All-iron hydrogenase: Synthesis, structure and properties of {2Fe3S}-assemblies related to the di-iron sub-site of the H-cluster

Article abstract of DOI:10.1039/b209690k

Tripodal dithiolate thioether ligands MeC(CH₂SH)₂CH₂SR (R = Me or Ph) provide a route to {2Fe3S}-complexes and syntheses are described. X-Ray crystal structures for two {2Fe3S}-pentacarbonyl derivatives and that for the first carbonyl cyanide are reported, together with temperature dependent ¹H-NMR. Moessbauer, FTIR and redox potential data. The NMR data establish fluctionality associated with inversion at the thioether sulfur in the carbonyl complexes. The Moessbauer data affirm that the coordination environment of the two iron atoms in a dicyanide bridging carbonyl intermediate are differentiated. Bridging carbonyl intermediates have been structurally and spectroscopically identified in resting and CO inhibited forms of the sub-site of all-iron hydrogenases; the observation of a thermally unstable {2Fe3S}-bridging carbonyl intermediate is discussed in this context.

Full text of DOI:10.1039/b209690k

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All-iron hydrogenase: synthesis, structure and properties of
{2Fe3S}-assemblies related to the di-iron sub-site of the H-cluster†
Mathieu Razavet, Sian C. Davies, David L. Hughes, J. Elaine Barclay, David J. Evans,
Shirley A. Fairhurst, Xiaoming Liu and Christopher J. Pickett*
Biological Chemistry Department, John Innes Centre, Norwich, UK NR4 7UH
Received 3rd October 2002, Accepted 3rd December 2002
First published as an Advance Article on the web 21st January 2003
Tripodal dithiolate thioether ligands MeC(CH2SH)2CH2SR (R = Me or Ph) provide a route to {2Fe3S}-complexes
and syntheses are described. X-Ray crystal structures for two {2Fe3S}-pentacarbonyl derivatives and that for the
first carbonyl cyanide are reported, together with temperature dependent 1H-NMR, Mössbauer, FTIR and redox
potential data. The NMR data establish fluctionality associated with inversion at the thioether sulfur in the carbonyl
complexes. The Mössbauer data affirm that the coordination environment of the two iron atoms in a dicyanide
bridging carbonyl intermediate are differentiated. Bridging carbonyl intermediates have been structurally and
spectroscopically identified in resting and CO inhibited forms of the sub-site of all-iron hydrogenases; the
observation of a thermally unstable {2Fe3S}-bridging carbonyl intermediate is discussed in this context.
understanding of the underlying chemistry of the enzyme may
Introduction
inform the design of new electrocatalytic systems for hydrogen
The di-iron sub-site of the H-cluster of all-iron hydrogenase
production or uptake pertinent to energy transduction
catalyses the reduction of protons to dihydrogen at very high
technology.
rates.1–6 The sub-site is extraordinary in that: (i) carbon
Earlier preparative studies of di-iron assemblies12–14 have
monoxide and cyanide ligands are essential structural elements;
focused on the synthesis of carbonyl cyanide derivatives of the
(ii) biologically unprecedent FeI oxidation states are probably
readily accessible {2Fe2S}-core whereas the sub-site is com-
involved in turnover; and (iii) interconversion of bridging and
prised of a {2Fe3S}-core with the three sulfur atoms trigonally
terminal CO ligands may play a key part in the catalysis at the
capping the iron atom which is proximal to the 4Fe4S-cluster,
binuclear centre.
Fig. 1a. In this paper we describe: (i) the synthesis of the first
X-Ray crystallographic data show that in the oxidised resting
synthetic {2Fe3S}-carbonyl and carbonyl cyanide compounds
state of the H-cluster (Hox) the distal Fe atom of the subsite is
and the preparation of the ligands which allow their assembly;
ligated by a water molecule or possesses a vacant coordination
(ii) X-ray crystallographic stuctural data for three of the di-iron
site, whereas in the carbon monoxide inhibited form, Hox(CO),
the site is occupied by CO. It is still unclear whether or not the
molecules, including that for the first {2Fe3S}-carbonyl cyan-
ide; and (iii) spectroscopic, dynamic and redox properties of the
2-position of the dithiolate ligand is an NH or CH2 group. The
assemblies, including 57Fe Mössbauer spectra pertinent to dif-
former is currently preferred on the basis of electron-density
ferential coordination at the di-iron centres. Some aspects of
arguments and enjoys a conjectural mechanistic role in relaying
this work have been described in earlier communications.18,23,24
protons to the distal Fe atom.3,7,8 A representation of the
The detailed mechanism of cyanation of the {2Fe3S}-core has
principle structural features of the sub-site in its paramagnetic
been the subject of a recent paper.25
resting state is provided by Fig. 1a which is a composite of
structural data from two independent crystallographic source
codes.1,2
Results and discussion
How the sub-site of the H-cluster catalyses proton reduction
Ligand syntheses
is the subject of extensive spectroscopic and mechanistic studies
of the enzyme,6,9–11 of synthetic assemblies,12–18 and in silico
models.19–22 These are driven in some part by the view that an
The reactions of the tripodal ligand H3CC(CH2SH)3 1 with
[Fe3(CO)12] were first investigated as a route to {2Fe3S}-
assemblies, following procedures which are known to give
{2Fe2S}-carbonyls with dithiols.26 The trithiol 1 was prepared
as described by Kolomyjec et al.27 However, it was found that 1
reacts with [Fe3(CO)12] to give intractable black materials. We
† Electronic supplementary information (ESI) available: crystal and
structure refinement data for complexes 4a, 4b and 5a. See http://
www.rsc.org/suppdata/dt/b2/b209690k/
Fig. 1 (a) Proposed structure of the H-cluster of Fe-only hydrogenases; this is a composite model combining features reported by Nicolet et al.
(PDB code 1HFE)1 and Peters et al. (code 1FEH).2 (b) Complex 4a. (c) Monocyanide derivative of complex 5a.
D a l t o n T r a n s . , 2 0 0 3 , 5 8 6 – 5 9 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
586

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Scheme 1 Synthesis of ligands and complexes 4a and 4b.
Scheme 2 Synthesis of {2Fe3S}-cyanide assemblies, with R = Me or Bn.
therefore sought to modify the reactivity of 1 by converting it to
{PhCH2SCH2C(Me)(CH2S)2}(CN)(CO)4] 5b. X-ray crystallo-
a dithiolate thioether. This required protecting two of the thiol
groups of 1 and leaving the third thiol free for conversion
to a thioether. This was achieved by reacting 1 with dimethyl-
disulfide which leads to oxidative coupling of two of the thiols
to give the cyclic disulfide 2 in good yield, Scheme 1. The driv-
ing force for the production of 2 is undoubtedly the formation
of a stable five membered ring with elimination of volatile
methylthiol.
graphic analysis of 5a shows that the thioether ligand remains
coordinated to one iron atom and that the cyanide substitution
of CO occurs at the other. Infra-red and Mössbauer data
are concordant with 5b possessing an analogous molecular
structure, as described below, Scheme 2.
These deceptively simple stoichiometric substitution reac-
tions mask much interesting chemistry which has been revealed
in a recent stopped-flow FTIR mechanistic study.25 Briefly, the
first step involves a fast regioselective attack by cyanide with
concerted decoordination of the distal methylthioether ligand.
This is followed by a slower re-binding of the thioether with
concerted dissociation of carbon monoxide to give the stable
{2Fe3S}-monocyanides, Scheme 2.
