English

• 0.   Introduction

  With increasing global attention to environmental protection, traditional fossil fuels have been difficult to meet the needs of sustainable development due to their inherent defects, such as high pollution intensity, significant damage to the atmosphere, and non- renewability^[1]. Therefore, the development of environmentally friendly and renewable energy has become the mainstream research direction in the energy field. As a highly promising renewable energy source, hydrogen has been regarded as an important alternative to traditional fossil fuels^[2]. Traditional hydrogen production methods, such as water electrolysis and petroleum pyrolysis, are energy-consuming and environmentally harmful. By contrast, photolysis of water using abundant solar energy offers a potential solution for producing hydrogen^[3-6]. The development of cost-effective photocatalysts with high catalytic activity and durability for the hydrogen evolution reaction is crucial for establishing a sustainable hydrogen economy.

  [FeFe]-hydrogenase isolated from anaerobic microorganisms is the most effective proton-reducing enzyme in nature and provides an important biomolecular template for developing green hydrogen production technology^[7-9]. However, the oxygen sensitivity and complex biosynthesis of these enzymes hampered their sustainable application for photocatalytic hydrogen evolution from water^[10].

  In this context, chemists have been working to structurally modify [FeFe]-hydrogenase to reconstruct a series of new model systems aimed at improving the stability of [FeFe]-hydrogenase while maintaining its catalytic capabilities^{[ 11-17]}. In this study, two new compounds, {Fe₂[(SCH₂CH₃)(SR)](CO)₆} (1 or 1′, which are the crystalline states from petroleum ether and dichloromethane solution, respectively) and {Fe₂[(SCH₂CH₃)(SR)](CO)₅(PPh₃)} (2) (R=2-cyanobenzyl) containing [FeFe]-hydrogenase unit were synthesized and characterized (Scheme 1). Their performance as photocatalytic water-splitting catalysts for hydrogen production under simulated sunlight was evaluated.

  Scheme 1

  

  Scheme 1.  Synthetic route of compounds 1 and 2

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  1.   Experimental

  1.1   Materials and instruments

  All reagents, solvents, and compound 2-(bromomethyl)benzonitrile (A) were obtained commercially and used without further purification. The precursor compound [(μ-S)₂Fe₂(CO)₆] (B) was synthesized according to literature procedures^[18]. ¹H NMR and ¹³C NMR spectra were recorded at 400 and 100 MHz, respectively, on a Bruker Avance spectrometer. Crystallographic data were collected using an Agilent SuperNova CCD single-crystal diffractometer. Fourier transform infrared (FTIR) spectra were recorded in a range of 400-4 000 cm^-1 using a Perkin-Elmer 1600 FTIR spectrometer. Ultraviolet-visible (UV-Vis) absorption spectra were measured using a Shimadzu UV-2600 double-beam UV-Vis Spectrophotometer. The powder X-ray diffraction (PXRD) data were collected using a D8 ADVANCE X-ray with Cu Kα radiation (λ=0.154 0 nm) at 40 kV and 40 mA. The data was collected in a range of 2θ=5°-50°. High-resolution mass spectra (HRMS) were recorded on a Thermo Fisher Q Exactive mass spectrometer.

  1.2   Syntheses of compounds 1 and 2

  The synthesis of compounds 1 and 2 is illustrated in Scheme 1.

