Diiron Carbonyl Complexes Bearing an N,C,S-Pincer Ligand

Aug 11, 2014 - The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka. City University, Sumiyoshi-ku, Osaka ...
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Diiron Carbonyl Complexes Bearing an N,C,S-Pincer Ligand: Reactivity toward Phosphines, Heterolytic Fe−Fe Cleavage, and Electrocatalytic Proton Reduction Masakazu Hirotsu,*,† Kiyokazu Santo,† Chiaki Tsuboi,† and Isamu Kinoshita†,‡ †

Graduate School of Science and ‡The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *

ABSTRACT: The thiolate-bridged diiron carbonyl complex [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3] (1) consists of two units, Fe(PyBPT)(CO)2 and Fe(CO)3, where the N,C,S-pincer ligand PyBPT is a doubly deprotonated form of 3′-(2″-pyridyl)1,1′-biphenyl-2-thiol. The two Fe complex units are connected through a thiolate S atom, π coordination, and an Fe−Fe bond. Diiron complex 1 reacted with 1 equiv of dimethylphenylphosphine to form the CO substitution product [{Fe(μ-PyBPTκ3N,C,S)(CO)2}Fe(CO)2(PMe2Ph)] (3) via the phosphine adduct [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3(PMe2Ph)] (2), which has a polarized Fe−Fe bond. A further reaction of 3 with PMe2Ph produced the N,C,S-pincer iron(II) complex trans-[Fe(PyBPT-κ3 N,C,S)(CO)(PMe 2 Ph) 2 ] (4) and the iron(0) complex trans-[Fe(CO)3(PMe2Ph)2]. 1,2-Bis(diphenylphosphino)benzene (dppbz) abstracted the Fe(CO)3 unit from 1 to give the dimeric diiron(II,II) complex [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}2] (7) and the iron(0) complex [Fe(CO)3(dppbz)]. The asymmetric bridging ligand PyBPT and coordination of the phosphines induce polarization of the Fe−Fe bond, which leads to the formation of the iron(II) and iron(0) complexes via heterolytic Fe−Fe cleavage. Electrochemical properties of 3 and 4 were investigated by cyclic voltammetry. Complex 3 showed two one-electron-reduction processes, the potentials of which are ca. 0.4 V more negative than those of 1. Electrocatalytic proton reduction by 3 was investigated, and the efficiency was comparable to that of 1.

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INTRODUCTION

1c), which undergoes protonation at Fe to form the hydride complex [Fe2H(μ-bdt)(CO)6]−. We recently reported that the carbon- and sulfur-bridged diiron carbonyl complex [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3] (1) (Figure 1d) and its related complexes show electrocatalytic activity for proton reduction of acetic acid, where PyBPT is a doubly deprotonated form of 3′-(2″-pyridyl)1,1′-biphenyl-2-thiol and acts as an N,C,S-pincer ligand.19,20 Complex 1 has an asymmetric structure consisting of two complex units, Fe(PyBPT)(CO)2 and Fe(CO)3, which are connected through a thiolate S atom, π coordination, and an Fe−Fe bond. The electrochemical study revealed that 1 shows two reversible redox couples for 10/1− and 11−/2−. The doubly reduced complex 12− undergoes protonation to form 1H− and catalyzes the proton reduction. The catalytic cycle for 1 is thought to be parallel to that for [Fe2(μ-bdt)(CO)6]. The thiolate-bridged diiron core in an asymmetric environment is a potential candidate for developing the proton reduction catalyst. Reactivity of the asymmetric diiron core is presumably different from that of the symmetric core.

Sulfur-bridged iron clusters are ubiquitous in metalloenzymes such as hydrogenases and nitrogenases.1−5 Structural transformations of the Fe−S cores have a crucial role in their enzymatic activities, as shown for a P cluster in the nitrogenase and a [4Fe-3S] cluster in the oxygen-tolerant [NiFe]hydrogenase.6,7 A [2Fe-2S] cluster is found in the active site of the [FeFe]-hydrogenase as a dithiolate-bridged diiron complex with carbonyl and cyanide ligands, in which two Fe atoms are spaced by ca. 2.6 Å (Figure 1a).8−11 It has been pointed out that fluxionality of the [2Fe-2S] core is important in the catalytic activity of [FeFe]-hydrogenases.3,12 The structural and functional features of the [FeFe]hydrogenase active site have been demonstrated by using diiron carbonyl complexes with bridging thiolate ligands. The diiron complexes [Fe2(μ-dithiolate)(CO)6] usually show electrocatalytic activity for proton reduction (Figure 1b).13−17 A catalytic cycle has been proposed for the proton reduction using [Fe2(μ-bdt)(CO)6] (bdt = 1,2-benzenedithiolate) as an electrocatalyst.15,16,18 The key feature is an asymmetric open structure with a single thiolate bridge and a bridging CO ligand of the doubly reduced complex [Fe2(μ-bdt)(CO)6]2− (Figure © 2014 American Chemical Society

Received: May 24, 2014 Published: August 11, 2014 4260

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607 nm in THF (Figure 2). Complex 3 proved to be a CO dissociation product from 2, as described below. The conversion of 2 to 3 was further supported by 1H and 31 1 P{ H} NMR experiments (Figures S2 and S3, Supporting Information): the conversion was ca. 90% after 1 h at 60 °C. Complexes 2 and 3 exhibit 31P resonances at 21.4 and 21.1 ppm in C6D6, respectively. The almost identical 31P chemical shifts suggest that the coordination site of PMe2Ph in 2 remains unchanged in 3. The 13C NMR spectrum of 3 gave four signals for carbonyl ligands, two of which show the 13C−31P coupling. On the other hand, complex 1 shows three signals for CO because of the fluxional behavior in Fe(CO)3. Therefore, the phosphine ligand initially binds to Fe of the Fe(CO)3 unit to form 2, and then a carbonyl ligand is dissociated from the Fe(CO)3(PMe2Ph) unit to form 3 (Scheme 1). Figure 1. Structures of (a) the H cluster of [FeFe]-hydrogenases in the oxidized state, (b) the dithiolate-bridged diiron complex [Fe2(μdithiolate)(CO)6], (c) the two-electron-reduced state of [Fe2(μbdt)(CO)6], and (d) PyBPT complex 1.

