Asymmetric mixed-valence complexes that consist of cyclometalated ruthenium and ferrocene: Synthesis, characterization, and electronic-coupling studies

Article abstract of DOI:10.1002/asia.201200900

Three bis-tridentate ferrocene-containing cyclometalated ruthenium complexes, [(Fcdpb)Ru(tpy)]⁺ (1⁺), [(Fctpy)Ru(dpb)] ⁺ (2⁺), and [(Fcdpb)Ru(Fctpy)]⁺ (3 ⁺), have been prepared and characterized, where Fcdpb is the 2-deprotonated form of 1,3-di(2-pyridyl)-5-ferrocenylbenzene, tpy is 2,2':6',2 -terpyridine, dpb is the 2-deprotonated form of 1,3-di(2-pyridyl)benzene, and Fctpy is 4'-ferrocenyl-2,2':6',2 - terpyridine. Single crystals of compounds 2⁺ and 3⁺ have been studied by X-ray analysis. Complexes 1⁺ and 2⁺ displayed two anodic redox waves, whilst three well-separated redox couples were observed for compound 3⁺. A combined experimental and computational study suggested that the ferrocene unit on the Fcdpb moiety in compounds 1 ⁺ and 3⁺ was oxidized first. In contrast, the order of the oxidation of ruthenium and ferrocene in complex 2⁺ was reversed. Metal-to-metal-charge-transfer transitions (MM'CT) have been observed for the singly oxidized states 1²⁺, 2²⁺, and 3²⁺ in the near-infrared region. Hush analysis showed that the metal-metal electronic couplings in compounds 1²⁺ and 3²⁺ were much stronger than those in compound 2²⁺. Copyright

Full text of DOI:10.1002/asia.201200900

FULL PAPER
DOI: 10.1002/asia.201200900
Asymmetric Mixed-Valence Complexes that Consist of Cyclometalated
Ruthenium and Ferrocene: Synthesis, Characterization, and Electronic-
Coupling Studies
Si-Hai Wu,[a] Jun-Jian Shen,[a] Jiannian Yao,[a] and Yu-Wu Zhong*[a, b]
Abstract: Three bis-tridentate ferro-
ied by X-ray analysis. Complexes 1+
and 2+ displayed two anodic redox
waves, whilst three well-separated
redox couples were observed for com-
pound 3+. A combined experimental
and computational study suggested that
the ferrocene unit on the Fcdpb moiety
in compounds 1+ and 3+ was oxidized
first. In contrast, the order of the oxi-
dation of ruthenium and ferrocene in
complex 2+ was reversed. Metal-to-
cene-containing cyclometalated ruthe-
nium complexes, [(Fcdpb)Ru
(1+), [(Fctpy)Ru(dpb)]+ (2+), and
[(Fcdpb)Ru
(Fctpy)]+ (3+), have been
(tpy)]+
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
G
metal-charge-transfer
transitions
prepared and characterized, where
Fcdpb is the 2-deprotonated form of
1,3-di(2-pyridyl)-5-ferrocenylbenzene,
tpy is 2,2’:6’,2“-terpyridine, dpb is the
2-deprotonated form of 1,3-di(2-pyri-
dyl)benzene, and Fctpy is 4’-ferrocenyl-
2,2’:6’,2”-terpyridine. Single crystals of
compounds 2+ and 3+ have been stud-
(MM’CT) have been observed for the
singly oxidized states 12+, 22+, and 32+
in the near-infrared region. Hush anal-
ysis showed that the metal–metal elec-
tronic couplings in compounds 12+ and
32+ were much stronger than those in
compound 22+.
Keywords: cyclometalation · ferro-
cene · mixed-valent compounds ·
redox-active compounds
nium
· ruthe-
Introduction
trochromic devices after their deposition onto electrode sur-
faces.[5]
Since the pioneering work of Creutz and Taube,[1] mixed-va-
lence (MV) systems have received tremendous interest over
the last three decades.[2] They are of interest for many rea-
sons. Studies of MV systems have provided useful informa-
tion regarding the charge delocalization in species with mul-
tiple redox sites and the influence of some key factors on in-
tramolecular electron-transfer processes between the indi-
vidual components.[3] This information has allowed the iden-
tification of promising candidates for molecular wires and
other components for molecular electronics.[4] MV compo-
nents with strong electronic coupling between redox sites
usually exhibit intense absorption in the near-infrared (NIR)
region, which makes them excellent materials for NIR elec-
Most of the reported MV systems have concentrated on
homobimetallic complexes that are bridged by an unsaturat-
ed organic bridge.[2,3] One of the most widely used redox
species is ferrocene (Fc),[6] which is characterized by a well-
defined ferrocene/ferrocenium (Fc0/+) redox pair. In addi-
tion, the synthesis and structural modification of ferrocene
derivatives are straightforward. A large number of bridged
bis-ferrocene derivatives are known and the electronic cou-
pling between individual metal centers can be finely
tuned.[6] Another interesting redox species for MV chemis-
try is cyclometalated ruthenium,[7] which features a chelate
ꢀ
ring that contains a Ru C bond. Owing to the presence of
an anionic carbon ligand, cyclometalated ruthenium com-
plexes often display much lower RuII/III redox potentials
than their noncyclometalated analogues. However, the po-
tential of the RuII/III couple can be varied by using different
auxiliary ligands.[8] Recent studies have demonstrated that
cyclometalated ruthenium complexes are useful for con-
structing homobimetallic MV systems[9] and dye-sensitized
solar cells.[10] Interestingly, these MV systems often display
strong metal–metal electronic coupling with the aid of an
anionic bis-carbon bridging ligand.
