Spin crossover in iron(II) dinuclear helicates and tetranuclear cages
Amongst the 151 discrete di- to poly-nuclear spin crossover (SCO) active assemblies of Fe(II) published before July 2020, only 19 are Fe(II) triply bridged dinuclear helicates (Fe2L3) and only 6 are edge bridged tetranuclear cages (Fe4L6). Brooker and coworkers have reported numerous examples of mononuclear SCO-active Fe(II) complexes of monotopic bidentate Rdpt (4-R-substituted-3,5-di(2-pyridinyl)-1,2,4-triazole) type ligands. This thesis builds on that foundation by developing new classes of ditopic Rdpt type ligands, for the self-assembly of new classes of SCO-active triply bridged dinuclear helicates (Fe2L3) and edge bridged tetranuclear tetrahedral cages (Fe4L6). Chapter 1 starts with a brief introduction to SCO. Then the 151 SCO-active discrete di- to poly-nuclear Fe(II) architectures, from dinuclear Fe2L3 to octanuclear Fe8L12, and the ligand designs used to access them, reported before July 2020, are comprehensively reviewed, followed by selected examples of Rdpt type Fe(II) complexes from the literature. Finally, the new class of ditopic Rdpt type ligands, which feature bis-bidentate azine-triazole or azole-triazole binding pockets (Rat) and the corresponding iron(II) complexes that are targeted in this thesis, are introduced. Chapter 2 presents the extension of our general protocol to the synthesis of the first generation of robust ditopic Rat ligands, which feature meta-phenylene linked (L2/4pym-meta) and para-phenylene linked (L2/4pym-para) bidentate azine-triazole binding pockets. The four ditopic Rat ligands enabled the assembly of two distinct supramolecular architectures, the structurally characterised pair of dinuclear helicates [FeII2Lnpym-meta3](BF4)4 and pair of tetranuclear tetrahedral cages [FeII4Lnpym-para6](BF4)8 (n = 2 and 4). Solution (UV-vis and Evans method 1H NMR spectroscopy) studies of all four complexes revealed that they remain low spin, confirming that coordination by three azine-triazole binding pockets imposes too strong a ligand field. In order to reduce the ligand field strength, in Chapter 3, the first Rat ligands to feature azoles, not azines, in the binding pocket were accessed. A pair of monotopic azole-triazole Rat ligands (L4NMe/SIm) was synthesised by extension of our general protocol. In contrast to the low spin tris(azine-triazole) coordinated Fe(II) complexes, these new tris(azole-triazole) coordinated complexes, [Fe(L4NMeIm)3]2+ and [Fe(L4SIm)3]2+, are high spin and SCO-active, respectively, in both solid and solution. This confirmed the expected reduced ligand field strength of the azoles compared with the azine analogues. Hence, in Chapters 4 and 5, these azole-triazole bidentate binding pockets were incorporated into a second generation of reduced ligand field ditopic Rat ligands, to enable the self-assembly of SCO-active dinuclear helicates (Chapter 4 meta-phenylene linker) and tetranuclear cages (Chapter 5 para-phenylene linker). Specifically, in Chapter 4, five second generation ditopic bis-bidentate Rat ligands, Lazole-meta (azole: 2 and 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), were prepared and shown to enable assembly of five structurally characterised SCO-active dinuclear helicates, [FeII2Lazole-meta3]4+. Variable temperature solution studies (UV-vis and Evans method NMR spectroscopy) and cyclic voltammetry studies in MeCN confirmed that all of the five helicates are SCO-active and that the choice of non-coordinated heteroatom influences the ligand field strength, SCO temperature and the redox potential of the Fe(II) centre. In general, the higher the high spin fraction present in the solution, the easier the Fe(II) centre was to oxidise. Furthermore, DOSY NMR spectroscopy was successfully used to characterise the dinuclear helicates regardless of the spin state. Chapter 5 presents the first pair of second generation ditopic Rat ligands (L4NMeIm-para and L4SIm-para) and the self-assembly of a pair of edge-bridged tetranuclear cages, HS [FeII4L4NMeIm-para6]8+ and SCO-active [FeII4L4SIm-para6]8+. Comparison of the variable temperature solid state magnetic and MeCN solution (UV-vis and Evans method NMR spectroscopy) studies of the cages and the analogous helicates confirmed that the meta-phenylene linked ligands in the dinuclear helicates provide a stronger ligand field than the para-phenylene linked ligands do in the tetranuclear cages. Finally, in Chapter 6, the key findings are summarised and some future directions are suggested.
Advisor: Brooker, Sally
Degree Name: Doctor of Philosophy
Degree Discipline: Chemistry
Publisher: University of Otago
Keywords: helicate; cage; azole-triazole; azine-triazole; DOSY; spin-crossover; Evans-method; redox
Research Type: Thesis