Diiron ethanedithiolate complexes with acetate ester: Synthesis, characterization and electrochemical properties

Journal of Organometallic Chemistry 870 (2018) 90e96

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Diiron ethanedithiolate complexes with acetate ester: Synthesis, characterization and electrochemical properties Meng Lian a, Jiao He b, Xiao-Yong Yu a, Chao Mu b, Xu-Feng Liu a, *, Yu-Long Li b, **, Zhong-Qing Jiang c a b c

School of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, China College of Chemistry and Environmental Engineering, Sichuan University of Science & Engineering, Zigong 643000, China Department of Physics, Key Laboratory of ATMMT Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 April 2018 Received in revised form 11 June 2018 Accepted 24 June 2018 Available online 25 June 2018

Five diiron ethanedithiolate complexes bearing acetate ester have been prepared and characterized. Esterification of complex [{m-SCH2CH(CH2OH)S}Fe2(CO)6] (1) with acetic acid in the presence of N,N0 dicyclohexylcarbodiimide and 4-dimethylaminopyridine gave the acetate ester [{m-SCH2CH(CH2O2CCH3) S}Fe2(CO)6] (2) in 94% yield. Further treatment of complex 2 with a monophosphine ligand triphenylphosphine, tricyclohexylphosphine, tris(2-methoxyphenyl)phosphine or tris(4-tolyl)phosphine in the presence of Me3NO$2H2O resulted in the formation of the corresponding monophosphine-substituted diiron derivatives [{m-SCH2CH(CH2O2CCH3)S}Fe2(CO)5L] (L ¼ PPh3, 3; PCy3, 4; P(C6H4OCH3-2)3, 5; P(C6H4CH3-4)3, 6) (Cy ¼ cyclohexyl) in 69e87% yields. The complexes 2e6 were characterized by elemental analysis and spectroscopy, together with X-ray crystallography. Furthermore, the electrochemical properties were investigated by cyclic voltammetry. © 2018 Elsevier B.V. All rights reserved.

Keywords: Diiron ethanedithiolate Acetate ester Monophosphine X-ray crystallography Electrochemistry

1. Introduction In recent years, research on the chemistry of diiron dithiolate complexes has been attracted much attention because these complexes resemble the active site of the natural enzyme [FeFe]hydrogenases, which can reversibly catalyze the protons reduction to a clean energy H2 in high efficiency in several microorganisms [1‒5]. The X-ray crystallographic structures of the active site of [FeFe]-hydrogenases were reported twenty years ago [6,7], as shown in Fig. 1, showing that the active site consists of a butterfly diiron propanedithiolate or azadithiolate cluster coordinated by terminal carbonyls, bridging carbonyl, cyanides and a cysteinyl ligand containing a cubane-like [Fe4S4] cluster. The structural information promoted chemists to design and synthesize a variety of model complexes for the biomimetic chemistry of the [FeFe]hydrogenases [8‒12]. For example, P-donor ligands [13‒16], Ndonor ligands [17‒19], S-donor ligands [20‒22] and N-heterocyclic carbene [23] have been introduced to the all-carbonyl complexes

* Corresponding author. ** Corresponding author. E-mail addresses: nkxfl[email protected] (X.-F. Liu), [email protected] (Y.-L. Li). https://doi.org/10.1016/j.jorganchem.2018.06.023 0022-328X/© 2018 Elsevier B.V. All rights reserved.

by carbonyl substitution. In our present study, we choose phosphine ligands as the substitute for the cyanide found in the active site of [FeFe]-hydrogenases because: i) the phosphine ligands are very easily available from commercial source; ii) the phosphinecontaining complexes are much more stable than those containing cyanide ligands; iii) the electronic effects of the phosphine ligands are very similar to those of cyanide ligands. The diiron ethanedithiolate complex bearing a hydroxy group [{m-SCH2CH(CH2OH)S}Fe2(CO)6] was produced by Pickett and coworkers in 2001 [24]. However, esterification of complex 1 with an acid is relatively scarce in the literature [25]. In order to develop new reactions of complex 1, we recently paid our attention to the esterification of complex 1 with acetic acid as well as the further reactions of the acetate ester 2 with four phosphine ligands by carbonyl substitution. Herein, in this paper, we report the synthesis, structural characterization and crystal structures of five diiron ethanedithiolate complexes 2e6 containing an acetate group. In addition, the electrochemical properties of these complexes were

