A Zwitterionic Inorganic Benzene Valence Isomer with Ï-Bonding

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A Zwitterionic Inorganic Benzene Valence Isomer with #-Bonding between Two #-Orbitals Bochao Su, Kei Ota, Kai Xu, Hajime Hirao, and Rei Kinjo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08025 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Journal of the American Chemical Society

A Zwitterionic Inorganic Benzene Valence Isomer with σBonding between Two π-Orbitals Bochao Su,† Kei Ota,† Kai Xu,‡ Hajime Hirao*,‡ and Rei Kinjo*,† †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore

‡

Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Supporting Information Placeholder ABSTRACT: Despite the large number of plausible isomers of benzene (C6H6), only four valence isomers [(CH)6] have been experimentally detected, all of which are nonionic and possess skeletal frameworks built from localized C–C bonds. Herein, we present the isolation of a diazatetraborabenzene analogue of a hypothetical zwitterionic valence isomer of benzene. Therein, two electrons are delocalized over the four boron atoms in the six-membered B4N2 ring, which is a result of the σ-bonding interaction between two odd-electron B–B π-orbitals. Simple treatment with a crown ether leads to the formation of a paramagnetic potassium-doped radical ion pair that exhibits a thermally populated triplet character.

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Since the first discovery of benzene by Faraday and the sub2 sequent identification of its cyclic structure by Kekulé, the chemistry of benzene and its associated concept of aromaticity have fascinated both synthetic and theoretical chemists 3,4 for over 190 years. Computational studies have identified more than 200 isomeric structures, all of which are at least 1 5,6 60 kcal∙mol‒ higher in energy than benzene. Four valence isomers that consist of (CH)6 units have been experimentally characterized, i.e., cis-Dewar benzene (II), benzvalene (III), 7-10 prismane (IV), and biscycloprop-2-enyl (V) (Figure 1A). Compounds I–V follow the octet rule as all the carbon atoms are tetravalent in the absence of a formal charge localization. Interestingly, the calculated natural resonance theory (NRT) weight values for benzene suggest a slight admixture of ionic 11 canonical forms in the electronic structure. The ionic character of benzene allows one to envision a number of zwitterionic valence isomers. For example, based on ionic resonance form I', it may be postulated that zwitterionic valence isomer 12VI (Figure 1B) could be formed via four-center two-electron 14 bonding interaction between two odd-electron π-orbitals 15-18 (Figure 1C). Thus far, however, such possible zwitterionic valence isomers have not yet been observed, neither experimentally nor computationally. Borazine (B3N3H6), also referred to as inorganic benzene, is highly stable although it does not exhibit aromaticity on ac19-21 count of its relatively limited electronic delocalization.

Figure 1. Benzene valence isomers. (A) Benzene (I) and its known nonionic valence isomers II–V. (B) Zwitterionic valence isomer VI is formed from the ionic resonance form I’ of benzene. (C) Four-center two-electron bonding interaction via σ-type overlap of two π-orbitals in I’.

This is probably due to the polarity of the B–N units, which in turn arises from the different number of valence electrons of the boron and nitrogen atoms and their respective electronegativity (χB = 2.04, χN = 3.04). Given that B and N are isolobal to cationic and anionic CH fragments, respectively, the incorporation of these elements in the framework of benzene may furnish inorganic analogues of the proposed zwitterionic valence isomers of benzene. Herein, we report the synthesis of diazatetraborinane 1 and its chemical reduc22 tion. The latter afforded the first valence isomer derivative of diazatetraborabenzene that is relevant to the envisioned zwitterionic benzene valence isomer VI (Figure 1). A neutral diazatetraborinane derivative featuring a sixmembered B4N2 heterocycle (1) was obtained from the deprotonation of 1,2-bis(anilide)-1,2-bis(dimethylamido)diborane with two equivalents of nbutyllithium, followed by treatment with bis(dimethylamido)diborondichloride (Figure 2). Tetra-

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Figure 2. Synthesis of diazatetraborinane 1 and its chemical reduction reactions. Reduction of 1 with potassium graphite (KC8), followed by addition of one and two equivalents of 18-crown-6 ether (18C6) afforded 2 and 3, respectively. When the reaction mixture is not immediately treated with crown ether after the reduction of 1, 4 is formed.

