Elemental carbon chain bridging two iron centers: Syntheses and spectroscopic properties of donor-acceptor [Fe]-C4-[Fe] complexes isolated in two different oxidation states. X-ray crystal structure of [Cp*(dppe)Fe-C4-Fe(CO)2Cp*]

Article abstract of DOI:10.1021/om970730o

The complexes Fe(η⁵-C₅R₅)(CO)₂(C=CC=CSiMe3) (R = Ph, 2a; R = Me, 2b) were obtained from the reaction of (η₅-C₅Ph₅)Fe(CO)₂Br (1a) or (Cp*)Fe(CO)₂I (Cp* = η⁵-C₅Me₅, 1b) with Me₃SiC≡CC≡CLi (1.05 equiv) in THF at -80 °C (70-75%). The butadiynyl complex (η⁵-C₅Me ₅)Fe(dppe)(C≡CC≡CSiMe₃) (2c) was prepared upon photolysis of 2b in a toluene/ acetonitrile mixture (95/5 v/v) in the presence of dppe (1,2-bis(diphenylphosphino)ethane) (85%). Treatment of complexes 2a,2b with 1.1 equiv of potassium fluoride in a 50/50 CH₃-OH/THF mixture at 20 °C afforded the terminal butadiyne complex (η⁵-C₅R₅)Fe(CO) ₂-(C≡CC≡CH) (R = Ph, 3a; R = Me, 3b), isolated in 85 and 95% yields respectively. Complex 3c was obtained by reaction of 2c with 0.2 equiv of Buⁿ₄NF in THF (95%). The mononuclear complexes 2a-c and 3a-c were characterized by cyclic voltammetry and IR, ¹H, ¹³C, and ³¹P NMR, and Mo?ssbauer spectroscopy. The binuclear complexes 5a,b were obtained (59-69%) in a one-step procedure by treatment of (η⁵-C₅R₅)Fe(CO) ₂(C≡CC≡CH) (3a,b) with 1 equiv of Cp*Fe(dppe)Cl (4) in the presence of KPF₆ and KOBu^t in methanol. The lower IR frequencies of the carbonyl groups in 5a,b relative to the isolated acceptor group in 2a,b and 3a,b are indicative of the electronic communication between the metal centers through the -C₄- spacer. In the complexes 5a,b, the polarization of the spacer was shown by the ¹³C NMR chemical shifts of the carbon of the -C₄- chain. The Mo?ssbauer spectrum of 5b establishes that the electron density at the two iron atoms is quite different. The structure of 5b has been determined by X-ray diffraction. Cyclic voltammograms of complexes 5a,b from -0.8 to -1.4 V/SCE displayed one fully reversible wave and a second one almost reversible, showing that the binuclear systems undergo two successive one-electron oxidations at the electrode. Comparison of both redox potentials and current ratio with those of the corresponding monomers 2a-c demonstrates that a strong electronic communication between the metal centers takes place across the all-carbon bridge. Treatment of the neutral complexes with 1 equiv of ferrocenium allowed isolation of the salts [5a,b] [PF₆] in 85-90% yield. Mo?ssbauer spectroscopy showed that both salts were trapped Fe(II)-Fe(III) mixed-valence compounds. The g tensors and couplings with the ³¹P nuclei are very close for both Fe(III) low-spin radicals. Intense absorption bands were observed at 810 and 829 nm for [5a][PF₆] and [5b][PF₆], respectively. A second absorption band was observed in the near-infrared for [5b][PF₆] (1600 nm) and ascribed to an ICT band. It allowed determination of the electronic coupling between the electron-poor and the electron-rich iron centers through the all-carbon spacer (V_ab = 0.021 eV).

Full text of DOI:10.1021/om970730o

5988
Organometallics 1997, 16, 5988-5998
Elem en ta l Ca r bon Ch a in Br id gin g Tw o Ir on Cen ter s:
Syn th eses a n d Sp ectr oscop ic P r op er ties of
Don or -Accep tor [F e]-C4-[F e] Com p lexes Isola ted in
Tw o Differ en t Oxid a tion Sta tes. X-r a y Cr ysta l Str u ctu r e
of [Cp *(d p p e)F e-C4-F e(CO)2Cp *]
Franc¸oise Coat,† Marie-Andre´e Guillevic,† Loic Toupet,‡ Fre´de´ric Paul,† and
Claude Lapinte*,†
UMR CNRS 6509 Organome´talliques et Catalyse and UMR CNRS 6626 Groupe Matie`re
Condense´e et Mate´riaux, Universite´ de Rennes I,
Campus de Beaulieu, 35042 Rennes Cedex, France
Received August 18, 1997X
The complexes Fe(η5-C5R5)(CO)2(CtCCtCSiMe3) (R ) Ph, 2a ; R ) Me, 2b) were obtained
from the reaction of (η5-C5Ph5)Fe(CO)2Br (1a ) or (Cp*)Fe(CO)2I (Cp* ) η5-C5Me5, 1b) with
Me3SiCtCCtCLi (1.05 equiv) in THF at -80 °C (70-75%). The butadiynyl complex (η5-
C5Me5)Fe(dppe)(CtCCtCSiMe3) (2c) was prepared upon photolysis of 2b in a toluene/
acetonitrile mixture (95/5 v/v) in the presence of dppe (1,2-bis(diphenylphosphino)ethane)
(85%). Treatment of complexes 2a ,2b with 1.1 equiv of potassium fluoride in a 50/50 CH3-
OH/THF mixture at 20 °C afforded the terminal butadiyne complex (η5-C5R5)Fe(CO)2-
(CtCCtCH) (R ) Ph, 3a ; R ) Me, 3b), isolated in 85 and 95% yields respectively. Complex
3c was obtained by reaction of 2c with 0.2 equiv of Bun NF in THF (95%). The mononuclear
4
complexes 2a -c and 3a -c were characterized by cyclic voltammetry and IR, H, 13C, and
1
31P NMR, and Mo¨ssbauer spectroscopy. The binuclear complexes 5a ,b were obtained (59-
69%) in a one-step procedure by treatment of (η5-C5R5)Fe(CO)2(CtCCtCH) (3a ,b) with 1
equiv of Cp*Fe(dppe)Cl (4) in the presence of KPF6 and KOBut in methanol. The lower IR
frequencies of the carbonyl groups in 5a ,b relative to the isolated acceptor group in 2a ,b
and 3a ,b are indicative of the electronic communication between the metal centers through
the -C4- spacer. In the complexes 5a ,b, the polarization of the spacer was shown by the
13C NMR chemical shifts of the carbon of the -C4- chain. The Mo¨ssbauer spectrum of 5b
establishes that the electron density at the two iron atoms is quite different. The structure
of 5b has been determined by X-ray diffraction. Cyclic voltammograms of complexes 5a ,b
from -0.8 to -1.4 V/SCE displayed one fully reversible wave and a second one almost
reversible, showing that the binuclear systems undergo two successive one-electron oxidations
at the electrode. Comparison of both redox potentials and current ratio with those of the
corresponding monomers 2a -c demonstrates that a strong electronic communication between
the metal centers takes place across the all-carbon bridge. Treatment of the neutral
complexes with 1 equiv of ferrocenium allowed isolation of the salts [5a ,b][P F 6] in 85-90%
yield. Mo¨ssbauer spectroscopy showed that both salts were trapped Fe(II)-Fe(III) mixed-
valence compounds. The g tensors and couplings with the 31P nuclei are very close for both
Fe(III) low-spin radicals. Intense absorption bands were observed at 810 and 829 nm for
[5a ][P F 6] and [5b][P F 6], respectively. A second absorption band was observed in the near-
infrared for [5b][P F 6] (1600 nm) and ascribed to an ICT band. It allowed determination of
the electronic coupling between the electron-poor and the electron-rich iron centers through
the all-carbon spacer (Vab ) 0.021 eV).
Elemental carbon chains constitute the simplest mo-
terminal group is conveyed through the carbon wire and
modifies the properties of the other end group. Thus,
the carbon spacer operates as a molecular wire permit-
ting electrons to flow between the redox centers. As a
consequence of this electronic communication, two
chemically identical redox-active end groups show two
different redox potentials. The separation between
them can be determined by voltammetry and constitutes
a measure of the electron conduction in this basic
electronic circuit. The intensity of this effect depends
upon both of the redox centers and the spacer. It is
lecular wires that one can imagine.1-3 In these linear
and rigid connectors the carbon atoms are sp-hybridized
with alternate single and triple bonds between the
carbon atoms. Among those, the carbon chains with
redox-active terminal end groups are particularly in-
teresting, since a change in the oxidation state of one
† UMR CNRS 6509 Organome´talliques et Catalyse.
‡ UMR CNRS 6626 Groupe Matie`re Condense´e et Mate´riaux.
X Abstract published in Advance ACS Abstracts, December 1, 1997.
(1) Rice, M. J .; Bishop, A. R.; Campbell, D. K. Phys. Rev. Lett. 1983,
51, 2136.
(2) Ward, M. D. Chem. Ind. (London) 1996, 568-573.
(3) Harriman, A.; Ziessel, R. Chem. Commun. 1996, 1707-1716.
significantly stronger for complexes having the general
4-9
formula LnM-Cx-MLn
than for those with other
S0276-7333(97)00730-9 CCC: $14.00 © 1997 American Chemical Society

Elemental Carbon Chain Bridging Two Iron Centers
Organometallics, Vol. 16, No. 26, 1997 5989
Sch em e 1
types of unsaturated bridges.10-14 In these molecular
devices, the redox potentials of the metal centers are
quite different from the potential of the mononuclear
isolated compounds. Moreover, the electrochemical
reversibility of the redox process can also be strongly
affected by electron delocalization between the metal
and the all-carbon bridge. Up to now, this effect had
never been evidenced in donor-acceptor organometallic
compounds with a large extent of delocalization of the
valence electrons.15
terms of redox potentials and chemical reversibility, (iv)
a spectroscopic study of the charge transfer and the
electronic coupling between the two redox centers in the
mixed valence complexes. The spectroscopic and redox
properties of the related mononuclear complexes are
given as well in a preliminary study.
Resu lts a n d Discu ssion
1. Syn t h esis a n d Ch a r a ct er iza t ion of t h e
Bu ta d iyn yl Com p lexes (C5R5)F e(L)2(CtCCtCR′).
Following a known procedure,18 the complexes Fe(η5-
C5R5)(CO)2(CtCCtCSiMe3) (R ) Ph, 2a ; R ) Me, 2b)
were obtained from the reaction of the corresponding
iron halide (η5-C5Ph5)Fe(CO)2Br (1a )19,20 or (η5-C5-
Me5)Fe(CO)2I (1b)21 with Me3SiCtCCtCLi22 (1.05 equiv)
in THF at -80 °C. The (trimethylsilyl)butadiynyliron
complexes 2a ,b were isolated after crystallization as
yellow powders in 70 - 75% yield (Scheme 1).
