Synthesis, structure, and electrochemical characterization of a mixed-ligand diruthenium(iii,ii) complex with an unusual arrangement of the bridging ligands

Article abstract of DOI:10.1039/c2dt32715e

A mixed-ligand metal-metal bonded diruthenium complex having the formula Ru₂(2,4,6-(CH₃)₃ap)₃(O ₂CCH₃)Cl where ap is the anilinopyridinate anion was synthesized from the reaction of Ru₂(O₂CCH ₃)₄Cl and H(2,4,6-(CH₃)₃ap), after which the isolated product was structurally, spectroscopically and electrochemically characterized. The crystal structure reveals an unusual arrangement of the bridging ligands around the dimetal unit where one ruthenium atom is coordinated to one anilino and two pyridyl nitrogen atoms while the other ruthenium atom is coordinated to one pyridyl and two anilino nitrogen atoms. To our knowledge, Ru₂(2,4,6-(CH₃) ₃ap)₃(O₂CCH₃)Cl is the only example of a mixed-ligand diruthenium complex of the type [Ru₂L ₃(O₂CCH₃)]⁺, where L is an unsymmetrical anionic bridging ligand that has been structurally characterized with a (2,1) geometric conformation of the bridging ligands, all others being (3,0) . The initial Ru₂⁵⁺ compound in CH₂Cl₂ or CH₃CN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) undergoes up to four one-electron redox processes involving the dimetal unit. The Ru₂^5+/4+ and Ru₂^5+/6+ processes were characterized under N₂ using thin-layer UV-visible spectroelectrochemistry and this data is compared to UV-visible spectral changes obtained during similar electrode reactions for related diruthenium compounds having the formula Ru₂L₄Cl or Ru₂L₃(O₂CCH₃)Cl where L is an anionic bridging ligand. Ru₂(2,4,6-(CH₃) ₃ap)₃(O₂CCH₃)Cl was also examined by UV-visible and FTIR spectroelectrochemistry under a CO atmosphere and two singly reduced Ru₂⁴⁺ species, [Ru₂(2,4,6- (CH₃)₃ap)₃(O₂CCH₃)(CO)Cl] ^- and Ru₂(2,4,6-(CH₃)₃ap) ₃(O₂CCH₃)(CO) were in situ generated for further characterization. The CO-bound complexes could be further reduced and exhibited additional reductions to their Ru₂³⁺ and Ru ₂²⁺ oxidation states.

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Synthesis, structure, and electrochemical
characterization of a mixed-ligand diruthenium(III,II)
complex with an unusual arrangement of the bridging
ligands†
Cite this: Dalton Trans., 2013, 42, 3571
Siyabonga Ngubane,a Karl M. Kadish,*a John L. Bear,*a Eric Van Caemelbecke,a,b
Antoine Thurierea and Kevin P. Ramirezb
A mixed-ligand metal–metal bonded diruthenium complex having the formula Ru2(2,4,6-(CH3)3ap)3-
(O2CCH3)Cl where ap is the anilinopyridinate anion was synthesized from the reaction of Ru2(O2CCH3)4Cl
and H(2,4,6-(CH3)3ap), after which the isolated product was structurally, spectroscopically and electroche-
mically characterized. The crystal structure reveals an unusual arrangement of the bridging ligands
around the dimetal unit where one ruthenium atom is coordinated to one anilino and two pyridyl nitro-
gen atoms while the other ruthenium atom is coordinated to one pyridyl and two anilino nitrogen
atoms. To our knowledge, Ru2(2,4,6-(CH3)3ap)3(O2CCH3)Cl is the only example of a mixed-ligand diruthe-
nium complex of the type [Ru2L3(O2CCH3)]+, where L is an unsymmetrical anionic bridging ligand that
has been structurally characterized with a “(2,1)” geometric conformation of the bridging ligands, all
5+
others being “(3,0)”. The initial Ru2 compound in CH2Cl2 or CH3CN containing 0.1 M tetra-n-butyl-
ammonium perchlorate (TBAP) undergoes up to four one-electron redox processes involving the dimetal
unit. The Ru25+/4+ and Ru25+/6+ processes were characterized under N2 using thin-layer UV-visible spectro-
electrochemistry and this data is compared to UV-visible spectral changes obtained during similar elec-
trode reactions for related diruthenium compounds having the formula Ru2L4Cl or Ru2L3(O2CCH3)Cl
where L is an anionic bridging ligand. Ru2(2,4,6-(CH3)3ap)3(O2CCH3)Cl was also examined by UV-visible
Received 13th November 2012,
Accepted 14th December 2012
4+
and FTIR spectroelectrochemistry under a CO atmosphere and two singly reduced Ru2 species,
[Ru2(2,4,6-(CH3)3ap)3(O2CCH3)(CO)Cl]− and Ru2(2,4,6-(CH3)3ap)3(O2CCH3)(CO) were in situ generated for
further characterization. The CO-bound complexes could be further reduced and exhibited additional
reductions to their Ru23+ and Ru22+ oxidation states.
