Synthesis, structural, spectroscopic, and electrochemical characterization of high oxidation state diruthenium complexes containing four identical unsymmetrical bridging ligands

Article abstract of DOI:10.1021/ic0499594

Six Ru₂⁶⁺ derivatives of the form Ru ₂(L)₄(C≡CC₆H₅)₂, where L = 2-Fap, 2,3-F₂ap, 2,4-F₂ap, 2,5-F₂ap, 3,4-F₂ap, or 2,4,6-F₃ap, are synthesized and characterized as to their electrochemical, spectroscopic, and/or structural properties. These compounds are synthesized from a reaction between LiC≡CC ₆H₅ and Ru₂(L)₄CI. Two of the investigated complexes exist in a (4,0) isomeric form while four adopt a (3,1) geometric conformation. These two series of geometric isomers are compared with previously characterized (4,0) Ru₂(ap)₄(C≡CC ₆H₅)₂, (4,0) Ru₂(F ₅ap)₄(C≡CC₆H₅)₂, and (3,1) Ru₂(F₅ap)4(C≡CC₆H ₅)₂. The overall data on the nine compounds thus provide an opportunity to systematically examine how the electrochemical and structural properties of these Ru₂⁶⁺ complexes vary with respect to isomer type and electronic properties of the bridging ligands.

Full text of DOI:10.1021/ic0499594

Inorg. Chem. 2004, 43, 4825−4832
Synthesis, Structural, Spectroscopic, and Electrochemical
Characterization of High Oxidation State Diruthenium Complexes
Containing Four Identical Unsymmetrical Bridging Ligands
,†
Karl M. Kadish,* Tuan D. Phan,† Li-Lun Wang,† Lingamallu Giribabu,† Antoine Thuriere,†
,†
Julien Wellhoff,† Shurong Huang,† Eric Van Caemelbecke,†,‡ and John L. Bear*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, and
Department of Chemistry, Houston Baptist UniVersity, Texas 77074-3298
Received January 8, 2004
Six Ru26+ derivatives of the form Ru2(L)4(CtCC H ) , where L ) 2-Fap, 2,3-F ap, 2,4-F ap, 2,5-F ap, 3,4-F ap, or
6
5 2
2
2
2
2
2,4,6-F ap, are synthesized and characterized as to their electrochemical, spectroscopic, and/or structural properties.
3
These compounds are synthesized from a reaction between LiCtCC H and Ru2(L)4Cl. Two of the investigated
6
5
complexes exist in a (4,0) isomeric form while four adopt a (3,1) geometric conformation. These two series of
geometric isomers are compared with previously characterized (4,0) Ru2(ap)4(CtCC H ) , (4,0) Ru2(F5ap)4-
6
5 2
(CtCC H ) , and (3,1) Ru (F ap)4(CtCC H ) . The overall data on the nine compounds thus provide an opportunity
6
5 2
2
5
6 5 2
to systematically examine how the electrochemical and structural properties of these Ru26+ complexes vary with
respect to isomer type and electronic properties of the bridging ligands.
Introduction
Ru2(L)4Cl, where L is one of the eight anionic ligands shown
in Chart 1. These complexes can also exist in up to four
different isomeric forms, (4,0), (3,1), (2,2)-cis, or (2,2)-trans,
depending upon the number and position of fluorine atoms
on the phenyl group of the anilinopyridinate anion, but in
virtually all cases, only one or two isomers are detected
for a given compound. These are the (4,0) and (3,1)
isomers.9,26-29
A large number of diruthenium complexes coordinated by
equatorial and axial ligands containing substantially different
σ and π donor/acceptor properties have been synthesized and
characterized as to their spectroscopic and electrochemical
properties.1-25 Among these compounds are a group of
substituted anilinopyridine bridged complexes of the type
* Author to whom correspondence should be addressed. E-mail:
kkadish@uh.edu.
(15) Lin, C.; Ren, T.; Valente, E. J.; Zubkowski, J. D. J. Chem. Soc., Dalton
Trans. 1998, 571.
(16) Bear, J. L.; Li, Y.; Cui, J.; Han, B.; Van Caemelbecke, E.; Phan, T.;
Kadish, K. M. Inorg. Chem. 2000, 39, 857.
(17) Bear, J. L.; Li, Y.; Han, B.; Van Caemelbecke, E.; Kadish, K. M.
Inorg. Chem. 2001, 40, 182.
† University of Houston.
‡ Houston Baptist University.
(1) Chakravarty, A. R.; Cotton, F. A. Inorg. Chim. Acta 1986, 113, 19.
(2) Malinski, T.; Chang, D.; Feldmann, F. N.; Bear, J. L.; Kadish, K. M.
Inorg. Chem. 1983, 22, 3225.
(3) Chakravarty, A. R.; Cotton, F. A. Polyhedron 1985, 4, 1957.
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10.1021/ic0499594 CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/10/2004
Inorganic Chemistry, Vol. 43, No. 16, 2004 4825

Kadish et al.
shown, the Ru27+, Ru25+, and Ru24+ forms of the compounds
are all stable on the cyclic voltammetry time scale, and the
measured E1/2 values are discussed in terms of the electronic
properties of the bridging ligands and the isomer type, (4,0)
or (3,1). These data are also compared to results obtained in
parallel studies carried out on Ru2(dpf)4(CtCC6H5)2 and its
derivatives, where dpf is the N,N′-diphenylformamidinate
anion.
