Trigonal Centers Research Articles

N-H Bond Activation of Ammonia by a Redox-Active Carboranyl Diphosphine.

In this work, we report the room-temperature N-H bond activation of ammonia by the carboranyl diphosphine 1-PtBu2-2-PiPr2-closo-C2B10H10 (1) resulting in the formation of zwitterionic 7-P(NH2)tBu2-10-P(H)iPr2-nido-C2B10H10 (2). Unlike the other phosphorus-based ambiphiles that require geometric constraints to enhance electrophilicity, the new mode of bond activation in this main-group system is based on the cooperation between electron-rich trigonal phosphine centers and the electron-accepting carborane cluster. As an exception among many other metal-based and metal-free systems, the N-H bond activation of gaseous ammonia or aqueous ammonium hydroxide by carboranyl diphosphine 1 proceeds with tolerance of air and water. Mechanistic details of ammonia activation were explored computationally by DFT methods, demonstrating an electrophilic activation of ammonia by the phosphine center. This process is driven by the reduction of the boron cluster followed by an ammonia-assisted deprotonation and proton transfer. A subsequent reaction of 2 and TEMPO results in the cleavage of all N-H and P-H bonds with the formation of a cyclic phosphazenium cation supported by an anionic cluster N(7-PtBu2-8-PiPr2)-nido-C2B9H10 (3). Transformations reported herein represent the first example of ammonia oxidation via triple hydrogen atom abstraction facilitated by a metal-free system.

Synthesis, characterization and reactivity of a Mn(III)–hydroxido complex as a biomimetic model for lipoxygenase

Manganese hydroxido (Mn–OH) complexes supported by a tripodal N,N′,N″-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido) ([poat]3−) ligand have been synthesized and characterized by spectroscopic techniques including UV–vis and electron paramagnetic resonance (EPR) spectroscopies. X-ray diffraction (XRD) methods were used to confirm the solid-state molecular structures of {Na2[MnIIpoat(OH)]}2 and {Na[MnIIIpoat(OH)]}2 as clusters that are linked by the electrostatic interactions between the sodium counterions and the oxygen atom of the ligated hydroxido unit and the phosphinic (P=O) amide groups of [poat]3−. Both clusters feature two independent monoanionic fragments in which each contains a trigonal bipyramidal Mn center that is comprised of three equatorial deprotonated amide nitrogen atoms, an apical tertiary amine, and an axial hydroxido ligand. XRD analyses of {Na[MnIIIpoat(OH)]}2 also showed an intramolecular hydrogen bonding interaction between the MnIII–OH unit and P=O group of [poat]3−. Crystalline {Na[MnIIIpoat(OH)]}2 remains as clusters with Na+---O interactions in solution and is unreactive toward external substrates. However, conductivity studies indicated that [MnIIIpoat(OH)]− generated in situ is monomeric and reactivity studies found that it is capable of cleaving C-H bonds, illustrating the importance of solution-phase speciation and its direct effect on chemical reactivity.Synopsis: Manganese–hydroxido complexes were synthesized to study the influence of H-bonds in the secondary coordination sphere and their effects on the oxidative cleavage of substrates containing C-H bonds.

Silyliumylidene Ion Stabilized by Two σ-Donating Ni(0)- and Pd(0)-Fragments.

A silyliumylidene ion 2 stabilized by two σ-donating Ni(0)- and Pd(0)-fragments was successfully synthesized. Due to the σ-donation of M→Si interactions, 2 presents a pyramidalized cationic silicon center with a localized lone pair. The additional coordination of basic Pd(0) fragment to the mono-Ni(0)-stabilized silyliumylidene 1 results in a higher HOMO level and an unchanged HOMO-LUMO gap and thus, 2 remains highly reactive. Interestingly, the coordination mode at the Si center is closely related to the nature of M-ligands. Indeed, the donor/donor-stabilized silyliumylidene ion 2 has been transformed into a donor/acceptor-stabilized ion 13, featuring a trigonal planar Si center with a vacant orbital, just via a ligand exchange reaction from PCy3/NHC toward PMe3.

Anti-Site Defects and Trigonal Center of Holmium in Y3Al5O12:Ho3+ Crystal According to the Results of Wideband EPR Spectroscopy

Anti-Site Defects and Trigonal Center of Holmium in Y3Al5O12:Ho3+ Crystal According to the Results of Wideband EPR Spectroscopy

Phosphinoborenium cations stabilized by N-heterocyclic carbenes: synthesis, structure, and reactivity.