[a] S.-H. Wu, J.-J. Shen, Prof. J. Yao, Prof. Dr. Y.-W. Zhong
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Photochemistry
Institute of Chemistry, Chinese Academy of Sciences
Bejing 100190 (Peopleꢀs Republic of China)
E-mail: zhongyuwu@iccas.ac.cn
Homepage: http://zhongyuwu.iccas.ac.cn/
[b] Prof. Dr. Y.-W. Zhong
Compared to homobimetallic complexes, heterobimetallic
MV systems are equally important and have received much
attention from both experimental and theoretical points of
view.[11] Herein, we present the synthesis, characterization,
and electronic-coupling studies of three heterometallic com-
State Key Laboratory of Coordination Chemistry
Nanjing University
Nanjing 210093 (Peopleꢀs Republic of China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200900.
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Yu-Wu Zhong et al.
plexes, [(Fcdpb)Ru
(tpy)]+ (1+), [(Fctpy)Ru
E
turally asymmetric, the connection mode between rutheni-
um and ferrocene can be varied. In complex 1+, the ferro-
cene motif and the ruthenium atom are bridged through
a benzene-1,4-diyl moiety, whereas, in complex 2+, they are
bridged through a pyridine-4-yl moiety, and complex 3+ has
two appended ferrocene moieties. We have recently report-
ed on asymmetric MV compounds that were composed of
triarylamine and cyclometalated ruthenium species[12] and
found that the connection mode between those two compo-
nents played an important role in determining the nature of
the electronic coupling between them. We considered that
a similar situation would happen in systems that were com-
posed of ferrocene and cyclometalated ruthenium. Thus,
complexes 1+–3+, with various connection modes, were de-
signed and synthesized. The electronic properties of these
complexes have been studied and compared with two ferro-
cene-containing noncyclometalated ruthenium complexes,
[(Fcdpb)Ru
ACHTUNGTRENNUNG
and cyclometalated ruthenium redox species, where Fcdpb
is the 2-deprotonated form of 1,3-di(2-pyridyl)-5-ferrocenyl-
benzene, tpy is 2,2’:6’,2“-terpyridine, dpb is the 2-deproton-
ated form of 1,3-di(2-pyridyl)benzene, and Fctpy is 4’-ferro-
cenyl-2,2’:6’,2”-terpyridine (Scheme 1). Considering that cy-
clometalated ruthenium complexes are readily accessible,
that their RuII/III potentials can be easily modulated, and
that they have very interesting electronic and spectroscopic
properties,[8–10] these systems would be of great interest.
Moreover, because cyclometalated ruthenium itself is struc-
[(Fctpy)Ru
C
(42+)
and
[Ru
A
(52+;
Scheme 1).[13]
We noticed that some heterobimetallic or multimetallic
systems that consisted of ferrocene and ruthenium compo-
nents have been reported previously. Indeed Taube, Henry,
ꢀ
Lewis, and co-workers reported the first Ru Fc heterobime-
tallic complex that was bridged by a cyano moiety in the
early 1980s.[14] The ruthenium component, [Ru
ACTHNUGTRNEUGN(NH3)5], was
found to be oxidized first and a weak metal-to-metal-
charge-transfer (MM’CT) band from Fc to RuIII was ob-
served at around 1100 nm in MeCN (emax <400mꢀ1 cmꢀ1). In
the mid-1990s, Sato et al. synthesized a series of covalently
connected heterobimetallic systems,[15] including some Ru
ꢀ
Fc complexes[15a] in which the Fc units were first oxidized
and the resulting MM’CT bands were quite intense (emax
ꢁ2000–4000mꢀ1 cmꢀ1, lmax ꢁ1550 nm). Later, Kwan and co-
workers studied two heterobimetallic complexes, [FcACHTUNGTRENNUNG
py)Ru
N
ACHTUNGNRET(NGNU PF6)2 and [FcACHTUTNGRENNUG(3-py)RuACHTUNEGTRG(NNUN NH3)5]ACHNUTGTRENNNUG
which Fc
N
ACHTUNGTRENNUNG
ferrocenylpyridine, respectively. They found that, in these
complexes, the ruthenium atom was oxidized first and the
resulting MM’CT band from Fc to RuIII was solvent depen-
dent, thus suggesting a moderate coupling between the two
metals. Long and co-workers synthesized a series of ferro-
cene-containing bis(acetylide) ruthenium complexes and
found that the oxidation of ferrocene occurred before that
of the ruthenium center.[17] A MM’CT band from RuII to
Fc+ was evident in the region between 1000 and 2000 nm. A
Scheme 1. Ferrocene-containing cyclometalated (1+–3+) and noncyclome-
talated ruthenium complexes (42+ and 52+).
heterobimetallic complex, [RuACHTNUTRGENNUG(CH=CHFc)Cl(CO)ACHTUNGTRENNUNG(PiPr3)2],
was recently investigated by Winter and co-workers, who
concluded that the first oxidation was associated with both
redox constituents but was more biased towards the ferro-
cene site.[18] Two distinct NIR absorption bands (1349 and
2164 nm) were observed for the singly oxidized form and
ꢀ
both of them were assigned to iron-centered d d type tran-
sitions instead of MM’CT transitions. In addition to these ef-
forts, other ferrocene-hybridized ruthenium complexes have
also been prepared and studied.[19] However, cyclometalated
ruthenium complexes have not been employed in this con-
text.
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Yu-Wu Zhong et al.
