Triply bridged dinuclear ruthenium complexes bearing alkylbis(2-pyridylmethyl)amine in the mixed-valence state of Ru(ii)-Ru(iii)

Article abstract of DOI:10.1039/C8DT03507E

Triply halogeno and methoxido-bridged dinuclear ruthenium complexes bearing a tridentate ancillary ligand, alkylbis(2-pyridylmethyl)amine (alkyl, ethyl and benzyl), in the Ru(ii)-Ru(iii) mixed-valence state were synthesized by reduction reactions of the trichloridoruthenium(iii) complex, fac-[Ru^IIICl₃(ebpma)], followed by chlorido-substitution and oxidation reactions in air. The conversion of the bridging ligands of the diruthenium complexes was also made possible through reduction of the dinuclear core. The electronic structures of the mixed-valence state were investigated by electron spin resonance (ESR), X-ray crystallography, electrochemical measurements and UV-vis-near infrared (NIR) spectroscopy. The mixed-valence state of all the triply bridged complexes was stable and classified as Class III.

Full text of DOI:10.1039/C8DT03507E

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Triply bridged dinuclear ruthenium complexes
bearing alkylbis(2-pyridylmethyl)amine in the
mixed-valence state of Ru(II)–Ru(III)†
Cite this: Dalton Trans., 2018, 47,
16182
T. Misawa-Suzuki,
K. Matsuya, T. Watanabe and H. Nagao
*
Triply halogeno and methoxido-bridged dinuclear ruthenium complexes bearing a tridentate ancillary
ligand, alkylbis(2-pyridylmethyl)amine (alkyl, ethyl and benzyl), in the Ru(II)–Ru(III) mixed-valence state
were synthesized by reduction reactions of the trichloridoruthenium(III) complex, fac-[RuIIICl3(ebpma)],
followed by chlorido-substitution and oxidation reactions in air. The conversion of the bridging ligands
of the diruthenium complexes was also made possible through reduction of the dinuclear core. The elec-
tronic structures of the mixed-valence state were investigated by electron spin resonance (ESR), X-ray
crystallography, electrochemical measurements and UV-vis-near infrared (NIR) spectroscopy. The mixed-
valence state of all the triply bridged complexes was stable and classified as Class III.
Received 29th August 2018,
Accepted 22nd October 2018
DOI: 10.1039/c8dt03507e
rsc.li/dalton
ities of a series of diruthenium complexes, which are com-
posed of different kinds of bridging moieties, are required.
Introduction
Multi-nuclear transition metal complexes have been attracting
In this study, the syntheses, structures, and electrochemical
attention in material conversion reaction fields, in which and spectroscopic properties of triply bridged mixed-valence
multi-electron transfer is required, such as water oxidation1,2 diruthenium complexes (Chart 1, C type) having two kinds of
and nitrogen fixation.3–7 Many types of, in particular, dinuc- bridging ligands, Cl−, Br−, and CH3O−, were investigated using
lear complexes of octahedral geometry having the bridging tridentate alkylbis(2-pyridylmethyl)amine (Rbpma; alkyl
=
moieties such as atoms, ions or molecules in the mixed- ethyl, benzyl, Chart 2)25–30 as a supporting ligand. The syn-
valence state have been synthesized and investigated on their thetic strategy for the change of bridging ligands was estab-
spectroscopic and electrochemical properties,8–11 in connec- lished as well, by reduction of the diruthenium core. Benzylbis
tion with biological systems, for instance. The development of (2-pyridylmethyl)amine (bbpma) was newly introduced to
dinuclear frameworks (Chart 1) and the control of the mixed- improve the solubility toward various organic solvents. Our
valence state are important for application in molecular elec- previous work about the ethylbis(2-pyridylmethyl)amine
tronics, devices, and quantum computing owing to the inert
properties and various oxidation states of these complexes.
While triply or doubly bridged diruthenium complexes having
only one kind of bridging ligand X (X = Cl, Br, I, OH, OR and
SR) with the {Ru2(μ-X)2}n+ or {Ru2(μ-X)3}n+ core have been
previously reported,12–22 there are still only a few diruthenium
complexes having two (or more) different kinds of bridging
ligands, such as [{RuCl(p-cymene)}2(μ-H)(μ-Cl)] (p-cymene;
1-methyl-4-(propan-2-yl)benzene)23 or [{Ru(Me3tacn)}2(μ-O)
(μ-P2O7)]+ (Me3tacn; trimethyltriazacyclononane).24 Thus, sys-
Chart 1 Dinuclear frameworks having bridging ligand(s).
tematic investigations on the electronic structures and reactiv-
Department of Materials and Life Sciences, Sophia University, 7-1 Kioicho,
Chiyodaku, Tokyo 102-8554, Japan. E-mail: h-nagao@sophia.ac.jp
†Electronic supplementary information (ESI) available: ESR spectra, NMR
spectra, X-ray structural analysis, electrochemical and spectroscopic data, UV-vis-
NIR spectra, and DFT calculations. CCDC 1861325, 1861084 and 1861326. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt03507e
Chart 2 Alkylbis(2-pyridylmethyl)amine (Rbpma; R = C2H5, ethyl, R =
CH2C6H5, benzyl).
