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Fully Delocalized (Ethynyl)(vinyl)phenylene-Bridged Diruthenium Radical Complexes

15 Oct 2010-Organometallics (American Chemical Society)-Vol. 29, Iss: 22, pp 5912-5918

Abstract: Diruthenium complexes (X)(dppe)2Ru−C≡C−1,4-C6H4−CH═CH−RuCl(CO)(PiPr3)2 (X = Cl, 1a; X = C≡CPh, 1b) containing an unsymmetrical (ethynyl)(vinyl)phenylene bridging ligand are compared to their symmet...
Topics: Phenylene (63%), Bridging ligand (57%), Delocalized electron (51%)

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r
2010 American Chemical Society
5912 Organometallics 2010, 29, 5912–5918
DOI: 10.1021/om1007133
Fully Delocalized (Ethynyl)(vinyl)phenylene-Bridged Diruthenium
Radical Complexes
Florian Pevny,
†,‡
Emmanuel Di Piazza,
§
Lucie Norel,
§
Malte Drescher,
Rainer F. Winter,*
,†,‡
and St
ephane Rigaut*
Institut f
ur Anorganische Chemie der Universit
at Regensburg, Universit
atsstrasse 31, D-93040 Regensburg,
Germany,
Fachbereich Chemie der Universit
at Konstanz, Universit
atsstrasse 10, D-78453 Konstanz,
Germany, and
§
UMR 6226 CNRS-Universit
e de Rennes 1, Sciences Chimiques de Rennes, Campus de
Beaulieu, F-35042, Rennes Cedex, France
Received July 20, 2010
Diruthenium complexes (X)(dppe)
2
Ru-CtC-1,4-C
6
H
4
-CHdCH-RuCl(CO)(P
i
Pr
3
)
2
(X = Cl,
1a;X=CtCPh, 1b) containing an unsymmetrical (ethynyl)(vinyl)phenylene bridging ligand are
compared to their symm etrical 1,4-bis(ethynyl)phenylene- and 1,4-divinylphenylene-bridged con-
geners and their mononuclear alkynyl precursors. Electrochemical and UV/vis/NIR, IR, and EPR
spectroscopic studies on the neutral complexes and their various oxidized forms indicate bridgi ng
ligand-centered oxidation processes and uniform charge and spin delocalization over both dislike
organoruthenium moieties despite differences in their intrinsic redox potentials. Comparison
between the chloro and the phenylacetylide-terminated derivatives 1a,b suggests further that the
conjugated organometallic π-system extends over the entire unsaturated backbone including the
terminal ligand at the alkynyl ruthenium site. This paves the way to even more extended π-conjugated
organoruthenium arrays for long-range electronic interactions.
Introduction
Within the field of organometallic “mixed-valent” chem-
istry, 1,4-diethynylphenylene has gained particular popular-
ity as a bridging ligand because of the ready availability of
the parent alkyne, its good ability to electronically couple the
bridged sites, and the stability it conveys to the oxidized
forms. At the same time, the 1,4-diethynylphenylene ligand
has turned out as an excellent example of Janus-headed
behavior with respect to Jørgensen’s original definition of a
noninnocent ligand. According to that definition, a ligand is
called noninnocent if it does not allow the oxidation state of
the metal to be defined.
1
This is well illustrated by the
comparison of diiron
2-8
and diruthenium
7,9-13
1,4-diethynyl-
phenylene-bridged complexes. Mostly metal-based oxida-
tion processes in the diiron systems contrast with bridging
ligand-dominated oxidations in the diruthenium ones. Thus,
while the “classical” description of a bridging ligand allow-
ing for electron exchange between the reduced and oxidized
termini in mixed-valent states may be adequate for most iron
systems, this is clearly not the case for their ruthenium
counterparts. The underlying reason is the lower energy of
the Ru 4d compared to the Fe 3d orbitals and the higher
ligand character of the highest occupied molecular orbital
(HOMO) resulting from the overlap of the metal dπ- and the
appropriate π-orbital of the carbon-rich bridging ligand.
