Reversible, fine performance tuning of an organometallic molecular wire by addition, ligand replacement and removal of dicobalt fragments

Article abstract of DOI:10.1002/ejic.201000661

Communication between the two iron centres in (dithienylethyne) diyl complex 1 can be finely tuned by reversible addition to, ligand replacement at and removal from the C=C moiety in 1 of dicobalt fragments Co ₂(CO)_n(PR₃)<

Full text of DOI:10.1002/ejic.201000661

SHORT COMMUNICATION
DOI: 10.1002/ejic.201000661
Reversible, Fine Performance Tuning of an Organometallic Molecular Wire by
Addition, Ligand Replacement and Removal of Dicobalt Fragments
Yuya Tanaka,[a] Takashi Koike,[a] and Munetaka Akita*[a]
Keywords: Metal–metal interactions / Mixed-valent compounds / Molecular devices / Molecular wires / Organometallic
compounds
Communication between the two iron centres in (dithienyl-
ethyne)diyl complex 1 can be finely tuned by reversible ad-
dition to, ligand replacement at and removal from the CϵC
ation for the two systems. In the case of the dicobalt adducts,
indirect communication via the dicobalt steppingstone can
be finely tuned by controlling the electronic structure of the
moiety in 1 of dicobalt fragments Co2(CO)n(PR3)6–n. Perform- dicobalt unit.
ance analysis reveals that disparate mechanisms are in oper-
tion” to control the communication between the two metal
Introduction
centres (Scheme 1) and picked up an acetylene-based diiron
molecular wire, 1 (Scheme 2), in which (i) the communica-
tion may be controlled by addition of dicobalt species to
the CϵC moiety and (ii) the effects brought about by the
dicobalt fragment can be further finely tuned by introduc-
ing appropriate ligands to it. The present study has also
revealed that different communication mechanisms operate
for 1 and its dicobalt adducts.
Downsizing is one of key issues in modern electronic
science and industry, and, as one of effective bottom-up
[1]
strategies for this issue, molecular electronics has attracted
increasing attention. If functions of an electronic circuit can
be represented by a combination of molecular components,
one would be able to obtain a circuit miniaturized to a
minimal level. A number of studies have been conducted
toward this goal, and, in organic-based systems, π-conju-
gated systems play central roles as devices carrying elec-
trons and holes.[2] Furthermore, combination with metal
fragments renders the systems more sophisticated, thanks
to their unique properties such as redox and magnetic fea-
tures. Organometallic molecular wires consisting of redox-
active metal centres connected by a π-conjugated bridge
have been studied extensively; as a result, many excellent
molecular wires with strongly interacting metal centres (e.g.
polyynediyl complexes) have been developed so far.[3] In ad-
dition to the most basic component, that is, wires, fine tun-
ing of the electronic communication between two or more
remote redox-active sites is demanded to develop molecular
parts such as switches, resistors and diodes.[4] Recently, ef-
ficient ON/OFF-type switches based on photo-, pH- and
ionochromic linkers have been developed by several re-
search groups including our group.[5–7]
Scheme 1. Reversible, fine tuning of metal–metal interaction.
Herein we describe reversible, fine tuning of the perform-
ance of an acetylene-based diiron molecular wire
(Scheme 1). To achieve the desired functions, the molecule
should be stable in at least three different states that can be
interconverted to each other. We have chosen “coordina-
[a] Chemical Resources Laboratory, Tokyo Institute of Technology,
R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Fax: +81-45-924-5230
E-mail: makita@res.titech.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000661.
Scheme 2. Synthesis of dicobalt adducts of 1.
Eur. J. Inorg. Chem. 2010, 3571–3575
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3571

Y. Tanaka, T. Koike, M. Akita
SHORT COMMUNICATION
the shifts of the CO vibrations to lower energies [νCO (KBr):
2067, 2033, 2001 (2a), 2009, 1984, 1957 (3a), 2001, 1972,
1945 (4a), 1898 (5a) cm–1] as well as the cathodic shifts of
the Co-centred redox processes (E1/2Co).
