LIN Hui-Min LI Ao XIAO Qi-Min LIU Xu-Feng② LI Yu-Long LIU Xing-Hi JIANG Zhong-Qing

a (School of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, China)

b (College of Chemistry and Environmental Engineering, Sichuan University of Science & Engineering, Zigong 643000, China)

c (College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China)

d (Department of Physics, Key Laboratory of ATMMT Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China)

ABSTRACT In this paper, a novel diiron ethane-1,2-dithiolate complex [Fe2(CO)4{κ2-(Ph2P)2(1,2-C6H4)}(μ- SCH2CH2S)] has been prepared and structurally characterized. Treatment of the parent complex [Fe2(CO)6(μ- SCH2CH2S)] with 1 equivalent of 1,2-bis(diphenylphosphino)benzene and Me3NO?2H2O as the oxidative agent gave the title complex in good yield. The title complex has been characterized by elemental analysis, IR, 1H NMR, 31P{1H} NMR, 13C{1H} NMR spectroscopy, and X-ray crystallography. X-ray crystal structure of the title complex contains a butterfly diiron cluster with a bridging ethane-1,2-dithiolate, four terminal carbonyls, and a chelating 1,2-bis(diphenylphosphino)benzene. In addition, electrochemical studies revealed that the title complex can catalyze the reduction of protons to H2 in the presence of acetic acid.

Keywords: diiron ethane-1,2-dithiolate, [FeFe]-hydrogenases, diphosphine ligand, X-ray crystallography, electrochemistry; DOI: 10.14102/j.cnki.0254-5861.2011-2502

1 INTRODUCTION

[FeFe]-hydrogenases are a class of natural enzymes that can reversibly catalyze the production or uptake of H2in some microorganisms[1]. Since the discovery of the active site of [FeFe]-hydrogenases, biomimetic chemistry on the active site of [FeFe]-hydrogenases has attracted more and more attention during the past two decades[2-4]. X-ray crystallographic studies revealed that the active site of [FeFe]-hydrogenases consists of a butterfly diiron cluster with a bridging dithiolate, carbonyls, cyanides, and a [Fe4S4]-connected cysteinyl ligand[5,6]. The bridging dithiolate was supposed to propane-1,3-dithiolate[7]or 2-aza- propane-1,3-dithiolate[8]. However, density functional theory (DFT) studies confirmed that it was 2-azapropane-1,3-di- thiolate due to the nitrogen atom playing an important role in the cleavage of H2[9]. Promoted by the structural information, chemists have designed and synthesized a great number of models for the active site of [FeFe]-hydrogenases[10-12]. Reactions of hexacarbonyl complexes with diphosphine ligands will give intermolecular bridging[13], intramolecular bridging[14], or chelating-diphosphine[15]containing analogues. Up to now, a variety of chelating-diphosphine containing diiron complexes have been reported in litera- ture[16-18]. However, to our knowledge, no corresponding reaction on the diphosphine ligand 1,2-bis(diphenylphos- phino)benzene has been reported. In order to develop new reactions on the diiron ethane-1,2-dithiolate complex [Fe2(CO)6(μ-SCH2CH2S)], we recently have started to investigate the reaction of the parent complex with 1,2-bis(diphenylphosphino)benzene. As a result, we obtained a novel complex with chelating 1,2-bis(diphenylphos- phino)benzene. Therefore, in this paper, we present the synthesis, characterization, and crystal structure of the diiron ethane-1,2-dithiolate complex [Fe2(CO)4{κ2-(Ph2P)2(1,2- C6H4)}(μ-SCH2CH2S)] as the model for the active site of [FeFe]-hydrogenases. Additionally, the electrochemical properties of the title complex were also studied by cyclic voltammetry.

2 EXPERIMENTAL

2. 1 Materials and methods

1,2-Bis(diphenylphosphino)benzene and Me3NO?2H2O were commercially available and used as received. Complex [Fe2(CO)6(μ-SCH2CH2S)][19]was prepared according to procedure. IR spectrum was recorded on a Nicolet MAGNA 560 FTIR spectrometer. NMR spectra were obtained on a Bruker Avance 500 MHz spectrometer. Elemental analysis was performed by a Perkin-Elmer 240C analyzer.

