Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H₂ Production

In this study, we reported the synthesis and structural characterization of a triphos-Co^II complex [(κ³-triphos)Co^II(CH₃CN)₂]²⁺ (1) and a triphos-Co^I-H complex [(κ²-triphos)HCo^I(CO)₂] (4). The facile synthetic pathways from 1 to [(κ³-triphos)Co^II(κ²-O₂CH)]⁺ (1') and [(κ³-triphos)Co^I(CH₃CN)]⁺ (2), respectively, as well as the interconversion between [(κ³-triphos)Co^I(CO)₂]⁺ (3) and 4 have been established. The activation energy barrier, associated with the dehydrogenation of a coordinated formate fragment in 1' yielding the corresponding 2 accompanied by the formation of H₂ and CO₂, was experimentally determined as 23.9 kcal/mol. With 0.01 mol % loading of 1, a maximum TON ∼ 1735 within 18 h and TOF ∼ 483 h^-1 for the first 3 h could be achieved. Kinetic isotope effect (KIE) values of 2.25 (k_HCOOH/k_DCOOH) and 1.36 (k_HCOOH/k_HCOOD) for the dehydrogenation of formic acid and its deuterated derivatives, respectively, implicate that the H-COOH bond cleavage is likely the rate-determining step. The catalytic mechanism proposed by density functional theory (DFT) calculations coupled with experimental ¹H NMR and gas chromatography-mass spectrometry (GC-MS) analysis unveils two competing pathways for H₂ production; specifically, deprotonating a HCOO-H bond by a proposed Co-H intermediate C and homolytic cleavage of the Co^II-H moiety of C, presumably via a dimeric Co intermediate D containing a [Co₂(μ-H)₂]²⁺ core, to yield the corresponding 2 and H₂.