Synthetic and electrochemical studies of [2Fe2S] complexes containing a 4-amino-1,2-dithiolane-4-carboxylic acid moiety

Article abstract of DOI:10.1002/ejic.201000619

In the search for novel [FeFe] hydrogenase model systems several di- and tripeptides containing an 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) moiety or N-phenylsulfonyl-Adt-OMe were treated with Fe₃(CO)₁₂. The resulting [FeFe] complexes were characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry and elemental analysis. All complexes were investigated by cyclic voltammetry, and their ability to catalyze the reduction of protons to dihydrogen, as a function of acid concentration, was studied in different solvents. The synthesis of [FeFe] hydrogenase model complexes, carrying an amino acid containing ligand, and their electrochemical properties are reported.

Full text of DOI:10.1002/ejic.201000619

FULL PAPER
DOI: 10.1002/ejic.201000619
Synthetic and Electrochemical Studies of [2Fe2S] Complexes Containing a
4-Amino-1,2-dithiolane-4-carboxylic Acid Moiety
Ulf-Peter Apfel,[a] Christian R. Kowol,[b] Enrico Morera,[c] Helmar Görls,[a] Gino Lucente,[c]
Bernhard K. Keppler,[b] and Wolfgang Weigand*[a]
Keywords: Iron / Sulfur / Hydrogenase / Amino acids / Electrocatalysis
In the search for novel [FeFe] hydrogenase model systems
several di- and tripeptides containing an 4-amino-1,2-dithiol-
ane-4-carboxylic acid (Adt) moiety or N-phenylsulfonyl-Adt-
OMe were treated with Fe3(CO)12. The resulting [FeFe] com-
plexes were characterized by 1H and 13C NMR spectroscopy,
mass spectrometry and elemental analysis. All complexes
were investigated by cyclic voltammetry, and their ability to
catalyze the reduction of protons to dihydrogen, as a function
of acid concentration, was studied in different solvents.
formationally restricted dithiolane was found in a natural
occurring dibrominated indole enamide, termed Kott-
amide E (Scheme 1) that was isolated from the ascidian
Pycnoclavella kottae.[9]
Introduction
Since the active site of the [FeFe] hydrogenase was eluci-
dated by Peters et al.[1] and Fontecilla-Camps et al.[2] vigor-
ous efforts have been undertaken to model the active site of
this enzyme. Numerous model complexes are known and
recent progress has been made in mimicking the specific
features of the active site of [FeFe] hydrogenase. In particu-
lar, the formation of the “rotated” state (formation of a
bridging CO ligand and a vacant coordination site on one
iron center),[3] the formation of terminal hydrides during
catalysis,[4] the establishment of an electron cascade towards
the [2Fe2S] subunit,[5] as well as the influence of proton
shuffle from an adjacent base towards the iron centres dur-
ing the electrocatalytic dihydrogen formation[6] are issues
that are in the focus of many research groups. However, so
far minor attention has been directed towards the use of
amino acid containing derivatives for mimicking the
enzymatic environment.[7] In continuation of our efforts on
[2Fe2S] complexes including an amino acid moiety, 4-
amino-1,2-dithiolane-4-carboxylic acid (Adt) derivatives
were selected as coordinating ligands. The ligand Adt ex-
hibits a dithiolane ring in the α-position and has been
known exclusively as a synthetic building block for a long
time.[8] But in 2003 the carboxamide derivative of this con-
Scheme 1. Kottamide E containing the carboxamide of Adt.
Since the reaction of carbonyliron complexes with di-
thiolanes generates stable double chair conformation com-
plexes, Adt could be an ideal precursor molecule for hydro-
genase models, offering an internal base, the capability to
form noncovalent interactions via the functional group of
the side chain[10] and a variety of possible derivatisations.
However, initial attempts with Adt containing [FeFe]
model complexes revealed no influence of the pendant base
on the catalytic ability of the complexes.[7c] Thus, novel Adt
derivatives with additional functional groups were synthe-
sized. A methionine and/or phenylalanine or proline moiety
was coupled to the Adt residue, thus building di- and tri-
[a] Institut für Anorganische und Analytische Chemie, Friedrich- peptidic ligands (Scheme 2). Furthermore, a N-sulfonyl de-
Schiller Universität Jena,
rivative of Adt offering an acidic NH proton has also been
August-Bebel-Straße 2, 07743 Jena, Germany
examined (Scheme 2). All dithiolane ligands were treated
with Fe3(CO)12 to afford the corresponding [2Fe2S] com-
plexes, and the ability of these compounds to act as [FeFe]
hydrogenase model complexes was investigated by cyclic
voltammetry. In addition, the electrocatalytical properties
of these complexes in different aprotic solvents were com-
pared.
