Synthesis of a series of diiron complexes based on a tetraethynylethene skeleton and related C6-enediyne spacers, (dppe)Cp*Fe- C≡CC(R)=C(R)C≡C-FeCp*(dppe): Tunable molecular wires

Article abstract of DOI:10.1021/om060403t

A series of 3,4-disubstituted (hex-3-ene-1,5-diyne-1,6-diyl)diron complexes with FeCp*(dppe) (Fe) end caps, Fe-C≡CC(R)=C(R)C=C-Fe (R = H, C≡C-SiMe₃, C≡C-H, C₆H₅, p-C ₆H₄CF₃), and the related (octa-3,5-diene-1,7- diyne-1,8-diyl)diron complex, Fe-C≡C-C(H)=C(H)C(H)=C(H)C≡C-Fe, has been prepared, and their performance as molecular wires has been evaluated. The enyne complexes have been synthesized via vinylidene intermediates, [Fe=C=C(H)C(R)=C(R)C(H)=C=Fe]²⁺, derived from the corresponding terminal alkynes or the Me3Si-protected precursors (XC≡CCC(R)=C(R)C=CX; X = H, SiMe₃), and the products have been characterized spectroscopically and crystallographically. The performance of the obtained dinuclear enyne complexes as molecular wires has been evaluated on the basis of the comproportionation constants (K_C) obtained by electrochemical measurements and the Vab values obtained from the spectral parameters of the intervalence charge transfer bands of the isolated monocationic radical species appearing in the near-IR region. As a result, the Cg-enediyne complexes turn out to be excellent molecular wires, with K_C values larger than 10 ⁸ as well as V_ab values larger than 0.35, belonging to class III compounds according to the Robin and Day classification and being comparable to the related polyynediyl complexes as well. It is notable that, in the enyne system, the performance can be readily tuned by introduction of appropriate substituents onto the olefinic part. Thus, diiron complexes containing an enyne spacer can be regarded as tunable molecular wires.

Full text of DOI:10.1021/om060403t

Organometallics 2006, 25, 5261-5275
5261
Synthesis of a Series of Diiron Complexes Based on a
Tetraethynylethene Skeleton and Related C6-Enediyne Spacers,
(dppe)Cp*Fe-CtCC(R)dC(R)CtC-FeCp*(dppe): Tunable
Molecular Wires§
Munetaka Akita,*,† Yuya Tanaka,† Chie Naitoh,† Takehiro Ozawa,† Nobuhiko Hayashi,†
Makoto Takeshita,† Akiko Inagaki,† and Min-Chul Chung‡
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan, and Department of Chemical Engineering, Sunchon National UniVersity,
315 Maegok-dong, Sunchon, Jeonnam 540-742, Republic of Korea
ReceiVed May 11, 2006
A series of 3,4-disubstituted (hex-3-ene-1,5-diyne-1,6-diyl)diron complexes with FeCp*(dppe) (Fe)
end caps, Fe-CtCC(R)dC(R)CtC-Fe (R ) H, CtC-SiMe3, CtC-H, C6H5, p-C6H4CF3), and the
related (octa-3,5-diene-1,7-diyne-1,8-diyl)diron complex, Fe-CtC-C(H)dC(H)C(H)dC(H)CtC-Fe,
has been prepared, and their performance as molecular wires has been evaluated. The enyne complexes
have been synthesized via vinylidene intermediates, [FedCdC(H)C(R)dC(R)C(H)dCdFe]2+, derived
from the corresponding terminal alkynes or the Me3Si-protected precursors (XCtCCC(R)dC(R)CtC-
X; X ) H, SiMe3), and the products have been characterized spectroscopically and crystallographically.
The performance of the obtained dinuclear enyne complexes as molecular wires has been evaluated on
the basis of the comproportionation constants (KC) obtained by electrochemical measurements and the
V
ab values obtained from the spectral parameters of the intervalence charge transfer bands of the isolated
monocationic radical species appearing in the near-IR region. As a result, the C6-enediyne complexes
turn out to be excellent molecular wires, with KC values larger than 108 as well as Vab values larger than
0.35, belonging to class III compounds according to the Robin and Day classification and being comparable
to the related polyynediyl complexes as well. It is notable that, in the enyne system, the performance can
be readily tuned by introduction of appropriate substituents onto the olefinic part. Thus, diiron complexes
containing an enyne spacer can be regarded as tunable molecular wires.
Scheme 1
Introduction
Dinuclear metal complexes connected by a linear π-conju-
gated bridging ligand have attracted increasing attention because
of their potential utility as molecular wires, which are indis-
pensable components for molecular electronics.1,2 In particular,
those with redox-active metal termini have been most exten-
sively studied. The electron-donating metal end caps can
stabilize the radical species resulting from oxidation, and their
performance as wires is conveniently evaluated in terms of their
electrochemical properties.3 Of the variety of structural motifs
of the linker studied so far, polyynediyl bridges ((CtC)n (An);
Scheme 1) have proven to be excellent, as revealed by the
§ This paper is dedicated to Professor John R. Shapley on the occasion
of his 60th birthday, in recognition of his outstanding contributions to
organometallic chemistry.
extensive studies by Lapinte on the FeCp*(diphosphine) com-
plexes (Cp* ) η5-C5Me5)4 and by other researchers on other
metal complexes.5-7 On the other hand, we have studied An-
type polyynediyldiiron complexes, Fp*-(CtC)n-Fp* (Fp* )
FeCp*(CO)2), from a different point of view, and we revealed
that a series of this type of compound with up to six CtC units
(C12) provided versatile precursors for polycarbon cluster
compounds.8 Taking into consideration these situations, we
turned our attention to higher order systems, i.e., two- and three-
dimensional communication systems, where diversity in the
structural motifs and the direction of the intermetallic com-
munication are enriched.
* To whom correspondence should be addressed. E-mail: makita@
res.titech.ac.jp.
† Tokyo Institute of Technology.
‡ Sunchon National University.
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10.1021/om060403t CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/23/2006

5262 Organometallics, Vol. 25, No. 22, 2006
Akita et al.
Scheme 2
Typical structural motifs for the core part of the two-
dimensional (2D) systems involve olefin and benzene frame-
works. Although their permetalated derivatives (M2CdCM2 and
C6M6) would be suitable for the study of intermetallic com-
munication, very few examples of such derivatives have been
reported so far. Previously we reported permetalated ethene
derivatives (B0; Scheme 1)8c,d,9 but the metal centers are
connected by additional metal-metal bonds, through which the
metal centers can also communicate. The lack of permetalated
derivatives without metal-metal bonds can be attributed to the
bulky metal fragment, which prevents multiple metalation of
the central carbon core. This disadvantageous point could be
overcome through insertion of a π-conjugated linker (e.g.,
acetylene unit) as shown in Scheme 1 to separate the metal
centers, as typically exemplified for the ethene derivatives (Bn
(n > 0)). The organic systems of the core parts of Bn (e.g. n )
1: tetraethynylethene (TEE)) and the benzene counterparts have
(4) (a) Paul, F.; Lapinte, C. Coord. Chem. ReV. 1998, 178, 431. (b) Le
Narvor, N.; Toupet, L.; Lapinte, C. Chem. Commun. 1993, 357. (c) Le
Narvor, N.; Toupet, L.; Lapinte, C. J. Am. Chem. Soc. 1995, 117, 7129. (d)
Le Narvor, N.; Lapinte, C. C. R. Acad. Sci., Ser. IIc: Chim. 1998, 1, 745.
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(f) Coat, F.; Guillevic, M. A.; Toupet, L.; Paul, F.; Lapinte, C. Organo-
metallics 1997, 16, 5988. (g) Paul, F.; Meyer, W. E.; Toupet, L.; Jiao, H.
J.; Gladysz, J. A.; Lapinte, C. J. Am. Chem. Soc. 2000, 122, 9405. (h) Jiao,
H. J.; Costuas, K.; Gladysz, J. A. Halet, J.-F.; Guillemot, M.; Paul, F.;
Lapinte, C. J. Am. Chem. Soc. 2003, 125, 9511. (i) Bruce, M. I.; Ellis, B.
G.; Gaudio, M.; Lapinte, C.; Melino, G.; Paul, F.; Skelton, B. W.; Smith,
M. E.; Toupet, L.; White, A. H. Dalton Trans. 2004, 1601. (j) Bruce, M.
I.; Costuas, K.; Davin, T.; Ellis, B. G.; Halet, J.-F.; Lapinte, C.; Low, P. J.;
Smith, M. E.; Skelton, B. W.; Toupet, L.; White, A. H. Organometallics
2005, 24, 3864. (k) Coat, F.; Lapinte, C. Organometallics 1996, 15, 477.
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Organomet. Chem. 2003, 683, 368. (m) Le Narvor, N.; Lapinte, C.
Organometallics 1995, 14, 634. (n) Weyland, T.; Lapinte, C.; Frapper, G.;
Calhorda, M. J.; Halet, J. F.; Toupet, L. Organometallics 1997, 16, 2024.
(o) Weyland, T.; Costuas, K.; Mari, A.; Halet, J. F.; Lapinte, C. Organo-
metallics 1998, 17, 5569. (p) Weyland, T.; Costuas, K.; Toupet, L.; Halet,
J. F.; Lapinte, C. Organometallics 2000, 19, 4228. (q) de Montigny, F.;
Argouarch, G. Costuas, K.; Halet, J. F.; Roisnel, T.; Toupet, L.; Lapinte,
C. Organometallics 2005, 24, 4558. (r) Ibn, G. S.; Paul, F.; Toupet, L. J.
Am. Chem. Soc. 2006, 128, 2463. (s) Le Stang, S.; Paul, F.; Lapinte, C.
Organometallics 2000, 19, 1035. (t) Roue, S.; Lapinte, C.; Bataille, T.
Organometallics 2004, 23, 2558. (u) Bruce, M. I.; de Montigny, F.; Jevric,
M.; Lapinte, C.; Skelton, B. W.; Smith, M. E.; White, A. H. J. Organomet.
Chem. 2004, 689, 2860. (v) Bruce. M. I.; Low, P. J.; Hartl, F.; Humphrey,
P. A.; de Montigny, F.; Jevric, M.; Lapinte, C.; Perkins, G. J.; Roberts, R.
L.; Skelton, B. W.; White, A. H. Organometallics 2005, 24. 5241. (w)
Guillaume, V.; Mahias, V.; Mari, A.; Lapinte, C. Organometallics 2000,
19, 1422. (x) Le Stang, S.; Paul, F.; Lapinte, C. Inorg. Chim. Acta 1999,
291, 403. (y) Le Stang, S.; Lenz, D.; Paul, F.; Lapinte, C. J. Organomet.
Chem. 1999, 572, 189. (z) Denis, R., Weyland, T., Paul, F.; Lapinte, C. J.
Organomet. Chem. 1997, 546, 615.
been studied extensively by Diederich10 and Vollhardt,11
respectively, and their chemical properties and extension to
higher systems have also been their research subjects. However,
very few studies on their metal complexes have been reported
so far.12
Herein we disclose results of our attempts at the preparation
of B1 and B2 type olefinic complexes bearing the FeCp*(dppe)
fragments (abbreviated as Fe throughout this paper). Although
the attempted preparation of 2D complexes has been unsuc-
cessful, we instead obtained a series of (hex-3-ene-1,5-diyne-
1,6-diyl)diiron complexes C. In comparison to the related A3-
type system consisting only of CtC units, which cannot be
functionalized, the properties of the C-type complexes can be
modified or tuned by introducing appropriate substituents (R)
onto the olefinic part, as verified by the present study.
Results and Discussion
Synthesis of Diiron Complexes with π-Conjugated Spac-
ers. (i) Feasible Synthetic Routes to Two-Dimensional (2D)
Systems. In principle, two routes (a and b) are feasible for the
construction of two-dimensional (2D) systems with FeCp*(dppe)
fragments (D), and the routes involve Fe-Ct or tC-C(sp2)
bond formation at the last stage (Scheme 2; the routes are
exemplified for a poly(ethynyl) derivative). Route a involves
deprotonation of the cationic vinylidene intermediate H, which
can be generated by interaction of the labile solvated cationic
(5) (a) Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J.-F.; Best, S. P.;
Heath, G. A. J. Am. Chem. Soc. 2000, 122, 1949. (b) Bruce, M. I.; Kelly,
B. D.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2000, 604, 150.
(c) Bruce, M. I.; Ellis, B. G.; Low, P. J.; Skelton, B. W.; White, A. H.
Organometallics 2003, 22, 3184. (d) Bruce, M. I.; Smith, M. E.; Skelton,
B. W.; White, A. H. J. Organomet. Chem. 2001, 637-639, 484.
(6) (a) Brady, M.; Weng, W.; Zhou, Y.; Seyler, J. W.; Amoroso, A. J.;
Arif, A. M.; Bohme, M.; Frenking, G.; Gladysz, J. A. J. Am. Chem. Soc.
1997, 119, 775. (b) Bartik, T.; Weng, W.; Ramsden, J. A.; Szafewrt, S.;
Falloon, S. B.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. Soc. 1998, 120,
11071. (c) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A.
J. Am. Chem. Soc. 2000, 122, 810. (d) Zheng, Q.; Gladysz, J. A. J. Am.
Chem. Soc. 2005, 127, 10508.
(10) See, for example: Tykwinski, R. R.; Diederich, F. Liebigs Ann./
Recl. 1997, 649. Diederich, F. Chem. Commun. 2001, 219. Diederich, F.
Chem. Rec. 2002, 2, 189.
(7) (a) Touchard, D.; Dixneuf, P. H. Coord. Chem. ReV. 1998, 178-
180, 409. (b) Fernandez, F. J.; Venkatesan, K.; Blacque, O.; Alfonso, M.;
Schmalle, H. W.; Berke, H. Chem. Eur. J. 2003, 9, 6192. (c) Ren, T.
Organometallics 2005, 24, 4854.
(8) (a) Akita, M.; Moro-oka, Y. Bull. Chem. Soc. Jpn. 1995, 68, 420.
(b) Akita, M.; Sakurai, A.; Chung, M.-C.; Moro-oka, Y. J. Organomet.
Chem. 2003, 670, 2. (c) Akita, M.; Sugimoto, S.; Tanaka, M.; Moro-oka,
Y. J. Am. Chem. Soc. 1992, 114, 7581. (d) Akita, M.; Sugimoto, S.;
Hirakawa, H.; Kato, S.; Terada, M.; Tanaka, M.; Moro-oka, Y. Oganome-
tallics 2001, 20, 1555.
(11) Diercks, R.; Armstrong, J. C.; Boese, R.; Vollhardt, K. P. C. Angew.
Chem., Int. Ed. Engl. 1986, 25, 270. Boese, R.; Green, J. R.; Mittendorf,
J.; Mohler, D. L.; Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1992,
31, 1643.
(12) As for the TEE system, Che reported synthesis and luminescence
properties of the Au complexes, C2(CtC-Au(PR3))4 and Low14c reported
µ-η2:η2-Co2 adducts: Lu, W.; Zhu, N.; Che, C. M. J. Organomet. Chem.
2003, 670, 11. For TEE-Pt2 complexes, see: Siemsen, P.; Gubler, U.;
Bosshard, C.; Gu¨nter, P. Diederich, F. Chem. Eur. J. 2001, 7, 1333. Recently
Bruce et al. reported the TEE complexes with the (µ3-C)Co3(CO)9 cluster
units. Bruce, M. I.; Zaitseva, N. A.; Low, P. J.; Skeleton, B. W.; White, A.
H. J. Organomet. Chem. 2006, 691, 4273.
(9) Byrne, L. T.; Hos, J. P.; Koutsantonis, G. A.; Skelton, B. W.; White,
A. H. J. Organomet. Chem. 2000, 598, 28.

