Facile Substitution of Bridging SO - ACS Publications

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Facile Substitution of Bridging SO22− Ligands in Re12 Bioctahedral Cluster Complexes Yakov M. Gayfulin,† Anton I. Smolentsev,†,‡ Svetlana G. Kozlova,†,‡ Igor N. Novozhilov,† Pavel E. Plyusnin,†,‡ Nikolay B. Kompankov,†,‡ and Yuri V. Mironov*,†,‡ †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Acad. Lavrentiev ave., 630090 Novosibirsk, Russia Novosibirsk State University, 2 Pirogova str., 630090 Novosibirsk, Russia

‡

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S Supporting Information *

ABSTRACT: Selective substitution of μ-SO22− groups by either O2− or Se2− ions occurs upon heating the bioctahedral rhenium cluster complex K6[Re12CS14(μ-SO2)3(CN)6] in air atmosphere or in the presence of a Se source, respectively, manifesting the remarkable lability of SO22− ligands bound to a transition-metal cluster. A series of compounds based on the new mixed-ligand anions, [Re 12 CS 14 (μ-O) 3 (CN) 6 ] 6− , [Re12CS14(μ-Se)3(CN)6]6−, and [Re12CS14(μ-O)3(OH)6]6−, were isolated and their solid-state structures were elucidated by single-crystal X-ray diffraction analysis. Along with the previously reported μ-sulfide clusters, the new species constitute a series of rhenium anionic complexes with the common formula [Re12CS14(μ-Q)3L6]6− (Q = O, S, Se, L = CN−; Q = O, S, L = OH−), within which the total charge and number of cluster valence electrons (CVEs) are constant. The article presents insights into the mechanistic and synthetic aspects of the substitution process, and it comprehensively discusses the influence of inner ligand environment on the structure, spectroscopic characteristics, and electrochemical behavior of the novel compounds.

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INTRODUCTION Over the past decades, the achievements of coordination chemistry have been considerably associated with the discovery and investigation of transition-metal clusters. Since 1964, when F. Cotton introduced the term “cluster” for compounds based on a framework of metal atoms linked by metal−metal bonds,1 the chemistry of clusters has developed rapidly, yielding a large number of fascinating compounds with a wide variety of interesting structures, chemical and physical properties, and applications.2 Cluster compounds with chalcogenide inner ligands represent typical examples of the so-called “high-valence clusters”.3 They are formed preferably by 4d and 5d metals of Groups 5−7. In particular, hundreds of rhenium chalcogenide compounds containing the cluster cores with different number of metal atoms, from two to nine, have been synthesized and intensively investigated up to the present time.3,4 Among them, the octahedral chalcogenide clusters with {Re6Q8}2+ (Q = S, Se, Te) cores have attracted much attention, because of the occurrence of potentially useful features, such as rich redox activity, luminescence, liquid crystallinity, and radio-opacity.5 More recently, the bioctahedral cluster anions [Re12(μ6-C)(μ3S)14(μ-S)3(L)6]n− (L = CN−, OH−, Br−, n = 6; L = SO32−, n = 12) were obtained and investigated.6 These anions are built from two {Re6S7L3} octahedral units connected face-to-face by one μ6-C4− and three μ-S2− ligands. In comparison with structurally related octahedral compounds of [Re6Q8L6] type, © 2017 American Chemical Society

the bioctahedral species contain two types of inner ligands (μS2− and μ3-S2−), which predetermines the difference in their properties. Structural, physical, and chemical properties of cluster compounds are strongly dependent on the ligands bound to the metal atoms. Therefore, tuning of the ligand environment constitutes a crucial stage in the development of principally new combinations of electronic structures and geometries, which are essential for the design of molecular building blocks with predefined properties. The terminal ligands (L) are often chemically active and can be substituted by various ions or molecules. In contrast, the inner ligands μ-Q and μ3-Q are strongly bound to the metal core and their replacement cannot be easily realized. The substitution of these sites may change the properties dramatically.7 It was assumed that the μ-S2− ligands within the {Re12CS14(μ-S)3} core may have a higher lability than μ3-S2− ones, because of their lower bonding energy with the Re atoms and high steric accessibility. Indeed, it has been recently shown that μ-S2− ligands can be selectively oxidized upon treatment with aqueous H2O2, forming the μSO 2 2− groups. 8 The resulting anion, [Re 12 CS 14 (μSO2)3(CN)6]6−, belongs to a narrow group of Re clusters containing bridging sulfur oxide ligands.9 Further investigations Received: July 25, 2017 Published: September 22, 2017 12389

