Metallosupramolecular chemistry of polydentate ligands and the solid state studies of diphenylcarbazide and dithizone

Described herein, is the synthesis and coordination chemistry of eight novel ligands L1-L8, and the solid state studies of diphenylcarbazide and dithizone. These ligands form metallosupramolecular assemblies upon coordination of transition metal ions resulting in a wide range of architectures.

Described in chapter two is a series of ligands that contain both N-donor and N-oxide donor atoms, L1-L4. Reaction of L1 with Cu2+ results in the formation of a mononuclear complex [Cu(L1)(ClO4)2(sol)] (solvent = MeCN or H2O), whereas L2 forms the dinuclear double helicate [Ni2(L2)2]4+ with Ni2+. Reaction of L3 with Cu2+ results in the formation of a head-to-tail dinuclear double helicate [Cu2(L3)2]4+. The N-oxide units imparts flexibility in the ligand strand and where the unoxidised ligand strand forms a circular helicate, the incorporation of an N-oxide unit allows the formation of the dinuclear double helicate. Reaction of L4 with Co2+ results in the formation of a tetranuclear circular helicate [Co4(L4)4]8+. Analogous complexes that contain ligands with a 1,3-phenyl spacer unit give pentanuclear circular helicates, whereas with a 1,3-phenol spacer the lower tetranuclear species is observed. The difference in the nuclearity of the circular helicates is due to the steric bulk of the methyl group on the central phenol spacer. In the dinuclear double complex formed with L3 the N-oxide unit allows the ligand to flex, whereas the steric bulk of the –OH unit in L4 is sufficiently large that even with the added flexibility that the N-oxide units imparts a double helicate cannot be formed.
Chapter three introduces a new class of polydentate ligands, L5-L7, these ligands consist of N-donor domains separated by a 1,3-phenol unit. The ligand L5 contains two identical tridentate N-donor domains, reaction of L5 with Zn2+ results in a tetranuclear circular helicate [Zn4(L5)4]8+. Within the structure all four Zn2+ ions are six-coordinate, arising from the coordination of two tridentate domains from two different ligand strands. Reaction of L6 with Ag+ results in the formation of the dinuclear double meso-helicate [Ag2(L6)2]2+. Reaction of L6 with Cd2+ produces a crystalline material that consists of both colourless and orange species. The colourless crystals correspond to the mononuclear complex [Cd(L6)2(MeCN)2]2+, whereas the orange crystals produce the dinuclear double meso-helicate [Cd2(L6)2]2+. This variation in self-assembly is a direct result of the –OH unit on the 1,3-phenol spacer; if the -OH unit is protonated the oxygen atom can only coordinate once and therefore the mononuclear complex forms, however deprotonation of one of the -OH unit results in the oxygen coordinating twice as a bridging donor to form the dinuclear complex. Both the [Cd(L6)2(MeCN)2]2+ and [Cd2(L6)2]2+ species are present in solution, under equilibrium conditions, varying the stoichiometry alters the predominant species. The ligand L7 is unsymmetrical, upon reaction with Co2+ the ligand partitions into two different binding sites; a tridentate N-donor domain and a tridentate domain consisting of the bidentate N-donor domain and the O-donor atom from the central 1,3-phenol spacer. The resulting dinuclear HH-[Co2(L7)2]3+ complex demonstrates that the two cobalt metal centres occupy different binding sites. Examining the solid state X-ray crystallographic data suggests that the two cobalt metal centres in the [Co2(L7)2]3+ complex occupy different oxidation states; Co2+ and Co3+ to give a mixed valence helicate. In an analogues fashion to L6, reaction of L7 with Zn2+ produces a crystalline material that consists of both colourless and orange species. The colourless crystals correspond to the mononuclear complex [Zn(L7)2]2+, whereas the orange crystals produce the dinuclear double helicate [Zn2(L6)2]3+. In the mononuclear [Zn(L7)2]2+ species the Zn2+ metal centre is coordinated by the tridentate N-donor domain of two different ligands. In the dinuclear [Zn2(L6)2]3+ species each Zn2+ metal centres is coordinated by the tridentate N-donor domain of one ligand and the tridentate domain, consisting of the bidentate N-donor and the O-donor from the central 1,3-phenol spacer, from another different ligand. The variation in the self-assembly is a direct result of the stoichiometry of the reaction; the formation of these two complexes is under the same equilibrium conditions of the previous L6 structures.
