Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing

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Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing Ammonia Yasuhiro Arikawa, Yuji Otsubo, Hiroki Fujino, Shinnosuke Horiuchi, Eri Sakuda, and Keisuke Umakoshi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12020 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Journal of the American Chemical Society

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. 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 reduction 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 parent nitrito ligand. Moreover, release of ammonia was detected.

INTRODUCTION

2e –

The inorganic nitrogen cycle is an elegant cycle, which compose 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 been also 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 researches 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 Scheme 1. NO Reduction Cycle on a Dinuclear Ruthenium Complex (2NO + 2H+ + 2e– -> N2O + H2O)

2+ Tp Ru N

N N Cl

O

RuTp N

O

N N Cl Tp Ru RuTp N N O O

(+ H +) H 2O N N Cl RuTp 2NO, Tp Ru + O 2H

N 2O

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 require multi-protons and multi-electrons for generating NH4+ at one time without releasing NO. The formation mechanism of NH4+ in CcNiR is proposed,19–21 but still unclear. Electrochemical reduction of NO2– have been widely explored.23 Pioneering works have demonstrated the nitrite reduction in solution (metal-cyclam (cyclam = 1,4,8,11-tetra-azacyclotetradecane,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 protoporhyrin,33 iron-alizarin complexone,34 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 afford

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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.

Ru1

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 bent-type nitrido-bridged complex.

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)}(µ-Cl)(µ-pz)]BF4 (1) with pyridine N-oxide gave a nitrito-bridged complex [(TpRu)2(µCl)(µ-NO2)(µ-pz)] (2) (48% yield), along with a dicationic pyridine N-oxide complex [{TpRu(OPy)}{TpRu(NO)}(µCl)(µ-pz)](BF4)2 (3) (24% yield) (Scheme 2).43 Their isolation indicates Scheme 2. Reaction of 1 with Pyridine N-oxide (BF 4) 2

BF 4 Tp Ru N

N N Cl

O

N + O– RuTp N

C Me 1 {Ru2(NO)}12

THF

N N Cl

Tp Ru N

RuTp

O

O 2 {Ru2}11

+

Tp Ru N

N N Cl

O

RuTp O N

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Ru2

Cl1 N1 O2 O1

Ru1

Ru2 Cl1 O2 N16

N1 O1

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.

the starting complex 3. These complexes are paramagnetic complexes, but confirmed by their MS spectra and Xray crystallographic analyses (Figure 1). Transformation to a Nitrido-bridged Dinuclear Complex through Two N–O Bond Cleavage. With the nitrito-bridged complex 2 in hand, reduction and protonation reactions of 2 were carried out. Treatment of 2 with 2 eq. of the reducing agent [Cp*2Fe] and 2 eq. of HBF4 afforded a nitrosyl-bridged complex [(TpRu)2(µCl)(µ-NO)(µ-pz)] (4) (79% yield) (Scheme 3), Scheme 3. Preparation of 5 by Reduction and Protonation Reactions of 2

3 {Ru 2(NO)}11

that the formation mechanism of 2 would include not only the substitution reaction, but also 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)(µpz)]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 open circuit 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 V or 0.34 V, respectively, indicates 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

N N Cl

Tp Ru N

RuTp

O

O 2 {Ru2}11

2 [Cp* 2Fe] 2 HBF 4

Tp Ru

CH 2Cl 2

N N Cl

RuTp

N

– H 2O – 2 [Cp* 2Fe]BF 4

O 4 {Ru2(NO)}13

2 HBF 4

1) 2 [Cp* 2Fe] 2) 4 HBF 4 BF 4 Tp Ru

N N N

Cl

BF 4

RuTp + 1/2 Tp Ru N

N N Cl

OH 2

5 {Ru2}8

RuTp OH

O {Ru2(NO)}11 (proposed structure)

