Bimetallic complexes of an unsymmetrical, dinucleating pyrazole-derived pincer ligand

Abstract

In this work, we describe the systematic synthesis and characterisation of homo- and heterobimetallic complexes of an unsymmetrical, dinucleating ligand. The pyrazole moiety of the ligand has a proton responsive NH that can be deprotonated by base therefore allowing for the stepwise synthesis of bimetallic complexes. The unsymmetrical nature of the ligand also allowed for a novel study into the pocket switching of metals in the different pockets. These complexes were all investigated as potential catalysts in transfer hydrogenation.

Chapter 2: Synthesis and characterisation of homometallic nickel complexes of PNNNNH ligand are reported and their reactivity explored. The redox properties of the mononickel complex in acetonitrile indicated two quasi-reversible reduction events, while the electrochemical profile of the dinickel complex in DMF indicated multiple redox events in its CV. The insolubility of these complexes particularly the dinickel complex prompted a co-ligand study in which the chloride ligands were substituted for nitrates and sulphates, but these led to the formation of trinickel bis-chelated complexes. These complexes were also investigated for their potential as electrocatalysts for proton reduction.

Chapter 3: The first examples of Ni-based heterobimetallic complexes are reported using the PNNNNH ligand. The proton-responsive nature of the pyrazole ligand core allows for the stepwise synthesis of heterobimetallic complexes via the monometallic nickel and ruthenium complexes. Although selectivity for the PNN pincer pocket is observed with Ni, the complexation to the bidentate NN-pocket is favoured with more sterically encumbered Ru complexes. The identification of the first dinuclear NiFe complex of the PNNNN systems was characterised by X-ray crystallography, but unfortunately a bulk product was never isolated. When the Ru analogues were used, soluble diamagnetic complexes were obtained. Although the two Ru precursors used in this study are closely related, they both gave different NiRu structures with the former leading to a NiRu complex with a bridging Cl ligand while the later gave a NiRu complex with no bridging ligand.

Chapter 4: A novel study of pocket switching is explored in this chapter. In Chapter 3, it was illustrated that Ru preferentially binds to the NN pocket rather than the PNN pocket, a process that could be led by steric and kinetic control. Coordination of Ru to the PNN site could be achieved thermally using high temperatures. Two mono RuPNN complexes were synthesised and served as precursors to pocket-switched heterobimetallic RuPNN,NiNN complexes. The pocket switched RuNi complexes were found to be more air- and moisture-stable compared to the NiRu complexes.

Chapter 5: In this chapter, we explored the reactivity of the homometallic and heterobimetallic complexes in transfer hydrogenation. The homometallic nickel complexes were ineffective in catalysis even in the presence of a diboron activator. The mono RuNN complex only showed 44% conversion of benzophenone after 24 hrs. However, the mono RuPNN complexes showed over 95% conversion within 2 hrs. Notably, when Ni was added, more than 2-fold reduction in conversion was observed for the heterobimetallic complexes. This suggests Ru is the catalytically active site and is most effective when it is coordinated in the pincer pocket. Show more