Control of molecular architecture by steric factors: mononuclear vs polynuclear manganese(iii) compounds with tetradentate N2O2 donor Schiff bases

Abstract

Three manganese(III) compounds, [Mn^III(vanoph)(DMF)(H₂O)]ClO₄ (1), [Mn^III(vanoph)(N₃)(H₂O)]·2H₂O (2) and [Mn^III(saloph)(μ_1,3-N₃)]_n (3), where H₂vanoph = N,N′-(1,2-phenylene)-bis(3-methoxysalicylideneimine), H₂saloph = N,N′-(1,2-phenylene)-bis(salicylideneamine) are tetradentate N₂O₂ ligands and DMF = N,N-dimethylformamide, have been prepared and characterised by elemental analysis, IR and UV–Vis spectroscopy and single-crystal X-ray diffraction studies. Compounds 1 and 2 are monomeric but compound 3 consists of a chain system with the repeating unit [Mn^III(saloph)(N₃)] bridged by μ-1,3 azide. Compound 1 crystallises in monoclinic space groupP2₁/n with cell dimensions of a = 11.1430(2), b = 16.3594(3), c = 15.4001(3) Å, β = 108.417(1), Z = 4 whereas compounds 2 and 3 crystallise in orthorhombic space groupsPbca and Pna2₁, respectively, with cell dimensions of a = 16.069(3), b = 15.616(3), c = 18.099(4) Å, Z = 8 (for 2) and a = 18.760(9), b = 13.356(5), c = 6.616(3) Å, Z = 4 (for 3). In all the compounds, Mn(III) has a six-coordinated pseudo-octahedral geometry in which O(2), O(3), N(1) and N(2) atoms of the deprotonated di-Schiff base constitute the equatorial plane. In both compounds 1 and 2, water molecules are present in the fifth coordination sites in the apical positions. The sixth coordination sites are occupied by one O atom of a solvent DMF in compound 1 and an N atom of azide in compound 2. The coordinated water initiates hydrogen-bonded networks in both compounds 1 and 2 to form well-isolated supramolecular dimers. At room temperature the χ_MT values for the compounds 1 and 2 remain almost constant until 30 K. Below this temperature, the χ_MT values drastically drop to 0.72 cm³ mol⁻¹ K for 1 and 0.52 cm³ mol⁻¹ K for 2. The best fits were obtained with J = −0.92 cm⁻¹, |D| = 2.05 cm⁻¹, g = 2.0 and R = 8.1 × 10⁻⁴ for 1 and J = −1.16 cm⁻¹, |D| = 2.05 cm⁻¹, g = 2.0 and R = 1.2 × 10⁻³ for 2. However, in compound 3, two axial positions are occupied by the azide ions. The Mn⋯Mn repeating distance is 6.616 Å along the chain. Magnetic characterisation shows that the μ_1,3-bridging azide ion mainly transmits an antiferromagnetic interaction (J = −6.36 cm⁻¹) between Mn(III) ions. The presence of two methoxy groups increases the steric crowding in the H₂vanoph moiety and thereby inhibits the formation of a polynuclear compound with this ligand.