Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing

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Cite This: J. Am. Chem. Soc. 2018, 140, 842−847

Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing Ammonia Yasuhiro Arikawa,* Yuji Otsubo, Hiroki Fujino, Shinnosuke Horiuchi, Eri Sakuda, and Keisuke Umakoshi Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Bunkyo-machi 1-14, Nagasaki 852-8521, Japan S Supporting Information *

ABSTRACT: The fundamental biogeochemical cycle of nitrogen includes cytochrome c nitrite reductase, which catalyzes the reduction of nitrite ions to ammonium with eight protons and six electrons (NO2− + 8H+ + 6e− → NH4+ + 2H2O). This reaction has motivated researchers to explore the reduction of nitrite. Although a number of electrochemical reductions of NO2− have been reported, the synthetic nitrite reduction reaction remains limited. To the best of our knowledge, formation of ammonia has not been reported. We report a three-step nitrite reduction cycle on a dinuclear ruthenium platform {(TpRu)2(μ-pz)} (Tp = HB(pyrazol-1-yl)3), producing ammonia. The cycle comprises conversion of a nitrito ligand to a NO ligand using 2H+ and e−, subsequent reduction of the NO ligand to a nitrido and a H2O ligand by consumption of 2H+ and 5e−, and recovery of the parent nitrito ligand. Moreover, release of ammonia was detected.

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INTRODUCTION The inorganic nitrogen cycle is an elegant cycle, which is composed of a diverse range of redox reactions mediated by some metalloenzymes (for example, nitrogenase).1−4 These reactions involve the reduction of dinitrogen (N2), nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), and nitrous oxide (N2O) and the oxidation of ammonium (NH4+) and hydroxylamine (NH2OH). These interesting systems as ideal models motivate many researchers to investigate their redox reactions. We have also been interested in the systems, especially nitric oxide reductase (NOR).5−14 NOR is a membrane-bound enzyme that catalyzes the 2e− reduction of nitric oxide to nitrous oxide using two protons (2NO + 2H+ + 2e− → N2O + H2O). In our continuing research of nitrosylruthenium chemistry, we have found an unusual N− N coupling of two NO ligands on a dinuclear ruthenium complex supported by Tp ligands.15 The use of this complex has succeeded in achieving a stepwise NO reduction cycle (Scheme 1).16−18 Next, we aimed to mimic the function of cytochrome c nitrite reductase (CcNiR).7,19−22 CcNiR catalyzes the 6e− reduction of NO2− to NH4+ with eight protons (NO2− + 8H+ + 6e− → NH4+ + 2H2O). Although the other type of nitrite reductase, which catalyzes one-electron reduction of NO2− to NO with two protons (NO2− + 2H+ + e− → NO + H2O),5,7,10,14,19 is also known, CcNiR requires multiprotons and multielectrons for generating NH4+ at one time without releasing NO. The formation mechanism of NH4+ in CcNiR is proposed19−21 but still unclear. Electrochemical reduction of NO2− has been widely explored.23 Pioneering works have demonstrated the nitrite reduction in solution [metal−cyclam (cyclam = 1,4,8,11© 2017 American Chemical Society

Scheme 1. NO Reduction Cycle on a Dinuclear Ruthenium Complex (2NO + 2H+ + 2e− → N2O + H2O)

tetraazacyclotetradecane,24,25 iron porphyrin,26,27 edta complexes,28 and iron-substituted polyoxotungstates29] or on functionalized electrodes, whose surfaces are modified by heme proteins,30−32 iron protoporphyrin,33 iron−alizarin complexone,34 and metallophthalocyanine complexes,35 affording a variety of reduction products (NH3, NH2OH, N2O, and N2) in different ratios. Although nitrite reduction on Pt electrodes in alkaline solution affords NH3 as the main or even only products, N2O and NH2OH are the dominant reaction products in acidic solution.23 In either case, as the problem of the electrochemical reduction, the selectivity and a large overpotential have been mentioned. Received: November 13, 2017 Published: December 19, 2017 842

