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

Six cadmium(II) complexes containing the N2O2 donor tetradentate asymmetrical Schiff base ligand 2-{[2-(dimethylamino)ethylimino]methyl}-6-methoxyphenol (HL5), namely, [((Cd3L2O4)-O-5)(2)]center dot CH3OH center dot H2O (I), [Cd(L-5)Cl](2)center dot CH3OH (2), [Cd-2(HL5)Cl-4](n) (3), {[Cd-3(H2L5)(2)Cl-8]center dot 2H(2)O} (4), [(H2L5)(2)](2+)center dot[Cd3Cl4](2-)center dot H2O (5), and [(H2L5)(2)](2+)center dot[CdCl4](2-) (6), have been synthesized using cadmium(II) chloride and the asymmetrical Schiff base ligand H L5 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(11). At pH = 8-9, a dinuclear cadmium(11) 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 in complex 6. The results further show that conversions of complex 1-5 can also be achieved by adjusting the pH value of the reaction solution, 1 --> 2(pH) (= 8) --> 5(pH = 3) and 5. Comparing these experimental results, it is clear that the pH plays a crucial role in the formation of the resulting structures, which simultaneously provides very effective strategies for constructing the Cd-II 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 2,2-diphenyl-1-picrylhydrazyl (DPPH) as a co-oxidant exhibited the stronger scavenging activity.