Letter pubs.acs.org/OrgLett
Tetraaza[1.1.1.1]m,p,m,p‑cyclophane Diradical Dications Revisited: Tuning Spin States by Confronted Arenes Ryohei Kurata, Daisuke Sakamaki, and Akihiro Ito* Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *
ABSTRACT: Diradical dications of two kinds of alternate-meta-para-linked tetraazacyclophanes in which p-phenylene (1) and 9,10-anthracenylene (2a) moieties are embedded in their macrocyclic backbones were successfully isolated as air-stable salts. The structures of 12+ and 2a2+ were elucidated by X-ray analysis, and significantly different types of structural deformation led to different spin density distributions due to the steric demand of the confronted arene moieties. The singlet−triplet energy gaps were determined to be +0.3 kcal mol−1 (+151 K) and −1.0 kcal mol−1 (−503 K) by SQUID measurements, indicating the triplet ground state for 12+ and the singlet ground state for 2a2+.
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therefore, the replacement of the p-phenylenes by the other bulkier arenes entails a conformational change of the macrocycle, thereby leading to a modulation of the exchange interaction.6 Inspired by the ability of multianthracene assemblies as a functional molecular system,7 we were interested in preparing the analogues 2a and 2b (Figure 1) containing 9,10anthracenylenes instead of p-phenylenes. In view of the fundamental importance of diradical chemistry, we reinvestigated the diradical dication of 1 and newly prepared the diradical dications of 2a and 2b to elucidate their intramolecular magnetic interactions, and we succeeded in the isolation and crystallographic characterization of the persistent diradical dications of 1 and 2a. DFT calculations were carried out at the B3LYP/6-31G* level of theory to check the frontier orbitals of 1 and 2a (Figure S1). As has already been reported for 1, the HOMO and HOMO−1 of 1 were composed of two non-disjoint (or coextensive) nonbonding MOs1c of m-phenylenediamine and a HOMO of benzene.4a In the case of 2a, the situation remained unchanged, except for the fact that the anthracenylene π-faces are almost perpendicular to the macrocyclic plane defined by four nitrogen atoms; thus, the 2p-orbitals on the amine centers were in almost parallel to the anthracenylene π-faces. Note that the LUMO of 1 was delocalized over the entire molecule, while that of 2a almost completely localized on the anthracenylene moieties. The orbital energy difference between the HOMO and HOMO−1 of 2a (0.23 eV) was larger than that of 1 (0.14 eV), thereby suggesting that 12+ prefers high-spin correlation in comparison of 2a2+. Macrocycle 1 was first synthesized by Hartwig and coworkers,4c and we have newly prepared compound 2b according to their procedure (see the Supporting Information). First, we tried to isolate the dication salt of 2b without methoxy groups on
iradicals have fascinated organic chemists for a long time, simply because they are considered (i) challenging to the nature that the pairing of electrons usually forms a chemical bond and (ii) a minimal model to provide fundamental insight into an intramolecular magnetic interaction and the related phenomena.1 Without consideration of stability, a vast number of organic diradical molecules with a positive or negative exchange interaction have been reported to date. However, several stable bis(triarylamine) diradical dication salts were recently isolated to clarify their structure−property relationship.2 In addition, (poly)macrocyclic oligoarylamines are expected to be promising hole- and spin-containing scaffolds for molecule-based electronics due to their multiredox activity and shape-persistency.3 Chemically oxidized dications of tetraazacyclophanes such as 1 (Figure 1), in which two redox active p-phenylenediamine
Figure 1. Alternate meta-para-linked tetraazacyclophanes 1, 2a, and 2b.
