[FeFe]-Hydrogenase active site models with relatively low reduction potentials: Diiron dithiolate complexes containing rigid bridges

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Inorganic Biochemistry Journal of Inorganic Biochemistry 102 (2008) 952–959 www.elsevier.com/locate/jinorgbio

[FeFe]-Hydrogenase active site models with relatively low reduction potentials: Diiron dithiolate complexes containing rigid bridges Ping Li a, Mei Wang a,*, Jingxi Pan a, Lin Chen a, Ning Wang a, Licheng Sun a,b,* a

State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116012, China b KTH School of Chemical Science and Engineering, Organic Chemistry, 10044 Stockholm, Sweden Received 20 September 2007; received in revised form 12 December 2007; accepted 14 December 2007 Available online 25 December 2007

Abstract Three diiron dithiolate complexes containing rigid and conjugated bridges, [l-SC6H4-2-(CO)S-l]Fe2(CO)6 (1), [2-l-SC5H3N-3-(CO)S-l]Fe2(CO)6 (2), and the PPh3-monosubstituted complex [l-SC6H4-2-(CO)S-l]Fe2(CO)5(PPh3) (1-P), were prepared as biomimetic models for the [FeFe]-hydrogenase active site. The structures of complexes 1 and 2 were determined by single crystal X-ray analysis, which shows that each complex features a rigid coplanar dithiolate bridge with a 2–3° deviation from the bisect plane of the molecule. The influence of the rigid bridge on the reduction potentials of complexes 1, 2 and 1-P was investigated by electrochemistry. The cyclic voltammograms of complexes 1 and 2 display large positive shifts for the primary reduction potentials, that is, 380–480 mV in comparison to that of the pdt-bridged (pdt = propane-1,3-dithiolato) complex (l-pdt)Fe2(CO)6 and 160–260 mV to that of the bdt-bridged (bdt = benzene-1,2-dithiolato) analogue (l-bdt)Fe2(CO)6. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Bioinorganic chemistry; Diiron complex; Electrochemistry; [FeFe]-hydrogenase; Rigid bridge

1. Introduction Hydrogenases are important enzymes in numerous microorganisms, which can catalyse hydrogen evolution or uptake. Among FeFe, NiFe, NiFeSe and FeS-cluster-free (Hmd) [1] hydrogenases reported so far, the [FeFe]-hydrogenases ([FeFe]Hases) are the most efficient biocatalysts for hydrogen evolution [2,3]. Crystallographic and IR spectroscopic studies on the two types of [FeFe]Hases, CpI (Clostridium pasteurianum) [4] and DdH (Desulfovibrio desulfuricans) [5], revealed that the active site of [FeFe]Hases *

Corresponding authors. Address: State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116012, China. Tel.: +86 411 88993886; fax: +86 411 83702185. E-mail address: [email protected] (M. Wang). 0162-0134/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2007.12.018

is comprised of a 2Fe2S butterfly structure (H-cluster), which contains diatomic ligands CO and CN, a cysteinylS ligand connecting to a 4Fe4S subcluster, and a three-atom linker (–CH2XCH2–, X = CH2, NH or NHþ 2 ) bridged between the two S atoms of the 2Fe2S H-cluster (Fig. 1). The resemblance in structure between the [FeFe]Hase active site and the well-known complexes [(l-SR)2Fe2(CO)6] draws intensive attention to the chemistry of such complexes. In recent years, a great progress has been achieved in structural and functional biomimics of the 2Fe2S subunit in the [FeFe]Hase active site. A large number of diiron dithiolate complexes, either with the alkylene bridges, e.g. pdt (propane-1,3-dithiolato) [6–23], edt (ethane-1,2-dithiolato) [24–26], and an adt (2-azopropane-1, 3-dithiolato) bridge [27–32], were prepared and characterized. Some diiron dithiolate complexes were reported catalytically active for electrochemical proton reduction

P. Li et al. / Journal of Inorganic Biochemistry 102 (2008) 952–959

NH (or CH2) Cys [S4Fe4]

S

S

NC OC

Fe C O

X S

L

S Fe

O

CO CN

CO L = − (Hred), H2O (HHox2O ), CO (Hox )

S CO

OC Fe OC OC

Fe CO CO

X = C(1); N(2)

Fig. 1. The active site of [FeFe]Hases (left) and synthetic models of the present work (right).