The butadiynyl complex (η5-C5Me5)Fe(dppe)-
(CtCCtCSiMe3) was prepared upon photolysis of 2b
in toluene. After 4 h of irradiation, the dark brown
solution was evaporated to dryness, and crystallization
We have previously reported the synthesis and the
characteristic features of the three-redox-state family
of compounds [Cp*(dppe)FeCtCCtCFe(dppe)Cp*]n+
-
[PF6-]n (n ) 0-2).5,6 This study, together with impor-
tant and complementary results obtained in a similar
rhenium series,7-9 has both revealed the very special
coupling properties of the -Cx- ligand in dinuclear
complexes and shown the versatility of the electronic
structure of the coordinated carbon chains. More
recently, the particular properties of the [M]-C4-[M]
assembly were illustrated with diruthenium com-
plexes.16 In these systems, the carbon bridge was
connected to two identical redox-active termini and
electron transfer occurred in a symmetrical way.
In contrast, a linear spacer bearing an electron
acceptor on one end and an electron donor on the other
should display preferential one-way electron transfer
and act as a rectifying component.17 Such molecular
devices can be seen as polarized molecular wires. If one
of the terminal groups is redox-active and stable at least
under two different oxidation states, these systems can
also constitute models for electro-switchable molecular
current rectifiers or NLO-active materials. In this
respect, we were interested in the properties of the
polarized D-C4-A organometallic assembly in which
the donor (D) and the acceptor (A) termini are orga-
noiron building blocks with quite different electronic
densities. The model that we designed in this work
comprises the electron-rich redox-active Cp*Fe(dppe)
center as the donor end, and the electron-withdrawing
(C5R5)Fe(CO)2 (R ) Ph, Me) units, which are not redox-
active in mononuclear complexes, on the other side.
Thus, we will now report (i) the synthesis of the -C4-
bridged bimetallic complexes [(C5Me5)(dppe)Fe-
(CtCCtC)Fe(CO)2(C5R5)]n+[PF6-]n (R ) Ph, n ) 0, 5a ;
R ) Ph, n ) 1, 5a [P F 6]; R ) Me; n ) 0, 5b; R ) Me, n
) 1, 5b[P F 6]), (ii) their full spectroscopic characteriza-
tions, including an X-ray crystal structure of the com-
pound 5b, (iii) a cyclic voltammetry study, showing the
strong metal-metal electronic communication both in
(4) Coat, F.; Lapinte, C. Organometallics 1996, 15, 477-480.
(5) Le Narvor, N.; Lapinte, C. J . Chem. Soc., Chem. Commun. 1993,
357-359.
(6) Le Narvor, N.; Toupet, L.; Lapinte, C. J . Am. Chem. Soc. 1995,
117, 7129-7138.
(7) Brady, M.; Weng, W.; Gladysz, J . A. J . Chem. Soc., Chem.
Commun. 1994, 2655-2656.
(8) Seyler, J .; Weng, W.; Zhou, Y.; Gladysz, J . A. Organometallics
1993, 12, 3802-3804.
(9) Brady, M.; Weng, W.; Zhou, Y.; Seyler, J . W.; Amoroso, A. J .;
Arif, A. M.; Bo¨hme, M.; Frenking, G.; Gladysz, J . A. J . Am. Chem. Soc.
1997, 119, 775-788.
(10) Ward, M. D. Chem. Soc. Rev. 1995, 121.
(11) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1-73.
(12) Crutchly, R. J . Adv. Inorg. Chem. 1994, 41, 273.
(13) Ribou, A. C.; Launay, J . P.; Sachtleben, M. L.; Li, H.; Spangler,
C. W. Inorg. Chem. 1996, 35, 3735-3740.
(14) Bunz, U. H. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 969-
971.
(15) Sato, M.; Shintate, H.; Kawata, Y.; Sekino, M. Organometallics
1994, 13, 1956-1962.
(16) Bruce, M. I.; Denisovich, L. I.; Low, P. J .; Peregudova, S. M.;
Ustynyuk, N. A. Mendeleev Commun. 1996, 200-201.
(17) Astruc, D. Electron Transfer and Radical Processes in Transi-
tion-Metal Chemistry; VCH: New York, 1995.
(18) Wong, A.; Kang, P. C. W.; Tagge, C. D.; Leon, D. R. Organo-
metallics 1990, 9, 1992-1994.
(19) Vey, S. M.; Pauson, P. L. J . Chem. Soc. 1965, 4312.
(20) Bre´gaint, P.; Hamon, J .-R.; Lapinte, C. J . Organomet. Chem.
1990, 398, C25-C28.
(21) Akita, M.; Terada, M.; Oyama, S.; Moro-Oka, Y. Organometal-
lics 1990, 9, 816-825.
(22) Holmes, A. B.; J ennings-White, C. L. D.; Schulthess, A. H.;
Akinde, B.; Walton, D. R. M. J . Chem. Soc., Chem. Commun. 1979,
840-842.

5990 Organometallics, Vol. 16, No. 26, 1997
Ta ble 1. IR Sp ectr a l Da ta for th e Or ga n oir on Bu ta d iyn yl Com p lexes (KBr /Nu jol, cm -1
Coat et al.
)
end groups
compd
νCO
νCtC
2a
2b
2c
3a
3b
3c
(C5Ph5)Fe(CO)2
(C5Me5)Fe(CO)2
(C5Me5)Fe(dppe)
(C5Ph5)Fe(CO)2
(C5Me5)Fe(CO)2
(C5Me5)Fe(dppe)
(C5Me5)Fe(dppe)
(C5Me5)Fe(dppe)
(C5Me5)Fe(dppe)
(C5Me5)Fe(dppe)
SiMe3
SiMe3
SiMe3
H
H
H
(C5Ph5)Fe(CO)2
(C5Me5)Fe(CO)2
(C5Ph5)Fe(CO)2
(C5Me5)Fe(CO)2
2024, 1984
2031, 2017, 1970
2185, 2129
2171, 2120
2165, 2090, 1980
2141
2142
2099, 1958
2102
2109
2033, 1988
2020, 1964
5a
2022, 1976
2013, 1952
2037, 1993
2032, 1989
5b
5a +
5b+
a
1967, 1906
a
The νCtC bond stretching is obscured by the carbonyl vibrations.
Ta ble 2. 13C NMR (20 °C, C6D6) Ch em ica l Sh ifts (p p m ) for th e C4 Ch a in a
end groups
compd
CR
Câ
Cγ
Cδ
2a
2b
2c
3a
3b
3c
5a
5b
(C5Ph5)Fe(CO)2
(C5Me5)Fe(CO)2
(C5Me5)Fe(dppe)
(C5Ph5)Fe(CO)2
(C5Me5)Fe(CO)2
(C5Me5)Fe(dppe)
(C5Me5)Fe(dppe)
(C5Me5)Fe(dppe)
SiMe3
SiMe3
SiMe3
H
H
H
99.8
106.3
142.2
94.1
102.9
136.6
114.4
107.4
99.4
96.5
102.3
99.7
92.6
94.7
96.2
72.8
73.4
75.1
108.3
102.4
71.7
69.5
69.7
55.9
54.1
50.5
86.9
68.3
95.0
100.7
108.7
108.5
(C5Ph5)Fe(CO)2
(C5Me5)Fe(CO)2
a
In the binuclear compounds the carbon atoms are named CR-δ starting from the more electron-rich iron center Cp*Fe(dppe).
of the solid residue from CH2Cl2/pentane afforded 2c
in 40% yield. We noted a strong dependency of the yield
of the reaction upon concentration of the starting
materials, which could be due to a competitive coordina-
tion of the terminal acetylenic function of the butadiynyl
chain. Addition of 5% (v/v) of CH3CN in toluene greatly
improved the procedure. Indeed, after irradiation, the
solution became red-orange this time, and washing with
cold ether provided 70% of 2c. The acetonitrile ligand
is believed to temporarily replace the photodisplaced
carbonyl groups before the diphosphine coordinates to
the metal. This contrasts with the photochemical
displacement of the carbonyl ligands by the dppm in the
related ethynyl complex (C5H5)Fe(dppm)(CtCR), where
assistance of a coordinating solvent is not required.23
Treatment of complexes 2a ,b with 1.1 equiv of potas-
sium fluoride in a 50/50 CH3OH/THF mixture at room
temperature gave the respective terminal butadiyne
complexes Fe(η5-C5Re5)Fe(CO)2(CtCCtCH) (3a ,b), iso-
lated as brown and yellow solids in 85 and 95% yields
respectively. Surprisingly, 2c does not react under the
above conditions even under reflux. However, we found
that catalytic cleavage of the trimethylsilyl group in 3c
wavenumbers are clearly suggestive of an increase in
the retrodonation from the metal center in π*CO or
π*CtC. If this interpretation fits very well with current
knowledge for the carbonyl groups, it is rather unex-
pected for the ethynyl and butadiynyl ligands. Indeed,
it has been reported that the filled/filled interactions
between the occupied metal dπ orbitals and π orbitals
of the -C2- or -C4- fragment was the dominant
interaction taking place in the CpFe(CO)2 series, ret-
rodonation occurring only to a negligible extent.27
However, in the more electron rich Cp*Fe(dppe) series
the present results, including the X-ray analysis of
complex 5b (vide infra), together with other IR data
obtained on Cp*Fe(dppe)(CtCC6H4X) complexes28 could
indicate that a weak retrodonation in the π* orbitals
takes place.
The 13C NMR resonances of the -C4- chain were
easily observed, and the chemical shifts of the carbon
atoms proved to be sensitive to the nature of the
terminal groups (Table 2). The carbon resonances of
the butadiynyl ligand can unequivocally be assigned on
the basis of the J CH and J CP coupling constants in the
case of the complex 3c. Indeed, the 13C resonances of
the -C4H fragment are located in the spectrum at δ 50.5
is readily effected in pure THF using 0.2 equiv of Bun
-
4
1
2
4
NF. This route, which provided 3c as an orange-red
solid in 95% yield, was previously used for the desily-
lation of similar organometallic compounds.18,24
(Cδ, d, J CH ) 248 Hz), 75.1 (Cγ, dt, J CH ) 50 Hz, J PC
3
) 3 Hz), 100.7 (Câ, d, J CH ) 5 Hz) and 136.6 (CR, t,
2J PC ) 38 Hz). Comparison of the J CH and J CP coupling
4
3
The mononuclear analytically pure complexes 2a -c
and 3a -c were characterized by cyclic voltammetry and
IR, 1H, 13C, and 31P NMR, and Mo¨ssbauer spectroscopy.
