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Aminophosphine-substituted diiron dithiolate complexes: Synthesis, crystal structure, and electrocatalytic investigation
Accepted Manuscript Aminophosphine-substituted diiron dithiolate complexes: Synthesis, crystal structure, and electrocatalytic investigation Yu-Long Li, Zhong-Yi Ma, Jiao He, Meng-Yuan Hu, Pei-Hua Zhao PII:
S0022-328X(17)30525-9
DOI:
10.1016/j.jorganchem.2017.09.014
Reference:
JOM 20089
To appear in:
Journal of Organometallic Chemistry
Received Date: 22 July 2017 Revised Date:
3 September 2017
Accepted Date: 5 September 2017
Please cite this article as: Y.-L. Li, Z.-Y. Ma, J. He, M.-Y. Hu, P.-H. Zhao, Aminophosphine-substituted diiron dithiolate complexes: Synthesis, crystal structure, and electrocatalytic investigation, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.09.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Aminophosphine-substituted diiron dithiolate complexes: Synthesis, crystal structure, and electrocatalytic investigation
1
College of Chemistry and Environmental Engineering, Sichuan University of Science
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& Engineering, Zigong 643000, P. R. China 2
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Yu-Long Li,1,* Zhong-Yi Ma,2 Jiao He,1 Meng-Yuan Hu,2 Pei-Hua Zhao1,2,*
School of Materials Science and Engineering, North University of China, Taiyuan
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030051, P. R. China
* Corresponding author. Email: [email protected] (Y.-L. Li);
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[email protected] (P.-H. Zhao).
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Abstract As
the
active
site
models
of
[FeFe]-hydrogenases,
two
new
aminophosphine-monosubstituted diiron dithiolate compelxes with the general formular Fe2(µ-pdt)(CO)5{PPh2NH(C6H4R-p)} (pdt = SCH2CH2CH2S; R = Br (1), Me (2)), were
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prepared by the oxidative decarbonylation of parent Fe2(µ-pdt)(CO)6 (A) with monophosphines Ph2PNH(C6H4Br-p) or Ph2PNH(C6H4Me-p) in the presence of decarbonylating agent Me3NO·2H2O in MeCN. Interestingly, the ambient temperature treatment of A and aminodiphosphine Ph2PN(C6H4Me-p)PPh2 resulted in the formation
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of the major chelated complex Fe2(µ-pdt)(CO)4{κ2-Ph2PN(C6H4Me-p)PPh2} (3) concomitant with the minor complex 2. All the complexes 1-3 have been well 31
P) spectroscopies, and X-ray
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characterized by elemental analysis, FTIR, NMR (1H,
crystallography. Additionally, the electrochemical and electrocatalytic properties of complexes 1-3 are studied using cyclic voltammetry, where the ratios of the catalytic current (icat) to the reductive peak current (ip) for similar complexes 1, 2, and parent A at
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10 mM HOAc exhibit a ranking of 2 ≈ 1 > A.
Keywords: Diiron dithiolate complexes; aminophosphine substitution; synthesis; crystal
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structure; electrocatalytic investigation.
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1. Introduction The synthetic and bioinorganic chemistry of diiron dithiolate complexes has received great attention over recent decades [1,2], due to the resemblance of their [Fe2S2] clusters to the active site of [FeFe]-hydrogenases that are a family of highly-efficient
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metalloenzymes for the catalytic proton reduction to dihydrogen (H2) [3,4]. The X-ray crystallographic and theoretical studies on [FeFe]-hydrogenases revealed in the followings: (i) their active site (called H-cluster) consists of a butterfly [Fe2S2] catalytic center subsite and a cubic [Fe4S4] electron transfer unit [5-7], which are jonited together
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through a S atom of cysteine; (ii) in H-cluster, the two Fe atoms in the butterfly [Fe2S2] subsite are bridged by a dithiolate cofactor and coordinated by several CO/CN ligands
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[5,6]; (iii) the brigding dithiolate cofactor was supposed to be a propanedithiolate (pdt, SCH2CH2CH2S) [6,8] or favorable azadithiolate (adt, SCH2NHCH2S) [9-11]. The active site structure of [FeFe]-hydrogenases is displayed in Chart 1.
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Chart 1, See pape 20
Inspired by the aforementioned structural information on the active site of [FeFe]-hydrogenases (Chart 1), synthetic chemists designed and prepared a wide range
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of diiron dithiolate complexes as the [FeFe]-hydrogenase models [12-22]. Among the synthesized model complexes, the aminophosphine-substituted diiron dithiolate
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complexes are of particular interest since aminophosphines with hydrophilic and basic sites are deemed to be valuable for the function modes of proton redution to dihydrogen [23-25]. To date, however, there are only a few examples of diiron dithiolate complexes with aminophosphine ligands, such as Fe2(µ-pdt)(CO)4{Ph2PN(R)PPh2} (R = CHMe2, CHCH=CH2, (CH2)2Me, (CH2)3Me, CH2CHMe2, C6H4Me-p, H, (CH2)2NMe2) [26-29] and Fe2(µ-pdt)(CO)5{Ph2PNH(R)} (R = (CH2)2NMe2, C6H4NH2-o, CMe3, Py, C6H4Cl-p, C6H4NO2-p, C6H4CO2Et-p) [27,30-32]. Meanwhile, to our knowledge, the investigation on the electrocatalytic H2-producing abilities of the aminophosphine-monosubstituted diiron dithiolate complexes has not been performed.
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ACCEPTED MANUSCRIPT In order to further develop the biomimetic chemistry of [FeFe]-hydrogenases and to investigate the electrocatalytic capabilites of diiron dithiolate complexes containing aminophosphine ligands, we launched this work. Herein, we present our results on the synthesis, crystal structures, and electrochemistry of three aminophosphine-subsitituted
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diiron dithiolate complexes 1-3. Notably, comparison for the electrocatalytic abilites of similar complexes 1, 2, and precursor A is studied using cyclic voltammetry. It should be noted that although complex 3 has been reported before [28], its crystal structrue is obtained for the first time and detailedly investigated since the chelated complex 3 with
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asymmetry of the diiron core may be regarded to be a desirable biomimetic model of
2. Experimental 2.1. Materials and Instruments
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[FeFe]-hydrogenases [33].
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All reactions and operations were carried out under a dry, oxygen-free nitrogen atmosphere with standard Schlenk and vacuum-line techniques. MeCN was distilled from CaH2 under N2. Me3NO·2H2O was commercially available and used as received. (A)
[34],
Ph2PNH(C6H4R-p)
(R
=
Br,
Me)
[35],
and
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Fe2(µ-pdt)(CO)6
Ph2PN(C6H4Me-p)PPh2 [36] were prepared according to the literature procedures.
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Preparative TLC was perform on glass plates (25 cm x 20 cm x 0.25 cm) coated with silica gel G (10-40 mm). Column chromatography was perforned on flash neutral Al2O3 (75-150 µm). FTIR spectra were recorded on a Nicolet 670 FTIR spectrometer. NMR (1H, 31P) spectra were obtained on a Bruker Avance 600 MHz spectrometer. Elemental analyses were obtained on a Perkin-Elmer 240C analyzer. 2.2. Synthesis 2.2.1. Synthesis of Fe2(µ-pdt)(CO)5{PPh2NH(C6H4Br-p)} (1) A mixture of Fe2(µ-pdt)(CO)6 (0.193 g, 0.5 mmol), Ph2PNH(C6H4Br-p) (0.214 g, 0.6 mmol), and Me3NO·2H2O (0.056 g, 0.5 mmol) was dissolved in MeCN (15 mL). The 4
ACCEPTED MANUSCRIPT reaction mixture was stirred at ambient temperature until the reaction was completed (ca. 0.5 h) by TLC monitor. After solvent was removed under vacuum, the residue was chromatographed by preparative TLC separation eluting with CH2Cl2/petroleum ether (2:3, v/v). From the main red band, complex 1 was obtained as a red solid. Yield: 43% (0.154 g). Anal. Calcd. (%) for C26H21BrFe2NO5PS2: C, 43.73; H, 2.96; N, 1.96. Found
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(%): C, 44.05; H, 3.43; N, 1.71. FTIR (KBr disk): νC≡O 2038 (vs), 1986 (vs), 1969 (vs), 1950 (vs), 1933 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.76, 7.44 (s, s, 4H, 6H, 2xPC6H5), 7.09, 6.41 (s, s, 2H, 2H, NC6H4O), 5.49 (br s, 1H, NH), 1.99 (s, 2H,
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2xSCHeHa), 1.70-1.53 (m, 4H, 2xSCHeHa and CH2) ppm. 31P NMR (243 MHz, CDCl3, 85% H3PO4): δ 92.95 (s) ppm.
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2.2.2. Synthesis of Fe2(µ-pdt)(CO)5{PPh2NH(C6H4Me-p)} (2)
Complex 2 was prepared by a similar procedure to complex 1, except that Ph2PNH(C6H4Me-p) (0.175 g, 0.6 mmol) was used instead of Ph2PNH(C6H4Br-p) (0.214 g, 0.6 mmol). Complex 2 was obtained as a red solid. Yield: 65% (0.211 g). Anal. Calcd. (%) for C27H24Fe2NO5PS2: C, 49.95; H, 3.73; N, 2.16. Found (%): C, 50.01; H,
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4.19; N, 2.48. FTIR (KBr disk): νC≡O 2030 (vs), 1983 (s), 1965 (vs), 1948 (vs), 1932 (vs). 1H NMR (600 MHz, CDCl3, TMS): δ 7.80, 7.42 (s, s, 4H, 6H, 2xPC6H5), 6.81, 6.45 (s, s, 2H, 2H, NC6H4), 5.37 (d, 2JPH = 16.2 Hz, 1H, NH), 2.16 (s, 3H, CH3), 1.96 (s,
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2H, 2xSCHeHa), 1.68-1.58 (m, 4H, 2xSCHeHa and CH2) ppm.