Fax: +49-3641-948-102
E-mail: wolfgang.weigand@uni-jena.de
[b] University of Vienna, Institute of Inorganic Chemistry,
Währingerstr. 42, 1090 Vienna, Austria
[c] Dpt. di Chimica e Tecnologie del Farmaco, “Sapienza” Uni-
versità di Roma,
P.le A. Moro 5, 00185 Roma, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000619.
View this journal online at
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Scheme 2. Adt containing ligands and the products of the reactions of these ligands with Fe3(CO)12.
trimethylamine N-oxide that should force an oxidative
cleavage of a CO group[11] and facilitate the coordination
of the thioether group.
Results and Discussion
The reaction of Boc-Adt-OMe (1) with thionyl chloride
generates the free amine, which was reacted in situ with 1-
Compound 2 was treated with sodium hydroxide to gen-
[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochlo- erate the associated carboxylic acid, which was used with-
ride (EDC), 1-hydroxybenzotriazole (HOBt) and N-(tert- out further purification for the conversion to the tripeptide
butoxycarbonyl)--methionine to afford the dipeptide 2 Boc-Met-Adt-Phe-OMe (4) via standard esterification pro-
(Scheme 2).[12] This molecule possesses a thioether group cedures using EDC, HOBt and -phenylalanine methyl es-
that enables the formation of [2Fe3S] clusters. The reaction ter.[12] Subsequent reaction of 4 with Fe3(CO)12 afforded the
of 2 with Fe3(CO)12 under reflux conditions results in the complex 5. Furthermore, the dipeptide Boc-Adt-Phe-OMe
formation of complex 3 in a moderate yield of 52%. The (6) and its corresponding [FeFe] complex 7 were synthe-
1
one- and two-dimensional H and 13C NMR spectra, the sized according to the same procedure. Additionally, com-
m/z values and isotopic distribution of the mass spectrome- plexes 3, 5 and 7 were treated with trifluoroacetic acid
try data, as well as elemental analysis revealed the molecu- (TFA) in an attempt to achieve N-Boc-deprotection, how-
lar structure of compound 3. However, no coordination of ever no reaction was observed and no complexes containing
the methionine sulfur to the iron centre could be observed a free amino group could be obtained. Thus, we decided to
either by thermal treatment or by the reaction of 3 with examine the properties of the dipeptide Boc-Pro-Adt-OMe
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Synthetic and Electrochemical Studies on [2Fe2S] Complexes
(8) (Scheme 2) in which, unlike in the model compounds 2,
4 and 6, linear side chains are not present. Dipeptide 8, due
to the presence of two consecutive cyclic amino acids,[13]
should prefer a compact folded backbone conformation
with reduced steric interferences in the side chain and there-
fore display distinct interaction capabilities. N-Boc-Proline
was treated with H-Adt-OMe to give 8 in moderate yield
(47%). Single crystals for X-ray diffraction analysis were
obtained by slow evaporation of a solution of 8 in chloro-
form (see Figure S1). The crystal structure of 8 showed that
it is cocrystallized with chloroform and has the expected
molecular structure (see Supporting Information). By
studying the bond lengths and angles for the Adt part of
the compound some interesting features become obvious.
The dithiolane ring exhibits two different C–S bond lengths
of 178.8(5) and 182.3(6) pm as well as two different C–S–S
angles of 92.66(19) and 90.20(19)° (see Table S1). Compari-
son of the torsion angle C–S–S–C in 1 and 8 revealed an
increase from 31.50 to 44.97°, whereas the S–S distance is
unchanged being 205.6(3) and 204.6(3) pm, respectively.[12]
These data are very similar to those observed for Boc-Adt-
Adt-NHMe where the torsion angle (C–S–S–C) is 42.6° and
steric reasons have been suggested for the distortion of the
dithiolane ring.[13] Unfortunately, all attempts to get single
crystals of any of the [FeFe] model complexes were unsuc-
cessful. Reaction of 8 with Fe3(CO)12 resulted in complex 9
(Scheme 2) in good yield (65%), but again attempts to de-
protect the peptide with standard reagents (TFA, thionyl
chloride, hydrogen chloride) failed. The deprotection of di-
peptide 8 with TFA and subsequent reaction with Fe3(CO)12
resulted in a mixture of species that could not be separated.
