Di-organoiron mixed valent complexes featuring "(η2- dppe)(η5-C5Me5)Fe" endgroups: Smooth class-III to class-II transition induced by successive insertion of 1,4-phenylene units in a butadiyne-diyl bridge

Article abstract of DOI:10.1021/ja057665r

The synthesis and study of a new redox family of symmetric dinuclear iron(II/III) complexes featuring "(η²-dppe) (η⁵-C₅Me₅)Fe(C≡C)" endgroups connected by a bis(diethynyl)-4,4′-biphenyl spacer are reported. The solid-state structures were determined (X-rays) for the homovalent Fe(II)/Fe(II) and Fe(III)/Fe(III) parents. In contrast, the mixed valent (MV) complex 5[PF₆] has a low thermodynamic stability (Kc around 10) and cannot be isolated in a pure form, but was studied in solution. According to the Robin and Day classification, it constitutes a remarkable example of well-behaved weakly coupled class-II organometallic MV compound. The photodriven metal-metal electron-transfer process takes place over ca. 16 A and corresponds to an electronic coupling of ca. 150 cm^-1 with a reorganization energy of ca. 6250 cm^-1 in dichloromethane. A similar investigation was also conducted in the near-IR range for the known and much more stable MV analogue 3[PF₆] featuring the 1,4-phenyl unit instead of the 4,4′-biphenyl one (K_c = 2.6 10⁴). The latter also exhibits a localized valency, but presents a very intense intervalence charge-transfer band (IVCT) with a cutoff on the low-energy side. A much stronger electronic coupling is derived (ca. 1700 cm^-1) from the band shape for this MV complex in the frame of the two-level model. Although slowed, the electron exchange is not disrupted by insertion of an additional para-phenylene moiety into a 1,4-diethynylaryl bridge. Thus, starting from a compound with a butadiyne-diyl spacer, stepwise paraphenylene insertions in the bridge produce a smooth Class-III to Class-II transition for the corresponding MV complexes.

Full text of DOI:10.1021/ja057665r

Published on Web 02/01/2006
Di-organoiron Mixed Valent Complexes Featuring
“(η2-dppe)(η5-C5Me5)Fe” Endgroups: Smooth Class-III to
Class-II Transition Induced by Successive Insertion of
1,4-Phenylene Units in a Butadiyne-Diyl Bridge
Safaa Ibn Ghazala,† Fre´de´ric Paul,*,† Loic Toupet,‡ Thierry Roisnel,§
Philippe Hapiot,| and Claude Lapinte*,†
Contribution from Organome´talliques et Catalyse: Chimie et Electrochimie Mole´culaires,
UMR CNRS 6509, Institut de Chimie, UniVersite´ de Rennes I, Campus de Beaulieu, Baˆt. 10C,
35042 Rennes Cedex, France, Groupe Matie`re Condense´e et Mate´riaux, UMR CNRS 6626,
UniVersite´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France,
Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR CNRS 6511,
Institut de Chimie, UniVersite´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France,
and Synthe`se et Electrosynthe`se Organique, UMR CNRS 6510, Institut de Chimie,
UniVersite´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
Received November 10, 2005; E-mail: frederic.paul@univ-rennes1.fr; claude.lapinte@univ-rennes1.fr
Abstract: The synthesis and study of a new redox family of symmetric dinuclear iron(II/III) complexes
featuring “(η2-dppe)(η5-C5Me5)Fe(CtC)” endgroups connected by a bis(diethynyl)-4,4′-biphenyl spacer are
reported. The solid-state structures were determined (X-rays) for the homovalent Fe(II)/Fe(II) and Fe(III)/
Fe(III) parents. In contrast, the mixed valent (MV) complex 5[PF6] has a low thermodynamic stability (Kc
around 10) and cannot be isolated in a pure form, but was studied in solution. According to the Robin and
Day classification, it constitutes a remarkable example of well-behaved weakly coupled class-II organo-
metallic MV compound. The photodriven metal-metal electron-transfer process takes place over ca. 16 Å
and corresponds to an electronic coupling of ca. 150 cm-1 with a reorganization energy of ca. 6250 cm-1
in dichloromethane. A similar investigation was also conducted in the near-IR range for the known and
much more stable MV analogue 3[PF6] featuring the 1,4-phenyl unit instead of the 4,4′-biphenyl one (Kc )
2.6 104). The latter also exhibits a localized valency, but presents a very intense intervalence charge-
transfer band (IVCT) with a cutoff on the low-energy side. A much stronger electronic coupling is derived
(ca. 1700 cm-1) from the band shape for this MV complex in the frame of the two-level model. Although
slowed, the electron exchange is not disrupted by insertion of an additional para-phenylene moiety into a
1,4-diethynylaryl bridge. Thus, starting from a compound with a butadiyne-diyl spacer, stepwise para-
phenylene insertions in the bridge produce a smooth Class-III to Class-II transition for the corresponding
MV complexes.
where two “(η2-dppe)(η5-C5Me5)Fe” redox-active termini are
Introduction
connected by a carbon-rich unsaturated spacer appear particu-
larly appealing as molecular wire models to study the bridge
properties.8
In the emerging field of molecular electronics,1 polynuclear
carbon-rich complexes featuring the electron-rich and redox-
active organometallic endgroups appear particularly interesting
from various perpectives, notably concerning information stor-
age and processing.2-6 From this point of view, the control (and
understanding) of electron transfer over nanometric distances
in a single molecule constitutes a decisive achievement. Ac-
cordingly, this area constitutes the center of active research from
various groups involved in carbon-rich organometallics over the
world.4-7 In this respect, dinuclear organoiron MV complexes
Since the seminal investigations on the Creutz-Taube salt,9
the spectroscopic study of dinuclear mixed-valent compounds
in the near-IR region has become a very useful way to get
decisive information on the electron-transfer process in MV
compounds and thereby also on the capability of a given bridge
to convey electrons.5,10-13 In most cases, the so-called Hush
model (or two-level model) initially developed by Hush in
196714 furnishes simple spectral criteria to categorize a given
MV complex according to the classification proposed the same
year by Robin and Day,15 and subsequently implemented by
other researchers.16
† Organome´talliques et Catalyse.
‡ Groupe Matie`re Condense´e et Mate´riaux.
§ Laboratoire de Chimie du Solide et Inorganique Mole´culaire.
| Synthe`se et Electrosynthe`se Organique.
(1) (a) Robertson, N.; Mc. Gowan, G. A. Chem. Soc. ReV. 2003, 32, 96-103.
(b) Caroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. Engl. 2002, 41,
4379-4400. (c) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-803.
As also established by others for related families of MV
compounds,12 we have shown that the polyyne bridge favors
9
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J. AM. CHEM. SOC. 2006, 128, 2463-2476
2463

A R T I C L E S
Chart 1
Ghazala et al.
very efficient electronic delocalization between the two metallic
centers in MV complexes 1a-c+ (Chart 1).8a,17 Such MV
complexes were previously categorized as Class-IIIA (1a+) or
borderline Class-II/Class-III. However, beyond eight carbon
atoms, the MV complexes become more and more unstable from
a kinetic standpoint and new structural variations have to be
devised in order to stabilize MV with longer bridges. We have
therefore started to investigate the use of aryl- and polyaryl units
in a systematic fashion.8a,18,19
Arylene-ethynylene units are usually quite more stable
kinetically than corresponding heteroaryl-ethynylene units20,21
and often allow an easy synthetic access to the desired
organoiron compounds, given the large number of aryl-based
(catalytic) cross-coupling protocols available.22,23 Moreover,
depending on the connectivity of the aryl units, various nonlinear
rigid geometries can be envisioned for the carbon-rich spacers
incorporating such units. However, as each structural variation
deeply modifies the electronic properties of the spacer and
thereby strongly affects the communication between the redox-
active endgroups, the effect of any such modification has to be
studied carefully. Thus, the inclusion of a 1,4-phenylene unit
into a polyyne-diyl bridge was shown to depress the electronic
transfer in the MV state 3[PF6] in comparison to a pure polyyne-
diyl bridge of same number of carbon atoms such as 1c[PF6],8a,24
(2) See for instance: (a) Cifuentes, M. P.; Humphrey, M. G.; Morall, J. P.;
Samoc, M.; Paul, F.; Roisnel, T.; Lapinte, C. Organometallics 2005, 24,
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2005, 24, 3966-3973. (c) Fillaut, J.-L.; Perruchon, J.; Blanchard, P.;
Roncali, J.; Gohlen, S.; Allain, M.; Migalska-Zalas, A.; Kityk, I. V.;
Sahraoui, B. Organometallics 2005, 24, 687-695. (d) Venkatesan, K.;
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Inorg. Chem. 2003, 42, 7086-7097. (g) Powell, C. E.; Humphrey, M. G.;
Cifuentes, M. P.; Morall, J. P.; Samoc, M.; Luther-Davies, B. J. Phys. Chem.
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L.; Evans, C. E. B.; Crutchley, R. J. Inorg. Chem. 1998, 37, 1880-1885.
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Prog. Inorg. Chem. 1967, 8, 391-444.
(15) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 247-422.
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Organomet. Chem. 2003, 683, 368-378. (b) Le Narvor, N.; Toupet, L.;
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A.; Halet, J.-F.; Lapinte, C. Organometallics 1998, 17, 5569-5579. (c)
Weyland, T.; Lapinte, C.; Frapper, G.; Calhorda, M. J.; Halet, J.-F.; Toupet,
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Chem. Soc. 2005, 127, 13354-13365. (d) Schull, T. L.; Kushmerick, J.
G.; Patterson, C. H.; George, C.; Moore, M. H.; Pollack, S. K.; Shashidhar,
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(19) de Montigny, F.; Argouarch, G.; Costuas, K.; Halet, J.-F.; Roisnel, T.;
Toupet, L.; Lapinte, C. Organometallics 2005, 24, 4558-4572.
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Le Stang, S.; Paul, F.; Lapinte, C. Organometallics 2000, 19, 1035-1043.
(21) Lapinte, C., unpublished results.
(22) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross-Coupling Reactions;
Wiley-VCH Verlag GmbH: Weinheim, 1998.
(23) (a) Courmarcel, J.; Le Gland, G.; Toupet, L.; Paul, F.; Lapinte, C. J.
Organomet. Chem. 2003, 670, 108-122. (b) Denis, R.; Weyland, T.; Paul,
F.; Lapinte, C. J. Organomet. Chem. 1997, 545/546, 615-618.
(8) (a) Paul, F.; Lapinte, C. Coord. Chem. ReV. 1998, 178/180, 427-505. (b)
Paul, F.; Lapinte, C. In Unusual Structures and Physical Properties in
Organometallic Chemistry; Gielen, M., Willem, R., Wrackmeyer, B., Eds.;
Wiley & Sons: Ldt: San-Francisco, 2002, pp 219-295.
