English

• Currently, great efforts have been paid to produce H₂ with noble-metal-free catalysts^[1]. In nature, [FeFe]-hydrogenases can efficiently catalyze the reversible reduction of proton to H₂ in some microorganisms^[2-5]. The X-ray structure of the catalytic center of the active site of [FeFe]-hydrogenases was found in 1998^[6] and 1999^[7], which is featuring a diiron subcluster coordinated by a bridging azadithiolato ligand^[8], carbonyls, cyanides, as well as a [Fe₄S₄]-linking cysteinyl ligand. While the diiron center is responsible for catalysis, the [Fe₄S₄] cluster is responsible for proton relay. Since the elucidation of the active site of [FeFe]-hydrogenases, a great number of diiron analogues have been synthesized, for example, diiron propane-1, 3-dithiolato complex [Fe₂(CO)₆(μ-SCH₂CH₂CH₂S)]^[9], diiron azadithiolato complex [Fe₂(CO)₆(μ-SCH₂NRCH₂S)]^[10] as the mimics for the bridging three-atom dithiolate. Furthermore, diiron ethane-1, 2-dithiolato complexes^[11] were synthesized due to their close relationship with the diiron propane-1, 3-dithiolato complex. Notably, cyanide^[12] or phosphine^[13] containing analogues were also synthesized by CO exchange reactions as mimics for the ligands found in the active site of [FeFe]-hydrogenases. It is noteworthy that the phosphine ligands, as well as the phosphine-containing analogues, are more stable than the cyanides and the cyanide-containing analogues. Moreover, recent studies have focused on the proton reduction electro-catalyzed by the diiron analogues to develop highly efficient catalysts^[14-18].

  Although a great number of [FeFe]-hydrogenases biomimics have been reported, the formate ester-containing derivative is still scarce in the literature. To further develop new diiron analogues as potential electrocatalysts for the production of H₂, we carried out this research on the synthesis of formate and phosphine-containing derivatives. In our earlier study, we reported a series of phosphine-containing analogues as the catalysts for H₂ production under electrochemical conditions^[19-20]. As a continuation, we have successfully prepared four diiron analogues by the CO exchange reactions. The structures of these complexes have been fully characterized including single-crystal X-ray diffraction analysis. More significantly, the electrochemical and electrocatalytic properties of these complexes were also studied and discussed. Herein, this paper presents the synthesis, characterization, X-ray crystallography, and electrochemistry of four diiron analogues bearing formate ester and phosphine ligands as the biomimics for the active site of [FeFe]-hydrogenases.

  1.   Experimental

  1.1   Materials and methods

  Triphenylphosphine, tricyclohexylphosphine, tris(2-methoxyphenyl)phosphine, tris(4-(trifluoromethyl)phenyl)phosphine, and Me₃NO∙2H₂O were commercially available and used as received. Complex [Fe₂(CO)₆ (μ-SCH₂CH(CH₂OOCH)S)] (1) was prepared according to the literature method^[21]. IR spectra were recorded on a Nicolet MAGNA 560 FTIR spectrometer. NMR spectra were obtained on a Bruker Avance 500 MHz spectrometer. Elemental analyses were performed by an Elementar Vario EL cube analyzer.

  1.2   Synthesis of complexes 2-5

  To a solution of complex 1 (0.043 g, 0.1 mmol) and the corresponding phosphine ligands (0.1 mmol) in CH₂Cl₂ (5 mL) was added a solution of Me₃NO·2H₂O (0.011 g, 0.1 mmol) in MeCN (5 mL). The mixture was stirred at room temperature for 1 h and the solvent was reduced on a rotary evaporator. The residue was subjected to TLC separation to collect the main red band. Complexes [Fe₂(CO)₅(L)(μ-SCH₂CH(CH₂OOCH)S)](L=PPh₃ (2), PCy₃ (3), P(2-C₆H₄OCH₃)₃ (4), P(4-C₆H₄CF₃)₃ (5)) were obtained as red solids.

  Complex 2. Yield: 80%. IR (KBr disk, cm^-1): ν_C≡O 2 046 (vs), 1 987 (vs), 1 976 (s), 1 961 (sh), 1 934 (m); ν_C=O 1 728 (m). ¹H NMR (500 MHz, CDCl₃): δ 7.94 (s, 1H, HCOO), 7.61 (s, 6H, PhH), 7.47 (s, 9H, PhH), 3.97-3.84 (m, 2H, OCH₂), 1.80 (s, 1H, SCH), 1.39-1.33 (m, 2H, SCH₂). ³¹P{¹H} NMR (200 MHz, CDCl₃, 85% H₃PO₄): δ 62.72 (s). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 214.54 (d, J_P-C=8.9 Hz, PFeC≡O), 214.38 (d, J_P-C=8.5 Hz, PFeC≡O), 209.91 (FeC≡O), 160.04 (C=O), 135.52 (d, J_P-C=39.4 Hz, i-PhC), 133.14 (d, J_P-C=11.2 Hz, o-PhC), 130.37 (s, p-PhC), 128.69 (d, J_P-C=9.5 Hz, m-PhC), 65.39 (OCH₂), 49.59 (SCH), 37.37 (SCH₂). Anal. Calcd. for C₂₇H₂₁Fe₂O₇PS₂(%): C, 48.82; H, 3.19. Found(%): C, 48.86; H, 3.61.

