Using naphthalene-2-thiolate ligands in the design of hydrogenase models with mild proton reduction overpotentials

Article abstract of DOI:10.1016/j.poly.2012.11.001

Diiron-carbonyl complexes coupled to naphthalene-2-thiolate ligands, [(μ-naphthalene-2-thiolato)₂Fe₂(CO)₆] (1) and [(μ-naphthalene-2-thiolato)₂Fe₂(CO) ₅PPh₃] (2), have been usefu

Full text of DOI:10.1016/j.poly.2012.11.001

Polyhedron 50 (2013) 164–168
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Using naphthalene-2-thiolate ligands in the design of hydrogenase models
with mild proton reduction overpotentials
a
b
Charles A. Mebi a, , Derek S. Karr , Bruce C. Noll
⇑
a Department of Physical Sciences, Arkansas Tech University, 1701 N. Boulder Ave., Russellville, AR 72801, USA
b Bruker AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Diiron–carbonyl complexes coupled to naphthalene-2-thiolate ligands, [(l-naphthalene-2-thiolato)2
Fe2(CO)6] (1) and [(l-naphthalene-2-thiolato)2Fe2(CO)5PPh3] (2), have been usefully prepared and struc-
Received 28 August 2012
Accepted 1 November 2012
Available online 12 November 2012
turally characterized. As models for the active site of hydrogenase enzymes, these compounds have been
examined as electrocatalysts for the reduction of proton to produce molecular hydrogen. In the presence
of acetic acid, 1 and 2 catalyze the electrochemical production of molecular hydrogen with mild overpo-
tentials of À0.54 and À0.51 V versus Fc/Fc+, respectively. The overpotential for compound 1 is 260 mV
Keywords:
Cluster compounds
Electrocatalysis
Hydrogenase model
Hydrogen production
Naphthalene-2-thiolate
Transition metal
smaller than that of the analogous compound, [(
l-SPh)2Fe2(CO)6].
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
lysts for the reduction of acetic acid to molecular hydrogen by cyc-
lic voltammetry and the results are discussed.
Iron–thiolato complexes are ubiquitous in biological systems.
They are involved in electron transfer and biocatalytic processes
[1]. For example, the active site (H-cluster, A: X = NH or CH2) of
the hydrogen producing enzyme, iron-only hydrogenase, contains
a diiron carbonyl cluster with a bridging thiolato ligand [2–4]. Bio-
organometallic chemistry focused on the study of models for the
H-cluster is of great interest. This interest is due to the need to de-
velop efficient and inexpensive catalysts for the generation of
2. Experimental
2.1. General methods
All syntheses were conducted under inert (N2) atmosphere
using Schlenk line techniques. The solvents, CH2Cl2, tetrahydrofu-
ran (THF), and CH3CN were purchased from VWR international
and used as obtained. nBu4NPF6, naphthalene-2-thiol, triphenyl-
phosphine, trimethylnitrogenoxide (Me3NO), and triirondodeca-
hydrogen,
a clean alternative to fossil fuels [5–7]. Simple
organometallic models of the H-cluster such as B and C have been
investigated (Scheme 1). To modulate the stereoelectronic and
electrocatalytic properties of models B and C, different thiolate li-
gands have been employed [8–40]. Of particular interest is the
carbonyl (Fe3(CO)12
) were obtained from Aldrich and used
without further purification. The 1H, 13C, and 31P NMR spectra were
obtained in CDCl3 on a Bruker 300 MHz instrument. IR spectra in
CH2Cl2 were recorded on a Nicolet FT/IR Magna spectrophotome-
ter. Melting point determinations were conducted using an electro
thermal melting point apparatus.
use of polyaromatic thiolate ligands providing
p-electronic sys-
tems which may influence the stability and electrochemical prop-
erties of the [Fe2(CO)6] core [22–24].
This report contains a study of type C models containing naph-
thalene-2-thiolate ligands. The synthesis and spectroscopic (IR and
2.2. Electrochemistry
NMR) characterization of [(
(1) is described. The substitution of CO for triphenylphosphine in
complex 1 to afford [( -naphthalene-2-thiolato)2Fe2(CO)5PPh3]
l-naphthalene-2-thiolato)2Fe2(CO)6]
Electrochemical studies on the complexes were conducted
using an Epsilon BAS potentiostat with glassy carbon working elec-
trode, platinum auxiliary electrode and Ag/AgCl reference elec-
trode. The glassy carbon and platinum electrodes were polished
with alumina paste and rinsed with distilled water and acetone.
