Influence of Dithiolate Bridges on the Structures and Electrocatalytic

5 days ago - Influence of Dithiolate Bridges on the Structures and Electrocatalytic Performance of Small Bite-Angle PNP-Chelated Diiron Complexes ...
0 downloads 0 Views 2MB Size
Download PDF
Article Cite This: Organometallics XXXX, XXX, XXX−XXX

pubs.acs.org/Organometallics

Influence of Dithiolate Bridges on the Structures and Electrocatalytic Performance of Small Bite-Angle PNP-Chelated Diiron Complexes Fe2(μ-xdt)(CO)4{κ2‑(Ph2P)2NR} Related to [FeFe]Hydrogenases Pei-Hua Zhao,*,† Meng-Yuan Hu,† Jian-Rong Li,† Zhong-Yi Ma,† Yan-Zhong Wang,† Jiao He,‡ Yu-Long Li,*,‡ and Xu-Feng Liu§ Organometallics Downloaded from pubs.acs.org by UNIV OF BRITISH COLUMBIA on 01/03/19. For personal use only.

†

School of Materials Science and Engineering, North University of China, Taiyuan, Shanxi 030051, P. R. China College of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong, Sichuan 643000, P. R. China § School of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo, Zhejiang 315211, P. R. China ‡

S Supporting Information *

ABSTRACT: As a further exploration of the asymmetrically substituted diiron models for the active site of [FeFe]hydrogenases, two new types of small bite-angle aminodiphosphine [(Ph2P)2NR; denoted as PNP in this study]chelated diiron N-phenyl-aza- and ethanedithioate complexes Fe2(μ-xdt)(CO)4{κ2-(Ph2P)2NR} (1a−1e) and (2a−2e), respectively, were successfully synthesized by the carbonyl substitution reactions of all-carbonyl diiron complexes Fe2(μxdt)(CO)6 (xdt = SCH2N(Ph)CH2S (adtNPh) and SCH2CH2S (edt)) with PNP (PNP = (Ph2P)2NR, R = CMe3, CH2CHMe2, (CH2)3Me, (CH2)3Si(OEt)3, and (CH2)3NMe2) in the presence of Me3NO·2H2O or UV irradiation. All the new complexes obtained above have been well characterized by elemental analysis, FT-IR, NMR spectroscopy, and particularly for 1a, 1b, 2a, and 2d by single-crystal X-ray diffraction analysis. By comparison, 31P{1H} NMR and X-ray crystallographic studies have clearly revealed that the change of the dithiolate bridge from adtNPh to edt has a significant influence on the coordination geometry of the chelating PNP ligands in Fe2S2 complexes, in which the basal−basal configuration in the adtNPh complexes 1a−1e is favorable whereas the apical−basal conformation in the edt complexes 2a−2e is main. In addition, the electrochemical properties of complexes 1b and 2b as a pair of representative counterparts are evaluated and compared by cyclic voltammetry in the absence and presence of HOAc as a proton source, indicating that they are found to be electrocatalytically active.

■

INTRODUCTION The discovery that [FeFe]-hydrogenases are a class of natural metalloenzymes for efficiently catalyzing the reversible conversion of proton to hydrogen (H2) has intensively inspired the biomimetic chemistry of diiron dithiolate complexes of the type [Fe2(μ-dithiolate)(CO)6−n(L)n].1,2 At the end of the last century, the structures of [FeFe]hydrogenases isolated from Clostridium pasteurianum and Desulfovibrio desulf uricans were determined by the highresolution single-crystal X-ray diffraction analysis.3,4 Further spectroscopic and theoretical investigations reveal that the active site of [FeFe]-hydrogenases (so-called H-cluster) consists of a butterfly shaped [Fe2S2] cluster (i.e., diiron subunit) and a cubane-like [Fe4S4] cluster that are attached together by a cysteine S atom (Figure 1).2−5 Meanwhile, the two Fe atoms in the diiron subunit are connected by a © XXXX American Chemical Society

Figure 1. Proposed structure of the active site of [FeFe]-hydrogenases.

dithiolate bridge and several diatomic CO/CN− ligands, in which the dithiolate cofactor has been definitively elucidated to be an azadithiolate (adtNH = SCH2N(H)CH2S) as a wellReceived: October 18, 2018

A

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Synthesis of the Target Complexes 1a−1e and 2a−2e in the Presence of Me3NO·2H2O

Scheme 2. Synthesis of the Target Complexes 1a−1e and 2a−2e under UV Irradiation (365 nm)

Inspired by these new findings and in continuation of our project on asymmetrically PNP-disubstituted diiron models related to [FeFe]-hydrogenases,33,34 we have fortunately prepared a new type of small bite-angle PNP-chelated diiron model complex Fe2(μ-adtNPh)(CO)4{κ2-(Ph2P)2NR} (1a−1e) with an adtNPh bridge (adtNPh = SCH2N(Ph)CH2S) in this work, which is structurally closer to the diiron subunit of [FeFe]-hydrogenase active site bearing an adtNH cofactor. To further assess the influence of the azadithiolate bridge on the structures and electrochemical properties of diiron model complexes, another new type of the corresponding PNPchelated diiron models Fe2(μ-edt)(CO)4{κ2-(Ph2P)2NR} (2a−2e) with an edt bridge (edt = SCH2CH2S) as reference complex is obtained. Herein, we report the facile synthesis, structural characterization, crystal structures, and electrochemical properties of two new types of asymmetrically PNP-chelated diiron N-phenyl-aza- and ethanedithioate complexes Fe2(μ-xdt)(CO)4{κ2-(Ph2P)2NR} (x = adtNPh and edt; R = CMe3, CH2CHMe2, (CH2)3Me, (CH2)3Si(OEt)3, and (CH2)3NMe2) as the active site models of [FeFe]-hydrogenases for comparison.

positioned base for relaying protons to and from the Fe core.6−8 It is especially proposed that the efficient catalysis occurs at a single coordination site on one Fe atom of the diiron subunit with the “rotated” geometry,9,10 which features a semibridging carbonyl and a vacant site around one Fe core (Figure 1). As a result of these studies mentioned above, a great number of diiron dithiolate complexes as the diiron subsite models of [FeFe]-hydrogenases were obtained over the past decade.11,12 Notably, density functional theory (DFT) calculations by Tye et al. suggest that asymmetric substitution of diiron dithiolate complexes might favor the formation of a “rotated” geometry in diiron model complexes similar to that found in the active reduced or oxidated states of [FeFe]-hydrogenases.13 Under the guidance of this prediction, many efforts have been always focused on the design and synthesis of unsymmetrically substituted diiron model complexes of the type [Fe2(μdithiolate)(CO)4(κ2-diphosphine)] with chelating diphosphines.14−27 Among these model complexes, a promising generation of asymmetrically chelated diiron dithiolate complexes with small bite-angle aminodiphosphines [(Ph2P)2NR; denoted as PNP in this study] has drawn remarkable attention recently. This is mainly based on the following fact that (i) ligands PNP with small P−N−P bite angle can be more readily prepared by a simple one-step reaction in contrast to the flexible backbone diphosphines such as dppv (Ph2PCH = CHPPh2), dppe (Ph2PCH2CH2PPh2), dmpe (Me2PCH2CH2PMe2), etc.28 and (ii) the pendant base of PNP is thought to be valuable for the functional models that can efficiently catalyze the reduction of proton to H2,29 and thus (iii) the PNP-substituted diiron complexes are supposed to an ideal alternative for the asymmetrically substituted diiron models of [FeFe]-hydrogenases. In this context, only a kind of small bite-angle PNP-chelated diiron model complex Fe2(μpdt)(CO) 4 {κ 2 -(Ph 2 P) 2 NR} with a pdt bridge (pdt = SCH2CH2CH2S) has been reported to date.31,30,32−34 Very recently, the electrochemical study demonstrated that the PNP-chelated diiron pdt complexes have a superior electrocatalytic ability for H2 production in contrast to their PNPbridged isomers.34

■

RESULTS AND DISCUSSION

Synthesis and Characterization of Complexes 1a−1e and 2a−2e. For comparison, two new types of the target model complexes Fe2(μ-xdt)(CO)4{κ2-(Ph2P)2NR} (1a−1e and 2a−2e) were prepared in satisfactory yields by the traditional oxidative decarbonylation reactions of all-carbonyl diiron complexes Fe2(μ-xdt)(CO)6 (xdt = adtNPh and edt) with small bite-angle PNP ligands ((Ph2P)2NR, R = CMe3, CH2CHMe2, (CH2)3Me, (CH2)3Si(OEt)3, and (CH2)3NMe2) in the presence of Me3NO·2H2O as COremoving agent, as shown in Scheme 1. Meanwhile, to further explore a new synthetic strategy to prepare the two types of model complexes described above, these new series of 1a−1e and 2a−2e can be smoothly obtained in moderate yields by the efficient UV irradiation treatments of precursors Fe2(μ-xdt)(CO)6 with different PNP ligands used above using an LED lamp emitting at 365 nm, as displayed in Scheme 2. B

