9-Iodo-o-carborane

Carboranylphosphines: B9-Substituted Derivatives with Enhanced Reactivity for the Anchoring to Dendrimers

Several ortho-carboranes bearing a phenoxy or a phenylamino group in the B9 position were prepared employing various protection and deprotection strategies. Following established protocols, dendritic compounds were synthesized from a hexachlorocyclotriphosphazene or thiophosphoryl chloride core, and possible anchoring options for the B9-substituted ortho-carboranes were investigated experimentally and theoretically (DFT). Furthermore, 1- or 1,2-phosphanyl-substituted carborane derivatives were obtained. The resulting diethyl-, diisopropyl-, di-tert-butyl-, diphenyl- or diethoxyphosphines bearing a tunable ortho-carborane moiety are intriguing ligands for future applications in homogeneous catalysis or the medicinal sector.

Tuning Phosphine Oxide-Substituted ortho-Carboranes for Improved Biphasic Electrochemical UO22+ Capture and Release

We report the synthesis and characterization of a series of new, tunable 1,2-bis(diarylphosphine oxide)-ortho-carboranes, derivatives of our previously reported uranyl (UO22+) capture agent 1,2-(Ph2PO)2-1,2-C2B10H10 (POCb). The series features new cage-substituted variants of POCb, namely, 9-I-POCb (POCbI), 9,12-I2-POCb (POCbI2), 9,12-Me2-POCb (POCbMe2), 9,12-Et2-POCb (POCbEt2), and 4,5,7,8,9,10,11,12-Me8-POCb (POCbMe8). Aryl-substituted variants include 1,2-((4-MeO-Ph)2PO)2-Cb ((OMe)POCb) and 1,2-((4-F-Ph)2PO)2-Cb ((F)POCb). The effects of electron-withdrawing (EWG) and electron-donating (EDG) groups on resulting carborane redox potentials were assessed using electrochemical means, and the resulting Lewis basicities were quantified using empirical and competition-based NMR experiments. In organic solution, carboranes substituted with EWGs exhibited weaker coordination to UO22+, whereas those with EDGs exhibited stronger coordination. Similar to the previously reported unsubstituted POCb, the tunable new series of carboranes were electrochemically reduced and used for the biphasic capture of UO22+ from an aqueous to an organic phase and back again (release) through electrochemical oxidation. Extraction and back-extraction efficiencies were determined by analyses of the aqueous phases by ICP-OES. While all reduced nido-carboranes efficiently extracted UO22+ in high yields (78-88%)─with seemingly no correlation to the aforementioned measured Lewis basicities─we found the back-extraction of UO22+ to be significantly improved from POCb and, surprisingly, more closely related to their hydrophobic rather than their Lewis basic properties.

On the Edge of the Known: Extremely Electron-Rich (Di)Carboranyl Phosphines

The syntheses of the first B9-connected carboranylphosphines (B9-Phos) featuring two carboranyl moieties as well as access to B9-Phos ligands with bulky electron-donating substituents, previously deemed unattainable, is reported. The electrochemical properties of the B9-Phos ligands were investigated, revealing the ability of the mesityl derivatives to form stabilized phosphoniumyl radical cations. The B9-Phos ligands display an extremely electron-releasing character surpassing that of alkyl phosphines and commonly used N-heterocyclic carbenes. This is demonstrated by their very small Tolman electronic parameters (TEPs) as well as extremely low P-Se coupling constants. Cone angles and buried volumes attest to the high steric demand exerted by the (di)carboranyl phosphines. The dicarboranyl phosphine Au^I complexes show superior catalytic performance in the hydroamination of alkynes compared to the monocarboranyl phosphine analogs.

