Halide Ion Mediated Aldehyde‐Amine Condensation Leading to Schiff–base and Cyclized Non‐Schiff–base Ligand Complexes of Cd^II: A Combined Experimental and Theoretical Investigation

Abstract

The reactions of pyridine‐2‐carbaldehyde with aminoalcohols N‐(2‐hydroxyethyl)ethylenediamine and 2‐(3‐aminopropylamino)ethanol in presence of CdIIX2 (X=Cl, Br, I) have been investigated. Structure analyses reveal that four complexes, namely [Cd2(L1)2(Br)4] (2), [Cd(L1)(I)2](3), [Cd(L1)(Br)2] (5) and [Cd(L2)(I)2] (6) are formed with the expected Schiff‐base ligands (E)‐2‐(2‐(pyridin‐2‐ylmethyleneamino)ethylamino)ethanol (L1) and (E)‐2‐(3‐(pyridin‐2‐ylmethyleneamino)propylamino)ethanol (L2). Interestingly, when X=Cl two dinuclear complexes of general formula [Cd2(L1C/L2C)2(Cl)4] (complex 1 and 4, respectively) have been structurally characterized, where the ligands are no longer Schiff‐bases but contain an imidazolidine/hexahydro‐pyrimidine cyclic part instead [L1C = 2‐(2‐Pyridin‐2‐yl‐imidazolidin‐1‐yl)‐ethanol and L2C = 2‐(2‐Pyridin‐2‐yl‐tetrahydro‐pyrimidin‐1‐yl)‐ethanol]. In order to understand whether the solid state structure is retained in solution various NMR analysis like 1H, 13C NMR and DEPT‐135 have been performed. When X=I, the equilibrium is shifted towards the formation of the Schiff‐base ligands, thus complexes 3 and 6 are obtained as sole products. However, with X=Br or Cl, a mixture of products is present, comprising complexes containing both Schiff‐bases as well as cyclized ligands. DFT calculations have been performed to rationalize the influence of the halide ligands on the reactivity of the Schiff‐base and the formation of the cyclized ligand.