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trochemical properties of complexes [l-SC6H4-2(CO)S-l]Fe2(CO)6 (1), [2-l-SC5H3N-3-(CO)S-l]Fe2(CO)6 (2), and the PPh3-monosubstituted complex [l-SC6H4-2(CO)S- l]Fe2(CO)5(PPh3) (1-P). As expected, complexes 1 and 2 display relatively low reduction potentials at 1.28 and 1.18 V vs. Fc/Fc+, respectively, and such low reduction potentials make light-driven electron transfer from the RuðbpyÞþ 3 species (the first oxidation event at 1.66 V vs. Fc/Fc+) [44,45] to the diiron complex thermodynamically feasible. 2. Experimental

to molecular hydrogen at ca. 1.0 to 1.2 V vs. NHE in the presence of strong acids [14–16,29,33–35], which contrasts sharply with the incredibly low potential and the mild condition (0.4 V vs. NHE at neutral pH) for the enzymatic proton reduction catalysed by the native [FeFe]Hases [36]. We are interested in building light-driven proton reduction catalytic systems, composed of a bio-inspired 2Fe2S model of the [FeFe]Hase active site and a light-harvesting component either by inter- or intramolecular combination [37]. For this purpose, the reduction property and the photo-stability of the diiron complexes are important factors and must be improved. One of our recent work is the preparation of some specially designed diiron complexes to lower the reduction potentials of the complexes and thereby to make electron transfer from a photosensitizer to a [2Fe2S] complex thermodynamically feasible. In general, the reported all-CO 2Fe2S structural models display the first reduction potentials in the range of 1.5 to 1.8 V vs. Fc/Fc+ in CH3CN [29,34,35]. Displacement of one or two CO ligands of diiron dithiolate complexes by other ligands makes the iron centers more protophilic and the reduction potentials of the complexes much more negative (1.9 to 2.7 V vs. Fc/Fc+) [7,13–20,33–35]. Although much effort has focused on adjusting the redox potential, the protophilicity, and the water solubility of diiron dithiolate model complexes by CO-displacement with various ligands, such as cyanide [6–8,28], tertiary phosphines and phosphites [9–15], N-heterocyclic carbenes [16–20], isocyanides [21,22], and 1,10-phenanthroline [23], little work has been reported on tuning the reactivity and the electrochemical property of the diiron center by varying the S-to-S linker [38–43]. It was reported that the bdtbridged (bdt = benzene-1,2-dithiolato) diiron dithiolate complex (l-bdt)Fe2(CO)6 showed the first reduction event at 1.44 V vs. Fc/Fc+ with a 220 mV positive shift in comparison to the pdt-bridged complex (l-pdt)Fe2(CO)6 [35], indicating that the reduction potential of the iron center in the diiron dithiolate complexes can be effectively tuned less negative by using the rigid and conjugated S-to-S linker. Therefore, we designed and prepared diiron complexes bearing rigid, conjugated and electron withdrawing S-to-S linkers, that is, SC6H4-2-(CO)S and 2-SC5H3N-3-(CO)S (Fig. 1). Herein, we report the preparation, the spectroscopic and crystallographic characterization, and the elec-

