Small Molecule Activation using a Metalloradical
Activation of small molecules is a challenging but important topic in chemistry. It can be directly utilized to generate sustainable and low-cost commodity chemicals and applied to new synthetic methodologies for various catalysis.
An article addressing this was recently published in Angewandte Chemie by a research group at KAIST.
The work describes an introduction of a novel methodology to activate small molecules using earth abundant transition metals. These researchers in the Department of Chemistry reported a coordinatively unsaturated nickel(I) metalloradical species which enables the activation of various small molecules, such as H2, CO2, C2H4 and NH2NH2 under ambient conditions.
In order to study the reactivity of a metalloradical species, a novel and rigid ligand was designed; its 3-coordinate nickel(I) complex was the successful synthesized.
The T-shaped Ni(I) complex reveals unique reactivity including homolytic cleavage of challenging σ-bonds at room temperature. These results demonstrate the open-shell type reaction using a metalloradical can be a powerful method to activate challenging small molecules.
Low-coordinate metal complexes are of great interest due to their unusual electronic structures and reactivities.
In a three-coordinate complex, a metal center generally possesses a trigonal planar geometry, although a T-shape geometry is electronically preferred in a d9 system.
In the latter case, the unpaired electron singly occupies a dx²-y² orbital, which is sterically exposed and thus is easily accessible for metalloradical chemistry.
Importantly, such T-shaped nickel(I) species are rare: their preparation is synthetically challenging but their reactivity is accordingly largely unveiled allowing for this research area to be better researched in the future.
Since the flexibility of the supporting ligand often times results in forming a Y-shape geometry or gives dimerization, the researchers, therefore, have sought specific molecule pieces in their design: a rigid acridane-based pincer-type PNP ligand (acriPNP– = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide).
From the one-electron reduction of a nickel(II) precursor (acriPNP)NiCl, a nickel(I) species (acriPNP)Ni was successfully generated and cleanly isolated.
Its solid-state X-ray crystallographic data shows a nickel center possessing a three-coordinate T-shaped geometry.
To explain the surprising reactivity an analysis of the electronics is important.
Thus, even with the available coordination site, the binding of σ-donor substrates (small molecules) is unfavourable due to the σ-antibonding character of a singly occupied dx²-y² orbital.
Having a sterically exposed half-filled orbital, bonding with the substrate occurs together with an inner-sphere single electron transfer from a single nickel center resulting in reduction of unsaturated molecules (C2H4 and CO2) and homolytic cleavage of various σ-bonds including those in the following molecules: H-H, H2N-NH2 and H3C-CN.
Angew. Chem., Int. Ed. 2017, 56, 9502-9506.
* lab webpage : https://sites.google.com/site/yunholab/