[FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar μ-(SCH2)2ER2Linkers (E=Ge and Sn)

Article abstract of DOI:10.1002/chem.201603843

Analogues of the [2Fe-2S] subcluster of hydrogenase enzymes in which the central group of the three-atom chain linker between the sulfur atoms is replaced by GeR₂and SnR₂groups are studied. The six-membered FeSCECS rings in these complexes (E=Ge or Sn) adopt an unusual conformation with nearly co-planar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to the steric interaction of the Me groups with the axial carbonyls of the Fe₂(CO)₆cluster and low torsional strain for GeMe₂and SnMe₂moieties owing to the long C?Ge and C?Sn bonds. Gas-phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character and, as supported by the computations, an increase in sulfur character in the predominantly metal–metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-type reduction mechanism (E=electron transfer and C=chemical process) in the absence of acid and catalysis of proton reduction in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe₂S₂(CO)₆cores bridged by CH₂SnR₂CH₂, R=Me, Ph, linkers were also obtained and characterized.

Full text of DOI:10.1002/chem.201603843

A Journal of
Accepted Article
Title: [FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar (µ-
SCH2)2ER2 Linkers (E = Ge and Sn)
Authors: Wolfgang Weigand, Hassan Abul-Futouh, Laith R.
Almazahreh, Takahiro Sakamoto, Nhu Y T. Stessman, Dennis
L. Lichtenberger, Richard S. Glass, Mohammad El-khateeb,
Philippe Schollhammer, and Grzegorz Mloston
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To be cited as: Chem. Eur. J. 10.1002/chem.201603843
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[FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar (µ-
SCH2)2ER2 Linkers (E = Ge and Sn)
Hassan Abul-Futouh,[a] Laith R. Almazahreh,*[a] Takahiro Sakamoto,[b] Nhu Y T. Stessman,[b] Dennis L.
Lichtenberger,*[b] Richard S. Glass,*[b] Mohammad El-khateeb,[c] Philippe Schollhammer,*[d] Grzegorz
Mloston,[e] and Wolfgang Weigand*[a]
Abstract: Analogues of the [2Fe-2S] subcluster of hydrogenase
enzymes in which the central group of the three-atom chain linker
between the sulfur atoms is replaced by GeR2 and SnR2 groups are
studied. The six-membered FeSCECS rings in these complexes (E =
Ge or Sn) adopt an unusual conformation with nearly coplanar
SCECS atoms perpendicular to the Fe–Fe core. Computational
modelling traces this result to steric interaction of the Me groups with
the axial carbonyls of the Fe2(CO)6 cluster and low torsional strain
for GeMe2 and SnMe2 moieties due to the long C–Ge and C–Sn
bond lengths. Gas phase photoelectron spectroscopy of these
complexes shows a shift of ionizations with substantial sulfur orbital
character to lower energies and, as supported by the computations,
an increase in sulfur character in the predominantly metal-metal
bonding HOMO. Cyclic voltammetry reveals that the complexes
follow an ECE-type reduction mechanism in the absence of acid,
and catalysis of proton reduction in the presence of acid. Two cyclic
cluster dimers featuring the sulfur atoms of two Fe2S2(CO)6 cores
bridged by CH2SnR2CH2, R = Me, Ph, linkers were also obtained
and characterized.
which is close to the thermodynamic potential. Proton relay to
the vacant axial iron site via agostic or hydrido-proton interaction
from the azadithiolate linker to the iron is thought to be
responsible for the high efficiency of catalytic hydrogen release
by the H cluster (ca. 104 turnover/s).[1,2,5]
Figure 1. The active H-cluster site of a [FeFe]-hydrogenase.[1]
Models for the active site [2Fe-2S] subcluster have been
extensively studied.[6] Although mixed valence analogues with a
semibridging carbonyl and rotated structure have been
reported,[7] the Fe(I)Fe(I) models typically do not exhibit these
essential features. However, three complexes with these
features have been reported.[8] To adopt these features up to
three requirements were denoted: (i) bulkiness of the dithiolato
bridge, (ii) desymmetrization of the di-iron system and (iii)
agostic interaction in two cases. Bulkiness in the dithiolato
bridge was previously proposed to favor these geometric
changes on the basis of computations[9] and, owing to steric
interactions between a methyl group and an apical CO in
Fe2(CO)6{(µ-SCH2)2CMe2}, the Fe(CO)3 moiety is twisted.[10] It
should be noted that the CH2CMe2CH2 bridge adopts a bent
geometry such that the FeS2(CH2)2CMe2 ring may be described
as a chair conformation with respect to one Fe and a boat
conformation with respect to the other Fe. Furthermore, this
chair/boat conformation undergoes ring inversion in analogy with
cyclohexane. Even in [2Fe-2S] models with all terminal CO
ligands acting as electrocatalysts for H2 production from weak
acids, the core reorganization may play a critical role. For
example, potential inversion, in which the first 1e– reduction
potential is more negative than second 1e– reduction potential,
occurs if there is an intervening reorganization.[11]
Introduction
The active H-cluster site of [FeFe]-hydrogenases (Figure
1) contains a [2Fe-2S] subcluster with a semi-bridging carbonyl
ligand between the iron centers.[1] This semi-bridging orientation
of the carbonyl ligand is thought to be important because it
opens an axial site on the iron atom that is distal to the [4Fe-4S]
cubane subcluster for protonation of the iron center. DFT studies
have proposed that protonation of the azadithiolate linker µ-
SCH2NHCH2S at the NH[1,2] group or protonation of the µ-S
atoms[3] provides low energy kinetic pathways for protonation of
the axial site and dihydrogen formation, which occurs with the
enzyme at a midpoint potential of -0.42 V(vs NHE)[4] at pH = 8.0,
[a]
[b]
Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-
Universität Jena, Humboldt Str. 8, 07743 Jena, GERMANY Email:
wolfgang.weigand@uni-jena.de
Department of Chemistry & Biochemistry, The University of Arizona,
Tucson, AZ, 85721, U.S.A. Email: rglass@email.arizona.edu
dlichten@email.arizona.edu
[c]
[d]
Chemistry Department, Jordan University of Science and
Technology, Irbid 22110, Jordan. Email: kateeb@just.edu.jo
UMR CNRS 6521, Université de Bretagne Occidentale, 6 avenue Le
Gorgeu, C.S. 93837, 29238 Brest-Cedex, France. Email:
philippe.schollhammer@univ-brest.fr
The synthetic models related to the structure of the H-
cluster may be divided into two categories: (i) Bioinspired
models with azadithiolato linkers[12] and (ii) artificial models with
abiological linkers μ-(XCH2)2Y in which the central atom/group Y
is CR2,[3d,12o,13] O,[13d,14] S,[15] Se,[16] SiR2[3e,17] or Ph-P=O[18] and X
could be S[12-18], Se[19], Te[19c,19d] or PR[20]. The use of these
[e]
Section of Heteroorganic Compounds, University of Lodz,
Narutowicza 68, 90-136, Lodz, Poland. Email: gmloston@uni.lodz.pl
Supporting information for this article is given via a link at the end of
the document.
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abiological linkers may provide valuable information and Results and Discussion
answers to basic questions concerning the role of the dithiolato
bridge as well as the effect of the unusual Y on the structural
and functional properties of such artificial H cluster mimics. For
example, Goy et al. recently reported a model with an especially
bulky Si-containing moiety, namely Fe2(CO)5(PPh3){(µ-
SCH2)2Si(1-silafluorene)}.[3e] In this case, protonation of the
sulfur atoms by triflic acid is favored by an interplay of the
presence of the Si atom, the steric bulkiness of the Si-containing
moiety and the presence of one σ-donor PPh3 ligand at the
diiron core. Previous suggestions of the relevance and
observation of protonation at sulfur have been made.[3] In
addition, we have previously shown that the electron richness of
thioether moieties, as judged by ionization energies determined
by photoelectron studies on Fe2(CO)6{(µ-SCH2)2SnR2}
complexes[21] and other compounds with SCSn groups,[22] is
dramatically increased by a C-Sn bond owing to a geometry-
dependent σ(Sn-C)↔3p(µ-S) filled-filled interaction.