The electrodes were connected to a cell that contains 10 mL of
the catalytic mixture (1 mM). A 0.1 M CH3CN solution of nBu4NPF4
l
(2) is reported. Compounds 1 and 2 are evaluated as electrocata-
⇑
Corresponding author. Tel.: +1 4793562151; fax: +1 4799640837.
E-mail address: cmebi@atu.edu (C.A. Mebi).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.11.001

C.A. Mebi et al. / Polyhedron 50 (2013) 164–168
165
doubly curved silicon crystal monochromator and a Mo K
focus sealed tube (k = 0.71073 Å). The structure was solved and re-
a
micro-
X
Cys
S
S
fined using the Bruker SHELXTL Software Package [41].
S
OH2
S
S
[4Fe4S]
NC
CO
CO
CO
OC
OC
Fe CO
Fe
Fe
Fe
3. Results and Discussion
C
O
CN
OC
OC
3.1. Preparation
B
A
The synthesis of [(l-naphthalene-2-thiolato)2Fe2(CO)6] is pre-
sented in Scheme 2. A 100 mL round bottom flask containing
naphthalene-2-thiol and Fe3(CO)12 was purged with nitrogen and
60 mL of freshly distilled THF added. The mixture was stirred under
reflux conditions for 1 h or until there was a change in color from
dark green to deep red. Removal of the solvent by rotary evaporation
followed by chromatographic separation on silica gel using hexanes
as eluent gave 1 in 90% yield. Red crystalline plates of 1 were
CO
Fe
L
S
OC
S
R
R
S
C
S
CO
OC
OC
Fe
Fe
Fe
CO
CO
CO
OC
CO
OC
obtained from hexanes solution of [(l-naphthalene-2-thiolato)2
1: L = CO; 2: L = PPh3
Fe2(CO)6] at À20 °C.
Scheme 1 also describes the substitution of one of the CO li-
Scheme 1.
gands on 1 for electron-donating triphenylphosphine to afford
[(l-naphthalene-2-thiolato)2Fe2(CO)5PPh3] (2). A dichloromethane
solution of 1 was added to an acetonitrile solution of Me3NO and
stirred for 10 min under nitrogen atmosphere. The resultant dark
red solution was then treated with triphenylphospine and stirred
for 40 min affording a red solution. Compound 2 was obtained in
40% yield as a red solid after removal of solvent followed by chro-
matographic separation on silica gel (Hexanes/CH2Cl2; 1:1).
Recrystalization of 2 was accomplished from hexanes solution of
was used as supporting electrolyte. Anhydrous solvent (acetoni-
trile) from Aldrich was used and purged with nitrogen before each
measurement. Electrocatalytic studies were conducted using gla-
cial acetic acid.
2.3. Synthesis of [(l-naphthalene-2-thiolato)2Fe2(CO)6] (1)
[(
l
-naphthalene-2-thiolato)2Fe2(CO)5PPh3] in a freezer at À20 °C
Triirondodecarbonyl (1.57 g, 3.12 mmol) and naphthalene-2-
thiol (1.00 g, 6.25 mmol) contained in a 100 mL flask were treated
with THF and refluxed for 1 h or until the color of the mixture
changed from green to red. The solvent of the solution was re-
moved by rotary evaporation affording an orange solid. Separation
of the residue by silica gel chromatography using hexanes as elu-
overnight affording red block crystals of 2.
3.2. Spectroscopy
Compounds 1 and 2 were characterized using spectroscopic
methods (IR and NMR). 1H NMR spectra of 1 and 2 in chloroform
ent gave [(
l
-naphthalene-2-thiolato)2Fe2(CO)6] in 90% yield
(1.68 g, Rf: 0.3). IR (CH2Cl2, CO) 2075, 2039, 2001 cmÀ1
.
