Quasi-One-Step Six-Electron Electrochemical Reduction of an

Article pubs.acs.org/IC

Quasi-One-Step Six-Electron Electrochemical Reduction of an Octahedral Hexanuclear Molybdenum(II) Cluster Sho Fujii,†,‡ Taishiro Horiguchi,‡ Soichiro Akagi,‡ and Noboru Kitamura*,†,‡ †

Department of Chemistry, Faculty of Science and ‡Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita-10, Nishi-8, Kita-ku, Sapporo, 060-0810, Japan S Supporting Information *

ABSTRACT: We report for the first time quasi-one-step six-electron electrochemical reduction of a new hexanuclear molybdenum(II) bromide cluster having terminal 3,5dinitrobenzoate ligands: [Mo6Br8(DNBA)6]2−. The electrochemical responses of the cluster were studied based on cyclic (CV), differential pulse, and normal pulse voltammetries, together with the analytical simulations of the CV and spectroelectrochemistry. CV simulations have revealed that the electrochemical reaction of the cluster proceeds in an EEEEEE scheme, and the potential differences between the two adjacent reduction steps are in the range of 15−30 mV. These potential differences indicate quite smooth and quasi-one-step six-electron reduction of the cluster.

■

INTRODUCTION Hexanuclear molybdenum(II) halide clusters ([Mo6X8L6]2−, X = halogen; L = X or organic ligands) have been widely studied due to their intense and long-lived luminescence properties and the roles in the building blocks for nanoarchtectures.1 In these clusters, a bridging ligand as X is substitution inert owing to the covalent-like bond between Mo and X, forming the {Mo6X8}4+ cluster core. In contrast, a terminal ligand (L) is labile and, thus, the cluster properties can be tuned synthetically by substituting L with an appropriate ligand, which affords a functional hexasubstituted molybdenum cluster.2−13 Electrochemical studies on [Mo6X8L6]2−, in particular, characteristics of electrochemical reduction, have rarely been reported in comparison to the works on the spectroscopic/ photophysical properties.7,9−12,14−16 It is worth pointing out that [Mo6X8L6]2− possesses a 24 metal electron (ME) system, where ME occupies the 12 frontier orbitals forming the closedshell structure.17,18 Therefore, an irreversible or quasi-reversible reduction wave is generally observed in the cyclic voltammograms of the cluster.3,19,20 In the case of Chevrel compounds in molybdenum(II) clusters possessing less than 24 ME, multistep reduction processes have been reported. For example, Magliocchi et al. have reported that [Mo6Se8(CN)6] (ME = 21) shows three reversible-reduction waves (one electron in each wave).21 However, it is difficult to demonstrate the multielectron reduction of [Mo6X8L6]2− having a 24 ME system. Furthermore, simultaneous one-step multielectron reduction of a 24 ME halide cluster is not a trivial task, although such properties are of primary importance for developments of electrochemical catalysts, capacitors, and electrode materials in batteries.22−24 Ligand-based reduction is one of the keys to demonstrate multielectron electrochemical reduction of [Mo6X8L6]2−. Previously, it has been reported that the hexarhenium(III) chalcogenide clusters (ME: 24) with terminal N-heterocyclic © 2016 American Chemical Society

ligands exhibit stepwise two reversible one-electron reduction waves, while that with 4,4′-bipyridine shows a one-step twoelectron reduction wave.25,26 Furthermore, the {Mo6I8}4+-core clusters with nitrophenolates as terminal ligands show twoelectron reduction through ligand-based reduction.12 Sasaki and Abe have reported multistep six-electron reduction of the hexarhenium(III) clusters with several pyridyl terminal ligands and demonstrated that the cluster with 5-(4-pyridyl)-10,15,20tritolylporphyrin as redox active terminal ligands show an almost one-step six-electron reduction.27,28 Nonetheless, simultaneous one-step multielectron reduction of a hexametal cluster beyond two electrons by simple ligands (L) has never been reported, which motivates us to design the cluster showing one-step multielectron reduction. In this paper, we report the electrochemical characteristics of new molybdenum bromide clusters containing terminal benzoate (BA) ligands ([Mo6Br8L6]2−, Chart 1) and demonstrate that the cluster with L = 3,5-dinitrobenzoate (DNBA) shows a quasi-one-step sixelectron reduction wave. Chart 1. Structures of a Hexamolybdenum(II) Cluster and Terminal Benzoate Ligands

Received: June 27, 2016 Published: September 29, 2016 10259

DOI: 10.1021/acs.inorgchem.6b01525 Inorg. Chem. 2016, 55, 10259−10266

Article

Inorganic Chemistry Chart 2. Quasi-Reversible EEEEEE Scheme

a

For the abbreviations, see the main text.

■

with the optical path length of the cell being 1 mm (BAS, SEC-C). The cell was set directly in a spectrophotometer (Hitachi, U-3900H), and the absorption change was monitored during the electrolysis under Ar-gas atmosphere. X-ray Diffraction Data Collection and Structure Determination. A crystal of 2 or 3 was mounted in a loop with a paratone oil. Diffraction data at 230 K for 2 and 150 K for 3 were collected on a Rigaku AFC-7R diffractometer using a Rigaku Mercury CCD area detector with graphite-monochromated Mo−Kα radiation (λ = 0.71075 Å). Cell parameters were retrieved using CrystalClear software32 and refined using CrystalClear on all observed reflections. The data were corrected for Lorentz and polarization effects. Diffraction data were collected and processed using CrystalClear. The structures were solved by direct method using SIR-2014.33 Structure refinements were conducted by the full-matrix least-squares techniques with SHELXL-2013.34 All non-hydrogen atoms were refined anisotropically. Semianalytical Simulation of CV. Simulations of a CV curve were performed on the basis of the Oldham and Myland method.31 For an EEEEEE process (Chart 2), we modified the method for an EE process by two successive quasi-reversible electron transfer steps reported in the literature.31 The surface flux densities of a solute before an electrochemical reaction (M0) and after a x-step reduced or oxidized solute (M±x) at an electrode surface, denoted as js0(t) and jsx(t) (x = 1, 2, ..., 6), respectively, are given by eq 1,