2.1. General procedure and materials All reactions and operations related to organometallic complexes were carried out under a dry, oxygen-free dinitrogen atmosphere with standard Schlenk techniques. All solvents were dried and distilled prior to use according to the standard methods. Commercially available materials, Fe(CO)5, 3H-1,2-benzodithiol-3-one, and 2-mercaptonicotinic acid, were reagent grade and used as received. Starting complex Fe2(CO)9 used in this study was prepared from Fe(CO)5 in acetic acid under irradiation of an Xe lamp (500 W). 3H-1,2-Dithiolo[3,4-b]pyridin-3-one was prepared according to the literature procedure [46]. The 1H and 31P NMR spectra were collected with a Bruker AVANCE II/ 400 spectrometer. Abbreviations used for the reported 1H NMR spectra are as follows: s = singlet, s br = singlet broad, d = doublet, t = triplet. Infrared spectra were recorded with a JASCO FT/IR 430 spectrophotometer. The intensity of reported IR signals are defined as m = medium, s = strong, vs = very strong. Elemental analyses were performed with an Elementar Vario EL III elemental analyzer. 2.2. Synthesis of [l-SC6H4-2-(CO)S-l]Fe2 (CO)6 (1) The mixture of Fe2(CO)9 (0.73 g, 2 mmol) and 3H-1,2benzodithiol-3-one (0.17 g, 1 mmol) in THF (20 mL) was stirred at room temperature for 1 h and then filtered. After removal of THF by evaporation under reduced pressure, the residue was purified by column chromatography on silica gel using petroleum ether and CH2Cl2 as gradient eluents. Product 1 was obtained from the second red band. Yield of 1: 0.35 g (78%). Recrystallization of 1 in the mixed solution of hexane and CH2Cl2 at 20 °C overnight gave single crystals suitable for X-ray diffraction analysis. Anal. Calc. for C13H4Fe2O7S2 (447.98): C, 34.85; H, 0.90. Found: C, 34.99; H, 0.94%. 1H NMR (CDCl3): 7.44 (d, JHH = 8.0 Hz, 1H), 7.33 (d, JHH = 7.6 Hz, 1H), 7.25 (d, JHH = 7.6 Hz, 1H), 7.18 (t, JHH = 7.6 Hz, 1H) ppm. IR (hexane): mCO 2082 (m), 2048 (s), 2012 (s) cm1. 2.3. Synthesis of [2-l-SC5H3N-3-(CO)S-l]Fe2(CO)6 (2) The mixture of Fe2(CO)9 (0.73 g, 2 mmol) and 3H-1, 2-dithiolo[3,4-b]pyridin-3-one (0.17 g, 1 mmol) in THF

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P. Li et al. / Journal of Inorganic Biochemistry 102 (2008) 952–959

(20 mL) was stirred at room temperature for 2 h and then filtered. The working up procedure is similar as that for 1. Yield of 2: 0.08 g (18%). Single crystals suitable for X-ray diffraction analysis were obtained by evaporation of the hexane and CH2Cl2 mixed solution of 2 at room temperature. Anal. Calc. for C12H3Fe2NO7S2 (448.97): C, 32.10; H, 0.67; N, 3.12. Found: C, 32.48; H, 0.69; N, 3.16%. 1H NMR (CDCl3): 8.32 (s br, 1H), 7.73 (d, JHH = 7.6 Hz, 1H), 7.22 (t, JHH = 5.2 Hz, 1H) ppm. IR (hexane): mCO 2084 (m), 2050 (s), 2014 (s) cm1. 2.4. Synthesis of [l-SC6H4-2-(CO)S-l]Fe2 (CO)5 (PPh3) (1-P) Compound PPh3 (0.26 g, 1 mmol) was added to a toluene solution (20 mL) of 1 (0.45 g, 1 mmol) in one portion at room temperature. The mixture was stirred for 0.5 h, and then the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel with petroleum ether and CH2Cl2 (1/1, v/v) as eluent. The main band was collected and the analytically pure product 1-P was obtained by cooling the concentrated solution to 20 °C overnight. Yield of 1-P: 0.56 g (82%). Anal. Calc. for C30H19Fe2O6PS2 (682.26): C, 52.81; H, 2.81. Found: C, 52.48; H, 2.86%. 1H NMR (CDCl3): 7.49–7.45 (m, 6H), 7.36–7.27 (m, 9H), 7.05 (d, JHH = 7.2 Hz, 1H), 6.91–6.87 (m, 2H), 6.82 (d, JHH = 7.6 Hz, 1H) ppm. 31 P{1H} NMR (CDCl3): 62.05 (s) ppm. IR (hexane): mCO 2057 (m), 2003 (s), 1955 (s) cm1. 2.5. X-ray crystal structure determination The single-crystal X-ray diffraction data were collected with a Bruker SMART Apex II diffractometer at ˚ ). Data 298(2) K using Mo Ka radiation (k = 0.71073 A processing were accomplished with the SAINT processing program. Absorption corrections were applied using SADABS. The structures were solved using direct method and refined by the full-matrix least-squares procedure with SHELXTL. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed geometrically and refined in a riding model. Crystal data and structure refinement details for complexes 1 and 2 were summarized in Table 1. 2.6. Electrochemistry Cyclic voltammograms were recorded on a BAS-100B electrochemical potentiostat using a three electrode cell at a scan rate of 100 mV s1 under argon atmosphere. The working electrode was a glass carbon disk (0.071 cm2) polished with 3 and 1 lm diamond pastes and sonicated in ion-free water for 10 min prior to use. The reference electrode was a non-aqueous Ag/Ag+ (0.01 M AgNO3) in CH3CN and the auxiliary electrode was a platinum wire. The acetonitrile (Aldrich, spectroscopy grade) used for electrochemical measurements was freshly distilled from

