A Single Organoiridium Complex Generating Highly Active Catalysts for both Water Oxidation and NAD⁺/NADH Transformations

Abstract

The complex [Cp*Ir(pica)Cl] (1; pica = picolinamidate = κ 2 -pyridine-2-carboxamide) was found to be an effective catalyst for both water oxidation to molecular oxygen and NAD + /NADH transformations, which are the key reactions of light-dependent natural photosynthesis. In particular, 1 exhibits high activity in water oxidation driven by CAN and NaIO 4 . With the former, the initial TOF exceeds that of [Cp*Ir(pic)Cl] (2; pic = picolinate = κ 2 -pyridine-2-carboxylate), which is the fastest iridium catalyst reported to date, whereas with NaIO 4 it compares well with the best catalysts. 1 exhibits top performances also in the hydrogenation of NAD + with HCOOK, leading to the regiospecific formation of 1,4-NADH (pH 7) with TOF = 143 h −1 , which is about 3 times higher than the previous highest value (54 h −1 ) reported for [Cp*Ir(4-(1H-pyrazol-1-yl-κN 2 )benzoic acid)(H 2 O)]SO 4 (3). The activity seems to be critically affected by the presence of the NH functionality, as indicated by its drop of about 1 order of magnitude when 2 (TOF = 17 h −1 ) was used as the catalyst instead of 1. 1 is also able to mediate the dehydrogenation of β-NADH, under slightly acidic conditions, as determined by NMR and GC measurements. Furthermore, an in-depth investigation carried out combining 1D, 2D, and diffusion NMR techniques indicate a remarkable speciation of β-NADH leading not only to the expected β-NAD + but also to α-NAD + , nicotinamide (NA), and 1,2,5,6-tetrahydronicotinamide (NAH 3 ). The formation of NAH 3 has been identified as the cause of the low TON values obtained with 1 and 3, because it consumes part of the produced H 2 .

Chemoenzymatic Hydrogen Production from Methanol through the Interplay of Metal Complexes and Biocatalysts

“…The metal‐catalyzed interaction between dihydrogen and nicotinamides, however, remained challenging and only recently the first effective systems appeared in the literature (Scheme ). Sharing the iridium half‐sandwich motif as a common feature, the groups of Sadler and Fukuzumi, as well as Bucci and Macchioni have reported individually systems capable of performing the acceptorless dehydrogenations of reduced nicotinamides. In addition to superior turnover frequencies, Fukuzumi's pyrazole‐based iridacycle showed high reversibility in this process and could hence be employed both for the hydrogen production from NADH and for the regeneration of NADH under hydrogen atmosphere.…”

“…In concert with formate dehydrogenase, methanol oxidation can be performed all the way to carbon dioxide by simultaneously generating three equivalents of NADH to be used in reductive enzymatic processes, or potentially also in H 2 ‐liberating chemoenzymatic approaches. The same group has also disclosed an alternative system in which the reduction potential of methanol as high‐density LOHC was utilized in aerobic biocatalytic oxygenation reactions exploiting alcohol oxidases as methanol‐active catalysts (Scheme bottom) …”

“…[30] The metalcatalyzed interaction between dihydrogen and nicotinamides, however, remained challenging and only recently the first effective systemsa ppeared in the literature (Scheme 5). Sharing the iridium half-sandwich motif as ac ommon feature, the groups of Sadler [27] and Fukuzumi, [31] as well as Bucci and Macchioni [32] have reported individually systemsc apable of performing the acceptorless dehydrogenationso fr educed nicotinamides.I na ddition to superior turnover frequencies, Fukuzumi's pyrazole-basedi ridacycle showed high reversibility in this process and could hence be employed both for the hydrogen production from NADH and for the regeneration of NADH under hydrogen atmosphere.Although some examples include hydrogen-production capabilities close to neutral conditions, the pH of the solutionsp layed am ajor role in all systems, and both reactions, the hydrogen evolution and the hydrogenation, showedh ighera ctivity at lower pH. Although being worthy of note in typical organometallic setting, pH optimal values below 5n eed to be considered as evere limitation in chemoenzymatic scenarios.…”

“…alcohol oxidases as methanol-active catalysts (Scheme 6 bottom). [32] The latter design,t aking advantage of the energetically favored aerobic methanol activation through alcohol oxidases at room temperature, was subsequently used in af irst chemoenzymatica pproach to generate hydrogen gas directly from methanol withoutt he requiremento fe xtensive heatinga nd/or substantial amountso fb ase (Scheme 7). [2] Combining an oxidase C. boidinii with ac atalase to remove the oxidase-derived side product hydrogen peroxide, the metal-enzyme cascade commenceswith the formal oxygenation of methanoltomethanediol.…”

The Middle‐Earth between Homogeneous and Heterogeneous Catalysis in Water Oxidation with Iridium

“…Successively, many other molecular Ir WOCs were reported, and many excellent and exhaustive reviews recently appeared in the literature , , . Herein, the focus is on the stability and activity of Ir WOCs, two strictly interlocked and crucial aspects.…”