Synthesis, Molecular Structures and Electrochemical Investigations of [FeFe]-Hydrogenase Biomimics [Fe2(CO)6-n(EPh3)n(μ-edt)] (E = P, As, Sb; n = 1, 2)

Article abstract of DOI:10.1002/ejic.201900891

A series of ethane-dithiolate (edt = S(CH₂)₂S) complexes [Fe₂(CO)₅(EPh₃)(μ-edt)] and [Fe₂(CO)₄(EPh₃)₂(μ-edt)] (E = P, As, Sb), biomimics of the core of [FeF

Full text of DOI:10.1002/ejic.201900891

DOI: 10.1002/ejic.201900891
Full Paper
Hydrogenase Biomimics
Synthesis, Molecular Structures and Electrochemical
Investigations of [FeFe]-Hydrogenase Biomimics
[Fe2(CO)6-n(EPh3)n(μ-edt)] (E = P, As, Sb; n = 1, 2)
Shishir Ghosh,[a,b,c] Ahibur Rahaman,[b] Georgia Orton,[c] Gregory Gregori,[a] Martin Bernat,[a]
Ummey Kulsume,[b] Nathan Hollingsworth,[a] Katherine B. Holt,[a] Shariff E. Kabir,[b] and
Graeme Hogarth*[c]
Abstract: A series of ethane-dithiolate (edt = S(CH2)2S) com- of the substituents. With L = CO or SbPh3 potential inversion is
plexes [Fe2(CO)5(EPh3)(μ-edt)] and [Fe2(CO)4(EPh3)2(μ-edt)] seen leading to a two-electron reduction, while for others (L =
(E = P, As, Sb), biomimics of the core of [FeFe]-hydrogenases, PPh3, AsPh3) a quasi-reversible one-electron reduction is ob-
have been prepared and structurally characterised. The intro- served. Protonation studies reveal that [Fe2(CO)5(PPh3)(μ-edt)] is
duced ligand(s) occupies apical sites lying trans to the iron-iron only partially protonated by excess HBF4·Et2O, thus ruling com-
bond. NMR studies reveal that while in the mono-substituted plexes [Fe2(CO)5(EPh3)(μ-edt)(μ-H)]+ out as a catalytic intermedi-
complexes the Fe(CO)3 moiety undergoes facile trigonal rota- ates, but [Fe2(CO)4(PPh3)2(μ-edt)] reacts readily with HBF4·Et2O
tion, the Fe(CO)2(PPh3) centres do not rotate on the NMR time- to produce [Fe2(CO)4(PPh3)2(μ-edt)(μ-H)]+. While all new com-
scale. The reductive chemistry has been examined by cyclic vol- plexes are catalysts for the reduction of protons in MeCN, their
tammetry both in the presence and absence of CO and the poor stability and relatively high reduction potentials does not
observed behavior is found to be dependent upon the nature make them attractive in this respect.
Introduction
1, dimerisation is suppressed under a CO atmosphere and Evans
and co-workers noted a scan rate dependency on the number
of electrons taken up, ranging smoothly between 1–2, with
slower scan rates leading to potential inversion and the uptake
of two-electrons.[17,18] In contrast the pdt analogue, 1*, shows
no such potential inversion and is limited to the uptake of ca.
one-electron except at very slow scan rates.[19] Density func-
tional theory (DFT) calculations suggest that potential inversion
in 1 results from a series of transformations as shown in
Scheme 1 (with calculated potentials).[17] The initially generated
Since the structural elucidation of the active site of [FeFe]-
hydrogenases[1–2] biomimetics of this enzyme have attracted
enormous attention, with a bewildering array of variants being
prepared and studied as electrocatalysts for proton-reduc-
tion.[3–14] Since the enzymes contain a three-atom dithiolate
bridge, the vast majority of studies have focused on propane-
dithiolate (pdt = S(CH2)3S) and amine-substituted (adt =
SCH2N(R)CH2S) dithiolate complexes, while in contrast, catalytic
studies on the seemingly similar ethane-dithiolate (edt =
S(CH2)2S) complexes as proton-reduction catalysts are more lim-
ited.
While edt- and pdt-bridged complexes have broadly similar
chemical and physical properties, closer inspection reveals
some important differences. For example while [Fe2(CO)6-
(μ-edt)] (1) and [Fe2(CO)6(μ-pdt)] (1*) are reduced at a similar
potential to generate a 35-electron radical anion, secondary
transformations lead to the formation of tetrairon complexes
resulting from either CO loss (edt)[15] or CO transfer (pdt).[16] For
–
(at –1.76 V) anion 1A can undergo a facile trigonal rotation to
give the so-called rotated isomer, 1B–. The latter has a calcu-
lated reduction potential of –1.34 V, thus allowing addition of
a second electron (potential inversion) with formation of a
bridging carbonyl and concomitant cleavage of an iron–sulfur
bond to give 12–. Thus the key chemical process which results
in potential inversion is the trigonal rotation of an Fe(CO)3
group and for 1 an activation barrier of ca. 51 kJ mol–1 has been
measured by VT NMR studies.[20]
While both 1 and 1* can act as proton-reduction cata-
lysts,[21–23] they are not basic enough to bind a proton except
by very strong acids[24] and thus reduction is the primary cata-
lytic process. A common strategy in the development of func-
tional [FeFe]-hydrogenase biomimics is the substitution of one
or more carbonyl by stronger electron-donating ligands; in-
creasing the basicity of the diiron centre and making it more
susceptible to protonation. For triarylphosphanes, substitution
is favoured at an apical site (trans to the Fe–Fe vector) and,
[a] Department of Chemistry, University College London,
20 Gordon Street, London, WC1H 0AJ, UK
[b] Department of Chemistry, Jahangirnagar University,
Savar, Dhaka-1342, Bangladesh
[c] Department of Chemistry, King's College London,
Britannia House, 7 Trinity Street, London SE1 1DB, UK
E-mail: graeme.hogarth@kcl.ac.uk
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900891.
