Crystal Structure, Hirshfeld Analysis, and DFT Calculations of Three Trinuclear Cu(II) Polymorphs

The crystal structure of the ferromagnetically-coupled Cu-3(II)-pyrazolato complex, (Bu4N)(2)[Cu-3(mu(3)-Cl)(2)(mu-4-NO2-pz)(3)Cl-3] (1a, pz = pyrazolato anion), was originally determined in the triclinic P-1 space group. By varying the recrystallization solvent and temperature, two additional true polymorphs were crystallized in the monoclinic P2(1)/n (1b) and orthorhombic Pbca (1c) space groups. Comparison of the metric parameters of the three polymorphs revealed only minor variations in their bond lengths and angles but clearly distinguishable packing patterns. The DFT calculations showed that, in vacuum, 1a had the lowest energetic minimum (also the densest of three polymorphs), whereas 1b and 1c lay at 6.9 kcal/mol and 7.8 kcal/mol higher energies. The existence of isolable 1b and 1c is, therefore, attributed to the intermolecular interactions analyzed by the Hirshfeld methods.