Mixed Alkali Metal/Transition Metal Coordination Polymers with the Mellitic Acid Hexaanion: 2-Dimensional Hexagonal Magnetic Nets

The hexaanion of mellitic acid, mel = (C₆(CO₂)₆)⁶⁻, links metal ions into extensively connected magnetic coordination polymers. Reaction of alkali metal mellitate salts, M₆(mel) (M = K, Rb), with M′Cl₂ precursors (M′ = Mn, Co, Ni) under mild (473 K) hydrothermal conditions yields an extensive family of isostructural 3-dimensional mixed alkali metal/transition metal polymers of general formula M₂[M′₂(mel)(OH₂)₂] (M/M′ = K/Mn (1a); K/Co (1b); K/Ni (1c); Rb/Mn (2a); Rb/Co (2b); Rb/Ni (2c)). These materials incorporate distorted 2-dimensional magnetic hexagonal nets with a honeycomb topology that are exclusively based on metal-carboxylate-metal bridging interactions. A further isostructural alkali metal-free Co²⁺ material with NH₄⁺ cations, (NH₄)₂[Co₂(mel)(OH₂)₂] (3), produced by reaction of H₆mel with [Co(NH₃)₆]Cl₃ is also presented. The magnetic susceptibility data for 1a−c, 2a−c, and 3 are presented. The susceptibility data for the Mn(II)- and Ni(II)-containing phases have been analyzed using a simple Mean Field Theory approach, and have been modeled using a high temperature series expansion. The comparative magnetism of the Co(II) phases is also presented, and is more complicated because of significant spin−orbit coupling effects.