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respective ligands and Me3NO·2H2O. Complex 5 was independ-
ently reported while this work was in progress.[26]
All were isolated in good yields as air-stable red crystalline
solids and are readily characterised on the basis of their IR spec-
tra. For mono-substituted complexes, the highest frequency
ν(CO) band for the edt complexes is consistently 3 cm–1 higher
in energy than the analogous pdt complex, suggesting that the
edt is slightly less electron-donating than pdt. Within each sub-
group there is a small increase in the frequency of this band
upon descending the group; 2 2048 cm–1 < 3 2050 cm–1 < 4
2051 cm–1. For disubstituted complexes 5–7 the highest fre-
quency ν(CO) band was observed at 1999 cm–1 for all three
complexes. 1H NMR spectra of mono-substituted 2–4 show two
different methylene environments; for 2 these appear as a pair
of A2B2X multiplets centered at δ = 1.92 and 1.21 in CD2Cl2 at
room temperature. Lowering the temperature results in a slight
broadening of each resonance but at all temperatures there
was no evidence of a second isomer in solution. Similarly, the
31P{1H} NMR spectrum of 2 consists of a sharp singlet at all
accessible temperatures, appearing at 62.9 ppm in CD2Cl2 at
room temperature. This shows that in solution they consist of
a single isomer. For disubstituted 5–7 the methylene protons
Scheme 1. Calculated potential inversion upon reduction of 1.
while increasing the basicity of the diiron centre, it also sub-
stantially increases the energy barrier for trigonal rotation at
the substituted site, such that this process is no longer accessi-
ble. While there are many examples of phosphane-substituted
hydrogenase biomimics, in contrast related substitution chemis-
try of the heavier group 15 elements have not been studied,
the only related example we can find in the literature being
[Fe2(CO)5(AsPh3)(μ-SMe)2].[25] Herein we report the synthesis
and structural characterisation of a series of 1 derivatives
[Fe2(CO)5(EPh3)(μ-edt)] (2–4) and [Fe2(CO)4(EPh3)2(μ-edt)] (5–7)
(E = P, As, Sb) together with studies of their reductive chemistry
as followed by cyclic voltammetry under both CO and argon
atmospheres.
1
are equivalent in the H NMR spectrum and the 31P{1H} NMR
spectrum of [Fe2(CO)4(PPh3)2(μ-edt)] (5) consists of a singlet at
all accessible temperatures, being observed at 61.3 ppm at
room temperature in CDCl3.
Structural studies. Molecular structures of 2–4 and 5–7 are
shown in Figure 1–Figure 2 respectively and structural parame-
ters, together with those for 1[27,28] and 2*,[29] are summarised
in Table 1. The mono-substituted phosphane and arsane com-
plexes are isostructural (monoclinic P21/c), while the antimony
analogue crystallises in the triclinic P-1 group but is structurally
very similar. Disubstituted 5–7 are isostructural (monoclinic
P21/n). Iron-iron bond lengths are within expected limits
spanning a small range between 2.4741(4)-2.5107(4) Å, the
shortest in [Fe2(CO)4(SbPh3)2(μ-edt)] (7) and the longest
[Fe2(CO)5(PPh3)(μ-edt)] (2). This suggests there is no significant
trans-influence. For comparison the Fe–Fe bond length in the
two polymorphs of 1 are; P21/n 2.505(2) Å[30] and P-1 2.502(1)
Å (–80 °C) and 2.497(4) Å (25 °C).[27,28] All Fe–Fe distances are
slightly shorter than found in [Fe2(CO)4(PPh3)2(μ-pdt)] (5*) [Fe–
Fe 2.5167(16) Å][31] and this slight shortening appears to be a
Results and Discussion
Synthesis and characterisation. Mono-substituted [Fe2(CO)5-
L(μ-edt)] (2–4; L = PPh3, AsPh3, SbPh3) were prepared as shown
in Scheme 2. For the phosphane adduct 2 we found that simply
refluxing 1 and PPh3 afforded the desired product, but for EPh3
derivatives 3–4 a more effective preparation involved use of
Me3NO·2H2O in MeCN as a CO-removal agent. Disubstituted
[Fe2(CO)4L2(μ-edt)] (5–7; L = PPh3, AsPh3, SbPh3) were prepared
upon heating MeCN solutions of 1 with a five-fold excess of the
Scheme 2. Synthesis of 2–7.
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Figure 1. Molecular structures of [Fe2(CO)5(PPh3)(μ-edt)] (2), [Fe2(CO)5(AsPh3)(μ-edt)] (3) and [Fe2(CO)5(SbPh3)(μ-edt)] (4).
Figure 2. Molecular structures of [Fe2(CO)4(PPh3)2(μ-edt)] (5), [Fe2(CO)4(AsPh3)2(μ-edt)] (6) and [Fe2(CO)4(SbPh3)2(μ-edt)] (7).
general feature of edt- vs. pdt-bridged complexes, which Pickett characterised complexes of the type [Fe2(CO)5(PAr3)(μ-dithiol-
has related to the strength of the Fe–Fe bond.[32] Iron–sulfur ate)][31,33] and [Fe2(CO)4(PAr3)2(μ-dithiolate)][26,34] the phos-
bond lengths and angles are unexceptional and cover a small phane(s) occupy apical sites. In [Fe2(CO)4(PPh3)(PMe3)(μ-edt)][35]
range. Iron-element bond lengths as expected increase by ca. the PPh3 also occupies an apical position, but the PMe3 is in a
0.1 Å upon going down the group, and the Fe–As and Fe–Sb basal site.
bond lengths of ca. 2.33 and 2.47 Å respectively. The most nota-
Trigonal rotation fluxionality. An important aspect of
ble feature of all structures is the apical coordination of the the reduction chemistry of 1 proposed by Felton et al.
introduced ligand(s) which lie approximately trans to the iron- (Scheme 1)[17] is the trigonal rotation of an Fe(CO)3 moiety upon
iron bond [Fe–Fe–E 151.74(2)–158.10(2)°]. In crystallographically reduction which then allows formation of a bridging carbonyl.
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Table 1. Selected structural parameters for 1–7 and [Fe2(CO)5(PPh3) (μ-pdt)] (2*).