Tunable Molecular Wires
Organometallics, Vol. 25, No. 22, 2006 5263
Scheme 3
iron species [FeCp*(dppe)(solvent)]+ with an appropriate poly-
ethynylated compound F (via a 1,2-H shift on G), as already
established by Lapinte.13 The use of a potentially explosive
polyethynylated compound F such as tetraethynylethene (TEE;
(HCtC)2CdC(CtCH)2)14a,b can be avoided by in situ genera-
tion from the corresponding SiR3-protected precursor E. The
TEE (1) and tetrakis(butadiynyl)ethene complexes (2) may be
obtained from the corresponding silyl derivatives 3 and 4, which
were reported by Diederich14 and by us,15 respectively. The other
access (route b) involves multiple Sonogashira coupling between
perhalo compounds and the polyynyl complex mediated by the
Pd-Cu catalyst system in amine solvent.16
(ii) Synthesis of Dinuclear Tetraethynylethene (TEE)
Complexes 6 and 7: Attempts at Synthesis of Tetranuclear
TEE (1) and Tetrakis(butadiynyl)ethene Derivatives (2). We
first examined reactions based on route a. A 1:4 mixture of
tetrakis(trimethylsilylethynyl)ethene (3)14 and Fe-Cl dissolved
in MeOH-THF was refluxed for 8 h in the presence of KF and
KPF6 (reaction I in Scheme 3),4,13 and subsequent extraction
with ether gave the purple product 6,17 which was characterized
as the ((E)-3,4-bis((trimethylsilyl)ethynyl)hex-3-ene-1,5-diyn-
1,6-diyl)diiron complex as described below. It is apparent that
the dinuclear TEE complex 6 resulted from partial metalation
of the tetrasilyl precursor 3 instead of the desired tetrasubsti-
tution leading to 1.
of the reaction product in less polar solvents (e.g. ether and
toluene) suggested formation of an H-type cationic vinylidene
intermediate (Scheme 2), which was subsequently treated with
KOBut to cause deprotonation. The red-purple product 717 thus
obtained was characterized as another dinuclear TEE complex,
((E)-3,4-diethynylhex-3-en-1,5-diyne-1,6-diyl)diron complex,
resulting from partial metalation, as described below. Again,
we could not obtain any evidence for the desired tri- and
tetrametalation. The ethynyl derivative 7 was also obtained by
desilylation of the Si-protected complex 6 (reaction IV).
Sonogashira coupling (route b; V) between the ethynyl
complex 8a4c and tetrahaloethene 9 in NEt3 resulted in recovery
of 8a (X ) Cl) or a complicated mixture of products (X ) I).
The failure in preparation of 1 apparently resulting from the
steric repulsion caused by the bulky Fe groups prompted us to
examine the butadiynyl derivative 2 with longer CtC-CtC
linkers, which separate the metal centers at a distance longer
than 10 Å.15 However, metalation of 4 with Fe-Cl (route a in
Scheme 2) afforded a complicated mixture of products, and
liberation of free dppe ligand suggested the occurrence of
decomposition. The Pd-Cu-catalyzed reaction of the butadiynyl
complex Fe-CtC-CtCH (8b)4f with X2CdCX2 9 (route b)
did not result in the desired Sonogashira reaction but in
dehydrogenative oxidative coupling of 8b to form the octatet-
raynediyl complex 10a,18 where the perhaloethene 9 might work
as oxidant. The µ-C8 complex 10a has already been reported
by Lapinte.4k,l
Because the incomplete metalation could be attributed to the
bulky substituents (Fe and SiMe3), as can be seen from its
molecular structure (see below), 3 was desilylated prior to the
metalation (reaction II). The desilylation of 3 was readily
effected by treatment with a methanolic K2CO3 solution
according to the procedure reported by Diederich.14b (Caution!
Because concentrated 5 is explosiVe, its solution must not be
dried.) The in situ generated 5 was subjected to reaction with
Fe-Cl/KPF6 in MeOH-THF (reaction III). The low solubility
(iii) (Hex-3-ene-1,5-diyne-1,6-diyl)diiron Complexes Con-
taining C6 Spacers and Their Derivatives.19 The formation
of the dinuclear TEE complexes 6 and 7 led us to extend our
research targets to the related dinuclear hex-3-ene-1,5-diyne-
1,6-diyl system (C6 bridge) and its longer congener, the octa-
3,5-dien-1,7-diyne-1,8-diyl system (C8 bridge), for which the
two synthetic approaches were also feasible (Scheme 4).
(13) Connelly, N.; Gamasa, M. P.; Gimeno, J.; Lapinte, C.; Lastra, E.;
Maher, J. P.; Le Narvor, N.; Rieger, A. L.; Rieger, P. H. J. Chem. Soc.,
Dalton Trans. 1993, 2575.
(14) (a) Rubin, Y.; Knobler, C. B.; Diederich, F. Angew. Chem., Int.
Ed. 1991, 30, 698. (b) Anthony, J.; Boldi, A. M.; Rubin, Y.; Hobi, M.;
Gramlich, V.; Knobler, K. B.; Seiler, P.; Diederich, F. HelV. Chim. Acta
1995, 78, 13. (c) Koentjoro, O. F.; Zuber, P.; Puschmann, H.; Goeta, A.
E.; Howard, J. A. K.; Low, P. J. J. Organomet. Chem. 2003, 670, 178.
(15) Ozawa, T.; Akita, M. Chem. Lett. 2004, 1180.
(16) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
50, 4467. (b) Sonogashira, K. In Handbook of Organopalladium Chemistry
for Organic Synthesis; Negishi, E., Ed.; Wiley: New York, 2002. (c)
Reference 4z. (d) Marsden, J. A.; Haley, M. M. In Metal-Catalyzed Cross-
Coupling Reactions; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: New
York, 2004.
(17) The purple color should arise from a trace amount of the monoca-
tionic species 6•+ and 7•+ presumably resulting from oxidation by
adventitious air during the prolonged refluxing.

