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1.
A class of polymeric compounds containing boron–boron triple bonds stabilized by N‐heterocyclic biscarbenes is proposed. Since a triply bonded B2 is related to its third excited state, the predicted macromolecule would be composed by several units of an electronically excited first‐row homonuclear dimer. Moreover, it is shown that the replacement of biscarbene with N2 or CO as spacers could change the bonding profile of the boron–boron units to a cumulene‐like structure. Based on these results, different types of diboryne polymers are proposed, which could lead to an unprecedented set of boron materials with distinct physical properties. The novel diboryne macromolecules could be synthesized by the reaction of Janus‐type biscarbenes with tetrabromodiborane, B2Br4, and sodium naphthalenide, [Na(C10H8)], similarly to Braunschweig’s work on the room temperature stable boron–boron triple bond compounds (Science, 2012 , 336, 1420).  相似文献   

2.
A triaminotriborane(3) was isolated as purple crystals through the reduction of (TMP)BCl2 (TMP=2,2,6,6‐tetramethylpiperidino) by sodium naphthalenide. Single‐crystal X‐ray diffraction and computational studies of the obtained triaminotriborane(3) revealed a bent structure of the [B(NR2)]3 chain. The bond lengths between the central and terminal boron atoms were similar to those observed in neutral diborene species. The multiple‐bonding character may be best described by a three‐center two‐electron π‐bond along the B3 chain. The distance between the two terminal boron atoms (2.177 Å) in the solid‐state structure implies a weak interaction between them. When an excess amount of Li was used as the reducing agent, the reaction yielded an unusual dianionic species. The isolation and characterization of these two reduction products are reported herein.  相似文献   

3.
Cyclic diboranes(4) based on a chelating monoanionic benzylphosphine linker were prepared through boron–silicon exchange between arylsilanes and B2Br4. Coordination of Lewis bases to the remaining sp2 boron atom yielded unsymmetrical sp3‐sp3 diboranes, which were reduced with KC8 to their corresponding trans‐diborenes. These compounds were studied with a combination of spectroscopic methods, X‐ray diffraction, and DFT calculations. PMe3‐stabilized diborene 6 was found to undergo thermal rearrangement to gem‐diborene 8 . DFT calculations on 8 reveal a polar boron–boron bond, and indicate that the compound is best described as a borylborylene.  相似文献   

4.
The realization of a phosphine‐stabilized diborene, Et3P?(Mes)B?B(Mes)?PEt3 ( 4 ), by KC8 reduction of Et3P?B2Mes2Br2 in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC‐stabilized diborene IMe?(Dur)B?B(Dur)?IMe ( 1 ). Importantly, both species feature unusual electron‐rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the B? B double bond system. Thus, the stronger σ‐donor strength and larger electronegativity of the NHC ligand results in an energetically higher‐lying HOMO, making 1 a stronger neutral reductant as 4 ( 1 : E1/2=?1.55 V; 4 : ?1.05 V), and a smaller HOMO–LUMO gap of 1 accompanied by a noticeable red‐shift of its lowest‐energy absorption band with respect to 4 . Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron‐centered radical cations ( 5 and 6 ).  相似文献   

5.
There is currently tremendous interest in the previously documented example of a stable species exhibiting a boron–boron triple bond (Science, 2012, 336, 1420). Notably, it has recently been stated using arguments based on force constants that this diboryne may not, in reality, feature a boron–boron triple bond. Here, we use advanced solid-state NMR and computational methodology in order to directly probe the orbitals involved in multiple boron–boron bonds experimentally via analysis of 11B–11B spin–spin (J) coupling constants. Computationally, the mechanism responsible for the boron–boron spin–spin coupling in these species is found to be analogous to that for the case of multiply-bonded carbon atoms. The trend in reduced J coupling constants for diborenes and a diboryne, measured experimentally, is in agreement with that known for alkenes and alkynes. This experimental probe of the electronic structure of the boron–boron multiple bond provides strong evidence supporting the originally proposed nature of the bonds in the diboryne and diborenes, and demonstrates that the orbitals involved in boron–boron bonding are equivalent to those well known to construct the multiple bonds between other second-row elements such as carbon and nitrogen.  相似文献   

