Investigation of the iron-carbon bonding for alkyl, alkynyl, carbene, vinylidene, and allenylidene complexes using 57Fe Moessbauer spectroscopy

Article abstract of DOI:10.1016/S0022-328X(98)00444-6

The synthesis of the iron allenylidene [Cp*(dppe)Fe(=C=C=C(OCH₃)CH₃)][BPh₄] (7) was achieved in one step, from the reaction of the Cp*(dppe)FeCl (8) with 1 equiv. of Me₃-Si-CC-CCH. By working in methanol and in the presence of NaBPh₄, the complex 7 was isolated in 85% yield as a dark green powder. Comparison of the Moessbauer parameters of a series of 16 compounds having the same Cp*(dppe)Fe backbone and a end-bound hydrocarbon ligand shows that it is possible to identify both the oxidation state of the metal (Fe^II vs. Fe^III) and the metal-carbon bonding (single vs. double bond) of the terminal hydrocarbon ligands using Moessbauer spectroscopy.

Full text of DOI:10.1016/S0022-328X(98)00444-6

Journal of Organometallic Chemistry 565 (1998) 75–80
Investigation of the iron–carbon bonding for alkyl, alkynyl, carbene,
57
vinylidene, and allenylidene complexes using Fe Mo¨ssbauer
spectroscopy1
Vale´rie Guillaume a, Patrice Thominot a, Franc¸oise Coat a, Alain Mari b, Claude Lapinte a,*
a UMR CNRS 6509 Organome´talliques et Catalyse: Chimie et Electrochimie Mole´culaires, Uni6ersite´ de Rennes 1, Campus de Beaulieu,
35042 Rennes Cedex, France
b Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex, France
Received 20 November 1997
Abstract
The synthesis of the iron allenylidene [Cp*(dppe)Fe(ꢀCꢀCꢀC(OCH3)CH3)][BPh4] (7) was achieved in one step, from the reaction
of the Cp*(dppe)FeCl (8) with 1 equiv. of Me3–Si–CꢁC–CꢁCH. By working in methanol and in the presence of NaBPh4, the
complex 7 was isolated in 85% yield as a dark green powder. Comparison of the Mo¨ssbauer parameters of a series of 16
compounds having the same Cp*(dppe)Fe backbone and a end-bound hydrocarbon ligand shows that it is possible to identify
both the oxidation state of the metal (FeII vs. FeIII) and the metal–carbon bonding (single vs. double bond) of the terminal
hydrocarbon ligands using Mo¨ssbauer spectroscopy. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Synthesis; Organoiron; Iron(II); Iron(III); Butadienyl; Alkyne; Carbene; Vinylidene; Allenylidene; Cumulene; Mo¨ss-
bauer spectroscopy
1. Introduction
been established that metallacumulenes act as interme-
diates in stoichiometric reactions [1,2] and are also
involved in many catalytic processes [3–11]. The low
solubility of many of these complexes and the difficulty
to establish their structure on the basis of 13C-NMR
and IR spectroscopic data prompted us to examine the
possibility to get complementary insights on the metal
carbon bonding by Mo¨ssbauer spectroscopy. Such an
approach might be particularly interesting when grow-
ing of single crystals is unfruitful.
After the discovery of complexes with alkyl, vinyl
and alkynyl |-bound ligands, the emergence of the
transition-metal carbene chemistry was one of the most
exciting areas in molecular chemistry research during
the last decades. Nowadays, the chemistry of metal–
vinylidene, allenylidene and cumulene complexes have
attracted considerable interest. Several synthetic meth-
ods are available to prepare this new class of com-
pounds which constitute potential building blocks for
the construction of polymeric materials or nanoscale
molecular devices as molecular wires. Moreover, it has
It is well known that 57Fe Mo¨ssbauer spectroscopy is
very efficient to identify the oxidation states of iron
[12–19]. This technique which usually requires a solid
powdered sample was used with success to characterize
many mononuclear, and polynuclear derivatives and
mixed-valence complexes. For instance, we have previ-
ously shown in the Cp*(dppe)Fe–R series that Mo¨ss-
* Corresponding author. e-mail: claude.lapinte@univ-rennes1.fr
1 Dedicated to Professor Michael Bruce on the occasion of his 60th
birthday, in recognition of his contributions to organometallic chem-
istry.
bauer spectroscopy is
a powerful tool for the
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00444-6

76
V. Guillaume et al. / Journal of Organometallic Chemistry 565 (1998) 75–80
Scheme 2.
