PNP-Chelated and -Bridged Diiron Dithiolate Complexes Fe2(μ-pdt

Apr 12, 2018 - PNP-Chelated and -Bridged Diiron Dithiolate Complexes Fe2(μ-pdt)(CO)4{(Ph2P)2NR} Together with Related Monophosphine Complexes for ...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

PNP-Chelated and -Bridged Diiron Dithiolate Complexes Fe2(μpdt)(CO)4{(Ph2P)2NR} Together with Related Monophosphine Complexes for the [2Fe]H Subsite of [FeFe]-Hydrogenases: Preparation, Structure, and Electrocatalysis Pei-Hua Zhao,*,† Zhong-Yi Ma,† Meng-Yuan Hu,† Jiao He,‡ Yan-Zhong Wang,† Xing-Bin Jing,† Hui-Yu Chen,† Zheng Wang,‡ and Yu-Long Li*,‡ †

School of Materials Science and Engineering, North University of China, Taiyuan 030051, People’s Republic of China College of Chemistry and Environmental Engineering, Sichuan University of Science & Engineering, Zigong 643000, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: As the [2Fe]H subsite models of [FeFe]-hydrogenases, a series of PNP-chelated and -bridged diiron dithiolate complexes 1a−f and 2a−f together with the three related monophosphine complexes 3a−c were prepared by the selective substitutions of the all-carbonyl complex Fe2(μpdt)(CO)6 (A, pdt = SCH2CH2CH2S) with aminodiphosphines (Ph2P)2NR (denoted as PNP) under different reaction conditions. The first UV irradiation of the toluene solutions of A with different PNP ligands (PNP = (Ph2P)2NR; R = (CH2)3Me, (CH2)3NMe2, (CH2)3Si(OEt)3, C6H5, C6H4OMe-p, C6H4CO2Me-p) readily afforded the target PNP-chelated complexes Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2NR} (1a−f), while the reflux of xylene solutions of A with the aforementioned PNP ligands produced the PNP-bridged complexes Fe2(μ-pdt)(CO)4{(μ-Ph2P)2NR} (2a−f). Comparatively, treatments of A and one type of PNP ligand with N-aryl substituents R (R = C6H5, C6H4OMe-p, C6H4CO2Me-p) in MeCN at room temperature in the presence of the decarbonylating agent Me3NO·2H2O formed the unexpected monophosphine complexes Fe2(μ-pdt)(CO)5{κ1-Ph2P(NHR)} (3a−c) and the minor chelated complexes 1d−f. All of the complexes 1a−f, 2a−f, and 3a−c have been characterized by elemental analysis, FT-IR, NMR spectroscopy, and particularly for 1a,b,d−f, 2b,d−f, and 3b by X-ray crystallography. Additionally, the electrochemical and electrocatalytic properties of complexes 1a and 2a as a pair of representative isomers have been evaluated and compared by cyclic voltammetry in MeCN as solvent in the absence and presence of HOAc as a proton source.

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INTRODUCTION

Scheme 1. Proposed Structure of the Active Site of [FeFe]hydrogenases

Over the past decades, the bioinorganic chemistry of diiron dithiolate complexes has attracted remarkable attention, mainly due to the realization that they closely resemble the two-iron subunit of the active site of [FeFe]-hydrogenases as highly efficient dihydrogen (H2) production catalysts in nature.1 Spectroscopic, crystallographic, and theoretical investigations have revealed that the active site of [FeFe]-hydrogenases (the so-called H-cluster, Scheme 1) contains a butterfly [Fe2S2] cluster (denoted as [2Fe]H) for the catalytically active center and a cubic [Fe4S4] subcluster for the electron transfer channel,2−4 which are linked together via a sulfur atom of the cysteine. In the H-cluster, the two iron atoms in the [2Fe]H subsite are coordinated by a dithiolate cofactor and several diatomic CO/CN− ligands. Meanwhile, the bridging dithiolate cofactor has been definitively determined to be an azadithiolate (adt, SCH2NHCH2S) in the enzyme,5 although it can be replaced by using chemically synthesized [2Fe]H cofactors to have either O or CH2 in place of NH.6,7 © XXXX American Chemical Society

Prompted by the well-elucidated structure of the H-cluster proposed above, there has so far been wide interest in the Received: January 16, 2018

A

DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Preparation of the Chelated Complexes 1a−f through UV Irradiation

Scheme 3. Preparation of the Bridged Complexes 2a−f

Scheme 4. Preparation of the Chelated Complexes 1a−f and Monophosphine Complexes 3a−c

{RPCH2NR′}2 that have been well developed by DuBois et al.22 In this context, Hogarth, Song, and we reported that only a few examples of PNP-chelated diiron complexes of the type Fe2(μ-pdt)(CO)4{(κ2-Ph2P)NR}, which can be regarded as an important class of the [2Fe]H subsite models of [FeFe]hydrogenases, were produced by the reactions of Fe2(μpdt)(CO)6 (A) with PNP ligands in the presence of Me3NO/ MeCN.23−26 On the basis of the new findings and to develop the synthetic methodology of [FeFe]-hydrogenase models, we have successfully explored an efficient way to prepare the unique and rare PNP-chelated diiron complexes using UV irradiation in this study. According to our new strategy, the new series of PNP-chelated complexes 1a−f can be readily prepared upon UV irradiation of the toluene solutions of A with different PNP ligands. Afterward, to perform a detailed comparative study of PNP-chelated and -bridged diiron models, we refluxed xylene solutions of A with the aforementioned PNP ligands to afford the target bridged complexes 2a−f, the isomers of 1a−f. Meanwhile, we found that the oxidative decarbonylation of A and one type of the PNP ligands used above with an N-aryl substituent (R) using Me3NO produced the unexpected monophosphine complexes 3a−c and the minor chelated complexes 1d−f. Most importantly, the spectroscopic, crystallographic, and electrochemical comparisons of PNP-chelated and -bridged isomeric pairs are well described. Herein, we report the facile synthesis, structural characterization, and electrochemical properties of a series of new PNP-chelated and

design and preparation of diiron dithiolate complexes as the [2Fe]H subsite models of [FeFe]-hydrogenases.8−10 Among these model complexes, diphosphine-chelated diiron complexes are considerably attractive.11−16 This is because that the asymmetry of the diiron center in the chelated complexes is a desirable feature of the biomimetic models of [FeFe]hydrogenases, as suggested by theoretical studies;17 meanwhile, the diphosphines as strong electron donors can make the diiron center more electron rich and thus more readily accept protons for fast catalytic H2 evolution. Currently, there are two main types of diphosphine ligands favoring the chelating coordination mode, which can be introduced into the butterfly [Fe2S2] cluster core to form the popular chelated complexes. One type is rigid two-carbon-backbone-containing diphosphines such as cis-bis(diphosphino)ethene (dppv) and its analogues.11−14 The other type is one-carbon-backbone-containing diphosphines such as 1,1‘-bis(diphosphino)methane (dppm) and its derivatives.15,16 However, relative to such diphosphines as dppv and dppm, the aminodiphosphines (Ph2P)2NR with a small P−N− P bite angle (PNP ligands) are of particular interest due to the fact that (i) PNP can be easily obtained by treatment of Ph2PCl and primary amines in the precense of Et3N,18,19 (ii) the pendant base in PNP is deemed to be valuable for the functional modes of proton reduction to dihydrogen,20,21 and (iii) the choice of their substituents in the (Ph2P)2NR ligands can readily tune the nature of the pendant N atom for proton relay during the catalysis, as similarly observed in the flexible base containing diphosphines of the type {R2PCH2}2NR′ and B

DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Comparisons of IR (νC≡O) and 31P{1H} NMR Data for Complexes 1a−f, 2a−f, and 3a−c Complex

IR for νC≡O (KBr disk, cm−1)

Fe2(μ-pdt)(CO)4{(κ -Ph2P)2N(CH2)3Me} (1a) Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(CH2)3NMe2} (1b) Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(CH2)3Si(OEt)3} (1c) Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2NC6H5} (1d) Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(C6H4OMe-p)} (1e) Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(C6H4CO2Me-p)} (1f) Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(CH2)3Me} (2a) Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(CH2)3NMe2} (2b) Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(CH2)3Si(OEt)3} (2c) Fe2(μ-pdt)(CO)4{(μ-Ph2P)2NC6H5} (2d) Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(C6H4OMe-p)} (2e) Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(C6H4CO2Me-p)} (2f) Fe2(μ-pdt)(CO)5{k1-Ph2P(NHC6H5)} (3a) Fe2(μ-pdt)(CO)5{k1-Ph2P(NHC6H4OMe-p)} (3b) Fe2(μ-pdt)(CO)5{k1-Ph2P(NHC6H4CO2Me-p)} (3c)

2014/1947/1935/1901 2013/1943/1935/1900 2020/1954/1935/1891 2016/1950/1940/1909 2013/1949/1928/1906 2017/1955/1930/1906 1993/1960/1926/1911 1995/1959/1921/1909 1989/1959/1920/1908 1995/1959/1925/1909 1996/1962/1933/1917 1995/1962/1926/1916 2037/1984/1951/1932 2042/1982/1954/1928 2043/1981/1956/1930

2

31

P{1H} NMR (CDCl3, 85% H3PO4, ppm) 98.01/112.55 98.00/112.72 97.64/112.61 97.90/117.45 98.52/117.86 100.19/119.73 119.16 119.60 119.44 123.99 124.40 123.92 ((CD3)2CO) 92.35 92.71 93.96

pdt)(CO)5{κ1-(Ph2P)2N(aryl)}] (C),24,26 and (iii) the species C undergoes two pathways, with one being observed for the hydrolysis of C with H2O to form the monophosphine complexes 3a−c and the other being observed for the intramolecular decarbonylation of C with one free phosphorus atom of the PNP ligand to produce the chelated complexes 1d−f. However, this proposed mechanism needs to be investigated further. Spectroscopic Characterization of Complexes 1a−f, 2a−f, and 3a−c. The as-obtained complexes 1a−f, 2a−f, and 3a−c are new except for 2a and are air-stable solids, which have been characterized by elemental analysis and various spectroscopic techniques. Their FT-IR and 31P{1H} NMR data are given in Table 1. The IR spectra of the chelated complexes 1a−f demonstrate four strong absorption bands in the range of 2020−1891 cm−1 for their carbonyl groups (Table 1), which are identical with those of the chelated complexes Fe2(μ-pdt)(CO)4{(κ2Ph2PCH2)2NR} (R = Me, (CH2)2Me) and Fe2(μ-pdt)(CO)4{(κ2-PhPCH2NPh)2} with base-containing diphosphines.30−32 By comparison, the IR spectra of the bridged complexes 2a−f show four strong absorption bands in the region of 1996−1907 cm−1 for their terminal carbonyls (Table 1), the first CO frequencies of which are further shifted toward lower energy relative to those of 1a−f. This observation demonstrates well that the energy of the first CO band attributed to the Fe(CO)2P unit in 2a−f is lower than that of the first CO band assigned to the Fe(CO)3 moiety in 1a−f.11a Meanwhile, the IR spectra of the monophosphine complexes 3a−c exhibit the CO absorption bands in the range of 2043− 1928 cm−1 (Table 1), which are shifted toward higher energy than those of 1a−f and 2a−f. This implies that the number of CO subsititution in 3a−c is less than those of 1a−f and 2a−f, that is, the former are monosubstituted relative to the latter. The 31P{1H} NMR spectra of the chelated complexes 1a−f display both a large singlet at ca. 98 ppm and a small singlet at ca. 112 (for 1a−c) or 117 (for 1d−f) ppm attributed to their dibasal and apical−basal isomers, respectively. Such an observation is consistent with the fact that the 31P{1H} NMR spectra usually contain a strong phosphorus signal in the region of 97−101 ppm for the dibasal isomer and a weak phosphorus signal in the range of 112−118 ppm for the apical−basal isomer, as observed in the similar chelated complexes Fe2(μpdt)(CO)4{(κ2-Ph2P)2NR} (R = CHMe2, CH2CHCH2,

-bridged diiron dithiolate complexes together with three new monophosphine complexes related to the [2Fe]H subsite of [FeFe]-hydrogenases.

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RESULTS AND DISCUSSION Preparation of Complexes 1a−f, 2a−f, and 3a−c. In order to explore an efficient way to prepare a new type of chelated diiron models, the six new PNP-chelated complexes Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2NR} (1a−f) were successfully synthesized in satisfactory yields through the first UV irradiation of the dry toluene solutions of Fe2(μ-pdt)(CO)6 (A) with different PNP ligands (PNP = (Ph2P)NR; R = (CH2)3Me, (CH2)3NMe2, (CH2)3Si(OEt)3, C6H5, C6H4OMep, C6H4CO2Me-p) using an LED lamp emitting at 365 nm, as shown in Scheme 2. Correspondingly, the series of PNP-bridged complexes Fe2(μ-pdt)(CO)4{(μ-Ph2P)2NR} (2a−f), the isomers of 1a− f, were prepared in 22−87% yields through the carbonyl substitution reactions of A with the PNP ligands used above in dry xylene at reflux, as shown in Scheme 3. In contrast to the new strategy described above, we also tried to prepare the chelated complexes 1a−f by the traditional oxidative decarbonylation, as displayed in Scheme 4. The roomtemperature treatments of A and one type of PNP ligands with N-alkyl substituents (R = (CH2)3Me, (CH2)3NMe2, (CH2)3Si(OEt)3) in dry MeCN in the presence of the decarbonylating agent Me3NO·2H2O afforded the sole products 1a−c in satisfactory yields. Similar reactions of A and the other type of PNP ligands with N-aryl substituents (R = C6H5, C6H4OMe-p, C6H4CO2Me-p) resulted in the obtainment of the target complexes 1d−f in very low yields and the unexpected formation of the monophosphine complexes Fe2(μ-pdt)(CO)5{κ1-Ph2P(NHR)} (3a−c) as the major products. In addition, although the true reason for the unexpected formation of 3a−c is unclear, we consider that both N-aryl substituents (R) in PNP ligands and water molecules in Me3NO·2H2O may play an important role in the generation of 3a−c on the basis of the experimental results observed from Schemes 2 and 4 as well as the reported similar cases.26−28 Thus, a possible pathway for the concomitant formation of 3a−c during the preparation of 1d−f is proposed as follows: (i) the reaction of A with 1 equiv of Me3NO·2H2O in dry MeCN gives the species [Fe2(μpdt)(CO)5L] (B, L = NCMe, NMe3),29 (ii) the intermediate B further reacts with (Ph2P)2N(aryl) to afford the species [Fe2(μC

DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 1. Molecular structures of complexes 1a,b,d−f. Hydrogen atoms and solvent molecules have been omitted for clarity.

CH2CHMe2, (CH2)2NMe2, C6H4Me-p).23−26 It can be noted that the ratios of the dibasal and apical−basal isomers in 1a−c,e (ca. 3:1) are slightly lower than those in 1d,f (ca. 4:1), demonstrating that the dibasal conformation is favored over the apical−basal form in solution and the isomer ratios can be affected by the steric and electronic nature of the PNP ligands. In contrast, the 31P{1H} NMR spectra of the bridged complexes 2a−f exhibit only one sharp singlet at ca. 119 (for 2a−c) or ca. 124 (for 2d−f) ppm for the two chemically equivalent phosphorus atoms of their two Fe(CO)2P units, which are shifted by more than 20 ppm downfield relative to the major singlet (ca. 98 ppm) given in the 31P{1H} NMR spectra of 1a− f. This is probably because the two P atoms of PNP ligands in 2a−f are each coordinated respectively to the two electronwithdrawing Fe(CO)2 groups, whereas the two P atoms in 1a− f are connected simultaneously to only one electron-withdrawing Fe(CO) moiety.25 It is worth pointing out that the phosphorus signals of 2a−c are shifted much upfield by about 5 ppm with respect to those of 2d−f, apparently owing to the stronger electron-donating capabilities of N-alkyl substituents of PNP ligands in 2a−c in comparison to those of the N-aryl substituents in 2d−f. Meanwhile, the 31P{1H} NMR spectra of

the monophosphine complexes 3a−c show only one singlet at 92.35, 92.71, and 93.96 ppm assigned to the respective coordinated-P atoms, which matches well with the 31P NMR signals in the range of 90−100 ppm reported in their analogues such as Fe2(μ-pdt)(CO)5{κ1-Ph2P(NHR)} (R = C6H4Cl-p, C6H4NO2-p, C6H4CO2Et-p, C6H4Br-p, C6H4Me-p).26,27 In addition, the 1H NMR spectra of 1a−f, 2a−f, and 3a−c all display the typical proton signals in the downfield region of 7.9−6.0 ppm for their phenyl groups and in the upfield region of 2.5−1.6 ppm for their pdt bridges. Obviously, in comparison to those of 1a−f and 2a−f, the 1H NMR spectra of 3a−c exhibit an additional broad singlet at 5.45 and 5.24 ppm as well as a doublet at 5.78 ppm with the coupling contact 2JPH = 17.4 Hz for their NH groups, respectively.26,27 This reveals that the molecular structures of 3a−c are distinct from those of 1a−f and 2a−f, which is further confirmed by the following X-ray single-crystal diffraction analysis. Molecular Structures of PNP-Chelated and -Bridged Complexes 1a,b,d−f and 2b,d−f Together with Monophosphine Complex 3b. The molecular structures of 1a,b,d−f, 2b,d−f, and 3b have been unambiguously determined by X-ray crystallography, as depicted in Figures 1−3. Their D

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Figure 2. Molecular structures of complexes 2b,d−f. Hydrogen atoms and solvent molecules have been omitted for clarity.

Ph2P)2X} (X = CH2, CMe2, NCHMe2, NCH2CHCH2, N(CH2)2NMe2, NC6H4Me-p).15,16,23−26 This is obviously different from the apical−basal geometry of diphosphines in the known chelated complexes Fe 2 (μ-pdt)(CO) 4 {(κ 2 Ph2PCH2)2X} (X = CH2, NMe, N(CH2)2Me) and Fe2(μpdt)(CO)4{(κ2-PhPCH2NPh)2}.13,30−32 Most importantly, another distinctive feature is that the small P−Fe−P bite angles in 1a−f (71−72°) are characteristic of dibasal chelated complexes with small-bite-angle diphosphines15,16,23−26 and are much smaller than the corresponding angles (92−95°) of the apical− basal chelated complexes with base-containing diphosphines.13,30−32 Additionally, the Fe1−Fe2 bond lengths in 1a (2.5892(8) Å), 1b (2.5900(9) Å), 1d (2.5957(6) Å), 1e (2.5961(9) Å), and 1f (2.5945(9) Å) are comparable to those of analogues where the diphosphines are the aforementioned (Ph2P)2X, (Ph2PCH2)2X, and (PhPCH2-

selected bond lengths and bond angles are given in Table S1 in the Supporting Information. As illustrated in Figure 1, the solid-state structures of the chelated complexes 1a,b,d−f all consist of a typical butterfly [Fe2S2] cluster, wherein each iron atom adopts a pseudosquare-pyramidal geometry, as reported for most of the known phosphine-substituted diiron dithiolate complexes. It is worth noting that the two P atoms of the PNP ligands in 1a,b,d−f lie in the dibasal site and are chelated asymmetrically to one of their two iron atoms in the crystalline state, thus resulting in the formation of the two chemically equivalent phosphorus atoms, which corresponds to a large phosphorus signal at ca. 98 ppm for the major dibasal isomer in CDCl3 solution in their 31P{1H} NMR spectra. Moreover, the dibasal coordination of PNP ligands in 1a,b,d−f are in good accordance with those observed in the similar chelated complexes Fe2(μ-pdt)(CO)4{(κ2E

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Figure 3. Molecular structure of complex 3b. Hydrogen atoms have been omitted for clarity.