5264 Organometallics, Vol. 25, No. 22, 2006
Akita et al.
Scheme 4
Three routes based on route a were planned for the (E)-C6
complex (E)-11. The direct (reaction VI) and stepwise Si-
substitution of (E)-12 (reactions VII and VIII) gave (E)-11 in
very low yields, although the monosubstituted intermediate (E)-
1318 (reaction VII) was obtained in 36% yield. Then the free
1-alkyne (E)-14 was generated in situ from the corresponding
Si precursor (E)-1220 according to the procedure reported by
Jia (reaction IX)21f but subsequent treatment with the Fe reagent
(reaction X) gave a mixture containing a trace amount of the
desired product (E)-11 along with (Z)-Fe-CtC-CHdCHCHd
CH2 (15).18,22
On the other hand, the desilylation method turned out to be
successful for the Z isomer ((Z)-11; reactions XI-XIII) and the
extended C8 complex with the diene part (17; reactions XV-
XVII), which were derived from the silyl-protected precursors
(Z)-1220 (via (Z)-16) and 1821d (via 19), respectively. The C8
complex 17 could also be obtained via the direct reaction of 18
with KF-KPF6 in MeOH-THF (reaction XIV). The vinylidene
intermediates (Z)-16 and (E,E)-20 could be isolated and
characterized by X-ray crystallography as described below.
Further extension to the C10 system Me3Si(CtC)2CHdCH(Ct
C)2SiMe3 (21) did not afford the desired product, Fe-(Ct
C)2CHdCH(CtC)2-Fe (22) but a mixture of unidentified
(18) Because the reaction products described in this paper were rather
featureless with respect to the spectroscopic data (e.g. one or two unique
protons in a molecule of MW > 1000, and overlap with the large Ph signals
of the dppe ligand), some of the intermediates and products, which could
not be characterized spectroscopically, were also subjected to X-ray
crystallography when single crystals were available. The crystallographically
characterized products are 10a, (E)-13, and 15, and their crystallographic
data are shown in the Supporting Information. Selected structural parameters
(in Å), FedC < cp-FedC, < cp-Fe-P1, cp-Fe-P2: 1.751, 120.3, 131.1,
134.0 (10a); 1.737, 118.7, 132.7, 131.9; 1.743, 118.6, 130.9, 133.9 ((E)-13
with two independent molecules); 1.748, 120.9, 129.7, 134.2 (15).
(19) Recently Ren reported related hex-3-ene-1,5-diyne-1,6-diyl com-
plexes containing a diruthenium fragment as the end caps: Shi, Y.; Yee,
G. T.; Wang, G.; Ren, T. J. Am. Chem. Soc. 2004, 126, 10552.
(21) (a) Etzenhouser, B. A.; Cavanaugh, M. D.; Spurgeon, H. N.;
Sponsler, M. B. J. Am. Chem. Soc. 1994, 116, 2221. (b) Etzenhouser, B.
A.; Chen, Q.; Sponsler, M. B. Organometallics 1994, 13, 4176. (c) Sponsler,
M. B. Organometallics 1995, 14, 1920. (d) Chung, M.-C.; Gi, X.;
Etzenhouser, B. A.; Spuches, A. M.; Rye, P. T.; Seetharaman, S. K.; Rose,
D. J.; Zubieta, J.; Sponsler, M. B. Organometallics 2003, 22, 3485. (e)
Liu, S. H.; Chen, Y.; Wan, K. L.; Wen, T. B.; Zhou, Z.; Lo, M. F.; Williams,
I. D.; Jia, G. Organometallics 2002, 21, 4984. (f) Liu, S. H.; Xia, H.; Wen,
T. B.; Zhou, Z.; Jia, G. Organometallics 2003, 22, 737. (g) Liu, S. H.; Hu,
Q.; Xue, P.; Wen, T. B.; Williams, I. D.; Jia, G. Organometallics 2005, 24,
769 and references therein. (h) Klein, A.; Lavastre, O.; Fiedler, J.
Organometallics 2006, 25, 635.
(20) (a) E isomer: Walker, J. A.; Bitler, S. P.; Wudl, F. J. Org. Chem.
1984, 49, 4733. (b) Z isomer: Vollhardt, K. P. C.; Winn, L. S. Tetrahedron
Lett. 1985, 26, 709. For the deslilylated compound 14, see: (c) Figeys, H.
P.; Gelbke, M. Tetrahedron Lett. 1970, 5139.

Tunable Molecular Wires
Organometallics, Vol. 25, No. 22, 2006 5265
Table 1. Selected Spectroscopic Data for Diiron Complexes
Scheme 5
1H NMR (δH/ppm)a
com-
plex
31P NMR
IRb (νCtC
/
C5Me5
dCH/othersc
(δP/ppm)a
cm-1
)
6
7
1.60
1.58
1.50
1.23
1.48
0.22 (SiMe3)
2.73 (tCH)
5.77
94.5
93.8
93.6
93.5
85.1
2128, 2021
2095, 2044, 2023d
2036, 2011
2024
(E)-11
(Z)-11
(Z)-16e
5.45
4.72 (m), 4.06 (dd,
7.2, 1.6)
(E,E)-17
1.52
1.49
6.03 (d, 13.4),
6.50 (dd, 13.4, 2.1)
4.94 (m), 5.12 (m),
5.24 (dd, 13.7, 2.5)
93.2
88.5
2018
20e
27a
27b
1.35
1.26
93.3
93.3
2016
2023
a 1H (at 200 MHz) and 31P NMR (at 83 MHz) were recorded in C6D6
products. In these cases as well, Sonogashira coupling was
unsuccessful.23
unless stated. b Recorded as KBr pellets. c Coupling patterns and coupling
constants (in Hz) are shown in parentheses. d νtC-H: 3318 cm-1 e NMR
.
in CD3CN.
(iv) Diaryl-Substituted C6 Complexes 27. To study the
substituent effect of the olefinic part, the diaryl-substituted C6
complexes 27 were prepared via route a (Scheme 5): i.e.,
metalation of the free diyne compounds (generated in situ from
the SiPri3 precursor 26) with the iron reagent followed by
deprotonation with KOBut (XVIII). The reaction was dependent
on the substituent (X): i.e., the reaction of the substrate 26 with
an electron-withdrawing substituent (26b) proceeded smoothly
compared to that of 26a (X ) H). Attempted reactions of the
p-Me and p-OMe derivatives were sluggish and afforded
inseparable mixtures of products.
Comments on the Synthesis of 2D Polyiron Complexes.
2D polyiron complexes are not always easily accessible, as
described above, owing to (1) the bulky Fe group hindering
multiple metalation and (2) the limited synthetic access.
The first problem may be overcome by using a spacer even
longer than the CtC-CtC bridge, and in fact, we recently
succeeded in preparation of a tetrairon complex with ethynylated
phenylene linkers, (Fe-CtC-p-C6H4)2CdC(p-C6H4CtC-
Fe)2.24 However, the use of such a long linker causes another
problem; the communication becomes worse as the metal centers
are separated.
As for the second, synthetic problem, the vinylidene method
(route a) is effective for introduction of a CtC-R group (e.g.
I and III) but cannot be always extended to the introduction of
a CtC-CtC-R group (e.g., reactions of 4 and 21; see above).
Although the reason is not clear, the failure could be ascribed
to the stability of the intermediate. In the case of the CtC-R
system, the vinylidene intermediate ([FedCdC(H)R]+) is the
only possible intermediate, whereas, in the case of the CtC-
CtC-R system, another intermediate such as [FedCdCdCd
C(H)R]+ may be formed in addition to the desired [FedCd
C(H)-CtC-R]+ species, perhaps leading to decomposition if
it is unstable.25
Spectroscopic and Crystallographic Characterization of
the Obtained Iron Complexes. (i) Diiron Complexes. The
obtained diiron complexes 6, 7, (E)- and (Z)-11, and 17 were
characterized by spectroscopic and/or crystallographic methods.
Their spectroscopic data are summarized in Table 1. Sym-
metrical, disubstituted olefin structures are evident from the
NMR data: i.e., (1) single sets of NMR resonances for the Cp*
(1H) and dppe ligands (31P) and the substituents on the olefinic
1
moiety (1H) and (2) the intensities of H NMR signals for the
Cp* ligand and the substituents. The presence of a CtC group
is evident from the IR absorptions around 2000 cm-1. The
configuration of the olefinic part was confirmed by X-ray
crystallography.
Molecular structures of the diiron complexes 6, 7, and 17
were determined by X-ray crystallography, and their ORTEP
views and selected structural parameters are shown in Figure 1
and Table 2, respectively. The three-legged piano-stool con-
figuration of the iron centers is evident from the bond angles
defined by the ligands and cp (the centroid of the Cp* ligand),
and no significant difference is observed for the series of the
complexes listed in Table 2.
For the TEE complexes 6 and 7, disorder is noted for the
central olefinic parts, which sit on centrosymmetric sites, and
this type of disorder is frequently observed for TEE derivatives,
including 4, as previously reported by us.15 While the disorder
hampers a detailed discussion on the bridging organic moiety,
(1) clear bond alternation is evident, (2) the E configuration
has been confirmed, and (3) structures of the core parts of 6
and 7 are essentially the same. Structural parameters for the
acetylide parts are comparable to those of related mononuclear
complexes, including 10a, (E)-13, and (Z)-15.18 From the
overviews it is apparent that the bulky Fe groups hinder
introduction of the third and fourth Fe groups.
E-Z isomerization was noted for the diene part in 17. When
a C6D6 solution of 17 left at room temperature was monitored
1
by H NMR (Cp* region; Figure 2), the single Cp* resonance
for the E,E isomer was partially converted to a pair of two
singlets assignable to the E,Z isomer and then to the single
resonance for the Z,Z isomer, and an equilibrium with a
composition of 60 (E,E):22 (E,Z):18 (Z,Z) was finally attained.
The isomers can also be characterized by the 1H NMR data for
the olefinic parts and the 31P NMR data (E,E isomer, δH 6.03,
6.53 (2H (doublet) × 2), δP 93.19; E,Z isomer, δH 6.77 (1H,
m), 5.77 (1H, m) (the other signals were not located), δP 93.63,
93.35; Z,Z isomer, δH 6.20, 5.61 (2H (doublet) × 2), δP 93.24)
(Figure 2c). The E,E configuration of the major isomer was
also verified on the basis of the large CHdCH coupling
(22) The formation mechanism of 15 is unknown.
(23) A 2:1 reaction between Fe-CtC-H (8a) and the 1,2-dihaloethene
(X)HCdCH(X) (23) resulted in a complicated mixture of products. On the
other hand, a 1:1 reaction gave the monosubstituted product Fe-CtC-
CHdCHX (24: X ) (E)-Cl (24a; 37%); X ) (E)- and (Z)-Br (24b; 22%)),
but the repeated alkynylation of 24 did not result in the substitution but in
a mixture of unidentified products (X ) Cl) or dehydrogenative oxidative
coupling of 8a giving the µ-C4 complex 10b4b,c (X ) Br), where the halide
24b worked as oxidant to be converted to the reduced product Fe-Ct
C-CHdCH2 (25).
3
constants for a simulated spectrum (CHAdCHBCHCdCHD: JAB
) 3JCD ) 14.35 Hz, 4JAC ) 4JBD ) -0.69 Hz, 5JAD ) 0.71 Hz,
5
6
3JBC ) 11.50 Hz; JP-H(A,D) ) 1.80 Hz, JP-H(B,C) ) 0.90 Hz).
The stereoisomerism was also noted by an X-ray crystal-
lographic analysis (Figure 1c). Initial refinement based on the
single component (E,E isomer) resulted in large Beq values for
the central olefinic carbon atoms, but the refinement was
(24) Tanaka, Y.; Ozawa, T.; Inagaki, A.; Akita, M., submitted.
(25) Bruce, M. I. Chem. ReV. 1998, 98, 2797; 1991, 91, 197.

