Structure, magnetism and optical properties of achiral and chiral two-dimensional oxalate-bridged anionic networks with symmetric and asymmetric ammonium cations

Abstract

A series of two-dimensional (2D) oxalate-based compounds, namely [N(n-C₄H₉)₄][M^IICr^III(ox)₃] (M^II = Mn, Fe; ox = C₂O₄²⁻) and [N(C₂H₅)(n-C₃H₇)(n-C₄H₉)(n-C₅H₁₁)][M^IIM^III(ox)₃] ((M^II, M^III) = (Mn, Cr), (Fe, Cr), (Mn, Fe)) were synthesised starting from racemic tris(oxalato)metalate: rac-[M^III(ox)₃]³⁻ (M^III = Cr, Fe). For Cr^III, the synthesis has been undertaken starting from resolved (Δ)- or (Λ)-[Cr^III(ox)₃]³⁻. The natural circular dichroism measurements assess the enantioselectivity of the synthesis. X-Ray powder diffraction analysis has revealed that, when racemic reagents are used to synthesise Mn^II containing compounds, a R3c achiral space group is found. In contrast a P6₃ chiral space group is found when starting from (Δ)- or (Λ)-[Cr^III(ox)₃]³⁻. Surprisingly, whatever the optical purity of the starting building block, all Fe^II containing compounds crystallise in the P6₃ chiral space group. The magnetic properties of the synthesised compounds confirm that these compounds are ferromagnets for M^III = Cr. For M^II = Mn, Θ ranges between 9 and 11 K and T_c equals 6 K. For M^II = Fe, Θ ranges between 14 and 16 K and T_c between 11 and 12 K. [N(C₂H₅)(n-C₃H₇)(n-C₄H₉)(n-C₅H₁₁)][Mn^IIFe^III(ox)₃] is an antiferromagnet with Θ = − 107 K and T_N = 29 K.