NPh)2,13,15,16,23−26,30−32 which are very close to those occurring in [FeFe]-hydrogenase enzymes (2.55−2.60 Å).2,3 This outcome implies that the steric and electronic characters of the iron centers in the chelated complexes 1a,b,d−f and their analogues might be very similar to those found in [FeFe]hydrogenase active sites. As depicted in Figure 2, the bridged complexes 2b,d−f each feature a butterfly-shaped [Fe2S2] cluster similar to that in their respective isomers 1b,d−f described above. Interestingly, the two P atoms of PNP ligands in 2b,d−f occupy the cisoid dibasal site and are symmetrically bridged between the two Fe atoms, which is quite consistent with the 31P{1H} NMR spectroscopic observation that there is only one sharp singlet for the two chemically equivalent phosphorus atoms. Furthermore, the cisoid dibasal coordination manners of PNP ligands in 2b,d−f match well with those reported in the known pdt-bridged diiro n comp lexes with bridging dip hosphine ligands.15,16,23,25,33−35 This may indicate that the cisoid dibasal configuration of diphosphines in 2b,d−f is thermodynamically favored due to the presence of the steady five-membered Fe1P1N1P2Fe2 ferracycle.25,34 It is worth noting that the remarkable structure change between 2b,d−f and their isomers 1b,d−f is the opening up of the P−N−P angle: that is to say, the P1−N1−P2 bond angles in 2b,d−f are significantly larger by 21.15, 21.93, 22.27, and 22.80° than those in 1b,d−f, respectively. This difference is obviously due to the fact that a less-steric five-membered Fe1P1N1P2Fe2 ferracycle is formed between the bridging PNP ligands and the two Fe atoms in 2b,d−f, whereas a large-steric four-membered FeP1N1P2 ferracycle is produced by the chelating PNP ligands and only one Fe atom in 1b,d−f. In addition, the Fe1−Fe2 bond lengths in 2b (2.4732(6) Å), 2d (2.4710(17) Å), 2e (2.4733(5) Å), and 2f (2.4750(4) Å) are closer to those of their pdt-type analogues15,16,23,25,33−35 but much shorter by 0.1168, 0.1247, 0.1228, and 0.1195 Å than those of isomers 1b,d−f, respectively. As shown in Figure 3, the monophosphine complex 3b has a known butterfly [Fe2S2] skeleton with five carbonyls, a propanedithiolate bridge, and an aminomonophosphine ligand. Obviously, the P atom of the ligand Ph2P(NHC6H4OMe-p) in 3b is bound in the apical position of the distorted-squarepyramidal coordination geometry of one iron atom, which is consistent with those found in the similar diiron monophosphine complexes.26,27,33,36,37 Notably, the Fe1−Fe2 bond

length in 3b (2.5006(10) Å) is slightly longer than that of the corresponding brigded complex 2e (2.4733(5) Å), both of which are similar to that of their precursor A (2.5103(11) Å) but considerably shorter than those of the chelated complex 1e (2.5772(13) Å) and the natural [FeFe]-hydrogenases (2.55− 2.60 Å).2,3,38 This comparison indicates well that the molecular features of the PNP-chelated diiron model complexes are favored over those of the related bridged and monophosphine derivatives in building biomimetic models for the active site of [FeFe]-hydrogenases. Electrochemical and Electrocatalytic Studies of PNPChelated and -Bridged Complexes 1a and 2a. In order to compare the PNP-chelated and -bridged complexes as the functional diiron models for catalytic proton reduction to H2, we selected the PNP-chelated and -bridged isomeric pair 1a and 2a as representative complexes and determined their electrochemical properties by cyclic voltammetry (CV). The CV studies of 1a and 2a were performed in 0.1 M n-Bu4NPF6/ MeCN solution at a scan rate of 0.1 V s−1 in the absence or presence of HOAc as a proton source. The relevant electrochemical data for complexes 1a and 2a are given in Table 2. Table 2. Relevant Electrochemical Data for Complexes 1a and 2a complex

Epc (V)

Epa1 (V)

ip (μA)

icat (μA)

icat/ip

1a 2a

−2.22 −2.19

−0.09 +0.33

21.12 42.10

157.60 184.54

7.46 4.38

First of all, the electrochemical properties of isomeric pair 1a and 2a have been studied without HOAc (0 mM), as displayed in Figure 4. For the chelated complex 1a (black line in Figure 4), the first reduction process takes place at −2.22 V and the first oxidation process occurs at −0.09 V (Table 2), which should be assigned respectively to the reduction of FeIFeI to Fe0FeI and the oxidation of FeIFeI to FeIIFeI.25,39−41 In contrast, for the bridged isomer 2a (blue line in Figure 4), the first reduction process occurs at −2.19 V and the first oxidation process takes place at +0.33 V (Table 2), ascribed respectively to the reduction of FeIFeI to Fe0FeI and the oxidation of FeIFeI to FeIIFeI.25,33,39−41 Meanwhile, it should be noted that the large difference between the redox properties of the isomeric pair is the easier oxidation of the chelated complex 1a in F

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[1a(H2)]+ ([(H2)FeII-FeI]+) accepts an electron to produce H2 and regenerate 1a to thereby complete the catalytic cycle. In addition, to further evaluate the electrocatalytic abilities for H2 production catalyzed by 1a and 2a, the ratio of the catalytic current (icat) to the reductive peak current (ip) without the added acid can be used as a marker to compare the catalytic efficiency of different complexes, in which higher values of icat/ ip indicate faster catalysis.25,43−45 It is seen from Table 2 that, at the highest concentration of HOAc (10 mM), the icat/ip value of the chelated complex 1a is equal to 7.46, which is apparently larger than that of the bridged isomer 2a (icat/ip = 4.38). To further confirm the electrocatalytic H2 evolution by 1a and 2a, we carried out controlled-potential electrolysis in MeCN solutions of 1a and 2a (1.0 mM) with excess HOAc (30 mM) at −2.24 V. During 1/2 h of the electrolysis process, total charge amounts of the corresponding 57.8 and 13.2 F mmol−1 of 1a and 2a passed, which correspond to theoretical turnover numbers (TONs) of 28.9 and 6.6, respectively. These results indicate that the chelated complex 1a has a better electrocatalytic H2 production ability in comparison to its bridged isomer 2a, which supports well the theoretical conclusion that “the asymmetry of the diiron center in the chelated complexes is a desirable feature of the biomimetic models”.17

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

comparison to the bridged isomer 2a, obviously due to the observation that the first oxidation peak of the former is shifted by 0.42 V toward more negative potentials in comparison to that of the latter (Table 2). Further electrocatalytic behaviors of proton reduction to H2 catalyzed by the isomeric pair 1a and 2a were investigated in the presence of HOAc (0−10 mM). As illustrated in Figure 5, the initial reduction peak currents of 1a and 2a increased dramatically and linearly upon consecutive additions of HOAc: i.e., the current intensities (icat) of the reduction peaks have a linear correlation relative to the concentration ([HOAc]) of the added acid (insert in Figure 5). Typically, such observations suggest an electrocatalytic process for the proton reduction to H2.25,42−44 Thus, the isomeric pair 1a and 2a is active for electrocatalytic H2 production with the weak acid HOAc. On the basis of the aforementioned observations and the previously reported similar case,25,42−44 we might propose the ECCE mechanism for H2 production catalyzed by 1a and 2a in the presence of HOAc. The proposed ECCE catalytic mechanism of 1a as a representative compound is described in the following: the neutral compound 1a (denoted [FeI-FeI]0) is first reduced at −2.22 V to form monoanion [1a]− ([Fe0FeI]−), and then it is protonated by HOAc to give the protonated species [1a(H)] ([HFeII-FeI]0). After [1a(H)] is further protonated by HOAc, the resulting monocation

