Hydrogenase biomimetics: structural and spectroscopic studies on diphosphine-substituted derivatives of Fe2(CO)6(μ-edt) (edt?=?ethanedithiolate) and Fe2(CO)6(μ-tdt) (tdt?=?1,3-toluenedithiolate)

Article abstract of DOI:10.1007/s11243-016-0097-5

Reactions of a series of diphosphines with Fe₂(CO)₆(μ-edt) (1) and Fe₂(CO)₆(μ-tdt) (2) have been explored, the nature of the products being highly dependent upon the diphosphine. Reaction of 1 with bis(diphenylp

Full text of DOI:10.1007/s11243-016-0097-5

Transit Met Chem (2016) 41:933–942
DOI 10.1007/s11243-016-0097-5
Hydrogenase biomimetics: structural and spectroscopic studies
on diphosphine-substituted derivatives of Fe2(CO)6(l-edt)
(edt 5 ethanedithiolate) and Fe2(CO)6(l-tdt)
(tdt 5 1,3-toluenedithiolate)
Shahed Rana1 Shishir Ghosh1 Md. Kamal Hossain1 Ahibur Rahaman1
•
•
•
•
Graeme Hogarth2 Shariff E. Kabir1
•
Received: 4 August 2016 / Accepted: 22 September 2016 / Published online: 12 October 2016
Ó Springer International Publishing Switzerland 2016
Abstract Reactions of a series of diphosphines with
Fe2(CO)6(l-edt) (1) and Fe2(CO)6(l-tdt) (2) have been
explored, the nature of the products being highly dependent
Fe2(CO)5(j1-dppf)(l-tdt) (10), respectively, which are
structurally similar to 3. All seven complexes have been
characterized by a combination of analytical and spectro-
scopic data, together with single-crystal X-ray diffraction
analysis for 3, 7, 9 and 10.
upon the diphosphine. Reaction of
1
with
bis(diphenylphosphino)methane (dppm) in acetonitrile at
82 °C affords monosubstituted Fe2(CO)5(j1-dppm)(l-edt)
(3) in which the phosphine occupies an apical site and
disubstituted Fe2(CO)4(l-dppm)(l-edt) (4) in which the
phosphine is acting as a bridging ligand, while a similar
reaction of 2 with dppm yields only Fe2(CO)4(l-dppm)(l-
Introduction
Over the past two decades, there has been intense interest
in the synthesis and structures of biomimetics of the active
site (H-cluster) of the [FeFe]-hydrogenase enzyme (Fig. 1)
tdt) (5). In contrast,
1
and
2
react with 1,2-
[1–8] by far the majority of which contain
a
bis(diphenylphosphino)ethane (dppe) to form the chelate
complex Fe2(CO)4(j2-dppe)(l-edt) (7) and Fe2(CO)4(j2-
dppe)(l-tdt) (8), respectively; in both the diphosphine
binds in an apical–basal fashion to a single iron atom. A
second product of the dppe reaction with 1 is tetranuclear
[Fe2(CO)5(l-edt)]2(j1,j1-dppe) (6) in which the diphos-
phine again occupies apical sites. Similar reactions of 1 and
2 with 1,10-bis(diphenylphosphino)ferrocene (dppf) give
single products, namely Fe2(CO)5(j1-dppf)(l-edt) (9) and
propanedithiolate (pdt) bridge [9–26]. The choice of the
latter stems from the crystallographic work which shows
that in the enzyme the two bridging sulfur atoms are linked
by three other non-hydrogen atoms, and also the relative
ease of preparation of pdt complexes as opposed to related
adt (SCH2N(R)CH2S) species [4–8]. There are, however,
some negative aspects to using pdt-bridged biomimetics.
For example, in the solid state the central methylene group
is often disordered over two sites and this can result in
relatively low-quality X-ray crystallographic data [20, 27].
In solution, it is well established that the central methylene
unit is fluxional at room temperature, ‘‘flipping’’ with a low
activation barrier [27–29]. Monitoring mechanistically
important protonation reactions can be problematic due to
this ‘‘flipping’’, since at the low temperatures required to
observe intermediate hydride species, methylene fluxion-
ality becomes slow on the NMR timescale, leading to
either a broadening of all resonances or, when sufficiently
slow, a duplication of resonances resulting from the pres-
ence of isomers [20, 28, 29]. In our own work on [Fe2(-
CO)3(l-j1,j2-triphos)(l-pdt)] (triphos = (Ph2PCH2CH2)2-
PPh), we have encountered this problem first hand [20].
Thus, cooling to 183 K in CD2Cl2 in order to carry out a
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-016-0097-5) contains supplementary
material, which is available to authorized users.
