Multiple Roles of Isocyanides in Palladium‐Catalyzed Imidoylative Couplings: A Mechanistic Study

Kinetic, spectroscopic and computational studies examining a palladium‐catalyzed imidoylative coupling highlight the dual role of isocyanides as both substrates and ligands for this class of transformations. The synthesis of secondary amides from aryl halides and water is presented as a case study. The kinetics of the oxidative addition of ArI with RNC‐ligated Pd⁰ species have been studied and the resulting imidoyl complex [(ArC=NR)Pd(CNR)₂I] (Ar=4‐F‐C₆H₄, R=tBu) has been isolated and characterized by X‐ray diffraction. The unprecedented ability of this RNC‐ligated imidoyl‐Pd complex to undergo reductive elimination at room temperature to give the amide in the presence of water and an F^?/HF buffer is demonstrated. Its behavior in solution has also been characterized, revealing an unexpected strong tendency to give cationic complexes, and notably [(ArC=NR)Pd(CNR)₃]⁺ with excess isocyanide and [(ArC=NR)Pd( )(CNR)]⁺ with bidentate phosphines ( ). These species may be responsible for catalyst deactivation and side‐reactions. Ab initio calculations performed at the DFT level allowed us to rationalize the multiple roles of RNC in the different steps of the catalytic cycle.     

Straightforward and Controlled Shape Access to Efficient Macrocyclic Imidazolylboronium Anion Receptors

A straightforward synthesis of air‐ and water‐stable bis‐cationic macrocyclic imidazolylboronium anion receptors is described herein. By taking advantage of the bulky and rigid 9‐borabicyclo[3.3.1]‐nonane (9‐BBN) attaching point and a well‐designed bis‐imidazolylaryl, highly stable dimeric imidazolylboronium macrocycles were synthesized. Additionally, NMR spectroscopy (¹H, DOSY, and HOESY), mass spectrometry (MS), and X‐ray diffraction studies revealed that these macrocyclic scaffolds can bind several monoanions with high association constants in DMSO, and are particularly sensitive for the MS detection of anions (with concentrations in the nm range). This anion/receptor interaction involves eight C?H binding sites, which include C ?H and unusual C_sp3?H hydrogen‐bonding donors.     

Paramagnetic NMR of Phenolic Oxime Copper Complexes: A Joint Experimental and Density Functional Study

¹H and ¹³C pNMR properties of bis(salicylaldoximato)copper(II) were studied in the solid state using magic‐angle‐spinning NMR spectroscopy and, for the isolated complex and selected oligomers, using density‐functional theory at the PBE0‐ //PBE0‐D3 level. Large paramagnetic shifts are observed, up to δ(¹H)=272 ppm and δ(¹³C)=1006 ppm (at 298 K), which are rationalised through spin delocalisation from the metal onto the organic ligand and the resulting contact shifts arising from hyperfine coupling. The observed shift ranges are best reproduced computationally using exchange‐correlation functionals with a high fraction of exact exchange (such as PBE0‐ ). Through a combination of experimental techniques and first‐principles computation, a near‐complete assignment of the observed signals is possible. Intermolecular effects on the pNMR shifts, modelled computationally in the dimers and trimers through effective decoupling between the local spins via A‐tensor and total spin rescaling in the pNMR expression, are indicated to be small. Addition of electron‐donating substituents and benzannelation of the organic ligand is predicted computationally to induce notable changes in the NMR signal pattern, which suggests that pNMR spectroscopy can be a sensitive probe for the spin distribution in paramagnetic phenolic oxime copper complexes.     

A Visible‐Light‐Triggered Conformational Diastereomer Photoswitch in a Bridged Azobenzene

Ketal‐substituted bridged azobenzenes have been synthesized; these display a symmetrical boat conformation with the ketal in pseudo‐equatorial positions. These bridged Z‐azobenzenes (Z₁) readily photoisomerize to the E‐isomer as well as another Z‐conformer (Z₂) with ketal function on the pseudo‐axial position upon irradiation at 406 nm. The two diastereomeric conformers display distinct physicochemical characteristics. Spectroscopic and NMR investigations supported that interconversion of two conformers occurs via the E‐isomer, with good photochemical quantum yield (Φ =0.45±0.03, Φ =0.33±0.05, Φ =0.37±0.06 and Φ =0.36±0.04). The system shows high photostability and no thermal equilibrium between the two stable Z₁ and Z₂ conformers.     

