Tuning of Metal–Metal Interactions in Mixed-Valence States of

May 13, 2014 - and Masa-aki Haga*. ,†. †. Department of ... Academy of Sciences, Beijing 100190, People,s Republic of China. §. Institute for Mol...
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Tuning of Metal−Metal Interactions in Mixed-Valence States of Cyclometalated Dinuclear Ruthenium and Osmium Complexes Bearing Tetrapyridylpyrazine or -benzene Takumi Nagashima,† Takuya Nakabayashi,† Takashi Suzuki,† Katsuhiko Kanaizuka,† Hiroaki Ozawa,† Yu-Wu Zhong,‡ Shigeyuki Masaoka,§ Ken Sakai,∥ and Masa-aki Haga*,† †

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan ‡ Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan ∥ Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan S Supporting Information *

ABSTRACT: New dinuclear ruthenium or osmium complexes with cyclometalated bonds in either tridentate bridging (BL) or ancillary ligands (L), [(L)M(BL)M(L)] (where M = Ru, Os; L = bis(Nmethylbenzimidazolyl)pyridine, -benzene; BL= tetrapyridylpyrazine (tppz), -benzene (tpb)), were synthesized, and their mixed-valencestate characteristics were investigated. All of the complexes showed successive one-electron redox processes, each of which correspond to M(II/III) (M = Ru, Os) or ligand reduction waves. In addition, an M(III/IV) couple was observed in cyclometalated [M2(bis(benzimidazolyl)benzene)2(BL)] complexes (M = Ru, Os). Effects of the cyclometalated bonds on the redox behaviors and the accessibility to the mixed-valence M(II)−M(III) dinuclear complexes are discussed. Introduction of a cyclometalated bond induced a large negative potential shift in the redox potentials of dinuclear ruthenium and osmium complexes, depending on either bridging or ancillary sites of the cyclometalated bonds: the change falls within the range of −1.0 to −1.2 V for the bridging sites and −0.65 to −0.7 V for the ancillary ones. This large negative potential shift arises from the strong electron-donating property of the phenyl anion in a metal−C bond. Replacing the ruthenium by osmium in the dinuclear complexes with the same bridging ligand results in an increase of the potential separation (ΔE(1)) and the comproportionation constant (Kcom) of the mixed-valence complexes having the tppz bridging ligand (ΔE(1) and Kcom values: Os > Ru); however, complexes having the tpb bridging ligand showed the opposite trend (ΔE(1) and Kcom: Os < Ru). In addition to the results of EPR and DFT calculation, it was found that the orbital energy levels of the central metal ion (namely, either Ru or Os) in the mixed-valence complex determines the degree of orbital mixing between metal dπ orbitals and bridging-ligand π or π* orbitals, which leads to either hole- or electron-transfer exchange mechanisms.

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INTRODUCTION

ruthenium (Ru) centers by varying the electron-donating or -accepting ability of bridging ligands or the length of conjugated groups such as polyacetylene (−CC−) or polyphenylene(−(Ph)n−).6,11 As a BL, monodentate, bidentate, and tridentate ligands have been studied. For these ligands, a bis(tridentate) coordination environment is attractive because diastereomers or enantiomers do not exist in the case where a symmetric tridentate ligand is used.14 Relevant to the Creutz−Taube complex, the pyrazine derivative 2,3,5,6-tetrakis(2-pyridyl)-

Mixed-valence dinuclear metal complexes having organic bridging ligands have been extensively studied from the viewpoint of potential candidates for applications in the field of molecular electronic devices such as molecular wires and logic gates.1−11 The degree of metal−metal interaction between metal ions is strongly dependent on the nature of the bridging ligands as well as the ancillary ligands.1,12 Since the pyrazinebridged mixed-valence complex [(NH3)5Ru(pyz)Ru(NH3)5]5+ was synthesized for the first time by Creutz and Taube,13 many diruthenium [(L)Ru(BL)Ru(L)] systems (where BL = bridging ligand, L = ancillary ligand) have been synthesized and studied in terms of the tuning of metal−metal interactions between two © XXXX American Chemical Society

Special Issue: Organometallic Electrochemistry Received: February 10, 2014

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Scheme 1. Chemical Structures of Complexes and Their Abbreviations

Figure 1. ORTEP drawings (50% probability level) of (a) side and (b) top views of the cationic moiety and (c) the Ru-tppz-Ru core in Ru14+.

coplanar conformation of five aromatic rings has been observed in terms of the steric repulsion of two pyridyl groups from the X-ray structures of [(L)Ru(tppz)Ru(L)], dinuclear Ru-tppz complexes are stable enough to allow their mixed-valence-state properties to be studied by electrochemical techniques.16,22,24,27,28 As a new family of tridentate bridging ligands, cyclometalated bridging ligands, which form a covalent

pyrazine (tppz) has been used as a bridging ligand to connect two metal ions. Extensive studies on ruthenium dinuclear complexes (having the formula [(L)Ru(tppz)Ru(L)] with different ancillary ligands have been reported.15−31 Recently, dyad ruthenium-based complexes bearing tppz bridging ligands have been applied to light-driven water oxidation catalysis.32 Even though the significant deformation of tppz from the B

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Table 1. Selected Bond Lengths (Å) for Ru14+(PF6−)4

carbon−metal bond, have attracted a great deal of interest.33−38 Dinuclear ruthenium complexes having a 1,2,4,5-tetrakis(2pyridyl)benzene dianion (tpb), 1,4-diethynylphenylene, and a 4,4′-biphenyl dianion are known to contain a cyclometalated bridging ligand and to increase the charge delocalization through redox-noninnocent character in the cyclometalated C− M bonds in the dinuclear complexes.35,37 We are interested in redox potential tuning of ruthenium and osmium complexes because the combination of two complexes with a potential gradient has the potential as a building unit to fabricate a hetero junction toward molecular rectification.10 In the present study, to elucidate how the introduction of cyclometalated bonds into the bridging or ancillary sites affects their redox potentials, dinuclear ruthenium and osmium complexes having a tppz or tpb bridging ligand have been synthesized and their electrochemical properties have been investigated. A comparison of the two series of complexes in Scheme 1, namely, Rua−Ruc and M1−M3 (M = Ru, Os), will provide new insight into the design of molecular wires based on highly coupled mixedvalence dinuclear metal complexes.

Ru1−N1 Ru1−N2 Ru1−N3 Ru1−N7 Ru1−N8 Ru1−N9

2.056(3) 1.960(3) 2.064(3) 2.058(3) 2.018(3) 2.070(3)

Ru2−N4 Ru2−N5 Ru2−N6 Ru2−N12 Ru2−N13 Ru2−N14

2.061(3) 1.951(3) 2.048(3) 2.074(3) 2.024(3) 2.073(3)

C(18)−C(17) −23.3(7)° (see Figure 1c). The intramolecular Ru−Ru distance is 6.597 Å, which is comparable to distances of reported tppz-bridged complexes such as cis-[Cl(dmbpy)Ru(tppz)Ru(dmbpy)Cl]2+ (6.558 Å),16 mixed-valent [Cl3Ru(tppz)RuCl3]− (6.504 Å),24 and [Ru2(1-(di-p-tolylamino)-3,5di(2-pyridyl)benzene)2(tppz)] (6.74 Å).31 The Ru−N(pz) distances (1.960(3) Å for Ru(1)−N(2) and 1.951(3) Å for Ru(2)−N(5)) are shorter than the trans-positioned Ru−N(py) distances with regard to bridged pyrazine in tppz (2.018(3) Å for Ru(1)−N(8) and 2.024(3) Å for Ru(2)−N(13)), which is attributed to a large contribution of dπ(Ru)−π*(pz) backbonding.16,24,28 Electrochemistry. The newly prepared dinuclear complexes showed rich electrochemistry. The cyclic voltammograms of all Ru/Os complexes are shown in Figure 2, and the electrochemical data are summarized in Table 2.