& Shishir Ghosh
sghosh_006@yahoo.com
& Graeme Hogarth
graeme.hogarth@kcl.ac.uk
& Shariff E. Kabir
skabir_ju@yahoo.com
1
Department of Chemistry, Jahangirnagar University,
Savar, Dhaka 1342, Bangladesh
2
Department of Chemistry, King’s College London, Britannia
House, 7 Trinity Street, London SE1 1DB, UK
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Transit Met Chem (2016) 41:933–942
recorded on a Shimadzu FTIR 8101 spectrophotometer,
while the NMR spectra were recorded on a DPX 400
instrument. The chemical shifts were referenced to residual
1
solvent resonances or external 85 % H3PO4 in H and 31P
spectra, respectively. Elemental analyses were performed
in the Microanalytical Laboratories of Wazed Miah Sci-
ence Research Centre at Jahangirnagar University.
Preparative thin-layer chromatography was carried out on
1-mm plates prepared from silica gel GF254 (type 60, E.
Merck) at Jahangirnagar University.
Reaction of 1 with dppm
Fig. 1 Active site of [FeFe]-hydrogenase
Fe2(CO)6(l-edt) (1) (100 mg, 0.268 mmol), Me3NO
(21 mg, 0.280 mmol) and dppm (104 mg, 0.271 mmol)
were heated in boiling acetonitrile (15 mL) for 30 min.
After cooling to room temperature, the volatiles were
removed under reduced pressure and the residue chro-
matographed by TLC on silica gel. Elution with hexane/
CH2Cl2 (7:3, v/v) developed two bands. The faster-moving
band afforded Fe2(CO)5(j1-dppm)(l-edt) (3) (135 mg,
69 %) as red crystals, while the slower-moving band gave
Fe2(CO)4(l-dppm)(l-edt) (4) [23] (17 mg, 9 %) as red
crystals after recrystallization from hexane/CH2Cl2 at 4 °C.
Fe2(CO)5(j1-dppm)(l-edt) (3): Anal. (%): Calcd. for
C32H26Fe2O5P2S2: C, 52.77; H, 3.60 %. Found: C, 53.29;
H, 3.71 %. IR (mCO, CH2Cl2): 2046 s, 1984 s, 1959 sh,
protonation experiment results in the freezing out of five
isomers varying as a function of both phosphine and pdt
orientations. The 31P{1H} NMR spectrum at this tempera-
ture is extremely complex (consisting of 15 resonances each
of which shows at least one coupling), and consequently,
attempts to monitor the protonation at this temperature were
fruitless. One possible partial solution to this problem is to
exchange the pdt-bridge for a far more rigid ligand such as
ethane- (edt), benzene- (bdt) or toluene (tdt) dithiolate, thus
simplifying the system and allowing important information
to be more easily abstracted [30–33].
In comparison with the volume of work on pdt-bridged
diiron hydrogenase biomimics, relatively little attention
has been paid to related models containing an edt-bridge
[32–35]. Recently, we have synthesized two edt-bridged
diiron complexes Fe2(CO)4(j2-diamine)(l-edt) containing
diamine ligands and investigated their electrocatalytic
properties [35]. Experimental studies reveal that these edt-
bridged complexes behave differently to their pdt-ana-
logues, showing small but significant differences in their
rates of protonation, leading to different mechanisms for
proton reduction [35]. This prompted us to extend our
investigations on phosphine-substituted edt-bridged diiron
complexes and also to develop related toluene (tdt) dithi-
olate chemistry. Herein we report details of these, together
with structural studies in order to compare with the anal-
ogous pdt derivatives.
1931 w cm-1
.
1H NMR (CDCl3): d 7.53 (m, 4H),
7.31–7.18 (m, 16H), 3.25 (dd, J 1.7, 8.0, 2H), 1.89 (m, 2H),
1.39 (m, 2H). 31P{1H} NMR (CDCl3): d 56.7 (d, J 86, 1P),
-24.3 (d, J 86, 1P).
Fe2(CO)4(l-dppm)(l-edt) (4) [23]: Anal. (%): Calcd. for
C31H26Fe2O4P2S2: C, 53.17; H, 3.74 %. Found: C, 54.49;
H, 3.79 %. IR (mCO, CH2Cl2): 1991 m, 1959 s, 1924 m,
1906 w cm-1. 1H NMR (CDCl3): d 7.49 (m, 8H), 7.29 (m,
12H), 3.88 (m, 1H), 3.34 (m, 1H), 2.40 (br. s, 2H), 2.34 (br.
s, 2H). 31P{1H} NMR (CDCl3): d 57.8 (s).
Reaction of 2 with dppm
An acetonitrile solution (15 mL) of Fe2(CO)6(l-tdt) (2)
(100 mg, 0.230 mmol), Me3NO (17 mg, 0.230 mmol) and
dppm (89 mg, 0.232 mmol) was heated to reflux for
30 min. The volatiles were removed by rotary evaporation,
and the residue chromatographed by TLC on silica gel.
