Mechanistic Investigations\non Cp*Co<sup>III</sup>-Catalyzed\nQuinoline Transfer Hydrogenation with Formic Acid

The\nmechanism of the quinoline transfer hydrogenation\n(TH) by aqueous\nHCOOH under the action of [Cp*Co(quinNH<sub>2</sub>)I]<sup>+</sup> (<b>A*</b>; quinNH<sub>2</sub> = 8-aminoquinoline) has been\ninvestigated by a combination of experiments and density functional\ntheory (DFT) calculations. Variable-temperature (−40 to 20\n°C) <sup>1</sup>H NMR in the absence of quinoline substrate shows\nrapid equilibration between <b>A*</b> and the formate complex\n[Cp*Co(quinNH<sub>2</sub>)(O<sub>2</sub>CH)]<sup>+</sup> (<b>B*</b>) upon the addition of HCOOH/NEt<sub>3</sub> in MeOH, yielding Δ<i>H</i>° = 1.49 ± 0.03 kcal mol<sup>–1</sup> and\nΔ<i>S</i>° = 1.92 ± 0.06 cal mol<sup>–1</sup> K<sup>–1</sup>. This equilibrium mixture slowly converts\nby decarboxylation and deprotonation to paramagnetic (<i>S</i> = 1) [Cp*Cp(quinNH<sub>2</sub>)] (<b>C*</b>), indirectly identified\nby derivatization to [Cp*Co(CN<sup><i>t</i></sup>Bu)<sub>2</sub>] and further I<sub>2</sub> oxidation to [Cp*Co(CN<sup><i>t</i></sup>Bu)<sub>2</sub>I](I<sub>3</sub>). The rate law of\nthe [Cp*Co(quinNH<sub>2</sub>)I]<sup>+</sup>-catalyzed 8-methylquinoline\n(8MQ) TH with HCOOH in D<sub>2</sub>O at 80 °C has order one\nfor substrate and catalyst and order zero for HCOOH, with a rate constant <i>k</i> = (1.52 ± 0.05) × 10<sup>–2</sup> s<sup>–1</sup> mol<sup>–1</sup> L. The quinoline (Q) TH with\nHCOOH in D<sub>2</sub>O at 80 °C (<i>k</i> = (2.04\n± 0.05) × 10<sup>–2</sup> s<sup>–1</sup> mol<sup>–1</sup> L) selectively yields tetrahydroquinoline doubly\nD-labeled at the C<sup>3</sup> position ([3,3-D<sub>2</sub>]-THQ).\nUnder the same conditions, DCOOD in D<sub>2</sub>O yields [2,3,3,4-D<sub>4</sub>]-THQ with <i>k</i> = (6.6 ± 0.6) × 10<sup>–3</sup> s<sup>–1</sup> mol<sup>–1</sup> L (KIE\n= <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> =\n3.1 ± 0.5), while DCOOD in H<sub>2</sub>O yields [2,4-D<sub>2</sub>]-THQ. DFT calculations of the Cp model system point to a catalytic\ncycle with both diamagnetic and paramagnetic intermediates. A key\naspect is that the transfer of the formate H atom as a hydride to\nthe metal center, converting [CpCo(quinNH<sub>2</sub>)(O<sub>2</sub>CH)]<sup>+</sup> (<b>B</b>) to [CpCo(quinNH<sub>2</sub>)H]<sup>+</sup> (<b>D</b>), is faster than its transfer as a proton\nto yield [CpCp(quinNH<sub>2</sub>)] (<b>C</b>). This is at variance\nwith the closely related complex with the 8-hydroxyquinoline ligand\n(<i>ACS Catal.</i> <b>2021,</b> <i>11,</i> 11906–11920), underlining the decisive roles of ligand and\nreaction medium in the selection of the dehydrogenation pathway.