A different approach was attempted with a sulfonyl group
bound to the Adt-OMe residue. Sulfonamides have been
known as the largest class of antimicrobial agents for a long
time, and have shown to be irreversible inhibitors of cysteine
proteases.[14] In addition to their importance in medicinal
chemistry,[15] the electron withdrawing properties of the sul-
fonyl group results in an acidic NH proton, which can be
deprotonated under mild conditions.[16] This was confirmed
by kinetic studies of the hydrolysis of p-nitrophenyl-o-meth-
Scheme 3. Hypothetical mechanistic pathway for the desired pro-
tonation of the [FeFe] core.
Electrochemistry
The cyclic voltammograms (Figure 1) of [FeFe] metal
complexes 3, 5, 7, 9 and 11 were recorded in order to ob-
serve the electrochemically induced reduction and oxidation
properties of these compounds, and to assess their ability
to catalyse the formation of dihydrogen from weak acids. A
comparison of the electrochemical data is shown in Table 1.
Figure 1. Cyclic voltammograms of complex 9 (1 m) recorded in
the presence of different concentrations of acetic acid.
Table 1. Electrochemical data[a] for the iron complexes discussed
herein.
Compound
Ep [FeIFeI]/ Ep [FeIFeI]/ Ep [Fe0FeI]/
[FeIIFeI]
[Fe0FeI]
[Fe0Fe0]
ylsulfonamidobenzoate[17] (pKa = 8.41 in acetonitrile) and Fe2(CO)6(Boc-Adt-OMe)[7c] +0.81[b]
–1.47[b]
–1.52[b]
–1.57[b]
–2.09[b]
–1.76[b]
n/a
3
5
7
+0.98[b]
+0.95[b]
p-nitrophenyl-p-methylsulfonamidobenzoate[18] (pKa = 7.34
in acetonitrile). Thus a proton shift from the sulfone NH
groups to the iron centres is possible. Inspired by these prop-
erties, the dipeptide 10 was synthesised and reacted with
Fe3(CO)12 in toluene to afford complex 11 in good yield
(83%) as an air and moisture stable red solid. To investigate
the possible influence of the sulfonamide group on the cata-
lytic mechanism of this complex, NMR investigations were
performed in the presence of CD3COOD. A decrease in the
intensity of the NH resonance at δ = 5.71 ppm should be
observed due to proton exchange with deuterium. This can
+0.71[b] and –1.51[b]
+0.94[b,c]
–1.89[b]
9
+0.8[b] and –1.48[b]
+0.97[b,c]
–1.81[b]
–1.72[b]
11
+0.8[b] and –1.48[b]
+1.0[b,c]
[a] Potentials given in VoltϮ0.01 vs. 0.01  Ag/Ag+ in CH3CN
with 0.10  [nBu4N][BF4]. [b] Irreversible wave. [c] Further oxi-
dation to the [FeIIFeII] state.
Complexes 3, 5, 7, 9 and 11 show quite similar electro-
occur either via a SN2 mechanism similar to that discussed chemical properties (Table 1) and as representative example
by Mandell et al. (Scheme 3)[19] or by dissociation of the N– a detailed description will be given for compound 9. Fol-
H bond and subsequent deuteration of the N atom. How- lowing a cathodic scan of complex 9 (Figure 1) starting at
ever, no change in the NMR resonance could be observed, 0.00 V vs. 0.01  Ag/Ag+ in CH3CN, the cyclic voltammog-
in agreement with IR measurements, with and without ace- ram reveals an irreversible reduction peak at Ep,red
tic acid being present.
=
–1.48 V that is attributable to the [FeIFeI]Ǟ[FeIFe0]– pro-
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cess. This irreversible signal suggests an EC (E = electro- very fast with protons from strong acids like TFA to give a
chemical reaction, C = chemical reaction) mechanism [{H–}FeIIFeI] assembly (Scheme 4).[22b] For low TFA con-
whereby the FeIFeI state is converted to [FeIFe0]– by a one- centrations (Ͻ 4 m) the reduction to [HFeIFeI] is followed
electron reduction followed by a fast change in the bonding by a second protonation ([{H2}FeIFeI]), resulting in the re-
properties within the molecule, which is in good agreement lease of dihydrogen at around –1.6 V. Thus, an ECEC
with literature results.[20] This change in bonding properties mechanism is suggested in the presence of TFA (Ͻ 4 m)
is best described by the cleavage of the Fe–Fe bond and/or in accordance with the results found by Pickett et al.[24] At
the appearance of a bridging carbonyl group.[20,21] At high concentrations of TFA (Ͼ 4 m) a new catalytic re-
–1.81 V a further small irreversible reduction wave can be duction signal can be observed, which continuously shifts
observed, and is attributed to the [FeIFe0]– Ǟ[Fe0Fe0]2– to more negative potentials, ending up at –1.86 V when the
process in accordance with data collected on Fe2(CO)6- concentration of TFA is 10 m. This behaviour can be ex-
(Boc-Adt-OMe).[7a] After reversal of the scan, an oxidation plained by further reduction of [{H2}FeIFeI] resulting in the
peak at –1.23 V occurs, 250 mV more positive than the formation of dihydrogen and regeneration of [Fe0FeI]–
[FeIFeI]Ǟ[FeIFe0]– reduction potential, suggesting that this (Scheme 4).[24]
signal does not arise from the simple re-oxidation of
[FeIFe0]– but from the oxidation of the rapidly formed reac-
tion product. At ca. +0.8–1.0 V the [FeIFeI] oxidation peak
can be observed. In addition, in the case of complexes 7
and 9 a second well separated oxidation wave occurs, which
can be ascribed to the [FeIFeII]+ Ǟ[FeIIFeII]2+ process.
