Supramolecular Coordination Cages Based on N-Heterocyclic Carbene-Gold(I) Ligands and Their Precursors: Self-Assembly,Structural Transformation and Guest-Binding Properties

The incorporation of functional groups into the cavity of discrete supramolecular coordination cages (SCCs) will bring unique functions and applications. Here, three dicarboxylate ligands (H₂ L1 Cl, H₂ L2 Cl and H₂ L3 Cl) containing N-heterocyclic carbene (NHC) precursors as linkers were introduced to construct SCCs by combining with two C₃-symmertic (CpZr)₃(μ₃-O)(μ₂-OH)₃ clusters as three-connect vertices, resulted in a series of rugby-like V₂E₃ (V=vertex, E=edge) type homoleptic cages ( SCC-1 , SCC-2 and SCC-3 ). However, V₄E₆-type tetrahedral cages ( SCC-4 and SCC-5 ), incorporating six Au-NHC moieties, were obtained when the corresponding NHC-gold(I) functionalized ligands (H₂ L1 ^Au, H₂ L2 ^Au) were applied. For the first time, we present a trackable CpZr-involved cage to cage conversion to generate a heteroleptic V₂E₃ cage ( SCC-6 ) from two homoleptic cages ( SCC-2 and SCC-5 ) with different geometries of V₂E₃ and V₄E₆. The heteroleptic assembly SCC-6 can also be formed upon a subcomponent displacement strategy. The structural transformation and reassembly processes were detected and monitored by ¹H NMR spectroscopy and electrospray-ionization mass spectrometry. The formation of heteroleptic assembly was further supported by single crystal X-ray diffraction analysis. Moreover, homoleptic cage SCC-2 possesses a trigonal bipyramidal cationic cavity allowing the encapsulation of a series of sulfonate anionic guests.     

Dimensional Matching versus Induced‐Fit Distortions: Binding Affinities of Planar and Curved Polyaromatic Hydrocarbons with a Tetragold Metallorectangle

A tetragold(I) rectangle‐like metallocage containing two pyrene‐bis‐imidazolylidene ligands and two carbazolyl‐bis‐alkynyl linkers is used for the encapsulation of a series of polycyclic aromatic hydrocarbons (PAHs), including corannulene. The binding affinities obtained for the encapsulation of the planar PAHs guests in CD₂Cl₂ are found to exponentially increase with the number of π‐electrons of the guest (1.3 > logK >6.6). For the bowl‐shaped molecule of corannulene, the association constant is much lower than the expected one according to its number of electrons. The molecular structure of the host–guest complex formed with corannulene shows that the molecule of the guest is compressed, while the host is expanded, thus showing an interesting case of artificial mutual induced‐fit arrangement.     

Electron-Rich,Lewis Acidic Diborane Meets N-Heterocyclic Aromatics: Formation and Electron Transfer in Cyclophane Boranes

Herein reported are the reactions of an electron-rich, Lewis acidic diborane with N-heterocyclic aromatics to give first members of an unprecedented family of highly charged cationic cyclophanes with diboranyl units. Tetracationic cyclophanes with 4,4‘-bipyridine/ 1,2-bis(4-pyridyl)ethylene and diboranyl units were synthesized and their redox chemistry was studied. Cyclisation of two diboranyl and two pyrazine units is accompanied by electron transfer from the diboranyl unit to the pyrazine. Our results pave the way for the integration of redox-active diboranyl units into cyclophanes and supramolecular structures.     

Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis,Structures, Steric Parameters,and Catalytic Activity

The synthesis and full characterization of new air-stable Ag^I and Cu^I complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag₂O afforded a collection of Ag^I complexes, and their first use as ligand transfer reagents led to novel isostructural Cu^I or Au^I complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new Cu^I complexes. The solid-state structures feature large N-C_NHC-N angles (118–128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%V_bur) values span easily in the 50–57 % range, and that one of (9-Dipp)CuBr complex (%V_bur=57.5) overcomes to other known erNHC–metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76–93 % product at the 0.5–2.5 mol % catalyst loading, proving the stability of Cu^I erNHC complexes at elevated temperatures (100 °C).     