EXPERIMENTAL SECTION

Materials. (TBA)2[Mo6Br14] (1) (TBA: tetra-n-butylammonium cation) was prepared according to the literature method.29 Benzoic acid (BA), 3,5-dinitrobenzoic acid (DNBA), and other chemicals were purchased from Wako Chemical Co. and used as received. Preparation of (TBA)2[Mo6Br8(BA)6] (2). A mixture of BA (1.0 g, 6.2 mmol) and silver oxide (0.46 mg, 2.0 mmol) in ethanol (50 mL) was stirred for 4 days at room temperature. The precipitates collected by filtration were washed with ethanol and dried in vacuo, affording silver benzoate (1.0 g). A mixture of 1 (200 mg, 92 μmol) and silver benzoate (200 mg, 870 μmol) in acetone (10 mL) was stirred for 1 week at room temperature in the dark. After the reaction, the precipitates were removed by filtration, and the solvent was removed under reduced pressure. Recrystallization of the crude product from an acetone−diethyl ether mixture gave orange crystals of the complex (167 mg, 75%). 1H NMR (270 MHz, acetone-d6): δ 8.03 (12H, d, J = 5.4 Hz), 7.38−7.32 (18H, m), 3.43 (16H, m), 1.82 (16H, m), 1.41 (16H, m), 0.96 (24H, t, J = 6.8 Hz) ppm. Anal. Calcd for C74H102N2O12Br8Mo6: C 36.6 ; H 4.24 ; N 1.15 ; Found: C 36.4 ; H 4.16 ; N 1.09. ESI-MS: 971 m/z [M]2−. Preparation of (TBA)2[Mo6Br8(DNBA)6] (3). Silver 3,5-dinitrobenzoate was prepared by the procedures analogous to those for 2. A mixture of 1 (200 mg, 92 μmol) and silver 3,5-dinitrobenzoate (179 mg, 560 μmol) in acetone (10 mL) was stirred for 1 week at room temperature in the dark. The precipitates were removed by filtration, and the solvent was removed under reduced pressure. Recrystallization of the crude product from an acetone−diethyl ether mixture gave yellow crystals of the complex (244 mg, 89%). 1H NMR (270 MHz, DMSO-d6): δ 8.97 (6H, s), 8.95 (12H, s), 3.21 (16H, m), 1.62 (16H, m), 1.36 (16H, m), 0.99 (24H, t, J = 6.8 Hz) ppm. Anal. Calcd for C74H90N14O36Br8Mo6: C 29.96 ; H 3.06 ; N 6.61 ; Br 21.55 ; Found: C 29.88 ; H 2.99 ; N 6.34 ; Br 21.03. ESI-MS: 1240 m/z [M]2−. Physical Measurements. 1H NMR spectra were recorded on a JEOL JNM-EX 270 spectrometer. Mass spectra were taken on Waters micromass LCT. Cyclic, differential pulse, and normal pulse voltammetries were conducted by using an ALS/701D electrochemical analyzer (BAS) at room temperature. The acetone solutions of 1, 2 and 3 (1.0 × 10−3 M (= mol/dm3)) containing 0.1 M tetra-nbutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte were deaerated by purging an Ar-gas stream prior to the experiments. The working, auxiliary, and reference electrodes were glassy carbon (BAS, area: 7.1 mm2), platinum wire, and Ag/AgNO3 electrodes, respectively. The electrode potential was calibrated with a ferrocene/ferrocenium (Fc/Fc+) redox couple in acetone and converted to the values vs Ag/AgCl (EAg/AgCl = EFc/Fc+ + 0.52 V).30 The initial electrode potential for a potential scan was set to the rest potential in each measurement. The half-wave potential (E1/2) was evaluated by the middle potential between the anodic and cathodic peak potentials (Epa and Epc, respectively): E1/2 = (Epa + Epc)/2. Simulations of cyclic voltammograms (CV) were performed on an Excel sheet based on the semianalytical method reported earlier.31 Spectroelectrochemistry experiments on 3 were performed by using a platinum-minigrid (80-mesh) working electrode in a thin-layer cell

j0s (t ) =

∓ I1(t ) FA

(1a)

jxs (t ) =

± Ix(t ) ∓ Ix + 1(t ) (x = 1, 2, ···, 5) FA

(1b)

j6s (t ) =

± I6(t ) FA

(1c)

where Ix(t) is the current at a time (t) from the start of a measurement, F is the Faraday constant, and A is the surface area of an electrode. The upper and lower signs correspond to the positive and negative potential scans in CV, respectively. The surface concentrations of the solutes (M0 and M±x) at an electrode, cs0(t) and csx(t), are given by eqs 2a and 2b, respectively, c0s(t ) = c0b +

cxs(t ) =

1 d −1/2 s j D0 dt −1/2 0

(2a)

1 d −1/2 s j (x = 1, 2, ··· , 6) Dx dt −1/2 x

(2b)

where cb0 is the bulk solute concentration and, D0 and Dx are the diffusion constants of M0 and M±x, respectively. The Butler−Volmer equation is given by eq 3, 10260


Article

Inorganic Chemistry ⎡ ± Ix(t ) cs ⎤ = [ξx(t )]αx ⎢cxs − 1(t ) − x ⎥ (x = 1, 2, ⋯ , 6) ξx(t ) ⎦ FAk°x ⎣ ⎛ ±F ⎜ξx(t ) = exp [Erev ∓ |vt ∓ Erev ± E(0)| − E°x ] ⎝ RT ⎞ (x = 1, 2, ⋯ , 6)⎟ ⎠

{

DNBA in acetone at room temperature in the dark required 1 week. In the early stage of the reaction, formation of partially substituted complexes was confirmed by the ESI-MS spectra of the reaction mixture, while the spectrum after the 1 week reaction showed the single signal attributed to 2 or 3, as the structures and purities of these clusters were confirmed by 1H NMR and elemental analysis. The solubility of 3 in acetone or acetonitrile was higher than the relevant value of 2. The X-ray diffraction data of 2 and 3 were collected at 230 and 150 K, respectively. Since the crystal of 2 was cracked at a low temperature, the measurements were performed at 230 K. The crystal structures of 2 and 3 are shown in Figure 1. The crystallographic data and the selected bond lengths of the complexes are summarized in Tables 1 and 2, respectively.