Results and Discussion
A diiron complex with the (dithienylethyne)diyl bridge,
[Fe–Th–CϵC–Th–Fe] (1) {Fe = Fe(η5-C5H4R)(dppe); R =
H (a-series), Me (b-series); Th = thiophene-2,5-diyl}, was
prepared by lithiation of bis(thien-2-yl)ethyne followed by
metalation with I-Fe(η5-C5H4R)(CO)2 and photochemical
ligand substitution with dppe.[8] (The b-series derivatives
were prepared in order to avoid the solubility problem.) X-
ray crystallographic analysis of 1b (Figure 1) reveals the co-
planar conformation of the two thiophene rings and an
Fe···Fe separation of 12.7 Å.[9] Cobalt adducts 2 and 3 were
obtained by reaction of 1 with [Co2(CO)8] and [Co2(CO)6-
(dppm)], respectively, in thf at room temperature.[10] Ligand
Communication performance of the obtained complexes
is evaluated on the basis of IVCT bands (MMCT appearing
in the near IR region) of 1e-oxidized monocationic spe-
cies.[12] In particular, the Vab coupling value, which is de-
rived from the spectral parameters of the IVCT bands, rep-
resents the extent of electronic interaction between the two
redox-active metal centres.[12c] A high-performance organo-
metallic molecular wire shows a large Vab value associated
with an intense, sharp IVCT band (with a large εmax and a
small ν1/2exp).[12a] Prior to the determination of the Vab
value, it is essential to classify the compound into one of
three classes (Robin–Day Classes I–III), because different
equations are applied to obtain the Vab value (see footnotes
[c,d] of Table 1); two Vab values for Class II and III com-
pounds are shown in Table 1.[13] For the Robin–Day classi-
fication, solvent dependency and half-height width of the
IVCT band are critical factors.[12c] For a high-performance
Class III compound, (1) the absorption maximum (νmax) of
the IVCT band is little affected by the solvent polarity, and
(2) the half-height width thereof (ν1/2exp) is narrower than
the value predicted on the basis of the Hush theory
(ν1/2calc). Comparison of the performances of compounds
belonging to different classes should be made with care by
taking into account the above-mentioned factors. Monoca-
tionic complexes 1a+, 3a+, 4a+ and 5a+ were obtained by
chemical 1e-oxidation of the corresponding neutral com-
plexes with 1 equiv. of [FeCp2][PF6], whereas the thermody-
namically less stable 2a+ was generated in situ by compro-
portionation of the isolable neutral (2a) and dicationic spe-
cies (2a2+).[8]
replacement
of
2
with
bidentate
phosphanes
[RЈ2PCH2PRЈ2: RЈ = Ph(dppm), Me(dmpm)] gave the
mono- (3, 4) and disubstituted (5) products. All complexes
show single sets of NMR spectroscopic signals for the η5-
C5H4R (1H) and dppe ligands (31P) at room temperature,
indicating symmetrical structures for 1–5 and the occur-
rence of fluxional behaviour of the ligands attached to the
Co centres in 3–5.
Figure 1. An ORTEP view of 1b drawn with thermal ellipsoids at
the 30% probability level. Hydrogen atoms are omitted for clarity.
