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The Effect of C-2 Substituents of Salicylaldehyde-Based Thiosemicarbazones on the Synthesis, Spectroscopy, Structures, and Fluorescence of Nickel(II) Complexes



The aim of this investigation was to study the bonding patterns, geometry, and fluorescence properties of nickel(II) complexes with salicylaldehyde-based thiosemicarbazones [(2-OH-5-R1-C6H3)C(R2)=NNHC(=S)NHR3, LH2] by invoking the effect of substituents at the C-2 and N-1 atoms of the thio ligands and by using bipyridines/phenanthrolines as auxiliary ligands. The methodology used for the preparation of complexes involved initial binding of NiII with a thio ligand in an organic solvent to yield an insoluble solid of empirical composition [NiL], which was reacted further with a N,N-donor ligand. In this way, we have prepared a series of dinuclear complexes and two mononuclear complexes with stoichiometries [Ni2L2(bipy)(D)] [bipy = 2,2′-bipyridine, L = L1 (R1, R2, R3 = H), D = H2O 1; L = L2 (R1, R2 = H, R3 = Me), D = CH3OH 2; L =L3 (R1, R2 = H, R3 = Et), D = CH3OH 4; L = L3, D = H2O 5; L = L4 (R1 = Me, R2 = H, R3 = Me), D = CH3OH 6], [Ni2L32(bipy)] (3), [Ni2L32(dmbipy)(H2O)] (7, dmbipy = 4,4′-dimethyl-2,2′-bipyridine), [Ni2L52(bipy)2] (8, R1 = NO2, R2 = H, R3 = Me), [NiL6(bipy)] (9, R1 = NO2, R2 = H, R3 = Et), [NiL7(bipy)(H2O)] (10, R1 = H, R2 = Me, R3 = Et), [Ni2L32(4,4′-bipy)] (11, 4,4′-bipy = 4,4′-bipyridine), and [Ni2L72(4,4′-bipy)] (12). The dinuclear complexes involve (i) O,N3,S chelation with bridging N2 atoms to form trigonal-bipyramidal/square-planar complexes with a five-coordinate and a four-coordinate Ni center (3, 5:4-coordination) and octahedral/square-planar complexes (1, 2, 47, 6:4-coordination), (ii) O,N3,S chelation with S bridges to form octahedral/octahedral complexes (8, 6:6-coordination), and (iii) O,N3,S chelation to form square-planar/square-planar complexes (11, 12, 4:4-coordiantion). The mononuclear complexes involve O, N3,S chelation and have square-pyramidal (9) and octahedral (10) geometries. Complexes 110 are paramagnetic with μeff value in the range 2.97–3.38 μB. Complexes 2 and 3 exhibit fluorescence emission.

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