Cyclic Amino(Ylide) Silylene: A Stable Heterocyclic Silylene with Strongly Electron‐Donating Character

An isolable heterocyclic silylene ( 4 ) with two different π‐donating substituents, namely a classical amino group and a more electropositive and stronger carbon‐based π‐donating phosphonium ylide, was synthesized and fully characterized. The combination of these two different π‐donating substituents confers high thermal stability and an unusual nucleophilic character on silylene 4 . Therefore, silylene 4 behaves as a strong donor ligand toward transition metals with a donating character comparable to N‐heterocyclic carbenes, in contrast to classical N‐heterocyclic silylenes, which generally present a weak donating character.     

Cyclic (Amino)(Phosphonium Bora‐Ylide)Silanone: A Remarkable Room‐Temperature‐Persistent Silanone

A silanone substituted by bulky amino and phosphonium bora‐ylide substituents has been isolated in crystalline form. Thanks to the exceptionally strong electron‐donating phosphonium bora‐ylide substituent, the lifetime at room temperature of the silanone is dramatically extended (t _1/2=4 days) compared to the related (amino)(phosphonium ylide)silanone VI (t _1/2=5 h), allowing easier manipulation and its use as precursor of new valuable silicon compounds. The interaction of silanone with a weak Lewis acid such as MgBr₂ increases further its stability (no degradation after 3 weeks at room temperature).     

An N‐Heterocyclic Silylene‐Stabilized Digermanium(0) Complex

The synthesis of an N‐heterocyclic silylene‐stabilized digermanium(0) complex is described. The reaction of the amidinate‐stabilized silicon(II) amide [LSiN(SiMe₃)₂] ( 1 ; L=PhC(NtBu)₂) with GeCl₂?dioxane in toluene afforded the Si^II–Ge^II adduct [L{(Me₃Si)₂N}Si→GeCl₂] ( 2 ). Reaction of the adduct with two equivalents of KC₈ in toluene at room temperature afforded the N‐heterocyclic carbene silylene‐stabilized digermanium(0) complex [L{(Me₃Si)₂N}Si→ Ge?Ge←Si{N(SiMe₃)₂}L] ( 3 ). X‐ray crystallography and theoretical studies show conclusively that the N‐heterocyclic silylenes stabilize the singlet digermanium(0) moiety by a weak synergic donor–acceptor interaction.     

Equilibrium Formation of Stable All‐Silicon Versions of 1,3‐Cyclobutanediyl

Main group analogues of cyclobutane‐1,3‐diyls are fascinating due to their unique reactivity and electronic properties. So far only heteronuclear examples have been isolated. Here we report the isolation and characterization of all‐silicon 1,3‐cyclobutanediyls as stable closed‐shell singlet species from the reversible reactions of cyclotrisilene c‐Si₃Tip₄ (Tip=2,4,6‐triisopropylphenyl) with the N‐heterocyclic silylenes c‐[(CR₂CH₂)(NtBu)₂]Si: (R=H or methyl) with saturated backbones. At elevated temperatures, tetrasilacyclobutenes are obtained from these equilibrium mixtures. The corresponding reaction with the unsaturated N‐heterocyclic silylene c‐(CH)₂(NtBu)₂Si: proceeds directly to the corresponding tetrasilacyclobutene without detection of the assumed 1,3‐cyclobutanediyl intermediate.     

Protic N‐Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton‐ and Electron‐Donating Abilities in a Pincer‐Type Framework

Evaluation of the acidity of proton‐responsive ligands such as protic N‐heterocyclic carbenes (NHCs) bearing an NH‐wingtip provides a key to understanding the metal–ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer‐type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron‐donating abilities. The synthesis is achieved by direct C?H metalation of 2‐(imidazol‐1‐yl)‐6‐(pyrazol‐3‐yl)pyridine with [RuCl₂(PPh₃)₃]. ¹⁵N‐Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron‐donating than pyrazole in both protonated and deprotonated forms.     