By far the majority of all systems investigated to date feature
two identical sets of metal atoms and co-ligands as the termini.
Complexes with two different metal end groups include
some ruthenium-palladium,
14
rhenium-platinum,
15
iron-
rhenium,
16,17
and iron-ruthenium
18
derivatives. In most of
*To whom correspondence should be addressed. E-mail: rainer.winter@
uni-konstanz.de.
(1) Jørgensen, C. K. Coord. Chem. Rev. 1966, 1, 164178.
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First publ. in: Organometallics 29 (2010), 22, pp. 5912–5918, DOI: 10.1021/om1007133
Konstanzer Online-Publikations-System (KOPS)
URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-136147

Article Organometallics, Vol. 29, No. 22, 2010 5913
these cases the two dislike metal moieties differ grossly in their
electronic properties. As a consequence of such redox asym-
metry, their mixed-valent radical cations arising from one-
electron oxidation display chargeandspinlocalizationonthe
more electron-rich metal-alk ynyl site. Notable exceptions
are the iron-ruthenium complexes Cp*(dppe)Fe-CtC-1,
4-C
6
H
4
-CtC-Ru(dppe)
2
Cl and Cp*(dppe)Fe-CtC-1,
4-C
6
H
4
-CtC-Ru(dppe)
2
(CtC-1,4-C
6
H
4
NO
2
)(Cp*=η
5
-
C
5
Me
5
; dppe =1,2-b is(diphenylphosphi no)ethane, Ph
2
PC
2
-
H
4
PPh
2
), where sizable electronic couplings of ca. 1100 cm
-1
have been deduced from the Hush-type analysis of their inter-
valence charge-transfer (IVCT) bands.
18
This is to be con-
trasted to a value of just 30 cm
-1
in the iron-rhenium complex
Cp*(dppe)Fe-CtC-1,4-C
6
H
4
-CtC-Re(bipy)(CO)
3
Cl
(bipy = 2,2
0
-bipyridine).
16
Divinylphenylene-bridged diiron
19
and diruthenium
20-25
complexes strongly resemble their diethynylphenylene-
bridged counterparts. Thus, some of us have described bridge-
dominated redox processes of 1,4-divinylphenylene-bridged
diruthenium complexes {(4-EtOOCpy)(PPh
3
)
2
(CO)ClRu}
2
-
( μ-CHdCH-1,4-C
6
H
4
-CHdCH) and {(P
i
Pr
3
)
2
(CO)ClRu}
2
-
( μ-CHdCH-1,4-C
6
H
4
-CHdCH).
22-24
Spectroscopic investi-
gations on their radical cations utilizing the charge-sensitive
Ru(CO) IR marker band and the resolved hyperfine splitting
patterns in the EPR spectrum disclosed full electron and spin
delocalization (or nearly so) over both vinyl rutheniu m entities.
Given the rather similar properties of the diethynylphenylene
and the divinylphenylene bridging ligands and of the
(X)(dppe)
2
Ru-CtCandthe(P
i
Pr
3
)
2
(CO)ClRu-CHdCH
moieties, it seemed of interest to prepare and investigate
unsymmetrical complexes that blend both these motifs into a
single system. Making use of the various spectroscopic labels
they offer allows us to address the degree of electron delocaliza-
tion at their various oxidation states, as has just successfully
been demonstrated for a vinyl-bridged ruthenium-ferrocene
system.
26
Herein we report our findings on (X)(dppe)
2
Ru-
CtC-1,4-C
6
H
4
-CHdCH-RuCl(CO)(P
i
Pr
3
)
2
(X = Cl, 1a;
X=CtCPh, 1b) and their alkynyl precursors (X)(dppe)
2
Ru-
CtC-1,4-C
6
H
4
-CtC-R(R=H,2a,b;R=SiMe
3
, 3a,b)en
route to still larger systems with enhanced conjugation over
long path lengths and increased ligand participation to the
“redox orbitals”.
