Exploration of CH•••π interactions involving the π-system of pseudohalide coligands in metal complexes of a Schiff-base ligand

A series of four mononuclear Schiff-base complexes, namely, [Zn(L)(NCS)(2)] (1), [Zn(L)(N-3)(2)] (2), [Cu(L)(NCS)(2)] (3) and [Ni (L)(2bpy)(NCS)](ClO4) (4), [where L = N,N-dimethyl-N'-(phenyl-pyridin-2-yl-methylene)-ethane-1,2-diamine and 2bpy = 2-benzoylpyridine] were synthesized with the aim of investigating the role of different non-covalent weak interactions responsible for the crystal packing of the complexes. All of them were structurally characterised by X-ray diffraction analysis. In addition to conventional CH3 center dot center dot center dot pi and pi center dot center dot center dot pi interactions, the importance of unconventional C-H center dot center dot center dot pi interactions in the crystal packing of compounds 1-4 was investigated by means of Hirshfeld surface analysis and DFT calculations. In these unconventional C-H center dot center dot center dot pi interactions, the pi-system (electron donor) is provided by the pseudohalide coligands. The interactions formed by the pi-system depend on the nature of the pseudohalide (N-3, NCO, NCS or NCSe) as demonstrated by molecular electrostatic potential calculations. Additionally, we have explored the photophysical properties of these complexes. Finally, we have combined a search in the Cambridge Structural Database and DFT energy calculations to analyse the rare ambidentate behaviour of SCN within the same complex.