Alkyl linker effects on the coordination topology of ditopic di(2-pyridylmethyl)amine carboxylate ligands with ZnII and CuII: polymers vs. macrocycles

A series of ditopic ω-di(2-pyridylmethyl)amine carboxylic acid ligands incorporating a range of n-alkyl linkers (C_nCOOH, n = 3–5, 7, 10 and 11) have been synthesised. Solution phase studies showed a 1 : 1 coordination stoichiometry between the ligands and M(ClO₄)₂·6H₂O (M = Zn^II or Cu^II) in all cases. The Zn^II and Cu^II complexes were subsequently crystallised by liquid–liquid diffusion and the solid-state structures investigated by X-ray crystallography. The crystal structures obtained are entirely consistent with the 1 : 1 metal–ligand ratio of the solution-phase adducts. However, the coordination geometries and complex topologies are dependent on the alkyl chain length of the ligand C_nCOOH. The Zn^II and Cu^II complexes of the short alkyl chain ligands (n ≤ 5) exhibit 1D coordination polymeric structures with somewhat different conformations for {[Zn(C₃COO)(H₂O)](ClO₄)·3.5H₂O}_n (1), {[Zn(C₄COO)(H₂O)]₄(ClO₄)₄·1.5H₂O}_n (2), {[Zn(C₅COO)(H₂O)](ClO₄)}_n (3), {[Cu(C₃COO)](ClO₄)·MeOH}_n (4), {[Cu(C₄COO)(H₂O)]₂(ClO₄)₂·2H₂O}_n (5) and {[Cu(C₅COO)(H₂O)](ClO₄)·2H₂O}_n (6). In contrast, the ligands with longer alkyl chains (n ≥ 7) participate in Zn₂L₂ metallomacrocyclic structures {[Zn(C₇COO)(H₂O)](ClO₄)}₂ (7), [Zn₂(C₁₀COO)₂(H₂O)₂](ClO₄)₂·2H₂O·MeOH (8) and {[Zn₂(C₁₁COO)₂(H₂O)₂][Zn₂(C₁₁COO)₂](ClO₄)₄·H₂O}_n (9). The formation of metallomacrocycles instead of the 1D coordination polymers is a persistent trend and, with identical crystal growth conditions and a non-coordinating anion employed, appears to be an effect of the longer alkyl chain.