Mechanochemical Synthesis of 3d Transition‐Metal–1,2,4‐Triazole Complexes as Precursors for Microwave‐Assisted and Thermal Conversion to Coordination Polymers with a High Influence on the Dielectric Properties

The complexes [MCl₂(TzH)₄] (M=Mn ( 1 ), Fe ( 2 ); TzH=1,2,4‐1H‐triazole) and [ZnCl₂(TzH)₂] ( 3 ) have been obtained by mechanochemical reactions of the corresponding divalent metal chloride and 1,2,4‐1H‐triazole. They were successfully used as precursors for the formation of coordination polymers either by a microwave‐assisted reaction or by thermal conversion. For manganese, the conversion directly yielded [MnCl₂TzH] ( 4 ), whereas for the iron‐containing precursor, [FeCl₂TzH] ( 6 ), was formed via the intermediate coordination polymer [FeCl(TzH)₂]Cl ( 5 ). For cobalt, the isotypic polymer [CoCl(TzH)₂]Cl ( 7 ) was obtained, but exclusively by a microwave‐induced reaction directly from CoCl₂. The crystal structures were resolved from single crystals and powders. The dielectric properties were determined and revealed large differences in permittivity between the precursor complexes and the rigid chain‐like coordination polymers. Whereas the monomeric complexes exhibit very different dielectric behaviour, depending on the transition metal, from “low‐k” to “high‐k” with the permittivity ranging from 4.3 to >100 for frequencies of up to 1000 Hz, the coordination polymers and complexes with strong intermolecular interactions are all close to “low‐k” materials with very low dielectric constants up to 50 °C. Therefore, the conversion procedures can be used to deliberately influence the dielectric properties from complex to polymer and for different 3d transition‐metal ions.     

The Relationship between NMR Chemical Shifts of Thermally Polarized and Hyperpolarized 89Y Complexes and Their Solution Structures

Recently developed dynamic nuclear polarization (DNP) technology offers the potential of increasing the NMR sensitivity of even rare nuclei for biological imaging applications. Hyperpolarized ⁸⁹Y is an ideal candidate because of its narrow NMR linewidth, favorable spin quantum number (I= ), and long longitudinal relaxation times (T₁). Strong NMR signals were detected in hyperpolarized ⁸⁹Y samples of a variety of yttrium complexes. A dataset of ⁸⁹Y NMR data composed of 23 complexes with polyaminocarboxylate ligands was obtained using hyperpolarized ⁸⁹Y measurements or ¹H,⁸⁹Y‐HMQC spectroscopy. These data were used to derive an empirical equation that describes the correlation between the ⁸⁹Y chemical shift and the chemical structure of the complexes. This empirical correlation serves as a guide for the design of ⁸⁹Y sensors. Relativistic (DKH2) DFT calculations were found to predict the experimental ⁸⁹Y chemical shifts to a rather good accuracy.     

Localized Mixed‐Valence and Redox Activity within a Triazole‐Bridged Dinucleating Ligand upon Coordination to Palladium

The new dinucleating redox‐active ligand ( L^H4 ), bearing two redox‐active NNO‐binding pockets linked by a 1,2,3‐triazole unit, is synthetically readily accessible. Coordination to two equivalents of Pd^II resulted in the formation of paramagnetic (S= ) dinuclear Pd complexes with a κ²‐N,N′‐bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin–Day Class II mixed‐valence within the redox‐active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one‐electron oxidation allowed for isolation of the corresponding cationic ligand‐based diradical species. SQUID (super‐conducting quantum interference device) measurements of these compounds revealed weak anti‐ferromagnetic spin coupling between the two ligand‐centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox‐active ligand framework thus allows for efficient electronic communication between the two binding pockets.     

Physicochemical and Electronic Properties of Cationic [6]Helicenes: from Chemical and Electrochemical Stabilities to Far‐Red (Polarized) Luminescence

The physicochemical properties of cationic dioxa ( 1 ), azaoxa ( 2 ), and diaza ( 3 ) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pK_R+=20.4, =?0.72 V) compared to its azaoxa 2 (pK_R+=15.2, =?0.45 V) and dioxa 1 (pK_R+=8.8, =?0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far‐red regions. From 1 to 3 , a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post‐functionalization reactions (12 examples, compounds 4 – 15 ). The electronic absorption is modulated from the orange to the far‐red spectral range (560–731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first‐principles calculations.     