  Complex 3. Yield: 59%. IR (KBr disk, cm^-1): ν_C≡O 2 042 (vs), 1 970 (vs), 1 956 (sh), 1 925 (s); ν_C=O 1 731 (m). ¹H NMR (500 MHz, CDCl₃): δ 8.07 (s, 1H, HCOO), 4.14 (s, 2H, OCH₂), 2.69-2.55 (m, 2H, CyH), 2.12 (s, 3H, CyH), 1.98 (s, 3H, CyH), 1.92-1.70 (m, 12H, CyH, and SCH), 1.58-1.56 (m, 4H, CyH and SCH₂), 1.47-1.44 (m, 3H, CyH), 1.31 (s, 9H, CyH). ³¹P{¹H} NMR (200 MHz, CDCl₃, 85% H₃PO₄): δ 70.58 (s). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 216.58 (d, J_P-C=11.2 Hz, PFeC≡O), 216.49 (d, J_P-C=11.5 Hz, PFeC≡O), 210.40 (FeC≡O), 160.33 (C=O), 65.82 (OCH₂), 50.79 (SCH), 39.30 (SCH₂), 38.32, 38.18, 30.58, 29.86, 27.92, 27.88, 27.84, 27.80, 26.40 (CyC). Anal. Calcd. for C₂₇H₃₉Fe₂O₇PS₂(%): C, 47.52; H, 5.76. Found(%): C, 47.28; H, 5.62.

  Complex 4. Yield: 88%. IR (KBr disk, cm^-1): ν_C≡O 2 042 (vs), 1 983 (vs), 1 973 (vs), 1 958 (sh), 1 932 (sh); ν_C=O 1 722 (m). ¹H NMR (500 MHz, CDCl₃): δ 7.91 (s, 1H, HCOO), 7.43 (s, 4H, PhH), 6.97, 6.89 (2s, 8H, PhH), 3.92, 3.76 (2s, 2H, OCH₂), 3.57 (s, 9H, 3OCH₃), 1.56 (s, 2H, SCH₂), 1.17 (m, 1H, SCH). ³¹P{¹H} NMR (200 MHz, CDCl₃, 85% H₃PO₄): δ 49.25 (s). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 215.72 (d, J_P-C=9.1 Hz, PFeC≡O), 215.53 (d, J_P-C=10.5 Hz, PFeC≡O), 210.61 (FeC≡O), 160.21 (C=O), 135.40, 131.86, 121.22, 119.79, 119.74, 111.13 (PhC), 65.68 (OCH₂), 54.70 (OCH₃), 49.16 (SCH), 37.38 (SCH₂). Anal. Calcd. for C₃₀H₂₇Fe₂O₁₀PS₂(%): C, 47.77; H, 3.61. Found(%): C, 47.46; H, 2.96.

  Complex 5. Yield: 72%. IR (KBr disk, cm^-1): ν_C≡O 2 056 (vs), 1 992 (vs), 1 944 (s); ν_C=O 1 722 (m). ¹H NMR (500 MHz, CDCl₃): δ 7.91 (s, 1H, HCOO), 7.76 (s, 12H, PhH), 3.94, 3.86 (2s, 2H, OCH₂), 1.78 (s, 1H, SCH), 1.28 (s, 2H, SCH₂). ³¹P{¹H} NMR (200 MHz, CDCl₃, 85% H₃PO₄): δ 64.85 (s). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 213.58, 209.06 (C≡O), 159.87 (C=O), 139.03, 138.75, 133.41, 133.32, 125.98 (PhC), 123.38 (d, J_F-C=270.0 Hz, CF₃), 65.17 (OCH₂), 49.93 (SCH), 37.49 (SCH₂). Anal. Calcd. for C₃₀H₁₈F₉Fe₂O₇PS₂(%): C, 41.50; H, 2.09. Found(%): C, 41.48; H, 2.12.

  1.3   X-ray crystal structure determination

  Single crystals were mounted on a Bruker D8 QUEST diffractometer. Data were collected at 296 K by using a graphite-monochromatic with Mo Kα radiation (λ=0.071 073 nm) in the ω-φ scan mode. Data collection and reduction were completed by APEX2. Absorption correction was performed by the SADABS program. Using OLEX2, the structures were solved by direct methods using the SHELXS program and refined by full-matrix least-squares techniques SHELXL on F ². Hydrogen atoms were located using the geometric method. Non-hydrogen atoms were refined with anisotropic thermal parameters. A disordered CH₂Cl₂ over two sites with an occupancy of 0.641 could be found in the structure of complex 4·CH₂Cl₂. Details of crystal data, data collections, and structure refinements are summarized in Table 1.

  Table 1

  