Results and Discussion
Single-Crystal Structures
Single crystals of compounds [2](PF6) and [3]ACHTUNTRGNE(UNG PF6) that were
Synthesis
U
Complexes [1]
(PF6), [2]
N
N
suitable for X-ray analysis[22] were obtained by the slow dif-
fusion of petroleum ether into a solution of the individual
complex in CHCl3. ORTEPs of these two complexes are
shown in Figure 1. The coordination geometry of the ruthe-
nium atoms is distorted octahedral and they are surrounded
by two tridentate ligands. One ligand is a NCN-type cyclo-
metalating ligand and the other is a NNN-type noncyclome-
as outlined in Scheme 2. A [PdACHTNUGTRNEUNG(PPh3)4]-catalyzed Suzuki
coupling between 3,5-di(2-pyridyl)bromobenzene[12] (6) and
ꢀ
ꢀ
talating ligand. The Ru C bond (Ru1 C1) in compounds
ꢀ
[2]
G
G
Figure 1. ORTEPs of a) [2]ACTHNURGTENN(UG PF6) and b) [3]ACHTUGNTREN(NUNG PF6); thermal ellipsoids are
set at 50% probability. Solvent molecules and anions are omitted for
clarity.
ꢀ
Scheme 2. Synthesis of compound 1+–3+; anions are PF6
.
ꢀ
ꢀ
ferroceneboronic acid afforded 1,3-di(2-pyridyl)-5-ferroce-
nylbenzene (7, FcdpbH) in 32% yield. The use of K3PO4 as
the base and dry 1,4-dioxane as the solvent was found to be
important for the success of this reaction. These conditions
have previously been used for Suzuki coupling reactions be-
tween ferroceneboronic acid and organic triflates.[20] No
product was isolated when using K2CO3 as the base in a mix-
ture of tetrahydrofuran (THF) and water. First, ligand 7 was
allowed to react with p-cymeneruthenium(II)–dichloride
dimer in the presence of KPF6 and NaOH in MeCN to yield
N bond that is opposite to the Ru C bond (Ru1 N1) is 2.03
and 2.02 ꢂ in compounds [2](PF6) and [3](PF6), respectively.
A
ACHTUNGTRENNUNG
ꢀ
Other Ru N bond lengths are in the range 2.07–2.10 ꢂ.
Similar structures of cyclometalated ruthenium complexes
have been reported previously.[8] In complex [2]
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
angles between the Cp ligand and the NCN and NNN li-
gands are 13.3 and 3.88, respectively.
a cyclometalated ruthenium intermediate, [8]
was then treated with tpy and Fctpy[13] to give compounds
[1](PF6) and [3](PF6) in 52% and 46% yield, respectively.
Similar methods have been reported for the synthesis of
tris-bidentate[10c] or bis-tridentate[21] cyclometalated rutheni-
um complexes. In a similar way, 1,3-di(2-pyridyl)benzene (9,
dpbH) was transformed into cyclometalated ruthenium in-
ACHTUNGTRENN(UGN PF6), which
Electrochemical Studies
G
E
The electronic properties of compounds 1+, 2+, and 3+ were
studied by electrochemical analysis and compared with com-
pounds 42+ and 52+ (Figure 2 and Table 1). Cyclic voltammo-
grams (CVs) of compounds 1+ and 2+ with wider potential
windows are provided in the Supporting Information, Fig-
ure S1–S3. Complex 1+ shows two widely separated anodic
couples at +0.41 and +0.74 V versus Ag/AgCl. The first
wave is ascribed to the Fc0/+ process and the second wave is
assigned to the RuII/III process. This assignment is consistent
with the fact that the Fc0/+ process of the pristine ligand (7)
termediate [10]
ACHTUNGTRENNUNG
give complex [2]ACHTUNGTRENNUNG
thesis and characterization data are given in the Experimen-
tal Section.
(E1/2ACHTNUGRTNEUNG
(Fc0/+)=0.53 V; see the Supporting Information, Fig-
ure S4) is slightly easier than that of a model cyclometalated
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Yu-Wu Zhong et al.
(the ferrocene unit that is substituted on the NNN ligand)
processes in complex 3+ should be the same as those in com-
plex 2+. Thus, the second and third waves in complex 3+ are
assigned to the RuII/III and Fc20/+ processes, respectively. If
this order was reversed, the E1/2ACTHNUTRGNEUNG
(RuII/III) value for complex
3+ should be much more positive than that in complex 1+,
owing to the presence of two electron-withdrawing ferroce-
nium units. However, the third wave of complex 3+ is locat-
ed at essentially the same potential as the RuII/III process of
complex 1+. On the other hand, when the RuII/III process
1/2ACHTNUTGRNEUNG
occurs before the Fc20/+ process, the E (RuII/III) value for
complex 3+ should be less positive than that in complex 1+
because the presence of an additional electron-donating Fc
moiety would make the oxidation of ruthenium more feasi-
ble. This fact supports the assignment of the second anodic
wave of complex 3+ to the RuII/III process. More conclusive
evidence for the assignment of the redox behavior of com-
plex 3+ comes from the spectroelectrochemistry experiments
(see below), which shows that the spectroscopic changes for
the single and double oxidations of complexes 1+ and 3+ are
basically the same. This result means that the first two oxi-
dation waves of these two complexes are of the same char-
acter.
