Diferrocenyl molecular wires. the role of heteroatom linkers

Article abstract of DOI:10.1021/ja101585z

Diferrocenyl molecular "wires", in which two ferrocenes are linked by a conjugated chain, allowing the communication of redox information between the ferrocenes, are a versatile platform on which to investigate notions of molecular conductivity. In this paper, we examine the role of heteroatoms=including O, P, S, and Se, as well as C atoms in various oxidation states=separated from the ferrocenes by intervening double bonds which minimize any steric effects. Surprisingly, oxygen is a better electronic mediator than sulfur, a phenomenon we attribute to superior molecular orbital overlap. These fundamental studies on redox coupling will help to guide the design of efficient organic conductors for organic electronics.

Full text of DOI:10.1021/ja101585z

Published on Web 07/08/2010
Diferrocenyl Molecular Wires. The Role of Heteroatom Linkers
Yu Li, Mira Josowicz, and Laren M. Tolbert*
School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic DriVe,
Atlanta, Georgia 30332-0400
Received March 4, 2010; E-mail: laren.tolbert@chemistry.gatech.edu
Abstract: Diferrocenyl molecular “wires”, in which two ferrocenes are linked by a conjugated chain, allowing
the communication of redox information between the ferrocenes, are a versatile platform on which to
investigate notions of molecular conductivity. In this paper, we examine the role of heteroatomssincluding
O, P, S, and Se, as well as C atoms in various oxidation statessseparated from the ferrocenes by intervening
double bonds which minimize any steric effects. Surprisingly, oxygen is a better electronic mediator than
sulfur, a phenomenon we attribute to superior molecular orbital overlap. These fundamental studies on
redox coupling will help to guide the design of efficient organic conductors for organic electronics.
Introduction
the donor and acceptor wave functions through the mediation
of the bridging component. The efficiency of such a mechanism
The notion of molecular “wires” has captured the imagination
of chemists and physicists in semiconductor nanotechnology.1
Missing are clear concepts of what this term means and how it
relates to structure and function. Although such wires are often
portrayed as single conductive paths, and the measurement of
single-molecule conductance has emerged as a rich and pro-
vocative research area, most often such wires are used in
ensembles connecting metallic surfaces.2 Moreover, such wires
often possess a low-dimensional π-conjugation backbone that
can carry electrons through a distance of several nanometers.3
Directly addressing the conductivity of a distinct molecule, for
instance, by assembling an electronic junction connected by the
molecular wire, is still fraught with problems. Therefore, an
indirect approach with a donor-bridge-acceptor motif remains
the most useful prototype for examining the capability of a single
molecule to support charge transfer. Molecules of this kind must
not only satisfy the dimensional requirements but also have the
appropriate electronic properties.
Electron transport across molecules is generally divided into
two alternative types: superexchange and hopping. In the
superexchange mechanism,4 the donor and acceptor orbitals are
assumed to overlap with the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
the bridging group, while the direct orbital overlaps between
the donor and acceptor are negligible. Thus, the coupling
between donor and acceptor is furnished by indirect mixing of
is determined by the effective overlap between donor and
acceptor. In the hopping mechanism, the electron (or hole)
transiently resides on the midway bridging group so that a
chemical intermediate is generated. Early efforts commonly have
treated them separately, but recent research has also shown that
both mechanisms can function simultaneously.5
Ferrocenes are ideal donor moieties, since, unlike conven-
tional aromatics, the HOMO is localized not in the π system
but in a metal-centered d orbital.6 In addition, the ease of organic
functionalization, the chemical stability of the neutral and
charged species, the low oxidation potential to allow effective
coupling with the organic component, and the diamagnetism in
the neutral state (thus enabling NMR characterization) make
ferrocenes one of the most employed candidates in implementing
the wire-bridged complex and for studying electronic com-
munication.7 The mixed valence (MV) state of the biferrocenyl
complex is of the most interest, since the charge mobility in
this system can be directly characterized by electrochemistry,
structural analysis, electronic spectroscopy, ESR, Mo¨ssbauer,
etc. We note in this regard that our studies are complementary
to previous studies in which electronic coupling between carbene
and nitrene centers is mediated by a chalcogenide.8
Previously we have shown that a solitonic bridge, i.e., a
polyenyl cation or anion, maintains electrical communication
(5) (a) Petrov, E. G.; May, V. J. Phys. Chem. A 2001, 105, 10176–10186.
(b) Sim, E. J. Phys. Chem. B 2004, 108, 19093–19093. (c) Weiss,
E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 5577–5584.
(6) Boccia, A.; Marrani, A. G.; Stranges, S.; Zanoni, R.; Alagia, M.; Cossi,
M.; Iozzi, M. F. J. Chem. Phys. 2008, 128, 154315/1–154315/9.
(7) Sundararaman, A.; Venkatasubbaiah, K.; Victor, M.; Zakharov, L. N.;
Rheingold, A. L.; Jaekle, F. J. Am. Chem. Soc. 2006, 128, 16554–
16565.
(1) (a) Chen, F.; Tao, N. J. Acc. Chem. Res. 2009, 42, 429–438. (b) Wang,
C.; Plsson, L.-O.; Batsanov, A. S.; Bryce, M. R. J. Am. Chem. Soc.
2006, 128, 3789–3799. (c) Flores-Torres, S.; Hutchison, G. R.;
Soltzberg, L. J.; Abrun˜a, H. D. J. Am. Chem. Soc. 2006, 128, 1513–
1522.
(2) (a) Chen, J.; Reed, M. A.; Asplund, C. L.; Cassell, A. M.; Myrick,
M. L.; Rawlett, A. M.; Tour, J. M.; Van Patten, P. G. Appl. Phys.
Lett. 1999, 75, 624–626. (b) Carroll, R. L.; Gorman, C. B. Angew.
Chem., Int. Ed. 2002, 41, 4378–4400. (c) Metzger, R. M. Chem. ReV.
2003, 103, 3803–3834.
