Mechanistic investigation of the formation of H2 from HCOOH with a dinuclear Ru model complex for formate hydrogen lyase

2.1. Materials and methods

All experiments were carried out under N2 or Ar atmosphere by using standard Schlenk techniques and a glovebox. Tetrahydrofuran (THF) was distilled from Na/benzophenone under N2 atmosphere prior to use. HCOOH, DCOOD, 40% NaOD/D2O, and dimethylsulfoxide (DMSO) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). HCOOD and DCOOH were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). These materials were used without further purification. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a JEOL JNM-AL300 spectrometer (JEOL, Tokyo, Japan) at 25 °C, in which the chemical shifts were referenced to tetramethylsilane (TMS) in chloroform-d 1 and DMSO in DMSO-d 6. UV-vis absorption spectra were recorded on a JASCO V-670 UV-Visible-NIR spectrophotometer (the light pass length was 1.0 cm). An IR spectrum was recorded on a Thermo Nicolet NEXUS 870 Fourier transform infrared (FTIR) instrument (Thermo Fisher Scientific, Massachusetts, USA) at 25 °C. Gas chromatographic (GC) analyses were conducted by a Shimadzu GC-8A (He carrier) (Shimadzu, Kyoto, Japan) with a MnCl2-alumina column (model: Shinwa OGO-SP) at -196 °C (liquid N2) for quantitative analyses of H2, HD, and D2 and by a Shimadzu GC-2014 (Ar carrier) (Shimadzu, Kyoto, Japan) with activated charcoal at 100 °C for quantitative analyses for H2, HD, D2, and CO2 using a thermal conductivity detector. Elemental analysis data were obtained by a PerkinElmer 2400II series CHNS/O analyzer (PerkinElmer, Massachusetts, USA) using Ar as the carrier gas. Dynamic light scattering measurements were conducted with a Malvern Zetasizer Nano (Malvern, Worcestershire, UK). X-band electron spin resonance (ESR) spectra were measured by a JEOL JES-FA200 spectrometer (JEOL, Tokyo, Japan) at -150 °C.

pH Adjustment. The pH of the solution was adjusted by using HCOOH and NaOH/H2O (1.0-7.0). The pD of the solution was adjusted by using HCOOH, DCOOH, HCOOD, DCOOD, and 40% NaOD/D2O (1.0-7.0), in which H+ concentration of HCOOH and DCOOH is negligible quantity compared to D+ concentration of D2O. In a pH (or pD) range of 1.0-7.0, the pH (or pD) values of the solutions were determined by a pH meter (model: TOA HM20 J; DKK-TOA, Tokyo, Japan) equipped with a pH combination electrode (model: TOA GST-5725C; DKK-TOA, Tokyo, Japan). Values of pD were corrected by adding 0.4 to the observed values (pD = pH meter reading + 0.4) [11,12].

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[Ru I 2 (CO) 4 (μ-HCOO) 2 (DMSO) 2 ] (1). A THF solution of [Ru0 3(CO)12] (1.00 g, 1.56 mmol) and HCOOH (12 mL, 0.32 mol) was refluxed for 3.5 h. The resulting solution was cooled to room temperature, to which was added DMSO (335 μL, 4.72 mmol) followed by its stirring for 30 min. The solvent was removed under reduced pressure to yield a yellow powder of 1, which was collected and dried in vacuo {yield: 85% based on the amount of [Ru0 3(CO)12] added}. 1H NMR (300 MHz, in chloroform-d 1, referenced to TMS): δ 8.23 (s, 2H, HCOO), 3.06 (s, 12H, (CH 3)2SO). Anal. Calcd for 1 (C10H14O10Ru2S2): C, 21.43; H, 2.52%. Found: C, 21.45; H, 2.22%.

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Typical procedure of H 2 evolution from HCOOH catalyzed by 1. In a 3.0 mL vial capped with a septum, a solution of HCOOH (2.60 mmol) in H2O (1.0 mL) was added to 1 (1.25 μmol). The pH of the resulting solution was adjusted to 1.0-7.0 and the solution was heated at 80 °C for 1 h. The gas above the solution within the vial was sampled with a gas-tight syringe (500 μL) and analyzed for H2 and CO2 by GC. No CO was observed. No nanoparticles were formed in the catalytic reaction, which was confirmed by dynamic light scattering measurements.

Isotope-labeling experiments for catalytic H 2 , HD, and D 2 evolution. In a 3.0 mL vial capped with a septum, DCOOH (2.60 mmol) in H2O (1.0 mL), HCOOD (2.60 mmol) in D2O (1.0 mL), or DCOOD (2.60 mmol) in D2O (1.0 mL) was added to 1 (1.25 μmol). The pH or pD of the resulting solution was adjusted to 3.5 by the addition of NaOH or NaOD, respectively, and it was stirred at 80 °C for 1 h. The gas above the solution within vial was sampled with a gas-tight syringe (500 μL) and analyzed for H2, HD, D2, and CO2 by GC. No CO was observed. No nanoparticles were formed in the catalytic reaction, which was confirmed by a dynamic light scattering measurements.

The initial rate of the catalytic H 2 evolution against the catalyst concentration. In a 3.0 mL vial capped with a septum, a solution of HCOOH (2.60 M) in H2O was added into 1 (0.63, 1.25, 2.5, or 5.0 mM). The pH of the resulting solution was adjusted to 3.5 and the solution was heated at 80 °C for 300 s. The gas above the solution within the vial was sampled with a gas-tight syringe (500 μL) and analyzed for H2 by GC. The catalytic reaction is first-order against the catalyst concentration.

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Reactivity of hydride species 2 toward proton of HCOOH. In a 3.0 mL vial capped with a septum, 10 equivalents of HCOOH (9.4 μL, 250 μmol) was added into a DMSO (2.0 mL) solution of 2 (vide infra), which was prepared from the reaction of 1 (14 mg, 25 μmol) with HCOONa (1.7 mg, 25 μmol) at 80 °C for 1 h. No H2 gas was formed, as confirmed by GC analysis. The same reaction was conducted using DMSO-d 6 (450 μL) instead of DMSO (2.0 mL), which was monitored by 1H NMR spectroscopy. No decrease of the hydride-derived peak of 2 was observed.

H + /D + exchange of hydride ligand of 2. In an NMR sample tube, a DMSO-d 6 solution (400 μL) of 1 (14 mg, 25 μmol) with HCOONa (1.7 mg, 25 μmol) was heated at 80 °C for 1 h to form 2, which was confirmed by 1H NMR spectroscopy. Then, D2O (50 μL) was added into the resulting DMSO-d 6 solution under N2 atmosphere. The H+/D+ exchange of hydride ligand of 2 was confirmed by 1H NMR spectroscopy with CH2Br2 as an internal standard to investigate the intensities of hydride- and formate-derived peaks.

X-ray crystallographic analysis of 1. A single crystal of 1 suitable for X-ray analysis was obtained from the diffusion of diethyl ether into its THF solution. Measurements were performed on a Rigaku/MSC Saturn CCD diffractometer (Rigaku, Tokyo, Japan) with confocal monochromated Mo-Kα radiation (λ = 0.7107 Å). Data were collected and processed using the CrystalClear program. All calculations were performed using the CrystalStructure crystallographic software package except for refinement, which was performed using SHELXL-97. Crystallographic data for 1 have been deposited at the Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC 1556459. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK {fax.: (+44)1223-336-033; e-mail: ku.ca.mac.cdcc@tisoped}.

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