Synthesis, Characterization, and Electrochemical Properties of

Article pubs.acs.org/Organometallics

Synthesis, Characterization, and Electrochemical Properties of Benzyloxy-Functionalized Diiron 1,3-Propanedithiolate Complexes Relevant to the Active Site of [FeFe]-Hydrogenases Li-Cheng Song,* Wei Gao, Xiang Luo, Zhi-Xuan Wang, Xiao-Jing Sun, and Hai-Bin Song State Key Laboratory of Elemento-Organic Chemistry, Department of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: A series of new benzyloxy-functionalized 1,3-propanedithiolate (PDT)-type model complexes (A and 1−7) have been synthesized and structurally characterized. The benzyloxy-functionalized all-carbonyl complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)6 (A) can be prepared by condensation reaction of 2benzyloxy-1,3-dibromopropane with the in situ generated (μ-LiS)2Fe2(CO)6, whereas it reacts with the in situ formed N-heterocyclic carbene 1-mesityl-3methylimidazol-2-ylidene (IMes/Me) to give the corresponding carbene monosubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(IMes/Me) (1). The PMe3-monosubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(PMe3) (2) can be prepared by substitution of the CO ligand in parent complex A with 1 equiv of PMe3 in the presence of Me3NO·2H2O, whereas PPh3-monosubstituted and PPh3-disubstituted complexes [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(PPh3) (3) and [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)2 (4) are prepared by reaction of A with 2 equiv of PPh3 under similar conditions. While the PPh3-disubstituted complex 4 can also be prepared by treatment of 3 in MeCN with 2 equiv of PPh3 in the presence of Me3NO·2H2O, treatment 4 with 2 equiv of PMe3 in refluxing toluene afforded unexpected PPh3/PMe3-disubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)(PMe3) (5). Particularly interesting is that although the reaction of A with 1 equiv of diphosphine dppe in refluxing toluene affords the dppe-chelated single-butterfly complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(dppe) (6), treatment of A in MeCN with 1 equiv of dppe in the presence of Me3NO·2H2O results in formation of the dppe-bridged double-butterfly complex {[(μSCH2)2CH(OCH2Ph)]Fe2(CO)5}2(dppe) (7). All new model complexes have been chatacterized by elemental analysis, spectroscopy, and particularly for 1 and 4−7 X-ray crystallography. Furthermore, complexes A, 3, and 4 have been found to be catalysts for HOAc proton reduction to H2 under electrochemical conditions.

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INTRODUCTION Hydrogenases are natural enzymes that can catalyze both reduction of proton to dihydrogen and the oxidation of dihydrogen to protons.1 These enzymes can be classified as two main groups on the basis of the metal contents in their active sites: [FeFe]-hydrogenases ([FeFe]Hases)2 and [NiFe]-hydrogenases ([NiFe]Hases).3 In recent years, [FeFe]Hases have drawn considerably more attention than [NiFe]Hases, largely owing to their unusual structures and particularly their catalytic function in the production of the “clean” and highly efficient fuel: dihydrogen.4−7 The X-ray crystallographic,8−11 FTIR spectroscopic,12−14 and theoretical15 studies indicated that the active site of [FeFe]Hases (so-called H-cluster) consists of a butterfly [Fe2S2] cluster core with one of its iron atoms linked to a cubic [Fe4S4] cluster through the sulfur atom of a Lcysteinyl ligand. There are also three other ligands, namely, CO, CN−, and dithiolate, coordinated to the two Fe atoms of the butterfly [Fe2S2] cluster core (Scheme 1). Encouraged by the structural studies on the H-cluster, synthetic chemists have prepared a variety of active site models for [FeFe]Hases,16,17 and some of them have been found to be catalysts for proton reduction to dihydrogen under electrochemical conditions.18−20 © 2012 American Chemical Society

Scheme 1. Basic Structure of H-Cluster Obtained from Protein Crystallographya

a

X = CH2, NH, or O.

In order to further develop the biomimetic chemistry of the natural enzyme [FeFe]Hases, we recently initiated a study on a new series of diiron PDT-type model complexes, in which a benzyloxy functionality is attached to the central bridgehead C atom of their PDT moiety and some electron-donating ligands Received: February 17, 2012 Published: March 27, 2012 3324

