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Dinuclear Ruthenium(III)−Ruthenium(IV) Complexes, Having a Doubly Oxido-Bridged and Acetato- or Nitrato-Capped Framework Tomoyo Suzuki,† Yutaka Suzuki,† Tatsuya Kawamoto,‡ Ryo Miyamoto,§ Shinkoh Nanbu,† and Hirotaka Nagao*,† †

Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 Japan ‡ Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan § Graduate School of Science and Technology, Hirosaki University, Bunkyo-cho, Hirosaki 036-8561, Japan S Supporting Information *

synthesized. The structure of the doubly oxido-bridged and nitrato-capped framework is determined by X-ray crystallography. The electronic structures, electrochemical and spectroscopic properties, reactions with Brønsted acids, and electrochemical behaviors in aqueous solutions were studied. Reactions of the singly oxido-bridged halogeno diruthenium complex, [{RuIII,IVCl2(ebpma)}2(μ-O)]PF6, with four equimolar amounts of AgL (L = CH3COO, NO3) in water−acetone under air afforded a triply bridged diruthenium complex having a doubly oxido-bridged and acetato- or nitrato-capped framework, 1 and 2, by the addition of NH4PF6 as the precipitant. The structure of 2 was determined by X-ray structural analysis, as shown in Figure 1 (Figure S1 and Table S1). For 1, a preliminary

ABSTRACT: Dinuclear ruthenium complexes in a mixedvalence state of RuIII−RuIV, having a doubly oxido-bridged and acetato- or nitrato-capped framework, [{Ru III,IV (ebpma)} 2 (μ-O) 2 (μ-L)](PF 6 ) 2 [ebpma = ethylbis(2-pyridylmethyl)amine; L = CH3COO− (1), NO3− (2)], were synthesized. In aqueous solutions, the diruthenium complex 1 showed multiple redox processes accompanied by proton transfers depending on the pH. The protonated complex of 1, which is described as 1H+, was obtained.

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onversion of small molecules into more useful and higherenergy chemicals using a metal complex catalyst under mild conditions has been paid much attention from the viewpoint of environmental concerns.1,2 Multinuclear frameworks of metal complexes that function as multielectron transferring sites and have appropriate electronic structures for the nature of targeting substrates are useful for material conversion systems. Singly and doubly oxido-bridged dinuclear complexes have been well studied as homogeneous catalysts for water oxidation3 and as biomimetic soluble methane monoxigenase,4 respectively. Several dinuclear complexes having a doubly oxido-bridged core, M2(μ-O)2, e.g., Mn−Mn,5 Fe−Mn,6 Fe−Fe,7 and Co−Co,8 have been synthesized and characterized in relation to oxidation reactions catalyzed by metal complexes. Diruthenium complexes having a doubly oxido- or hydroxido-bridged core, Ru2(μ-O)2 or Ru2(μ-OH)2,9,10 and triply bridged diruthenium complexes, in which a bidentate ligand is capped between metal centers with the doubly oxido- or hydroxido-bridged core, have been reported.11,12 In our previous work on dinuclear frameworks using a tridentate ancillary ligand, ethylbis(2-pyridylmethyl)amine (ebpma), triply chlorido- and/or methoxido-bridged diruthenium complexes of RuII−RuIII, [{Ru(ebpma)}2(μCl)n(μ-OMe)3−n]2+ (n = 1, 3),13,14 and singly oxido-bridged complexes of RuIII−RuIV, [{RuX2(ebpma)}2(μ-O)]+ (X = Cl, Br),14 have been synthesized. Their structures and reactivities, as well as their competency for functioning as molecular conversion reaction sites, have been of interest. In this work, diruthenium complexes of RuIII−RuIV having the doubly oxido-bridged core with capping of acetato or nitrato, [{RuIII,IV(ebpma)}2(μ-O)2(μL)](PF6)2 [L = CH3COO− (1), NO3− (2)], have been © XXXX American Chemical Society

Figure 1. Thermal ellipsoid plots of the crystal structure of 2 shown at the 50% probability level. For clarity, H atoms and two PF6− counteranions are omitted.

