Deprotonation-Induced Enantioselective Aggregation and Deprotonation-Induced Ligand Rearrangement of Copper(II) Complexes Yield 1D Homochiral and Heterochiral Chains and a Cyclic Tetramer, Respectively

The copper(II) complexes of protonated pentadentate Schiff-base ligands with [Cu(H2Ln)](ClO4)2 formula (n =
3−6, 3−6) have been synthesized and characterized (H2L3 = N-((2-methylimidazol-4-yl)methylene)-3-aminpropyl-N‘-((2-methylimidazol-4-yl)methylene)-4‘-aminobutylamine, H2L4 = N-((2-phenylimidazol-4-yl)methylene)-3-aminpropyl-N‘-((2-phenyl-imidazol-4-yl)methylene)-4‘-aminobutylamine, H2L5 = N,N‘-bis((2-phenylimidazol-4-yl)methylene)-3,3‘-diaminodipropylmethylamine, H2L6 = N-((2-methylimidazol-4-yl)methylene)-2-aminoethyl-N‘-((2-methylimidazol-4-yl)methylene)-3‘-aminopropylamine. The mono-deprotonated complexes 3‘, 4‘, and 5‘
contain one imidazole and one imidazolate groups per unit and are Δ (clockwise) or Λ (anticlockwise)
enantiomorphs due to the spiral arrangement of the ligand around copper(II) ion. They function as chiral building
components for a self-assembly process resulting from the formation of hydrogen bonds between the imidazole
and imidazolate groups of adjacent units to yield 1D zigzag-chain structures. The distance between hydrogen-bonded nitrogen atoms is 2.81(2), 2.832(9), and 2.875(9) Å for 3‘, 4‘, and 5‘, respectively. The crystal lattice of
3‘ yielded either ...ΔΔΔ... or ...ΛΛΛ... isotactic 1D zigzag-chains, while the crystal lattices of 4‘ and 5‘ yielded
...ΔΛΔΛ... syndiotactic 1D zigzag-chains. In 3‘, two adjacent methyl groups at the 2-position connected by hydrogen
bond array in the same direction, thus allowing homochiral aggregation of the complex molecules in a 1D chain.
On the other hand, in 4‘ and 5‘, two adjacent bulky phenyl groups require opposite orientations, thus allowing
heterochiral aggregation. Enantioselective aggregation with homochirality or heterochirality can thus be controlled
with suitable substituents. While its mononuclear precursor 6 is pentacoordinated with the N5 donor set of the
pentadentate ligand H2L6, the deprotonated complex 6‘ has an imidazolate-bridged tetranuclear cyclic structure
with a Cu−Cu distance of 6.086(2) Å. The ligand in 6‘ is tetradentate and includes an hexahydropyrimidine ring
resulting from a deprotonation induced rearrangement reaction.