Figure 2. CVs (a–c) and DPVs (d–f) of compounds [1]
ACHTUNGTREN(NUNG PF6) (a,d), [2]-
A
U
Table 1. Electrochemical data.[a]
Compound
E1/2 (anodic) [V]
E1/2 (cathodic) [V]
Fc0/+
RuII/III
A
G
N
+0.41
+0.69
+0.40,
+0.75
+0.59
+0.74
+0.58
+0.60
ꢀ1.54
ꢀ1.52
ꢀ1.55
E
N
Oxidation of the ferrocene units in compounds 42+ and
E
ACHTUNGTRENNUNG
1/2ACHTNUTGRNEUNG
52+ (E (Fc0/+)=0.59 and 0.58 V versus Ag/AgCl, respective-
[b]
E
E
)
+1.42
+1.39
ꢀ1.21,
ꢀ1.52
ꢀ1.22,
ꢀ1.52
ꢀ1.51
–
ly)[13] is more difficult than that of the Fc0/+ moiety in com-
plex 1+ (+0.41 V) and the Fc10/+ group in complex 3+
(+0.40 V). This result reflects the electron-donating nature
N
)
+0.58
of the cyclometalated RuII component. However, the E1/2
-
C
G
–
+0.56
–
–
FcdpbH (7)
Fctpy
+0.53
+0.65
AHCTUNGTRENNUNG
(Fc0/+) values for complexes 42+ and 52+ are less positive
–
than the Fc0/+ value of complex 2+ (+0.69 V) and the Fc20/+
value of complex 3+ (+0.75 V), thus indicating that the oxi-
dized cyclometalated RuIII component is more electron-
[a] The potential is reported as the E1/2 value versus Ag/AgCl. Potentials
versus Fc0/+ can be derived by subtracting 0.45 V. [b] See reference [13].
[c] See references [8a] and [8e].
withdrawing than the [RuII
ACTHNUTRGNEN(UG tpy)2] unit. At more positive po-
tentials, complexes 1+, 2+, and 3+ all exhibit some irreversi-
ble oxidation peaks (see the Supporting Information, Fig-
ure S1–S3), possibly caused by ferrocene decomposition.
Similar irreversible waves have been found in noncyclome-
talated complexes 42+ and 52+.[13] In the cathodic scan, they
all show a ligand-based reduction couple, which is associated
with the reduction of an individual NNN ligand.
ruthenium
complex,
[(dpb)Ru
(tpy)]+
(E1/2ACTHGNUTERNNUG
(RuII/III)=
0.56 V).[8a,e] The Fc0/+ process in complex 1+ becomes much
easier, owing to a combined inductive and electron delocali-
zation effect that is caused by the ruthenium component.
On the other hand, the RuII/III process in compound 1+ be-
comes more difficult than that in [(dpb)RuACTHNUTRGNEUNG
(tpy)]+, owing to
the presence of an electron-withdrawing ferrocenium unit.
This assignment is also corroborated by computational re-
sults (see below).
Density Functional Theory (DFT) Calculations
Complex 2+ displays two anodic waves at +0.58 and
DFT calculations of complexes 1+, 2+, and 3+ were per-
formed at the B3LYP/LANL2DZ/6-31G*/vacuo level to
assist in the understanding of their electronic structures (for
details, see the Experimental Section). The starting coordi-
nates were taken from the X-ray data of the cationic frag-
+0.69 V versus Ag/AgCl. Considering that the E
N
)
)
value of [(dpb)Ru
G
U
value for ligand Fctpy (0.65 V; see the Supporting Informa-
tion, Figure S5), the oxidation of the ruthenium component,
mixed with some amount of ligand oxidation, is believed to
take place prior to that of the ferrocene unit. Three well-
separated redox pairs at +0.40, +0.60, and +0.75 V are evi-
dent for complex 3+ in the same region. By comparing Fig-
ure 2a–c, it is safe to assign the first wave to the oxidation
of the ferrocene unit on the NCN ligand (Fc1; Figure 1b).
However, the assignment of the other two waves is more
difficult. We believe that the order of the RuII/III and Fc20/+
ment of compounds [2]ACHTUNGRTENNUG(PF6) and [3]ACHTUNGTRNEN(GUN PF6). The Supporting
Information, Figures S6–S8 show isodensity plots of selected
frontier orbitals along with energy diagrams. The LUMOs
and closely spaced LUMO+1 orbitals of all of the com-
plexes are associated with the individual NNN ligand. Their
LUMO+2 orbitals, which are much higher lying, are all do-
minated by the NCN ligand. These results are in agreement
with above electrochemical assignment that the first catho-
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Yu-Wu Zhong et al.
dic waves are associated with the reduction of the NNN
ligand.
cribed to Fe-based MLCT transitions. This assignment is
supported by TDDFT analysis of complexes 1+ and 2+ (see
the Supporting Information, Table S1 and Figure S9). The
predicted S6 and S7 excitations of complex 2+ at 532 and
531 nm are responsible for the experimentally observed
shoulder bands between 560 and 600 nm. These two excita-
tions are associated with charge transfer from the
HOMOꢀ1, HOMOꢀ2, and HOMOꢀ3 orbitals of complex
2+; these orbitals have both ruthenium and iron character
(see the Supporting Information, Figure S7). However, this
feature is not present in complex 1+.
The orbital compositions of the frontier-occupied orbitals
differ from each other: The closely spaced HOMO and
HOMOꢀ1 in complex 1+ are mainly associated with the fer-
rocene segment and the HOMO level has some contribu-
tions from the cyclometalated phenyl ring and ruthenium
atom (see the Supporting Information, Figure S6). The
HOMOꢀ2 orbital has contributions from across the whole
Fc–phenyl–Ru array. The low-lying HOMOꢀ3 orbital is do-
minated by the ruthenium component. These orbital ar-
rangements may suggest that the ferrocene unit in complex
1+ may firstly be oxidized during the anodic scan.
