Postsynthetic Metalation of Bipyridyl-Containing Metal–Organic Frameworks for Highly Efficient Catalytic Organic Transformations

We have designed highly stable and recyclable single-site solid catalysts via postsynthetic metalation of the 2,2′-bipyridyl-derived metal–organic framework (MOF) of the UiO structure (bpy-UiO). The Ir-functionalized MOF (bpy-UiO-Ir) is a highly active catalyst for both borylation of aromatic C–H bonds using B₂(pin)₂ (pin = pinacolate) and <i>ortho</i>-silylation of benzylicsilyl ethers; the <i>ortho</i>-silylation activity of the bpy-UiO-Ir is at least 3 orders of magnitude higher than that of the homogeneous control. The Pd-functionalized MOF (bpy-UiO-Pd) catalyzes the dehydrogenation of substituted cyclohexenones to afford phenol derivatives with oxygen as the oxidant. Most impressively, the bpy-UiO-Ir was recycled and reused 20 times for the borylation reaction without loss of catalytic activity or MOF crystallinity. This work highlights the opportunity in designing highly stable and active catalysts based on MOFs containing nitrogen donor ligands for important organic transformations

Metal–Organic Frameworks Stabilize Solution-Inaccessible Cobalt Catalysts for Highly Efficient Broad-Scope Organic Transformations

New and active earth-abundant metal catalysts are critically needed to replace precious metal-based catalysts for sustainable production of commodity and fine chemicals. We report here the design of highly robust, active, and reusable cobalt-bipyridine- and cobalt-phenanthroline-based metal–organic framework (MOF) catalysts for alkene hydrogenation and hydroboration, aldehyde/ketone hydroboration, and arene C–H borylation. In alkene hydrogenation, the MOF catalysts tolerated a variety of functional groups and displayed unprecedentedly high turnover numbers of ∼2.5 × 10⁶ and turnover frequencies of ∼1.1 × 10⁵ h^–1. Structural, computational, and spectroscopic studies show that site isolation of the highly reactive (bpy)­Co­(THF)₂ species in the MOFs prevents intermolecular deactivation and stabilizes solution-inaccessible catalysts for broad-scope organic transformations. Computational, spectroscopic, and kinetic evidence further support a hitherto unknown (bpy^•–)­Co^I(THF)₂ ground state that coordinates to alkene and dihydrogen and then undergoing σ-complex-assisted metathesis to form (bpy)­Co­(alkyl)­(H). Reductive elimination of alkane followed by alkene binding completes the catalytic cycle. MOFs thus provide a novel platform for discovering new base-metal molecular catalysts and exhibit enormous potential in sustainable chemical catalysis

Bipyridine- and Phenanthroline-Based Metal–Organic Frameworks for Highly Efficient and Tandem Catalytic Organic Transformations via Directed C–H Activation

We report here the synthesis of a series of robust and porous bipyridyl- and phenanthryl-based metal–organic frameworks (MOFs) of UiO topology (BPV-MOF, mBPV-MOF, and mPT-MOF) and their postsynthetic metalation to afford highly active single-site solid catalysts. While BPV-MOF was constructed from only bipyridyl-functionalized dicarboxylate linker, both mBPV- and mPT-MOF were built with a mixture of bipyridyl- or phenanthryl-functionalized and unfunctionalized dicarboxylate linkers. The postsynthetic metalation of these MOFs with [Ir­(COD)­(OMe)]₂ provided Ir-functionalized MOFs (BPV-MOF-Ir, mBPV-MOF-Ir, and mPT-MOF-Ir), which are highly active catalysts for tandem hydrosilylation of aryl ketones and aldehydes followed by dehydrogenative <i>ortho</i>-silylation of benzylicsilyl ethers as well as C–H borylation of arenes using B₂pin₂. Both mBPV-MOF-Ir and mPT-MOF-Ir catalysts displayed superior activities compared to BPV-MOF-Ir due to the presence of larger open channels in the mixed-linker MOFs. Impressively, mBPV-MOF-Ir exhibited high TONs of up to 17000 for C–H borylation reactions and was recycled more than 15 times. The mPT-MOF-Ir system is also active in catalyzing tandem dehydrosilylation/dehydrogenative cyclization of <i>N</i>-methylbenzyl amines to azasilolanes in the absence of a hydrogen acceptor. Importantly, MOF-Ir catalysts are significantly more active (up to 95 times) and stable than their homogeneous counterparts for all three reactions, strongly supporting the beneficial effects of active site isolation within MOFs. This work illustrates the ability to increase MOF open channel sizes by using the mixed linker approach and shows the enormous potential of developing highly active and robust single-site solid catalysts based on MOFs containing nitrogen-donor ligands for important organic transformations

