Octahedral {Ta6I12} Clusters | Inorganic Chemistry - ACS Publications

Jun 24, 2019 - Taisia Sukhikh. [a]. , Enrico Benassi. [d]. ,. Alexandra R. Tarkova. [c] ... 3.359. 3.655. 2.465. 2.576. μ/mm. -1. 15.463. 17.376. 10...
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Article Cite This: Inorg. Chem. 2019, 58, 9028−9035

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Octahedral {Ta6I12} Clusters Maxim V. Shamshurin,*,† Maxim A. Mikhaylov,† Taisia Sukhikh,† Enrico Benassi,‡,§ Alexandra R. Tarkova,∥ Alexey A. Prokhorikhin,∥ Evgeniy I. Kretov,∥ Michael A. Shestopalov,†,⊥ Pavel A. Abramov,†,⊥ and Maxim N. Sokolov*,†,⊥ †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Akad. Lavrentiev Ave., Novosibirsk 630090, Russian Federation Department of Chemistry, Hexi University, Zhangye 734000, People’s Republic of China § Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 10 Tianshui Middle Road, Chengguan Qu, Lanzhou 730000, People’s Republic of China ∥ National Medical Research Center named after E.N. Meshalkin, 15 Rechkunovskaya str., Novosibirsk 630055 Russian Federation ⊥ Novosibirsk State University, 2 Pirogova Str., Novosibirsk 630090, Russian Federation

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‡

S Supporting Information *

ABSTRACT: Ta powder reacts with I2 at 650 °C with the formation of Ta6I14, which belongs to the family of {M6(μ-X)12} clusters. It undergoes aquation with the formation of the intensely colored [Ta 6I 12 (H 2O)6 ] 2+ . The crystal structure was determined for [Ta 6 I 12 (H 2 O) 6 ](BPh 4 ) 2 ·xH 2 O (Ta−Ta 2.9322(6) Å, Ta−I 2.8104(7) Å, Ta−O 2.3430(5) Å). With DMF, [Ta6I12(DMF)6]I2· xDMF was isolated (Ta−Ta 2.9500(2) Å, Ta−I 2.8310(4) Å, Ta−O 2.2880(7) Å). Cyclic voltammetry of [Ta6I12(H2O)6]2+ shows two consecutive quasi-reversible one-electron oxidations (E1/2 0.61 and 0.92 V vs Ag/AgCl). Reaction of Ta6I14 with Bu4NCN yields (Bu4 N) 4 [Ta 6 I12 (CN) 6 ]·xCH3 CN (Ta−Ta 2.9777(4) Å, Ta−I 2.8165(6) Å, Ta−C 2.2730(7) Å). Quantum chemical calculations reproduce well the experimental geometry of the aqua complex and show the essentially Ta-centered nature of both the HOMO and LUMO. The long-term stability of [Ta6I12(H2O)6]2+ solutions can be greatly enhanced in the presence of polystyrenesulfonate (PSS), which forms nanoparticle associates with the aqua complex in water (ca. 1 cluster per 3 PSS monomeric units).

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INTRODUCTION Halide-bridged octahedral clusters of group 5 heavier transition metals (Nb, Ta) consist of six metal atoms, octahedrally arranged by multicenter metal−metal bonding, with a halide bridge along each of the 12 octahedral edges, thus giving a {M6X12} cluster core, plus one terminal ligand at each metal, leading to discrete species [M6X12L6]z, where L may or may not be the same as X (Figure 1a). Typically, X is either Cl or Br. Despite the low formal oxidation states of the metal (ranging from 2.33 to 2.67) and high oxophilicity of Nb and Ta, these clusters are remarkably robust toward hydrolysis and air oxidation. The chemistry of these clusters is just over 100 years old1 and is currently experiencing renewed interest due to the accessibility of several oxidation states of the cluster cores which can be exploited for the creation of various chargetransfer salts, chemically modified surfaces, and extended functional solids. Modern developments have been repeatedly reviewed in various depths.2−4 Recently the scope of this chemistry has been expanded by the introduction of methoxide, CH3O−, as bridging ligands, as well as by use of organic solvents and ionic liquids as reaction media.5−8 Chihara and Kamiguchi have © 2019 American Chemical Society

shown that solid halide clusters of Nb and Ta, taken as [M6Cl12(H2O)4Cl2], are strong Brønsted acids and catalyze isomerization of olefins, dehydrohalogenation of alkyl halides, ring-attachment isomerization and dealkylation of diethylbenzenes, dehydration of alcohols, alkylations, heterocyclizations, and other organic reactions at elevated temperatures.9,10 Within the scope of cluster materials, tetrathiafulvaleniumbased organic cation radicals form salts with paramagnetic [Nb6Cl18]3−, in which there is mixing of the spins of the cluster and cation.11,12 [Nb6Cl12Br6]3−, when it is absorbed on gold layers, transfers the electron density from the cluster to the metal surface, while solutions of [Nb6Cl18]3− are capable of dissolving gold films.13 These and other findings demonstrate that these clusters can efficiently modify gold and silver surfaces. A binary cluster tantalum chloride, Ta 6 Cl 1 5 = [Ta6Cli12Cla‑a6/2], is capable of reversible Li uptake to form LiTa6Cl15; the electrochemical system is stable over at least 1500 cycles.14 The photophysical and redox properties of such Received: February 14, 2019 Published: June 24, 2019 9028