moieties are tethered by two m-phenylenes (or 2,7-naphthalenes) as a typical ferromagnetic coupling unit, are well-known stable high-spin organic molecules,4 and the pure triplet state was proved by the pulsed EPR studies of the diradical dication at low temperatures.5 However, the strengths of exchange coupling (J) or the singlet−triplet energy gaps (ΔES−T = 2J) of these diradicals have still not been experimentally determined. The conformational rigidity of the macrocycle 1, which is roughly derived from the confronted p-phenylenes, can ensure a definite exchange coupling between the radical spins, and © 2017 American Chemical Society
Received: April 24, 2017 Published: June 1, 2017 3115
DOI: 10.1021/acs.orglett.7b01229 Org. Lett. 2017, 19, 3115−3118
Letter
Organic Letters the m-phenylene bridge. However, we failed to isolate the salt of 2b2+ due to gradual decomposition upon treatment with more than 1 equiv of AgSbF6 as an oxidant, as is apparent from the decay of intervalence charge-transfer (IVCT) band around 0.57 eV (Figure S2). Instead, we prepared 2a in which methoxy groups are introduced on the m-phenylene bridge to prevent degradation of the oxidized species. Consequently, 2a showed the hypsochromic shift of the IVCT band upon addition of 2 equiv of AgSbF6, thus providing an evidence of the generation of the dication 2a2+ (Figure S3).8 Finally, the dicationic salts of 1 and 2a, i.e., 12+·2SbF6− and 2a2+·2SbF6−, were successfully prepared and isolated by treatment with 2 equiv of AgSbF6 in CH2Cl2. The isolated salts were stable under ambient conditions and can be stored under aerobic conditions at room temperature. The absorption spectra of 12+·2SbF6− and 2a2+·2SbF6− in CH2Cl2 are shown in Figure S4. Both dications displayed characteristic IVCT bands in the lowest energy region: 12+· 2SbF6− showed a relatively sharp IVCT band around 1.31 eV (945 nm); 2a2+·2SbF6− exhibited a rather broad IVCT band around 0.75 eV (1650 nm). Qualitatively, the IVCT band of 12+· 2SbF6− is similar to that of the radical cation of tetraanisyl-pphenylenediamine,9 while that of 2a2+·2SbF6− is similar to that of the radical cation of the tetraanisyl-m-phenylenediamine derivative (Figure S5). Note that the IVCT band of 9,10bis(dianisylamino)anthracene radical cation (3•+) is also observed in the similar energy region (0.64 eV).10 However, the intense next lowest energy band (1.6 eV) corresponding to the localized aminium radical band9 is not detected for 2a2+ but for 3•+, strongly suggesting that the generated spins in 2a2+ are dynamically delocalized within the m-phenylenediamine moieties. From these observations, it can be inferred that the two generated spins are delocalized within the p-phenylenediamine moieties in 12+, while they are confined within the mphenylenediamine moieties in 2a2+. These observations are confirmed by the X-ray crystallographic studies. Single crystals suitable for X-ray structural analyses were obtained from CH2Cl2/toluene solution for both 12+·2SbF6− and 2a2+·2SbF6− (Figures 2 and 3). X-ray structures of the neutral species 1 and 2a were also determined for comparison (Figures 2 and 3). The first important point is that the dihedral angle between the confronted arene (9,10-anthracenylene or pphenylene) π-faces and the macrocyclic plane defined by the four nitrogen atoms increased on going from 2a to 2a2+, whereas it decreased from 1 to 12+. Furthermore, a noticeable shortening of one of three N−C bond lengths around each nitrogen center was observed in N−Cp‑phenylene for 12+ and N−Cm‑phenylene for 2a2+, while the three N−C bond lengths are almost the same value in 1 and 2a. In addition, the averaged O−Cm‑phenylene bond length of the methoxy groups substituted on the m-phenylene bridges decreased on going from 1.371 Å (2a) to 1.340 Å (2a2+). This is explainable by mesomeric effect of the methoxy groups substituted on the m-phenylene bridges. Judging from these observations, it can be safely said that (i) a semiquinoidal deformation of the p-phenylenediamine moieties takes place for 12+, thus indicating that the generated charge and spins are delocalized within the two p-phenylenediamine moieties; (ii) a structural deformation of the m-phenylenediamine moieties are seen for 2a2+, which is a clear sign of charge and spin localization within the two meta-phenylenediamine moieties. The bond length alternation (BLA) parameter is often used to measure the extent of quinoidal deformation of benzene rings. The BLA parameter is defined as the difference between the averaged a bond length and the averaged b bond lengths in Figure 2c. As
Figure 2. ORTEP diagrams, selected bond lengths, dihedral angles, and the bond-length alternation (BLA) values of (a) 1 and (b) 12+·2SbF6− as determined from X-ray structural analysis at 144 K. Hydrogen atoms are omitted for clarity; nitrogen, oxygen, fluorine, and antimony atoms are colored in blue, red, green, and black, respectively. The thermal ellipsoids are shown at the 50% probability level. (c) Bonds named a and b are used to calculate the BLA parameters of these molecules (see the text).
