Catalysis of organic reactions using immobilised transition metal nanoparticles

Abstract

The synthesis and properties of polymer immobilised ionic liquids (PIILs) have been extensively studied. In particular, the Knight-Doherty group has explored and developed novel functionalised PIIL systems as supports for transition metal- and nanoparticles-based catalysts for a range of chemical reactions. Chapter 2 describes the completion of a study of iridium-catalysed aromatic C-H borylation under homogeneous conditions, which does not involve PIIL but was used to gain experience in catalysis. In situ generated Ir-complexes, formed from [Ir(COD)(acac)] or [Ir(COD)(OMe)]2 and tmphen catalysed the C-H borylation of diethyl phenylphosphonate (A, R = H) with B2pin2 or HBpin under mild conditions but were nonselective, producing a mixture of 3-, 3,5-di- and 4-boryl substituted aryl phosphonates (B). In contrast, aryl phosphonates carrying an existing 3-substituent undergo C-H borylation with very high regioselectivity to give the product in which the boronate ester is meta to both existing substituents. The resulting meta-substituted arylboronic esters were then utilised as the nucleophilic coupling partner for the Pd-catalysed Suzuki–Miyaura cross-coupling reaction with a variety of aryl halides to prepare a diverse range of meta-phosphonate substituted biphenyls (C). Chapter 3 builds upon the work carried out in a paper published by our research group in PIIL supported AuNPs for the reduction of nitroarenes. The impregnation of a phosphine-decorated PIIL with KAuCl4 was anticipated to result in oxidation of the phosphine and reduction of the Au(III) to Au(I). A series of 31P NMR experiments were performed with triphenylphosphine, as a model compound, which confirmed that treatment with KAuCl4 results in formation of a transient Ph3PCl2 species, and ultimately the major products Ph3PO and Ph3PAuCl, together with a small amount of Ph3PAuCl3. The PIIL Au(I) phosphine complex D (AuCl, Z = PPh2) and Au(III) phosphine oxide complex D (AuCl4 - , Z = O=PPh2) were prepared, together with the in situ generated Au(I) containing phosphine oxide D (AuCl, Z = O=PPh2) PIIL in order to investigate the effect of gold precursor and the heteroatom donor in the polymer support on the catalyst performance and selectivity for reduction of nitroarenes by the derived nanoparticle-containing PIILs E. These catalytic systems reduce nitrobenzene under mild reaction conditions and with low catalyst loading into three distinct products; N-phenylhydroxylamine (NOH), azoxybenzene (N2O), and aniline (NH2). Using water as the reaction solvent the phosphine oxide based Au(III) and Au(I) precursors were both extremely active and selective for reduction of nitrobenzene (NO2) to form the thermodynamically unfavourable N-phenylhydroxylamine (NOH) with either NaBH4 or (CH3)2NH·BH3 as the stoichiometric hydrogen source. Changing the solvent system from water to ethanol under similar reaction conditions provided azoxybenzene (N2O) as the sole product with NaBH4 but the reaction was unselective when using (CH3)2NH·BH3 as hydrogen source. Finally, by increasing the number of equivalents of NaBH4, using longer reaction time and higher temperature, aniline (NH2) was afforded in almost quantitative yields. In chapter 4, a strongly hydrophobic decyl hydrocarbon chain was incorporated into the polymer to investigate the influence of replacing the strongly hydrophilic PEG. Both PdNP@PPh2-PEGPIIL (F) and PdNP@PPh2-N-decylPIIL (G) are efficient and selective systems for quinoline hydrogenation in the presence of NaBH4 or (CH3)2NH·BH3 under mild conditions and delivered 100% selectivity for 1,2,3,4-tetrahydroquinoline (THQ). The polyethylene glycol modified catalyst (PdNP@PPh2-PEGPIIL, F) had a higher activity than the N-decyl analogue (PdNP@PPh2-NdecylPIIL, G). In chapter 5, PIIL supported PtNPs were found to catalyse the aqueous hydrolysis of NaBH4 efficiently under mild conditions to produce hydrogen. The composition of the polymer effects the efficiency of the catalyst. PtNP@PPh2-PEGPIIL (H) was found to be more active as compared to PtNP@PPh2-N-decylPIIL (I). A maximum initial TOF of 169 moleH2.molcat-1 .min-1 with PtNP@PPh2-PEGPIIL (H) loading of 0.08 mol% was achieved under mild conditions. This represents one of the highest TOF values reported for the aqueous hydrolysis of NaBH4 in the presence of a PtNP-based system. Arrhenius plots indicated an activation energy (Ea) of 23.9 kJ mol-1 for the aqueous hydrolysis of NaBH4 catalysed by PtNP@PPh2-PEGPIIL (H) and 35.6 kJ mol-1 for PtNP@PPh2-N-decylPIIL (I). The mechanism of the reaction was explored by primary kinetic isotope effect, using either H2O or D2O which produced a kH/kD of 1.8 for hydrolysis catalysed by PtNP@PPh2-PEGPIIL and 2.1 for PtNP@PPh2-N-decylPIIL. These values are consistent with cleavage of the O-H bond being involved in the rate determining step. The PtNP@PPh2-PEGPIIL was recycled up to five times. The drop in conversion to 70% after the fifth cycle was ascribed to poisoning by the accumulated byproduct, metaborate.