Elution with hexane/CH2Cl2 (7:3, v/v) developed only one
band which yielded Fe2(CO)4(l-dppm)(l-tdt) (5) (95 mg,
54 %) as red crystals after recrystallization from hexane/
CH2Cl2 at 4 °C.
Experimental
Materials and methods
All reactions were carried out under a nitrogen atmosphere
using standard Schlenk techniques unless otherwise stated.
Reagent-grade solvents were dried by the standard proce-
dures and were freshly distilled prior to use. Fe2(CO)6(l-
edt) (1) [36] and Fe2(CO)6(l-tdt) (2) [36] were synthesized
following published procedures. Infrared spectra were
Fe2(CO)4(l-dppm)(l-tdt) (5): Anal. (%): Calcd. for C36-
H28Fe2O4S2P2: C, 56.71; H, 3.70 %. Found: C, 57.25; H,
3.76 %. IR (mCO, CH2Cl2): 1994 s, 1964 s, 1930 s,
1911 w cm-1. 1H NMR (CDCl3): d 7.30 (m, 20H), 6.32 (d, J
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935
8.4, 1H), 6.94 (d, J 8.4, 1H), 7.51 (s, 1H), 3.72 (m, 1H), 3.49 (m,
1H), 2.05 (s, 3H). 31P{1H} NMR (CDCl3): d 56.9 (s).
was added Me3NO (21 mg, 0.280 mmol), and the mixture
was heated to reflux for 1 h. After cooling to room tem-
perature, the solvent was removed by rotary evaporation,
and the residue chromatographed by TLC on silica gel.
Elution with hexane/CH2Cl2 (1:1, v/v) developed one band
which yielded Fe2(CO)5(j1-dppf)(l-edt) (9) (145 mg,
60 %) as red crystals after recrystallization from hexane/
CH2Cl2 at 4 °C.
Reaction of 1 with dppe
A toluene solution (15 mL) of Fe2(CO)6(l-edt) (1)
(100 mg, 0.268 mmol), Me3NO (42 mg, 0.559 mmol) and
dppe (161 mg, 0.404 mmol) was heated to reflux for
30 min. The volatiles were removed under reduced pres-
sure, and the residue chromatographed by TLC on silica
gel. Elution with hexane/CH2Cl2 (7:3, v/v) developed two
bands. The faster-moving band afforded [Fe2(CO)5(l-
edt)]2(j1,j1-dppe) (6) [34] (27 mg, 19 %) as red crystals,
while slower-moving band yielded Fe2(CO)4(j2-dppe)(l-
edt) (7) (48 mg, 25 %) as green crystals after recrystal-
lization from hexane/CH2Cl2 at 4 °C.
Fe2(CO)5(j1-dppf)(l-edt) (9): Anal. (%): Calcd. for
C41H32Fe3O5P2S2: C, 54.82; H, 3.59 %. Found: C, 55.39;
H, 3.66 %. IR (mCO, CH2Cl2): 2046 s, 1984 s, 1931 w
1
cm-1. H NMR (CDCl3): d 7.55–7.14 (m, 20H), 6.45 (d, J
6.4, 1H), 6.14 (s, 1H), 6.03 (d, J 6.4, 1H), 4.52 (br. s, 1H),
4.39 (br. s, 1H), 4.35 (br. s, 1H), 4.29 (br. s, 1H), 4.21 (br.
s, 1H), 4.15 (br. s, 1H), 3.94 (br. s, 1H), 3.77 (br. s, 1H),
1.83 (s, 3H). 31P{1H} NMR (CDCl3): d 53.5 (s, 1P), -16.3
(s, 1P).
[Fe2(CO)5(l-edt)]2(j1,j1-dppe) (6) [34]: Anal. (%):
Calcd. for C40H32Fe4O10P2S4: C, 44.23; H, 2.97 %. Found:
C, 44.64; H, 3.03 %. IR (mCO, CH2Cl2): 2047 s, 1984 s,
Reaction of 2 with dppf
1
1929 w cm-1. H NMR (CDCl3): d 7.47–7.32 (m, 20H),
2.40 (m, 4H), 1.85 (m, 4H), 1.30 (m, 4H). 31P{1H} NMR
(CDCl3): d 58.4 (s).
An acetonitrile solution (15 mL) of Fe2(CO)6(l-tdt)
(100 mg, 0.230 mmol), Me3NO (17 mg, 0.230 mmol) and
dppf (128 mg, 0.231 mmol) was heated to reflux for 1 h.
The volatiles were removed by rotary evaporation, and the
residue chromatographed by TLC on silica gel. Elution
with hexane/CH2Cl2 (9:1, v/v) developed only one band
which yielded Fe2(CO)5(j1-dppf)(l-tdt) (10) (120 mg,
54 %) as red crystals after recrystallization from hexane/
CH2Cl2 at 4 °C.
