Difference in reactivity of triel halides EX3 towards ferrocene

Article abstract of DOI:10.1016/j.ica.2007.03.049

A facile route to the Fe²⁺-arene complex [(C₆H₆)FeCp][AlBr₄] is the reaction of ferrocene with AlBr₃ in benzene. The structure of the Fe²⁺-arene complex [(C₆H₆)FeCp][AlBr₄] · C₆H₆ was found to be isomorphous with those of [FeCp₂][ECl₄] · C₆H₆ (E = Al, Ga). The crystal structures of the [FeCp₂][AlCl₄] · C₆H₆ (E = Al, Ga) presented here show structural features which are different from those of solvent-free ferrocenium salts [FeCp₂][ECl₄] (E = Al, Ga, Fe). However, the cell parameters of solvent-free ferrocenium tetrafluoroborate [FeCp₂][BF₄] are also quite different from those of solvent-free [FeCp₂][ECl₄] (E = Al, Ga, Fe). In contrast to the eclipsed conformation in solvent-free [FeCp₂][ECl₄] (E = Al, Ga, Fe) the cyclopentadiene rings in [FeCp₂][BF₄] and [FeCp₂][ECl₄] · C₆H₆ (E = Al, Ga) are in a staggered conformation.

Full text of DOI:10.1016/j.ica.2007.03.049

Inorganica Chimica Acta 360 (2007) 3323–3329
www.elsevier.com/locate/ica
Difference in reactivity of triel halides EX3 towards ferrocene
Stefan Scholz, Matthias Scheibitz, Frauke Schodel, Michael Bolte,
¨
*
Matthias Wagner, Hans-Wolfram Lerner
Institut fu¨r Anorganische Chemie, Johann Wolfgang Goethe-Universita¨t Frankfurt am Main, Max-von-Laue-Straße 7, D-60438 Frankfurt, Germany
Received 31 January 2007; received in revised form 28 March 2007; accepted 31 March 2007
Available online 11 April 2007
Abstract
A facile route to the Fe2+-arene complex [(C6H6)FeCp][AlBr4] is the reaction of ferrocene with AlBr3 in benzene. The structure of the
Fe2+-arene complex [(C6H6)FeCp][AlBr4] Æ C6H6 was found to be isomorphous with those of [FeCp2][ECl4] Æ C6H6 (E = Al, Ga). The
crystal structures of the [FeCp2][AlCl4] Æ C6H6 (E = Al, Ga) presented here show structural features which are different from those of
solvent-free ferrocenium salts [FeCp2][ECl4] (E = Al, Ga, Fe). However, the cell parameters of solvent-free ferrocenium tetrafluoroborate
[FeCp2][BF4] are also quite different from those of solvent-free [FeCp2][ECl4] (E = Al, Ga, Fe). In contrast to the eclipsed conformation
in solvent-free [FeCp2][ECl4] (E = Al, Ga, Fe) the cyclopentadiene rings in [FeCp2][BF4] and [FeCp2][ECl4] Æ C6H6 (E = Al, Ga) are in a
staggered conformation.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Triel halides; Ferrocenium; Isomorphism
1. Introduction
Up to now, only ferrocenium salts with bulky counteÀr-
ions such as AlCl4À; GaCl4À; FeCl4À; FeBr4
PF6
À
;
;
A number of studies have shown that an electrophilic
substitution of ferrocene takes place in the presence of
BX3 (X = Cl, Br) [1–3]. The mono and diborylated
ferrocene (1 and 2), therefore, are accessible in high yield
by treatment of ferrocene with 1 and 2 equiv. of BBr3,
respectively [2]. On the other hand, borylation with an
excess of BBr3 results in the formation of tetraborylated
ferrocene 3 [3]. However, our efforts to synthesize the
related ferrocene derivatives with GaCl2 substituents on
the Cp ring failed [4]. When an equimolar mixture of
ferrocene and GaCl3 is treated with benzene, blue green
crystals of [FeCp2][GaCl4] and red platelets of the
multidecker sandwich complex 4 were obtained, as
depicted in Scheme 1 [4]. This leads to the conclusion
that a redox reaction, by which ferrocene is oxidized to
ferrocenium cation and Ga(III) is reduced to Ga(I), takes
place.
SbCl4À; SbF6À, had been characterized by X-ray crystallog-
raphy [4–10]. The cyclopentadiene rings in those ferroce-
nium compounds have various conformations (staggered,
gauche or eclipsed) which usually change with temperature.
The structures of ferrocenium salts normally exhibit a stag-
gered conformation at high and an eclipsed conformation
of the cyclopentadiene rings at low temperature (e.g.
[FeCp2][FeCl4]) [5,11]. In contrast to that, [FeCp2][PF6]
reveals a phase change from a staggered disordered to an
eclipsed conformation of the cyclopentadiene rings at high
temperature [9]. It is interesting to note that the structure of
solvent-free ferrocenium salts [FeCp2][ECl4] (E = Al, Ga,
Fe) is isomorphous.
The purpose of this paper is to investigate the reactivity
of ferrocene towards AlBr3 and to give more information
about the structures of ferrocenium salts of the type [Fe-
Cp2][EX4]. We herein report the solvent-free structure of
[FeCp2][BF4]. In the presence of benzene, cocrystals of ben-
zene and the ferrocenium complexes [FeCp2][ECl4]
(E = Al, Ga) have been isolated. The structure of
*
Corresponding author. Tel.: +49 69 79829152; fax: +49 69 79829260.
E-mail address: lerner@chemie.uni-frankfurt.de (H.-W. Lerner).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.03.049

