Conformational Flexibility of Dibenzobarrelene-Based PC(sp³)P Pincer Iridium Hydride Complexes: The Role of Hemilabile Functional Groups and External Coordinating Solvents

Bifunctional dibenzobarrelene-based PC­(sp3)P pincer iridium complex 1 is known as an efficient catalyst in acceptorless dehydrogenation of alcohols and hydrogenation/hydroformylation of alkenes. In order to shed light on the mechanism of the hydrogen formation/activation, we performed variable-temperature IR and NMR (1H, 31P) analysis of intra- and intermolecular interactions involving a hydride ligand and hydroxymethyl cooperating group in 1 and its analogues. The results of the spectroscopic measurements in different media (dichloromethane, toluene, DMSO, and mixed solvents) were compared with the quantum chemical (DFT/M06 and B3PW91) calculations. The obtained data imply flexibility of the dibenzobarrelene-based scaffold, unprecedented for conventional pincer ligands. Both the CH2OH-substituted complex 1 and its COOEt analogue 3 prefer facial configuration of the PCP ligand with a P–Ir–P angle of ca. 100°. Such geometries are stabilized by Ir···O interaction with the dangling functional group and differ by the mutual arrangement of the H and Cl ligands. The complexes show dynamic equilibrium between the two most stable fac-isomers, which can be transformed into the meridional ones in the presence of coordinating additives (CH3CN, DMSO, or CO, but not Et3N). The process is reversible for CH3CN but irreversible for DMSO and CO, in agreement with the Lewis basicity of these molecules.基于二苯并桶烯（dibenzobarrelene）的双功能PC(sp³)P钳形铱配合物1，已被证实为可高效催化醇的无受体脱氢反应以及烯烃的加氢/氢甲酰化反应的催化剂。为阐明氢气生成与活化的反应机制，我们针对配合物1及其类似物中涉及氢配体与羟甲基协同基团的分子内及分子间相互作用，开展了变温红外（IR）与核磁共振（NMR，含¹H、³¹P谱）分析。将不同介质（二氯甲烷、甲苯、二甲基亚砜（DMSO）及混合溶剂）中的光谱测试结果，与采用DFT/M06与B3PW91方法的量子化学计算结果进行了对比。所得数据表明，该二苯并桶烯基骨架具有传统钳形配体前所未有的结构柔性。带有CH₂OH取代基的配合物1，以及其COOEt取代的类似物3，均倾向于采用PCP配体为面式构型的结构，其P–Ir–P键角约为100°。此类几何结构通过悬垂官能团与Ir…O之间的相互作用得以稳定，二者的差异仅在于氢配体与氯配体的相对排布方式。该类配合物在两种最稳定的面式异构体之间存在动态平衡；在配位添加剂（乙腈CH₃CN、二甲基亚砜DMSO或一氧化碳CO，而非三乙胺Et₃N）存在时，这两种异构体可转化为经式异构体。该转化过程对于乙腈CH₃CN是可逆的，而对于二甲基亚砜DMSO与一氧化碳CO则是不可逆的，这与这些分子的路易斯碱性（Lewis basicity）相符。