M. Lian et al. / Journal of Organometallic Chemistry 870 (2018) 90e96

X

OC

Fe

OC NC

Cys

S

S

S Fe

C

[Fe4S4]

CN

91

C ¼ 39.4 Hz, i-PhC), 133.17 (d, JP-C ¼ 9.5 Hz, o-PhC), 130.32 (s, p-PhC), 128.63 (d, JP-C ¼ 11.2 Hz, m-PhC), 65.97 (OCH2), 50.06 (SCH), 37.35 (SCH2), 20.76 (CH3) ppm. Anal. Calcd. for C28H23Fe2O7PS2: C, 49.58; H, 3.42. Found: C, 49.52; H, 3.26%.

2.4. Synthesis of [{m-SCH2CH(CH2O2CCH3)S}Fe2(CO)5P(C6H11)3] (4)

CO

O X = CH2, NH Fig. 1. The active site of [FeFe]-hydrogenases (H cluster).

investigated by cyclic voltammetry, showing that these complexes can catalyze the protons reduction to H2 in the presence of acetic acid under electrochemical conditions. 2. Experimental 2.1. Materials and methods Acetic acid, N,N0 -dicyclohexylcarbodiimide (DCC), 4dimethylaminopyridine (DMAP), triphenylphosphine, tricyclohexylphosphine, tris(2-methoxyphenyl)phosphine, tris(4-tolyl)phosphine and Me3NO$2H2O were available commercially and used as received. Complex 1 [24] was prepared according to literature procedures. IR spectra were recorded on a Nicolet MAGNA 560 FTIR spectrometer. NMR spectra were obtained on a Bruker 500 MHz spectrometer. Elemental analyses were performed by a PerkinElmer 240C analyzer. 2.2. Synthesis of [{m-SCH2CH(CH2O2CCH3)S}Fe2(CO)6] (2) To a solution of 1 (0.201 g, 0.5 mmol), DMAP (0.024 g, 0.2 mmol) and acetic acid (0.036 g, 0.6 mmol) in CH2Cl2 (10 mL) was added DCC (0.124 g, 0.6 mmol). The mixture was stirred at room temperature for 12 h and then the solvent was reduced on a rotary evaporator. The residue was subjected to TLC separation using CH2Cl2/ petroleum ether ¼ 2:3 (v/v) as eluent. From the main yellow band, 0.208 g (94%) of complex 2 was obtained as a red solid. IR (CH2Cl2, cm1): nC≡O 2077 (vs), 2037 (vs), 2008 (vs), 1990 (vs); nC¼O 1744 (m). 1H NMR (500 MHz, CDCl3): 4.02 (d, J ¼ 6.5 Hz, 2H, OCH2), 2.90 (quint, J ¼ 6.7 Hz, 1H, SCH), 2.68 (dd, J ¼ 7.7, 13.2 Hz, 1H, SCH2), 2.09 (s, 3H, CH3), 1.92 (dd, J ¼ 5.7, 13.2 Hz, 1H, SCH2) ppm. 13C{1H} NMR (125 MHz, CDCl3): 208.08 (C≡O), 170.41 (C¼O), 65.67 (OCH2), 51.93 (SCH), 38.71 (SCH2), 20.76 (CH3) ppm. Anal. Calcd. for C11H8Fe2O8S2: C, 29.76; H, 1.82. Found: C, 30.05; H, 2.02%. 2.3. Synthesis of [{m-SCH2CH(CH2O2CCH3)S}Fe2(CO)5(PPh3)] (3) To a solution of 2 (0.044 g, 0.1 mmol) and triphenylphosphine (0.026 g, 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 then the solvent was reduced on a rotary evaporator. The residue was subjected to TLC separation using CH2Cl2/petroleum ether ¼ 3:2 (v/v) as eluent. From the main red band, 0.059 g (87%) of complex 3 was obtained as a red solid. IR (CH2Cl2, cm1): nC≡O 2048 (vs), 1987 (vs), 1937 (m); nC¼O 1740 (m). 1H NMR (500 MHz, CDCl3): 7.60e7.57 (m, 6H, PhH), 7.44e7.43 (m, 9H, PhH), 3.79e3.75 (m, 2H, OCH2), 2.00 (s, 3H, CH3), 1.78 (quint, J ¼ 6.7 Hz, 1H, SCH), 1.43 (s, 2H, SCH2) ppm. 31P{1H} NMR (200 MHz, CDCl3, 85% H3PO4): 62.91 (s) ppm. 13C{1H} NMR (125 MHz, CDCl3): 214.65 (d, JP-C ¼ 9 Hz, PFeC≡O), 214.41 (d, JPC ¼ 8.1 Hz, PFeC≡O), 209.91 (FeC≡O), 170.26 (C¼O), 135.58 (d, JP-