DOI: 10.1021/jacs.7b12020 J. Am. Chem. Soc. 2018, 140, 842−847

Article

Journal of the American Chemical Society The study of synthetic nitrite reduction remains limited. In the fundamental study, the coordinated NO2− or NO ligand has been reduced to an NH3 on mononuclear ruthenium complexes.36−40 However, as synthetic nitrite reduction cycles, two representative examples have been reported. One is a mononuclear molybdenum complex [MoCl(NO)(S4)] [S4 = 2,2′-(ethylenedithio)dibenzenthiolate], which shows a stepwise reduction cycle of NO to NH2OH (NO + 3H+ + 3e− → NH2OH).41 The other is a heterobimetallic CoMg complex supported by a diamine−dioxime ligand, which exhibits a stepwise and electrocatalytic reduction of NO2− to N2O (2NO2− + 6H+ + 4e− → N2O + 3H2O).42 However, both systems did not lead to further reduction producing NH3. Herein, we report a full NO2− reduction cycle on a dinuclear ruthenium complex producing NH3 through an unusual benttype nitrido-bridged complex.

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RESULTS AND DISCUSSION Synthesis of a Nitrito-Bridged Dinuclear Complex. To investigate the nitrite reduction cycle, we initially tried to prepare a nitrito-bridged dinuclear ruthenium complex. The substitution reaction of [{TpRu(NCMe)}{TpRu(NO)}(μCl)(μ-pz)]BF4 (1) with pyridine N-oxide gave a nitritobridged complex [(TpRu)2(μ-Cl)(μ-NO2)(μ-pz)] (2) (48% yield), along with a dicationic pyridine N-oxide complex [{TpRu(OPy)}{TpRu(NO)}(μ-Cl)(μ-pz)](BF4)2 (3) (24% yield) (Scheme 2).43 Their isolation indicates that the

Figure 1. Structures of 2 (top) and the cationic part of 3 (bottom), with ellipsoids drawn at the 50% probability level. The disordered minor atoms of 2, counter BF4 ions of 3, crystallization solvents, and hydrogen atoms are omitted for clarity.

Scheme 2. Reaction of 1 with Pyridine N-Oxide

Scheme 3. Preparation of 5 by Reduction and Protonation Reactions of 2

formation mechanism of 2 would include not only the substitution reaction but also the redox reactions. Complex 2 would be produced by substitution with pyridine N-oxide, generating a putative monocationic pyridine N-oxide complex [{TpRu(OPy)}{TpRu(NO)}(μ-Cl)(μ-pz)]BF4, release of the pyridine, and N−O coupling of the NO ligand, followed by reduction reaction with the monocationic intermediate (Scheme S1). The last redox reaction is rationalized by cyclic voltammetry of 2 and 3 (Figure S1). According to the opencircuit potentials (2, 0.25 V; 3, 0.55 V), the reduction or oxidation potentials of the cationic intermediates of 2 and 3 are 0.71 or 0.34 V, respectively, indicating that two cationic intermediates can undergo reduction or oxidation reactions with each other. The formation mechanism is also supported by the reaction of 3 with [Cp*2Fe], affording 2 and the starting complex 3. These complexes are paramagnetic complexes but are confirmed by their MS spectra and X-ray crystallographic analyses (Figure 1). Transformation to a Nitrido-Bridged Dinuclear Complex through Two N−O Bond Cleavages. With the nitrito-bridged complex 2 in hand, reduction and protonation reactions of 2 were carried out. Treatment of 2 with 2 equiv of the reducing agent [Cp*2Fe] and 2 equiv of HBF4 afforded a nitrosyl-bridged complex [(TpRu)2(μ-Cl)(μ-NO)(μ-pz)] (4) (79% yield) (Scheme 3), which has already been prepared.44 Since NO is available upon acidification of nitrite due to the

