Multimetallic Complexes Supported by an Unsymmetrical Imidazopyrimidine-Based Ligand: Synthesis, Characterization, and Catalytic Studies

Abstract

Bimetallic catalysts containing two metals in close proximity harness cooperative effects that enable enhanced or unique reactivity in comparison to traditional monometallic catalysts. The development of such catalysts relies on the development of binucleating ligands that support their assembly and modulate key parameters, synthetic routes to access bimetallic complexes, and continued exploration of their catalytic properties. In this regard, heterobimetallic catalysts are particularly underdeveloped due to synthetic challenges associated with incorporating two different metal centers selectively. This thesis explores the synthesis of heterobimetallic complexes using a novel unsymmetrical ligand design. In ‎Chapter 2, imidazopyrimidine-based ligands are introduced as a novel motif for binucleating ligand design. The imidazopyrimidine motif was selected for its ease of synthesis and inherently unsymmetrical nature. A representative ligand was synthesized in high yield from readily available starting materials, and the route was successfully extended to multigram scale. In ‎Chapter 3, the coordination chemistry of this ligand was investigated through the synthesis of homobimetallic complexes. Dinickel(II), dicopper(II), and dipalladium(II) complexes were prepared and characterized to assess the structural influence of the imidazopyrimidine motif. Serendipitously, trinickel(II) and tricobalt(II) complexes were also prepared and characterized, demonstrating the ability of imidazopyrimidine-based ligands to potentially accommodate variable nuclearities. Key structural features, such as the metal-metal distances, were evaluated and compared with literature complexes. ‎Chapter 4 focuses on the synthesis of heterobimetallic complexes supported by an imidazopyrimidine-based ligand. One-step syntheses of nickel(II)/copper(II) and cobalt(II)/copper(II) complexes were achieved, including both binuclear and trinuclear complexes. NMR studies revealed that the heterometallic complexes were thermodynamically favoured. Competition reactions analyzed by ESI-MS demonstrated that the selective formation of heterometallic complexes was driven in part by the preferential binding of copper(II) to one of the coordination sites on the ligand. Attempts to access other heterobimetallic combinations, including nickel(II)/palladium(II) or copper(II)/palladium(II), were unsuccessful. In ‎Chapter 5, the Glaser-Hay coupling is explored using a dicopper complex supported by an imidazopyrimidine-based ligand. Compared to related monometallic catalysts, the dicopper complex exhibited a consistently reduced reaction rate, as determined by NMR studies.