) complex [RhL(1)(acac)] 1 (acac = acetylacetonate) and dinuclear complexes [Rh(2)L(2)(acac)(2)] 2 and [Rh(2)L(2)Cl(2)(CO)(2)] 3 have been isolated. The fluxial behaviour of the ligand in the mono- and di-nuclear rhodium complexes in solution was studied by dynamic H-1 and P-31 NMR spectroscopy, showing hindered rotation in the biarylmethane units. The crystal structure of complex 2, obtained by X-ray analysis, reveals its dinuclear nature and an 'unfolded' geometry. Complexes 1 and 2 catalyse the hydroformylation of cyclohexene with average turnover frequencies of 428 and 344 mm(-1) h(-1), respectively, over 4 h. A notable increase was observed in turnover frequency during the course of reaction. High-pressure (20 bar H-2-CO) IR and H-1 and P-31 NMR studies with complex 1 revealed a single rhodium hydride species in solution.