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[1 with combining macron] (# 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, H2.5TTTPC, is suggested.