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

Abstract We report the mechanistic investigation of catalytic H2 evolution from formic acid in water using a formate-bridged dinuclear Ru complex as a formate hydrogen lyase model. The mechanistic study is based on isotope-labeling experiments involving hydrogen isotope exchange reaction.


OPEN ACCESS
[turnover frequency {TOF = (mol of evolved H 2 /mol of catalyst) per hour} = 857 h −1 ]. This extremely fast reaction rate, however, prevented us from investigating the mechanism of H 2 formation.
As part of our efforts to investigate the mechanism of H 2 /CO 2 formation from HCOOH, we now report a dinuclear Ru I complex [Ru I 2 (CO) 4 (μ-HCOO) 2

(DMSO) 2 ]
(1) that catalyzes the above reaction at a significantly slower reaction rate (TOF = 13.1 h −1 ). Here we disclose the detailed mechanism of H 2 formation from HCOOH based on isotope-labeling experiments.

Materials and methods
All experiments were carried out under N 2 or Ar atmosphere by using standard Schlenk techniques and a glovebox. Tetrahydrofuran (THF) was distilled from Na/benzophenone under N 2 atmosphere prior to use. HCOOH, DCOOD, 40% NaOD/D 2 O, 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 ( 1 H 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 MnCl 2 -alumina column (model: Shinwa OGO-SP) at -196 °C (liquid N 2 ) for quantitative analyses of H 2 , HD, and D 2 and by a Shimadzu GC-2014 (Ar carrier) (Shimadzu, Kyoto, Japan) with activated charcoal at 100 °C for quantitative analyses for H 2 , HD, D 2 , and CO 2 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/H 2 O (1.0-7.0). The pD of the solution was adjusted by using HCOOH, DCOOH, HCOOD, DCOOD, and 40% NaOD/D 2 O (1.0-7.0), in which H + concentration of HCOOH and DCOOH is negligible quantity compared to D + concentration of D 2 O. 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].
[Ru I 2 (CO) 4 (μ-HCOO) 2  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 H 2 O (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 H 2 and CO 2 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 H 2 O (1.0 mL), HCOOD (2.60 mmol) in D 2 O (1.0 mL), or DCOOD (2.60 mmol) in D 2 O (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 H 2 , HD, D 2 , and CO 2 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 H 2 O 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 H 2 by GC. The catalytic reaction is first-order against the catalyst concentration.
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 H 2 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 1 H 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 1 H NMR spectroscopy. Then, D 2 O (50 μL) was added into the resulting DMSO-d 6 solution under N 2 atmosphere. The H + /D + exchange of hydride ligand of 2 was confirmed by 1 H NMR spectroscopy with CH 2 Br 2 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.   12 ] with HCOOH and DMSO in THF under a N 2 atmosphere, which was then characterized by X-ray analysis, and 1 H NMR, UV-vis, and IR spectroscopies (Figures 1-4). Both Ru atoms adopt a distorted octahedral geometry in which the Ru ion is ligated by two C(CO), an S(DMSO), two O(μ-formate), and an adjacent Ru atom (Figure 1). Two Ru atoms are tethered by a Ru-Ru bond and two formate ligands. The bond distance of Ru1-Ru2 {2.6654(3) Å} is similar to those of other formate-bridged dinuclear Ru complexes (2.679 and 2.720 Å) [13,14].     (Figure 4).

