Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes

Reductive
elimination (RE) is a critical step in many catalytic processes.
The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl)
from Pd­(II) species is considerably faster than RE of saturated alkyl
groups. Pd­(II) dimethyl complexes ligated by chelating diimine ligands
are stable toward RE unless subjected to a thermal or redox stimulus.
Herein, we report the spontaneous RE of ethane from (azpy)­PdMe2 complexes and the unique role of the redox-active azopyridine
(azpy) ligands in facilitating this reaction. The (azpy)­PdMe2 complexes are air- and moisture-stable in the solid form, but they
readily produce ethane upon dissolution in polar solvents at temperatures
from 10 °C to room temperature without the need for an external
oxidant or elevated temperatures. Experimental and computational studies
indicate that a bimolecular methyl transfer precedes the reductive
elimination step, where both steps are facilitated by the redox-active
azopyridine ligand.

还原消除（Reductive Elimination，RE）是诸多催化过程中的关键步骤。从钯(II)物种上发生的不饱和基团（芳基、乙烯基与乙炔基）的还原消除，其速率远快于饱和烷基基团的还原消除。由螯合二亚胺配体配位的钯(II)二甲基配合物，在未经受热或氧化还原刺激的情况下，对还原消除表现出稳定性。本文报道了偶氮吡啶（azopyridine，azpy）配位的钯(II)二甲基[(azpy)PdMe₂]配合物发生乙烷的自发还原消除过程，以及氧化还原活性偶氮吡啶配体在促进该反应中发挥的独特作用。这类(azpy)PdMe₂配合物的固体形态在空气与湿气中均保持稳定，但将其溶解于极性溶剂后，在10 ℃至室温的温度范围内无需外部氧化剂或高温条件，即可快速生成乙烷。实验与计算研究表明，还原消除步骤前存在双分子甲基转移过程，且这两个步骤均由氧化还原活性偶氮吡啶配体所促进。