Article pubs.acs.org/IC
Solid-State Synthesis and Structure of the Enigmatic Ammonium Octaborate: (NH4)2[B7O9(OH)5]·3/4B(OH)3·5/4H2O Doinita Neiner,† Yulia V. Sevryugina,‡ and David M. Schubert*,† †
Research & Innovation Department, U.S. Borax Inc, Rio Tinto Borates, Greenwood Village, Colorado 80111, United States Department of Chemistry & Biochemistry, Texas Christian University, Fort Worth, Texas 76129, United States
‡
S Supporting Information *
ABSTRACT: The compound known since the 19th century as ammonium octaborate was structurally characterized revealing the ammonium salt of the ribbon isomer of the heptaborate anion, [B7O9(OH)5]2−, with boric acid and water molecules. Of composition (NH4)2B7.75O12.63·4.88H2O, it approximates the classical ammonium octaborate composition (NH 4 ) 2 B 8 O 13 ·6H 2 O and has the structural formula {(NH4)2[B7O9(OH)5]}4·3B(OH)3·5H2O. It spontaneously forms at room temperature in solid-state mixtures of ammonium tetraborate and ammonium pentaborate. It crystallizes in the monoclinic space group P21/c with a = 11.4137(2) Å, b = 11.8877(2) Å, c = 23.4459(3) Å, β = 90.092(1)°, V = 3181.19(8) Å3, and Z = 2 and contains wellordered ammonium cations and [B7O9(OH)5]2− anions and disordered B(OH)3 and H2O molecules linked by extensive H bonding. Expeditious solid-state formation of the heptaborate anion under ambient conditions has important implications for development of practical syntheses of industrially useful borates.
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composition (NH4)2B8O13·6H2O is formed when ammonium pentaborate and ammonium tetraborate are mixed in the solid state, but no further progress was made on resolving the nature of this compound until now.12 Here we report the single-crystal X-ray structure of this enigmatic compound, showing it to have the structural formula {(NH4)2[B7O9(OH)5]}4·3B(OH)3·5H2O (1), corresponding to the oxide formula 8(NH4)2O·31B2O3·39H2O. This compound contains the ribbon isomer of the isolated heptaborate anion, [B7O9(OH)5]2−, shown in Figure 1. This isolated anion was not described until 2006. It was first observed in the alkali metal salts Cs2[B7O9(OH)5] and Rb2[B7O9(OH)5] prepared by hydrothermal syntheses involving prolonged heating at 150 °C.13,14
INTRODUCTION Borate compounds in which boron is bound exclusively to oxygen have many industrial uses important to modern society. These include agricultural micronutrients that increase food and energy crop yields and the manufacture of glasses, ceramics, fire retardants, lubricants, and durable and energysaving building materials.1,2 Although relatively minor on the scale of industrial borates, such as borax pentahydrate and boric acid, ammonium borates are nevertheless among the borates produced in amounts greater than 1000 tons per year. Ammonium pentaborate, NH4[B5O6(OH)4]·2H2O, and ammonium tetraborate, (NH4)2[B4O5(OH)4]·2H2O, are used in electrolytic capacitors, fire retardant coatings, lubricants, and in the anodizing of aluminum for aerospace and other applications. In addition to industrial ammonium borates, the minerals ammonioborite, (NH4)3[B15O17(OH)8]·3H2O, and larderellite, NH4[B5O6(OH)2]∞·H2O, are well-characterized inorganic natural products.3−6 Chemical literature dating back to at least the 19th century contains references to an unusual compound referred to as ammonium octaborate having the composition (NH4)2B8O13· xH2O, or equivalent oxide formula (NH4)2O·4B2O3·xH2O, where x is typically given as 5 or 6.7 This compound was identified in detailed studies of the (NH4)2O−B2O3−H2O phase diagram done in the 1920s and 1960s, but the true identity of this compound has been a longstanding mystery.8−11 Unpublished work done in the Rio Tinto Borax company more than four decades ago showed that a compound of approximate © XXXX American Chemical Society
Figure 1. Ribbon isomer of the heptaborate anion, [B7O9(OH)5]2−. Received: May 24, 2016
A
DOI: 10.1021/acs.inorgchem.6b01243 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry We found that 1 can be prepared at room or higher temperatures via the solid-state reaction of a simple mixture of ammonium tetraborate and ammonium pentaborate in a mole ratio that provides a stoichiometric excess of ammonia. The presence of the unusual heptaborate anion in a compound known for well over a century and its spontaneous formation at room temperature from simple mixtures of common reagents has important implications for the development of practical synthetic routes to industrially useful borate compounds.