Co‐Adsorption of O2 and CO on Au2−: Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2O2(CO)n]− (n=2–6)

The co‐adsorption of O₂ and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au₂O₂(CO)_n]^? (n=2–6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au₂O₂(CO)_n]^? (n=2–6) complexes were characterized to have a core structure involving one CO and one O₂ molecule co‐adsorbed on Au₂^? with the other CO molecules physically tagged around. The CO stretching frequency of the [Au₂O₂(CO)]^? core ion is observed around =2032–2042 cm^?1, which is about 200 cm^?1 higher than that in [Au₂(CO)₂]^?. This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O₂ and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co‐adsorbed O₂ and CO on Au₂^? may have relevance to the elementary reaction steps on real gold catalysts.     

En Route to Stimuli‐Responsive Boron‐, Nitrogen‐, and Sulfur‐Doped Polycyclic Aromatic Hydrocarbons

Replacing both meso carbon atoms of the polycyclic aromatic hydrocarbon (PAH) bisanthene by boron atoms creates an efficient blue fluorophore with a strong electron‐accepting character. The corresponding meso‐B,S‐doped bisanthene exhibits a solvent‐dependent green‐to‐orange photoluminescence and undergoes a reversible reduction at E =?2.06 V (vs. FcH/FcH⁺). After oxidation of the sulfur atom, the resulting sulfoxide emits in the blue range of the spectrum, shows only negligible solvatochromism, and a reversible redox transition at E =?1.74 V. Several related B, N‐ and B, S‐containing PAHs have been prepared following the same modular synthetic procedure and are also described herein. In order to systematically compare their optoelectronic properties, all products have been investigated by cyclic voltammetry as well as UV/Vis absorption/emission spectroscopy.     

Hollow Gold Cages and Their Topological Relationship to Dual Fullerenes

Golden fullerenes have recently been identified by photoelectron spectra by Bulusu et al. [S. Bulusu, X. Li, L.‐S. Wang, X. C. Zeng, PNAS 2006 , 103, 8326–8330]. These unique triangulations of a sphere are related to fullerene duals having exactly 12 vertices of degree five, and the icosahedral hollow gold cages previously postulated are related to the Goldberg–Coxeter transforms of C₂₀ starting from a triangulated surface (hexagonal lattice, dual of a graphene sheet). This also relates topologically the (chiral) gold nanowires observed to the (chiral) carbon nanotubes. In fact, the Mackay icosahedra well known in gold cluster chemistry are related topologically to the dual halma transforms of the smallest possible fullerene C₂₀. The basic building block here is the (111) fcc sheet of bulk gold which is dual to graphene. Because of this interesting one‐to‐one relationship through Euler's polyhedral formula, there are as many golden fullerene isomers as there are fullerene isomers, with the number of isomers N_iso increasing polynomially as ). For the recently observed , , and we present simulated photoelectron spectra including all isomers. We also predict the photoelectron spectrum of . The stability of the golden fullerenes is discussed in relation with the more compact structures for the neutral and negatively charged Au₁₂ to Au₂₀ and Au₃₂ clusters. As for the compact gold clusters we observe a clear trend in stability of the hollow gold cages towards the (111) fcc sheet. The high stability of the (111) fcc sheet of gold compared to the bulk 3D structure explains the unusual stability of these hollow gold cages.     

A Convenient Approach To Synthesize o‐Carborane‐Functionalized Phosphorescent Iridium(III) Complexes for Endocellular Hypoxia Imaging

The structure–property relationship of carborane‐modified iridium(III) complexes was investigated. Firstly, an efficient approach for the synthesis of o‐carborane‐containing pyridine ligands a – f in high yields was developed by utilizing stable and cheap B₁₀H₁₀(Et₄N)₂ as the starting material. By using these ligands, iridium(III) complexes I – VII were efficiently prepared. In combination with DFT calculations, the photophysical and electrochemical properties of these complexes were studied. The hydrophilic nido‐o‐carborane‐based iridium(III) complex VII showed the highest phosphorescence efficiency (abs. =0.48) among known water‐soluble homoleptic cyclometalated iridium(III) complexes and long emission lifetime (τ=1.24 μs) in aqueous solution. Both of them are sensitive to O₂, and thus endocellular hypoxia imaging of complex VII was realized by time‐resolved luminescence imaging (TRLI). This is the first example of applying TRLI in endocellular oxygen detection with a water‐soluble nido‐carborane functionalized iridium(III) complex.     