Table 1 Crystal data and structure refinements for 1 and 2 Complex

1

2

Formula Mw Crystal system Space group ˚) a (A ˚) b (A ˚) c (A ˚ 3) V (A Z l (mm1) Crystal size (mm) hmin/max (°) Reflections collected Unique data/parameters Rint GOF on F2 R1 [I > 2r(I)] wR2[I > 2r(I)] Residual electron density ˚ 3) (e A

C13H4Fe2O7S2 447.98 Orthorhombic P212121 8.2714(11) 11.9718(17) 16.314(2) 1615.5(4) 4 2.086 0.28  0.28  0.46 2.50–27.48 6840 3486/217 0.0203 0.986 0.0306 0.0643 0.264, 0.240

C12H3Fe2NO7S2 448.97 Orthorhombic P212121 8.2581(3) 11.8094(4) 16.2662(6) 1586.33(10) 4 2.126 0.08  0.20  0.26 2.77–27.50 6234 3532/217 0.0247 0.948 0.0355 0.0686 0.335, 0.217

CaH2 under nitrogen. A solution of 0.05 M nBu4NPF6 (Fluka, electrochemical grade) in CH3CN was used as electrolyte. All potentials reported here are versus Fc/Fc+ couple. 2.7. Laser flash photolysis Nanosecond transient absorption measurements were performed on a LP-920 pump–probe spectroscopic setup (Edinburgh). The excitation source was the unfocused third harmonic (532 nm, 7 ns fwhm) output of a Nd:YAG laser (Continuum surelite II); the probe light source was a pulsexenon lamp. The signals were detected by Edinburgh analytical instruments (LP900) and recorded on a Tektronix TDS 3012B oscilloscope and computer. All samples in flash photolysis experiments were deaerated with argon for ca. 20 min. 3. Results and discussion 3.1. Preparation and spectroscopic characterization of 1, 2 and 1-P The diiron complexes containing ethylene- and benzene1,2-dicarbonyl S-to-S linkers were reported previously without electrochemical investigation [47,48], while the diiron dithiolate complexes with the SC6H4-2-(CO)S and 2-SC5H3N-3-(CO)S linkers have not been found in literature. It was reported that the reaction of 1,2-benzodithiole-3-thione with 2 equiv. of Fe2(CO)9 in THF afforded a [4Fe3SC] cluster [49]. In contrast, treatment of 3H-1,2-benzodithiol-3-one or 3H-1,2-dithiolo[3,4-b]pyridin-3-one with 2 equiv. of Fe2(CO)9 in THF gave diiron dithiolate complexes 1 and 2 containing rigid bridges, respectively (Scheme 1). Complex 1 with an SC6H4-2-(CO)S

P. Li et al. / Journal of Inorganic Biochemistry 102 (2008) 952–959

955

Scheme 1.

linker was obtained in a good yield, while the yield of complex 2 was relatively low when the benzene ring in the rigid bridge was changed to a pyridine ring, although the complete consume of 3H-1,2-dithiolo[3,4-b]pyridin-3-one was observed by TLC. In addition to complex 2 the reaction of Fe2(CO)9 and 3H-1,2-dithiolo[3,4-b]pyridin-3-one generated a fair amount of precipitate, which is a mixture. The structure of the precipitate is still unknown. The PPh3-monosubstituted complex 1-P was synthesized in high yield by CO-displacement of 1 in toluene at room temperature. Complexes 1, 2 and 1-P were characterized by IR, NMR and elemental analysis. The results of elemental analyses for the three complexes are in good agreement with the supposed compositions. The IR data for 1, 2 and 1-P are summarized in Table 2 together with the data for the analogous bdt- and pdt-bridged diiron complexes for a ready comparison. The shifts of the mCO bands can reflect the variation of electron density on the iron center. The average values of the mCO bands for complexes 1 and 2 shift by 13 and 15 cm1 to higher frequency in comparison to that of (l-pdt)Fe2(CO)6, and the mCO absorption bands of 1 and 2 show 4–6 cm1 blue-shifts compared to the three mCO bands for (l-bdt)Fe2(CO)6 with a rigid phenylene bridge. The displacement of a CO ligand by a better donor, a PPh3 ligand, results in large red-shifts of the mCO bands for 1-P in comparison to those for its parent complex 1. The red-shift value (DmCOav = 44 cm1) for 1 to 1-P is

