Experimental and Theoretical Predictors for Redox Potentials of Bispyridinylidene Electron Donors

• Published in: Chemphyschem 2024-08, Vol.25 (16), p.e202400092-n/a
• Main Authors: Khor, Chun Keat, Calhoun, Larry A., Neville, John J., Dyker, C. Adam
• Format: Article
• Language: English
• Subjects: bispyridinylidene; correlation; density functional calculations; NMR; NMR spectroscopy; Nuclear magnetic resonance; Organic chemistry; Oxidation; redox potential; Reducing agents; Solvation
• ISSN: 1439-4235; 1439-7641; 1439-7641
• DOI: 10.1002/cphc.202400092

Bispyridinylidenes are neutral organic molecules capable of two‐electron oxidation at a range of redox potentials that are widely tunable by choice of substituent, making them attractive as homogeneous organic reductants and active materials in redox flow batteries. In an effort to readily predict the redox potentials of this important class of compounds, we have developed correlations between the experimental redox potentials and both experimental and theoretical predictors. On the experimental side, we show that multinuclear NMR chemical shifts of related pyridinium ions correlate well with the redox potentials of bispyridinylidenes, with R2 and standard errors (S) reaching 0.9810 and 0.048 V, respectively, when the 13C (N‐CH3) and 1H (ortho) chemical shifts are used together. Theoretical studies of the bispyridinylidenes and their doubly oxidized bipyridinium ions gave a range of predictively valuable equations at various levels of computational cost. This ranged from a simple model using only the EHOMO of the bispyridinylidenes (R2=0.9689; S=0.060 V), to a more computationally intensive model which include solvation effects for both redox states which gave the highest predictive value for all methods (R2=0.9958; S=0.022 V). This work will guide further studies of this important class of molecules. Two‐electron redox potentials for bispyiridinylidenes 2 vary widely with the R substituent. Here they are correlated to a variety of experimental NMR parameters for related pyridinium derivatives 1, as well as theoretical parameters for 2 and 22+ to derive predictively useful equations for this important class of molecule.