Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H2Production

In this study, we reported the synthesis and structural characterization of a triphos-CoIIcomplex [(κ3-triphos)CoII(CH3CN)2]2+(1) and a triphos-CoI-H complex [(κ2-triphos)HCoI(CO)2] (4). The facile synthetic pathways from 1to [(κ3-triphos)CoII(κ2-O2CH)]+(1′) and [(κ3-triphos)CoI(CH3CN)]+(2), respectively, as well as the interconversion between [(κ3-triphos)CoI(CO)2]+(3) and 4have been established. The activation energy barrier, associated with the dehydrogenation of a coordinated formate fragment in 1′yielding the corresponding 2accompanied by the formation of H2and 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–1for the first 3 h could be achieved. Kinetic isotope effect (KIE) values of 2.25 (kHCOOH/kDCOOH) and 1.36 (kHCOOH/kHCOOD) 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 H2production; specifically, deprotonating a HCOO–H bond by a proposed Co–H intermediate Cand homolytic cleavage of the CoII–H moiety of C, presumably via a dimeric Co intermediate Dcontaining a [Co2(μ-H)2]2+core, to yield the corresponding 2and H2.