5266 Organometallics, Vol. 25, No. 22, 2006
Akita et al.
Figure 1. ORTEP views of the diiron acetylide complexes (a) 6, (b) 7, (c) 17, and (d) the cation of 6+PF6- drawn with thermal ellipsoids
at the 30% probability level. The disordered core parts are also shown.
successfully achieved by taking into account the E,E and Z,Z
isomers with a composition of 66:34. It is surprising that linkers
with different configurations span the two metal centers
separated at the same distance, suggesting flexibility of the
olefinic linker. E-Z isomerization is a problem, which can be
associated with olefinic linkers, and the present result provides
an example for this problem. This problem could be avoided
by using cyclic dienes with a fixed s-cis-E,E configuration such
as a thiophene linker.4s No evidence for stereoisomerism was
observed for the mono-olefinic complexes 6, 7, and 11; the E

Tunable Molecular Wires
Organometallics, Vol. 25, No. 22, 2006 5267
Table 2. Selected Structural Parameters for Diiron Complexesa
-
6
7
6+PF6
17
16
20
Fe‚‚‚Fe
9.923(1)
2.159(1)
2.166(2)
2.098(4)-2.130(5)
1.741
1.878(5)
1.212(6)
1.443(7)
1.35(2)
9.8944(5)
9.848(4)
2.202(1)
2.204(1)
2.117(4)-2.153(4)
1.756
1.841(4)
1.231(6)
1.423(8)
1.34(1)
11.778(2)
2.187(2)
2.186(2)
2.096(6)-2.137(6)
1.740
1.896(7)
1.227(9)
9.214(1)
11.6260(5)
2.238(2)
2.207(2)
2.113(6)-2.176(6)
1.776
1.723(6)
1.321(8)
1.428(9)
Fe1-P1
Fe1-P2
Fe-C(Cp*)
Fe-cpb
Fe1-C1
C1-C2
C2-C3
C3-C3A*
C3-C4
C4-C5
others
2.1751(5)
2.1711(7)
2.096(2)-2.134(2)
1.741
1.858(9)
1.249(9)
1.427(4)
1.370(5)
1.440(4)
1.164(9)
2.219(1)
2.235(1)
2.132(4)-2.175(4)
1.780
1.740(4)
1.329(5)
1.412(9)
1.36(1)
1.43(1)
1.479(9)
1.217(8)
1.827(6) (C5-Si1)
1.47(2)
1.19(2)
1.84(2) (C5-Si1)
1.41(1)
1.348(8)
1.51(3) (C4-C4*)
1.41(1) (C4-C4*)
cp-Fe1-C1b
cp-Fe1-P1b
cp-Fe1-P2b
C1-Fe1-P1
C1-Fe1-P2
P1-Fe1-P2
Fe1-C1-C2
C1-C2-C3
C2-C3-C3A*
C3*-C3-C4
C2-C3-C4
C3-C4-C5
others
119.6
134.7
131.2
83.9(1)
121.2
131.4
134.1
119.9
130.9
131.7
120.7
130.0
134.7
85.2(2)
84.1(2)
85.63(6)
177.8(6)
175.9(5)
120.6
130.4
129.6
89.7(1)
87.5(1)
85.96(4)
174.4(3)
125.0(5)
126.2(7)
119.9
130.3
134.7
92.0(2)
86.3(2)
85.48(6)
171.4(5)
124.2(5)
84.1(3)
82.0(2)
86.14(2)
174.2(7)
176.6(5)
122.9(4)
119.3(3)
117.8(3)
167.7(3)
90.5(1)
83.2(1)
85.82(5)
172.6(4)
167.1(5)
122.5(8)
111(1)
123.5(8)
166(2)
170(1)
86.3(2)
84.50(5)
176.4(5)
163.0(7)
120.3(8)
113.3(7)
126.3(7)
164.0(6)
176.2(6)
(C4-C5-Si1)
108.6(8)
125.3(5)
116(1)
(C3-C4-C4*)
124.9(6)
(C3-C4-C4*)
(C4-C5-Si1)
θ/θ′c
1 (0)/0(1)d
1.5(6)/1(2)e
19(1)/12(1)f
a Interatomic distances in Å and bond angles in deg. b Centroid of the Cp* ring. c Dihedral angles of the olefinic part. d C2-C3-C3*-C4*/C2-C3A-
C3A*-C4. e C2-C3-C3*-C4*/C2A-C3A-C3A*-C4. f C2-C3-C3*-C4B.
Scheme 6
with respect to the crystallographic centrosymmetric inversion
center, as shown in Figure 3a. (E)-16 could be also isolated
from reaction VI (without addition of KOBut) and characterized
by its NMR data.26
Performance as Molecular Wires. The performance of the
obtained diiron complexes as molecular wires was evaluated
on the basis of their electrochemical properties and near-IR
observation of their monocationic radical species resulting from
one-electron oxidation.
(i) Cyclic Voltammetry of Diiron Complexes. The extent
of delocalization over a π-conjugated system can be conve-
niently evaluated on the basis of the comproportionation constant
(KC), the equilibrium constant between a mixed-valence state
and two different non-mixed-valence states (Scheme 6).27 In
the system with a large KC value, delocalization of the unpaired
electron of the 1e-oxidized species over the π-conjugated system
leads to a stable mixed-valence state. The KC value can be
determined according to the equation KC ) exp[(∆E)F/RT],
where ∆E stands for the separation of the first and second
and Z isomers of 11 are distinguishable by their spectroscopic
features (Table 1) and show discernible electrochemical features
(see below).
(ii) Vinylidene Intermediates. The isolated vinylidene
intermediates showed the characteristic highly deshielded CR
signals (13C NMR) coupled with the two P nuclei (triplet; δC
368.2 ((Z)-16) and 367.2 ((E,E)-20))4c,25 and were also char-
acterized by X-ray crystallography (Figure 3 and Table 2). The
structure of the vinylidene linkage (FedCdC) is evident from
the bond alternation pattern, which is different from those
observed for the Fe-CtC moieties in the acetylide complexes
described above. The three-legged piano-stool structure of the
Fe parts is similar to that of the acetylide complexes, whereas
the slightly elongated Fe-ligand (P and cp) distances can be
ascribed to decreased back-donation to the ligand caused by
the π-acidic vinylidene ligand as well as the formal positive
charge on the metal center. The Z configuration of the diene
moiety in (Z)-16 reveals that reactions XI-XIII proceed with
retention of the configuration, while the molecule is disordered
2
oxidation processes (∆E ) |E1/21 - E1/2 | (in V)). Electrochemi-
cal data for 6, 7, (E)- and (Z)-16, (E,E)-17, and 27a,b are
summarized in Table 3, and cyclic voltammograms for selected
iron complexes are shown in Figure 4. The voltammograms
contain two reversible redox waves, as judged by (1) ic/ia values
close to unity and (2) Ea-Ec separations close to the ideal value
(57 mV at 298 K), and large KC values on the order of 108-
109 (Table 3) calculated from the separations as large as 556
mV reveal that the diiron complexes are efficient molecular
wires belonging to class III according to the Robin and Day
classification.28
(ii) Preparation of Mono- and Dicationic Diiron Com-
plexes. The two well-separated reversible redox processes of
the diiron complexes 6, 7, (E)- and (Z)-11, 17, and 27a,b
(26) δH (CD3CN): 7.7-7.1 (m, Ph), 4.52, 4.02 (2H × 2, m, dCH), 2.8,
2.4 (2H × 2, m, PCH2), 1.24 (30H, s, Cp*). δP (CD3CN): 89.2.
(27) For detailed discussions, see, for example, refs 4a,c, 6a, and 21a,b.
(28) A significant solvent effect was noted. Geiger et al. recently pointed
out the critical dependence of KC on the solvent and electrolyte: Barrie´re,
F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980 and references therein.

5268 Organometallics, Vol. 25, No. 22, 2006
Akita et al.
Figure 2. 1H NMR monitoring of the isomerization of (E,E)-17 (observed in C6D6): (a) changes in the Cp* region; (b) plots of the isomer
ratio as a function of time; (c) olefinic region of an equilibrated isomeric mixture.
(J+PF6-) and dicationic complexes (J2+(PF6-)2), respectively
Scheme 7
(Scheme 7). The oxidized species traced the same cyclic
voltammogram as the corresponding neutral counterparts. IR
and UV-vis-near-IR spectra for the representative examples
are compared in Figure 4 together with the CV traces.
The monocationic, odd-electron species (J+) are paramag-
netic, and their intense νCtC vibrations (Figure 4 and Table 3)
appearing in the energy region lower than those of the neutral
counterparts reveal that delocalization of the unpaired electron
over the metal-carbon linkages (J+ in Scheme 7) causes a
decrease in the bond order of the CtC parts.
The monooxidized species of 6 (6+PF6-) could also be
characterized by X-ray crystallography (Figure 1d). Although
the two independent disorder problems associated with the
central olefinic moiety and the Me3SiCtC part hampered
detailed discussion of the structure, the molecular structure could
be verified. The geometrical parameters of 6+ are roughly
comparable to those of the neutral counterpart 6 as compared
in Table 3, but the bond alternation in 6+ becomes less apparent
suggested that the monooxidized species might be stable enough
compared to that in 6, in accord with the delocalized resonance
structures J+ shown in Scheme 7. In particular, shortening of
the Fe-C distance (1.841(4) Å; cf. 6, 1.878(5) Å) clearly
indicates its multiple-bond character. In addition, a decrease of
the bond order of the central CdC moiety causes a significant
to be isolated and characterized. In addition, an intervalence
charge-transfer band for the isolated monocationic radical
species provides another measure for the performance as
molecular wires. Treatment of the diiron complexes J with 0.9
or 1.9 equiv of FcPF6 (Fc ) FeCp2) in THF gave stable mono-

Tunable Molecular Wires
Organometallics, Vol. 25, No. 22, 2006 5269
Figure 3. ORTEP views of the cations of the cationic diiron vinylidene complexes (a) (Z)-16 and (b) 20 drawn with thermal ellipsoids at
the 30% probability level. The disordered core part of (Z)-16 is also shown.
Table 3. Electrochemical, Near-IR, and IR Data for the Monocationic Diiron Complexes Fe-CtCC(R)dC(R)CtC-Fea
electrochemical datab
IVCT band (near-IR) for monocationd
E1/2
/
|Ea - Ec|/
mV
∆E/
ν
max/cm-1
(ꢀ/M-1 cm-1
ν
1/2/cm-1
ν1/2
/
Vab ,
calcd
II g
c
complex (R)
mV
ic/ia
mV
KC
)
(r/Å)f
cm-1 e
3695
VabIII h/eV
ν
CtC/cm-1 i
6e (CtC-SiMe3)
-930
-408
-907
-408
-1021
-529
-989
-513
-871
-595
-1092
-557
-972
-417
49
61
57
64
54
62
54
63
49
57
84
75
62
57
1.0
1.1
1.0
1.0
0.9
1.0
0.9
1.2
0.9
1.0
1.0
0.9
1.0
0.9
522
6.8 × 108
2.7 × 108
2.1 × 108
1.2 × 108
4.7 × 104
1.4 × 109
3.0 × 109
5910 (1.32 × 104)
5695 (2.27 × 103)
2249 (9.85)
2675 (9.85)
0.11, 0.36
2128, 1968, 1884
7e (CtCH)
(E)-11 (H)
(Z)-11 (H)
499
492
477
276
536
556
3627
0.048, 0.35
1954, 1890
6305 (1.60 × 104)
5181 (5.65 × 103)
6427 (2.2 × 104)
6266 (1.9 × 104)
2488 (9.85)
2664 (11.78)
1905 (9.85)
1769 (9.84)
3961
3459
3853
3805
0.13, 0.39
0.060, 0.32
0.13, 0.40
0.12, 0.39
1995, 1893
1970, 1904
1980, 1892
1980, 1893
(E,E)-17j
27a (C6H5)
27b (p-CF3C6H4)
a Fe ) FeCp*(dppe). b Conditions: [complex] ) ∼2 × 10-3 M, [Bu4NPF6] ) 0.1 M, in CH2Cl2, at 278 K, scan rate 100 mV/s. c KC ) exp[38.97(∆E)]
(at 298 K). d Observed in CH3CN. ν1/2calcd ) (2310νmax
)
.
f r for the C6 complexes is taken from the Fe‚‚‚Fe separation of the cation 6+, and that for the
e
1/2
C8 complex is taken from the neutral (E,E)-17. VabII (estimated as a class II compound) ) (2.55 × 10-6)(νmaxꢀ‚1/2
)
1/2r-1 (eV). VabIII (estimated as a class
g
h
III compound) ) (1/2)νmax (cm-1) ) (6.20 × 10-5)νmax (eV). i For monocations; observed as KBr pellets. j Fe-CtCCHdCHCHdCHCtC-Fe.
distortion of the olefinic part from a planar structure, as can be
seen from the dihedral angles θ and θ′ (>10°; cf. θ/θ′ for 6
and 7, ∼0). The Fe-ligand (dppe and cp) distances in the
monocationic species 6+ are between those of the neutral species
6 and the dicationic vinylidene species 16 and 20, supporting
the discussion on the elongation based on the decreased back-
donation in cationic species (see above).
The dicationic species (J2+) are diamagnetic, as indicated by
their NMR signals appearing in the normal diamagnetic region,29
indicating that the two unpaired electrons (J′2+) couple with
each other to lead to the cumulenic structure (J2+) as the
dominant resonance contributor. The cumulenic structure is
(29) Complexes 6 and 7 showed slightly broad NMR spectra, but the
signals appeared in the normal range.

