Cationic tetrylenes as Single-Center Ambiphile ligands for Cooperative N-H bond activation and hydroamination catalysis

• Applicant: Terrance Hadlington, Ph.D.
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 470323245

Amines, which are indispensable building blocks in chemical synthesis, are produced on industrial scale through multi-step stoichiometric processes, thus generating considerable waste and using excessive energy. A process which overcomes this is the 100% atom efficient addition of ammonia to alkenes and alkynes. Homogenous catalytic systems for the selective synthesis of primary amines through hydroamination are essentially unknown, historically due to major challenges in the activation of N-H bonds at a transition metal (TM) center. The selective synthesis of secondary amines from primary amines and unactivated alkenes are also extremely rare on similar grounds. These two aspects of hydroamination therefore still stand as major challenges in catalysis to this day. This project aims to tackle these challenging processes through the development of a novel concept in ligand design and bond activation, namely cationic Single-Center Ambiphile ligands. These ligands can simultaneously act as σ-donors and strong π-acceptors, remaining Lewis-acidic when in the coordination sphere of a TM. This leads to a unique metal-ligand cooperative mechanism in N-H bond activation, where substrate binding occurs at the ligand as opposed to the metal, and so proceeding through an ‘Umpoled’ bond activation. The unique activation mechanism would lead to a TM-hydride complex through N-H bond scission, which can subsequently react with unsaturated substrates. We will develop and define this unique bond activation process, and take steps towards utilizing it in the catalytic hydroamination of alkenes with ammonia, to form primary amines, and in the subsequent selective generation of secondary amines. To achieve this, the present project will develop a novel class of cationic tetrylenes, which have the capacity to act as Single-Center Ambiphile ligands, specifically focusing on GeII and SnII, in conjunction with low-cost, highly abundant late first-row transition metals, Fe, Co, and Ni. Through differing metal-ligand combinations, as well as steric and electronic modifications of the ligands, we will develop a broad, well-defined set of systems for controlled N-H bond activation in ammonia and higher amines, and extend this to well-defined catalytic processes. To further define bond-activation mechanisms via which Single-Center Ambiphile ligand-TM systems can operate, we will also study the ‘Umpoled’ scission of further protic and hydridic E-H substrates (E = O, P, B, Si, H, etc.), as well as reactivity towards key catalytic substrates such as CO and CO2, in order to understand the effects of bond polarity in bond-activation by Single-Center Ambiphile systems. As a whole, this project aims to lay the foundations for the novel Single-Centre Ambiphile ligand concept, to ultimately develop systems which counter the poor atom economy of common industrial processes for amine synthesis.

• DFG Programme: Research Grants