In the presence of acetic acid (AcOH) the initial re-
duction potential at –1.48 V remains constant. Thus, prior
to the reduction no protonation of compound 9 takes place
(this is equally true for 3, 5, 7 and 11), suggesting no in-
volvement of the ligand amide groups as proton donors.
However, with continuous addition of acetic acid a steadily
increasing current at around –2.2 V emerged, indicating the
electrocatalytic formation of dihydrogen.[22] In agreement
with the literature, the second reduction step that creates
Figure 2. Cyclic voltammograms of complex 9 (1 m) recorded in
the presence of different concentrations of trifluoroacetic acid.
[Fe0Fe0]2– is followed by two protonation reactions leading
to the generation of dihydrogen and the simultaneous re-
generation of the initial [FeIFeI] state (Scheme 4).[22a]
To obtain further information about the parameters that
affect the potential at which dihydrogen is formed, and the
redox properties of the [FeFe] complexes, cyclic voltamme-
try measurements of complex 9 were repeated in the aprotic
solvents dimethylformamide (DMF), dimethyl sulfoxide
(DMSO) and dichloromethane (CH2Cl2) in the presence of
5 and 10 mmol AcOH, the solvent leads to an alteration in
the acid strength [pKa = 13.2 in DMF; pKa = 12.3 in
DMSO] as compared to CH3CN.[25,26] To allow for com-
parison with the data obtained in CH3CN, the reduction
potentials of AcOH in these solvents were investigated in
the absence of the complex (Table 2). In contrast to the
Scheme 4. Catalytic mechanisms for the electrochemical reactions
strong acetic acid reduction signal at around –2.3 V in
of [FeFe] complexes in the presence of a weak (grey) and strong
CH3CN, the respective spectra in DMF and DMSO re-
acids (black). The Ep values reported are for compound 9.
vealed almost no dihydrogen development up to –2.7 and
–2.9 V vs. 0.01  Ag/Ag+ in CH3CN, respectively, values
When TFA (pKa = 12.65 in CH3CN)[23] is used in the that are near to the solvent cut off potentials. In the case
voltammetry experiments instead of acetic acid (pKa = 22.6 of the CH2Cl2 voltammogram a small reduction peak at
in CH3CN),[23] no shift in the [FeIFeI]Ǟ[FeIFe0]– reduction around –2.1 V can be observed (solvent cut off at around
potential could be observed (Figure 2), confirming that –2.2 V).
TFA is not able to remove the Boc protecting group from
Without the addition of AcOH the redox potential of the
the proline nitrogen, as also seen in the synthesis experi- [FeIFeI]Ǟ[FeIFe0]– reduction of complex 9 is similar in all
ments. Nevertheless, the cyclic voltammograms (Figure 2) investigated solvents. Addition of 5 mmol AcOH to a DMF
are strongly altered compared to those recorded in the pres- solution of 9 results in a continuous increase in the current
ence of AcOH. It is known that, in contrast to weak ac- starting at ca. –2.0 V, but not in a distinct reduction signal
ids,[22] the one-electron reduction product [Fe0FeI] can react (Figure 3). Furthermore, the intensity of the catalytic cur-
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Synthetic and Electrochemical Studies on [2Fe2S] Complexes
Table 2. Comparison of electrochemical reduction of AcOH in dif-
Conclusions
ferent solvents in the absence and in the presence of complex 9.[a]
Diiron dithiolato compounds containing Adt have been
prepared in good yields, and can be considered as [FeFe]
hydrogenase model compounds. The N-Boc-protected
amino acid backbones of these novel di- and tripeptidic
complexes do not influence the electrochemical formation
of dihydrogen in the presence of AcOH or TFA. This be-
haviour was also observed in the case of a sulfonamide
complex containing an acidic NH proton. A comparative
study of the electrochemical properties of compound 9 in
DMSO, DMF, CH2Cl2 and CH3CN revealed a very similar
redox response of the complex in all solvents, but the dihy-
drogen formation strongly depends on the solvent. Only in
the case of CH2Cl2 is the catalytical current associated with
dihydrogen formation comparable to that observed with the
standard solvent CH3CN, whereas DMF and DMSO do
not appear to be suitable solvents for the characterisation
of the electrocatalytical properties of [2Fe2S] complexes.