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RESULTS AND DISCUSSION Syntheses and Characterizations. Two synthetic routes, depending on whether the bridging ligand is tppz or tpb, were used to prepare new dinuclear ruthenium/osmium complexes. For the tppz ligand, Ru14+ and Ru22+ were synthesized by the reaction of [Cl2(EtOH)M(tppz)MCl3] (M = Ru, Os) in ethylene glycol or glycerol with an ancillary L ligand. An appropriate 1/2 stoichiometric ratio of the starting tppz complex and L in the presence of AgOTf, where L is bis(Nmethylbenzimidazolyl)pyridine (Mebip) or bis(Nmethylbenzimidazolyl)benzene (Mebib), was used. For the preparation of the dinuclear complexes, the reaction of [Ru(L)Cl3] with tppz under microwave-assisted irradiation gave many byproducts, and their purification was a laborious task. On the other hand, for the tpb bridging ligand, the microwave-assisted reaction of [Ru(Mebip)Cl2(CH3CN)] with tpb at a ratio of 2:1 proceeded smoothly without formation of byproducts, clarified by 1H NMR of the as-prepared product. The complexes were purified by Sephadex LH-20 gel-filtration chromatography using a 2/1 mixture of acetonitrile and methanol as an eluent. In the case of the tpb bridging system, the oxidized form of dinuclear complexes or mixed-valence Ru(II)−Ru(III) and Os(III)−Os(III) complexes was obtained during column chromatography under ambient conditions. The M(II)−M(II) state of the complexes (M = Ru, Os) was obtained by adding reducing agents such as Na2S2O4 and NH2NH2·H2O. X-ray Structure of Ru14+(PF6−)4. A single crystal of Ru14+(PF6−)4 was obtained by recrystallization of the complex from a water/acetonitrile mixture (1/1 v/v). ORTEP drawings of Ru14+ are shown in Figure 1. Crystal data and selected bond lengths are given in Table S1 (Supporting Information) and Table 1, respectively. Each ruthenium center possesses a distorted-octahedral geometry and is surrounded by tridentate tppz and Mebip ligands. The complexes are significantly distorted by twisting of the tppz bridging ligand in comparison to the planarity, although similar distortion of the structures in tppz-bridged metal complexes has been reported.16,22,24,27,28 The twisted pyrazine ring with alternate upward and downward displacements of the pyridyl groups in the tppz-bridged ligand is clear at torsion angles of Cpyr−Cpyz−Cpyz−Cpyr bonds: i.e., C(8)−C(7)−C(19)−C(20) −29.7(7)° and C(5)−C(6)−

Figure 2. Cyclic voltammograms of Ru/Os complexes in CH3CN/0.1 M TBAPF6 at room temperature. Working and counter electrodes are glassy carbon and platinum, respectively.

The cyclic voltammogram of Ru14+ shows two well-defined one-electron oxidation waves at 0.84 and 1.06 V vs Fc+/Fc and two one-electron reduction waves at −0.67 and −1.21 V vs Fc+/Fc. In consideration of the redox behavior of similar tppzbridged ruthenium dinuclear complexes reported previously,15−19,23,27,29,31 the oxidation waves correspond to Ru(II)−Ru(II)/Ru(II)−Ru(III) and Ru(II)−Ru(III)/Ru(III)− Ru(III) processes, respectively, and the reduction waves are tppz/tppz− and tppz−/tppz2− processes. For two successive one-electron-reduction processes on the bridging tppz ligand due to a strong π acidity, it has ben pointed out that the potential separation between the two waves for the ligand reflects the rough measure of the spin-pairing energy in the C

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Table 2. Electrochemical Data for Ru1−3n+ and Os1−3n+ Complexes along with the Relevant Pyridine Derivatives Ruan+− Rucn+ a E1/2/V vs Fc+/Fc (ΔEp/mV) ligand reduction complex 4+

Ru1 Os14+ Ru22+ Os22+ Ru33+ Os34+ Rua4+ c Rub2+ c Ruc2+ c

tppz−/2−

Mebip0/− −1.78 (60) −1.79 (57)b b

−1.21 −1.18 −1.61 −1.60

(58) (61) (60) (59)

tppz0/− −0.67 −0.70 −1.07 −1.09

(56) (62) (53) (59)

−1.82 (69) −1.76 (70) −1.25 (60) −1.61 (60)

−0.76 (60) −1.14 (60)

MII−MII/MIII−MII 0.84 0.49 0.16 −0.12 −0.40 −0.53 1.03 0.21 −0.30

(60) (65) (59) (59) (60) (56) (75) (55) (−)

MIII−MII/MIII−MIII 1.06 0.84 0.46 0.33 0.07 −0.19 1.35 0.49 0.13

(69) (65) (65) (70) (60) (54) (85) (60) (−)

MIII−MIII/MIV−MIII

1.27 0.97 1.12 0.73

(irr) (irr) (71) (62)

MIV−MIII/MIV−MIV

1.44 (83) 1.11 (91)

a Data collected in CH3CN (0.1 M TBAPF6). bWhen the potential was scanned negatively up to −2.1 V, another reduction process was observed at around −1.95 V. However, adsorption of the reduced complex took place, which was accompanied by a desorption peak at around −1.6 V. c Reference 18.

LUMO.22,39 The potential separations between the first and second reduction potentials are 0.54, 0.54, 0.48, and 0.51 V for the present complexes, Ru14+, Os14+, Ru22+, and Os22+, respectively, which is close to the reported values for Rua4+ (0.49 V), Rub2+ (0.47 V), and [Cl(L)Ru(tppz)Ru(L)Cl] (L = β-diketonato ligand) (0.53−0.56 V).28 The reduction processes of ancillary Mebip ligands for Ru14+, Os14+, Ru33+, and Os34+ occurred at −1.78, −1.79, −1.82, and −1.76 V vs Fc+/Fc, respectively, shown in Figure 2. The reduction processes for Ru14+ and Os14+ were associated with the strong adsorption of the reduced species, resulting in a large desorption peak for the reverse scan (see CV in Figure S21 in the Supporting Information). The corresponding osmium analogue, Os14+, exhibits four similar waves within the experimental potential range from +1.5 to −2.2 V vs Fc+/Fc, except the two oxidation processes of Os(II)−Os(II)/Os(II)−Os(III) and Os(II)−Os(III)/Os(III)− Os(III) are negatively shifted by 0.35 and 0.22 V, respectively, in comparison to those of Ru14+. On the other hand, the two reduction processes of Os14+ stay almost unchanged within ±0.03 V. When the cyclometalated bond was introduced into the ancillary ligand by using Mebib, the oxidation potentials of Ru22+ appeared at +0.16 and +0.46 V. Similarly, two oneelectron-reduction processes of Ru22+ occurred on tppz at potentials of −1.07 and −1.61 V. Both the oxidation and reduction potentials of Ru22+ revealed large negative shifts in comparison to those of Ru14+, while the potential shifts (∼0.6 V) for the oxidations are larger than those for the reductions (∼0.4 V). These shifts are clearly attributable to the significant increase in electron density on ruthenium centers owing to the strong electron-donating ability of the cyclometalated Mebib ligands. Recently, the redox chemistry of the analogous [(L)Ru(tppz)Ru(L)]n+ (where L = 2,2′:2″,6-terpyridine (tpy, N∧N∧N coordination mode), 2,6-bis(2′-pyridyl)phenyl (bpp, N∧C∧N coordination mode)), namely, Rua4+ and Rub2+ in Scheme 1, was reported.18 The only difference between the benzimidazole and pyridine groups are the ancillary ligands in the Ru complexes. While the Rua4+ and Rub2+ complexes showed redox behaviors similar to those of the Ru14+ and Ru22+ complexes, the oxidation potentials in Ru14+ are negatively shifted by almost 0.2 V in comparison to those of the complex Rua4+. However, the difference between the oxidation potentials of Ru22+ and Rub2+ is small (∼0.05 V), which indicates that peripheral substituents such as benzimi-

