Studies on the Bridge Dependence of Bis(triarylamine) Diradical

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Studies on the Bridge Dependence of Bis(triarylamine) Diradical Dications: Long-Range #-Conjugation and #-# Coupling Systems Shuyu Li, Ningning Yuan, Yong Fang, Chao Chen, Lei Wang, Rui Feng, Yue Zhao, Haiyan Cui, and Xinping Wang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00003 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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The Journal of Organic Chemistry

Studies on the Bridge Dependence of Bis(triarylamine) Diradical Dications: Long-Range π-Conjugation and π-π Coupling Systems Shuyu Li,§,† Ningning Yuan,§,† Yong Fang,§ Chao Chen,§ Lei Wang,§ Rui Feng,§ Yue Zhao,§ Haiyan Cui*,§,‡ and Xinping Wang*,§ §

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center

of Advanced Microstructures, Nanjing 210023, China ‡

Institution Jiangsu Key Laboratory of Pesticide Science, College of Sciences, Nanjing Agricultural University, Nanjing 210095,

China

ABSTRACT: Three bis(triarylamine) dications bridged with 2,2’-bithienyl group (12+), biphenyl group (22+) and [2,2]paracyclophane (32+) have been successfully isolated. The electronic structures of 12+ – 32+ show bridge dependence. Magnetic studies and DFT calculations show that dications 12+ – 32+ possess an open-shell singlet ground state with a thermally excited triplet state. Dication 32+ has rather small singlet–triplet energy gap and could basically be regarded as a nearly pure diradical. Diradical dications 12+ – 32+ represent rare examples of diradicaloids with intramolecular long-range π-conjugation or π-π coupling interaction. INTRODUCTION Stable organic diradicaloids1 have received significant attention because of their unique chemical bonding and interesting physical properties, thus are potential functional materials in molecular electronics. Thiele’s,2a,2e Chichibabin’s2b,2e and Müller’s hydrocarbons2c,2d are representative hydrocarbon diradicals in π-conjugated systems. However, the high reactivity of these hydrocarbon species and their derivatives has hindered the practical application as functional materials. Fortunately, using electronegative nitrogen atoms instead of carbon centers and employing silver salts with weakly coordinating anions as oxidizing agents could lead to more stable

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dicationic nitrogen analogues (Scheme 1).1i Following this strategy, our group has successfully isolated a series of stable nitrogen analogues of Thiele’s, Chichibabin’s and Müller’s hydrocarbons, showing more diradical character than the corresponding hydrocarbons.3 Noteworthily, unusual phenomena were observed such as thermally controlled singlet-triplet energy gap and magnetic bistability.3e,f The previous work clearly shows that the bridging units play important roles in tuning the diradical character of nitrogen-doped dications, as well. Scheme 1. Nitrogen analogues of Thiele′s (n = 1), Chichibabin′s (n = 2) and Müller′s (n = 3) Hydrocarbons.

In the past few decades, considerable attentions have been paid to intermolecular π stacking of radical and radical-ion systems.4 Nevertheless, diradicals in intramolecular π-π coupling systems have rarely been studied probably for the reason of their high reactivity. To the best of our knowledge, only a few examples of diradicals with intramolecular π-π space interactions have been reported.5 [2.2]paracyclophane is a simple scaffold, in which the electronic interactions are based on through-bond (σ) and through-space (π-π) couplings without direct π-conjugation.6 In order to investigate how the diradical character changes with the different bridging systems, we now plan to extend the length of the bridge to four aryl groups. Herein, we reported three bis(triarylamine) dications bridged with 2,2’-bithienyl group (12+), biphenyl group (22+) and [2,2]paracyclophane (32+). Dications 12+ – 32+ were investigated by single crystal X-ray diffraction, UV-vis, electron paramagnetic resonance spectroscopy (EPR), and superconducting quantum interference device (SQUID) measurements, in conjunction with density functional theory (DFT) calculations.

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RESULTS AND DISCUSSION Scheme 2. Neutral bis(triarylamine)s 1 – 3.

