Interconversion between (3,1) and (4,0) isomers of Ru2(L) 4X complexes where L is 2-anilinopyridinate or 2-(2,4,6- trifluoroanilino)pyridinate anion and X = Cl- or C≡CC 5H4N-

Article abstract of DOI:10.1021/ic800787p

A reaction between the (4,0) isomer of Ru₂(ap)₄Cl and LiC≡CC₅H₄N leads to a (3,1) isomer of Ru ₂(ap)₄(C≡CC₅H₄N)₂ 1 (ap = anilinopyridinate anion), whereas a reaction involving the (3,1) isomer of Ru₂(F₃ap)₄Cl and TBACl·H₂O leads to (4,0) Ru₂(F₃ap)₄Cl 2 (F₃ap = 2-(2,4,6-trifluoroanilino)pyridinate anion). To our knowledge, these are the first documented examples for isomeric conversion involving diruthenium compounds with tetracarboxylate-type structures. The structural, electrochemical, and spectroscopic properties of 1 and 2 were examined. The reversible Ru₂^5+/6+ process of (3,1) [Ru ₂(F₃ap)₄Cl]⁺ is located at 0.62 V in CH₂Cl₂, 0.1 M TBAP but shifts to 0.29 V upon formation of (3,1) Ru₂(F₃ap)₄Cl₂ in CH ₂Cl₂ containing chloride from added TBACl·H ₂O and shifts even further to E_1/2 = 0.10 V after generation of (4,0) Ru₂(F₃ap)₄Cl₂ in solution. The 190 mV potential difference between the Ru₂ ^6+/5+ redox couples of (3-1) Ru₂(F₃ap) ₄Cl₂ and (4,0) Ru₂(F₃ap) ₄Cl₂ in chloride-containing media can be compared to a smaller potential difference of only 60 mV between the Ru₂ ^6+/5+ redox couples of (3,1) Ru₂(F₃ap) ₄Cl and (4,0) Ru₂(F₃ap)₄Cl in CH₂Cl₂ containing 0.1 M tetrabutylammonium Perchlorate (TBAP) as supporting electrolyte. The larger ΔE_1/2 in the case of the bis-chloride complexes in solutions containing 0.1 M TBACl·H ₂O can be accounted for in large part by structural differences that manifest themselves in different strengths of axial coordination to the Ru ₂⁵⁺ form of the compounds.

Full text of DOI:10.1021/ic800787p

Inorg. Chem. 2008, 47, 7775-7783
Interconversion between (3,1) and (4,0) Isomers of Ru2(L)4X Complexes
where L is 2-Anilinopyridinate or 2-(2,4,6-Trifluoroanilino)pyridinate
Anion and X ) Cl- or C≡CC5H4N-
Minh Nguyen,† Tuan Phan,‡ Eric Van Caemelbecke,†,§ Wiroaj Kajonkijya,† John L. Bear,*,†
and Karl M. Kadish*,†
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, Department of
Chemistry, Texas Southern UniVersity, Houston, Texas 77004, and Houston Baptist UniVersity,
7502 Fondren Road, Houston, Texas 77074-3298
Received April 30, 2008
A reaction between the (4,0) isomer of Ru2(ap)4Cl and LiC≡CC5H4N leads to a (3,1) isomer of Ru2(ap)4(C≡CC5H4N)2
1 (ap ) anilinopyridinate anion), whereas a reaction involving the (3,1) isomer of Ru2(F3ap)4Cl and TBACl · H2O
leads to (4,0) Ru2(F3ap)4Cl 2 (F3ap ) 2-(2,4,6-trifluoroanilino)pyridinate anion). To our knowledge, these are the
first documented examples for isomeric conversion involving diruthenium compounds with tetracarboxylate-type
structures. The structural, electrochemical, and spectroscopic properties of 1 and 2 were examined. The reversible
Ru25+/6+ process of (3,1) [Ru2(F3ap)4Cl]+ is located at 0.62 V in CH2Cl2, 0.1 M TBAP but shifts to 0.29 V upon
formation of (3,1) Ru2(F3ap)4Cl2 in CH2Cl2 containing chloride from added TBACl · H2O and shifts even further to
E1/2 ) 0.10 V after generation of (4,0) Ru2(F3ap)4Cl2 in solution. The 190 mV potential difference between the
Ru26+/5+ redox couples of (3,1) Ru2(F3ap)4Cl2 and (4,0) Ru2(F3ap)4Cl2 in chloride-containing media can be compared
to a smaller potential difference of only 60 mV between the Ru26+/5+ redox couples of (3,1) Ru2(F3ap)4Cl and (4,0)
Ru2(F3ap)4Cl in CH2Cl2 containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The
larger ∆E1/2 in the case of the bis-chloride complexes in solutions containing 0.1 M TBACl · H2O can be accounted
for in large part by structural differences that manifest themselves in different strengths of axial coordination to the
Ru25+ form of the compounds.
Introduction
substituted anilinopyridinate anion may exist in up to four
different isomeric forms, which are represented as the (4,0),
(3,1), (2,2)-cis, and (2,2)-trans conformations schematically
A large number of diruthenium and dirhodium complexes
with tetracarboxylate-type structures with symmetrical, un-
symmetrical, or mixed anionic bridging ligands have been
synthesized and characterized as to their structural and
physicochemical properties.1–48 Compounds of the type M2L4
where M ) Ru or Rh, L is an anilinopyridinate (ap) or
5+
shown in Chart 1 for the case of the Ru2 derivatives.
