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 [(kappa(3)-triphos)Co-II(CH3CN)(2)](2+) (1) and a triphos-Co-I-H complex [(kappa(2)-triphos)HCoI(CO)(2)] (4). The facile synthetic pathways from 1 to [(kappa(3)-triphos)Co-II(kappa(2)-O2CH)](+) (1 ') and [(kappa(3)-triphos)Co-I(CH3CN)](+) (2), respectively, as well as the interconversion between [(kappa(3)-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 H-2 and CO2, was experimentally determined as 23.9 kcal/mol. With 0.01 mol % loading of 1, a maximum TON similar to 1735 within 18 h and TOF similar to 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 H-1 NMR and gas chromatography-mass spectrometry (GC-MS) analysis unveils two competing pathways for H-2 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 [Co-2(mu-H)(2)](2+) core, to yield the corresponding 2 and H-2.