Carbon- and Sulfur-Bridged Diiron Carbonyl Complexes Containing N

Oct 19, 2012 - Masakazu Hirotsu , Kiyokazu Santo , Chiaki Tsuboi , and Isamu Kinoshita. Organometallics 2014 33 (16), 4260-4268. Abstract | Full Text ...
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Carbon- and Sulfur-Bridged Diiron Carbonyl Complexes Containing N,C,S-Tridentate Ligands Derived from Functionalized Dibenzothiophenes: Mimics of the [FeFe]-Hydrogenase Active Site Masakazu Hirotsu,*,† Kiyokazu Santo,† Hideki Hashimoto,†,‡,§ and Isamu Kinoshita†,‡,§ †

Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka, 558-8585, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan § The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, Sumiyoshi-ku, Osaka, 558-8585, Japan ‡

S Supporting Information *

ABSTRACT: Photochemical reactions of [Fe(CO)5] with dibenzothiophene (DBT) derivatives bearing a N-donor group produced a series of C,S-bridged diiron carbonyl complexes [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3], as previously reported for 4-(2′-pyridyl)dibenzothiophene (L1), where L′ represents the N,C,S-tridentate ligands L1′−L5′, formed by C−S bond cleavage of L1−L5, respectively. The DBT derivatives used in this study have Schiff base or oxazoline moieties at the 4position: L2 = PhCH2NCH-DBT, L3 = 2MeOC6H4CH2NCH-DBT, L4 = (S)-PhC(Me)HNCHDBT, L5 = (R)-4-isopropyl-2-oxazolinyl-DBT. The diiron complexes were characterized by NMR, absorption, and circular dichroism spectroscopy, and the dinuclear structures bridged by thiolate S and aryl C atoms were established by X-ray crystallography. The diiron complex [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] consists of two units, Fe(L′)(CO)2 and Fe(CO)3: the latter unit is located on a thiolate-containing metallacycle in the former one. The chiral Schiff base ligand precursor L4 gave a 55:45 mixture of two diastereomers for [{Fe(μ-L4′-κ3N,C,S)(CO)2}Fe(CO)3], while chiral L5 with an (R)-4-isopropyl-2oxazolinyl group afforded [{Fe(μ-L5′-κ3N,C,S)(CO)2}Fe(CO)3] in a 9:1 diastereomeric ratio. The diiron carbonyl complexes of the N,C,S-tridentate ligands (L1′L5′) showed two reversible redox couples for [Fe2(μ-L′)(CO)5]0/− and [Fe2(μL′)(CO)5]−/2−. The two-electron-reduced forms undergo protonation and act as electrocatalysts for proton reduction of acetic acid in acetonitrile.

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INTRODUCTION Thiolate iron complexes with carbonyl ligands are found in the active site of hydrogenases, which catalyze the formation and consumption of dihydrogen in biological systems.1 The active site of [FeFe]-hydrogenases (H-cluster) consists of a diiron center bridged by a dithiolate ligand and a {4Fe4S} cubane cluster.1−3 The diiron center has carbonyl and cyanide ligands, and one of the two Fe ions is connected to the {4Fe4S} cluster via a cysteinyl residue (Figure 1). A binding site for substrates is

in the diiron unit, and the {4Fe4S} cluster is involved in electron transfer in the catalytic cycle of [FeFe]-hydrogenases.4 In the oxidized form of the H-cluster (HOX), there is a vacant or an exchangeable coordination site at the distal iron center of the diiron unit, which is presumed to have the FeIIFeI oxidation state in the asymmetric environment.1d,3−5 Synthetic mimics of the [FeFe]-hydrogenase active site have been studied using diiron(I,I) carbonyl complexes of dithiolate ligands because of the structural resemblance to the diiron unit of the Hcluster.1,6−11 A well-studied diiron system is the dithiolate hexacarbonyl complexes [Fe2(μ-dithiolate)(CO)6],6−10 some of which were found to catalyze electrochemical reduction of protons.8−10 To design efficient synthetic models for the H-cluster, dithiolate-bridged diiron complexes derived from [Fe2(μdithiolate)(CO)6] have been widely investigated.11−16 Key features of the synthetic models include (1) a coordination site for substrate binding, (2) stabilization of the low-spin FeII state

Figure 1. Structures of (a) the H-cluster of [FeFe]-hydrogenases in the oxidized state and (b) complex 1. © 2012 American Chemical Society

Received: August 30, 2012 Published: October 19, 2012 7548

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proposed in the catalytic cycle,4 (3) a mediator for electron transfer such as a {4Fe4S} cluster, and (4) a protonation site to assist catalysis.13 The steric environment and the oxidation state of the diiron center can be controlled by substitution of CO with other strong σ-donor ligands, and mixed-valent FeIIFeI dithiolate complexes with phosphines or N-heterocyclic carbenes were reported as a model for the diiron unit in the HOX state.14,15 The active site models with the cubane cluster have been synthesized by using tripod thiolate ligands.16 On the other hand, diiron model complexes with different bridging atoms such as P, N, and Se have been explored.17−19 Electrocatalytic activity of the diphosphido-, diimido-, and diselenido-bridged complexes for proton reduction was also reported. We recently reported that the photochemical reaction of 4(2′-pyridyl)dibenzothiophene (PyDBT, L1) with [Fe(CO)5] produced the thiolato-bridged diiron carbonyl complex [{Fe(μL1′-κ3N,C,S)(CO)2}Fe(CO)3] (1), where L1′ is a meridional N,C,S-tridentate ligand formed by the C−S bond cleavage of L1 (eq 1).20 The N,C,S-tridentate ligand forms two fused

complexes [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3], similar to 1, where L′ represents N,C,S-tridentate ligands L1′−L5′, formed by C−S bond cleavage of L1−L5, respectively. A high diastereoselectivity was observed for the cyclometalation of chiral L5 with an (R)-4-isopropyl-2-oxazolinyl group, which provides information on the absolute configuration around Fe. The C,S-bridged diiron complexes [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] showed two well-separated reduction processes. We further report the electrocatalytic ability of these complexes for proton reduction.

