A New Phenolato-Bridged Dinuclear Manganese(II) Complex as a Turn-On Fluorosensor for Zn <sup>2</sup>+ Ions Via Mn <sup>2</sup>+ Ion Replacement

Abstract

A new synthesized and structurally characterized phenolato bridged di-nuclear manganese(II) complex, formulated as [Mn2L(OAc)2(DMF)2]PF6 (1) (where HL = 4-methyl-2,6-bis-[(2-morpholin-4-yl-ethylimino)-methyl]-phenol, and OAc = acetate anion) can act as a turn-on Zn2+ sensing probe through in situ formation of the correspondent dinuclear Zn(II) complex, [Zn2L(OAc)2]PF6 (2) via direct Mn2+ ion replacement in in MeOH/DMF (1/14) medium. This selective and specific fluorescent change due to addition of Zn2+ ions to the solution of 1 enables to detect Zn2+ ion in dimethylformamide (DMF). The fluorescent dinuclear zinc(II) complex 2 has been achieved also through the direct reaction between Zn2+ ions and HL. Both the complexes 1 and 2 have been characterized using physico-chemical and spectroscopic tools, and finally by single crystal X-ray crystallography for a detailed structural determination. The crystallographic investigation indicates that the Mn(II) and Zn(II) in the complexes exhibit a distorted octahedral and distorted trigonal bipyramidal coordination geometry, respectively. Interestingly, complex 1 showed a very weak fluorescence in solution and no emission in solid state, whereas complex 2 gave a strong bluish-green coloured fluorescence in both solution and solid state, which may be due to the distinct packing of complex 2 compared to 1, as well as to the intermolecular hydrogen bonding and C–H---π, C–H---N, C–H---O interactions in the crystal packing, likely playing crucial roles for the fluorescence behavior.