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, [(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 the 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 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 3 → 4_pH=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-dipheny1-1-picrylhydrazy1 (DPPH) as a co-oxidant exhibited the stronger scavenging activity.