Electron densities at the Fe and Co centres are estimated
by CV and IR spectroscopic measurements. Two Fe-centred
redox waves appear in the range –1200 to –400 mV
Complex 1a+ shows intense NIR bands in the region
(Table 1).[11] Attachment of the Co2(CO)6 fragment to 1a 3000–8000 cm–1 (Figure 2a), which are attributed to the Fe-
Fe1
Fe2
causes slight anodic shifts of E1/2
and E1/2
(the first to-Fe IVCT bands, as noted for the related Fe(η5-
and second redox potential for the iron centres, respec- C5R5)(dppe) complexes.[3a,14] These NIR bands have been
tively), whereas ligand replacement of the resultant adduct successfully deconvoluted into three Gaussian curves, of
2a by the electron-donating diphosphane ligands causes sig- which the most intense, lowest energy band (band I; νmax
=
nificant cathodic shifts of E1/2Fe1 and E1/2Fe2 (increasing or- 4240 cm–1) is used for the determination of Vab.[3a,14] Be-
der of the electron densities at the Fe centres: 2a Ͻ 3a Ͻ cause (1) νmax of this IVCT band is little affected by the
4a Ͻ 5a) in accord with the increasing order of the electron solvent polarity (less than 100 cm–1)[8,12c,15] and (2) the half-
exp
densities at the dicobalt centres estimated on the basis of height width of this band (ν1/2
= 1220 cm–1) is signifi-
Table 1. Electrochemical and NIR data for complexes 1a–5a.[a]
Complex E1/2Fe1 /mV E1/2Fe2 /mV E1/2Co /mV ∆ECo/Fe2 /mV νmax /cm–1 εmax /–1cm–1 ν1/2exp /cm–1 ν1/2calcd.[b] /cm–1
V V
II [c] /cm–1 III[d] /cm–1
ab ab
1a
2a
3a
4a
5a
–620
–595
–736
–755
–1039
–432
–483
–571
–567
–681
–
–
4240
7650
8295
7660
6060
22320
1900
5120
5380
10660
1220
2646
3562
3086
2550
3130
4204
4377
4206
3741
551
550
1091
1000
1138
2120
3825
4148
3830
3030
Ͼ400 [e]
21
Ͼ883
592
455
369
–112
–312
[a] The CV data are for the neutral species, and the NIR spectroscopic data are for the corresponding monocationic species. Conditions
for electrochemical measurements: [complex] = ≈ 1.0ϫ10–3 , [NBu4·PF6] = 0.1  at 293 K, scan rate = 100 mV/sec. E1/2 values are
Fe2
Co
referenced against the FeCp2/[FeCp2]+ couple. E1/2Fe1, E1/2
and E1/2 are the first and second redox potentials for the iron centres
and the redox potential for the dicobalt centre, respectively. ∆ECo/Fe2 is the difference between E1/2 and E1/2Fe2. [b] ν1/2calcd. (in cm–1) =
Co
1/2
II
III
(2310·νmax
)
[c] Vab (estimated as a Class II compound) = 2.06ϫ10–2 (νmax =·εmax·ν1/2
)
exp 1/2·r–1 (r: M···M distance).[9] [d] Vab (esti-
.
Co
mated as a Class III compound) = (1/2)·νmax =. [e] The E1/2 process overlaps with the FeIII–FeIII/FeIII–FeIV process.
3572
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 3571–3575

Reversible, Fine Tuning of an Organometallic Molecular Wire
calc
exp
cantly narrower than the predicted value (ν1/2
=
ν1/2 are noted for the series 2a+–5a+ with the only excep-
exp
3130 cm–1),[13] it is concluded that complex 1a+ falls in the tion of ν1/2 of 2a+. Because high performance is associ-
Robin–Day Class III (fully delocalized system) with an Vab ated with an intense, sharp IVCT band (with a large εmax
value of 2120 cm–1, and thus the two metal centres therein and a small ν1/2exp) leading to a large Vab value, the two
strongly interact with each other.
factors (εmax and ν1/2exp) are also considered to be criteria
for the classification. The monotonous changes of εmax and
ν1/2exp and the Class III performance of 1a+ discussed above
exp
(with by far the largest εmax and smallest ν1/2 values) re-
veal the order of the wire-like performance as follows: 1a+
(Class III) ϾϾ 5a+ (Class IIB) Ͼ 4a+ Ͼ 3a+ Ͼ 2a+ (Class
IIA). This order turns out to be consistent with that esti-
mated on the basis of the ESR parameters for 1a+–
5a+,[16,17] and can furthermore be correlated to the increas-
ing order of the electron densities at the Co centres dis-
cussed above.
Thus, the communication between the two iron centres
can be controlled by coordination of the dicobalt species to
the CϵC moiety in 1 and can be further tuned by introduc-
tion of a phosphane ligand with the appropriate electron-
donating ability.