  1.2.1   Synthesis of compound 1

  Under N₂ atmosphere, B (500 mg, 1.45 mmol) and anhydrous THF (20 mL) were added to a Schlenk flask. Triethylborohydride (2.9 mL, 2.9 mmol) was added dropwise to the reaction system at -78 ℃. The reaction was maintained at -78 ℃ for 1 h, after which A (340 mg, 1.74 mmol) was added. After stirring for an additional hour, the reaction mixture slowly warmed to room temperature. The solvent was then removed, and the resulting red solid was purified by column chromatography [dichloromethane/petroleum ether (PE) as eluent, 1∶20, V/V]. Compound 1 was isolated as a red solid (520 mg, 62% yield). Slow evaporation of the petroleum ether solution of compound 1 at 4 ℃ afforded red single crystals of compound 1. However, crystallization of compound 1 from a dichloromethane solution resulted in a completely different crystalline form, designated 1′. ¹H NMR (400 MHz, CDCl₃): δ 7.60-7.62 (d, J=8.0 Hz, 1H), 7.55 (s, 1H), 7.36 (s, 2H), 3.46 (s, 2H), 2.52-2.54 (d, J=8.0 Hz, 2H), 1.46 (s, 3H). Selected FTIR data (KBr pellets, cm^-1): 2 225 (m), 2 071 (s), 2 028 (vs), 1 993 (vs), 1 487 (m), 617 (m). HRMS (m/z): Calcd. for C₁₆H₁₁Fe₂NO₆S₂Na [M+Na]⁺, 511.861 9; Found, 511.858 5.

  1.2.2   Syntheses of compound 2

  Under N₂ atmosphere, 1 (140 mg, 0.27 mmol) was added to a 100 mL reaction flask. Then, degassed dichloromethane (15 mL) and trimethylamine oxide (Me₃NO, 40 mg, 0.3 mmol) were added, and the mixture was stirred for 20 min. After that, triphenylphosphine (70 mg, 0.27 mmol) was added to the above reaction solution, and the mixture was stirred for another 4 h. After removing the solvent, the crude product was purified by silica gel chromatography (dichloromethane/petroleum ether as eluent, 1∶20, V/V) to obtain 2 as deep red solids (120 mg, 76% yield). ¹H NMR (400 MHz, CDCl₃): δ 7.66-7.62 (d, J=16 Hz, 4H), 7.45 (s, 9H), 7.40 (s, 6H), 2.26-2.12 (m, 2H), 1.20-1.16 (t, J=16 Hz, 3H). Selected FTIR data (KBr pellets, cm^-1): 2 224 (m), 2 070 (s), 2 027 (vs), 1 993 (vs), 1 486 (m), 1 448 (m), 616 (m). HRMS (m/z): Calcd. for C₃₃H₂₆O₅N Fe₂PS₂Na [M+Na]⁺, 745.958 1; Found, 745.954 5.

  1.3   Crystal structure determination

  Slow evaporation of the petroleum ether solution or DCM (dichloromethane) solution of compound 1 at 4 ℃ gave red single crystals 1 and 1′, respectively, suitable for X-ray diffraction. The red single crystal of compound 2 for X-ray diffraction was obtained by slowly diffusing petroleum ether into its DCM solution at 4 ℃. Crystal data were collected with Mo Kα radiation (λ=0.071 073 nm) on a CCD diffractometer. Cell parameters were retrieved and refined using the SMART and SAINT software, respectively. The SADABS program was applied for absorption corrections. The structures were solved by direct methods with the SHELXL-97 program package. All the non-hydrogen atoms were in the Fourier maps and refined with anisotropic parameters. Crystal data are summarized in Table 1. Selected bond lengths (nm) and bond angles (°) are listed in Table 2.

  Table 1

  