5270 Organometallics, Vol. 25, No. 22, 2006
Akita et al.
Figure 4. Cyclic voltammograms, IR spectra, and UV-vis-near-IR spectra of selected diiron complexes. (the asterisks denote solvent
noises).
supported by the disappearance of the νCtC vibrations (Figure
4) as well as the highly deshielded 13C NMR signals (e.g. δC
286.4 ((Z)-112+), 274.0 (172+)) characteristic of the R-carbon
atom of cumulenylidene metal complexes.6a,25
bands in the near-IR region (∼1500 nm; Figure 4), in contrast
to the neutral and dicationic species, which are silent in that
region. For comparison, two values (VabII (calculated as assumed
to be class II) and VabIII (calculated as assumed to be class III))
are given in Table 3. The metal-metal separation (r) for the
C6 complexes is taken from that of 6+PF6-, and that for the C8
complex 17+ is taken from the neutral counterpart 17. In all
cases, (i) the ν1/2 values are smaller than the critical values and
(ii) the large Vab values support the notion that the complexes
are efficient molecular wires belonging to class III, in accord
with the electrochemical evaluation discussed above.30
(iv) Evaluation of the Performance of the Diiron Com-
plexes as Molecular Wires. The KC and Vab values for the
obtained complexes are compared with those of the related
complexes reported previously by Lapinte and Sponsler (Table
4). As for the complexes obtained by the present study, the trend
observed for the KC values are essentially the same as those
observed for the VabII and VabIII values. The following discussion,
therefore, is based on the KC values for the sake of simplicity.
The KC values for the C6 complexes 6, 7, 11, and 27 are in
the range of (0.12-3.0) × 109, close to the region for class III
Robin-Day compounds, while the KC value for the C8 complex
17 diminishes by the order of 10-5. The dependence of KC on
r was already pointed out by Gladysz6c and Lapinte (e.g. see
30-32 in Table 4);4 the KC value diminishes exponentially as
a function of the distance between the two metal centers: i.e.,
the number of the carbon atoms spanning the two redox-active
metal centers.
(iii) Near-IR Spectra of One-Electron-Oxidized Species.27
The stability of the mixed-valence monocation obtained by one-
electron oxidation of the neutral compound can be estimated
on the basis of the KC value obtained by electrochemical analysis
(see above). Furthermore, in case the monocation is stable
enough to be isolated, it can be subjected to near-IR measure-
ment. A mixed-valence monocation shows a broad intense
intervalence charge transfer (IVCT) band in the near-IR region,
which is assigned as the π-π transition peculiar to such a highly
delocalized system. The effective electronic coupling parameter
Vab is another measure for the stability of the delocalized system
and can be obtained on the basis of the spectroscopic parameters
of the absorption. The Vab value for class II Robin-Day
compounds can be calculated according to the Hush formula:
Vab ) (2.55 × 10-6)(νmaxꢀν1/2
) ,
1/2r-1 (in eV), where νmax, ꢀ, ν1/2
and r denote the absorption maximum, absorption coefficient,
half-height width, and distance between the two redox active
centers, respectively. On the other hand, for class III Robin-
Day compounds, for which the half-height width (ν1/2) is
1/2
narrower than the critical value (2310νmax
)
(in cm-1), the
electronic coupling can be evaluated using the equation Vab
)
(1/2)νmax (in cm-1) ) 6.20 × 10-5 νmax (in eV). Although no
theoretical relationship between the KC and Vab values is
established, an efficient molecular wire shows larger KC and
Vab values.
(30) The νmax values were not dependent on the solvent polarity (e.g.
(Z)-11: νmax) 6250 cm-1 (in CH2Cl2), 6305 cm-1 (MeCN)), supporting
the classification as class III.27
Near-IR data for the isolated monocations are summarized
in Table 3. The monocationic species show broad intense IVCT

Tunable Molecular Wires
Organometallics, Vol. 25, No. 22, 2006 5271
Table 4. Comparison of the KC and Vab Values for Diiron Complexes
X in Fe-CtCXCtC-Fea
nb
KC
VabII/eVd
VabIII/eVe
ref
c
(Me3SiCtC)CdC(CtCSiMe3)
(HCtC)CdC(CtCH)
HCdCH
HCdCH
(Ph)CdC(Ph)
(p-CF3C6H4)CdC(p-C6H4CF3)
HCdCHHCdCH
6
7
6
6
6
6
6
6
8
6.8 × 108
2.7 × 108
2.1 × 108
1.2 × 108
1.4 × 109
3.0 × 109
4.7 × 104
0.11
0.048
0.36
0.35
f
f
f
f
f
f
f
(E)-11
(Z)-11
27a
27b
(E,E)-17
0.13
0.13
0.12
0.060
0.39
0.40
0.39
0.32
c
Y in Fe-Y-Fea
nb
KC
VabII/eVd
VabIII/eVe
ref
CHdCHCHdCH
C(OMe)dCH-CHdC(OMe)
CtCCtC
28
29
30
31
32
33
34
35
4
4
4
6
8
8
8
8
1.5 × 1010
2.2 × 107
1.6 × 1012
1.0 × 109
2.0 × 107
5.8 × 105
2.6 × 104
1.3 × 106
0.24
0.11
0.19
0.49
0.54
0.47
21a
4a,w
4b,c
4a
4k,l
4s
CtCCtCCtC
CtCCtCCtCCtC
CtCC4H2SCtCg
CtC-p-C6H4CtC
CtCC14H8CtCh
0.32
0.12
0.31
0.25
0.27
0.096
0.076
0.038
4m,r
4q
a Fe ) FeCp*(dppe). b The number of carbon atoms spanning the two Fe centers. c Bu4NPF6 (electrolyte) in CH2Cl2. d Estimated as class II compounds
(see text).27 e Estimated as class III compounds (see text).27 f Present work. g C4H2S ) thiophene-2,5-diyl. C14H8 ) anthracene-9,10-diyl.
h
The E and Z isomers of the C6H2 complex, (E)- and (Z)-11,
are distinguishable by the spectroscopic (NMR and IR) and
electrochemical features, and no isomerization is observed, in
contrast to the C8 complex 17 with the diene linker. Taking
into account the molecular structure of the C6 precursor (Z)-
16, the bulky ancillary ligands (Cp* and dppe) may engage with
each other to freeze the olefin isomerization. The KC values for
the two isomers are comparable, but the E isomer with a longer
metal-metal distance shows a slightly larger KC value.31
The VabIII values show the tendency that we expected (0.40 (27a)
> 0.39 (27b)), although the difference is not significant.
A comparison of the performance of the CtC and CdC units
can be made. As already pointed out by Sponsler21c for the C4
system, the CtC unit is better than the CdC unit, as can be
seen from 28 and 30 (KC(28)/KC(30) ) 1.1 × 10-2). The same
tendency is observed for the C6 system with one CdC unit (KC-
(11)/KC(31) ) 1.2 × 10-1) and the C8 system with two CdC
units (KC(17)/KC(32) ) 2.4 × 10-3), and thus, replacement of
one CtC unit in a certain conjugated bridge by the CdC unit
would cause an exponential decrease of the KC value (at least
10-1 times smaller by one replacement). The C8 complex 17
lies between complexes 33 and 34 with the aromatic linker in
the central part, where the two iron centers are also separated
by eight carbon atoms. The most significant feature of the enyne
system is that the performance can be tuned by introducing
appropriate substituents on the CdC part. As typically exempli-
fied by the series of C6 complexes obtained by the present study,
introduction of the aryl groups onto the CdC part improves
the performance 25 times greater than for the parent complex
11.
As far as the C6 complexes (hex-3-ene-1,5-diyne-1.6-diyl
complexes) Fe-CtC-(R)CdC(R)CtC-Fe, are concerned,
a substituent effect (R) is evident: KC for R ) aryl (27) > Ct
C-R′ (6, 7) > H (11). Because no apparent relationship with
the σp parameter (-0.01 (Ph) ≈ H (0) < 0.23 (CtC-H))32 is
present, the high performance of the aryl derivatives 27 should
be ascribed to the delocalization of the odd electron over the
π-system extended to the R groups. In addition, the effect of
the CF3 group in 27b is notable. The introduction of the CF3
groups causes substantial shifts of the two redox potentials to
the positive side, in accord with the electron-withdrawing nature
of the CF3 group, but the KC value of 27b is twice as large as
that of the parent compound 27a, though not significant, in
contrast to our expectation that the electron-deficient cation-
radical species would be destabilized by an electron-withdrawing
group to show a smaller KC value. Although very few studies
on the related substituent effect have appeared, it was reported
that introduction of the OMe groups at the 1- and 4-positions
of the butadiene-1,4-diyl system (KC(29; OMe)/KC(28; H) )
1.5 × 10-3) caused a drastic decrease of the KC values (Table
4).21d The large difference has been interpreted in terms of
stabilization of the dication by neutralization of the positive
charges by the π-electron-donating OMe substituents. In the
case of the CF3 complex 27b, the electron-withdrawing CF3
group should destabilize not only the monocation but also the
dication. When the shifts caused by the replacement of H by
CF3 are compared, the CF3 substitution influences the second
redox process (Fe(II)-Fe(III) f Fe(III)-Fe(III): -557 mV
(27a) f -417 mV (27b); 140 mV) more significantly than the
initial redox process (Fe(II)-Fe(II) f Fe(II)-Fe(III): -1092
mV (27a) f -972 mV (27b); 120 mV), leading to a larger
separation of the two redox processes; i.e., a larger KC value.
Conclusion
A series of the redox-active diiron complexes consisting of
combinations of the CtC and CdC units has been prepared
and characterized by spectroscopic and crystallographic meth-
ods. The C6 hex-3-ene-1,5-diyne-1,6-diyl complexes with sub-
stituents at the C3 and C4 carbon atoms show two well-
separated, reversible redox waves, suggesting formation of stable
monocationic, mixed-valence states upon one-electron oxidation.
The comproportionation constants KC exceed 108, leading to
the classification to class III compounds according to the Robin
and Day classification. All of the diiron complexes obtained
by the present study afford isolable mono- and dications upon
chemical oxidation with the ferrocenium cation, and the isolated
monocations exhibit intense absorptions in the near-IR region,
which are ascribed to the intervalence charge transfer (IVCT)
band of a highly delocalized π-conjugated system. The Vab
values derived from the IVCT bands are so large that the diiron
complexes belong to class III Robin-Day compounds. In
comparison with a related polyynediyl system consisting only
of the CtC unit, the performance is slightly inferior, but the
performance can be tuned by introduction of appropriate
substituents onto the olefinic carbon atoms (Scheme 1). The
(31) In the Z isomer, the engaging of the bulky Fe fragments may distort
the central olefinic part from a planar structure to cause a decrease of the
overlap of the π-system or the Fe-CR orbital interaction.
(32) Johnson, C. D. The Hammett Equation; Cambridge University
Press: Cambridge, U.K., 1973.

