Elucidating the intimate mechanism of NAD⁺ hydrogenation with phosphonic acid catalysed by Cp*Ir(pyridine-2-sulfonamidate) complexes

Abstract

Cp*Ir(pyridine-2-sulfonamidate) complexes undergo pyridine displacement when catalysing nicotine amide dinucleotide hydrogenation in the presence of phosphonic acid, thus explaining the molecular origin of their improved performance.

Electrochemical NADH Regeneration Mediated by Pyridine Amidate Iridium Complexes Interconverting 1,4- and 1,6-NADH

“…Consequently, the second-order rate constant k 2 was found to be 170 M –1 s –1 for 1 and 447 M –1 s –1 for 2 . Importantly, such values are nicely consistent with those obtained in the chemical regeneration of NADH catalyzed by iridium pyridine-2-sulfonamidate complexes (473 < k 2 < 1040 M –1 s –1 ), previously reported by us. , …”

“…39,57,58 As far as the latter is concerned, best catalytic performances were obtained with iridium pyridine-2-sulfonamidate catalysts 57,58 that, in combination with H−P(O)(OH) as a hydride source, approach the catalytic activity of the most efficient enzymatic systems. Indepth experimental and computational mechanistic investigations 58 clearly indicate the formation of the Ir−H intermediate, which occurs in two steps, namely the generation of a coordinative vacancy on the metal and P−H bond activation, as the turnover limiting step. The hydrogenation of NAD + by Ir−H was found to be at least 2 or 3 orders of magnitude faster than Ir−H formation.…”

“…The hydrogenation of NAD + by Ir−H was found to be at least 2 or 3 orders of magnitude faster than Ir−H formation. 58 We hypothesized that some members of the pyridine amidate iridium(III) class of complexes, satisfying the stringent requirements discussed above, might be excellent catalysts in the indirect electrochemical regeneration of NADH because the slow step of the cycle, i.e., Ir−H generation, could be facilitated/stimulated by the reduction of Ir(III) at the cathode. Indeed, our hypothesis resulted to be correct Both compounds showed an irreversible reduction event in the range of −0.6 to −0.8 V vs normal hydrogen electrode (NHE) (Figures 1a,b and S1) with the reductive peak currents (i d ) being linearly dependent on the square root of the scan rate, in agreement with a diffusive process (Figure 1c).…”

“…This different dependence of i cat /i d on NAD + concentration might be ascribed to the lower tendency of 2 to generate the out-ofcycle NAD + /catalyst adduct with respect to 1. 39,57,58,63 Indeed, the formation of such out-of-cycle species is strongly favored in the latter by the establishment of a hydrogen bond between the carbonyl group of NAD + and the −NH functionality of the pyridine amidate ligand. Direct 1 H NOESY NMR evidence for the formation of the 1−NAD + adduct was previously reported by our group.…”

A cobalt molecular catalyst for hydrogen evolution reaction with remarkable activity in phosphate buffered water solution

“…It can be hypothesized that the superior performances of 1 is due to the presence of the –NH 2 functionality that binds H 2 PO 4 − anion, properly orienting it in a way to facilitate the proton transfer to Co–H ( vide infra ). 87,88 The significant lower activity of 2 compared to 1 is also consistent with such interpretation since having a –NHMe in place of –NH 2 clearly disfavors hydrogen bonding with H 2 PO 4 − , at least statistically. The higher activity of 4 with respect to 3 might be attributed to the greater ability of the former to undergo hydrogen bonding with H 2 PO 4 − , using the lone pairs of the coordinated carboxylic oxygen atom.…”