16182 | Dalton Trans., 2018, 47, 16182–16189
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
(ebpma) ligand system has revealed that Rbpma coordinates to electrode at 25 °C. Controlled potential electrolysis (CPE)
the metal center in either facial or meridional fashion and the experiments were carried out in a three-compartment cell
geometry of ruthenium complexes bearing ebpma is controlled with the platinum gauze working electrode in the first com-
by the combination of the co-existing ligands.31 The nitrosyl- partment, the auxiliary electrode in the second compartment,
ruthenium complex containing an electron donating alkoxido and the reference electrode in the third compartment using a
ligand RO−, [Ru(NO)(OR)(ebpma)]2+, favors a meridional con- Huso polarograph Model 312 and 343B digital coulometer
figuration due to the strong interactions between the at 25 °C. At the end of each measurement, ferrocene (E1/2
=
π-accepting NO and RO− which locates at the trans position to 0.074 V) was added as an internal standard to correct redox
NO, through the metal center,31 while the trichlorido- potentials.
ruthenium(III) complex, [RuIIICl3(ebpma)], is more stable in the
Syntheses of supporting ligands and ruthenium complexes
facial geometry than in the meridional form.
Reduction reactions of fac-[RuIIICl3(ebpma)] afforded new RuCl3·nH2O (content of Ru: 40.46 wt%) was purchased from
complexes with the facial-type configuration accompanied by Furuya Kinzoku Inc. All solvents and chemicals were pur-
dissociation of chlorido ligands. In methanol in air, one chlo- chased as dehydrated grade and reagent grade, respectively,
rido and two methoxido ligands bridged between the two and were used without further purification. Ethylbis(2-pyridyl-
ruthenium centers, resulting in the formation of a triply methyl)amine (ebpma) was prepared by the procedure
bridged complex in the mixed-valence state of Ru(II)–Ru(III), previously reported in the literature.31 fac-[RuIIICl3(ebpma)],31
[{RuII,III(ebpma)}2(μ-Cl)(μ-OCH3)2]2+ ([1]2+), whose mixed- fac-[RuII(NCCH3)3(ebpma)]31 and [1](PF6)2 were synthesized
33
valency was classified as Class III according to the Robin–Day as reported earlier.
classification.32 The crystal structure of [1]2+ also suggested
Benzylbis(2-pyridylmethyl)amine (bbpma)
strong interactions between the two ruthenium centers.33
Systematic investigation on such a mixed-valence complex 2-Picolylchloride hydrochloride (4.20 g, 25 mmol) and benzyla-
having different kinds of bridging ligands has not achieved mine (1.47 cm3, 13 mmol) were dissolved in water (30 cm3)
success so far. Herein we would like to report the investigation and the solution was warmed at 60 °C for an hour. An aqueous
and comparison of the electronic structures of a series of solution of sodium hydroxide (5.0 mol dm−3, 10 cm3) was
mixed-valence diruthenium complexes of Ru(II)–Ru(III), whose added dropwise within 30 min and the solution was warmed
ruthenium centers are bridged by halogeno (Cl− and Br−) and for a further 1 hour. The resultant solution was cooled down to
methoxido ligands, bearing tridentate ebpma or bbpma.
room temperature and extracted with 40 cm3 of dichloro-
methane three times. The extract was dehydrated with MgSO4.
Then, the precipitated insoluble materials were filtered and
the obtained reddish-orange solution was evaporated to afford
a reddish-brown solution. Yield: 3.70 g (99%).
Experimental
General procedures
1H NMR (400 MHz, CD2Cl2): δ 3.66 (s), 3.76 (s), 7.13 (m),
Elemental analyses were performed with a PerkinElmer 2400-II 7.22 (m), 7.31 (t, J = 7.63), 7.42 (d, J = 7.02), 7.57 (d, J = 7.93),
instrument. FAB MS spectra and high-resolution MS spectra 7.65 (td, J = 6.87), 8.48 (d, J = 6.41). 13C NMR (400 MHz,
were recorded with a JEOL JMS-700 spectrometer with m-nitro- CD2Cl2): δ 58.76, 60.23, 122.22, 123.07, 127.31, 128.56, 129.20,
benzyl alcohol as the matrix. 1H and 13C NMR spectra were 136.63, 139.63, 149.23, 160.22. High-resolution MS: 290.17
recorded with a JEOL AL400 spectrometer. IR spectra were (M + H+).
recorded on a Shimadzu IR Affinity-1 spectrophotometer using
samples prepared as KBr disks. UV-vis-NIR spectra were
fac-[RuIIIBr3(ebpma)]
recorded on a Shimadzu MultiSpec-1500 system and on a fac-[RuII(NCCH3)3(ebpma)](PF6)2 (316 mg, 430 μmol) was sus-
UV-3100 system using a quartz cell of 1 cm or 1 mm path pended in water–ethanol (1/1, v/v, 60 cm3) and hydrobromic
length. The effective magnetic moment was measured by acid (48%, 1.0 cm3) was added. The mixture was refluxed for
Gouy’s method at 300 K using Hg[Co(SCN)4] as a calibrant. 1.5 hours and the obtained yellow solution was concentrated
X-band ESR spectra were recorded with a JEOL JESFA300 ESR by heating until 3 cm3. The mixture was allowed to stand in a
spectrometer at 77 K in the form of frozen acetone or powder. fridge. The reddish-purple precipitate was collected by fil-
Electrochemical measurements such as cyclic voltammetry tration, washed with water, ethanol, and diethyl ether and
(CV) and normal pulse voltammetry (NPV) were carried out in dried in vacuo. Yield: 170 mg (74%).
acetonitrile solutions containing 0.10 mol dm−3 tetraethyl-
Elemental analysis. Calcd for C14H17N3Br3Ru: C 29.97,
ammonium perchlorate (TEAP) or tetrabutylammonium per- H 3.05, N 7.41%. Found: C 29.60, H 3.02, N 7.40%.
chlorate (TBAP) as the supporting electrolyte using the Pt disk
fac-[RuIIICl3(bbpma)]
working electrode (ϕ = 1.6 mm) and an Ag|0.01 mol dm−3
AgNO3 (CH3CN) reference electrode using a BAS 100B/W This complex was synthesized in a similar manner to that of
electrochemical analyzer at 25 °C. Hydrodynamic voltammo- the
corresponding
complex
bearing
ebpma,
fac-
grams (HDVs) were obtained with the rotating platinum disk [RuIIICl3(ebpma)].31 RuCl3·nH2O (500 mg, 42.01 wt%) was dis-
working electrode (ϕ = 3.0 mm) using the BAS RDE-2 rotating solved in water–ethanol (2/3, v/v, 50 cm3) and refluxed until
This journal is © The Royal Society of Chemistry 2018
Dalton Trans., 2018, 47, 16182–16189 | 16183

View Article Online
Paper
Dalton Transactions
the color of the solution turned dark blue (‘Ru-blue’ solution).
