Different Degrees of Electron Delocalization in Mixed Valence Ru-Ru-Ru Compounds by Cyanido-/Isocyanido-Bridge Isomerism

Article abstract of DOI:10.1002/anie.201806157

The two stable pairs of trimetallic compounds trans-[Cp*(dppe)Ru(μ-NC)Ru(dmap)₄(μ-CN)Ru(dppe)Cp*][PF₆]_n (1[PF₆]_n, n=2, 3; Cp=1,2,3,4,5-pentamethylcyclopentadiene; dppe=1,2-bis-(diphenylphosphino)ethane; dmap= 4-dimethylaminopyridine) and trans-[Cp*(dppe)Ru(μ-CN)Ru(dmap)₄(μ-NC)Ru(dppe)Cp*][PF₆]_n (2[PF₆]_n, n=2, 3), which demonstrate cyanide/isocyanide isomerism, have been synthesized and fully characterized. 1³⁺[PF₆]₃ and 2³⁺[PF₆]₃ are the one-electron oxidation products of 1²⁺[PF₆]₂ and 2²⁺[PF₆]₂, respectively. The results suggest that 1[PF₆]₃ is a class III mixed valence compound, whereas 2[PF₆]₃ might be an unusually symmetrical class II–III mixed valence compound composed of the two asymmetrical delocalized Ru^III?NC?Ru^II mixed valence subunits.

Full text of DOI:10.1002/anie.201806157

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Accepted Article
Title: Different Degree of Electron Delocalization in Mixed-Valence Ru-
Ru-Ru Compounds via Cyanido-/Isocyanido-Bridge Isomerism
Authors: Yu-Ying Yang, Xiao-Quan Zhu, Sheng-Ming Hu, Shao-Dong
Su, Lin-Tao Zhang, Yue-Hong Wen, Xin-Tao Wu, and Tian-Lu
Sheng
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Different Degree of Electron Delocalization in Mixed-Valence Ru-
Ru-Ru Compounds via Cyanido-/Isocyanido-Bridge Isomerism
Yu-Ying Yang,[a][b] Xiao-Quan Zhu,[a] Sheng-Ming Hu,[a] Shao-Dong Su,[a][b] Lin-Tao Zhang,[a][b] Yue-
Hong Wen,[a] Xin-Tao Wu[a] and Tian-Lu Sheng*[a]
Abstract: The two stable pairs of trimetallic compounds trans-
[Cp*(dppe)Ru(µ-NC)Ru(dmap)4(µ-CN)Ru(dppe)Cp*][PF6]n (1[PF6]n,
n =2, 3; Cp* = 1,2,3,4,5-pentamethylcyclopentadiene; dppe = 1,2-
bis- (diphenylphosphino)ethane) and trans-[Cp*(dppe)Ru(µ-CN)Ru-
compounds, but most of which belong to Class II systems. For
cyanido-bridged MV compounds,[2h, 7] there have been even few
well-characterized examples of fully delocalized (class III) MV
systems. In 2014, Baraldo and co-workers reported the first
example of class III cyanido-bridged species which was
confirmed by in situ NIR and IR spectroscopy and supported by
DFT calculations.[8] Most recently, we have reported an example
of delocalized cyanide-bridged MV compound induced by
thermal electron transfer.[9] Unfortunately, however, their
molecular structures were not obtained. Thus, isolation and fully
characterization of Class III cyanido-bridged MV compounds are
still challengeable.
(dmap)4(µ-NC)Ru(dppe)Cp*][PF6]n (2[PF6]n,
n
=2, 3) with
cyanide/isocyanide isomerism have been synthesized and fully
characterized. 13+[PF6]3 and 23+[PF6]3 are the one-electron
oxidation products of 12+[PF6]2 and 22+[PF6]2, respectively. The
investigated results suggest that 1[PF6]3 is a class III mixed-valence
compound. Whereas, 2[PF6]3 might be an unusually symmetrical
class II-III mixed-valence compound composed of the two
asymmetrical delocalized RuIII-NC-RuII mixed-valence subunits.