Fe2(CO)4(j2-dppe)(l-edt) (7): Anal. (%): Calcd. for
C32H28Fe2O4P2S2: C, 53.80; H, 3.95 %. Found: C, 54.32; H,
4.03 %. IR (mCO, CH2Cl2): 2021 s, 1950 s, 1906 w cm-1. 1H
NMR (CDCl3): d 7.89 (m, 4H), 7.48 (m, 6H), 7.28–7.20 (m,
10H), 2.82 (m, 2H), 2.62 (m, 2H), 1.74 (m, 2H), 1.21 (m, 2H).
31P{1H} NMR (CDCl3): major isomer: d 92.9 (s). minor
isomer: d 76.3 (s). major/minor = 11:1.
Fe2(CO)5(j1-dppf)(l-tdt) (10): Anal. (%): Calcd. for
C46H34Fe3O5P2S2: C, 57.53; H, 3.57 %. Found: C, 58.05;
Reaction of 2 with dppe
H, 3.62 %. IR (mCO
,
CH2Cl2): 2049 s, 1989 s,
1
An acetonitrile solution (15 mL) of Fe2(CO)6(l-tdt) (2)
(100 mg, 0.230 mmol), Me3NO (17 mg, 0.230 mmol) and
dppe (92 mg, 0.231 mmol) was heated to reflux for 10 min.
The volatiles were removed by rotary evaporation, and the
residue chromatographed by TLC on silica gel. Elution with
hexane/CH2Cl2 (4:1, v/v) developed only one band which
yielded Fe2(CO)4(j2-dppe)(l-tdt) (8) (25 mg, 14 %) as red
crystals after recrystallization from hexane/CH2Cl2 at 4 °C.
Fe2(CO)4(j2-dppe)(l-tdt) (8): Anal. (%): Calcd. for
C37H30Fe2O4S2P2: C, 57.24; H, 3.90 %. Found: C, 57.97;
1932 m cm-1. H NMR (CDCl3): d 7.56-7.15 (m, 20H),
6.58 (d, J 8.4, 1H), 6.15 (s, 1H), 5.94 (d, J 8.4, 1H), 4.45
(br. s, 1H), 4.39 (br. s, 1H), 4.35 (br. s, 1H), 4.29 (br. s,
1H), 4.21 (br. s, 1H), 4.15 (br. s, 1H), 3.94 (br. s, 1H), 3.77
(br. s, 1H), 1.84 (3H, s). 31P{1H} NMR (CDCl3): d 53.5 (s,
1P), -16.3 (s, 1P).
X-ray crystallography
Single crystals of 3, 7, 9 and 10 suitable for X-ray
diffraction were grown by slow diffusion of hexane into a
dichloromethane solution at 4 °C. All geometric and
crystallographic data were collected at 150(2) K on a
Bruker SMART APEX CCD diffractometer using Mo-Ka
H, 3.98 %. IR (mCO
,
CH2Cl2): 2021 s, 1950 m,
1906 m cm-1. H NMR (CDCl3): d 8.03 (d, J 8.4, 1H),
7.97 (s, 1H), 7.81 (m, 4H), 7.49 (m, 6H), 7.28 (m, 2H), 7.14
(m, 8H), 6.96 (d, J 8.4, 1H), 2.82 (m, 2H), 2.62 (m, 2H),
2.37 (s, 3H). 31P{1H} NMR (CDCl3): major isomer: d 89.3
(s); minor isomer: d 34.3 (s). major/minor = 8:1.
1
˚
radiation (k = 0.71073 A) [37]. Data reduction and inte-
gration were carried out with SAINT? [38], and absorption
corrections were applied using the program SADABS [39].
The structures were solved by direct methods and refined
by full-matrix least squares on F2 [40, 41]. All non-hy-
drogen atoms were refined anisotropically. The hydrogen
atoms were placed in the calculated positions, and their
Reaction of 1 with dppf
To an acetonitrile solution (15 mL) of Fe2(CO)6(l-edt) (1)
(100 mg, 0.268 mmol) and dppf (149 mg, 0.269 mmol)
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Transit Met Chem (2016) 41:933–942
thermal parameters were linked to those of the atoms to
which they were attached (riding model). The SHELXTL
PLUS version 6.10 program package was used for structure
solution and refinement [41]. Final difference maps did not
show any residual electron density of stereochemical sig-
nificance. The details of the data collection and structure
refinement are given in Table 1.