zolyl and pyridyl groups in the ancillary ligand showed little effect on the ruthenium ions in the dinuclear cyclometalatedruthenium system. This result is supported by DFT calculations described later. Figure 3 showes the rotating-disk voltammo-

Figure 3. Rotating -disk voltammograms of Ru14+ (blue solid line), Ru33+ (red solid line), and Os34+(red dotted line) complexes in CH3CN/0.1 M TBAPF6 at room temperature. The working and counter electrodes are glassy carbon and platinum, respectively.

gram of Ru33+ along with those of Ru14+ and Os34+. One cathodic and three anodic waves are observed for Ru33+ in the potential region of −0.5 to +1.5 V vs Fc/Fc+. This observation is in sharp contrast to the two anodic waves observed for Ru14+ (Figure 3). Further, the cyclic voltammogram of Ru33+ shows three well-defined one-electron-oxidation waves at +0.07, +1.12, and +1.44 V vs Fc+/Fc and two one-electron reductions at −0.40 and −1.82 V vs Fc/Fc+. In consideration of the reported potentials,35 the first oxidation (at +0.07 V) and the first reduction (at −0.40 V) are assigned to Ru(II)−Ru(III)/ Ru(III)−Ru(III) and Ru(II)−Ru(II)/Ru(II)−Ru(III) couples, respectively, and the Ru33+ complex exists as a mixed-valence Ru(II)−Ru(III) state. The two further oxidations at +1.12 and +1.44 V correspond to the Ru(III)−Ru(III)/Ru(III)−Ru(IV) and Ru(III)−Ru(IV)/Ru(IV)−Ru(IV) processes, respectively. On the other hand, the reduction at −1.82 V may be attributable to the reduction of one of the Mebip ligands. In the case of the Os34+ complex, the rotating-disk voltammogram revealed two cathodic waves at −0.53 and −0.19 V and two anodic waves at +0.73 and +1.11 V in the potential region of −0.5 to +1.5 V. An additional cathodic wave corresponding to the reduction of the Mebip ligand was D

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observed at −1.76 V. The isolated Os34+ complex therefore possesses an Os(III)−Os(III) state, which is consistent with the chemical formula of [Os34+](PF6−)4 obtained from the result of elemental analysis. The stability of the mixed-valence complexes can be generally estimated from the separation of the electrochemical potentials, ΔE, for E(1) (M(II)−M(III)/M(III)−M(III)) and E(2) (M(II)−M(II)/M(II)−M(III)) couples (M = Ru, Os) and the comproportionation constant Kcom, which can be obtained from the equation Kcom

Table 4. UV−Vis Absorption Spectra with Different Oxidation States Obtained by Spectroelectrochemistry in CH3CN with 0.1 MTBAPF6 as the Supporting Electrolyte complex Ru1

n+

6+ Os1n+

6+

Table 3. Potential Separation ΔE and Values of the Comproportionation Constant Kcom complex Ru14+ Os14+ Ru22+ Os22+ Ru33+ Os34+ Rua4+ Rub2+ Ruc2+

0.22 0.35 0.30 0.45 0.47 0.34 0.32 0.28 0.43

Kcom(1) 3.7 8.3 1.2 4.1 8.9 8.3 2.6 5.4 1.9

× × × × × × × × ×

103 105 105 107 107 105 105 104 107

4+ 5+

Table 3 gives the ΔE and Kcom values for the investigated ruthenium and osmium complexes. Comparing the potential

ΔE(2)/V (=E4 − E3)

4+ 5+

⎛ nF ΔE1/2 ⎞ = exp⎜ ⎟ ⎝ RT ⎠

ΔE(1)/V (=E2 − E1)

n

a

Ru2n+

2+ 3+

Kcom(2)

4+ Os2n+

2+ 3+

0.32 0.38

2.6 × 10 2.7 × 106 5

4+ Ru3n+

separations (ΔE(1)) of the ruthenium and osmium complexes makes it possible to classify the complexes into two types, depending on the bridging ligand in the dinuclear complexes; namely, one type is M = Os > Ru for M1 and M2, and the other type is M = Os < Ru for M3. Interestingly, the trend of potential separation ΔE(2) for M3 is reverse: i.e., M = Os > Ru. From the electrochemical measurements, the presence of cyclometalated bond(s) in the bridging ligand induced a large negative shift in the oxidation potentials, which makes it possible to isolate the mixed-valence complex of Ru3. When the metal ion M was changed from M = Ru to M = Os in the dinuclear complex [(L)M(BL)M(L)], the values of peak separation ΔE(1) upon metal oxidation processes increased for the tppz bridging system but decreased for the tpb bridging system. UV−vis Absorption Spectra and Spectroelectrochemistry. UV−vis absorption spectral data are given in Table 4. For the tppz complexes of Ru14+, Os14+, Ru22+, and Os22+, the MLCT bands appear in the visible region of 400−600 nm, which represents features characteristic of this type of Ru(II) or Os(II) polypyridine complex in addition to intraligand ππ* transitions of tppz and bis(benzimidazolyl)pyridine around 310−350 nm. For the tpb complexes, the isolated states of Ru33+ and Os34+ are mixed-valent Ru(II)−Ru(III) and homovalent Os(III)−Os(III), respectively. Broad bands with several shoulders were therefore observed. To shed light on the electronic structure of the complexes with different oxidation states, particularly the mixed-valence state, spectroelectrochemical experiments were performed for all of the present complexes to obtain the spectral change by applying the potentials. Since well-separated one-electron-redox processes occurred in the M(II)−M(II), M(II)−M(III), and

Os3n+

2+

formal oxidation statea Ru(II)− Ru(II) Ru(II)− Ru(III) Ru(III)− Ru(III) Os(II)− Os(II) Os(II)− Os(III) Os(III)− Os(III) Ru(II)− Ru(II) Ru(II)− Ru(III) Ru(III)− Ru(III) Os(II)− Os(II) Os(II)− Os(III) Os(III)− Os(III) Ru(II)− Ru(II)

3+

Ru(II)− Ru(III)

