Terpyridine–Triarylborane Conjugates for the Dual Complexation of Zinc(II) Cation and Fluoride Anion

A series of ditopic terpyridine–triarylborane conjugates (1–3) in which 4′-ethynylterpyridine is linked to the para, meta, and ortho positions of the phenyl ring of dimesitylphenylborane (Mes2PhB), respectively, were prepared to investigate the dual complexation behavior of the conjugates toward Zn­(II) cation and fluoride anion. The crystal structures of the corresponding Zn­(II) complexes (L·ZnCl2, L = 1–3) reveal the formation of a 1:1 adduct between ZnCl2 and a conjugate, with a five-coordinate Zn­(II) center bound to three nitrogen atoms and two chlorine atoms. In particular, the structure of ortho-substituted 3·ZnCl2 in comparison with that of 3 indicates the presence of π–π interactions between the mesityl ring and ethynylene–pyridine fragment in 3·ZnCl2. UV/vis absorption and fluorescence spectra of 1–3 display low-energy bands mainly assignable to a π­(Ar) → pπ(B) (Ar = Mes and/or phenylene–ethynylene) charge transfer (CT) transition. The transition in Zn­(II) complexes has a π­(Mes) → π*­(Ar) (Ar = terpyridine–ethynylene) intramolecular CT nature with red shifts of both the absorption and emission bands in comparison to those of free conjugates. These spectroscopic features are further supported by TD-DFT calculations. UV/vis absorption and fluorescence titration experiments of 1–3 toward Zn­(II) and fluoride ion, respectively, show that while the absorption and fluorescence bands underwent gradual quenching upon addition of fluoride, the addition of ZnCl2 gave rise to the red shifts of both bands. Fluoride titration experiments of Zn­(II) complexes also resulted in gradual quenching of both the absorption and emission bands accompanied by the disappearance of emission color. Sequential addition of ZnCl2 and fluoride to the conjugates reproduced the above binding behavior with an emission color change from deep blue to sky blue to dark.