comparable with that (DmCOav = 49 cm1) for the PPh3monosubstituted diiron complex reported previously [13]. Complex 1-P displays a 19 cm1 blue-shift for the mCOav in comparison to that of the analogous complex (lpdt)Fe2(CO)5(PPh3). 3.2. Molecular structures of complexes 1 and 2 The molecular structures of complexes 1 and 2 were determined by X-ray crystal diffraction. The ORTEP drawings are shown in Figs. 2 and 3. Selected bond lengths and angles are listed in Table 3. Crystallographic studies show the structural resemblance of the framework of 1 and 2 to previously reported 2Fe2S models [6–14]. The central structure of the 2Fe2S core is in a butterfly conformation and each Fe atom is coordinated with a pseudo-square-

Table 2 IR data in the mCO region for 1, 2, 1-P, and related diiron complexesa Complex

mCO (cm1)

mCOav (cm1) Ref.

(l-bdt)Fe2(CO)6

2079 2006 2075 1993 2082 2012 2084 2014 2057 1955 2044 1931

2043

[50]

2034

[6]a

2047

This work

2049

This work

2005

This work

1986

[13]b

(l-pdt)Fe2(CO)6 1 2 1-P (l-pdt)Fe2(CO)5(PPh3) a b

(m), 2044 (s), (vs) (m), 2035 (s), (s) (m), 2048 (s), (s) (m), 2050 (s), (s) (s), 2003 (s), (m) (s), 1984 (s), (m)

The spectra were measured in hexane. In CH3CN.

Fig. 2. ORTEP drawing of 1 with thermal ellipsoids at the 30% probability level.

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P. Li et al. / Journal of Inorganic Biochemistry 102 (2008) 952–959 Table 3 ˚ ) and angles (deg) for 1 and 2 Selected bond lengths (A Complex Bond lengths Fe(1)–Fe(2) Fe(1)–S(1) Fe(1)–S(2) Fe(2)–S(1) Fe(2)–S(2) S(1)  S(2) Fe(1)–Cav Fe(2)–Cav Bond angles Fe(1)–S(1)–Fe(2) Fe(1)–S(2)–Fe(2) S(1)–Fe(1)–S(2) S(1)–Fe(2)–S(2) C(1)–Fe(1)–Fe(2) C(1)–Fe(1)–C(2) C(1)–Fe(1)–C(3) C(2)–Fe(1)–C(3) C(6)–Fe(2)–Fe(1) C(6)–Fe(2)–C(4) C(6)–Fe(2)–C(5) C(4)–Fe(2)–C(5)

1

2 2.5212(6) 2.2551(9) 2.2369(9) 2.2585(10) 2.2421(8) 3.050 1.794(4) 1.801(1)

67.92(3) 68.51(3) 85.52(3) 85.32(3) 147.29(12) 100.36(18) 98.88(17) 89.73(16) 146.87(12) 98.75(18) 99.21(16) 91.35(18)

2.5152(7) 2.2511(11) 2.2407(10) 2.2614(10) 2.2502(8) 3.071 1.791(4) 1.800(4) 67.75(3) 68.12(3) 86.27(4) 85.80(3) 148.36(15) 99.5(2) 98.34(18) 89.70(19) 146.85(13) 97.9(2) 99.80(18) 91.72(19)

˚ longer than that of the Fe–S(2) bonds due 0.011–0.017 A to weaker electron donating capability of the S(1) attached to the carbonyl group than that of the S(2) bonded to a phenyl group. The rigid dithiolate bridge is the special feature for complexes 1 and 2 compared to reported pdt- and adt-bridged diiron complexes. The calculated plane of the SRS bridge is nearly a bisect plane of the molecular structure. Except for two Fe(CO)3 units all atoms are in the plane with the aver˚ for 1 and 0.0159 A ˚ for 2. The age deviation of 0.0289 A drawing of complex 2 along the direction of S(1) to S(2) is shown in Fig. 3 (bottom). The angle between the calculated plane of the rigid bridge and the Fe–Fe bond deviates from 90° by 2.7° for 1 and 3.2° for 2, resulting in the asymmetric molecular structures. Complexes 1 and 2 are both crystallized in a chiral space group, orthorhombic P212121. 3.3. Electrochemistry of complexes 1, 2 and 1-P