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SUMMARY AND CONCLUSIONS In this work, we explored an efficient way to prepare the new series of PNP-chelated diiron dithiolate complexes 1a−f, which are considered as desirable biomimetic models for the [2Fe]H subsite of [FeFe]-hydrogenases. Complexes 1a−f can be easily obtained in satisfactory yields through the first UV irradiation reactions of precursor A with different PNP ligands in dry toluene. Correspondingly, the bridged counterparts 2a−f were prepared by the reflux reactions of A with the same PNP ligands in dry xylene. Meanwhile, we found that the roomtemperature treatments of A and one type of N-aryl-containing PNP ligands in the presence of Me3NO/MeCN afforded the chelated complexes 1d−f in very low yields and the monophosphine complexes 3a−c as major products, whereas similar reactions of A and the other type of N-alkyl-containing PNP ligands produced the sole chelated complexes 1a−c in satisfactory yields. These new complexes 1a−f, 2a−f, and 3a−c have been well characterized by elemental analysis, various spectra, and X-ray crystallography. The IR (νC≡O) data suggest that the order of

Figure 5. Cyclic voltammograms of 1a and 2a (1.0 mM) with HOAc (0, 2, 4, 6, 8, 10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1. Insert: plots of icat (μA) versus HOAc concentration (mM). All potentials are versus the ferrocene/ferrocenium (Fc/Fc+) couple. G

DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX

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Organometallics the first CO absorption frequency is described as follows: bridged complexes 2a−f (ca. 1990 cm−1) < chelated complexes 1a−f (ca. 2020 cm−1) < monophosphine complexes 3a−c (ca. 2040 cm−1). A 31P{1H} NMR spectroscopic study reveals that the chelated complexes 1a−f exist as a mixture of dibasal and apical−basal isomers in solution, whereas the bridged counterparts 2a−f have only one species. An X-ray crystallographic investigation indicates that the chelated complexes 1a,b,d−f adopt an unsymmetrical dibasal form and display a very small P−Fe−P bite angle in the solid state, while the bridged counterparts 2b,d−f have a cisoid dibasal geometry and contain a steady five-membered Fe1P1N1P2Fe2 ferracycle. Most siginficantly, X-ray comparisons of the four isomeric pairs 1b/2b, 1d/2d, 1e/2e, and 1f/2f indicate that (i) the Fe−Fe bond lengths of PNP-chelated complexes are closer to those found in [FeFe]-hydrogenase enzymes relative to their bridged isomers, which supports well the theory that asymmetric substitution of strong donor ligands results in the formation of better synthetic diiron models of the [FeFe]-hydrogenase active site as suggested by Tye, Darensbourg, and Hall,17 and (ii) the P−N−P bond angles of PNP ligands have remarkably increased by ca. 22° from chelating to bridging geometry, clearly confirming the result that the PNP-chelated complexes can convert to the PNP-bridged isomers at higher temperature as reported by Hogarth and Song.23,25 In addition, electrochemical and electrocatalytic comparisons of the isomeric pair 1a and 2a have shown that the former has higher icat/ip and TON values in comparison to the latter for electrocatalytic H2 evolution using HOAc as a proton source. This probably reveals that the PNP-chelated diiron model has the superior electrocatalytic ability for proton reduction to H2 in contrast to the PNP-bridged model.

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P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 112.55 (s, apical− basal isomer, 25%), 98.01 (s, basal−basal isomer, 75%) ppm. Method ii. A mixture of Fe2(μ-pdt)(CO)6 (0.194 g, 0.5 mmol), (Ph2P)2N(Bu-n) (0.266 g, 0.6 mmol), and Me3NO·2H2O (0.067 g, 0.6 mmol) was dissolved in dry MeCN (20 mL). The reaction mixture was stirred at ambient temperature for 0.5 h until TLC monitoring showed that the starting material Fe2(μ-pdt)(CO)6 had been completely consumed. Aftet the same workup as in method i, complex 1a was obtained as a brown-red solid (0.250 g, 65% yield) and identified by comparison with authentic samples (method i) using TLC, IR, and NMR techniques. TLC analysis indicates that the brown-red solids obtained from methods i and ii show the same Rf value. Preparation of Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(CH2)3NMe2} (1b). Complex 1b was prepared by a procedure similar to that for complex 1a (methods i and ii), except that (Ph2P)2N(CH2)3NMe2 (0.080 g, 0.18 mmol and 0.283 g, 0.6 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.080 g, 0.18 mmol and 0.266 g, 0.6 mmol) and the residue was subjected to flash column chromatography on silica gel with CH2Cl2/MeOH (20/1, v/v) as eluent. From the main brown-red band, complex 1b was obtained as a brown-red solid. Method i. Yield: 0.062 g (52%). Anal. Calcd for C36H38Fe2N2O4P2S2: C, 54.02; H, 4.78; N, 3.50. Found: C, 54.20; H, 4.57; N, 3.26. FT-IR (KBr disk): νC≡O 2013 (vs), 1943 (s), 1935 (s), 1900 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.93−7.39 (m, 20H, 4 × PC6H5), 3.13−2.90 (m, 2H, NCH2), 2.38−1.29 (m, 16H, (SCH2)2CH2) and CH2CH2N(CH3)2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 112.72 (s, apical−basal isomer, 25%), 98.00 (s, basal−basal isomer, 75%) ppm. Method ii. Yield: 0.356 g (89%). The target product 1b was identified by comparison with authentic samples (method i) using TLC, IR, and NMR techniques. TLC analysis indicates that the brown-red solids obtained from methods i and ii show the same Rf value. Preparation of Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(CH2)3Si(OEt)3} (1c). Complex 1c was prepared by a procedure similar to that for complex 1a (methods i and ii), except that (Ph2P)2N(CH2)3Si(OEt)3 (0.080 g, 0.18 mmol and 0.354 g, 0.6 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.080 g, 0.18 mmol and 0.266 g, 0.6 mmol) and the residue was subjected to flash column chromatography on silica gel with EtOAc/petroleum ether (1/10, v/v) as eluent. From the main brown-red band, complex 1c was obtained as a brown-red solid. M e th o d i . Y ie l d 0. 06 0 g ( 4 4% ) . A na l . C a lc d fo r C40H47Fe2NO7P2S2Si: C, 52.24; H, 5.15; N, 1.52. Found: C, 52.36; H, 5.29; N, 1.69. FT-IR (KBr disk): νC≡O 2020 (vs), 1954 (vs), 1935 (vs), 1891 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.54− 7.48 (m, 10H, 2 × PC6H5), 7.16−7.05 (m, 10H, 2 × PC6H5), 3.38 (s, 6H, 3 × OCH2), 2.84−2.73 (m, 2H, NCH2), 1.95−1.84 (m, 4H, 2 × SCHeHa and CH2), 1.51−1.05 (m, 4H, 2 × SCHeHa and NCH2CH2), 0.84 (s, 9H, 3 × OCH2CH3), 0.002 (s, 2H, SiCH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 112.61 (s, apical−basal isomer, 25%), 97.64 (s, basal−basal isomer, 75%) ppm. Method ii. Yield 0.252 g (55%). The target product 1c was identified by comparison with authentic samples (method i) using TLC, IR, and NMR techniques. TLC analysis indicates that the brown-red solids obtained from methods (i) and (ii) show the same Rf value. Preparation of Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2NC6H5} (1d). Complex 1d was prepared by a procedure similar to that for complex 1a (method i), except that (Ph2P)2NC6H5 (0.083 g, 0.18 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.080 g, 0.18 mmol) and the residue was chromatographed by preparative TLC separation with CH2Cl2/ petroleum ether (1/2, v/v) as eluent. From the main brown-red band, complex 1d was afforded as a brown-red solid (0.059 g, 50% yield). Anal. Calcd for C37H31Fe2NO4P2S2: C, 56.15; H, 3.95; N, 1.77. Found: C, 56.39; H, 4.22; N, 1.96. FT-IR (KBr disk): νC≡O 2016 (vs), 1950 (s), 1940 (s), 1909 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.83−7.36 (m, 20H, 4 × PC6H5), 6.98−6.55 (m, 5H, NC6H5), 2.17− 2.07 (m, 4H, 2 × SCHeHa and CH2), 1.78−1.57 (m, 2H, 2 × SCHeHa) 31

