Decoration of N-heterocyclic cores with bisphosphonates: synthetic access to mono- and di-substituted s-triazine linkers, and their structurally diverse coordination networks.

The synthesis and characterization of six new phosphonate linkers and their coordination networks are reported in this paper. All have the s-triazine heterocyclic core as a common structural feature and display mono- or di-substitution by aminoalkyl-phosphonate grafts. The phosphonate-bearing, amine-containing nucleophiles used for the substitution were amino-bis(methylenephosphonate) (ABMP), (3-amino-1-hydroxypropane-1,1-diyl)bis(phosphonic acid) (pamidronate, PAM), (4-amino-1-hydroxy-1-phosphonobutyl)phosphonic acid (alendronate, ALE), and (6-amino-1-hydroxyhexane-1,1-diyl)bis(phosphonic acid) (neridronate, NER). ABMP features a secondary amine, and PAM, ALE, and NER feature primary amines. The products isolated were Tr-1ABMP, Tr-1PAM, Tr-2PAM, Tr-1ALE, Tr-2ALE, and Tr-2NER (Tr = triazine; the numbers 1 and 2 indicate mono- and di-substitution). These ligands were used to synthesise coordination networks with alkaline earth metal ions. The crystal structures of three derivatives, namely Sr-Tr-1ABMP (2D), Ca-Tr-1PAM (1D), and Sr-Tr-2NER (1D), reveal the rich coordination chemistry and potential of these ligands. Tr-1PAM and Tr-2NER use only their phosphonate moieties for coordination, whereas Tr-1ABMP employs also the N and O atoms from the triazine ring. None of the structures display π-π stacking interactions. The structures of the metal derivatives are analysed and compared to several triazine-containing literature examples to determine the electronic and protonation state of the triazine ring. The triazine ring and external N atoms in the compounds Ca-Tr-1PAM, Sr-Tr-1ABMP, and Sr-Tr-2NER are planar, in agreement with the extended planarity concept. The protonation state of the triazine ring depends on the synthesis pH. The triazine ring is neutral (non-protonated) in Sr-Tr-1ABMP, but one of the N atoms is protonated in Ca-Tr-1PAM and Sr-Tr-2NER. This research establishes a promising entry to the coordination chemistry of phosphonate-substituted triazine rings. Such ligands can enrich the phosphorus synthetic chemist's "toolbox" for the creation of new coordination networks with diverse structure types.