Solvent
Ep (H+/H2)
Ep (H+/H2) in the presence of 9
CH3CN
DMF
DMSO
CH2Cl2
–2.3
shoulder at –2.1
Ͻ –2.7
Ͻ –2.9
–2.1
no distinct reduction peak
no distinct reduction peak
shoulder at –2.1
[a] Potentials in V vs. 0.01  Ag/Ag+ in CH3CN containing 0.10 
[nBu4N][BF4] as the supporting electrolyte.
rent increases only slightly when the acid concentration is
increased from 5 to 10 equiv., and nearly no difference
could be observed when the concentration is changed from
10 to 15 mmol AcOH. With DMSO as the solvent, 9 exhib-
ited similar electrochemical behaviour as in DMF solution,
again no correlation between the AcOH concentration and
the catalytic current was observed. The scan recorded in
CH2Cl2 shows a shoulder at around –2.1 V associated with
the catalytic transformation of protons to dihydrogen (Fig-
ure 3), and a stepwise increase of the catalytic current.
Thus, with respect to the characterisation of the electrocata-
lytical properties, only CH2Cl2 is comparable to CH3CN,
however with the limitation that the cut off potential is far
more positive at around –2.2 V vs. 0.01  Ag/Ag+ in
CH3CN.
Experimental Section
General: All reactions were carried out in an argon atmosphere.
Toluene and THF were dried with KOH and distilled from sodium/
benzophenone. Chemicals were received from Fluka or Acros and
used without further purification. Compounds 1, 2, 4 and 6 as well
as Boc-Pro-OH were synthesized following literature pro-
cedures.[12,27] Thin layer chromatography (TLC) was performed on
Merck silica gel 60 F254 plates (detection under UV light at
254 nm) and FC (flash chromatography) on Fluka silica gel 60.
1H NMR and 13C{1H} NMR spectra were recorded on a Bruker
AVANCE 200 MHz or 400 MHz spectrometer, whereby the split-
ting of the proton resonances are defined as s (singlet), d (doublet),
t (triplet) and m (multiplet). Chemical shifts are given in parts per
million with reference to an internal standard, CHCl3 or SiMe4.
Infrared spectra were obtained from KBr pellets with a Perkin–
Elmer 2000 FT-IR instrument or Perkin–Elmer 983 spectropho-
tometer. The intensity of the signals are assigned as vs (very
strong), s (strong), m (medium) and w (weak). Electron impact
mass spectrometry (MS) was carried out at 70 eV with a Finnigan
SSQ710 or Q-TOF Micro instrument (Micromass, Waters) with de-
sorption electron ionisation (DEI), fast atom bombardment (FAB)
or electron spray ionisation (ESI) mode. Expected and experimen-
tal isotope distributions were compared. Elemental analyses (C, H,
N, S) were carried out on a LECO CHNS-931 instrument or on a
Fisons EA-1108 apparatus. Prior to the elemental analyses, all sam-
ples were dried under high vacuum for at least one day, and the
remaining amounts of hexane are in agreement with the amounts
1
determined from H NMR spectroscopy.
[(Boc-Met-Adt-OMe)Fe2(CO)6] (3): Boc-Met-Adt-OMe (2) (25 mg,
0.061 mmol) and Fe3(CO)12 (31 mg, 0.061 mmol) were dissolved in
dry toluene (20 mL) and refluxed for 30 min. The solvent was evap-
orated under reduced pressure and the crude product was purified
1
by FC (THF/hexane = 1:3); yield 22 mg (52%) as a red solid. H
NMR (200 MHz, CDCl3): δ = 6.82 (m, 1 H, NHBoc), 4.90 (d,
3JH,H = 8.2 Hz, 1 H, NH-Adt), 4.30 (m, 1 H, CH), 3.66 (s, 3 H,
OCH3), 3.06 and 2.77 (2d, 2JH,H = 14.2 Hz, 2 H, CCHAHBS), 2.52
3
2
(t, JH,H = 7.1 Hz, 2 H, CH2SCH3), 2.40 and 2.27 (2d, JH,H
=
13.4 Hz, 2 H, CCHAHBS), 2.11 (s, 3 H, SCH3), 2.02–1.83 (m, 2 H,
CH2CH) 1.41 [s, 9 H, C(CH3)3] ppm. 13C NMR (50 MHz, CDCl3):
δ = 207.2 (CO), 172.0 [C(O)OCH3], 170.7 [C(O)NH], 156.2
Figure 3. Cyclic voltammograms of complex 9 (1 m) recorded in
the presence of acetic acid in DMF (top) and CH2Cl2 (bottom).