The attached dicobalt fragments in 2–5 can be removed
upon treatment with NBu4F or OǟNMe3 (Scheme 2).[20]
Thus, the dicobalt fragment can be attached to and re-
moved from 1 in a reversible manner. In other words, the
wire-like performance of the organometallic molecular wire
1 can be controlled by addition, ligand substitution and re-
moval of the dicobalt unit.
The significantly different features of the IVCT bands
observed for 1a+ and 2a+–5a+ suggest disparate mecha-
nisms operating for the two systems. The IVCT band for
1a+ arises from the Fe-to-Fe MMCT, because 1a+ contains
the two iron centres as the unique redox-active sites. On
Figure 2. (a) NIR spectrum for 1a+ and its deconvoluted Gaussian
curves and (b) NIR spectra for 2a+–6a+ (observed in CH2Cl2).
Cobalt adducts 2a+–5a+ exhibit IVCT bands that are re- the other hand, two pathways are feasible for the dicobalt
markably different from those of 1a+ (Figure 2b; the ab- adducts 2a+–5a+, that is, the direct Fe–Fe MMCT as ob-
sorptions at greater than 10000 cm–1 are LMCT bands), be- served for 1a+ and the indirect Fe–Co–Fe MMCT, where
cause of the different IVCT mechanisms (see below). For the dicobalt unit works like a steppingstone. Of the two
example, the IVCT bands for 2a+–5a+ are considerably mechanisms, it turns out that the latter process is at work
weaker than those of 1a+, and the absorption maxima for 2a+–5a+, because monocationic species [(µ-η2:η2-Fe–
(νmax) are shifted to higher energies. The order of the sol- Th–CϵC–Th–H)Co2(CO)4(dppm)] (6a), a monoiron deriv-
vent dependency is determined to be as follows: 3a+ Ͼ 4a+ ative of 3a, for which direct Fe–Fe transition is not feasible
Ͼ 5a+[8] (2a+ can not be examined because of its low sta- but Fe–Co transition is feasible, has a NIR absorption band
bility).
that is close in shape to that of 3a+ (Figure 2b). It should
Compound 5a+ has been assigned to Class IIB, although be also noted that (1) εmax of the monoiron species 6a+ is
the little solvent dependency and the half-height width about half of that of the diiron species 3a+ (Figure 2b) and
(ν1/2exp) of the IVCT band narrower than ν1/2
suggest its (2) the transition is characterized as an IVCT band as re-
calc
assignment to Class III. One debatable point is the con- vealed by the linear relationship between νmax and (1/n2 –
siderably large ν1/2 value (2550 cm–1) compared to those 1/Ds2) observed in solvents with different polarities (n and
exp
of typical Class III compounds (e.g. 1a+: 1220 cm–1). Re- Ds are denoted by the optical and statistical dielectric con-
cently, this type of compounds has been categorized into stants of the solvent).[8,21]
the subclass “Class IIB”, as discussed by Brunschwig et
Electron transfer process of dinuclear species has been
al.,[12b] that is, the electronic coupling is underestimated by interpreted in terms of the diagram involving two overlap-
the Hush treatment (VabII) and overestimated by the Class ping potential energy curves.[22] A simplified diagram for a
III treatment (VabIII).[14a]
Class II dinuclear species is shown in Scheme 3a. Photo-
The solvent-dependent species 2a+–4a+ belong to Class chemical excitation of the ground state A by absorption of
II
II (Table 1). Complex 2a+ shows the smallest Vab value the IVCT transition energy (νIVCT) and subsequent thermal
II
(550 cm–1). Although the Vab values for 3a+ (1091 cm–1) relaxation following the potential curve finally lead to B to
4a+ (1000 cm–1) and 5a+ (1138 cm–1) are comparable, a mo- accomplish the electron transfer between the two iron
notonous increase of εmax and a monotonous decrease of centres. For the three-component dicobalt adducts 2–5,
Eur. J. Inorg. Chem. 2010, 3571–3575
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3573

Y. Tanaka, T. Koike, M. Akita
SHORT COMMUNICATION
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
electron transfer can be explained by the potential energy
curve diagrams shown in Scheme 3b.[23] The IVCT transi-
tion from the ground state C followed by thermal relaxation Supporting Information (see footnote on the first page of this arti-
cle): Full synthetic and spectroscopic details of 1–5.
