Metal–Organic Framework Nodes Support Single-Site Magnesium–Alkyl Catalysts for Hydroboration and Hydroamination Reactions

Here we present the first example of a single-site main group catalyst stabilized by a metal–organic framework (MOF) for organic transformations. The straightforward metalation of the secondary building units of a Zr-MOF with Me₂Mg affords a highly active and reusable solid catalyst for hydroboration of carbonyls and imines and for hydroamination of aminopentenes. Impressively, the Mg-functionalized MOF displayed very high turnover numbers of up to 8.4 × 10⁴ for ketone hydroboration and could be reused more than 10 times. MOFs can thus be used to develop novel main group solid catalysts for sustainable chemical synthesis

Trivalent Zirconium and Hafnium Metal–Organic Frameworks for Catalytic 1,4-Dearomative Additions of Pyridines and Quinolines

We report the quantitative conversion of [M^IV₆(μ₃-O)₄(μ₃-OH)₄Cl₁₂]^6– nodes in the MCl₂-BTC metal–organic framework into the [M^III₆(μ₃-O)₄(μ₃-ONa)₄H₆]^6– nodes in M^IIIH-BTC (M = Zr, Hf; BTC is 1,3,5-benzenetricarboxylate) via bimetallic reductive elimination of H₂ from putative [M^IV₆(μ₃-O)₄(μ₃-OH)₄H₁₂]^6– nodes. The coordinatively unsaturated M^IIIH centers in M^IIIH-BTC are highly active and selective for 1,4-dearomative hydroboration and hydrosilylation of pyridines and quinolines. This work demonstrated the potential of secondary building unit transformation in generating electronically unique and homogeneously inaccessible single-site solid catalysts for organic synthesis

Cerium-Hydride Secondary Building Units in a Porous Metal–Organic Framework for Catalytic Hydroboration and Hydrophosphination

We report the stepwise, quantitative transformation of Ce^IV₆(μ₃-O)₄(μ₃-OH)₄­(OH)₆(OH₂)₆ nodes in a new Ce-BTC (BTC = trimesic acid) metal–organic framework (MOF) into the first Ce^III₆(μ₃-O)₄(μ₃-OLi)₄(H)₆(THF)₆Li₆ metal-hydride nodes that effectively catalyze hydroboration and hydrophosphination reactions. CeH-BTC displays low steric hindrance and electron density compared to homogeneous organolanthanide catalysts, which likely accounts for the unique 1,4-regioselectivity for the hydroboration of pyridine derivatives. MOF nodes can thus be directly transformed into novel single-site solid catalysts without homogeneous counterparts for sustainable chemical synthesis

Confinement of Ultrasmall Cu/ZnO_<i>x</i> Nanoparticles in Metal–Organic Frameworks for Selective Methanol Synthesis from Catalytic Hydrogenation of CO₂

The interfaces of Cu/ZnO and Cu/ZrO₂ play vital roles in the hydrogenation of CO₂ to methanol by these composite catalysts. Surface structural reorganization and particle growth during catalysis deleteriously reduce these active interfaces, diminishing both catalytic activities and MeOH selectivities. Here we report the use of preassembled bpy and Zr₆(μ₃-O)₄(μ₃-OH)₄ sites in UiO-bpy metal–organic frameworks (MOFs) to anchor ultrasmall Cu/ZnO_<i>x</i> nanoparticles, thus preventing the agglomeration of Cu NPs and phase separation between Cu and ZnO_<i>x</i> in MOF-cavity-confined Cu/ZnO_<i>x</i> nanoparticles. The resultant Cu/ZnO_<i>x</i>@MOF catalysts show very high activity with a space–time yield of up to 2.59 g_MeOH kg_Cu^–1 h^–1, 100% selectivity for CO₂ hydrogenation to methanol, and high stability over 100 h. These new types of strong metal–support interactions between metallic nanoparticles and organic chelates/metal-oxo clusters offer new opportunities in fine-tuning catalytic activities and selectivities of metal nanoparticles@MOFs

