About the synthesis of the prismane [NEt4]2[Fe6S6I6]

Article abstract of DOI:10.1016/S0020-1693(98)00301-6

In the literature there are conflicting views as to whether the iron-sulfur prismane [NEt₄]₂[Fe₆S₆I₆] can be prepared in a one-pot synthesis from iron, sulfur, iodine and tetraethylammonium iodide. The cluster can be prepared in this way, provided the work-up conditions are rigorously controlled. The spectroscopic properties of [NEt₄]₂[Fe₆S₆I₆] prepared in a one-pot synthesis are compared with those of a sample prepared by the oxidative transformation of [NEt₄]₂[Fe₄S₄I₄] and to those of other iodo-iron-sulfur clusters.

Full text of DOI:10.1016/S0020-1693(98)00301-6

Inorganica Chimica Acta 284 (1999) 296±299
Note
About the synthesis of the prismane [NEt4]2[Fe6S6I6]
a,*
David J. Evans , Gabriel Garcõa , M. Dolores Santana , M. Carmen Torralba
b
b
b
Â
aNitrogen Fixation Laboratory, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK
b
 Â
Departamento de Quõmica Inorganica, Universidad de Murcia, 30071 Murcia, Spain
Received 12 March 1998; accepted 22 June 1998
Abstract
In the literature there are con¯icting views as to whether the iron±sulfur prismane [NEt4]2[Fe6S6I6] can be prepared in a one-pot synthesis
from iron, sulfur, iodine and tetraethylammonium iodide. The cluster can be prepared in this way, provided the work-up conditions are
rigorously controlled. The spectroscopic properties of [NEt4]2[Fe6S6I6] prepared in a one-pot synthesis are compared with those of a sample
prepared by the oxidative transformation of [NEt4]2[Fe4S4I4] and to those of other iodo±iron±sulfur clusters. # 1999 Elsevier Science S.A.
All rights reserved.
È
Keywords: Iron complexes; Sulfur complexes; Clusters complexes; Mossbauer spectroscopy
2
‡
1. Introduction
3‰Fe4S4X4Š ‡ 2‰FeꢀC5H5† Š
2
2
! 2‰Fe6S6X6Š ‡ 2FeꢀC5H5†
‰Fe6S6X6Š ‡ 2X ! ‰Fe4S4X4Š ‡ ‰Fe2S2X4Š
(2)
(3)
2
An extensive literature exists on the synthesis of iron±
sulfur clusters [1±4]. In 1984 Pohl and coworkers. reported
the synthesis of the prismane cluster anion [Fe6S6I6]2
2
2
2
,
In this paper we report the synthesis of [NEt4]2[Fe6S6I6]
by an adaptation of the Coucouvanis method and show that
the same material can be obtained by the method of Pohl
[5,6,8] if the work-up conditions are controlled.
according to Eq. (1), isolated as the tetraethylammonium
salt [5,6]. It was suggested that the formation of the cluster is
via lower-charged species, which were readily soluble in
dichloromethane but which have never been isolated. Also,
dependent on the molar ratios of the reactants, other iodo±
iron±sulfur clusters should be accessible by the same route.
However, Coucouvanis et al. were unable to repeat the
reported synthesis and suspected that [Fe6S6I6]2 was only
a minor byproduct since they obtained [FeI4] in excellent
yield. They also reported a general procedure for the synth-
esis of [Fe6S6X6]2 (X ˆ Cl, Br) anions based on the
oxidation of [Fe4S4X4]2 clusters in dichloromethane [7]
(Eq. (2)). Moreover, the same authors reported that solutions
of the prismane dianions in polar solvents (MeCN, DMF) are
unstable. The transformation of [Fe6S6X6]2 to [Fe4S4X4]2
clusters and unidenti®ed byproducts is evident in the
observed changes in the electronic spectra. In the presence
of excess X the transformation occurs quantitatively [7]
according to Eq. (3). The same transformation occurs for the
iodo-prismane [8].
2. Experimental
All operations were performed under dinitrogen and
solvents were dried as appropriate before use. The materials
[NEt4]2[Fe2S2I4] [9], [NEt4]2[Fe4S4I4] [9], [NEt4]3[Fe6S6I6]
[10] and [Fe(C5H5)2][FeCl4] [11] were prepared by pub-
È
lished procedures. Mossbauer spectra were recorded on an
ES-Technology MS105 spectrometer at 77 K, using a 925
MBq 57Co source in a rhodium matrix, and were referenced
against iron foil at 298 K. All measurements were at zero
®eld on samples in the solid state. Variable temperature
magnetic susceptibility measurements were made on poly-
crystalline samples in a ®eld of 0.1 T using a Quantum
Design SQUID magnetometer. The susceptometer was cali-
brated with (NH4)2Mn(SO4)2.12H2O. Correction for dia-
magnetism was estimated from Pascal constants as
700 Â 10 6 cm3 mol 1. UV±Vis spectra and infrared
spectra were recorded on a Hitachi 2000Vand Perkin-Elmer
16F PC FT-IR spectrophotometer, respectively.
2
6Fe ‡ 6S ‡ 2I2 ‡ 2I ! ‰Fe6S6I6Š
(1)
*Corresponding author. Tel.: +44-1603-456900; fax: +44-1603-454970;
e-mail: dave.evans@bbsrc.ac.uk
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00301-6

