Ligand-centered oxidations and electron derealization in a tetranuclear complex of a tetradonor-substituted olefin

Article abstract of DOI:10.1021/om8003338

The tetrakis(4-styryl)ethene (TSTE^4-)-bridged tetraruthenium complex [{(PⁱPr₃)₂(CO)ClRu}₄{μ ₄-(CH=CHC₆-H₄)₄(C=C)}] undergoes four consecutive oxidations at low potential. The ligand-dominated nature of these processes is confirmed by spectroscopic and quantum-chemical investigations.

Full text of DOI:10.1021/om8003338

Copyright 2008
Volume 27, Number 14, July 28, 2008
American Chemical Society
Communications
Ligand-Centered Oxidations and Electron Delocalization in a
Tetranuclear Complex of a Tetradonor-Substituted Olefin
Michael Linseis,† Rainer F. Winter,*,† Biprajit Sarkar,‡ Wolfgang Kaim,‡ and
Stanislav Za´lisˇ§
Institut fu¨r Anorganische Chemie, UniVersita¨t Regensburg, UniVersita¨tsstrasse 31, 93040 Regensburg,
Germany, Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart,
Germany, and J. HeyroVsky´ Institute of Physical Chemistry V. V. i., Academy of Sciences of the Czech
Republic, DolejsˇkoVa 3, Prague, Czech Republic
ReceiVed April 15, 2008
Summary: The tetrakis(4-styryl)ethene (TSTE4-)-bridged tet-
four consecutive metal-centered oxidations, during which the
raruthenium complex [{(PiPr3)2(CO)ClRu}4{µ4-(CHdCHC6-
bridging ligand acquires only a small fraction δ of the positive
H4)4(CdC)}] undergoes four consecutiVe oxidations at low
potential. The ligand-dominated nature of these processes is
confirmed by spectroscopic and quantum-chemical inVestigations.
charge(s): i.e., [{Cp*(dppe)FeII}4-n{Cp*(dppe)FeIII-δ/n}n{µ4-
(CtCC6H4-4)4(CdC)}δ]n+. Here we report on the tetraruthe-
nium complex [{(PiPr3)2(CO)ClRu}4{µ4-(CHdCHC6H4)}4-
(CdC)}] (1), which features the tetrakis(4-styryl)ethene
(TSTE4-) ligand (Chart 1).
The ability of the easily reducible tetracyanoethylene (TCNE)
ligand to bridge up to four metal centers and to adopt any
oxidation state between 0 and -2 has generated a unique
coordination chemistry1,2 with potential applications in molec-
ular magnetism, electrochromism, and molecular computing.3,4
The coordination chemistry of electroactive, tetradonor-
substituted olefins is much less explored but may hold similar
potential. The tetrairon complex [{Cp*(dppe)FeII}4{µ4-(CtCC6-
H4-4)4(CdC)}] of the (4-ethynylphenyl)ethene ligand [(CtCC6-
H4-4)4CdC]4- is a recent example.5 This compound undergoes
Complex 1 was prepared as outlined in Scheme 1.6 Treatment
of the alkyne (HCtCC6H4)4(CdC)7,8 with 4.08 equiv of
[HRu(CO)Cl(PiPr3)2] gave 1 as a deep orange-red solid in 85%
yield. 1H NMR spectroscopy shows the presence of four
equivalent vinyl groups with signals at δ 8.42 (HR) and 5.90
(Hꢀ) that display well-resolved H-H and P-H couplings of
13.4 and 0.9 Hz (2JP-H) and 2.1 Hz (3JP-H), respectively, and
two signals of an AB system at δ 6.77 and 6.71 ppm for the
p-phenylene groups. In 13C NMR spectra the resonance signals
of the ruthenium-bonded vinyl groups appear at 150.8 (CR) and
134.8 ppm (Cꢀ), again with well-resolved coupling to two
equivalent phosphorus nuclei each. The central ethylene carbon
atoms are observed at 139.8 ppm, while the phosphorus nuclei
give rise to a sharp singlet at δ 38.3 ppm in 31P{1H} NMR
spectroscopy (see Figure S1, Supporting Information).
