Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Coordinated Guanidine Derivatives
The formation of hydrogen-bonded supramolecular 1-D chains and 2-D sheets by planar metal-containing centres co-ordinated by members of the family of hydrogen-bonding ligands, biuret (bu), carbamoylguanidine (cg), biguanide (bg) and their substituted derivatives has been reviewed. Biurets co-ordinate transition metal centres through two carbonyl oxygens, carbamoylguanidines through one imino nitrogen and one carbonyl oxygen and biguanides through two imino nitrogens. Planar cations with the hydrogen-bonding potential to form 1-D chain or 2-D sheet structures are formed by bidentate ligands with square planar metal centres, especially Ni2+ and Cu2+, when the metal:ligand ratio is 1:2 [ML2]n+ and by tetradentate ligands when the metal:ligand ratio is 1:1 [ML]n+. The majority of the literature abstracted for the review reports biguanide-type co-ordination. Only a limited number of authors have considered biuret-type co-ordination and even fewer have investigated carbamoylguanidine-type co-ordination. Where comparable data are available, trends can be discerned and common structural themes and supramolecular synthons identified. Thus [ML2]n+ species containing two bidentate biguanides often use their N-H donors to act as four-connecting cations forming hydrogen-bonded 2-D sheets with (4,4) topologies. Modification of the two-connecting anion-mediator leads to networks of different size and cohesion. Chloride and bromide form relatively small, fragile, frameworks whereas nitrate and tetrafluoroborate form larger, more robust, networks. Similarly [ML]n+ species containing a single tetradentate biguanide often use their N-H donors to form anion mediated 1-D chains. Depending on the relative orientations of the cations, the chains are linked by hydrogen-bonding contacts through anions and/or solvent molecules to form either 1-D ribbons or 2-D sheets. These [ML]n+ cations have also been shown to bind the pyrimidine bases cytosine and thymine in elegant hydrogen-bonded 2-D sheet architectures.
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