Phosphorus Ligand Imaging with Two‐Photon Fluorescence Spectroscopy: Towards Rational Catalyst Immobilization

Abstract

Catalyst recovery is an important topic in homogeneous catalysis, since product/catalyst separation is one of the main obstacles towards application of this type of catalysis. So far, several strategies for catalyst recycling have been explored, but a general strategy remains elusive. [1][2][3] A widely studied approach to facilitate catalyst/product separation is attachment of homogeneous catalysts to soluble or insoluble supports, which can consist of organic polymers [4][5][6][7] or dendrimers, [8,9] inorganic materials, [10,11] or hybrids thereof. Inorganic materials have proved to be particularly suited as solid supports for homogeneous catalysts because of their physical strength and chemical inertness, and many such immobilized catalyst systems have been reported. Common drawbacks, however, are the generally lower activity and selectivity compared to the homogeneous counterpart. Surprisingly, however, an in-depth investigation of the effect of the immobilization process on the performance of the catalyst appears to be lacking. To gain more insight into immobilization of transition metal catalysts we set out to design a method for the detection of bis-phosphine ligands on surfaces. The ligand studied acts as a fluorescence probe, and detection is achieved by two-photon excitation fluorescence microscopy. In principle, this allows the immobilization product to be imaged with high spatial resolution (down to single molecules). Here we report the findings of the first study on the immobilization process employing an intrinsically fluorescent ligand imaged on a submicrometer level, which indicated that precondensation of ligands takes place prior to immobilization under the standard immobilization conditions, a conclusion supported by analysis of the liquid phase. These results directly translate to simple procedures that do not have these precondensation problems. The resulting immobilized catalysts show superior performance in hydroformylation catalysis, and open the way toward rational catalyst immobilization.Two-photon excitation (TPE) fluorescence microscopy has shown astonishing potential, but application has been mainly restricted to imaging biological samples.[12] Twophoton absorption (TPA) is a process in which two photons are absorbed simultaneously. Such a process only occurs at a very high flux of photons by focusing a pulsed near-infrared laser, thus restricting the excitation to a very small focal volume, with no appreciable off-focal fluorescence. Generally, the two-photon selection rule yields low background fluorescence, and hence high contrast in the images can be produced. [12,13] As part of a broader effort to study the immobilization of transition metal catalysts, [14][15][16][17] we were curious whether catalyst immobilization could be studied by fluorescence imaging. For this purpose we envisioned the class of xanthene-based phosphorus ligands, also known as Xantphos, suitable for various reasons. Metal complexes of these ligands have outstanding catalytic properties in various catalytic ...

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An efficient, soluble, and recyclable multiwalled carbon nanotubes-supported TEMPO for oxidation of alcohols

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