[Fe]-Hydrogenase - ACS Publications - American Chemical Society

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Cite This: Inorg. Chem. 2017, 56, 15216−15230

Studies on Chemical Reactivity and Electrocatalysis of Two Acylmethyl(hydroxymethyl)pyridine Ligand-Containing [Fe]Hydrogenase Models (2-COCH2‑6-HOCH2C5H3N)Fe(CO)2L (L = η1‑SCOMe, η1‑2-SC5H4N) Li-Cheng Song,*,†,‡ Liang Zhu,† Fu-Qiang Hu,† and Yong-Xiang Wang† Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China

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ABSTRACT: On the basis of preparation and characterization of [Fe]-H2ase models (2-COCH2-6-HOCH2C5H3N)Fe(CO)2L (A, L = η1-SCOMe; B, L = η1-2-SC5H4N), the chemical reactivities of A and B with various electrophilic and nucleophilic reagents have been investigated, systematically. Thus, when A reacted with 1 equiv of MeCOCl in the presence of Et3N in MeCN to give the η2-SCOMe-coordinated acylation product (2-COCH2-6-MeCO2CH2C5H3N)Fe(CO)2(η2-SCOMe) (1), treatment of A with excess HBF4·Et2O in MeCN gave the cationic MeCN-coordinated complex [(2-COCH2-6-HOCH2C5H3N)Fe(CO)2(MeCN)](BF4) (2). In addition, when 2 was treated with 1 equiv of 2,6-(p-4-MeC6H4)2C6H3SK or PPh3 in CH2Cl2 to give the thiophenolato- and PPh3-substituted derivatives (2-COCH2-6-HOCH2C5H3N)Fe(CO)2[2,6-(p-MeC6H4)2C6H3S] (3) and [(2COCH2-6-HOCH2C5H3N)Fe(CO)2(PPh3)](BF4) (4), treatment of B with 1 equiv of PMe3 or P(OMe)3 in THF afforded the phosphine- and phosphite-substituted complexes (2-COCH2-6-HOCH2C5H3N)(η1-2-SC5H4N)Fe(CO)2L (5, L = PMe3; 6, L = P(OMe)3). Interestingly, in contrast to A, when B reacted with excess HBF4·Et2O in MeCN to afford the BF3 adduct [2COCH2-6-HO(BF3)CH2C5H3N]Fe(CO)2(η1-2-SC5H4N) (7), reaction of B with 1 equiv of p-MeC6H4COCl in the presence of Et3N in MeCN gave not only the expected 2-acylmethyl-6-p-toluoyloxomethylpyridine-containing complex (2-COCH2-6-pMeC6H4CO2CH2C5H3N)Fe(CO)2(η2-2-SC5H4N) (8), but also gave the unexpected 2-toluoyloxovinyl-6-toluoyloxomethylpyridine-containing complex (2-p-MeC6H4CO2C2H-6-p-MeC6H4CO2CH2C5H3N)Fe(CO)2(η2-2-SC5H4N) (9). While the possible pathways for the novel reactions leading to complexes 1, 2, and 7−9 are suggested, the structures of complexes B, 1−4, and 6−9 were unambiguously confirmed by X-ray crystallography. In addition, model complexes A and B have been found to be catalysts for proton reduction to H2 from TFA under CV conditions.

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INTRODUCTION

molecule), a pyridinol N atom, and the acyl C atom (Scheme 2a). The successful elucidation of the active site structure of [Fe]H2ase and its unique catalytic function have spurred chemists to design and synthesize quite a number of biomimetic models,10−29 and even a few of them have been found to be catalysts for H2 activation30,31 and proton reduction to H2 under CV conditions.32−34 In recent years, we have prepared a series of 2-acylmethyl-6-hydroxymethylpyridine ligand-containing [Fe]-H2ase models,21,23 in which the model complex (2-

In nature there are three major types of hydrogenases (H2ases): [NiFe]-, [FeFe]-, and [Fe]-H2ases. While [NiFe]- and [FeFe]H2ases catalyze the reversible redox reaction of dihydrogen with protons and electrons, the [Fe]-H2ase (also called Hmd) catalyzes the stereospecific hydride transfer from H2 to C14a of methenyl-tetrahydromethanopterin (methenyl-H4MPT+) to give methylene-tetrahydromethanopterin (methylene-H4MPT) and H+ (Scheme 1).1−3 The spectroscopic4−6 and, particularly, the protein crystallographic7−9 studies indicated that the active site of [Fe]-H2ase consists of a single Fe atom coordinated by two cis CO ligands, a cysteine S atom, one solvent molecule (probably a water © 2017 American Chemical Society

Received: October 8, 2017 Published: November 30, 2017 15216

DOI: 10.1021/acs.inorgchem.7b02582 Inorg. Chem. 2017, 56, 15216−15230

Article

Inorganic Chemistry Scheme 1. Catalytic Function of [Fe]-H2ase

Scheme 3

Scheme 2. (a) Active Site of [Fe]-H2ase and (b) Precursor Models

whether the two H atoms in each of the CH2CO and CH2O groups are diastereotopic or not under the determined conditions. In addition, the 1H NMR spectra of A and B displayed one additional singlet at 10.52 and 13.87 ppm for their hydroxy groups, while the 13C{1H} NMR spectra each exhibited two singlets in the region 208.1−221.1 ppm for their terminal carbonyl C atoms and one singlet at 255.8 and 256.2 ppm for their acyl carbonyl C atoms, respectively.16,24 Like the previously reported A (Figure S1),21 the molecular structure of B (Figure 1) was successfully determined by X-ray

COCH2-6-HOCH2C5H3N)Fe(CO)2(η1-SCOMe) (A) previously reported in our communication21 and the hitherto unreported model complex (2-COCH2-6-HOCH2C5H3N)Fe(CO)2(η1-2-SC5H4N) (B) (Scheme 2b) are of considerable interest since they faithfully replicate all the coordination atoms in the first coordination sphere of the Fe center in [Fe]H2ase.7−9 Note that more exact structural mimics containing the 2-acylmethyl-6-hydroxypyridine ligand were previously reported by the Hu group18 and our group.23 To further develop the new synthetic methodology for [Fe]-H2ase mimics and to prepare the new type of [Fe]-H2ase mimics, we carried out a study on reactions of A and B with some electrophilic and nucleophilic reagents. As a result, the studied reactions have allowed us to obtain a series of the new type of [Fe]-H2ase models. In addition, we also carried out a study on electrocatalysis of A and B, which has proved that both A and B can serve as catalysts for proton reduction to H2 under CV conditions. Herein, we report the interesting results obtained from these studies.

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RESULTS AND DISCUSSION Synthesis and Characterization of Starting Model Complexes (2-COCH2-6-HOCH2C5H3N)Fe(CO)2L (A, L = η1-SCOMe; B, L = η1-2-SC5H4N). In our previously published communication, the synthesis and structural characterization of model complex A were briefly reported.21 However, we further found that complex A could also be prepared by a modified method in high yield (94%). This method involves the reaction of iodide complex (2-COCH2-6-HOCH2C5H3N)Fe(CO)2I (C) with slightly excessive sodium thioacetate in THF at room temperature; in addition, model complex B could be similarly prepared by treatment of complex C with sodium 2mercaptopyridinate under the same conditions in 93% yield (Scheme 3). Complexes A and B are slightly air-sensitive yellow solids and were characterized by elemental analysis and various spectroscopy techniques. The IR spectra of A and B showed two very strong absorption bands in the range 2028−1956 cm−1 for their two terminal carbonyls, one strong band at 1665 and 1660 cm−1 for their acyl carbonyl groups, respectively.16,24 The 1H NMR spectra of A and B displayed a singlet at 4.26 and 4.22 ppm for their CH2CO groups and two doublets in the range 5.10−5.25 ppm for their CH2O groups, depending upon

Figure 1. Molecular structure of B with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity except the hydroxy H3 atom. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−C(10) 1.920(2), Fe(1)−N(1) 1.940(2), Fe(1)−O(3) 2.1054(17), Fe(1)−S(1) 2.3639(7); N(1)−Fe(1)−S(1) 88.11(6), C(10)−Fe(1)−S(1) 89.37(8), N(1)−Fe(1)−O(3) 79.46(8), N(1)− Fe(1)−C(10) 84.97(10).

crystallography. Both iron(II) centers of A and B are six coordinate, and their acylmethylpyridyl and hydroxymethylpyridyl units construct a five-membered ferracycle, respectively. In addition, the two terminal CO ligands of A and B occupy the positions cis to their acyl ligands; the sulfur atoms are located in the positions trans to one of their two CO ligands, and the acyl ligands in A and B are coordinated trans to their hydroxy 15217


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Inorganic Chemistry groups.7,8 It follows that complexes A and B faithfully replicate all the coordination atoms in the first coordination sphere of the Fe(II) center in [Fe]-H2ase,7−9 and therefore, they could be regarded as good models for the active site of [Fe]-H2ase. In addition, it should be noted that in the crystal structures of A and B there exists a type of intramolecular hydrogen bond35,36 (note that in our communication,21 this type of hydrogen bond in A was not indicated). While the hydrogen bond in A (Figure S1) is formed by interaction of the O3 atom of the thioaceto ligand with its hydroxy H4 atom (bond length O4−H4····O3 = 2.879 Å, bond angle ∠O4−H4····O3 = 126.50°), the hydrogen bond in B (Figure 1) is constructed by interaction of the N2 atom of the 2-mercaptopyridinato ligand with its hydroxy H3 atom (the bond length O3−H3····N2 = 2.556 Å, bond angle ∠O3−H3···N2 = 164.08°). Interestingly, such a type of hydrogen bonding might play an important role in stabilization of the molecular structures of A and B,35,37,38 and it would be also helpful in understanding the natural hydrogen bonding present in [Fe]-H2ase.39,40 Reaction of Starting Complex A with MeCOCl To Give Acylation Product (2-COCH2-6-MeCO2CH 2C5H 3N)Fe(CO)2(η2-SCOMe) (1). Interestingly, it was found that treatment of the tridentate 2-acylmethyl-6-hydroxymethylpyridine ligand-containing and monodentate thioacetyl ligandcontaining complex A with an equimolar amount of acetyl chloride in the presence of Et3N in MeCN gave the bidentate 2-acylmethyl-6-acetoxymethylpyridine ligand-containing and bidentate thioacetyl ligand-containing acylation product 1 in 83% yield, unexpectedly (Scheme 4).

