Spodium bonds and metal–halogen···halogen–metal interactions in propagation of monomeric units to dimeric or polymeric architectures

In this work we report on crystal structures of a heteroleptic coordination polymer [Hg2(HL)Cl4]n (1) where the nitrogen atom of the peripheral pyridine fragment is linked to another HgCl2 molecule and two discrete mononuclear heteroleptic complexes [Hg(HL)I2] (2) and [Mn(HL)Cl2]·MeOH (3·MeOH), which were obtained through self-assembling of N'-(1-(pyridin-2-yl)ethylidene)isonicotinohydrazide (HL) with HgCl2, HgI2 or MnCl2, respectively. HL coordinates the HgII salts in its keto-form, while MnCl2 is chelated by a zwitterionic form of HL confirmed by the solid state IR spectroscopy. The studied here ligand molecule has a tendency to involve the 4-pyridyl nitrogen atom in coordination bonding which is correlated with the ability to transfer the electric charge and production of a zwitterionic form. The crystal structure and Hirshfeld surface data analysis show a great importance of spodium bonds and metal–halogen⋯halogen–metal interactions. A prominent consequence of lack of σ- or π-hole interactions in case of MnII ion is the isolation of metal center from the external contacts. Apart form this in 3·MeOH the halogen⋯halogen interactions are absent. It seems that the σ- or π-hole regions formed at the HgII cation in 1 and 2 favor the participation of coordinated Cl or I atoms in the M–Hal⋯Hal'–M' halogen⋯halogen interactions, whereas in complex 3·MeOH there are no metal or halogen centered σ-hole interactions. Thus, the 1D coordination polymer in 1 is stabilized by the Hg⋯Cl spodium bonding. Additionally, the observed intermolecular halogen⋯halogen interactions in 1 provide cross-linking of the 1D coordination polymers, yielding a 2D supramolecular double-layered sheet. In 2 an additional contact between halogen and mercury atoms results in a dimeric unit. These dimers are linked into a 1D polymer by halogen⋯halogen interactions between metal bounded iodine atoms. The DFT theoretical studies were applied to analyze the Hg⋯X non-covalent spodium interactions that govern the formation of the 1D polymeric structure in 1 and self-assembled dimers in 2. © 2021