The procedure was similar to that of 3 except tricyclohexylphosphine (0.028 g, 0.1 mmol) was used instead of triphenylphosphine; 0.048 g (69%) of complex 4 was obtained as a red solid. IR (CH2Cl2, cm1): nC≡O 2045 (vs), 1979 (vs), 1925 (m); nC¼O 1740 (m). 1H NMR (500 MHz, CDCl3): 4.09e3.91 (m, 2H, OCH2), 2.64e2.51 (m, 2H, SCH2), 2.08 (s, 3H, CH3), 1.98e1.29 (m, 33H, 3C6H11), 1.68e1.64 (m, 1H, SCH) ppm. 31P{1H} NMR (200 MHz, CDCl3, 85% H3PO4): 70.72 (s) ppm. 13C{1H} NMR (125 MHz, CDCl3): 216.66 (d, JP-C ¼ 10.9 Hz, PFeC≡O), 216.53 (d, JP-C ¼ 10.9 Hz, PFeC≡O), 210.45 (FeC≡O), 170.56 (C¼O), 66.32 (OCH2), 51.25 (SCH), 39.22 (SCH2), 38.32, 38.19, 30.59, 29.86, 27.89, 27.82, 26.93, 26.41 (C6H11), 20.83 (CH3) ppm. Anal. Calcd. for C28H41Fe2O7PS2: C, 48.29; H, 5.93. Found: C, 48.12; H, 6.05%. 2.5. Synthesis of [{m-SCH2CH(CH2O2CCH3)S}]Fe2(CO)5P(C6H4OCH32)3] (5) The procedure was similar to that of 3 except tris(2methoxyphenyl)phosphine (0.035 g, 0.1 mmol) was used instead of triphenylphosphine; 0.063 g (82%) of complex 5 was obtained as a red solid. IR (CH2Cl2, cm1): nC≡O 2043 (vs), 1985 (vs), 1935 (sh); nC¼O 1734 (m). 1H NMR (500 MHz, CDCl3): 7.40 (t, J ¼ 7.5 Hz, 4H, PhH), 6.94 (br. s, 4H, PhH), 6.85 (q, J ¼ 4 Hz, 4H, PhH), 3.76 (dd, J ¼ 6.7, 11.2 Hz, 1H, OCH2), 3.67 (dd, J ¼ 7, 12 Hz, 1H, OCH2), 3.54 (s, 9H, 3OCH3), 1.98 (s, 1H, SCH), 1.97 (s, 3H, CH3), 1.43 (s, 2H, SCH2) ppm. 31P{1H} NMR (200 MHz, CDCl3, 85% H3PO4): 49.47 (s) ppm. 13C {1H} NMR (125 MHz, CDCl3): 215.81 (d, JP-C ¼ 8.9 Hz, PFeC≡O), 215.53 (d, JP-C ¼ 10.1 Hz, PFeC≡O), 210.62 (FeC≡O), 170.29 (C¼O), 160.23, 131.80, 119.72, 111.10 (PhC), 66.21 (OCH2), 54.69 (OCH3), 49.54 (SCH), 37.36 (SCH2), 20.77 (CH3) ppm. Anal. Calcd. for C31H29Fe2O10PS2: C, 48.46; H, 3.80. Found: C, 48.56; H, 3.99%. 2.6. Synthesis of [{m-SCH2CH(CH2O2CCH3)S}Fe2(CO)5P(C6H4CH34)3] (6) The procedure was similar to that of 3 except tris(4-tolyl) phosphine (0.030 g, 0.1 mmol) was used instead of triphenylphosphine; 0.053 g (74%) of complex 6 was obtained as a red solid. IR (CH2Cl2, cm1): nC≡O 2046 (vs), 1985 (vs), 1937 (sh); nC¼O 1735 (m). 1 H NMR (500 MHz, CDCl3): 7.45 (dd, J ¼ 8, 10.5 Hz, 6H, PhH), 7.21 (d, J ¼ 7 Hz, 6H, PhH), 3.78 (d, J ¼ 7 Hz, 2H, OCH2), 2.39 (s, 9H, 3PhCH3), 2.00 (s, 3H, CH3), 1.76 (quint, J ¼ 6.5 Hz, 1H, SCH), 1.32 (d, J ¼ 6.5 Hz, 2H, SCH2) ppm. 31P{1H} NMR (200 MHz, CDCl3, 85% H3PO4): 60.34 (s) ppm. 13C{1H} NMR (125 MHz, CDCl3): 214.88 (d, JP-C ¼ 9.1 Hz, PFeC≡O), 214.65 (d, JP-C ¼ 8.6 Hz, PFeC≡O), 210.08 (FeC≡O), 170.24 (C¼O), 140.45 (s, p-PhC), 133.08 (d, JP-C ¼ 11.5 Hz, o-PPhC), 132.56 (d, JP-C ¼ 421.5 Hz, i-PhC), 129.29 (d, JP-C ¼ 10 Hz, m-PhC), 66.00 (OCH2), 49.93 (SCH), 37.41 (SCH2), 21.37 (PhCH3), 20.73 (CH3) ppm. Anal. Calcd. for C31H29Fe2O7PS2: C, 51.69; H, 4.06. Found: C, 51.44; H, 3.87%. 2.7. X-ray structure determination Single crystals of 2e6 suitable for X-ray diffraction analysis were grown by slow evaporation of CH2Cl2/hexane solutions at 4  C. A single crystal of 2e6 was mounted on a Bruker D8 QUEST CCD diffractometer. Data were collected at 296 K using a graphite monochromator with Mo Ka radiation (l ¼ 0.71073 Å) in the u-f