NMR spectrum displayed two singlets at 2.20 and 2.23 ppm for the two Me groups in its MeCO2 and MeCOS groups, respectively. The 13C{1H} NMR spectrum of 1 showed two singlets at 210.5 and 221.2 ppm for its terminal carbonyl C atoms and one singlet at 256.5 ppm characteristic of its acyl carbonyl C atom. The molecular structure of complex 1 was also confirmed by the X-ray crystallographic study (Figure 2). This complex

Figure 2. Molecular structure of 1 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−C(3) 1.903(5), Fe(1)−N(1) 2.034(4), Fe(1)−O(6) 2.135(3), Fe(1)−S(1) 2.3621(15); C(3)−Fe(1)−O(6) 162.86(17), C(3)−Fe(1)−S(1) 95.47(15), N(1)−Fe(1)−O(6) 91.14(13), N(1)−Fe(1)−S(1) 88.23(11).

Scheme 4

indeed contains an Fe(II) center that is coordinated by a bidentate 2-acylmethyl-6-acetoxymethylpyridine ligand, a bidentate thioacetyl ligand, and two cis terminal carbonyl ligands. While the acylmethylpyridine moiety forms a five-membered ferracycle with its Fe(II) center, the η2-SCOMe ligand constructs a four-membered ferracycle with its Fe(II) center. In addition, the acyl ligand of 1 is trans to the O atom of its thioacetyl ligand, while the S atom of its thioacetyl ligand is trans to one of its two terminal CO ligands. Similar to A and B, complex 1 could be also regarded as a good model for the active site of [Fe]-H2ase since all the coordinated atoms around its Fe(II) center are completely the same as those around the Fe(II) center in the active site of [Fe]-H2ase.7−9 Reaction of A with HBF4·Et2O in MeCN To Give MeCNCoordinated Complex [(2-COCH2-6-HOCH2C5H3N)Fe(CO)2(MeCN)](BF4) (2). More interestingly, when A reacted with excess HBF4·Et2O in MeCN at room temperature, the MeCN-coordinated cationic complex 2 was obtained in 68% yield, unexpectedly (Scheme 5). As shown in Scheme 5, the possible pathway for formation of complex 2 also includes two elementary reaction steps. The first step involves protonation of the S atom of the η1-SCOMe ligand in A to generate the cationic intermediate m2. This step is very similar to the protonation step of the S atom in the Cys176-S ligand suggested for the heterolytic cleavage of H2 catalyzed by the active site in [Fe]-H2ase.41 The second step

As shown in Scheme 4, the conversion from complex A to product 1 includes two elementary reaction steps. The first step involves the acylation of the 6-hydroxymethyl group in A in the presence of Et3N to give intermediate m1. The second step involves the intramolecular ligand exchange in m1 between the coordinated oxomethyl O atom in the 6-acetyloxymethyl group and the uncoordinated carbonyl O atom of the η1-SCOMe ligand. It follows that in A the coordination ability of the 6hydroxymethyl O atom is greater than that of the carbonyl O atom of the η1-SCOMe ligand, but in m1 the coordination ability of the carbonyl O atom of η1-SCOMe is greater than that of the oxomethyl O atom. Complex 1 is a slightly air-sensitive yellow solid and was characterized by elemental analysis and spectroscopy. For instance, the IR spectrum showed one strong band at 1655 cm−1 for its acylmethyl carbonyl and two strong bands at 2043 and 1977 cm−1 for its terminal carbonyls. In addition, the 1H 15218


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Inorganic Chemistry Scheme 5

includes the intermolecular ligand exchange reaction of the protonated MeCOSH+ ligand in m2 with one molecule of solvent MeCN. Complex 2 is also a slightly air-sensitive yellow solid and was fully characterized by elemental analysis, spectroscopy, and Xray diffraction analysis. The IR spectrum showed two strong absorption bands at 2051 and 1986 cm−1 for its two terminal CO ligands, one strong band at 1681 cm−1 for its acyl carbonyl ligand, and one medium band at 2301 cm−1 for its cyano group in the MeCN ligand. The 1H NMR spectrum displayed one singlet at 2.40 ppm for its methyl group in MeCN and a set of doublet/doublet peaks at 4.56 and 4.70 ppm for its methylene group in the acylmethyl substituent. The 13C{1H} NMR spectrum showed one singlet at −4.1 ppm for its methyl group in MeCN, two singlets at 201.4 and 202.5 ppm for its terminal carbonyl C atoms, and one singlet at 250.9 ppm for its acyl carbonyl C atom. The molecular structure of 2 was further confirmed by X-ray crystallography (Figure 3). Interestingly, complex 2, in contrast to its neutral precursor A, is a cationic complex, which contains one cation [(2-COCH2-6-HOCH2C5H3N)Fe(CO)2(MeCN)]+ and one BF4− anion. However, the geometrical structure of the cationic moiety of complex 2 is almost the same as that of its precursor complex A, except that the 1e− ligand η1-SCOMe in A is replaced by the 2e− ligand MeCN. It is worth pointing out that in 2 there exists an intramolecular type of hydrogen bond between the hydroxy H3 atom in its cation and the nearest F1 atom in its anion BF4− (the bond length O3−H3···F1 = 2.704 Å, bond angle ∠O3−H3···F1 = 158.40°). Reaction of Complex 2 with 2,6-(p-MeC6H4)2C6H3SK or PPh3 To Give the Corresponding Ligand-Substituted Products (2-COCH 2 -6-HOC 5 H 3 N)Fe(CO) 2 [2,6-(pMeC 6H 4) 2C 6H 3S] (3) and [(2-COCH 2-6-HOC 5 H 3 N)Fe(CO)2(PPh3)](BF4) (4). We further found that the MeCNcoordinated cationic complex 2 could undergo substitution reactions with an equimolar 2,6-(p-MeC6H4)2C6H3SK or PPh3 in CH2Cl2 to afford the corresponding thiophenolato ligandmonosubstituted neutral complex 3 and PPh3-monosubstituted cationic complex 4 in 63% and 69% yields, respectively (Scheme 6). Complexes 3 and 4, like their precursor 2, are slightly airsensitive yellow solids and were fully characterized by elemental analysis and spectroscopy. For instance, in their IR spectra there are two strong absorption bands in the range 2046−1953 cm−1 for their two terminal CO ligands, and one strong band at 1666 and 1659 cm−1 for their acyl ligands, respectively. The 1H

Figure 3. Molecular structure of 2 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity except the hydroxy H3 atom. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−C(12) 1.9313(12), Fe(1)−N(1) 1.9671(11), Fe(1)−O(3) 2.1423(10), Fe(1)−N(2) 1.9415(11); N(1)−Fe(1)−N(2) 86.56(4), C(12)−Fe(1)−N(2) 84.72(5), N(2)−Fe(1)−O(3) 78.92(4), N(1)− Fe(1)−O(3) 86.49(4).

Scheme 6

NMR spectrum of 3 showed one multiplet between 3.63 and 3.74 ppm for its CH2CO group and one multiplet between 4.67 and 4.77 ppm for its CH2O group, whereas 4 displayed two doublets at 3.39 and 4.09 ppm for its CH2CO group and two doublets at 3.89 and 4.95 ppm for its CH2O group. In addition, the 13C{1H} NMR spectrum of 4 displayed two doublets in the range 200−210 ppm for its terminal carbonyl C atoms due to the coupling with its phosphine P atom, while its 31P{1H} NMR spectrum exhibited one singlet at 32.8 ppm for the P atom in its phosphine ligand. The X-ray crystallographic study revealed that complexes 3 and 4 (Figures 4 and 5) are indeed the thiophenolato 2,6-(pMeC6H4)2C6H3S ligand- and PPh3 ligand-monosubstituted derivatives of the MeCN-coordinated complex 2, respectively. Complex 3 is isostructural with neutral complex A, while complex 4 is isostructural with its cationic precursor 2. In addition, while complex 3 does not have any intramolecular hydrogen bond, complex 4 possesses the intramolecular 15219


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Inorganic Chemistry

phosphite-monosubstituted products 5 and 6 were produced in 70% and 73% yields, respectively (Scheme 7). Scheme 7

Figure 4. Molecular structure of 3 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity except the hydroxy H4 atom. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−C(3) 1.907(2), Fe(1)−N(1) 1.9619(19), Fe(1)−O(4) 2.2026(16), Fe(1)−S(1) 2.3250(7); C(3)−Fe(1)−O(4) 162.23(8), C(3)−Fe(1)−S(1) 92.89(6), N(1)−Fe(1)−O(4) 77.35(6), N(1)− Fe(1)−S(1) 83.76(5).