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 rea-


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sonable. 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. Since complex 4 seems to be moderately reduced, protonation of 4 was performed (Scheme 3). Treatment of 4 with 2 eq. of HBF4 gave [(TpRuCl){TpRu(OH2)}(µ-N)(µpz)]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 and TpRu(OH2), are bridged by one pyrazolyl and one nitrido ligands. 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 µnitrido 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, since 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)º). 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)º55; [Nb]–N–[Nb], 145.2(1)º).56 The 1H NMR spectrum of 5 shows diamagnetic signals assignable to distinct seven 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)}(µ-N)(µ-pz)]BF4 (5’), whose structure includes a transoid orientation of the NCMe ligand relative to the Cl ligand (Figure 2).

Ru1 Ru2

N1 O1

Cl1

N16 Ru1

Ru2 N1

Cl1

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.

Additional insight into the bridging nitrido ligand of 5 was obtained from DFT calculation, in which the X-ray structure was used as an initial geometry. The frontier orbitals are shown in Figure 3. Although the 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. Figure 3. Isosurface representations of the HOMO (left) and LUMO (right) for 5 at the B3LYP/6-31G(d,p) + SDD level of DFT.

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 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 nitroxylbridged (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 an one-pot reaction directly from 2, reactions of 2 with [Cp*2Fe] (2 eq.), followed by addition of HBF4 (4 eq.) was carried out (Scheme 3). Isolation of 5 was 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 close the full nitrite reduction cycle. Complex 5 was allowed to react with [Cp*2Fe] and [nBu4N][NO2], followed by addition of HBF4 (Scheme 4). 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. Scheme 4. Reproduction of 2 from 5 BF 4 1) 3 [Cp* 2Fe] 2) excess [ nBu 4N][NO 2] N N 3) 3 HBF 4 Cl Tp Ru Tp Ru RuTp RuTp CH 2Cl 2 N O Cl OH 2 O 5 {Ru 2}8 2 {Ru 2}11 N N N

CONCLUSION In summary, we report a three-step nitrite reduction cycle on the dinuclear ruthenium complex (Scheme 5). In the first step, 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. One-pot conversion of nitrito to nitrido ligand was 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. And release of ammonia was detected under the similar reaction conditions. Therefore, we achieved the synthetic nitrite reduction cycle on the dinuclear ruthenium complex. Scheme 5. Nitrite Reduction to Ammonia on a Dinuclear Ruthenium Complex (NO2– + 7H+ + 6e–-> NH3 + 2H2O)

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N N Cl Tp Ru RuTp N O

NH 3 H 2O

O 3H +, 3eNO 2–

2H+, 2e-

2

H 2O

+ Tp Ru

N N N

Cl

RuTp

Tp Ru

N N Cl

RuTp

N

OH 2

O 4

5

2H+, e -

EXPERIMENTAL SECTION General Procedures. All reactions were carried out under N2 or Ar unless otherwise noted, and subsequent work-up 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 TMS 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 Perkin Elmer 2400II elemental analyzer. Cyclic voltammetry was recorded at room temperature with a BAS ALS-600C electrochemical analyzer by use of a platinum-disk working electrode, a platinumwire 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 µ-Cl)(µ µpz)]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 20h. After evaporation to dryness, the residue was separated on column chromatography with a silica gel by use of CH2Cl2-acetone (50/1) as an eluent to give [(TpRu)2(µ-Cl)(µ-NO2)(µ-pz)] (2) as an orange solid (12.3 mg, 48%). The use of CH2Cl2acetone (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)]+). Elemental analysis (%) 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, pel-