Elemental analysis. Calcd for C28H34N6ClBr2Ru2P2F12:
To the Ru-blue solution, hydrochloric acid (35–37%, 4.0 cm3) C 30.26, H 3.18, N 7.18%. Found: C 30.53, H 3.24, N 6.93%.
and bbpma (590 mg, 2.04 mmol) were added in this order and FAB MS: 904 (M–PF6), 759 (M–2PF6).
the solution was refluxed for 10 hours. The obtained solution
[{RuII,III(ebpma)}2(μ-Br)(μ-OCH3)2](PF6)2 ([4](PF6)2)
was concentrated to 10 cm3 by heating and allowed to stand in
a fridge overnight. A red-brown precipitate was collected by fil- fac-[RuIIIBr3(ebpma)] (150 mg, 264 μmol) and zinc (100 mg,
tration, washed with water, ethanol, and diethyl ether in this 1.53 mmol) were suspended in methanol (15 cm3) and stirred
order and dried in vacuo. Yield: 751 mg (76%).
for 2 days at room temperature. To the suspension, 10 cm3 of
Elemental analysis. Calcd for C19H19N6Cl3Ru: C 45.93, H water was added to dissolve the precipitated green solid and
3.85, N 8.46%. Found: C 45.74, H 3.72, N 8.46%. FAB MS: 521 the remaining zinc was removed by filtration. NH4PF6 (150 mg,
(M + Na).
920 μmol) was dissolved in the obtained green solution and
the volume of the solution was reduced by using a rotary evap-
orator until a green precipitate appeared. The precipitate was
[{RuII,III(ebpma)}2(μ-Cl)3](PF6)2 ([2](PF6)2)
(a) [{RuII,III(ebpma)}2(μ-Cl)(μ-OMe)2](PF6)2 ([1](PF6)2; 100 mg, collected by filtration, washed with water, ethanol, and diethyl
96 μmol) was dissolved in acetone (15 cm3) under an argon ether and dried in vacuo. Yield: 125 mg (87%).
atmosphere and hydrochloric acid (35–37%, 0.05 cm3) was
Elemental analysis. Calcd for C30H40N6O2BrRu2P2F12: C
added and the mixture was stirred for 10 min at room tempera- 33.10, H 3.70, N 7.72%. Found: C 33.06, H 3.72, N 7.67%. FAB
ture. To the green solution, 10 cm3 of water containing MS: 853 (M–2PF6).
NH4PF6 (200 mg, 1.23 mmol) were added and the volume was
[{RuII,III(ebpma)}2(μ-Cl)2(μ-OCH3)](PF6)2 ([5](PF6)2)
reduced until a blue precipitate appeared. The blue precipitate
was collected by filtration, washed with water, ethanol, and [{RuII,III(ebpma)}2(μ-Cl)3](PF6)2 ([2](PF6)2; 50 mg, 47 μmol) was
diethyl ether and dried in vacuo. Yield: 82 mg (81%).
dissolved in acetone–methanol (2/1, v/v, 15 cm3) and zinc
(b) fac-[RuIIICl3(ebpma)] (150 mg, 345 μmol) and zinc (50 mg, 0.76 mmol) was added, and then the mixture was
(250 mg, 3.82 mmol) were suspended in acetone (15 cm3). To stirred for 5 min at room temperature. After the remaining
the suspension, hydrochloric acid (35–37%, 0.5 cm3) was zinc was filtered, the reddish-brown solution was oxidized in
added and the mixture was stirred for 6 hours at room tem- air until it turned green. After NH4PF6 (50 mg, 310 μmol) and
perature to obtain a reddish-brown solution. After the remain- water (10 cm3) were added, the volume of the solution was
ing zinc was removed, the solution was allowed to stand for reduced until a blue precipitate appeared by using a rotary
two hours in air to obtain a green solution. To the green solu- evaporator. The obtained blue solid was collected by filtration
tion, 20 cm3 of water and NH4PF6 (300 mg, 1.84 mmol) were and washed with water, ethanol, and diethylether and dried
added and the solution was evaporated until a blue precipitate in vacuo. Yield: 29 mg (58%).
was obtained. The blue precipitate was collected by filtration,
Elemental analysis. Calcd for C29H37N6O1Cl2Ru2P2F12:
washed with water, ethanol, and diethylether and dried C 33.22, H 3.56, N 8.01%. Found: C 33.12, H 3.57, N 7.90%.
in vacuo. Yield: 155 mg (85%).
FAB MS: 904 (M–PF6), 759 (M–2PF6−).
Elemental analysis. Calcd for C28H34N6Cl3Ru2P2F12:
C 31.94, H 3.25, N 7.98%. Found: C 32.17, H 3.39, N 7.63%.
[{RuII,III(bbpma)}(μ-Cl)(μ-OCH3)2](PF6)2·2H2O ([6](PF6)2·2H2O)
FAB MS: 765 (M − 2PF6). The effective magnetic moment (μeff
of the blue complex, [2](PF6)2, at 25 °C; 1.94μB.