  Table 1.  Crystal data and structure refinements details for complexes 2-5

  下载: 导出CSV

  Parameter	2	3	4·CH2Cl2	5
  Empirical formula	C27H21Fe2O7PS2	C27H39Fe2O7PS2	C31H29Cl2Fe2O10PS2	C30H18F9Fe2O7PS2
  Formula weight	664.23	682.37	839.23	868.23
  Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic
  Space group	P21/c	P21/c	P1	P1
  a / nm	0.910 23(19)	1.004 12(7)	0.903 91(6)	0.932 4(3)
  b / nm	3.009 5(6)	1.361 45(10)	1.296 03(8)	1.132 4(3)
  c / nm	1.097 8(3)	2.342 62(17)	1.719 77(11)	1.694 0(5)
  α / (°)			111.420(3)	73.151(7)
  β / (°)	104.691(7)	102.167(2)	95.611(3)	78.328(8)
  γ / (°)			103.086(3)	84.259(8)
  V / nm3	2.909 0(11)	3.130 6(4)	1.790 3(2)	1.674 8(8)
  Z	4	4	2	2
  Dc / (g·cm-3)	1.517	1.448	1.557	1.722
  μ / mm-1	1.238	1.152	1.174	1.135
  F(000)	1 352.0	1 424.0	856.0	868.0
  Crystal size / mm	0.34×0.32×0.3	0.32×0.24×0.18	0.32×0.28×0.26	0.18×0.16×0.14
  2θ range / (°)	4.82-50.048	4.864-50.186	5.028-55.112	4.724-50.218
  Indices range	-10 ≤ h ≤ 10, -35 ≤ k ≤ 35, -13 ≤ l ≤ 13	-11 ≤ h ≤ 11, -15 ≤ k ≤ 16, -27 ≤ l ≤ 27	-11 ≤ h ≤ 11, -16 ≤ k ≤ 16, -22 ≤ l ≤ 22	-11 ≤ h ≤ 11, -13 ≤ k ≤ 13, -20 ≤ l ≤ 20
  Reflection collected	57 645	46 723	31 213	32 102
  Independent reflection	5 096 (Rint=0.060 1)	5 559 (Rint=0.064 7)	8 209 (Rint=0.044 2)	5 888 (Rint=0.040 0)
  Data, restraint, parameter	5 096, 32, 352	5 559, 29, 352	8 209, 38, 455	5 888, 0, 460
  Goodness of fit on F 2	1.070	1.049	1.033	1.042
  Final R indices [I > 2σ(I)]	0.088 9, 0.207 9	0.071 0, 0.181 8	0.053 9, 0.138 8	0.051 6, 0.131 0
  Final R indices (all data)	0.100 5, 0.215 3	0.100 5, 0.199 1	0.084 2, 0.156 2	0.068 8, 0.142 1
  Largest diff peak and hole / (e·nm-3)	1 450, -1 193	1 320, -956	920, -802	720, -394

  1.4   Electrochemical experiment

  Electrochemical properties were studied by cyclic voltammetry (CV) in the MeCN solution. Electrochemical measurements were carried out under nitrogen using a CHI 620 Electrochemical workstation. As the electrolyte, ⁿBu₄NPF₆ was recrystallized several times from a CH₂Cl₂ solution by the addition of hexane. CV curves were obtained in a three-electrode cell with a glassy carbon electrode (diameter: 3 mm) as the working electrode, a platinum wire as the counter electrode, and a non-aqueous Ag/Ag⁺ electrode as the reference electrode. The potential scale was calibrated against the Fc/Fc⁺ couple and reported versus this reference system.

  2.   Results and discussion

  2.1   Synthesis and characterization

  As depicted in Scheme 1, CO exchange of the starting complex 1 with the corresponding phosphine ligand assisted by the decarbonylation agent Me₃NO·2H₂O gave the products 2-5 in 59%-88% yields. Complexes 2-5 are air-stable red solids and soluble in medium polar solvents such as CH₂Cl₂ and CHCl₃. We have identified the structures of complexes 2-5 by elemental analysis, IR, and NMR spectroscopy. In the IR spectra, three to five bands ranging from 2 056 to 1 925 cm^-1 were observed for complexes 2-5, which can be designated to the stretching vibrations of carbonyls C≡O, shifting to lower energies relative to all-carbonyl complex [Fe₂(CO)₆{μ-SCH₂CH(CH₂OOCCH₃)S}] (2 077, 2 037, 2 008, 1 990 cm^-1)^[21] due to the better donor of the phosphine ligands^[22]. In addition, the C≡O vibrations of complexes 2-5 were close to other monosubstituted analogues^[23], but smaller than phosphine-chelated analogues^[24]. In the ¹H NMR spectra of complexes 2-5, a single resonance at ca. δ 8.0 is ascribed to the formate ester proton. The ³¹P{¹H} NMR spectra of complexes 2-5 displayed a singlet at δ 70.58-49.25 for the coordinated phosphine ligands, in accord with phosphine-containing analogues^[25-27]. The ¹³C{¹H} NMR spectra of complexes 2-5 showed a single resonance at ca. δ 160 for the ester carbonyl.

  Scheme 1

  

  Scheme 1.  Synthesis of complexes 2-5

  下载: 全尺寸图片 幻灯片

  2.2   Solid-state structure

  To determine the solid-state structures, high-quality single crystals were obtained and analyzed by X-ray diffraction analysis. The molecular structures of complexes 2-5 drawn as thermal ellipsoids and capped sticks are shown in Fig. 1 and the selected bond lengths and angles are given in Table 2. Complexes 2 and 3 crystallize in monoclinic space group P2₁/c whereas complexes 4·CH₂Cl₂ and 5 crystallize in triclinic space group P1. Fig. 1 shows that the molecular structures of complexes 2-5 are composed of a diiron unit with a bridged ethane-1, 2-dithiolate containing a formate ester group, five terminal carbonyls, and a phosphine ligand. The position of the phosphine ligands is apical to the pseudo-octahedral coordination arrangement of the Fe atom, in good agreement with monosubstituted analogues^[28-30]. The Fe1—Fe2 bond lengths of complexes 3 (0.251 09(12) nm) and 4·CH₂Cl₂ (0.251 59(7) nm) are slightly longer than those of complexes 2 (0.249 96(15) nm) and 5 (0.250 93(10) nm), probably due to the better donor of tricyclohexylphosphine and tris(2-methoxyphenyl)phosphine. Moreover, the Fe1—Fe2 bond lengths of complexes 2-5 are much shorter than those in natural [FeFe]-hydrogenases (0.255-0.262 nm)^[6-7]. The orientation of the formate ester group in complexes 2-5 shows some differences due to the effect of the phosphine ligand.