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S. Scholz et al. / Inorganica Chimica Acta 360 (2007) 3323–3329
Br2B
Br2B
[(C6H6)FeCp][AlBr4] Æ C6H6 was found to be isomorphous
with those of benzene solvated [FeCp2][ECl4] (E = Al, Ga).
BBr2
BBr2
BBr2
BBr2
BBr2
Fe
Fe
Fe
2. Experimental
1
2
3
2.1. General considerations
+BBr3
All experiments were carried out under dry argon or
nitrogen using standard Schlenk techniques. Benzene and
tetrahydrofuran were distilled from sodium/benzophenone.
CH2Cl2 was dried over molecular sieves and stored under
dry nitrogen.
Fe
2.2. Synthesis of ferrocenium tetrafluoroborate
[FeCp2][BF4]
+GaCl3
Ferrocene (1.8 mmol, 0.35 g) was dissolved in 10 ml of
tetrahydrofuran and a solution of AgBF4 (1.6 mmol,
0.32 g) in 10 ml tetrahydrofuran was added at À78 ꢁC.
The formation of a silver mirror was observed immedi-
ately. After filtration the solvent was evaporated in vacuo.
Then the residue was extracted in 7 ml CH2Cl2. X-ray
suitable single crystals of ferrocenium tetrafluoroborate
were obtained from this solution at ambient temperature
(0.40 g, 87%).
Fe GaCl4
+
Fe
Cl
Cl
Cl
2.3. Synthesis of ferrocenium tetrachloroaluminate
[FeCp2][AlCl4]
Ga
Ga
Cl
Cl
Ga
Fe
A solution of Cl2 (2.0 mmol) in CH2Cl2 (10 ml) was
added dropwise under stirring to a slurry of ferrocene
(1.8 mmol, 0.34 g) and AlCl3 (1.7 mmol, 0.23 g) in CH2Cl2
(10 ml). After stirring for 8 h, the precipitate was removed
by filtration and the solvent was evaporated in vacuo.
Then the residue was extracted in 10 ml of benzene. X-
ray suitable single crystals of ferrocenium tetrachloroa-
lumniate were obtained from this solution at ambient
temperature (0.61 g, 82%).
Cl
Ga
Cl
Cl
Fe
Cl
Cl
Cl
Ga
Ga
Cl
4
2.4. Synthesis of ferrocenium tetrachlorogallate
[FeCp2][GaCl4]
Scheme 1. Reacitivity of BBr3 and GaCl3 towards ferrocene.
Ferrocene (1.8 mmol, 0.34 g) was dissolved in 10 ml of
benzene and a solution of GaCl3 (2.5 mmol, 0.45 g) was
added dropwise at ambient temperature. The solution
became cloudy immediately. After 12 h at ambient temper-
ature, X-ray suitable single crystals of [FeCp2][Ga-
Cl4] Æ C6H6 were obtained (0.76 g, 88%).
AlCp3, cyclopentadiene and cyclopentadiene dimer were
detected.
Dibromoborylferrocene (0.3 mmol, 0.11 g) was dissolved
in 0.8 ml of benzene and AlBr3 (0.03 mmol, 0.08 g) was
added at ambient temperature. After 12 h at ambient tem-
perature, X-ray suitable single crystals of [(C6H6)Fe-
Cp][AlBr4] Æ C6H6 were obtained from the reaction solution.
[(C6H6)FeCp][AlBr4]: 1H NMR (CD2Cl2): d 5.39 (s, Cp)
7.41 (s, C6H6).
2.5. Synthesis of [(C6H6)FeCp][AlBr4] Æ C6H6
A mixture of ferrocene (0.5 mmol, 0.09 g) and AlBr3
(1.0 mmol, 0.26 g) in 5 ml of benzene was heated in a sealed
tube for 2 h of 120 ꢁC. After cooling to ambient tempera-
ture, crystals of [(C6H6)FeCp][AlBr4] Æ C6H6 were obtained
(0.22 g, 70%). In the NMR spectra of the reaction solution
signals of Cp-containing substances such as, among others,
2.6. Crystal structure determination
Suitable single crystals of [FeCp2][BF4], [FeCp2]-
[ECl4] Æ C6H6 (E = Al, Ga) and [(C6H6)FeCp][Al-
Br4] Æ C6H6 were selected and attached in a perfluoropolye-