Scheme 1. Reactions of 1 with PMe2Ph

Therefore, we investigated reactions of 1 toward phosphines in this study. The Fe−Fe bond underwent heterolytic cleavage to form iron(0) and PyBPT-iron(II) complexes. A phosphine adduct and a monosubstitution product were isolated as intermediates. Electrochemical properties and the electrocatalytic activity for proton reduction were investigated by using the phosphine-PyBPT complexes of iron.

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RESULTS AND DISCUSSION Reactions of 1 with Dimethylphenylphosphine. A purple solution of 1 in tetrahydrofuran (THF) was treated with 1 equiv of PMe2Ph to give a dark purple-brown solution. The color of the solution gradually changed to dark green at room temperature. The 1:1 phosphine adduct [{Fe(μ-PyBPT)(CO)2}Fe(CO)3(PMe2Ph)] (2) formed in the initial stage of the reaction. Complex 2 was isolated as dark purple-brown crystals in quantitative yield. An electronic absorption spectrum of 2 in THF showed an absorption band at 550 nm, which is red-shifted from that of 1 (521 nm, THF) as shown in Figure 2.

The molecular structures of 2 and 3 were confirmed by single-crystal X-ray analyses (Figure 3). Selected bond distances and angles are listed in Table 1. Both complexes have a PMe2Ph ligand at the iron center of the Fe(CO)3 unit in 1, which is consistent with the NMR study. Complex 2 consists of Fe(PyBPT)(CO) 2 and Fe(CO)3(PMe2Ph) units, which are connected through an Fe− Fe bond and a bridging thiolate S atom. The Fe1−Fe2 distance (2.7437(5) Å) of 2 is significantly larger than those of complex 1 and related diiron complexes with N,C,S-tridentate ligands (2.50−2.55 Å).19,20 The five-coordinate geometries around the two Fe atoms in 2, excluding the Fe−Fe bond, give the structural index parameters τ: 0.37 for Fe(PyBPT-N,C,S)(CO)2 and 0.54 for Fe(CO)3(PMe2Ph)(PyBPT-S); τ = (β − α)/60 (α and β are the two largest bond angles around the metal ion, β > α).21 The geometrical distortions around Fe are caused by the Fe−Fe bonding. Unlike complex 1, there is no interaction between the central phenyl ring of PyBPT and Fe2 in 2. The interplanar angle between two phenyl rings in PyBPT of 2 (22.85(12)°) is smaller than those of 1 and related complexes (ca. 40°) because of the absence of the π coordination. The Fe1−S1 distance (2.1680(6) Å) is shorter than the Fe2−S1 distance (2.2539(7) Å). The dinuclear structure of the substitution product 3 consisting of Fe(PyBPT)(CO)2 and Fe(CO)2(PMe2Ph) units is quite similar to that of the starting complex 1. The

Figure 2. Electronic absorption spectra of 1, 2, 3, and 4 in THF.

These absorption bands are assigned to metal-to-ligand charge transfer (MLCT) transitions (vide infra). The spectrum of 2 changed at room temperature with isosbestic points (Figure S1, Supporting Information). The final product [{Fe(μ-PyBPT)(CO)2}Fe(CO)2(PMe2Ph)] (3) was isolated by the 1:1 reaction of 1 with PMe2Ph at 60 °C for 2 h in 74% yield as dark green crystals, which show an MLCT absorption band at 4261

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and similar to that of 1 because of the regeneration of the π coordination. The bond lengths of Fe1−S1 (2.2383(8) Å) and Fe2−S1 (2.2612(8) Å) of 3 are in the normal range of the diiron complexes with N,C,S-tridentate ligands derived from functionalized dibenzothiophenes (Fe1−S, 2.23−2.25 Å; Fe2− S, 2.26−2.28 Å).19,20 The structure of the Fe2S core is not affected by the substitution of PMe2Ph for CO, while the replacement of the π coordination by PMe2Ph, forming 2, results in the deformation of the core. It is clear that the CO substitution reaction from 1 to 3 with PMe2Ph proceeds via phosphine adduct 2 as an intermediate (Scheme 1). The formation of 2 was quite fast even at room temperature, while the elimination of CO was relatively slow. The π coordination of the central phenyl ring is absent in 2 and recovers in the CO substitution product 3. The facile binding of PMe2Ph to the Fe(CO)3 unit of 1 is indicative of a low-energy barrier for the dissociation of the π coordination. The Fe−Fe bond of 1 is not cleaved in the ligand substitution reaction. In the case of the diiron dithiolate complexes [Fe2(μ-dithiolate)(CO)6], the proposed pathways for the ligand substitution involve homolytic cleavage and formation of the Fe−Fe bond.22−24 A further reaction of 3 with PMe2Ph in C6D6 was monitored by 31P{1H} NMR spectroscopy (Figure S4, Supporting Information). When 1 equiv of PMe2Ph was added to a solution of 3, no significant change was observed at room temperature. After heating at 60 °C for 2 h, 31P signals for 3 and free PMe2Ph decreased, and two major signals appeared at 27.8 and 50.0 ppm together with a few minor signals. When an additional 2 equiv of PMe2Ph was added and heated at 60 °C, the intensity of the two major signals increased and the signals of 3 and PMe2Ph almost disappeared. In the 1H NMR spectrum, three methyl signals were observed at 1.63 (t, J = 3.8 Hz), 0.80 (t, J = 4.0 Hz), and 1.58 ppm (d, 2JPH = 8.3 Hz) in a ca. 1:1:2 ratio (Figure S5, Supporting Information). The doublet 1H signal at 1.58 ppm and the 31P signal at 50.0 ppm were assigned to the bis(phosphine) iron(0) complex trans[Fe(CO)3(PMe2Ph)2].25 The remaining signals are assignable to the PyBPT iron(II) complex trans-[Fe(PyBPT-κ3N,C,S)(CO)(PMe2Ph)2] (4), in which two PMe2Ph ligands are in trans positions to each other. Indeed, the 1:4 reaction of 1 with PMe2Ph at 60 °C gave 4 as red-brown crystals in 78% yield (Scheme 2). The two PMe2Ph ligands are chemically equivalent in solution, and the proton signals of the two enantiotopic methyl groups split into a doublet of doublets with two nearly equal coupling constants, 2JPH and 4JPH (ca. 4 Hz). The