DOI: 10.1021/acs.inorgchem.7b01889 Inorg. Chem. 2017, 56, 12389−12400

Article

Inorganic Chemistry

575 (1300, sh), 790 nm (365 mol−1dm3cm−1, sh). Elemental analysis calcd (%) for C7H24N6S14K6O15Re12: C 2.51, H 0.72, N 2.51, S 13.40; found: C 2.37, H 0.85, N 2.38, S 13.24; EDS: K:Re:S = 5.9:12.0:14.2. (Ph4P)6[Re12CS14(μ-O)3(CN)6]·6H2O (1a). Compound 1 (0.1 g, 0.03 mmol) was dissolved in 15 mL of H2O, and then a solution of Ph4PCl (0.12 g, 0.32 mmol) in 15 mL of H2O was added with stirring. The quickly precipitated brown powder was separated by centrifugation, washed with H2O, and dried in air. Yield: 0.14 g (93%). IR (ν, cm−1): 3430 (m, νas(OH)), 3392 (m, νs(OH)), 3050, 2955, 2924, 2855 (m, ν(CH)), 2109 (s, ν(CN)), 1633 (m, δ(HOH)), 1583 (s), 1480 (m), 1435 (vs), 1340 (m), 1313 (m), 1188 (m), 1160 (m), 1105 (vs), 1024 (m), 994 (s), 932 (m), 845 (m), 753 (m), 748 (sh, m, νa(Re−μ-O)), 718 (vs), 685 (vs), 672 (w, νs(Re−μ-O)), 615 (m), 569 (m, δ(Re−μO)), 525 (vs), 421 (m, ν(ReS)). Elemental analysis calcd (%) for C151H132N6P6S14O9Re12: C 35.96, H 2.64, N 1.67, S 8.90; found: C 35.70, H 2.54, N 1.65, S 8.96; EDS: P:Re:S = 6.0:12.0:13.8. K[Cu(NH 3 )(H 2 O) 5 ][Cu(NH3 ) 6 ] 0.5 [Cu(NH 3 ) 4 Re 12 CS 14 (μ-O) 3 (CN) 6 ]· 5H2O (2). A solution of CuCl2·2H2O (10 mg, 0,059 mmol) in 5 mL of aqueous ammonia was layered on a solution of 1 (20 mg, 0,006 mmol) in H2O (5 mL) in a thin glass tube. After 4 days, the brown crystals of 2 were formed on the tube walls. For X-ray diffraction (XRD) study, a few crystals were selected directly from the mother solution. The crystals are quite unstable outside the mother solution, because of the loss of NH3 and H2O at room temperature. Yield: 15 mg (75%). IR (ν, cm−1): 3574 (s, νas(OH)), 3317 (s, νs(OH)), 2145 (m, ν(CN)), 2113 (s, ν(CN)), 1595 (s, δ(HOH)), 1232 (s, δs(HNH)), 964 (w), 891 (w), 746 (m, νa(Re−μ-O)), 679 (s, ρr(NH3), νs(Re−μ-O)), 574 (m, δ(Re−μ-O)), 409 (s, ν(ReS)); EDS: Cu:K:Re:S = 2.7:0.9:12.0:13.9. K6[Re12CS14(μ-Se)3(CN)6]·20H2O (3). K6[Re12CS14(μ-SO2)3(CN)6] (0.4 g, 0.12 mmol), KSeCN (0.5 g, 3.45 mmol) and deoxygenated H2O (0.20 mL) were mixed in the glass ampule. The ampule was filled with Ar, sealed, and heated at 160 °C for 6 h. After cooling, the brown reaction product was washed by stirring with three 15 mL portions of CH3CN. A brown powder of 3 was collected and dried in air. Yield: 0.32 g (71%). IR (ν, cm−1): 3566 (broad, vs, ν(OH)), 2112 (vs, ν(CN)), 1610 (s, δ(HOH)), 1041 (w), 975 (w), 885 (m); UV/vis (H2O): λmax (ε) = 325 (17 200, sh), 365 (12 460, sh), 450 (4940, sh), 525 (2650, sh), 592 (1230, sh), 750 nm (445 mol−1 dm3 cm−1). Elemental analysis calcd (%) for C7H40N6S14Se3K6O20Re12: C 2.27, H 1.09, N 2.28, S 12.12; found: C 2.42, H 0.95, N 2.45, S 11.96; EDS: K:Re:S:Se = 6.1:12.0:13.8:2.8. (Ph4P)6[Re12CS14(μ-Se)3(CN)6]·5H2O (3a). Compound 3a was synthesized by metathesis reaction using 0.1 g (0.027 mmol) of compound 3 following a similar procedure as for 1a. Yield: 0.135 g (96%). IR (ν, cm−1): 3427 (m, νas(OH)), 3377 (m, νs(OH)), 3055, 2957, 2926, 2855 (m, ν(CH)), 2120 (s, ν(CN)), 1626 (m, δ(HOH)), 1585 (s), 1481 (m), 1435 (vs), 1339 (m), 1314 (m), 1186 (m), 1161 (m), 1105 (vs), 1024 (m), 995 (s), 930 (sh, m), 845 (m), 752 (m), 719 (vs), 687 (vs), 615 (m), 525 (vs), 424 (m, ν(ReS)); elemental analysis calcd (%) for C151H130N6P6S14Se3O5Re12: C 34.78, H 2.51, N 1.61, S 8.61; found: C 34.88, H 2.52, N 1.37, S 8.70; EDS: P:Re:S:Se = 6.1:12.0:13.7:2.9. K6[Re12CS14(μ-O)3(OH)6]·12H2O (4). K6[Re12CS14(μ-SO2)3(CN)6] (0.4 g, 0.12 mmol), KOH (1.0 g, 17.86 mmol), and H2O (0.2 mL) was placed in a GC vessel. The vessel was placed into a furnace and heated to 240 °C. After 30 min, the vessel was air-cooled to the ambient temperature. The brown reaction mixture was dissolved in 40 mL of hot H2O; the solution then was filtered, slowly evaporated to a volume at which the crystallization begins (ca. 20 mL), and cooled to room temperature. The remaining solution was removed and the black crystals of 4 were washed with 10 mL of an EtOH/H2O mixture (8/1 (v/v)) and EtOH and then dried in air. Yield: 0.26 g (65%). 17O NMR (D2O, 20 °C, 500 MHz): δ 188.82 (s, terminal OH), 164.26 (s, μ-O); IR (ν, cm−1): 3549 (s, νas(OH)), 3474 (vs, νs(OH)), 1620 (s, δ(HOH)), 1518 (s), 1298 (s), 858 (s), 752 (s, νa(Re−μ-O)), 690 (m, νs(Re−μ-O)), 573 (m, δ(Re−μ-O)), 475 (s), 424 (s, ν(ReS)); UV/vis (H2O): λmax (ε) = 325 (9170, sh), 370 (6000, sh), 417 (3690, sh), 850 nm (205 mol−1 dm3 cm−1). Elemental analysis calcd (%) for CH30S14K6O21Re12: C 0.36, H 0.91, S 13.61; found: C 0.32, H 0.84, S 13.60; EDS: K:Re:S = 6.1:12.0:14.3.

revealed that μ-SO22− ligands can be oxidized by air oxygen or reduced by sulfide and selenide ions to form μ-SO32− or μ-SO2− ligands, respectively, without undesirable destruction of the cluster core.10 It is noteworthy that, during these transformations, the μ-bridging groups retain their formal oxidation state (2−), which causes the resulting anions to be of the same total charge (6−) and the same CVE number (46). Thus, the unprecedented selective reactivity of inner μ-S2− and μ-SO2 2− ligands within the bioctahedral rhenium chalcocyanide clusters has brought us to the detailed study of their chemistry. Based on the [Re12CS14(μ-SO2)3(CN)6]6− anion, we developed a new versatile synthetic route for obtaining the bioctahedral Re clusters with mixed inner ligand environmentsthe compounds which proved to be inaccessible by any of the common approaches. As a result of μ-SO22− ligands substitution, a series of compounds based on the neutral cluster cores {Re12CS14(μ-O)3}0 and {Re12CS14(μ-Se)3}0 was synthesized and characterized. The present study highlights the mechanistic and synthetic aspects of the substitution process and addresses the structural and spectroscopic features of the novel compounds, along with their electrochemical properties. For the sake of completeness, the structure−property relationships in the complete series of anions [Re12CS14(μ-Q)3(L)6]6− (Q = O, S, Se, L = CN; Q = O, S, L = OH) are discussed using the data of X-ray structural analysis, UV/vis spectroscopy, cyclic voltammetry, and DFT calculations.