Dihydrogen Generation from Amine/Boranes: Synthesis,FT‐ICR,and Computational Studies

A Fourier transform ion cyclotron resonance spectrometry (FT‐ICR) study of the gas‐phase protonation of ammonia‐borane and sixteen amine/boranes R¹R²R³N? BH₃ (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H₂) are R¹R²R³N? BH₄⁺ and R¹R²R³N? BH₂⁺, respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6‐311+G(d,p)] level) of the thermodynamic stabilities of the R¹R²R³N? BH₃ adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R¹R²R³N? BH₄⁺ ions is characterized by a three‐center, two‐electron bond between B and a loosely bound H₂ molecule. Unexpected features of some R¹R²R³N? BH₄⁺ ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.     

Ti‐Substituted Boranes as Hydrogen Storage Materials: A Computational Quest for the Ideal Combination of Stable Electronic Structure and Optimal Hydrogen Uptake

Hydrogen storage : A family of Ti‐substituted boranes (see figure) having optimum electronic structures and the ability to absorb hydrogen has been designed computationally. Substantial binding energies and gravimetric densities of hydrogen storage show their potential as hydrogen storage materials. The computational study invites experimental synthesis of the novel borane family and offers a guide to searching for new hydrogen storage materials.

     

Unveiling Luminescent IrI and RhI N-Heterocyclic Carbene Complexes: Structure,Photophysical Specifics,and Cellular Localization in the Endoplasmic Reticulum

Complexes of Rh^I and Ir^I of the [M(COD)(NHC)X] type (where M=Rh or Ir, COD=1,5-cyclooctadiene, NHC=N-heterocyclic carbene, and X=halide) have recently shown promising cytotoxic activities against several cancer cell lines. Initial mechanism of action studies provided some knowledge about their interaction with DNA and proteins. However, information about their cellular localization remains scarce owing to luminescence quenching within this complex type. Herein, the synthesis of two rare examples of luminescent Rh^I and Ir^I [M(COD)(NHC)I] complexes with 1,8-naphthalimide-based emitting ligands is reported. All new complexes are comprehensively characterized, including with single-crystal X-ray structures. Steric crowding in one derivative leads to two distinct rotamers in solution, which apparently can be distinguished both by pronounced NMR shifts and by their respective spectral and temporal emission signatures. When the photophysical properties of these new complexes are exploited for cellular imaging in HT-29 and PT-45 cancer cell lines, it is demonstrated that the complexes accumulate predominantly in the endoplasmic reticulum, which is an entirely new finding and provides the first insight into the cellular localization of such Ir^I(NHC) complexes.     

Relative Stability of closo‐closo,closo‐nido,and nido‐nido Macropolyhedral Boranes: The Role of Orbital Compatibility

The concept of orbital compatibility is used to explain the relative energies of different macropolyhedral structural patterns such as closo‐closo, closo‐nido, and nido‐nido. A large polyhedral borane condenses preferentially with a smaller polyhedron owing to orbital compatibility. Calculations carried out at the B3LYP/6‐31G* level show that the macropolyhedron closo(12)‐closo(6) is the most preferred structural pattern among the face‐sharing closo‐closo systems. The relative stabilities of four‐shared‐atom closo‐closo, three‐shared‐atom closo‐closo, three‐shared‐atom closo‐nido, edge‐sharing closo‐nido, and edge‐sharing nido‐nido structures are in accordance with the difference in the number of vertices of the individual polyhedra of the macropolyhedra. When the difference in the number of vertices of the individual polyhedra is large, the stability of the macropolyhedra is also large. Calculations further show that the orbital compatibility plays an important role in deciding the stability of the macropolyhedral boranes with more than two polyhedral units. The dependence of the orbital compatibility on the relative stability of the macropolyhedron varies with other factors such as inherent stability of the individual polyhedron and steric factors.     

Modulating Hierarchical Micro/Mesoporosity by a Mixed Solvent Approach in Al-MOF: Stabilization of MAPbBr3 Quantum Dots

Various hierarchical micro/mesoporous MOFs based on {[Al(μ-OH)(1,4-NDC)]⋅H₂O} ( MOF1 ) with tunable porosities (pore volume and surface area) have been synthesized by assembling Al^III and 1,4-NDC (1,4-naphthalenedicarboxylate) under microwave irradiation by varying water/ethanol solvent ratio. Water/ethanol mixture has played a crucial role in the mesopore generation in MOF1M₂₅ , MOF1M₅₀ , and MOF1M₇₅ , which is achieved by in situ formation of water/ethanol clusters. By adjusting the ratio of water/ethanol, the particle size, surface area and micro/mesopore volume fraction of the MOFs are controlled. Furthermore, reaction time plays a critical role in mesopore formation as realized by varying reaction time for the MOF with 50 % ethanol ( MOF1M₅₀ ). Additionally, hierarchical MOF ( MOF1M₅₀ ) has been used as a template for the stabilization of MAPbBr₃ (MA=methylammonium) perovskite quantum dots (PQDs). MAPbBr₃ PQDs are grown inside MOF1M₅₀ , where mesopores control the size of PQDs which leads to quantum confinement.     