MeO

OMe

S N

N S

MeO

OMe

1

MeO

OMe

N

N

MeO

OMe

2

MeO

OMe

N

N

MeO

3

OMe

Neutral precursor 1 (Scheme 2, top) was synthesized according to the similar procedures in reported literatures7a,7c from bis(4-methoxyphenyl)-4-(2-thienyl)phenylamine.7d Compound 2 (Scheme 2, middle) was prepared via cross-coupling amination of 1,4’’-diamino-p-quaterphenylene8 with 4-bromoanisole. Compound 3 (Scheme 2, bottom) was obtained according to the published process.7b Upon reaction with two equiv Ag[Al(ORF)4] (ORF = OC(CF3)4)9 in CH2Cl2, 1 – 3 were oxidized to dications 12+·2[Al(ORF)4]-, 22+·2[Al(ORF)4]and 32+·2[Al(ORF)4]- in considerable yields, respectively. These dication salts were air insensitive and thermally stable in solid state and in solution at ambient temperature.

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Figure 1. Ortep drawing of 12+ (a), 22+ (b) and 32+ (c) with ellipsoids given at the 50% probability level. Hydrogen atoms were

omitted for clarity.

Single crystals of 12+·2[Al(ORF)4]- – 32+·2[Al(ORF)4]- suitable for X-ray crystallographic analysis were obtained from CH2Cl2 at -30 °C. Structures of dications 12+ – 32+ were illustrated in Figure 1, and selected parameters are given in Table 1 – 2. The three diradical salts crystalize in the monoclinic P21/n or P21/c space groups. Each nitrogen atom in 12+ – 32+ is nearly coplanar with three neighboring carbon atoms. Dication 12+ shows a nearly planar geometry between the two nitrogen atoms, exibiting great π-conjugation effect. The bridging four phenyl groups are twisted with dihedral angles of ca. 8.5° (for 22+) and ca. 40° (for 32+). In 32+, the silghtly bent phenyl rings in the [2,2]paracyclophanyl moiety are face-to-face with a distance of ca. 2.9 Å, indicating π-π interaction in the molecule. The average bond length to the phenyl (N–C1 1.358(5) Å) are much shorter than that to the peripheral aryl ring systems (N–Ar 1.439(5) Å) in 12+ (Table 1). And for 22+ and 32+, the average N–C1 bond lengths (~1.42 Å) are very close to the average N–Ar bond length (~1.41 Å) (Table 2), supporting more diradical character than 12+. The C4–C7 bond length between ring A and ring B of bridging moiety of dication 12+ (1.402(5) Å) is much shorter than the neutral compound 1 (1.479 Å)7e (Table 1). The C4–C7 bond lengths of 22+ (1.487(9) Å) and 32+ (1.489(6) Å) are slightly longer than those of a typical biphenyl single bond (1.48 Å). Reduced bond-length alteration (BLA) of the bridged aryl rings is a signature of diradical character in hydrocarbon diradicals.2e,10,11 BLAA is defined as the difference between the average of C1–C2, C3–C4, C4–C5

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and C1–C6 bond lengths and the average of C2–C3 and C5–C6 bond lengths in the ring A of 12+ – 32+, and BLAB is defined as the difference between the average of C7–C8, C9–C10, C10–C11 and C7–C12 bond lengths and the average of C8–C9, C11–C12 bond lengths in the ring B of 22+ and 32+ in this work. The BLAs (BLAA 0.071 for 12+, BLAA 0.024 and BLAB 0.006 for 22+, and BLAA 0.02 and BLAB 0.004 for 32+) are greatly declined from 12+ to 32+, displaying rising diradical nature and great bridging effect. Table 1. Selected experimental and calculated bond lengths (Å), relative energy (kcal/mol) and BLA values for 12+.

∆EX–OSd X-ray b

CS

OS Tc a

c

5.67

avg.N-Ar

avg. N-C1

C4-C7

avg. C2-C3 and C5-C6

avg. C1-C2, C3-C4 C4-C5 and C1-C6

BLAAa

1.439(5)

1.358(5)

1.402(5)

1.355(5)

1.426(5)

0.071

1.425

1.381

1.427

1.374

1.424

0.05

0

1.416

1.403

1.447

1.380

1.415

0.035

1.86

1.413

1.409

1.452

1.382

1.413

0.031

BLA = bond length alteration, i.e. BLAA is the difference between the average of C1–C2, C3–C4, C4–C5 and C1–C6 bond lengths and the average of C2–C3 and C5–C6 bond

lengths in the ring A. bCalculated at the level of B3LYP/6-31G(d). cCalculated at the level of UB3LYP/6-31G(d). dX = CS (closed-shell singlet), OS (open-shell singlet), or T (triplet).

Table 2. Selected experimental and calculated bond lengths (Å), relative energy (kcal/mol) and BLA values for 22+ and 32+.