31
P NMR (243 MHz,
CDCl3, 85% H3PO4): δ 91.77 (s) ppm. Synthesis
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2.2.3.
of
Fe2(µ-pdt)(CO)5{PPh2NH(C6H4Me-p)}
(2)
and
Fe2(µ-pdt)(CO)4{κ2-Ph2PN(C6H4Me-p)PPh2} (3) A mixture of Fe2(µ-pdt)(CO)6 (0.193 g, 0.5 mmol), Ph2PN(C6H4Me-p)PPh2 (0.285 g, 0.6 mmol), and Me3NO·2H2O (0.056 g, 0.5 mmol) was dissolved in MeCN (20 mL). The reaction mixture was stirred at ambient temperature until the reaction was completed (ca. 1.5 h) by TLC monitor. After solvent was removed under vacuum, the residue was separated by column chromatography on flash neutral Al2O3 eluting with CH2Cl2/petroleum ether (1:2, v/v). From the first red band, complex 2 was obtained as a red solid (0.073 g, 23% yield), the single crystals of which can be afforded by slow
5
ACCEPTED MANUSCRIPT evaporation of the CH2Cl2/hexane solution in the refrigerator. The experimental procedure was monitored by TLC, indicating that complex 2 shows the same Rf value as the target product obtained in the Experimental section “2.2.2.”. From the second red band, complex 3 was obtained as a dark-red solid (0.132 g, 33% yield). Anal. Calcd. (%) for C38H33Fe2NO4P2S2: C, 56.67; H, 4.13; N, 1.74; Found (%): C, 56.91; H, 4.57; N,
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2.04. FTIR (KBr disk): νC≡O 2010 (vs), 1944 (s), 1928 (vs), 1905 (vs). 1H NMR (600 MHz, CDCl3, TMS): δ 7.82-7.36 (m, 20H, 4xPC6H5), 6.69, 6.45 (s, s, 4H, NC6H4), 2.15 (s, 3H, CH3), 2.08 (s, 2H, 2xSCHeHa), 1.79-1.43 (m, 4H, 2xSCHeHa and CH2) ppm. 31P
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NMR (243 MHz, CDCl3, 85% H3PO4): δ 97.38 (s, basal-basal isomer, 75%), 116.95 (s,
2.3. X-ray structure determination
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apical-basal isomer, 25%) ppm.
Single crystals of 1-3 suitable for X-ray diffraction analysis were grown by slow evaporation of the CH2Cl2/hexane solution at 5oC. The crystals were mounted on a Bruker-CCD diffractometer. Data were collected at 150(2) and 123(2) K using a graphite monochromator with Cu-Kα radiation (λ = 1.54178 Å) in the ω-φ scanning
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mode. The structure was solved by direct methods using the SHELXS-97 program [37] and refined by full-matrix least-squares techniques (SHELXL-97) on F2 [38]. Hydrogen atoms were located using the geometric method. The crystallographic parameters, data
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collection, and structure refinement details are summarized in Table 1.
Table 1, See in page 25
2.4. Electrochemical and electrocatalytic experiments Electrochemical and electrocatalytic properties of 1-3, and A were studied by cyclic voltammetry (CV) in MeCN solutions. 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/Ag+
6
ACCEPTED MANUSCRIPT electrode as the reference electrode. The potential scale was calibrated against the Fc/Fc+ couple and reported versus this reference system.
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3. Results and Discussion 3.1. Synthesis of complexes 1-3
As shown in Scheme 1, two new aminophosphine-monosubstituted complexes 1 and
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2 were prepared in moderate yields by the decarbonylating reaction of the parent Fe2(µ-pdt)(CO)6 (A) with the monophosphines Ph2PNH(C6H4R-p) (R = Br and Me)
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using Me3NO·2H2O as CO-removing agent. Most interestingly, we found that the room temperature reaction of A and aminodiphosphine Ph2PN(C6H4Me-p)PPh2 in the presence of 1.1 equivalents of Me3NO·2H2O can afford not only the chelated complex 3 but also by-product 2 in the yields of 33% and 23%, respectively.
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Scheme 1, See page 21
It is worth pointing out that Hogarth and co-workers reported the similar oxidative decarbonylation of A and Ph2PN(C6H4Me-p)PPh2 in MeCN solution to obtain the sole
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product 3, during which it shows the initial formation of an intermediate with strong absorption at ca. 2040 cm-1 for a monodentate pentacarbonyl complex as revealed by IR
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spectroscopy [28]. Based on the aforementioned observation and the previously similar cases [32,39], a possible pathway for the formation of 3 together with by-product 2 can be proposed (Scheme 2). As depicted in Scheme 2, reaction of A with one equivalent of decarbonylating agent Me3NO·2H2O in MeCN gives the species B {Fe2(µ-pdt)(CO)5L, L = NCMe or NMe3} [40], followed by the treatment of Ph2PN(C6H4Me-p)PPh2 to afford the species C {Fe2(µ-pdt)(CO)5[PPh2N(C6H4Me-p)PPh2]} [28]. And then, the species C undergoes two pathways: (i) the hydrolysis of C with H2O produces complex 2 and (ii) the intramolecular decarbonylation of C with the free phosphorus atom of
7
ACCEPTED MANUSCRIPT ligand in the presence of Me3NO/MeCN forms the possible intermediate D to afford complex 3. It should be noted that this proposed mechanism need to be further studied.
3.2. Spectroscopic characterization of complexes 1-3
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Scheme 2, See page 22
All the complexes 1-3 are air-stable red solids and were well characterized by IR and NMR spectroscopies as well as elemental analysis. The IR spectra of 1 and 2 show five
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strong absorption bands in the ranges of 2038-1932 cm-1 for their carbonyl groups, which are apparently shifted toward lower frequency as compared to their all-carbonyl
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precursor A (2074, 2036, 1995 cm-1) [41]. This is probably because that the electron-donating abilities of the aminophosphines Ph2PNH(C6H4R-p) (R = Br and Me) are stronger than the carbonyl ligands [32]. Meanwhile, the highest carbonyl absorption band of 1 is shifted by 8 cm-1 to lower energy relative to that of 2. This is possibly due to the fact that the Br substituent of ligand Ph2PNH(C6H4Br-p) in 1 is more
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electron-withdrawing than the Me substituent of ligand Ph2PNH(C6H4Me-p) in 2, which leads to the weaker electron density on the Fe atoms of 1 with respect to that of 2 owing to the effect of back-donating π-bonding from Fe to terminal CO [30-32]. In addition, in
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the IR spectrum of 3, the carbonyl absorption bands in the region of 2010-1905 cm-1 are further shifted toward much lower energy as compared to those of 1 and 2. This result
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strongly indicates that the iron-carbonyl groups of 3 is further substituted by the phosphorus atom of aminophosphine in comparison to those of 2. The 1H NMR spectra of 1-3 display two kinds of the similar proton peaks in the
downfield regions of 7.8-7.4 and 7.0-6.4 ppm for the monosubstituted phenyl moieties (P(C6H5)2) and the disubstituted one (NC6H4), respectively. Furthermore, in the 1H NMR spectra of 2 and 3, there is a sharp singlet at ca. 2.16 ppm assigned to their methyl groups. Most importantly, the 1H NMR spectra of 1 and 2 show an addtional broad singlet at 5.49 ppm and doublet at 5.37 ppm with a coupling contact 2JPH = 16.2 Hz for
8
ACCEPTED MANUSCRIPT their NH groups [33]. These observations well demonstrate that the structures of 1 and 2 are apparatently different from that of 3. The
31
P NMR spectra of 1 and 2 give only a singlet at 92.95 and 91.77 ppm
resepctively for the phosphorus atom coordinated to one of iron centres, which is well in
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accordance with the phosphorus signals in the range of 90-100 ppm observed in the known analogues Fe2(µ-pdt)(CO)5{Ph2PNH(R)} (R = (CH2)2NMe2, C6H4NH2-o, CMe3, Py, C6H4Cl-p, C6H4NO2-p, C6H4CO2Et-p) [27,30-32]. However, in the
31
P NMR
spectrum of 3, two sharp singlets at 97.38 and 116.95 ppm are respectively ascribed to
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the basal-basal isomer and apical-basal one in ca. 3:1 ratio (as measured by integration of the correspoding phosphorus signals). This assignment is according to the fact that
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the 31P NMR spectra usually consist of a large singlet in the region of 97-101 ppm and a small singlet in the range of 113-118 ppm for the dibasal and apical-basal isomers in the different ratios, as observed in the reported aminodiphosphine-chelated complexes Fe2(µ-pdt)(CO)4{κ2-Ph2PN(R)PPh2} (R = CHMe2, CHCH=CH2, CH2CHMe2, C6H4Me, and (CH2)2NMe2) [26,28,29].
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3.3. X-ray crystal structure of complexes 1-3 The molecular structures of 1-3 are definitely confirmed by X-ray crystallography,
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Table 2.
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which are illustrated in Figs. 1-3. The selected bond distances and angles are listed in
Figs. 1-3, See pages 29-31 Table 2. See page 26
As depicted in Figs. 1-3, the monosubstituted complexes 1 and 2 contain a typical butterfly [Fe2S2] cluster with five carbonyls plus one monodentate aminophosphine, whereas the chelated complex 3 features a well-known butterfly [Fe2S2] skeleton bearing four carbonyls as well as one chelated aminodiphosphine. Furthermore, in 1 and 2, the monophosphines (Ph2PNH(C6H4R); R = Br, Me) lie in the apical position of the square-pyramidal coordination geometry of one of two iron atoms, which match well
9
ACCEPTED MANUSCRIPT with those of the known PR3-monosubstituted diiron propanedithiolate complexes [42-46]. For 3, the diphosphine (Ph2PNH(C6H4Me)PPh2) spans the basal-basal site coordinated to one Fe1 atom, which enables the two phosphorus atoms to be equivalent as confirmed by the existence of a large singlet at 116.95 ppm in the 31P spectrum of 3
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in CDCl3 solution. The Fe1-Fe2 bond distances of 1 (2.5000(7) Å) for and 2 (2.4954(9) Å for) are somewhat different relative to precursor A (2.5103(11) Å) [34] but considerably shorter than those found in the natural [FeFe]-hydrogenases (2.55-2.60 Å) [5,6]. The Fe-P bond
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distances (2.2089(10) Å for 1 and 2.2055 (12) Å for 2) are closer to that of one analogue Fe2(µ-pdt)(CO)5{Ph2PNH(C6H4Cl-p)} (2.2038(6) Å) [32] but slightly different from
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those observed in the other analogues Fe2(µ-pdt)(CO)5{Ph2PNH(R)} (R = (CH2)2NMe2, 2.2160(13) Å [30]; C6H4NH2-o, 2.2139(14) Å [30]; CMe3, 2.2348(8) Å [27]; Py, 2.220(2) Å [31]). Furthermore, the average Fe-CCO bond distances (1.781 Å for 1 and 1.779 Å for 2) in Fe(CO)2P units are slightly shorter than the corresponding distances (1.792 Å for 1 and 1.794 Å for 2) in Fe(CO)3 moieties, possibly because that the higher
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electron density of the coordinated-Fe core leads to the stronger electron back-donation from Fe atom to CO ligands relative to that of the uncoordinated-Fe core [47,48]. In addition, the C-S-Fe angles in one boat-conformational six-membered ring of the propanedithiolate unit (C24-S2-Fe2/C26-S1-Fe2 in 1 and C22-S2-Fe1/C20-S1-Fe1 in 2)
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are 3.0-5.5o larger than the corresponding angles in the other chair-conformational six-membered ring (C24-S2-Fe1/C26-S1-Fe1 in 1 and C22-S2-Fe2/C20-S1-Fe2 in 2).