Oxidative Titration and NIR-Transition Analysis
The HOMO of complex 2+ is dominated by the cyclome-
talated phenyl ring and the ruthenium atom (see the Sup-
porting Information, Figure S7); this result is commonly ob-
served in many monometallic cyclometalated ruthenium
complexes.[8] The low-lying HOMOꢀ1, HOMOꢀ2,
HOMOꢀ3, and HOMOꢀ4 levels have contributions from
both the ferrocene segment and the ruthenium components.
The HOMO, HOMOꢀ1, and HOMOꢀ2 in complex 3+ (see
the Supporting Information, Figure S8) resemble those in
complex 1+. The HOMOꢀ3 and HOMOꢀ4 in complex 3+
are mainly associated with the ruthenium component,
whereas the HOMOꢀ5 and HOMOꢀ6 are dominated by
the ferrocene unit on the NNN ligand side.
To study the electronic coupling effect, complexes 1+, 2+,
and 3+ were oxidized by either chemical or electrochemical
methods and changes in their corresponding absorption
spectra were recorded. Figure 4 shows the changes in the ab-
sorption spectra of complex 1+ upon the gradual addition of
a solution of SbCl5 in CH2Cl2, which has previously been
used for the generation of organic MV systems.[23] When up
to one equivalent of oxidant was added, the MLCT transi-
tions in the visible-light region decreased a little with the
concomitant emergence of two distinct absorption bands in
the NIR region (Figure 4a). When more SbCl5 was added,
the MLCT transitions continued to decrease and the two
new NIR bands were found to decrease as well (Figure 4b).
When a solution of complex 1+ in MeCN was titrated
against cerium ammonium nitrate (CAN) in a similar
way,[24] very similar spectroscopic changes were recorded
(see the Supporting Information, Figure S9). The NIR bands
Spectroscopic Studies
The absorption spectra of compounds [1]
[3](PF6) in CH2Cl2 were compared with those of model com-
pounds [(dpb)Ru (PF6)2 (Figure 3). Complex
(tpy)]+ and [4]
1+ shows a very similar absorption pattern to that of
[(dpb)Ru
(tpy)]+ in the visible-light region, albeit with
ACHTUGTNRNENUG(PF6), [2]ACHTUNGTNER(NUGN PF6), and
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
a slightly red-shifted absorption maximum (510 versus
498 nm). These bands are ascribed to the Ru-based metal-
to-ligand-charge-transfer (MLCT) transitions. In addition to
the Ru-based MLCT transitions at around 510 nm, com-
plexes 2+ and 3+ display some shoulder bands on their
lower-energy side. Similar transitions are also evident in the
absorption spectrum of complex 42+. According to previous
resonance-Raman measurements,[13c] these bands are as-
Figure 4. Changes in the absorption spectra of [1]ACTHNUGTRNEUNG(PF6) in CH2Cl2 upon
Figure 3. Absorption spectra of [(Fcdpb)Ru
(dpb)]+ (2+), [(Fcdpb)Ru(Fctpy)]+ (3+), [(dpb)Ru
[(Fctpy)Ru
(tpy)]2+ (42+) in CH2Cl2.
G
a) single- and b) double oxidation by gradually adding SbCl5. Inset in
(a): Gaussian-fitting of the NIR bands (in wavenumbers) of the singly
oxidized form.
E
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
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Yu-Wu Zhong et al.
of the singly oxidized state (12+) show almost the same ab-
sorption patterns and energies in CH2Cl2 or in MeCN
(Figure 5), which suggests that the electron is largely delo-
calized across the dimetallic unit.
Figure 5. Comparison of the NIR spectra of compounds 12+ and 32+; * de-
notes artifacts that are due to an imperfect compensation of the back-
ground.
Figure 6. Changes in the absorption spectra of [2]ACTHNUGTRNEUNG(PF6) in CH2Cl2 upon
a) single- and b) double oxidation by gradually adding SbCl5. Inset in
(a): Gaussian-fitting of the NIR bands (in wavenumbers) of the singly
oxidized form. Inset in (b): Enlarged spectra in the NIR region.
Complex 3+ exhibited similar spectroscopic changes upon
single and double oxidation with the gradual addition of
a solution of SbCl5 in CH2Cl2 (see the Supporting Informa-
tion, Figure S11). The NIR bands of the singly oxidized
state (32+) are slightly red-shifted and more intense than
those of complex 12+ (Figure 5). The appearance and disap-
pearance of the NIR bands of complexes 1+ and 3+ upon
stepwise oxidation could also be realized by electrolysis by
using a transparent indium-tin-oxide (ITO) glass electrode
(see the Supporting Information, Figure S12 and S13). For
instance, when the applied potential was gradually increased
from +0.32 to +0.52 V and from +0.55 to +0.70 V versus
Ag/AgCl during the spectroelectrochemical measurements
of complex 3+ (corresponding to the first and second oxida-
tion event), similar NIR bands were found to increase and
decrease again (see the Supporting Information, Fig-
ure S13). Upon further increasing the potential, the absorp-
tion bands in the visible-light region decreased a little. This
result supports the previous electrochemical assignment of
complex 3+, in which three anodic waves at +0.40, +0.60,
and +0.75 V are due to the Fc10/+, RuII/III, and Fc20/+ pro-
cesses, respectively.