Interception of Elusive Cationic Hf–Al and Hf–Zn Heterobimetallic Adducts with Mixed Alkyl Bridges Featuring Multiple Agostic Interactions

Heterobimetallic complexes with inequivalent bridging alkyl chains are very often invoked as key intermediates in many catalytic processes, yet their interception and structural characterization are lacking. Such complexes have been prepared from reactions of the cationic cyclometalated hafnocene [Cp^PrCp Hf][B(C₆F₅)₄] ( 1 ) with main group metal alkyls to afford the corresponding hetero-bridged cationic products, [Cp^PrCp Hf(μ-R)E(R)_n][B(C₆F₅)₄] (E=Al or Zn; R=Me, Et, or iBu). NMR and DFT studies demonstrate that both bridging alkyls establish agostic interactions with Hf, which are appreciably stronger for ethyl rather than methyl groups. Hf–Al and Hf–Zn distances are surprisingly short and only slightly longer than computed Hf–Al or Hf–Zn single bond lengths (2.80 Å). Finally, a reaction of [Cp^PrCp Hf(μ-Me)Zn(Me)][B(C₆F₅)₄] with excess ZnMe₂ yields an unprecedented heterotrimetallic species, [(Cp^Pr)₂Hf(μ-Me)(ZnMe)(μ₃-CH₂)ZnMe][B(C₆F₅)₄], the detailed structure of which is elucidated by a combination of NMR spectroscopic methods and molecular calculations.     

The Quadruple Bonding in C2 Reproduces the Properties of the Molecule

Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C₂ is bonded by a quadruple bond that can be distinctly characterized by valence‐bond (VB) calculations. We demonstrate that the quadruply‐bonded structure determines the key observables of the molecule, and accounts by itself for about 90 % of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply‐bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ‐type bond, the bond strength of which is estimated as 17–21 kcal mol^?1. Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol^?1, respectively, above the quadruply‐bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply‐bonded model is underscored by “predicting” the properties of the ³ state. C₂’s very high reactivity is rooted in its fourth weak bond. Thus, carbon and first‐row main elements are open to quadruple bonding!     

Structure and electrochemical properties for complexes of nitrocompounds with inorganic ions: A theoretical approach

Reduction and oxidation (redox) reactions are widely used for removal of nitrocompounds from contaminated soil and water. Structures and redox properties for complexes of nitrocompounds, such as 2,4,6‐trinitrotoluene (TNT), 2,4‐dinitrotoluene (DNT), 2,4‐dinitroanisole (DNAN), and 5‐nitro‐2,4‐dihydro‐3H?1,2,4‐triazol‐3‐one (NTO), with common inorganic ions (Na⁺, Cl^?, ) were investigated at the SMD(Pauling)/PCM(Pauling)/MPWB1K/TZVP level of theory. Atoms in molecules (AIM) theory was applied to analyze the topological properties of the bond critical points involved in the interactions between the nitrocompounds and the ions. Topological analyses show that intermolecular interactions of the types O(N)…Na⁺, C‐H…Cl^?( ), and C…Cl^?( ) may be discussed as noncovalent closed‐shell interactions, while N‐H···Cl^?( ) hydrogen bonds are partially covalent in nature. Complexation causes significant decrease of redox activity of the nitrocompounds. Analysis of the reduction potentials of the complexes obtained through application of the Pourbaix diagram of an iron/water system revealed that sodium complexes of NTO might be reduced by metallic iron. © 2016 Wiley Periodicals, Inc.     