structure similar to that of 2 was obtained because of the size and quality of the single crystals. The structural parameters of the Ru2(μ-O)2 core [Ru−O, 1.929(3)−1.933(3) Å; Ru−O−Ru, 78.15(10) and 78.10(10)°; O−Ru−O, 101.65(11) and 101.76(11)°] were similar to those of the reported diruthenium complex of RuIII−RuIV having a doubly oxido-bridged and Received: April 11, 2016

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DOI: 10.1021/acs.inorgchem.6b00890 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

In UV−vis and NIR spectroscopy in CH3CN (Figure S14), two intense absorption bands and one broad weak band were observed. Spectroelectrochemical measurements of 1 and 2 were performed in CH3CN using an optically transparent thin-layer electrode cell. The RuIII−RuIII complexes that were formed by one-electron reduction of 1 and 2 were unstable and gradually changed during reductive CPE. In oxidative CPE, similar spectral changes of both complexes with isosbestic points were observed, indicating that the oxidized forms of the RuIV−RuIV state were stable during these experiments (Figures S15 and S16). Weak broad bands at 677 nm for 1 and 586 nm for 2 were newly observed with a decreasing weak broad band at 762 nm for 2. The broad band around 700 nm was observed for a similar RuIV−RuIV complex having the Ru2(μ-O)2 core.11 Thus, the bands of 1 and 2 at 339 and 318 nm were assigned to metal-to-ligand charge transfer, those at 977 and 762 nm represented a transition from the highest occupied frontier orbital to the lowest unoccupied molecular orbital, which were both mainly contributed by the Ru2(μ-O)2 core.5 CV curves of 1 in 2:1 (v/v) water−acetonitrile mixed solutions, as shown in Figure S12, revealed three or four distinctive redox waves in all pH regions. These waves were attributed to stepwise one- or two-electron redox processes of the metal centers. The redox potentials depend on the pH of the solutions, explained by the contribution of proton transfer(s) with reduction and oxidation processes. Profiles of CV reversibly changed in the pH region between 1.5 and 12.5, indicating that the complex was stable without decomposition. For complex 2, electrochemical measurements in aqueous solutions could not be identified in detail because of the reactivity of 2. A Pourbaix diagram, a plot of the redox potential versus pH, is described in Figure 3.16 The expected oxidation states of the Ru centers and

carbonato-capped framework, [{Ru(dtne)}(μ-O) 2 (μO2CO)]PF6 [dtne = 1,2-bis(1,4,7-triazacyclononan-1-yl)ethane].11 Comparison of structural parameters between 2 and reported triply bridged complexes of RuIII−RuIII having the Ru2(μ-OH)2 core, [{Ru-(tacn)}(μ-OH)2(μ-O2CCH3)]I3 (tacn = 1,4,7-triazacyclononane)12d and [{Ru(tacn)}(μ-OH)2(μO2CO)]Br2,11 revealed longer Ru−O lengths and similar Ru− O−Ru and O−Ru−O angles. The present diruthenium complex had a shorter Ru−Ru distance [2.4334(4) Å] compared to that of the diruthenium complexes described above, suggesting interaction between two Ru centers. The capping nitrate anion coordinated without much distortion (O−N−O, 118.56, 120.71, and 120.72°) to form the triply bridged framework. The values of the effective magnetic moment μeff at 295 K were 2.19 μB for 1 and 2.17 μB for 2, accounting for one unpaired electron (Table S3). Electron spin resonance (ESR) spectra were measured at 77 K in frozen acetone−toluene solutions (Figure S2). Clear ESR signals were obtained with g values of 2.22, 2.10, and 2.00 for 1 and 2.17, 2.11, and 2.00 for 2. From these measurements, one unpaired electron was confirmed and the electronic structure was identified to be RuIII−RuIV with antiferromagnetic electronic coupling. Density functional theoretical (DFT) calculations for 2 were performed using the Gaussian 09 program with unrestricted B3LYP/LANL2DZ/ccpVDZ (Table S4). The result suggested that the electron density of the highest occupied frontier molecular orbital (161aα; see Figure S3) of 2 was contributed by the diruthenium core, Ru2(μO)2. Cyclic voltammetry (CV) curves of 1 and 2 in acetonitrile show one reversible oxidation process and stepwise reversible and irreversible reduction ones (Figure 2). Analysis of the

