Synthetic Tuning of Redox, Spectroscopic, and Photophysical

Aug 9, 2016 - (N.K.), *E-mail: [email protected]. .... Only recently was it demonstrated that the [{Mo6I8}L6]2– complexes with fluorinated and poly...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/IC

Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8}4+ Core Cluster Complexes by Terminal Carboxylate Ligands Maxim A. Mikhailov,† Konstantin A. Brylev,†,‡ Pavel A. Abramov,† Eri Sakuda,§,# Soichiro Akagi,∥ Akitaka Ito,⊥ Noboru Kitamura,*,§,∥ and Maxim N. Sokolov*,†,‡ †

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 3 Acad. Lavrentiev Prosp., 630090 Novosibirsk, Russia ‡ Novosibirsk State University, 2 Ul. Pirogova, 630090 Novosibirsk, Russia § Department of Chemistry, Faculty of Science, Hokkaido University, 060-0810 Sapporo, Japan ∥ Department of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering, Hokkaido University, 060-0810 Sapporo, Japan ⊥ Department of Chemistry, Graduate School of Science, Osaka City University, 558-8585 Osaka, Japan # Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, 852-8521 Nagasaki, Japan S Supporting Information *

ABSTRACT: The reactions between the tetra-n-butylammonium salt of [{Mo6I8}I6]2− and silver carboxylates RCOOAg (R = CH3 (1), C(CH3)3 (2), α-C4H3O (3), C6H5 (4), α-C10H7 (5), or C2F5 (6)) in CH2Cl2 afforded new carboxylate complexes [{Mo6I8}(RCOO)6]2−. The complexes were characterized by X-ray single-crystal diffraction and elemental analysis, cyclic/ differential pulse voltammetry, and IR, NMR, and UV−visible spectroscopies. The emission properties of the complexes 1−6, and those of the earlier reported complexes with R = CF3 (7) and n-C3F7 (8), were studied both in acetonitrile solution and in the solid state. In deaerated CH3CN at 298 K, all of the complexes 1−8 exhibit intense and long-lived emission with the quantum yield and lifetime being 0.48−0.73 and 283−359 μs, respectively. The oxidation (Eox)/reduction (Ered) potentials of the complexes correlate linearly with the pKa value of the terminal carboxylate ligands L = RCOO (pKa(L)). Reflecting the pKa(L) dependences of Eox/Ered, the emission energy (νem) of the complexes was also shown to correlate with pKa(L). The present study successfully demonstrates synthetic tuning of the redox, spectroscopic, and photophysical characteristics of a {Mo6I8}4+-based cluster complex with pKa(L).

■

INTRODUCTION Octahedral halide clusters of molybdenum(II) with the general formula [{Mo6(μ3-X)8}L6]n (X = Cl−, Br−, I−; L = inorganic or organic ligands; {Mo6(μ3-X)8}4+ is the cluster core)1 emit relatively long-lived (some tens or even hundreds of microseconds!) red phosphorescence, which can be used for energy transformation and photooxidation,2 generation of singlet oxygen,3 as well as for sensing.4 The emissive behavior of the clusters depends both on X and L, and a study on a series of the clusters with various X and L is therefore prerequisite to understanding of these relationships. The coordination chemistry of the [{Mo6X8}L6]2− complexes with X = Cl and Br has been studied extensively, and a large variety of the complexes with L being O-donor (carboxylates, alkoxo, sulfonates, nitrophenolates, nitrate, nitrite),3c−e,5 N-donor (CH3CN, N3−, NCO−, NCS−, NCSe−, pyridines),6 C-, P-, and S-donor (cyanides, alkynyles, phosphines, thiolates)6c,7 ligands have © 2016 American Chemical Society

been reported. In numerous papers by various research groups it has been shown that {Mo6X8}4+-based complexes (X = Cl, Br) emit bright red phosphorescence upon UV−visible excitation. By contrast, few studies have reported on the luminescence properties of {Mo6I8}4+-based clusters. Only recently was it demonstrated that the [{Mo6I8}L6]2− complexes with fluorinated and polyarene carboxylates, closo-dicarbaborane C- or adamantane-1-carboxylates, as L showed more intense and longlived emission in comparison with the emission from other {M6X8} clusters (M = Mo or W and X = Cl or Br; M = Re and X = S, Se, or Te).3b,d,e,5d Organometallic complex [{Mo6I8}(C CC(O)OMe)6]2−, p-toluenesulfonate [{Mo6I8}(OTs)6]2−, and nitrophenolate complexes [{Mo6I8}(OR)6]2− (R = C6H4-p(NO2) or C6H3-2,4-(NO2)2), and inorganic nitrato [{Mo6I8}Received: April 26, 2016 Published: August 9, 2016 8437

