Isolable Bis(triarylamine) Dications: Analogues of Thiele's

Article pubs.acs.org/accounts

Isolable Bis(triarylamine) Dications: Analogues of Thiele’s, Chichibabin’s, and Mü ller’s Hydrocarbons Gengwen Tan and Xinping Wang* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China CONSPECTUS: Since the pioneering work by Thiele and Chichibabin, who synthesized the first diradicals bridged by phenylene and biphenylene groups in 1904 and 1907, respectively, numerous efforts have been devoted to synthesizing stable diradicals during the last few decades, and several strategies have been developed to stabilize these highly reactive diradicals. In this Account, we describe the synthesis and characterization of isolable bis(triarylamine) dications, nitrogen analogues of Thiele’s, Chichibabin’s, and Müller’s hydrocarbons, which represent facilely accessible, stable diradicals by replacing carbinyl centers with isoelectronic aminium centers. Along with discussing the molecular structures and electronic structures of the isolated bis(triarylamine) dications, their spectroscopic and magnetic properties are also introduced. Since 2011, we have reported the stabilization of a variety of radical cations bearing the weakly coordinating anion Al(ORF)4− (RF = polyfluorinated alkyl group), which we have recently successfully applied for the stabilization and crystallization of bis(triarylamine) dications, analogues of Thiele’s, Chichibabin’s, and Müller’s hydrocarbons. Prior to our and Kamada’s work, there have been only three stable bis(triarylamine) dications isolated in the solid state. The facile access of bis(triarylamine) dications in their crystalline forms allowed us to pursue a deep investigation of their solid-state structures, electronic structures, and physical properties. Similar to their hydrocarbon analogues, bis(triarylamine) dications possess characteristic resonance structures between open-shell singlet (OS) diradicals and closed-shell (CS) quinoidal forms. The combination of single-crystal X-ray diffraction (XRD) analysis and density functional theory (DFT) calculations has proved to be a robust strategy to gain a better understanding of the electronic structures of the obtained diradicals. The structural parameters obtained from XRD analysis reflect the overall contribution of each resonance structure to the crystal structure. The comparison of the parameters from the crystal structures with those from DFT calculations for the pure electronic configurations (CS, OS, and triplet states) affords an overview of the ground-state structures of the diradicals. To justify the “degree” of singlet diradical character, the diradical parameter y is applied, which is estimated by the occupancy of the lowest unoccupied natural orbital (LUNO) having antibonding nature (y = 0 for the closed-shell and y = 1 for the pure singlet diradical). In addition, magnetic susceptibility measurements serve as a practical experimental method to determine the singlet−triplet energy gaps of the isolable diradical dications. Through detailed studies on isolable bis(triarylamine) dications, magnetic bistability caused by intramolecular electron-exchange interactions was observed. Moreover, we also found that the singlet−triplet energy gaps of the diradicals could be thermally controlled. These investigations highlight the potential of bis(triarylamine) dications as building blocks for functional materials. (n = 3, Scheme 1).11,12 These hydrocarbons possess characteristic resonance structures between open-shell singlet diradicals and closed-shell quinoidal forms (Scheme 1). However, the intrinsic instability of these hydrocarbons prevents their further investigation and practical application. Thus, extensive efforts have been devoted to preparing their derivatives and analogues with enhanced stability and enhanced diradical character.4−7,13 Replacing the carbon centers with nitrogen atoms and losing two electrons to form stable dicationic nitrogen analogues has proved to be one of the most effective ways for the facile isolation of stable bis(triarylamine) dications.

1. INTRODUCTION Diradicals, which possess two unpaired electrons, are of great interest to understand the nature of chemical bonds and have promising applications in nonlinear optics, molecular electronics, and organic spintronics.1−7 Particularly, stable diradicals delocalized over π-conjugated systems have attracted much attention due to their potential application as functional materials.1−7 In 1904 and 1907, shortly after the synthesis of the triphenylmethyl radical by Gomberg,8 Thiele and Chichibabin reported the synthesis of the hydrocarbons depicted in Scheme 1, which are named Thiele’s hydrocarbon (n = 1)9 and Chichibabin’s hydrocarbon (n = 2).10 Since then, a series of carbon-centered π-conjugated diradicals have been synthesized,7 including Müller’s hydrocarbon reported in 1941 © 2017 American Chemical Society