On the other hand, it is important to investigate the factors
influencing intervalence charge transfer (IVCT) of cyanido-
bridged MV compounds, including the nature of the metals and
their ligands, the shape and length of the chains and the
orientation of the cyanido bridges.[2h] To date, much efforts have
been devoted to the investigations on the former two influencing
factors.[2h, 7] However, it is relatively less known but significant
how IVCT is influenced by the orientation of the cyanido
bridges.[10] Particularly，there have been even few reports about
influences of M'-NC-M-CN-M' → M'-CN-M-NC-M' isomerism on
IVCT of cyanidometal-bridged MV compounds. Herein, we
report the stable pairs of MV compounds [Cp*(dppe)RuIII(µ-NC)-
Electron transfer is one of the most fundamental processes in
chemistry, physics and biology.[1] Mixed-valence (MV)
compounds are excellent model systems for understanding the
electron transfer processes and have attracted a great deal of
attentions over the past decades.[2] Furthermore, MV
compounds have applications in various fields ranging from
biology to electronic devices such as quantum cellular
automata,[3] molecule switch[4] and phase-coherent logic.[5] To
interpret electron transfer processes in MV compounds, some
2d, 6]
important theories have been developed.[2a,
According to
Robin and Day, any MV compounds can be classified into one of
three classes: Class I, the electronic interaction is null or very
weak; Class II, the electronic interaction is moderate, but the
charge is still localized; and Class III, the electronic interaction is
very strong, the charge is delocalized.[2a] Both classes II and III
compounds generally exhibit an absorption in the visible or near-
infrared region. Generally, the two classes can be distinguished
based on the following two criterions: First, class II MV
compounds exhibit a broad and weak absorption, but class III
RuII(dmap)4(µ-CN)RuII(dppe)Cp*][PF6]3
(1[PF6]3)
and
[Cp*(dppe)RuII(µ-CN)RuIII(dmap)4(µ-NC)RuII(dppe)Cp*][PF6]3
(2[PF6]3) with cyanide/isocyanide isomerism. The investigations
show that the former belongs to class III species, and the latter
might be a class II-III MV complex which is composed of two
asymmetrical delocalized RuIII-NC-RuII MV units.
mixed-valence compounds show
a
narrow and intense
absorption; Second, the energy of the absorption of class II
mixed-valence compounds is solvent dependent, but that of
class III mixed-valence compounds is solvent independent. In
addition to the Robin and Day classification, Meyer et al.
proposed a new category of the borderline class II-III systems
which exhibit an intermediate behavior between class II and
class III.[2d] The energy of the absorption of class II-III mixed-
valence compounds is solvent independent, but the charge is
localized.[2i] To date, it has been reported a huge amount of MV
Figure 1. Cyclic voltammogram of 1[PF6]2 recorded in CH2Cl2/0.1M
[TBA]PF6 at 100 mV s-1 scan rate.
[a]
Y. Y. Yang, Dr. X. Q. Zhu, S. M. Hu, S. D. Su, L. T. Zhang, Dr. Y.
H. Wen, Prof. X. T. Wu and Prof. T. L. Sheng
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Science,
Fuzhou, Fujian 350002, P.R. China
E-mail: tsheng@fjirsm.ac.cn
Y. Y. Yang, S. D. Su, L. T. Zhang
School of Chemical Sciences, University of Chinese Academy of
Sciences, Beijing 100049, P.R. China
In this work, we first synthesized the two isomeric divalent
compounds,
trans-[Cp*(dppe)RuII(µ-NC)RuII(dmap)4(µ-
CN)RuII(dppe)Cp*][PF6]2 (1[PF6]2) and trans-[Cp*(dppe)RuII(µ-
CN)RuII(dmap)4(µ-NC)RuII(dppe)Cp*][PF6]2 (2[PF6]2), which
differ only in the orientation of the bridging CN. That is, 1[PF6]2
and 2[PF6]2 exist in the Ru-NC-Ru-CN-Ru and Ru-CN-Ru-NC-
Ru forms, respectively. 12+ was synthesized by the reaction of
[b]
Supporting information for this article is given via a link at the end
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Figure 2. (a) Molecular structure (top) and computed spin density (central Ru, 0.236; terminal Ru, 0.319 and 0.347) (bottom, 0.0004 e/bohr3) of 13+; (b) Molecular
structure (top) and computed spin density (central Ru,0.597; terminal Ru, 0.161 and 0.163) (bottom, 0.0004 e/bohr3) of 23+ (the calculated ground-state energy
difference between 13+ and 23+ is 0.68 eV) . Hydrogen atoms, [PF6]- anions, and solvent molecules have been omitted for clarity. Light blue, Ru; pink, P; white, C;
blue, N.