(8) (a) Matsuda, K.; Yamagata, T.; Seta, T.; Iwamura, H.; Hori, K. J. Am.
Chem. Soc. 1997, 119, 8058–8064. (b) Minato, M.; Lahti, P. M. J.
Phys. Org. Chem. 1994, 7, 495–502. (c) Itoh, K.; Takui, T.; Teki, Y.;
Kinoshita, T. Mol. Cryst. Liq. Cryst. 1989, 176, 49–66. (d) Nimura,
S.; Kikuchi, O.; Ohana, T.; Yabe, A.; Kaise, M. Chem. Lett. 1994, 9,
1679–82.
(3) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.;
Chidsey, C. E. D.; Feldberg, S. W. Science 2001, 291, 1519–1523.
(4) (a) Anderson, P. W. Phys. ReV. 1959, 115, 2–13. (b) Marcus, R. A.
J. Chem. Phys. 1965, 43, 679–701.
9
10374 J. AM. CHEM. SOC. 2010, 132, 10374–10382
10.1021/ja101585z  2010 American Chemical Society

Diferrocenyl Molecular Wires
A R T I C L E S
up to 13 carbon atoms.9 However, the use of such bridges is
complicated by the ionic nature of the substrates. In contrast, it
has been very common to use a number of heteroatoms (silicon,
phosphorus, or sulfur) to bind redox-active surfaces. In this case,
the pair of electrons on the heteroatom serves to substitute for
the more mobile electron pair (or hole) within a soliton. Little
discussion, however, has taken place over the nature of the
conduction path. How well do such linking groups facilitate or
block electronic communication? In this study we seek to
establish the nature of that charge transport.
If we compare a diferrocenyl “soliton” molecule (Fc-
(CH)5Fc)- with an isoelectronic molecule containing a hetero-
atom (FcCHdCHXCHdCHFc), we see that the HOMO has a
striking similarity to that for the solitonic molecule. However,
the redox potential of the replacement group would be expected
to have a profound effect, as would the polarizability of the
substituted group. We were concerned that the rather na¨ıve views
of resonance extant in the community lead to the assumption
that resonance is equally effective whether through a double
bond, a triple bond, heteroatom, an aromatic ring, or a
heteroaromatic ring. Since the range of molecules studied by
various methods does not allow ready comparison, we deter-
mined to compare a range of molecules of type FcCHd
CHXCHdCHFc, in which the group X was varied across the
common range of linking groups studied but for which the
vinylene groups CHdCH served to minimize steric effects
which would otherwise intervene.
Scheme 1. Mechanism of Charge Injection and Propagation in
Diferrocenyl Chalcogenides
As stated earlier, the effectiveness of organic wires using the
prototype binuclear complexes largely relies on efficient cou-
pling between the d orbital of the metal and the frontier p orbital
of the bridging fragment. We expect that electron transfer from
one metal to another will be more efficient when the energy
gap between the metal orbital and organic fragment is narrowed
to allow enhanced charge delocalization. In the present series,
the X group, in one sense, serves to tune the relevant energy
levels (Scheme 1).
The subjects of our study are represented by the general
formula FcCHdCHXCHdCHFc (1-X), with X representing the
common range of linking groups used either to attach directly
to an electrode or between delocalized moieties. These consist
of X ) S (1-S), O (1-O) Se (1-Se), CH2 (1-CH2), (CH2)2 (1-
C2H4), CtC (1-C2), CHdCH (1-C2H2), PPh (1-PPh), P(dO)Ph
(1-POPh), P(dS)Ph (1-PSPh), CdO (1-CO), CH+ (1-CH+),
“none” (1-Ø; i.e., 1,4-diferrocenyl-1,3-butadiene) (see Figure
1).
Results
In examining the capability of the bridging moiety -CHd
CHXCHdCH- in supporting intramolecular electronic interac-
tions, our methods involve electrochemistry and near-infrared
(near-IR) absorption spectroscopy of the mixed-valence species.
If we consider a bis-ferrocenyl molecule for which the con-
necting bridge is insulating, then the half-wave potential of each
ferrocene is the same, save for an entropy effect,10 and we
observe a single two-electron wave. Conversely, an effective
bridge alters the second half-wave potential, and the difference
between the two potentials is an indication of the effectiveness
of the wire. In addition, intervalence electron transfer can be
detected and quantitatively studied from the characteristics of
the intervalence band of mixed-valence complexes. Basically,
the intervalence transition is considered as a special case of
charge transfer transition, the reduced site (Red) being the donor
group, while the oxidized site (Ox) is the acceptor group. The
mixing of the two electronic states Red-Ox and Ox-Red is due
to the electronic coupling term Vab, which is also responsible
for the intensity of the intervalence transition. Thus, Vab and
the derived delocalization parameter R can be determined by
the following formulas11 according to Hush’s theoretical
treatment:
Synthesis. The systems described below were synthesized by
a combination of Wittig (route A) and Horner-Emmons-
Wadsworth (HEW, route B) coupling reactions (Figure 2). Initial
syntheses with the Wittig approach produced mixtures of cis
and trans isomers, due to the nonstereoselectivity of the
conventional Wittig reaction, while the HEW approach produced
the trans isomers exclusively, as indicated by single-crystal
X-ray diffraction.12 Thus, assignment of the stereochemistry was
made straightforwardly by analysis of the NMR coupling
constants (Jtrans ≈ 15 Hz, Jcis ≈ 11 Hz) and confirmation by the
X-ray data of the crystallized samples.
Addition of 2 equiv of ferrocenecarboxaldehyde (FcCHO)
to the bis(ylide) of the appropriate bis(triphenylphosphonium-
methyl)-X dibromide, i.e., (BrPh3P+CH2)2X·Br-2, produced a
number of the desired compounds with reasonable yields (X )
CH2, (CH2)2, C2H2, S, O). In each case, a mixture of cis (Z)
and trans (E) isomers was obtained, as evidenced by NMR
spectroscopy. Separation of the isomers by column chromatog-
raphy or recrystallization proved very difficult and was only
partially successful for some of the compounds.
