Greasy tails switch 1D-coordination [{Zn2(OAc)4(4′-(4-ROC6H4)-4,2′:6′,4′′-tpy)}n] polymers to discrete [Zn2(OAc)4(4′-(4-ROC6H4)-4,2′:6′,4′′-tpy)2] complexes

Abstract

The homologous series of 4′-(4-ROC₆H₄)-4,2′:6′,4′′-tpy ligands with R = Me, Et, ⁿPr, ⁿBu, ⁿpentyl, ⁿhexyl, ⁿheptyl, ⁿoctyl, ⁿnonyl and ⁿdecyl (1–10, respectively) are reported, including single crystal structures of 6 and 7. Reactions of zinc(II) acetate with 1–10 have been investigated using room temperature crystallization methods (diffusion or layering). For ligands with the shortest alkoxy substituents, 1-dimensional coordination polymers [{Zn₂(OAc)₄(L)}_n] (L = 1, 2 or 3) are formed. In each polymer, the 4′-(4-ROC₆H₄)-4,2′:6′,4′′-tpy ligands bind zinc through the two outer pyridine donors. The polymer structures are similar with the n-propyl chain adopting a folded conformation in [{Zn₂(OAc)₄(3)}_n] which allows it to fit in the cavity occupied by methyl or ethyl groups in [{Zn₂(OAc)₄(1)}_n] and [{Zn₂(OAc)₄(2)}_n]. Reaction between 5 and Zn(OAc)₂·2H₂O gives both the coordination polymer [{2Zn₂(OAc)₄(5)·2H₂O}_n] and the discrete complex [Zn₂(OAc)₄(5)₂]. Although the zig-zag form of the polymer chain in [{2Zn₂(OAc)₄(5)·2H₂O}_n] mimics those in [{Zn₂(OAc)₄(L)}_n] (L = 1, 2 or 3), packing interactions differ and the wider separation of the chains in a sheet results in the incorporation of water molecules in the lattice. π-Stacking between pyridine rings in [{Zn₂(OAc)₄(L)}_n] (L = 1, 2 or 3) produces infinite assemblies in contrast to isolated tetradecker π-stacks in [{2Zn₂(OAc)₄(5)·2H₂O}_n]. This assembly is replicated in [{4Zn₂(OAc)₄(7)·3H₂O}_n] (n-heptoxy substituents). In contrast, the n-hexoxy-containing coordination polymer crystallizes with acetic acid in the lattice; [{Zn₂(OAc)₄(6)·MeCO₂H}_n] consists of zig-zag polymer chains which π-stack in a manner which is unique among the other polymers. Further lengthening of the alkoxy chain favours the formation of [Zn₂(OAc)₄(L)₂] (L = 8, 9 or 10) which are analogues of [Zn₂(OAc)₄(5)₂]. In each, the 4′-(4-ROC₆H₄)-4,2′:6′,4′′-tpy ligand is monodentate. The alkoxy chains are in extended (or close to extended) conformations and pack into planar sheets with interdigitated chains. Pockets in the sheets are occupied by methyl groups of {Zn₂(OAc)₄} units in the adjacent sheet in a ball-and-socket assembly motif. The study shows that coordination polymers [{Zn₂(OAc)₄(L)}_n] in which π-stacking are the dominant interactions are favoured for small alkoxy substituents (ligands 1–3); for ligands 8–10, discrete complexes [Zn₂(OAc)₄(L)₂] in which van der Waals interactions dominate are observed. In the intermediate range (ligands 5–7), the preference between the two structure types appears to be marginal.