Phosphinoborenium cations stabilized by N-heterocyclic carbenes (NHCs) were synthesized via the reaction of bromo(phosphino)boranes with NHCs. Their structures were investigated by heteronuclear magnetic resonance spectroscopy, X-ray diffraction, and density functional theory calculations. They possess a planar trigonal boron center directly bonded with the pyramidal phosphanyl group (PR2) and can be treated as cationic phosphinoboranes. The reactivity of the selected NHC-phosphinoborenium cation was tested toward AuCl·SMe2 and Ph2PCl. In both reactions, the titled compound acted as a phosphido group donor under heterolytic cleavage of the P-B bond. Control experiments with parent phosphinoborane emphasized differences between the reactivity of low-coordinate neutral and cationic species with P-B functionality.

C-C Bond Activation of Cyclopropanes Enabled by Phosphine-Catalyzed In Situ Formation of High-Strain Methylenecycopropane Intermediate.

An effective strategy for the ring-opening/elaboration of cyclopropanes by phosphine catalyst is documented, providing the 2,4-pentadiene sulfonamides and isoindolines in moderate to good yields. The key to the success of this reaction is phosphine-catalyzed introduction of a trigonal center into cyclopropanes, which results in the formation of higher ring strain cyclopropylidenemethyl phosphonium salt. Moreover, this methodology is employed as the key step for the synthesis of bioactive molecules.

Gallium vacancy --- shallows donor complexes in n-GaAs doped with elements of group VI Te or S (R e v i e w)

The results of studies of Ga vacancy --- shallow donor (Te or S atom substituting As atom in the lattice site closest to the vacancy VGa) complexes by methods of photoluminescence under resonant polarized excitation and piezospectroscopy are considered. An analysis of the experimental data in the classical dipole approximation shows that the symmetry of the complexes is lower than the trigonal one and their optical properties are explained in the model of a monoclinic defect with a symmetry plane of the \011\ type, in which the initial trigonal axis and the axis of the optical dipole lie. The reason for the lowering of the symmetry of the initially trigonal center is the distortions arising from the Jahn-Teller effect. A microscopic model is proposed that relates the behavior peculiarities of the optical properties of the VGaTeAs complex under uniaxial deformations and temperature changes with the structure of the electronic levels and the change in the charge state of the complex. The differences in this behavior observed for the VGaTeAs complex are analyzed and possible reasons for their appearance are discussed. Keywords: vacancy complexes in GaAs, Jahn-Teller effect, photoluminescence under resonant excitation, piezospectroscopy.

Coupling of Erbium-Implanted Silicon to a Superconducting Resonator

Erbium-implanted silicon is promising for both photonic and quantum-technology platforms, since it possesses both telecommunications and integrated-circuit processing compatibility. However, several different $\mathrm{Er}$ centers are generated during the implantation and annealing process, the presence of which could hinder the development of these applications. When $\mathrm{Si}$ is coimplanted with ${10}^{17}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ $\mathrm{Er}$ and ${10}^{20}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ $\mathrm{O}$ ions, and the appropriate annealing process is used, one of these centers, which is present at higher $\mathrm{Er}$ concentrations, can be eliminated. Characterization of samples with $\mathrm{Er}$ concentrations of ${10}^{17}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is limited by the sensitivity of standard electron paramagnetic resonance (EPR) instruments. The collective coupling strength between a superconducting (SC) $\mathrm{Nb}\mathrm{N}$ lumped-element resonator and a ${10}^{17}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ $\mathrm{Er}$-implanted $\mathrm{Si}$ sample at 20 mK is measured to be about 1 MHz, which provides a basis for the characterization of low-concentration $\mathrm{Er}$-implanted $\mathrm{Si}$ and for future networks of hybrid quantum systems that exchange quantum information over the telecommunication network. Of six known $\mathrm{Er}$-related EPR centers, only one trigonal center couples to the SC resonator.

