Metallacrown ethers with a symmetric bis(phosphite) ligand derived from 1,2-bis-(2-hydroxyethoxy)benzene: Synthesis, characterization and hydroformylation of styrene

To better understand the effects of ligand configuration on hydroformylation reactions carried out in the presence of LiBPh4·3dme (dme = 1,2-dimethoxyethane), a conformationally restrained bis(phosphite) ligand derived from 1,2-bis-(2-hydroxyethoxy)benzene, {[(2,2′-O2C12H8)P(C2H4O2)]2C6H4}, 1, has been prepared and its Rh(I) metallacrown ether complex has been evaluated as a catalyst for the hydroformylation of styrene. Both the activity and regioselectivity of the catalyst are sensitive to the amount of the LiBPh4·3dme added with the activity decreasing by 16% and the regioselectivity for the iso increasing by 9% at a 8:1 LiBPh4:Rh ratio. Model complexes for the octahedral, cis-Mo(CO)4(1), 2, and square planar, cis-PtCl2(1), 3, and cis-PdCl2(1), 4, complexes in the catalytic cycle has been have been studied using multinuclear NMR spectroscopy and X-ray crystallography. Although the X-ray crystal structure of 2 suggests that the metallacrown ether ring could adopt a configuration capable of binding alkali metal cations, this does not appear to occur in a dichloromethane-d2 solution of 2 because no shift in the 31P NMR resonance 2 is observed upon the addition of an excess of LiBPh4·3dme. The 31P{1H} NMR spectra of chloroform-d solutions of 2 (in the presence of a catalytic amount of HgCl2) and of 4 and the X-ray crystal structures of the complexes indicate that the bis(phosphite) ligands are cis coordinated in these complexes in both the solution and in the solid state. This is particularly surprising for 4 because related PdCl2{Ph2P(CH2CH2O)nCH2CH2PPh2} (n = 3–5) complexes exhibit both cis–trans and monomer–oligomer equilibria in solution.