Chart 1
Ru2(dpf)4(CtC)x (x ) 2 or 4) has been used as a
precursor30 for the synthesis of nanomolecular assemblies
with the aim of generating conducting polymers as well as
nonlinear optical systems, and one can anticipate that both
the electronic effect of the bridging ligands and the arrange-
ment of the bridging ligands around the dimetal core, i.e.,
(4,0) or (3,1), may alter the conducting and/or nonlinear
optical properties of synthesized molecules that contain a
Ru2(L)4(CtC)x unit where L is ap or a substituted ap
bridging ligand.
Experimental Section
Chemicals and Reagents. High-purity nitrogen was purchased
from Matheson-Trigas Co. and was passed through anhydrous
calcium sulfate and potassium hydroxide pellets to remove trace
oxygen and water prior to use. GR grade dichloromethane, n-hexane
(Aldrich), absolute dichloromethane (Fluka) for electrochemical
measurements, and absolute ethyl alcohol (McCormick, Inc.) for
recrystallization were used without purification. Lithium phenyl-
acetylide (LiCtCC6H5) in 0.1 M THF was purchased from Aldrich
and used as received. Tetra-n-butylammonium perchlorate (TBAP)
was purchased from Fluka Chemical Co., twice recrystallized from
ethyl alcohol, and stored in a vacuum oven at 40 °C for at least
one week prior to use. Silica gel (Merck 230-400 mesh 60 Å)
and anhydrous THF were all purchased from Aldrich and used as
received.
Physical Measurements. Cyclic voltammetry was carried out
using either an EG&G Princeton applied research (PAR) model
173 or a 263 potentiostat/galvanostat. A three-electrode system was
used and consisted of a glassy carbon working electrode, a platinum
wire counter electrode, and a homemade saturated calomel electrode
(SCE) as the reference electrode. The SCE was separated from the
bulk of the solution by a fritted-glass bridge of low porosity that
contained the solvent/supporting electrolyte mixture. All potentials
are referenced to the SCE, and all measurements were carried out
at room temperature.
Most Ru25+ complexes of this type can undergo two metal-
centered oxidations and one metal-centered reduction under
7+
a N2 atmosphere,28 giving compounds with Ru26+, Ru2
,
4+
and Ru2 cores. However, under a CO atmosphere,29 five
oxidation states may be electrogenerated for the same series
of compounds. These are Ru26+, Ru25+, Ru24+, Ru23+, and
2+
Ru2
.
Linear free energy relationships and isomer effects on the
structural and electrochemical properties of low oxidation
state anilinopyridinate (ap) and substituted ap complexes
have both been reported,28,29 and it was of interest to examine
high oxidation state compounds with the same type of
bridging ligands and the same isomer type. This is described
in the current Article, which builds upon earlier reported
structural and electrochemical results for other related
compounds which have four identical unsymmetrical bridg-
ing ligands.9,24
Six new Ru2
6+
derivatives of the form Ru2(L)4-
UV-visible spectra were recorded with a Hewlett-Packard model
8453 diode array spectrophotometer. IR spectra were recorded on
a FTIR Nicolet 550 Magna-IR spectrophotometer. Elemental
analyses were carried out by Atlantic Microlab, Inc., Norcross, GA.
Mass spectra were recorded either on a Finnigan TSQ 700
instrument at the University of Texas, Austin or on an Applied
Biosystem Voyager DE-STR MALDI-TOF mass spectrometer
equipped with a nitrogen laser (337 nm) at the University of
Houston Mass Spectrometry Laboratory.
Synthesis of Starting Materials. The three (4,0) isomers of
Ru2(L)4Cl (L ) ap, 2,5-F2ap, or 3,4-F2ap), the four (3,1) isomers
of Ru2(L)4Cl (L ) 2-Fap, 2,3-F2ap, 2,4-F2ap, or 2,4,6-F3ap) as well
as (4,0) Ru2(ap)4(CtCC6H5)2 were prepared as described in the
literature.24,26-29
(CtCC6H5)2, where L ) 2-Fap, 2,3-F2ap, 2,4-F2ap, 2,5-F2-
ap, 3,4-F2ap, or 2,4,6-F3ap, are synthesized and characterized
in the present Article as to their electrochemical, spectro-
scopic, and/or structural properties. The data on these six
complexes are then compared with previously characterized
(4,0) Ru2(ap)4(CtCC6H5)2, (4,0) Ru2(F5ap)4(CtCC6H5)2, and
(3,1) Ru2(F5ap)4(CtCC6H5)2. Four of the nine compounds
exist in a (4,0) isomeric form while five adopt a (3,1)
geometric conformation. These two series of compounds thus
provide a large enough number of derivatives to examine
how the electrochemical and structural properties of these
6+
Ru2 complexes vary with respect to isomer type and
electronic properties of the bridging ligands. As will be
(29) Kadish, K. M.; Phan, T.; Giribabu, L.; Shao, J.; Wang, L.-L.; Thuriere,
(30) Wong, K.-T.; Lehn, J.-M.; Peng, S.-M.; Lee, G.-H. Chem. Commun.
2000, 2259.
A.; Van Caemelbecke, E.; Bear, J. L. Inorg. Chem. 2004, 43, 1012.