  Figure 1

  

  Figure 1.  Molecular structures of complexes 2 (a), 3 (b), 4‧CH₂Cl₂ (c), and 5 (d) drawn as ORTEP with a 30% probability level and capped sticks

  下载: 全尺寸图片 幻灯片

  Table 2

  

  Table 2.  Selected bond lengths (nm) and angles (°) for complexes 2-5

  下载: 导出CSV

  Bond	2	3	4·CH2Cl2	5
  Fe1—Fe2	0.249 96(15)	0.251 09(12)	0.251 59(7)	0.250 93(10)
  Fe1—S1	0.225 1(3)	0.225 5(2)	0.225 51(11)	0.224 32(13)
  Fe1—S2	0.226 6(3)	0.225 6(2)	0.225 12(12)	0.225 66(14)
  Fe2—S1	0.224 7(2)	0.224 49(19)	0.226 05(10)	0.226 35(13)
  Fe2—S2	0.226 5(2)	0.227 5(2)	0.224 92(10)	0.227 89(15)
  Fe2—P1	0.224 94(18)	0.228 16(16)	0.227 38(11)	0.224 29(12)
  S1—Fe1—Fe2	56.17(6)	55.89(5)	56.24(3)	56.55(3)
  S1—Fe1—S2	79.84(11)	79.50(8)	79.52(4)	80.27(5)
  S2—Fe1—Fe2	56.50(6)	56.71(5)	55.97(3)	56.83(4)
  S1—Fe2—Fe1	56.31(7)	56.27(5)	56.04(3)	55.78(4)
  S2—Fe2—Fe1	56.55(7)	55.99(5)	56.05(3)	55.99(4)
  S2—Fe2—S1	79.95(12)	79.31(8)	79.45(4)	79.37(5)
  P1—Fe2—Fe1	156.16(7)	159.76(6)	155.57(4)	155.87(4)
  Fe1—S1—Fe2	67.52(8)	67.84(6)	67.72(3)	67.67(4)
  Fe2—S2—Fe1	66.95(8)	67.31(6)	67.98(3)	67.18(4)

  2.3   Electrochemistry

  We carried out CV studies in 0.1 mol·L^{-1 n}Bu₄NPF₆ as the supporting electrolyte in MeCN to evaluate the redox activity. The CV curves of complexes 1-5 are shown in Fig. 2 and the electrochemical data are given in Table 3. As shown in Fig. 2 and Table 3, CV curves of complexes 2-5 displayed one irreversible reduction process in a range of -1.71 to -1.93 V as well as one irreversible oxidation process in a range of 0.33 to 0.73 V. According to the electrochemical behavior of complex [Fe₂(CO)₆(μ-SCH₂CH₂CH₂S)] as described by Darensbourg and co-workers^[31], the reduction process can be ascribed to the one-electron reduction from Fe^ⅠFe^Ⅰ to Fe⁰Fe^Ⅰ whereas the oxidation process can be ascribed to the one-electron oxidation from Fe^ⅠFe^Ⅰ to Fe^ⅡFe^Ⅰ. Nevertheless, the CV curve of complex 1 only had one irreversible reduction process at -1.67 V and we assume that the oxidation process is so positive that beyond the solvent window. It is worth pointing out that the reduction peaks for complexes 2-5 moved negatively in comparison with complex 1 in terms of the enhancement of the electron density on the diiron center because the phosphine ligand is a better donor than CO^[22]. The reduction and oxidation process of complex 5 are very close to complex [Fe₂(CO)₆(μ-SCH₂CH₂CH₂S)] (-1.74 and 0.74 V versus Fc/Fc⁺)^[31]. However, the corresponding values of complexes 2-4 shifted cathodically with respect to complex 5 and complex [Fe₂(CO)₆(μ-SCH₂CH₂CH₂S)], suggesting that the electron-withdrawing trifluoromethyl group can reduce the electron density on the diiron center.

  Figure 2

  

  Figure 2.  CV curves of complexes 1 (black line), 2 (red line), 3 (blue line), 4 (pink line), and 5 (green line) in <0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  下载: 全尺寸图片 幻灯片

  Table 3

  

  Table 3.  Electrochemical data for complexes 1-5

  下载: 导出CSV

  Complex	Epc / V	Epa / V	Overpotential / V	TOF* / s-1
  1	-1.67	—	0.74	26.94
  2	-1.84	0.47	0.79	17.86
  3	-1.93	0.33	0.81	5.46
  4	-1.92	0.34	0.72	5.46
  5	-1.74	0.73	0.80	22.47
  * TOF: turnover frequency

  Further studies on the electrocatalytic H₂ evolution were undertaken by adding a weak proton acid HOAc into the MeCN solution. As shown in Fig. 3, upon the addition of HOAc, the current height of the reduction peak for complex 1 at -1.67 V was slightly increased, however, a catalytic wave at -2.19 V can be found and the current height was significantly increased with continuous addition of HOAc, indicative of H₂ evolution catalyzed by complex 1^[31-33]. It should be noted that the catalytic wave shifted positively by 0.31 V as compared to the HOAc without the diiron complex (blank)^[19]. Similarly, as shown in Fig. 4, 6, and 7, complexes 2, 4, and 5 could also act as electrocatalysts for H₂ production in the presence of HOAc. However, we found that upon the addition of HOAc into complex 3, the reduction peak at -1.93 V shifted positively by 40 mV (Fig. 5), in agreement with the case obtained from [Fe₂(CO)₆{(μ-SCH₂)₂N(4-NO₂C₆H₄)}] by Wang and co-workers^[34]. Shoulder peaks around -2.00 V for complexes 1 (Fig. 3), 2 (Fig. 4), and 4 (Fig. 6) could be observed and we assume that this may be attributed to the dissociation of the ester group under acidic conditions.