In contrast, the Horner-Wadsworth-Emmons (HEW) reac-
tion13 stereospecifically produced (E)-alkenes. In general, the
synthetic precursors, bis(diethylphosphinylmethylene) deriva-
tives ([(EtO)2P(O)CH2]2X), were prepared through the Arbuzov
reaction by refluxing a dibromide with triethyl phosphite. In
the HEW olefination step, heating was essential to allow the
reaction to proceed and to promote generation of double bonds
in E form.
Vab ) (2.05 × 10-2)(Vmax
ε
max∆V1/2
)
1/2/RMM
(1)
(2)
R ) Vab/νmax
where Vab is the coupling (cm-1), εmax is the maximum extinction
coefficient, νmax is the band position (cm-1), ∆ν1/2 is the full
width at half-maximum (cm-1), and RMM is the metal-metal
distance (Å).
When the X group was a heteroatom such as S or Se, the
HEW reactions proceeded smoothly at room temperature but
(9) Tolbert, L. M.; Zhao, X. J. Am. Chem. Soc. 1997, 119, 3253–3258.
(10) For an extended discussion, see: Flanagan, J. B.; Margel, S.; Bard,
A. J.; Anson, F. C. J. Am. Chem. Soc. 1978, 100, 4248–4253.
(11) (a) Hush, N. S. Coord. Chem. ReV. 1985, 64, 135–57. (b) Hush, N. S.
Prog. Inorg. Chem. 1967, 8, 391–444.
(12) Unless specifically indicated, the absence of stereochemical prefixes
means that the compounds were a mixture of E,E and E,Z isomers.
(13) (a) Horner, L.; Hoffmann, H. M. R.; Wippel, H. G. Ber. Bunsen-Ges.
Phys. Chem. 1958, 91, 61–63. (b) Wadsworth, W. S., Jr.; Emmons,
W. D. J. Am. Chem. Soc. 1961, 83, 1733–1738.
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10375

A R T I C L E S
Li et al.
Figure 1. Ferrocenes used in this study.
Figure 2. Synthesis of FcCHdCHXCHdCHFc.
Figure 4. Synthesis of 1-C2, 1-CH+, and 1-CO.
was conveniently obtained by the treatment of 1-PPh with 1
equiv of iodine and a trace of water in THF. These two
transformations could be easily observed by 1H NMR spectros-
copy, in which the para phenyl proton was noticeably shifted
to low field upon oxygenation or thionylation. Interestingly,
whereas the stereochemistry of 1-PSPh basically inherited that
of the antecedent compound 1-PPh, 1-POPh was produced
exclusively as the E,E adduct (see Figure 3).
Bis(2-ferrocenyl)hexa-1,5-dien-3-yne (1-C2) was prepared
through a modified Stephans-Castro reaction16 (Figure 4). (1-
Bromovinyl)ferrocene (FcCHdCHBr) was first prepared via
Wittig coupling of (bromomethyl)triphenylphosphonium bro-
mide17 with ferrocenecarboxaldehyde.18 Cuprous iodide was
added to a mixture of bis(triphenylphosphine)palladium dichlo-
ride and a diethylamine solution of FcCHdCHBr under an inert
atmosphere followed by passing a slow current of acetylene
gas. After workup and column chromatography, the symmetrical
disubstituted acetylene 1-C2 was obtained as a bright red solid
in very good yield (85%). The final product was characterized
Figure 3. Synthesis of 1-PPh, 1-PSPh, and 1-POPh.
only afforded an improved E,E stereochemistry (approximately
70%) in comparison with the corresponding Wittig reactions.
When the same reactions were conducted under reflux, however,
nearly 95% E,E adduct was observed, as evidenced by 1H NMR
spectroscopy. The pure E,E isomer could be conveniently
isolated by recrystallization from CH2Cl2/hexane (1:1). Our
efforts to use HEW reactions for the remaining compounds to
achieve exclusive E,E product were unsuccessful.
1
by H NMR spectroscopy as a mixture of E and Z isomers.
Finally, 1-CH+BArF- was prepared as described previously
19
for the tetrafluoroborate salt 1-CH+BF4
.
-
Whereas most of the bis(2-ferrocenylvinyl) compounds
prepared here were fairly stable both in the solid/liquid state
and in solution, there were a few exceptions. Fc(CHdCH)2Fc
(1-Ø) and Fc(CHdCH)3Fc (1-C2H2) were observed to have low
stability in solution. An unknown brick red solid was isolated
from solution after standing, which has been reported by
The preparation of the phosphorus-substituted ferrocenes used
a modification of Kobayashi’s procedure (Figure 3).14 Phe-
nylphosphine was added to 2 equiv of ethynylferrocene15 to
1
give the double 1:2 adduct 1-PPh in high yield. The H NMR
spectrum of the isolated product showed predominantly trans
vinylene protons with minor cis vinylene contributions. Stirring
this phenylphosphine derivative with excessive elemental sulfur
in THF under argon furnished the phenylphosphine sulfide
1-PSPh in high yield (Figure 3). The phosphine oxide 1-POPh
(16) (a) Sonogashira, K.; Tohda, Y.; N. H., Tetrahedron Lett. 1975, 4467–
4470. (b) Stephens, R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 3313–
3315.
(17) Driscoll, J. S.; Grisley, D. W.; Pustinger, J. V.; Harris, J. E.; Matthews,
C. N. J. Org. Chem. 1964, 29, 2427–2431.
(18) Barlow, S.; Henling, L. M.; Day, M. W.; Schaefer, W. P.; Green,
J. C.; Hascall, T.; Marder, S. R. J. Am. Chem. Soc. 2002, 124, 6285–
6296.
(14) Kobayashi, E.; Obata, T.; Asohima, S.; Furukawa, J. Polym. J. 1993,
25, 1049–1057.