Complexes 5 and 6, like their precursor B, are slightly airsensitive yellow solids. Consistent with their structures shown in Scheme 7, their IR spectra showed one very strong absorption band at 1926 and 1943 cm−1 for their terminal CO ligands and one strong band at 1648 and 1624 cm−1 for their acyl carbonyl ligands, respectively. The 1H NMR spectra of 5 and 6 displayed one doublet at 1.11 and 3.58 ppm for their methyl groups in their PMe3 and P(OMe)3 ligands, whereas their 31P{1H} NMR spectra exhibited one singlet at 33.8 and 2.58 ppm for their phosphine and phosphite P atoms, respectively. In addition, the 13C{1H} NMR spectra of 5 and 6 displayed one doublet in the region 216.3−217.2 ppm for their terminal carbonyl C atoms and one doublet in the range 270.4−275.4 ppm for their acyl carbonyl C atoms due to the coupling with their P atoms, respectively. The X-ray diffraction analysis confirmed that the molecular structure of 6 (Figure 6) includes an Fe(II) center that is coordinated by a tridentate 2-acylmethyl-6-hydroxymethylpyridine ligand, one monodentate 2-mercaptopyridinato ligand,

Figure 5. Molecular structure of 4 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity, except the hydroxy H3 atom. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−P(1) 2.3356(10), Fe(1)−N(1) 1.962(2), Fe(1)−O(3) 2.108(2), Fe(1)−C(10) 1.927(3); N(1)−Fe(1)−P(1) 93.31(6), N(1)−Fe(1)−O(3) 77.24(8), C(10)−Fe(1)−N(1) 84.07(10), C(10)−Fe(1)−P(1) 92.30(8).

hydrogen bond formed by interaction of its hydroxy H3 atom with the nearest F4 atom in its BF4− anion. The bond length O3−H3···F4 is 2.595 Å, and the bond angle ∠O3−H3···F4 is 154.82°, which are very close to the corresponding values reported for such a type of hydrogen bond.35,36 Reaction of Starting Complex B with PMe3 or P(OMe)3 To Give the Corresponding Ligand-Substituted Products (2-COCH2-6-HOCH2C5H3N)Fe(CO)(η1-2-SC5H4N)L (5, L = PMe3; 6, L = P(OMe)3). Similar to the substitution reaction of cationic complex 2 with PPh3 described above, when neutral complex B was treated with an equimolar PMe3 or P(OMe)3 in THF, the corresponding phosphine- and

Figure 6. Molecular structure of 6 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity except the hydroxy H5 atom. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−P(1) 2.1598(16), Fe(1)−N(1) 1.961(3), Fe(1)−O(5) 2.131(3), Fe(1)−C(9) 1.918(5); N(1)−Fe(1)−P(1) 92.08(12), N(1)−Fe(1)−O(5) 78.09(13), C(9)−Fe(1)−O(5) 162.71(17), S(1)−Fe(1)−P(1) 172.64(6). 15220


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Inorganic Chemistry one terminal CO ligand, and one P(OMe)3 ligand. While the acyl ligand is trans to the hydroxy O5 atom, the P(OMe)3 ligand is trans to the 2-mercaptopyridinato S1 atom. In addition, this molecule has an intramolecular hydrogen bond O5−H5···N2 in which the bond length is 2.555 Å and bond angle is 170.74°. Reaction of B with HBF4·Et2O To Give BF3 Adduct [2COCH2-6-HO(BF3)CH2C5H3N]Fe(CO)2(η1-2-SC5H4N) (7). To compare the chemical reactivity of the starting complexes A and B toward HBF4·Et2O, we carried out the reaction of complex B with excess HBF4·Et2O in MeCN at room temperature. As a result, to our surprise, the reaction resulted in the formation of novel complex 7, an adduct of the hydroxymethyl O atom of complex B with BF3 in 65% yield (Scheme 8). Apparently, this Scheme 8

Figure 7. Molecular structure of 7 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity except the hydroxy H2 atom. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−C(3) 1.934(3), Fe(1)−N(1) 1.960(2), B(1)−O(4) 1.490(4), Fe(1)−S(1) 2.3455(10); C(3)−Fe(1)−N(1) 84.03(10), C(3)− Fe(1)−S(1) 85.77(8), N(1)−Fe(1)−O(4) 78.45(7), B(1)−O(4)− Fe(1) 123.71(15).

hydrogen bond (the bond length = 2.960 Å, bond angle = 150.80°) and simple N2−H2····F1 hydrogen bond (the bond length = 2.999 Å, bond angle = 145.38°). In addition, it is worth noting that the O4····H2, H2−N2, and H2····F1 distances in the simple O4····H2−N2 and N2−H2····F1 hydrogen bonds of 7 are 2.141, 0.901, and 2.213 Å, respectively. To the best of our knowledge, complex 7 is the first prepared and crystallographically characterized mononuclear Fe complex in which BF3 is added to O atom of a hydroxy group, although a similar dinuclear Fe complex was previously prepared by another method.43 Reaction of B with p-MeC6H4COCl To Give Single and Double Acylation Products (2-COCH2-6-pMeC6H4CO2CH2C5H3N)Fe(CO)2(η2-2-SC5H4N) (8) and (2p-MeC6H4CO2C2H-6-p-MeC6H4CO2CH2C5H3N)Fe(CO)2(η22-SC5H4N) (9). More interestingly, starting complex B was further found to react with an equimolar p-toluoyl chloride in MeCN in the presence of Et3N to afford not only the expected 2-acylmethyl-6-p-toluoyloxomethylpyridine ligand-containing complex 8, but also the unexpected 2-p-toluoyloxovinyl-6-ptoluoyloxomethylpyridine ligand-containing complex 9 in 60% and 20% yields, respectively (Scheme 9). Obviously, the formation of single acylation product 8 could be suggested to involve two elementary reaction steps (Scheme 9). The first step, similar to that suggested for forming intermediate m1 described above, involves acylation reaction of 6-hydroxymethyl group in B with the aid of Et3N to generate intermediate m4. The second step involves the intramolecular ligand exchange in m4 between the coordinated oxomethyl O atom in the p-toluoyloxomethyl group and the uncoordinated N atom of the η1-2-mercaptopyridinato ligand. Now, the question is how the double acylation product 9 was formed under such conditions. To answer this question, a possible pathway that involves the following two elementary reaction steps is proposed. That is, the first step includes isomerization of the keto form of the 2-acylmethyl group (this group is trans to the N atom of η2-2-SC5H4N ligand) into its enol form in

result indicates the remarkably different influence of η1-SCOMe and η1-2-SC5H4N ligands upon the chemical reactivity of A and B. That is, as described above, when A reacted with HBF4·Et2O under the same conditions, the MeCN-coordinated cationic complex 2 was produced. The possible pathway for formation of complex 7 is shown in Scheme 8, which includes two elementary reaction steps. The first step, in contrast to that for formation of complex 2, involves protonation of the pyridine N atom of η1-2-SC5H4N ligand in B to form the cationic intermediate m3. The second step involves the intramolecular nucleophilic attack of the hydroxymethyl O atom in m3 at the B atom of the BF4− anion followed by releasing one molecule of HF. Complex 7 is a slightly air-sensitive yellow solid. The IR spectrum of 7 showed two strong absorption bands at 2031 and 1966 cm−1 for its terminal carbonyls, and one band at 1666 cm−1 for its acyl carbonyl. The 1H NMR spectrum displayed two doublets at 5.02 and 5.29 ppm for its CH2O group and one multiplet between 4.26 and 4.41 ppm for its acylmethyl group. The 13C{1H} NMR spectrum exhibited two singlets at 207.0 and 211.6 ppm for its terminal carbonyl C atoms and one singlet at 259.3 ppm for its acyl carbonyl C atoms, respectively. Figure 7 shows the molecular structure of complex 7 determined by X-ray diffraction analysis. As can be seen in Figure 7, this complex is indeed an adduct of Lewis acid BF3 (derived from HBF4·Et2O)42,43 with the hydroxy O atom of its precursor B. Particularly interesting is that in complex 7 there exists a type of three-centered (also termed bifurcated) hydrogen bond,44,45 consisting of the simple O4···H2−N2 15221


