Carboxy-functionalized dithiolate di-iron complexes related to the active site of Fe-only hydrogenase

Article abstract of DOI:10.1016/j.jorganchem.2007.06.042

Reactions of the di-iron complex [Fe₂(μ-S)₂(CO)₆]^2- with carboxy-functionalized dihalide derivatives (XCH₂)₂R (X = Cl, R = NC₆H₄CH₂CO₂CH₃; X = Br, R = C₆H₃COOH, C₆H₃COON(COCH₂)₂) gave new functionalized dithiolate di-iron complexes [Fe₂(μ-SRS)(CO)₆] (R = (CH₂)₂NC₆H₄CH₂CO ₂CH₃ (1), (CH₂)₂C₆H₃COOH (2), (CH₂)₂C₆H₃COON(COCH ₂)₂ (3)) in low yields. The azadithiolate complex 1 has been characterized by a single crystal X-ray diffraction analysis and studied by electrochemical methods.

Full text of DOI:10.1016/j.jorganchem.2007.06.042

Journal of Organometallic Chemistry 692 (2007) 4177–4181
www.elsevier.com/locate/jorganchem
Note
Carboxy-functionalized dithiolate di-iron complexes related
to the active site of Fe-only hydrogenase
b
a
a
a
´ ´
´
Vijendran Vijaikanth , Jean-Franc¸ois Capon , Frederic Gloaguen , Franc¸ois Y. Petillon ,
a,
a
Philippe Schollhammer *, Jean Talarmin
a
UMR CNRS 6521, Chimie, Electrochimie Mole´culaires et Chimique Analytique, UFR Sciences et Techniques Universite´ de Bretagne Occidentale,
CS 93837, 29238 BREST-Cedex 3, France
b
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N1N4
Received 22 May 2007; received in revised form 9 June 2007; accepted 9 June 2007
Available online 29 June 2007
Abstract
Reactions of the di-iron complex [Fe2(l-S)2(CO)6]2À with carboxy-functionalized dihalide derivatives (XCH2)2R (X = Cl,
R = NC6H4CH2CO2CH3; X = Br, R = C6H3COOH, C6H3COON(COCH2)2) gave new functionalized dithiolate di-iron complexes
[Fe2(l-SRS)(CO)6] (R = (CH2)2NC6H4CH2CO2CH3 (1), (CH2)2C6H3COOH (2), (CH2)2C6H3COON(COCH2)2 (3)) in low yields. The
azadithiolate complex 1 has been characterized by a single crystal X-ray diffraction analysis and studied by electrochemical methods.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Hydrogenase; Di-iron complex; Carboxy-functionalized dithiolate; Molecular structure; Electrochemistry
1. Introduction
substrates having ester functionnality with amine groups
of a surface-modified electrode [3]. Recently, we decided
to explore this strategy to attach an organometallic
2Fe2S site through an adequately funtionalized dithiolate
bridge to an electrode (Scheme 1) [4].
Hydrogen has been identified as a potential clean and
renewable energy source for the future. In the last few
years, structural determinations of the [FeFe] hydrogenase,
one of the enzymes that catalyze the reversible reaction
2H+ + 2eÀ = H2, have prompted several fundamental
studies on synthetic organometallic iron–sulfur assemblies.
These works were developed in the hope to get a better
understanding of the chemistry of the di-iron active sub-
site and to determine the minimal structural requirements
for hydrogenase activity in view to provide alternative cat-
alysts and electrocatalysts for efficient hydrogen produc-
tion [1]. Studies on immobilized catalysts are of
importance in this context [2]. Immobilization of a mole-
cule onto an electrode can be achieved through, for exam-
ple, the formation of a covalent amide link by coupling
This led us to investigate the syntheses of a series of hexa-
carbonyl di-iron complexes with carboxy-functionalized
dithiolate bridges. Such systems can be obtained by reacting
dithiols with [Fe3(CO)12] and carboxy-substituted dithiol di-
iron systems have been recently reported [5]. The synthesis of
di-iron dithiolate complexes can be also performed by func-
tionalization of the di-iron disulfide species [Fe2(l-S)2-
(CO)6]2À by treatment with various organic halides [6–10].
In addition, straightforward condensation of [Fe2(l-SH)2-
(CO)6] with RNH2 and CH2O or RN(CH2OH)2 affords also
N-substituted azadithiolate compounds [11]. We wish to
report here our attempts to prepare new carboxy-functional-
ized di-iron compounds by addition of dihalide derivatives
(XCH2)2R [X = Cl, R = NC6H4CH2CO2CH3; X = Br,
R = C6H3COOH, C6H3COON(COCH2)2] to dinuclear
*
Corresponding author.
anionic species [Fe2(l-S)2(CO)6]2À
.
E-mail address: schollha@univ-brest.fr (P. Schollhammer).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.06.042