S. Scholz et al. / Inorganica Chimica Acta 360 (2007) 3323–3329
3325
Table 1
Crystal data and structure refinement for [FeCp2][BF4], [FeCp2][AlCl4] Æ C6H6, [FeCp2][GaCl4] Æ C6H6, and [(C6H6)FeCp][AlBr4] Æ C6H6
[FeCp2][BF4]
[FeCp2][AlCl4] Æ C6H6
[FeCp2][GaCl4] Æ C6H6
[(C6H6)FeCp][AlBr4] Æ C6H6
Formula
Fw
C10H10BF4Fe
272.84
C16H16AlCl4Fe
432.92
C16H16Cl4FeGa
475.66
C17H17AlBr4Fe
623.78
Crystal system
Space group
Temperature (K)
orthorhombic
Cccm
monoclinic
Cc
monoclinic
Cc
monoclinic
Cc
173(2)
100(2)
0.71073
173(2)
173(2)
˚
Radiation (MoKa) (A)
0.71073
7.524(2)
11.954(4)
12.346(4)
90
90
90
1110.4(6)
4
1.632
0.71073
11.8120(10)
15.1740(10)
11.2130(10)
90
106.293(6)
90
1929.1(3)
4
0.71073
11.9314(8)
15.7460(10)
11.6751(7)
90
108.378(5)
90
2081.6(2)
4
˚
a (A)
11.8318(10)
15.0759(14)
11.1965(10)
90
106.754(7)
90
1912.4(3)
4
1.504
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (Mg/mÀ3
)
1.638
2.695
948
1.990
8.441
1192
l (mmÀ1
F(000)
)
1.377
548
1.386
876
Crystal size (mm)
h Range for data collection (ꢁ)
Index ranges
0.13 · 0.12 · 0.12
3.60–27.38
À9 6 h 6 9,
À15 6 k 6 15,
À15 6 l 6 15
3224
0.36 · 0.35 · 0.28
3.60–27.75
À14 6 h 6 15,
À19 6 k 6 19,
À14 6 l 6 14
13030
0.38 · 0.22 · 0.12
2.24–28.84
À15 6 h 6 15,
À19 6 k 6 19,
À14 6 l 6 13
13758
0.28 · 0.28 · 0.26
2.22–28.77
À15 6 h 6 15,
À19 6 k 6 20,
À15 6 l 6 15
17327
Number of reflections collected
Number of independent reflections
Rint
663
0.0627
4235
0.0478
3732
0.0389
4670
0.0400
Completeness to h = 24.76ꢁ (%)
Absorption correction
99.1
empirical
98.9
89.1
empirical
89.9
empirical
semi-empirical from
equivalents
0.6976 and 0.6353
Maximum and minimum
transmission
0.8521 and 0.8412
0.7381 and 0.4274
0.2176 and 0.2009
Refinement method
full-matrix least-squares
on F2
full-matrix least-squares
on F2
full-matrix least-squares full-matrix least-squares
on F2
on F2
Data/restraints/parameters
Goodness-of-fit on F2
663/0/41
1.027
4235/2/200
1.027
3732/2/199
1.089
4670/24/205
1.042
R1, wR2 (I > 2r(I))
R1, wR2 (all data)
0.0518, 0.1283
0.1034, 0.1533
0.0684, 0.1759
0.0718, 0.1811
0.70(3)
1.587 and À1.308
0.0318, 0.0609
0.0397, 0.0635
0.091(12)
0.429 and À0.331
0.0444, 0.0823
0.0669, 0.0909
0.127(16)
0.660 and À0.505
Absolute structure parameter
Largest difference in peak and hole
0.176 and À0.583
À3
˚
(e A
CCDC
)
632409
632410
170257
632408
ther oil to a thin glass fiber. Intensity measurement was car-
ried out using a STOE IPDS-II two circle diffractometer.
The structures were solved with direct methods [12] and
refined with full-matrix least-squares techniques [13]. All
non-hydrogen atoms were refined with anisotropic dis-
placement parameters. Hydrogen atoms were placed at cal-
culated positions and refined using a riding model. Crystal
data, data collection parameters, and refinement statistics
for [FeCp2][BF4], [FeCp2][ECl4] (E = Al, Ga) and
[(C6H6)FeCp][AlBr4] Æ C6H6 are listed in Table 1.
described in the literature [14,15]. The most common
methods are the reactions of Al/AlCl3 mixtures and aro-
matic compounds with CpFe(CO)Br or ferrocene. Now
we have found a facile route to the Fe2+-arene complex
[(C6H6)FeCp][AlBr4] by the reaction of ferrocene with
benzene in the presence of AlBr3 at 120 ꢁC (Scheme 2).
Single crystals of [(C6H6)FeCp][AlBr4], suitable for X-
ray diffraction, were grown from the reaction solution
at ambient temperature. In the NMR spectra of the reac-
tion solution signals of Cp-containing substances such as,
among others, AlCp3, cyclopentadiene and cyclopentadi-
ene dimer were detected. It is interesting to note that
treatment of dibromoborylferrocene with AlBr3 in ben-
zene also leads to the formation of [(C6H6)FeCp][AlBr4]
(Scheme 2). Several studies have revealed that Fe2+-arene
complexes of the type [(C6H6)FeCp]+ are convenient
sources for Fe(I) complexes [16].
3. Results and discussion
3.1. Reaction of ferrocene with AlBr3
To date, several procedures for the preparation of
Fe2+-arene cation of type [(arene)FeCp]+ have been

