Pyrrolinium-substituted Persistent Zwitterionic Ferrocenate Derivative Enabling the Application of Ferrocene Anolyte

Song, Hayoung; Kwon, Giyun; Citek, Cooper; Jeon, Seungwon; Kang, Kisuk; Lee, Eunsung

doi:10.26434/chemrxiv.12687416.v3

Cited by 3 publications

(9 citation statements)

References 19 publications

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“…Previous examples of ruthenocene derivatives with one and more imidazolium substituents showed irreversible one-electron reduction, 7c but the cyclic voltammograms of [4a]PF6 and [4b]PF6 demonstrated reversible one-electron reduction at E1/2 = −2.36 V (ipa/ipc = 0.96) and E1/2 = −1.72 V (ipa/ipc = 0.95) (vs. Fc/Fc + ) in 0.1 M [Bu4N]PF6/MeCN solution, respectively (Figures 5b, S11, and S12). These results are fairly similar to the reduction potentials of our previous ferrocene derivatives bearing ZCp ligands ([(IZCp)FeCp]PF6: −2.40 V and [(PZCp)FeCp]PF6: −1.69 V vs. Fc/Fc + ), 13 thereby implying ZCp ligand-centered reduction for M(II)-metallocene derivatives. Unfortunately, the one-electron oxidation of [4a]PF6 and [4b]PF6 was irreversible (Epc = 0.77 V for [4a]PF6 and 0.75 V for [4b]PF6 vs. Fc/Fc + ) even in non-coordinating media, such as [Bu4N][BAr F 4] electrolyte in DCM, to prevent the decomposition pathway of ruthenocenium (Figure S13).…”

Section: Introductionsupporting

confidence: 88%

“…In this context, our group previously reported a novel application of pyrrolinium-substituted persistent zwitterionic ferrocenate as an anolyte in redox flow batteries; here, the one-electron reduction of ferrocene, which is known to be highly unstable at room temperature, 12 was stabilized via derivatization with sterically hindered cationic N-heterocycle substituents. 13 However, the synthetic utility of our strategy is highly limited to ferrocene because it requires the direct functionalization of ferrocenium via the nucleophilic attack of N-heterocyclic carbenes (NHCs) to access the redox non-innocent Cp ligand. Therefore, we sought to design new redox-innocent ligands by preparing NHC-derived zwitterionic Cp compounds for direct complexation with transition metals.…”

Section: Introductionmentioning

confidence: 99%

“…The red-shifted and stronger UV absorbance of 3b at 514 nm compared with that of 3a at 436 nm reflects favorable metal-to-ligand charge transfer (MLCT) owing to the stronger π-accepting redox domain of the pyrrolinium substituent compared with that of the imidazolium substituent (Figure S5). Based on our previous report on zwitterionic ferrocenate derivatives, 13 we expected that the sterically hindered ZCP ligands 1a and 1b could be applied to the reversible reduction of other sandwich complexes as redox non-innocent ligands. Thus, we synthesized the ruthenocene derivatives [4a]PF6 and [4b]PF6 by the simple reaction of 1a and 1b with [Ru(CH3CN)3Cp]PF6 to investigate their ligand properties; [4a]PF6 and [4b]PF6 were obtained in high yield (Figure 4).…”

Section: Introductionmentioning

confidence: 99%

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IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Lig-ands as Electron Reservoirs for Sandwich Complexes

Choi

Yoo

Song

et al. 2022

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Numerous developments in the electronic and steric modification of cyclopentadienyl (Cp) ligands have led to a corresponding growth in their applications. Herein, we present two new redox non-innocent Cp compounds, imidazolium- and pyrrolinium-substituted zwitterionic Cps (IZCp 1a and PZCp 1b). The Cps were obtained in high yields and could be handled easily owing to their high stability in air and moisture, unlike other known Cps. The redox non-innocent behavior of the Cp ligands was demonstrated via experimental and computational analyses of their half-sandwich and sandwich complexes. During cyclic voltammetry, the molybdenum half-sandwich (3a, 3b) and ruthenium sandwich ([4a]PF6, [4b]PF6) complexes of IZCp and PZCp showed reversible one-electron reduction at a low potential (E1/2 ranging from −1.7 to −2.7 V vs. Fc/Fc+). This phenomenon has not been studied previously because of the high instability of the reduced complexes. DFT calculations for the reduced complexes ([3a]‾, [3b]‾, 4a, and 4b) confirmed that their spin density was mainly delocalized over the ligand center (70%–90%). EPR analysis of the isolated K[3b] and 4b indicated their ligand-centered radical structures. Furthermore, the CO stretching frequencies of K[3b] (νCO 1871.35, 1748.10, and 1698.80 cm−1) revealed that the redox non-innocent [PZCp]‾ ligand is the strongest electron-donating Cp ligand among previously reported CpMo(CO)3 derivatives (νCO > 1746 cm−1).