Direct B-H Functionalization of Icosahedral Carboranes via Hydrogen Atom Transfer

The efficient and selective functionalization of icosahedral carboranes (C₂B₁₀H₁₂) at the boron vertexes is a long-standing challenge owing to the presence of 10 inert B-H bonds in a similar chemical environment. Herein, we report a new reaction paradigm for direct B-H functionalization of icosahedral carboranes via B-H homolysis enabled by a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy. Both the HAT process of the carborane B-H bond and the resulting boron-centered carboranyl radical intermediate have been confirmed experimentally. The reaction occurs at the most electron-rich boron vertex with the lowest B-H bond dissociation energy (BDE). Using this strategy, diverse carborane derivatization, including thiolation, selenation, alkynylation, alkenylation, cyanation, and halogenation, have been achieved in satisfactory yields under a photoinitiated condition in a metal-free and redox-neutral fashion. Moreover, the synthetic utility of the current protocol was also demonstrated by both the scale-up reaction and the construction of carborane-based functional molecules. Therefore, this methodology opens a radical pathway to carborane functionalization, which is distinct from the B-H heterolytic mechanism in the traditional strategies.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

N-Ligand-Enabled Aromatic Nucleophilic Amination of 1,2-Diaryl-o-Carboranes with (R2 N)2 Mg for Selective Synthesis of 4-R2 N-o-Carboranes and 2-R2 N-m-Carboranes

The nucleophilic aromatic BH substitution reaction of carboranes is uncommon, compared to the electrophilic one. This work reported a pyridine-enabled transition-metal-free regioselective nucleophilic aromatic cage B(4)-H amination of 1,2-diaryl-o-carboranes with magnesium bisamides, giving a series of B(4)-aminated o-carboranes. DFT calculations showcased a stepwise B-N formation/B-H cleavage process, in which Mg-H formation/cage closure is the rate-determining step. Unprecedentedly, in the presence of 4,4'-di-tert-butyl-2,2'-dipyridyl (dtbpy), a tandem B(4)-amination/cage isomerization reaction of o-carboranes was discovered for the facile preparation of B(2)-aminated m-carboranes. Control experiments indicated that magnesium complex, bidentate ligand (dtbpy) and reaction temperature were crucial in the cage isomerization process. This direct nucleophilic aromatic cage B-H amination reaction offers an alternative strategy for selective amination of o- and m-carboranes.

Exploring the Reactivity of B‐Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH₂ and tBuPH₂ react with 9‐iodo‐m‐carborane yielding B9‐connected secondary carboranylphosphines 1,7‐H₂C₂B₁₀H₉‐9‐PHR (R=2,4,6‐Me₃C₆H₂ (Mes; 1  a), tBu (1  b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1  a, b resulted in the zwitterionic compounds 1,7‐H₂C₂B₁₀H₉‐9‐PHR(p‐C₆F₄)BF(C₆F₅)₂ (2  a, b) through nucleophilic para substitution of a C₆F₅ ring followed by fluoride transfer to boron. Further reaction with Me₂SiHCl prompted a H−F exchange yielding the zwitterionic compounds 1,7‐H₂C₂B₁₀H₉‐9‐PHR(p‐C₆F₄)BH(C₆F₅)₂ (3  a, b). The reaction of 2  a, b with one equivalent of R'MgBr (R’=Me, Ph) gave the extremely water‐sensitive frustrated Lewis pairs 1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)B(C₆F₅)₂ (4  a, b). Hydrolysis of the B−C₆F₄ bond in 4  a, b gave the first tertiary B‐carboranyl phosphines with three distinct substituents, 1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄H) (5  a, b). Deprotonation of the zwitterionic compounds 2  a, b and 3  a, b formed anionic phosphines [1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)BX(C₆F₅)₂]⁻[DMSOH]⁺ (R=Mes, X=F (6  a), R=tBu, X=F (6  b); R=Mes, X=H (7  a), R=tBu, X=H (7  b)). Reaction of 2  a, b with an excess of Grignard reagents resulted in the addition of R’ at the boron atom yielding the anions [1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)BR’(C₆F₅)₂]⁻ (R=Mes, R’=Me (8  a), R=tBu, R’=Me (8  b); R=Mes, R’=Ph (9  a), R=tBu, R’=Ph (9  b)) with [MgBr(Et₂O)_n]⁺ as counterion. The ability of the zwitterionic compounds 3  a, b to hydrogenate imines as well as the Brønsted acidity of 3  a were investigated.

Synthesis and Crystal Structure of 9,12-Dibromo-ortho-Carborane

Synthesis, NMR spectral data and crystal structure of 9,12-dibromo derivative of ortho-carborane are reported.

