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

Abstract

Here, we report the imidazolium-/pyrrolinium-substituted persistent zwitterionic ferrocenate derivatives, which were characterized by electron paramagnetic resonance (EPR) and 57 Fe Mössbauer spectroscopy. Additional theoretical studies on these zwitterionic ferrocenate derivatives clearly explain the origin of their thermal stability and the orbital interactions between iron and imidazolium-/pyrrolinium-substituted zwitterionic cyclopentadienyl ligand. Exploiting the facile Fe(II/I) redox chemistry, we successfully demonstrated that the ferrocene derivative can be applied as an example of derivatized ferrocene anolyte for redox-flow batteries. These zwitterionic ferrocenate derivatives will not only deepen our understanding of the intrinsic chemistry of ferrocenate but have the potential to open the way for the rational design of metallocenate derivatives for various applications. electrochemistry based on Fe(I)/Fe(II) ferrocene redox couples is feasible in practical battery operation and is enabled by the stereoelectronic stabilization of pyrrolinium-substituted zwitterionic ferrocenate derivative. ConclusionsFerrocene, one of the most representative compounds in the discipline of organometallic chemistry, attracted much attention because of its extremely high thermodynamic, chemical, and Fe(II/III) redox stability. However, the applications using the extremely negative voltage of ferrocene have not been reported to date due to the fast dissociation of the cyclopentadienyl anion ligands from Fe(I). Thus, an Fe(I) biscyclopentadiene complex remained elusive for a long time. Here, we have shown that zwitterionic ferrocenate derivatives 2a and 2b can be prepared by the one-electron reduction of 1a and 1b at ambient conditions, as confirmed by several complementary spectroscopic and theoretical methods. Additionally, a potential application in a redox-flow battery was successfully demonstrated using 1b as an example of ferrocene anolyte, in part due to the high degree of stabilization of low-valent state by the cAAC moiety. These imidazolium-/pyrrolinium-substituted ferrocene derivatives may deepen our understanding of the electrochemical behavior of ferrocene compounds and open the way to the rational design of ferrocenate derivatives. ASSOCIATED CONTENTSupporting Information. Experimental details, X-ray crystallographic analysis, Computational details, and other characterization data. The crystallographic data for 1a and 1b are deposited in the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers CCDC 1940623 and 1940622. This material is available free of charge via the Internet at http://pubs.acs.org.

Forever young: the first seventy years of ferrocene

“…Relatively stable ferrocenates were obtained by the reduction of octamethyl-1,1′diphosphaferrocene, [Fe(η 5 -C 5 Me 4 P) 2 ], 162 and ferrocenes equipped with cationic imidazolium and pyrrolinium substituents provide zwitterionic ferrocenates. 163,164 As a substituent, the ferrocenyl group is a strong electron donor. However, its oxidation turns it into an electron-withdrawing moiety, as illustrated by the Hammett σ p constant of −0.18 for the ferrocenyl group and 0.29 for the corresponding ferrocenium.…”

Metallocene Anions: From Electrochemical Curiosities to Isolable Complexes

“…A cyclic voltammogram of 15 performed at room temperature showed irreversible reduction to 14 at −2.31 V vs Fc +/0 , and the cyclic voltammogram of 17 showed reversible reduction to 16 at −1.69 V vs Fc +/0 , indicating that 11 has higher stability in solution than 14 [58] . The thermal stability of 14 and 16 in solution in toluene was investigated; monitoring by UV‐Vis spectroscopy showed a half‐life of 10 h for 14 and several days for 16 .…”

“…In this section, we compile derivatized d-block metallocene anions that have been isolated to date, 1, 2, 3, 5, 6, 7, 9, 10-M, 12-M, and the related zwitterions 14 and 16 (Figure 1). [50][51][52][53][54][55][56][57][58] We include discussions of synthetic pathways and characterisation, with salient information about each complex compiled in [50][51][52][53][54][55][56][57][58] Table 2, including structural parameters, electronic structures and redox potentials. Metallocenes and their anions can adopt a variety of electronic configurations (Figure 2), here we give exemplar frontier orbital electron occupancies based upon the MO diagram of ferrocene, but note that the order and occupancies of these orbitals can vary markedly from those depicted here due to ligand field effects.…”

“…Finally, in 2021, Kang, Lee and co‐workers reported the synthesis of two zwitterionic complexes [Fe(Cp)(Cp Imi )] ( 14 , Cp Imi= C 5 H 4 (C 3 N 2 H 2 ‐N,N ′ ‐(C 6 H 3 ‐2,6‐ i Pr 2 ) 2 )) and [Fe(Cp)(Cp Pyr )] ( 16 , Cp pyr =C 5 H 4 (C 4 N‐2,2,4,4‐Me 4 ‐N‐(C 6 H 3 ‐2,6‐ i Pr 2 )) [58] . Complexes 14 and 16 were prepared by the separate reductions of the respective parent cationic species [Fe(Cp)(Cp Imi )][PF 6 ] ( 15 ) and [Fe(Cp)(Cp Pyr )][PF 6 ] ( 17 ) with KC 8 in Et 2 O (Scheme 17).…”

“…Figure 1. Synthetically described d-block derivatized metallocene anions reported to date, 1, 2, 3, 5, 6, 7, 9, 10-M, 12-M, and the related zwitterions 14 and 16; Dipp = C 6 H 3 -2,6-iPr 2 [50][51][52][53][54][55][56][57][58]. …”