Communication pubs.acs.org/IC
Prototropic μ‑H8,9 and μ‑H9,10 Tautomers Derived from the [nido5,6‑C2B8H11]− Anion Oleg L. Tok,† Zdenka Růzǐ čková,‡ Aleš Růzǐ čka,‡ Drahomír Hnyk,† and Bohumil Štíbr*,†
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, Husinec-Ř ež 1001, Czech Republic Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
† ‡
S Supporting Information *
zwitterionic tautomer has thus far been established by an X-ray diffraction study.3
ABSTRACT: Reported is an unusual tautomeric behavior within the [nido-5,6-C2B8H11]− (1a−) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-nido5,6-C2B8H10-μ8,9]− (2d−) and [5,6-Me2-nido-5,6-C2B8H9μ9,10]− (3b−), which differ in the positioning of the openface hydrogen bridge. Their structures have been determined by X-ray diffraction analyses. The 3b−structure is stabilized by intermolecular interaction involving Et3NH+ and B8−B9 and H8 atoms in the solid phase; however, its dissolution in CD3CN causes instant conversion to the more stable [5,6-Me2-nido-5,6-C2B8H9μ8,9]− (2b−) tautomer. The dynamic electron-correlationbased MP2/6-31G* computations suggest that the parent [nido-5,6-C2B8H11-μ8,9]− (2a−) tautomer is 3.9 kcal·mol−1 more stable than the [nido-5,6-C2B8H11-μ9,10]− (3a−) counterpart and the μ8,9 structure 2− is therefore the most stable tautomeric form in the solution, as was also demonstrated by multinuclear (1H, 11B, and 13C) NMR measurements on the whole series of C-substituted compounds.
This Communication outlines the isolation of two Csubstituted derivatives of the [nido-5,6-C2B8H11]− (1a−) anion that may be classified as skeletal tautomers. This unprecedented case of constitutional isomerism occurs solely within the same carborane cage; both tautomers can be isolated in the pure state, structurally established, and mutually interconverted. It has been long known that deprotonation of the parent dicarbadecaborane nido-5,6-C2B8H12 (1a) by basic reagents leads to the conjugated 1a− anion,4 but it has never been clearly established which of the two nonequivalent bridging H atoms is actually abstracted. As shown in Scheme 1 (path A), the deprotonation reaction between the neutral 5,6-R2,R1-nido-5,6C2B8H10 dicarbaboranes and NMe4OH in water precipitates the corresponding conjugate [5,6-R2,R1-nido-5,6-C2B8H9]−NMe4+ salts, which can be crystallized conveniently from MeCN−
T
automerism is a special case of isomerism in which compounds differ in the positioning of one or more H atoms. Prototropic tautomers are sets of isomeric protonation states with the same empirical formula and total charge. There are numerous examples of tautomerism in organic chemistry,1 of which the two outlined below may perhaps illustrate the typical situation:
Scheme 1. Two Modes of Deprotonation of the Neutral 5,6R2,R1-nido-5,6-C2B8H10 Compounds and Interconversion of the [5,6-R2,R1-nido-5,6-C2B8H9]− (2− and 3−) Tautomers
In organic chemistry, tautomers are constitutional isomers that readily interconvert with each other in solutions. As far as we are aware, there has been only one isolated example of tautomerism in nonmetallic carborane chemistry, the one involving one of the amine H atoms in amino-substituted tricarbollides that, depending on the solvent polarity, can reside either on the amine N atom or on the cluster. Both of these tautomeric structures sharply differ in their NMR spectra and can be isolated in the solid state,2 but only the structure of the © XXXX American Chemical Society
Received: August 26, 2016
A
DOI: 10.1021/acs.inorgchem.6b02076 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Et2O mixtures. As illustrated in Figure 1, the structure of the [6Ph-nido-5,6-C2B8H10]−NMe4+ derivative thus prepared has been
Figure 3. Intercomparison of the authentic 11B NMR (coupled and decoupled) spectra of anions 2a− and 2b− (Et3NH+ salts in CD3CN) with those computed by the GIAO-MP2/II//MP2/6-31G* method for the tautomeric 2a−/2b− and 3a−/3b− pairs (for numbering, see Scheme 1).