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let): 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+). Elemental analysis (%) 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 µ-Cl)(µ µpz)](BF4)2 (3) with [Cp*2Fe]. Complex 3 (20.0 mg, 0.02 mmol) and [Cp*2Fe] (9.0 mg, 0.028 mmol) was dissolved in acetone (10 mL), and the mixture was stirred at room temperature for 20h. After evaporation to dryness, the residue was separated on column chromatography with a silica gel to 2 (10.3 mg, 67%) and the starting complex 3 (5.0 mg, 25%). Reaction of [(TpRu)2(µ µ-Cl)(µ µ-NO2)(µ 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 1h. HBF4·Et2O (18 µL, 0.109 mmol) was added to the solution, and the mixture was stirred for 15min. 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], a 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 µ-Cl)(µ µ-NO)(µ 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 20h, 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)(µpz)]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. 1 H 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, 4-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-OH2]+). Elemental analysis (%) 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 µN)(µ 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 1h. After dropwise addition of HBF4·Et2O (9.0 µL, 54.9 µmol), the mixture was stirred at room temperature for 20h. 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 µ-N)(µ µ-pz)]BF4 (5) with MeCN. Complex 5 (20.6 mg, 24.2 µmol) was dissolved in MeCN and stirred at room temperature for 20h. After evaporation to dryness, the residue was separated on column chromatography with a silica gel by use of 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). 13C{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]+). Elemental analysis (%) 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 µ-Cl)(µ µ-NO2)(µ µ-pz)] (2) µ-N)(µ µ-pz)]BF4 (5). from [(TpRuCl){TpRu(OH2)}(µ 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 dropwisely added to the mixture. After stirring for 4h, 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 stirring for 5min, [Cp*2Fe] (12.1 mg, 37.1 µmol) in CH2Cl2 (2.0 mL) was added, and the mixture was stirred for 4h. A solution of excess N,N,N’N’-tetramethyl-1,8-naphthalenediamine (protonsponge) (78.0 mg, 364 µmol) in MeCN/H2O (2.5/0.5 mL) was added to the mixture. After stirring for 1h, 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 stirring for 30min, 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 Tables 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), respectively. Diffraction data were collected at -180ºC under a stream of cold N2 gas on a Rigaku RA-Micro7 HFM instrument equipped with a Rigaku Saturn724+ CCD detector by using graphitemonochromated Mo-Kα radiation. The intensity images were obtained at the exposure of 16.0 s/º (2·MeCN and 5·2iPrOH), 4.0 s/º (3·MeCN), and 8.0 s/º (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 well define it. 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). Computational Methods. All calculations were performed using the Gaussian 09 package and the B3LYP hybrid functional. Ruthenium was treated with the SDD basis set, while other elements were treated with the 631G(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.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Proposed formation mechanisms; Spectroscopic data; cyclic voltammogram data (2 and 3); a table for the crystal data; cartesian coordinates for DFT optimized structure of 5 (PDF) Crystal data (CIF)

AUTHOR INFORMATION

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*[email protected]

ORCID Yasuhiro Arikawa: 0000-0001-6727-2340 Shinnosuke Horiuchi: 0000-0002-5692-6377 Eri Sakuda: 0000-0001-9882-7628 Keisuke Umakoshi: 0000-0002-2027-6333

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Number 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.