(15) Luo, S. J.; Liu, Y. H.; Liu, C. M.; Liang, Y. M.; Ma, Y. X. Synth.
Commun. 2000, 30, 1569–1572.
(19) See: Zhao, X. Ph.D. Thesis, Georgia Institute of Technology, 1996
and the Supporting Information.
9
10376 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Diferrocenyl Molecular Wires
A R T I C L E S
Spangler as being due to decomposition or polymerization.20
Thus, during purification procedures, fast recrystallization or
flash chromatography was always performed.
As discussed, some of the compounds were obtained as E,Z
isomeric mixtures. For instance, 1-C2, 1-PPh, and 1-Ø contained
all three configurational isomers, as evidenced by NMR spec-
troscopy. The initial Wittig approach almost always afforded
E,Z isomeric mixtures, even though the E,E isomer was the
major component. Efforts to separate the isomers by slow
chromatography or recrystallization turned out to be either
extremely tedious or fruitless. Attempted isomerization to the
thermodynamically more stable products using conventional
methods, e.g., catalytic amounts of I2 or p-toluenesulfonic acid
led to decomposition. However, the presence of both E,Z
mixtures (Wittig) and E,E mixtures (HEW) allowed us to
unambiguously assign stereochemistry and compare the proper-
ties of pure isomers with those of the mixtures (see below). In
all cases, the electrochemical and spectroscopic properties were
indistinguishable. Additionally, our earlier study9 on the difer-
rocenyl complex bridged by a polymethine cation has revealed
that the synthetic precursor 1-CH2, a geometric mixture, was
converted to the thermodynamically favorable E,E cationic form
upon hydride extraction as shown:
Figure 5. Cyclic voltammograms of 1-S in a CH2Cl2 solution with
Bu4N+PF6- (top) or Bu4N+BArF- (bottom) as supporting electrolyte. Scan
rate: 50 mV/s.
Na+BArF- with Bu4N+Br- readily afforded the electrolyte
Bu4N+BArF-. This ammonium salt was recrystallized several
times from CH2Cl2/Et2O and passed through an alumina column
(CH2Cl2) prior to use.
In performing the electrochemistry, our major concern was
whether the stereochemistry would affect the electrochemical
behavior, leading to incorrect conclusions. Fortunately, the use
of both Wittig and HEW methodologies provided more than
five bis(ferrocenylvinyl) compounds as both isomeric mixtures
and E,E products. Furthermore, we also made both (E,E,E)- and
(E,Z,E)-1-C2H2. With this sample library at hand, we were able
to conduct comparative studies of the impact of stereochemistry
on the corresponding electrochemical behaviors. No electro-
chemical variations with the steric arrangement of the double
bonds were observed.26
An obvious question which arose from this lack of depen-
dence on stereochemistry was whether this reflected the
underlying electronics or simply an electron-transfer-mediated
isomerization during the cathodic process. To test this, a small
amount of the isomeric mixture of 1-S under an argon
atmosphere was first electrochemically exposed to a constant
potential 200 mV positive of the second oxidation wave of 1-S.
The brown solution was immediately exposed to a constant
potential 400 mV negative of the first oxidation potential. After
column chromatography, almost 90% of 1-S was recovered. The
NMR of the isolated product indicated exclusive formation of
the E,E form (see Figure 6).
With these controls in place, we obtained the polarographic
first and second oxidation potentials of all compounds 1-X and
report them in Table 1, using the Fc/Fc+ couple as reference.
This simplest member of the series, X ) CH2 (1-CH2), showed
a peak separation of 115 mV which disappeared upon extension
to X ) CH2CH2 (1-C2H4). In contrast, use of the triple bond
CtC (1-C2) as the connecting group led to a diminution of the
coupling relative to CHdCH (1-C2H2). Similarly, and surpris-
ingly, in the chalcogenide series O, S, and Se, oxygen was most
effective in maintaining conjugation, despite its higher oxidation
potential. Conversely, the electron-poor carbonyl chain (X )
CO, 1-CO) actually produced an effective bridge. Moreover,
Electrochemistry. Previous cyclic voltammetric studies in our
-
laboratory used conventional Bu4N+PF6-, BF4-, or ClO4 as
the supporting electrolyte.9 More recently, the use of less
-
-
nucleophilic counterions, B(C6F5)4 and B[3,5-C6H3(CF3)2]4
(BArF-), has been found to give wider peak separations than
the conventional anions.21,22 The improved electrochemistry
arises from the weakly coordinating nature of the bulky anions.23
Geiger and co-workers have performed extensive studies on the
fluoroarylborate anions; the reported larger peak separation was
ascribed to the low ion-pairing effect.24 In our hands, the BArF-
anion provided improved solubility and enhanced peak separa-
tions and was used for all the electrochemistry in this study.
As an example of the counterion effect, 1-S exhibited a single
two-electron oxidation wave (-50 mV vs Fc/Fc+) in a solution
-
of Bu4N+PF6 in CH2Cl2 but two well-separated one-electron
waves (∆E ) 180 mV) when the supporting electrolyte was
Bu4N+BArF- (see Figure 5). For these reasons, we selected
Bu4N+BArF- and the oxidatively more stable solvent CH2Cl2
as the electrochemical media throughout this work. The synthetic
precursor Na+BArF- was easily prepared from commercially
available 3,5-bis(trifluoromethyl)bromobenzene via a multiple-
step one-pot reaction in 90% yield.25 Ion metathesis of
(20) Ribou, A.-C.; Launay, J.-P.; Sachtleben, M. L.; Li, H.; Spangler, C. W.
Inorg. Chem. 1996, 35, 3735–3740.
(21) Barrie`re, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U. T.;
Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262–7263. (b) LeSuer,
R. J.; Geiger, W. E. Angew. Chem., Int. Ed. 2000, 112, 254–256.
(22) Olson, E. J.; Boswell, P. G.; Givot, B. L.; Yao, L. J.; Buehlmann, P.