Fe2(CO)5(j1-dppm)(l-edt) (3) and Fe2(CO)4(l-dppm)(l-
edt) (4) in 69 and 9 % yields, respectively, after chro-
matographic separation and workup (Scheme 1). In
contrast, a similar reaction between Fe2(CO)6(l-tdt) (2)
and dppm yielded only Fe2(CO)4(l-dppm)(l-tdt) (5) in
54 % yield (Scheme 2), indicating that the removal of
the second carbonyl from the molecule by phosphine is
relatively faster in tdt-complex 2 as compared to that in
edt-complex 1. Complex 4 has been previously reported
and structurally characterized by Kabir and Hogarth,
isolated from the reaction between 1 and dppm in boil-
ing toluene as the only product [23]. Here we have
characterized 3 and 5 by a combination of analytical and
spectroscopic data together with single-crystal X-ray
diffraction studies for 3.
Results and discussion
Reactions of 1 and 2 with dppm
Me3NO-initiated reaction between Fe2(CO)6(l-edt) (1)
and dppm in acetonitrile resulted in the isolation of
Table 1 Crystal data and structure refinement details for compounds (3), (7), (9) and (10)
Complex
3ÁCH2Cl2
7
9
10
Empirical formula
Formula weight
Temperature (K)
C33H28Cl2Fe2O5P2S2
813.21
C32H28Fe2O4P2S2
714.30
C41H32Fe3O5P2S2
898.28
C46H34Fe3O5P2S2
960.34
150(2)
150(2)
150(2)
150(2)
˚
Wavelength (A)
0.71073
0.71073
0.71073
0.71073
Triclinic
P-1
Crystal system
Space group
Triclinic
P-1
Monoclinic
P21/n
Monoclinic
P21/c
Unit cell dimensions
˚
a (A)
11.7308(10)
11.7655(10)
12.5708(10)
86.9620(10)
81.9960(10)
80.7460(10)
1694.9(2)
12.0601(12)
15.1211(16)
17.2738(18)
90
11.9306(15)
17.230(2)
13.2014(17)
13.7267(18)
14.0341(18)
112.962(2)
107.333(2)
103.637(2)
2048.2(5)
˚
b (A)
˚
c (A)
18.615(2)
a (°)
b (°)
c (°)
90
98.281(2)
102.550(3)
90
90
3
˚
Volume (A )
3117.2(6)
3735.2(8)
Z
2
4
4
2
Density (calculated) (Mg/m3)
Absorption coefficient (mm-1
F(000)
1.593
1.522
1.597
1.557
)
1.272
1.204
1.393
1.276
828
1464
1832
980
Crystal size (mm3)
h range for data collection (°)
Index ranges
0.14 9 0.12 9 0.03
2.44 to 28.29
-15 B h B 15,
-15 B k B 15,
-16 B l B 16
14,494
0.18 9 0.16 9 0.08
2.95–28.31
-15 B h B 15,
-19 B k B 20,
-23 B l B 22
26,144
0.04 9 0.03 9 0.01
2.20–28.28
-15 B h B 15,
-24 B k B 22,
-24 B l B 24
32,465
0.22 9 0.10 9 0.08
2.67–28.23
-17 B h B 17,
-17 B k B 17,
-18 B l B 18
16,950
Reflections collected
Independent reflections [Rint
Max. and min. transmission
Data/restraints/parameters
Goodness of fit on F2
]
7729 [Rint = 0.0261]
0.9628 and 0.8420
7729/0/415
1.115
7432 [Rint = 0.0377]
0.9099 and 0.8125
7432/0/379
1.000
8952 [Rint = 0.1279]
0.9862 and 0.9464
8952/0/478
1.127
9167 [Rint = 0.0301]
0.9048 and 0.7666
9167/0/523
1.029
Final R indices [I [ 2r(I)]
R1 = 0.0419,
wR2 = 0.1294
R1 = 0.0514,
wR2 = 0.1450
0.880 and -0.767
R1 = 0.0387,
wR2 = 0.0893
R1 = 0.0546,
wR2 = 0.0952
0.597 and -0.408
R1 = 0.0956,
wR2 = 0.1589
R1 = 0.1468,
wR2 = 0.1776
0.921 and -0.683
R1 = 0.0434,
wR2 = 0.0997
R1 = 0.0595,
wR2 = 0.1058
0.634 and -0.330
R indices (all data)
-3
)
˚
Largest diff. peak and hole (e A
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Transit Met Chem (2016) 41:933–942
937
Scheme 1 Reactions of Fe2(CO)6(l-edt) (1) with various diphosphines
Scheme 2 Reactions of Fe2(CO)6(l-edt) (2) with various diphosphines
123

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Transit Met Chem (2016) 41:933–942
The solid-state molecular structure of 3 is depicted in
displays two well-separated doublets at 56.7 and
Fig. 2 with the caption containing selected bond distances
and angles. The molecule contains a diiron framework
bridged by an edt ligand in addition to five terminal car-
bonyls and a dangling dppm ligand which completes the
coordination sphere of the molecule. The Fe–Fe bond
˚
distance of 2.5052(6) A in 3 is the same as that of the
˚
parent hexacarbonyl 1 [2.5032 (5) A] [42] (within experi-
-24.3 ppm (J 86 Hz) for the coordinated and non-coordi-
1
nated 31P nuclei, respectively. The H NMR spectrum is
not very informative, but it shows separate resonances for
the methylene moieties of dppm and edt ligands in addition
to phenyls proton resonances in the aromatic region.
