Stable Radical Ion Pairs Induced by Single Electron Transfer: Frustrated Versus Nonfrustrated

Open AccessCCS ChemistryCOMMUNICATIONS30 Aug 2022Stable Radical Ion Pairs Induced by Single Electron Transfer: Frustrated Versus Nonfrustrated Shanshan Kong†, Shuxuan Tang†, Tao Wang, Yu Zhao, Quanchun Sun, Yue Zhao and Xinping Wang Shanshan Kong† State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu , Shuxuan Tang† State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu , Tao Wang State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu , Yu Zhao State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu , Quanchun Sun State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu , Yue Zhao State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu and Xinping Wang *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu https://doi.org/10.31635/ccschem.022.202202306 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Single electron transition reactions between amines (Lewis base) and B(C6F5)3 (Lewis acid) in cooperation with benzoquinones gave rise to a frustrated radical pair 3 and a nonfrustrated radical pair 4. Both of them were isolated as stable crystals and studied by single-crystal X-ray diffraction, superconducting quantum interference device measurements, electron paramagnetic resonance, nuclear magnetic resonance, and UV–vis spectroscopy. Antiferromagnetic exchange coupling was observed among both 3 and 4. Radical anion and cation are basically separated in 3, while 4 featured a relatively strong anion-cation π–π stacking interaction. This work demonstrated that the Lewis acid coupled electron transfer is an efficient way to prepare stable radical ion pairs. Download figure Download PowerPoint Introduction Radical ion pairs (RIPs) have great importance in fundamental research, as they are essential species in organic reactions, catalytic chemistry, photosynthetic energy transduction, and quantum information science.1–40 RIPs can be induced by single electron transfer (SET) between an electron donor (D) and an electron acceptor (A) (Scheme 1) by photolytic, thermolytic, and radiolytic processes. They can be divided into frustrated and nonfrustrated, according to the absence or existence of intermolecular electronic interaction between the radical cation and anion. RIPs are usually unstable, as they are quenched readily by the electron transfer back from the radical anion to the radical cation. Thus, the number of reported stable RIPs is limited.35–40 One boron-centered RIP is structurally characterized,39 which is viewed as a frustrated RIP, while all other stable RIPs are tetracyanoquinodimethane-based and nonfrustrated.35–38,40 Scheme 1 | Formation of intermolecular RIPs by SET between a D and an A. Download figure Download PowerPoint A frustrated radical ion pair (FRIP) can be generated using non-classic frustrated Lewis pairs (FLPs) by SET from the Lewis base (donor) to the Lewis acid (acceptor) (Scheme 2).6–23 FRIPs have been found to play critical roles as the intermediates and reactants in the FLP radical mechanism.13–30 Until now, different kinds of FRIPs have been produced by SETs,13–17 including the photoinduced SET processes,8,9,13 which has led to a comprehensive review recently. However, all reported FRIPs are transient or persistent in solution and are restricted to spectroscopic characterizations,13–23 except the one boron-centered FRIP.39 Scheme 2 | Formation of unstable and stable RIPs by SET. Download figure Download PowerPoint As a well-known electron acceptor, model Lewis acid, B(C6F5)3 has been widely studied in areas including organic syntheses, hydrogenation reactions, and activation reactions.41–48 In 2013, we observed a SET from an organic molecule to B(C6F5)3.49 Unfortunately, the reduction product, [B(C6F5)3]•–, underwent a solvent quench and was probably involved in a four-coordinate borate such as [ClB(C6F5)3]– or [HB(C6F5)3]–. In 2017, Stephan and co-workers14 came up with a radical-pair mechanism for FLP reactivity, where a transient frustrated radical pair (FRP) is formed by SET and plays an important role in following reactions. It is worthy of note that [B(C6F5)3]•– is stabilized as a diborylated semiquinone dianion upon adding