Tuning of the ionization potential of paddlewheel diruthenium(ii, ii) complexes with fluorine atoms on the benzoate ligands

Article abstract of DOI:10.1039/c0dt00956c

A series of paddlewheel diruthenium(ii, ii) complexes with various fluorine-substituted benzoate ligands were isolated as THF adducts and structurally characterized: [Ru₂(F_xPhCO₂) ₄(THF)₂] (F_x

Full text of DOI:10.1039/c0dt00956c

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PAPER
Tuning of the ionization potential of paddlewheel diruthenium(II, II) complexes
with fluorine atoms on the benzoate ligands†
Hitoshi Miyasaka,* Natsuko Motokawa, Ryo Atsuumi, Hiromichi Kamo, Yuichiro Asai and Masahiro Yamashita
Received 4th August 2010, Accepted 18th October 2010
DOI: 10.1039/c0dt00956c
A series of paddlewheel diruthenium(II, II) complexes with various fluorine-substituted benzoate
ligands were isolated as THF adducts and structurally characterized: [Ru2(FxPhCO2)4(THF)2]
-
(FxPhCO2 = o-fluorobenzoate, o-F; m-fluorobenzoate, m-F; p-fluorobenzoate, p-F;
2,6-difluorobenzoate, 2,6-F2; 3,4-difluorobenzoate, 3,4-F2; 3,5-difluorobenzoate, 3,5-F2;
2,3,4-trifluorobenzoate, 2,3,4-F3; 2,3,6-trifluorobenzoate, 2,3,6-F3; 2,4,5-trifluorobenzoate, 2,4,5-F3;
2,4,6-trifluorobenzoate, 2,4,6-F3; 3,4,5-trifluorobenzoate, 3,4,5-F3; 2,3,4,5-tetrafluorobenzoate,
2,3,4,5-F4; 2,3,5,6-tetrafluorobenzoate, 2,3,5,6-F4; pentafluorobenzoate, F5). By adding fluorine atoms
on the benzoate ligands, it was possible to tune the redox potential (E1/2) for [Ru2II,II]/[Ru2II,III]+ over a
wide range of potentials from -40 mV to 350 mV (vs. Ag/Ag+ in THF). 2,3,6-F3, 2,3,4,5-F4, 2,3,5,6-F4
and F5 were relatively air-stable compounds even though they are [Ru2II,II] species. The redox potential
in THF was dependent on an electronic effect rather than on a structural (steric) effect of the o-F
atoms, although more than one substituent in the m- and p-positions shifted E1/2 to higher potentials in
relation to the general Hammett equation. A quasi-Hammett parameter for an o-F atom (so) was
estimated to be ~0.2, and a plot of E1/2 vs. a sum of Hammett parameters including so was linear. In
addition, the HOMO energy levels, which was calculated based on atomic coordinates of solid-state
structures, as well as the redox potential were affected by adding F atoms. Nevertheless, a steric
contribution stabilizing their static structures in the solid state was present in addition to the electronic
effect. On the basis of the electronic effect, the redox potential of these complexes is correlated to the
HOMO energy level, and the electronic effect of F atoms is the main factor controlling the ionization
potential of the complexes with ligands free from the rotational constraint, i.e. complexes in solution.
for designing electronic modules in aggregated charge-transfer
Introduction
systems.8–12 This is because the two oxidation states [Ru2
]
II,II
and [Ru2II,III]+, which possess unpaired spins with S = 113 and
S = 3/2, respectively, in a degenerated orbital set of (d*p*2)
and considerably large zero-field splitting (D ª +250 cm-1 and
+60 cm-1, respectively), are relatively stable and can be isolated.1
Recently, our group has focused on assemblies of [Ru2II,II] with
7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and its derivatives
to design a charge-transfer system with metal–organic frameworks
(MOFs), where [Ru2II,II] and TCNQ are regarded as a one-electron
(1-e-) donor (D) and acceptor (A), respectively. Although no in-
terunit charge transfer occurs in a D2A two-dimensional assembly,
[{Ru2(O2CCF3)4}2(TCNQ)],8 which is neutral, substitution with
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4)
Paddlewheel complexes with metal–metal bonds have been exten-
sively studied over the past four decades.1 The most fascinating
aspect of paddlewheel complexes is their rich electrochemistry:
the complexes undergo both oxidation and reduction at the
dimetal center with systematical changes in the metal–metal
bond length in relation to the arrangement of electrons in the
frontier orbitals sp2dd*p*2s* without considerable structural
change. Among such paddlewheel dimetal complexes, a family
of diruthenium complexes with bridging carboxylate ligands,2
[Ru2(RCO2)4]n+ (n = 0 or 1; abbreviated as [Ru2]n+), are interesting
for studying the magnetic properties of their assemblies3–7 and
∑
-
led to charge transfer forming TCNQF4 due to the higher
electron affinity of TCNQF4 than that of TCNQ.10 In addition,
this compound exhibited long-range antiferromagnetic ordering at
a Ne´el temperature TN = 95 K owing to intralayer ferromagnetic
ordering and interlayer antiferromagnetic ordering. These results
clearly indicate that [Ru2II,II] complexes are capable of acting
not only as a coordination-acceptor to form MOFs with TCNQ
Department of Chemistry, Graduate School of Science, Tohoku Uni-
versity, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi, 980-8578,
Japan. E-mail: miyasaka@agnus.chem.tohoku.ac.jp; Fax: +81-22-795-
6548; Tel: +81-22-795-6545
† Electronic supplementary information (ESI) available: Experimen-
tal details. CCDC reference numbers 766135–766150. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00956c
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groups but also as charge-transfer insulators or 1-e- donors even
in the same D2A system. In other words, tuning of the ionization
potential (ID) of [Ru2II,II] and the electron affinity (EA) of the
TCNQ unit makes it possible to design D/A MOFs with various
electronic and spin states. For this strategy, [Ru2II,II] complexes with
benzoate ligands are good candidates for donor units, of which ID
can be systematically tuned by adding substituents on the phenyl
ring. A few of these complexes are charge-transfer magnets when
combined with the appropriate TCNQ derivatives.12,14 However,
only a few examples of these [Ru2II,II] complexes have been
reported to date15–23 likely because it is difficult to isolate and
characterize them, and to the best of our knowledge, a systematic
2,3,5,6-F4, and F5, of which E1/2 > 300 mV, were considerably
stable even in air. Finally, molecular orbital (MO) calculations
based on their crystal structures were performed to compare their
HOMO energy levels with their redox potentials. In this paper, the
syntheses, characterizations, and electrochemistry in comparison
to energy levels of MOs of these complexes, which would afford
us useful information to know their redox nature related to ID, are
summarized.