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Section: Introductionsupporting

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Lig-ands as Electron Reservoirs for Sandwich Complexes

Choi

Yoo

Song

et al. 2022

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“…Interestingly, in the latter case, we confirmed the unexpected C-H substitution of ferrocene in the reaction between ferrocenium and NHCs with the formation of the [NHC-H] + by-product. 20 Similarly, Hansmann et al reported the C-H substitution of pyrylium 21 and pyridinium 22 using 2 eq. of NHCs to produce dearomatized organic super-electron donors with the [NHC-H] + by-product.…”

Section: Direct Observation Of the Transient [Nhc]mentioning

confidence: 96%

“…The oxidation of NHCs has been extensively studied in our laboratory. Recently, we reported the reactivity of NHC toward oxidizing agents, including nitric oxide (NO), 18 tetracyanoethylene (TCNE), 19 and ferrocenium 20 . Interestingly, in the latter case, we confirmed the unexpected C-H substitution of ferrocene in the reaction between ferrocenium and NHCs with the formation of the [NHC-H] + by-product.…”

Section: Direct Observation Of the Transient [Nhc]mentioning

confidence: 99%

Eliminative Nucleophilic Aromatic Substitution of Hydrogen (SNArH): An Alternative Mechanism Explaining Reactivity of N-heterocyclic Carbene (NHC) toward Tritylium

Song

Lee

2022

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Despite a recent proposal on the mechanism of a single-electron transfer (SET) process between tritylium and 2,6-bis(diisopropylphenyl)imidazol-2-ylidene (IPr) based on evidence of transient IPr radical cation intermediate ([IPr]●+) formation, such oxidation is still contentious because of the high oxidation potential of N-heterocyclic carbenes. Our experimental analysis indicates that the appearance of a deep purple color, previously considered to be from transient [IPr]●+, originates from a zwitterionic intermediate (3a), not a radical cation. Here, we propose an alterna-tive mechanism for the reaction involving tritylium and IPr: the eliminative nucleophilic aromatic substitution of hydrogen (SNArH). This mechanism is noteworthy in two respects: (1) for showing the first example of SNArH without any vicarious leaving group and (2) for explaining how [NHC–H]+ can be generated without the formation of tran-sient [NHC]●+, which has been frequently proposed as an intermediate for the reaction between NHC and oxidants. These results also show that a transient strong single-electron donor (3a) could be generated by the eliminative SNArH mechanism for oxidants using NHCs, which is a more feasible explanation for the reactivity of NHCs with oxidants.

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Persistent Radicals Derived from N-Heterocyclic Carbenes for Material Applications

2022

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Metrics & MoreArticle RecommendationsCONSPECTUS: Persistent radicals are potential building blocks of novel materials in many fields. Recently, highly stable persistent radicals are considered to be within reach, thanks to several radical stabilization strategies such as spin delocalization and steric protection. N-Heterocyclic carbene (NHC)-derived substituents can be attached to a radical center for these purposes, as illustrated by numerous NHC-stabilized radicals reported in the last two decades. This Account describes our recent work on developing NHC-derived persistent radicals, as well as their prospective applications. Considering that NHCs not only stabilize radicals but also reversibly interact with gas molecules, in 2015 our group reported NHC-nitric oxide (NHC−NO) radicals produced by reversibly trapping nitric oxide (NO) radical gas in NHCs. The resultant compounds were loaded into biocompatible poly(ethylene glycol)-block-poly(caprolactone) (PEG-b-PCL) micelles and injected into tumor-bearing mice. Then, NO release was triggered by high-intensity focused ultrasound irradiation of the tumor tissue. Furthermore, the NHC− NO radicals could also serve as a platform to generate other organic radicals such as oxime ether or iminyl radicals. Apart from medicine-related applications, radicals stabilized by NHCs can be used as energy storage materials. In this context, the triazenyl radical containing two NHC units reported by our laboratory could be a cathode active material in batteries, as an organic alternative to LiCoO 2 . The subsequently prepared unsymmetrical triazenyl radical derivatives were applied as anolytes in nonaqueous allorganic redox flow batteries. In addition, a ferrocene-based redox flow battery anolyte was obtained by introducing NHC-derived substituents that effectively stabilize the ferrocenate derivatives previously reported only at low temperatures. The batteries containing NHC-supported radicals exhibited high energy efficiency and insignificant radical decomposition over multiple cycles. Finally, toward developing air-persistent organic radicals for flexible devices and MRI contrasting agents, we also highlight our recent air-and physiologically stable organic radicals derived from NHCs. Coordination of tris(pentafluorophenyl)borane to the NHC− NO radical produced a new radical cation that is stable in an organic solvent under air for several months. The readily accessible 1,2dicarbonyl radical cations generated by the reaction of NHCs with oxalyl chloride are remarkably persistent even in an aqueous solution for several months. They are also highly stable even under physiological conditions, making them particularly attractive potential candidates for organic MRI contrast agents. We hope that this Account will serve as a guide for the future development of stable NHC-derived organic radicals and draw the attention of the synthetic community to their potential applications in material science.

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Pyrrolinium-substituted Persistent Zwitterionic Ferrocenate Derivative Enabling the Application of Ferrocene Anolyte

Cited by 3 publications

References 19 publications

IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Lig-ands as Electron Reservoirs for Sandwich Complexes

IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Lig-ands as Electron Reservoirs for Sandwich Complexes

Eliminative Nucleophilic Aromatic Substitution of Hydrogen (SNArH): An Alternative Mechanism Explaining Reactivity of N-heterocyclic Carbene (NHC) toward Tritylium

Persistent Radicals Derived from N-Heterocyclic Carbenes for Material Applications

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