benzoquinone. In 2018, Erker and co-workers50 found that the SET among B(C6F5)3, benzoquinone, and selected reductants such as TEMPO [(CH2)3(CMe2)2NO], Ph3C• or Cp*2Fe afforded stable diborylated semiquinone radical anion salts. Very recently, we found that incorporation of –B(C6F5)2 fragments into a D–A–D molecule could lead to the formation of a diradical with an intramolecular ion pairing state.51 Inspired by this result, and by the unique combination property of benzoquinones,14,50 together with our finding in 2013,49 we speculated that introducing benzoquinones as a bridge in FLP reactions involving B(C6F5)3 might prevent [B(C6F5)3]•– from decomposition and generate a stable RIP. Herein, we report two stable RIPs, 3 and 4, both generated through the SETs between selected amines (Lewis base) and B(C6F5)3 (Lewis acid), in cooperation with benzoquinones, featuring frustrated ( 3) and nonfrustrated ( 4) structures, respectively. Both 3 and 4 were isolated as stable crystals and characterized by single-crystal X-ray diffraction (XRD), electron paramagnetic resonance (EPR), UV–vis, nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies, and superconducting quantum interference device (SQUID) measurements, in conjunction with theoretical calculations. Results and Discussion Synthesis of 3 and 4 Both 3 and 4 were synthesized through one-pot reactions between amines ( 1 and 2) and B(C6F5)3 in the presence of benzoquinones (Scheme 3). Upon addition of CH2Cl2, the reaction mixtures immediately turned into dark red for the synthesis of 3 and dark blue for 4. The resulting mixture of 3 was vacuumed and extracted with toluene. The toluene solution of 3 and CH2Cl2 solution of 4 were filtered, and the filtrates were concentrated for crystallization at −25 °C. Both 3 and 4 were isolated as dark purple crystals, which were thermally stable under anaerobic conditions at room temperature. Scheme 3 | Formation of RIPs 3 and 4. Download figure Download PowerPoint Crystal structure analysis Complexes 3 and 4 crystallize in the triclinic space group P–1 and monoclinic space group P21/n, respectively. Their geometries are illustrated in Figure 1a,b and Supporting Information Figures S1–S4, with selected bond distances shown in Tables 1 and 2.a The average bond distances N–Cib, Coa–Cob in the amine fragment of 3 (i.e., 3-cation) decreased compared to the neutral precursor 1.52,53 The geometry of doubly O-borylated quinone radical anion in 3 (i.e., 3-anion) was similar to previously reported monoradical anions.50 Bonds Cya–Cyb, O–Cxa, and O–Cxb in 3-anion was longer than the precursor benzoquinone, consistent with the coordination effect on the O atoms and the aromatic recovery at the benzene ring (Table 1). Similar to 3, bonds N–Cia, N–Cib, and Coa–Cob in 4-cation were shorter than the precursor 2, while bonds O–Cxa, O–Cxb, and Cya–Cyb in 4-anion were elongated in comparison with the precursor benzoquinone (Table 2).54,55 These results indicated that both cations and anions in 3 and 4 featured semiquinone structures, suggesting the delocalization of the electron spin density. There were no π–π interactions in compound 3, indicating that 3 featured a frustrated structure. However, intermolecular π–π stacking interaction between the neighboring cations and anions was observed in the crystal structure of 4 ( Supporting Information Figure S5), with a distance of 3.72 Å between the geometric centers of the bridging benzene rings of 4-cation and 4-anion. The existence of π–π interaction in 4 revealed that it is nonfrustrated. Figure 1 | Thermal ellipsoid (30%) drawings of (a) 3 and (b) 4. Hydrogen atoms are not shown for clarity. Download figure Download PowerPoint Table 1 | Average Bond Distances (Å) in 3 and Precursors Exp. 153 p-Benzoquinone52 Cia–Cia′ 1.433 1.480 — N–Cib 1.370 1.410 — Coa–Cob 1.365 1.387 — Cia–Coa and Cib–Cob 1.411 1.400 — N–Ar 1.433 1.428 — O–Cxa and O–Cxb 1.297 — 1.223 Cya–Cyb 1.353 — 1.344 Cxa–Cya and Cxb–Cyb 1.423 — 1.474 O–B 1.531 — — Note: Exp, exposure. Table 2 | Average Bond Distances (Å) in 4 and Precursors Exp. 254 9,10-Anthraquinone55 N–Cia and N-Cib 1.357 1.413 — Coa–Cob 1.356 1.390 — Cia–Coa and Cib–Cob 1.420 1.397 — N–Me 1.471 1.438 — O–Cxa and O–Cxb 1.310 — 1.2236 Cya–Cyb 