Hetero-bimetallic M(II)/Hg(II) complexes (M = Ni and Cu) with unsymmetrical Schiff base and ancillary thiocyanate ligands: synthesis, crystal structures, DFT calculations and Hirshfeld surface analysis

Abstract

Two new heterobimetallic complexes, [(CuL)₂Hg(SCN)₂] (1) and [(NiL)Hg(μ₁,₃-SCN)(SCN)]₂ (2), were synthesized by the reaction of Hg(SCN)₂ with [CuL] and [NiL] as “metalloligands” toward Hg, where H₂L is the unsymmetrical Schiff base = N-naphthylidene-N'-salicylidene-2-aminobenzylimine. Both complexes were fully characterized and their crystal structures were determined by single-crystal X-ray diffraction. In the heterotrinuclear centrosymmetric complex 1, the central Hg atom, located on an inversion center, exhibits a distorted octahedral coordination environment. It is bonded to four bridging phenoxo oxygen atoms from two [CuL] metalloligands and two mutually trans S-bound terminal thiocyanate anions. In the heterotetranuclear complex 2, each Hg(II) ion adopts a distorted square-pyramidal HgO2S₂N geometry, being coordinated by two bridging phenoxo oxygen atoms of one [NiL] unit, one S-bound terminal thiocyanate, and one S and one N atom of two bridging thiocyanate ligands. The Hg‧‧‧N in complex 2 is a noncovalent contact that exemplifies an intramolecular spodium bond (SpB). The Cu(II) and Ni(II) centers display distorted square-planar coordination geometries. The crystal packing of both complexes is mainly governed by intermolecular π···π stacking interactions of the coordinated Schiff base ligand, leading to two-dimensional stacked network architectures. Density functional theory (DFT) calculations indicate a triplet ground state for 1 and a singlet ground state for 2. Hirshfeld surface analyses (HSA) were employed to visualize the dominant non-covalent intermolecular interactions and their contributions to the crystal packing. The HSA and the two-dimensional fingerprint plots (2D-FP) reveal that H···H and C···H/H···C contacts are the principal interactions stabilizing the crystal lattices. The electrochemical behavior of the compounds is also reported and discussed.