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)CoII(κ2-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.