  Figure 3

  

  Figure 3.  CV curves of complex 1 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片

  Figure 4

  

  Figure 4.  CV curves of complex 2 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片

  Figure 5

  

  Figure 5.  CV curves of complex 3 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片

  Figure 6

  

  Figure 6.  CV curves of complex 4 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片

  Figure 7

  

  Figure 7.  CV curves of complex 5 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片

  To better understand the catalytic activities for proton reduction, overpotential and turnover frequency (TOF) were calculated by the reported methods^[35-36], and the results are also illustrated in Table 3. It can be seen that complex 4 had the lowest overpotential concerning the other four complexes and complex 1 possessed the highest TOF of 26.94 possibly due to the lower electron density on diiron subsite. However, the TOF for complex 4 was the same as complex 3 and they were both much lower than the other two complexes bearing phosphine ligands. Among those complexes with phosphine ligands, complex 5 had the highest TOF, indicating that the diiron complex bearing electron-withdrawing group functionalized phosphine is more favorable for proton reduction than that bearing electron-donating group. Further, the electronic effect of tricyclohexylphosphine is very close to tris(2-methoxyphenyl)phosphine. Additionally, the catalysis of complexes 1-5 was compared by the dependence of peak current of the electrocatalysis versus the addition of HOAc concentration (Fig. 8). It can be viewed that the electrocatalysis of complex 1 was faster than other complexes, in agreement with the TOFs calculation.

  Figure 8

  

  Figure 8.  Plots of the peak current of electrocatalysis versus the concentration of added HOAc for complexes 1-5

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In conclusion, we have detailed the synthesis and characterization of four diiron ethane-1, 2-dithiolato pentacarbonyl complexes bearing phosphine ligands as the mimics for the active site of [FeFe]-hydrogenases. The electrochemical properties of these complexes were studied by CV together with electrocatalytic H₂ production by adding HOAc. The results have shown that these complexes can act as catalysts for the reduction of protons to H₂ under electrochemical conditions. The catalytic activities for electrochemical proton reduction were estimated and the results showed that complex 4 had the lowest overpotential and complex 1 had the best catalytic efficiency among these complexes.

  
  
• 1. [1]

     Han Z J, Qiu F, Eisenberg R, Holland P L, Krauss T D. Robust photogeneration of H₂ in water using semiconductor nanocrystals and a nickel catalyst[J]. Science, 2012, 338:  1321-1324. doi: 10.1126/science.1227775

  2. [2]

     Rauchfuss T B. Diiron azadithiolates as models for the [FeFe]-hydrogenase active site and paradigm for the role of the second coordination sphere[J]. Acc. Chem. Res., 2015, 48:  2107-2116. doi: 10.1021/acs.accounts.5b00177

  3. [3]

     Tard C, Pickett C J. Structural and functional analogues of the active sites of the [Fe]-, [NiFe]-, and [FeFe]-hydrogenases[J]. Chem. Rev., 2009, 109:  2245-2274. doi: 10.1021/cr800542q

  4. [4]

     Li Y L, Rauchfuss T B. Synthesis of diiron? dithiolato carbonyl complexes[J]. Chem. Rev., 2014, 116:  7043-7077.

  5. [5]

     Schilter D, Camara J M, Huynh M T, Hammes-Schiffer S, Rauchfuss T B. Hydrogenase enzymes and their synthetic models: The role of metal hydrides[J]. Chem. Rev., 2016, 116:  8693-9749. doi: 10.1021/acs.chemrev.6b00180

  6. [6]

     Peters J W, Lanzilotta W N, Lemon B J, Seefeldt L C. X-ray crystal structure of the Fe-only hydrogenase (CpI) from Clostridium Pasteurianum to 1.8 angstrom resolution[J]. Science, 1998, 282:  1853-1858. doi: 10.1126/science.282.5395.1853

  7. [7]

     Nicolet Y, Piras C, Legrand P, Hatchikian C E, Fontecilla-Camps J C. Desulfovibrio Desulfuricans iron hydrogenase: The structure shows unusual coordination to an active site Fe binuclear center[J]. Structure, 1999, 7:  13-23. doi: 10.1016/S0969-2126(99)80005-7

  8. [8]

     Fan H J, Hall M B. A capable bridging ligand for Fe-only hydrogenase: Density functional calculations of a low-energy route for heterolytic cleavage and formation of dihydrogen[J]. J. Am. Chem. Soc., 2001, 123:  3828-3829. doi: 10.1021/ja004120i

  9. [9]

     Lyon E J, Georgakaki I P, Reibenspies J H, Darensbourg M Y. Carbon monoxide and cyanide ligands in a classical organometallic complex model for Fe-only hydrogenase[J]. Angew. Chem. Int. Ed., 1999, 38:  3178-3180. doi: 10.1002/(SICI)1521-3773(19991102)38:21<3178::AID-ANIE3178>3.0.CO;2-4

