Selective Redox Activation of H2 or O2 in a [NiRu] Complex by

Dec 26, 2012 - ... and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, ... E-mail: [email protected]...
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Selective Redox Activation of H2 or O2 in a [NiRu] Complex by Aromatic Ligand Effects Kyoungmok Kim,†,‡ Takahiro Kishima,‡ Takahiro Matsumoto,†,‡ Hidetaka Nakai,†,‡ and Seiji Ogo*,†,‡,§ †

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan § Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: We present two closely related series of a [NiFe] hydrogenase analogue. Based on a [NiRu] core, these complexes demonstrate inactivity, H2 activation, or O2 activation depending only on the nature of the Ru-coordinated aromatic ligand. It is demonstrated that even small changes made to this aromatic ligand can modulate the catalytic activity of the complex. Structural, electrochemical, kinetic, and thermodynamic studies reveal that differences in activation and binding modes of the substrates, combined with differences in σ donation and lability of the aromatic ligands, result in abrupt changes in catalytic activity.

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INTRODUCTION

Dihydrogen is currently drawing much interest from the science and engineering community as a promising replacement for fossil fuels.1 Indeed, dihydrogen is already used for fuel by some bacteria by means of the hydrogenase (H2ase) class of enzymes,2 which are currently undergoing intense scrutiny both for direct application and for the clues they can give for artificial analogues. The practical application of H2ases has been hampered, however, by their sensitivity to O2, which deactivates their active centers.2−10 Notably, a few H2ases based on [NiFe] centers exhibit O2 tolerance.5−10 Currently, it is believed that active center-bound O2 is reduced by a proximal FeS cluster and thereafter removed as H2O.7−10 This reaction is of great significance, since it is a direct complement to the oxidation of H2 into 2H+ and 2e− that is the normal function of these enzymes. O2 tolerance was also apparent in our [NiRu] model of H2ases (5a; Figure 1), which reproduces all of the main chemical features of natural [NiFe]H2ases.11 By replacing the Ru-coordinated hexamethylbenzene (η6-C6Me6, HMB) ligand with a more strongly σ-donating pentamethylcyclopentadienyl (η5-C5Me5, Cp*) ligand, we were even able to activate O2 and reduce it to H2O.12 We were subsequently successful in producing a basic fuel cell by coupling the oxidation of H2 by the HMB analogue (5a or 5b; Figure 1) to the reduction of O2 by the Cp* analogue (9a or 9c; Figure 1)in other words, we could harness the reaction of H2 and O2 to produce electricity.13 While this fuel cell is less efficient than conventional platinum-based cells, there is a great deal of scope for improvement. © 2012 American Chemical Society

Figure 1. Activation of H2 or O2 with the two series of [NiIIRuII] aqua complexes 1a−5a (L = benzene-derived aromatic ligand, n = 2) and 6a−9a (L = cyclopentadienyl (Cp)-derived aromatic ligand, n = 1).

With these improvements in mind, we have undertaken a systematic study of the H2-activating and O2-activating forms of these strongly related catalysts (Figure 1). The H2-activating forms all incorporate benzene derivatives coordinated to the Ru center. Differing numbers of methyl moieties change the σReceived: August 29, 2012 Published: December 26, 2012 79

dx.doi.org/10.1021/om300833m | Organometallics 2013, 32, 79−87

Organometallics

Article

(H2O)3](NO3)2 (C11H22N2O9Ru): C, 30.91; H, 5.19; N, 6.55. Found: C, 30.62; H, 4.99; N, 6.45. [RuII(η5-C5Me4H)(CH3CN)3](PF6). To a solution of the RuII complex [RuII(η6-C6H6)Cl2]2 (600 mg, 1.2 mmol) in EtOH (25 mL) were added K2CO3 (829 mg, 6.0 mmol) and C5Me4H2 (2.55 g, 21 mmol). The mixture was heated to 60 °C with rapid stirring and kept overnight. The mixture was cooled to room temperature and filtered through Celite. The brown filtrate was concentrated to 10 mL, and then to the resulting solution was added an aqueous solution (8.0 mL) of NH4PF6 (815 mg, 5.0 mmol). A brown precipitate was collected by filtration and redissolved in 75 mL of CH3CN. The solution was irradiated by an USHIO Optical ModuleX (Deep UV 500, BA-M500) for 24 h. The solvent was removed and dried in vacuo (yield 13% based on [RuII(η6-C6H6)Cl2]2). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.67 (s, 12H, C5Me4H), 4.30 (s, 1H, C5Me4H). FT-IR (cm−1, KBr disk): 3473 (O−H), 2899 (aliphatic C−H), 1530 (aromatic CC), 1460 (aromatic CC), 1432 (aromatic CC), 840 (PF6). Anal. Calcd for [RuII(η5-C5Me4H)(CH3CN)3](PF6)·CH3CN (C17H25N4PF6Ru): C, 38.42; H, 4.74; N, 10.54. Found: C, 38.65; H, 4.71; N, 10.43. [NiII(μ-SR)2RuII(H2O)(η6-C6H6)](NO3)2 ([1a](NO3)2). An aqueous solution (30 mL) of the RuII complex [RuII(η6-C6H6)(H2O)3](NO3)2 (179 mg, 0.50 mmol) was added to an aqueous solution (30 mL) of the Ni complex [NiII(μ-SR)2] (140 mg, 0.50 mmol). The mixture was stirred at room temperature for 2 h. The solvent was evaporated to yield a red-brown powder, which was dried in vacuo (yield 95% based on [RuII(η6-C6H6)(H2O)3](NO3)2). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.77−1.82, 1.97−2.11, and 2.82−3.41 (m, 14H, −CH2−), 2.76 (s, 6H, N−CH3), 6.31 (s, 6H, C6H6). ESI-MS (in H2O): m/z 520.0 ([1a − H2O + NO3]+; relative intensity (I) = 100% in the range of m/z 200−2000). FT-IR (cm−1, KBr disk): 3473 (O−H), 2935 (aliphatic C−H), 1638 (aromatic CC), 1462 (aromatic CC), 1438 (aromatic CC), 1384 (NO3). Anal. Calcd for [1a](NO3)2·0.5H2O (C15H29N4O7.5S2NiRu): C, 29.57; H, 4.80; N, 9.20. Found: C, 29.60; H, 4.91; N, 9.34. [NiII(μ-SR)2RuII(H2O)(η6-C6MeH5)](NO3)2 ([2a](NO3)2). Complex 2a was synthesized by the same method as described for the synthesis of 1a except [RuII(η6-C6MeH5)(H2O)3](NO3)2 was used (yield 92% based on [RuII(η6-C6MeH5)(H2O)3](NO3)2). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.79−2.52 and 2.82−3.58 (m, 14H, −CH2−), 2.13−2.32 (s, 3H, C6MeH5), 2.71 (s, 6H, N−CH3), 5.57− 6.61 (m, 5H, C6MeH5). ESI-MS (in H2O): m/z 534.0 ([2a − H2O + NO3]+; I = 100% in the range of m/z 200−2000). FT-IR (cm−1, KBr disk): 3490 (O−H), 2930 (aliphatic C−H), 1631 (aromatic CC), 1437 (aromatic CC), 1384 (NO3). Anal. Calcd for [2a](NO3)2·2H2O (C16H34N4O9S2NiRu): C, 29.55; H, 5.27; N, 8.61. Found: C, 29.49; H, 5.33; N, 8.63. [NiII(μ-SR)2RuII(H2O)(η6-C6Me4H2)](NO3)2 ([3a](NO3)2). Complex 3a was synthesized by the same method as described for the synthesis of 1a, except [RuII(η6-C6Me4H2)(H2O)3](NO3)2 was used (yield 92% based on [RuII(η6-C6Me4H2)(H2O)3](NO3)2). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.83−2.47 and 2.85−3.36 (m, 14H, −CH2−), 2.31−2.73 (s, 12H, C6Me4H2), 2.63 (s, 6H, N−CH3), 5.78 (s, 2H, C6Me4H2). ESI-MS (in H2O): m/z 576.0 ([3a − H2O + NO3]+; I = 100% in the range of m/z 200−2000). FT-IR (cm−1, KBr disk): 3444 (O−H), 2927 (aliphatic C−H), 1639 (aromatic CC), 1462 (aromatic CC), 1384 (NO3). Anal. Calcd for [3a](NO3)2 (C19H36N4O7S2NiRu): C, 34.77; H, 5.53; N, 8.54. Found: C, 34.72; H, 5.60; N, 8.68. [NiII(μ-SR)2RuII(H2O)(η6-C6Me5H)](NO3)2 ([4a](NO3)2). Complex 4a was synthesized by the same method as described for the synthesis of 1a, except [RuII(η6-C6Me5H)(H2O)3](NO3)2 was used (yield 94% based on [RuII(η6-C6Me5H)(H2O)3](NO3)2). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.66−1.90, 2.36−2.43, and 2.76− 3.36 (m, 14H, −CH2−), 1.95−2.28 (s, 15H, C6Me5H) 2.63 (s, 6H, N−CH3), 5.75 (s, 1H, C6Me5H). ESI-MS (in H2O): m/z 590.1 ([4a − H2O + NO3]+; I = 100% in the range of m/z 200−2000). FT-IR (cm−1, KBr disk): 3441 (O−H), 2931 (aliphatic C−H), 1639 (aromatic CC), 1462 (aromatic CC), 1384 (NO3). Anal. Calcd

