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

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journal contribution
posted on 2024-01-13, 14:10 authored by Chou-Pen Tsai, Chih-Yao Chen, Yi-Lin Lin, Jen-Chen Lan, Ming-Li Tsai
In this study, we reported the synthesis and structural characterization of a triphos-CoII complex [(κ3-triphos)­CoII(CH3CN)2]2+ (1) and a triphos-CoI-H complex [(κ2-triphos)­HCoI(CO)2] (4). The facile synthetic pathways from 1 to [(κ3-triphos)­CoII2-O2CH)]+ (1′) and [(κ3-triphos)­CoI(CH3CN)]+ (2), respectively, as well as the interconversion between [(κ3-triphos)­CoI(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 (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 H2 production; specifically, deprotonating a HCOO–H bond by a proposed Co–H intermediate C and homolytic cleavage of the CoII–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.