  10. [10]

      Li H X, Rauchfuss T B. Iron carbonyl sulfides, formaldehyde, and amines condense to give the proposed azadithiolate cofactor of the Fe-only hydrogenases[J]. J. Am. Chem. Soc., 2002, 124:  726-727. doi: 10.1021/ja016964n

  11. [11]

      Liu X F, Jiang Z Q, Jia Z J. Synthesis, characterization and crystal structures of tetrairon ethanedithiolate complexes containing bridging bidentate phosphine ligands[J]. Polyhedron, 2012, 33:  166-170. doi: 10.1016/j.poly.2011.11.032

  12. [12]

      Lawrence J D, Li H X, Rauchfuss T B, Bénard M, Rohmer M M. Diiron azadithiolates as models for the iron-only hydrogenase active site: Synthesis, structure, and stereoelectronics[J]. Angew. Chem. Int. Ed., 2001, 40:  1768-1771. doi: 10.1002/1521-3773(20010504)40:9<1768::AID-ANIE17680>3.0.CO;2-E

  13. [13]

      Mejia-Rodriguez R, Chong D, Reibenspies J H, Soriaga M P, Darensbourg M Y. The hydrophilic phosphatriazaadamantane ligand in the development of H₂ production electrocatalysts: iron hydrogenase model complexes[J]. J. Am. Chem. Soc., 2004, 126:  12004-12014. doi: 10.1021/ja039394v

  14. [14]

      Li Z M, Xiao Z Y, Xu F F, Zeng X H, Liu X M. Enhancement in catalytic proton reduction by an internal base in a diiron pentacarbonyl complex: Its synthesis, characterization, inter-conversion and electrochemical investigation[J]. Dalton Trans., 2017, 46:  1864-1871. doi: 10.1039/C6DT04409C

  15. [15]

      Zhong W, Wu L, Jiang W D, Li Y L, Mookan N, Liu X M. Proton-coupled electron transfer in the reduction of diiron hexacarbonyl complexes and its enhancement on the electrocatalytic reduction of protons by a pendant basic group[J]. Dalton Trans., 2019, 48:  13711-13718. doi: 10.1039/C9DT02058F

  16. [16]

      Xiao Z Y, Zhong W, Liu X M. Recent developments in electrochemical investigations into iron carbonyl complexes relevant to the iron centres of hydrogenases[J]. Dalton Trans., 2022, 51:  40-47. doi: 10.1039/D1DT02705K

  17. [17]

      Orton G R F, Ringenberg M R, Hogarth G. Biomimics of [FeFe]-hydrogenases incorporating redox-active ligands: Ferrocene-bridged dithiolate complexes [Fe₂(CO)₆(μ-EC₅H₄FeC₅H₄E)] (E=S, Se)[J]. J. Organomet. Chem., 2022, 978:  122472. doi: 10.1016/j.jorganchem.2022.122472

  18. [18]

      Orton G R F, Ghosh S, Alker L, Sarker J C, Pugh D, Richmond M G, Hartl F, Hogarth G. Biomimics of [FeFe]-hydrogenases incorporating redox-active ligands: Synthesis, redox properties and spectroelectrochemistry of diiron-dithiolate complexes with ferrocenyl-diphosphines as Fe₄S₄ surrogates[J]. Dalton Trans., 2022, 51:  9748-9769. doi: 10.1039/D2DT00419D

  19. [19]

      Yan L, Yang J, Lü S, Liu X F, Li Y L, Liu X H, Jiang Z Q. Phosphine-containing diiron propane-1, 2-dithiolate derivatives: Synthesis, spectroscopy, X-ray crystal structures, and electrochemistry[J]. Catal. Lett., 2021, 151:  1857-1867. doi: 10.1007/s10562-020-03450-2

  20. [20]

      Liu X F, Ma Z Y, Jin B, Wang D, Zhao P H. Substituent effects of tertiary phosphines on the structures and electrochemical performances of azadithiolato-bridged diiron model complexes of [FeFe]-hydrogenases[J]. Appl. Organomet. Chem., 2022, 36:  e6751.

  21. [21]

      Lian M, He J, Yu X Y, Mu C, Liu X F, Li Y L, Jiang Z Q. Diiron ethanedithiolate complexes with acetate ester: Synthesis, characterization and electrochemical properties[J]. J. Organomet. Chem., 2018, 870:  90-96. doi: 10.1016/j.jorganchem.2018.06.023

  22. [22]

      Li P, Wang M, He C J, Li G H, Liu X Y, Chen C N, Å kermark B, Sun L C. Influence of tertiary phosphanes on the coordination configurations and electrochemical properties of iron hydrogenase model complexes: Crystal structures of [(μ-S₂C₃H₆)Fe₂(CO)_6-nL_n] (L=PMe₂Ph, n=1, 2; PPh₃, P(OEt)₃, n=1)[J]. Eur. J. Inorg. Chem., 2005, :  2506-2513.

  23. [23]

      Yan L, Hu K, Liu X F, Li Y L, Liu X H, Jiang Z Q. Diiron ethane-1, 2-dithiolate complexes with 1, 2, 3-thiadiazole moiety: Synthesis, X-ray crystal structures, electrochemistry and fungicidal activity[J]. Appl. Organomet. Chem., 2021, 35:  e6084.