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EXPERIMENTAL SECTION

Materials and Methods. The starting cluster salt K6[Re12CS14(μSO2)3(CN)6] was prepared using the reported procedure.8 Other reagents and solvents were used as purchased. Elemental analysis was performed on a EuroVector EA3000 analyzer. Fourier transform infrared (FT-IR) spectra in the range of 4000−400 cm−1 were recorded in KBr pellets on a Bruker Scimitar FTS 2000 spectrometer. Energy dispersion spectroscopy (EDS) was performed on a Hitachi TM-3000 electron microscope that was equipped with a Bruker Nano EDS analyzer. UV/vis spectra in the wavelength range of 300−1050 nm were recorded on a PerkinElmer Helios Lambda 3 spectrophotometer. Cyclic voltammetry was performed on a Metrohm Computrace 797 VA voltammetry analyzer, using a three-electrode system that was composed of a glassy carbon (GC) working electrode, a Pt counter electrode, and KCl-saturated Ag/AgCl reference electrode. Investigations were conducted for 1 × 10−3 M solutions of cluster salts in 0.1 M solution of Bu4NClO4 in CH3CN at scan rates of 100 mV s−1. 17O NMR measurements were made on a Bruker Avance III 500 MHz NMR spectrometer at the p/2 pulse duration of 12.6 μs and the relaxation delay of 0.1 s. Tetramethylsilane (TMS) was used as a reference. Synchronous thermal analysis (STA) was made on NETZSCH STA 449F1 Jupiter device combined with a quadrupole mass spectrometer QMS 403 D Aëolos. Syntheses. K6[Re12CS14(μ-O)3(CN)6]·12H2O (1). K6[Re12CS14(μSO2)3(CN)6] (0.4 g, 0.12 mmol), KOH (1.0 g, 17.86 mmol) and H2O (0.2 mL) were placed in a GC vessel. The vessel was placed into a furnace and heated to 220 °C. After 10 min, the vessel was air-cooled to room temperature, then the brown reaction mixture was stirred with EtOH/H2O (8/1 (v/v), 15 mL) for 15 min. After removing the solution, the residual brown oil was dissolved in 20 mL of H2O with the addition of KCN (0.4 g, 6.15 mmol). The resulting solution was filtered, evaporated to a volume at which the precipitation begins (ca. 12 mL), and cooled to room temperature. A brown precipitate of 1 was filtered, washed with 10 mL of a EtOH/H2O mixture (8/1 (v/v)) and EtOH, and dried in air. Yield: 0.35 g (83%). 17O NMR (D2O, 20 °C, 500 MHz): δ 174.07 (s, μ-O); IR (ν, cm−1): 3576 (vs, νas(OH)), 3420 (vs, νs(OH)), 2117 (vs, ν(CN)), 1597 (s, δ(HOH)), 1112 (m), 1039 (w), 972 (w), 883 (m), 750 (m, νa(Re−μ-O)), 675 (w, νs(Re−μO)), 565 (m, δ(Re−μ-O)), 419 (s, ν(ReS)); UV/vis (H2O): λmax (ε) = 325 (21 000, sh) 350 (15 870, sh), 365 (13 700, sh), 440 (4970, sh), 12390

DOI: 10.1021/acs.inorgchem.7b01889 Inorg. Chem. 2017, 56, 12389−12400

empirical formula formula weight, M space group Z a (Å) c (Å) V (Å3) T (K) μMo Kα (mm−1) Dcalc (g cm−3) crystal size (mm) θ scan range (deg) ranges of h, k, and l measured reflns independent reflns observed reflns [I > 2σ(I)] parameters refined goodness of fit, GooF R[F2 > 2σ(F2)] wR(F2) Δρ (e Å−3), min/max

3 C7H40K6N6O20Re12S14Se3 3642.85 P63/mmc 2 11.0727(4) 28.2368(12) 2998.1(3) 150(2) 29.892 4.035 0.08 × 0.08 × 0.02 2.12−27.56 −13 ≤ h ≤ 13; −13 ≤ k ≤ 14; −36 ≤ l ≤ 36 15 127 1362 1195 80 1.067 0.0397 0.1048 −2.556/4.417

2 C7H44Cu2.50KN14O13Re12S14 3413.75 P4/mbm 4 20.0724(7) 14.8049(7) 5964.9(4) 150(2) 25.714 3.801 0.20 × 0.10 × 0.06 1.99−26.37 −24 ≤ h ≤ 19; −23 ≤ k ≤ 25; −18 ≤ l ≤ 18 37 474 3290 2826 184 1.054 0.0355 0.0919 −1.505/5.464

Table 1. Selected Crystallographic Data, Data Collection, and Structure Refinement Details for 2−4 CH30K6O21Re12S14 3296.09 P63/m 2 13.8914(6) 13.9960(8) 2339.0(2) 150(2) 32.122 4.680 0.15 × 0.02 × 0.02 2.23−27.49 −10 ≤ h ≤ 18; −18 ≤ k ≤ 13; −7 ≤ l ≤ 18 8647 1867 1311 105 1.008 0.0490 0.1349 −2.959/2.228

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DOI: 10.1021/acs.inorgchem.7b01889 Inorg. Chem. 2017, 56, 12389−12400

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Inorganic Chemistry

Table 2. Characteristic Interatomic Distances and Re−μ-Q−Re Angles (deg) for [Re12CS14(μ-Q)3(CN)6]6− (Q = O, S, Se) and [Re12CS14(μ-O)3(OH)6]6− Cluster Anions Value/Range; Average bond/angle

[Re12CS14(μ-O)3(CN)6]6−

Reout−Reouta Rein−Rein Rein−Reout Re−Reb Re−μ3-S Re−μ-Q Re−μ6-C Re−CCN Re−OOH

2.5924(7)−2.6017(8); 2.596(4) 2.6725(6)−2.6746(8); 2.6732(8) 2.6244(6)−2.6293(6); 2.627(2) 2.8263(11)−2.8368(8); 2.833(5) 2.402(4)−2.440(3);2.42(2) 2.004(10)−2.041(8); 2.02(2) 2.092(8)−2.102(2); 2.095(5) 2.109(14)−2.121(11); 2.117(6)

Bond Distance (Å) 2.5983(7) 2.5955(10) 2.6919(6) 2.6789(9) 2.6255(5) 2.6203(6) 2.9133(8) 2.9425(10) 2.396(2)−2.423(2) 2.399(3)−2.421(3); 2.410(8) 2.380(3) 2.497(2) 2.1301(4) 2.1346(5) 2.118(11) 2.12(2)

89.7(6)

Bond Angle (deg) 75.47(10) 72.20(7)

Re−μ-Q−Re

[Re12CS14(μ-S)3(CN)6]6−

[Re12CS14(μ-Se)3(CN)6]6−

[Re12CS14(μ-O)3(OH)6]6− 2.5876(11) 2.6807(10) 2.6194(8)−2.6199(8); 2.6197(3) 2.8204(10) 2.393(4)−2.445(4); 2.42(2) 2.053(11) 2.0938(5) 2.078(10) 86.8(6)

Atoms Reout and Rein belong to opposite “outward” and “inward” faces of the {Re6} octahedra, with regard to the μ6-C atom. bDistances between Re atoms in the {Re6C} prism. a

