Models of the iron-only hydrogenase: Reactions of [Fe2(CO)6(μ-pdt)] with small bite-angle diphosphines yielding bridge and chelate diphosphine complexes [Fe2(CO)4(diphosphine)(μ-pdt)]

Article abstract of DOI:10.1016/j.jorganchem.2007.05.050

Reactions of [Fe₂(CO)₆(μ-pdt)] (1) (pdt = SCH₂CH₂CH₂S) and small bite-angle diphosphines have been studied. A range of products can be formed being dependent upon the nature of the diphosphine and reaction conditions. With bis(diphenylphosphino)methane (dppm), thermolysis in toluene leads to the formation of a mixture of bridge and chelate isomers [Fe₂(CO)₄(μ-dppm)(μ-pdt)] (2) and [Fe₂(CO)₄(κ²-dppm)(μ-pdt)] (3), respectively. Both have been crystallographically characterised, 3 being a rare example of a chelating dppm ligand in a first row binuclear system. At room temperature in MeCN with added Me₃NO · 2H₂O, the monodentate complex [Fe₂(CO)₅(κ¹-dppm)(μ-pdt)] (4) is initially formed. Warming 4 to 100 °C leads the slow conversion to 2, while oxidation (on alumina) gives [Fe₂(CO)₅(κ¹-dppmO)(μ-pdt)] (5). With bis(dicyclohexylphosphino)methane (dcpm), heating in toluene cleanly affords [Fe₂(CO)₄(μ-dcpm)(μ-pdt)] (6). With Me₃NO · 2H₂O in MeCN the reaction is not clean as the phosphine is oxidised but monodentate [Fe₂(CO)₅(κ¹-dcpm)(μ-pdt)] (7) can be seen spectroscopically. With 1,2-bis(diphenylphosphino)benzene (dppb) and cis-1,2-bis(diphenylphosphino)ethene (dppv) the chelate complexes [Fe₂(CO)₄(κ²-dppb)(μ-pdt)] (8) and [Fe₂(CO)₄(κ²-dppv)(μ-pdt)] (9), respectively are the final products under all conditions, although a small amount of [Fe₂(CO)₅(κ²-dppvO)(μ-pdt)] (10) was also isolated. Protonation of 2 with HBF₄ affords a cation with poor stability while with the more basic diiron centre in 6 readily forms the stable bridging-hydride complex [(μ-H)Fe₂(CO)₄(μ-dcpm)(μ-pdt)][BF₄] (11) which has been crystallographically characterised.

Full text of DOI:10.1016/j.jorganchem.2007.05.050

Journal of Organometallic Chemistry 692 (2007) 3957–3968
www.elsevier.com/locate/jorganchem
Models of the iron-only hydrogenase: Reactions
of [Fe2(CO)6(l-pdt)] with small bite-angle diphosphines
yielding bridge and chelate diphosphine complexes
[Fe2(CO)4(diphosphine)(l-pdt)]
*
Fatima I. Adam, Graeme Hogarth , Idris Richards
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
Received 23 April 2007; received in revised form 16 May 2007; accepted 31 May 2007
Available online 19 June 2007
Abstract
Reactions of [Fe2(CO)6(l-pdt)] (1) (pdt = SCH2CH2CH2S) and small bite-angle diphosphines have been studied. A range of products
can be formed being dependent upon the nature of the diphosphine and reaction conditions. With bis(diphenylphosphino)methane
(dppm), thermolysis in toluene leads to the formation of a mixture of bridge and chelate isomers [Fe2(CO)4(l-dppm)(l-pdt)] (2) and
[Fe2(CO)4(j2-dppm)(l-pdt)] (3), respectively. Both have been crystallographically characterised, 3 being a rare example of a chelating
dppm ligand in a first row binuclear system. At room temperature in MeCN with added Me3NO Æ 2H2O, the monodentate complex
[Fe2(CO)5(j1-dppm)(l-pdt)] (4) is initially formed. Warming 4 to 100 ꢀC leads the slow conversion to 2, while oxidation (on alumina)
gives [Fe2(CO)5(j1-dppmO)(l-pdt)] (5). With bis(dicyclohexylphosphino)methane (dcpm), heating in toluene cleanly affords
[Fe2(CO)4(l-dcpm)(l-pdt)] (6). With Me3NO Æ 2H2O in MeCN the reaction is not clean as the phosphine is oxidised but monodentate
[Fe2(CO)5(j1-dcpm)(l-pdt)] (7) can be seen spectroscopically. With 1,2-bis(diphenylphosphino)benzene (dppb) and cis-1,2-bis(diph-
enylphosphino)ethene (dppv) the chelate complexes [Fe2(CO)4(j2-dppb)(l-pdt)] (8) and [Fe2(CO)4(j2-dppv)(l-pdt)] (9), respectively
are the final products under all conditions, although a small amount of [Fe2(CO)5(j2-dppvO)(l-pdt)] (10) was also isolated. Protonation
of 2 with HBF4 affords a cation with poor stability while with the more basic diiron centre in 6 readily forms the stable bridging-hydride
complex [(l-H)Fe2(CO)4(l-dcpm)(l-pdt)][BF4] (11) which has been crystallographically characterised.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Iron-only hydrogenase; Diphosphine; Dithiolate; Diiron; Chelating; dppm
1. Introduction
One particularly widely studied complex is [Fe2(CO)6(l-
pdt)] (1) (pdt = SCH2CH2CH2S) [3–8], since in the iron-
only hydrogenase a three atom unit bridges the two sulfur
atoms. Initially this was thought to be a propane chain [9],
although recent studies suggest that it more likely contains
a central NH unit [10]. A key role of the two-iron unit of
the H-active cluster is to promote the reduction of protons
to dihydrogen, which most probably involves proton bind-
ing to the diiron unit, possibly to one of the iron atoms.
Clearly, any complex which aims to model this active site
should be basic enough to bind a proton; unfortunately this
does not apply to 1.
While being known for over forty years [1], there has
recently been a tremendous resurgence in interest in the
chemistry of dithiolate-bridged diiron complexes of the
type [Fe2(CO)6(l-SR)2]. This has resulted primarily from
the realisation that they closely resemble the two-iron unit
of the H-cluster active site of iron-only hydrogenases [2].
*
Corresponding author.
E-mail address: g.hogarth@ucl.ac.uk (G. Hogarth).
0022-328X/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.05.050

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F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
Cys
X
S
S
S
S
L
L
OC
CO
OC
CO
S
S
S
OC
CO
ap
S
S
S
[4Fe4S]
S
Fe
Fe
Fe
Fe
Fe
Fe
OC
OC
L
OC
OC
CO
CO
CO
CO
Fe
Fe
Fe
Fe
OC
OC
CO
L
L
L
OC
NC
CN
CO
ba
CO
CO
ba
transoid dibasal
cisoid dibasal
diapical
One approach adopted to circumvent this problem is the
substitution of one or more carbonyls for more basic phos-
phine ligands. This has the effect of increasing the electron-
density at the diiron centre, thus enhancing proton binding.
For example, Darensbourg and co-workers have prepared
[Fe2(CO)4(PMe3)2(l-pdt)] and shown that it can be readily
protonated [4b]. The addition of phosphines to the diiron
centre, however, introduces a further level of complexity
into the system as isomers can result. Phosphines can sub-
stitute at either apical or basal sites with the former being
generally preferred. For disubstitution, six possible confor-
mations are possible (Chart 1). Two monodentate phos-
phines always substitute at different metal centres and the
diapical and transoid dibasal conformations are most
common. Thus, in the solid-state both [Fe2(CO)4(PTA)2-
(l-pdt)] (PTA = 1,3,5-triaza-7-phosphaadamantane) [4f]
and [Fe2(CO)4(PMe3)2(l-pdt)] [4b] adopt the transoid
dibasal configuration, while in contrast in [Fe2(CO)4-
(PMe2Ph)2(l-pdt)] and [Fe2(CO)4(PPh3)2(l-pdt)] both
phosphines occupy apical positions [8a].