∆EX–OSd avg N–Ar 22+

X-ray CSb OS

c

c

T

CS

OS Tc a

avg C1–C2, C3–C4,

and C5–C6

C4–C5 and C1–C6

BLAAa

avg C7–C8, C9–C10,

avg C8–C9

BLABa

C10–C11 and C7–C12 and C11–C12

1.412(3)

1.410(3)

1.487(9) 1.376(3)

1.400(3)

0.024

1.390(10)

1.384(9)

0.006

1.417

1.397

1.461

1.380

1.416

0.036

1.415

1.382

0.033

0

1.411

1.415

1.475

1.385

1.410

0.025

1.408

1.388

0.020

1.386

0.3

c

avg C2–C3 C4–C7

10.2

32+e X-ray b

avg N–C1

12.16

1.411

1.416

1.475

1.410

0.024

1.408

1.388

0.020

1.406(6)

1.424(5)

1.489(6) 1.376(6)

1.396(6)

0.02

1.393(6)

1.397(6)

0.004

1.405

1.414

1.473

1.413

0.029

1.408

1.398

0.01

1.384

0

1.417

1.411

1.480

1.386

1.410

0.024

1.407

1.398

0.009

0.004

1.417

1.410

1.480

1.386

1.410

0.024

1.407

1.398

0.009

BLA = bond length alteration, i.e. BLAA is the difference between the average of C1–C2, C3–C4, C4–C5 and C1–C6 bond lengths and the average of C2–C3 and C5–C6 bond

lengths in the ring A, BLAB is the difference between the average of C7–C8, C9–C10, C10–C11 and C7–C12 bond lengths and the average of C8–C9, C11–C12 bond lengths in the ring B. bCalculated at the level of B3LYP/6-31G(d). cCalculated at the level of UB3LYP/6-31G(d). dX = CS (closed-shell singlet), OS (open-shell singlet), or T (triplet). e

Dashed single bonds only exist in 32+.

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Full geometry optimizations of 12+ – 32+ were performed at the (U)B3LYP/6−31G(d) level and the obtained stationary points were characterized by frequency calculations.12 The broken-symmetry approach is applied for open-shell singlet calculations and spin contamination errors have been corrected by approximate spin-projection method.13 Relative energies, bond lengths and their alternations (BLAs) of the optimized closed-shell singlets (CS), open-shell singlets (OS) and pure diradical triplets (T) are listed in Table 1 for 12+, Table 2 for 22+ and 32+. The single-crystal structure of 12+ showed that the structural parameters are close to those of the calculated closed-shell singlet state (12+-CS, Table 1). However, the signal-silence of the 1H NMR spectrum and the calculated relative energies indicate that the dication 12+ may possess diradical character. The experimental bond lengths and BLAs of 22+ and 32+ are found to agree better with those calculated for OS or T configurations than with those for the CS. The diradical character y is defined as the occupancy of the lowest unoccupied natural orbital (LUNO) of the open-shell singlets and represents the ‘‘degree’’ of the diradical character ( y = 0 for closed-shell and y = 1 for pure diradical).11 The diradical character y of 22+–OS (0.93) is larger than that of nitrogen analogues of Chichibabin’s hydrocarbon (0.88)3b and close to that of nitrogen analogues of Müller’s hydrocarbon (0.93).3f Whereas, the y value of 32+–OS is 0.98, far exceeding that of other π-conjugated systems, implying that replacement of the π-conjugated systems with a bridge containing π-π coupling system (32+–OS) leads to dramatically enhanced singlet diradical character. And the y value of 12+–OS (0.74) is much smaller than that of 22+–OS and 32+–OS, indicating less diradical character than the two dications. This may be ascribed to the electron-richness14b of thienyl groups and the nearly planar geometry of the bridge groups. While for 22+, the complete co-planarization is suppressed by the steric repulsion between the hydrogen atoms in ortho-positions of biphenyl moiety. From 12+ to 32+, the calculated energy gaps ∆ECS-OS between the open-shell singlet states and closed-shell singlet states increase (12+ 5.67 kcal/mol, 22+ 10.20 kcal/mol, 32+ 12.16 kcal/mol), while the singlet–

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triplet energy gaps ∆ET-OS decline (12+ 1.86 kcal/mol, 22+ 0.30 kcal/mol, 32+ 0.004 kcal/mol), thus revealing bridge dependence. The ∆ET-OS (0.004 kcal/mol) value for 32+ is rather small and could be thermally excited to the higher energy triplet state facilely. Consequently, 32+ can be regarded as owning almost degenerate singlet and triplet states, or as nearly pure diradicals.14 Barlow group recently reported bisstyryl(hetero)arenes-bridged bis(diarylamino) dications in solution,10,14b which vary from closed-shell singlets (electron-rich bridge) to the more-or-less biradical (electron-poor bridge), exhibiting bridge dependence as well but different coupling ability. Figure 2. Spin densities, HOMO(α) and LUMO(α) of 12+–OS (a, b, c), 22+–OS (d, e, f) and 32+–OS (g, h, i) calculated at the

UB3LYP/6-31G(d) level.