Synthesis and Characterization of R2Sn(CH2I)2
The compound Ph2Sn(CH2I)2 was synthesized following a
similar procedure applied for Me2Sn(CH2I)2.[23] The reaction of
Ph2SnCl2 with two equiv. of in situ generated IZnCH2I afforded
Ph2Sn(CH2I)2 in ca. 48% yield after purification by column
chromatography as a colorless liquid (Scheme 2).
Scheme 2. Synthesis of Ph2Sn(CH2I)2.
We report in the present paper the synthesis of models
with μ-(SCH2)2ER2 linkers, where the central atom E is Ge or Sn.
Here the steric hindrance can be varied by judicious choice of R.
We now study in a systematic manner the molecular structures
and electronic structures of a series of complexes depicted in
Scheme 1, where complexes 1 (R = H or Me)[10b,13c] and 2 (R =
Me)[17] have been reported previously.
The compound Ph2Sn(CH2I)2 was characterized by 1H and
1
13C{1H} NMR spectroscopy and mass spectrometry. Its H NMR
spectrum (CDCl3, 400.08 MHz) exhibits a singlet at 2.41 ppm
with Sn satellites (2J{117Sn,1H} = 20.9 Hz and 2J{119Sn,1H} = 41.8
Hz) for the CH2 hydrogen atoms. The hydrogen atoms at the
meta and para positions of the Ph groups show a multiplet in the
region of 7.38-7.43 ppm and the hydrogen atoms located at the
ortho position resonate in the region of 7.55-7.60 ppm. The
13C{1H} NMR spectrum shows a singlet at -10.53 ppm due to the
CH2 carbon atoms. The Ph groups show multiplets at 126.0-
128.8 (meta and para C atoms), 136.85 ppm (ortho C atoms)
and 139.31 ppm (ipso C atoms) in the 13C{1H} NMR spectrum. In
addition, the DEI-MS spectrum shows peaks at m/z 479
[PhSn(CH2I)2]+, 415 [Ph2SnCH2I]+, 351 [PhSn(CH2I)(CH2)]+ and
273 [Ph2Sn]+.
Scheme 1. A series of [FeFe]-hydrogenase mimics containing group IV
elements with increasing size of bridgehead atoms in the dithiolate linker. The
drawings do not depict the variations in the geometries of the linkages and the
Fe2(CO)6 portions of the structures that are revealed in this study.
Synthesis and Characterization of the Iron Complexes
The reaction of in situ generated (µ-LiS)2Fe2(CO)6 with one
equivalent of R2Sn(CH2I)2 afforded Fe2(CO)6{(µ-SCH2)2SnR2} (R
= Me, 4a, 25 % yield and Ph, 4b, 18 %) as air-stable red solids
as well as unexpected tetranuclear products [Fe2(CO)6{(µ-
SCH2)2SnR2}]2 (R = Me, 5a, 9 % yield, and R = Ph, 5b, traces)
as air-stable orange solids (Scheme 3). On the other hand, we
were able to isolate only Fe2(CO)6{(µ-SCH2)2GeMe2} (3) in 15 %
yield from the reaction of (µ-LiS)2Fe2(CO)6 with one equivalent of
Me2Ge(CH2Cl)2, but not a tetranuclear product. Complexes 3, 4a,
4b, and 5a were characterized by means of 1H, 13C{1H} NMR
and IR spectroscopic techniques as well as mass spectrometry,
elemental analysis and X-ray crystallography. Because of the
very low yield of 5b, we were only able to characterize it by X-
ray crystallography.
This series provides insight into the effect of Ge and Sn on
the molecular structure of the Fe(I)Fe(I) complexes and their
reorganization energies on forming formal Fe(I)Fe(II) ions, as
well as the effect of the C–Ge and C–Sn bonds on the electron-
richness of the adjacent S atoms. Within the series in Scheme 1,
only the models 3 and 4 reveal a unique planarity when R = Me,
also in comparison to all of the previously reported models that
show the usual chair/boat conformation of the two fused
FeS2C2E rings. We herein illuminate the origin of the planarity
and its impact on the electron richness of the S atoms.
Furthermore, we report the effect of germanylation and
stannylation on the redox properties and describe the
electrocatalytic behavior of these complexes toward reduction of
protons from moderate and strong acids.
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The tetranuclear complex 5a. The FAB-MS spectrum of
5a shows the parent ion peak at m/z 1041 [M]+ and the
consecutive loss of six CO ligands. The IR spectrum exhibits
absorption bands at 1991, 2031 and 2071 cm-1 for terminal CO
ligands. The 13C{1H} NMR spectrum displays peaks at -8.1 (CH3),
1.1 (CH2) and 209.0 ppm (CO). An apparent singlet at 2.00 ppm
1
was observed at room temperature in the H NMR spectrum for
all of the CH2 hydrogen atoms in the macrocycle and another
singlet at 0.36 ppm due to the CH3 groups.
The Dinuclear Models
Molecular Structures. As can be seen from Figure 2,
each iron core of 3, 4a and 4b can be best described as a
distorted octahedron in which the central atom is Fe surrounded
by three terminal CO ligands in facial fashion as well as two S
atoms that bridge both iron atoms. The bicyclic [2Fe-2S]
structure in these complexes reveals a butterfly conformation.
Scheme 3. Syntheses of complexes 3-5b.
The bridgehead E atoms of the linker μ-(SCH2)2ER2 (ER2
GeMe2, SnMe2, SnPh2) of the complexes is surrounded by
atoms in distorted tetrahedral fashion.
=
The dinuclear complex 3. The ESI-MS spectrum of 3
shows the parent ion peak at m/z 476 [M]+ as well as the
consecutive loss of the CO ligands at m/z 448 [M - CO]+, 420 [M
- 2CO]+, 392 [M - 3CO]+, 364 [M - 4CO]+, 334 [M - 5CO]+ and
308 [M - 6CO]+. The IR spectrum (CH2Cl2 solution) of 3 displays
four absorption bands at υ(CO) = 1990 (s), 1998 (s), 2032 (vs)
and 2071 (s) cm-1 for terminal CO ligands. The 13C{1H} NMR
spectrum of 3 exhibits singlets at 0.13, 6.5 and 207.95 ppm for
the CH3, CH2 and CO carbon atoms, respectively. In the 1H
NMR spectrum, two singlets are observed at 0.26 and 1.66 ppm
due to the CH3 and CH2 protons, respectively.
The dinuclear complexes 4a and 4b. The DEI-MS
spectra of 4a and 4b show the parent ion peak at m/z 520 [M]+
and 646 [M]+, respectively, as well as the consecutive loss of the
CO ligands. The IR spectrum (CH2Cl2 solution) of 4a displays
four vibration bands at υ(CO) = 1989 (s), 1997 (s), 2032 (vs) and
2070 (s) cm-1 whereas three υ(CO) bands are observed at 1989
(vs), 2033 (vs) and 2066 (s) cm-1 in case of 4b. The υ(CO)
wavenumbers (CH2Cl2 solution) slightly shift to smaller values
ongoing from 1 (R = Me; 1990 (s), 2000 (s), 2032 (vs), 2073 (s)
cm-1), 2 (R = Me; 1990 (s),1998 (s), 2032 (vs), 2072 (s) cm–1)
and 3 to 4a. The 13C{1H} NMR spectrum of 4a shows singlets at
-7.1, 1.6 and 207.0 ppm for the CH3, CH2 and CO carbon atoms;
respectively, while that of 4b exhibits a singlet at 1.7 ppm for the
CH2 groups as well as peaks due to the Ph substituents in the
region of 128.7-130.6 ppm (meta and para C atoms) and 136.7-
138.7 ppm (ipso and ortho C atoms). The CO ligands of 4b
resonate at 208.6 and 209.1 ppm, which is in contrast to the
13C{1H} NMR spectrum of 4a that shows only one signal due to
the CO ligands. The 1H NMR spectra of 4a and 4b exhibit a
singlet at 1.79 ppm (4a) and 2.27 ppm (4b), respectively, for the
CH2 protons. Complex 4a shows a singlet at 0.25 ppm due to
Figure 2. Molecular structures (50% probability) of 3, 4a and 4b.
