Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Ground Triplet Spirobiradical: 2,2′,7,7′-Tetra(tert-butyl)9,9′(10H,10′H)‑spirobiacridine-10,10′-dioxyl Takuya Kanetomo,† Kana Ichihashi,‡ Masaya Enomoto,† and Takayuki Ishida*,‡ †
Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
‡
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S Supporting Information *
ABSTRACT: A new spirobiradical, 2,2′,7,7′-tetra(tert-butyl)-9,9′(10H,10′H)-spirobiacridine-10,10′-dioxyl, was prepared. The crystallographic analysis clarified the D2d molecular structure, suggesting the degeneracy of SOMOs. The magnetic study revealed that intramolecular ferromagnetic coupling was operative with 2J/kB = +23(1) K. To the best of our knowledge, the ferromagnetic coupling parameter is the largest ever reported for a paramagnetic spiro compound.
S
two fragment SOMOs may interact with each other when they are doubly antisymmetric (AA) to both planes (Figure 1a).9,12 Accordingly, it is necessary for a high-spin spirobiradical that each SOMO be symmetric to the bisecting plane [AS or SA (Figure 1b)]. In fact, the spirobi(radical cation) [1 (Scheme 1)] with symmetric fragment SOMOs shows intramolecular ferromagnetic coupling (2J/kB = +3.2 K in the Ĥ = −2JS1̂ ·S2̂ convention).9
piro-junctioned compounds have a unique cruciform-like structure and have been of considerable interest for application as building blocks for covalent−organic frameworks (COFs),1 metal−organic frameworks (MOFs),2 organic light-emitting diodes (OLEDs),3 and thermal-switching materials.4 In particular, the orthogonal arrangement (D2d) has attracted a great deal of attention in magnetochemistry, e.g., radical compounds5−9 and metal complexes.10,11 When each π electron system has one paramagnetic spin in a spirobiradical, only the spiro junction does not suffice to realize a ground triplet state. The ground spin state is regulated by the two orthogonal π systems or spiro conjugation.12 The former favors a triplet state (Stotal = 1) resulting from the degeneracy of singly occupied molecular orbitals (SOMOs) derived from the two π systems, whereas the latter would break the degeneracy of two SOMOs and thus give a singlet state (Stotal = 0).12 The two contributions depend on the orbital symmetry in the fragment MOs of mutually perpendicular π systems (Figure 1). These MOs are classified by orbital symmetry with respect to two planes P and Q (inset of Figure 1). The
Scheme 1. Compounds 1−3
To improve 2J, we should focus on the following points in molecular design: (i) enhancing the spin densities at four surrounding Csp2 atoms and (ii) decreasing the distance between the mutually perpendicular π systems. From these points of view, an N,N′-dioxyl biradical of 9,9′(10H,10′H)spirobiacridine (2)13 is thought to be promising for this purpose. Changing from N-phenyl (1) to N-oxyl (2) makes the spin more limitedly delocalized at the acridine core. Furthermore, the spiro center of 2 is a carbon atom, and thus, the two π planes are closer to each other than those of 1 having a silicon atom as a spiro center. Hence, the intramolecular ferromagnetic coupling of 2 is assumed to be
Figure 1. Interaction of two fragment MOs that are (a) antisymmetric to both planes P and Q and (b) symmetric to the bisecting plane. “A” and “S” denote antisymmetric and symmetric, respectively, and the left and right positions correspond to symmetry to planes P and Q, respectively. © XXXX American Chemical Society
Received: March 13, 2019
A
DOI: 10.1021/acs.orglett.9b00901 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
N2−O2 bond lengths are 1.293(2) and 1.291(2) Å, respectively, which are close to the corresponding values of 2 [1.290(1) and 1.283(2) Å, respectively]13 and typical of 9,10dihydroacridin-10-oxyls.17,18 The N1−C1−C6−C5, N1− C13−C8−C9, N2−C22−C27−C26, and N2−C33−C28− C29 torsion angles are 178.4(1)°, −175.0(1)°, −179.1(1)°, and 179.8(1)°, respectively. These findings suggest that both acridine rings possess a slight butterfly-like curvature (Figures S4 and S5). The shortest interatomic distance of 5.325(2) Å for the O1···O2 bond (Figure 2c) is larger than the sum of the van der Waals radii (3.04 Å).19 This value is also larger than those of 2 [3.176(2) and 3.184(2) Å].13 We can conclude here that the bulky tert-butyl groups were successfully introduced to suppress the intermolecular contacts. The ESR spectrum of 3 measured in toluene at rt is shown in Figure 3a. The g value of 3 was determined to be 2.0054,
