Unique syndio-selectivity in CO/styrene copolymerization reaction catalyzed by palladium complexes with 2-(2′-oxazolinyl)-1,10-phenanthrolines

Abstract

The reaction of the neutral Pd complex [Pd(CH₃)Cl(cod)] with the potentially terdentate 2-oxazolinyl phenanthroline ligands 1–3 affords the corresponding cationic dinuclear Pd-complexes 1a–3a, which can be isolated in the solid state in good yields. By treatment with AgPF₆ the complexes 1a–3a were converted into the corresponding hexafluorophosphate derivatives 1b–3b, where both the ligand units feature a terdentate coordination around the two Pd-centres with the phenanthroline fragment of each unit displaying a chelate coordination to one Pd-centre, while the corresponding oxazolinyl pendant acts as a bridging ligand towards the second Pd-centre. The persistence of this dimeric structure of 1b–3b in CD₂Cl₂ solution was confirmed by ¹⁵N-NMR experiments at natural abundance, which clearly show the binding to the metal of all of the nitrogen donors, as well as the overall C₂ symmetry of the compound. In consequence of the different strengths of the relevant ion-pair, the dimeric structure of the complex undergoes partial fragmentation in the case of the chloride derivatives 1a–3a, as evidenced from the ¹⁵N-NMR spectra. Complexes 1b–3b are active catalysts in styrene alternate carbonylation, where, under very mild conditions (30 °C and 1 atm of CO), they provide oligomers with 3–5 repetitive units as the exclusive or prevailing product. When traces of the CO/styrene polyketones are also formed, their ¹³C-NMR characterization shows that they are stereochemically homogeneous with a unique syndio-tacticity. This result implies that Pd-complexes able to induce a complete enantioface discrimination in the insertion step of the alkene during the catalytic cycle of the styrene alternate carbonylation have been produced for the first time.