Understanding Precatalyst Activation and Speciation in Manganese-Catalyzed C-H Bond Functionalization Reactions

Area of Science:

• Organometallic Chemistry
• Catalysis
• Spectroscopy

Background:

• Manganese complexes are crucial catalysts for C-H bond functionalization.
• Understanding the initial species formed after precatalyst activation is key to controlling reaction outcomes.
• Time-resolved spectroscopy offers insights into rapid reaction intermediates.

Purpose of the Study:

• To investigate the transient species formed during manganese-catalyzed C-H functionalization.
• To elucidate the kinetic factors governing ligand substitution after precatalyst activation.
• To correlate substrate properties with reaction pathways and product selectivity.

Main Methods:

• Time-resolved infrared (TR-IR) spectroscopy to monitor species formation on nanosecond to microsecond timescales.
• Use of specific manganese precatalysts with cyclometalated ligands ([Mn(C^N)(CO)4]).
• Density Functional Theory (DFT) calculations to probe substrate-ligand affinities.

Main Results:

• Light-induced CO dissociation from precatalysts generates initial solvent complexes ([Mn(C^N)(CO)3(toluene)]).
• Subsequent solvent substitution by nitrogen-containing ligands (2a or 2b) occurs on the nanosecond timescale.
• Competitive reactions with phenylacetylene lead to either alkyne insertion or ligand substitution, depending on the substrate (2a vs. 2b).
• DFT calculations suggest differing substrate affinities for manganese dictate reaction pathways.

Conclusions:

• Speciation immediately following precatalyst activation is a kinetically controlled process.
• Solvent coordination is the initial step, followed by kinetically controlled ligand substitution.
• The observed differences in reaction outcomes are attributed to the varying affinities of substrates for the manganese center.