4+

Ru(III)− Ru(III) Os(II)− Os(II)

2+ 3+ 4+

Os(II)− Os(III) Os(III)− Os(III)

λmax/nm (ε/103 M−1 cm−1) 314 (65.4), 352 (73.4), 383 (30.7), 410 (29.1) , 573 (38.3) 314 (71.9), 348 (62.3), 548 (17.8), 2924 (0.96)b 320 (68.5), 363 (53.0), 467 (7.0), 793 (5.9), 1857 (0.84) 316 (81.9), 357 (93.0), 434 (31.0), 572 (32.9) , 800 (11.7) 316 (94.9), 357 (82.2), 547 (30.7), 895 (6.2) 316 (100.3), 359 (79.3), 495 (23.9), 655 (16.9) 305 (113.2), 365 (31.6), 396 (26.2), 449 (20.9), 641 (38.6), 941 (2.4) 385 (42.7), 618 (25.2), 797 (4.3), 1520 (1.36) 394 (49.2), 500 (13.7), 606 (10.1), 894 (3.8) 299 (79.3), 360 (35.1), 458 (31.4), 618 (38.8) , 859 (8.58), 1315 (0.43), 1567 (0.63) 378 (35.6), 414 (32.0), 580 (27.6), 1276 (0.32), 1936 (0.19) 392 (42.9), 553 (17.2), 736 (6.5), 1936 (0.57) 308 (76.4), 318 (92.8), 351 (47.4), 388 (24.6) , 408 (26.2), 484 (29.1), 527 (25.2), 616 (18.2), 688 (17.4), 856 (10.4) 317 (90.3), 344 (71.2), 354 (81.1), 443 (30.0) , 537 (16.2), 628 (10.3), 667 (8.9), 976 (13.7), 1141 (25.3), 1586 (3.2) 320 (86.3), 332 (40.7), 356 (65.5), 526 (38.4) , 620 (17.7) 303 (57.9), 335 (33.4), 349 (39.6), 409 (15.0), 492 (24.4), 574 (14.6), 720(10.7), 992(3.5) 313(49.5), 333(33.1), 354(40.3), 464(19.7), 575(12.7), 723(10.4), 968(6.2), 2038 (2.24) 307 (47.2), 316 (46.8), 352 (44.0), 404 (18.3) , 531 (22.4), 567 (20.0), 1789 (1.29)

a

Boldface type indicates the original state of the complex in air. bThe value calculated from the Gaussian band shape.

M(III)−M(III) states, clear spectral changes were obtained for all the complexes. The resulting spectroelectrochemical data are summarized in Table 4, along with spectral data for the isolated state. Oxidative spectroelectrochemistry of Ru14+ revealed that the first oxidation at +0.95 V led to the diminishing intensity of the MLCT band at 573 nm and, at the same time, the appearance of a broad intervalence charge-transfer (IVCT) band envelope at around 2500 nm (Figure 4). The maximum wavelength and half-bandwidth of the IVCT band were obtained by the deconvolution of the high-energy side of the band envelope, assuming a Gaussian band shape. During the first oxidation, isosbestic points at 361 and 548 nm appeared. Further oxidation under spectroelectrochemical conditions at +1.3 V resulted in complete loss of this MLCT band at 573 nm and a new appearance of an LMCT band at 793 nm in addition to a decrease of the ππ* band at 352 nm. One-electron oxidation of Os14+ at +0.6 V led to the disappearance of the 3MLCT absorption band at 800 nm and a shift of absorption maximum from 572 to 547 nm. E

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Figure 4. UV−vis spectroelectrochemistry of Ru14+ in CH3CN/0.1 M TBAPF6 at +0.95 V (a) and +1.30 V (b) at room temperature and NIR spectra of first-oxidized Ru15+ (red line) and second-oxidized Ru16+ states (blue line) (c).

Figure 5. Temporal UV−vis−NIR spectral changes of Ru33+ at applied potentials of −0.6 V vs Fc/Fc+ (a) and +0.6 V (b) during thin-layer spectroelectrochemistry in CH3CN/0.1 M TBAPF6 at room temperature.

Furthermore, the ππ* band decreased at 357 nm, and a new ππ* band increased at 316 nm. In addition, new broad bands appeared at 1500, 2100, and 2500 nm in the near-infrared region. The second one-electron oxidation at 1.0 V resulted in the MLCT band at 547 nm diminishing and several broad peaks appearing at 495 and 655 nm. In addition, upon the second oxidation, the two bands at 1500 and 2100 nm in the near-infrared region increased, but the band around 2500 nm decreased, indicating that the latter band can be assigned as an IVCT band. Spectroelectrochemical features of Ru22+ and Os22+ are similar to those of Ru14+ and Os14+, except for the longer wavelength shift of MLCT bands. Typical spectra of Ru33+ are shown in Figure 5. Since the isolated Ru33+ possesses a mixed-valent Ru(II)−Ru(III) state, the first reduction of Ru33+ at −0.6 V generated the Ru(II)− Ru(II) state. After one-electron reduction, the relatively strong NIR bands at 976, 1141, and 1586 nm disappear, and visible absorption bands appear at 688 and 856 nm (as shown in Figure 5). The NIR spectral feature of the original Ru33+ shown in Figure 6 is similar to that of the relevant complex, Ruc3+,35 in which multiple NIR transitions were observed. The NIR spectra of Ru33+ can be deconvoluted into a sum of three Gaussian-shaped bands, in which two of three bands revealed remarkably narrow half-height bandwidth, Δν1/2. The NIR

Figure 6. Deconvolution of multiple IVCT bands of Ru33+.

charge-transfer band with the highest intensity revealed almost the same wavelengths for both Ru33+ (1141 nm) and Ruc3+ (1147 nm), which can be assigned as the charge-transfer transitions from the ruthenium centers to the bridging phenyl ring. The strong NIR transition is indicative of greater charge delocalization in Ru33+, which falls into the Robin-Day class III category. In addition, the visible spectral features of the oneelectron-reduced Ru33+ consists of many partially overlapped bands in comparison to the spectra of tppz-bridged dinuclear Ru(II)−Ru(II) complexes such as Ru14+ and Ru22+. The F

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Figure 7. UV−vis−NIR spectroelectrochemistry of Os34+ in CH3CN/0.1 M TBAPF6 at room temperature: (blue line) original spectrum; (red line) spectrum at −0.3 V vs Fc/Fc+; (black line) spectrum at −0.7 V vs Fc/Fc+.