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[tBuOC(O)NH], 80.7 [C(CH3)3], 61.5 (SCH2CCH2S), 53.4 (OCH3), tracted with 10% citric acid, saturated NaHCO3 and water. After
53.1 (CHNHBoc), 30.3 (CH2CH), 30.0 (SCH2CCH2S), 29.7 drying the solution with Na2SO4 and removal of the solvent under
(CH2SCH3), 28.3 [C(CH3)3], 15.2 (SCH3) ppm. MS (FAB in meta- reduced pressure, the crude product was purified by FC (ethyl acet-
nitrobenzyl alcohol): m/z = 691 [M + H]+. IR (KBr): ν = 3423 (m), ate/hexane = 35:65); yield 318 mg (47%) as a white solid. 1H NMR
˜
2960 (m), 2924 (m), 2853 (m), 2076 (vs), 2036 (vs), 1996 (vs), 1740 (400 MHz, CDCl3): δ = 7.95 (s, 1 H, NH), 4.29 (m, 1 H, CH), 3.74
(s), 1686 (s) cm–1. C21H26Fe2N2O11S3·1.7hexane (690.35): calcd. C (s, 3 H, OCH3), 3.52 (m, 2 H, NCH2), 3.33 (m, 4 H, SCH2CCH2S),
44.78, H 6.00, N 3.35, S 11.50; found C 44.56, H 6.19, N 3.42, S 2.32–2.01 (m, 1 H, CHCH2CH2), 1.85 (m, 3 H, CHCH2CH2 and
11.76.
CHCH2CH2), 1.45 [s, 9 H, C(CH3)3] ppm. 13C NMR (100 MHz,
CDCl3): δ = 172.1 [C(O)OCH3], 170.1 (CONH), 155.8 (tBuO-
CONH), 80.8 [C(CH3)3], 70.7 (SCH2CCH2S), 60.1 (CH), 53.2
(OCH3), 47.6 (NCH2), 47.0 (SCH2CCH2S), 30.8 (CHCH2CH2),
28.3 [C(CH3)3], 24.3 (CHCH2CH2) ppm. MS (DEI): m/z = 376
[(Boc-Met-Adt-Phe-OMe)Fe2(CO)6] (5): Boc-Met-Adt-Phe-OMe
(4) (44 mg, 0.081 mmol) and Fe3(CO)12 (41 mg, 0.081 mmol) were
dissolved in dry THF (40 mL) and refluxed for 45 min. The solvent
was evaporated under reduced pressure and the crude product was
purified by FC (THF/hexane = 1:1); yield 29 mg (42%) as a red
solid. 1H NMR (400 MHz, CDCl3): δ = 7.71–7.14 (m, 5 H,
Haromatic), 6.98 (s, 1 H, NH-Adt), 5.09 (m, 1 H, NHBoc), 4.84 [m,
1 H, CHC(O)OCH3], 4.71 (m, 1 H, CHNHBoc), 4.22 (s, 3 H,
[M]+. IR (KBr): ν = 3274 (s), 3219 (s), 3054 (m), 2975 (s), 2877
˜
(m), 1748 (s), 1682 (s), 1543 (s), 1479 (m), 1424 (s), 1289 (s) cm–1.
C15H24N2O5S2 (376.50): calcd. C 47.85, H 6.43, N 7.44, S 17.03;
found C 48.05, H 6.69, N 7.17, S 17.04.