Compound
1[27]
2
3
4
5
6
7
2*[29]
Fe–Fe
Fe–L
2.502(1)
2.5107(4)
2.2382(6)
2.5010(8)
2.3316(7)
2.5034(4)
2.4727(3)
2.5074(5)
2.2427(6)
2.2264(6)
2.2643(6)
2.2594(6)
2.2617(6)
2.2590(6)
158.10(2)
154.05(2)
67.32(2)
2.4858(5)
2.3214(4)
2.3362(5)
2.2597(7)
2.2654(7)
2.2586(7)
2.2625(7)
153.43(2)
156.38(2)
66.64(2)
2.4741(4)
2.4722(4)
2.4688(3)
2.2598(6)
2.2570(6)
2.2636(6)
2.2640(6)
154.57(1)
153.17(1)
66.43(2)
2.5427(6)
2.2566(9)
Fe–S(1)
Fe–S(2)
Fe–Fe–L
2.340(1)
2.228(1)
2.245(1)
2.241(1)
2.2559(6)
2.2498(6)
2.2547(7)
2.2593(6)
154.58(2)
2.252(1)
2.259(1)
2.257(1)
2.252(1)
152.69(3)
2.2510(6)
2.2538(6)
2.2559(6)
2.2535(6)
151.74(2)
2.2700(9)
2.2598(9)
2.264(1)
2.263(1)
Fe–S(1)–Fe
Fe–S(2)–Fe
68.11(4)
67.80(4)
67.73(2)
67.59(2)
67.33(3)
67.38(3)
67.52(2)
67.44(2)
67.37(2)
66.71(2)
66.25(2)
It is not possible to probe this directly in the reduced form due HBF4·Et2O to a CH2Cl2 solution of 2 resulted in a slight lighten-
both to the limited stability of the anion and also its para- ing of the solution and the slow formation (over ca. 4 h) of a
magnetic nature. Consequently we probed the fluxionality of new product characterised by IR bands at 2110s, 2064s, 2053sh,
the Fe(CO)3 sub-unit in 2 making the assumption that reduction 2021m and 2018m cm–1. Reactions with 3 and 4 also led to the
would not significantly affect this (see below). For comparison, similar changes in the IR spectra, however, spectral changes
Darensbourg and co-workers have previously shown that were not so clean and some decomposition was apparent. Thus,
Fe(CO)3 trigonal rotation occurs in both 1 and 1* and is facile as all appear to behave in the same way, we focused later stud-
although free energies of activation vary significantly at ca. 51 ies on 2. Addition of ca. 5 equivalents of HBF4·Et2O to a CD2Cl2
and 35–36 kJ mol–1 respectively, as estimated from VT 13C{1H} solution of 2 resulted in a slight lightening of the red solution
NMR measurements.[36] The low-field region of the room tem- and both 1H and 31P{1H} NMR spectra showed the formation of
perature 13C{1H} NMR spectrum of 2 in CD2Cl2 shows a doublet a small amount (ca. 5 %) of a new complex characterised by a
at 215.2 ppm (JPC 13.5 Hz) attributed to the equivalent pair of high-field doublet at δ = –17.6 (JPH = 2.2 Hz) in accord with
basal carbonyls on the substituted iron centre and a broad sin- formation of the bridging hydride [Fe2(CO)5(PPh3)(μ-edt)(μ-H)]-
glet at 210.4 ppm for the Fe(CO)3 group, the carbonyls of which [BF4] (2H+ap). After a short time (ca. 5 min) a second hydride
are interconverting rapidly on the NMR timescale. Upon cooling resonance appeared at δ = –16.8 (JPH = 20.0 Hz) and after
to 233 K, no significant change was observed for the low-field 10 min the two hydride resonances were of approximately
carbonyl doublet, but the higher field signal split into two sepa- equal intensity. As coupling between apical phosphanes and
[32]
rate singlet resonances at 212.7 and 206.5 in a 2:1 ratio. From bridging hydrides is often small
and on the basis of the
these studies (Tc = 278 K, Δν = 625 Hz) we estimate a free larger JPH coupling constant we assign this second species to
energy of activation of 44.5 2 kJ mol–1 for the trigonal rota- 2H+ba (Scheme 3). After 4 h the ratio of the two hydrides was
tion in 2. This is in accord with the value obtained for 1 and still approximately equal but after longer periods (20 h) only
suggests that phosphane substitution, while completely chang- 2H+ba was present. Unfortunately, these later spectra were
ing the nature of the fluxionality at the substituted iron atom, broad, indicative of formation of either a paramagnetic species
has relatively little impact upon the trigonal rotation of the and/or decomposition. At all times the major resonance (ca.
Fe(CO)3 group. Related experiments with 3 and 4 gave free en- 90 %) in the 31P{1H} NMR spectrum was that of 2, but a later
ergies of activation of 46.3 2 and 44.5 2 kJ mol–1 showing times (20 h) a singlet at 5.9 ppm characteristic of free PPh3 was
that while Fe(CO)3 trigonal rotation in 2–4 is slightly lower in also observed. We rationalize these observations as shown in
energy than in 1, there is little effect upon changing the group Scheme 3. The Fe2 in 2 is not very basic and only at low pH
15 element. For disubstituted [Fe2(CO)4(PPh3)2(μ-edt)] (5), the
carbonyl region of the 13C{1H} NMR spectrum did not vary with
temperature showing that trigonal rotation of Fe(CO)2L groups
has a high energy barrier.
are there detectable amounts of cationic hydrides. The initially
formed 2H+ap rearranges via
a trigonal twist of the
Fe(CO)2(PPh3) moiety to give 2H+ba characterised by the rela-
tively large JPH value. Formation of two isomers of 2H+ was
unexpected as isomerisation requires a trigonal twist which we
Protonation studies. Extensive protonation studies have
been carried out on a range of hydrogenase biomimics[32,36–38] have shown is a high energy process. However it is not unprece-
with bridging hydrides almost always being observed. We have dented and we note that Rauchfuss has recently reported that
carried out a protonation studies on 2–7. Addition of excess protonation of [Fe2(CO)4(PPh3)2(μ-S2)] affords a mixture of iso-
Scheme 3. Protonation of 2 by HBF4·Et2O.