Complex 5 can be readily characterized by its IR spectra
which show four absorption bands: three sharp and strong
absorptions at 1994, 1964 and 1930 cm-1, and a weak
absorption at 1911 cm-1. It shows a single resonance in the
31P{1H} NMR spectrum at 56.9 ppm since both the phos-
phorus atoms of the dppm ligand have an identical envi-
ronment in this molecule. In addition to resonances in the
aromatic region for the phenyl protons of the dppm ligand,
the 1H NMR spectrum of 5 also shows a multiplet at
3.49 ppm (integrated to two protons) and a singlet at
2.05 ppm (integrated to three protons) which are attributed
to the methylene and methyl protons of the dppm and tdt
ligands, respectively.
mental error). The Fe–S bond distances of 3 are also very
similar to those observed in 1 [42]. The dppm ligand is
apically bonded to a single iron atom using one phosphorus
atom, thus acting as a monodentate ligand, and the Fe–P
˚
bond distance of 2.2258(8) A in 3 is within the range
reported for related diiron-dithiolate complexes [19–29].
Almost a decade ago, Sun and co-workers reported the
crystal structure of the pdt and adt analogues of 3, namely
Fe2(CO)5(j1-dppm)(l-pdt) and Fe2(CO)5(j1-dppm)(l-
SCH2N(R)CH2S) (R = CH2CH2CH3) [43]. While in the
adt analogue the dppm is bound to an apical site akin to
that observed in 3, the diphosphine is found to be bonded to
a single iron through a basal coordination site in the pdt
complex [43]. The solution spectroscopic data of 3 also
indicate that the solid-state structure persists in solution.
The carbonyl region of the IR spectrum shows character-
istic absorptions pattern of Fe2(CO)5(phosphine)(l-dithio-
late) complexes, while the 31P{1H} NMR spectrum
Reactions of 1 and 2 with dppe
Similar reactions described as above between 1 and dppe
yielded the tetrairon [Fe2(CO)5(l-edt)]2(j1,j1-dppe) (6)
[34] and diiron Fe2(CO)4(j2-dppe)(l-edt) (7) in 19 and
25 % yields, respectively, whereas reaction between 2 and
Fig. 2 Solid-state molecular structure of Fe2(CO)5(j1-dppm)(l-edt)
(3). Ring hydrogen atoms are omitted for clarity. [Selected bond
distances (A) and angles (°): Fe(1)–Fe(2) 2.5052(6), Fe(1)–P(1)
2.2258(8), Fe(1)–S(1) 2.2491(8), Fe(1)–S(2) 2.2481(8), Fe(2)–S(1)
2.2518(8), Fe(2)–S(2) 2.2525(8); P(1)–Fe(1)–Fe(2) 153.67(3), P(1)–
Fe(1)–S(1) 106.31(3), P(1)–Fe(1)–S(2) 104.42(3), P(1)–Fe(1)–C(1)
97.40(10), P(1)–Fe(1)–C(2) 98.22(10)]
˚
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939
dppe afforded only the chelate analogue Fe2(CO)4(j2-
dppe)(l-tdt) (8) in 14 % yield (Schemes 1 and 2). We
could not isolate any complex similar to 3 from the reac-
tion between 1 and dppe in which the dppe ligand is
coordinated in monodentate fashion. The increased flexi-
bility due to the presence of an extra methylene group in
the backbone helps dppe to coordinate with a second diiron
unit, which is not possible for dppm. The tetrairon complex
6 has been previously reported and structurally character-
ized [34]. Here, we have characterized 7 and 8 by a com-
bination of analytical and spectroscopic data together with
X-ray diffraction analysis for 7. Both 7 and 8 show three
absorption bands in the carbonyl region of their IR spectra,
which is characteristic of Fe2(CO)4(j2-diphophine)(l-
1
dithiolate) complexes. The H NMR spectra of both com-
plexes are uninformative, showing resonances attributed to
the methylene protons of the dppe ligand in addition to
other proton resonances. Their 31P{1H} NMR spectra
indicate that both exist in two isomeric forms in solution
due to trigonal twist of the Fe(CO)(j2-dppe) unit com-
monly observed in this type of complexes (Fig. 3)
[23, 24, 29]. Both display a large singlet (92.9 ppm for 7
and 89.3 ppm for 8) together with a smaller singlet
(76.3 ppm for 7 and 34.3 ppm for 8) in their 31P{1H} NMR
spectra which are ascribed to the basal–apical and dibasal
isomers of 7 and 8, being formed in an approximate 11:1
and 8:1 ratio, respectively. Such isomerism has been pre-
viously observed in complexes of this type with exchange
between isomers being slow on the NMR timescale
[23, 24, 29]. The major isomer in solution for diphosphines
with flexible backbones such as dppe utilized in this study
which are able to span apical and basal sites is the basal–
apical isomer [24, 29]. Complex 7 is also found to adopt
basal–apical configuration in the solid state (see later)
which is in accord with this observation.