Tuning Lewis Acidity of Metal–Organic Frameworks via Perfluorination of Bridging Ligands: Spectroscopic, Theoretical, and Catalytic Studies

The Lewis acidity of metal–organic frameworks (MOFs) has attracted much research interest in recent years. We report here the development of two quantitative methods for determining the Lewis acidity of MOFsbased on electron paramagnetic resonance (EPR) spectroscopy of MOF-bound superoxide (O₂^•–) and fluorescence spectroscopy of MOF-bound <i>N</i>-methylacridone (NMA)and a simple strategy that significantly enhances MOF Lewis acidity through ligand perfluorination. Two new perfluorinated MOFs, Zr₆-fBDC and Zr₆-fBPDC, where H₂fBDC is 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid and H₂fBPDC is 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxylic acid, were shown to be significantly more Lewis acidic than nonsubstituted UiO-66 and UiO-67 as well as the nitrated MOFs Zr₆-BDC-NO₂ and Zr₆-BPDC-(NO₂)₂. Zr₆-fBDC was shown to be a highly active single-site solid Lewis acid catalyst for Diels–Alder and arene C–H iodination reactions. Thus, this work establishes the important role of ligand perfluorination in enhancing MOF Lewis acidity and the potential of designing highly Lewis acidic MOFs for fine chemical synthesis

Tuning Lewis Acidity of Metal–Organic Frameworks via Perfluorination of Bridging Ligands: Spectroscopic, Theoretical, and Catalytic Studies

The Lewis acidity of metal–organic frameworks (MOFs) has attracted much research interest in recent years. We report here the development of two quantitative methods for determining the Lewis acidity of MOFsbased on electron paramagnetic resonance (EPR) spectroscopy of MOF-bound superoxide (O₂^•–) and fluorescence spectroscopy of MOF-bound <i>N</i>-methylacridone (NMA)and a simple strategy that significantly enhances MOF Lewis acidity through ligand perfluorination. Two new perfluorinated MOFs, Zr₆-fBDC and Zr₆-fBPDC, where H₂fBDC is 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid and H₂fBPDC is 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxylic acid, were shown to be significantly more Lewis acidic than nonsubstituted UiO-66 and UiO-67 as well as the nitrated MOFs Zr₆-BDC-NO₂ and Zr₆-BPDC-(NO₂)₂. Zr₆-fBDC was shown to be a highly active single-site solid Lewis acid catalyst for Diels–Alder and arene C–H iodination reactions. Thus, this work establishes the important role of ligand perfluorination in enhancing MOF Lewis acidity and the potential of designing highly Lewis acidic MOFs for fine chemical synthesis

Transformation of Metal–Organic Framework Secondary Building Units into Hexanuclear Zr-Alkyl Catalysts for Ethylene Polymerization

We report the stepwise and quantitative transformation of the Zr₆(μ₃-O)₄(μ₃-OH)₄(HCO₂)₆ nodes in Zr-BTC (MOF-808) to the [Zr₆(μ₃-O)₄(μ₃-OH)₄Cl₁₂]^6– nodes in ZrCl₂-BTC, and then to the organometallic [Zr₆(μ₃-O)₄(μ₃-OLi)₄R₁₂]^6– nodes in ZrR₂-BTC (R = CH₂SiMe₃ or Me). Activation of ZrCl₂-BTC with MMAO-12 generates ZrMe-BTC, which is an efficient catalyst for ethylene polymerization. ZrMe-BTC displays unusual electronic and steric properties compared to homogeneous Zr catalysts, possesses multimetallic active sites, and produces high-molecular-weight linear polyethylene. Metal–organic framework nodes can thus be directly transformed into novel single-site solid organometallic catalysts without homogeneous analogs for polymerization reactions

Titanium(III)-Oxo Clusters in a Metal–Organic Framework Support Single-Site Co(II)-Hydride Catalysts for Arene Hydrogenation

