Two Cu-Based Phosphonate Metal–Organic Frameworks as Efficient Water-Splitting Photocatalysts

Abstract

Two novel three-dimensional metal–organic frameworks (MOFs) based on the photoactive pyrene tetraphosphonate ligand and copper (denoted as IEF-8 and IEF-9) have been hydrothermally synthesized and fully characterized (XRD, FTIR, TGA, SEM, XPS, etc.). Their crystal structures were unveiled by single-crystal X-ray diffraction. Remarkably, these materials exhibit coordinatively unsaturated copper (II) sites, free −PO3H2 and −PO3H acidic groups, and good thermal and chemical stability. Further, their optoelectronic characterization evidenced a photoresponse suitable for photocatalysis. In this sense, the photocatalytic activity of pyrene phosphonate MOFs was evaluated for the first time for the challenging hydrogen evolution reaction. In particular, IEF-8 exhibited a catalytic efficiency higher than that of the benchmarked Ti carboxylate photocatalyst MIL-125(Ti)–NH2, producing 1800 μmol·g–1 after 22 h under UV–vis irradiation in the absence of any co-catalyst. Furthermore, this material presented good reusability (at least up to 4 cycles), preserving its activity and structural integrity.

Intrinsic Specific Activity Enhancement for Bifunctional Electrocatalytic Activity toward Oxygen and Hydrogen Evolution Reactions via Structural Modification of Nickel Organophosphonates

“…Moreover, the 3D framework of compound II is well stabilized via intermolecular O–H···O and C–H···O hydrogen-bonding interactions (Table S4 and Figure S4, Supporting Information). From the literature survey, it is found that similar one-dimensional metal–phosphonate chains are often observed with various transition metals, especially the metal centers with octahedral coordination environment. ,, Such one-dimensional Fe, Co, or Ni-phosphonate chains are stabilized in the presence of a templated aliphatic amine. When the metal center is changed from a 3d n -element to 3d 10 -Zn, the coordination environment becomes tetrahedral with three-dimensional zinc phosphonate structures. ,,,, Higher dimensional Zn–phosphonate structures are stabilized with templated linear aliphatic amines or various pyrazine derivatives .…”

“…From the literature survey, it is found that similar one-dimensional metal–phosphonate chains are often observed with various transition metals, especially the metal centers with octahedral coordination environment. ,, Such one-dimensional Fe, Co, or Ni-phosphonate chains are stabilized in the presence of a templated aliphatic amine. When the metal center is changed from a 3d n -element to 3d 10 -Zn, the coordination environment becomes tetrahedral with three-dimensional zinc phosphonate structures. ,,,, Higher dimensional Zn–phosphonate structures are stabilized with templated linear aliphatic amines or various pyrazine derivatives . If the amine molecules are modified from aliphatic to aromatic amines, the nitrogen atoms intend to bind with the metal centers to improve the dimensionality .…”

“…Among the inorganic–organic hybrid family, metal organophosphonate has emerged as a flourishing class of hybrid framework with organic and inorganic moieties altogether. − Remarkably, organophosphonic acid plays an important role to form highly stable hybrid materials as it can possess the promising capability to bind various metal centers due to a plenty of flexible coordinating oxygen sites. ,,, Organophosphonate ligand is also capable to show different valence states from monovalent to pentavalent at different pH which can make a broad range of coordination networks at different pH. − A great diversity of metal phosphonates can also be obtained by using variable inorganic and organic moieties as secondary ligands. − Hence, the dimensionality as well as porosity can be enhanced by making amendments to the reaction conditions such as pH, reaction time, solvent, structure directing agent, and supportive ligands.…”

“…In today’s scenario, dual-ligand approach has been taken into consideration for the synthesis of novel hybrid frameworks. − In general, metal phosphonates incorporated with different types of auxiliary ligands have given rise to distinct novel architectures of the hybrid frameworks. − Earlier, various N-containing aliphatic or aromatic ligands were utilized to form metal phosphonate hybrid structures. Such auxiliary ligands in hybrid framework structures can act as templates or structure-directing agents, and they can also coordinate with different metal centers to generate various molecular structures to 3D hybrid metal phosphonate frameworks. − ,− Among various N-containing auxiliary ligands, pyrazine-based metal phosphonate hybrid structures are very less explored until date. − The diverse alignment of the pyrazine ligand, which often binds with different metal centers, as well as the flexible coordination of phosphonate moieties not only provides new metal phosphonate architectures but also offers a versatile platform for designing active electrocatalysts to promote water-splitting reactions. However, it is evident that most of the metal phosphonate structures possess marginally less porosity compared to CPs and MOFs due to the incorporation of template agents within their pore channels. ,, Despite the low porosity of hybrid metal phosphonate frameworks, the specific role of the incorporated auxiliary ligands in metal phosphonate frameworks toward energy-related applications has not been examined so far, though a very few metal phosphonate structures have been introduced as efficient electrocatalysts for water-splitting technologies. ,,, …”

Copper Phosphinate Complexes as Molecular Precursors for Ethanol Dehydrogenation Catalysts

“…Applying molecular precursors to prepare structure-controlled catalysts with homogeneously dispersed catalytic species on the support is a well-known and widely used method. − However, there is a lack of information on applying phosphorus-containing metal complexes (phosphates, phosphonates, and phosphinates) as precursors to nonoxidative ethanol dehydrogenation catalysts. Numerous studies have been conducted on copper phosphates − and phosphonates. − However, the application of phosphinate ligands for the formation of copper complexes has seen limited exploration, with a modest number of copper complexes based on phosphinate ligands present to date. − A small amount of the copper complexes with macrocyclic ligands possessing pendant phosphinate groups are also being studied for potential medical applications. − The formation of insoluble polymeric compounds is commonplace in such systems, which is a factor that limits their potential application as molecular precursors. Typical strategies employed for the isolation of molecular species include the utilization of bulky ligands, ,…”