(9) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1973, 95, 1086-1094.
9
2464 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Di-organoiron Mixed Valent Complexes
A R T I C L E S
Scheme 1
a
Table 1. Infrared Data for Selected Complexes in CH2Cl2 Solution (cm-1
)
b
Fe(II)
Fe(II)/Fe(III)
Fe(III)
∆νC
≡C
cpnd
νC
≡C
νC
≡C
νC
≡C
Fe(II)/Fe(III)
ref
[Fe]C≡C-(C6H4)2-C≡C[Fe] (5)
2051
2043
1979
/
2016
1934
/
1991
-60
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[Fe]C≡C-C6H4-C6H5 (6)
2052
2051
1991
1987
-61
-64
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[Fe]C≡C-C6H4-C≡C[Fe] (3)
[Fe]C≡CPh (4)
2053
2021
1988
-49c
29
[Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. a Solid state νC≡C values obtained in KBr for isolated complexes are given in the Experimental Section. b Fe(II) vs Fe(III)
νC≡C difference (PF6 counterion). c Mean difference: two bands were observed for the Fe(III) parent, presumably due to Fermi coupling.29
-
while the 9,10-anthryl unit in 2[PF6] recently appeared as a
promising alternative to this unit regarding electronic delocal-
ization.19
the evolution of the electronic coupling (Hab) between these and
related MV complexes such as 1a[PF6] or 2[PF6].
Results and Discussion
Further cis-insertion of 1,4-phenylene units in the bridge was
next envisioned as a straightforward way to obtain MV
complexes with extended bridges. However, a dramatic decrease
in the electronic coupling was expected for these compounds,12a,25
especially with “(η2-dppe)(η5-C5Me5)Fe” endgroups prone to
exhibit highly metal-centered frontier molecular orbitals.26,27
This was also more likely considering the existence of various
conformers with nonplanar bridge conformations, evidently less
favorable to the electron-transfer process.12a,28 The study of the
complex 5[PF6] featuring a 2,2′-biphenyl unit in the bridge
(Chart 1) revealed a quite larger electronic coupling than
anticipated. More remarkably, we presently show that a progres-
sive transition from class-III to class-II can be achieved by suc-
cessive 1,4-phenylene insertions in the carbon-rich bridge, when
proceeding from 1a[PF6] to 5[PF6]. For the first time, we also
provide here an interpretation of the complex band shape of
the intervalence charge transfer band (IVCT) in strongly coupled
compounds such as 3[PF6]. Accordingly, the reader will find
in the following (i) the synthesis and characterization of the
new redox family of dinuclear complexes 5/5+/52+ and of the
corresponding new family of mononuclear complexes 6/6+, used
as benchmarks for the characterization of 5/5+/52+, (ii) the study
of the MV complex 5[PF6] in the near-IR domain along with
that of the known MV complex 3[PF6] in the same spectral
range for comparison purposes and (iii) a short discussion on
Synthesis and Characterization of the New Organoiron-
(II) Complexes. The synthesis of the desired dinuclear Fe(II)
complex 5 was achieved following the reaction sequence
depicted in Scheme 1. The trimethylsilyl-protected organic
precursor 8 of the bridge was classically obtained following a
Sonogashira coupling procedure from the commercial 4,4′-
dibromo-1,1′-biphenyl 7 (95%). This compound was then
deprotected with potassium carbonate giving the terminal bis-
alkyne 9 in good yields (93%). The latter was reacted with the
chloride complex (η2-dppe)(η5-C5H5)FeCl (10) to form the
corresponding bis-vinylidene complex 5-v[BPh4]2, which was
only briefly characterized by NMR and infrared spectroscopies
before being deprotonated in situ to give 5 in 63% overall yield.
This complex has a poor solubility in most organic solvents,
but could nevertheless be fully characterized by NMR, by ESI-
MS, and a satisfactory elemental analysis was obtained. 31P
NMR reveal the highly symmetric nature of this compound,
and the single signal observed at 101.1 ppm is characteristic of
an acetylide Fe(II) complex. The proton spectrum clearly
confirms the expected 1-1 ratio between the C5Me5 ligand and
1,4-phenylene units. Due to the very low solubility of the
complex mentioned above, the characteristic triplet (2JCP ca. 40
Hz) and singlet signatures for the symmetric acetylide linkage
could only be detected with difficulty at 142.3 and 120.4 ppm
by 13C NMR, respectively.18c,26 As for the 1,4-phenyl analogue
3, a single stretching frequency is detected for the two alkynyl
units at very similar wavenumbers (Table 1), which are also
very close to the value recorded for the corresponding mono-
nuclear complex 4. Notably, Raman spectroscopy reveals that
the weak absorption observed at 1949 cm-1 (Figure 1) does
not correspond to the symmetric νCtC mode.29 This infrared
absorption rather corresponds to a combination or harmonic
(24) Le Narvor, N.; Lapinte, C. Organometallics 1995, 14, 634-639.
(25) Kim, Y.; Lieber, C. M. Inorg. Chem. 1989, 28, 3990-3992.
(26) (a) Costuas, K.; Paul, F.; Toupet, L.; Halet, J.-F.; Lapinte, C. Organome-
tallics 2004, 23, 2053-2068. (b) Denis, R.; Toupet, L.; Paul, F.; Lapinte,
C. Organometallics 2000, 19, 4240-4251.
(27) Paul, F.; Toupet, L.; The´pot, J.-Y.; Costuas, K.; Halet, J.-F.; Lapinte, C.
Organometallics 2005, 24, 5464-5478.
(28) Woitellier, S.; Launay, J.-P.; Joachim, C. Chem. Phys. 1989, 131, 481-
488.
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A R T I C L E S
Ghazala et al.
Figure 1. Infrared spectra for 5 (a) and 52+ (b) complexes and for an
equimolar mixture of 5 and 52+ (c) in dichloromethane (KBr window).
Figure 3. ORTEP representations of complex 6 at 50% probability level.
Figure 4. UV-vis spectra for 5 and 52+ complexes and for an equimolar
mixture of 5 and 52+ in dichloromethane.
chloride complex 10 to give the vinylidene complex 6-v[PF6]
which was subsequently deprotonated in situ. This complex was
fully characterized by usual spectroscopies, by ESI-MS, and
a satisfactory elemental analysis was obtained as well. 31P NMR
gives the characteristic singlet at 101.5 ppm expected for an
acetylide Fe(II) complex, while the acetylide linkage is signaled
by a characteristic triplet (2JCP ) 39 Hz) and singlet at 139.3
and 121.0 ppm, respectively, in the 13C NMR spectrum. A single
stretching frequency is also detected at 2056 cm-1 for the νC≡C
(Table 1). In addition, red crystals could be grown from
dichloromethane/n-pentane mixtures and the structure of 6 could
be solved (Figure 3).
Figure 2. ORTEP representations of the two conformations of complex 5
in the asymmetric units at 50% probability level.
mode. Comparison with the infrared data obtained for 3 (Table
1) suggest that the introduction of a second 1,4-phenylene unit
does not markedly perturb the νCtC stretches. Thus, the vibronic
coupling between the νCtC through the phenylene linker must
remain relatively small in both complexes.
Finally, very small red crystals could be grown from THF-
pentane mixtures for 5 and a X-ray structure could be obtained
which confirms the identity of the compound (see Figure 2).
The synthesis of the mononuclear Fe(II) acetylide complex
6 containing the 4-biphenyl unit was realized following a similar
reaction sequence (Scheme 2). Again, a Sonogashira procedure
was used to generate the silyl-protected biphenyl acetylene 12
from commercial 4-bromobiphenyl (11), followed by a de-
silylation step. The resulting alkyne (13) was reacted with the
The UV-vis spectra of these new compounds were recorded
(Table 2). For each of them, a broad absorption was observed
near 410 nm, which corresponds to a MLCT band (Figure 4).26
Accordingly, this band is not found on the absorption spectrum
of the silylated organic bridge precursor. The MLCT absorption
for 5 is bathochromically shifted by 30 nm relative to 6, as ex-
pected from the more electron-rich nature of the dinuclear com-
plex, resulting in more energetic filled (donor) d metal-centered
nonbonding MOs and thereby in closer MLCT (excited) states.
Cyclic Voltammetry. Cyclic voltammograms were recorded
for the oxidation of the dinuclear compounds 5 and 6, and
Scheme 2
9
2466 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Di-organoiron Mixed Valent Complexes
A R T I C L E S
Table 2. UV-vis Data for Selected Fe(II) and Fe(III) Complexes in CH2Cl2
1
cpnd
absorptions in nm (10-3ꢀ in M-1 cm-
)
ref
Me3Si-C≡C-(C6H4)2-C≡C-SiMe3 (7)
[Fe]C≡C-(C6H4)2-C≡C[Fe] (5)
303 (37.5)
275 (sh, 42.9), 432 (40.2)
278 (sh, 50.4), 334 (sh, 33.0), 464 (sh, 6.8), 623 (3.8), 726 (8.4)
275 (sh, 34.1), 401 (17.5)
280 (sh, 88.2), 370 (sh, 7.2), 440 (sh, 3.0), 613 (2.0), 710 (4.1)
261 (sh, 34.7), 395 (sh, 24.2), 411 (25.4), 525 (3.4)
264 (sh, 46.8), 424 (8.7), 547 (17.0), 702 (7.6)
275 (sh, 51.1), 420 (8.0), 619 (sh, 5.4), 702 (42.8), 769 (sh, 7.1)
277 (sh, 14.5), 350 (13.6)
261 (sh, 32.6); 280 (sh, 27.4); 301 (sh, 18.8); 342 (sh, 5.9); 379
(sh, 3.6); 575 (sh, 2.3); 662 (3.1)
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26b
{[Fe]C≡C-(C6H4)2-C≡C[Fe]}2+ (52+
[Fe]C≡C-C6H4-C6H5 (6)
)
[[Fe]C≡C-C6H4-C6H5]+ (6+)
[Fe]C≡C-C6H4-C≡C[Fe] (3)
{[Fe]C≡C-C6H4-C≡C[Fe]}+ (3+)
{[Fe]C≡C-C6H4-C≡C[Fe]}2+ (32+
)
[Fe]C≡CPh (4)
{[Fe]C≡CPh}+ (4+)
27
[Fe] ≡ (η2-dppe)(η5-C5Me5)Fe.