1H NMR
(CDCl3): 7.60–7.86 (m); 7.27–7.52 (m). 13C NMR (CDCl3): 209,
140–126 ppm. Elemental Anal. Calc.: C, 52.20; H, 2.36; S, 10.72.
Found: C, 52.21; H, 2.36; S, 11.24%. Melting point: decomposes at
164 °C.
Fe3(CO)12
HS
Reflux
1 hr
THF
2.4. Synthesis of [(l-naphthalene-2-thiolato)2Fe2(CO)5PPh3] (2)
A dichloromethane solution of 1 (101 mg, 0.168 mmol, 15 mL)
was transferred into an acetonitrile solution of Me3NO (18.7 mg,
0.168 mmol, 15 mL) and stirred for 10 min under nitrogen atmo-
sphere. To the dark red solution was added triphenylphosphine
(44.0 mg, 0.168 mmol) dissolved in 5 mL of CH2Cl2 to afford an or-
ange mixture. The solution was stirred at room temperature for 1 h
and the solvent removed by rotary evaporation. The residue was
separated by chromatography on preparative silica gel TLC plate
(solvent: Hexanes/CH2Cl2; 1:1, Rf: 0.6). Compound 2 was obtained
as red solid in 40% yield (55.7 mg). IR (CH2Cl2, CO) 2050, 1990,
CO
Fe
CO
S
OC
S
Fe
CO
OC
CO
1) Me3NO / CH3CN
2) PPh3 / DCM
1960 cmÀ1 1H NMR (CDCl3): 7.61–7.98 (m); 7.31–7.50 (m), 7.0–
.
7.12 (m). 13C NMR (CDCl3): 210, 140–126 ppm. 31P NMR
(121 MHz, CDCl3, 85% H3PO4): 56.7 (s) ppm. Elemental Anal. Calc.:
C, 62.03; H, 3.52; S, 7.70. Found: C, 61.85; H, 4.1; S, 7.78%. Melting
point: decomposes at 140 °C.
CO
Fe
PPh3
OC
S
2.5. Crystal structure determination
S
Fe
Red block-like specimen of 2, approximate dimensions
0.250 Â 0.250 Â 0.400 mm, was used for the X-ray crystallographic
analysis. The X-ray intensity data were measured on a Bruker
SMART X2S benchtop diffractometer system equipped with a
CO
OC
CO
Scheme 2. Synthesis of compounds 1 and 2.

166
C.A. Mebi et al. / Polyhedron 50 (2013) 164–168
Table 2
Summary of data collection, solution, and refinement parameters for 2.
2
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
43H29Fe2O5PS2
832.45
triclinic
P1
ꢀ
11.2438(5)
11.4387(5)
16.7992(8)
79.708(2)
74.0790(10)
65.2540(10)
1881.90(15)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (Mg/cm3)
Absorption coefficient (mmÀ1
F(000)
1.469
0.970
852
)
Reflections collected
18479
h (°)
Index ranges
2.23–25.01
À13 6 h 6 13, À13 6 k 6 13,
À19 6 l 6 19
6561 (0.0274)
Independent reflections (Rint
)
Coverage of independent reflections 98.8%
Absorption correction
Largest difference peak and hole
Maximum and minimum
transmission
Multiscan
0.299 and À0.256 e ÅÀ3
Fig. 1. IR spectra of 1 and 2 in CH2Cl2.
0.7935 and 0.6976
Final R indices [I > 2
r
(I)]
R1 = 0.0269, wR2 = 0.0660
R1 = 0.0329, wR2 = 0.0698
1.035
R indices (all data)
Table 1
Goodness-of-fit (GOOF) on F2
Infrared data for 1 and 2.
CCDC
898238
Compound
Infrareda
(co)/cmÀ1
À1b
~
m
m
(co)/cm
2038
2000(38)c
1
2
2075, 2039, 2001
2050, 1990, 1960
Table 3
Selected bond lengths [Å] and bond angles [°] for 2.
a
Recorded in DCM.
Bond lengths (Å)
Bond angles (o)
b
~
m
is average of IR values.
~
c
~
~
.