Experimental
General procedures and materials
investigation of their electrochemistry to determine indirectly ID
II,II
has never been done. Here, we introduce a series of [Ru2
]
Except for 2,3,6-F3, 2,3,4,5-F4, 2,3,5,6-F4 and F5, all synthetic
procedures were performed under anaerobic conditions using
standard Schlenk techniques. All chemicals were purchased as
reagent grade from commercial sources. THF, n-hexane, N,N-
dimethylaniline (NDMA), and methanol were purchased from
Wako Pure Chemical Industries, and except for methanol,
dried and distilled with common drying agents under a nitro-
complexes with various fluorine-substituted benzoate ligands,
-
[Ru2(FxPhCO2)4(THF)2] (FxPhCO2 = o-fluorobenzoate, o-F; m-
fluorobenzoate, m-F; p-fluorobenzoate, p-F; 2,6-difluorobenzoate,
2,6-F2; 3,4-difluorobenzoate, 3,4-F2; 3,5-difluorobenzoate, 3,5-F2;
2,3,4-trifluorobenzoate, 2,3,4-F3; 2,3,6-trifluorobenzoate, 2,3,6-F3;
2,4,5-trifluorobenzoate, 2,4,5-F3; 2,4,6-trifluorobenzoate, 2,4,6-
F3; 3,4,5-trifluorobenzoate, 3,4,5-F3; 2,3,4,5-tetrafluorobenzoate,
2,3,4,5-F4; 2,3,5,6-tetrafluorobenzoate, 2,3,5,6-F4; pentafluo-
robenzoate, F5) (Chart 1).
24
gen atmosphere before use. [Ru2II,III(O2CCH3)4(THF)2]BF4 and
[Ru2II,III(O2CCH3)4Cl]25 were prepared according to previously
reported procedures.
Syntheses of [Ru2II,II(O2CPh-o-F)4(THF)2] (o-F), [Ru2II,II(O2CPh-
II,II
m-F)4(THF)2] (m-F), [Ru2II,II(O2CPh-p-F)4(THF)2] (p-F), [Ru2
-
(O2CPh-2,6-F2)4(THF)2] (2,6-F2), [Ru2II,II(O2CPh-3,4-F2)4(THF)2]
(3,4-F2), [Ru2II,II(O2CPh-3,5-F2)4(THF)2] (3,5-F2), [Ru2II,II(O2CPh-
2,3,4-F3)4(THF)2] (2,3,4-F3), and [Ru2II,II(O2CPh-2,4,6-F3)4-
(THF)2] (2,4,6-F3)
The compounds were synthesized using a method based on a
report by Furukawa and Kitagawa.22 Only the procedure for o-F
is described: [Ru2II,III(O2CCH3)4(THF)2]BF4 (335 mg, 0.5 mmol)
and o-fluorobenzoic acid (280 mg, 2.0 mmol) were refluxed in
15 mL of NDMA for 12 h. After removal of the solvent in vacuo,
the brown residue was washed with n-hexane (10 mL ¥ 3) and
dissolved in a minimum amount of THF. The red solution was
filtered and then layered with n-hexane to obtain brown crystals
of o-F after one week or more. Yield: 68%. Elemental analysis (%)
calcd for C36H32F4O10Ru2: C 47.90, H 3.57. Found: C 47.80, H
3.57. IR (KBr): n(CO2) = 1554, 1402 cm-1. For m-F, yield: 67%.
Elemental analysis (%) calcd for C36H32F4O10Ru2: C 47.90, H 3.57.
Found: C 47.60, H 3.58. IR (KBr): n(CO2), 1558, 1396 cm-1. For p-
F, yield: 68%. Elemental analysis (%) calcd for C36H32F4O10Ru2: C
47.90, H 3.57. Found: C 47.91, H 3.78. IR (KBr): n(CO2) = 1554,
1407 cm-1. For 2,6-F2, yield: 63%. Elemental analysis (%) calcd
for C36H28F8O10Ru2: C 44.36, H 2.90. Found: C 44.08, H 3.01. IR
(KBr): n(CO2) = 1570, 1406 cm-1. For 3,4-F2, yield: 40%. Elemental
analysis (%) calcd for C36H28F8O10Ru2: C 44.36, H 2.90. Found: C
44.32, H 2.84. IR (KBr): n(CO2) = 1562, 1394 cm-1. For 3,5-F2,
the reaction time should be taken for 14 h. Yield: 53%. Elemental
analysis (%) calcd for C36H28F8O10Ru2: C 44.36, H 2.90. Found: C
44.36, H 3.16. IR (KBr): n(CO2) = 1564, 1400 cm-1. For 2,3,4-F3,
yield: 52%. Elemental analysis (%) calcd for C36H24F12O10Ru2: C
41.31, H 2.31. Found: C 41.39, H 2.45. IR (KBr): n(CO2) = 1571,
1400 cm-1. For 2,4,6-F3, yield: 60%. Elemental analysis (%) calcd
for C36H24F12O10Ru2: C 41.31, H 2.31. Found: C 41.40, H 2.52. IR
(KBr): n(CO2) = 1570, 1406 cm-1.
Chart 1
Fluorine atoms on benzoic acid change its acidity, i.e., pKa,
depending on the position on the ring and the number of
substituents. The redox potentials of these complexes vary dramat-
ically in relation to the pKa of the F-substituted benzoate ligands.
Interestingly, the collective law between the redox potential, and
the pKa of this series strongly depends on the moiety at the
o-position (non-, mono-, and di-o-F), leading to a co-linear
dependency with the m- and p-F groups in relation to non-,
mono-, and di-o-substituted subgroups. Therefore, on the basis
of a Hammett analysis, we were able to determine a Hammett
constant for o-F, so ª 0.2, in this series. The present compounds
encompass a wide range of redox potentials from -40 to 350 mV
vs. Ag/Ag+ in THF, giving an ID-tunable series of a 1-e- donor
[Ru2II,II]. Furthermore, it was found that 2,3,6-F3, 2,3,4,5-F4,
674 | Dalton Trans., 2011, 40, 673–682
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Syntheses of [Ru2II,II(O2CPh-2,4,5-F3)4(THF)2] (2,4,5-F3) and
[Ru2II,II(O2CPh-3,4,5-F3)4(THF)2] (3,4,5-F3)
were conducted using a Quantum Design SQUID magnetometer
(MPMS-XL or -5S) in the temperature range of 1.8–300 K, where
the applied magnetic fields were 1 T for o-F and m-F and 0.1 T
for the other complexes. Corrections for diamagnetism and for
the samples wrapped in vinyl capsules were applied using Pascal’s
constants.27 Cyclic voltammograms (CVs) were recorded in THF
with tetra-n-butylammonium hexafluorophosphate n-Bu4N(PF6)
(0.1 M) as supporting electrolyte under a nitrogen atmosphere
using a BAS CV-50 W voltammetric analyzer. At the beginning of
the measurements, CVs of the solvent with only supporting elec-
trolyte were measured. To this solution were added the compounds
([Compound] = 1 ¥ 10-3 M), and the CVs were acquired using a
glassy carbon electrode as the working electrode, a Pt counter
electrode, and an Ag/AgNO3 reference electrode. Finally, CV
potentials were adjusted with the ferrocene/ferrocenium couple
used as an internal standard (Fc/Fc+ = 213 mV (DE = 91 mV) in
THF vs. Ag/Ag+).