} (3)

kox

is the formal heterogeneous electron transfer rate constant where and v is a potential scan rate. E(0) and Erev are the starting and reversal potentials, respectively, and R is the gas constant. On the basis of a semi-integral algorithm in eq 4, d −1/2 I(t ) = dt −1/2 ⎛ ⎜I(< t ) = ⎜ ⎝

δ [I(t ) + I(< t )]

N−1

⎛ N − ns ⎞ 1 t ⎟ , w0 = 1, w1 = , ⋯ , wns ⎠ N 2 ns = 1 ⎞ 2n − 1 = s wns − 1⎟⎟ 2ns ⎠

∑ wnsI ⎜⎝

(4)

where δ is a time interval, wns is the weight factor for a weighted sum, ns is a summation index and N (= t/δ) is an interval number, the Butler−Volmer equation in eq 3 can be rewritten as in eq 5: ± I1(t ) = c0b ∓ FAk°1[ξ1(t )]α1

δ /D0 F

[I1 + I1(< t )] ∓

δ /D1 Fξ1(t )

[I1 + I1(< t ) − I2 − I2(< t )]

(5a)

δ /Dx − 1 ± Ix(t ) [Ix − 1 + Ix − 1(< t ) − Ix − Ix(< t )] αx = ± FAk°x [ξx(t )] FA δ /Dx ∓ [Ix + Ix(< t ) − Ix + 1 − Ix + 1(< t )] FAξx(t ) (x = 2, 3, ⋯ , 5) (5b) δ /D5 ± I6(t ) =± [I5 + I5(< t ) − I6 − I6(< t )] FAk°6 [ξ6(t )]α6 FA ∓

δ /D6 FAξ6(t )

[I6 + I6(< t )]

(5c)

Solving the simultaneous equations in eq 5, one can simulate Ix(t). The sum of Ix(t) represents a CV curve as given by eq 6: I(t ) = I1(t ) + I2(t ) + I3(t ) + I4(t ) + I5(t ) + I6(t )

(6)

Estimation of the Diffusion Coefficient of 3. The diffusion coefficient of a solute, D, is described by an empirical formula35 as in eq 7, D = 7.4 × 10−8

(x′M )1/2 T ηV 0.6

Figure 1. X-ray crystal structures of (a) (TBA)2[Mo6Br8(BA)6] (2) and (b) (TBA)2[Mo6Br8(DNBA)6] (3) at the 50% probability levels. Hydrogen atoms and TBA involved in the crystals are omitted for clarity. For 2, acetone as a recrystallization solvent involved is also omitted. Color legends: C (gray), O (red), N (blue), Br (yellow), Mo (purple).

(7)

where x′ is an association parameter of a solute (3 in acetone; 1.0), M is the molecular weight of a solvent, T is temperature (K), η is the viscosity of a solution, and V is the molar volume of a solute. For the calculation of D, V was estimated based on the formula weight and density of 3 evaluated by the X-ray diffraction data.

■

The nature of the terminal carboxylate (BA or DNBA) does not influence the {Mo6Br8}4+-core structures. For example, the average Mo−Mo bond lengths in 2 and 3 are 2.625 and 2.630 Å, respectively, while the average Mo−Br bond lengths are 2.603 Å for 2 and 2.600 Å for 3. On the other hand, the Mo−O bond length observed for 2 (2.092 Å) was shorter than that for 3 (2.115 Å). This difference is caused by that of the σ-donating

RESULTS AND DISCUSSION The new complexes 2 and 3 were prepared according to the literature9,29 and obtained in good yields as described in the Experimental Section. The complete substitution reaction of the six Br terminal ligands in (TBA)2[Mo6Br14] by BA or 10261


Article

Inorganic Chemistry Table 1. Selected Crystallographic Data for (TBA)2[Mo6Br8(BA)6] (2)·2 acetone, and (TBA)2[Mo6Br8(DNBA)6] (3) chemical formula formula weight cryst syst space group a/Å b/Å c/Å β/° V/Å3 Z ρ calcd/g cm−3 R1 wR2 GOF

2·2 acetone

3

C80H114Br8Mo6N2O14 2542.66 monoclinic P21/n (No. 14) 13.450(5) 23.923(8) 14.435(5) 98.121(4) 4598(3) 2 1.836 0.0555 0.1674 1.077

C74H90N14O36Br8Mo6 2966.47 monoclinic P21/n (No. 14) 14.775(4) 22.208(5) 14.907(3) 97.717(3) 4847(2) 2 2.032 0.0507 0.1122 1.101

λ = 0.71075 Å, R1 = Σ∥Fo| − |Fc∥/Σ|Fo|, wR2 = [Σw(Fo2 − Fc2)2/ Σw(Fo2)2]1/2.

a

Table 2. Selected Bond Lengths (Å) for (TBA)2[Mo6Br8(BA)6] (2) and (TBA)2[Mo6Br8(DNBA)6] (3) Mo−Mo Mo−Br Mo−O

2

3

2.619−2.632 ave. 2.625 2.589−2.625 ave. 2.603 2.083−2.099 ave. 2.092

2.620−2.635 ave. 2.630 2.583−2.613 ave. 2.600 2.094−2.132 ave. 2.115

Figure 2. Cyclic voltammograms of 1 (a), 2 (b), and 3 (c) (1.0 × 10−3 M) in acetone containing 0.1 M TBAPF6. Scan rate = 100 mV/s. A scale bar = 25 μA for (a−c). (d) Differential pulse voltammogram (DPV) of 3. DPV conditions: pulse amplitude = −50 mV; pulse width =50 ms; the time measured from the pulse rise = 25 ms; scan rate = 5 mV/s.

