Assembly of a Planar, Tricyclic B 4 N 8 Framework With s -Indacene Structure

tetrahydrazidotetraborane was obtained in a two-step procedure involving self-assembly of a dilithiodiborate with B 4 N 8 framework and subsequent oxidation of the phenylborate moieties to boranes and biphenyl using Fe(II) as an oxidant.


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neutral, formally 16-π-electron, tricyclic tetrahydrazidotetraborane was obtained in a two-step procedure involving self-assembly of a dilithiodiborate with B4N8 framework and subsequent oxidation of the phenylborate moieties to boranes and biphenyl using Fe(II) as an oxidant.
The chemistry and properties of borazines, the "inorganic" analogues of benzene, were intensely investigated in the mid sixties. 1 Borazine research has experienced a resurgence in the last few years, driven by the applications as precursors to boron nitride fibres and ceramics, 2 as well as molecular materials. 3Boron derivatives of hydrazines have received considerably less attention.However, a few cyclic tetraazadiborinanes have been synthesized and structurally characterized. 4 A limited number of molecules with bi and tricyclic BN frameworks have been reported as well, 5 including analogues of phenalene, 5b naphthalene 5f and Dewar 20 benzene.5e We were interested in studying the ligand properties of the planar 6-π-electron system 1, 6a which was obtained through quantitative self-assembly of monomethylhydrazine with PhB(NMe2)2, according to a reported procedure.6b Deprotonation of 1 with KH followed by methylation with MeI resulted in clean replacement of the more acidic ring proton in 1 with a methyl group.The disappearance of the signal corresponding to the ring proton of 1 at 7.48 ppm in the 1 H NMR spectrum clearly indicated the regiochemistry of the reaction.The lithium amide produced by 30 deprotonation of the exocyclic nitrogen in 2 with LiTMP dimerized through N-B bond formation, producing the dilithium salt of a tricyclic borate dianion 3. The 1 H and 13 C NMR spectra confirmed the presence of two inequivalent phenyl groups and three inequivalent methyl groups, while the 11 B NMR spectrum featured 35 signals corresponding to the borane and the borate moieties at 28.5 and 2.7 ppm, respectively.The chemical shift of the 7 Li NMR signal (-2.18 ppm) is typical for Li(thf)4 + , suggesting the presence of solvent separated ion pairs in THF solution. 7 crystallographic determination for 3 revealed a 40 centrosymmetric structure featuring a tricyclic, annelated B4N8 skeleton containing two B2N3 rings connected through a B2N4 ring (Figure 1).As indicated by the NMR spectra, the skeleton contains two borane and two borate moieties.to the borate center.The coordination sphere of the lithium ion is completed by an intramolecular contact involving the ipso carbon of the phenylborate moiety, and a THF molecule.Oxidation of 3 with FeCl2(thf)2 in THF produced the neutral BN tricycle 4 in low (22 %) but reproducible isolated yield.The other probable product of this reaction, FePh2(thf)2, subsequently decomposed to metallic iron and biphenyl, which were identified in the reaction mixture.A similar decomposition pathway was reported for FePh2(PEt3)2 at temperatures above 0 °C. 9The 1 H and 13 C NMR spectra of 4 featured the resonances expected for one 65 phenyl group and three inequivalent methyl groups, and the signals corresponding to the two inequivalent boron centers merged into a broad singlet at 25.1 ppm.The use of a stoichiometric quantity of I2 as oxidizing agent instead of FeCl2(thf)2 did not result in the formation of 4. The electrochemical oxidation of tetraphenylborate to boric acid and biphenyl in aqueous conditions at a potential of 0.216 V vs. SCE has been reported, 10 however, the reaction of Li[BPh4] with FeCl2(thf)2 failed to produce BPh3.Thus, the formation of the stable polycycle appears to be an essential thermodynamic contributor to the oxidation of the phenylborate 3 to biphenyl and the borane 4.
The X-ray diffraction study of 4 reveals two very similar, centrosymmetric, independent molecules containing planar B4N8 frameworks with s-indacene structure (Figure 2).The tricyclic skeleton is regular, with all B-N and N-N bonds having the same length, between 1.423(3) and 1.441(2) Å.This length is characteristic of a partial multiple bond character for N-B, as observed in borazines (1.42 -1.44 Å), 11 and for a single N-N bond (vide supra).The phenyl substituents show little deviation from the ring plane (5.3°), while the C-N bonds form angles of 22.7 -31.1° 85 with this plane, in an all-trans arrangement.The all-carbon analog of 4, s-indacene, is an unstable molecule with formally antiaromatic character according to the Hückel rule (12-π-electrons). 12Its more stable substituted derivatives 13 can be reduced to formally aromatic, 14-π-electron dianions. 14It could therefore be expected that a two 90 electron oxidation or reduction of 4 would yield a stable dication or dianion, respectively, satisfying the Hückel condition of aromaticity.In order to investigate this hypothesis, theoretical investigations were performed for 4 as well as for its doubly reduced and oxidized forms.The geometry optimized structure of 4 is in excellent agreement with the data from the structural determination (Figure 2).The analysis of Kohn-Sham orbitals of 4 confirms that it is only formally a 16-π-electron system: though the tricyclic B4N8 framework is essentially planar, the geometry around all methyl 100 substituted nitrogen atoms is pyramidal, giving rise to MOs with only partial π-like character (see Electronic Supplementary Information).The HOMO is π-antibonding through all four N-N linkages, which readily explains the observed long bond distances indicative of single bonds.

