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

The complex [Cp*Ir(pica)Cl] (1; pica = picolinamidate = κ2-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 NaIO4. With the former, the initial TOF exceeds that of [Cp*Ir(pic)Cl] (2; pic = picolinate = κ2-pyridine-2-carboxylate), which is the fastest iridium catalyst reported to date, whereas with NaIO4 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-κN2)benzoic acid)(H2O)]SO4 (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 (NAH3). The formation of NAH3 has been identified as the cause of the low TON values obtained with 1 and 3, because it consumes part of the produced H2.