* To whom correspondence should be addressed. E-mail: rainer.winter@
chemie.uni-regensburg.de.
† Universita¨t Regensburg.
‡ Universita¨t Stuttgart.
§ J. Heyrovsky´ Institute of Physical Chemistry v.v.i.
(1) Kaim, W.; Moscherosch, W. Coord. Chem. ReV. 1994, 129, 157.
(2) Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 2508.
(3) Miller, J. S.; Epstein, A. J. Coord. Chem. ReV. 2000, 207, 651.
(4) Wang, G.; Zhu, H.; Fan, J.; Slebodnik, C.; Yee, G. T. Inorg. Chem.
2006, 45, 1406.
The low ν˜(CO) value of 1909 cm-1 and the low oxidation
potentials of 1 attest to its electron-rich character. Cyclic
voltammetry of 1 shows two reversible couples at -0.020 and
+0.415 V vs the ferrocene/ferrocenium standard (see Figure
(5) Tanaka, Y.; Ozawa, T.; Inagaki, A.; Akita, M. Dalton Trans. 2007,
928.
10.1021/om8003338 CCC: $40.75
2008 American Chemical Society
Publication on Web 06/20/2008

3322 Organometallics, Vol. 27, No. 14, 2008
Communications
further conversion to fully oxidized 14+ (Figure 1). There is
only one CO band for 1 (ν˜(CO) 1909 cm-1) and 14+ (ν˜(CO)
1968 cm-1) but a broad asymmetric band for 12+. According
to spectral deconvolution this band results from overlapping
individual absorptions at 1924 and 1942 cm-1 (Supporting
Information, Figure S3). Any combination of two out of the
three pathways of electron delocalization in such systems
(diagonal, lateral, or cross conjugation,; see Chart S1 of the
Supporting Information) would render each of the four styryl
ruthenium moieties equivalent.7,12,13 The presence of two
pairs of electronically different carbonyl ruthenium moieties
in 12+ thus signals that only one of these pathways is effective
on the fast IR time scale of about 10-12 s. The overall CO
band shift of 57 cm-1 between neutral 1 and fully oxidized
14+ is significantly smaller than that of ca. 130-150 cm-1
that would be expected of a metal-centered oxidation process
and that of 81 cm-1 for [{(PiPr3)2(CO)ClRu}2(µ-CHdCHC6-
H4CHdCH-1,4)],14 where also one electron per vinyl ruthe-
nium moiety is released. This is a consequence of the more
extended π-chromophore of 1, which leads to an exception-
ally strong ligand contribution to the redox orbitals.
Chart 1
S2, Supporting Information). Comparison of the slopes of i vs
t1/2 plots in chronoamperometry and of the step heights in
steady-state voltammetry to those of decamethylferrocene
according to the method of Baranski9 established that each
anodic couple involves the transfer of two electrons. The peak-
to-peak separations, particularly of the first couple, are notably
smaller than that of decamethylferrocene under identical condi-
tions. This signals that there is only a small splitting between
the first and the second oxidations underlying the first wave
and between the third and the fourth oxidations that give rise
to the second wave. Similar behavior has been noted for metal-
free tetrakis(4-dimethylaminophenyl)ethene.10 Half-wave po-
tentials as determined by digital simulation11 are -0.028 V (0
f +), -0.013 V (+ f 2+), 0.400 V (2+ f 3+), and 0.434
V (3+ f 4+), from which comproportionation constants Kc,1
) 1.8 ( 0.2, Kc,2 ) (9.6 ( 0.8) × 106, and Kc,3 ) 3.8 ( 0.6
are calculated (eqs 1–4).
The comproportionation constant of one-electron-oxidized 1+,
while small at Kc,1 ) 1.8 ( 0.2, is still large enough to allow
for its detection by ESR spectroscopy. The room-temperature
ESR spectrum (Supporting Information, Figure S4) consists of
an unresolved isotropic signal at g ) 2.0157. When the
temperature is lowered to 110 K, a rhombic splitting of the g
tensor is observed with individual g tensor components gx )
2.072, gy ) 2.034, and gz ) 2.014 ( gav ) 2.040, ∆g ) 0.058).
