Benzimidazole-diamide (bida) Pincer Chromium Complexes: Structures and Reactivity

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry
• Inorganic Synthesis

Background:

• The development of redox-active ligands is crucial for advancing catalysis and materials science.
• Pincer ligands offer unique coordination environments and stability for metal complexes.
• The bida ligand, a novel hemilabile pincer, was serendipitously discovered and exhibits potential for proton/H atom reactivity.

Purpose of the Study:

• To synthesize and characterize novel chromium complexes incorporating the bida ligand.
• To investigate the reactivity of these complexes with various reagents, including azides and N-chlorosuccinimide.
• To explore the electronic structure and redox properties of the resulting chromium species.

Main Methods:

• Synthesis of the disodium salt of bida (bida)Na₂(THF).
• Salt metathesis reactions with chromium and vanadium precursors.
• Ligand substitution and oxidation reactions.
• Spectroscopic characterization (NMR, UV-Vis) and X-ray crystallography.
• Computational studies including Mulliken spin density analysis.

Main Results:

• Successful synthesis of various bida-chromium complexes, including (bida)CrCl(THF), (bida)CrCl(PMe₂Ph), and (bida)CrR(THF).
• Formation of imido-chromium species, (bida)ClCr═NR, through oxidation with azides.
• Observation of an insertion product, (bida)Cr(κ²-N,N-ArN₃Nph), and a deprotonated ligand species, (bia)CrCl₂(THF).
• Metric parameters remained constant across different oxidation states (Cr(III) to Cr(V)).
• Electronic structure analysis revealed antiferromagnetic coupling and complex ground state admixtures.

Conclusions:

• The bida ligand demonstrates versatility in coordinating to chromium and facilitating diverse reactivity.
• Chromium complexes with the bida ligand exhibit complex electronic structures with evidence of redox noninnocence.
• The methylene group of the bida ligand plays a role in reactivity, enabling transformations like deprotonation and HAT reactions.
• This work expands the scope of pincer ligand chemistry and provides insights into the electronic properties of high-valent chromium species.