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

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 2/ a 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 Al 3+ ions in aqueous DMF media. Importantly, we have reported the first structurally characterized six-coordinate dinuclear Al 3+ complex ([Na(Al 2 L 2 )·2H 2 O·4DMF], complex 1 (where ligand, L = sensor 1 ). Complex 1 crystallizes in the space group P 1&cmb.macr;. X-ray single crystal diffraction studies for complex 1 reveal that each Al 3+ 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 Na 2 EDTA 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 Al 3+ ions under UV light. A two arm-containing amido Schiff base named bis((2-hydroxynaphthalen-1-yl) methylene)-1,1′-biphenyl-2,2′-dicarbohydrazide was structurally characterized and used for the selective recognition of Al 3+ ions through "off-on" fluorescence signalling behaviour.