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Article

Inorganic Chemistry

and C5094). The Φem values of the complexes in deaerated CH3CN were evaluated with Absolute Photo-Luminescence Quantum Yield Measurement System (Hamamatsu Photonics, C9920−03), composed of an excitation Xenon light source (the excitation wavelength was set at 380 nm), an integrating sphere, and a red-sensitive multichannel photodetector (Hamamatsu Photonics, PMA-12). To be confident of the accuracy of the quantum yields and lifetime values of the complexes, the studies on the luminescence properties for all the complexes were repeated several times to check the reproducibility of the results. The errors in determining Φem and τem were ±0.01 and ±5 μs (±2−3% of the measured lifetime), respectively. Synthesis of (Bu4N)2[{Mo6I8}(CH3COO)6] (1). To a CH2Cl2 solution (10 mL) of (Bu4N)2[{Mo6I8}I6] (200 mg, 0.07 mmol), solid CH3COOAg (70 mg, 0.42 mmol) was added. The resulting slurry was stirred for 1 d in a flask covered with aluminum foil and then filtered, and the solvent was removed in vacuum to leave a red oily residue. The oil was triturated with diethyl ether for solidification. Single crystals suitable for X-ray analysis were obtained by slow diffusion of diethyl ether vapors into CH2Cl2 solution of the complex. Yield: 70 mg (40%). Anal. Calcd for Mo6I8C44H90N2O12: C, 21.7; H, 3.7; N, 1.2. Found C, 21.8; H, 3.8; N, 1.2%. 1H NMR (deuterated dimethyl sulfoxide (DMSO-d6), normalized to 24 H from the methyl groups in Bu4N+, ppm): δ = 1.76 ((s), 18 H, CH3). 13C NMR (DMSO-d6, ppm): δ = 175.9 ((s), CO), δ = 25.2 ((s), CH3). IR (KBr, cm−1): 3436w, 2960m, 2932m, 2873m, 1616s, 1521w, 1493w, 1469w, 1357s, 1301s, 1008m, 938m, 883w, 796w, 747w, 639m, 503m. UV−vis in CH3CN, λmax, nm (ε, mol−1 dm3 cm−1): 400 sh (4326), 349 (5654), 293 sh (10 212). Synthesis of (Bu4N)2[{Mo6I8}((CH3)3CCOO)6] (2). The compound and its single crystals were obtained in a similar way from (Bu4N)2[{Mo6I8}I6] (114 mg, 0.04 mmol) and (CH3)3CCOOAg (65 mg, 0.31 mmol). Yield: 80 mg (61%). Anal. Calcd for Mo6I8C62H126N2O12 for solventfree composition: C, 27.8; H, 4.7; N, 1.0. Found C, 27.3; H, 4.5; N, 1.0%. 1 H NMR (CD3CN, normalized to 24 H of the methyl groups in Bu4N+, ppm): δ = 1.03 ((s), 54 H, CH3). 13C NMR (CD3CN, ppm): δ = 183.4 ((s), CO), δ = 39.3 ((s), tertiary carbon atom), δ = 27.7 ((s), CH3). IR (KBr, cm−1): 3424w, 3386w, 2961s, 2898m, 2873m, 2524w, 2193w, 2104w, 1770w, 1703w, 1619s, 1553m, 1477s, 1458m, 1390s, 1316s, 1210s, 1060w, 1027w, 956w, 883w, 802w, 778w, 741w, 662w, 594m, 564w, 487w, 415w. UV−vis in CH3CN, λmax, nm (ε, mol−1 dm3 cm−1): 395 sh (5871), 352 (7544), 250 sh (75 067), 231 (85 277). Synthesis of (Bu4N)2[{Mo6I8}(C4H3OCOO)6] (3). The complex was obtained in a similar way from (Bu4N)2[{Mo6I8}I6] (170 mg, 0.06 mmol) and the silver salt of α-furanecarboxylic acid, C5H3O3Ag (80 mg, 0.36 mmol). Yield: 108 mg (66%). Anal. Calcd for Mo6I8C62H90N2O18: C 27.1; H 3.3; N 1.0. Found C 26.8; H 3.3; N 1.0%. 1H NMR (CD3CN, normalized to 24 H from the methyl groups in Bu4N+, ppm): δ = 7.49 ((s), 6 H, C5−H), δ = 6.836.81 ((m), 6 H, C3−H), δ = 6.446.43 ((m), 6 H, C4−H).13C NMR (DMSO-d6, ppm): δ = 164.1 ((s), CO), δ = 149.9 ((s), C5), δ = 144.5 ((s), C2), δ = 114.1 ((s), C4), δ = 112.6 ((s), C3). IR (KBr, cm−1): 3463w, 3107w, 2960m, 2935m, 2872m, 1740w, 1681w, 1626s, 1565m, 1476m, 1386m, 1322s, 1221w, 1185s, 1134m, 1074w, 1007w, 931w, 883w, 788m, 776m, 615w, 563w, 454w, 411w. UV−vis in CH3CN, λmax, nm (ε, mol−1 dm3 cm−1): 395 sh (4942), 335 sh (6932), 255 (110499), 235 sh (91914). Synthesis (Bu4N)2[{Mo6I8}(C6H5COO)6]·2CH2Cl2 (4). The complex was obtained by employing (Bu4N)2[{Mo6I8}I6] (200 mg, 0.07 mmol) and C6H5COOAg (110 mg, 0.48 mmol). Yield: 90 mg (43%). Anal. Calcd for Mo6I8C74H102N2O12 for solvent-free composition: C, 31.7; H, 3.7; N, 1.0. Found: C, 31.8; H, 3.9; N, 1.1% (vacuum-dried sample). 1H NMR (CD3CN, normalized to 24 H from the methyl groups in Bu4N+, ppm): δ = 7.94 ((d), 12 H, o-protons, J3H−H = 7 Hz), δ = 7.447.36 (the multiplets from m- and p-protons, 18H). 13C NMR (DMSO-d6, ppm): δ = 171.4 ((s), CO), 136.7 ((s), C4), 130.8 ((s), C2, C6), 129.4 ((s), C1), 128.2 ((s), C3, C5). IR (KBr, cm−1): 3461m, 3057m, 2959m, 2934m, 2872m, 1615s, 1575m, 1485w, 1447w, 1380w, 1322s, 1298s, 1168m, 1130m, 1065w, 1024m, 879w, 827w, 815w, 715m, 667m, 571m, 417m. UV−vis in CH3CN, λmax, nm (ε, mol−1 dm3 cm−1): 395 sh (7572), 347 (9640), 250 sh (144 261), 231 (191 250). Synthesis of (Bu4N)2[{Mo6I8}(C10H7COO)6] (5). The complex was obtained from (Bu4N)2[{Mo6I8}I6] (170 mg, 0.06 mmol) and the silver

(NO3)6]2− and azido [{Mo6I8}(N3)6]2− complexes have been also found to exhibit relatively intense emission with sharp (i.e., narrow bandwidth) emission spectra.5f,7f It is worth noting that while replacement of the external chloride ligands in [{Mo6Cl8}Cl6]2− by thiolates drastically decreased the emission quantum yield (Φem) and lifetime (τem),7c,d switching from {Mo6Cl8}4+ to {Mo6I8}4+ and introduction of fluorine atoms into thiolates resulted in a significantly increased τem, as shown for (Bu4N)2[{Mo6I8}(SC6F4H)6].7g Obviously, the photophysical characteristics observed for a given Mo(II) octahedral complex are complicated functions of the following three factors: the nature of (i) the donor atom in the terminal (external) ligands, (ii) the capping (inner) halide ligands, and (iii) the organic substituents (with different electronic and steric effects) within a given external ligand set. To shed light on the relationships between these three factors and the spectroscopic/photophysical properties of the complexes, systematic study is absolutely necessary. In the present work, we attempted to fix the first factor mentioned above by focusing on a series of [{Mo6I8}L6]2− (L = RCOO−) clusters and to discuss the effects of nature of L on the spectroscopic/ photophysical properties of the complexes. In this paper we report the synthesis and redox, spectroscopic, and photophysical properties of a series of {Mo6I8}4+-core clusters with both fluorinated and nonfluorinated aromatic and aliphatic terminal carboxylate ligands (L = RCOO−) and demonstrate that the pKa of the carboxylate ligand plays decisive role in determining the redox, spectroscopic, and photophysical properties of the cluster complexes.

