Synthesis of Pyridine- and Pyrazine-BF₃ Complexes and Their Characterization in Solution and Solid State

Following the discovery of the redox-active 1,4-bis-BF₃-quinoxaline complex, we undertook a structure-activity study with the objective to understand the active nature of the quinoxaline complex. Through systematic synthesis and characterization, we have compared complexes prepared from pyridine and pyrazine derivatives, as heterocyclic core analogues. This paper reports the structural requirements that give rise to the electrochemical features of the 1,4-bis-BF₃-quinoxaline adduct. Using solution and solid-state NMR spectroscopy, the role of aromatic ring fusion and nitrogen incorporation in bonding and electronics was elucidated. We establish the boron atom location and its interaction with its environment from 1D and 2D solution NMR, X-ray diffraction analysis, and ¹¹B solid-state NMR experiments. Crystallographic analysis of single crystals helped to correlate the boron geometry with ¹¹B quadrupolar coupling constant (C_Q) and asymmetry parameter (n_Q), extracted from ¹¹B solid-state NMR spectra. Additionally, computations based on density functional theory were performed to predict electrochemical behavior of the BF₃-heteroaromatic complexes. We then experimentally measured electrochemical potential using cyclic voltammetry and found that the redox potentials and C_Q values are similarly affected by electronic changes in the complexes.