donating properties of the ligands, which changes the pKa of the H2O ligand, which in turn functions as a Lewis base to extract the first proton from a coordinated H2 molecule. The O2activating forms all incorporate cyclopentadienyl (η5-C5H5, Cp) derivatives with differing numbers of methyl attachments. As well as being significantly stronger σ donors than benzenederived ligands, the Cp ligands carry a negative charge that changes the overall charge on the complex. In this paper, we report on the trends in reactivity toward H2 and O2 activation in H2O. We describe structural, electrochemical, kinetic, and thermodynamic properties of these systems. Finally, we propose details of the mechanism of H2 and O2 activation in these systems.

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EXPERIMENTAL SECTION

Materials and Methods. All experiments were carried out under a N2 atmosphere by using standard Schlenk techniques and a glovebox. CH3CN was distilled from CaH2 under a N2 atmosphere and stored over 4 A molecular sieves. D2O (99.9%) was purchased from Cambridge Isotope Laboratories, Inc., and CH3CN and EtCN were purchased from Wako Pure Chemical Industries, Ltd. H2, H2/N2 (75/ 25, 50/50, and 28/72%), O2, and O2/N2 gas mixture (75/25, 50/50, and 30/70%) were purchased from Sumitomo Seika Chemicals Co., Ltd. The RuII chloro complexes [RuII(η6-C6H6)Cl2]2 and [RuII(η6C6Me5H)Cl2]2,14a,b RuII mononuclear complexes [RuII(η6-C6H6)(H 2 O) 3 ](NO 3 ) 2 , [Ru II (η 6 -C 6 MeH 5 )(H 2 O) 3 ](NO 3 ) 2 , [Ru II (η 6 C6Me4H2)(H2O)3](NO3)2,14c,d [RuII(η5-C5H5)(CH3CN)3](PF6),14e and [RuII(η5-C5MeH4)(CH3CN)3](PF6),14f the NiII complex [NiII(μSR)2] ((μ-SR)2 = N,N′-dimethyl-3,7-diazanonane-1,9-dithiolato),14g and [NiRu] complexes [NiII(μ-SR)2RuII(H2O)(η6-C6Me6)](NO3)2 ([5a](NO3)2),11a [NiII(μ-SR)2RuII(μ-H)(η6-C6Me6)](NO3) ([5b](NO3)),11a [NiII(μ-SR)2RuII(μ-D)(η6-C6Me6)](NO3) ([D-labeled 5b](NO3)),11a [NiII(μ-SR)2RuII(H2O)(η5-C5Me5)](NO3) ([9a](NO3)),12 and [NiII(μ-SR)2RuIV(η2-O2)(η5-C5Me5)](NO3) ([9c](NO3))12 were prepared by the methods described in the literature. 1 H NMR spectra were recorded on a JEOL JNM-AL300 spectrometer at 25 °C. 1H NMR experiments in D2O were performed in a NMR tube (diameter 1.5 mm) containing 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (TSP, 100 mM, as the reference with the methyl proton or carbon resonance set at 0.00 ppm) dissolved in D2O. Electrospray ionization mass spectrometry (ESIMS) data were obtained by a JEOL JMS-T100LC AccuTOF instrument. Infrared (IR) spectra of solid compounds in KBr disks were recorded on a Thermo Nicolet NEXUS 8700 FT-IR instrument from 650 to 4000 cm−1 using 2 cm−1 standard resolution at 25 °C. UV/visible spectra were recorded on an Otsuka Electronics MCPD2000 photodiode array spectrometer with an Otsuka Electronics optical fiber attachment and a JASCO V-670 UV/visible/near-IR spectrophotometer (light path length 1.0 cm). Elemental analysis data were obtained by a PerkinElmer 2400II series CHNS/O analyzer. The pH values of the solutions were determined by a pH meter (TOA HM25G) equipped with a pH combination electrode (TOA GST5725C) and a pH meter (IQ Scientific Instruments, Inc., IQ200) equipped with a stainless steel micro pH probe (IQ Scientific Instruments, Inc., PH15-SS). The pH of the solution was adjusted by using 25 mM Na2HPO4/KH2PO4 solution (pH 7.0). [RuII(η6-C6Me5H)(H2O)3](NO3)2. To a suspension of the RuII complex [RuII(η6-C6Me5H)Cl2]2 (427 mg, 1.0 mmol) in H2O (900 mL) was added AgNO3 (527 mg, 3.1 mmol) in H2O (10 mL). The solution was stirred at room temperature for 12 h, and the precipitate was removed by filtration. The solvent of the resulting solution was evaporated to yield a yellow powder, which was dried in vacuo (yield 97% based on [RuII(η6-C6Me5H)Cl2]2). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.98−2.12 (m, 15H, C6Me5H), 5.43 (s, 1H, C6Me5H). FT-IR (cm−1, KBr disk): 3443 (O−H), 2975 (aliphatic C−H), 1541 (aromatic CC), 1525 (aromatic CC), 1482 (aromatic CC), 1384 (NO3). Anal. Calcd for [RuII(η6-C6Me5H)80