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V. Vijaikanth et al. / Journal of Organometallic Chemistry 692 (2007) 4177–4181
2.91 ppm for the four protons of the N-hydroxysuccinimide
(CO)3
Fe
O
C
ester group in 3. The dissymmetry of the bridge in 3 is
shown by the observation in CDCl3 of a doublet and a
doublet of doublet at 3.18 ppm and 3.93 ppm, respectively,
for S–CH2–Ar groups.
S
S
NH
R
Fe
(CO)3
Scheme 1.
The formulation of 1 was confirmed by a X-ray diffrac-
tion study of a single crystal obtained from hexane/dichlo-
romethane solution. The analysis indicates, as expected, a
distorted square-pyramidal coordination at each 18-elec-
tron iron centre and the well-known butterfly structure
found in related dithiolate di-iron complexes [6–9,11,12]
2. Results and discussion
Complexes [Fe2(l-SRS)(CO)6] [R = (CH2)2NC6H4CH2-
CO2CH3 (1), (CH2)2C6H3COOH (2), (CH2)2C6H3COON-
(COCH2)2 (3)] were obtained in low yields by treatment
of [Fe2(CO)6(l-S)2]2À with dihalide (XCH2)2R [X = Cl,
R = NC6H4CH2CO2CH3; X = Br, R = C6H3COOH, C6-
H3COON(COCH2)2] following a known procedure [6,7]
(Scheme 2). Compounds 1–3 were characterized by IR
˚
(Fig. 1a). 1 contains a single Fe–Fe bond [2.5044(8) A]
which is bridged by the two sulfur atoms of the azapropan-
edithiol group. The structure of 1 features two fused
six-member rings, Fe2–S1–C7–N1–C8–S2 and Fe1–S1–
C7–N1–C8–S2, which adopt boat and chair conformation,
respectively. The N-carboxyl group lies in an axial position
and the sum of C–N–C angles (C7–N1–C8, C8–N1–C9, C9–
N1–C7) around N1 is 356.7ꢁ, indicating a noticeable flatten-
ing of the trigonal pyramid about N1. It is worth noting that
the proton of one bridging methylene {N–CH2–S}(C7) is
involved in a bifurcated hydrogen bond to one oxygen atom
1
and H NMR spectroscopy. Three typical strong bands
between 2075 and 1998 cmÀ1 are observed in the terminal
carbonyl region of the infrared spectrum of the complexes
in tetrahydrofuran. In addition, the detection of new bands
between 1800 and 1700 cmÀ1 accords with the presence of
carboxy-functionalized groups. 1H NMR spectra of 1–3
confirm the nature of the dithiolate bridge {(SCH2)2R}.
Indeed, the 1H NMR spectrum of 1 in CD3CN shows
two doublets related to the aromatic group (7.20 and
6.79 ppm) and three singlets with intensities of 4, 3 and 2
for both S–(CH2)2–N (4.41 ppm) and CH2CO2CH3 (3.62
˚
(O7) of the carboxy function (H7Á Á ÁO7 = 2.454(3) A)
(Fig. 1(b)).
The electrochemical behavior of 1 has been studied in
acetonitrile. A CH3CN–Bu4NPF6 solution of 1 is reduced
at À1.57 V versus Fc+/0 (Fig. 2), consistent with the data
reported for other di-iron complexes bearing an azadithio-
late bridge substituted with an aryl group [7d]. Compounds
where the substituent of the azadithiolate bridge is an alkyl
group are reduced at a potential about 0.1 V more negative
[11e]. It is also relevant to note that the reduction of the
azadithiolate-bridged complex 1 occurs at a potential about
0.1 V less negative and appears chemically less reversible
than that of the corresponding propanedithiolate-bridged
derivative under otherwise similar experimental conditions.
Upon addition of 1 molar equiv. toluenesulfonic acid
(HOTs, pKa ꢀ 8), the voltammogram shows two new reduc-
tion events at À1.32 V and À1.53 V, respectively (Fig. 2).
The first reduction peak is ascribed to the reduction of the
protonated form of 1 (the most likely protonation site in 1
being the nitrogen atom of the azadithiolate bridge [7d]).
The origin of the second reduction peak cannot be explained
at present, although similar observations have recently been
reported (Fig. 4 bottom in Ref. [11c]). Upon increasing the
acid concentration, the height of the first reduction wave
increases and shifts to negative potentials merging with
the second reduction wave. Such an electrochemical behav-
ior could result from an electrocatalytic proton reduction
[7d].
1
and 3.54 ppm) groups, respectively. H NMR spectra of
2 and 3 display between 8.13 and 6.25 ppm three signals
(two doublets and one singlet) assigned to the aromatic
protons of the dithiolate bridge. The nature of the carboxyl
group is revealed by the observation of a broad acidic pro-
ton signal at 11.4 ppm for 2, and that of a singlet at
R
2-
S
S
S
S
X
R
X
(CO)3Fe
Fe(CO)3
(CO)3Fe
Fe(CO)3
(X = Cl or Br)
1-3
CH2
N
CH2COO CH3
(1),
(2),
R =
CH2
COOH
CH2
CH2
O
In summary, attempts to synthesize carboxy-function-
alized di-iron dithiolate complexes have been developed
in this work, but it appeared difficult to obtain them from
the di-iron disulfide [Fe(CO)6(S)2]2À. Despite this, three
new members of the hexacarbonyl di-iron dithiolate series
were isolated and characterized. The grafting of the
N-hydroxysuccinimide ester compound 3 via an amide
COO
CH2
N
(3)
O
CH2
Scheme 2.

