Packing Problem (Z′=2) in Mercuric Halide Complexes Containing the Most Stable Polymorph of Ester Ligands; Directionalized by Robust C−H∙∙∙πe Synthon

The number of formula units in the unit cell and the number of symmetry-independent units in a crystal structure are known as Z and Z′, respectively. In general, Z′ will be 1 but when it is greater than 1, the situation referred as a packing problem in crystallography. [1] The ability of a compound to exist in more than one arrangement in the solid state is defined as polymorphism and if the same molecule in two different polymorphic forms adopts clearly different conformations this is called conformational polymorphism. [2] Intermolecular interactions can determine the outcome in some part of the crystallization process since the interactive donor part of the molecule is engaged in covalent bonding. As we noted in our previous study, if one compound has two or more polymorphs, the most stable one may control derivatives of the compound. [3] In this study, with crystal engineering in mind, the organic ligands phenyl nicotinate (L1) and phenyl pyrazine-2-carboxylate (L2) were synthesized. For each ligand, two polymorphs were systematically obtained (Ia, Ib and IIa, IIb). For L2 three new mercury (II) halide complexes, [Hg(L1)2Cl2] (1), [Hg(L1)2Br2] (2), [Hg(L1)2I2] (3) and for L2 two new mercury (II) halide complexes, [Hg(L2)2Cl2] (4), [Hg(L2)2Br2] (5), were synthesized. All of the compounds were fully characterized using FT-IR, TGA, DSC, mass spectrometry (only for ligands), CHNOS elemental analyses, NMR, PXRD and SCXRD. Following our previous CSD study, the directionality of C−H∙∙∙π interactions has also been investigated. Our results show a high tendency for C−H∙∙∙π interaction but in a special way. On the basis of our observations, C−H units tend to interact with center of ring’s bonds rather than the center of the ring. We named the interactions C−H∙∙∙πe (e is abbreviation for edge) and Cg (center of gravity), respectively. C−H∙∙∙πe using Cg is defined for the first time in recent study. The Cn−Hn tend to interact with neighboring Cg of Cn, even after applying the corrected parameters in the CSD search. This interaction acts as a Lego , meaning electron deficiency for one of them can be compensated with electron withdrawing of another one. In mercuric halide complexes 1,2 and 3 (which included the most stable conformational polymorph of L1), the C−H∙∙∙πe remains as a robust interaction and makes the high Z′ (2). Due to the different conformations for the ligands, we have formulated them as ML′LX2 with different interactions in L and L′. In L2 one of the carbons in pyridine ring was exchanged with nitrogen (pyrazine) and the packing of polymorph I (the thermodynamic crystal) was controlled by an alternative arrangement of the phenol and pyrazine rings. For 4 and 5 chemical formulas can be symbolized by M(LX)2 with Z′ is 0.5 owing to the strong C−H∙∙∙N in pyrazine ring and similar interactions for ligands in both sides (in lack of a C−H∙∙∙πe interaction).