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This concept paper aims to summarize recent research on the design and synthetic applications of metal-organic frameworks (MOFs)-based hydrogen atom transfer (HAT) catalysts. Functional MOFs have emerged as highly promising heterogeneous catalysts, leveraging their tunable, ordered structure and abundant active sites to enhance catalytic efficiency in various reactions. HAT, a versatile method for generating active open-shell radical species through selective hydrogen abstraction, has been well explored in homogeneous photocatalysis. In contrast, heterogeneous HAT catalysis remains less developed, albeit their great potential in industry. This paper will provide a timely overview of the advancements in MOFs-based HAT photocatalysis, classified according to catalytic centers including aromatic ketones, polyoxometalates, xanthene dyes, thiols, and alcohols, highlighting their design principles and practical applications in photocatalytic organic synthesis.

The selective difunctionalization of N-heterocyclic carbene (NHC) boranes with alkenes has been achieved via decatungstate and thiol synergistic catalysis. The catalytic system also allows stepwise trifunctionalization, leading to complex NHC boranes with three different functional groups which are challenging to prepare by other methods. The strong hydrogen-abstracting ability of the excited decatungstate enables the generation of boryl radicals from mono- and di-substituted boranes for realizing borane multifunctionalization. This proof-of-principle research provides a new chance for fabricating unsymmetrical boranes and developing boron-atom-economic synthesis.

Photoinduced borylation is becoming a fascinating and growing research field in synthetic chemistry. On the other hand, electron donor–acceptor (EDA) complex photoactivation has emerged as an alternative strategy to generate active open-shell species without the need for photosensitizers. Suitable organoboron compounds can serve as the electron donor or acceptor to form the corresponding electron donor–acceptor complexes, which then undergo photoinduced intracomplex single electron transfer (SET) to generate carbon- or boron-centered radicals for the construction of new C−B bonds. In this Concept paper, we review recent advances in photoinduced borylation via the EDA complex photoactivation strategy surveying the relevant literature until December 2022. © 2023 Wiley-VCH GmbH.

Photoinduced metal–organic framework (MOF) enabled heterogeneous thiol catalysis has been achieved for the first time. MOF Zr-TPDCS-1, consisting of Zr6-clusters and TPDCS linkers (TPDCS=3,3′′,5,5′′-tetramercapto[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylate), effectively catalyzed the borylation, silylation, phosphorylation, and thiolation of organic molecules. Upon irradiation, the fast electron transfer from TPDCS to Zr6-cluster is believed to facilitate the formation of the thiyl radical, a hydrogen atom transfer catalyst, which competently abstracts the hydrogen from borane, silane, phosphine, or thiol for generating the corresponding element radical to engender the chemical transformations. The elaborate control experiments evidenced the generation of thiyl radicals in MOF and illustrated a radical reaction pathway. The gram-scale reaction worked well, and the product was conveniently separated via centrifugation and vacuum with a turnover number (TON) of ≈3880, highlighting the practical application potential of heterogeneous thiyl-radical catalysis. © 2023 Wiley-VCH GmbH.

Air-stable amine- and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehydrogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Wheland-like radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of alpha-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Metal-organic frameworks (MOFs), bridging metal-containing second building units by organic ligands, have provided a versatile platform for the design of heterogeneous catalysis. In the last decade, various photo-responsible MOFs have been designed and prepared, exhibiting outperformed catalytic performance. Among them, MOFs featuring organic photosensitizers for photoinduced organic transformations remain less investigated, due probably to the large steric demands and low symmetry of typical organic dyes. In view of the promising potential of integrating versatile MOFs with organic photosensitizers, this Concept summarizes recent advances in this research area.

Ammonia gas, NH3, is a cheap and widely used industrial feedstock, which has received tremendous research interests in its functionalization. This work reports a breakthrough in catalytic selective cage B(3)-H amination of o-carboranes with NH3 via Ir-catalyzed B-H/N-H dehydrocoupling, offering convenient and efficient access to a series of 3-NH2-o-carborane derivatives in moderate to high isolated yields with a broad substrate scope. The employment of readily available NH3 gas as the aminating reagent with H2 as the sole byproduct endows the protocol economy, practicability, and environmental friendliness. A plausible reaction mechanism is proposed on the basis of control experiments.

