1,2,4,5-Tetrazine-Boron Halide Adduct Formation Unraveled in Solution and Solid State.

1,2,4,5-Tetrazines are electron-deficient heterocycles with four nitrogen atom-localized lone pairs potentially acting as Lewis bases. While it has been shown that tetrazines, as electronically amphiphilic ligands, undergo anion-π, π-π, and transition metal-based adduct formation reactions, adducts of tetrazines with main group metal Lewis acids have not yet been structurally characterized. We report spectroscopic and single crystal X-ray diffraction (SC-XRD) analyses of tetrazine-boron halide adducts in solution and in the solid state, respectively. With the aid of (diffusion-ordered) nuclear magnetic resonance (DOSY-)NMR spectroscopy dynamic equilibria between tetrazine-boron halide adducts have been proven, while solid state structures show localized 1:1 tetrazine-boron halide adducts.