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) towards CuII, ZnII, CdII, HgII and PbII 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 CuII has the highest affinity for all three ligands, followed by ZnII, HgII, PbII and CdII. 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•½MeNO2•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 ZnII complexes than in the CdII complexes in both media. The results have been examined by considering the ratio Irel(ZnII)/Irel(CdII), within the emerging idea that the relative strength of the CHEF effect for the small ZnII ion as compared to larger CdII ion might be determined by steric crowding in the corresponding complexes with quinoline-based fluorescent chemosensors.