stronger than that of 1. In fact, the electron spin resonance (ESR) and density functional theory (DFT) MO calculation on 2 indicated the ground triplet state (Stotal = 1). However, the intramolecular ferromagnetic interaction was hidden in the solid state owing to close radical−radical contacts from two neighboring molecules; in other words, strong intermolecular antiferromagnetic interaction occurred. The head-to-tail (NO)2 arrangement forming a practically covalent dimer14 seems to be an obstacle to insight into intramolecular exchange interaction. In this study, we have introduced four tert-butyl substituents at the para positions to the radical position, which disturbs intermolecular contact. tert-Butylation would also improve the stability of the radical center and solubility in organic solvents, because of the steric bulkiness. Moreover, tert-butyl groups might be accommodated in the void space in crystals, to purge solvated molecules, which sometimes cause the deterioration of solid-state physical properties after desolvation. We synthesized a new spirobiradical, 2,2′,7,7′-tetra(tertbutyl)-9,9′(10H,10′H)-spirobiacridine-10,10′-dioxyl (3), according to the manner developed by Ooishi and co-workers.15 After the precursor diamine was prepared, 3 was synthesized by oxidation of the diamine with m-chloroperbenzoic acid. Although 2 gradually decomposed in a solution phase, as clarified by the presence of doublet impurities in the magnetic measurements, 3 was stable enough to be treated under ambient conditions, thus affording high-purity specimens. Polycrystalline 3 also can be stored in a freezer under nitrogen for two years without any decomposition. The product was characterized by means of elemental, spectroscopic, and X-ray crystallographic analyses (Supporting Information). Compound 3 crystallizes in monoclinic space group P21/n.16 The crystal structure and molecular packing diagram are depicted in Figure 2. Two dihydroacridine skeletons are connected by an sp3-hybridized carbon atom to form a cruciform-shaped structure, as indicated by a dihedral angle of 89.44(1)° between the two acridine planes. The N1−O1 and
Figure 3. (a) X-Band ESR spectrum for 3 in a degassed toluene solution at rt. (b) Frozen-solution ESR spectra of 3 in toluene at 150 K. The top spectrum is the experimental spectrum, and the bottom spectrum the simulated spectrum. For the optimized parameters in the simulation, see the text. The inset shows the half-field absorption at 100 K.
which is typical for nitroxides. The hyperfine splitting constant aN was characterized to be ∼0.44 mT, and the five-line pattern is compatible with the presence of two nitrogen (I = 1) atoms. However, aH values could hardly be evaluated from the unresolved line shape partly because of line broadening due to dipolar and exchange interactions within a molecule.13,14b,20 The frozen-solution ESR spectra of 3 in toluene at 150 K displayed a zero-field splitting (ZFS) structure (Figure 3b). A forbidden signal with |ΔmS| = 2 appeared at half-field (inset of Figure 3b). The ZFS parameters were deduced from simulation with the WIN-EPR SimFonia software21 as |D|/hc = 4.03 × 10−3 cm−1, |E|/hc = 3.73 × 10−4 cm−1, gxx = gyy = 2.003, and gzz = 2.006 for 3. The point dipole approximation, 2D = 3g2μB/r3, with the experimental |D| gave a dipole−dipole distance of 6.9 Å. This value almost reproduces the crystallographically determined distance of 7.1 Å, defined between the centers of the NO bonds. This dipole−dipole separation of 3 is shorter than that of 1 (8.0 Å),9 indicating the advantage of the introduction of a small spiro carbon atom as well as the effective suppression of radical spin delocalization outside the acridine skeleton. A negligible center signal due to monoradicals guarantees the biradical purity. From a close look at the fine structure, additional signals are also found at E = 0 positions, which can be assigned to highly symmetrical species. For details of the analysis, see the Appendix in the Supporting Information.
Figure 2. Crystal structures of 3 (a) viewed almost along the O1−N1 direction and (b) viewed almost along the O2−N2 direction. (c) Molecular arrangement in a unit cell with Z = 4. Thermal ellipsoids for non-hydrogen atoms are drawn at the 50% probability level. Hydrogen atoms have been omitted for the sake of clarity. B
DOI: 10.1021/acs.orglett.9b00901 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 5. Spin density maps drawn from the DFT MO calculation for (a) the ground triplet and (b) the excited singlet states of 3. The geometrical parameters are from the crystallographic results. Blue and white lobes stand for positive and negative spin densities, respectively.
