Abstract
The ability of Ni(II) coordinated 4-pyrrolyl-3,5-di(2-pyridyl)-1,2,4-triazole (pldpt) to establish multiple anion-pi interactions is analyzed. Experimentally, such complexes were previously shown to form strong anion-it interactions, including "pi-pocket" and "pi-sandwiched" motifs, in the crystal lattice. In the latter, the triazole ring is "sandwiched" by two anions forming a ternary anion-pi-anion assembly (pi-sandwich) which, surprisingly, gave about 0.2 angstrom shorter anion-pi 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-pi-anion interaction is slightly less energetically favorable than the binary anion-pi 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 pi-acidic Ni(II)-coordinated triazole ring), the presence of nonclassical C-H center dot center dot center dot anion hydrogen bonds can offset this, making the short anion-pi sandwich interactions the most favorable solid state conformation.