  Table 1.  Crystallographic data for compounds 1, 1′, and 2

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  Parameters	1	1'	2
  Formula	C16H11Fe2NO6S2	C16H11Fe2NO6S2	C33H26O5NFe2PS2
  Formula weight	489.08	489.08	723.34
  Crystal system	Triclinic	Monoclinic	Triclinic
  Space group	P1	C2/c	P1
  a / nm	0.813 3(6)	3.678 1(9)	1.177 9(7)
  b / nm	0.886 0(7)	0.929 4(3)	1.239 9(5)
  c / nm	1.518 4(10)	1.155 0(3)	1.273 9(7)
  α / (°)	93.614(6)		84.283(4)
  β / (°)	99.979(6)	96.225(2)	63.253(6)
  γ / (°)	114.064(7)		81.230(4)
  V / nm3	0.973 1(12)	3.925 4(19)	1.641 2(15)
  Z	2	8	2
  Dc / (g·cm-3)	1.669	1.655	1.464
  μ / mm-1	1.736	14.140	1.101
  F(000)	492.0	1 968.0	740.0
  Crystal size / mm	0.27×0.08×0.05	0.29×0.15×0.11	1.0×0.76×0.13
  Reflection collected	9 070	8 672	15 532
  Rint	0.034 3	0.034 0	0.044 9
  Data, Nres, Npara	45 693, 0, 245	3 690, 0, 245	6 237, 0, 398
  GOF on F 2	1.069	1.029	1.104
  R1b, wR2c [I > 2σ(I)]	0.036 2, 0.076 6	0.034 3, 0.079 5	0.045 3, 0.109 4
  R1, wR2 (all data)	0.049 2, 0.086 1	0.050 3, 0.087 0	0.061 6, 0.120 7
  a Nres=number of restraints, Npar=number of parameters; b R1=∑||Fo|-|Fc||/∑|Fo|; c wR2={∑[w(Fo2-Fc2)2/(Fo2)2]}1/2.

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) for compounds 1, 1′, and 2

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  1
  Fe1—Fe2	0.251 2(6)	Fe1—S1	0.227 0(7)	Fe1—S2	0.223 7(7)
  Fe2—S1	0.227 5(7)	Fe2—S2	0.226 2(7)	C11—Fe1	0.179 6(3)
  C12—Fe1	0.178 6(3)	C13—Fe1	0.180 0(3)	C14—Fe2	0.179 3(3)
  C15—Fe2	0.178 8(3)	C16—Fe2	0.181 4(3)
  O1—C11—Fe1	177.010(234)	O2—C12—Fe1	178.967(259)	O3—C13—Fe1	178.886(296)
  O4—C14—Fe2	179.236(279)	O5—C16—Fe2	178.881(294)	O6—C15—Fe2	178.347(274)
  C11—Fe1—C12	96.750(117)	C11—Fe1—C13	99.262(126)	C11—Fe1—Fe2	152.680(83)
  C12—Fe1—C13	92.336(134)	C12—Fe1—Fe2	98.576(84)	C14—Fe2—C15	91.284(139)
  C14—Fe2—C16	100.541(143)	C14—Fe2—Fe1	101.431(101)	C15—Fe2—C16	98.704(140)
  C15—Fe2—Fe1	102.499(102)	C16—Fe2—Fe1	148.917(106)	C14—Fe2—S2	157.031(101)
  C14—Fe2—S1	86.832(94)	C15—Fe2—S2	93.762(103)	S1—Fe2—Fe1	56.34(2)
  1′
  Fe1—Fe2	0.250 8(6)	Fe1—S1	0.227 3(9)	Fe1—S2	0.224 5(8)
  Fe2—S1	0.227 4(8)	Fe2—S2	0.225 8(8)	C11—Fe1	0.180 2(3)
  C12—Fe1	0.180 1(3)	C13—Fe2	0.179 2(3)	C14—Fe2	0.180 0(3)
  C15—Fe2	0.178 4(3)	C16—Fe1	0.178 1(4)
  C14—Fe2—S1	102.494(100)	C14—Fe2—S2	104.245(107)	C13—Fe2—Fe2	98.486(101)
  C13—Fe2—S1	87.668(103)	C13—Fe2—S2	154.270(111)	C15—S1—Fe1	58.714(56)
  C15—S1—Fe2	8.427(47)	C16—S2—Fe1	37.887(71)	C16—S2—Fe2	77.239(72)
  Fe2—S2—Fe1	67.696(25)	Fe1—S1—Fe2	66.968(25)	C11—Fe1—S1	110.316(116)
  C11—Fe1—S2	101.912(107)	S1—Fe1—Fe2	56.536(23)	S1—Fe1—S2	80.302(30)
  C12—Fe1—S1	86.446(94)	S1—Fe2—S2	80.017(28)	S2—Fe2—Fe1	55.916(22)
  2
  Fe1—Fe2	0.251 9(7)	Fe1—S1	0.227 5(10)	Fe1—S2	0.227 1(10)
  Fe2—S1	0.227 3(9)	Fe2—S2	0.225 3(10)	C31—Fe1	0.177 3(4)
  C32—Fe1	0.178 1(4)	C33—Fe1	0.181 1(4)	C29—Fe2	0.176 5(4)
  C30—Fe2	0.177 3(4)	P1—Fe2	0.223 9(11)
  S1—Fe1—Fe2	56.320(31)	C33—Fe1—S2	106.936(103)	C32—Fe1—S1	87.582(117)
  Fe2—S2—Fe1	67.655(37)	C31—Fe1—S1	159.106(116)	P1—Fe2—S1	105.987(46)
  S2—Fe1—Fe2	55.824(33)	C29—Fe2—S2	91.722(131)	C33—Fe1—Fe2	153.522(161)
  P1—Fe2—S2	102.321(43)	C32—Fe1—S2	153.490(128)	S1—Fe2—S2	80.114(39)
  S2—Fe1—Fe2	55.824(33)	P1—Fe2—Fe1	152.324(42)	C32—Fe1—S1	87.582(117)
  Fe2—S1—Fe1	67.269(35)	C30—Fe2—S1	88.140(136)	C29—Fe2—S1	156.035(129)
  S2—Fe2—Fe1	56.521(33)	C31—Fe1—S2	91.485(133)	P1—Fe2—C30	98.096(138)