Fig. 3. (top) ORTEP drawing of 2 with thermal ellipsoids at the 30% probability level, (bottom) ORTEP drawing of 2 along the direction of S(1) to S(2).

pyramidal geometry. The deviations of the iron atoms from ˚ for Fe(1) and the 2S2C-formed basal plane are 0.38 A ˚ 0.37 A for Fe(2). Similar crystal structures and geometry parameters were found for the two complexes except for the difference in the atoms of the C(13) in 1 and the N(1) ˚ for 1 and in 2. The Fe–Fe distances, 2.5212(6) A ˚ 2.5152(7) A for 2, are in good agreement with those ˚ ) found for other all-CO diiron complexes (2.48–2.52 A [6,27–32]. The average distance of the Fe–S(1) bonds is

Cyclic voltammograms (CV) were recorded in cathodic direction as shown in Fig. 4, to evaluate the effect of the rigid bridges on the redox property of the iron centers in the diiron complexes. The first reduction event of complex 1 takes place at 1.28 V vs. Fc/Fc+. Complex 2 shows the reduction event at 1.18 V with 100 mV anodic shift relative to that for complex 1. The primary reduction potentials of 1 and 2 are 380–480 mV less negative than that for the propanedithiolate complex (l-pdt)Fe2(CO)6 (1.66 V vs. Fc/Fc+) [35]. Insertion of a carbonyl group between one of the S atoms and the phenylene unit results in 160 mV anodic shift as compared to that for (lbdt)Fe2(CO)6 (1.44 V vs. Fc/Fc+) [35]. It is noticeable that both complexes 1 and 2 display completely irreversible

P. Li et al. / Journal of Inorganic Biochemistry 102 (2008) 952–959

Fig. 4. Cyclic voltammograms of complexes 1, 2 and 1-P at a scan rate of 100 mV s1.

reduction events, while the reduction of the phenylenebridged analogue appears chemically reversible. It shows that not only the FeIFeI complexes 1 and 2 but also their reduced species are less stable than the phenylene-bridged diiron complex. Replacement of a CO ligand in 1 by PPh3 leads to a 190 mV negative shift of the reduction event to 1.47 V. The current height in the CVs of complexes 1, 2 and 1-P is compared with that in the CV of [(l-pdt)Fe2(CO)6] with identical concentration and under the same measuring condition. The approximately equal current height in the CVs of complexes 1, 2 and 1-P and [(l-pdt)Fe2(CO)6] suggests that the primary reduction of complexes 1, 2 and 1-P is a one-electron process (FeIFeI/ FeIFe0). The electrochemical results indicate that in addition to introduction of non-CO ligands to the iron center, variation of the S-to-S bridge can also effectively tune the reduction potentials of diiron model complexes. The current height of the first reduction event for complex 1 in CV apparently increases upon addition of 1 equiv HOAc (Fig. 5), but it does not grow further with successive addition of 2–10 mM HOAc and only the current height of

957

the second reduction peak grows apparently. Similar phenomena were found for complex 2. As reported for pdtand bdt-bridged all-CO complexes, complexes 1 and 2 do not display electrocatalytic activity for proton reduction of HOAc at the first reductive events, while the second reduction events at 2.06 V vs. Fc/Fc+ are catalytic active. The control experiment was made in the absence of the diiron complex under the same measuring condition and the blank curve is also shown in Fig. 5 for a comparison. The electrolysis of complex 1 with controlled potential at 2.1 V generated small bubbles on the surface of the glass carbon disk electrode (surface area, 1.17 cm2), which were identified as H2 by gas chromatograms on GC7890 instrument with nitrogen as carrier gas. To compare the proton reduction properties, the overpotentials of the complexes with different S-to-S linkers were calculated, that is, 0.60 V for 1, 0.52 V for 2, 0.89 V for (l-pdt)Fe2(CO)6, and 0.64 V for (l-bdt)Fe2(CO)6 [51]. As expected, complexes 1 and 2 possess lower overpotentials for electrocatalytic proton reduction of HOAc, but the current height of the second reduction event of complexes 1 and 2 is less sensitive than those of the pdt- and bdt-bridged diiron complexes towards the concentration of the weak acid. 3.4. Laser flash photolysis þ