dx.doi.org/10.1021/om300136b | Organometallics 2012, 31, 3324−3332

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Scheme 2

solid. The IR spectrum of 1 showed three absorption bands in the range 2033−1955 cm−1 for its terminal carbonyls. The highest νCO value among the three bands is shifted by 47 cm−1 toward lower energy relative to that of parent complex A. This is because an N-heterocyclic carbene is a stronger σ-donor than CO with negligible π-accepting ability.25 The 1H NMR spectrum of 1 displayed two singlets at 6.84 and 6.96 ppm for hydrogen atoms in the CHCH unit of the imidazole ring, two singlets at 2.00 and 2.36 ppm for hydrogen atoms of CH3 groups in its mesityl group, and a singlet at 3.86 ppm for hydrogen atoms of the CH3 group connected to the N atom. It also exhibited two multiplets at ca. 1.4 and 2.7 ppm for the axial and equatorial hydrogen atoms in its two SCH2 groups, respectively.22,23 Synthesis and Spectroscopic Characterization of Monophosphine-Substituted Complexes 2−5. It was further found that (i) treatment of A in MeCN with 1 equiv of PMe 3 in the presence of decarbonylating agent Me3NO·2H2O26 resulted in formation of the PMe3-monosubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(PMe3) (2) in 47% yield; (ii) treatment of A with 2 equiv of PPh3 under similar conditions gave PPh3monosubstituted complex [(μ-SCH 2 ) 2 CH(OCH 2 Ph)]Fe2(CO)5(PPh3) (3) and PPh3-disubstituted complex [(μSCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)2 (4) in 63% and 12% yields, respectively; (iii) disubstituted complex 4 could also be obtained by treatment of 3 with 2 equiv of PPh3 in the presence of Me3NO·2H2O in MeCN at room temperature in a much higher (80%) yield; and (iv) treatment of disubstituted complex 4 with 2 equiv of PMe3 in refluxing toluene did not produce the corresponding trisubstituted complex [(μSCH2)2CH(OCH2Ph)]Fe2(CO)3(PPh3)2(PMe3), but instead afforded the unexpected disubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)(PMe3) (5) in 75% yield (Scheme 4). It is apparent that the easy formation of the disubstituted ligand-exchange product 5 might be attributed to ligand PMe3 having less steric hindrance and stronger electron-donating ability than PPh3. While 2 is an air-stable red oil and 3 is an air-stable red solid, complexes 4 and 5 are slightly air-sensitive red solids. All these new complexes have been characterized by elemental analysis and IR, 1H NMR, and 31P NMR spectroscopy. For example, the IR spectra of monosubstituted complexes 2 and 3 showed three to four absorption bands in the range 2043−1910 cm−1 for their terminal carbonyls, whereas disubstituted complexes 4 and 5 displayed three to four bands in the lower region 1998−1915 cm−1 for their terminal carbonyls. The highest νCO values of 2−5 are shifted by 37−82 cm−1 toward lower energy relative to that of parent complex A. All these observations are consistent with phosphine ligands being stronger σ-donors than CO.27 While the 1 H NMR spectra of 2−5 exhibited their

such as phosphine and N-heterocyclic carbene (NHC) are bound to the iron atoms in their diiron subsite (vide infra). It should be noted that the introduction of a benzyloxy functional group and some stronger donor ligands in such model complexes is to make their bridgehead C atom-attached oxygen atom and the iron atoms in their diiron subsite more electronrich and thus easily accepting of protons for the catalytic H2 production. Herein we report the synthesis, structural characterization, and some electrochemical properties of these new diiron PDT-type model complexes.

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RESULTS AND DISCUSSION Synthesis and Spectroscopic Characterization of Benzyloxy-Functionalized Complex A and Its NHCSubstituted Complex 1. We found that complex A could be prepared by in situ treatment of (μ-LiS) 2Fe 2 (CO) 6 generated from (μ-S2)Fe2(CO)6 and Et3BHLi21 with 2benzyloxy-1,3-dibromopropane in THF from −78 °C to room temperature in 35% yield (Scheme 2). Complex A is an air-stable red solid, which was characterized by elemental analysis and spectroscopy. The C/H analytical data are in good agreement with its composition, whereas its IR spectrum showed six absorption bands in the range 2080−1928 cm−1 for its terminal carbonyls. In addition, the 1H NMR spectrum of A displayed a triplet at 1.54 ppm and a doublet at 2.79 ppm for the axial and equatorial hydrogen atoms, respectively, in its two SCH2 groups, and it displayed a multiplet at about 2.85 ppm for the hydrogen atom attached to its bridgehead C atom.22,23 Interestingly, parent complex A could react with Nheterocyclic carbene IMes/Me (generated in situ by reaction of its precursor 1-mesityl-3-methylimidazolium salt IMes/Me·HI with n-BuLi)24 in THF at room temperature to afford the corresponding NHC-monosubstituted complex [(μSCH2)2CH(OCH2Ph)]Fe2(CO)5(IMes/Me) (1), albeit in low yield (9%) (Scheme 3). Complex 1 is also an air-stable red Scheme 3