Received: May 6, 2017 Published: July 21, 2017 1997

DOI: 10.1021/acs.accounts.7b00229 Acc. Chem. Res. 2017, 50, 1997−2006

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Accounts of Chemical Research

Using [Al(ORF)4]− as the counterions, we successfully isolated a variety of bis(triarylamine) diradical dications, the nitrogen analogues of Thiele’s, Chichibabin’s, and Muller’s hydrocarbons, in crystalline forms. The high stability of these dications allowed us to pursue a deep investigation of their solid-state structures and magnetic properties. In this Account, we systematically discuss the synthesis and characterization of the stable nitrogen analogues of the corresponding carboncentered diradicals based on work by our group and others since 2013. We target to provide instruction on the design of stable nitrogen-based diradical dications. We hope this Account will inspire innovative synthetic strategies and material designs in triarylamine-based diradicals and polyradicals and promote more synergistic experimental and theoretical collaborations to face the exciting opportunities and great challenges in radical chemistry.

Scheme 1. Thiele’s, Chichibabin’s, and Müller’s Hydrocarbons

The higher stability of bis(triarylamine) dications is likely attributed to the higher electronegativity of the nitrogen atom in comparison to a carbon atom. Prior to 2013, only three stable bis(triarylamine) dications had been reported (Scheme 2). In 2006, Barlow and co-workers isolated bis(triarylamine) dications 12+ and 22+ having closedshell singlet ground states and determined their structures by single-crystal X-ray diffraction (XRD), which represented the first solid-state structures of bis(triarylamine) dications.14 In 2003, Tanaka and co-workers reported the synthesis of the first stable bis(triarylamine) dication 32+ fused by a spiro structural motif, which featured a triplet ground state, but its crystal structure was not determined.15,16 In addition, several persistent bis(triarylamine) dications have been feasibly generated by electrochemical or chemical oxidation, and they feature either a singlet or a triplet ground state, as determined by electron paramagnetic resonance (EPR) spectroscopy or theoretic calculations; however, their solid-state structures are still elusive.17−23 These limited developments are a solid testimony that the chemistry of bis(triarylamine) dications is still in an early stage of study. During the last six years, we have been working on a project involving the synthesis and characterization of stable radical cations. We found that the weakly coordinating anions [Al(ORF)4]− (RF = polyfluorinated alkyl group)24,25 are exceptionally useful in the stabilization and crystallization of radical cations.26−29 For instance, triarylphosphine radical cations,26 a tetraaryldiphosphine radical cation,27 and oddelectron-bonded sulfur28 and selenium radical cations29 were successfully isolated with the aid of [Al(ORF)4]−, and their structures were elucidated by XRD analysis.

2. THE NITROGEN ANALOGUES OF THIELE’S HYDROCARBON The nitrogen analogues of Thiele’s hydrocarbons, 42+−102+, were obtained as crystalline solids through the two-electron oxidation of the corresponding neutral precursors with 2 molar equiv of Ag[Al(ORF)4] (ORF = OC(CF3)3)30 or a combination of AgSbF6 and Li[Al(ORMe)4] (ORMe = OC(CF3)2Me)30 in CH2Cl2 (Scheme 3, Table 1).31,32 Their molecular structures were determined by XRD (except 42+, Figure 1); the important geometric parameters as well as the corresponding calculation results are summarized in Table 1. All the nitrogen atoms exhibit a trigonal planar geometry to the three bonded carbon atoms, and the two planes around the nitrogen atoms are nearly coplanar (α, Figure 1). The angle (θ, Table 1) between the α and β planes, made up of the central bridge, increases from 52+ to 62+ to 72+ (21.6°, 33.0°, and 56.2°, respectively), which is likely owed to the increasing steric repulsion from the central linking unit. The two phenothiazine units in 82+ and 92+ are almost coplanar (Figure 2) and nearly orthogonal to the central aromatic bridges (86.4° for 82+ and 89.6° for 92+). The average N−C bond lengths in 52+ and 62+ (1.346(3) Å in 52+ and 1.354(3) Å in 62+) to the bridged moieties are substantially shorter than that in 72+ (1.405(4) Å). Moreover, the bond-length alternation (BLA) values of the central benzene rings in 52+ and 62+ are 0.092 and 0.087, respectively, which are close to that of Thiele’s

