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(ClO4)2, Ni(NO3)2, and Cd(NO3)2. In {[Co(L1)(MeCN)2](ClO4)2}∞ (1), {[Ni(L1)(NO3)2]}∞ (2), and {[Cd2(L1)(MeCN)(H2O)(NO3)4]·H2O}∞ (3·H2O), 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·H2O 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](ClO4)2·H2O}∞ (4·H2O) and {[Cd(L2)(H2O)(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 ClO4- 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.