Syntheses, crystal structures and magnetic properties of cyano- and phenoxide-bridged Fe(iii)Mn(iii) tetramers containing fac-Fe(iii) tricyanides and Mn(iii) Schiff bases

Abstract

Three cyanide-linked Fe(III)Mn(III) bimetallic clusters, [(Tp)Fe(CN)₃]₂[Mn(acphen)]₂ [1; acphen = N,N′-ethylenebis(2-hydroxyacetophenylideneiminato) dianion], [(Tp)Fe(CN)₃]₂[Mn(5-Bracphen)]₂ [2; 5-Bracphen = N,N′-ethylenebis(5-bromo-2-hydroxyacetophenylideneiminato) dianion], and [(Tp)Fe(CN)₃]₂[Mn(salen)]₂·6H₂O [3; salen = N,N′-ethylenebis(salicylidineiminato) dianion], were prepared by self-assembling a facial [(Tp)Fe(CN)₃]⁻ [Tp = hydrotris(pyrazolyl)borate] precursor and respective Mn(III) Schiff bases. X-Ray crystal structure analyses revealed that each complex is composed of a central Mn(III) dimer doubly linked by phenoxides of the tetradentate N₂O₂Schiff bases and the terminal [(Tp)Fe(CN)₃]⁻groups connecting to the center through cyanides. Using the spin Hamiltonian H = −2J₁(S_Fe1·S_Mn1 + S_Mn1a·S_Fe1a) − 2J₂(S_Mn1·S_Mn1a), where J₁ stands for the exchange coupling constant between Fe(III) (S_Fe = 1/2) and Mn(III) (S_Mn = 2) through cyanides and J₂ between Mn(III) ions via phenoxides, the best fits corresponded to J₁ = 2.61 cm⁻¹ and J₂ = 0.85 cm⁻¹ for 1, J₁ = 2.50 cm⁻¹ and J₂ = 1.30 cm⁻¹ for 2, and J₁ = −2.15 cm⁻¹ and J₂ = 0.55 cm⁻¹ for 3. The phenoxide routes transmit ferromagnetic interactions in all cases, while ferromagnetic or antiferromagnetic couplings occur through the cyanide linkage. Although the geometric parameters relevant to the magnetic Fe–CN–Mn pathways are analogous to each other their magnetic natures are varied across the compounds, which supports that a degree of orbital overlap is quite sensitive to a subtle structural change in the present system.