3325


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Scheme 4

corresponding organic groups, their 31P NMR spectra showed one singlet at 26.50 ppm for the P atom in the PMe3 ligand of 2 and one singlet at 65.04 ppm for the P atom in the PPh3 ligand of 3, two singlets at 61.60 and 62.35 ppm for the P atoms in two PPh3 ligands of 4, and two singlets at 27.31 and 62.08 ppm for the P atoms in the PMe3 and PPh3 ligands of 5, respectively. Synthesis and Spectroscopic Characterization of Diphosphine-Substituted Complexes 6 and 7. More interestingly, parent complex A could react with 1 equiv of diphosphine dppe in refluxing toluene to afford the dppechelated single-butterfly complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(Ph2PCH2CH2PPh2) (6) in 18% yield, whereas A reacted with 1 equiv of dppe in the presence of Me3NO·2H2O in MeCN at room temperature to give the dppe-bridged double-butterfly complex {[(μ-SCH 2 ) 2 CH(OCH 2 Ph)]Fe2(CO)5}2(Ph2PCH2CH2PPh2) (7) in 16% yield (Scheme 5). Complexes 6 and 7 are air-stable solids, which were also characterized by elemental analysis and IR, 1H NMR, and 31P NMR spectroscopy. For instance, their IR spectra showed three to four absorption bands in the range 2046−1891 cm−1 for their terminal carbonyls. Similar to monophosphine-substituted complexes 2−5, the highest νCO values of diphosphinesubstituted 6 and 7 are shifted respectively by 64 and 34 cm−1 toward lower energy relative to that of parent complex A due to the same reason indicated above.27 The 1H NMR spectrum of 6 exhibited two multiplets at ca. 1.4 and 2.5 ppm for the axial and equatorial hydrogen atoms in its two SCH2 groups, whereas 7 displayed one triplet and one doublet at 1.31 and 2.29 ppm for the axial and equatorial hydrogen atoms in its four SCH2 groups, respectively.22,23 The 31P NMR spectra of 6 and 7 showed a singlet at 91.02 and 59.32 ppm for the P atoms in the chelated and bridged dppe ligands of 6 and 7, respectively. Crystal Structures of 1 and 4−7. The molecular structures of 1 and 4−7 were investigated by X-ray crystallography. While their ORTEP drawings are depicted in Figures 1−5, Table 1 lists their selected bond lengths and angles. As can be seen intuitively from Figures 1−5, all these complexes contain a benzyloxy group-substituted propanedithiolate ligand bridged between two iron atoms to form two

Scheme 5

fused six-membered rings with a chair and a boat conformation. In addition, the benzyloxy group and one hydrogen atom are attached to the bridgehead C atom (namely, C7 for 1, C22 for 4, C9 for 5, C6 for 6, and C7/C7A for 7) via the common equatorial and axial bonds of the two fused six-membered rings, respectively. Apparently, this is in order to avoid the strong steric repulsions of the bulky benzyloxy group with the apical CO and the substituted ligands in these complexes. Similar cases were also observed in other bridgehead C atomsubstituted PDT-type model complexes.22,23 Also, as can be seen intuitively from Figures 1−3, the substituted ligand IMes/Me in 1 and the two PPh3 in 4 are all located in apical positions of the square-pyramidal geometries of the corresponding iron atoms, whereas the PPh3 and PMe3 ligands in 5 lie in an apical and a basal positions of its two iron atoms, respectively. In addition, as shown in Figures 4 and 5, the two Ph2P moieties of 3326


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pounds.19,20,29 In addition, in contrast to A and 3, Figure 8 shows that when the first 2 mM HOAc was added, both peaks of the first and second reductions of 4 increased remarkably and continuously increased linearly with increments of the acid concentration. Such double electrocatalytic processes for proton reduction to H2 catalyzed by similar [FeFe]-hydrogenase model complexes were also previously observed.30 Particularly noteworthy is that the first electrocatalytic process catalyzed by 4 is most likely to have an ECCE electrochemical mechanism,31 whereas its second electrocatalytic process probably has an EECC electrochemical mechanism caused by an electrocatalytic reaction involving a species derived from 4.30 Finally, it is worth pointing out that the two catalytic potentials of 4 are all less negative than the two corresponding potentials of A and 3 as a result of the existence of two stronger electrondonating PPh3 ligands.

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Figure 1. Molecular structure of 1 with 30% probability level ellipsoids. Hydrogen atoms are omitted for clarity.

