Crystallographic insights into (ImHR)2[TcBr6] (Im = imidazole, R = H, Me, Et): non-covalent interactions and mass spectrometry

Abstract

The structures of three Tc(IV) complexes (ImHR)₂[TcBr₆] (Im = imidazol-2-yl, R = H, Me, Et) are described. The compound (ImHMe)₂[TcBr₆] was obtained in two distinct polymorphic forms (space groups P and P2₁2₁2), arising from differences in the network of non-covalent interactions. Increasing the length of the hydrocarbon substituent at the imidazole ring results in a systematic increase in the proportion of H⋯H contacts within the crystal lattice, accompanied by a decrease in the fraction of Br⋯H/H⋯Br interactions responsible for hydrogen bonding. In the imidazolium and methylimidazolium (triclinic) structures, halogen–halogen interactions were identified that link the anions into one-dimensional chains. Additional non-covalent interactions, including π-stacking and anion–π contacts, are also present in the structures. A temperature increase from 100 to 296 K does not result in significant changes in the noncovalent interaction network. Four types of synthons of group-7 hexahalogenides, formed via anion–anion interactions, were found: a dimer, linear (the most common) and zigzag chains, and a 2D network. ESP surface analysis revealed only minor variations in electron density distribution among group-7 MBr₆²⁻ anions, but pronounced differences upon transition from MBr₆²⁻ to MCl₆²⁻. Laser ionization mass-spectrometry was performed for hexachloro- and hexabromotechnetates. The mass spectra of hexachlorotechnetates exhibit multiplets characteristic of reduced low-valent technetium clusters containing up to three metal atoms, along with oxychloride anions. In contrast, laser ionization of hexabromotechnetates does not lead to oxidative processes; the spectra contain multiplets assigned to cluster anions incorporating up to five metal atoms. The positions of these multiplets are independent of the nature of the counterion.