EXPERIMENTAL SECTION

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 and xylene were distilled under N2 from sodium/benzophenone ketyl. Me3NO·2H2O was commercially available and was used as received. (Ph2P)2NR (R = (CH2)3Me, (CH2)3NMe2, (CH2)3Si(OEt)3, C6H5, C6H4OMe-p, C6H4CO2Me-p) and Fe 2 (μ-pdt)(CO) 6 (A) were prepared according to the literature.19,38 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) and neutral Al2O3 (75−150 μm). Elemental analyses were 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. Preparation of Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(CH2)3Me} (1a). Method i. A mixture of Fe2(μ-pdt)(CO)6 (0.058 g, 0.15 mmol) and (Ph2P)2N(CH2)3Me (0.080 g, 0.18 mmol) was dissolved in dry toluene (90 mL) in a Pyrex Schlenk tube. The reaction mixture was photolyzed using an LED lamp at 365 nm for 2 h until TLC monitoring showed that the starting material Fe2(μ-pdt)(CO)6 had been completely consumed. The resulting black-red solution was concentrated under vacuum to afford a black-red residue. The residue was separated by flash column chromatography on neutral Al2O3 with CH2Cl2/petroleum ether (1/2, v/v) as eluent. From the main brownred band, complex 1a was obtained as a brown-red solid (0.055 g, 48% yield). Anal. Calcd for C35H35Fe2NO4P2S2: C, 54.49; H, 4.57; N, 1.82. Found: C, 54.65; H, 4.78; N, 1.95. FT-IR (KBr disk): νC≡O 2014 (vs), 1947 (s), 1935 (s), 1901 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.72−7.46 (m, 20H, 4 × PC6H5), 2.93 (s, 2H, NCH2), 2.13− 1.71 (m, 6H, (SCH2)2CH2), 1.33−0.63 (m, 7H, CH2CH2CH3) ppm. H

DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX

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Organometallics ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 117.45 (s, apical−basal isomer, 20%), 97.90 (s, basal−basal isomer, 80%) ppm. Preparation of Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(C6H4OMe-p)} (1e). Complex 1e was prepared by a procedure similar to that for complex 1a (method i), except that (Ph2P)2N(C6H4OMe-p) (0.089 g, 0.18 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.080 g, 0.18 mmol) and the residue was chromatographed by preparative TLC separation with CH2Cl2/petroleum ether (1/2, v/v) as eluent. From the main brown-red band, complex 1e was afforded as a brown-red solid (0.059 g, 48% yield). Anal. Calcd for C38H33Fe2NO5P2S2: C, 55.56; H, 4.05; N, 1.71. Found: C, 55.79; H, 4.32; N, 1.86. FT-IR (KBr disk): νC≡O 2013 (vs), 1949 (vs), 1928 (vs), 1906 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.76−7.35 (m, 20H, 4 × PC6H5), 6.64−6.45 (m, 4H, NC6H4), 3.65 (s, OCH3), 2.21−2.11 (m, 4H, 2 × SCHeHa and CH2), 1.80−1.71 (m, 2 × SCHeHa) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 117.86 (s, apical−basal isomer, 25%), 98.52 (s, basal−basal isomer, 75%) ppm. Preparation of Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(C6H4CO2Me-p)} (1f). Complex 1f was prepared by a procedure similar to that for complex 1a (method i), except that (Ph2P)2N(C6H4CO2Me-p) (0.093 g, 0.18 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.080 g, 0.18 mmol) and the residue was chromatographed by preparative TLC separation with CH2Cl2/petroleum ether (1/2, v/v) as eluent. From the main brown-red band, complex 1f was obtained as a brown-red solid (0.057 g, 45% yield). Anal. Calcd for C39H33Fe2NO6P2S2: C, 55.14; H, 3.92; N, 1.65. Found: C, 55.36; H, 4.21; N, 1.83. FT-IR (KBr disk): νC≡O 2017 (vs), 1955 (vs), 1930 (vs), 1906 (vs) cm−1; νC(O)OMe 1722 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.84− 7.63 (m, 10H, 2 × PC6H5), 7.55 (d, J = 7.8 Hz, 2H, NC6H4), 7.50− 7.38 (m, 10H, 2 × PC6H5), 6.71−6.54 (m, 2H, NC6H4), 3.77 (s, 3H, OCH3), 2.15−2.01 (4H, 2 × SCHeHa and CH2), 1.77−1.71 (m, 2H, 2xSCHeHa) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 119.73 (s, apical−basal isomer, 18%), 100.19 (s, basal−basal isomer, 72%) ppm. Preparation of Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(CH2)3Me} (2a). A mixture of Fe2(μ-pdt)(CO)6 (0.194 g, 0.5 mmol) and (Ph2P)2N(CH2)3Me (0.266 g, 0.6 mmol) was dissolved in dry xylene (50 mL). The reaction mixture was refluxed (138 °C) for 6 h until TLC monitoring indicated that the starting material Fe2(μ-pdt)(CO)6 was completely consumed. After the solvent was removed in vacuo, the residue was chromatographed by preparative TLC separation with CH2Cl2/petroleum ether (2/3, v/v) as eluent. From the main red band, complex 2a was afforded as a red solid (0.191 g, 50% yield). Anal. Calcd for C35H35Fe2NO4P2S2: C, 54.49; H, 4.57; N, 1.82. Found: C, 54.73; H, 4.36; N, 1.99. FT-IR (KBr disk): νC≡O 1993 (vs), 1960 (vs), 1926 (vs), 1911 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.71−7.42 (m, 20H, 4 × PC6H5), 2.75 (s, 2H, NCH2), 2.07 (s, 4H, 2 × SCHeHa and CH2), 1.84 (s, 2H, 2 × SCHeHa), 0.40−0.14 (m, 7H, CH2CH2CH3) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 119.16 (s) ppm. Preparation of Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(CH2)3NMe2} (2b). Complex 2b was prepared by a procedure similar to that for complex 2a, except that (Ph2P)2N(CH2)3NMe2 (0.283 g, 0.6 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.266 g, 0.6 mmol) and the residue was subjected to flash column chromatography on silica gel with CH2Cl2/MeOH (20/1, v/v) as eluent. From the main red band, complex 2b was afforded as a red solid (0.348 g, 87% yield). Anal. Calcd for C36H38Fe2N2O4P2S2: C, 54.02; H, 4.78; N, 3.50. Found: C, 54.24; H, 5.05; N, 3.71. FT-IR (KBr disk): νC≡O 1995 (vs), 1959 (vs), 1921 (vs), 1909 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.73−7.43 (m, 20H, 4 × PC6H5), 2.82 (s, 2H, NCH2), 2.09 (s, 4H, 2 × SCHeHa and CH2), 1.86 (s, 2H, 2 × SCHeHa), 1.73 (s, 6H, N(CH3)2), 1.40 (s, 2H, NCH2), 0.40 (s, 2H, NCH2CH2CH2N) ppm. 31 1 P{ H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 119.60 (s) ppm. Preparation of Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(CH2)3Si(OEt)3} (2c). Complex 2c was prepared by a procedure similar to that for complex 2a, except that (Ph2P)2N(CH2)3Si(OEt)3 (0.354 g, 0.6 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.266 g, 0.6 mmol) and the residue was chromatographed by preparative TLC separation with CH2Cl2/petroleum ether (2/1, v/v) as eluent. From the main red