Synthesis and Catalytic Properties of Novel Ruthenacarboranes Based on nido-[5-Me-7,8-C2B9H10]2− and nido-[5,6-Me2-7,8-C2B9H9]2− Dicarbollide Ligands

The effect of methyl substituents in the lower belt of dicarbollide ligands on the redox potential of ruthenacarboranes based thereof, as well as the ability of the metallacarboranes obtained to catalyze radical polymerization with atom transfer were studied. For this purpose, a new approach to the synthesis of closo-ruthenacarboranes based on substituted dicarbollide ligands was developed and six new complexes 3,3-(Ph2P(CH2)4PPh2)-3-H-3-Cl-9-Me-12-X-closo-3,1,2-RuC2B9H9, 3,3,8-(Ph2P(CH2)4PPh-μ-(C6H4-o))-3-Cl-9-Me-12-X-closo-3,1,2-RuC2B9H8 and 3,3,4,8-(Ph2P(CH2)4P-μ-(C6H4-o)2)-3-Cl-9-Me-9-X-closo-3,1,2-RuC2B9H7 (X = H, Me) were synthetized and characterized by single crystal X-ray diffraction, NMR and ESR spectroscopy and MALDI TOF mass-spectrometry. Comparison of the values of the redox potentials of the synthesized ruthenium complexes in 1,2-dichloroethane with the values previously found for the corresponding ruthenacarboranes based on the parent dicarbollide anion showed that the introduction of methyl substituents into the carborane cage led to a decrease in the redox potentials of the complexes, which made them more preferable catalysts for ATRP. Test experiments on the polymerization of MMA showed that the synthesized ruthenacarboranes were effective catalysts for ATRP, the most active being the complex with two methyl groups and two ortho-phenylenecycloboronated fragments.

Free three-dimensional carborane carbanions

Carbon atom functionalization via generation of carbanions is the cornerstone of carborane chemistry. In this work, we report the synthesis and structural characterization of free ortho-carboranyl [C2B10H11]-, a three-dimensional inorganic analog of the elusive phenyl anion that features a "naked" carbanion center. The first example of a stable, discrete C(H)-deprotonated carborane anion was isolated as a completely separated ion pair with a crown ether-encapsulated potassium cation. An analogous approach led to the isolation and structural characterization of a doubly deprotonated 1,1'-bis(o-carborane) anion [C2B10H10]2 2-, which is the first example of a discrete molecular dicarbanion. These reactive carbanions are key intermediates in carbon vertex chemistry of carborane clusters.

1,12-Diiodo-Ortho-Carborane: A Classic Textbook Example of the Dihalogen Bond

The crystal structure of 1,12-diiodo-ortho-carborane 1,12-I2-1,2-C2B10H10 was determined by single crystal X-ray diffraction. In contrary to earlier studied 1,12-dibromo analogue 1,12-Br2-1,2- C2B10H10, its crystal packing is governed by the presence of the intermolecular I⋯I dihalogen bonds between the iodine atom attached to the carbon atom (acceptor) and the iodine atom attached to the antipodal boron atom (donor) of the carborane cage. The observed dihalogen bonds belong to the II type and are characterized by classical parameters: shortened I⋯I distance of 3.5687(9) Å, C–I⋯I angle of 172.61(11)° and B–I⋯I angle of 92.98(12)°.

Transition metal-mediated B(4)-H hydroxylation/halogenation of o-carboranes bearing a 2-pyridylsulfenyl ligand

The introduction of the 2-pyridylsulfenyl directing group to o-carboranes allowed either B(3)-Ir or B(4)-Ir bond formation using a steric effect strategy. Moreover, the reactivity of the B(4)-Rh o-carborane complexes with small molecules was probed by reactions with N-bromosuccinimide, N-iodosuccinimide and O₂. Rhodium-mediated B(4)-hydroxylation and B(4)-halogenation which are seldom reported have been achieved under practical and mild conditions.

