Complexes of Fe(II), Ni(II), and Zn(II) with a Tetradentate Naphthalene Diimine–Dipyrazole Ligand: Influence of a Metal Ion and Anions on the Luminescence Properties

Luminescent first-row transition metal complexes offer distinct advantages over traditional organic fluorophores for anion sensing, exhibiting greater structural diversity, tunable luminescence properties through ligand and metal selection, and often enhanced sensitivity and selectivity in detecting anions. Thus, we report the synthesis and characterization of three metal complexes, [FeII(H2L)(H2O)(BF4)]BF4·MeCN (1), [NiII(H2L)(MeCN)2](BF4)2·H2O (2), and [ZnII(H2L)(BF4)]BF4·MeCN (3), incorporating a tetradentate diimine–dipyrazole ligand containing a naphthalene unit (H2L). Single-crystal X-ray diffraction (SCXRD) reveals distorted coordination spheres with three narrower and one wider cis-angles and a “face-to-face” arrangement of the pyrazole units, facilitating bifurcated hydrogen bonding between the pendant NHpyrazole moieties and solvent or anion molecules. VT-magnetic measurements confirmed a high spin state (S = 2) for the Fe complex, while the Ni complex exhibits orbital contribution to the g value. The luminescent quantum yield of the complexes in dry and deoxygenated MeCN solutions were evaluated and compared to the 2,3-diaminonaphthalene (2,3-DAN) synthon. Metal coordination influences the emission intensity, following the trend: Zn(II) > Ni(II) > Fe(II). The Zn complex displays concentration-dependent emission due to the J-excimer formation at higher concentrations, which was corroborated in the solid state by SCXRD and detected in the gas and solution phases. Furthermore, the Zn complex shows selective luminescence quenching upon addition of F– in dilute (1.0 × 10–5 M) and neutral MeCN solutions, attributed to deprotonation of the NH pyrazole unit. This selective response is not observed for other anions tested except for the triethylamine base.