Light-driven electron transfer from the RuðbpyÞ3 species, generated by reductive quenching of the excited state of RuðbpyÞ2þ 3 , to the 2Fe2S complexes 1 and 2 was detected in CH3CN/H2O (3/1, v/v) using ascorbic acid (H2A) as electron donor. Laser excitation (532 nm, pulse width 2þ 7 ns) of the deoxygenated solution of RuðbpyÞ3 and þ H2A resulted in the instant formation of RuðbpyÞ3 , which is a long-lived species (s = 16.9 ls) [37]. The lifetime of þ RuðbpyÞ3 was shortened to 1.7 ls in the presence of complex 1 (1  104 M, Fig. 6). The kinetic constant for electron transfer from RuðbpyÞþ 3 to 1 was determined to be

0 mM 1 mM 2 mM −2.06 4 mM 10 mM 10 mM HOAc without 1

120

i / μA

80

−1.28

40

0 1

0

-1 -2 + E / V vs. Fc/Fc

-3

Fig. 5. Cyclic voltammograms of 1 in the presence of HOAc (0–10 mM) and the blank curve in the absence of 1 with 10 mM HOAc at a scan rate of 100 mV s1.

Fig. 6. Kinetic traces at 520 nm obtained following flash photolysis of a deoxygenated mixture of CH3CN/H2O (3:1, v/v) containing (a) 0.03 mM RuðbpyÞ2þ 3 , 0.1 M H2A; (b) the same as (a) but with 0.1 mM complex 1. The two decay curves are normalized for easier comparison.

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6.0  109 M1 S1 by varying the concentration of 1 from 0.025 to 0.15 mM in the triad system, which is somewhat faster than the previously reported rate constants for elecþ tron transfer from RuðbpyÞ3 to the well-known diiron model complexes (l-pdt)Fe2(CO)6 (3.9  109 M1 S1) and [{(l-SCH2)2NCH2C6H5}Fe2(CO)6] (3.5  109 M1 S1) [37]. The concomitant kinetics for the decay of I 0 RuðbpyÞþ 3 and the buildup of the Fe Fe species further supports the conclusion that one-electron transfers from þ RuðbpyÞ3 to 1. No good transient absorption spectrum was obtained for complex 2 due to its poor photo-stability under irradiation. An apparent color change of the solution was observed after irradiation of 2 in CH3CN/H2O for 1 min and a precipitate appeared immediately, while the solution of complex 1 remained clear and the color of the solution did not display observable change during the flash photolysis measurement. The result indicates that complex 1 is more photo-stable than complex 2. 4. Conclusion

Swedish Research Council, the Swedish Energy Agency and the K&A Wallenberg Foundation for financial support of this work. Appendix A. Supplementary material CCDC 652825 (1) and 652826 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/ data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10. 1016/j.jinorgbio.2007.12.018. References [1] [2] [3] [4] [5]

The rigid and conjugated bridges can be readily introduced to the diiron dithiolate complexes, which show relatively low reduction potentials for the FeIFeI to FeIFe0 process. The results indicate that the S-to-S bridge can apparently influence the redox property of the diiron dithiolate model complexes, providing another approach, besides CO-displacement, to tune the reduction potentials of candidate catalysts for electro- and photo-chemical proton reduction to molecular hydrogen. The driving force for light-induced electron transfer from photosensitizer to diiron complex is enhanced by lowing the reduction potential of the diiron complex, resulting in a somewhat faster electron transfer rate for complex 1 in comparison to that for well-known pdt- and adt-bridged all-CO diiron complexes.

[14]

5. Abbreviations

[15]

pdt bdt edt adt THF bpy CV NHE Hmd Fc/Fc Hases

+

propane-1,3-dithiolato benzene-1,2-dithiolato ethane-1,2-dithiolato 2-azopropane-1,3-dithiolato tetrahydrofuran bipyridyl cyclic voltammogram normal hydrogen electrode H2-forming methylenetetrahydromethanopterin dehydrogenase ferrocene/ferrocenium hydrogenases

Acknowledgements We are grateful to the Chinese National Natural Science Foundation (Grant Nos. 20471013 and 20633020), the

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