in a manner similar to the dinuclear system leads to the
intermediary state D. Subsequent thermally induced elec-
tronic transition from the other iron centre to the dicobalt
unit leads to the other ground state E to accomplish the
electron transfer between the two iron centres via the dico-
balt unit. In this context, the energy gap (∆E) is a key factor
for the determination of Vab, because a diminution of ∆E
brings about an increase in the electronic coupling Vab. ∆E
can be experimentally estimated by the CV data, that is, ∆E
is equivalent to the difference of the redox potentials for
the iron and cobalt centres (∆ECo/Fe2).[24] As summarized
in Table 1, the order of the ∆ECo/Fe2 values are 2a Ͼ 3a Ͼ
4a Ͼ 5a. Thus a compound with a small ∆ECo/Fe2 value
gives a large Vab value (5a Ͼ 4a Ͼ 3a Ͼ 2a), in accord with
the experimental results described above. As a result of
these electronic effects, the communication performance
(Vab) of the dicobalt adducts can be finely tuned by choos-
ing a ligand with appropriate electron-donating ability. ∆E
can be correlated to the densities at the dicobalt unit, which
Acknowledgments
The financial support from the Japanese government (Grants-in-
Aid for Scientific Research: Nos. 18065009, 20044007 and
50167839; M. A.) and the Japan Society for the Promotion of Sci-
ence (Y. T.) are gratefully acknowledged. We are also grateful to
Dr. F. Paul (University of Rennes 1) for the helpful discussion on
the ESR analysis.
[1] J. Jortner, M. A. Ratner, Molecular Electronics, Blackwell Sci-
ence, Oxford, 1997; A. Aviram, M. Ratner, Ann. NY Acad. Sci.
1998, 852; M. Ratner, Nature 2000, 404, 137; B. L. Feringa
(Ed.), Molecular Switches, Wiley-VCH, Weinheim, 2001; J. M.
Tour, Acc. Chem. Res. 2000, 33, 791; N. Robertson, G. A.
Mc Gowan, Chem. Soc. Rev. 2003, 32, 96; M. A. Reed, T. Lee
(Eds.), Molecular Nanoelectronics, American Scientific Publish-
ers, Stevenson Ranch, CA, 2003; M. C. Petty, Molecular Elec-
tronics: From Principles to Practice, Wiley, New York, 2008; K.
Szacilowski, Chem. Rev. 2008, 108, 3481.
Co
[2] V. Coropceanu, J. Cornil, D. A. S. Filho, Y. Olivier, R. Silbey,
J.-L. Brédas, Chem. Rev. 2007, 107, 926.
can be estimated by E1/2 and νCO as discussed above; in
other words, an electron-donating ligand induces better
communication between the two metal centres.
[3] a) F. Paul, C. Lapinte, Coord. Chem. Rev. 1998, 178–180; F.
Paul, C. Lapinte, Coord. Chem. Rev. 1998, 427; b) M. I. Bruce,
P. J. Low, Adv. Organomet. Chem. 2004, 50, 231; c) S. Szafert,
J. A. Gladysz, Chem. Rev. 2006, 106, PR1; d) T. Ren, Chem.
Rev. 2008, 108, 4185–4207; e) M. Akita, T. Koike, Dalton
Trans. 2008, 3523; f) P. F. H. Schwab, M. D. Levin, J. Michl,
Chem. Rev. 1999, 99, 1863; g) P. F. H. Schwab, J. R. Smith, J.
Michl, Chem. Rev. 2005, 105, 1197.
[4] C. Joachim, J. K. Gimzewski, A. Aviram, Nature 2000, 408,
541; M. Irie, Chem. Rev. 2000, 100, 1685.
[5] S. Fraysee, C. Coudret, J.-P. Launay, Eur. J. Inorg. Chem. 2000,
1581; G. Guirado, C. Christophe, J.-P. Launay, J. Phys. Chem.
