An Achiral Tetradentate Cis‐α‐Coordinating NCCN Ligand Gives Rise to a Configurationally Stable Chiral‐at‐Iron Complex for Enantioselective Catalysis

TL;DR

Scientists designed a new ligand that creates a stable chiral iron complex for enantioselective catalysis, using an achiral starting material.

Contribution

First chiral-at-iron catalyst derived from an achiral tetradentate ligand for asymmetric catalysis.

Findings

A linear tetradentate NCCN ligand was synthesized with two mesoionic carbene donors.

The iron complex showed configurational stability and enantioselective catalytic activity in C(sp3)−H amination and Cannizzaro reactions.

Abstract

Ligand design plays a crucial role in developing chiral transition metal complexes with enhanced or new catalytic properties. Here, we report our progress toward a new class of linear tetradentate NCCN ligands incorporating strongly σ‐donating carbene moieties. The NCCN ligand coordinates to iron(II) in a cis‐α topology, with two pyridine donors occupying the apical positions and two 1,2,3‐triazolin‐5‐ylidene mesoionic carbene (MIC) donors in the equatorial plane. Two acetonitrile ligands complete the octahedral coordination sphere, and their lability provides the basis for the observed catalytic activity. Notably, the two strongly σ‐donating MIC groups create a strong ligand field, which is critical for achieving configurational stability of the metal‐centered stereogenicity. This design strategy thus enabled the first chiral‐at‐iron catalyst derived from an achiral tetradentate ligand, which was applied to catalytic and enantioselective C(sp3)−H amination and a Cannizzaro reaction.

The design and synthesis of a linear tetradentate NCCN ligand incorporating two 1,2,3‐triazolin‐5‐ylidene mesoionic carbene donors, and its application to asymmetric iron catalysis is reported. This work represents the first example of an iron complex derived from a cis‐α‐coordinating tetradentate NCCN ligand, and notably, the first chiral iron catalyst derived from an achiral tetradentate ligand.

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