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Table 1. Comparisons of IR (νC≡O) and 31P{1H} NMR Data of Complexes 1a−1e and 2a−2e complex

IR for νC≡O (cm−1)

Fe2(μ-adt )(CO)4{κ -(Ph2P)2N(CMe3)} (1a) Fe2(μ-adtNPh)(CO)4{κ2-(Ph2P)2N(CH2CHMe2)} (1b) Fe2(μ-adtNPh)(CO)4{κ2-(Ph2P)2N(CH2)3Me} (1c) Fe2(μ-adtNPh)(CO)4{κ2-(Ph2P)2N(CH2)3Si(OEt)3} (1d) Fe2(μ-adtNPh)(CO)4{κ2-(Ph2P)2N(CH2)3NMe2} (1e) Fe2(μ-edt)(CO)4{κ2-(Ph2P)2N(CMe3)} (2a) Fe2(μ-edt)(CO)4{κ2-(Ph2P)2N(CH2CHMe2)} (2b) Fe2(μ-edt)(CO)4{κ2-(Ph2P)2N(CH2)3Me} (2c) Fe2(μ-edt)(CO)4{κ2-(Ph2P)2N(CH2)3Si(OEt)3} (2d) Fe2(μ-edt)(CO)4{κ2-(Ph2P)2N(CH2)3NMe2} (2e)

2018/1948/1884 2019/1954/1914 2019/1954/1910 2018/1954/1902 2018/1953/1907 2017/1964/1941/1919 2016/1946/1929/1908 2016/1959/1934/1906 2016/1946/1910 2012/1945/1933/1899

NPh

2

31

P{1H} NMR (δ/ppm, isomer ratio) 119.92/104.47 (3:7) 111.24/100.76 (1:1) 110.81/97.03 (1:1) 111.28/97.47 (1:3) 111.27/97.89 (3:7) 119.55/109.78 (10:1) 114.05/106.27 (19:1) 113.21/103.31 (10:1) 112.86/103.01 (10:1) 113.69/103.91 (10:1)

Figure 2. Molecular structures of the adtNPh complexes 1a (left) and 1b (right) with thermal ellipsoids at 30% probability. Hydrogen atoms and solvent molecules have been omitted for clarity.

attributed to the basal−basal isomer (Table 1), which is in good accordance with those reported in the PNP-chelated diiron pdt complexes Fe2(μ-pdt)(CO)4{κ2(Ph2P)2NR}.31,30,32−34 In general, we found that the 31P{1H} NMR chemical shifts for the small bite-angle PNP ligands in the Fe2S2 complexes are related to stereochemistry: the P signal at δ > 110 ppm for PNP ligands bound in the apical− basal geometry, whereas the P signal at δ < 110 ppm for PNP ligands bound in the basal−basal fashion. Furthermore, as measured by integration of phosphorus peaks in the 31P{1H} NMR spectra, the ratios of apical−basal versus basal−basal isomers in the adtNPh complexes 1a−1e are in the range of 1:1−1:3, which is apparently lower than those in the edt complexes 2a−2e (ratios are from 10:1 to 19:1). Thus, the ratios of the two isomers in solution exhibit a conversion from basal−basal conformation to apical−basal configuration as the dithiolate bridge is changed from adtNPh to edt. This well indicates that the variation of the dithiolate bridge may induce the ratios of the apical−basal and basal−basal isomers in solution formed by the steric and electronic nature of the dithiolate bridge in the Fe2S2 species, as predicted by the IR spectroscopy mentioned above. At the same time, the 1H NMR spectra of 1a−1e and 2a−2e all exhibit one or two groups of the typical aromatic proton signals in the downfield region of δ 8.0−7.4 ppm assigned to their phenyls in PNP ligands. For 1a−1e, there are the additional downfield proton signals in the range of δ 7.2−6.7 ppm for the bridgehead-N-substituted phenyls and the

All the new complexes 1a−1e and 2a−2e are air-metastable brown-red and dark-green solids, respectively, which are well characterized by elemental analysis, FT-IR, and 1H and 31 1 P{ H} NMR techniques. The corresponding spectroscopic data are summarized in Table 1 for comparison. The IR spectra of 1a−1e and 2a−2e all display three to four absorption bands in the range of 2019−2012 cm−1 for their terminal carbonyls (Table 1), the first carbonyl band of which is strong and appears at approximately 2020 cm−1 that is consistent with the observation in the known diphosphinechelated diiron pdt complexes Fe2(μ-pdt)(CO)4(κ2-diphosphine).19−21,26,27,31,30,32−34 In their IR spectra, one νC≡O absorption band with the lowest wavenumber around 1900 cm−1 corresponds to the Fe(CO)(PNP) moiety, while the other two or three νC≡O absorption bands at the higher wavenumbers in the region of 1929−2019 cm−1 are associated with the Fe(CO)3 unit.18 It should be noted that relative to the edt complexes 2a−2e, the first νC≡O absorption bands in the adtNPh complexes 1a−1e are shifted toward the higher energy, apparently due to the electron-withdrawing character of the aromatic azadithiolate bridge. This result implies that the change of the dithiolate bridge may tune the back-donating πbonding electron from the iron core to the antibonding π*orbital of the terminal carbonyl, which might further influence the structure and redox property of the Fe2S2 system. The solution 31P{1H} NMR spectra of 1a−1e and 2a−2e all show a broad singlet at δ 110.81−119.92 ppm ascribed to the apical−basal isomer and a singlet at δ 97.03−109.78 ppm C

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Figure 3. Molecular structures of the edt complexes 2a (left) and 2d (right) with thermal ellipsoids at 30% probability. Hydrogen atoms have been omitted for clarity.

Table 2. Key Bond Lengths and Bond Angles of Complexes 1a, 1b, 2a, and 2d and Their Related Chelate Analogues for Comparison complex

ligand site

Fe−Fe (Å)

P−Fe−P (deg)

P−N−P (deg)

Fe2(μ-adtNPh)(CO)4{κ2-(Ph2P)2N(CMe3)} (1a) Fe2(μ-adtNPh)(CO)4{κ2-(Ph2P)2N(CH2CHMe2)} (1b) Fe2(μ-edt)(CO)4{κ2-(Ph2P)2N(CMe3)} (2a) Fe2(μ-edt)(CO)4{κ2-(Ph2P)2N(CH2)3Si(OEt)3} (2d) Fe2(μ-adtNR)(CO)4{κ2-(Ph2PCH2)2N(CH2)2Me} (R = CH2Ph) Fe2(μ-adtNR)(CO)4{κ2-(Ph2PCH2)2} (R = CH2Ph) Fe2(μ-adtNR)(CO)4{κ2-(Ph2PCH2)2} (R = CHMe2) Fe2(μ-adtNR)(CO)4{κ2-(Ph2PCH2)2} (R = (CH2)2OMe) Fe2(μ-adtNR)(CO)4{κ2-(Ph2PCH2)2} (R = C5H9) Fe2(μ-edt)(CO)4{κ2-(Ph2PCH2)2N(CH2)2Me} Fe2(μ-edt)(CO)4{κ2-(Ph2PCH2)2} Fe2(μ-edt)(CO)4{κ2-(Ph2PCHCHPPh2)}

apical−basal basal−basal apical−basal apical−basal apical−basal apical−basal apical−basal apical−basal apical−basal apical−basal apical−basal apical−basal

2.5329(12) 2.5946(6) 2.5121(5) 2.5050(16) 2.5825(10) 2.5445(11) 2.5451(8) 2.5453(12) 2.583(2) 2.5618(7) 2.5296(9) 2.5249(9)

71.92(6) 71.51(3) 72.25(3) 72.20(7) 92.59(6) 86.95(6) 87.84(4) 87.74(6) 88.06(9) 92.98(3) 89.10(5) 87.83(5)