Results and Discussion
The synthesis of complexes 1a an d 1b was achieved
by combini ng equimo lar amounts of the known alkynyl
complexes (X)(dppe)
2
Ru-CtC-1,4-C
6
H
4
-CtC-H(2a,
b)
12,27
and the hydride ruthenium complex RuClH(CO)-
(P
i
Pr
3
)
2
28
in dichloromethane (Scheme 1). These reactions
involve the regio- and stereospecific insertion of a terminal
alkyne into the Ru-H bond of the hydride complex and
provide 1,2-disubstituted vinyl ligands with a trans disposi-
tion of the metal atom and the aryl substituent.
28-31
Despite
their mechanistic intricacy,
32
alkyne insertions are usually
complete within 15 min and afford the unsymmetrically
bridged enynyl complexes in >90% yields. The presence of
both the ruthenium vinyl and ruthenium ethynyl end groups
in 1a,b follows from the observation of the typical Ru-CtC
and Ru-CHdCH resonance signals at 113.6 (1a) or 117.4
(1b) (Ru-CtC), 148.0 (1a) or 148.4 ( 1b) (Ru-CH), and
Scheme 1
(17) Jiao, H.; Costuas, K.; Gladysz, J. A.; Halet, J.-F.; Guillemot, M.;
Toupet, L.; Paul, F.; Lapinte, C. J. Am. Chem. Soc. 2003, 125, 9511
9522.
(18) Gauthier, N.; Olivier, C.; Rigaut, S.; Touchard, D.; Roisnel, T.;
Humphrey, M. G.; Paul, F. Organometallics 2008, 27, 10631072.
(19) Field, L. D.; George, A. V.; Malouf, E. Y.; Hambley, T. W.;
Turner, P. Chem. Commun. 1997, 133134.
(20) Wu, X. H.; Jin, S.; Liang, J. H.; Yong, Z.; Yu, G.-a.; Liu, S. H.
Organometallics 2009, 28, 24502459.
(21) Seetharaman, S. K.; Chung, M.-C.; Englich, U.; Ruhlandt-Senge,
K.; Sponsler, M. B. Inorg. Chem. 2007, 46,561567.
(22) Maurer, J.; Winter, R. F.; Sarkar, B.; Fiedler, J.; Z
ali
s, S. Chem.
Commun. 2004, 19001901.
(23) Maurer, J.; Sarkar, B.; Kaim, W.; Winter, R. F.; Z
ali
s, S.
Chem.;Eur. J. 2007, 13, 1025710272.
(24) Maurer, J.; Sarkar, B.; Schwederski, B.; Kaim, W.; Winter,
R. F.; Z
ali
s, S. Organometallics 2006, 25, 37013712.
(25) Z
ali
s, S.; Winter, R. F.; Kaim, W. Coord. Chem. Rev. 2010, 254,
13831396.
(26) Kowalski, K.; Linseis, M.; Winter, R. F.; Zabel, M.; Z
ali
s, S.;
Kelm, H.; Kr
uger, H.-J.; Sarkar, B.; Kaim, W. Organometallics 2009, 28,
41964209.
(27) Hurst, S. K.; Cifuentes, M. P.; McDonagh, A. M.; Humphrey,
M. G.; Samoc, M.; Luther-Davies, B.; Asselberghs, I.; Persoons, A.
J. Organomet. Chem. 2002, 642, 259267.
(28) Werner, H.; Esteruelas, M. A.; Otto, H. Organometallics 1986, 5,
2295.
(29) Werner, H.; Meyer, U.; Peters, K.; von Schnering, H. G. Chem.
Ber. 1989, 122, 20892107.
(30) Hill, A. F. In Comprehensive Organometallic Chemistry II;
Shriver, D. E.; Bruce, M. I., Eds.; Pergamon: Oxford, 1995; Vol. 7,
pp 399-411.