Tris(carbene) Stabilization of Monomeric Magnesium Cations: A Neutral, Nontethered Ligand Approach

Herein, we describe the syntheses and structural characterization of bis(carbene)- and tris(carbene)-stabilized organomagnesium cations. The reaction of the N-heterocyclic carbene (NHC) stabilized Grignard reagent (iPrNHC)2Mg(Me)(Br) (1) and Na[BArF4] (iPrNHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, ArF = 3,5-bis(trifluoromethyl)phenyl) in chlorobenzene yields exclusively the bis(NHC)-stabilized dication [{(iPrNHC)2Mg}2(μ-Me)2][(BArF4)2] (2). If the reaction is performed in ethereal or nonpolar arene solvents, 2 undergoes Schlenk-type rearrangements to tris(NHC)-stabilized cations [(iPrNHC)3Mg(Me)][BArF4] (3[BArF4]) and [(iPrNHC)3Mg(Br)][BArF4] (4[BArF4]). These monomeric cations 3[A] and 4[A] (A = BArF4, BPh4) can be independently prepared as single pure products in high yields using common hydrocarbon solvents. The electronic influence of tris(carbene) stabilization is further evidenced by an NHC-mediated ionization of magnesium bromide in the absence of abstraction reagents. The reaction between the sterically unencumbered 1,3,4,5-tetramethylimidazol-2-ylidene (MeNHC) ligand and (MeNHC)2MgBr2 (7) resulted in two geometrically unique cations of the type [(MeNHC)3MgBr][Br]: complex 8a bearing a weakly coordinating bromide anion resulting in a trigonal bipyramidal magnesium center, and complex 8b bearing a noncoordinating bromide anion where the magnesium atom resides in a tetrahedral coordination environment. All isolated complexes were characterized by NMR spectroscopy and single-crystal X-ray diffraction, and their bonding was investigated by density functional theory (DFT).

Парамагнитные центры V-=SUP=-2+-=/SUP=- в иттрий-алюминиевом гранате

The study of the EPR spectrum of Y3Al5O12:V crystals allowed us to determine the parameters of the fine and hyperfine structure of the V2+ trigonal centers. The existence of V2+ triclinic centers that arose as a result of association of V2+ with defects that reduce symmetry was discovered.

Pushing the Lewis Acidity Boundaries of Boron Compounds With Non‐Planar Triarylboranes Derived from Triptycenes

AbstractBending the planar trigonal boron center of triphenylborane by connecting its aryl rings with carbon or phosphorus linkers gave access to a series of 9‐boratriptycene derivatives with unprecedented structures and reactivities. NMR spectroscopy and X‐ray diffraction of the Lewis adducts of these non‐planar boron Lewis acids with weak Lewis base revealed particularly strong covalent bond formation. The first Lewis adduct of a trivalent boron compounds with the Tf2N− anion illustrates the unrivaled Lewis acidity of these species. Increasing the pyramidalization of the boron center and using a cationic phosphonium linker resulted in an exceptional enhancement of Lewis acidity. Introduction of a phosphorus and a boron atom at each edge of a triptycene framework, allowed access to new bifunctional Lewis acid‐base 9‐phospha‐10‐boratriptycenes featuring promising reactivity for the activation of carbon‐halogen bonds.

Pushing the Lewis Acidity Boundaries of Boron Compounds With Non-Planar Triarylboranes Derived from Triptycenes.

Bending the planar trigonal boron center of triphenylborane by connecting its aryl rings with carbon or phosphorus linkers gave access to a series of 9-boratriptycene derivatives with unprecedented structures and reactivities. NMR spectroscopy and X-ray diffraction of the Lewis adducts of these non-planar boron Lewis acids with weak Lewis base revealed particularly strong covalent bond formation. The first Lewis adduct of a trivalent boron compounds with the Tf2 N- anion illustrates the unrivaled Lewis acidity of these species. Increasing the pyramidalization of the boron center and using a cationic phosphonium linker resulted in an exceptional enhancement of Lewis acidity. Introduction of a phosphorus and a boron atom at each edge of a triptycene framework, allowed access to new bifunctional Lewis acid-base 9-phospha-10-boratriptycenes featuring promising reactivity for the activation of carbon-halogen bonds.

EPR study of fluorite containing different concentration of ceria

The variation of EPR spectra has been investigated in CaF2 single crystals doped with ceria depending on the content of CeO2 and methods of the crystal growth. The concentration of CeO2 changed from 0.5% CeO2 up to 15% CeO2. For the low concentrations of CeO2 the spectra are mainly due to the usual tetragonal or trigonal EPR centers. For the sample with 5 at% CeO2 grown in argon atmosphere by the Bridgman technique the alternative tetragonal center with g|| = 0.73, and g┴ = 2.6 prevails. The possible models of EPR centers are briefly described.