In solution the situation is potentially more complex
since the different isomers may be in equilibrium as the
well-known trigonal-twist of Fe(CO)2L fragments can
occur with only low activation barriers, and potentially
leads to an increase in complexity of the system. One
potential way of simplifying the situation is to use diphos-
phine ligands capable of offering a more rigid coordination
environment. Thus, the cisoid dibasal conformation is not
seen with monodentate phosphines presumably because it
is sterically unfavourable with respect to the transoid diba-
sal geometry. However, a diphosphine capable of bridging
between two metal centres should favour this geometry
while making the alternative diapical and transoid dibasal
arrangements inaccessible. In this regard, bis(diphenyl-
phosphino)methane (dppm) is an ideal choice as it exhibits
a high propensity to bridge metal–metal vectors leading to
the formation of a stable five-membered rings [11]. Like-
wise, diphosphines with rigid two-carbon backbones such
as cis-bis(diphenylphosphino)ethene (dppv) and 1,2-
bis(diphenylphosphino)benzene (dppb) should favour
unsymmetrical chelating coordination thus affording elec-
S
S
S
S
S
OC
L
OC
L
OC
CO
L
S
Fe
Fe
Fe
Fe
Fe
Fe
OC
OC
OC
OC
OC
CO
CO
L
L
CO
L
apical-basal
unsymmetrical dibasal
unsymmetrical apical-basal
Chart 1.
tronic and steric asymmetry to the diiron centre. Herein
we report details of the reaction of 1 with a number of
small bite-angle diphosphines showing that the three
disubstituted conformations inaccessible with monoden-
tate phosphines are all available with diphosphines pro-
vided the correct choice of ligand is made. While this
publication was in preparation two contributions in this
˚
area were reported. Thus, Liu, Akermark, Sun and co-
workers have detailed aspects of the reaction of 1 with
dppm [12], and De Gioia, Rauchfuss and co-workers
reported the formation of [Fe2(CO)4(j2-dppv)(l-pdt)]
[13]. Further, in closely related work we have recently
shown that reaction of 1 with bis(2-diphenylphosphino-
ethyl)phenylphosphine (triphos) affords the novel tricar-
bonyl complex, [Fe2(CO)3(l,j1,j2-triphos)(l-pdt)], in
which the triphosphine chelates to one metal centre in a
apical–basal manner, while also bridging the diiron centre
via a dibasal coordination [14].
2. Results and discussion
2.1. With bis(diphenylphosphino)methane (dppm)
Heating a toluene solution of 1 with a slight excess of
dppm results in a slow reaction in which the hexacarbonyl
was consumed over 16 h. After washing with hexanes in
order to remove any unreacted starting materials, the IR
spectrum showed the relatively clean formation of
[Fe2(CO)4(l-dppm)(l-pdt)] (2) characterised by a singlet
at d 53.2 in the 31P NMR spectrum. A second minor reso-
nance was also observed at d 12.5 being attributed to the
isomeric chelate complex, [Fe2(CO)4(j2-dppm)(l-pdt)] (3)
(see below).
S
S
S
OC
CO
S
OC
S
Fe
CO
OC
CO
S
dppm, Δ
+
Fe
Fe
Fe
Fe
Fe
OC
Ph2P
OC
OC
CO
PPh2
PPh2
- 2 CO
OC
OC
CO
CO
Ph2P
1
2
3

F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
3959
Crystallisation of 2 upon slow diffusion of methanol into
a dichloromethane solution gave small orange blocks
shown to be [Fe2(CO)4(l-dppm)(l-pdt)] (2) (Fig. 1). These
contain one whole and two half molecules of 2 in the asym-
metric unit, together with half a molecule of hexane and
half a molecule of methanol. Differences in the independent
molecules of 2 are small, and for comparison data are
shown only for the unique whole molecule. Frustratingly,
despite many efforts we failed to isolate significant amounts
of the pure chelate isomer [Fe2(CO)4(j2-dppm)(l-pdt)] (3).
However, after heating 1 and a slight excess of dppm in tol-
uene for 4 h, chromatography gave a red band which com-
prised mainly 4 (see below) but also contained significant
amounts of 3. The latter is easily detected by IR spectros-
copy, showing a distinctive strong band at 2023 cmÀ1. For-
tunately, crystallisation of this mixture upon slow diffusion
of hexane into a concentrated diethyl ether solution
afforded a small number of red blocks of [Fe2(CO)4-
(j2-dppm)(l-pdt)] (3) suitable for X-ray diffraction (Fig. 2).
Both isomers show the expected dithiolate-bridged
diiron core. In 2 the diphosphine bridges the iron–iron vec-
tor in a cisoid dibasal fashion which renders the two thio-
late bridges inequivalent. This arrangement is adopted in
other diiron dithiolate complexes [15]. As alluded to in
the introduction, while this manuscript was in preparation,
the independent synthesis and structural characterisation
of 2 was reported [12]. Our preparation excludes the use
of Me3NO which results in higher yields (76% vs. 27%).
In 3 the diphosphine binds to a single iron atom occupying
the two basal positions and rendering the two dithiolate-
bridges equivalent and while in 2 the two sulphurs bind
approximately symmetrically to the two iron atoms, in 3
Fig. 2. Molecular structure of [Fe2(CO)4(j2-dppm)(l-pdt)] (3) with
selected bond lengths (A) and angles (ꢀ); Fe(1)–Fe(2) 2.5879(7), Fe(1)–
P(1) 2.2123(11), Fe(1)–P(2) 2.2217(11), Fe(1)–S(1) 2.2222(10), Fe(1)–S(2)
2.2250(10), Fe(2)–S(1) 2.2614(11), Fe(2)–S(2) 2.2650(11), P(1)–Fe(1)–S(1)
156.89(4), P(2)–Fe(1)–S(2) 163.34(4), P(1)–Fe(1)–P(2) 74.55(4), P(1)–C(8)–
P(2) 93.53(17), Fe(1)–S(1)–Fe(2) 70.50(3), Fe(1)–S(2)–Fe(2) 70.39(3).
˚
they are quite asymmetric, lying closer to the phosphine
bound metal centre [Fe(1)–S(av) 2.224(2), Fe(2)–S(av)
˚
2.263(2) A]. While there are a number of examples of dppm
acting in a chelating manner in polynuclear complexes con-
taining the larger second and third row transition metals
[16], the only crystallographically characterised example
of this behaviour at a first row metal centre is in [CpMo-
(CO)2(l-H)(l-PPh2)Mn(j2-dppm)(CO)2] [17].
In 3 both phosphorus atoms occupy basal sites and this
seems unusual since apical substitution is generally pre-
ferred. For example, both [Fe2(CO)5(PPh3)(l-pdt)] and
[Fe2(CO)5(PPhMe2)(l-pdt)] [8a] are substituted in the api-
cal positions. The adoption of the dibasal geometry may
be favoured in order to minimise adverse steric interactions
between the phenyl rings and dithiolate-bridge. The bite-
angle of 74.55(4)ꢀ for the diphosphine in 3 is very small,
but in line with values found in dppm-chelate complexes
[16,17]. Its adoption requires a significant decrease in the
preferred dibasal angle as can be seen when compared with
that at the unsubstituted iron centre [C(3)–Fe(2)–C(4)
90.85(19)ꢀ] [4a]. There is also a significant reduction in
the P–C–P angle to 93.53(17)ꢀ upon chelation which can
be compared with that of 114.3(2)ꢀ in 2. In 3 this leads to
the close contact of the two phosphorus nuclei [P(1)–P(2)
˚
2.6854(12) A] when compared to that in the 2 [P(1)–P(2)
˚
3.094(3) A].
