Palladium(II) halide complexes of N,N,S-tridentate Schiff bases: synthesis, structural studies in solution and solid state, and analysis of Hirshfeld surfaces

The synthesis and characterization of bromo- and iodo complexes of Pd(II) containing tridentate anionic Schiff bases, derived from the ring-opening of 2-R-2-(2-pyridyl)benzothiazoline precursors (R = H, 1; R = Me, 2; R = C₆H₅, 3; R = C₅H₄N, 4) is described. The [Pd(Lⁿ)X] complexes [for X = Br: n = 1 (1a), n = 2 (2a), n = 3 (3a), and n = 4, (4a); for X = I: n = 1 (1b), n = 2 (2b), n = 3 (3b), and n = 4 (4b)] were characterized in solution and in solid state by NMR and IR spectroscopy and by elemental analyses. In all complexes, the corresponding anionic Schiff ligand {Lⁿ}⁻ displayed a κ²NκS-tridentate coordination mode. X-ray diffraction studies of [Pd(L³)Br] (3a), [Pd(L³)I] (3b), [Pd(L⁴)Br] (4a), and [Pd(L⁴)I] (4b) complexes showed the formation of two five-membered chelate rings, with a distorted square planar geometry around the Pd(II) cation. Crystal structures of 3a, 3b, 4a, and 4b are stabilized by non-classical C–H···π, C–H···S, and C–H···X hydrogen bonding. The Hirshfeld surface analysis showed that the H···H contacts are predominant, and the C–H···S or C–H···π interactions were the major contributions in the stacking of the molecular complexes.