Hydrogen-Bonded and Halogen-Bonded: Orthogonal Interactions for the Chloride Anion of a Pyrazolium Salt

In the hydrochloride of a pyrazolyl-substituted acetylacetone, the chloride anion is hydrogen-bonded to the protonated pyrazolyl moiety. Equimolar co-crystallization with tetrafluorodiiodobenzene (TFDIB) leads to a supramolecular aggregate in which TFDIB is situated on a crystallographic center of inversion. The iodine atom in the asymmetric unit acts as halogen bond donor, and the chloride acceptor approaches the σ-hole of this TFDIB iodine subtending an almost linear halogen bond, with Cl···I = 3.1653(11) Å and Cl···I-C = 179.32(6)°. This contact is roughly orthogonal to the N-H···Cl hydrogen bond. An analysis of the electron density according to Bader's Quantum Theory of Atoms in Molecules confirms bond critical points (bcps) for both short contacts, with ρbcp = 0.129 for the halogen and 0.321eÅ-3 for the hydrogen bond. Our halogen-bonded adduct represents the prototype for a future class of co-crystals with tunable electron density distribution about the σ-hole contact.