Dm
is average shift = m1
À
m2
Fe1–Fe2 2.5215(4) S1–Fe1–Fe2 56.865(15) Fe2–S1–Fe1 66.814(16)
Fe1–S1
Fe1–S2
Fe2–S1
Fe2–S2
Fe2–P
2.2827(6) S1–Fe2–Fe1 56.321(15) Fe2–S2–Fe1 67.518(16)
2.2705(5) S2–Fe2–Fe1 56.306(14) P1–Fe2–S2
2.2970(5) S2–Fe1–Fe2 56.176(14) P1–Fe2–S1
103.835(19)
112.08(2)
show peaks for the aromatic protons (8.0–7.3 ppm). 31P NMR spec-
trum of 2 confirms the presence of the PPh3 ligand in complex 2
with a chemical shift at 56.7 ppm as a singlet. The 13C NMR spectra
of 1 and 2 show peaks ascribed to the CO ligands at ꢀ210 ppm
including a set of peaks between 140 and 126 ppm.
2.2671(5) S1–Fe1–S2
2.2540(5) S1–Fe2–S2
80.141(18) P1–Fe2–Fe1 156.670(18)
79.909(19)
enzyme (2.6 Å) [2]. The X-ray diffraction analysis of 2 reveals a but-
terfly structure (Fig. 2) with equatorial–axial spatial orientation of
the naphthyl groups tethered to the iron–carbonyl moiety.
The infrared spectra of 1 and 2 recorded in dichloromethane are
reported in Fig. 1. IR spectrum of 1 contains three peaks at 2075,
2039 and 2001 cmÀ1 characteristic of stretching vibrational modes
of terminal metal carbonyls. These bands are consistent with those
reported [8–34] for similar organometallic compounds indicating
the presence of Fe2(CO)6 unit. Compound 2 also shows three IR
peaks at 2050, 1990 and 1960 cmÀ1. These peaks are shifted to
lower wavenumbers compared to 1 as a result of the electron-
donating property of PPh3 ligand (enhanced back-donation into
~
3.4. Electrochemistry
Cyclic voltammograms (CVs) of compounds 1 and 2 were re-
corded in acetonitrile. The relevant parameters are contained in
Table 4. The CVs of compounds 1 and 2 contain two interesting
electrochemical events; an irreversible reduction (1, Epc = À1.33 V;
2, Epc = À1.49 V versus Fc/Fc+) assigned to [FeI–FeI] ? [FeI–Fe0] and
an irreversible oxidation (1, Epa = +0.61 V versus Fc/Fc+; 2,
Epa = +0.31 V versus Fc/Fc+) ascribed to [FeI–FeI] ? [FeI–FeII]. The
reduction of compound 1 occurs at a less negative potential and
its oxidation at a more positive potential than compound 2. This
is in accord with the substitution of CO for PPh3 making the [Fe–
Fe] center of 2 more electron-rich than 1. The assignments and
shifts in potentials are consistent with results of similar models re-
ported in the literature [9]. The effect of the naphthalene rings on
the redox property of compound 1 can be clearly seen in compar-
the p⁄ orbital of CO in 2). The average shift (
to be ꢀ38 cmÀ1 (see Table 1).
Dm) was determined
3.3. X-ray crystallography
The spectroscopic data obtained for the products in Scheme 1,
allow for the assignment of the suggested structures for 1 and 2.
Their compositions and structures were further ascertained by ele-
mental analyses and X-ray crystallographic study. Red crystals of 1
and 2 were obtained from hexanes solutions of the compounds at
À20 °C. Block-like crystals 2 suitable for X-ray diffraction were ob-
tained and subjected to X-ray diffraction studies. Crystallographic
data for 2 are presented in Table 2 and structural parameters are
contained in Table 3. The Fe–Fe bond length for 2 is 2.5215(4). This
value is close to that reported for the active site of the hydrogenase
ison with the complexes, [(
and [( -SPh)2Fe2(CO)6]. The first reduction potential of complex 1
exhibits an anodic shift of about 190 mV in comparison to that of
[( -naphthalene-1,8-dithiolate)Fe2CO6] complex [8] and a positive
shift (110 mV) to that of [( -SPh)2Fe2(CO)6] [25b].
l-naphthalene-1,8-dithiolate)Fe2CO6]
l
l
l

C.A. Mebi et al. / Polyhedron 50 (2013) 164–168
167
Fig. 4. Cyclic voltammograms of 2 (1 mM) in 0.1 M Bu4NPF6/CH3CN at 100 mV/s in
the presence of increasing amounts of acetic acid (0, 8, 14, 30, 50 mM).