CONCLUSIONS We have prepared a series of new benzyloxy-functionalized butterfly Fe/S cluster complexes (A and 1−7) as the active site models of [FeFe]Hases. While A is prepared by ring-closure reaction of PhCH2OCH(CH2Br)2 with (μ-LiS)2Fe2(CO)6, its CO substitution derivatives 1−7 are prepared by reactions of A with PMe3, PPh3, and dppe under the corresponding conditions. From a synthetic view, the ring-closure method to give parent complex A, the ligand exchange method between 4 and PMe3 to afford 5, and the formation of two types of dppesubstituted derivatives 6 and 7 are of particular interest. The molecular structures of complexes 1 and 4−7 have been confirmed by X-ray crystal diffraction analysis. Particularly noteworthy is that the two monophosphine ligands PPh3 in 4 are coordinated to its two Fe atoms with a symmetrically substituted apical/apical coordination mode, whereas the monophosphine ligands PPh3 and PMe3 in 5 are coordinated to its two Fe atoms with an apical/basal coordination mode. In addition, the diphosphine ligand dppe in 6 is chelated to one of its two Fe atoms in its single-butterfly cluster core with an apical/basal coordination mode, whereas the dppe ligand in 7 is bridged respectively to two Fe atoms in its two butterfly cluster cores with an apical/apical coordination mode. It is worth pointing out that complexes A, 3, and 4 have been proved to be catalysts for proton reduction to hydrogen under electrochemical conditions. Further studies on electrocatalytic mechanisms for such H2 production processes will be carried out in the near future in our laboratory.

dppe in 6 are indeed chelated to one iron atom (Fe2) in its single-butterfly cluster core, and those two PPh2 moieties in 7 are bridged to two iron atoms (Fe2/Fe2A) in its two butterfly subcluster cores, respectively. Although the Fe−Fe bond lengths of 1 (2.5223 Å), 4 (2.5319 Å), 5 (2.5534 Å), 6 (2.545 Å), and 7 (2.5167 Å) are very close to the corresponding bond length of the reduced form of [FeFe]Hases (2.55 Å),11 they are obviously shorter than that of the oxidized form (2.62 and 2.60 Å)8,9 of [FeFe]Hases. Electrochemistry of A, 3, and 4. The electrochemical properties of parent complex A and its phosphine-substituted complexes 3 and 4 were investigated by using cyclic voltammetry. It is shown that parent complex A exhibits one quasi-reversible reduction at −1.61 V, one irreversible reduction at −2.12 V, and one irreversible oxidation at +0.70 V (see Table 2). All three redox events could be assigned to the one-electron processes from FeIFeI to FeIFe0, FeIFe0 to Fe0Fe0, and FeIFeI to FeIFeII couples, respectively (supported by the calculated value of 1.10 Farady/equiv obtained by study of the bulk electrolysis of a MeCN solution of A at −1.80 V).28 Similarly, complex 3 displays one quasi-reversible reduction at −1.63 V, one irreversible reduction at −2.41 V, and one irreversible oxidation at +0.40 V, whereas 4 shows two irreversible reductions at −1.96/−2.26 V and one irreversible oxidation at +0.07 V (see Table 2). The first and second reduction peaks of 3 and 4 could also be assigned to the oneelectron reduction processes from FeIFeI to FeIFe0 and FeIFe0 to Fe0Fe0 couples, whereas the oxidation peaks of 3 and 4 could be attributed to their one-electron oxidation processes from the FeIFeI to FeIFeII couple, respectively (supported by the calculated values of 1.11 and 1.05 Farady/equiv obtained by study of the bulk electrolysis of a MeCN solution of 3 at −1.85 V and 4 at −2.20 V, respectively). The cyclic voltammograms of A, 3, and 4 in the presence of HOAc and without HOAc (for comparison) are presented in Figures 6−8. As shown in Figures 6 and 7, when the first 2 mM HOAc was added, the first reduction peaks of A and 3 increased slightly, but they did not continuously increase with subsequent addition of the acid. However, in contrast to this, their second reduction peaks grew up remarkably with continuous addition of the acid. Such observations are consistent with those previously reported for proton reduction catalyzed by similar [FeFe]-hydrogenase model com-

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EXPERIMENTAL SECTION

General Comments. All reactions were performed using standard Schlenk and vacuum-line techniques under highly prepurified N2. Toluene and THF were distilled under N 2 from sodium/ benzophenone ketyl, whereas MeCN was distilled under N2 from CaH2. (μ-S2)Fe2(CO)6,32 2-benzyloxy-1,3-dibromopropane,33 1-mesityl-3-methylimidazolium salt,24 and 1,2-bis(diphenylphosphino)ethane (dppe)34 were prepared according to the published methods. Et3BHLi (1 M in THF), n-BuLi (2.5 M in hexane), PMe3 (1 M in toluene), PPh3, and other chemicals were purchased from commercial suppliers and used as received. Preparative TLC was carried out on glass plates (25 × 15 × 0.25 cm) coated with silica gel G (10−40 μm). IR spectra were recorded on a Bio-Rad FTS 135 infrared spectrophotometer. 1H and 31P NMR spectra were taken on a Bruker Avance 300 NMR or a Varian Mercury Plus 400 NMR spectrometer. Elemental analyses were performed with an Elementar Vario EL analyzer. Melting points were determined on a Yanaco MP-500 apparatus and were uncorrected. 3327