Fig. 1. Molecular structure of [Fe2(CO)4(l-dppm)(l-pdt)] (2) with
selected bond lengths (A) and angles (ꢀ); Fe(1)–Fe(2) 2.5076(9), Fe(1)–
P(1) 2.2134(13), Fe(2)–P(2) 2.2104(13), Fe(1)–S(1) 2.2531(12), Fe(2)–S(1)
2.2489(12), Fe(1)–S(2) 2.2597(13), Fe(2)–S(2) 2.2592(13), P(1)–Fe(1)–S(1)
85.12(4), P(2)–Fe(2)–S(1) 85.56(5), P(1)–Fe(1)–S(2) 152.90(5), P(2)–Fe(2)–
S(2) 153.26(5), Fe(1)–S(1)–Fe(2) 67.70(4), Fe(1)–S(2)–Fe(2) 67.41(4),
C(1)–Fe(1)–Fe(2) 152.62(15), P(2)–C(8)–P(1) 114.3(2).
NMR spectra of 2 show significant temperature depen-
dence. At room temperature the 1H NMR spectrum shows
two broad methylene resonances associated with the l-pdt
ligand suggesting that the ligand ‘‘flips’’ rapidly at this tem-
perature and this is in accord with the observation of a sin-
glet in the 31P NMR spectrum. Upon cooling the latter
˚

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F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
broadens and collapses and at 213 K is replaced by two well-
resolved AB doublets (JPP 54 Hz) suggesting that now the
flipping of the central methylene is slow on the NMR time-
scale. From the coalescence temperature (243 K) a free
station has occurred via a crystallographic study. We inde-
pendently carried out this reaction under essentially
identical conditions (although we added the diphosphine
before Me3NO Æ 2H2O and stirred for 14 h rather than
4 h). In our hands, the reaction leads to the formation
of 4 (JPP 84 Hz) as the major product, but two further
products were also seen in the crude reaction mixture.
Small amounts (ca. 5%) of the tetracarbonyl chelate 3 were
seen but the bridge isomer 2 was notably absent. A second
minor product (ca. 15%) appeared as doublets at d 55.7
and 23.1 (JPP 19 Hz). In an attempt to separate these
products the mixture was chromatographed on alumina
which resulted in the isolation of a pure sample of 4 along
with substantial amounts of [Fe2(CO)5(j1-dppmO)(l-pdt)]
(5), characterised by a phosphorus–phosphorus coupling
constant of 19 Hz. Initial formation of 5 presumably
results from oxidation of 4 by Me3NO Æ 2H2O since dppm
itself is oxidised very slowly by this reagent. Isolation of
significant amounts of 5 after chromatography suggests
that some degree of oxidation of 4 has been facilitated
on the column. Complex 5 is presumed to be the basal
isomer and in this instance a second isomer was not
observed.
energy of activation is estimated at 45 1 kJ molÀ1
.
S
S
OC
CO
OC
CO
S
S
Fe
Fe
Fe
Fe
OC
Ph2P
OC
Ph2P
CO
PPh2
CO
PPh2
methylene flip
Significant changes also occur to the 1H NMR spectrum
upon cooling being associated with the inequivalence of all
six l-pdt protons, but signal overlap and resolution
prevented full assignment. At room temperature the two
methylene protons of the diphosphine ligand are also
inequivalent being consistent with the solid-state structure.
Upon warming, however, these broaden and collapse into a
single broad resonance at 333 K. From the coalescence tem-
perature (315 K) a free energy of activation is estimated at
61 1 kJ molÀ1. While this high energy process could
involve either dissociation of one end of the bridging dppm
ligand or the reversible cleavage of Fe–Fe or Fe–S bonds,
we favour a double trigonal-twist of the diphosphine in
the basal site. The trigonal-twist process is known to occur
in l-pdt and related diiron dithiolate complexes although
here a major difference is the requirement for both phospho-
rus atoms to undergo the process simultaneously. This is not
unexpected since the movement of one of the phosphorus
atoms (as shown) would be expected to lead to a significant
strain at the backbone carbon in the intermediate state.
O
S
S
PPh2
PPh2
OC
S
OC
S
CO
CO
Al2O3
Fe
Fe
CO
Fe
Fe
CO
OC
OC
OC
OC
P
P
Ph2
Ph2
4
5
Heating 4 for a prolonged period results in a second car-
bonyl loss and formation of 2. In a similar manner to that
observed for 2, cooling a CD2Cl2 solution of 4 results in a
gradual broadening of the phosphorus resonances such
that at 213 K each doublet splits into two separate signals
(Jpp unresolved) in an approximate 2:1 ratio. We attribute
these changes to the flipping of the pdt ligand. Intuitively,
one would expect the minor low temperature species to be
that with the central methylene moiety orientated away
from the substituted iron centre, however it is noteworthy
that in the solid-state it is this conformation which is found
[12].
double
trigonal-twist
S
OC
S
CO
S
OC
CO
S
Fe
Fe
Fe
Fe
OC
Ph2P
CO
PPh2
Ph2P
OC
PPh2
CO
S
OC
CO
S
Since apical substitution is generally favoured in these
diiron l-pdt complexes, we were somewhat surprised the
apically substituted isomer of 4 was not observed at room
temperature or below. In order to probe whether this
transformation was accessible at higher temperatures, a
d8-toluene of 4 was warmed to 373 K while monitoring
by 31P NMR spectroscopy. No significant changes were
seen over this temperature range, although the high tem-
perature (373 K) phosphorus–phosphorus coupling con-
stant of 78 Hz was slightly smaller than that of 90 Hz
seen at room temperature. In these complexes the methy-
lene protons of the l-pdt are typically poorly resolved at
room temperature, appearing as what seems to be a series
of overlapping broad singlets. At 373 K, however, 4 shows
Fe
Fe
Ph2P
OC
CO
PPh2
˚
While this manuscript was in preparation, Liu, Aker-
mark, Sun and co-workers reported the synthesis of the
monodentate diphosphine complex, [Fe2(CO)5(j1-dppm)-
(l-pdt)] (4) (77%) from the reaction of 1 with dppm and
Me3NO Æ 2H2O in MeCN [12],1 confirming that basal sub-
1
Throughout this contribution JPP values have been incorrectly
measured, being consistently overestimated by not considering that the
frequency of 31P at ca. 400 MHz is 161.977 MHz.

F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
3961
three well defined quintets (two overlapping). This is con-
sistent with the fast flipping of the pdt ligand but also
requires that the phosphine is averaged over either both
basal positions or all three possible sites, since if the phos-
phine remains in a single basal site all six methylene pro-
tons of the pdt ligand remain inequivalent. To further
probe the phosphine position in solution we recorded a
13C NMR spectrum. The key feature of this is the observa-
tion of only two carbonyl resonances (ratio 3:2) showing
that at room temperature the carbonyls at both iron sites
are mobile. Hence, while the solid-state structure shows
phosphine coordination at a basal site, in solution the situ-
ation is clearly more complex.
S
OC
CO
S
dcpm,Δ
Fe
Fe
1
OC
Cy2P
CO
PCy2
- 2 CO
6
As with dppm, the reaction of 1 with dcpm was also
carried out in MeCN in the presence of Me3NO Æ 2H2O.
In this case, however, the reaction was not clean, the
major reaction product being doubly oxidised dcpm.
The major diiron containing component was tentatively
identified on the basis of 31P NMR data as [Fe2(CO)5-
(j1-dcpm)(l-pdt)] (7) being characterised by doublets at
d 62.9 and À9.2 (JPP = 23.8 Hz). Generation of 7 is in line
with the IR crude spectra and it is probably the basal iso-
mer since the dcpm ligand is more sterically demanding
than dppm. It is not clear why the oxidised variant of 7
is not seen, and this suggests that phosphine oxidation
must be slower once coordinated to iron. A number of
other minor products were seen in the crude reaction mix-
ture but the bridging dcpm complex 6 was noticeably
absent.
2.2. With bis(dicyclohexylphosphino)methane (dcpm)
Heating a toluene solution of 1 and dcpm for 16 h results
in the clean formation of [Fe2(CO)4(l-dcpm)(l-pdt)] (6) in
high yield. Formation of the bridge isomer was immediately
clear from the IR spectrum which showed four carbonyl
absorptions at 1975s, 1942vs, 1906s, 1889w cmÀ1. These
compare well with those for 2 at 1988s, 1957vs, 1921s,
1908sh cmÀ1, being shifted to lower wavenumbers by ca.
