Copper–halide bonds as magnetic tunnels; structural, magnetic and theoretical studies of trans-bis(2,5-dibromopyridine)dihalo copper(ii) and trans-bis(2-bromopyridine)dibromo copper(ii)

Abstract

The magneto-structural correlations in Cu(25dbp)₂X₂ and Cu(2bp)₂Br₂, where 25dbp = 2,5-dibromopyridine, X = Cl or Br, and 2bp = 2-bromopyridine have been investigated. The supramolecular structures of Cu(25dbp)₂X₂ are based on C–Br⋯X–Cu halogen bonding interactions. The temperature dependence of the susceptibility of Cu(25dbp)₂Cl₂ is best represented by an antiferromagnetic chain model. This agrees with the supramolecular structure; Cu(25dbp)₂Cl₂ molecules form linear chains of Cu–Cl⋯Cl–Cu structural units. In contrast, the magnetic susceptibility of Cu(25dbp)₂Br₂ is best represented by a ferromagnetic chain model; Cu(25dbp)₂Br₂ molecules form a chain structure based on Cu⋯Br2⋯Br–Cu where Br2 is the bromine atom on position 2 of the 2,5-dibromopyridine ligand. The susceptibilities of Cu(2bp)₂Br₂ indicate that the magnetic exchange interaction is very weak, even though Cu(2bp)₂Br₂ forms an isostructural chain structure similar to that observed in Cu(25dbp)₂Br₂. This paramagnetic behavior might be due to the presence of competitive antiferromagnetic and ferromagnetic exchange pathways. The shortest Cu–Br⋯Br–Cu distance is 4.5 Å in Cu(2bp)₂Br₂, where Br is the bromide ligand, whereas the shortest inter-bromide ligand distance is 5.4 Å in Cu(25dbp)₂Br₂. The two-bromide exchange pathway is known to be antiferromagnetic. This analysis is supported by DFT/B3LYP calculations. Calculations show that there is a significant spin density on the halide ligands, whereas, only a small spin density resides on Br2 (organic bromine). Also, the calculated spin density surface shows the presence of a spin end-cap along the Cu–X bond, which can rationalize the two-halide exchange pathway. In addition, theoretical calculations reveal that the two-halide exchange pathway in CuX₂L₂ compounds is stronger than the corresponding interaction in [CuX₄²⁻] compounds.