Trioxotriangulene: Air- and Thermally Stable Organic Carbon-Centered Neutral π-Radical without Steric Protection
28 May 2018-Bulletin of the Chemical Society of Japan (The Chemical Society of Japan)-Vol. 91, Iss: 6, pp 922-931
TL;DR: Based on the design of electronic-spin structure of polycyclic carbon-centered π-radicals, the authors realized a peculiarly stable neutral π radical without bulky substituent groups, 4,8,12-trioxotriangulene (TOT).
Abstract: To stabilize organic neutral radicals, which are usually very unstable chemical species in air atmosphere, “steric protection” is the most general and indispensable method. Based on the design of electronic-spin structure of polycyclic carbon-centered π-radicals, we have for the first time realized a peculiarly stable neutral π-radical without bulky substituent groups, 4,8,12-trioxotriangulene (TOT), whose decomposition point is higher than 240 °C in the solid state under air. This remarkably high air-stability as a neutral radical is achieved by spin-delocalization originating from the symmetric and expanded π-electronic structure of TOT. The oxo-functionalities also highly contribute to the high stability through electronic-spin modulation, where the largest electronic spin density located at the central carbon atom further decreases the spin densities of the peripheral carbon atoms. In the solution state, TOT is in the equilibrium between the monomer and highly symmetric π-dimer, as stabilized by the f...
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TL;DR: The role of van der Waals interactions in pancake bonded dimers, chains, and other aggregates is different from closed shell vdW aggregates: here the Pauli repulsions reduce the attractive dispersion interaction significantly.
Abstract: A category of parallel π-stacking interaction, termed pancake bonding, is surveyed. The main characteristics are: the interaction occurs among radicals with highly delocalized π-electrons in their singly occupied molecular orbitals (SOMOs), the contact distances in the π-stacking direction are shorter than the typical van der Waals distances, and the stabilization obtained by the bonding combination of the SOMO orbitals leads to direct atom-to-atom overlap with strong orientational preferences. These atypical intermolecular interactions contain a component of electron sharing between the radicals that can be viewed as covalent-like. Pancake bonded dimers characteristically have low-lying singlet and triplet states and show characteristic interlayer vibrational modes. Pancake bonded aggregates serve as molecular components in many conducting and other functional organic materials. The role of van der Waals (vdW) interactions in pancake bonded dimers, chains, and other aggregates is different from closed shell vdW aggregates: here the Pauli repulsions reduce the attractive dispersion interaction significantly. Fluxionality between π- and σ-bonded aggregates often occur in the context of pancake bonding. Both experimental and computational aspects are reviewed.
142 citations
TL;DR: The aim herein is to highlight the recent achievements in radicals in chemistry, materials science, and flexible electronics, and further bridge the gap between these three disciplines.
Abstract: In the last few years, air-stable organic radicals and radical polymers have attracted tremendous attention due to their outstanding performance in flexible electronic devices, including transistors, batteries, light-emitting diodes, thermoelectric and photothermal conversion devices, and among many others. The main issue of radicals from laboratory studies to real-world applications is that the number of known air-stable radicals is very limited, and the radicals that have been used as materials are even less. Here, the known and newly developed air-stable organic radicals are summarized, generalizing the way of observing air-stable radicals. The special electric and photophysical properties of organic radicals and radical polymers are interpreted, which give radicals a wide scope for various of potential applications. Finally, the exciting applications of radicals that have been achieved in flexible electronic devices are summarized. The aim herein is to highlight the recent achievements in radicals in chemistry, materials science, and flexible electronics, and further bridge the gap between these three disciplines.
117 citations
TL;DR: This Minireview highlights these newly explored, stable carbon-centered radicals, with a focus on porphyrinoid-stabilized radicals because of their remarkable spin delocalization abilities.
Abstract: Organic radicals can potentially play important roles in functional materials owing to an unpaired electron, but they are usually highly reactive and difficult to use. Therefore, stabilization of organic radicals is important. Among organic radicals, carbon-centered radicals are promising because of their trivalent nature, which enables structural diversity and elaborate designs, but they are also less stable because of the reactivities towards carbon-carbon bond formation and atmospheric oxygen. Recently, stable carbon-centered radicals across diverse molecular platforms have been increasingly explored. This Minireview highlights these newly explored, stable carbon-centered radicals, with a focus on porphyrinoid-stabilized radicals because of their remarkable spin delocalization abilities.
91 citations
15 May 2019
TL;DR: In this article, the oxygen reduction reaction activities of trioxotriangulene derivatives, which are stable neutral radicals with high redox abilities, were characterized via rotating disk electrode measurements in alkaline aqueous solution.