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

Mixed N‑Heterocyclic Carbene–Bis(oxazolinyl)borato Rhodium and Iridium Complexes in Photochemical and Thermal Oxidative Addition Reactions

In order to facilitate oxidative addition chemistry of <i>fac</i>-coordinated rhodium­(I) and iridium­(I) compounds, carbene–bis­(oxazolinyl)­phenylborate proligands have been synthesized and reacted with organometallic precursors. Two proligands, PhB­(Ox^Me2)₂(Im^<i>t</i>BuH) (H­[<b>1</b>]; Ox^Me2 = 4,4-dimethyl-2-oxazoline; Im^<i>t</i>BuH = 1-<i>tert</i>-butylimidazole) and PhB­(Ox^Me2)₂(Im^MesH) (H­[<b>2</b>]; Im^MesH = 1-mesitylimidazole), are deprotonated with potassium benzyl to generate K­[<b>1</b>] and K­[<b>2</b>], and these potassium compounds serve as reagents for the synthesis of a series of rhodium and iridium complexes. Cyclooctadiene and dicarbonyl compounds {PhB­(Ox^Me2)₂Im^<i>t</i>Bu}­Rh­(η⁴-C₈H₁₂) (<b>3</b>), {PhB­(Ox^Me2)₂Im^Mes}­Rh­(η⁴-C₈H₁₂) (<b>4</b>), {PhB­(Ox^Me2)₂Im^Mes}­Rh­(CO)₂ (<b>5</b>), {PhB­(Ox^Me2)₂Im^Mes}­Ir­(η⁴-C₈H₁₂) (<b>6</b>), and {PhB­(Ox^Me2)₂Im^Mes}­Ir­(CO)₂ (<b>7</b>) are synthesized along with To^MM­(η⁴-C₈H₁₂) (M = Rh (<b>8</b>); M = Ir (<b>9</b>); To^M = tris­(4,4-dimethyl-2-oxazolinyl)­phenylborate). The spectroscopic and structural properties and reactivity of this series of compounds show electronic and steric effects of substituents on the imidazole (<i>tert</i>-butyl vs mesityl), effects of replacing an oxazoline in To^M with a carbene donor, and the influence of the donor ligand (CO vs C₈H₁₂). The reactions of K­[<b>2</b>] and [M­(μ-Cl)­(η²-C₈H₁₄)₂]₂ (M = Rh, Ir) provide {κ⁴-PhB­(Ox^Me2)­₂Im^{Mes^′}CH₂}­Rh­(μ-H)­(μ-Cl)­Rh­(η²-C₈H₁₄)₂ (<b>10</b>) and {PhB­(Ox^Me2)₂Im^Mes}­IrH­(η³-C₈H₁₃) (<b>11</b>). In the former compound, a spontaneous oxidative addition of a mesityl <i>ortho</i>-methyl to give a mixed-valent dirhodium species is observed, while the iridium compound forms a monometallic allyl hydride. Photochemical reactions of dicarbonyl compounds <b>5</b> and <b>7</b> result in C–H bond oxidative addition providing the compounds {κ⁴-PhB­(Ox^Me2)₂Im^{Mes^′}CH₂}­RhH­(CO) (<b>12</b>) and {PhB­(Ox^Me2)₂Im^Mes}­IrH­(Ph)­CO (<b>13</b>). In <b>12</b>, oxidative addition results in cyclometalation of the mesityl <i>ortho</i>-methyl similar to <b>10</b>, whereas the iridium compound reacts with the benzene solvent to give a rare crystallographically characterized <i>cis</i>-[Ir]­(H)­(Ph) complex. Alternatively, the rhodium carbonyl <b>5</b> or iridium isocyanide {PhB­(Ox^Me2)₂Im^Mes}­Ir­(CO)­CN^<i>t</i>Bu (<b>15</b>) reacts with PhSiH₃ in the dark to form the silyl compound {PhB­(Ox^Me2)₂Im^Mes}­RhH­(SiH₂Ph)­CO (<b>14</b>) or {PhB­(Ox^Me2)₂Im^Mes}­IrH­(SiH₂Ph)­CN^<i>t</i>Bu (<b>17</b>). These examples demonstrate the enhanced thermal reactivity of {PhB­(Ox^Me2)₂Im^Mes}-supported iridium and rhodium carbonyl compounds in comparison to tris­(oxazolinyl)­borate, tris­(pyrazolyl)­borate, and cyclopentadienyl-supported compounds