Figure 3. ORTEP drawings of (a) 2 and (b) 3 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) for 2 and 3 Fe(1)−Fe(2) Fe(1)−S(1) Fe(1)−N(1) Fe(1)−C(7) Fe(1)−C(18) Fe(1)−C(19) Fe(2)−P(1) Fe(2)−S(1) Fe(2)−C(7) Fe(2)−C(8) Fe(1)···C(20) Fe(2)−C(20) Fe(2)−C(21) Fe(2)−C(22) S(1)−Fe(1)−C(7) N(1)−Fe(1)−C(7) S(1)−Fe(1)−N(1) C(7)−Fe(1)−C(18) Fe(1)−S(1)−Fe(2) interplanar anglea interplanar angleb

2

3

2.7437(5) 2.1680(6) 1.9898(18) 1.995(2) 1.802(2) 1.745(3) 2.2397(6) 2.2539(7)

2.5631(6) 2.2383(8) 1.992(2) 1.951(3) 1.803(3) 1.754(3) 2.2188(8) 2.2612(8) 2.129(3) 2.348(3) 2.865(3) 1.753(3) 1.764(3)

1.791(2) 1.780(2) 1.822(2) 93.01(6) 82.74(8) 154.06(6) 176.51(9) 76.68(2) 22.85(12) 1.09(13)

87.81(8) 81.79(10) 159.40(7) 164.50(13) 69.45(2) 40.17(7) 1.42(14)

Scheme 2. Reaction of 1 with 4 equiv of PMe2Ph

a

Interplanar angles were calculated as the dihedral angle between the least-squares planes of C(1)−C(6) and C(7)−C(12). bInterplanar angles were calculated as the dihedral angle between the least-squares planes of C(7)−C(12) and C(13)−C(17), N(1).

Fe(CO)2(PMe2Ph) unit is bound to the Fe(PyBPT)(CO)2 unit through a thiolate S atom, π coordination, and an Fe−Fe bond. The Fe1−Fe2 bond (2.5631(6) Å) of 3 is shorter than that of 2 4262

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3. The phenylpyridine moiety of 4 is also twisted (16.22(13)°), and the central phenyl ring deviates from the equatorial coordination plane (S1, C7, N1, C18, Fe1) by 19.95(10)°. These structural deviations stem from the strain of the sixmembered thiolate-containing metallacycle. In 1−3, the phenylpyridine moieties are almost planar; therefore the thiolate bridge and the π coordination reduce the ring strain of the six-membered metallacycle. The conversion of 3 to 4 requires 3 equiv of PMe2Ph. Since the structural and spectroscopic properties of 3 are similar to those of 1, the reaction of 3 with the first PMe2Ph molecule probably gives the bis(dimethylphenylphosphine) intermediate [{Fe(μ-PyBPT)(CO)2}Fe(CO)2(PMe2Ph)2] (A) (Figure 5a),

presence of the CO ligand was confirmed by IR spectroscopy (1890 cm−1), and the 13C signal of CO was observed at δ 215.0 as a triplet with 2JCP = 20 Hz. These spectral features are similar to those of iron(II) complexes with imine-phenyl chelating ligands (L), trans-[Fe(L)(CH3)(CO)(PMe3)2].26 In the 13 C{1H} NMR spectrum of 4 the PyBPT carbon donor atom attached to Fe also gives a triplet signal (δ 192.5, 2JCP = 28 Hz). The crystal structure of mononuclear complex 4 is depicted in Figure 4, and selected bond distances and angles are listed in

Figure 5. (a) Possible intermediate A for the reaction of 3 with PMe 2 Ph. (b) Migration of CO and dissociation of [Fe(CO)3(PMe2Ph)2] from A.

Figure 4. ORTEP drawing of 4 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity.

which is similar to 2. Intermediate A was not observed in the NMR experiments; therefore, the subsequent reactions with 2 equiv of PMe2Ph are faster than the first step. A hypothetical mechanism involves migration of CO to the electron-rich Fe(CO)2(PMe2Ph)2 unit and cleavage of S−Fe and Fe−Fe bonds, which lead to dissociation of [Fe(CO)3(PMe2Ph)2] (Figure 5b). The resulting coordinatively unsaturated sites on Fe(PyBPT)(CO) are readily occupied by the two PMe2Ph molecules to give 4. Reactivity toward Triphenylphosphine. A C6D6 solution of complex 1 and PPh3 (1 equiv), which is more sterically demanding than PMe2Ph, in a sealed NMR tube gave an equilibrium mixture (Scheme 3). The reaction of 1 with 3 equiv

Table 2. Selected Bond Distances (Å) and Angles (deg) for 4 Fe(1)−P(1) 2.2665(6) Fe(1)−S(1) 2.2884(5) Fe(1)−C(7) 1.9872(17) S(1)−C(1) 1.762(2) P(1)−Fe(1)−P(2) S(1)−Fe(1)−N(1) C(7)−Fe(1)−C(18) S(1)−Fe(1)−C(7) N(1)−Fe(1)−C(7) Fe(1)−S(1)−C(1) Fe(1)−C(7)−C(8) Fe(1)−C(7)−C(12) interplanar anglea interplanar angleb

Fe(1)−P(2) 2.2341(6) Fe(1)−N(1) 1.9876(15) Fe(1)−C(18) 1.7591(19) C(18)−O(1) 1.164(2) 174.58(2) 173.82(5) 177.96(8) 92.22(6) 82.17(7) 109.43(7) 131.23(14) 111.82(14) 35.72(9) 16.22(13)

Scheme 3. Reaction of 1 with PPh3

a

Interplanar angles were calculated as the dihedral angle between the least-squares planes of C(1)−C(6) and C(7)−C(12). bInterplanar angles were calculated as the dihedral angle between the least-squares planes of C(7)−C(12) and C(13)−C(17), N(1).