trans-Ru(dmap)4(CN)2 with Cp*Ru(dppe)Cl in methanol, and the
isomer 22+ was obtained by reacting Cp*Ru(dppe)CN with
Ru(dmap)6Cl2 in ethanol. Electrochemical explorations of 12+
and 22+ in dichloromethane were performed, as shown in
Figures 1 and S2. The electrochemical data are summarized in
Table S1. Cyclic voltammogram (CV) of 12+ exhibits three
chemically reversible one-electron oxidation waves (Figure 1).
The first two waves (0 and 0.36 V vs. Cp2Fe3+/2+) are attributed
to the oxidation processes of the terminal Ru centers, while the
last one (0.53 V vs. Cp2Fe3+/2+) is due to the oxidation process of
the central Ru center. The large potential separation of 0.36 V
between the first two oxidations indicate the high stability of 13+
with the comproportionation constant (KC) of 1.72 × 106. CV of
22+ also shows three chemically reversible redox waves (Figure
S1). The first redox wave (-0.20 V vs. Cp2Fe3+/2+) is attributed to
the oxidation process of the central RuII, while the other two
waves (0.66 and 0.78 V vs. Cp2Fe3+/2+) are due to the oxidation
processes of the two terminal RuII. The second separation of
0.12V in 22+ suggests the presence of electronic interaction
between the two terminal Ru centers. It should be noted,
however, that the small potential separation of 0.12 V does not
suggest that 23+ must be a localized mixed valence species.[2i, 11]
Based on the electrochemical measurement, the reaction of 12+
with 1 equiv. of Cp2Fe(PF6) gave rise to the stable MV
n = 2, 3, Cp = 1,3-cyclopentadiene) as reference compounds
were also prepared in the similar way to 2[PF6]3 excepting for
CpRu(dppe)CN instead of Cp*Ru(dppe)CN (Supporting
Information). The electrochemical data for 3[PF6]2 are also
included in Table S1. The redox behavior of 3[PF6]2 (Figure S3)
is similar to that of 2[PF6]2 but with the poorer redox reversible
waves for the two terminal Cp(dppe)Ru fragments. All
compounds 1[PF6]n, 2[PF6]n and 3[PF6]n (n = 2, 3) were
characterized by elemental analyses, single crystal X-ray
diffraction analyses, and IR, UV-vis-NIR and EPR spectra.
The structural analyses revealed that the two pairs of
isomers 12+/22+ and 13+/23+ crystallize in the space group Pc and
C2/c, respectively. Whereas, both compounds 32+ and 33+
crystallize in the space group P21/c. The crystal structures of 1n+,
2n+ (n = 2, 3) are similar differing only in bond lengths and in
orientation of the bridging CN. Thus, only molecular structures of
13+ and 23+ are shown in Figure 2, and the other structures are
shown in Figures S4-S7 in Supporting Information.
Crystallographic details and selected bond lengths and angles
are summarized in Tables S2-S6 in Supporting Information. In
13+, the presence of a more counter anion of PF6- per compound
confirms that it is the one-electron oxidized form of 12+.