Table 3. Electrochemical Data for Selected Complexes
cpnd
E
°
(V)a
∆E
°
(V)
ic/ia
ref
[Fe]C≡C-(C6H4)2-C≡C[Fe] (5) -0.17b
0.06
1
1
1
1
1
1
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-0.11b
[Fe]C≡C-C6H4-C6H5 (6)
-0.16
/
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8a
[Fe]C≡C-C6H4-C≡C[Fe] (3)
-0.27 c
-0.01 c
-0.15
0.26
[Fe]C≡CPh (4)
/
26
Figure 5. ORTEP representations of complex 52+ at 50% probability level.
[Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. a All E values in V vs SCE Conditions:
CH2Cl2 solvent, 0.1 M (NnBu4)(PF6) supporting electrolyte, 20 °C, Pt
electrode, sweep rate 0.100 V s-1. The ferrocene/ferricinium (Fc/Fc+)
complex was used as an internal reference for potential measurements.
b E° values extracted by simulation. c E° values presently corrected for Fc/
Fc+ at 0.460 V vs SCE in CH2Cl2.33
compared to the results obtained for mononuclear complexes 3
and 4 (Table 3). Two separate monoelectronic and reversible
waves are observed for 3 in contrast to 5, which exhibits a single
two-electron reversible wave. The determination of the two
standard potentials E°1 and E°2 is straightforward in the case
of two separated waves. These can be derived as the midpoints
between the anodic and cathodic peaks of each one-electron
wave. In the case of a single two-electron wave, as observed
for 5, the two standard potentials can also be derived from the
location of the midpoint between the anodic and cathodic peaks
and the distance between them (∆Ep), provided the kinetics of
the electron-transfer processes do not affect the cyclic voltam-
metric response.30-32 This condition was checked by investigat-
ing the variation of ∆Ep as a function of the scan rate, and we
found that only negligible variations are observed for scan rates
below 1 V‚s-1. Thus, ∆Ep tends toward a limit (85 mV) that
corresponds to the thermodynamics of the electron-transfer
processes which leads to a difference ∆E° (V) ) 62 mV (use
of a working curve after simulation of the voltammogram),
evidencing that the equilibrium constant for the compropor-
tionation reaction (equation 1) is around 11-12 for 5 (see
Supporting Information). This is quite close to the statistical
value of 4 and reflects the weak intramolecular electronic
interaction taking place between the two metal end-groups in
the MV compound 5+. For complex 6, one redox wave
corresponding to the Fe(III/II) oxidation is observed at -0.16
V vs ECS. Expectedly, this potential is very close to the first
oxidation potential E°1 of 5 (ca. -0.17 V), in line with a more
Figure 6. ORTEP representations of complex 6[TCNQ] at 50% probability
level.
difficult oxidation process for the mononuclear complex, which
is slightly less electron-rich than the dinuclear relative.
5 + 5++ a 5+ + 5+
(1)
It is also interesting to note that the small variation of ∆Ep
with the scan rate (<1 V.s-1) indicates that the kinetics of the
electron transfer are relatively fast,31 and thus, that the reorga-
nization energies associated with the change of the redox state
are small, as expected from previous work on related com-
pounds.26a
Synthesis and Characterization of Organoiron(III) Com-
plexes. The isolation of the corresponding mono- and dioxidized
Fe(III) congeners 6+ and 52+ was achieved from 6 and 5 by
chemical oxidation using 1 and 2 equiv. of ferricinium
hexafluorophosphate, respectively (Scheme 3).
Small dark crystals could be grown from dichloromethane/
n-pentane mixtures for 5[PF6]2 and a X-ray structure could be
obtained which confirms the identity of the compound (see
Figure 5). Using TCNQ as an oxidant in place of ferricinium
hexafluorophosphate allowed the isolation of 6[TCNQ]. Dark
green crystals of this compound could be grown from dichlo-
romethane/n-pentane mixtures, and the solid-state structure was
solved (Figure 6).
(29) Paul, F.; Mevellec, J.-Y.; Lapinte, C. J. Chem. Soc., Dalton Trans. 2002,
1783-1790.
(30) Myers, R. L.; Shain, I. Anal. Chem. 1969, 41, 980.
(31) (a) Hapiot, P.; Kispert, L. D.; Konovalov, V. V.; Saveant, J.-M. J. Am.
Chem. Soc. 2001, 123, 6669-6677. (b) Andrieux, C. P.; Save´ant, J.-M. J.
Electroanal. Chem. 1974, 57, 27-33.
These green Fe(III) complexes were characterized by infrared
spectroscopy (Table 1) and cyclic voltammetry. For all Fe(III)
complexes, a single weak νC≡C is observed near 1990 cm-1, in
(32) Guerro, M.; Carlier, R.; Boubekeur, K.; Lorcy, D.; Hapiot, P. J. Am. Chem.
Soc. 2003, 125, 3159-3167.
9
J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2467

A R T I C L E S
Scheme 3
Ghazala et al.
Table 4. ESR spectra for compounds in frozen CH2Cl2/1,2-C2H4Cl2 solutions at 80 K
cpnd
g1
g2
g3
∆g
ref
{[Fe]C≡C-(C6H4)2-C≡C[Fe]}2+ (52+
{[Fe]C≡C-(C6H4)2-C≡C[Fe]}+ (5+)
[[Fe]C≡C-C6H4-C6H5]+ (6+)
)
2.035 (∆Hpp ca. 160 G)a
/
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24
2.418
2.134
2.032
1.976
1.975
0.442
0.464
/
0.168
0.489
2.439
{[Fe]C≡C-C6H4-C≡C[Fe]}2+ (32+
{[Fe]C≡C-C6H4-C≡C[Fe]}+ (3+)
{[Fe]C≡CPh}+ (4+)
)
2.032 (∆Hpp ca. 160 G)a
2.199
2.464
2.049
2.033
2.031
1.975
27,34
[Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. a Pure solid sample of 52+ (no signal corresponding to a hypothetical forbidden ∆ms ) 2 transition detected around g )
4.5).
accordance with the expected decrease in acetylide bond order
The Mo¨ssbauer spectra also clearly reveal the change in the
oxidation state of the iron centers between 5 and 5[PF6]2 as
well as between 6 and 6[PF6] (Table 5). Indeed, while the slight
differences in isomeric shift are less informative with such
arylacetylide organoiron compounds,26b the quadrupolar splitting
is nevertheless diagnostic of an Fe(II) center in 5 and 6, and of
an Fe(III) center in 5[PF6]2 and 6[PF6].34,35 The 1H NMR spectra
of 5[PF6]2 and 6[PF6] are also very typical of paramagnetic Fe-
(III) complexes.36,37 Notably, the characteristic broad cyclopen-
upon oxidation.24,29 Again, Raman spectroscopy reveals that the
weak absorption observed at 1952 cm-1 (Figure 1) does not
correspond to the symmetric stretching motion of the alkyne
vibrators in 5[PF6]2.
The UV-vis spectra of these new compounds 5 and 6 were
also recorded (Table 2). The broad absorption observed near
700 nm (Figure 4) which corresponds to a LMCT band in these
Fe(III) compounds explains the darker color of the complexes.27
This LMCT absorption is bathochromically shifted of ca. 20
nm for 52+ relative to 6+, likewise to what was observed
between 5 and 6. This time, this trend can be rationalized by
the dicationic nature of the 5[PF6]2, resulting in less energetic
virtual (accepting) d MOs localized on the metal center,
therefore in closer excited LMCT states.
The presence of unpaired electron(s) in 6+ and in 52+ is
confirmed by clear ESR signatures (Table 4 and Figure 7). Thus,
a rhombic signal, typical of a metal-centered radical, was
observed at 80° K in dichloromethane/1,2-dichloroethane glasses
for 6+, while a broad, seemingly isotropic, signal was obtained
at the same temperature for 52+ at g ) 2.035 (∆H ) 160 G).
Upon coming back to room temperature, this signal is still
observed, but becomes thinner (∆H ) 60 G) and shifts toward
ge. While the latter signal is not typical of metal centered radicals
but is rather suggestive of an organic side-product, the spectra
initially recorded at 77 K or obtained with pure solid samples
of 52+ (Table 4, Figure 7) is clearly in line with ESR signatures
previously obtained for related Fe(III)/Fe(III) diradicals.18b
1
tadiene resonance is located at ca. -10 ppm for both the H
NMR spectra of 5[PF6]2 and 6[PF6] in chloroform, which
suggests the presence of roughly one unpaired electron on the
metallic endgroups.36 Indeed, for dinuclear Fe(III) complexes
exhibiting strong antiferromagnetic coupling this resonance is
usually detected at higher fields.17b Thus, while antiferromag-
netic coupling between the unpaired spins is expected in 5[PF6]2
according to Ovchinnikov’s rule38 or molecular orbital (MO)
considerations,8b the latter must be quite weak. In line with this
hypothesis, Evans measurements in dichloromethane solution
for 6[PF6] and 5[PF6]2 give 1.8 ( 0.4 µB and 2.7 ( 0.4 µB,
close to the values expected for doublet (1.73 µB) and triplet
(33) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
(34) Connelly, N. G.; 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, 2575-2578.
(35) Guillaume, V.; Thominot, P.; Coat, F.; Mari, A.; Lapinte, C. J. Organomet.
Chem. 1998, 565, 75-80.
(36) Roger, C.; Hamon, P.; Toupet, L.; Rabaaˆ, H.; Saillard, J.-Y.; Hamon, J.-
R.; Lapinte, C. Organometallics 1991, 10, 1045-1054.
(37) Paul, F., work in progress.
9
2468 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Di-organoiron Mixed Valent Complexes
A R T I C L E S
In the near-IR range, a weak and broad absorption is detected
around 1850 nm (5400 cm-1) for both compounds (Table 6).
Both transitions take place roughly at the same energy than as
previously observed for 4[PF6]. Thus, this absorption, absent
in the Fe(II) precursors, likely corresponds to a forbidden ligand
field (LF) transition localized at the Fe(III) end of the complexes
(SOMO-2/SOMO transition).18c,27,41 The transition in 5[PF6]2
is therefore doubly degenerate. In line with this interpretation,
the near-IR transition of complex 5[PF6]2 exhibits roughly a
doubled intensity in comparison to that of the mononuclear
complex 6[PF6].
X-ray Structures of 5, 5[PF6]2, 6 and 6[TCNQ]. The
compound 5 (Figures 2) crystallizes in the P-1 symmetry group,
with two molecules in the asymmetric unit (Table 7). The
presence of two molecules with slightly different conformations
and bond lengths in the asymmetric unit reveals the sizable
structural influence of the packing on the molecular geometry.
This suggests that some caution should be exercised when trying
to interpret weak structural changes in terms of intramolecular
influences only. Notably, this phenomenon is not connected to
the presence of THF or n-pentane molecules as solvates, since
a very similar solid state arrangement is observed in crystals
grown from nitrobenzene-diethyl ether mixtures, but featuring
only water molecules as solvates (see Supporting Information).