  1.4   Photocatalytic water-splitting experiment

  The experiments were conducted in a H₂O/CH₃CN (3 mL+3 mL) solvent system, utilizing compounds 1 and 2 (8.62 μmol) as the photocatalyst, respectively. Ir[dF(CF₃)ppy]₂(dtbbpy)PF₆ (1.78 μmol) (dF(CF₃)ppy= 3, 5-difluoro-2-[5-(trifluoromethyl)pyridin-2-yl]phenyl, dtbbpy=4, 4'-di-tert-butyl-2, 2'-bipyridine) as the photosensitizer, and ascorbic acid (300 μmol) as the sacrificial electron donor. The reaction was irradiated with a 300 W xenon lamp in the full wavelength range. The generated hydrogen was quantitatively measured using a gastight syringe and analyzed via gas chromatography (Techcomp 7890-Ⅱ).

  2.   Results and discussion

  2.1   Structure description of compound 1

  Compound 1 crystallizes in the triclinic space group P1 (Table 1) from petroleum ether solution. As shown in Fig.1a, the crystal structure of 1 displays a distorted butterfly structure with [Fe₂S₂] unit as the core, one ethyl group on one sulfur atom, and one 2- cyanobenzyl group on the other sulfur atom. Each of the iron centers is coordinated with three terminal carbonyl ligands. There are metal-metal interactions [Fe1—Fe2 0.251 2(6) nm] and typical Fe—S covalent bonds in the active center of 1. The Fe—S bonds range from 0.224 7(7) to 0.227 5(7) nm, and the Fe—C bonds vary from 0.178 6(3) to 0.181 4(3) nm, which are comparable to those in the previous reports^[18-20]. The S1…S2 distance is 0.291 8(11) nm. The dihedral angle between the planes S2—Fe1—Fe2 and S1—Fe1—Fe2 is 78.199(24)°. Detailed structural analysis revealed that there exist strong intermolecular C—H…O (H3^ⅲ…O1^ⅱ 0.245 nm, ∠C3—H3^ⅲ…O1^ⅱ=157°; H1^ⅰ…O1^ⅱ 0.260 nm, ∠C1—H1^ⅰ…O1^ⅱ=167°) and C—H…N (H4^ⅱ…N1^ⅰ 0.261 nm, ∠C4—H4^ⅱ…N1^ⅰ=143°) (Symmetry codes: ^ⅰ x, y, z; ^ⅱ -1+x, y, z; ^ⅲ 1-x, 2-y, 1-z) hydrogen bonding interactions between adjacent three molecules. These intermolecular hydrogen bonds lead to the coplanarity of two benzene rings on two neighboring molecules arranged parallelly along the crystallographic a-axis with a dihedral angle of 0°, as shown in Fig.1b and 1c. The packing along the a-axis results in 1D parallelogram tunnels, which were further stacked along the crystallographic b-axis, but no π…π interaction is observed.

