May halogen bonding interactions compete with Cu•••Cl semi-coordinate bonds? Structural, magnetic and theoretical studies of two polymorphs of trans-bis(5-bromo-2-chloro pyridine)dichlorocopper(ii) and trans-bis(2,5-dichloropyridine)dichlorocopper(ii)

Two polymorphs of Cu(25dcp)(2)Cl-2 [Cu(25dcp)(2)Cl-2_p1 and Cu(25dcp)(2)Cl-2_p2] and Cu(5b2cp)(2)Cl-2 [Cu(5b2cp)(2)Cl-2_p1 and Cu(5b2cp)(2)Cl-2_p2] were prepared and characterized by single crystal X-ray diffraction (25dcp = 2,5-dichloropyridine and 5b2cp = 5-bromo-2-chloropyridine). The four structures crystallize in the monoclinic crystal system: three of them (Cu(25dcp)(2)Cl-2_p1, Cu(5b2cp)(2)Cl-2_p2 and Cu(25dcp)(2)Cl-2_p2) crystallize in the C2/c space group and the fourth one (Cu(5b2cp)(2)Cl-2_p1) in the C2/m space group. Cu(5b2cp)(2)Cl-2_p1 and Cu(25dcp)(2)Cl-2_p1 are structurally isomorphous. The formation of two polymorphs for each complex is a result of competition between ClCu semi-coordinate bonds and C-X center dot center dot center dot Cl-Cu (X = Cl or Br) halogen bonding interactions. The formation of C-XCl-Cu halogen bonding alone resulted in crystallization of Cu(25dcp)(2)Cl-2_p1 and Cu(5b2cp)(2)Cl-2_p1, whereas the formation of the Cu center dot center dot center dot Cl semi-coordinate bonds resulted in the formation of Cu(25dcp)(2)Cl-2_p2 and Cu(5b2cp)(2)Cl-2_p2. To our knowledge, these are the first examples in which the competition between the halogen bonding interactions and the semi-coordinate Cu center dot center dot center dot Cl resulted in the formation of different polymorphs. The calculated electrostatic potential was used to rationalize the formation of the different polymorphs. The magnetic properties of Cu(5b2cp)(2)Cl-2_p1 were studied; it is found to obey an antiferromagnetic chain model. This magnetic behavior was rationalized using the two-halide exchange pathway. Structurally, Cu(5b2cp)(2)Cl-2_p1 forms a chain structure based on Cu-Cl center dot center dot center dot Cl-Cu interactions. The mapped electron density with spin density showed the presence of a spin-density-end-cap along the Cu-Cl bond. This spin-density-end-cap corroborates the observed antiferromagnetic interactions.