Valeria van Axel Castelli

Molecular Recognition of Carbonyl Compounds by Uranyl-salophen Based Neutral Receptors Driven by Van Der Waals Forces

The complexation of the salophen-uranyl metallocleft 2 and of its half-cleft analogue 3 with enones and other carbonyl compounds was assessed in chloroform by UV-Vis titration and, occasionally, by FT-IR measurements. Complexes with receptors 2 and 3 are in all cases more stable than those with the control unsubstituted uranyl-salophen 1 , showing that in addition to the primary binding force provided by coordination of the carbonyl oxygen to the uranium, a significant driving force for complexation, typically in the range of 2-3 kcal/mol, results from van der Waals interactions of the guest with the aromatic walls. Replacement of the phenyl group in 3 with larger aromatic residues to give 4 and 5 , led to enhanced complex stabilities, due to more extended contact surfaces between host and guest.

THE URANYL UNIT AS ELECTROPHILIC CATALYST OF ACYL TRANSFER REACTIONS

The kinetics of the base induced alcoholysis of a series of metallomacrocycles in which the salophen-uranyl unit is connected by polymethylene bridges of variable length to a pyrogallol monoacetate unit have been investigated in the MeO−Me4N+/MeOH and EtO−Me4N+/EtOH base-solvent systems. Analysis of rate data collected over a wide range of base concentrations, coupled with equilibrium measurements for complex formation between the alkoxide ion and the uranyl centre, showed that the ester cleavage processes are the results of two competing pathways involving alkoxide ion attack on uncomplexed and alkoxide-complexed substrates. Rate data related to the former pathway show that the proximal uranyl centre activates the ester carbonyl towards nucleophilic addition, thus behaving as an immobilized Lewis acid catalyst. The size of the acceleration critically depends on both solvent nature and distance of the ester function from the metal electrophile.

Catalysis of the addition of benzenethiol to 2-cyclohexen-1-ones by uranyl-salophen complexes: a catalytic metallocleft with high substrate specificity

The base induced addition of benzenethiol to 2-cyclohexen-1-one and its 4,4-, 5,5- and 6,6-dimethyl derivatives is catalysed by a salophen-uranyl based metallocleft 2 in chloroform solution with high turnover efficiency and low product inhibition. Analysis of rate data coupled with equilibrium measurements for complexation of the catalyst with the enone reactants and addition products shows that the catalytic mechanism involves the three main steps typical of single-substrate enzymatic processes, namely substrate binding and recognition, transformation of the bound substrate, and release of the reaction product. Unlike the reference salophen-uranyl 1, catalyst 2 is endowed with a structured binding site responsible for a high degree of substrate specificity among the investigated enones, due to recognition of their shape and size.