acid−base equilibrium,23 the appearance of the nitrosyl ligand in 4 is reasonable. Subsequent reduction reaction led to the formation of 4. When this reaction was carried out in the absence of [Cp*2Fe], a N−O bond cleavage complex [{TpRu(OH)}{TpRu(NO)}(μ-Cl)(μ-pz)]BF4 ({Ru2(NO)}11), which is proposed by IR and ESI-MS data, was precipitated. The transformation of 2 to 4 consumed two protons and two electrons; if the reaction is described as the following equation (NO2− + 2H+ + e− → NO + H2O), the remaining one electron is reserved in the dinuclear complex. As complex 4 seems to be moderately reduced, protonation of 4 was performed (Scheme 3). Treatment of 4 with 2 equiv of HBF4 gave [(TpRuCl){TpRu(OH2)}(μ-N)(μ-pz)]BF4 (5) (47% yield) as a green powder. The structural assignment of 5 is confirmed by a single-crystal X-ray diffraction, and the X-ray structure of 5 is shown in Figure 2. Two fragments, TpRuCl 843

DOI: 10.1021/jacs.7b12020 J. Am. Chem. Soc. 2018, 140, 842−847

Article

Journal of the American Chemical Society

Figure 3. Isosurface representations of the HOMO (left) and LUMO (right) for 5 at the B3LYP/6-31G(d,p) + SDD level of DFT.

contribution of the μ-nitrido nitrogen atom in HOMO is very low, LUMO is mainly a σ-type orbital on the nitrogen atom. This indicates that reduction is required for inducing Lewis basic character of the nitrogen atom. In addition to the isolation of the nitrido-bridged complex 5 in the protonation reaction of 4, the oxidized complex [{TpRu(OH)}{TpRu(NO)}(μ-Cl)(μ-pz)]BF4 ({Ru2(NO)}11), which results from the oxidation of 4, was detected from the remaining mixture. This means that reduction occurred during the formation of 5. Conversion of 4 to 5, where the NO ligand in 4 is reduced to the N3− ligand in 5 with concomitant production of the H2O ligand, includes a two protons−five electrons process (NO + 2H+ + 5e− → N3− + H2O). The five electrons would be supplied from the metal centers (4e−) and the oxidation reaction of 4 (1e−). The formation mechanism of 5 is unclear, but similar protonation of a nitrosyl-bridged dinuclear molybdenum complex has been reported, where a nitroxyl ligand (HNO) bridging two metal atoms in a κ1:η2 fashion has been observed.57 Hence, after the nitroxyl-bridged (κ1:η2) intermediate would be generated upon protonation along with the formation of the terminal Cl ligand, additional protonation would induce a N−O bond cleavage of the nitroxyl ligand to result in the bridging nitrido ligand and water, although timing of one-electron reduction is not clear (Scheme S2). Considering the conversion from 2 to 4 to 5, four protons and two electrons are required from the outside. Therefore, to prepare 5 as a one-pot reaction directly from 2, reactions of 2 with [Cp*2Fe] (2 equiv), followed by addition of HBF4 (4 equiv), were carried out (Scheme 3). Isolation of 5 failed due to the difficulty of separation of [Cp*2Fe]BF4 (side product), but complex 5 was produced (24% NMR yield). Closing a Nitrite Reduction Cycle. Reproduction of parent 2 from 5 using three protons, three electrons, and a nitrite ion with release of NH3 closed the full nitrite reduction cycle. Complex 5 was allowed to react with [Cp*2Fe] and [nBu4N][NO2], followed by addition of HBF4 (Scheme 4).