(31) Maurer, J.; Linseis, M.; Sarkar, B.; Schwerderski, B.; Niemeyer,
M.; Kaim, W.; Z
ali
s, S.; Anson, C.; Zabel, M.; Winter, R. F. J. Am.
Chem. Soc. 2008, 130, 259268.
(32) Marchenko, A. V.; G
erard, H.; Eisenstein, O.; Caulton, K. G.
New J. Chem. 2001, 25, 12441255.

5914 Organometallics, Vol. 29, No. 22, 2010 Pevny et al.
134.4 (1a) or 134.9 (1b) (Ru-CHdCH) ppm and of the four
resonance signals of an unsymmetrically substituted 1,4-
phenylene ring in
13
C NMR spectroscopy, from the typical
resonance signals of the CHdCH protons of the vinyl group
at 8.40 (Ru-CHdCH) and 5.95 (Ru-CHdCH) ppm and
the presence of the phenylene, dppe, and P
i
Pr
3
protons in the
correct integral ratios, and from the two sharp singlets at δ =
49.5 (1a) or 53.8 (1b) (dppe) and 38.5 (P
i
Pr
3
) ppm in
31
P
NMR spectroscopy. The CHdCH
13
C NMR values are to be
compared to those of the symmetrically substituted
{(P
i
Pr
3
)
2
(CO)ClRu}
2
(μ-CHdCH-1,4-C
6
H
4
-CHdCH) (5)
at δ = 148.5 (Ru-CH) and 134.5 (Ru-CHdCH) ppm.
Complex 1b also features the C
β
resonance signal of the
phenylacetylide ligand, which was disseminated from that of
the bridging ligand by virtue of appropriate HMBC and
HSQC pulse sequences. Attempts to obtain similar com-
plexes from the butadiynyl and hexatriynyl complexes Cl-
(dppe)
2
Ru-(CtC)
n
H(n = 2, 3) gave, however, only
intractable mixtures.
Electrochemistry in the CH
2
Cl
2
/NBu
4
PF
6
(0.1 M) sup-
porting electrolyte revealed stepwise oxidations of com-
plexes 1a,b in two fully reversible one-electron processes
(Figure 1). Half-wave potentials are provided in Table 1
along with those of their symmetrically substituted bis-
(ethynyl)- and bis(vinyl)-substituted counterparts 4 and 5
and those of their protected and deprotected alkynyl pre-
cursors 3a and 2a,b. Comparison of the half-wave potentials
of the pairs of complexes 1a,b or 2a,b shows that chloro by
phenylacetylide substitution exerts only a small influence on
the redox potentials, as is expected on the basis of their rather
similar electrochemical ligand parameters P
L
of -1.19 (Cl
-
)
and -1.22 (PhCtC
-
).
33,34
Comparison of 1a,b with the
symmetrically substituted complexes 4 and 5 reveals a clear
dependence of the redox potential of the one organometallic
subunit on the identity of the other. Thus, the potential of the
first oxidation of 1a,b is higher than that that in {Cl(dppe)
2
-
Ru}
2
(μ-CtC-1,4-C
6
H
4
-CtC) (4), while that for the second
oxidation is lower than in {(P
i
Pr
3
)
2
(CO)ClRu}
2
(μ-CHdCH-
1,4-C
6
H
4
-CHdCH) (5). A comparison of the half-wave
potential of the mononuclear styryl complex (PhCHdCH)RuCl-
(CO)(P
i
Pr
3
)
2
(E
1/2
= þ0.28 V) with the second oxidation
potentials of 1a,b (þ0.140 and þ0.155 V, respectively) further
suggests that electron donation from the CtC-RuCl(dppe)
2
“substituent” and further extens ion of the ligand’s π-system
overcompensate for the effects of electron loss from one-
electron oxidation.
31
The potential separation between the
individual half-wave potentials of ca. 360 mV is, however,
nearly identical to that in 4 and appreciably larger than in 5.