shown in Figures 2 and 3, the large BLA values of 12+ displayed a clear quinoidal deformation of the p-phenylene moiety. On the other hand, the BLA values of the central six-membered ring in 9,10-anthracenylene moieties for both 2a and 2a2+ exhibited a negligible change, thereby indicating that 9,10-anthracenylene moieties remain almost intact even after the oxidation. In contrast, the BLA values for the m-phenylene bridges became considerably larger for 2a2+, whereas those of both 1 and 12+ remained almost unchanged. Overall, these observations elucidated that the generated spins are delocalized over the entire p-phehylenediamine moiety in 12+, while they are confined within the m-phenylenediamine moiety in 2a2+ (Figure 4). This picture agrees well with the fact that SbF6− counteranions in 12+· 2SbF6− and 2a2+·2SbF6− locate in the vicinity of p-phenylene moieties and m-phenylene bridges, respectively (Figures 2 and 3116
DOI: 10.1021/acs.orglett.7b01229 Org. Lett. 2017, 19, 3115−3118
Letter
Organic Letters
compare the structural modification of the corresponding diradical dications due to the difference of counteranions (Figures S6 and S7). As a whole, the molecular structure in 12+·2[B(C6F5)4]− salt is similar to that in 12+·2SbF6− salt, while the distortion of the molecule was relaxed in 2a2+·2[B(C6F5)4]−. This may be considered a counteranion effect: SbF6− acts as a “hard” anion to strongly polarize the molecular cation, whereas [B(C6F5)4]− is known as one of weakly coordinating anions.11 EPR spectra were recorded for a frozen CH2Cl2 solution of 12+·2SbF6− and 2a2+·2SbF6− salts at 123 K (Figure S8). The observed spectra exhibited typical fine structures and half-field resonances for forbidden transitions, which provide clear evidence for the existence of triplet species of 12+ and 2a2+. From the detected fine structures, we obtained the zero-field splitting parameter 2|D| of 10.7 mT for 12+ and 12.1 mT for 2a2+. Average distances between the two radical centers were estimated to be 8.0 Å for 12+ and 7.7 Å for 2a2+ within the point dipole approximation. SQUID measurements were carried out to elucidate the singlet−triplet energy gap, ΔES−T (Figure 5). The 12+·2SbF6−
Figure 3. ORTEP diagrams, selected bond lengths, dihedral angles, and the bond-length alternation (BLA) values of (a) 2a and (b) 2a2+· 2SbF6−, as determined from X-ray structural analysis at 93 K. Hydrogen atoms are omitted for clarity; nitrogen, oxygen, fluorine, and antimony atoms are colored in blue, red, green, and black, respectively. The thermal ellipsoids are shown at the 50% probability level.