Figure 2. Structures of the cationic part of 5 (top) and 5′ (bottom), with ellipsoids drawn at the 50% probability level. Crystallization solvents and hydrogen atoms are omitted for clarity.

and TpRu(OH2), are bridged by one pyrazolyl ligand and one nitrido ligand. The original bridging Cl ligand is transformed to the terminal Cl ligand. The formation of the bridging nitrido ligand is consistent with short Ru−N distances (1.741(5) and 1.752(5) Å), which are similar to those of other dinuclear ruthenium μ-nitrido complexes.45−52 The range of the Ru−N distances in the symmetric binding mode of the μ-nitrido ligand is 1.716−1.827 Å (av. 1.75 Å). The Ru−N distances of a parent imido (NH)-bridged dinuclear ruthenium complex are 1.818(6) and 1.952(6) Å.53 The short distances can rule out the possibility of a μ-oxido ligand because Ru−O distances of the similar complex [(TpRu)2(μ-4-Br-pz)(μ-Cl)(μ-O)] are 1.898(4) and 1.904(3) Å.15 It is noteworthy that the Ru−N− Ru moiety has a bent form (144.0(3)°). To the best of our knowledge, there are only three examples of bent μ-nitrido dinuclear complexes ([Fe]−N−[Fe], 142.4(1) 54 and 135.9(3)°;55 [Nb]−N−[Nb], 145.2(1)°).56 The 1H NMR spectrum of 5 shows diamagnetic signals assignable to seven distinct sets of peaks of the pyrazolyl groups (two Tp and one bridging pyrazolyl ligands), indicating an unsymmetrical dinuclear complex. A broad singlet signal (δ = 1.60 ppm) for the OH2 ligand almost disappeared on addition of D2O to the NMR sample. A SQUID magnetic susceptibility measurement of 5 shows its diamagnetism without strong antiferromagnetic interactions between two Ru centers. The ESI-MS spectrum also supports the formulation of 5. Moreover, the μ-nitrido form was confirmed by substitution reaction of the OH2 ligand in 5 with MeCN, affording [(TpRuCl){TpRu(NCMe)}(μN)(μ-pz)]BF4 (5′), whose structure includes a transoid orientation of the NCMe ligand relative to the Cl ligand (Figure 2). Additional insight into the bridging nitrido ligand of 5 was obtained from density functional theory (DFT) calculation, in which the X-ray structure was used as an initial geometry. The frontier orbitals are shown in Figure 3. Although the

Scheme 4. Reproduction of 2 from 5

Crystallization from the reaction mixture isolated 2 in 32% yield. Furthermore, NH3 was detected (41% yield) in the reaction of 5 with HBF4 and [Cp*2Fe] (the amount of ammonia was determined by the indophenol method). Therefore, the nitrite reduction cycle was completed. 844

DOI: 10.1021/jacs.7b12020 J. Am. Chem. Soc. 2018, 140, 842−847

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Journal of the American Chemical Society

