Cobalt(ii) coordination polymers with 4′-substituted 4,2′:6′,4′′- and 3,2′:6′,3′′-terpyridines: engineering a switch from planar to undulating chains and sheets

Abstract

Two new ligands 4′-(1H-imidazol-4-yl)-4,2′:6′,4′′-terpyridine (3) and 4′-(4-dimethylaminophenyl)-3,2′:6′,3′′-terpyridine (4) are described. Structure determination of 3·CHCl₃ reveals the assembly of hydrogen-bonded chains of molecules of 3; the preference for NH_imidazole⋯N_tpy over NH_imidazole⋯N_imidazole hydrogen bonds is consistent with the relative basicities of the heterocyclic rings. Reactions of Co(NCS)₂ with 4′-phenyl-4,2′:6′,4′′-terpyridine (1), 4′-(4-ethynylphenyl)-4,2′:6′,4′′-terpyridine (2) or 3, produce two-dimensional networks. In each, the Co²⁺ ion is in an octahedral trans-{Co(N_tpy)₄(NCS)₂} environment. [{2Co(1)₂(NCS)₂·5H₂O}_n] and [{Co(3)₂(SCN)₂·2MeOH}_n] exhibit (4,4) nets. In [{Co(2)₂(NCS)₂·0.67C₂H₄Cl₂·MeOH·H₂O}_n], a (6,3) net is present. In all three coordination networks, there is substantial twisting of the 4,2′:6′,4′′-terpyridine backbone and as a consequence, the dominant packing interactions are not the face-to-face π-stacking of tpy domains that are ubiquitous in many solid-state structures containing metal-bound tpy domains. As a preliminary investigation of the effects of altering the directionality of the donor set in the terpyridine domain, Co(SCN)₂ was reacted with ligand 4, and the one-dimensional coordination polymer [{Co(4)(MeOH)₂(NCS)₂}_n] was isolated. Ligand 4 adopts a trans,trans-arrangement and the zig-zag chains are undulating. The buckled sheets that result from the intermeshing of the chains contrast with the planar sheets observed in [{Cd(5)(OH₂)₂(ONO₂)(O₂NO)·H₂O}_n] (5 = 4′-(4-dimethylaminophenyl)-4,2′:6′,4′′-terpyridine). The observed packing interactions suggest that the change from planar to undulating chains and sheets on going from 5 to 4 is a consequence of optimizing face-to-face π-stacking interactions.