Table 3. Electrochemical Parameters of 1, 2, and 3 E1/2, oxa/V 1 2 3

property of the terminal ligand. DNBA with two electronwithdrawing nitro-groups is a weak donor (pKa = 2.8) to an Mo ion compared to BA (pKa = 4.2). It has been reported that the Mo−O bond length in [Mo6Br8L6]2− (L = 3,4,5-trimethoxybenzoate (TMBA)) is 2.095 Å.4 Since the pKa value of TMBA (pKa ≈ 4.2) is almost the same with that of BA (4.2), the similar Mo−O bond length between those in 2 and the TMBA cluster will be the reasonable consequence. It has been also reported that the Mo−O bond length in the {Mo6Br8}4+-core cluster with trifluoroacetate (pKa ≈ 0.2) or heptafluorobutyrate (pKa ≈ 0.2)36 as the terminal ligands is 2.112 or 2.111 Å, respectively. 9,14 These results support that a stronger carboxylate donor ligand gives rise to the shorter Mo−O bond length in [Mo6Br8(carboxylate)6]2−. Cyclic voltammograms of 1, 2, and 3 in acetone are shown in Figure 2, and the relevant electrochemical parameters are summarized in Table 3. The cluster, 1, 2, or 3 showed the reversible one-electron oxidation wave at E1/2, ox = 1.47, 1.27, or 1.52 V (vs Ag/AgCl), respectively. The one-electron reversible oxidation wave observed for 1 agrees very well with that in acetonitrile.19,37 The ratio of the cathodic peak current (ipc) to the anodic peak current (ipa) was ca. 1.0 irrespective of L. The peak separation of the oxidation wave (ΔEp) was 71 or 73 mV for 2 or 3, respectively, which was slightly larger than the ideal ΔEp value for a one-electron reversible wave: 59 mV.38 The one-electron oxidation waves observed for 1−3 are assigned to that of the {Mo6Br8}4+-core due to the similarities of the E1/2, ox value to that of [Mo6Br8Br6]2−.37 Density functional theory calculations also demonstrated that the

1.47 1.27 1.52

ΔEpb/mV 73 71 73

ipc/ipa

E1/2, reda/V

ΔEpb/mV

ipa/ipc

0.99 0.95 0.96

−1.48 (irr.) −1.65c (irr.) −0.84 (6e)

116

0.99

c

a

Electrochemical measurements were carried out in acetone. An Ag/ AgCl electrode in a satd. aq. NaCl solution was used as a reference electrode. bΔEp = Epa − Epc. cPotential peak value.

electrochemical oxidation of [Mo6Br8L6]2− (L = F, Cl, or Br) was responsible for that of the {Mo6Br8}4+-core.39 The E1/2, ox value of 3 (1.52 V) is shifted to the positive potential direction by 0.25 V relative to that of 2 (1.27 V), which is attributed to the presence of the two electron-withdrawing NO2 groups in 3. Since the pKa values of BA and DNBA are 4.2 and 2.8, respectively, as mentioned before, the weaker σ-donating ability of DNBA results in the stabilization of the highest-energy molecular orbital (HOMO) energy level of 3 relative to that of 2, giving rise to the positive shift of E1/2, ox of 3 compared to that of 2. Similar behaviors with those of 2 and 3 have been reported for E1/2, ox of the hexarhenium(III) clusters with terminal N-heterocyclic ligands ([Re6S8Cl4L′2]2−), where the linear correlation between pKa of L′ and the HOMO energy level has been reported: the decrease in the pKa value of L′ stabilizes the HOMO energy level of the cluster.26 The changes in the Mo−O bond lengths in 3 relative to those in 2 mentioned before will also influence the HOMO energy level. On the other hand, an irreversible reduction peak was observed for 1 or 2 at E1/2, red = −1.48 or −1.65 V, respectively. In contrast, a reversible reduction wave was observed for 3 at E1/2, red = −0.84 V with the peak current (87 μA) being 4.4 times larger than the relevant oxidation current at E1/2, ox = 1.52 V (20 μA) as seen clearly in Figure 2c. Differential pulse 10262


Article

Inorganic Chemistry voltammetry of 3 showed a single-peak without any shoulder peak (Figure 2d). However, the ΔEp value (116 mV) of the redox couple of 3 at −0.84 V was much larger than the ideal ΔEp value for an n-electron transfer reaction (59/n mV). These results indicate that the electrochemical reduction of 3 accompanies multiple electron transfer steps at around E1/2, red = −0.84 V. To elucidate the number of an electron transferred in the reduction of 3, we evaluated the limiting currents in the oxidation (Ilim, ox, 1e−) and reduction waves (Ilim, red, ne−) based on normal pulse voltammetry as the results were shown in Figure 3. Ilim is given by Ilim = nFDA1/2C/(πtd)1/2, where n is

Figure 3. Normal pulse voltammograms (NPV) of 3 observed by sweeping the potential toward the positive (a) and negative directions (b). Dotted lines indicate the residual currents. NPV conditions: pulse amplitude = 50 mV; pulse width = 50 ms; the time measured from the pulse rise = 25 ms; scan rate = 5 mV/s.