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Inorganic Chemistry Scheme 9

which the enol moiety is trans to the S atom of the η2-2SC5H4N ligand (such a type of conversion is presumably due to the different trans effects of η2-2-SC5H4N ligand in 8 and 9) to give intermediate m5. Then, in the second step the enol form’s hydroxy group of m4 undergoes a second acylation to give the double acylation product 9 (Scheme 9). Apparently, the keto− enol equilibrium is in favor of the keto form since the yield of 8 is considerably higher than that of 9. Complexes 8 and 9 are also slightly air-sensitive yellow solids, and their structures were characterized by elemental analysis and spectroscopy. For example, the IR spectrum of 8 showed two strong absorption bands at 2030 and 1965 cm−1 for its two terminal carbonyls, one strong band at 1721 cm−1 for its ester carbonyl, and one strong band at 1660 cm−1 for its acylmethyl carbonyl, whereas complex 9 displayed two strong bands at 2038 and 1986 cm−1 for its two terminal CO ligands and one strong band at 1724 cm−1 for its two ester carbonyls. In addition, the 1H NMR spectrum of 8 showed one singlet at 2.41 ppm for its methyl group, whereas 9 displayed two singlets at 2.42 and 2.44 ppm for its two methyl groups. The 13C{1H} NMR spectrum of 8 displayed two singlets at 209.7 and 214.4 ppm for its two terminal carbonyl C atoms and one singlet at 263.0 ppm for its acylmethyl carbonyl C atoms; complex 9 exhibited the expected singlets in the range 213−216 ppm for its terminal carbonyl C atoms, but no 13C{1H} NMR signal was observed for the acylmethyl carbonyl, which is completely consistent with the original 2-acylmethyl substituent in B being converted into the 2-p-toluoyloxovinyl substituent. Both molecular structures of 8 and 9 were unambiguously confirmed by X-ray crystallography. As can be seen in Figure 8, complex 8 contains a bidentate 2-acylmethyl-6-toluoyloxomethylpyridine ligand, a bidentate 2-mercaptopyridinato ligand, and two terminal cis-orientated terminal CO ligands. While one CO ligand is trans to the N1 atom of the 2,6-disubstituted pyridine, another CO ligand is trans to the S1 atom of the 2mercaptopyridinato ligand. In addition, the 2-acylmethylpyridine moiety forms a five-membered ferracycle with its Fe(II) center, whereas the 2-mercaptopyridinato ligand constructs a four-membered ferracycle with its Fe(II) center. So, the structure of 8 is very similar to that of complex 1. Figure 9 shows that complex 9 contains a bidentate 2-(p-toluoyloxovinyl)-6-p-toluoyloxomethylpyridine ligand, a bidentate 2mercaptopyridinato ligand, and two cis-orientated terminal CO ligands. However, in contrast to complex 8, one of the two

Figure 8. Molecular structure of 8 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−C(10) 1.959(3), Fe(1)−N(1) 2.088(3), Fe(1)−N(2) 2.072(2), Fe(1)−S(1) 2.3659(9); C(10)−Fe(1)−N(1) 83.02(12), N(1)−Fe(1)−S(1) 88.43(7), C(10)−Fe(1)−N(2) 161.88(12), N(2)−Fe(1)−S(1) 69.66(7).

CO ligands is trans to the N2 atom of the 2,6-disubstituted pyridine, and another one is trans to the N1 atom of 2mercaptopyridinato ligand. It should be noted that in complex 9 the bond lengths between C18 and O6 (1.320 Å) and between C17 and C18 (1.322 Å) are consistent with the former being a CO single bond and the latter being a CC double bond, respectively. Electrochemical Properties and Electrocatalytic H2Producing Ability of A and B. Up to now, the electrochemical properties and electrocatalytic H2-producing ability for some [Fe]-H2ase models have been investigated by us and the others.25,32−34 That we are engaged in such a study is mainly because (i) the electrocatalytic H2 production by proton reduction belongs to one of the important chemical reactivities displayed by [Fe]-H2ase models, and (ii) such a study is related to a search for the earth-abundant metal Fe-based biomimetic catalysts used for production of H2, a “clean” and renewable 15222


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Inorganic Chemistry

processes (Figures S2 and S3), which might be assigned to the FeII/I reduction and FeIII/II oxidation, respectively;49,50 (ii) all the redox events of A and B are irreversible at the scan rates from 25 to 2500 mV s−1; and (iii) all the reduction events are diffusion-controlled, since plots of the reduction peak currents of A and B versus the square root of the scan rates (25−2500 mV s−1) are linearly correlated (Figure S4).51 Interestingly, complexes A and B were further found to have the electrocatalytic H2-producing ability from trifluoroacetic acid (TFA) under CV conditions. As can be seen in Figures 11

Figure 9. Molecular structure of 9 with ellipsoids drawn at a 30% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Fe(1)−C(18) 1.925(2), Fe(1)−N(2) 2.0651(17), Fe(1)−N(1) 2.0024(16), Fe(1)−S(1) 2.4165(6); C(18)−Fe(1)−N(2) 81.06(8), N(2)−Fe(1)−S(1) 100.23(5), N(1)−Fe(1)−N(2) 85.76(7), N(1)−Fe(1)−S(1) 69.40(5). Figure 11. Cyclic voltammograms of A (1.0 mM) with TFA in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1 (reverse scans were partly truncated for clarity).

energy source.46−48 Since the starting complexes A and B very much structurally resemble the first coordination sphere of the Fe(II) center of [Fe]-H2ase, we chose them to study their electrochemical properties and electrocatalytic H2-producing ability. First, their cyclic voltammograms were determined in MeCN solution (1 mM) with n-Bu4NPF6 as the supporting electrolyte. As shown in Figure 10, complexes A and B each

Figure 12. Cyclic voltammograms of B (1.0 mM) with TFA in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1 (reverse scans were partly truncated for clarity).

Figure 10. Cyclic voltammograms of A and B (1.0 mM) in 0.1 M nBu4NPF6/MeCN at a scan rate of 0.1 V s−1. Arrows indicate the starting potential and scan direction.

and 12, upon addition of increasing amounts of TFA to MeCN solution of A or B, the corresponding cyclic voltammograms displayed remarkably increased cathodic currents near the reduction potential. Apparently, such an observation is typical of an electrocatalytic proton reduction process.32−34 To evaluate the electrocatalytic activity of A and B, we determined their turnover frequencies (TOFs) to be 40 and 97 s−1 (Figure S5), respectively, by taking the successive cyclic voltammograms of their reaction mixtures for which the concentration of

displayed one irreversible reduction wave at Epc = −1.99 and −2.03 V and one irreversible oxidation wave at Epa = 0.53 and 0.47 V, respectively. It follows that the redox potentials of complex A were slightly shifted toward a positive direction relative to those of B. This implies that η1-SCOMe in A is a slightly stronger electron-withdrawing ligand than η1-2-SC5H4N in B. Further study on the electrochemical properties indicated that (i) all the redox events of A and B are one-electron 15223


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Inorganic Chemistry

reactions with the same reagent or the same type of reagent under identical conditions, such as the following: (i) While reaction of A with aliphatic acyl chloride MeCOCl in the presence of Et3N results in formation of complex 1 in which the original tridentate ligand 2-COCH2-6-HOCH2C5H3N and monodentate ligand SCOMe in A are converted to the acylated bidentate ligand 2-COCH2-6-MeCO2CH2C5H3N and bidentate ligand SCOMe, respectively, treatment of B with aromatic acyl chloride p-MeC6H4COCl in the presence of Et3N affords not only complex 8 (in which the original tridentate ligand 2COCH2-6-HOCH2C5H3N and monodentate ligand SC5H4N in B are converted to the singly acylated bidentate ligand 2COCH2-6-p-MeC6H4CO2CH2C5H3N and bidentate ligand SC5H4N), but also complex 9 (in which those original tridentate and bidentate ligands in B are converted to the doubly acylated bidentate ligand 2-p-MeC6H4CO2C2H-6-pMeC6H4CO2CH2C5H3N and bidentate ligand SC5H4N). (ii) While reaction of A with HBF4·Et2O in MeCN produces the MeCN-coordinated cationic complex 2 in which the original tridentate 2-COCH2-6-HOCH2C5H3N ligand in A remains unchanged but the original monodentate SCOMe ligand is replaced by solvent MeCN, treatment of B with HBF4·Et2O in MeCN yields the BF3 adduct 7 in which the original monodentate SC5H4N ligand in B remains unchanged, but the original tridentate ligand 2-COCH2-6-HOCH2C5H3N is turned to the Lewis acid BF3-containing tridentate ligand (2COCH2-6-HO(BF3)CH2C5H3N. All the model complexes 1, 2, and 7−9 produced from the unexpected reactions are novel, and the possible reaction pathways are suggested. In addition, the structural characterizations by spectroscopy and X-ray crystallography have proven that all complexes A, B, and 1−9 could be regarded as the structural models for [Fe]-H2ase due to their structural similarity with the active site of [Fe]-H2ase. More interestingly, model complexes A and B have been found to be electrocatalysts for proton reduction to H2 from TFA under CV conditions. In addition, compared to A, complex B should be regarded as a better catalyst, since the TOF and TON values of B are greater than those corresponding to A and the overpotential of B is smaller than that of A.

TFA was systematically increased until the TOF value remained constant (Figure 13).

Figure 13. Dependence of TOF for A (▲) and B (■) (1.0 mM) upon TFA concentration in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1.