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mers with apical-apical and apical-basal arrangement of the tron transfer step. Figure 3 shows the normalised CVs for the
PPh3 ligands.[26b] Thus while the trigonal twist is slow on the four complexes in CO-saturated electrolyte at slow (20 mV s–1)
NMR timescale it is accessible on the synthesis timescale and and fast (5 V s–1) scan rates. Normalisation was carried out by
2H+ must be the thermodynamic product. Nevertheless, for dividing currents by the square root of the scan rate (Table 2).
ba
the purposes of electrocatalytic studies (see below), at all acid
concentrations only small amounts of [Fe2(CO)5(EPh3)(μ-H)-
(μ-edt)]+ will be present, thus the predominant electrocatalytic
process must be initiated by reduction.
Addition of a slight excess of HBF4·Et2O to a CH2Cl2 solution
of 5 resulted in the slow formation of a new species character-
ised by ν(CO) resonances at 2060vs, 2040m and 2000s cm–1; the
shift of ca. 60 cm–1 to higher wavenumbers being consistent
with formation of [Fe2(CO)4(PPh3)2(μ-H)(μ-edt)][BF4] (5H+). Re-
lated experiments with 6–7 showed similar changes and solu-
tions of these cations were relatively stable over a period of
1
hours. Low temperature H NMR studies (–20 °C) showed that
addition of HBF4·Et2O to a solution of 5 in CD2Cl2 resulted in
appearance of a singlet at δ = –17.1 ppm. Pickett and co-work-
ers have studied the protonation chemistry of [Fe2(CO)4(PMe3)2-
(μ-edt)] (8) and [Fe2(CO)4(PMe3)2(μ-pdt)] (8*) in detail.[32] Both
exist as a mixture of apical-apical and apical-basal isomers. They
found that the rate of protonation of 8 was ca. 10 times slower
than that of 8* and also that protonation of 8 was more compli-
cated than that of 8*, affording four spectroscopically charac-
terised isomers of [Fe2(CO)4(PMe3)2(μ-edt)(μ-H)]+ (8H+). In only
one of these, the apical-apical isomer, does the hydride appear
as a singlet. This isomer is present immediately after protona-
Figure 3. Normalised CVs at 20 mV s–1 (black) and 5 V s–1 (grey) for 0.5 m
M
tion being generated from apical-apical 8 and on this basis the
product of protonation of 5 is characterised as the apical-apical
isomer [Fe2(CO)4(PPh3)2(μ-H)(μ-edt)][BF4] (5H+) (Scheme 4). Un-
like the protonation of 2, we did not observe any further
changes upon standing. This is likely due to the high rotational
barrier of the Fe(CO)2(PPh3) groups which precludes secondary
rearrangements observed for the PMe3 analogue 8. The basicity
of the diiron centers in 5 and 8 can be measured (to some
extent) by the relative positions of their ν(CO) bands in the IR
spectrum; the highest frequency bands appearing at 1999 and
1982 cm–1 respectively, highlighting the significantly more basic
nature of the PMe3-substituted center. Thus, in the presence of
strong acids the major species present in solution for these
disubstituted complexes is the protonated apical-apical isomer.
complex in CO-saturated 0.1
M TBAPF6/MeCN: a) 1; b) 2; c) 3 and d) 4. Vertical
double headed arrows indicate normalised current of 20 μA V–1/2 s1/2 for each
set of CVs.
Figure 3 a shows similar behavior for 1 as noted previously
by Best[15] and Evans.[17] At all scan rates reduction takes place
at –1.75 V vs. Fc/Fc+ however normalised peak current varies
from 8.5 × 10–5 A V–1/2 s1/2 at 20 mV s–1, consistent with a two
electron reduction, to 4 × 10–5 A V–1/2 s1/2 at 5 V s–1, which is
tending towards a one electron reduction. Reduction appears
reversible at slow scan rates, with an oxidation peak at –1.63 V
and iox/ired = 0.8. At the high scan rate there are two oxidation
peaks, one at –1.63 V and the other at –1.2 V. These results are
consistent with a potential inversion mechanism, as described
by Felton et al.[17] In a CO atmosphere at slow scan rate, the
trigonal rotation described in Scheme 1 takes place on the elec-
trochemical timescale, allowing uptake of a second electron at
the same or less negative potential. At faster scan rates there is
insufficient time for the structural change to take place, hence
the reduction remains a one-electron process, as indicated by
the smaller normalised reduction current. The one-electron re-
duction appears to be somewhat reversible, as indicated by the
oxidation peak at –1.63 V. However, the magnitude of this peak
is smaller than the reduction peak and taken with the addi-
tional oxidation peak at –1.2 V suggests some lack of stability
of the anion.
Scheme 4. Protonation of 5 with HBF4.
Electrochemistry. Cyclic voltammetry (CV) of 1–4 were car-
ried out in CO or argon-saturated MeCN/0.1
M TBAPF6 solutions
over scan rates 10 mV s–1 to 10 V s–1. The product of the one
electron reduction in each case is the 35 electron anion, from
which rapid loss of CO results in a more stable 33-electron
Figure 3b-d show the effect of substitution on the redox
product. Hence in argon-saturated solution the reduction of all properties of 1–4. The potential at which the complex under-
complexes is irreversible over all scan rates (Fig. S1). In CO- goes reduction is influenced by the substituting ligand, with
saturated electrolyte CO ligand loss is suppressed, allowing for the more σ-donating PPh3 resulting in Ered at –1.9 V (at 5 V s–1)
study of further reduction of the anion formed in the first elec- for 2.[39] In contrast reduction takes place at less negative po-
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Table 2. Peak reduction potentials and normalised peak reduction currents for 2–4 under a CO atmosphere at different scan rates.
