Synthesis and coordination properties of quinoline pendant arm derivatives of [9]aneN3 and [9]aneN2S as fluorescent zinc sensors

The coordination chemistry of three new quinoline pendant arm derivatives of [9]aneN3 (L1, L2) and [9]aneN2S (L3) toward Cu(II), Zn(II), Cd(II), Hg(II), and Pb(II) has been investigated both in solution and in the solid state. The protonation constants for L1-L3 and stability constants with the aforementioned metal ions have been determined potentiometrically in 0.10 M NMe4Cl MeCN/H2O (1:1 v/v) solution at 298.1 (0.1 K; the measured values show that Cu(II) has the highest affinity for all three ligands, followed by Zn(II), Hg(II), Pb(II), and Cd(II). For each metal ion considered, 1:1 complexes with L1-L3 have also been isolated in the solid state and [Cu(L1)](BF4)2 (1), [Zn(L1)](BF4)2 (2), [Cd(L1)](ClO4)2 (3), [Hg(L1)](NO3)2 (4), [Pb(L1)](ClO4)2.MeCN (5), [Zn2Cl2(L2)2](BF4)2.1/2MeNO2.H2O (6), [Cu(L3)](ClO4)2 (7), [Zn(L3)(NO3)]NO3 (8), [Cd(L3)(NO3)0.82Cl0.18]NO3 (9), and [Hg(L3)](ClO4)2.MeCN (10) have also been characterized by X-ray crystallography. The optical response of L1-L3 to the presence of the above-mentioned metal ions has been investigated in MeCN/H2O (1:1 v/v) and H2O solutions. All three ligands show a stronger “OFF-ON” CHEF (chelation enhancement of fluorescence) effect in the Zn(II) complexes than in the Cd(II) complexes in both media. The results have been examined by considering the ratio I-rel(Zn(II))/I-rel(Cd(II)), within the emerging idea that the relative strength of the CHEF effect for the small Zn(II) ion as compared to larger Cd(II) ion might be determined by steric crowding in the corresponding complexes with quinoline-based fluorescent chemosensors.