Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H 2 Production.

In this study, we reported the synthesis and structural characterization of a triphos-Co II complex [(κ 3 -triphos)Co II (CH 3 CN) 2 ] 2+ ( 1 ) and a triphos-Co I -H complex [(κ 2 -triphos)HCo I (CO) 2 ] ( 4 ). The facile synthetic pathways from 1 to [(κ 3 -triphos)Co II (κ 2 -O 2 CH)] + ( 1' ) and [(κ 3 -triphos)Co I (CH 3 CN)] + ( 2 ), respectively, as well as the interconversion between [(κ 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 CO 2 , 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 1 H 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 (μ-H) 2 ] 2+ core, to yield the corresponding 2 and H 2 .