OCH3), 3.71 (m, 4 H, SCH2C), 3.49 (m, 2 H, CH2Ph), 3.12 (m, 2 [(Boc-Pro-Adt-OMe)Fe2(CO)6] (9): Boc-Pro-Adt-OMe (8) (103 mg,
H, CH2SCH3), 2.58 (m, 2 H, CH2CH2S), 2.10 (s, 3 H, SCH3), 1.49 0.27 mmol) and Fe3(CO)12 (138 mg, 0.27 mmol) were dissolved in
[s, 9 H, C(CH3)3] ppm. 13C NMR (100 MHz, CDCl3): δ = 207.3 dry toluene (40 mL) and refluxed for 90 min. The solvent was evap-
(CO), 171.5 [C(O)OCH3], 167.7 [CHC(O)NH], 155.9 [OC(O)NH], orated under reduced pressure and the crude product was purified
132.3, 130.8, 129.9, 128.2 (Caromatic), 80.8 [C(CH3)3], 68.1 by FC (THF/hexane, 1:1); yield 116 mg (65%) as a red solid. 1H
(SCH2CCH2S), 53.8 (CHCH2Ph), 53.6 (CHNHBoc), 52.3 (OCH3), NMR (200 MHz, CDCl3): δ = 7.89 (s, 1 H, NH), 4.38 (m, 1 H,
37.9 (CH2Ph), 30.3 (SCH2CH2CH), 30.2 (SCH2CCH2S), 29.3 CH), 3.62 (s, 3 H, OCH3), 3.28 (m, 2 H, NCH2), 2.57 (m, 2 H,
(CH2SCH3), 28.2 [C(CH3)3], 15.2 (SCH3) ppm. MS (Micro-ESI in SCHAHB), 2.24 (m, 3 H, SCHAHB and CHCH2CH2), 1.85 (m, 3
CHCl /CH OH): m/z = 860 [M + Na]+. IR (KBr): ν = 3418 (s), H, CHCH2CH2 and CHCH2CH2), 1.47 [s, 9 H, C(CH3)3] ppm. 13
C
˜
3
3
2955 (m), 2924 (s), 2850 (m), 2074 (vs), 2035 (vs), 1989 (vs), 1931 NMR (50 MHz, CDCl3): δ = 207.3 (CO), 172.4 [C(O)OCH3], 171.1
(w), 1693 (vs), 1514 (m), 1504 (m)
cm–1. [C(O)NH], 156.6 (tBuOCONH), 80.7 [C(CH3)3], 61.6
C30H35Fe2N3O12S3·0.5hexane (837.53): calcd. C 45.01, H 4.81, N (SCH2CCH2S), 59.2 (CH), 53.2 (OCH3), 46.7 (NCH2), 29.4
4.77, S 10.92; found C 45.11, H 4.98, N 4.62, S 11.05.
(SCH2CCH2S), 26.3 [C(CH3)3], 24.6 (CHCH2CH2), 22.3
(CHCH2CH2) ppm. MS (Micro-ESI in CH2Cl2/CH3OH): m/z =
[(Boc-Adt-Phe-OMe)Fe2(CO)6] (7): Boc-Adt-Phe-OMe (6) (25 mg,
0.059 mmol) and Fe3(CO)12 (30 mg, 0.059 mmol) were dissolved in
dry toluene (20 mL) and the dark green solution was stirred under
reflux for 30 min. The solvent was evaporated under reduced pres-
sure and the crude product was purified by FC (THF/hexane =
679 [M + Na]+. IR (KBr): ν = 2966 (w), 2930 (w), 2867 (w), 2076
˜
(vs), 2036 (vs), 1999 (vs), 1742 (m), 1639 (m), 1410 (m) cm–1.
C21H24Fe2N2O11S2·0.3hexane (656.27): calcd. C 40.15, H 4.17, N
4.11, S 9.40; found C 40.24, H 4.21, N 4.06, S 9.30.