The average E-C-S bond angles (122.42(13)º (3),
121.96(15)º (4a) and 119.7(2)º (4b)) deviate significantly from
the ideal tetrahedral angle as was observed in various
Fe2(CO)6{(µ-SCH2)2SiR2} models, 118.22(12)-122.05(13)º.[17]
The Fe-Fe bond lengths in 3 (2.5128(4) Å), 4a (2.5249(5) Å) and
4b (2.5158(7) Å) are slightly shorter than those of the H-cluster,
2.55-2.62 Å,[2,24] but comparable to those in the complexes with
the μ-(SCH2)2SiR2 linker (ca. 2.518 Å).[17] The average Fe-CO
bond lengths are 1.800(2), 1.802(3), and 1.798(4) Å in 3, 4a,
1
the CH3 protons in the H NMR spectrum, while the spectrum of
complex 4b displays a multiplet in the region of 7.35-7.70 ppm
attributed to the Ph protons.
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and 4b, respectively. The average Fe-S bond lengths (2.2535(6) Å (3), 2.2561(8) Å (4a) and 2.2582(10) Å (4b)) are comparable to
those in Fe2(CO)6{(µ-SCH2)2SiMe2} (2.2582(6) Å).[17]
Geometry of the dithiolato linker
All of the previously reported [FeFe]-hydrogenase models adopt a chair/boat conformation (Figure 3) of the two fused six-
membered FeSCECS rings in the solid state as well as in solution and many complexes such as 1, R = H or Me, undergo FeSCECS
ring inversion[10b] with the E atom exchanging places on either side of the molecule. The flap angle (α) formed from the intersection
between the C2E and S2C2 planes, as shown in Figure 3, lies within the range of 118° > α > 160° in various [FeFe]-hydrogenase
models. Only one complex, namely Fe2(CO)6{(µ-SCH2)2Si(1-silafluorene)}[3e], has a large angle α = 171°. In the sofa conformation α =
180º and the -SCECS- moiety is planar. Table 1 lists the flap angle α for the homologous series with CMe2, SiMe2, GeMe2 and SnMe2
in the bridge as well as angles related to the Fe(CO)3 distortion and twisting. Remarkably, the -SCECS- moiety is nearly planar for E
= Ge and Sn. That is, the flap angles α deviate by only 4.8° (E = Ge) and 6.4° (E = Sn) from planarity. In addition, their solution NMR
spectra show equivalent Me groups for E = Ge and Sn. This may result from a planar structure or rapid equilibration (ring inversion)
1
of nonplanar isomers (as observed in the room temperature H NMR spectrum of 1, R = Me)[10b]. However, the Me groups in 4a
remain equivalent even at -90°C.
Figure 3. The chair/boat and planar conformations and the definition of the angle α.
To ascertain whether the variations in these structures are solid state effects or inherent to the molecules, the gas phase
structures were computed and compared in Table 1 and S-1 for the lowest energy conformers. When R is Me, there is a general
increase in the flap angle α for E going down the group with an angle of about 135° when E = C and approaching nearly 180° when E
= Ge (only in the solid state structure), E = Sn (in both of the solid and gas phase structures). The flap angle in the molecular
structure for the molecule with ER2 = GeMe2 seems anomalous, being slightly greater than the angle for ER2 = SnMe2, but
examination of the crystal packing suggests that intermolecular interactions are influencing this angle in the solid state. This
suggestion is supported by computation of the distortion energies. The gas phase structures (computational) reproduce the flap
angles observed in the solid state structures within 2° for E = C, Si, and Sn, and place the flap angle for E = Ge intermediate to those
for Si and Sn. The computations also account well for the solution carbonyl stretching frequencies (Figure S-1). These results
indicate that the large flap angles for the heavier elements of group IV are an inherent property of the isolated molecules, but can be
influenced by solid state packing forces especially for E = Ge and Sn.
The change in the R group also has a significant influence on the structure. When E = Sn, the flap angle α for R = Ph is about
20° less than that for R = Me. It is noted in the solid state structure of 4b that one phenyl group is oriented approximately edge-on to
an apical carbonyl while the other is face-on to the other apical carbonyl. The computations reproduce this orientation, but give a
larger flap angle than observed in the solid state structure, and here again the crystal packing indicates intermolecular interactions
between the phenyl rings are also at play. When E = C the comparison for R = H with R = Me is problematic because of disorder in
the structure with CH2 as the central bridge group. The gas phase structures obtained computationally give a much smaller flap angle
of 126.6° for R = H than for R = Me, which is close to the ideal angle of 127° for a perfectly staggered conformation of a cyclohexane
ring. Taken together the above observations indicate that steric factors play a major role in determining the flap angle α. As a
computational test for E = Sn, the R groups were changed to H atoms, and the optimum -SCSnCS- structure bent substantially from
planarity (Table 1).
The steric characteristics of the ER2 groups distort the symmetry of the Fe2(CO)6 portion of the molecules. This is reflected in
the bending of an apical carbonyl away from steric interaction with the ER2 group and a rotation of an Fe(CO)3 group away from the
mirror plane of the molecule. The Fe-Fe-C(O) angles and (O)C-Fe-Fe-C(O) dihedral angles for the apical carbonyls of these
molecules are listed in Table 1. The steric effect of the ER2 group on the metal carbonyl is observed most clearly in the deflection of
the apical carbonyl that is proximal to the ER2 group. As pointed out previously by Darensbourg,[10] there is an appreciable increase in
the distortion from ER2 = CH2 to CMe2. Unfortunately, the disorder in the solid state structure for ER2 = CH2 prevents comparison of
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Table 1. Influence of ER2 groups (E = group IV element) toward planarity of the S2C2E linker and distortion of the Fe2(CO)6 symmetry in Fe2(CO)6{(µ-
SCH2)2ER2} complexes.
ER2
CH2
CMe2
SiMe2
GeMe2
SnMe2
SnPh2
SnH2
Solid Phase Structures
Flap angle α,[a]
°
137.1[d]
135.7
150.0
175.2
173.6
152.5
–
–
Fe–Fe–C(O) [b], °
148.3/148.3[d]
Δ = NA[d]
159.9/148.1
Δ = 11.8
152.8/144.7
Δ = 8.1
147.4/146.4
Δ = 1.0
146.2/145.1
Δ = 1.1
149.5/144.4
Δ = 5.1
Ca–Fe–Fe–Ca [c], °
0.0[d]
6.5
0.0
7.9
5.2
7.5
–
Gas Phase Structures (Computational)
Flap angle α, °
126.6
137.5
148.0
159.6
172.8
161.6
141.9
Fe–Fe–CCO [a], °
151.5/146.0
159.2/146.1
154.1/144.0
149.2/144.1
144.8/142.8
148.6/142.9
151.0/144.3
Δ = 5.5
Δ = 13.1
Δ = 10.1
Δ = 5.1
Δ = 2.0
Δ = 5.7
Δ = 6.7
Ca–Fe–Fe–Ca [b], °
0.0
0.8
2.2
0.3
0.1
0.1
0.6
[a] See Figure 3 for illustration of the flap angle α. [b] Fe–Fe–C angle for the apical carbonyl proximal to the ER2 group / Fe–Fe–C angle for the apical carbonyl
distal to the ER2 group (see Figure 4), Δ is the difference between these angles. [c] Dihedral twist angle of the apical carbonyl carbon atoms across the Fe–Fe
bond. [d] Disorder in the crystal for the case of ER2 = CH2 averages the structure to C2v symmetry, which masks any symmetry distortion.
the deflection of the carbonyls proximal and distal to the CH2 group. However, the computations support the increasing deflection of
the proximal carbonyl from the CH2 to the CMe2 complex. The dimethyl complexes show that the deflection of the apical carbonyl
decreases from E = C to Si to Ge to Sn, both in the molecular structures and in the gas phase structures from the computations.