DOI: 10.1039/c2dt32715e
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isomeric forms that have been labeled as having (4,0), (3,1),
Introduction
(2,2)-cis and (2,2)-trans conformations9,13,28–30 but generally
A large number of metal–metal bonded diruthenium com- only one or two isomers have been isolated in a given syn-
plexes with anionic bridging ligands have been synthesized thesis. For example, in the case of substituted ap ligands (ap =
and characterized as to their structural, electrochemical and 2-anilinopyridinate anion), Ru2(2,6-F2ap)4Cl and Ru2(2,4,6-
spectroscopic properties.1–18 The compounds which have been F3ap)4Cl were isolated as a mixture of (3,1) and (4,0) isomers,13
investigated in most detail are represented as [Ru2L4]n+ and while Ru2(2-Fap)4Cl exists exclusively in its (3,1) isomeric
[Ru2(L)x(L′)4−x
metrical or unsymmetrical anionic bridging ligand and L′ is isomer.28 The fact that two different geometric arrangements
usually an acetate ligand.2,19–27
are formed for Ru2L4Cl complexes with 2-CH3ap and 2-Fap
]
n+, where x = 1, 2 or 3, n = 0 or +1, L is a sym- form13 and Ru2(2-CH3ap)4Cl is obtained only as the (4,0)
The totally substituted diruthenium complexes with unsym- anionic ligands is surprising since substitution occurs at the
metrical bridging ligands can exist in up to four different ortho position of the anilinopyridinate bridging ligands in
both Ru25+ compounds.
In an attempt to better understand the factors that control
the isomeric form in the final substitution product, i.e.
Ru2L4Cl, we opted to use a sterically crowded bridging ligand
with electron donating substituents, i.e. 2,4,6-(CH3)3ap (see
Chart 1). This study reports that only three of the acetate
aDepartment of Chemistry, University of Houston, Houston, Texas 77204-5641, USA.
E-mail: kkadish@uh.edu, jbear@uh.edu
bHouston Baptist University, Houston, Texas 77074-3298, USA
†CCDC 910526. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt32715e
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was used as the reference electrode. This electrode was sepa-
rated from the bulk of the solution by a fritted glass bridge
containing the non-aqueous solvent with TBAP as the support-
ing electrolyte. The solvent was saturated with N2 gas prior to
taking measurements. Ferrocene (Fc) was used as an internal
standard for all measurements. The half-wave potential of the
Fc/Fc+ couple under our conditions was E1/2 = 0.49 V.
Thin-layer cyclic voltammograms and UV-visible spectra
were obtained using a cell whose design has been reported in
the literature.31 The cell consists of a platinum gauze sand-
wiched by two glass sides in order to reduce the diffusion of
the species from the electrode. Measurements were carried out
with a scan rate of 20 mV s−1. All experiments were performed
in degassed non-aqueous solvents containing TBAP as a sup-
porting electrolyte. A stream of high purity nitrogen was main-
tained over the solution while making the measurements. The
applied potential was controlled by an EG&G Model 173 poten-
tiostat which was coupled with a Hewlett-Packard model 8453
diode array spectrophotometer. Thin-layer time-dependent
infrared spectroelectrochemical measurements were per-
formed using a FTIR Nicolet 550 Magna-IR spectrophotometer
with a specially constructed light-transparent FTIR spectro-
electrochemical cell.32
Mass spectra were obtained at the University of Houston
with an Applied Biosystem Voyager DE-STR MALDI-TOF mass
spectrometer equipped with a nitrogen laser (337 nm) or with
a high-resolution hybrid tandem VG analytical model 70-SEQ
(EEQQ geometry) mass spectrometer. A standard fast atom
bombardment (FAB) source was used in the latter case with
m-nitrobenzyl alcohol (NBA) as the liquid matrix. Elemental
Analyses were carried out by Atlantic Microlab Inc., Norcros,
Georgia.
Chart 1
groups on Ru2(O2CCH3)4Cl are substituted by the newly inves-
tigated bridging ligand. The final reaction product which is
formulated as Ru2(2,4,6-(CH3)3ap)3(O2CCH3)Cl was obtained
in good yield. Its crystal structure was determined and its
electrochemistry and spectroelectrochemistry investigated in
CH2Cl2 and CH3CN under N2 as well as CH2Cl2 under a CO
atmosphere. The data are then discussed in light of what has
been reported for other related metal–metal bonded diruthe-
nium complexes of the type Ru2L4Cl and Ru2(L)x(OAc)4−xCl
where L is a substituted ap ligand.2 For simplicity, the nota-
tions Ru2(OAc)4Cl and Ru2L3(OAc)Cl will be used in the
manuscript for Ru2(O2CCH3)4Cl and Ru2(2,4,6-(CH3)3ap)3-
(O2CCH3)Cl, respectively.
Experimental
Chemicals and reagents
Ultra high purity nitrogen (N2) and carbon monoxide (CO)
were purchased from Matheson-Trigas. N2 was passed through
a glass column packed with anhydrous calcium sulfate and
potassium hydroxide pellets to remove traces of moisture and
oxygen before use while CO was used without further purifi-
cation. Hexane (95% min, Aldrich), acetone (GC grade, EM
Science), methanol (99.9% anhydrous, Aldrich), acetic acid
(GR, EM Science), and dichloromethane (99.8%, EMD Chemi-
cals) were used as received. Tetra-n-butylammonium perchlo-
rate (TBAP) was purchased from Fluka Chemicals Co. and
stored in a vacuum oven at 40 °C before use. 2-Bromopyridine
(C5H4NBr), 2,4,6-trimethylaniline ((CH3)3–C6H4NH2), ruthe-
nium chloride trihydrate, lithium chloride and CD2Cl2 (99.8%
atom in D, for NMR measurements) were purchased from
Aldrich and used as received.