C 2007, 111, 2770; Y. Tanaka, T. Koike, M. Akita, Chem. Com-
mun. 2007, 1169; K. Motoyama, T. Koike, M. Akita, Chem.
Commun. 2008, 5812; Y. Liu, C. Lagrost, K. Costuas, N. Tou-
char, H. Le Bozec, S. Rigaut, Chem. Commun. 2008, 6117; Y.
Lin, J. Yuan, M. Hu, J. Cheng, J. Yin, S. Jin, S. H. Liu, Organo-
metallics 2009, 28, 6402.
[6] H. Tannai, K. Tsuge, Y. Sasaki, Inorg. Chem. 2005, 44, 5206;
C. D. Pietro, S. Campagna, M. T. Gandolfi, R. Ballardini, S.
Fnni, W. R. Browne, J. G. Vos, Inorg. Chem. 2002, 41, 2871; M.
Haga, M. M. Ali, S. Koseki, K. Fujimoto, A. Yoshimura, K.
Nozaki, T. Ohno, K. Nakajima, D. J. Stufkens, Inorg. Chem.
1996, 35, 3335.
Scheme 3. Energy diagrams for (a) di- and (b) tricomponent sys-
tems of Class II. Vab: electronic coupling; νIVCT: IVCT transition
energy; ∆E: energy gap between the Fe and Co centres; ∆G: ther-
mal electron-transfer barrier.
[7] Y. Ie, T. Kawabata, T. Kaneda, Y. Aso, Chem. Lett. 2006, 35,
1366.
Conclusions
[8] See Supporting Information.
We have succeeded in fine tuning the communication be-
tween the two metal centres in the organometallic molecu-
lar wire 1 by attachment and removal of an appropriate
dicobalt fragment (Scheme 1). It is notable that: (1) the
wire-like performance of the derivatives varies in the range
from Robin–Day Class IIA (2) to Class III (1); and (2) 1
and the cobalt adducts 2–5 can be interconverted in a re-
versible and facile manner. In the case of the dicobalt ad-
ducts, the indirect communication via the dicobalt step-
pingstone can be finely tuned by controlling the electronic
structure of the dicobalt unit.
[9] Crystallographic data for 1b: C78H74Cl8Fe2P4S2 (1b·4CH2Cl2),
¯
Mr = 1594.67; triclinic space group P1; a = 9.3537(13) Å, b =
12.1333(15) Å, c = 17.988(2) Å; α = 81.632(6)°; β = 84.916(6)°;
γ = 67.604(5)°; V = 1866.1(4) Å3; Z = 1; ρcalcd. = 1.419 gcm–3;
µ = 0.860 mm–1; λ = 0.71073 Å; T = –60 °C; total data col-
lected = 15426; R1 = 0.0538 [5126 observed reflections with
Fo2Ͼ2σ(Fo2)]; wR2 = 0.1520 for 424 variables and all 7796
unique reflections.
[10] Preliminary X-ray crystallographic structure analysis of 3b re-
vealed the equatorial coordination of the dppm ligand as well
as the value of 7.34 Å (r for the dicobalt adducts; Table 1) for
the average Fe···Co separation.
[11] KC (comproportionation constant) values are as follows: ∆E
(in mV)/KC = 188/1.5ϫ103 (1a), 112/79 (2a), 165/6.1ϫ102 (3a),
189/1.5ϫ103 (4a), and 358/1.1ϫ106 (5a).
CCDC-775289 (1b) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
3574
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 3571–3575

Reversible, Fine Tuning of an Organometallic Molecular Wire
[12] a) N. S. Hush, Prog. Inorg. Chem. 1967, 8, 391; b) B. S.
Brunschwig, C. Creutz, N. Sutin, Chem. Soc. Rev. 2002, 31,
168; c) K. D. Demadis, C. M. Hartshorn, T. J. Meyer, Chem.
Rev. 2001, 101, 2655.
[13] C. Creutz, H. Taube, J. Am. Chem. Soc. 1969, 91, 3988.