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The Papical-Fe-Fe angles (P1-Fe2-Fe1 in 1 and P1-Fe1-Fe2 in 2) are 5.67o and 9.71o wider than the Capical-Fe-Fe ones (C19-Fe1-Fe2 in 1 and C27-Fe2-Fe1 in 2), respectively. Thus, the differences in the aforementioned angles demonstrate that the six-membered iron dithiacyclohexane ring (FeS2C3) of the propanedithiolate groups is pushed away from the apical position of the monophosphine ligands and towards the Fe(CO)3 site in the solid state of 1 and 2. This outcome implies that the steric repulsion between the apical-monophosphine ligand and the bridgehead structural unit is strong, since the monophosphines (Ph2PNH(C6H4R); R = Br, Me) in 1 and 2 are cis to their bridgehead carbon atoms (C25 for 1 and C21 for 2) as shown in Figs. 1 and 2.
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ACCEPTED MANUSCRIPT The Fe1-Fe2 bond distance (2.5877(13) Å) in dibasal complex 3 is slightly shorter than its analogues Fe2(µ-pdt)(CO)4{κ2-Ph2PN(R)PPh2} (R = CHMe2, 2.6236(5) Å [26,28];
CHCH=CH2, 2.6042(4) Å [26,28]; (CH2)2NMe2), 2.6010(7) Å [29]) but very
similar to those observed in the natural [FeFe]-hydrogenases (2.55-2.60 Å) [5,6]. This
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result indicate that the steric and electronic character of the iron centers in 3 might be much close to those of [FeFe]-hydrogenases relative to the other dibasal complexes with aminodiphosphines [26-29]. Another notable feature is that the very small bite angle of P1-Fe1-P2 (71.90(6)o) in 3 is characteristic of the dibasal-chelated complexes with the P-X-P
bite-angle bis(diphenylphosphino) ligands
such
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small
as
dppm
and
aminodiphosphines, for example, Fe2(µ-pdt)(CO)4{κ2-dppm} (74.55(4)o) [26,28] and CHCH=CH2,
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Fe2(µ-pdt)(CO)4{κ2-Ph2PN(R)PPh2} (R = CHMe2, 71.32(2)o [26,28];
71.68(2)o [26,28]; (CH2)2NMe2), 71.48(3)o [29]). Moreover, the very small P1-Fe1-P2 bite angle leads to the quite short P1-P2 distance (2.5779(18) Å) in 3, which is probably due to the significant strain in the aminodiphosphine ligand coordinated to the one iron center in the basal-basal manner [28].
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3.4. Electrochemical and electrocatalytic studies of 1-3 In order to evaluate the ability to catalyze proton reduction to dihydrogen, cyclic voltammetry (CV) studies of 1 and 2 were performed in MeCN solution at a scan rate of
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100 mV s-1 as displayed in Fig. 4. The relevant electrochemcal and electrocatalytic data are listed in Table 3. Complexes 1 and 2 exhibit an irreversible reduction peak at -1.80
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and -1.82 V, respectively, assigned to the [FeIFeI] + e- → [FeIFe0] process [30,41], whereas the second reduction peaks may be beyond the electrochemical solvent window of MeCN. It should be noted that the observed reduction peaks of 1 and 2 are shifted towards more negative potentials by 150 and 160 mV respectively, relative to that of their parent complex A (Table 3). This is probably because that the electron-donationg capabilities of the aminophosphines (Ph2PNH(C6H4R); R = Br, Me) in 1 and 2 are obviously stronger that the carbonyls [32], as revealed by their IR spectra. Meanwhile, the reduction peak of 2 is more negative by 20 mV than that of 1, indicating that the iron core of 2 is slightly harder to reduce owing to the better donor character of ligand
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ACCEPTED MANUSCRIPT Ph2PNH(C6H4Me-p) in 2 than Ph2PNH(C6H4Br-p) in 1 [30]. In addition, the CV investigation of 3 is also carried out in the same electrochemical condition (as illustrated in Fig. 2S in Supporting Information). Complex 3 shows an observed reduction peak at -2.21 V [28], which is shifted by ca. 410 mV towards more negative potential as
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compared to those of 1 and 2. This might reveal that the iron cores in the disubstituted complex 3 are not much easier to reduce with respect to those of the monosubstituted
the latter as indicated by their IR spectra.
Fig. 4, See page 32
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Table 3. See page 27
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complexes 1 and 2, because of the stronger electron density of the former than those of
The electrochemical behaviors for the proton reduction to H2 catalyzed by 1-3 were studies on the sequential addition of HOAc (0-10 mM) in MeCN solution. As shown in Fig. 4, the initial reduction peak currents of 1 and 2 increase slightly when HOAc was
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added, but they do not grow steadily with the sequential addition of HOAc. However, a new reduction peak for 1 and 2 at ca. -2.40 V appears and the corresponding peak currents grow dramatically with the sequential addition of HOAc, wherein the current intensities (icat) of the new peaks have a linear correlation relative to the concentration
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([HOAc]) of the added acid (insert in Fig. 4). Similarly, in the case of 3, the initial reduction peak currents at ca. -2.21 V increase linearly with sequential acid addition
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(Fig. 2S). Consequently, these observations suggest that a typically catalytic process takes place [29,49-51]. That is, complexes 1-3 are active for the electrocatalytic H2 evolution in the presence of HOAc. According
to
the
above-mentioned
electrochemical
results
and
the
previously-reported similar cases [49-51], we might propose the EECC or ECEC (E = electrochemical; C = chemical) mechanisms for H2 production catalyzed by the aminophosphine-monosubstituted complexes 1 and 2. The two proposed catalytic
mechanisms of 1 as representative complex are illustrated in Scheme 3. One EECC process is described as follows: complex 1 is firstly reduced at -1.80 V to give
12
ACCEPTED MANUSCRIPT monoanion 1-. Then 1- is further reduced at -2.40 V to form dianion 12-, which can be protonated by weak acid HOAc to form the protonated species 1H-. Finally, the species 1H- receives proton to produce molecular H2 and regenerate 1 to thereby complete the catalytic cycle. The other ECEC process is stated in the following: the first step is also
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to produce monoanion 1- via the reduction of 1 at -1.80 V. However, due to the increase of HOAc concentration, the hydride-containing species 1H may be generated through the protonation of 1-. Afterwards, the species 1H accepts electron to reduce at -2.19 V,
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followed by the treatment with proton to finish the catalytic cycle.
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Scheme 3, See Page 23
Especially, to evaluate the electrocatalytic abilities for H2 production catalyzed by similar complexes 1 and 2, we use the ratio of the catalytic current (icat) to the reductive peak current (ip) in the absence of the added acid as a marker and compare the electrocatalytic abilities of 1, 2, and parent A, where the higher value of icat/ip indicates
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the faster catalysis [29,50,51]. It can be seen from Table 3 that the icat/ip values of 1, 2, and A upon 10 mM HOAc exhibit a ranking of 2 (9.38) ≈ 1 (9.28) > A (7.33), in which the icat/ip values of 1 and 2 are approximately equal but apparently larger than that of A. Such results indicate that 1 and 2 are a kind of the better electrocatalyst in the presence
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of HOAc relative to the all-carbonyl complex A, indicative of the considerable influence of aminophosphine ligands on the redox properties of the iron cores in the
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ligand-substituted diiron dithiolate complexes [41].
4. Conclusions
In this work, we found that the aminophosphine-monosubstituted complexes 1 and 2 can be easily prepared by oxidative decarbonylating reaction of precursor A and monophosphines Ph2PNH(C6H4R-p) (R = Br and Me). Interestingly, the chelated complex 3 together with by-product 2 can be simultaneously obtained from the room
13
ACCEPTED MANUSCRIPT temperature treatment of A with aminodiphosphine Ph2PN(C6H4Me-p)PPh2 in the presence of CO-removing agent Me3NO·2H2O. Meanwhile, complexes 1-3 are well characterized by elemental analysis and various spectroscopies. X-ray crystallographic studies demonstrate that the Ph2PNH(C6H4R-p) ligands in 1 and 2 occupy the apical site
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in the square-pyramidal geometry of one iron atom, whereas the Ph2PN(C6H4Me-p)PPh2 ligand in 3 adopts the expected dibasal geometric arrangement coordiated to one iron atom. Furthermore, X-ray crystallographic study of 3 displays that the Fe-Fe distance (2.5877(13) Å) is very similar to those observed in the natural [FeFe]-hydrogenases
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(2.55-2.60 Å). This possibly implies that the steric and electronic character of the iron centers in 3 bearing the chelated aminodiphosphine might be much close to those found
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in [FeFe]-hydrogenases, which match well with the theoretical conclusion that “asymmetric substitution of strong donor ligands is the most viable method of making better synthetic diiron complexes that will serve as both structural and functional models of the active site of iron-only hydrogenase” as suggested by Tye, Darensbourg, and Hall [33].