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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1, 4, 5, 6, and 7 1 Fe(1)−S(2) Fe(1)−C(16) Fe(2)−S(2) S(1)−C(6) O(6)−C(7) C(16)−Fe(1)−S(2) S(2)−Fe(1)−S(1) S(1)−Fe(1)−Fe(2) Fe(2)−S(1)−Fe(1) C(9)−O(6)−C(7)

2.2605(11) 1.974(4) 2.2536(11) 1.821(3) 1.448(4) 105.14(10) 84.39(4) 55.97(3) 67.58(3) 113.1(2)

Fe(1)−S(1) Fe(1)−Fe(2) Fe(2)−S(1) O(6)−C(9) C(6)−C(7) C(16)−Fe(1)−S(1) S(2)−Fe(1)−Fe(2) S(2)−Fe(2)−S(1) Fe(2)−S(2)−Fe(1) C(8)−C(7)−C(6)

2.2740(11) 2.5223(8) 2.2613(12) 1.418(4) 1.512(5) 108.05(11) 55.90(3) 84.84(4) 67.94(3) 112.7(3)

Fe(1)−C(2) Fe(1)−S(1) Fe(1)−Fe(1A) S(1)−Fe(1A) O(3)−C(22) C(2)−Fe(1)−P(1) P(1)−Fe(1)−S(1A) P(1)−Fe(1)−Fe(1A) S(1A)−Fe(1)−Fe(1A) C(23)−O(3)−C(22)

1.785(2) 2.2721(9) 2.5319(8) 2.2764(7) 1.390(4) 97.51(8) 106.85(3) 156.667(18) 56.09(3) 130.8(3)

Fe(1)−S(2) Fe(1)−Fe(2) Fe(2)−S(2) S(1)−C(8) O(5)−C(9) P(1)−Fe(1)−S(1) S(2)−Fe(1)−Fe(2) P(2)−Fe(2)−S(2) Fe(1)−S(1)−Fe(2) C(11)−O(5)−C(9)

2.2555(11) 2.5534(9) 2.2549(11) 1.824(4) 1.441(5) 166.71(5) 55.51(3) 108.25(4) 68.55(4) 113.9(3)

4 Fe(1)−C(1) Fe(1)−P(1) Fe(1)−S(1A) S(1)−C(21) O(3)−C(23) C(1)−Fe(1)−P(1) P(1)−Fe(1)−S(1) S(1)−Fe(1)−S(1A) S(1)−Fe(1)−Fe(1A) Fe(1)−S(1)−Fe(1A)

1.764(2) 2.2503(7) 2.2764(7) 1.823(3) 1.279(6) 94.38(8) 109.81(3) 83.22(4) 56.26(2) 67.65(3)

Fe(1)−P(1) Fe(1)−S(1) Fe(2)−P(2) Fe(2)−S(1) O(5)−C(11) P(1)−Fe(1)−S(2) S(2)−Fe(1)−S(1) S(1)−Fe(1)−Fe(2) P(2)−Fe(2)−S(1) Fe(2)−S(2)−Fe(1)

2.2287(14) 2.2621(14) 2.2430(12) 2.2717(12) 1.428(5) 90.69(4) 84.23(4) 55.90(3) 103.98(5) 68.96(4)

Fe(1)−S(2) Fe(1)−Fe(2) Fe(2)−P(2) Fe(2)−S(2) O(5)−C(8) S(2)−Fe(1)−S(1) S(1)−Fe(1)−Fe(2) S(1)−Fe(2)−S(2) S(2)−Fe(2)−Fe(1) Fe(2)−S(1)−Fe(1)

2.261(2) 2.545(2) 2.200(2) 2.265(2) 1.432(7) 84.40(9) 55.64(6) 84.51(10) 55.71(6) 68.38(6)

Fe(1)−S(1) Fe(1)−Fe(2) Fe(2)−S(1) O(6)−C(9) S(1)−C(6) S(1)−Fe(1)−Fe(2) P(1)−Fe(2)−S(2) P(1)−Fe(2)−Fe(1) S(2)−Fe(2)−Fe(1) Fe(1)−S(1)−Fe(2)

2.2503(12) 2.5167(9) 2.2574(11) 1.402(5) 1.831(4) 56.20(3) 106.58(4) 152.53(3) 55.88(3) 67.88(4)

5

6 Fe(1)−S(1) Fe(2)−P(1) Fe(2)−S(1) S(1)−C(5) O(5)−C(6) S(2)−Fe(1)−Fe(2) P(1)−Fe(2)−P(2) S(1)−Fe(2)−Fe(1) C(8)−O(5)−C(6) Fe(1)−S(2)−Fe(2)

2.269(2) 2.195(2) 2.260(2) 1.823(5) 1.434(6) 55.86(5) 85.79(7) 55.98(6) 114.0(5) 68.43(6)