Results and discussion
The IR spectrum of 1 shows the stretching frequencies of CO coordinated to Ru I centers at 1950, 1994, and 2041 cm −1 (Figure 2), which are lower than the free CO stretching frequency (2143 cm −1 ). This weakened CO bond is also caused by the back donation from the low-valent Ru I to the CO ligand. The 1 H NMR signals observed in the diamagnetic region revealed that 1 is diamagnetic, which originates from an antiferromagnetic exchange interaction between two Ru I centers       (Figure 5(a)). The hydride-derived peak is typical of those found in hydride-bridged Ru complexes [16][17][18][19]. The dimer structure of 2 with an antiferromagnetic exchange interaction between two Ru I centers, was suggested by the signals observed in the diamagnetic region of the 1 H NMR spectrum and its ESR silent character.
We have confirmed that the hydride ligand of 2 has a protic character rather than hydridic character based on the following investigations. We observed 2 being unreactive toward proton of HCOOH, i.e. dihydrogen gas was not formed via protonation of the hydride ligand, which was confirmed by 1 H NMR spectroscopy and GC analysis. Then, we observed an H + /D + exchange of the hydride ligand of 2, as confirmed by 1 H NMR spectroscopy ( Figure 5). The intensity of hydridederived peak of 2 only decreased by the addition of D 2 O into DMSO-d 6 solution of 2 ( Figure 5), meaning that the hydride ligand underwent the H + /D + exchange with D + .
Complex 1 is a precursor to catalyze the pH-dependent conversion of HCOOH to H 2 and CO 2 in water at pH 1.0-7.0 ( Figures 6-10 and Table 1). H 2 and CO 2 gases were detected by GC. No nanoparticles were formed in the catalytic reaction, which was confirmed by dynamic light scattering measurements. Figure 6 shows the time-dependent profile of turnover numbers (TONs, mol of H 2 evolved/mol of 1) of H 2 evolution in the reaction of 1 with an excess amount of HCOOH in water at 80 °C. The pH-dependent TON shows a maximum around pH 3.5 (Figure 7). We investigated the dependence of initial rate for H 2 production against the concentration of 1 (0.63-5.0 mM). This linear correlation clearly indicates that the catalytic reaction is first-order against the catalyst concentration ( Figure 8).
The bis(μ-formate) Ru I 2 complex 1 can convert to a (hydride)(formate) species 2 with evolution of CO 2 in DMSO at 80 °C in the presence of 1 equivalent of HCOONa. β-Hydrogen elimination is expected to be assisted by the metal centers to release CO 2 and form the hydride ligand [15]. The 1 H NMR spectrum of 2 in DMSO-d 6 shows hydride-and formate-derived signals at -12.4 and 7.76 ppm, respectively, with the same  Figure 9. a proposed reaction mechanism for the conversion of hcooh to h 2 and co 2 catalyzed by dinuclear ru complexes in h 2 o.  Figure 10. a proposed reaction mechanism for the hydrogen isotope exchange reaction in the conversion of hcood to h 2 , hd, d 2 , and co 2 in d 2 o catalyzed by dinuclear ru complexes (entry 3 of Table 1).

Conclusions
Isotope-labeling experiments suggest that the formation of dihydrogen gas from formic acid takes place via H + / D + exchange of the protic hydride ligands and reductive elimination from the protic hydride ligands of the dinuclear Ru complex in H 2 O/D 2 O.

Disclosure statement
No potential conflict of interest was reported by the authors. In order to investigate the reaction mechanism of the dehydrogenation of HCOOH, deuterium isotope-labeling experiments were conducted as shown in Table 1. Entries 2 (DCOOH in H 2 O at pH 3.5) and 3 (HCOOD in D 2 O at pD 3.5) show the TONs of 5.2 and 6.7, respectively, which are lower than that of 13.1 in the entry 1 (HCOOH in H 2 O at pH 3.5). These results suggest that the cleavage of C-H bond in HCOOH and O-H bond in H 2 O should be involved in the rate-determining step. The lowest TON value of 2.4 in entry 4 (DCOOD in D 2 O at pD 3.5) can be expected by the results of entries 2 and 3. According to the results of entries 1-4, we propose the reaction mechanism shown in Figure 9. The bis(μ-formate) complex 1 is converted to the (hydride)(formate) species 2 with the release of CO 2 . The subsequent decarboxylation of 2 is expected to result in formation of a dihydride species A, which can be proposed owing to the previously reported analogous structures [20][21][22][23][24]. Then reductive elimination of H 2 can yield a low-valent species B. Protonation of B would provide a monohydride species C, which can bind HCOOto return to 2. This proposed mechanism involves protonation step of B and binding step of HCOO -, with a pK a of 3.75, to C, which causes a maximum TON around pH 3.5 because acidic conditions facilitate the protonation step and basic conditions deprotonating HCOOH facilitate the binding step of HCOOto C.
We also investigated the hydrogen isotope exchange reaction by observation of evolved dihydrogen gases in Table 1. The entry 2 shows H 2 (7%) and HD (93%) are evolved in the reaction of 1 with DCOOH in H 2 O. This result clearly indicates that dihydrogen gases (H 2 and HD) originate from H of H 2 O and D of DCOOH. The evolved H 2 is a key product to elucidate the reaction mechanism since H 2 comes from only H + in H 2 O, which enables us to exclude the possibility of protonation of the hydride ligand to evolve dihydrogen gas. The same is true of entry 3. In this context, we can propose the reaction mechanism of the hydrogen isotope exchange reaction, involving the H + /D + exchange of the protic hydride ligand, causing reductive elimination of dihydrogen gas, as shown in Figure 10 [25].