The structure shows that the two aryl rings of the biphenyl are
coplanar, despite the steric interactions existing between the 3,5-
and 3′,5′- hydrogen atoms of the 4,4′-biphenyl spacer. Note-
worthy, this phenomenon is observed for the two molecules in
the asymmetric units in the absence of any π-stacking interac-
tions between adjacent biphenyl moieties, well-known to
stabilize such conformations in the solid state.42 The corre-
sponding dication 5[PF6]2 (Figure 5) also crystallizes in the P-1
symmetry group, with one molecule in the asymmetric unit
(Table 7). This solid-state structure again exhibits a coplanar
conformation for the biphenyl spacer. The significant changes
in bond lengths induced by oxidation include a slight expansion
of the coordination spheres around the iron centers, as well a
slight shortening of the Fe-C bond length (See Supporting
Information). These changes closely match those stated for 6
and 6[TCNQ] (see after). Also, note that for both 5 and 5[PF6]2
the intramolecular Fe-Fe distances (16.18 and 16.11 Å,
respectively) are longer than the shortest intermolecular Fe-
Fe separations (8.75 and 10.64 Å respectively).
Figure 7. ESR spectrum of (a) 6[PF6] and (b) a mixture of 5[PF6] and
5[PF6]2 in 1,2-CH2Cl2/C2H4Cl2 (1:1) at 80 K. (c) ESR spectrum of a pure
solid sample of 5[PF6]2 at 77 K.
(2.83 µB) compounds, respectively, using the spin only formula
(eq 2) and taking the free electron ge () 2.023) value for g.39
Thus, 5[PF6]2 essentially behaves as an unpaired diradical in
solution at ambient temperature in line with no or weak
antiferromagnetic coupling between unpaired spins through the
bis-4,4′-ethynyl-(1,1′-biphenyl) spacer. Note that in the case of
3[PF6]2, the existence of antiferromagnetic coupling under
similar conditions was previously suggested by Evans measure-
ments and more firmly established by magnetic susceptibility
measurements.24,40
The mononuclear compounds 6 and 6[TCNQ] (Figures 3 and
6) crystallize in the P-1 symmetry group, with two and one
molecule(s) in the asymmetric unit (Table 7), respectively. As
µ ) g[S(S + 1)]1/2
(2)
Table 5. 57Fe Mo¨ssbauer Fitting Parameters at 80 K for Selected Complexes
δ
∆
EQ
λ
1
1
1
cpnd
(mm.s-
)
(mm.s-
)
(mm.s-
)
refs
{[Fe]C≡C-(C6H4)2-C≡C[Fe]} (5)
{[Fe]C≡C-(C6H4)2-C≡C[Fe]}[PF6]2 (5[PF6]2)
[[Fe]C≡C-C6H4-C6H5] (6)
0.255
0.257
0.275
0.256
0.28
0.25
0.265
0.239
1.985
0.894
2.029
0.898
2.02
0.90
2.020
0.911
0.134
0.203
0.133
0.118
n.r.a
n.r.a
/
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34
34
24
24
[[Fe]C≡C-C6H4-C6H5]+ (6[PF6])
[[Fe]C≡C-C6H5] (4)
[[Fe]C≡C-C6H5]+ (4[PF6])
[Fe]C≡C-C6H4-C≡C[Fe] (3)b
{[Fe]C≡C-C6H4-C≡C[Fe]}2+(3[PF6]2)b
/
a Not reported. b Value recorded at 78 K.
9
J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2469

A R T I C L E S
Ghazala et al.
Table 6. Near-IR Data for Selected Fe(III) Complexes in CH2Cl2
1
absorptionsa in cm-
ν1/
2
1
1 c
cpnd
(ꢀ
b in M-1 cm-
)
(cm-
)
ref
{[Fe]C≡C-(C6H4)2-C≡C[Fe]}2+ (52+
)
5450 (130)
5550 (80)
5500 (45)
5417 (94)
1400
1500
1580
1500
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27
[[Fe]C≡C-C6H4-C6H5]+ (6+)
{[Fe]C≡C-C6H4-C≡C[Fe]}2+ (32+
)
{[Fe]C≡CPh}+ (4+)
[Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. a Values ( 50 cm-1
.
b Values ( 10 M-1cm-1 c Values ( 200 cm-1
. .
Table 7. Crystal Data, Data Collection, and Refinement Parameters for 5, 5[PF6]2, 6, and 6[TCNQ]
1.5(5)
‚2/3(C4H8O)‚
cpnd
1/2(C5H12
)
5[PF6]2
‚
2(CH2Cl2)
6
6[TCNQ]
‚
1/4(CH2Cl2)
formula
fw
temp (K)
cryst. syst.
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V(Å3)
C140.50H147Fe3O1.50P6
2212.95
120(1)
triclinic
P-1
13.5647(3)
15.8974(3)
27.9560(4)
88.863(1)
78.054(1)
79.310(1)
5694.5(2)
2
C90H90Cl4F12Fe2P6
1838.94
120(1)
triclinic
P-1
10.4160(2)
14.1614(2)
15.0200(2)
98.182(1)
99.956(1)
103.838(1)
2079.4(1)
1
C50H48P2Fe1
766.67
293(2)
triclinic
P-1
14.7336(3)
16.8011(3)
17.9380(3)
86.810(1)
67.920(1)
82.818(1)
4082.3(1)
4
C62.25H48.5N4P2Cl0.5Fe1,
992.10
120(1)
triclinic
P-1
11.1235(3)
11.9397(3)
20.8638(6)
98.742(2)
95.508(2)
98.512(2)
2688.6
Z
D
2
(calcd) (g cm-3
)
1.268
1.469
1.247
1.225
crystal size (mm)
F(000)
0.22 × 0.16 × 0.1
0.32 × 0.28 × 0.18
0.32 × 0.22 × 0.20
0.30 × 0.30 × 0.08
2340
948
1616
1037
diffractometer
KappaCCD
(Nonius)
MoKR
KappaCCD
(Nonius)
MoKR
KappaCCD
(Nonius)
MoKR
KappaCCD
(Nonius)
MoKR
radiation
abs. coef. (mm-1
)
0.507
0.667
0.482
0.407
data collection: θmax (°),
frames, Ω rotation (°),
seconds/frame
27.2,
10,
1.0,
27.0,
10,
2.0,
27.0,
280,
2.0,
27.0,
342,
1.2,
10
30
120
40
Θ range
h k l range
2.57-26.00
0/16
1.40-26.00
0/12
2.35-26.73
0/18
2.42-27.57
0/14
-19/19
-33/34
79572
-17/16
-18/17
55632
-20/21
-20/22
92577
-15/15
-27/26
47838
number total refl.
number unique refl.
22683
8158
17314
12379
number obs. refl. [I > 2σ(I)]
restraints/parameters
w ) 1/[σ2(Fo)2+(aP)2+bP]
16000
0/1373
a ) 0.1127
7276
514
a ) 0.0652
11822
0/956
a ) 0.0783
8666
0/650
a ) 0.0806
2
2
(where P ) [Fo +Fc ]/3)
b ) 9.5157
0.068
b ) 2.2205
0.042
b ) 1.0811
0.048
b ) 2.6631
0.060
final R
Rw
0.186
0.115
0.123
0.149
R indices (all data)
0.104
0.048
0.083
0.098
R
w (all data)
goodness of fit/F2 (Sw)
∆Fmax (e Å-3
∆Fmin (e Å-3
0.215
1.087
1.168
-0.889
0.122
1.101
0.819
-0.573
0.148
1.017
0.373
-0.370
0.172
1.025
0.922
-0.510
)
)
mentioned above, the bond lengths and angles compare with those
of 5 and 5[PF6]2 (See Supporting Information), and are also in
good accordance with crystallographic data available for rela-
ted mononuclear Fe(II) and Fe(III) compounds.23,26,27 The changes
in the coordination sphere occurring upon oxidation have been
discussed in detail elsewhere.26,27 The two aryl rings in the bi-
phenyl moiety are tilted from an angle of ca. 30°, regardless of
the oxidation state of the metal center, suggesting that the latter
has only a negligible electronic influence on this angle in 6/6+.
The geometric features of the TCNQ counterion in 6[TCNQ]
are also quite usual, a slight dissymmetry being observed be-
tween the cyano bonds on opposite exocyclic carbon atoms on the
central ring, which can be imparted to a weak stacking π-interac-
tion taking place (distance C57-C57 ) 3.186 Å) between TCNQ
pairs in the elementary cell. Apart from that, no specific inte-
rmolecular interaction takes place and a dichloromethane solvate
partially occupies the voids between molecules in 6[TCNQ].
(38) Ovchinnikov, A. O. Theor. Chim. Acta (Berl.) 1978, 47, 297-304.
(39) (a) Kahn, O. Molecular Magnetism; VCH Publisher Inc.: New York
Weinheim Cambridge, 1993. (b) Evans, D. F. J. Chem. Soc. 1959, 2003-
2004. (c) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
(40) Le Narvor, N.; Lapinte, C. C. R. Acad. Sci., T. 1, Ser. IIc 1998, 745-749.
(41) Le Stang, S.; Paul, F.; Lapinte, C. Inorg. Chim. Acta 1999, 291, 403-425.
(42) (a) Leme´e, M. H.; Toupet, L.; De´lugeard, Y.; Messager, J. C.; Cailleau, H.
Acta Crystallogr. 1987, B43, 446-470. (b) Saito, K.; Atake, T.; Chihara,
H. Acta Crystallogr. 1987, B43, 383-385. (c) One referee suggested that
apparent planarity could result from rapid interconversion between slightly
twisted conformations (see Baudour; J.-L. Acta Crystallogr. 1991, B47,
935-949 for details). Presently, the anisotropic refinements reveal nothing
unusual, as expected for a truly planar biphenyl unit in the solid state.