Approximately, half of the structurally characterized
diruthenium and dirhodium complexes synthesized in our
laboratory over the last 25 years with four identical unsym-
metrical bridging ligands have been isolated as a mixture of
several isomers, the most common of which possessed (4,0)
and (3,1) configurations that seem to be synthetically favored.
The other half of the structurally characterized compounds
were isolated only in a single isomeric form, again either
(4,0) or (3,1) but not both.
*
To whom correspondence should be addressed. E-mail:
kkadish@uh.edu.
†
University of Houston.
Texas Southern University.
Houston Baptist University.
‡
§
(1) Ying, J.-W.; Cordova, A.; Ren, T. Y.; Xu, G.-L.; Ren, T. Chem.sEur.
J. 2007, 13, 6874.
Numerous M2(L)4 complexes with ap or substituted-ap
bridging ligands have also been examined in our laboratory
over the years as to their reactivity with small mol-
ecules,14,21–24,30,32,35,37,41,42,45 examples of which include
(2) Chen, W.-Z; Fanwick, P. E.; Ren, T. Organometallics 2007, 26, 4115.
(3) Chen, W.-Z; Protasiewicz, J. D.; Davis, S. A.; Updegraff, J. B.; Ma,
L.-Q.; Fanwick, P. E.; Ren, T. Inorg. Chem. 2007, 46, 3775.
(4) (a) Cotton, F. A.; Lin, C.; Murillo, C. A Acc. Chem. Res. 2001, 34,
759. (b) Cotton, F. A.; Lin, C.; Murillo, C. A. Proc. Natl. Acad. Sci.,
U.S.A. 2002, 99, 4810.
23,30
NO,45 CO,35 C≡CC6H5 ,
[C≡C-C≡C-Si(CH3)3]-,14
-
10.1021/ic800787p CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 17, 2008 7775
Published on Web 08/01/2008

Nguyen et al.
CN-,21,24 or NCN2-.22 In each case, the isomeric configu-
ration of the parent M2(L)4 complex was retained after
formation of the axially coordinated compound and this was
true independent of the nature of the anionic bridging
Chart 1
ligand(s), the axial ligands, or the dimetal oxidation state,
5+
which, in the case of diruthenium, was usually Ru26+, Ru2
,
4+
or Ru2 in the final reaction product. Although, different
isomers were obtained upon changing the solvent of crystal-
lization for Os2(ap)4Cl2,49 to the best of our knowledge there
have been no reports in the literature where the binding of
an axial ligand to Ru2(L)4 or Rh2(L)4 would trigger a change
in its isomeric form, that is, a conversion of a (4,0) isomer
to a (3,1) isomer or vice-versa. Thus, we were recently
surprised to observe two different examples of isomeric
transformations while examining the ligand binding reactivity
of (4,0) Ru2(ap)4Cl and (3,1) Ru2(F3ap)4Cl. One transforma-
tion involved a (4,0) to a (3,1) isomeric conversion of
Ru2(ap)4Cl and the other a (3,1) to (4,0) isomeric conversion
of Ru2(F3ap)Cl. It was believed that only the (4,0) isomer
of Ru2(ap)4Cl could be synthesized,12 and thus the possibility
for synthesizing or isolating Ru2(ap)4Cl in any isomeric form
other than (4,0) had not previously been suggested. In
contrast, (3,1) Ru2(F3ap)4Cl is synthesized as a mixture of
the (4,0) and (3,1) isomers. These can then be separated (with
some difficulty), but a conversion between the two isomeric
forms of the compound has never been observed to occur
after isolation of a given Ru2(F3ap)4Cl compound in an
isomerically pure form. Thus, our observations of these
transformations, which are described in the present article,
were totally unexpected and might provide insights into the
synthesis of other known tetracarboxylate-type diruthenium
complexes in previously unreported isomeric forms. Both
isomeric conversions, that is, (4,0) to (3,1) and (3,1) to (4,0),
are described in the present article and a structural, electro-
chemical, and spectroscopic characterization of the reaction
products is presented.
(5) (a) Barral, M. C.; Gallo, T.; Herrero, S.; Jimenez-Aparicio, R.; Torres,
M. R.; Urbanos, F. A. Chem.sEur. J. 2007, 13, 10088–10095. (b)
Barral, M. C.; Gonzalez-Prieto, R.; Jimenez-Aparicio, R.; Priego, J. L.;
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Experimental Section
Chemicals and Reagents. Ultra-high purity nitrogen was
purchased from Matheson-Trigas. GR graded dichloromethane,
diethyl ether, ethyl acetate, and absolute dichloromethane (for
electrochemical and UV-vis spectroelectrochemical measure-
ments), from EMD, VWR, Fluka or Aldrich, tetra-n-butylammo-
nium chloride monohydrate (TBACl ·H2O) from J. T. Baker and
tetraethylammonium chloride (TEACl) from Sigma were used as
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2000, 2259.
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Urbanos, F. A.; Villagran, D.; Wang, X. J. Am. Chem. Soc. 2007,
129, 12666–12667.
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T.; Takahashi, N.; Yamashita, M.; Dunbar, K. R. J. Am. Chem. Soc.
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Sugimoto, K.; Sugiura, K.; Yamashita, M. Bull. Chem. Soc. Jpn. 2006,
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2002, 21, 5919.
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Kadish, K. M. Inorg. Chem. 2000, 39, 857.
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Inorg. Chem. 2001, 40, 1663.
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Inorg. Chem. 2001, 40, 182.