J. Electroanal. Chem. 2010, 639, 154–160.
(23) Strauss, S. H. Chem. ReV. 1993, 93, 927–942.
(24) (a) Camire, N.; Mueller-Westerhoff, U. T.; Geiger, W. E. J. Orga-
nomet. Chem. 2001, 637-639, 823–826. (b) Barriere, F.; Geiger, W. E.
J. Am. Chem. Soc. 2006, 128, 3980–3989.
(25) (a) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith,
M. D. Inorg. Chem. 2001, 40, 3810–3814. (b) Bahr, S. R.; Boudjouk,
P. J. Org. Chem. 1992, 57, 5545–5547.
(26) A lone exception was when X was o-phenylene. This “exception which
proves the rule” will be discussed in a future paper.
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10377

A R T I C L E S
Li et al.
Figure 6. Electrochemical isomerization/equilibration of 1-S.
Table 1. Electrochemical Dataa, b (from Cyclic Voltammetry) for
Compounds 1-X
1-X
X
E1, mV
E2, mV
∆E, mV
Kc
1-Ø
none
CHdCH
CH+
CtC
CH2
CH2CH2
CdO
O
S
Se
PPh
P(S)Ph
P(O)Ph
-35
-80
-75
50
190
100
210c
175
90
225
180
285b
125
115
0
6300
1100
65100
130
1-C2H2
1-CH+
1-C2
Figure 7. Spectrum of the radical cation of 1-C2H4.
The mixed-valence spectra of bis-ferrocene compounds are
often complicated by the overlapping of the coexisting absorp-
tions arising from different types of electronic transitions in the
low-energy area. This is especially the case for the current
mixed-valence species herein due to the large variety of the
bridging ligands CHdCHXCHdCH employed. Therefore,
spectral deconvolution in the frequency mode is generally
required to resolve the discrete bands into Gaussian components.
Such curve fittings were conveniently fulfilled with the Origin
peak fitting module (PFM).29 This analysis is provided in more
detail in the Discussion.
1-CH2
1-C2H4
1-CO
1-O
1-S
1-Se
1-PPh
1-PSPh
1-POPh
-25
88
0
0
+110
-90
-50
+50
-20
+60
+140
235
110
130
125
160b
210
240
125
200
180
75
130
2400
1100
19
180b
150
100
1100b
340
50
a Conditions: electrolyte, Bu4N+BArF- (0.1 M); working electrode,
Pt; reference electrode, Ag/AgI, corrected to Fc; scan rate, 50 mV/s.
b Relative to Fc. c Irreversible oxidation wave.
One-electron oxidation of 1-CH2 and 1-C2H4 produced a
single new absorption at 775 nm (see Figure 7). Similarly,
1-POPh and 1-PSPh produced new absorptions at 710 and 760
nm, also with good isosbestic points (see Figure 8). In contrast,
the chalcogenides 1-S and 1-Se exhibited longer wavelength
absorptions, with λmax values of 1090 and 1070 nm, respectively
(Figure 9).
Finally, for the purposes of comparison, the radical cation
spectra of 1-Ø and 1-C2H2, previously reported by Spangler
and co-workers,20 were recorded (Figure 10). These are es-
sentially identical with those reported.
Crystal Structures. Single crystals of the linear hydrocarbon
and heteroatom-incorporated molecules proved to be very difficult
to obtain. Aside from that, relatively low solubility (e.g.,
Fc(CHdCH)2Fc and Fc(CHdCH)3Fc), poor solid morphology due
to stereoisomeric mixtures (e.g., 1-PPh), and flexible backbones
could also account for the poorly organized features for compounds
of this kind. For these reasons, (E,E)-1-S and (Z,Z)-1-PSPh were
the only linearly bridged compounds providing crystals suitable
for X-ray crystallographic analysis. The relevant Fe-Fe distances,
both through-bond and through-space, are provided in Table 2,
and ORTEP plots are given in Figure 11.
The most remarkable structural feature of the Z,Z isomer is
its pseudo-coplanarity along the conjugation path, including the
conjugated rings Cp1 and Cp2 attached to the vinyl groups.
The disposition of the two ferrocenes is anti. The phenylene
ring attached to the phosphorus atom is nearly perpendicular to
the bridging plane and takes a face-to-edge orientation to one
ferrocenyl group. This steric arrangement certainly has important
consequences in minimizing the steric conflict between the aryl
and metallocenyl groups and contributes to the nondistorted
conjugation pathway. While most of the bond lengths and angles
due to the lack of reversible oxidation waves, we exclude 1-PPh
from further discussion.
Using the equilibrium [Fc-B-Fc] + [Fc-B-Fc]2+ h 2 [Fc-B-
Fc]+, we were able to calculate the conproportionation constant
Kc for the radical cations in each case. These are also entered
in Table 1.
Spectroscopy. In order to achieve partially oxidized substrates
to compare with our electrochemical results, we found chemical
oxidation to be more convenient, to be easier to manipulate,
and to provide more reproducible results. Thus, we adapted this
method for all the mixed-valence absorption studies, using the
data provided by Connelly and Geiger.27 We determined that
-
-
Fc+PF6 and Ar3N•+SbCl6 (TBA or “magic blue,” Ar )
p-BrC6H4) were very good oxidation reagents for the diferro-
cenyl compounds, especially Fc+PF6-. The mild oxidation
strength of Fc+PF6 (0.0 V vs Fc/Fc+) would oxidize the first
-
ferrocene moiety but not the second one for most compounds,
which had a negatively shifted first oxidation and a higher
second oxidation potential than the Fc/Fc+ couple. When a
stronger oxidant was required, TBA was used (0.76 V vs Fc/
Fc+). It is noteworthy that both the oxidants and their reduced
forms did not have absorptions in the low-energy region which
could have interfered with the mixed-valence spectra. The
general procedure is described in the Supporting Information.