The FTIR spectral pattern of 5a,b in the CO region is almost
identical to that of the unsymmetrical phosphine derivative
[(Me3P)(CO)2Fe(SC3H6S)Fe(CO)2(CN)]Ϫ isolated by Rauchfuss
et al.28 This is expected as both complexes possess similar ligand
differentiation at the iron centres. The rather small ϩ10 cmϪ1
shift in the average ν(CO) for 5a,b vis a vis the trimethyl-
phosphine complex attests to the SMe ligand behaving as a
remarkably strong electron donor ligand at the FeI site.
The reaction of the carbonyl 4a with two or more equivalents
of cyanide proceeds via formation of the monocyanide 5a to
give a moderately stable bridging carbonyl dicyanide inter-
mediate 6a which at room temperature slowly rearranges to the
thermally stable all-terminal carbonyl species 7a, Scheme 2.
The final stable product 7a has nearly identical spectroscopic
properties to those of the {2Fe2S}-dicyanide [Fe2(SC3H6S)-
The unprotected thiol group of
2 was deprotonated
and reacted with either methyl iodide or benzyl bromide to
produce the corresponding thioethers, Scheme 1. Deprotection
by reduction of the disulfide bond with LiAlH4 followed by
protonation affords the dithiol methyl and benzyl thioethers 3a
and 3b respectively, Scheme 1.
Synthesis of {2Fe3S}-carbonyls and their cyanide derivatives
The dithiol thioether ligands 3a and 3b react with [Fe3(CO)12] at
90 ЊC in toluene to afford the complexes [Fe2{MeSCH2C(Me)-
(CH2S)2}(CO)5] 4a and [Fe2{PhCH2SCH2C(Me)(CH2S)2}-
(CO)5] 4b respectively which were purified by silica gel
chromatography, Scheme 1. X-Ray crystallographic analysis of
4a and 4b established the formation of the {2Fe3S}-core with
differential ligation of the two iron atoms, as discussed more
fully below.
The carbonyl complexes 4a and 4b react with one equivalent
of [Et4N][CN] to afford the monocyanide salts [Et4N][Fe2-
{MeSCH2C(Me)(CH2S)2}(CN)(CO)4] 5a and [Et4N][Fe2-
D a l t o n T r a n s . , 2 0 0 3 , 5 8 6 – 5 9 5
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Table 1 Mössbauer parameters (mm sϪ1) of {2Fe2S}-carbonyl/cyanide and {2Fe3S}-carbonyl/cyanide assemblies at 80 K
i.s.
q.s.
26, a
[Fe2(SC3H6S)(CO)6]
0.04(1)
0.03(1)
0.01(1)
0.01(1)
0.03(1)
0.12(1)
0.03(1)
0.12(1)
0.05(1)
0.11(1)
0.03(1)
0.11(1)
0.03(1)
0.09(1)
0.03(1)
0.04(1)
0.03(1)
0.04(1)
0.02(1)
0.02(1)
0.87(1)
0.70(2)
0.95(1)
0.75(1)
0.94(1)
0.25(1)
0.98(1)
0.27(1)
0.92(3)
0.44(3)
0.78(3)
0.39(2)
1.13(1)
0.68(2)
0.98(1)
1.02(1)
1.09(4)
0.86(4)
1.14(1)
0.85(2)
23, a
[Fe2(SCH2C(CH2OH)S)(CO)6]
[Fe2{MeSCH2C(Me)(CH2S)2(CO)5], 4ab
[Fe2{PhCH2SCH2C(Me)(CH2S)2(CO)5], 4bc
[Et4N][Fe2{MeSCH2C(Me)(CH2S)2}(CN)(CO)4], 5ac
[Et4N][Fe2{PhCH2SCH2C(Me)(CH2S)2}(CN)(CO)4], 5ba
[Et4N]2[Fe2(µ-CO){MeSCH2C(Me)(CH2S)2}(CN)2(CO)3], 6ac
[K(18-crown-6)]2[Fe2{MeSCH2C(Me)(CH2S)2}(CN)2(CO)4], 7ac
[Et4N]2[Fe2{PhCH2SCH2C(Me)(CH2S)2}(CN)2(CO)4], 7bc
[Et4N]2[Fe2(SC3H6S)(CN)2(CO)4]
14, c
14, c
[Et4N]2[Fe2(SCH2C(CH2OH)S)(CN)2(CO)4]
a Solid state samples. b Frozen acetone solution. c Frozen acetonitrile solution.
(CN)2(CO)4]2Ϫ 12–14
,
thus the FTIR spectra are superimposable,
differentiated Fe atoms, each ligated by a single cyanide ligand,
both complexes show a strong electronic transition at 353 nm,
and Mössbauer isomer shift (i.s.) and quadrupole splitting (q.s.)
parameters are all very similar, Table 1. This is as expected with
the formal remote substitution at the 2-bridgehead position of
the 1,3-propane dithiolate having little electronic influence on
the di-iron assembly.
Fig. 2.
Although unstable at room temperature, the intermediate 6a
can be generated from 5a at low temperature under conditions
whereby its isomerisation to 7a is extremely slow. This has
enabled its characterisation by FTIR, UV-visible and Möss-
bauer spectroscopy. The pertinent spectroscopic observations
on 6a are:
Fig. 2 (a) Proposed structure of Hox–CO, the partially oxidised and
CO inhibited form of the sub-site, X = CH2 or NH and (b) the
intermediates 6a,b.
(i) a medium intensity FTIR band at 1780 cmϪ1 which is
more than 100 cmϪ1 lower in wavenumber than the group of
terminal ν(CO) bands in 6a;
Crystal structure description of {2Fe3S}-assemblies 4a, 4b and
5a
(ii) the absence of an intense absorption in the 330–380 nm
region, observed in the parent material 5a at 367 nm;
(iii) two distinct cyanide bands at 2074 and 2085 cmϪ1; and
(iv) two overlapping 57Fe Mössbauer quadrupole split
doublets.
In the crystals of 4a there are two independent molecules,
virtually identical in all aspects, Fig. 1b. The core structures of
these molecules are also very similar in shape and size to those
of 4b and 5a. Molecular dimensions of the four molecules are
listed in Table 2, the structure of 4b is shown in Fig. 3 and that
of 5a in Fig. 1c.
In each complex, both iron atoms are five-coordinate with
square pyramidal patterns. The square base in every case com-
prises two thiolate sulfur atoms, S(1) and S(2), and two carbon
The low energy FTIR band at 1780 cmϪ1, and its position
relative to the terminal bands centred near 1920 cmϪ1, is charac-
teristic of a bridging carbonyl group. Intense absorption in the
330–380 nm region is characteristic of both {2Fe2S}- and
{2Fe3S}-systems possessing metal–metal bonds. Thus observ-
ations (i) and (ii) are concordant with the replacement of the
Fe–Fe bond by a bridging carbonyl group. Observations (iii)
and (iv) establish that the two iron atoms in 6a have different
coordination environments. This spectroscopy and the estab-
lished stoichiometry of the cyanation reactions, taken together,
are fully in accord with the closed-shell dianion 6a being
[Fe2(µ-CO){MeSCH2C(Me)(CH2S)2}(CN)2(CO)3]2Ϫ, Scheme 2.
Thioether ligation stabilises the formation of a ‘ketonic’ bridg-
ing carbonyl,25,29 and accounts for failure to observe bridging
carbonyl intermediates in the cyanation reactions of the
{2Fe2S}-complex [Fe2(SC3H6S)(CO)6].
The complex 4b reacts with cyanide under similar conditions
in an analogous fashion giving successively 5b, the inter-
mediate 6b and finally 7b, Scheme 2. From the close similarity
of the FTIR spectra of 6b to that of 6a together with the
rearrangement to 7b, we infer that 6a and b are structural
analogues.
