Synthesis, structures and electrochemical properties of amino-derivatives of diiron azadithiolates as active site models of Fe-only hydrogenase

Article abstract of DOI:10.1016/j.ica.2005.10.053

The preparation and characterization of two novel amino-incorporated sulfur-bridged dinuclear iron (I) complexes of the type [NR(μ-SCH ₂)₂]Fe₂(CO)₆, one being amino protected [N(CH₂CH₂NHTs)(μ-SCH₂) ₂]Fe₂(CO)₆ (8) and the other [(μ- SCH ₂)₂Fe₂(CO)₆NCH₂CH ₂N (μ-SCH₂)₂]Fe₂(CO)₆ (9) are described. These two complexes are readily prepared in a S_N2 manner between double lithium anion and bis(chloromethyl) amine derivatives. The structures of 8 and 9 were characterized by IR, ¹H, ¹³C NMR, MS and HRMS spectra and further determined by X-ray analyses. Protonation of complex 8 gave the mono N-protonated product, while for 9 the protonation occurred in both of the N atoms. The redox properties were evaluated by cyclic voltammograms. It was shown that these two complexes can catalyze electrochemical reduction of proton to molecular hydrogen.

Full text of DOI:10.1016/j.ica.2005.10.053

Inorganica Chimica Acta 359 (2006) 1071–1080
www.elsevier.com/locate/ica
Synthesis, structures and electrochemical properties of
amino-derivatives of diiron azadithiolates as active site models of
Fe-only hydrogenase
a
a,
b
c
a
Weiming Gao , Jianhui Liu *, Chengbing Ma , Linhong Weng , Kun Jin ,
b
d
a,e,
*
˚
Changneng Chen , Bjo¨rn Akermark , Licheng Sun
a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Zhongshan Road 158-40, Dalian 116012, PR China
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Fuzhou 350002, PR China
b
c
Chemistry Department, Fudan University, Handan Road 220, Shanghai 200433, PR China
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Sweden
d
e
KTH Chemistry, Organic Chemistry, Royal Institute of Technology, 10044 Stockholm, Sweden
Received 27 June 2005; received in revised form 16 October 2005; accepted 26 October 2005
Available online 18 January 2006
Abstract
The preparation and characterization of two novel amino-incorporated sulfur-bridged dinuclear iron (I) complexes of the type [NR(l-
SCH2)2]Fe2(CO)6, one being amino protected [N(CH2CH2NHTs)(l-SCH2)2]Fe2(CO)6 (8) and the other [(l- SCH2)2Fe2(-
CO)6NCH2CH2N (l-SCH2)2]Fe2(CO)6 (9) are described. These two complexes are readily prepared in a SN2 manner between double
1
lithium anion and bis(chloromethyl) amine derivatives. The structures of 8 and 9 were characterized by IR, H, 13C NMR, MS and
HRMS spectra and further determined by X-ray analyses. Protonation of complex 8 gave the mono N-protonated product, while for
9 the protonation occurred in both of the N atoms. The redox properties were evaluated by cyclic voltammograms. It was shown that
these two complexes can catalyze electrochemical reduction of proton to molecular hydrogen.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Bioinorganic chemistry; Catalysis; Hydrogen production; Iron–sulfur complex
1. Introduction
SCH2NHCH2S–) had been proposed as two types of
bridging dithiolate ligands based on the central atom
(carbon or nitrogen) [7]. Very recent crystallographic
studies with higher resolution [7], together with DFT cal-
culations [8], suggested that ADT or the N-substituted
equivalent (Fig. 1(a)) was the likely structure subunit.
Nevertheless, the sulfur bridged binuclear complexes con-
taining either PDT or ADT features the model structural
aspects of the Fe-only hydrogenase. The architecture
shown in Fig. 1(b) was the leading model for the active
site of hydrogenase, based on which much work has been
performed including the structure modifications by ligand
exchange and bridge variation [9–18]. The incorporation
of suitably functionalized moieties at defined sites facili-
tates the study to mimic the behavior of metal–sulfur
Since the first crystal structure of the Fe-only hydrog-
enase was reported in 1998 [1], the chemistry of iron–sul-
fur complexes has found significant applications in close
mimics to the active site of this natural system [2–5]. The
X-ray crystallographic studies revealed that the active
site of Fe-only hydrogenase, also called H-cluster,
consisted of a 4Fe4S cubane structure bridged with
cysteine-S to a 2Fe2S subunit [1,6]. PDT (propanedithio-
late, –SCH2CH2CH2S–) and ADT (azadithiolate,
–
*
Corresponding authors. Tel./fax: +86 411 8370 2185.