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CH2Cl2/acetone (1/1) as an eluent led to [{TpRu(OPy)}{TpRu(NO)}(μ-Cl)(μ-pz)](BF4)2 (3) as a red solid (8.5 mg, 24%). In the NMR experiment, pyridine as a byproduct was detected. Complex 2: IR (KBr, pellet) ν(BH) 2488 (w) cm−1; FAB-MS (m/z) 777.1 ([M]+), 761.1 ([M − O]+), 747.2 ([M − (NO)]+). Anal. Calcd (%) for C21H23N15B2ClO2Ru2: C, 32.47; H, 2.98; N, 27.05. Found: C, 32.77; H, 2.73; N, 26.82. Complex 3: IR (KBr, pellet) ν(BH) 2532 (m); ν(NO) 1911 (s); ν(BF) 1122−1056 (s) cm−1; ESI-MS (m/z) 943.2 ([M + BF4]+), 856.2 ([M]+), 761.1 ([M − OPy]+), 428.2 ([M]2+). Anal. Calcd (%) for C26H28N16B4ClF8O2Ru2: C, 30.34; H, 2.74; N, 21.77. Found: C, 30.13; H, 2.64; N, 21.44. Reaction of [{TpRu(OPy)}{TpRu(NO)}(μ-Cl)(μ-pz)](BF4)2 (3) with [Cp*2Fe]. Complex 3 (20.0 mg, 0.02 mmol) and [Cp*2Fe] (9.0 mg, 0.028 mmol) were dissolved in acetone (10 mL), and the mixture was stirred at room temperature for 20 h. After evaporation to dryness, the residue was separated on column chromatography with silica gel to 2 (10.3 mg, 67%) and the starting complex 3 (5.0 mg, 25%). Reaction of [(TpRu)2(μ-Cl)(μ-NO2)(μ-pz)] (2) with [Cp*2Fe] and HBF4·Et2O. A mixture of 2 (39.8 mg, 0.0512 mmol) and [Cp*2Fe] (40.6 mg, 0.124 mmol) in CH2Cl2 (20 mL) was stirred at room temperature for 1 h. HBF4·Et2O (18 μL, 0.109 mmol) was added to the solution, and the mixture was stirred for 15 min. After evaporation to dryness, the residue was purified by silica column chromatography using CH2Cl2 as an eluent to afford [(TpRu)2(μCl)(μ-NO)(μ-pz)] (4) (30.8 mg, 79%). This complex was identified by the spectroscopic comparison of the authentic sample. When this reaction was carried out in the absence of [Cp*2Fe], the N−O bond cleavage complex [{TpRu(OH)}{TpRu(NO)}(μ-Cl)(μ-pz)]BF4 ({Ru2(NO)}11) was precipitated. This complex was identified by IR (ν(NO) 1910 cm−1) and ESI-MS ((m/z) 778.0 ([M]+)) spectroscopies, but the purification was unsuccessful. Reaction of [(TpRu)2(μ-Cl)(μ-NO)(μ-pz)] (4) with HBF4·Et2O. Complex 4 (76.5 mg, 0.101 mmol) was dissolved in CH2Cl2 (30 mL) and treated with HBF4·Et2O (33 μL, 0.201 mmol). After the mixture was stirred at room temperature for 20 h, the mixture was evaporated to dryness and extracted with THF. The filtrate was chromatographed on a silica gel column using CH2Cl2/acetone (10/1) as an eluent to give [(TpRuCl){TpRu(OH2)}(μ-N)(μ-pz)]BF4 (5) as a green solid (40.4 mg, 47%). From the residue, the existence of the oxidized complex [{TpRu(OH)}{TpRu(NO)}(μ-Cl)(μ-pz)]BF4 ({Ru2(NO)}11) was identified by IR and ESI-MS spectroscopies. Complex 5: IR (KBr, pellet) ν(BH) 2517 (m); ν(BF) 1121−1053 (s) cm−1; 1H NMR (300 MHz, CDCl3) δ 8.54 (brs, 1H, pz), 8.49 (brs, 1H, pz), 8.01 (brs, 2H, pz), 7.81 (d, 2H, J = 1.9 Hz, pz), 7.79 (d, 1H, J = 2.5 Hz, pz), 7.75 (d, 2H, J = 2.3 Hz, pz), 7.67 (d, 1H, J = 2.3 Hz, pz), 7.59 (d, 1H, J = 2.3 Hz, pz), 7.53 (d, 1H, J = 2.3 Hz, pz), 7.09 (brs, 1H, pz), 6.91 (brs, 1H, pz), 6.54 (t, 1H, J = 2.3 Hz, pz), 6.51− 6.48 (m, 3H, pz), 6.42 (t, 1H, J = 2.2 Hz, pz), 6.29 (t, 1H, J = 2.4 Hz, pz), 6.27 (t, 1H, J = 2.4 Hz, pz), 1.60 (brs, 2H, OH2); 13C{1H} NMR (CD2Cl2) δ 145.3 (pz), 145.0 (pz), 144.7 (pz), 144.3 (pz), 143.8 (pz), 143.8 (pz), 143.7 (pz), 143.2 (pz), 138.9 (pz), 138.5 (pz), 137.5 (pz), 137.2 (pz), 136.5 (pz), 136.0 (pz), 107.8 (4-pz), 107.8 (4-pz), 107.7 (4-pz), 107.7 (4-pz), 107.5 (4-pz), 107.4 (4-pz), 107.2 (4-pz); ESI-MS (acetone) (m/z) 803.0 ([M − H2O + Me2CO]+), 762.9 ([M] + ), 744.9 ([M − OH 2 ] + ). Anal. Calcd (%) for C21H25N15B3ClF4ORu2·(C3H8O)1.5: C, 32.59; H, 3.97; N, 22.36. Found: C, 32.39; H, 3.71; N, 22.07. One-Pot Synthesis of [(TpRuCl){TpRu(OH2)}(μ-N)(μ-pz)]BF4 (5). A mixture of 2 (10.0 mg, 12.9 μmol) and [Cp*2Fe] (9.3 mg, 28.5 μmol) in CH2Cl2 (10 mL) was stirred at room temperature for 1 h. After dropwise addition of HBF4·Et2O (9.0 μL, 54.9 μmol), the mixture was stirred at room temperature for 20 h. After removal of the volatiles and extraction with distilled THF, the filtrate was filtered and evaporated to dryness to give a brown solid with the inseparable [Cp*2Fe]BF4. The yield (24%) was determined by 1H NMR using a C6Me6 internal standard. Reaction of [(TpRuCl){TpRu(OH2)}(μ-N)(μ-pz)]BF4 (5) with MeCN. Complex 5 (20.6 mg, 24.2 μmol) was dissolved in MeCN and stirred at room temperature for 20 h. After evaporation to dryness, the