dx.doi.org/10.1021/om300833m | Organometallics 2013, 32, 79−87

Organometallics

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for [4a](NO3)2·2H2O (C20H42N4O9S2NiRu): C, 34.00; H, 5.99; N, 7.93. Found: C, 34.30; H, 5.81; N, 8.20. [NiII(μ-SR)2RuII(H2O)(η5-C5H5)](PF6) ([6a](PF6)). Complex 6a was synthesized by the same method as described for the synthesis of 1a, except [RuII(η5-C5H5)(CH3CN)3](PF6) was used (yield 78% based on [RuII(η5-C5H5)(CH3CN)3](PF6)). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.64−1.70, 1.88−2.53, and 2.71−3.50 (m, 14H, −CH2−), 2.65 (s, 6H, N−CH3), 6.35 (s, 5H, C5H5). ESI-MS (in H2O): m/z 444.9 ([6a − H2O]+; I = 100% in the range of m/z 200−2000). FT-IR (cm−1, KBr disk): 3436 (O−H), 2939 (aliphatic C−H), 1639 (aromatic CC), 1477 (aromatic CC), 1431 (aromatic CC), 840 (PF6). Anal. Calcd for [6a](PF6)·CH3CN (C16H30N3OS2PF6NiRu): C, 29.60; H, 4.66; N, 6.47. Found: C, 29.56; H, 4.44; N, 6.20. [NiII(μ-SR)2RuII(H2O)(η5-C5MeH4)](PF6) ([7a](PF6)). Complex 7a was synthesized by the same method as described for the synthesis of 1a, except [RuII(η5-C5MeH4)(CH3CN)3](PF6) was used (yield 89% based on [RuII(η5-C5MeH4)(CH3CN)3](PF6)). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.87−1.98, 2.26−2.36, and 2.87−3.77 (m, 14H, −CH2−), 2.74 (s, 6H, N−CH3), 2.07 (s, 3H, C5MeH4), 6.12 (s, 4H, C5MeH4). ESI-MS (in H2O): m/z 459.0 ([7a − H2O]+; I = 100% in the range of m/z 200−2000). FT-IR (cm−1, KBr disk): 3437 (O−H), 2941 (aliphatic C−H), 1588 (aromatic C C), 1474 (aromatic CC), 1435 (aromatic CC), 839 (PF6). Anal. Calcd for [7a](PF6) (C15H29N2OS2PF6NiRu): C, 28.95; H, 4.70; N, 4.50. Found: C, 28.78; H, 4.88; N, 4.61. [NiII(μ-SR)2RuII(H2O)(η5-C5Me4H)](PF6) ([8a](PF6)). Complex 8a was synthesized by the same method as described for the synthesis of 1a, except [RuII(η5-C5Me4H)(CH3CN)3](PF6) was used (yield 87% based on [RuII(η5-C5Me4H)(CH3CN)3](PF6)). 1H NMR (300 MHz, in D2O, referenced to TSP, 25 °C): δ 1.83−1.93, 2.25−2.45, and 2.71−3.38 (m, 14H, −CH2−), 2.58 (s, 6H, N−CH3), 1.67 (s, 12H, C5Me4H), 5.82 (s, 1H, C5Me4H). ESI-MS (in H2O): m/z 501.0 ([8a − H2O]+; I = 100% in the range of m/z 200−2000). FT-IR (cm−1, KBr disk): 3396 (O−H), 2922 (aliphatic C−H), 1593 (aromatic C C), 1460 (aromatic CC), 1435 (aromatic CC), 840 (PF6). Anal. Calcd for [8a](PF6)·CH3CN (C20H38N3OS2PF6NiRu): C, 34.05; H, 5.43; N, 5.96. Found: C, 34.35; H, 5.15; N, 5.87. Typical Procedure for H2 Activation with [NiIIRuII] Aqua Complexes 3a and 4a To Form [NiIIRuII] Hydride Complexes 3b and 4b. H2 was bubbled through H2O (5.0 mL, pH 7.0) of the [NiIIRuII] aqua complexes [3a](NO3)2 (131 mg, 0.20 mmol) and [4a](NO3)2 (134 mg, 0.20 mmol) under a N2 atmosphere at room temperature to form the [NiIIRuII] hydride complexes [NiII(H2O)(μSR)2(μ-H)RuII(η6-C6Me4H2)](NO3) ([3b](NO3)) and [NiII(H2O)(μ-SR)2(μ-H)RuII(η6-C6Me5H)](NO3) ([4b](NO3)), respectively. The ESI-MS results in H2O showed m/z 515.1 ([3b − H2O]+; I = 100% in the range of m/z 200−2000) for 3b and m/z 529.1 ([4b − H2O]+; I = 100% in the range of m/z 200−2000) for 4b (Figure S3 in the Supporting Information). Typical Procedure for D2 Activation with [NiIIRuII] Aqua Complexes 3a and 4a To Form D-Labeled 3b and D-Labeled 4b. D2 was bubbled through H2O (5.0 mL, pH 7.0) of the [NiIIRuII] aqua complexes [3a](NO3)2 (131 mg, 0.20 mmol) and [4a](NO3)2 (134 mg, 0.20 mmol) under a N2 atmosphere at room temperature to form the deuteride species [Ni II (H 2 O)(μ-SR) 2 (μ-D)Ru II (η 6 C6Me 4H2)](NO3) ([D-labeled 3b](NO3)) and [NiII(H2O)(μSR)2(μ-D)RuII(η6-C6Me5H)](NO3) ([D-labeled 4b](NO3)), respectively. The ESI-MS results in H2O showed m/z 516.1 ([D-labeled 3b − H2O]+; I = 100% in the range of m/z 200−2000) for D-labeled 3b and m/z 530.1 ([D-labeled 4b − H2O]+; I = 100% in the range of m/z 200−2000) for D-labeled 4b (Figure S4 in the Supporting Information). Typical Procedure for O2 Activation with [NiIIRuII] Aqua Complexes 6a−8a To Form [NiIIRuIV] Peroxo Complexes 6c−8c. O2 was bubbled through aqueous solutions (30 mL) of the [NiIIRuII] aqua complexes [6a](PF6) (66.9 mg, 0.11 mmol), [7a](PF6) (68.4 mg, 0.11 mmol), and [8a](PF6) (73.1 mg, 0.11 mmol) to afford the [NiIIRuIV] peroxo complexes [NiII(μ-SR)2RuIV(η2-O2)(η5-C5H5)](PF6) ([6c](PF6)), [NiII(μ-SR)2RuIV(η2-O2)(η5-C5MeH4)](PF6)