E-mail addresses: liujh@dlut.edu.cn (J. Liu), lichengs@kth.se (L. Sun).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.10.053

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W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
2.2. Crystal structure determination
(or NH)
CH2
X
CH2 (or NH)
CO
S4Fe4
S
S
S
S
S
OC
Diffraction data of compound 8 and 9 were collected on
a Siemens Smart CCD diffractometer equipped with graph-
ite monochromated Mo Ka radiation (k = 0.71073 A). The
Cys
OC
Fe
Fe
CO
CN
Fe
Fe
OC
OC
CO
˚
C
NC
O
CO
structures were solved by direct methods and subsequent
difference Fourier syntheses, and refined by the full-matrix
least squares on (F2) by using the SHELXL-97 program. The
hydrogen atoms were placed in calculated positions and
were held riding on their parent non-hydrogen atoms dur-
ing the subsequent refinement calculations. All non-hydro-
gen atoms were refined with anisotropic thermal
displacement parameters.
a
b
Fig. 1. The active site of Fe-only hydrogenase (a) and its leading model
structure (b).
sites in hydrogenase. For the ADT series, N-alkyl, N-
benzyl and N-substituted phenyl analogues have been
reported [19,20]. Among them the N-benzyl substituted
Fe2S2 system, which had only one carbon atom between
the N atom and the phenyl group, shows better behavior
for catalyzing protons to produce hydrogen [20]. Such a
conformation forces the benzyl moiety into a pseudo-
equatorial position and keeps the N lone-pair pointing
towards an iron nucleus. This prompted us to design a
new kind of ADT complex with NH2CH2CH2– as the
N substituent featured with a terminal free amino group
connected to the substituent by a two carbon unit.
Through this free amino group, a photosensitizer could
potentially be attached to the active site model of
hydrogenase [21,22]. The present work will report the
synthesis of Ts protected amino derivatives 8, and the
formation of the expected dimer 9 as well as their struc-
ture characterizations. Both compounds are found to be
good catalysts for electrochemical reduction of protons
to molecular hydrogen.
2.3. Procedures
2.3.1. Compound 2
A solution of p-tosyl chloride (20 g, 0.1 mol) in benzene
(50 mL) was added to ethylenediamine (1) (67 mL, 1.0 mol)
and the mixture was stirred for 4.5 h at 40 °C. Then ben-
zene was removed on a rotary evaporator. The crude prod-
uct was purified by crystallization in water to give
compound 2 as a white solid (16.8 g, 79%). 1H NMR
(CDCl3): d 2.43 (s, 3H), 2.81 (t, J = 11.2 Hz, 2H), 2.97 (t,
J = 11.2 Hz, 2H), 7.31 (d, J = 8.4 Hz, 2H), 7.75 (d,
J = 8.4 Hz, 2H). MS (API-ES): m/z 215 [M + H]+.
2.3.2. Complex 8
Method A:
A mixture of compound 2 (2.14 g,
10 mmol) and paraformaldehyde (0.6 g, 20 mmol) in
THF (40 mL) was stirred for 4 h. It was cooled to 0 °C
and then freshly distilled SOCl2 (2.84 mL, 40 mmol)
was added dropwise. After further stirring for 1.5 h,
THF and unreacted SOCl2 were removed under vacuum,
and the residue was added to (LiS)2Fe2(CO)6 (6)
(1 mmol) in THF (40 mL) at ꢀ78 °C. The reaction mix-
ture was stirred for 1 h at this temperature and then
warmed to rt. After filtration, the solvent was removed
by evaporation on a rotary evaporator. The crude prod-
uct was purified by column chromatography with
CH2Cl2/hexane (1/1 v/v) as eluent to give 8 (288 mg,
49%) as a red solid. IR (KBr): m(CO) 2073, 2029,
2. Experimental
2.1. Reagents and instruments
All reactions and operations related to organometallic
complexes were carried out under a dry, oxygen-free nitro-
gen atmosphere with standard Schlenk techniques. All sol-
vents were dried and distilled prior to use according to the
standard methods. Commercially available chemicals,
including perchloric acid (only used for the UV–Vis mea-
surement), paraformaldehyde, p-tosyl chloride, ethylene
diamine and Fe(CO)5 were used without further purifica-
tion. The reagents LiEt3BH and triflic acid were purchased
from Aldrich. (l-LiS)2Fe2(CO)6 (6) and (l-HS)2Fe2(CO)6
(7) were prepared according to the literature procedures
[23] and they were used in situ immediately for the further
reaction.