IR spectra featured νCtC and νCO bands (Table 1). It is
noteworthy that IR spectra of the complexes 2a -c
which bear a trimethylsilyl group at one end of the
butadiynyl ligand exhibit one more CtC band stretch-
ing than the corresponding complexes 3a -c. Three
vibrational modes are observed for the carbonyl ligands
of 2b and for the νCtC bands of 2c. These unusual
observations could possibly result from the coupling of
one of the expected normal modes with another oscil-
lator.25,26 A decrease of both the νCtC and νCO frequen-
cies with increasing electronic density at the iron center
is observed for the series of compounds 2 and 3. These
constants establishes that J CP is larger than J CP,
4
whereas the J CH coupling constant was too small to be
observed. For 3a ,b similar attributions were made. An
3
upfield resonance was attributed to Câ (δ 99.7, J CH
)
5.8 Hz) relative to CR (δ 94.1, s) in compound 3a .
Accordingly, in the case of the trimethylsilyl homologue
(23) Gamasa, M. P.; Gimeno, J .; Lastra, E.; Lanfranchi, M.; Tirip-
icchio, A. J . Organomet. Chem. 1991, 405, 333-345.
(24) Sun, Y.; Taylor, N. J .; Carty, A. J . Organometallics 1992, 11,
4293-4300.
(25) Wojtkowiak, B. Ann. Chim. 1964, 9, 5-24.
(26) Fontaine, M.; Chauvelier, J .; Barchewitz, P. Bull. Soc. Chim.
Fr. 1962, 359, 2145-2150.
(27) Lichtenberger, D. L.; Renshaw, S. K.; Wong, A.; Tagge, C. D.
Organometallics 1993, 12, 3522-3526.
(28) Denis, R.; Weyland, T.; Paul, F.; Lapinte, C. J . Organomet.
Chem., in press.

Elemental Carbon Chain Bridging Two Iron Centers
Organometallics, Vol. 16, No. 26, 1997 5991
Ta ble 3. Electr och em ica l Da ta for Com p ou n d s
Sch em e 2
2-5a
compd
∆Ep
E0
ic/ia
∆E0
2a
2b
2c
3a
3b
3c
5a
-0.97
+1.15
+0.00
-0.87
-1.30
0.00
-0.28
+0.93
-0.36
+0.74
NR
NR
1
NR
NR
0.48
1
0.8
1
0.9
0.09
0.09
0.06
0.07
0.07
0.07
1.21
1.10
5b
a
All E and ∆E values in V vs SCE. NR ) nonreversible.
Conditions: CH2Cl2 solvent; 0.1 [nBu4N][PF6] supporting
M
electrolyte; 20 °C; Pt electrode; sweep rate 0.100 V s-1. The
ferrocene-ferrocenium couple (0.460 V vs SCE) was used as an
internal reference for the potential measurements.
2a , an upfield resonance was attributed as well to Câ
(δ 99.7) relative to CR (δ 94.4), since those carbon
resonances were observed at a very close position. A
more intuitive assignment was proposed for the other
compounds. Thus, substitution of the trimethylsilyl
group with a hydrogen atom or the substitution of an
ancillary ligand at the iron center results in variations
of the chemical shifts of the carbon atoms of the chain.
This can be taken as an indication of good communica-
tion between the end groups and the butadiyne π-sys-
tem.
2. Syn th esis a n d ch a r a cter iza tion of th e bi-
n u clea r µ-bu ta d iyn d iyl com p lexes (C5Me5)(d p p e)-
F e(CtCCtC)F e(CO)2(C5R5), (R ) P h , 5a ; R ) Me,
5b). The complexes 5a ,b were prepared in a one-step
procedure involving the complexation of the butadiyne
ligand of 3a ,b onto the [(Cp*)Fe(dppe)]+ fragment in the
presence of KPF6 to favor the iron-chlorine bond dis-
sociation.31,32 In situ deprotonation of the cationic
intermediate, resulting in the complexation of the
σ-metalated butadiyne at the Cp*Fe(dppe) unit, is
subsequently achieved by addition of 1 equiv of a strong
base.6,29 Thus, treatment of Fe(η5-C5Re5)Fe(CO)2-
(CtCCtCH) (3a ,b) KPF6, and KOBut in methanol with
1 equiv of Cp*Fe(dppe)Cl (4) produced a brown solution
from which the binuclear µ-butadiynediyl complexes
(C5Me5)(dppe)Fe(CtCCtC)Fe(CO)2(C5R5) (R ) Ph, 5a ;
R ) Me, 5b) were isolated in 59-69% yield (Scheme 2).
Subsequent crystallization gave analytically pure 5a
and 5b, which were characterized by IR, multinuclear
NMR, and Mo¨ssbauer spectroscopy. Several synthetic
routes to binuclear diynyl complexes have been re-
ported. Except for the synthesis of the [Fe]-C4-[Fe]
and [Re]-C4-[Re] symmetric compounds,6,33 which
were obtained upon oxidative coupling of the corre-
sponding transition-metal ethynyl derivatives, all other
C4 complexes were obtained from preformed diynyl
building blocks.18,34-39 More recently, a procedure based
The cyclic voltammograms (CV) of the butadiyne
complexes (C5R5)Fe(CO)2(CtCCtC)-SiMe3 (2a ,b) from
-1.5 to +1.5 V display one wave at a platinum electrode
(dichloromethane, 0.1 M tetrabutylammonium hexaflu-
orophosphate, 0.100 V s-1, 20 °C, Table 3). The very
different electrochemical behaviors observed for these
two complexes illustrate the opposite electronic effects
of the C5Ph5 and C5Me5 cyclic ligands, which stabilize
respectively the low and high oxidation states. Thus,
the very electron-poor complex (C5Ph5)Fe(CO)2-
(CtCCtCSiMe3) (2a ) shows an irreversible reduction
wave at -0.99 V corresponding to the Fe(II)/Fe(I) redox
system. The CV of the complex (C5Me5)Fe(CO)2-
(CtCCtCSiMe3) (2b) exhibits an irreversible oxidation
wave at +1.18 V attributed to the Fe(II)/Fe(III) redox
couple. Replacement of the two CO ligands by dppe
stabilizes the 17-electron iron(III), and a fully reversible
oxidation wave is observed for the complex (C5Me5)Fe-
a
(CO)2(CtCCtCSiMe3) (2c) with an ip /ipc current ratio
of unity (E0 ) -0.01 V vs SCE). As in the previous
ethynyl series, the presence of a bulky terminal end
group is still required to stabilize the iron(III) butadiy-
nyl radical cation, since the compound 3c shows only a
partially reversible oxidation process (Table 3).6,29
The Mo¨ssbauer spectral parameters of a recrystallized
sample of the mononuclear complexes 2b,c and 3c
recorded at zero field (80 K) are typical of pure iron(II)
centers with a quadrupole splitting weakly dependent
on the ligands coordinated to the metal (Table 4). A
significant increase of the isomeric shift from 0.017 to
0.245 mm/s associated with an increase of the electron
density at the iron center. The Mo¨ssbauer parameters
of the iron(II) and iron(III) sites in the Cp*Fe(dppe)R
series are usually weakly dependent on the tempera-
ture.30 This general trend is confirmed for the butadiy-
nyl derivatives in the case of complex 2c.
(30) Roger, C.; Hamon, P.; Toupet, L.; Rabaaˆ, H.; Saillard., J .-Y.;
Hamon, J .-R.; Lapinte, C. Organometallics 1991, 10, 1045-1054.
(31) Bruce, M. I.; Swincer, A. G. Adv. Organomet. Chem. 1987, 52,
3940.
(32) Bruce, M. I. Chem. Rev. 1991, 91, 197-257.
(33) Zhou, Y.; Seyler, J . W.; Weng, W.; Arif, A. M.; Gladysz, J . A. J .
Am. Chem. Soc. 1993, 115, 8509-8510.
(34) Sonogashira, K.; Kataoka, S.; Takahashi, S.; Hagihara, N. J .
Organomet. Chem. 1978, 160, 319-327.
(35) Fyfe, H. B.; Mlekuz, M.; Zargarian, D.; Taylor, N. J .; Marder,
T. B. J . Chem. Soc., Chem. Commun. 1991, 188-189.
(36) Stang, P. J .; Tykwinski, R. J . Am. Chem. Soc. 1992, 114, 4411-
4412.
(37) Crescenzi, R.; Lo Sterzo, C. Organometallics 1992, 11, 4301-
4305.
(38) Rappert, T.; Nu¨rnberg, O.; Werner, H. Organometallics 1993,
12, 1359-1364.
(39) Bruce, M. I.; Hinterding, P.; Tiekink, E. R. T.; Skelton, B. W.;
White, A. H. J . Organomet. Chem. 1993, 450, 209218.
(29) Connelly, N. G.; Gamasa, M. P.; Gimeno, J .; Lapinte, C.; Lastra,
E.; Maher, J . P.; Narvor, N. L.; Rieger, A. L.; Rieger, P. H. J . Chem.
Soc., Dalton Trans. 1993, 2575-2578.

5992 Organometallics, Vol. 16, No. 26, 1997
Ta ble 4. 57F e Mossba u er F ittin g P a r a m eter s for Com p ou n d s 2-5
δ, mm/s ∆EQ, mm/s Γ, mm/s
Coat et al.
% area
compd
T (K)
FeII
FeIII
FeII
FeIII
FeII
FeIII
FeII
100
100
100
100
FeIII
2b
2c
2c
3c
5b
80
80
293
80
0.017
0.245
0.261
0.255
0.257
0.029
0.192
-0.033
-0.048
-0.028
-0.050
2.030
1.934
1.992
1.920
1.973
1.974
1.978
1.958
1.922
1.990
1.967
0.125
0.126
0.122
0.130
0.121
0.133
0.100
0.160
0.160
0.132
0.230
80
49.96
50.04
49.91
50.09
49.8
48.5
49.0
5b
293
5a +
5b+
5b+
80
80
293
0.160
0.206
0.152
0.865
0.922
0.838
0.300
0.216
0.179
50.2
49.8
49.9
on CuI-catalyzed reactions in diethylamine was re-
ported. This route allowed the preparation of homo- and
heterobimetallic complexes with a C4 bridge.40 Starting
from the preformed iron butadiynyl complexes 3a ,b, we
found a smooth route to the homobinuclear derivatives
5a ,b which required neither metal catalyst assistance
nor a very strong deprotonating agent such as butyl-
lithium. The procedure has also been extended to the
preparation of heterobinuclear -Cx- complexes that
contain both rhenium and iron end groups.41
A significant improvement of the synthesis of 5b was
found by starting from 2b and following a one-pot
procedure. The (trimethylsilyl)butadiynyl organoiron
complex 2b was solubilized in a THF/methanol mixture
(50/50) before being refluxed in the presence of a small
excess of anhydrous potassium fluoride. The resulting
dark suspension was cooled to -50 °C, and 2.2 volumes
of methanol, the organoiron complex 4, KPF6, and KOt-
Bu were added. After the mixture was stirred overnight
and slowly warmed to room temperature, a brick red
solution was obtained, from which 5b was isolated (85%
yield).