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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes. The dibenzothiophene−Schiff base ligand precursors L2−L4 were prepared by condensation of 4-formyldibenzothiophene22 and the primary amines benzylamine, 2-methoxybenzylamine, and (S)-(−)-α-methylbenzylamine, respectively. Irradiation of a tetrahydrofuran (THF) solution of [Fe(CO)5] and the ligand precursor L2 or L3 afforded a deep purple solution, from which dark purple crystals of [{Fe(μ-L2′-κ3N,C,S)(CO)2}Fe(CO)3] (2) or [{Fe(μ-L3′-κ3N,C,S)(CO)2}Fe(CO)3] (3) were obtained, respectively (eq 2). In the 1H NMR spectra, an AB

metallacycles including Pyridine N and thiolate S donor atoms.20,21 An Fe(CO)3 unit in 1 is bound to the thiolatecontaining metallacycle to form a carbon- and sulfur-bridged dinuclear structure with an Fe−Fe bond, which is similar to the diiron unit of the [FeFe]-hydrogenase active site (Figure 1). The central phenyl ring of L1′ acts as a strong σ-donor for the Fe atom in the metallacycle plane, while the other Fe atom in the Fe(CO)3 unit is stabilized by π coordination of the central phenyl ring. This coordination mode makes the diiron center asymmetric; therefore, the two Fe sites are clearly differentiated by one-electron oxidation or reduction. Furthermore, dissociation of the π-coordinated phenyl ring can provide a vacant coordination site on the Fe(CO)3 unit. A potential advantage of the N,C,S-tridentate ligand system is that the structures and properties of the active site mimics can be controlled by replacement of the pyridyl group in L1 with other coordinating groups. In this work, we used dibenzothiophene (DBT) derivatives L2−L5, presented in Chart 1, as ligand precursors: a Schiff base or an oxazolinyl group is incorporated instead of the pyridyl group in L1. Photoreactions of the ligand precursors with [Fe(CO)5] afforded a series of C,S-bridged diiron carbonyl

pattern was observed for two diastereotopic benzyl methylene protons. Electronic absorption spectra of acetonitrile solutions of 2 and 3 showed an intense absorption band at 525 nm (ε = 3100 dm3 mol−1 cm−1) and 519 nm (ε = 2900 dm3 mol−1 cm−1), respectively, which are similar to that of 1 in dichloromethane (524 nm, ε = 3400 dm3 mol−1 cm−1).20 Furthermore, in the previous paper, we reported that the analogous complex [{Fe(μ-bpyBPT-κ4N,N′,C,S)(CO)}Fe(CO)3], where bpyBPT is a tetradentate ligand derived from 6-(4″-dibenzothienyl)-2,2′-bipyridine, exhibits an absorption band at 544 nm (ε = 5800 dm3 mol−1 cm−1) in dichloromethane.20 The red shift and the increase of absorption coefficient of the bpyBPT complex are consistent with the assignment of this feature to metal-to-ligand charge transfer (MLCT) transitions (vide infra). These results suggest that L2 and L3 undergo C−S bond cleavage to give dinuclear iron complexes of the N,C,S-tridentate ligands L2′ and L3′, respectively. In the photoreactions, the coordination of the Schiff base N atoms accelerates the C−S bond cleavage reactions of DBT derivatives, as reported for the reactions with the 2-pyridyl precursor L1.20,21 The molecular structure of 3 was confirmed by a singlecrystal X-ray structure analysis (Figure 2). Selected bond distances and angles are given in Table 1, and schematic structures are illustrated in Figure 3. Complex 3 consists of two iron atoms, a tridentate-N,C,S ligand, and five carbonyl ligands. The C−S-cleaved ligand L3′ forms a six-membered thiametallacycle with a thiolate S atom and a five-membered azametallacycle with a Schiff base N atom. The 2-methoxybenzyl group on N is disordered over two orientations. The thiolate S and central aryl C atoms bridge two iron centers: one has two CO

Chart 1

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Figure 3. Schematic structures of the diiron complex [{Fe(μ-L′κ3N,C,S)(CO)2}Fe(CO)3]. Figure 2. ORTEP drawing of 3 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity.

ligands, and the other has three CO ligands. The Fe1−S1 bond (2.2434(9) Å) is slightly shorter than the Fe2−S1 bond (2.2686(9) Å). The Fe1−C7 bond in the tridentate ligand plane is 1.971(3) Å, while Fe2 is bound to the central benzene ring in an η2 mode (Fe2−C7, 2.141(3) Å; Fe2−C8, 2.444(3) Å). The two units are further linked through an Fe−Fe bond (2.5075(7) Å). A weak interaction between Fe(1) and C(16), which completes an Fe-NCSC3 octahedral geometry, was observed (Fe1···C16 = 2.500(4) Å, Fe2−C16−O3 = 166.3(3)°). The enantiopure ligand precursor L4 was readily obtained from 4-formyldibenzothiophene and (S)-(−)-α-methylbenzylamine. The photoreaction of [Fe(CO)5] with L4 produced a 55:45 mixture of two isomers with the formula [{Fe(μ-L4′κ3N,C,S)(CO)2}Fe(CO)3] (4a,b), which were obtained as a 1:1 mixture after recrystallization (eq 3). The X-ray crystal structure analysis of the product revealed that two isomers 4a,b exist in an asymmetric unit (Figure 4). Complexes 4a,b have different absolute configurations at the Fe centers; therefore, they are diastereomers arising from the (S)-αmethylbenzyl group and the chiral iron centers.

Selected bond distances and angles for 4a,b are summarized in Table 1 together with those of 1 and 3, and definitions for the geometric parameters are given in Figure 3. The structures of the two metallacycles formed by the N,C,S-tridentate ligand L′ are quite similar for 1, 3, and 4a,b. The lengths of the Fe(a)−C(c) bonds are similar to those of the azametallacycles in iron complexes containing a C,N-bidentate Schiff base ligand.23 The Fe−C(carbonyl) bond trans to C(c) is longer than that cis to C(c) by ca. 0.05 Å, which is attributable to the

Table 1. Selected Bond Distances (Å) and Angles (deg) for 1, 3, and 4 Fe(a) −S Fe(a)−N Fe(a)−C(c) Fe(a)−C(cis to C(c)) Fe(a)−C(trans to C(c)) Fe(a)−C(e) Fe(b)−S Fe(b)−C(c) Fe(b)−C(d) Fe(b)−C(e) Fe(a)−Fe(b) C(c)−Fe(a)−S C(c)−Fe(a)−N S−Fe(a)−N Fe(a)−S−Fe(b) Fe(a)−C(c)−Fe(b) Fe(a)−C(e)−Fe(b) Fe(a)−C(e)−O Fe(b)−C(e)−O interplanar anglec b

1a

3

4a

4b

2.2379(5), 2.2330(5) 1.9915(16), 1.9873(16) 1.9523(19), 1.9563(19) 1.757(2), 1.762(2) 1.801(2), 1.812(2) 2.697(2), 2.725(2) 2.2634(6), 2.2692(6) 2.146(2), 2.152(2) 2.362(2), 2.3573(19) 1.773(2), 1.776(2) 2.5481(4), 2.5455(4) 88.43(5), 88.37(5) 82.29(7), 82.17(7) 159.83(5), 160.95(5) 68.952(18), 68.852(18) 76.73(6), 76.41(6) 65.77(7), 64.94(7) 123.70(17), 124.08(18) 170.5(2), 171.0(2) 42.29(9), 41.05(8)