Scheme 1.
readily accessible in 1982 with the paper by Selegue
which illustrated for the first time that propargylic
alcohols can be converted into allenylidene ligands in
the coordination sphere of an electron rich ruthenium
complex [24]. More recently, a large number of
mononuclear allenylidene compounds were reported for
a large variety of transition metals [25–29]. The iron
allenylidene complex 7 was prepared according to a
one-step original procedure.
identification of the oxidation state of the metal in
iron-alkyl and iron-alkynyl complexes [20–22]. In this
series of compounds with a |-bound hydrocarbon lig-
and, the iron(II) derivatives (Scheme 1, type IA, IB, and
IC, Scheme 1) exhibit a quadrupole splitting close to 2
mm s−1, whereas the quadrupole splitting of their
17-electron iron(III) analogues (type IIA and IIB) is
lower than 1 mm s−1 (Table 1).
Owing to the growing interest for compounds with
multiple metal–carbon bonds, the characterization of
metal–carbon double bonding in iron–carbene, iron–
vinylidene and iron–allenylidene (Scheme 2, type III,
n=0, 1, 2) compounds by 57Fe Mo¨ssbauer spectroscopy
was the goal of this work. The target molecules used in
this study were all chosen in the same Cp*(dppe)Fe
series and are depicted on Scheme 3. Their syntheses
were previously reported for almost all of them. How-
ever, iron allenylidene complexes (n=3) were unknown
in this series, and for the purpose of this spectroscopic
study we designed and synthesised the iron allenylidene
[Cp*(dppe)Fe{ꢀCꢀCꢀC(OCH3)CH3}][BPh4] (7).
Treatment of the chloro complex Cp*(dppe)FeCl (8)
and NaBPh4 in methanol with 1 equiv. of Me3–Si–CꢁC–
CꢁCH produced after 16 h a deep-green solution
from
which
the
allenylidene
complex
[Cp*(dppe)Fe(ꢀCꢀCꢀC(OCH3)CH3)][BPh4] (7) was iso-
lated as a green forest powder in 85% yield (Scheme 4).
Subsequent crystallization gave analytically pure 7
1
(78%) which was characterized by IR, H-, 13C-, 31P-
NMR and UV–vis spectroscopies. The infrared spec-
trum revealed two strong allenylidene CꢀCꢀC stretches
at 1938 and 1947 cm−1 in the solid state (Nujol) and a
single band stretching in solution at 1943 cm−1
(CH2Cl2). The observation of two band stretches in the
solid state could be due either to the presence of
isomorphous microcrystals or to a restricted rotation in
the allenylidene fragment giving different rotamers
quenched in the crystal lattice (vide infra). The 1H-
NMR spectrum (CDCl3) displays a singlet at l 1.41 for
the methyl of the Cp* ring and two singlets at l 1.71
and 3.41 for the protons of the methyl and methoxy
groups of the allenylidene ligand, respectively. A
2. Results and discussion
2.1. Synthesis of the methyl methoxymethyl
allenylidene complex
[Cp*(dppe)Fe{ꢀCꢀCꢀC(OCH3)CH3}][BPh4] (7)
Metal–allenylidene complexes were reported for the
first time in 1976 [23], but these complexes became
Table 1
+
Mo¨ssbauer parameters in mm s−1 for the Fe(II) and Fe(III) complexes of the [Cp*(dppe)FeL]n n[X−] series at 80 K
L
Compounds
n
Group
l
DEQ
Ref.