  Figure 1

  

  Figure 1.  (a) Crystal structure of 1, 1′ and 2 (30% probability displacement ellipsoids); (b) Packing diagrams of compounds 1, 1′ and 2; (c) Intermolecular hydrogen bonds indicated by orange dashed lines

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  Unexpectedly, the crystallization of compound 1 from dichloromethane solution resulted in a completely different monoclinic space group C2/c, named as 1′ (Table 1). Interestingly, 1′ displayed a distinctly different conformation from 1 due to the presence of a —CH₂— group and the free rotation of C—S and C—C σ-bonds, as shown in Fig.1A. A pair of molecules are connected by two sets of intermolecular C—H…O (H5^ⅰ…O5^ⅱ 0.260 nm, ∠C5—H5^ⅰ…O5^ⅱ=139°) hydrogen bonds and arranged parallelly along crystallographic b-axis as shown in Fig. 1b and 1c. The parallelly arranged molecules extend along crystallographic a- and b-axis to form layers, which are further linked along crystallographic c-axis through C—H…N (H6^ⅱ…N1^ⅲ 0.287 nm, ∠C6—H6^ⅱ…N1^ⅲ=147°) (Symmetry codes: ^ⅰ 0.5+x, 1.5-y, 0.5+z; ^ⅱ 1-x, 1+y, 1.5-z; ^ⅲ 1-x, 1-y, 2-z) hydrogen bonding interactions.

  Compound 2 also crystallizes in the triclinic space group P1 (Table 1). Due to the replacement of CO by P(Ph)₃ on the Fe2 center, as shown in Fig.1a, the Fe2—C bond lengths [0.176 5(4)-0.177 3(4) nm] of the other two CO groups are slightly shorter than those in compound 1. In addition, the Fe—Fe [0.251 93(7) nm] and Fe—S2 [0.227 1(10)-0.225 3(10) nm] bond lengths of 2 are slightly longer than those in 1. The Fe1—C and Fe—S1 bond lengths are comparable to those in 1. PPh₃, as a better σ donor, has a strong electron-donating ability, which has a subtle effect on the coordination environment of the Fe—Fe center and is hoped to have a positive influence on its catalytic activity. Two neighboring molecules are interacted through C—H…N (H6^ⅰ…N6^ⅱ 0.276 nm, ∠C6—H6^ⅰ…N6^ⅱ=131°) (Symmetry codes: ^ⅰ 1-x, 1-y, 2-z; ^ⅱ x, y, 1+z) hydrogen bonds and arranged parallelly along crystallographic a-axis. They are further stacked along the crystallographic c-axis.

  2.2   FTIR spectra

  Compounds 1 and 2 were also characterized by FTIR spectroscopy (Fig.2). Absorption peaks at 2 228 cm^-1 can be attributed to the stretching vibrations of the —CN group. The IR spectra of compound 1 presented characteristic CO stretching vibration bands at 2 071, 2 027, and 1 989 cm^-1, consistent with the spectral features of other reports^[21-22]. However, the CO absorption bands of compound 2 at 2 041, 1 990, 1 968, and 1 934 cm^-1 shifted to lower energies compared with those of 1 due to the coordination of PPh₃.

  Figure 2

  

  Figure 2.  FTIR spectra of compounds 1 and 2

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  2.3   UV-Vis absorption spectra

  UV-Vis spectroscopy of compound 1 exhibited strong and moderate absorptions at 229 and 334 nm and very weak absorption at around 470 nm in the DCM solution. Compound 2 exhibited strong and moderate absorptions at 229 and 373 nm and very weak absorption at around 500 nm in the DCM solution (Fig.3a). The lower energy absorptions of 2 red-shifted 30-40 nm compared with those of 1. Tauc plots were also determined by calculations (Fig.3b). The band-gap energies (E_g) were estimated from the x-axis intercept at (Ahν)²=0. The E_g values of compounds 1 and 2 were 3.16 and 2.86 eV. The different UV-Vis absorptions and E_g should result in different photocatalytic capabilities.