hydrofuran (THF) was added to a solid mixture of 1 and five equivalents of potassium graphite (KC8) at room temperature, which immediately afforded a dark purple solution. After filtration, a stoichiometric amount of 18-crown-6 ether (18C6) was added to the solution, which resulted in an immediate color change from dark purple to dark blue. By slow diffusion of hexane into the blue solution at ‒26 °C, single crystals of dianionic product 2 were obtained in 68% yield. 1 The H NMR spectrum of 2 shows only a broad singlet at 2.21 11 ppm, which corresponds to the protons of NMe2. In the B NMR spectrum, a broad signal at 12.9 ppm was observed, which is upfield-shifted relative to that of 1 (39.5 ppm), and thus indicative of an increase in the electron density on the boron centers. A single-crystal X-ray diffraction analysis confirmed the ionic nature of 2, in which two potassium cations are trapped by 18C6, whereas the C2-symmetric dianionic fragment contains two six-membered B4N2 units that interact with one potassium ion each. Both potassium atoms are bridged by a molecule of THF (Figure 3A). Each B4N2 ring exhibits a distorted boat shape, whereby both N atoms are bent toward the coordinating potassium atom (sum of the bond angles around the N atoms: ΣN1 = 360.0°; ΣN2 = 359.0°). The N atoms of the NMe2 groups at the B2 and B4 atoms display a pyramidal geometry (ΣN4 = 345.9°; ΣN6 = 345.0°), whereas those at the B1 and B3 atoms retain a planar geometry. The distance between the K1 and K2 atoms (3.660 Å) is well within the sum of the van der Waals radii of two potassium atoms (5.6 Å). The B1−B4 and B2−B3 bonds are not

parallel to each other; i.e., they include a torsion angle of 27.2°. The B1−B4 (1.700(9) Å) and B2−B3 (1.673(9) Å) bond distances are shorter than those in 1 (1.732(3) Å and 1.717(3) Å, respectively) but slightly longer than the reported B–B bond distances of diborane radical anions (1.636(7)−1.661(5) 23-27 The distances between these boron atoms, i.e., B1−B2 Å). (2.201 Å), B1−B3 (3.038 Å), B2−B4 (2.330 Å), and B3−B4 (2.176 Å), are longer than typical B−B single bonds (1.72 Å). However, density functional theory (DFT) calculations performed on 2 at the M06-2X/6-31G(d) level of theory indicated bonding interactions between these four B atoms. The highest occupied molecular orbital (HOMO) and the HOMO‒1 correspond to σ-type bonding orbitals formed between two π(B– B) orbitals in the six-membered B4N2 rings (Figure 3B). Thus, the four B atoms in each B4N2 ring share two electrons, which is consistent with the results of a natural population analysis (NPA), suggesting that the B atoms in 2 are less positively charged (B1: 0.63; B2: 0.48; B3: 0.51; B4: 0.47) than those in 1 (B1: 0.71; B2: 0.79). A natural bond orbital (NBO) analysis yielded the following Wiberg bond index (WBI) values: B1−B2 (0.17), B1−B3 (0.06) B1−B4 (1.02), B2−B3 (1.06), B2−B4 (0.34), and B3−B4 (0.23), which conﬁrm that the bonding interaction between the B2 and B4 atoms is stronger than that between the B1 and B3 atoms. Indeed, an atoms-inmolecules (AIM) analysis revealed the existence of a bond critical point between the B2 and B4 atoms (Figure 3C). In the solid state, 2 is stable for several days at room temperature, while it decomposes into an unidentified mixture after 6 hours in solution.

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Figure 3. Crystallographic characterization and computational results on the molecules prepared in this work. (A) Solid-state structures of 1 (top left), the dianionic part of 2 (top right), 3 (bottom left), and 4 (bottom right) (hydrogen atoms, solvent molecules, and the counterion in 2 are omitted for clarity). All ellipsoids are shown at 50% probability. (B) Plots of the HOMO (left) and HOMO‒1 (right) of 2. (C) The topology of the Laplacian distribution of the charge density in the six-membered B4N2 ring of 2 with bond critical points (blue dots). (D) Plot of the SOMO of 3-D.