Generation and Characterization of the Mixed Valence
Complex 5+. The mixed valence (MV) complex 5+ has been
9
2470 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Di-organoiron Mixed Valent Complexes
A R T I C L E S
Table 8. Near-IR Data for Selected 5[PF6] and 3[PF6] in CH2Cl2
1
ν
max in cm-
(
ν1/
)
dab
(Å)c
(
ν1/ )
Hab
(cm-
2
exp
2 theo
1
a
1
b
1
d
1
cpnd
band
(
ꢀ
in M-1 cm-
)
(cm-
)
(cm-
)
)
[Fe]C≡C-(C6H4)2-C≡C[Fe]+ (5+)
B
5000 (240)
1650
1650e
1650f
3750
4050e
4410f
2120
16.1
16.1
11.6
3381
3480e
3565f
3800
4050e
4120f
3040
/
/
/
5250 (290)e
5500 (160)f
6250 (560)
7090 (660)e
7350 (625)f
C
A
145g
176e,g
182f,g
[Fe]C≡C-C6H4-C≡C[Fe]+ (3+)
4000 (12700)
590g
2000h
B
C
D
6500 (3400)
9000 (600)
5400 (300)
2110
2110
1580
/
/
/
3880
4560
3530
/
/
/
[Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. a Values ( 10 M-1cm-1
.
b Values ( 50 cm-1 c Evaluated from X-ray structures or calculated from DFT. d Calculated
.
following eq 4. e In dichloromethane/acetone (1:1) mixture. f In neat acetone. g Calculated following eq 3. h Calculated following eq 11.
Figure 9. Near-IR spectra for mixtures of 5/5[PF6]2 in dichloromethane
(a), in acetone (b) and in a 1:1 mixture of both solvents (c).
(Table 2) has also been introduced in the deconvolution. Among
the new bands detected, the first one (B) at 5000 cm-1 is rather
weak while the second one (C) at 6250 cm-1 is significantly
more intense. The former is attributed to a ligand field (LF)
transition, also called interconfigurational (IC) transition by
Demadis et al.,10 by analogy with the absorptions previously
detected in related Fe(III) aryl acetylides.27 This transition is
red-shifted of ca. 450 cm-1 relative to similar processes
occurring at 5450 cm-1 for 52+ and at 5417 and 5500 cm-1 for
the mononuclear complexes 4+ or 6+ respectively (Table 8).
The band (C) in Figure 8 is believed to correspond to an
intervalence charge-transfer transition (IVCT). Its width at half-
Figure 8. Near-IR spectra for an equimolar mixture of 5 (2 10-3 M) and
5[PF6]2 (2 10-3 M) in dichloromethane and proposed de-convolution.
generated following the comproportionation reaction given in
eq 1, by mixing equimolar amounts of 5 and 52+. However, the
electrochemical data reveals that 5+ is not sufficiently stable to
exist in solution as a single product within this equilibrium.
Thus, once generated in solution, it will equilibrate with ca.
18% of 5 and 18% of 52+, and the occurrence of these two
species has to be taken into consideration during the ensuing
spectroscopic studies.
height (ν
Hush model ((ν1/2 theo
)
1/2 exp, is well in line with predictions based on the
From these solutions, two new infrared bands could be
detected in the acetylide stretching region which presumably
correspond to the MV complex at 2043 and 1979 cm-1 (Table
1 and Figure 1). Also, after thawing a solution containing
equimolar mixtures of 5 and 5[PF6]2, a new rhombic signal can
be detected by ESR in addition to the signal of the diradical
5[PF6]2, with main g-components at g1 ) 1.976, g2 ) 2.134
and g3 ) 2.418 (Table 4). This signal resembles that observed
for 6[PF6] (g1 ) 1.975, g2 ) 2.032, and g3 ) 2.439) and is
typical of an Fe(III)-centered radical in cationic piano-stool
acetylide complexes.27 The anisotropy of the signal is also very
close to that previously observed for related mononuclear
complexes such as 6[PF6] or 4[PF6] (Figure 7 and Table 4).
Finally, in the near-IR range a new broad absorption centered
at ca. 6500 cm-1 shows up which is not present in solutions of
pure 5 or 5[PF6]2 complexes. De-convolution of the spectrum
in this spectral range using Gaussian functions reveals the
presence of two new overlapping transitions attributable to 5+
in addition to the weak transition (A) at 5450 cm-1 expected
for 52+ in equilibrium with 5+ (Figure 8 and Table 8). The onset
of a band (D) which corresponds to the strong absorptions in
the visible range at ca. 726 nm for both 5+ and 52+complexes
)
in Table 8).14b,16a,43 Notably, band B is
much narrower than predicted on the basis of eq 4 and presents
an experimental ν1/2 in the range usually observed for such
transitions.27
The influence of the solvent polarity on these bands was also
briefly investigated. However, due to the low solubility of 5 in
common polar solvents, the use of such solvents resulted in the
rapid precipitation of this complex, driving the comproportion-
ation equilibrium (eq 1) toward the left. Among the polar
solvents tested, only acetone proved suitable to this experiment,
since the precipitation of 5 in neat acetone occurred much more
slowly (after 10 mn) and the measurements could be performed
directly after dissolution. While remaining essentially qualitative
in neat acetone, the data obtained from dichloromethane, acetone
and a 1:1 mixture of these solvents clearly establish a solva-
tochromic behavior for the band envelope of the broad near-IR
absorption (Figure 9). Spectral data extracted from these
experiments (Table 8) reveals that both Gaussian bands B and
C are shifted toward higher energies when the polarity of the
(43) The so-called “semiclassical” treatment has been used to deconvolute this
band.16a
9
J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2471

A R T I C L E S
Ghazala et al.
medium increases, but that band C is much more strongly
affected by this change than is band B. Notably, there is also a
concomitant increase in band C bandwidth when it shifts to
higher energy, as expected for an IVCT process following the
Hush model (equations 3 and 4). In contrast, despite the slight
shift of band B its bandwidth does apparently not change, further
confirming that this transition has nothing to do with an IVCT
process. Note, however, that the bandwidth found for 5[PF6] in
neat acetone is larger than theoretically expected on the basis
of eq 3. In the absence of identified causes for additional line
broadening, this might be attributed to the experimental
uncertainties associated with this measurement. Finally, there
is also an apparent enhancement in the relative intensity of band
C in the more polar media tested.
Figure 10. Near-IR spectra for 3[PF6] (2 10-4 M) in dichloromethane and
proposed deconvolution.
Hab ) (2.06 × 10-2/dab)(ꢀmax
ν
max ∆ν1/2)1/2
(3)
(4)
MV complexes possessing several electrons on metal d sublevels
in pseudo-octahedral environments might give rise to three
optical electron-transfer processes when the metal-metal
interaction mediated by the bridging ligand is strong and when
these levels lie close in energy to the SOMO/SOMO-1 levels
which usually give rise to the lowest-energy intervalence transfer
process.47 Multiple optical electron-transfer processes were also
recently envisioned by Sponsler and co-workers as possible
sources of sidebands for related dinuclear MV complexes
presenting related IVCT bands.48 This is indeed particularly true
for MV complexes containing “(η2-dppe)(η5-C5Me5)Fe” end-
groups, since the SOMO-2 to SOMO-4 are largely metal-
centered with respectively strong dxz and dx2-y2 character and
usually lie quite close in energy to the SOMO/SOMO-1 levels
which presents a strong dyz character (the z-axis being defined
along the Fe-Fe axis, with the y-axis pointing toward the
permethylated cyclopentadienyl ligand).27 We presently checked
this possibility, and the de-convolution of the near-IR-absorption
was attempted using the minimum number of sub-bands to
accurately fit the spectrum, starting from its most intense part.49
Since no bridge-centered oxidation is expected for 3[PF6] in
the ground state (GS) based on previous characterizations,24 the
lowest-energy allowed transition (A) was considered to cor-
respond to the “normal” metal-metal optical electron-transfer
(IVCT). Consequently, the first sub-band (A) at 4000 cm-1 was
then supposed to obey the two-level model and to exhibit a
half-width (ν1/2) corresponding to eq 4. A half-width of 3040
cm-1 was therefore used for this curve. However, such a
Gaussian sub-band cannot fit the low-energy side of the
spectrum (Figure 10) and has to be truncated. This is not
surprising since similar cutoffs are often observed with strongly
coupled MV complexes when the electronic coupling (Hab)
becomes close to half of the reorganization energy (λ/2).11,16a,47
Accordingly, we have truncated this first sub-band using a
1/2
(ν1/2)theo ) (2310.νmax
)
Study of the Near-IR Absorptions of the Mixed Valence
Complex 3+. For comparison purposes, we have also recorded
the near-IR spectra of the MV dinuclear complex 3[PF6] under
similar conditions, synthesized as previously described.24 This
time, the MV complex can be considered as being the only
species in solution (Kc ) 2.6 104) according to an equilibrium
similar to that given in eq 1. A strong near-IR absorption is
observed which obviously corresponds to an overlay of several
peaks. It is much stronger than that observed for 5[PF6] and
presents an apparent maximum at lower energy (ca. 4340
cm-1).44 We verified that this strong near-IR-absorption is
specific to 3[PF6]. Thus, this feature disappears upon oxidation
or reduction and we have verified that it can be obtained either
from the isolated MV complex 3[PF6] or from 3[PF6] generated
in situ by comproportionation of 3 and 3[PF6]2.
De-convolution of the near-IR absorption in Gaussian sub-
bands proved less straightforward than for 5[PF6]. Indeed,
correct fits can be obtained in different ways. For instance, the
observed spectrum can be fitted with a single transition
presenting a partly resolved vibronic progression (see Supporting
Information).45 To the best of our knowledge, vibronic progres-
sions observed so far on IVCTs involved fewer vibronic sub-
bands than needed here.11,46 Moreover, such vibronic progres-
sions on the IVCT band were observed only for fully delocalized
(class-III) MV compounds, and 3[PF6] is not a fully delocalized
MV complex (see below).
Alternatively, the de-convolution of the near-IR spectrum of
3[PF6] can be attempted using distinct overlapping transitions.
Meyer and co-workers10 have conclusively shown that inorganic
(44) Notably, the apparent absorption maximum is roughly twice as intense (ca.
13300 vs 5940 M-1cm-1) and located at a lower energy (ca. 4120 vs 4960
cm-1) than reported previously with a sample of 3[PF6].8a We believe that
this is due to the poor sensitivity of the spectrometer previously used in
this spectral range.
(47) Al-Noami, M.; Yap, G. P. A.; Crutchley, R. J. Inorg. Chem. 2004, 43,
1770-1778.
(48) Chung, M.-I.; Gu, X.; Etzenhouser, B. A.; Spuches, A. M.; Rye, P. T.;
Seetharaman, S. K.; Rose, D. J.; Zubieta, J.; Sponsler, M. B. Organome-
tallics 2003, 22, 3485-3494.
(49) An electron-transfer process (LMCT) to form an excited state with a bridge-
localized oxidation could also be envisioned with 3[PF6] to rationalize one
of the observed bands (B or C). Such processes were recently demonstrated
to occur in strongly coupled organic MV complexes featuring phenylene
bridges and produce very similar band shapes in the near-IR domain, when
not fully resolved.50 While such an explanation cannot yet be disregarded,
we favour the former hypothesis, since MLCT processes occur usually
above 14 000 cm-1 in mononuclear model complexes and are believed to
be also observed in this region with 3[PF6] (Table 2) based on DFT
computations.19
(45) In line with the recent contribution of Bailey and co-workers,46 the spectrum
can be fitted using a series of Gaussian curves of decreasing intensity (see
Supporting Information); the main peak would be at 3950 cm-1 with a
vibronic progression of ca. 1000 cm-1, each sub-band exhibiting a
significantly smaller half-width than expected based on Hush model (ca.