)
fac-[RuIIICl3(bbpma)] (100 mg, 200 μmol) and zinc were sus-
pended in methanol (50 cm3) and stirred for 72 hours at room
temperature. The remaining zinc was removed and a green
solution was obtained. To the solution, 10 cm3 of ethanol con-
[{RuII,III(ebpma)}2(μ-Cl)(μ-Br)2](PF6)2 ([3](PF6)2)
[{RuII,III(ebpma)}2(μ-Cl)(μ-OMe)2](PF6)2 ([1](PF6)2; 100 mg, 96 μmol) taining NH4PF6 (200 mg, 1.30 mmol) was added and the solu-
was dissolved in acetone (15 cm3) under an argon atmosphere tion was slowly concentrated in air. The precipitated green
and hydrobromic acid (48%, 0.05 cm3) was added. The solution solid was collected by filtration and washed with water. For
was stirred for 10 min at room temperature. To the obtained purification, the solid was dissolved in acetonitrile and an
green solution, NH4PF6 (200 mg, 1.23 mmol) and 20 cm3 of water insoluble matter was removed. The obtained filtrate was dried
were added and the volume of the solution was reduced until a to obtain a black powder. Yield: 75 mg (64%).
green precipitate was obtained. The green solid was collected
Elemental analysis. Calcd for C40H48N6O4ClRu2P2F12:
by filtration and washed with water, ethanol, and diethyl ether C 39.89, H 4.02, N 6.98%. Found: C 39.97, H 4.00, N 6.94%.
and dried in vacuo. The green solid was purified by precipitat- FAB MS: 1024 (M–PF6), 879 (M–2PF6), 439 {(M–2PF6)/2}.
ing in acetone (10 cm3) to remove the red-purple solid, fac-
X-ray crystallography
[RuIIIBr3(ebpma)], by filtration. To the obtained green filtrate,
5 cm3 of water was added and the green solution was con- Single crystals of three diruthenium complexes, [{RuII,III
densed until a green precipitate appeared. The green solid was (ebpma)}2(μ-Cl)3](PF6)2·(CH3)2CO ([2](PF6)2·(CH3)2CO; CCDC
collected by filtration and washed with water, ethanol, and 1861325†), [{RuII,III(ebpma)}2(μ-Cl)(μ-Br)2](PF6)2·0.5H2O·(CH3)2CO
diethyl ether in this order and dried in vacuo. Yield: 30 mg ([3](PF6)2·0.5H2O·(CH3)2CO; CCDC 1861084†), and [{RuII,III
(27%).
(ebpma)}2(μ-Br)(μ-OCH3)2](PF6)2·H2O ([4](PF6)2·H2O; CCDC
16184 | Dalton Trans., 2018, 47, 16182–16189
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
1861326†), were obtained by diffusion of diethyl ether into the one methoxido-bridged complex, [5](PF6)2, was formed
acetone solutions at room temperature. Each single crystal was through a reduction reaction of [2](PF6)2. A reaction of [1](PF6)2
platelet shaped and the size was 0.200 × 0.160 × 0.030 mm with hydrobromic acid in acetone afforded a corresponding
for [2](PF6)2·(CH3)2CO, 0.07
×
0.07
×
0.03 mm for one chlorido- and two bromido-bridged complex, [3](PF6)2. In
[3](PF6)2·0.5H2O·(CH3)2CO and 0.160 × 0.100 × 0.030 mm for methanol, through reduction reactions of fac-[RuIIIBr3(ebpma)]
[4](PF6)2·H2O, respectively. The colour was blue for or fac-[RuIIICl3(bbpma)], one halogeno and two methoxido-
[2](PF6)2·(CH3)2CO and green for [3](PF6)2·0.5H2O·(CH3)2CO bridged complexes, [4](PF6)2 or [6](PF6)2, respectively, were
and [4](PF6)2·H2O. The intensity data were collected on a obtained. One-electron reduction of the Ru(III) center induces
Rigaku Mercury CCD diffractometer, using graphic monochro- wakening of the bridging ligands, resulting in substitution to
matized MoKα radiation (0.71069 Å). All the calculations were another ligand in the reaction system, followed by oxidation to
carried out using the Crystal Structure software package stabilize the dinuclear structure.
(Version 4.3, Single Crystal Structure Analysis Software).
The electronic structures of the series of triply bridged com-
Structures were solved by direct methods, expanded using plexes were found to be in the Ru(II)–Ru(III) state and the elec-
Fourier techniques and refined using full-matrix least-squares tronic spin was delocalized over the {Ru2(μ-X)(μ-Y)(μ-Z)} core by
techniques.
magnetic susceptibility measurements, ESR spectra, NIR
spectra and cyclic voltammetry, which will be discussed in
detail in later sections.
Density functional theory (DFT) calculations
All the calculations were performed with the Gaussian09
program.34 The geometry was optimized at the B3LYP level of
density functional theory with the SDD (Ru)-6-31++G(d,p)(C,H,
O,N) basis sets.
Structural comparison of diruthenium complexes
The structures of the diruthenium cations showed a distorted
octahedral geometry around the ruthenium centers, as shown
in Fig. 1–3. The crystallographic data are summarized in
Table S2† and the selected structural parameters are listed in
Tables 1 and S3.† Each ruthenium center was coordinated by
one amine and two pyridines of the ebpma ligand in the fac-
manner and by three bridging ligands. For the triply chlorido-
bridged complex, [2]2+, two of the three bridging Cl− ligands
were located at the trans position to the amine group on one
ruthenium center (Ru1) and pyridyl on the other ruthenium
center (Ru2), while for [3]2+ and [4]2+, one halogeno ligand,
Results and discussion
Synthesis and characterization of dinuclear ruthenium
complexes
Reduction of fac-[RuIIICl3(ebpma)] at the Ru(III) center in
methanol induced dissociation of chlorido ligands to give a
dinuclear complex with the triply bridged framework, in which
one chlorido and two methoxido ligands bridged between the
ruthenium centers, [1](PF6)2.33 Conversion reactions among
the dinuclear ruthenium complexes in the mixed-valence state
of Ru(II)–Ru(III) are shown in Scheme 1. A triply chlorido-
bridged diruthenium complex, [2](PF6)2, was obtained in
acetone containing hydrochloric acid, instead of methanol.35
The composition of the bridging ligands could be controlled
by the solvent and co-existing species in the solution. In a
methanol-containing acetone solution, a two chlorido- and
Fig. 1 Structure of [2]2+ with thermal ellipsoids at 50% probability.