4826 Inorganic Chemistry, Vol. 43, No. 16, 2004

High Oxidation State Diruthenium Complexes
Table 1. Crystal Data and Data Collection and Processing Parameters
for the (3,1) Isomers of 3 and 5
Synthesis of Ru2(L)4(CtCC6H5)2 Complexes. Ru2(L)4-
(CtCC6H5)2 (L ) 2-Fap, 2,3-F2ap, 2,4-F2ap, 2,5-F2ap, 3,4-F2ap,
or 2,4,6-F3ap) was synthesized by stirring a mixture of Ru2(L)4Cl
and Li(CtCC6H5) in deaerated THF in a 1:25 molar ratio under
N2 for 10 h. During this time, the color gradually changed from
dark green to dark red. Upon exposure to air, the color of the
solution changed to dark blue within 30 min. Evaporation of the
solvent yielded in each case a dark blue residue, which was purified
by silica gel column chromatography using a mixture of CH2Cl2/
n-hexane (3/7, v/v) to give Ru2(L)4(CtCC6H5)2.
(4,0) Ru2(3,4-F2ap)4(CtCC6H5)2 (1). Yield: 60%. Mass spectral
data [m/e, (fragment)]: 1228 [Ru2(3,4-F2ap)4(CtCC6H5)2]+, 1126
[Ru2(3,4-F2ap)4(CtCC6H5)]+, 1022 [Ru2(3,4-F2ap)4]+. Anal. Calcd
for C60H38N8F8Ru2: C, 63.82; H, 3.09; N, 9.93. Found: C, 63.72;
H, 3.04; N, 9.91. IR (cm-1): 2080 [ν(CtC)]. UV-vis spectrum
in CH2Cl2 [λmax, nm (ꢀ × 10-3, M-1 cm-1)]: 623 (5.9), 1031 (2.0).
(4,0) Ru2(2,5-F2ap)4(CtCC6H5)2 (2). Yield: 35%. Mass spectral
data [m/e, (fragment)]: 1228 [Ru2(2,5-F2ap)4(CtCC6H5)2]+, 1126
[Ru2(2,5-F2ap)4(CtCC6H5)]+, 1022 [Ru2(2,5-F2ap)4]+. Anal. Calcd
for C60H38N8F8Ru2: C, 63.82; H, 3.09; N, 9.93. Found: C, 63.74;
H, 3.02; N, 9.88. IR (cm-1): 2081 [ν(CtC)]. UV-vis spectrum
in CH2Cl2 [λmax, nm (ꢀ × 10-3, M-1 cm-1)]: 622 (6.4), 1030 (1.8).
(3,1) Ru2(2-Fap)4(CtCC6H5)2 (3). Yield: 58%. Mass spectral
data [m/e, (fragment)]: 1154 [Ru2(2-Fap)4(CtCC6H5)2]+, 1053
[Ru2(2-Fap)4(CtCC6H5)]+, 952 [Ru2(2-Fap)4]+. IR (cm-1): 2084
[ν(CtC)]. UV-vis spectrum in CH2Cl2 [λmax, nm (ꢀ × 10-3, M-1
cm-1)]: 369 (4.9), 615 (3.2).
(3,1) Ru2(2,4-F2ap)4(CtCC6H5)2 (4). Yield: 69%. Mass spectral
data [m/e, (fragment)]: 1228 [Ru2(2,4-F2ap)4(CtCC6H5)2]+, 1126
[Ru2(2,4-F2ap)4(CtCC6H5)]+, 1022 [Ru2(2,4-F2ap)4]+. Anal. Calcd
for C60H38N8F8Ru2: C, 63.82; H, 3.09; N, 9.93. Found: C, 63.91;
H, 3.10; N, 9.90. IR (cm-1): 2081 [ν(CtC)]. UV-vis spectrum
in CH2Cl2 [λmax, nm (ꢀ × 10-3, M-1 cm-1)]: 455 (4.0), 627 (3.1).
(3,1) Ru2(2,4,6-F3ap)4(CtCC6H5)2 (5). Yield: 52%. Mass
spectral data [m/e, (fragment)]: 1296 [Ru2(2,4,6-F3ap)4(Ct
CC6H5)2]+, 1195 [Ru2(2,4,6-F3ap)4(CtCC6H5)]+, 1094 [Ru2(2,4,6-
F3ap)4]+. IR (cm-1): 2080 [ν(CtC)]. UV-vis spectrum in CH2Cl2
[λmax, nm (ꢀ × 10-3, M-1 cm-1)]: 466 (8.1), 634 (12.3), 892 (3.1).
(3,1) Ru2(2,3-F2ap)4(CtCC6H5)2 (6). Yield: 68%. Mass spectral
data [m/e, (fragment)]: 1228 [Ru2(2,3-F2ap)4(CtCC6H5)2]+, 1126
[Ru2(2,3-F2ap)4(CtCC6H5)]+, 1022 [Ru2(2,3-F2ap)4]+. Anal. Calcd
for C60H38N8F8Ru2: C, 63.82; H, 3.09; N, 9.93. Found: C, 63.99;
H, 3.15; N, 9.87. IR (cm-1): 2079 [ν(CtC)]. UV-vis spectrum
in CH2Cl2 [λmax, nm (ꢀ × 10-3, M-1 cm-1)]: 450 (3.8), 630 (2.8)
X-ray Crystallography of Compounds 3 and 5. Single-crystal
X-ray crystallographic studies were performed at the University of
Houston X-ray Crystallographic Center. Each sample was placed
in a steam of dry nitrogen gas at -50 °C in a random position.