The dianions 6a,b have a particular significance in the con-
text of the di-iron subsite of all-iron hydrogenase which in its
CO inhibited form has a {2Fe3S}-core, a bridging CO and two
Fig. 3 View of a molecule of 4b. Hydrogen atoms have been omitted
for clarity. In molecules of 4a and 5a, C(31) is a terminal methyl group,
but their cores are essentially as that of 4b. O(15) becomes N(15) in the
case of 5a.
D a l t o n T r a n s . , 2 0 0 3 , 5 8 6 – 5 9 5
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Table 2 Selected molecular dimensions in the complexes 4a, 4b and 5a
4a, mol. A
4a, mol. B
4b
5a
Fe(1) ؒ ؒ ؒ Fe(2)
2.5086(9)
2.5158(10)
2.4969(8)
2.5378(9)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(1)–C(13)
Fe(1)–C(14)
Fe(1)–C(15)
2.2560(12)
2.2549(13)
1.795(5)
1.779(5)
1.795(6)
2.2537(13)
2.2506(13)
1.794(7)
1.787(6)
1.770(6)
2.2574(11)
2.2617(11)
1.779(5)
1.784(5)
1.782(4)
2.2412(12)
2.2486(13)
1.765(4)
1.761(4)
1.941(4)a
Fe(2)–S(1)
Fe(2)–S(2)
Fe(2)–S(3)
Fe(2)–C(24)
Fe(2)–C(25)
2.2512(13)
2.2614(12)
2.2508(14)
1.764(5)
2.2505(14)
2.2563(13)
2.2567(14)
1.765(5)
2.2516(10)
2.2594(11)
2.2347(10)
1.764(4)
2.2599(11)
2.2646(14)
2.2547(11)
1.752(4)
1.755(5)
1.772(6)
1.767(4)
1.761(4)
S(2)–Fe(1)–S(1)
85.94(4)
103.06(14)
156.22(17)
86.65(15)
102.88(15)
86.49(17)
157.87(18)
100.6(2)
86.34(5)
97.06(17)
160.7(2)
87.73(18)
103.60(18)
86.81(17)
154.7(3)
102.1(3)
101.5(3)
90.8(2)
85.49(4)
102.77(15)
157.09(17)
86.59(15)
104.02(15)
87.23(15)
154.12(14)
100.1(2)
86.60(4)
99.42(14)
160.04(15)
87.92(13)
104.12(14)
88.71(13)
158.10(13)
100.5(2)
C(13)–Fe(1)–S(1)
C(14)–Fe(1)–S(1)
C(15)–Fe(1)–S(1)
C(13)–Fe(1)–S(2)
C(14)–Fe(1)–S(2)
C(15)–Fe(1)–S(2)
C(14)–Fe(1)–C(13)
C(15)–Fe(1)–C(13)
C(14)–Fe(1)–C(15)
99.1(2)
92.0(2)
101.7(2)
90.6(2)
97.68(18)
89.25(18)
S(1)–Fe(2)–S(2)
S(3)–Fe(2)–S(1)
C(24)–Fe(2)–S(1)
C(25)–Fe(2)–S(1)
S(3)–Fe(2)–S(2)
C(24)–Fe(2)–S(2)
C(25)–Fe(2)–S(2)
C(24)–Fe(2)–S(3)
C(25)–Fe(2)–S(3)
C(25)–Fe(2)–C(24)
85.91(5)
96.95(5)
162.11(16)
87.28(16)
96.14(5)
89.09(14)
160.34(16)
100.66(16)
102.98(16)
91.8(2)
86.28(5)
94.63(5)
161.31(17)
88.16(17)
97.08(5)
89.87(16)
160.45(18)
103.99(16)
102.05(17)
89.4(2)
85.68(4)
100.37(4)
158.84(13)
88.69(13)
90.78(4)
90.00(15)
164.93(13)
100.39(13)
103.99(13)
90.24(19)
85.78(5)
94.34(4)
162.84(13)
88.99(15)
98.42(5)
90.75(14)
159.35(16)
102.79(13)
101.90(16)
88.4(2)
Fe(2)–S(1)–Fe(1)
Fe(1)–S(2)–Fe(2)
67.64(4)
67.49(4)
67.91(4)
67.86(4)
67.25(3)
67.05(3)
68.64(4)
68.43(4)
Displacement (Å) towards apical ligand of iron atoms from square base mean-plane
Fe(1)
Fe(2)
0.392(1)
0.317(1)
0.380(1)
0.327(1)
0.4171(9)
0.3075(9)
0.3674(9)
0.3231(9)
Angle (Њ) between normals to square-base mean-planes
74.11(5)
Torsion angles (Њ), C(4)–C(3)–S(3)–C(31)
–104.9(4)
75.04(5)
72.49(4)
76.67(4)
–100.8(4)
–134.3(3)
–99.4(3)
a Cyanide ligand.
atoms (of carbonyl ligands in every case except in complex 5a
where C(15) is of a cyanide ligand). The Fe(1) iron atoms are
displaced ca. 0.39 Å from its base plane towards the apical
carbonyl ligand, and the Fe(2) atoms ca. 0.32 Å towards the
thioether S(3) atom. The square planes are hinged sharply
through S(1) and S(2) and the two iron atoms can thus form an
Fe–Fe bond with length between 2.497(1) Å in 4b to 2.538(1) Å
in 5a; the angles between the normals to the two base planes
range correspondingly from 72.5(1) to 76.7(1)Њ. The S3 ligand
sits symmetrically about a pseudo-mirror plane above the
Fe2(CO)5 or Fe2(CO)4(CN) unit, with the C(31) group project-
ing out one side or the other from that plane; the C(4)–C(3)–
S(3)–C(31) torsion angles are ca. Ϫ101Њ in the 4a and 5a mole-
cules, and rather larger, Ϫ134.3(3)Њ, in 4b where C(31) is the
methylene carbon of the benzyl group.
replacement also changes the overall charge on the complex,
from zero to anionic (Ϫ1). The Fe ؒ ؒ ؒ Fe distance is lengthened
in this complex, and all other coordination distances about
Fe(1) are correspondingly shorter; the dimensions about Fe(2)
appear little affected by the substitution.
The structural similarities between the enzyme sub-site and
the model compounds are evident from Fig. 1. For example, the
Fe ؒ ؒ ؒ Fe bond distance in 5a and the average bridging Fe–S
bond lengths are similar to those distances estimated from the
protein crystallographic data, which are 2.6 and 2.3 Å respect-
ively, but the thioether Fe–S distance in 5a at 2.25 Å is some-
what shorter than the corresponding sub-site Fe–Scysteine
distance in the enzyme which is ca. 2.5 Å.1,2
Solution dynamics of carbonyls 4a and 4b
The principal variations in dimensions in this series of
complexes result from the replacement of the carbonyl
ligand C(15)–O(15) by a cyanide ligand in complex 5a; this
Rauchfuss and co-workers described a fluctional process for the
dithiolate complex [Fe2(SC3H6S)(CO)6] which is associated with
D a l t o n T r a n s . , 2 0 0 3 , 5 8 6 – 5 9 5
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flipping of the propane backbone.13 This process is not possible
for 4a or 4b because the backbone is locked into position by the
coordinated thioether group. However, 4a and 4b do show
dynamic solution processes which are associated with inversion
at the thioether sulfur. The evidence for this is as follows.