  Figure 3

  

  Figure 3.  (a) UV-Vis absorption spectra of compounds 1 and 2; (b) Tauc plot showing the band gap energy determined from the optical absorption spectra of 1 and 2

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  2.4   PXRD pattern

  The PXRD patterns of compounds 1 and 2 were also determined. The experimental patterns were well consistent with the simulated ones, which verified the phase purity of the samples.

  Figure 4

  

  Figure 4.  PXRD patterns of compounds 1 and 2

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  2.5   Photocatalytic hydrogen production

  The structural characteristics and optical properties of compounds 1 and 2 make them promising candidates for photocatalysts in photocatalytic hydrogen production from water splitting. As shown in Fig.5 and Table 3, the amounts of H₂ produced by compounds 1 and 2 within 3 h were 316.8 and 705.0 μmol, respectively, achieving catalytic efficiency of 25.1 and 37.9 μmol·mg^-1·h^-1 and turnover numbers (TONs) of 36.8 and 81.8^[23]. By comparison, the hydrogen production ability of 2 was significantly higher than that of 1, due to the substitution of CO by PPh₃. This finding is consistent with the previous report^[18].

  Figure 5

  

  Figure 5.  Photocatalytic hydrogen evolution in the presence of compounds 1 and 2

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  Table 3

  

  Table 3.  Three-hour hydrogen production data for compounds 1 and 2

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  Compound	$ {V}_{{H}_{2}} $ / μL	$ {n}_{{H}_{2}} $ / μmol	Catalytic efficiency / (μmol·mg-1·h-1)	TON
  1	7 096.3	316.8	25.1	36.8
  2	15 792.0	705.0	37.9	81.8

  2.6   Density functional theory calculations

  Density functional theory (DFT) calculations for compounds 1 and 2 were performed using Gaussian 03W at the B3LYP level. The calculation for compound 1 was based on crystallographic atomic coordinates. The HOMO and LUMO energy levels, along with their difference, are presented in Fig.6 and Table 4. The HOMOs and LUMOs of 1 primarily distribute on the [Fe-Fe]-hydrogenase unit, rather than on the benzene rings. Compared to the HOMOs, the LUMOs of 2 are more spread out and could be distributed on the PPh₃ group. The HOMO and LUMO energy levels are -6.17 and -2.22 eV, respectively, with an energy gap of 3.95 eV for 1, and -5.31 and -1.71 eV, respectively, with an energy gap of 3.60 eV for 2. The narrower energy gap shows that 2 has higher hydrogen production activity than 1, consistent with the photochemical experiment of hydrogen production.

  Figure 6

  

  Figure 6.  HOMOs and LUMOs of compounds 1 (top) and 2 (bottom)

  Fe, C, S, P, O, and N are shown in purple, gray, yellow, orange, red, and blue, respectively.

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  Table 4

  

  Table 4.  HOMO and LUMO energy levels for compounds 1 and 2

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  Compound	HOMO / eV	LUMO / eV	ΔELUMO-HOMO / eV
  1	-6.17	-2.22	3.95
  2	-5.31	-1.71	3.60

  3.   Conclusions

  In this study, two [FeFe]-hydrogenase-containing compounds with 2-cyanobenzyl groups were synthesized and characterized using single-crystal X-ray diffraction, FTIR, UV-Vis spectroscopy, and DFT calculation. Photochemical H₂ generation experiments demonstrated that the catalytic efficiency of compound 2 (37.9 μmol·mg^-1·h^-1) was apparently higher than that of 1 (25.1 μmol·mg^-1·h^-1) due to the substitution of CO by PPh₃. Compound 2 can serve as an effective photocatalyst for hydrogen production from water splitting under simulated sunlight. This research offers new insights into the design and development of novel [FeFe]-hydrogenase model catalysts.

