General Methods for Molecule Immobilization

There are several approaches to immobilizing transition-metal complexes on solid supports. One way to present them is to group on the type of interaction between the support and the molecular species to be grafted. In this context, three different groups of support-molecule interactions can be considered [I-10]  

The most important advantage of this method is that molecular species are bonded via chemical bonds and experience almost no leaching from the support, as far as the metal complex ligation and all the bonds are adequately stable in catalytic media. Nevertheless, an important drawback of this method is the large preparative effort, since it involves a multistep procedure that usually includes not only the functionalization of the ligands coordinated to the metal, but also the grafting of spacers onto the support. Furthermore, the covalent bonding of metal complexes, directly or via spacers, can alter, to different extents, the electronic density within the metal, which in turn, can alter the catalytic performance of the catalysts in a way that may be difficult to foresee [3-10]. 

The other two methodologies for immobilization of metal complexes, physical adsorption and electrostatic interaction, belong to the group of noncovalent interactions between the support and the metal complex [1-10]. In the former case it includes n-n interactions, van der Waals and hydrogen bonds, and hydrophobic-hydrophilic interactions between the support and the complex. In the second method, there is an electrostatic interaction between the support and the complex, and consequently, there must be charges of opposite signals between the support and complex. The noncovalent interactions can also occur directly between the support and the complex or via the use of spacers in the latter case the interaction between the spacer and the complex can be of different in nature, covalent or noncovalent. 

The great advantage of noncovalent immobilization methodologies is the easy preparation of these systems compared to covalent bonding, but their sensitivity to solvents is an important weakness which can cause extensive leaching of the active phase by simple manipulation of the experimental conditions in preparation of the immobilized complexes or during catalytic reactions. 

Encapsulation implies physical entrapment of the metal complex within the pores of the support, and as a starting point it is supposed that no other interactions between the support and the metal complex exist besides the physical confinement 

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