The reader should note here that the redox-splitting of 1a,b is not
an adequate measure for electron delocalization in the radical
cations due to the mainly ligand-centered redox processes and
the differing organometallic end groups. The likewise unsymmet-
rically substituted Cp*(dppe)Fe-CtC-1,4-C
6
H
4
-CtC-
Ru(dppe)
2
Cl with E
1/2
values of -0.70 V (“Fe
II/III
”) and
þ0.05 V (“Ru
II/III
”) may serve as a further point of reference.
18
The mutual influence of each of the individual metal organic end
groups on the thermodynamic stabilities of 1a,b in their various
oxidation states suggests a fair deal of orbital mixing within the
entire {Ru}-CtC-1,4-C
6
H
4
-CHdCH -{Ru
0
}entity.
Owing to the rather substantial splitting of individual
redox potentials, the intermediate, monooxidized radical
cations 1a
þ
and 1b
þ
constitute thermodynamically stable
species with comproportionation constants, K
c
, of about 1
10
6
(Table 1). They could thus be generated by electrolysis
inside a thin-layer electrolysis cell at a potential past the 0/þ
wave as ruby to purple-red species and were characterized by
their specific IR, UV/vis/NIR, and EPR signatures. Gratify-
ingly, 1a,b possess the charge sensitive ν(CtO) IR label at
the vinyl ruthenium subunit, as well as ν(CtC) and the
various combinations of C-H bending and CdC stretching
modes of the 1,4-(ethynyl)(vinyl)phenylene subunit. These
labels are indicative of how oxidation affects the charge
densities at the vinyl metal terminus and the bridge. IR data
for the complexes 1a,b in their various oxidation states are
compiled in Table 2 and compared to those of symmetrical
4
nþ
and 5
nþ
(n = 0, 1, or 2). Of note are a substantial
bleaching and a slight red shift by 4 cm
-1
of the ν(CtC) band
at 2065 cm
-1
of 1a along with the emergence of a new CtC
band at 1967 cm
-1
and a blue shift of the Ru(CO) band by
19 cm
-1
during the first oxidation process. Another striking
feature is the growth of intense phenylene-based absorptions
in the 1580 to 1490 cm
-1
range and of a band at 1157 cm
-1
(Figures 2 and S1 of the Supporting Information). Similar
observations have been reported for the radical cations of the
symmetrical diethynyl and the divinyl phenylene-bridged
complexes 4 and 5 and are a token of the strong participation
of the bridge to the overall oxidation process.
9,22,24
Most
Figure 1. Voltammetric scans of complexes 1a (upper curve)
and 1b (lower curve) in CH
2
Cl
2
/NBu
4
PF
6
(0.1 M) at rt and v =
0.1 V/s.
Table 1. Half-Wave Potentials of Enynyl Complexes 1a,b,
Symmetrically Bridged 4 and 5, and the Alkynyl Precursors 2a,b
and 3a
E
1/2
0/þ
[V]
a
E
1/2
þ/2þ
[V] ΔE
1/2
[V]
b
K
c
c
1a -0.22 þ0.14 0.36 1.2 10
6
1b -0.195 þ0.155 0.35 8.3 10
5
4 -0.33 þ0.01 0.34 5.6 10
5
5 -0.075 þ0.175 0.25 1.7 10
4
2a 0.035
2b -0.015
3a 0.025
a
Potentials measured in CH
2
Cl
2
/NBu
4
PF
6
(0.1 M); potentials are
referenced to the Cp
2
Fe
0/þ
couple as an internal reference.
b
Difference
between half-wave potentials.
c
Comproportionation constant for the
reaction A
2þ
þ A / 2A
þ
as calculated by the expression K
c
=
exp{FΔE
1/2
/(RT)}.
(33) Pombeiro, A. J. L. New J. Chem. 1997, 21, 649660.
(34) Pombeiro, A. J. L. Eur. J. Inorg. Chem. 2007, 14731482.

