Bimetallic Cooperativity of Nickel and Cobalt–Group 13 Pincer Complexes via Polarized Metal–Metal Bonds

Abstract

The underpinnings of modern industrial chemistry necessitate the use of transition metals to catalyze a wide variety of challenging chemical transformations. Precious metals, such as platinum, palladium, rhodium, and iridium are an essential component to many of these processes and have dominated due to the high catalytic activity and predictable two-electron redox. In the area of homogenous catalysis, precious metals have been an indispensable tool in many industrial processes. The scarcity of these metals and their high environmental impact have prompted a more recent focus on earth-abundant 3d metals. Earth-abundant transition metals are a critical component in many difficult biological transformation and offer untapped potential for homogenous catalysis. However, unlike precious metals, base metal catalysis tends to suffer from unproductive one-electron pathways which often hinder performance.Metal-ligand cooperativity has emerged as a premier strategy to perform catalysis with base metals. Cooperative ligand supports promote two-electron chemistry with base metal by working in concert during catalysis. Main group metalloligand scaffolds have shown promise in enhancing and turning on new types of reactivity at transition metals. Among these, Lewis acidic group 13 metals have garnered much attention due to their ability to act as strong σ-acceptors and potential for ambiphilic character. In this regard, rational ligand design was used to synthesize group 13 pincer-type ligands aimed towards cooperative bond activations involving first-row transition metals. Two new bimetallic systems have been developed with an open ligand framework that allows direct access to the M−group 13 interaction and exhibit a diverse array of cooperative σ-bond activations. Specifically, the group 13 support embedded in the pincer framework functions to bind substrate, stabilize low-valent metal states, and stabilize intermediates during reactivity. This strategy enabled two-site C–H, C–X, and H–H cleavage in which both metal ions participate. A key finding of this study is the unique bond polarization towards the transition metal which is induced by the electropositive group 13 element. A Ni–alane moiety is isolated which undergoes a reversible transformation between a Z and X-type ligand via aryl group transfer between the two metals. Mechanistic investigations identify that the Ni−Al bond is formed or broken as necessary during metal-ligand cooperativity. Additionally, by systematically varying the ligand frameworks and group 13 element, structure property relationships are found, highlighting important principles in the design of pincer-type group 13 metalloligands. A pair of isostructural Co–Al and Co–Ga pincer complexes are isolated which feature an X-type aluminyl or gallyl ligands. The isostructural nature of this pair allows for direct comparison of the effects of the supporting metal for reactivity. Further investigation of the duo reveals disparate reactivity with a much more active Al ion compared to Ga. Collectively, experimental and theoretical results show that the Lewis acidic metalloligand is key to the observed reactivity