CH3
CH2OCH3
–CꢁC–H
2a
2b
4a
4b
4c
4d
3
1
5
6a
6b
7
2a+
2b+
4b+
4c+
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
I
I
I
I
I
I
I
I
III
III
III
III
II
II
II
II
0.15
0.13
0.276
0.27
0.28
1.95
1.97
1.987
2.02
2.00
[20]
[20]
This work
This work
This work
[37]
This work
This work
This work
This work
This work
This work
[20]
–CꢁC–(C6H5)
–CꢁC–tBu
–CꢁC–CꢁC–SiMe3
–C(ꢀCH2)OCH3
CO
ꢀC(CH3)OCH3
ꢀCꢀCH2
ꢀCꢀC(C6H5)H
ꢀCꢀCꢀC(CH3)OCH3
CH3
0.25
1.93
0.323
0.196
0.216
0.086
0.116
0.160
0.35
0.25
0.25
0.26
1.904
1.842
1.223
1.032
1.118
1.451
0.76
0.95
0.90
0. 85
CH2OCH3
[20]
This work
This work
–CꢁC–(C6H5)
–CꢁC–tBu

V. Guillaume et al. / Journal of Organometallic Chemistry 565 (1998) 75–80
77
Scheme 3.
2
deshielded Ch triplet (lC 268.2, JPC=34 Hz), a Ci (lC
156.6) singlet and a Ck (lC 152.4) unresolved multiplet,
typical of a metalla-allenylidene, were observed in the
13C-NMR.
[M]ꢀCꢀCH(CꢁCH) [30]. Moreover, elegant quenching
reactions of ruthenium butatrienylidene intermediates
[(PPh3)2CpRuꢀCꢀCꢀCꢀCH2] generated by reaction of
the buta-1,3-diyne with [(PPh3)2CpRu(THF)] were re-
ported from M.I. Bruce’s group [1,2]. On the other
hand, the synthesis of ruthenium allenylidene com-
plexes from the reaction of a dichloro ruthenium
derivative with terminal diynes in methanol also sup-
ports the proposed mechanism for the formation of the
iron compound 7 [29].
By analogy with the reaction of the trimethylsily-
lacetylene with the chloro iron complex Cp*(dppe)FeCl
(8) which provided the vinylidene derivative
[Cp*(dppe)Fe(ꢀCꢀCH2)][BPF6] under the same con-
ditions (6a, Eq. (1)),[22] one can assume the forma-
tion of the butatrienylidene complex [Cp*(dppe)
Fe(ꢀCꢀCꢀCꢀCH2)][BPF6] results from the primary
product of the reaction of 8 with the trimethylsilyl
1,3-butadiyne (Eq. (2)). A subsequent addition of a
methanol molecule to the complex 7 yields the isolated
allenylidene 7 (Eq. (3)).
2.2. In6estigation of the iron–carbon bonding by
Mo¨ssbauer spectroscopy
The Mo¨ssbauer spectra of microcrystalline samples
of the iron complexes depicted on Scheme 3 are charac-
teristic of pure organoiron complexes (Table 1, Fig. 1).