2.2434(9) 1.985(2) 1.971(3) 1.758(3) 1.813(3) 2.500(4) 2.2686(9) 2.141(3) 2.444(3) 1.781(4) 2.5075(7) 87.39(9) 81.66(11) 156.58(8) 67.52(3) 75.03(10) 69.38(12) 124.1(3) 166.3(3) 39.75(10)

2.2474(13) 1.994(4) 1.976(5) 1.771(5) 1.817(6) 2.404(5) 2.2754(13) 2.146(5) 2.421(5) 1.786(5) 2.5149(9) 87.36(15) 81.7(2) 158.66(12) 67.56(4) 75.07(18) 72.06(17) 124.4(4) 162.9(4) 38.40(19)

2.2414(14) 2.001(4) 1.970(5) 1.769(5) 1.819(6) 2.597(5) 2.2765(13) 2.139(5) 2.353(5) 1.780(6) 2.5438(9) 86.87(15) 82.03(19) 160.41(12) 67.52(3) 75.03(10) 69.38(12) 124.1(3) 166.3(3) 40.00(18)

a c

Data from ref 20. Two independent molecules exist in an asymmetric unit. bFe(a), Fe(b), C(c), C(d), and C(e) are defined in Figure 3. Interplanar angles were calculated as the dihedral angle between the least-squares planes 1 and 2 presented in Figure 3. 7550

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solution. The 13C{1H} NMR spectra of 1−3 show an intense signal and two weak signals in the carbonyl region, suggesting that the two carbonyl ligands in the Fe(L′)(CO)2 unit are static and the three carbonyl ligands in the Fe(CO)3 unit are fluxional. It has been pointed out that fluxionality of the Fe2 unit in the H-cluster of [FeFe]-hydrogenases is an important factor for catalysis.1,25 The two isomers 4a,b were partially separated by column chromatography with silica gel or Florisil (n-hexane/ethyl acetate, 10/1), although this process is accompanied by decomposition of the complexes to form the precursor L4. UV−vis absorption spectra of the early and later fractions were quite similar to each other. To check the separation of 4a,b, the circular dichroism (CD) and absorption spectra were measured. In silica gel column chromatography, the early elution fraction showed a positive CD band at 550 nm, while the later fraction showed a negative CD band at 560 nm (Figure S11, Supporting Information): these bands are assignable to the MLCT transitions. When Florisil was used as an adsorbent, the CD spectra showed that two isomers were eluted in the reverse order. The CD patterns in the range of 380800 nm displayed almost enantiomeric features for the early and later fractions. The 1H NMR spectra indicated that the early fraction contains only 4a or 4b and the later fraction is a mixture of 4a and 4b (Figure S7, Supporting Information). The major complex in the early fraction of Florisil column chromatography can be assigned to 4a by comparing the CD spectrum of the fraction with that of 5a (vide infra). No interconversion between 4a and 4b was observed for the separated products in C6D6 at 70 °C for 2 h; therefore, the Fe(CO)3 unit is tightly bound to the thiolate-containing metallacycle. Irradiation of the separated solution resulted in a 55:45 mixture of the two isomers, which suggests that the interconversion between 4a and 4b is induced by dissociation of carbonyl ligands. Diastereoselective Synthesis of 5a,b. To improve the diastereoselectivity, (R)-2-(4-dibenzothienyl)-4-isopropyloxazoline (L5),26 which was prepared from 4-formyldibenzothiophene and D-valinol,27 was used as a precursor for the N,C,Stridentate ligand L5′. The photochemical reaction of [Fe(CO)5] with L5 was performed in toluene to afford a purple solution containing two diastereomers of [{Fe(μ-L5′-κ3N,C,S)(CO)2}Fe(CO)3], 5a,b (eq 4). The 1H NMR spectrum of the

Figure 4. ORTEP drawings of 4 with thermal ellipsoids at the 50% probability level. The two isomers 4a (a) and 4b (b) exist in an asymmetric unit. Hydrogen atoms are omitted for clarity.

trans influence induced by the strong σ-donating character of the carbanion C donor.21,24 The dinuclear structure of [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] is considered as consisting of two units, Fe(L′)(CO)2 and Fe(CO)3: the latter unit is located on a thiolate-containing metallacycle in the former one. Excluding the Fe−Fe bond, the coordination geometries of the two units can be described as follows (Figure 3). The Fe(L′)(CO)2 unit has a five-coordinate square-pyramidal geometry with an apical CO ligand and a basal plane composed of L′ and CO, and the sixth coordination site is weakly coordinated by a CO ligand from the Fe(CO)3 unit. The geometry around Fe in the Fe(CO)3 unit is a trigonal bipyramid with axial S and C donor atoms: the three equatorial positions are occupied by two CO ligands and the π coordination of the phenyl ring. The L′ ligands lose the planarity by the π coordination, as described by the dihedral angles between planes 1 and 2 (Table 1 and Figure 3). In the solid state, complexes 1, 3, and 4a,b show similar C,S-bridged Fe2 core structures. The Fe−Fe bonds are in the range of 2.5075(7)2.5481(4) Å. However, the Fe(a)···C(e) distances significantly decrease in the order 1 (2.697(2), 2.725(2) Å) > 4b (2.597(5) Å) > 3 (2.500(4) Å) > 4a (2.404(5) Å). The Fe···CO interaction, which increases from 1 to 4a, competes with the π coordination of the central phenyl ring. The Fe(b)− C(d) distances for 3 (2.444(3) Å) and 4a (2.421(5) Å) are larger than those for 1 (2.362(2), 2.3573(19) Å) and 4b (2.353(5) Å). In addition to the variable Fe(a)···C(e) and Fe(b)−C(d) distances in the solid state, fluxional behavior was observed in

reaction mixture showed that the ratio of major and minor isomers was 9:1. After purification by column chromatography with silica gel, the products were obtained as a purple solid in a diastereomeric ratio of 15:1 (Figure S8, Supporting Information). Figure 5 shows absorption and CD spectra of a mixture of 5a,b with the 15:1 diastereomeric ratio. A broad absorption 7551

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Figure 5. Absorption (top) and CD (bottom) spectra of a 15:1 mixture of 5a and 5b in acetonitrile.