The IR spectra of the complexes 5a ,b display two
strong νCO band stretchings and one νCtC absorption.
The substitution of the terminal H in complexes 3a ,b
by the Cp*Fe(dppe) unit lowers the IR frequency of the
carbonyl groups and reveals thereby the occurrence of
electronic communication between the metal centers
through the -C4- spacer. Considering that the chemi-
cal shift of the Cp* ligand in the butadiynyl complexes
3b,c are observed at δ 1.34 and 1.44, respectively, the
two Cp* resonances observed at δ 1.47 and 1.54 in the
1H NMR spectrum of the complex 5b were attributed
to the methyl groups of the C5 ring bound to the Cp*Fe-
(dppe) and Cp*Fe(CO)2 building blocks, respectively.
F igu r e 1. X-ray crystal structure of the complex (C5-
Me5)(dppe)Fe(CtCCtC)Fe(CO)2(C5Me5) (5b).
upfield shift of the CR carbon at the remote end of the
bridge. Again, this shows that the electronic effects are
transmitted from one metal center to the other through
the carbon chain.
As already pointed out above, the isomeric shift of the
Mo¨ssbauer doublet for the mononuclear complexes 2b,c
strongly depends on the nature of the ancillary ligand
coordinated to the iron centers. Accordingly, the Mo¨ss-
bauer spectrum of the diiron compound 5b recorded at
zero field (80 K) displays two well-separated doublets
with the same areas (Table 4). Both doublets have
almost the same quadrupole splitting. This is charac-
teristic of piano-stool iron(II) complexes with a very
weak temperature dependence. Depending upon the
electron density at the iron centers, the isomeric shifts
of the two doublets are quite different (δ ) 0.257/0.192
and 0.029/-0.033 mm s-1 at 80/293 K, respectively), in
agreement with a strong polarization of this homobi-
nuclear compound. The isomeric shifts at δ 0.257 and
0.029 are assignated to the iron centers of the Cp*Fe-
(dppe) and Cp*Fe(CO)2 building blocks, respectively, on
the basis of the isomeric shift determined for the
mononuclear complexes 2b,c. In contrast with the
behavior of the mononuclear species, the isomeric shift
of the binuclear complex 5b is temperature dependent
and the separation between the two doublets is almost
constant.
Crystals of 5b were grown by vapor diffusion of
n-pentane in a toluene solution of the binuclear complex.
The unit cell contains four molecules. The molecular
structure of compound 5b is shown in Figure 1, and the
X-ray data conditions are summarized in Table 5.
Selected bond distances and bond angles are collected
in Table 6. Complex 5b crystallizes in the monoclinic
space group P21/n. The two metal centers clearly adopt
a pseudooctahedral geometry, as generally observed for
the piano-stool complexes, with the Cp* ring occupying
The carbon atoms of the linkage were named CR-δ
,
starting from the more electron rich iron building block
Cp*Fe(dppe), and the 13C carbon resonances of the
butadiynyl ligand were again assigned on the basis of
4
the J CP coupling constants, assuming that J CP is larger
3
than J CP as seen for 3c (vide supra). On this basis a
more upfield resonance was attributed to the Cγ carbon
atom Cγ (δ 107.7) than to Câ (δ 108.3) in complex 5a .
The Câ (δ 108.5) carbon of 5b is also observed at upper
field relative to CR (δ 107.4). The difference between
the chemical shift of the CR and Cδ carbon atoms
indicates that the C4 linkage is polarized. The replace-
ment of the phenyl groups of the pentaphenylcyclopen-
tadienyl species by methyl substituents induces an
(40) Bruce, M. I.; Ke, M.; Low, P. J . Chem. Commun. 1996, 2405-
2406.
(41) Gladysz, J . A.; Lapinte, C.; Paul, F.; Meyer, W. E. Work in
progress.

Elemental Carbon Chain Bridging Two Iron Centers
Organometallics, Vol. 16, No. 26, 1997 5993
Ta ble 5. Exp er im en ta l Cr ysta llogr a p h ic
Da ta for 5b
compounds (M ) Ru, Re, Rh) with a butadiynediyl
linkage have been reported.39,46,47 It has been shown
that the CtC carbon triple bonds range between 1.19
and 1.21 Å and the central carbon-carbon single bond
between 1.37 and 1.39 Å. Concerning the [Fe]-C4-[Fe]
linkage of 5b, the X-ray data reveal shorter FesC and
CtC bond lengths on the electron-rich side of the
molecule than on the electron-poor moiety. This is in
agreement with the polarization of the electron density
from the donor Fe(P)2 center to the acceptor Fe(CO)2
group already suggested by 13C NMR data. The com-
parison of the iron-carbon and carbon-carbon bond
distances has to be made with caution in the limit of
the accuracy of the X-ray data. However, the differences
in the bonding patern along the [Fe]-C4-[Fe] axis could
also be the result of effective π* back-bonding on the
Fe(P)2 side of the complex, as previously suggested from
the IR data (vide supra). Thus, the length of the C37-
Fe1 bond (1.886(9) Å) seems to be shorter than the
C(40)-Fe(2) bond distance (1.90(1) Å), in agreement
with an increased bond order on the more electron rich
metal side. Comparison of the bond angles also shows
an increase of the linearity of the Fe-C4-Fe axis on
this side of the molecule. Curiously, the C(37)-C(38)
triple bond length (1.21(1) Å) is very close to the
carbon-carbon triple bond of the butadiyne (1.218(2)
Å), whereas the C(39)-C(40) triple bond (1.24(1) Å) is
significantly longer than the distance observed in the
organic molecule.48-50 The length of the C(38)-C(39)
single bond (1.36(1) Å) is shorter than the C-C bond
distance of the butadiyne (1.384(2) Å). A distance
similar to those determined for the central C-C bond
of 5b was observed in the case of the mixed-valence
complex [Cp*(dppe)Fe(CtCCtC)Fe(dppe)Cp*][PF6] (6).6
Discussion of the bond length in this system is not
straightforward. One should not forget that retrodo-
nation occurs in the empty π* orbital set of the whole
butadiynediyl ligand, whereas the metal-dπ-buta-
diynediyl-π interactions are filled/filled type interac-
tions.27 Both types of interactions are possible in this
donor-acceptor polarized [Fe]-CtCCtC-[Fe′] assem-
bly. As a result, a strong electronic communication
between the two organometallic fragments is believed
to occur via the carbon bridge. In the complex 5b the
distance between the two nonequivalent iron atoms is
7.57 Å. Cyclic voltammogram measurements were
performed to determine the mutual interaction between
the metal centers over this distance.
formula
fw
C52H54Fe2P2O2
884.65
cryst syst
space group
a, Å
monoclinic
P21/c
21.099(5)
13.868(4)
15.924(9)
104.60(3)
4509(2)
4
b, Å
c, Å
â, deg
V, Å3
Z
D
calcd, g/cm3
1.303
F(000)
1856
7.504
294
µ(Mo KR), cm-1
T, K
cryst size, mm
max 2θ, deg
scan
0.12 × 0.12 × 0.28
50
ω/2θ ) 1
60
t
max/measure, s
variance of standards
range of hkl
no. of rflns measd
no. od rflns obsd (I > σ(I))
Rint (from merging equiv rflns)
R (isotropic)
0.9%
-23 to +22; 0-15; 0-17
6854
3240 (1.5σ)
0.027
0.11
difference Fourier
final R
0.63-0.32
0.073
Rw
0.064
w ) 1/σ(Fo)2 ) [σ2(I) +
(0.04Fo2)2]-1/2
Sw
2.99
0.40, 0.05
residual density (∆/σ), e Å-3
Ta ble 6. Selected Bon d Dista n ces (Å) a n d
An gles (d eg) for 5b
Fe(1)-P(1)
2.190(3)
2.172(3)
1.76(1)
1.73(1)
1.740(3)
1.706(3)
Fe(1)-C(37)
Fe(2)-C(40)
C(37)-C(38)
C(38)-C(39)
C(39)-C(40)
C(1)-C(2)
1.886(9)
1.90(1)
1.21(1)
1.36(1)
1.24(1)
1.38(2)
Fe(1)-P(2)
Fe(2)-C(51)
Fe(2)-C(52)
Fe(1)-Cp*(centroid 1)
Fe(2)-Cp*(centroid 2)
P(1)-Fe(1)-P(2)
P(1)-Fe(1)-C(37)
P(2)-Fe(1)-C(37)
85.7(1) C(51)-Fe(2)-C(52)
83.8(3) C(51)-Fe(2)-C(40)
83.4(3) C(52)-Fe(2)-C(40)
95.3(5)
87.3(5)
90.4(5)
Fe(1)-C(37)-C(38) 177.8(8) Fe(2)-C(40)-C(39) 172.0(1)
C(37)-C(38)-C(39) 175(1)
C(38)-C(39)-C(40) 178.4(9) Fe(2)-C(52)-O(2)
Fe(2)-C(51)-O(1)
176(1)
178(1)
three coordination sites and the carbon of the -C4-
bridge and the phosphorus atoms of the dppe or the two
carbons of the carbonyl ligands occupying the other
three sites. The C(51)-Fe(2)-C(52) angle is signifi-
cantly more opened than the P(1)-Fe(1)-P(2) angle.
The bond lengths and angles for both organoiron build-
ing blocks compare well with the data determined for
other mononuclear complexes in the FeCp*(dppe) or
FeCp*(CO)2 series.21,42-45 The volumes occupied by the
two organoiron units are quite different and the steric
protections of the two iron centers are very different.
The relative orientation of the two organometallic
building blocks defined by the planes of the two Cp*
rings gives an angle of 126.9(4)°.
3. Cyclic Volta m m etr y An a lysis of th e Bin u clea r
Diir on µ-Bu ta d iyn ed iyl Com p lexes (C5Me5)(d p p e)-
F e(CtCCtC)F e(CO)2(C5R5) (R ) P h , 5a ; R ) Me,
5b). The initial scans in the CV of the butadiyne-
bridged binuclear complexes 5a ,b from -0.8 to +1.4 V
display two oxidation waves in dichloromethane, show-
ing that, at the electrode surface (Figure 2), the neutral
dimers undergo two successive one-electron oxidations
to yield the mono- and dications, respectively. The data
for peak potentials and current ratio compiled in Table
3 are quite similar for 5a and 5b. The ∆Ep values are
The mixed-valence complex 5b is, to our knowledge,
the first example of a donor-acceptor [MLn]-CtCCtC-
[ML′n′] to be structurally characterized. However,
several solid-state structures of symmetric bimetallic
(46) Yam, V. W.-W.; Lau, V. C.-Y.; Cheung, K.-K. Organometallics
1996, 15, 1740-1744.