EPR and optical spectroscopy of Yb3+ ions in LiCaAlF6 single crystals

EPR and optical spectroscopy of trigonal Yb3+ centers in LiCaAlF6 are reported. The results of these experiments make it possible to conclude that Yb3+ ions replace host cation sites Ca2+ forming mainly one type of paramagnetic center of trigonal symmetry. Positions of lines in optical spectra were specified of the observed center. The crystal field parameters of trigonal center Yb3+ were determined. Structural models of the observed center are proposed. It is shown that the substitution of the Yb3+ ion for the Ca2+ ion leads to the considerable compression of the YbF6 octahedron along the trigonal axis of the center.

Multi-modal imageability and degradation characteristics of high-borate glass systems for transient embolization

Two glass series were developed based on the substitution of a monovalent glass modifier for a di- or trivalent ion in a high borate glass system. The BRS series consists of compositions 0.70 B2O3–0.30-X Rb2O – X SrO, and the BRG series consists of compositions 0.70 B2O3–0.30-X Rb2O – X Ga2O3, where 0.00 ≤ X ≤ 0.10 in increments of 0.2. All glasses were characterized in order to examine their composition-structure-property relationships, and to assess their potential for use as degradable, radiopaque embolic agents. Glasses were melt quenched, and evaluated in terms of structural changes (11B MAS-NMR, density, and glass transition temperature), changes in radiopacity (both CT and MRI), and changes in degradation timeframes under simulated physiological conditions. Structural analysis determined there was no change in the B4% of the BRS series, despite a linear increase in density and Tg. The strontium acts as a crosslinker, creating a more hydrolytically stable network, which leads to longer dissolution times. Conversely, the BRG series displayed a linear decrease in the B4%, a decrease in density (Tg data not available), yet a slight increase in hydrolytic stability is also observed. Gallium most likely acts more as a glass former than a glass modifier, thereby sequestering oxygens from the tetrahedral boron centers, creating trigonal B3 centers and tetrahedral gallium. All glasses were found to be imageable on CT (intensity > 3200 HU at 120 kVp), and invisible on clinical MR imaging modalities.

EPR spectra and magnetization of XY-type rare-earth ions in pyrochlores Y2Ti2O7: RE3+ (RE=Yb, Er)

The results of studies of Y$_{2}$Ti$_2$O$_7$ single crystals doped with Er$^{3+}$ and Yb$^{3+}$ ions by means of electron paramagnetic resonance (EPR) and dc-magnetometry are reported. EPR signals of the trigonal centers with the characteristic hyperfine structure of Er$^{3+}$ or Yb$^{3+}$ ions were observed. Field dependences of magnetization of single crystals for magnetic fields directed along the crystallographic axes and temperature dependences of magnetic susceptibilities were measured. Spin Hamiltonian parameters ($g$-factors and parameters of the hyperfine interaction) for Er$^{3+}$ and Yb$^{3+}$ ions were obtained from analysis of experimental data. The registered EPR spectra and magnetization curves were successfully reproduced by simulations in framework of the crystal-field approach, in particular, with an account for hybridization of ground 4f$^{13}$ configuration of Yb$^{3+}$ ions with the charge transfer states.

Supramolecular Pt(II) and Ru(II) Trigonal Prismatic Cages Constructed with a Tris(pyridyl)borane Donor.

We report a novel example of supramolecular cages containing a Lewis acidic trigonal boron center. Self-assembly of the tris(pyridyl)borane donor 1 with diruthenium (2) or platinum (3), as an electron acceptor, furnished boron-containing trigonal prismatic supramolecular cages 5 and 6, which were characterized by 1H NMR and electrospray ionization time-of-flight mass spectroscopy and X-ray crystallography. The molecular structure of cage 5 was confirmed as a trigonal prismatic cage with an inner dimension of about 400 Å3. The fluoride binding properties of borane ligand 1 and Pt cage 6 were studied. UV/vis absorption titration studies demonstrated that the boron center of cage 6 undergoes strong binding interaction with the fluoride ion, with an estimated binding constant of 1.3 × 1010 M-2 in acetone based on the 1:2 binding isotherm. The binding was also confirmed by 1H NMR titration. Photoluminescence titration studies showed that cage 6 emitted borane-centered fluorescence (τ = 2.21 ns), which was gradually quenched upon addition of fluoride. When excess fluoride was added to a solution of 6, however, dissociation of the pyridyl ligand from the Pt(II) center was observed.