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In addition, the electrochemical studies of 1-3 show that the reduction peak of 3 is shifted towards more negative potentials relative to those of 1 and 2, demonstrating that the chelated complex 3 is difficult to reduce but should be easily protonated to produce H2 in comparison to the monosubstituted complexes 1 and 2. Notably, the
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electrocatalytic investigations of 1, 2, and A upon 10 mM HOAc indicate that the icat/ip values of 1 and 2 are much larger than that of A. This result reveals that the introduction
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of the aminophosphine ligands can considerably enhance the electrocatalytic capabilities of 1 and 2 for proton reduction to H2.
Supplementary materials CCDC 1563379-1563381 (1-3) contain the supplementary crystallographic data for this
paper.
These
data
can
be
obtained
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
14
or
free from
of the
charge
via
Cambridge
ACCEPTED MANUSCRIPT Crystallographic Data Centre, 12 Union Road Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgements
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We are grateful to the National Natural Science Foundation of China (No. 21301160, 21501124), the Natural Science Foundation of Shanxi Province (No. 201701D121035), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of
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[39] X.F. Liu, Inorg. Chim. Acta. 378 (2011) 338–341.
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[44] P.H. Zhao, Y.Q. Liu, G.Z. Zhao, Polyhedron 53 (2013) 144–149. [45] Y.C. Liu, T.H. Yen, Y.J. Tseng, C.H. Hu, G.H. Lee, M.H. Chiang, Inorg. Chem. 51 (2012) 5997–5999. [46] P.H. Zhao, X.H. Li, Y.F. Liu, Y.Q .Liu, J. Coord. Chem. 67 (2014) 766–778. [47] G. Durgaprasad, R. Bolligarla, S.K. Das, J. Organomet. Chem. 691 (2011) 3097–3105.
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Lichtenberger, J. Organomet. Chem. 694 (2009) 2681–2699.
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[50] G.A.N. Felton, C.A. Mebi, B.J. Petro, A.K. Vannucci, D.H. Evans, R.S. Glass, D.L.
[51] R.X. Li, X.F. Liu, T. Liu, Y.B. Yin, Y. Zhou, S.K. Mei, J. Yan, Electrochim. Acta
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237 (2017) 207–216.
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Scheme captions Chart 1 The proposed active site of [FeFe]-hydrogenase Scheme 1 Synthesis of complexes 1-3. Scheme 2 Possible pathway for the formation of 2 and 3.
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Scheme 3 The proposed EECC or ECEC catalytic mechanisms for H2 production catalyzed by the representative complex 1 in the presence of HOAc (note that the pdt,
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CO, and Ph2PNH(C6H4Br-p) groups are omitted for clarity).
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Chart 1
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Scheme 1
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C6H4Me-p S
OC
Fe OC
Fe
A
Me3NO(2H2O), MeCN OC -CO2, -H2O
CO
OC
CO
Fe
OC
CO
S
S
L Fe
Ph2PN(C6H4Me-p)PPh2 OC -L
OC
CO
CO
Fe
OC
B
S
S
Ph2 N P Fe
C
PPh2
CO
CO
H2O hydrolysis
2
C6H4Me-p
OC
S
S
OC
Ph2 N P
PPh2
-L
3
Fe
Fe
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Me3NO, MeCN decarbonylation
L
OC
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(L = NCMe, NMe3)
CO
D
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(L = NCMe, NMe3)
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Scheme 2
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OC
S
22
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Scheme 3
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Table captions Table 1 Crystal data and structural refinements details for 1-3. Table 2 Selected bond distances (Å) and angles (°) for 1-3.
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Table 3 The relevant electrochemical and electrocatalytic data for 1, 2, and A.[a]
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ACCEPTED MANUSCRIPT Table 1 1
2
3•2CH2Cl2
Empirical formula
C26H21BrFe2NO5PS2
C27H24Fe2NO5PS2
C40H37Cl4Fe2NO4P2S2
Formula weight
714.15
649.28
975.29
Temperature (K)
123(2)
150(2)
150(2)
Wavelength (Å)
1.54178
1.54178
1.54178
Crystal system
monoclinic
orthorhombic
triclinic
Space group
P21/c
Pbca
a (Å)
9.2296(4)
11.7424(4)
b (Å)
27.2635(12)
16.8134(3)
c (Å)
12.0427(6)
28.3519(7)
α (°)
90
90
β (°)
112.339(6)
90
90
90
2802.9(3)
5597.5(3)
2106.13(16)
8
2
1.541
1.538
10.570
9.845
2656.0
996.0
V (Å ) Z
4 -3
Dcalc (g cm )
1.692
-1
11.3408(6)
11.9941(5)
16.5349(5) 90.635(3)
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3
P-1
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γ (°)
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Complex
106.722(4) 101.381(4)
12.225
F(000)
1432.0
Crystal size (mm)
0.21 × 0.20 × 0.19
0.19 × 0.18 × 0.17
0.19 × 0.17 × 0.16
θmin, θmax (°)
6.484, 132.004
9.702, 132.002
7.538, 132.002
Reflections collected/unique
11128/4867
11093/4841
11845
0.0478
0.0681
0.0856
-6 ≤ h ≤ 10
-12 ≤ h ≤ 13
-13 ≤ h ≤ 13
-32 ≤ k ≤ 28
-18 ≤ k ≤ 19
-14 ≤ k ≤ 13
-14 ≤ l ≤ 13
-32 ≤ l ≤ 33
-19 ≤ l ≤ 16
99.9
99.6
99.2
4867/0/347
4841/0/344
7300/6/497
1.005
0.991
1.019
R1/wR2 [I > 2σ(I)]
0.0460/0.1147
0.0571/0.1380
0.0945/0.2458
R1/wR2 (all data)
0.0511/0.1193
0.0742/0.1503
0.1098/0.2726
Largest difference peak/ hole (e A-3)
1.15/-0.74
1.02/-0.78
1.79/-1.59
Rint
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hkl Range
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µ (mm )
Completeness to θmax (%)
Data/restraints/parameters
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Goodness-of-fit (GOF) on F
2
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ACCEPTED MANUSCRIPT Table 2
Complex 3 Fe(1)-Fe(2) Fe(2)-C(33) Fe(2)-C(34) Fe(2)-C(32) P(1)-Fe(1)-Fe(2) P(2)-Fe(1)-Fe(2) S(1)-Fe(1)-S(2) S(1)-Fe(2)-S(2)
2.5877(13) 1.778(7) 1.798(8) 1.798(7) 115.04(6) 106.88(6) 86.54(6) 84.32(6)
Fe(2)-C(22) Fe(2)-C(23) C(19)-Fe(1)-Fe(2) C(26)-S(1)-Fe(1) C(26)-S(1)-Fe(2) Fe(1)-S(1)-Fe(2) Fe(1)-S(2)-Fe(2)
1.776(4) 1.786(4) 150.12(14) 109.94(14) 115.27(17) 66.83(3) 67.07(3)
Fe(1)-P(1) N(1)-P(1)
2.2055(12) 1.695(4)
Fe(1)-C(23) Fe(1)-C(24) C(27)-Fe(2)-Fe(1) C(20)-S(1)-Fe(1) C(20)-S(1)-Fe(2) Fe(1)-S(2)-Fe(2) Fe(1)-S(1)-Fe(2)
1.778(5) 1.779(5) 146.91(18) 114.84(17) 109.50(18) 67.07(4) 66.88(4)
Fe(1)-P(1) Fe(1)-P(2) P(1)-P(2) Fe(1)-C(38) P(1)-Fe(1)-P(2) P(1)-N(1)-P(2) Fe(1)-S(1)-Fe(2) Fe(1)-S(2)-Fe(2)
2.1952(17) 2.1956(17) 2.5779(18) 1.744(7) 71.90(6) 95.8(3) 70.45(5) 70.34(5)
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2.4954(9) 1.786(5) 1.788(5) 1.808(6) 156.62(5) 114.92(18) 111.44(19) 85.23(5) 84.79(4)
2.2089(10) 1.690(3)
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Complex 2 Fe(1)-Fe(2) Fe(2)-C(25) Fe(2)-C(26) Fe(2)-C(27) P(1)-Fe(1)-Fe(2) C(22)-S(2)-Fe(1) C(22)-S(2)-Fe(2) S(1)-Fe(1)-S(2) S(1)-Fe(2)-S(2)
Fe(2)-P(1) N(1)-P(1)
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2.5000(7) 1.783(4) 1.794(4) 1.799(4) 155.79(4) 111.55(15) 115.29(13) 84.19(3) 84.83(4)
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Complex 1 Fe(1)-Fe(2) Fe(1)-C(20) Fe(1)-C(21) Fe(1)-C(19) P(1)-Fe(2)-Fe(1) C(24)-S(2)-Fe(1) C(24)-S(2)-Fe(2) S(1)-Fe(1)-S(2) S(1)-Fe(2)-S(2)
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Epc (V)
ip (µA)
icat (µA)[b]
icat/ip
1
-1.80
27.03
250.97
9.28
-1.82
20.53
192.60
9.38
-1.65
38.41
281.69
7.33
2 A
[c]
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[a] All the data for 1, 2 and A (1 mM) are versus the Fc/Fc+ couple in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV s-1. [b] icat is the catalytic current at the highest concentration of HOAc (10 mM).
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[c] Cyclic voltammogram of A (1 mM) is shown in Figure S1 (seen in Supporting Information).
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Figure captions Fig. 1 Molecular structure of 1 (hydrogens are omitted for clarity). Fig. 2 Molecular structure of 2 (hydrogens are omitted for clarity). Fig. 3 Molecular structure of 3 (Solvent and hydrogens are omitted for clarity).
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Fig. 4. Cyclic voltammogram of 1 and 2 (1.0 mM) with HOAc (0, 2, 4, 6, 8, 10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV s-1, 1 for (a) and 2 for (b). Insert:
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Plots of icat (mA) versus HOAc concentration (mM) for 1 and 2.
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Fig. 1
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Fig. 3
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Highlights
Three aminophosphine-substituted diiron dithiolate compelxes 1-3 were prepared by the oxidative decarbonylation.
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All the complexes have been well characterized by elemental analysis, FTIR, NMR (1H, 31P) spectroscopies, and X-ray crystallography.