Scheme 2. Stable Bis(triarylamine) Dications 12+−32+

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72+ (0.002) is greatly reduced in comparison to those of 52+ and 62+, indicating the diradical nature of 72+. In the dications 82+ and 92+, the N−Ci and N−Ci′ bonds have similar lengths, and their BLA values (0.008 and 0.040, respectively) of the bridging units are considerably smaller than that of Thiele’s hydrocarbon (0.103).33 The crystal structure of 102+ was determined at 100 and 200 K, and the one measured at 100 K is shown in Figure 3. The molecular packing modes at 100 and 200 K are almost identical, and there is no substantial difference in the anion− cation interactions between the crystal structures at 100 and 200 K. However, some of the structural parameters of 102+ at 100 and 200 K differ greatly. For instance, the angles (θ) between the α and β planes are 58.1° and 61.5° at 100 and 200 K, respectively (Table 1). In addition, there is a crucial difference between the BLAs of the central phenyl groups at 100 (0.016) and 200 K (0.006), and they are much less than that of Chichibabin’s hydrocarbon (0.052), suggesting a strong diradical character of 102+ at both temperatures. The N−C bond lengths to the central bridge at 100 and 200 K are similar (1.433(3) and 1.443(4) Å, respectively), which are much larger than those in 52+−72+. The electronic structures and magnetic properties of 42+−102+ were investigated by EPR spectroscopy and superconducting quantum interference device (SQUID) measurements.

Scheme 3. Structurally Characterized Examples of the Nitrogen Analogues of Thiele’s Hydrocarbons

hydrocarbon (0.103),33 suggesting they feature quasi-quinoidal structures. In contrast, the BLA value of the central bridge in

Table 1. Selected Experimental and Calculated Results for 52+−102+

ΔEX−OSa (kcal/mol) 52+

62+

72+

82+

92+

102+

X-ray CSa,d OSa,d Ta,d X-ray CS OS T X-ray CS OS T X-ray CS OS T X-ray CS OS T X-ray (100 K) X-ray (200K) CS OS T

0 6.0 1.82 0 3.26 3.64 0 1.47 14.8 0 0.0038 16.4 0 0.0013

8.1 0 0.4

avg Co−Co′

BLAb

θ (deg)

1.346 1.362

1.344(2) 1.365

0.092 0.071

21.6

1.429 1.354(3) 1.380 1.400 1.439 1.405(4) 1.394 1.437 1.445 1.456(2) 1.454 1.455 1.455 1.454(10) 1.457 1.459 1.459 1.433(3) 1.443(4) 1.420 1.451 1.453

1.390 1.356(3) 1.372 1.384 1.406 1.432(5) 1.435 1.445 1.447 1.390(2) 1.398 1.396 1.396 1.436(3) 1.446 1.449 1.449 1.394(4) 1.397(5) 1.399 1.410 1.411

0.015 0.087 0.066 0.043 0.002 0.002 0.009 0.027 0.034 0.008 0.002 0.001 0.001 0.040 0.037 0.041 0.041 0.016 0.006 0.027 0 0.002

avg N−Ci

yc

33.0 0.60

56.2 0.85

86.4 0.993

89.6 0.995

58.1 61.5

a X = CS (closed-shell singlet), OS (open-shell singlet), or T (triplet). bBLA is defined as the difference between the average of all the Ci−Co and Ci′−Co′ bond lengths and the average of the Co−Co′ bond lengths. cDiradical character; y values were calculated at the UBH and HLYP/6-31G(d) level. dOptimized at the level of (U)B3LYP/6-31G(d).

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Figure 1. Molecular structures of 52+ (a), 62+ (b), and 72+ (c) and their side views.

Figure 2. Molecular structures of 82+ (a) and 92+ (b).

Scheme 4. Structurally Characterized Examples of the Nitrogen Analogues of Chichibabin’s Hydrocarbons

Figure 3. Molecular structure of 102+ at 100 K.

Figure 4. (a) Temperature dependence of χMT for the crystals of 102+ measured in the sweep mode at the scan rate of 1 K min−1 from 100 to 150 K. (b) Powder EPR spectra of 102+ at 100 K (in black) and 200 K (in red).