band, complex 2c was afforded as a red solid (0.102 g, 22% yield). Anal. Calcd for C40H47Fe2NO7P2S2Si: C, 52.24; H, 5.15; N, 1.52. Found: C, 52.39; H, 4.98; N, 1.73. FT-IR (KBr disk): νC≡O 1989 (vs), 1959 (vs), 1920 (vs), 1908 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.69−7.41 (m, 20H, 4 × PC6H5), 3.41 (s, 6H, 3 × OCH2), 2.69 (s, 2H, NCH2), 2.04 (s, 4H, 2 × SCHeHa and CH2), 1.83 (s, 2H, 2 × SCHeHa), 1.25 (s, 2H, NCH2CH2CH2Si), 0.96 (s, 9H, 3 × OCH2CH3), 0.49 (s, 2H, SiCH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 119.44 (s) ppm. Preparation of Fe2(μ-pdt)(CO)4{(μ-Ph2P)2NC6H5} (2d). Complex 2d was prepared by a procedure similar to that for complex 2a, except that (Ph2P)2NC6H5 (0.277 g, 0.6 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.266 g, 0.6 mmol) and the residue was purified by preparative TLC separation with CH2Cl2/petroleum ether (1/2, v/ v) as eluent. From the main red band, complex 2d was afforded as a red solid (0.262 g, 66% yield). Anal. Calcd for C37H31Fe2NO4P2S2: C, 56.15; H, 3.95; N, 1.77. Found: C, 56.34; H, 4.22; N, 1.98. FT-IR (KBr disk): νC≡O 1995 (vs), 1959 (vs), 1925 (s), 1909 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.88 (s, 4H, 2 × PC6H5-o), 7.42− 7.38 (m, 10H, 2 × PC6H5), 7.17−7.02 (m, 6H, 2 × PC6H5-m,p), 6.75−6.09 (m, 5H, NC6H5), 2.44−2.10 (m, 6H, (SCH2)2CH2) ppm. 31 1 P{ H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 123.99 (s) ppm. Preparation of Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(C6H4OMe-p)} (2e). Complex 2e was prepared by a procedure similar to that for complex 2a, except that (Ph2P)2N(C6H4OMe-p) (0.295 g, 0.6 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.266 g, 0.6 mmol) and the residue was purified by preparative TLC separation with CH2Cl2/petroleum ether (1/2, v/v) as eluent. From the main red band, complex 2e was afforded as a red solid (0.211g, 51% yield). Anal. Calcd for C38H33Fe2NO5P2S2: C, 55.56; H, 4.05; N, 1.71. Found: C, 55.38; H, 4.29; N, 1.92. FT-IR (KBr disk): νC≡O 1996 (vs), 1962 (vs), 1933 (vs), 1917 (vs) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.85 (s, 4H, 2 × PC6H5-o) 7.41 (s, 10H, 2 × PC6H5), 7.19−7.06 (m, 6H, PC6H5m,p), 6.07−5.98 (m, 4H, NC6H4), 3.54 (s, 3H, OCH3), 2.41−2.08 (m, 6H, (SCH2)2CH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 124.40 (s) ppm. Preparation of Fe2(μ-pdt)(CO)4{(μ-Ph2P)2N(C6H4CO2Me-p)} (2f). Complex 2f was prepared by a procedure similar to that for complex 2a, expept that (Ph2P)2N(C6H4CO2Me-p) (0.311 g, 0.6 mmol) was used instead of (Ph2P)2N(CH2)3Me (0.266 g, 0.6 mmol) and the residue was purified by preparative TLC separation with CH2Cl2/petroleum ether (1/2, v/v) as eluent. From the main red band, complex 2f was afforded as a red solid (0.104 g, 25% yield). Anal. Calcd for C39H33Fe2NO6P2S2: C, 55.14; H, 3.92; N, 1.65. Found: C, 55.37; H, 4.19; N, 1.83. FT-IR (KBr disk): νC≡O 1995 (vs), 1962 (vs), 1926 (vs), 1916 (s) cm−1; νC(O)OMe 1733 (m) cm−1. 1H NMR (600 MHz, (CD3)2CO, TMS): δ 7.93 (s, 4H, PC6H5-o), 7.53−7.43 (m, 10H, and 2 × PC6H5), 7.26−7.11 (m, 6H, PC6H5-m,p), 6.16−6.08 (m, 4H, NC6H4), 3.55 (s, 3H, OCH3), 2.52−2.10 (m, 6H, (SCH2)2CH2) ppm. 31P{1H} NMR (243 MHz, (CD3)2CO, 85% H3PO4): δ 123.92 (s) ppm. Preparation of Fe2(μ-pdt)(CO)5{κ1-Ph2P(NHC6H5)} (3a) and Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(C6H5)} (1d). A mixture of Fe2(μpdt)(CO)6 (0.194 g, 0.5 mmol), (Ph2P)2NC6H5 (0.277 g, 0.6 mmol), and Me3NO·2H2O (0.067 g, 0.6 mmol) was dissolved in dry MeCN (20 mL). After the reaction mixture was stirred at room temperature for 1.5 h and monitored by TLC analysis and solvent was removed under vacuum, the residue was chromatographed by preparative TLC separation with CH2Cl2/petroleum ether (1/2, v/ v) as eluent. From the first red band, complex 3a was obtained as a red solid (0.163 g, 51% yield). Anal. Calcd for C26H22Fe2NO5PS2: C, 49.16; H, 3.49; N, 2.20. Found: C, 49.33; H, 3.74; N, 2.34. FT-IR (KBr disk): νC≡O 2037 (vs), 1984 (vs), 1968 (vs), 1951 (s), 1932 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.78−7.41 (m, 10H, 2 × PC6H5), 6.99−6.54 (m, 5H, NC6H5), 5.45 (br s, 1H, NH), 1.96 (s, 2H, 2 × SCHeHa), 1.58 (s, 4H, 2 × SCHeHa and CH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 92.35 (s) ppm. From the second brown-red band, complex 1d was afforded as a brown-red solid (0.029 g, 7% yield) and identified by comparison with authentic samples (UV irradiation) using TLC, IR, and NMR techniques. TLC I

DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics analysis indicates that the brown-red solids obtained from the two methods show the same Rf value. Preparation of Fe2(μ-pdt)(CO)5{κ1-Ph2P(NHC6H4OMe-p)} (3b) and Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(C6H4OMe-p)} (1e). Complexes 3b and 1e were formed by a procedure similar to that for complexes 3a and 1d, except that (Ph2P)2N(C6H4OMe-p) (0.295 g, 0.6 mmol) was used instead of (Ph2P)2NC6H5 (0.277 g, 0.6 mmol). From the first red band, complex 3b was obtained as a red solid (0.150 g, 45% yield). Anal. Calcd for C27H24Fe2NO6PS2: C, 48.74; H, 3.64; N, 2.11. Found: C, 48.90; H, 3.83; N, 2.00. FT-IR (KBr disk): νC≡O 2042 (vs), 1982 (vs), 1954 (s), 1928 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.78−7.41 (m, 10H, 2 × PC6H5), 6.53 (s, 4H, NC6H4), 5.24 (br s, 1H, NH), 3.67 (s, 3H, OCH3), 1.95 (s, 2H, 2 × SCHeHa), 1.59 (s, 4H, 2 × SCHeHa and CH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 92.71 (s) ppm. From the second brown-red band, complex 1e was afforded as a brown-red solid (0.104 g, 25% yield) and identified by comparison with authentic samples (UV irradiation) using TLC, IR, and NMR techniques. TLC analysis indicates that the brown-red solids obtained from the two methods show the same Rf value. Preparation of Fe2(μ-pdt)(CO)5{κ1-Ph2P(NHC6H4CO2Me-p)} (3c) and Fe2(μ-pdt)(CO)4{(κ2-Ph2P)2N(C6H4CO2Me-p)} (1f). Complexes 3c and 1f were formed by a procedure similar to that for complexes 3a and 1d, except that (Ph2P)2N(C6H4CO2Me-p) (0.311 g, 0.6 mmol) was used instead of (Ph2P)2NC6H5 (0.277 g, 0.6 mmol). From the first red band, complex 3c was afforded as a red solid (0.077 g, 22% yield). Anal. Calcd for C28H24Fe2NO7PS2: C, 48.51; H, 3.49; N, 2.02. Found: C, 48.32; H, 3.70; N, 2.26. FT-IR (KBr disk): νC≡O 2043 (vs), 1981 (vs), 1956 (s), 1930 (s) cm−1; νC(O)OMe 1714 (s) cm−1. 1H NMR (600 MHz, CDCl3, TMS): δ 7.78−7.75 (m, 4H, 2 × PC6H5-o), 7.70 (d, 2H, 2JHH = 8.4 Hz, NC6H4), 7.43 (s, 6H, 2 × PC6H5-m,p), 6.55 (d, 2H, 2JHH = 8.4 Hz, NC6H4), 5.78 (d, 2JPH = 17.4 Hz, 1H, NH), 3.81 (s, OCH3), 2.02−2.00 (m, 2H, 2 × SCHeHa), 1.71−1.61 (m, 4H, 2 × SCHeHa and CH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 93.96 (s) ppm. From the second brown-red band, complex 1f was obtained as a brown-red solid (0.018 g, 4% yield) and identified by comparison with authentic samples (UV irradiation) using TLC, IR, and NMR techniques. TLC analysis indicates that the brown-red solids obtained from the two methods show the same Rf value. X-ray Crystal Structure Determination. Single crystals of complexes 1a,b,d−f, 2b,d−f, and 3b suitable for X-ray diffraction analysis were grown by slow evaporation of their CH2Cl2/hexane solutions at 5 °C. The crystals were mounted on a Bruker-CCD diffractometer. Data were collected at 123(2), 150(2), and 296(2) K using a graphite monochromator with Cu Kα or Mo Kα radiation (λ = 1.54178 or 0.71073 Å) in the ω−φ scanning mode. The structures were solved by direct methods using the SHELXS-97 program46 and refined by full-matrix least-squares techniques (SHELXL-97) on F2.46 Hydrogen atoms were located using the geometric method. Details of crystallographic data and structure refinement for 1a,b,d−f, 2b,d−f, and 3b are summarized in Tables S2 and S3 in the Supporting Information. Electrochemistry. Electrochemical and electrocatalytic properties of complexes 1a and 2a 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.

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Selected bond lengths and bond angles for 1a,b,d−f, 2b,d−f, and 3b, details of crystallographic data and structure refinement for 1a,b,d−f, 2b,d−f, and 3b, and all IR and NMR spectra for 1a−f, 2a−f, and 3a−c (PDF) Accession Codes

CCDC 1580377−1580384, 1580386, and 1590237 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]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for P.-H.Z.: [email protected]. *E-mail for Y.-L.L.: [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.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (No. 21301160 and 21501124), the Natural Science Foundation of Shanxi Province (No. 201701D121035 and 2014021019-7), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Shanxi Province, the Science & Technology Department of Sichuan Province (18YYJC1071), and the Education Department of Sichuan Province (18ZA0337) for financial support. We also thank Dr. Bao-Ping Lu at Taiyuan Normal University for assistance with the electrochemical experiments.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00030. J

DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.organomet.8b00030 Organometallics XXXX, XXX, XXX−XXX