The radiation used was Mo KR monochromatized by a highly
ordered graphite crystal. Final cell constants as well as other
information pertinent to data collection and structure refinement
are listed in Table 1.
The measurement for 3 was made with a Nicolet R3m/V
automatic diffractometer, while the measurement for 5 was made
with a Siemens SMART platform diffractometer equipped with a
1K CCD area detector. A hemisphere of data 1271 frames at 5 cm
detector distance was collected using a narrow-frame method with
scan widths of 0.30° in ω and an exposure time of 35 s/frame (3)
or 25 s/frame (5). The first 50 frames were remeasured at the end
of data collection to monitor instrument and crystal stability, and
the maximum correction on I was <1%. The data were integrated
using the Siemens SAINT program, with the intensities corrected
for Lorentz factor, polarization, air absorption, and absorption
Ru2(2-Fap)4(CtCC6H5)2
(3)
Ru2(F3ap)4(CtCC6H5)2
(5)
space group
cell constant
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
P1h triclinic
P21/c monoclinic
12.0286(7)
14.4427(9)
17.7049(11)
110.649(1)
97.758(1)
103.353(1)
2720.1(3)
C60H42N8F4Ru2.CH2Cl2
1238.08
11.0146(5)
19.7799(9)
24.1941(11)
90.00
101.145(1)
90.00
5171.7(4)
C60H34N8F12Ru2
1297.09
4
1.666
0.678
0.71073
223
0.0235
mol formula
fw (g/mol)
Z
2
F
calcd (g/cm3)
1.512
0.715
0.71073
223
0.0457
0.1297
µ (cm-1
)
λ (Mo KR) (Å)
temp (K)
R (Fo)a
Rw (Fo)b
0.0126
2
a R ) ∑|Fo| - |Fc|/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
because of variation in the path length through the detector
faceplate. A Ψ-scan absorption correction was applied on the basis
of the entire data set. Redundant reflections were averaged. Final
cell constants were refined using 4332 reflections for 3 and 6241
reflections for 5 having I > 10σ(I). The Laue symmetries were
determined to be 1h for compound 3 and 2/m for compound 5, and
from the systematic absences noted, the space groups were shown
to be P1h for 3 and P21/c for 5.
Results and Discussion
Synthesis and Reaction Mechanism for Compounds
6+
1-6. The air-stable Ru2 complexes, formulated as Ru2-
(L)4(CtCC6H5)2 (L ) 2-Fap, 2,3-F2ap, 2,4-F2ap, 2,5-F2ap,
3,4-F2ap, or 2,4,6-F3ap), were synthesized and characterized
with respect to their electrochemical and/or structural proper-
ties. The exact stoichiometry of the reaction depends on the
amount of added LiCtCC6H5, and the bis-adduct Ru2(L)4-
(CtCC6H5)2 is favored over the monoadduct Ru2(L)4-
(CtCC6H5) when a large excess of LiCtCC6H5 is used.9,11
Therefore, a large excess of LiCtCC6H5 (25 mmol) was
reacted with Ru2(L)4Cl (1 mmol) to maximize the yield of
Ru2(L)4(CtCC6H5)2. The sequence of reactions shown in
eqs 1-3 has been proposed for the synthesis of Ru2(L)4-
(CtCC6H5)2 (L ) dpf, ap, or F5ap),9,15,24 and these three
reactions can also account for synthesis of the Ru2(L)4-
(CtCC6H5)2 derivatives examined in the present study.
Ru2(L)4Cl + CtCC6H5- f Ru2(L)4(CtCC6H5) + Cl- (1)
Ru2(L)4(CtCC6H5) + CtCC6H5- f
-
Ru2(L)4(CtCC6H5)2 (2)
Ru2(L)4(CtCC6H5)2- + air f Ru2(L)4(CtCC6H5)2 (3)
Equation 1 involves the replacement of the Cl- anion by
one phenylacetylide anion, and this reaction is followed by
the addition of a second phenylacetylide ion in a trans
arrangement as shown in eq 2. The air-stable Ru26+ complex,
Ru2(L)4(CtCC6H5)2, is then obtained by air oxidation of the
Inorganic Chemistry, Vol. 43, No. 16, 2004 4827

Kadish et al.
Table 2. Selected Average Bond Lengths (Å) and Bond Angles (deg) for the (4,0) and (3,1) Isomers of Ru2(L)4(CtCC6H5)2
(4,0) isomer
(3,1) isomer
L
apa
F5apb
2-Fap (3)
2,4,6-F3ap (5)
F5apb
Bond Lengths (Å)
Ru-Ru
2.4707(8)
2.050
2.067
1.988
1.203
2.441(1)
2.072
2.065
1.953
1.211
2.4689(6)
2.020
2.130
2.002
1.203
2.4640(2)
2.034
2.101
1.984
1.206
2.475(1)
2.075
2.050
1.983
1.214
Ru-Na
Ru-Np
Ru-C
C-C
Bond Angles (deg)
Ru-Ru-C
163.4
85.60
87.00
22.40
171.4
87.65
85.91
20.40
161.0
90.31
83.08
18.20
164.3
88.31
85.78
17.90
163.2
90.48
84.21
14.40
Ru-Ru-Na
Ru-Ru-Np
N-Ru-Ru-N
a Taken from ref 24. b Taken from ref 9.
compounds 3 and 5, Ru1 is coordinated to an axial
phenylacetylide group and to three pyridyl nitrogen atoms
and one anilino nitrogen atom, while Ru2 is coordinated to
three anilino nitrogen atoms, one pyridyl nitrogen atom, and
an axial phenylacetylide ligand.