■

EXPERIMENTAL SECTION

Chemicals and Spectroscopic/Photophysical Measurements. (Bu4N)2[{Mo6I8}I6]8 was used as staring material throughout. The silver salts of the carboxylic acids were prepared by reacting RCOOH (employed as purchased) with the stoichiometric amount of Ag2O (freshly prepared from AgNO3 and NaOH) in CH3CN. Other chemicals purchased from various suppliers (Sigma-Aldrich, Wako Co, Ltd) were used without further purification. Acetonitrile for spectroscopic and electrochemical measurements was distilled prior to use. IR spectra (4000−400 cm−1) and UV/vis spectra of the complexes were recorded on an IFS-85 Bruker spectrometer and a Hitachi U-3300 spectrophotometer, respectively. Elemental analyses were performed by the analytical service of the Nikolaev Institute of Inorganic Chemistry (Novosibirsk). Electrospray ionization mass spectrometry (ESI-MS) was performed on a quadrupole−hexapole time-of-flight (Q-TOF 1) mass-spectrometer with an orthogonal Z-spray electrospray interface (Micromass, Manchester, U.K). Nitrogen was employed as drying and nebulizing gas. The isotopic patterns were compared with the calculated patterns with MassLynx 4.0 program. NMR spectra were recorded on a 500 MHz Bruker Avance 500 plus spectrometer. Electrochemical measurements of the complexes in CH3CN (1 × 10−4 M (= mol/dm3)) in the presence of tetra-n-butylammonium hexafluorophosphate (0.1 M) were conducted with a BAS ALS-701D electrochemical analyzer equipped with a glassy carbon disk, Pt wire, and Ag/Ag+ as working, counter, and reference electrodes, respectively. Photoluminescence measurements were conducted at 298 K throughout the study. The powdered samples of the complexes were placed between two nonfluorescent glass plates. The absorbance of acetonitrile solutions was set below 0.05 at the excitation wavelength of 355 nm. The solutions of the complexes in CH3CN in quartz cuvettes were deaerated by purging with an Ar gas stream for 30 min, and then the cuvettes were sealed. The samples were excited with 355 nm laser pulses (6 ns duration, LOTIS TII, LS-2137/3). Corrected emission spectra were recorded on a red-sensitive multichannel photodetector (Hamamatsu Photonics, PMA-11). For emission decay measurements, the emission was analyzed by a streakscope system (Hamamatsu Photonics, C4334 8438

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Article

Inorganic Chemistry salt of 1-naphthoic acid, α-C10H7COOAg (100 mg, 0.36 mmol). Yield: 110 mg (59%). Anal. Calcd for Mo6I8C98H114N2O12: C, 37.9; H, 3.7; N, 0.9. Found: C, 37.6; H, 3.6; N, 0.9%. 1H NMR (CD3CN, normalized to 24 H from the methyl groups in Bu4N+, ppm): δ = 8.93 ((d), 6 H, (C8− H), J3H−H = 8.45 Hz), δ = 7.957.87 ((m), 18 H), δ = 7.537.47 ((m), 18 H). 13C NMR (DMSO-d6, ppm): δ = 173.1 ((s), CO), δ = 135.1 ((s), C10), δ = 133.8 ((s), C4), δ = 131.2 ((s), C9), δ = 130.3 ((s), C2), δ = 128.5 ((s), C5), δ = 128.2 ((s), C1), δ = 127.5 ((s), C7), δ = 126.6 ((s), C6), δ = 125.9 ((s), C8), δ = 125.5 ((s), C3). IR (KBr, cm−1): 3455w, 3044w, 2958m, 2932m, 2871m, 1732w, 1613s, 1506w, 1457w, 1380w, 1302s, 1251m, 1210m, 1146m, 1068w, 1004w, 877w, 786 m, 652m, 592w, 533w, 483w, 421w. UV−vis in CH3CN, λmax, nm (ε, mol−1 dm3 cm−1): 400 sh (2922), 293 (41 962), 250 sh (47 313), 221 (179 786). Synthesis of (Bu4N)2[{Mo6I8}(C2F5COO)6] (6). The complex was obtained from (Bu4N)2[{Mo6I8}I6] (200 mg, 0.07 mmol) and C2F5COOAg (133 mg, 0.49 mmol). Yield: 120 mg (56%). Anal. Calcd for Mo6I8C50H72N2O12F30: C, 19.7; H, 2.4; N, 0.9%. Found C, 19.8; H, 2.6; N, 0.9%. 19F NMR (DMSO-d6, ppm): δ1 = −507.6 (s), δ2 = −545.3 (s). IR (KBr, cm−1): 2967m, 2939m, 2880m, 2085w, 1699s, 1534w, 1490m, 1466w, 1381m, 1317s, 1261w, 1211s, 1163s, 1025s, 880w, 809m, 726m, 658w, 623w, 584w, 539w, 437w. UV−vis in CH3CN, λmax, nm (ε, mol−1 dm3 cm−1): 392 (4903), 342 (5861), 292 (9656), 241 (61 777), 219 (94 160). Synthesis of (Bu4N)2[{Mo6I8}(CF3COO)6] (7) and (Bu4N)2[{Mo6I8}(C3F7COO)6] (8). These complexes were prepared according to the published procedures.3b,5d X-ray Crystallography. The diffraction data for complexes 1 and 3−6 were collected on a Bruker Apex Duo diffractometer with Mo Kα radiation (λ = 0.710 73 Å) by doing ω and φ scans of narrow (0.5°) frames. All structures were solved by direct methods and refined by fullmatrix least-squares treatment against |F|2 in anisotropic approximation with SHELX-2014/7 software packages in ShelxLe program.9 Absorption corrections were applied empirically with SADABS.10 In the structure of the benzoate complex (4) an unrefined electronic density from highly disordered solvent molecules was found. It was treated with the SQUEEZE option of the PLATON suite of programs.11 Following this treatment 81 electrons per formula unit were found, which is very close to the value expected from two molecules of CH2Cl2 (84 e).