1.428 — 1.4045 Cxa–Cya and Cxb–Cyb 1.445 — 1.4855 O–B 1.547 — — Note: Exp, exposure. EPR measurement and analysis Both solids 3 and 4 could be well-redissolved in CH2Cl2. Compound 3 is NMR silent ( Supporting Information Figure S6), while the NMR signal of the hydrogen atoms in 4-anion was broader and weaker compared with that of 9,10-anthraquinone ( Supporting Information Figure S7). The IR spectra of 3 and 4 are shown in Supporting Information Figures S15 and S16. The peaks absorption band at about 1640 cm−4 correspond to the stretching vibrations of C = 0. The solution EPR spectrum of 3 only revealed a hyperfine coupling signal for 3-cation (Figure 2a). The g factor of 3-cation was simulated to be 2.0033, and the isotropic hyperfine coupling constant is A(14N) = 4.5 G (12.6 MHz). The EPR signal of 3-anion was not observed, which might result from a low coupling constant or weak signal intensity. In contrast, the solution EPR spectrum of 4 displayed signals for both 4-cation and 4-anion (Figure 2b and Supporting Information Figure S11). The best simulation results were obtained with g = 2.0034, A(1H) = 1.95, 6.55 G (5.46, 18.36 MHz), A(14N) = 7.00 G (19.61 MHz) for 4-cation, and g = 2.0040 for 4-anion. The hyperfine coupling splitting of 4-anion was not obtained. The solution EPR spectra of 3 and 4 confirmed their RIP structures. Moreover, half-field signals for Δms = ±2 forbidden transitions were observed from the solid samples of 3 and 4 at both room temperature and low temperature of 88 K ( Supporting Information Figures S8, S9, S12, and S13), indicating the magnetic exchange coupling interaction in the RIPs. The EPR measurements on the CH2Cl2 frozen solutions of 3 and 4 are listed in Supporting Information Figures S10 and S14. Both spectra exhibited broad single peaks without anisotropic signals, probably due to the low anisotropy of the organic radicals in 3 and 4. The EPR measurements conditions are listed in Supporting Information Table S2. Figure 2 | Solution EPR spectra of 1 × 10−4 M (a) 3 and (b) 4 in CH2Cl2 at room temperature exposure (EXP, ν = 9.8209 GHz for 3 and 9.4185 GHz for 4) and the simulation (SIM) spectrum. Download figure Download PowerPoint SQUID measurement and analysis To further investigate the magnetic properties of 3 and 4, variable temperature magnetic susceptibility measurements were carried out (Figure 3). The χMT versus T plot of 3 is 0.674 cm3 mol−1 K at 300 K and decreased gradually upon cooling, reaching 0.542 cm3 mol−1 K at 1.8 K. This phenomenon indicated the relatively weak antiferromagnetic interaction between the cation and anion in 3, attributed to the long distance between them, as shown in the frustrated crystal structure of 3. The SQUID results of 4 were different from that of 3. The χMT versus T plot was 0.712 cm3 mol−1 K at 300 K, which decreased upon cooling, suggesting the relatively strong antiferromagnetic exchange coupling in 4, consistent with the π–π stacking interaction between the cation and anion in the nonfrustrated crystal structure of 4. Figure 3 | χMT versus T plots for the powder samples of 3 and 4 in the SQUID measurements. Download figure Download PowerPoint DFT calculation and analysis Theoretical calculations were performed to get a deep insight into the SET processes and the electronic structures of 3 and 4. Cation-anion pairs were selected from the crystal structures of 3 and 4 and optimized at the (U)CAM-B3LYP/6-31G(d) level. The stationary points were checked by frequency calculations. Radical pairs were viewed as a diradical species with an open-shell singlet (OS) or a triplet state (T).32 Closed-shell singlets (CS) were also optimized for comparison. For the optimization of OS states, broken-symmetry approaches were applied. The calculated bond distances are listed in Supporting Information Tables S6 and S7, which were in good agreement with the single-crystal XRD results. For both 3 and 4, the spin density was mainly delocalized on the central bridging benzene rings of the amine parts and the O-borylated semiquinone parts (Table 3 and Figure 4a,b). The calculated spin Hamiltonian parameters of 3 and 4 are listed in Supporting Information Table S5. The calculated charge distribution of the OS states for 3 