Synthesis of a calix[6]arene-derived diphosphite, its palladium and platinum complexes, and the remarkable activity of (syn-calix[6]arene diphosphite)Pd(CH3) (CH3CN)OTf in carbon monoxide and ethene copolymerization

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 co nformation the phosphorus lone pairs of the phosphite have an exo orientati on with respect to the cavity of the calixarene backbone. The fluxional beh avior 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 cia coordinating ligand toward palladium(II) and platinum (II). Two syn diphosphite ligands coordinate to palladium(0), and the geome try of the complex is probably a distorted tetahedron. The X-ray structure of(syn-1)PdCl2 demonstrated the cis coordination mode and the C-2 symmetry of the ligand. The cationic complex (syn-1)Pd(CH3)(CH3CN)OTf is the first r eported complex based on a diphosphite ligand that shows catalytic activity in the copolymerization of carbon monoxide and ethene. Turnover frequencie s were 850-5300 mol mol(-1) h(-1) (25 degrees C, 20 bar carbon monoxide/eth ene). The [M-n] of all tested polymers was approximately 34 000 with a PDL of 2.3. From C-13 NMR spectral data we conclude that hydrolysis of the acyl intermediate to a carboxylic acid is the most important chain-transfer mec hanism. To a lesser extent (15% of all chain ends) beta-H elimination of th e alkyl intermediate gave rise to vinyl end groups. All elementary seeps in chain propagation have been monitored by IR spectroscopy and low-temperatu re NMR experiments.