1
Variable temperature H NMR of 4a and 4b in deuterated
dichloromethane revealed the presence of a dynamic process, as
shown for the methyl derivative by Fig. 4 and for the benzyl
derivative by Fig. 5. The static spectra were recorded for the Me
and benzyl derivatives at low temperature. Fig. 6 shows the
experimental data for 4a at 233 K together with the spectrum
simulated using the program gNMR.30 The dynamic process of
4a can be represented as a combination of two independent
spin systems. Namely, two inter-converting ABCD spin systems
1
Fig. 6 (a) Experimental and (b) simulated H NMR spectra of 4a at
233 K.
which, as the temperature is raised, coalesce into two identical
AB spin systems i.e. ½ (ABCD) ϩ ½ (ABCD)Ј
isolated AB pair which coalesces to an A2 spin system, Fig. 4.
The energetics of the common AB A2 system for 4a and 4b
was not analysed because of overlap with the ABCD system.
An additional AB A2 system is observed in 4b, Fig. 5. Chem-
2AB, plus an
ical shift and integration parameters show that the ABCD
system is associated with the methylene groups attached to the
bridging sulfur atoms and the isolated AB pair with the methyl-
ene group attached to the thioether ligand for both 4a and 4b.
The additional AB
A2 system of 4b, centred at 4.05 ppm in
Fig. 5, is correspondingly associated with the methylene of the
S–CH2Ph group.
The interconverting spins systems are readily explained by
inversion at the thioether sulfur as represented in Fig. 7. The
second AB
A2 system of 4b must arise from restricted
rotation about the SCH2Ph group leading to inequivalence of
the methylene protons when the sulfur inversion is frozen-out.
The activation free energies ∆G‡ for the inversion process of
4a and 4b were determined by simulation of spectra over a
range of temperature. A typical plot is illustrated by Fig. 8
which shows ln k/T versus 1/T for 4b using values of k deter-
mined by simulation of the ABCD system. Table 3 summarises
the ∆G‡ data obtained by this method. It is evident from this
table that the energetics for the inversion of 4a and 4b are quite
Fig. 4 Variable temperature 1H NMR spectrum of 4a in CD2Cl2
(–SCH3 protons at 2.62 ppm not shown).
similar with ∆G‡ (and ∆H‡) differing by less than 10 kJ molϪ1
Included in the table are data for the AB A2 spin-system
.
which is associated with restricted rotation around the
S–CH2Ph bond. The magnitude of ∆G‡ is close to that for
inversion process of 4b and this can be explained by the activ-
ation free-energy barrier to rotation being larger than that for
inversion, i.e. rotation is frozen out at a higher temperature
than the inversion which is consequently rate limiting. However,
∆H‡ and ∆S‡ components of the free-energy of activation for
the two spin systems differ substantially suggesting that the
barrier to rotation may be lower or comparable with that for
inversion, Table 3.
Solid and solution state Mössbauer parameters
Mössbauer parameters for the {2Fe2S}-carbonyl/cyanide and
{2Fe3S}-carbonyl/cyanide complexes are given in Table 1. The
isomer shift (i.s.) values of the {2Fe2S}-hexacarbonyl com-
plexes are consistent with those reported earlier for related
complexes.31,32 Previously, we fitted the solid state spectrum of
[Fe2(SC3H6S)(CO)6] to one doublet.14 However, a more rigorous
analysis shows a better fit of the spectrum by two, overlapping,
quadrupole split doublets with similar i.s. but different q.s.. The
origin of the differentiation of the iron sites in the solid state is
thought to be the positioning of the propanedithiolate CH2
above one of the iron atoms. In solution this differentiation is
lost, due to the fluctionality of the propanedithiolate unit,13 and
Fig. 5 Variable temperature 1H NMR spectrum of 4b in CD2Cl2.
Peak at 1.97 ppm is CH3CN.
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Table 3 Activation parameters for spin systems obtained from the simulation30
4a, ABCD
AB
4b, ABCD
AB
4b (centred at 4.05 ppm), AB
A2
∆H‡/kJ molϪ1
58.8
8.8
56.2
64.9
47.9
50.6
82.0
109.5
49.3
∆S‡/J KϪ1 molϪ1
∆G‡298/kJ molϪ1
Fig. 7 Schematic representation of the ABCD
AB and AB
A2 systems for both 4a and 4b (view from the apical methyl on top of the ligand).
The parameters for one of the doublets are similar to those of
the [Fe2(SC3H6S)(CO)6] complexes, those of the other doublet
are significantly different, exhibiting a larger i.s. and smaller
q.s., and can therefore be assigned to the iron atom ligated by
the thioether sulfur. The replacement of a terminal carbonyl
group (π-acceptor) by a thioether lowers the s-electron density
at the metal, which is reflected by the increase in i.s., and
replacement of a π-acceptor by a non-π-acceptor causes a
decrease in q.s. There is no significant difference between
the parameters obtained for 4b in frozen solution and in the
solid state. The Mössbauer spectrum of 4b was also recorded at
300 K, the change in i.s. was as expected and the q.s. was
shown to be independent of temperature (i.s. = Ϫ0.03(1),
0.05(1); q.s. = 1.03(1), 0.31(1) mm sϪ1).
The mono-cyanide derivatives 5a and 5b and the carbonyl
bridged, dicyano-derivative 6a also show two well-defined
doublets in frozen solution as observed for 4a and 4b. This is
fully consistent with thioether sulfur being ligated to one iron
atom and the formal oxidation state of the iron atoms remain-
ing iron(). Importantly, when complex 6a isomerises to 7a in
which there is no carbonyl bridge and no ligation by thioether
sulfur and therefore essentially undifferentiated iron atoms, a
single doublet is observed with parameters similar to those of
the doublets of [Fe2(SC3H6S)(CN)2(CO)4]2Ϫ and [Fe2(SCH2C-
(CH2OH)S)(CN)2(CO)4]2Ϫ, Table 1.
Fig. 8 Eyring plot obtained from estimated values of k, for the ABCD
system of 4b.
a single quadrupole split doublet is hence observed (i.s. =
0.03(1); q.s. = 0.81(1) mm sϪ1).
The spectra of the {2Fe3S}-pentacarbonyls 4a and 4b in
frozen acetone solution show two, well defined, overlapping,
quadrupole split doublets, Fig. 9; one of the iron atoms is now
differentiated from the other by binding of a thioether sulfur.
The Mössbauer parameters at 4.2 K for the 2Fe subcluster in
iron-only hydrogenase from two organisms have been reported.
The reduced 2Fe subcluster from C. pasteurianum has an i.s.
and a q.s. respectively of 0.08 and 0.87 mm s .
Ϫ1 10 The oxidised,
Fig. 9 Selected Mössbauer spectra recorded at 80 K (frozen solution samples). (a) [Fe2{MeSCH2C(Me)(CH2S)2}(CO)5] (acetone) 4a,
(b) [Et4N]2[Fe2(µ-CO){MeSCH2C(Me)(CH2S)2}(CN)2(CO)3] (acetonitrile) 6a.
