Understanding the forces that govern packing: A density functional theory and structural investigation of anion-π-anion and nonclassical C-H⋯anion interactions

The ability of Ni(II) coordinated 4-pyrrolyl-3,5-di(2-pyridyl)-1,2,4- triazole (pldpt) to establish multiple anion-π interactions is analyzed. Experimentally, such complexes were previously shown to form strong anion-π interactions, including "π-pocket" and "π-sandwiched" motifs, in the crystal lattice. In the latter, the triazole ring is "sandwiched" by two anions forming a ternary anion-π-anion assembly (π-sandwich) which, surprisingly, gave about 0.2 Å shorter anion-π distances than in binary assemblies (where only one side of the triazole participates in the anion binding), indicating the possibility of cooperativity. In depth analysis, using dispersion-corrected density functional theory (DFT, BP86-D/def2-TZVP level of theory), shows that this ternary anion-π-anion interaction is slightly less energetically favorable than the binary anion-π interactions in isolation. Hence, the sandwich interaction is not cooperative (E _coop is positive), but, as E _coop contributes less than 1.5% of the total interaction energy (which is dominated by the strong electrostatic attraction of the anions to the highly π-acidic Ni(II)-coordinated triazole ring), the presence of nonclassical C-H⋯anion hydrogen bonds can offset this, making the short anion-π sandwich interactions the most favorable solid state conformation.