Table 2. Complex 4 has a six-coordinate octahedral geometry with a dianionic N,C,S-pincer ligand, PyBPT, which occupies three equatorial positions. Two PMe2Ph ligands are located in axial positions, and a carbonyl ligand is in a trans position to the central C donor of the pincer ligand. The Fe−N, Fe−C, and Fe−S bond distances of Fe-PyBPT in 4 are comparable to those of the Fe(PyBPT)(CO)2 unit in thiolate-bridged complexes 1−3, except for Fe1−S1 in 2. The PyBPT ligand in 4 deviates from a planar structure in spite of the absence of a thiolate bridge and π coordination. The biphenyl interplanar angle (35.72(9)°) in the PyBPT ligand of 4 is larger than that of S-bridged complex 2 and is somewhat smaller than those of C,S-bridged complexes 1 and

of PPh3 was monitored by 1H and 31P{1H} NMR spectroscopy (Figures S6 and S7, Supporting Information). One equivalent of PPh3 was consumed at room temperature, and 1 was converted to the 1:1 adduct [{Fe(μ-PyBPT)(CO)2}Fe(CO)3(PPh3)] (5), which showed a 31P resonance at δ 56.8. When the solution was heated at 60 °C, the solution color changed from dark brown to moss green. A new signal due to the substitution product [{Fe(μ-PyBPT)(CO) 2 }Fe(CO)2(PPh3)] (6) appeared in the 31P{1H} NMR spectrum at δ 57.4 without further consumption of PPh3. The ratio of 5 and 6 was 1:1.7 after heating at 60 °C for 6 h. The reverse reaction was also observed after the reaction solution was left at 4263

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room temperature for 3 weeks (5:6 = 1:0.6). A further reaction of 6 with PPh3 is inhibited by steric hindrance of the coordinated phosphine and the lower nucleophilicity of entering PPh3 compared with PMe2Ph. The reaction of mono(dimethylphenylphosphine) complex 3 with PPh3 was slow even at 80 °C. Reaction of 1 with 1,2-Bis(diphenylphosphino)benzene. When the 1:1 reaction of 1 with 1,2-bis(diphenylphosphino)benzene (dppbz) in C6D6 was performed at 60 °C, the 31P{1H} NMR spectrum showed a new signal at 94.1 ppm, which was assigned to the iron(0) complex [Fe(CO)3(dppbz)].27 After heating at 60 °C for 12 h, most of dppbz was converted to [Fe(CO)3(dppbz)] (Figure S8, Supporting Information). However, the 1H NMR spectrum of the resulting dark green solution showed the formation of several complexes with PyBPT (Figure S9, Supporting Information). When the reaction was performed in toluene at 80 °C for 24 h, the solution color changed from purple to dark brown and then to red-brown, and orange crystals of the dimeric iron(II,II) complex [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}2] (7) were precipitated (40% yield, Scheme 4). Complex 7 was characterized by NMR and IR spectroscopy, and the structure was determined by X-ray crystallography (Figure 6, Table 3).

Table 3. Selected Bond Distances (Å) and Angles (deg) for 7 Fe(1)−S(1) Fe(1)−S(1)* Fe(1)−N(1) Fe(1)−C(7) Fe(1)−C(18) Fe(1)−C(19) S(1)−C(1) Fe(1)···Fe(1)* S(1)−Fe(1)−S(1)* S(1)−Fe(1)−N(1) C(7)−Fe(1)−C(18) S(1)*−Fe(1)−C(19) S(1)−Fe(1)−C(7) N(1)−Fe(1)−C(7) Fe(1)−S(1)−Fe(1)* Fe(1)−S(1)−C(1) interplanar angleb interplanar anglec

Aa

Ba

2.2514(8) 2.3432(8) 1.988(2) 1.998(3) 1.818(3) 1.751(3) 1.776(3) 3.4828(8) 81.44(3) 166.94(7) 178.26(12) 169.46(9) 92.03(8) 82.89(10) 98.56(3) 110.11(10) 28.76(13) 3.82(19)

2.2509(7) 2.3352(7) 2.002(2) 1.994(3) 1.818(3) 1.766(3) 1.775(3) 3.4784(8) 81.36(3) 166.30(7) 177.40(11) 167.59(9) 92.22(8) 82.57(10) 98.64(3) 109.86(9) 28.99(14) 2.39(19)

a

There are two independent molecules, A and B, in an asymmetric unit, and A is displayed in Figure 6. bInterplanar angles were calculated as the dihedral angle between the least-squares planes of C(1)−C(6) and C(7−C(12). cInterplanar angles were calculated as the dihedral angle between the least-squares planes of C(7)−C(12) and C(13)− C(17), N(1).

Scheme 4. Reaction of 1 with dppbz

PyBPT)(CO)2}2]. The Fe(1)−S(1) and Fe(2)−S(2) bond lengths are shorter than Fe(1)−S(1)* and Fe(2)−S(2)*. Similar Fe2S2 core structures have been found in the dinuclear Fe(II) cyanocarbonyl complex of 1,2-benzenedithiolate [{Fe(μbdt)(CN)(CO)2}2]2− and [{Fe(CO)2(‘S3’)}2] (‘S3’ = bis(2mercaptophenyl)sulfide(2−)).29,30 In the reaction of 1 with dppbz, the Fe(CO)3 unit in 1 is abstracted as the iron(0) complex [Fe(CO)3(dppbz)]. Formation of complex 7 involves dimerization of the remaining iron(II) complex unit Fe(PyBPT)(CO)2. The dimerization process could compete with formation of the C2 isomer of 7 and coordination of unreacted dppbz, as shown for the reactions of [{Fe(CO)2(‘S3’)}2] with phosphines, which results in a reduction of the yield of 7.28,30 DFT Calculations. The oxidation states of the two iron centers in 1−3 are ambiguous (FeIFeI or FeIIFe0) because of the direct Fe−Fe bond. The formation of iron(II) complexes 4 and 7 with iron(0) phosphine carbonyl complexes implies that complexes 1−3 behave as a diiron(II,0) complex with a polarized Fe−Fe bond. Although the Fe−Fe bond of 2 is longer than those of 1 and 3, the metal−metal bond distance is not a measure of the metal−metal dative bonding.31 Therefore, we performed density functional theory (DFT) calculations of 1−3 at the B3LYP/6-311+G(d,p) level. Mononuclear complex 4 was also investigated for comparison. MO surfaces of HOMO and LUMO for 1−4 are shown in Figure 7. In complexes 1−3, the HOMO is largely located between two Fe centers, which indicates that the HOMO has an Fe−Fe bonding character. The HOMO also represents π-back-bonding interactions from Fe in the phosphine-binding site toward CO (and PMe2Ph). The LUMO is delocalized over the π system of the pyridylphenyl moiety and Fe in a similar pattern for 1−3. The time-dependent (TD) DFT calculations suggested that the transition wavelengths are 556 nm (f = 0.0453), 515 nm ( f = 0.0320) for 1; 562 nm (f = 0.0281), 533 nm ( f = 0.0593) for 2;

Figure 6. ORTEP drawing of 7 with thermal ellipsoids at the 50% probability level. One of the two independent molecules is displayed. Hydrogen atoms are omitted for clarity.