A deeper insight into the factors that determine this trend is obtained by imagining a split of the molecules into the dithiolato
bridges and the Fe2(CO)6 fragments and computing the distortion energies of each fragment, as depicted in Figure 4. These
distortion energies are thus in the absence of mutual steric and electronic effects between the fragments. The fragments are
terminated by hydrogen atoms to satisfy the valence. The flap angle α of the linker is varied from 127°, which is the optimum angle of
a fully staggered chair-like conformation, to 180°. Note that at α = 180° the E-C bonds of the EMe2 groups are in a fully eclipsed
conformation with the C-H bonds of the CH2 groups. For CMe2, the planar structure of the linker with the eclipsed bonds is disfavored
by 7.7 kcal/mol, but this value falls by more than half for each substitution down the group (SiMe2, 2.8; GeMe2, 1.3 kcal/mol; SnMe2,
0.5 kcal/mol). The dramatic decrease in this strain energy for the planar structures is attributed primarily to the longer E-C bonds that
diminish the eclipsed non-bonded repulsions.
The EMe2 groups are within van der Waals distance interactions with the axial carbonyls of the Fe2(CO)6 fragment even at the
planar geometries. The optimized structures in which the linkers are constrained to be planar give distances from the hydrogen atoms
of EMe2 atom to the axial carbonyl O atom of 2.86 Å for CMe2, 2.87 Å for SiMe2, 2.88 Å for GeMe2, and 2.94 Å for SnMe2, whereas
the van der Waals distance between these atoms implemented in the MM3 force field[25] is 3.44 Å and in the UFF force field is 3.19
Å.[26] This interaction forces a mutual distortion of each fragment.
The bottom of Figure 4 shows the energy cost of distorting the Fe2(CO)6 portion of the molecule as a function of the axial Fe-
Fe-C(O) angle through the maximum values obtained in the crystal structures and computations. When EMe2 is CMe2, there is a
strong driving force toward the staggered linker conformation (α = 127°) that pushes a distortion of the Fe2(CO)6 portion of the
molecule. This driving force toward the staggered conformation of the linker rapidly diminishes down the group of SiMe2, GeMe2, and
SnMe2. The potential energies when EMe2 is
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Figure 4. Top: illustration of the conceptual split of the molecule into the linker portion and the [2Fe-2S] portion for separate evaluation of the conformational
energies, with definition of the distortion angle α from the fully staggered conformation (α = 127°) to the planar form (α = 180°) where the C-H bonds of the CH2
groups are eclipsed with the E-C bonds of the EMe2 group, and definition of the Fe-Fe-C(O) distortion angle. Middle: distortion energies of the linker as a function
of the degree of planarity α. Bottom: distortion energy of the metal carbonyl as a function of the deflection of the proximal carbonyl away from the linker methyl
group.
GeMe2 and SnMe2 are very flat in the region of α = 180°, making the structures obtained from crystal structures susceptible to
packing forces as mentioned earlier.
Table 2 shows the calculated activation energies for the flap of the EMe2 group from one side of the molecule to the other, in
which the geometry where α = 180° is the transition state. For comparison, the case of CMe2 has a smaller energy than the case of
CH2 in agreement with previous work.[10] The E = Si, Ge, and Sn cases have very small energies for the reasons given above,
consistent with the low-temperature NMR spectrum. Geometries also were optimized using continuum solvation models for the NMR
solvents THF, dichloromethane, and acetone, and these computations gave little difference in the relative conformational energies
compared to the gas-phase computations.
Table 2. Calculated activation energies E‡ for fluxional processes (kcal/mol).
ER2
CH2
9.9
CMe2
5.6
SiMe2
1.0
GeMe2
0.2
SnMe2
0.1
Flap
Rotate proximal
Rotate distal
7.1
4.1
4.8
5.7
6.0
10.4
10.8
10.1
5.6
5.9
The twist of the Fe(CO)3 group gives a less clear indication of the steric interaction with the linker than the deflection of the axial
carbonyl. The gas phase computations yield very little twist, and both the crystal structures and the computations do not show regular
trends with increasing size of the ER2 group. Nonetheless, the larger ER2 groups may make the Fe(CO)3 group more prone to
rotation, and the rotations found in the crystal structures may be sensitive to intermolecular interactions. Table 2 includes the
calculated activation energies for rotation of the Fe(CO)3 groups. For the familiar case of 1, where ER2 is CH2, the computations
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indicate the energy barrier for equilibration of the CO groups between axial and basal positions is about 10 kcal/mol, and this value
compares well with the experimental activation energy of 10.4 kcal/mol determined from variable temperature NMR.[13c,27]
Comparison of the energy for rotation of the Fe(CO)3 group proximal to the ER2 group of the linker with the rotation of the Fe(CO)3
group distal to the ER2 group gives an indication of the effect of the steric interaction of the ER2 group. The smallest rotation energy
for the proximal Fe(CO)3 group and greatest difference from the rotation energy for the distal Fe(CO)3 group is found for ER2 = CMe2.
The Ge and Sn examples have essentially no difference between the proximal and distal Fe(CO)3 rotation energies as expected from
the near-planar geometries of these linkers.
The near planar geometries of the -SCECS- bridges in 3 and 4a are remarkable in comparison to the conformations of
cyclohexane. The six-membered FeSCECS moiety is comparable to the cyclohexane conformation with 5-coplanar atoms A.
Although initially calculated by molecular mechanics[28] to be higher in energy than the half-chair conformation B, subsequent
calculations suggest that they are comparable in energy[29] and provide the transition state for conversion of the chair to twist
conformation[30] of cyclohexane with a 10.8 kcal/mol
barrier.[31] This is comparable to the 9.9 kcal/mol barrier for the similar process to A of the [2Fe-2S] complex with ER2 = CH2 shown in
Table 2. Nevertheless, it is this conformation that is adopted in 4a and almost as well in 3. Previous studies on conformational
analysis of cyclohexanes with higher Group IV elements in the ring are limited, but there have been studies on silacyclohexanes[32] as
well as a silathia analogue[33], germacyclohexanes[34] and stannacyclohexanes[35]. The main conclusion of these studies is that there
is a modest flattening of the chair conformation made possible by the greater C-Si than C-C bond length and consequent decease in
Si-C repulsive torsional interactions as well as decreased ring inversion barriers. This flattening is taken to remarkable extremes in 4a
and 3. It is interesting to note that Me∙∙∙apical CO repulsion in 1 (R = Me) is predominantly relieved by Fe(CO)3 geometry changes,
but with 4a and 3 it induces geometry changes in the -SCECS- flap. As mentioned previously in support of the Me∙∙∙apical CO
interaction as the driver in this distortion, calculations on 4 (R = H) are shown in Table 1 and reveal an α value of only 141.9°. Also
note that comparison of the flap angles of 1 (R = H) and 1 (R = Me) reflect this interaction as well, although the molecular structures
cannot be compared because of disorder in 1 (R = H).
Photoelectron Spectroscopy
Gas-phase ultraviolet photoelectron spectroscopy (UPS) offers a direct experimental probe of the valence electron energies in
the absence of intermolecular interactions, and as such can provide information on the electronic structure variations through this
series as the central bridgehead atom changes from C to Si to Ge to Sn. The technique is relevant to the study of hydrogenase
mimics because the ionization of these molecules give direct access to the formal [Fe(I)Fe(II)] oxidation state in the proposed
catalytic cycle of the enzyme.[2] In addition, the well-defined energy quantities provide experimental validation for the computational
methods utilized in this study. The He I spectra of 1-3 (R = Me) in the energy region from 7 to 12 eV were collected and compared
with the spectrum of 4a from a previous study[21] in Figure 5.
Figure 5. He I photoelectron spectra of 1-4a. The black arrows point to the calculated vertical ionizations energies (IEV). The red arrows correspond to the global
minimum cation structures (IEA0), and blue and green arrows correspond to local minima (IEA1 and IEA2) of less stable cation structures (see text and Figure 7).
