Insights into the mechanism of photosynthetic H₂ evolution catalyzed by a heptacoordinate cobalt complex

Abstract

Shining light on the high activity of a heptacoordinate cobalt complex for photosynthetic hydrogen evolution: an integrative approach.

Rationalizing Photo‐Triggered Hydrogen Evolution Using Polypyridine Cobalt Complexes: Substituent Effects on Hexadentate Chelating Ligands

“…Isodensity plots of the “redox active orbitals” (Figure S16) and the calculated Mulliken charges (Table S1) show that a consistent fraction of the added electronic charge is localized on the metal center. However, the BPY2 ligand (nomenclature given in Figure S15) is also largely involved in the process, similarly to what was found for C0 in our previous work [16] . The second reduction(s), at more negative potentials, is assigned to reduction(s) centered on the bipyridine ligands, and essentially on BPY1, as it is apparent in Figure S16.…”

“…The second reduction(s), at more negative potentials, is assigned to reduction(s) centered on the bipyridine ligands, and essentially on BPY1, as it is apparent in Figure S16. In this case, Mulliken charges do not indicate additional electronic density on the cobalt atom [16] . As to the second reduction process, a single two‐electron wave is observed for all complexes except for C1 , for which two well separated one‐electron waves are observed at similar potentials (Table 1).…”

“…Addition of trifluoroacetic acid (TFA) to an acetonitrile solution of each cobalt complex triggers the appearance of irreversible waves (Figure 2 for C1 – C4 , Figure S4 for C0’ ), which can be attributed to catalytic proton reduction to dihydrogen, as demonstrated from bulk electrolysis experiments with the parent complex C0’ [16] . For complexes C2 – C4 (Figure 2b–d), a single, intense catalytic wave emerges at potential close to the Co II /Co I redox process and displays mid‐wave potentials ( E cat ) [19] that fall at more positive values than the Co II /Co I reduction event and are appreciably independent of the TFA concentration (Table 2).…”

“…This can be taken as an indication [20] that, similarly to C0’ , [16] for complexes C2 , C3 , and C4 an ECEC (where E indicates a reduction and C a protonation event) catalytic mechanism is operating (Scheme 3a), which implies two alternated reduction/protonation steps before hydrogen elimination. Interestingly, the overpotential ( η ) for proton reduction (Table 2) is comparable in the case of complexes C0’ , C2 , and C4 , while it is substantially larger in the case of complex C3 , thus suggesting that the substitution on the bipyridines has a stronger impact on the overpotential required for proton reduction than the substitution on the pyridine.…”

Co/Ni-polyoxotungstate photocatalysts as precursor materials for electrocatalytic water oxidation

“…[11][12][13][14][15][16] Among the attractive and low cost 3d transition metal water splitting catalysts, cobalt-based homogeneous catalysts and POMs keep attracting intense attention. [17][18][19][20][21][22] Specically, Co-POMs have recently been applied as co-catalysts on photoanodes or for enhanced performance in composite systems. [23][24][25][26][27][28] .…”