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EXPERIMENTAL SECTION
Materials and Methods. Ammonium pentaborate and boric acid were products of U.S. Borax, Inc. Ammonium tetraborate (archaically referred to as ammonium biborate) was prepared by reaction in aqueous slurry of boric acid with ammonium hydroxide in the appropriate molar ratio followed by filtering and air drying the crystalline product. Ammonium hydroxide was obtained from Rocky Mountain Reagents of Golden, Colorado. Powder X-ray diffraction (PXRD) data were collected on a Phillips X’Pert diffractometer equipped with a graphite monochromator using Cu Kα radiation (λ = 1.5418 Å). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves were collected simultaneously at a scan rate of 5 °C·min−1 under a flow of N2 using a TA Instruments SDT 600 Q thermoanalyzer. Infrared spectra were collected for powder samples using a Bruker Alpha Fourier transform infrared (FTIR) spectrometer equipped with a single reflection diamond attenuated total reflectance module. Scanning electron microscopy (SEM) micrographs were captured using a JEOL JEM2500SE scanning electron microscope at the Colorado School of Mines Electron Microscopy Laboratory, a user facility. The instrument was operated at 200 kV using powder samples affixed to carbon tape mounted on an Al stub. Solid-state magic-angle spinning (MAS) 11 1 B[ H] NMR data were recorded at 160.412 MHz on a Varian 500 spectrometer at the Pacific Northwest National Laboratory, Richland, Washington, using a 5 mm ZrO rotor operated with a spin rate of 20 kHz. Chemical shifts were referenced to BF3·OEt2 (δ = 0 ppm) at 20 °C. Synthesis of 1. Microcrystalline samples of 1 were prepared from mixtures of ammonium tetraborate and ammonium pentaborate in a mole ratio that supplies excess ammonia. In a typical experiment (NH 4 ) 2 [ B 4 O 5 (OH) 4 ] ·2H 2 O ( 8.56 g, 32.5 mmol) and NH4[B5O6(OH)4]·2H2O (9.80 g, 36.0 mmol) were intimately mixed with mortar and pestle, placed in a sealed tube, and maintained at 60 °C for 20 h. The product was identified by PXRD and standard borate colorimetric titration. Anal. Calcd for 8(NH4)2O·31B2O3·39H2O, 12.7% (NH4)2O, 65.8% B2O3; Found, 12.9% (NH4)2O, 65.1% B2O3. Compound 1 can also be obtained by mixing ammonium tetraborate with ammonium pentaborate in the same ratio as above and placing the mixture in a humidity chamber at room temperature for 20 h. In another example, (NH4)2[B4O5(OH)4]·2H2O (1.697 g, 6.44 mmol) and NH4[B5O6(OH)4]·2H2O (1.403 g, 5.16 mmol) were mixed with two drops of water and ball milled with a high-energy Spex mill at room temperature for 1 h. Single crystals of 1 used for X-ray crystallography were prepared by mixing (NH4)2[B4O5(OH)4]·2H2O (6.50g, 24.68 mmol) and NH4[B5O6(OH)4]·2H2O (6.07g, 22.30 mmol) and maintaining the mixture in a sealed glass tube at 60 °C for 5 d. Figure 2 shows an SEM micrograph of a crystal of 1 that formed in a sealed tube experiment. Single-Crystal X-ray Data Collection and Structure Determination. A colorless prism-shaped crystal of 1 was mounted on a glass fiber, and data were collected on an Agilent Technologies SuperNova Atlas S2 CCD diffractometer using Cu Kα radiation (λ = 1.541 84 Å). The CrysAlisPro software package was used for data collection and data integration.15 Data were corrected for absorption effects using the empirical absorption correction with spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. The structure was solved and refined using the CrysAlisPro and Bruker SHELXTL software packages.16
