A Nucleophilic Chromium(V) Dioxo Radical Anion

Area of Science:

• Organometallic Chemistry
• Inorganic Chemistry
• Coordination Chemistry

Background:

• Chromium complexes with nitrogen-based ligands are of interest due to their diverse reactivity and applications.
• The synthesis and characterization of high-valent chromium species, particularly chromium(V), remain challenging.
• Understanding the electronic and structural properties of these complexes is crucial for advancing catalysis and materials science.

Purpose of the Study:

• To synthesize and characterize novel chromium(VI) and chromium(V) complexes.
• To investigate the reactivity of a chromium(V) dioxo complex as a nucleophile.
• To explore the structural, magnetic, and spectroscopic properties of the resulting monooxo chromium(V) derivatives.

Main Methods:

• Oxidation of a tris(amido)chromium(III) precursor to a chromium(VI) dioxo complex.
• One-electron reduction of the chromium(VI) complex using cobaltocene to form a cobaltocenium salt.
• Nucleophilic reactions of the chromium(V) dioxo complex with electrophiles (silyl chlorides, acyl chlorides, methyl triflate).
• Structural characterization (X-ray crystallography), magnetic susceptibility measurements, NMR spectroscopy, and cyclic voltammetry.

Main Results:

• Successful synthesis of a purple chromium(VI) dioxo complex, O2Cr(NRAr)2, in 83% yield.
• Isolation of the cobaltocenium salt of the chromium(V) dioxo anion, [CoCp2][O2Cr(NRAr)2], in 89% yield.
• The chromium(V) dioxo anion exhibits paramagnetic behavior consistent with one unpaired electron.
• The chromium(V) dioxo complex reacts as a nucleophile to yield three rare monooxo chromium(V) complexes: OCr(OSiPh3)(NRAr)2 (86%), OCr[OC(O)Ph](NRAr)2 (34%), and OCr(OMe)(NRAr)2 (76%).
• Structural characterization of two monooxo complexes reveals short Cr-O(oxo) bond distances (1.611(6) and 1.588(3) Å).
• NMR data indicate restricted rotation around Cr-N bonds in the monooxo complexes.

Conclusions:

• Novel chromium(VI) and chromium(V) complexes have been synthesized and characterized.
• The chromium(V) dioxo complex demonstrates valuable nucleophilic reactivity, providing access to rare monooxo chromium(V) derivatives.
• The structural and magnetic properties of these complexes offer insights into the chemistry of high-valent chromium species.