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Table 4 Primary redox potentials of {2Fe3S}-carbonyl/cyanide assemblies referenced against calomel, at 25 ЊC, in MeCN (0.1 M [NBu4][BF4])
Entry
Compound
Epred/V
Epox/V
23
1
2
3
4
5
6
7
8
9
10
11
12
13
14
[Fe2(SCH2C(CH2OH)S)(CO)6]
[Fe2(SC3H6S)(CO)6]
Ϫ1.28
Ϫ1.32
1.3
1.15
0.67
0.77
0.13
0.17
0.12
0.46
Ϫ0.06
Ϫ0.10b
Ϫ0.25
Ϫ0.26
Ϫ0.08
Ϫ0.11
26,35
[Fe2{MeSCH2C(Me)(CH2S)2}(CO)5], 4a
[Fe2{PhCH2SCH2C(Me)(CH2S)2(CO)5], 4b
Ϫ1.38b
Ϫ1.36b
Ϫ1.97
16, c
[Fe2(SC3H6S)(CO)5(PMe3)]
[Fe2{MeSCH2C(Me)(CH2S)2}(CN)(CO)4]Ϫ, 5a
[Fe2{PhCH2SCH2C(Me)(CH2S)2}(CN)(CO)4]Ϫ, 5b
Ϫ1.83
Ϫ1.83
Ϫ1.84
Ϫ2.25
≤ Ϫ2.40
≤ Ϫ2.40
≤ Ϫ2.40
Ϫ2.36
16, c
[Fe2(SC3H6S)(CN)(CO)5]Ϫ
[Fe2(SC3H6S)(CN)(CO)4(PMe3)]Ϫ
16, c
[Fe2(µ-CO){MeSCH2C(Me)(CH2S)2}(CN)2(CO)3]2Ϫ, 6aa
[Fe2{MeSCH2C(Me)(CH2S)2}(CN)2(CO)4]2Ϫ, 7a
[Fe2{PhCH2SCH2C(Me)(CH2S)2}(CN)2(CO)4]2Ϫ, 7b
[Fe2(SCH2C(CH2OH)S)(CN)2(CO)4]2Ϫ
14
14,16
[Fe2(SC3H6S)(CN)2(CO)4]2Ϫ
Ϫ2.33
a Recorded at Ϫ45 ЊC. b Partially reversible. c Corrected to SCE.
Fig. 10 Evolution of HOMO and LUMO of {2Fe3S}-assemblies as qualitatively measured by solution oxidation and reduction potentials Epox and
Epred versus SCE respectively.
CO inhibited 2Fe subcluster of D. vulgaris has i.s. 0.13 and 0.17,
and q.s. 0.65 and 0.70 mm s
Ϫ1 11 The i.s. range of 0.11–0.15 mm
bridging carbonyl, mixed-valence, hydrido and catalytically
active intermediates. Detailed mechanistic aspects of these, as
probed by a range of electrochemical and spectroelectro-
chemical methods will be descibed in detail in a subsequent
paper; here we confine ourselves to a discussion of trends in
potentials, noting that Ep values are indicative rather than
thermodynamic measures of relative redox orbital energies.
The general effect of successive substitution of CO for CNϪ
is that Ep and Ep are shifted to more negative values by
around 400–500 mV for both {2Fe2S}- and {2Fe3S}-assemblies
as illustrated by Fig. 10 and evident from the Table 4 (compare
entries 2,8; 3,6; and 4,7). This is most likely a predominantly
electrostatic effect which shifts the HOMO–LUMO redox
manifold in tandem as the negative charge density on the com-
plex is increased, Fig. 10. Substitution of CO by CNϪ at closed-
shell mononuclear centres usually invokes a shift of around
1000 mV, as quantified by the ligand parameter PL, which is 0.0
and 1.00 V for CO and CNϪ respectively.33 This smaller shift in
the binuclear systems is consistent with delocalisation of charge
over the two iron centres and this is supported by the general
shift in all the FTIR terminal carbonyl frequencies to lower
values upon substitution of CO by CNϪ.
.
sϪ1 (adjusted to 77 K‡) for the 2Fe subcluster of the oxidised,
CO inhibited form of the enzyme active site has been
compared11 to the values we have previously reported14 for
{2Fe2S}-assemblies. As the i.s. of the synthetic iron()/iron()
assemblies were smaller than those observed in the enzyme, an
iron()/iron() assignment was favoured (i.e. reduced 2Fe sub-
cluster = iron()/iron(); oxidised 2Fe subcluster = iron()/
iron()). However, it is clear from this work that i.s. values
from the more closely analogous {2Fe3S}-synthetic assemblies
fall well within the range found for the 2Fe subcluster. The
Mössbauer parameters of the enzyme 2Fe subcluster do
not, therefore, exclude the possibility of an iron()/iron() (i.e.
reduced 2Fe subcluster = iron()/iron(); oxidised 2Fe subcluster
= iron()/iron()) system in iron-only hydrogenase.
ox
red
Primary oxidation and reduction potentials of {2Fe3S}-systems
The primary oxidation and reduction peak potentials, Ep,
of the various {2Fe3S}-complexes were measured by cyclic
voltammetry in MeCN–0.1M [NBu4][BF4] and are given in
Table 4 together with comparative literature data for related
{2Fe2S}-systems. All of the primary processes of the {2Fe3S}-
complexes are irreversible or at best partially reversible at room-
temperature and moderate scan-rates (< 200 mV sϪ1) and give
rise to a diverse following chemistry. This includes formation of
Formally replacing neutral CO by a neutral pendant
red
thioether ligand leads to a relatively small negative shift in Ep
of less than 100 mV but a considerable negative shift in Epox of
nearly 500 mV, Table 4 and Fig. 10. Thus the LUMO, which is
almost certainly an Fe–Fe anti-bonding (σ*) orbital,34 can have
little CO or thioether ligand character. In contrast, replacing
the strong π-acid CO by the thioether ligand significantly per-
turbs the HOMO which is evidently very sensitive to the Fe
‡ 0.2 mm sϪ1 was subtracted from the i.s. recorded at 4.2 K to compare
with the 77 K values reported in this work.40
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592

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atom environment. That the methyl or benzyl thioether ligand
acts as a moderately good donor ligand as indicated by FTIR
supporting electrolyte [NBu4][BF4] (0.1 M in MeCN and 0.2 M
in CH2Cl2) was prepared following a standard laboratory
procedure.
ox
data discussed above is reinforced by comparison of the Ep
potentials for 5a,b with [Fe2(SC3H6S)(CN)(CO)4(PMe3)]Ϫ,16
Table 4 (entries 6, 7 and 9).
Crystal structure analyses
The analysis of [Fe2{MeSCH2C(Me)(CH2S)2}(CO)5], complex
4a, is described here. The analyses of the other samples fol-
lowed very similar procedures. Crystal data for the three
complexes are listed in Table S1 (ESI).†
Conclusions
In summary, we have described:
(i) the synthesis of tripodal dithiolate thioether ligands which
provide a route to {2Fe3S}-species,
(ii) the X-ray crystallographic structures of carbonyl and
carbonyl cyanide species exemplifying the {2Fe3S}-framework,
(iii) fluctionality involving inversion at the thioether sulfur
ligand,
(iv) the formation of an intermediate possessing a {2Fe3S}-
core, a bridging carbonyl group and differentiated iron atoms
each ligated by a cyanide ligand, these structural elements
closely relate to those of the di-iron sub-site of hydrogenase,
(v) Mössbauer data which show differentiated iron-sites in
the di-iron systems and which provide i.s. and q.s. parameters
pertinent to the H-cluster of all-iron hydrogenase.
Bridging/terminal carbonyl interconversions evidently take
place during turnover of the hydrogenase system with accom-
panying making or breaking of the metal–metal bond.5,25 Here
we note that bridging carbonyl formation may provide a means
of exposing an enzymic site at which the substrate binds (water/
proton) whereas reformation of a metal–metal bond, with CO
switching into a terminally bonded mode may provide the
mechanism for desorbing the product (dihydrogen). The
rearrangement of the bridging carbonyl intermediate 6a to the
Fe–Fe bonded terminal CO isomer 7a, Scheme 2,25 has some
analogy with the latter possibility. Thus the necessity of a
binuclear centre in the natural system can be explained by the
need to support terminal/bridging CO interconversions.