Synthesis of starting compound
Ru2(O2CCH3)4Cl was synthesized according to a procedure
reported in the literature.33
Synthesis of H(2,4,6-(CH3)3ap)
2-Bromopyridine (5.53 g, 35.0 mmol) and 2,4,6-trimethyl-
aniline (9.47 g, 70.0 mmol) were heated under N2 for 6 hours
at 175 °C. Upon cooling, a light brown solid was formed. A
10% NaOH solution (w/v) was added to the flask and the
unreacted materials were steam distilled to remove excess
aniline. The compound was extracted with CH2Cl2 and the
resulting organic layer dried over MgSO4. The solvent was
Physical measurements
Cyclic voltammetry was performed with a three-electrode removed with a rotary evaporator and the compound was
system using an IBM Model EC225 voltammetric analyzer or recrystallized several times in hexanes to give white rhomb-
an EG&G Princeton Applied Research (PAR) Model 173/175 shaped crystals in 67% yield. Mass spectral data [m/z, (frag-
potentiostat/signal generator, a PAR Model 174A/175 polaro- ment)]: 212 [H((CH3)3ap)]+; 197 [H((CH3)2ap)]+; 182 [H-
graphic analyzer/universal programmer and a Houston Instru- (CH3ap)]+. Anal. Calcd for C14H16N2: C, 79.20; H, 7.60; N,
1
ment Model 2000 or EG&G Princeton Applied Research (PAR) 13.19. Found: C, 79.40; H, 7.64; N, 13.22. H NMR (CD2Cl2): δ
Model REO151 X-Y recorder. A glassy carbon electrode was 8.08 (d, 1H), 7.37 (t, 1H), 6.96 (s, 2H), 6.60 (t, 1H), 6.14 (bs,
used as a working electrode. A Pt wire was used as a counter 1H), 6.03 (d, 1H), 2.30 (s, 3H), 2.17 (s, 6H) ppm (key: d,
electrode and a homemade saturated calomel electrode (SCE) doublet; t, triplet; bs, broad singlet).
3572 | Dalton Trans., 2013, 42, 3571–3580
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Synthesis of Ru2L3(OAc)Cl
Table 1 Crystal data and structure refinement for Ru2(2,4,6-(CH3)3ap)3(OAc)
Cl·EtOH
The reaction between Ru2(O2CCH3)4Cl and H(2,4,6-(CH3)3ap)
followed procedures described in the literature.34,35 Ru2-
(O2CCH3)4Cl (0.43 g, 0.91 mmol) and H(2,4,6-(CH3)3ap)
(3.86 g, 18.16 mmol) were placed in a round-bottom flask and
heated for 10 hours at 140 °C under N2. The green compound
obtained upon cooling was extracted with CH2Cl2. After
removal of the solvent, the excess ligand was sublimed at
140 °C under N2. The crude product was subjected to column
chromatography and eluted with CH2Cl2. A blue-green band
was isolated and further purified on a second column with a
mixture acetone–n-hexane 3 : 7 (v/v) as eluent. The title com-
pound was obtained in 36% yield. Mass spectral data [m/z,
(fragment)]: 931 [Ru2(2,4,6-(CH3)3ap)3(O2CCH3)Cl]+; 896
[Ru2(2,4,6-(CH3)3ap)3(O2CCH3)]+; 837 [Ru2(2,4,6-(CH3)3ap)3]+
622 [Ru2(2,4,6-(CH3)3ap)2]+. Anal. Calcd for C44H48N6ORu2Cl:
C, 56.72; H, 5.19; N, 9.02. Found: C, 56.71; H, 5.13; N, 9.42.
UV-vis spectrum in CH2Cl2 [λmax nm (10−3 ε, mol−1 L cm−1)]:
425 (2.7), 468 (2.2), 656 (6.0), 1033 (0.8). μeff (298 K) = 3.90 μB.
Empirical formula
C46H54ClN6O3Ru2
Fw (g mol−1
)
976.54
223(2)
0.71073
Pnma (orthorhombic)
Temp (K)
λ (Mo Kα) (Å)
Space group
Cell constant
a (Å)
b (Å)
c (Å)
15.1363(8)
14.0416(7)
20.7216(10)
90
90
90
4404.1(4)
4
α (°)
β (°)
γ (°)
V (Å3)
Z
ρcalcd (g cm−3
)
1.473
0.793
2004
μ (mm−1
F(000)
)
Crystal color and shape
Dark blue-green flat column
0.45 × 0.10 × 0.05
1.67 ≤ 2θ ≤ 25.07
0≤ h ≤18, 0 ≤ k ≤ 16, 0 ≤ l ≤ 24
22 652/4366 [R(int) = 0.0649]
99.8%
Crystal size (mm3)
Collection range (°)
Limiting indices
Reflections collected/unique
Completeness to theta = 25.07
Absorption correction
Max. and min. transmission
Refinement method
Number of parameters refined
Goodness-of-fit on F2
Final R indices [I > 4σ(I)]
R indices (all data)
Empirical
X-ray crystallography
1.0000 and 0.7444
Full-matrix least-squares of F2
301
Dark blue-green flat single crystals of Ru2L3(OAc)Cl·EtOH were
obtained by slow evaporation from an absolute ethanol solu-
tion. X-ray crystallographic measurements were carried out at
the University of Houston’s MRSEC/TcSUH shared experi-
mental facilities. All measurements were made with a Siemens
SMART platform diffractometer equipped with a 4K CCD APEX
II detector. A hemisphere of data (1271 frames at 6 cm detector
distance) was collected using a narrow-frame algorithm with
scan widths of 0.30° in ω and an exposure time of 40 s per
frame. The data were integrated using the Bruker-Nonius
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 ψ
scan absorption correction was applied based on the entire
data set. Redundant reflections were averaged. Final cell con-
stants were refined using 8016 reflections having I > 10σ(I).