2
3
4
1
v
Epa(1)
–1.77
–1.94
–1.88
–1.88
–1.87
–1.86
–1.91
–1.91
–1.90
–2.04
–2.00
ipa(1) × 10–5
3.935
3.260
2.492
3.264
3.585
3.332
3.247
3.404
v
Epa(1)
–1.64
–1.65
–1.65
–1.64
–1.66
–1.76
–1.74
–1.78
–1.83
–1.82
–1.89
ipa(1) × 10–5
2.474
2.384
2.296
2.209
2.050
2.583
2.741
2.645
v
Epa(1)
–1.62
–1.62
–1.63
–1.65
–1.65
–1.66
–1.65
–1.65
–1.67
–1.71
–1.72
ipa(1) × 10–5
6.100
5.669
4.695
–4.767
4.813
3.936
3.509
3.120
2.721
v
Epa(1)
–1.70
–1.79
–1.81
–1.75
–1.76
–1.74
–1.75
–1.81
–1.78
–1.84
–1.84
ipa(1) × 10–5
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
9.3625
8.099
7.227
7.449
6.236
5.269
4.583
4.151
4.132
3.675
3.433
3.316
2.317
2.474
2.337
2.872
2.522
2.500
2.489
tentials for 3 and 4 (ca. –1.75 V at 5 V s–1). The SbPh3 complex tron-density on the diiron centre (Figs. S4–5). Thus, successive
(Figure 3d) is the only one which shows a similar response to 1 replacement of CO for PPh3 leads to a decrease in oxidation
and exhibits potential inversion. At slow scan rates a reversible potential by ca. 0.4 V per substitution, with similar reductions
electron transfer is observed, with normalised peak currents being observed for the heavier homologues. Unexpectedly for
tending towards a two electron process, while at faster rates 2–4, oxidation decreased slightly with increasing molecular
a one-electron reduction followed by two oxidation peaks at weight, while for 5–7 oxidation potentials are (within experi-
different potentials is noted, exactly as observed for 1. In con- mental error) the same. The stability of the oxidation product
trast, reduction of 2–3 (Figure 3b, Figure 3c) is mainly irreversi- also varied significantly. Thus, while oxidation of both 1 and 2
ble and the normalised reduction current remains constant over is totally irreversible, oxidation of 5 shows good reversibility in
all scan rates. This suggests a different reduction mechanism is both argon and CO. Oxidation of 6–7 was again irreversible,
taking place and that there is no potential inversion for these and in each case a second smaller oxidation wave at ca. + 0.25 V
complexes. In both cases the reduction peak consists of two was observed.
overlapping responses and the normalised peak current is con-
sistent with a process of between one and two electrons. The
overlapping response suggests an initial one-electron reduction
is followed by further reduction of the anion or a decomposi-
tion product at more negative potentials. The difference be-
tween this mechanism and the potential inversion mechanism
is that any structural change taking place upon the first reduc-
tion step does not lead to facilitated uptake of a second elec-
tron at the same or less negative potential. All of the complexes
exhibit an oxidation peak at a similar potential at fast scan rates
(1, 3 and 4 at –1.2 V; 2 at –1.4 V) suggesting it can be attributed
to oxidation of a similar species in each case, probably a prod-
uct of the structural rearrangement or decomposition of the
anion. Exhaustive electrolysis of edt complexes is reported to
result in dimer formation,[15] hence the oxidation peak may be
attributed to such a species. The more negative potential for
the oxidation peak in the case of 2 suggests the responsible
species is PPh3 substituted.
The electrochemical reduction response of the disubstituted
complexes 5–7 were also investigated under argon and CO at-
mospheres. All showed a single reduction peak at more nega-
tive potentials than their mono-substituted counterparts: 5
–2.18 V; 6 –1.92 V and 7 –1.79 V. Normalised peak currents for
their reduction was found to be independent of scan rate in all
cases, indicating that potential inversion did not take place. The
absence of potential inversion for 7 (Fig. S2) as compared to 4
in a CO atmosphere demonstrates that in 4 the rotation of the
Fe(CO)3 unit rather than FeSb(CO)2 results in the structural rear-
rangement, consistent with the NMR results and calculations.
Proton-reduction catalysis. Both 1 and 1* have been shown
to catalyse the reduction of protons to hydrogen at their first
reduction potential and mechanisms have been probed.[40–42]
Chiang and co-workers have prepared [Fe2(CO)6(μ-H)(μ-edt)]+
(1H+) from reaction of [H(SiEt3)2][B(C6F5)4] with 1, and from 1H+
generated and spectroscopically characterised catalytic interme-
diates [Fe2(CO)6(μ-H)(μ-edt)](1H), [Fe2(CO)5(μ-CO)(μ-H)(μ-edt)]–
(1H–), [Fe2(CO)5(μ-CO)(μ-H)(μ-edtH)] (1–2HA) and [HFe2(CO)5-
(μ-CO)(μ-H)(μ-edtH)] (1–3HA+). On the basis of these observa-
tions and detailed DFT calculations[40–42] it has proved possible
to elucidate likely mechanistic pathways,[42]
a (somewhat
abridged) mechanistic scheme for 1 being shown in Scheme 5.
Reduction of 1 occurs at ca. –1.76 V and the generated anion
–
1A is readily protonated to give 1H. This in turn is readily re-
duced at a potential (–1.57 V) lower than that of the reduction
potential of 1 to afford 1H+. The second reduction leads to
major structural changes including scission of an Fe–S bond
and trigonal rotation about one Fe(CO)3 moiety leading to for-
mation of a bridging carbonyl. The generated thiolate anion is
basic and readily protonates to give 1–2HA which contains hy-
dridic and acidic protons. This cannot, however, eliminate
hydrogen as the spatial distribution of the two protons is not
optimal and a series of rearrangements proceeding via inver-
sion at sulfur and hydride migration to a terminal position gen-
erate isomer 1–2HB from which a feasible route to hydrogen
elimination is possible and 1 is regenerated. A second pathway
is also accessible resulting from the further protonation of 1–
+
2HA to yield 1–3HA which after isomerisation and hydrogen
loss affords 1-H+. In all but the strongest acids the equilibrium
Complexes 2–7 all undergo oxidation (in MeCN), the poten- between this cationic hydride and 1 will lie towards the latter,
tial of which changes substantially upon increasing the elec- thus regenerating it for further catalytic cycles.
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Scheme 5. Proposed proton-reduction processes for 1 (for all complexes apart from 1 the Fe–Fe interaction is given as a dashed line as little is understood
about its precise nature).