These two compounds were synthesized step-by-step via a ligand-
substitution process involving [Ru2II,III]+, followed by reduction
to the final [Ru2II,II] products.26 As a representative example, the
procedure for 2,4,5-F3 is described: [Ru2II,III(O2CCH3)4Cl] (200 mg,
0.42 mmol) and 2,4,5-trifluorobenzoic acid (599 mg, 3.4 mmol)
were refluxed in a 1 : 1 solution of MeOH–H2O (20 mL) for 12 h
under aerobic conditions to give [Ru2II,III(O2CPh-2,4,5-F3)4Cl].
The obtained red precipitate was collected by filtration, washed
with water, and dried in vacuo. Without further purification, a
solution of the crude product and Zn powder (52 mg, 0.80 mmol)
in THF (10 mL) was stirred for 24 h under a nitrogen atmosphere.
During this time, most of the solid dissolved. The reddish-colored
solution was filtered, and the filtrate was layered with n-hexane
to obtain brown crystals of 2,4,5-F3 after one week or more.
Yield: 45%. Elemental analysis (%) calcd for C36H24F12O10Ru2:
C 41.31, H 2.31. Found: C 41.27, H 2.37. IR (KBr): n(CO2) =
1573, 1386 cm-1. For 3,4,5-F3, yield: 60%. Elemental analysis (%)
calcd for C36H24F12O10Ru2: C 41.31, H 2.31. Found: C 41.11, H
2.41. IR (KBr): n(CO2) = 1529, 1415 cm-1.
Crystallographic analyses
Crystal data were collected on a Rigaku CCD diffractometer
(Saturn 70 or VariMax) with graphite-monochromated Mo-
˚
Ka radiation (l = 0.71070 A). The structures were solved
Syntheses of [Ru2II,II(O2CPh-2,3,6-F3)4(THF)2] (2,3,6-F3),
[Ru2II,II(O2CPh-2,3,4,5-F4)4(THF)2] (2,3,4,5-F4),
[Ru2II,II(O2CPh-2,3,5,6-F4)4(THF)2] (2,3,5,6-F4), and
[Ru2II,II(O2CPh-F5)4(THF)2] (F5)
using direct methods (SIR97).28 The non-hydrogen atoms were
refined anisotropically, except for some disordered solvent atoms,
which were refined isotropically. The hydrogen atoms were only
introduced for non-disordered carbon atoms at fixed positions,
because H atoms on disordered atoms seem to be not accurate in
their position (in addition, electron density with less than 1e- is
inconsequential for the accuracy of other atoms). The position-
disordered F atoms in o-F, m-F, and 3,4-F2 and C atoms for THF in
3,5-F2, 3,4,5-F3, and F5 were first calculated with free occupancies
to find subequal proportions of temperature factor between the
possible two sites, and then adequate occupancies for the two sites,
which come to 1, were determined to two places of decimals. Full-
matrix least-squares refinements on F2 converged with unweighted
and weighted agreement factors of R1 = RꢀFo|-|Fcꢀ/R|Fo| (I
For these compounds, the first part of the syntheses, i.e.,
the ligand-substitution process of [Ru2II,III(O2CCH3)4Cl], was
performed under anaerobic conditions similar to the above
method. However, the reduction of the Ru center occurred at the
same time as the ligand substitution. As a representative example,
the procedure for 2,3,6-F3 is described: [Ru2II,III(O2CCH3)4Cl]
(474 mg, 1.0 mmol) and 2,3,6-trifluorobenzoic acid (704 mg,
4.0 mmol) were refluxed in MeOH (100 mL) for 85 h under aerobic
conditions. Then the solution was cooled to room temperature,
and the solvent was evaporated under vacuum. After washing
with degassed water and drying again, the residue was dissolved
in a minimal amount of THF under a nitrogen atmosphere. The
reddish-colored solution was filtered and then layered with n-
hexane to obtain brown crystals of 2,3,6-F3 after one week or more.
Yield: 56%. Elemental analysis (%) calcd for C36H24F12O10Ru2:
C 41.31, H 2.31. Found: C 41.05, H 2.49. IR (KBr): n(CO2) =
1581, 1402 cm-1. For 2,3,4,5-F4, yield: 47%. Elemental analysis
(%) calcd for C36H20F16O10Ru2: C 38.65, H 1.80. Found: C 38.69,
H 2.01. IR (KBr): n(CO2) = 1529, 1414 cm-1. For 2,3,5,6-F4,
yield: 58%. Elemental analysis (%) calcd for C36H20F16O10Ru2: C
38.65, H 1.80. Found: C 38.54, H 1.88. IR (KBr): n(CO2) = 1589,
1398 cm-1. For F5, yield: 60%. ESI MS (MeOH) m/z: 1070.73
[[Ru2(F5PhCO2)4]+Na]+, 1220.62 [[Ru2(F5PhCO2)4]+Na+NaI]+,
1370.52 [[Ru2(F5PhCO2)4]+Na+2NaI]+, 1520.41 [[Ru2(F5-
PhCO2)4]+Na+3NaI]+, 1670.30 [[Ru2(F5PhCO2)4]+Na+4NaI]+.
IR (KBr): n(CO2) = 1596, 1400 cm-1.
> 2.00s(I) and all data), and wR2 = [Rw(Fo -Fc )2/Rw(Fo2)2]1/2
(all data), respectively. In some final analyses, relatively large
residual electron densities remain around Ru centers, which are
possibly due to satellite peaks of Ru atoms. A Sheldrick weighting
scheme was used, where the f ¢ and f ¢ scattering factor values were
refined in four places of decimals (the values were rounded in the
CIF). All calculations were performed using the CrystalStructure
crystallographic software package.29 The crystal data and details
of all compounds are summarized in Table 1.