Scheme 1 Conversion scheme of triply bridged complexes.
Fig. 2 Structure of [3]2+ with thermal ellipsoids at 50% probability.
Dalton Trans., 2018, 47, 16182–16189 | 16185
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
Dalton Transactions
Ru(III) (2.606–2.6515 Å).17,38 The bridging Ru–Cl distances
(2.3936(19)–2.4108(18)) were similar to those of mixed-valence
complexes containing the triply chlorido-bridged framework,
{Ru2(μ-Cl)3} (2.3511–2.5268 Å).17,19–21,38 The Ru–O(alkoxido)
lengths, 2.068(3)–2.07072(4) Å, were longer than those in the
doubly alkoxido-bridged unit of Ru(III)–Ru(III), {Ru2(μ-OR)2}
(R; CH3, C2H5, 2.022–2.042 Å),15,36,37 and shorter than those in
the corresponding doubly alkoxido-bridged moiety of Ru(II)–
Ru(II) (2.065–2.114 Å).39,40 The Ru–Cl–Ru angles (70.09(2)–
73.44(5)°) were larger than those of the previously reported
one chlorido- and two methoxido-bridged complex, [1]2+,
(67.51(3)°),33 and smaller than those of the doubly chlorido-
bridged complex of Ru(II)–Ru(II) (98.70–99.70°) and almost con-
sistent with those of the triply chlorido-bridged complexes of
Ru(II)–Ru(III) (70.2–87.16°).17,19–21,42,43 The structural compari-
sons indicated that the vacant d orbitals of the halogeno
ligand had an influence on these structural features, in par-
ticular, the Ru–L–Ru (L; bridging ligand) angles, and stabiliz-
ation of the mixed-valency of Ru(II)–Ru(III). The Ru–O(alkox-
ido)–Ru angles in the one bromido- and two methoxido-
bridged complex, [4]2+, were 81.75(9)° and larger than those of
the three chlorido-bridged one, [2]2+. The angles were smaller
than those of the doubly alkoxido-bridged complexes of Ru(II)–
Ru(II) (91.5–104.6°)39–41 and Ru(II)–Ru(III) (100.13°),22 and
closer to those of the corresponding Ru(III)–Ru(III) complexes
(79.6–81.86°).15,36,37 These structural parameters of the present
triply bridged diruthenium complexes indicated the strong
interactions between the two ruthenium centres.
Fig. 3 Structure of [4]2+ with thermal ellipsoids at 50% probability.
Table 1 Structural parameters of triply bridged diruthenium complexes
[2]2+
[3]2+
[4]2+
[1]2+ (ref. 33)
X
Y
Z
Cl
Cl
Cl
Br
Br
Cl
OCH3
OCH3
Br
OCH3
OCH3
Cl
Bond lengths/Å
Ru⋯Ru
2.875(2)
2.8959(8)
2.5216(5)
2.5217(3)
2.4121(11)
2.7072(4)
2.068(2)
2.069(3)
2.5427(6)
2.6898(3)
2.071(2)
2.071(2)
2.4200(9)
Ru–X
2.408(2)
2.4016(19)
2.417(3)
Ru–Y
Ru–Z
Bond angles/°
Ru–X–Ru
Ru–Y–Ru
73.12(4)
73.44(5)
73.03(5)
70.09(2)
70.09(2)
73.78(4)
81.75(9)
81.75(9)
64.329(16)
80.96(7)
80.96(7)
67.53(2)
Ru–Z–Ru
Cl− or Br−, was at the trans position to the amine on both Ru1
and Ru2 and two Br− or CH3O− were trans to pyridine, respect-
ively. Because of the order of σ-donating nature, CH3O− > Br− >
Cl−, the location of these bridging ligands is determined in
relation to the position of σ-donating amines and two
π-accepting pyridines in the ebpma ligand.
The structural parameters of the ebpma ligand were almost
identical to those of the fac-form ebpma complexes as com-
pared in Table S3.†31,33 The Ru⋯Ru distances of the triply
bridged complexes of Ru(II)–Ru(III) in this work, [1]2+–[4]2+
(2.6898(3)–2.8959(8) Å), were within the range of the triply
bridged mixed-valence complexes of Ru(II)–Ru(III), in which
strong interactions between the Ru centers exist
(2.753–3.325 Å),14,17–19 and were slightly longer than those of
the isovalent doubly alkoxido-bridged complexes of Ru(III)–
Mixed-valency of diruthenium complexes
The Ru(II)–Ru(III) mixed-valence electronic structure was evalu-
ated by magnetic susceptibility measurements at 298 K, elec-
tron spin resonance (ESR) spectroscopy at 77 K in frozen
acetone, electrochemical measurements and UV-vis-NIR spec-
troscopy in CH3CN. The spectroscopic and electrochemical
properties are summarized in Table 2.
The effective magnetic moments μeff were 1.79μB for
[1](PF6)2 and 1.94μB for [2](PF6)2 at 300 K and the ESR spectra
at 77 K indicated that an unpaired electronic spin was deloca-
lized on the {Ru2(μ-X)(μ-Y)(μ-Z)} core (Fig. S1 and Table S1†).