Effect of Bridging Ligands and Metal Ions on the Metal−Metal Interaction. The peak separation ΔE is often used as a measure of the extent of metal−metal interaction. The ΔE values for the ruthenium complexes increased in the order Ru14+ < Ru22+ < Ru33+; however, for the osmium complexes, the trend changed in the order Os34+ ≤ Os14+ < Os22+. This trend in ΔE values of metal ions (namely, ruthenium and osmium) derives from the change in the degree of orbital mixing between ruthenium and osmium dπ orbitals and bridging ligand π/π* orbitals of tppz or tpb; in other words, the ΔE value for the ruthenium complex with bridging tpb, i.e., Ru33+, is larger than that with the tppz complex, i.e., Ru14+. On the other hand, in the case of the osmium complexes, the ΔE value for tpb-bridged Os34+ is slightly smaller than that for tppz-bridged Os14+. When the central metal ions in a dinuclear complex having the tppz bridging ligand are changed from ruthenium to osmium, the ΔE value for Os14+ is larger than those for Ru14+; however, in the case of the complex having tpb-bridged complexes, the ΔE value for the Ru33+ complex is larger than that for the Os34+ complex. This kind of changeover of peak separation ΔE by means of a bridging ligand on dinuclear complexes of ruthenium and osmium has been reported4,34,42,43 and interpreted in terms of the change in the degree of interactions between metal dπ orbitals and bridging π/π* orbitals from the viewpoint of two electronic coupling mechanisms: namely, electron transfer and hole transfer superexchange (Scheme 2). In consideration of the two extreme cases for the energy level diagram in Scheme 2,

present spectral features arise from the existence of closely located LUMO, LUMO+1, and LUMO+2 orbital levels due to the strong mixing of ruthenium dπ orbitals and cyclometalated N∧C∧N ligands. The one-electron oxidation of Ru33+ at +0.6 V caused the disappearance of the multiple NIR bands at around 1141 nm. Since the isolated Os34+ has a Os(III)−Os(III) state, two successive one-electron reductions provide Os(II)−Os(III) and then Os(II)−Os(II) states. Figure 7 shows the spectral changes accompanied by the reductions. After the first one-electron reduction of Os34+ at −0.35 V, in addition to the MLCT bands at 575 and 723 nm in the visible region, multiple NIR bands appear at 968 and 2038 nm. Further reduction at −0.7 V led to the disappearance of the NIR band at 2038 nm and the MLCT bands appear at 492, 574, and 992 nm. EPR Spectra. EPR spectroscopy can provide useful information on the spin delocalization of metal complexes.40 Complex Ru15+, generated by one-electron oxidation of Ru14+ with cerium ammonium nitrate, exhibits an axial EPR signal at 77 K in frozen CH3CN (Figure 8). The electron g factor g1, g2,

Scheme 2. Energy-Level Diagrams for Two Different Interactions of Metal (M) dπ Orbitals and Bridging-Ligand (BL) π/π* Orbitalsa

Figure 8. EPR spectra of Ru15+ and Ru33+ in CH3CN at 77 K.

and g3 values are 2.347, 2.347, and 1.800, respectively. The anisotropy Δg (=g1 − g3) equals 0.547. The isotropic g factor ⟨g⟩ (=[(g12 + g22 + g32)/3]1/2) is calculated to be 2.180. In contrast, complex Ru33+ has a rhombic EPR signal at 77 K. The g1, g2, and g3 values are 2.346, 2.136, and 1.943, respectively. The Δg and ⟨g⟩ values of Ru33+ are 0.403 and 2.148, respectively. The smaller g anisotropy and isotropic g factor of Ru33+ with respect to those of Ru15+ suggest some amount of spin delocalization on the bis-cyclometalating bridging ligand in Ru33+. No distinct EPR signals were recorded for complex Ru2 and the other three Os complexes. The EPR silence of mixedvalent osmium complexes is well-known because of their strong spin−orbital coupling.41

a

The electronic coupling takes place via (a) an electron transfer mechanism or (b) a hole transfer mechanism.

G

dx.doi.org/10.1021/om500142t | Organometallics XXXX, XXX, XXX−XXX

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HOMO and HOMO-1 of Ru14+ and Ru22+ are localized on ruthenium dπ orbitals and terminal Mebip or Mebib π orbitals. In particular, for Ru22+ the HOMO/HOMO-1 are localized on both ruthenium dπ and the π orbitals in cyclometalated central benzene moieties. As a result, the HOMO/HOMO-1 energy levels of Ru22+ rose significantly in comparison to the orbital energy levels of Ru14+. The LUMO and LUMO+1 of Ru14+ and Ru22+ are composed of the combination of ruthenium dπ orbitals with bridging tppz π* orbitals. Therefore, the metal− metal interaction occurs mainly through an electron transfer mechanism, as shown in Scheme 2a. On the other hand, the HOMO and HOMO-1 of Ru32+ are mainly composed of the combination of ruthenium dπ orbitals with cyclometalated bridging ligand π orbitals, which suggests that the metal−metal interaction takes place through a hole-transfer mechanism using the HOMO (see Scheme 2b). The LUMO+1 of Ru32+ reveals the orbital contribution of the ruthenium dπ orbitals and terminal Mebip π orbitals and no orbital distribution on the cyclometalated bridging tpb moiety, while the LUMO shows the feature of the combination of ruthenium dπ orbitals with cyclometalated bridging ligand π orbitals almost the same as that of Ru14+ and Ru22+. Therefore, the sharp contrast in HOMO and LUMO+1 between the tppz-bridged Ru14+ and Ru22+ and the tpb-bridged Ru32+ leads to different orbital mixing, which was clearly seen in the results of DFT calculations (see Figure 9). The ruthenium 4dπ orbitals thus interacted strongly with the tpb π orbitals; on the other hand, the osmium 5dπ orbitals strongly mixed with the tppz π*orbitals, thereby determining the degree of strength of the metal−metal interaction. For the dinuclear ruthenium complexes bearing a tppz or tpb bridging ligand, a characteristic IVCT band(s) was observed in the NIR region. On the other hand, the NIR bands for the corresponding osmium complexes revealed relatively complicated spectral changes during the electrolysis because of the overlap among LMCT, 3MLCT, and dπdπ transitions in the IVCT band. The IVCT band for the osmium complexes (except Os33+) therefore could not be assigned. From an analysis of the IVCT band in the ruthenium complexes, the half-width Δν1/2 (at 300 K) and the degree of electronic coupling between two metal centers HAB were obtained from the equations

the mixing between metal dπ(M1 and M2) and bridging ligand π* is ascendant for the Os 5dπ level in comparison to the Ru 4dπ level (Scheme 2a); on the other hand, the mixing between metal dπ(M1 and M2) and bridging ligand π is dominant for the Ru 4dπ level (Scheme 2b). Consequently, the change in the degree of orbital mixing is responsible for the changeover of peak separation ΔE by the bridging ligand on the dinuclear ruthenium and osmium complexes. To shed light on the metal−metal interaction, DFT calculations were carried out using a BLYP functional with the SDD(M) basis set with the effective core potential and the 6-31G* basis set for C, H, and N atoms with the Gaussian 03 program package. Figure 9

Figure 9. Orbital-energy diagrams and molecular orbitals of Ru14+, Ru22+, and Ru32+ in CH3CN. The HOMO−LUMO gap is shown by an arrow and is given in eV.

shows energy diagrams of three dinuclear ruthenium complexes in acetonitrile with the Ru(II)−Ru(II) oxidation state and the feature of each molecular orbital HOMO-1, HOMO, LUMO, and LUMO-1, respectively. The HOMO and LUMO energy levels become higher in the order Ru14+ < Ru22+ < Ru32+, which is consistent with experimentally measured redox potentials. The calculated values of HOMO energy in acetonitrile show a good linear relationship with the oxidation potential for the Ru(II)−Ru(II)/Ru(II)−Ru(III) process (see Table 2 and Figure S22 in the Supporting Information). The

Δν1/2 = (2310νmax )1/2 ⎡ εmax Δν1/2 ⎤1/2 ⎡ νmax ⎤ HAB = 2.05 × 10−2⎢ ⎥ ⎢ ⎥ ⎣ νmax ⎦ ⎣ r ⎦

