Evaluating the Origins of Aerobic Oxidation Catalysis with TAM-3, a MOF with Accessible Co(II) Sites and Large Pores

TL;DR

This paper studies TAM-3, a metal-organic framework (MOF), to understand how its structure affects catalytic activity in oxidation reactions.

Contribution

The study introduces TAM-3, a MOF with large pores and accessible Co(II) sites, and uses kinetic experiments to clarify the role of interstitial metal ions in catalysis.

Findings

TAM-3 promotes aerobic C–H oxidation and olefin epoxidation.

Kinetic experiments show interstitial metal ions do not significantly contribute to oxidation reactions.

The study emphasizes the importance of kinetic analysis in understanding MOF catalytic activity.

Abstract

Metal-organic frameworks
(MOFs) are attractive platforms that merge
concepts of homogeneous and heterogeneous catalysis. Catalyst design
and optimization are enabled by an array of synthetic methods that
offer independent control over the local chemical structure of lattice-embedded
metal ions (i.e., ligand identity and geometry) and the long-range
materials properties (i.e., porosity). Establishing the origin of
catalytic activity in MOF-promoted reactions remains a significant
challenge: The relative rates of catalyst turnover and substrate diffusion
dictate the extent to which interstitial sites are accessible and
operational in catalysis. To minimize the contributions of surface
sites in catalysis, materials with large pore dimensions are often
sought, however, the impact of pore expansion on the origins of catalytic
activity is similarly challenging to establish. Here, we describe
TAM-3, a Co­(II) based MOF with accessible metal sites supported by
a facially coordinating tris-tetrazole ligand set.
TAM-3 features large channel-like pores (17 × 23 Å) and
promotes aerobic C–H oxidation and olefin epoxidation. Using
a set of simple kinetics experiments, based on the analysis of kinetic
isotope effects and olefin oxidation diastereoselectivities, we demonstrate
that despite the large pores, interstitial metal ions do not significantly
contribute to the observed substrate oxidation. This study highlights
the importance of conducting kinetic experiments to assess the origin
of apparent catalytic activity with MOFs and the challenge of harnessing
reactive oxidants with microporous catalyst materials.

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