DOI: 10.1021/acs.inorgchem.9b00364 Inorg. Chem. 2019, 58, 9028−9035

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Inorganic Chemistry

discrete cluster complexes, [Ta6I12(H2O)6]2+, [Ta6I12(DMF)6]2+, and [Ta6I12(CN)6]4−, by cluster excision reactions. These compounds can serve as convenient entries into unexplored areas of {Ta6I12} clusters.

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EXPERIMENTAL SECTION

General Remarks. All reagents were obtained from commercial sources and used without additional purification. IR spectra were recorded on a IFS-85 Bruker spectrometer (4000−400 cm−1) and on a VERTEX 80 Fourier spectrometer (600−100 cm−1). UV/vis spectra were recorded on a Cary 60 UV−vis spectrophotometer (Agilent). Elemental analysis was carried out by the analytical service of the Nikolaev Institute of Inorganic Chemistry (Novosibirsk) on a Euro EA 3000 spectrometer. The size and morphology of Ta 2 O 5 nanoparticles were characterized by transmission electron microscopy (TEM) using a Libra 120 instrument (Zeiss). Thermogravimetric Analysis. Synchronic thermal analysis (STA) were carried out on a STA 449F1 Jupiter instrument (NETZSCH, Germany). The measurements were made in a synthetic air flow (80% vol Ar, 20% vol O2) in the temperature range of 30− 1200 °C using a heating rate of 10 °C min−1, a gas flow rate of 20 mL min−1 Ar and 5 mL min−1 O2, and open Al2O3 crucibles. Cyclic Voltammetry. Cyclic voltammetry was run on VA 797 Computrace equipment from Metrohm, in a three-electrode cell. The working electrode was Pt with a butt-end working surface of 3 mm in diameter, and a Pt wire was auxiliary electrode. As a reference electrode a saturated AgCl electrode was used, connected to the working solution with a key. A 10 mL electrolytic cell was purged with a stream of Ar before each measurement. Dry acetonitrile and Bu4NClO4 (Sigma-Aldrich) were used as the solvent and supporting electrolyte, respectively. Cyclic voltammograms were recorded at scan speeds ranging from 0.1 to 1.5 V/s. ESI-MS. Atmospheric pressure chemical ionization (APCI) mass spectra were recorded on an Agilent 6130 Quadrupole MS fitted with a multimode ionization source (MM). The MM was tuned for atmospheric pressure chemical ionization. The optimized parameters of the ion source were as follows: mobile phase (solvent), HPLC grade acetonitrile at a flow rate 0.4 mL/min; drying gas temperature, 350 °C; drying gas flow, 5 L/min; vaporizer temperature, 250 °C; nebulizer pressure, 20 psig; capillary voltage, 2000 V; charging voltage, 2000 V. The analysis was performed in the m/z range 300− 3000 Da; negative and positively charged ions were observed simultaneously using the SCAN mode. Analyte solutions in acetonitrile (5 μL, concentration ∼10−4 g/mL) were introduced into the mobile phase. The chromatographic system of the mass spectrometer did not use a separation column. Instead, a filter column (ZORBAX Eclipse Plus-C18 4-Pack Narrow Bore Guard Column 2.1 × 12.5 mm 5 μm) was used to protect against large particles. In other words, only a mass spectrometric analysis was performed (without chromatographic separation). XRD. The diffraction data were collected on a Bruker Apex Duo diffractometer with Mo Kα radiation (λ = 0.71073 Å), by doing φ and ω scans of narrow (0.5°) frames at 150 K. The structure was solved by direct methods and refined by full-matrix least-squares treatment against |F|2 in anisotropic approximation with SHELX 2017/126 programs set in Olex 2 (version 1.2; compiled 2018.05.29 svn.r3508 for OlexSys, GUI svn.r5506).27 Absorption corrections were applied empirically with the SADABS program.28 Crystallographic data and refinement details are given in Table S1. The main geometric parameters are summarized in Table S2. Further details may be obtained from the Cambridge Crystallographic Data Center on quoting the depository numbers CCDC 1885843 (3), 1885845 (4), 1885844 (5), and 1885846 (6). Copies of this information may be obtained free of charge from http://www.ccdc.cam.ac.uk. X-ray Computed Tomography Radiopacity Studies. The radiopacity investigations for freshly prepared, 1 week old, and 2 month old solutions of compound 5 were performed using a Toshiba Aquilion 64 CT scanner with CT-scan parameters 120 kV and 40 mA. A 0.4 mL portion of a 3.75 × 10−3 M stock solution of the clusters

