Tuning Molecular Structures Using Weak Noncovalent Interactions: Theoretical Study and Structure of trans -Bis(2-chloropyridine)dihalocopper(II) and trans -Bis(3-chloropyridine)dibromocopper(II)

The effect of the weak noncovalent interactions (C-H center dot center dot center dot X-Cu, C-Y center dot center dot center dot Cu, and C-Y center dot center dot center dot H-C) on the molecular structure of Cu(nYP)(2)X-2 (where nYP denotes the n-halopyridine ligand, n = 2 or 3, X = Cl- or Br-, and Y = H, F, Cl, or Br) has been investigated using the DFT/B3LYP method. The molecular structure of Cu(nYP)(2)X-2 was optimized using two different starting geometries; the two Y groups are (a) L-cis arrangement and (b) L-trans arrangement with respect to each other, L for ligand. The optimized molecular structures of the Cu(nYP)(2)X-2 structures indicate that the L-cis isomer is more stable than corresponding L-trans one by avg = 9.45 kJ/mol (range 4.29-13.15 kJ/mol). The analysis of theoretical results indicates the strength of the noncovalent interactions follows the order C-Y center dot center dot center dot H-C < C-Y center dot center dot center dot Cu < C-H center dot center dot center dot X-Cu. The L-cis isomer is stabilized by C-H center dot center dot center dot X-Cu interactions, in contrast, the L-trans isomer is stabilized by C-Y center dot center dot center dot H-C and C-Y center dot center dot center dot Cu. There is no perfect agreement (L-trans and L-cis-isomerism) between the optimized structures and the solid-state molecular structures. This is possibly because the optimization process ignores the effect of intermolecular interactions, and the energy difference between each L-trans and L-cis corresponding isomer is quite small. The structures of Cu(2CP)(2)X-2 and Cu(3CP)(2)Br-2 have been determined. An infinite [Cu(3CP)(2)Br-2](n) chain structure forms based on the Cu center dot center dot center dot Br semicoordinate bond, whereas the semicoordinate bond connects the molecular species of Cu(2CP)2X2 to form a dimer structure. The chains of Cu (3CP)(2)Br-2 are subsequently linked via C-Cl center dot center dot center dot Br-Cu halogen bonding interactions besides the weak C-H center dot center dot center dot X-Cu hydrogen bonding interactions in the three-dimensional structure. The C-Cl center dot center dot center dot X-Cu interactions are absent in Cu(2CP)(2)X-2, and the dimer structures of Cu(2CP)(2)X-2 are linked via C-Cl center dot center dot center dot Cl-C interactions to form chain structures. This competition would indicate that C-Cl center dot center dot center dot X-Cu and C-CI center dot center dot center dot Cl-C are of comparable strength. Another interesting observation, even though the two Cu(2CP)(2)X-2 structures are isomorphous, is that the symmetrical C-Cl center dot center dot center dot Cl-C halogen bonding interactions play the dominant role in developing Cu(2CP)(2)Br-2 crystal structures. In contrast, the perpendicular C-Cl center dot center dot center dot Cl-C halogen bonding interactions play the dominant role in the case of Cu(2CP)(2)Cl-2.