Indeed, all these spectra displayed a single quadrupole
doublet. As previously observed for complexes 2a and
2b [20] and also for many ferrocene derivatives [31], the
Mo¨ssbauer parameters are almost temperature indepen-
dent between 77 and 300 K. For this reason, the spectra
here reported were all recorded at 80 K. Considering
the Mo¨ssbauer quadrupole splittings of these com-
pounds (Table 1), it is possible to classify them in three
different groups. All the complexes having a 18-electron
iron(II) center connected to a p1-bound carbon ligand
belong to the first group. Their DEQ is close to 2 mm
(8)[(Cp*)(dppe)FeCl]
NH PF
4
+Me3SiCꢁCH “ 6[(Cp*)(dppe)Fe(ꢀCꢀCH2)][PF6]
MeOH
(6a)
(1)
(8)[(Cp*)(dppe)FeCl]+Me3SiCꢁC
–CꢁCHNa BPh
“
4{[(Cp*)(dppe)Fe(ꢀCꢀCꢀCꢀCH2)][BPh4]}
4
MeOH
(2)
{[(Cp*)(dppe)Fe(ꢀCꢀCꢀCꢀCH2)][BPh4])+MeOH“7
(3)
Whereas the formation of a vinylidene derivative re-
quires a 1,2 H-shift according to Eq. (1), the formation
of a butatrienylidene compound from 1,3-butadiyne
involves a 1,4 H-shift. Meanwhile butatrienylidene
complexes were never isolated nor spectroscopically
characterized to date, their existence is not inconsistent
with the result of ab initio calculations. Indeed, it was
possible to predict that the free energy of
s
−1. In this group of compounds, the quadrupole split-
ting is independent of the hybridization of the carbon
bound to the iron atom and weakly depends on the
electronic effect of the hydrocarbon ligand. Thus, in the
case of the cationic complex [Cp*(dppe)Fe(CO)][PF6]
(1), the less electron rich complex of this group, one can
note that the quadrupole splitting (DEQ=1.842 mm
[M]ꢀCꢀCꢀCꢀCH2
should
be
close
that
of

78
V. Guillaume et al. / Journal of Organometallic Chemistry 565 (1998) 75–80
Scheme 4.
s
−1) is not far from the values observed for the neutral
In the case of the 17-electon iron(III) radical cations
having an p1-bound hydrocarbon ligand, it was ob-
served that the quadrupole splitting is systematically
below 1 mm s−1 (see Table 1). Once again, similar DEQ
values are observed with hydrocarbon ligands having
either a sp3 or a sp hybridized carbon atom. These
complexes constitute the group II of the compounds
here considered, and they can be identified by their
doublet separation in the Mo¨ssbauer spectrum. More-
over, one can note that the isomeric shift of 2a+ and
2b+ are significantly weaker than those of 2a and 2b,
whereas almost the same l values are observed for the
couple of compounds 4b, 4c and 4b+, 4c+. This
indicates that the ethynyl ligand should participate to
the delocalization of the odd electron. As a result, the
diminution of the electron density at the metal center
resulting of the one-electron oxidation should be partly
balanced in 4b+, 4c+ by the ethynyl fragment.
More interesting are the Mo¨ssbauer parameters of
the carbene, vinylidene and allenylidene iron derivatives
5–7. These compounds with a FeꢀC double bond con-
stitute the third group of complexes in this study, and
have never been the subject of a Mo¨ssbauer spectro-
scopic investigation. The quadrupole splittings of these
iron(II) centers are well differentiated from those of the
iron(II) and iron(III) parent compounds of the groups I
and II. As a consequence, the quadrupole splitting
(DEQ) is diagnostic of the bond order of the iron–car-
bon bond between the metal and the end-bound carbon
ligands, at least in this Cp*(dppe)Fe series. Moreover,
the quadrupole separation is significantly higher for the
compounds having a methoxy group on the Ch (5) or
Ck (7) carbon atom that for the vinylidene 6a and 6b.
This might be related to the strength of the y bond
between the iron center and the Ch atom. Indeed, the
contribution of the mesomeric form B (Scheme 5) has
been recognized for a long time in the description of the
electronic structure of Fischer-type carbene complexes.
Such a structure also supports the observed restricted
rotation about the carbene carbon–oxygen bond in
many of these derivatives [32–35].
complexes 2–4. Generally, in an homogenous series of
iron compounds the isomeric shift is related to the
electron density at the metal center. This is apparently
not verified for the complexes of group I which do not
constitute an homogeneous enough family to allow
such a statement to be made.