92

M. Lian et al. / Journal of Organometallic Chemistry 870 (2018) 90e96

scanning mode. Data collection and reduction were used by APEX2 [26]. Absorption correction was performed by SADABS program [27]. Using OLEX2 [28], the structure was solved by direct methods using the SHELXS-97 program [29] and refined by full-matrix leastsquares techniques SHELXL-97 [29] on F2. Hydrogen atoms were located using the geometric method. Details of crystal data, data collections and structure refinement are summarized in Table 1.

CH2OH S

S

OC OC OC

Fe

Fe 1

CO CO CO

CH2O2CCH3

acetic acid DCC, DMAP

OC OC

S

S

OC

Fe

Fe 2

CO CO CO

Scheme 1. Synthesis of complex 2.

2.8. Electrochemical experiment Electrochemical properties of the complexes 2e6 were studied by cyclic voltammetry (CV) in CH2Cl2 solution. Electrochemical measurements were carried out under nitrogen using a CHI 620 Electrochemical work station. As the electrolyte, n-Bu4NPF6 was recrystallized multiple times from a CH2Cl2 solution 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/Agþ electrode as the reference electrode. The potential scale was calibrated against the Fc/Fcþ couple and reported versus this reference system. 3. Results and discussion 3.1. Synthesis and characterization of complex 2 As shown in Scheme 1, esterification of complex 1 with 1.2 equivalents of acetic acid in the presence of the dehydrating agent DCC and the catalyst DMAP in CH2Cl2 resulted in the formation of acetate ester 2 in very good yield (94%). The complex 2 is both air and moisture stable red solid, soluble in CH2Cl2 and THF. The structure of complex 2 has been fully characterized by elemental analysis, spectroscopy and X-ray crystallography. The IR spectrum shows four absorption bands in the region of 2077e1990 cm1 for the stretching vibrations of the six terminal carbonyls, comparable

