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

Abstract

A new bidentate phosphite, {bis[2-(diphenoxyphosphinoxy)-1-naphthyl]methyl}benzene L¹ and a tetradentate phosphite, 1,4-bis{bis[2-(diphenoxyphosphinoxy)-1-naphthyl]methyl}benzene L² were prepared in a facile two-step procedure involving condensation of 2-naphthol with respectively benzaldehyde or terephthaladehyde, followed by treatment with chlorodiphenoxyphosphine. The corresponding monuclear rhodium(I) complex [RhL¹(acac)]1(acac = acetylacetonate) and dinuclear complexes [Rh₂L²(acac)₂]2 and [Rh₂L²Cl₂(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 ¹H and ³¹P 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 H₂–CO) IR and ¹H and ³¹P NMR studies with complex 1 revealed a single rhodium hydride species in solution.