DFT) calculations predicted intense transitions at 515 ( f = 0.0323) and 556 nm ( f = 0.0236) for 5a and 511 (f = 0.0277) and 551 nm (f = 0.0218) for 5b, which are mainly composed of transitions from HOMO to LUMO and from HOMO to LUMO+1. Therefore, the experimentally observed absorption band at 516 nm was assigned to the MLCT transitions from Fe−Fe bonding orbitals to π* orbitals of the N,C,S-tridentate ligand. The calculated rotatory strengths were compared with the experimental CD spectrum (Figure S18, Supporting Information): the spectrum in the range of 400−600 nm was similar to the data for 5a rather than those for 5b. Thus, the major product for the photoreaction of [Fe(CO)5] with L5 was tentatively assigned to 5a, which was calculated to be more stable than 5b. Electrochemistry. Electrochemical properties of diiron complexes 1−3, 4 (4a and 4b, 1:1), and 5 (major 5a and minor 5b) were investigated by cyclic voltammetry. The cyclic voltammograms of 1, 3, and 5 in acetonitrile are shown in Figure 7 and those of 2 and 4 in Figures S19 and S20

band at 516 nm (ε = 2400 dm3 mol−1 cm−1) is assignable to MLCT transitions (vide infra). In this MLCT region, the CD spectrum showed a negative CD band at 546 nm and a positive CD band at 451 nm, which mainly reflect the absolute configurations around the iron centers in the major product. To estimate the structures of major and minor products, density functional theory (DFT) calculations of 5a,b were performed at the B3LYP/6-311+G(d,p) level.28 The optimized structures of 5a,b are presented in Figures S14 and S16 (Supporting Information), respectively. The calculated total energy of 5a was lower than that of 5b by 4.164 kJ mol−1, which is reasonable since the ratio of 5a and 5b in the crude product was 9:1. The higher energy of 5b is attributable to the steric repulsion between the isopropyl group and the carbonyl ligands. Molecular orbitals calculated for 5a are displayed in Figure 6. The HOMO is distributed over two Fe atoms, indicating Fe−

Figure 7. Cyclic voltammograms of 1 (black line), 3 (blue line), and 5 (red line) (5.0 × 10−4 M) in CH3CN containing 0.10 M Bu4NPF6: scan rate, 100 mV s−1; working electrode, glassy carbon; auxiliary electrode, platinum wire; reference electrode, Ag/Ag+. Potentials are versus ferrocenium/ferrocene (Fc+/Fc).

Table 2. Electrochemical Data of 1−5a complex 1 2 3 4 5

reduction E1/2/V (ΔEp/mV)b −1.88 −1.89 −1.88 −1.91 −1.90

(67), (68), (63), (68), (66),

−1.27 −1.25 −1.26 −1.27 −1.23

(66) (73) (65) (74) (81)

oxidation Epa/Vc 0.09, 0.12, 0.11, 0.12, 0.10,

0.38 0.38 0.37 0.38 0.39

a

Data from cyclic voltammetric measurements in acetonitrile with 0.1 M Bu4NPF6 as a supporting electrolyte: scan rate, 100 mV s−1; working electrode, glassy carbon; auxiliary electrode, platinum wire; reference electrode, Ag/Ag+. bPotentials are given vs E°′(Fc+/Fc); ΔEp = peak to peak separation. cEpa vs E°′(Fc+/Fc).

(Supporting Information). The electrochemical data are summarized in Table 2. Although the N,C,S-tridentate ligands used in this study have three different types of N donor atoms, pyridine, Schiff base, and oxazoline, their complexes gave quite similar voltammograms. Furthermore, the voltammogram of 4 suggests isomers 4a,b have the same redox properties. Two reversible one-electron-reduction processes were observed at around E1/2 = −1.25 and −1.90 V vs E°′(Fc+/Fc), which are assigned to [Fe2(μ-L′)(CO)5]0/− and [Fe2(μ-L′)(CO)5]−/2− couples, respectively. In the oxidation process, two oxidation peaks with no corresponding cathodic peaks appeared at ca. 0.1

Figure 6. Plots of (a) HOMO, (b) HOMO−1, (c) LUMO, and (d) LUMO+1 of 5a (isovalue = 0.04).

Fe bonding. The HOMO−1 is delocalized over two Fe atoms and L5′ through dπ−pπ interactions. The LUMO and LUMO +1 are centered on the central phenyl and thiophenol moieties of the L5′ ligand, respectively. The π system of L5′ largely contributes to the unoccupied molecular orbitals. The molecular orbitals of 5b are similar to those of 5a (Figure S17, Supporting Information). The time-dependent DFT (TD7552

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appeared at a potential more positive than that in the absence of acetic acid, and the corresponding cathodic peak does not appear during the reverse scan. This suggests that the twoelectron-reduced species 32− undergoes protonation to form 3H−. A further scan to −2.6 V shows a new reduction wave at ca. −2.3 V, which is attributable to the reduction of 3H−. Complexes 1 and 5 exhibited similar electrochemical behavior in the presence of acetic acid. Figure 9 shows the dependence of the voltammograms on the concentration of acetic acid. The second reduction wave at −1.9 V shifted to positive potentials with increasing concentrations of acetic acid, which indicates that 3H− is formed by an EC mechanism. The peak current at ca. −2.3 V increases with an increase in the acid concentrations. In the acid concentrations of 15 equiv, the second reduction peak was observed. The peak is shifted to negative potentials with the acid concentrations. The control experiments in the absence of the diiron complexes indicated that these reduction waves result from electrocatalytic proton reduction with the protonated two-electron-reduction product. Electrocatalytic generation of dihydrogen from weak acid was reported by using the diiron(I,I) complex [Fe2(μ-bdt)(CO)6].9 The catalyst is the one-electron-reduced species [Fe2(μbdt)(CO)6]−, and an ECEC mechanism, including [Fe2(μbdt)(CO)6]2−, [Fe2(μ-bdt)(μ-H)(CO)6]−, and [Fe2(μ-bdt)(μH)(CO)6]2−, was suggested. Although the protonation site in [Fe2(μ-L′)(CO)5]2− is not clear, a similar ECEC mechanism is possible for the N,C,S-tridentate ligand system. The overpotentials (ca. −0.8 V) of 1, 3, and 5 are larger than that of [Fe2(μ-bdt)(CO)6] (−0.57 V). The efficiency of the proton reduction is not dependent on the type of N donor groups, which is consistent with similar solution properties including redox potentials and a fluxional Fe(CO)3 unit.