Cyclic voltammograms of the diruthenium complexes in
CH3CN showed two reversible one-electron reduction and oxi-
dation waves at 25 °C in the potential window (Table 2 and
Table 2 Electrochemical and spectroscopic properties of triply bridged Ru(II)–Ru(III) complexes
ebpma
bbpma
[6](PF6)2
33
Ancillary ligand
[2](PF6)2
[3](PF6)2
[4](PF6)2
[5](PF6)2
[1](PF6)2
λmax (ε)/nm (mol−1 dm3 cm−1
)
328 (7150)
709 (6500)
1500 (500)
4170, 2000
5710, 3900
0.84
328 (10 200)
732 (7540)
1590 (300)
3920, 1700
5620, 3800
0.78
370 (12 000)
757 (8280)
1590 (300)
3900, 1300
5520, 3800
0.23
351 (7890)
693 (7000)
1590 (200)
3960, 800
5770, 3800
0.50
370 (10 400)
734 (7870)
1520 (300)
4000, 1500
5610, 3900
0.22
365 (9910)
728 (8210)
Δ˜ν (found, calcd)/cm−1
E1/2/V at 298 K
0.26
−0.61
5.1
−0.14
380
−0.10
7.8
−0.64
5.2
−0.41
24
−0.65
5.2
Kc (×1014
)
16186 | Dalton Trans., 2018, 47, 16182–16189
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
tation on the delocalized dinuclear core.8,9 The band
maximum in the visible region was independent of the polarity
of the solvent: CH2Cl2, (CH3)2CO, CH3CN or CH3OH, as indi-
cated in Tables S6 and S7.† The maximum absorption band
λmax and the MLCT band were exhibited in a shorter wave-
length for those containing three bridging halogeno ligands,
328 nm for [2]2+ and [3]2+, compared to those containing one
or two halogeno ligands, 351 nm for [5]2+ and 365–370 nm for
[1]2+, [4]2+ and [6]2+, indicating that the dπ(Ru) orbital is rela-
tively more stabilized for [2]2+ and [3]2+ compared to the
others. The other two bands shown in the NIR region
suggested no explicit dependency of both λmax and molar
absorption coefficient ε on the combination of bridging
ligands. To put the discussion one step forward, the isolation
and investigation of the corresponding triply bridged Ru(II)–
Ru(III) complexes without the bridging halogeno ligand, which
has vacant d orbitals for an electron to pass through, seem to
be important.
Fig. 4 Cyclic voltammograms in CH3CN at room temperature
(a) [4](PF6)2, (b) [1](PF6)2, (c) [5](PF6)2, (d) [2](PF6)2, (e) [3](PF6)2 and
(f) [6](PF6)2.
Spectroelectrochemical experiments for [2]2+ and [3]2+ using
an Optically Transparent Thin-Layer Electrode (OTTLE) cell at
room temperature showed spectral changes as shown in
Fig. S9.† One-electron reduction reactions from the Ru(II)–Ru(III)
state in acetone containing 0.1 mol dm−3 TEAP as an electro-
Fig. 4). These waves were assigned to one-electron oxidation lyte afforded isovalent Ru(II)–Ru(II) species, showing isosbestic
and reduction processes from the Ru(II)–Ru(III) state, respect- points, with a gradual decrease of the weak band around
ively, by the wave analyses of hydrodynamic voltammetry or 700–730 nm and an increase of a new intense MLCT band
normal pulse voltammetry (Fig. S4–S7, Tables S4 and S5†). The around 410–440 nm, which was attributed to the electronic
differences in redox potentials, as summarized in Table 2, transitions from dπ(Ru) to π*(py). These isovalent species were
were due to the electrostatic nature of the bridging halogeno reversibly changed to the original Ru(II)–Ru(III) state
(Cl−, Br−) and methoxido ligands, since the methoxido ligand accompanied by spectroelectrochemical re-oxidation. The
shows more σ-electron donating nature than the chlorido stability of the one-electron oxidized and reduced species of
ligand. Accompanied by substitution of the one bridging halo- the Ru(II)–Ru(III) complexes was suggested by the reversibility
geno to the methoxido ligand, the redox potential shifts by ca. of the one-electron reduction and oxidation waves in the CV
0.3 V (e.g. [1]2+ and [5]2+), while only a slight change was measurement and the reversible spectral changes during redox
shown by the difference between the Cl− and Br− ligand ([2]2+ reactions. The comproportionation constants Kc were esti-
and [3]2+; ca. 50 mV), or between the ebpma and bbpma ligand mated by the difference in potentials between two reversible
([1]2+ and [6]2+; 30 mV). In the Kc value, which reveals the stabi- redox couples of Ru(III)–Ru(III)/Ru(III)–Ru(II) and Ru(III)–Ru(II)/
lity of the mixed-valence state, no clear relationship with the Ru(II)–Ru(II).10 The values were in the order of 1014–1016. The
kind of bridging ligand or un-bridging Rbpma ligand was analyses of the half width at the half maximum (HWHM) of
shown.
