Abstract:
The magneto-structural correlations in Cu(25dbp)2X2 and Cu(2bp)2Br2, where 25dbp = 2,5dibromopyridine, 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.