15–16 cmÀ1, suggesting that the diiron centre is consider-
ably more basic. Significantly, under these conditions no
evidence was seen for the formation of a chelate isomer.
In order to confirm the nature of 6 a crystallographic study
was carried out (Fig. 3). The structure contains four inde-
pendent diiron molecules (and two molecules of dichloro-
methane) in the asymmetric unit but the former vary only
slightly in their coordination geometry and only one is
shown. General features are the same as seen for 2, the
diphosphine binding in a cisoid dibasal fashion.
S
PCy2
OC
S
dcpm, MeCN
Me3NO.2H2O
CO
Cy2P
PCy2
1
+
Fe
Fe
CO
O
OC
OC
O
P
Cy2
7
2.3. With dppb and dppv
Heating 1 with either of the more rigid diphosphines,
1,2-bis(diphenylphosphino)benzene (dppb) or cis-1,2-
bis(diphenylphosphino)ethene (dppv), in toluene for two
days led to the slow formation of the chelate complexes
[Fe2(CO)4(j2-dppb)(l-pdt)] (8) and [Fe2(CO)4(j2-dppv)-
(l-pdt)] (9), respectively as the major reaction products.
Both were readily characterised as chelate complexes on
the basis of their IR spectra which are characteristic. Thus,
8 shows three bands in dichloromethane at 2019vs, 1950s
and 1905w cmÀ1, and 9 is similar; 2020vs, 1950s and
1912w cmÀ1. In the 31P NMR spectra both showed two
singlets in an approximate 10:1 ratio, the low-field reso-
nance being most intense. While this manuscript was in
preparation, De Gioia, Rauchfuss and co-workers reported
the independent synthesis of 9 [13]. Their data generally
agree with our own, although we do not see a resonance
in the 1H NMR spectrum at d 4.3 which they have assigned
with the vinylic protons. We observe no peaks in this
region and assume that the vinylic protons are hidden by
the aromatic resonances as indeed they are in the free
ligand.
Fig. 3. Molecular structure of one of the four independent molecules of
˚
[Fe2(CO)4(l-dcpm)(l-pdt)] (6) with selected bond lengths (A) and angles
(ꢀ); Fe(1)–Fe(2) 2.5259(10), Fe(1)–P(1) 2.2486(14), Fe(2)–P(2) 2.2477(15),
Fe(1)–S(1) 2.2701(14), Fe(2)–S(1) 2.2539(14), Fe(1)–S(2) 2.2654(14),
Fe(2)–S(2) 2.2640(15), P(1)–Fe(1)–S(1) 86.74(5), P(2)–Fe(2)–S(1)
87.98(5), P(1)–Fe(1)–S(2) 153.52(6), P(2)–Fe(2)–S(2) 150.58(6), Fe(1)–
S(1)–Fe(2) 67.88(4), Fe(1)–S(2)–Fe(2) 67.79(4), C(1)–Fe(1)–Fe(2)
155.13(17), P(2)–C(5)–P(1) 114.6(3).

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F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
Ph2
P
Ph2
P
S
OC
S
S
S
dppv, Δ
dppb, Δ
OC
1
Fe
Fe
Fe
Fe
- 2 CO
- 2 CO
OC
OC
P
OC
P
Ph2
Ph2
CO
OC
CO
9
8
Single crystals of 8 suitable for X-ray crystallography
were grown upon slow diffusion of hexane into a dichloro-
methane solution. The molecular structure (Fig. 4) reveals
the apical–basal coordination of the 1,2-bis(diphenyl-
phosphino)benzene ligand. The bite-angle of the diphos-
phine [P(1)–Fe(2)–P(2)] at 86.80(3)ꢀ is significantly larger
than that of 74.55(4)ꢀ found in the dibasal 3. In 8, the angle
between the dibasal positions [P(1)–Fe(2)–C(4)] is
89.14(10)ꢀ showing that the dppb ligand could sterically
occupy both sites and suggesting that the dibasal geometry
found in 3 is a result of the extremely small bite-angle of
the dppm ligand. Unlike 3, the sulphur atoms in 8 bridge
the diiron vector approximately symmetrically and it is also
noteworthy that the central methylene unit of the pdt
ligand is proximal to the substituted iron atom.
De Gioia and Rauchfuss investigated 9 by variable tem-
perature 31P NMR spectroscopy. While at room tempera-
ture two singlets are observed, cooling to À90 ꢀC results
in the appearance of six signals associated with four iso-
mers. These are proposed to result from the different orien-
tations of the central methylene group of the pdt ligand
(either proximal or distal to the phosphine-substituted
metal centre) and the orientation of the diphosphine (diba-
sal or basal–apical). We have previously observed very
similar behaviour for the triphos complex, [Fe2(CO)3-
(l,j1,j2-triphos)(l-pdt)] [14]. For 9, the apical–basal con-
formation is preferred (ca. 82%) at low temperature.
The room temperature 31P NMR spectrum of 8 in
CD2Cl2 shows two singlets at d 90.3 and 82.3 (ca. 10:1)
which we attribute to apical–basal (8a) and dibasal (8b) iso-
mers, respectively. Cooling results in a gradual broadening
of the major singlet resonance d 90.3 in the 31P NMR spec-
trum. This collapses into the baseline around 213 K to be
replaced by two sets of singlets in a ca. 13:1 ratio at
178 K; the major pair being observed at d 94.5 and 88.1
and the minor set at d 93.9 and 90.7. The line-width of
the major signals at half-height is 17 Hz at this temperature
and thus the phosphorus–phosphorus coupling (which
could not be resolved) must be small. This splitting of the
single high temperature resonance is associated with the
freezing out of the trigonal-twist between basal and apical
sites and for the major component a free energy of activa-
tion is estimated at 38 1 kJ molÀ1. The observation of
two low temperature conformations relates to the different
possible orientations of the central methylene unit of the
pdt ligand and similar to 4, in the solid-state this is found
to be proximal to the substituted iron centre in 8 and we
suppose that this is the major solution component. Unfor-
tunately, we could not ascertain the precise fate of the
minor room temperature component of 8, although at
178 K a small singlet was still apparent at d 83.5 which
may be due to a dibasal isomer.
Ph2
P
S
S
S
S
OC
OC
CO
Fe
Fe
Fe
Fe
OC
OC
OC
PPh2
P
Ph2
OC
CO
Ph2P
8b
8a
Reactions of dppb and dppv with 1 were also carried out
in MeCN with added Me3NO Æ 2H2O. In both cases, for-
mation of the desired tetracarbonyls was shown to be ca.
80% complete within 2 h at room temperature. In order
to complete the conversions, the mixtures were heated
overnight at 60 ꢀC. This synthetic method is preferable to
toluene reflux as work-up is simple and yields are higher.
When 9 prepared by this method was purified by column
chromatography, significant amounts of [Fe2(CO)5-
(j1-dppvO)(l-pdt)] (10) were also isolated. This was shown
Fig. 4. Molecular structure of [Fe2(CO)4(j2-dppb)(l-pdt)] (8) with
selected bond lengths (A) and angles (ꢀ); Fe(1)–Fe(2) 2.5382(6), Fe(2)–
P(1) 2.2008(9), Fe(2)–P(2) 2.1843(9), Fe(2)–S(1) 2.2616(9), Fe(2)–S(2)
2.2587(9), Fe(1)–S(1) 2.2698(9), Fe(1)–S(2) 2.2752(9), P(1)–Fe(2)–S(1)
93.59(3), P(1)–Fe(2)–S(2) 166.17(3), P(2)–Fe(2)–S(2) 106.47(3), P(2)–
Fe(2)–S(1) 112.18(3), P(2)–Fe(2)–Fe(1) 157.34(3), P(1)–Fe(2)–P(2)
86.80(3), Fe(1)–S(1)–Fe(2) 68.13(3), Fe(1)–S(2)–Fe(2) 68.09(3).
