Self‐Assembled Supramolecular Bidentate Ligands for Aqueous Organometallic Catalysis

Abstract

Supramolecular chemistry is one of the fastest growing areas of chemistry and greatly contributes to the development of numerous new innovative concepts. This is especially true in the field of organometallic catalysis where host-guest interactions have proved to be a powerful tool to elaborate new strategies aimed at improving the performances of catalytic entities.[1] Among them, the design of supramolecular bidentate ligands appeared as an elegant approach to stabilize organometallic complexes under catalytic conditions and constitutes a possible way to access a library of ligands.[2] For that purpose, chelating ligands with an appropriate bite angle [3] have been elaborated by the self-assembly of monophosphanes or monophosphites. These assemblies were found to be efficient as supramolecular bidentate ligands in numerous reactions.[4] However, all these reactions were performed in organic media that did not constitute an ecological alternative in the context of green chemistry. Our experience with cyclodextrins (CDs) and their use in aqueous solutions [5] led us to design a water-soluble supramolecular bidentate ligand starting from a well-chosen b-cyclodextrin/ sulfonated phosphane couple.[6] Our approach relies on the self-assembly of the 6 I -amino-6 I -deoxycyclomaltoheptaose (1) [7] and the disodium bis(3-sulfonatophenyl)(4-tert-butylphenyl)phosphane (2) [8] (Scheme 1). Indeed, we postulated that the inclusion of 2 in the cavity of 1 by the NH 2 -containing face would result in the formation of a rigid chelating bidentate ligand, with the nitrogen and the phosphorus atoms on the same side of the supramolecular edifice. Herein, we report the results of our investigations on the host-guest interaction between 1 and 2 and the ability of the resulting supramolecular complex to coordinate platinum complexes. The efficiency of the obtained organoplatinum complexes was demonstrated in a hydrogenation reaction of a water-soluble allylic alcohol.The formation of inclusion complexes between 1 and 2 was confirmed by NMR spectroscopy measurements. A 1:1 stoichiometry was revealed by the NMR continuous-variation method, and the association constant between 1 and 2 was found to be 40 000 m À1 at 25 8C (see Supporting Information). The geometry of the supramolecular entity was deduced from 2D T-ROESY NMR experiments with an equimolar mixture of 1 and 2. The absence of strong correlation between hydrogen atoms H A or H B and H3 and the presence of a correlation between H B and H6 unambiguously proved the inclusion of 2 by the primary face of 1. Moreover, cross-peaks between the tBu protons of 2 and the inner protons H3 and H5 of 1 confirmed the penetration of the aromatic cycle of 2 in the cavity of 1. Thus, the obtained supramolecular chelating complex reveals the expected structural features to coordinate a metal as the phosphorus and nitrogen atoms are spatially close.More interestingly, subsequent addition of K 2 [PtCl 4 ] (1 equiv) to the supramolecular complex 1·2 (1 equiv) gave three different complexes (3, 4, an...

Supramolecular catalysis. Part 1: non-covalent interactions as a tool for building and modifying homogeneous catalysts

“…Machut et al demonstrated that hydrophobic interactions between b-cyclodextrin derivatives and water-soluble phosphines enable the construction of self-assembled ligands in water. 198 A b-cyclodextrin in which one of the primary hydroxyl groups has been replaced by an amino substituent was used as the host (61, Fig. 17a).…”

Hydroformylation in Aqueous Biphasic Media Assisted by Molecular Receptors

“…The art of synthesizing efficient molecular receptors and phosphines has been of tremendous help in the understanding of the catalytic process and now opens fascinating opportunities in the area of biphasic aqueous catalysis. For example, the formation of inclusion complexes between modified CDs and appropriate water soluble phosphines can be advantageously used to design new water soluble self-assembled supramolecular bidentates ligands for aqueous organometallic catalysis [95,96]. …”

Chemically Modified Cyclodextrins: An Attractive Class of Supramolecular Hosts for the Development of Aqueous Biphasic Catalytic Processes

“…In this last case, the local bulkiness around the metallic centre of the obtained supramolecular adducts become too high and a steric decongestion by dissociation of the RAME--CD/phosphine inclusion complex from the metal occurs, leading to phosphine lowcoordinated complexes as schematically represented in Figure 8. The ability of phosphine 1 to form very stable inclusion complexes with chemically modified CDs has also been used to design water-soluble supramolecular bidentate ligand for aqueous organometallic catalysis [71]. Thus, the inclusion of phosphine 1 into the cavity of 6 I -amino-6 I -deoxycyclomaltoheptaose by the NH 2 -containing face resulted in the formation of a rigid chelating bidentate ligand, with the nitrogen and the phosphorus atoms on the same side of the supramolecular edifice.…”