Development of New Ligands for Homogeneous Transition Metal Catalysts

Abstract

This thesis describes the development of new nitrogen donor ligands for homogeneous transition metal catalysis. This includes the synthesis of a macrocyclic ligand with amidate functionality and its palladium complexes, as well as the synthesis and study of acyclic ligands with pyridinium amidate and amide functionality and the palladium complexes of these ligands. Some aspects of the coordination chemistry of the macrocyclic ligand, H4LnBu, which contains pyridine carboxamide groups, were investigated. It was found that palladium metal can be selectively inserted in to head or tail region of macrocycle depending on the conditions employed. Palladium(II) migrates from the tail to the head region on heating in the presence of bases like DBU, NH2Lac[7] and n-butyl amine. The water molecule coordinated to palladium in Pd(H2LnBu)(OH2) can be displaced by nitrogen donor ligands such as N-(3-aminopropyl)caprolactam (NH2Lac), nbutylamine (NH2Bu), and DBU. DBU coordinated to palladium in the head region of the macrocycle can also be displaced by N-(3-aminopropyl)caprolactam and nbutylamine. Hydrolysis of DBU to give NH2Lac occurs during the formation of Pd(H2LnBu)(NH2Lac[7]) from either H4LnBu or Pd(OAc)(H3LnBu). However the coordinated DBU in Pd(H2LnBu)(DBU) does not undergo hydrolysis. Studies showed that it is the acetic acid that is formed during the coordination of the palladium acetate to the macrocycle that catalyses the hydrolysis of DBU. A new class of nitrogen donor ligands, the pyridinium amidates, was studied and found to be strong σ-donor ligands. These were synthesised by deprotonation of the corresponding N-protonated compounds either by using sodium hydride or aqueous sodium hydroxide base. DFT calculations and X-ray crystallography results clearly indicate that the pyridinium amidate ligands exist predominantly in imine resonance form and there is restricted rotation about exocyclic C=N. The strong σ-donor properties of this type of ligand were evidenced by the very low average ν(CO) value displayed by cis-Rh(L1{R2})(CO)2Cl and this is amongst the lowest average ν(CO) value recorded for rhodium complexes of the type cis-Rh(L)(CO)2Cl, where L is a neutral ligand. The rhodium complex, Rh(L1{R2})(CO)2Cl, was formed by direct reaction of rhodium carbonyl chloride dimer, [RhCl(CO)2]2, with the pyridinium amidate ligand. However pure palladium complexes could be prepared more conveniently through the reaction between Pd(COD)Cl2 and the N-protonated ligand after addition of the base DBU, which presumably serves to generate the pyridinium amidate in situ. Long Pd-Cl bond distances were found in complexes where the trans ligand was a pyridinium amidate nitrogen. This indicates this nitrogen donor ligand typically has a strong trans influence. The comparison of these pyridinium amidate ligands with pyridinium amide and NHC ligands showed that the structural and electronic properties of all these ligands are very similar. In order to compare the reactivity of pyridinium amidate ligands with closely related pyridinium amide ligands, new pyridinium amide ligands analogous to pyridinium amidates were prepared by an alternative route to those previously reported in literature. This involved a solid state reaction between 4-chloro-N-methyl pyridium triflate and the corresponding amine at 200 to 250oC followed by deprotonation of the N-protonated ligands with sodium hydride or sodium hydroxide in dichloromethane. Palladium complexes of these ligands were prepared by employing similar method to the one used for the synthesis of pyridinium amidate palladium complexes. The new palladium complexes were investigated for catalytic activity in the Heck-Mizoroki and Suzuki-Miyaura coupling reactions. Comparative studies showed that pyridinium amidate ligands are better supporting ligands for above mentioned coupling reactions than pyridinium amides. The stability of the complexes under catalytic conditions was judged on the basis of the formation of visible palladium black during the reactions. It was found that palladium pyridinium amidate complexes were more stable than palladium pyridinium amide complexes under the conditions used for the Heck- Mizoroki and Suzuki-Miyaura coupling conditions. It was also found that the presence of a chelating pyridyl group clearly improves the stability of the palladium pyridinium amidate complexes under these conditions.