Bioinspired, heterogeneous amino acid complexes for benign oxidation catalysis

Abstract

Metalloenzymes catalyse the most fundamental reactions in organic chemistry
from oxidation of hydrocarbons to complex C-C bond forming reactions with exceptional
selectivity. Mimicking the active site of a metalloenzyme by immobilising well-defined
amino acids containing catalytically active transition metal centres based on transitionmetals
on a robust inorganic framework, affords powerful catalysts that can be utilised in
oxidation reactions. Porous aluminosilicates, mesoporous silicas and polymers offer
suitable supports for single-site bio-derived catalysts. Dispersion of catalytically active
centers within porous solids with high surface area improves site-isolation which is essential in
catalytic processes. These materials can be created from a range of methodologies and the
different strategies used for immobilisation can greatly affect the nature of the active
catalyst. The routes by which these catalysts are immobilised have given the potential to
derivatize inorganic porous hosts and organic polymer structures with amino acids for
complexation to metal centres. These bio-derivatized frameworks offer advantages over
the homogeneous counterparts in terms of easy separation, recover and recyclability and can
carry out selective oxidation reactions with great effectiveness.

Herein, heterogenous bioinspired complexes of two amino acids; proline and
valine with a series of transition metals (Fe, Cu) were synthesised and immobilised within
zeolite cages, mesoporous silica MCM-41 and polystyrene. The preparation methods
allowed the synthesis of materials with varying loadings of immobilized active sites. The structural
information obtained by spectral and elemental analysis suggested tetrahedral geometry for iron
complexes and distorted square planar geometry for copper complexes. Both amino acids
coordinated to metal ions through the nitrogen atom of amino group and oxygen atom of
carboxylate group via dissociation of the acidic proton as bidentate N,O-donors.

The resulting biomimetic complexes were employed as catalysts for oxidation of
cyclohexane, cyclohexene, benzyl alcohol and dimethyl sulfide, using molecular dioxygen (O2),
tert-butyl hydroperoxide (TBHP) and acetylperoxyborate (APB) as oxidants. The observed trends
in catalytic activity showed that the metal loading and separation of the active sites played key role
in the selective oxidation reactions. By decreasing the loading of metal active centres, their spatial
separation increased which strongly enhanced the activity of catalysts. The decrease in metal
active site content resulted in significant increase in TON and TOF. The product selectivity was
dependent on the nature of oxidant, hydrophobicity/hydrophilicity of the support, loading of metal
active centres and the metal/substrate ratio.

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Identifiers

ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

ORCID for Eugen Stulz: orcid.org/0000-0002-5302-2276

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