1992 cmꢀ1 1H NMR (CDCl3): d (ppm) 2.44 (s, 3H),
;
2.79 (s, 2H), 2.86 (s, 2H), 3.42 (s, 4H), 7.32 (d,
J = 6.4 Hz, 2H), 7.70 (d, J = 6.4 Hz, 2H). 13C NMR d
(ppm) 21.73, 29.87, 40.92, 52.74, 56.05, 127.17, 130.05,
136.79, 144.03, 207.75. MS (API-ES): m/z: 619.1
[M + Cl]ꢀ. HRMS (ESI): m/z [M ꢀ H]ꢀ calcd for
582.8690, found: 582.8705. Elemental analysis for
C17H16Fe2N2O8S3, Calc. C, 34.95; H, 2.76; N, 4.80.
Found: C, 35.44; H, 2.56; N, 4.82%.
Infrared spectra were recorded on a JASCO FT/IR 430
spectrophotometer. H and 13C NMR were collected on a
1
Varian INOVA 400 NMR spectrometer. Mass spectra
were recorded on an HP1100 MSD instrument. HRMS
were recorded on a HPLC-Q-Tof MS (Micro) system.
Gas chromatography was performed with a GC7890
instrument under isothermal conditions with nitrogen as
a carrier gas and a thermal conductivity detector (TCD).
Method B: The mixture of complex 2 (2.14 g, 10 mmol)
and paraformaldehyde (0.6 g, 20 mmol) in THF (40 mL)
was stirred for 4 h. Then it was cooled to 0 °C and the
THF solution (20 mL) of (HS)2Fe2(CO)6 (7) (0.5 mmol)
was added. The suspension was stirred for 5 h at room tem-
perature to afford complex 8 (146 mg, 50%).

W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
1073
2.3.3. Complex 9
2.5. Electrochemistry
Ethylenediamine (1) (0.67 mL, 10 mmol) and parafor-
maldehyde (0.6 g, 20 mmol) were stirred in THF (20 mL)
until the solution became transparent. It was cooled to
0 °C and then freshly distilled SOCl2 (2.84 mL, 40 mmol)
was added dropwise to the mixture and stirred for 1 h.
The solvent was removed by vacuum, then the addition
of (LiS)2Fe2(CO)6 (6) in the same manner as for 8 gave 9
as a dark red solid (200 mg, 50%). IR (KBr): m(CO) 2068,
Acetonitrile (Aldrich, spectroscopy grade) used for per-
formance of electrochemistry was dried with molecular
sieve (4 A) and then freshly distilled from CaH2 under
˚
N2. A solution of 0.05 M n-[Bu4N]ꢀ[PF6]+ (Fluka, electro-
chemical grade) in CH3CN was used as electrolyte.
Electrochemical measurements were recorded using a
BAS-100W electrochemical potentiostat. The electrolyte
solution was degased by bubbling with dry argon for
10 min before measurement. Cyclic voltammograms were
obtained in a three-electrode cell under argon. The work-
ing electrode was a glassy carbon disc (diameter 3 mm)
successively polished with 3 and 1 lm diamond pastes
and sonicated in ion-free water for 10 min. The reference
electrode was a non-aqueous Ag/Ag+ electrode (1.0 mM
AgNO3 in CH3CN) and the auxiliary electrode was a
platinum wire.
1988, 1961 cmꢀ1 1H NMR (CDCl3): d (ppm) 2.64 (s,
;
4 H), 3.54 (s, 8H) 13C NMR (CDCl3): d 36.55, 53.22,
208.39. MS (API-ES): m/z 834.7 [M + Cl]ꢀ. HRMS
(ESI): m/z [M + H]+ calcd for 800.6749, found: 800.6730.
Elemental analyisis for C18H12Fe4N2O12S4, Calc: C,
27.03; H, 1.51; N, 3.50. Found: C, 27.19; H, 1.65; N, 3.74%.
2.3.4. Protonation of 8 and 9
A small amount of 8 (2–3 mg) was dissolved in an
CD3CN (0.5 mL) in an NMR-tube, and then two drops
of triflic acid were added directly to the solution for
in situ 1H NMR analysis. The solvent was removed by vac-
uum to get the red solid of 8H+ for IR analysis (KBr):
3. Results and discussion
m(CO) 2091, 2039, 2027 cmꢀ1
.
