Preparação e caracterização de bases de Schiff e complexos metálicos a partir de quitosana e derivados de salicilaldeído

Biopolymeric Schiff bases were prepared from chitosan and salicylaldehyde and its 5-bromo, 5-chloro, 5-methyl, 5-methoxy and 5-nitro derivatives, under conditions optimized to improve the degree of substitution (DS).From 1 H NMR data, DS (%) = 78.7,51.3, 43.7, 28.1, 22.1 and 17.5, for 5-methoxy, salicylaldehyde, 5-nitro, 5-chloro, 5-methyl, and 5-bromo derivatives, respectively.The differences had been attributed to inductive and resonance effects of each substituent.From these ligands, copper (II) and nickel (II) complexes were synthesized with chitosan and all the biopolymeric Schiff bases.Chitosan, ligands and complexes were characterized by Fourier transform vibrational spectroscopy in the infrared region (FTIR) and thermal analytical techniques (thermogravimetry, TGA; derivative thermogravimetry, DTG and differential thermal analysis, DTA).Chitosan presented a deacetylation degree, DD = 75.6%,determined by 1 H NMR, presenting the characteristic bands of this biopolymer in the FTIR spectra as well as decomposition in two exothermic steps after dehydration.TGA/DTG and DTA curves of the bases revealed that they are less stable than chitosan.This stability is closely related to the nature of substituent than the DS.From TGA curves and DS values it was possible to calculate the bases composition which agreed with elemental analysis data.Complexes presented coordination via oxygen atom of aldehydes aromatic rings and the iminic nitrogen, based on the FTIR results, being the copper (II) complexes apparently stronger the those from nickel (II), if one consider the displacements in the vibration frequencies of the C=N and C-O phen bonds.The copper (II) and nickel (II) complexes also presented decomposition in two steps after dehydration, being the first step observed in lower temperatures when compared to those of the free bases.In some cases it was possible to observe that the decomposition steps in the DTG curves are split suggesting different decomposition for regions of the biopolymer in which the complexes is present and for the unmodified ones, once such phenomena occurred mainly in complexes derived from the bases with lower DS.The residues of decomposition for such complexes were CuO and NiO, according to X-ray diffractograms and their contents permitted to calculate the amount of each metal present in the complex leading to conclude that almost all the sites containing the Schiff bases on the biopolymeric matrix was complexed in both cases.