Fluoride Binding and Crystal‐Field Analysis of Lanthanide Complexes of Tetrapicolyl‐Appended Cyclen

Lanthanide complexes of tetrapicolyl cyclen displayed remarkably high affinities for fluoride (log K≈5) in water, and were shown to form 1:1 complexes. The behaviour of these systems can be rationalised by changes to the magnitude of the crystal‐field parameter, . However, such changes are not invariably accompanied by a change in sign of this parameter: for early lanthanides, the N8 donor set with a coordinated axial water molecule ensures that the magnetic anisotropy has the opposite sense to that observed in the analogous dehydrated lanthanide complexes.     

Pressure Dependence and Kinetic Isotope Effects in the Absolute Rate Constant for Methoxy Radical Reacting with NO2

We have investigated the kinetics for the reaction CH₃O? + NO₂ in N₂ bath gas. The rate constants are well‐fit by the Troe expression over the temperature (250–335 K) and pressure range (30–700 Torr) investigated. The termolecular rate constant is given by cm⁶ molecule^?2 s^?1, and the rate constant at the high‐pressure limit is given by cm³ molecule^?1 s^?1. We also studied the kinetics of the reaction of CD₃O? + NO₂ as a function of temperature and pressure under similar conditions as those for CH₃O? + NO₂. The resulting low‐ and high‐pressure limiting rate constants are cm⁶ molecule^?2 s^?1 and cm³ molecule^?1 s^?1, respectively. The rate constants for the two isotopologues track each other closely as the high‐pressure limit is approached. The present results agree with most previous results at 295 K over a range of pressures, but there is substantial disagreement about the temperature dependence.     

Formal Nickelate(−I) Complexes Supported by Group 13 Ions

Formal nickelate(?I) complexes bearing Group 13 metalloligands (M=Al and Ga) were isolated. These 17 e^? complexes were synthesized by one‐electron reduction of the corresponding Ni⁰→M^III precursors, and were investigated by single‐crystal X‐ray diffraction, EPR spectroscopy, and quantum chemical calculations. Collectively, the experimental and computational data support: 1) the strengthening of the Ni?M bond upon one‐electron reduction, and 2) the delocalization of the unpaired spin across the Ni and M atoms. An intriguing electronic configuration is revealed where three valence electrons occupy two σ‐type bonding interactions: Ni(3d )²→M and σ‐(Ni?M)¹. The latter is an unusual Ni?M σ‐bonding molecular orbital that comprises primarily the Ni 4p_z and M np_z/ns atomic orbitals.     

Effect of tetra‐n‐butylammonium bromide salt on the cationic polymerization of dimethylketene and on the thermal properties of the obtained polyketones

The cationic polymerization of dimethylketene is achieved in dichloromethane at ?30 °C, using a stoichiometric mixture of aluminum bromide (AlBr₃) and tetra‐n‐butylammonium bromide (n‐Bu₄N⁺Br^?) as initiator. Characterizations by ¹H and ¹³C NMR show that the resulting polymers have a perfect polyketonic microstructure. Capillary viscosity, DSC, and SEC analysis show that for a constant monomer/initiator ratio, polymers synthesized in the presence of tetra‐n‐butylammonium bromide are more crystalline and have better properties than those produced only with AlBr₃. Melting temperatures, inherent viscosities and average molecular weights are systematically higher. A good linearity is observed between ln (inherent viscosity) versus ln for the system with n‐Bu₄N⁺Br^?, showing a good control of the molecular weight by the initial feed ratio. The effect of this compound suggests a reversible equilibrium between active and dormant species, which limits the transfer and/or termination reactions, and enables a better control of the cationic polymerization. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1493–1499     

Oblate versus Prolate Electron Density of Lanthanide Ions: A Design Criterion for Engineering Toroidal Moments? A Case Study on {LnIII6} (Ln=Tb,Dy, Ho and Er) Wheels

We report four new complexes based on a {Ln^III₆} wheel structure, three of which possess a net toroidal magnetic moment. The four examples consist of {Tb^III₆} and {Ho^III₆} wheels, which are rare examples of non Dy^III based complexes possessing a toroidal magnetic ground state, and a {Dy^III₆} complex which improves its toroidal structure upon lowering the crystallographic symmetry from trigonal (R ) to triclinic (P ). Notably the toroidal moment is lost for the trigonal {Er^III₆} analogue. This suggests the possibility of utilizing the popular concept of oblate and prolate electron density of the ground state M_J levels of lanthanide ions to engineer toroidal moments.