Self-assembly of Sm(iii) Schiff base complexes: crystal structure, luminescence and antibacterial inhibition activity

Abstract

Two new Sm(III) Schiff base complexes have been synthesized and fully characterized from the novel ligands: L1, benzoic acid pyridin-2-ylmethylene-hydrazide and L2, 2-[(4-methoxy-phenylimino)-methyl]-phenol. The reaction of Sm(III) ions with the SB ligands and β-diketone units, such as hexafluoroacetylacetone (hfac) and 3-benzoyl-1,1,1-trifluoroacetone (tba), afforded [Sm(L1)₂(hfac)₂]₄Cl₄·8H₂O (Sm1) and [Sm(L2)₂(tba)₂(NO₃)] (Sm2), respectively. Interestingly, Sm1 exhibits a supramolecular mesocate assembly from four symmetrical Sm(III) complexes. Each Sm(III) center defines a corner of the mesocate with Sm⋯Sm separations of 10.291 and 11.634 Å. The assembly is stabilized by non-covalent interactions from two chloride ions and two water molecules trapped inside the assembly. The excitation of the Sm(III) complexes in solution displayed a broad ligand-centred (LC) emission centred at ∼450 nm in addition to the luminescence lines of Sm(III) from ⁴G_5/2 → ⁶H_j (j = 5/2, 7/2, 9/2 and 11/2) transitions. Sm1 displayed cyan light emission at CIE coordinates of (0.332, 0.345), while Sm2 displayed efficient red emission with an intense photoluminescence quantum yield. Furthermore, the antibacterial properties of the Sm(III) complexes have been also investigated using the inhibition zone diameter method and SEM images. The results highlighted the efficient antibacterial activity of Sm1 and Sm2 against some Gram-positive and Gram-negative bacteria. Interestingly, Sm2 exhibits the lowest MIC values. The structural design of ligand L2 would induce the lipophilic character of the complex and its ability to form hydrogen bonding interactions within the active pockets of the bacteria.