Table 5. Parameters Obtained from the IVCT Band for the Mixed-Valence M(II)−M(III) Complexes complex

νmax/cm−1 (λmax/nm)

Ru15+ Os15+ Ru23+ Os23+ Ru33+

3420 (2924) a 6580 (1520) a 10363 (965) 8764 (1141) 6305 (1586) 4907 (2038)

Os33+ a

εmax/M−1 cm−1

Δν1/2(obsd)/cm−1

Δν1/2(calcd)/cm−1

r/Å

HAB/cm−1 (HAB(delocalized)b/cm−1)

960

3830

2810

6.60

350

1360

1313

3898

6.60

338

11 418 22 620 3200 2240

1875 1073 3342 2203

4892 4499 3703 3298

6.60 6.60 6.60 6.84

1470 (5181)b 1440 (4382)b 774 (3152)b 469

The IVCT band could not be assigned. bHab(delocalized) was calculated from the equation Hab = 1/2νmax. H

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Scheme 3. Chemical Structures of Three Bis-Cyclometalating Bridging Ligands in Dinuclear Ruthenium Complexes

where εmax is the extinction coefficient of the IT band (in M−1 cm−1), Δν1/2 is the bandwidth at half-height, νmax is the absorption maximum (cm−1), and r is the distance between metal centers. The parameters for the degree of electronic coupling are summarized in Table 5. The HAB values were calculated under the assumption that Hush formulas can be used for localized mixed-valence systems. For Ru33+ in Table 5, the HAB values for a fully delocalized case are given in parentheses. The Hab values for ruthenium complexes increased in the order Ru15+ ≈ Ru23+ < Ru33+, which reflects the degree of metal−metal interaction. The mixed-valence tpb-bridged Ru33+ exhibited multiple NIR bands, and similar spectral features in the bis-cyclometalated bridged complex Ruc3+ 35 and [Cl3Ru(tppz)RuCl3]− 24 have been reported. Complex 33+ has been classified as a Robin−Day class III system by the redox-noninnocent nature of the bridging tpb ligand35 and [Cl3Ru(tppz)RuCl3]− as a Robin−Day class II−III.24 For Ru33+, the bandwidths at halfheight of the IVCT band are narrower than the theoretical value; therefore, Ru33+ falls into a Robin−Day class II−III system. Beley et al.,34 Yao et al.,44 and others45,46 reported the synthesis and electronic coupling within mixed-valence dinuclear ruthenium and osmium complexes bridged by biscyclometalating ligands of dpb and tppyr, as shown in Scheme 3. One of the common characteristics of mixed-valence ruthenium complexes is the presence of an IVCT band with large molar absorption coefficient (ε of (2.5−3.4) × 104 M−1 cm−1) in the near-infrared region. Since the present IVCT bands can be mainly assigned to the transitions between ruthenium dπ orbitals and cyclometalated bridged ligand π* orbitals, the common structural feature for Ru(II)−(biscyclometalated BL)−Ru(III) will provide a new guideline for designing new NIR markers for mixed-valence molecular devices. Effect of Ancillary Ligands on the Metal−Metal Interaction. In comparison to the electrochemical properties of dinuclear ruthenium analogues Ruan+−Rucn+ with terpyridine as terminal ligands (see Scheme 1), the oxidation potentials of Ru1n+−Ru3n+ were shifted to negative potential (0.2−0.3 V). This negative potential shift results from the stronger σ-donating and weaker π-accepting abilities of the benzimidazolyl groups on the terminal ligands in comparison to those of pyridine groups in the tpy ligand. However, once the cyclometalated Ru−C bonds were introduced into the dinuclear complexes, the potential difference between Ru22+ and Rub2+ or between Ru33+ and Ruc2+ became smaller (0.05− 0.1 V) (see Table 2). It should be noted that the introduction of cyclometalated Ru−C bonds leads to an increase in the potential differences ΔE(1) (=E2 − E1) of Ru22+ and Ru33+ in comparison to those of Rub2+ and Ruc2+ (see Scheme 1),18,35 suggesting that the Ru−Ru interaction becomes stronger in Ru22+ and Ru33+. Recently, a similar dependence of peak

separation ΔE on the electronic nature of the ancillary ligands in bis-cyclometalated ruthenium dinuclear complexes bridged by 3,3′,5,5′-tetrakis(N-methylbenzimidazol-2-yl)biphenyl was observed.37 Effect of Metal Oxidation State on Potential Difference, ΔE. Introducing a cyclometalated bond to dinuclear complexes affords access to higher oxidation states of M(III/ IV) in Ru33+ and Os34+, and a new mixed-valence M(III)− M(IV) state was obtained electrochemically. Of course, the formulation for M(III)−M(IV) is still a formal oxidation-state description: M(II)−(BL)−M(II)→M(II)−(BL)−M(III) + e−

E1

M(II)−(BL)−M(III)→M(III)−(BL)−M(III) + e−

E2

M(III)−(BL)−M(III)→M(III)−(BL)−M(IV) + e−

E3

M(III)−(BL)−M(IV)→M(IV)−(BL)−M(IV) + e−

E4

Here, the potential differences ΔE(1) (i.e., E2 − E1) and ΔE(2) (i.e., E4 − E3) provide the indicator of stability of mixedvalent M(II)−M(III) and M(III)−M(IV), respectively. It is interesting to note that ΔE(1) of Ru33+ is larger than that of Os34+; however, ΔE(2) of Ru33+ is slightly smaller than that of Os34+. Since the metal orbital energy levels are stabilized by the metal-centered oxidation, orbital mixing between dπ levels with the occupied tpb π levels is increased for Os34+ in comparison to Ru33+ in the case of the M(III)−M(IV) state. The degree of metal−metal interaction in the cyclometalated bridging tpb system therefore varied along with changing oxidation states from M(II/III) to M(III/IV). Several examples of mixedvalence Ru(III)−Ru(IV) species having strong σ-donating ligands such as acetylacetonate have been reported,47,48,40 although these Ru(III)−Ru(IV) species are still limited. Of course, further detailed study will be required to determine the charge configuration in the resonance forms of M(III)− (phenylene)−M(IV) and M(III)−(phenylene+)−M(III).