2
2
Computational details
Molecular orbital (MO) calculations of the present complexes
were carried out using the density functional theory (DFT)
formalism with the unrestricted spin option, as implemented in
the Gaussian03 software,30 with the B3LYP (UB3LYP) exchange–
correlation functional. The quasi-relativistic pseudo-potentials of
the Ru atoms proposed by Hay and Wadt31 with 16 valence
electrons were employed, and LANL2TZ basis sets with po-
larization (LANL2TZ(f))32 for Ru atoms and LANL2DZ basis
sets with polarization and diffuse functions (LANL2DZpd)33
for C, H, F and O atoms associated with the pseudo-potential
Physical measurements
Infrared spectra were measured as KBr disks on a Jasco FT-IR 620
spectrophotometer. ESI MS was measured on a Bruker Daltonics
APEXIII spectrometer. Magnetic susceptibility measurements
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Table 1 Crystallographic data of [Ru2(FxPhCO2)4(THF)2]
o-F m-F
p-F
2,6-F2
3,4-F2
3,5-F2
2,3,4-F3
Formula
C36H32O10F4Ru2 C36H32O10F4Ru2 C36H32O10F4Ru2 C36H28O10F8Ru2 C36H28O10F8Ru2 C36H28O10F8Ru2 C36H24O10F12Ru2
Formula weight
Crystal system
Space group
902.78
Monoclinic
C2/c
19.188(7)
9.225(3)
20.991(8)
90
902.78
Monoclinic
P21/n
8.655(5)
19.076(9)
10.793(6)
90
902.78
Monoclinic
P21/c
9.0627(19)
17.772(3)
11.225(2)
90
974.74
Orthorhombic
P212121
12.4495(5)
15.2218(7)
18.5927(8)
90
974.74
Monoclinic
P21/c
9.090(2)
18.451(4)
11.068(3)
90
974.74
Monoclinic
P21/n
10.634(3)
10.835(3)
15.883(5)
90
1046.70
Monoclinic
P21/c
9.1898(5)
17.7022(9)
11.4563(9)
90
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
111.679(5)
90
3453(2)
4
101.521(6)
90
1746.0(16)
2
109.466(2)
90
1704.6(6)
2
90
90
3523.4(3)
4
108.293(3)
90
1762.5(7)
2
95.196(4)
90
1822.4(9)
2
105.975(4)
90
1791.74(19)
2
3
˚
V/A
Z
Crystal size/mm
0.08 ¥ 0.05 ¥
0.02
0.75 ¥ 0.08 ¥
0.05
0.25 ¥ 0.20 ¥
0.15
0.15 ¥ 0.13 ¥
0.13
0.25 ¥ 0.10 ¥
0.10
0.20 ¥ 0.15 ¥
0.10
0.09 ¥ 0.04 ¥
0.04
T/K
93(1)
1.737
1808.00
0.71070
9.553
15156
4728
0.066
93(1)
1.717
904.00
0.71070
9.446
14039
3978
103(1)
1.759
904.00
0.71070
9.676
13003
3868
0.023
93(1)
1.837
1936.00
0.71070
9.593
28681
7908
0.017
7908
93(1)
1.837
968.00
0.71070
9.588
12930
3942
93(1)
1.776
968.00
0.71070
9.273
14413
4151
103(1)
1.940
1032.00
0.71070
9.659
14298
4024
Dc/g cm-3
F000
˚
l/A
m(Mo-Ka)/cm-1
Data measured
data unique
Rint
0.088
3978
0.110
3942
0.0729
3761
0.018
4024
No. of observations 3004
3868
No. of variables
R1 (I > 2.00s(I))a
R (all reflections)a
271
245
251
506
287
264
283
0.0393
0.0418
0.0449
0.0660
0.1076
0.888
—
766148
0.0313
0.0376
0.0821
1.019
—
766150
0.0197
0.0206
0.0441
1.066
0.155(13)
766142
0.0698
0.0801
0.1853
1.009
—
766145
0.0531
0.0891
0.1212
1.019
—
766146
0.0218
0.0290
0.0490
1.113
—
766136
wR2 (all reflections)b 0.1178
GOF
Flack parameter
CCDC No.
1.003
—
766149
2,3,6-F3
2,4,5-F3
2,4,6-F3
3,4,5-F3
2,3,4,5-F4
2,3,5,6-F4
F5
Formula
C36H24O10F12Ru2 C36H24O10F12Ru2 C36H24O10F12Ru2 C36H24O10F12Ru2 C36H20O10F16Ru2 C36H20O10F16Ru2 C36H16O10F20Ru2
Formula weight
Crystal system
Space group
1046.70
Monoclinic
P21/c
9.6162(17)
11.352(2)
16.766(3)
90
100.046(2)
90
1802.1(6)
2
1046.70
Monoclinic
P21/c
9.2572(16)
19.413(3)
10.1835(17)
90
103.330(2)
90
1780.7(5)
2
1046.70
Monoclinic
P21/c
8.7241(5)
12.0006(8)
17.9403(8)
90
101.429(2)
90
1841.01(18)
2
1046.70
Monoclinic
P21/c
8.882(3)
17.855(4)
12.627(3)
90
109.561(3)
90
1886.9(8)
2
1118.66
Monoclinic
P21/n
12.666(3)
17.274(4)
17.523(4)
90
107.667(4)
90
3653.3(15)
4
1118.66
Monoclinic
P21/c
8.8140(17)
16.926(3)
13.077(3)
90
109.572(2)
90
1838.2(6)
2
1190.62
Triclinic
P1
¯
˚
a/A
10.838(4)
11.755(4)
15.517(5)
77.607(9)
81.127(11)
86.496(10)
1907.0(11)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Crystal size/mm
0.50 ¥ 0.20 ¥
0.10
0.10 ¥ 0.08 ¥
0.03
0.20 ¥ 0.10 ¥
0.08
0.46 ¥ 0.05 ¥
0.05
0.32 ¥ 0.13 ¥
0.04
0.65 ¥ 0.12 ¥
0.08
0.47 ¥ 0.15 ¥
0.14
T/K
93(1)
1.929
1032.00
0.71070
9.603
14200
4048
0.043
93(1)
1.952
1032.00
0.71070
9.718
14500
4072
0.056
4072
93(1)
1.888
1032.00
0.71070
9.400
11977
4135
0.027
4135
193(1)
1.842
1032.00
0.71070
9.172
15034
4297
0.0227
4297
93(1)
2.034
2192.00
0.71070
9.697
28689
8194
93(1)
2.021
1096.00
0.71070
9.635
13200
4129
93(1)
2.073
1160.00
0.71070
9.501
15600
8337
0.0259
8337
Dc/g cm-3
F000
˚
l/A
m(Mo-Ka)/cm-1
Data measured
Data unique
Rint
0.035
8194
0.097
4129
No. of observations 4048
No. of variables
R1 (I > 2.00s(I))a
R (all reflections)a
272
272
283
275
597
290
615
0.0273
0.0322
0.0471
0.0622
0.1130
1.123
—
766140
0.0267
0.0335
0.0655
1.027
—
766141
0.0389
0.0482
0.1036
1.004
—
766144
0.0523
0.0774
0.1438
1.009
—
766135
0.0620
0.0676
0.1644
1.070
—
766137
0.0282
0.0372
0.0605
1.109
—
766147
wR2 (all reflections)b 0.0721
GOF
Flack parameter
CCDC No.
