Mechanistic Studies of the Insertion of CO2 into Palladium(I) Bridging Allyl Dimers
Mechanistic Studies of the Insertion of CO2 into Palladium(I) Bridging Allyl Dimers
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DOI：
10.1021/om201163k
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发表时间：
2012-01-09
期刊：
影响因子：
2.8
通讯作者：
Schmeier, Timothy J.
中科院分区：
文献类型：
--
作者：
Hruszkewycz, Damian P.;Wu, Jianguo;Schmeier, Timothy J.
与单体ETA(1)-Pd烯丙基作为亲核剂和单体ETA(3)-Pd烯丙基作为亲电体相比，人们对Pd络合物与桥联烯丙基配体的反应活性知之甚少。最近，我们证明了含有两个桥联烯丙基配体的HI二聚体与一个当量的二氧化碳反应，形成一个桥烯丙基和一个桥联羧酸配体的物种。在这项工作中，我们制备了三类不同类型的Pd-I桥联烯丙基二聚体：(I)含有两个桥联烯丙基配体的二聚体，(Ii)含有一个桥联烯丙基和一个桥式氯配体的二聚体，以及(Iii)含有一个桥联烯丙基和一个桥式羧酸配体的二聚体。用X-射线单晶衍射法对三个基团的配合物进行了表征，并对它们的结构进行了比较。具有两个桥联烯丙基配体的配合物具有最长的Pd桥联烯丙基键长，这是因为相反的桥联烯丙基配体的高反式影响。对于这些物种，HOMO几乎完全位于桥联的4，1配体上，而对于氯和羧酸桥联的物种，HOMO主要是基于Pd的。用实验和理论相结合的方法研究了三种不同类型的桥联烯丙基二聚体与二氧化碳的反应活性。具有一个桥联烯丙基和一个桥联氯配体的络合物和具有一个桥烯丙基和一个桥联羧酸配体的络合物不插入二氧化碳，因为该反应在热力学上是不利的。相反，在大多数情况下，二氧化碳与含有两个桥联烯丙基配体的物种的反应很容易，涉及桥联烯丙基配体对亲电性二氧化碳的亲核攻击。二氧化碳插入的另一种途径，涉及单体/二聚体平衡，可以在弱配位配体存在的情况下发生。总体而言，我们的结果表明，尽管桥联烯丙基配体在羧酸盐和氯桥联物种中可能是不反应的，但具有两个桥联烯丙基配体的配合物可以像单体ETA(1)-Pd烯丙基一样起到亲核作用。
In contrast to the chemistry of momomeric eta(1)-Pd allyls, which act as nucleophiles, and monomeric eta(3)-Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demonstrated that HI dimers containing two bridging allyl ligands react with one equivalent of CO2 to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of Pd-I bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high trans influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging 4,1 ligands, whereas for chloride- and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO2. Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO2 because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO2 with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO2. An alternative pathway for CO2 insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylate and chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric eta(1)-Pd allyls.