As far as the main driving force behind the equilibrium shift in eq 1 and (2) is precipitation of poorly soluble AgX, the use of the iodide complex as precursor is especially favorable, since AgI has a particularly low solubility and can be easily separated by filtration. Starting from (Bu4N)2[{Mo6I8}I6] and appropriate silver carboxylates, we successfully prepared complexes with acetate (1), pivalate (2), α-furanecarboxylate (3), benzoate (4), α-naphtoate (5), and perfluoropropionate (6) anions as the terminal ligands. The tetrabutylammonium salts of the complexes 1−6 are air-stable solids soluble in common organic solvents such as acetone, CH2Cl2, CH3CN, N,N-dimethylformamide, and DMSO. The reactions are slow and proceed stepwise, as exemplified by the ESI mass spectrum from the reaction mixture (Bu4N)2[{Mo6I8}I6] and α-C10H7COOAg. In addition to the expected peak from [{Mo6I8}(C10H7COO)6]2− (m/z 1309.5), less intense peaks from partially substituted species [{Mo6I8}(C10H7COO)6−xIx]2− (x = 1, 2, 3) were observed. In addition, the ESI-mass peaks from [{Mo6I8}(C10H7COO)6−xClx]2− (x = 1, 2, 3) are observed, demonstrating partial I/Cl metathesis of the external I− ligands in CH2Cl2.

■

CRYSTAL STRUCTURES The main structural unit in 1−6 is a cluster anion [{Mo6I8}(RCOO)6]2− (main geometric parameters are listed in Table S2), which possesses a common mode of orientation of the carboxylate ligands as shown in Figure 1.

■

RESULTS AND DISCUSSION Synthesis. The preparative routes to the carboxylate cluster complexes [{M6X8}(OOCR)6]2− commonly employ (i) substitution of a stronger acid for weaker acid, (ii) substitution of a more volatile acid by a less volatile one, and (iii) replacement of a weaker coordinating (less basic) ligand by a stronger (more basic) one. The first approach was used to convert methoxide [{Mo 6 Cl 8 }(OCH 3 ) 6 ] 2− complex into [{Mo 6 Cl 8 }(CpFeC5H4COO)6]2−.12 In the second approach, trifluoroacetic acid CF3COOH expels HCl gas from [{Mo6Cl6}Cl8]2− giving rise to the formation of [{Mo6Cl8}(CF3COO)6]2−.5b The third route was used to convert a precursor triflate [{Mo6Cl8}(CF3SO3)6]2− into [{Mo6Cl8}L6]2− by reacting it with L = Cl−, Br−, I−, SCN−, CH3O−, CH3COO−, CH3C6H4SO3−, and CF3COO−.5a Alternatively, halide abstraction of the terminal halide from [{M6X8}X6]2− (X = Cl−, Br−, or I−) with Ag+ in the presence of other ligands can be used. Thus, [{Mo6X8}(NO3)6]2− or [{Mo6X8}(RCOO)6]2− are obtained in good yields from [{Mo6X8}X6]2− according to eq 1 or (2), respectively.3b,5a,c,d,f,13 [{Mo6X 8}X 6]2 − + 6AgNO3 ⇔ [{Mo6X 8}(NO3)6 ]2 − + 6AgX

Figure 1. General view of [Mo6I8(OOCR)6]2−.

The spatial arrangement of the carboxylate ligands is mainly determined by steric or packing effects in the crystal without significant intermolecular interactions. However, it is worth noting that in the structure of 6 (R = C2F5) four carboxylate groups are coplanar with the four corresponding molybdenum atoms (Figure 2). The complexes prepared in the present study prefer monoclinic P21/n space group. Several other known carboxylates (Bu 4 N) 2 [{Mo 6 I 8 }(C 3 F 7 COO) 6 ], 5 d (Bu 4 N) 2 [{W 6 Cl 8 }(C3F7COO)6],14 (Bu4N)2[{Mo6I8}(pyrene-1-COO)6], and (Bu4N)2[{Mo6I8}(anthracene-9-COO)6 15 also crystallized in a monoclinic space group, but (Bu4N)2[{Mo6I8}(CF3COO)6]3b and Cs2[{Mo6I8}(C2F5COO)6]·2C4H10O5g are triclinic. It means that crystal packing of these tetrabutylammonium salts is dictated by the packing of the [{M6X8}L6]2− cluster anions that prevents any possible secondary interactions. In the case of 2 all attempts to find right conditions for growing well-diffracting single crystals failed, and we were able to obtain only a model with the following unit cell parameters: C2/c, a = 31.8591(13) Å, b = 15.6210(6) Å, c = 26.2114(12) Å, β = 124.176(2)°. However,

(1)

[{Mo6X 8}X 6]2 − + 6RCOOAg ⇔ [{Mo6X 8}(RCOO)6 ]2 − + 6AgX (R = CF3, C2F5, n‐C3F7)

(2) 8439

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Article

Inorganic Chemistry

reduction potentials of the complex (Ered({Mo6I8}4+/{Mo6I83+}) from +0.5 to +1.1 V and from −1.5 to −1.9 V (vs Ag/Ag+), respectively. It is anticipated that the redox potential of a given complex is governed by the pKa value of the carboxylate ligand (pKa(L), see Table 1) through the change in the σ-donating ability of L to the {Mo6I8}4+ cluster core. In practice, we obtained good linear relationships between pKa(L) and the redox potential of the complex as shown in Figure 4. The slope values of the plots

Figure 2. Coplanar orientation of the four carboxylic units in the structure of 6.

the structural model for 2 is consistently supported with analytical and NMR data. Electrochemistry. Figure 3 shows the cyclic voltammograms of 1−8 in acetonitrile. Figure 4. pKa(L) dependences of the Eox and Ered values of [{Mo6I8}L6]2−: (a) for the {Mo6I8}5+/{Mo6I8}4+ couple; (b) for the {Mo6I8}4+/{Mo6I8}3+ couple.

are −103 and −73 mV/pKa(L) for the data on Eox (correlation coefficient (cc) = 0.891) and Ered (cc = 0.833), respectively. For a series of [{Re6S8}Cl4L′2]4− (L′ = N-heteroaromatic ligand), Yoshimura et al. reported that the Eox({Re6S8}2+/{Re6S8}3+) value of the complex increased with a decrease in pKa(L′),16 which is very similar to our results on Eox as shown in Figure 4. Since a decrease in pKa(L) reflects the decrease in the σ-donating ability of L, the highest occupied molecular orbital (HOMO) energy level of the complex should also decrease with a decrease in pKa(L), and thus, the Eox value as a measure of the HOMO energy of the complex is anodically shifted in the positive direction with a decrease in pKa(L). To the best of our knowledge, this correlation between pKa(L) and Ered has never been reported for [{M6X8}L6]n (M = Mo(II), W(II), or Re(III); X = Cl−, Br−, I−, S2−, Se2−; L = Cl−, Br−, I−, N-heteroaromatics, carboxylate, and so forth), and ours is the first demonstration of linear relationship between pKa(L) and Ered for a series of any complexes with the general formula [{M6X8}L6]n. The positive

Figure 3. Cyclic voltammograms of [{Mo6I8}L6]2− in acetonitrile at 298 K.