For weak coupling compounds, 0.1-1.0 equiv of oxidant was
used to minimize the possible disproportionation. Fc+PF6
-
absorbs at ca. 622 nm with an extinction coefficient of 374 cm-1
M-1,28 which is essentially negligible compared to the absorption
of the ferrocenium moiety of the mixed-valence species herein
and the low-energy absorptions in the near-IR region.
(27) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877–910.
(28) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1971,
93, 3603–3612.
(29) http://www.mpassociates.gr/software/distrib/science/microcal/pfm.html.
9
10378 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Diferrocenyl Molecular Wires
A R T I C L E S
Figure 8. Spectral evolution during one-electron oxidation of 1-POPh (left) and 1-PSPh (right).
Figure 9. Absorption spectra of the monocationic species of 1-S (left) and 1-Se (right).
Figure 10. Absorption spectra of the monocationic species of 1-Ø and 1-C2H2.
Table 2. Selected Steric Parameters for Bis(2-ferrocenylvinyl)
angle (100.5°) is smaller than expected, resulting in a shorter
C(12)-C(13) through-space distance (2.77 Å), analogous to that
of 1-S.
Compounds
1-S
1-PSPh
vinyl,vinyl
Fc to Fc
Fe-Fe (space), Å
Fe-Fe (bond), Å
Cp1-Cp2 (dihedral), deg
E,E
Z,Z
Discussion
anti
9.77
13.1
52.7
anti
9.43
13.3
3.16
Our original objective was to control the conjugation pathway
by using a (trans) vinylidene bridge between the ferrocene and
the heteroatom to maintain consistency among the various
molecules. Although we were unable to obtain sufficient X-ray
data to confirm this consistency, it is useful to note that both
the through-space and through-bond Fe-Fe distance for 1-S
and 1-PSPh are almost identical, despite having quite different
lie in the normal range, the C(10)-C(11)-C(12) and C(13)-
C(14)-C(15) angles are 133.8 and 133.0°, respectively, which
helps to maintain coplanarity. In contrast, the C(12)-P(1)-C(13)
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10379

A R T I C L E S
Li et al.
13. The major broad Gaussian component is apparently ascribed
to the IVCT transition between the metal centers; the adjacent
weak band centered at ca. 8000 cm-1 (1250 nm) is assigned to
the LMCT derived from the electron transition from the bridging
ligand to the FeIII d orbital. Another weak band appearing at
higher energy around 10 000 cm-1 (1000 nm) was necessary
for good spectral fits and has been noticed in other diferrocenyl
systems.20 Unfortunately, the literature is somewhat deficient
in this area, because of the fact that nearly all the relevant reports
associated with ferrocenium complexes cut the low-energy
absorption spectrum at 2200 nm, presumably avoiding the strong
solvent interferences depicted above. However, it has been well
established that the RuIII and OsIII complexes often demonstrate
interconfigurational (IC) transition bands derived from the dπ
f dπ transitions between the Kramers doublets,31 as illustrated
in Figure 12. The IC bands are nominally parity or LaPorte
forbidden but gain intensity through spin-orbit coupling and
metal-ligand mixing. They appear in the near-IR (between 4000
to 6000 cm-1) as narrow (∆ε1/2 < 1000 cm-1), relatively weak
bands,32 which are almost the same characteristics as the band
we observed in the ferrocenium complexes. Therefore, we
tentatively assigned this band, centered at 2400 nm, to an IC
transition.
While group A includes compounds not showing the IVCT
band in the mixed-valence state, group B contains those
exhibiting an IVCT transition. The former compounds clearly
belong to class I under the Robin-Day classification,33 which
are quasi-insulating systems. For group B compounds, the
strengths of electronic coupling could be analyzed from their
spectral parameters by the Hush formula12 (eqs 1 and 2).
Group A includes compounds in which X ) P(dO)Ph,
P(dS)Ph, CH2, CH2CH2, CdO. It basically contains nearly all
of the linearly “interrupted” compounds, which do not have
favorable resonance forms for supporting electron transfer. The
sample spectra are shown in Figures 8 and 9, where Figure 8
demonstrates the spectral evolution of 1-POPh and 1-PSPh
upon progressive oxidation with 0.2-2.0 equiv of oxidant. As
can be seen, for each mixed-valence species, only a weak low-
energy band with λmax falling in the range of ca. 770-1280 nm
is revealed, which is assigned to the bridging ligand to FeIII
LMCT transition. The spectral data are collected in Table 3.
Obviously, the absorption maxima and intensities of the LMCT
bands are both associated with the extent of conjugation and
electron density on the bridging ligand. Therefore, the LMCT
band of 1-S+ (Figure 9a) appears at the longest wavelength (λmax
1275 nm) with the greatest intensity (εmax ) 1750 L M-1 cm-1)
due to the longest conjugation in this series. In contrast, the
corresponding εmax of 1-POPh+ is very low (300 L M-1 cm-1),
due to the low electron density on the bridging ligand caused
by the electron-withdrawing PdO group (Table 3). For each
case of the group A compounds, except the LMCT band, no
additional absorption, i.e. an IVCT band, is observed in the near-
IR, which implies that this group of compounds has rather
insulating bridges. This conclusion seems inconsistent with the
electrochemical results, where most of the compounds demon-
strated a peak separation, implying the existence of electronic
Figure 11. ORTEP plots of (E,E)-1-S (top) and (Z,Z)-1-PSPh (bottom).
couplings. The subject of through-space vs through-bond
electronic coupling has been the subject of much study.30
However, the peak separation (∆E), which is routinely used to
address the degree of electronic coupling between the metal
centers, must be a result of a through-bond effect. When no
obvious distortion is present in the bridging fragment, the E,Z
configurational difference should play a very small role under
such a through-bond mechanism. In this context, it is obviously
significant whether we consider distances in terms of metal-metal
distances or the ferrocene C atoms at the attachment point. Since
ferrocene ground states are characterized by metal-centered
HOMOs, we prefer to use the former. This is also consistent
with the Hush formalism. Use of the latter would change the
distances considered but not the conclusions.
