IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Ligands as Electron Reservoirs for Sandwich Complexes.

A long-sustained effort of systematic steric and electronic modification of cyclopentadienyl (Cp) ligands has enabled them to find wide-ranging, valuable applications. Herein, we present two novel Cp ligands: imidazolium- and pyrrolinium-substituted zwitterionic Cps (IZCp and PZCp), whose key utility is redox non-innocence─the ability to participate cooperatively with the metal center in redox reactions. Through the simple metalation of ZCps, the Cr(0) and Mo(0) half-sandwich complexes (IZCp)Cr(CO)3, (PZCp)Cr(CO)3, (IZCp)Mo(CO)3, and (PZCp)Mo(CO)3, respectively, as well as the Ru(II) sandwich complexes [(IZCp)RuCp]PF6 and [(PZCp)RuCp]PF6 were prepared. The sandwich complexes were fully characterized and showed by cyclic voltammetry reversible one-electron reduction at E1/2 potentials ranging from -1.7 to -2.7 V vs Fc/Fc+. These values are unusually low and have not been observed with other Cp ligands due to the instability of the reduced complexes. Density functional theory (DFT) calculations for the reduced sandwich derivatives with IZCp and PZCp showed their spin densities to be highly delocalized over their ZCp ligand moieties (70-90%). Electron paramagnetic resonance (EPR) analysis of the isolated K[(PZCp)Mo(CO)3] and (PZCp)RuCp also indicated a high degree of ligand-localized radical character. Thus, the IZCp and PZCp ligands act as electron reservoirs to sustain these sandwich complexes in highly reduced states. At the same time, the CO stretching frequencies of K[(PZCp)Mo(CO)3]: νCO 1871, 1748, and 1699 cm-1, rank the [PZCp]- ligand as the strongest electron-donating Cp ligand among the reported CpMo(CO)3 derivatives, whose νCO > 1746 cm-1. In addition, these redox non-innocent Cps were obtained in high yields and found to be practically air- and moisture-stable, unlike typical Cps.

Persistent Radicals Derived from N-Heterocyclic Carbenes for Material Applications.

Persistent radicals are potential building blocks of novel materials in many fields. Recently, highly stable persistent radicals are considered to be within reach, thanks to several radical stabilization strategies such as spin delocalization and steric protection. N-Heterocyclic carbene (NHC)-derived substituents can be attached to a radical center for these purposes, as illustrated by numerous NHC-stabilized radicals reported in the last two decades.This Account describes our recent work on developing NHC-derived persistent radicals, as well as their prospective applications. Considering that NHCs not only stabilize radicals but also reversibly interact with gas molecules, in 2015 our group reported NHC-nitric oxide (NHC-NO) radicals produced by reversibly trapping nitric oxide (NO) radical gas in NHCs. The resultant compounds were loaded into biocompatible poly(ethylene glycol)-block-poly(caprolactone) (PEG-b-PCL) micelles and injected into tumor-bearing mice. Then, NO release was triggered by high-intensity focused ultrasound irradiation of the tumor tissue. Furthermore, the NHC-NO radicals could also serve as a platform to generate other organic radicals such as oxime ether or iminyl radicals. Apart from medicine-related applications, radicals stabilized by NHCs can be used as energy storage materials. In this context, the triazenyl radical containing two NHC units reported by our laboratory could be a cathode active material in batteries, as an organic alternative to LiCoO2. The subsequently prepared unsymmetrical triazenyl radical derivatives were applied as anolytes in nonaqueous all-organic redox flow batteries. In addition, a ferrocene-based redox flow battery anolyte was obtained by introducing NHC-derived substituents that effectively stabilize the ferrocenate derivatives previously reported only at low temperatures. The batteries containing NHC-supported radicals exhibited high energy efficiency and insignificant radical decomposition over multiple cycles. Finally, toward developing air-persistent organic radicals for flexible devices and MRI contrasting agents, we also highlight our recent air- and physiologically stable organic radicals derived from NHCs. Coordination of tris(pentafluorophenyl)borane to the NHC-NO radical produced a new radical cation that is stable in an organic solvent under air for several months. The readily accessible 1,2-dicarbonyl radical cations generated by the reaction of NHCs with oxalyl chloride are remarkably persistent even in an aqueous solution for several months. They are also highly stable even under physiological conditions, making them particularly attractive potential candidates for organic MRI contrast agents. We hope that this Account will serve as a guide for the future development of stable NHC-derived organic radicals and draw the attention of the synthetic community to their potential applications in material science.