Article Organometallics, Vol. 29, No. 22, 2010 5915
importantly, the ν(CtC) bands of 1a
þ
appear at almost the
same positions as in 4
þ
, while ν(CtO) of 1a
þ
is likewise
close to its position in 5
þ
. From this we conclude that both
different ruthenium sites in 1a
þ
have about the same charge
densities as in their symmetrical counterparts 4
þ
and 5
þ
.
Since these latter radical cations have both been found to be
strongly delocalized systems, this must also be the case for
1a
þ
. We are thus dealing with a bridge-centered radical
cation with a fraction of the unipositive charge almost evenly
delocalized over two different organometallic end groups.
The same obviously holds for 1b
þ
. Oxidized alkynyl pre-
cursors 2a,b
þ
have just one ν(CtC) band at ca. 1900 cm
-1
and thus at much lower energies than for 1a,b
þ
(Figures S2
and S3 of the Supporting Information).
More evidence toward full delocalization comes from the
electronic spectra. Radical cations 4
þ
and 5
þ
present rather
sharp and intense low-energy absorption bands. These have
been assigned as the SOMO-n f SOMO transitions within an
extended open-shell organometallic chromophore involving
some charge transfer from the metal end groups to the central
arene part of the bridge.
11,24
The close resemblance of those
features to those of oxidized purely organic counterparts
35,36
has already been commented on.
24
Similar bands of identical
appearance and like origin are observed in 1a,b
þ
at energies
intermediate between those of the bordering symmetrical
complexes (Table 3, Figures 3 and S4 of the Supporting
Information). The distinct red shift of the band at lowest energy
upon replacing the trans-chloro by the trans-phenylacetylide
ligand shows that the organometallic π-system of 1a,b
nþ
extends over the entire (X)-Ru-CtC-1,4-C
6
H
4
-CHd
CH-Ru unit. Similar “radical” bands of the oxidized ruthe-
nium alkynyl precursors 2a,b
þ
and 3a
þ
are less intense and
appear at higher energies. As was found for the dinuclear
complexes, replacement of the chloride by the phenylacetylide
co-ligand produces a distinct red shift (Figures S5 and S6 of
the Supporting Information).
Table 2. Characteristic IR Data for Complexes 1a,b, 4, 5, 2a,b, and 3a in Their Various Oxidation States (1,2-C
2
H
4
Cl
2
/NBu
4
PF
6
(0.2 M))
ν(CtC) ν(CtO) aryl
a
1a 2065(m) 1910(s) 1565(m), 1527(w), 1497(w), 1486(m), 1482(s)
1a
þ
2061(w), 1967(m) 1929(vs) 1581(m), 1559(m), 1521(m), 1513(m), 1493(s), 1483(m), 1157(vs)
1a
2þ
1888(m) 1977(s) 1586(w), 1576(w), 1560(w)
1b 2059(s), 1981(w) 1910(s) 1591(w), 1563(w), 1528(w)
1b
þ
2054(w), 1973(m) 1927(vs) 1579(s), 1524(s), 1512(s), 1492(vs), 1155(vs)
1b
2þ
2047(m), 1900(m) 1974(m)
4 2071(m)
4
þ
2068(w), 1966(s)
4
2þ
1918(m)
5 1910 1573(m), 1561(m)
5
þ
1932
b
1519(m), 1503(s), 1481(s)
5
2þ
1991
2a 2067(s), 2038(w), 1968(m) 1577(s)
2a
þ
1901(s) 1595(m)
3a 2149(m), 2065(s) 1573(s)
3a
þ
1905(s) 1594(m), 1166(m)
a
Combinations of CdC stretching and CH-bending modes.
b
Data for the “delocalized conformer”.
Figure 2. IR spectroscopic changes during the first oxidation of
complex 1a (1a f 1a
þ
, 1,2-C
2
H
4
Cl
2
/NBu
4
PF
6
, rt).