96.4(3) 97.47(13) 96.90(10) 99.2(3)

relatively upfield proton signals in the region of δ 4.9−3.0 ppm for the N/S-linked methenes in the adtNPh bridge. For 2a−2e, two sets of the characteristic aliphatic proton signals appear at about δ 1.7 and 1.1 ppm, which are attributed to their methenes in the edt bridge. In addition to the aliphatic proton signals mentioned above, other aliphatic proton signals for methyls, methenes, and methylenes in PNP ligands are also observed in the upfield region of their 1H NMR spectra. Molecular Structures of Complexes 1a, 1b, 2a, and 2d. The molecular structures of 1a, 1b, 2a, and 2d are unambiguously determined by single-crystal X-ray diffraction analyses, as displayed in Figures 2 and 3. The key bond lengths and bond angles for 1a, 1b, 2a, and 2d and their related chelate complexes are listed in Table 2 for comparison. As depicted in Figure 2, the solid-state molecules of the adtNPh complexes 1a and 1b each contain a typical butterfly Fe2S2 framework and a chelating PNP ligand to one Fe core, as observed in the reported diphosphine-chelated diiron dithiolate complexes of the formula [Fe2(μ-dithiolate)(CO)4(κ2-disphosphine)].14−27,31,30,32−34 It is worth noting that the two P atoms of the chelating PNP ligand in 1a bind in an apical−basal site at the Fe2 atom, whereas those in 1b lie in a basal−basal position at the Fe2 atom, in which the former is similar to only a few known flexible backbone diphosphinechelated diiron complexes Fe2(μ-adtNR)(CO)4(κ2-disphosphine) with an adtNR bridge (Table 2),14−16 while the latter is analogous to all of the reported small bite-angle PNP-

ref. this this this this 14 15 15 15 16 14 17 18

work work work work

chelated diiron complexes Fe2(μ-pdt)(CO)4{κ2-(Ph2P)2NR} with a pdt bridge.31,30,32−34 Obviously, the change in the coordination geometry of the chelating PNP ligands in diiron adtNR and pdt complexes should be ascribed to the different steric effect between the bridgehead amine/methene in adtNR/ pdt bridges and the pendant N-substituent in PNP ligands. Most notably, a distinctive feature is that the small P−Fe−P bite angles (ca. 72°) in the adtNPh complexes 1a and 1b are well accordant with the known pdt complexes with chelating small bite-angle diphosphines like PNP and dppm (Ph2PCH2PPh2),31,30,32−34 while in contrast the reported adtNR complexes with chelating flexible backbone diphosphines such as (Ph2PCH2)2N(Pr-n) and dppe are characterized by a slightly larger P−Fe−P angle (ca. 90°) (Table 2).14−16 Meanwhile, the Fe1−Fe2 bond lengths in 1a (2.5329(12) Å) and 1b (2.5946(6) Å) are comparable to those found in the known diiron dithiolate complexes with chelating bis(diphenylphosphino) ligands.14−18,31,30,32−34 Additionally, the NPh groups in the adt bridge occupy an axial position at one N atom and the sum of the C−N−C angles around the other N atom is changed from 359.7 (1a) to 355.6 (1b) (see Table S1). As shown in Figure 3, the molecular molecules of the edt complexes 2a and 2d consist of a diiron core ligated by a bridging ethanedithiolate (edt), a chelating aminodiphosphine (PNP), and four terminal carbonyls (CO), which is structurally consistent with the diphosphine-chelated diiron edt complexes Fe2(μ-edt)(CO)4(κ2-disphosphine) reported D

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics previously.14,17,18 Compared to 1a and 1b described above, the chelating PNP ligands in 2a and 2d both occupy the apical− basal conformation at the Fe2 atom, as found in several other diiron edt complexes with chelating diphosphines (Table 2).14,17,18 This is in agreement with the observation that the apical−basal isomer is favored over the basal−basal isomer in solution (isomeric ratio = 10:1 to 19:1; see Table 1) as shown by the 31P{1H} NMR spectra of 2a−2e. These findings indicate that the apical−basal configuration of the chelating diphosphines in diiron edt complexes is kinetically more favorable relative to the basal−basal conformation, probably attributed to the less steric interplay between the edt bridge and the bis(diphenylphosphino) ligands. Clearly, the edt complexes 2a and 2d have the characteristic small P−Fe−P bite angles (ca. 72°) similar to the adtNPh complexes 1a and 1b but are different from the known diiron edt complexes with chelating flexible backbone diphosphines such as (Ph2PCH2)2N(Pr-n), dppe, and dppv (Table 2).14,17,18 Interestingly, the Fe1−Fe2 bond lengths in 2a (2.5121(5) Å) and 2d (2.5050(16) Å) are somewhat shorter than those in 1a and 1b (Table 2), as observed in the known diiron edt versus adtNR complexes with the same chelating diphosphines like (Ph2PCH2)2N(Pr-n) and dppe (Table 2).14,15,17 This outcome well demonstrates that the stronger rigidity of the edt bridge with respect to the adtNR one results in the formation of the shorter Fe−Fe bond in diiron complexes. On the basis of these studies described above, we can find that (i) the change of the dithiolate bridge may affect the coordination geometry (apical−basal or basal−basal) of the chelating diphosphines and the Fe−Fe bond distance of the diiron cores, (ii) the kinds of diphosphines can tune the P− Fe−P bite angle in Fe 2 S 2 complexes, and (iii) the aforementioned results are in good accordance with the formation of the different ratios of the apical−basal versus basal−basal isomers in solution as observed in the 31P{1H} NMR spectra. Electrochemical and Electrocatalytic Studies of Complexes 1b and 2b. For comparison with the effect of the dithiolate bridge on the electrocatalytic performance of small bite-angle PNP-chelated diiron model complexes related to the active site of [FeFe]-hydrogenases, we selected the representative counterparts 1b and 2b with the adtNPh and edt bridges, respectively, and evaluated their electrochemical properties without or with the weak acetic acid (HOAc) as proton source by using cyclic voltammetry (CV). The relevant electrochemical data for 1b and 2b are given in Table 3.

Figure 4. Cyclic voltammograms of 1b (1.0 mM, solid line) and 2b (1.0 mM, dashed line) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1. All potentials are versus ferrocene/ferrocenium (Fc0/+) couple.

irreversibly at −2.23 V (Table 3), attributed to the reduction process of FeIFeI to Fe0FeI similar to 1b. Of importance to note is that the reduction peak potential (Epc) of 1b is shifted positively by 130 mV in comparison to that of 2b, suggesting that the former is more easily reduced than the latter. This is probably due to the lower electron density on the iron center in 1b than that in 2b, as indicated by the decrease of the first carbonyl frequency from 2019 cm−1 in 1b to 2016 cm−1 in 2b in their IR spectra (Table 1). Further electrocatalytic abilities of proton reduction to H2 catalyzed by 1b and 2b were studied in the presence of HOAc (0−10 mM) by using CV techniques. As displayed in Figure 5, the initial reduction peak currents of 1b and 2b increased dramatically upon the consecutive additions of HOAc, wherein the plots of the current intensities (icat) versus the acid concentration ([HOAc]) are linearly correlated (inset in Figure 5). This well indicates that a typical catalytic process for proton reduction to H2 happens;32,34,38−40 that is to say, model complexes 1b and 2b are active for the electrocatalytic H2 evolution under weak acid condition. Meanwhile, on the basis of the aforementioned results and the previously reported similar cases,32,34,38−40 we might suggest an ECCE mechanism for H2 production catalyzed by 1b and 2b in the presence of HOAc. The proposed ECCE catalytic process of 1b as a representative is described as follows: the neutral compound 1b (marked as [FeI−FeI]0) is first reduced at −2.10 V to form monoanion [1b]− ([Fe0−FeI]−), and then, it is protonated by HOAc to give protonated species [1b(H)] ([HFeII−FeI]0). After [1b(H)] is further protonated by HOAc, the resulting monocation [1b(H2)]+ ([(H2)FeII−FeI]+) accepts an electron to produce H2 and regenerates 1b to thereby complete a catalytic cycle. Most importantly, the overpotential and turnover frequency (TOF) are investigated in order to give a further measure of the catalytic activities of 1b and 2b as the hydrogen evolution catalysts. The overpotentials for 1b and 2b are determined by calculating the difference between the theoretical half-wave reduction potential for HOAc reduction (E1/2T) and the observed potential at half-maximum of the catalytic current (Ecat/2) for HOAc reduction in the presence of 1b and 2b,32,41,42 i.e., overpotential = |E1/2T − Ecat/2|. From Table 3, the overpotentials for H2 production catalyzed by 1b and 2b in MeCN solution with 10 mM HOAc are approximately 0.60

Table 3. Relevant Electrochemical Data for Complexes 1b and 2b complex

Epc (V)

Ecat/2 (V)

overpotential (V)

icat/ip

k (TOF) (s−1)