(Ph4P)6[Re12CS14(μ-S)3(CN)6]·5H2O (5). Compound 5 was synthesized by metathesis reaction using 0.1 g (0.028 mmol) of K6[Re12CS14(μ-S)3(CN)6]·20H2O,6a following a similar procedure as for 1a. Yield: 0.135 g (94%). IR (ν, cm−1): 3430 (m, νas(OH)), 3390 (m, νs(OH)), 3053, 2957, 2924, 2855 (m, ν(CH)), 2110 (s, ν(CN)), 1620 (m, δ(HOH)), 1584 (s), 1435 (vs), 1336 (m), 1317 (m), 1188 (m), 1161 (m), 1105 (vs), 1020 (m), 995 (s), 931 (m), 841 (m), 754 (m), 721 (vs), 687 (vs), 615 (m), 524 (vs), 417 (m, ν(ReS)). Elemental analysis calcd (%) for C151H130N6P6S17O5Re12: C 35.74, H 2.58, N 1.66, S 10.74; found: C 35.98, H 2.63, N 1.62, S 10.70; EDS: P:Re:S = 6.1:12.0:16.7. X-ray Crystallography. The collection of single-crystal XRD data for compounds 2−4 was made using a Bruker X8 Apex CCD automatic four-circle diffractometer that was equipped with a 4K CCD area detector (Mo Kα, graphite monochromator, φ and ω scans). The intensities were measured with the standard φ/ω-scan techniques. Semiempirical absorption corrections were made using the SADABS program.11 The structures were solved by direct methods and refined by full-matrix least-squares techniques using the SHELXTL software.12 All non-hydrogen atoms were refined anisotropically. H atoms of NH3 groups in 2 were calculated by geometrical methods and refined as riding; H atoms of OH groups in 4 were located from a difference Fourier map and refined with Uiso(H) = 1.2Ueq(O); H atoms of solvate H2O molecules were not located. Crystallographic data, as well as details of data collection and structure refinement for compounds 2−4, are given in Table 1. Selected bond lengths, in comparison with those in the “parent” anion [Re12CS14(μ-S)3(CN)6]6−, are listed in Table 2. CCDC 1542389−1542391 contains the crystallographic data for compounds 2−4, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/structures. Computational Details. Theoretical calculations were realized in ADF2013 programs for the [Re12CS14(μ-O)3(CN)6]6−, [Re12CS14(μS) 3 (CN) 6 ] 6− , [Re 12 CS 14 (μ-Se) 3 (CN) 6 ] 6− , and [Re 12 CS 14 (μO)3(OH)6]6− cluster anions within spin-restricted formalism.13 Geometry optimizations were done with the Becke−Perdew general gradient approximation (GGA) exchange-correlation density functional (BP86).14 For all elements, a relativistic valence triple-ζ Slatertype basis set with an augmented polarization function (TZP) without core potential was applied. The scalar relativistic effects were taken into account by using a zero-order regular approximation (ZORA)15 Hamiltonian. Calculated vibrational spectra show no imaginary frequencies (see Figures S1−S4 in the Supporting Information). The optimized atomic coordinates are listed in Tables S1−S4 in the Supporting Information. The charges on the atoms were calculated by using the Bader method.16 The experimental values of extinction indicate that the observed electronic transitions in UV/vis spectra are dipole-allowed electronic

transitions. In order to simulate the electronic absorption spectra, the 20 lowest-lying dipole-allowed electronic transitions were computed for the [Re12CS14(μ-O)3 (CN)6 ] 6− , [Re12CS14(μ-Se) 3(CN) 6] 6− , [Re12CS14(μ-O)3(OH)6]6−, and [Re12CS14(μ-S)3(OH)6]6− anions within the time-dependent density functional theory (TD-DFT)17 by employing the LB94 functional18 and the same basis set as that used for geometry optimization. The iterative Davidson method was used.19 Solvent effects on transition energies were calculated with the Conductor like Screening Model (COSMO)20 with water as a solvent, using default surface parameters. The bonding energy, geometry optimization, harmonic frequencies and other parameters which are available within this model were also calculated. The most experimentally distinctive absorption bands at 700−900 nm were analyzed, because only the long-wavelength electronic transitions correlate with those calculated within the one-electron approximation.

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RESULTS AND DISCUSSION Synthesis and Reaction Mechanism. The formation of a brown product was first observed during the heating of violet cluster salt K6[Re12CS14(μ-SO2)3(CN)6] with KOH at 220 °C under an air atmosphere. Isolation and characterization of the reaction product revealed that the original μ-SO22− groups were selectively substituted by O2− ligands to yield the new cluster anion [Re12CS14(μ-O)3(CN)6]6−. To the best of our knowledge, only a limited number of Re clusters (of various nuclearities) with oxo ligands have been reported to date.21 Most likely, this is due to the lack of suitable methods for their synthesis from simple starting materials. A literature survey shows that the observed reaction represents an unprecedented example of the substitution lability of SO22− ligands bound to a transition-metal cluster. It is known that polynuclear and cluster compounds have a tendency to be coordinated by SO2 in bridging μ,η1 or μ,η2 position, where the ligand formally donates two electrons to form covalent bonds with metal atoms.22 As a result, μ-SO2 ligands usually display chemical inertness. In order to shed light on the reaction pathway, a dry powder of K6[Re12CS14(μSO2)3(CN)6] was heated in O2/Ar (20% v/v) atmosphere and the process was investigated using the combination of TG-DSC thermal analysis and evolved gas analysis−mass spectrometry (EGA-MS). During the decomposition, the sample loses the residual H2O (m/z = 18, not shown) and CH3CN (m/z = 12, 15, and 26) molecules in a wide temperature range (up to ∼200 °C) (see Figure 1). In the next stage, at temperatures of 250− 12392

DOI: 10.1021/acs.inorgchem.7b01889 Inorg. Chem. 2017, 56, 12389−12400

Article

Inorganic Chemistry

Figure 2. FT-IR spectra of K6[Re12CS14(μ-SO2)3(CN)6] before heating (violet), after 20 min at 250 °C (green), and after 60 min at 300 °C (red).

and 1117 cm−1 can be attributed to the ν4 and ν3 vibrations of free SO42− ion, while the band at 1034 cm−1 is typical for coordinated SO32− group. The relatively weak band at 745 cm−1 can be assigned to asymmetric stretching vibrations of the Re−μ-O−Re bonds. Upon longer heating at 300 °C, a strong band at 914 cm−1 appears, indicating the formation of ReO4− ions (ν3), as a result of oxidative decomposition of the cluster core. According to the previous study, the μ-SO22− groups are easily converted to μ-SO32− groups (with the formation of Re− O−S−Re bridges) by atmospheric oxygen in aqueous and CH3CN solutions at room temperature.8,10 Moreover, there was a mass-spectrometric evidence for formation of a small amount of SO42−-containing clusters. Combining these data with the current observations, one can suppose that heating of the K6[Re12CS14(μ-SO2)3(CN)6] salt in the presence of oxygen causes oxidation of μ-SO22− groups to μ-SO32−, both in solution and in the solid state. Further disproportionation of μSO32− groups to noncoordinated SO42− ions and sulfur dioxide can proceed as either a solid-state reaction or a melt reaction. This leads to the cluster core decomposition with the formation of ReO4− ions due to the direct oxidation of Re centers by oxygen. An alternative pathway to the decomposition of μSO32− groups is the formation of μ-O2− ion and SO2 molecule, which correlates with the EGA-MS data. This process can produce the μ-O-containing clusters without direct interaction of Re centers with oxygen. In the presence of H2O or/and in an alkaline medium, the formation of μ-O-containing clusters presumably becomes the main process, even at relatively low temperatures, because of the fixation of liberated SO2 as HSO3− and SO32− ions. Therefore, the interaction of μ-SO22− groups with oxygen in an alkaline solution includes the following steps: 1 μ‐SO2 2 − + O2 → μ‐OSO2 2 − 2

Figure 1. Thermogravimetry−differential scanning calorimetry (TGDSC) curves and evolved gas analysis−mass spectroscopy (EGA-MS) of K6[Re12CS14(μ-SO2)3(CN)6] in O2/Ar (20% (v/v)) atmosphere.