to those of the all-carbonyl complexes [(m-SCH2CH2CH2S)Fe2(CO)6] (2072, 2033, 1993 cm1) [30] and [(m-SCH2CH2S)Fe2(CO)6] (2079, 2039, 2009, 1996 cm1) [31]. The 1H NMR spectrum displays a doublet at d 4.02 ppm for the hydrogens of the methylene group attached to the ester group. The 13C{1H} NMR spectrum exhibit a singlet at d 208.08 ppm for the six terminal carbonyls. The molecular structure of complex 2 has been determined by X-ray diffraction analysis. While the ORTEP view is shown in Fig. 2, the selected bond distances and angles are provided in Table 2. Complex 2 crystallizes in monoclinic space group P21/c with four molecules in the unit cell and one molecule in the asymmetric unit. As shown in Fig. 2, complex 2 features a diiron cluster ligated by a bridging ethanedithiolate containing an acetate ester and six terminal carbonyls. The Fe1‒Fe2 bond distance [2.4967 (11) Å] is actually the same as that of complex 1 [2.4998 (6) Å] [32] as well as those in some all-carbonyl diiron complexes [33]. 3.2. Synthesis and characterization of complexes 3‒6 As shown in Scheme 2, the monosubstituted diiron complexes could be prepared by carbonyl substitution of the parent complex 2 with 1 equivalent of the corresponding phosphine ligands in the presence of Me3NO$2H2O as the decarbonylating agent in 69e87% yields. The complexes 3e6 are both air and moisture stable red solids, soluble in CH2Cl2 and THF, which have been characterized by

Table 1 Crystal data and structure refinement details for 2e6. Complex

2

3

4

5

6

Empirical formula Formula weight Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) a ( ) b ( ) g ( ) V (Å3) Z Dcalc (g$cm3) m (mm1) F (000) Crystal size (mm3) Radiation 2q range ( ) hkl range

C11H8Fe2O8S2 443.99 296 (2) monoclinic P21/c 13.223 (2) 9.3524 (13) 13.1588 (18) 90 90.703 (4) 90 1627.2 (4) 4 1.812 2.074 888.0 0.22  0.18  0.16 MoKa (l ¼ 0.71073) 5.336 to 53.754 ‒14  h  16 ‒11  k  11 ‒16  l  16 13493 3484 [Rint ¼ 0.0822] 3484/0/209 1.088 0.0649/0.1756 0.0914/0.1961 0.89/‒1.42

C28H23Fe2O7PS2 678.25 296 (2) triclinic P-1 8.8979 (7) 11.4570 (9) 16.7014 (13) 70.748 (4) 88.335 (4) 68.648 (4) 1489.0 (2) 2 1.513 1.211 692.0 0.32  0.24  0.22 MoKa (l ¼ 0.71073) 5.084 to 55.068 ‒11  h  11 ‒14  k  14 ‒21  l  21 49154 6846 [Rint ¼ 0.0311] 6846/0/362 1.140 0.0401/0.1170 0.0595/0.1354 0.62/‒0.76

C28H41Fe2O7PS2 696.40 296 (2) monoclinic P21/c 9.7851 (6) 16.5105 (11) 20.3464 (13) 90 103.105 (2) 90 3201.5 (4) 4 1.445 1.128 1456.0 0.26  0.22  0.16 MoKa (l ¼ 0.71073) 4.274 to 50.05 ‒11  h  11 ‒19  k  19 ‒24  l  23 52116 5628 [Rint ¼ 0.0364] 5628/0/402 1.277 0.0425/0.1344 0.0548/0.1517 0.73/‒1.13

C31H29Fe2O10PS2 768.33 296 (2) triclinic P-1 9.7208 (12) 10.3102 (13) 17.547 (2) 93.985 (3) 101.647 (3) 90.842 (4) 1717.5 (4) 2 1.486 1.066 788.0 0.22  0.2  0.16 MoKa (l ¼ 0.71073) 4.454 to 50.184 ‒11  h  11 ‒12  k  12 ‒20  l  20 36324 6074 [Rint ¼ 0.0580] 6074/0/429 1.073 0.0421/0.1227 0.0528/0.1399 1.11/‒0.60

C31H29Fe2O7PS2 720.33 296 (2) monoclinic C2/c 24.067 (2) 13.3227 (11) 21.8398 (19) 90 111.328 (2) 90 6523.0 (10) 8 1.467 1.111 2960.0 0.2  0.18  0.14 MoKa (l ¼ 0.71073) 4.32 to 55.19 ‒31  h  31 ‒17  k  17 ‒28  l  28 66253 7530 [Rint ¼ 0.0721] 7530/0/392 1.131 0.0539/0.1430 0.0852/0.1647 0.99/‒0.82

Reflections collected Independent reflections Data/restraints/parameters Goodness of fit on F2 Final R indexes (I>2s(I)) Final R indexes (all data) Largest diff peak and hole/e Å3

M. Lian et al. / Journal of Organometallic Chemistry 870 (2018) 90e96

93

Table 2 Selected bond distances (Å) and angles ( ) for 2e6.