To examine the catalytic efficiency of A and B, we further determined their overpotentials in MeCN according to Artero’s relationship (note that overpotential is defined as the difference between the potential at half-maximum of the catalytic current and the theoretical half-wave reduction potential ET1/2 of the acid).52−54 Since the ET1/2 of 300 mM TFA in MeCN is −0.64 V52 and the half-wave potentials of A and B determined under the same conditions are −1.70 and −1.60 V, the overpotentials for H2 production catalyzed by A and B should be 1060 and 960 mV, respectively (Figures S6 and S7). Obviously, the overpotentials of A and B are close to those of the previously reported [Fe]-H2ase model complexes Fe(RNPyS4)(CO)33 (note that the reported overpotentials of these complexes are 670−750 mV obtained by Evans’ relationship,55,56 but the corresponding overpotetials obtained by Artero’s overpotentials are 910−990 mV). In order to confirm the electrocatalytic H2 production catalyzed by A and B, the bulk electrolysis of a MeCN solution of A or B (0.5 mM) was carried out with a large excess TFA (15 mM) at −2.0 V. During 0.5 h of the bulk electrolysis, a total of 26.4 F/mmol for A and 32.4 F/mmol for B passed, which corresponds to turnover numbers (TONs) of 13.2 and 16.2, respectively. Gas chromatography showed that the yields of H2 produced during the bulk electrolysis are all above 94%. It follows that complex B should be regarded as a better electrocatalyst than A for proton reduction to H2 from TFA under the given conditions, since the TOF/TON values of B are larger than those of A and the overpotential value of B is smaller than that of A.

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EXPERIMENTAL SECTION

General Comments. All reactions were carried out using standard Schlenk and vacuum-line techniques under an atmosphere of highly purified nitrogen or argon. MeCN and CH2Cl2 were distilled under N2 from CaH2, while THF was distilled from sodium/benzophenone ketyl. Acetyl chloride, p-MeC6H4COCl, HBF4·Et2O (50−55% in Et2O), PPh3, P(OMe)3, PMe3 (1 M in THF), and some other materials are available commercially and used without further purification. Iodide complex (2-COCH2-6-HOCH2C5H3N)Fe(CO)I,21 NaSCOMe,57 and 2-NaSC5H4N58 were prepared according to the published methods. 2,6-(p-MeC6H4)2C6H3SK was prepared by reaction of 2,6-(p-MeC6H4)2C6H3SH59 with 1 equiv of t-BuOK in THF at room temperature. Solid IR spectra were recorded on a BioRad FTS 135 infrared spectrophotometer. 1H, 13C{1H}, and 31P{1H} NMR spectra were obtained on a Bruker Avance 400 NMR spectrometer. Elemental analyses were performed on an Elementar Vario EL analyzer. Melting points were determined on a SGW X-4 microscopic melting point apparatus and were uncorrected. Preparation of (2-COCH2-6-HOCH2C5H3N)Fe(CO)2(η1-SCOMe) (A). A solution of iodide complex (2-COCH2-6-HOCH2C5H3N)Fe(CO)2I (0.078 g, 0.20 mmol) and NaSCOMe (0.029 g, 0.3 mmol) in THF (10 mL) was stirred at room temperature for 4 h. After volatiles were removed at reduced pressure, the residue was subjected to column chromatography (silica gel). Elution with CH2Cl2/acetone (10:1, v/v) developed a yellow band, from which A (0.063 g, 94%) was obtained

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SUMMARY AND CONCLUSIONS Having prepared [Fe]-H2ase models A and B, a comparative study on some reactions of A and B has been performed in order to see the influence of the ligands η1-SCOMe in A and η12-C5H4N in B upon their chemical reactivities and to prepare the new type of [Fe]-H2ase models. Interestingly, it has been found that the two structurally different ligands may enable A and B to undergo not only the expected ligand exchange reactions, but also to undergo the following unexpected 15224


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Inorganic Chemistry as a yellow solid. Mp 129−130 °C. Anal. Calcd for C12H11FeNO5S: C, 42.75; H, 3.29; N, 4.15. Found: C, 43.00; H, 3.42; N, 4.31. IR (KBr disk): νOH 3512 (w); νC≡O 2028 (vs), 1965 (vs); νCH2C=O 1665 (s); νSC=O 1608 (s) cm−1. 1H NMR (400 MHz, d6-acetone): 2.24 (s, 3H, CH3), 4.26 (s, 2H, CH2CO), 5.15, 5.25 (dd, AB system, J = 15.2 Hz, 2H, CH2O), 7.55 (d, J = 7.2 Hz, 1H, 3-H of C5H3N), 7.66 (d, J = 6.8 Hz, 1H, 5-H of C5H3N), 8.05 (t, J = 7.0 Hz, 1H, 4-H of C5H3N), 10.52 (s, 1H, OH) ppm. 13C{1H} NMR (100 MHz, d6-acetone): 37.0 (s, CH3), 65.8 (s, CH2CO), 70.3 (s, CH2O), 120.1−161.2 (m, C5H3N), 207.9 (s, SCO), 212.3, 221.1 (2s, CO), 255.8 (s, CH2CO) ppm. Preparation of (2-COCH2-6-HOCH2C5H3N)Fe(CO)2(η1-2-SC5H4N) (B). The same procedure as that for A was followed, except that 2NaSC5H4N (0.040 g, 0.3 mmol) was utilized in place of NaSCOMe. Complex B (0.069 g, 93%) was obtained as a yellow solid. Mp 135− 136 °C. Anal. Calcd for C15H12FeN2O4S: C, 48.41; H, 3.25; N, 7.53. Found: C, 48.27; H, 3.34; N, 7.48. IR (KBr disk): νOH 3562 (w); νC≡O 2020 (vs) 1956 (vs); νC=O 1660 (s) cm−1. 1H NMR (400 MHz, d6acetone): 4.22 (s, 2H, CH2CO), 5.10, 5.23 (dd, AB system, J = 16.8 Hz, 2H, CH2O), 6.92 (t, J = 6.0 Hz, 1H, 5-H of SC5H4N), 7.27 (d, J = 8.0 Hz, 1H, 3-H of SC5H4N), 7.37 (t, J = 8.2 Hz, 1H, 4-H of SC5H4N), 7.46 (d, J = 7.6 Hz, 1H, 3-H of C5H3N), 7.61 (d, J = 7.6 Hz, 1H, 5-H of C5H3N), 7.97 (t, J = 8.0 Hz, 1H, 4-H of C5H3N), 8.02 (d, J = 4.8 Hz, 1H, 6-H of SC5H4N), 13.87 (s, 1H, OH) ppm. 13C{1H} NMR (100 MHz, d6-acetone): 65.0 (s, CH2CO), 68.4 (s, CH2O), 118.1−174.9 (m, C5H3N, SC5H4N), 208.1, 213.0 (2s, CO), 256.2 (s, CH2CO) ppm. Preparation of (2-COCH2-6-MeCO2CH2C5H3N)Fe(CO)2(η2-SCOMe) (1). To a solution of A (0.274 g, 0.81 mmol) in MeCN (25 mL) was added Et3N (0.11 mL, 0.81 mmol) at 0 °C, and then the resulting mixture was stirred at this temperature for 30 min. To this stirred mixture was added MeCOCl (0.057 mL, 0.81 mmol), and then the new mixture was warmed to room temperature and stirred at this temperature for 3 h. After volatiles were removed at reduced pressure, the residue was subjected to column chromatography (silica gel). Elution with petroleum ether/acetone (3:1, v/v) developed a yellow band, from which 1 (0.254 g, 83%) was obtained as a yellow solid. Mp 83−84 °C. Anal. Calcd for C14H13FeNO6S: C, 44.35; H, 3.46; N, 3.69. Found: C, 44.10; H, 3.54; N 3.90. IR (KBr disk): νC≡O 2043 (vs), 1977 (vs); νOC=O 1736 (vs); νCH2C=O 1655 (vs); νSC=O 1605 (m) cm−1. 1 H NMR (400 MHz, d6-acetone): 2.20, 2.23 (2s, 6H, CH3CO2, CH3COS), 4.07, 4.62 (dd, J = 20.0 Hz, 2H, CH2CO), 5.39, 5.47 (dd, AB system, J = 16.0 Hz, 2H, CH2OCO), 7.66−7.71 (m, 2H, 3,5-H of C5H3N), 8.10 (t, J = 8.0 Hz, 1H, 4-H of C5H3N) ppm. 13 C{1H} NMR (100 MHz, d6-acetone): 20.1, 34.8 (2s, CH3), 61.1 (s, CH2CO), 66.0 (s, CH2OCO), 121.5−161.6 (m, C5H3N), 169.7 (s SCO), 207.2 (s, OCO), 210.5, 221.2 (2s, CO), 256.5 (s, CH2CO) ppm. Preparation of [(2-COCH2-6-HOCH2C5H3N)Fe(CO)2(MeCN)](BF4) (2). To a solution of complex A (0.174 g, 0.51 mmol) in MeCN (10 mL) was added HBF4·Et2O (0.14 mL, 1.0 mmol), and then the resulting mixture was stirred at room temperature for 10 min. Volatiles were removed at reduced pressure to give a viscous solid. After this solid was washed with Et2O and dried under vacuum, product 2 (0.136 g, 68%) was obtained as a yellow solid. Mp 173 °C (dec). Anal. Calcd for C12H11BF4FeN2O4: C, 36.97; H, 2.84; N, 7.19. Found: C, 36.75; H, 2.68; N, 6.98. IR (KBr disk): νOH 3447 (w); νN≡C 2301 (m); νC≡O 2051 (vs), 1986 (vs); νC=O 1681 (vs) cm−1. 1H NMR (400 MHz, d6acetone): 2.40 (s, 3H, CH3), 4.56, 4.70 (dd, J = 22.0 Hz, 2H, CH2C O), 5.45−5.54 (m, 2H, CH2O), 7.68 (d, J = 8.0 Hz, 1H, 3-H of C5H3N), 7.77 (d, J = 8.0 Hz, 1H, 5-H of C5H3N), 8.18 (t, J = 8.0 Hz, 1H, 4-H of C5H3N) ppm. 13C{1H} NMR (100 MHz, d6-DMSO): −4.1 (s, CH3), 58.6 (s, CH2CO), 62.9 (s, CH2O), 113.9, 115.6, 134.9, 153.9 (4s, C5H3N, NC), 201.4, 202.5 (2s, CO), 250.9 (s, CH2CO) ppm. Preparation of (2-COCH 2 -6-HOCH 2 C 5 H 3 N)Fe(CO) 2 [2,6-(pMeC6H4)2C6H3S] (3). To a solution of complex 2 (0.130 g, 0.33 mmol) in CH2Cl2 (10 mL) was added 2,6-(p-MeC6H4)2C6H3SK (0.100 g, 0.33 mmol). The resulting mixture was stirred at room temperature for 1 h, and then, the mixture was subjected to column