Figure 2. SEM micrograph of a crystal of 1. The crystal system originally appeared orthorhombic and gave reasonable orthorhombic data (Rint ≈ 0.034). Careful study of the data images and intensities revealed that the crystal system is monoclinic (Rint = 0.027), space group P21/c, but with β = 90.09°, and is twinned about a mirror perpendicular to a*. Refinement of the twin law revealed a 0.45:0.55 ratio. The asymmetric unit contains two independent [B7O9(OH)5]2− anions and four NH4+ cations, fully occupied and ordered. The crystal structure also contains cocrystallized water and boric acid molecules. Two crystallographically independent boric acid molecules with total occupancy of 1.5 per asymmetric unit both display symmetrygenerated disorder. In addition, one boric acid is disordered over two orientations modeled with an occupancy ratio of 0.25:0.25. There are three crystallographically independent water molecules in the asymmetric unit with total occupancy 2.5: (a) one H2O molecule is ordered with full occupancy; (b) the oxygen atom of a second H2O molecule is disordered over two positions modeled with an occupancy ratio of 0.5:0.5; (c) the third water molecule displays symmetrygenerated disorder and is additionally disordered over two positions refined with an occupancy ratio of 0.25:0.25. All of the non-hydrogen atoms were refined with anisotropic thermal parameters except for oxygen atoms in the partially occupied boric acid and partially occupied water molecule. All of the hydrogen atoms were included at geometrically idealized positions except for those belonging to ammonium cations and water molecules. In the latter, H atoms were located in difference Fourier maps and were refined individually. It was not possible to locate hydrogen atoms of the water molecule with 0.5 occupancy disordered over two orientations. Spectroscopy. Figure 3 shows the FTIR spectrum of 1 along with the spectra of the ammonium borate starting materials individually and freshly mixed. The IR spectrum of 1 features bands characteristic of BO3 and BO4 groups (800−1050 cm−1) and bridging B−O−B groups (1175−1350 cm−1). Broad absorptions in the 2600−3600 cm−1 region are those expected for B−OH hydroxyl groups, water, and ammonium cations. The solid-state 11B[1H] NMR spectrum of 1, shown in Figure 4, displays the broad resonances of the three-coordinate boron atoms of the heptaborate anion and boric acid showing quadrupolar splitting and a narrower resonance of the four-coordinate boron atoms of the heptaborate anion at 1.92 ppm.17 Thermal Analysis. TGA and DSC of 1 performed in the 20−1000 °C temperature range show decomposition with release of water and ammonia in three broad weight-loss events. Initial loss of free water begins just above room temperature with a maximum at 82 °C (ca. 5.5 wt %). This is followed by further loss of water via condensation of B− OH groups of boric acid and the heptaborate anion in a broad complex B
DOI: 10.1021/acs.inorgchem.6b01243 Inorg. Chem. XXXX, XXX, XXX−XXX
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tration ranges.8−11 This suggests that crystallization from solutions of appropriate composition might be used for larger-scale preparation of 1. However, such solutions readily supersaturate, and even with seeding, ammonium pentaborate or tetraborate are often obtained as kinetic products instead of 1, making this method impractical. Alternatively, we found that solid-state synthesis can be used to reliably prepare 1. Compound 1 forms at room temperature when ammonium tetraborate and ammonium pentaborate are combined as a solid-state mixture in the presence of moisture. A description of this reaction based on unpublished work performed in the Rio Tinto Borax company was given in a 1980 review of ammonium borates using equivalent oxide formulas as eq 1.12 Figure 3. IR spectrum of 1 (top) shown together with the spectra of ammonium tetraborate and ammonium pentaborate individually and freshly mixed.