Complex 7 is composed of two Fe(PyBPT)(CO)2 units, which are bridged by two thiolate S atoms of the PyBPT ligands. The dimeric structure is similar to that of the PyBPT ruthenium complex Ci-[{Ru(μ-PyBPT)(CO)2}2], except that the metal−ligand bonds in 7 are ca. 0.1 Å shorter than those in the ruthenium complex.28 There are no significant differences in the ligand structures: the biphenyl interplanar angles are 28.76(13)° and 28.99(14)° for 7 and 31.15(12)° for Ci-[{Ru(μ4264

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A, the Fe−Fe bond is more polarized than that of 2 and undergoes heterolytic cleavage. Electrochemistry. Electrochemical studies were conducted for complexes 3 and 4 in acetonitrile by cyclic voltammetry (Figure 8, Table 4). Complex 3 shows two one-electron-

Figure 8. Cyclic voltammograms of (a) 3 (5.0 × 10−4 M) and (b) 4 (4.0 × 10−4 M) in CH3CN containing 0.10 M Bu4NPF6: scan rate, 0.1 V s−1; working electrode, glassy carbon; auxiliary electrode, platinum wire; reference electrode, Ag/Ag+. Potentials are versus ferrocenium/ ferrocene (Fc+/Fc).

Table 4. Electrochemical Data of 1, 3, and 4a

Figure 7. Plots of the HOMO (left) and LUMO (right) of (a) 1, (b) 2, (c) 3, and (d) 4 (isovalue = 0.04).

reduction

oxidation

complex

E1/2/V (ΔEp/mV)b

E1/2/V (ΔEp/mV)b

1c 3 4

−1.88 (67), −1.27 (66) −2.25 (91),f −1.67 (66) −2.64,e −2.51,e −1.83d

0.09,d 0.38d −0.30,d 0.10,d 0.32,d 0.84d −0.46 (62), 0.60 (82),f 0.82d

a

Data from cyclic voltammetric measurements in acetonitrile with 0.10 M Bu4NPF6 as a supporting electrolyte; scan rate, 0.1 V s−1; working electrode, glassy carbon; auxiliary electrode, platinum wire; reference electrode, Ag/Ag+. bPotentials are given vs Eo′(Fc+/Fc); ΔEp, peak to peak separation. cData from ref 20. dEpa vs Eo′(Fc+/Fc). eEpc vs Eo′(Fc+/Fc). fScan rate, 1 V s−1.

and 602 nm ( f = 0.0755), 581 nm ( f = 0.0236) for 3, which are in good agreement with the experimental data: 1, 521 nm; 2, 550 nm; 3, 607 nm (Figure 2). These transitions are related to the charge transfer from Fe−Fe bonding orbitals to π* orbitals of PyBPT because the main components are transitions from HOMO to LUMO and from HOMO to LUMO+1. The red shift by the ligand substitution is due to a decrease of the ligand field strength. The corresponding MLCT is not observed for mononuclear complex 4 (Figure 2). The TD-DFT calculations for 4 suggested that the absorption band at 389 nm is related to the transition from HOMO−1 to LUMO, which is a π* orbital localized on a phenylpyridine moiety of PyBPT (381 nm, f = 0.0412). To evaluate the charge density on the Fe atoms, natural population analysis was performed for 1−3 by using the 6311G(d,p) and 6-311+G(d,p) basis sets (Table S11, Supporting Information). Although the calculated atomic charges are affected by the diffuse functions, the difference in atomic charge between two Fe atoms in 2 is significantly larger than those in 1 and 3. The σ donation character of PMe2Ph makes Fe in the Fe(CO)3 site of 2 more electron-rich than that of 1. This effect is weakened in 3 because the d electrons of the Fe atom are delocalized over the PyBPT ligand through the π coordination. Thus, the polarization of the Fe−Fe bond in 2 is significantly larger than that in 1 and 3. In the bis(phosphine) intermediate

reduction processes at E1/2 = −1.67 and −2.25 V vs E°′(Fc+/ Fc), which are assigned to 30/− and 3−/2−, respectively. Small waves observed at −1.53, −1.16, and −1.01 V disappeared in the reverse scan at −1.9 V, and the first reduction process is reversible. The anodic wave for the second reduction process was small at a scan rate of 0.1 V s−1 and clearly observed at higher scan rates (Figure S26, Supporting Information). The anodic to cathodic peak current ratios, ipa/ipc, for the first and second reduction processes were 0.92 and 0.75, respectively, at a scan rate of 2 V s−1. The redox potentials for the 30/− and 3−/2− couples are more negative than those of 1 by ca. 0.4 V because of the electron-donating ability of PMe2Ph. The first oxidation peak at −0.30 V is also shifted negatively by ca. 0.4 V. The coordination of the phosphine ligand to the Fe(CO)3 site affects both iron centers through the Fe−Fe bond. The cyclic voltammogram of 4 showed two oxidation processes at −0.46 and 0.60 V, which are assigned to FeIII/ FeII and FeIV/FeIII couples, respectively. The FeIII/FeII process is reversible, while the second anodic wave is followed by an anodic wave at 0.82 V at a scan rate of 0.1 V s−1. Almost reversible redox couples were observed at higher scan rates (Figure S27). In the reduction process, two cathodic peaks 4265

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were observed at −2.51 and −2.64 V, which can be assigned to 40/1− and 41−/2−, respectively. Complex 3 exhibited electrocatalytic ability for the proton reduction of organic acids, as reported for dithiolate-bridged diiron carbonyl complexes.13−17 Figure 9 shows voltammo-

the Fe−Fe bond. The highly polarized Fe−Fe bond is heterolytically cleaved to form the iron(II) and iron(0) complexes. These reactions imply the Fe−Fe dative bond character in the starting complex. Polarization of Fe−Fe bonds in diiron complexes can be induced by an asymmetrically bridged thiolate ligand. The extent of bond polarization changes during ligand substitution reactions, which proceed smoothly. More importantly, the polarized Fe−Fe bond provides an electron-rich iron site, which could act as a proton binding site in proton reduction catalysis. Phosphine complex 3 showed catalytic activity for the proton reduction. The catalytic efficiency of 3 is comparable to that of 1, which can be attributable to the similarity in the reaction mechanism and the polarization of the Fe−Fe bond.