(42) Weyland, T.; Lapinte, C.; Frapper, G.; Calhorda, M. J .; Halet,
J .-F.; Toupet, L. Organometallics 1997, 16, 2024-2031.
(43) Hamon, P.; Toupet, L.; Hamon, J .-R.; Lapinte, C. J . Chem. Soc.,
Chem. Commun. 1994, 931-932.
(47) Gevert, O.; Wolf, J .; Werner, H. Organometallics 1996, 15,
2806-2809.
(48) Beer, M. J . Chem. Phys. 1956, 25, 745-750.
(49) Coles, B. F.; Hitchcock, B. P.; Watson, D. R. M. J . Chem. Soc.,
Dalton Trans. 1975, 442-445.
(50) Rubin, Y.; Lin, S. S.; Knobler, C. B.; Anthony, J .; Boldi, A. M.;
Diederich, F. J . Am. Chem. Soc. 1991, 113, 6943-6949.
(44) Mahias, V.; Cron, S.; Toupet, L.; Lapinte, C. Organometallics
1996, 15, 5399-5408.
(45) Goddard, R.; Howard, J .; Woodward, P. J . J . Chem. Soc., Dalton
Trans. 1974, 2025-2027.

5994 Organometallics, Vol. 16, No. 26, 1997
Coat et al.
Sch em e 3
Sch em e 4
could be explained by the participation of the electron
of the SOMO on the stabilization of both the mixed-
valence species and the dications [5a ,b][P F 6]2. The
delocalization of the odd electron on the metal center
but also on the carbon atoms of the -C4- spacer could
explain the diminution of the oxidation potential as well
as the increased reversibility of the second oxidation
process (Scheme 3). In fact, in the two oxidation
processes, the electron should be removed from highly
delocalized molecular orbitals all over the LnFe-C4-
FeL′n′ assembly and stabilized by participation of a
cumulenic resonance structure depicted by the canonical
form B in Scheme 4. However, despite a dramatic
increase, the ic/ia current ratio is below unity for the
second redox process and, as a consequence, the dica-
tions [(C5Me5)(dppe)Fe(CtCCtC)Fe(CO)2(C5R5)][PF6]2
(5a ,b)2+ are not thermally stable enough to be consid-
ered as accessible synthetic targets.
4. Syn t h eses a n d P r op er t ies of t h e Mixed -
Va len ce Com p lexes [(C5Me5)(d p p e)F e(CtCCtC)-
F e(CO)2(C5R 5)][P F 6] (R ) P h , 5a [P F 6]; R ) Me,
5b[P F 6]). On the basis of the CV results, oxidation 5
was carried out with [Cp2Fe][PF6]. The addition of 0.95
equiv of the oxidizing agent to 5 in CH2Cl2 resulted in
a rapid color change from dark red to brown. After
precipitation by pentane or diethyl ether, the Fe(III)-
Fe(II) complexes [5a ,b][P F 6] were isolated as deep
green or dark brown powders in 85-90% yields (Scheme
2). Both salts are stable to heat and air for short periods
of time, and their CV scans exhibit two waves identical
with those of their Fe(II)-Fe(II) parents. These com-
plexes were characterized by elemental analysis and
Mo¨ssbauer, ESR, IR, UV-vis, and NIR spectroscopy.
It is well-documented that 57Fe Mo¨ssbauer spectros-
copy allows identification of the oxidation states of the
iron centers.52 The Mo¨ssbauer parameters of mono-
nuclear and binuclear half-sandwich iron species are
very different for Fe(II) and Fe(III).6,30,52 The Mo¨ss-
bauer spectra of the complexes 5a ,b and [5a ,b][P F 6]
were run at 80 and 293 K, and the parameters (Table
4) show that the oxidation takes place on the Cp*Fe-
(dppe) side of the molecule. Expectedly, the spectra of
the salts [5a ,b][P F 6] clearly exhibit two doublets for
localized Fe(II)-Fe(III) mixed-valence systems. From
•
F igu r e 2. Cyclic voltammograms for (C5Me5)(dppe)Fe-
(CtCCtC)Fe(CO)2(C5R5) (a, R ) Ph; b, R ) Me) and
related mononuclear complexes in 0.1 M [nBu4N][PF6]/CH2-
Cl2 (Pt electrode; V vs SCE; scan rate 0.100 V/s; 20 °C).
in the range 0.06-0.07 V, and the second redox process
becomes reversible when the scan rate increases. For
c
a
both compounds 5a and 5b the current ratio ip /ip ) 1
for the first oxidation waves indicates the chemical
reversibility of the first electron transfer and the
c
thermal stability of the monocations (5a ,b)+. The ip /
a
ip current ratio, less than unity, ranges from 0.8 to 0.9
(scan rate 0.1 V s-1) for the second electron transfer,
indicating that the dications (5a ,b)2+ slowly decompose
at the platinum electrode. The comparison of the CV
parameters with those of the corresponding monomers
2a -c confirms the strong electronic communication
between the metal centers across the all-carbon con-
nector. Moreover, the role of the electron-donating-
C4-bridge is illustrated by comparison of the oxidation
potential of the mononuclear electron-rich complex 2c.
The first oxidation potentials of the binuclear com-
pounds 5a ,b are located at a potential 0.3 V less positive
than for 2c.
The quasi-reversibility of the second redox process is
the result of the communication between the two iron
building blocks across the connector but also constitutes
a new illustration of the electron-donating role of the
-C4- bridge. The comparison of the CV waves and the
potential of the second almost-reversible redox process
of 5a and 5b with the redox properties of 2a and 2b
evidences a dramatic change (Figure 2). The increase
of the reversibilty of the second oxidation process
associated with an important diminution of the potential
indicates a strong stabilization of the dioxidized species
[5a ,b][P F 6]2. On the other hand, it is also noteworthy
that the potentials measured for 5a and 5b are rather
close (∆E ) 0.15 V), whereas it was shown that the
replacement of the methyl substituents on the cyclo-
pentadienyl ring by phenyl groups decreases the oxida-
tion potential of mononuclear compounds by 0.5 V.51 The
original redox properties of these binuclear complexes
(51) Bre´gaint, P.; Hamon, J .-R.; Lapinte, C. J . Organomet. Chem.
1990, 398, C25-C28.
(52) Greenwood, N. N. Mo¨ssbauer Spectroscopy; Chapman and
Hall: London, 1971.

Elemental Carbon Chain Bridging Two Iron Centers
Organometallics, Vol. 16, No. 26, 1997 5995
Ta ble 7. ESR Da ta for [Cp *F e(d p p e)R]•+ in a
CH2Cl2/ClCH2CH2Cl (1/1) Gla ss a t 77 K
Ta ble 8. Visible a n d Nea r -In fr a r ed Sp ectr a l Da ta
for th e Mixed -Va len ce Com p lexes [5a ,b][P F 6],
[6][P F 6], a n d [7][P F 6]
compd
R
g1
g2
g3
λmax
1/Dop - 1/Ds (nm) (M-1 cm-1
10-3ꢀmax
νmax
from ref 30 -H
1.9944
1.987
1.9719
2.0430 2.4487
2.034 2.457
2.0325 2.4857
2.0214 2.3474
compd
solvent
)
(cm-1
)
from ref 29 -CtCH
2b+
5a +
5b+
-CtCCtCSiMe3
[5a ][P F 6] CH2Cl2
[5b][P F 6] CH2Cl2
[5b][P F 6] CH3COCH3
[5b][P F 6] CH3CN
[5b][P F 6] MeOH
[6][P F 6] CH2Cl2
[7][P F 6] CH2Cl2
0.382
0.383
0.493
0.526
0.536
0.382
0.382
810
a
2.0
12 345
-CtCCtCFe(CO)2(C5Ph5) 1.9867
-CtCCtCFe(CO)2(C5Me5) 1.9865a 2.0497 2.3343
829
1600
829
1622
829
1609
829
1620
575
663
619
845
1302
619
829
8.3
0.36
12 062
6 250
12 062
6 165
12 062
6 215
12 062
6 172
17 391
15 082
16 155
11 834
7 541
a
A1 ) 11.67 G.
0.27
0.27
the QS values it can be concluded that the electron
vacancy is mainly localized on the Cp*Fe(dppe) orga-
nometallic fragment. Characteristic parameters of a 17-
electron low-spin iron(III) metal center are observed,
whereas the isomeric shift and quadrupole spliting of
the (C5R5)Fe(CO)2 moieties correspond to pure iron(II)
centers. The very similar values observed for the IS and
QS of the (C5Me5)Fe(CO)2 unit in 2b, 5b, and [5b][P F 6]
indicate that the electronic structure of this iron center
is not very sensitive to the nature of the fragment
σ-bound at the remote end of the -C4- linkage or, at
least, that the Mo¨ssbauer spectroscopy is less suited
than the IR spectroscopy to reveal these interactions
(vide infra).
0.22
17.39
15.08
4.80
3.2
12
7.87
3.79
[7][P F 6]2 CH2Cl2
0.382
16 155
12 062
a
Near-IR spectrum not recorded.
organometallic building blocks. The 18-electron Cp*Fe-
(dppe) group is more electron donating than the hydro-
gen atom and produces a weak diminution of the
carbonyl frequencies of the carbonyl ligands coordinated
at the other iron center through the -C4- connector.
In contrast, the 17-electron radical cation [Cp*Fe-
(dppe)]+ produces an increase of the carbonyl frequen-
cies. This also supports a weak contribution of the
resonance from D to the description of the electronic
structure of the mixed-valence complex [5a ,b][P F 6].
Consequently, the one-electron oxidation/reduction pro-
cess might allow control of the molecular dipolar mo-
ment. Such a property is of interest for the design of
electrochemically switchable NLO/active materials.