400 °C, the sample mass increases. This indicates partial oxidation of the μ3-S2− ligands to oxidized forms, followed by elimination of gaseous SO2, which was detected as a molecular ion at m/z = 64 and a fragment ion at m/z = 48. Above 300 °C, the oxidation of terminal CN− ligands occurs yielding N2, CO, and CO2 (m/z = 12, 28, 44). The most interesting part of the TG curve is the release of gaseous SO2 in a temperature range of 200−300 °C. These temperatures are definitely insufficient to cause the oxidation of μ3-S2− ligands within the cluster core; thus, the formation of a small amount of SO2 can possibly be associated with the partial oxidation of μ-SO22− ligands. In the temperature range of 400−700 °C, the gaseous products (SO2 and CO2) are intensively released, which is accompanied by a small decrease in sample mass (Δm = 6.6%). This indicates completion of the oxidation of μ3-S2− and CN− ligands. However, the Δm value at this stage is much lower than that calculated for the loss of all μ3-S2− and CN− ligands (18.5%), which can possibly be explained by the formation of intermediate rhenium sulfides. The increase of the mass loss rate at 700−900 °C indicates the beginning of the formation of volatile Re2O7. Finally, upon heating from 900 °C to 1000 °C, the sample loses most of its weight by elimination of Re2O7 (and, probably, K2O which vaporizes at ∼900 °C)23 and intensively releases SO2 due to the oxidation of rhenium sulfides. The residual (4.18% at 1010 °C) is most probably composed of nonvolatile K2CO3. Heating of K6[Re12CS14(μ-SO2)3(CN)6] at 220−250 °C in air for 20 min yields brown, partially water-soluble amorphous product. FT-IR spectrum of the product shows the presence of intensive characteristic bands at 618, 1034, and 1117 cm−1 (see Figure 2). According to the literature data,24 the bands at 618

μ‐OSO2 2 − + 2OH− → μ‐O2 − + SO32 − + H 2O

This assumption fully agrees with the literature data on the thermal behavior of inorganic sulfites. In particular, the sulfites are known to decompose via two pathways: (i) with formation 12393

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bond between the Cu2+ ion and one of the μ-O2− ligands of the cluster moiety (Figure 3). The Cu−O bond length of 2.31(2) Å

of a metal oxide and sulfur dioxide, and (ii) with formation of a metal oxide, metal sulfate, and sulfur dioxide.23 In our case, the μ-O-containing clusters formally play the role of oxide species. Apart from oxidation reactions, the K 6 [Re 12 CS 14 (μSO2)3(CN)6] salt was tested for ligand substitution upon treatment with KSeCN (a convenient Se source)25 in the presence of H2O. The reaction afforded the μ-Se-containing product (3) in a good yield. The compound 3 is isostructural with the sulfide analogue K6[Re12CS14(μ-S)3(CN)6]·20H2O, the parent of all {Re12} derivatives. The outstanding benefit of this reaction is the possibility of obtaining the bioctahedral rhenium selenide clusters. Unlike the sulfide analogues, these clusters were inaccessible for a long time, because they cannot be produced by simple high-temperature reactions of ReSe2 or ReS2/ReSe2 mixtures with KCN. In this regard, the facile substitution of μ-SO22− groups in the presence of KSeCN opens the way to the chemistry of new type of bioctahedral rhenium clusters with μ-Se2− ligands. The substitution of terminal ligands is most commonly used to modify the properties of cluster compounds. Accordingly, the ability of a metal cluster to change the terminal ligand environment is an important characteristic. The replacement of CN− ligands usually requires rather hard conditions, such as low-temperature melts or solvothermal synthesis, and can only be conducted with relatively stable clusters. In the case of the [Re12CS14(μ-S)3(CN)6]6− anion, it was shown previously that CN− ligands can be substituted by OH− groups by treatment of the corresponding potassium salt with molten KOH at 300 °C.6b In order to investigate this type of ligand modification, the potassium salts of the new anions [Re 12 CS 14 (μO)3(CN)6]6− and [Re12CS14(μ-Se)3(CN)6]6− (compounds 1 and 3, respectively) were tested in reactions with KOH melts. The reaction of 1 with KOH at 240 °C led to the substitution of all CN− ligands and formation of compound 4 with a high yield. In contrast, a similar reaction conducted with compound 3 resulted in the cluster core decomposition and no cluster products were isolated from the reaction mixture. This fact illustrates that the overall stability of bioctahedral rhenium clusters is significantly dependent on the inner ligand environment. Crystal Structures. Despite the crystalline nature of a bulk of compound 1, we could not succeed in obtaining single crystals, even after several attempts, suitable for X-ray structural analysis. Therefore, in order to structurally investigate the [Re12CS14(μ-O)3(CN)6]6− anion, we employed a different approach. As was shown earlier, the bioctahedral Re cluster anions can often be isolated by crystallization from aqueous ammonia solutions containing the d-metal cations, such as Cu2+,8 Cd2+,10 and Ni2+.26 Following this idea, we used compound 1 in the reaction with Cu(II) ammine complexes, which allowed us to successfully crystallize compound 2. The crystals of compound 3 were formed during the synthesis and obtained directly from reaction mixture. Finally, the crystals of compound 4 were obtained by slow evaporation of its aqueous solution. Structure of K[Cu(NH 3 )(H 2 O) 5 ][Cu(NH 3 ) 6 ] 0 . 5 [Cu(NH3)4Re12CS14(μ-O)3(CN)6]·5H2O (2). Compound 2 crystallizes in the tetragonal crystal system, space group P4/mbm. The most interesting part of the structure is the bulky [Cu(NH 3 ) 4 Re 12CS 14 (μ-O) 3 (CN) 6 ] 4− anion. It has a point symmetry of C2v and is formally built up from two oppositely charged fragments, {Cu(NH 3 ) 4 } 2+ and {Re 12 CS 14 (μO)3(CN)6}6−, which are bound together through the Cu−O

Figure 3. Structure of the [Cu(NH3)4Re12CS14(μ-O)3(CN)6]4− anion in 2. Dashed lines indicate the longer Re−Re bonds. Atom O5 belongs to the half-occupied H2O molecule. Thermal ellipsoids are drawn at the 50% probability level.