Crystals are deep red, irregular fragments of diamond-
shaped plates. One, ca. 0.29 × 0.17 × 0.14 mm, was sealed in a
capillary. After preliminary photographic examination, this was
transferred to an Enraf-Nonius CAD4 diffractometer (with
monochromated radiation) for determination, at room temper-
ature, of accurate cell parameters (from the settings of 25 reflec-
tions, θ = 10–11Њ, each centred in four orientations) and for
measurement of diffraction intensities (3528 unique reflections
to θmax = 21Њ, the limit of measurable intensities; of these, 2614
were ‘observed’ with I > 2σI).
During processing, corrections were applied for Lorentz-
polarisation effects, a small deterioration correction, absorp-
tion (by semi-empirical ψ-scan methods) and to eliminate
negative net intensities (by Bayesian statistical methods). The
structure was determined by the direct methods routines in the
SHELXS program36 and refined by full-matrix least-squares
methods, on F 2, in SHELXL.37 There are two essentially identi-
cal, independent molecules in the crystal. In one molecule, one
of the carbonyl oxygen atoms is disordered over two distinct
sites. Hydrogen atoms were included in idealised positions, and
the methyl groups were allowed to rotate about the C–C bonds.
The non-hydrogen atoms were refined with anisotropic thermal
parameters and the H-atom Uiso values were set to ride on the
Ueq values of the parent carbon atoms. At the conclusion of the
refinement, wR2 = 0.061 and R1 = 0.046,37 for all 3528 reflections
2
weighted w = σϪ2(Fo ); for the ‘observed’ data only, R1 = 0.028.
In the final difference map, the highest peaks (to ca. 0.24
e ÅϪ3) were close to a carbonyl ligand.
Experimental
Scattering factors for neutral atoms were taken from ref. 38.
Computer programs used in this analysis have been noted above
or in Table 4 of ref. 39 and were run on a DEC-AlphaStation
200 4/100 in the Biological Chemistry Department, John Innes
Centre.
Complex 5a was unstable at room temperature and the
intensity data were recorded at 140 K.
CCDC reference numbers 195090–195092.
See http://www.rsc.org/suppdata/dt/b2/b209690k/ for crystal-
lographic data in CIF or other electronic format.
General
All experiments, including ligand synthesis, were carried out
under inert atmosphere of nitrogen. Solvents were dried using
standard procedures and distilled under nitrogen atmosphere.
[Fe3(CO)12] was purchased from Alfa Aesar, all other reagents
were purchased from Aldrich. 1,1,1-Tris(mercaptomethyl)-
ethane was prepared according to the literature method.27
Microanalysis: Microanalysis Service, University of East
Anglia, Norwich. NMR: JEOL Lambda 400 MHz. FTIR:
Bio-Rad FTS-7 spectrometer equipped with a sealed cell for
liquid analysis. Mass spectrometry: School of CPES, University
of Sussex. Metal analysis: samples were prepared by dissolv-
ing metal complex in HNO3 and water, samples of known con-
centration were sent for analysis to Southern Water, Sussex
Laboratory, Brighton.
Syntheses
MeC(CH2S)2CH2SH, (4-methyl-[1,2]dithiolan-4-yl)methane-
thiol, 2. (CH3)2S2 (2.51 mL, 28 mmol) was added to 1,1,1-tris-
(mercaptomethyl)ethane27 (4.7 g, 28 mmol) in 200 mL freshly
distilled methanol under nitrogen atmosphere. Addition of
potassium tert-butoxide (31 mg, 0.28 mmol) produced a change
from colourless to yellow after 5 min. The mixture was stirred
for an hour and degassed every ten minutes to liberate CH3SH.
Methanol was evaporated and the crude material was distilled
to give a yellow oil (2.8 g, 60%), bp 150–152 ЊC (0.7 mm).
Microanalysis: C5H10S3, found (calc): C, 36.48 (36.12); H,
Mössbauer
Spectra were recorded on an ES-Technology MS-105 spectro-
meter with a 200 MBq 57Co source in a rhodium matrix at
ambient temperature. Spectra were referenced to a 25 µm iron
foil at 298 K. Parameters were obtained by fitting the data with
Lorentzian lines.
1
6.14 (6.06); S, 57.6 (57.9)%. H NMR (400 MHz, C6D5CD3):
3
δ 0.73 (3H, s, CH3), 0.89 (1H, t, J 9 Hz, SH), 2.07 (2H, d,
Electrochemistry
3J 9 Hz, CH2SH), 2.27 [2H, d, 2J 11.2 Hz, (CHHSSCHH)], 2.48
[2H, d, 2J 11.2 Hz, (CHHSSCHH )]. FTIR (KBr): ν(SH)
2551(m, br) cmϪ1. EI-MS: m/z 166 {M}ϩ, 133 {M Ϫ SH}ϩ, 119
{M Ϫ CH2SH}ϩ.
A two-compartment cell fitted with a vitreous carbon disc
working electrode, a platinum wire secondary electrode and
either an SCE or Ag wire/ferrocene internal standard reference
system was employed for cyclic voltammetric measurements.
Experiments were carried out using a Hi-Tek DT 2101 poten-
tiostat interfaced with a Hi-Tek PPR1 waveform generator and
data were recorded on a Philips PM 8041 X–Y recorder. The
MeC(CH2SH)2CH2SMe, 2-methyl-2-methylsulfanylmethyl-
propane-1,3-dithiol, 3a. MeC(CH2S)2CH2SH, 2 (5.06 g, 30
mmol), was dissolved in 200 mL freshly distilled THF under
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593

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nitrogen atmosphere. NaH (1.26 g of 60% suspension, 31
mmol) was cautiously added over 30 min. After 20 min the
mixture turned white, foaming gently. MeI (1.87 mL, 30 mmol)
was added and the mixture turned back to yellow. This was
stirred for an hour. THF was removed and the residue was
quenched by 50 mL NH4Cl saturated water. 300 mL ether was
added and this was washed 3 times with 300 mL water. The
organic extract was dried (MgSO4) and the solvent removed
under vacuum to give a yellow oil. 1H NMR (400 MHz,
C6D5CD3): δ 0.90 (3H, s, CCH3), 1.70 (3H, s, SCH3), 2.22 (2H,
by 20 mL MeCN. Acetonitrile was in turn evaporated
down, leaving a brown–red powder (0.67 g, 49%). Single
crystals were obtained by dissolving this powder in 50 mL
hexane and leaving the solution in the fridge for two days.
Microanalysis: C11H12O5S3Fe2(0.1hexane) found (calc): C,
31.60 (31.59); H, 3.17 (3.05); S, 18.41 (21.83)%. 1H NMR
(400 MHz, CD2Cl2): δ 0.90 (3H, s, CH3), 1.65 [2H, d, 2J 14 Hz,
2
2(CCHHS)], 2.05 (2H, br, CH2SCH3), 2.23 [2H, d, J 14 Hz,
2(CCHHS)], 2.64 (3H, s, SCH3). FTIR (in CH3CN): ν(CO)
1925w, 1979(s, br) and 2046m cmϪ1. EI-MS: m/z 432 {M}ϩ,
404{M Ϫ CO}ϩ, 376{M Ϫ 2CO}ϩ, 348{M Ϫ 3CO}ϩ,
320{M Ϫ 4CO}ϩ, 292{M Ϫ 5CO}ϩ, 180{MeC(CH2S)2CH2-
SMe}ϩ, 176{Fe2S2}ϩ.
2
s, CH2SCH3), 2.38 [2H, d, J 11.5 Hz, (CHHSSCHH )], 2.65
[2H, d, 2J 11.5 Hz, (CHHSSCHH)].
