Mechanistic Investigations on Cp*CoIII-Catalyzed Quinoline Transfer Hydrogenation with Formic Acid

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(CNtBu)2] and further I2 oxidation to [Cp*Co(CNtBu)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 = kH/kD = 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.