1b 2b

−2.10 −2.23

−1.98 −2.05

0.60 0.67

10.45 8.47

21.19 13.92

The redox properties of 1b and 2b were performed in nBu4NPF6/MeCN solution in the absence of HOAc by means of CV, as shown in Figure 4. For the adtNPh complex 1b (solid line in Figure 4), the first reduction event occurs irreversibly at −2.10 V (Table 3), which should be ascribed to the reduction process of FeIFeI to Fe0FeI based on the previously similar cases.32,34−37 In contrast, for the edt counterpart 2b (dashed line in Figure 4), the first reduction event takes place E

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Figure 5. Cyclic voltammograms of 1b (1.0 mM, left) and 2b (1.0 mM, right) with HOAc (0, 2, 4, 6, 8, and 10 mM) in 0.1 M n-Bu4NPF6/MeCN solution at a scan rate of 0.1 V s−1. Inset: plots of icat (μA) vs [HOAc] (mM). All potentials are versus the ferrocene/ferrocenium (Fc0/+) couple.

and 0.67 V, respectively, according to the following fact that (i) the value of E1/2T for HOAc in MeCN is −1.38 V41,42 and (ii) the catalytic half-wave potentials (Ecat/2) of 1b and 2b determined for HOAc with the highest concentration (10 mM) are −1.98 and −2.05 V, respectively (see Figures S1 and S2). It should be therefore noted that the adtNPh complex 1b may well improve the catalytic activities for H2 production by causing a 70 mV decrease of the overpotential in contrast to the edt complex 2b. The turnover frequencies (TOF) for 1b and 2b are estimated through the kinetics (rate constant, k) of hydrogen evolution reaction catalyzed by them, which are determined by plotting the concentration of the added acetic acid ([HOAc]) against the ratio of the catalytic current (icat) and the peak current (ip) in the presence and absence of acetic acid from Figure 5 (see Figures S3 and S4).43,44 For 1b and 2b, there is a linear relationship between the acid concentration ([HOAc]) and the icat/ip ratio as the concentration of HOAc is from 0 to 10 mM (see Figures S3 and S4), indicating that the catalytic pathway is a second-order dependence on the acid concentration.42−44 At this linear level, the k value (TOF) of hydrogen evolution reaction for 1b and 2b can be calculated by using the following equation:

■

CONCLUSIONS

■

EXPERIMENTAL SECTION

In summary, we have successfully prepared and structurally characterized two types of novel small bite-angle PNP-chelated diiron complexes 1a−1e and 2a−2e with the adtNPh and edt bridges, respectively, which can be regarded as the desirable asymmetrically substituted biomimetic models for the diiron subunit of the [FeFe]-hydrogenase active site. Complexes 1a− 1e and 2a−2e can be readily obtained through not only the oxidative decarbonylation reactions but also the UV irradiation treatments of two precursors Fe2(μ-xdt)(CO)6 with different PNP ligands in the presence of Me3NO·2H2O and LED lamp emitting at 365 nm, respectively. Most notably, a comparative study on the influence of dithiolate bridges on the molecular structures and electrochemical performance of 1a−1e and 2a−2e was investigated by various spectroscopies, X-ray crystallography, and cyclic voltammetry. In contrast to the edt complexes 2a−2e, the adtNPh complexes 1a−1e demonstrate the following results: (i) their first νC≡O IR absorption bands are shifted toward the higher energy, clearly caused by the stronger electronwithdrawing ability of the adtNPh bridge than the edt bridge; (ii) the dibasal isomers in them are all favorable in solution while the PNP ligands in 1a and 1b show the apical−basal and dibasal manners in the solid state, probably due to the larger steric interplay between the adtNPh bridge and the PNP ligands; (iii) they show a more positive reduction potential, a lower overpotential, and a greater catalytic frequency (TOF) in the hydrogen evolution reaction under weak acid and electrochemical condition, thus implying that the small biteangle PNP-chelated diiron models with an adt bridge could have superior electrocatalytic activity of proton reduction to H2 in comparison to their analogous models with other bridges such as edt or pdt.

icat /i p = n/0.4463 RT (k[H+]2 )/Fν , where R is ideal gas constant (K−1 mol−1), T is temperature (K), F is Faraday’s constant (C mol−1), ν is scan rate (V s−1), and n is number of electrons transferred (n = 2 for hydrogen evolution reaction).42−44 It is seen from Table 3 that, at the highest concentration of HOAc (10 mM), the icat/ip value of 1b reaches 10.45 whereas that of 2b is 8.47 (see Figures S3 and S4); correspondingly, the k value (TOF) of the former is calculated to be 21.19 s−1, which is significantly larger than that of the latter (k = 13.92 s−1). This result demonstrates that the adtNPh complex 1b has a greater turnover frequency of the electrocatalytic proton reduction to H2 relative to its edt counterpart 2b, possibly resulting from the synergistic contribution of the amine in the dithiolate bridge and PNP ligand to the catalytic reaction. In addition, we also performed the CV curves of other counterparts 1a vs 2a, 1c vs 2c, 1d vs 2d, and 1e vs 2e without or with HOAc (see Table S3 and Figures S5−S8) and found that they have similar electrochemical as well as electrocatalytic behaviors to the aforementioned representatives 1b vs 2b under the same CV conditions.

Materials and Methods. 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, whereas toluene is distilled under N2 from sodium/benzophenone ketyl. Me3NO·2H2O was commercially available and used as received. Ligands (Ph2P)2NR (R = CMe3, CH2CHMe2, (CH2)3Me, (CH2)3Si(OEt)3, and (CH2)3NMe2) and complexes Fe2(μ-xdt)(CO)6 (xdt = adtNPh, edt) were prepared according to the literature procedures.28,45,46 Preparative TLC was performed on glass plates (25 cm × 20 cm × 0.25 cm) coated with silica gel G (10−40 mm). Flash column chromatography was performed on silica gel (45−75 μm). Elemental analyses were F

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

MHz, CDCl3, 85% H3PO4): δ 111.28 (br s, apical−basal isomer, 24%), 97.47 (s, basal−basal isomer, 76%) ppm. Method II. Yield: 0.111 g (74%). TLC analysis indicates that the brown-red solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2019 (vs), 1954 (vs), 1903 (m) cm−1. Complex 1e (R = (CH2)3NMe2). Method I. Yield: 0.145 g (66%). Anal. Calcd for C41H41Fe2N3O4P2S2: C, 56.12; H, 4.71; N, 4.79%. Found: C, 55.99; H, 4.99; N, 4.62%. FT-IR (KBr disk): νC≡O 2018 (vs), 1953 (vs), 1907 (m) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.80−7.38 (m, 20H, 2 × P(C6H5)2), 7.24 (s, 2H, NC6H5-o), 6.64 (m, 3H, NC6H5-m,p), 4.24−3.97 (m, 4H, 2 × SCH2N), 3.08 (m, 2H, NCH2), 2.05−1.96 (m, 8H, NCH2, and 2 × NCH3), 1.41 (s, 2H, NCH2CH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 111.27 (br s, apical−basal isomer, 30%), 97.89 (s, basal−basal isomer, 70%) ppm. Method II. Yield: 0.112 g (85%). TLC analysis indicates that the brown-red solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2018 (vs), 1955 (vs), 1909 (m) cm−1. General Procedure for Preparation of Fe2(μ-edt)(CO)4{(κ2Ph2P)2N(R)} (2a−2e). Method I. A dry MeCN solution (15 mL) of Fe2(μ-edt)(CO)6 (0.25 mmol), Me3NO·H2O (0.375 mmol), and (Ph2P)2N(R) (0.375 mmol) was stirred at room temperature for about 45 min to give a color change of the solution from red to dark green. After the reaction, volatiles were removed under reduced pressure. The residue was subjected to flash column chromatography on silica gel eluting with petroleum ether/dichloromethane (3:2, v/v) for 1a−1c and preparative TLC separation using petroleum ether/ ethyl acetate (5:1, v/v) for 1d as well as dichloromethane/methanol (20:1, v/v) for 1e as eluent. Collecting the main dark-green band afforded a dark-green solid as the target edt complex. Method II. A toluene solution (100 mL) of Fe2(μ-edt)(CO)6 (0.150 mmol) and (Ph2P)2N(R) (0.225 mmol) was stirred in a Pyrex Schlenk tube with an LED lamp at 365 nm for 2 h to give a green solution until the starting material Fe2(μ-edt)(CO)6 disappeared as monitored by TLC. After the same workup as in the method I for 2b−2e and the recrystallization from the mixture of petroleum ether/ dichloromethane (10:1, v/v) for 2a, a dark-green solid was collected and identified by comparison with authentic sample (method I) using TLC monitoring and an IR spectrum. Complex 2a (R = CMe3). Method I. Yield: 0.056 g (29%). Anal. Calcd for C34H33Fe2NO4P2S2: C, 53.92; H, 4.39; N, 1.85%. Found: C, 53.71; H, 4.63; N, 2.13%. FT-IR (KBr disk): νC≡O 2017 (vs), 1964 (vs), 1941 (vs), 1919 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 8.06 (m, 8H, 2 × P(C6H5-o)2), 7.49 (m, 12H, 2 × P(C6H5-m,p)2), 1.62 (s, 2H, 2 × SCHeHa), 1.05 (s, 2H, 2 × SCHeHa), 0.92 (s, 9H, 3 × CH3) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 119.55 (s, apical−basal isomer, 88%), 109.78 (s, basal−basal isomer, 12%) ppm. Method II. Yield: 0.043 g (38%). TLC analysis indicates that the dark-green solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2017 (vs), 1963 (vs), 1941 (vs), 1919 (s) cm−1. Complex 2b (R = CH2CHMe2). Method I. Yield: 0.128 g (68%). Anal. Calcd for C34H33Fe2NO4P2S2: C, 53.92; H, 4.39; N, 1.85%. Found: C, 53.79; H, 4.67; N, 2.08%. FT-IR (KBr disk): νC≡O 2016 (vs), 1946 (vs), 1929 (vs), 1908 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.69−7.46 (m, 20H, 2 × P(C6H5)2), 2.75 (s, 2H, NCH2), 1.77 (s, 2H, 2 × SCHeHa), 1.59 (s, 1H, NCH2CH), 1.12 (s, 2H, 2 × SCHeHa), 0.44 (s, 6H, 2 × CH3) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 114.05 (br s, apical−basal isomer, 95%), 106.27 (s, basal−basal isomer, 5%) ppm. Method II. Yield: 0.043 g (38%). TLC analysis indicates that the dark-green solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2014 (vs), 1947 (vs), 1928 (vs), 1908 (s) cm−1. Complex 2c (R = (CH2)3Me). Method I. Yield: 0.115 g (61%). Anal. Calcd for C34H33Fe2NO4P2S2: C, 53.92; H, 4.39; N, 1.85%. Found: C, 53.75; H, 4.61; N, 2.03%. FT-IR (KBr disk): νC≡O 2016 (vs), 1959 (vs), 1934 (vs), 1906 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.66−7.46 (m, 20H, 2 × P(C6H5)2), 2.95 (s, 2H, NCH2), 1.77 (s,