Spin density distributions on peripheral aryl rings, nitrogen atoms, and bridging aryl rings in 12+–OS, 22+–OS and 32+–OS dications also show the bridge dependence. Spin densitiy of 12+–OS (Figure 2a) is delocalized over the whole molecule. Different from 12+, the spin densities of 22+ and 32+ are mainly distributed on two triarylamine units (Figure 2d for 22+ and Figure 2g for 32+), which manifest more diradical character of 22+ and 32+ than 12+. Comparing to 32+–OS with the π-π coupling system in bridging moiety, the spin density of 22+–OS is further delocalized to the bridging π-conjugation rings, supporting that 32+ has a more remarkable diradical character than

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22+. Figure 3. The powder EPR spectra of (a) 12+, (b)22+ and 32+ (c) at 298 K; (d) χMT versus T curve in the SQUID measurements for the powder of 12+, 22+ and 32+, and the fitting plot of 22+ and 32+ obtained with the Bleaney–Bowers equation. (b)

(a)

(c)

(d)

The diradical characters of 12+ – 32+ were further confirmed by EPR (Figure 3a–3c) and SQUID measurements (Figure 3d). The average g factors are 2.0041, 2.0017 and 2.0019 for 12+ – 32+, respectively. The EPR spectra of 12+ – 32+ have not been simulated because clear hyperfine splitting was unobserved. The forbidden half-field transitions (∆ms = ± 2) were clearly observed for 12+ and 22+, indicating that 12+ and 22+ are diradical dications. The intensity of forbidden transition is largely dependent on the distance between the two unpaired electrons. The unobserved half-field transition of 32+ may derive from a large separation of the two spins. Compared to Barlow’ dications,10,14b 12+ – 32+ possess a series of bridges with approx. equal length but give very different magnetic properties. SQUID measurements of 12+ – 32+ in powder form were conducted at 5–360 K, and the signals of 22+ and 32+ can carefully fit with the Bleaney–Bowers equation and Hamiltonian H = - 2J S1 S2 (S1 = S2 = 1/2) at 5–

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300 K,15 and the discrepancy above 300 K may be caused by volatilization of solvent molecules (Figure 3d). The results give small singlet–triplet energy gaps (∆ET–S = 0.1 kcal mol-1 for 22+ and 0.05 kcal mol-1 for 32+). And the declined trend agrees with that of the calculated values (0.3 kcal mol-1 for 22+ and 0.004 kcal mol-1 for 32+) at the (U)B3LYP/6-31G(d) level. The increasing molar susceptibilities with temperature suggest that 22+ and 32+ both have an open-shell singlet ground state (rather than triplet), which can be thermally excited to its triplet excited state at room temperature, resulting from the small singlet–triplet energy gaps, consistent with the calculations. The SQUID measurement for 12+ could not been simulated, but an obvious increasing susceptibility above 300 K can be observed, verifying its open-shell singlet ground state. The nearly signal-silent susceptibility below 300K may result from the large singlet–triplet energy gap. Figure 4. UV-vis absorption spectrum of 12+–32+ (1 × 10-4 M in CH2Cl2) at 25 °C.

The maximum absorption of 12+ (1311 nm) is significantly red-shifted compared to 22+ (750 nm) and 32+ (753 nm) (Figure 4). And the absorption curves of 22+ and 32+ are similar to that of the Barlow’s diradical,14b including the low-energy shoulders. TD-DFT calculations on the open-shell singlet geometry of 12+ – 32+ at the UB3LYP/6-31G(d) level indicate that the NIR absorptions are due to HOMO(α) → LUMO(α) and HOMO (β) → LUMO(β) transitions for 12+, HOMO-2(α) → LUMO(α) and HOMO-2(β) → LUMO(β) transitions for 22+ and

