Manganese (II) Coordination Complexes Involving Nitronyl Nitroxide Radicals

Abstract

Three new complexes involving nitronyl nitroxide and PhCOO- or N3- ligands have been synthesized and structurally and magnetically characterized. These compounds are formulated as Mn(L)4(X)2·nH2O, L = 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PNN) or 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (PN) and X = PhCOO- or N3-. Compound A [Mn(PNN)2(PhCOO)2(H2O)2] presents the following structural parameters: triclinic, space group P1̄ (No. 2), a = 6.859(1) Å, b = 11.271(2) Å, c = 13.978(6) Å, α = 88.88(2)°, β = 89.38(2)°, γ = 78.28(2)°, Z = 1. The complex has a centrosymmetric distorted octahedral geometry in which the manganese ion is bound to two radical ligands through the nitrogen atom of the pyridine rings, two benzoate groups, and two water molecules. The two compounds B [Mn(PNN)4(N3)2] and C [Mn(PN)4(N3)2] are isostructural with the following structural parameters: B; triclinic, space group P1̄ (No. 2), a = 7.177(4) Å, b = 13.767(3) Å, c = 13.928(4) Å, α = 90.20(2)°, β = 102.94(4)°, γ = 91.86(4)°, Z = 1; C, triclinic, space group P1̄ (No. 2), a = 7.004(2) Å, b = 13.885(1) Å, c = 14.036(2) Å, α = 90.34(1)°, β = 101.42(2)°, γ = 92.92(1)°, Z = 1. They adopt a centrosymmetric tetragonally distorted octahedral geometry in which the manganese ion is bound to four radical ligands through the nitrogen atom of the pyridine rings, and the azido groups occupy the apical positions. For all three compounds A, B, and C intramolecular ferromagnetic interactions between the Mn(II) ion and the nitronyl nitroxide radicals are observed, but at very low temperatures, intermolecular antiferromagnetic interactions dominate.