Synthesis and properties of isomeric pyridyl-containing chitosan derivatives

Here we report on the method of synthesis in gel of a new heterocyclic aminopolymer-N-2-(4-pyridyl)ethylchitosan (4-PEC) via direct addition of 4-vinylpyridine to chitosan that yields a derivative with the substitution degree (DS) up to 0.8. The comparison of reactivity, thermal, spectroscopic, and sorption properties of a new derivative and its isomer N-2-(2-pyridyl)ethylchitosan (2-PEC) is presented. 2-PEC has higher sorption capacity and forms more stable chelates with [PdCl4]2− and [PtCl6]2− ions than 4-PEC, but the latter shows higher selectivity to noble metals ions in the presence of Cl− ions. A gradual increase of the sorption capacities and the affinity coefficient for Cu2+ and Ni2+ in the row chitosan < 4-PEC < 2-PEC was related to the increase of electron donor nitrogen atoms content and chelating properties of 2-PEC. A nearly negligible increase of the 4-PEC sorption capacity for Ag+, as compared to plain chitosan, was suggested to be dependent on the difference in complexation models for 2-PEC and 4-PEC derivatives. The density functional theory (DFT) calculations have shown that the “pendant” model of the complex with Ag(I) is energetically favorable only for 2-PEC derivative, while in cases of chitosan and 4-PEC only “bridge” complexes can be formed that results in lower sorption capacity.