Mixture of Azolium Tetraphenylborate with Isopropylthioxanthone: A New Class of N-Heterocyclic Carbene (NHC) Photogenerator for Polyurethane,Polyester, and ROMP Polymers Synthesis

In the search of smarter routes to control the conditions of N-heterocyclic carbene (NHCs) formation, a two-component air-stable NHC photogenerating system is reported. It relies on the irradiation at 365 nm of a mixture of 2-isopropylthioxanthone (ITX) with 1,3-bis(mesityl)imidazoli(ni)um tetraphenylborate. The photoinduced liberation of NHC is evidenced by reaction with a mesitoyl radical to form an NHC-radical adduct detectable by electron spin resonance spectroscopy. The NHC yield can be determined by ¹H NMR spectroscopy through the formation of a soluble and stable NHC–carbodiimide adduct. To deprotonate the azolium salt and liberate the NHC, a mechanism is proposed in which the role of base is played by ITX radical anion formed in situ by a primary photoinduced electron-transfer reaction between electronically excited ITX (oxidant) and BPh₄⁻ (reductant). An NHC yield as high as 70 % is achieved upon starting with a stoichiometric ratio of ITX and azolium salt. Three different photoNHC-mediated polymerizations are described: synthesis of polyurethane and polyester by organocatalyzed step-growth polymerization and ring-opening copolymerization, respectively, and generation of polynorbornene by ring-opening metathesis polymerization using an NHC-coordinated Ru catalyst formed in situ.     

Pt(II) Complexes with Azolate‐containing Bidentate Chelate: Design,Photophysics, and Application

This review is aimed at the current research progression of a unique class of Pt(II ) metal complexes bearing at least one azolate‐containing bidentate chelate. The azole fragment can link to a neutral heteroaromatic entity or another azole and form bidentate chelates, such as monoanionic 3‐pyridyl‐1H ‐pyrazole and derivatives, dianionic 3,3′‐bi‐1H ‐pyrazole, 3,3′‐(1‐methylethylidene)‐bis‐1H ‐pyrazole, and their analogs. These azole‐containing chelates readily react with a variety of Pt(II) reagents to afford the corresponding bis‐bidentate Pt(II) complexes. Most of them were highly emissive in solution, doped polymer matrix, thin film, and even as crystal or powder, due to the high ligand field strength exerted by these chelates and their high propensity in forming the singular square‐planar architecture and intermolecular aggregates with substantially strengthened Pt⋯Pt interaction, according to their structural design. Therefore, they hold bright prospects in academic research and future optoelectronic applications such as organic light‐emitting diodes.     

N-Heterocyclic Olefins as Electron Donors in Combination with Triarylborane Acceptors: Synthesis,Optical and Electronic Properties of D–π–A Compounds

N-heterocyclic olefins (NHOs), relatives of N-heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π-electron donating ability, NHOs were attached to triarylborane π-acceptors (A) giving donor (D)–π–A compounds 1 – 3 . In addition, an enamine π-donor analogue ( 4 ) was synthesized for comparison. UV–visible absorption studies show a larger red shift for the NHO-containing boranes than for the enamine analogue, a relative of cyclic (alkyl)(amino) carbenes (CAACs). Solvent-dependent emission studies indicate that 1 – 4 have moderate intramolecular charge-transfer (ICT) behavior. Electrochemical investigations reveal that the NHO-containing boranes have extremely low reversible oxidation potentials (e.g., for 3 , =−0.40 V vs. ferrocene/ferrocenium, Fc/Fc⁺, in THF). Time-dependent (TD) DFT calculations show that the HOMOs of 1 – 3 are much more destabilized than that of the enamine-containing 4 , which confirms the stronger donating ability of NHOs.     