Table 3. UV/Vis/NIR Data for Complexes 1a,b, 4,
9
5,
24
2a,b,
and 3a in Their Various Oxidation States 1,2-C
2
H
4
Cl
2
/NBu
4
PF
6
(0.2 M)
λ
max
[nm] (energy in cm
-1
;
extinction coefficient [M
-1
3
cm
-1
])
1a 251 (39 840; 44 700), 368 (27 174; 32 500)
1a
þ
264 (37 879; 41 000), 525 (19 048; 20 200),
1340 (7463; 33 000)
1a
2þ
273 (36 630; 43 500), 400 (25 000; 6200),
719 (13 908; 35 700)
1b 364 (27 473; 33 000)
1b
þ
535 (18 692; 12 500), 1491 (6707; 20 500)
1b
2þ
408 (24 510; 7350), 633 (15 798; 12 400),
715 (13 986; 12 900), 835 (11 976; 13 300),
1024 (9766; 12 000)
2a 249 (40 161; 40 200), 362 (27 624; 22 800)
2a
þ
270 (37 037; 43 200), 390 (25 641; 12 200),
615 (16 260; 2400), 861 (11 614; 9600),
1090 (9174; 1550)
2b 364 (27 473; 30 720)
2b
þ
261 (38 314; 50 360), 301 (33 223; 25 190),
318 (31 447; 24 320), 342 (29 240; 18 980),
627 (15 949; 1620), 1157 (8643; 8010)
3a 254 (39 370; 40 010), 368 (27 174; 26 640)
3a
þ
269 (37 175; 41 970), 407 (24 570; 17 380),
616 (16 234; 2360), 869 (11 507; 10 330)
4 245 (40 820; 9100), 276 (36 230; 3100),
370 (27 030; 4400),
4
þ
268 (37 313; 2600), 490 (20 410; 2900),
535 (18 690; 3500), 1526 (6553; 3800)
4
2þ
278 (35 970; 4500), 620 (16 130; 2100),
788 (12 690; 6000), 1170 (8547; 1000)
5 353 (28 329; 10 300), 405 (24 691; 2630),
503 (19 881; 1330)
5
þ
346 (28 900; 10 300), 585 (17 095; 4270),
1255 (7968; 4110)
5
2þ
266 (37594; 9060), 430 (23 256; 3230),
624 (16 026; 5360)
(35) Rauscher, U.; B
assler, H.; Bradley, D. D. C.; Hennecke, M.
Phys. Rev. B 1990, 42, 98309836.
(36) Deussen, M.; B
assler, H. Chem. Phys. 1992, 164, 247257.

5916 Organometallics, Vol. 29, No. 22, 2010 Pevny et al.
EPR spectroscopy is a powerful tool for elucidating the
metal versus bridge character of a paramagnetic species.
Genuine Ru(III) species are usually EPR inactive at room
temperature due to rapid spin-lattice relaxation and exhibit
axially or rhombically split g-tensors with large g-anisotro-
pies at low temperatures as solids or frozen solutions and
average g-values that differ strongly from the free electron value
g
e
of 2.0023. Paramagnetic ruthenium alkynyl complexes
Cl(dppe)
2
Ru-CtC-C
6
H
4
-X-4
þ37
or Cp*(dppe)Ru-CtC-
C
6
H
4
-X-4
þ38
generally behave similarly with the exception of
those with the most electron-releasing substituents X (X =
NMe
2
,NH
2
, OMe), for which lower g-anisotropies at 80 K and
isotropic signals in fluid solution were observed. In contrast,
organic radicals typically give isotropic signals at room tem-
perature and as solids or in frozen solution at g-values close to
g
e
. Complexes 4
þ9
and 5
þ24
are rare examples of oxidized
organometallic ruthenium complexes that are EPR active
in fluid solution and at room temperature.
31,37-39
Average
g-values close to g
e
and revealingly low (4
þ
) or altogether absent
(5
þ
) g-anisotropies in frozen solution evidence their dominant
organic character with the major spin densities on the bridging
ligand. Radical cations 1a,b
þ
were obtained by chemical oxida-
tion with one equivalent of ferrocenium hexafluorophosphate.