(NH4)2 O·2B2O3 ·4H 2O + 2(NH4)2 O·5B2O3 ·4H 2O
(1)
→ 3(NH4)2 O·4B2O3 ·6H 2O + 2H 2O
We found in practice that a molar ratio of ammonium tetraborate to ammonium pentaborate that provides an excess of volatile ammonia is required for this reaction to proceed to completion, at least in a short time period. The identity of the product determined in this work as {(NH4)2[B7O9(OH)5]}4· 3B(OH)3·5H2O and the reaction stoichiometry given in the experimental section gives eq 2 as a more accurate representation of this reaction. 3.25(NH4)2 [B4 O5(OH)4 ]·2H 2O + 3.60NH4[B5O6 (OH)4 ]· 2H 2O → {(NH4)2 [B7O9(OH)5 ]}4 · 3B(OH)3 · 5H 2O + 2.10NH3 + 8.95H 2O
(2)
As observed by PXRD analysis, an intimate mixture of ammonium tetraborate and ammonium pentaborate in the molar ratio given by eq 2, when placed in a humid environment at room temperature, begins converting into 1 within 100 min. As illustrated by the PXRD patterns shown in Figure 5,
Figure 4. MAS 11B[1H] NMR spectrum of 1. process that is complete by ca. 250 °C (ca. 16 wt %). A third event associated with loss of ammonia and further water (deammoniation) occurs between 250 and 400 °C (ca. 13 wt %). The total observed weight loss is close to the theoretical value of 34.15 wt %. No further events were observed in the DSC scan up to 1000 °C.
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RESULTS AND DISCUSSION The primary boron species occurring in concentrated aqueous solution in the intermediate pH regime are B(OH) 3, [B5O6(OH)4]−, [B3O3(OH)4]−, [B4O5(OH)4]2−, [B3O3(OH)5]2−, and [B(OH)4]−, listed in order of increasing alkalinity.1,2 Refined industrial borates, including borax pentahydrate and decahydrate, Na2[B4O5(OH)4]·xH2O (x = 3, 8), ammonium tetraborate, and ammonium pentaborate, are salts of these primary polyborate species or, in some cases, further condensed forms of them as found in the commercially important zinc borate Zn[B3O4(OH)3]∞.18 Although many new borate compounds containing unusual polyborate anions have been reported in recent years, most were produced under hydrothermal conditions that are impractical for industrial scale manufacture.19 Studies of the phase relationships in the (NH4)2O−B2O3− H2O system show that 1 is a stable phase in contact with highly concentrated ammonium borate solutions in specific concen-
Figure 5. PXRD patterns for a mixture of ammonium tetraborate and pentaborate after exposure to a humid environment for 20 h. with reference pattern for 1 below.
conversion to 1 is complete after ∼20 h. The XRD patterns in Figure 6 show that the same mixture when heated at 60 °C in a sealed tube without added humidity converted almost completely into 1 in 4 h and to a higher crystallinity form of 1 in 20 h. Also, as illustrated in Figure 7, this reaction can be performed in a ball mill within 1 h if a small amount of water is added. We also observe formation of 1 by XRD in samples of C
DOI: 10.1021/acs.inorgchem.6b01243 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 6. PXRD patterns for a mixture of ammonium tetraborate and ammonium pentaborate maintained at 60 °C in a sealed tube for 1, 4, and 20 h with reference pattern for 1 below.
Figure 8. Asymmetric unit in 1 showing crystallographically ordered NH4+ cations and [B7O9(OH)5]2− anions and disordered boric acid and water molecules (left).
molecules. The two nonequivalent heptaborate anions display similar geometries, shown in Figure 9, and contain six-
Figure 7. PXRD patterns for mixtures of ammonium tetraborate and pentaborate ball milled for 1 h: (A) without added water, and (B) with a small amount of added water, with the reference pattern for 1 below.