LiAlH4 (2.05 g, 54 mmol) was dissolved in 200 mL distilled
ether under nitrogen atmosphere. MeC(CH2S)2CH2SMe
(4.95 g, 27 mmol) was added slowly at such rate that moderate
reflux occurred. After 15 h at room temperature, the mixture
was cooled at 0 ЊC. 60 mL degassed water was carefully added
as the mixture foamed a lot, and this was followed by 100 mL
degassed H2SO4 (10% solution). The ether layer was washed
3 times with 200 mL degassed water. The organic extract was
dried (MgSO4) and the solvent removed under reduced pres-
sure. The crude material was distilled to give MeC(CH2SH)2-
CH2SMe as a colourless oil (2.7 g, 55%), bp 160 ЊC (0.7 mm).
Microanalysis: C6H14S3, found (calc): C, 39.26 (39.56); H, 7.70
[Fe2{PhCH2SCH2C(Me)(CH2S)2}(CO)5],
4b.
[Fe3CO12]
(0.95 g, 1.9 mmol) was dissolved in 70 mL freshly distilled tolu-
ene under nitrogen atmosphere. MeC(CH2SH)2CH2SCH2Ph
(0.49 g, 1.9 mmol) was added to the mixture and this was
stirred at 90 ЊC for 1 h 30 min. The brown–red solution was
allowed to cool to room temperature and was filtered through
silica gel (70–230 mesh). A dark brown–red fraction was
collected by elution with more toluene. Solvent was removed
under vacuum, leaving a brown–red powder (0.7 g, 73%). The
powder was dissolved in 20 mL MeCN and slow evaporation of
the solvent under a stream N2 led to the deposition of crystal-
line material. Microanalysis: C19H16O5S3Fe2, found (calc): C,
1
(7.59); S, 53.0 (52.8)%. H NMR (400 MHz, CD2Cl2): δ 1.03
(3H, s, CH3), 1.32 (2H, t, 3J 9 Hz, SH), 2.13 (3H, s, SCH3), 2.63
3
1
(2H, s, CH2SCH3), 2.63 [4H, d, J 9 Hz, (HSCH2CCH2SH)].
40.26 (40.16); H, 3.57 (3.15); S, 15.9 (18.9)%. H NMR (400
EI-MS: m/z 182 {M}ϩ, 61 {CH2SCH3}ϩ.
MHz, CD2Cl2): δ 0.90 (3H, s, CH3), 1.70 [2H, d, J 14 Hz,
2
2
2(CCHHS)], 1.82 (2H, s, CH2SCH2Ph), 2.28 [2H, d, J 14 Hz,
MeC(CH2SH)2CH2SCH2Ph,
2-methyl-2-methylsulfanyl-
2(CCHHS)], 4.12 (2H, s, SCH2Ph), 7.40 (5H, m, SCH2C6H5).
benzylpropane-1,3-dithiol, 3b. MeC(CH2S)2CH2SH, 2 (1.40 g,
8.4 mmol), was dissolved in 200 mL freshly distilled THF
under nitrogen atmosphere. NaH (0.354 g of 60% suspension,
8.8 mmol) was cautiously added over 30 min. After 20 min the
mixture turned white, foaming gently. PhCH2Br (1.44 mg,
8.4 mmol) was added and the mixture remained cloudy. This
was stirred for 30 min. THF was removed and the residue
was quenched by 50 mL NH4Cl saturated water. 300 mL
ether was added and this was washed 3 times with 300 mL
water. The organic extract was dried (MgSO4) and the solvent
FTIR (in CH3CN): ν(CO) 1925w, 1981(s, br), 2046m cmϪ1
.
EI-MS: m/z 508 {M}ϩ, 480{M Ϫ CO}ϩ, 452{M Ϫ 2CO}ϩ,
424{M Ϫ 3CO}ϩ, 396{M Ϫ 4CO}ϩ, 368{M Ϫ 5CO}ϩ, 267{Fe2-
SSCH2Ph}ϩ, 231{Fe2SCH2C(Me)CH2S}ϩ, 176{Fe2S2}ϩ.
[Et4N][Fe2{MeSCH2C(Me)(CH2S)2}(CN)(CO)4],
5a.
[Fe2{MeSCH2C(Me)(CH2S)2(CO)5] (26 mg, 0.06 mmol) was
dissolved in 2 mL freshly distilled acetonitrile. [Et4N]CN (37
mg, 0.24 mmol) was dissolved in 2 mL MeCN from which 500
µL (0.06 mmol) were syringed and added to the brown–red
solution containing the complex. The mixture turned immedi-
ately to bright red before returning slowly to brown–red. After
1 h 30 min examination by solution FTIR showed essentially
quantitative conversion to 5a. The solvent was removed under
vacuum and the resulting oil was triturated with 5 mL of
diethyl ether to give a brown powder which was removed by
fitration and dried in vacuo. Single crystals suitable for X-ray
analysis were obtained by slow diffusion of diethyl ether into
an MeCN solution at 4 ЊC over 1 month. Microanalysis:
C19H32N2O4S3Fe2.H2O, found (calc): C, 39.60 (39.44); H, 5.93
(5.88); N, 4.52 (4.84)%. FTIR (in CH3CN): ν(CO) 1904s, 1940s,
1978s and ν(CN) 2085m cmϪ1. FAB-MS: m/z, 413 {[Et4N]-
[Fe2S(CNH)(CO)4]}ϩ.
1
removed under vacuum to give a brown–yellow oil. H NMR
(400 MHz, CDCl3): δ 1.25 (3H, s, CCH3), 2.68 (2H, s,
2
CH2SCH2Ph), 2.88 [2H, d, J 11.3 Hz, (CHHSSCHH )], 3.05
[2H, d, 2J 11.3 Hz, (CHHSSCHH)], 3.75 (2H, s, SCH2Ph), 7.30
(5H, m, SCH2C6H5),
LiAlH4 (0.60 g, 15.8 mmol) was dissolved in 200 mL distilled
ether under nitrogen atmosphere. MeC(CH2S)2CH2SCH2Ph
(2.2 g, 7.9 mmol) was added slowly at such rate that moderate
reflux occurred. After 15 h at room temperature, the mixture
was cooled at 0 ЊC. 60 mL degassed water was carefully added
as the mixture foamed a lot, and this was followed by 100 mL
degassed H2SO4 (10% solution). The ether layer was washed
3 times with 200 mL degassed water. The organic extract was
dried (MgSO4) and the solvent removed under reduced pres-
sure. The crude material was distilled to give MeC(CH2SH)2-
CH2SCH2Ph as a colourless oil (0.67 g, 31%), bp 105 ЊC (0.002
[Et4N][Fe2{PhCH2SCH2C(Me)(CH2S)2}(CN)(CO)4],
5b.
[Fe2{PhCH2SCH2C(Me)(CH2S)2(CO)5] (60 mg, 0.12 mmol) was
dissolved in 4 mL freshly distilled acetonitrile. [Et4N]CN
(55 mg, 0.35 mmol) was dissolved in 1 mL MeCN from which
330 µL (0.12 mmol CNϪ) were syringed and added to the
brown–red solution containing the complex. The mixture
turned immediately to bright red before returning slowly to
brown–red. After 1 h 30 min examination by solution FTIR
showed essentially quantitative conversion to 5b. The solvent
was removed under vacuum and the resulting oil was triturated
with 5 mL of diethyl ether to give a brown powder which was
removed by fitration and dried in vacuo. Microanalysis:
C25H36N2O4S3Fe2ؒH2O, found (calc): C, 46.03 (45.84); H, 5.84
(5.85); N, 4.40 (4.27); S, 14.86 (14.66)%. FTIR (in CH3CN):
ν(CO) 1904s, 1940s, 1978s and ν(CN) 2084m cmϪ1. ESI-MS:
m/z 499 {M–4CO–CN}Ϫ.
