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 (<b>1</b>), [Cd(L⁵)Cl]₂·CH₃OH (<b>2</b>), [Cd₂(HL⁵)Cl₄]<i>_n</i> (<b>3</b>), {[Cd₃(H₂L⁵)₂Cl₈]·2H₂O}_<i>n</i> (<b>4</b>), [(H₂L⁵)₂]²⁺·[CdCl₄]²⁻·H₂O (<b>5</b>), and [(H₂L⁵)₂]²⁺·[CdCl₄]²⁻ (<b>6</b>), 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 <b>1</b> formed at pH = 10 exhibits a rare zero-dimensional structure of trinuclear cadmium(II). At pH = 8−9, a dinuclear cadmium(II) complex <b>2</b> is formed. The reaction at pH = 5−7 leads to two one-dimensional structures of <b>3</b> and <b>4</b>. A further decrease of the pH to 3−5 results in a zero-dimensional structure <b>5</b>. Owing to the departure of lattice water molecules in the crystal, complex <b>5</b> at room temperature can gradually undergo single-crystal-to-single-crystal transformation to result in complex <b>6.</b> The results further show that conversions of complex <b>1</b>−<b>5</b> can also be achieved by adjusting the pH value of the reaction solution, <b>1</b> → <b>2</b>_pH=8 → <b>5</b>_pH=3 and <b>3</b> → <b>4</b>_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 (<b>1</b>−<b>6</b>) make them potentially useful photoactive materials. Furthermore, six Schiff base cadmium complexes (<b>1</b>−<b>6</b>), with 2,2-dipheny1-1-picrylhydrazy1 (DPPH) as a co-oxidant exhibited the stronger scavenging activity.