The Laue symmetry was determined to be mmm, and from the
systematic absences noted the space group was shown to be
either Pna2(1) or Pnma (orthorhombic). These along with other
information pertinent to data collection and refinement, are
listed in Table 1. The asymmetric unit of Ru2L3(OAc)Cl·EtOH
consists of one-half Ru2L3(OAc)Cl molecule and one-half
ethanol solvent molecule, both situated about a mirror plane.
The solvent molecules are massively disordered, with at least
six main orientations spread over each crystallographic site.
1.08
R1 = 0.0307, wR2 = 0.0806
R1 = 0.0494, wR2 = 0.0948
0.0649
R (Fo)a
Rw (Fo)b
0.025
a R = ∑||Fo| − |Fc||/∑|Fo|. b Rw = [∑w(|Fo| − |Fc|)2/∑w|Fo|2]1/2
.
Results and discussion
Synthesis
The reaction between Ru2(OAc)4Cl and H(2,4,6-(CH3)3ap) was
initially expected to produce a substitution product of the type
Ru2L4Cl where all four acetate groups had been replaced by
the incoming bridging ligand, L.13,28,35 However, this was not
the case under molten conditions, where only three of the four
OAc− ligands were replaced in the substitution reaction, giving
Ru2(2,4,6-(CH3)3ap)3(OAc)Cl as the final isolated diruthenium
product. Generally, at temperatures above the melting point of
the ligand, complete displacement of the four acetates is
observed,9,13,28–30 but this was not the case in synthesis of the
compound reported in the present study.
Molecular structure
The three most probable orientations comprising the asym- The ORTEP diagrams of Ru2L3(OAc)Cl are shown in Fig. 1. A
metric unit were refined using ideal rigid body models, with projection view along the Ru(2)–Ru(1)–Cl unit is included to
occupancies roughly based on comparison of isotropic displa- show the bulkiness of the 2,4,6-(CH3)3ap ligand (Fig. 1b) in
cement parameters. Two of the refined positions seem fairly addition to the side view structure (Fig. 1a). The coordination
accurate since they show reasonable hydrogen bonding to Cl. of Ru1 and Ru2 is essentially octahedral and square pyramidal,
The third position does not seem to be accurate since it does respectively with three “substituted ap ligands” arranged in a
not show hydrogen bonding and gives short H⋯H contacts trans fashion and one acetate bridging ligand forming the
with the main molecule; however all attempts to shift the equatorial plane. Ru1 is coordinated to the chloride axial
location to a more reasonable position failed.
ligand, two pyridyl nitrogens, one anilino nitrogen and one
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Table 2 Selected average bond lengths and bond angles of Ru2L3(OAc)Cl
Bond length (Å)
Ru(1)–Ru(2)
Ru(1)–Cl
2.2917(6)
2.4726(14)
2.028
a
Ru(2)–Nab
Ru(1)–Np
2.113
Ru(1)–O(1)
Ru(2)–O(2)
2.075(3)
2.061(3)
Bond angle (°)
Ru(2)–Ru(1)–Cl
Ru(1)–Ru(2)–Na
Ru(2)–Ru(1)–Np
Ru(2)–Ru(1)–O(1)
Ru(1)–Ru(2)–O(2)
178.22(4)
92.55
87.99
90.99(11)
87.56(11)
a Na: anilino nitrogen. b Np: pyridyl nitrogen.
Selected bond lengths and bond angles of Ru2L3(OAc)Cl are
listed in Table 2. The Ru–Ru bond length falls within the Ru–
Ru bond distances of other diruthenium compounds with
similar mixed bridging ligands such as Ru2(ap)2(OAc)2(Hap)Cl
(2.308(1) Å)27 and Ru2(2-Fap)2(OAc)2(ac)Cl (2.2945(7) Å) (ac =
acetone).2 The Ru–Cl bond length is 2.4726(14) Å and this
value can be compared to the Ru–Cl bond distances of
Ru2(ap)4Cl (2.437(7) Å),35 Ru2(2-CH3ap)4Cl (2.487 Å)28 and
Ru2(2-Fap)4Cl (2.461(7) Å),2 all of which have either four ap or
four “ap-type” bridging ligands.
Some structural changes are observed as one of the two
acetate groups in Ru2L2(OAc)2Cl, where L = ap27 or 2-Fap,2 is
substituted by one ap-type bridging ligand. For example, the
two Ru–O bond distances of the title compound are longer than
the average Ru–O bond lengths in Ru2(ap)2(OAc)2(Hap)Cl27
(2.045 Å) and Ru2(2-Fap)2(OAc)2(ac)Cl2 (2.041 Å). This can be
attributed to the strong σ-donation properties of the 2,4,6-
(CH3)3ap ligand. There is also a slight increase of the Ru(1)–
Ru(2)–Na bond angle, a slight decrease of the Ru(2)–Ru(1)–Np
bond angle and a slight increase of the Ru(2)–Ru(1)–O(1) bond
angle in the title compound as compared to the other two
mixed-ligand diruthenium complexes.
