Unexpected Structure of a Helical N 4 -Schiff-Base Zn(II) Complex and Its Demetallation: Experimental and Theoretical Studies.

A new Zn-N ₄ -Schiff base L=((±)-trans-N,N'-Bis(2-aminobenzylidene)-1,2-diaminocyclohexane) complex was synthesized and fully characterized, showing an unexpected self-assembled double-stranded helicate structure. The X-ray crystal analysis of the Zn ₂ L ₂ complex ((C ₄₀ H ₄₄ N ₈ Zn ₂ ,CH ₂ Cl ₂ , a=14.2375(3) Å, b=16.7976(4) Å, c=16.1613(4) Å, monoclinic, P2 ₁ /n, Z=4) shows a centrosymmetrical structure in which zinc atoms are in distorted tetrahedral environments, revealing an M- (R, R) left-handed helicity in its asymmetric unit. However, it was observed that this dinuclear complex is thermodynamically unstable in the presence of small water amounts and undergoes demetallation into free N4-Schiff base ligand and ZnO nanoparticles. This hydrolysis process was thoroughly identified and monitored through detailed ¹ H NMR, DOSY NMR analysis. The reaction mechanism of this demetallation event was elucidated by using the DFT method, involving an activation energy smaller than 13 kcal/mol. Besides, a theoretical mechanism of the demetallation process is given for the first time.
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