Figure 9. Heptaborate anion, [B7O9(OH)5]2−, in 1.
boric acid after exposure to ammonia vapors with ammonium tetraborate forming as the primary product. The formation of uncommon borate anions in solid-state reactions has been reported previously, including the isomeric heptaborate anion discussed below.20 Although an abundant species in aqueous borate solutions, the [B3O3(OH)4]− triborate monoanion is encountered as an isolated anion in only a small number of natural and synthetic compounds.21−24 While difficult to crystallize from solution, the potassium salt of this anion forms spontaneously at room temperature when potassium tetraborate and potassium pentaborate are mixed in the presence of moisture, according to eq 3.23
membered B3O3 rings that are nearly planar. The outer rings exhibit slight half-chair conformations resulting from small outof-plane displacements of the four-coordinate boron atoms shared with the central ring. This is more pronounced for one of the anions, which contains four-coordinate boron atoms that are 0.16 and 0.17 Å displaced from the mean planes of the other five ring atoms. The mean B−O distance is 1.47 Å for four-coordinate boron and 1.37 Å for three-coordinate boron for each heptaborate anion. All of the unit cell constituents are linked by extensive H bonding. As shown in Figure 10, the heptaborate anions form pairs of H bonds (1.83−2.06 Å) with adjacent anions along the crystallographic a axis. In addition, heptaborate anions form interanion H bonds (1.82 Å) along the crystallographic b axis, as shown in Figure 11. Two of the four ammonium cations form H bonds only with heptaborate B−O−B oxygen atoms and B−OH oxygen atoms. One ammonium cation forms H bonds with anion and boric acid hydroxyl oxygen atoms and a water molecule. A fourth ammonium cation forms H bonds
K 2[B4 O5(OH)4 ]·2H 2O + K[B5O6 (OH)4 ]·2H 2O→ 3K[B3O3(OH)4 ] + 2H 2O
(3)
Structure of 1. The asymmetric unit of 1, shown in Figure 8, contains two crystallographically nonequivalent [B7O9(OH)5]2− anions, four nonequivalent NH4+ cations, 1.5 disordered boric acid molecules, and 2.5 disordered water D
DOI: 10.1021/acs.inorgchem.6b01243 Inorg. Chem. XXXX, XXX, XXX−XXX
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CONCLUSIONS Most new borate compounds reported in recent years have resulted from syntheses performed under hydrothermal conditions that are not practical for large-scale industrial production. In this and previous work we have shown that unusual borate species that are not prevalent in borate solutions can form selectively in solid-state reactions under mild nonhydrothermal conditions. This suggests avenues to pursue for the large-scale practical synthesis of new and potentially useful borate compounds going beyond the now common practice of hydrothermal synthesis.
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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.6b01243. Crystallographic information file for 1. (CIF)
Figure 10. View along the crystallographic a axis showing pairs of H bonds (dotted lines) linking adjacent heptaborate anions in 1.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 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.
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ACKNOWLEDGMENTS We would like to thank Dr. L. Daniels at Agilent Technologies for X-ray data collection and Dr. J. Linehan of the Pacific Northwest National Laboratory, Richland, WA, for running the 11 B NMR experiment.
Figure 11. View along the crystallographic b axis showing H bonds linking adjacent heptaborate anions in 1.
with one anion hydroxyl group and two water molecules. All of these contacts are within the 1.88−2.07 Å range. Compound 1 contains the ribbon isomer of the [B7O9(OH)5]2− anion. Another isomer referred to as the O+ isomer, shown in Figure 12, was previously described by us and
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Figure 12. O+ isomer of the heptaborate anion, [B7O9(OH)5]2−.