CONCLUSION In summary, we report a three-step nitrite reduction cycle on the dinuclear ruthenium complex (Scheme 5). In the first step, Scheme 5. Nitrite Reduction to Ammonia on a Dinuclear Ruthenium Complex (NO2− + 7H+ + 6e− → NH3 + 2H2O)

the nitrito ligand in 2 was converted to the NO ligand using 2H+ and 1e−. In the second step, the resulting NO ligand in 4 was transformed to the nitrido ligand by consumption of 2H+ and 5e−, including participation of the metal’s electrons. Onepot conversion of nitrito to nitrido ligand also succeeded. In the last step, the nitrito ligand in 2 was recovered by the reaction of 5 with a protonic acid and a reducing agent in the presence of a nitrite ion. Release of ammonia was detected under the similar reaction conditions. Therefore, we achieved the synthetic nitrite reduction cycle on the dinuclear ruthenium complex.

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EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under N2 or Ar unless otherwise noted, and subsequent workup manipulations were performed in air. The starting complex [{TpRu(NCMe)}{TpRu(NO)}(μ-Cl)(μ-pz)]BF4 (1) was prepared according to the previously reported method.44 Organic solvents and all other reagents were commercially available and used without further purification. NMR spectra were recorded on a Varian Gemini-300 and a JEOL JNM-AL-400 spectrometer. 1H and 13C{1H} NMR chemical shifts are quoted with respect to tetramethylsilane and the solvent signals, respectively. Infrared spectra in KBr pellets were obtained on a JASCO FT-IR-4100 spectrometer. UV/vis spectra were recorded on a Jasco V-560 spectrophotometer. Fast atom bombardment mass spectra (FAB-MS) and electrospray mass spectra (ESI-MS) were carried out on a JEOL JMS-700N and a Waters ACQUITY SQD MS system, respectively. Elemental analyses (C, H, N) were performed on a PerkinElmer 2400II elemental analyzer. Cyclic voltammetry was recorded at room temperature with a BAS ALS-600C electrochemical analyzer by using a platinum disk working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. Cyclic voltammograms at a scan rate of 10 mV/s were recorded for 1.0 mM test solutions of the complexes in CH2Cl2 with 0.1 M [nBu4N][PF6] as a supporting electrolyte. Reaction of [{TpRu(NCMe)}{TpRu(NO)}(μ-Cl)(μ-pz)]BF4 (1) with Pyridine N-Oxide. A mixture of 1 (30.0 mg, 0.034 mmol) and pyridine N-oxide (30.0 mg, 0.32 mmol) in THF (15 mL) was refluxed for 20 h. After evaporation to dryness, the residue was separated on column chromatography with silica gel by using CH2Cl2/acetone (50/1) as an eluent to give [(TpRu)2(μ-Cl)(μNO2)(μ-pz)] (2) as an orange solid (12.3 mg, 48%). The use of 845