According to the characteristics of their long-wavelength
absorptions, the FcCHdCHXCHdCHFc compounds can be
divided into two groups: A and B. Group A consists of those
molecules (1-CH2, 1-C2H4, 1-PSPh, 1-POPh) with short-
wavelength (<900 nm) absorptions. These absorptions are very
close to the absorptions of ferrocenium itself (675 nm); therefore,
we assign those absorptions to a metal-centered interconfigu-
rational (IC) d-d absorption. To deal with the more complex
absorptions for the other molecules, we adopt the treatment of
Spangler et al.20 for the hydrocarbon-bridged compounds 1-Ø
and 1-C2H2. For such mixed-valence species, several absorption
bands are observed in the visible and near-IR region, including
Cp-to-FeIII charge transfer, bridging ligand π-π* transition,
bridging ligand to metal charge transfer (LMCT), intervalence
charge transfer (IVCT), and the aforementioned IC transition.
In contrast to the simplicity of the visible bands, the
absorptions in the near-IR are not straightforward (the vertical
lines appearing beyond 2200 nm are from solvent absorption
exceeding the compensation capacity of the double-beam
spectrophotometer). The broad envelope in the near-IR is
deconvoluted using four Gaussians as shown in Figures 12 and
(31) (a) Sen, J.; Taube, H. Acta Chem. Scand., Ser. A 1979, 33, 125–135.
(b) Hill, N. J. J. Chem. Soc., Faraday Trans. 2 1972, 68, 427–434.
(32) (a) Demadis, K. D.; El-Samanody, E.-S.; Coia, G. M.; Meyer, T. J.
J. Am. Chem. Soc. 1999, 121, 535–544. (b) Kober, E. M.; Goldsby,
K. A.; Narayana, D. N. S.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105,
4303–4309.
(30) Sun, D.-L.; Rosokha, S. V.; Lindeman, S. V.; Kochi, J. K. J. Am.
Chem. Soc. 2003, 125, 15950–15963.
(33) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247–
422.
9
10380 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Diferrocenyl Molecular Wires
A R T I C L E S
Figure 12. Deconvoluted spectra of (a) Fc(CHdCH)2Fc+ (b) and Fc(CHdCH)3Fc+.
with the different experimental conditions used in developing
and measuring the mixed-valence species: i.e., spectroelectro-
chemical20 and chemical (this work) oxidation. First, the IVCT
νmax (or λmax) values of the current results are shifted (ca. 300
cm-1) to lower energy. It is well-known that, according to
Marcus theory for outer-sphere electron transfer,11b,35 the IVCT
bands can be altered significantly by external factors such as
solvent, ion pairing, concentration of counterion, temperature,
etc. That is, λIVCT ) λin + λout, where λin and λout are the inner-
and outer-sphere reorganization energies in the light-induced
electron transfer process, respectively. Most likely, the red shifts
of the current IVCT bands come from the low concentration of
-
PF6 (ca. 1-3 × 10-4 M, essentially the same as that of the
mixed-valence species) present, in comparison to the high
concentration of counterion (0.1 M Bu4N+PF6-) used in
electrospectroscopic measurements. Second, the calculated εmax
of IVCT is at least 25% greater than the literature result due to
the advantages of chemical oxidation over electrochemical
oxidation in moderating the underestimation of εmax as discussed
earlier. This in turn contributed to the higher Vab and R values.
Interestingly, the mixed-valence absorption of 1-S+ in the
near-IR (Figure 9) is much broader than a normal LMCT band.
Whereas the predominant absorption centered at ca. 1090 nm
is clearly ascribed to a LMCT transition, there is a noticeable
band buried by the tailing segment of the LMCT band, which
indeed possesses the IVCT feature upon spectral deconvolution.
Although this IVCT band is relatively weak in terms of both
intensity (εmax ) 440 L M-1 cm-1) and calculated electronic
coupling constant (Vab ) 202 cm-1), it is the first example
demonstrating that a heteroatom can also serve as the supporting
element for electron transfer in the context of a photodriven
process. Thus, 1-S belongs to group B. On the basis of this, we
expect that 1-PPh+ and 1-O+ should also present IVCT bands
similar to that of [1-S]+, since both compounds showed even
greater ∆E values in electrochemistry. Unfortunately, these two
mixed-valence species are chemically unstable, and no absorp-
tions could be observed in the low-energy area.
Figure 13. Spectral decomposition of the 1-S+ absorption spectrum.
Table 3. LMCT Band Parameters, Obtained from the Spectral
Deconvolution, of the Mixed-Valence Species of Group A
Compounds
compd
ν
max, cm-1
λ
max, nm
ε
max, M-1 cm-1
∆ν, cm-1
[1-CH2]+
12 850
12 850
13 980
13 430
778
778
715
745
420
420
300
230
3020
3020
2630
2670
[1-C2H4]+
[1-POPh]+
[1-PSPh]+
coupling. However, it is important to realize that interpretation
of ∆E data is less straightforward than that of IVCT data because
the former depends on stabilizing factors in the FeII/FeII, FeII/
FeIII, and FeIII/FeIII species, on through-space electrostatic and
through-bond inductive effects, and on solvation.34 The lack of
IVCT band in the mixed-valence species above suggests that
there is no intrinsic correlation between ∆E value and interva-
lence transition.
In addition to 1-S and 1-Se, group B also includes compounds
in which X ) “-”, CHdCH, i.e., systems with π conjugation.
The mixed-valence spectra for the latter are shown in Figure
12, and the spectroscopic data obtained after spectral decon-
volution are gathered in Table 4. Fc(CHdCH)2Fc (1-Ø) and
Fc(CHdCH)3Fc (1-C2H2) have been previously studied by
Spangler and co-workers.20 The reported results are also
included in Table 4. In comparison with the literature, our
current data present two major differences, which we associate
Finally, it is worth mentioning that the appearance, band
shape, absorption maximum, and intensity of all the bands
discussed above are very case-dependent for the series of mixed-
valence species herein. The IVCT transition is of the most
interest, since it provides the spectral information associated
with the strength of the electronic interaction. However, we must
caution that the assignments remain somewhat tentative.
