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.