REFERENCES (1) Stein, L. Y.; Klotz, M. G., Curr. Biol. 2016, 26, R94–R98. (2) Thamdrup, B., Annu. Rev. Ecol., Evol. Syst. 2012, 43, 407– 428. (3) Canfield, D. E.; Glazer, A. N.; Falkowski, P. G., Science 2010, 330, 192–196. (4) Gruber, N.; Galloway, J. N., Nature 2008, 451, 293–296. (5) Wasser, I. M.; de Vries, S.; Moënne-Loccoz, P.; Schröder, I.; Karlin, K. D., Chem. Rev. 2002, 102, 1201–1234. (6) Wright, A. M.; Hayton, T. W., Inorg. Chem. 2015, 54, 9330– 9341. (7) Timmons, A. J.; Symes, M. D., Chem. Soc. Rev. 2015, 44, 6708–6722. (8) Chakraborty, S.; Reed, J.; Sage, J. T.; Branagan, N. C.; Petrik, I. D.; Miner, K. D.; Hu, M. Y.; Zhao, J.; Alp, E. E.; Lu, Y., Inorg. Chem. 2015, 54, 9317–9329. (9) Berto, T. C.; Speelman, A. L.; Zheng, S.; Lehnert, N., Coord. Chem. Rev. 2013, 257, 244–259. (10) Xu, N.; Yi, J.; Richter-Addo, G. B., Inorg. Chem. 2010, 49, 6253–6266. (11) Schopfer, M. P.; Wang, J.; Karlin, K. D., Inorg. Chem. 2010, 49, 6267–6282. (12) Goodrich, L. E.; Paulat, F.; Praneeth, V. K. K.; Lehnert, N., Inorg. Chem. 2010, 49, 6293–6316. (13) Tavares, P.; Pereira, A. S.; Moura, J. J. G.; Moura, I., J. Inorg. Biochem. 2006, 100, 2087–2100. (14) Averill, B. A., Chem. Rev. 1996, 96, 2951–2964. (15) Arikawa, Y.; Asayama, T.; Moriguchi, Y.; Agari, S.; Onishi, M., J. Am. Chem. Soc. 2007, 129, 14160–14161. (16) Arikawa, Y.; Matsumoto, N.; Asayama, T.; Umakoshi, K.; Onishi, M., Dalton Trans. 2011, 40, 2148–2150. (17) Arikawa, Y.; Onishi, M., Coord. Chem. Rev. 2012, 256, 468– 478. (18) Suzuki, T.; Tanaka, H.; Shiota, Y.; Sajith, P. K.; Arikawa, Y.; Yoshizawa, K., Inorg. Chem. 2015, 54, 7181–7191. (19) Maia, L. B.; Moura, J. J. G., Chem. Rev. 2014, 114, 5273–5357. (20) Bykov, D.; Neese, F., Inorg. Chem. 2015, 54, 9303–9316. (21) Einsle, O.; Messerschmidt, A.; Huber, R.; Kroneck, P. M. H.; Neese, F., J. Am. Chem. Soc. 2002, 124, 11737–11745. (22) Simon, J., FEMS Microbiol. Rev. 2002, 26, 285–309. (23) Rosca, V.; Duca, M.; de Groot, M. T.; Koper, M. T. M., Chem. Rev. 2009, 109, 2209–2244. (24) Taniguchi, I.; Nakashima, N.; Matsushita, K.; Yasukouchi, K., J. Electroanal. Chem. 1987, 224, 199–209. (25) Taniguchi, I.; Nakashima, N.; Yasukouchi, K., J. Chem. Soc., Chem. Commun. 1986, 1814–1815.

Corresponding Author


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(26) Barley, M. H.; Meyer, T. J., J. Am. Chem. Soc. 1986, 108, 5876–5885. (27) Goodrich, L. E.; Roy, S.; Alp, E. E.; Zhao, J.; Hu, M. Y.; Lehnert, N., Inorg. Chem. 2013, 52, 7766–7780. (28) Rhodes, M. R.; Barley, M. H.; Meyer, T. J., Inorg. Chem. 1991, 30, 629–635. (29) Toth, J. E.; Anson, F. C., J. Am. Chem. Soc. 1989, 111, 2444– 2451. (30) Lin, R.; Bayachou, M.; Greaves, J.; Farmer, P. J., J. Am. Chem. Soc. 1997, 119, 12689–12690. (31) Mimica, D.; Zagal, J. H.; Bedioui, F., J. Electroanal. Chem. 2001, 497, 106–113. (32) Jiang, H.-J.; Yang, H.; Akins, D. L., J. Electroanal. Chem. 2008, 623, 181–186. (33) Younathan, J. N.; Wood, K. S.; Meyer, T. J., Inorg. Chem. 1992, 31, 3280–3285. (34) Zhang, J.; Lever, A. B. P.; Pietro, W. J., Inorg. Chem. 1994, 33, 1392–1398. (35) Chebotareva, N.; Nyokong, T., J. Appl. Electrochem. 1997, 27, 975–981. (36) Armor, J., Inorg. Chem. 1973, 12, 1959–1961. (37) Bottomley, F.; Mukaida, M., J. Chem. Soc., Dalton Trans. 1982, 1933–1937. (38) Murphy, W. R.; Takeuchi, K. J.; Meyer, T. J., J. Am. Chem. Soc. 1982, 104, 5817–5819. (39) Murphy, W. R.; Takeuchi, K.; Barley, M. H.; Meyer, T. J., Inorg. Chem. 1986, 25, 1041–1053. (40) See for example of other transition metal complexes: Legzdins, P.; Nurse, C. R.; Rettig, S. J., J. Am. Chem. Soc. 1983, 105, 3727–3728. (41) Sellmann, D.; Seubert, B., Angew. Chem. Int. Ed. 1992, 31, 205–207. (42) Uyeda, C.; Peters, J. C., J. Am. Chem. Soc. 2013, 135, 12023– 12031. (43) In order to better understand the redox states, a modified notation {M2(NO)x}n based on the Enemark-Feltham electron