The visible spectroscopy of transition-metal complexes
is generally discussed with the assumption that the
various spectroscopic transitions are classified as metal
centered (MC), ligand centered (LC), metal to ligand
charge transfer (MLCT), or ligand to metal charge
transfer (LMCT). It was recently pointed out that this
description constitutes a simplified picture.53 This is
particularly true for the σ-butadiynediyl-bridged transi-
tion-metal complexes, for which there is a large orbital
mixing. The LC and MLCT configurations could lie very
close in energy, as in some cyclometalated complexes.54,55
Thus, assignment of the transition in the UV-vis
spectrum of the neutral complexes 5a ,b and of the
corresponding mixed-valence binuclear cations [5a ,b]-
[P F 6] is not straightforward. A maximum is observed
around 210 nm, at the edge of the solvent absorption
and features a great deal of shoulders alongside that
spread over the visible range. The energy of these π-π*
ligand-centered transitions is difficult to evaluate with
precision, since no clear maxima could be observed
above 350 nm. No other maximum is observed for the
neutral complexes. An intense absorption at the limit
between the visible range and near-infrared range is
present in the spectrum of the oxidized dimers (Table
8). A similar observation of an intense absorption in
the 700-900 nm region was previously done for related
X-Band ESR spectra of the binuclear radical cations
[5a ,b•+] dissolved in a glass of CH2Cl2/ClCH2CH2Cl (1/
1) were run at 80 K. The ESR spectra exhibited three
well-separated features corresponding to the three
components of the g tensor (Table 7). In both spectra
the resolution of the high-field component into a 1/2/1
triplet was observed, in agreement with a hyperfine
coupling with two equivalent 31P nuclei. The g tensors
and 31P couplings are very close to those previously
observed for related iron(III) d5 low-spin compounds.
The large value usually observed for the g3 tensor in
this type of piano-stool iron(III) complexes was at-
tributed to the energetic proximity of the singly occupied
2
2
HOMO of predominant dx -y character with the doubly
occupied levels.30 However, it is noteworthy that the
g3 values observed for the binuclear one-electron radi-
cals 5a •+ and 5b•+ are significantly weaker than those
previously observed for the closely related mononuclear
radical [Cp*Fe(dppe)(CtCH)]•+ (Table 7).29 The ESR
spectrum of the butadiynyl iron radical [Cp*Fe(dppe)-
(CtCCtCH)]•+ also exhibits a similar spectrum. The
comparison of these values suggests that the HOMO in
the binuclear species would be more separated from the
levels of the SOMO orbitals. This could be the conse-
quence of a repulsive interaction between the d orbitals
of two metal centers across the carbon bridge, in
agreement with the more negative oxidation potential.
The IR data in Table 1 show that the -CtC- bond
stretching is observed in the Fe(II)-Fe(III) complex [5b]-
[P F 6] at a significantly lower frequency than in the
mononuclear species 5b, indicating the σ-complexation
of the two termini of the butadiynediyl bridge. In
agreement with the Mo¨ssbauer and ESR data the
canonical structure A (Scheme 3) mainly contributes to
the description of the mixed-valence complexes [5a ,b]-
[P F 6]. Additionally, the IR data support a weak
contribution of the resonance structures B and C.
Comparisons of the νCO bond stretching of the binuclear
neutral and monocationic complexes 5a ,b and [5a ,b]-
[P F 6] with those of the mononuclear species 2a ,b or
3a ,b provide the most direct indication of the opposite
electronic effect of the Cp*Fe(dppe) and [Cp*Fe(dppe)]+
(53) Serroni, S.; J uris, A.; Campagna, S.; Venturi, M.; Denti, G.;
Balzani, V. J . Am. Chem. Soc. 1994, 116, 9086-9091.
(54) Maestri, M.; Balzani, V.; Deuschel-Cornioley, C.; Von Zelewsky,
A. Adv. Photochem. 1992, 17, 1.
(55) Schmid, B.; Garces, F. O.; Watts, R. J . Inorg. Chem. 1994, 33,
9-14.

5996 Organometallics, Vol. 16, No. 26, 1997
Coat et al.
2.06 × 10-2
iron(III) complexes. For instance, the visible spectrum
of the mononuclear iron(III) derivative [Cp*Fe(dppe)-
(CtC-C5H4)][PF6] [6][P F 6]56 and of the binuclear
compounds [Cp*(dppe)Fe-CtCCtC-Fe(dppe)Cp*][PF6]n
(n ) 1, 2; [7][P F 6]n ) show an intense absorption in the
visible range57 (Table 8). Since these bands have not
been observed in the spectrum of their iron(II) relatives,
we have tentatively attributed them to a ligand to metal
charge transfer. However, as far as the -C4- carbon
chain is concerned, the localized MO approach may no
longer be valid and transitions between poorly localized
orbitals can be involved.
1/2
Vab
)
(ꢀmax
ν
max∆ν1/2
)
(1)
Rab
νmax is the transition energy, and ν1/2 is the full width
at half-maximum in cm-1
.
From the near-IR data, the electronic parameter was
calculated to be Vab ) 0.021 eV. A much larger value
was found for the symmetrical mixed-valence complex
[7][P F 6] (Vab ) 0.47 eV),6 showing that the overlap
between the the electronic wave functions of the donor
and acceptor groups decrease abruptly with the energy
difference between the donor and acceptor centers. We
established also that the position of the maximum of
the ICT band was virtually independent of the solvent.
For the mixed-valence compounds the independence of
the energy of the ICT with the solvent is generally
interpreted as an indication of a weak inner-sphere and
outer-sphere reorganization energy associated with the
intramolecular electron transfer.62 Comparison of the
geometry of the iron(II) center in complex 5b with that
of the iron(III) compound [Cp*Fe(dppe)(CH2OCH3)]-
[PF6]63 indicates that the reorganization of the coordi-
nation sphere of the Cp*Fe(dppe) backbone should be
weak upon the one-electron-oxidation process. More-
over, the large protection of the Fe-C4-Fe axis provided
by the bulky ligands bound at the metal centers strongly
contributes to minimize the reorganization of the solvent
molecules during the intramolecular electron transfer.
In conclusion, the electrochemical and spectroscopic
studies performed on the donor-acceptor homobinuclear
system [Fe]-C4-[Fe′]n+ show that the properties of the
iron units in the molecular assemblies are different from
their properties in individual organoiron complexes.
Clearly, contribution of the four-carbon spacer takes
place in the properties of the dinuclear species. In our
mind, the key role played by the all-carbon spacer is
2-fold. First, it provides rigidity to the molecular
backbone, and second, it makes possible new properties
that the individual components do not possess. These
properties originate from the large degree of covalency
of the metal-carbon bonds, which favors mixing of the
orbitals of the carbon linkage with those of the transi-
tion metals. Finally, such molecules can be considered
as interesting models of polarized molecular wires.
More importantly, we have also stated that the initial
polarization could be reversed upon oxidation, thereby
opening the possibility of external control or switching
of the polarity. We believe that this kind of system has
a high potential for the elaboration of new electronic
devices.
The ICT character of the absortion at 829 nm was
excluded by the observation of the electronic spectrum
of the unstable dioxidized species [5b][P F 6]2. The CV
data indicated that the dioxidized complex decomposes
at 20 °C in less than a few minutes; however, it could
be generated in situ. Thus, the complex [5b][P F 6] was
directly oxidized in the spectroscopic cell by addition of
silver hexafluorophosphate to a CH2Cl2 solution. Spec-
troscopic monitoring of the solution allowed observation
of a shift of the absorption band to higher frequencies
(762 nm) followed by a rapid disappearance of the
absorption due to decomposition of the dioxidized com-
plex, in agreement with the CV data. This gives no
information regarding the intensity of the real extinc-
tion coefficient of the band at 762 nm, since the
decomposition of the dication is concomitant with its
formation. However, the similarity between the absorp-
tion spectra of [5b][P F 6] and [5b][P F 6]2 is strongly
indicative of the LMCT character of this transition.
Examination of the near-infrared domain with close
scrutiny, using a concentrated solution (10-2 M) of the
very soluble mixed-valence complex [5b][P F6], did allow
detection of another absorption at 1600 nm. This very
broad and weak band is absent both in the neutral
compound 5b and in the in situ generated derivative
[5b][P F 6]2. It was shown that pseudooctahedral 17e
complexes of manganese or chromium sometimes dis-
play very low energy electronic absorptions in the near-
infrared.58 We confirmed that the mononuclear iron(III)
17e species [6][P F 6] did not display such an electronic
transition in the near-IR region.59 Consequently, we
ascribed the observed band at 1600 nm to an interva-
lence charge transfer (ICT) in [5b][P F 6]. The ICT
bands of mixed-valence compounds can furthermore
allow the calculation of the electronic coupling param-
eter Vab. This parameter represents the adiabaticity of
the electron transfer between the two redox centers, or
in other words, it can be seen as the mixing between
the HOMO orbitals of the metal centers with the
π-orbital of the carbon chain.17,60,61 For the class II
valence-trapped asymmetric compounds the electronic
coupling is weak. It is accessible from the Hush formula
(Vab in cm-1, eq 1), in which Rab is the distance between
the metal centers in Å, ꢀmax is the extinction coefficient,
Exp er im en ta l Section
Gen er a l Da ta . Reagent grade tetrahydrofuran (THF),
diethyl ether, and n-pentane were dried and distilled from
sodium benzophenone ketyl prior to use. Pentamethylcyclo-
pentadiene was prepared according to the published proce-
dure,64 and other chemicals were used as received. All the
manipulations were carried out under an argon atmosphere
using Schlenk techniques or in a J acomex 532 drybox under
nitrogen. The FTIR spectra were recorded using a Nicolet
instrument (Model 205) and KBr windows. Routine NMR
spectra were recorded using a Bruker AW 80 MHz spectrom-
(56) Weyland, T. Synthesis and Properties of Bi- and Trinuclear
Organoiron Complexes Having Different Oxidation States. PhD Dis-
sertation, University of Rennes 1, 1997; p 231.
(57) Weyland, T.; Lapinte, C. Unpublished results (see Table 8).
(58) Atwood, C. G.; Geiger, W. E. J . Am. Chem. Soc. 1994, 116,
10849-10850.
(59) Weyland, T.; Lapinte, C. Unpublished results.
(60) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391-444.
(61) Sauvage, J .-P.; Collin, J .-P.; Chambron, J .-C.; Guillerez, S.;
Coudret, C. Chem. Rev. 1994, 94, 993-1019.
(62) Creutz, C.; Taube, H. J . Am. Chem. Soc. 1973, 95, 1086-1094.
(63) Roger, C.; Toupet, L.; Lapinte, C. J . Chem. Soc., Chem.
Commun. 1988, 713-714.
(64) Kohl, F. X.; J utzi, P. J . Organomet. Chem. 1983, 243, C37-
C38.