Revisiting the Reaction of IPr with Tritylium: An Alternative Mechanistic Pathway.

Despite a recent proposal on the mechanism of a single-electron transfer (SET) process between tritylium and 2,6-bis(diisopropylphenyl)imidazol-2-ylidene (IPr) based on evidence of transient IPr radical cation intermediate ([IPr]⋅+ ) formation, such oxidation is still contentious because of the high oxidation potential of N-heterocyclic carbenes. Our experimental analysis indicates that the appearance of deep purple color, previously considered to be from transient [IPr]⋅+ , originates from a zwitterionic intermediate (3 a), not a radical cation. Here, we propose an alternative mechanism for the reaction involving tritylium and IPr. This mechanism is noteworthy for explaining how [NHC-H]+ can be generated without the formation of transient [NHC]⋅+ , which has been frequently proposed as an intermediate for the reaction between NHC and oxidants. These results also show that a transient strong single-electron donor (3 a) could be generated by the alternative mechanism for oxidants using NHCs, which is a more feasible explanation for the reactivity of NHCs with oxidants.

Uncovering Nitrosyl Reactivity at N-Heterocyclic Carbene Center.

N-heterocyclic carbenes (NHCs) have garnered much attention due to their unique properties, such as strong σ-donating and π-accepting abilities, as well as their transition-metal-like reactivity toward small molecules. In 2015, we discovered that NHCs can react with nitric oxide (NO) gas to form radical adducts that resemble transition metal nitrosyl complexes. To elucidate the analogy between NHC and transition metal NO adducts, here we have undertaken a systematic investigation of the electron- and proton-transfer chemistry of [NHC-NO]⋅ (N-heterocyclic carbene nitric oxide radical) compounds. We have accessed a suite of compounds, comprised of [NHC-NO]+ , [NHC-NO]- , [NHC-NOH]0 , and [NHC-NHOH]+ species. In particular, [NHC-NO]- was isolated as potassium and lithium ion adducts. Most interestingly, a monomeric potassium [NHC-NO]- compound was isolated with the assistance of 18-crown-6, which is the first instance of a monomeric alkali N-oxyl compound to the best of our knowledge. Our results demonstrate that [NHC-NO]⋅ exhibits redox behavior broadly similar to metal nitrosyl complexes, which opens up more possibilities for utilizing NHCs to build on the known reactivity of metal complexes.

Cobalt-Catalyzed Formation of Grignard Reagents via C-O or C-S Bond Activation.

C(aryl)-OMe bond functionalization catalyzed by cobalt(II) chloride in combination with a nacnac-type ligand and magnesium as a reductant is reported. Borylation and benzoylation of aryl methoxides are demonstrated, and C(aryl)-SMe bond borylation can be achieved under similar conditions. This is the first example of achieving these transformations using cobalt catalysis. Mechanistic studies suggest that a Grignard reagent is generated as an intermediate in a rare example of a magnesiation via a C-O bond activation reaction. Indeed, an organomagnesium species could be directly observed by electrospray ionization mass spectroscopic analysis. Kinetic experiments indicate that a heterogeneous cobalt catalyst performs the C-O bond activation.

Mechanistic Studies for Pd(II)(O2) Reduction Generating Pd(0) and H2O: Formation of Pd(OH)2 as a Key Intermediate.

Molecular oxygen (O2) remains to be an ideal yet underutilized feedstock for the oxidative transformation of organic substrates and renewable energy systems such as fuel cells. Palladium (Pd) has shown particular promise in enabling these applications. The present study describes a Pd-mediated O2 reduction to water via C-H activation of 9,10-dihydroanthracene (DHA) by a Pd(II) η2-peroxo complex 1O2. The reaction yields stoichiometric anthracene and Pd(0) product 1 and is notable in two respects. First, plots of concentrations of the reaction participants over time have distinctly sigmoidal shapes, indicating that conversion accelerates over time and implying autocatalysis. Second, the reaction proceeds via a genuine monometallic Pd(II) dihydroxide 1(OH)2 directly observed to grow and decay as an intermediate. Confirming its role as an intermediate, the dihydroxide 1(OH)2 was found to mediate C-H oxidation of DHA on par in activity with the peroxo compound 1O2. Mechanistic studies with density functional theory (DFT) calculations suggested that both 1O2 and 1(OH)2 react with DHA by hydrogen atom transfer (HAT) and that autocatalysis in the 1O2 reaction results from oxidative addition of the initial Pd(II) complex 1O2 to the Pd(0) product 1. This reaction forms a transient bis(µ-oxo) Pd(II) dimer 1O21 that is more active in the HAT oxidation of DHA than the initial 1O2.