chromatography (silica gel). Elution with petroleum ether/acetone (3:1, v/v) developed a brown-yellow band, from which 3 (0.114 g, 63%) was obtained as a yellow solid. Mp 135 °C (dec). Anal. Calcd for C30H25FeNO4S: C, 65.34; H, 4.57; N, 2.54. Found: C, 65.09; H, 4.67; N, 2.57. IR (KBr disk): νOH 3669 (m); νC≡O 2017 (vs), 1953 (vs); νC=O 1666 (s) cm−1. 1H NMR (400 MHz, d6-acetone): 2.37 (s, 6H, 2CH3), 3.63−3.74 (m, 2H, CH2CO), 4.67−4.77 (m, 2H, CH2O), 7.13−7.42 (m, 13H, C6H3, 2C6H4, 3,5-H of C5H3N), 7.90 (t, J = 8.0 Hz, 1H, 4-H of C5H3N) ppm. 13C{1H} NMR (100 MHz, CDCl3): 20.2 (s, CH3), 62.2 (s, CH2CO), 66.0 (s, CH2O), 116.5−159.1 (m, C6H3, C6H4, C5H3N), 204.1, 209.6 (2s, CO), 250.6 (s, CH2CO) ppm. Preparation of [(2-COCH2-6-HOCH2C5H3N)Fe(CO)2(PPh3)](BF4) (4). To a solution of complex 2 (0.078, 0.20 mmol) in CH2Cl2 (10 mL) was added PPh3 (0.053 g, 0.20 mmol). The resulting mixture was stirred at room temperature for 6 h, and then volatiles were removed at reduced pressure to give a yellow solid. After this, the solid was washed successfully with Et2O and hexane/CH2Cl2 (10:1, v/v), and finally dried under vacuum to give product 4 (0.072 g, 69%) as a yellow solid. Mp 148 °C (dec). Anal. Calcd for C28H23BF4FeNO4P: C, 55.03; H, 3.79; N, 2.29. Found: C, 54.93; H, 3.85; N, 2.41. IR (KBr disk): νOH 3646 (m); νC≡O 2046 (vs), 1986 (vs); νC=O 1659 (s) cm−1. 1 H NMR (400 MHz, d6-acetone): 3.39, 4.09 (dd, J = 20.0 Hz, 2H, CH2CO), 3.89, 4.95 (dd, J = 18.0 Hz, 2H, CH2O), 7.30−7.56 (m, 17H, 3C6H5, 3,5-H of C5H3N), 7.97 (t, J = 8.0 Hz, 1H, 4-H of C5H3N) ppm. 13C{1H} NMR (100 MHz, CDCl3): 63.9 (s, CH2C O), 68.1 (s, CH2O), 118.6−161.9 (m, C6H5, C5H3N), 201.2 (d, JC−P = 50.3 Hz, CO), 208.8 (d, JC−P = 32.0 Hz, CO), 261.3 (d, JC−P = 22.0 Hz, CH2CO) ppm. 31P{1H} NMR (162 MHz, d6-acetone, 85% H3PO4): 32.8 (s, PPh3) ppm. Preparation of (2-COCH2-6-HOCH2C5H3N)Fe(CO)(η1-2-SC5H4N)(PMe3) (5). To a solution of complex B (0.075 g, 0.20 mmol) in THF (10 mL) was added PMe3 (0.2 mL, 0.20 mmol). The solution was stirred at room temperature for 8 h. After volatiles were removed at reduced pressure, the residue was subjected to column chromatography (silica gel). Elution with CH2Cl2/acetone (5:1, v/v) developed a yellow band, from which 5 (0.059 g, 70%) was obtained as a yellow solid. Mp 139−140 °C. Anal. Calcd for C17H21FeN2O3PS: C, 48.59; H, 5.04; N, 6.67. Found: C, 48.71; H, 5.14; N, 6.43. IR (KBr disk): νOH 3645 (w); νC≡O 1926 (vs); νC=O 1648 (s) cm−1. 1H NMR (400 MHz, d6-acetone): 1.11 (d, JP−H = 9.6 Hz, 9H, 3CH3), 3.82, 3.92 (dd, AB system, J = 21.2 Hz, 2H, CH2CO), 5.04, 5.10 (dd, AB system, J = 16.8 Hz, 2H, CH2O), 6.73 (t, J = 5.6 Hz, 1H, 5-H of SC5H4N), 7.15−7.23 (m, 2H, 3-H and 4-H of SC5H4N), 7.35 (d, J = 7.6 Hz, 1H, 3-H of C5H3N), 7.52 (d, J = 8.0 Hz, 1H, 5-H of C5H3N), 7.83−7.87 (m, 2H, 4-H of C5H3N and 6-H of SC5H4N), 14.25 (s, 1H, OH) ppm. 13C{1H} NMR (100 MHz, d6-acetone): 14.2 (s, CH3), 64.7 (s, CH2CO), 67.4 (s, CH2O), 116.1−179.2 (m, C5H3N, SC5H4N), 217.1 (d, JC−P = 30.9, CO), 275.3 (d, JC−P = 26.8, CH2CO) ppm. 31 1 P{ H} NMR (162 MHz, d6-acetone, 85% H3PO4): 33.8 (s, PMe3) ppm. Preparation of (2-COCH2-6-HOCH2C5H3N)Fe(CO)(η1-2-SC5H4N)[P(OMe)3] (6). To a solution of complex B (0.075 g, 0.20 mmol) in THF (10 mL) was added P(OMe)3 (24 μL, 0.20 mmol). The resulting mixture was stirred at room temperature for 8 h. After volatiles were removed at reduced pressure, the residue was subjected to column chromatography (silica gel). Elution with CH2Cl2/acetone (5:1, v/v) developed a yellow band, from which 6 (0.068 g, 73%) was obtained as a yellow solid. Mp 132−133 °C. Anal. Calcd for C17H21FeN2O6PS: C, 43.61; H, 4.52; N, 5.98. Found: C, 43.36; H, 4.76; N, 6.00. IR (KBr disk): νOH 3522 (m); νC≡O 1943 (vs); νC=O 1624 (s) cm−1. 1H NMR (400 MHz, d6-acetone): 3.58 (d, JP−H = 10.8 Hz, 9H, 3CH3), 3.92, 4.04 (dd, AB system, J = 20.8 Hz, 2H, CH2CO), 5.04, 5.15 (dd, AB system, J = 16.4 Hz, 2H, CH2O), 6.80 (t, J = 4.4 Hz, 1H, 5-H of SC5H4N), 7.23−7.26 (m, 2H, 3-H and 4-H of SC5H4N), 7.34 (d, J = 7.6 Hz, 1H, 3-H of C5H3N), 7.52 (d, J = 8.0 Hz, 1H, 5-H of C5H3N), 7.86 (t, J = 7.8 Hz, 1H, 4-H of C5H3N), 7.92 (d, J = 5.2 Hz, 1H, 6-H of SC5H4N), 14.45 (s, 1H, OH) ppm. 13C{1H} NMR (100 MHz, d6acetone): 52.4 (s, CH3), 64.9 (s, CH2CO), 67.7 (s, CH2O), 116.4− 178.5 (m, C5H3N, SC5H4N), 216.5 (d, JC−P = 41.8, CO), 270.6 (d, 15225


Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details for B, 1, and 2 mol formula mol wt cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/g cm−3 abs coeff/mm−1 F(000) index ranges

no. reflns no. indep reflns 2θmax/deg R Rw GOF largest diff peak, hole/e Å−3

B

1

2

C15H12FeN2O4S 372.18 monoclinic P121/n1 7.2501(16) 11.375(2) 18.841(4) 90 99.350(5) 90 1533.1(5) 4 1.612 1.140 760 −9 ≤ h ≤ 8 −13 ≤ k ≤ 14 −24 ≤ l ≤ 24 19196 3535 55.162 0.0341 0.1099 1.182 1.241/−0.445