D.J. Evans et al. / Inorganica Chimica Acta 284 (1999) 296±299
297
Table 1
Low frequency infrared absorbance (cm 1)a for [NEt4]2[Fe6S6I6] (I) and other iodo±iron±sulfur clusters
[NEt4]2[Fe6S6I6] (I)
[NEt4]2[Fe2S2I4]
[NEt4]2[Fe4S4I4]
466
466
466
452
452
452
394
382
292
288
238
225
228
432
420
415
322
344
263
220
a
As Nujol mulls between polyethylene discs.
Table 2
È
2.1. Preparation of [NEt4]2[Fe6S6I6] (I) by oxidative
transformation
a
Mossbauer parameters in the solid state and zero field
T (K)
i.s.
q.s.
hwhmb
[NEt4]2[Fe2S2I4] (0.166 g, 14.84 mmol) and [Fe(C5H5)2]-
[FeCl4] (0.038 g, 0.099 mmol) were stirred together in
dichloromethane (40 ml) for 4 h. The volume of the solvent
was reduced in vacuo to 20 ml and diethyl ether (20 ml)
added. After storage at 253 K for 2 days a black solid was
collected by ®ltration and washed with diethyl ether. Yield
65%.
[NEt4]2[Fe6S6I6] (I)
77
0.49
0.49
0.43
0.37
0.36
0.48
0.51
0.42
1.03
0.64
0.66
0.67
1.04
0.90
1.06
0.00
0.16c
0.23c
0.26
0.24
0.12
0.21
0.13
0.15
185
298
77
[NEt4]2[Fe2S2I4]
[NEt4]2[Fe4S4I4]
[NEt4]3[Fe6S6I6]
[NEt4][FeI4]
77
77
77
a mmsÆ1, Æꢁ0.01.
2.2. Preparation of [NEt4]2[Fe6S6I6] (I) by one-pot
synthesis and of mixtures II±VI
b hwhm ˆ half width at half maxima.
c Ratio 1:2.
All syntheses followed the same initial procedure. A
mixture of iron (0.84 g, 15 mmol), sulfur (0.48 g, 15 mmol),
iodine (1.27 g, 5 mmol) and tetraethylammonium iodide
(1.29 g, 5 mmol) in tetrahydrofuran (100 ml) was re¯uxed
for 48 h. The resultant solution was ®ltered hot. The product
mixtures were isolated using the following procedures:
the method of preparation for the analogous chloro- and
bromo-derivatives [7]. Cluster I is stable to 510 K under
an atmosphere of dinitrogen and decomposes slowly in air
at room temperature. It is soluble in acetonitrile, dimethyl-
sulfoxide and dimethylformamide, and sparingly soluble
in tetrahydrofuran and dichloromethane, but insoluble in
diethyl ether. An acetonitrile or dichloromethane solution
of the cluster shows a major UV±Vis absorbance at 360 nm
consistent with those of [Fe6S6X6]2 (in dichloromethane):
X ˆ Cl, 287 nm; X ˆ Br, 326 nm, and a minor peak at
702 nm. The infrared spectrum is different to that of
[NEt4]2[Fe4S4I4] and [NEt4]2[Fe2S2I4] (Table 1).
I.
The filtrate was concentrated in vacuo to 15 ml then
dichloromethane (25 ml) was added, after 2 days at
253 K a black solid was obtained or the filtrate was
concentrated in vacuo to 15 ml, dichloromethane
(15 ml) was added, after standing overnight at 253 K
a black solid was isolated.
È
II. The filtrate was taken to dryness and to the residue
was added dichloromethane (25 ml). After 3 days at
253 K a black solid was isolated.
III. The filtrate was evaporated to dryness in vacuo and
the residue was isolated as a black solid.
IV. The filtrate was concentrated in vacuo to 10 ml,
dichloromethane (15 ml) was added, the mixture
stirred for 2 h then, after 2 days at 253 K, a black
solid was collected.
The Mossbauer spectral parameters at three temperatures
are collected in Table 2. At 298 and 185 K, a single quad-
rupole split doublet is observed. The isomer shift (i.s.) and
quadrupole splitting (q.s.) values are similar to those
observed for [Fe6S6Cl6]2 (i.s. ˆ 0.39, q.s. ˆ 0.66 mms
,
1
at 200 K) [12] and [Fe6S6Br6]2 (i.s. ˆ 0.44, q.s. ˆ
0.70 mms 1, averaged values above 100 K) [13]. Pre-
viously, it was demonstrated that, on cooling [Fe6S6X6]2
È
below 50 K, the Mossbauer spectrum resolves into three
V. The filtrate from the isolation of I was allowed to stand
for several days and further black solid was collected.
VI. To product mixture II MeCN (20 ml) was added, the
mixture was filtered and the filtrate concentrated in
vacuo to 10 ml, diethyl ether (5 ml) was added and a
black solid was obtained.
overlapping doublets. This reversible effect was associated
with a structural distortion at the lower temperatures, giving
È
three chemically distinct iron sites [13]. Krockel et al. [14]
have expanded this explanation in terms of temperature-
dependent valence delocalisation. At the lowest tempera-
tures two iron atoms can be considered to be in the oxidation
state ‡3, two in the oxidation state ‡2, and two in between.
At higher temperatures (above 150 K), when a single
3. Results and discussion
È
Mossbauer doublet is observed, all six iron atoms appear
equivalent with an oxidation state of ‡2.67. For [NEt4]2-
[Fe6S6I6] we observe the beginnings of this resolution at
77 K, the spectrum at this temperature being best ®tted by
The [NEt4]2[Fe6S6I6] (I) prismane cluster can be prepared
by the oxidative transformation of [Fe4S4I4]2 following