1H NMR (CD3CN): d
3.1. Synthesis and spectral characterizations
(ppm) 2.40 (s, 3H), 3.06 (t, J = 5.2 Hz, 2H), 3.22–3.29
(m, 4H), 4.29 (d, J = 12.8 Hz, 2H), 7.40 (d, J = 8.0 Hz,
2H), 7.71 (d, J = 8.4 Hz, 2H). Complex 9 was protonated
in a similar manner to afford 9H22þ: IR (KBr): m(CO)
Very recently, Sun and co-workers reported an amino-
functionalized model of the diiron subsite with ADT-
bridge [19]. For our cases, due to the two equal free amino
reactive sites in the starting ethylenediamine, we at first
wished to protect one of them with the most widely used
tosyl group, and then removed it after the reaction with
Fe–S complex 6 or 7. Azadithiolate 8 could be prepared
through either [(l-S)2Fe2(CO)6]2ꢀ or (l-HS)2Fe2(CO)6 by
reacting respectively with halogenated or hydroxymethyl-
amine. Thus, ethylenediamine was mono-protected by
heating with TsCl to give the mono amine 2. This com-
pound was allowed to react with formaldehyde to form
the bis(hydromethyl) derivative 3a. The hydroxyl groups
were then replaced by chloro groups by reaction with
freshly distilled SOCl2 to give the bis(chloromethyl) inter-
mediate 4a. This was used without isolation in the next
step. The sulfur-centered dianion [(l-S)2Fe2(CO)6]2ꢀ (6)
which was generated by the reaction of (l-S)2Fe2(CO)6
(5) with LiBEt3H, reacted with 4a to afford our desired
complex 8. The dianion 6 could also be transformed to
thiol complex 7 by protonation with CF3CO2H. Complex
8 could also be prepared directly from 7 and the diol 3a
in ca. 50% yield. Attempts to produce the free amino ana-
logue by removing the tosyl group under a variety of con-
ditions failed. For example, TBAF (tetrabutylammonium
fluoride) gave only decomposed dark material, and HBr
in phenol caused protonation of bridged amine. At this
stage, diamine 1 with two free amino groups was directly
reacted with (HCHO)n in a molar ratio 1:2 in the hope that
the complete regio selectivity can be realized to give mono-
(1,1-bischloromethyl) ethylenediamine. However this reac-
tion took place without any regioselectivity and gave only
the product 4b after chlorization. Accordingly, the Fe2S2
1
2095, 2067, 2025 cmꢀ1; H NMR (CD3CN): d (ppm) 3.21
(t, J = 11.6 Hz, 2H), 3.38 (s, 2H), 4.08 (d, J = 12.0 Hz, 2H).
2.4. Crystal data for 8 and 9
See Table 1.
Table 1
X-Ray crystallographic data for 8 and 9
8
9
Empirical formula
Fw
C
584.20
17H16Fe2N2O8S3
C18H12Fe4N2O12S4
799.98
Crystal system
Space group
a (A)
triclinic
monoclinic
P21/n
7.69140(10)
18.4673(5)
9.7645(3)
90.00
94.1990(10)
90.00
1383.22(6)
2
ꢀ
P1
˚
6.9917(19)
12.885(3)
13.660(4)
94.276(4)
104.641(4)
102.394(4)
1152.0(5)
2
1.684
293(2)
1.576
592
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
qcalc (g mꢀ3
T (K)
)
1.921
298
2.419
796
l (mmꢀ1
F(000)
)
Reflections measured
Reflections/Rint
Goodness-of-Fit (on F2)
R1 [I > 2r(I)]
4845
3973/0.0380
1.038
4070
2398/0.0277
1.047
0.0764
0.0725
wR2 (all data)
0.1985
0.2407

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W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
˚
dimer 9 was obtained by reaction of 4b with 6. The com-
urianum) (ca. 2.6 A) [1,6,7], but still in good agreement with
1
pounds 8 and 9 were characterized by H and 13C NMR,
the structural data of Fe–Fe bonds (2.49–2.51 A) found in
˚
MS and HRMS spectra.