Figure 4. Experimental (red dotted line) and simulated (black solid line) reduction (a) and oxidation (b) waves of 3 (1.0 × 10−3 M) in acetone containing 0.1 M TBAPF6. Parameters used are as follows: electrode area = 7.1 mm2, diffusion coefficient, D = 7.7 × 10−10 m2/s, capacitance = 10 μF, scan rate = 0.1 V/s, formal heterogeneous rate constants: ko1, ko2, ko3, ko4, ko5, and ko6 = 2.0 × 10−4 m/s, Eo1 = −0.790 V, Eo2 = −0.805 V, Eo3 = −0.835 V, Eo4 = −0.850 V, Eo5 = −0.875 V, Eo6 = −0.905 V, transfer coefficient: α = 0.5. The CV waves (I1 − I6) reported by the different colors correspond to deconvolution of the simulated CV curve by the EEEEEE process. For the oxidation, the following values were employed: diffusion coefficient, D = 7.7 × 10−10 m2/s, capacitance 33 μF, scan rate 0.1 V/s, formal heterogeneous rate constants, koox = 2.0 × 10−4 m/s, Eoox = 1.520 V, and transfer coefficient, α = 0.5.

electron stoichiometry, A is an electrode area, and td is the time measured from the pulse rise. C and D are the concentration and diffusion coefficient of 3 in acetone, respectively. The limiting current ratio (Ilim, red/Ilim, ox) thus determined was 5.9, demonstrating that the reduction of 3 at around −0.84 V accompanied six electron transfer: n = 6. The results indicate that a highly negative charged cluster, [Mo6Br8(DNBA)6]8−, is produced by the electrochemical reaction. To obtain further information on the electrochemical mechanism, we conducted semianalytical simulations31 of the reduction wave of 3. The model used was an EEEEEE process (see Chart 2 and Experimental Section). The simulated data on the experimentally observed CV curve is shown in Figure 4a, where the simulation has been conducted by assuming D of 3 to be 7.7 × 10−10 m2/s. The D value was in good agreement with that estimated by an empirical formula (eq 7) based on the cluster size (6.6 × 10−10 m2/s).35 The formal heterogeneous electron transfer rate constants, kox (x = 1−6), were evaluated to be the same at 2.0 × 10−4 m/s, and these values demonstrated the redox reaction of 3 was quasi-reversible: the rate of electron transfer becomes comparable to the mass transport rate.38,40 The transfer coefficient, α, was 0.5. The reduction potentials of 3 were then simulated to be Eo1 = −0.790, Eo2 = −0.805, Eo3 = −0.835, Eo4 = −0.850, Eo5 = −0.875, and Eo6 = −0.905 V (vs Ag/AgCl). The deconvolved waves are also shown in Figure 4a. The potential differences between any of the two adjacent formal potentials are quite small (15−30 mV). It has been reported that successive electron transfer of a molecule containing identical noninteracting redox sites follows simple statistics governed by an entropic factor.41,42 Each formal potential difference is given by eq 8, Exored − E1/2 = −(RT /F ) ln[xred /(nsite − xred + 1)]

values calculated based on the experimentally observed values (Eox) and eq 8 were plotted against xred in Figure S1 in the Supporting Information. The experimentally observed values (Eox) were slightly larger than the relevant theoretical values evaluated by eq 8. The results indicate that, in addition to the entropic factor, weak electrostatic interactions between the DNBA redox sites in 3 also contribute to the Eox values. The difference in the formal potentials between the first and the most negative reduction peaks in a molecule having reducing sites (nsite) is given by eq 9. E1o − Enosite = (2RT /F ) ln nsite

(9)

In the present system (nsite = 6 for 3), the (E 1 − E 6) value is 115 mV, while eq 9 demonstrates the value to be 92 mV. The difference (23 mV) would be caused by the solvation entropy41 and/or the electrostatic repulsion between the DNBA anion sites in 3, and, thus, the average potential difference in in the six electron transfer steps is 4.6 mV. An E process was applied to simulate the oxidation wave of 3 (Figure 4b) and, the Eoox value was estimated to be 1.520 V and other parameters for the simulation were the same with those for the reduction. The simulation curve is in good agreement with the observed curve. The oxidation reaction of 3 is therefore quasi-reversible. The results demonstrate that the electrochemical responses of 2 and 3 are totally different with one another. We suppose o

(8)

where nsite and xred are the numbers of a reducing site in a molecule and the site reduced, respectively. The (Eoxred − E1/2) 10263

o


Article

Inorganic Chemistry

electrostatic repulsion between the ligands upon electrochemical reduction. The electrostatic repulsion energy between two electric charges, Ee, correlates inversely to the distance between the two charges, d: Ee ∝ 1/d. As a result, the Ee value predicted for 3 would be almost half of that of [Fe(bpy)3]2+, leading to quasi one-step six-electron reduction of 3, while [Fe(bpy)3]2+ shows three successive reduction waves. The Ee value between the DNBA anion sites (9.0 Å) in acetone with the dielectric constant of 20.7 was roughly calculated to be ca. 80 mV, which was larger than the predicted value mentioned above: 23 mV.48 We suppose the electrostatic repulsion between the DNAB anion sites in 3, not via the cluster core, would be reduced more or less by ion pairing between the cluster and the supporting electrolyte, leading to the small potential difference (23 mV). Upon electrochemical reduction, second, the electron(s) would localize on the electronwithdrawing NO2 group(s) in 3, apart from the carboxylate group as the coordination site.47 Furthermore, the LUMO of 3 (−0.84 V) would localize primarily on the DNBA group(s) as described before. In the case of electrochemical reduction of 3, therefore, six electrons would be injected almost independently and instantaneously to the DNBA groups, leading to quasi-onestep six-electron reduction of 3. The six-electron reduction of 3 without a ligand−ligand interaction is, therefore, quite unique and extraordinary among various transition metal complexes hitherto reported.