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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. NMR spectra were recorded on a JEOL Lambda 300, a Bruker AVANCE 300, or a Bruker AVANCE 600 FT-NMR spectrometer at room temperature. IR spectra were recorded on a JASCO FT/IR-6200 spectrometer with an attenuated total reflection (ATR) accessory. 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 a 450 W high-pressure Hg lamp (Ushio UM-452) placed in a water-cooled quartz jacket. Preparation of [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3] (1). Complex 1 was prepared by a modification of a literature procedure.19 A quartz glass sample tube with a Teflon valve was charged with PyDBT (200 mg, 0.765 mmol), [Fe(CO)5] (300 mg, 1.53 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 10 h, during which time the reaction solution was degassed every 3 h. The color of the solution changed to purple. The volatiles were removed under reduced pressure to leave a purple solid, which was dissolved in THF (3 mL) and then filtered through Celite. The filtrate was concentrated to a small volume, and n-hexane was layered on top of the solution. After 1 day at room temperature, the resulting dark purple crystals of 1 were collected by decantation and washed with n-hexane (175 mg, 43%). Purification of the crude product by column chromatography (silica gel, n-hexane/dichloromethane, 2:1) gave an identical yield. The yield was improved by prolonged irradiation (20 h) up to 52%. 1H NMR (300 MHz, C6D6): δ 6.02 (ddd, JHH = 7.4, 5.8, 1.4 Hz, 1H, 5-py), 6.40 (td, JHH = 7.4, 1.4 Hz, 1H, 4-BPT), 6.57 (ddd, JHH = 8.1, 7.4, 1.5 Hz, 1H, 4-py), 6.65 (ddd, JHH = 7.7, 7.4, 1.3 Hz, 1H, 5-BPT), 6.73 (dd, JHH = 7.7, 1.4 Hz, 1H, 6-BPT), 6.81−6.94 (m, 4H, 3-py, 3, 5′, 4′-BPT), 7.92 (dd, JHH = 7.8, 1.7 Hz, 1H, 6′-BPT), 8.74 (ddd, JHH = 5.8, 1.5, 0.8 Hz, 1H, 6-py). 13C{1H} NMR (75.5 MHz, C6D6): δ 118.9 (3-py), 119.3 (4′-BPT), 122.7 (5-py), 124.9 (1′-BPT), 125.8 (4-BPT), 127.1, 128.2, 129.4, 129.8 (6-BPT), 133.7 (2-BPT), 135.8 (4-py), 138.9 (6′BPT), 144.9 (1-BPT), 155.1 (6-py), 157.8 (3′-BPT), 163.1 (2-py), 167.9 (Fe-C(2′-BPT)), 212.9 (Fe(PyBPT)(CO)2), 214.14 (Fe(PyBPT)(CO)2), 214.22 (Fe(CO)3). Anal. Calcd for C22H11Fe2NO5S: C, 51.50; H, 2.16; N, 2.73. Found: C, 51.66; H, 2.18; N, 2.66. IR (ATR): νCO/cm−1 2030, 1965, 1934, 1920. UV−vis: λmax(THF)/nm 521 (ε/dm3 mol−1 cm−1 3300). [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3(PMe2Ph)] (2). To a purple solution of 1 (20 mg, 0.039 mmol) in THF (1 mL) was added PMe2Ph (6.0 μL, 0.042 mmol) in THF (0.3 mL) with stirring. The solution was stirred at room temperature for 5 min. The resulting dark purple-brown solution was layered with n-hexane (25 mL) and stored at −30 °C. After 3 days, dark brown crystals were collected by decantation and washed with n-hexane (25 mg, 96%). 1H NMR (300

Figure 9. Cyclic voltammograms of 3 (0.50 mM) in the presence of (a) acetic acid and (b) monochloroacetic acid in CH3CN containing 0.10 M Bu4NPF6. Concentration of acid: 0, black line; 0.50 mM, red; 1.0 mM, purple; 1.5 mM, brown; 2.5 mM, pink; 5.0 mM, blue; 10 mM, green; scan rate, 0.1 V s−1; working electrode, glassy carbon; auxiliary electrode, platinum wire; reference electrode, Ag/Ag+. Potentials are versus Fc+/Fc.

grams of 3 (0.50 mM) in the presence of acetic acid and chloroacetic acid (0−10 mM). The catalytic current for the proton reduction was observed at −2.3 V for acetic acid. Both the catalytic current and the reduction potential are similar to those for 1 under the same conditions.20 In the case of chloroacetic acid, the voltammogram of 3 shows a catalytic current at −2.1 V and an additional catalytic peak at more negative potentials. The acid concentration dependence of the catalytic reduction process at −2.1 V is similar to that observed for the reduction of acetic acid at −2.3 V. The shift of the reduction potential is consistent with the trend of the standard potential for reduction of the acid.32 The proton reduction catalyzed by 3 occurs after one-electron reduction, while 1 acts as a catalyst after two-electron reduction. However, the similar efficiency of 1 and 3 as electrocatalysts implies that the catalytic mechanism is similar to each other. Probably, the one-electronreduced form takes part in the catalytic cycle, and the protonated form of the two-electron-reduced species, which has a hydride ligand, is a key intermediate as reported for [Fe2(μ-bdt)(CO)6].16,18

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CONCLUSIONS A diiron complex bearing an N,C,S-pincer ligand, [{Fe(μPyBPT-κ3N,C,S)(CO)2}Fe(CO)3] (1), readily reacts with PMe2Ph to give a CO substitution product, [{Fe(μ-PyBPTκ3N,C,S)(CO)2}Fe(CO)2(PMe2Ph)] (3) via a phosphine adduct, [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3(PMe2Ph)] (2). Complexes 1 and 3 have π coordination between the central phenyl ring of PyBPT and the Fe(CO)3 unit, but the π coordination disappears in intermediate 2. A reaction of 3 with an additional 3 equiv of PMe2Ph gave the mononuclear iron(II) complex trans-[Fe(PyBPT-κ3N,C,S)(CO)(PMe2Ph)2] (4) and the iron(0) complex trans-[Fe(CO)3(PMe2Ph)2]. 1,2-Bis(diphenylphosphino)benzene abstracts the Fe(CO)3 unit as [Fe(CO)3(dppbz)] to produce a diiron(II,II) complex with no phosphine ligands, [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}2] (7). Coordination of phosphines to the Fe(CO)3 site and dissociation of the π coordination increase the polarization of 4266