Fig. 1 ORTEP diagram of Ru2L3(OAc)Cl (a) side-view projection and (b) projec-
tion view along the Ru(2)–Ru(1)–Cl unit. Thermal ellipsoids are 40% equiprob-
ability envelopes with hydrogen atoms omitted for clarity.
oxygen atom while Ru2 is coordinated to two anilino nitro-
gens, one pyridyl nitrogen and one oxygen atom. Assuming
that the putative intermediate in the ligand replacement reac-
tion is Ru2L2(OAc)2Cl, where the bound 2,4,6-(CH3)3ap ligands
(L) have the same trans geometric arrangement as that of an
earlier characterized Ru2(2-Fap)2(OAc)2Cl derivative,2 the
addition of a third 2,4,6-(CH3)3ap ligand on the dimetal unit
would then require the bridging ligand to “flip” to give a
“(2,1)” trans conformational framework reported in this work
instead of a “(3,0)” isomer obtained for Ru2(admp)3(O2CCH3)Cl23
and Ru2(chp)3(O2CCH3)Cl,27 both of which were structurally
characterized. Actually, two (2,1) isomers Ru2L3(OAc)Cl are
theoretically possible, one with two pyridines cis to each other
and the other trans, as observed in the current study. Neither
isomer has previously been reported in the literature and,
to our knowledge, the title compound is the only example
Electrochemistry in CH2Cl2 and CH3CN under N2
Cyclic voltammograms of Ru2L3(OAc)Cl in CH2Cl2 and CH3CN
containing 0.1 M TBAP under N2 are shown in Fig. 2. The com-
pound undergoes two one-electron reductions and two one-
electron oxidations within the potential window of the solvent.
All four reactions involve the dimetal unit, as has been
reported for related Ru2L4Cl compounds.2,20,28 The first
6+/7+
reduction (Ru25+/4+) and two oxidations (Ru25+/6+) and (Ru2
)
are reversible in both solvents but the second reduction in
CH2Cl2 is irreversible due to a following coupled chemical
reaction. The change in solvent from CH2Cl2 to CH3CN leads
to a shift of all E1/2 values and an increase in reversibility of
4+/3+
the Ru2
process (the second reduction) as illustrated in
Fig. 2. The reversible first reduction of Ru2L3(OAc)Cl occurs at
−0.60 V in CH3CN and is anodically shifted by 210 mV from
the E1/2 value for the same reaction in CH2Cl2 (−0.81 V)
whereas the first and second reversible oxidations of the
of
a
mixed-ligand diruthenium complex of the type
[Ru2(L)3(O2CCH3)]+ that has been structurally characterized
with a “(2,1)” geometric conformation framework.
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Fig. 2 Cyclic voltammogram of Ru2L3(OAc)Cl in (a) CH2Cl2 and (b) CH3CN, 0.1
M TBAP. Scan rate = 0.1 V s−1. The process with an asterisk is assigned to a
product of a reaction following the Ru24+/3+ process of the compound.
Fig. 3 Plot of E1/2 vs. log[CH3CN] for the first oxidation of Ru2L3(OAc)Cl in
CH2Cl2, 0.1 M TBAP.
diruthenium compound are cathodically shifted by 70 and
80 mV, respectively, upon going from CH2Cl2 to CH3CN (see concentration. The resulting correlation is shown in Fig. 3
Fig. 2). The potential separation between the first reduction where a plot of E1/2 for the first oxidation vs. log[CH3CN] con-
and first oxidation amounts to 1.29 V in CH2Cl2 and this sists of two linear portions; the first has a slope of −63 mV
difference in potentials can be compared to a separation of while the second has a zero slope, with the same E1/2 value
1.32 V between the same two reactions of Ru2(2-Fap)3(OAc)Cl2 (0.41 V) being measured in CH3CN, 0.1 M TBAP as in CH2Cl2–
under the same solution conditions. A slightly smaller poten- CH3CN mixtures containing 1.0 M added CH3CN. The data in
tial separation of 1.26 V is observed in CH3CN between the Fig. 3 suggests that one CH3CN ligand binds to the Ru26+ form
5+/4+
5+/6+
Ru2
and Ru2
processes of Ru2(2-Fap)3(OAc)Cl16 but, as of the compound at low concentrations of added acetonitrile
seen in Fig. 2, this separation decreases from 1.29 V in CH2Cl2 while at higher concentrations of CH3CN in the CH2Cl2–
5+
to 1.01 V in CH3CN, thus suggesting a stabilization of the CH3CN mixture, a single CH3CN ligand also binds to the Ru2
6+
4+
Ru2 and Ru2 oxidation states of Ru2L3(OAc)Cl by solvent form of the compound. This is in agreement with formulation
coordination to the dimetal center of the electrooxidized and of the title compound as Ru2L3(OAc)Cl(CH3CN) in CH3CN, 0.1
electroreduced compounds.
When the negative potential sweep in CH2Cl2 is reversed data discussed in later sections of the manuscript.
after the second reduction (Fig. 2a), a new reoxidation peak A titration was also carried out for the first reduction of
M TBAP. The same formulation is suggested by the UV-visible
appears on the reverse scan at Epa = −1.02 V. This new peak is Ru2L3(OAc)Cl in CH2Cl2–CH3CN mixtures but the cyclic vol-
indicated by an asterisk in the figure and occurs at all tempera- tammograms were ill-defined in the mixed solvent system,
tures when scanning past the second reduction process, but is being reversible only in neat CH2Cl2, or neat CH3CN contain-
not observed when the scan is reversed at about −1.40 V, i.e. ing 0.1 M TBAP. Nonetheless, the fact that E1/2 for reduction of
after the first reduction (dashed line in Fig. 2a). This result Ru2L3(OAc)Cl is easier by 210 mV in CH3CN than in CH2Cl2
4+
suggests that the “new” oxidation peak at Epa = −1.02 V can be can only be accounted for by axial coordination of the Ru2
attributed to a species formed in a chemical reaction after the electroreduction product with the solvent molecule.