and 0.4 V. These electrochemical data are indicative of the similarity of the C,S-bridged diiron core structures in solution. Furthermore, the N donor atoms of pyridine, Schiff base, and oxazoline moieties have similar electronic effects. Reduction processes of a variety of thiolate-bridged diiron(I,I) complexes with carbonyl ligands have been investigated by cyclic voltammetry. The dithiolato-bridged complex with 1,3propanedithiolate (pdt) [Fe2(μ-pdt)(CO)6] shows an initial quasi-reversible one-electron reduction at Epc = ca. −1.7 V vs E°′(Fc+/Fc).12b,29 A second reduction is irreversible and does not take place until −2.2 V. In the case of the 1,2ethanedithiolate (edt) complex [Fe2(μ-edt)(CO)6], the initial reduction (Epc = ca. −1.7 V) changes from a one-electron to a two-electron process as the scan rate is slowed.25 The twoelectron process is attributable to an intramolecular structural rearrangement that results in the second reduction at a potential more positive than that for the first reduction. On the other hand, the 1,2-benzenedithiolate (bdt) complex [Fe2(μ-bdt)(CO)6] is reduced to its dianion in a reversible two-electron process with closely spaced individual potentials (E1/2 = −1.32 V), which is also attributed to the potential inversion.9 In the diiron complexes studied here, the tridentate ligand L′ asymmetrically bridges two Fe centers through S and C atoms. As a result of the static nature of L′, the reduction processes are quite different from those for [Fe2(μ-edt)(CO)6], [Fe2(μ-pdt)(CO)6], and [Fe2(μ-bdt)(CO)6]. The initial reduction potentials of [{Fe(μ-L′)(CO)2}Fe(CO)3] are more positive than those of [Fe2(μ-edt)(CO)6] and [Fe2(μ-pdt)(CO)6] and similar to that of [Fe2(μ-bdt)(CO)6]. The second reduction process is reversible and occurs at a more negative potential. This suggests that the diiron complexes [{Fe(μL′)(CO)2}Fe(CO)3] do not alter the essential structure even after two-electron reduction. The electrocatalytic ability of 1, 3, and 5 for the proton reduction of acetic acid was investigated. The standard potential of acetic acid (−1.46 V vs E°′(Fc+/Fc)) is more negative than that of the initial reduction process and more positive than that of the second reduction process of these complexes.30 Figure 8 shows cyclic voltammograms of 3 in the presence of 1 equiv of acetic acid. The initial reduction of 3 to 3− is not affected on the reverse scan at −1.5 V. When the scan was performed at −2.1 V, a second reduction peak (3−/32−)

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CONCLUSIONS We have demonstrated that diiron carbonyl complexes bearing N,C,S-tridentate ligands (L′), [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3], are produced via C−S bond cleavage of DBT derivatives assisted by the N donor group of Schiff base or oxazoline moieties, as previously reported for pyridylsubstituted dibenzothiophenes.20,21 Because the DBT derivatives studied here are readily obtained from 4-formyldibenzothiophene, a variety of diiron complexes with N,C,S-tridentate ligands are available by this procedure. The chiral character of [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] was investigated by the chiral Schiff base precursor L4 and the chiral oxazoline precursor L5. Diastereoselective formation of diiron complexes was observed for L5. The CD spectral feature in the MLCT region (500550 nm) is related to the absolute configuration around Fe. The thiolato- and aryl-bridged diiron complexes [{Fe(μ-L′κ3N,C,S)(CO)2}Fe(CO)3] can be considered as an asymmetric model of the active site of [FeFe]-hydrogenases. The electrochemical investigations of the diiron complexes revealed reversible two-step one-electron redox couples for [Fe2(L′)(CO)5]0/− and [Fe2(L′)(CO)5]−/2−. The one-electron-reduced forms do not react with acetic acid, while the two-electronreduced forms undergo protonation and act as electrocatalysts for proton reduction of acetic acid in acetonitrile. The clear difference between [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] and [Fe2(μ-dithiolate)(CO)6] is the fluxional behavior in solution. The C,S-bridged complexes have rigid metallacycle structures, and the Fe(CO)3 unit is located on the thiolate-containing metallacycle. The electrocatalytic proton reduction ability of

Figure 8. Cyclic voltammograms of 3 (5.0 × 10−4 M) in CH3CN containing 0.10 M Bu4NPF6 in the absence of acetic acid (black line) and in the presence of 5.0 × 10−4 M acetic acid (blue, green, and red lines) and a control solution containing 5.0 × 10−4 M acetic acid in the absence of 3 (red dashed line): scan rate, 100 mV s−1; working electrode, glassy carbon; auxiliary electrode, platinum wire; reference electrode, Ag/Ag+. Potentials are versus Fc+/Fc. 7553

dx.doi.org/10.1021/om300826y | Organometallics 2012, 31, 7548−7557

Organometallics

Article

Figure 9. Cyclic voltammograms of (a) 1 (0.50 mM), (b) 3 (0.50 mM), and (c) 5 (0.50 mM) in CH3CN containing 0.10 M Bu4NPF6 in the absence of acetic acid (black line) and in the presence of acetic acid (0.50 mM, red; 1.0 mM, purple; 1.5 mM, brown; 2.5 mM, pink; 5.0 mM, blue; 10 mM, green) and a control solution containing acetic acid (5.0 mM) in the absence of complexes (blue dashed line): scan rate, 100 mV s−1; working electrode, glassy carbon; auxiliary electrode, platinum wire; reference electrode, Ag/Ag+. Potentials are versus Fc+/Fc. Insets: plots of the catalytic peak current measured from current in the absence of acetic acid. MHz, CDCl3): δ 5.05 (s, 2H, CH2), 7.27−7.43 (m, 2H), 7.44−7.59 (m, 7H), 7.65 (dd, J = 7.3, 1.1 Hz, 1H), 7.90−7.97 (m, 1H), 8.17− 8.23 (m, 1H), 8.27 (dd, J = 7.8, 1.2 Hz, 1H), 8.71 (t, J = 1.3 Hz, 1H). Anal. Calcd for C20H15NS: C, 79.70; H, 5.02; N, 4.65. Found: C, 79.41; H, 5.00; N, 4.65. Preparation of L3. 4-Formyldibenzothiophene (300 mg, 1.4 mmol) and 2-methoxybenzylamine (0.54 mL, 4.2 mmol) were refluxed in methanol (25 mL) for 2 h. The resulting yellow solution was concentrated and then cooled to 0 °C to give a pale yellow precipitate. Recrystallization from dichloromethane−methanol afforded a pale yellow solid of L3 (408 mg, 84%). 1H NMR (300 MHz, CDCl3): δ 3.89 (s, 3H, OCH3), 5.05 (s, 2H, CH2), 6.92 (d, J = 8.2 Hz, 1H), 7.00 (td, J = 7.5, 0.9 Hz, 1H), 7.28 (m, 1H), 7.44−7.51 (m, 2H), 7.52−7.62 (m, 2H), 7.66 (dd, J = 7.4, 1.2 Hz, 1H), 7.89− 7.97 (m, 1H), 8.18−8.24 (m, 1H), 8.27 (dd, J = 7.8, 1.2 Hz, 1H), 8.70 (t, J = 1.3 Hz, 1H). Anal. Calcd for C21H17NOS: C, 76.10; H, 5.17; N, 4.23. Found: C, 75.70; H, 5.33; N, 4.31. Preparation of L4. 4-Formyldibenzothiophene (400 mg, 1.86 mmol) and (S)-(−)-α-methylbenzylamine (0.48 mL, 3.7 mmol) were refluxed in methanol (15 mL) for 2 h. The resulting yellow solution was concentrated and then cooled to 0 °C to give a pale yellow precipitate, which was filtered and washed with methanol. Compound L4 was obtained as a white solid (503 mg, 86%). 1H NMR (300 MHz, CDCl3): δ 1.71 (d, 3JHH = 6.6 Hz, 3H, CH(Me)Ph), 4.74 (q, 3JHH = 6.6 Hz, 1H, CH(Me)Ph), 7.23−7.30 (m, 1H), 7.35−7.43 (m, 2H), 7.44−7.54 (m, 2H), 7.56 (d, J = 7.6 Hz, 1H), 7.60−7.69 (m, 3H), 7.95−8.01 (m, 1H), 8.17−8.24 (m, 1H), 8.26 (dd, J = 7.8, 1.2 Hz, 1H), 8.68 (s, 1H). Anal. Calcd for C21H17NS: C, 79.96; H, 5.43; N, 4.44. Found: C, 79.61; H, 5.44; N, 4.42. Preparation of (R)-2-(4-Dibenzothienyl)-4-isopropyloxazoline (L5).26,27 (R)-2-Amino-3-methyl-1-butanol (D-valinol; 103 mg, 1.0 mmol) was dissolved in dichloromethane (6 mL), and 4 Å molecular sieves (1.5 g) and 4-formyldibenzothiophene (212 mg, 1.0 mmol) were added. The mixture was stirred for 24 h. NBromosuccinimide (178 mg, 1.0 mmol) was added, and the solution was stirred for 1 h. After removal of molecular sieves, the yellow suspension was poured into a saturated aqueous NaHCO3 solution (40 mL). The organic layer was separated, washed with H2O (10 mL), dried over Na2SO4, and filtered. The pale yellow filtrate was concentrated and purified by column chromatography (silica gel, 2 × 18 cm, n-hexane/ethyl acetate, 4/1). The product was obtained as a pale yellow solid. Yield: 149 mg (50%). The 1H NMR spectral data matched with those reported by Voituriez and Schulz.26 1H NMR (300 MHz, CDCl3): δ 1.03 (d, 3JHH = 6.7 Hz, 3H, CH(CH3)2), 1.14 (d, 3 JHH = 6.7 Hz, 3H, CH(CH3)2), 1.92 (septet of doublets, 3JHH = 6.7, 6.7 Hz, 1H, CH(CH3)2), 4.18−4.34 (m, 2H, CH2CHiPr), 4.50 (m, 1H, CH2CHiPr), 7.43−7.52 (m, 2H, C6-H, C7-H, C8-H, or C9-H), 7.53 (dd, 3JHH = 7.6, 7.8 Hz, 1H, C2-H), 7.87−7.96 (m, 1H, C6-H,