Highly Stable 1,2-Dicarbonyl Radical Cations Derived from N-Heterocyclic Carbenes.

Stable organic radicals have been of great academic interest not only in the context of fundamental understanding of reactive intermediates but also because of their numerous applications as functional materials. Apart from the early examples of triphenylmethyl and TEMPO derivatives, reports on air- and water-stable organic radicals are scarce, and their development remains a challenge. Herein, we present the design and synthesis of a novel organic radical based on a 1,2-dicarbonyl scaffold supported by N-heterocyclic carbenes (NHCs). The presented radical cations exhibit remarkable stability toward various harsh conditions, such as the presence of reactive chemicals (reductants, oxidants, strong acids, and bases) or high temperatures, by far exceeding the stability of triphenylmethyl and TEMPO radicals. In addition, physiological conditions including aqueous buffer and blood serum are tolerated. The steric and electronic stabilization provided by the two NHC moieties enabled the successful design of the highly stable radical.

Indol-2-ylidene (IdY): Ambiphilic N-Heterocyclic Carbene Derived from Indole*.

The synthesis of ambiphilic N-heterocyclic carbene ligand, indol-2-ylidene (IdY, A), is described. A series of indolenium precursors (2 a-f) were prepared on a gram scale in good yields. Trapping experiments with elemental selenium, [RhCl(cod)]2 and CuCl provided the expected carbene adducts. Further computational and spectroscopic studies supported the ambiphilicity of IdY, which lies between cyclic (alkyl)(amino)carbenes (CAAC-5) and cyclic (amino)(aryl)carbene (CAArC). The copper complexes (6) show high percent buried volume (% Vbur = 58.1) and allow for carboboration of terminal alkynes within 30 minutes in a demonstration of synthetic utility with good yields and high regioselectivity.

Generation of N-H Imines from α-Azidocarboxylic Acids through Ru-Catalyzed Decarboxylation.

A new method for the synthesis of N-H imines from α-azidocarboxylic acids was developed, which proceeds through decarboxylative C-C bond cleavage catalyzed by a commercial diruthenium complex ([CpRu(CO)2]2) under visible light irradiation at room temperature within several minutes. The reactive products undergo condensation, which forms cyclic trimers (2,4,6-trialkylhexahydro-1,3,5-triazines) or linear N,N'-bis(arylmethylidene)arylmethanediamines in quantitative yields. Alternatively, the N-H imines can be trapped with benzylamine and 2-(aminomethyl)aniline, providing stable N-benylimines and tetrahydroquinazolines, respectively. Subsequent oxidation of tetrahydroquinazolines produced quinazolines.

Construction of Stable Metal-Organic Framework Platforms Embedding N-Heterocyclic Carbene Metal Complexes for Selective Catalysis.

We report a bottom-up approach to immobilize catalysts into MOFs, including copper halides and gold chloride in a predictable manner. Interestingly, the structures of MOFs bearing NHC metal complexes maintained a similar 4-fold interpenetrated cube. They exhibited exceptionally high porosity despite the interpenetrated structure and showed good stability in various solvents. Moreover, these MOFs possess high size activity depending on the size of the substrates in various reactions, compared to homogeneous catalysis. Also, the high catalytic activity of MOFs can be preserved 4 times without significant loss of crystallinity. Incorporation of the various metal complexes into MOFs allows for the preparation of functional MOFs for practical applications.

Fixation of Dinitrogen at an Asymmetric Binuclear Titanium Complex.

A new type of dititanium dinitrogen complex supported by a triphenolamine (TPA) ligand is reported. Analysis by single-crystal X-ray diffraction and Raman and NMR spectroscopy reveals different coordination geometries for the two titanium centers. Hence, coordination of TPA and a nitrogen ligand results in trigonal-bipyramidal geometry, while an octahedral titanium center is obtained upon additional coordination of an ethoxide generated upon C-O bond cleavage in a diethyl ether solvent molecule. The titanium complex successfully generates ammonia in the presence of an excess amount of PCy3HI and KC8 in 154% yield (per titanium atom). A titanium complex with a bulkier TPA does not form a dinitrogen complex, and mononuclear titanium dinitrogen complexes were not accessible, presumably because of the high tendency of early transition metals to form binuclear dinitrogen complexes.