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MHz, C6D6): δ 1.16 (d, 2JHP = 8.8, 3H, PMe2Ph), 1.21 (d, 2JHP = 8.8, 3H, PMe2Ph), 6.33 (ddd, JHH = 7.3, 5.8, 1.4 Hz, 1H, 5-py), 6.86−7.01 (m, 5H), 7.07−7.20 (m, 3H), 7.21 (t, JHH = 7.7 Hz, 1H, 5′-BPT), 7.41 (d, br, JHH = 8.0 Hz, 1H), 7.63 (dd, J = 7.7, 1.0 Hz, 1H), 7.90−7.98 (m, 3H), 9.29 (ddd, JHH = 5.8, 1.5, 0.8 Hz, 1H, 6-py). 31P{1H} NMR (121.5 MHz, C6D6): δ 21.4. Anal. Calcd for C30H22Fe2NO5PS· 0.2THF: C, 55.57; H, 3.57; N, 2.10. Found: C, 55.82; H, 3.76; N, 2.13. IR (ATR): ν CO /cm −1 2027(w), 1968, 1931,1894. UV−vis: λmax(THF)/nm 550 (ε/dm3 mol−1 cm−1 4800). [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)2(PMe2Ph)] (3). To a purple solution of 1 (20 mg, 0.039 mmol) in THF (2 mL) was added PMe2Ph (6.0 μL, 0.042 mmol) in THF (1 mL) with stirring. The solution was stirred at 60 °C for 2 h to give a dark green solution. The solution was concentrated to dryness. The residue was dissolved in a small amount of toluene, layered with n-hexane (25 mL), and stored at −30 °C. After 3 days, dark green crystals were collected by decantation and washed with n-hexane (18 mg, 74%). 1H NMR (300 MHz, C6D6): δ 1.46 (d, 2JHP = 8.4, 3H, PMe2Ph), 1.52 (d, 2JHP = 8.5, 3H, PMe2Ph), 6.02 (ddd, JHH = 7.3, 5.8, 1.4 Hz, 1H, 5-py), 6.44 (ddd, JHH = 7.5, 7.0, 1.7 Hz, 1H, 4-BPT), 6.59 (ddd, JHH = 8.0, 7.3, 1.5 Hz, 1H, 4-py), 6.57−6.68 (m, 2H, 5, 6-BPT), 6.81 (ddd, JHH = 7.5, 1.3, 0.4 Hz, 1H, 3BPT), 6.87 (dd, JHH = 8.4, 6.9 Hz, 1H, 5′-BPT), 6.98 (d, br, JHH = 8.0 Hz, 1H, 3-py), 6.98−7.14 (m, 5H, PMe2Ph), 7.04 (d, JHH = 6.9 Hz, 1H, 4′-BPT), 7.41 (d, br, JHH = 8.4 Hz, 1H, 6′-BPT), 9.02 (ddd, JHH = 5.8, 1.5, 0.8 Hz, 1H, 6-py). 13C{1H} NMR (100 MHz, C6D6): δ 18.4 (d, 1JCP = 27 Hz, PMe2Ph), 18.8 (d, 1JCP = 28 Hz, PMe2Ph), 117.3, (d, JCP = 2 Hz), 117.9, 118.6, 121.5, 122.8, 126.2, 127.0, 128.3, 128.4, 128.7 (d, JCP = 2 Hz), 128.96, 129.05, 129.3, 129.5, 134.6, 135.4 (d, JCP = 2 Hz), 139.0 (d, JCP = 1 Hz), 140.6 (d, 1JCP = 39 Hz, PMe2Ph), 146.2 (d, JCP = 2.5 Hz), 155.3, 157.3, 164.0, 176.3 (d, 2JCP = 3.5 Hz, Fe-C(2′BPT)), 215.2 (CO), 216.1 (d, 2JCP = 2 Hz, CO), 216.3 (d, 2JCP = 21 Hz, CO), 219.2. 31P{1H} NMR (121.5 MHz, C6D6): δ 21.1. Anal. Calcd for C29H22Fe2NO4PS: C, 55.89 H, 3.56; N, 2.25. Found: C, 56.09; H, 3.70; N, 2.23. IR (ATR): νCO/cm−1 = 1979, 1941, 1912. UV−vis: λmax(THF)/nm 607 (ε/dm3 mol−1 cm−1 3200). trans-[Fe(PyBPT-κ3N,C,S)(CO)(PMe2Ph)2] (4). To a purple solution of 1 (20 mg, 0.039 mmol) in THF (2 mL) was added PMe2Ph (23 μL, 0.16 mmol) in THF (1 mL) with stirring. The solution was stirred at 60 °C for 8 h, during which time the color of the solution changed from dark brown to dark green and then dark brown. The solution was concentrated to dryness. The residue was washed with n-hexane to give a brown solid. The solid was dissolved in toluene (1.5 mL), layered with n-hexane (30 mL), and stored at −30 °C. After 1 day, redbrown crystals were collected by decantation and washed with nhexane (19 mg, 78%). 1H NMR (300 MHz, C6D6): δ 0.80 (dd, 2JHP = 4 JHP = 4.0, 6H, PMe2Ph), 1.63 (dd, 2JHP = 4JHP = 3.8, 6H, PMe2Ph), 5.48 (ddd, JHH = 7.3, 5.8, 1.4 Hz, 1H, 5-py), 6.35 (tm, JHH = 7.7 Hz, 1H, 4-py), 6.62−6.71 (m, 9H, 3-py, PMe2Ph), 6.71−6.80 (m, 2H, PMe2Ph), 6.95−7.09 (m, 2H, 4,5-BPT), 7.28−7.41 (m, 2H, 4′,5′BPT), 7.69 (dm, JHH = 5.8 Hz, 1H, 6-py), 7.81 (dd, J = 7.8, 1.6 Hz, 1H, 6-BPT), 8.08 (ddd, J = 7.5, 1.3, 0.5 Hz, 1H, 6′-BPT) 8.15 (dd, JHH = 7.7, 1.6 Hz, 1H, 3-BPT). 13C{1H} NMR (75.5 MHz, C6D6): δ 11.7 (t, 1JCP = 3JCP = 15 Hz, PMe2Ph),13.8 (t, 1JCP = 3JCP = 15 Hz, PMe2Ph), 117.0 (3-py), 119.9 (5-py), 120.8 (t, JCP = 2 Hz, 4′-BPT), 122.1 (5-BPT), 123.7 (t, JCP = 2 Hz, 5′-BPT), 125.6 (5-BPT), 127.5 (t, JCP = 4 Hz, PMe2Ph), 127.6 (PMe2Ph), 129.2 (t, JCP = 4 Hz, PMe2Ph), 129.8 (6-BPT), 132.4 (4-py), 134.6 (3-BPT), 138.0 (t, 1JCP = 14 Hz, PMe2Ph), 142.2 (1-BPT), 143.6 (2-BPT), 145.9 (t, JCP = 4 Hz, 3′BPT), 147.7 (t, JCP = 2 Hz, 1′-BPT), 153.2 (6-py), 166.7 (2-py), 192.5 (t, 2JCP = 28 Hz, Fe-C(2′-BPT)), 215.0 (t, 2JCP = 20 Hz, CO). Anal. Calcd for C34H33FeNOP2S: C, 65.71; H, 5.35; N, 2.25. Found: C, 65.58; H, 5.42; N, 2.24. 31P{1H} NMR (121.5 MHz, C6D6): δ 27.7. IR (ATR): νCO/cm−1 1890. UV−vis: λmax(THF)/nm 389 (ε/dm3 mol−1 cm−1 6300). Reaction of 1 with PPh3. An NMR tube with a Teflon valve was charged with 1 (4.8 mg, 0.0094 mmol), PPh3 (7.4 mg, 0.028 mmol), and C6D6 (0.5 mL). When the resulting dark brown solution was heated at 60 °C, the color changed to dark green. The reaction was monitored by 1H and 31P{1H} NMR spectroscopy. Data for 5: 31 1 P{ H} NMR (121.5 MHz, C6D6) δ 56.8. Data for 6: 31P{1H} NMR