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The He I UPS spectra and ionization band assignments of similar compounds have been reported previously.[19c,36] The broad
first ionization band ranging from 7.5 to 8.7 eV represents the Fe-Fe σ bond and six occupied “t2g-like” 3d orbitals that are oriented for
backbonding to the carbonyl ligands on the two distorted-octahedral Fe centers, mixed with some S lone pair p orbitals. Ionization in
the region from roughly 8.7 to 9.2 eV is primarily sulfur p in character mixed with significant Fe character, and another primarily sulfur
p ionization is around 9.5 eV. The grey lines in Figure 5 trace the shift of these ionizations due to the neighboring effects from E = C
to E = Sn.[21] Additional ionizations ranging from roughly 9.2 to 12.0 eV, consist of C, Si, Ge and Sn character interacting with S
character and Fe d orbitals.
The leading edges of the first ionization bands correspond to ionizations from the HOMO, starting at about 7.5 eV for all
molecules. The HOMOs of 1 (R = CMe2) and 4a (ER2 = SnMe2) are compared in Figure 6 (Figure S-2 for the HOMOs of 2, R = Me,
and 3). In all cases the HOMO is primarily the Fe-Fe bond. In 1 the tilt of the CMe2 group displaces the orbital more toward the distal
Fe atom, whereas in 4a the nearly planar linker between the S atoms (large α angle) creates a more symmetric orbital. Also note the
increase in S character in the HOMO of 4a with some antibonding interaction with the linker atoms, both of which can facilitate
protonation of the [2Fe-2S] cluster. The S character in the HOMO increases in the series E = C, Si, Ge, Sn (Figure S-2) due to the
increasing instability of the E orbitals and E–C sigma bonds that impart both an inductive and overlap destabilization of the S lone
pair that then mixes with the metal-metal bond density in an antibonding fashion in the HOMO.
The calculated ionization energies corresponding to removal of an electron from the HOMO of these molecules are indicated by
a series of arrows on the photoelectron spectra shown in Figure 5. The vertical ionization energies (IEV), calculated by the ΔSCF
difference in energy from the neutral molecule to the cation, without change in geometry, are found to decrease from 7.98 eV for 1 to
0
7.86 eV for 2 to 7.77 eV for 3 and to 7.68 eV for 4a (Table 3). The adiabatic ionization energies (IEA ) of these compounds are
calculated by the ΔSCF method with full geometry optimization of the cations as shown in Figure 7 for 4a.
Figure 6. The HOMOs of 1, R = Me, and 4a (isosurface value ±0.05).
Table 3. Calculated first ionization energies and reorganization energies of
Fe2(CO)6{(µ-SCH2)2EMe2} complexes (all energies in eV).
ER2
CMe2
7.98
7.23
7.58
N/A[b]
0.75
SiMe2
7.86
7.26
7.48
7.61
0.60
GeMe2
7.77
7.28
7.43
7.52
0.49
SnMe2
7.68
7.27
7.37
7.42
0.41
IEV
0
IEA
1
IEA
2
IEA
λ[a]
[a] Reorganization energy with ionization, IEV – IEA0. [b] Local minimum for
CMe2 not available. Geometry relaxes automatically to the global minimum.
Figure 7. The cation structures and relative energies of 4a, Fe2(CO)6{(µ-SCH2)2SnMe2}.
For all four complexes the global minimum IEA0 conformation of the cation has a rotated Fe(CO)3 group with a bridging carbonyl
1
and the dimethyl unit bends toward the vacant site. The alternative conformation (corresponding to IEA ) with the dimethyl unit in
close proximity to the non-rotated apical terminal CO (Figure 7 for 4a and Figure S-3 for molecules 1-3) is a local minimum at higher
energy. The difference in ionization energy from IEA1 to IEA0 for these two conformers is a combination of the stabilization gained from
release of the steric interaction of the linker with the apical carbonyl and the stabilization gained from the contribution of the agostic
C-H bond to the Fe atom. The energy difference decreases down the series from E = C (0.35 eV) to Si (0.22 eV) to Ge (0.15 eV) to
Sn (0.10 eV).
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2
Another local minimum, IEA , with an all terminal CO conformation was found for molecules 2, 3, and 4a. However, all attempts
to find a local minimum for the cation of molecule 1 (R = Me) with all terminal carbonyl ligands resulted in collapse to the global
minimum conformation with the bridging carbonyl. This is a consequence of the stronger driving force to the rotated Fe(CO)3 structure
when ER2 is CMe2. The energy difference between the cation at the neutral molecule geometry (IEV) and the optimized structure of
0
the cation (IEA ) is the reorganization energy λ. Substantial reduction of reorganization energy was found, 0.75 eV for 1 (R = Me),
0.60 eV for 2, 0.49 eV for 3 and 0.41 eV for 4a, from C to Sn consistent with the indications of decreasing strain down the series.
The Tetranuclear Models
The X-ray diffraction analyses confirmed the tetranuclear structures of 5a and 5b as macrocycles in which each Fe2S2(CO)6 unit
has nonequivalent axial (a) and equatorial (e) -CH2- units (Figure 8). That is, the stereochemistry of the linkers connected to the sulfur
atoms in 5a is e,a,e',a' (the unprimed labels correspond to one [2Fe-2S] cluster and the primed labels to the other [2Fe-2S] cluster).
While the average Fe–Fe, Fe–CO and Fe–S bond lengths in 5a (2.5224(6), 1.797(3) and 2.2638(8) Å, respectively) and 5b
(2.5165(5), 1.803(3) and 2.2616(7) Å, respectively) are comparable to those in 4a and 4b, the Sn–C–S bond angles in 5a
(107.32(14)º-114.08(14)º) and 5b (111.19(13)-112.33(13)º) are closer to the ideal tetrahedral angle. Examples of tetranuclear
complexes in which two butterfly [2Fe-2S] clusters are part of a macrocycle are rare in the literature and are reported only when the
dithiolato linker contains more than three atoms between the two sulfur atoms (-S-CH2-(X)n-CH2-S-, n > 1).[37] In our case, the
formation of these macrocycles (5a and 5b) is allowed by the longer dithiolato linker chain -S-CH2-Sn-CH2-S- due to the large size of
the Sn atom and hence the relatively long Sn–CH2 bond (average: 2.155(3) for 5a and 2.156(3) for 5b Å). Of particular relevance is
the tetranuclear macrocycle C[37a] in which two Fe2S2(CO)6 moieties are connected by CH2(CH2OCH2)2CH2 units forming a 24-
membered ring with e,a,e',a'-stereochemistry similar to 5a.
Figure 8. Molecular structures (50% probability) of complexes 5a (to the top) and 5b (to the bottom).
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The structures of the Fe2S2(CO)6 units of 5a and C are comparable. However, there is a 14-membered ring in 5a which results
in the Fe2S2(CO)6 units being much closer to each other than those in C. The intramolecular distances between Fe and S atoms in
one unit and those in the other unit for 5a are listed in Table S-3. As seen in Table S1, the shortest Fe∙∙∙S distances are 4.841 and
4.874 Å. All of the Fe∙∙∙S distances involving the Fe of one Fe2S2(CO)6 and a S of the other cluster in C are greater than 7.39 Å.
1
The molecular structure of 5a shows that for each Fe2S2(CO)6 unit there is an a and e CH2 substituent. Nevertheless, its H
NMR spectrum shows a singlet for the CH2 groups despite their nonequivalence. This observation is surprising because compound
D[21], in which one CH2SnMe3 substituent is a and the other is e, shows two singlets for the nonequivalent CH2 groups at room
temperature.[38]
It is possible that the a and e positions in 5a interconvert at room temperature. To test this possibility, we have performed VT 1H NMR
spectroscopic measurements and computational studies and found that the observation of one resonance signal arises from
coincidental overlap of the temperature-dependent signals rather than interconversion of a and e substituents (for more details see
the Supporting information, including Figure S-4).
