Hydrogen Bonding Patterns and Supramolecular Structure of 4,4′-Bipyrazolium Salts

The crystal structures of 18 inorganic salts of 4,4′-bipyrazolium [H₂bpz]²⁺ and 3,3′,5,5′-tetramethyl-4,4′-bipyrazolium [H₂Me₄bpz]²⁺ (bpz = 4,4′-bipyrazole; Me₄bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole) involving Cl⁻, I⁻, I₃⁻, PdCl₄²⁻, Cu₂Cl₆²⁻, Re₂Cl₈²⁻, SiF₆²⁻, TaF₆⁻, Zr₂F₁₂⁴⁻, (BeF₃⁻)_n, IO₃⁻, ClO₄⁻, S₂O₆²⁻, HSO₄⁻, and H₂PO₄⁻ ions were determined by X-ray diffraction. Primary supramolecular organization of the bipyrazolium salts originates in strong hydrogen bonding between multiple NH cationic donors and O, F, Cl, I anionic acceptors following three main modes, which support linear joints of the cationic moieties: {(Hpz⁺)₂(A⁻)₂}, {(Hpz⁺)₂(AX₂⁻)₂}- two pyrazolium moieties joined by a one-atom and three-atom bridging fragment respectively, and {(Hpz⁺)(AX₂⁻)} - a single pyrazolium moiety “capped” by a three-atom anionic fragment. These modes provide suitable and characteristic supramolecular synthons for the rational design of hydrogen bonded pyrazolium frameworks. The control over dimensionality of the structure is feasible through proper choice of the anion, its charge, and configuration of the acceptor atoms. A relatively high number of hydrogen bond acceptor atoms of the anions (TaF₆⁻, Zr₂F₁₂⁴⁻, ClO₄⁻) results in bifurcation of NH···X bonding. Weaker CH···X hydrogen bonding and slipped π/π interactions are relevant for the secondary supramolecular organization.