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

## TL;DR

A new electrochemical method using a biazulene core allows for determining Hammett constants and studying molecular effects with high precision.

## Contribution

A versatile electrochemical platform for determining Hammett constants and analyzing π-communication in functionalized molecules.

## Key findings

- The 6,6′-biazulene core enables accurate Hammett constant determination via electrochemical reduction.
- Intramolecular hydrogen bonding effects and substituent influences are quantified using NMR and IR spectroscopy.
- The method reveals long-range electron donor/acceptor effects across the biazulenic π-linker.

## Abstract

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 σp 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

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O⋯H–S) and enables determining effective σ constants (σeff) 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(13COtrans) vs. δ(13CN) and δ(13COcis) vs. δ(13CN) 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.

Functionalization along the molecular axis of 6,6′-biazulene enables electrochemical Hammett analysis, precise redox tunability, and probing of long-range π-communication.