The Ru-Ru bond lengths of 3 and 5 are 2.469(6) Å and
2.464(2) Å, respectively. These values are comparable to
those of the ap24 and the two F5ap9 complexes and are within
the range of 2.316-2.599 Å for Ru-Ru bond distances of
6+
other reported Ru2 compounds of the type Ru2(L)4(X)2,
where L ) DiMeODMBA (N,N′-dimethyl-3,5-dimethoxy-
benzamidinate) or DmAniF (di-m-methoxyphenylformamidi-
- 31,32
nate) and X ) Cl- or (CtC)2Si(CH3)3 .
As shown in
Table 2, there is no obvious correlation between the
substituent effect of the bridging ligand and the bond lengths
or bond angles in the (3,1) isomers of Ru2(L)4(CtCC6H5)2.
For example, the Ru-Ru-C bond angle increases from
161.0° to 164.3° upon going from L ) 2-Fap to 2,4,6-F3ap
but decreases from 164.3° to 163.2° upon going from 2,4,6-
F3ap to F5ap. The Ru2(L)4 framework is less distorted when
it has more electron-withdrawing groups on the bridging
ligands as shown by the fact that, in the (4,0) isomer series,
the average N-Ru-Ru-N torsion angle follows the order:
ap (22.40°) > F5ap (20.40°) while in the (3,1) isomer series
it follows the order: 2-Fap (18.20°) > 2,4,6-F3ap (17.90°)
> F5ap (14.40°). One might have predicted an opposite
relationship if the steric hindrance between the axial and
bridging ligands is proposed to account for the twist of the
structural frame.
Figure 1. Molecular structures for the (3,1) isomer of (a) Ru2(2-Fap)4-
(CtCC6H5)2 (3) and (b) Ru2(2,4,6-F3ap)4(CtCC6H5)2 (5). H and/or F atoms
have been omitted for clarity.
5+
Ru2 species as shown in eq 3. The last step in this
mechanism is consistent with the fact that in each case the
solution turns from red to blue upon exposing the solution
to air (see Experimental Section).
The final bis-ligated Ru26+ product was purified by silica
gel column chromatography and obtained in yields ranging
from 52 to 69%; no increase in the yield was observed when
the mixture was left to react for a longer time.
Molecular Structures of Compounds 3 and 5. Selected
average bond lengths and average bond angles for com-
pounds 3 and 5 are summarized in Table 2, along with data
for (4,0) Ru2(ap)4(CtCC6H5)2, (4,0) Ru2(F5ap)4(CtCC6H5)2,
and (3,1) Ru2(F5ap)4(CtCC6H5)2. The molecular structures
of compounds 3 and 5 are illustrated in Figure 1a,b,
respectively. All intramolecular bond lengths and bond angles
as well as other structural data of the two compounds are
given in the Supporting Information. The coordination of
each Ru atom is essentially octahedral with four “substituted
ap” bridging ligands forming the equatorial plane. For
6+
The Ru-Ru bond distances of Ru2 complexes bridged
by ap and substituted ap ligands are significantly shorter than
the Ru-Ru bond distances of Ru26+ compounds with dpf or
substituted dpf ligands, and this can be attributed to the
features of the ap ligand itself since the N-C-N angle of
the ap ligand is smaller than that of the dpf ligand (see Chart
5+
2). A similar trend was also observed in the case of Ru2
complexes with ap or dpf ligands.13,28 The Ru-Ru bond
lengths in Ru2(L)4(CtCC6H5)2 (L ) ap, 2-Fap, 2,4,6-F3ap,
or F5ap) are, however, comparable to those of Ru2[((m-MeO)-
DMBA)4(CtCC6H5)2 (2.447(9) Å) and Ru2(DEBA)4-
(CtCC6H5)2 (2.458(9) Å) (where ((m-MeO)DMBA) is N,N′-
(31) Xu, G.-L.; Jablonski, C. G.; Ren, T. J. Organomet. Chem. 2003, 683,
388.
(32) Xu, G.; Ren, T. Inorg. Chem. 2001, 40, 2925-2927.