Figure 1. Molecular structure (ORTEP view, 30% probability level) of the [6-Ph-nido-5,6-C2B8H10-μ8,9]−Et4N+ tautomer. Selected intracluster bond lengths (Å) and angles (deg): C5−C6 1.475(5), B7−B8 1.977(6), B8−B9 1.748(6), B9−B10 1.639(6), C5−B10 1.723(5), C5−B1 1.703(5), C5−B2 1.732(5), C6−B2 1.669(5), C6−B7 1.511(5), B4− B9 1.742(6), B1−B3 1.825(6), B1−B10 1.786(7); B7−C6−C5 113.8(3), C6−C5−B1 114.6(3), C6−C5−B2 62.1(2), B8−B9−B10 106.6(3), B4−B9−B10 64.9(3), B2−B1−B4 113.9(3). There are two pairs of R and S enantiomorphs in the unit cell.
determined via an X-ray diffraction analysis, pointing unambiguously to the presence of a single [6-Ph-nido-5,6-C2B8H10-μ8,9]− (2d−) tautomer in the anionic part. Path B of Scheme 1 involves generation of the triethylammonium salts of nido anions [5,6R2,R1-nido-5,6-C2B8H9]−, which were prepared by adding a slight excess of Et3N to hexane solutions of the corresponding neutral counterpart 1. Surprisingly, an X-ray diffraction study on [5,6Me2-nido-5,6-C2B8H9]−Et3NH+ revealed that the anionic part consisted only of the inverse tautomer, [5,6-Me2-nido-5,6C2B8H9-μ9,10]− (3b−) (see Figure 2). The 11B NMR spectra of the [5,6-R2,R1-nido-5,6-C2B8H9]− anions (Et3NH+ salts; see Figure 3) in a CD3CN solution consist of eight different resonances, in agreement with the absence of symmetry. The signals can be assigned unambiguously by 11 B−11B COSY experiments5 to individual cluster vertexes, with
the common feature being the broadening of the B9 and B10 resonances, probably due to μ-hydrogen interactions. However, the spectra in Figure 3 are consistent only with the μ8,9 structure 2b− (for the spectra of other anions 2−, see the Supporting Information), which is strongly supported by the excellent agreement between the experimental and GIAO-MP2/II// MP2/6-31G*-computed shifts for anions 2a− and 2b− in Figure 3. The data calculated for the inverse μ9,10-tautomeric structures 3a− and 3b− remarkably deviate from those found by the experiment, especially for positions B9, B3, B8, and B10 flanking the would-be bridging sites. The variable-temperature NMR measurements in the range −40 to −90 °C did not show any significant changes in the 1H and 11B NMR spectra attributable to the interconversion of two tautomeric forms in solution. It is therefore evident that tautomer 3b− can be isolated only in the solid phase as a consequence of its stabilization via a nonbonding interaction between the neighboring cell Et3NH+ proton and the B8−B9 bond, combined with dihydrogen interaction,6 as shown in Figure 4. Diluted electron density in an extremely long Et3N− H+ distance (1.07 Å) is a driving force for contact with the H8
Figure 2. Molecular structure (ORTEP view, 50% probability level) of the [5,6-Me2-nido-5,6-C2B8H9-μ9,10]−Et3NH+ tautomer. Selected bond lengths (Å) and angles (deg): C12−C6 1.507(3), C11−C5, 1.516(4), C5−C6 1.454(4), B7−B8 1.864(4), B8−B9 1.645(4), B9−B10 1.781(4), C6−B7 1.520(4), C5−B10 1.825(4), C5−B1 1.663(4), C5−B2 1.714(4), C6−B2 1.682(4), B4−B9 1.744(4), B1−B3 1.801(4), B1−B10 1.807(4); B7−C6−C5 114.4(2), C6−C5−B1 113.5(2), C6− C5−B2 63.53(17), B8−B9−B10 105.3(2), B4−B9−B10 60.21(16), B2−B1−B4 114.4(2). There is a pair of R and S enantiomorphs in the unit cell.