1200 cm-1). Presently, this progression could have corresponded to a strong
infrared-active mode observed at 1020 cm-1 in 3[PF6]. Note however that
such a correspondence is not required given the existence of “missing mode
effects” (MIME).46 Note also that other vibronic deconvolutions can be
obtained, especially when using less energetic progressions.
(46) Bailey, S. E.; Zink, J. I.; Nelsen, S. F. J. Am. Chem. Soc. 2003, 125, 5939-
5947.
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Di-organoiron Mixed Valent Complexes
A R T I C L E S
straight line in order to obtain the better fit of the low-energy
side. As seen in Figure 10, the truncation of the real spectrum
occurs more progressively certainly due to quantum effects
“rounding off the corners”.11 At this stage, the fit can be
completed with two additional sub-bands, B and C, at 6500 and
9000 cm-1, respectively, presenting the same half-width and
cutoff as band A (B and C; see Table 8 for details). In line
with Meyer’s proposal, this implies that these sub-bands
correspond to IVCT processes with comparable reorganization
energies to that of band A. Furthermore, in such a case, the
energies of the three optical metal-metal electron transfers
(EIVCT(1) - EIVCT(3)) are roughly related to the energies of two
LF transitions ELF(1) and ELF(2) by eqs 5-7.10 According to
eqs 5-7 and to the values found for EIVCT(2) and EIVCT(3)
(Table 8), the energies of these LF transitions should take place
at ca. 2500 and 5000 cm-1, which is in line with independent
estimates of these processes.27 Unfortunately, the high intensities
of sub-bands A and B preclude the detection of the “forbidden”
LF(2) transition at 5000 cm-1 on the experimental spectrum
with confidence, while the other “forbidden” transition (LF(1))
lies outside the spectrometer range. Nevertheless, it is apparent
that the use of an additional weak sub-band (D) at ca. 5400
cm-1 does significantly improve the fit (Figure 10). While the
fit obtained with these four sub-bands (A-D) is evidently quite
approximate in comparison with the real spectrum, we consider
it to be satisfactory in regard to the various approximations made
(notably the similar reorganization energies and linear cutoff
used for sub-bands A-C).
is also consistent with the Γ parameter determined according
to eqs 8 and 9.16a The later (Γ ) 0.013) gives also a measure of
the degree of delocalization of the unpaired electron in 5[PF6].
Γ ) (1-θ)
(8)
(9)
1/2
θ ) (ν1/2)exp/(2310.νmax
)
The electronic coupling derived for this complex (eq 5) proves
quite sizable for two metal centers situated 16 Å apart.5,10-13
The significant improvement in the electronic coupling (+25%)
found for 5[PF6] in acetone is quite surprising and should be
considered with caution given the experimental uncertainties
associated with the detection of weak IVCT bands and the poor
solubility of 5 in this solvent (see above).51 In the absence of
additional data, one could also tentatively relate this observation
to a solvent-induced increase of the comproportionation constant
(Kc in eq 1) in more polar media resulting in an apparent increase
of the IVCT band. More interestingly, the unambiguous class-
II categorization of 5[PF6] means that the νmax experimentally
found for band B (6250 cm-1) corresponds to the reorganization
energy (λ) of the optical electron-transfer process. Given its
relative magnitude in comparison to the electronic coupling
(2Hab/λ ) 0.046), no cutoff of the IVCT band is expected, as
observed.
While the valency seems localized at IR time scales (10-12
s), the ESR signal obtained for 5[PF6] is quite typical of a single
Fe(III) center and shows only a slightly diminished anisotropy
(∆g ) 0.442) in comparison to the mononuclear complex 6[PF6]
(∆g ) 0.464). This slight change could actually be induced by
the electronic exchange process between metal centers, since
the rate of electron transfer (kET) is calculated to be around 5
109 s-1 at 300 K using eq 10 (nonadiabatic electron transfer)
for a weakly coupled MV complex (see below).16a,53 Whatever
the rate of the exchange, the ESR data unambiguously shows
that the unpaired electron is dominantly metal-centered, further
emphasizing a class-II categorization for 5[PF6].
EIVCT(1) ≈ λ
(5)
(6)
(7)
EIVCT(2) ≈ λ + ELF(1)
E
IVCT(3) ≈ λ + ELF(2)
Regardless which deconvolution is considered, the strong
intensity of the near-IR absorption reveals a rather large
transition moment for 3[PF6] which, in turn, implies a strongly
allowed (IVCT) process corresponding to a strongly coupled
MV complex. In line with this hypothesis, the near-IR spectrum
is only marginally affected by a change in the solvent polarity
(see Supporting Information). As for 5[PF6] (see above), the
very low solubility of the neutral complex (3) in common
solvents restricted us to the use of a few dichloromethane-solvent
mixtures for this investigation.
[2 (Hab)2/h][π3/λRT]1/2exp[-(λ - 2Hab)2/(4λRT)] (10)
Hab ) (νmax)/2
(11)
While the data for 5[PF6] are in accordance with a mixed-valent
(MV) complex presenting a localized valency, the data for 3+
is strongly reminiscent of that recently obtained with 2+ (Chart
1), proposed to be a Class-IIIA MV complex.16a,19 Accordingly,
the high intensity of the near-IR (IVCT) transition and its
distinctly truncated shape on the low-energy side is suggestive
of a strongly coupled MV complex.11,46-47,54,55 However, 3[PF6]
is not fully delocalized (class-III) at the time scale of the
molecular vibrations (10-13 s) since two νC≡C modes are
observed in solution and in the solid state (KBr pellets), in line
with an overall dissymmetric structure (i.e., partly localized
valency) at ambient temperatures.24 As previously supposed,8a
Electronic Delocalization in the Mixed Valence Complex
5+ versus 3+. The observation of two νC≡C for the MV complex
5+, while only one had previously been detected for both 5 and
5
2+ evidences a localization of the valence at the time scale of
the infrared spectrometry (ca. 10-12 s) and strongly suggests
that we are not dealing here with a class-III MV complex.15,16a
In line with this observation, the electronic spectrum of 5+ in
the UV-vis range is merely a superposition of the absorption
observed for 5 and 52+ (Figure 4). While the detection of an
IVCT band in the near-IR domain indicate that 5[PF6] is not a
class-I MV compound, the negative solvatochromic behavior
evidenced for the IVCT band (Figure 9) establishes 5[PF6] as
a class-II MV complex. Notably, this shift to higher energies
for more polar solvents and the simultaneous increase in half-
width conforming to eq 4 is exactly what is expected for a
weakly coupled MV complex or class-IIA complex.14b,16a This
(50) Lambert, C.; Amthor, S.; Schelter, J. J. Phys. Chem. 2004, 108, 6474-
6486.
(51) Note that solvent-dependent electronic couplings have been occasionally
reported for organic52 or inorganic47 MV compounds.
(52) Nelsen, S. F.; Konradsson, A. E.; Telo, J. P. J. Am. Chem. Soc. 2005, 127,
920-925.
(53) A very similar result is also obtained when the corresponding equation for
an adiabatic electron transfer is considered, with a nuclear factor of 5 ×
1012 s-1
.
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A R T I C L E S
Ghazala et al.
3[PF6] must belong to the class-IIB and therefore constitutes a
limiting case between class-II and class-III with (Hab e λ/2).16a
The reorganization energy for the optical electron transfer of
lowest energy (A) in this complex is given by νmax (λeff ) 4000
cm-1). It is lower than that for 5[PF6] (λ ) 6250 cm-1), as
expected when going to a more delocalized and shorter MV
complex like 3[PF6]. An electronic coupling of 1700 cm-1 and
a Γ parameter of 0.30 can be derived from the experimental
bandwidth (equation 12), which implies a significantly larger
electronic delocalization than in 5[PF6].56,57 In line with the
negligible solvatochromy of band (A), the electronic coupling
is close to the higher limit of Hab derived in Table 8 (452 cm-1
< Hab < 2000 cm-1). This value also suggest a diminished
electronic delocalization relative to the 9,10-anthryl analogue
2[PF6], as was to be expected from published data.58,59
The near-IR absorption of the known and much more stable
MV analogue 3[PF6] presenting the 1,4-phenyl unit instead of
the 4,4′-biphenyl one (Kc ) 2.6 104) was also closely reexam-
ined. We consider it as mainly resulting from the overlap of
three optical electron-transfer processes involving lower lying
d electrons. Under such an assumption, an electronic coupling
of ca. 1700 cm-1 can be derived from the band shape for this
MV complex in the frame of the two-level model. Whatever
the assignments made, the redox centers are more strongly
coupled in this second compound, which lies much closer to
the borderline between class-II and class-III than 5[PF6].
Consequently, this work clearly demonstrates that the electron
exchange is not dramatically depressed by insertion of a second
para-phenylene moiety into a 1,4-dietynylaryl bridge, but simply
slowed, as expected when going from 3[PF6] to 5[PF6]. Thus,
the successive insertion of 1,4-phenylene units in the middle
of the butadiyne-diyl bridge of the Class-IIIA MV complex
1a[PF6] results in a progressive transition toward a Class-IIA
(5[PF6]) MV complex, via a Class-IIB (3[PF6]) MV complex.
Γ ) 1/2 - (λ - 2 Hab)/(ν1/2)theo
(12)
Conclusions
In this contribution, we have reported on the synthesis and
characterization of a new family of binuclear acetylide com-
plexes featuring electron-rich “(η2-dppe)(η5-C5Me5)Fe(CtC)”
endgroups with a 4,4′-biphenyl spacer inserted in the carbon-
rich bridge. Although no splitting of the Fe(III/II) redox wave
was discernible in the cyclic voltammogram, the compropor-
tionation constant of 5[PF6] could be measured by cyclic
voltammetry (Kc around 10). With such a low value, the
selective isolation of the MV complex 5[PF6] proved not
possible, but the latter could be characterized in solution by
various spectrometries. An IVCT band was detected in the near-
IR domain, evidencing the existence of a photodriven metal-
metal electron-transfer process between metal centers ca. 16 Å
apart. The reorganization energy of this intramolecular process
amounts to ca. 6250 cm-1 in dichloromethane and an electronic
coupling of ca. 150 cm-1 could also be derived for this process.60
Notably, this study reveals that 5[PF6] constitutes an(other)
example of well behaved weakly coupled class-IIA organome-
tallic MV compound according to the Robin and Day clas-
sification and, considering the high consistency of the experi-
mental data gathered for this compound, justifies a posteriori
the use of a two-level model to interpret the experimental data.