5272 Organometallics, Vol. 25, No. 22, 2006
Akita et al.
pressure, and THF (20 mL) and KOBut (100 mg) were added to
the residue. The mixture was stirred for 2 h, and the volatiles were
removed under reduced pressure. The product was extracted with
ether and passed through an alumina plug. After removal of the
volatiles under reduced pressure, the residue was washed with
MeCN. Crystallization of the residue from toluene-ether gave 7
as deep purple crystals (263 mg, 0.202 mmol, 28% based on 3).
FD-MS: m/z 1300 (M+). Anal. Calcd for C82H80P4Fe2: C, 75.70;
H, 6.20. Found: C, 75.50; H, 6.31.
(ii) Reaction IV. A THF solution (20 mL) containing 6 (194
mg, 0.134 mmol) and Bu4NF (1 M THF solution, 0.05 mL, 0.05
mmol) was stirred for 2 h. After removal of the volatiles under
reduced pressure the residue was washed with pentane at -78 °C
and crystallized from THF-pentane to give 7 (143 mg, 0.110 mmol,
82%).
present study suggests that the stability of the monocation may
be tuned by delocalization of the radical cation over the attached
substituents.
Functionalization of the olefinic part would lead to a
sophisticated system. For example, introduction of functional
groups responsive to the environment (e.g. pH, heat, and
irradiation) would be able to introduce a switching function.
Studies targeting such a system as well as higher order systems
(Scheme 1) are now under way, and the results will be reported
in forthcoming papers.
Experimental Section
General Methods. All manipulations were carried out under an
inert atmosphere by using standard Schlenk tube techniques. THF,
ether, hexane (Na-K alloy), CH2Cl2 (P2O5), and ROH (Mg(OR)2;
R ) Me, Et) were treated with appropriate drying agents, distilled,
Preparation of (E)-11. (i) Reaction VI. A mixture of (E)-12
(150 mg, 0.68 mmol), Fe-Cl (1.019 g, 1.63 mmol), KPF6 (300
mg, 1.63 mmol), and KF (95 mg, 1.63 mmol) dissolved in MeOH
(50 mL)-THF (5 mL) was stirred for 24 h at room temperature.
After removal of the volatiles under reduced pressure the residue
was washed with pentane. KOBut (153 mg, 1.36 mmol) and THF
(10 mL) were added to the residue, and the resultant mixture was
stirred for 30 min. The product was extracted with ether and passed
through an alumina column. After removal of the volatiles under
reduced pressure the residue was washed with MeCN. Crystalliza-
tion of the residue from ether-pentane gave (E)-11 as a red powder
(8 mg, 0.064 mmol, 1%). (E)-11: FD-MS m/z 1252 (M+).36a
(ii) Reactions VII and VIII. A mixture of (E)-12 (106 mg, 0.58
mmol), Fe-Cl (300 mg, 0.58 mmol), KPF6 (107 mg, 0.58 mmol),
and KF (34 mg, 0.58 mmol) dissolved in MeOH (50 mL)-THF (5
mL) was stirred for 6 h at room temperature. The product was
extracted with ether and passed through an alumina column. After
removal of the volatiles under reduced pressure, the residue was
extracted with MeCN, and cooling to -30 °C gave (E)-13 as orange
crystals (147 mg, 0.20 mmol, 34%). (E)-13: δH (C6D6) 6.66 (dt, J
) 15.6, 2.3, Fe-CtC-CHd), 5.73 (d, J ) 15.6, dCHCtC-
SiMe3), 1.43 (s, Cp*), 0.24 (s, SiMe3); δP (C6D6) 92.6; δC (C6D6)
155.9 (t, JCP ) 40, CR), 129.0 (s, CtC), 121.8 (d, JCP ) 6, CtC),
108.9 (d, JCP ) 9, CtC), 108.7 (d, JCH ) 163, dCH), 94.4 (s,
CtC), 88.2 (s, C5Me5), 31.0 (m, PCH2), 10.4 (q, J ) 126, C5Me5),
0.4 (q, J ) 119, SiMe3); IR (KBr) 2113, 2016 cm-1; FD-MS 736
(M+). Anal. Calcd for C45H50SiP2Fe2: C, 73.36; H, 6.84. Found:
C, 73.55; H, 7.12. Repeated metalation of (E)-13 gave a trace
amount of (E)-11.
Preparation of (Z)-11. (i) (Z)-16 (Reactions XI and XII). A
THF solution (5 mL) of (Z)-12 (560 mg, 2.55 mmol) was slowly
added to Bu4NF (1.0 M THF solution 5 mL, 5.0 mmol) dissolved
in ethylene glycol (4 mL), and the mixture was stirred for 3 h at
room temperature. Most of the THF was removed by a rotary
evaporator at room temperature, and the desilylated (Z)-14 was
collected in a trap immersed in a liquid N2 bath by vacuum
distillation. To the distillate was added THF (5 mL) at -78 °C.
The THF solution of (Z)-14 was slowly added to a mixture of Fe-
Cl (1.03 g, 1.65 mmol) and KPF6 (334 mg, 1.7 mmol) suspended
in MeOH (50 mL)-THF (5 mL), and the resultant mixture was
stirred for 1 day at room temperature to give a gray-green solution.
Then the volatiles were removed under reduced pressure, the residue
was extracted with CH2Cl2, and the extract was passed through a
Celite plug. The volume of the solution was reduced under vacuum,
and (Z)-16 was obtained as deep green crystals (1.01 g, 0.65 mmol,
79%) after crystallization from CH2Cl2-THF-pentane. (Z)-16: δC
(CDCl3) 368.2 (t, J ) 35, FedC), 134-128 (Ph), 120.2 (d, J )
154, dCH), 107.1 (d, J ) 159, dCH), 100.8 (C5Me5), 29.5 (m,
1
and stored under argon. H and 13C NMR spectra were recorded
on Bruker AC-200 (1H, 200 MHz; 31P, 81 MHz) and JEOL EX-
400 spectrometers (1H, 400 MHz; 13C, 100 MHz). Chemical shifts
(downfield from TMS (1H and 13C) and H3PO4 (31P)) and coupling
constants are reported in ppm and in Hz, respectively. Solvents for
NMR measurements containing 0.5% TMS were dried over
molecular sieves, degassed, distilled under reduced pressure, and
stored under Ar. IR and UV-vis-near-IR spectra were obtained
on a JASCO FT/IR 5300 spectrometer and a JASCO V-570
spectrometer, respectively. ESI-MS and FD/FAB-MS spectra were
recorded on a ThermoQuest Finnigan LCQ Duo mass spectrometer
and a JEOL JMS-700 mass spectrometer, respectively. For cationic
complexes, M stands for the molecular weight for the cationic
fragment. Electrochemical measurements were made with a BAS
100B/W analyzer. Fe-Cl,33 3,14c 4,15 8a,4c 8b,4f (E)-12,20 (Z)-12,20
18,21e 21,21g 26,34 and FcPF6 were prepared according to the
35
published procedures. Other chemicals were purchased and used
as received.
1:4 Reaction between 3 and Fe-Cl (Reaction I): Preparation
of 6. A mixture of 3 (165 mg, 0.40 mmol), Fe-Cl (1.00 g, 1.60
mmol), KF (93 mg, 1.6 mmol), and KPF6 (294 mg, 1.60 mmol)
was dissolved in MeOH (130 mL)-THF (13 mL) refluxed for 7 h,
upon which it turned purple. Then the volatiles were removed under
reduced pressure, and the resultant residue was extracted with ether.
The ethereal extract was passed through an alumina plug, and the
filtrate was evaporated under reduced pressure. The resultant residue
was washed with MeCN at room temperature and then with pentane
at -78 °C. The obtained purple powder was dried in vacuo: 6
(399 mg, 0.28 mmol, 69%). UV-vis (in THF): λmax/nm (ꢀmax/M-1
cm-1) 578 (3.05 × 104), 800 (4.0 × 103). FD-MS: m/z 1446 (M+).
Anal. Calcd for C88H96Si2P4Fe2: C, 73.12; H, 6.69. Found: C,
72.91; H, 6.50.
Preparation of 7. (i) Reactions II and III. A methanolic
solution (12 mL) containing 3 (300 mg, 0.728 mmol) and K2CO3
(48 mg, 0.35 mmol) was stirred for 30 min and then extracted with
pentane (5 mL × 3). The pentane layer containing 5 was washed
with water (3 mL × 3), dried over Na2SO4, and concentrated to
ca. 5 mL under reduced pressure. (Caution! Because a concentrated
sample of compound 5 is explosiVe, a small amount of pentane
should be left.) Fe-Cl (300 mg, 0.47 mmol) and KPF6 (89 mg,
0.47 mmol) were weighed into a separate Schlenk tube and
dissolved in MeOH (63 mL)-THF (7 mL). To the mixture was
added the pentane solution of crude 5, and the resultant mixture
was stirred for 24 h. Then the volatiles were removed under reduced
(33) Roger, C.; Hamon, P.; Toupet, L.; Rabaa, L. Saillard, J.-Y.; Hamon,
J.-R.; Lapinte, C. Organometallics 1991, 10, 1045.
(34) Jones, G. B.; Wright, J. M.; Hynd, G.; Huber, R. S.; Mathews, J.
E. J. Am. Chem. Soc. 2000, 122, 1937.
(35) Gray, H. B.; Hendrickson, D. N.; Sohn, Y. S. Inorg. Chem. 1971,
10, 1559. Duggan, D. M.; Henrickson, D. N. Inorg. Chem. 1975, 14, 955.
(36) (a) Elemental analysis could not be performed due to the low yield.
(b) Undesired products were characterized only by spectroscopy and
crystallography. (c) An analytically pure sample could not be obtained,
despite several attempts.

