Cation-/Ligand-Induced Solvent-Assisted Transformations of Zn(II) and Cu(II) Complexes Featuring Single-Pocket Multidentate Chelating Members

A new family of single-pocket metal complexes bearing O,N,O-tridentate and O,N-bidentate chelating members {Cu, 1b (P2₁/n); Ni, 1c (C2/c); Mn, 1d (I2/a); Cu, 2b; and Ni, 2c (both P2₁/c)}, starting from synthesized and fully characterized Zn(II) (1a; I2/a) and Cu(II) (2a; C₂) precursors, were conveniently prepared via cation-induced solvent-assisted and ligand-induced solvent-assisted transformations. Herein, we show multistep solvent-assisted transformations from cis-1a → trans-1b → cis-1c → cis-1d, as well as all-trans 2a → 2b → 2c. All processes are one-way irreversible, as substantiated by thermodynamic aspects (enthalpies based on Gibbs free energies) derived from density functional theory calculations. On the other hand, complex 2a′ (C2/c; a polymorphic form of 2a) was obtained through a routine synthetic procedure. The compounds have been established by various spectroscopic techniques (infrared, UV-vis, ESI-MS, ¹H, and ¹³C NMR), elemental analysis, and X-ray crystallography. Single-crystal X-ray studies reveal that complexes 1a-d exhibit a pseudo-octahedral geometry around each metal center, with 2a displaying a four-coordinate seesaw geometry Cu(II) sphere (Addison parameter; τ = 0.42), while 2a′ (τ = 0.00), 2b (τ = 0.00), and 2c (τ = 0.00) possess a perfect square-planar configuration around each metal center. Furthermore, distortion is stabilized by the presence of peripheral O-donor atoms from the bulky −OMe group, and by virtue of its size, increased bond lengths and angles are accommodated. Ligand substitution induced coordination geometry transformation from quasi-square-planar 2a to perfect square-planar 2b. Assessment of the metric parameter shows that the distances between the two Cu-O_methoxy are all largely positive due to Jahn-Teller distortion, indicating an unprecedented tetragonal bipyramidal geometry in 1b.