(34) Nelsen, S. F.; Trieber, D. A., II; Ismagilov, R. F.; Teki, Y. J. Am.
Chem. Soc. 2001, 123, 5684–5694.
(35) Marcus, R. A. J. Chem. Phys. 1965, 43, 679–701.
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10381

A R T I C L E S
Li et al.
Table 4. IVCT and LMCT Band Parameters, Obtained from the Spectral Deconvolution, and Calculated Hush Parameters for the
Mixed-Valence Species of Group Ba Compounds
b
,
d
compd
ν
max, cm-1
λ
max, nm
ε
max, M-1 cm-1
∆ν, cm-1
Rmm
Å
Vab,c cm-1
R
Fc(CHdCH)2Fc+ (1-Ø+)
5290
5500
5660
6010
6020
9257g
5270
9265g
1960
1820
1770
1660
1660
1080
1900b
1079
2280
1570
2590
2100
440
965
160
3900
4340
4100
3800
3640
4176
4200
4330
9.21f
9.21f
483
430
436
390
206
0.091
Fc(CHdCH)2Fc+
0.078
0.077
0.065
0.034
0.034
e
11.54f
11.54f
9.78
+
Fc(CHdCH)3Fc+ (1-C2H2
)
Fc(CHdCH)3Fc+
e
[1-S]+
[1-Se]+
h
h
990
a See text. b Spatial iron-iron distance obtained from the crystal structure if not otherwise stated. c Hush coupling constant V ) (2.06 ×
10-2)(εmax∆v1/2νmax
)
1/2/Rmm
.
d Delocalization efficient R ) Vab/νmax
.
e Obtained from ref 20. f Obtained from theoretical calculations. g LMCT transition as
described in text. h Not determined.
and the carbonyl group is equivalent to a triple bond. Phos-
phorus, in contrast, overlaps these two groups, with a modest
preference for the phosphine sulfide. We suspect that high-level
calculations are required to unravel this distinction. Again, we
cannot distinguish the effect of the heteroatom itself from its
+
Figure 14. Homoconjugation in 1-CH2
.
associated solvation shell, but we do not expect the latter to
change this order, given the subtle differences in structure among
the various diferrocenes.
With these observations, it is possible to discuss the transmis-
sion characteristics of the individual moieties X for a charge
localized on the end of the molecule. The differences in first
and second oxidation potentials, ∆Eox, have often been used as
measures of the attenuation factor. As Nelsen has pointed out,
such measures can be complicated by screening due to solva-
tion.34 Indeed, we ourselves have observed a significant differ-
ence in ∆E as a function of counterion, and we cannot conclude
that the BArF group leads to elimination of solvation effects
on the cation. Thus, the ∆Eox we obtain may be attenuated by
such effects. Nevertheless, within this approximation, ∆Eox
becomes a first-order measure of that attenuation factor (see
Table 1). For instance, lengthening the chain from butadiene
(1-Ø, ∆Eox ) 225 mV) to hexatriene (1-C2H2, ∆Eox ) 180 mV)
reduces the coupling by 45 mV, while an ethane bridge (1-
C2H4) reduces the coupling to 0. In contrast, a triple bond (1-
C2, ∆Eox ) 1250 mV) has a 100 mV attenuation and thus is a
much poorer coupling moiety. Similarly, a CdO group, while
increasing both oxidation potentials, maintains a significant
coupling, suggesting simple mixing of lower-lying orbitals may
not be a significant factor. Somewhat surprisingly, the methylene
bridge (1-C2H2, ∆Eox ) 115 mV) retains significant coupling.
This molecule is reminiscent of “interrupted” polyacetylene36
and suggests that some homoconjugation is possible for the
radical cation (see Figure 14).37
The near-IR absorption wavelengths mirror these trends.
Molecules with significant IVCT bands show high transmission
characteristics, although oxygen and sulfur, with only LMCT
charge-transfer characteristics, are also effective. We associate
the presence of an IVCT band with a superexchange mechanism,
while the LMCT band indicates the presence of a discrete
ligand-centered radical-ion state and the possibility of hopping.
In the latter case, oxidation of the iron center is a dynamic
process which our electrochemical measurements cannot resolve.
Conclusions
These studies present validation for the use of sulfur as an
electrically equivalent moiety for charge transport in unsaturated
chains, although oxygen has superior transmission characteris-
tics, presumably because of its readily accessible 2p electrons.
In contrast, the triple bond exhibits a severe attenuation, which
makes its extensive use in molecular wires problematic. In a
forthcoming paper, we will examine the role of aromatic and
heteroaromatic rings in such systems.
Acknowledgment. We thank the U.S. Department of Energy
through Grant No. FG05-85ER45194 for continuing financial
support.
For heteroatoms, we see that oxygen (1-O, ∆Eox ) 180 mV)
retains the most coupling, diminishing in the order O < S < Se,
with oxygen having somewhat better transmission characteristics
than a double bond, while sulfur is equivalent to a double bond
Supporting Information Available: Text giving experimental
information, including synthetic details, characterization data,
and details of spectroscopic manipulations and acquisition, and
CIF files giving crystallographic data for (E,E)-1-S and (Z,Z)-
1-PSPh. This material is available free of charge via the Internet
at http://pubs.acs.org.
(36) Surjan, P. R.; Kuzmany, H. Phys. ReV. B: Condens. Matter Mater.
Phys. 1986, 33, 2615–24.
(37) For leading references, see: (a) Paddon-Row, M. N.; Shephard, M. J.;
Jordan, K. D. J. Phys. Chem. 1993, 97, 1743–1745. (b) Weng, H.;
Du, X.-M.; Roth, H. D. J. Am. Chem. Soc. 1995, 117, 135–140.
JA101585Z
9
10382 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010