Rhodium(I) complexes of robust phosphites derived from calix[4]arenes and their application in the hydroformylation of 1-hexene

Abstract

The monophosphites derived from calix[4]arene (L_aa) and p-tert-butylcalix[4]arene (L_bbb) react with [Rh(CO)₂(acac*)] (acac* = Bu^tCOCHCOBu^t) to give the mononuclear complexes [Rh(CO)(L_aa)(acac*)] and [Rh(CO)(L_bbb)(acac*)] respectively. The crystal structure of [Rh(CO)(L_bbb)(acac*)] shows the calixarene conformation to have aryl groups in {down, out, up, up} orientations with one aryl blocking the axial site at the square planar metal. Treatment of [Rh₂(μ-Cl)₂(CO)₄] with L_bbb gives an equilibrium mixture of products which have been assigned to the cis/trans isomers of [Rh₂(μ-Cl)₂(CO)₂(L_bbb)₂]. The crystal structure of cis-[Rh₂(μ-Cl)₂(CO)₂(L_bbb)₂] has a folded dimer geometry with both the concave and convex faces of the dimer partly blocked by the calixarene phosphite ligand. The rhodium complexes of L_aa and L_bbb are very active and chemoselective catalysts for the hydroformylation of hexene but the regioselectivity is low.