This Thesis describes the synthesis and characterisation of a series of nitrogen-thioether ligands, known as amplectors, and their Co(III) complexes. Hexadentate N4S2 amplectors were formed from various dithiol molecules giving ligands with linear dithio-hinges, EtN4S2amp, PrN4S2amp, BuN4S2amp and PeN4S2amp, and elaborate hinges, (rac- and meso-) 1,2-Me2EtN4S2amp, XyN4S2amp and BzN4S2amp (see List of Abbreviations for IUPAC names of all ligands presented). The synthetic strategy for these molecules was adapted to give a pentadentate ligand N4Samp. The amplectors, EtN4S2amp, PrN4S2amp, BuN4S2amp, (rac- and meso-) l,2-Me2EtN4S2amp, XyN4S2amp and N4Samp were isolated as their Co(III) complexes using cation exchange chromatography. The possibility that amplector complexes could be used as synthetic precursors for encapsulating ligands was investigated.
Crystal structures of the linear dithio-hinged amplector complexes, [Co(EtN4S2amp)]Cl(C1O4)2 (orthorhombic space group Pbcn with a = 7.6249(5) Å, b = 16.779(3) Å, c = 17.6830(10) Å, Z = 4 and R = 0.0471), [Co(PrN4S2amp)]Cl(C1O4)2.2H2O (monoclinic space group P21/n with a = 10.3340(10) Å, b = 15.389(2) Å, c = 16.216(3) Å, β= 99.810(10) °, Z = 4 and R = 0.0596) and [Co(BuN4S2amp)]Cl2(C1O4).2H2O (monoclinic space group P21/c with a = 10.8730(10) Å, b = 18.4840(10) Å, c = 13.636(3) Å, β= 112.311(9) °, Z = 4 and R = 0.0545), were obtained. These complexes, characterised by NMR (1H, 13C) spectroscopy, mass spectrometry and electrochemistry, displayed properties which can be attributed to the increasing strain about the dithio-hinge with increasing size of this chelate ring observed in structural parameters.
Crystal structures were obtained for Co(III) amplector complexes with hinge moieties other than linear dithio-groups ([Co(rac-1,2-Me2EtN4S2amp)]C12(C1O4).2H2O: monoclinic space group P21/c with a = 11.5490(10) Å, b= 14.563(2) Å, c = 15.3650(10) Å, β= 91.278(7) °, Z = 4 and R = 0.0366; [Co(XyN4S2amp)]Br3.3H2O: monoclinic space group P21/n with a = 23.719(5) Å, b = 9.7912(9) Å, c = 24.073(2) Å, β= 99.626(7) °, Z = 4 and R = 0.0591; [Co(N4Samp)Cl](ZnCl4): monoclinic space group P21/c with a = 7.861(2) Å, b = 15.432(2) Å, c = 16.856(2) Å, β= 98.64(2) °, Z = 4 and R - 0.0510). The use of 2,3-butanedithiol as the precursor dithio-moiety introduced chiral centres to the amplector ligand topology. The diastereoisomers of [Co(l,2-Me2EtN4S2amp)]3+ were isolated using Sephadex cation exchange resin eluting with 0.1 M Na2SO4 (pH 4). The structural and spectroscopic properties of [Co(XyN4S2amp)]3+ indicates the rigid aromatic dithio-chelate ring induces a high level of strain. The Co(III) complex of the pentadentate ligand N4Samp in aqueous solution (pH 1 - 7) exhibits ligand exchange at the position trans to the thioether donor. These complexes were spectroscopically characterised by NMR ( 1H, 13C) and ESI-MS, and investigated using electrochemical techniques.
UV-visible spectroscopy of the Co(III) amplector complexes was performed using Nafion films at room and low temperatures (≤ 14 K). For most of these complexes, spin allowed (1A1g -> 1T1g, 1T2g) and spin forbidden d-d transitions (1A1g -> 3T1g, 3T2g) were observed at low temperature. This allowed for the unique determination of ligand field parameter l0Dq and Racah parameters B and C. Other methods used for the calculation of these parameters were explored. It was found that the assumption C = 6B is most appropriate for low spin Co(III) complexes in determining spectrophotometric parameters when the spin allowed d-d transitions are only observed. Similar conclusions were made for another low spin d6 system, Fe(II) complexes with nitrogen-thioether ligands, where the crystal structure of the rigorously low spin complex [Fe(AMN3S3sarH)](C1O4)3.3H2O (monoclinic space group P21/n with a = 18.724(4) Å, b = 9.1458(5) Å, c = 19.172(3) Å, β= 115.753(7) °, Z = 4 and R = 0.0490) was obtained.