Fig. 1. Selected Mo¨ssbauer spectra at 80 K of (a) Cp*(dppe)Fe–
CꢁC–H (4a); (b) [Cp*(dppe)FeꢀCꢀC(Ph)H][PF6] (6a); (c)
[Cp*(dppe)FeꢀCꢀCꢀC(Me)OMe][BPh4] (7); (d) [Cp*(dppe)Fe–CꢁC–
Ph][PF6] (4b+).
Scheme 5.

V. Guillaume et al. / Journal of Organometallic Chemistry 565 (1998) 75–80
79
2) [22,36,37]. Similar systems were also isolated in
different oxidation states using rhenium, [38–40] ruthe-
nium [41] and tungsten [42] metal centers. In the case of
dicationic complexes (n=4, z=2) a cumulenic struc-
ture (Scheme 7, type IV) was attributed to the rhenium
and tungsten compounds, whereas we found a polyyne
structure with two 17-electron end-capping metal cen-
ters (type V) in the Cp*(dppe)Fe series. This versatility
of the electronic structure of all-carbon chains also
emphasizes the interest for implementing the set of
spectroscopic tools allowing the determination of the
metal–carbon bonding and makes Mo¨ssbauer spec-
troscopy a particular interesting technique in this
respect.
Scheme 6.
Additionally, comparison between the quadrupole
splittings of the carbene (5) and the allenylidene (7)
complexes revealed an important increase of DEQ from
1.223 to 1.451 mm s−1. As a consequence of this
experimental result, one can suppose that the double
bond character of the FeꢀC bond in a hetero-carbene
might be stronger than in the homologous hetero-al-
lenylidene. In other words, the weight of the contribu-
tion of the mesomeric form D (Scheme 6) in the
description of the electronic structure of the allenyli-
dene 7 might be stronger than those of the form B in
the description of the electronic structure of the
methoxycarbene 5. To our knowledge, comparison be-
tween the rotation barriers of heterocarbene and het-
eroallenylidene complexes has never been the subject of
discussion in the literature.
At this point, it is important to emphasize the inter-
est in the characterization of the metal–carbon bonding
for an unsaturated end-bound hydrocarbon ligand us-
ing Mo¨ssbauer spectroscopy because this technique is
complementary to infrared spectroscopy. Indeed, the
alkynyl and vinylidene ligands are easily characterized
by IR (i.e. wCꢁC of 4a, 1910 cm−1; wCꢀC of 6a 1612
cm−1) but it is almost impossible to make the distinc-
tion between the ethynyl and allenylidene ligands using
this spectroscopy (i.e. wCꢀCꢀC=1943 cm−1 for 7). As a
result of our investigation on sixteen mononuclear iron
complexes of the Cp*(dppe)Fe series using Mo¨ssbauer
spectroscopy, it can be concluded that the Mo¨ssbauer
spectra recorded at zero field constitute a very useful
mean to determine both the oxidation state of the iron
center (FeII vs. FeIII) and the metal–carbon bond order
of end-bound hydrocarbon ligands in microcrystallised
samples. Additionally, Mo¨ssbauer is a very sensitive
spectroscopy and the presence of a single iron species in
the sample can easily be checked by the observation of
a single Mo¨ssbauer doublet.
3. Experimental section
3.1. General data
Reagent grade tetrahydrofuran (THF), diethylether
and n-pentane were dried and distilled from sodium
benzophenone ketyl prior to use. Pentamethylcyclopen-
tadiene was prepared according to the published proce-
dure [43] and other chemicals were used as received. All
the manipulations were carried out under an argon
atmosphere using Schlenk techniques or in a Jacomex
532 dry box under nitrogen. The FTIR spectra were
recorded using Nicolet instrument (Model 205) and
KBr windows. High field NMR spectra experiments
were performed on a multinuclear Bruker 300 MHz
instrument (AM300WB). Chemical shifts are given in
parts per million relative to tetramethylsilane (TMS) for
1H- and 13C-NMR spectra, H3PO4 for 31P-NMR spec-
tra. Elemental analyses were performed at the Center
for Microanalyses of the CNRS at Lyon-Solaise,
France. The syntheses of the compounds 1 [45], 2 [20],
3 [46], 4a [22], 4b [47], 4c [37], 5 [46], 6a [22], 6b [47],
2+ [20], and 4b+ [47] were reported elsewhere.