Figure 4. Temperature dependence of the product χmT, measured at 5000 Oe for 3. A solid line stands for a simulated curve. For details, see the text. The inset shows the magnetization curve measured at 1.8 K. A solid line is drawn with a Brillouin function of S = 1.
affording 2J/kB = +30.26 K. Note that this calculated value almost reproduced the experimental value.
The magnetic susceptibility was measured for polycrystalline 3 at 1.8−300 K on a SQUID susceptometer (Figure 4). The χmT value at 300 K of 0.797 cm3 K mol−1 is close to the value of 0.750 cm3 K mol−1 expected for a species having two magnetically isolated radical centers (g = 2; S = 1/2). Upon cooling, the χmT value increased around 70 K and reached 0.939 cm3 K mol−1 at 16 K, which is near the theoretical triplet value of 1.00 cm3 K mol−1 (g = 2; Stotal = 1). This finding implies the presence of ferromagnetic interaction. From the crystallographic study clarifying that the specimen consists of discrete molecules, the major exchange coupling should be assigned to intramolecular interaction. The χmT value decreased upon further cooling and approached 0.564 cm3 K mol −1 at 1.8 K, suggesting the presence of minor antiferromagnetic coupling. The experimental data of 3 were analyzed with the Bleaney−Bowers equation22 (eq 1) based on the singlet−triplet model. A Weiss mean-field parameter (zj) is introduced for intermolecular interaction. Ä ÉÑ NAg 2μB 2 ÅÅÅÅ ÑÑÑ 2 T ÅÅ ÑÑ χm T = Å Ñ kB ÅÅÇ 3 + exp( −2J /kBT ) ÑÑÖ T − zj /kB (1) The best fit curve was given with 2J/kB = +22.8(10) K, g = 2.0054 (fixed), and zj/kB = −1.12(2) K. To the best of our knowledge, the ferromagnetic coupling is the strongest ever reported in paramagnetic spiro-junctioned compounds.8−11 The inset of Figure 4 shows the magnetization curve of 3, measured at 1.8 K. The magnetization reached 1.94 μB at 7 T, being consistent with the ground ferromagnetic Stotal = 1 state with g = 2 (2.0 μB). The experimental curve fell somewhat below the calculated curve from the Brillouin function with S = 1 and g = 2.0054 (a solid line). This finding indicates that weak intermolecular antiferromagnetic interaction is operative, which agrees well with the decrease in the χmT value observed in a low-temperature region below ∼16 K (Figure 4). The DFT MO calculation on 3 was performed using the atomic coordination determined from the crystallographic study. The self-consistent-field energies at the UB3LYP/6311+G(2d,p) level were EBS = −1851.376377264 au with ⟨S2⟩BS = 0.2495 and ET = −1851.376461217 au with ⟨S2⟩T = 2.0007, where BS and T stand for the broken symmetry singlet state23 and the triplet state, respectively. The spin densities of the singlet and triplet states are mapped on the molecule skeleton (Figure 5), which are approximately compatible with the SOMO lobes. The exchange coupling constant is calculated according to Yamaguchi’s equation (eq 2),24
J=
E BS − E T 2
⟨S ⟩T − ⟨S2⟩BS
(2)
In this study, we assumed that 3 would possess the ground triplet state by the mechanism as shown in Figure 1, and actually it has been completely evidenced by the structural and magnetic studies on isolated 3. To confirm the mechanism, we investigated the distribution of molecular orbitals around the frontier orbitals for the triplet state of 3 (Tables S1 and S2 and Figure S6). First, the HOMO−1 and HOMO for α spins displayed the degeneracy of two SOMOs of 3. These orbital lobes are practically the same as that of SOMO for DFT geometry-optimized 9,9-dimethyl-9,10-dihydroacridin-10-oxyl as a model compound for a half-portion of 3 (for details, see the Supporting Information). The β-LUMO and LUMO+1 for 3 also exhibited the degeneracy with essentially the same surface of LUMO of the model compound (Table S2). On the other hand, we observed bonding [MO 152 and 164 for α and β spins (Tables S1 and S2)] and antibonding orbitals (MO 161 and 170 for α and β spins) owing to the spiro conjugation through the AA fragment MOs (Figures S8 and S9), according to this “S” and “A” formalism. They are well split. Therefore, the choice of the acridinoxyl chromophore is rationalized on the basis of the orbital orthogonality between the fragmental SOMOs. In conclusion, a new spiro-junctioned biradical compound was prepared. The intermolecular interaction is suppressed owing to tert-butylation, as designed. By means of SQUID magnetometry, the presence of intramolecular ferromagnetic coupling was successfully proven. The coordination bond formation ability of nitroxide oxygen atoms25 will open a new research field of MOFs and COFs using spirobinitroxides.