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S. Scholz et al. / Inorganica Chimica Acta 360 (2007) 3323–3329
for this study. The synthesis of [FeCp2][AlCl4], however,
was achieved by the reaction of ferrocene with Cl2 in
CH2Cl2 in the presence of AlCl3 at ambient temperature
(Scheme 3). Recrystallization from benzene yields
blue-green blocks of [FeCp2][AlCl4] as benzene solvate.
On the hand, side X-ray quality crystals of the ferrocenium
salt [FeCp2][GaCl4] Æ C6H6 were accessible from the reac-
tion of GaCl3 and ferrocene in benzene (Scheme 1).
Fe
+AlBr3
+C6H6
-AlCp3
3.3. X-ray crystallographic structures
Fe AlBr4
The molecular structures of complexes [FeCp2][BF4],
[FeCp2][AlCl4] Æ C6H6,
[FeCp2][GaCl4] Æ C6H6,
and
[(C6H6)FeCp][AlBr4] Æ C6H6 are shown in Figs. 1–6.
+AlBr3
+C6H6
BBr2
Fe
Scheme 2. Routes to the Fe2+-arene complex [(C6H6)FeCp][AlBr4] by the
reaction of ferrocene and dibromoborylferrocene with AlBr3.
3.2. Syntheses of ferrocenium complexes of type
[FeCp2][BF4]
Oxidation of ferrocene with silver(I) salts is
a
common route to ferrocenium cations, and this method
was used, as shown in Scheme 3, to prepare [FeCp2][BF4]
Fig. 1. ORTEP plot of [FeCp2][BF4] in the solid state. Displacement
ellipsoids are drawn at the 50% probability level.
Fe BF4
+AgBF4
-Ag
Fe
+AlCl3
+0.5 Cl2
Fe AlCl4
Scheme 3. Syntheses of the ferrocenium complexes [FeCp2][BF4] and
[FeCp2][AlCl4].
Fig. 2. Crystal packing diagram for [FeCp2][BF4].

S. Scholz et al. / Inorganica Chimica Acta 360 (2007) 3323–3329
3327
Fig. 3. ORTEP plot of [FeCp2][AlCl4] with one molecule C6H6 in the solid
state. Displacement ellipsoids are drawn at the 50% probability level.
Fig. 5. ORTEP plot of [FeCp2][GaCl4] with one molecule C6H6 in the
solid state. Displacement ellipsoids are drawn at the 50% probability level.
Fig. 6. Plot of [(C6H6)FeCp][AlBr4] with one molecule C6H6 in the solid
state. The cyclopentadienyl ligand of [(C6H6)FeCp] cation is disordered
over two positions.
Fig. 4. Crystal packing diagram for [FeCp2][AlCl4] Æ C6H6.
We obtained single crystals of [FeCp2][BF4], which are
free of solvent, from CH2Cl2 at ambient temperature.
Crystallization from benzene at ambient temperature yields
X-ray quality crystals of [FeCp2][GaCl4] Æ C6H6 and [Fe-
Cp2][AlCl4] Æ C6H6, respectively. Single crystals of [(C6H6)-
FeCp][AlBr4] Æ C6H6, suitable for X-ray diffraction, were