([7c](PF6)), and [NiII(μ-SR)2RuIV(η2-O2)(η5-C5Me4H)](PF6) ([8c](PF6)), respectively. The ESI-MS results in H2O showed m/z 447.0 ([6c]+; I = 100% in the range of m/z 200−2000) for 6c, m/z 491.0 ([7c]+; I = 100% in the range of m/z 200−2000) for 7c, and m/z 533.0 ([8c]+; I = 100% in the range of m/z 200−2000) for 8c (Figure S5 in the Supporting Information). Kinetic Measurements. Kinetic measurements of H2 activation with [NiIIRuII] aqua complexes 3a−5a in H2O (pH 7.0) at 20−40 °C were carried out by monitoring the spectral change of the absorption band at 298 nm. H2 or H2/N2 gas mixture (H2/N2 ratio 75/25, 50/50, or 28/72%) was bubbled through H2O (pH 7.0, 4.9 mL) for 30 min to give an H2O solution with a constant concentration of H2. The [NiIIRuII] aqua complexes ([3a](NO3)2 (2.1 mM), [4a](NO3)2 (1.5 mM), and [5a](NO3)2 (2.0 mM)) in H2O (0.1 mL) were added to H2- or H2/N2-bubbled H2O solutions (4.9 mL) in a Schlenk flask (10 mL) at 20−40 °C. The final concentrations of [NiIIRuII] aqua complexes and H2 are given in Figures S6−S8 and Table S1 in the Supporting Information. Pseudo-first-order rate constants (kobs) were determined by least-squares curve fitting. The reaction rate of O2 activation with the [NiIIRuII] aqua complexes 6a−9a in EtCN at −20 °C was followed by the spectral change at 390 nm (6a, 7a, and 9a) or 400 nm (8a). O2 or O2/N2 gas mixture (O2/N2 ratio 75/25, 50/50, or 30/70%) was bubbled through EtCN (4.9 mL) for 30 min to give a EtCN solution with a constant concentration of O2. The [NiIIRuII] aqua complexes ([6a](PF6) (8.0 mM), [7a](PF6) (8.0 mM), [8a](PF6) (8.5 mM), and [9a](NO3) (10 mM)) in EtCN (0.1 mL) were added to O2- or O2/N2-bubbled EtCN solutions (4.9 mL) in a Schlenk flask (10 mL) at −20 °C. The final concentrations of the [NiIIRuII] aqua complexes and O2 are given in Figures S9−S12 and Table S2 in the Supporting Information. Pseudo-first-order rate constants (kobs) were determined by least-squares curve fitting. The concentration of H2 in H2-saturated H2O and O2 in O2saturated EtCN solution was determined on the basis of an equation in the literature,15 and each concentration was calculated as a ratio of H2 in H2/N2 gas mixture or O2 in O2/N2 gas mixture. Electrochemical Analysis. Electrochemical measurements were performed in an CH3CN solution of the [NiIIRuII] aqua complexes 1a−9a (1.0 mM) with nBu4NPF6 (100 mM) as a supporting electrolyte on a BAS660A electrochemical analyzer using a carbon working electrode at room temperature (Figures S17 and S18 and Table S3 in the Supporting Information). The cyclic voltammograms are collected using a scan rate of 200 mV s−1. E1/2 values are taken as the average of the voltages of maximum current for the forward and reverse electrochemical processes. Potentials are reported in volts versus NHE, which are referenced to the potential of the Fc+/Fc (ferrocenium/ferrocene) couple (+400 mV versus NHE).16 Procedure for the pH Titration of [NiIIRuII] Aqua Complexes 1a−5a by UV/Vis Spectroscopy. The pH of an H2O solution of 1a−5a (5.0 mM) was adjusted to 1.8 with 0.10 M HNO3/H2O. The titration of the resulting solution with 0.01−10 M NaOH/H2O was monitored by UV/vis spectroscopy. The pKa values of 1a−5a were determined as 7.8, 7.9, 8.2, 8.3, and 8.5, respectively (Figure S19 and Table S4 in the Supporting Information). X-ray Crystallographic Analysis. [NiII(μ-SR)2RuII(CH3CN)(η6C6Me4H2)](CF3SO3)2 ([3d](CF3SO3)2) and [NiII(μS R) 2 Ru I I (CH 3 CN)(η 6 -C 6 M e 5 H)][Na 2 (CF 3 SO 3 ) 4 ] ([4d][Na2(CF3SO3)4]) were prepared by replacing the NO3− counteranion in [3a](NO3)2 and [4a](NO3)2 with CF3SO3− by addition of NaCF3SO3 in CH3CN. X-ray-quality crystals of [3d](CF3SO3)2 and [4d][Na2(CF3SO3)4] were then prepared by slow diffusion of diethyl ether into CH3CN solutions of these compounds. Crystallographic data for [3d](CF3SO3)2 and [4d][Na2(CF3SO3)4] have been deposited with the Cambridge Crystallographic Data Center as Supplementary Publication Nos. CCDC 894836 and 894837. Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44) 1223336-033; e-mail, [email protected]). Measurements were made on a Rigakui/MSC Saturn CCD diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71070 Å). All calculations 81

