Supramolecular control of transition metal complexes in water by a hydrophobic cavity: a bio-inspired strategy

Abstract

Supramolecular chemistry in water is a very challenging research area. In biology, water is the universal solvent where transition metal ions play major roles in molecular recognition and catalysis. In enzymes, it participates in substrate binding and/or activation in the heart of a pocket defined by the folded protein. The association of a hydrophobic cavity with a transition metal ion is thus a very appealing strategy for controlling the metal ion properties in the very competitive water solvent. Various systems based on intrinsically water-soluble macrocyclic structures such as cyclodextrins, cucurbituryls, and metallo-cages have been reported. Others use calixarenes and resorcinarenes functionalized with hydrophilic substituents. One approach for connecting a metal complex to these cavities is to graft a ligand for metal ion binding at their edge. Early work with cyclodextrins has shown Michaelis-Menten like catalysis displaying enhanced kinetics and substrate-selectivity. Remarkable examples of regio- and stereo-selective transformation of substrates have been reported as well. Dynamic two-phase systems for transition metal catalysis have also been developed. They rely on either water-transfer of the metal complex through ligand embedment or synergistic coordination of a metal ion and substrate hosting. Another strategy consists in using metallo-cages, which provide a well-defined hydrophobic space, to stabilize metal complexes in water. When the cages can host simultaneously a substrate and a reactive metal complex, size- and regio-selective catalysis was obtained. Finally, construction of a polydentate coordination site closely interlocked with a calixarene or resorcinarene macrocycle has been shown to be a very fruitful strategy for obtaining metal complexes with remarkable hosting properties. For each of these systems, the synergism resulting from the biomimetic association of a hydrophobic cavity and a metal ion is discussed within the objective of developing new tools for either selective molecular recognition (with analytical perspectives) or performant catalysis, in water.

Cyclodextrin-based PNN supramolecular assemblies: a new class of pincer-type ligands for aqueous organometallic catalysis

“…14 Concurrently to the strategies developed in organic media, we demonstrated that the supramolecular approach was applicable to water-soluble bidentate ligands through hydrophobic effects using an appropriate cyclodextrin (CD)/phosphane combination. 15,16,17,18 More precisely, we showed that CDs functionalized on their primary face by nitrogen donor groups could partially include within their cavity a water-soluble sulfonated phosphane to form supramolecular PN ligands. The resulting self-assembled entities were capable of coordinating platinum and rhodium complexes in a κ 2 -P,N coordination mode, the CD simultaneously acting as a first-and secondsphere ligand (Scheme 1).…”

Expanding the boundary of biocatalysis: design and optimization of in vitro tandem catalytic reactions for biochemical production

“…It has been found that supramolecular assemblies could stabilize reactive metal complexes and increase their turn over numbers (Brown et al, 2011). Additionally, supramolecular complexes enable the catalytic activity of several transition metal catalysts in water by providing a bio-inspired hydrophobic cavity (Bistri et al, 2015; Kaphan et al, 2015; Pluth et al, 2007). Importantly, supramolecular cage prevents the direct interaction of transition metal with enzymes, thereby averting their mutual inactivation (Kohler et al, 2013; Wang et al, 2013).…”

Tandem Reactions Combining Biocatalysts and Chemical Catalysts for Asymmetric Synthesis

“…Creation of supramolecular hosts with hydrophobic cavities and a transition metal complex is a very appealing strategy for controlling the metal complex properties in competitive water solvents containing proteins and other cellular components [111]. Similarly, artificial metalloenzymes have been further developed to integrate transition-metal catalysts into cascade reactions with other biocatalysts and foster bioorthogonal transformations catalyzed by metal complexes in living cells.…”