This characterizes 1+ as a metal-stabilized but mainly ligand-
centered paramagnetic species, as is also the case for other vinyl
ruthenium complexes with π-conjugated substituents attached
to the vinyl group.14–16
In the electronic spectrum of 1 the bands of parent tetrakis(4-
ethynylphenyl)ethene8 are preserved but undergo a bathochromic
shift of 8100 and 4100 cm-1, respectively (Supporting Informa-
tion, Figure S5). This goes along with an approximate doubling
in molar absorptivity. Both these observations point to efficient
electronic conjugation across the entire organometallic chro-
mophore. Upon oxidation to the dication 12+ the 390 nm band
of 1 decreases in intensity and is gradually replaced by intense,
broad absorptions that extend over the low-energy part of the
optical spectrum and the near-IR. Distinct maxima are observed
at 8400, 13 680, and 15 080 cm-1 (Figure 2). The intermediate
radical monocation, while not separately detected in this regime,
gives rise to a characteristic absorption at about 4730 cm-1
which intensifies during the initial stages of the electrolysis and
then gradually disappears, while the absorption bands of 12+
continue to grow (Supporting Information, Figure S6). Further
oxidation with slow scanning through the second wave generates
the tetracation 14+. It is another strongly absorbing species with
peak maxima at 11 610, 14 870, and 17 660 cm-1 and thus has
energies distinctly higher than those of the corresponding
dication but considerably lower energies in comparison to those
Kc,1
1
2+ + 10 a 21+
(1)
(2)
Kc,2
1
3+ + 1+ a 212+
Kc,3
1
4+ + 12+ a 213+
(3)
(4)
Kc ) exp (nF∆E1⁄2 ⁄ RT)
Stepwise oxidation inside a thin-layer electrolysis cell
induced a blue shift of the CO band of 1 during the
conversion of the neutral form to the dication 12+ and its
(6) All synthetic work was performed with dry solvents under a dry
nitrogen atmosphere. Synthesis of 1: a solution of 97 mg (0.227 mmol) of
tetrakis(4-ethynylphenyl)ethene in 10 mL of CH2Cl2 was added dropwise
with stirring to a solution of 450 mg (0.926 mmol) of [HRu(CO)Cl(PiPr3)2]
in 10 mL of CH2Cl2. After addition was complete, the solution was
concentrated to 1.5 mL under vacuum and 25 mL of methanol was added.
The resulting red solid was isolated by filtration, washed with three 5 mL
portions of methanol, and dried under vacuum to give 457 mg (0.192 mmol,
84.7%) of 1 as a red powder. Selected spectroscopic data of complex 1: 1H
3
3
NMR (400 MHz, CD2Cl2, 298 K) δ 8.42 (dt, 4H, JH-H )13.4 Hz, JP-H
3
) 0.9 Hz, Ru-CH), 6.77, 6.71 (each d, 8H, JH-H ) 8.2 Hz, C6H4), 5.90
3
4
(dt, 4H, JH-H ) 13.4 Hz, JP-H ) 2.1 Hz, Ru-CHdCH), 2.74 (m, 24H,
PCH(CH3)2), 1.29 (m, 144H, PCH(CH3)2); 13C NMR (100.6 MHz, CD2Cl2,
2
2
298 K): δ 203.4 (t, JP-C ) 13.1 Hz, CO), 150.8 (t, JP-C ) 10.8 Hz,
4
Ru-CH), 140.5 (s, CdCC), 139.8 (s, CdC), 137.0 (t, JP-C ) 2.0 Hz,
3
Ru-CHdCHC), 134.8 (t, JP-C ) 3.4 Hz, Ru-CHdCH), 131.7, 123.3
(each s, (CH styryl)), 24.8 (vt, JP-C ) 9.8 Hz, CH(CH3)2), 21.1, 19.9 (each
s, CH(CH3)2); 31P{1H} NMR (CD2Cl2, 121.5 MHz, 298 K) δ 38.3 (s); IR
(KBr, ν˜ in cm-1): 2962, 2930, 2873 (m, ν˜CH), 1909 (s, ν˜CO), 1598, 1567,
1536, 1504 (m, ν˜CdC, aryl, vinyl, ethene); UV/vis (CH2Cl2, λmax (εmax in L
mol-1 cm-1)) 312 (sh, 47 000), 352 (86 000), 400 (sh, 46 000), 525 (2800).
