Unveiling the mechanistic implications of water oxidation reactions boosted by guanidine proton relays: a chemical-electrochemical-chemical pathway and a non-concerted proton-electron transfer

Abstract

Concerted or non-concerted are two competitive pathways involved in proton-coupled electron transfer–driven artificial water oxidation reactions (WORs).

Zwitterionic hydrogen-bonded organic framework (ZHOF): a metal-free water oxidation proton relay in alkaline and neutral media

“…Here, no detectable difference (H z D) is sensed by the proton relay system at low driving forces with the limited rate of ET, which was not seen in our previous metallic-supported guanidine relay. 19 Proceeding potentials to the more positive values cause the observable differences, although still, KIE falls into the SKIE region with no PT in RDS. The lagging of J D at the higher driving forces may be related to the acceleration of ET that discriminates a difference in the bond-breaking of O-D/O-H, knowing the fact that the shorter bond length of O-D is energetically unfavourable in PCET than the O-H bond.…”

“…Deuterium/hydrogen kinetic isotope effects (KIEs) studies were used to divulge which PT or ET are involved in the ratedetermining step (RDS) of WOR performed on FGF/ZHOF, as well as the ability of the proton translocation as a part of PCET. 19,43 The KIE value is calculated using:…”

“…44 The KIE values larger than 1.5 denotes a primary-isotope effect (PKIE), in which PT is involved in RDS or, at least, in one of the steps affecting the kinetic of the reaction. 45 The KIE #1.5, secondary kinetic isotope effect (SKIE), connotes no tangible PT(O-H bond cleavage) is involved in the RDS of water oxidation reaction, 19,46 which approves the acceleration of the proton transfer. A comparative H/D isotopic study is conducted to designate the role of FGF/ZHOF and post-metalated FGF/ZHOF (FGF/ HOF@Ni) in boosting WOR.…”

“…5B (red plot) shows some deviation from linearity, a criterion of the non-concertedness during PCET. 19 In addition to the PI studies, pH-dependency on RHE scale behaviour gives a deep insight into PCET identity (concerted or non-concerted) and the reaction order of the catalytic processes proceeding by FGF/ZHOF. Changing the thermodynamic driving force against pH is regarded by considering current density versus potential at the RHE scale.…”

“…Another one is the dense and orderly arranged proton donor/ acceptor allowing a sufficient degree of reorientation freedom of PSs and carriers within 3D un-/shaped channels, which can be helpful for interfacial proton shuttling in heterogeneous catalysis. 18,19 Theoretically, these frameworks can accelerate the translocation of PSs through an efficient proton hopping (Grotthuss mechanism) with smaller activation energy (E a < 0.4 eV). 18,20 A proton hops from one site to the next through the exchange of a hydrogen bond with a covalent bond.…”