The EPR silence of 42+−62+ in solution and in the solid state as well as their diamagnetism in the SQUID measurements of the crystalline samples demonstrate that they exhibit singlet ground states with relatively large single−triplet energy gaps. In contrast, the powder samples of 72+−102+ are EPR active, and the characteristic half-field resonance signals corresponding to the forbidden Δms = 2 transitions were observed, suggesting 2000


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Table 2. Selected Experimental and Calculated Bond Lengths (Å) and Diradical Character (y) for the Nitrogen Analogues of Chichibabin’s Hydrocarbon

2+

12

142+

152+

162+

172+

182+

192+

X-ray CSa,d OSa,d Ta,d X-ray CS OS T X-ray CS OS T X-ray CS OS T X-ray CS OS T X-ray CS OS T X-ray CS OS T

avg N−Ci

avg Cp1−Cp2

avg Ci−Co and Cm−Cp

avg Co−Cm

BLAb

yc

1.336(10) 1.352 1.363 1.374 1.404(5) 1.378 1.412 1.423 1.339(4) 1.368 1.403 1.418 1.365(6) 1.374 1.411 1.421 1.383(7) 1.386 1.419 1.425 1.401(2) 1.396 1.424 1.430 1.459(5) 1.455 1.456 1.456

1.42(1) 1.428 1.451 1.474 1.467(8) 1.441 1.472 1.482 1.414(4) 1.434 1.470 1.482 1.422(9) 1.439 1.474 1.482 1.457(7) 1.448 1.477 1.483 1.410(2) 1.405 1.420 1.423 1.47(3)

1.417(12) 1.429 1.420 1.412 1.422(6) 1.428 1.413 1.409 1.431(4) 1.432 1.414 1.409 1.426(7) 1.428 1.411 1.408 1.407(7) 1.423 1.408 1.406 1.417(3) 1.427 1.418 1.416 1.392 1.401 1.401 1.401

1.373(10) 1.377 1.384 1.390 1.380(6) 1.375 1.388 1.392 1.357(4) 1.369 1.383 1.387 1.363(6) 1.371 1.385 1.388 1.370(7) 1.375 1.387 1.389 1.389(2) 1.389 1.399 1.400 1.390 1.394 1.394 1.394

0.044(2) 0.052 0.036 0.022 0.042 0.053 0.025 0.017 0.074 0.063 0.031 0.022 0.063 0.057 0.026 0.020 0.037 0.048 0.021 0.017 0.0028 0.035 0.014 0.011 0.002 0.002 0.002 0.007

0.615

0.77

0.47 0.81

0.61 0.85

0.79 0.88

0.999

a X = CS (closed-shell singlet), OS (open-shell singlet), or T (triplet). bBLA is defined as the difference between the average of all the Ci−Co and Cm−Cp bond lengths and the average of the Co−Cm bond lengths. cDiradical character; y values were calculated at the UBH and HLYP/6-31G(d) level. dOptimized at the level of (U)B3LYP/6-31G(d).

Scheme 5. Synthesis of the Singlet Diradical 124+ and Its Resonance Structure 12A4+

The SQUID magnetometry of the powder crystalline samples of 72+−92+ affords 2J values of −489.98 (−1.40 kcal mol−1), −6.07 (−0.017 kcal mol−1), and −0.75 cm−1 (−0.0021 kcal mol−1), respectively, by fitting the curves using the Bleaney−Bowers equation with the Hamiltonian H = −2JS1S2 (S1 = S2 = 1/2).34 The experimental results agree well with the singlet−triplet energy gaps obtained by DFT calculations (Table 1). The calculated y values increase from 52+ to 92+, suggesting an increasing

that the spin-triplet states are thermally accessible under the measurement conditions. The zero-field parameters of 72+ are D = 13.4 mT (12.5 × 10−3 cm−1) and E = 1.28 mT (1.19 × 10−3 cm−1) according to the spectrum simulation. The average spin−spin distance is estimated to be 5.9 Å from the D value, which is slightly longer than the intramolecular N···N distance (5.6 Å) in the crystal structure, suggesting the delocalization of the two unpaired electrons over the peripheral aromatic groups. 2001


