6,6'-Biazulenic core as a platform for unlocking Hammett constants via electrochemical free-energy relationships

Quantitative appreciation of electronic substituent effects is beneficial across the chemical sciences, from reaction kinetics and catalysis to functional materials design and enzymatic processes. While Hammett parameters (σ constants) constitute the most widely invoked paradigm, their determination for structurally complex functional groups is often impractical by traditional empirical means. Herein, we demonstrate that the reversible one-step, 2-e^- reduction of linearly functionalized 1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene provides a simple electrochemical readout of effective Hammett constants. By design, the values obtained align closely with the conventional σ _p descriptors pertaining to benzenoid systems. This approach not only helps reevaluate previously reported Hammett values but also quantifies the role of intramolecular hydrogen bonding (e.g., C[double bond, length as m-dash]O⋯H-S) and enables determining effective σ constants (σ _eff) for "designer" functional groups, such as -SAuPPh₃ and -NCCr(CO)₅. Moreover, the long-range net electron donor/acceptor influence of the substituents -S^-, -SAuPPh₃, -SH, -SCH₂CH₂CO₂CH₂CH₃, and -NCCr(CO)₅ on the [(-NC)Cr(CO)₅] ¹³C NMR reporter across the 6,6'-biazulenic π-linker was unveiled through inverse-linear δ(¹³ CO_trans) vs. δ(¹³ CN) and δ(¹³ CO_cis) vs. δ(¹³ CN) correlations. The π-communication along the molecular axis of the 6,6'-biazulenic scaffold was further confirmed via Reflection-Absorption Infrared (RAIR) spectroscopic analysis of [(OC)₅Cr(η¹-2-isocyano-2'-mercapto-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] self-assembled on the Au(111) surface. By uniting redox tunability with rigorous linear free-energy correlations, this work offers both a versatile molecular platform and a straightforward electrochemical strategy for expanding and refining the Hammett parameter domain.