Figure 4. Stabilization of the 3b− tautomer in the solid state via intermolecular nearest-neighbor interactions involving the Et3NH+ proton and B8, B9, and H8 atoms. B
DOI: 10.1021/acs.inorgchem.6b02076 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry atom. This H···H+ distance is 2.27 Å and represents, along with the B8−H+ distance of 2.18 Å, the shortest nonbonding cation− anion interaction. However, these interactions are removed upon dissolution in CD3CN (see path C of Scheme 1) when the tautomer 3b− is instantly converted into the more stable 2b− form. As depicted in Figure 3, MP2/6-31G* computations on the 3a− → 2a− conversion suggest a B9H2-containing transition state (TS) for this process, with the energy gap between 3a− and 2a− being 3.9 kcal·mol−1. Note that the TS has an imaginary frequency of −631.2 cm−1, which predominantly exhibits a wagging motion of the migrating hydrogen. Tautomers 2 and 3 can also be easily interconverted in a chemical manner (see Scheme 1). For example, treatment of the solid [5,6-Me2-nido-5,6-C2B8H9-μ9,10]−NEt3H+ with aqueous NMe4OH releases NEt3 to generate the solid [5,6-Me2-nido5,6-C2B8H9-μ8,9]−NMe4+ (2b−), while acidification of the latter NMe4+ salt with CF3CO2H leads to the neutral 5,6-Me2-nido-5,6C2B8H10 (1b; reverse path A), which, in turn, provides solid tautomer 3b− upon reaction with NEt3 in hexane (path B). In conclusion, isolated in pure form in the solid state and structurally characterized have been two tautomeric 2d− and 3b− anions that differ both in the nature of the countercation and in the positioning of the open-face bridging hydrogen. The two tautomers are interrelated by the common neutral carborane of type 1. This exceptional tautomerism occurs solely within the cluster, in contrast to the exocluster−cluster tautomerism previously reported.2 The less stable tautomer 3b− is capable of existencing only in the solid phase, thanks to intermolecular stabilization. This exceptional case has no parallel in the whole area of carborane chemistry,7 most probably even in the organic field.1 Moreover, the two tautomers can be chemically interconverted but do not equilibrate in solution. General Procedure for the Preparation of [5,6-R2,R1-nido5,6-C2B8H9-μ8,9]−NMe4+ Tautomers (2−). The synthesis is exemplified for R1 = Ph and R2 = H (2d−). A solution of 6-Phnido-5,6-C2B8H11 (1d; 100 mg, 0.5 mmol) in hexane (15 mL) was added dropwise to a mixture of a 1 M solution of NMe4OH (5 mL) and water (25 mL) under intensive stirring for 2 h. The yellowish precipitate was then isolated by filtration, washed with degassed water, and then vacuum-dried to isolate 130 mg (96%) of crude [6-Ph-nido-5,6-C2B8H10-μ8,9]NMe4 (2d−). This was dissolved in CH3CN (∼10 mL), and Et2O (∼10 mL) was carefully syringed onto the surface to get crystals suitable for Xray analysis upon longer standing. For 11B NMR spectra, see Figure 3. General Procedure for the Preparation of [5,6-R2,R1-nido5,6-C2B8H9-μ9,10]−NEt3H+ Tautomers (3−) in the Solid State. The synthesis is exemplified for R1 and R2 = Me (3b−). A solution of 1b (95 mg, 0.63 mmol) in hexane (15 mL) was treated with a slight excess of NEt3 (∼1 