˚

F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
3963
to be a mixture of isomers in an approximate 3:1 ratio
characterised by phosphorus–phosphorus coupling con-
stants of 11–12 Hz. We suggest that the major isomer 10a
contains the phosphine in a basal site.
PPh2
O
S
S
O
PPh2
OC
S
OC
S
CO
PPh2
CO
Fe
Fe
CO
Fe
Fe
OC
OC
P
Ph2
OC
OC
CO
10a
10b
2.4. Protonation of [Fe2(CO)4(l-R2PCH2PR2)(l-pdt)]
(R = Ph, Cy) (2 and 6)
Fig. 5. Molecular structure of the cation in [(l-H)Fe2(CO)4(l-dcpm)(l-
pdt)][BF4] (11) with selected bond lengths (A) and angles (ꢀ); Fe(1)–Fe(1A)
˚
2.531(2), Fe(1)–P(1) 2.291(3), Fe(1)–S(1) 2.265(4), Fe(1)–S(2) 2.259(3),
P(1)–Fe(1)–S(1) 153.63(11), P(1)–Fe(1)–S(2) 85.67(14), Fe(1)–S(1)–Fe(1A)
67.93(13), Fe(1)–S(2)–Fe(1A) 68.12(12), C(1)–Fe(1)–Fe(2) 114.9(3), C(2)–
Fe(1)–Fe(1A) 144.0(4), P(1)–C(6)–P(1A) 122.5(10).
A key function of the diiron unit in the iron-only
enzyme is its ability to bind protons. Addition of concen-
trated HCl to a methanol suspension of 2 did not result
in the formation of a protonated product, consistent with
the hypothesis that the dppm ligand is not a strong enough
electron–donor to render the diiron centre basic enough.
Addition of an excess of HBF4 Æ Et2O to a dichloromethane
solution of 2 in air did lead to significant changes in the
carbonyl region of the IR spectrum, albeit over a pro-
longed timescale. Thus, absorptions at 1988s, 1957vs,
1921s, 1908sh in 2 were replaced over 3d with new peaks
at 2053w, 2034vs, 2000s, 1965w. All attempts to isolate this
new species were unsuccessful.
poor as the crystal was only weakly diffracting, and the
included half molecule of diethylether and a disordered
BF4 anion both of which could only be poorly modeled.
Nevertheless, the main structural features of the cation
could be discerned. The diiron unit lies on a plane of symme-
try with the central carbon of pdt ligand being disordered
over two sites (50:50). The metal–metal bond at
˚
2.531(2) A is slightly elongated as compared to 6 [Fe–Fe(av)
˚
2.520 A] and the diiron centre is more open as shown by the
In contrast, addition of a drop of HBF4 Æ Et2O to a
dichloromethane solution of 6 resulted in an immediate
colour change from orange to yellow with the clean and
quantitative formation of [(l-H)Fe2(CO)4(l-dcpm)(l-
pdt)][BF4] (11). The carbonyl region of the IR spectrum
displays three absorptions at 2047m, 2029vs, 1993s cmÀ1
the shift to higher wavenumbers of ca. 80 cmÀ1 confirming
the formation of a three-centre two-electron Fe2H unit.
The appearance of a singlet in the 31P NMR spectrum sug-
gests that the symmetrical nature of the diiron centre is
retained upon protonation and formation of the hydride
was confirmed with the observation of a low-field triplet
increasing bond angle between the basal carbonyl and trans
thiolate sulfur; C(1)–Fe(1)–S(2) 170.6(3)ꢀ in 11 as compared
to C(2)–Fe(1)–S(1) 152.3ꢀ and C(4)–Fe(2)–S(1) 162.9ꢀ in 6.
Three pairs of protonated–unprotonated pdt complexes
have previously been crystallographically characterised,
being derived from [Fe2(CO)4(PMe3)2(l-pdt)], [Fe2(CO)4-
(PMe2Ph)2(l-pdt)] and [Fe2(CO)4(j2-Ph2PCH2CH2PPh)2-
(l-pdt)] [4c,4d,7e,8a]. In all cases a lengthening of the
iron–iron vector is noted, but here protonation is also asso-
ciated with significant ligand rearrangements; for example,
diapically substituted [Fe2(CO)4(PMe2Ph)2(l-pdt)] rear-
ranges to dibasal [(l-H)Fe2(CO)4(PMe2Ph)2(l-pdt)][PF6]
[4d,8a]. Transformation of 6–11 occurs without significant
rearrangement of the diphosphine ligand and thus the coor-
dination geometry about the metal centre.
1
at Àd 14.54 (J 26.8 Hz) in the H NMR spectrum.
S
OC
CO
S
S
OC
CO
HBF4.Et2O
S
[BF4]
Fe
Fe
Fe
Fe
OC
Cy2P
CO
PCy2
OC
Cy2P
3. Conclusions
CO
PCy2
H
It is clear from the work described herein and from that
of others [7e,12,13] that reactions of [Fe2(CO)6(l-pdt)] (1)
with diphosphines can lead to a range of products the nat-
ure of which is dependent upon both the backbone flexibil-
ity of the diphosphine and the reaction conditions
employed. For diphosphines with a single methylene back-
bone (dppm and dcpm), symmetrically disubstituted cisoid
complexes are thermodynamically favoured. In separate
6
11
In order to confirm the formation of a bridging hydride
and to probe any structural changes occurring upon proton-
ation, single crystals of 11 grown from diethyl ether and
hexane were analysed by X-ray crystallography (Fig. 5).
Unfortunately, the overall quality of the X-ray data was

3964
F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
work we have shown that this complexity increases upon
lengthening the methylene backbone, such that for
1,3-bis(diphenylphosphino)propane (dppp) a range of co-
existing products including chelate, bridged and linked
binuclear complexes are available [18]. In contrast, for
the more rigid diphosphines (cis-dppv and dppb), the
asymmetrically disubstituted complexes are exclusively
formed, although reaction times can be long. An aim of
our research is to prepare such asymmetrically substituted
complexes [19] and investigate their reactivity towards cya-
nide and proton sources. Complexes 8 and 9 fulfil this
requirement as do complexes derived from the reaction of
1 amino-diphosphines, Ph2PN(R)PPh2 and these studies
will be reported in the near future.
orange blocks. For 2: IR m(CO) (CH2Cl2): 1988s, 1957vs,
1
1921s, 1908sh cmÀ1; H NMR (CDCl3, 298 K): d 7.80–
7.10 (m, br, 20H, Ph), 3.70 (br, 1H, PCH2), 3.21 (br, 1H,
PCH2), 2.17 (br, 4H, SCH2), 1.97 (br, 2H, CH2); 31P{1H}
NMR (CDCl3, 298 K): d 53.2 (s); (CD2Cl2, 213 K): 54.6
(d, J 54.6), 51.4 (d, J 54.6). Anal. Calc. for Fe2P2S2O4-
C32H28 Æ 0.5CH2Cl2: C, 51.55; H, 3.83. Found: C, 51.66;
H, 3.93%.
A toluene solution (80 cm3) of 1 (0.20 g, 0.52 mmol) and
dppm (0.26 g, 0.68 mmol) was refluxed for 5 h. Volatiles
were removed under reduced pressure, the crude reaction
mixture was dissolved in a minimum of dichloromethane
(ca. 5 cm3) and absorbed onto deactivated alumina. Chro-
matography on alumina in hexane gave unreacted 1 and
dppm. Eluting with Et2O:hexane 1:4 gave a red band which
afforded 0.08 g of a red solid. This was shown to be a mix-
ture of 3 and 4 (approximate ratio 1:4 by 31P NMR spec-
troscopy). Crystallisation of this mixture upon slow
diffusion of hexane into a diethyl ether solution afforded
a small number of red blocks of [Fe2(CO)4(j2-dppm)-
(l-pdt)] (3). Unfortunately, there were insufficient for ele-
mental analysis. Eluting with Et2O:hexane 1:1 gave an
orange band which afforded 2 (0.10 g, 27%). For 3: IR
4. Experimental
All reactions were carried out under a nitrogen atmo-
sphere in dried degassed solvents unless otherwise stated.