Due to the symmetry of both 8 and 9, the H NMR
spectrum was simple and consistent with the X-ray struc-
other diiron azadithiolates [12,13,24]. For complex 8, the
twisted flexible chain of ethylenediamine gives the p-tosyl
group a different orientation from that of phenyl and ben-
zyl-substituents of the ADT-bridged diiron complexes
[19,20]. By viewing N(1)–C(9)–C(10)–N(2), the torsion
angle 65.9° accounts for the gauche conformation with
rotation around the central C9–C10 bond. This indicates
that steric factors are important for the rotational energy
barriers. N(1) has an approximately tetrahedral shape, with
three bonds and a lone-pair. The lone-pair lies directed
towards the adjacent carbonyl ligand, i.e. C(1) in 8. The
conformation about N(1) is well described by the S(2)–
C(8)–N(1)–C(9) torsion angle 145.9°. This trans conforma-
tion has the lone-pair pointing towards C(1). In the solid
state, the nonbonding Cꢁ ꢁ ꢁN distance between the ADT
nitrogen atom and the nearest carbonyl carbon atom is
almost consistent with the benzyl-substituted analogue
1
1
ture. Both complexes exhibited broadened signals in H
NMR. The most intense peaks were observed in MS
spectra at m/z 585 and 619 for 8 corresponding to
[M + H]+ (positive mode) and [M + Cl]ꢀ (negative
mode), and at 834.7 [M + Cl]ꢀ for 9. The IR spectra
of 8 and 9 in KBr showed three strong CO bands in
the region of 1960–2080 cmꢀ1, identical to the carbonyl
stretching pattern for terminal C–O bonds. In compari-
son with 9, the bands for complex 8 red shifted by 5–
30 cmꢀ1. The reaction routes are outlined in Schemes 1
and 2.
3.2. Molecular structure
˚
The crystal structures of 8 and 9 are shown in Figs. 2
and 3 respectively with atom labeling scheme, and selected
bond distances and bond angles are listed in Tables 2–5.
The central FeI atoms in both complexes displayed similar
distorted octahedral geometry, each being coordinated by
three terminal carbonyl C atoms, two bridging S atoms
and the other Fe atom. The Fe–Fe distances of 8 and 9
are somewhat shorter than those in the structures of H2-
uptake enzyme from DdHase (Desulfovibrio desulfuricans)
and H2-evolving enzyme from CpHase (Clostridium paste-
(3.01 and 3.02 A, respectively), which is significantly
˚
shorter than that of phenyl-substituted analogues (3.5 A)
[19,25]. Actually, the environment of N(1) is similar to that
of benzyl-substituted ADT-bridged diiron complexes.
Whereas in the gauche arrangement, the lone-pair is direc-
ted away from that carbonyl group, as found in compound
9 which is consistent with the usual phenyl-substituted ana-
logues (Table 2). The torsion angle of S(2)–C(8)–N(1)–C(9)
is about 82.0°. The nonbonding Cꢁ ꢁ ꢁN distance from the
nearest carbonyl carbon atom to the ADT nitrogen atom
Ts
Ts
Ts
NH2
NH
NH
NH
TsCl
SOCl2
(HCHO)n
THF, 4h
THF, rt 1.5 h
benzene, 40 ˚C, 4.5 h
NH2
NH2
N
N
OH OH
Cl
Cl
1
2
3a
4a
H
Li
S
H
S
Li
S
S
S
S
CF3COOH
THF, -78 ˚C, 0.5 h
LiEt3BH
THF, -78 ˚C, 0.5 h
OC
OC
OC
CO
CO
OC
CO
CO
OC
CO
CO
Fe
Fe
CO
Fe
Fe
Fe
Fe
OC
OC
OC
OC
CO
CO
5
6
7
Ts
HN
N
+
4a
3a
7
+
6
rt, 5 h
-78 ˚C, 1 h
S
S
OC
OC
OC
CO
CO
Fe
Fe
CO
8
Scheme 1. The synthetic procedure of complex 8.

W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
1075
OH OH
N
Cl
Cl
NH2
N
SOCl2
rt, 1 h
(HCHO)n
rt, 1.5 h
6
-78 ˚C, 1 h
NH2
N
N
OH OH
3b
Cl
Cl
1
4b
OC
OC
CO
CO
Fe
S
Fe
OC
CO
S
N
N
S
S
OC
CO
CO
Fe
Fe
OC
OC
CO
9
Scheme 2. The synthetic procedure of complex 9.
Fig. 3. ORTEP (ellipsoids at 30% probability) diagram of 9.
Fig. 2. ORTEP (ellipsoids at 30% probability) diagram of 8.
˚
(3.69 A) is even longer than that of the phenyl-substituted
structure, with an inversion centre at the mid-point of the
C(9)–C(9A) bond. When observing N(1)–C(9)–C(9A)–
N(1A), this chain does not twist like that of 8, rather it
takes the zig-zag form.
analogues.
