Rational Design of Improved Ru Containing Fe-Based Metal-Organic Framework (MOF) Photoanode for Artificial Photosynthesis

Metal-organic frameworks (MOFs) of an MIL-101-supported iridium(III) complex as efficient photocatalysts in the three-component alkoxycyanomethylation of alkenes

Metal-organic frameworks (MOFs) exhibit intriguing physicochemical properties due to their manageable structure, abundant porosity, and uniform pore size, which provide ideal environments for photocatalysts to achieve highly efficient photocatalysis. In this work, fac-Ir(ppy)3 is directly anchored to MOFs of MIL-101 with different morphologies via Friedel-Crafts alkylation, affording various MIL-101-supported fac-Ir(ppy)3 without the molecular modification of fac-Ir(ppy)3. The as-fabricated photocatalysts possess high specific surface areas (785-962 m2 g-1), pore volumes (0.42-0.47 cc g-1) and uniform pore sizes (∼1.9 nm). The luminescence properties of anchored fac-Ir(ppy)3 including emission lifetime, band gap energy and quantum yield are enhanced by fabricating a hollow interior and double shell in the frameworks of MIL-101 through etching with acetic acid. In the visible light-induced three-component alkoxycyanomethylation of styrenes with bromoacetonitriles and methanol, comparable catalytic activities (66-90%) to homogeneous fac-Ir(ppy)3 (69-90%) are achieved at room temperature. Furthermore, owing to the good chemical and mechanical stability of the catalyst, no significant decrease in yield (<2%) is observed over ten catalytic cycles. Overall, this study provides a mass/charge transfer-enhanced platform for supported photocatalysts to achieve highly efficient synthesis of fine chemicals in the field of heterogeneous photocatalysis.

Construction of efficient Pb(II) carboxylate catalysts for the oxygen and hydrogen evolution reactions

The development of cost-effective and efficient electrocatalysts can solve the problems associated with the production of energy via water-splitting reactions. In this work, we have focused on two lead-based coordination polymers (CPs), namely, {[Pb2(TPBN)(HBTC)2]2·2.5H2O}n (CP1) and {[Pb2(TPBN)(NDC)2]·H2O}n (CP2), that were synthesized by self-assembly method at room temperature in good yields. The two-dimensional structures of CP1 and CP2 were determined by single-crystal X-ray diffraction studies. Their phase purity and thermal stability were confirmed by powder X-ray diffraction and TGA analysis, respectively. In addition to this, Hirshfeld surface analysis of CP1 and CP2 revealed the key differences in their intermolecular interactions. Both CP1 and CP2 were employed for HER and OER. It has been found that the change in the carboxylate from BTC to NDC resulted in better electrocatalytic activity towards water-splitting reactions. This may be attributed to the presence of more π character in NDC compared to BTC, which makes the electron flow much easier for HER process. CP1 and CP2 showed overpotential values of -0.58 V and -0.55 V, respectively, in 1 M H2SO4 to reach a 10 mA cm-2 current density with Tafel slopes of 31 mV dec-1 and 25 mV dec-1, respectively. For the OER process, CP1 and CP2 exhibited overpotentials of 590 mV and 470 mV, respectively, in 1 M KOH at a current density of 50 mA cm-2 with Tafel slope values of 81 mV dec-1 and 56 mV dec-1, respectively. Turnover frequency (TOF) values of CP1 and CP2 were 1.05 s-1 and 3.21 s-1 for OER and 1.97 s-1 and 9.65 s-1 for HER, respectively. These results indicate that both CPs can act as highly efficient electrocatalysts for clean energy production.

Unexpected Photodriven Linker-to-Node Hole Transfer in a Zirconium-Based Metal-Organic Framework

Zr6(μ3-O)4(μ3-OH)4 node cores are indispensable building blocks for almost all zirconium-based metal-organic frameworks. Consistent with the insulating nature of zirconia, they are generally considered electronically inert. Contrasting this viewpoint, we present spectral measurements and calculations indicating that emission from photoexcited NU-601, a six-connected Zr-based MOF, comes from both linker-centric locally excited and linker-to-node charge-transfer (CT) states. The CT state originates from a hole transfer process enabled by favorable energy alignment of the HOMOs of the node and linker. This alignment can be manipulated by changing the pH of the medium, which alters the protonation state of multiple oxy groups on the Zr-node. Thus, the acid-base chemistry of the node has a direct effect on the photophysics of the MOF following linker-localized electronic excitation. These new findings open opportunities to understand and exploit, for energy conversion, unconventional mechanisms of exciton formation and transport in MOFs.

Intermolecular O-O Bond Formation between High-Valent Ru-oxo Species

Despite extensive research on water oxidation catalysts over the past few decades, the relationship between high-valent metal-oxo intermediates and the O-O bond formation pathway has not been well clarified. Our previous study showed that the high spin density on O in RuV=O is pivotal for the interaction of two metal-oxyl radical (I2M) pathways. In this study, we found that introducing an axially coordinating ligand, which is favorable for bimolecular coupling, into the Ru-pda catalyst can rearrange its geometry. The shifts in geometric orientation altered its O-O bond formation pathway from water nucleophilic attack (WNA) to I2M, resulting in a 70-fold increase in water oxidation activity. This implies that the I2M pathway is concurrently influenced by the spin density on oxo and the geometry organization of the catalysts. The observed mechanistic switch and theoretical studies provide insights into controlling reaction pathways for homogeneous water oxidation catalysis.