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

The monophosphites derived from calix[4]arene (L-a) and p-tert-butylcalix[4]arene (L-b) react with [Rh(CO)(2)(acac*)] (acac* = (BuCOCHCOBut)-C-t) to give the mononuclear complexes [Rh(CO)(L-a)(acac*)] and [Rh(CO)(L-b)(acac*)] respectively. The crystal structure of [Rh(CO)(L-b)(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 [Rh-2(mu-Cl)(2)(CO)(4)] with L-b gives an equilibrium mixture of products which have been assigned to the cis/trans isomers of [Rh-2(mu-Cl)(2)(CO)(2)(L-b)(2)]. The crystal structure of cis-[Rh-2(mu-Cl)(2)(CO)(2)(L-b)(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-a and L-b are very active and chemoselective catalysts for the hydroformylation of hexene but the regioselectivity is low.