Elemental Carbon Chain Bridging Two Iron Centers
Organometallics, Vol. 16, No. 26, 1997 5997
eter. High-field NMR experiments were performed on a
multinuclear Bruker 300 MHz instrument (AM300WB). Chemi-
cal shifts are given in parts per million relative to tetrameth-
yielded a dark red solid that was washed twice with 2 mL of
cold ether (-40 °C). The remaining red powder was identified
as complex 2c (1.100 g, 70%). Anal. Calcd for C43H48FeP2Si:
C, 72.67; H, 6.81. Found: C, 72.82; H, 7.24. FT-IR (KBr/Nujol,
cm-1): ν 2166 (w, CtC); 2090 (s, CÃ); 1980 (m, CÃ). FTIR
(KBr/CH2Cl2, cm-1): ν 2162 (w, CtC); 2081 (s, CtC); 1977
(m, CtC). 31P NMR (121 MHz, C6D6): δP 99.6 (s). 1H NMR
(300 MHz, C6D6): δH 7.93-6.99 (m, 20H, Ph); 2.48, 1.69 (2m,
4H, PCH2); 1.39 (s, 15H, C5Me5); 0.24 (s, 9H, SiMe3). 13C NMR
1
ylsilane (TMS) for H and 13C NMR spectra and H3PO4 for 31P
NMR spectra. Cyclic voltammograms were recorded using a
PAR 263 instrument and a saturated calomel electrode.
Potentials are given in volts vs SCE, and the ferrocene-
ferrocenium couple was used as an internal calibrant for the
potential measurements (E0 ) 0.460 V vs SCE).65 X-Band ESR
spectra were recorded on a Bruker ESP-300E spectrometer.
Mo¨ssbauer spectra were recorded with a 2.5 × 10-2 Ci (9.25
× 108 Bq)57Co source using a symmetric triangular sweep
mode.66 Elemental analyses were performed at the Center for
Microanalyses of the CNRS at Lyon-Solaise, France.
F e(η5-C5P h 5)(CO)2(CtCCtCSiMe3) (2a ). A 2.2 M com-
mercial solution of MeLi‚LiBr (0.855 mL, 1.88 mmol) in diethyl
ether was syringed into a cooled solution (-80 °C) of Me3-
SiCtCCtCSiMe3 (0.365 g, 1.88 mmol) in 15 mL of THF with
stirring. The translucent solution was slowly warmed to room
temperature (20 °C). After approximately 4 h of total stirring,
this solution was cooled and Fe(η5-C5Ph5)(CO)2Br (1.0 g, 1.57
mmol) in THF was subsequently added at -80 °C. Then the
cold reaction medium was warmed to ambient temperature
overnight and the solvent was removed in vacuo. The pre-
cipitate was extracted with a mixture of n-pentane and diethyl
ether (50/50), and the extract was filtered on a degassed
alumina plug before being concentrated to dryness. The
remaining pale brown solid was washed with 10 mL of
n-pentane and dried in vacuo, giving 0.800 g of 2a (75%). Anal.
Calcd for C44H34FeO2Si: C, 77.87; H, 5.05. Found: C, 78.50;
H, 4.89. FT-IR (KBr/Nujol, cm-1): ν 1984, 2024 (s, CO); 2129,
2185 (s, CtC). 1H NMR (300 MHz, C6D6): δH 0.12 (s, 9H, Si-
(CH3)3); 6.5-7.5 (m, 25H, 5C6H5). 13C NMR (75 MHz, C6D6):
δC 212.8 (s, CO); 125-135 (m, 5C6H5); 104.0 (s, C5Ph5); 99.8
(s, CtCCtCSiMe3); 99.4 (s, CtCCtCSiMe3); 92.6 (s,
CtCCtCSiMe3); 71.1 (m, CtCCtCSiMe3); 0.35 (q, 1J CH ) 120
Hz, Si(CH3)3).
2
(75 MHz, C6D6): δC 142.2 (t, J PC ) 38 Hz, CtCCtCSiMe3);
3
139.2-126.3 (m, Ph); 102.3 (t, J PC ) 2 Hz, CtCCtCSiMe3);
4
96.2 (t, J PC ) 3 Hz, CtCCtCSiMe3); 89.1 (s, C5Me5); 64.7 (s,
CtCCtC-SiMe3); 33.1-28.9 (m, CH2); 10.7 (q, 1J CH ) 126 Hz,
1
C5Me5); 1.8 (q, J CH ) 119 Hz, SiMe3).
F e(η5-C5P h 5)(CO)2(CtCCtCH) (3a ). Potassium fluoride
(0.040 g, 0.73 mmol) was added to a solution of Fe(η5-C5Ph5)-
(CO)2(CtCCtCSiMe3) (0.500 g, 0.73 mmol) in
a 20 mL
methanol/THF mixture (50/50). The solution was refluxed
overnight. After removal of the solvents under vacuum, the
residue was extracted with toluene. Solvent was then evapo-
rated from the extract, and the remaining solid was washed
twice with 10 mL of n-pentane before being dried in vacuo to
yield 0.300 g of brown 3a (85%). Anal. Calcd for C41H26FeO2:
C, 81.20; H, 4.32. Found: C, 81.10; H, 5.00. FT-IR (KBr/Nujol,
cm-1): ν 3301 (w, CtCH); 2141 (w, CtC); 2033, 1988 (s, CO).
1H NMR (300 MHz, C6D6): δH 6.5-7.5 (m, 25H, 5C6H5); 1.25
(s, 1H, CtCCtCH). 13C NMR (75 MHz, C6D6): δC 213.0 (s,
3
CO); 125-135 (m, 5C6H5); 103.7 (s, C5Ph5); 99.7 (d, J CH ) 5
2
Hz, CtCCtCH); 94.1 (s, CtCCtCH); 72.8 (d, J CH ) 51 Hz,
1
CtCCtCH); 55.9 (d, J CH ) 259 Hz, CtCCtCH).
F e(η5-C5Me5)(CO)2(CtCCtCH ) (3b ). In
a Schlenk
tube wrapped with aluminum foil, Fe(η5-C5Me5)(CO)2-
(CtCCtCSiMe3) (1.530 g, 4.18 mmol) was solubilized in a 60
mL THF/methanol mixture (50/50) and refluxed for 3.5 h in
the presence of a slight excess of anhydrous potassium fluoride
(0.279 g, 4.98 mmol). The brown solution was cooled to
ambient temperature, extracted with four 50 mL portions of
diethyl ether, and filtered. Subsequent evacuation of the
solvent, washing with 10 mL of n-pentane at -80 °C, and
drying in vacuo yielded 1.040 g of crude yellow-brown 3b (85%
yield). Pure 3b could be obtained after recrystallization of the
crude precipitate from hot diethyl ether at -80 °C as a bright
yellow powder (∼50% yield). The compound is air stable only
in the solid form and for short periods of time. It is also light
sensitive, especially in solution. It should be stored in the dark
in an inert atmosphere. Anal. Calcd for C16H16FeO2: C, 64.89;
H, 5.45. Found: C, 65.05; H, 5.56. FT-IR (KBr/CH2Cl2, cm-1):
F e(η5-C5Me5)(CO)2(CtCCtCSiMe3) (2b). To a 15 mL
solution of LiCtCCtCSiMe3 in THF, prepared as described
above from 4.350 mL of MeLi‚LiBr (9.55 mmol) and 1.600 g of
Me3SiCtCCtCSiMe3 (8.24 mmol), was added 2.750 g of Fe(η5-
C5Me5)(CO)2I (7.41 mmol) in 50 mL of THF at -80 °C with
stirring. The reaction medium was slowly warmed to ambient
temperature overnight. Then the solvent was evaporated and
the remaining brown oil diluted with 70 mL of diethyl ether.
The pale brown suspension, shielded from light (aluminum
foil), was added to 70 mL of n-pentane and eluted quickly on
a degassed alumina column (10 cm) preloaded with 100 mL of
a mixture of n-pentane and diethyl ether (50/50). Concentra-
tion of this solution in vacuo yielded 2.306 g of crude yellow-
brown 2b (85% yield). Pure 2b can be obtained after recrys-
tallization of the crude compound from 10 mL of n-pentane at
-80 °C as a yellow powder (∼70% total yield). The compound
2b is air stable in the solid form for short periods of time. It is
a light-sensitive compound, especially in solution. It should
be stored in the dark under an inert atmosphere. Anal. Calcd
for C19H24FeO2Si: C, 61.96; H, 6.57. Found: C, 62.23; H, 6.57.
FT-IR (KBr/CH2Cl2, cm-1): ν 2171, 2119 (w, CtC); 2032, 2016
(m, CO); 1977 (s, CO). 1H NMR (300 MHz, C6D6): δH 1.28 (s,
15H, C5Me5), 0.18 (s, 9H, SiMe3). 13C NMR (75 MHz, C6D6):
δC 214.3 (s, CO); 106.3 (s, CtCCtCSiMe3); 97.4 (s, C5Me5);
96.5 (s, CtCCtCSiMe3); 94.7 (s, CtCCtCSiMe3); 69.5 (s,
1
ν 3303 (w, CtCH); 2141 (m, CtC); 2026, 1976 (s, C≡O). H
NMR (300 MHz, C6D6): δH 1,34 (s, 15H, C5Me5); 1.32 (s, 1H,
CtCCtCH). 13C NMR (75 MHz, C6D6): δC 214.3 (s, CO); 102.9
(s, CtCCtCH); 97.4 (s, C5); 95.0 (d, 3J CH ) 6 Hz, CtCCtCH);
2
1
73.4 (d, J CH ) 51 Hz, CtCCtCH); 54.1 (d, J CH ) 216 Hz,
1
CtCCtCH); 9.5 (q, J CH ) 128 Hz, C5Me5).
F e (η5-C5Me5)(η2-d p p e)(CtCCtCH) (3c). To a solution
of Fe(η5-C5Me5)(dppe)(CtCCtCSiMe3) (0.900 g, 1.27 mmol) in
THF (20 mL) was added 0.20 equiv of tetrabutylammonium
fluoride (TBAF). The solution was stirred for 4 h at room
temperature. The solvent was then removed under vacuum
and the residue was extracted with a mixture of toluene and
ether (20/80). After evaporation, the solid was washed twice
with 2 mL of cold ether and dried in vacuo to give 0.770 g
of the deep red complex 3c (95%). Anal. Calcd for
C40H40FeP2‚1/2Et2O: C, 74.67; H, 6.71. Found: C, 74.25; H,
6.24. FT-IR (KBr/Nujol, cm-1): ν 3297 (m, CtCH); 2099 (s,
CtC),1958 (w, CtC). IR (KBr/CH2Cl2, cm-1): ν 3300 (m,
CtCH); 2095 (s, CtC), 1955 (w, CtC). 31P NMR (121 MHz,
C6D6): δP 99.5 (s). 1H NMR (300 MHz, C6D6): δH 7.99-6.98
(m, 20H, Ph); 2.54, 1.74 (2m, 4H, CH2); 1.44 (s, 15H, C5Me5);
1.39 (s, 1H, CtCCtCH). 13C NMR (75 MHz, C6D6): δC 139.3-
1
1
CtCCtCSiMe3); 9.5 (q, J CH ) 128 Hz, C5Me5); 0.6 (q, J CH
)
117 Hz, SiMe3).
F e(η5-C5Me5)(η2-d p p e)(CtCCtC-SiMe3) (2c). Photolysis
(with a Hanovia lamp equipped with a quartz jacket, 450 W,
250 nm) of Fe(η5-C5Me5)(CÃ)2(CtCCtC-SiMe3) (0.800 g, 2.17
mmol) in a mixture of toluene and acetonitrile (95/5) in the
presence of bis(diphenylphosphino)ethane (0.865 g, 2.17 mmol)
over 4 h gave a red-orange solution. Removal of the solvents
(65) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910.