We report the design of a bifunctional metal–organic layer (MOL), Hf12-Ru-Co, composed of [Ru(DBB)(bpy)2]2+ [DBB-Ru, DBB=4,4′-di(4-benzoato)-2,2′-bipyridine; bpy=2,2′-bipyridine] connecting ligand as a photosensitizer and Co(dmgH)2(PPA)Cl (PPA-Co, dmgH=dimethylglyoxime; PPA=4-pyridinepropionic acid) on the Hf12 secondary building unit (SBU) as a hydrogen-transfer catalyst. Hf12-Ru-Co efficiently catalyzed acceptorless dehydrogenation of indolines and tetrahydroquinolines to afford indoles and quinolones. We extended this strategy to prepare Hf12-Ru-Co-OTf MOL with a [Ru(DBB)(bpy)2]2+ photosensitizer and Hf12 SBU capped with triflate as strong Lewis acids and PPA-Co as a hydrogen transfer catalyst. With three synergistic active sites, Hf12-Ru-Co-OTf competently catalyzed dehydrogenative tandem transformations of indolines with alkenes or aldehydes to afford 3-alkylindoles and bisindolylmethanes with turnover numbers of up to 500 and 460, respectively, illustrating the potential use of MOLs in constructing novel multifunctional heterogeneous catalysts.

Here we report the design of a bifunctional metal–organic layer (MOL), Hf-EY-Fe, by bridging eosin Y (EY)-capped Hf6 secondary building units (SBUs) with Fe-TPY (TPY = 4′-(4-carboxyphenyl)[2,2′:6′,2′′-terpyridine]-5,5′′-dicarboxylate) ligands. With the organic dye EY as an efficient photosensitizer and TPY-Fe(OTf)2 as the catalytic center, Hf-EY-Fe efficiently catalyzes aminotrifluoromethylation, hydroxytrifluoromethylation, and chlorotrifluoromethylation of alkenes. Hf-EY-Fe also catalyzes the synthesis of CF3-substituted derivatives of large bioactive molecules such as rotenone, estrone, and adapalene with sizes of up to 2.2 nm. The proximity between EY and iron centers and their site isolation in Hf-EY-Fe enhance catalytic activity while inhibiting their mutual deactivation, leading to high turnover numbers of up to 1840 and good recyclability of the MOL catalyst.

We disclose herein the first example of iron-catalyzed regioselective intramolecular C–H/B–H dehydrogenative coupling, affording unprecedented C,B-substituted carborane-fused phenanthroline derivatives. The 8-aminoquinoline type auxiliaries not only serve as the bidentate directing groups but also ingeniously become the core part of the final products. The mechanistic hypothesis includes B–H activation, directing group rotation promoted by trans effect, C–H activation, and reductive elimination.

Tandem catalytic reactions improve atom- and step-economy over traditional synthesis but are limited by the incompatibility of the required catalysts. Herein, we report the design of bifunctional metal-organic layers (MOLs), HfOTf-Fe and HfOTf-Mn, consisting of triflate (OTf)-capped Hf6 secondary building units (SBUs) as strong Lewis acidic centers and metalated TPY ligands as metal active sites for tandem catalytic transformations using O2 and CO2 as coreactants. HfOTf-Fe effectively transforms hydrocarbons into cyanohydrins via tandem oxidation with O2 and silylcyanation whereas HfOTf-Mn converts styrenes into styrene carbonates via tandem epoxidation and CO2 insertion. Density functional theory calculations revealed the involvement of a high-spin FeIV (S = 2) center in the challenging oxidation of the sp3 C-H bond. This work highlights the potential of MOLs as a tunable platform to incorporate multiple catalysts for tandem transformations. ©

Ir-catalyzed cascade dehydrogenative CH/BH and BH/OH cross-coupling of carboranyl carboxylic acid with readily available benzoic acid has been achieved, leading to the facile synthesis of previously unavailable carborano-coumarin in a simple one-pot process. Two cage B—H, one aryl C—H and one O—H bonds are activated to construct efficiently new B—C and B—O bonds. The cascade cyclization can stop at the first B—H/C—H cross-coupling step by tuning the reaction conditions, resulting in a series of α-carboranyl benzoic acid and aryl carborane derivatives. Control experiments indicate that B—H/C—H dehydrocoupling proceeds preferentially over B—H/O—H dehydrocoupling, and both directing groups and oxidants are crucial for this reaction. An iridium(V) intermediate is proposed to be involved in the catalytic cycle.