  24. [24]

      Zhao P H, Hu M Y, Li J R, Ma Z Y, Wang Y Z, He J, Li Y L, Liu X F. Influence of dithiolate bridges on the structures and electrocatalytic performance of small bite-angle PNP-chelated diiron complexes Fe₂(μ-xdt)(CO)₄{κ²-(Ph₂P)₂NR} related to [FeFe]-hydrogenases[J]. Organometallics, 2019, 38:  385-394. doi: 10.1021/acs.organomet.8b00759

  25. [25]

      Gao W M, Ekström J, Liu J H, Chen C N, Eriksson L, Weng L H, Å kermark B, Sun L C. Binuclear iron-sulfur complexes with bidentate phosphine ligands as active site models of Fe-hydrogenase and their catalytic proton reduction[J]. Inorg. Chem., 2007, 46:  1981-1991. doi: 10.1021/ic0610278

  26. [26]

      刘旭锋, 徐博, 徐航, 李玉龙. 膦配体取代的乙基乙撑双铁配合物的合成、晶体结构及电化学催化性能[J]. 无机化学学报, 2022,38,(12): 2521-2529. LIU X F, XU B, XU H, LI Y L. Diiron butane-1, 2-dithiolate complexes with phosphine ligands: Preparation, crystal structures, and electrochemical catalytic performance[J]. Chinese J. Inorg. Chem., 2022, 38(12):  2521-2529.

  27. [27]

      Jin B, Tan X, Zhang X X, Wang Z Y, Qu Y P, He Y B, Hu T P, Zhao P H. Substituent effects in carbon-nanotube-supported diiron monophosphine complexes for hydrogen evolution reaction[J]. Electrochim. Acta, 2022, 434:  141325. doi: 10.1016/j.electacta.2022.141325

  28. [28]

      Chen F Y, He J, Yu X Y, Wang Z, Mu C, Liu X F, Li Y L, Jiang Z Q, Wu H K. Electrocatalytic properties of diiron ethanedithiolate complexes containing benzoate ester[J]. Appl. Organomet. Chem., 2018, 32:  e4549. doi: 10.1002/aoc.4549

  29. [29]

      Lin H M, Li J R, Mu C, Li A, Liu X F, Zhao P H, Li Y L, Jiang Z Q, Wu H K. Synthesis, characterization, and electrochemistry of monophosphine‐containing diiron propane‐1, 2‐dithiolate complexes related to the active site of [FeFe]‐hydrogenases[J]. Appl. Organomet. Chem., 2019, 33:  e5196.

  30. [30]

      Hu M Y, Zhao P H, Li J R, Gu X L, Jing X B, Liu X F. Synthesis, structures, and electrocatalytic properties of phosphine-monodentate, -chelate, and -bridge diiron 2, 2-dimethylpropanedithiolate complexes related to [FeFe]-hydrogenases[J]. Appl. Organomet. Chem., 2020, 34:  e5523.

  31. [31]

      Chong D, Georgakaki I P, Mejia-Rodriguez R, Sanabria-Chinchilla J, Soriaga M P, Darensbourg M Y. Electrocatalysis of hydrogen production by active site analogues of the iron hydrogenase enzyme: structure/function relationships[J]. Dalton Trans., 2003, :  4158-4163.

  32. [32]

      Gloaguen F, Lawrence J D, Rauchfuss T B. Biomimetic hydrogen evolution catalyzed by an iron carbonyl thiolate[J]. J. Am. Chem. Soc., 2001, 123:  9476-9477. doi: 10.1021/ja016516f

  33. [33]

      Vannucci A K, Wang S, Nichol G S, Lichtenberger D L, Evans D H, Glass R S. Electronic and geometric effects of phosphatriazaadamantane ligands on the catalytic activity of a [FeFe] hydrogenase inspired complex[J]. Dalton Trans., 2010, 39:  3050-3056. doi: 10.1039/B921067A

  34. [34]

      Liu T B, Wang M, Shi Z, Cui H G, Dong W B, Chen J S, Å kermark B, Sun L C. Synthesis, structures and electrochemical properties of nitro- and amino-functionalized diiron azadithiolates as active site models of Fe-only hydrogenases[J]. Chem.-Eur. J., 2004, 10:  4474-4479. doi: 10.1002/chem.200400004

  35. [35]

      Fourmond V, Jacques P A, Fontecave M, Artero M. H₂ Evolution and molecular electrocatalysts: Determination of overpotentials and effect of homoconjugation[J]. Inorg. Chem., 2010, 49:  10338-10347. doi: 10.1021/ic101187v

  36. [36]

      Tatematsu R, Inomata T, Ozawa T, Masuda H. Electrocatalytic hydrogen production by a Nickel(Ⅱ) complex with a phosphinopyridyl ligand[J]. Angew. Chem. Int. Ed., 2016, 55:  5247-5250. doi: 10.1002/anie.201511621

• 

  Scheme 1  Synthesis of complexes 2-5

  下载: 全尺寸图片 幻灯片
  

  Figure 1  Molecular structures of complexes 2 (a), 3 (b), 4‧CH₂Cl₂ (c), and 5 (d) drawn as ORTEP with a 30% probability level and capped sticks

  下载: 全尺寸图片 幻灯片
  

  Figure 2  CV curves of complexes 1 (black line), 2 (red line), 3 (blue line), 4 (pink line), and 5 (green line) in <0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  下载: 全尺寸图片 幻灯片
  

  Figure 3  CV curves of complex 1 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  CV curves of complex 2 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  CV curves of complex 3 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片
  

  Figure 6  CV curves of complex 4 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片
  

  Figure 7  CV curves of complex 5 with HOAc in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1

  c_complex=1.0 mmol·L^-1.