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F. F. D.; Santos, A. M.; Ko¨hler, K.; Ku¨hn, F. E. Eur. J. Inorg. Chem.
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Inorg. Chem. 1997, 36, 5449.
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A.; Van Caemelbecke, E.; Kadish, K. M.; Ren, T. Inorg. Chem. 2003,
42, 6230.
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Xiang, O.; Dunbar, K. R. Inorg. Chem. 2002, 41, 1532.
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7776 Inorganic Chemistry, Vol. 47, No. 17, 2008

(3,1) and (4,0) Isomers of Ru2(L)4X Complexes
received. Tetra-n-butylammonium perchlorate (TBAP) purchased
from Fluka was recrystallized from ethyl alcohol and stored in a
vacuum oven at 40 °C for a few weeks prior to use. 2-(2,4,6-
trifluoroaniline), (C6H4NF3), 2-bromopyridine (C5H4BrN), 4-ethy-
nylpyridine hydrochloride (C7H5N·HCl), lithium chloride (LiCl),
methyl lithium (CH3Li), ruthenium chloride hydrate (RuCl3 ·3H2O),
and CDCl3 (99.8% atom in D for NMR measurements) were
purchased from Aldrich and used without additional purification.
Silica gel (Merck 230-400, mesh 60 Å) was purchased from
Sorbent Technologies, Inc. and used as received. Ru2(ap)4Cl and
the (3,1) isomer of Ru2(F3ap)4Cl were synthesized as described in
the literature.41
Physical Measurements. Cyclic voltammetry and rotating disk
voltammetry were carried out with an EG&G model 263A
potentiostat/galvanostat. A Pine Instrument model AFMSR rotator
was used to control the rotation rate of the RDE. A three-electrode
system was used for all electrochemical measurements and consisted
of a glassy carbon or platinum disk working electrode, a platinum
wire auxiliary 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
containing the solvent/supporting electrolyte mixture. All potentials
are referenced to the SCE, and measurements were carried out at
room temperature. UV-vis spectroelectrochemical experiments
were performed with a homemade spectroelectrochemical thin-layer
cell50 and a Hewlett-Packard model 8453 diode array spectropho-
tometer.
was added and stirred under a nitrogen atmosphere at room
temperature. Degassed THF was then introduced and the mixture
was placed in an ice-bath at a temperature close to 0 °C for 10 min
prior to adding dropwise 1.6 M MeLi (2 mL) as the solution color
changed from green to purple. The solution was stirred for another
30 min prior to adding Ru2(ap)4Cl (0.91 g, 0.100 mmole), then
gradually brought to room temperature and stirred overnight, after
which the solvent was removed and the crude product extracted
with ethyl acetate and water (1:1 v/v). The organic layer was then
concentrated and subjected to silica gel column chromatography
using acetone:hexanes (1:1 v/v) as eluent. A blue band was observed
and collected to yield the title compound with a 70% yield (Rf )
0.4). 1H NMR (300 MHz, CDCl3, 20 °C, δ): 8.4 (dd, 4H), 8.3 (dd,
4H), 7.3 (m, 4H), 7.1 (m, 16H), 6.9 (m, 4H), 6.7 (m, 4H), 6.4 (m,
4H), 6.3 (m, 4H). Mass spectral data [m/e, (fragment)]: 1083.4
[Ru2(ap)4(C≡CC5H4N)2]+, 982 [Ru2(ap)4(C≡CC5H4N)]+, 915.2
[Ru2(ap)3(C≡CC5H4N)2]+, 879.3 [Ru2(ap)4]+. Anal. Calcd for
C58H44N10Ru2: C, 64.31; H, 4.09; N, 12.93. Found: C, 64.25; H,
4.41; N, 12.21. UV-vis spectrum in CH2Cl2: λmax, nm (ꢀ × 10-3
,
M-1 cm-1): 607 (8.9) 1040 (2.4). IR: νC≡C ) 2076 cm-1
.
Synthesis of (4,0) Ru2(F3ap)4Cl 2. The title compound was
prepared by mixing the (3,1) isomer of Ru2(F3ap)4Cl with
TBACl·H2O in a 1:1000 molar ratio in CH2Cl2 at room temperature
for 18 h. The solvent was evaporated and the brown slurry crude
product was dissolved in Et2O before being extracted with water.
The organic layer was collected and concentrated down to give a
residue which was subjected to silica gel column chromatography
using acetone as eluent. The solvent was then removed, and the
brown green title compound was recovered in 95% yield. The
spectral properties of the compound are identical to what has been
previously reported for (4,0) Ru2(F3ap)4Cl.41
1H NMR measurements were recorded at room temperature on
a General Electric QE-300 Plus spectrometer and were referenced
to tetramethylsilane (TMS). Mass spectra were obtained with an
Applied Biosystem Voyager DE-STR MALDI-TOF mass spec-
trometer equipped with a nitrogen laser (337 nm) at the University
of Houston Mass Spectrometry Laboratory. Elemental analysis was
carried out by Atlantic Microlab, Inc., GA.
X-ray Crystallography of (3,1) Ru2(ap)4(C≡CC5H4N)2
1
and (4,0) Ru2(F3ap)4Cl 2. Single-crystal X-ray crystallographic
Synthesis of (3,1) Ru2(ap)4(C≡CC5H4N)2 1. Into a three-neck
studies were performed at the University of Houston X-ray
round-bottom flask, 4-ethynyl pyridine ·HCl (0.14 g, 1.00 mmole)
Crystallographic
Center.
