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

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 center dot CHCl3 reveals the assembly of hydrogen-bonded chains of molecules of 3; the preference for NHimidazole center dot center dot center dot N-tpy over NHimidazole center dot center dot center dot 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)center dot 5H(2)O(n)] and [Co(3)(2)(SCN)(2)center dot 2MeOH(n)] exhibit (4,4) nets. In [Co(2)(2)(NCS)(2)center dot 0.67C(2)H(4)Cl(2)center dot MeOH center dot 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 pi-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)center dot 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 pi-stacking interactions.