7 Fe(1)−S(2) Fe(2)−P(1) Fe(2)−S(2) O(6)−C(7) C(6)−C(7) S(2)−Fe(1)−Fe(2) S(1)−Fe(2)−S(2) S(1)−Fe(2)−Fe(1) C(9)−O(6)−C(7) Fe(1)−S(2)−Fe(2)

Preparation of [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)6 (A). A red solution of (μ-S2)Fe2(CO)6 (0.344 g, 1.00 mmol) in THF (25 mL) was cooled to −78 °C, and then Et3BHLi (2.0 mL, 2.00 mmol) was added. The mixture was stirred at −78 °C for 15 min to give an emerald green solution containing (μ-LiS)2Fe2(CO)6. After 2-

2.2534(11) 2.2394(11) 2.2703(12) 1.434(4) 1.507(5) 56.52(3) 84.50(4) 55.93(3) 114.0(3) 67.60(4)

benzyloxy-1,3-dibromopropane (0.308 g, 1.00 mmol) was added, the new mixture was warmed to room temperature and stirred at this temperature for 12 h. Volatiles were removed in vacuo, and the residue was subjected to TLC separation using CH2Cl2/petroleum ether (1:3 v/v) as eluent. From the main red band, A was obtained as a red solid 3328


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Figure 3. Molecular structure of 5 with 30% probability level ellipsoids. Hydrogen atoms are omitted for clarity. (0.174 g, 35%), mp 59−60 °C. Anal. Calcd for C16H12Fe2O7S2: C, 39.05; H, 2.46. Found: C, 38.85; H, 2.59. IR (KBr disk): νCO 2080 (vs), 2033 (vs), 2007 (vs), 1981 (vs), 1960 (vs), 1928 (m) cm−1. 1H NMR (400 MHz, CDCl3, TMS): 1.54 (t, 2Ha, JHaHe = JHaHa′ = 11.2 Hz), 2.79 (d, 2He, JHeHa = 12.0 Hz), 2.82−2.90 (m, 1Ha′), 4.46 (s, 2H, OCH2), 7.31−7.35 (m, 5H, C6H5) ppm (Ha and He represent the axially and equatorially bonded H atoms in CH2S groups, whereas Ha′ and He′ represent those axially and equatorially bonded to the bridgehead C atom). Preparation of [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(IMes/Me) (1). To a stirred suspension of 1-mesityl-3-methylimidazolium salt IMes/Me·HI (0.818 g, 2.50 mmol) in THF (20 mL) was slowly added a solution of n-BuLi (2.5 M in hexane) (1.10 mL, 2.75 mmol) to give a yellowish solution. After stirring at room temperature for an additional

Figure 5. Molecular structure of 7 with 30% probability level ellipsoids. Hydrogen atoms are omitted for clarity. 10 min, A (0.246 g, 0.50 mmol) was added and the new mixture was stirred at room temperature for 4.5 h. The resulting deep-red solution was evaporated to dryness under vacuum. The residue was subjected to TLC separation using CH2Cl2/petroleum ether (1:1.5 v/v) as eluent. From the main red band, 1 was obtained as a red solid (0.031 g, 9%), mp 180−181 °C. Anal. Calcd for C28H28Fe2N2O6S2: C, 50.62; H, 4.25; N, 4.22. Found: C, 50.41; H, 4.13; N, 4.11. IR (KBr disk): νCO 3329


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Table 2. Electrochemical Data of A, 3, and 4a complex

Epc/V

Epa/V

A

−1.61 −2.12 −1.63 −2.41 −1.96 −2.26

+0.70

3 4 a

+0.40 +0.07

All potentials versus Fc/Fc+.