Figure 5. Plot of χMT versus T for (a) 12+·2SbF6− and (b) 2a2+·2SbF6− at 0.1 T. The dashed curve represents the best theoretical fit.
salt exhibited the χMT value of 0.82 emu K mol−1 at 300 K, exceeding 0.75 emu K mol−1 expected for two uncoupled spins of 1/2, and the value gradually increased with decreasing temperature, thus indicating an intramolecular ferromagnetic interaction for 12+. The rapid decrease of χMT value at low temperatures is due to the intermolecular antiferromagnetic interaction (the Weiss constant (θ): −5 K). In sharp contrast with 12+·2SbF6−, the χMT value for 2a2+·2SbF6− decreased with decreasing temperature and finally approached zero around 100 K, indicative of an intramolecular antiferromagnetic interaction for 2a2+. The ΔES−T values for 12+·2SbF6− and 2a2+·2SbF6− were determined to be 0.3 kcal mol−1 and −1.0 kcal mol−1, respectively, from fitting with the Bleaney−Bowers singlet− triplet model equation12 for the measured data (Figure 5). In 12+· 2SbF6−, each spin is delocalized over the p-phenylenediamine moiety, and the two spins are ferromagnetically coupled with each other through the m-phenylene bridge as a ferromagnetic coupling unit. On the other hand, in 2a2+·2SbF6−, each spin is confined within each m-phenylenediamine moiety, and two spin centers are separated by the perpendicular 9,10-anthracenylene
Figure 4. Schematic drawings of spin density distributions for 12+ and 2a2+.
3). Obviously, the present contrasting results were mainly owing to the different types of confronted arenes within the macrocyclic framework: the p-phenylene effectively connects the πconjugation, while the 9,10-anthracenylene efficiently disconnects the π-conjugation. Fortunately, we also obtained the single crystals of [B(C6F5)4]− salts for 12+ and 2a2+, that is, 12+· 2[B(C6F5)4]− and 2a2+·2[B(C6F5)4]−, thus allowing us to 3117
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moieties, as seen in the crystal structure, and this spin density distribution resulted in the singlet ground state of 2a2+. From the DFT computations based on the broken-symmetry approach,13 the ΔES−T values for 12+ and 2a2+ were estimated to be +0.8 and −0.8 kcal mol−1, respectively, in good correlation with the experimental values. In summary, we isolated the stable salts of the diradical dications of alternate-meta-para-linked tetraazacyclophanes, 1, in which the intramolecular exchange interaction remained undetermined for a long time, and 2a bearing 9,10anthracenylene moieties in the macrocyclic backbone. Such isolated diradical dication salts were fully characterized from a structural viewpoint by means of X-ray crystal structure analysis as well as from both electronic and magnetic standpoint on the basis of EPR spectroscopy, SQUID measurements, and DFT investigations. The two kinds of salts, 12+·2SbF6− and 2a2+· 2SbF6−, exhibited a remarkable difference in spin density distribution: in 12+·2SbF6−, the generated spins were delocalized over the two p-phenylenediamine moieties and ferromagnetically coupled with each other; on the other hand, in 2a2+·2SbF6−, the generated spins were separated into the two m-phenylenediamine moieties via perpendicularly confronted 9,10-anthracenylene moieties and antiferromagnetically coupled with each other. These findings provide new insight into the design of diradical molecular systems.
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REFERENCES
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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.orglett.7b01229. Crystallographic data for 1 (CIF) Crystallographic data for 2a (CIF) Crystallographic data for 12+(SbF6− salt) (CIF) Crystallographic data for 12+([B(C6F5)4]− salt) (CIF) Crystallographic data for 2a2+(SbF6− salt) (CIF) Crystallographic data for 2a2+([B(C6F5)4]− salt) (CIF) Synthetic details, NMR, UV−vis−NIR, EPR, and DFT calculations, and X-ray structural data for 1, 2a, and their dication salts (PDF)
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Akihiro Ito: 0000-0002-8698-0032 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (JSPS KAKENHI Grant No. JP15H00734). We are grateful to Dr. Takashi Matsumoto (Rigaku Corp.) for X-ray structural analysis. Elemental analyses were performed by the Center for Organic Elemental Microanalysis, Kyoto University. Numerical calculations were partly performed at the Supercomputer System of Kyoto University (Japan) and the Research Center for Computational Science in Okazaki (Japan). 3118
DOI: 10.1021/acs.orglett.7b01229 Org. Lett. 2017, 19, 3115−3118