Single
crystals
of
(3,1)
Ru2(ap)4(C≡CC5H4N)2 were obtained by slow diffusion of acetone
in hexanes. Single crystals of (4,0) Ru2(F3ap)4Cl were formed by
slow diffusion of dichloromethane in hexanes, and all collected
data agree within experimental error with what has been reported
in the literature.41
(35) Kadish, K. M.; Phan, T. D.; Giribabu, L.; Shao, J.; Wang, L.-L.;
Thuriere, A.; Van Caemelbecke, E.; Bear, J. L. Inorg. Chem. 2004,
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X.; Wilkinson, C. C. Inorg. Chem. 2004, 43, 8373.
All X-ray data measurements of 1 were made with a Siemens
SMART platform diffractometer equipped with a 1K CCD area
detector. A hemisphere of data (1271 frames at a 5 cm detector
distance) was collected using a narrow-frame method with scan
widths of 0.30% in omega and an exposure time of 30 s/frame.
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 due to variation
in the path length through the detector faceplate. A psi scan
absorption correction was applied based on the entire data set.
Redundant reflections were averaged. Final cell constants were
refined using 4191 reflections having I > 10/s (I), and these, along
with other information pertinent to data collection and refinement,
are listed in Table 1. The Laue symmetry was determined to be
2/m, and from the systematic absences noted the space group was
shown unambiguously to be P21/n. The two terminal pyridine
acetylide groups were found to be disordered over two slightly
different orientations, and this was treated by refinement of ideal
pyridine rings at each location.
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Inorganic Chemistry, Vol. 47, No. 17, 2008 7777

Nguyen et al.
Table 1. Crystal Data, Data Collection, and Processing Parameters for
(3,1) Ru2(ap)4(C≡CC5H4N)2 1
compound
mol formula
fw (g/mol)
space group
cell constant
a (Å)
Ru2(ap)4(C≡CC5H4N)2 1
C58H44N10Ru2
1083.17
P21/n
10.6816(7)
19.1358(13)
23.4822(16)
90
98.163(1)
90
4751.2(6)
4
1.514
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
Figure 1. ORTEP diagram of (3,1) Ru2(ap)4(C≡CC5H4N)2 1. Hydrogen
V (Å3)
atoms are being omitted for clarity.
Z
Fcalcd (g/cm3)
Table 2. Selected Bond Lengths (Angstroms) and Bond Angles
(Degrees) for (3,1) Ru2(ap)4(C≡CC5H4N)2 1
µ (mm-1
)
0.688
λ(Mo KR) (Å)
temp (K)
R (Fo)a
0.71073
223(2)
0.0650
0.0340
Bond Lengths (angstroms)
Ru-Ru
Ru-N1
Ru-N2
Ru-N3
Ru-N4
2.4543(5)
2.085(4)
2.003(4)
2.172(4)
1.985(4)
Ru-Caxial
Ru-N5
Ru-N6
Ru-N7
Ru-N8
1.96(2)
2.037(4)
2.064(4)
2.175(4)
1.977(4)
Rw (Fo)b
a R ) ∑|Fo| -|Fc|/∑|Fc|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2
.
Results and Discussion
Bond Angles (degrees)
84.32(10)
Ru-Ru-N1
Ru-Ru-N2
Ru-Ru-N3
Ru-Ru-N4
Ru-Ru-N5
Ru-Ru-N6
Ru-Ru-N7
Ru-Ru-N8
Ru-Ru-Caxial
162.7(5)
174(2)
177(3)
Synthesis. The complex Ru2(ap)4(C≡CC5H4N)2 1 was
prepared by reacting 4-ethynylpyridine hydrochloride with
CH3Li/Et2O prior to mixing with (4,0) Ru2(ap)4Cl as shown
in eq 1.
90.52(11)
78.68(9)
Ru-C-C
C45-Ru-Ru-52
N1-Ru-Ru-C52
N1-Ru-Ru-N2
N3-Ru-Ru-N4
N5-Ru-Ru-N6
N7-Ru-Ru-N8
93.26(10)
88.81(11)
83.20(10)
78.81(10)
94.42(11)
104(3)
16.83(15)
26.42(15)
24.35(15)
22.05(15)
bond angles of the compound are summarized in Table 2.