Salicylaldimine-Based Metal–Organic Framework Enabling Highly Active Olefin Hydrogenation with Iron and Cobalt Catalysts

A robust and porous Zr metal–organic framework, sal-MOF, of UiO topology was synthesized using a salicylaldimine (sal)-derived dicarboxylate bridging ligand. Postsynthetic metalation of sal-MOF with iron­(II) or cobalt­(II) chloride followed by treatment with NaBEt₃H in THF resulted in Fe- and Co-functionalized MOFs (sal-M-MOF, M = Fe, Co) which are highly active solid catalysts for alkene hydrogenation. Impressively, sal-Fe-MOF displayed very high turnover numbers of up to 145000 and was recycled and reused more than 15 times. This work highlights the unique opportunity of developing MOF-based earth-abundant catalysts for sustainable chemical synthesis

Isoreticular Metal–Organic Frameworks Confined Mononuclear Ru-Hydrides Enable Highly Efficient Shape-Selective Hydrogenolysis of Polyolefins

Upcycling nonbiodegradable plastics such as polyolefins is paramount due to their ever-increasing demand and landfills after usage. Catalytic hydrogenolysis is highly appealing to convert polyolefins into targeted value-added products under mild reaction conditions compared with other methods, such as high-temperature incineration and pyrolysis. We have developed three isoreticular zirconium UiO-metal–organic frameworks (UiO-MOFs) node-supported ruthenium dihydrides (UiO-RuH2), which are efficient heterogeneous catalysts for hydrogenolysis of polyethylene at 200 °C, affording liquid hydrocarbons with a narrow distribution and excellent selectivity via shape-selective catalysis. UiO-66-RuH2 catalyzed hydrogenolysis of single-use low-density polyethylene (LDPE) produced a C12 centered narrow bell-shaped distribution of C8–C16 alkanes in >80% yield and 90% selectivity in the liquid phase. By tuning the pore sizes of the isoreticular UiO-RuH2 MOF catalysts, the distribution of the products could be systematically altered, affording different fuel-grade liquid hydrocarbons from LDPE in high yields. Our spectroscopic and theoretical studies and control experiments reveal that UiO-RuH2 catalysts enable highly efficient upcycling of plastic wastes under mild conditions owing to their unique combination of coordinatively unsaturated single-site Ru-active sites, uniform and tunable pores, well-defined porous structure, and superior stability. The kinetics and theoretical calculations also identify the C–C bond scission involving β-alkyl transfer as the turnover-limiting step