59Co NMR spectroscopy was performed for numerous cobalt(III) nitrogen-thioether complexes, including the amplector complexes. Changes in the 59Co NMR chemical shift (δCo) were ascribed to the size and strain of chelate rings, and the type of donor set. The linewidth (ν1/2) of the 59Co NMR resonance was found to reflect both the symmetry of the donor set and the rigidity of the ligand. The relationship between 59Co NMR chemical shifts and spectrophotometric parameters was explored. It was found that a linear correlation between the magnetogyric ratio (γCo) and the product of the nephelauxetic ratio (βRacah) and the wavelength of the 1A1g → 1T1g transition (ΔE-1(1A1g → 1T1g) occurs for the majority of nitrogen-thioether complexes studied. The data for a number of complexes depart from the line of best fit, the departures attributed to either deviations of donor atoms from octahedral geometry or changes in π-electron distribution, particularly when donors other than amines are trans to thioethers.
Several by-products from the synthesis of [Co(EtN4S2amp)]3+ were isolated. The crystal structure of [Co(EtN3OS2ampH)]Cl(C1O4)4 (orthorhombic space group Pna21 with a = 17.7310(10) Å, b= 16.7660(10)Å, c = 7.620(2) Å, Z = 4 and R = 0.0462) is nearly identical to that of [Co(EtN4S2amp)]Cl(C1O4)2. The precursor for the ligand EtN3OS2amp was most likely formed during the reaction of potassium phthalimide and the tetratosyl molecule in the synthetic scheme for EtN4S2amp. The pKa of the coordinated hydroxyl group for [Co(EtN3OS2ampH)]3+ was found to be 2.8(2) by potentiometric, 13C and 59Co NMR, spectrophotometric and electrochemical methods. Two other coordination modes for EtN3OS2ampH were discovered with the crystal structures of endo-[Co(η5-HOEtN3S2amp)Cl]Cl(PF6).1/2H2O(monoclinic space group C2/c with a = 30.357(5) Å, b = 6.908(10) Å, c = 21.547(5) Å, β= 102.07(2) °, Z = 8 and R = 0.0485) and exo-[Co(η5-HOEtN3S2amp)Cl]Cl(C1O4) (triclinic space group P 1 with a = 7.1319(8) Å, b = 10.5300(9) Å, c= 14.864(2) Å, α= 74.432(8) °, β= 99.810(10) °, γ= 82.573(9) °, Z = 2 and R = 0.0566) obtained. For these examples, the hydroxyl group is not coordinated while the three amine donors have a meridonial arrangement whereas the amines are positioned facially for [Co(EtN3OS2ampH)]3+ . Another by-product isolated was a pendant arm eleven membered macrocyclic structure, referred to as dmatue. This macrocycle probably formed by the oxidation of the hydroxyl group in EtN3OS2ampH to the aldehyde by Co(NO3)2 followed by a metal template assisted intramolecular condensation to form an imine. The crystal structure of [Co(dmatue)Cl](C1O4)2-H2O (monoclinic space group P21/a with a = 14.079(4) Å, b = 10.515(2) Å, c= 15.043(2) Å, β=- 96.18(2) °, Z = 4 and R = 0.0384) was obtained. Characterisation of these by-products by NMR (1H, 13C, 59Co) and UV-visible spectroscopy and mass spectrometry was performed where possible.
Encapsulation with the [Co(EtN3OS2ampH)]3+ template was achieved by a mixed aldehyde reaction of isobutyraldehyde and paraformaldehyde with base in DMF. The crystal structure of [Co(Me6docosadieneN4S2)](C1O4)3.3H2O (monoclinic space group P21/c with a = 12.7000(10) Å, b= 16.6370(10) Å, c = 16.102(2) Å, β = 94.700(10) °, Z = 4 and R = 0.0680) shows the imine bonds are located cis to one another and the [Co(EtN3OS2ampH)]3+ entity differs little after encapsulation. Electrochemistry shows the Co3+/2+ redox couple is reversible for the encapsulated complex while the metal centred couple for the precursor [Co(EtN3OS2ampH)]3+ exhibits irreversible behaviour.