the lower-energy NIR bands at around 2500 nm are assigned
2+
ꢀ
to Fe-centered d d-type transitions. As far as complex 2 is
concerned, the observed weak NIR band at 1910 nm is as-
signed to the FeII!RuIII MM’CT transition. The NIR bands
of complexes 12+, 22+, and 32+ were fitted to two Gaussian
functions (insets in Figure 4a and 6a; also see the Support-
ing Information, Figure S11) and the parameters for the
MM’CT bands are given in Table 2. According to Hush the-
ory,[11n,o,25] the predicted half-width for the intervalence
charge-transfer bands of asymmetric MV systems, Dn1/2(theo)
,
equals [2310
AHCTUNGTRENNUNG
ference between the two oxidation-state isomers. Although
the exact n0 value cannot be directly derived from electro-
chemical data, it can be estimated to an upper limit by the
difference between the redox potentials of the two centers
in the molecules.[11] From the CV data (Figure 2), the poten-
tial differences in complexes 12+, 22+, and 32+ are 0.33, 0.11,
and 0.20 V, respectively. These values correspond to n0 =
2660, 890, and 1610 cmꢀ1, respectively. Thus, the theoretical
lower limits of Dn1/2(theo) for complexes 12+, 22+, and 32+ are
Figure 6 shows the changes in the absorption spectra of
complex 2+ in CH2Cl2 upon oxidative titration with SbCl5.
In the single-oxidation process, the MLCT transitions in the
visible-light region decreased and the appearance of a shal-
low and broad absorption in the NIR region was evident. In
the double-oxidation process, both the MLCT and NIR
bands decreased in intensity. Enlarged NIR spectroscopic
changes are shown in the inset of Figure 6b. This new ab-
sorption band at around 1000 nm is assigned to ferroceni-
um-associated ligand-to-metal-charge-transfer transitions.
On the basis of this spectroscopic analysis and that of pre-
Table 2. Parameters for the NIR transitions in the singly oxidized states
[a]
12+–32+
.
12+
22+
32+
lmax [nm]
nmax [cmꢀ1
1780
5620
2860
2830
2790
1910
5230
370
2490
3160
1825
5480
4160
2740
2990
]
emax [mꢀ1 cmꢀ1
]
Dn1/2(obsv) [cmꢀ1
]
]
Dn1/2(theo) [cmꢀ1
rab [ꢂ][b]
7.96
550
7.80
180
8.00
640
HMM’ [cmꢀ1
]
[c]
[14–18]
ꢀ
viously reported heterobimetallic Ru Fc systems,
the
[a] These data were obtained from chemical oxidation in CH2Cl2.
major NIR bands of complexes 12+ and 32+ (around
1800 nm) are assigned to RuII!FeIII MM’CT transitions and
ꢀ
[b] DFT-optimized Ru Fe distance. [c] Calculated according to the Hush
formula.
Chem. Asian J. 2013, 8, 138 – 147
143
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www.chemasianj.org
Yu-Wu Zhong et al.
2790, 3160, 2990 cmꢀ1, respectively. For complexes 12+ and
Conclusions
32+, the observed half-widths (Dn1/2(obsd)) are very close to
the Dn1/2(theo) values. For complex 22+, the observed half-
width is narrower than the Dn1/2(theo) value. However, its
MM’CT band is rather weak (emax =370mꢀ1 cmꢀ1). Because
the NIR bands of complexes 12+, 22+, and 32+ are somewhat
weak and broad, all three complexes are assigned to Robin–
Day class II systems.[26] According to the Hush formula, H=
In conclusion, three bis-tridentate complexes that consist of
ferrocene and cyclometalated ruthenium have been success-
fully prepared and characterized. A combined experimental
and computational study has shown that the electronic cou-
pling between the iron and ruthenium centers is strongly de-
pendent on the connection mode between two redox-active
sites. When ferrocene is connected onto the cyclometalating
(NCN) ligand, a charge transfer from the ruthenium to the
iron center occurs in the singly oxidized state. On the other
hand, when the ferrocene unit is connected onto the noncy-
clometalating (NNN) ligand, the charge transfer is reversed
and the electronic coupling is much weaker. These studies
will be of interest and importance for the design and synthe-
sis of new redox-asymmetric MV systems. Compared to
ACTHNUTRGNEUNG
0.0206ꢃ(emaxnmaxDn1/2)1/2/(rab),[11n,p,25] where rab is taken to be
ꢀ
the Ru Fe distance, the electronic-coupling parameters
(HMM’) in CH2Cl2 are estimated to be 550, 180, and 640 cmꢀ1
for complexes 12+, 22+, and 32+, respectively. This result sug-
gests that the metal–metal electronic couplings in complexes
12+ and 32+ are much stronger than those in complex 22+.
DFT Computations of the Singly Oxidized States
[14–19]
ꢀ
other reported heterobimetallic Ru Fe complexes,
one
DFT calculations were performed on the singly oxidized
states 12+, 22+, and 32+. The Mulliken spin-density plots
(aꢀb) of these complexes (Figure 7) show that the ferrocene
advantage of this system is that the electronic nature of the
cyclometalated ruthenium center can be easily varied by at-
taching different terminal ligands,[8c,d,f,9k] which would, in
turn, influence the intermetallic electronic coupling. Such
studies are currently underway in our laboratory.
Experimental Section
Synthesis
General
NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer.
Spectra are reported in ppm relative to residual protons of the deuterat-
ed solvent (1H NMR: d=7.26 ppm for CDCl3 and d=1.92 ppm for
CD3CN). MS was performed on Bruker Daltonics Inc. Apex II FT-ICR
or Autoflex III MALDI-TOF mass spectrometers. The matrix for
MALDI-TOF measurements was a-cyano-4-hydroxycinnamic acid. Ele-
mental analysis was carried out on a Flash EA 1112 or Carlo Erba 1106
analyzer at the Institute of Chemistry, the Chinese Academy of Sciences.