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The B4N8 framework is signifcantly nonplanar in the anion [4] 2- due to occupation of a MO with B-N antibonding character.Hence, a two-electron reduction of 4 leads to a structure which does not fulfill the general conditions of aromaticity.On the other hand, the two-electron oxidation of 4 yields a dication with a nearly planar 110 structure because two electrons are removed from the N-N antiboding HOMO (see Electronic Supplementary Information).The minor geometric distortions present in the system arise from steric interactions, as evidenced by the geometry optimized structure of a hydrogen substituted tricycle [B4N8H8] 2+ , displaying 115 a perfectly planar Hückel aromatic geometry.Interestingly, the closed shell singlet state SCF solution of [4] 2+ has an internal instability indicating that the singlet ground state of this dication has nitrogen-centered diradical character.Hence, the open-shell singet diradical state of [4] 2+ was modeled using broken symmetry 120 formalism and the calculated structure was compared to the lowest energy triplet state.The calculations showed that the two spin states are very close in energy, with the formally aromatic open-shell singlet state being ca. 5 kJ mol -1 lower than the triplet state.
Unfortunatelly the experimental search for a stable oxidation 125 product of 4 remained fruitless.Cyclovoltammetric measurements were performed in THF in the range of -2.5 to 1.5 V, and they showed two irreversible oxidation steps at 0.35 and 0.90 V vs. SCE (-0.21 and 0.34 V vs. Fc).Chemical oxidation with [Cp2Fe]PF6 produced Cp2Fe and a solid that was insoluble in organic solvents 130 and was not further characterized.Radical species were not detected by EPR upon in-situ electrochemical oxidation.We note that stable radicals with s-indacene-like frameworks are wellknown in the chalchogen-nitrogen chemistry. 19his work was supported by the Natural Sciences and Engineering All operations were conducted under strict exclusion of air and moisture.Solvents were dried prior to use and methylhydrazine was dried over CaH2.Methylated hydrazines are highly toxic and probable carcinogens, and their handling requires special precautions; all residues were neutralized using commercial bleach solution.Synthesis of 2: A solution of 1 6 (0.700 g, 2.65 mmol) in THF (20 mL) was slowly added to a suspension of KH (0.106 g, 2.65 mmol) in THF (15 mL).KH dissolved with evolution of hydrogen producing a yellow solution that was stirred at ambient temperature for another 2 h.MeI (0.165 mL, 2.65 mmol) was added to the mixture and the colourless suspension was stirred for an additional hour and then concentrated under vacuum to ca. 3 mL.Hexane (30 mL) was added and the KI by-product was filtered off.The solvent was subsequently removed in vacuo, leaving behind the product as a colourless powder (638 mg, 88 %). 1 H NMR (400 MHz, C6D6, 25 °C): δ 2.32 (d, 3H, 3 JHH = 6.4 Hz, HNCH3), 2.88 (s, 6H, (NCH3)2), 3.76 (q, 1H, 3 JHH = 6.4 Hz, HNCH3), 7.25 -7.34 (m, 6H, m-+ p-C6H5), 7.74 (d, 4H, 3 JHH = 6.6 Hz, o-C6H5); 13  Synthesis of 3: A solution of lithium 2,2,6,6-tetramethylpiperidide, LiTMP was prepared from 1.6 M n-butyllithium in hexane (1.12 ml, 1.80 mmol) and tetramethylpiperidine, TMP, (0.254 g, 1.80 mmol) in THF (3 mL).The solutions of 2 (0.500 g, 1.80 mmol) in THF (3 mL) and LiTMP 165 were pre-cooled to -35 °C and mixed, yielding an orange solution that was stored at -35 °C for a day.Subsequently it was allowed to warm to room temperature and volatiles were removed in vacuo, leaving behind a yellow residue that was washed twice with hexane (30 mL) and dried under vacuum.The product was obtained as a colourless powder (388 mg, was added to solid FeCl2(thf)2 20 (38.0 mg, 0.141 mmol).The dark mixture was allow to stand at room temperature for a day, and was then concentrated to 1 mL and cooled to -35 °C for several days until colourless crystals of 4 formed and were separated by decantation (10 mg, 22 %). 1  Cyclovoltammetry of 4: The measurements were performed in THF at an analyte concentration of 1mM, and using 0.1 M [nBu4N]PF6 as a supporting electrolyte.A PARstat 2273 potentiostat was used at scan rates between 50 to 1000 mVs -1 .The cell had a platinum disk working