Fig. 2. ORTEP view of 2 with 30% probability level ellipsoids.

elemental analysis, spectroscopy and X-ray crystallography. The IR spectra of the complexes 3e6 show three absorption bands in the region of 2048e1925 cm1, ascribed to the stretching vibrations of the terminal carbonyls, shifted to lower frequencies with respect to those of the parent complex 2 due to the phosphine ligands having stronger electron-donating than CO [34‒36]. The 1H NMR spectra of complexes 3 and 4 display a multiplet in the region of d 3.79e3.75 or 4.09e3.91 ppm, respectively, for the protons of methylene group attached to the ester group, whereas the 1H NMR spectra complexes 5 and 6 display two doublet/doublet at d 3.76 and 3.67 ppm or a doublet at d 3.78 ppm for the corresponding protons. This is possibly due to their different conformations in the solution. The 31 1 P{ H} NMR spectra of the complexes 3e6 demonstrate a singlet in the region of d 49.47e70.72 ppm, in good agreement with phosphine-substituted diiron analogous [37,38], but notably larger than those of the corresponding free ligands because the phosphorus atom of the phosphine ligands coordinated to the iron atom. In contrast to complex 2, the 13C{1H} NMR spectra of complexes 3e6 exhibit two doublets at ca. d 214 ppm for the terminal carbonyls of PFe(CO)2 group and a singlet at ca. d 210 ppm for the terminal carbonyls of Fe(CO)3 group because of the coupling between phosphorus and carbon. While the X-ray crystal structures of the complexes 3e6 are shown in Fig. 3‒6, the selected bond distances and angles are provided in Table 2. Complexes 3 and 5 crystallize in triclinic, whereas complexes 4 and 6 crystallize in monoclinic. As shown in Fig. 3‒6, complexes 3e6 feature a diiron cluster ligated by an acetate ester containing ethanedithiolate, five terminal carbonyls and a monophosphine ligand. The monophosphine ligands are located in an apical position of the distorted square-pyramidal geometry of the iron center, in consistent with the monophosphine-containing

2 Fe1‒Fe2 Fe1‒S2 Fe2‒S2 C8‒C9 S1‒Fe1‒Fe2 S2‒Fe1‒Fe2 S1‒Fe2‒S2 Fe2‒S1‒Fe1 C9‒C8‒C7 C10‒O7‒C9 3 Fe1‒Fe2 Fe1‒S2 Fe2‒S2 C6‒C7 S1‒Fe1‒Fe2 S2‒Fe1‒Fe2 S2‒Fe2‒Fe1 P1‒Fe2‒Fe1 C6‒C7‒C8 C9‒O6‒C8 4 Fe1‒Fe2 Fe1‒S2 Fe2‒S2 C6‒C7 S1‒Fe1‒Fe2 S2‒Fe1‒S1 S2‒Fe2‒Fe1 P1‒Fe2‒Fe1 C8‒C7‒C6 C9‒O6‒C8 5 Fe1‒Fe2 Fe1‒S2 Fe2‒S2 C6‒C7 S1‒Fe1‒Fe2 S2‒Fe1‒S1 S2‒Fe2‒Fe1 P1‒Fe2‒Fe1 C8‒C7‒C6 C9‒O6‒C8 6 Fe1‒Fe2 Fe1‒S2 Fe2‒S2 C6‒C7 S1‒Fe1‒Fe2 S2‒Fe1‒S1 S2‒Fe2‒Fe1 P1‒Fe2‒Fe1 C8‒C7‒C6 C9‒O6‒C8

2.4967 (11) 2.2434 (16) 2.2425 (16) 1.441 (10) 56.24 (4) 56.17 (4) 80.25 (6) 67.67 (5) 114.0 (7) 113.4 (7)

Fe1‒S1 Fe2‒S1 C7‒C8 C10‒C11 S1‒Fe1‒S2 S1‒Fe2‒Fe1 S2‒Fe2‒Fe1 Fe1‒S2‒Fe2 C8‒C9‒O7 O7‒C10‒C11

2.2402 (16) 2.2440 (16) 1.527 (10) 1.461 (10) 80.31 (6) 56.09 (4) 56.20 (4) 67.64 (5) 106.5 (6) 112.8 (7)

2.4945 (6) 2.2596 (9) 2.2677 (8) 1.501 (5) 56.41 (2) 56.72 (2) 56.41 (2) 157.42 (3) 114.5 (3) 115.1 (3)