6.
We explore the possibility that a 3‐electron‐2‐center bonding exists in cycloimmonium ylides. To detect this bonding in a polyatomic system, 3‐electron‐1‐hole density operators, characterizing a Pauling 3‐electron bond, are used in the framework of second quantization formalism. The weights of 3‐electron resonance structures are calculated and compared with the weights of 2‐electron structures for the ylide bond of pyridinium dicyanomethylide; the correlations of (↑↓) and (↑) electronic events, involved in the 3‐electron resonance structures, are also investigated. The calculations are performed in various approximation levels, and both orthogonal and nonorthogonal natural atomic orbitals are adopted. All calculations show that a 3‐electron bond exists between N and C atoms of ylide bond, but this bonding is not extended in C atoms of the pyridinium group. The interactions of α,β electrons (at the configuration interaction [CI] level) increase the localization of electrons, the weights of 3‐electron resonance structures, and thus the probability for 3‐electron bonding. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004  相似文献   

7.
A simple way of rationalizing the structures of cyclic, bicyclic, and tricyclic sulfur–nitrogen species and their congeners is presented. Starting from a planar tetrasulfur tetranitride with 12π electrons, we formally derived on paper a number of heterocyclic eight‐membered 10π electron species by reacting the 3p orbitals of two opposite sulfur centers with one radical each, or by replacing these centers by other atoms with five (P) or four (Si, C) valence electrons. This led to planar aromatic 10π electron systems, nonplanar bicyclic structures with a transannular S?S bond, and tricyclic structures by bridging the planar rings with an acceptor or donor unit. The final structures depend on the number of π electrons in the bridges. Intermediate biradicals are stabilized by Jahn–Teller distortion, giving transannular S?S bonds between the NSN units. This procedure may be summarized by two rules, which provide a rationale for the structures of a large number of sulfur–nitrogen‐based molecules. The long bonds between the NSN units show a p character of >95 %. The qualitative results have been compared with known molecular structures and the results of B3LYP/cc‐pVTZ calculations as well as CASSCF and CASVB calculations. B3LYP/cc‐pVTZ calculations have also provided the UV/Vis spectra and the NICS values of the planar 10π systems.  相似文献   

8.
Symmetric‐ and asymmetric hexaarylbenzenes (HABs), each substituted with three electron‐donor triarylamine redox centers and three electron‐acceptor triarylborane redox centers, were synthesized by cobalt‐catalyzed cyclotrimerization, thereby forming compounds with six‐ and four donor–acceptor interactions, respectively. The electrochemical‐ and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption‐ and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor–acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak‐separation is greatly influenced by the conducting salt, owing to ion‐pairing and shielding effects. Consequently, the peak‐separations cannot be interpreted in terms of the electronic couplings in the generated mixed‐valence species. Transient‐absorption spectra, fluorescence‐solvatochromism, and absorption spectra show that charge‐transfer states from the amine‐ to the boron centers are generated after optical excitation. The electronic donor–acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge‐transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60 ns). This result was confirmed by steady‐state fluorescence‐anisotropy measurements, which further indicated symmetry‐breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge‐transfer character in the donor–acceptor chromophores vanishes, as observed in both the absorption‐ and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen‐bonding interactions between water molecules and the fluoride anions.  相似文献   

9.
Is one electron sufficient to bring about significant σ bonding between two atoms? The chemist’s view on the chemical bond is usually tied to the concept of shared electron pairs, and not too much experimental evidence exists to challenge this firm belief. Whilst species with the unusual one‐electron σ‐bonding motif between homonuclear atoms have so far been identified mainly by spectroscopic evidence, we present herein the first crystallographic characterization, augmented by a detailed quantum‐chemical validation, for a radical anion featuring a B?B one‐electron‐two‐center σ bond.  相似文献   

10.
Novel subphthalocyanine analogues that display strong absorption in the green region have been synthesized by using a boron template cyclotrimerization of maleonitrile derivatives. The spectroscopic properties of these macrocycles indicate that, like subphthalocyanines, they have 14 π electrons and are aromatic compounds with a conical shape. The removal of the three fused benzene rings from the subphthalocyanine skeleton produces a 75–80 nm blue shift of the Q‐band and a slight lowering of the absorption coefficients for this band. In addition, the reduction of the π system from 18 to 14 electrons that accompanies progression from porphyrazines to subporphyrazines causes a hypsochromic shift of the Q‐band of around 100 nm. Subporphyrazines that are peripherally functionalized with six thioether chains, and in which the sulfur atoms are attached directly to the pyrrole moieties, exhibit optical features that may be explained in terms of the extension of π conjugation over the six thiolene groups, as well as strong π donation from the sulfur lone pairs to the macrocycle. These two effects are quantitatively and qualitatively very similar to those observed for porphyrazines that possess the same type of substitution. In addition, the mesomorphic behavior at low temperatures of a macrocycle that is substituted with six thiododecyl chains was demonstrated by using differential scanning calorimetry and optical polarising microscopy.  相似文献   