For complex 9, it has an absolutely symmetric structure
of C2v. Complex 9 has a crystallographically symmetrical

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W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
Table 2
3.3. Protonation of model complexes
Selected crystal data among different kinds of N atoms in ADT diiron
complexes
The protonation reactions of compounds 8 and 9 to
form 8H+ and 9H2 were followed by changes in the H
1
2þ
Compound
Torsion
Cꢁ ꢁ ꢁN
Bond angle (°)
distancea (A)
(–C–N–C–)b
˚
angle (°)
NMR spectra upon addition of triflic acid to CD3CN solu-
tion of the respective complex. In the case of complex 8, no
peaks at d < 0 exist in the 1H NMR, showing that the pro-
tonation occurred on the N atom rather than to form l-H
complex. Most of the peaks are generally shifted to lower
magnetic field than those of 8. However, the methylene
protons in proximity of the ADT nitrogen (–NCH2S–)
were split from 3.43 ppm (singlet) into 3.06 ppm (triplet)
and 4.29 ppm (doublet) in a ratio of 1:1. The protons (–
NCH2CH2NTs) were shifted from 2.83 ppm (doublet) to
the region 3.23–3.29 ppm (multiplet). The carbonyl fre-
quencies in the IR spectrum of 8H+ were also shifted 10–
18 cmꢀ1 to higher in frequency energy relative to those of
8, which was consistent with the observations made during
the protonation of a related complex [12]. Protonation of 8
was accompanied by a color change from orange to yellow.
The UV absorption maximum was shifted from 328 to
333 nm, and an isosbestic point was preserved at 320 nm,
which indicated the formation of a single protonation
product at different concentrations [20]. The changes in
1HNMR, IR and UV absorptions of 9 were similar to those
(S–C–N–C)a
8
145.9/147.3
82.0/83.2
159.80/155.83
89.54/96.78
3.01
3.69
3.02
3.50
115.0
115.6
114.0
113.0
9
[20]
[19]
a
From the nearest carbonyl carbon atom to the ADT nitrogen atom.
ADT nitrogen atom [19,20].
b
Table 3
Selected bond lengths (A) and angles (°) for complex 8
˚
Fe(1)–C(2)
Fe(1)–C(3)
Fe(1)–C(1)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(1)–Fe(2)
1.778(10)
1.798(12)
1.816(12)
2.249(3)
2.250(3)
2.5152(18)
Fe(2)–C(5)
Fe(2)–C(4)
Fe(2)–C(6)
Fe(2)–S(2)
Fe(2)–S(1)
1.802(12)
1.796(11)
1.801(12)
2.250(3)
2.245(3)
C(2)–Fe(1)–C(3)
C(2)–Fe(1)–S(1)
C(3)–Fe(1)–S(1)
C(1)–Fe(1)–S(1)
C(2)–Fe(1)–S(2)
C(3)–Fe(1)–S(2)
C(1)–Fe(1)–S(2)
S(1)–Fe(1)–S(2)
89.7(5)
88.5(3)
158.4(3)
102.8(4)
158.6(3)
88.3(4)
C(4)–Fe(2)–C(6)
C(4)–Fe(2)–S(1)
C(6)–Fe(2)–S(1)
C(5)–Fe(2)–S(1)
C(4)–Fe(2)–S(2)
C(6)–Fe(2)–S(2)
C(5)–Fe(2)–S(2)
S(1)–Fe(2)–S(2)
100.0(5)
88.6(3)
104.7(3)
152.0(3)
162.7(3)
97.2(3)
1
of 8 during the protonation. The H NMR and the kinetic
100.3(4)
85.52(10)
87.8(4)
85.60(10)
UV absorption are shown in Figs. 4, 5 and 6, 7,
respectively.
On addition of triflic acid, the methylene protons of 9
(–NCH2S–) in the 1H NMR were changed from 3.54
ppm (singlet) to 3.21 ppm (triplet) and 4.08 ppm (dou-
blet) in a ratio of 1:1. The peak for the chain protons
(–NCH2CH2NH–) was shifted from 2.64 to 3.38 ppm.
Independent of whether an equivalent amount or an
excess of triflic acid was added, the behavior in the
1H NMR is the same. Mainly due to the limitation of
solubility of compound 9, no stepwise protonation
was observed under the conditions in our experiment
Table 4
˚
Hydrogen bonds for complex 8 [A and °].