Phosphines were purchased from Aldrich and used as sup-
plied and [Fe2(CO)6(l-pdt)] was prepared as described in
the literature [3]. NMR spectra were run on a Bruker
AMX400 spectrometer and referenced internally to the
residual solvent peak (1H) or externally to P(OMe)3 (31P).
Infrared spectra were run on Nicolet 205 or Shimadzu
8700 FT-IR spectrometers in a solution cell fitted with cal-
cium fluoride plates, subtraction of the solvent absorptions
being achieved by computation. Fast atom bombardment
mass spectra were recorded on a VG ZAB-SE high resolu-
tion mass spectrometer and elemental analyses were per-
formed in house. Chromatography was carried out on
deactivated alumina support. The crude mixtures were dis-
solved in a small amount (ca. 5 ml) of dichloromethane,
absorbed onto alumina (ca. 10 g) and dried. This was then
added to the top of the wet chromatography column.
m(CO)(C6H14): 2023vs, 1952 s, 1917m cmÀ1 1H NMR
;
(CDCl3, 298 K): d 7.80–7.24 (m, br, 20H, Ph), 4.78 (br,
1H, PCH2), 4.22 (br, 1H, PCH2), 2.28 (br, 2H, SCH2),
2.16 (br, 2H, SCH2), 2.01 (br, 2H, CH2); 31P{1H} NMR
(CDCl3, 298 K): d 12.5 (s).
4.2. Reaction of 1 and dppm in MeCN: synthesis of
[Fe2(CO)5(j1-dppm)(l-pdt)] (4) and [Fe2(CO)5-
(j1-dppmO)(l-pdt)] (5)
To an MeCN solution (10 cm3) of 1 (0.10 g, 0.24 mmol)
and dppm (0.10 g, 0.26 mmol) at room temperature was
slowly added an MeCN solution (5 cm3) of Me3NO Æ 2H2O
(0.058 g, 0.52 mmol). This resulted in a colour change from
orange to red. IR m(CO)(CH2Cl2): 2044s, 1982br,
1928w cmÀ1. The mixture was stirred for 14 h after which
volatiles were removed under reduced pressure giving a
dark red solid. IR m(CO)(CH2Cl2): 2044s, 2017w, 1982br,
4.1. Thermolysis of 1 and dppm in toluene: synthesis
of [Fe2(CO)4(dppm)(l-pdt)] (2–3)
A toluene solution (80 cm3) of 1 (0.20 g, 0.52 mmol) and
dppm (0.26 g, 0.68 mmol) was refluxed for 16 h leading to a
colour change from orange to dark red. After cooling to
room temperature volatiles were removed by rotary evapo-
ration to give an oily red solid which was washed with
hexane (3 · 10 cm3) to remove a small amount of unreacted
1 and excess dppm. After drying an orange-red solid was
1926w, cmÀ1
.
31P NMR (CDCl3, 298 K): d À57.4 (d, J
84.5, 4), 55.7 (d, J 18.9, 5), 25.1 (s, dppmO2), 23.1 (d, J
18.9, 5), 12.5 (s, 3), À25.7 (d, J 84.5, 4). Ratio
3:4:5:dppmO2 ca. 1:10:3:1.5. In an attempt to convert 4
into 3 we carried out a similar reaction at 60 ꢀC. To an
MeCN solution (15 cm3) of 1 (0.25 g, 0.65 mmol) and
dppm (0.25 g, 0.65 mmol) was added an MeCN solution
(5 cm3) of Me3NO Æ 2H2O (0.058 g, 0.52 mmol). The
mixture was stirred at room temperature for 1 h and then
heated in a graphite bath maintained at 58 ꢀC for 14 h.
After cooling to room temperature, volatiles were removed
under reduced pressure to give a dark red solid. IR
m(CO)(CH2Cl2): 2043s, 1982br, 1959w, 1919w cmÀ1; IR
obtained shown to be
a mixture of isomers of
[Fe2(CO)4(l-dppm)(l-pdt)] (0.28 g, 76%). Alternatively,
the crude reaction mixture was dissolved in a minimum
of dichloromethane (ca. 5 cm3) and absorbed onto deacti-
vated alumina. Chromatography on alumina in hexane
gave an orange band eluting with hexane which gave a
small amount (0.03 g) of unreacted 1. Eluting with
Et2O:hexane 1:1 gave an orange band which afforded
[Fe2(CO)4(l-dppm)(l-pdt)] (2) as a bright orange solid
(0.21 g, 57%). Crystallisation from the slow diffusion of
methanol into a dichloromethane solution gave 2 as small
m(CO)(C6H14): 2048, 2042, 2023, 1986, 1966, 1952 cmÀ1
.
The crude reaction mixture was dissolved in a minimum
of dichloromethane (ca. 5 cm3) and absorbed onto deacti-

F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
3965
vated alumina. Chromatography on alumina in hexane
gave a yellow band eluting with hexane yielding a small
quantity of 1. Eluting with hexane:Et2O (9:1) gave a red
band which afforded [Fe2(CO)5(j1-dppm)(l-pdt)] (4) as a
red powder (0.29 g, 60%). Eluting with hexane:Et2O (4:1)
gave an orange band which afforded a small amount of a
dark orange solid (ca. 30 mg) shown to be a mixture of 4,
5 and 3 (ratio ca. 8:4:1) by 31P NMR spectroscopy. Eluting
with hexane:Et2O (7:3) gave a second orange band which
afforded 2 (0.08 g, 17%). Eluting CH2Cl2:Et2O (3:7) gave
an orange-brown band which afforded [Fe2(CO)5-
(j1-dppmO)(l-pdt)] (5) as a brown solid (0.12 g, 24%).
Pure 4 was formed upon slow addition of hexanes to a con-
centrated dichloromethane solution. For 4: IR m(CO)
small amount (0.02 g) of unreacted 1. Eluting with
Et2O:hexane 1:9 gave an orange band which afforded
[Fe2(CO)4(l-dcpm)(l-pdt)] (6) as a bright orange solid
(0.30 g, 84%). Crystallisation from dichloromethane–meth-
anol afforded large orange plates suitable for X-ray diffrac-
tion. For 6: IR m(CO) (CH2Cl2): 1975s, 1942vs, 1906s,
1
1889w cmÀ1; H NMR (CDCl3): d 2.49 (dt, J 13.9, 10.2,
1H, CH2), 2.53–1.23 (m, 45H, Cy + CH2); 31P{1H} NMR
(CDCl3):
d
61.6 (s). Anal. Calc. for Fe2P2S2O4-
C32H52 Æ 0.5CH2Cl2: C, 49.96; H, 6.80. Found: C, 49.72;
H, 6.80%.
4.4. Reaction of 1 and dcpm in MeCN: identification
of [Fe2(CO)5(j1-dcpm)(l-pdt)] (7)
1
(CH2Cl2): 2043m, 1981vs, 1961sh, 1924w cmÀ1; H NMR
(CDCl3, 298 K): d 7.62 (t, J 8.5, 4H, Ph), 7.29–7.20 (m,
16H, Ph), 3.30 (dd, J 8.9, 2.3, 2H, PCH2), 1.85 (br, 2H,
SCH2), 1.62 (br, 4H, SCH2); 1H NMR (d8-toluene,
373 K): d 7.61 (brt, J 5.8, 4H, Ph), 7.18 (brt, J 6.0, 4H,
Ph), 6.91 (brs, 12H, Ph), 3.29 (d, J 8.4, 2H PCH2P), 1.57
(quin, J 7.0, 2H, SCH2), 1.54 (quin, J 7.0, 2H, SCH2),
1.32 (quin, J 5.8, CH2); 13C{1H} NMR (CD2Cl2, 298 K):
d 215.21 (dd, J 14.0, 2.3, 2CO), 210.29 (s, 3CO), 138.70
(dd, J 15.5, 7.2), 136.40 (dd, J 35.6, 3.8), 133.20 (s),
133.13 (d, J 1.3), 133.03 (d, J 1.3), 132.99 (s), 130.52 (d, J
2.3), 129.00 (s), 128.75 (d, J 7.1), 128.56 (d, J 9.4), 33.61
(dd, J 31.3, 21.9, PCH2P), 30.72 (s, CH2), 22.86 (s,
2SCH2); 31P{1H} NMR (CDCl3, 298 K): d 57.7 (d, J
84.6, Fe–P), À26.1 (d, J 84.6, P); 31P{1H} NMR (CD2Cl2,
298 K): d 59.2 (d, J 86.3), À24.3 (d, J 86.3); 31P{1H} NMR
(CDCl3, 213 K): d 61.5 (br), 57.7 (br), À25.9 (br), À28.2
(br); 31P{1H} NMR (d8-toluene, 298 K): d 63.3 (d, J
89.7), À20.4 (d, J 89.7); 31P{1H} NMR (d8-toluene,
373 K): d 63.1 (d, J 78.1), À18.9 (d, J 78.1). Anal. Calc.
for Fe2P2S2O5C33H28 Æ 0.5CH2Cl2: C, 51.24; H, 3.70.