According to the CV results, one of the terminal Ru2+ of 12+
should be first oxidized into Ru3+, that is from RuII-RuII-RuII (12+)
to RuII-RuII-RuIII (13+). However, the cation structure of 13+ is
centrosymmetric, the central Ru locates in the center of the
inversion and the two terminal Ru are strictly equivalent,
suggesting 13+ may be fully delocalized. Furthermore, the bond
lengths of Ru-P (av. 2.300(1) Å), Ru-CCN (av. 2.076(4) Å) and
Ru-NCN (av. 2.077(4) Å) in 13+ are virtually indistinguishable from
those in 12+ (av. 2.299(3), 2.060(12) and 2.068(9) Å) within
compound
trans-[Cp*(dppe)RuIII(µ-NC)RuII(dmap)4(µ-
CN)RuII(dppe)Cp*][PF6]3 (1[PF6]3), and the reaction of 22+ with
1 equiv. of Cp2Fe(PF6) resulted in the stable isomeric MV
compound
trans-[Cp*(dppe)RuII(µ-CN)RuIII(dmap)4(µ-
NC)RuII(dppe)Cp*][PF6]3 (2[PF6]3). In order to probe the
electronic characteristics of compound 23+, the analogues trans-
[Cp(dppe)(µ-CN)Ru(dmap)4(µ-NC)Ru(dppe)Cp][PF6]n (3[PF6]n,
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experimental error. The similar phenomenon was also observed
in the Ru-N distance of [RuIII(bpy)3][PF6]3 vs [RuII(bpy)3][PF6]2
and was explained by the high electron self-exchange rate
between RuII and RuIII.[12] Herein, the indistinguishable bond
lengths may further suggest the presence of the electron
delocalization along Ru-NC-Ru-CN-Ru in 13+, strongly supported
by the IR, UV/Vis/NIR and EPR spectra (see below). In contrast
to 12+, based on the CV results, the central Ru2+ of 22+ should be
first oxidized into Ru3+, resulting in 23+ with a symmetrical RuII-
RuIII-RuII arrangement. Thus, the crystal structure of 23+ may be
easily centrosymmetric in crystallization. Indeed, the single-
crystal X-ray diffraction analyses showed that the cation
structure of 23+ is centrosymmetric and the oxidized RuIII locates
in the center of the inversion. Several changes in individual bond
lengths can be discerned upon oxidation, although 23+ and 22+
present common features. Particularly, the Ru-CCN and Ru-NCN
bond lengths in 23+ are shorter about 0.04 and 0.03 Å than 22+,
respectively, whereas the C≡N bond lengths are longer about
0.02 Å in the former. This may result from the electronic
delocalization between the terminal ruthenium and the central
ruthenium[13] or throughout Ru-CN-Ru-NC-Ru.
in both sites approaches the averaged value. That is, the
oxidation state of the central RuIII decreases and that of the
terminal RuII increases in compound 23+.
Comparison of the electronic spectra of the compounds
further reveals their intrinsic electronic characteristics. As shown
in Figure 3, the UV/Vis/NIR spectra of 12+ is similar to 22+ and
both of them contain only one absorption band at about 28570
cm-1 and do not exhibit any absorption band in Vis/NIR region.
Upon oxidation, the spectrum of 13+ displays three new bands in
Vis-NIR region, and the band at about 28570 cm-1 in 12+
disappears. The two bands of 13+ at 14869 cm-1 and 12556 cm-1
are attributed to the LMCT of the central RuIII.[8,14] The similar
bands can also be observed in 23+ (17168 cm-1, 14427 cm-1) but
are blue-shifted, which result from the oxidation of the central
RuII into RuIII. The IVCT band of 13+ appears around λmax = 5045
cm-1 (Table S7) and is narrow and asymmetrical. This intense
and narrow absorption with a cutoff, which are Gaussian on the
high energy side and skewed on the low energy side, resembles
that of other class III systems.[2d, 2i, 8] These results indicate that
the odd electron of 13+ may be delocalized and the central
ruthenium shows some natures like RuIII. Moreover, the IVCT
energy is solvent independent (Figure S9). This is also the
expected behavior for a class III MV compound,[8, 15] suggesting
the IVCT band arises from the electron resonate in the system
rather than IVCT. The IVCT band of 23+ appears around λmax
=
8870 cm-1 (Table S7), which is the ET processes between the
terminal RuII and the central RuIII. This band is also cutoff and
solvent independent (Figure S10), indicating that 23+ may be a
class II-III or class III MV compound. The reference compound
33+ shows similar LMCT and IVCT absorption, but the MLCT
band of terminal Ru still remain (Figure S11). However, the
MLCT band of 2 has disappear when the 22+ was oxidized into
23+. Those results indicate that the electron of RuII of 33+ is more
localized, which is supported by the result of the DFT
calculations (Table S8).