The ORTEP diagram of 1 shows that the compound has four
ap ligands in an equatorial position and two C≡CC5H4N-
anions in axial positions, the latter of which are bound to
the ruthenium atom via the terminal carbon of the acetylide
group to complete a square-pyramidal geometry for both Ru1
and Ru2. As illustrated in Figure 1, Ru1 is coordinated to
three pyridyl nitrogen atoms, one anilino nitrogen atom, and
one axial carbon atom, whereas Ru2 is coordinated to one
pyridyl nitrogen atom, three anilino nitrogen atoms, and one
axial carbon atom. The structural data in Figure 1 shows
that 1 adopts a (3,1) isomeric conformation, indicating that
an isomeric transformation has occurred upon reaction with
(4,0) Ru2(ap)4Cl. Remarkably, this is the only reported
example of a Ru2 complex bridged by four ap ligands, which
exists in a (3,1) isomeric form. The Ru-Ru bond length in
1 is 2.4543(5) Å, and this value is much larger than the
Ru-Ru bond length in the parent compound Ru2(ap)4Cl
(2.275(3) Å) (Table 3). Such an elongation of the Ru-Ru
bond distance has already been reported upon conversion of
After Ru2(ap)4Cl was allowed to react with the pyridyl
acetylide anion, the solution was exposed to air and the color
changed from brown-purple to deep blue. A blue product
was previously observed upon air oxidation of the related
Ru26+ complexes, Ru2(L)4(C≡CC6H5)2 (L ) 2-Fap, 2,3-F2ap,
2,4-F2ap, 2,5-F2ap, or 2,4,6-F3ap),30 thus suggesting that air
oxidation must also occur with the formation of Ru2(ap)4-
(C≡CC5H4N)2 1. As earlier indicated, all previously syn-
thesized Ru2(L)4(C≡CC6H5)2 complexes retain the isomeric
form of the starting Ru2(L)4 compound but this was not the
case for 1. Because Ru2(L)4(C≡CC6H5)2 and 1 are both
obtained via air oxidation, it is thus likely that the isomeric
conversion of 1 occurs before the sample was exposed to
air. The diruthenium complex (4,0) Ru2(F3ap)4Cl 2 was
prepared by reacting (3,1) Ru2(F3ap)4Cl with excess
TBACl · H2O (eq 2). Interestingly, the conversion of (3,1)
Ru2(F3ap)4Cl to (4,0) Ru2(F3ap)4Cl is not observed when
TBACl · H2O is replaced by TEACl in the experimental
procedure. One plausible explanation is that TEACl is
anhydrous and that water may be required; however, there
was also no isomeric conversion when water had been added
to a CH2Cl2 solution of (3,1) Ru2(F3ap)4Cl, which had been
left to react overnight with 0.5 M TEACl.
6+
Ru2(ap)4Cl to the Ru2 complex, Ru2(ap)4(C≡CC6H5)2
(where the Ru-Ru distance is 2.4707(8) Å). Other bond
lengths of 1, the parent compound Ru2(ap)4Cl, and
Ru2(ap)4(C≡CC6H5)2 for comparison purposes are given in
Table 3.
The average Ru-Na (Na ) anilino nitrogen) bond length
in 1 (2.008Å) is shorter than that in Ru2(ap)4(C≡CC6H5)2
(2.050 Å), whereas the opposite trend occurs for the average
Ru-Np (Np ) pyridyl nitrogen) bond length. This might be
accounted for by the fact that 1 has a (3,1) isomeric form,
whereas Ru2(ap)4(C≡CC6H5)2 retains a (4,0) isomeric con-
formation; however, an isomeric effect on the average
(3,1) Ru2(F3ap)4Cl excess TBACl·H O8 (4,0) Ru2(F3ap)4Cl (2)
2
Molecular Structure. 1 crystallizes in the monoclinic unit
cell with space group P21/n. The ORTEP diagram of 1 is
illustrated in Figure 1, whereas selected bond lengths and
7778 Inorganic Chemistry, Vol. 47, No. 17, 2008

(3,1) and (4,0) Isomers of Ru2(L)4X Complexes
Table 3. Selected Bond Lengths (Angstroms) and Bond Angles (Degrees) of (3,1) Ru2(ap)4(C≡CC5H4N)2 1, (4,0) Ru2(ap)4Cl, and (4,0)
Ru2(ap)4(C≡CC6H5)2
30
(3,1) Ru2(ap)4(C≡CC5H4N)2 1
(4,0) Ru2(ap)4Cl41
(4,0) Ru2(ap)4(C≡CC6H5)2
Bond Lengths (angstroms)
Ru-Ru
2.4543(5)
2.008
2.118
2.275(3)
2.026
2.104
2.4707(8)
2.050
2.067
a
Ru-Na
Ru-Np
Ru-X
a
1.962
(X)C2C5H4N)
(X)C2C5H4N)
2.437 (X ) Cl)
1.988
(X)C2C6H5)
Bond Angles (degrees)
87.90/89.00
a
Ru-Ru-Na/Np
Ru-Ru-Xa
a Averaged values.
90.35/82.66
167.2
85.60/87.00
180.0 (X ) Cl), 162.4 (X ) C2C6H5)
Ru-Na and Ru-Np bond lengths was not seen when
comparing the (3,1) and (4,0) isomers of
1030-1031 nm under similar solution conditions. As the first
6+/7+
oxidation of 1 proceeds (the Ru2
process), the 607 nm
Ru2(F5ap)4(C≡CC6H5)2.23,30 The Ru-X bond length in 1 is
1.962 Å, a value similar to the Ru-X bond length in
Ru2(F5ap)4(C≡CC6H5)2 (1.988 Å). In 1, the average
Ru-Ru-Na bond angle is greater than the aver-
age Ru-Ru-Np bond angle (Table 3). A different trend is
seen in the case of Ru2(F5ap)4(C≡CC6H5)2, but this might
be explained in terms of the change in isomeric form of the
compound since other (3,1) isomers of Ru2(L)4(C≡CC6H5)2
also possess an average Ru-Ru-Na bond angle, which is
greater than the average Ru-Ru-Np bond angle. Finally,
the Ru-Ru-C bond angle of 1 is 167.2°, a value close to
the 161.0-164.3° values found in other (3,1) isomers of
Ru2(L)4(C≡CC6H5)2.30
band decreases in intensity and a broad low intensity band
grows in between 400 and 1000 nm. These spectral changes
contrast with what occurs upon oxidation of (3,1) or (4,0)
30
Ru2(L)4(C≡CC6H5)2 where the singly oxidized products
are characterized by three well-defined absorption bands
between 400 and 1000 nm. These differences might be
accounted for by the different type of axial ligands, pyridyl
vs phenyl.