For the triply chlorido-bridged complex, [2]2+, which shows III state, according to Hush’s theory,44 in which ν (found)
the IVCT band of [2]2+, [3]2+ and [4]2+ also supported the Class
˜
1/2
a one-electron redox couple of Ru(II)–Ru(III)/Ru(II)–Ru(II) at was smaller than ν1/2 (calcd). These results revealed that all the
˜
−0.14 V (vs. Ag|0.01 mol dm−3 AgNO3), a corresponding Ru(II)– complexes were categorized into Class III along with the ana-
Ru(II) species was isolated through a reduction reaction with lyses of the IVCT bands observed in the NIR region in elec-
zinc in acetone.35 An attempt to isolate the corresponding tronic transition spectra, according to the Robin–Day
Ru(III)–Ru(III) species of [1]2+, which shows a lower one-electron classification.32
oxidation potential of Ru(III)–Ru(III)/Ru(II)–Ru(III) at +0.22 V, has
The electronic structures of [2]2+ [3]2+ and [4]2+ were theor-
also been successful and a study describing this is in progress. etically studied by DFT calculations and the optimized struc-
The diruthenium complexes showed characteristic absorp- tures of frontier orbitals with MOs are shown in Fig. S10–S14.†
tion bands in the UV-vis region and very weak absorption The contributions by the ruthenium centers, halogeno ligand(s)
bands around 1500 nm in the NIR region in CH3CN (Table 2) and methoxido ligands to each MO are listed in Table S8.† For
and in CH2Cl2 (Fig. S8 and Table S6†). The weak absorption the SOMO of these three mixed-valence complexes, contri-
bands in the NIR region were also found in triply bridged butions from the {Ru2(μ-X)(μ-Y)(μ-Z)} core, which consists of
mixed-valence complexes having the {RuII(μ-X)3RuIII} (X = Cl, the Ru centers and the three bridging ligands, were large, sup-
Br)12,13 and {RuII(μ-OH)3RuIII}13 core, in which both the elec- porting that the one-electron oxidation and reduction reac-
tronic transitions were attributed to intervalence-type exci- tions occur at the dinuclear core.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans., 2018, 47, 16182–16189 | 16187

View Article Online
Paper
Dalton Transactions
Conclusions
Notes and references
New mixed-valence diruthenium complexes, whose metal
centers were triply bridged by halogeno and methoxido
ligands bearing a tridentate ligand, ebpma or bbpma, [{RuII,
III(Rbpma)}2(μ-X)(μ-Y)(μ-Z)](PF6)2 (X, Y = Cl−, Br− or CH3O−),
were synthesized through the reduction reactions of fac-
[RuIIIX3(ebpma)] (X = Cl−, Br−) to the Ru(II) state, accompanied
by the dissociation of chlorido ligand(s) under certain reaction
conditions. By the synthetic strategy using reduction of the
dinuclear core to control the reactivity of the bridging moiety,
a wide range of diruthenium complexes having different kinds
of bridging ligands could be designed and synthesized. The
interconversion among the triply bridged complexes was also
established by inducing dissociation of the bridging ligand(s)
through reduction of the Ru(II)–Ru(III) center to Ru(II)–Ru(II) or
reactions with acids. These kinds of bridging ligands could
be controlled by the reaction conditions such as solvent and
co-existing species in the reaction mixture. The Ru(II)–Ru(III)
1 B. Gerey, E. Gouré, J. Fortage, J. Pécaut and M.-N. Collomb,
Coord. Chem. Rev., 2016, 256, 1 and the references therein.
2 X. Liu and F. Wang, Coord. Chem. Rev., 2012, 256, 1115 and
the references therein.
3 H. Tanaka, Y. Nishibayashi and K. Yoshizawa, Acc. Chem.
Res., 2016, 49, 987.
4 I. Čorićand and P. L. Holland, J. Am. Chem. Soc., 2016, 138,
7200.
5 S. F. McWilliams and P. L. Holland, Acc. Chem. Res., 2015,
48, 2059.
6 B. M. Hoffman, D. Lukoyanov, Z.-Y. Yang, D. R. Dean and
L. C. Seefeldt, Chem. Rev., 2014, 114, 4041 and the refer-
ences therein.
7 J. M. Smith, A. R. Sadique, T. R. Cundari, K. R. Rodgers,
G. L. Rodgers, R. J. Lachicotte, C. J. Flaschenriem,
J. Vela and P. L. Holland, J. Am. Chem. Soc., 2006, 128,
756.
mixed-valence state was classified as
a
Class III state,
8 R. Modak, Y. Sikdar, S. Mandal and S. Goswami, Inorg.
Chem. Commun., 2013, 37, 193.
9 B. Mabad, J.-P. Tuchagues, Y. T. Hwang and
D. N. Hendrickson, J. Am. Chem. Soc., 1985, 107, 2801.
according to the Robin–Day classification32 and Hush’s
theory,44 for all those series of diruthenium complexes
herein, which is quite a rare case. X-ray crystallography
revealed that the shape of the diruthenium core structure, 10 W. Kaim and B. Sarkar, Coord. Chem. Rev., 2007, 251, 584
{Ru2(μ-X)(μ-Y)(μ-Z)}, and the position of the bridging ligand and the references therein.
against the ebpma ligand were dependent on the kind of 11 W. Kaim and G. K. Lahiri, Angew. Chem., Int. Ed., 2007, 46,
bridging ligand; the core structure of triply halogeno- 1778 and the references therein.
bridged complexes, [2]2+ and [3]2+, resembled each 12 N. S. Hush, J. K. Beattie and V. M. Ellis, Inorg. Chem., 1984,
other and were relatively different from the one halogeno- 23, 3339.
and two methoxido-bridged complexes, [{RuII,III(ebpma)}2 13 P. Neubold, B. S. P. C. D. Vedova, K. Wieghardt, B. Nuber
(μ-X)2(μ-OCH3)]2+ (X = Cl; [1]2+, Br; [4]2+). The more σ-donat-
and J. Weiss, Inorg. Chem., 1990, 29, 3355.
ing methoxido ligand coordinates to the trans position 14 R. S. Armstrong, W. A. Horsfield and K. W. Nugent, Inorg.
toward the pyridine of ebpma. An electronic spin on the Chem., 1990, 29, 4551.
complex cation was delocalized over the {Ru2(μ-X)(μ-Y)(μ-Z)} 15 H. Miyasaka, H. Chang, K. Mochizuki and S. Kitagawa,
core to stabilize the Ru(II)–Ru(III) mixed-valence state, which Inorg. Chem., 2001, 40, 3544.
was almost identical to the Ru2.5–Ru2.5 state, regardless of 16 N. Matsuura, T. Kawamoto and T. Konno, Bull. Chem. Soc.
the kind or combination of bridging ligand. The dinuclear Jpn., 2006, 79, 297.
core structure, {Ru2(μ-X)(μ-Y)(μ-Z)}, and the electronic pro- 17 W. A. Clucas, R. S. Armstrong, I. E. Buys, T. W. Hanbley and
perties such as redox potentials and electronic transition K. W. Nugent, Inorg. Chem., 1996, 35, 6789.
energy of dinuclear complexes could be controlled by the 18 W. Kaim, C. Titze, A. Klein, A. Knödler and S. Zalis,
combination of bridging ligand(s).