Figure 8. Cyclic voltammograms of 4 (1.0 mM) with HOAc (0−10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV·s−1. 12.4 Hz, JHeHa′ = 3.6 Hz), 2.85−2.93 (m, 1Ha′), 4.46 (s, 2H, OCH2), 7.28−7.35 (m, 5H, C6H5) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 26.50 (s) ppm. Preparation of [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(PPh3) (3) and [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)2 (4). Method (i), Preparation of 3 and 4 from Parent Complex A: To a red solution of A (0.246 g, 0.50 mmol) in MeCN (15 mL) were added PPh3 (0.262 mL, 1.00 mmol) and Me3NO·2H2O (0.056 g, 0.50 mmol). The mixture was stirred at room temperature for 1 h until complete consumption of A, as indicated by TLC. The resulting deep red solution was evaporated to dryness under vacuum. The residue was subjected to TLC separation using CH2Cl2/petroleum ether (1:3 v/v) as eluent. From the two major red bands, 3 (0.228 g, 63%) and 4 (0.058 g, 12%) were obtained as a red solid, respectively. 3: mp 135−136 °C. Anal. Calcd for C33H27Fe2O6PS2: C, 54.57; H, 3.75. Found: C, 54.69; H, 3.75. IR (KBr disk): νCO 2043 (vs), 1981 (vs), 1948 (s), 1933 (s) cm−1. 1H NMR (400 MHz, CDCl3, TMS): 1.36 (t, 2Ha, JHaHe = JHaHa′ = 12.0 Hz), 2.13−2.18 (m, 1Ha′), 2.39 (dd, 2He, JHeHa = 12.8 Hz, JHeHa′ = 4.0 Hz), 3.63 (s, 2H, OCH2), 6.96−7.22 (m, 5H, C6H5CH2), 7.43−7.68 (m, 15H, (C6H5)3P) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 65.04 (s) ppm. 4: mp 185−186 °C. Anal. Calcd for C50H42Fe2O5P2S2: C, 62.51; H, 4.41. Found: C, 62.47; H, 4.56. IR (KBr disk): νCO 1998 (vs), 1954 (vs), 1937 (vs) cm−1. 1H NMR (300 MHz, CDCl3, TMS): 1.71−2.00 (m, 5H, OCH, 2SCH2), 3.31 (s, 2H, OCH2), 6.73−7.71 (m, 35H, 7C6H5) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 61.60 (s), 62.35 (s) ppm. Method (ii), Preparation of 4 from Monosubstituted Complex 3: To a red solution of 3 (0.363 g, 0.50 mmol) in MeCN (15 mL) were added PPh3 (0.262 mL, 1.00 mmol) and Me3NO·2H2O (0.056 g, 0.50 mmol). After the mixture was stirred at room temperature for 12 h and the same workup as that described above, 4 (0.384 g, 80%) was obtained. Preparation of [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)(PMe3) (5). To a red solution of 4 (0.482 g, 0.50 mmol) in toluene (15 mL) was added PMe3 (1.0 mL, 1.00 mmol), and then the mixture was stirred at reflux for 4.5 h. After removal of solvent at reduced pressure, the residue was subjected to TLC separation using CH2Cl2/petroleum ether (1:3 v/v) as eluent. From the main red band, 5 was obtained as a red solid (0.292 g, 75%), mp 89−90 °C. Anal. Calcd for C35H36Fe2O5P2S2: C, 54.28; H, 4.69. Found: C, 54.36; H, 4.68. IR (KBr disk): νCO 1998 (s), 1986 (vs), 1948 (vs), 1915 (vs) cm−1. 1H NMR (300 MHz, CDCl3, TMS): 0.85 (t, 2Ha, JHaHe = JHaHa′ = 7.5 Hz), 1.40 (d, 9H, JPH = 9.0 Hz, 3CH3), 2.13 (dd, 2He, JHeHa = 12.3 Hz, JHeHa′ = 3.9 Hz), 2.72−2.81 (m, 1Ha′), 4.14 (s, 2H, OCH2), 7.02−7.19 (m, 5H, C6H5CH2), 7.34−7.68 (m, 15H, (C6H5)3P) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 27.31 (s), 62.08 (s) ppm. Preparation of [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(dppe) (6). To a red solution of A (0.100 g, 0.20 mmol) in toluene (15 mL) was added dppe (0.081 g, 0.20 mmol), and then the mixture was stirred at reflux for 4.5 h. After solvent was removed at reduced pressure, the residue was subjected to TLC separation using CH2Cl2/petroleum

Figure 6. Cyclic voltammograms of A (1.0 mM) with HOAc (0−10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV·s−1.

Figure 7. Cyclic voltammograms of 3 (1.0 mM) with HOAc (0−10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 100 mV·s−1. 2033 (vs), 1973 (vs), 1955 (vs) cm−1. 1H NMR (400 MHz, CDCl3, TMS): 1.36−1.50 (m, 2Ha), 2.00 (s, 6H, 2o-CH3 of C6H2), 2.36 (s, 3H, p-CH3 of C6H2), 2.66−2.69 (m, 2He), 3.86 (s, 3H, NCH3), 4.18− 4.33 (m, 3H, 1Ha′, OCH2), 6.84 (s, 1H, MeNCHCH), 6.96 (s, 1H, MesNCHCH), 7.00 (s, 2H, 2m-H of Mes), 7.12−7.30 (m, 5H, C6H5) ppm. Preparation of [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(PMe3) (2). To a red solution of A (0.246 g, 0.50 mmol) in MeCN (15 mL) were added PMe3 (0.50 mL, 0.50 mmol) and Me3NO·2H2O (0.056 g, 0.50 mmol). The mixture was stirred at room temperature for 2.5 h to give a deep-red solution. Volatiles were removed in vacuo, and the residue was subjected to TLC separation using CH2Cl2/petroleum ether (1:5 v/v) as eluent. From the main red band, 2 was obtained as a red oil (0.128 g, 47%). Anal. Calcd for C18H21Fe2O6PS2: C, 40.02; H, 3.92. Found: C, 40.16; H, 4.09. IR (KBr disk): νCO 2034 (vs), 1944 (vs), 1910 (vs) cm−1. 1H NMR (400 MHz, CDCl3, TMS): 1.47 (d, 9H, JPH = 9.6 Hz, 3CH3), 1.51 (d, 2Ha, JHaHe = 11.6 Hz), 2.81 (dd, 2He, JHeHa = 3330


