UC San Diego
Coordination polymers and building blocks based on ditopic heteroscorpionate ligands
- Author(s): Santillan, Guillermo A.
- et al.
Ditopic heteroscorpionate ligands emerge as attractive from the point of view of crystal engineering because of their potential coordination versatility. By placing the donor group in a phenyl-substituted heteroscorpionate in a para or meta position rather than an ortho arrangement, we change the ligand from a facially coordinating tripodal one favoring mononuclear complexes, into one capable of bridging interactions-possibly leading to multidimensional metallosupramolecular structures. To this end, using our standard synthetic methodology, we have prepared three new ditopic heteroscorpionate ligands that have either OH or COOH groups located on the meta or para positions of the aromatic ring. We showed that the new heteroscorpionate ligands, designated L3c (m-carboxy) and L4c (p-carboxy), can adopt a variety of coordination modes toward Cu(II), Co(II), Zn(II), and Ni(II) which depend on the coordination geometry preferences of the metal ion, solvent polarity, the presence of anions, and pH. Under high pH conditions, the dominant species are dinuclear complexes of stoichiometry M₂L₂Z₂ or M₂L₄. These building blocks can be linked together to form interesting coordination polymers. By substitution of the simple monodentate "Z" ligands with ditopic linkers such as terephthalate or biphenyldicarboxylate, we developed 1- and 2-D polymeric structures. An alternate strategy for producing higher dimensional structures is to make use of H-bonding. We have found that as the pH is lowered so that either a mixture of protonated and deprotonated or completely protonated species are present, mononuclear metal complexes form where the coordination of the ligand is strictly through the pyrazole nitrogens leaving the uncoordinated carboxylates to engage in H-bonding interactions between molecules in the solid state. Similar chemistry was obtained by using ditopic ligand designated as L5v, where the presence of different anions (i.e. BF₄-С , CF₃SO₃-С, NO₃-С, or SbF₆-С) lead to different crystal packing schemes due to different H-bonding patterns that depend on the relative charge delocalization and/or shape of the anion. Since the metal monomers (tectons) possess chirality due to the asymmetric coordination of the achiral ligands to the metal Ni (II) or Ag (I), all of the solid-state materials were isolated as racemic mixtures or as mesohelical structures