11.
The types of sulfur bonding—as sulfane or sulfide—encountered in the molecules of maingroup elements are almost unknown in the chemistry of metal complexes, where the sulfur atoms function instead as two-electron donors by bridging two metal atoms, as four-electron donors by bridging three or four metal atoms, or as six-electron donors by incorporation between four metal atoms. In such complexes, the metal-metal bond can be modified over a wide range by chemical or electrochemical variation of the number of electrons present. The readiness with which polynuclear complexes containing metals and sulfur undergo redox reactions is also utilized by Nature in the active sites of some redox proteins.  相似文献   

12.
This article proposes to bridge two fields, namely organometallics and quantum chemical topology. To do so, Palladium‐catalyzed reductive elimination is studied. Such reaction is a classical elementary step in organometallic chemistry, where the directionality of electrons delocalization is not well understood. New computational evidences highlighting the accepted mechanism are proposed following a strategy coupling quantum theory of atoms in molecules and electron localization function topological analyses and enabling an extended quantification of donated/back‐donated electrons fluxes along reaction paths going beyond the usual Dewar–Chatt–Duncanson model. Indeed, if the ligands coordination mode (phosphine, carbene) is commonly described as dative, it appears that ligands lone pairs stay centered on ligands as electrons are shared between metal and ligand with strong delocalization toward the latter. Overall, through strong trans effects coming from the carbon involved in the reductive elimination, palladium delocalizes its valence electrons not only toward phosphines but interestingly also toward the carbene. As back‐donation increases during reductive elimination, one of the reaction key components is the palladium ligands ability to accept electrons. The rationalization of such electronic phenomena gives new directions for the design of palladium‐catalyzed systems. © 2015 Wiley Periodicals, Inc.  相似文献   

13.
The Electron Localization Function (ELF) describes chemical bonding through localized pairs of electrons and gives a quantum mechanical basis to the representation of the chemical bond as a line. Computer graphics produce illustrative and intuitive pictures of the shell structure of atoms, ionic, covalent, and coordinative bonds, or multiple bonds and lone pairs of electrons. Resonance formulas, delocalized bonds, and the space occupied by electron pairs can be visualized in the same manner.  相似文献   

14.
A new qualitative treatment of the bonding in ozone is presented. It is based upon a combination of several simple concepts: the nonparticipation of the pairs of electrons tightly held in the atomic 2s orbitals; simple overlap of the 2p orbitals to form sigma bonds; interaction of three 2p orbitals to yield bonding and nonbonding pi molecular orbitals that are populated by electron pairs; and van der Waals repulsion between the two terminal oxygen atoms forcing these atoms apart to yield the bond angle of 117° as a compromise. Both the assumptions and the resulting bonding picture are in accord with the photoelectron spectroscopic data, the results from sophisticated molecular orbital calculations, and the common physical properties of ozone.  相似文献   

15.
The Me‐cAAC:‐stabilized dimer of silicon disulfide (SiS2) has been isolated in the molecular form as (Me‐cAAC:)2Si2S4 ( 2 ) at room temperature [Me‐cAAC:=cyclic alkyl(amino) carbene]. Compound 2 has been synthesized from the reaction of (Me‐cAAC:)2Si2 with elemental sulfur in a 1:4 molar ratio under oxidative addition. This is the smallest molecular unit of silicon disulfide characterized by X‐ray crystallography, electron ionization mass spectrometry, and NMR spectroscopy. Structures with three sulfur atoms arranged around a silicon atom are known; however, 2 is the first structurally characterized silicon–sulfur compound containing one terminal and two bridging sulfur atoms at each silicon atom. Compound 2 shows no decomposition after storing for three months in an inert atmosphere at ambient temperature. The bonding of 2 has been further studied by theoretical calculations.  相似文献   