Large-Scale Synthesis of a Niche Olefin Metathesis Catalyst Bearing an Unsymmetrical N-Heterocyclic Carbene (NHC) Ligand and its Application in a Green Pharmaceutical Context

A large-scale synthesis of known Ru olefin metathesis catalyst VII featuring an unsymmetrical N-heterocyclic carbene (NHC) ligand with one 2,5-diisopropylphenyl (DIPP) and one thiophenylmethylene N-substituent is reported. The optimised procedure does not require column chromatography in any step and allows for preparation of up to 0.5 kg batches of the catalyst from simple precursors. The application profile of the obtained catalyst was studied in environmentally friendly dimethyl carbonate (DMC). Although VII exhibited low efficiency in cross-metathesis (CM) with electron-deficient partners, good to excellent results were noted for substrates featuring easy to isomerise C−C double bonds. This includes polyfunctional substrates of medicinal chemistry interest, such as analogues of psychoactive 5F-PB-22 and NM-2201 and two PDE5 inhibitors—Sildenafil and Vardenafil. Finally, a larger scale ring-closing metathesis (RCM) of a Vardenafil derivative was conducted in DMC, allowing for straightforward isolation of the expected product (23 g) in high yield and with low Ru contamination level (7.7 ppm).     

N-Heterocyclic Silylenes as Ligands in Transition Metal Carbonyl Chemistry: Nature of Their Bonding and Supposed Innocence

A study on the reactivity of the N-heterocyclic silylene Dipp₂NHSi (1,3-bis(diisopropylphenyl)-1,3-diaza-2-silacyclopent-4-en-2-yliden) with the transition metal complexes [Ni(CO)₄], [M(CO)₆] (M=Cr, Mo, W), [Mn(CO)₅(Br)] and [(η⁵-C₅H₅)Fe(CO)₂(I)] is reported. We demonstrate that N-heterocyclic silylenes, the higher homologues of the now ubiquitous NHC ligands, show a remarkably different behavior in coordination chemistry compared to NHC ligands. Calculations on the electronic features of these ligands revealed significant differences in the frontier orbital region which lead to some peculiarities of the coordination chemistry of silylenes, as demonstrated by the synthesis of the dinuclear, NHSi-bridged complex [{Ni(CO)₂(μ-Dipp₂NHSi)}₂] ( 2 ), complexes [M(CO)₅(Dipp₂NHSi)] (M=Cr 3 , Mo 4 , W 5 ), [Mn(CO)₃(Dipp₂NHSi)₂(Br)] ( 9 ) and [(η⁵-C₅H₅)Fe(CO)₂(Dipp₂NHSi-I)] ( 10 ). DFT calculations on several model systems [Ni(L)], [Ni(CO)₃(L)], and [W(CO)₅(L)] (L=NHC, NHSi) reveal that carbenes are typically the much better donor ligands with a larger intrinsic strength of the metal–ligand bond. The decrease going from the carbene to the silylene ligand is mainly caused by favorable electrostatic contributions for the NHC ligand to the total bond strength, whereas the orbital interactions were often found to be higher for the silylene complexes. Furthermore, we have demonstrated that the contribution of σ- and π-interaction depends significantly on the system under investigation. The σ-interaction is often much weaker for the NHSi ligand compared to NHC but, interestingly, the π-interaction prevails for many NHSi complexes. For the carbonyl complexes, the NHSi ligand is the better σ-donor ligand, and contributions of π-symmetry play only a minor role for the NHC and NHSi co-ligands.     

Influence of Electronic Effects on the Reactivity of Triazolylidene‐Boryl Radicals: Consequences for the use of N‐Heterocyclic Carbene Boranes in Organic and Polymer Synthesis

A small library of triazolylidene‐boranes that differ only in the nature of the aryl group on the external nitrogen atom was prepared. Their reactivity as hydrogen‐atom donors, as well as that of the corresponding N‐heterocyclic carbene (NHC)‐boryl radicals toward methyl acrylate and oxygen, was investigated by laser flash photolysis, molecular orbital calculations, and ESR spin‐trapping experiments, and benchmarked relative to the already known dimethyltriazolylidene‐borane. The new NHC‐boranes were also used as co‐initiators for the Type I photopolymerization of acrylates. This allowed a structure–reactivity relationship with regard to the substitution pattern of the NHC to be established and the role of electronic effects in the reactivity of NHC‐boryl radicals to be probed. Although their rate of addition to methyl acrylate depends on their electronegativity, the radicals are all nucleophilic and good initiators for photopolymerization reactions.