Crystal engineering is an emerging field of research that is concerned with the rational design of molecular building blocks and preparation of crystalline materials with the desired properties. In this regard, crystal engineering is closely related to supramolecular chemistry and non-covalent interactions play a key role in the supramolecular chemistry. Hydrogen bond, halogen bond and π-based interactions are the most important non-covalent interactions used in various self-assembly of supramolecular aggregates.1- 4 In the present work the synthesis of new mercury (II) iodide complexes including carboxamide ligand (N-(1,3-thiazole-2-yl)-2pyrazinecarboxamide using two different synthetic methods were performed and characterized by IR spectroscopy, elemental analysis and x-ray crystal structure analysis. X-ray crystallographic analysis of complex [Hg2I4(L)2] (1) obtained from the conventional solution-based method, indicated that the complex (1) has a polymeric structure and iodine atoms are located as a bridge between the mercury atoms. Also according to the data obtained from x-ray diffraction analysis, complex [Hg2I4(L)2] crystallizes in orthorhombic Pna21 space group and the asymmetric unit of this compound consists of two metallic mercury(II) and two carboxamide ligands. Powder X-ray diffraction has been exploited to investigate the structure of mercury (II) iodide complex prepared by the environmentally friendly solvent-free method (2) and the results showed that the complex (2) has a different crystal structure

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