dx.doi.org/10.1021/om300833m | Organometallics 2013, 32, 79−87

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515.1 for [3b − H2O]+, m/z 529.1 for [4b − H2O]+, and m/z 543.1 for [5b − H2O]+) (Figure S3 in the Supporting Information). When D2 was used instead of H2 in order to establish the origin of the hydrido ligand, the corresponding deuteride species [NiII(H2O)(μ-SR)2(μ-D)RuII(η6-C6Me4H2)]+ (D-labeled 3b), [NiII(H2O)(μ-SR)2(μ-D)RuII(η6-C6Me5H)]+ (D-labeled 4b), and [Ni II (H 2 O)(μ-SR) 2 (μ-D)Ru II (η 6 C6Me6)]+ (D-labeled 5b) were formed, which were confirmed by ESI-MS (m/z 516.1 for [D-labeled 3b − H2O]+, m/z 530.1 for [D-labeled 4b − H2O]+, and m/z 544.1 for [D-labeled 5b − H2O]+) (Figure S4 in the Supporting Information). The 1H NMR spectra show that the sharp signals derived from the aqua complexes 3a and 4a in a diamagnetic region change to broad signals derived from the hydride complexes 3b and 4b in a paramagnetic region by the reaction of the aqua complexes with H2. The results suggest that a low-spin NiII center in a squareplanar structure changes to a high-spin NiII center in an octahedral structure, in which the hydrido ligand could bridge NiII and RuII centers similar to the case for 5b. Such spin crossover behavior has already been reported for 5b.11a Though complexes 3a−5a could activate H2, no reaction with O2 was observed. While similar experiments demonstrated that compounds 6a−9a could not activate H2, experiments with O2 did show activation. Spectral changes in ESI-MS and UV/vis and 1H NMR spectroscopy upon reaction with O2 illustrate that 6a−9a could activate O2 to give side-on η2-peroxo complexes via oxidative addition of O2: [NiII(μ-SR)2RuIV(η2-O2)(η5-C5H5)]+ (6c), [NiII(μ-SR)2RuIV(η2-O2)(η5-C5MeH4)]+ (7c), [NiII(μSR) 2 Ru I V (η 2 -O 2 )(η 5 -C 5 Me 4 H)] + (8c), and [Ni I I (μSR)2RuIV(η2-O2)(η5-C5Me5)]+ (9c). The structure of the side-on η2-peroxo complex 9c has been previously established.12 UV/vis spectra of these complexes show an increase of the intense absorption bands derived from η2-peroxo complexes around 390 nm. The ESI-MS shows η2-peroxo complexes (m/z 477.0 for [6c]+, m/z 491.0 for [7c]+, m/z 533.0 for [8c]+, and m/z 547.1 for [9c]+) from O2 activation (Figure S5 in the Supporting Information). Similar [NiRu] η2-peroxo complexes have been also synthesized from O2 activation.18 The difference in activation ability of the Cp-bearing complexes as compared to that of the benzene-bearing complexes is attributed to the much greater σ-donating ability of the Cp-derived ligands. The electron density and redox potential of the RuII center is therefore regulated by the selected aromatic ligand, allowing for selectivity between H2 and O2 activation. Electrochemical Studies. We already knew that a Rucoordinated H2O molecule with high basicity was necessary for the heterolytic activation of H2, and it seemed highly likely that strong σ donation to the Ru center would be necessary for stabilizing O2-bound intermediates.11,12 Since both of these properties would be highly dependent on the electron density at the Ru center, we carried out cyclic voltammetry measurements to assess the redox potential of the Ru center. We determined the redox potentials (E1/2) of RuII/RuI in the [NiRu] complexes 1a−9a (Figure 2 and Figures S17 and S18 and Table S3 in the Supporting Information). In an CH3CN solution, all [NiIIRuII] complexes display a quasi-reversible voltammogram in the E1/2 range of −0.626 to −1.380 V (all values are calculated versus NHE), in the ranges of E1/2 values for the Ru complexes [RuII(tpy)2]2+ (tpy = 2,2′:6′,2″terpyridine, E1/2 = −1.21 V), [RuII(tpy)(pydppx)]2+ (pydppx = 3-(pyrid-2′-yl)-11,12-dimethyldipyrido[3,2-a:2′,3′-c]-

were performed using the teXsan crystallographic software package of Molecular Structure Corp.

