Synthesis of a Calix[6]arene-Derived Diphosphite, Its Palladium and Platinum Complexes, and the Remarkable Activity of (<i>s</i><i>yn</i>-Calix[6]arene diphosphite)Pd(CH₃)(CH₃CN)OTf in Carbon Monoxide and Ethene Copolymerization

Abstract

Two noninterconvertable conformations of calix[6]arene diphosphite (1) were synthesized starting from calix[6]arene and PCl3. The conformations differ in the orientation of the phosphite moieties (syn and anti). In the syn conformation the phosphorus lone pairs of the phosphite have an exo orientation with respect to the cavity of the calixarene backbone. The fluxional behavior shown by syn-1 in solution is described as a up−up−out ↔ out−up−up interconversion of the calix[6]arene backbone. The syn diphosphite behaves as an exclusively cis coordinating ligand toward palladium(II) and platinum(II). Two syn diphosphite ligands coordinate to palladium(0), and the geometry of the complex is probably a distorted tetrahedron. The X-ray structure of (syn-1)PdCl2 demonstrated the cis coordination mode and the C2 symmetry of the ligand. The cationic complex (syn-1)Pd(CH3)(CH3CN)OTf is the first reported complex based on a diphosphite ligand that shows catalytic activity in the copolymerization of carbon monoxide and ethene. Turnover frequencies were 850−5300 mol mol-1 h-1 (25 °C, 20 bar carbon monoxide/ethene). The 〈Mn〉 of all tested polymers was approximately 34 000 with a PDI of 2.3. From 13C NMR spectral data we conclude that hydrolysis of the acyl intermediate to a carboxylic acid is the most important chain-transfer mechanism. To a lesser extent (15% of all chain ends) β-H elimination of the alkyl intermediate gave rise to vinyl end groups. All elementary steps in chain propagation have been monitored by IR spectroscopy and low-temperature NMR experiments.