is substantially longer than the bonds with NH3 molecules (all of the same length, 2.030(11) Å), which define the equatorial plane of the Cu2+ coordination polyhedron. The remaining Cu2+ coordination site is occupied by the water oxygen atom at a distance of 2.51(5) Å. Overall, the Cu2+ center has a typical distorted six-coordinate geometry. Interestingly, the Cu2+ ions have a prominent affinity for the N-donor ligands in aqueous solutions. However, the formation of 2 demonstrates the possible coordination of Cu2+ ions by oxo ligands, even in the presence of cyanide groups of cluster anions and aqueous ammonia medium. This suggests that sterically available inner ligands can be potentially used as coordination sites for the construction of cluster-based coordination polymers. The geometrical parameters of the [Re 12 CS 14 (μO)3(CN)6]6− fragment can be compared with those of the [Re12CS14(μ-S)3(CN)6]6− anion, which have been determined in many studies and, therefore, are the most reliable. As seen from Table 2, the overall geometry of octahedral subcores shows almost no dependence on the type of μ-bridging ligand. This is best illustrated by the Re−Re and Re−μ3-S bond lengths, which fall into the very narrow intervals. However, the substitution of larger μ-S2− ligands by smaller μ-O2− ones expectedly led to the shortening of the Re−Re distances between {Re6} octahedra by ∼0.1 Å. The mean Re−μ-O bond length is 2.02(2) Å, and the Re−μ-O−Re angle is 89.7(6)°, which are comparable with the corresponding values found in other high-valency Re clusters containing Re−μ-O and Re−μ3O bonds. For example, the Re−μ-O bond lengths in octahedral clusters [Re6(μ-O)12L6] lie in the range of 1.848(5)−2.119(5) Å, while the Re−μ-O−Re angles vary from 82.1(2)° to 92.3(1)°.27 The mean Re−μ3-O distances in several known oxide octahedral clusters are ∼2.09 Å,21 which are somewhat longer than the Re−μ-O values. The cationic part of the structure includes ammine [Cu(NH3)6]2+ and aqua-ammine [Cu(NH3)(H2O)5]2+ complex cations with a typical distorted octahedral geometry; the latter share their H2O ligands with the K+ cations. The structure 12394

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Figure 4. Structures of (a) [Re12CS14(μ-Se)3(CN)6]6− anion in 3 and (b) [Re12CS14(μ-O)3(OH)6]6− anion in 4. Thermal ellipsoids are drawn at the 50% probability level. H atoms of terminal OH− groups are omitted for the sake of clarity.

Electronic Structure. The electronic structures of anions in 2−4 were calculated and then compared with that previously reported for the [Re12CS14(μ-S)3(CN)6]6− and [Re12CS14(μS)3(OH)6]6− anions.6a,b According to the calculations, the bonding energies (ΔE) of the [Re12CS14(μ-O)3(CN)6]6−, [Re12CS14(μ-S)3(CN)6]6−, and [Re12CS14(μ-Se)3(CN)6]6− anions are characterized by the negative values of −314.7, −309.1, and −307.2 eV, respectively. The optimized geometrical parameters (Tables S1−S4) are in good agreement with the experimental data. In the case of three cyanide clusters, the A2′ highest occupied molecular orbitals (HOMOs) consist mainly of p orbitals from μ-Q ligands and 5d orbitals from inner Re atoms (see Figure 5). The respective contribution of p orbitals increases gradually, from ∼32% for Q = O to ∼48% for Q = S and then to ∼56% for Q = Se, which correlates with the increase in the corresponding ionic radii. The contribution of 5d orbitals from Re atoms is ∼31% for all complexes. The

is characterized by significant contribution of hydrogen bonds playing an important role in the lattice stabilization. Structure of K6[Re12CS14(μ-Se)3(CN)6]·20H2O (3). Compound 3 is isostructural with the μ-sulfide analogue, K6[Re12CS14(μ-S)3(CN)6]·20H2O,6a which represents the “parent” of a bioctahedral Re cluster family. It crystallizes in the hexagonal crystal system, space group P63/mmc. The structure can be described as a framework consisting of K+ cations and [Re12CS14(μ-Se)3(CN)6]6− anions (D3h point symmetry; see Figure 4a) interconnected by multiple K−N, K−S, and K−Se ionic bonds of normal lengths. Since most of the K+ cations share their sites with water molecules of crystallization, the structure contains many hydrogen bonds of O−H···N and O−H···O types, stabilizing the crystal lattice. Similarly to the μ-O-containing anion in 2, the overall geometry of the μ-Se-containing anion is only slightly affected by the μligand substitution. Some changes are observed in the central prismatic unit {Re3(μ6-C)(μ-Se)3Re3}, where the inclusion of relatively large selenide ligands resulted in elongation of the Re−Re distances between the octahedral {Re6} units from 2.9133(8) Å to 2.943(1) Å, as compared with the [Re12CS14(μS)3(CN)6]6− anion. The Re−μ-Se bond lengths of 2.497(2) Å are slightly shorter than those found in cluster compounds containing the Re−μ3-Se bonds.28 (A more appropriate comparison with the Re−μ-Se distances cannot be made because of the lacking of literature data.) Structure of K6[Re12CS14(μ-O)3(OH)6]·12H2O (4). Crystals of compound 4 belong to the hexagonal system, space group P63/ m. This compound has also an isostructural μ-sulfide counterpart, K6[Re12CS14(μ-S)3(OH)6]·12H2O.6b Since the replacement of terminal CN− ligands by OH− groups does not affect the cluster core, the geometry of [Re12CS14(μO)3(OH)6]6− anion (C3h point symmetry; see Figure 4b) is very close to that of the [Re12CS14(μ-O)3(CN)6]6− anionic fragment in compound 2 (Table 2). The structure of 4 is an ionic framework composed of K + and [Re 12 CS 14 (μO)3(OH)6]6− ions, in which the latter provide all their μ-O2− and OH− ligands for the formation of bonds with alkali-metal cations. In addition, both μ-O2− and OH− ligands are involved in strong hydrogen bonding with water molecules of crystallization; therefore, the contribution of these noncovalent interactions cannot be excluded.

