Magneto-structural studies of monohydroxo-ridged dicopper(II) complexes M[Cu₂L₂(OH)]·2H₂O (M=Na⁺ (1) and K⁺ (2); H₂L=2,6-bis[N-(phenyl)carbamoyl]pyridine). Effect of Cu---OH---Cu bridge angle on antiferromagnetic coupling

Abstract

Using a tridentate bis-amide ligand 2,6-bis[N-(phenyl)carbamoyl]pyridine (H₂L), in its deprotonated form, two new monohydroxo-bridged dicopper(II) complexes M[Cu₂L₂(OH)]·2H₂O (M=Na⁺ (1) and K⁺ (2)) have been prepared and characterised by a number of methods, including X-ray crystallography. Each copper(II) ion is terminally coordinated by one pyridyl and two amide nitrogen donors. The two copper(II) centres are bridged by a hydroxo group, with each copper(II) centre assuming a distorted square planar geometry. The observation of short Cu---N_py and long Cu---N_amide bonds is caused by the steric requirement of the ligand. Interestingly, each cation Na⁺/K⁺ is coordinated to four different [Cu₂L₂(OH)]^- units through the amide O-donors, in an uncommon distorted tetrahedral coordination environment. Temperature-dependent magnetic susceptibility measurements revealed that the compounds have S=0 ground state with singlet-triplet energy separation, 2J=-334 and -296 cm^-1 for 1 and 2, respectively. The larger Cu---OH---Cu bridge angle in 1 (131.1(6)°) causes better antiferromagnetic exchange coupling than that in 2 (125.7(6)°).