A New Trend in Phenalenyl Chemistry: A Persistent Neutral Radical, 2,5,8-Tri-tert-butyl-1,3-diazaphenalenyl, and the Excited Triplet State of the Gablesyn-Dimer in the Crystal of Column Motif

Abstract

Stable gable radical: 1,3-Diazaphenalenyl radical, a typical example of an isoelectronic mode of modification for phenalenyl, has been isolated for the first time as a crystalline solid by introducing bulky substituents (see picture, tert-butyl groups are omitted from the crystal structure). The gable syn dimer with a column motif shows an extremely strong antiferromagnetic exchange coupling of 2J/kB=−4.19(2)×103 K. Phenalenyl (1) is a highly symmetric (D3h) odd alternating-hydrocarbon π radical, found as early as 1956,1 and it still plays an important role as a building block for spin-mediated molecular functions in organic molecule-based magnets,2 and organic metal and conducting materials.3 Recent progress in phenalenyl chemistry has been made in the isolation of the radical itself in the crystalline state by employing bulky substituents (24a and perchlorophenalenyl4b), with the exploration of amphoteric redox systems5 which have intriguing potential applications, such as organic molecular batteries,6 and with the synthesis of novel phenalenyls with extended conjugation, such as compound 3.7 1,3-Diazaphenalenyl (4) is a typical example of the isoelectronic mode of heteroatomic modification for phenalenyl. Successful isolation of 24 aided by the steric hindrance induced by the bulky groups at the β-positions (2-, 5-, and 8-positions) has encouraged us to synthesize 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl (6), which has led to the isolation of such a radical as crystalline solid for the first time, in contrast to a pioneering attempt by Sabanov et al. for compound 5.8 The synthesis, electronic structure, and a gable syn-dimerization of 6 in the crystal structure, which has a column motif, are reported here. Interestingly, the column structure motif is a counterpart of the herringbone one of homoatomic phenalenyl 2.1 The precursor, 2,5,8-tri-tert-butyl-1,3-diazaphenalene (7; Scheme 1), was prepared from tert-butylated dinitronaphthalene 89 in two steps: 1) reduction with Sn, SnCl2⋅2 H2O under acidic conditions, 2) condensation with tBuCHO followed by the dehydrogenation with Pd/C catalyst.10 Treatment of 7 with active PbO2 and recrystallization gave 6 as green crystals (Scheme 1).11 The radical 6 in the crystal is stable in the absence of air. In air the radical decomposes slowly, but most of it remains unchanged for weeks, thus showing higher stability than 2.4a The increased stability resulting from the heteroatomic modification in the π conjugation is contrary to the claim by Sabanov et al. for 5.8 Synthetic route for 6. Reagents and conditions: a) Sn (4.3 equiv), SnCl2⋅2 H2O (7.9 equiv), conc. HCl:AcOH (1:1), 100 °C, 5 h, 82 %; b) tBuCHO (1.3 equiv), 3 mol % Pd/C, xylene, reflux, 3.5 h, 77 %; c) PbO2 (5 equiv), degassed benzene, RT, 1.5 h, recrystallized from hexane, 42 %. To clarify the bulk magnetic properties of the crystalline state of 6, the magnetic susceptibility χp of a polycrystalline sample was measured from 1.8 to 350 K at 0.1 T.12 Below 250 K 6 exhibits a weak paramagnetism from impurity radicals (ca. 0.1 %) presumably arising from lattice defects. The result shows that molecular assemblies of 6 in the crystal are in the spin-singlet state, thus revealing the dimerization of 6. Dissolution of this solid sample in organic solvents gave the strong ESR signals characteristic of 6, confirming that 6 remains stable and unchanged in the crystal under inert atmosphere. On raising the temperature to 350 K, a small but significant increase in χpT was observed, which suggests the possible existence of thermally accessible paramagnetic states. To identify such a thermally activated paramagnetic state, cw-ESR (cw=continuous wave) spectra were measured for the solid sample of 6 (Figure 1 a1). In addition to the central intense signal from the monoradical impurities, fine-structure triplet-state ESR signals (ΔMS=±1 allowed transitions) with axial symmetry are clearly seen. The spectral simulation yielded spin-Hamiltonian parameters S=1, g=2.004, |D|/hc=0.0173 cm−1, and |E|/hc<10−3 cm−1. The temperature dependence of the triplet signal intensity was best described by a singlet–triplet model13 with an energy gap of 2 J/kB=−4.19(2)×103 K (Figure 1 b1). The results seemingly resemble those for 2 (S=1, g=2.003, |D|/hc=0.0167 cm−1, |E|/hc<10−3 cm−1, and 2 J/kB=−3.34(3)×103 K), but the values for 6 are larger than those for 2. The comparison suggests that 6 also forms a dimer structure with a face-to-face staggered arrangement. Such an arrangement ensures minimum steric repulsion between the tert-butyl groups, but the bonding character of the dimerization of 6 is different from that in 2. a) Observed triplet-state ESR spectrum of the powder sample of 6 at 350 K. The x, y, and z denote the canonical absorption peaks. The microwave frequency used is 9.638615 GHz. b) Temperature dependence of the triplet signal intensity ○=observed, —=calculated; 2J/kB=−4.19×103 K. The X-ray