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RESULTS AND DISCUSSION Synthesis and Characterization of [NiIIRuII] Aqua Complexes 1a−9a. Each [NiIIRuII] aqua complex was obtained by the reaction of the RuII complex [RuII(L)(H2O)3](NO3)2 (L = η6-C6H6, η6-C6MeH5, η6-C6Me4H2, η6-C6Me5H, η6-C6Me6) or [RuII(L)(CH3CN)3](X) (L = η5-C5H5, η5C5MeH4, η5-C5Me4H, X = PF6 or L = η5-C5Me5, X = NO3) with an equal amount of the NiII complex [NiII(μ-SR)2] ((μSR)2 = N,N′-dimethyl-3,7-diazanonane-1,9-dithiolato) in H2O at 25 °C. The series of dinuclear [NiIIRuII] aqua complexes were isolated as red-brown powders for [1a−5a](NO3)2 or dark brown powders for [6a−8a](PF6) and [9a](NO3). These complexes were characterized by ESI-MS, 1H NMR and IR spectroscopy, elemental analysis, and X-ray analysis. Recrystallization from diethyl ether diffusion into CH3CN solutions of the CF3SO3− salts of 3a and 4a gave orange crystals suitable for X-ray analysis. The H2O ligands of 3a and 4a were replaced by an CH3CN ligand to form the corresponding CH3CN-coordinated complexes 3d and 4d, respectively (Figures S1 and S2 in the Supporting Information). The structures of 3d and 4d correspond directly to that of [Ni I I (μ-SR) 2 Ru II (CH 3 CN)(η 6 -C 6 Me 6 )](NO 3 ) 2 ([5d](NO3)2).17 In 3d and 4d, the Ni center, with a square-planar geometry, and the Ru center, with a distorted-octahedral geometry, are bridged by the thiolato parts of the (μ-SR)2 ligand. The Ru atom is surrounded by one CH3CN, one aromatic ligand (η6-C6Me4H2 for 3d and η6-C6Me5H for 4d), and one metalloligand [NiII(μ-SR)2]. The interatomic distances between the Ni and Ru atoms for 3d and 4d are 3.1566(4) and 3.178(1) Å, respectively, similar to that of 5d (3.1664(4) Å). The other structural parameters in 3d and 4d, such as the Ni− S−Ru angle and Ni−ligand and Ru−ligand distances, are also almost the same as those of 5d. Reactivity of [NiIIRuII] Aqua Complexes 1a−9a toward H2 and O2. Reactivity of the [NiIIRuII] aqua complexes 1a−9a with H2 and O2 were investigated in H2O (pH 7.0) at 25 °C. H2 or O2 gas was bubbled through stirred solutions of the series of [NiIIRuII] complexes. Each reaction was monitored by ESIMS (Figures S3−S5 in the Supporting Information) and UV/ vis measurements (Figures S6−S12 in the Supporting Information). For 1a and 2a, bearing η6-C6H6 and η6-C6MeH5, respectively, we could not observe any spectral changes, signifying that no reactions occur between H2 or O2 and 1a or 2a. Reactions with H2 were observed for 3a−5a, however, and were monitored by ESI-MS and UV/vis and 1H NMR spectroscopy. We were able to identify the resulting complexes as the corresponding [NiIIRuII] hydride complexes [NiII(H2O)(μ-SR) 2 (μ-H)Ru II (η 6 -C 6 Me 4 H 2 )] + (3b), [Ni II (H 2 O)(μSR) 2 (μ-H)Ru II (η 6 -C 6 Me 5 H)]+ (4b), and [Ni II (H 2 O)(μSR)2(μ-H)RuII(η6-C6Me6)]+ (5b). The activity of compounds 3a−5a is believed to result from the greater σ-donating ability of their aromatic ligands, in comparison to the corresponding ligands in 1a and 2a. The resulting hydride complexes demonstrate successful heterolytic cleavage of H2. The spectroscopic features of the hydride complex 5b were obtained, while the structure has been reported previously.11a UV/vis spectra of these complexes show a decrease of the intense absorption bands derived from the aqua complexes (λmax 298 nm). The ESI-MS shows the hydride species (m/z 82

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3b−5b, respectively. By following a decrease of the absorption band at 298 nm, we conducted kinetic analysis for H2 activation (Figure 3a and Figures S6−S8 in the Supporting Information). The H2-activation processes of 3a−5a obey pseudo-first-order kinetics over 5 half-lives (v = kobs[NiRu complex]).

Figure 2. Redox potentials (E1/2) of RuII/RuI in the [NiIIRuII] aqua complexes 1a−5a (L = benzene-derived aromatic ligand, n = 2) and 6a−9a (L = Cp-derived aromatic ligand, n = 1). E1/2 (V versus NHE): −0.626 (1a), −0.650 (2a), −0.718 (3a), −0.743 (4a), −0.778 (5a), −1.352 (6a), −1.357 (7a), −1.374 (8a), and −1.380 (9a). The redox potentials (E1/2) of 1a−9a were determined in CH3CN ([NiIIRuII] complex, 1.0 mM; nBu4NPF6, 100 mM) at 25 °C (Figures S17 and S18 and Table S3 in the Supporting Information).

phenazine, E1/2 = −0.99 V), [RuII(pydppx)2]2+ (E1/2 = −0.96 V), [RuII(tpy)(pydppn)]2+ (pydppn =3-(pyrid-2′-yl)-4,5,9,16tetraazadibenzo[a,c]naphthacene, E1/2 = −0.70 V), and [RuII(pydppn)2]2+ (E1/2 = −0.66 V) in the literature.19 As the electron-donating strength of the aromatic ligands increased, E1/2 values of RuII/RuI in 1a−9a were shifted to the negative side (Figure 2 and Figures S17 and S18 and Table S3 in the Supporting Information). The RuII centers coordinated by strong electron donors are more easily oxidized than those coordinated by weak electron donors, as evidenced by a negative shift of E1/2 from the η6 neutral ligand with no substituents in 1a (E1/2 = −0.626 V) to the six-methyl-groupsubstituted ligand in 5a (E1/2 = −0.778 V) and from the η5 anionic ligand with no substituent in 6a (E1/2 = −1.352 V) to the five-methyl ligand in 9a (E1/2 = −1.380 V). The results are consistent with previously reported data for the Ru complexes [RuII(bpy)2(dppx)]2+ (dppx = 11,12-dimethyldipyrido[3,2a:2′,3′-c]phenazine, E1/2 = −0.81 V) and [RuII(bpy)2(dppz)]2+ (dppz = dipyrido[3,2-a:2′,3′-c]phenazine, E1/2 = −0.73 V), which demonstrate that E1/2 values of RuII complexes with electron-donating substituents are shifted to the negative side.20 The large gap between 5a and 6a reflects the much stronger σ donation of the Cp-derived ligands in comparison to those of the benzene-derived ligands. The active derivatives for H2 activation are 3a−5a, and the range of the redox potential of RuII/RuI for H2 activation is between −0.718 and −0.778 V. The active derivatives for O2 activation are 6a−9a, and the range of redox potential of RuII/RuI for O2 activation is between −1.352 and −1.380 V. We investigated the detailed kinetic and thermodynamic analyses on the activation of H2 and O2 as described below. Kinetic Studies of H2 and O2 Activation. We investigated the effect of aromatic electron-donating ability on the rate of activation of H2 and O2. As mentioned above, the reactions of 3a−5a with H2 in H2O at pH 7.0 resulted in a change of the UV/vis spectra to form the corresponding hydride complexes

Figure 3. (a) UV/vis spectral change for the reaction of [5a](NO3)2 (0.040 mM) with H2 (0.32 mM) in H2O (pH 7.0) at 30 °C. Inset: time profile of the absorbance at 298 nm and first-order kinetic plots for the change in absorbance at 298 nm of [5a](NO3)2. (b) Plot of kobs against concentration of H2 (0.18, 0.32, 0.48, and 0.64 mM) for the reaction of [5a](NO3)2 with H2 in H2O (pH 7.0) at 30 °C.