Isr. J. Chem., 2001, 41, 145.
19 M. N. Hughes, D. O’Reardon, R. K. Poole,
M. B. Hursthouse and M. Thornton-pett, Polyhedron, 1987,
6, 1711.
20 I. S. Thorburn, S. J. Rettig and B. R. James, Inorg. Chem.,
1986, 25, 234.
Conflicts of interest
There are no conflicts to declare.
21 L. Quebatte, E. Solari, R. Scopelliti and K. Severin,
Organometallics, 2005, 24, 1404.
22 E. Eskelinen, T.-J. J. Kinnunen, M. Haukka and
T. A. Pakkanen, Eur. J. Inorg. Chem., 2002, 1169.
23 J. Chen, X. Chen, C. Zhu and J. Zhu, J. Mol. Catal. A: Chem.,
2014, 394, 198.
Acknowledgements
We show our deep appreciation to Prof. Dr Shigeki Kuwata and
Yoshihito Kayaki (Tokyo Institute of Technology) for the NMR 24 Y. Miyazato, T. Wada, M. Ohba and N. Matsushita, Chem.
measurements, Prof. Dr Ryo Miyamoto (Hirosaki University) Lett., 2016, 45, 1388.
for the comments on ESR signals and Prof. Dr Shinkoh Nanbu 25 J. Mola, I. Romero, M. Rodriguez, F. Bozoglian, A. Poater,
(Sophia University) for the DFT calculations.
M. Solâ, T. Parella, J. Benet-Buchholz, X. Fontrodona and
16188 | Dalton Trans., 2018, 47, 16182–16189
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
A. Llobet, Inorg. Chem., 2007, 46, 10707 and the references
therein.
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT,
2009.
26 C. Baffert, I. Romero, J. Pécaut, A. Llobet, A. Deronzier and
M.-N. Collomb, Inorg. Chim. Acta, 2004, 357, 3430.
27 C. Mantel, A. K. Hassan, J. Pécaut, A. Deronzier,
M.-N. Collomb and C. Duboc-Toia, J. Am. Chem. Soc., 2003,
125, 12337.
28 I. Romero, L. Dubois, M.-N. Collomb, A. Deronzier, 35 T. Suzuki, K. Matsuya, T. Kawamoto and H. Nagao,
J.-M. Latour and J. Pécaut, Inorg. Chem., 2002, 41, 1795. Eur. J. Inorg. Chem., 2014, 722.
29 C. E. Dubé, D. W. Wright, S. Pal, P. J. Bonitatebus Jr. and 36 S. Kar, N. Chanda, S. M. Mobin, A. Datta, F. A. Urbanos,
W. H. Armstrong, J. Am. Chem. Soc., 1998, 120, 3704.
30 S. Pal, M. M. Olmstead and W. H. Armstrong, Inorg. Chem.,
1995, 34, 4708.
31 Y. Shimizu, S. Fukui, T. Oi and H. Nagao, Bull. Chem. Soc.
Jpn., 2008, 81, 1285.
V. G. Puranik, R. Jimenez-Aparicio and G. K. Lahiri, Inorg.
Chem., 2004, 43, 4911.
37 T. Hashimoto, Y. Kawamoto, Y. Ishitobi, T. Sasaki,
Y. Fukuda, A. Yamagishi, H. Sato, K. Shimizu and
T. Hayashita, Chem. Lett., 2007, 1174.
32 Mixed Valency Systems-Applications in Chemistry, Physics and 38 R. M. Christie, G. A. Heath, S. A. MacGregor, M. Schröder
Biology, ed. K. Prassides, Kluwer Academic Publishers,
and L. J. Yellowlees, J. Chem. Soc., Chem. Commun., 1990,
Dordrecht, 1991.
1445.
33 K. Matsuya, S. Fukui, Y. Hoshino and H. Nagao, Dalton 39 D. Bardwell, L. Horsburgh, J. C. Jeffery, L. Joulié,
Trans., 2009, 7876.
34 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. D. Ward, I. Webster and L. J. Yellowlees, J. Chem. Soc.,
Dalton Trans., 1996, 2527.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, 40 D. Bardwell, J. C. Jeffery, L. Joulié and M. D. Ward, J. Chem.
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, Soc., Dalton Trans., 1993, 2255.
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, 41 S. D. Loren, B. K. Campion, R. H. Heyn, T. D. Tilley,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
B. E. Bursten and L. W. Luth, J. Am. Chem. Soc., 1989, 111,
4712.
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, 42 M. Murali and M. Palaniandavar, J. Chem. Soc., Dalton
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, Trans., 2006, 730.
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, 43 A. M. Joshi, I. S. Thorburn, S. J. Rettig and B. R. James,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, Inorg. Chim. Acta, 1992, 198–200, 283.
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, 44 Advances in Inorganic Chemistry, ed. H. J. Emeleus and
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
A. G. Sharpe, Academic Press, New York, 1960, vol. 2.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans., 2018, 47, 16182–16189 | 16189