5+/4+
second reduction of the compound. Although the nature of
The Ru2
reactions of Ru2L4Cl where L = 2-CH3ap, 2-Fap
the chemical reaction involving doubly reduced Ru2L3(OAc)Cl and 2,4,6-F3ap in CH2Cl2–PhCN mixtures were previously
in CH2Cl2 was not investigated in the present study, we shown to shift anodically from E1/2 values measured in
suggest that it is due to the transient formation of a metal– CH2Cl2, and this was interpreted by a coordination of one
carbon bonded diruthenium derivative, as was demonstrated solvent molecule (PhCN) to the Ru24+ form of each compound.
for Rh2(dpf)4(R) after reduction of Rh2(dpf)4, (dpf = N,N′- The magnitude of the potential shift with solvent binding
diphenylformamidinate anion) in THF containing added of involving coordination of CH3CN is smaller than in the
alkyl iodides (RI).36
present study, amounting to only 100–140 mV,37 and the larger
5+/4+
Benzonitrile (PhCN) is also known to axially bind to the (210 mV) shift in the Ru2
process of Ru2L3(OAc)Cl upon
4+
Ru2 unit of Ru2L4 where L = a substituted ap ligand37 and changing the solvent from CH2Cl2 to CH3CN can be attributed
solvent binding should also occur in the case of CH3CN. To to a stronger coordination of CH3CN than PhCN.
6+
verify this point, we added small aliquots of CH3CN to
The binding of CH3CN to the Ru2 form of Ru2L3(OAc)Cl
Ru2L3(OAc)Cl in CH2Cl2, 0.1 M TBAP and monitored changes can also be accounted for by a stronger binding ability of
6+
in the cyclic voltammogram as a function of the added CH3CN CH3CN than PhCN to the Ru2 form of the compound
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(actually PhCN seems not to bind at all to the singly oxidized CH3CN containing 0.2 M TBAP. As illustrated in Fig. 5a, the
[Ru2L4Cl]+).37 The structure of the currently studied diruthe- major absorption band at 656 nm in CH2Cl2 decreases in
nium complex may also play a role in the strong binding of intensity and shifts to 609 nm as the compound is converted
4+
CH3CN, since Ru2L3(OAc) is not as sterically crowded around to its Ru2 form. This contrasts with what occurs during the
5+/4+
the dimetal core as other previously investigated Ru2L4Cl com- Ru2
process in CH3CN, 0.2 M TBAP where the 663 nm
plexes. The dimetal unit of Ru2L3(OAc)Cl is also more electron band of the reduced compound decreases in intensity as a new
rich than other related previously investigated diruthenium band grows in at 490 nm (Fig. 5b). The Ru24+ products are pro-
complexes.2,9,28,37
posed to be Ru2L3(OAc) in CH2Cl2 and Ru2L3(OAc)(CH3CN) in
CH3CN. The presence of one bound solvent molecule in
CH3CN, 0.1 M TBAP is consistent with the electrochemical
data in both CH3CN, 0.1 M TBAP and CH2Cl2–CH3CN mixtures
(see earlier discussion). Differences in UV-visible spectra have
also been reported in the literature between related Ru24+ com-
plexes of the type Ru2(L)4 and Ru2(L)4(S), where S = solvent and
L = 2-CH3ap, 2-Fap and 2,4,6-F3ap.37
UV-visible spectroscopy in CH2Cl2 and CH3CN
Fig. 4 shows UV-visible spectra of Ru2L3(OAc)Cl in CH2Cl2 and
CH3CN. In both solvents the compound is characterized by
one strong absorption band (at 656 nm in CH2Cl2 and 663 nm
in CH3CN) due to the charge transfer transition π (ligand,
metal) → π* (Ru2) or σ* (Ru2) as previously assigned to related
Ru25+ complexes,4 and three less intense bands as indicated in
the figure. The overall shape of the spectrum is not signifi-
cantly different in the two solvents, but variations are seen in
the 400–500 and 900–1100 nm regions, thus suggesting two
different forms of the compound in these solvents.
An earlier study of related diruthenium compounds with
substituted ap ligands suggested that a solvent molecule does
not displace the chloride axial ligand when Ru2L4Cl is dis-
solved in PhCN37 and this is most likely also the case for
Ru2L3(OAc)Cl in CH3CN. On the other hand, CH3CN is able to
bind to Ru2L3(OAc)Cl trans to the axial chloride, and this could
account for the differences in UV-vis spectra between the two
5+/6+
The UV-visible spectral changes measured during the Ru2
oxidation process of Ru2L3(OAc)Cl in CH2Cl2 and CH3CN con-
taining 0.2 M TBAP resemble each other. In both solvents, the
major absorption band of the initial compound vanishes as a
new red-shifted absorption band develops (Fig. 5c and 5d).
There are multiple isosbestic points, indicating the lack of a
spectrally detectable intermediate during the oxidation. The
6+
Ru2 species generated in the two solvents exhibit similar
UV-visible spectra and the 9 nm shift in λmax upon going from
CH2Cl2 to CH3CN can be attributed to the presence of one
bound CH3CN molecule, thus providing additional evidence
for the proposed electron transfer mechanism shown in
Scheme 1.
5+
solvents in Fig. 4. Solvent binding to the Ru2 form of the
compound is also implied by data from the electrochemically
monitored titration in CH2Cl2 (Fig. 3).
