The activity of three [Cp*IrL(n)] (Cp* = pentamethylcyclopentadienyl) archetypal catalysts ([Cp*Ir (bpy)Cl]Cl (1, bpy = 2,2'-bipyridine), [Cp*Ir(bzpy)(NO(3))] (2, bzpy = 2-benzoylpyridine) and [Cp*Ir(H(2)O)(3)](NO(3))(2) (3)) for water oxidation to molecular oxygen was compared using cerium(IV) ammonium nitrate as a sacrificial oxidant. Kinetic studies were carried out by: i) measuring the depletion of Ce(4+) through UV-Vis spectroscopy, ii) directly detecting the evolved oxygen through the Clark electrode and iii) measuring the volume of the evolved oxygen. The kinetics of Ce(4+) consumption were zero-order in Ce(4+) for catalysts 2 and 3, while they were first-order for 1. The order with respect to catalyst was 1 for 1 and 2 while it was 1.5 for 3. As a consequence, 2 (TOF(max) = 14.4 min(-1)) and 3 (TOF(max) = 50.4 min(-1)) were found to be the most active catalysts at low and high catalyst concentration, respectively, while the performance of 1 (TOF(max) = 8.6 min(-1)) increased with increasing the concentration of Ce4(+.) 1 and 3 were found to be the most robust catalysts at low (3.1 mu M, TON = 1240) and high (7.0 mu M, TON = 4042) catalyst concentration, respectively. In situ NMR studies were performed under exactly the same conditions of the catalytic experiments. It was observed that Cp* underwent an oxidative degradation, ultimately leading to acetic, formic and glycolic acids. Several Ir-containing intermediates of the degradation process were intercepted and fully characterized in solution through 1D- and 2D-NMR experiments. DFT and NMR studies indicated that the degradation proceeds via an initial double oxidative functionalization of both the quanternary carbon and proton of a Cp* C-CH(3) moiety

We report on three Ir(iii) molecular catalysts for water oxidation: 1, [Cp*Ir(ppy)Cl]; 2, [Cp*Ir(bzpy)NO(3)]; 3, [Cp*Ir(H(2)O)(3)](NO(3))(2). 2 and 3 are water-soluble and show a long-term activity ca. 2 and 3 times higher than 1. It is remarkable that 3, having the simplest structure, is the catalyst with the highest activity.

The reaction of [Cp*Ir(bzpy)NO3 ] (1; bzpy=2-benzoylpyridine, Cp*=pentamethylcyclopentadienyl anion), a competent water-oxidation catalyst, with several oxidants (H2 O2 , NaIO4 , cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI-MS techniques provided evidence for the formation of many species that all had the intact Ir-bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen-Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a η(3) interaction with iridium (2 a). Formal addition of H2 O to 2 a or H2 O2 to 1 leads to 2 b, in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X-ray single-crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal (2 c). A η(3) interaction between the three non-oxygenated carbons of "ex-Cp*" and iridium is also present in both 2 b and 2 c. Isolated 2 b and mixtures of 2 a-c species were tested in water-oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min(-1) ).

Complexes [Pd(η1,η2-C8H12OMe)bipy]+X- (2a−f) (where X = BPh4 - (a), CF3SO3 - (b), BF4 - (c), PF6 - (d), SbF6 - (e), and B(3,5-(CF3)2C6H3)4 - (f); bipy = 2,2‘-bipyridine; C8H12OMe = cyclooctenylmethoxy group) were synthesized by the reaction of the dimer [Pd(η1,η2-C8H12OMe)Cl]2 (1) with the bipy ligand in methanol containing Y+X- salts. They were characterized in solution by multinuclear and multidimensional NMR spectroscopy. The solid-state structure of complex 2d was obtained by X-ray single-crystal investigation. The catalytic activity of complexes 2 toward CO/styrene copolymerization in methylene chloride was tested and related to the type of counterion. The order of the catalytic activity of complexes 2a−f is the following:  BPh4 - ≪ CF3SO3 - < BF4 - < PF6 - < SbF6 - < B(3,5-(CF3)2C6H3)4 -. If the copolymerization reactions are carried out in the presence of an excess of the bipy ligand, the anion effect is less important and the order is the following:  BPh4 - ≪ CF3SO3 - < BF4 - ≈ B(3,5-(CF3)2C6H3)4 - ≈ PF6 - ≈ SbF6 -. The interionic structure of all complexes was investigated in CD2Cl2 at room and low temperature by 19F{1H} HOESY and 1H NOESY NMR spectroscopies. In solution, the counterions are located above or below the bipy ligand shifted toward the pyridine ring trans to the Pd−C σ bond, while in the crystal structure of 2d, they are settled sideways to the cationic moiety. The best anion in catalysis is the least strong coordinating one that shows the weakest interionic contacts in the 19F{1H} HOESY or 1H NOESY NMR spectra. The dynamic process that exchanges the two pyridyl rings was investigated by variable-temperature NMR spectroscopy in CD2Cl2. The activation parameters were determined. ΔG ⧧ 298 values range from 54 to 58 kJ/mol. The negative values of ΔS ⧧ (−58/−108 J K-1 mol-1), for all compounds, with the exception of 2f, suggest an associative mechanism.

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 .