The electrochemical and electrocatalytic properties of complexes 1-3 are studied using cyclic voltammetry, where the ratios of the catalytic current (icat) to the
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HOAc exhibit a ranking of 2 ≈ 1 > A.
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reductive peak current (ip) for similar complexes 1, 2, and parent A at 10 mM
S0022-328X(17)30525-9
DOI:
10.1016/j.jorganchem.2017.09.014
Reference:
JOM 20089
To appear in:
Journal of Organometallic Chemistry
Received Date: 22 July 2017 Revised Date:
3 September 2017
Accepted Date: 5 September 2017
Please cite this article as: Y.-L. Li, Z.-Y. Ma, J. He, M.-Y. Hu, P.-H. Zhao, Aminophosphine-substituted diiron dithiolate complexes: Synthesis, crystal structure, and electrocatalytic investigation, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.09.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Aminophosphine-substituted diiron dithiolate complexes: Synthesis, crystal structure, and electrocatalytic investigation
1
College of Chemistry and Environmental Engineering, Sichuan University of Science
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& Engineering, Zigong 643000, P. R. China 2
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Yu-Long Li,1,* Zhong-Yi Ma,2 Jiao He,1 Meng-Yuan Hu,2 Pei-Hua Zhao1,2,*
School of Materials Science and Engineering, North University of China, Taiyuan
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030051, P. R. China
* Corresponding author. Email: [email protected] (Y.-L. Li);
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[email protected] (P.-H. Zhao).
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Abstract As
the
active
site
models
of
[FeFe]-hydrogenases,
two
new
aminophosphine-monosubstituted diiron dithiolate compelxes with the general formular Fe2(µ-pdt)(CO)5{PPh2NH(C6H4R-p)} (pdt = SCH2CH2CH2S; R = Br (1), Me (2)), were
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prepared by the oxidative decarbonylation of parent Fe2(µ-pdt)(CO)6 (A) with monophosphines Ph2PNH(C6H4Br-p) or Ph2PNH(C6H4Me-p) in the presence of decarbonylating agent Me3NO·2H2O in MeCN. Interestingly, the ambient temperature treatment of A and aminodiphosphine Ph2PN(C6H4Me-p)PPh2 resulted in the formation
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of the major chelated complex Fe2(µ-pdt)(CO)4{κ2-Ph2PN(C6H4Me-p)PPh2} (3) concomitant with the minor complex 2. All the complexes 1-3 have been well 31
P) spectroscopies, and X-ray
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characterized by elemental analysis, FTIR, NMR (1H,
crystallography. Additionally, the electrochemical and electrocatalytic properties of complexes 1-3 are studied using cyclic voltammetry, where the ratios of the catalytic current (icat) to the reductive peak current (ip) for similar complexes 1, 2, and parent A at
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10 mM HOAc exhibit a ranking of 2 ≈ 1 > A.
Keywords: Diiron dithiolate complexes; aminophosphine substitution; synthesis; crystal
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structure; electrocatalytic investigation.
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1. Introduction The synthetic and bioinorganic chemistry of diiron dithiolate complexes has received great attention over recent decades [1,2], due to the resemblance of their [Fe2S2] clusters to the active site of [FeFe]-hydrogenases that are a family of highly-efficient
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metalloenzymes for the catalytic proton reduction to dihydrogen (H2) [3,4]. The X-ray crystallographic and theoretical studies on [FeFe]-hydrogenases revealed in the followings: (i) their active site (called H-cluster) consists of a butterfly [Fe2S2] catalytic center subsite and a cubic [Fe4S4] electron transfer unit [5-7], which are jonited together
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through a S atom of cysteine; (ii) in H-cluster, the two Fe atoms in the butterfly [Fe2S2] subsite are bridged by a dithiolate cofactor and coordinated by several CO/CN ligands
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[5,6]; (iii) the brigding dithiolate cofactor was supposed to be a propanedithiolate (pdt, SCH2CH2CH2S) [6,8] or favorable azadithiolate (adt, SCH2NHCH2S) [9-11]. The active site structure of [FeFe]-hydrogenases is displayed in Chart 1.
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Chart 1, See pape 20
Inspired by the aforementioned structural information on the active site of [FeFe]-hydrogenases (Chart 1), synthetic chemists designed and prepared a wide range
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of diiron dithiolate complexes as the [FeFe]-hydrogenase models [12-22]. Among the synthesized model complexes, the aminophosphine-substituted diiron dithiolate
AC C
complexes are of particular interest since aminophosphines with hydrophilic and basic sites are deemed to be valuable for the function modes of proton redution to dihydrogen [23-25]. To date, however, there are only a few examples of diiron dithiolate complexes with aminophosphine ligands, such as Fe2(µ-pdt)(CO)4{Ph2PN(R)PPh2} (R = CHMe2, CHCH=CH2, (CH2)2Me, (CH2)3Me, CH2CHMe2, C6H4Me-p, H, (CH2)2NMe2) [26-29] and Fe2(µ-pdt)(CO)5{Ph2PNH(R)} (R = (CH2)2NMe2, C6H4NH2-o, CMe3, Py, C6H4Cl-p, C6H4NO2-p, C6H4CO2Et-p) [27,30-32]. Meanwhile, to our knowledge, the investigation on the electrocatalytic H2-producing abilities of the aminophosphine-monosubstituted diiron dithiolate complexes has not been performed.
3
ACCEPTED MANUSCRIPT In order to further develop the biomimetic chemistry of [FeFe]-hydrogenases and to investigate the electrocatalytic capabilites of diiron dithiolate complexes containing aminophosphine ligands, we launched this work. Herein, we present our results on the synthesis, crystal structures, and electrochemistry of three aminophosphine-subsitituted
RI PT
diiron dithiolate complexes 1-3. Notably, comparison for the electrocatalytic abilites of similar complexes 1, 2, and precursor A is studied using cyclic voltammetry. It should be noted that although complex 3 has been reported before [28], its crystal structrue is obtained for the first time and detailedly investigated since the chelated complex 3 with
SC
asymmetry of the diiron core may be regarded to be a desirable biomimetic model of
2. Experimental 2.1. Materials and Instruments
M AN U
[FeFe]-hydrogenases [33].
TE D
All reactions and operations were carried out under a dry, oxygen-free nitrogen atmosphere with standard Schlenk and vacuum-line techniques. MeCN was distilled from CaH2 under N2. Me3NO·2H2O was commercially available and used as received. (A)
[34],
Ph2PNH(C6H4R-p)
(R
=
Br,
Me)
[35],
and
EP
Fe2(µ-pdt)(CO)6
Ph2PN(C6H4Me-p)PPh2 [36] were prepared according to the literature procedures.
AC C
Preparative TLC was perform on glass plates (25 cm x 20 cm x 0.25 cm) coated with silica gel G (10-40 mm). Column chromatography was perforned on flash neutral Al2O3 (75-150 µm). FTIR spectra were recorded on a Nicolet 670 FTIR spectrometer. NMR (1H, 31P) spectra were obtained on a Bruker Avance 600 MHz spectrometer. Elemental analyses were obtained on a Perkin-Elmer 240C analyzer. 2.2. Synthesis 2.2.1. Synthesis of Fe2(µ-pdt)(CO)5{PPh2NH(C6H4Br-p)} (1) A mixture of Fe2(µ-pdt)(CO)6 (0.193 g, 0.5 mmol), Ph2PNH(C6H4Br-p) (0.214 g, 0.6 mmol), and Me3NO·2H2O (0.056 g, 0.5 mmol) was dissolved in MeCN (15 mL). The 4
ACCEPTED MANUSCRIPT reaction mixture was stirred at ambient temperature until the reaction was completed (ca. 0.5 h) by TLC monitor. After solvent was removed under vacuum, the residue was chromatographed by preparative TLC separation eluting with CH2Cl2/petroleum ether (2:3, v/v). From the main red band, complex 1 was obtained as a red solid. Yield: 43% (0.154 g). Anal. Calcd. (%) for C26H21BrFe2NO5PS2: C, 43.73; H, 2.96; N, 1.96. Found
RI PT
(%): C, 44.05; H, 3.43; N, 1.71. FTIR (KBr disk): νC≡O 2038 (vs), 1986 (vs), 1969 (vs), 1950 (vs), 1933 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.76, 7.44 (s, s, 4H, 6H, 2xPC6H5), 7.09, 6.41 (s, s, 2H, 2H, NC6H4O), 5.49 (br s, 1H, NH), 1.99 (s, 2H,
SC
2xSCHeHa), 1.70-1.53 (m, 4H, 2xSCHeHa and CH2) ppm. 31P NMR (243 MHz, CDCl3, 85% H3PO4): δ 92.95 (s) ppm.
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2.2.2. Synthesis of Fe2(µ-pdt)(CO)5{PPh2NH(C6H4Me-p)} (2)
Complex 2 was prepared by a similar procedure to complex 1, except that Ph2PNH(C6H4Me-p) (0.175 g, 0.6 mmol) was used instead of Ph2PNH(C6H4Br-p) (0.214 g, 0.6 mmol). Complex 2 was obtained as a red solid. Yield: 65% (0.211 g). Anal. Calcd. (%) for C27H24Fe2NO5PS2: C, 49.95; H, 3.73; N, 2.16. Found (%): C, 50.01; H,
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4.19; N, 2.48. FTIR (KBr disk): νC≡O 2030 (vs), 1983 (s), 1965 (vs), 1948 (vs), 1932 (vs). 1H NMR (600 MHz, CDCl3, TMS): δ 7.80, 7.42 (s, s, 4H, 6H, 2xPC6H5), 6.81, 6.45 (s, s, 2H, 2H, NC6H4), 5.37 (d, 2JPH = 16.2 Hz, 1H, NH), 2.16 (s, 3H, CH3), 1.96 (s,
EP
2H, 2xSCHeHa), 1.68-1.58 (m, 4H, 2xSCHeHa and CH2) ppm.
