A set of iridium(I) and iridium(III) complexes is reported with triazolylidene ligands that contain pendant benzoxazole, thiazole, and methyl ether groups as potentially chelating donor sites. The bonding mode of these groups was identified by NMR spectroscopy and X-ray structure analysis. The complexes were evaluated as catalyst precursors in transfer hydrogenation and in acceptorless alcohol oxidation. High-valent iridium(III) complexes were identified as the most active precursors for the oxidative alcohol dehydrogenation, while a low-valent iridium(I) complex with a methyl ether functionality was most active in reductive transfer hydrogenation. This catalyst precursor is highly versatile and efficiently hydrogenates ketones, aldehydes, imines, allylic alcohols, and most notably also unpolarized olefins, a notoriously difficult substrate for transfer hydrogenation. Turnover frequencies up to 260 h were recorded for olefin hydrogenation, whereas hydrogen transfer to ketones and aldehydes reached maximum turnover frequencies greater than 2000 h. Mechanistic investigations using a combination of isotope labeling experiments, kinetic isotope effect measurements, and Hammett parameter correlations indicate that the turnover-limiting step is hydride transfer from the metal to the substrate in transfer hydrogenation, while in alcohol dehydrogenation, the limiting step is substrate coordination to the metal center.

The cyclometalated platinum complex [PtMe(bhq)(dppy)] (1), in which bhq = benzo{h}quinoline and dppy = 2-(diphenylphosphino)pyridine, was prepared by the reaction of [PtMe(SMe(2))(bhq)] with 1 equiv of dppy at room temperature. Complex 1 contains one free pyridyl unit and was readily characterized by multinuclear NMR spectroscopy and elemental microanalysis. The reaction of complex 1 with 1 equiv of [Ag(CH(3)CN)(4)]BF(4) gave the cyclometalated cluster complex [Pt(2)Me(2)(bhq)(2)(mu-dppy)(2)Ag(2)(mu-acetone)](BF(4))(2) (2) in 70% yield. The crystal structure of complex 2 was determined by X-ray crystallography, indicating a rare example of a butterfly cluster with a Pt(2)Ag(2) core in which the Ag atoms occupy the edge-sharing bond. In solution, the bridging acetone dissociates from the cluster complex 2, but as shown by NMR spectroscopy, the Pt(2)Ag(2) core is retained in solution and a dynamic equilibrium is suggested to be established between the planar and butterfly skeletal geometries.

ExperimentalThe 1 H, 13 C and 195 Pt NMR spectra were recorded on a Bruker Avance DRX 500-MHz spectrometer. The operating frequencies and references, respectively, are shown in parentheses as follows:1 H (500MHz, tetramethylsilane, SiMe 4 ), 13 C ((125 MHz, TMS), and 195 Pt (107 MHz, aqueous Na 2 PtCl 4 ). The chemical shifts and coupling constants are in parts per million and hertz, respectively. Electrospray ion mass spectra (ESI-MS) were recorded on a Hewlett-Packard Series 1100 spectrometer. 1-(tert-Butyl)imidazole and 1-(isopropyl)-imidazole were synthesized according to literature procedures. The non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were positioned geometrically and refined with the riding model approximation, with Uiso(H) = 1.2 or 1.5 Ueq(C). All calculations were carried out using the PLATON [Spek, A. L. Acta Cryst. 1990, A46, C34. PLATON, A Multipurpose Crystallographic Tool, Utrecht University, Utrecht, The Netherlands ]. For the molecular graphics the program SHELXTL was used. For complex 2, a series of similarity and same restraints were applied to model the disordered acetone molecule. Bis(imidazolium) SaltsThe 1-alkyl-imidazoles and dibromomethane(1/2 equiv) were heated neat overnight in a pressure tube at 120 °C. The resulting solid was triturated with CH 2 Cl 2 and washed with diethyl ether. Preparation of [PtMe 2Ag , 24.4; H, 4.1; N, 6; Found: C, 24.6; H, 4; N, 6.2

The first continuous flow process has been developed for the synthesis of a superior trifluoromethylating reagent, “ligandless” CuCF3, from fluoroform, by far the best CF3 source in terms of availability, cost, and atom economy. Optimization of the residence time and feed rates for CHF3 (gas), the cuprating reagent (premade from CuCl and t-BuOK in a 1:2 ratio) in DMF, and the stabilizer (Et3N·3HF) at 23 °C and atmospheric pressure has allowed for the continuous production of CuCF3 in consistently high yields of up to 94%. The thus produced CuCF3 has been shown to be as highly efficient a trifluoromethylating agent as the one from the previously developed batch process.