Found: C, 51.84; H, 4.02%. For 5: IR m(CO) (CH2Cl2):
To an MeCN solution (10 cm3) of 1 (0.10 g, 0.26 mmol)
and dcpm (0.107 g, 0.26 mmol) was added Me3NO Æ 2H2O
(0.076 g, 0.64 mmol) in MeCN (5 cm3) Me3NO Æ 2H2O
(0.076 g, 0.64 mmol) resulting in a colour change from
orange to dark red. The reaction was stirred at room tem-
perature for 14 h and volatiles removed under reduced
pressure to give a dark red solid. IR m(CO) (CH2Cl2):
2038s, 1980vs, 1961m cmÀ1 31P{1H} NMR (CDCl3): d
;
62.7 (d, J 23.8, 7), 49.5 (s, dcpmO2), À9.3 (d, J 23.8, 7).
Ratio 7: dcpmO2 ca. 2:1.
4.5. Synthesis of [Fe2(CO)4(j2-dppb)(l-pdt)] (8)
A toluene solution (80 cm3) of 1 (0.21 g, 0.54 mmol) and
dppb (0.244 g, 0.55 mmol) was heated at reflux for 2 d to
give a mud-green solution. After cooling to room tempera-
ture, the solvent was removed under vacuum and the
resulting solid washed with hexanes (3 · 25 cm3) giving a
dark brown powder. This was dissolved in a minimum
amount of dichloromethane and deactivated alumina (ca.
12 g) added. The solvent was removed under vacuum and
the resulting solid purified by column chromatography.
Eluting with hexane:Et2O (9:1) afforded an orange band
which gave [Fe2(CO)4(j2-dppb)(l-pdt)] (8) (0.113 g, 26%)
as a dry brown powder. A dark brown band then eluted
with hexane:Et2O (1:1) also yielded a further batch of 8
(0.115 g, 26%). Alternatively, to an MeCN solution
(20 cm3) of 1 (0.10 g, 0.26 mmol) and dppb (0.115 g,
0.26 mmol) at room temperature was added an MeCN
solution (15 cm3) of Me3NO Æ 2H2O (0.062 g, 0.56 mmol)
resulting in a colour change from orange to dark red.
The mixture was stirred at room temperature for 2 h and
then placed in a graphite bath maintained at 60 ꢀC for
14 h. After cooling to room temperature, volatiles were
removed under vacuum giving the crude product as a dark
red solid after washing with hexanes. This was then dis-
solved in dichloromethane, filtered under gravity and vola-
tiles removed to give 8 as a red-brown solid (0.14 g, 69%).
Recrystallisation upon slow diffusion of hexane into a
concentrated dichloromethane solution gave crystals suit-
able for X-ray crystallography. For 8: IR m(CO) (CH2Cl2):
1
2044m, 1982vs, 1961sh, 1928w cmÀ1; H NMR (CDCl3,
298 K): d 7.70 (t, J 8.7, 4H, Ph), 7.51 (t, J 8.7, 4H, Ph),
7.38 (d, J 6.4, 2H, Ph), 7.32–7.22 (m, 10H, Ph), 3.55 (dd,
J 10.1, 8.0, 2H, PCH2), 1.94,1.92,1.90 (brs, 2H, SCH2),
1.73,1.72,1.69,1.66,1.64 (brs, 4H, SCH2); 31P{1H} NMR
(CDCl3, 298 K): d 55.7 (d, J 18.8, Fe–P), 23.1 (d, J 18.8,
P@O). Anal. Calc. for Fe2P2S2O6C33H28 Æ 0.5C6H14: C,
53.93; H, 4.37. Found: C, 53.47; H, 4.35%.
4.3. Thermolysis of 1 and dcpm in toluene: synthesis
of [Fe2(CO)4(l-dcpm)(l-pdt)] (6)
A toluene solution (20 cm3) of 1 (0.20 g, 0.52 mmol) and
dcpm (0.20 g, 0.49 mmol) was refluxed for 16 h leading to a
colour change from orange to dark red. After cooling to
room temperature volatiles were removed by rotary evapo-
ration to give an oily red solid which was washed with hex-
ane (3 · 10 cm3) and dried. The crude reaction mixture was
dissolved in a minimum of dichloromethane (ca. 5 cm3)
and absorbed onto deactivated alumina. Chromatography
on alumina in hexane gave an orange band which gave a
2020s, 1949m, 1905w cmÀ1 31P NMR (CDCl3 298 K): d
;

3966
F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
88.1 (s), 80.5 (s) ratio ca. 10:1; 31P NMR (CD2Cl2 298 K): d
90.3 (s), 82.3 (s); 31P NMR (CD2Cl2 178 K): d 94.5 (s), 88.1
(s), 93.9 (s), 90.8 (s), ratio ca. 13:1; H NMR (CDCl3): d
4.7. Synthesis of [(l-H)Fe2(CO)4(dcpm)(l-pdt)][BF4]
(11)
1
7.76–7.01 (m, 24H, Ar), 1.99–1.96 (m, 2H, SCH2), 1.66,
1.63, 1.60 (brs, 4H, SCH2); mass spectrum (FAB) m/z
776 (M+, 1.05%), 748 (M+ÀCO, 0.30%), 720 (M+À2CO,
0.20%), 694 (M+À3CO, 0.70%), 664 (M+À4CO, 0.95%),
154 (100%). Anal. Calc. for Fe2P2S2O4C37H30 Æ 2CH2Cl2:
C, 49.47; H, 3.60. Found: C, 50.30; H, 3.52%.
To a dichloromethane solution (10 cm3) of 6 (50 mg,
0.068 mmol) was added one drop of HBF4 Æ Et2O. An
immediate lightening of the orange solution was apparent
and an IR spectrum showed the clean formation of [(l-H)-
Fe2(CO)4(l-dcpm)(l-pdt)][BF4] (11). The solution was
allowed to sit in air for 4 h and no further changes were
observed. Removal of volatiles gave an oily orange solid
which was dried and washed with diethyl ether (1 cm3). This
was quite strongly coloured and was thus layered with hex-
ane. Slow mixing of the two solvents lead to the formation
of crystals suitable for X-ray diffraction. The ether insoluble
material was washed with hexane and dried to afford 11 as a
pale orange solid (0.50 g, 89%). IR m(CO) (CH2Cl2): 2047m,
4.6. Synthesis of [Fe2(CO)4(j2-dppv)(l-pdt)] (9) and
[Fe2(CO)5(j1-dppvO)(l-pdt)] (10)
A toluene solution (50 cm3) of 1 (0.20 g, 0.53 mmol) and
cis-dppv (0.208 g, 0.52 mmol) was refluxed for 2 d to give a
dark red solution. After cooling to room temperature
volatiles were removed under reduced pressure and the
crude material washed with hexane (3 · 50 cm3) to give a
dark red powder consisting predominantly of [Fe2(CO)4-
(j2-dppv)(l-pdt)] (9) (0.26 g, 69%). Extraction into diethyl
ether followed by filtration and removal of volatiles gave a
purer product. Some of the desired product was extracted
with hexane and upon cooling was precipitated as a red-
brown solid. Crystallisation from slow diffusion of hexane
into a concentrated dichloromethane solution gave a
brown crystalline material suitable for analysis. Alterna-
tively, an MeCN solution (10 cm3) of Me3NO Æ 2H2O
(0.06 g, 0.54 mmol) was added to a stirred MeCN solution
(15 cm3) of 1 (0.104 g, 0.27 mmol) and cis-dppv (0.105 g,
0.264 mmol) over 10 min, resulting in an instant colour
change from orange to dark red. The reaction was stirred
at room temperature for 2 h and then at 60 ꢀC for 14 h.