0.970
—
766138
2
a R1 = R = RꢀFo| - |Fcꢀ/R|Fo|. b wR2 = [Rw(Fo - Fc2)2/Rw(Fo2)2]1/2
.
676 | Dalton Trans., 2011, 40, 673–682
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were adopted. In the calculations, spin polarization with SZ
=
1 (spin multiplicity of 3) for all [Ru2] units was used. The atomic
coordinates determined by using X-ray crystallography with those
for [Ru2(PhCO2)4(THF)2] being taken from ref. 22 were used. In
the case of disordered fluorine atoms on the benzoate groups, the
fluorine atoms with major occupancy were used, and H atoms
were added instead of vacant positions.
Results and discussion
The one-step synthetic method reported by Furukawa and
Kitagawa,22 in which ligand-substitution and reduction of the
diruthenium center of [Ru2II,III(CH3CO2)4(THF)2]+ occurring in
N,N-dimethylaniline (NDMA), was used to synthesize most of
the [Ru2II,II] complexes with mono-, di-, and tri-F-substituted
benzoate ligands. However, in the cases of 2,4,5-F3 and 3,4,5-
F3, the [Ru2II,II] complexes reacted with 4,4¢-methylenebis(N,N-
dimethylaniline) (NDMA-me-NDMA), which formed from the
oxidation of NDMA, to afford [Ru2II,II(FxPhCO2)4(NDMA-me-
NDMA)2] even after several attempts to purify it with THF–n-
hexane for modifying from NDMA-me-NDMA to THF (2,4,5-F3-
NDMA: CCDC-766139; 3,4,5-F3-NDMA: CCDC 766143; Table
S1 and Fig. S1†). Therefore, a step-by-step procedure via the
isolation of [Ru2II,III(FxPhCO2)4Cl], followed by reduction with
Zn powder,26 was used to prepare the THF-adducts of these
complexes. Note that this procedure should be applicable to the
preparation of other [Ru2II,II] compounds even if two steps are
required. In the cases of 2,3,5,6-F4 and F5, the NDMA method
afforded only [Ru2II,II(CH3CO2)4(THF)2] in high yield without
-
substitution of the CH3CO2 ligands, which is formed from
the reduction of the starting material. Interestingly, the ligand-
substituted [Ru2II,II] complexes for these species were obtained
as methanol-adducts from the ligand-substitution reaction of
[Ru2II,III(CH3CO2)4Cl] with methanol in air, which were then
modified to obtain the desired complexes by crystallizing in
THF under an N2 atmosphere. Although complexes 2,3,6-F3 and
2,3,4,5-F4 could be synthesized by using the one-step reaction in
NDMA, they were produced in low yield. However, they as well
as 2,3,5,6-F4, and F5 could be prepared in high yield by using the
aerobic reaction in MeOH. Even under aerobic conditions, which
are usually disadvantageous for the syntheses of [Ru2II,II] species,
Fig. 1 ORTEP drawings of (a) o-F, (b) 2,6-F2, (c) 2,4,6-F3, (d) 2,3,5,6-F4,
and (e) F5 (one unit in an asymmetric set), where the dotted bonds for
F atoms represent disordered atomic positions (50% probability thermal
ellipsoids; symmetry operations (*) -x-1/2, -y-1.5, -z+1 for o-F; -x+1,
-y+1, -z for 2,4,6-F2; -x+2, -y+2, -z+1 for 2,3,5,6-F4; -x+2, -y+1, -z+1
for F5).
II,III
+
the reduction of [Ru2
] occurred simultaneously with ligand
substitution, indicating that fluorine atoms of 2,3,6-F3, 2,3,4,5-
F4, 2,3,5,6-F4, and F5 act as strong electron-withdrawing groups,
stabilizing the [Ru2II,II] state in air (see electrochemistry).
been shown to be scarcely dependent on the valence state of the Ru
II,III
+
centers, although the formal Ru–Ru bond order is 2.5 for [Ru2
]
and 2 for [Ru2II,II]. On the basis of the [Ru2II,III]+ complexes in Table
2, the Ru–Ru bond lengths are basically in the range of 2.25–
All compounds were structurally characterized by using single
crystal X-ray crystallography. Fig. 1 shows ORTEP drawings of
o-F, 2,6-F2, 2,4,6-F3, 2,3,5,6-F4, and F5 (the other complexes are
shown in Fig. S2†), and the relevant bond lengths around the Ru
centers are summarized in Table 2 together with those of several
representative [Ru2II,II],4b,15,1618–20,22,23 and [Ru2II,III]+ complexes.20,34–36
Except for 2,6-F2 and 2,3,4,5-F4, the complexes have an inversion
center on the midpoint of the Ru–Ru vector. Complex F5 contains
two structurally-independent units in a unit cell, which are very
similar to each other (one of them was depicted in Fig. 1e), even
if one of them has a disordering form in the part of THF moieties
(Fig. S3†). The Ru–Ru bond lengths of complexes are in the range
˚
2.29 A. In other words, the bond length does not provide proof
for the valence state of the compounds. The bonds that reflect
the valence state of the Ru centers are the Ru–O(carboxylate)
bonds (Ru–Oeq), of which one set of d frontier orbitals ([dd*]2)
2
2
made from the dx