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HOMO-4 (α) → LUMO(α) and HOMO-4 (β) → LUMO(β) transition for 32+ (Figure S13−S15 and Table S2 in the supporting information), in accordance with the experimental absorption. In summary, we have successfully isolated three long-bridged bis(triarylamine) diradical dications 12+ – 32+, showing bridge dependence. Their geometries and electronic structures have been fully characterized by single-crystal X-ray diffraction, EPR, SQUID and UV-Vis absorption spectroscopy, in conjunction with theoretical calculations. Dications 12+ – 32+ feature open-shell singlet ground states with thermally excited triplet states. Dication 32+, with π-π coupling system in the molecule, has a rather small singlet–triplet energy gap, thus could basically be regarded as a nearly pure diradical. Dications 12+ – 32+ represent the rare examples of diradicaloids with long-range π-conjugation or intra π-π coupling interaction. This work demonstrates the significance of different bridging systems, such as π-conjugated and π-π coupling system, in tuning the electronic structures of bis(triarylamine) dications. Exploration of the application of these systems and other long bridged diradicaloids is under way.

EXPERIMENTAL SECTION General considerations. All operations were carried out under an atmosphere of dry argon or nitrogen by using modified Schlenck line and glovebox techniques. All solvents were freshly distilled from Na and degassed prior to use. The 1H NMR and

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C NMR spectroscopic data were recorded on a Bruker DRX 400 and 500 MHz NMR

spectrometers. UV-Vis spectra were recorded on the Lambda 750 spectrometer. Element analyses were performed at Shanghai Institute of Organic Chemistry, the Chinese Academy of Sciences and Center of Mordern Analysis, Nanjing University. EPR spectra were obtained using Bruker plus-6/1 X-band variable-temperature apparatus. Magnetic measurements were performed using a Quantum Design SQUID VMS magnetometer with a field of

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0.1T. The X-ray single crystal diffraction data were collected on Bruker D8 CMOS detectors at 123 K. Compounds

bis(4-methoxyphenyl)-4-(2-thienyl)phenylamine,7d

1,4’’-diamino-p-quaterphenylene,8

37b

and

Ag[Al(ORF)4] (ORF = OC(CF3)4)9 were prepared by the published procedures. Compound 1 was synthesized according to the similar producedures in the published literatures7a,7c and the NMR data are in good agreement with the values in the reported literature.7e Synthesis of neutral compound 1. A solution of Cu(ClO4)2 hexahydrate (0.65 g, 1.8 mmol) in CH3CN (20 ml) was added to a solution of the bis(4-methoxyphenyl)-4-(2-thienyl)phenylamine (0.45 g, 1.2 mmol) in CH3CN (40 ml) at room temperature, and the mixture was stirred for 12 h. Solid K2CO3 (4.25 g, 30.8 mmol) was added, and stirring was continued for 2 h. Filtered and the solids were washed three times with chloroform (3 × 50 ml). The combined organic extracts were filtered through a short pad of alumina. The solvent was removed and the crude product was chromatographed on silica gel using a mixture of hexane and ethyl acetate (20: 1) as eluent to give 1. Orange powders were collected (0.33 g, 71%). 1H NMR (400 MHz, CDCl3): δ 7.39 (d, J = 8.5 Hz, 4H, Ar-H), 7.07 (d, J = 8.6 Hz, 12H, Ar-H), 6.91 (d, J = 8.1 Hz, 4H, Ar-H), 6.84 (d, J = 8.8 Hz, 8H, Ar-H), 3.80 (s, 12H, OCH3) ppm. 13C NMR (100 MHz, CDCl3): δ 155.0, 147.2, 142.0, 139.5, 134.5, 125.7, 125.2, 123.1, 121.3, 119.4, 113.7, 54.5 ppm. Elemental analysis for C48H40N2O4S2 (%): Calcd: C, 74.59; H, 5.22; N, 3.62; Found: C 74.64, H 5.84, N 3.30. m.p. 150-152°C. Synthesis of neutral compound 2. A mixture of 1,4’’-diamino-p-quaterphenylene (0.21 g, 0.6 mmol), 4-bromoanisole (0.49 g, 2.6 mmol), sodium-tert-butoxide (0.35 g, 3.6 mmol), Pd2(dba)3 (6.9 mg, 8 µmol), and PtBu3 (0.1 ml of a 10% solution in n-hexane, 43 µmol) in dry toluene was stirred under nitrogen atmosphere at 110 °C overnight. The volatiles were removed under vacuum, and the residue was dissolved in dichloromethane and washed with water. The organic layer was dried over MgSO4 and the solvent was removed. Crude product was