Figure 1. Views of (a) generic [M6X12L6] cluster complexes, (b) [Ta6I12(H2O)6]2+ (in 2 and 3), (c) [Ta6I12(DMF)6]2+ (in 4), and (d) [Ta6I12(CN)6]4− (in 5 and 6).

clusters are of considerable interest for converting solar energy to chemical energy. Thus, irradiation of {Ta6Br12}2+ clusters with visible light in acidic solutions leads to photooxidation of the cluster into {Ta6Br12}3+ and to stoichiometric hydrogen production.15 Ta6Br14·7H2O, whose {Ta6Br12}2+ core has a high electron count, demonstrates, at higher X-ray energies, multiplied X-ray attenuation relative to organic iodinated contrast agents and thus is a potential radiographic contrast agent.16,17 It is used for phase determination of isomorphous protein derivatives in biomacromolecular crystallography, and this use is to be increased as larger structures and assemblies (membrane proteins, ribosomes, proteasomes) are studied; for example, it was used to determine the structures of RuBisCO and transcetolase, proteasome core complex, RNA polymerase, and cytokinin-specific binding protein (CSBP).18−23 From this point of view, the {Ta6I12} clusters certainly deserve attention, since the unique combination of 18 heavy elements in a single cluster core offers unique possibilities of making new X-ray contrast agents with high radiopacity. Previous works reported only the preparation and structural characterization of Ta6I14, while its reactivity remained unexplored. Apparently, it was first prepared in 1939 by disproportionation of in situ made TaI5 with Ta at 500 °C in low yield, as a black powder. Although no chemical composition was given, the product reportedly produced relatively stable green solutions in water, highly suggestive of [Ta6I12(H2O)6]2+, characterized in the present work (vide infra).24 Individual Ta6I14 was prepared in 1965 using the reduction of TaI5 with Ta or Al. It is isotypic with Nb6Cl14 and can be described, using Schäfer notation, as [{Ta6Ii10Ii‑a2/2}Ia‑i2/2Ia‑a4/2].25 It dissolves in acetone to give green solutions, but this, to the best of our knowledge, is about all that has been known regarding the solution chemistry of this Ta iodide prior to our work. In this contribution we report a straightforward preparation of Ta6I14 and its conversion into a useful series of 9029