31
P NMR (243 MHz,
CDCl3, 85% H3PO4): δ 91.77 (s) ppm. Synthesis
AC C
2.2.3.
of
Fe2(µ-pdt)(CO)5{PPh2NH(C6H4Me-p)}
(2)
and
Fe2(µ-pdt)(CO)4{κ2-Ph2PN(C6H4Me-p)PPh2} (3) A mixture of Fe2(µ-pdt)(CO)6 (0.193 g, 0.5 mmol), Ph2PN(C6H4Me-p)PPh2 (0.285 g, 0.6 mmol), and Me3NO·2H2O (0.056 g, 0.5 mmol) was dissolved in MeCN (20 mL). The reaction mixture was stirred at ambient temperature until the reaction was completed (ca. 1.5 h) by TLC monitor. After solvent was removed under vacuum, the residue was separated by column chromatography on flash neutral Al2O3 eluting with CH2Cl2/petroleum ether (1:2, v/v). From the first red band, complex 2 was obtained as a red solid (0.073 g, 23% yield), the single crystals of which can be afforded by slow
5
ACCEPTED MANUSCRIPT evaporation of the CH2Cl2/hexane solution in the refrigerator. The experimental procedure was monitored by TLC, indicating that complex 2 shows the same Rf value as the target product obtained in the Experimental section “2.2.2.”. From the second red band, complex 3 was obtained as a dark-red solid (0.132 g, 33% yield). Anal. Calcd. (%) for C38H33Fe2NO4P2S2: C, 56.67; H, 4.13; N, 1.74; Found (%): C, 56.91; H, 4.57; N,
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2.04. FTIR (KBr disk): νC≡O 2010 (vs), 1944 (s), 1928 (vs), 1905 (vs). 1H NMR (600 MHz, CDCl3, TMS): δ 7.82-7.36 (m, 20H, 4xPC6H5), 6.69, 6.45 (s, s, 4H, NC6H4), 2.15 (s, 3H, CH3), 2.08 (s, 2H, 2xSCHeHa), 1.79-1.43 (m, 4H, 2xSCHeHa and CH2) ppm. 31P
SC
NMR (243 MHz, CDCl3, 85% H3PO4): δ 97.38 (s, basal-basal isomer, 75%), 116.95 (s,
2.3. X-ray structure determination
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apical-basal isomer, 25%) ppm.
Single crystals of 1-3 suitable for X-ray diffraction analysis were grown by slow evaporation of the CH2Cl2/hexane solution at 5oC. The crystals were mounted on a Bruker-CCD diffractometer. Data were collected at 150(2) and 123(2) K using a graphite monochromator with Cu-Kα radiation (λ = 1.54178 Å) in the ω-φ scanning
TE D
mode. The structure was solved by direct methods using the SHELXS-97 program [37] and refined by full-matrix least-squares techniques (SHELXL-97) on F2 [38]. Hydrogen atoms were located using the geometric method. The crystallographic parameters, data
AC C
EP
collection, and structure refinement details are summarized in Table 1.
Table 1, See in page 25
2.4. Electrochemical and electrocatalytic experiments Electrochemical and electrocatalytic properties of 1-3, and A were studied by cyclic voltammetry (CV) in MeCN solutions. 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/Ag+
6
ACCEPTED MANUSCRIPT electrode as the reference electrode. The potential scale was calibrated against the Fc/Fc+ couple and reported versus this reference system.
RI PT
3. Results and Discussion 3.1. Synthesis of complexes 1-3
As shown in Scheme 1, two new aminophosphine-monosubstituted complexes 1 and
SC
2 were prepared in moderate yields by the decarbonylating reaction of the parent Fe2(µ-pdt)(CO)6 (A) with the monophosphines Ph2PNH(C6H4R-p) (R = Br and Me)
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using Me3NO·2H2O as CO-removing agent. Most interestingly, we found that the room temperature reaction of A and aminodiphosphine Ph2PN(C6H4Me-p)PPh2 in the presence of 1.1 equivalents of Me3NO·2H2O can afford not only the chelated complex 3 but also by-product 2 in the yields of 33% and 23%, respectively.
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Scheme 1, See page 21
It is worth pointing out that Hogarth and co-workers reported the similar oxidative decarbonylation of A and Ph2PN(C6H4Me-p)PPh2 in MeCN solution to obtain the sole
EP
product 3, during which it shows the initial formation of an intermediate with strong absorption at ca. 2040 cm-1 for a monodentate pentacarbonyl complex as revealed by IR
AC C
spectroscopy [28]. Based on the aforementioned observation and the previously similar cases [32,39], a possible pathway for the formation of 3 together with by-product 2 can be proposed (Scheme 2). As depicted in Scheme 2, reaction of A with one equivalent of decarbonylating agent Me3NO·2H2O in MeCN gives the species B {Fe2(µ-pdt)(CO)5L, L = NCMe or NMe3} [40], followed by the treatment of Ph2PN(C6H4Me-p)PPh2 to afford the species C {Fe2(µ-pdt)(CO)5[PPh2N(C6H4Me-p)PPh2]} [28]. And then, the species C undergoes two pathways: (i) the hydrolysis of C with H2O produces complex 2 and (ii) the intramolecular decarbonylation of C with the free phosphorus atom of
7
ACCEPTED MANUSCRIPT ligand in the presence of Me3NO/MeCN forms the possible intermediate D to afford complex 3. It should be noted that this proposed mechanism need to be further studied.
3.2. Spectroscopic characterization of complexes 1-3
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Scheme 2, See page 22
All the complexes 1-3 are air-stable red solids and were well characterized by IR and NMR spectroscopies as well as elemental analysis. The IR spectra of 1 and 2 show five
SC
strong absorption bands in the ranges of 2038-1932 cm-1 for their carbonyl groups, which are apparently shifted toward lower frequency as compared to their all-carbonyl
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precursor A (2074, 2036, 1995 cm-1) [41]. This is probably because that the electron-donating abilities of the aminophosphines Ph2PNH(C6H4R-p) (R = Br and Me) are stronger than the carbonyl ligands [32]. Meanwhile, the highest carbonyl absorption band of 1 is shifted by 8 cm-1 to lower energy relative to that of 2. This is possibly due to the fact that the Br substituent of ligand Ph2PNH(C6H4Br-p) in 1 is more
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electron-withdrawing than the Me substituent of ligand Ph2PNH(C6H4Me-p) in 2, which leads to the weaker electron density on the Fe atoms of 1 with respect to that of 2 owing to the effect of back-donating π-bonding from Fe to terminal CO [30-32]. In addition, in
EP
the IR spectrum of 3, the carbonyl absorption bands in the region of 2010-1905 cm-1 are further shifted toward much lower energy as compared to those of 1 and 2. This result
AC C
strongly indicates that the iron-carbonyl groups of 3 is further substituted by the phosphorus atom of aminophosphine in comparison to those of 2. The 1H NMR spectra of 1-3 display two kinds of the similar proton peaks in the
downfield regions of 7.8-7.4 and 7.0-6.4 ppm for the monosubstituted phenyl moieties (P(C6H5)2) and the disubstituted one (NC6H4), respectively. Furthermore, in the 1H NMR spectra of 2 and 3, there is a sharp singlet at ca. 2.16 ppm assigned to their methyl groups. Most importantly, the 1H NMR spectra of 1 and 2 show an addtional broad singlet at 5.49 ppm and doublet at 5.37 ppm with a coupling contact 2JPH = 16.2 Hz for
8
ACCEPTED MANUSCRIPT their NH groups [33]. These observations well demonstrate that the structures of 1 and 2 are apparatently different from that of 3. The
31
P NMR spectra of 1 and 2 give only a singlet at 92.95 and 91.77 ppm
resepctively for the phosphorus atom coordinated to one of iron centres, which is well in
RI PT
accordance with the phosphorus signals in the range of 90-100 ppm observed in the known analogues Fe2(µ-pdt)(CO)5{Ph2PNH(R)} (R = (CH2)2NMe2, C6H4NH2-o, CMe3, Py, C6H4Cl-p, C6H4NO2-p, C6H4CO2Et-p) [27,30-32]. However, in the
31
P NMR
spectrum of 3, two sharp singlets at 97.38 and 116.95 ppm are respectively ascribed to
SC
the basal-basal isomer and apical-basal one in ca. 3:1 ratio (as measured by integration of the correspoding phosphorus signals). This assignment is according to the fact that
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the 31P NMR spectra usually consist of a large singlet in the region of 97-101 ppm and a small singlet in the range of 113-118 ppm for the dibasal and apical-basal isomers in the different ratios, as observed in the reported aminodiphosphine-chelated complexes Fe2(µ-pdt)(CO)4{κ2-Ph2PN(R)PPh2} (R = CHMe2, CHCH=CH2, CH2CHMe2, C6H4Me, and (CH2)2NMe2) [26,28,29].
TE D
3.3. X-ray crystal structure of complexes 1-3 The molecular structures of 1-3 are definitely confirmed by X-ray crystallography,
AC C
Table 2.
EP
which are illustrated in Figs. 1-3. The selected bond distances and angles are listed in
Figs. 1-3, See pages 29-31 Table 2. See page 26
As depicted in Figs. 1-3, the monosubstituted complexes 1 and 2 contain a typical butterfly [Fe2S2] cluster with five carbonyls plus one monodentate aminophosphine, whereas the chelated complex 3 features a well-known butterfly [Fe2S2] skeleton bearing four carbonyls as well as one chelated aminodiphosphine. Furthermore, in 1 and 2, the monophosphines (Ph2PNH(C6H4R); R = Br, Me) lie in the apical position of the square-pyramidal coordination geometry of one of two iron atoms, which match well
9
ACCEPTED MANUSCRIPT with those of the known PR3-monosubstituted diiron propanedithiolate complexes [42-46]. For 3, the diphosphine (Ph2PNH(C6H4Me)PPh2) spans the basal-basal site coordinated to one Fe1 atom, which enables the two phosphorus atoms to be equivalent as confirmed by the existence of a large singlet at 116.95 ppm in the 31P spectrum of 3
RI PT
in CDCl3 solution. The Fe1-Fe2 bond distances of 1 (2.5000(7) Å) for and 2 (2.4954(9) Å for) are somewhat different relative to precursor A (2.5103(11) Å) [34] but considerably shorter than those found in the natural [FeFe]-hydrogenases (2.55-2.60 Å) [5,6]. The Fe-P bond
SC
distances (2.2089(10) Å for 1 and 2.2055 (12) Å for 2) are closer to that of one analogue Fe2(µ-pdt)(CO)5{Ph2PNH(C6H4Cl-p)} (2.2038(6) Å) [32] but slightly different from
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those observed in the other analogues Fe2(µ-pdt)(CO)5{Ph2PNH(R)} (R = (CH2)2NMe2, 2.2160(13) Å [30]; C6H4NH2-o, 2.2139(14) Å [30]; CMe3, 2.2348(8) Å [27]; Py, 2.220(2) Å [31]). Furthermore, the average Fe-CCO bond distances (1.781 Å for 1 and 1.779 Å for 2) in Fe(CO)2P units are slightly shorter than the corresponding distances (1.792 Å for 1 and 1.794 Å for 2) in Fe(CO)3 moieties, possibly because that the higher
TE D
electron density of the coordinated-Fe core leads to the stronger electron back-donation from Fe atom to CO ligands relative to that of the uncoordinated-Fe core [47,48]. In addition, the C-S-Fe angles in one boat-conformational six-membered ring of the propanedithiolate unit (C24-S2-Fe2/C26-S1-Fe2 in 1 and C22-S2-Fe1/C20-S1-Fe1 in 2)
EP
are 3.0-5.5o larger than the corresponding angles in the other chair-conformational six-membered ring (C24-S2-Fe1/C26-S1-Fe1 in 1 and C22-S2-Fe2/C20-S1-Fe2 in 2).