Electrochemical Properties
Table 4 summarizes the electrochemical reduction and oxidation potentials from the cyclic voltammetric measurements of the
complexes Fe2(CO)6{(µ-SCH2)2EMe2} (1-4a). Complex 1, R = Me shows the occurrence of only one quasi-reversible reduction,
whereas complexes 2-4a exhibit one partially reversible and one irreversible (irr.) reduction events at low scan rates. Increasing the
scan rate v (see Figures 9 and S-5 to S-7) results in disappearance of the irreversible reduction event (the more cathodic peak in the
cyclic voltammograms of 2-4a) and an increase of the anodic-to-cathodic peak current ratio (Ipa/Ipc) of their first reduction wave. These
observations can be readily explained in terms of irreversible follow-up reactions occurring after the first reduction process, forming
side products.
Table 4. Summary of the redox features of complexes Fe2(CO)6{(µ-
SCH2)2EMe2} (1-4a) in 0.1 M CH2Cl2-[n-Bu4N][PF6] solution measured at v =
0.2 V∙s-1 using glassy carbon disk (A = 0.072 cm2). Potentials E are given in
volts (V) and referenced to the ferrocenium/ferrocene couple (Fc+/Fc).
[a]
[b]
[a]
[c]
[d]
Complex
Ered1
E1/2
Ered2
Eox,rot
Eox1
1
2
-1.75 (Epc), -1.61 (Epa
-1.71 (Epc), -1.61 (Epa
-1.72 (Epc), -1.62 (Epa
-1.68 (Epc), -1.57 (Epa
)
)
)
)
-1.68
−
+0.19
+0.21
+0.15
+0.13
+0.81
+0.77
+0.74
+0.70
-1.66 -1.84 (Epc) irr.
-1.67 -1.85 (Epc) irr.
-1.62 -2.20 (Epc) irr.
3
4a
[a] Ered1 and Ered2 are the potentials for the first and the second reductions,
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where Epc and Epa are the cathodic and anodic scan peak potentials. [b] E1/2
is the half-wave potential for the first reduction event. Because the Ipa/Ipc
value (anodic to cathodic peak currents ratio) at 0.2 V∙s-1 of the complexes 2-
4a is less than 1, the E1/2 can be considered as approximated values. [c]
Eox,rot is the potential of a weak oxidation peak attributed to initial oxidation to
the rotated structure as discussed previously for related compounds.[19a] [d]
Eox1 is the potential of the primary oxidation peak. A second oxidation peak
(Eox2) is observed for complexes 1, 2 and 4a as a shoulder at the first
oxidation peak, where Eox2 is more positive than Eox1
.
Figure 9. Cyclic voltammetry of (a) 0.84 mM Fe2(CO)6{(µ-SCH2)2GeMe2} (3) and (b) 0.84 mM Fe2(CO)6{(µ-SCH2)2SnMe2} (4a) in 0.1 M CH2Cl2-[n-Bu4N][PF6]
solution under Ar at various scan rates. Glassy carbon electrode (A = 0.072 cm2). E is in V against the Fc+/Fc couple. The arrows indicate the scan direction.
The primary oxidation peak of these complexes (Eox1) shows an overall cathodic shift of 110 mV on going from complex 1 to 2
to 3 to 4a. This cathodic shift is a result of an increased electron density at the di-iron core and destabilization of the HOMO by the S
lone pairs as discussed above, which eases the oxidation process. The potentials Eox1 correlate linearly with the computed vertical
ionization energies from Table 3 with an excellent coefficient of determination R2 = 0.995 (Figure S-11). Correlation of Eox1 with IEv is
consistent with a previous study of related compounds in which this oxidation event was attributed to initial oxidation to a species with
all-terminal carbonyls similar to the structure of the neutral molecule.[19a] Also similar to the previous study, a very weak oxidation
event is observed in the vicinity of 0.1 to -0.2 V that is consistent with oxidation to the cation with a rotated Fe(CO)3 group and a
bridging carbonyl ligand (Figure 7). The trend in this potential down the series is less dramatic and clearly due to the decreasing
reorganization energies of the cations.
On going from 1 to 4a the reduction potentials are less cathodic. That is, stannylated model 4a is the easiest to reduce in
comparison with the models 1, 2 and 3. This is due to reorganization and stabilization of the reduced species as previously shown in
the case of electrochemical reduction of [FeFe]-hydrogenase models with [2Fe-2X] cores, where X = S, Se and Te.[6i,19] The reduction
of the [2Fe-2X] core in this series of models becomes easier on going from X = S to Se to Te. Thus, these results show how the
stannylation can tune the redox features of the [2Fe2S] core of the model complexes.
One important aspect of mechanistic investigation of proton reduction cycle catalyzed by a model complex is the determination
of the number of electrons involved in the reduction of the model in the absence of a proton source. Studying the current function (Ipc
/
c∙v1/2, c = complex concentration) of the reduction process at slow and fast scan rates is a useful method to provide mechanistic
information.[39] Figure 10 shows that the Ipc / c∙v1/2 value of the reduction of complexes 1-4a starts to significantly increase as the scan
rate decreases beyond 5 V∙s-1. This scan rate dependence of the current function suggests that the reduction of complexes 1-4a
follows an ECE-type mechanism at slow scan rates; E = electron transfer and C = chemical process.[6i,39,40] Based on theoretical and
experimental studies on various [FeFe]-hydrogenase models including 1 and 2 as well, the intervening chemical process of the ECE
reduction mechanism of 3 and 4a might also involve similar core reorganization such that one Fe-S bond is broken and one Fe(CO)3
unit rotates and locates one of its CO ligands in a semi-bridging position, leaving an open site at the iron atom.[6i,11,12b,17] We have
recently illustrated how the steric clash between the CMe2 group and the proximal CO in 1 lowers the barrier of Fe(CO)3 rotation
during the reduction process and hence leads to ECE reduction mechanism at slow scan rates. That the intervening core
reorganization is a facile process during reduction of complexes 2-4a can also be explained in terms of the steric bulkiness of their
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central EMe2 groups of the dithiolato linkers; notwithstanding the planarity of 3 and 4a (see Table 2 for the barriers of Fe(CO)3
rotation of the complexes in the ground Fe(I)Fe(I) states).[6i]
Figure 10. The scan rate dependence of the current function of the primary reduction peaks of complexes 1-4a in 0.1 M CH2Cl2-[n-Bu4N][PF6] solution under Ar
atmosphere. Glassy carbon electrode (A = 0.072 cm2). The dashed line represents the current function expected for a one-electron process assuming D ≈ 9 × 10-6
cm2∙s-1, a value calculated for various [FeFe]-hydrogenase models[6i,18a]
.
Electrocatalysis
Furthermore, we have tested the electrochemical behavior of the models 3 and 4a in the presence of the acids CF3CO2H
(pKaMeCN = 12.7)[41] and CF3SO3H (pKaMeCN = 0.7)[41]. The presence of CF3CO2H in the solution of 3 (or 4a) shifts their primary cathodic
peak by ~110 mV to less negative potentials, which is a typical observation when protonation of a reduced species takes
place.[3e,11a,42] As evident from Figure 11, complexes 3 and 4a reveal a catalytic behavior via two processes: process I at Epcat ≈ -1.78
cat
cat
V for 3 and 4a and process II at Ep ≈ -1.91 V or -1.89 V for 3 or 4a, respectively, where Ep is the peak potential of the catalytic
waves in Figure 11. The studies of the electrocatalytic properties of 3 and 4a using the very strong acids HBF4∙Et2O (pKaDCE = -10.3,
DCE = Dichloroethane)[43] or CF3SO3H (pKaDCE = -11.4)[43] were possible only at low acid concentrations because of the significant
direct reduction of these strong acids at the glassy carbon electrode (Figures S-8 to S-10). As shown in Figures S-8 to S-10,
protonation follows reduction of 3 or 4a at low acid concentration and catalysis occurs at potential more negative than that of the
primary reduction waves of these model complexes.
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Figure 11. Cyclic voltammetry of complexes (a) 0.632 mM 3 and (b) 0.825 mM 4a in 0.1 M CH2Cl2-[n-Bu4N][PF6] solution in the presence of various equivalents of
CF3CO2H at 0.2 V∙s-1. Glassy carbon electrode (A = 0.072 cm2). E is in V against the Fc+/Fc couple. The arrows indicate the scan direction.
