Structure, Optical and Magnetic Properties of Two Isomeric 2-Bromomethylpyridine Cu(II) Complexes [Cu(C 6 H 9 NBr) 2 (NO 3 ) 2 ] with Very Different Binding Motives

Two isomeric 2-bromomethylpyridine Cu(II) complexes [Cu(C 6 H 9 NBr) 2 (NO 3 ) 2 ] with 2-bromo-5-methylpyridine (L 1 ) and 2-bromo-4-methylpyridine (L 2 ) were synthesized as air-stable blue materials in good yields. The crystal structures were different with [Cu(L 1 ) 2 (NO 3 ) 2 ] ( CuL 1 ) crystallizing in the monoclinic space group P 2 1 /c, while the 4-methyl derivative CuL 2 was solved and refined in triclinic P 1 ¯ . The orientation of the Br substituents in the molecular structure ( anti ( CuL 1 ) vs. syn ( CuL 2 ) conformations) and the geometry around Cu(II) in an overall 4 + 2 distorted coordination was very different with two secondary (axially elongated) Cu–O bonds on each side of the CuN 2 O 2 basal plane in CuL 1 or both on one side in CuL 2 . The two Br substituents in CuL 2 come quite close to the Cu(II) centers and to each other (Br⋯Br ~3.7 Å). Regardless of these differences, the thermal behavior (TG/DTA) of both materials is very similar with decomposition starting at around 160 °C and CuO as the final product. In contrast to this, FT-IR and Raman frequencies are markedly different for the two isomers and the UV–vis absorption spectra in solution show marked differences in the π–π* absorptions at 263 ( CuL 2 ) or 270 ( CuL 1 ) nm and in the ligand-to-metal charge transfer bands at around 320 nm which are pronounced for CuL 1 with the higher symmetry at the Cu(II) center, but very weak for CuL 2 . The T -dependent susceptibility measurements also show very similar results ( µ eff = 1.98 µ B for CuL 1 and 2.00 µ B for CuL 2 and very small Curie–Weiss constants of about −1. The EPR spectra of both complexes show axial symmetry, very similar averaged g values of 2.123 and 2.125, respectively, and no hyper-fine splitting.