2. 2 Synthesis of the title complex

To a solution of [Fe2(CO)6(μ-SCH2CH2S)] (0.186 g, 0.5 mmol) and 1,2-bis(diphenylphosphino)benzene (0.223 g, 0.5 mmol) in CH2Cl2(20 mL) was added a solution of Me3NO·2H2O (0.056 g, 0.5 mmol) in MeCN (20 mL). The mixture was stirred at room temperature for 1 h and then the solvent was reduced on a rotary evaporator and the residue was subjected to TLC separation using CH2Cl2/petroleum ether (v/v= 1:2) as eluent. From the main red band, 0.325 g (85%) of the title complex was obtained as a red solid. IR (CH2Cl2, cm-1):vC≡O(shè)2023 (vs), 1955 (vs), 1943 (vs).1H NMR (500 MHz, CDCl3): 7.70 (s, 4H, PhH), 7.61 (s, 2H, PhH), 7.48 (s, 2H, PhH), 7.43 (s, 6H, PhH), 7.32 (t,J= 7 Hz, 2H, PhH), 7.22 (t,J= 7 Hz, 4H, PhH), 7.16 (t,J= 8 Hz, 4H, PhH), 1.90 (d,J= 7.5 Hz, 2H, SCH2), 1.62 (d,J= 7.5 Hz, 2H, SCH2) ppm.31P{1H} NMR (200 MHz, CDCl3, 85% H3PO4): 91.08 (s) ppm.13C{1H} NMR (125 MHz, CDCl3): 216.35 (t,2JP-C= 18 Hz, P2FeCO), 212.35 (FeCO), 147.72, 147.37 (dd,2JP-C= 35.2 Hz,1JP-C= 43 Hz,1-C6H4C), 136.45 (d,JP-C= 31.7 Hz,i-C6H5C), 135.52, 135.18 (dd,2JP-C= 3.5 Hz,1JP-C= 42.2 Hz, 2-C6H4C), 132.77 (t,2JP-C= 11.4 Hz,o-C6H5C), 132.33 (d,2JP-C= 13.9 Hz, 3,6-C6H4C), 130.07 (d,3JP-C= 3.9 Hz, 4,5-C6H4C), 129.82, 129.52 (2s,p-C6H5C), 128.31 (d,3JP-C= 9.4 Hz,m-C6H5C), 132.77 (d,3JP-C= 9.1 Hz,m-C6H5C), 35.54 (d,JP-C= 6.7 Hz, SCH2) ppm. Anal. Calcd. (%) for C36H28Fe2O4P2S2: C, 56.72; H, 3.70. Found (%): C, 56.98; H, 3.80.

2. 3 X-ray structure determination

A single crystal of the title complex was mounted on a Bruker D8 QUEST diffractometer. Data were collected at 296 K by using a graphite-monochromatic with MoKαradiation (λ= 0.71073 ?) in theω-φscan mode. Data collection and reduction were used by APEX2[20]. Absorption correction was performed by SADABS pro- gram[21]. Using OLEX2[22], the structure was solved by direct methods using the SHELXS program[23]and refined by full-matrix least-squares techniques SHELXL[23]onF2. Hydrogen atoms were located using the geometric method. Non-hydrogen atoms were refined with anisotropic thermal parameters. Crystal data for C36.5H29ClFe2O4P2S2(Mr= 804.81 g/mol): monoclinic system, space groupP21/c,a= 16.430(9),b= 11.258(7),c= 21.501(13) ?,β=108.761(16)°,V= 3766(4) ?3,Z= 4,T= 296(2) K,μ(MoKα) = 1.074 mm-1,Dc= 1.420 g/cm3, 71065 reflections measured (4.47≤2θ≤54.95°), 8624 unique (Rint= 0.0775,Rsigma= 0.0433) which were used in all calculations. The finalR= 0.0536 (I> 2σ(I)) andwR= 0.1817 (all data).

2. 4 Electrochemical experiment

Electrochemical properties of the title complex were studied by cyclic voltammetry (CV) in MeCN solution. Electrochemical measurements were carried out under nitrogen using a CHI 620 Electrochemical work station. As the electrolyte, n-Bu4NPF6was recrystallized several times from a CH2Cl2solution by the addition of hexane. CV scans were obtained in a three-electrode cell with a glassy carbon electrode (3 mm diameter) as the working electrode, a platinum wire as the counter electrode, and a nonaqueous Ag/Ag+ electrode as the reference electrode. The potential scale was calibrated against the Fc/Fc+ couple and reported versus this reference system.

3 RESULTS AND DISCUSSION

3. 1 Synthesis and characterization

The title complex can be prepared by carbonyl substitu- tion of the parent complex [Fe2(CO)6(μ-SCH2CH2S)] with 1,2-bis(diphenylphosphino)benzene and Me3NO·2H2O in CH2Cl2/MeCN solution in 85% yield. The title complex is an air-stable red solid and soluble in CH2Cl2, which has been characterized by elemental analysis and spectroscopy. The IR spectrum shows three absorption bands in the region of 2023～1943 cm-1, which is characteristic of the stretching vibrations of terminal carbonyls C≡O(shè), shifting to lower frequencies as compared to those of the parent complex (2079, 2039, 1996 cm-1)[19]due to the diphosphine ligand having stronger electron-donating than CO[24]. In addition, theνC≡O(shè)values are red-shifted relative to those of monophosphine-substituted analogues[25-27], but similar to the previously-reported diphosphine-chelated analogues[17,18]. The1H NMR spectrum shows two doublets atδ1.90 and 1.62 ppm for the methylene protons of SCH2group. The31P{1H} NMR spectrum shows a single resonance atδ91.08 ppm, in good agreement with diphosphine-chelated analogues[17,18], but notably larger than that of the free ligand due to the different coordination environment. It is interesting to find out that the13C{1H} NMR spectrum shows a triplet atδ216.35 ppm for the one terminal carbonyl of P2FeCO because of the coupling between the two phosphorus atoms of 1,2-bis(diphenylphosphino)benzene and the carbon atom of CO and a singlet atδ212.35 ppm for the three terminal carbonyls of Fe(CO)3.