Electrochemistry in CH2Cl2 under a CO atmosphere
In the present study, neither the IR nor the UV-visible spec-
trum of Ru2L3(OAc)Cl under CO indicates the binding of CO to
The UV-visible spectral changes which occur during the
5+/4+
5+/6+
Ru2
and Ru2
processes of Ru2L3(OAc)Cl were moni-
5+
Ru2 in the absence of an applied potential. A similar lack of
tored by thin-layer spectroelectrochemistry in CH2Cl2 and
CO binding was seen in the case of Ru2(dpf)3(O2CCH3)Cl
(dpf is the N,N′-diphenylformamidinate anion) and the pres-
ence of the chloride anion was proposed to inhibit the axial
5+
binding of CO to the Ru2 complex.38 In fact, the only
example in the literature for CO being axially bound to a stable
mixed-ligand Ru2 occurs for Ru2(dpf)3(OAc)BF4,38 which has
5+
a weakly bound BF4− anion.
A
cyclic voltammogram showing the reduction of
Ru2L3(OAc)Cl in CH2Cl2, 0.1 M TBAP under a CO atmosphere
is illustrated in Fig. 6 along with a cyclic voltammogram of
Ru2(ap)4Cl under the same experimental conditions for com-
parison purposes. Ru2L3(OAc)Cl exhibits four electroreduc-
tions labeled, as I′′, I, II and III in Fig. 6a but only process II is
reversible. The first and second reductions (Processes I′′ and I)
are located at Epc = −0.50 and −0.84 V while the third and
fourth reductions (II and III) are seen at E1/2 = −0.95 and Epc
=
−2.02 V for a scan rate of 0.1 V s−1
.
The cyclic voltammogram of Ru2L3(OAc)Cl under CO
(Fig. 6a) can be compared to the cyclic voltammogram of
Ru2(ap)4Cl (Fig. 6b) under the same solution conditions. As
was previously reported in the literature,30 the latter com-
pound undergoes three reductions labeled as I, II, and III in
Fig. 6b. A key difference between Ru2L3(OAc)Cl and Ru2(ap)4Cl
Fig. 4 UV-visible spectra of Ru2L3(OAc)Cl in (a) CH2Cl2 and (b) CH3CN.
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Fig. 5 Time-dependent UV-visible spectral changes of Ru2L3(OAc)Cl upon the first reduction in (a) CH2Cl2 and (b) CH3CN and the first oxidation in (c) CH2Cl2 and
(d) CH3CN containing 0.2 M TBAP.
Scheme 1 Electrode reactions of Ru2L3(OAc)Cl in CH2Cl2 and CH3CN (S).
5+/4+
is that only one reduction process precedes the reversible
In a previous study,30 the Ru2
processes of Ru2(L)4Cl
reaction II in the case of Ru2(ap)4Cl, but two reduction pro- (L is ap or a substituted ap anion) in CH2Cl2, 0.1 M TBAP
cesses are seen prior to the reversible reaction II in the case of under CO were shown to generally shift positively by 10–80 mV
5+/4+
Ru2L3(OAc)Cl. It was earlier reported30 that the Ru2
with respect to the same electrode processes under N2, the
reduction of Ru2(ap)4Cl under CO (process I) was coupled with only exception being in the case of Ru2(F5ap)4Cl where the
the oxidation process I′ and this is shown by the dotted line in electrode reaction under CO was shifted negatively by
Fig. 6b. In a similar manner, the reduction process I′′ of 40–70 mV, i.e. the reduction under CO was more difficult than
Ru2L3(OAc)Cl at Ep = −0.50 V is coupled with the oxidation under N2. For all of the investigated diruthenium compounds,
5+/4+
process I′ at Ep = 0.06 V, as shown by the dotted line in Fig. 6a, including Ru2(F5ap)4Cl, the Ru2
process was proposed to
5+
thus suggesting that processes I′′ and I both involve a Ru2 to involve a dissociation of the chloride axial ligand after electron
4+
Ru24+ conversion in the case of Ru2L3(OAc)Cl. Further evidence transfer and the final Ru2
product was formulated as
for this assignment is given by the fact that processes I′′ and I Ru2L4(CO).30 Process I in Fig. 6a is only 30 mV more negative
5+/4+
have similar peak currents, the sum of which is approximately than the Ru2
process under N2 (Fig. 2), suggesting that
equal to the peak current of process II (see Fig. 6a).
this electron transfer can be described by a reduction of
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Fig. 6 Cyclic voltammograms of (a) Ru2L3(OAc)Cl and (b) Ru2(ap)4Cl in CH2Cl2,
0.1 M TBAP under CO. Scan rate = 0.1 V s−1
.
Ru2L3(OAc)Cl to Ru2L3(OAc)(CO).9,30 Although process I′′ also
involves a Ru25+/Ru2 reaction, it is 310 mV easier than the
4+
Fig. 7 In situ FTIR absorbance spectral changes of Ru2L3(OAc)Cl in CH2Cl2, 0.1
M TBAP under CO atmosphere upon (a) the first reduction (process I’’, Fig. 6a)
and (b) the second reduction (process II, Fig. 6a).
same electrode reaction under N2. This result might be
accounted for by the occurrence of a chemical reaction prior to
the electron transfer which would lower the electron density
on the dimetal unit. A probable chemical reaction could be the
binding of a CO molecule trans to the chloride axial ligand,
thus generating the transient electroactive Ru25+ species in sol-
ution, Ru2L3(OAc)(CO)Cl.