and dxy orbitals of the two Ru atoms mainly
-y
form the bond. Another set of dd* orbitals is used for the Ru–Ru
bond and an electron in this d* orbital is directly related to the
valence state of the metals (vide infra). The electron configurations
of the valence states are s p2 d d p*2 s for [Ru2II,II] (S = 1)
2
4
2
*2
2
*0
and s p2 d d p*2 s for [Ru2II,III]+ (S = 3/2) since the d* and p*
orbitals are degenerate. In other words, if an electron is added or
removed from the d* orbital of the Ru–Ru bond during reduction
or oxidation, respectively, the Ru–Oeq bond is affected, and the
2
4
2
*1
2
*0
II,II
˚
of 2.27–2.28 A similar to those of previously reported [Ru2
]
compounds (see Table 2). However, the Ru–Ru bond length has
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Table 2 Relevant bond lengths around Ru centers in [Ru2II,II(FxPhCO2)4(X)2] and related compounds (where Oeq means oxygen atoms of equatorial
positions and Xax is axially-coordinating atoms) and dihedral angles defined by LS planes composed of atom sets of phenyl group (Plane 1) and carboxyl
groups and diruthenium (Plane 2)
Dihedral angles between
Plane 1 and Plane 2/◦
˚
˚
˚
Compound
Ru–Ru/A
Averaged Ru–Oeq/A
Ru–Xax/A
Set-1
Set-2
Set-3
Set-4
Average Ref
[Ru2II,II] compounds
[Ru2II,II(CH3CO2)4(THF)2]
[Ru2II,II(CH3CO2)4(H2O)2]
[Ru2II,II(EtCO2)4(Me2CO)2]
[Ru2II,II(C10H15CO2)4(MeOH)2]
2.261(3)
2.262(3)
2.260(3)
2.2809(9)
2.266(1)
2.060
2.068
2.068
2.017
2.062
2.391(5)
2.335(4)
2.363(5)
2.287(5)
2.348a
15b
15b
15b
19
II,II
[Ru2 {L-C6H5(OH)CHCO2}4-
18
(H2O)2]
2.265(1)
2.069
2.066(2)b
2.073
2.064(3)
2.064(3)
2.068c
2.065
2.064
2.063
2.068
2.062
2.068
2.067
2.065
2.061
2.065
2.064
2.0728
2.066
2.070
2.068
2.071
2.067
2.068
2.065
2.070
2.068
2.351a
[Ru2II,II(CF3CO2)4]•
2.2679(5)b
2.276(3)
2.363(2)b
2.268(6)
2.670(3)
2.347(3)
2.136(5)c
2.341(4)
2.3696(24)
2.331(5)
2.347(2)
2.340(2)
2.342(2)
2.312(2)
2.331(2)
2.3537(15)
2.332a
23
16
23
23
4b
22
20
20
22
22
[Ru2II,II(CF3CO2)4(THF)2]
[Ru2II,II(CF3CO2)4(C16H16)]•
2.2993(5)
2.2613(7)
2.300(2)c
2.2642(8)
2.2689(11)
2.2727(10)
2.2672(3)
2.2633(7)
2.2649(8)
2.2669(2)
2.2691(4)
2.2677(2)
2.2706(2)
2.2744(4)
2.2708(8)
2.2712(2)
2.2719(2)
2.2703(3)
2.2723(2)
2.2767(5)
2.2774(4)
2.2731(3)
2.2754(7)
2.2804(8)
[Ru2II,II(CF3CO2)3(C2H5CO2)]
[Ru2II,II(CF3CO2)4(Tempo)2]
[Ru2II,II(PhCO2)4(THF)2]
[Ru2II,II(p-MePhCO2)4(THF)2]
[Ru2II,II(p-MePhCO2)4(CH3CN)2]
[Ru2II,II(NapCO2)4(THF)2]
[Ru2II,II(AntCO2)4(THF)2] (unit 1)
(unit 2)
o-F
37.489
3.168
17.691
19.521
15.782
32.675
14.040
8.082
17.685
41.925
15.118
76.474
27.779
21.556
26.341
33.434
38.666
—
—
—
31.249
—
—
—
—
—
—
—
35.452
—
—
—
—
—
43.651
—
—
—
—
—
—
—
18.028
—
—
27.59
11.34
17.63
37.39
16.92
14.10
20.15
51.55
13.28
64.16
23.00
26.20
31.51
29.64
40.33
m-F
p-F
19.476
41.986
19.798
20.112
22.621
61.174
11.441
51.836
18.224
29.769
36.677
25.842
42.000
2,6-F2
3,4-F2
2.354(3)
2.310(5)
2.3334(11)
2.3022(17)
2.329(2)
2.2829(16)
2.307(2)
2.299a
3,5-F2
2,3,4-F3
2,3,6-F3
2,4,5-F3
2,4,6-F3
3,4,5-F3
2,3,4,5-F4
2,3,5,6-F4
F5 (unit 1)
(unit 2)
2.298(2)
2.318(2)
2.308(3)
—
—
[Ru2II,III]+ compounds
[Ru2II,III(CH3CO2)4(H2O)2]BF4
[Ru2II,III(p-
2.248(1)
2.2618(16)
2.00
2.06
2.31a
2.258(6)
34
20
MePhCO2)4(THF)2]BF4
[Ru2II,III(PhCO2)4Cl]
[Ru2II,III(CH3CO2)4Cl]
2.290(1)
2.287(2)
2.019
2.017
2.532(1)
2.577(1)
35
36
a Averaged value. b at 100 K. c at 201 K.
upon cooling, decreased smoothly to be in the range of 9.6 ¥ 10-3–
2.0 ¥ 10-2 cm3 K mol-1 at 1.8 K. However, the c values increased
with a decrease in the temperature to about 100 K and then reached
a plateau, followed by an increase at temperatures below 10 K.
These features of the c and cT vs. T plots are consistent with
those for isolated [Ru2II,II] complexes with an S = 1 ground state
affected by strong zero-field splitting (ZFS; D ª 230–320 cm-1). The
increase in c at low temperatures was ascribed to a paramagnetic
[Ru2II,III]+ species with S = 3/2. The magnetic data were simulated
by using a Curie paramagnetic model with S = 1 taking into
account ZFS, temperature-independent paramagnetism (TIP), an
impurity with S = 3/2 (r), and intermolecular interactions (zJ)
commonly used for magnetically isolated or weakly interacting
˚
Ru–Oeq bond lengths vary in the range of 2.00–2.02 A and 2.06–
2.07 A for [Ru2
]
and [Ru2II,II], respectively.1 In the case of
the present compounds, the bond lengths are about 2.06–2.07 A,
consistent with [Ru2II,II]. The bond lengths involving the axial THF
II,III
+
˚
˚
II,II
˚
(Ru–Oax) of ca. 2.3–2.4 A are also characteristic of the [Ru2
]
valence state.
