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The iridium(III) complexes [Ir(H)(Cl)(κ2-NSitBu2)(κ2-bipyMe2)] (2) and [Ir(H)(OTf)(κ2-NSitBu2)(κ2-bipyMe2)] (3) (NSitBu2 = {4-methylpyridine-2-yloxy}ditertbutylsilyl) have been synthesized and characterized including X-ray studies of 3. A comparative study of the catalytic activity of complexes 2, 3, [Ir(H)(OTf)(κ2-NSitBu2)(coe)] (4), and [Ir(H)(OTf)(κ2-NSitBu2)(PCy3)] (5) (0.1 mol%) as catalysts precursors for the solventless formic acid dehydrogenation (FADH) in the presence of Et3N (40 mol%) at 353 K has been performed. The highest activity (TOF5 min ≈ 3260 h−1) has been obtained with 3 at 373 K. However, at that temperature the FTIR spectra show traces of CO together with the desired products (H2 and CO2). Thus, the best performance was achieved at 353 K (TOF5 min ≈ 1210 h−1 and no observable CO). Kinetic studies at variable temperature show that the activation energy of the 3-catalyzed FADH process is 16.76 kcal mol−1. Kinetic isotopic effect (5 min) values of 1.6, 4.5, and 4.2 were obtained for the 3-catalyzed dehydrogenation of HCOOD, DCOOH, and DCOOD, respectively, at 353 K. The strong KIE found for DCOOH and DCOOD evidenced that the hydride transfer from the C–H bond of formic acid to the metal is the rate-determining step of the process. [ABSTRACT FROM AUTHOR]

Using a low loading of the iridium(III) complexes [Ir(CF3SO3)(κ2-NSiiPr)2] (1) (NSiiPr = (4-methylpyridin-2-yloxy)diisopropylsilyl) and [{Ir(κ2-NSiMe)2}2(μ-CF3SO3)2] (2) (NSiMe = (4-methylpyridin-2-yloxy)dimethylsilyl) in the presence of Et3N, it has been possible to achieve the solventless selective dehydrogenation of formic acid. The best catalytic performance (TOF5 min ≈ 2900 h−1) has been achieved with 2 (0.1 mol%) and Et3N (40 mol% to FA) at 373 K. Kinetic studies at variable temperatures show that the activation energy of the 2-catalyzed process at 353 K is 22.8 ± 0.8 kcal mol−1. KIE values of 1.33, 2.86, and 3.33 were obtained for the 2-catalyzed dehydrogenation of HCOOD, DCOOH, and DCOOD, respectively, in the presence of 10 mol% of Et3N at 353 K. These data show that the activation of the C–H bond of FA is the rate-determining step of the process. A DFT mechanistic study for the catalytic cycle involving hydride abstraction from the formate anion by the metal, assisted by a molecule of formic acid, and heterolytic H2 formation has been performed. Moreover, the presence of Ir-formate intermediates was identified by means of NMR studies of the catalytic reactions in thf-d8 at 323 K. In all the cases, the decomposition of the catalyst to give unactive crystalline iridium NPs was observed. [ABSTRACT FROM AUTHOR]

The Ir–Si bond distances reported for Ir-(fac-κ3-NSiNOPy) and Ir-(fac-κ3-NSiN4MeOPy) species (NSiNOPy = bis(pyridine-2-yloxy)methylsilyl and NSiN4MeOPy = bis(4-methyl-pyridine-2-yloxy)methylsily) are in the range of 2.220–2.235 Å. These values are in the lowest limit of the Ir–Si bond distances found in the Cambridge Structural Database (CSD). To understand the origin of such remarkable shortening, a computational study of the bonding situation of representative examples of Ir-(fac-κ3-NSiN) species has been carried out. It is found that the Ir–Si bond can be described as an electron-sharing (i.e. covalent) bond. Despite that, this bond is highly polarized and as a result, the contribution of the electrostatic attractions to the bonding is rather significant. Indeed, there exists a linear relationship (R2 = 0.97) between the Ir–Si bond distance and the extent of the computed electrostatic interactions, which indicates that the ionic contribution to the bonding is mainly responsible for the observed Ir–Si bond shortening. [ABSTRACT FROM AUTHOR]

Iridium(III) complexes of the general formula [Ir(X)(κ2-NSiiPr2)2] (NSiiPr2 = (4-methyl-pyridine-2-yloxy)diisopropylsilyl; X = Cl, 3; CF3SO3, 5; CF3CO2, 6) have been prepared and fully characterized, including X-ray diffraction studies and theoretical calculations. The presence of isopropyl substituents at the silicon atom favours the monomeric structure found in complexes 3 and 5. The short Ir–Si bond distances (2.25–2.28 Å) indicate some degree of base-stabilized silylene character of the Ir–Si bond in 3, 5 and 6 assisted by the 2-pyridone moiety. However, the shortening of these Ir–Si bonds might be a consequence of the constrained 2-pyridone geometry, and consequently the silyl character of these bonds can not be excluded. A DFT theoretical study on the nature of the Ir–Si bonds has been performed for complex 3 as well as for four other iridium complexes finding representative examples of different bonding situations between Ir and Si atoms: silylene, base-assisted silylene (both with an anionic base and with a neutral base), and silyl bonds, using the topological properties of the electron charge density. The results of these studies show that the Ir–Si bonds in Ir–NSiiPr2 complexes can be considered as an intermediate between the base-stabilized silylene and silyl cases, and therefore they have been proposed as 2-pyridone-stabilized iridium silylene/silyl bonds. [ABSTRACT FROM AUTHOR]

Reaction of [Ir(μ-Cl)(COE)2]2 (COE = cis-cyclooctene) with tris(3,5-dimethylpyrazol-1-yl)methane (MeTpm) affords [IrCl(κ1-N-MeTpm)(COD)] (1) (COD = 1,5-cyclooctadiene). The formation of 1 implies the transfer dehydrogenation of a COE ligand to give COD and COA (cyclooctane). A mechanistic proposal based on DFT calculations that explains this iridium promoted process has been disclosed. Additionally, reactivity studies have allowed the preparation and characterization, including determination of the molecular structures of a number of iridium complexes with the MeTpm ligand in κ1, κ2 or κ3-N coordination modes. Moreover, the first example of an Ir-cyclooctyl complex featuring hydride and carbonyl ligands, whose solid state structure has been determined by X-ray diffraction methods, is reported. [ABSTRACT FROM AUTHOR]

The reaction of (4-methyl-pyridin-2-iloxy)ditertbutylsilane (NSitBu–H, 1) with [IrCl(coe)2]2 affords the iridium(iii) complex [Ir(H)(Cl)(κ2-NSitBu)(coe)] (2), which has been fully characterized including X-ray diffraction studies. The reaction of 2 with AgCF3SO3 leads to the formation of species [Ir(H)(CF3SO3)(κ2-NSitBu)(coe)] (3). The iridium complexes 2 and 3 are effective catalysts for the reduction of formamides with HSiMe2Ph. The selectivity of the reduction process depends on the catalyst. Thus, by using complex 2, with a chloride ancillary ligand, it has been possible to selectively obtain the corresponding O-silylated hemiaminal by reaction of formamides with one equivalent of HSiMe2Ph, while complex 3, with a triflate ligand instead of chloride, catalyzed the selective reduction of formamides to the corresponding methylamine. [ABSTRACT FROM AUTHOR]