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Accounts of Chemical Research diradical character (Table 1).35 These results suggest that the electronic structure and single−triplet gap of the bis(triarylamine) dication can be tuned by changing the bridging unit. It is noteworthy that 92+ possesses almost degenerate singlet and triplet states in terms of the small singlet−triplet energy gap, in line with the results previously reported by Okada and co-workers.19 The magnetic properties of diradical 102+ are rather interesting, showing a thermal hysteresis loop at a temperature range from 118 to 131 K, as observed from the temperaturedependent magnetic susceptibility measurements (Figure 4a). Moreover, the powder EPR spectra display typical broad tripletstate signals (Figure 4b). The temperature-dependent spin susceptibility determined by solid-state EPR spectroscopy shows a similar hysteresis loop in comparison to that observed in the SQUID measurements. The two phases in the loop correspond to two different singlet states of the diradical dication, and the singlet−triplet energy gaps are estimated to be 2J = −1.06 and −0.54 kcal mol−1 for the low-temperature and high-temperature phases, respectively, according to the SQUID measurements. Compound 102+ represents the first example of magnetic bistability mainly caused by intramolecular electron-exchange interactions, in contrast to those of organic radicals attributed to intermolecular interactions, such as radical stacks or dimers.36−50 It is noteworthy the slight modification of the bridging units from an unsubstituted benzene in 42+ to tetramethylbenzene in 102+ greatly changes the electronic structure; that is, 42+ has a closed-shell singlet ground state, but 102+ features an open-shell singlet ground state.

triplet carbenes.51 The oxidation of the allenic precursor 11 with 2 molar equiv of (4-BrC6H4)3N•+[SbCl6] in CH2Cl2 afforded a dark purple solid, which was determined to be 124+[SbCl4]4 by XRD (Scheme 5). The central moiety of 124+ is almost coplanar (Figure 5), and the bond length (1.42(1) Å) between the two acridinium cation moieties is in the range of a typical biphenyl single bond (1.48 Å) and a CC double bond (1.34 Å), suggesting the contribution of both the quinoidal form of 12A4+ with a closedshell structure and a singlet diradical structure form (Scheme 5), which is similar to the phenomenon of Chichibabin’s hydrocarbon.33 DFT calculations of 124+ at the BHandHLYP/ 6-31G(d) level reveal that the open-shell singlet state has the lowest energy and thus is the dominate resonance structure contributing to the overall structure. The energy gaps ΔEOS−CS = −13.7 kcal mol−1 and ΔEOS−T = −8.9 kcal mol−1 are obtained after correction by the approximate spin-projection (AP) method.52,53 The diradical character y is 0.615, indicating that 124+ has an intermediate diradical character between a pure singlet diradical and a closed-shell species. The open-shell singlet ground state of 124+ was further confirmed by SQUID measurements.54 The bis(triarylamine) dication 142+ was unexpectedly obtained by the reaction of 13 with 1/2 molar equiv of Ag[Al(ORF)4] (Scheme 6).55 Species 142+ could also be straightforwardly prepared by the two-electron oxidation of the neutral compound 14 with 2 molar equiv of Ag[Al(ORF)4]. The dication 142+ has a slightly curved configuration (Figure 6). The average N−C bond length to the biphenyl moiety

3. THE NITROGEN ANALOGUES OF CHICHIBABIN’S HYDROCARBON Several stable nitrogen analogues of Chichibabin’s hydrocarbons bearing different substituents were synthesized and fully characterized (Scheme 4, Table 2). The singlet diradical tetracation salt 124+[SbCl4]4 was isolated serendipitously by Kamada and co-workers in 2013 during the synthesis of stable

Figure 6. Molecular structure of 142+ and its side view.

(1.404(5) Å) is shorter than those to the peripheral aryl ring systems (1.442(5) Å). The bond length (1.467(8) Å) between the two triarylamine moieties is slightly shorter than a typical biphenyl single bond but much longer than a typical double bond, suggesting that 142+ has the resonance structures shown in Scheme 6. The open-shell singlet diradical nature of 142+ is

Figure 5. Molecular structure of 124+.

Scheme 6. Synthesis of the Singlet Diradical 142+ and Its Resonance Structure

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2J = −2.8 and −2.58 kcal mol−1, were estimated from the SQUID measurements on 172+ and 182+, respectively, which are in accordance with the calculation results (ΔEOS−T = −0.7 and −1.43 kcal mol−1, respectively). The two phenothiazine moieties in 192+ are almost coplanar (Figure 8), and they are nearly orthogonal with the biphenyl

further supported by theoretical calculations, which show a small singlet−triplet energy gap, ΔEOS−T = −1.0 kcal mol−1, and a moderate y value (0.77), higher than that of 124+ (0.615).51 In addition, the 1H NMR spectrum of 142+ reveals broad signals at room temperature, which become sharper upon cooling, further supporting its singlet diradical nature. The dications 152+−192+ were synthesized by the oxidations of the neutral bis-triarylamines with 2 molar equiv of silver salts bearing WCAs.31,56,57 The dication 182+ bridged by a pyrene moiety was recently reported by Ito and co-workers.57 The dication 152+ has a bent geometry, whereas 162+ and 172+ have planar structures (Figure 7).56 The average N−C bond

