Mechanism of CO Displacement from an Unusually Labile Rhenium Complex: An Experimental and Theoretical Investigation

The displacement of a CO ligand from an unusually labile
rhenium
carbonyl complex containing a bidentate carboxyaldehyde pyrrolyl ligand
by PPh3 and pyridine has been investigated. The reaction
is found to proceed by an associative, preequilibrium mechanism. Theoretical
calculations support the experimental data and provide a complete
energetic profile for the reaction. While the Re–CO bond is
found to be intrinsically weak in these complexes, it is postulated
that the unusual lability of this species is due to the presence of
a weak aldehyde Re–O link that can easily dissociate to open
a coordination site on the metal center and accommodate an incoming
ligand prior to CO loss. The resulting intermediate complex has been
identified by IR spectroscopy. The presence of the hemilabile pyrrolyl
ligand provides a lower-energy reaction channel for the release of
CO and may be of relevance in the design of CO-releasing molecules.

本研究探究了三苯基膦（PPh3）与吡啶（pyridine）取代含双齿羧醛吡咯基配体（bidentate carboxyaldehyde pyrrolyl ligand）的异常活泼羰基铼配合物中一氧化碳（CO）配体的反应。实验证实该反应遵循缔合预平衡机理（associative preequilibrium mechanism）。理论计算（theoretical calculations）结果验证了实验数据，并完整描绘了该反应的能量历程剖面。尽管此类配合物中的铼-羰基（Re–CO）键本征键强较弱，但研究推测该物种表现出异常活泼性的根源在于存在弱的醛基-铼-氧（Re–O）配位作用：该作用可轻易解离，从而在金属中心开放配位位点，并在CO解离前容纳进攻配体。所生成的中间配合物已通过红外（IR）光谱完成鉴定。这种半稳定吡咯基配体（hemilabile pyrrolyl ligand）的存在为CO的释放提供了低能反应通道，该发现或可为CO释放分子（CO-releasing molecules）的设计提供参考。