The electron localization/delocalization can influence the
intramolecular vibrational motions. Therefore, electron
movements can be monitored by IR spectra. The IR spectra of
12+ and its isomer 22+ display a cyanide stretching vibration
absorption at 2050 cm-1 (Figure 4a) and 2058 cm-1 (Figure 4b),
respectively. It has been reported that for the localized trinuclear
asymmetric cyanido-bridged systems (RuIII-NC-RuII-CN-RuII or
FeIII-NC-RuII-CN-FeII), two cyanide stretching vibrations are
observed.[7e, 7f, 16] Upon oxidation, however, the spectra of the
MV species 13+ exhibits only a broad absorption at 1970 cm-1
Figure 3. UV/Vis/NIR spectra of (a) 12+ (black) and 13+ (red); (b) 22+ (pink) and
23+ (blue) in CH2Cl2.
(Figure 4a), the characteristic of class III MV systems[2c, 8, 15b, 17]
,
indicating the two CN are equivalent on the timescale of IR
spectroscopy. The IR spectrum of 23+ exhibits also one cyanide
stretching vibration absorption at 2000 cm-1 (Figure 4b). This
single sharp ν(CN) band might be explained by the symmetric
structure of 23+ [RuII-CN-RuIII-NC-RuII], because similar results
have been observed in the localized symmetric cyanido-bridged
As the reference compound of 23+, the molecular structure of
33+ is very similar to that of 23+, both compounds contain the
same central RuIII. Upon replacement of Cp* by Cp from 23+ to
33+, their most remarkable difference is the Ru1III-NCN and Ru2
-
II
[7e, 7f]
system (RuIII-NC-RuII-CN-RuIII and FeIII-NC-RuII-CN-FeIII).
CCN/Ru3II-CCN bond lengths. It was found that Ru1III-NCN bond
length is 0.04 Å longer in 23+ (av. 2.046(6) Å) than in 33+ (av.
2.000(6) Å) , and the Ru2II-CCN/Ru3II-CCN bond length is longer
by 0.01 Å in the former [av. 1.984(5) Å in 23+ and av. 1.970(8) Å
in 32+], pointing to the presence of the electron delocalization in
compound 23+, so that the oxidation state of the ruthenium atom
Hence, the MV compound 23+ might be at the borderline of class
II and class III, namely class II-III species defined by Meyer.[2d]
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supported by the change of the selected bond lengths of 23+
(Table S5) at 100K and 293K. Based on the variable
temperature EPR behaviours and the strcutural data at different
temperature, 23+ might be classified as a localized-to-delocalized
(Class II–III) compound. The EPR of the more localized MV
compound 33+ (Figure S14), however, displays anisotropic
signals even at room temperature, suggesting the unpaired
electron density is mainly located in the central RuIII, which is
consistent with the result of DFT (Table S8). In conclusion, 13+ is
a class III MV compound, and 23+ might be regarded as a class
II-III compound composed of the two asymmetrical delocalized
RuII-CN-RuIII subunits. That is, the central ruthenium center in
13+ is to mediate electron communication between the two
terminal ruthenium centers, whereas the central ruthenium
center in 23+ plays the role of the electron acceptor. Therefore,
the electron transfer process of 13+ over the three metal centers
could be described as three resonant states (RuIII-NC-RuII-CN-
RuII ↔ RuII-NC-RuIII-CN-RuII ↔ RuII-NC-RuII-CN-RuIII); For the
MV compound 23+, the electron transfer process could be
described as RuII-CN-RuIII-NC-RuII ↔ RuIII-CN-RuII-NC-RuII or
RuII-CN-RuII-NC-RuIII.
Figure 4. IR spectra of 12+ (red) and 13+ (black) (a),22+ (green) and 23+ (blue)
(b) in CH2Cl2.