Summary
A mixed-ligand complex of the type Ru2L3(OAc)Cl was success-
fully synthesized and structurally characterized by X-ray crystal-
lography, revealing a “(2,1)” geometric conformation in the
The product of the reduction in process I′′ is formulated as
[Ru2L3(OAc)Cl(CO)]−, and would be expected to have a lower
4+
5+
electron density on the Ru2 dimetal unit than in the case of
solid state. Ru2L3(OAc)Cl is a paramagnetic Ru2 complex
[Ru2L3(OAc)Cl]− because of the π-back donation by the CO
axial group. If the Cl− anion did not readily dissociate, one
might then observe a CO vibration for both Ru24+ products, i.e.
[Ru2L3(OAc)Cl(CO)]− and Ru2L3(OAc)(CO). This seems to be the
case as shown by the presence of two CO vibrations at 1918
and 1898 cm−1 in the IR spectrum obtained after the first
reduction of the title compound in CH2Cl2, 0.2 M TBAP under
a CO atmosphere (see Fig. 7a).
with three unpaired electrons, that can be reversibly converted
6+
7+
to its Ru24+, Ru2 and Ru2 oxidation states in CH2Cl2 con-
4+/3+
taining 0.1 M TBAP under an N2 atmosphere. A Ru2
process is also observed to occur under the above experimental
conditions. This reaction is irreversible in CH2Cl2 owing to a
3+
coupled chemical reaction that involves the Ru2 species and
the solvent itself but the process becomes reversible when
4+
CH3CN axially coordinates to the Ru2 form of Ru2L3(OAc)Cl.
5+
6+
The second reduction of Ru2L3(OAc)Cl in CH2Cl2, 0.1 M
TBAP under a CO atmosphere (i.e. process II in Fig. 6a) is
Acetonitrile will also bind to the Ru2 and Ru2 forms of
Ru2L3(OAc)Cl in neat CH3CN and this axial ligand coordi-
nation most likely occurs trans to the bound chloride axial
ligand. A similar type of axial coordination also occurs with
CO under an applied potential and two diruthenium(II,II)
species, [Ru2L3(OAc)(CO)Cl]− and Ru2L3(OAc)(CO), are pro-
posed to exist in CH2Cl2, 0.1 M TBAP under CO.
4+
3+
attributed to a Ru2 to Ru2 electrode reaction and, as was
the case for the previously examined diruthenium compounds
with similar bridging ligands,9,30 is proposed to involve the
addition of a second axial CO molecule to give [Ru2L3(OAc)-
(CO)2]−. As shown by the IR spectrum in Fig. 7b, the bis-CO
3+
Ru2 complex exhibits νCO stretches at 1874 and 1822 cm−1
,
A binding of CO trans to the chloride axial ligand has not
been reported for other metal–metal bonded diruthenium
complexes with ap or ap-type bridging ligands and can be
attributed, in the present study, to the fact that the (2,4,6-
(CH3)3ap) anion has a much richer electron density than other
values which fall in the range of previously investigated
diruthenium CO complexes.30 Thus, the electron transfer
mechanism in CH2Cl2 under CO is proposed to occur as
shown in Scheme 2.
3578 | Dalton Trans., 2013, 42, 3571–3580
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Scheme 2 Electrode reactions of Ru2L3(OAc)Cl in CH2Cl2 under CO (the anionic ligands have been omitted for clarity).
previously utilized anionic bridging ligands, thus allowing
more π-back donation between the dimetal unit and the
incoming group. It could also be due to the fact that
Ru2L3(OAc)Cl has only three ap-type ligands at the dimetal per-
iphery as compared to four in the case of the other investi-
gated diruthenium complexes. This might also be a key factor
7 L. Villalobos, Z. Cao, E. Fanwick Phillip and T. Ren, Dalton
Trans., 2012, 41, 644.
8 M. Nguyen, T. Phan, E. Van Caemelbecke, X. Wei, J. L. Bear
and K. M. Kadish, Inorg. Chem., 2008, 47, 4392.
9 K. M. Kadish, M. Nguyen, E. Van Caemelbecke and
J. L. Bear, Inorg. Chem., 2006, 45, 5996.
because it would help minimize steric hindrance at the axial 10 K. M. Kadish, T. D. Phan, L.-L. Wang, L. Giribabu,
position trans to the chloride axial ligand. Further studies are
now in progress in our laboratory to elucidate these points.
A. Thuriere, J. Wellhoff, S. Huang, E. Van Caemelbecke and
J. L. Bear, Inorg. Chem., 2004, 43, 4825.
11 B. Han, J. Shao, Z. Ou, T. D. Phan, J. Shen, J. L. Bear and
K. M. Kadish, Inorg. Chem., 2004, 43, 7741.
12 K. M. Kadish, B. Han, J. Shao, Z. Ou and J. L. Bear, Inorg.
Chem., 2001, 40, 6848.
13 J. L. Bear, J. Wellhoff, G. Royal, E. Van Caemelbecke,
S. Eapen and K. M. Kadish, Inorg. Chem., 2001, 40,
2282.
14 J. L. Bear, Y. Li, B. Han, E. Van Caemelbecke and
K. M. Kadish, Inorg. Chem., 2001, 40, 182.
Acknowledgements
The support of the Robert A. Welch Foundation (K.M.K., Grant
E-680) is gratefully acknowledged. E.V.C. and K.R. also thank
the Robert A. Welch Foundation for their HBU departmental
grant (Grant BF-0016). We also thank Dr Magnus U. Legemah
for growing the crystals of the compound and Dr J. D. Korp for
performing the X-ray analyses.
15 J. L. Bear, Y. Li, B. Han, E. Van Caemelbecke and
K. M. Kadish, Inorg. Chem., 1996, 35, 3053.
16 J. L. Bear, Y. Li, B. Han and K. M. Kadish, Inorg. Chem.,
1996, 35, 1395.
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