To prevent the potassium atoms from interacting with the six-membered B4N2 rings, two equivalents of 18C6 were added immediately after the reduction reaction. The reduction of 1 with KC8 (5 equiv.) in THF, followed by treatment with 18C6 (2 equiv.), afforded a dark blue solution, which is NMR silent even at low temperature (−60 °C). Slow diﬀusion of hexane into this THF solution afforded dark needle-shaped crystals of 3 in 63% yield. We found that 3 could also be prepared by adding 18C6 to a THF solution of 2. An X-ray diffraction analysis at 103 K revealed the involvement of an anionic B4N2 unit and two potassium atoms that are coordinated by 18C6 and two molecules of THF in the unit cell (Figure 3A). The shortest distance between the B4N2 anion and the + [K(18C6)(THF)2] counterions is greater than 3.0 Å, indicating weak interactions. The boat-shaped geometry of the B4N2 ring of 3, as well as its structural parameters, is similar to those of 2, suggesting that the four B atoms in 3 share electrons, as seen in 2. The paramagnetic nature of 3 was confirmed by means of electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetic measurements. The EPR spectrum of 3 shows a broad unresolved resonance both in THF solution (g = 2.00476; −90 ~ 20 °C) and in the solid state (g = 2.00484; −150 ~ 20 °C) (Figure S13). The observed temperature dependence of both the EPR spectrum of the THF solution and the magnetic susceptibility for the crystalline 28 sample of 3 fits well with the Bleaney–Bowers equation, from which a singlet ground state was estimated with the −1 −1 single-triplet gaps (2J) of −419.15 cm (−1.2 kcal∙mol ) by the −1 EPR measurements (Figure S13) and −647.08 cm (−3.7 −1 kcal∙mol ) by the SQUID magnetic measurements (Figure S14), respectively. This observation agrees reasonably well with the DFT results, which show that the singlet ground −1 state 3-S is by 7.8 kcal∙mol more stable than the triplet state. Significantly, the most stable triplet state consists of a doublet state of 3 (3-D) and a neutral potassium atom [K(18C6)(THF)2] rather than a triplet state of 3 (3-T). Such + alkali metal-doped radical ion pairs of the type M2⦁ A⦁‒ (M = alkali metal; A = organic molecule) are considered to be prospective molecular materials with large nonlinear optical 29 (NLO) responses. A comparison of the calculated and experimental geometric parameters for 3 suggests that, even at 103 K, 3 should be best described as a thermally populated 30 triplet state. Indeed, a triplet was still detected experimentally even at 103 K by the SQUID measurements. These re+ sults indicate the significant role of K . While the localization 2of two negative charges in the (N2B4) core must be essentially disfavored, such electronic state is allowed if there is a + potassium cation in close proximity, because a K cation stabilizes the negative charges and thus the closed-shell singlet + state 2. Without K , the electrons will get delocalized in a wider rage to gain greater stabilization, which may lead to the triplet state 3. The singly occupied molecular orbital (SOMO) of 3-D corresponds to the σ-type bonding orbital formed between two π(B–B) orbitals (Figure 3D), confirming that the four B atoms in the B4N2 ring share one electron. An NBO analysis yielded WBI values between those boron atoms of B1−B2 (0.11), B1−B3 (0.03), B2−B4 (0.18), and B3−B4 (0.11). The spin density is mainly localized on the two B atoms (B1: 0.09; B2: 0.31; B3: 0.08; B4: 0.31). The synthesis and isolation of 3-D via independent control reactions have not been successful thus far. When crown ether was not added immediately after the reduction of 1, the dark purple solution gradually turned into

a colorless solution, indicative of thermal isomerization (Figure S19−S21). After work-up, i.e., treatment with two equivalents of 12C4, single crystals of 4 were obtained in 71% yield, and an X-ray diffraction study revealed the dianionic tetraborane structure of 4. Compound 4 represents a rare 31 example of long-chain homocatenated tetraboranes. It seems quite likely that 4 was formed via a ring-opening reac32,33 tion coupled with an electron-precise B–B bond formation from an in-situ-generated dianionic intermediate (Figure S22). This notion is consistent with the results of the NBO and AIM analyses of 2, which show bonding interactions among the four B atoms, especially between the B2 and B4 atoms. In conclusion, we have demonstrated that a diazatetraborbenezene analogue 2 of a hypothetical zwitterionic valence isomer of benzene can be synthesized by simple reduction of diazatetraborinane 1 with KC8 followed by the addition of 2 eq 18-crown-6 ether. X-ray analysis and DFT calculation disclose that two electrons are delocalized over the four boron atoms in the six-membered B4N2 ring with exhibiting a σbonding interaction between two B-B π-orbitals, which is further supported by the generation of the long-chain homocatenated tetraborane 4. Treatment of 1 with twice amount of 18-crown-6 ether for 2 leads to the formation of a paramagnetic potassium-doped radical ion pair 3 that exhibits a thermally populated triplet character. These results in their entirety show that this method represents a useful strategy for the construction of benzene valence isomers that are not accessible via conventional hydrocarbon systems, indicating that this method may potentially lead to a variety of molecules with unique bonding.

ASSOCIATED CONTENT Supporting Information Synthesis, NMR spectra, and crystallographic data (CIF) of 14; EPR spectroscopy; SQUID magnetic measurements; UVVis spectroscopy; Cyclic voltammograms; Computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes B.S. and K.O. contributed equally to the study. The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by Nanyang Technological University (NTU), Singapore, and the Singapore Ministry of Education (MOE2015-T2-032) (R.K.). H.H. acknowledges financial support from City University of Hong Kong (7200534 and 9610369). We thank Dr. Y. Li (NTU) for assistance in X-ray analyses.

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