Mo¨ssbauer spectra were recorded with a 2.5×10−2
C (9.25×108 Bq) 57Co source using a symmetric trian-
gular sweep mode
3.2. 57Fe Mo¨ssbauer spectroscopy
The 57Fe Mo¨ssbauer spectra were obtained by using
a constant acceleration spectrometer previously de-
scribed with a 50 mCi 57Co source in a Rh matrix [49].
The sample temperature was controlled by a Oxford
MD306 cryostat and a Oxford ITC4 temperature con-
2.3. Conclusion and perspecti6es
Concomitantly to our activity on the mononuclear
polyyne, carbene, vinylidene, allenylidene and more
generally cumulene iron compounds, we have an ongo-
ing interest in the synthesis and physical properties of
bimetallic complexes where the metals are connected by
an all-carbon chain. These iron compounds which have
+
the general formula [LnFe–(CꢁC)x–FeL% ]z z[X−] have
n
been prepared for different lengths of the carbon link-
age (4BxB8) and different oxidation states (z=0, 1,
Scheme 7.

80
V. Guillaume et al. / Journal of Organometallic Chemistry 565 (1998) 75–80
[14] M.F. Moore, S.R. Wilson, M.J. Cohn, T.Y. Dong, T. Kampara,
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troller. Computer fitting of the Mo¨ssbauer data to
Lorenzian line shapes were carried out with a previ-
ously reported computer program [44,50]. The isomer
shift values are reported relative to iron foil at 298 K
and are not corrected for the temperature-dependent
second-order Doppler shift. The Mo¨ssbauer sample cell
consists of 2 cm diameter cylindrical plexiglass holder.
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2700.
3.2.1. Cp*(dppe)Fe(ꢀCꢀCꢀC(OCH3)CH3) (7)
To a solution (20°C) of Cp*(dppe)FeCl (0.430 g, 0.69
mmol) in methanol (40 ml) was added 1.25 equiv. of
trimethylsilyl-1,3-butadiyne [48] and 1.1 equiv. of
NaBPh4. The mixture was stirred for 16 h and the
resulting green solution was evaporated to dryness. The
crude residue was extracted with dichloromethane, then
concentrated to 5 ml, and diethyl ether (50 ml) was
slowly added to precipitate 0.570 g (85%) of 7 as a dark
green powder. Anal. Calcd. for C65H65BFeOP2. 0.25
CH2Cl2: C, 77.44; H, 6.52. Found: C, 77.64; H, 6.21.
FT-IR (Nujol/CH2Cl2 cm−1) w 1938, 1947/1943 (s,
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1
CꢀCꢀC). H-NMR (300 MHz, CDCl3) lH 7.4–6.8 (m,
20H, 4 C6H5); 3.41 (s, 3H, OCH3); 2.66, 2.32 (m, 4H,
CH2); 1.71 (s, 3H, ꢀC(OMe)CH3
13C-NMR (75 MHz, CDCl3) lC 268.2 (t, JCP=34 Hz,
Ch); 156.6 (s, Ci); 152.4 (m, Ck); 97.0 (s, C5Me5); 59.5
(q, JCH=147 Hz, OCH3); 32.7, 28.5 (2m, CH2); 27.4
6 ); 1.41 (s, 15H, C5Me5).
2
6
1
1
1
(q, JCH=130 Hz, ꢀC(OMe)CH3); 10.0 (q, JCH=128
Hz, C5Me5). 31P-NMR (121 MHz, CDCl3) lC 93.2 (s).
UV–vis (nm, (m, M−1 cm−1), CH2Cl2) 455 (3260).
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Organometallics (in press).
Acknowledgements
Dr F. Paul (Rennes) is gratefully acknowledged for
stimulating discussions.
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