Anal. Calcd (found) for C110H192Cl4O4P8Ru4: C, 55.69 (55.43); H, 8.16
(8.53).
(12) Hilger, A.; Gisselbrecht, J.-P.; Tykwinski, R. R.; Boudon, C.;
Schreiber, M.; Martin, R. E.; Lu¨thi, H. P.; Gross, M.; Diederich, F. J. Am.
Chem. Soc. 1997, 119, 2069.
(13) Giuffreda, M. G.; Bruschi, M.; Lu¨thi, H. P. Chem. Eur. J. 2004,
10, 5671.
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Za´lisˇ, S. Organometallics 2006, 25, 3701.
(15) Maurer, J.; Winter, R. F.; Sarkar, B.; Fiedler, J.; Za´lisˇ, S. Chem.
Commun. 2004, 1900.
(7) Sengupta, S. Synlett 2004, 1191.
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M.; Kaim, W.; Za´lisˇ, S.; Anson, C.; Zabel, M.; Winter, R. F. J. Am. Chem.
Soc. 2008, 130, 259.
(11) Rudolph, M.; Feldberg, S. DigiSim3, Version 3.03, DigiSim3,
Version 3.03; Bioanalytical Systems, Inc.,1994.

Communications
Organometallics, Vol. 27, No. 14, 2008 3323
Scheme 1. Synthesis of Complex 1
tions induces a large torsion of the Ar2C moieties around the
central CC bond, a coplanar arrangement of at least one aryl
ring with the plane defined by the central carbon and the attached
ipso carbon atoms of each CAr2 subunit, and quinoidal distortion
of the aryl subsituents.23,24 DFT-optimized structures of model
complex 1′ and its various oxidized forms (Supporting Informa-
tion, Figure S8 and Table S1) show that TSTE4--bridged
tetraruthenium complexes behave in the same manner. The
central CC bond lengthens from 1.371 Å in 1′ to 1.458 Å in
the dication 1′2+ and to 1.484 Å in the tetracation 1′4+. This is
accompanied by an increasing deviation of the central ethylene
group from planarity and a concomitant decrease of the torsional
angles between the methylene planes and the planes of the
attached aryl rings. Vibrational analysis of 1′ and oxidized 1′2+
and 1′4+ reproduces the average CO band shifts (28 cm-1 vs
24 cm-1 for the first oxidation, 33 cm-1 vs 35 cm-1 for the
second oxidation) and the presence of two separate, intense
absorptions for 1′2+ but underestimates the experimental splitting
(5 cm-1 vs 18 cm-1) (Supporting Information, Table S2). The
calculations on 1′2+ also indicate that the geminally disposed
styryl moieties A and B on one hand and C and D on the other
hand are pairwise equivalent but different from the other pair
(Supporting Information, Figure S8). This suggests that the
geminal delocalization pathway is the most efficient one.
Figure 3 shows that the highest occupied molecular orbital
(HOMO) of 1′ is delocalized across the π system of the bridging
ligand with smaller contributions from Ru 4dxz orbitals (12%).
The closely lying HOMO-1, HOMO-2, and HOMO-3 orbitals
have negligible contributions from the central ethylene group
and are mainly composed of the styryl π orbitals with overall
Ru contributions of 28, 30, and 36%, respectively. The LUMO
of 1 is an antibonding combination of π orbitals of the bridging
ligand (6% Ru) (Supporting Information, Figure S9).
Figure 1. IR spectroelectrochemistry of complex 1 in 1,2-C2H4Cl2/
NBu4PF6: spectroscopic changes upon oxidation (a) of 1 to 12+
and (b) of 12+ to 14+
.