The spectral changes during reduction of (3,1)
6+/5+
Ru2(ap)4(C≡CC5H4N)2 1 at -0.8 V (the Ru2
process)
are also shown in part b of Figure 2. Like in the case of
oxidation, the 607 and 1040 nm bands decrease in intensity
during reduction as new bands grow in at 358, 530, and 820
nm. Similar UV-vis spectral changes are observed during
Electrochemistry of (3,1) Ru2(ap)4(C≡CC5H4N)2 1. Part
a of Figure 2 shows a cyclic voltammogram of 1 in CH2Cl2,
0.1 M TBAP whereas Table 4 summarizes half-wave
potentials for each redox reaction of this compound along
with data for (4,0) Ru2(ap)4Cl and (4,0) Ru2(ap)4(C≡CC6H5)2
the Ru26+/Ru2 reaction of Ru2(ap)4(C≡CC6H5)2.30
5+
Isomeric Change of (3,1) Ru2(F3ap)4Cl to (4,0)
Ru2(F3ap)4Cl Monitored by Electrochemistry and
UV-Vis Spectroscopy. The stepwise addition of
TBACl · H2O to CH2Cl2 solutions containing (3,1)
Ru2(F3ap)4Cl and 0.1 M TBAP results in a change in the
electro-oxidation behavior, which initially depends upon the
concentration of Cl- in solution and then, at higher concen-
trations of Cl-, upon the time elapsed since the preparation
of the solution. Three distinctly different half-wave potentials
are observed for oxidation of (3,1) Ru2(F3ap)4Cl under the
different experimental conditions (Figure 3). The one-electron
oxidation of freshly prepared (3,1) Ru2(F3ap)4Cl in CH2Cl2
containing only 0.1 M TBAP is located at E1/2 ) 0.62 V, as
reported in the literature41 but immediately after adding high
concentrations of TBACl ·H2O to solution the potential shifts
negatively from 0.62 to 0.29 V. The same current-voltage
curve is reproducibly obtained in CH2Cl2 with 1000 eq
TBACl · H2O upon multiple scans over 25-30 min but
changes begin to occur at longer times as a new reversible
redox process grows in at E1/2 ) 0.10 V, whereas currents
for the process at 0.29 V decrease in magnitude.
under
the
same
solution
conditions.
(3,1)
Ru2(ap)4(C≡CC5H4N)2 1 undergoes one reversible oxidation
and two reversible reductions, and this is also the case for (4,0)
Ru2(ap)4(C≡CC6H5)2.30 The electron transfer processes of the
newly synthesized 1 at E1/2 ) 0.69, -0.52, and -1.44 V are
all metal-centered and assigned to the Ru27+/6+, Ru26+/5+, and
Ru25+/4+ redox couples, respectively. These assignments are
based on what has been reported for related (3,1) and (4,0)
derivatives of Ru2(L)4(C≡CC6H5)2 where L ) ap, Fap, F3ap,
or F5ap30 and are consistent with the potential separation
between the first reduction and oxidation of 1 (1.21 V), which
is virtually identical to the HOMO-LUMO gap of 1.23 V for
the parent compound, (4,0) Ru2(ap)4Cl (Table 4). The elec-
troreduction process of 1 (E1/2 ) -1.44 V) is more than 200
mV easier than the electroreduction of (4,0)
Ru2(ap)4(C≡CC6H5)2 (Epc ) -1.67 at 0.1 V/s), and this can
be accounted for by the fact that the pyridyl groups decrease
the electron density more on the diruthenium unit than do the
phenyl groups.
UV-Vis
Spectroelectrochemistry
of
(3,1)
Ru2(ap)4(C≡CC5H4N)2 1. The UV-vis spectrum of 1 in
CH2Cl2, 0.2 M TBAP before and after oxidation or reduction
at a controlled potential is shown in part b of Figure 2. The
initial compound has a major band at 607 nm and a weaker
band at ∼1040 nm. This spectrum is similar to spectra of
(3,1) or (4,0) Ru2(L)4(C≡CC6H5)2 derivatives with substituted
ap bridging ligands, which have bands at 622-623 and
In contrast to the oxidation, the potentials for reduction
of (3,1) Ru2(F3ap)4Cl and/or its reaction products after
addition of TBACl · H2O remain relatively constant at about
-0.65 V under all experimental conditions. This is consistent
5+/4+
with earlier reports that the Ru2
process of Ru2(L)4Cl
or Ru2(L)4X compounds is independent of the isomer
Inorganic Chemistry, Vol. 47, No. 17, 2008 7779

Nguyen et al.
Figure 2. (a) Cyclic voltammogram of (3,1) Ru2(ap)4(C≡CC5H4N)2 1 in CH2Cl2, 0.1 M TBAP at a scan rate of 0.10 V/s and (b) UV-vis spectral changes
of 1 in CH2Cl2, 0.2 M TBAP upon controlled potential (i) oxidation at Eapp ) 1.0 V and (ii) reduction at Eapp ) -0.8 V.
Table 4. Half-Wave Potentials for Redox Reactions of (4,0)
electroactive compound have the same number of coordi-
Ru2(ap)4Cl,41 (4,0) Ru2(ap)4(C≡CC6H5)2,30 and (3,1)
nated Cl- axial ligands.51 The electrooxidation mechanism
Ru2(ap)4(C≡CC5H4N)2 1 in CH2Cl2, 0.1 M TBAP
in each case can then only be described by eq 3 because the
E1/2 (V vs SCE)
5+/6+
alternate possibility of a Ru2
process involving
a
7+/6+
6+/5+
5+/4+
compound
ox state Ru2
Ru2
Ru2
∆
Ru2(F3ap)4Cl and [Ru2(F3ap)4Cl]+ is only seen in the absence
of added chloride.