There are two possibilities for the initial binding of H2, as shown in Figure 4. Path A involves an immediate, irreversible binding of H2 to both the Ru center and the H2O ligand. Path B involves a preliminary, reversible binding of H2 to the Ru center before irreversible rearrangement to the Ru−OH2 binding mode. To determine the correct path, we determined H2-concentration-dependent kobs values. The pseudo-first-order rate constant (kobs) is proportional to the concentration of H2 for all three complexes 3a−5a (Figure 3b and Figure S13 in the Supporting Information). These linear plots allow us to determine the reaction mechanism as a simple bimolecular process, i.e. path A, and the rate equation as v = k2[H2][NiRu complex]. A 16e− Ru complex is well-known as a highly reactive complex for H2 activation,2f which allows us to propose that an H2O ligand is released from the RuII center before interaction with H2, similar to the H2-activation mechanism proposed for [NiFe]H2ases.2 The removal step of the H2O ligand could be fast, which is not involved in the ratedetermining step. Although the H2 addition step is thought to involve the reversible formation of an undetected H2coordinated complex on the basis of theoretical studies21 and previous hydrogen chemistry,2,22 the kinetic data from this study support the path A mechanism at the present time 83

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Figure 4. Possible mechanisms for H2 activation with 3a−5a via paths A and B. The kinetic results support the path A mechanism. Electron counts for the Ru center are depicted.

(Figure 4). The values of second-order rate constants (k2) were obtained as the slope (Figure 3b and Figure S13 and Table S1 in the Supporting Information). A deuterium kinetic isotope effect (KIE, kH/kD) of dihydrogen activation with 5a was determined as 0.86 at 20 °C by using D2 instead of H2. The observed inverse KIE indicates that the formation of the metal−H or H2O−H bond is involved in the rate-determining step, rather than H−H bond cleavage. A similar inverse KIE (0.71) has been observed for dihydrogen activation in an [FeFe]H2ase model system.23 The rate of the reaction was faster in [NiIIRuII] complexes with more strongly electron-donating ligands (k2 = 7.8 M−1 s−1 for 5a > 3.0 M−1 s−1 for 4a > 1.4 M−1 s−1 for 3a at 30 °C; Figure 5a), which corresponds directly to the pKa of the H2O ligands, as discussed below. The redox potential (E1/2) of the Ru center depends on the pKa value of the H2O ligand, and a linear correlation between E1/2 and pKa is observed (Figure 5b). The linear relationship indicates that a higher electron density of the Ru center causes lower Lewis acidity of the Ru center, followed by higher Lewis basicity of the H2O ligand. There is also a linear correlation between log k2 and the pKa (Figure 5c). The linear correlation clearly reveals that the H2O ligand acts as a Lewis base to cleave H2 via heterolytic activation. A combination of these two linear plots (Figure 5b,c) could yield the linear plot of log k2 against E1/2 (Figure 5a). Our previous studies of H2 activation showed the H2 molecule to coordinate to the Ru center, whereupon the H2O ligand acts as a base to abstract H+ from H2, leaving H− bound to the Ru center.11 In this model, there is a balance between the need for a Ru center with a high Lewis acidity for coordinating the initial H2 and subsequent H− and the need for an H2O molecule with a Lewis basicity high enough to abstract the H+ from H2. Our studies suggest that this balance depends on the nature of the aromatic ligandstrong σ donation to the Ru center will increase the electron density on the H2O ligand and increase its Lewis basicity, while at the same time decreasing the Lewis acidity of the metal center, preventing it from coordinating to the H2 and stabilizing the subsequent H− ligand.

Figure 5. (a) Plot of log k2 against E1/2(RuII/RuI) in 3a−5a. (b) Plot of pKa against E1/2(RuII/RuI) in 1a−5a. (c) Plot of log k2 against pKa in 3a−5a. The rate constants (k2) were determined in H2O (pH 7.0) at 30 °C (Table S1 in the Supporting Information). The redox potentials (E1/2) were determined in CH3CN ([NiIIRuII] complex, 1.0 mM; nBu4NPF6, 100 mM) at 25 °C (Figure 2 and Table S3 in the Supporting Information). The pKa values were determined in H2O ([NiIIRuII] complex, 5.0 mM) (Figure S19 and Table S4 in the Supporting Information).

To determine activation parameters (activation enthalpy ΔH⧧ and activation entropy ΔS⧧), the temperature-dependent rate constants in H2O at pH 7.0 over a range of temperatures at 293−313 K at 5 K intervals are investigated (Table S1 in the Supporting Information). On the basis of the Eyring plots for 3a−5a as shown in Figure 6, the activation parameters were estimated (Table 1). The activation enthalpy (ΔH⧧) becomes more favorable as the pKa value of the H2O ligand increases, since higher Lewis basicity causes higher bond energy of the formed H2O−H bond. The activation entropy (ΔS⧧) becomes more favorable as the number of methyl groups decreases, due to possibilities of steric hindrance of the methyl group. The positive values of ΔS⧧, obtained from 3a−5a, suggest that the 84

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Figure 6. Eyring plots for H2 activation with 3a−5a (filled circles) and D2 activation with 5a in which D2 was used instead of H2 (open circles). The rate constants (k2) are summarized in Table S1 in the Supporting Information.

Table 1. Activation Parameters for H2 Activation with 3a−5a in H2O at pH 7.0 [NiIIRuII] complex

ΔH⧧ (kJ mol−1)

ΔS⧧ (J mol−1 K−1)