All of the complexes exhibited quasi-reversible oxidation and irreversible reduction cyclic voltammogram (CV) waves, and the redox potentials of the complexes (Table 1) were evaluated by differential pulse voltammetry (Figure S1). The entries in Table 1 demonstrate that a variation of L in [{Mo6I8}L6]2− gives rise to the changes in the oxidation (Eox({Mo6I8}4+/{Mo6I85+}) and

Table 1. Electrochemical and Ultraviolet−Visible Absorption Data on 1−8 in Deaerated Acetonitrile

8440

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Article

Inorganic Chemistry

(14.3 × 104) and 231 nm (19.1 × 105) for 4, and 293 nm (4.2 × 104) and 221 nm (18.0 × 104) for 5. All of the complexes including 7 and 8 show intense and longlived emission in both deaerated CH3CN solution (Figure 6) and solid phases (Figure S2).

shift of Ered with a decrease in pKa(L) demonstrates clearly that Ered of the complex is responsible for the {Mo6I8}4+/{Mo6I83+} couple, whose energy as a lowest-energy unoccupied molecular orbital (LUMO) is governed by the electron density in the {Mo6I8}4+ core of the complex through the σ-donating ability of L. The results shown in Table 1 and Figure 4 demonstrate explicitly that the redox characteristics of [{Mo6I8}L6]2− can be controlled by changing the nature of the terminal carboxylate ligands L. Since the linear dependences of Eox and Ered with pKa(L) in Figure 4 demonstrate that both HOMO and LUMO energies are governed by pKa(L), the (Eox − Ered) values listed in Table 1, corresponding to the HOMO−LUMO energy gap of a given complex, should reflect the spectroscopic and photophysical properties of the complexes, which will be discussed in the following section.17 Spectroscopic and Photophysical Properties. The absorption spectra of (Bu4N)2[{Mo6I8}L6] in CH3CN are shown in Figure 5, together with the enlarged spectra of the absorption envelopes of the complexes (wavelength (λ) > 300 nm) in the inset.

Figure 6. Emission spectra of [{Mo6I8}L6]2− in deaerated CH3CN at 298 K. The emission intensity of the complex is normalized to that at the maximum wavelength.

The emission decay profiles are shown in Figures S3 (for CH3CN solutions) and S4 (for solids). There is no dissociation of the carboxylate in the solutions, affecting the nature of the species, responsible for the emission, as can be seen from NMR (no signals from free carboxylates) and ESI-MS (only peaks from hexacarboxylate complexes). The spectroscopic and photophysical parameters (emission maximum wavelength (λem), emission maximum wavenumber (νem), the full width at halfmaximum (fwhm) of the emission spectrum, Φem, and τem (with the corresponding amplitudes (A) in the solid phases) as well as the radiative (kr) and nonradiative (knr) decay rate constants for solutions) are listed in Table 2. Among the eight [{Mo6I8}L6]2− clusters, the λem, Φem, and τem values of 7 in deaerated CH3CN at room temperature have been reported to be 673 nm, 1.0, and 182 μs, respectively.3b However, the Φem and τem values determined in the present study were lower (0.67) and longer (333 μs) than the values given in ref 3b. To check the Φem value of 7, we evaluated the Φem values of the other seven complexes by using 7 as standard with Φem = 1.0.3b The Φem values of 1−5 thus evaluated ranged from 0.7 to 0.9, while that of 6 exceeded 1.0, indicating that the previously reported value3b was questionable. In the present study, therefore, we use the Φem values of 1−8 in deaerated CH3CN and (aerated) solid phases evaluated by the photodetector equipped with an integration sphere (see Experimental Section). As seen in Table 2, the Φem values of 6 and 7 in deaerated CH3CN are 0.73 and 0.67, respectively, and those of the other complexes are in the range of 0.48−0.62. It is worth pointing out that the Φem value of 8 reported in the previous paper (0.59)5d is confirmed by the present experiments as well: Φem = 0.60. We believe that the Φem value of 7 as determined in the present study is more reliable.3b The Φem being 1.0 means that the complex after excitation emits all of the photons absorbed. But for a compound with such a large Stokes shifts and a long emission lifetime (phosphorescence), it is highly probable that the excitation energy would be lost during the transitions and changes of the spin states of the excited states (S1 and/or T1); that is, it is reasonable to expect Φem to be below 1. We can suppose that using such standard as cresyl violet could be the reason for an error in the relative quantum yield

Figure 5. Absorption spectra of [{Mo6I8}L6}2− in CH3CN at 298 K. (inset) Enlarged spectra of the absorption envelopes.

The absorption maximum wavelengths (λabs) and the relevant molar absorption coefficients (ε, M−1 cm−1) are listed in Table 1. The complexes show broad and featureless absorption in the λ region longer than 350 nm (inset in Figure 5) and structured bands in the UV region. The absorption of the complexes at λ > 350 nm can be assigned to the {Mo6I8}4+ core-centered d−d transitions, based on the theoretical studies carried on [{Mo6X8}Y6]2− (X = Cl−, Br−, or I−; Y = F−, Cl−, Br−, or I−) by Ramirez-Tagle et al.18 Although we cannot discuss the absorption characteristics in the visible region in detail owing to the featureless absorption bands of the complexes with nonfluorinated carboxylates, the complexes with the perfluorinated carboxylates (6, 7, and 8) exhibit clear absorption maxima at ∼395 nm, which are shorter than those of other complexes; see Table 1. Since 6, 7, and 8 possess the smallest pKa(L) among the selected RCOOH (0.18 to ca. 0.52−0.67), these hypsochromic shifts can be explained by destabilization of the HOMO/LUMO energies in 6, 7, and 8 relative to those of other carboxylate complexes as predicted from the (Eox − Ered) values of the complexes; see Table 1 and Figure 4. In the UV region, however, the complexes with aromatic carboxylates (3, 4, and 5) show characteristic and relatively intense absorption bands assigned to the π−π* transitions in the aromatic terminal ligands: λabs (ε) = 337 nm (0.69 × 104) and 255 nm (11.1 × 104) for 3, 252 nm 8441