Tunable Molecular Wires
Organometallics, Vol. 25, No. 22, 2006 5273
PCH2), 10.1 (q, J ) 126, C5Me5). Anal. Calcd for C81H88F12P6Cl6-
Fe2 (16‚3CH2Cl2): C, 54.14; H, 4.94. Found: C, 54.05; H, 4.93.
(ii) (Z)-11 (Reactions XI-XIII: without Isolation of (Z)-16).
A solution of (Z)-16 was generated by starting from (Z)-12 (560
mg, 2.55 mmol) as described above. The obtained gray-green
solution was evaporated to dryness under reduced pressure, and
the residue was washed with pentane (50 mL). Then the obtained
residue was treated with KOBut (185 mg, 1.65 mmol) in THF (50
mL) for 30 min to give a dark red solution. After the volatiles were
removed under reduced pressure, the products were extracted with
ether and passed through an alumina plug. Concentration and
crystallization from ether-pentane at -30 °C gave (Z)-11 as deep
red crystals (750 mg, 0.60 mmol, 73%). (Z)-11: δC (C6D6) 139.4
(t, J ) 38, Fe-C), 141-126 (Ph), 122.5 (s, Fe-CtC), 112.8 (d,
J ) 154, dCH), 87.9 (C5Me5), 31.2 (m, PCH2), 10.4 (C5Me5); UV-
vis (in THF) ꢀmax/M-1cm-1 (λmax/nm) 404 (5.7 × 103); FD-MS m/z
1252 (M+). Anal. Calcd for C76H80P4Fe2: C, 74.27; H, 6.56.
Found: C, 74.52; H, 6.80.
CH), 95.0, 87.7, 83.0, 73.8 (Ct), -0.2 (q, J ) 128, Me); IR (KBr)
2198, 2181, 2098 cm-1; FAB-MS m/z 268 (M+).
Desilylation of (E)- and (Z)-21. To a stirred ethylene glycol
(0.5 mL)-THF (2.5 mL) solution containing Bu4NF (1 M THF
solution, 0.9 mL, 0.9 mmol) was added (E)-21 (113 mg, 0.42 mmol)
dropwise, and the resultant mixture was stirred for 3 h at room
temperature and then vacuum-transferred to another flask cooled
at -78 °C. Most of the volatiles were removed with a water
aspirator to leave the desilylated product as oil contaminated with
THF and HOCH2CH2OH. Because the product might be explosive,
concentration was stopped and the product was analyzed by NMR.
The Z isomer can also be generated in essentially the same manner.
(E)-HCtC-CtC-CHdCHCtC-CtCH: δH (CDCl3) 6.13 (2H,
s, dCH), 2.64 (2H, tCH); δC (CDCl3) 123.2 (d, J ) 162, dCH),
74.8 (d, J ) 257, tCH), 73.0, 67.8, 63.7 (s × 3, CtC). Z isomer:
δH (CDCl3) 5.98 (s, dCH), 2.67 (s, tCH); δC 121.9 (d, J ) 171,
dCH), 75.0 (d, J ) 261, CH), 72.2, 68.0, 63.7 (s × 3, CtC).
Preparation of 24. 24a. To a mixture of 8 (500 mg, 0.81 mmol),
PdCl2(PPh3)2 (58 mg, 0.082 mmol), CuI (33 mg, 0.17 mmol), and
(E)-23a (X ) Cl; 0.63 mL, 8.1 mmol) was added degassed NHPri2
(25 mL), and the mixture was refluxed for 8 h. After consumption
of 8 was checked by TLC, the volatiles were removed under reduced
pressure, the residue was extracted with toluene, and the extract
was filtered. After removal of the volatiles the residue was extracted
with ether and passed through an alumina pad. Evaporation gave a
red oily product, which was triturated with pentane at -78 °C to
give 24a as a red powder (201 mg, 0.30 mmol, 37%). 24a: δH
(C6D6) 6.36 (dt, J ) 14.0, 2.0, tCCHd), 6.03 (J ) 14.0, dCHCl);
δP (C6D6) 92.8; δC (C6D6) 145.4 (t, J ) 35, Fe-Ct), 128-140
(Ph), 120.2 (d, J ) 153, CdC), 119.1 (d, J ) 189, CdC), 115.0
(s, Fe-CtC), 87.9 (s, C5Me5), 31.0 (t, J ) 22, PCH2), 10.4 (q, J
Preparation of (E,E)-17: (i) Reaction XIV. A mixture of 18
(70 mg, 0.29 mmol), Fe-Cl (358 mg, 0.58 mmol), KPF6 (105 mg,
0.57 mmol), and KF (40 mg, 0.69 mmol) dissolved in MeOH (50
mL)-THF (5 mL) was refluxed for 8 h. After removal of the
volatiles, the residue was washed with ether and extracted with
THF. The THF extract was passed through an alumina plug, and
the residue obtained by removal of the volatiles under reduced
pressure was washed with MeCN and crystallized from THF-ether
to give (E,E)-17 as red-orange crystals (105 mg, 0.082 mmol, 28%
based on 18). (E,E)-17: UV-vis (in THF) λmax/nm (ꢀmax/M-1 cm-1
)
452 (1.6 × 104); FD-MS m/z 1278 (M+). Anal. Calcd for C98H100P4-
Fe2 (17‚3(benzene): C, 77.77; H, 6.66. Found: C, 77.36; H, 6.72.
(ii) Reactions XV-XVII. To an ethanolic solution (3 mL) of
19 (50 mg, 0.20 mmol) were added water (0.36 mL) and 50%
NaOH solution (0.36 mL), and the resultant mixture was stirred
for 3 h. Then the desilylated 18 was extracted with pentane, and
the extract was evaporated under reduced pressure and dissolved
in THF. (Caution! Because a concentrated sample of compound
19 may be explosiVe, a small amount of pentane should be left.)
Fe-Cl (152 mg, 0.24 mmol) and KPF6 (45 mg, 0.24 mmol) were
weighed into a separate Schlenk tube and dissolved in MeOH (20
mL)-THF (2 mL). To the mixture was added the crude THF
solution of 19, and the resultant mixture was stirred for 24 h. Then
the volatiles were removed under reduced pressure. The residue
was washed with ether and crystallized from MeCN-ether, giving
(E,E)-17 as dark yellow-green crystals (44 mg, 0.028 mmol, 14%
based on 18). (E,E)-17: δC (CD3CN) 367.2 (t, JCP ) 33, FedC),
134-128 (Ph), 125.0 (d, JCH ) 152, dCH), 124.3 (d, JCH ) 152,
dCH), 116.3 (d, JCH ) 148, dCH), 101.1 (C5Me5), 29.8 (m, PCH2),
10.2 (q, JCH ) 127, C5Me5); ESI-MS m/z 1424 (M+ - H + PF6),
640 (M2+). To a THF solution of 20 (29 mg, 0.023 mmol) was
added KOBut (10 mg, 0.1 mmol), and the mixture was stirred for
30 min. Then the volatiles were removed under reduced pressure,
and the resultant residue was extracted with benzene and crystallized
from benzene-ether, giving (E,E)-17 (23 mg, 0.018 mmol, 78%).
Preparation of 21. Me3SiCtC-CtCH37 (323 mg, 2.65 mmol),
23b (E,Z mixture, 0.10 mL, 1.26 mmol), Pd(PPh3)4 (146 mg, 0.21
mmol), and CuI (26 mg, 0.14 mmol) dissolved in degassed NEt3
(20 mL) were stirred for 16 h at room temperature. After removal
of the volatiles under reduced pressure the products were extracted
with pentane and separated by silica gel chromatography to give
(E)-21 (196 mg, 0.71 mmol, 56%) and (Z)-21 (95 mg, 0.34 mmol,
27%) as yellow oils. (E)-21: δH (CDCl3) 6.11 (2H, s, dCH), 0.21
(18H, s, SiMe3); δC (CDCl3) 123.0 (d, J ) 170, dCH), 94.9, 87.4,
81.4, 74.7 (Ct), -0.3 (q, J ) 128, Me); IR (KBr) 2201, 2097
cm-1; FAB-MS m/z 268 (M+). (Z)-21: δH (CDCl3) 5.96 (2H, s,
dCH), 0.23 (18H, s, SiMe3); δC (CDCl3) 121.5 (d, J ) 171, d
) 123, C5Me5); IR (KBr) 2027 cm-1 36b
.
24b. To a mixture of 8 (300 mg, 0.48 mmol), PdCl2(PPh3)2 (35
mg, 0.049 mmol), and CuI (20 mg, 0.10 mmol) were added THF
(2 mL), 23b (E and Z mixture; 0.82 mL, 0.96 mmol), and degassed
NEt3 (5 mL), and the mixture was stirred for 40 h at room
temperature. After consumption of 8 was checked by TLC, the
volatiles were removed under reduced pressure, the residue was
extracted with ether, and the extract was passed through an alumina
plug. After removal of the volatiles the residue was extracted with
pentane. The red oily residue obtained by evaporation was triturated
with pentane at -78 °C to give 24b as a red powder (76.0 mg,
0.11 mmol, 22%). 24b (a 2:1 mixture of E and Z isomers): m/z
1180 (M+). E isomer: δH (C6D6) 6.59 (dt, J ) 13.3, 2.1, tCCHd
), 6.08 (d, J ) 13.3, dCHBr), 1.44 (s, Cp*); δP (C6D6) 92.7. Z
isomer: δH (C6D6) 6.42 (dt, J ) 6.6, 2.0, tC-CHd), 5.59 (d, J
) 6.6, dCHBr), 1.52 (s, Cp*). δP (C6D6) 93.1.
Reaction between 24b and 8. Reaction among 24b (83 mg,
0.115 mmol), 8 (71 mg, 0.115 mmol), Pd(PPh3)4 (8.3 mg, 0.0012
mmol), and CuI (4 mg, 0.02 mmol) in NEt3 for 45 h at room
temperature followed by workup as described for 24b gave a
1
mixture of 25 and 10b, as revealed by H NMR. 25: δH (C6D6)
6.20 (ddt, J ) 17.0, 10.4, 2.2, tCCHd), 5.22 (dd, J ) 17.0, 3.3,
CH2), 4.93 (dd, J ) 10.4, 3.3, dCH2), 1.44 (s, Cp*); δP (C6D6)
93.5. Complex 10b was identified by comparison with an authentic
sample.4b,c
Preparation of 27 (Reaction XIX). As a typical example, the
preparative procedures for 27a are described. To a THF solution
(5 mL) of 26a (244 mg, 0.45 mmol) cooled in a dry ice-acetone
bath was added Bu4NF (1 M THF solution, 1 mL, 1.0 mmol), and
the mixture was stirred for 30 min. Then water was poured into
the mixture and the product was extracted with pentane. The pentane
layer was separated, and the solvent was removed under reduced
pressure. To the residue were added THF (15 mL)-MeOH (1.5
mL), Fe-Cl (353 mg, 0.56 mmol), and KPF6 (119 mg, 0.65 mmol)
under argon, and the mixture was stirred for 31 h. After removal
of the volatiles the residue was washed with toluene. Then KOBut
(37) Bartik, B.; Dembinski, R.; Bartik, T.; Arif, A. M.; Gladysz, J. A.
New J. Chem. 1997, 21, 739.