Complexes 2–7 all facilitate catalytic proton-reduction in and this has allowed a comparison of their chemical and elec-
MeCN when HBF4·Et2O is used as the proton source (Figure S3). trochemical properties. In all the EPh3 ligands lie in apical sites,
For 2–4 proton-reduction occurs at the first reduction potential trans to the Fe–Fe bond, a common feature in complexes of
consistent with initial formation of a 35-electron anion, which
presumably protonates to afford 2–4H analogous to 1H
(Scheme 5). Following this, further steps cannot be unambigu-
ously determined but given that the diiron centre in 2–4 is not
very basic and each has an Fe(CO)3 moiety able to undergo a
trigonal rotation then a similar process to that proposed for 1
seems reasonable. For 5 there are clearly two potentials at
which H2 is generated, the first reduction potential of the com-
plex at ca. –2.1 V and a second smaller wave at ca. –1.7 V which
we associate with the one electron reduction of 5H+. Thus, both
channel through the putative 35-electron complex 5H. A major
difference between this and 1–4H is that lack of a rotatable
Fe(CO)3 unit and hence a similar process to that discerned for
1 seems unlikely. This may also explain the instability of 5 in
the catalytic system as the electrode was rapidly deposited with
an insoluble film resulting from complex decomposition.
For 6–7 voltammograms are of poor quality and little can be
discerned regarding the catalytic process except to say that
there is some evidence that they follow a similar process to 5
but reduced intermediates appear to be even less stable. Thus,
overall the proton-reduction behaviour of 2–7 is disappointing
and given that it occurs at a higher potential than for 1 they
are sadly not worth further study in this respect.
this type. Substitution significantly alters the barrier to trigonal
rotation of the Fe(CO)2(EPh3) moiety, such that at accessible
temperatures the EPh3 ligand is also fixed in an apical site in
solution, although the trigonal rotation of the Fe(CO)3 units in
2–4 are relatively unperturbed. This likely has important impli-
cations to their electrochemistry since a key feature of the re-
ductive chemistry of [Fe2(CO)6(μ-edt)] (1) is trigonal rotation
upon one-electron reduction leading to a new isomer with a
bridging carbonyl that can undergo potential inversion. For the
disubstituted complexes 5–7 no evidence of potential inversion
was found, consistent with the inability of the respective 35-
electron anions to undergo trigonal rotation. For mono-substi-
tuted complexes 2–3 (L = PPh3, AsPh3) a quasi-reversible one-
electron reduction is observed and we could find no evidence
of potential inversion. For 4 (L = SbPh3) potential inversion was
seen leading to a two-electron reduction. At this point we are
unable to unambiguously determine whether potential inver-
sion results from a similar process as calculated for 1
(Scheme 1)[17] or whether it results from loss of the relatively
weakly bound SbPh3. While 2–7 are all catalytic for the reduc-
tion of HBF4·Et2O in MeCN, their performance as compared to
many other [FeFe]-hydrogenase biomimics is poor. At the outset
of this work we were somewhat surprised, given the vast
amount of research on diiron dithiolate complexes carried out
over the past five decades,[43] that complexes with heavy group
15 substituents were virtually unexplored[26] and the ease of
synthesis and good stability of AsPh3 and SbPh3 derivatives of
Conclusions
We have successfully prepared and crystallographically charac-
terised a series of ethanedithiolate complexes [Fe2(CO)5(EPh3)- 1 suggests that this may still be an area which should be exam-
(μ-edt)] (2–4) and [Fe2(CO)4(EPh3)2(μ-edt)] (5–7) (E = P, As, Sb) ined in further depth.
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mother solution. IR (υCO, CH2Cl2): ν = 1999vs, 1952m, 1935s cm–1
;
Experimental Section
˜
1H NMR (CDCl3): δ = 7.56 (m, 12H), 7.37 (m, 18H), 0.62 (s, 4H); 31P{1H}
NMR (CDCl3): 61.3 (s) ppm; Anal. calcd. for Fe2S2P2O4C42H34: C,
59.68, H, 3.98; Found C, 60.02, H, 4.08.
General considerations. Unless otherwise noted, all the reactions
were carried out under a nitrogen atmosphere using standard
Schlenk techniques. Reagent-grade solvents were dried using
appropriate drying agents and distilled prior to use by standard
methods. Infrared spectra were recorded on a Shimadzu FTIR 8101
spectrophotometer. NMR spectra were recorded on a Bruker DPX
400 instrument. Mass spectra were recorded on a Varian Mat 312
mass spectrometer. Elemental analyses were performed by Micro-
analytical Laboratories, University College London. The diiron com-
plex [Fe2(CO)6(μ-edt)] (1)[27] was prepared by literature methods.
Synthesis of [Fe2(CO)4(AsPh3)2(μ-edt)] (6): An MeCN solution
(25 mL) of 1 (50 mg, 0.134 mmol), AsPh3 (164 mg, 0.536 mmol) and
Me3NO·2H2O (58 mg, 0.53 mmol) was vigorously stirred and re-
fluxed for 4 h. The solvent was removed and the solid was dried
and then redissolved in a minimum amount of CH2Cl2 and ab-
sorbed onto silica. Flash column chromatography afforded a trace
of 1 with hexanes. Elution with diethyl ether afforded two bands
the second being collected and identified as 6. Recrystallization
Synthesis of [Fe2(CO)5(PPh3)(μ-edt)] (2): PPh3 (0.702 g, 2.68 mmol)
was added to a toluene solution (20 mL) of 1 (0.50 g, 1.34 mmol)
and the mixture was refluxed for 5 h. The resulting solution was
cooled and the solvent removed via rotary evaporation giving a
rust powder which was washed with hexanes and dried. The com-
ponents of the product mixture were then separated using column
chromatography with silica gel using a 1:3 mixture of CH2Cl2/hex-
ane as eluent. Three bands were present, the product being the
second red band. It was recrystallised upon dissolving in CH2Cl2 and
layering with hexanes, which when left overnight, afforded dark-
red diamond shaped crystals of [Fe2(CO)5(PPh3)(μ-edt)] (2) (182 mg,
from CH2Cl2 and hexanes afforded dark-red platelet crystals of 6
1
(87 mg, 70 %). IR (υCO, CH2Cl2): ν = 1999vs, 1954m, 1935s cm–1; H
˜
NMR (CDCl3): δ = 7.53 (m, 12H), 7.38 (m, 18H), 0.90 (s, 4H); 13C{1H}
NMR (CDCl3): δ = 215.4, 137.4, 133.8, 132.7, 129.7, 128.9, 128.7,
128.5, 33.3; Anal. calcd. for Fe2S2As2O4C42H34: C, 53.48, H, 3.53;
Found C, 54.34, H, 3.69.