Abstract: Development of a structurally well-defined small molecule with a high oxygen reduction reaction catalytic activity is a key approach for the bottom-up design of a metal-free carbon-based catalysts for metal-air batteries and fuel cells. In this paper, we characterize the oxygen reduction reaction activities of trioxotriangulene derivatives, which are stable neutral radicals with high redox abilities, via rotating disk electrode measurements in alkaline aqueous solution. Among trioxotriangulene derivatives having various substituent groups, N-piperidinyl-substituted derivative mixed with acetylene black shows a high catalytic activity with the two-electron transferring process exceeding other derivatives and quinones. To reveal the correlation between molecular structure and catalytic activity, we discuss substituent effects on the redox ability of trioxotriangulene derivatives, and demonstrate that a molecule with electron-donating groups yields relatively higher catalytic activities. Catalytic oxygen reduction is an important process for clean energy production. Here the catalytic activity of trioxotriangulenes for the oxygen reduction reaction is shown to be correlated with their redox potential, offering a potential route to rationally tune their catalytic activity.
43 citations
TL;DR: Dynamic covalent bonds by stable radical species are ideal platforms for simple, facile, and clean rearrangements of chemical bonds without the need for catalysts and the formation of byproducts.
Abstract: Reversible covalent bond formation/scission systems, referred to as Dynamic Covalent Chemistry (DCC), have received significant interest in view of the molecular systems, which offer feasible “error-correction” of the targeted chemical structures and the rearrangement of chemical bonds into the proper manner during the synthetic processes. DCC has been widely designed and developed with molecules in chemical equilibria where a set of reactants and a product are both in closed shell molecules. Within a few years, the concept of DCC systems has been extended to the formation of a covalent bond between a set of stable radicals, utilizing a common feature of radical species: the ease of the bond cleavage and formation. Generally, the coupling reactions among radical species are thermodynamically favorable in a down-hill manner with no or extremely small energetic barriers, and the barrier of the dissociation reactions can be minimized by properly designing the radical species. This review highlights the examples of the radicals showing reversible oligomerization–dissociation behavior, which have been or will potentially be utilized as building blocks in DCC, and the recent development of molecular self-assembly based on reversible radical coupling and homolytic cleavage reactions.
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16,894 citations
TL;DR: The use of absolute magnetic shieldings, computed at ring centers with available quantum mechanics programs, are proposed as a new aromaticity/antiaromaticity criterion to establish NICS as an effective aromaticity criterion.
Abstract: The ability to sustain a diatropic ring current is the defining characteristic of aromatic species.1-7 Cyclic electron delocalization results in enhanced stability, bond length equalization, and special magnetic as well as chemical and physical properties.1 In contrast, antiaromatic compounds sustain paratropic ring currents3 despite their localized, destabilized structures.1-7 We have demonstrated the direct, quantitative relationships among energetic, geometrical, and magnetic criteria of aromaticity in a wide-ranging set of aromatic/antiaromatic fivemembered rings.5a While the diamagnetic susceptibility exaltation (Λ) is uniquely associated with aromaticity, it is highly dependent on the ring size (area2) and requires suitable calibration standards.6 Aromatic stabilization energies (ASEs) of strained and more complicated systems are difficult to evaluate. CC bond length variations in polybenzenoid hydrocarbons can be just as large as those in linear conjugated polyenes.2 The abnormal proton chemical shifts of aromatic molecules are the most commonly employed indicators of ring current effects.1 However, the ca. 2-4 ppm displacements of external protons to lower magnetic fields are relatively modest (e.g., δH ) 7.3 for benzene vs 5.6 for dC-H in cyclohexene). In contrast, the upfield chemical shifts of protons located inside aromatic rings are more unusual. The six inner hydrogens of [18]annulene, for example, resonate at -3.0 ppm vs δ ) 9.28 for the outer protons. This relationship is inverted dramatically in the antiaromatic [18]annulene dianion, C18H18, where δ ) 20.8 and 29.5 (in) vs. -1.1 (out).8 Similar demonstrations of paratropic ring currents in antiaromatic compounds are well documented.3,8,9 Chemical shifts of encapsulated 3He atoms are now employed as experimental and computed measures of aromaticity in fullerenes and fullerene derivatives.10 While the rings of most aromatic systems are too small to accommodate atoms internally, the chemical shifts of hydrogens in bridging