The mechanism of the quinoline transfer hydrogenation (TH) by aqueous HCOOH under the action of [Cp*Co(quinNH₂)I]⁺ (A*; quinNH₂ = 8-aminoquinoline) has been investigated by a combination of experiments and density functional theory (DFT) calculations. Variable-temperature (−40 to 20 °C) ¹H NMR in the absence of quinoline substrate shows rapid equilibration between A* and the formate complex [Cp*Co(quinNH₂)(O₂CH)]⁺ (B*) upon the addition of HCOOH/NEt₃ in MeOH, yielding ΔH° = 1.49 ± 0.03 kcal mol^–1 and ΔS° = 1.92 ± 0.06 cal mol^–1 K^–1. This equilibrium mixture slowly converts by decarboxylation and deprotonation to paramagnetic (S = 1) [Cp*Cp(quinNH₂)] (C*), indirectly identified by derivatization to [Cp*Co(CN^tBu)₂] and further I₂ oxidation to [Cp*Co(CN^tBu)₂I](I₃). The rate law of the [Cp*Co(quinNH₂)I]⁺-catalyzed 8-methylquinoline (8MQ) TH with HCOOH in D₂O at 80 °C has order one for substrate and catalyst and order zero for HCOOH, with a rate constant k = (1.52 ± 0.05) × 10^–2 s^–1 mol^–1 L. The quinoline (Q) TH with HCOOH in D₂O at 80 °C (k = (2.04 ± 0.05) × 10^–2 s^–1 mol^–1 L) selectively yields tetrahydroquinoline doubly D-labeled at the C³ position ([3,3-D₂]-THQ). Under the same conditions, DCOOD in D₂O yields [2,3,3,4-D₄]-THQ with k = (6.6 ± 0.6) × 10^–3 s^–1 mol^–1 L (KIE = k_H/k_D = 3.1 ± 0.5), while DCOOD in H₂O yields [2,4-D₂]-THQ. DFT calculations of the Cp model system point to a catalytic cycle with both diamagnetic and paramagnetic intermediates. A key aspect is that the transfer of the formate H atom as a hydride to the metal center, converting [CpCo(quinNH₂)(O₂CH)]⁺ (B) to [CpCo(quinNH₂)H]⁺ (D), is faster than its transfer as a proton to yield [CpCp(quinNH₂)] (C). This is at variance with the closely related complex with the 8-hydroxyquinoline ligand (ACS Catal. 2021, 11, 11906–11920), underlining the decisive roles of ligand and reaction medium in the selection of the dehydrogenation pathway.

中文翻译：

---

Cp*CoIII 催化的喹啉转移加氢与甲酸的机理研究


通过实验和密度泛函理论 （DFT） 计算的结合，研究了在 [Cp*Co（quinNH₂）I]⁺（A*;quinNH₂ = 8-氨基喹啉）作用下，HCOOH 水溶液转移氢化 （TH） 的机理。在没有喹啉底物的情况下，变温（-40 至 20 °C）¹H NMR 显示，在 MeOH 中加入 HCOOH/NEt₃ 后，A* 和甲酸盐络合物 [Cp*Co（quinNH₂）（O₂CH）^]+ （B*） 之间快速平衡，产生 ΔH° = 1.49 ± 0.03 kcal mol^–1 和 ΔS° = 1.92 ± 0.06 cal mol^–1 K^–1.这种平衡混合物通过脱羧和去质子化缓慢转化为顺磁性 （S = 1） [Cp*Cp（quinNH₂）] （C*），通过衍生化为 [Cp*Co（CN^tBu）₂] 和进一步 I₂ 氧化为 [Cp*Co（CN^tBu）₂I]（I₃） 来间接鉴定。在 80 °C 下，在 D₂O 中，含 HCOOH 的 [Cp*Co（quinNH₂）I]⁺-催化的 8-甲基喹啉 （8MQ） TH 的速率定律对于底物和催化剂为 1 阶，对于 HCOOH 为 0 阶，速率常数 k = （1.52 ± 0.05） × 10^–2 s^–1 mol^–1 L。在 80 °C 的 D₂O 中含 HCOOH 的喹啉 （±Q） TH × 10^–2 s^–1 mol^–1 L） 选择性地产生在 C³ 位点 （[3,3-D₂]-THQ） 处双 D 标记的四氢喹啉。 在相同条件下，D₂O 中的 DCOOD 产生 [2,3,3,4-D₄]-THQ，k = （6.6 ± 0.6） × ^10-3 s^–1 mol^–1 L （KIE = k_H/k_D = 3.1 ± 0.5），而 H₂O 中的 DCOOD 产生 [2,4-D₂]-THQ。Cp 模型系统的 DFT 计算表明，抗磁和顺磁中间体都存在催化循环。一个关键方面是甲酸盐 H 原子作为氢化物转移到金属中心，将 [CpCo（quinNH₂）（O₂CH）]⁺ （B） 转化为 [CpCo（quinNH₂）H]⁺ （D），比它作为质子转移产生 [CpCp（quinNH₂）] （C） 更快。这与与 8-羟基喹啉配体 （ACS Catal.2021,11， 11906–11920），强调了配体和反应介质在脱氢途径选择中的决定性作用。