Solid samples of 1a,b
þ
provided strong isotropic signals at room
temperature with g-values intermediate between those of 4
þ
and 5
þ
(Figures 4 and S7, S8 of the Supporting Information,
Table 4). At 103 K the spectrum of 1a
þ
still remained isotropic,
while simulations of the broader signal of 1b
þ
indicatedanaxial
splitting (Table 4). Slightly different values were measured for
their CH
2
Cl
2
/NBu
4
PF
6
solutions, where only 1b
þ
proved to be
EPR active at rt.
In keeping with the idea of full delocalization, dioxidized
species 1a,b
2þ
display their Ru(CO) band at lower energy as
in the divinylphenylene-bridged 5
2þ
. This may be ascribed to
the higher electron density at the (X)(dppe)
2
Ru
δþ
moiety
compared to the Cl(CO)(P
i
Pr
3
)
2
Ru
δ
0
þ
one at the opposite
end, where δ and δ
0
symbolize the fractions of a unit charge at
the respective ruthenium co-ligand entities. Moreover, the
visible spectrum of 1a
2þ
features a prominent absorption
band at an energy that is again intermediate between those of
similar bands of 4
2þ
and 5
2þ
(Figure 5). As a curiosity, we
note that the dioxidized phenylacetylide complex 1b
2þ
has a
series of overlapping bands in its visible spectrum, leading to
plateau-like absorption over the range 625 to 1050 nm
(Figure 6, Table 3), which may indicate transitions from a
larger manifold of lower-lying occupied donor orbitals
spreading over the entire Ph-CtC-Ru-CtCAr
0
unit into
the emptied bridge-based, delocalized frontier orbital
(the HOMO of neutral 1b). The lower energy component
at 9900 cm
-1
is at much lower energy than that in 1a
2þ
(13 900 cm
-1
).
In conclusion, we have found that the unsymmetrically
bridged (X)(dppe)
2
Ru-CtC-1,4-C
6
H
4
-CHdCH-RuCl-
(CO)(P
i
Pr
3
)
2
þ
(X = Cl, PhCtC) complexes are bridge-
centered radical cations featuring an extended X-Ru-
CtC-1,4-C
6
H
4
-CHdCH-Ru organometallic π-system
Figure 3. UV/vis/NIR spectroscopic changes during the first
oxidation of complex 1a (1a f 1a
þ
, 1,2-C
2
H
4
Cl
2
/NBu
4
PF
6
, rt).
Figure 4. EPR spectra of chemically oxidized 1b
þ
in CH
2
Cl
2
at
rt (upper left) and at 107 K (upper right) and of 1a
þ
as a solid at
rt (lower left) and at 107 K (lower right).
Table 4. EPR Parameters of Chemically Oxidized 1a
þ
and 1b
þ
under Various Conditions
T =rt T = 103 K
1a
þ
solution no signal 2.0191
solid 2.0289 2.0327
1b
þ
solution 2.0568 g
^
= 2.1290, g
)
= 2.0176,
Æg
av
æ = 2.0554
a
solid 2.0365 2.0383
a
Data based on the spectrum simulation.
44
Figure 5. UV/vis/NIR spectroscopic changes during the second
oxidation of complex 1a (1a
þ
f 1a
2þ
,1,2-C
2
H
4
Cl
2
/NBu
4
PF
6
,rt).
(37) Paul, F.; Ellis, B. G.; Bruce, M. I.; Toupet, L.; Roisnel, T.;
Costuas, K.; Halet, J.-F.; Lapinte, C. Organometallics 2006, 25, 649665.
(38) Paul, F.; da Costa, G.; Bondon, A.; Gauthier, N.; Sinbandhit, S.;
Toupet, L.; Costuas, K.; Halet, J.-F.; Lapinte, C. Organometallics 2007,
26, 874896.
(39) Maurer, J.; Sarkar, B.; Zalis, S.; Winter, R. F. J. Solid State
Electrochem. 2005, 9, 738749.

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