4828 Inorganic Chemistry, Vol. 43, No. 16, 2004

High Oxidation State Diruthenium Complexes
Chart 2
Table 3. Comparison of Ru-Ru Bond Lengths (Å) of the (4,0) and
(3,1) Isomers of Ru2(L)4Cl and Ru2(L)4(CtCC6H5)2 Complexes, Where
L Is ap or a Substituted ap Ligand
bond length (Å)
isomer type
ligand, L
ap
2-Fap
2,4,6-F3ap
Ru2(L)4Cl
Ru2(L)4(CtCC6H5)2
(4,0)
(3,1)
2.275(3)
2.286(3)
2.284(6)
2.470(3)
2.469(6)
2.464(2)
Figure 2. Cyclic voltammograms of (4,0) Ru2(L)4(CtCC6H5)2 (L ) ap,
3,4-F2ap, 2,5-F2ap or F5ap) in CH2Cl2 containing 0.1 M TBAP. Scan rate
) 0.1 V/s. Dashed line shows current-voltage curve obtained upon scanning
the potential beyond the second reduction of the compound.
dimethyl-m-methoxybenzamidinate and DEBA is N,N′-
dialkylbenzamidinate).23,31
The nature of the axial ligand L in Ru2(ap)4(L) complexes
is known to affect the Ru-Ru bond length. For example,
Ru2(ap)4(CtCR) has a Ru-Ru bond length of 2.362(5) Å
when R ) Si(CH3)3 and 2.3234(7) Å when R ) CH2OCH3.33
Both values are slightly larger than in Ru2(ap)4Cl, a
compound with a Ru-Ru bond length of 2.275(3) Å, thus
showing a slight elongation of the Ru-Ru bond upon going
from L ) Cl- to L ) CtCR-. A slight increase in the Ru-
Ru bond distance is also observed upon changing the
oxidation state from Ru25+ to Ru26+. For example, the Ru-
Ru bond length of Ru2(ap)4Cl is 0.03 Å shorter than that in
[Ru2(ap)4Cl]+.34 However, neither the change in oxidation
state from Ru25+ to Ru26+ nor the change in the type of axial
ligand from Cl- to CtCR- can account for the large increase
of 0.018-0.195 Å seen upon going from Ru2(L)4Cl to Ru2-
(L)4(CtCC6H5)2, where L ) ap, 2-Fap, or 2,4,6-F3ap (see
Table 3). This elongation of the Ru-Ru bond has been
explained for the case of L ) ap in terms of a change in the
electronic configuration from σ2π4δ2π*2δ* for the five-
coordinate complex to π4δ2π*4 for the six-coordinate acetyl-
Figure 3. Cyclic voltammograms of (3,1) Ru2(L)4(CtCC6H5)2 (L )
2-Fap, 2,4-F2ap, 2,3-F2ap, 2,4,6-F3ap or F5ap) in CH2Cl2 containing 0.1 M
TBAP. Scan rate ) 0.1 V/s.
-
ide derivative with two CtCC6H5 ligands on the diruthe-
nium core.24
A similar conclusion can be proposed to rationalize the
elongation of the Ru-Ru bond in the case of the 2-Fap and
2,4,6-F3ap derivatives. This result is also consistent with the
fact that these two compounds, as well as all other investi-
gated Ru2(L)4(CtCC6H5)2 complexes, have active NMR
spectra, which is expected if these diruthenium derivatives
all possess the electronic configuration π4δ2π*.4
Electrochemistry of Ru2(L)4(CtCC6H5)2. The redox
behavior of (4,0) Ru2(ap)4(CtCC6H5)2 and compounds 1-6
was investigated by cyclic voltammetry in the noncoordi-
nating solvent CH2Cl2 containing 0.1 M TBAP, and the data
were compared to the (4,0) and (3,1) isomers of Ru2(F5ap)4-
(CtCC6H5)2.9 Cyclic voltammograms of (4,0) Ru2(ap)4-
(CtCC6H5)2, (4,0) Ru2(F5ap)4(CtCC6H5)2, 1 and 2 are
shown in Figure 2, while cyclic voltammograms of com-
pounds 3-6 are illustrated in Figure 3 along with (3,1)
Ru2(F5ap)4(CtCC6H5)2. Half-wave potentials of the inves-
tigated complexes are listed in Table 4 along with half-wave
potentials for the (4,0) and (3,1) isomers of Ru2(F5ap)4-
(CtCC6H5)2.
6+
Five of the nine electrochemically examined Ru2 com-
plexes (1-4 and 6) undergo four one-electron metal-centered
processes (two oxidations and two reductions), while for the
ap derivative, the two isomers of F5ap, and compound 5,
only three metal-centered processes are seen, one of which
is an oxidation and the others are reductions. A stepwise
(33) Zou, G.; Alvarez, J. C.; Ren, T. J. Organomet. Chem. 2000, 596, 152.
(34) Cotton, F. A.; Yokochi, A. Inorg. Chem. 1997, 36, 567.
Inorganic Chemistry, Vol. 43, No. 16, 2004 4829

Kadish et al.
Table 4. Half-Wave Potentials (V vs SCE) in CH2Cl2 Containing 0.1 M TBAP and UV-Vis Spectral Data of Ru2(L)4(CtCC6H5)2 in CH2Cl2
redox reactions
λ
max, nm (ꢀ × 10-3, M-1 cm-1
)
Σσa
ligand, L
Ru2
Ru2
Ru2
Ru2
band I
band II
band III
ꢀI/II
ꢀII/II
8+/7+
7+/6+
6+/5+
5+/4+
(4,0) Isomer
-1.67b
-1.45
0.00
0.40
0.58
1.22
ap
0.55
-0.54
452(6.9)
633(5.6)
623(5.9)
629(6.4)
650
1011(3.0)
1031(2.8)
1030(3.4)
1.23
1.87
2.11
1.85
3,4-F2ap (1)
2,5-F2ap (2)
F5ap
1.42
1.43
0.70
0.76
0.90
-0.45
-0.42
-0.05
-1.39
-1.18
450
1.13
(3,1) Isomer
-1.54
0.24
0.30
0.54
0.58
1.22
2-Fap (3)
1.31
1.38
0.62
0.70
0.78
0.80
1.00
-0.58
-0.56
-0.42
-0.40
-0.13
469(4.9)
455(4.0)
466(2.0)
450(3.8)
480
615(3.2)
627(3.1)
634(3.1)
630(2.8)
650
1.53
1.30
0.64
1.40
0.62
2,4-F2ap (4)
2,4,6-F3ap (5)
2,3-F2ap (6)
F5ap
-1.50
-1.47
892(0.8)
1055
3.90
4.00
1.50
-1.40
-1.15
b
a Taken from ref 34. Epc value.