crystal-structure analysis14 gives a rationale for the difference in the D and 2 J/kB values between 2 and 6. Figure 2 a and b2 show ORTEP views of the syn-dimer of 6 with a gable structure. In the gable syn-dimer the shortest distance between the α-carbon sites is 0.215 nm and the longest one between the α-carbon and the nitrogen sites is 0.379 nm, in contrast to the pancake-type stacking of symmetric pure π-dimerization for 2 (0.320–0.332 nm).4a The gable syn-dimer structure for 6 and asymmetric bonding nature formed in the dimer are induced by the symmetry-breaking incorporation of nitrogen atoms at the 1- and 3-positions. The dimer packing mode is a column structure for 6 (see Supporting Information), which is important in organic metals and conducting materials,3a,3b whereas the herringbone structure for 2 does not give properties that are relevant for these areas.4a ORTEP views of the gable syn-dimer of 6. a) Top view and a schematic representation; the nitrogen sites are located in terms of the most probable face-to-face arrangement; *1: the shortest C−C distance and *2: the longest distance between the α-carbon and nitrogen sites (see text). b) Side view; the tert-butyl groups are omitted for clarity. The observed very low E value (|E|/hc<10−3 cm−1) for 6 reflects axially symmetric nature of the dipolar spin–spin interaction in the excited triplet state of the dimer. The assumed dimer structure, however, does not straightforwardly lead to the electronic structure with a threefold or greater axial symmetry because of the symmetry lowering caused by the incorporated nitrogen atoms. Indeed, 13C cross-polarization magic-angle spinning (CPMAS) solid-state NMR measurements of the molecular crystal at room temperature indicated the lower symmetric (C1) dimerization of 6 in the singlet ground state. A rationale for the low E value is that the dipolar spin–spin interaction within the syn-dimer molecule undergoes averaging and smearing out of a small amount of the departure from the axial symmetry as a result of inhomogeneous hyperfine broadening in the fine-structure spectra, which gives rise to an apparent high symmetry with a more than threefold rotation axis. To evaluate the heteroatomic effects of 6 induced by a pair of nitrogen atoms, the π-spin-density distribution of 6 was determined by liquid-phase ESR and ENDOR/TRIPLE spectroscopy. Under inert atmosphere, 6 in a toluene solution was extremely stable in contrast to 5.8 The ESR spectrum is interpreted by 1H-, 14N-, and 13C (1.1 %) hyperfine couplings as summarized in Table 1. The π-spin densities on the nitrogen and carbon sites were determined by the observed couplings with the help of McConnell and Heller–McConnell equations, respectively. To determine the π-spin densities on the other carbon sites without protons, the Fraenkel–Karplus equation was applied. Such an empirical treatment gave all the π-spin densities, as depicted in Figure 33. It appears that a robust π-spin polarization similar to the parent phenalenyl radical 1 is maintained in 6 in spite of the heteroatomic modification, which gives rise to the alternation of the signs of the π spin densities between the neighboring sites. By comparing the spin densities of 6 with those of 2,4a it appears that the spin densities of the 1- and 3-positions decrease appreciably, while those of the 4-, 6-, 7-, and 9-positions increase, that is, these four sites become more “active” following the heteroatomic perturbation. Nevertheless, the presence of the bulky substituents at the 5- and 8-positions contributes to the chemical stabilization of the 1,3-diazaphenalenyl π system. aH [mT] aN [mT] g Value 4, 9 6, 7 2-tBu 5, 8-tBu 1, 3 2.0033 −0.638 −0.715 +0.006 +0.021 +0.292 aC [mT] 2 4, 9 5, 8 6, 7 10, 11 12 13 −0.550 +0.880 −0.820 +1.087 −0.654 −0.885 +0.081 π-spin-density distribution of 6 a) experimentally determined and b) calculated by DFT method using Gaussian 94 (SVWN/6–31G*//SVWN/6-31G*). Open and filled circles denote positive and negative spin densities, respectively. The spin structure of 6 is in harmony with such a gabled face-to-face arrangement of the syn-dimer in the crystal. In such an arrangement maximum orbital overlaps occur between the α-carbon sites with largest and next-largest spin densities, which causes a strong bonding interaction (2 J/kB=−4.19(2)×103 K) between the radicals in the syn-dimer of 6. The reflection spectroscopy and the refined X-ray crystal-structural analysis of 6 and other azaphenalenyls are underway. To our knowledge, the syn-dimer of 6 is not only the first example that exhibits a column crystal-structure motif with pseudo π dimerization between neutral organic radicals,15 but also diverse potentials, such as those of organic conducting materials. In contrast to the herringbone motif for the pure π-dimer 2, the finding of the gable syn-dimer with the column motif for 6 derived by the heteroatomic modification should be of interest in crystal engineering and organic materials science. Supporting information for this article is available on the WWW under http://www.angewandte.com or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.