(121.5 MHz, C6D6) δ 57.4. The NMR spectra are shown in Figures S6 and S7 (Supporting Information). Ci-[{Fe(μ-PyBPT-κ3N,C,S)(CO)2}2] (7). A sample tube with a Teflon valve was charged with 1 (20 mg, 0.039 mmol), 1,2-bis(diphenylphosphino)benzene (18 mg, 0.040 mmol), and toluene (3 mL). The purple solution was heated at 80 °C for 24 h. The color of the solution changed to red-brown, and orange crystals were deposited. The crystals were collected by decantation, washed with n-hexane, and dried under reduced pressure (5.9 mg, 40%). 1H NMR (300 MHz, C6D6): δ 5.85 (ddd, JHH = 7.4, 5.6, 1.3 Hz, 1H, 5-py), 5.89 (dd, JHH = 7.7, 1.4 Hz, 1H, 3-BPT), 6.50 (td, JHH = 7.4, 1.3 Hz, 1H, 4BPT), 6.75 (ddd, JHH = 8.1, 7.4, 1.5 Hz, 1H, 4-py), 7.08 (ddd, JHH = 8.0, 7.1, 1.4 Hz, 1H, 5-BPT), 7.33 (d, br, JHH = 8.1 Hz, 1H, 3-py), 7.62 (t, JHH = 7.7 Hz, 1H, 5′-BPT), 7.67 (d, br, JHH = 5.6 Hz, 1H, 6-py), 7.89 (d, br, JHH = 7.7 Hz, 2H, 4′- or 5′-BPT, 6-BPT), 8.29 (d, br, JHH = 7.8 Hz, 1H, 4′- or 5′-BPT). Anal. Calcd for C38H22Fe2N2O2S2: C, 61.15; H, 2.97; N, 3.75. Found: C, 60.53; H, 3.07; N, 3.77. Single crystals suitable for X-ray diffraction studies were obtained from a dichloromethane solution. X-ray Crystallography. Diffraction data were collected on a Rigaku AFC11/Saturn 724+ CCD diffractometer for 2 and a Rigaku AFC7/Mercury CCD diffractometer for 3, 4, and 7, with monochromated Mo Kα radiation (λ = 0.7107 Å). The data were processed and corrected for Lorentz and polarization effects using the CrystalClear software package.33 The analyses were carried out using the WinGX software.34 Absorption corrections were applied using the Multi Scan method. The structures were solved using direct methods (SIR9735) and refined by full-matrix least-squares on F2 using SHELXL97.36 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were found in difference Fourier maps and refined isotropically. Crystallographic data are summarized in Tables S1 and S2 (Supporting Information). Computational Details. Structures of complexes 1−4 were optimized by DFT calculations using the Gaussian 03 program package.37 The B3LYP density functional method and the 6311+G(d,p) basis set were used for the calculations. The initial models were obtained from the crystal structures. Optimized structures and selected molecular orbitals of 1−4 are shown in Figures S18−S25 (Supporting Information), and their molecular coordinates are listed in Tables S3−S6 (Supporting Information). Harmonic vibrational frequencies were calculated for the optimized geometries using the same level of theory. Time-dependent DFT calculations were performed using the optimized structures at the B3LYP/6-311+G(d,p) level. Calculated electronic transitions for 1−4 are given in Tables S7−S10 (Supporting Information). Electrochemistry. Cyclic voltammetric 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, a Ag/Ag+ (0.01 M AgNO3, 1 M = 1 mol dm−3) reference electrode, and a platinum wire, respectively. Acetonitrile solutions of 3 and 4 were prepared in concentrations of 5.0 × 10−4 and 4.0 × 10−4 M, respectively, and 0.10 M tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) was used as the supporting electrolyte. 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.

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving absorption spectra, NMR data for 1−7, crystallographic data for 2−4 and 7, computational details for 1−4, and cyclic voltammograms for 3 and 4. This material is available free of charge via the Internet at http://pubs.acs.org. 4267

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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 Japan Society for the Promotion of Science.

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REFERENCES

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dx.doi.org/10.1021/om500558h | Organometallics 2014, 33, 4260−4268