3. 2 X-ray crystal structure

High-quality crystals of the title complex were obtained by recrystallization from slow evaporation of CH2Cl2/hexane solution and analyzed by X-ray diffraction analysis. The ORTEP view is shown in Fig. 1 and the selected bond lengths and angles are presented in Table 1. The hydrogen bond parameters are shown in Table 2. The title complex crystallizes in monoclinic space groupP21/cwith four molecules in the unit cell and one molecule in the asymmetric unit. As shown in Fig. 1, the molecular structure of the title complex consists of a butterfly diiron ethane-1,2-dithiolate cluster with four terminal carbonyls and a chelating diphosphine ligand 1,2-bis(diphenylphos- phino)benzene. The diphosphine ligand occupies an apical- basal position of the distorted square pyramidal geometry of the Fe(2) atom, consistent with 1,2-bis(diphenylphos- phino)ethane-chelated diiron propane-1,3-dithiolate complex [Fe2(CO)4{κ2-(Ph2PCH2CH2PPh2)}(μ-SCH2CH2CH2S)][28]and PNP-chelated diiron ethane-1,2-dithiolate analogues[18], but different from PNP-chelated diiron propane-1,3-dithio- late analogues[15,17]probably due to the steric repulsion. The Fe(1)-Fe(2) bond length (2.5364(13) ?) is longer than that of the parent complex (2.505(2) ?)[29]due to the diphosphine ligand having stronger electron-donating than CO, but shorter than those in natural [FeFe]-hydrogenases (2.55～2.62 ?)[5,6]. In addition, the Fe(1)-Fe(2) bond length is slightly longer than those of the monosubstituted deriva- tives[30], but a little shorter than that of complex [Fe2(CO)4{κ2-(Ph2PCH2CH2PPh2)}(μ-SCH2CH2CH2S)] (2.547(7) ?)[28].

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Table 2. Hydrogen Bonds for the Title Complex (? and °)

Fig. 1. ORTEP view of the title complex with 30% probability level ellipsoids

Intramolecular hydrogen bond C(18)-H(18)···S(1) between the benzene and sulfur with the distance of 2.90 ? is observed in the crystal packing.

3. 3 Electrochemistry

The electrochemical properties of the title complex were studied by CV in MeCN solution. The title complex shows one quasi-reversible reduction peak at -2.19 V, which can be assigned to the reduction of FeIFeIto FeIFe0[31], shifting negatively by 0.49 V as compared to the first reduction peak of the parent complex (-1.70 V) due to the diphosphine ligand 1,2-bis(diphenylphosphino)benzene having stronger electron-donating than CO. In addition, the title complex has one irreversible oxidation peak at +0.08 V, which can be assigned to the oxidation of FeIFeIto FeIFeII[31], shifting negatively by 0.8 V relative to the oxidation peak of the parent complex. It is obvious that the current intensities of the reduction and oxidation peaks will grow while the concentration of the title complex increases. In addition, the good linearity of the current intensities to the scan rates, 100～500 mV·S-1, suggests that the reduction and oxidation processes are diffusion control. In order to examine the title complex as electrocatalyst for H2production, a weak acid HOAc was added and the system was investigated by CV. As shown in Fig. 2, upon addition of HOAc, the reduction peak at -2.19 V is increased remarkably with sequential adding of HOAc. The sharp increase in the current intensity suggests that the title complex can catalyze the reduction of protons to H2in the presence of HOAc[31,32]. According to afore- mentioned observations as well as the previously-reported similar cases[31,33], we might propose the ECCE (E = electrochemical, C = chemical) mechanism for H2evolution catalyzed by the title complex in the presence of HOAc. The title complex is reduced at -2.19 V to form a monoanion species [FeIFe0]-. Due to the chelating of the diphosphine ligand, the monoanion species [FeIFe0]-can be protonated by HOAc to give the species [FeIFeIIH]. Then, species [FeIFeIIH] accepts another proton to give [FeIFeIIH2]+. Finally, species [FeIFeIIH2]+is reduced at ca. -2.30 V to produce H2and regenerates [FeIFeI] to thereby complete the catalytic cycle.

Fig. 2. Cyclic voltammogram of the title complex (1.0 mM) with HOAc (0～10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV·s-1

4 CONCLUSION

In summary, a novel diiron ethane-1,2-dithiolate complex with chelating 1,2-bis(diphenylphosphino)benzene was prepared and characterized by spectroscopy as well as by single-crystal X-ray diffraction analysis. CV studies showed that the title complex can electrocatalyze the reduction of protons to H2in the presence of HOAc. Moreover, a possible mechanism for the H2production was proposed. However, more evidence is still needed for an accurate mechanism. Further studies on the catalytic mechanism are in progress in our laboratory.