Figure 8. Molecular structure of 192+.

bridge (83.5°).31 The C−C bond (1.47(3) Å) between the diphenyl groups is quite close to that of a typical biphenyl single bond (1.48 Å), suggesting a small contribution of the quinoidal form. Moreover, the BLA of the central biphenyl bridge is 0.002, substantially smaller than that of Chichibabin’s hydrocarbon.33 In addition, the SQUID measurements gave a very small singlet−triplet gap 2J = −6.76 cm−1 (−0.019 kcal mol−1), suggesting that a singlet is the ground state of 192+ or nearly degenerate with the triplet state.

4. THE NITROGEN ANALOGUE OF MÜ LLER’S HYDROCARBON The nitrogen analogue of Müller’s hydrocarbon 202+ was isolated by treatment of the neutral compound with 2 molar equiv of Ag[Al(ORF)4] (Scheme 7).58 The magnetic susceptibility Scheme 7. Nitrogen Analogue of Müller’s Hydrocarbon

Figure 7. Molecular structures of 152+ (a), 162+ (b), 172+ (c), and 182+ (d).

lengths to the diphenyl units increase from 152+ to 182+, while the BLA values decrease (Table 2). The BLA values of 172+ and 182+ are less than that of Chichibabin’s hydrocarbon (0.052),33 suggesting the diradical characters of 172+ and 182+. The comparison of the experimental and calculated structural parameters clearly reveals that 152+ and 162+ have closed-shell structures, whereas 172+ and 182+ have open-shell singlet ground states. This result shows that the ground-state electronic structures of these species are tunable with different substituents or bridging units. The dications 152+ and 162+ are EPR silent in both solution and the solid state, and their SQUID measurements in powdered forms only show diamagnetism, consistent with their closed-shell structures.56 However, the powder samples of 172+ and 182+ are EPR active, and a Δms = 2 resonance signal attributed to the spin-triplet state is observed at 320 K for 172+. The zero-field parameters of D = 9.45 mT (8.84 × 10−3 cm−1) and E = 0.92 mT (8.60 × 10−4 cm−1) were determined by spectral simulation of the Δms = 1 resonance for 172+;56 however, no well-resolved EPR signals were observed for 182+.57 From the D value, the average spin−spin distance in 172+ is estimated to be 6.6 Å, which is much smaller than the distance (9.8 Å) between the two N atoms in the solid-state structure. Moreover, the singlet−triplet energy gaps,

measurements showed that the singlet−triplet energy gap of 202+ could be controlled by the temperature. The χmT value decreases from 300 to 170 K and then suddenly increases to a maximum at 130 K, followed by a gradual decrease to zero at 50 K (Figure 9a). This indicates the presence of two singlet states at low temperature (LT) and high temperature (HT), and the singlet−triplet gaps are estimated to be 2J = −400.57 cm−1 (−1.14 kcal mol−1) and 872.84 cm−1 (−2.49 kcal mol−1) for the LT and HT phases, respectively. The EPR spectra at 123 and 290 K also display two different broad signals with g factors of 2.00259 and 2.00213, respectively (Figure 9b), consistent with the corresponding critical temperatures determined in the SQUID measurements. This phenomenon can be interpreted by the structural differences at 123 and 200 K (Figure 10). At 200 K, the central terphenyl backbone is nearly planar, while it is sigmoidal with a large torsion angle (26.76°) at 123 K (Table 3), and the N−Cterphenyl bond lengths at 200 K (1.366(4) Å) and 123 K (1.392(3) Å) are considerably different. The calculated y values, via geometry optimizations from the X-ray structures at 123 and 200 K, are 0.89 and 0.79, respectively, showing different 2003


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Figure 9. (a) The χmT versus T plot for the crystals of 202+, and the fitting plots via the Bleaney−Bowers equation. (b) The powder EPR spectra of 202+ at 123 and 290 K.