5274 Organometallics, Vol. 25, No. 22, 2006
6‚6CH2Cl2
Akita et al.
Table 5. Crystallographic Data
7
10a‚2Et2O
13
15‚2MeCN
formula
formula wt
cryst syst
space group
a/Å
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
C94H108Si2P4Cl12Fe2
1955.08
triclinic
C82H80P4Fe2
1301.12
monoclinic
P21/c
17.4725(9)
11.0643(4)
17.9715(9)
90
106.615(3)
90
3329.2(3)
2
C88H98O2P4Fe2
1423.33
triclinic
P1h
10.919(8)
12.218(10)
15.67(1)
72.04(3)
79.77(3)
87.96(3)
1956(2)
1
C45H50SiP2Fe
736.77
monoclinic
C2/c
47.2573(8)
18.2182(3)
20.4735(3)
90
102.0703(9)
90
17236.8(5)
16
1.136
0.479
68 068
C46H50N2P2Fe
748.71
monoclinic
P21/n
20.80(1)
10.369(7)
20.79(1)
90
115.02(2)
90
4062(4)
4
P1h
15.247(1)
15.708(2)
12.407(2)
111.850(1)
105.193(4)
62.000(7)
2421.3(5)
1
Z
d
calcd/g cm-3
1.341
0.668
17 456
1.298
0.577
27 107
1.208
0.498
14 859
1.224
0.483
25 365
µ/mm-1
no. of diffractions
collected
no. of variables
R1 for data with
I > 2σ(I)
515
410
433
895
460
0.0783 (for 6596
data)
0.2105 (for all
9894 data)
0.0461 (for 6093
data)
0.1379 (for all
7490 data)
0.0603 (for 3791
data)
0.1778 (for all
8090 data)
0.0587 (for 9097
data)
0.1733 (for all
19 024 data)
0.0540 (for 3217
data)
0.1765 (for all
8436 data)
wR2
16‚4CH2Cl2
17‚3C6H6
20‚6MeCN
6+PF6-‚MeCN
formula
formula wt
cryst syst
space group
a/Å
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
C82H90F12P6Cl8Fe2
1884.75
triclinic
C98H100P4Fe2
1513.46
monoclinic
P21/n
14.977(8)
12.689(9)
22.77(2)
90
108.30(3)
90
4108(4)
2
C92H102N6F12P6Fe2
1817.38
monoclinic
P21/c
12.932(4)
20.910(6)
17.410(3)
90
108.19(1)
90
4472(2)
2
C90H99NF6Si2P5Fe2
1631.50
rhombohedral
R3h
19.06(1)
19.06(1)
19.06(1)
91.07(1)
91.07(1)
91.07(1)
6918(6)
3
P1h
11.107(2)
11.477(3)
18.169(4)
102.282(7)
103.041(12)
100.894(10)
2135.3(8)
1
Z
d
calcd/g cm-3
1.466
0.771
17 294
1.223
0.477
26 162
1.349
0.505
34 841
1.175
0.480
44 961
µ/mm-1
no. of diffractions
collected
no. of variables
R1 for data with
I > 2σ(I)
507
478
535
497
0.0632 (for 6014
data)
0.1773 (for all
8893 data)
0.0637 (for 3008
data)
0.2002 (for all
8875 data)
0.0808 (for 5101
data)
0.2179 (for all
9449 data)
0.0797 (for 6025
data)
0.2400 (for all
10 529 data)
wR2
(150 mg, 1.33 mmol) and THF (15 mL) were added, and the
mixture was stirred for 30 min. Removal of the volatiles, extraction
with toluene, filtration through an alumina plug, and precipitation
from toluene-MeCN gave 27a (180 mg, 0.13 mmol, 28%) as a
red powder. 27a: UV-vis (in THF) λmax/nm (ꢀmax/M-1 cm-1) 452
(2.1 × 104); FD-MS m/z 1405 (M+). Anal. Calcd for C90H88P4Fe2:
C, 76.92; H, 6.31. Found: C, 77.07; H, 6.18. 27b: UV-vis (in
THF) λmax/nm (ꢀmax/M-1 cm-1) 222 (6.5 × 104), 506 (1.3 × 104);
FD-MS m/z 1540 (M+).36c
Electrochemical Measurements. CV and DPV measurements
were performed with a Pt electrode for CH2Cl2 solutions of the
samples (∼2 × 10-3 M) in the presence of an electrolyte (Bu4-
NPF6: 0.1 M) at room temperature under an inert atmosphere. The
scan rates were 100 mV/s (CV) and 20 mV/s (DPV). After the
measurements, ferrocene (Fc) was added to the mixture and the
potentials were calibrated with respect to the Fc/Fc+ redox couple.
Preparation of Mono- and Dicationic Species. As typical
examples, procedures for the cationic species derived from 6 are
described. Most of the mono- and dications were characterized
spectroscopically.
(ii) Preparation of 62+(PF6-)2. Reaction of 6 with 1.9 equiv of
[FeCp2]PF6 gave 62+(PF6-)2 as deep blue crystals.
6+PF6-: UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1‚cm-1) 798 (2.5
× 104), 1692 (1.3 × 104). Anal. Calcd for C88H96F6Si2P5Fe2: C,
66.46; H, 6.08. Found: C, 66.14; H, 6.14. 62+(PF6-)2: IR, no
characteristic band; UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1 cm-1
)
264 (1.5 × 104), 432 (4.5 × 103), 672 (2.8 × 103). 7+PF6-: Anal.
Calcd for C82H80F6P5Fe2: C, 68.11; H, 5.50. Found: C, 67.88; H,
5.21. (Z)-11+PF6-: UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1 cm-1):
404 (5.0 × 103), 628 (1.5 × 104), 1586 (1.6 × 104). (Z)-112+
-
(PF6-)2: δH (CD2Cl2) 8.08 (2H, m, dCH), 7.60-6.70 (Ph), 4.15,
3.13 (2H × 2, CH2P), 1.30 (30H, s, Cp*); δP (CD2Cl2) 75.2; IR,
no characteristic band; UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1
cm-1) 270 (7.6 × 103), 400 (1.5 × 103), 646 (1.3 × 104). (E,E)-
17+PF6-: UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1‚cm-1) 266 (3.1
× 104), 452 (8.3 × 103), 682 (3.9 × 104), 1930 (5.7 × 103); ESI-
MS m/z 1278 (M+ + PF6). (E,E)-172+(PF6-)2: δH (CD3CN) 8.74
(2H, m, dCH), 7.55-6.82 (Ph), 6.54 (2H, m, dCH), 3.80, 2.95
(2H × 2, CH2P), 1.28 (30H, s, Cp*); δP (CD3CN) 78.2; IR 1905
cm-1; UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1 cm-1) 266 (3.1 ×
104), 452 (8.3 × 103), 682 (3.9 × 104), 1930 (5.7 × 103); ESI-MS
m/z 1423 (M+ + PF6), 639 (M2+). 27a+PF6-: UV-vis (in CH3-
CN) λmax/nm (ꢀmax/M-1 cm-1) 406 (9.7 × 103), 636 (1.9 × 104),
(i) Preparation of 6+PF6-. A mixture of 6 (50.0 mg, 0.035
mmol) and FcPF6 (8.2 mg, 0.025 mmol) was stirred for 3 h at room
temperature. After removal of the volatiles under reduced pressure
the residue was washed with ether and recrystallized from CH2-
Cl2-ether, giving 6+PF6- (37 mg, 0.023 mmol, 93%) as deep green
crystals.
1556 (2.2 × 104); FD-MS 1423 (M+ + PF6), 639 (M2+). 27a2+
-
(PF6-)2: δH (CD3CN) 7.71-6.35 (Ph), 2.90-2.46 (2H × 2, CH2P),
1.03 (30H, s, Cp*); δP (CD3CN) 82.6; IR 1630 cm-1; UV-vis (in

Tunable Molecular Wires
Organometallics, Vol. 25, No. 22, 2006 5275
CH3CN) λmax/nm (ꢀmax/M-1 cm-1) 270 (2.6 × 104), 384 (3.4 ×
103), 668 (1.6 × 104); ESI-MS m/z 1549 (M+ + PF6), 1404 (M+).
27b+PF6-: UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1 cm-1): 392
(1.6 × 104), 654 (2.1 × 104), 1596 (1.9 × 104); FD-MS m/z 1686
(M+ + PF6), 1540 (M+). 27b2+(PF6-)2: δH (CD3CN) 7.76-6.63
(Ph), 3.05, 2.68 (2H × 2, CH2P), 0.99 (30H, s, Cp*); δP (CD3CN)
7: The molecule sat on a crystallographic inversion center. The
disordered central part was refined, taking into account two
components (C1,2,3,5:C1A,2A,3A,5A ) 0.76:0.24).
10a: The molecule sat on a crystallographic inversion center.
13: A unit cell contained two independent molecules, one of
which was found to be disordered and refined, taking into account
two components (C4B-C5B-C6B:C4C-C5C-C6C ) 0.64:0.36).
The C4C, C5C, and C6C atoms were refined isotropically, and
hydrogen atoms attached to the disordered part were not included
in the refinement.
80.9; IR 1614 cm-1; UV-vis (in CH3CN) λmax/nm (ꢀmax/M-1 cm-1
)
268 (4.0 × 104), 410 (6.7 × 103), 654 (4.3 × 104); FD-MS m/z
1686 (M+ + PF6), 1540 (M+).
X-ray Crystallography. Diffraction measurements were made
on a Rigaku RAXIS IV imaging plate area detector with Mo KR
radiation (λ ) 0.710 69 Å) at -60 °C. Indexing was performed
from 3 oscillation images, which were exposed for 3 min. The
crystal-to-detector distance was 110 mm (2θmax ) 55°). In the
reduction of data, Lorentz and polarization corrections and empirical
absorption corrections were made.38 Crystallographic data and
results of structure refinements are given in Table 5.
The structural analysis was performed on an IRIS O2 computer
using the teXsan structure solving program system obtained from
the Rigaku Corp., Tokyo, Japan.39 Neutral scattering factors were
obtained from the standard source.40
The structures were solved by a combination of direct methods
(SHELXS-86)41 and Fourier synthesis (DIRDIF94).42 Least-squares
refinements were carried out using SHELXL-9741 (refined on F2)
linked to teXsan. Unless otherwise stated, all non-hydrogen atoms
were refined anisotropically; methyl hydrogen atoms were refined
using riding models and fixed at the final stage of the refinement,
and other hydrogen atoms were fixed at the calculated positions.
Structural data for 10a, 13, and 15 are included in the Supporting
Information.
6: The molecule sat on a crystallographic inversion center. The
C1 atom and one of the CH2Cl2 solvate molecules were found to
be disordered. The C1 atom was refined anisotropically, taking into
account two components (C1:C1A ) 0.55:0.45). The CH2Cl2
solvate molecule was refined isotropically, taking into account two
components (Cl5-C73-Cl6:Cl5a-C73a-Cl6a ) 0.65:0.35), and
the hydrogen atoms attached to the disordered solvate molecule
were not included in the refinement.
16: The molecule sat on a crystallographic inversion center. The
disordered central part was refined, taking into account two
components (C3-H2-H3:C3A-H2A-H3A ) 1:1). One of the
CH2Cl2 solvate molecules was also found to be disordered and was
refined isotropically, taking into account two components (Cl3-
C71-Cl4:Cl3A-C71A-Cl4A ) 0.58:0.42), and hydrogen atoms
attached to the disordered part were not included in the refinement.
17: The molecule sat on a crystallographic inversion center. The
disordered central part was refined, taking into account two
components (C4:C4A ) 0.658:0.342), and hydrogen atoms attached
to the disordered part were not included in the refinement.
20: The molecule sat on a crystallographic inversion center.
6+PF6-: The molecule sat on a crystallographic inversion center
and was found to be disordered. The olefinic part (C3-C3*:C3A-
C3A* ) 0.76:0.24) and a part of the Me3SiCtC part (C4,5,7,8-
Si1:C4B,5B,7B,8B,Si1B ) 0.58:0.42) disordered independently
were refined anisotropically, taking into account minor components.
The MeCN solvate molecule was refined isotropically. Hydrogen
atoms attached to the disordered parts and the solvate molecule
were not included in the refinement. The PF6 anions sat on two
6-fold inversion sites, and only two-thirds of the anions could be
located. This should be a result of disorder of the anion, and the
other disordered anion could not be located. Another possible
interpretation is that the crystal consists of a 2:1 disordered mixture
of 6+PF6- and 6. Because, however, the single crystals did not show
the νCtC vibration for 6, the latter possibility has been eliminated.
Acknowledgment. We are grateful to Dr. Toshiro Takao
1
(Tokyo Institute of Technology) for his help for the H NMR
simulation. This research was financially supported by the
Ministry of Education, Culture, Sports, Science and Technology
of the Japanese Government (Grant-in-Aid for Scientific
Research on Priority Areas, No. 18065009) and the Japan
Society for Promotion of Science and Technology (Grant-in-
Aid for Scientific Research (B), No. 15350032), which are
gratefully acknowledged.
(38) Higashi, T. Program for Absorption Correction; Rigaku Corp.,
Tokyo, Japan, 1995.
(39) teXsan: Crystal Structure Analysis Package, version 1.11; Rigaku
Corp., Tokyo, Japan, 2000.
(40) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, U.K., 1975; Vol. 4.
(41) (a) Sheldrick, G. M. SHELXS-86: Program for Crystal Structure
Determination; University of Go¨ttingen, Go¨ttingen, Germany, 1986. (b)
Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refinement;
University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(42) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF
Program System; Technical Report of the Crystallography Laboratory;
University of Nijmegen, Nijmegen, The Netherlands, 1992.
Supporting Information Available: Tables, figures, and CIF
files giving crystallographic results. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM060403T