DOI: 10.1021/acs.inorgchem.9b00364 Inorg. Chem. 2019, 58, 9028−9035

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Inorganic Chemistry was (3 × 2)-fold diluted and placed in a flat-bottomed cell culture multiwell plate (0.3 mL) alongside pure H2O, as a negative radiolucency reference. Synthesis. Ta6I14 (1). A stoichiometric mixture of Ta powder (1.83 g, 10.1 mmol) and freshly sublimed crystalline I2 (3.00 g, 11.8 mmo;, atomic ratio Ta:I = 6:7) were heated in an evacuated quartz tube for 2 h at 450 °C, and then the temperature was raised to 655 °C and the heating was continued for 70 h. The furnace with the tube was cooled at a rate of 20 °C/h. The tube was opened carefully. The product was a dark violet solid mass, which according to XPRD contained mainly of Ta6I14 with some TaOI2 as a minor phase. This product was used in subsequent reactions without purification. IR (4000−100 cm−1): 180 s; 147 s.29 [Ta6I12(H2O)6]I2·x14H2O (2). To Ta6I14 (4.83 g) in a conical flask was added 200 mL of cold (≤15 °C) water. The flask was stoppered with a rubber cork and was sonicated in a supersonic bath for 3 h. The green suspension thus obtained was carefully decanted into an open 3 L beaker. The remaining solid in the same flask was sonicated again in 350 mL of water, to give a dark green solution. The solution was transferred into the same beaker, and the remaining solid was repeatedly sonicated until a new portion of water did not produce a green coloration. The united extracts were filtered through a thick filter paper, to produce 2.5 L of a dark-green solution. The solution was rotavaporated; it is important to keep the temperature in the bath below 40 °C. A dark green powder was obtained. Yield: 4.15 g (86%). EDX Ta:I = 6.0:14.7 (atomic ratio). IR (4000−100 cm−1): 3561 s; 3262 m; 3144 w; 1638 s; 1589 s; 1234 m; 1152 m; 868 w; 644 m; 498 w; 447 w; 184 s; 148 s. UV−vis, H2O: ε(640 nm) = 3230 L/(mol cm), ε(749 nm) = 2600 L/(mol cm). [Ta6I12(H2O)6](BPh4)2 (3). To 2 mL of a saturated aqueous solution of [Ta6I12(H2O)6]I2·x14H2O was addeda solution of NaBPh4 (2 equiv) in 0.5 mL of water. The resulting solution was kept in a closed vial at 4 °C. Dark green crystals of [Ta6I12(H2O)6](BPh4)2, suitable for X-ray analysis, were collected in 4 days. IR (KBr, 4000−400 cm−1): 3451 s; 2887 s; 2824 w; 2106 s; 1969 w; 1628 m; 1469 s; 1452 s; 1351 s; 1284 s; 1249 s; 1105 s; 960 s; 836 s; 528 w; 465 w. [Ta6I12(DMF)6]I2·DMF (4). In a flask, Ta6I14 was extracted with 300 mL of DMF upon stirring at room temperature for 2 days. To the filtered green extract was added an equal volume of diethyl ether, followed by addition of 60 mL of acetone. The resulting mixture was kept in a stoppered flask at 4 °C. A crop of dark green crystals, suitable for X-ray analysis, was collected in 1 day. Yield: 48%. Anal Calcd for Ta6I14(C3H7NO)7: C, 7.5; H, 1.46; N, 2.9. Found: C, 7.8; H, 1.6; N, 3.1. IR (4000−400 cm−1): 3130 w; 1638 s; 1490 m; 1428 s; 1362 s; 1245 w; 1115 m; 1060 w; 683 s; 416 w. (Bu4N)4[Ta6I12(CN)6]·CH3CN (5). To 250 mg of Ta6I14(0.095 mol) in a glass tube was added 310 mg (1.14 mmol) of Bu4NCN, dissolved in 2 mL of freshly distilled CH3CN. The tube was evacuated, sealed, and kept for 12 h at 100 °C. After the tube was cooled and opened, a dark green solution was filtered from a dark solid. Slow diffusion of diethyl ether vapors yielded dark green crystals, suitable for X-ray analysis, overnight. Yield 0.195 g, 51%. IR (4000−100 cm−1): 3483 m; 3416 m; 2957 s; 2870 s; 2100 s; 1630 m; 1465 s; 1379 s; 1321 w; 1242 w; 1166 m; 1150 m; 1107 m; 1062 m; 1028 m; 921 w; 879 s; 793 w; 737 s; 673 w; 530 w; 386 w; 326 s; 183 s; 142 s; 106 w. Anal. Calcd for C72H147I12N11Ta6: C, 22.9; H, 3.92; N, 4.08. Found: C, 24.0; H, 4.1; N, 3.9. (Ph4P)4[Ta6I12(CN)6] (6). A solution of KCN (2.492 g, 0.038 mol) dissolved in 600 mL of water was added to 2500 mL of a freshly prepared 4 mM solution of [Ta6I12(H2O)6]I2·x14H2O. The resulting solution was stirred in air at room temperature for 4 h, followed by addition of a saturated aqueous solution of 1.254 g of Ph4PBr (0.003 mol). A green precipitate separated, leaving a virtually colorless solution. The precipitate was collected by centrifugation (15 min, 7000 rpm) and redissolved in 500 mL of CH3CN, and this solution was filtered. Slow diffusion of diethyl ether vapors in a closed box into the filtrate yielded black-green crystals of the product, suitable for Xray analysis. Yield: 1.191 g, 19%. IR (KBr, 4000−400 cm−1): 3410 w; 3053 w; 2110 m (CN); 1626 w; 1584 w; 1482 m; 1435 s; 1337 w; 1314 w; 1187 w; 1163 w; 1107 w; 1027 w; 995 m; 850 w; 753 s; 722