D–Hꢁ ꢁ ꢁA
d(D–H)
0.86
d(Hꢁ ꢁ ꢁA)
2.13
d(Dꢁ ꢁ ꢁA)
2.982(10)
\(DHA)
N(2)–H(2A)ꢁ ꢁ ꢁO(8)a
172.8
a
Symmetry transformations used to generate equivalent atoms: ꢀx,
ꢀy + 1, ꢀz.
and the protonation occurred for the two
concomitantly.
N
Table 5
Selected bond lengths (A) and angles (°) for complex 9
a
˚
3.4. Electrochemistry
Fe(1)–C(2)
Fe(1)–C(3)
Fe(1)–C(1)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(1)–Fe(2)
1.801(10)
1.800(10)
1.807(10)
2.264(2)
2.282(2)
2.5062(17)
Fe(2)–C(5)
Fe(2)–C(4)
Fe(2)–C(6)
Fe(2)–S(2)
Fe(2)–S(1)
1.797(9)
1.802(10)
1.803(11)
2.262(2)
2.264(2)
The formal potentials for complexes 8 and 9 had been
determined by cyclic voltammetry in CH3CN for the eval-
uation of their redox properties (Fig. 8). Complex 8 exhib-
ited clearly an irreversible oxidation couple at 0.64 V and a
quasi-reversible reduction couple at ꢀ1.49 V (versus Ag/
AgNO3). Complex 9 exhibited two irreversible oxidation
peaks at 0.66 and 0.87 V, respectively, and an irreversible
reduction peak at ꢀ1.51 V, although the solubility of 9
was poor in CH3CN. Both the one oxidation peak for 8
and the two oxidation peaks for 9 were assigned to one
electron process FeIFeI/FeIIFeI. The reduction waves
observed for 8 and 9 were described as one electron pro-
cesses FeIFeI/Fe0FeI. This set of data followed the general
trend discussed for that of PDT and EDT (SCH2CH2S)
C(2)–Fe(1)–C(3)
C(2)–Fe(1)–C(1)
C(3)–Fe(1)–C(1)
C(2)–Fe(1)–S(1)
97.7(4)
92.9(4)
96.8(5)
89.5(3)
C(5)–Fe(2)–C(4)
C(5)–Fe(2)–C(6)
C(4)–Fe(2)–C(6)
C(5)–Fe(2)–S(2)
91.1(4)
98.3(4)
101.7(5)
162.0(3)
C(3)–Fe(1)–S(1)
C(1)–Fe(1)–S(1)
C(2)–Fe(1)–S(2)
C(3)–Fe(1)–S(2)
C(1)–Fe(1)–S(2)
S(1)–Fe(1)–S(2)
a
101.2(3)
161.4(4)
147.8(3)
114.4(3)
84.3(3)
C(4)–Fe(2)–S(2)
C(6)–Fe(2)–S(2)
C(5)–Fe(2)–S(1)
C(4)–Fe(2)–S(1)
C(6)–Fe(2)–S(1)
S(2)–Fe(2)–S(1)
89.1(4)
99.3(3)
88.0(3)
155.2(4)
103.0(4)
84.34(8)
83.90(9)
A: ꢀx + 1, ꢀy, ꢀz + 1.

W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
1077
Fig. 4. The 1HNMR spectra of (–NCH2S–) and (–NCH2CH2NH–) for complexes 8 and 8H+.
Fig. 5. The 1H NMR spectra of (–NCH2S–) and (–NCH2CH2NH–) for complexes 9 and 9H
.
2þ
2
bridged diiron complexes [26–31] and especially that of [(l-
SCH2)2N(4-XC6H4)]Fe2(CO)6 (X = NO2 or NH2) [19]
(Table 6). Compared to the (l-PDT)Fe2(CO)6, the oxida-
tion potential of 8 for FeIFeI/FeIIFeI was less positive;
while for 9, one of the oxidation potentials for FeIFeI/FeII-
FeI was less positive, the other was a little more positive.
There seemed to be a mutual effect between the two
Fe2S2 structures. We could clearly see that complexes 8
and 9 would be easier to reduce than (l-PDT)Fe2(CO)6
and more difficult to reduce than [(l-SCH2)2N(4-
XC6H4)]Fe2(CO)6 (X = NO2 or NH2). The difference of
inductive effects between aliphatic and aromatic chains
could explain for the varying electron density of iron cores.
Complex 8 was a little more readily to reduce than 9, per-
haps because of a relatively stronger inductive effect of
TsNH.
