Abstract
Molecular switches, discrete molecules capable of reversible switching, are of interest to researchers due to their relevance in fields such as molecular materials, memory devices, and nanoscience. Two significant areas of research in recent years are the prediction of their switching properties (i.e. the conditions to cause switching), and the incorporation of secondary reporting and/or switching mechanisms into the molecules, both of which are investigated herein.
In Chapter 1 a general introduction to spin crossover in iron(II) and cobalt(II) systems is presented. Recent advances in linking redox active units, and fluorescent moieties, to SCO active molecules are discussed. A brief introduction to reversible electrochemistry follows, and finally, the systems targeted in the present study are introduced.
In Chapter 2 the synthesis of a new, non-symmetric, ligand, Lpyrtz-tpy (4-(p-tolyl)-3-pyridyl-5-(2,2′:6′,2′′-terpyridine)-1,2,4-triazole), which comprises a bidentate pyridyl-triazole binding pocket and a tridentate terpyridine binding pocket, is described. A series of monometallic ‘metalloligand’ complexes, [MII(Lpyrtz-tpy)2]X2 (X = BF4 and MII = FeII, CoII, RuII, or X = PF6 and MII = CoII), is characterised. All feature reversible redox, and, in the case of the CoII complexes, spin crossover behaviour. Utilising the free pyridyl-triazole binding pocket, a series of heterometallic metallopolymers is synthesised by linking the metalloligands by CuI or AgI centres.
In Chapter 3 the formation of two heteroleptic Ru(II) metalloligands derived from Lpyrtz-tpy, [Ru(tpy)(Lpyrtz-tpy)](PF6)2 and [Ru(tpy)(Lpyrtz-tpy)(MeCN)](PF6)2, is described, along with the redox properties thereof. Utilising the free binding pockets of these complexes and appropriate 3d transition metal ions, a series of discrete, heterometallic, complexes is synthesised. Electrochemical and magnetic studies of these complexes are presented.
In Chapter 4 the synthesis of five Ru(II) complexes from a previously reported family of azine-substituted 1,2,4-triazole-based ligands (the Lazine family), [RuII(Lazine)3](PF6)2, is described. Electrochemical characterisation of the complexes reveals that the choice of azine donor (pyridine and the diazines) significantly impacts the potential at which the RuII/RuIII redox couple appears. DFT calculations on the complexes, along with the FeII and OsII analogues, and on the ligands, opens a method of predicting, pre-synthesis, the oxidation potential of the redox process for a new complex in the family.
In Chapter 5 the synthesis of a fluorescent pyrene-functionalised 1,3,4-oxadiazole ligand, Lpyrox-pyrene (2-(pyrene-1-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole), is presented. A set of three Fe(II) complexes, [FeII(Lpyrox-pyrene)2(NCE)2] (E = S, Se, BH3), is characterised in the solid state, including magnetic measurements, Mössbauer, single crystal structure determination, and fluorescence measurements. The solvent dependent nature of the SCO behaviour is discussed.
In Chapter 6 the complexation of a series of tetradentate Schiff-base ligands, derived from the condensation of 1,3-diaminopropane and two equivalents of various azole-aldehydes (2-thiazolecarboxaldehyde (L2thiazole), 4-thiazolecarboxaldehyde (L4thiazole), or 4-oxazolecarboxaldehyde (L4oxazole)) is investigated. The complexes, [FeII(Lazole)(NCS)2] show significantly different magnetic behaviour depending on the azole bound to the Fe(II) centre.