C14H13FeNO6S 379.17 monoclinic P21/c 8.5877(17) 11.795(2) 15.719(3) 90 103.10(3) 90 1550.7(5) 4 1.624 1.136 776 −10 ≤ h ≤ 9 −14 ≤ k ≤ 14 −18 ≤ l ≤ 18 11511 2723 50.016 0.0570 0.1545 0.905 0.596/−0.885

C12H11BF4FeN2O4 389.89 monoclinic P21/n 13.882(2) 7.9460(10) 15.145(2) 90 114.230(6) 90 1523.4(4) 4 1.700 1.055 784 −22 ≤ h ≤ 22 −12 ≤ k ≤ 12 −24 ≤ l ≤ 24 27259 6705 69.944 0.0383 0.0955 1.062 0.509/−0.603

as a yellow solid. Mp 89−90 °C. Anal. Calcd for C23H18FeN2O5S: C, 56.34; H, 3.70; N, 5.71. Found: C, 56.15; H, 3.91; N, 5.57. IR (KBr disk): νC≡O 2030 (vs), 1965 (vs); νOC=O 1721 (s); νCH2C=O 1660 (s) cm−1. 1H NMR (400 MHz, d6-acetone): 2.41 (s, 3H, CH3), 3.96, 4.61 (dd, J = 20.0 Hz, 2H, CH2CO), 4.94, 5.78 (dd, J = 14.0 Hz, 2H, CH2OCO), 6.78 (d, J = 8.0 Hz, 1H, 3-H of SC5H4N), 6.96 (t, J = 8.0 Hz, 1H, 5-H of SC5H4N), 7.34 (d, J = 8.0 Hz, 2H, 3,5-H of C6H4), 7.41 (t, J = 8.0 Hz, 1H, 4-H of SC5H4N), 7.66 (t, J = 8.0 Hz, 2H, 3,5-H of C5H3N), 7.91 (d, J = 8.0 Hz, 2H, 2,6-H of C6H4), 8.03 (t, J = 8.0 Hz, 1H, 4-H of C5H3N), 8.46 (d, J = 8.0 Hz, 1H, 6-H of SC5H4N) ppm. 13C{1H} NMR (100 MHz, d6-acetone): 21.6 (s, CH3), 63.7 (s, CH2CO), 67.7 (s, CH2OCO), 118.9−166.3 (m, C6H4, C5H3N, SC5H4N), 177.7 (s, CH2OCO), 209.7, 214.4 (2s, CO), 263.0 (s, CH2CO) ppm. Electrochemical and Electrocatalytic Experiments. Acetonitrile (HPLC grade) was purchased from Amethyst Chemicals. For electrochemical and electrocatalytic experiments in MeCN, a 0.1 M solution of n-Bu4NPF6 was used as supporting electrolyte. n-Bu4NPF6 electrolyte was dried in an oven at 110 °C for at least 24 h. Argon was sparged through the solutions for at least 10 min before measurements. The measurements were made using a BAS Epsilon potentiostat. All voltammograms were obtained in a three-electrode cell with a 3 mm diameter glassy carbon working electrode, a platinum counter electrode, and a Ag/Ag+ (0.01 M AgNO3/0.1 M n-Bu4NPF6 in MeCN) reference electrode under an atmosphere of Ar. The working electrode was polished with 0.05 μm alumina paste and sonicated in water for about 10 min. All potentials are quoted against the Fc/Fc+ potential. Gas chromatography was performed with a Shimadzu gas chromatograph GC-2014 under isothermal conditions with nitrogen as a carrier gas and a thermal conductivity detector. X-ray Crystal Structure Determinations of B, 1−4, and 6−9. Single crystals of B, 1, 3, 8, and 9 suitable for X-ray diffraction analysis were grown by slow diffusion of hexane into their CH2Cl2 solutions at −5 °C, while those of 2 were grown by slow evaporation of its CH2Cl2/hexane solution at room temperature, those of 4 and 7 by slow diffusion of hexane into their acetone solutions at room temperature, and those of 6 by slow diffusion of hexane into its acetone solutions at −15 °C, respectively. A single crystal of B or 4

JC−P = 37.0, CH2CO) ppm. 31P{1H} NMR (162 MHz, d6-acetone, 85% H3PO4): 2.6 (s, P(OMe)3) ppm. Preparation of [2-COCH2-6-HO(BF3)CH 2C5H3N]Fe(CO)2(η1-2SC5H4N) (7). To a solution of complex B (0.124 g, 0.33 mmol) in MeCN (20 mL) was added HBF4·Et2O (0.09 mL, 0.67 mmol). The resulting mixture was stirred at room temperature for 5 min. Volatiles were removed at reduced pressure to give a yellow viscous solid. After this, the solid was washed with Et2O and dried under vacuum, and product 7 (0.191 g, 65%) was obtained as a yellow solid. Mp 168−169 °C. Anal. Calcd for C15H12BF3FeN2O4S: C, 40.76; H, 3.19; N, 6.34. Found: C, 40.50; H, 3.17; N, 6.11. IR (KBr disk): νC≡O 2031 (vs), 1966 (vs); νC=O 1666 (s) cm−1. 1H NMR (400 MHz, d6-acetone): 4.26−4.41 (m, 2H, CH2CO), 5.02, 5.29 (dd, J = 20.0 Hz, 2H, CH2O), 7.40−8.15 (m, 7H, C5H3N, SC5H4N), 13.07 (s, 1H, OH) ppm. 13C{1H} NMR (100 MHz, d6-DMSO): 64.5 (s, CH2CO), 68.1 (s, CH2O), 113.2−168.0 (m, C5H3N, SC5H4N), 207.0, 211.6 (2s, CO), 259.3 (s, CH2CO) ppm. Preparation of (2-COCH2-6-p-MeC6H4CO2CH2C5H3N)Fe(CO)2(η2SC5H4N) (8) and (2-p-MeC6H4CO2C2H-6-p-MeC6H4CO2CH2C5H3N)Fe(CO)2(η2-SC5H4N) (9). To a solution of complex B (0.150 g, 0.40 mmol) in MeCN (20 mL) was added Et3N (0.056 mL, 0.40 mmol) at 0 °C, and then the resulting mixture was stirred at this temperature for 30 min. To this mixture was added p-MeC6H4COCl (0.053 mL, 0.40 mmol). The new mixture was warmed to room temperature and stirred at this temperature for 4 h. After volatiles were removed at reduced pressure, the residue was subjected to column chromatography (silica gel). Elution with petroleum ether/acetone (5:1, v/v) developed a yellow band, from which 9 (0.049 g, 20%) was obtained as a yellow solid. Mp 114−115 °C. Anal. Calcd for C31H24FeN2O6S: C, 61.19; H, 3.98; N, 4.60. Found: C, 61.31; H, 4.12; N, 4.88. IR (KBr disk): νC≡O 2038 (vs), 1986 (vs); νOC=O 1724 (vs) cm−1. 1H NMR (400 MHz, d6-acetone): 2.42, 2.44 (2s, 6H, 2CH3), 6.09−6.18 (m, 2H, CH2OCO), 6.77−8.16 (m, 16H, 2C6H4, C5H3N, SC5H4N, CH CFe) ppm. 13C{1H} NMR (100 MHz, d6-acetone): 21.6 (s, CH3), 69.1 (s, CH2OCO), 118.3−167.7 (m, C6H4, C5H3N, SC5H4N, CHCFe), 180.6 (s, CH2OCO), 213.4, 214.3, 215.4 (3s, CO) ppm. Further elution with petroleum ether/acetone (3:1, v/v) developed a yellow band, from which 8 (0.117 g, 60%) was obtained 15226


Article

Inorganic Chemistry Table 2. Crystal Data and Structure Refinements Details for 3, 4, and 6 mol formula mol wt cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/g cm−3 abs coeff/mm−1 F(000) index ranges


3

4

6

C30H25FeNO4S·2CH2Cl2 721.27 monoclinic P121/n1 14.978(3) 15.256(3) 15.098(3) 90 111.74(3) 90 3204.6(13) 4 1.495 0.907 1480 −19 ≤ h ≤ 19 −20 ≤ k ≤ 20 −19 ≤ l ≤ 19 31562 7643 55.876 0.0451 0.1152 1.108 0.625/−1.052

C28H23BF4FeNO4P 611.10 triclinic P1̅ 9.477(3) 10.988(4) 13.144(5) 94.111(10) 97.610(11) 90.346(11) 1353.1(18) 2 1.500 0.680 624 −11 ≤ h ≤ 10 −13 ≤ k ≤ 13 −15 ≤ l ≤ 15 12793 4687 50.04 0.0410 0.1079 1.070 1.243/−0.676

C17H21FeN2O6PS 468.24 orthorhombic P212121 8.1745(4) 15.5296(8) 16.039(8) 90 90 90 2044.34(18) 4 1.521 0.954 968 −9 ≤ h ≤ 10 −13 ≤ k ≤ 19 −20 ≤ l ≤ 19 12020 4237 52.996 0.0414 0.1018 1.033 0.550/−0.383