others as alkylammonium salts.20,25,26 Gas-phase calculations by Beckett and co-workers indicate that the two isomers have similar energies, with the ribbon isomer being more stable by just 10 kJ·mol−1.26 Notably, we prepared the O+ isomercontaining compound [H3N(CH2)7NH3][B7O9(OH)5]·H2O by the solid-state reaction given by eq 4, which proceeds at room temperature.20 H 2N(CH 2)7 NH 2 + 7B(OH)3 → [H3N(CH 2)7 NH3][B7O9(OH)5 ]·H 2O + 6H 2O
REFERENCES
(1) Schubert, D. M. Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, online Apr 15, 2011; pp 1−68. doi: 10.1002/0471238961.0215181519130920.a01.pub3. (2) Schubert, D. M. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co, online Feb 10, 2015; pp 1−32. doi: 10.1002/14356007.a04_263.pub2. (3) Janda, R.; Heller, G.; Pickardt, J. Z. Kristallogr. - Cryst. Mater. 1981, 154, 1−9. (4) Becker, P.; Held, P.; Bohatý, L. Cryst. Res. Technol. 2000, 35, 1251−1262. (5) Merlino, S.; Sartori, F. Science 1971, 171, 377−379. (6) Merlino, S.; Sartori, F. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 2264−2270. (7) Storer, F. H. First Outlines of a Dictionary of Solubilities of Chemical Substances; University Press, Welch, Bigelow and Co: Cambridge, U.K, 1864; p 69. (8) Sborgi, U.; Ferri, L. Mem. R. Accad. Ital. Cl. Sci. Fis., Nat. Ser. 1922, 5, 570−591. (9) Heller, G. J. Inorg. Nucl. Chem. 1965, 27, 2345−2350. (10) Tolédano, P. Bull. Soc. Chim. Soc. Fr. 1966, 6, 1933−1941. (11) Tolédano, P.; Hébrard-Matringe, M.-A. Bull. Soc. Chim. Soc. Fr. 1969, 5, 1462−1465. (12) Bowden, G. In Mellor’s Comprehensive Treatise on Inorganic and Theoretical Chemistry, Part A; Boron-Oxygen Compounds, Longman: London, U.K, 1980; Vol. V, p 687. (13) Liu, Z.-H.; Li, L.-Q. Cryst. Growth Des. 2006, 6, 1247−1249. (14) Liu, Z.-H.; Li, L.-Q.; Zhang, W.-J. Inorg. Chem. 2006, 45, 1430− 1432.
(4) E
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Inorganic Chemistry (15) CrysAlisPro, v. 1.171.37.35; Agilent, 2014. (16) Sheldrick, G. M. SHELXTL, v. 2008/4; Bruker AXS, Inc: Madison, WI, 2008. (17) Beckett, M. A.; Strickland, G. C.; Varma, K. S.; Hibbs, D. E.; Hursthouse, M. B.; Malik, K. M. A. J. Organomet. Chem. 1997, 535, 33−41. (18) Schubert, D. M.; Alam, F.; Visi, M. Z.; Knobler, C. B. Chem. Mater. 2003, 15, 866−871. (19) Beckett, M. A. Coord. Chem. Rev. 2016, 1 DOI: 10.1016/ j.ccr.2015.12.012. (20) Schubert, D. M.; Visi, M. Z.; Khan, S.; Knobler, C. B. Inorg. Chem. 2008, 47, 4740−4745. (21) Dal Negro, A.; Pozas, J. M. M.; Ungaretti, L. Am. Mineralog 1975, 879−883. (22) Schubert, D. M.; Visi, M. Z.; Knobler, C. B. Inorg. Chem. 2008, 47, 2017−2023. (23) Salentine, C. G. Inorg. Chem. 1987, 26, 128−132. (24) Beckett, M. A.; Horton, P. N.; Hursthouse, M. B.; Timmis, J. L. RSC Adv. 2013, 3, 15185−15191. (25) Beckett, M. A.; Horton, P. N.; Hursthouse, M. B.; Timmis, J. L.; Varma, K. S. Dalton Trans. 2012, 41, 4396−4403. (26) Beckett, M. A.; Davies, R. A.; Thomas, C. D. Comput. Theor. Chem. 2014, 1044, 74−79.
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DOI: 10.1021/acs.inorgchem.6b01243 Inorg. Chem. XXXX, XXX, XXX−XXX