1
mm). H NMR (400 MHz, CD2Cl2): δ 0.90 (3H, s, CH3), 1.11
(2H, t, 3J 9 Hz, SH), 2.46 (2H, s, CH2SCH2Ph), 2.47 [4H, d, 2J 9
Hz, (HSCH2CCH2SH)], 3.63 (2H, s, SCH2Ph), 7.24 (5H, m,
SCH2C6H5). EI-MS: m/z 258 {M}ϩ.
[Fe2{MeSCH2C(Me)(CH2S)2}(CO)5], 4a. [Fe3CO12] (1.63 g,
3.2 mmol) was dissolved in 70 mL freshly distilled toluene
under nitrogen atmosphere. MeC(CH2SH)2CH2SMe (0.58 g,
3.2 mmol) was added to the mixture and this was stirred at
80 ЊC for 1 h 30 min. The brown–red solution was allowed
to cool to room temperature and was filtered through silica
gel (70–230 mesh). A dark brown–red fraction was collected
by elution with more toluene, leaving a red material on top
of column. Solvent was removed under vacuum and replaced
D a l t o n T r a n s . , 2 0 0 3 , 5 8 6 – 5 9 5
594

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4 A. J. Pierik, M. Hulstein, W. R. Hagen and S. P. J. Albracht,
Eur. J. Biochem., 1998, 258, 572–578.
Generation of [Et4N]2[Fe2(ꢀ-CO){MeSCH2C(Me)(CH2S)2}-
(CN)2(CO)3], 6a. [Fe2{MeSCH2C(Me)(CH2S)2(CO)5] (33 mg,
0.07 mmol) was dissolved in 3 mL freshly distilled acetonitrile.
[Et4N]CN (30 mg, 0.19 mmol) was added to the brown–red
solution and the mixture turned immediately to bright red
before returning slowly to brown–red. After 50 min at room
temperature, the mixture was transferred to a freezer at Ϫ10 ЊC.
After 12 days in the freezer, the mixture had turned red and was
shown by infrared to be the thermally unstable CO-bridged
intermediate. FTIR (in CH3CN): ν(µ-CO) 1780(m, br), ν(CO)
5 Y. Nicolet, A. L. de Lacey, X. Vernede, V. M. Fernandez,
E. C. Hatchikian and J. C. Fontecilla-Camps, J. Am. Chem. Soc.,
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7884.
1879s, 1919s, 1958s, ν(CN) 2074m and 2084m cmϪ1
.
11 A. S. Pereira, P. Tavares, I. Moura, J. J. G. Moura and B. H. Huynh,
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14 A. Le Cloirec, S. P. Best, S. Borg, S. C. Davies, D. J. Evans,
D. L. Hughes and C. J. Pickett, Chem. Commun., 1999, 2285–
2286.
15 E. J. Lyon, I. P. Georgakaki, J. H. Reibenspies and M. Y.
Darensbourg, J. Am. Chem. Soc., 2001, 123, 3268–3278.
16 F. Gloaguen, J. D. Lawrence, M. Schmidt, S. R. Wilson and
T. B. Rauchfuss, J. Am. Chem. Soc., 2001, 123, 12518–12527.
17 H. X. Li and T. B. Rauchfuss, J. Am. Chem. Soc., 2002, 124, 726–
727.
[K(18-crown-6)]2[Fe2{MeSCH2C(Me)(CH2S)2}(CN)2(CO)4],
7a. [Fe2{MeSCH2C(Me)(CH2S)2(CO)5] (22 mg, 0.05 mmol) was
dissolved in 4 mL freshly distilled acetonitrile. A [K(18-crown-
6)]CN solution was prepared by dissolving KCN (75 mg, 1.15
mmol) and 18-crown-6 ether (0.30g, 1.15 mmol) in 3 mL MeCN
from which 295 µL (0.11 mmol CNϪ) were syringed and added
to the brown–red solution containing the complex. It was
stirred at room temperature for 17 h during which time the
mixture gradually turned to bright red. FTIR spectroscopy of
the solution showed stoichiometric conversion to the dianion.
Solvent evaporation and 4 successive washing steps with diethyl
ether led to the isolation of a red solid. FTIR (in CH3CN):
ν(CO) 1870(sh), 1884s, 1922s, 1963s, ν(CN) 2075m cmϪ1
.
18 M. Razavet, S. C. Davies, D. L. Hughes and C. J. Pickett,
Chem. Commun., 2001, 847–848.
FAB-MS: m/z, 413 {[K(18-crown-6)][Fe2{MeSCH2C(Me)-
(CH2S)2(CN)2(CO)4]}ϩ, 648 {[K(18-crown-6)][Fe2{MeSCH2-
C(Me)(CH2S)2(CNH)(CN)]}ϩ. Iron analysis, found (calc): 23.0
(23.4) mg LϪ1. The tetraethyl ammonium salt was prepared in a
similar fashion using [Et4N]CN. Microanalysis: C28H52N4O4S3-
Fe2ؒCH3CN, found (calc): C, 47.91 (47.51); H, 7.41 (7.26); N,
8.77 (9.23)%.
19 Z. X. Cao and M. B. Hall, J. Am. Chem. Soc., 2001, 123, 3734–
3742.
20 H. J. Fan and M. B. Hall, J. Am. Chem. Soc., 2001, 123, 3828–
3829.
21 M. Bruschi, P. Fantucci and L. De Gioia, Inorg. Chem., 2002, 41,
1421–1429.
22 Z. P. Liu and P. Hu, J. Am. Chem. Soc., 2002, 124, 5175–5182.
23 M. Razavet, A. Le Cloirec, S. C. Davies, D. L. Hughes and
C. J. Pickett, J. Chem. Soc., Dalton Trans., 2001, 3551–3552.
24 M. Razavet, S. J. Borg, S. J. George, S. P. Best, S. A. Fairhurst and
C. J. Pickett, Chem. Commun., 2002, 700–701.
25 S. J. George, Z. Cui, M. Razavet and C. J. Pickett, Chem.: Eur. J.,
2002, 8, 4037–4046.
[Et4N]2[Fe2{PhCH2SCH2C(Me)(CH2S)2}(CN)2(CO)4],
7b.
[Fe2{PhCH2SCH2C(Me)(CH2S)2(CO)5] (48 mg, 0.94 mmol)
was dissolved in 4 mL freshly distilled acetonitrile. [Et4N]CN
(94 mg, 0.60 mmol) was dissolved in 1 mL MeCN from which
340 µL (0.20 mmol CNϪ) were syringed and added to the
brown–red solution containing the complex. It was stirred at
room temperature for 17 h while the mixture gradually turned
to bright red. Solvent was evaporated and solid was obtained by
washing the oily product 4 times with CH2Cl2 and drying under
vacuum for 4 h. Microanalysis: C34H56N4O4S3Fe2ؒCH2Cl2,
found (calc): C, 47.84 (47.86); H, 6.88 (6.61); N, 6.11 (6.38)%.
FTIR (in CH3CN): ν(CO) 1870(sh), 1984s, 1922s, 1963s, ν(CN)
2075m cmϪ1. ESI-MS: m/z 534 {M Ϫ 2(Et4Nϩ) ϩ 2(Hϩ)}Ϫ, 506
{M Ϫ (2Et4Nϩ) ϩ (2Hϩ) Ϫ CO}Ϫ, 450 {M Ϫ (2Et4Nϩ) Ϫ 2CO
Ϫ CN}Ϫ, 394 {M Ϫ (2Et4Nϩ) Ϫ 4CO Ϫ CN}Ϫ.
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Acknowledgements
We thank the BBSRC and the John Innes Foundation for
supporting this work. J. R. Sanders is thanked for helpful
discussion.
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