AC C
The Papical-Fe-Fe angles (P1-Fe2-Fe1 in 1 and P1-Fe1-Fe2 in 2) are 5.67o and 9.71o wider than the Capical-Fe-Fe ones (C19-Fe1-Fe2 in 1 and C27-Fe2-Fe1 in 2), respectively. Thus, the differences in the aforementioned angles demonstrate that the six-membered iron dithiacyclohexane ring (FeS2C3) of the propanedithiolate groups is pushed away from the apical position of the monophosphine ligands and towards the Fe(CO)3 site in the solid state of 1 and 2. This outcome implies that the steric repulsion between the apical-monophosphine ligand and the bridgehead structural unit is strong, since the monophosphines (Ph2PNH(C6H4R); R = Br, Me) in 1 and 2 are cis to their bridgehead carbon atoms (C25 for 1 and C21 for 2) as shown in Figs. 1 and 2.
10
ACCEPTED MANUSCRIPT The Fe1-Fe2 bond distance (2.5877(13) Å) in dibasal complex 3 is slightly shorter than its analogues Fe2(µ-pdt)(CO)4{κ2-Ph2PN(R)PPh2} (R = CHMe2, 2.6236(5) Å [26,28];
CHCH=CH2, 2.6042(4) Å [26,28]; (CH2)2NMe2), 2.6010(7) Å [29]) but very
similar to those observed in the natural [FeFe]-hydrogenases (2.55-2.60 Å) [5,6]. This
RI PT
result indicate that the steric and electronic character of the iron centers in 3 might be much close to those of [FeFe]-hydrogenases relative to the other dibasal complexes with aminodiphosphines [26-29]. Another notable feature is that the very small bite angle of P1-Fe1-P2 (71.90(6)o) in 3 is characteristic of the dibasal-chelated complexes with the P-X-P
bite-angle bis(diphenylphosphino) ligands
such
SC
small
as
dppm
and
aminodiphosphines, for example, Fe2(µ-pdt)(CO)4{κ2-dppm} (74.55(4)o) [26,28] and CHCH=CH2,
M AN U
Fe2(µ-pdt)(CO)4{κ2-Ph2PN(R)PPh2} (R = CHMe2, 71.32(2)o [26,28];
71.68(2)o [26,28]; (CH2)2NMe2), 71.48(3)o [29]). Moreover, the very small P1-Fe1-P2 bite angle leads to the quite short P1-P2 distance (2.5779(18) Å) in 3, which is probably due to the significant strain in the aminodiphosphine ligand coordinated to the one iron center in the basal-basal manner [28].
TE D
3.4. Electrochemical and electrocatalytic studies of 1-3 In order to evaluate the ability to catalyze proton reduction to dihydrogen, cyclic voltammetry (CV) studies of 1 and 2 were performed in MeCN solution at a scan rate of
EP
100 mV s-1 as displayed in Fig. 4. The relevant electrochemcal and electrocatalytic data are listed in Table 3. Complexes 1 and 2 exhibit an irreversible reduction peak at -1.80
AC C
and -1.82 V, respectively, assigned to the [FeIFeI] + e- → [FeIFe0] process [30,41], whereas the second reduction peaks may be beyond the electrochemical solvent window of MeCN. It should be noted that the observed reduction peaks of 1 and 2 are shifted towards more negative potentials by 150 and 160 mV respectively, relative to that of their parent complex A (Table 3). This is probably because that the electron-donationg capabilities of the aminophosphines (Ph2PNH(C6H4R); R = Br, Me) in 1 and 2 are obviously stronger that the carbonyls [32], as revealed by their IR spectra. Meanwhile, the reduction peak of 2 is more negative by 20 mV than that of 1, indicating that the iron core of 2 is slightly harder to reduce owing to the better donor character of ligand
11
ACCEPTED MANUSCRIPT Ph2PNH(C6H4Me-p) in 2 than Ph2PNH(C6H4Br-p) in 1 [30]. In addition, the CV investigation of 3 is also carried out in the same electrochemical condition (as illustrated in Fig. 2S in Supporting Information). Complex 3 shows an observed reduction peak at -2.21 V [28], which is shifted by ca. 410 mV towards more negative potential as
RI PT
compared to those of 1 and 2. This might reveal that the iron cores in the disubstituted complex 3 are not much easier to reduce with respect to those of the monosubstituted
the latter as indicated by their IR spectra.
Fig. 4, See page 32
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Table 3. See page 27
SC
complexes 1 and 2, because of the stronger electron density of the former than those of
The electrochemical behaviors for the proton reduction to H2 catalyzed by 1-3 were studies on the sequential addition of HOAc (0-10 mM) in MeCN solution. As shown in Fig. 4, the initial reduction peak currents of 1 and 2 increase slightly when HOAc was
TE D
added, but they do not grow steadily with the sequential addition of HOAc. However, a new reduction peak for 1 and 2 at ca. -2.40 V appears and the corresponding peak currents grow dramatically with the sequential addition of HOAc, wherein the current intensities (icat) of the new peaks have a linear correlation relative to the concentration
EP
([HOAc]) of the added acid (insert in Fig. 4). Similarly, in the case of 3, the initial reduction peak currents at ca. -2.21 V increase linearly with sequential acid addition
AC C
(Fig. 2S). Consequently, these observations suggest that a typically catalytic process takes place [29,49-51]. That is, complexes 1-3 are active for the electrocatalytic H2 evolution in the presence of HOAc. According
to
the
above-mentioned
electrochemical
results
and
the
previously-reported similar cases [49-51], we might propose the EECC or ECEC (E = electrochemical; C = chemical) mechanisms for H2 production catalyzed by the aminophosphine-monosubstituted complexes 1 and 2. The two proposed catalytic
mechanisms of 1 as representative complex are illustrated in Scheme 3. One EECC process is described as follows: complex 1 is firstly reduced at -1.80 V to give
12
ACCEPTED MANUSCRIPT monoanion 1-. Then 1- is further reduced at -2.40 V to form dianion 12-, which can be protonated by weak acid HOAc to form the protonated species 1H-. Finally, the species 1H- receives proton to produce molecular H2 and regenerate 1 to thereby complete the catalytic cycle. The other ECEC process is stated in the following: the first step is also
RI PT
to produce monoanion 1- via the reduction of 1 at -1.80 V. However, due to the increase of HOAc concentration, the hydride-containing species 1H may be generated through the protonation of 1-. Afterwards, the species 1H accepts electron to reduce at -2.19 V,
SC
followed by the treatment with proton to finish the catalytic cycle.
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Scheme 3, See Page 23
Especially, to evaluate the electrocatalytic abilities for H2 production catalyzed by similar complexes 1 and 2, we use the ratio of the catalytic current (icat) to the reductive peak current (ip) in the absence of the added acid as a marker and compare the electrocatalytic abilities of 1, 2, and parent A, where the higher value of icat/ip indicates
TE D
the faster catalysis [29,50,51]. It can be seen from Table 3 that the icat/ip values of 1, 2, and A upon 10 mM HOAc exhibit a ranking of 2 (9.38) ≈ 1 (9.28) > A (7.33), in which the icat/ip values of 1 and 2 are approximately equal but apparently larger than that of A. Such results indicate that 1 and 2 are a kind of the better electrocatalyst in the presence
EP
of HOAc relative to the all-carbonyl complex A, indicative of the considerable influence of aminophosphine ligands on the redox properties of the iron cores in the
AC C
ligand-substituted diiron dithiolate complexes [41].
4. Conclusions
In this work, we found that the aminophosphine-monosubstituted complexes 1 and 2 can be easily prepared by oxidative decarbonylating reaction of precursor A and monophosphines Ph2PNH(C6H4R-p) (R = Br and Me). Interestingly, the chelated complex 3 together with by-product 2 can be simultaneously obtained from the room
13
ACCEPTED MANUSCRIPT temperature treatment of A with aminodiphosphine Ph2PN(C6H4Me-p)PPh2 in the presence of CO-removing agent Me3NO·2H2O. Meanwhile, complexes 1-3 are well characterized by elemental analysis and various spectroscopies. X-ray crystallographic studies demonstrate that the Ph2PNH(C6H4R-p) ligands in 1 and 2 occupy the apical site
RI PT
in the square-pyramidal geometry of one iron atom, whereas the Ph2PN(C6H4Me-p)PPh2 ligand in 3 adopts the expected dibasal geometric arrangement coordiated to one iron atom. Furthermore, X-ray crystallographic study of 3 displays that the Fe-Fe distance (2.5877(13) Å) is very similar to those observed in the natural [FeFe]-hydrogenases
SC
(2.55-2.60 Å). This possibly implies that the steric and electronic character of the iron centers in 3 bearing the chelated aminodiphosphine might be much close to those found
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in [FeFe]-hydrogenases, which match well with the theoretical conclusion that “asymmetric substitution of strong donor ligands is the most viable method of making better synthetic diiron complexes that will serve as both structural and functional models of the active site of iron-only hydrogenase” as suggested by Tye, Darensbourg, and Hall [33].
