A unique -conjugated benzothiadiazole-ortho-vanillin ligand (HL), characterized by single crystal X-ray diffraction and DFT calculations, has been prepared by condensation between 4-amino-benzothiadiazole (BTD) and ortho-vanillin. Its reaction with cobalt(II) acetate afforded the complex of formula [CoL2]•CH2Cl2 (1), for which the coordination environment of the cobalt centre is a distorted octahedron and the ligand acts as a monoanionic tridentate NNO chelate in its phenolate form. Intermolecular - stacking interactions between the -conjugated BTD units provides an antiferromagnetic coupling pathway, as indicated by the analysis of the dc magnetic measurements of a crystalline sample of complex and supported by DFT type calculations. The static magnetic behaviour of 1 is analysed according to spinorbit coupling and zero-field splitting models. Remarkably, the complex exhibits slow relaxation of the magnetization under applied magnetic field being thus a new example of fieldinduced mononuclear single-molecule magnet (SMM).