Six rhenium hydride complexes, [(6,6′-R₂-bpy)Re(CO)₃H] (bpy = 2,2′-bipyridine, R = OEt, OMe, NHMe, Me, F, Br), were synthesized. These complexes insert CO₂ to form rhenium formate complexes of the type [(6,6′-R₂-bpy)Re(CO)₃{OC(O)H}]. All the rhenium formate species were characterized using X-ray crystallography, which revealed that the bpy ligand is not coplanar with the metal coordination plane containing the two nitrogen donors of the bpy ligand but tilted. A solid-state structure of [(6,6′-Me₂-bpy)Re(CO)₃H] determined using MicroED also featured a tilted bpy ligand. The kinetics of CO₂ insertion into complexes of the type [(6,6′-R₂-bpy)Re(CO)₃H] were measured experimentally and the thermodynamic hydricities of [(6,6′-R₂-bpy)Re(CO)₃H] species were determined using theoretical calculations. A Brønsted plot constructed using the experimentally determined rate constants for CO₂ insertion and the calculated thermodynamic hydricities for [(6,6′-R₂-bpy)Re(CO)₃H] revealed a linear free energy relationship (LFER) between thermodynamic and kinetic hydricity. This LFER is different to the previously determined relationship for CO₂ insertion into complexes of the type [(4,4′-R₂-bpy)Re(CO)₃H]. At a given thermodynamic hydricity, CO₂ insertion is faster for complexes containing a 6,6′-substituted bpy ligand. This is likely in part due to the tilting observed for systems with 6,6′-substituted bpy ligands. Notably, the 6,6′-(NHMe)₂-bpy ligand could in principle stabilize the transition state for CO₂ insertion via hydrogen bonding. This work shows that if only the rate of CO₂ insertion into [(6,6′-(NHMe)₂-bpy)Re(CO)₃H] is compared to [(4,4′-R₂-bpy)Re(CO)₃H] systems, the increase in rate could be easily attributed to hydrogen bonding, but in fact all 6,6′-substituted systems lead to faster than expected rates.

中文翻译：

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与从 [（6,6′-R2-bpy）Re（CO）3H] 的氢化物转移相关的线性自由能关系：识别次级配位球中氢键效应的警示故事


合成了六种氢化铼配合物 [（6,6′-R_2-bpy）Re（CO）₃H] （bpy = 2,2′-联吡啶，R = OEt， OMe， NHMe， Me， F， Br）。这些配合物插入 CO₂ 形成 [（6,6′-R 2-bpy）Re（CO）₃{OC（O）H}] 型甲酸铼配合物。使用 X 射线晶体学对所有甲酸铼物种进行了表征，结果表明 bpy 配体与包含 bpy 配体的两个氮供体的金属配位面不共面，而是倾斜的。使用 MicroED 测定的 [（6,6′-Me_2-bpy）Re（CO）₃H] 的固态结构也具有倾斜的 bpy 配体。通过实验测量了 CO₂ 插入 [（6,6′-R 2-bpy）Re（CO）₃H] 型复合物中的动力学，并使用理论计算确定了 [（6,6′-R_2-bpy）Re（CO）₃H] 物种的热力学水合度。使用实验确定的 CO₂ 插入速率常数和计算的 [（6,6′-R 2-bpy）Re（CO）₃H] 的热力学水力度构建的 Brønsted 图揭示了热力学和动力学水力之间的线性自由能关系 （LFER）。该 LFER 与先前确定的 CO₂ 插入 [（4,4′-R 2-bpy）Re（CO）₃H] 型复合物的关系不同。在给定的热力学水力下，对于包含 6,6′-取代的 bpy 配体的复合物，CO₂ 插入更快。这可能部分是由于在具有 6,6′ 取代的 bpy 配体的系统中观察到的倾斜。 值得注意的是，6,6′-（NHMe）_2-bpy 配体原则上可以通过氢键稳定 CO₂ 插入的过渡态。这项工作表明，如果仅将 CO₂ 插入 [（6,6′-（NHMe）2-bpy）Re（CO）₃H] 的速率与 [（4,4′-R_2-bpy）Re（CO）₃H] 系统进行比较，速率的增加可以很容易地归因于氢键，但实际上所有 6,6′-取代的系统都会导致比预期更快的速率。