Design, Synthesis, and Characterization of Multimetallic Complexes Supported by an Imidazopyrimidine-Based Trinucleating Ligand

Abstract

Transition metal catalysis has revolutionized chemical synthesis for decades and has allowed for the development of several Nobel prize-winning chemical reactions and processes. These catalysts, however, usually rely on the use of rare Earth metals such as platinum-group metals, mainly palladium, leading to economic and sustainability concerns. Recent studies on the use of Earth-abundant elements nickel, cobalt, and copper have revealed that these metals have the potential of offering low-cost alternatives to the traditional catalysts. Furthermore, these metals can access many more states, allowing for new and complementary reactivities to be achieved. Whilst transition metal catalysis is a large and impactful field, the majority of known catalysts are monometallic in nature. A compelling yet much underexplored area is the use of multimetallic complexes. Several studies and reviews have highlighted the beneficial effect of having multiple metal centers held in proximity. These sorts of systems often display improved catalytic performances over their monometallic counterparts. Synergy or metal-metal cooperativity between the centers is usually responsible for these observations, sometimes allowing for multielectron processes that are simply not possible with traditional monometallic catalysts. In terms of trimetallics, there is a paucity of ligand systems that can reliably produce a precise and controlled arrangement of the three metal centers in a way that is useful in catalysis. This is due to most relying on flexible organic frameworks tied to a symmetric node, additionally excluding them from heterometallic applications.

Herein is reported a new trinucleating ligand framework, bpipp, specifically designed to enforce close proximity among three metal centers upon complexation. Based on the inherently unsymmetric imidazopyridmine backbone, the ligand features a tridentate pincer-like binding pocket with two additional bidentate binding pockets. This approach utilizes scalable synthetic methods to create a rigid ligand scaffold that precisely controls the spatial arrangement of the metals. The versatility of this ligand is demonstrated through the synthesis of several trimetallic complexes of Ni(II), Cu(II), Co(II); fully characterized by NMR spectroscopy, ESI-HRMS, and X ray crystallography. Notably, our ligand design achieves remarkably short metal-metal distances ranging from 3.3–3.5 Å, significantly closer than most reported trimetallic systems. This structural feature establishes an ideal platform for investigating genuine three-metal cooperative effects in catalysis.