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Abstract
This thesis describes the synthesis of hybrid Rh(I), Ir(I) and Ir(III) complexes containing either bidentate ligands with carbene-triazole, pyrazole-triazole motifs or C2-asymmetric monodentate carbene ligands on a series of carbon surfaces, i.e. graphene, carbon black and glassy carbon. A simple and versatile method of immobilising transition metal complexes directly onto the surface was established. All hybrid complexes were characterised using X-ray photoelectron spectroscopy, thermogravimetric analysis, inductively coupled plasma-optical emission spectroscopy/mass spectrometry, Raman spectroscopy, transmission or scanning electron microscopy or cyclic voltammetry. A series of analogous homogeneous complexes to the hybrid catalysts were also prepared and fully characterised.
The hybrid and homogeneous complexes were tested as catalysts for C X (X = Si, N or H) bond forming reactions and their catalytic activities were evaluated. Mechanistic investigations of homogeneous Ir(I and III) catalysed reactions were conducted using NMR spectroscopy techniques, i.e. variable temperature, deuteration and parahydrogen induced polarisation (PHIP and SABRE). The Rh(I) hybrid (2.18-2.20) and homogeneous (2.13) complexes containing a bidentate carbene-triazole ligand were highly efficient for promoting the hydrosilylation of a disubstituted alkyne. The Rh(I) hybrid catalyst on graphene (2.20) and carbon black (2.19) were highly recyclable for at least 10 catalytic cycles with no significant loss of Rh from the carbon surface as indicated by ICP-MS. Homogeneous and hybrid Ir(III) (3.4a-f, 3.14), Rh(I) (2.12-2.13, 2.18) and a related series of in situ generated Rh(I) (3.8a-e) catalysts containing bidentate carbene-triazole ligands showed unprecedented efficiency at catalysing C-N bond formation reactions between alcohols and amines via hydrogen borrowing.
Homogeneous Ir(I) (4.1 and 4.13) and Rh(I) (2.12-2.13) complexes and their analogous hybrid catalysts (2.19, 4.18, 4.4) containing bidentate C.N and N,N ligands were capable of catalysing parahydrogen induced hyperpolarisation (PHIP) hydrogenation of alkynes. The catalytic efficiencies of Ir(I) complexes 4.7a-d containing C2 asymmetric monodentate carbene ligands for catalysing signal amplification by reversible exchange (SABRE) reactions with a series of pyridine derivatives (4.20-4.22) as hyperpolarisation targets were established. A highly efficient SABRE catalyst that transferred hyperpolarisation to 1H and 13C resonances was generated from complex 4.7c, and improved signal enhancement was observed compared to when using benchmark C2-symmetric catalyst 4.7e.