[3a](NO3)2 [4a](NO3)2 [5a](NO3)2

113 105 94.6

130 110 84.6

rate-determining step is dominated by dissociation of H3O+ with the formation of the metal−H and H2O−H bonds, which correspond to the inverse KIE described above. The two series of our compounds demonstrate these twin effects. In the case of the benzene-ligated complexes (1a and 2a), the σ donation of the aryl ring is too weak to make the H2O act as an efficient base, while in the case of the complexes with Cp-derived ligands, the σ donation of the aryl ring is so strong that the Ru center can no longer act as a strong Lewis acid. For O2 activation, the reactions of complexes 6a−9a with O2 in EtCN at −20 °C obey pseudo-first-order kinetics over 5 halflives (v = kobs[NiRu complex]) (Figure 7a and Figures S9−S12 in the Supporting Information). There is a possibility that the H2O ligand is replaced by EtCN to form the corresponding EtCN-coordinated complexes 6e−9e when the aqua complex is dissolved in EtCN. The pseudo-first-order rate constant (kobs) was then plotted against the concentration of O2 to afford a Michaelis−Menten type saturation curve, as shown in Figure 7b and Figures S14−S16 in the Supporting Information. This is a highly significant finding for O2-reduction chemistry in contrast with the linear correlation of kobs against [O2] in the literature24 and indicates a Michaelis−Menten type pathway, i.e. path B, for O2 complexation (Figure 8). The double-reciprocal plot (Figure 7c), from 1/kobs = 1/k2 + (1/Kk2)(1/[O2]), yielded the equilibrium constant K (k1/k−1) and the rate constant k2 for the oxidation process (Table 2 and Table S2 in the Supporting Information). As shown in Figure 9, there is a linear correlation between log k2 in O2 reduction and the redox potential (E1/2) of the Ru center in 6a−9a. The rate of the reaction with O2 in the [NiIIRuII] aqua complexes 6a−9a was accelerated as the electron donation of ligands decreased, which is consistent with the tendency in the previous literature for the reaction rate of O2 activation to increase with the electron-withdrawing substituent (k2 = 840 M−1 s−1 for [NiII(TpMe2)(SC6H4NO2)] versus k2 > 5000 M−1 s−1 for [NiII(TpMe2,Br)(SC6H4NO2)],

Figure 7. (a) UV/vis spectral change for the reaction of [6a](PF6) (0.16 mM) with O2 (9.3 mM) in EtCN at −20 °C. Inset: time profile of the absorbance at 390 nm and first-order kinetic plots for the change in absorbance at 390 nm of [6a](PF6). (b) Plot of kobs against concentration of O2 (2.8, 4.7, 7.0, and 9.3 mM) for the reaction of [6a](PF6) with O2 in EtCN at −20 °C. (c) Double-reciprocal plot of kobs against concentration of O2 for the reaction of [6a](PF6) with O2 in EtCN at −20 °C.

where TpMe2 = hydrotris(3,5-dimethylpyrazol-1-yl)borate and Tp Me2,Br = hydrotris(4-bromo-3,5-dimethylpyrazol-1-yl)borate).25 The most obvious difference between O2 and H2 activation is that the O2 molecule coordinates in an η2 fashion as a peroxo ligand and is activated homolytically. This binding mode and radical activation mechanism completely change the requirements made of the aromatic ligand. First, the peroxo species results in a high-valent RuIV center and it is well-known that strong σ donation stabilizes such high-valent metal centers. Only the complexes bearing Cp-derived ligands have access to such strong σ donation, as evidenced by the inactivity of all benzene-bearing complexes. Second, the η2-bound O2 molecule results in a seven-coordinate RuII center, as seen in the X-ray 85

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CONCLUSIONS We have produced two series of a [NiRu]-based H2ase biomimic by employing either benzene-derived or Cp-derived aromatic ligands. The two series are separated by differences in charge, steric volume, and σ donation from the aromatic ligand, while members of a series are separated to a lesser extent by σdonation ability and, in the case of the Cp-bearing series, lability. These differences result in their participation in either of two very different activation mechanisms for H2 and O2: irreversible complexation of H2 followed by heterolytic activation versus Michaelis−Menten type binding of O2 followed by homolytic activation. These results demonstrate that (1) a change in ligand environment can switch a [NiFe]H2ase analogue between H2 activation and O2 activation, (2) this system is an excellent model for probing the behavior of natural H2ases, and (3) there is clear scope for improving the properties of these catalysts for use in molecular fuel cells (Figure 10). We are confident,

Figure 8. Possible mechanisms for O2 activation with 6a−9a (X = H2O) or 6e−9e (X = EtCN) via path A and B. The kinetic results support the path B mechanism. Electron counts for Ru centers are depicted.

Table 2. Rate Constants (k2) for H2 Activation via Path A of [3a](NO3)2, [4a](NO3)2, and [5a](NO3)2 (Figure 4) and Rate Constants (k2) and Equilibrium Constants (K) for O2 Activation via Path B of [6a](PF6), [7a](PF6), [8a](PF6), and [9a](NO3) (Figure 8) [NiIIRuII] complex [3a](NO3)2a [4a](NO3)2a [5a](NO3)2a [6a](PF6)b [7a](PF6)b [8a](PF6)b [9a](NO3)b a

K (k1/k−1)

848 146 1466 1941

k2 1.4 3.0 7.8 4.3 3.6 1.6 9.0

−1

M s−1 M−1 s−1 M−1 s−1 × 10−2 s−1 × 10−3 s−1 × 10−4 s−1 × 10−5 s−1

In H2O (pH 7.0) at 30 °C. bIn EtCN at −20 °C. Figure 10. Correlation between log k2 and E1/2(RuII/RuI) of [NiRu] complexes.

therefore, that molecular catalysts will soon find real applications in power cell technology and that biomimetic chemistry will continue to prove its worth in cross-disciplinary innovation.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S19 and Tables S1−S4, giving ORTEP drawings and additional data, and CIF files, giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Plot of log k2 against E1/2(RuII/RuI) in 6a−9a. The rate constants (k2) were determined in EtCN at −20 °C (Table S2 in the Supporting Information). The redox potentials (E1/2) were determined in CH3CN ([NiIIRuII] complex, 1.0 mM; nBu4NPF6, 100 mM) at 25 °C (Figure 2 and Table S3 in the Supporting Information).

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-92-802-2818. Fax: +81-92-802-2823. E-mail: [email protected].

structure.12 To allow this binding mode, the aromatic ligand must move and possibly temporarily reduce its coordination number with the RuII center. In other words, aromatic ligands with greater labilities are better able to allow access to the η2bound peroxo complex. This requirement for greater lability runs counter to the requirement for greater σ donation and explains the correlation between log k2 and E1/2 of the Ru center seen for O2 activation (Figure 9).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the World Premier International Research Center Initiative (WPI), grants-in-aid 23655053, 24750058, and 24109016 (Scientific Research on Innovative 86

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Areas “Stimuli-responsive Chemical Species”) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and the Basic Research Programs CREST Type, “Development of the Foundation for NanoInterface Technology” from the JST of Japan.

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dx.doi.org/10.1021/om300833m | Organometallics 2013, 32, 79−87