Organometallic catalysis within self-assembled supramolecular structures

Abstract

The initial aim of this thesis was to substitute traditional solvents characterized by high environmental impact, with more environmentally friendly and health compatible aqueous media. This approach is relevant to recent trends in green chemistry, especially when considering industrial scale productions. The transfer in water of processes normally performed in organic solvents is accomplished by means of solubilizing agents such as surfactants, that are cheap and extensively used in several industrial processes, first of all in the formulation of detergents. The use of surfactants is of great interest in terms of “greening up” chemical processes especially because the general current method to run aqueous homogeneous and biphasic catalytic reactions is to modify catalysts with water soluble tags to make them soluble in water. This involves energy, money and time consuming synthetic procedures. Moreover the modified catalyst can display different sterics and electronics compared to the original one that may not be favorable for its performance. The use of surfactant-based nano-aggregates (micelles) falls also within an innovative research field concerning not only the possibility of switching from organic to aqueous phase, but also the tuning of the chemical reaction selectivity exploiting the tridimensional scaffold built by micelles around hydrophobic catalysts and substrates. The initial objective was then widened including a comprehensive study on the effect of supramolecular self-assembled hosts, such as micelles, in reactions mediated by organometallic catalysts. The inclusion of organometallic complexes within nanometric supramolecular aggregates would allow a fine tuning of the selectivity on the basis of shape, dimensions and affinity of substrates with the host, similarly to the interaction between a substrate and the complex peptide backbone of an enzyme. At the beginning the attention was focused on the hydration of nitriles, in order to use the aqueous medium also as reactant and avoid the co-solvent approach. The catalysts used were a series of RuII complexes and the major goal was the development of a highly active system without modifying the catalyst structure and using milder conditions than the traditional ones. Subsequently the work continued with the application of micellar systems to the Baeyer-Villiger oxidation of cyclic ketones, extensively studied in the past in our lab in common organic solvents, but poorly studied in aqueous medium, except in its enzymatic version. This oxidation reaction presents both activity and selectivity issues: high activities are difficult to achieve especially for intrinsically less reactive cyclic six-members ring ketones, while selectivity is a general problem to overcome in oxidation. Micellar systems were tested in order to solve both challenges by virtue of the confinement of catalyst and substrates inside the supramolecular structure. The idea was expanded to supramolecular hosts combined with organometallic complexes. The use of a self-assembling capsule was evaluated during a six-months stage in the research group of prof. Joost N.H. Reek at the Van’t Hoff Institute of Molecular Sciences, University of Amsterdam. The capsule of choice was the C-undecylcalix[4]resorcinarene hexamer, for which no examples of organometallic catalyst encapsulation has been reported so far. The objective of this part of the work was to mimic enzyme behavior developing new catalytic entities capable to address general issues like product and substrate selectivity. The initial interest focused on the formation of a supramolecular catalyst, exploiting the large cavity of this capsule in which a metal complex could be encapsulated, followed by the extension of this new system to the hydration of alkynes as a model reaction. Water is vital for the formation of the nano-capsule and this drove the attention again to hydration reactions. Compatibility with the catalytic nature of the encapsulated organometallic complex was solved. Albeit further studies are required to optimize this unusual catalytic system and to eventually scale up the micellar processes, there are some fundamental concepts that have been proven not only to comply with environmental issues, but also towards the creation of high-performance catalysts with a new catalysis concept.

L'utilizzo di nano-aggregati a base di tensioattivi (micelle) implica non soltanto la possibilità di passare da mezzo organico a mezzo acquoso, ma anche la modulazione della selettività di una reazione sfruttando la struttura tridimensionale costruita dalla micella stessa attorno a catalizzatori e substrati idrofobi. Questa tesi include uno studio completo sull'effetto di 'hosts' supramolecolari auto-assemblanti, come le micelle, in reazioni mediate da catalizzatori organometallici. L'inclusione di complessi organometallici all'interno di sistemi micellari ha permesso di variare l'attività e la selettività (sulla base di forma, dimensione e affinità del substrato con l'host supramolecolare) di reazioni com el'idratazione di nitrili e l'ossidazione di Baeyer-Villiger di chetoni ciclici. L'idea è stata poi estesa ad una capsula supramolecolare per la quale non era mai stata riportata l'inclusione di complessi organometallici. E' stato così sviluppato un nuovo catalizzatore supramolecolare, incapsulando un complesso di oro all'interno di questo host. Successivamente questo sistema è stato testato nell'idratazione di alchini come reazione modello, osservando un'insolita selettività sia di prodotto sia di substrato. Nonostante siano richiesti ulteriori studi per ottimizzare questi sistemi catalitici innovativi, in questa tesi sono stati dimostrati alcuni concetti fondamentali che riguardano non soltanto la realizzazione di processi ecocompatibili, ma anche la creazione di catalizzatori altamente performanti nell'ottica di sviluppare una nuova visione della catalisi.