5+
(4,0) Ru2(ap)4Cl
(4,0) Ru2(ap)4-
(C≡CC6H5)2
Ru2
Ru2
∼1.33
0.37
-0.86
1.23
1.09
-1.67b
6+
6+
0.55
-0.54
-0.52
(3,1) [Ru2(F3ap)4Cl2]- h (3,1) Ru2(F3ap)4Cl2 + e- (3)
(3,1) Ru2(ap)4-
(C≡CC5H4N)2 1
Ru2
0.69
-1.44
1.21
a Potential gap in V between the Ru2
and Ru2
couples in the
6+/5+
5+/4+
As will be described below, the change in the reversible
potential from 0.29 to 0.10 V occurs only after an air
oxidation of (3,1) Ru2(F3ap)4Cl2 at longer times and a
following rearrangement of the bridging ligands on the singly
oxidized species to give (4,0) Ru2(F3ap)Cl2 which is in its
7+/6+
6+/5+
case of Ru2(ap)4Cl and the Ru2
and Ru2
couples in the case of
b
the other two compounds. Epc at a scan rate of 0.1 V/s.
type23,41 and also independent of the anionic axial ligand
X, which usually binds weakly to Ru25+, the only exception
being in the case of CN-.24
6+
Ru2 form.
The invariance of E1/2 for the processes at E1/2 ) 0.29 in
Figure 3 with changes in [Cl-] is consistent with a mecha-
nism where the neutral and singly oxidized forms of the
(50) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1489.
(51) Crow, D. R. Polarography of Metal Complexes; Academic Press:
London, 1969.
7780 Inorganic Chemistry, Vol. 47, No. 17, 2008

(3,1) and (4,0) Isomers of Ru2(L)4X Complexes
Figure 3. Cyclic voltammograms of (3,1) Ru2(F3ap)4Cl in CH2Cl2 with
added TBACl ·H2O and time-dependence in the presence of 1000 equiv
TBACl·H2O. Scan rate ) 0.1 V/s.
Time-dependent changes are also observed in the UV-vis
spectrum of the initial (3,1) Ru2(F3ap)4Cl complex as the
(4,0) isomer is formed and this is shown in Figure 4.
Immediately after addition of 1000 eq TBACl · H2O to
solution, the UV-vis spectrum is characterized by three
absorption bands at λmax ) 418, 478, and 777 nm, a spectrum
that closely resembles the UV-vis spectrum of (3,1)
Ru2(F3ap)4Cl in neat CH2Cl2.41 Over the first 25 min, the
three absorption bands of the initial compound decrease
slightly in intensity and a new shoulder peak develops at
895 nm. There are two isosbestic points at 615 and 816 nm,
thus suggesting that over this time interval only two species
are present in equilibria, the most likely of which are (3,1)
Ru2(F3ap)4Cl and (3,1) [Ru2(F3ap)4Cl2]- on the basis of the
electrochemical data that shows a reversible oxidation at 0.29
V, which is independent of [Cl-].
Figure 4. UV-vis spectral changes of (3,1) Ru2(F3ap)4Cl in CH2Cl2, 1000
equiv TBACl ·H2O with respect to time for (a) time ) 0-25 min and (b)
25-500 min.
suggesting that only two species are present in equilibrium
from 25 to 500 min. The final spectrum after 500 min
exhibits absorption bands at 446, 580, and 895 nm and is
reminiscent of the 466, 634, and 895 nm bands for
Ru2(F3ap)4(C≡CC6H5)2,30 therefore suggesting that the final
6+
product contains a Ru2 core and further suggesting that
the process at 0.10 V is not an oxidation of Ru25+ but rather
the reduction of a Ru26+ species generated in solution. This
is indeed the case, as demonstrated by rotating disk volta-
mmograms of a similar solution obtained as a function of
time. These voltammograms are shown in Figure 5 and
parallel what is observed in the time-resolved electrochemical
and spectroscopic measurements of (3,1) Ru2(F3ap)4Cl under
the same solution conditions.
From 25 to 500 min, a more significant set of changes is
observed in the UV-vis spectrum (Figure 4). Two new bands
grow in at λmax ) 446 and 580 nm, while, at the same time,
the lower energy absorption band decreases significantly in
intensity. There is an isosbestic point at 630 nm, again
Inorganic Chemistry, Vol. 47, No. 17, 2008 7781

Nguyen et al.
above the i ) 0 line, both of which will increase with the
square root of rotation rate, ω1/2, for diffusion-controlled
electron transfer processes.52 As seen in Figure 5, the RDE
response of a freshly prepared (3,1) Ru2(F3ap)4Cl solution
in the presence of high [Cl-] (top green curve in Figure 5)
is that for a reversible electrooxidation at E1/2 ) 0.29 V. A
plot of the maximum peak current versus ω1/2 (Figure S1 in
the Supporting Information) is linear, consistent with the
transfer of 1.0 electron to give the singly oxidized species
and no other redox processes are observed between -0.2
and +0.6 V versus SCE.