5+
conversion between compounds with Ru27+, Ru26+, Ru2
,
4+
or Ru2 cores can therefore be electrochemically ac-
complished as shown by eqs 4-6 for all of the investigated
8+
compounds, while Ru2 can be seen only for compounds
1-4 and 6 (eq 7). The ap, F5ap derivatives, and 5 cannot be
8+
converted to a Ru2 oxidation state under our utilized
experimental conditions. The notation for the compound is
n+
given below as (L)4Ru2 where n ) 4, 5, 6, 7, or 8.
(L)4Ru26+ + e- a (L)4Ru2
(L)4Ru25+ + e- a (L)4Ru2
(4)
(5)
(6)
(7)
5+
4+
(L)4Ru26+ a (L)4Ru27+ + e-
(L)4Ru27+ a (L)4Ru28+ + e-
Plots of E1/2 vs the sum of substituent constants (Σσ)35
for the three redox reactions given by eqs 4-6 of both (4,0)
and (3,1) isomers of Ru2(L)4(CtCC6H5)2 are shown in Figure
4a,b, respectively. The dependence of E1/2 on the electronic
effect of the substituents can be quantified by linear least-
squares fit of the data to the Hammett relationship as shown
in eq 8:36,37
∆E1/2 ) E1/2(X) - E1/2(H) ) 4ΣσF
(8)
Figure 4. Linear free energy relationships for the redox reactions of (a)
(4,0) isomers and (b) (3,1) isomers of Ru2(L)4(CtCC2H5)2.
where F is the reactivity constant. Because there are four
equivalent bridging ligands on each investigated Ru2
6+
complex, 4Σσ is used in eq 8. The reactivity constants (F)
and the correlation coefficients (R) for the plots in Figure 4
are listed in Table 5 along with the values of the substituted
dpf complexes with the same two CtCC6H5- axial ligands.15
The good correlation coefficient (R) values in Table 5
indicate that the same electron-transfer mechanism occurs
for each redox process in the two series of isomers.
As shown in Table 5, the F values of Ru2(L)4(CtCC6H5)2
range from 70 to 112 mV. The exact value depends on the
redox couple in both series of isomers, but different trends
are seen for the (4,0) and (3,1) isomers. For the (4,0) isomers,
the F values of the Ru25+/4+ and Ru26+/5+ redox couples are
Table 5. Comparison of F Values (mV) for the (4,0) and (3,1) Isomers
of Ru2(ap-type)4(CtCC6H5)2 and Ru2(dpf-type)4(CtCC6H5)2, with R
Values of Linear-Square Fit Given in Parentheses
redox reaction
5+/4+
6+/5+
7+/6+
compounds
Ru2
Ru2
Ru2
(4,0) isomers
(3,1) isomers
substituted dpfa
98 (0.98)
85 (0.97)
81 (0.99)
102 (0.94)
112 (0.99)
92 (0.99)
70 (0.98)
90 (0.97)
72 (0.99)
a Taken from ref 15.
of similar magnitude and both are larger than the F value
7+/6+
for the Ru2
process, while for the (3,1) isomers, the F
value of the Ru26+/5+ redox couple is larger than that of the
and Ru2
The F value for the Ru2
5+/4+
7+/6+
Ru2
processes.
(35) Zuman, P. Substituent Effects in Organic Polarography; Plenum
Press: New York, 1967.
(36) Zuman, P. The Elucidation of Organic Electrode Process; Academic
Press: London, 1967.
7+/6+
6+/5+
and Ru2
redox couples
also increases upon going from the (4,0) to (3,1) isomers
while the F value of the Ru25+/4+ process shows an opposite
(37) Hammett, L. P. Physical Organic Chemistry; Wiley: New York, 1970.
4830 Inorganic Chemistry, Vol. 43, No. 16, 2004

High Oxidation State Diruthenium Complexes
trend (see Table 5), thus suggesting that the substituent effect
of the bridging ligands also varies with the isomer type. This
5+
is also the case for the parent Ru2 complexes.28
The F values for a given redox process of the substituted
ap and substituted dpf complexes in Chart 2 depend on the
symmetry of the bridging ligands, i.e., symmetrical vs
unsymmetrical as shown by comparing the F values of
n+/(n-1)+
6+
Ru2
for Ru2 complexes bridged by substituted ap
6+
ligands to those of Ru2 complexes containing substituted
dpf bridging ligands (see Table 5). Overall, the data in Table
5 suggest that the substituted ap complexes usually exhibit
a greater substituent effect than the substituted dpf deriva-
tives, and a similar conclusion was also reached in the case
5+
of Ru2 complexes with the same bridging ligands (ap or
dpf).28
The proposed electronic configuration π4δ2π*4 of all
Ru2(L)4(CtCC6H5)2 derivatives implies that the reduction
of each compound involves the addition of an electron to
the first available antibonding δ* orbital, while the oxidation
involves the removal of an electron from the π* orbital. As
discussed above, only the first oxidation of the compounds
differs significantly in the F values upon going from the (4,0)
to (3,1) isomers, thus suggesting that the substituent effect
on the energy level of the π* orbital is more sensitive to the
isomer type than the substituent effect on the energy level
of the δ* orbital.