s; 688 s; 637 w; 526 s. Anal. Calcd for C102H80I12N6P4Ta6: C, 29.7; H, 1.9; N, 2.0. Found: C, 29.3; H, 1.9; N, 2.1. [Ta6I12(H2O)6]@PSS (7). A 250 mL portion of an aqueous solution of [Ta6I12(H2O)6]I2 (0.0005 mol/L) was mixed with 120 mg of PSS (2200 kDa), and this suspension was stirred for 6 h to achieve complete solubilization of PSS. Ethanol (∼90 mL) was added, causing rapid formation of a colloid solution (Tindal effect). The solution was centrifuged (15 min, 16000 rpm) to give a green precipitate, which was separated and air-dried for 2 days. Anal. Calcd for {Na[Ta6I12(H2O)6](C8H7SO3)3}n: C, 8.82; H, 1.01; S, 2.94. Found: C, 8.7; H, 1.2; S, 2.8. EDX: Ta/I = 5.6/12. Computational Details. The cluster [Ta6I12(H2O)6]2+ was computationally investigated at three different levels of theory: Hartree−Fock self-consistent field (HF-SCF), second-order Møller− Plesset (MP2) perturbation theory, and density functional theory (DFT). The triple-ζ Ahlrichs Def2-TZVPP30 basis set was used; the choice of basis set was driven by previous studies31 on Ta compounds. For the DFT calculations, five different exchange-correlation functions were tested: B3LYP,32 CAM-B3LYP,33 PBE0,34 M06-2X,35 and ωB97X[D].36 Empirical dispersion was included by mean of Grimme dispersion with Becke−Johnson damping (GD3BJ)37 along with the B3LYP, CAM-B3LYP, and PBE0 functionals. Geometry optimization was performed without symmetry constraints in both the gas and solvated phase (water), followed by frequency calculation (in harmonic approximation). No negative frequency was found. Thermochemical quantities (at T = 298.15 K and p = 1 atm) and IR intensities were also computed. Solvent effects were taken into account via the implicit polarizable continuum model in its integral equation formalism (IEF-PCM).38 For geometry optimizations and frequency calculations, the PCM molecular cavity was built according to the universal force field (UFF)39 radii within the value used in the last implementation of the PCM (based on a continuum surface charge formalism). The standard values for dielectric constant and refractive index were always assumed. The UV−vis absorption spectrum for the equilibrium geometry optimized at the DFT M06-2X/Def2-TZVPP level was calculated using time-dependent density functional theory (TD-DFT) accounting for S0 → Sn (n = 1−30). The energy of the first 30 triplet states was also computed. The nature of the vertical excited electronic state was analyzed both under vacuum and in the solvated phase. This investigation was performed by employing the long-range corrected functional ω-B97-X[D] coupled with the Def2-TZVPP basis sets. For the solvated phase, state-specific SS treatment of the solvent effects was also considered, within both the nonequilibrium and equilibrium solvation regimes, along with the linear-response approach (UFF parametrization for the construction of the PCM cavity). The integration grid for the electronic density for topological and RDG analysis was set to 150 radial shells and 974 angular points. For the rest of the calculations, the integration grid was set as 99 radial shells and 590 angular points. The convergence criteria of the selfconsistent field were set to 1 × 10−12 for the RMS change in the density matrix and 1 × 10−10 for the maximum change in the density matrix. The convergence criteria for optimizations were set to 2 × 10−6 au for maximum force, 1 × 10−6 au for RMS force, 6 × 10−6 au. for maximum displacement, and 4 × 10−6 au for RMS displacement. All calculations were performed using the GAUSSIAN G16.A03 software package.40

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RESULTS AND DISCUSSION Synthesis. The standard procedure for preparation of Ta6I14 is based upon reduction of TaI5 with Ta at high temperature; use of a temperature gradient allows the preparation of X-ray-quality single crystals. To avoid manipulations with highly moisture sensitive TaI5, the synthesis is run in a three-section ampule using a special furnace to provide a different temperature for each section. The reaction is complete in 1 week.25 We have simplified the preparation, running firsta reaction between Ta and I2 at 400 9030

DOI: 10.1021/acs.inorgchem.9b00364 Inorg. Chem. 2019, 58, 9028−9035

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Table 1. Comparison between Computed and Experimental (SCXRD) Bond Distances in [Ta6I12(H2O)6]2+a

a

interatomic distance

computed

experimentalb

Ta−Ta Ta−I Ta−O

2.92016 ± 0.00603 2.85874 ± 0.00967 2.39108 ± 0.00221

2.9322(6) 2.8104(7) 2.3430(5)

Data are reported in units of 10−10 m. b[Ta6I12(H2O)6](BPh4)2

changes its coordination environment from six I ligands (all bridging, Ta6I14 is a halide bridged three-dimensional cluster coordination polymer, as indicated by the above Schäfer notation) to six terminal water molecules. Care should be taken at this stage to keep the solution as cold as possible; attempts to accelerate the aquation by heating lead to rapid decoloration and formation of hydrated Ta2O5. We successfully used sonication to assist in the formation of [Ta6I12(H2O)6]2+. From these solutions, a green solid, [Ta6I12(H2O)6]I2·x14H2O (2), can be isolated. Crystalline [Ta6I12(H2O)6](BPh4)2·xH2O (3) can be obtained from these solutions by introduction of NaBPh4. Another strongly coordinating solvent, DMF, dissolves Ta6I14 with the formation of air-stable green solutions, from which single crystals of [Ta6I12(DMF)6]I2·xDMF (4) were isolated and structurally characterized. Cluster excision from Ta6I14 is also efficient with Bu 4 NCN, which in CH 3 CN gives green crystals of (Bu4N)4[Ta6I12(CN)6]·xCH3CN (5) in acceptable yield. In a more cumbersome procedure, [Ta6I12(H2O)6]I2·x14H2O reacts with KCN and PPh4Br to give (Ph4P)4[Ta6I12(CN)6] (6) in a low yield; both cyanide salts were obtained as single crystals and structurally characterized.

Figure 2. Time-dependent change in optical density of 2 and 5. Initial concentrations: for 2, 0.3117 M; for 7, 0.3402 M.

