Unique syndio-selectivity in CO/styrene copolymerization reaction catalyzed by palladium complexes with 2-(2′-oxazolinyl)-1,10-phenanthrolines
作者:Angelo Meduri、Daniela Cozzula、Angela D'Amora、Ennio Zangrando、Serafino Gladiali、Barbara Milani
DOI:10.1039/c2dt30157a
日期:——
The reaction of the neutral Pd complex [Pd(CH3)Cl(cod)] with the potentially terdentate 2-oxazolinyl phenanthroline ligands 1â3 affords the corresponding cationic dinuclear Pd-complexes 1aâ3a, which can be isolated in the solid state in good yields. By treatment with AgPF6 the complexes 1aâ3a were converted into the corresponding hexafluorophosphate derivatives 1bâ3b, 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 1bâ3b 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 1aâ3a, as evidenced from the 15N-NMR spectra. Complexes 1bâ3b are active catalysts in styrene alternate carbonylation, where, under very mild conditions (30 °C and 1 atm of CO), they provide oligomers with 3â5 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.
中性钯络合物[Pd(CH3)Cl(cod)]与潜在的三齿 2-噁唑啉基菲罗啉配体 1â3 反应,生成了相应的阳离子双核钯络合物 1aâ3a,这些络合物可以在固态下以良好的产率分离出来。用 AgPF6 处理后,络合物 1aâ3a 转化为相应的六氟磷酸盐衍生物 1bâ3b,其中两个配体单元都以三齿配位为特征,围绕两个 Pd 中心,每个单元的菲罗啉片段与一个 Pd 中心螯合配位,而相应的噁唑啉基垂饰则作为桥接配体与第二个 Pd 中心配位。自然丰度下的 15N-NMR 实验证实了 1bâ3b 在 CD2Cl2 溶液中的这种二聚体结构的持久性,该实验清楚地显示了所有氮供体与金属的结合以及化合物的整体 C2 对称性。由于相关离子对的强度不同,在氯化物衍生物 1aâ3a 的情况下,复合物的二聚结构会发生部分碎裂,这一点从 15N-NMR 光谱中可以得到证明。络合物 1bâ3b 是苯乙烯交替羰基化反应的活性催化剂,在非常温和的条件下(30 °C、1 atm CO),它们能提供具有 3â5 个重复单元的低聚物作为唯一或主要产物。当也形成痕量的 CO/苯乙烯聚酮时,其 13C-NMR 特征显示,它们在立体化学上是同质的,具有独特的联合接触性。这一结果表明,在苯乙烯交替羰基化催化循环过程中,钯络合物能够在烯烃插入步骤中诱导完全的对映体区分,这是首次生产出这种钯络合物。