Understanding the Forces That Govern Packing: A Density Functional Theory and Structural Investigation of Anion-pi-Anion and Nonclassical C-H···Anion Interactions

Abstract

[eng] 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 (Ecoop is positive), but, as Ecoop 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.