Deeper Understanding of Mononuclear Manganese(IV)-Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)-Hydroxide Complex.

Binding of Lewis acidic metal ions and Brønsted acid at the metal-oxo group of high-valent metal-oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal-oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)-oxo complex binding triflic acid, {[(dpaq)MnIV(O)]-HOTf}+ (1-HOTf). First, 1-HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn-O stretching frequencies of 1-HOTf (e.g., νMn-O at ∼793 cm-1) and 1-Mn+ (Mn+ = Ca2+, Zn2+, Lu3+, Al3+, or Sc3+) and the Lewis acidities of H+ and Mn+ ions, suggesting that H+ and Mn+ bind at the metal-oxo moiety of [(dpaq)MnIV(O)]+. Interestingly, a single-crystal structure of 1-HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)-oxo complex but a Mn(IV)-hydroxide complex, [(dpaq)MnIV(OH)](OTf)2 (4), with a Mn-O bond distance of 1.8043(19) Å and a Mn-O stretch at 660 cm-1. More interestingly, 4 reverted to 1-HOTf upon dissolution, demonstrating that 1-HOTf and 4 are interconvertible depending on the physical states, such as 1-HOTf in solution and 4 in isolated solid. The reactivity of 1-HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1-Mn+ complexes; an interesting correlation between the Mn-O stretching frequencies of 1-HOTf and 1-Mn+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

Programmable Synthesis of Silver Wheels.

The synthesis of sandwich-shaped multinuclear silver complexes with planar penta- and tetranuclear wheel-shaped silver units and a central anion, [Agn(2-HPB)2(A-)](OTf-)n-1, nAgA, n = 4 or 5 and A- = OH- or F- or Cl-, is reported, along with complete spectroscopic and structural characterization. An NMR mechanistic study reveals that silver complexes were formed in the following order: 2Ag → 3AgH2O → 5AgOH → 4AgOH. The central hydroxides in 4AgOH and 5AgOH exhibit exotic physical properties due to the confined environment inside the complex. The size of these silver wheels can be tuned by changing the central anion or extracting/adding one silver atom. This study provides the facile way to synthesize discrete wheel-shaped multinuclear silver complexes and provides valuable insights into the dynamics of the self-assembly process.

Superacid-Mediated Functionalization of Hydroxylated Cucurbit[n]urils.

Herein we report a facile transformation of hydroxylated cucurbit[n]uril (CB[n], n = 6 and 7) to other functionality-conjugated CB[n]s by nucleophilic substitution of the hydroxyl group with a wide range of nitriles and alcohols. The reaction proceeds efficiently via generation of a superelectrophilic carbocation on the CB framework from hydroxylated CB[n]s under superacidic conditions. One of the resulting CB[n] derivatives with reactive functionality, monocarboxylated CB[7], is efficiently conjugated to an enzyme (horseradish peroxidase, HRP) by amide coupling. This provides a CB[7]-conjugated functional biomaterial (CB[7]-HRP) that selectively detects proteins labeled with a guest, adamantylammonium (AdA), based on bioorthogonal high-affinity host-guest interactions between CB[7] and AdA. We demonstrated the potential of overcoming the limitations in preparing reactive functional CB[n] derivatives, enabling the exploration of novel bioapplications of CB[n]-based host-guest chemistry with new CB[n]-conjugated functional materials.

Structural Control of Metal-Organic Framework Bearing N-Heterocyclic Imidazolium Cation and Generation of Highly Stable Porous Structure.

Building a well-defined porous structure with permanent porosity in a metal-organic framework (MOF) is significant to achieve selective adsorption of guest molecules and its practicality. There have been various efforts to develop new functional frameworks, but chasing both advantages (functions and stability) is always challenging. Herein, we report a highly stable and porous 3D MOF having an imidazolium group. By adjusting counteranions of the imidazolium group, we could prepare two distinct MOFs. The 2D MOF constructed by applying bicarbonate anion inhibited three-dimensional growth and showed N2 adsorption with BET results of 330 m2/g. However, the 3D MOF (Imi-3D) formed by applying chloride anion on the imidazolium group evolved into a 4-fold interpenetrated cube structure. The Imi-3D has high porosity in spite of the folded nature of the structure, with BET results of 1400 m2/g. Interestingly, unlike the usual trend of unstable zinc carboxylate based MOFs, the multiple π-π interactions of the Imi-3D presents a unique tolerance toward various solvents, especially toward water, for 6 months and without decomposition or collapse of the framework. Moreover, since the porous 3D MOF consists of imidazolium groups, the MOF shows a cationic feature with strong adsorption toward anionic guests in aqueous media.