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CONCLUSION Novel Robin−Day class II−III mixed-valent dinuclear ruthenium or osmium complexes with cyclometalated bonds either in tridentate bridging (BL) or ancillary (L) ligands, [(L)M(BL)M(L)] (M = Ru, Os; L = bis(Nmethylbenzimidazolyl)pyridine, -benzene; BL = tetrapyridylpyrazine (tppz), -benzene (tpb)) were synthesized and examined by means of electrochemical measurements, EPR, and DFT calculations. The dinuclear complexes [(L)M(BL)M(L)] (M = Ru, Os) exhibited rich redox series. Introduction of a cyclometalated bond in dinuclear metal complexes induced a large negative potential shift, which made it possible to isolate the air-stable mixed-valent Ru(II)−Ru(III) complex Ru33+. Because of a strong electron delocalization through the 1,4phenylene dianion in two Ru−C bonds and strong orbital mixing between Ru dπ and ligand π orbitals, the Ru33+ complex I

dx.doi.org/10.1021/om500142t | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

[Ru22+](PF6−)2. To 10 mL of ethylene glycol were added [Cl2(EtOH)Ru(tppz)RuCl3] (50.4 mg, 62.0 μmol) and Mebib (50.0 mg, 0.148 mmol). The mixture was heated with microwave radiation (power 650 W) for 20 min. After the mixture was cooled to room temperature, 80 mL of water and an excess of KPF6 were added. The resulting precipitate was collected by filtration and washed with water. The obtained soild was subjected to gel-filtration chromatography on Sephadex LH-20 (eluent 2/1 MeCN/MeOH) to give 41.9 mg of [Ru22+](PF6−)2 (43%). 1H NMR (CD3CN, 500 MHz): δ 8.66 (d, 2H, J = 8.2 Hz), 8.59 (d, 2H, J = 7.8 Hz), 7.75 (t, 1H, J = 7.8 Hz), 7.67 (d, 2H, J = 7.4 Hz), 7.56 (t, 2H, J = 7.8 Hz), 7.52 (d, 2H, J = 8.1 Hz), 7.18 (t, 2H, J = 6.6 Hz), 7.08(m, 2H), 6.45 (d, 4H, J = 6.6 Hz), 4.44 (s, 6H). ESI-TOF-MS (CH3CN): m/z 633.13 [M − 2PF6]2+ (calcd for C68H48N14Ru2 633.12). Anal. Calcd for C68H50F12N14P2Ru2: C, 52.02; H, 3.10; N, 12.37. Found: C, 52.51; H, 3.24; N, 12.51. [Os22+](PF6−)2. To 10 mL of ethylene glycol were added [Cl2(EtOH)Os(tppz)OsCl3] (50.2 mg, 0.51 mmol) and Mebib (38.0 mg, 0.112 mmol). The mixture was heated with microwave radiation (power 650 W) for 30 min. After the mixture was cooled to room temperature, 100 mL of water and an excess of KPF6 were added. The resulting precipitate was collected by filtration and washed with water. The obtained solid was subjected to gel filtration chromatography on Sephadex LH-20 (eluent 2/1 MeCN/MeOH) to give 29.1 mg of [Os22+](PF6−)2 (33%). 1H NMR (CD3CN, 500 MHz): δ 8.73 (d, 2H, J = 8.0 Hz), 8.56 (d, 2H, J = 7.8 Hz), 7.69 (d, 2H, J = 5.7 Hz), 7.49 (m, 3H), 7.37 (t, 2H, J = 7.8 Hz), 7.06 (dt, 4H, J = 7.8 Hz, 6.6 Hz), 6.36 (t, 2H, J = 7.2 Hz), 6.20 (d, 2H, J = 8.0 Hz), 4.48 (s, 6H). ESI-TOF-MS (CH3CN): m/z 722.19 [M − 2PF6]2+ (calcd for C68H48N14Os2 722.18). Anal. Calcd for C68H50F12N14Os2P2: C, 46.78; H, 2.92; N 11.38. Found: C, 47.11; H, 2.91; N, 11.37. [Ru32+](PF6−)2. A mixture of [Ru(Mebip)Cl2(CH3CN)] (28 mg, 52.0 μmol) and tpb (0.46 g, 1.4 mmol) in glycerol (30 mL) was irradiated by microwave (650 W, multimode) in an on-and-off manner (10 times of 30 s irradiation). After the mixture was cooled to room temperature, water (50 cm3) and an excess of KPF6 were added to give a brown precipitate. After it was filtered, the obtained solid was purified by Sephadex LH 20 column chromatography (eluent MeCN/ MeOH 1/1 v/v). The resulting eluate was reduced by mixing with aqueous Na2S2O4 solution. After addition of KPF6, the product was collected. Yield: 24 mg (65%). 1H NMR (DMSO-d6, 500 MHz): δ 9.07 (d, 4H, J = 8.0 Hz), 8.42 (t, 2H, J = 8.0 Hz), 8.16 (d, 4H, J = 8.0 Hz), 7.78 (d, 4H, J = 5.2 Hz), 7.42 (t, 4H, J = 7.7 Hz), 7.30 (d, 4H, J = 5.7 Hz), 7.22 (t, 4H, J = 7.7 Hz), 6.87 (d, 4H, J = 8.6 Hz), 6.73 (t, 4H, J = 6.6 Hz), 6.45 (t, 4H, J = 7.7 Hz), 4.59 (s, 12H). ESI-TOF-MS (CH3CN): m/z 422.09 [M − 3PF6]3+ (calcd for C68H47N14Ru2 422.08). Anal. Calcd for C68H50F12N14P2Ru2·H2O: C, 51.91; H, 3.33; N, 12.46. Found: C, 51.72; H, 3.18; N, 12.15. [Ru33+](PF6−)3. When the synthesis was carried out without the addition of aqueous Na2S2O4 solution, the mixed-valence complex was isolated as a pure form in air. Anal. Calcd for C68H50F18N14P3Ru2: C, 48.04; H, 2.96; N, 11.53. Found: C, 48.07; H, 3.02; N, 11.50. [Os34+](PF6−)4. To 5 mL of ethylene glycol were added [Os(Mebip)Cl3] (76.2 mg, 0.120 mmol) and tpb (21.0 mg, 0.054 mmol). The mixture was heated with microwave radiation (power 650 W) for 15 min. After the mixture was cooled to room temperature, 30 mL of water and an excess of KPF6 were added. The resulting precipitate was collected by filtration and washed with water. The obtained solid was subjected to flash column chromatography on silica gel (eluent 8/1/1 MeCN/H2O/aqueous 0.5 M KNO3) followed by anion exchange with KPF6 to give 33.5 mg of [Os34+](PF6−)4 (35%). ESI-TOF-MS (CH3CN): m/z 481.47 [M − 3PF6]3+ (calcd for C68H47N14Os2 481.45). Anal. Calcd for C68H50F24N14Os2P4: C, 40.58; H, 2.58; N, 9.24. Found: C, 40.36; H, 2.49; N, 9.69. X-ray Crystallography. A high-quality single crystal of [Ru14+](PF6−)4 (red, plate) was mounted on a glass fiber and transferred to a cold nitrogen stream, and X-ray diffraction data were collected using a Bruker SMART APEXII ULTRA CCD-detector diffractometer on a rotating anode (Mo Kα radiation, graphite monochromator, λ = 0.71073 Å). Corrections for absorption were made by SADABS.50 The structure was determined by direct methods using SHELXS-97 and

revealed strong multiple IVCT bands. In particular, a large molar absorption coefficient of Ru33+ at 1141 nm (ε = 22620 M−1 cm−1) can be used as a good NIR indicator. When ruthenium in dinuclear [(L)Ru(BL)Ru(L)] complexes is replaced by osmium, the comproportionation constant for BL = tppz (ΔE(1) values: Os > Ru) is increased; in contrast, that for BL = tpb (ΔE(1): Os < Ru) decreased. These results can be explained by hole- and electron-transfer exchange mechanisms reflecting the difference in interactions between metal dπ orbitals and bridging-ligand π or π* orbitals, which was clearly seen in the results of DFT calculations. The large potential difference between Ru14+ and Ru33+ makes it possible to design a rectification device based on the molecular junction from Ru1n+−Ru3n+ units on an electrode.