Carborane Guests for Cucurbit[7]uril Facilitate Strong Binding and On-Demand Removal

High-affinity guests have been reported for the macrocyclic host cucurbit[7]uril (CB[7]), enabling widespread applications, but hindering CB[7] materials from being returned to their guest-free state for reuse. Here, we present polyhedral boron clusters (carboranes) as strongly binding, yet easily removable, guests for CB[7]. Aided by a Pd-catalyzed coupling of an azide anion, we prepared boron-functionalized 9-amino-ortho-carborane that binds to CB[7] with a K_a ≈ 10¹⁰ M^-1. Upon basic treatment, ortho-carborane readily undergoes deboronation to yield anionic nido-carborane, a poor guest for CB[7], facilitating recovery of guest-free CB[7]. We showcase the utility of the modified ortho-carborane guest by recycling a CB[7]-functionalized resin. With this report, we introduce stimuli-responsive decomplexation as an additional consideration in the design of high-affinity host-guest complexes.

One-Pot Synthesis of B-Aryl Carboranes with Sensitive Functional Groups Using Sequential Cobalt- and Palladium-Catalyzed Reactions

The simple and efficient method was developed for the one-pot synthesis of B-substituted aryl derivatives of ortho-carborane with functional groups sensitive to organolithium and organomagnesium reagents using 9-iodo-ortho-carborane and generated in situ organozinc compounds. The method proposed was used to prepare a series of 9-aryl-ortho-carboranes, including those containing nitrile and ester groups, 9-RC6H4-1,2-C2B10H11 (R = p-Me, p-NMe2, p-OCH2OMe, o-OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, and p-COOEt). It was demonstrated that the same approach can be used for synthesis of diaryl derivatives of ortho-carborane 9,12-(RC6H4)2-1,2-C2B10H10 (R = H, p-Me). The solid-state structures of 9-RC6H4-1,2-C2B10H11 (R = p-NMe2, p-OCH2OMe, o-OMe, o-CN, p-CN, m-COOEt, and p-COOEt) and 9,12-(p-MeC6H4)2-1,2-C2B10H10 were determined by single crystal X-ray diffraction.

The stability of group 10 metal POCOP pincer complexes: decomposition/reconstruction pathways of the pincer backbone

Organometallic chemists usually like to use the word robust to describe pincer ligand frameworks in metal pincer complexes. Although most transition metal pincer complexes are thermally stable, the pincer backbone frameworks can still decompose under certain circumstances. In order to explore the stability of the bis(phosphinite) (POCOP) pincer backbone in transition metal pincer complexes, group 10 metal POCOP pincer complexes were exposed to different nucleophilic and electrophilic conditions, respectively. It was found that the POCOP pincer backbone is stable under intermolecular nucleophilic conditions but cannot survive intramolecular nucleophilic attack; the POCOP pincer backbone is also stable under weak electrophilic conditions but the backbone can be completely destroyed by strong Lewis acids such as AlCl3. Possible decomposition/reconstruction pathways of the POCOP pincer ligand framework were proposed.

Enlargement of a Modular System-Synthesis and Characterization of an s-Triazine-Based Carboxylic Acid Ester Bearing a Galactopyranosyl Moiety and an Enormous Boron Load

The amount of boron accumulated in tumor tissue plays an important role regarding the success of the boron neutron capture therapy (BNCT). In this article, we report a modular system, combining readily available starting materials, like glycine, 1,3,5-triazine and the well-known 9-mercapto-1,7-dicarba-closo-dodecaborane(12), as well as α-d-galactopyranose for increased hydrophilicity, with a novel boron-rich tris-meta-carboranyl thiol.

Nucleophilic substitution: a facile strategy for selective B-H functionalization of carboranes

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.

Controlled functionalization of o-carborane via transition metal catalyzed B–H activation

This review summarizes recent advances in transition metal catalyzed vertex-specific BH functionalization of o-carborane for controlled synthesis of its derivatives.