  
  
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  Scheme 1  Synthetic route of compounds 1 and 2

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  Figure 1  (a) Crystal structure of 1, 1′ and 2 (30% probability displacement ellipsoids); (b) Packing diagrams of compounds 1, 1′ and 2; (c) Intermolecular hydrogen bonds indicated by orange dashed lines

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  Figure 2  FTIR spectra of compounds 1 and 2

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  Figure 3  (a) UV-Vis absorption spectra of compounds 1 and 2; (b) Tauc plot showing the band gap energy determined from the optical absorption spectra of 1 and 2

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  Figure 4  PXRD patterns of compounds 1 and 2

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  Figure 5  Photocatalytic hydrogen evolution in the presence of compounds 1 and 2

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  Figure 6  HOMOs and LUMOs of compounds 1 (top) and 2 (bottom)

  Fe, C, S, P, O, and N are shown in purple, gray, yellow, orange, red, and blue, respectively.

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  Table 1.  Crystallographic data for compounds 1, 1′, and 2

  Parameters	1	1'	2
  Formula	C16H11Fe2NO6S2	C16H11Fe2NO6S2	C33H26O5NFe2PS2
  Formula weight	489.08	489.08	723.34
  Crystal system	Triclinic	Monoclinic	Triclinic
  Space group	P1	C2/c	P1
  a / nm	0.813 3(6)	3.678 1(9)	1.177 9(7)
  b / nm	0.886 0(7)	0.929 4(3)	1.239 9(5)
  c / nm	1.518 4(10)	1.155 0(3)	1.273 9(7)
  α / (°)	93.614(6)		84.283(4)
  β / (°)	99.979(6)	96.225(2)	63.253(6)
  γ / (°)	114.064(7)		81.230(4)
  V / nm3	0.973 1(12)	3.925 4(19)	1.641 2(15)
  Z	2	8	2
  Dc / (g·cm-3)	1.669	1.655	1.464
  μ / mm-1	1.736	14.140	1.101
  F(000)	492.0	1 968.0	740.0
  Crystal size / mm	0.27×0.08×0.05	0.29×0.15×0.11	1.0×0.76×0.13
  Reflection collected	9 070	8 672	15 532
  Rint	0.034 3	0.034 0	0.044 9
  Data, Nres, Npara	45 693, 0, 245	3 690, 0, 245	6 237, 0, 398
  GOF on F 2	1.069	1.029	1.104
  R1b, wR2c [I > 2σ(I)]	0.036 2, 0.076 6	0.034 3, 0.079 5	0.045 3, 0.109 4
  R1, wR2 (all data)	0.049 2, 0.086 1	0.050 3, 0.087 0	0.061 6, 0.120 7
  a Nres=number of restraints, Npar=number of parameters; b R1=∑||Fo|-|Fc||/∑|Fo|; c wR2={∑[w(Fo2-Fc2)2/(Fo2)2]}1/2.

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  Table 2.  Selected bond lengths (nm) and bond angles (°) for compounds 1, 1′, and 2