Table 3. Crystal Data and Structure Refinements Details for 7−9 mol formula mol wt cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/g cm−3 abs coeff/mm−1 F(000) index ranges


7

8

9

C15H12BF3FeN2O4S 439.99 monoclinic P21/n 11.351(4) 12.858(5) 11.926(4) 90 96.082(7) 90 1730.7(11) 4 1.689 1.047 888 −14 ≤ h ≤ 12 −16 ≤ k ≤ 16 −15 ≤ l ≤ 15 15660 4125 55.806 0.0425 0.1046 1.074 0.614/−0.506

C23H18FeN2O5S 490.30 monoclinic P21/n 9.2940(10) 22.767(3) 20.271(2) 90 91.109(4) 90 4288.5(8) 8 1.519 0.839 2016 −12 ≤ h ≤ 11 −30 ≤ k ≤ 30 −26 ≤ l ≤ 26 49272 10285 56.290 0.0667 0.1203 1.181 0.452/−0.517

C31H24FeN2O6 S 608.43 monoclinic P21/n 11.6188(14) 8.5406(11) 28.639(4) 90 93.274(2) 90 2837.2(6) 4 1.424 0.652 1256 −14 ≤ h ≤ 15 −9 ≤ k ≤ 11 −36 ≤ l ≤ 37 25161 6752 50.484 0.0434 0.1132 1.080 0.340/−0.612

monochromator with Mo Kα radiation (λ = 0.71073) in the ω scanning mode at 293 K, data of 3 were collected using a confocal monochromator with Mo Kα radiation (λ = 0.71073) in the ω scanning mode at 113 K. A single crystal of 2 or 8 was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Saturn 724 CCD, and their data were collected using a confocal

was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Pilatus 200 K, and their data were collected using a confocal monochromator with Mo Kα radiation (λ = 0.71073) in the ω scanning mode at 113 K. A single crystal of 1 or 3 was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Saturn70 CCD. While data of 1 were collected using a confocal 15227


Article

Inorganic Chemistry monochromator with Mo Kα radiation (λ = 0.71075) in the ω scanning mode at 113 K. A single crystal of 7 or 9 was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Saturn 724 CCD, and the data were collected using a confocal monochromator with Mo Kα radiation (λ = 0.71073) in the ω−ϕ scanning mode at 113 K. A single crystal of 6 was mounted on a Bruker SMART diffractometer equipped with an APEX II CCD area detector, and its data were collected using a graphite monochromator with Mo Kα radiation (λ = 0.71073) in the ω−ϕ scanning mode at 296 K. Data collection, reduction, and absorption correction were performed by the CRYSTALCLEAR program.60 All structures were solved by direct methods using the SHELXS-97 program61 and refined by full-matrix least-squares techniques (SHELXL-97)62 on F2. Hydrogen atoms were located by using the geometric method. Details of crystal data, data collections, and structure refinements are summarized in Tables 1−3.

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(5) Shima, S.; Lyon, E. J.; Sordel-Klippert, M.; Kauβ, M.; Kahnt, J.; Thauer, R. K.; Steinbach, K.; Xie, X.; Verdier, L.; Griesinger, C. The Cofactor of the Iron−Sulfur Cluster Free Hydrogenase Hmd: Structure of the Light-Inactivation Product. Angew. Chem., Int. Ed. 2004, 43, 2547−2551. (6) Shima, S.; Lyon, E. J.; Thauer, R. K.; Mienert, B.; Bill, E. Mössbauer Studies of the Iron−Sulfur Cluster-Free Hydrogenase: The Electronic State of the Mononuclear Fe Active Site. J. Am. Chem. Soc. 2005, 127, 10430−10435. (7) Shima, S.; Pilak, O.; Vogt, S.; Schick, M.; Stagni, M. S.; MeyerKlaucke, W.; Warkentin, E.; Thauer, R. K.; Ermler, U. The Crystal Structure of [Fe]-Hydrogenase Reveals the Geometry of the Active Site. Science 2008, 321, 572−575. (8) Hiromoto, T.; Ataka, K.; Pilak, O.; Vogt, S.; Stagni, M. S.; MeyerKlaucke, W.; Warkentin, E.; Thauer, R. K.; Shima, S.; Ermler, U. The crystal structure of C176A mutated [Fe]-hydrogenase suggests an acyliron ligation in the active site iron complex. FEBS Lett. 2009, 583, 585−590. (9) Tamura, H.; Salomone-Stagni, M.; Fujishiro, T.; Warkentin, E.; Meyer-Klaucke, W.; Ermler, U.; Shima, S. Crystal Structures of [Fe]Hydrogenase in Complex with Inhibitory Isocyanides: Implications for the H2-Activation Site. Angew. Chem., Int. Ed. 2013, 52, 9656−9659. (10) For reviews, see: (a) Dey, S.; Das, P. K.; Dey, A. Mononuclear iron hydrogenase. Coord. Chem. Rev. 2013, 257, 42−63. (b) Schultz, K. M.; Chen, D.; Hu, X. [Fe]-Hydrogenase and Models that Contain Iron-Acyl Ligation. Chem. - Asian J. 2013, 8, 1068−1075. (c) Wright, J. A.; Turrell, P. J.; Pickett, C. J. The Third Hydrogenase: More Natural Organometallics. Organometallics 2010, 29, 6146−6156. (11) Guo, Y.; Wang, H.; Xiao, Y.; Vogt, S.; Thauer, R. K.; Shima, S.; Volkers, P. I.; Rauchfuss, T. B.; Pelmenschikov, V.; Case, D. A.; Alp, E. E.; Sturhahn, W.; Yoda, Y.; Cramer, S. P. Characterization of the Fe Site in Iron−Sulfur Cluster-Free Hydrogenase (Hmd) and of a Model Compound via Nuclear Resonance Vibrational Spectroscopy (NRVS). Inorg. Chem. 2008, 47, 3969−3977. (12) Wang, X.; Li, Z.; Zeng, X.; Luo, Q.; Evans, D. J.; Pickett, C. J.; Liu, X. The iron centre of the cluster-free hydrogenase (Hmd): lowspin Fe(II) or low-spin Fe(0)? Chem. Commun. 2008, 3555−3557. (13) Liu, T.; Li, B.; Popescu, C. V.; Bilko, A.; Pérez, L. M.; Hall, M. B.; Darensbourg, M. Y. Analysis of a Pentacoordinate Iron Dicarbonyl as Synthetic Analogue of the Hmd or Mono-Iron Hydrogenase Active Site. Chem. - Eur. J. 2010, 16, 3083−3089. (14) Royer, A. M.; Salomone-Stagni, M.; Rauchfuss, T. B.; MeyerKlaucke, W. Iron Acyl Thiolato Carbonyls: Structural Models for the Active Site of the [Fe]-Hydrogenase (Hmd). J. Am. Chem. Soc. 2010, 132, 16997−17003. (15) Turrell, P. J.; Wright, J. A.; Peck, J. N. T.; Oganesyan, V. S.; Pickett, C. J. The Third Hydrogenase: A Ferracyclic Carbamoyl with Close Structural Analogy to the Active Site of Hmd. Angew. Chem., Int. Ed. 2010, 49, 7508−7511. (16) Chen, D.; Scopelliti, R.; Hu, X. [Fe]-Hydrogenase Models Featuring Acylmethylpyridinyl Ligands. Angew. Chem., Int. Ed. 2010, 49, 7512−7515. (17) Chen, D.; Scopelliti, R.; Hu, X. Reversible Protonation of a Thiolate Ligand in an [Fe]-Hydrogenase Model Complex. Angew. Chem., Int. Ed. 2012, 51, 1919−1921. (18) Hu, B.; Chen, D.; Hu, X. Synthesis and Reactivity of Mononuclear Iron Models of [Fe]-Hydrogenase that Contain an Acylmethylpyridinol Ligand. Chem. - Eur. J. 2014, 20, 1677−1682. (19) Hu, B.; Chen, X.; Gong, D.; Cui, W.; Yang, X.; Chen, D. Reversible CO Dissociation of Tricarbonyl Iodide [Fe]-Hydrogenase Models Ligating Acylmethylpyridyl Ligands. Organometallics 2016, 35, 2993−2998. (20) Tanino, S.; Ohki, Y.; Tatsumi, K. An Iron(II) Carbonyl Thiolato Complex Bearing 2-Methoxy-Pyridine: A Structural Model of the Active Site of [Fe] Hydrogenase. Chem. - Asian J. 2010, 5, 1962−1964. (21) Song, L.-C.; Xie, Z.-J.; Wang, M.-M.; Zhao, G.-Y.; Song, H.-B. Biomimetic Models for the Active Site of [Fe]Hydrogenase Featuring an Acylmethyl(hydroxymethyl)pyridine Ligand. Inorg. Chem. 2012, 51, 7466−7468.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02582. Molecular structure of A (Figure S1), part of the electrochemical and electrocatalytic figures of A and B (Figures S2−S7), and IR and NMR spectra of B, 1−4, and 6−9 (Figures S8−S40) (PDF) Accession Codes

CCDC 1573505−1573513 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Li-Cheng Song: 0000-0003-0964-8869 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Ministry of Science and Technology of China (973 program 2014CB845604) and the National Natural Science Foundation of China (21472095 and 21772106) for financial support.

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REFERENCES

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