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Article

Inorganic Chemistry Table 2. Spectroscopic and Photophysical Properties of 1−8 in Deaerated Acetonitrile Solutions and in the Solid States

a

Relative quantum yield.

determination because cresyl violet3b has much narrower and blue-shifted emission spectrum than the compound 7. In our study, the quantum yield was determined without any standard by the Absolute Photo-Luminescence Quantum Yield Measurement System, which is supposed to give more accurate Φem values. The emission lifetimes of the complexes (1−8) in deaerated CH3CN range from 283 to 359 μs, that of 8 being slightly longer (359 μs) than the reported value (303 μs).5d Since the excited triplet state of a transition metal complex is very likely to be quenched by a small amount of oxygen, determination of the absolute τem value is in general difficult. Nonetheless, we suppose that the longer τem value of 8 (359 μs) observed in the present study is more accurate. Similarly, the τem value of 7 determined in the present study (333 μs) looks more likely to be the correct value than that reported in ref 3b (182 μs). Quite recently AmelaCortes et al. reported the λem and Φem values of solid Cs2[{Mo6I8}(C2F5COO)6] to be 650 nm and 0.35, respectively,5g which is very close to the values determined for the powdered Bu4N+ salt (6): λem = 661 nm and Φem = 0.32. We are thus convinced that accurate measurements were performed in the present study and that the photophysical properties of 1−8 as listed in Table 2 are highly reliable. In 2011, the emission properties of [{Mo 6 X 8 }(nC3F7COO)6]2− (X = Cl−, Br−, I− (8)) in deaerated CH3CN at 298 K were reported. Complex 8 exhibits intense and long-lived emission compared to the relevant Cl (τem < 0.01 and τem = 1.4/ 0.4 μs) and Br complexes (0.36 and 206 μs).5d The present study has revealed that other {Mo6I8}4+-based carboxylate complexes also show intense and long-lived emission irrespective of R at L,

as seen in Table 2, which is a very important observation for the future synthetic design of highly luminescent {Mo6X8}4+-based clusters with terminal carboxylate ligands. As another important emission characteristics, 8 shows short-wavelength emission (λem = 668 nm) with a relatively small fwhm value (2390 cm−1) of the emission spectrum, compared to the relevant values of the Cl (λem = 745 nm and fwhm = 3980 cm−1) and Br complexes (715 nm and 4160 cm−1).5d The data in Table 2 demonstrate explicitly that such emission characteristics of the {Mo6I8}4+-based complexes are also retained in the series of [{Mo6I8}L6]2− complexes irrespective of L: λem = 668−711 nm and fwhm =2340−2440 cm−1. In a previous paper,5d we suggested that the difference in the emission characteristics between 8 and its {Mo 6 Cl8 } 4+ - and {Mo 6Br8 }4+-based analogues could be explained by the differences in the zero-magnetic-field splitting (zfs) parameters in the emitting excited triplet states of the clusters.19 Indeed, we confirmed recently that the temperature dependences of the emission spectrum and lifetime of 8 are quite different from those of the relevant Cl and Br complexes, suggesting large differences in the zfs parameters between the I and Cl/Br clusters. We will report detailed photophysical properties of [{Mo6X8}(n-C3F7COO)6]2− (X = Cl, Br, and I) in a separate publication.20 As the factor governing the emission properties of [{Mo6I8}L6]2− in CH3CN, the linear relationships between Eox/Ered and pKa(L) in Figure 4 suggest the importance of pKa(L) in determining the emission properties of the complexes. As shown in Figure 7, we obtained linear relationships between the emission energy (νem in wavenumbers) and pKa(L) with the 8442

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Article

Inorganic Chemistry

Figure 7. pKa(L) dependence of the emission energies of [{Mo6I8}L6]2− in deaerated CH3CN at 298 K.

slope and cc values of the plot being −190 cm−1/pKa(L) and 0.983, respectively. These results demonstrate clearly that the Eox and Ered values of the complex correspond nicely to the relevant HOMO and LUMO energy levels, respectively. For a transition metal complex, Eox and Ered are determined by the metal oxidation and ligand reduction, respectively. Since the lowest-energy excited state of the cluster possesses a metal-to-ligand charge transfer (MLCT) character, Eox and Ered correspond to HOMO and LUMO, respectively, and the emission (phosphorescence) maximum energy (νem) of the complex correlates very well with (Eox − Ered) as a measure of the HOMO−LUMO gap. In the present case, as seen in Figure 5S, both Eox and Ered are under the strong influence of L (pKa(L)) indicating Eox and Ered can be used as a “measure” of the HOMO and LUMO energies, respectively. However, the (Eox − Ered) value itself cannot predict the νem value: νem and (Eox − Ered) are in the ranges of (1.74−1.86) eV and (2.45−2.60) V, respectively. Since the (Eox − Ered) value is a measure of the HOMO−LUMO energy gap of the cluster complexes, the νem value of the complex correlates with pKa(L) through the linear relationships between pKa(L) and Eox/Ered in Figure 4. The pKa(L) dependences of Φem and τem of [{Mo6I8}L6]2− in deaerated CH3CN at 298 K are shown in Figures 8a,b, respectively. Obviously, the pKa(L) value does not govern directly the Φem and/or τem values of the complex. Nevertheless, the pKa(L) value roughly correlates with Φem (cc = 0.557) and τem of the complexes (cc = 0.861), although some data deviate from the linear relationships. Since the νem value correlates linearly with pKa(L) as seen in Figure 7, a plausible explanation for the results in Figure 8 will be the energy gap dependence of the nonradiative rate constant (knr) of the complex (ln knr ∝ −νem),21 by which the Φem and τem values are influenced the relations: Φem = kr/(kr + knr) = krτem and τem = 1/(kr + knr). In practice, the energy gap dependence of ln knr has been reported for [{W6X8}Y6]2− (X and Y are Cl−, Br−, or I−) and [{Re6Q8}L″nL‴6−n]z (Q = S, Se, or Te; L″ and L‴ = X, CN, SCN, phosphine, N,N-dimethylformamide, and so forth) complexes.22 The energy gap plot for the present system is shown in Figure 9. The positive correlation between ln knr and νem in Figure 9 (cc = 0.701) might partially explain the results in Figure 8, although the data on 6, 7, and 8 with perfluorinated aliphatic carboxylate ligands are scattered. Since these three complexes show the same νem value (14.97 × 103 cm−1), while the Φem and τem values vary from 0.60 to 0.73

Figure 8. pKa(L) dependences of the emission quantum yield (a) and lifetime (b) of [{Mo6I8}L6]2− in deaerated CH3CN at 298 K.