  1
  Fe1—Fe2	0.251 2(6)	Fe1—S1	0.227 0(7)	Fe1—S2	0.223 7(7)
  Fe2—S1	0.227 5(7)	Fe2—S2	0.226 2(7)	C11—Fe1	0.179 6(3)
  C12—Fe1	0.178 6(3)	C13—Fe1	0.180 0(3)	C14—Fe2	0.179 3(3)
  C15—Fe2	0.178 8(3)	C16—Fe2	0.181 4(3)
  O1—C11—Fe1	177.010(234)	O2—C12—Fe1	178.967(259)	O3—C13—Fe1	178.886(296)
  O4—C14—Fe2	179.236(279)	O5—C16—Fe2	178.881(294)	O6—C15—Fe2	178.347(274)
  C11—Fe1—C12	96.750(117)	C11—Fe1—C13	99.262(126)	C11—Fe1—Fe2	152.680(83)
  C12—Fe1—C13	92.336(134)	C12—Fe1—Fe2	98.576(84)	C14—Fe2—C15	91.284(139)
  C14—Fe2—C16	100.541(143)	C14—Fe2—Fe1	101.431(101)	C15—Fe2—C16	98.704(140)
  C15—Fe2—Fe1	102.499(102)	C16—Fe2—Fe1	148.917(106)	C14—Fe2—S2	157.031(101)
  C14—Fe2—S1	86.832(94)	C15—Fe2—S2	93.762(103)	S1—Fe2—Fe1	56.34(2)
  1′
  Fe1—Fe2	0.250 8(6)	Fe1—S1	0.227 3(9)	Fe1—S2	0.224 5(8)
  Fe2—S1	0.227 4(8)	Fe2—S2	0.225 8(8)	C11—Fe1	0.180 2(3)
  C12—Fe1	0.180 1(3)	C13—Fe2	0.179 2(3)	C14—Fe2	0.180 0(3)
  C15—Fe2	0.178 4(3)	C16—Fe1	0.178 1(4)
  C14—Fe2—S1	102.494(100)	C14—Fe2—S2	104.245(107)	C13—Fe2—Fe2	98.486(101)
  C13—Fe2—S1	87.668(103)	C13—Fe2—S2	154.270(111)	C15—S1—Fe1	58.714(56)
  C15—S1—Fe2	8.427(47)	C16—S2—Fe1	37.887(71)	C16—S2—Fe2	77.239(72)
  Fe2—S2—Fe1	67.696(25)	Fe1—S1—Fe2	66.968(25)	C11—Fe1—S1	110.316(116)
  C11—Fe1—S2	101.912(107)	S1—Fe1—Fe2	56.536(23)	S1—Fe1—S2	80.302(30)
  C12—Fe1—S1	86.446(94)	S1—Fe2—S2	80.017(28)	S2—Fe2—Fe1	55.916(22)
  2
  Fe1—Fe2	0.251 9(7)	Fe1—S1	0.227 5(10)	Fe1—S2	0.227 1(10)
  Fe2—S1	0.227 3(9)	Fe2—S2	0.225 3(10)	C31—Fe1	0.177 3(4)
  C32—Fe1	0.178 1(4)	C33—Fe1	0.181 1(4)	C29—Fe2	0.176 5(4)
  C30—Fe2	0.177 3(4)	P1—Fe2	0.223 9(11)
  S1—Fe1—Fe2	56.320(31)	C33—Fe1—S2	106.936(103)	C32—Fe1—S1	87.582(117)
  Fe2—S2—Fe1	67.655(37)	C31—Fe1—S1	159.106(116)	P1—Fe2—S1	105.987(46)
  S2—Fe1—Fe2	55.824(33)	C29—Fe2—S2	91.722(131)	C33—Fe1—Fe2	153.522(161)
  P1—Fe2—S2	102.321(43)	C32—Fe1—S2	153.490(128)	S1—Fe2—S2	80.114(39)
  S2—Fe1—Fe2	55.824(33)	P1—Fe2—Fe1	152.324(42)	C32—Fe1—S1	87.582(117)
  Fe2—S1—Fe1	67.269(35)	C30—Fe2—S1	88.140(136)	C29—Fe2—S1	156.035(129)
  S2—Fe2—Fe1	56.521(33)	C31—Fe1—S2	91.485(133)	P1—Fe2—C30	98.096(138)

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  Table 3.  Three-hour hydrogen production data for compounds 1 and 2

  Compound	$ {V}_{{H}_{2}} $ / μL	$ {n}_{{H}_{2}} $ / μmol	Catalytic efficiency / (μmol·mg-1·h-1)	TON
  1	7 096.3	316.8	25.1	36.8
  2	15 792.0	705.0	37.9	81.8

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  Table 4.  HOMO and LUMO energy levels for compounds 1 and 2

  Compound	HOMO / eV	LUMO / eV	ΔELUMO-HOMO / eV
  1	-6.17	-2.22	3.95
  2	-5.31	-1.71	3.60

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