Metal–organophosphine and metal–organophosphonium frameworks with layered honeycomb-like structures

Phosphanotriylbenzenecarboxylic acid (ptbcH₃; P(C₆H₄-p-CO₂H)₃) and its methyl phosphonium iodide derivative (mptbcH₃I; {H₃CP(C₆H₄-p-CO₂H)₃}I) have been used as organic building blocks in reaction with Zn(II) salts to obtain a series of related two-dimensional coordination polymers with honeycomb-like networks. The variable coordination number and oxidation states available to phosphorus have been exploited to produce a family of related phosphine coordination materials (PCMs) using a single ligand precursor. The phosphine carboxylate trianion, ptbc³⁻, reacted with Zn(II) to form 3,3-connected undulating hexagonal sheets based on tetrahedral P and Zn nodes, where Zn–ptbc = 1 : 1. When hydroxide was used as an additional framework ligand, Zn₄(OH)₂ clusters were obtained. The clusters support 6,3-connected bilayers that consist of pairs of fused hexagonal sheets (Zn–ptbc = 2 : 1) with intra-layer pore spaces. The Zn₄(OH)₂ clusters are also coordinated by solvent, which was preferentially displaced when the bilayer material was synthesized in the presence of ethylene diamine. Treatment of ptbc³⁻ with MeI resulted in methylation of the phosphine to give the P(V) phosphonium iodide salt derivative. The formally dianionic methylphosphonium tricarboxylate building block, mptbc²⁻, has the same trigonal-pyramidal bridging geometry as the parent phosphine. However, mptbc²⁻ reacted with Zn(II) on a 1 : 1 stoichiometric ratio to give an unusual trilayer sheet polymer that is based exclusively on 3-connected nodes. Solid-state ³¹P NMR studies confirmed that the phosphine ligands were resistant to oxidation upon solvothermal reaction under aerobic conditions.