Fig. 2. Thermal ellipsoid representation of 2. Thermal ellipsoids drawn at the 50%
probability level. All hydrogen atoms are omitted for clarity.
Table 4
Electrochemical data for 1 and 2.
Compound
Epc/V
Epa/V
Ecat
Overpotential
1
À1.33
À1.49
À1.52
À1.44
+0.61
+0.31
+0.87
+0.81
À2.00
À1.97
À2.00
À2.26
À0.54
À0.51
À0.54
À0.80
2
3a
4b
a
3 is [(
4 is [(
l
l
-naphthalene-1,8-dithiolate)Fe2(CO)6], Refs. [8] and [23].
-SPh)2Fe2(CO)6], Refs. [9] and [25b].
b
Fig. 5. Cyclic voltammograms of
1 (blue) 2 (red) recorded under the same
conditions: (1 mM) in 0.1 M Bu4NPF6/CH3CN at 100 mV/s in the presence of
50 mM acetic acid. (Colour online.)
peak at À1.66 V versus Fc/Fc+. This peak may be due a new com-
plex formed by the substitution of CO in 1 for acetonitrile. The
overpotential of the catalysts have been calculated as described
by Evans et al. [9] and presented in Table 4. Compounds 1 and 2
have overpotentials of À0.54 and À0.51 V (versus Fc/Fc+) respec-
tively. These values compare favourably with those reported for
similar models [(naphthalene-1,8-dithiolate)Fe2(CO)6] (À0.54 V
versus Fc/Fc+) [23] and [(
[9,25b]. As observed in Fig. 5, although the reduction of 1 occurs
at a less negative potential than 2, compound 2 catalyzes the
reduction of protons at a slightly less negative potential.
l
-SPh)2Fe2(CO)6] (À0.80 V versus Fc/Fc+)
4. Conclusion
We have synthesized and structurally characterized two diiron–
carbonyl complexes containing polyaromatic thiolate ligands,
Fig. 3. Cyclic voltammograms of 1 (1 mM) in 0.1 M Bu4NPF6/CH3CN at 100 mV/s in
the presence of increasing amounts of acetic acid (0, 6, 14, 30, 50, 70 mM).
[(l
-naphthalene-2-thiolato)2Fe2(CO)6] and [(
l-naphthalene-2-
thiolato)2Fe2(CO)5PPh3]. The reduction of [(
l
-naphthalene-
2-thiolato)2Fe2(CO)6] produced a new species attributed to the
3.5. Electrocatalytic production of hydrogen
substitution of CO by solvent molecule. Upon reduction,
[(
stable to CO substitution and generate hydrogen from acetic acid
at a slightly lower overpotential than [( -naphthalene-2-thiolato)2
Fe2(CO)6]. Our results also show that the proton reduction overpo-
tential for [( -naphthalene-2-thiolato)2Fe2(CO)6] is 260 mV smaller
than that of the analogous compound, [( -SPh)2Fe2(CO)6]. This
study illustrates the potential of naphthalene-2-thiolate ligand to
modulate redox and electrocatalytic properties of the iron–carbonyl
moiety.
l-naphthalene-2-thiolato)2Fe2(CO)5PPh3] is shown to be more
Compounds 1 and 2 were examined as electrocatalyts for the
reduction of proton to molecular hydrogen. The results are pre-
sented in Figs. 3 and 4. Cyclic voltammograms of 1 and 2 in the
presence of acetic acid show a new peak at À2.00 V and À1.97 V
versus Fc/Fc+. The current at these reduction potentials increases
with the amount of acetic acid added. This is attributed to the
reduction of proton to molecular hydrogen [9]. Interestingly, the
cyclic voltammogram of 1 in the presence of acid show another
l
l
l

168
C.A. Mebi et al. / Polyhedron 50 (2013) 164–168
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CCDC 898238 contains the supplementary crystallographic
data for compound 2. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
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deposit@ccdc.cam.ac.uk.
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