Boron Difluoride Adducts of a Flexidentate Pyridine-Substituted Formazanate Ligand: Property Modulation via Protonation and Coordination Chemistry

The synthesis and characterization of a flexidentate pyridine-substituted formazanate ligand and its boron difluoride adducts, formed via two different coordination modes of the title ligand, are described. The first adduct adopted a structure that was typical of other boron difluoride adducts of triarylformazanate ligands and contained a free pyridine subsituent, while the second was formed via chelation of nitrogen atoms from the formazanate backbone and the pyridine substituent. Stepwise protonation of the pydridine-functionalized adduct, which is essentially non-emissive, resulted in a significant increase in the fluorescence quantum yield up to a maximum of 18%, prompting study of this adduct as a pH sensor. The coordination chemistry of each adduct was explored through reactions with nickel(II) bromide [NiBr2(CH3CN)2], triflate [Ni(OTf)2] and 1,1,1,4,4,4-hexafluoroacetylacetonate [Ni(hfac)2(H2O)2] salts. Coordination to nickel(II) ions altered the physical properties of the boron difluoride formazanate adducts, including red-shifted absorption maxima and less negative reduction potentials. Together, these studies have demonstrated that the physical and electronic properties of boron difluoride adducts of formazanate ligands can be readily modulated through protonation and coordination chemistry