the origin of the results would be the difference in the energy level of the lowest-energy unoccupied molecular orbital (LUMO) between BA and DNBA in [Mo6Br8L6]2−. It has been reported that LUMO of 1 lies in the {Mo6Br8}4+-cluster core,39,43 and electron injection into LUMO would decompose 1, resulting in the irreversible reduction wave. On the other hand, the irreversible reduction wave was observed for 2 (E1/2, red = −1.65 V) at a more negative potential than that of 1 (−1.48 V), indicating that LUMO of 2 would also lie more or less in the {Mo6Br8}4+-cluster core. In contrast, 3 exhibits the reversible and six-electron reduction wave (E1/2, red = −0.84 V) more positive than that of 1 or 2. This demonstrates that LUMO of 3 would not lie in the cluster core, but in the DNBA terminal ligands. In practice, spectroelectrochemistry experiments shown in Figure 5 indicate that the spectrum of 3 at E =

■

CONCLUSION We demonstrated quasi-one-step electrochemical six-electron reduction of the hexanuclear molybdenum cluster containing terminal 3,5-dinitrobenzoate ligands, 3. CV simulations of 3 indicated that the potential difference between the adjacent reduction peaks was in the range of 15−30 mV. The Eo1 − Eo6 (= 115 mV) was slightly larger than the value (92 mV) expected by simple statics (entropic factor). The potential difference mentioned above (23 mV) would be caused by the solvation entropy and/or the electrostatic repulsion between the DNBA anions in 3. The single reduction wave observed in the CV of 3 will be explained by the following reasons. First, the large center-to-center distance between the adjacent DNAB ligands (∼9.0 Å) would diminish the electrostatic repulsion between the DNBA anions upon electrochemical reduction. Second, the LUMO of 3 lies primarily on the DNBA terminal ligand(s) and the electron would localize more or less on the NO2 groups, which will impede the electronic interaction between the terminal ligands via the {Mo6Br8}4+-core in 3. To the best of our knowledge, this is the first demonstration for quasi-one-step six-electron electrochemical reduction in a molecular system. Further design of L and X (Cl or I) in [Mo6X8L6]2− will develop new and novel nanoarchitectures with unique electrochemical properties.

Figure 5. UV−visible absorption spectra of 3 in acetone containing 0.1 M TBAPF6 under controlled electrolyses at 0 (dotted line) and −1.0 V (vs Ag/AgCl, solid line) in Ar-gas atmosphere. Inset shows the difference spectrum. The optical path length of the cell is 1 mm.

−1.0 V is best characterized by that of the DNBA anion radical as judged by the analogy of the data in Figure 5 to the reported spectrum,44 although we have failed to record the spectrum of a free 3,5-dinitrobenzoate anion owing to the irreversible electrochemical reduction on the Pt electrode. We conclude that the electrochemical reduction of 3 proceeds through the LUMO level of DNBA. In ligand-based reduction of a transition metal complex, the reduction peak generally splits into several peaks owing to the electronic interactions between the ligands and/or the ligand(s) and the metal.45 For example, it has been reported that the electrochemical reduction wave of [Fe(bpy)3]2+ (bpy = 2,2′bipyridine) in acetonitrile splits into three peaks due to threeelectron reduction, and the potential difference between the first and second or second and third reduction peaks is 180 or 260 mV, respectively.46 Furthermore, Sasaki and Abe have reported that [Re6S8{5-(4-pyridyl)-10,15,20-tritolylporphyrin}6]2+ shows almost one-step six-electron reduction, since the perpendicular arrangements between the coordinated pyridyl and porphyrin units as the terminal ligands in the cluster prevent a ligand−ligand interaction via the {Re6S8}2+cluster core.28 On the other hand, although there is no perpendicular arrangement between DNBA in 3, the cluster shows the almost single six-electron reduction wave as described above. We suppose two possible reasons. First, the center-to-center distance between the two adjacent DNBA groups in 3 evaluated by the crystal structure is ∼9.0 Å, while that between the two bpy in [Fe(bpy)3]2+ is ∼4.5 Å, whose difference between the two complexes should reflect that of the

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01525. Statistically estimated formal potential values (PDF) Crystallographic information files (CIF1 and CIF2) 10264