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chromatographed on silica gel using a mixture of hexane and dichloromethane (5:1) as eluent to give 2. A slightly yellow solid (0.22 g, 48%) was obtained. 1H NMR (500 MHz, CDCl3): δ 7.67 (m, 8H, Ar-H), 7.48 (d, 4H, J = 10.5 Hz, Ar-H), 7.13 (d, 8H, J = 10.0 Hz, Ar-H), 7.03(d, 4H, J = 10.0 Hz, Ar-H), 6.87 (d, 8H, J = 11.0Hz, Ar-H), 3.82 (s, 12H, OCH3). 13C NMR (125 MHz, CDCl3): δ 156.0, 148.2, 140.8, 139. 7, 138.8, 132.5, 127.4, 127.2, 126.8, 126.7, 120.7, 114.8, 55.5 ppm. Elemental analysis for C52H44N2O4 (%): Calcd: C, 82.08; H, 5.83; N, 3.68; Found: C 82.06, H 6.30, N 3.37. m.p. 95-97°C. Synthesis of Dication Salt 12+·2[Al(ORF)4]-. Under anaerobic and anhydrous conditions, a mixture of 1 (71.3 mg, 0.092 mmol) and Ag[Al(ORF)4] (0.20 g, 0.19 mmol) in CH2Cl2 (≈ 50 ml) was stirred at room temperature overnight. The resultant solution was filtered to remove the gray precipitate (Ag metal). The filtrate was then concentrated and stored at around -30 oC for 1 day to afford blue-green crystals of 12+·2[Al(ORF)4]-. Yield: 0.13 g, 51 %; m.p. 241-243°C. UV-Vis (CH2Cl2): λmax = 1311 nm, 644 nm (shoulder); Elemental analysis for C80H40N2O12Al2F72S2 (%): Calcd: C 35.49, H 1.49, N 1.03; Found: C 35.71, H 1.57, N 1.06. Synthesis of Dication Salt 22+·2[Al(ORF)4]-. Under anaerobic and anhydrous conditions, a mixture of 2 (63.2 mg, 0.083 mmol) and Ag[Al(ORF)4] (0.21 g, 0.19 mmol) in CH2Cl2 (≈ 50 ml) was stirred at room temperature overnight. The resultant solution was filtered to remove the gray precipitate (Ag metal). The filtrate was then concentrated and stored at -30 oC for 1 day to afford blue-green crystals of 22+·2[Al(ORF)4]-. Yield: 81.2 mg, 36 %; m.p. 258-260°C. UV-Vis (CH2Cl2): λmax = 750 nm; Elemental analysis for C84H44N2O12Al2F72 (%): Calcd: C, 37.43; H, 1.65; N, 1.04; Found: C 37.65, H 1.80, N 1.20. Synthesis of Dication Salt 32+·2[Al(ORF)4]-. Under anaerobic and anhydrous conditions, a mixture of 3 (0.13 g, 0.16 mmol) and Ag[Al(ORF)4] (0.33 g, 0.31 mmol) in CH2Cl2 was stirred at room temperature overnight. The resultant solution was filtered to remove the gray precipitate (Ag metal). The filtrate was then concentrated and

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stored at around -30oC for 1 day to afford green crystals of 32+·2[Al(ORF)4]-. Yield: 0.16 g, 35%; UV-Vis (CH2Cl2): λmax = 753 nm. Elemental analysis for C88H50N2O12Al2F72 (%): calcd: C, 38.45; H, 1.83; N, 1.02; found: C 38.48, H 1.79, N, 1.06. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.xxx Crystallographic data for 12+·2[Al(ORF)4]-, 22+·2[Al(ORF)4]- and 32+·2[Al(ORF)4]- (CIF) NMR spectra and theoretical calculations (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Xinping Wang: 0000-0002-1555-890X Haiyan Cui: 0000-0002-9002-7877 Author Contributions †

These authors contributed equally to this work.

NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS

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The authors thank the Major State Basic Research Development Program (grant 2016YFA0300404, X.W.), the National Natural Science Foundation of China (grants 21525102, 21690062, X.W.; 21402094, H.C.) and the China Postdoctoral Science Foundation (grant 2017M611774, H.C.) for financial support. We are grateful to the High Performance Computing Center of Nanjing University for doing the numerical calculations in this paper on its IBM Blade cluster system. REFERENCES (1) (a) Breher, F. Coord. Chem. Rev. 2007, 251, 1007. (b) Casado, J.; Ortiz, R. P.; Navarrete, J. T. L. Chem. Soc. Rev. 2012, 41, 5672.

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TABLE OF CONTENT

17


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