positions have long been used as aromaticity and antiaromaticity probes.9 δLi+ can be employed similarly, with the advantage that Li+ complexes with individual rings in polycyclic systems can be computed.4,11 We now propose the use of absolute magnetic shieldings, computed at ring centers (nonweighted mean of the heavy atom coordinates) with available quantum mechanics programs,12 as a new aromaticity/antiaromaticity criterion. To correspond to the familiar NMR chemical shift convention, the signs of the computed values are reversed: Negative “nucleus-independent chemical shifts” (NICSs) denote aromaticity; positive NICSs, antiaromaticity (see Table 1 for selected results). Figure 1, a plot of NICSs vs the ASEs for our set of five-membered ring heterocycles,5a provides calibration. The equally good correlations with magnetic susceptibility exaltations and with structural variations establish NICS as an effective aromaticity criterion. Unlike Λ,6 NICS values for [n]annulenes (Table 1) show only a modest dependence on ring size. The 10 π electron systems give significantly higher values than those with 6 π electrons. The antiaromatic 4n π electron compounds, cyclobutadiene (27.6), pentalene (18.1), heptalene (22.7), and planar D4h cyclooctatetraene (30.1), all show highly positive NICSs. Like the Li+-complex probe,4 the NICS evaluates the aromaticity and antiaromaticity contributions of individual rings in polycyclic systems. Scheme 1 (HF/6-31+G*, data from Table 1) shows NICSs for selected examples. The benzenoid aromatic NICSs provide evidence both for localized and “perimeter” models. The naphthalene (1) NICS (-9.9) resembles that of benzene (-9.7), as do the NICSs for the outer rings of phenanthrene (2) (-10.2) and triphenylene (3); the aromaticity of the central rings of the latter two are reduced. The NICS of the central ring of anthracene (4) (-13.3) exceeds the benzene value in contrast to the outer ring NICS (-8.2). Remarkably, the NICS (-7.0) for the seven-membered ring of azulene (5) is very close to that of the tropylium ion (-7.6 ppm), whereas the azulene five-membered ring NICS (-19.7) is even larger in magnitude than that of the cyclopentadienyl anion (-14.3). The four-membered rings in benzocyclobutadiene (6) (NICS ) 22.5) and in biphenylene (7) (19.0) are antiaromatic, but less so than cyclobutadiene itself (27.6). The six-membered rings in these polycycles are still aromatic, but their NICSs (-2.5 (1) (a) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and Antiaromaticity; Wiley: New York, 1994. (b) Garratt, P. J. Aromaticity; Wiley: New York, 1986. (c) Eluidge, J. A.; Jackman, L. M. J. Chem. Soc. 1961, 859. (2) Schleyer, P. v. R.; Jiao, H. Pure Appl. Chem. 1996, 28, 209. (3) Pople, J. A.; Untch, K. G. J. Am. Chem. Soc. 1966, 88, 4811. (4) Jiao, H; Schleyer, P. v. R. AIP Conference Proceedings 330, E.C.C.C.1, Computational Chemistry; Bernardi, F., Rivail, J.-L., Eds.; American Institute of Physics: Woodbury, New York, 1995; p 107. (5) (a) Schleyer, P. v. R.; Freeman, P.; Jiao, H.; Goldfuss, B. Angew. Chem., Int. Ed. Engl. 1995, 34, 337. (b) Jiao, H.; Schleyer, P. v. R. Unpublished IGLO results. (c) Kutzelnigg, W.; Fleischer, U.; Schindler, M. In NMR: Basic Princ. Prog.; Springer: Berlin, 1990; Vol. 23, p 165. (6) Dauben, H. J., Jr.; Wilson, J. D.; Laity, J. L. In Non-Benzenoid Aromatics; Synder, J., Ed.; Academic Press, 1971; Vol. 2, and references cited. The partitioning of ring current or ring current susceptabilitites among various rings in polycyclic syestems were considered earlier, e.g., by Aihara (Aihara, J. J. Am. Chem. Soc. 1985, 207, 298 and refs cited) and by Mallion (Haigh, C. W.; Mallion, J. Chem. Phys. 1982, 76, 1982). (7) Fleischer, U.; Kutzelnigg, W.; Lazzeretti, P.; Mühlenkamp, V. J. Am. Chem. Soc. 1994, 116, 5298. (8) Sondheimer, F. Acc. Chem. Res. 1972, 5, 81. (9) (a) Hunandi, R. J. J. Am. Chem. Soc. 1983, 105, 6889. (b) Pascal, R. A., Jr.; Winans, C. G.; Van Engen, D. J. Am. Chem. Soc. 1989, 111, 3007. (10) (a) Bühl, M.; Thiel, W.; Jiao, H.; Schleyer, P. v. R.; Saunders, M.; Anet, F. A. L. J. Am. Chem. Soc. 1994, 116, 7429 and references cited. (b) Bühl, M.; van Wüllen, C. Chem. Phys. Lett. 1995, 247, 63. The authors have shown that the negative absolute shielding in the center of C60 is nearly the same as δ3He, computed at the same level. (11) Paquette, L. A.; Bauer, W.; Sivik, M. R.; Bühl, M.; Feigel, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 8776. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, ReVision B.2; Gaussian Inc., Pittsburgh, PA, 1995. Figure 1. Plot of NICSs (ppm) vs the aromatic stabilization energies (ASEs, kcal/mol)5a for a set of five-membered ring heterocycles, C4H4X (X ) as shown) (cc ) 0.966). 6317 J. Am. Chem. Soc. 1996, 118, 6317
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TL;DR: In this article, the paramagnetic resonance spectrum of copper acetate is anomalous in that it resembles that of an ion of spin 1, and its intensity decreases as the temperature is lowered.
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1,850 citations
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