Article

Inorganic Chemistry

■

(12) Mikhailov, M. A.; Brylev, K. A.; Virovets, A. V.; Gallyamov, M. R.; Novozhilov, I.; Sokolov, M. N. Complexes of {Mo6I8} with nitrophenolates: synthesis and luminescence. New J. Chem. 2016, 40, 1162−1168. (13) Mikhailov, M. A.; Brylev, K. A.; Abramov, P. A.; Sakuda, E.; Akagi, S.; Ito, A.; Kitamura, N.; Sokolov, M. N. Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8}4+ Core Cluster Complexes by Terminal Carboxylate Ligands. Inorg. Chem. 2016, 55, 8437−8437. (14) Kirakci, K.; Kubát, P.; Langmaier, J.; Polívka, T.; Fuciman, M.; Fejfarová, K.; Lang, K. A comparative study of the redox and excited state properties of (nBu4N)2[Mo6X14] and (nBu4N)2[Mo6X8(CF3COO)6] (X = Cl, Br, or I). Dalton Trans. 2013, 42, 7224−7232. (15) Kirakci, K.; Fejfarová, K.; Kuče rák ová, M.; Lang, K. Hexamolybdenum Cluster Complexes with Pyrene and Anthracene Carboxylates: Ultrabright Red Emitters withthe Antenna Effect. Eur. J. Inorg. Chem. 2014, 2014, 2331−2336. (16) Kirakci, K.; Kubát, P.; Kučeráková, M.; Šícha, V.; Gbelcová, H.; Lovecká, P.; Grznárová, P.; Ruml, T.; Lang, K. Water-soluble octahedral molybdenum cluster compounds Na2[Mo6I8(N3)6] and Na2[Mo6I8(NCS)6]: Syntheses, luminescence, and in vitro studies. Inorg. Chim. Acta 2016, 441, 42−49. (17) Hughbanks, T.; Hoffmann, R. Molybdenum Chalcogenides: Clusters, Chains, and Extended Solids. The Approach to Bonding in Three Dimensions. J. Am. Chem. Soc. 1983, 105, 1150−1162. (18) Zietlow, T. C.; Hopkins, M. D.; Gray, H. B. Electronic Spectroscopy and Photophysics of d4 Clusters. J. Solid State Chem. 1985, 57, 112−119. (19) Nocera, D. G.; Gray, H. B. Electrochemical Reduction of Molybdenum(II) and Tungsten(II) Halide Cluster Ions. Electrogenerated Chemiluminescence of Mo6C142‑. J. Am. Chem. Soc. 1984, 106, 824−825. (20) Prokopuk, N.; Shriver, D. F. A One-Dimensional Array of Clusters: Na2Mo6Cl8(O2CC5H4FeCp)6 ·CH3OH. Inorg. Chem. 1997, 36, 5609−5613. (21) Magliocchi, C.; Xie, X.; Hughbanks, T. Cyanide-Melt Synthesis of Reduced Molybdenum Selenide Clusters. Inorg. Chem. 2004, 43, 1902−1911. (22) Rakowski DuBois, M. R.; DuBois, D. L. Development of Molecular Electrocatalysts for CO2 Reduction and H2 Production/ Oxidation. Acc. Chem. Res. 2009, 42, 1974−1982. (23) Praneeth, V. K. K.; Ringenberg, M. R.; Ward, T. R. Redox-Active Ligands in Catalysis. Angew. Chem., Int. Ed. 2012, 51, 10228−10234. (24) Abruña, H. D.; Kiya, Y.; Henderson, J. C. Batteries and electrochemical capacitors. Phys. Today 2008, 61, 43−47. (25) Yoshimura, T.; Umakoshi, K.; Sasaki, Y.; Sykes, A. G. Synthesis, Structures, and Redox Properties of Octa(μ3 -sulfido)hexarhenium(III) Complexes Having Terminal Pyridine Ligands. Inorg. Chem. 1999, 38, 5557−5564. (26) Yoshimura, T.; Umakoshi, K.; Sasaki, Y.; Ishizaka, S.; Kim, H.B.; Kitamura, N. Emission and Metal- and Ligand-Centered-Redox Characteristics of the Hexarhenium(III) Clusters trans- and cis[Re6(μ3-S)8Cl4(L)2]2‑, Where L Is a Pyridine Derivative or Pyrazine. Inorg. Chem. 2000, 39, 1765−1772. (27) Itasaka, A.; Abe, M.; Yoshimura, T.; Tsuge, K.; Suzuki, M.; Imamura, T.; Sasaki, Y. Octahedral Arrangement of Porphyrin Moieties around Hexarhenium(III) Cluster Cores: Structure of (μ3Selenido)hexa-(5-(4-pyridyl)-10,15,20-tritolylporphyrin)hexarhenium(III) (2+). Angew. Chem., Int. Ed. 2002, 41, 463−466. (28) Sasaki, Y.; Abe, M. Ligand−Ligand Redox Interaction through Some Metal-Cluster Units. Chem. Rec. 2004, 4, 279−290. (29) Sheldon, J. C. Bromo- and iodo-molybdenum(II) compounds. J. Chem. Soc. 1962, 410−415. (30) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.; Masters, A. F.; Phillips, L. The Decamethylferrocenium/ Decamethylferrocene Redox Couple: A Superior Redox Standard to the Ferrocenium/Ferrocene Redox Couple for Studying Solvent

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government for the support of the research (No. 26248022, Grant-in-Aid for Scientific Research (A)) to N.K. We thank Prof. M. Kato, Assoc. Prof. A. Kobayashi, and Assist. Prof. M. Yoshida at Hokkaido University for X-ray diffraction measurements.

■

REFERENCES

(1) Cordier, S.; Grasset, F.; Molard, Y.; Amela-Cortes, M.; Boukherroub, R.; Ravaine, S.; Mortier, M.; Ohashi, N.; Saito, N.; Haneda, H. Inorganic Molybdenum Octahedral Nanosized Cluster Units, Versatile Functional Building Block for Nanoarchitectonics. J. Inorg. Organomet. Polym. Mater. 2015, 25, 189−204. (2) Méry, D.; Plault, L.; Ornelas, C.; Ruiz, J.; Nlate, S.; Astruc, D.; Blais, J.-C.; Rodrigues, J.; Cordier, S.; Kirakci, K.; Perrin, C. From Simple Monopyridine Clusters [Mo6Br13(Py-R)][n-Bu4N] and Hexapyridine Clusters [Mo6X8(Py-R)6][OSO2CF3]4 (X = Br or I) to Cluster-Cored Organometallic Stars, Dendrons, and Dendrimers. Inorg. Chem. 2006, 45, 1156−1167. (3) Szczepura, L. F.; Ketcham, K. A.; Ooro, B. A.; Edwards, J. A.; Templeton, J. N.; Cedeño, D. L.; Jircitano, A. J. Synthesis and Study of Hexanuclear Molybdenum Clusters Containing Thiolate Ligands. Inorg. Chem. 2008, 47, 7271−7278. (4) Molard, Y.; Dorson, F.; Cîrcu, V.; Roisnel, T.; Artzner, F.; Cordier, S. Clustomesogens: Liquid Crystal Materials Containing Transition-Metal Clusters. Angew. Chem., Int. Ed. 2010, 49, 3351− 3355. (5) Garreau, A.; Massuyeau, F.; Cordier, S.; Molard, Y.; Gautron, E.; Bertoncini, P.; Faulques, E.; Wery, J.; Humbert, B.; Bulou, A.; Duvail, J.-L. Color Control in Coaxial Two-Luminophore Nanowires. ACS Nano 2013, 7, 2977−2987. (6) Amela-Cortes, M.; Paofai, S.; Cordier, S.; Folliot, H.; Molard, Y. Tuned red NIR phosphorescence of polyurethane hybrid composites embedding metallic nanoclusters for oxygen sensing. Chem. Commun. 2015, 51, 8177−8180. (7) Kirakci, K.; Kubát, P.; Dušek, M.; Fejfarová, K.; Šícha, V.; Mosinger, J. í.; Lang, K. A Highly Luminescent Hexanuclear Molybdenum Cluster − A Promising Candidate toward Photoactive Materials. Eur. J. Inorg. Chem. 2012, 2012, 3107−3111. (8) Johnston, D. H.; Gaswick, D. C.; Lonergan, M. C.; Stern, C. L.; Shriver, D. F. Preparation of Bis(tetrabuty1ammonium) Octa(μ3chloro)hexakis(trifluoromethanesulfonato)-octahedra-hexamolybdate(2-), (Bu4N)2[Mo6Cli8(CF3SO3)a6]: A Versatile Starting Material for Substituted Mo(II) Clusters Containing the [Mo6Cli8]4+ Core. Inorg. Chem. 1992, 31, 1869−1873. (9) Sokolov, M. N.; Mihailov, M. A.; Peresypkina, E. V.; Brylev, K. A.; Kitamura, N.; Fedin, V. P. Highly luminescent complexes [Mo6X8(nC3F7COO)6]2‑ (X = Br, I). Dalton Trans. 2011, 40, 6375−6377. (10) Sokolov, M. N.; Mikhailov, M. A.; Brylev, K. A.; Virovets, A. V.; Vicent, C.; Kompankov, N. B.; Kitamura, N.; Fedin, V. P. Alkynyl Complexes of High-Valence Clusters. Synthesis and Luminescence Properties of [Mo6I8(CCC(O)OMe)6]2−, the First Complex with Exclusively Organometallic Outer Ligands in the Family of Octahedral {M6X8} Clusters. Inorg. Chem. 2013, 52, 12477−12481. (11) Efremova, O. A.; Shestopalov, M. A.; Chirtsova, N. A.; Smolentsev, A. I.; Mironov, Y. V.; Kitamura, N.; Brylev, K. A.; Sutherland, A. J. A highly emissive inorganic hexamolybdenum cluster complex as a handy precursor for the preparation of new luminescent materials. Dalton Trans. 2014, 43, 6021−6025. 10265