Fe1‒S1 Fe2‒S1 Fe2‒P1 C9‒C10 S1‒Fe1‒S2 S1‒Fe2‒Fe1 S2‒Fe2‒S1 Fe1‒S1‒Fe2 O6‒C8‒C7 O6‒C9‒C10

2.2484 (9) 2.2522 (8) 2.2456 (8) 1.463 (7) 79.62 (3) 56.27 (2) 79.37 (3) 67.32 (2) 109.4 (3) 112.8 (4)

2.5034 (6) 2.2566 (11) 2.2444 (9) 1.498 (6) 56.64 (3) 79.41 (4) 56.44 (3) 159.80 (3) 114.9 (4) 118.7 (4)

Fe1‒S1 Fe2‒S1 Fe2‒P1 C9‒C10 S2‒Fe1‒Fe2 S1‒Fe2‒Fe1 S2‒Fe2‒S1 Fe1‒S1‒Fe2 O6‒C8‒C7 O6‒C9‒C10

2.2605 (11) 2.2699 (10) 2.2691 (9) 1.487 (8) 55.98 (3) 56.28 (3) 79.47 (4) 67.09 (3) 112.5 (4) 112.5 (6)

2.5078 (7) 2.2539 (10) 2.2522 (10) 1.502 (6) 56.48 (3) 79.79 (4) 56.22 (3) 154.84 (3) 112.9 (4) 120.9 (4)

Fe1‒S1 Fe2‒S1 Fe2‒P1 C9‒C10 S2‒Fe1‒Fe2 S1‒Fe2‒Fe1 S2‒Fe2‒S1 Fe1‒S1‒Fe2 O6‒C8‒C7 O6‒C9‒C10

2.2456 (11) 2.2610 (10) 2.2649 (9) 1.454 (8) 56.15 (3) 55.90 (3) 79.50 (3) 67.62 (3) 109.7 (4) 112.4 (5)

2.5112 (8) 2.2593 (11) 2.2538 (11) 1.506 (6) 56.00 (3) 79.59 (4) 56.29 (3) 155.71 (4) 111.5 (4) 113.8 (5)

Fe1‒S1 Fe2‒S1 Fe2‒P1 C9‒C10 S2‒Fe1‒Fe2 S1‒Fe2‒Fe1 S2‒Fe2‒S1 Fe1‒S1‒Fe2 O6‒C8‒C7 O6‒C9‒C10

2.2546 (12) 2.2488 (11) 2.2515 (10) 1.498 (11) 56.09 (3) 56.22 (3) 79.83 (4) 67.78 (3) 107.2 (4) 109.7 (8)

diiron analogues [39‒41]. The Fe1‒Fe2 bond distance of complex 6 [2.5112 (8) Å] is larger than those of complexes 3e5 [2.4956 (6) Å for 3, 2.5034 (6) Å for 4, 2.5078 (7) Å for 5], suggesting that tris(4tolyl)phosphine is stronger electron-donating than other ligands, but shorter than those of disubstituted complexes [16,42] as well as those in [Fe2(CO)6 (m-StBu) (m-h2-RCH2C ¼ CH2)] (R ¼ acridone)

CH2O2CCH3

OC OC OC

S

S

Fe

Fe 2

CH2O2CCH3 Me3NO

CO CO CO

L

OC OC OC

S Fe

L = PPh3, 3 PCy3, 4 P(C6H4OCH3-2)3, 5 Fe CO P(C6H4CH3-4)3, 6 CO

S

L

Scheme 2. Synthesis of complexes 3e6.

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Fig. 3. ORTEP view of 3 with 30% probability level ellipsoids.

Fig. 5. ORTEP view of 5 with 30% probability level ellipsoids.

Fig. 6. ORTEP view of 6 with 30% probability level ellipsoids.

Fig. 4. ORTEP view of 4 with 30% probability level ellipsoids.

