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Chelate ring size effect as a factor of selective fluorescent recognition of $Zn^{2+}$ ions by pyrrolo[2,3-b]quinoxaline with a substituted 2-oyridyl group receptor
Chelate ring size effect as a factor of selective fluorescent recognition of $Zn^{2+}$ ions by pyrrolo[2,3-b]quinoxaline with a substituted 2-oyridyl group receptor
Chelate ring size effect as a factor of selective fluorescent recognition of $Zn^{2+}$ ions by pyrrolo[2,3-b]quinoxaline with a substituted 2-oyridyl group receptor
author:
Ostrowska Katarzyna , Musielak Bogdan , Szneler Edward , Gryl Marlena , Stadnicka Katarzyna , Dudek Łukasz
Analysis of the spectral properties and structural
di
ff
erences of two turn-on ratiometric
fl
uorescent receptors for
Zn
2+
and Cd
2+
ions, derivatives of pyrrolo[2,3-
b
]quinoxaline
(
2
), and earlier published
3
(Ostrowska et al.
CrystEngComm
2015
,
17
, 498
−
502) was performed. Both ligands are
E
/
Z
push
−
pull ole
fi
ns interconverting at room temperature, with
barriers to rotation about enamine double bonds, from
E
to
Z
isomers of 19.3
±
0.1 and 16.9
±
0.3 kcal/mol and from
Z
to
E
of 16.9
±
0.3 and 15.7
±
0.2 kcal/mol, respectively.
Diastereoisomers (
E
)-
2
and (
Z
)-
2
were isolated and charac-
terized by X-ray structural analysis. The formation of complexes
by (
E
/
Z
)-
2
with acetates and acetylacetonates of Zn
2+
and Cd
2+
was monitored by UV
−
vis,
fl
uorescence, and
1
H NMR titrations
in acetonitrile, respectively. X-ray structural analysis for isolated [(
E
)-
2
]
2
Zn in relation to earlier published (
E
)-
3
-ZnOAc revealed
the formation of a six-coordinated zinc ion with six- and four-membered bis-chelate rings by (
E
)-
2
. The chelate e
ff
ect increases
the ligand a
ffi
nity for Zn
2+
(log
β
12
= 12.45) and causes the elongation of nitrogen
−
metal bonds. Extension of the coordination
cavity size allows coordination of a cadmium ion. The introduction of a
fl
exible ethylene linker between the
fl
uorophore and
ionophore pyridyl groups in
3
signi
fi
cantly a
ff
ects the selectivity of zinc-ion recognition. The distorted tetrahedral geometry of
(
E
)-
3
-ZnOAc with a four-coordinated zinc ion appears to be the most preferred because of the short donor
−
zinc distance with a
1:1 binding mode. The formation of the small coordination cavity size with six-membered bis-chelate rings provides an e
ff
ective
overlap of zinc and donor orbitals, precluding the coordination of a cadmium ion in the same manner as zinc.
affiliation:
Wydział Chemii : Zakład Krystalochemii i Krystalofizyki, Wydział Chemii : Pracownia Spektroskopii NMR, Wydział Chemii : Zakład Chemii Organicznej