Mixed-valence copper(i,ii) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines: from ionic structures to coordination polymers

Abstract

Two pyrimidine-based ligands, 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(morpholino)pyrimidine (L²) and 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-phenoxypyrimidine (L³), and a series of mixed-valence copper(I,II) halide complexes, [Cu(L²)₂Br]₂[Cu₂Br₄] (1), [Cu(L²)₂Cl][CuCl₂] (2), and [Cu₂L³Br₃]_n (3), have been synthesized. The complex [Cu(L²)₂Br]₂[Cu₂Br₄] was prepared by the reaction of L² with CuBr₂ in a 1 : 1 molar ratio in MeCN. Its chlorido-analogue, the complex [Cu(L²)₂Cl][CuCl₂], was synthesized by the reaction between L², CuCl₂ and CuCl in a 2 : 1 : 1 molar ratio in MeCN. The ligand L² acts as a chelating one. In the structures of the complexes [Cu(L²)₂Br]₂[Cu₂Br₄] and [Cu(L²)₂Cl][CuCl₂] the Cu²⁺ ion is in the cationic part of the complex whereas the Cu⁺ ion is located in the anionic part. The best way to synthesize the mixed-valence 1D coordination polymer [Cu₂L³Br₃]_n is to react CuBr₂ with L³ in a 2 : 1 molar ratio in the MeCN/CHCl₃ mixture on heating. In the structure of [Cu₂L³Br₃]_n the ligand L³ shows chelating/bridging tridentate coordination. This is the first example of the tridentate coordination of 4-(1H-pyrazol-1-yl)-6-R-pyrimidines. The striking difference between the coordination behavior of L² and L³ (chelating bidentate vs. chelating/bridging coordination) is related with the possibility of rotation of the 6-phenoxy group around the C–O bond which makes the N¹ pyrimidine atom less sterically hindered, enabling it to participate in metal ion binding. Importantly, all copper ions in [Cu₂L³Br₃]_n show similar tetrahedral environments, CuNBr₃ and CuN₂Br₂, which is extremely rare for mixed-valence copper(I,II) compounds. The ligands L² and L³ show blue emission which is quenched upon their coordination to copper ions. The 1D coordination polymer [Cu₂L³Br₃]_n shows high thermal stability and unusual solvent-occlusion properties. The role of the substituents favoring the formation of the mixed-valence copper(I,II) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines is discussed.