obtained on a PerkinElmer 240C analyzer. FT-IR spectra were recorded on a Nicolet iS 10 FT-IR spectrometer. NMR (1H, 31P{1H}) spectra were obtained on a Bruker Avance 600 MHz spectrometer. General Procedure for Preparation of Fe2 (μ-adt NPh )(CO)4{(κ2-Ph2P)2N(R)} (1a−1e). Method I. A dry MeCN solution (15 mL) of Fe2(μ-adtNPh)(CO)6 (0.250 mmol), Me3NO·H2O (0.375 mmol), and (Ph2P)2N(R) (0.375 mmol) was stirred at room temperature for about 1 h to give a color change of the solution from red to brown-red. After this reaction, volatiles were removed under reduced pressure. The residue was subjected to flash column chromatography on silica gel eluting with petroleum ether/dichloromethane (3:2, v/v) for 1a−1c and preparative TLC separation using petroleum ether/ethyl acetate (5:1, v/v) for 1d as well as dichloromethane/methanol (20:1, v/v) for 1e as eluent. Collecting the main red band afforded a brown-red solid as the target adtNPh complex. Method II. A toluene solution (100 mL) of Fe2(μ-adtNPh)(CO)6 (0.150 mmol) and (Ph2P)2N(R) (0.225 mmol) was stirred in a Pyrex Schlenk tube with an LED lamp at 365 nm for 2 h to give a red solution until the starting material Fe2(μ-adtNPh)(CO)6 disappeared as monitored by TLC. After the same workup as in method I, a brown-red solid was collected and identified by comparison with authentic sample (method I) using TLC monitoring and an IR spectrum. Complex 1a (R = CMe3). Method I. Yield: 0.100 g (48%). Anal. Calcd for C40H38Fe2N2O4P2S2: C, 56.62; H, 4.51; N, 3.30%. Found: C, 56.33; H, 4.71; N, 3.51%. FT-IR (KBr disk): νC≡O 2018 (vs), 1948 (vs), 1884 (m) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 8.06 (m, 8H, 2 × P(C6H5-o)2), 7.48 (m, 12H, 2 × P(C6H5-m,p)2), 7.11 (s, 2H, NC6H5-o), 6.60 (m, 3H, NC6H5-m,p), 4.09−3.75 (m, 4H, 2 × SCH2N), 0.88 (s, 9H, 3 × CH3) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 119.92 (s, apical−basal isomer, 30%), 104.47 (s, basal−basal isomer, 70%) ppm. Method II. Yield: 0.057 g (45%). TLC analysis indicates that the brown-red solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2018 (vs), 1947 (vs), 1884 (m) cm−1. Complex 1b (R = CH2CHMe2). Method I. Yield: 0.130 g (61%). Anal. Calcd for C40H38Fe2N2O4P2S2: C, 56.62; H, 4.51; N, 3.30%. Found: C, 56.41; H, 4.78; N, 3.15%. FT-IR (KBr disk): νC≡O 2019 (vs), 1954 (vs), 1914 (m) cm−1. 1H NMR (600 MHz, acetone-d6, TMS): δ 7.87−7.43 (m, 20H, 2 × P(C6H5)2), 7.16 (s, 2H, NC6H5-o), 6.73 (m, 3H, NC6H5-m,p), 4.16 (m, 2H, 2 × SCHeHaN), 2.96 (m, 2H, 2 × SCHeHaN), 2.77 (d, J = 13.6 Hz, 2H, NCH2), 1.43 (s, 1H, NCH2CH), 0.57 (s, 6H, 2 × CH3) ppm. 31P{1H} NMR (243 MHz, acetone-d6, 85% H3PO4): δ 111.24 (br s, apical−basal isomer, 54%), 100.76 (s, basal−basal isomer, 46%) ppm. Method II. Yield: 0.050 g (39%). TLC analysis indicates that the brown-red solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2019 (vs), 1954 (vs), 1913 (m) cm−1. Complex 1c (R = (CH2)3Me). Method I. Yield: 0.127 g (60%). Anal. Calcd for C40H38Fe2N2O4P2S2: C, 56.62; H, 4.51; N, 3.30%. Found: C, 56.41; H, 4.80; N, 3.48%. FT-IR (KBr disk): νC≡O 2019 (vs), 1954 (vs), 1910 (m) cm−1. 1H NMR (600 MHz, acetone-d6, TMS): δ 7.81−7.43 (m, 20H, 2 × P(C6H5)2), 7.15 (s, 2H, NC6H5-o), 6.72 (m, 3H, NC6H5-m,p), 4.13 (m, 2H, 2 × SCHeHaN), 3.16 (m, 2H, 2 × SCHeHaN), 2.78 (s, 2H, NCH2), 1.43 (s, 2H, NCH2CH2), 1.00 (m, 2H, CH2CH3), 0.64 (m, 3H, CH3) ppm. 31P{1H} NMR (243 MHz, acetone-d6, 85% H3PO4): δ 110.81 (br s, apical−basal isomer, 52%), 97.03 (s, basal−basal isomer, 48%) ppm. Method II. Yield: 0.089 g (70%). TLC analysis indicates that the brown-red solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2018 (vs), 1955 (vs), 1908 (m) cm−1. Complex 1d (R = (CH2)3Si(OEt)3). Method I. Yield: 0.159 g (64%). Anal. Calcd for C45H50Fe2N2O7P2S2Si: C, 54.22; H, 5.06; N, 2.81%. Found: C, 54.32; H, 5.33; N, 2.68%. FT-IR (KBr disk): νC≡O 2018 (vs), 1954 (vs), 1902 (m) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.97−7.38 (m, 20H, 2 × P(C6H5)2), 7.18 (s, 2H, NC6H5-o), 6.64 (m, 3H, NC6H5-m,p), 4.94−3.98 (m, 4H, 2 × SCH2N), 3.59 (s, 6H, 3 × OCH2), 2.93 (m, 2H, NCH2), 1.51 (s, 2H, NCH2CH2), 1.05 (s, 9H, 3 × OCH2CH3), 0.23 (m, 2H, SiCH2) ppm. 31P{1H} NMR (243 G