298
D.J. Evans et al. / Inorganica Chimica Acta 284 (1999) 296±299
two overlapping doublets of intensities of 1:2. The change
from a two-quadrupole split doublet to a one-quadrupole
split doublet spectrum with temperature is fully reversible.
[FeI4] [16]. Under these preparative conditions the
[NEt4]2[Fe6S6I6] prismane can be prepared by the
one-pot synthesis.
È
The Mossbauer parameters are different to those measured
The following preparative methods led to product mix-
È
for [NEt4]2[Fe2S2I4], [NEt4]2[Fe4S4I4] and the reduced
prismane [NEt4]3[Fe6S6I6], Table 2.
tures (Scheme 1) as detected by Mossbauer and infrared
spectroscopy. If the reaction mixture is taken to dryness
before adding dichloromethane, as previously reported
[5,6], then a mixture of prismane and [NEt4][FeI4] is
The room temperature effective magnetic moment for
I is 7.2 BM, a value lower than that expected for two
iron(II)- and four iron(III)-isolated high spin tetrahedral
iron atoms. When cooled, the effective magnetic moment
decreases to 2.5 BM at 2 K. These features indicate that
an intramolecular antiferromagnetic interaction occurs in
prismane I. The magnetic moment for the chloro-anologue,
[NEt4]2[Fe6S6Cl6], at room temperature, was estimated to
be 3.1 BM by NMR measurements in solution [7], a value
much lower than that of I. It is known that structural
distortions and the nature of the peripheric ligands can
signi®cantly affect the magnitude of the magnetic coupling
[15] and the observed difference can be attributed to such
effects.
We have repeated the synthetic procedure of Pohl and by
variation of work-up conditions (see Section 2) can prepare,
reproducibly, either [NEt4]2[Fe6S6I6] (I) or various product
mixtures II±IV, summarised in Scheme 1. Cluster I was
precipitated as a black solid, 40% yield, by addition of
dichloromethane to the tetrahydrofuran reaction solution
È
obtained II. By Mossbauer spectroscopy, assuming an
equivalent recoil-free fraction for each type of iron atom,
then the mixture composition is approximately two tetra-
iodoferrite to one prismane. Product mixture III, obtained
when tetrahydrofuran solvent is completely removed, has
the same composition. This helps to explain the dif®culties
of others [7] to reproduce the published preparation. When
tetrahydrofuran is removed to a small volume prior to the
addition of a smaller volume of dichloromethane than used
in the isolation of I, and the mixture is stirred for 2 h before
standing for 2 days, then a mixture of [NEt4]2[Fe2S2I4] and
[NEt4][FeI4] is obtained IV. A similar mixture V is obtained
from the ®ltrate obtained after collection of I, if it is allowed
to stand for several days. On recrystallisation of mixture II
from MeCN/Et2O, [NEt4]2[Fe2S2I4] is obtained as the only
product VI.
To conclude, the prismane [NEt4]2[Fe6S6I6] can be
obtained by a one-pot synthesis provided the work-up
conditions are rigorously controlled. We have shown that
if care is not taken various mixtures can easily form. The
ease with which such clusters as those described above can
be readily formed from elemental iron, sulfur and iodine
with iodide, may support the suggestion that such metal±
sulfur clusters could have formed during the very early
stages of evolution [17,18].
È
after concentration. The infrared (Table 1), Mossbauer
(Table 2), and UV±Vis spectra are equivalent to those
of I prepared by the alternative route. The reversible
È
dependence of the variation of Mossbauer complexity with
temperature was again observed. There was no evidence
of contamination by [NEt4][FeI4]. The infrared absorbance
1
at 238 cm
is common to the prismane and [NEt4]-
Scheme 1. (a) Oxidative transformation; (b) conc. ‡ CH2Cl2, 2 days, collect; (c) conc. ‡ CH2Cl2, overnight, collect; (d) to dryness ‡ CH2Cl2, 3 days,
collect; (e) to dryness; (f) conc. ‡ CH2Cl2, stir 2 h, 2 days, collect; (g) filtrate after collection of I, several days, collect; (h) II ‡ CH3CN, conc. ‡ Et2O,
collect.

