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Pseudosolvent effects on interfacial catalysis: exploring local environments in nanoconfined spaces

     It is well known that solvents affect the stability of molecules, intermediates, transition states and intermolecular interactions to control chemical reactivity. While such effects are well understood in homogeneous processes, little is known about their behavior in confined interfacial systems, where molecular crowding may play an important role. We have observed that immobilizing organic groups around catalytic nanoparticles leads to significant changes in the selectivity of reactions. For example the Ni-catalyzed hydrogenation of fatty acids gives mainly products of molecular fragmentation (hydrocracking) when the catalyst is loaded in nanopores; however if the pores are modified with amines the selectivity changes completely giving long chain hydrocarbons with very little fragmentation.(37) While the effect of amines in this example may be of steric or electronic nature, immobilized groups may also act by defining specific environments around the active site. In this sense, groups covalently bound to the surface can surround incoming reactant molecules in a solvent-like fashion. Thus, we can expect these surface groups to have the same type of effects on molecular guests as solvents do. For instance, several researchers have identified unusual phenomena arising from hydrophobic domains created by organic groups inside nanopores, however these effects have not been quantified and systematically investigated. Using solvatochromic probes we have been able to measure the local polarity within organofunctionalized nanopores and develop structure-property relationships that let us precisely tune the effective dielectric properties at liquid-solid interfaces.(52) We have also demonstrated that the local polarity inside functionalized nanopores is different from the bulk solvent that surrounds them. We can now use this understanding to control catalytic reactions and other solvent dependent processes such as drug delivery or biosensing.(23, 34) For example, we have shown how the selective TEMPO-catalyzed aerobic oxidation of primary alcohols can be precisely tuned by adjusting the polarity of the local environment.(52) Importantly, while the reaction requires an organic solvent to maximize yields, this approach allows performing the whole process in water, using a suspension of particles with low-polarity pores as nanoreactors.  These systems aim to emulate the environmental control achieved in the active pockets of enzymes.

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