Figure 5. Frontier orbitals of the [Re12CS14(μ-Se)3(CN)6]6− anion. 12395

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Inorganic Chemistry calculated HOMO−LUMO gaps are 1.36, 1.08, and 0.89 eV for Q = O, S, and Se, respectively. [LUMO = lowest unoccupied molecular orbital.] The A2″ LUMO of these clusters contain the similar set of atomic orbitals with a higher contribution of 5d orbitals from inner Re atoms (40%−50%) and a lower contribution of p orbitals from μ-Q ligands (14%, 26%, and 32% for Q = O, S, and Se, respectively). The bonding energies of the [Re12CS14(μ-O)3(OH)6]6− and [Re12CS14(μ-S)3(OH)6]6− anions are −272.2 and −266.8 eV, respectively. The electronic structures of these anions are similar to those of the cyanide analogues. Particularly, the A′2 HOMO of the [Re12CS14(μ-O)3(OH)6]6− anion consists mostly of 2p orbitals of μ-O (∼40%) and Re 5d orbitals (∼36%). For the [Re12CS14(μ-S) 3(OH)6]6− anion, the corresponding contributions of 3p orbitals from S atoms and 5d orbitals from Re atoms in A2′ HOMO are ∼77% and 19%. The calculated HOMO−LUMO gaps are 1.19 and 0.98 eV for these clusters. As it was shown earlier, the central μ6-C atom in bioctahedral Re clusters is characterized by sp2-hybridization.29 The electron localization function (ELF) maps of the discussed clusters indicate that this hybridization state remains unchanged. The contribution of 2p orbitals of μ6-C atom in HOMO and LUMO is significant, being ∼17% for all clusters. The partial charges of the μ-Q atoms decrease as expected in the Q = O, S, Se row. The corresponding values are given as follows: −1.014 e and −1.022 e for Q = O in the [Re12CS14(μO)3(CN)6]6− and [Re12CS14(μ-O)3(OH)6]6− anions; −0.701 e for Q = S in the [Re12CS14(μ-S)3(CN)6]6− anion; and −0.623 e for Q = Se in the [Re12CS14(μ-Se)3(CN)6]6− anion. Generally, it can be seen that the frontier molecular orbitals are mainly contributed by atomic orbitals from the atoms constituting the {Re3(μ6-C)(μ-Q)3Re3} central structural unit. The contributions of outer Re atoms, μ3-S and terminal ligands are negligible. This feature undoubtedly has a large influence on the chemical and spectroscopic properties of bioctahedral cluster complexes, which distinguishes them from structurally related octahedral clusters with the {Re6Q8} cores. The electronic structures of {Re12CS14(μ-Q)3} core-based complexes change in a rather regular manner, depending on the type of μ-Q ligand. In particular, the contribution of Q atoms in HOMO increases along the O < S < Se row, while the HOMO−LUMO gap gradually decreases. In addition, the partial charges of the Q atoms predictably diminish in the same order. Spectroscopy. The [Re12CS14(μ-Q)3(CN)6]6− anion-based compounds are colored brown or red-brown. Their electronic absorption spectra in aqueous solutions are similar (see Figure 6a). The absorption bands in the wavelength region of 300− 700 nm are represented by a set of shoulders lying at ∼315, 330, 360, 445, 505, and 610 nm. The shoulders are characterized by extinction values gradually decreasing from ∼103 to 102 M−1 cm−1, in order of magnitude. The shape and position of shoulders are similar for all cluster anions, and the type of bridging ligand has significant influence on the intensity of these bands. In the red region, one can see the broad bands of low intensity. In contrast with previously described shoulders, both the shape and intensity of these bands are dependent on the type of ligand Q. According to the calculations, these bands are associated with the allowed electronic transition from HOMO − n to LUMO + m (see Table 3). As mentioned above, the composition of the molecular orbitals is characterized by the presence of p and d

Figure 6. UV/vis spectra of cluster anions: (a) [Re12CS14(μQ)3(CN)6]6− (Q = O, S, Se) and (b) [Re12CS14(μ-Q)3(OH)6]6− (Q = O, S) in aqueous solutions of their potassium salts.

atomic orbitals of μ-Q ligands and inner Re atoms, respectively. Therefore, electronic transitions can be characterized as an electron transfer from the valence p orbitals of the Q atoms to the valence d orbitals of the Re atoms and vice versa; thus, transitions can be assigned to mixed ligand-to-metal (LMCT) and metal-to-ligand (MLCT) charge transfers. Figure 6b shows a comparison of the UV/vis spectra of [Re12CS 14 (μ-O) 3(OH) 6 ]6− and [Re12 CS 14(μ-S)3 (OH) 6]6− anions. One can see that the μ-O-containing anion has a lower extinction coefficient in all wavelength ranges. The characteristic absorption bands are observed at 325, 370, and 700−900 nm. Similar to the μ-S and μ-Se-containing (CN)6 cluster anions, the broad band at 700−900 nm appears to be the most sensitive to the type of μ-bridging ligand, shifting by ∼75 nm to longer wavelengths. The calculations correlate well with experimental data. Note that spectroscopic data available to date for the Re oxoclusters are very limited. The possible reason for this is that the selective synthesis of high-nuclear (octahedral) μ-O-containing clusters was reported only recently,27 while the common routes for obtaining μ3-O-derivatives imply the nonselective partial substitution of chalcogenide or halogenide ligands.21 The IR spectra of 1, 2, and 4 revealed that the Re−O vibrations in all cases appear as three bands in the wavelength regions of 744− 12396

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Table 3. Excitation Energies, Oscillator Strengths (f), and Main Transition Configurations for the Transitions in the LongestWave Absorption Bands for the Cluster Anions Considered Computed at the ZORA/COSMO/TD-LB94/TZP//ZORA/BP86/ TZP Level cluster anion

excitation 6−

[Re12CS14O3(CN)6] [Re12CS14S3(CN)6]6− [Re12CS14Se3(CN)6]6− [Re12CS14O3(OH)6]6− [Re12CS14S3(OH)6]6−

1.41 2.50 1.64 1.33 1.54

eV eV eV eV eV

(880 (495 (794 (933 (807

nm) nm) nm) nm) nm)

f

transition

0.008 0.014 0.011 0.008 0.038

(99.7%) HOMO → LUMO (86%) HOMO − 1 → LUMO + 5; (14%) HOMO − 1 → LUMO (99.5%) HOMO − 3 → LUMO (99.8%) HOMO → LUMO (98.9%) HOMO → LUMO + 2

752 for νa(Re−μ-O), 678−690 for νs(Re−μ-O), and 565−575 cm−1 for δ(Re−μ-O). These values are shifted to the shorter wavelengths, in comparison with the literature data on octahedral clusters [Re6(μ-O)12(3-Mepy)6]0/1+ (701, 681, and 510 cm−1)27a and [Re6Se4O4Cl6]4− (650, 620, and 433 cm−1),21f which correlates with a greater molecular weight of the bioctahedral {Re12CS14(μ-O)3}0 core. The 17O NMR spectrum obtained for 1 in an aqueous solution revealed that the [Re12CS14(μ-O)3(CN)6]6− anion adopts D3h symmetry, with the singlet signal of μ-O atoms observed at 174.07 ppm. As for the [Re12CS14(μ-O)3(OH)6]6− anion in 4, the μ-O and OH signals were found at 164.25 and 188.82 ppm, respectively. The integral ratio of these signals is 1.18:2.00, which is close to the corresponding estimated ratio of 1.00:2.00. Cyclic Voltammetry. The redox properties of octahedral rhenium chalcocyanide clusters are well-studied and worthy of mention. The [Re6Q8(CN)6]4−/3− anions exhibit one fully reversible redox couple in CH3CN with E1/2 = 0.55 vs SCE for Q = S and 0.33 V for Q = Se.4c This couple corresponds to the transition between the diamagnetic, luminescent form with 24 CVE and the paramagnetic, nonluminescent form with 23 CVE. The bioctahedral anions [Re12CS14(μ-Q)3(CN)6]6− can be represented as dimers composed of two octahedral fragments containing 46 CVE. In this work, we examined the redox behavior of [Re12CS14(μ-Q)3(CN)6]n− (Q = O, S, Se) anions in acetonitrile solutions of their salts 1a, 3a, and 5. The voltammogram of [Re12CS14(μ-O)3(CN)6]6− anion displays one quasi-reversible wave with Epa = 0.32 V vs Ag/ AgCl (Figure 7a). This wave indicates, most probably, oneelectron oxidation of the cluster core, followed by its irreversible transformation. In the voltammogram of [Re12CS14(μ-S)3(CN)6]6− anion, a similar, but reversible, oneelectron redox process was found with E1/2 = 0.32 V (ΔE = 120 mV; see Figure 7b). As in the case of [Re12CS14(μO)3(CN)6]6− anion, this wave corresponds to the oxidation and formation of a [Re12CS14(μ-S)3(CN)6]5−/6− couple. In addition, the cyclic voltammogram of the [Re12CS14(μS)3(CN)6]6− anion is framed at both sides by quasi-reversible waves located at E1/2 = −0.78 and 0.60 V (see Figures S5a and S5b in the Supporting Information) and a series of multielectronic irreversible processes. Quasi-reversible waves did not appear if the potential stopped before the irreversible processes, so they are characteristic most probably for the decomposition products. Surprisingly, the CV curve for the [Re 12 CS 14 (μSe)3(CN)6]6− anion is characterized by the absence of reversible processes at positive voltage. Instead, the reversible redox process is observed at E1/2 = −1.42 V (ΔE = 80 mV). This wave corresponds to the one-electron reduction and formation of a [Re12CS14(μ-Se)3(CN)6]6−/7− couple. Cyclic voltammogram at the wider potential range shows the presence