Transition-Metal-Catalyzed Selective Cage B-H Functionalization of o-Carboranes

Carboranes are a class of carbon-boron molecular clusters with unusual thermal and chemical stabilities. They have been proved as very useful building blocks in supramolecular design, optoelectronics, nanomaterials, boron neutron capture therapy agents and organometallic/coordination chemistry. Thus, the functionalization of o-carboranes has received growing interests. Over the past decades, most of the works in this area have been focused on cage carbon functionalization as the weakly acidic cage C-H proton can be readily deprotonated by strong bases. In sharp contrast, selective cage B-H activation/functionalization among chemically very similar ten B-H vertices is very challenging. Considering the differences in electron density of ten cage B-H bonds in o-carborane and the nature of transition metal complexes, we have tackled this selectivity issue by means of organometallic chemistry. Our strategy is as follows: using electron-rich transition metal catalysts for the functionalization of the most electron-deficient B(3,6)-H vertices (bonded to both cage CH vertices); using electron-deficient transition-metal catalysts for the functionalization of relatively electron-rich B(8,9,10,12)-H vertices (with no bonding to both cage CH vertices); and using the combination of directing groups and electrophilic transition metal catalysts for the functionalization of B(4,5,7,11)-H vertices (bonded to only one cage CH vertex). Successful applications of such a strategy result in the preparation of a large variety of cage B-functionalized carboranes in a regioselective and catalytic manner, which are inaccessible by other means. It is believed that as this field progresses, other cage B-functionalized carboranes are expected to be synthesized, and the results detailed in this concept article will further these efforts.

Phosphine-catalyzed cage carbon functionalization of o-carborane: facile synthesis of alkenylcarboranes

Phosphines can efficiently catalyze alkenylation of o-carborane with electron-deficient alkynes, which represents the first example of organocatalysis in o-carborane functionalization.

Palladium catalyzed/silver tuned selective mono-/tetra-acetoxylation of o-carboranes via B–H activation

Silver tuned the selective mono-/tetra-acetoxylation of o-carboranes via palladium catalyzed B–H activation.

Palladium catalyzed selective mono-arylation of o-carboranes via B–H activation

Palladium catalyzed selective mono-arylation of o-carborane on B(8) and B(9).

Palladium catalyzed regioselective mono-alkenylation of o-carboranes via Heck type coupling reaction of a cage B–H bond

Palladium catalyzed electrophilic B–H activation for regioselective mono-alkenylation of o-carborane on B(8) and B(9).

Reaction of 13-vertex carborane μ-1,2-(CH2)4-1,2-C2B11H11 with nucleophiles: linkage effect on product formation

The length of C,C'-linkage has a great influence on the reactivity of 13-vertex carboranes. Reaction of 1,2-(CH2)4-1,2-C2B11H11 (1a) with Et2NH gave a 1:1 adduct nido-7-NEt2H-μ-1,3-(CH2)4-1,3-C2B11H11 (2). Compound 1a reacted with Me2NLi or Et2NLi to afford nido-[9-Nu-μ-7,8,10-(CH2)4CCH-B11H10](-) (Nu = NMe2, [3](-); Nu = NEt2, [4](-)). Complex [4](-) was also obtained by deprotonation of 2. Treatment of 1a with MeOH/base generated nido-[3-OMe-μ-1,2-(CH2)4-1,2-C2B11H11](-) ([5](-)) at room temperature, which was converted to nido-[μ-7,8-(CH2)4CHB(OMe)2-7-CB10H11](-) ([6](-)) upon heating in the presence of Et3N. Complex [6](-) was oxidized by H2O2 to the corresponding alcohol [μ-7,8-(CH2)4CHOH-7-CB10H11](-) ([7](-)) or hydrolyzed to the boronic acid [μ-7,8-(CH2)4CHB(OH)2-7-CB10H11](-) ([8](-)). Reaction of 1a with (4-MeC6H4)SNa produced a CB11(-) anion closo-[μ-1,2-(CH2)4CHS(4-MeC6H4)-1-CB11H10](-) ([9](-)). The above complexes were fully characterized by (1)H, (13)C, and (11)B NMR spectroscopic data and elemental analyses. Molecular structures of 1-[7](-) and [9](-) were further confirmed by single-crystal X-ray analyses.

Extremely Electron-Rich, Boron-Functionalized, Icosahedral Carborane-Based Phosphinoboranes

We have prepared the first examples of B9-connected trivalent aryl and alkyl phosphinoborane species via Pd-catalyzed phosphination of 9-iodo-meta-carborane. Our studies highlight the unique electronic features of the B9-connected meta-carboranyl moiety as compared to its C1-based analogue. This work suggests that the B9-functionalized meta-carboranyl substituent in these ligands exhibits more electron-releasing character than any other known carbon-based substituent, ultimately laying the foundation for a new class of phosphine ligands with extremely electron-rich character.