A bifunctional 2,2′:6′,2″-terpyridine-based ligand for ratiometric Cu(II) sensing

Synthesis and characterization of the bifunctional sensor receptor ligand N-([2,2′:6′,2″-terpyridine]-4′-yl)methyl)-N-propylacrylamide (1) and the model ligand N-([2,2′:6′,2″-terpyridine]-4′-yl)methyl)-N-propylisobutyramide (2) are described. Ligand 1 is a receptor for Cu(II) that is copolymerizable with N-isopropylacrylamide giving a ratiometric sensor of weakly bound Cu(II) in environmental waters. Ligand 2 is a model for copolymerized 1 whereby the reactive acrylamide group is replaced by isobutyramide. Solution speciation of complexes of Cu(II) and Zn(II) with 2 were investigated spectroscopically and their solid-state structures were studied through single-crystal X-ray diffraction. Solution UV–vis and fluorescence studies show a preference of 2 toward Cu(II) over Na(I), Zn(II), Cu(II), Co(II), Mn(II), Ni(II), Cd(II), and Pb(II) in accord with the Irving–Williams series and other coordination principles. Solution speciation determined in a weakly coordinating aqueous-organic (60 : 40 DMF/H2O) medium indicates 1 : 1 Cu(II):2 binding as desired in that formation of [Cu(2)2]2+ would crosslink the polymer sensor. The crystal structures of [Cu(2)(NO3)2] and [Zn(2)(NO3)2]·MeOH·1/2Et2O display distorted octahedral geometries where 2 coordinates meridionally and two nitrate groups occupy the remaining sites around the metal center.