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

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 is confirmed by 15N-NMR experiments at natural abundance, which clearly show the binding to the metal of all the nitrogen donors as well as the overall C2 symmetry of the compound. In consequence of the different strength 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.