Steric Effects in the Aerobic Oxidation of π-Allylnickel(II) Complexes with <i>N</i>-Heterocyclic Carbenes

π-Allylchloro(NHC)nickel(II) complexes were synthesized and their reactions with O2 were studied. Ligand steric effects were found to determine the difference between rapid oxidation of the allyl group to produce bis-μ-hydroxonickel complexes and no observable reaction. The ability of the metal−NHC bond to rotate correlates with the ability of the complex to react with O2. In the limiting cases, conformationally restricted complexes are stable to O2 and complexes with rapid Ni−NHC bond rotation react rapidly with O2. Complexes with intermediate conformational flexibility were found to exhibit lesser reactivity with O2. On the basis of the observed inertness of complexes with saddle-shaped ligands to O2, we propose the adoption of a nonplanar geometry upon reaction with O2 to be required. The issue of conformational flexibility versus rigidity is expected to directly impact the catalytic behavior of metal−NHC complexes.本研究合成了π-烯丙基氯化（N-杂环卡宾，N-Heterocyclic Carbene, NHC）合镍(II)配合物，并对其与氧气（O₂）的反应进行了探究。研究发现，配体的空间位阻效应决定了反应路径的分野：一是烯丙基快速氧化生成双μ-羟桥合镍配合物，二是无可观测到的反应发生。金属-NHC键的旋转能力与该配合物与氧气的反应活性直接相关。在极端情形下，构象受限的配合物对氧气表现出稳定性，而Ni-NHC键快速旋转的配合物则可与氧气快速反应；具备中等构象灵活性的配合物，其与氧气的反应活性相对较弱。基于鞍形配体修饰的配合物对氧气呈惰性这一实验现象，我们提出：该类配合物与氧气发生反应时，需采取非平面几何构型。配体构象灵活性与刚性的权衡问题，将直接影响金属-NHC配合物的催化行为。