[{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] is probably related to the fluxionality of the Fe(CO)3 unit. The structure shift from the η2:η1 to the η1:η1 bridging mode of the central phenyl ring might be possible during the catalytic reduction. Further studies will focus on the introduction of new coordinating functional groups in DBT derivatives as well as substitution of CO ligands, as reported for [Fe2(μ-dithiolate)(CO)6].

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EXPERIMENTAL SECTION

General Procedures. All manipulations were performed using a glovebox under an atmosphere of oxygen-free dry nitrogen or standard Schlenk techniques under a nitrogen atmosphere. Dried solvents were purchased from Nacalai Tesque, Inc., or Kanto Chemical Co., Inc., and were purified by passing through a column of activated alumina under an inert atmosphere in a glovebox. [{Fe(μ-L1′-κ3N,C,S)(CO)2}Fe(CO)3] (1) was prepared according to a literature procedure.20 NMR spectra were recorded on a JEOL Lambda 300 or a Bruker AVANCE 300 FT-NMR spectrometer at room temperature. IR spectra were recorded on a JASCO FT/IR-6200 spectrometer with an attenuated total reflection (ATR) accessory. Circular dichroism spectra were measured on a JASCO J-720W spectropolarimeter. Elemental analyses were performed by the Analytical Research Service Center at Osaka City University on J-SCIENCE LAB JM10 or FISONS Instrument EA1108 elemental analyzers. Photolysis was carried out using an Ushio UM-452 450W high-pressure Hg lamp placed in a water-cooled quartz jacket. Preparation of 4-Formyldibenzothiophene. 4-Formyldibenzothiophene was prepared by a modification of a literature procedure.22 Dibenzothiophene (4.00 g, 21.6 mmol) was dissolved under N2 in dry diethyl ether (80 mL). The solution was cooled to −78 °C, and secbutyllithium (1.0 M) in cyclohexane/n-hexane (32 mL, 32 mmol) was added to the solution. The solution was warmed to room temperature and stirred for 3 h. The solution was cooled to 0 °C, and N,Ndimethylformamide (8 mL, 104 mmol) was added. The solution was warmed to room temperature and stirred for 1 h. A 1 M HCl solution (20 mL) was added to the solution, and an ether layer was separated. The ether layer was dried over Na2SO4 and filtered. The solvent was removed under reduced pressure. The yellow residue was recrystallized from dichloromethane−methanol to afford pale yellow crystals (1.81 g, 40%). 1H NMR (300 MHz, CDCl3): δ 7.48−7.58 (m, 2H), 7.69 (t, J = 7.6 Hz, 1H), 7.94−8.00 (m, 1H), 8.01 (dd, J = 7.4, 1.2 Hz, 1H), 8.19− 8.28 (m, 1H), 8.45 (dd, J = 7.9, 1.2 Hz, 1H), 10.30 (s, 1H, CHO). Preparation of L2. 4-Formyldibenzothiophene (600 mg, 2.8 mmol) and benzylamine (0.64 mL, 7.8 mmol) were refluxed in methanol (25 mL) for 2 h. The yellow solution was concentrated and then cooled to 0 °C. The resulting yellow oil was washed with methanol to afford L2 as a yellow oil (570 mg, 68%). 1H NMR (300 7554