Fig. 4 Solid-state molecular structure of Fe2(CO)4(j2-dppe)(l-edt)
(7). Ring hydrogen atoms are omitted for clarity. [Selected bond
distances (A) and angles (°): Fe(1)–Fe(2) 2.5502(5), Fe(1)–P(1)
2.1822(6), Fe(1)–P(2) 2.2089(6), Fe(1)–S(1) 2.2445(6), Fe(1)–S(2)
2.2392(6), Fe(2)–S(1) 2.2530(6), Fe(2)–S(2) 2.2529(7); P(1)–Fe(1)–
Fe(2) 151.34(2), P(1)–Fe(1)–S(1) 103.62(2), P(1)–Fe(1)–S(2)
105.11(2), P(1)–Fe(1)–P(2) 86.91(2), P(1)–Fe(1)–C(1) 96.33(8)]
˚
coordination sites in the solid state with a bite angle of
86.91(2)° which is quite similar to 87.7(4)° observed in
Fe2(CO)4(j2-dppe)(l-pdt) [44]. The Fe–Fe and Fe–P bond
˚
distances in 7 [Fe(1)–Fe(2) 2.5502(5) A, Fe(1)–P(1)
˚
˚
2.1822(6) A and Fe(1)–P(2) 2.2089(6) A] are also quite
similar to those reported for its pdt analogue Fe2(CO)4(j2-
˚
dppe)(l-pdt) [44] [Fe(1)–Fe(2) 2.547(7) A, Fe(1)–P(1)
˚
2.190(1) A and Fe(1)–P(2) 2.231(1) A].
˚
The solid-state molecular structure of 7 is shown in
Fig. 4 with the caption containing selected bond distances
and angles. The molecule consists of a diiron core ligated
by four carbonyls, a dppe and a bridging edt ligand. The
molecule structure of 7 is very similar to that of its pdt
analogue, Fe2(CO)4(j2-dppe)(l-pdt) [44]. The diphosphine
chelates one iron, occupying the apical and one of the basal
Reactions of 1 and 2 with dppf
Recently, redox-active ligands containing a ferrocene
moiety have been successfully used in diiron biomimics as
a surrogate of the ferrodoxin (Fe4S4) cluster by several
groups [25, 45, 46], which prompted researchers to syn-
thesize and study more biomimetic models containing
redox-active ligands based on ferrocene [31, 47, 48]. A
couple of years ago, we examined the reaction of
Fe2(CO)6(l-pdt) with dppf in boiling toluene and observed
that the tetrairon complex [Fe2(CO)5(l-pdt)]2(j1,j1-dppf)
formed very quickly which converted into the diiron
complex Fe2(CO)4(l-dppf)(l-pdt) upon prolonged heating
at the same temperature [25]. Very recently, Kaur-Ghu-
maan et al. [31] studied the reactivity of the tdt-complex 2
with dppf and reported that room temperature reaction
between 2 and dppf (in 2:1 molar ratio) in the presence of
Ph2
P
S
S
S
S
CO
OC
OC
OC
Ph2
Fe
Fe
Fe
Fe
P
P
OC
Ph2
P
CO
OC
OC
Ph2
basal-apical
dibasal
Fig. 3 Basal–apical and dibasal isomers of Fe2(CO)4(j2-dppe)(l-
dithiolate) complexes
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Transit Met Chem (2016) 41:933–942
Fig. 5 Solid-state molecular structure of Fe2(CO)5(j1-dppf)(l-edt)
(9). Ring hydrogen atoms are omitted for clarity. [Selected bond
distances (A) and angles (°): Fe(1)–Fe(2) 2.5164(13), Fe(1)–P(1)
2.2348(19), Fe(1)–S(1) 2.2510(19), Fe(1)–S(2) 2.2540(19), Fe(2)–
S(1) 2.2439(19), Fe(2)–S(2) 2.2463(19); P(1)–Fe(1)–Fe(2) 152.59(6),
P(1)–Fe(1)–S(1) 105.32(7), P(1)–Fe(1)–S(2) 104.53(7), P(1)–Fe(1)–
C(1) 100.5(2), P(1)–Fe(1)–C(2) 94.9(2)]
˚
one equivalent of Me3NOÁ2H2O yielded three products: the
tetrairon complex [Fe2(CO)5(l-tdt)]2(j1,j1-dppf) and the
diron complexes Fe2(CO)5(j1-dppf)(l-tdt) (10) and
Fe2(CO)5(j1-dppfO)(l-tdt). The latter formed due to the
oxidation of dppf by Me3NOÁ2H2O [31]. Here, we have
investigated the reactivity of both 1 and 2 toward dppf
using slightly different reaction conditions and obtained a
single product in each case. Thus, the equimolar reaction
between 1 or 2 and dppf in the presence of one equivalent
of Me3NO in boiling acetonitrile only led to the formation
of Fe2(CO)5(j1-dppf)(l-edt) (9) (60 %) or Fe2(CO)5(j1-
dppf)(l-tdt) (10) [31] (54 %), respectively, after chro-
matographic separation and workup (Schemes 1 and 2).