and 4 was consistent with the corresponding ionic valence of the cations and anions (Table 3). These results further confirmed the RIPs electronic structures of both species. Moreover, the orbital distributions of highest occupied molecular orbital [HOMO](α) and lowest unoccupied molecular orbital [LUMO](β) for 3 ( Supporting Information Table S8), and HOMO(β) and LUMO(α) for 4 were matchable ( Supporting Information Table S10), respectively, indicating the electron-loss from HOMOs of precursors 1 and 2, which results in corresponding LUMOs of FRPs ( Supporting Information Tables S9 and S11). The calculated singlet-triplet energy gaps for 3 and 4 were −1.3 and −584.4 cal mol−1, respectively ( Supporting Information Tables S3 and S4). The relatively low calculated energy gap of 3 revealed its nearly degenerated spin state, consistent with the SQUID result. Complex 4 featured an OS ground state with considerable electronic coupling. The UV–vis spectrum of 3 in dilute CH2Cl2 solution displayed an intense and broad absorption band at 1500 nm ( Supporting Information Figure S17), close to the reported UV–vis spectrum of 3-cation.53 The UV–vis spectrum of 4 exhibited two peaks at ∼ 570 and 620 nm ( Supporting Information Figure S17), also similar to the reported absorption spectrum of 4-cation.56 Table 3 | Calculated Spin Density and Charge Distribution Spin Density Charge Cation Anion Cation Anion 3–OS 1.00 −1.00 0.86 −0.86 3–CS — — −0.08 0.08 3–T 1.00 1.00 0.86 –0.86 4–OS −0.98 0.98 0.84 −0.84 4–CS — — 0.17 −0.17 4–T 1.00 1.00 0.85 −0.85 Note: OS, open-shell singlet; CS, closed-shell singlet; T, triplet. Figure 4 | Calculated spin density distribution of (a) 3 and (b) 4. Isovalue = 0.002. Download figure Download PowerPoint Conclusion The SET between amines (Lewis base) and B(C6F5)3 (Lewis acid) in cooperation with benzoquinones gave rise to two different RIPs as stable crystals, which were fully characterized by multiple spectroscopic techniques and single-crystal XRD. Experimental studies revealed the antiferromagnetic exchange coupling among both species. This work demonstrates that the Lewis acid coupled electron transfer is an efficient way to prepare stable RIPs. Isolation and structural illustration of such species will give insight into the photoelectric process of D–A materials in the fields of an organic light-emitting diode, organic field-effect transistor, and organic solar cell. Footnote a X-ray data were obtained for 3 and 4 are listed in Supporting Information Table S1. CCDC 2167335 and 2166107 contain the supplementary crystallographic data for this report and could be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC). Supporting Information Supporting Information is available free of charge at https://doi.org/10.31635/ccschem.022.202202306 and includes crystallographic data for 3 and 4, synthesis procedures, selected crystal parameters, 1H NMR measurements, EPR measurements, IR measurements, UV–vis measurements, and theoretical calculations. Conflict of Interest There is no conflict of interest to report. Author Contributions All authors have approved the final version of the manuscript. Acknowledgments We thank the National Key R&D Program of China (grant no. 2018YFA0306004) and the National Natural Science Foundation of China (grant no. 21525102) for their financial support. The DFT calculations were performed at the High-Performance Computing Centre of Nanjing University. References 1. Lubitz W.; Lendzian F.; Bittl R.Radicals, Radical Pairs and Triplet States in Photosynthesis.Acc. Chem. Res.2002, 35, 313–320. Google Scholar 2. 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Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentNot Yet AssignedSupporting Information Copyright & Permissions© 2022 Chinese Chemical SocietyKeywordsLewis acidnonfrustrated Lewis pairsradical ion pairsfrustrated Lewis pairssingle electron transferAcknowledgmentsWe thank the National Key R&D Program of China (grant no. 2018YFA0306004) and the National Natural Science Foundation of China (grant no. 21525102) for their financial support. The DFT calculations were performed at the High-Performance Computing Centre of Nanjing University. Downloaded 386 times PDF DownloadLoading ...