Highly Enantioselective Zirconium-Catalyzed Cyclization of Aminoalkenes

Aminoalkenes are catalytically cyclized in the presence of cyclopenta­dienyl­bis­(oxazolinyl)­borato group 4 complexes {PhB­(C₅H₄)­(Ox^R)₂}­M­(NMe₂)₂ (M = Ti, Zr, Hf; Ox^R = 4,4-dimethyl-2-oxazoline, 4<i>S</i>-isopropyl-5,5-dimethyl-2-oxazoline, 4<i>S</i>-<i>tert</i>-butyl-2-oxazoline) at room temperature and below, affording five-, six-, and seven-membered N-heterocyclic amines with enantiomeric excesses of >90% in many cases and up to 99%. Mechanistic investigations of this highly selective system employed synthetic tests, kinetics, and stereochemistry. Secondary aminopentene cyclizations require a primary amine (1–2 equiv vs catalyst). Aminoalkenes are unchanged in the presence of a zirconium monoamido complex {PhB­(C₅H₄)­(Ox^{4<i>S</i>‑<i>i</i>Pr,Me₂})₂}­Zr­(NMe₂)Cl or a cyclopenta­dienyl­mono­(oxazolinyl)­borato zirconium diamide {Ph₂B­(C₅H₄)­(Ox^{4<i>S</i>‑<i>i</i>Pr,Me₂})}­Zr­(NMe₂)₂. Plots of initial rate versus [substrate] show a rate dependence that evolves from first-order at low concentration to zero-order at high concentration, and this is consistent with a reversible substrate–catalyst interaction preceding an irreversible step. Primary kinetic isotope effects from substrate conversion measurements (<i>k</i>′_obs^(H)/<i>k</i>′_obs^(D) = 3.3 ± 0.3) and from initial rate analysis (<i>k</i>₂^(H)/<i>k</i>₂^(D) = 2.3 ± 0.4) indicate that a N–H bond is broken in the turnover-limiting and irreversible step of the catalytic cycle. Asymmetric hydroamination/cyclization of <i>N</i>-deutero-aminoalkenes provides products with higher optical purities than obtained with <i>N</i>-proteo-aminoalkenes. Transition state theory, applied to the rate constant <i>k</i>₂ that characterizes the irreversible step, provides activation parameters consistent with a highly organized transition state (Δ<i>S</i>^⧧ = −43(7) cal·mol^–1 K^–1) and a remarkably low enthalpic barrier (Δ<i>H</i>^⧧ = 6.7(2) kcal·mol^–1). A six-centered, concerted transition state for C–N and C–H bond formation and N–H bond cleavage involving two amidoalkene ligands is proposed as most consistent with the current data

Single-Site Cobalt Catalysts at New Zr₁₂(μ₃‑O)₈(μ₃‑OH)₈(μ₂‑OH)₆ Metal–Organic Framework Nodes for Highly Active Hydrogenation of Nitroarenes, Nitriles, and Isocyanides

We report here the synthesis of a robust and porous metal–organic framework (MOF), Zr₁₂-TPDC, constructed from triphenyl­dicarboxylic acid (H₂TPDC) and an unprecedented Zr₁₂ secondary building unit (SBU): Zr₁₂(μ₃-O)₈­(μ₃-OH)₈­(μ₂-OH)₆. The Zr₁₂-SBU can be viewed as an inorganic node dimerized from two commonly observed Zr₆ clusters via six μ₂-OH groups. The metalation of Zr₁₂-TPDC SBUs with CoCl₂ followed by treatment with NaBEt₃H afforded a highly active and reusable solid Zr₁₂-TPDC-Co catalyst for the hydrogenation of nitroarenes, nitriles, and isocyanides to corresponding amines with excellent activity and selectivity. This work highlights the opportunity in designing novel MOF-supported single-site solid catalysts by tuning the electronic and steric properties of the SBUs

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