Synthesis, reactivity and catalysis of heterobimetallic PNNN complexes

Abstract

This thesis explores the chemistry of a dinucleating, unsymmetrical phosphino pyridyl 1,8-naphthyridine PNNN ligand, 2-((di-tert-butylphosphino)methyl)-7-(2-pyridinyl)-1,8-naphthyridine, with the aim of preparing heterobimetallic complexes and investigating their reactivity and catalytic activity in hydrogenation reactions. Chapter 2 details the development of monometallic PNNN precursors using ruthenium. Complexation with commonly used ruthenium triphenylphosphine precursors was unsuccessful due to formation of mixtures and unstable products. Alternatively, ruthenium arene precursors allowed the isolation of N,N- and P,N-bound monoruthenium complexes, with N,N vs P,N selectivity dictated primarily by sterics. This selective synthesis of P,N and N,N monoruthenium complexes provides a promising platform for stepwise synthesis of heterobimetallic complexes. Chapter 3 covers ruthenium-copper heterobimetallic complexes. Synthesis of the heterobimetallic complexes could be achieved through the reaction of monoruthenium PNNN complexes with copper precursors. A dearomatised ruthenium-copper complex was isolated, which induced ruthenium-copper proximity. A dicopper-dichloride complex was synthesised to allow a comparative catalytic study between homo- and heterobimetallic PNNN systems. A preliminary investigation of metal-ligand cooperative dihydrogen activation was carried out using the dearomatised ruthenium-copper complex, which indicated that this stable ruthenium-copper combination may be incompatible with direct hydrogenation. In Chapter 4, the coordination of nickel precursors is assessed. The formation of heterobimetallic complexes with P,N-Ni;N,N-Ru and P,N-Ru;N,N-Ni were achieved through complexation of nickel source with two monoruthenium precursors. The synthesis of P,N-Ni;N,N-Ru and P,N-Ru;N,N-Ni complexes, respectively, is the first example of metal exchange between the two binding pockets within a 2,7-disubstituted-1,8-naphthyridine ligand system. The similar structural features of these ruthenium-nickel heterobimetallic complexes enabled the investigation of impacts of cooperativity between the ligand and different metals and provide a comparative system for evaluating their catalytic performance relative to their monometallic analogues. Finally, Chapter 5 examines the catalytic potential of PNNN complexes in hydrogenation reactions. Ruthenium and ruthenium-copper complexes were assessed in transfer hydrogenation of acetophenone, where the monoruthenium complex was found to be the most efficient catalyst. The copper centre remained catalytically inactive and sterically hinders substrate access in ruthenium-copper heterobimetallic systems, thereby decreasing reactivity. Possible metal-metal cooperativity was also examined in semi-hydrogenation of diphenylacetylene. Inclusion of a more active nickel centre led to a significant increase in catalytic activity using ruthenium-nickel complexes compared to both monoruthenium and the ruthenium-copper analogue. The enhanced catalytic activity may be due to nickel being the active site, or it may be induced by metal-metal cooperativity between ruthenium and nickel centres. The P,N-Ni;N,N-Ru complex delivers faster and more selective conversion than the P,N-Ru;N,N-Ni complex. Although preliminary studies were unable to determine to origin of these differences, these are promising system for further study to probe the role of metal-metal or even metal-metal-ligand cooperativity in heterobimetallic complexes. Show more