After cooling to room temperature, volatiles were removed
under reduced pressure to give a dark red solid. This was
adsorbed onto deactivated alumina (ca. 5 g) and column
chromatography carried out. Eluting with hexane:Et2O
(3:2) gave a brown band which afforded 9 (0.057 g, 29%).
Eluting with hexane:Et2O (1:9) gave a pink band which
afforded [Fe2(CO)5(j1-dppvO)(l-pdt)] (10) (0.036 g, 18%)
as a pale red solid. For 9: IR m(CO) (CH2Cl2): 2020vs,
1
2029vs, 1993s cmÀ1; H NMR (CD2Cl2): d 4.78 (q, J 11,
1H, CH2), 3.68–1.15 (m, 45H, Cy + CH2), 14.54 (t, J
26.8, 1H, l-H); 31P{1H} NMR (CD2Cl2): d 70.9 (s).
4.8. X-ray data collection and solution
Single crystals were mounted on glass fibres and all geo-
metric and intensity data were taken from these samples
using a Bruker SMART APEX CCD diffractometer using
graphite-monochromated Mo Ka radiation (k = 0.71073
˚
A) at 150 2 K. Data reduction was carried out with
SAINT PLUS and absorption correction applied using the pro-
gramme SADABS. Structures were solved by direct methods
and developed using alternating cycles of least-squares
refinement and difference-Fourier synthesis. For 3 all
non-hydrogen atoms were refined anisotropically. The
structure of 2 contains one whole and two half diiron mol-
ecules. The central methylene carbon, C(6), of the pdt
group in the whole molecule was disordered (50:50) over
two sites. The structure also contained half molecules of
hexane and methanol. In all cases these atoms were refined
only isotropically. Hydrogens were generally placed in cal-
culated positions (riding model). The hydrogen atoms on
the solvate molecules and the pdt methylene carbons were
not included. Complex 6 contains four full diiron molecules
and two molecules of dichloromethane in the asymmetric
unit. All atoms were refined anisotropically. The crystal
of 11 was only weakly diffracting. The central methylene
carbon, C(4), of the pdt group was disordered (50:50) over
two sites and the fluorines of the BF4 were disordered over
three sites (45:45:10). A solvate diethylether was best mod-
eled at 25% site occupancy with atoms lying in a plane of
symmetry. The x and z coordinates were fixed for refine-
ment as two of the atoms, C(22) and C(23) kept drifting
away from the central oxygen. The hydrogen atoms on
the solvate molecule, the pdt methylene carbons and the
bridging hydride were not included. Structure solution used
SHELXTL PLUS V6.10 program package.
1950s, 1912w cmÀ1
;
31P NMR (CDCl3 298 K): d 96.0 (s),
1
82.1 (s) ratio ca. 10:1; H NMR (CDCl3): d 8.10–6.98 (m,
22H, Ar + CH@CH), 1.71 (brs, 2H, SCH2), 1.41 (brs,
2H, SCH2), 1.25 (brs, 2H, CH2); mass spectrum (FAB)
m/z 726 (M+, 15%), 176 (100%). Anal. Calc. for Fe2P2-
S2O4C33H28 Æ CH2Cl2: C, 50.31; H, 3.70. Found: C, 49.57;
H, 4.13%. For 10: IR m(CO) (CH2Cl2): 2043s, 1987vs,
1962s, 1937w cmÀ1 31P NMR (CDCl3 298 K): d 55.3 (d,
;
J 10.9, Fe–P, 10a), 54.5 (d, J 11.0, Fe–P, 10b), 17.5 (d, J
10.9, P@O, 10a), 17.3 (d, J 11.0, P@O, 10b) ratio 10a:10b
ca. 3:1; 1H NMR (CDCl3): d 7.80–6.98 (m, 22H,
Ar + CH@CH), 2.28 (brs, 1H, SCH2, 10a), 2.17 (brs, 1H,
SCH2, 10b), 1.75 (brs, 2H, SCH2, 10a), 2.28 (brs, 1H,
SCH2, 10a), 1.43 (brs, 4H, 10a and 10b); mass spectrum
(FAB) m/z 726 (M+, 15%), 176 (100%). Anal. Calc. for
Fe2P2S2O6C34H28: C, 52.98; H, 3.66. Found: C, 52.81; H,
4.44%.
Crystallographic data for 2[Fe2(CO)4(l-dppm)-
(l-pdt)] Æ 0.5C6H14 Æ 0.5MeOH (2). Orange block, dimen-
sions 0.18 · 0.13 · 0.05 mm, orthorhombic, space group

F.I. Adam et al. / Journal of Organometallic Chemistry 692 (2007) 3957–3968
3967
˚
˚
˚
Pnma, a = 16.1052(19) A, b = 40.759(5) A, c = 21.395(3) A,
Appendix A. Supplementary material
V = 14045(3) A , Z = 4, F(000) 6056, dcalc = 1.399 g cmÀ3
,
3
˚
l = 1.072 mmÀ1. 120128 reflections were collected, 17467
unique [Rint = 0.0992] of which 11245 were observed
[I > 2.0r(I)]. At convergence, R1 = 0.0687, wR2 = 0.1616
[I > 2.0r(I)] and R1 = 0.1148, wR2 = 0.1843 (all data), for
822 parameters.
CCDC 614428, 614429, 641156, 639867 and 641658 con-
tain the supplementary crystallographic data for 2, 3, 6, 8
and 11. These 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. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2007.05.050.
Crystallographic data for [Fe2(CO)4(j2-dppm)(l-pdt)]
(3). Red block, dimensions 0.14 · 0.13 · 0.11 mm, triclinic,
˚
˚
space group P1bar, a = 10.3556(13) A, b = 12.1406(15) A,
˚
c = 12.9298(16) A, a = 91.174(2)ꢀ, b = 98.599(2)ꢀ, c =
102.055(2)ꢀ, V = 1569.6(3) A , Z = 2, F(000) 732,
3
˚
dcalc = 1.511 g cmÀ3, l = 1.195 mmÀ1. 12622 reflections
were collected, 7042 unique [Rint = 0.0378] of which 5039
were observed [I > 2.0r(I)]. At convergence, R1 = 0.0543,
wR2 = 0.1253 [I > 2.0r(I)] and R1 = 0.0826, wR2 = 0.1387
(all data), for 379 parameters.
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10.1181(13) A, b = 10.9537(14) A, c = 17.301(2) A, b =
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˚
106.262(2)ꢀ, V = 1840.7(4) A , Z = 2, F(000) 880,
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´
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˚
˚
˚
a = 28.225(8) A, b = 14.318(4) A, c = 10.887(3) A, V =
4400(2) A , Z = 2, F(000) 6920, dcalc = 1.238 g cmÀ3
,
3
˚
l = 0.868 mmÀ1. 12742 reflections were collected, 4879
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2.0r(I)]. At convergence, R1 = 0.1150, wR2 = 0.2787 [I >
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Acknowledgements
˚
We thank Akkas Miah and Chao Quan for help in the
early stages of this work and The Nuffield Foundation
for Nuffield Science Bursaries to Schools and Colleges for
awards to both as part of the London Science Festival
2003 (Operation Michael Faraday). We thank Dr. Abil
Aliev for his help with the variable temperature NMR
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