Figure 2. CV curves of 1 (upper) and 2 (bottom) in acetonitrile containing tetraethylammonium perchlorate at 25 °C. Figure 3. Pourbaix diagram of 1.

hydrodynamic voltammetry (Figures S4−S7) and controlled potential electrolysis (CPE) experiments for reversible oxidation and reduction waves (Figures S8−S11) indicated that these processes were assigned to one-electron redox couples, RuIII− RuIV/RuIII−RuIII and RuIV−RuIV/RuIII−RuIV, respectively. Thus, the formal oxidation state of both complexes is concluded to be RuIII−RuIV. The large difference of the redox potentials between two reversible waves reveals a stable mixed-valence state (KC = 2.7 × 1022 for 1 and 3.7 × 1024 for 2; Table S5), classified into class III according to Robin and Day classification.15 The RuIII− RuIII species in CH3CN changed and ligand-substitution reactions occurred (Figure S9), and the RuIV−RuIV state was stable during the oxidative CPE experiment (Figure S11).

chemical states of the bridging moieties were described in the diagram [1 is shown as RuIII−RuIV and (O,O)]. The pHindependent regions reveal only a change of the oxidation states, and the slopes of the linear relationship in the pH-dependent regions indicate the number of protons and electrons in the redox processes. The diagram indicates that nine species having a (μ-O)2, (μ-O)(μ-OH), (μ-OH)2, (μ-OH)(μ-OH2), or (μOH2)2 bridging core are expected to exist in the pH regions of the diagram with oxidation states of RuIV−RuIV, RuIII−RuIV, RuIII−RuIII, RuII−RuIII, and RuII−RuII. Notably, in a pH lower than 1.5, a protonated complex of 1, [{RuIII,IV(ebpma)}2(μO)(μ-OH)(μ-O2CCH3)]3+ (1H+), exists and can be isolated. B

DOI: 10.1021/acs.inorgchem.6b00890 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Reactions of 1 with Brønsted acids HX (X− = Cl−, ClO4−, CF3SO3−, or 1/2SO42−) in water−acetone under air were investigated. The protonated species 1H+ was isolated from a 3:7 (v/v) water−acetone solution containing Brønsted acid and identified as the RuIII−RuIV state by the magnetic susceptibility measurement at room temperature (μeff = 1.92 μB; Table S3). In the CV measurements in CH3CN, the protonated complex 1H+ showed three waves at a more positive region than those of 1 (Figure S13a). By the addition of Bu4NOH to the solution of 1H+, a similar CV profile of 1 was obtained, and the further addition of CF3SO3H reversibly afforded the original CV (Figure S13). These CV changes agreed with the results of electrochemical measurements in a water−acetonitrile solution. In reactions of 2 with Brønsted acids, decomposition of the nitratocapped framework occurred to give a nitrosylruthenium complex. Studies of the details of the products and differences of the reactivities between 1 and 2 are in progress. New dinuclear ruthenium complexes of RuIII−RuIV, composed of a doubly oxido-bridged Ru2(μ-O)2 core and one capped anion, [{RuIII,IV(ebpma)}2(μ-O) 2(μ-L)]2+ (L = CH3COO− and NO3−), were synthesized and fully characterized. Detailed studies of the redox behaviors in aqueous solutions of 1 provided stability of the expected forms at certain pH conditions. The protonated complex 1H+ having the oxido- and hydroxidobridged and acetato-capped framework (μ-O)(μ-OH)(μO2CCH3) in the state of RuIII−RuIV could be isolated under acidic conditions. The properties of the diruthenium complexes having a doubly oxido-bridged core are important in order to understand the rules of multinuclear metal complexes and to develop a multielectron and multicenter reaction site that functions as a redox reaction mediator for material conversion reactions. Further reactions of these diruthenium complexes and syntheses of diruthenium complexes in alternative oxidation states are in progress.

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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/acs.inorgchem.6b00890. Synthetic procedures, characterization data, X-ray crystallographic data, magnetic susceptibility, ESR spectra, DFT calculations, electrochemical measurements, and spectroscopic properties (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00890 Inorg. Chem. XXXX, XXX, XXX−XXX