In-depth structural analysis of lanthanoid coordination networks based on a flexible tripodal zwitterionic isonicotinate ligand

Abstract

Crystallizing metal–organic frameworks (MOFs) has been studied using a tripodal pyridinecarboxylic acid derivative ligand and selected lanthanoid salts. The zwitterionic ligand, 1,1′,1′′-((2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene))tris(pyridin-1-ium-4-carboxylate) (TTTPC) introduced as a bromide salt, forms coordination networks in aqueous environments and under ambient conditions with neodymium bromide, trifluoromethanesulfonate (OTf) or acetate (OAc). Seven structures are elucidated in detail using single crystal X-ray crystallography. TTTPC NdBr₃, TTTPC NdBr₂OTf, TTTPC NdBr(OTf)₂ and TTTPC Nd(OTf)₃ are porous 3D networks with similar ligand–metal and ligand–anion interactions, but with different anion distributions, coordination modes and/or crystal systems. Freshly prepared TTTPC NdBr₂OAc is a porous 2D network, and otherwise has the same attributes, but it transforms into a 3D network upon drying. All network solids crystallize in the space group P (# 2), except TTTPC NdBr(OTf)₂ which crystallizes in P2₁/c (# 14). Compounds retain their crystallinity under vacuum, and a crystal structure for an evacuated sample of TTTPC NdBr₂OTf is presented. Thermal analysis of network solids shows that upon heating, all solids exhibit solvent loss and withstand decomposition up to or over 300 °C. In addition to Nd networks, synthesis and crystal structures of several exactly or almost isostructural systems with other lanthanoids are presented. These include TTTPC YbBr₃, TTTPC LnBr₂OTf (Ln = La, Sm, Eu, Gd and Tb), TTTPC LnBr(OTf)₂ (Ln = Sm, Eu, and Tb), TTTPC Yb(OTf)₃ and TTTPC₂Sm₂(OTf)₆. The synthesis and crystal structures of Ln(NO₃)₃ (Ln = La, Nd, and Y) are also briefly discussed as a separate, yet similar, system. Finally, a revised crystal structure of the protonated bromide TTTPC ligand, H_2.5TTTPC, is suggested.