DOI: 10.1021/jacs.7b12020 J. Am. Chem. Soc. 2018, 140, 842−847

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Journal of the American Chemical Society

Computational Methods. All calculations were performed using the Gaussian 09 package and the B3LYP hybrid functional. Ruthenium was treated with the SDD basis set, whereas other elements were treated with the 6-31G(d,p) basis set. The initial geometry of 5 was extracted from the crystallographic result and then computationally optimized for the singlet ground state of the molecule in the gas phase, aiming to determine the HOMO and LUMO energies.

residue was separated on column chromatography with silica gel by using CH2Cl2/acetone (10/1) as an eluent to give [(TpRuCl){TpRu(NCMe)}(μ-N)(μ-pz)]BF4 (5′) as a brown solid (19.2 mg, 91%). Complex 5′: IR (KBr, pellet) ν(BH) 2514 (m); ν(BF) 1122− 1054 (s) cm−1; 1H NMR (300 MHz, CDCl3) δ 8.63 (d, 1H, J = 2.1 Hz, pz), 8.08 (d, 1H, J = 2.1 Hz, pz), 7.98 (d, 1H, J = 2.1 Hz, pz), 7.94 (d, 1H, J = 2.2 Hz, pz), 7.85−7.84 (m, 3H, pz), 7.75 (d, 1H, J = 2.3 Hz, pz), 7.73 (t, 2H, J = 2.4 Hz, pz), 7.70 (t, 2H, J = 2.8 Hz, pz), 7.61 (d, 1H, J = 2.2 Hz, pz), 7.32 (d, 1H, J = 2.2 Hz, pz), 6.59 (t, 1H, J = 2.3 Hz, pz), 6.55 (t, 1H, J = 2.3 Hz, pz), 6.48 (t, 1H, J = 2.3 Hz, pz), 6.43 (t, 1H, J = 2.5 Hz, pz), 6.42 (t, 1H, J = 2.8 Hz, pz), 6.35 (t, 1H, J = 2.2 Hz, pz), 6.24 (t, 1H, J = 2.3 Hz, pz), 2.49 (s, 3H, CH3); 13 C{1H} NMR (CDCl3) δ 147.5 (pz), 145.4 (pz), 144.8 (pz), 144.4 (pz), 144.1 (pz), 143.7 (pz), 143.4 (pz), 143.0 (pz), 138.5 (pz), 137.5 (pz), 137.4 (pz), 137.2 (pz), 136.5 (pz), 136.0 (pz), 131.2 (CH3CN), 109.0 (4-pz), 108.0 (4-pz), 108.0 (4-pz), 107.9 (4-pz), 107.7 (4-pz), 107.4 (4-pz), 106.4 (4-pz), 4.86 (CH3CN); ESI-MS (CH3CN) (m/z) 785.9 ([M]+), 744.9 ([M − OH2]+). Anal. Calcd (%) for C23H26N16B3ClF4Ru2: C, 31.66; H, 3.00; N, 25.68. Found: C, 31.34; H, 2.92; N, 24.93. Reproduction of [(TpRu)2(μ-Cl)(μ-NO2)(μ-pz)] (2) from [(TpRuCl){TpRu(OH2)}(μ-N)(μ-pz)]BF4 (5). Complex 5 (19.4 mg, 22.8 μmol), nBu4NNO2(27.0 mg, 0.0936 mmol), and [Cp*2Fe] (24.3 mg, 0.0745 mmol) were dissolved in CH2Cl2 (10 mL), and HBF4· Et2O (10.5 μL, 64.0 μmol) was added dropwise to the mixture. After being stirred for 4 h, the reaction solution turned brownish-green. The mixture was evaporated to dryness, followed by crystallization from MeCN/ether to give complex 2 (5.6 mg, 32%). After column chromatographic purification of the mother liquor, the unknown complex was also isolated. Procedure for Detection of NH3. Under Ar, to a solution of complex 5 (10.0 mg, 11.8 μmol) in CH2Cl2 (2.0 mL) HBF4·Et2O (6.0 μL, 36.6 μmol) was added. After being stirred for 5 min, [Cp*2Fe] (12.1 mg, 37.1 μmol) in CH2Cl2 (2.0 mL) was added, and the mixture was stirred for 4 h. A solution of excess N,N,N′N′tetramethyl-1,8-naphthalenediamine (proton sponge) (78.0 mg, 364 μmol) in MeCN/H2O (2.5/0.5 mL) was added to the mixture. After being stirred for 1 h, all volatiles were vacuum transferred into a collection tube. An Et2O solution of HCl (1 M, 3.0 mL) was added to the collection tube before it was sealed and warmed to room temperature. After being stirred for 30 min, the solvents were removed under reduced pressure. The remaining residue in the tube was dissolved in H2O (25 mL; volumetric flask), and an aliquot (2.5 mL) of this solution was analyzed for the presence of NH3 (present as NH4Cl) by the indophenol method.58 Quantification was performed with UV−vis spectroscopy by analyzing absorbance at 625 nm. X-ray Crystal Structure Determinations. Crystallographic data are summarized in Table S1. X-ray quality single crystals were obtained from (MeCN + CH2Cl2)/ether (for 2·MeCN), (MeCN + MeOH)/ether (for 3·MeCN), (iPrOH + CH2Cl2)/ether (for 5· 2iPrOH), and MeCN/ether (for 5′·MeCN). Diffraction data were collected at −180 °C under a stream of cold N2 gas on a Rigaku RAMicro7 HFM instrument equipped with a Rigaku Saturn724+ CCD detector by using graphite-monochromated Mo Kα radiation. The intensity images were obtained at the exposure of 16.0 s/deg (2· MeCN and 5·2iPrOH), 4.0 s/deg (3·MeCN), and 8.0 s/deg (5′· MeCN). The frame data were integrated using a Rigaku CrystalClear program package, and the data sets were corrected for absorption using a REQAB program. The calculations were performed with a CrystalStructure software package. The structures were solved by direct methods and refined on F2 by the full-matrix least-squares methods. For 2·MeCN, the NO2 moiety was disordered over two positions with occupancy factors of 0.85/0.15 and constrained and restrained. For 3·MeCN, owing to serious disorder problems of the crystallization solvent, we were not able to define it well. Therefore, a SQUEEZE/PLATON technique was applied. Anisotropic refinement was applied to all non-hydrogen atoms except for the disordered atoms. Hydrogen atoms for all structures were put at calculated positions, except for those of the OH2 ligand (5·2iPrOH).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b12020. Proposed formation mechanisms; spectroscopic data; cyclic voltammogram data (2 and 3); table for the crystal data; cartesian coordinates for DFT-optimized structure of 5 (PDF) Crystal data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Yasuhiro Arikawa: 0000-0001-6727-2340 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant No. JP17K05813 and by the priority research project of Nagasaki University. We are grateful to T. Nakayama at Nagasaki University for his technical assistance, and Prof. M. Ohba at Kyushu University for a SQUID magnetic susceptibility measurement.

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