Zinc complexes of 3-pyrimidinyl-benzo-1,2,4-thiadiazine

The synthesis of the novel redox active ligand 3-(2′,6′-pyrimidine)-benzo-1,2,4-thiadiazine (pmbtdaH) is reported. The radical pmbtda can be prepared by in situ 1e− oxidation and its radical character confirmed by EPR spectroscopy, along with DFT calculations of the spin density distribution. Reaction of pmbtdaH with ZnCl2·2H2O in a 1:1 mol ratio afforded the dinuclear complex Zn2Cl4(pmbtdaH)2 (1) in which the two trigonal bipyramidal metal centres are linked via μ2-bridging Cl− ligands while the pmbtdaH ligand binds in an N,N′-chelate fashion via N(4) and the pyrimidinyl N(1) atom. The outcome of the reaction of pmbtdaH with Zn(hfac)2 (hfac = hexafluoroacetylacetonate) proved sensitive to the crystallization method; the mononuclear octahedral complex Zn(hfac)2(pmbtdaH) (2) in which the ligand coordinates in a similar N,N′-chelate fashion to that observed in 1 was isolated by layering a concentrated CH2Cl2 solution with hexane. Conversely the same reaction mixture afforded the tetranuclear complex Zn4(hfac)6(pmbtdaox)2 (3) when left undisturbed in a sealed vial. In 3 the ligand is both deprotonated and oxidized forming an anionic ligand in which there are two potential N,N′-chelate pockets and an additional S-oxide donor. The pmbtdaox− ligand bridges between a Zn(hfac)2 and a Zn(hfac) unit to form Zn2(hfac)3(pmbtdaox). Two such Zn2(hfac)3(pmbtdaox) units dimerise through the S-oxide oxygen atom to form a centrosymmetric tetranuclear Zn4(hfac)6(pmbtdaox)2 cluster. The structures of 1–3 have been determined by X-ray diffraction, elemental analysis, IR and UV–Vis spectroscopies as well as 1H NMR spectroscopy for complex 2.

Syntheses, steric hindrance effects, luminescent properties and TD-DFT calculations for a series of copper(I) iodide coordination complexes

A series of copper(I) coordination complexes, CuI(Phen)[2-(Dpp)bp] (1) (Phen = phenanthroline, 2-(Dpp)bp = 2-(Diphenylphosphino)-biphenyl), Cu2I2(Phen)[2-(Dpp)bp] (2), CuI(2-PBI)[2-(Dpp)bp] (3) and (2-PBI = 2-(pyridin-2-yl)-1H-benzo[d]imidazole) and CuI(Bipy)[2-(Dpp)bp] (Bipy = 2,2′-bipyridine) (4) have been synthesized. X-ray crystal structure studies revealed that complexes 1, 3 and 4 showed mononuclear structures with the copper atoms coordinated by iodide, a chelating nitrogen-donor ligand, and a monodentate phosphine ligand. However, the coordination centers display different distortions of their tetrahedral geometries, according to the steric hindrance of the bulky phosphine ligands. Complex 2 has a dinuclear structure, with trigonal and tetrahedral coordination centers. Variations in the aromatic system of the N-heterocyclic ligands result in different luminescence properties. Thus, the emission maxima for these complexes range from 580 to 642 nm, with lifetimes of τ = 0.6–0.9 and 1.6–4.2 μs. TD-DFT calculations reveal the origin of the luminescence to be metal–ligand charge transfer, as well as halogen–ligand charge transfer. The optical absorption spectra and thermal stabilities of the complexes have also been studied.

Slow magnetic relaxation in a {CoIICo} complex containing a high magnetic anisotropy trigonal bipyramidal CoII centre.

We report a trinuclear mixed-valence {CoIICoIII2} complex, where the CoII centre adopts a trigonal bipyramidal geometry, leading to a large, easy-plane magnetic anisotropy and field-induced slow magnetic relaxation with a Raman-like relaxation process.