The solution of (3,1) Ru2(F3ap)4Cl was then left to stand
for 5 h and the RDE measurements repeated (black curve in
Figure 5). Under these conditions, the maximum oxidation
currents for the process at E1/2 ) 0.29 V decrease by about
50% and a new reversible reduction process is observed at
E1/2 ) 0.10 V. This parallels what is seen in Figure 3 and
indicates an approximately equimolar mixture of the (3,1)
Ru2(F3ap)4Cl isomer (in its bis-chloride form) and the newly
generated Ru26+ product, the latter being formulated as (4,0)
Ru2(F3ap)4Cl2 on the basis of the spectroscopic and electro-
chemical data and a structural characterization of the isolated
reaction product after workup (Experimental Section).
Finally, after 24 h in solution, there is a nearly complete
conversion of (3,1) Ru2(F3ap)4Cl to its singly oxidized (4,0)
isomer and the RDE (red line in Figure 5) is then character-
ized by a reduction at E1/2 ) 0.10 V with little to no oxidation
currents at the more positive potential of 0.29 V. This is
again consistent with what is observed by electrochemistry
and UV-vis spectroscopy under the same solution condi-
tions. The maximum current for reduction of the homoge-
neously generated species in solution, formulated as (4,0)
Ru2(F3ap)4Cl2, is proportional to the square root of the
rotation rate indicating diffusion control and the number of
electrons transferred is calculated as n ) 1 on the basis of a
Levich plot shown in Figure S1 in the Supporting Informa-
tion.
Figure 5. Rotating disk voltammograms of (3,1) Ru2(F3ap)4Cl in CH2Cl2
containing 0.1 M TBAP and 1000 equiv TBACl ·H2O immediately after
solution preparation (t ) 0 min) and after 5 or 24 h of elapsed time. Rotation
rate ) 1600 rpm.
Scheme 1. Conversion of (3,1) Ru2(L)4Cl to (4,0) Ru2(L)4Cl in CH2Cl2
Solutions Where L ) F3apa
6+
The Ru2 form of the diruthenium compound formed
upon air oxidation is stable in solution for several days but
all attempts to isolate this oxidized species for further
characterization led only to a final product which was
structurally characterized as (4,0) Ru2(F3ap)4Cl (Experimental
Section). The X-ray structure of the isolated product is
identical to that which was earlier published for (4,0)
Ru2(F3ap)4Cl synthesized by another method.41 Electro-
chemical and spectroscopic properties of previously char-
acterized (4,0) Ru2(F3ap)4Cl also match the properties of the
product isolated and structurally characterized after air
oxidation of (3,1) Ru2(F3ap)4Cl in CH2Cl2 containing
TBACl ·H2O. Thus electrochemical, spectroscopic, and struc-
tural data are self-consistent and proposed reaction pathways
for the conversion of (3,1) Ru2(F3ap)Cl to (4,0) Ru2(F3ap)4Cl
are summarized in Scheme 1, where species in solution are
indicated by a box around the formula and the other species
are generated at the electrode surface.
a The initial oxidation of (3,1) Ru2(L)4Cl or (3,1) [Ru2(L)4Cl2]- converts
to a reduction of (4,0) Ru2(L)4Cl2 after >10 hrs in the presence of
TBACl·H2O.
One key advantage of the RDE measurement is its ability
to uniquely differentiate an oxidation from a reduction
reaction, the former having maximum limiting diffusion
currents below the i ) 0 line and the later having currents
(52) Bard, A. J.; Faulker, L. R. Electrochemical Methods: Fundamentals
and Applications; John Wiley & Sons: New York, 2000; 2nd Edition.
7782 Inorganic Chemistry, Vol. 47, No. 17, 2008

(3,1) and (4,0) Isomers of Ru2(L)4X Complexes
It was earlier reported that only small differences are
Ru2(CH3CO2)4Cl and HF3ap, can be subsequently formed
as the sole product isolated from the major (3,1) isomer, thus
eliminating the need for a time-consuming and costly
separation process to obtain the (4,0) isomer, which can then
be further studied as to its reactivity.
5+/6+
observed between the Ru2
processes of (3,1) and (4,0)
Ru2(F3ap)4Cl in noncoordinating media, E1/2 values for
electrooxidation being 0.62 and 0.69 V, respectively, in
CH2Cl2.41 Much larger differences in E1/2 are seen for the
6+/5+
Ru2
process of (3,1) Ru2(F3ap)4Cl2 and (4,0)
Ru2(F3ap)4Cl2, and this can be accounted for by structural
differences between the two diruthenium compounds, which
leads to different strengths of axial coordination to the Ru2
Acknowledgment. Support from the Robert A. Welch
Foundation (J.L.B., Grant E-918; K.M.K., Grant E-680) is
gratefully acknowledged. T.P. acknowledges support from
a TSU Seed Grant and a Departmental Welch Grant. We
also thank Dr. J. D. Korp for X-ray analyses.
5+
form of the two isomers.
In summary, we have shown two examples of isomeric
conversions among diruthenium complexes. One type of
isomeric conversion involves a change in configuration from
(4,0) to (3,1) and is observed when (4,0) Ru2(ap)4Cl reacts
with LiC≡CC5H4N. The other isomeric change involves a
conversion of (3,1) Ru2(F3ap)4Cl to (4,0) Ru2(F3ap)4Cl in
the presence of excess TBACl · H2O. The fact that (4,0)
Ru2(F3ap)4Cl, a minor product in the reaction between
Supporting Information Available: X-ray crystallographic data
for compound 1 (CIF format) and Levich plots from RDE data for
the Ru25+/6+ reactions of the (3,1) and (4,0) isomers. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC800787P
Inorganic Chemistry, Vol. 47, No. 17, 2008 7783