(66) Varret, F.; Mariot, J .-P.; Hamon, J .-R.; Astruc D. Hyperfine
Interact. 1988, 39, 67-75.
2
126.3 (m, Ph); 136.6 (t, J PC ) 38 Hz, CtCCtCH); 100.7 (d,
2
3J CH ) 5 Hz, CtCCtCH); 88.5 (s, C5Me5); 75.1 (dt, J CH ) 50

5998 Organometallics, Vol. 16, No. 26, 1997
Coat et al.
4
1
Hz, J PC ) 3 Hz, CtCCtCH); 50.5 (d, J CH ) 248 Hz,
solution of the complex at 20 °C (4 weeks). Crystal data and
details of measurements are reported hereafter. Diffraction
data were collected at 294 K on a CAD4 Enraf-Nonius
1
CtCCtCH); 33.0-28.9 (m, CH2); 10.3 (q, J CH ) 126 Hz, C5-
Me5).
{F e (η5-C5Me 5)(η2-d p p e )}(µ2-CtCCtC){F e (η5-C5P h 5)-
(CO)2} (5a ). Fe(η5-C5Me5)(η2-dppe)(Cl) (0.500 g, 0.78 mmol),
automated diffractometer with graphite-monochromated Mo
KR radiation. Crystal data collection and refinement param-
eters are given in Table 5. The cell constants and orientation
matrix for data collection were obtained from a least-squares
refinement using a set of 25 high-θ reflections. After Lorentz
and polarization corrections the structure was solved with SIR-
92, which revealed many non-hydrogen atoms of the structure.
The remaining ones were found after several scale factor and
difference Fourier calculations. After isotropic (R ) 0.11) and
then anisotropic refinement (R ) 0.089), many hydrogen atoms
were found by a difference Fourier calculation (between 0.63
and 0.32 e Å-3), and the remaining ones were set in theoretical
positions. The whole structure was refined by full-matrix
least-squares techniques (use of F magnitude; x, y, z, âij for
Fe, P, O, and C atoms and x, y, z fixed for H atoms; 524
variables and 3240 observations). Atomic scattering factors
from ref 67 were used. The calculations were performed on a
Silicon Graphics Indy computer with the MOLEN package
(ENRAF-Nonius, 1990).68
[{F e(η5-C5Me5)(η2-d p p e)}(µ2-CtCCtC){F e(η5-C5P h 5)-
(CO)2}][P F 6] ([5a ][P F 6]). [Fe(C5H5)2][PF6] (0.125 g, 0.92
equiv) was added to a solution of {Fe(dppe)(η5-C5Me5)}(µ2-
CtCCtC){Fe(η5-C5Ph5)(CO)2} (0.500 g, 0.42 mmol) in 10 mL
of dichloromethane at -80 °C. Stirring was continued for 2
h, while the reaction medium was warmed to room tempera-
ture. The solution was then concentrated, and [5a ][P F 6] was
precipitated as a deep green solid by addition of excess diethyl
ether. After washing with ether and drying in vacuo, 0.480 g
of pure product was obtained (86%). Anal. Calcd for
Fe(η5-C5Ph5)(CO)2(CtCCtCH) (0.500 g, 0.78 mmol), BuOK
t
(0.100 g, 0.93 mmol), and KPF6 (0.173 g, 0.93 mmol) were
suspended in 10 mL of methanol and the solution was stirred
for 12 h at 40 °C. After removal of the solvent, the residue
was extracted with toluene and the extract concentrated in
vacuo. Addition of n-pentane precipitated a solid. Double
washing with 10 mL of n-pentane and drying under vacuum
allowed the isolation of 0.400 g of pure gray-green 5a (59%).
Anal. Calcd for C77H64Fe2O2P2: C, 77.39; H, 5.40. Found: C,
76.7; H, 5.6. FT-IR (KBr/Nujol, cm-1): ν 2102 (s, CtC); 2022,
1976 (s, CO). UV (CH2Cl2): λmax (ꢀ/103 dm3 M-1 cm-1) 543 nm
(1.5). 31P NMR (121 MHz, C6D6): δP 100.6 (s, dppe). 1H NMR
(300 MHz, C6D6): δH 6.5-7.5 (m, 25H, 5C6H5); 1.48 (s, 15H,
C5Me5). 13C{1H} NMR (75 MHz, C6D6): δC 214.6 (s, CO); 125-
2
135 (m, 5C6H5); 114.4 (t, J CP ) 42 Hz, CRtCCtC); 108.7 (t,
3J CP ) 3 Hz, CRtCCtC); 108.3 (t, J CP ) 2 Hz, CRtCCtC);
4
103.2 (s, C5Ph5); 88.0 (s, C5Me5); 86.9 (s, CRtCCtC); 28-34
(m, CH2(dppe)); 10.5 (q, J CH ) 126 Hz, C5Me5).
1
{F e(η5-C5Me 5)(η2-d p p e)}(µ2-CtCCtC){F e(η5-C5Me5)-
(CO)2} (5b). Fe(η5-C5Me5)(η2-dppe)Cl (0.340 g, 0.54 mmol),
Fe(η5-C5Me5)(η2-dppe)(CtCCtCH) (0.200 g, 0.54 mmol), and
KPF6 (0.110 g, 0.59 mmol) were suspended at -40 °C in 60
mL of a methanol/THF mixture (50/50). A slight excess of KOt-
Bu (0.075 g, 0.66 mmol) was added to this cold solution under
argon, and the reaction medium was warmed to room tem-
perature with vigorous stirring over 12 h. The solvents were
evacuated, and the brown precipitate was extracted with
toluene. The extract was concentrated in vacuo to ∼10 mL,
and a red-orange product was precipitated by addition of 90
mL of n-pentane. This suspension was decanted, washed twice
with 20 mL of n-pentane at -80 °C, and dried under vacuum,
yielding 0.597 g of red complex 5b (69%). Anal. Calcd for
C
77H64F6Fe2O2P3: C, 69.02; H, 4.81. Found: C, 70.23; H, 4.67.
[{F e(η5-C5Me5)(η2-d p p e)}(µ2-CtCCtC){F e(η5-C5Me5)-
(CO)2}][P F 6] ([5b][P F 6]). [Fe(C5H5)2][PF6] (0.185 g, 0.92
equiv) was added to a solution of {Fe(η5-C5Me5)(dppe)}(µ2-
CtCCtC){Fe(η5-C5Me5)(CO)2} (5b; 0.540 g, 0.62 mmol) in 10
mL of dichloromethane. Stirring was maintained 1 h at room
temperature, and the solution was concentrated in vacuo to
∼2 mL. Addition of 50 mL of n-pentane allowed precipitation
of a solid. Subsequent washing with four 5 mL portions of
diethyl ether and drying under vacuum yielded 0.500 g of pure
[5b][P F 6] as a dark brown product (89%). Anal. Calcd for
C
52H54Fe2O2P2: C, 70.60; H, 6.15. Found: C, 70.69; H, 6.30.
FT-IR (KBr/CH2Cl2, cm-1): ν 2102 (vw, CtC); 2015, 1962 (s,
CO). 31P NMR (121 MHz, C6D6): δP 100.6 (s, dppe). 1H NMR
(300 MHz, C6D6): δH 8.16-7.02 (m, 20H, Ph); 2.74, 1.74 (2m,
4H, CH2(dppe)); 1.54 (s, 15H, {Fe(η5-C5Me5)(CO)2}); 1.48 (s,
15H, {Fe(η5-C5Me5)(η2-dppe)}). 13C NMR (75 MHz, C6D6): δC
3
216.0 (s, CO); 140.4-126.2 (m, Ph); 108.5 (t, J CP ) 2 Hz,
C
52H54F6Fe2O2P3: C, 60.66; H, 5.29. Found: C, 60.31; H, 5.38.
2
4
IR (KBr/CH2Cl2, cm-1): ν 2039, 1985 (m, CO); 1906 (s broad,
CtC).
CRtCCtC); 107.4 (t, J CP ) 41 Hz, CRtCCtC); 102.4 (t, J CP
) 3 Hz, CRtCCtC); 96.6 (s, {Fe(η5-C5Me5)(CO)2}); 87.8 (s,
{Fe(η5-C5Me5)(η2-dppe)}); 68.3 (s, CRtCCtC); 33.3, 29.2 (m,
1
CH2); 10.5, 9.8 (2s, J CH ) 126, 128 Hz, Cp*).
Ack n ow led gm en t. We are grateful to Dr. A. Mari
(Toulouse, France) for Mo¨ssbauer facilities and to Dr.
J . Maher (Bristol, England) for ESR assistance.
On e-P ot Syn th esis of 5b fr om 2b. In a Schlenk flask
wrapped with aluminum foil Fe(η5-C5Me5)(CO)2(CtCCtCTMS)
(0.360 g, 0.98 mmol) was solubilized in a 20 mL mixture of
THF and methanol (50/50) and this solution refluxed for 4 h
in the presence of 1.2 equiv of anhydrous potassium fluoride
(0.070 g, 1.20 mmol). The heterogeneous dark mixture was
cooled to -50 °C and Fe(η5-C5Me5)(η2-dppe)Cl (0.610 g, 0.99
mmol), KPF6 (0.200, 1.08 mmol), and KOtBu (0.130 g, 1.16
mmol) were subsequently added under argon. The reaction
medium was then warmed to room temperature with stirring
over 12 h and the deep red precipitate that was formed in the
medium was decanted. Drying in vacuo yielded 0.585 g of
crude 5b (∼70%). Purer 5b can be obtained after recrystal-
lization from a toluene/n-pentane mixture (0.440 g, 50%).
X-r a y Cr ysta llogr a p h y of 5b. Crystals for 5b were
obtained by slow vapor diffusion of n-pentane in a toluene
Su p p or tin g In for m a tion Ava ila ble: Complete tables of
atomic coordinates and their estimated standard deviations,
bond lengths and angles, and general temperature factor
expressions for 5b[P F 6] (12 pages). Ordering information is
given on any current masthead page.
OM970730O
(67) International Tables for X-ray Crystallography; Kynoch Press
(present distributor D. Reidel, Dordrecht, The Netherlands): Birming-
ham, U.K., 1974; Vol. IV.
(68) Enraf-Nonius Molecular Structure Determination Package;
MolEN, version 1990; Enraf-Nonius, Delft, The Netherlands.