mmol) and the mixture left standing for 2 days, during which time yellowish crystals deposited on the walls. These were isolated by filtration, washed with hexane, dried in a vacuum, and identified by an X-ray diffraction study as [5,6-Me2-nido-5,6-C2B8H9-μ9,10]NEt3H (3b−). Dissolution of 3b− in CD3CN led instantly to its conversion to 2b−, as assessed by the 11B NMR spectra (see Figure 3). The same experiments for all R1 and R2 carried out in CD3CN led to solutions of inverse tautomers 2− (for comparisons of the 11B NMR spectra, see Figure 3 and the Supporting Information). Protonation of [5,6-R2,R1-nido-5,6-C2B8H9-μ8,9]−NMe4+ Tautomers (2−). The reaction is exemplified for R1 = Ph and R2 = H (2d−). A suspension of [6-Ph-nido-5,6-C2B8H10μ8,9]NMe4 (2d−; 68 mg, 0.25 mmol) in CH2Cl2 or hexane (10
mL) was treated with CF3CO2H (∼0.5 mL) under shaking for 1 h and intensive cooling in an ice bath. The mixture was washed with water (2 × 10 mL), dried with Na2SO4, and evaporated in vacuo to give 6-Ph-nido-5,6-C2B8H11 (3d; 45 mg, 90%), as identified by 11B NMR. This can be converted into [6-Ph-nido5,6-C2B8H10-μ9,10] NEt3H (3d−), as in the preceding experiment.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02076. CIF file for 2d− (CIF) CIF file for 3b− (CIF) NMR spectra for other anions of structure 2−, further experimental and computation details, and X-ray crystallography (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Grant Agency of the Czech Republic (Project 16-01618S). DEDICATION Dedicated to the memory of Prof. Dr. Igor T. Chizhevsky, our good friend. REFERENCES
(1) Antonov, L. Tautomerism: Methods and Theories; Wiley-VCH: Weinheim, Germany, 2013. (2) Bakardjiev, M.; Holub, J.; Hnyk, D.; Císařová, I.; Londesborough, M. G. S.; Perekalin, D. S.; Štíbr, B. Structural Dualism in the Zwitterionic 7-RR′NH-nido-7,8,9-C3B8H10 Tricarbollide Series. A Unique Example of Absolute Tautomerism. Angew. Chem., Int. Ed. 2005, 44, 6222−6226. (3) Štíbr, B.; Holub, J.; Císařová, I.; Teixidor, F.; Viňas, C.; Fusek, J.; Plzák, Z. The Derivatives of the 7,8,9-Series of Tricarbollides. Preparation and Structural Characterization of the 11-Vertax Tricatbaboranes 7-L-nido-7,8,9-C3B8H10 (L= amines). Inorg. Chem. 1996, 35, 3635−3642. (4) Plešek, J.; Heřmánek, S. Chemistry of boranes xxxiv. Oxidation of 7,8-Dicarba-nido-decaborate to 5,6-Dicarba-nido-decaborane(12) and its Dehydrogenation to 1,2-Dicarba-closo-decaborane (10). Collect. Czech. Chem. Commun. 1974, 39, 821−826. (5) Venable, T. L.; Hutton, W. C.; Grimes, R. N. Two-dimensional Boron-11-Boron-11 Nuclear Magnetic Resonance Spectroscopy as a Probe of Polyhedral Structure: Application to Boron hydrides, cCarboranes, Metallaboranes, and Metallacarboranes. J. Am. Chem. Soc. 1984, 106, 29−37. (6) Fanfrlík, J.; Hnyk, D.; Lepšík, M.; Hobza, P. Interaction of Heteroboranes with Biomolecules. Part 2. The Effect of various Metal Vertices and exo-Substitutions. Phys. Chem. Chem. Phys. 2007, 9, 2085− 2093. (7) Grimes, R. N. Carboranes, 2nd ed.; Elsevier Science: New York, 2011.
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DOI: 10.1021/acs.inorgchem.6b02076 Inorg. Chem. XXXX, XXX, XXX−XXX