Titania (TiO₂) is widely used in the chemical industry as an efficacious catalyst support, benefiting from its unique strong metal–support interaction. Many proposals have been made to rationalize this effect at the macroscopic level, yet the underlying molecular mechanism is not understood due to the presence of multiple catalytic species on the TiO₂ surface. This challenge can be addressed with metal–organic frameworks (MOFs) featuring well-defined metal oxo/hydroxo clusters for supporting single-site catalysts. Herein we report that the Ti₈(μ₂-O)₈(μ₂-OH)₄ node of the Ti-BDC MOF (MIL-125) provides a single-site model of the classical TiO₂ support to enable Co^II-hydride-catalyzed arene hydrogenation. The catalytic activity of the supported Co^II-hydride is strongly dependent on the reduction of the Ti-oxo cluster, definitively proving the pivotal role of Ti^III in the performance of the supported catalyst. This work thus provides a molecularly precise model of Ti-oxo clusters for understating the strong metal–support interaction of TiO₂-supported heterogeneous catalysts

Electron Crystallography Reveals Atomic Structures of Metal–Organic Nanoplates with M₁₂(μ₃‑O)₈(μ₃‑OH)₈(μ₂‑OH)₆ (M = Zr, Hf) Secondary Building Units

Nanoscale metal–organic frameworks (nMOFs) have shown tremendous potential in cancer therapy and biomedical imaging. However, their small dimensions present a significant challenge in structure determination by single-crystal X-ray crystallography. We report here the structural determination of nMOFs by rotation electron diffraction (RED). Two isostructural Zr- and Hf-based nMOFs with linear biphenyldicarboxylate (BPDC) or bipyridinedicarboxylate (BPYDC) linkers are stable under intense electron beams to allow the collection of high-quality RED data, which reveal a MOF structure with M₁₂(μ₃-O)₈(μ₃-OH)₈(μ₂-OH)₆ (M = Zr, Hf) secondary building units (SBUs). The nMOF structures differ significantly from their UiO bulk counterparts with M₆(μ₃-O)₄(μ₃-OH)₄ SBUs and provide the foundation for clarifying the structures of a series of previously reported nMOFs with significant potential in cancer therapy and biological imaging. Our work clearly demonstrates the power of RED in determining nMOF structures and elucidating the formation mechanism of distinct nMOF morphologies

Single-Site Cobalt Catalysts at New Zr₈(μ₂‑O)₈(μ₂‑OH)₄ Metal-Organic Framework Nodes for Highly Active Hydrogenation of Alkenes, Imines, Carbonyls, and Heterocycles

We report here the synthesis of robust and porous metal–organic frameworks (MOFs), M-MTBC (M = Zr or Hf), constructed from the tetrahedral linker methane-tetrakis­(<i>p</i>-biphenylcarboxylate) (MTBC) and two types of secondary building units (SBUs): cubic M₈(μ₂-O)₈(μ₂-OH)₄ and octahedral M₆(μ₃-O)₄(μ₃-OH)₄. While the M₆-SBU is isostructural with the 12-connected octahedral SBUs of UiO-type MOFs, the M₈-SBU is composed of eight M^IV ions in a cubic fashion linked by eight μ₂-oxo and four μ₂-OH groups. The metalation of Zr-MTBC SBUs with CoCl₂, followed by treatment with NaBEt₃H, afforded highly active and reusable solid Zr-MTBC-CoH catalysts for the hydrogenation of alkenes, imines, carbonyls, and heterocycles. Zr-MTBC-CoH was impressively tolerant of a range of functional groups and displayed high activity in the hydrogenation of tri- and tetra-substituted alkenes with TON > 8000 for the hydrogenation of 2,3-dimethyl-2-butene. Our structural and spectroscopic studies show that site isolation of and open environments around the cobalt-hydride catalytic species at Zr₈-SBUs are responsible for high catalytic activity in the hydrogenation of a wide range of challenging substrates. MOFs thus provide a novel platform for discovering and studying new single-site base-metal solid catalysts with enormous potential for sustainable chemical synthesis