Complex 10 has been previously characterized by analyt-
ical and spectroscopic data by Kaur-Ghumaan et al. [31].
Here we have performed single-crystal X-ray diffraction
analysis of both dppf-substituted complexes 9 and 10, the
results of which are shown in Figs. 5 and 6.
Both molecules contain a diiron core ligated by five
carbonyls, a dithiolate ligand and a dppf ligand. The Fe–Fe
˚
bond distance of 2.5164(13) A in 9 is slightly longer than
˚
that in 10 [2.4877(6) A] since the latter is bridged by
comparatively more rigid tdt ligand. The dppf ligand is
bonded to an iron through the apical coordination site in
j1-fashion, thus acting as a monodentate phosphine in both
molecules. The Fe–P bond distance of both molecules is
˚
identical within the experimental error [2.2348(19) A in 9
˚
and 2.2374(8) A] and is similar to those observed in
related dppf-substituted diiron-dithiolate complexes such
1
˚
as Fe2(CO)5(j -dppfO)(l-tdt) [2.2419(6) A] [31] and
˚
Fe2(CO)4(l-dppf)(l-pdt) [2.2468(6) A] [25]. In both, the
geometry around each iron can be best described as dis-
torted square pyramidal which is a common feature
observed in diiron-dithiolate complexes, and the Fe–S bond
distances observed in 9 and 10 are quite similar to those
found in related complexes [25, 31]. The solution
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Transit Met Chem (2016) 41:933–942
941
Fig. 6 Solid-state molecular structure of Fe2(CO)5(j1-dppf)(l-tdt)
(10). Ring hydrogen atoms are omitted for clarity. [Selected bond
distances (A) and angles (°): Fe(1)–Fe(2) 2.4877(6), Fe(1)–P(1)
2.2374(8), Fe(1)–S(1) 2.2854(9), Fe(1)–S(2) 2.2696(8), Fe(2)–S(1)
2.2812(9), Fe(2)–S(2) 2.2844(9); P(1)–Fe(1)–Fe(2) 152.96(3), P(1)–
Fe(1)–S(1) 103.82(3), P(1)–Fe(1)–S(2) 103.99(3), P(1)–Fe(1)–C(4)
98.98(10), P(1)–Fe(1)–C(5) 97.49(9)]
˚
spectroscopic data of both complexes are in accord with the
solid-state structure. The IR spectra of both show three
absorption bands in the carbonyl region characteristic of
Fe2(CO)5(phosphine)(l-dithiolate) complexes, while their
31P{1H} NMR spectra display two well-separated singlets
for the coordinated and non-coordinated phosphorus of the
dppf ligand.
Supplementary data
ORTEP diagrams of the solid-state molecular structures of
3, 7, 9 and 10 are given in Figs. S1–S4. Crystallographic
data for the structural analyses have been deposited with
the Cambridge Crystallographic Data Centre. CCDC
1497271, CCDC 1497272, CCDC 1497361 and CCDC
1497358 contain supplementary crystallographic data for
compound 3, 7, 9 and 10, respectively. These data can be
obtained free of charge from the Director, CCDC, 12
Union Road, Cambridge, CB2 1 EZ, UK (Fax: ?44-1223-
336033; Email: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.ac.uk).
Conclusions
The reactivity of two diiron-dithiolate complexes,
Fe2(CO)6(l-edt) (1) and Fe2(CO)6(l-tdt) (2), toward three
diphosphines (dppm, dppe and dppf) have been investi-
gated, which resulted in the isolation of a number of
products. This reveals that the nature of the products is
highly sensitive to the diphosphine used in the study. As we
know, the dppm ligand in which the donor phosphorus
atoms are separated by single CH2 unit prefers bridging
coordination mode, thus leading to the formation of prod-
ucts where the two iron atoms are bridged by the diphos-
phine. In contrast, the dppe in which there are two CH2
units between phosphorus atoms formed products where
the diphosphine chelates a single iron. The dppf ligand
shows only monodentate coordination mode in the present
study, since prolonged heating at high temperature is
usually required to bind both donor atoms of dppf with the
diiron core [25].
Acknowledgments This research has been partly sponsored by the
Ministry of Education, Government of the People’s Republic of
Bangladesh.
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