D.J. Evans et al. / Inorganica Chimica Acta 284 (1999) 296±299
299
[5] W. Saak, G. Henkel, S. Pohl, Angew. Chem., Int. Ed. Engl. 23 (1984)
150.
Acknowledgements
[6] W. Saak, G. Henkel, S. Pohl, Angew. Chem. 96 (1984) 153.
[7] D. Coucouvanis, M.G. Kanatzidis, W.R. Dunham, W.R. Hagen, J.
Am. Chem. Soc. 106 (1984) 7998.
The Biotechnology and Biological Sciences Research
Council, UK, is thanked for funding this work in part,
Â
together with the Direccion General de Investigacion Cien-
tõ®ca y Tecnica (PB94-1148) and the Ministerio de Educa-
Â
[8] W. Saak, S. Pohl, Z. Naturforsch., Teil B 40 (1985) 1105.
[9] G.B. Wong, M.A. Bobrik, R.H. Holm, Inorg. Chem. 17 (1978)
578.
Â
Â
Â
cion y Ciencia (grant to MCT), Spain. Professor M. Julve
and Dr F. Lloret of the Universitat de Valencia, Spain, are
[10] M.G. Kanatzidis, W.R. Hagen, W.R. Dunham, R.K. Lester, D.
Coucouvanis, J. Am. Chem. Soc. 107 (1985) 953.
[11] E.P. Paulus, L. Schafer, J. Organomet. Chem. 144 (1978) 205.
Á
thanked for measuring magnetic susceptibility. Mrs J.E.
Barclay and Mr A. Rufete are thanked for experimental
assistance in the early stages of this work.
È
[12] M. Krockel, A.X. Trautwein, M. Grodzicki, V. Papaefthymiou, A.
Kostikas, A.F. Arendsen, W.R. Hagen, in: I. Ortalli (Ed.), Conf. Proc.
Italian Chemical Society, ICAME 95, 50, 1996, p. 843.
[13] D. Coucouvanis, M.G. Kanatzidis, A. Salifoglou, W.R. Dunham, A.
Simopoulos, J.R. Sams, V. Papaefthymiou, A. Kostikas, C.E.
Strouse, J. Am. Chem. Soc. 109 (1987) 6863.
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