Organometallics

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ether (2:1 v/v) as eluent. From the main brown band, 6 was obtained as a brown solid (0.030 g, 18%), mp 178−179 °C. Anal. Calcd for C40H36Fe2O5P2S2: C, 57.57; H, 4.35. Found: C, 57.35; H, 4.36. IR (KBr disk): νCO 2016 (vs), 1950 (vs), 1934 (vs), 1891 (s) cm−1. 1H NMR (300 MHz, CDCl3, TMS): 1.36−1.45 (m, 2Ha), 2.22−2.32 (m, 1Ha′), 2.45−2.69 (m, 6H, 2He, 2PCH2), 3.54 (s, 2H, OCH2), 6.82− 7.73 (m, 25H, 5C6H5) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 91.02 (s) ppm. Preparation of {[(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5}2(dppe) (7). To a red solution of A (0.100 g, 0.20 mmol) in MeCN (10 mL) were added dppe (0.081 g, 0.20 mmol) and Me3NO·2H2O (0.023 g, 0.20 mmol). The mixture was stirred at room temperature for 10 h to give a red solution. Volatiles were removed in vacuo, and the residue was subjected to TLC separation using CH2Cl2/petroleum ether (1:2 v/v) as eluent. From the main red band, 7 was obtained as a red solid (0.022 g, 17%), mp 180 °C (dec). Anal. Calcd for C56H48Fe4O12P2S4: C, 50.70; H, 3.65. Found: C, 50.63; H, 3.88. IR (KBr disk): νCO 2046 (vs), 1981 (vs), 1933 (s) cm−1. 1H NMR (400 MHz, CDCl3, TMS): 1.31 (t, 4Ha, JHaHe = JHaHa′ = 11.6 Hz), 2.29 (d, 4He, JHeHa = 11.6 Hz), 2.34−2.40 (m, 2Ha′), 2.65 (s, 4H, 2PCH2), 4.00 (s, 4H, OCH2), 7.09− 7.57 (m, 30H, 6C6H5) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 59.32 (s) ppm. X-ray Structure Determinations of 1 and 4−7. Single crystals of 1 and 4−7 suitable for X-ray diffraction analysis were grown by slow evaporation of their CH2Cl2/petroleum ether solutions at about −4 °C. Each crystal was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Saturn 70CCD. Data were collected at room temperature, using a confocal monochromator with Mo Kα radiation (λ = 0.71070 or 0.71073 Å) in the ω−ϕ scanning mode. Data collection, reduction, and absorption correction were performed with the CRYSTALCLEAR program.35 The structures were solved by direct methods using the SHELXS-97 program36 and refined by fullmatrix least-squares techniques (SHELXL-97)37 on F2. Hydrogen atoms were located by using the geometric method. Details of crystal data, data collections, and structure refinements are summarized in Tables 3 and 4, respectively, in the Supporting Information. Electrochemistry. A solution of 0.1 M n-Bu4NPF6 in MeCN (Fisher Chemicals, HPLC grade) was used as electrolyte in all cyclic voltammetric experiments. The electrolyte solution was degassed by bubbling with N2 for at least 10 min before measurement. Electrochemical 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, platinum counter electrode, and Ag/Ag+ (0.01 M AgNO3/0.1 M n-Bu4NPF6 in MeCN) reference electrode under a N2 atmosphere. The working electrode was polished with 1 μm alumina paste and sonicated in water for about 10 min prior to use. Bulk electrolyses were carried out under N2 on a glassy carbon rod (A = 2.9 cm2) in a two-compartment, gastight, H-type electrolysis cell containing 25 mL of MeCN. All potentials are quoted against the ferrocene/ferrocenium (Fc/Fc+) potential. Gas chromatography was performed with a Shimadzu GC2014 gas chromatograph under isothermal conditions with nitrogen as a carrier gas and a thermal conductivity detector.

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ACKNOWLEDGMENTS We are grateful to 973 (2011CB935902), the National Natural Science Foundation of China (21132001, 20972073), and the Tianjin Natural Science Foundation (09JCZDJC27900) for financial support.

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ASSOCIATED CONTENT

S Supporting Information *

Full tables of crystal data, atomic coordinates, thermal parameters, and bond lengths and angles for 1 and 4−7 as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3331


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