Figure 9. Energy gap plot for the knr value of [{Mo6I8}L6]2− in deaerated CH3CN at 298 K.

and from 333 to 359 μs, respectively, we suggest that the presence of the perfluorinated carboxylate ligands gives rise to the change in the nonradiative decay mode(s) in the emissive excited triplet states of the complexes compared to those of other complexes. Further detailed experiments and analysis of the photophysical data of the complexes are necessary. Nonetheless, the results summarized in Figures 7, 8, and 9 convincingly demonstrate that the spectroscopic and photophysical characteristics of [{Mo6I8}L6]2− can be tuned successfully by the pKa value of the terminal carboxylate ligand. We anticipated that the absorption data would also correlate with the pKa of the ligands, since the donating ability of L might govern the longest wavelength λ and ε values. However, no such correlation could be found (Figure S6). 8443

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Inorganic Chemistry

■

■

CONCLUSIONS We have synthesized six new tetra-n-butylammonium salts of {Mo6I8}4+-based complexes with a series of terminal carboxylate ligands with the general formula of [{Mo6I8}(RCOO)6]2−. The complexes have been characterized by X-ray single-crystal diffraction, elemental analysis, and IR, NMR, UV/vis, and luminescence spectroscopies. The redox potentials of the complexes were evaluated by cyclic and differential pulse voltammetry techniques. All the carboxylate complexes including the previously reported complexes, 7 and 8, display bright and long-lived red phosphorescence in both deaerated acetonitrile solutions and solids. In particular, the Φem and τem values in deaerated acetonitrile range from 0.48 to 0.73 and from 283 to 359 μs, respectively, demonstrating fantastic performance of a {Mo6I8}4+-based cluster as an inorganic luminescent chromophore. The most important finding in the present study is the linear pKa(L) dependences between the Eox/Ered and νem values. These results indicate that the HOMO (Eox)/LUMO (Ered) energy levels of a carboxylate complex can be tuned by changing pKa(L), and thus, the emission energy of the complex can be tuned: (Eox − Ered) = HOMO/LUMO energy gap ∝ νem. The present findings offer scope for further synthetic manipulation of the redox, spectroscopic, and photophysical characteristics of {Mo6X8}4+-based cluster complexes (X = Cl−, Br−, or I−) by the changing the pKa value of the terminal ligand.

■

REFERENCES

(1) Prokopuk, N.; Shriver, D. F. Advances in Inorganic Chemistry; Sykes, A. G., Ed.; Academic Press, 1998; Vol. 46, pp 1−49. (2) Jackson, J. A.; Mussell, R. D.; Nocera, D. G. Inorg. Chem. 1993, 32, 4643−4645. (3) (a) Jackson, J. A.; Turro, C.; Newsham, M. D.; Nocera, D. G. J. Phys. Chem. 1990, 94, 4500−4507. (b) Kirakci, K.; Kubat, P.; Dusek, M.; Fejfarova, K.; Sicha, V.; Mosinger, J.; Lang, K. Eur. J. Inorg. Chem. 2012, 2012, 3107−3111. (c) Kirakci, K.; Kubat, P.; Langmaier, J.; Polivka, T.; Fuciman, M.; Fejfarova, K.; Lang, K. Dalton Trans. 2013, 42, 7224− 7232. (d) Kirakci, K.; Šícha, K.; Holub, J.; Kubát, P.; Lang, K. Inorg. Chem. 2014, 53, 13012−13018. (e) Kirakci, K.; Kubát, P.; Fejfarová, K.; Martinčík, J.; Nikl, M.; Lang, K. Inorg. Chem. 2016, 55, 803−809. (4) (a) Elistratova, J.; Mikhailov, M.; Burilov, V.; Babaev, V.; Rizvanov, I.; Mustafina, A.; Abramov, P.; Sokolov, M.; Konovalov, A.; Fedin, V. RSC Adv. 2014, 4, 27922−27930. (b) Amela-Cortes, M.; Paofai, S.; Cordier, S.; Folliot, H.; Molard, Y. Chem. Commun. 2015, 51, 8177− 8180. (5) (a) Johnston, D. H.; Gaswick, D. C.; Lonergan, M. C.; Stern, C. L.; Shriver, D. F. Inorg. Chem. 1992, 31, 1869−1873. (b) Harder, K.; Preetz, W. Z. Anorg. Allg. Chem. 1992, 612, 97−100. (c) Braack, P.; Simsek, M. K.; Preetz, W. Z. Anorg. Allg. Chem. 1998, 624, 375−380. (d) Sokolov, M. N.; Mihailov, M. A.; Peresypkina, E. V.; Brylev, K. A.; Kitamura, N.; Fedin, V. P. Dalton Trans. 2011, 40, 6375−6377. (e) Sokolov, M. N.; Mikhailov, M. A.; Abramov, P. A.; Fedin, V. P. J. Struct. Chem. 2012, 53, 197−201. (f) Efremova, O. A.; Shestopalov, M. A.; Chirtsova, N. A.; Smolentsev, A. I.; Mironov, Y. V.; Kitamura, N.; Brylev, K. A.; Sutherland, A. J. Dalton Trans. 2014, 43, 6021−6025. (g) Amela-Cortes, M.; Molard, Y.; Paofai, S.; Desert, A.; Duvail, J.-L.; Naumov, N. G.; Cordier, S. Dalton Trans. 2016, 45, 237−245. (h) Mikhailov, M. A.; Brylev, K. A.; Virovets, A. V.; Gallyamov, M. R.; Novozhilov, I. N.; Sokolov, M. N. New J. Chem. 2016, 40, 1162−1168. (i) Efremova, O. A.; Brylev, K. A.; Vorotnikov, Y. A.; Vejsadova, L.; Shestopalov, M. A.; Chimonides, G. F.; Mikes, P.; Topham, P. D.; Kim, S.-J.; Kitamura, N.; Sutherland, A. J. J. Mater. Chem. C 2016, 4, 497−503. (6) (a) Ehrlich, G. M.; Warren, C. J.; Haushalter, R. C.; DiSalvo, F. J. Inorg. Chem. 1995, 34, 4284−4286. (b) Simsek, M. K.; Preetz, W. Z. Anorg. Allg. Chem. 1997, 623, 515−523. (c) Simsek, M. K.; Bublitz, D.; Preetz, W. Z. Anorg. Allg. Chem. 1997, 623, 1885−1891. (d) Bublitz, D.; Preetz, W.; Simsek, M. K. Z. Anorg. Allg. Chem. 1997, 623, 1−7. (e) Pilet, G.; Cordier, S.; Golhen, S.; Perrin, C.; Ouahab, L.; Perrin, A. Solid State Sci. 2003, 5, 1263−1270. (f) Mery, D.; Plault, L.; Nlate, S.; Astruc, D.; Cordier, S.; Kirakci, K.; Perrin, C. Z. Anorg. Allg. Chem. 2005, 631, 2746−2750. (g) Mery, D.; Plault, L.; Ornelas, C.; Ruiz, J.; Nlate, S.; Astruc, D.; Blais, J. C.; Rodrigues, J.; Cordier, S.; Kirakci, K.; Perrin, C. Inorg. Chem. 2006, 45, 1156−1167. (h) Kirakci, K.; Kubát, P.; Kučeráková, M.; Šícha, V.; Gbelcová, H.; Lovecká, P.; Grznárová, P.; Ruml, T.; Lang, K. Inorg. Chim. Acta 2016, 441, 42−49. (7) (a) Hamer, A. D.; Smith, T. J.; Walton, R. A. Inorg. Chem. 1976, 15, 1014−1017. (b) Saito, T.; Nishida, M.; Yamagata, T.; Yamagata, Y.; Yamaguchi, Y. Inorg. Chem. 1986, 25, 1111−1117. (c) Szczepura, L. F.; Ketcham, K. A.; Ooro, B. A.; Edwards, J. A.; Templeton, J. N.; Cedeno, D. L.; Jircitano, A. J. Inorg. Chem. 2008, 47, 7271−7278. (d) Szczepura, L. A.; Edwards, J. A.; Cedeno, D. L. J. Cluster Sci. 2009, 20, 105−112. (e) Saito, T.; Yamamoto, N.; Nagase, T.; Tsuboi, T.; Kobayashi, K.; Yamagata, T.; Imoto, H.; Unoura, K. Inorg. Chem. 1990, 29, 764−770. (f) Sokolov, M. N.; Mikhailov, M. A.; Brylev, K. A.; Virovets, A. V.; Vicent, C.; Kompankov, N. B.; Kitamura, N.; Fedin, V. P. Inorg. Chem. 2013, 52, 12477−12481. (g) Sokolov, M. N.; Mikhailov, M. A.; Virovets, A. V.; Brylev, K. A.; Bredikhin, R. A.; Maksimov, A. M.; Platonov, V. E.; Fedin, V. P. Russ. Chem. Bull. 2013, 62, 1764−1767. (8) Kirakci, K.; Cordier, S.; Perrin, C. Z. Anorg. Allg. Chem. 2005, 631, 411−416. (9) Hübschle, C. B.; Sheldrick, G. M.; Dittrich, B. J. Appl. Crystallogr. 2011, 44, 1281−1284. (10) Sheldrick, G. M. SADABS, Program for empirical X-ray absorption correction; Bruker-Nonius, 1990. (11) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7−13. (12) Prokopuk, N.; Shriver, D. F. Inorg. Chem. 1997, 36, 5609−5613.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01042. Further details may be obtained from the Cambridge Crystallographic Data Center on quoting the depository numbers CCDC 1027966 (1), 1027964 (3), 1472152 (4), 1027965 (5), 1472153 (6). Copies of this information may be obtained free of charge from http://www. ccdc.cam.ac.uk. X-ray crystallographic information (CIF) Crystallographic data and refinement details, summary of main geometric parameters with comparison to the published data (PDF)