[2.534 (2) Å] [43] and natural [FeFe]-hydrogenases [6,7]. One cyclohexyl group in complex 4 is disordered, whereas the oxygen of ester carbonyl in complex 5 is disordered over two sites with the occupancy of 0.56, indicating that the isomers may exist in the solid state of complexes 4 and 5. 3.3. Electrochemical properties of the complexes 2e6 The electrochemical properties of the complexes 2e6 were investigated by cyclic voltammetry (CV) in CH2Cl2 solution. The electrochemical data of complexes 2e6 are listed in Table 3. The CV curve of complex 2 is shown in Fig. 7, whereas the CV curves of complexes 3e6 are displayed in Figs. S1‒S4 (see supporting information). As shown in Fig. 7, complex 2 displays one quasi-reversible reduction process at 1.66 V which can be ascribed to the reduction of FeIFeI to FeIFe0. The result is very similar to the electrochemically irreversible reduction observed for the complex [(m-SCH2CH2S) Fe2(CO)6] (1.7 V), in agreement with the values obtained by

Darensbourg and co-workers [44,45]. Meanwhile, complex 2 displays one irreversible oxidation process at þ1.0 V, which can be ascribed to the oxidation of FeIFeI to FeIFeII. Furthermore, the first reduction potential of 2 is more positive than those of complexes 3e6, obviously due to the fact that phosphine ligands are stronger electron-donating than CO ligand. Moreover, complex 2 has been found to have the catalytic ability for protons reduction to H2 in the presence of HOAc under CV conditions. As shown in Fig. 7, upon addition of the first

Table 3 Electrochemical data of 2e6. complex

Epc1 [V]

Epa [V]

2 3 4 5 6

‒1.66 ‒1.84 ‒1.92 ‒1.88 ‒1.94

þ1.0 þ0.49 þ0.34 þ0.49 þ0.36

M. Lian et al. / Journal of Organometallic Chemistry 870 (2018) 90e96

95

Provincial Natural Science Foundation (No. LY16B020009), the National Natural Science Foundation of China (No. 21501124), Science & Technology Department of Sichuan Province (No. 2018JY0235), Education Department of Sichuan Province (No. 18ZA0337) and the Sichuan University of Science & Engineering (2014RC05, 201710622016). Appendix A. Supplementary data CCDC 1835979e1835983 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Supplementary data related to this article can be found at https://doi.org/10.1016/j.jorganchem.2018.06.023. References Fig. 7. Cyclic voltammogram of complex 2 (1.0 mM) with HOAc (0e10 mM) in 0.1 M nBu4NPF6/CH2Cl2 at a scan rate of 100 mV s1.

2 mM HOAc to the solution of complex 2, the first reduction peak at 1.66 V was slightly increased, whereas the second reduction potential of 2 is shifted positively to 2.23 V in the presence of 2 mM HOAc. It is noteworthy to point out that the current peaks at 2.23 V increased markedly with increasing concentration of HOAc, indicating that complex 2 has the ability for the electrocatalytic reduction of protons to dihydrogen. In addition, according to the abovementioned electrochemical observations of 2, an EECC (E ¼ electrochemical; C ¼ chemical) catalytic mechanism can be proposed for the electrocatalytic H2 production of 2 in the presence of the weak acid [ [5] [45‒48]], although more evidence is still needed for an accurate mechanism. The electrochemical behaviors of 3‒ ‒6 were also examined (Figs. S1‒S4). The results show that complexes 3e6 can also be regarded as electrocatalysts for hydrogen production in the presence of acetic acid. In addition, to further evaluate the electrocatalytic abilities for H2 production catalyzed by complexes 2e6, the ratios of the catalytic current (icat) to the reductive peak current (ip) without the added acid have been used as a marker to compare the catalytic efficiency of different complexes [49]. The results show that the icat/ip value of complex 5 is equal to 12.8, which is apparently larger than those of complexes 2e4 and 6.

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

4. Conclusion In summary, we have presented an acetate ester 2, prepared by esterification of the starting material 1 with acetic acid in the presence of DCC and DMAP, together with the derivatives 3e6, prepared by carbonyl substitution of complex 2 with the corresponding monophosphine ligands in the presence of Me3NO$2H2O. X-ray crystallographic studies revealed that the molecular structure of complex 2 is composed of a diiron ethanedithiolate cluster with six terminal carbonyls and an acetate group. However, the molecular structures of complexes 3e6 contains five terminal carbonyls and apicallycoordinated monophosphine ligands. Due to the coordination of different phosphine ligands, the Fe‒Fe bond distances of complexes 4e6 are longer than that of complexes 2 and 3. In addition, the electrochemical properties of complexes 2e6 were investigated by cyclic voltammetry, showing that these complexes can produce H2 in the presence of HOAc under electrochemical conditions. Acknowledgments The authors gratefully acknowledge the support of the Zhejiang

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