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Organometallics

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C7-H, C8-H, or C9-H), 8.00 (dd, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, 1H, C1-H or C3-H), 8.13−8.22 (m, 1H, C6-H, C7−H, C8-H, or C9-H), 8.29 (dd, 3JHH = 7.8 Hz, 4JHH = 1.2 Hz, 1H, C1-H or C3-H). Preparation of [{Fe(μ-L2′-κ3S,C,N)(CO)2}Fe(CO)3] (2). A quartz glass sample tube with a Teflon valve was charged with L2 (100 mg, 0.33 mmol), Fe(CO)5 (135 μL, 0.66 mmol), and tetrahydrofuran (THF; 25 mL). The yellow solution was degassed three times using a freeze−pump−thaw method, and then the tube was sealed. The solution was irradiated with a high-pressure Hg lamp for 12 h, during which time the reaction solution was degassed every 3 h. The solution turned purple. The solvent was removed to leave a purple solid. Dark purple crystals of 2 were obtained by recrystallization from THF−nhexane (32 mg, 16%). 1H NMR (300 MHz, C6D6): δ 4.50 (d, 2JHH = 13.9 Hz, 1H, CH2), 4.78 (d, 2JHH = 13.9 Hz, 1H, CH2), 6.32 (ddd, J = 7.5, 6.9, 1.7 Hz, 1H), 6.55−6.81 (m, 5H), 7.00−7.22 (m, 6H), 7.89 (dd, J = 8.4, 0.8 Hz, 1H),. 1H NMR (300 MHz, CDCl3): δ 5.15 (d, 2 JHH = 14.1 Hz, 1H, CH2), 5.24 (d, 2JHH = 14.1 Hz, 1H, CH2), 6.70− 6.77 (m, 1H), 6.88−7.03 (m, 3H), 7.20−7.30 (m, 2H), 7.32−7.47 (m, 5H), 8.14 (s, br, 1H), 8.30 (dd, J = 8.1, 1.3 Hz, 1H). 13C{1H} NMR (75.5 MHz, CDCl3): δ 69.2 (CH2), 123.0, 125.2, 126.4, 126.9, 127.9, 128.6, 128.8, 129.0, 129.2, 129.5, 133.0, 136.2, 140.0, 144.3, 158.3, 169.7, 170.7, 211.6 (CO), 211.7 (CO), 213.7 (CO). Anal. Calcd for C25H15Fe2NO5S·0.5CH2Cl2: C, 51.42; H, 2.71; N, 2.35. Found: C, 51.41; H, 2.88; N, 2.38. IR (ATR): νCO/cm−1 2030, 1994, 1964, 1943, 1892. UV−vis: λmax(CH3CN)/nm 525 (ε/dm3 mol−1 cm−1 3100). Preparation of [{Fe(μ-L3′-κ3S,C,N)(CO)2}Fe(CO)3] (3). A quartz glass sample tube with a Teflon valve was charged with L3 (100 mg, 0.29 mmol), Fe(CO)5 (205 μL, 1.00 mmol), and THF (25 mL). The yellow solution was degassed three times using a freeze−pump−thaw method, and then the tube was sealed. The solution was irradiated with a high-pressure Hg lamp for 14 h, and the reaction solution was degassed after 3 h of irradiation. The solution turned purple. The solvent was removed to leave a purple solid, which was purified by column chromatography (silica gel, n-hexane/dichloromethane, 5/2). A purple band was collected and concentrated to afford a purple solid (59 mg, 35%). Dark purple crystals of 3 were obtained by recrystallization from THF−n-hexane. 1H NMR (300 MHz, C6D6): δ 3.20 (s, 3H, OMe), 4.87 (d, br, 2JHH = 14.7 Hz, 1H, CH2), 5.03 (dd, 2 JHH = 14.7, 4JHH = 1.2 Hz, 1H, CH2NCH), 6.34 (ddd, J = 7.6, 6.6, 2.1 Hz, 1H), 6.45 (dd, J = 8.2, 0.85 Hz, 1H), 6.56 (dd, J = 6.8, 1.0 Hz, 1H, C4′-H or C6′-H), 6.58−6.65 (m, 2H), 6.73 (dd, 3JHH = 8.4, 6.8 Hz, 1H, C5′-H), 6.76 (ddd, J = 7.5, 1.1, 0.6 Hz, 1H), 6.84 (td, J = 7.5, 1.1 Hz, 1H), 7.02−7.13 (m, 2H), 7.42 (t, 4JHH = 1.2 Hz, 1H, CH2N CH), 7.87 (dd, J = 8.4, 1.0 Hz, 1H, C4′-H or C6′-H). 1H NMR (300 MHz, CDCl3): δ 3.79 (s, 3H, OMe), 5.19 (d, 2JHH = 15.2 Hz, 1H, CH2), 5.24 (d, 2JHH = 15.2 Hz, 1H, CH2NCH), 6.71−6.78 (m, 1H), 6.90−7.07 (m, 5H), 7.15−7.23 (m, 2H), 7.35−7.42 (m, 2H), 7.95 (s, br, 1H), 8.27 (m, 1H). 13C{1H} NMR (75.5 MHz, CDCl3): δ 55.4 (OCH3), 64.4 (CH2), 110.9, 120.9, 123.3, 124.4, 125.1, 125.7, 126.8, 127.8, 129.0, 129.5, 130.3, 131.4, 133.1, 139.5, 144.4, 158.1, 158.6, 169.4, 170.2, 211.7 (CO), 212.2 (CO), 214.0 (CO). Anal. Calcd for C26H17Fe2NO6S: C, 53.55; H, 2.94; N, 2.40. Found: C, 54.30; H, 3.21; N, 2.35. IR (ATR): νCO/cm−1 2025, 1990, 1963, 1940, 1891. UV−vis: λmax(CH3CN)/nm 519 (ε/dm3 mol−1 cm−1 2900). Preparation of [{Fe(μ-L4′-κ3S,C,N)(CO)2}Fe(CO)3] (4a,b). A quartz glass sample tube with a Teflon valve was charged with L4 (100 mg, 0.32 mmol), Fe(CO)5 (200 μL, 0.96 mmol), and THF (25 mL). The yellow solution was degassed three times using a freeze− pump−thaw method, and then the tube was sealed. The solution was irradiated with a high-pressure Hg lamp for 13 h, during which time the reaction solution was degassed after 3 h of irradiation. The solution turned purple. The solvent was removed to leave a purple solid, which was purified by column chromatography (silica gel, n-hexane/ dichloromethane, 5/2). A purple band was collected and concentrated to afford a purple solid as a mixture of 4a and 4b (39 mg, 22%). Dark purple crystals suitable for single-crystal X-ray analysis were obtained by recrystallization from THF−n-hexane. 1H NMR (300 MHz, C6D6): δ 1.38 (d, 3JHH = 6.8 Hz, 3H, CHMe), 1.55 (d, 3JHH = 6.9 Hz, 3H, CHMe), 5.03 (q, br, 3JHH = 6.9 Hz, 1H, CHMe), 5.17 (q, br, 3JHH = 6.8 Hz, 1H, CHMe), 6.32 (ddd, 1H), 6.34 (ddd, 1H), 6.52−6.81 (m,