Experimental Section
General Data. All manipulations were carried out under inert
atmospheres. Solvents or reagents were used as follows: Et2O and
n-pentane, distilled from Na/benzophenone; CH2Cl2, distilled from CaH2
and purged with argon; HN(iPr)2, distilled from KOH and purged with
argon; aryl bromides (Acros, >99%), opened/stored under Ar. The [(η5-
C5H5)2Fe][PF6] ferricinium salt was prepared by previously published
procedures.33 Transmittance-FTIR spectra were recorded using a Bruker
IFS28 spectrometer (400-4000 cm-1). Raman spectra of the solid
samples were obtained by diffuse scattering on the same appartus and
recorded in the 100-3300 cm-1 range (Stokes emission) with a laser
excitation source at 1064 nm (25 mW) and a quartz separator with a
FRA 106 detector. Near-IR (near-IR) spectra were recorded using a
Bruker IFS28 spectrometer, using a Nernst Globar source and a KBr
separator with a DTGS detector (400-7500 cm-1) or tungsten source
and a quartz separator with a Peltier-effect detector (5200-12500 cm-1
)
or on a Cary 5 spectrometer. UV-Visible spectra were recorded on an
UVIKON XL spectrometer. ESR spectra were recorded on a Bruker
EMX-8/2.7 (X-band) spectrometer. Elemental analyses were performed
at the “Centre Regional de Mesures Physiques de l′Ouest” (C. R. M.
P. O., University of Rennes).
Synthesis of the Binuclear Bis-vinylidene Fe(II) complex [{(η2-
dppe)(η5-C5Me5)Fe}2(dCdCH-4,4′-{1,1′-(C6H4)2}sHCdCd)]-
[BPh4]2 (5v[BPh4]2). In a Schlenk tube, 0.50 g of 4,4′-bis-ethynyl-
2,2′-biphenyl (2.47 mmol), 3.08 g of (η2-dppe)(η5-C5Me5)FeCl (4.94
mmol) and 1.80 g of [Na][BPh4] (5.26 mmol) were introduced under
argon. A mixture of MeOH/THF (30/10 mL) was added and the dark
green suspension was stirred for 32 h. The solvent was then removed
in vacuo and the residue was extracted with CH2C12. The extract was
concentrated and n-pentane was added to precipitate the [{(η2-dppe)-
(η5-C5Me5)Fe}2(dCdCH-1,4-{1,1′-(C6H4)2}sHCdCd)][BPh4]2 vi-
nylidene complex (5v[BPh4]2) as a pale brown powder which was
subsequently washed with diethyl ether and dried in vacuo. Yield 80%.
FT-IR (KBr, ν in cm-1) 1616 (m, FedCdC). 31P NMR (81 MHz,
(54) Low, P. J.; Paterson, M. A. J.; Puschmann, H.; Goeta, A. E.; Howard, J.
A. K.; Lambert, C.; Cherryman, J. C.; Tackley, D. R.; Leeming, S.; Brown,
B. Chem. Eur. J. 2004, 10, 83-91.
(55) Szeghalmi, A. V.; Erdmann, M.; Engel, V.; Schmitt, M.; Amthor, S.;
Kriegisch, V.; No¨ll, G.; Stahl, R.; lambert, C.; Leusser, D.; Stalke, D.;
Zabel, M.; Popp, J. J. Am. Chem. Soc. 2004, 126, 7834-7845.
(56) Although the two-level approach (equations 3-4 and 11) might constitute
a poor model to accurately derive the energy of the electronic coupling for
localized MV compounds when several diabatic states interact with the
(degenerate) GS,57 this approach remains correct provided that the four
closely lying interconfigurational diabatic (LF) states couple only weakly
with the two degenerate diabatic ground states and provided the Born-
Oppenheimer approximation still holds. The first condition is certainly
fulfilled for 3[PF6] considering the poor overlap and the energetical gaps
between the different metal-centred frontier molecular orbitals presenting
a different d character.27
1
CDCl3, δ in ppm) 88.5 (s, 2 P, dppe). H NMR (200 MHz, CDCl3, δ
in ppm) 7.70-6.75 (m, 84 H, HAr); 6.42 (d, 4 H, 4JPH ) 8.2 Hz, HAr);
(57) Coropceanu, V.; Malagoli, M.; Andre´, J. M.; Bre´das, J. L. J. Am. Chem.
Soc. 2002, 124, 10519-10530.
(58) (a) Lambert, C.; No¨ll, G.; Schelter, J. Nat. Mater. 2002, 1, 69-73. (b)
Gao, L.-B.; Zhang, L.-Y.; Shi, L.-X.; Chen, Z.-N. Organometallics 2005,
24, 1678-1684. (c) Fraysse, S.; Coudret, C.; Launay, J.-P. J. Am. Chem.
Soc. 2003, 125, 5880-5888.
4
5.18 (t, 2 H, JPH ) 4.6 Hz, CdCH); 2.25-2.85 (2m, 8 H, PCH2);
1.30 (s, 30 H, C5Me5). 13C NMR (50 MHz, CDCl3, δ in ppm) 363.4 (t,
2JCP ) 27 Hz, FedCdC); 164.6 (q, JBC ) 50 Hz, BsCAr); 135.7-
1
125.7 (12 s, CAr); 136.8, 125.9, 122.1 (s, CAr(BPh4)), 126.4 (s, FedCd
(59) Karafiloglou, P.; Launay, J. P. Chem. Phys. 2003, 289, 231-242.
(60) As suggested by one referee, the value of 145 cm-1 found is certainly not
“unusually high” for the bridge featuring a 2,2′-biphenyl spacer. This is
also indicated by the plot of the logarithm of Hab versus the number of
bonds between the irons atoms in the series 1a[PF6], 3[PF6], and 5[PF6]
which appears reasonably linear (see Supporting Information), the value
for 5[PF6] being slightly below the value expected.
CH); 99.8 (s, C5Me5); 28.3 (m, PCH2); 9.4 (s, C5Me5).
Synthesis of the Binuclear Bis-alkyny1 Fe(II) Complex {(η2-
dppe)(η5-C5Me5)Fe}2(CtC-4,4′-{1,1′-(C6H4)2}CtC) (5). In a Schlenk
tube, 3.00 g of bis-viny1idene complex 5v[BPh4]2 (1.76 mmol) and
0.40 g of tBuOK (3.54 mmol) and THF were introduced under argon.
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2474 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Di-organoiron Mixed Valent Complexes
A R T I C L E S
The dark red solution was stirred for 7 h and the solvent was removed
in vacuo. The residue was extracted with toluene and the extract was
concentrated, before addition of n-pentane to precipitate {(η2-dppe)-
(η5-C5Me5)Fe}2(CtC-4,4′-{1,1′-(C6H4)2}CtC) the alkynyl complex (5)
as a red powder which was subsequently dried in vacuo. Yield 90%.
Dark red crystals of 5 could be grown by (i) allowing diethyl ether to
slowly diffuse into a nitrobenzene solution of the compound or (ii) by
Ar(A1)). 31P NMR (81 MHz, C6D6, δ in ppm) 101.5 (s, 2 P, dppe). 1H
NMR (200 MHz, CD2Cl2, δ in ppm) 7.97 (m, 4 H, HAr); 7.60-7.20
(m, 19 H, HAr); 6.75 (d, 2 H, HAr); 2.75-2.03 (2 m, 4 H, PCH2); 1.46
(s, 15 H, C5Me5). 13C NMR (125 MHz, CDCl3, δ in ppm) 139.3 (t,
2JCP ) 39 Hz, Fe-CtC); 138.4-126.6 (16 s, CAr); 121.3 (s, Fe-C≡C);
88.2 (s, C5Me5); 29.5 (m, PCH2); 10.1 (s, C5Me5).
Synthesis of the Mononuclear Alkyny1 Fe(III) Complex [(η2-
dppe)(η5-C5Me5)Fe(CtC-4-{(C6H4)-1,1′-(C6H5)}][PF6] (6[PF6]). To
a suspension of 0.25 g of the bis alkynyl complex 6 (0.33 mmol), 0.10
g of [Fe(η5-C5H5)2][PF6] (0.30 mmol) was added in 15 mL dichlo-
romethane, resulting in an instantaneous darkening of the solution.
Stirring was maintained 3 h at room temperature and the solution was
concentrated in vacuo to ca. 5 mL. Addition of 50 mL of n-pentane
allowed precipitation of a dark solid. Decantation, subsequent washing
with 2 × 3 mL portions diethyl ether and drying under vacuum yielded
the desired [{(η2-dppe)(η5-C5Me5)Fe}(CtC-4-{(C6H4)-1,1′-(C6H5)}]-
[PF6] complex (6[PF6]) as an analytically pure sample. Yield 91%. FT-
IR (KBr, ν in cm-1) 1989 (s, CtC); 1599 (vs, Ar(A1)). 1H NMR (200
MHz, CDCl3, δ in ppm) 31.0 (Hbip); 11.0 (Hbip); 8.0-6.3 (Hdppe); 3.7
(Hdppe); 0.9 (Hdppe); -0.2 (Hbip); - 0.6 (Hbip); -2.8 (Hdppe); -10.4
(C5Me5); -44.0(Hbip).
Synthesis of the Mononuclear Alkyny1 Fe(III) Complex [(η2-
dppe)(η5-C5Me5)Fe(CtC-4-{(C6H4)-1,1′-(C6H5)}][TCNQ] (6[TCNQ]).
To a suspension of 0.210 g of the alkynyl complex 6 (0.27 mmol) in
15 mL of THF, 0.055 g of TCNQ (0.27 mmol) was added, resulting in
an instantaneous darkening of the solution. Stirring was maintained 3
h at room temperature and the green solution was concentrated in vacuo
to ca. 5 mL. Addition of 50 mL of n-pentane allowed precipitation of
a dark solid. Decantation and subsequent washing with 2 × 3 mL
portions of n-pentane and drying under vacuum yielded the desired
[{(η2-dppe)(η5-C5Me5)Fe}(CtC-4-{(C6H4)-1,1′-(C6H5)}][TCNQ] com-
plex (6[TCNQ]). Yield 90%. Small dark crystals of 6[TCNQ] could
be grown after layering a dichloromethane solution of the complex
with n-pentane.
slow evaporation of a THF/n-pentane solution of 5. MS (FAB+,
m-NBA) m/z 1378.4383 ([M]+, 100%), m/z calcd. for [C88H86P456Fe2
]
+
) 1378.4379. Anal. Calcd. for C88H86P4Fe2: C, 76.63; H, 6.29.