V. Vijaikanth et al. / Journal of Organometallic Chemistry 692 (2007) 4177–4181
4179
˚
Fig. 1. (a) A view of a molecule of 1 showing 30% ellipsoids (b) hydrogen bonding between two molecules in the unit cell. Selected distances and angles (A
and ꢁ): Fe1–Fe2 2.5044(8), Fe1–S1 2.2634(10), Fe1–S2 2.2678(11), Fe2–S1 2.2588(11); Fe2–S2 2.2703(10), S2–C8 1.863(3), S1–C7 1.859(4), N1–C9
1.408(4), N1–C8 1.414(4), N1–C7 1.433(4), O8–C16 1.333(4), O8–C17 1.429(5), O7–C16 1.176(4), C8–S2–Fe1 113.14(12), C8–S2–Fe2 107.99(12), Fe1–S2–
Fe2 66.99(3), C7–S1–Fe2 109.79(12), C7–S1–Fe1 112.19(11), Fe2–S1–Fe1 67.26(3), C9–N1–C8 120.8(3), C9–N1–C7 121.5(3), C8–N1–C7 114.4(3), C16–
O8–C17 115.4(3), N1–C7–S1 116.2(2), N1–C8–S2 115.1(2), O7–C16–O8 124.1(4), O7–C16–C15 126.4(3), O8–C16–C15 109.5(3).
Literature methods were used for the preparation of
3,4-bis(bromomethyl)benzoic acid and its N-hydroxy-
succinimide ester [13,14], NH2C6H4CO2CH3 [15] and
[Fe2S2(CO)6] [16]. N,N-di(chloromethyl)amine(ClCH2)2-
NC6H4CH2CO2CH3 was obtained from reaction of parafor-
maldehyde with NH2C6H4CH2CO2CH3, followed by
chlorination with thionyl chloride, according to a procedure
previously described [7a]. All other reagents were purchased
commercially. Chemical analyses were performed by the Ser-
vice de Microanalyse du CNRS, Gif sur Yvette (France). The
1H NMR spectra were recorded at room temperature with a
Bruker AC 300 or AMX 400 spectrometers and were refer-
enced to SiMe4. The infrared spectra were recorded on a Nic-
olet Nexus Fourier transform spectrometer. Cyclic
voltammetry experiments were carried out as recently
Fig. 2. Cyclic voltammograms of a N2-purged 2 mM solution of complex
described elsewhere [18].
1 in CH3CN–Bu4NPF6 with increasing concentration of HOTs. First
negative going scan at 0.1 V sÀ1 recorded at a glassy carbon disk 0.3 cm in
diameter.
3.2. Syntheses of [Fe2(l-SRS)(CO)6]
(R = (CH2)2NC6H4CH2CO2CH3 (1),
(CH2)2C6H3COOH (2), (CH2)2C6H3COON(COCH2)2
(3))
link to a carbon electrode has been reported previously
[4]. This study revealed the decomposition of the modified
electrode in acidic media and seems to end up this
approach.
In a typical procedure, a red solution of [Fe2(l-
S2)(CO)6] (344 mg, 1 mmol) in THF (20 mL) was treated
at À78 ꢁC with a 1 mol LÀ1 solution of superhydride,
LiBEt3H, in THF (2 mL, 2 mmol). This mixture was stirred
10 min and the solution turned to green. A solution
of 1.5 equiv. of a dihalide reagent [(ClCH2)2NC6H4-
CH2CO2CH3: 393 mg; (BrCH2)2C6H3COOH: 462 mg;
(BrCH2)2C6H3COON(COCH2)2: 593 mg] in THF (5 mL)
was then added and the coloration of the mixture changed
readily to red. This solution was allowed to warm to room
3. Experimental
3.1. General procedures
All reactions were performed under an atmosphere of
argon or dinitrogen using conventional Schlenk techniques.
Solvents were deoxygenated and dried by standard methods.