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics 2H, 2 × SCHeHa), 1.26 (s, 2H, NCH2CH2), 1.13 (s, 2H, 2 × SCH e H a ), 0.93 (s, 2H, NCH 2 CH 2 CH 2 ), 0.63 (s, 3H, NCH2CH2CH2CH3) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 113.21 (br s, apical−basal isomer, 91%), 103.31 (s, basal− basal isomer, 9%) ppm. Method II. Yield: 0.041 g (36%). TLC analysis indicates that the dark-green solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2017 (vs), 1958 (s), 1943 (vs), 1910 (s) cm−1. Complex 2d (R = (CH2)3Si(OEt)3). Method I. Yield: 0.166 g (73%). Anal. Calcd for C39H45Fe2NO7P2S2Si: C, 51.72; H, 5.01; N, 1.55%. Found: C, 51.83; H, 5.28; N, 1.77%. FT-IR (KBr disk): νC≡O 2016 (vs), 1946 (vs), 1910 (m) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.65 (m, 8H, 2 × P(C6H5-o)2), 7.43 (m, 12H, 2 × P(C6H5-m,p)2), 3.60 (q, J = 6.6 Hz, 6H, 3 × OCH2), 2.97 (t, J = 7.2 Hz, 2H, NCH2), 1.76 (m, 2H, 2 × SCHeHa), 1.42 (s, 2H, NCH2CH2), 1.13 (m, 2H, 2 × SCHeHa), 1.05 (t, J = 6.6 Hz, 9H, 3 × OCH2CH3), 0.20 (t, J = 7.2 Hz, 2H, SiCH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 112.86 (br s, apical−basal isomer, 91%), 103.01 (s, basal− basal isomer, 9%) ppm. Method II. Yield: 0.043 g (32%). TLC analysis indicates that the dark-green solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2017 (vs), 1948 (vs), 1913 (m) cm−1. Complex 2e (R = (CH2)3NMe2). Method I. Yield: 0.181 g (92%). Anal. Calcd for C35H36Fe2N2O4P2S2: C, 53.45; H, 4.61; N, 3.56%. Found: C, 53.32; H, 4.86; N, 3.38%. FT-IR (KBr disk): νC≡O 2012 (vs), 1945 (vs), 1933 (vs), 1899 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.67−7.46 (m, 20H, 2 × P(C6H5)2), 3.04 (s, 2H, NCH2), 1.96 (s, 6H, 2 × CH3), 1.76 (s, 2H, 2 × SCHeHa), 1.62 (s, 2H, NCH2), 1.39 (s, 2H, NCH2CH2), 1.13 (s, 2H, 2 × SCHeHa) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 113.69 (br s, apical−basal isomer, 91%), 103.91 (s, basal−basal isomer, 9%) ppm. Method II. Yield: 0.034 g (29%). TLC analysis indicates that the dark-green solids obtained from methods I and II display the same Rf value. FT-IR (KBr disk): νC≡O 2013 (vs), 1944 (vs), 1932 (vs), 1899 (s) cm−1. X-ray Crystal Structure Determination. Single crystals of complexes 1a, 1b, 2a, and 2d suitable for X-ray diffraction analysis were grown by slow evaporation of the CH2Cl2/hexane solution at −20 °C. The crystals were mounted on a Bruker-CCD diffractometer. Data were collected at 296(2) K using a graphite monochromator with Mo Kα radiation (λ = 0.71073 Å) in the ω−φ scanning mode. The structure was solved by direct methods using the SHELXS-97 program and refined by full-matrix least-squares techniques (SHELXL-97) on F2.47 Hydrogen atoms were located using the geometric method. Details of crystallographic data and structure refinement for 1a, 1b, 2a, and 2d are summarized in Table S2. Electrochemistry. Electrochemical and electrocatalytic properties of complexes 1b and 2b were studied by cyclic voltammetry (CV) in MeCN solution. 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+ electrode as the reference electrode. The potential scale was calibrated against the Fc/ Fc+ couple and reported versus this reference system.

■

Accession Codes

CCDC 1865617−1865619 and 1865625 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Pei-Hua Zhao: 0000-0002-5480-6128 Yu-Long Li: 0000-0002-5579-3269 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Nos. 21301160 and 21501124), the Natural Science Foundation of Shanxi Province (No. 201701D121035), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi Province (No. 201802078), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Shanxi Province, and the Natural Science Foundation of Zhejiang Province (No. LY19B020002) for the financial support of this work.

■

REFERENCES

(1) (a) Li, Y. L.; Rauchfuss, T. B. Synthesis of Diiron(I) Dithiolato Carbonyl Complexes. Chem. Rev. 2016, 116, 7043−7077. (b) Song, L. C. Investigations on Butterfly Fe/S Cluster S-Centered Anions (μS−)2Fe2(CO)6, (μ-S−)(μ-RS)Fe2(CO)6, and Related Species. Acc. Chem. Res. 2005, 38, 21−28. (2) (a) Tard, C.; Pickett, C. J. Structural and Functional Analogues of the Active Sites of the [Fe]-, [NiFe]-, and [FeFe]-Hydrogenases. Chem. Rev. 2009, 109, 2245−2274. (b) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Hydrogenases. Chem. Rev. 2014, 114, 4081− 4148. (c) Rauchfuss, T. B. Diiron Azadithiolates as Models for the [FeFe]-Hydrogenase Active Site and Paradigm for the Role of the Second Coordination Sphere. Acc. Chem. Res. 2015, 48, 2107−2116. (3) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. XRay Crystal Structure of the Fe-Only Hydrogenase (Cpl) from Clostridium Pasteurianum to 1.8 Angstrom Resolution. Science 1998, 282, 1853−1858. (4) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, E. C.; FontecillaCamps, J. C. Desulfovibrio Desulfuricans Iron Hydrogenase: The Structure Shows Unusual Coordination to an Active Site Fe Binuclear Center. Structure 1999, 7, 13−23. (5) (a) Le Cloirec, A.; Best, S. P.; Davies, S. C.; Evans, D. J.; Hughes, D. L.; Pickett, C. J.; et al. A Di-Iron Dithiolate Possessing Structural Elements of the Carbonyl/Cyanide Sub-Site of the H-Centre of FeOnly Hydrogenase. Chem. Commun. 1999, 2285−2286. (b) Pandey, A. S.; Harris, T. V.; Giles, L. J.; Peters, J. W.; Szilagyi, R. K. Dithiomethylether as a Ligand in the Hydrogenase H-Cluster. J. Am. Chem. Soc. 2008, 130, 4533−4540. (c) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Structure/Function Relationships of [NiFe]- and [FeFe]-Hydrogenases. Chem. Rev. 2007, 107, 4273−4303. (6) Erdem, Ö . F.; Schwartz, L.; Stein, M.; Silakov, A.; KaurGhumaan, S.; Huang, P.; Ott, S.; Reijerse, E. J.; Lubitz, W. A Model of the [FeFe] Hydrogenase Active Site with a Biologically Relevant Azadithiolate Bridge: A Spectroscopic and Theoretical Investigation. Angew. Chem., Int. Ed. 2011, 50, 1439−1443.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00759. Selected crystallographic data for 1a, 1b, 2a, and 2d (Tables S1−S2); additional electrochemical studies of 1b and 2b (Figures S1−S8, Table S3); IR and NMR (1H, 31P) spectra of 1a−1e and 2a−2e (Figures S9− S38) (PDF) H

