Abstract
Spin switchable materials and CO2 reduction are two areas of chemical research that address modern society’s demands for nanoscale technologies and renewable energy supplies, respectively. To understand both phenomena better and to fine tune the desired property, families of metal complexes of polydentate ligands were designed and synthesised in this thesis, and some of the most interesting complexes were surface immobilised using the Langmuir Blodgett technique.
Chapter 1 provides an introduction to the spin crossover (SCO) phenomenon, as well as various methods of monitoring SCO in solid state and solution. Selected literature examples of predictably tuning SCO in solution by establishing linear correlations of switching temperature (T1/2) with various parameters are presented and discussed. A range of methods of surface immobilisation of the SCO complexes, with a key focus on the Langmuir Blodgett technique, is discussed along with selected examples from the literature. The focus then shifts to providing an introduction to CO2 reduction, with a key focus on earth abundant molecular based catalysts for photocatalytic CO2 reduction.
In Chapter 2 the synthesis of the literature tetradentate HLH-OH ligand (from the 2:1 reaction of pyridine-2-carboxylaldehyde with 1,3-diamino-2-propanol) and five new analogues with alkyl tails of various lengths, from C10 to C22, added by alkylation of the alcohol moiety of HLH-OH to form LH-OR ligands, where R= C10, C16, C18, C20 and C22, are described. The synthesis, solution magnetic properties and surface immobilisation attempts of the corresponding family of complexes [FeII(LH-OR)(NCBH3)2], are described. The solution SCO T1/2 was tuned by two ways (a) by increasing the solvent polarity (P’) the T1/2 increases (b) in increasing the tail length from C10 to C22 T1/2 decreases. Linear correlations are obtained between T1/2 vs chain length and polarity index (P’) (R2=0.98 and 0.99) respectively. The compounds with longer alkyl tails from C18, C20 and C22 showed a small window of thermal hysteresis in solution.
In Chapter 3 the synthesis and characterisation of two tetradentate Schiff base ligands HLX-OH (X= H, Br), formed from the 2:1 reaction of pyridine-2-carboxylaldehyde or 5-bromopyridine-2-carboxylaldehyde with 1,3-diamino-2-propanol, and the corresponding six mononuclear complexes [FeII(LX-OH)(NCE)2] where E= BH3, Se and S are described. The T1/2 is tuned in three different ways, by the choice (a) substituent X in the ring (b) NCE co-ligand (c) solvent. An excellent linear correlation was obtained in the case of T1/2 vs P’ (R2=0.99). A measure of the ligand field strength of the three NCE co-ligands was obtained by measuring 15N-NMR chemical shifts of the coordinating nitrogen (δNA) for three different ligands E=BH3, Se and S in D2O. The BH3>Se>S trend in ligand field strength is established in solution. The obtained chemical shifts δNA are correlated with the T1/2 (R2=0.74 and 0.80).
In Chapter 4 the synthesis and characterisation of one new adptUrea ligand and one previously known adptPeg and corresponding two mononuclear complexes [FeII(adptPeg/Urea)2(NCBH3)2] are reported. The complex [FeII(adptPeg)2(NCBH3)2] was previously synthesised and solid state magnetics was obtained. The complex synthesis was repeated to obtain solution magnetic data. The SCO property and the influence of choice of tail in tuning T1/2 are studied both in solid state and solution. Multilayers (40 layers) of the [FeII(adptUrea)2(NCBH3)2] tailed complex are deposited onto a CaF2 solid support using Langmuir Blodgett technique and the infrared spectrum of the thin film on the surface, with a strong CH stretching around 2800-3000 cm-1 and a broad NH stretching around 3300 cm-1, confirmed the complex is intact.
In Chapter 5 a pair of literature 3d4f heterometallic CuII3TbIII complexes noncyclic [CuII3Tb(HU)3(H2O)4(NO3)2](NO3)2·H2O and macrocyclic [CuII3Tb(LPr)(MeOH)(H2O)2(NO3)2](NO3)2·3H2O, were re-made and tested for photocatalytic CO2 reduction. The effect of the macrocyclic vs acyclic systems on the catalytic activity and the lifetime of the catalysts are reported. The use of an external proton source trifluoroethanol to enhance the activity and increase the selectivity is also reported.
Finally, in Chapter 6 a summary of the key findings is provided.