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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.orglett.9b00901. Experimental details, 1H and 13C NMR spectra of all new compounds (Figures S1−S3), crystallographic data, computational studies, and the Appendix (ESR analysis) (PDF) C
DOI: 10.1021/acs.orglett.9b00901 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Accession Codes
(13) Ishida, T.; Ooishi, M.; Ishii, N.; Mori, H.; Nogami, T. Polyhedron 2007, 26, 1793. (14) (a) Capiomont, A.; Chion, B.; Lajzerowicz, J. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1971, 27, 322. (b) Kurokawa, G.; Ishida, T.; Nogami, T. Chem. Phys. Lett. 2004, 392, 74. (15) Ooishi, M.; Seino, M.; Imachi, R.; Ishida, T.; Nogami, T. Tetrahedron Lett. 2002, 43, 5521. (16) Selected crystallographic data for 3 are as follows: C41H48N2O2, fw 600.84, monoclinic, P21/n, a = 10.794(2) Å, b = 19.316(4) Å, c = 16.518(3) Å, β = 90.760(11)°, V = 3443.6(12) Å3, Z = 4, dcalc = 1.159 g cm−3, μ(Mo Kα) = 0.070 mm−1, R(F) [I > 2σ(I)] = 0.0478, Rw(F2) (all data) = 0.1182, goodness of fit = 1.069, and T = 100 K for 7873 unique reflections. (17) Nakagawa, M.; Ishida, T.; Suzuki, M.; Hashizume, D.; Yasui, M.; Iwasaki, F.; Nogami, T. Chem. Phys. Lett. 1999, 302, 125. (18) (a) Seino, M.; Akui, Y.; Ishida, T.; Nogami, T. Synth. Met. 2003, 133−134, 581. (b) Imachi, R.; Ishida, T.; Suzuki, M.; Yasui, M.; Iwasaki, F.; Nogami, T. Chem. Lett. 1997, 26, 743. (19) Bondi, A. J. Phys. Chem. 1964, 68, 441. (20) (a) Poole, C. P.; Farach, H. A. Bull. Magn. Reson. 1979, 1, 162. (b) Ishida, T.; Iwamura, H. J. Am. Chem. Soc. 1991, 113, 4238. (c) Roques, N.; Gerbier, P.; Schatzschneider, U.; Sutter, J.-P.; Guionneau, P.; Vidal-Gancedo, J.; Veciana, J.; Rentschler, E.; Guérin, C. Chem. - Eur. J. 2006, 12, 5547. (d) Okazawa, A.; Terakado, Y.; Ishida, T.; Kojima, N. New J. Chem. 2018, 42, 17874. (21) Wever, R. T. WIN-EPR SimFonia, version 1.2; Bruker Instruments: Billerica, MA, 1995. (22) Bleaney, B.; Bowers, D. K. Proc. Phys. Soc., London, Sect. A 1952, 65, 667. (23) Neese, F. J. Phys. Chem. Solids 2004, 65, 781. (24) Yamaguchi, K.; Kawakami, T.; Takano, Y.; Kitagawa, Y.; Yamashita, Y.; Fujita, H. Int. J. Quantum Chem. 2002, 90, 370. (25) (a) Demir, S.; Jeon, I.-R.; Long, J. R.; Harris, T. D. Coord. Chem. Rev. 2015, 289−290, 149. (b) Kanetomo, T.; Yoshitake, T.; Ishida, T. Inorg. Chem. 2016, 55, 8140. (c) Liu, X.; Zhang, Y.; Shi, W.; Cheng, P. Inorg. Chem. 2018, 57, 13409.
CCDC 1883767 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Takuya Kanetomo: 0000-0001-5970-9477 Takayuki Ishida: 0000-0001-9088-2526 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Assoc. Prof. Daisuke Shiomi (Osaka City University, Osaka, Japan) for valuable advice in the analysis of frozen-solution ESR. Ms. Chihiro Matsuhashi and Prof. Takashi Hirano (The University of Electro-Communications) kindly assisted with the HRMS measurements. T.K. was financially supported by the JSPS program for Research Fellowships for Young Scientist (17J08718).
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REFERENCES
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DOI: 10.1021/acs.orglett.9b00901 Org. Lett. XXXX, XXX, XXX−XXX