Ferroceneboronic acid was purchased from Aladdin Reagent (China).
Figure 7. Spin-density plots.
Synthesis of 1,3-Di(2-pyridyl)-5-ferrocenylbenzene (7)
A mixture of 3,5-di(2-pyridyl)bromobenzene[12] (155 mg, 0.50 mmol), fer-
roceneboronic acid (138 mg, 0.60 mmol), [PdACTHUNRTGNEUNG(PPh3)4] (15.0 mg,
unit on the NCN ligand side is predominantly responsible
for the spins of complexes 12+ and 32+. However, the spin
population of complex 22+ is dominated by the ruthenium
component. These results are in agreement with the above
electrochemical assignment and spectroscopic analysis. That
is, the ferrocene unit on the NCN ligand side of complexes
1+ and 3+ and the ruthenium site of complex 2+ are respon-
sible for their first one-electron-oxidation processes.
TDDFT calculations have been performed for complexes
12+ and 22+ at the same level of theory (see the Supporting
Information, Table S2 and Figure S14). However, the S1 ex-
citation of 12+ was predicted at a negative energy. In the
low-energy region of complex 22+, the oscillator strengths of
all of the predicted excitations are either zero or negligible;
this result means that these methods are not suitable for the
TDDFT calculations of complexes 12+ and 22+. We will look
for other suitable methods in the future.
0.013 mmol), and K3PO4 (212 mg, 1.0 mmol) in freshly distilled 1,4-diox-
ane (20 mL) was heated at reflux for 24 h under a nitrogen atmosphere.
After the reaction was complete, the mixture was cooled to RT and the
solvent was removed under reduced pressure. The residue was subjected
to flash column chromatography on silica gel (CH2Cl2/EtOAc, 7:1) to
yield compound 7 as an orange solid in 32% yield (65 mg). 1H NMR
(400 MHz, CDCl3): d=4.08 (s, 5H), 4.41 (t, J=1.2 Hz, 2H), 4.90 (t, J=
1.2 Hz, 2H), 7.34–7.37 (m, 2H), 7.89 (t, J=6.0 Hz, 2H), 8.02 (d, J=
8.0 Hz, 2H), 8.24 (s, 2H), 8.59 (s, 1H), 8.73 ppm (s, 2H); 13C NMR
(100 MHz, CDCl3): d=66.9, 69.1, 69.7, 85.1, 120.8, 122.3, 123.4, 125.3,
136.7, 140.0, 140.6, 149.7, 157.4 ppm; MS (EI): m/z: 416 [M]+; HRMS
(EI): m/z calcd for C26H20N2Fe: 416.0976; found: 416.0981.
Synthesis of Intermediate [8]
ACHTUNGRTEN(NUNG PF6)
ligand 7 (41.6 mg, 0.10 mmol) was added to a suspension of [{RuCl2ACHTUNGTRENNUNG(p-
cymene)}2] (37.0 mg, 0.060 mmol), KPF6 (36.8 mg, 0.20 mmol), and
crushed NaOH (4.0 mg, 0.10 mmol) in dry MeCN (10 mL). The resulting
mixture was stirred at 508C for 20 h under a nitrogen atmosphere and
the solvent was then removed under reduced pressure. The residue was
subjected to flash column chromatography on neutral Al2O3 (MeCN).
The yellow band was collected and the solvent was removed to give [8]-
Chem. Asian J. 2013, 8, 138 – 147
144
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www.chemasianj.org
Yu-Wu Zhong et al.
A
coil was used as the counter electrode. All potentials were referenced to
a Ag/AgCl electrode in a saturated aqueous solution of NaCl without
regard for the liquid-junction potential.
E
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
This sample was used directly in the next transformation without further
purification or characterization.
Spectroscopic Measurements
All optical UV/Vis absorption spectra were obtained on a TU-1810DSPC
spectrometer (Beijing Purkinje General Instrument Co. Ltd.) at RT in
the stated solvents with a conventional 1.0 cm quartz cell. UV/Vis/NIR
spectra were recorded on a PE Lambda 750 UV/Vis/NIR spectrophotom-
eter. Oxidative spectroelectrochemistry was performed in a thin-layer
cell (optical length: 0.2 cm), in which an ITO glass electrode was set in
the indicated solvent that contained the compound to be studied (about
1ꢃ10ꢀ4 m) and 0.1m Bu4NClO4 as the supporting electrolyte. Platinum
wire and Ag/AgCl in a saturated aqueous solution of NaCl were used as
the counter electrode and the reference electrode, respectively. The cell
was placed into the spectrophotometer to monitor spectroscopic changes
during electrolysis.
Synthesis of Complex [1]
ACHTUNGRTEN(NUNG PF6)
Ligand 2,2’:6’,2“-terpyridine (10.0 mg, 0.040 mmol) was added to a solu-
tion of [8](PF6) (20.0 mg, 0.025 mmol) in DMF (10 mL). The resulting
ACHTUNGTRENNUNG
mixture was heated at reflux for 5 h under a nitrogen atmosphere before
being cooled to RT. The solvent was removed under reduced pressure
and the residue was purified by flash column chromatography on silica
gel (CH2Cl2/MeCN, 1:1) to give [1]ACHTUNGRTENUNG(PF6) as a purple solid in 52% yield
(12 mg). 1H NMR (400 MHz, CD3CN): d=4.25 (s, 5H), 4.50 (m, 2H),
5.09 (m, 2H), 6.64 (t, J=7.2 Hz, 2H), 6.97–7.01 (m, 4H), 7.16 (s, 2H),
7.61–7.71 (m, 4H), 8.24 (d, J=8 Hz, 3H), 8.41 (d, J=5.6 Hz, 4H),
8.74 ppm (d, J=8.4 Hz, 2H); MS (MALDI): m/z: 750.2 [MꢀPF6]+; ele-
mental analysis calcd (%) for C41H30F6N5FePRu·2H2O: C 52.92, H 3.68,
N 7.53; found: C 52.64, H 3.45, N 7.61.
