C 2-Symmetric Pseudopeptidic Schiff Bases Drive Discrete M2L2 Complexes: Coordination, Stability, and Photophysical Response.

We report the design and synthesis of two new C 2-symmetric, multifunctional pseudopeptidic ligands derived from L-valine and L-phenylalanine that integrate dual Schiff-base units (one per amino-acid fragment) and an aromatic central spacer. These ligands were rationally engineered to favor the self-assembly of discrete M2L2 architectures by combining C 2 symmetry with preorganized, convergent donor sets. Their metal-binding behavior, in particular toward Zn(II) and Cu(II), was elucidated by potentiometric titrations, UV-Vis spectrophotometry, and single-crystal X-ray diffraction. The combined data demonstrate a strong propensity to form dimeric M2L2 species, with particularly pronounced stabilization for Zn(II). X-ray analysis of the Zn2L2 complex reveals a pseudo-octahedral environment in which each Zn(II) is coordinated by imine nitrogen donors together with amide and phenolate oxygen atoms, with the required donor set completed cooperatively by two different ligand subunits, thus enforcing the targeted dimeric topology. Complex formation is accompanied by fluorescence enhancement for Zn(II), marked changes in the circular dichroism signatures of the Schiff-base chromophores, and characteristic d-d transition bands for Cu(II) and Ni(II) complexes. These results establish a modular pseudopeptidic platform for programming discrete M2L2 metallosupramolecular assemblies and their associated chiroptical and emissive responses.