In order to further verify our classification, EPR spectra were
run for the two mixed-valence compounds 13+ and 23+ in solid at
various temperatures ranging from 150 K to 291 K. For the
comparison purpose, The solid EPR spectrum of 33+ was
measured at 291 K. The EPR spectra of 13+ at different
temperatures contain only isotropic signal with no hyperfine
coupling (Figure 5a). The g values at each temperature are
almost a constant of about 2.003 (Figure S13a) which is smaller
than Cp*Ru(dppe) mononuclear metal (g ~ 2.1)[18], typical of free
electron (g = 2.0023). Compound 13+ with such an isotropic
feature is in stark contrast to previously reported MV trinuclear
cyanide-bridged compounds, which exhibit anisotropic
features.[7e] The characters of free electron and temperature-
independent behavior of the EPR spectra between 150 and 291
K further confirm the fully delocalized class III nature of 13+.[19, 20]
In contrast, the EPR spectra of 23+ exhibit anisotropic feature at
low temperatures (Figure 4b, for more details, see Figure S13b),
suggesting the unpaired electron on 23+ may be localized or the
rate of electron transfer is slower than the EPR timescal. With
the temperature increasing, the anisotropic feature fades away.
When the temperature goes to 291 K, the anisotropic feature
disappears and only the isotropic signal can be observed.
Moreover, the g value (2.005) at 291 K is close to that of free
electron (g = 2.0023). This indicates that intramolecular electron-
transfer rate of 23+ is faster than the EPR timescale at 291 K. In
this case, the unpaired electron on 23+ is delocalized, the DFT
calculations support this nature of 23+ (Figure 2 and Table S8).
Thereore, the unpaired electron on 23+ may be localized at low
temperature and delocalized at high temperature, which is
Figure 5.Temperature-dependent EPR spectra of 13+ (a) and 23+ (b).
In summary, we have synthesized and structurally
characterized two isomeric stable trinuclear MV compounds 13+
and 23+ with different orientation of the CN bridge. Therein, the
strong evidences reveal that 13+ is the fully delocalized (class III)
cyanidometal-bridged MV compound, which is the first example
of the crystallographic characterized class III cyanido-bridged
MV systems. Upon isomerization of Ru-NC-Ru-CN-Ru → Ru-
CN-Ru-NC-Ru, 23+ might behave class II-III cyanide-bridged MV
species which is composed of the two asymmetric delocalized
RuII-CN-RuIII subunits. It should point out, however, that one
could not completely exclude the other possibility for the
assignment of 23+.[21] From 13+ to 23+, it seems that the electron
delocalization throughout Ru-Ru-Ru is devided evenly into two
electron delocalization parts from the central ruthenium center.
This may have implication in designing molecular conducting
devices.
Acknowledgements
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G. E. Pieslinger, P. Albores, L. D. Slep, L. M. Baraldo, Angew. Chem.
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We sincerely thank the referees for the valuable comments. We
thank the National Science Foundation of China (21773243), the
973 Program (2014CB845603), and the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB20010200) for financial support. We also thank the staff of
NCPSS beamline BL17B beamline at National Center for Protein
Sciences Shanghai and Shanghai Synchrotron Radiation Facility,
Shanghai, People's Republic of China, for assistance during
data collection. We also sincerely thank Dr. Jia-hui Yang (Bruker
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Keywords: cyanido-/isocyanido bridge • electron transfer •
mixed-valennce compounds • Class II-III • Class III
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10.1002/anie.201806157
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Class II-III to Class III: 23+
behaves class II-III cyanide-
bridged MV species which is
composed of the two asymmetric
delocalized RuII-CN-RuIII subunits.
Upon isomerization of Ru-CN-Ru-
NC-Ru → Ru-NC-Ru-CN-Ru, the
strong evidences reveal that 13+ is
the fully delocalized (class III)
Y. Y. Yang, X. Q. Zhu, S. M. Hu, S.
D. Su, L. T. Zhang, X. T. Wu and T.
L. Sheng*
Page No. – Page No.
Different Degree of Electron
Delocalization in Mixed-Valence
Ru-Ru-Ru
Compounds
via
Cyanido-/Isocyanido-Bridge
Isomerism
cyanidometal-bridged
compound.
MV
This article is protected by copyright. All rights reserved.