Here we report the design of an enzyme-inspired metal–organic framework (MOF), 1-OTf-Ir, by installing strong Lewis acid and photoredox sites in engineered mesopores. Al-MOF (1), with mixed 2,2′-bipyridyl-5,5-dicarboxylate (dcbpy) and 1,4-benzenediacrylate (pdac) ligands, was oxidized with ozone and then triflated to generate strongly Lewis acidic Al-OTf sites in the mesopores, followed by the installation of [Ir(ppy)2(dcbpy)]+ (ppy = 2-phenylpyridine) sites to afford 1-OTf-Ir with both Lewis acid and photoredox sites. 1-OTf-Ir effectively catalyzed reductive cross-coupling of N-hydroxyphthalimide esters or aryl bromomethyl ketones with vinyl- or alkynyl-azaarenes to afford new azaarene derivatives. 1-OTf-Ir enabled catalytic synthesis of anticholinergic drugs Pheniramine and Chlorpheniramine.

Copper-catalyzed electrochemical selective cage B–H oxygenation of o-carboranes has been achieved for the first time. Under a constant electric current (4.0 mA) at room temperature, copper-catalyzed cross-coupling of carboranyl amides with lithium phenolates results in the formation of B(4,5)-diphenolated o-carboranes via direct B–H activation, whereas the use of lithium tert-butoxide affords B(4)-monooxygenated products. This reaction does not require any additional chemical oxidants and generates H2 and a lithium salt as byproducts. Control experiments indicated that a high-valent Cu(III) species is likely involved in the reaction process.

A Pd-catalyzed three-component carbonylative-annulation of 1-hydroxy-o-carborane, internal alkyne and carbon monoxide has been achieved via direct and regioselective cage B−H activation. A class of C,B-substituted carborano-coumarin derivatives with potential applications in pharmaceuticals were facilely prepared in moderate to high yields with excellent chemoselectivity and regioselectivity. A plausible reaction mechanism including CO insertion, electrophilic B−H metalation, alkyne insertion and reductive elimination was proposed.

We report the design of a new multifunctional metal–organic layer (MOL), Hf12-Ir-OTf, comprising triflate (OTf)-capped Hf12 secondary building units (SBUs) and photosensitizing Ir(DBB)[dF(CF3)ppy]2+ [DBB-Ir-F, DBB = 4,4′-di(4-benzoato)-2,2′-bipyridine; dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine] bridging ligands. Hf12-Ir-OTf effectively catalyzed dehydrogenative cross-couplings of heteroarenes with ethers, amines, and unactivated alkanes with turnover numbers of 930, 790, and 950, respectively. Hf12-Ir-OTf also competently catalyzed late-stage functionalization of bioactive and drug molecules such as caffeine, Fasudil, and Metyrapone. The superior catalytic performance of Hf12-Ir-OTf over a mixture of photoredox catalyst and stoichiometric amounts of Brønsted acids or substoichiometric amounts (20 mol %) of Lewis acids is attributed to the close proximity (1.2 nm) between photoredox and Lewis acid catalysts in Hf12-Ir-OTf, which not only facilitates the reaction between the carbon radical and the activated heteroarene but also accelerates the electron transfer from the nitrogen radical intermediate to the Ir(IV) species in the catalytic cycle.

An efficient Ir-catalyzed selective B–H/C–H dehydrocoupling between o-carboranes and (hetero)aryls has been achieved based on a mixed directing-group strategy, leading to a wide variety of carborane-(hetero)arene combinations. Control experiments indicate that aryl C–H activation proceeds preferentially over the B–H one. Thus, a plausible reaction pathway for dehydrogenative cross-coupling is proposed after the characterization of a key intermediate.

A traceless bidentate directing group guided copper catalyzed direct cage B(4,5)–H disulfenylation of o-carboranes has been achieved, leading to a series of B(4,5)–disulfenylated o-carborane derivatives in high yields with excellent regioselectivity. The in situ departure of the 8-aminoquinoline bidentate auxiliary circumvents additional processes for directing group removal, thus enhancing the atom-/step-economy.