°C to ensure the formation of TaI524 and then raising the temperature to 655 °C, where excess Ta reduces TaI5 into Ta6I14.25 Our tests show that, under these conditions, a 90 h heating span suffices to produce Ta6I14 (1) on a preparative scale: out of the solid thus produced at least 86% can be extracted into water to give [Ta6I12(H2O)6]2+ with a limit of concentration equal to 0.004 mol/L. According to XRPD, the rest is composed mainly of TaOI2. Ta6I14 slowly aquates with the formation of the cluster aqua complex [Ta6I12(H2O)6]2+, producing intense dark green solutions. The aquation is a classical excision reaction, in which the cluster core {Ta6I12}

Figure 3. (a) TEM image of a 2 week old solution of 7. (b) CT image for solutions of 7, iohexol, and water. (c) Dependence of radiopacity vs concentration for differently aged solutions of 7. (d) Dependence of radiopacity vs concentration for Iohexol. 9031

DOI: 10.1021/acs.inorgchem.9b00364 Inorg. Chem. 2019, 58, 9028−9035

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Figure 4. Comparison between computed and experimental IR spectra. No scaling factor was applied.

The relative ease of ligand substitution in the [Ta6I12L6] series (L = H2O, CH3CN, DMF, CN−) can be rapidly assessed from electrospray mass spectrometry. For a sample of [Ta6I12(H2O)6]I2·x14H2O dissolved in CH3CN, only signals from [Ta6I12(CH3CN)6]2+ (m/z 1427.4) and [Ta6I12(CH3CN)5]2+ (m/z 1406.8) appear, indicating rapid and complete ligand exchange. In contrast, solutions of [Ta6I12(DMF)6]I2 and (Ph4P)4[Ta6I12(CN)6] in the same solvent give the expected peaks of their respective cluster units: [Ta6I12(DMF)4]2+ (m/z 1451.2), [Ta6I12(DMF)3]2+ (m/z 1413.6), [Ta6I12(DMF)2]2+ (m/z 1377.2), and {(Ph4P)[Ta6I12(CN)6]}2−(m/z 1552.3). In the first case the ligand is eliminated during the mass spectra recording and in the last case the parent [Ta6I12(CN)6]4− undergoes one-electron oxidation under the experimental conditions. Thus, at least qualitatively, the ease of ligand substitution decreases in the order H2O > CH3CN > DMF > CN. Crystal Structures. The view of the aqua complex as found in [Ta6I12(H2O)6](BPh4)2·xH2O (3) is shown in Figure 1b. Twelve Ta−Ta bonds fall within a narrow limit of 2.9221(6)− 2.9432(6) Å. This can be compared with the 2.8848(3)− 2.8984(3) Å range reported for [Ta6Br12(H2O)6](BPh4)2· x4H2O.41 This difference reflects a well-known matrix effect expansion of the M6 octahedron when the electronegativity of halide bridges decreases and their size increases.2 Small variations in the Ta−I (2.7934(7)−2.8325(7) Å) and Ta−O (2.315(5)−2.366(5) Å) bond lengths indicate negligible structural distortion. In [Ta6Br12(H2O)6](BPh4)2·x4H2O the Ta−O distances (2.245(6)−2.335(7) Å; average 2.29[5] Å) are generally shorter, reflecting the weaker matrix effect of 12 bridging bromide ligands.41 Probably lengthening of the Ta−O bonds in 3 permits an avoidance of overlap between the bridging and terminal ligands and eases the strain within the overcrowded cluster framework. It is interesting to compare the geometry of the {Ta6I12O6} core in 3 and in the DMF complex [Ta6I12(DMF)6]I2·xDMF (4) (Figure 1c). In the latter, only the Ta−I distances (2.7992(6)−2.8346(6) Å)

Figure 5. (a) Scheme of frontier molecular orbitals calculated at the DFT M06-2X/Def2-TZVPP level and isosurface plot with |isovalue| = 0.02 au. (b) Comparison between computed and experimental UV− vis spectra.

remain virtually the same after substitution of DMF for H2O, while the Ta−O (2.225(5)−2.253(6) Å) and Ta−Ta (2.9416(4)−2.9637(5) Å) are appreciably shorter and longer, respectively. A shorter Ta−O distance in the case of DMF agrees with its higher donor number (26.6 vs 18.0 for water).42 Being a stronger donor, DMF increases electron density on the d orbitals and, in accordance with ligand field theory, raises the energy of d orbitals and reduces their M−M bonding character. Unfortunately, this observation can not be generalized for other cluster Nb and Ta halides. The whole series [M6X12(DMF)6]X2 (M = Nb, Ta; X = Cl, Br) was reported, but its characterization was limited to elemental analysis and IR spectra.43 Introduction of a very strong donor, the negatively charged CN− ligand (Figure 1d), causes even more significant lengthening of the Ta−Ta bonds (2.9745(5)− 2.9951(5) Å in 5 and 2.9777(4)−2.9986(4) Å in 6). Again, the Ta−I bonds are hardly affected, falling virtually in the same range as in 3 and 4. For comparison, in (Me4N) 4[Ta 6Cl12(CN) 6] the Ta−Ta bond length was reported to be 2.8894(5) Å (average), and the Ta−C bond length was 2.258(10) Å (average).44 These values again reflect a matrix effect: going from X = Cl to X = I in the 9032

