pH-Dependent Assembly and Conversions of Six Cadmium(II)-Based Coordination Complexes

Six cadmium(II) complexes containing N2O2 donor tetradentate asymmetrical Schiff base ligand 2-{(2-(dimethylamino)ethylimino)methyl}-6-methoxyphenol (HL5), namely, ((Cd3L52Cl4)2)·CH3OH·H2O (1), (Cd(L5)Cl)2·CH3OH (2), (Cd2(HL5)Cl4)n (3), {(Cd3(H2L5)2Cl8)·2H2O}n (4), ((H2L5)2)2+·(CdCl4)2-·H2O (5), and ((H2L5)2)2+·(CdCl4)2- (6), have been synthesized using cadmium(II) chloride and asymmetrical Schiff base ligand HL5 under different pH conditions at room temperature. The diverse structures show the marked sensitivity of the structural chemistry of the tetradentate asymmetrical Schiff base ligand HL5. Complex 1 formed at pH = 10 exhibits a rare zero- dimensional structure of trinuclear cadmium (II). At pH = 8-9, a dinuclear cadmium (II) complex 2 is formed. The reaction at pH = 5-7 leads to two one-dimensional structures of 3 and 4. A further decrease of the pH to 3-5 results in a zero-dimensional structure 5. Owing to the departure of lattice water molecules in the crystal, complex 5 at room temperature can gradually undergo single-crystal-to-single-crystal transformation to result complex 6. The results further show that conversions of complex 1 to 5 can also be achieved by adjusting the pH value of the reaction solution, 1 2pH=8 5pH=3 and 3 4pH=5. Comparing these experimental results, it is clear that the pH plays a crucial role in the formation of the resulting structures, which simultaneously provide very effective strategies for constructing the CdII compounds with N2O2 donor tetradentate asymmetrical Schiff base ligand. The strong fluorescent emissions of the six compounds (1-6) make them potentially useful photoactive materials. Furthermore, six Schiff base cadmium complexes (1-6), with DPPH (2,2-dipheny1-1-picrylhydrazy1) as a co-oxidant exhibited the stronger scavenging activity.