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EXPERIMENTAL SECTION

General Procedures. NMR spectra were recorded in the designated solvent on using a JNM-ECA 599 (JEOL) spectrometer. Spectra are reported in ppm from residual protons of the deuterated solvent for 1H NMR. MS data were obtained with a Micromass ESITOF mass spectrometer. UV−vis and NIR spectra were recorded on an Agilent 8453 or a Hitachi U-4000 UV−vis spectrophotometer. Electrochemical measurements were carried out with an ALS/CHI Model 660A Electrochemical Analyzer. A three-electrode cell system was used. Microanalysis was carried out using a PerkinElmer 240C elemental analyzer at Chuo University. Materials. Ruthenium trichloride trihydrate (W. C. Heraeus GmbH) and osmium trichloride trihydrate (Wako-Chemical) were used without further purification. The starting complexes [Cl2(EtOH)M(tppz)MCl3] (M = Ru, Os) ande [Ru(Mebip)Cl2(CH3CN)] were synthesized by following the reported procedure.16,20,49 The other synthetic reagents and solvents were purchased from commercial suppliers and used without further purification. For the electrochemical measurements, acetonitrile was purified twice by distillation over P 2 O 5 , and tetra-n-butylammonium hexafluorophosphate (TBAPF6; TCI) was purified by recrystallization from EtOH. [Ru14+](PF6−)4. To 20 mL of dry acetone were added [Cl2(EtOH)Ru(tppz)RuCl3] (50 mg, 61 μmol) and AgOTf (79 mg, 0.31 mmol). The mixture was refluxed for 24 h before it was cooled to room temperature. The precipitate was removed by Celite filtration, and the filtrate was concentrated to dryness. To the blue-violet residue were added Mebip (41 mg, 0.12 mmol) and ethylene glycol (10 mL). The mixture was then refluxed for 3 h. After it was cooled to room temperature, the resulting solution was diluted by addition of water (10 mL). After addition of an excess of KPF6, the resulting precipitate was collected by filtration. The crude product was purified by column chromatography on Sephadex LH 20 (eluent CH3CN/MeOH, 1/1 v/ v) to give 90 mg of the pure complex as the second of four eluted bands (96%). Crystals suitable for X-ray analysis were obtained by the diffusion of ether into an acetonitrile solution. 1H NMR (500 MHz, CD3CN): δ 9.00 (d, J = 8.6 Hz, 4H), 8.78 (d, J = 8.0 Hz, 4H), 8.68 (t, J = 8.3 Hz, 2H), 7.88 (d, J = 5.2 Hz, 4H), 7.79 (t, J = 7.2 Hz, 4H), 7.70 (d, J = 8.6 Hz, 4H), 7.44 (t, J = 6.3 Hz, 4H), 7.33 (t, J = 7.7 Hz, 4H), 6.70 (t, J = 7.7 Hz, 4H), 6.53 (d, J = 8.0 Hz, 4H), 4.53 (s, 12H). ESITOF-MS (CH 3CN): m/z 317.55 [M − 4PF6] 4+ (calcd for C66H46N16Ru2, 317.56). Anal. Calcd for C66H50F24N16P4Ru2·H2O: C, 42.45; H, 2.81; N, 12.00. Found: C, 42.39; H, 2.66; N, 11.92. [Os14+](PF6−)4. This Os dinuclear complex was synthesized in a manner similar to that for [Ru14+](PF6−)4, except that [Cl2(EtOH)Os(tppz)OsCl3] was used instead of [Cl2(EtOH)Ru(tppz)RuCl3]. A dark green product was obtained. Yield: 82%. 1H NMR (500 MHz, CD3CN): δ 8.95 (d, J = 8.6 Hz, 4H), 8.33 (d, J = 8.0 Hz, 4H), 8.15 (t, J = 8.3 Hz, 2H), 7.83 (d, J = 5.2 Hz, 4H), 7.68 (d, J = 8.0 Hz, 4H), 7.62 (t, J = 8.6 Hz, 4H), 7.36 (dt, J = 6.3 Hz, 8H), 6.64 (t, J = 7.7 Hz, 4H), 6.48 (d, J = 8.6 Hz, 4H), 4.53 (s, 12H). ESI-TOF-MS (CH3CN): m/z 362.09 [M −4PF6] 4+ (calcd for M C66H46N16Os2, 362.09). Anal. Calcd for C66H50F24N16P4Os2: C, 39.10; H, 2.49; N, 11.05. Found: C, 39.15; H, 2.54; N, 10.83. J

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refined anisotropically on F2 with SHELXL-97.51 All hydrogen atoms (except those of water molecules) were located in their idealized positions, with methyl C−H bond lengtsh of 0.98 Å and aromatic C− H bond lengths of 0.95 Å, and included in the refinement using a riding-model approximation. The data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam. ac.uk/data_request/cif. Electrochemical Measurements. All cyclic voltammetry (CV) measurements were taken using a CHI620D potentiostat with a onecompartment electrochemical cell under an atmosphere of nitrogen. A glassy-carbon electrode with a diameter of 0.3 mm was used as the working electrode. The electrode was polished prior to use with 0.05 μm alumina and rinsed thoroughly with water and acetone. A largearea platinum-wire coil was used as the counter electrode. All potentials were measured on a saturated Ag/AgNO3 (0.01 M AgNO3 in 0.1 M TBAPF6 in CH3CN) electrode as a reference and converted to a ferrocenium/ferrocene (Fc+/Fc) couple without regard for the liquid junction potential. All measurements were carried out in acetonitrile with 0.1 M TBAPF6 as the supporting electrolyte. Oxidative Spectroelectrochemistry. Spectroelectrochemistry was performed in a thin-layer cell (optical length 0.2 cm) in which a platinum-mesh working electrode was used. Platinum wire and Ag/ AgNO3 (0.01 M AgNO3 in 0.1 M TBAPF6 in CH3CN) were used as a counter electrode and a reference electrode, respectively. The cell was placed into the spectrometer to monitor the spectral changes during electrolysis. Computational Methods. DFT calculations were carried out using the B3LYP exchange correlation functional as implemented in the Gaussian 03 program package.52 The electronic structures of the complexes were determined using a general basis set with the Los Alamos effective core potential SDD(M) for ruthenium and 6-31G* for other atoms under vacuum.53−56

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ASSOCIATED CONTENT

A table, text, figures, and a CIF file giving crystallographic data for Ru14+(PF6−)4, 1H NMR, ESI-TOF-MS, and UV−vis spectra of all of the complexes, spectroelectrochemistry data, and DFT calculation results of Ru1n+−Ru3n+ and Os1n+− Os3n+ complexes under vacuum and an xyz file of all computed molecule Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data have also been deposited with the Cambridge Structure Data Base as CCDC 985877 (complex Ru14+(PF6−)4). These can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html.

AUTHOR INFORMATION

Corresponding Author

*E-mail for M.H.: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

This work was supported by the Ministry of Education, Culture, Sports, Science and Technology by a Grant-in-Aid for Priority Area “Coordination Programming” (no. 21108003). M.H. acknowledges financial support from the Institute of Science and Engineering at Chuo University. H.O. acknowledges a Grant for Basic Science Research Projects from the Sumitomo Foundation. We both acknowledge Prof. Hirotoshi Mori at Ochanomizu University and Ms. Kie Yamada for their support with regard to the DFT calculations. K

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