10H), 7.02−7.19 (m, 10H), 7.44 (d, 4JHH = 0.9 Hz, 1H, CHNCH), 7.60 (d, 4JHH = 1.0 Hz, 1H, CHN=CH), 7.89 (dd, J = 8.5, 1.0 Hz, 1H), 7.90 (dd, J = 8.4, 1.1 Hz, 1H). 1H NMR (300 MHz, CDCl3): δ 8.32− 8.25 (m, 2H), 8.18 (s, 1H), 8.00 (s, 1H), 7.45−7.31 (m, br, 10H), 7.22−7.19 (m, 5H), 7.04−6.88 (m, 6H), 6.78−6.70 (m, 2H), 5.54− 5.40 (m, 2H), 1.97 (d, J = 6.9 Hz, 3H), 1.87 (d, J = 6.9 Hz, 3H). 13 C{1H} NMR (75.5 MHz, C6D6): δ 21.27 (CH3), 21.88 (CH3), 71.43 (CHNCH), 71.53 (CHNCH), 122.46, 123.10, 125.13, 125.16, 126.11, 126.25, 127.07, 127.43, 129.05, 129.12, 129.17, 129.63, 129.67, 133.05, 133.15, 140.00, 140.13, 140.40, 141.40, 144.35, 144.45, 158.67, 158.73, 168.24, 168.68, 169.03, 169.47, 212.14 (CO), 212.24 (CO), 212.67 (CO), 212.87 (CO), 213.95 (CO). Anal. Calcd for C26H17Fe2NO5S: C, 55.06; H, 3.02; N, 2.47. Found: C, 55.54; H, 3.43; N, 2.32. IR (ATR): νCO/cm−1 2027, 1990, 1972, 1960, 1946, 1909, 1873. UV−vis: λmax(CH3CN)/nm 523 (ε/dm3 mol−1 cm−1 2700). Preparation of [{Fe(μ-L5′-κ3S,C,N)(CO)2}Fe(CO)3] (5a,b). A quartz glass sample tube with a Teflon valve was charged with L5 (60 mg, 0.20 mmol), [Fe(CO)5] (80 mg, 0.41 mmol), and toluene (10 mL). The pale yellow solution was degassed three times using a freeze−pump−thaw method, and then the tube was sealed. The solution was irradiated with a high-pressure Hg lamp for 25 h, during which time the reaction solution was degassed after 6 and 14 h of irradiation. The solution turned purple. The solvent was removed to leave a purple solid, which was purified by column chromatography (silica gel, n-hexane/dichloromethane, 2/1). A purple band was collected and concentrated. The resulting purple residue was dissolved in n-pentane (5 mL), filtered with Celite, and then concentrated to dryness to afford a purple solid (39 mg, 35%). 1H NMR (300 MHz, C6D6): δ 0.44 (d, 3JHH = 7.0 Hz, 3H, CH(CH3)2), 0.52 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2), 2.39 (septet of doublet, 3JHH = 6.9, 2.9 Hz, 1H, CH(CH3)2), 3.48 (dd, 2JHH = 8.5 Hz, 3JHH = 9.3 Hz, 1H, CH2), 3.69 (ddd, 3JHH = 9.3, 5.3, 2.9 Hz, 1H, CHCH(CH3)2), 3.77 (dd, 2JHH = 8.5 Hz, 3JHH = 5.3 Hz, 1H, CH2), 6.40 (ddd, 3JHH = 7.6, 6.1 Hz, 4JHH = 2.7 Hz, 1H, C4-H or C5-H), 6.60−6.67 (m, 2H, C3-H, C4-H, C5-H, or C6-H), 6.71 (dd, 3JHH = 8.5, 6.9 Hz, 1H, C5′-H), 6.84 (m, 1H, C3-H or C6-H), 7.11 (dd, 3JHH = 6.9 Hz, 4JHH = 1.1 Hz, 1H, C4′-H), 7.88 (dd, 3JHH = 8.5 Hz, 4JHH = 1.1 Hz, 1H, C6′-H). 13C{1H} NMR (75.5 MHz, C6D6): δ 13.7, 18.5, 30.0, 69.5, 70.8, 123.8, 125.5, 125.9, 127.2, 128.2, 129.4, 129.8, 133.4, 140.5, 144.2, 144.7, 166.4, 170.3, 213.8 (Fe(L4′)(CO)2), 214.7 (Fe(CO)3). Anal. Calcd for C23H17Fe2NO6S: C, 50.49; H, 3.13; N, 2.56. Found: C, 50.83; H, 3.50; N, 2.46. IR (ATR): νCO/cm−1 2030, 1988, 1941. UV−vis: λmax(CH3CN)/nm 516 (ε/dm3 mol−1 cm−1 2400). Electrochemistry. Cyclic voltammetric (CV) measurements were performed at room temperature using an ALS/CHI600A voltammetric analyzer (Bioanalytical Systems Inc.). Working, reference, and counter electrodes were a glassy-carbon-disk electrode with a diameter of 3 mm (Bioanalytical Systems Inc.), a Ag/Ag+ (0.01 M AgNO3, 1 M = 1 mol dm−3) reference electrode, and a platinum wire, respectively. Sample solutions in acetonitrile were prepared with a concentration of 5 × 10−4 M, and the supporting electrolyte was Bu4NPF6 (0.1 M). The observed potentials were corrected using the redox potential of ferrocenium/ferrocene (Fc+/Fc) obtained under the same conditions. The sample solutions were degassed using N2 prior to each measurement. X-ray Crystal Structure Determination of 3 and 4. A single crystal of 3 or 4 was mounted on a glass fiber. The diffraction data were collected on an AFC7/CCD Mercury diffractometer. The data were processed and corrected for Lorentz and polarization effects using the CrystalClear software package.31 The analyses were carried out using the WinGX software.32 Absorption corrections were applied using the Multi Scan method. The structures were solved using direct methods (SHELXS9733 for 3, SIR9734 for 4) and refined by full-matrix least squares on F2 using SHELXL97.33 Crystallographic data are summarized in Table S1 (Supporting Information). All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were located on calculated positions with C−H(aromatic) = 0.95 Å and C−H(methyl) = 0.98 Å and were refined using a riding model with Uiso(H) = 1.2[Ueq(C)]. 7555

dx.doi.org/10.1021/om300826y | Organometallics 2012, 31, 7548−7557

Organometallics

Article

Computational Details. Structures of complexes 5a,b were optimized using the Gaussian 03 program package.35 The B3LYP density functional method and the 6-31G(d,p), 6-31+G(d,p), and 6311+G(d,p) basis sets were used for the calculations. The optimized structures of 5a,b are shown in Figures S14 and S16 (Supporting Information), and their molecular coordinates are given in Tables S4 and S5 (Supporting Information), respectively. Selected molecular orbitals are presented in Figures S15 and S17 (Supporting Information). Harmonic vibrational frequencies were calculated for the optimized geometries using the same level of theory. The zeropoint energy and thermal contributions at 298.15 K and 1.0 atm to the gas-phase free energy were obtained from the harmonic vibrational frequencies without scaling factors. The calculated data and the energy difference between 5a and 5b are summarized in Table S7 (Supporting Information). Time-dependent DFT calculations were performed using the optimized structures at the B3LYP/6-311+G(d,p) level to analyze the absorption and CD spectral data. Calculated electronic transitions for 5a,b are given in Tables S8 and S9 (Supporting Information), respectively.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files, figures, and tables giving crystallographic data for 3 and 4, NMR data for 2−5, absorption spectra of 2−4, CD spectra of 4, crystal structures and selected bond lengths and angles for 3 and 4, and computational details for 5. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-6-6605-2519. Fax: +81-6-6690-2753. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 22550064) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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