Figure 2. UV/vis/near-IR spectroelectrochemistry of complex 1 in
1,2-C2H4Cl2/NBu4PF6: spectroscopic changes upon oxidation (a)
of 1 to 12+ and (b) of 12+ to 14+
.
of neutral 1 (Figure 2). The intermediate trication 13+ is also
observed by virtue of a low-energy absorption at 5060 cm-1 at
the border of the near-IR and mid-IR regions (Supporting
Information, Figure S7). Stepwise reduction at any stage restored
neutral 1 such that the 1 a 12+ a 14+ redox system constitutes
a reversible cycle. The stepwise oxidation of mainly the bridging
ligand makes 1 a strongly chromophoric organometallic violene/
cyanine hybrid17 with a delocalized π system and a large number
of closely spaced occupied energy levels below the HOMO.
Quantum chemical density functional (DFT) calculations on
the simplified PH3-substituted model 1′ were conducted in order
to understand the structural, electronic, and spectroscopic
properties and their dependence on the overall oxidation
state.18–22 Stepwise oxidation of tetraarylethenes to their dica-
TD DFT calculations on 12+ retrace the main features of the
experimental electronic spectrum well, although transition
energies are somewhat overestimated (Supporting Information,
Figure S10). Intense low-energy transitions b1A and c1A
correspond mainly to the excitations HOMO f LUMO and
HOMO-2 f LUMO of 12+, respectively. The composite low-
energy band can thus be characterized as involving both π f
π* and MLCT components. The band h1A calculated at around
(17) Hu¨nig, S.; Kemmer, M.; Wenner, H.; Barbosa, F.; Gscheidt, G.;
Perepichka, I. F.; Ba¨uerle, P.; Emge, A.; Peters, K. Chem. Eur. J. 2000, 6,
2618.
(20) Hariharan, P. H.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(21) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123.
(22) Gaussian 03, Revision C.02; www.gaussian.com.
(23) Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am.
Chem. Soc. 1998, 120, 6931.
(18) The hybrid B3LYP functional19 together with 6-31G* polarized
double-ꢁ basis sets20 for C, N, H, and O atoms and effective core
pseudopotentials and corresponding optimized sets of basis functions for
Ru atoms21 were used in DFT calculations.22 Frequency analysis and TD
DFT calculations were done at optimized structures.
(24) Bock, H.; Na¨ther, C.; Havlas, Z. J. Chem. Soc., Chem. Commun.
1995, 1111.
(19) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

3324 Organometallics, Vol. 27, No. 14, 2008
Communications
constitutes a strongly electrochromic25–27 system with high
absorptivities in the low-energy part of the visible and the near-
infrared (near-IR) regions at the dication and tetracation levels.
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft (Grant No. Wi 1262/7-1) and the Czech
Academy of Sciences (Grant No. KAN100400702) and
Ministry of Education of the Czech Republic (Grant No. OC
139) for their financial support of this work.
1
Supporting Information Available: Figures showing the H,
13C, and 31P NMR spectra, the cyclic voltammogram, and the
spectral deconvolution of the IR band of 12+, ESR spectra of 1+,
UV/vis spectra of 1 and its parent alkyne, vis/near-IR spectra
showing the presence of the 1+ and 13+ intermediates, a comparison
of the TD-DFT calculated and experimental electronic spectra of
2+, selected frontier orbitals of 1′, and a chart with possible
Figure 3. Contour plot of the HOMO of 1.
1
pathways of electron delocalization in 12+ and tables with calculated
structure parameters and ν˜(CO) values of 1′ in its various oxidation
states. This material is available free of charge via the Internet at
http://pubs.acs.org.
19 000 cm-1 is assigned as the excitation from the HOMO-1
to the LUMO+1 of 12+
.
In summary, we have prepared the organometallic tetrakis(4-
styryl)ethene tetraanion (TSTE4-)-bridged tetraruthenium com-
plex 1, which undergoes a series of four reversible ligand-
centered oxidations. The low oxidation potentials and the
reversibility of all oxidation processes are a further demonstra-
tion of the impressive stabilizing effect of vinyl ruthenium
moieties on organic π systems, rivaling that of the dimethyl-
amino group. Moreover, the 1 a 12+ a 14+ redox series
OM8003338
(25) Monk, P. M. S.; Mortimer, R. J.; Kosseinsky, D. R. Electro-
chromism; VCH: Weinheim, Germany, 1995.
(26) Mortimer, R. J. Chem. Soc. ReV. 1997, 26, 147.
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