A proof-of-principle study of cascade dehydrogenative cross-coupling of carboranyl carboxylic acid with readily available benzamide has been achieved, resulting in the facile synthesis of previously inaccessible carborano-isoquinolinone derivatives in a simple one-pot process, in which two cage B–H, one aryl C–H, and one N–H bond were sequentially activated to construct efficiently new B–C and B–N bonds, respectively. Under suitable reaction conditions, such cascade cyclization can be stopped at the first B–H/C–H cross-coupling step to give a series of α-carboranyl benzamides, suggesting the preferential occurrence of B–C cross-coupling over that of B–N. The carboxylic acid directing group plays a key role in the B–C cross-coupling step, which is then removed through in situ decarboxylation. The CV results combined with control experiments indicate that high-valent Ir(V)-species may be involved in the reaction pathways, which is crucial for such cascade dehydrogenative cross-coupling reactions. The isolation and structural identification of a key intermediate, its controlled transformations, and deuterium labeling experiments support a new Ir-nitrene-mediated amination for B–H/N–H dehydrocoupling.

Vertex-specific functionalization of carboranes has received considerable research interest, due to the valuable application of carborane derivatives in medicine, coordination/organometallic chemistry and materials. In comparison with a protic cage C–H bond, cage B–H is hydridic and generally less polar, with a bond dissociation energy of ca. 108 kcal mol−1. These features make B–H activation quite different from that of the C–H bond. In addition, selectivity among ten very similar BH vertices in o-carborane is challenging yet crucial to the effective construction of carborane-based functional molecules. To address these issues, a brand new strategy of cage B–H nucleophilic substitution has recently been presented and developed for straightforward and regioselective cage B–H functionalization. The regioselectivity can be controlled by the electronic/steric properties of the cage carbon substituents. This Frontier article highlights the recent advancement in the nucleophilic cage B–H substitution of carboranes.

Metal–organic layers (MOLs) have recently emerged as a novel class of molecular two-dimensional (2D) materials with significant potential for catalytic applications. Herein we report the design of a new multifunctional MOL, Hf12-Ir-Ni, by laterally linking Hf12 secondary building units (SBUs) with photosensitizing Ir(DBB)[dF(CF3)ppy]2+ [DBB-Ir-F, DBB = 4,4′-di(4-benzoato)-2,2′-bipyridine; dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine] bridging ligands and vertically terminating the SBUs with catalytic Ni(MBA)Cl2 [MBA = 2-(4′-methyl-[2,2′-bipyridin]-4-yl)acetate] capping agents. Hf12-Ir-Ni was synthesized in a bottom-up approach and characterized by TEM, AFM, PXRD, TGA, NMR, ICP-MS, UV–vis, and luminescence spectroscopy. The proximity between photosensitizing Ir centers and catalytic Ni centers (∼0.85 nm) in Hf12-Ir-Ni facilitates single electron transfer, leading to a 15-fold increase in photoredox reactivity. Hf12-Ir-Ni was highly effective in catalytic C–S, C–O, and C–C cross-coupling reactions with broad substrate scopes and turnover numbers of ∼4500, ∼1900, and ∼450, respectively.

A proof-of-concept example of catalytic regioselective cage B(8)–H functionalization of o-carboranes has been disclosed for the first time. Under the help of an acylamino directing group at cage B(3), a series of B(8)-arylated, B(4,7,8)-triarylated and B(4,7,8)-trifluorinated o-carborane derivatives were conveniently prepared. On the basis of isolation of a key intermediate, deuterium labeling experiments and DFT calculations, a reaction mechanism involving a high-valent palladium induced “cage-walking” from B(4) to B(8) vertex is proposed to account for the regioselective B(8)–H activation.

Carboranes, a class of carbon–boron molecular clusters, are often viewed as three-dimensional analogues of benzene. They are finding increasing applications as useful functional building blocks in materials science, medicine, organometallic/coordination chemistry and more. Thus, functionalization of carboranes has received considerable attention. In comparison with the weakly acidic cage C–H bonds that can be readily functionalized, selective cage B–H functionalization among ten chemically similar BH vertices in o-carboranes is very challenging. Only in the recent few years, considerable progress has been made in transition metal catalyzed vertex-specific BH functionalization. This review summarizes recent advances in this research area.