DOI: 10.1021/acs.inorgchem.9b00364 Inorg. Chem. 2019, 58, 9028−9035

Article

Inorganic Chemistry [Ta6X12(CN)6]4− family significantly lengthens the Ta−Ta bond by 0.1 Å and, to a lesser degree, the Ta−C bond (2.26− 2.28 Å in 5 and 6). Electrochemical Studies. Cyclic voltammetry of 2 in water shows two consecutive quasi-reversible waves corresponding to two one-electron redox processes among [Ta6I12(H2O)6]2+, [Ta6I12(H2O)6]3+, and [Ta6I12(H2O)6]4+. The first wave has E1/2 = 0.61 V and the second wave 0.92 V (vs Ag/AgCl). Both values reflect the unexpectedly high oxidation stability of the {Ta6I12}2+ state. The {Ta6Br12} clusters are easier to oxidize: E 1 / 2 va lue s fo r [Ta6Br12(H2O)6]3+/[Ta6Br12(H2O)6]2+ and [Ta6Br12(H2O)6]4+/[Ta6Br12(H2O)6]3+ are 0.35 and 0.65 V, respectively (vs SCE). The {Ta6Cl12} clusters are easiest to oxidize in the series, with E1/2= 0.25 and 0.59 V (vs SCE) for [Ta6Cl12(H2O)6]3+/[Ta6Cl12(H2O)6]2+ and [Ta6Cl12(H2O)6]4+/[Ta6Cl12(H2O)6]3+, respectively. Thus, [Ta6I12(H2O)6]2+ can coexist with [Ta6Br12(H2O)6]3+ and [Ta6Cl12(H2O)6]4+. With their easier oxidation with an increase in the electronegativity of the bridging halides the {Ta6X12} clusters show a tendency exactly opposite to that of the halide-capped {M6X8} clusters (M = Mo, W; X = Cl, Br, I). For the electron-precise {M6X8} clusters, the E1/2 values systematically decrease on going from Cl to I. This difference reflects the different natures of the HOMO orbitals in both cluster families and different contributions of the M and X AOs to the HOMO orbitals. Since compound 2 contains 6 tantalum and 14 iodine atoms (i.e. 20 heavy atoms) and is soluble in water, it is promising in terms of X-ray contrast properties. However, the aqua complex [Ta6I12(H2O)6]2+ in unstable in water solutions in air and decays by oxidation in several days (in about 2 weeks the concentration decreases by 1 order of magnitude), with the formation of hydrated Ta2O5 and HI. During the decomposition of the aqua cluster the pH changes from 3.09 to 2.71. No polyiodides were observed. Solutions of 4−6 in organic solvents are more stable; organic solvents are incompatible with biomedical applications. In order to increase the stability of the aqua complex in water, a highly water-soluble anionic polymer, namely sodium polystyrenesulfonate (PSS), was used to bind and protect the [Ta6I12(H2O)6]2+ units. This approach was successfully used to maintain easily hydrolyzable hexamolybdenum cluster complexes in the aqueous phase.45 Due to its negative charge, PSS can strongly bind cluster cations such as positively charged [Ta6I12(H2O)6]2+. This is electrostatic binding without direct coordination. In the reverse mode, negatively charged clusters, such as the tetraanionic hexarhenium cluster complex [Re6S8(OH)6]4−, can be electrostatically bound to polyethylenimine.46 Addition of PSS (2200 kDa) to an aqueous solution of [Ta6I12(H2O)6]I2, followed by precipitation with ethanol, resulted in the formation of [Ta6 I12 (H2 O) 6]@PSS (7). According to elemental analysis the obtained material contained one cluster unit per three monomeric styrenesulfonate links. Since only two links are required for neutralization of the positive charge of [Ta6I12(H2O)6]2+, a Na+ ion must be incorporated per each [Ta6I12(H2O)6] unit, thus giving the final composition {Na[Ta6I12(H2O)6](C8H7SO3)3}n. As expected, the aqueous solutions of 7 are more stable than [Ta6I12(H2O)6]I2 without addition of PSS (Figure 2). After 2 weeks of solution aging of 7 the absorption at 640 nm was 6 times higher for 7 than for 2. Obviously, the decay in the presence of PSS is slower and tends to become linear, while without PSS the decay is much

