Crystallographic elucidation of an aluminium-bound amido Schiff base chemosensor: a selective turn-on fluorescent chemosensor for Al³⁺ ions

Abstract

We have synthesized a biphenyl-derived two arm-containing amido Schiff base, bis((2-hydroxynaphthalen-1-yl) methylene)-[1,1'-biphenyl]-2,2'-dicarbohydrazide (sensor 1), having hard donors to facilitate chelation with hard metal centers. The crystal structure of sensor 1 reveals that it crystallizes in the monoclinic system with the space group <i>I</i>2/<i>a</i> and shows several types of intra/inter-molecular H-bond interactions, which stabilized the crystal lattice. The sensing property of sensor 1 towards different metal ions has been demonstrated using different analytical techniques. In fact, sensor 1 shows high fluorescence selectivity and sensitivity towards Al³⁺ ions in aqueous DMF media. Importantly, we have reported the first structurally characterized six-coordinate dinuclear Al³⁺ complex ([Na(Al₂L₂)·2H₂O·4DMF], complex 1 (where ligand, L = sensor 1). Complex 1 crystallizes in the space group <i>P</i>1̄. X-ray single crystal diffraction studies for complex 1 reveal that each Al³⁺ ion is hexa-coordinated by four oxygen and two nitrogen atoms from each arm of the two ligands. The sodium ion has penta-coordination in a highly distorted trigonal bipyramidal geometry, surrounded by two μ-bridging naphtholate oxygen atoms and three solvent DMF oxygen atoms. Upon addition of Na₂EDTA to complex 1, no spectral change or naked-eye colour change was observed. Furthermore, "test kits" coated with sensor 1 showed successful selective detection of Al³⁺ ions under UV light.