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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)

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posted on 2011-12-07, 00:00 authored by Firas Awwadi, Roger D. Willett, Brendan Twamley
The effect of the weak noncovalent interactions (C–H···X–Cu, C–Y···Cu, and C–Y···H–C) on the molecular structure of Cu(nYP)2X2 (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)2X2 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)2X2 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···H–C < C–Y···Cu < C–H···X–Cu. The L-cis isomer is stabilized by C–H···X–Cu interactions, in contrast, the L-trans isomer is stabilized by C–Y···H–C and C–Y···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)2X2 and Cu(3CP)2Br2 have been determined. An infinite [Cu(3CP)2Br2]n chain structure forms based on the Cu···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)2Br2 are subsequently linked via C–Cl···Br–Cu halogen bonding interactions besides the weak C–H···X–Cu hydrogen bonding interactions in the three-dimensional structure. The C–Cl···X–Cu interactions are absent in Cu(2CP)2X2, and the dimer structures of Cu(2CP)2X2 are linked via C–Cl···Cl–C interactions to form chain structures. This competition would indicate that C–Cl···X–Cu and C–Cl···Cl–C are of comparable strength. Another interesting observation, even though the two Cu(2CP)2X2 structures are isomorphous, is that the symmetrical C–Cl···Cl–C halogen bonding interactions play the dominant role in developing Cu(2CP)2Br2 crystal structures. In contrast, the perpendicular C–Cl···Cl–C halogen bonding interactions play the dominant role in the case of Cu(2CP)2Cl2.