more rapid and the decay curve appears exponential. This finding indicates the strong ability of PSS to stabilize the cluster complex in water. For practical purposes one can assume that solution of [Ta6I12(H2O)6]@PSS are stable for 2 days. Despite the increase in stability in the presence of PSS, slow formation of Ta2O5 still occurs. PSS prevents the precipitation but stabilizes a colloidal solution of Ta2O5. According to a transmission microscope image (Figure 3a) for a 2 week solution of 7 the particle size of Ta2O5 is about 0.1−0.2 μm. Both freshly prepared and 2 week old solutions of 7 were utilized for measurement of the X-ray attenuation using computerized tomography. Figure 3b,c demonstrates the dependence of the radiopacity on a Hounsfield unit scale vs concentration. The X-ray attenuation has a linear dependence on the concentration, and the slope can be used to compare Xray attenuation of different compounds. The slope for fresh and 2 week old solutions of 7 are similar and equal to (6.73 ± 0.17) × 104 and (6.49 ± 0.08) × 104 HU L mol−1, correspondingly. These results demonstrate the high radiopacity of the cluster complex [Ta6I12(H2O)6]I2, comparable with that for [W6I14]2− and [Re6Te8(CN)6]2−.47,48 In order to compare the radiopacity with currently used standard contrast agents, a radiopacity study for iohexol (Omnipaque, GE Healthcare) was also carried out (Figure 3d). The slope for the iohexol solution is equal to (0.77 ± 0.01) × 104 HU L mol−1, being 8.4−8.7 times less than those for hexatantalum clusters, in good agreement with literature data.47,48 Computational Results for [Ta6I12(H2O)6]2+. Quantum mechanical DFT calculations were performed for the aqua complex [Ta6I12(H2O)6]2+. Table 1 summarizes the comparison between the optimized structure and the experimental data. A good agreement was found. The optimized geometry was used to compute the IR spectrum of the cluster. In particular, in Figure 4 the FIR region is depicted. The computational method used shows an extremely accurate level of predictibility of both frequencies and intensities. The band at about 150 cm−1 consists of the convolution of six modes involving the metal core and iodine atoms. Between 180 and 220 cm−1 19 modes are observed, for which the involvement of the ligand is more important. Figure 5a shows the scheme of the frontier molecular orbitals (from H-3 to L+1). We observe an agreement with respect to the previous literature49 except for H-3, which had been described as a nondegenerate orbital of a1g symmetry, whereas in the present study eg was found with all the employed levels of theory, except HF-SCF. The HF-SCF calculation shows a loss of degeneracy as reported in the literature; the energy difference between the two orbitals is ∼30 meV. This discrepancy is probably due to the lack of electronic correlation in the HF-SCF theory. We are therefore inclined to believe that the description of the ground-state electronic structure of the investigated Ta cluster needs 18 cluster-based electrons, instead of 16 as proposed in the previous descriptions. Concerning the optical properties, the UV−vis absorption spectrum was computed at the TD-DFT level, accounting for the solvent effects. In the region between 600 and 800 nm (Figure 5b) two main bands are observed; more precisely, at 756 nm (f = 0.7512), the electronic transition shows a main HOMO−LUMO character (coeff = 0.70232), whereas the band at 641 nm (f = 0.9628) is mainly HOMO−LUMO+1 (coeff = 0.6805). From the analysis of the frontier molecular 9033

DOI: 10.1021/acs.inorgchem.9b00364 Inorg. Chem. 2019, 58, 9028−9035

Inorganic Chemistry

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orbitals involved in the two transitions (depicted in Figure 5a) we can conclude that these two electronic bands are local excitations localized on the cluster core; ligands do not show any involvement in the absorption. Also in this case we observe an excellent agreement between computational and experimental results.

CONCLUSIONS In this work we have demonstrated that aqaution of Ta6I14 under strictly controlled conditions is a viable way to obtain the moderately stable aqua complex [Ta6I12(H2O)6]2+, which possesses very high radiopacity. As it is, unexpectedly, more stable toward oxidation than its Cl and Br analogues, its decomposition into Ta2O5 and I− in aqueous solutions is triggered by hydrolysis of Ta−I−Ta bridging units. The stability of [Ta6I12(H2O)6]2+ is significantly enhanced in the presence of polystyrenesulfonate. Both Ta6I14 and the aqua complex react with DMF with the formation of [Ta6I12(DMF)6]2+ and with CN− to give [Ta6I12(CN)6]4−. These complexes are stable in organic solvents, showing no traces of hydrolysis and oxidation. To conclude, this work has provided a convenient entry into previously unknown coordination chemistry of the {Ta6I12} clusters. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00364. Crystallographic data, refinement details, and main geometric parameters (PDF) Accession Codes

CCDC 1885843−1885846 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, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.V.S.: [email protected]. *E-mail for M.N.S.: [email protected]. ORCID

Michael A. Shestopalov: 0000-0001-9833-6060 Pavel A. Abramov: 0000-0003-4479-5100 Maxim N. Sokolov: 0000-0001-9361-4594 Author Contributions

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

The work has been supported by Russian Foundation for Basic Research (No. 18-33-20056). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Siberian Branch of the Russian Academy of Sciences (SB RAS) Siberian Supercomputer Center is gratefully acknowledged for providing supercomputer facilities. 9034

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