Synthesis of [Fe2(CO)4(SbPh3)2(μ-edt)] (7): An MeCN solution
(40 mL) of 1 (100 mg, 0.27 mmol), SbPh3 (379 mg, 1.076 mmol) and
Me3NO·2H2O (119 mg, 1.07 mmol) was stirred and refluxed for 4 h.
Work-up and chromatography as described above gave 7 as a red
59 %). IR (ν(CO); CH2Cl2): ν = 2048vs, 1988vs, 1935m cm–1
.
31P{1H}
˜
crystalline solid (201 mg, 73 %). IR (υCO, CH2Cl2): ν = 1999vs, 1956m,
1
˜
NMR: (CDCl3): δ = 63.5 ppm. H NMR (CDCl3): δ = 7.47 (m, 9H), 7.60
(m, 6H), 1.14 (m, 2H), 1.88 (m 2H); 13C{1H} NMR (CD2Cl2); (298 K)
215.18 (d, J 13.5, 2CO), 210.4 (brs, 3CO), 135.81 (d, J 39.7), 133.10
(d, J 11.3), 130.21 (d, J 1.7), 128.58 (d, J 9.6), 34.92 (s) ppm; (223 K)
215.48 (d, J 8.0, 2CO), 212.71 (s, 2CO), 206.50 (s, CO), 135.61 (d, J
39.9), 133.12 (d, J 11.4), 130.40 (s), 128.79 (d, J 9.5), 34.87 (s) ppm;
Anal. calcd. for Fe2S2P1O5C25H19: C, 49.50, H, 3.14; Found C, 49.26,
H, 3.12.
1
1937s cm–1; H NMR (CDCl3): δ = 7.57 (m, 12H), 7.41 (m, 18H), 1.36
(s, 4H); 13C{1H} NMR (CDCl3): δ = 215.2, 136.3, 135.1, 134.2, 133.7,
129.9, 129.6, 129.3, 128.9, 128.6, 35.3; Anal. calcd. for
Fe2S2Sb2O4C42H34: C, 49.12, H, 3.41; Found C, 49.36, H, 3.35.
Electrochemistry. Electrochemical measurements were made using
a μ-Autolab III potentiostat. CVs were recorded in MeCN with 0.1
M
[Bu4N][PF6] in a conventional three-electrode cell under an argon
or CO atmosphere and are referenced to the Fc/Fc+ couple. A glassy
carbon electrode (3.0 mm diameter) was used as the working elec-
trode, polished on a wet felt polishing pad with alumina gel and
dried before each experiment. The counter electrode was a plati-
num wire and the quasi-reference electrode was a solid silver wire
(both ca. 1.0 mm diameter).
Synthesis of [Fe2(CO)5(AsPh3)(μ-edt)] (3): An MeCN solution
(20 mL) of 1 (50 mg, 0.13 mmol), AsPh3 (42 mg, 0.14 mmol) and
Me3NO (10 mg, 0.13 mmol) was heated to reflux for 2 h. The solvent
was removed under reduced pressure and the residue was chroma-
tographed by TLC on silica gel. Elution with hexane developed two
bands. The faster moving band gave unreacted 1, while the slower
moving band afforded [Fe2(CO)5(AsPh3)(μ-edt)] (3) (42 mg, 48 %) as
red crystals after recrystallization from hexane/CH2Cl2 at 4 °C.
X-ray crystallography. Single crystals of 2–7 suitable for X-ray dif-
fraction were grown by slow diffusion of hexane into a dichloro-
methane solution at 4 °C. All geometric and crystallographic data
were collected at 150 K on a Bruker SMART APEX CCD diffractome-
ter using Mo-Kα radiation (λ = 0.71073 Å). Data reduction and inte-
gration was carried out with SAINT+[44] and absorption corrections
were applied using the program SADABS. Structures were solved
by direct methods and developed using alternating cycles of least-
squares refinement and difference-Fourier synthesis. All non-
hydrogen atoms were refined anisotropically. For 2 hydrogen atoms
were located in different maps and refined independently, for 3–4
hydrogen atoms were placed in the calculated positions and their
thermal parameters linked to those of the atoms to which they
were attached (riding model). The SHELXTL PLUS V6.10 program
package was used for structure solution and refinement.[45] Final
difference maps did not show any residual electron density of
stereochemical significance. The details of the data collection and
structure refinement are given in Table 3 and a full list of bond
lengths and angles are given in Tables S1–6 respectively.
1
IR (υCO, CH2Cl2): ν = 2049vs, 1985vs, 1933m cm–1. H NMR (CDCl3):
˜
δ = 7.53 (m, 6H), 7.43 (m, 9H), 1.92 (m, 2H), 1.32 (m, 2H); Anal. calcd.
for Fe2S2PAs1O5C25H19: C, 46.15, H, 2.92; Found C, 46.02, H, 2.86.
Synthesis of [Fe2(CO)5(SbPh3)(μ-edt)] (4): To an MeCN solution
(20 mL) of [Fe2(CO)6(μ-edt)] (50 mg, 0.13 mmol) was added SbPh3
(48 mg, 0.14 mmol) and Me3NO (10 mg, 0.13 mmol) and the reac-
tion mixture was then heated to reflux for 2 h. A similar chromato-
graphic separation and workup described as above gave
[Fe2(CO)5(SbPh3)(μ-edt)] (4) (48 mg, 53 %) as red crystals after re-
crystallization from hexane/CH2Cl2 at 4 °C. IR (υCO, CH2Cl2): ν =
˜
2050vs, 1986vs, 1933m cm–1. 1H NMR (CDCl3): δ = 7.56 (m, 6H), 7.45
(m, 9H), 2.03 (m, 2H), 1.66 (m, 2H); Anal. calcd. for Fe2S2Sb1O5C25H19
:
C, 43.06, H, 2.73; Found C, 42.85, H, 2.63.
Synthesis of [Fe2(CO)4(PPh3)2(μ-edt)] (5): An MeCN solution
(40 mL) of 1 (100 mg, 0.269 mmol), PPh3 (280 mg, 1.076 mmol) and
Me3NO·2H2O (119 mg, 1.07 mmol) was stirred and refluxed for 3 h.