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S. Scholz et al. / Inorganica Chimica Acta 360 (2007) 3323–3329
˚
also grown from
temperature.
a
benzene solution at ambient
12.346(4) A and a = b = c = 90ꢁ. The structural parameters
of cocrystals of benzene and the ferrocenium complexes
[FeCp2][ECl4] (E = Al, Ga), which crystallize in the mono-
clinic space group Cc, are similar to those of the cobaltoce-
nium derivative [CoCp2][GaCl4] Æ C6H6 [4]. Therefore, these
ferrocenium compounds are isomorphous with the cobalt-
ocenium complex [CoCp2][GaCl4] Æ C6H6. Surprisingly, the
structure of the Fe2+-arene complex [(C6H6)FeCp][Al-
Br4] Æ C6H6 was found to be isomorphous with those of [Fe-
Cp2][ECl4] Æ C6H6 (E = Al, Ga) and [CoCp2][GaCl4] Æ C6H6
as well. The complex [(C6H6)FeCp][AlBr4] Æ C6H6 reveals a
structure in which the cyclopentadienyl ligand of the
[(C6H6)FeCp] cation is disordered over two positions, which
have an occupancy ratio of roughly 1:1.
X-ray structure analyses of the solvent-free ferrocenium
compounds [FeCp2][ECl4] (E = Al, Ga, Fe) reveal that all
of them crystallize in orthorhombic space group Pnma
[4,5]. However, the cell parameters of solvent-free ferroce-
nium tetrafluoroborate are quite different from those of
[FeCp2][ECl4] (E = Al, Ga, Fe). The cell dimensions of
˚
˚
[FeCp2][BF4] are a = 7.524(2) A, b = 11.954(4) A, c =
The ferrocenium salts listed in Table 1 possess quite sim-
˚
ilar Fe–C bond lengths in the solid (range of 2.064 A for
˚
[FeCp2][AlCl4] and 2.095 A for [FeCp2][BF4]). The average
values for the C–C bonds of Cp rings of these ferrocenium
˚
complexes are found to be between 1.385 A for [Fe-
˚
Cp2][AlCl4] to 1.417 A for [FeCp2][BF4] and are therefore
comparable with those of ferrocene.
The Fe-CGCp (CG: center of gravity) distances found of
the ferrocenium salts [FeCp2][BF4] and [FeCp2]-
˚
[ECl4] Æ C6H6 (E = Al, Ga) range between 1.691 A
Fig. 7. Cp ligand rotation of [FeCp2][BF4] away from the eclipsed
conformation.
˚
{[FeCp2][GaCl4]} and 1.711 A {[FeCp2][BF4]}, whereas
the structure of [(C6H6)FeCp][AlBr4] features the Fe-
˚
CGbenzene distance of 1.529 A and Fe-CGCp distance of
˚
1.637 A, respectively. The shorter Fe(II) arene contacts in
[(C6H6)FeCp][AlBr4] are the result of stronger p interac-
tions of Fe(II) with arenes than those of Fe(III) in ferroc-
enium compounds.
In contrast to the Cp rings of solvent-free [FeCp2][ECl4]
(E = Al, Ga, Fe) the Cp ligands of [FeCp2][BF4], [Fe-
Cp2][ECl4] Æ C6H6, (E = Al, Ga) and [CoCp2][GaCl4] Æ
C6H6 are rotated away from the eclipsed conformation,
as shown in Figs. 7–9.
4. Supplementary material
CCDC 632409, 632410, 170257 and 632408 contain the
supplementary crystallographic data for {[FeCp2][BF4]},
Fig. 8. Cp ligand rotation of [FeCp2][AlCl4] Æ C6H6 away from the
eclipsed conformation.
Fig. 9. Cp ligand rotation of [FeCp2][GaCl4] Æ C6H6 away from the eclipsed conformation.

S. Scholz et al. / Inorganica Chimica Acta 360 (2007) 3323–3329
3329
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{[FeCp2][AlCl4] Æ C6H6}, {[FeCp2][GaCl4] Æ C6H6} and
{[(C6H6)FeCp][AlBr4] Æ C6H6}. The data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.
ac.uk.
Acknowledgements
We are grateful to the University of Frankfurt for finan-
cial funding. M.W. is grateful to the ‘‘Deutsche Fors-
chungsgemeinschaft’’ (DFG) for financial support.
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