counting formalism is described, where n stands for the number of electrons in the two metal d and π*NO orbitals (or n represents the number of d electrons on the two metal when the NO is formally considered to be NO+). (44) Arikawa, Y.; Ikeda, A.; Matsumoto, N.; Umakoshi, K., Dalton Trans. 2013, 42, 11626–11631. (45) Griffith, W. P.; McManus, N. T.; Skapski, A. C., J. Chem. Soc., Chem. Commun. 1984, 434–435. (46) Haukka, M.; Venalainen, T.; Ahlgren, M.; Pakkanen, T. A., Inorg. Chem. 1995, 34, 2931–2936. (47) Jüstel, T.; Bendix, J.; Metzler-Nolte, N.; Weyhermüller, T.; Nuber, B.; Wieghardt, K., Inorg. Chem. 1998, 37, 35–43. (48) Sellman, D.; Gottschalk-Gaudig, T.; Heinemann, F. W., Inorg. Chim. Acta 1998, 269, 63–72. (49) Bonomo, L.; Solari, E.; Scopelliti, R.; Floriani, C., Angew. Chem. Int. Ed. 2001, 40, 2529–2531. (50) Sun, X.-R.; Liang, J.-L.; Che, C.-M.; Zhu, N.; Zhang, X. X.; Gao, S., Chem. Commun. 2002, 2090–2091. (51) Matsumura, S.; Shikano, K.; Oi, T.; Suzuki, N.; Nagao, H., Inorg. Chem. 2008, 47, 9125–9127. (52) Cheung, W.-M.; Chiu, W.-H.; de Vere-Tucker, M.; Sung, H. H. Y.; Williams, I. D.; Leung, W.-H., Inorg. Chem. 2017, 56, 5680–5687. (53) Yi, X.-Y.; Ng, H.-Y.; Williams, I. D.; Leung, W.-H., Inorg. Chem. 2011, 50, 1161–1163. (54) Brown, S. D.; Mehn, M. P.; Peters, J. C., J. Am. Chem. Soc. 2005, 127, 13146–13147. (55) Brown, S. D.; Peters, J. C., J. Am. Chem. Soc. 2005, 127, 1913–1923. (56) Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli, C.; Chiesi-Villa, A., J. Am. Chem. Soc. 2000, 122, 3652– 3670. (57) Alvarez, M. A.; García, M. E.; García-Vivó, D.; Ruiz, M. A.; Toyos, A., Inorg. Chem. 2015, 54, 10536–10538. (58) Weatherburn, M. W., Anal. Chem. 1967, 39, 971–974.



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TOC NO2 – + 7H+ + 6e– → NH3 + 2H2 O O N O Ru Ru Cl

NH 3, H 2O

2H+, 2e-

3H+, 3e-, NO 2–

H 2O Cl OH 2 Ru

Ru

O

+

N Ru

N

Cl

Ru

2H+, e -


8

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Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing

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