Computational Methods
DFT calculations were performed by using the B3LYP exchange correla-
tion functional[27] and implemented in the Gaussian 03 program pack-
age.[28] The electronic structures of the complexes were determined by
using a general basis set with the Los Alamos effective core potential
LanL2DZ basis set for ruthenium and 6-31G* for the other atoms in
vacuo.[29]
Synthesis of Complex [3]
To a solution of [8]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
added ligand 4’-ferrocenyl-2,2’:6’,2“-terpyridine[13] (16.0 mg, 0.040 mmol).
The resulting mixture was heated at reflux for 5 h under a nitrogen at-
mosphere before cooling to RT. The solvent was removed under reduced
pressure and the residue was purified by flash column chromatography
on silica gel (CH2Cl2/MeCN, 1:4) to give [3]ACHTNURTGNEUNG(PF6) as a purple solid in
46% yield (10 mg). 1H NMR (400 MHz, CD3CN): d=4.23 (s, 5H), 4.30
(s, 5H), 4.46 (m, 2H), 4.70 (t, J=1.2 Hz, 2H), 5.06 (m, 2H), 5.32 (t, J=
1.2 Hz, 2H), 6.69 (t, J=6 Hz, 2H), 6.96 (t, J=9.2 Hz, 2H), 7.13 (d, J=
5.6 Hz, 4H), 7.64 (t, J=6.4 Hz, 2H), 7.71 (t, J=8.4 Hz, 2H), 8.26 (d, J=
8 Hz, 2H), 8.45 (s, 2H), 8.55 (d, J=8 Hz, 2H), 8.78 ppm (s, 2H); MS
(MALDI): m/z: 934.3 [MꢀPF6]+; elemental analysis calcd (%) for
C51H38F6N5Fe2PRu·2H2O: C 54.95, H 3.80, N 6.28; found: C 54.65, H 3.41,
N 6.62.
Acknowledgements
This work was supported the National Natural Science Foundation of
China (21002104 and 21271176), the National Basic Research 973 pro-
gram of China (2011CB932301), the Scientific Research Foundation for
Returned Overseas Chinese Scholars, the State Education Ministry of
China, and the Institute of Chemistry, Chinese Academy of Sciences
(“100 Talent Program”, CMS-PY-201230).
Synthesis of Intermediate [10]
ACHTUNGRTEN(NUNG PF6)
According to the same procedure for the synthesis of [8]CAHTNUGTRNEN(GU PF6), intermedi-
[1] a) C. Creutz, H. Taube, J. Am. Chem. Soc. 1969, 91, 3988; b) C.
Creutz, H. Taube, J. Am. Chem. Soc. 1973, 95, 1086.
ate [10](PF6) was prepared from 1,3-di(2-pyridyl)benzene (34.8 mg,
0.15 mmol), [{RuCl2A(p-cymene)}2] (55.0 mg, 0.090 mmol), KPF6 (55.8 mg,
0.15 mmol), and crushed NaOH (6.0 mg, 0.15 mmol) in 47% yield, as-
suming a chemical structure of [(Fcdpb)Ru(p-cymene)(CH3CN)](PF6).
MS (MALDI): m/z: 464.1 [(dpb)Ru
(p-cymene)]+. This sample was used
ACHTUNGTRENNUNG
CHTUNGTRENNUNG
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ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Synthesis of Complex [2]
ACHTUNGRTEN(NUNG PF6)
A
solution of ligand 4’-ferrocenyl-2,2’:6’,2“-terpyridine[13] (16.0 mg,
0.040 mmol) and [10](PF6) (12.0 mg, 0.020 mmol) in DMF (10 mL) was
ACHTUNGTRENNUNG
heated at reflux for 5 h under a nitrogen atmosphere. After cooling to
RT, the solvent was removed under reduced pressure. The residue was
purified by flash column chromatography on silica gel (CH2Cl2/MeCN,
1:1) to give [2]ACHTUNGTRENNUNG
(PF6) as a purple solid in 45% yield (8 mg). 1H NMR
(400 MHz, CD3CN): d=4.31 (s, 5H), 4.71 (m, 2H), 5.32 (m, 2H), 6.65 (t,
J=6.4 Hz, 2H), 6.98 (t, J=6 Hz, 2H), 7.12 (d, J=4.8 Hz, 4H), 7.60 (t,
J=8.4 Hz, 2H), 7.70 (t, J=8 Hz, 2H), 8.10 (d, J=8 Hz, 2H), 8.33 (s,
2H), 8.53(d, J=8 Hz, 2H), 8.79 ppm (s, 2H); MS (MALDI): m/z: 750.2
[MꢀPF6]+; elemental analysis calcd (%) for C41H30F6N5FePRu·H2O:
C 53.96, H 3.53, N 7.67; found: C 53.90, H 3.59, N 7.65.
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indices R1=0.0881 and wR2=0.2056; R indices (all data) R1=
0.1185
and
wR2=0.2237.
CCDC 900951
([2]
G
and
CCDC 900952 ([3]AHCTUNGTR(EUNNNG PF6)) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif..
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Received: September 26, 2012
Published online: October 30, 2012
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