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (7) Berggren, G.; Adamska, A.; Lambertz, C.; Simmons, T. R.; Esselborn, J.; Atta, M.; Gambarelli, S.; Mouesca, J. M.; Reijerse, E.; Lubitz, W.; Happe, T.; Artero, V.; Fontecave, M. Biomimetic Assembly and Activation of [FeFe]-Hydrogenases. Nature 2013, 499, 66−69. (8) Bethel, R. D.; Darensbourg, M. Y. Enzymes Activated by Synthetic Components. Nature 2013, 499, 40−41. (9) Silakov, A.; Wenk, B.; Reijerse, E. J.; Lubitz, W. 14N HYSCORE Investigation of the H-cluster of [FeFe] Hydrogenase: Evidence for a Nitrogen in the Dithiol Bridge. Phys. Chem. Chem. Phys. 2009, 11, 6592−6599. (10) Olsen, M. T.; Rauchfuss, T. B.; Wilson, S. R. Role of the Azadithiolate Cofactor in Models for [FeFe]-Hydrogenase: Novel Structures and Catalytic Implications. J. Am. Chem. Soc. 2010, 132, 17733−17740. (11) (a) Gloaguen, F.; Rauchfuss, T. B. Small Molecule Mimics of Hydrogenases: Hydrides and Redox. Chem. Soc. Rev. 2009, 38, 100− 108. (b) Darensbourg, M. Y.; Weigand, W. Sulfoxygenation of Active Site Models of [NiFe] and [FeFe] Hydrogenases − A Commentary on Possible Chemical Models of Hydrogenase Enzyme Oxygen Sensitivity. Eur. J. Inorg. Chem. 2011, 2011, 994−1004. (c) Schilter, D.; Camara, J. M.; Huynh, M. T.; Hammes-Schiffer, S.; Rauchfuss, T. B. Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides. Chem. Rev. 2016, 116, 8693−8749. (12) (a) Qian, G. F.; Zhong, W.; Wei, Z. H.; Wang, H. L.; Xiao, Z. Y.; Long, L.; Liu, X. M. Diiron Hexacarbonyl Complexes Bearing Naphthalene-1,8-Dithiolate Bridge Moiety as Mimics of the Sub-Unit of [FeFe]-Hydrogenase: Synthesis, Characterisation and Electrochemical Investigations. New J. Chem. 2015, 39, 9752−9760. (b) Lunsford, A. M.; Beto, C. C.; Ding, S. D.; Erdem, Ö . F.; Wang, N.; Bhuvanesh, N.; Hall, M. B.; Darensbourg, M. Y. Cyanide-bridged Iron Complexes as Biomimetics of Tri-iron Arrangements in Maturases of the H Cluster of the Di-iron Hydrogenase. Chem. Sci. 2016, 7, 3710−3719. (c) Liu, J. G.; Zhang, F. J.; Zhang, A. L.; Tong, Q. X.; Tung, C.-H.; Wang, W. G. New Class of Hydrido Iron(II) Compounds with cis-Reactive Sites: Combination of Iron and Diphosphinodithio Ligand. Chem. - Asian J. 2016, 11, 2271−2277. (d) Zhao, P. H.; Gray, D. L.; Rauchfuss, T. B. Rational Synthesis of the Carbonyl(perthiolato)diiron [Fe2(S3CPh2)(CO)6] and Related Complexes. Eur. J. Inorg. Chem. 2016, 2016, 2681−2683. (e) Li, Z. M.; Xiao, Z. Y.; Xu, F. F.; Zeng, X. H.; Liu, X. M. Enhancement in Catalytic Proton Reduction by an Internal Base in a Diiron Pentacarbonyl Complex: Its Synthesis, Characterisation, InterConversion and Electrochemical Investigation. Dalton Trans 2017, 46, 1864−1871. (f) He, J.; Deng, C. L.; Li, Y.; Li, Y. L.; Wu, Y.; Zou, L. K.; Mu, C.; Luo, Q.; Xie, B.; Wei, J.; Hu, J. W.; Zhao, P. H.; Zheng, W. A New Route to the Synthesis of Phosphine-Substituted Diiron Aza- and Oxadithiolate Complexes. Organometallics 2017, 36, 1322− 1330. (g) Abul-Futouh, H.; Almazahreh, L. R.; Harb, M. K.; Görls, H.; El-khateeb, M.; Weigand, W. [FeFe]-Hydrogenase H-Cluster Mimics with Various -S(CH2)nS- Linker Lengths (n = 2−8): A Systematic Study. Inorg. Chem. 2017, 56, 10437−10451. (h) Carlson, M. R.; Gray, D. L.; Richers, C. P.; Wang, W. G.; Zhao, P. H.; Rauchfuss, T. B.; Pelmenschikov, V.; Pham, C. C.; Gee, L. B.; Wang, H.; Cramer, S. P. Sterically Stabilized Terminal Hydride of a Diiron Dithiolate. Inorg. Chem. 2018, 57, 1988−2001. (i) Yu, X.; Pang, M. F.; Zhang, S. N.; Hu, X. L.; Tung, C.-H.; Wang, W. G. Terminal Thiolate-Dominated H/D Exchanges and H2 Release: Diiron ThiolHydride. J. Am. Chem. Soc. 2018, 140, 11454−11463. (13) Tye, J. W.; Darensbourg, M. Y.; Hall, M. B. De Novo Design of Synthetic Di-Iron(I) Complexes as Structural Models of the Reduced Form of Iron−Iron Hydrogenase. Inorg. Chem. 2006, 45, 1552−1559. (14) Cheng, M. L.; Wang, M.; Zheng, D. H.; Sun, L. Effect of the Sto-S Bridge on the Redox Properties and H2 Activation Performance of Diiron Complexes Related to the [FeFe]-Hydrogenase Active Site. Dalton Trans 2016, 45, 17687−17696. (15) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Electron-Transfer-Catalyzed Rearrangement of Unsymmetrically Substituted Diiron Dithiolate

Complexes Related to the Active Site of the [FeFe]-Hydrogenases. Inorg. Chem. 2007, 46, 9863−9872. (16) Si, Y.; Chen, H.; Chen, C. N. [1,2-Bis(diphenyl-Phosphanyl)ethane-2κ2P,P’]- tetracarbonyl-1κ3C,2κC-(μ-2-cyclopentyl-2-azapropane-1,3-dithiolato-1:2κ4S,S’:S,S’)diiron(II)(Fe-Fe). Acta Crystallogr., Sect. E: Struct. Rep. Online 2011, E67, m1500−m1501. (17) (a) Liu, X. F. Substitution Reactions of Diiron Dithiolate Complexes with Phosphine or Isocyanide Ligands. J. Organomet. Chem. 2014, 750, 117−124. (b) Rana, S.; Ghosh, S.; Hossain, M. K.; Rahaman, A.; Hogarth, G.; Kabir, S. E. Hydrogenase Biomimetics: Structural and Spectroscopic Studies on Diphosphine-substituted Derivatives of Fe2(CO)6(μ-edt) (edt = ethanedithiolate) and Fe2(CO)6(μ-tdt) (tdt = 1,3-toluenedithiolate). Transition Met. Chem. 2016, 41, 933−942. (18) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.; van der Vlugt, J. I.; Wilson, S. R. Chelate Control of Diiron(I) Dithiolates Relevant to the [FeFe]-Hydrogenase Active Site. Inorg. Chem. 2007, 46, 1655−1664. (19) Karnahl, M.; Tschierlei, S.; Erdem, O. F.; Pullen, S.; Santoni, M.-P.; Reijerse, E. J.; Lubitz, W.; Ott, S. Mixed-valence [Fe(I)Fe(II)] Hydrogenase Active Site Model Complexes Stabilized by a Bidentate Carborane Bis-phosphine Ligand. Dalton Trans 2012, 41, 12468− 12477. (20) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J.; Pichon, R.; Kervarec, N. Evidence for the Formation of Terminal Hydrides by Protonation of an Asymmetric Iron Hydrogenase Active Site Mimic. Inorg. Chem. 2007, 46, 3426−3428. (21) Adam, F. I.; Hogarth, G.; Kabir, S. E.; Richards, I. Models of the Iron-Only Hydrogenase: Synthesis and Protonation of Bridge and Chelate Complexes [Fe2(CO)4{Ph2P(CH2)nPPh2}(μ-pdt)] (n = 2− 4) − Evidence for a Terminal Hydride Intermediate. C. R. Chim. 2008, 11, 890−905. (22) Song, L. C.; Gao, W.; Luo, X.; Wang, Z. X.; Sun, X. J.; Song, H. B. Synthesis, Characterization, and Electrochemical Properties of Benzyloxy-Functionalized Diiron 1,3-Propanedithiolate Complexes Relevant to the Active Site of [FeFe]-Hydrogenases. Organometallics 2012, 31, 3324−3332. (23) Wang, W. G.; Rauchfuss, T. B.; Moore, C. E.; Rheingold, A. L.; De Gioia, L.; Zampella, G. Crystallographic Characterization of a Fully Rotated, Basic Diiron Dithiolate: Model for the Hred State? Chem. - Eur. J. 2013, 19, 15476−15479. (24) Munery, S.; Capon, J.-F.; De Gioia, L.; Elleouet, C.; Greco, C.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J.; Zampella, G. New FeIFeI Complex Featuring a Rotated Conformation Related to the [2Fe]H Subsite of [Fe-Fe] Hydrogenase. Chem. - Eur. J. 2013, 19, 15458−15461. (25) Goy, R.; Bertini, L.; Elleouet, C.; Görls, H.; Zampella, G.; Talarmin, J.; De Gioia, L.; Schollhammer, P.; Apfel, U.-P.; Weigand, W. A Sterically Stabilized FeI-FeI Semi-Rotated Conformation of [FeFe] Hydrogenase Subsite Model. Dalton Trans 2015, 44, 1690− 1699. (26) Adam, F. I.; Hogarth, G.; Richards, I. Models of the Iron-Only Hydrogenase: Reactions of [Fe2(CO)6(μ-pdt)] with Small Bite-Angle Diphosphines Yielding Bridge and Chelate Diphosphine Complexes [Fe2(CO)4(diphosphine)(μ-pdt)]. J. Organomet. Chem. 2007, 692, 3957−3968. (27) Ghosh, S.; Sanchez, B. E.; Richards, I.; Haque, M. N.; Holt, K. B.; Richmond, M. G.; Hogarth, G. Biomimetics of the [FeFe]Hydrogenase Enzyme: Identification of Kinetically Favoured ApicalBasal [Fe2(CO)4(μ-H){κ2-Ph2PC(Me2)PPh2}(μ-pdt)]+ as a ProtonReduction Catalyst. J. Organomet. Chem. 2016, 812, 247−258. (28) (a) Cao, Q. Y.; Chen, Y. H.; Liu, J. H.; Gao, X. C. Novel Luminescent Europium(III) Complexes Covalently Bonded to Bis(phosphino)amine Oxide Functionalized MCM-41. Inorg. Chem. Commun. 2009, 12, 48−51. (b) Gallo, V.; Mastrorilli, P.; Nobile, C. F.; Braunstein, P.; Englert, U. Chelating versus Bridging Bonding Modes of N-Substituted Bis(diphenylphosphanyl)amine Ligands in Pt Complexes and Co2Pt Clusters. Dalton Trans 2006, 2342−2349. I