■

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (N.K.) *E-mail: [email protected]. (M.N.S.) Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS N.K. acknowledges a grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government for the support of the research No. 26248022 (Grant-in-aid for Scientific Research (A)). Also, K.A.B. thanks the Japan Society for the Promotion of Science for a Post-Doctoral Fellowship for Foreign Researchers. The work has been supported by Russian Foundation for Basic Research Grant No. 16-33-60016. 8444

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445

Article

Inorganic Chemistry (13) Prevot, M.; Amela-Cortes, M.; Manna, S. K.; Lefort, R.; Cordier, S.; Folliot, H.; Dupont, L.; Molard, Y. Adv. Funct. Mater. 2015, 25, 4966−4975. (14) Abramov, P. A.; Rogachev, A. V.; Mikhailov, M. A.; Virovets, A. V.; Peresypkina, E. V.; Sokolov, M. N.; Fedin, V. P. Russ. J. Coord. Chem. 2014, 40, 259−267. (15) Kirakci, K.; Fejfarová, K.; Kučeráková, M.; Lang, K. Eur. J. Inorg. Chem. 2014, 2014, 2331−2336. (16) Yoshimura, T.; Umakoshi, K.; Sasaki, Y.; Ishizaka, S.; Kim, H.-B.; Kitamura, N. Inorg. Chem. 2000, 39, 1765−1772. (17) (a) Lide, D. R CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2009. (b) http://www.chemicalbook.com/. (18) Ramirez-Tagle, R.; Arratia-Pérez, R. Chem. Phys. Lett. 2008, 460, 438−441. (19) Kitamura, N.; Ueda, Y.; Ishizaka, S.; Yamada, K.; Aniya, M.; Sasaki, Y. Inorg. Chem. 2005, 44, 6308−6313. (20) Akagi, S.; Sakuda, E.; Brylev, K. A.; Mikhailov, M. A.; Sokolov, M. N.; Kitamura, N. manuscript in preparation. (21) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583− 5590. (b) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444−2453. (c) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952−957. (22) (a) Zietlow, T. S.; Nocera, D. G.; Gray, H. B. Inorg. Chem. 1986, 25, 1351−1353. (b) Yoshimura, T.; Ishizaka, S.; Sasaki, Y.; Kim, H.-B.; Kitamura, N.; Naumov, N. G.; Sokolov, M. N.; Fedorov, V. E. Chem. Lett. 1999, 1121−1122. (c) Gray, T. G.; Rudzinski, C. M.; Meyer, E. E.; Holm, R. H.; Nocera, D. G. J. Am. Chem. Soc. 2003, 125, 4755−4770. (d) Yoshimura, T.; Chen, Z.-N.; Itasaka, A.; Abe, M.; Sasaki, Y.; Ishizaka, S.; Kitamura, N.; Yarovoi, S. S.; Solodovnikov, S. F.; Fedorov, V. E. Inorg. Chem. 2003, 42, 4857−4863.

8445

DOI: 10.1021/acs.inorgchem.6b01042 Inorg. Chem. 2016, 55, 8437−8445