state. Structural, magnetic, and theoretical studies have shown that the singlet−triplet gaps of these dications could be tuned not only by changing the bridging units, such as 42+−72+, but also by substituent effects, such as 152+−172+. Strikingly, magnetic bistability caused by intramolecular electron-exchange interactions and thermally controllable singlet−triplet energy gaps were observed for the diradical dications 102+ and 202+, respectively. Despite the progress already made, the exploration of isolable triarylamine-based diradicals and polyradicals is still in its infancy.59 Recently, Casado and co-workers applied carbon-bridged oligo(phenylene vinylene)s as linkers to construct bis(triarylamino) dications, and they found that the ground state could be switched from quinoidal closed-shell to open-shell diradicals by increasing the size of the bridging unit.60

Figure 10. Molecular structures of 202+ measured at 123 K (top) and 200 K (below).

■

degrees of diradical character in the LT and HT phases. Dication 202+ is the first example of a thermally controllable singlet−triplet gap for a diradical in the solid state.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

5. CONCLUSION The successful synthesis and characterization of stable bis(triarylamine) diradical dications has been a considerable advance in radical chemistry. Bis(triarylamine) dications are primarily isolated by two-electron oxidation of the corresponding neutral compounds with silver salts of weakly coordinating anions (WCAs), highlighting WCAs as counterions to stabilize reactive cations. The stable nature of these dications in comparison to their hydrocarbon analogues allows us to pursue a deep investigation of their properties, especially in the solid

ORCID

Gengwen Tan: 0000-0002-6972-2197 Xinping Wang: 0000-0002-1555-890X Notes

The authors declare no competing financial interest. Biographies Gengwen Tan was born in Jiangxi, China, in 1987. He obtained his Ph.D. in 2015 from Technische Universität Berlin, Germany.

Table 3. Selected Experimental and Calculated Bond Lengths (Å), Torsion Angle (deg) and Diradical Character (y) for 202+

X-ray (200 K) X-ray (123 K) CSa,d OSa,d Ta,d

avg N−Ar

avg N−Cterphenyl

bond k

avg b and e

bond j

θ

BLAAb

BLABb

yc

1.430(5) 1.414(3) 1.424 1.414 1.413

1.366(4) 1.392(3) 1.383 1.411 1.415

1.446(5) 1.468(3) 1.451 1.477 1.479

1.362(5) 1.376(3) 1.375 1.385 1.386

1.374(6) 1.379(3) 1.377 1.388 1.389

3.78 26.76 17.38 32.17 34.06

0.053 0.031 0.047 0.026 0.024

0.022 0.023 0.045 0.021 0.018

0.79 0.089 0.93

a

X = CS (closed-shell singlet), OS (open-shell singlet), or T (triplet). bBLAA and BLAB are defined as the difference between the average length of the longitudinal bonds (a, c, d and f in A; h, h′, i, and i′ in B) and the average length of the transverse bonds (b and e in A; j and j′ in B) in rings A and B, respectively. cDiradical character; y values were calculated at the UBH and HLYP/6-31G(d) level. dOptimized at the level of (U)B3LYP/6-31G(d). 2004


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Afterwards, he received a fellowship from International Postdoctoral Program of Chinese Scholarship Council and joined the research group of Prof. Xinping Wang in Nanjing University. Since then, he has been working as a postdoctoral fellow researching the chemistry of main-group-element-based radicals. He has coauthored more than 20 research papers in peer reviewed international journals. Xinping Wang was born in Zhejiang, China, in 1974. He obtained his doctorate in 2007 from the University of New Brunswick, Canada. He then carried out his postdoctoral research at the University of California at Davis from 2007 to 2010. Afterwards, he joined the faculty of the School of Chemistry and Chemical Engineering at Nanjing University. In 2015, he received the award of the National Science Fund for Distinguished Young Scholars and was appointed as a Cheung Kong Scholar. He has coauthored more than 40 research papers in peer reviewed international journals. His current research interest focuses on designing and synthesizing novel radicals with interesting functional properties.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21525102 and 21690062, X.W., and 21601082, G.T.), the Major State Basic Research Development Program (Grant 2016YFA0300404, X.W.), and the Natural Science Foundation of Jiangsu Province (Grant BK20140014, X.W.) for financial support. We are grateful to the High Performance Computing Center of Nanjing University for the numerous calculations mentioned in this Account.

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Isolable Bis(triarylamine) Dications: Analogues of Thiele's

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