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V. Vijaikanth et al. / Journal of Organometallic Chemistry 692 (2007) 4177–4181
temperature. The solvent was then evaporated and
the resulting residue was extracted with diethylether
(2 · 20 mL). After evaporation of Et2O the solid was puri-
fied by chromatography on silica gel column with mixtures
of hexane–dichloromethane or dichloromethane–THF as
eluents. Compound 1 was eluted with hexane:dichloro-
methane (50:50 and 40:60) solutions and 2 and 3 were
eluted with a dichloromethane–THF (95:5) mixture. 1–3
were obtained as red solids (yields after workup: 1,
140 mg, 26%; 2, 70 mg, 14%;3, 40 mg, 7%).
Appendix A. Supplementary material
CCDC 648154 contains the supplementary crystallo-
graphic data for 1. These data can be obtained free of
charge
via
http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.jorganchem.2007.06.042.
(ClCH2)2NC6H4CH2CO2CH3: 1H NMR (300 MHz,
CDCl3, d): 7.29 (d, 2H, JHH = 8.8 Hz, C6H4), 7.20 (d,
2H, JHH = 8.8 Hz, C6H4), 5.53 (s, 4H, N(CH2)2S2), 3.69
(s, 3H, CH2CO2CH3), 3.60 (s, 2H, CH2CO2CH3).
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