An Unexpected Tetragonal Network of Seven-Connected Er6 Clusters Stabilized by Multiple Halogen Bonds through Cooperative Effects

Halogen bonding is increasingly recognized as a further elaboration of supramolecular effects between coordination moieties. Here, we report two isostructural trigonal prismatic Er 6 coordination clusters for which one has no halogen on the ligand ( E )- N ′-(2-hydroxy-3-ethoxybenzylidene)pyrazine-2-carbohydrazide (H 2 oepch) ( 1 ), whereas the other has a bromine on the o -vanillin component ( E )- N ′-(5-bromo-2-hydroxy-3-methoxybenzylidene)pyrazine-2-carbohydrazide (H 2 opch-Br) ( 2 ). This allows us to gauge the effect of the supramolecular interactions on network formation via halogen bonding. Furthermore, the topological analysis on the network in ( 2 ) reveals a completely novel and unexpectedly 7-connected uninodal network that has to conform with the symmetry imposed by the tetragonal space group P 4̅2 1 c . In addition, in-depth theoretical calculations were performed to investigate the strength of the halogen bonding interactions and reveal cooperative effects unprecedented for inorganic coordination clusters.