Conclusions
We report here dinuclear models for the H-cluster featuring unique planarity of their dithiolato linkers (µ-SCH2)2ER2 (E = Ge or
Sn and R = Me or Ph) and tetranuclear cyclic dimers with a 14-membered ring and unusual temperature dependent 1H NMR spectra.
As a result of the repulsive steric interaction between the EMe2 group and the apical carbonyl ligand in Fe2(CO)6{(µ-SCH2)2EMe2}
complexes, the central six-membered FeSCECS ring preferentially adopts an unusual conformation with five near-coplanar atoms (–
SCECS–) when E = Ge or Sn in dramatic contrast with a chair/boat conformation when E = C or Si. Computations of the complexes
in the gas phase support the notion that this near-planarity is the lowest energy conformer. Rings with E = Ge and Sn have less
torsional strain and are more deformable than those with E = C. The smaller strain energy also means that the Fe(CO)3 group
proximal to the EMe2 methyl group is less driven to rotate and exchange the apical and basal carbonyl ligands when E = Ge and Sn.
On ionization the reorganization energy of the cation to form the rotated structure with a semibridging CO substantially
decreases in the series C through Sn again because of the decreasing release of strain energy down the series. The sulfur ionization
energies are substantially lowered as the bridging atom in Fe2(CO)6{(µ-SCH2)2EMe2} complexes changes in the series C, Si, Ge, Sn
owing to increasing orbital interaction between the σ C–E bond and the S lone pair p-orbital. Consequently, the sulfur character in the
HOMO increases down the series. This denotes greater electron-richness of sulfur in the series going from C to Si to Ge to Sn. This
increase in the electron density of the [2Fe-2S] core is also reflected by the electrochemical oxidation potentials, where Eox shifts
toward less positive values on going from C to Si to Ge to Sn. Nonetheless, the reduction potential E1/2 anodically shifts in this series
with the Sn-containing complex the easiest to reduce due to diffences in reorganization energy and anion stabilization. Thus, the Sn-
containing complex is the easiest to oxidize and reduce.
In the absence of proton source, the complexes Fe2(CO)6{(µ-SCH2)2EMe2} (E = Ge or Sn) follow an ECE mechanism of
reduction similar to the cases of E = C and Si. These complexes catalyze the reduction of protons from moderate and strong acids.
Experimental Section
Materials and Techniques. All reactions were performed using standard Schlenk and vacuum-line techniques under an inert gas (argon or nitrogen).
The 1H and 13C{1H} spectra were recorded with a Bruker Avance 200 MHz spectrometer. Chemical shifts are given in parts per million with reference to
internal SiMe4 or CHCl3. The mass spectrum was recorded with a Finnigan MAT SSQ 710 instrument. The IR spectra were measured with a Perkin–
Elmer System 2000 FT-IR spectrometer. Elemental analysis was performed with a Leco CHNS-932 apparatus. Silica gel 60 (0.015–0.040 mm) was
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used for column chromatography, and TLC was performed by using Merck TLC aluminum sheets (Silica gel 60 F254). Solvents from Fisher Scientific
and other chemicals from Acros were used without further purification. All solvents were dried and distilled prior to use according to standard methods.
The starting materials (µ-S)2Fe2(CO)6[44], Me2Ge(CH2Cl)2[45] and Me2Sn(CH2I)2[23] were prepared according to literature procedures.
Synthesis of Ph2Sn(CH2I)2. Into a 100 mL three-necked round-bottom flask, Cu(OAc)2∙H2O (0.026 g, 0.13 mmol) and CH3CO2H (6.0 mL) were mixed
and heated to 90-100 °C while stirring. To this hot solution, granular zinc (30-mesh, 1.700 g, 26.0 mmol) was added and the mixture was stirred for 2
min while heating. CH3CO2H was removed from the settled Cu-Zn couple using a syringe and fresh CH3CO2H (6.0 mL) was added. The mixture was
stirred hot for 2 min. CH3CO2H was again removed and the couple was cooled, washed with Et2O (3 × 7.0 mL) and dried by a stream of N2. The deep
grey/black colored couple was covered with THF (7.0 mL) and a few drops of CH2I2 were added to initiate the reaction. When the color appears purple,
THF (11.0 mL) was added followed by dropwise addition of CH2I2 (2.09 mL, 6.96 g, 26.0 mmol) in THF (6.0 mL) using an addition funnel. The addition
should be with a rate that maintains the temperature at 40 °C. The addition took 1 h and mild heating was required during the last 15 min. This THF
solution of IZnCH2I was then cooled to ~ 0 °C and filtered under N2 into a three necked round bottom flask. A solution of diphenyltin dichloride
(Ph2SnCl2) (1.788 g, 5.2 mmol) in THF (6.0 mL) was added dropwise to the in situ generated IZnCH2I during 1 h at 40 °C. For an additional 2 h, the
reaction mixture was stirred at 40 °C. The cloudy, deep purple reaction mixture was diluted with equal volume of benzene (30 mL), extracted with 5 %
HCl (4 × 60.0 mL). The organic phase was dried with Na2SO4, concentrated and purified by column chromatography (SiO2 / hexane:CH2Cl2, 1:1) to
afford Ph2Sn(CH2I)2 (1.393 g, 48 % yield) as a colorless oil containing semi-crystalline particles of the same compound.
DEI-MS (m/z): 479 [PhSn(CH2I)2]+, 415 [Ph2SnCH2I]+, 351 [PhSn(CH2I)(CH2)]+ and 273 [Ph2Sn]+. 13C{1H} NMR (400.08 MHz, CDCl3): δ –10.53 (s, CH2),
126.0-128.78 (m, CCHCHCH), 136.85 (m, CCHCHCH), 139.31 (m, CCHCHCH). 1H NMR (400.08 MHz, CDCl3): δ 2.41 (s with Sn satellites,
2J{117Sn,1H} = 20.9 Hz, 2J{119Sn,1H} = 41.8 Hz, 4H, CH2), 7.38-7.43 (m, 4H, CCHCHCH), 7.55-7.60 (m, 2H, CCHCHCH).
Synthesis of Fe2(CO)6{µ-(SCH2)2GeMe2} (3). A red solution of (µ-S2)Fe2(CO)6 (150 mg, 0.436 mmol) in THF (10 mL) was cooled to –78 ºC and
treated dropwise with Et3BHLi (0.9 mL, 0.872 mmol, 1.0 M in THF) to give a dark green solution. After stirring the solution for ~ 20 min at –78 ºC,
Me2Ge(CH2Cl)2 (88 mg, 0.436 mmol) in THF (3 mL) was added. The mixture was stirred for 18 h while slowly warming up to room temperature, giving
rise to a dark red solution. Solvent removal was performed under N2 using a vacuum transfer line. The residue was extracted several times with
pentane and the extracts were collected and dried under N2 using a vacuum transfer line. The red residue was then purified by column chromatography
(SiO2 / hexane) to give complex 3 (31 mg, 15 % yield).
Anal. Calcd for C10H10Fe2O6S2Ge: C, 25.30 %; H, 2.12 %; S, 13.51 %. Found: C, 25.28 %; H, 2.14 %; S 13.54 %. Micro-ESI-MS (m/z): 476 [M]+, 448
[M - CO]+, 420 [M - 2CO]+, 392 [M - 3CO]+, 364 [M - 4CO]+, 334 [M - 5CO]+, 308 [M - 6CO]+. IR (CH2Cl2): 1989 (CO), 1996 (CO), 2031 (CO), 2070 (CO)
cm–1 13C{1H} NMR (300 MHz, CD2Cl2): δ 0.13 (CH3), 6.5 (CH2), 207.95 (CO). 1H NMR (300 MHz, CD2Cl2): δ 0.26 (s, 6H, CH3), 1.66 (s, 4H, CH2).
.