1:3); yield 28 mg (67%) as a red solid. 1H NMR (200 MHz, N-Phenylsulfonyl-Adt-OMe (10): Thionyl chloride (17 µL,
3
CDCl3): δ = 7.24–6.96 (m, 5 H, Haromatic), 6.64 (d, JH,H = 7.8 Hz, 0.23 mmol) was added to a solution of Boc-Adt-OMe (1) (64 mg,
1 H, NHCH), 4.69 (m, 2 H, NHBoc and NHCH), 3.66 (s, 3 H, 0.23 mmol) in dry MeOH (1.5 mL) at 0 °C. The solution was
3
OCH3), 3.04 (d, JH,H = 4.4 Hz, 2 H, CHCH2), 2.92 and 2.76 (2d, heated to 50 °C and stirred for 3 h. Removal of the solvent under
2JH,H = 14 Hz, 2 H, CHAHBS), 2.40 and 2.20 (2d, 2JH,H = 14.6 Hz, reduced pressure afforded H-Adt-OMe·HCl (50 mg), which was
2 H, CHAHBS), 1.41 [s, 9 H, C(CH3)3] ppm. 13C NMR (50 MHz, used in the next step without further purification. H-Adt-OMe·HCl
CDCl3): δ = 207.0, 206.6 (CO), 171.5 [C(O)NH], 170.7 [C(O)- was suspended in dry CH2Cl2 (2 mL) and phenylsulfonyl chloride
OCH3], 153.3 [tBuOC(O)NH], 135.5, 129.2, 127.2, 125.5 (Caromatic), (36 µL, 0.28 mmol) and triethylamine (0.11 mL, 0.79 mmol) were
81.8 [C(CH3)3], 61.6 (SCH2CCH2S), 53.4 (NHCH), 52.4 (OCH3), added. The mixture was stirred at room temp. for 24 h and then at
37.9 (CHCH2), 30.3 (SCH2CCH2S), 28.1 [C(CH3)3] ppm. MS 40 °C for 4 h. After evaporation of the solvent under reduced pres-
(Micro-ESI in CHCl ): m/z = 729 [M + Na]+. IR (KBr): ν = 3371 sure, the residue was purified on silica gel (3 g) using CH2Cl2 as
˜
3
(m), 3315 (m), 2955 (m), 2927 (m), 2855 (w), 2074 (vs), 2036 (vs), the eluent; yield 38 mg (51%) as a white solid. 1H NMR (400 MHz,
2000
(vs),
1750
(s),
1690
(s),
1649
(s)
cm–1. CDCl3): δ = 7.52–7.94 (m, 5 H, Haromatic), 5.71 (s, 1 H, NH), 3.67
C25H26Fe2N2O11S2·0.6hexane (706.33): calcd. C 45.32, H 4.57, N (s, 3 H, OCH3), 3.61 (d, JH,H = 12.0 Hz, 2 H, 2ϫSCHAHB), 3.42
3.70, S 8.46; found C 45.46, H 4.56, N 3.42, S 8.10.
(d, JH,H = 12.0 Hz, 2 H, 2ϫSCHAHB) ppm. 13C NMR (100 MHz,
CDCl3) 141.2, 133.1, 129.1, 127.1 (Caromatic), 73.5
δ
=
Boc-Pro-Adt-OMe (8): Thionyl chloride (130 µL) was added to a
solution of Boc-Adt-OMe (1) (500 mg, 1.79 mmol) in methanol
(10 mL) at 0 °C. The solution was heated to 50 °C and stirred for
6 h, followed by removal of the solvent under reduced pressure. The
remaining residue (HCl·H-Adt-OMe) was dissolved in dimethyl-
formamide (10 mL) at 0 °C and 1-hydroxybenzotriazole (HOBt)
(SCH2CCH2S), 53.4 (OCH3), 47.3 (SCH2CCH2S) ppm. MS (ESI):
m/z = 342 [M + Na]+. IR (KBr): ν = 3250 (m), 3226 (s), 1747 (vs),
˜
1453 (s), 1324 (m), 1295 (s), 1154 (s), 1087 (s) cm–1. C11H13NO4S3
(319.44): calcd. C 41.36, H 4.10, N 4.39, S 30.12; found C 41.31,
H 4.11, N 4.42, S 30.16.
(306 mg, 2.26 mmol), 1-(3-dimethylaminopropyl)-3-ethylcarbodi- [(N-Phenylsulfonyl-Adt-OMe)Fe2(CO)6] (11): N-Phenylsulfonyl-
imide hydrochloride (EDC) (380 mg, 1.98 mmol) and Boc-Pro-OH Adt-OMe (10) (7 mg, 0.022 mmol) and Fe3(CO)12 (12 mg,
(488 mg, 2.26 mmol) were added to the solution. After 20 min at 0.022 mmol) were dissolved in dry toluene (20 mL) and refluxed
0 °C triethylamine (0.32 mL, 2.30 mmol) was added, and the solu- for 1.5 h. The solvent was evaporated under reduced pressure and
tion was stirred for additional 17 h at room temperature. Water the dark brown crude product was purified by FC (THF/hexane =
(3 mL) was added and after 30 min the solution was twice extracted 1:3); yield 11 mg (83%) of a red solid. 1H NMR (200 MHz,
with ethyl acetate (20 mL). The combined organic phases were ex-
CDCl3): δ = 7.79–7.58 (m, 5 H, Haromatic), 3.67 (s, 1 H, NH), 3.52
5084
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Eur. J. Inorg. Chem. 2010, 5079–5086

Synthetic and Electrochemical Studies on [2Fe2S] Complexes
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Crystal Data for 8: C15H24N2O5S2·2CHCl3, Mr = 615.22 gmol–1,
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Received: June 3, 2010
Published Online: October 4, 2010
5086
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Eur. J. Inorg. Chem. 2010, 5079–5086