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Plots of the peak current of electrocatalysis versus the concentration of added HOAc for complexes 1-5

  下载: 全尺寸图片 幻灯片

  Table 1.  Crystal data and structure refinements details for complexes 2-5

  Parameter	2	3	4·CH2Cl2	5
  Empirical formula	C27H21Fe2O7PS2	C27H39Fe2O7PS2	C31H29Cl2Fe2O10PS2	C30H18F9Fe2O7PS2
  Formula weight	664.23	682.37	839.23	868.23
  Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic
  Space group	P21/c	P21/c	P1	P1
  a / nm	0.910 23(19)	1.004 12(7)	0.903 91(6)	0.932 4(3)
  b / nm	3.009 5(6)	1.361 45(10)	1.296 03(8)	1.132 4(3)
  c / nm	1.097 8(3)	2.342 62(17)	1.719 77(11)	1.694 0(5)
  α / (°)			111.420(3)	73.151(7)
  β / (°)	104.691(7)	102.167(2)	95.611(3)	78.328(8)
  γ / (°)			103.086(3)	84.259(8)
  V / nm3	2.909 0(11)	3.130 6(4)	1.790 3(2)	1.674 8(8)
  Z	4	4	2	2
  Dc / (g·cm-3)	1.517	1.448	1.557	1.722
  μ / mm-1	1.238	1.152	1.174	1.135
  F(000)	1 352.0	1 424.0	856.0	868.0
  Crystal size / mm	0.34×0.32×0.3	0.32×0.24×0.18	0.32×0.28×0.26	0.18×0.16×0.14
  2θ range / (°)	4.82-50.048	4.864-50.186	5.028-55.112	4.724-50.218
  Indices range	-10 ≤ h ≤ 10, -35 ≤ k ≤ 35, -13 ≤ l ≤ 13	-11 ≤ h ≤ 11, -15 ≤ k ≤ 16, -27 ≤ l ≤ 27	-11 ≤ h ≤ 11, -16 ≤ k ≤ 16, -22 ≤ l ≤ 22	-11 ≤ h ≤ 11, -13 ≤ k ≤ 13, -20 ≤ l ≤ 20
  Reflection collected	57 645	46 723	31 213	32 102
  Independent reflection	5 096 (Rint=0.060 1)	5 559 (Rint=0.064 7)	8 209 (Rint=0.044 2)	5 888 (Rint=0.040 0)
  Data, restraint, parameter	5 096, 32, 352	5 559, 29, 352	8 209, 38, 455	5 888, 0, 460
  Goodness of fit on F 2	1.070	1.049	1.033	1.042
  Final R indices [I > 2σ(I)]	0.088 9, 0.207 9	0.071 0, 0.181 8	0.053 9, 0.138 8	0.051 6, 0.131 0
  Final R indices (all data)	0.100 5, 0.215 3	0.100 5, 0.199 1	0.084 2, 0.156 2	0.068 8, 0.142 1
  Largest diff peak and hole / (e·nm-3)	1 450, -1 193	1 320, -956	920, -802	720, -394

  下载: 导出CSV

  Table 2.  Selected bond lengths (nm) and angles (°) for complexes 2-5

  Bond	2	3	4·CH2Cl2	5
  Fe1—Fe2	0.249 96(15)	0.251 09(12)	0.251 59(7)	0.250 93(10)
  Fe1—S1	0.225 1(3)	0.225 5(2)	0.225 51(11)	0.224 32(13)
  Fe1—S2	0.226 6(3)	0.225 6(2)	0.225 12(12)	0.225 66(14)
  Fe2—S1	0.224 7(2)	0.224 49(19)	0.226 05(10)	0.226 35(13)
  Fe2—S2	0.226 5(2)	0.227 5(2)	0.224 92(10)	0.227 89(15)
  Fe2—P1	0.224 94(18)	0.228 16(16)	0.227 38(11)	0.224 29(12)
  S1—Fe1—Fe2	56.17(6)	55.89(5)	56.24(3)	56.55(3)
  S1—Fe1—S2	79.84(11)	79.50(8)	79.52(4)	80.27(5)
  S2—Fe1—Fe2	56.50(6)	56.71(5)	55.97(3)	56.83(4)
  S1—Fe2—Fe1	56.31(7)	56.27(5)	56.04(3)	55.78(4)
  S2—Fe2—Fe1	56.55(7)	55.99(5)	56.05(3)	55.99(4)
  S2—Fe2—S1	79.95(12)	79.31(8)	79.45(4)	79.37(5)
  P1—Fe2—Fe1	156.16(7)	159.76(6)	155.57(4)	155.87(4)
  Fe1—S1—Fe2	67.52(8)	67.84(6)	67.72(3)	67.67(4)
  Fe2—S2—Fe1	66.95(8)	67.31(6)	67.98(3)	67.18(4)

  下载: 导出CSV

  Table 3.  Electrochemical data for complexes 1-5

  Complex	Epc / V	Epa / V	Overpotential / V	TOF* / s-1
  1	-1.67	—	0.74	26.94
  2	-1.84	0.47	0.79	17.86
  3	-1.93	0.33	0.81	5.46
  4	-1.92	0.34	0.72	5.46
  5	-1.74	0.73	0.80	22.47
  * TOF: turnover frequency

  下载: 导出CSV
•