Iminophosphorano‐Substituted Bispyridinylidenes: Redox Potentials and Substituent Constants from Tolman Electronic Parameters

Abstract

Bispyridinylidenes (BPYs) have emerged as an important class of neutralo rganic electron donors, with redox potentials that vary widely with choice of substituent. Methods to predict the effect of substitution on the redox potential are therefore highly desirable. Here we show that the redox potential of BPYs featuring iminophosphoranos ubstituents (R 3 P = N-), which representt he most reducing class of BPYs, can be predicted basedo n the well-knownT olman electronic parameter(TEP) for the respective phosphine fragment (R 3 P). Moreover,b uilding on earlier work relatingr edox potentials to Hammett-type substituent constants, it is now possible to quantitatively predict s p + values for iminophosphoranos ubstituents from TEP values. Theser esultsp rovide ap ath for precisely tailoringr edoxp otentials of iminophosphorano-substituted BPYs, but also give quantitative descriptors for how these highly versatile iminophosphorano substituents can impactt he properties of any molecular scaffold. Scheme1.Reductions of iodoester 4 to products 5 or 6 in varying isolated yields depending on the strengthoft he OEDused (1, 2,o r3).

Ester-Substituted Bispyridinylidenes: Double Concerted Two-Electron Bipolar Molecules for Symmetric Organic Redox Flow Batteries

“…Analysis of the blue solid by 1 H, 13 C, and COSY NMR spectroscopy in C 6 D 6 (Figures S5−S7) was consistent with the formation of the neutral compound BPYME, but we noted that the 1 H and 13 C NMR spectra showed only broad signals for a single isomer, instead of sharp peaks for both E and Z isomers as found for most other N−Me bispyridinylidenes. [48][49][50]58,59 Based on the earlier discussion of the longer C2−C2′ bond in BPYME, and the likely involvement of "push-pull" resonance structures (BPYME′), it is believed that the CO 2 Me group induces a low barrier to E/ Z isomerization via C2−C2′ bond rotation, which combines with the rotation of the ester functional group to give rise to the broad resonances we observe by NMR spectroscopy.…”

“…Redox processes can depend strongly on the choice of electrolyte solvent and salt, and modulation from two sequential one-electron redox processes to a concerted twoelectron process can be achieved. 31,53−55 We therefore investigated DMF as an alternate solvent, which has been shown to promote concerted two-electron oxidation of other BPYs, 48,50 and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a supporting salt, which features the more Scheme 1. Redox Processes for BPYME and Related Monopyridine Derivative 1 strongly coordinating Li + cation.…”

“…The inspiration to investigate diaklyester-substituted bispyridinylidenes came from our extensive experience with bispyridinylidenes − and previous electrochemical studies on pyridinium ions substituted with electron withdrawing groups . Indeed, a closely spaced sequential two-electron redox process for the BPYME 2+ /BPYME couple (Scheme ) has already been established, and though the low potential electrochemical activity of this scaffold was not reported, we were optimistic that a two-electron couple for BPYME/BPYME 2– could also be accessible owing to the electron withdrawing ester groups.…”

“…Redox processes can depend strongly on the choice of electrolyte solvent and salt, and modulation from two sequential one-electron redox processes to a concerted two-electron process can be achieved. ,− We therefore investigated DMF as an alternate solvent, which has been shown to promote concerted two-electron oxidation of other BPYs, , and lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) as a supporting salt, which features the more strongly coordinating Li + cation. This cation has been shown to enable concerted two-electron reduction processes for carbonyl containing compounds such as quinones while at the same time causing a positive shift of the redox process because of stabilization provided by strong ionic interactions between ions. , When the TBAPF 6 /DMF supporting electrolyte was used (Figure b), the reduction from BPYME to BPYME 2– is observed as two sequential one-electron processes (−1.98 and −2.17 V vs Ag/Ag + ) similar to what occurred in MeCN/TBAPF 6 , but the oxidation to BPYME 2+ -2BF 4 – occurs as an effectively concerted two-electron process at a single potential (−0.89 V vs Ag/Ag + , with a peak-to-peak separation (Δ E p ) of 100 mV; i pc / i pa = 0.98).…”

Bis(Iminophosphorano)‐Substituted Pyridinium Ions and their Corresponding Bispyridinylidene Organic Electron Donors

“…Of the various OED scaffolds that have been reported, bispyridinylidenes (BPYs, including 1 – 3 ) have been the most widely studied. Derivatives of these two‐electron donors are generally easy to prepare and have a wide range of redox potentials [35] . For example, the E 1/2 for disubstituted BPY derivatives spans more than 1 V, ranging from −0.26 V vs SCE (acetonitrile) for 2 b , [36,37] to −1.55 V vs SCE (dimethylformamide) for 2 e [38,39] .…”