Stable Organic Radicals Derived from N-Heterocyclic Carbenes.

Over the past decade, numerous stable organic and main-group radicals have been synthesized from N-heterocyclic carbenes (NHCs). The structure of NHCs, in particular, offers electronic stabilization that helps to delocalize the unpaired electron over the molecule. In addition, the sterically bulky substituents of NHCs protect the radical center to prevent detrimental reactions such as dimerization. These advantages enable the straightforward synthesis and characterization of various interesting organic radicals starting from NHCs, which are, these days, widely available. NHCs have enabled the structural characterization of various organic radicals over the past decade, including α-carbonyl, propargyl, oxyallyl, and triazenyl radicals as notable examples. This minireview summarizes the advances in this field, mainly focusing on the preparation and stability as well as the properties and applications of NHC-derived organic radicals.

Clinical and radiologic outcomes after arthroscopic rotator cuff repair: Single-row versus Speed-Bridge technique.

We compared clinical outcome between the Speed- Bridge technique and single-row techniques in patients with full-thickness rotator cuff tears and figured out the patterns of retear by computed tomography (CT) arthrogram and ultrasonography follow-up. In total 209 patients with full-thickness rotator cuff tears who underwent arthroscopic rotator cuff repair and were followed up for at least 2-year were enrolled retrospectively (group 1: single-row repair, group 2: Speed-Bridge repair). Pre- and postoperative data were reviewed to assess clinical and radiologic outcomes. There were no significant differences in clinical outcome between the 2 groups. The retear rates of medium and large-sized rotator cuff tear groups were higher in group 1 than in group 2 (p < 0.05). There was no significant difference in the medial row failure rate between the 2 groups. Present study showed that the knotless suture Bridge technique may be a considerable alternative method for treating full-thickness rotator cuff-tears. Level of evidence: Level III, Retrospective comparative study.

Oxime Ether Radical Cations Stabilized by N-Heterocyclic Carbenes.

N-heterocyclic carbene (NHC) nitric oxide (NHCNO) radicals, which can be regarded as iminoxyl radicals stabilized by NHCs, were found to react with a series of silyl and alkyl triflates to generate the corresponding oxime ether radical cations. The structures of the resulting oxime ether radical cations were determined by X-ray crystallography, along with EPR and computational analysis. In contrast, lutidinium triflate produced a 1:1 mixture of [NHCNO+ ][OTf- ] and [NHCNHOH+ ][OTf- ] upon the reaction with NHCNO. This study adds an important example of stable singlet carbenes for stabilizing main-group radicals because of their π-conjugating effect, the synthesis and structures of which have not been reported previously.

Coumaraz-2-on-4-ylidene: Ambiphilic N-Heterocyclic Carbenes with a Tunable Electronic Structure.

Herein, a coumaraz-2-on-4-ylidene (1) as a new example of an ambiphilic N-heterocyclic carbene, having electronic properties that can be fine-tuned, is reported. The N-carbamic and aryl groups on the carbene carbon center provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the 77 Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine-tuned in a quantitative and predictable way by using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be applicable to eliciting novel properties in main-group and transition-metal chemistry.

Separation of Acetylene from Carbon Dioxide and Ethylene by a Water-Stable Microporous Metal-Organic Framework with Aligned Imidazolium Groups inside the Channels.

Separation of acetylene from carbon dioxide and ethylene is challenging in view of their similar sizes and physical properties. Metal-organic frameworks (MOFs) in general are strong candidates for these separations owing to the presence of functional pore surfaces that can selectively capture a specific target molecule. Here, we report a novel 3D microporous cationic framework named JCM-1. This structure possesses imidazolium functional groups on the pore surfaces and pyrazolate as a metal binding group, which is well known to form strong metal-to-ligand bonds. The selective sorption of acetylene over carbon dioxide and ethylene in JCM-1 was successfully demonstrated by equilibrium gas adsorption analysis as well as dynamic breakthrough measurement. Furthermore, its excellent hydrolytic stability makes the separation processes highly recyclable without a substantial loss in acetylene uptake capacity.