Redox-active pyridine(diimine) (PDI) iron catalysts promote the reversible [2 + 2] cycloaddition of alkenes and dienes to cyclobutane derivatives that have applications ranging from fuels to chemically recyclable polymers. Metallacycles were identified as key intermediates, and spin crossover from the singlet to the triplet surface was calculated to facilitate the reductive coupling step responsible for the formation of the four-membered ring. In this work, a series of sterically and electronically differentiated PDI ligands was studied for the [2 + 2] cycloaddition of ethylene and butadiene to vinylcyclobutane. Kinetic studies revealed that the fastest and slowest turnover were observed with equally electron-deficient supporting ligands that either feature phenyl-substituted imine carbon atoms (^MeBPDI) or a pyrazine core (^MeP^ZDI). While the oxidative cyclization was comparatively slow for both catalysts, the rate of reductive coupling─determined by stoichiometric ¹³C₂H₄ labeling studies─correlated with the turnover frequencies. Two-state density functional theory studies and the distinct electronic structures of related (^iPrBPDI) and (^iPrP^ZDI) iron methyl complexes revealed significantly different ligand field strengths due to either diminished ligand σ-donation (^MeBPDI) or promoted metal π-backbonding (^MeP^ZDI). Spin acceleration, leading to fast reductive coupling and catalytic turnover, was promoted in the case of the weaker ligand field and depends on both the nature and position of the electron-withdrawing group. This study provides strong evidence for the role of two-state reactivity in C(sp³)–C(sp³) bond formation and insights on how ligand design either promotes or inhibits spin acceleration in earth-abundant metal catalysis.

中文翻译：

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未活化烯烃和二烯的铁催化 [2 + 2] 环加成中的配体场敏感自旋加速

氧化还原活性吡啶（二亚胺）（PDI）铁催化剂促进烯烃和二烯与环丁烷衍生物的可逆[2 + 2]环加成，其应用范围从燃料到化学可回收聚合物。金属环被确定为关键中间体，并计算了从单线态到三线态表面的自旋交叉，以促进负责形成四元环的还原偶联步骤。在这项工作中，研究了一系列空间和电子差异的 PDI 配体，用于乙烯和丁二烯与乙烯基环丁烷的 [2 + 2] 环加成。动力学研究表明，同样缺电子的支持配体观察到最快和最慢的周转，这些配体要么具有苯基取代的亚胺碳原子（^Me BPDI），要么具有吡嗪核心（^Me P ^Z DI）。虽然两种催化剂的氧化环化相对较慢，但还原偶联速率（由化学计量¹³ C ₂ H ₄标记研究确定）与周转频率相关。双态密度泛函理论研究和相关 ( ^iPr BPDI) 和 ( ^iPr P ^Z DI) 铁甲基配合物的不同电子结构揭示了由于配体 σ 供体 ( ^Me BPDI) 减少或金属 π 增强而导致的配体场强显着不同-背键合（^Me P ^Z DI）。自旋加速导致快速还原偶联和催化转换，在较弱配体场的情况下得到促进，并且取决于吸电子基团的性质和位置。这项研究为两态反应性在 C(sp ³ )–C(sp ³ ) 键形成中的作用提供了强有力的证据，并深入了解配体设计如何促进或抑制地球丰富的金属催化中的自旋加速。