Competition vs. Cooperativity of I⋅⋅⋅Omorpholinyl and I⋅⋅⋅Cl−M Halogen Bonds in Cocrystals of Zinc(II) and Copper(II) Coordination Compounds Carrying Multiple Acceptor Sites

In order to explore a strategy for synthesizing halogen-bonded metal-organic cocrystals by utilizing metal complexes whose pendant chloride group and the morpholinyl oxygen atom enables halogen bonding, we have synthesized four pentacoordinated Cu(II) and Zn(II) complexes of the MCl₂L general formula (L=imines prepared by the condensation reaction of 4-aminoethylmorpholine with 2-pyridinecarboxyaldehide or 2-acetylpyridine). The prepared metal complexes were further cocrystallized with selected iodoperfluorinated benzenes. Out of 20 combinations, 14 experiments yielded crystals suitable for single-crystal X-ray diffraction. Structural analysis revealed that in 7 cocrystals halogen bonds are formed both with morpholinyl oxygen as well as with chloride atoms. In 6 cocrystals only I⋅⋅⋅Cl halogen bonds are present, while only one cocrystal exclusively featured I⋅⋅⋅O_morpholinyl halogen bonds. We observed 5 halogen bonding motifs to the MCl₂ moiety, in which each chloride atom can be an acceptor of one halogen bond, two, or none at all. The most common motif in our work (6 cocrystals) is where one chlorine atom is an acceptor of one halogen bond, while the other chlorine atom does not participate in halogen bonding. The crystal packing in the prepared cocrystals is directed by halogen-bonded architectures which are either zero-, one- or two-dimensional.