Described in chapter four is the potentially pentadentate N-donor ligand L8, which comprises of a bidentate and tridentate binding domains separated by a 1,3-pyrene spacer. Reaction of L8 with Cu2+ results in the formation of a tetranuclear circular helicate [Cu4(L8)4]8+. Each of the Cu2+ ions adopts a 5-coordinate geometry formed by the coordination of the bidentate domain of one ligand strand and the tridentate domain of a different ligand strand, resulting in a head-to-tail tetranuclear circular helicate. The formation of this head-to-tail circular helicate is a result of the 1,3-pyrene spacer preventing the formation of the linear double stranded assemblies and secondly the stereoelectronic preference of Cu2+.
Chapter five reports the solid state studies of diphenylcarbazide and dithizone, which are both useful reagents for the colorimetric determination of a variety of different metal ions. Examination of the scientific literature over the past 100 years shows that the coordination chemistry of DPC and DPTC is inconsistent, with literature sources proposing contradictory and non-definitive explanations, this chapter aims to extend the knowledge surrounding these reagents by isolating crystals. DPC reacts with Cd2+ to form the mononuclear species [Cd(DPC)2]2+ the two ligands are coordinating through both the N-donor and O-donor domains. The discrepancies surrounding the DPC reaction is whether the redox reactions between the metal and ligand occur, upon reaction of DPC and Cd2+ the metal does not oxidise the ligand. Reaction of DPC and Cu2+ is more complex than the previous Cd2+ reaction, the resulting [Cu3OH(OH2)3(DPTO)6]5+ structure comprises of six ligands and three metal ions. DPC undergoes oxidative intramolecular cyclisation to form the nitrogen containing heterocycle 2,3-diphenyltetrazolium-5-olate (DPTO) and coordinates the Cu2+ metal centre in two different modes: via both the oxygen and amide nitrogen atoms or by the bridging carbonyl unit. The [Cu3OH(OH2)3(DPTO)6]5+ structure is also generated when reacting DPCO with Cu2+. Unfortunately a crystal of a chromium or vanadium complex with DPC was not achieved; however the cyclised ligand was isolated,
highlighting that the oxidation and cyclisation of DPC is important in the coordination chemistry of these ions. Reaction of the sulphur derivative DPTC with various metal ions results in the deprotonation of the ligand to form the monoanionic species, which coordinates the metal ions via the S-donor and azo N-donor atoms. Reaction of DPTC with Hg2+ to form the mononuclear complex [Hg(DPTC)2]. The simple mononuclear complex involves two DPTC ligands coordinating the four-coordinate Hg2+ ion as a bidentate donor via the N-donor and S-donor atoms. The reaction of DPTC with both Hg2+ and Ag+ results in an interesting structure containing two Ag+, two Hg2+ and four DPTC ligands. The DPTC appears to first react with Hg2+ to form the previous [Hg(DPTC)2] complex, this then acts as a bidentate ligand, coordinating via the S-donor atom and the Hg2+ itself to form the [Hg2Ag2(DPTC)4(acetone)2(ClO4)2] complex. The reaction of DPTC with Cu2+, whether the anion is perchlorate or tetrafluoroborate, results in a very interesting structure, which comprises of eight DPTC ligands and eight Cu+ metal ions. The reaction of Cu2+ with DPTC results in the metal ion reducing to Cu+ and simultaneously the DPTC deprotonates to form the monoanionic form. The counter-anion acts as a template and the formation of the “Cu8” is a result of the presence of the anion. Reaction of copper (II) acetate with DPTC results in the [Cu2(DPTC)2(DPTCO)] complex. The structure contains three ligands and two reduced distorted tetrahedral Cu+ ions. Each Cu+ ion has four-coordinate geometry arising from the coordination of two different forms of the DPTC ligands. Two of the ligands present are the monoanionic DPTC, coordinating via the S-donor and terminal N-donor azo atoms. Whereas the third ligand has completely oxidised to form DPTCO, coordinating via both the terminal N-donor azo N-donor atoms, the sulphur atom bridges both of the metal ions.