expt 1.57 2.44 1.57 1.46 1.57

eV eV eV eV eV

(790 (508 (790 (850 (790

nm) nm) nm) nm) nm)

Figure 7. Cyclic voltammograms of cluster anions (a) [Re12CS14(μO)3(CN)6]6−, (b) [Re12CS14(μ-S)3(CN)6]6−, and (c) [Re12CS14(μSe)3(CN)6]6− in acetonitrile vs Ag/AgCl electrode.

of multielectron irreversible wave located at Epc = −1.80 and quasi-reversible wave at Epa = 0.50 V (see Figures S5c and S5d 12397

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compounds represent a remarkable class of high-nuclear Re complexes whose chemistry, although not yet developed, is definitely different from that of the easily accessible sulfide {Re12CS14(μ-S)3} analogues. By using a combination of methods, it is found that the nature of inner μ-Q ligand is the key factor influencing the chemical and physical properties of the novel cluster compounds. In particular, the geometrical characteristics, spectroscopic, redox, and chemical properties of [Re12CS14(μ-Q)3(CN)6]6− anions change regularly across the entire series with Q = O, S, Se. The {Re12CS14(μ-Q)3} corebased compounds may be considered as convenient building blocks for the design of cluster-based materials with predefined properties, such as hybrid nanocomposites (copolymers and liquid crystals), nanoparticles, and functional surfaces. We anticipate that the approach developed in this study can find even wider applicationbeing expanded to other transitionmetal clusters, it would undoubtedly enrich the toolbox for selective modification of their inner ligand environments.

in the Supporting Information). The waves have high intensities and are characterized by a pronounced change of their baseline angles. These transitions can be attributed to the multielectron irreversible reduction and oxidation of the cluster anion, accompanied by its decomposition. As one can see, the redox properties of {Re12CS14(μ-Q)3}type anions are highly dependent on the nature of the Q ligand and do not correlate with those of the {Re6Q8}-type octahedral clusters. This feature distinguishes the bioctahedral Re clusters from some related species, e.g., [Co12S16(PEt3)10][TCNQ]2 (TCNQ = tetracyanoquinodimethane),30 [Cr12S16(PEt3)10],31 [Mo12Q16(PEt)10] (Q = S, Se),32 and [Re12Se16(PEt3)8(MeCN)2]4+,33 where dimerization occurs through the formation of {M2Q2} rhomboidal bridges. Furthermore, this finding confirms that delocalization of frontier molecular orbitals occurs between the {Re6} octahedral fragments and these molecular orbitals mainly involve electrons that are shared by atoms of the {Re3(μ6-C)(μ-Q)3Re3} central prismatic unit. The formal oxidation states of the Re atoms in the [Re12CS14(μ-Q)3(CN)6]6− clusters is 3+ for 10 atoms and 4+ for the remaining 2 atoms. In view of the above, one can propose that the Re(IV) atoms are localized in the bridging part and the outer Re atoms keep Re(III) oxidation states during the redox conversions. Thus, the spectroscopic and redox properties of bioctahedral rhenium clusters are determined by the composition and geometrical structure of the metal−metal-bonded (μ6-C)-centered prism. It is known that the first oxidation and reduction potentials correlate with the energies of HOMO and LUMO, respectively.34 Both electronic structures and calculated energy levels of HOMO and LUMO are similar among the series of the [Re12CS14(μ-Q)3(CN)6]6− complexes. The (μ-O) and (μS)-containing complexes display similar redox behavior, notably oxidation at ∼0.3 V, but the redox behavior of (μ-Se)containing cluster is quite different. We propose that the difference may be regarded as being related to structural transformations in the cluster cores. Oxidation of the [Re12CS14(μ-S)3(CN)6]8− anion to the 6− form (from 48 CVE to 46 CVE) causes a dramatic shortening (by ∼0.3 Å) of the distances between two {Re6} fragments due to the strong antibonding character of the Re···Re interactions along the edges of the central prismatic unit.35 Thereby, the reduction of all [Re12CS14(μ-Q)3(CN)6]6− anions should result in a distancing of the {Re6} fragments from each other. This geometrical distortion should be less pronounced in case of the [Re 12 CS 14 (μ-O) 3 (CN) 6 ]6− and [Re 12 CS 14(μ-S) 3 (CN) 6 ] 6− anions, because of the smaller ionic radius of μ-O and μ-S ligands and relatively short characteristic Re−Q bond lengths. In contrast, the large μ-Se ligand may promote the increase of Re···Re distances. This assumption is supported by the fact that reversible reduction of the [Re12CS14(μ-O)3(CN)6]6− anion is not observed and a reversible oxidation wave in the positive potential region for the [Re12CS14(μ-Se)3(CN)6]6− anion is also absent.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01889. Optimized coordinates, energy levels of frontier molecular orbitals, calculated infrared spectra, and the additional CV data (PDF) Accession Codes

CCDC 1542389−1542391 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yakov M. Gayfulin: 0000-0002-6378-0409 Svetlana G. Kozlova: 0000-0001-7114-8676 Pavel E. Plyusnin: 0000-0002-7494-6240 Yuri V. Mironov: 0000-0002-8559-3313 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grant of Russian Foundation for Basic Research (Project RFBR No. 16-33-60046).

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CONCLUSIONS In summary, we developed a new synthetic approach for obtaining the bioctahedral Re clusters with mixed-ligand neutral cores {Re12CS14(μ-Q)3} (Q = O, Se). This approach utilizes the outstanding substitution lability of μ-SO2 ligands within the [Re12CS14(μ-SO2)3(CN)6]6− anion-containing compounds, which makes it possible to replace them selectively under relatively mild conditions. The {Re12CS14(μ-Q)3} core-based 12398

DOI: 10.1021/acs.inorgchem.7b01889 Inorg. Chem. 2017, 56, 12389−12400

Article

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