Proton reduction catalyzed by 8 and 9 was studied by
cyclic voltammograms in the presence of a strong acid,
CF3SO3H (0–10, 30 mM,) in CH3CN (Fig. 9). The first
reduction peak appeared at about ꢀ1.18 V both in 8
and 9 when 2.0 mM CF3SO3H was added; at the same
time the two oxidation peaks for 9 at 0.66 and 0.88 V
disappeared, and
a
new oxidation wave at 0.75 V
2þ
appeared. The good solubility of 9H2 in CH3CN made
the two Fe2S2 subunits equal, giving just a single oxida-
tion peak. The current intensities of all reduction peaks
grew further with the increase of the acid concentration
and the peaks gradually moved to more negative poten-
tial. A sharp increase of current intensity occurred,
which reached ca. 900 lA for 8 and 9, almost 10 times
more than that of [(l-SCH2)2N(4-NO2C6H4)]Fe2(CO)6
when the concentration of acid increased to 30 mM

1078
W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
1.5
1.0
0.5
0.0
60
40
20
0
-20
-40
-60
-80
250
300
350
400
450
1.0
0.5
0.0
-0.5
E/V
-1.0
-1.5
-2.0
Wavelength (nm)
a
Fig. 6. UV/Vis spectra of 8 (1 mM) in CH3CN. Arrows indicate spectra
changes upon addition of increasing amounts of acid (concentrations of
perchloric acid: from 1 to 7 mM).
15
10
5
2.5
2.0
1.5
1.0
0.5
0.0
0
-5
-10
-15
1.5
1.0
0.5
0.0
-0.5
E/V
-1.0
-1.5
-2.0
-2.5
b
Fig. 8. Cyclic voltammograms of 8 (a) and 9 (b), 2.0 mM in 0.05 M n-
Bu4NPF6/CH3CN at a scan rate of 100 mV/s.
250
300
350
400
450
Table 6
Wavelength (nm)
Electrochemical data of complexes 8 and 9a
ox1
Fig. 7. UV/Vis spectra of 9 (1 mM) in CH3CN. Arrows indicate spectra
changes upon addition of increasing amounts of acid (concentrations of
perchloric acid: from 1 to 7 mM).
Compound
E
ðV Þ
Epc, Epa (V) Epc, Epa (V)
pirr
I;I
II;I
Fe2ð Þ=ð
+0.64
Þ
Fe2(I,I)/(0,I
)
Fe2(0,I)/(0,0
)
8
9
ꢀ1.49
ꢀ1.36
+0.66; +0.87 ꢀ1.51
[19]. It seems that the spacial factors and electron den-
sity differences of aliphatic and aromatic chain caused
the difference of protonation efficiency. Of course, all
of these features were indicative of catalytic proton
reduction. Without the diiron complex, reduction of pro-
tons at the carbon electrode was slow at the potential of
the reduction peak. In the case of both 8 and 9, we
could observe bubbles get out from the surface of the
glassy carbon disc when enough CF3SO3H was added
during the CV measurement. These bubbles were
molecular hydrogen which had been identified by GC
as before with a similar catalyst [20]. Based on the data
of structure and electrochemistry, we propose the
catalytic cycle mechanism as the CECE (chemical–elec-
trochemical–chemical–electrochemical) process [19,20].
ꢀ1.42
[(l-SCH2)2N-
(4-NO2C6H4)]Fe2(CO)6
+0.78
+0.80
+0.84
ꢀ1.34
ꢀ1.28
ꢀ1.71
ꢀ1.63
[(l-SCH2)2N-
(4-NH2C6H4)]Fe2(CO)6
ꢀ1.48
ꢀ1.23
(l-PDT)Fe2(CO)6
ꢀ1.57
ꢀ1.47
Epc = ꢀ2.11
All potentials in Table 4 are vs. Ag/Ag+ (0.01 M AgNO3 in CH3CN).
a
4. Conclusion
Two amino-substituted diiron azadithiolate complexes
[N(CH2CH2NHTs)(l-SCH2)2]Fe2(CO)6 (8) and [(l-
SCH2)2Fe2(CO)6NCH2CH2N(l-SCH2)2]Fe2(CO)6 (9) have

W. Gao et al. / Inorganica Chimica Acta 359 (2006) 1071–1080
1079
700
600
500
400
300
200
100
0
Cambridge CB2 1EZ, UK (fax: +441223/336-033 or e-
mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
We are grateful to the Ministry of Science and Technol-
ogy of China and the Chinese National Natural Science
Foundation (Grant No. 20128005) for financial support
of this work. We are also grateful to the Swedish Energy
Agency and the Swedish Research Council for support.
We thank Ms. Rong Zhang for the HRMS measurements.
-100
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