Syntheses and Studies of Ruthenium and Rhodium Pyridinium Amidate Complexes as Potential CO releasing molecules

Abstract

This thesis describes the coordination complexes formed through reaction between a new class of neutral nitrogen donor ligands, N-(pyridinylidene)amides (PYAs), and the organometallic ruthenium [(η⁶-p-cymene)RuCl₂]₂ and rhodium [Cp*RhCl₂]₂ dimers.The fully characterised complexes [(η⁶-p-cymene)Ru¹¹(N-(1-benzylpyridin- 4(1H)ylidene)picolinamide)Cl]Cl(1), [Chloro(η⁶-p-cym)(picolinamidato)ruthenium(II)] (2) (as a byproduct the derivative with the hydrolysed ligand) and [(Cp*RhIII(N-(1- benzylpyridin-4(1H)ylidene)picolinamide)Cl]Cl (4) were synthesised from these components. N-(Pyridinylidene)amide (PYA) ligands act as bidentate (N,O) or (N,N) coordinating bidentate ligands to the Ru and Rh metal centres, respectively. This was confirmed by the X-ray crystal structures of complexes 1, 2 and 4. Upon carbonylation of 1 and 4, they formed the ruthenium and rhodium carbonyl derivates [(η⁶-pcymene) RuII(N-(1-benzylpyridin-4(1H)ylidene)picolinamide)(CO)][SO₃CF₃]₂ (7) and [Cp*RhIII(N-(1-benzylpyridin-4(1H)ylidene)picolinamide)(CO)][SO₃CF₃]₂ (8). Both carbonyl complexes showed the presence of a terminal CO ligand with a strong IR band around 1974 cm⁻¹ corresponding to the CO stretching frequency. The myoglobin assay on the CO releasing ability of the complexes at pH 7.3 and 6.5 were conducted. The release of CO from the ruthenium and rhodium carbonyl complexes and from [RuCl2(CO)₃]₂ (CORM-2) as a positive control were studied by UV/Vis spectroscopy by measuring the conversion of deoxy-myoglobin (deoxy-Mb) to carbonmonoxy myoglobin (MbCO). Upon CO release from the metal complexes, the absorption peak in the Q band region of deoxy-Mb at 558 nm gradually decreases in intensity with the appearance of two new bands of MbCO adducts at 540 and 578 nm demonstrating conversion of Mb to MbCO. PYAs act as molecular triggers for CO release from the CO complexes by controlling their electron donor abilities towards the transition metal ions from very strong donating to only weakly donating through the protonation of a remote uncoordinated nitrogen atom at lower pH environment as present in some tissues in the human body. Therefore, this is a new approach to pH sensitive CO-releasing molecules which can exploit pH differences in biological tissues to increase the selectivity of such compounds.