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In addition, the electrochemical studies of 1-3 show that the reduction peak of 3 is shifted towards more negative potentials relative to those of 1 and 2, demonstrating that the chelated complex 3 is difficult to reduce but should be easily protonated to produce H2 in comparison to the monosubstituted complexes 1 and 2. Notably, the
EP
electrocatalytic investigations of 1, 2, and A upon 10 mM HOAc indicate that the icat/ip values of 1 and 2 are much larger than that of A. This result reveals that the introduction
AC C
of the aminophosphine ligands can considerably enhance the electrocatalytic capabilities of 1 and 2 for proton reduction to H2.
Supplementary materials CCDC 1563379-1563381 (1-3) contain the supplementary crystallographic data for this
paper.
These
data
can
be
obtained
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
14
or
free from
of the
charge
via
Cambridge
ACCEPTED MANUSCRIPT Crystallographic Data Centre, 12 Union Road Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgements
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We are grateful to the National Natural Science Foundation of China (No. 21301160, 21501124), the Natural Science Foundation of Shanxi Province (No. 201701D121035), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of
References
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[1] L.C. Song, Acc. Chem. Res. 38 (2005) 21–28.
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[5] J.W. Peters, W.N. Lanzilotta, B.J. Lemon, L.C. Seefeldt, Science 282 (1998)
[6] Y. Nicolet, C. Piras, P. Legrand, E.C. Hatchikian, J.C. Fontecilla-Camps, Structure 7 (1999) 13–23.
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[22] X. Yu, C.-H. Tung, W.G. Wang, M.T. Huynh, D.L. Gray, S. Hammes-Schiffer, T.B. Rauchfuss, Organometallics 36 (2017) 2245–2253. [23] R.M. Henry, R.K. Shoemaker, R.H. Newell, G.M. Jacobsen, M.R. DuBios, D.L. DuBios, Organometallics 24 (2005) 2481–2491.
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[51] R.X. Li, X.F. Liu, T. Liu, Y.B. Yin, Y. Zhou, S.K. Mei, J. Yan, Electrochim. Acta
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Scheme captions Chart 1 The proposed active site of [FeFe]-hydrogenase Scheme 1 Synthesis of complexes 1-3. Scheme 2 Possible pathway for the formation of 2 and 3.
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Scheme 3 The proposed EECC or ECEC catalytic mechanisms for H2 production catalyzed by the representative complex 1 in the presence of HOAc (note that the pdt,
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CO, and Ph2PNH(C6H4Br-p) groups are omitted for clarity).
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Chart 1
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Scheme 1
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C6H4Me-p S
OC
Fe OC
Fe
A
Me3NO(2H2O), MeCN OC -CO2, -H2O
CO
OC
CO
Fe
OC
CO
S
S
L Fe
Ph2PN(C6H4Me-p)PPh2 OC -L
OC
CO
CO
Fe
OC
B
S
S
Ph2 N P Fe
C
PPh2
CO
CO
H2O hydrolysis
2
C6H4Me-p
OC
S
S
OC
Ph2 N P
PPh2
-L
3
Fe
Fe
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Me3NO, MeCN decarbonylation
L
OC
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(L = NCMe, NMe3)
CO
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Scheme 2
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OC
S
22
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Scheme 3
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Table captions Table 1 Crystal data and structural refinements details for 1-3. Table 2 Selected bond distances (Å) and angles (°) for 1-3.
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Table 3 The relevant electrochemical and electrocatalytic data for 1, 2, and A.[a]
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2
3•2CH2Cl2
Empirical formula
C26H21BrFe2NO5PS2
C27H24Fe2NO5PS2
C40H37Cl4Fe2NO4P2S2
Formula weight
714.15
649.28
975.29
Temperature (K)
123(2)
150(2)
150(2)
Wavelength (Å)
1.54178
1.54178
1.54178
Crystal system
monoclinic
orthorhombic
triclinic
Space group
P21/c
Pbca
a (Å)
9.2296(4)
11.7424(4)
b (Å)
27.2635(12)
16.8134(3)
c (Å)
12.0427(6)
28.3519(7)
α (°)
90
90
β (°)
112.339(6)
90
90
90
2802.9(3)
5597.5(3)
2106.13(16)
8
2
1.541
1.538
10.570
9.845
2656.0
996.0
V (Å ) Z
4 -3
Dcalc (g cm )
1.692
-1
11.3408(6)
11.9941(5)
16.5349(5) 90.635(3)
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P-1
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γ (°)
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Complex
106.722(4) 101.381(4)
12.225
F(000)
1432.0
Crystal size (mm)
0.21 × 0.20 × 0.19
0.19 × 0.18 × 0.17
0.19 × 0.17 × 0.16
θmin, θmax (°)
6.484, 132.004
9.702, 132.002
7.538, 132.002
Reflections collected/unique
11128/4867
11093/4841
11845
0.0478
0.0681
0.0856
-6 ≤ h ≤ 10
-12 ≤ h ≤ 13
-13 ≤ h ≤ 13
-32 ≤ k ≤ 28
-18 ≤ k ≤ 19
-14 ≤ k ≤ 13
-14 ≤ l ≤ 13
-32 ≤ l ≤ 33
-19 ≤ l ≤ 16
99.9
99.6
99.2
4867/0/347
4841/0/344
7300/6/497
1.005
0.991
1.019
R1/wR2 [I > 2σ(I)]
0.0460/0.1147
0.0571/0.1380
0.0945/0.2458
R1/wR2 (all data)
0.0511/0.1193
0.0742/0.1503
0.1098/0.2726
Largest difference peak/ hole (e A-3)
1.15/-0.74
1.02/-0.78
1.79/-1.59
Rint
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µ (mm )
Completeness to θmax (%)
Data/restraints/parameters
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2
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Complex 3 Fe(1)-Fe(2) Fe(2)-C(33) Fe(2)-C(34) Fe(2)-C(32) P(1)-Fe(1)-Fe(2) P(2)-Fe(1)-Fe(2) S(1)-Fe(1)-S(2) S(1)-Fe(2)-S(2)
2.5877(13) 1.778(7) 1.798(8) 1.798(7) 115.04(6) 106.88(6) 86.54(6) 84.32(6)
Fe(2)-C(22) Fe(2)-C(23) C(19)-Fe(1)-Fe(2) C(26)-S(1)-Fe(1) C(26)-S(1)-Fe(2) Fe(1)-S(1)-Fe(2) Fe(1)-S(2)-Fe(2)
1.776(4) 1.786(4) 150.12(14) 109.94(14) 115.27(17) 66.83(3) 67.07(3)
Fe(1)-P(1) N(1)-P(1)
2.2055(12) 1.695(4)
Fe(1)-C(23) Fe(1)-C(24) C(27)-Fe(2)-Fe(1) C(20)-S(1)-Fe(1) C(20)-S(1)-Fe(2) Fe(1)-S(2)-Fe(2) Fe(1)-S(1)-Fe(2)
1.778(5) 1.779(5) 146.91(18) 114.84(17) 109.50(18) 67.07(4) 66.88(4)
Fe(1)-P(1) Fe(1)-P(2) P(1)-P(2) Fe(1)-C(38) P(1)-Fe(1)-P(2) P(1)-N(1)-P(2) Fe(1)-S(1)-Fe(2) Fe(1)-S(2)-Fe(2)
2.1952(17) 2.1956(17) 2.5779(18) 1.744(7) 71.90(6) 95.8(3) 70.45(5) 70.34(5)
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2.4954(9) 1.786(5) 1.788(5) 1.808(6) 156.62(5) 114.92(18) 111.44(19) 85.23(5) 84.79(4)
2.2089(10) 1.690(3)
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Complex 2 Fe(1)-Fe(2) Fe(2)-C(25) Fe(2)-C(26) Fe(2)-C(27) P(1)-Fe(1)-Fe(2) C(22)-S(2)-Fe(1) C(22)-S(2)-Fe(2) S(1)-Fe(1)-S(2) S(1)-Fe(2)-S(2)
Fe(2)-P(1) N(1)-P(1)
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2.5000(7) 1.783(4) 1.794(4) 1.799(4) 155.79(4) 111.55(15) 115.29(13) 84.19(3) 84.83(4)
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Complex 1 Fe(1)-Fe(2) Fe(1)-C(20) Fe(1)-C(21) Fe(1)-C(19) P(1)-Fe(2)-Fe(1) C(24)-S(2)-Fe(1) C(24)-S(2)-Fe(2) S(1)-Fe(1)-S(2) S(1)-Fe(2)-S(2)
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Epc (V)
ip (µA)
icat (µA)[b]
icat/ip
1
-1.80
27.03
250.97
9.28
-1.82
20.53
192.60
9.38
-1.65
38.41
281.69
7.33
2 A
[c]
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[a] All the data for 1, 2 and A (1 mM) are versus the Fc/Fc+ couple in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV s-1. [b] icat is the catalytic current at the highest concentration of HOAc (10 mM).
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[c] Cyclic voltammogram of A (1 mM) is shown in Figure S1 (seen in Supporting Information).
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Figure captions Fig. 1 Molecular structure of 1 (hydrogens are omitted for clarity). Fig. 2 Molecular structure of 2 (hydrogens are omitted for clarity). Fig. 3 Molecular structure of 3 (Solvent and hydrogens are omitted for clarity).
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Fig. 4. Cyclic voltammogram of 1 and 2 (1.0 mM) with HOAc (0, 2, 4, 6, 8, 10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV s-1, 1 for (a) and 2 for (b). Insert:
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Plots of icat (mA) versus HOAc concentration (mM) for 1 and 2.
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Highlights
Three aminophosphine-substituted diiron dithiolate compelxes 1-3 were prepared by the oxidative decarbonylation.
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All the complexes have been well characterized by elemental analysis, FTIR, NMR (1H, 31P) spectroscopies, and X-ray crystallography.
The electrochemical and electrocatalytic properties of complexes 1-3 are studied using cyclic voltammetry, where the ratios of the catalytic current (icat) to the
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HOAc exhibit a ranking of 2 ≈ 1 > A.
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reductive peak current (ip) for similar complexes 1, 2, and parent A at 10 mM