Rhodium-Based Metal–Organic Polyhedra Assemblies for Selective CO₂ Photoreduction

Abstract

Heterogenization of molecular catalysts via their immobilization within extended structures often results in a lowering of their catalytic properties due to a change in their coordination sphere. Metal-organic polyhedra (MOPs) are an emerging class of well-defined hybrid compounds with a high number of accessible metal sites organised around an inner cavity, making them appealing candidates for catalytic applications. Here we demonstrate a design strategy that enhances the catalytic properties of dirhodium paddlewheels heterogenized within a MOP (Rh-MOP) and its three-dimensional assembled supramolecular structures, which proved to be very efficient catalysts for the selective photochemical reduction of carbon dioxide to formic acid. Surprisingly, the catalytic activity per Rh atom is higher in the supramolecular structures than in its molecular sub-unit Rh-MOP or in Rh-MOF, and yields turnover frequencies of up to 60 h -1 and production rates of approx. 76 mmole formic acid per gram of catalyst per hour, unprecedented in heterogeneous photocatalysis, demonstrating the high potential of MOPs as catalysts for the photoreduction of CO 2 . This opens new perspectives for the electronic design of discrete molecular architectures with accessible metal sites for the production of solar fuels. The enhanced catalytic activity is investigated by XPS spectroscopy and electrochemical characterization, showing that self-assembly into supramolecular polymers increases the electron density on the active site, making the overall reaction thermodynamically more favourable. The catalyst can be recycled without loss of activity and with no change of its molecular structure as shown by pair distribution function analysis.

Heterogenized Molecular Rhodium Phosphine Catalyst within Metal-Organic Framework for Ethylene Hydroformylation

“…This technique has been already shown to be applicable even to amorphous MOF materials 52 and has been recently successfully used to assess the structural integrity of MOFsupported polyoxometalate catalysts 53 as well as Rh active sites in MOP-based materials. 54 It has also been recently used to detect highly loaded single site metals trapped by catechol-benzoate ligands into MOF-808 (approx. 0.3-0.4 Fe or Cu per Zr).…”

Macromolecular Rhenium–Ruthenium Complexes for Photocatalytic CO2 Conversion: From Catalytic Lewis Pair Polymerization to Well-Defined Poly(vinyl bipyridine)–Metal Complexes

“…In this context, common turnover numbers for the photocatalytic reduction of CO 2 with supramolecular rhenium–ruthenium catalysts and BIH as the sacrificial electron donor range from values between 100 and the mid-1000s. , Despite the apparent contradiction to the previously stated beneficial effect of Re­(I)–Re­(I) interactions, the trend of PVBpy 5/95 exhibiting the highest turnover numbers can be most likely attributed to the stabilizing effect of Ru­(II) photosensitizing units in the catalyst system . In this respect, high concentrations of ruthenium moieties in photocatalytic systems protect the catalytic sites as well as other Ru­(II) centers from deactivation of the OERS by overexcitation, as will be discussed in more detail later. ,, Additionally, PVBpy 5/95 exhibits the highest Ru­(II)/Re­(I) ratio (see Table ), which increases the probability of a close proximity between a catalytic reduction site and photosensitizers. All catalysts have long lifetimes of over seven days, and in the case of PVBpy 5/95 , the catalyst showed activity even after 30 days of irradiation (see the Supporting information, Figure S25).…”

“…38 In this respect, high concentrations of ruthenium moieties in photocatalytic systems protect the catalytic sites as well as other Ru(II) centers from deactivation of the OERS by overexcitation, as will be discussed in more detail later. 3,38,39 Additionally, PVBpy 5/95 exhibits the highest Ru(II)/Re(I) ratio (see Table 2), which increases the probability of a close proximity between a catalytic reduction site and photosensitizers. All catalysts have long lifetimes of over seven days, and in the case of PVBpy 5/95 , the catalyst showed activity even after 30 days of irradiation (see the Supporting information, Figure S25).…”

Unravelling the molecular structure and confinement of an or-ganometallic catalyst heterogenized within amorphous porous polymers

“…To visualize the contributions from the molecular complex, we applied a differential PDF (d-PDF) approach for the analysis of our materials. 53 While the application of PDF and d-PDF has been extended to study the molecular structure of adsorbed species in crystalline MOFs and COFs, 30,[54][55][56][57] and to assemblies of metal−organic polyhedra, 10 to the best of our knowledge no study concerning the molecular structure of an organometallic complex within an amorphous polymer has been reported yet.…”

“…In fact, the heterogenization of molecular complexes within porous supports allows for reactivities, productivities and selectivities inaccessible in the homogeneous phase due to the controlled confinement of the active site into a three-dimensional porous framework. [1][2][3][4][5][6][7][8][9][10] The knowledge acquired about the homogeneous molecular catalysts structure-activity relationship is now so pre-cise that computational chemistry can be used to predict reactivity and selectivity. 11,12 Unfortunately, this level of maturity has not yet been reached for heterogenized molecular catalysts.…”