Found: C, 76.78; H, 6.53. FT-IR (KBr, ν in cm-1) 2051 (s, CtC);
1597 (vs, Ar(A1)). Raman (ν, cm-1) 2057, 2030 (vw, CtC); 1590 (vs,
Ar(A1)). 31P NMR (81 MHz, CDCl3, δ in ppm) 101.1 (s, 2P, dppe). 1H
NMR (500 MHz, C7D8, δ in ppm) 8.05 (m, 8 H, 4 HAr); 7.46 (d, 3JHH
) 7.2 Hz, 4 H, HAr); 7.75-7.00 (m, 36 H, HAr); 2.05-2.75 (2m, 8 H,
PCH2); 1.47 (s, 30 H, C5Me5). 13C NMR (75 MHz, CD3C6D5, δ in
ppm) 142.3 (t, 2JCP ) 39 Hz, Fe-CtC); 140.0-120.4 (12 s, CAr); 120.4
(s, Fe-C≡C); 88.4 (s, C5Me5); 29.5 (m, PCH2); 9.9 (s, C5Me5).
Synthesis of the Binuclear Bis-alkyny1 Fe(III) Complex [{(η2-
dppe)(η5-C5Me5)Fe}2(CtC-4,4′-{1,1′-(C6H4)2}CtC)][PF6]2 (5[PF6]2).
To a 1.00 g suspension of the bis-alkynyl complex 5 (0.74 mmol) in
50 mL dichloromethane, 0.49 g of [Fe(η5-C5H5)2][PF6] (1.48 mmol)
was added, resulting in an instantaneous darkening of the solution.
Stirring was maintained 3 h at room temperature and the solution was
concentrated in vacuo to ca. 5 mL. Addition of 50 mL of n-pentane
allowed precipitation of a dark solid. Decantation and subsequent
washing with 2 × 3 mL portions of toluene followed by 2 × 3 mL
diethyl ether and drying under vacuum yielded the desired [{(η2-dppe)-
(η5-C5Me5)Fe}2(CtC-4,4′-{1,1′-(C6H4)2}CtC)][PF6]2 (5[PF6]2) com-
plex as an analytically pure sample. Small dark crystals of 5[PF6]2 could
be grown after layering a dichloromethane solution of the complex
with n-pentane. Yield 94%. FT-IR (KBr, ν in cm-1) 1988 (s, CtC);
1593 (vs, Ar(A1)). Raman (ν, cm-1) 2016 (w, CtC); 1593 (vs, Ar-
1
(A1)). H NMR (200 MHz, CDCl3, δ in ppm) 22.0 (Hbip); 7.9-6.4
(Hdppe); 3.8 (Hdppe); 2.0 (Hdppe); -2.4 (Hdppe); -9.7 (C5Me5); -37.0 (Hbip).
Electrochemical Experiments. All the cyclic voltammetry experi-
ments were carried out at 20 ( 0.1 °C using a cell equipped with a
jacket allowing circulation of water from the thermostat. The working
electrode was either a 1 mm diameter platinum or gold disk. It was
carefully polished before each set of voltammograms with 1 µm
diamond paste and ultrasonically rinsed in absolute ethanol. Electro-
chemical instrumentation consisted of a Tacussel GSTP4 programmer
and of home-built potentiostat equipped with a positive feedback
compensation device.61 The data were acquired with a 310 Nicolet
oscilloscope. The counter electrode was a Pt wire and the reference
electrode an aqueous saturated calomel electrode with a salt bridge
containing the supporting electrolyte. The SCE electrode was checked
against the ferrocene/ferricinium couple (considering the following E°
values: E° ) +0.460V/SCE in dichloromethane) before and after each
experiment.
Synthesis of the Mononuclear Fe(II) Vinylidene Complex [(η2-
dppe)(η5-C5Me5)Fe(dCdCH-4-{(C6H4)-1,1′-(C6H5)})][PF6] (6v-
[PF6]). In a Schlenk tube, 0.22 g of 4-ethynyl-1,1′-biphenyl (1.23
mmol), 0.70 g of (η2-dppe)(η5-C5Me5)FeCl (1.12 mmol) and 0.22 g of
of KPF6 (1.19 mmol) were introduced in 40 mL of a mixture of MeOH/
THF (3:1). The dark green suspension was stirred for 16 h. The solvent
was removed in vacuo and the residue was extracted with CH2C12.
The extract was concentrated in vacuo and n-pentane was added to
precipitate the vinylidene complex [{(η2-dppe)(η5-C5Me5)Fe}(dCdCH-
4-{(C6H4)-1,1′-(C6H5)][PF6] (6v[PF6]) as a pale brown powder which
was subsequently washed with diethyl ether and dried. Yield 80%. MS
(FAB+, m-NBA) m/z 767.2651([M+1]+, 100%), m/z calc for [C50H47P256
-
Fe]+ ) 767.2659. FT-IR (KBr, ν in cm-1) 1618 (m, FedCdC). 31P
NMR (81 MHz, CDCl3, δ in ppm) 88.3 (s, 2 P, dppe), -143.0
(septuplet, 1JPF ) 713 Hz, 1 P, PF6). 1H NMR (200 MHz, CDCl3, δ in
ppm) 7.65-7.20 (m, 27 H, HAr); 6.44 (d, 2 H, HAr); 5.17 (t, 1 H, 4JPH
) 3.8 Hz, CdCH); 2.25-2.85 (2m, 4 H, PCH2); 1.64 (s, 30 H, C5Me5).
Numerical simulations of the voltammograms were performed with
the commercial BAS Digisim Simulator 3.162 using the default
numerical options with the assumption of planar diffusion. At low scan
rate, the convection effect was taken into account by considering a
pseudo hydrodynamic-diffusion regime (w ) 1.4 rad s-1, vk ) 0.001
cm2s-1). Butler-Volmer law was considered for the electron-transfer
kinetics transfer. The transfer coefficient, R, was taken as 0.5 with equal
diffusion coefficients for the all the species (D ) 10-5 cm2.s-1).
Crystallography. Crystals of 5‚(C4H8O)‚3/4(C5H12), 5[PF6]2‚2(CH2-
Cl2), 6 and 6[TCNQ]‚1/4(CH2Cl2) were obtained as described above.
The samples were studied on a NONIUS Kappa CCD with graphite
monochromatized MoKR radiation. The cell parameters were obtained
with Denzo and Scalepack with 10 frames (psi rotation: 1° per
Synthesis of Mononuclear Fe(II) Alkynyl Complex (η2-dppe)-
(η5-C5Me5)Fe(CtC-4-{(C6H4)-1,1′-(C6H5)}) (6). In a Schlenk tube,
0.161 g (0.21 mmol) of vinylidene complex 6v[PF6], 0.030 g of tBuOK
(0.27 mmol) was introduced in ca. 20 mL THF. The dark red solution
was stirred for 5 h. The solvent was then removed in vacuo and the res-
idue was extracted with toluene. The extract was concentrated and n-
pentane was added to precipitate the (η2-dppe)(η5-C5Me5)Fe(CtC-4-
{(C6H4)-1,1′-(C6H5)}) complex (5) as an orange powder which was
subsequently dried in vacuo. Yield 92%. Red crystals of 5 could be grown
after layering a dichloromethane solution of the complex with n-pentane.
MS (FAB+, m-NBA) m/z 766.2597 ([M]+, 100%), m/z calc for
[C50H46P256Fe]+ ) 766.2581. Anal. Calcd. For C50H48P2Fe: C, 78.33;
H, 6.31. Found: C, 78.32; H, 6.28. FT-IR (KBr, ν in cm-1) 2056 (vs,
CtC); 1594 (s, Ar(A1)). Raman (ν, cm-1) 2059 (m, CtC); 1597 (vs,
(61) Garreau, D.; Save´ant, J.-M. J. Electroanal. Chem. 1972, 35, 309-331.
(62) Rudolph, M.; Reddy, D. P.; Felberg, S. W. Anal. Chem. 1994, 66, 589A-
560A.
9
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A R T I C L E S
Ghazala et al.
frames).63 The data collection64 (2θmax, number of frames, Ω rotation,
scan rate and HKL range are given in Table 7) provided reflections
for 5‚(C4H8O)‚3/4(C5H12), 5[PF6]2‚2(CH2Cl2), 6 and 6[TCNQ]‚1/4(CH2-
Cl2). Subequent data reduction with Denzo and Scalepack63 gave the
independent reflections (Table 7). The structures were solved with SIR-
97 which revealed the non-hydrogen atoms.65 After anisotropic refine-
ment, the remaining atoms were found in Fourrier difference maps.
The complete structures were then refined with SHELXL9766 by the
full-matrix least-squares technique (use of F square magnitude; x, y, z,
âij for Fe, P, C, N, and/or O atoms, x, y, z in riding mode for H atoms
with variables “N(var.)”, observations and “w” used as defined in Table
7). Atomic scattering factors were taken from the literature.67 ORTEP
views of 5, 52+, 6 and 6[TCNQ] were realized with PLATON98.68
Acknowledgment. We thank J.-Y. The´pot for experimental
assistance in the ESR measurements and the CNRS for financial
support.
Supporting Information Available: Figure showing the
superposition of the simulated and experimental CVs of 5.
Tables of selected bond lengths and angles for 5, 5[PF6]2, 6
and 6[TCNQ]. Figures of one alternative (vibronic progression)
deconvolution and of the solvent-induced changes of the NIR
absorption of 3[PF6]. A plot of log(Hab) vs number of bonds
between iron atoms. Full details of the X-ray structures of 5
crystallized from nitrobenzene-diethyl ether mixtures, including
tables of atomic positional parameters, bond distances and
angles, anisotropic and isotropic thermal displacement param-
eters (25 pages, print/PDF). The CIF files 5‚(C4H8O)‚3/4(C5H12),
5[PF6]2‚2(CH2Cl2), 6 and 6[TCNQ]‚1/4(CH2Cl2). This material
is available free of charge via the Internet at http://pubs.acs.org.
(63) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter, C. W.,
Sweet, R. M., Eds.; Academic press: London, 1997; Vol. 276, pp 307-
326.
(64) Nonius, B. V. Kappa CCD Software; Delft: The Netherlands, 1999.
(65) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.;
Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Chem.
1998, 31, 74-77.
(66) Sheldrick, G. M. SHELX97-2. Program for the refinement of crystal
structures; University of Go¨ttingen: Germany, 1997.
(67) Reidel: D. International Tables for X-ray Crystallography; Kynoch
Press: present distrib. D. Reidel, Dordrecht: Birmingham, 1974; Vol. IV.
(68) Spek, A. L. PLATON. A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 1998.
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