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

(43) Pool, D. H.; DuBois, D. L. [Ni(P2PhN2Ar)2(NCMe)][BF4]2 as an Electrocatalyst for H2 Production: P2PhN2Ar = 1,5-(Di(4(thiophene-3-yl)phenyl)-3,7-diphenyl-1,5- diaza-3,7-diphosphacyclooctane). J. Organomet. Chem. 2009, 694, 2858−2865. (44) Hemming, E. B.; Chan, B.; Turner, P.; Corcilius, L.; Price, J. R.; Gardiner, M. G.; Masters, A. F.; Maschmeyer, T. [Fe(C5Ar5)(CO)2Br] Complexes as Hydrogenase Mimics for the Catalytic Hydrogen Evolution Reaction. Appl. Catal., B 2018, 223, 234−241. (45) Li, H. X.; Rauchfuss, T. B. Iron Carbonyl Sulfides, Formaldehyde, and Amines Condense To Give the Proposed Azadithiolate Cofactor of the Fe-Only Hydrogenases. J. Am. Chem. Soc. 2002, 124, 726−727. (46) Jiang, X. J.; Long, L.; Wang, H. L.; Chen, L. M.; Liu, X. M. Diiron Hexacarbonyl Complexes as Potential CO-RMs: CO-Releasing Initiated by a Substitution Reaction with Cysteamine and Structural Correlation to the Bridging Linkage. Dalton Trans 2014, 43, 9968− 9975. (47) Sheldrick, G. M. A. Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122.

(29) (a) Henry, R. M.; Shoemaker, R. K.; Newell, R. H.; Jacobsen, G. M.; DuBois, D. L.; DuBois, M. R. Stereochemical Control of Iron(II) Complexes Containing a Diphosphine Ligand with a Pendant Nitrogen Base. Organometallics 2005, 24, 2481−2491. (b) Henry, R. M.; Shoemaker, R. K.; DuBois, D. L.; DuBois, M. R. Pendant Bases as Proton Relays in Iron Hydride and Dihydrogen Complexes. J. Am. Chem. Soc. 2006, 128, 3002−3010. (30) Adam, F. I.; Hogarth, G.; Richards, I.; Sanchez, B. E. Models of the Iron-Only Hydrogenase: Structural Studies of Chelating Diphosphine Complexes [Fe2(CO)4(μ-pdt)(κ2P,P’-diphosphine)]. Dalton Trans 2007, 2495−2498. (31) Ghosh, S.; Hogarth, G.; Hollingsworth, N.; Holt, K. B.; Richards, I.; Richmond, M. G.; Sanchez, B. E.; Unwin, D. Models of the Iron-Only Hydrogenase: A Comparison of Chelate and Bridge Isomers of Fe2(CO)4{Ph2PN(R)PPh2}(μ-pdt) as Proton-Reduction Catalysts. Dalton Trans 2013, 42, 6775−6792. (32) Song, L. C.; Wang, Y. X.; Xing, X. K.; Ding, S. D.; Zhang, L. D.; Wang, X. Y.; Zhang, H. T. Hydrophilic Quaternary AmmoniumGroup-Containing [FeFe]-Hydrogenase Models: Synthesis, Structures, and Electrocatalytic Hydrogen Production. Chem. - Eur. J. 2016, 22, 16304−16314. (33) Li, Y. L.; Ma, Z. Y.; He, J.; Hu, M. Y.; Zhao, P. H. Aminophosphine-Substituted Diiron Dithiolate Complexes: Synthesis, Crystal Structure, and Electrocatalytic Investigation. J. Organomet. Chem. 2017, 851, 14−21. (34) Zhao, P. H.; Ma, Z. Y.; Hu, M. Y.; He, J.; Wang, Y. Z.; Jing, X. B.; Chen, H. Y.; Wang, Z.; Li, Y. L. PNP-Chelated and -Bridged Diiron Dithiolate Complexes Fe2(μ-pdt)(CO)4{(Ph2P)2NR} Together with Related Monophosphine Complexes for the [2Fe]H Subsite of [FeFe]-Hydrogenases: Preparation, Structure, and Electrocatalysis. Organometallics 2018, 37, 1280−1290. (35) Chong, D.; Georgakaki, I. P.; Mejia-Rodriguez, R.; SanabriaChinchilla, J.; Soriaga, M. P.; Darensbourg, M. Y. Electrocatalysis of Hydrogen Production by Active Site Analogues of the Iron Hydrogenase Enzyme: Structure/Function Relationships. Dalton Trans 2003, 4158−4163. (36) Tye, J. W.; Lee, J.; Wang, H. W.; Mejia-Rodriguez, R.; Reibenspies, J. H.; Hall, M. B.; Darensbourg, M. Y. Dual Electron Uptake by Simultaneous Iron and Ligand Reduction in an NHeterocyclic Carbene Substituted [FeFe] Hydrogenase Model Compound. Inorg. Chem. 2005, 44, 5550−5552. (37) Mazzoni, R.; Gabiccini, A.; Cesari, C.; Zanotti, V.; Gualandi, I.; Tonelli, D. Diiron Complexes Bearing Bridging Hydrocarbyl Ligands as Electrocatalysts for Proton Reduction. Organometallics 2015, 34, 3228−3235. (38) Mejia-Rodriguez, R.; Chong, D.; Reibenspies, J. H.; Soriaga, M. P.; Darensbourg, M. Y. The Hydrophilic Phosphatriazaadamantane Ligand in the Development of H2 Production Electrocatalysts: Iron Hydrogenase Model Complexes. J. Am. Chem. Soc. 2004, 126, 12004− 12014. (39) Felton, G. A. N.; Mebi, C. A.; Petro, B. J.; Vannucci, A. K.; Evans, D. H.; Glass, R. S.; Lichtenberger, D. L. Review of Electrochemical Studies of Complexes Containing the Fe2S2 Core Characteristic of [FeFe]-Hydrogenases Including Catalysis by These Complexes of the Reduction of Acids to Form Dihydrogen. J. Organomet. Chem. 2009, 694, 2681−2699. (40) Li, R. X.; Liu, X. F.; Liu, T.; Yin, Y. B.; Zhou, Y.; Mei, S. K.; Yan, J. Electrocatalytic Properties of [FeFe]-Hydrogenases Models and Visible-Light-Driven Hydrogen Evolution Efficiency Promotion with Porphyrin Functionalized Graphene Nanocomposite. Electrochim. Acta 2017, 237, 207−216. (41) Fourmond, V.; Jacques, P.-A.; Fontecave, M.; Artero, V. H2 Evolution and Molecular Electrocatalysts: Determination of Overpotentials and Effect of Homoconjugation. Inorg. Chem. 2010, 49, 10338−10347. (42) Tatematsu, R.; Inomata, T.; Ozawa, T.; Masuda, H. Electrocatalytic Hydrogen Production by a Nickel(II) Complex with a Phosphinopyridyl Ligand. Angew. Chem., Int. Ed. 2016, 55, 5247−5250. J

DOI: 10.1021/acs.organomet.8b00759 Organometallics XXXX, XXX, XXX−XXX