Magnetism of Linear [Ln₃]⁹⁺ Oxo-Bridged Clusters (Ln = Pr, Nd) Supported inside a [R₃PR′]⁺ Phosphonium Coordination Material

Two new isostructural phosphine coordination materials, Ln-PCM-21 (Ln = Pr, Nd), have been obtained using a tris­(p-carboxylated) methyltriphenylphosphonium ligand that is formally dianionic when triply deprotonated, allowing access to materials based on uncommon metal-to-ligand ratios. The polymers of the formula [Ln3(mptbc)4]­X·solv (X = Cl–, NO3–) are cationic and contain unusual, linear oxo-bridged [Ln3]9+ clusters. Magnetic susceptibility data for both the Pr and Nd analogues has been compared to models based on three contrasting approaches.