Magnetic data correlated well with the orbital states of the spin
ground state of [Ru2II,II] compounds. The temperature dependence
of the dc susceptibilities of all compounds was measured on
polycrystalline samples in the temperature range of 1.8–300 K at
1 T for o-F and m-F and 0.1 T for the other complexes (the c and
cT vs. T plots are shown in Fig. S4†). The cT values at 300 K were
determined to be in the range of 0.74–1.07 cm3 K mol-1, which,
678 | Dalton Trans., 2011, 40, 673–682
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Table 3 Magnetic parameters of [Ru2(FxPhCO2)4(THF)2] with g = 2.00
(fix) and zJ¢ = 0 (fix)
Compounds D/kB/K TIP [¥ 10-6 cm3 mol-1] r [¥ 10-3] Ra [¥10-8]
o-F
333(1)
366(1)
449(1)
341(1)
356(1)
385.7(5) 215(5)
358.1(4) 695(4)
367(1)
370.3(6) 298(6)
374.5(6) 341(6)
251(14)
476(15)
287(9)
347(15)
362(12)
2.84(3)
3.90(5)
6.91(1)
4.03(3)
2.46(6)
1.243(9)
0.511(8)
3.18(3)
1.11(1)
4.73(3)
6.24(4)
4.77(1)
0.516(8)
0.046(7)
40.1
41.4
9.47
49.4
19.8
4.52
3.99
52.7
6.41
4.33
28.4
12.4
4.33
2.67
m-F
p-F
2,6-F2
3,4-F2
3,5-F2
2,3,4-F3
2,3,6-F3
2,4,5-F3
2,4,6-F3
3,4,5-F3
2,3,4,5-F4
2,3,5,6-F4
F5
610(17)
375(1)
762(14)
368.2(8) 619(8)
345.5(4) 530(5)
396.5(4) 388(4)
a R = R(ccalc - cobs)2/Rcobs
.
2
[Ru2II,II] complexes.13,23,37 The intermolecular interaction (zJ) was
introduced in the framework of the mean-field approximation
but not required to obtain adequate fitting (so, zJ ª 0 for all
complexes). The best fit of parameters for all compounds are listed
in Table 3, where the g value was fixed to 2.00. The estimated D
value is typical for general [Ru2II,II] species.17,18,21,23
Understanding the effects of the F-substituents on the redox
properties is the main subject of this report. Cyclic voltammo-
grams (vs. Ag/Ag+) of each compound acquired in N2-saturated
THF solutions with n-Bu4N(PF6) as the supporting electrolyte are
shown in Fig. 2. For all compounds, a reversible one-electron
redox wave, assigned to [Ru2II,II]/[Ru2II,III]+, was observed with
Ic/Ia ª 1 and DEp ª 47–183 mV. It should be emphasized that
the redox potential of the present series was observed over a wide
range of potentials from -40 to ca. 350 mV, showing that the
substituent had an effect on the redox properties of the complexes.
The electrochemical data are summarized in Table 4 together with
pKa of the corresponding F-substituted benzoic acids and the
summation of the Hammett constants for m- and p-F groups,
R(xsm + ysp), where sm = 0.337, sp = 0.06238 and x and y the are
numbers of m- and p-F-substituents, respectively.
Fig. 2 Cyclic voltammograms of a series of [Ru2II,II(FxPhCO2)4(THF)2]
in THF containing 0.1 M n-Bu4N(PF6) under N2.
Because there are F atoms at the o-positions in some complexes,
it can be difficult to evaluate the compounds using a unified
methodology, such as a general Hammett analysis. Therefore, in
order to compare all of the redox potentials, the half-wave poten-
tial (E1/2) of the [Ru2II,II]/[Ru2II,III]+ redox wave of the complexes
and [Ru2(PhCO2)4(THF)2]22 were plotted as a function of pKa of
the corresponding benzoic acids (Fig. 3). The correlation between
E1/2 and pKa of the ligands is seemingly complicated. However,
the ligands could be classified into three groups independent of
the other substituent positions: non-, mono-, and di-o-substituted
complexes. These classifications show that: (1) The pKa value,
which represents the electronic state at the terminal oxygen atoms
of the carboxylate ligands, is strongly affected by the presence of o-
F atoms. The pKa ranges for each group are 3.5–4.2, 2.5–3.3, and
1.5–2.5 for the non-, mono-, and di-o-substituted carboxylates,
respectively. (2) In the respective groups, E1/2 is dependent on
the electronic effect of m- and p-F groups, which obeys the
general Hammett law (vide infra), leading to a linear relationship
within each group and similar slopes. (3) The separation between
neighboring lines (DpKa) is almost identical, meaning that the o-F
Fig. 3 Plot of half-wave redox potential (E1/2) vs. pKa, where E1/2 are
values measured in THF with a Ag/Ag+ reference electrode. The red,
green, and blue solid lines represent the linear least-squares fitted lines for
non-, mono-, and di-o-F substituted groups, respectively.
atoms could have an electronic effect rather than a steric effect at
least in solution. If a steric effect due to o-F atoms is dominant,
DpKa between the non- and mono-o-F subgroups would be much
larger than that between lines of the mono- and di-o-F groups.
E1/2 was plotted as a function of R(xsm + ysp), as shown
in Fig. 4a. Linear trends in respective non-, mono-, and di-o-F
groups as well as the pKa dependence were observed, although
mismatches in the data from the linear least-squares fitted lines
This journal is
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Table 4 Electrochemical data of [Ru2(FxPhCO2)4(X)2] measured in THF containing 0.1 M n-Bu4N(PF6) under N2 (mV vs. Ag/Ag+)a (nd = no referential
data)
Compounds
Ea/mV
Ec/mV
E
1/2/mV
DEp/mV
Ia/Ic
pKa of FxPhCO2H
R(xsm+ysp)b
R(xsm+ysp+zso)c
[Ru2II,II(PhCO2)4(THF)2]d
nd
84
99
11
nd
60
35
42
-39
51
144
178
209
336
214
172
274
315
360
358
nd
98
nd
4.20
3.27
3.86
4.14
2.34
3.80
3.59
2.87
2.00
2.87
2.28
3.46
2.53
1.66
1.60
0
0
0
o-F
-14
-15
-89
-14
52
125
139
277
164
99
213
246
308
287
1.00
0.99
0.99
1.02
0.96
1.08
1.06
1.01
1.01
1.10
1.06
1.09
1.00
0.99
0.200
0.337
0.062
0.400
0.399
0.674
0.599
0.737
0.599
0.462
0.736
0.936
1.074
1.136
m-F
114
100
129
183
105
139
117
99
145
121
137
104
142
0.337
0.062
0
p-F
2,6-F2
3,4-F2
3,5-F2
2,3,4-F3
2,3,6-F3
2,4,5-F3
2,4,6-F3
3,4,5-F3
2,3,4,5-F4
2,3,5,6-F4
F5
115
235
230
278
394
263
244
334
383
412
429
0.399
0.674
0.399
0.337
0.399
0.062
0.736
0.736
0.674
0.736
a The ferrocene/ferrocenium couple, Fc/Fc+ = 213 mV (Ic/Ia ª 1, DEp = 91 mV), was observed at the same condition described in the Experimental
Section in the text. b sm = 0.337, sp = 0.062 referred from ref. 38 c so = 0.2 was experimentally estimated in this work. d Ref. 22.
by the electronic effect of the o-F atoms. Thus, we estimated a
Hammett constant for o-F (so) to be 0.2 and revised Fig. 4a
as a function of R(xsm + ysp + zso) (Fig. 4b), where z is 0
to 2 for the non-, mono-, and di-o-F groups, respectively. The
relationship was found to be linear. It should be pointed out that
the electronic effect of the o-F atoms affecting the redox properties
of [Ru2II,II] complexes is intermediate between those of the m- and
p-F atoms. Thus, by adding fluorine atoms on the benzoate ligand,
II,II
we were able to tune systematically the redox properties of [Ru2
complexes.