[14] a) S. I. Ghazala, F. Paul, L. Toupet, T. Roisnel, P. Hapiot, C.
Lapinte, J. Am. Chem. Soc. 2006, 128, 2463; b) F. de Montigny,
G. Argouarch, K. Costuas, J.-F. Halet, T. Roisnel, L. Toupet,
SOMO,[18] and ∆g decreases as the rate of the intramolecular
electron transfer increases in a homologous series of mixed-
valence compounds.[19,3a] The tendencies of the two parameters
(1a+ Ͻ 5a+ Ͻ 4a+ Ͻ 3a+ Ͻ 2a+) are in accord with the result
obtained from the NIR spectroscopic data. ∆g = 0.213 (2a+)
Ͼ 0.130 (3a+) Ͼ 0.113 (4a+) Ͼ 0.091 (5a+) Ͼ 0.084 (1a+); giso
= 2.092 (2a+) Ͼ 2.068 (3a+) Ͼ 2.057 (4a+) Ͼ 2.052 (5a+) Ͼ
2.042 (1a+).
C. Lapinte, Organometallics 2005, 24, 4558; c) Y. Tanaka, J. A. [18] F. D. Montigny, G. Argouarch, K. Costuas, J.-F. Halet, T. Rois-
Shaw-Taberlet, F. Justaud, O. Cador, T. Roisnel, M. Akita, J.-
R. Hamon, C. Lapinte, Organometallics 2009, 28, 4656.
nel, L. Toupet, C. Lapinte, Organometallics 2005, 24, 4558.
[19] T.-Y. Dong, D. N. Hendrickson, C. G. Pierpont, M. F. Moore,
J. Am. Chem. Soc. 1986, 108, 963.
[15] S. Stang, F. Paul, C. Lapinte, Organometallics 2000, 19, 1035.
[16] P. Hamon, F. Justaud, O. Cador, P. Hapiot, S. Rigaut, L. Tou- [20] a) D. S. Davis, S. C. Shadinger, Tetrahedron Lett. 1999, 40,
pet, L. Ouahab, H. Stueger, J.-R. Hamon, C. Lapinte, J. Am.
Chem. Soc. 2008, 130, 17372; T.-Y. Dong, D. N. Hendrickson,
C. G. Pierpont, M. F. Moore, J. Am. Chem. Soc. 1986, 108,
963.
7749; b) G. B. Jones, J. M. Wright, T. M. Rush, G. W.
Plourde II, T. F. Kelton, J. E. Mathews, R. S. Huber, J. P. Da-
vidson, J. Org. Chem. 1997, 62, 9379.
[21] M. J. Powers, T. J. Meyer, J. Am. Chem. Soc. 1978, 100, 4393.
[22] R. A. Marcus, N. Sutin, Biochim. Biophys. Acta 1985, 811, 265.
[23] D. M. D’Alessandro, F. R. Keene, Chem. Rev. 2006, 106, 2270;
Y. Zhu, O. Clot, M. O. Wolf, G. P. A. Yap, J. Am. Chem. Soc.
1998, 120, 1812; B. W. Pfennig, A. B. Bocarsly, J. Phys. Chem.
[17] The extent of delocalization of the radical centre over the
metal-π-conjugated system can also be estimated on the basis
of ESR parameters. Complex 1 displays three components of
g tensors expected for pseudo-octahedral d5 low spin FeIII com-
plexes, indicating that the radical cation is mainly localized on
the iron centre on the ESR measurement time scale (ca. 10–9 s).
The cobalt adducts show a couple of slightly broad g tensors,
which also arise from FeIII. The isotropic g value (giso) and the
anisotropy tensor (∆g) are measures for the delocalization. The
giso value is an indicator of the metal character of the
1992, 96, 226.
Fe1
[24] Because, in general, E1/2
is significantly influenced by the
wire-like performance, E1/2Fe2, which is hardly affected by the
wire-like performance, is used instead.
Received: June 16, 2010
Published Online: July 16, 2010
Eur. J. Inorg. Chem. 2010, 3571–3575
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3575