Synthesis of Fe2(CO)6{µ-(SCH2)2SnMe2} (4a) and [Fe2(CO)6{µ-(SCH2)2SnMe2}]2 (5a). A red solution of (µ-S2)Fe2(CO)6 (185 mg, 0.538 mmol) in THF
(15 mL) was cooled to –78 ºC and treated dropwise with Et3BHLi (1.0 mL, 1.08 mmol 1.0 M in THF) to give a dark green solution. After stirring the
solution for ~ 20 min at –78 ºC, Me2Sn(CH2I)2 (184 mg, 0.427 mmol) in THF (3 mL) was added. The mixture was stirred for 18 h while slowly warming
up to room temperature, giving rise to a dark red solution. Solvent removal was performed under N2 using a vacuum transfer line. The residue was
extracted several times with pentane and the extracts were collected and dried under N2 using a vacuum transfer line. The red residue was then
purified by column chromatography (SiO2 / pentane) and two red bands were identified being for 4a (56 mg, 25 % yield) and 5a (20 mg, 9 % yield),
respectively.
Complex 4a: Anal. Calcd for C10H10Fe2O6S2Sn: C, 23.07 %; H, 1.94 %; S, 12.32 %. Found: C, 22.98 %; H, 1.93 %; S 12.36 %. Micro-ESI-MS (m/z):
1
520 [M]+. IR (CH2Cl2): 2063 (CO), 2031(CO), 1991 (CO), 1974 (CO) cm–1. 13C{1H} NMR (200 MHz, CD2Cl2): δ –7.1 (CH3), 1.6 (CH2), 207.0 (CO). H
NMR (200 MHz, CD2Cl2): δ 0.25 (s, 6H, CH3), 1.79 (s, 4H, CH2).
Complex 5a: Anal. Calcd for C20H20Fe4O12S4Sn2: C, 23.07 %; H, 1.94 %; S, 12.32 %. Found: C, 23.10 %; H, 1.94 %; S 12.35 %. Micro-ESI-MS (m/z):
1041 [M]+. FAB-MS (m/z): 1041, 1013, 957, 929, 901, 873. IR (CHCl3): 2071 (CO), 2031 (CO), 1991 (CO) cm–1. 13C{1H}NMR (200 MHz, CD2Cl2): δ –8.1
(CH3), 1.1 (CH2), 209 (CO). 1H NMR (200 MHz, CD2Cl2): δ 0.36 (s, 12H, CH3), 2.00 (s, 8H, CH2).
Synthesis of Fe2(CO)6{µ-(SCH2)2SnPh2} (4b) and [Fe2(CO)6{µ-(SCH2)SnPh2}]2 (5b). A red solution of (µ-S2)Fe2(CO)6 (95 mg, 0.276 mmol) in THF
(15 mL) was cooled to –78 ºC and treated dropwise with Et3BHLi (0.6 ml, 0.553 mmol, 1.0 M in THF) to give a dark green solution. After stirring the
solution for ~ 20 min at –78 ºC, Ph2Sn(CH2I)2 (307 mg, 0.553 mmol) in THF (3 mL) was added. The mixture was stirred for 18 h while slowly warming
up to room temperature, giving rise to a dark red solution. Solvent removal was performed under N2 using a vacuum transfer line. The residue was
extracted several times with pentane and the extracts were collected and dried under N2 using a vacuum transfer line. The red residue was then
purified by column chromatography (SiO2 / pentane) and two red bands were identified being for 4b as a red oil (32 mg, 18 % yield) and 5b (traces),
respectively.
Complex 4b: Anal. Calcd for C20H14Fe2O6S2Sn: C, 37.25 %; H, 2.19 %; S, 9.94 %. Found: C, 37.23 %; H, 2.20 %; S 9.96 %. Micro-ESI-MS (m/z): 646
[M]+. IR (CH2Cl2): 2066 (CO), 2032 (CO), 1989 (CO) cm–1 13C{1H} NMR (200 MHz, CD2Cl2): δ 1.70 (CH2), 128.7-130.6 (c,d-CPh), 136.7-138.7 (a,b-CPh),
.
208.6 (CO), 209.1 (CO). 1H NMR (200 MHz, CD2Cl2): δ 2.27 (s, 4H, CH2), 7.35-7.70 (m, 10H, 2C6H5).
Photoelectron Spectroscopy: Photoelectron spectra were recorded using an instrument that features a 360 mm radius hemispherical analyzer
(McPherson),[46] with a custom-designed photon source, sample cells, detection, and control electronics. Calibration and data analysis were described
previously.[47] All samples sublimed cleanly, with no visible changes in the spectra during data collection after initial observation of ionizations from the
diiron compounds. The sublimation temperatures (at 10-6 Torr) were 52–77, 64–84 and 62–87 °C for compounds 1, 2 and 3, respectively.
Density Functional Theory (DFT) Computations: DFT computations were carried out with the Amsterdam density functional (ADF2014.06)
package.[48,49] The geometries and energies were refined in the gas phase and in solution with the PBE functional[50] with dispersion corrections
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10.1002/chem.201603843
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according to the method of Grimme using the BJ damping function (PBE-D3-BJ).[51] All calculations utilized a triple-ζ Slater type orbital (STO) basis set
with one polarization function (TZP) and relativistic effects by the zero-order regular approximation (ZORA).[52,53] The frozen-core approximation was
used for the inner core of all heavy atoms. Diffuse density fitting was employed at the quadruple-ζ level with four polarization functions (QZ4P), and the
Becke grid was used with 9402 points.[54] Geometries were further optimized in the solvents tetrahydrofuran, dichloromethane, and acetone according
to the COSMO model with default parameters.[55] Figures of the optimized geometries and molecular orbital plots were created with the program
VMD.[56]
Structure Determinations. The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer using graphite-
monochromated Mo-K radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semi-empirical basis
using multiple-scans[57-59]. The structures were solved by direct methods (SHELXS[60]) and refined by full-matrix least squares techniques against Fo2
(SHELXL-97[60]). The hydrogen atoms of 3 and 5b were located by difference Fourier synthesis and refined isotropically. All other hydrogen atoms
were included at calculated positions with fixed thermal parameters. All non-hydrogen atoms were refined anisotropically[60]. Crystallographic data as
well as structure solution and refinement details are summarized in Table S-4. MERCURY was used for structure representations.
Cyclic Voltammetry. These experiments do not involve corrections for the iR drop. Cyclic voltammetric experiments were performed in a three-
electrode cell using a Radiometer potentiostat (μ-Autolab Type-III or an Autolab PGSTAT 12) driven by the GPES software. The working electrode
consisted of a vitreous carbon disk (A = 0.072 cm2) that was polished on a felt tissue with alumina before each CV scan. The Ag/Ag+ reference
electrode was separated from the analyte by a CH2Cl2–[n-Bu4N][PF6] bridge. All the potentials are quoted against the ferrocene-ferrocenium couple
(Fc+/Fc); ferrocene was added as an internal standard at the end of the experiments.
Acknowledgements
The authors (TS, DLL and RSG) gratefully acknowledge support of this work by the U. S. National Science Foundation (Grant No.
1111718 and 1111570). L. A. and H. A. are grateful to the Deutscher Akademischer Austausch Dienst (DAAD) for scholarships. G. M.
and W. W. thank the National Science Center (Cracow, PL) for financial support (Grant Maestro-3; Dec-2012/06/A/ST5/00219).
Keywords: hydrogenase • conformational analysis • sulfur • electron-richness • reorganization energies • photoelectron • DFT •
Electrocatalysis
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Hydrogenase-mimic Conformations*
With E = Ge or Sn the six-membered
FeSCECS rings adopt an unusual
conformation with SCECS atoms
almost coplanar in contrast to the
chair/boat conformation when E = C
or Si.
Hassan Abul-Futouh, Laith R.
Almazahreh ,* Takahiro Sakamoto, Nhu
Y T. Stessman, Dennis L.
Lichtenberger,* Richard S. Glass,*
Mohammad El-khateeb, Philippe
Schollhammer,* Grzegorz Mloston and
Wolfgang Weigand*
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[FeFe]-Hydrogenase Models with
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Title
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