16.
To introduce halogen‐bond interactions between a cation and an anion, a novel FeIII complex from iodine‐substituted ligands involving a paramagnetic nickel dithiolene anion was prepared and characterized. The compound exhibited the synergy between a spin‐crossover transition and a spin‐Peierls‐like singlet formation. The halogen‐bond interactions between the iodine and the sulfur atoms stabilized the paramagnetic state of π‐spins and played a crucial role in the synergistic magnetic transition between d‐ and π‐spins. In addition, the compound showed the light‐induced excited spin state trapping effect.  相似文献   

17.
12‐Chloro‐12‐n‐butyl‐1,11‐dioxa‐4,8‐ dithia‐12‐stannacyclododecane ( 3a ) and 12‐chloro‐ 12‐n‐butyl‐1,4,8,11‐tetrathia‐12‐stannacyclododecane ( 3b ) have been prepared by reacting n‐butyltin trichloride with 1,11‐dioxa‐4,8‐dithiaundecane and 1,4,8,11‐tetrathiaundecane, respectively. Complexes 3a,b were characterized by elemental analyses, IR, electron impact mass spectrometry, and multinuclear NMR (1H, 13C, and 119Sn). The spectroscopic data are consistent with bonding of the ligands through both sulfur and oxygen atoms in 3a and through all sulfur atoms in 3b to the Sn(IV) center. We suggest hexacoordination around the Sn atoms. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:451–453, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20040  相似文献   

18.
By using combination of bond‐order–length–strength (BOLS) correlation, the tight‐binding (TB) approach, and zone‐selective photoelectron spectroscopy (ZPS), we were able to resolve local bond relaxation and the associated 4f7/2 core‐level shift of Au atomic clusters, Au(100, 110, 111) skins, and Au foils exposed to ozone for different lengths of time. In addition to quantitative information, such as local bond length, bond energy, binding‐energy density, and atomic cohesive energy, the results confirm our predictions that bond‐order deficiency shortens and stiffens the bond between undercoordinated atoms, which results in local densification and quantum entrapment of bonding electrons. The entrapment perturbs the Hamiltonian, and hence, shifts the core‐level energy accordingly. ZPS also confirms that oxidation enhances the effect of atomic undercoordination on the positive 4f7/2 energy shift, with the associated valence electron polarization contributing to the catalytic ability of undercoordinated Au atoms.  相似文献   

19.
Iminosulfane(phosphane)carbon(0) derivatives (iSPCs; Ar3P→C←SPh2(NMe); Ar=Ph ( 1 ), 4‐MeOC6H4 ( 2 ), 4‐(Me2N)C6H4 ( 3 )) have been successfully synthesized and the molecular structure of 3 characterized. Carbone 3 is the first thermally and hydrolytically stable carbone stabilized by phosphorus and sulfur ligands. DFT calculations reveal the electronic structures of 1 – 3 , which have two lone pairs of electrons at the carbon center. First and second proton affinity values are theoretically calculated to be in the range of 286.8–301.1 and 189.6–208.3 kcal mol?1, respectively. Cyclic voltammetry measurements reveal that the HOMO energy levels follow the order of 3 > 2 > 1 and the HOMO of 3 is at a higher energy than those of bis(chalcogenane)carbon(0) (BChCs). The reactivities of these lone pairs of electrons are demonstrated by the C‐diaurated and C‐proton‐aurated complexes. These results are the first experimental evidence of phosphorus‐ and sulfur‐stabilized carbones behaving as four‐electron donors. In addition, the reaction of hydrochloric salts of the carbones with Ag2O gives the corresponding AgI complexes. The resulting silver(I) carbone complexes can be used as carbone transfer agents. This synthetic protocol can also be used for moisture‐sensitive carbone species.  相似文献   

20.
Despite the fundamental importance of radical‐anion radical‐cation pairs in single‐electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have highly electron‐rich B? B double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron‐centered radical‐anion radical‐cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one‐electron reduction. However, since the reduction potentials of all know diborenes (E1/2=?1.05/?1.55 V) were not sufficiently negative to reduce MesBC4Ph4 (E1/2=?1.69 V), a suitable diborene, IiPr?(iPr)B?B(iPr)?IiPr, was tailor‐made to comply with these requirements. With a halfwave potential of E1/2=?1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4Ph4 by SET to afford a stable boron‐centered radical‐anion radical‐cation pair.  相似文献   

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