Solvent was removed and the solid was absorbed into silica from
CH2Cl2 and eluted with hexane/CH2Cl2 (3:1) using flash silica gel
column chromatography. Elution with hexane/CH2Cl2 gives the
product from the column, which can be recrystallised from CH2Cl2
layered over with hexane to afford very dark-red crystals of 5
CCDC 1947948 (for 2), 1947949 (for 3), 1947950 (for 4), 1947951 (for
5), 1947952 (for 6) and 1947953 (for 7) contain the supplementary
crystallographic data for this paper. These data can be obtained
(170 mg, 68 %). A second crop of crystals can be obtained from the free of charge from The Cambridge Crystallographic Data Centre.
Eur. J. Inorg. Chem. 0000, 0–0
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Table 3. Crystallographic data and structure refinement for 2–7.
Compound
2
3
4
5
6
7
empirical formula
formula weight
crystal system
space group
a [Å]
C25H19Fe2O5PS2
606.19
monoclinic
P21/c
C25H19Fe2O5AsS2
650.14
monoclinic
P21/c
C25H19Fe2O5SbS2
696.97
triclinic
C42H34Fe2O4P2S2
840.45
monoclinic
P21/n
C42H34Fe2O4As2S2
928.35
monoclinic
P21/n
C42H34Fe2O4Sb2S2
1022.01
monoclinic
P21/n
P1
¯
9.181(1)
9.250(2)
8.1452(7)
14.861(2)
15.160(2)
15.539(2)
b [Å]
c [Å]
α (°)
17.214(2)
16.607(2)
90
17.192(3)
16.682(3)
90
9.2822(8)
18.572(2)
90.898(1)
15.794(2)
17.143(2)
90
15.807(2)
17.191(2)
90
15.977(2)
17.311(2)
90
ꢀ (°)
γ (°)
103.627(2)
90
2550.8(5)
4
1.579
1.398
102.835(3)
90
2586.7(8)
4
1.669
2.586
102.835(3)
99.878(1)
1346.1(2)
109.173(2)
90
3800.5(9)
109.822(2)
90
3875.4(8)
110.419(2)
90
4027.8(8)
Volume [Å3]
Z
2
4
4
4
D
calc (Mg m–3
)
1.720
2.247
1.469
1.000
1.591
2.593
1.685
2.177
μ (Mo Kα) [mm–1
]
F(000)
1232
1304
688
1728
1872
2016
crystal size [mm]
θ range (°)
limiting indices
0.36 × 0.24 × 0.20
2.57–28.30
–12 ≤ h ≥ 12,
–21 ≤ k ≥ 22,
–21 ≤ l ≥ 21
20664
0.18 × 0.16 × 0.08
2.37–28.27
–12 ≤ h ≥ 12,
–22 ≤ k ≥ 22,
–22 ≤ l ≥ 22
21212
0.48 × 0.36 × 0.28
2.54–28.28
–10 ≤ h ≥ 10,
–12 ≤ k ≥ 11,
–24 ≤ l ≥ 24
10987
0.42 × 0.38 × 0.26
2.56–28.27
–18 ≤ h ≥ 18,
–20 ≤ k ≥ 20,
–22 ≤ l ≥ 22
31518
0.26 × 0.18 × 0.08
1.92–28.26
–19 ≤ h ≥ 19,
–20 ≤ k ≥ 20,
–22 ≤ l ≥ 22
32307
0.25 × 0.20 × 0.18
1.89–28.25
–20 ≤ h ≥ 20,
–21 ≤ k ≥ 20,
–22 ≤ l ≥ 22
33342
reflections collected
independent reflections (Rint
max. and min. transmission 0.7673 and 0.6330
)
5902 (0.0397)
6034 (0.0735)
0.8198 and 0.6532
6034/0/392
0.917
5855 (0.0182)
0.5718 and 0.4118
5855/0/392
1.078
8867 (0.0465)
0.7811 and 0.6789
8867/0/605
0.904
9067 (0.0465)
0.8194 and 0.5521
9067/0/605
0.928
9290 (0.0257)
0.6954 and 0.6122
9290/0/605
1.012
data/restraints/parameters
5902/0/392
0.998
goodness of fit on F2
final R indices [I > 2σ(I)]
R1 = 0.0352,
wR2 = 0.0819
R1 = 0.0493,
wR2 = 0.0876
) 0.631 and –0.806
R1 = 0.0446
wR2 = 0.0833
R1 = 0.0855,
wR2 = 0.0955
1.073 and –1.034
R1 = 0.0257
wR2 = 0.0652
R1 = 0.0281,
wR2 = 0.0664
1.062 and –1.056
R1 = 0.0327
wR2 = 0.0652
R1 = 0.0513,
wR2 = 0.0679
0.499 and –0.447
R1 = 0.0310
wR2 = 0.0621
R1 = 0.0490,
wR2 = 0.0647
0.569 and –0.526
R1 = 0.0214
wR2 = 0.0476
R1 = 0.0279,
wR2 = 0.0486
0.531 and –0.549
R indices (all data)
largest peak and hole (e Å–3
[15]
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Acknowledgments
We thank King's College London (GO) and the Commonwealth
Scholarship Commission (SG) for the provision of PhD student-
ships.
Keywords: Hydrogenase · Biomimic · Pnictides ·
Substitution · Trigonal rotation
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Received: August 20, 2019
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Hydrogenase Biomimics
S. Ghosh, A. Rahaman, G. Orton,
G. Gregori, M. Bernat, U. Kulsume,
N. Hollingsworth, K. B. Holt,
S. E. Kabir, G. Hogarth* ................. 1–11
A series of [FeFe]-hydrogenase bio- the pnictide to be systematically
mimics [Fe2(CO)5(EPh3)(μ-edt)] and probed for the first time; changes to
[Fe2(CO)4(EPh3)2(μ-edt)] (E = P, As, Sb) IR spectra and redox potentials being
have been prepared and structurally correlated with changes in the nature
characterised allowing variations in of the pnictide atom
Synthesis, Molecular Structures and
Electrochemical Investigations of
[FeFe]-Hydrogenase
Biomimics
[Fe2(CO)6-n(EPh3)n(μ-edt)] (E = P, As,
Sb; n = 1, 2)
DOI: 10.1002/ejic.201900891
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