]
In order to determine the energy levels of molecular orbitals in
the present compounds, calculations based on density functional
theory (DFT) at the B3LYP (UB3LYP) level with basis functions
LANL2TZ(f)32 for Ru and LANL2DZpd33 for other atoms, where
the atomic coordinates determined from X-ray crystallography
were used, an Sz = 1 (spin multiplicity of 3) spin state was
assumed because of their paramagnetic character confirmed by
using magnetic measurements, and the charge of molecules was
zero, were carried out using Gaussian 03. As a representative
example, the energy levels of the frontier orbitals involving the
p* and d* orbitals on the diruthenium unit of 2,6-F2 are shown in
Fig. 5 (Fig. S5† for others). Taking into account the correlations
between electrons, on the basis of the calculations using the
UB3LYP function with a spin multiplicity of 3 (Sz = 1), the
configurations of the a and b electrons are given,39 and the HOMO
level corresponds to d*(b), which is most likely involved in the
[Ru2II,II]/[Ru2II,III]+ couple. The list of energy levels calculated for
each complex is given in Table S2.†
Fig. 6 shows a plot of the HOMO (d* for b spin) energy
levels vs. pKa of the ligands. We observed a similar trend to
that in the plot of E1/2 vs. pKa in Fig. 3, in which three groups
made up of non-, mono-, and di-o-F substituted complexes
obeying quasi-linear relationships dependent on the presence of
m- and p-F substituents, where the solid lines represent the least-
squares fitted lines for each group. Thus, the effect of the F
atoms corresponds to the energy levels estimated by using DFT
calculations based on solid-state structures. The real HOMO
level is directly associated with ID of the complexes. Therefore,
understanding the relationship between the calculated HOMO
Fig. 4 Plots of half-wave redox potential (E1/2) vs. (a) R(xsm + ysp) and
(b) R(xsm + ysp + zso), where sm = 0.337, sp = 0.062, and so = 0.2 and
x, y, and z are numbers of m-, p-, and o-F-substitutions, respectively. The
solid lines in (a) represent the linear least-squares fitted line for respective
subgroups, and the solid line in (b) represents that for all data.
occurred. Considering the results of the pKa dependence, only the
number of o-F atoms had an effect. Namely, the separation among
the lines of the non-, mono-, and di-o-F groups is mainly caused
680 | Dalton Trans., 2011, 40, 673–682
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Fig. 5 Frontier orbitals associated with p* and d* orbitals of the
diruthenium unit of 2,6-F2 and their energy levels (eV), where d* for b
electron corresponds to HOMO level. See Fig. S5† and Table S2† for the
other complexes.
Fig. 7 Plot of HOMO (d*(b)) level vs. E1/2, where the solid line represents
the least-squares linear fitting line for the non-o-F group in relation to a
standard line based mainly on the electronic effect of the F atoms.
words, complexes with small dihedral angles (< 23◦) are located
near the line, although there are a few exceptions (2,6-F2 and 2,4,6-
F3). As seen in the plots of E1/2 vs. pKa and Hammett constants,
the main effect of o-substitution on the redox potential was found
to be an electronic effect in solution. This fact suggests that the
phenyl groups of the FxPhCO2 ligands rotate freely in solution
without significant barrier even in mono- and di-o-F complexes.
Consequently, the steric factor due to substitution at the ortho
position does not have an effect in solution. On the other hand, the
DFT calculations directly reflect a static state structure. Therefore,
steric effects should be included in the results. Thus, the HOMO
energy levels of complexes in solution media are expected to be
near the standard line in Fig. 7.
Fig. 6 Plot of HOMO (d*(b)) level vs. pKa. The red, green, and blue solid
lines represent the least-squares linear fit lines for non-, mono-, and di-o-F
substituted groups, respectively.
Conclusions
level in the solid state and E1/2 in solution is very convenient for
estimating the real HOMO level of the complexes in redox reaction
media (i.e., in solution). In the plot of the HOMO (d* for b spin)
energy levels vs. pKa (Fig. 6), lines for the mono- and di-o-F groups
are not parallel to that for the non-o-F group, whereas they are
almost parallel to each other in the plot of E1/2 vs. pKa (see Fig. 3).
This is most likely because there is a structural (steric) effect in
the estimated energy levels in addition to the electronic effect due
to the use of the solid-state atomic coordinates. A plot of the
HOMO energy level vs. E1/2 is shown in Fig. 7, where the solid
line is the least-squares fitted (LS) line for the non-o-F group. We
define the LS line for the non-o-F group as a standard line to
show the electronic effect of the F atoms. Some of complexes are
considerably far from the standard line, which could be due to both
electronic and steric effects occurring at the same time. Although
quantitative analysis of the steric effect is nearly impossible, we
could find a rough trend where the complexes that lie far from
the standard line tend to have a large dihedral angle defined by
the plane of phenyl group (plane 1) and the plane composed of a
carboxy group and a diruthenium unit (plane 2) (Table 2). In other
A series of [Ru2II,II(FxPhCO2)4(THF)2] was synthesized using
comparatively-simple synthetic routes, and the addition of fluorine
groups on the benzoate ligands made it possible to tune the redox
potential over a wide range from -40 mV to 350 mV (in THF).
Thus, although most of the compounds were air-sensitive, 2,3,6-
F3, 2,3,4,5-F4, 2,3,5,6-F4, and F5, which have E1/2 > 300 mV, were
relatively stable in air.
II,III
+
The redox potential of the [Ru2II,II]/[Ru2
] redox couple in
THF, which is a general reaction medium, is strongly dependent
on the electronic effect of o-F atoms, whereas substituents at
the m- and p-positions shift the redox potentials in the positive
direction in relation to the general Hammett law. The F atoms
affected both the redox potential and the HOMO energy levels.
However, since the HOMO energy levels were calculated using
atomic coordinates of the solid-state structures, they contained
contributions from steric effects. Thus, the HOMO energy level of
the present complexes correlated to the redox potential. Namely,
the main factor determining ID of complexes in a solution, in
which the phenyl rings can freely rotate, is the electronic effect of
the F atoms.
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The authors acknowledge Prof. Brian K. Breedlove, Tohoku Uni-
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for Scientific Research from the Ministry of Education, Culture,
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682 | Dalton Trans., 2011, 40, 673–682
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