Synthesis and Coordination Chemistry of Ditopic Ligands Capable of Coordinating Metal Ions and Interacting with Anions

The aim of this research was to synthesise a series of novel organic multidentate ligands which contain N-donor domains for the coordination of metal ions and amide or amine hydrogen atoms which are capable of interaction with anions. It was envisaged that incorporation of these two binding units would produce a system where the metal ions would control the ability of the ligand to interact with anions or vice versa.
Ligand 1 contains a tetradentate N-donor domain formed by a central bipyridine, two thaizole units and two amide units attached in the 4,4’-position of the bipyridine unit. Reaction of this with divalent metal ions results in a mono-nuclear complex where the metal is bound by the N-donor atoms and the amides interact with a variety of anions. Reaction with monovalent metal ions results in the formation of a dinuclear double helicate with the metal again coordinated by the N-donor domains and the anions interacting with the amide hydrogen atoms. This results in a polymeric assembly in the solid state.
Ligand 2 contains an identical tetradentate domain comprised of the same N-donor units; however the single amides in the 4,4’-position have been removed and a diamide attached in the 3,3’-position of the bipyridine unit. Reaction of [L2 with divalent cations results in a similar mono-nuclear species. The metal centre is coordinated by the N-donor atoms and one of the acetyl units from two adjoining ligands with the counter ions undergoing interactions with the diamide hydrogen atoms. Coordination of the same ligand with a monovalent cation resulted in a di-nuclear double helicate, each metal centre is fulfilled by the N-donor atoms of the ligand strand and the hydrogen atoms of the diamide units interact with anions. This too results in a polymeric assembly in the solid state.
Ligands 3 and 4 contain the iso-structural tetradentate N-donor domain seen in [L1] and [L2] but their functionality in the 3,3’-position differ. Ligand 3 contains a urea group while ligand 4 has a single amide group attatched to an indole unit. Coordination of [L3] and a divalent metal ion results in the formation of a mono-nuclear species with the metal ion bound by the central bipyridine and the N-donor of two thaizole units. Furthermore each of the urea groups in the 3,3’-position undergo favourable interactions with the perchlorate counter ions. A solid state structure of Ligand 4 was only successful with a monovalent cation resulting in the formation of a dinuclear double stranded species. Each metal centre exhibits a distorted trigonal planar geometry through coordination with a pyridine and thiazole ring of one strand and a single thiazole ring of another. The indole and amide of each ligand strand undergo two sets of interactions; anion interactions through the amide and indole hydrogen atoms as well as complementary intermolecular interactions between the indole N···H units of one ligand and the carbonyl C···O units of another complex. Both [L3] and [L4] exhibit long range order through favourable anion-NH interactions however [L4] also displays complimentary indole / acetyl interactions to develop a larger aggregate species.
In all these cases the resultant complex is independent upon which anion is used. However,
this is not the case with ligand 5. Reaction of [L5] with Cu(BF4)2 or Cu(ClO4)2 gave a dinuclear double helicate with a cleft within the helicate assembly in which an anion is bound. However, reaction of this with half an equivalent of either sulphate (SO4 2-) or dihydrogen phosphate (H2PO4 -) results in the formation of a different dinuclear double helicate whereby the cleft is occupied by either a dihydrogen phosphate or sulphate anion which bridges the metal centres. Further addition of sulphate results in no change of the ESI-MS indicating the dinuclear double helicate persist however addition of one equivalent of di-hydrogen phosphate leads to the formation of a pentanuclear circular helicate. Each metal centre is
coordinated by the pyridine and thiazole units of two different ligand strands and a single Cu···O interaction from one of the dihydrogen phosphates.
The inclusion of three dihydrogen phosphates into the centre of the assembly as well as a series of phosphate-ligand and phosphate-phosphate interactions leads to the dimerization of the structure with another set of phosphates from a second assembly.
Further reaction of this dinuclear species with one equivalent of (Bu4N)NO3 resulted in the formation of a hexanuclear circular meso-helicate (or mesocate). In this structure each Ndonor domain of a thiazole and pyridine ring coordinate two different Cu2+ metal centres. Each metal centre exhibits a distorted octahedral arrangement with two ligand strands completing 4 of its 6 coordination sites, the remaining sites are occupied by two O-donors of a nitrate anion. In addition an amine of each ligand strand points into the centre of the complex creating a cavity capable of hosting two nitrate anions.
Ligand 6 is made up of the same bis-bidentate donors as ligand 5 with the addition of a nitrogen atom into the central phenyl spacer. On reaction of [L6] with a divalent metal ion (e.g. Cu(II)) a simple mono-nuclear structure is observed. Although a mono-nuclear assembly is expected, it is interesting that even a simple change in the ligand strand can have a
dramatic affect on the self-assembly process. When a central 1,3-phenylene spacer is employed (i.e. [L5]) a dinuclear double helicate is formed, however, when a 1,3-pyridine unit is contained within the ligand strand (i.e. [L6]) a simple mono-nuclear species is produced.