Probing anion-π interactions in 1-D Co(II), Ni(II), and Cd(II) coordination polymers containing flexible pyrazine ligands

Two flexible thioether-containing heterocyclic ligands bis(2-pyrazylmethyl) sulfide (L1) and 2-benzylsulfanylmethylpyrazine (L2) have arene rings with differing π-acidities which were used to probe anion-π binding in five 1-D coordination polymers formed from the metal salts Co(CIO₄) ₂, Ni(NO₃)₂, and Cd(NO₃) ₂. In {[Co(L1)(MeCN)₂](CIO₄)₂} _∞ (1), {[Ni(L1)(NO₃)₂]}_∞ (2), and {[Cd₂(L1)(MeCN)(H₂O)(NO₃) ₄]·H₂O}_∞ (3·H ₂O), the symmetrical ligand L1 was bound facially to the metal center and was bridged through a pyrazine donor to an adjacent metal forming a polymer chain. The folding of L1 formed U-shaped π-pockets in 1 and 3·H ₂O which encapsulated free and bound anions, respectively. The anions interacted with the π-acidic centers in a variety of different binding modes including anion-π-anion and π-anion-π sandwiching. A wider π-pocket was formed in 2 which also contained anion-π interactions. The polymer chains in 2 were interdigitated through a rare type of complementary T-shaped N(pyrazine)⋯π interaction. In {[Co(L2)(H₂O) ₃](CIO₄)₂·H₂O} _∞ (4·H₂₀) and {[Cd(L2)(H ₂O)(NO₃)₂]}_∞ (5), the unsymmetrical ligand L2 chelated the metal center and bridged through a pyrazine donor to an adjacent metal forming a polymer chain. The ligand arrangement resulted in the anions in both structures being involved in only anion-π-anion sandwich interactions. In 4·H₂O, the noncoordinated CIO₄^- anions interacted with only one chain while in 5 the coordinated NO₃^- anions acted as anion-π supramolecular synthons between chains. Comparison between the polymers formed with ligands L1 and L2 showed that only the more π-acidic ring was involved in the anion-π interactions.