Article

Inorganic Chemistry Effects on the Thermodynamics of Electron Transfer. J. Phys. Chem. B 1999, 103, 6713−6722. (31) Oldham, K. B.; Myland, J. C. Modelling cyclic voltammetry without digital simulation. Electrochim. Acta 2011, 56, 10612−10625. (32) Molecular Structure Corp., Orem, UT, 2001. (33) Burla, M. C.; Caliandro, R.; Carrozzini, B.; Cascarano, G. L.; Cuocci, C.; Giacovazzo, C.; Mallamo, M.; Mazzone, A.; Polidori, G. Crystal structure determination and refinement via SIR2014. J. Appl. Crystallogr. 2015, 48, 306−309. (34) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (35) Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Dilute Solutions. AIChE J. 1955, 1, 264−270. (36) Henne, A. L.; Fox, C. J. Ionization Constants of Fluorinated Acids. J. Am. Chem. Soc. 1951, 73, 2323−2325. (37) Maverick, A. W.; Najdzionek, J. S.; MacKenzie, D.; Nocera, D. G.; Gray, H. B. Spectroscopic, Electrochemical, and Photochemical Properties of Molybdenum(II) and Tungsten(II) Halide Clusters. J. Am. Chem. Soc. 1983, 105, 1878−1882. (38) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley and Sons Publishers: New York, 2001. (39) Ramirez-Tagle, R.; Arratia-Pérez, R. Electronic structure and molecular properties of the [Mo6X8L6]2‑ ; X = Cl, Br, I; L = F, Cl, Br, I clusters. Chem. Phys. Lett. 2008, 460, 438−441. (40) Matsuda, H.; Ayabe, Y. Zur Theorie der Randles-Sevčikschen Kathodenstrahl-Polarographie. Z. Elektrochem. 1955, 59, 494−503. (41) Ammar, F.; Savéant, J.-M. Thermodynamics of successive electron transfers. Entropy effects in the series of polynitro compounds. J. Electroanal. Chem. Interfacial Electrochem. 1973, 47, 115−125. (42) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. Electron transfer to and from molecules containing multiple, noninteracting redox centers. Electrochemical oxidation of poly(vinylferrocene). J. Am. Chem. Soc. 1978, 100, 4248−4253. (43) Robinson, L. M.; Bain, R. L.; Shriver, D. F.; Ellis, D. E. Effect of Coordination Environment on the Electronic Structure and Properties of Mos-Based Systems: A Density Functional Treatment. Inorg. Chem. 1995, 34, 5588−5596. (44) Atifi, A.; Ryan, M. D. Electrochemistry and Spectroelectrochemistry of 1,4-Dinitrobenzene in Acetonitrile and RoomTemperature Ionic Liquids: Ion-Pairing Effects in Mixed Solvents. Anal. Chem. 2014, 86, 6617−6625. (45) Vlček, A. A. Ligand Based Redox Series. Coord. Chem. Rev. 1982, 43, 39−62. (46) Saji, T.; Aoyagui, S. Polarographic studies on bipyridine complexes: I. Correlation between reduction potentials of iron(II), ruthenium(II) and osmium(II) complexes and those of free ligands. J. Electroanal. Chem. Interfacial Electrochem. 1975, 58, 401−410. (47) Hernández-Muñoz, L. S.; González, F. J.; González, I.; Goulart, M. O. F.; Abreu, F. C. d.; Ribeiro, A. S.; Ribeiro, R. T.; Longo, R. L.; Navarro, M.; Frontana, C. Revisiting the electrochemical formation, stability and structure of radical and biradical anionic structures in dinitrobenzenes. Electrochim. Acta 2010, 55, 8325−8335. (48) The authors thank one of the reviewers for the important comments on the electrostatic interaction energy between the DNBA terminal ligands in 3.

10266


Quasi-One-Step Six-Electron Electrochemical Reduction of an

Recommend Documents