UV-Vis Studies. The UV-vis absorption features of the
investigated compounds are given in Table 4, and the UV-
vis spectra of the (4,0) and (3,1) isomers of Ru2(L)4-
(CtCC6H5)2 are given in the Supporting Information Figures
S1 and S2, respectively. The number of absorption bands
depends on both the type of isomer and the type of bridging
ligands. For instance, in the case of (4,0) isomer, the 3,4-
F2ap and 2,5-F2ap complexes exhibit bands II and III while
the F5ap derivative shows bands I and II and the ap derivative
shows all three bands (I, II, and III). The ratio of ꢀ values
between bands I and II, ꢀI/II, is 1.23 for the ap derivative,
and this is similar to a value of 1.13 for the ꢀI/II of the F5ap
derivative. The ꢀII/III values of the 3,4-F2ap and 2,5-F2ap are
also virtually the same, but they differ from the ꢀII/III value
of the ap complex.
Somewhat similar relationships are also seen for com-
pounds having a (3,1) conformation, as shown in Table 4.
All of the (3,1) isomeric complexes exhibit bands I and II
with the exception of the F3ap and F5ap derivatives, which
exhibit an extra band III of weak intensity. However, the
ratio of molar absorptivities between bands I and II of the
(3,1) isomeric complexes possessing four redox reactions
differs from those of the (3,1) isomeric compounds that
exhibit only three redox reactions. For example, the values
of ꢀI/II for the 2-Fap, 2,4-F2ap, and 2,3-F2ap complexes range
from 1.30 to 1.53, while the values of ꢀI/II for the F3ap and
F5ap derivatives are 0.64 and 0.62, respectively.
Figure 5. Thin-layer UV-visible spectral changes of (a) (4,0) Ru2(3,4-
F2ap)4(CtCC6H5)2 and (b) (3,1) Ru2(F3ap)4(CtCC6H5)2 in CH2Cl2, 0.2
M TBAP upon the first oxidation.
7+
are therefore self-consistent, and both suggest that each Ru2
species has the electronic configuration of π4δ2π*3 indepen-
dent of the isomer type and number of oxidations seen by
cyclic voltammetry. One may therefore wonder why a formal
Ru28+/7+ process is observed for compounds 1-4 and 6 but
not for the other investigated compounds. The electronic
configuration of the Ru27+ species indicates that the second
oxidation will involve the removal of one electron from a
π* orbital. It has been reported that the two π* orbitals can
have different energy levels,38 thus suggesting that the
removal of one electron from the lower energy π* orbital is
more difficult to access, and this could explain why the ap,
F3ap, and F5ap complexes undergo only one oxidation.
However, further theoretical studies will be needed to
confirm this interpretation of the data.
Summary
6+
Six Ru2 complexes of the type Ru2(L)4(CtCC6H5)2,
which exist in (3,1) or (4,0) isomeric form, were examined
as to their electrochemical, spectroscopic, and/or structural
properties. The structural data suggest that the Ru2(L)4
framework is not significantly affected by the bridging ligand
Similar UV-vis spectral changes are observed upon
oxidation of (4,0) Ru2(3,4-F2ap)4(CtCC6H5)2 and (3,1)
Ru2(F3ap)4(CtCC6H5)2, although the (4,0) isomer undergoes
two oxidations while the (3,1) isomer undergoes only one
(Figure 5). The spectral data and the electrochemical results
(38) Wesemann, J. L.; Chisholm, M. H. Inorg. Chem. 1997, 36, 3258.
Inorganic Chemistry, Vol. 43, No. 16, 2004 4831

Kadish et al.
5+
or the isomer type; a similar trend was observed for the Ru2
gratefully acknowledged. We thank Dr. J. D. Korp for
performing the X-ray analysis.
derivatives with ap or substituted ap bridging ligands. As is
the case for the ap derivative, the Ru26+ complexes with two
CtCC6H5 axial ligands have much longer Ru-Ru bond
distances than their parent Ru2 compounds. All of the
investigated complexes undergo two one-electron reductions
and one or two one-electron oxidations. The first oxidation
and both reductions follow linear free energy relationships
between E1/2 and the Hammett parameter of the substituents
on the bridging ligands.
-
5+
Supporting Information Available: X-ray crystallographic files
in CIF format for structural determination of (3,1) Ru2(2-Fap)4-
(CtCC6H5)2 (4) and (3,1) Ru2(2,4,6-F3ap)4(CtCC6H5)2 (6). UV-
visible spectra of investigated complexes are given in Figure S1
(4,0 isomer) and Figure S2 (3,1 isomer). This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. The support of the Robert A. Welch
Foundation (J.L.B., Grant E-918; K.M.K., Grant E-680) is
IC0499594
4832 Inorganic Chemistry, Vol. 43, No. 16, 2004