The Lightest Element Phosphoranylidene: NHC‐Supported Cyclic Borylidene–Phosphorane with Significant B=P Character

A borylidene–phosphorane, the lightest phosphoranylidene, which is stabilized by an N ‐heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong B→P π‐back bonding related to the less electronegative boron compared to phosphorus.     

Reaction of an N‐Heterocyclic Carbene‐Stabilized Silicon(II) Monohydride with Alkynes: [2+2+1] Cycloaddition versus Hydrogen Abstraction

An in depth study of the reactivity of an N‐heterocyclic carbene (NHC)‐stabilized silylene monohydride with alkynes is reported. The reaction of silylene monohydride 1 , tBu₃Si(H)Si←NHC, with diphenylacetylene afforded silole 2 , tBu₃Si(H)Si(C₄Ph₄). The density functional theory (DFT) calculations for the reaction mechanism of the [2+2+1] cycloaddition revealed that the NHC played a major part stabilizing zwitterionic transition states and intermediates to assist the cyclization pathway. A significantly different outcome was observed, when silylene monohydride 1 was treated with phenylacetylene, which gave rise to supersilyl substituted 1‐alkenyl‐1‐alkynylsilane 3 , tBu₃Si(H)Si(CH?CHPh)(C?CPh). Mechanistic investigations using an isotope labelling technique and DFT calculations suggest that this reaction occurs through a similar zwitterionic intermediate and subsequent hydrogen abstraction from a second molecule of phenylacetylene.     

Synthesis and Rearrangement of Stable NHC→Silylene Adducts and Their Unique Reactivity towards Cyclohexylisocyanide

The syntheses and reactivity of the two N‐heterocyclic carbene (NHC)→ silylene complexes 2 and 4 have been investigated. The latter are easily accessible by reaction of the zwitterionic, N‐heterocyclic silylene LSi: 1 [L=Ar‐N‐C(=CH₂)CH?C(Me)‐N‐Ar, Ar=2,6‐iPr₂C₆H₃] with 1,3,4,5‐tetramethylimidazol‐2‐ylidene and 1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene, respectively. While compound 2 undergoes facile rearrangement above ?20 °C to give the unsymmetrical N‐heterocyclic silylcarbene 3 , the derivative 4 remains unchanged even after boiling in benzene. The remarkable reactivity of 3 and 4 towards cyclohexylisocyanide has been examined which leads in a unique series of C? H, Si? H, and C? N bond activations to the new triaminosilanes 5 and 6 , respectively. The novel compounds 3 , 4 , 5 , and 6 were fully characterized by ¹H, ¹³C, and ²⁹Si NMR spectroscopy, EI‐MS, elemental analysis, and single‐crystal X‐ray diffraction.     

Chemoselectivity in the Reaction of 2‐Diazo‐3‐oxo‐3‐phenylpropanal with Aldehydes and Ketones

The chemoselectivity in the reaction of 2‐diazo‐3‐oxo‐3‐phenylpropanal ( 1 ) with aldehydes and ketones in the presence of Et₃N was investigated. The results indicate that 1 reacts with aromatic aldehydes with weak electron‐donating substituents and cyclic ketones under formation of 6‐phenyl‐4H‐1,3‐dioxin‐4‐one derivatives. However, it reacts with aromatic aldehydes with electron‐withdrawing substituents to yield 1,3‐diaryl‐3‐hydroxypropan‐1‐ones, accompanied by chalcone derivatives in some cases. It did not react with linear ketones, aliphatic aldehydes, and aromatic aldehydes with strong electron‐donating substituents. A mechanism for the formation of 1,3‐diaryl‐3‐hydroxypropan‐1‐ones and chalcone derivatives is proposed. We also tried to react 1 with other unsaturated compounds, including various olefins and nitriles, and cumulated unsaturated compounds, such as N,N′‐dialkylcarbodiimines, phenyl isocyanate, isothiocyanate, and CS₂. Only with N,N′‐dialkylcarbodiimines, the expected cycloaddition took place.     

A Stable N‐Hetero‐Rh‐Metallacyclic Silylene

A cyclic (amino)metal‐substituted dicoordinated silylene derivative has been synthesized and fully characterized. Of particular interest is that the N‐hetero‐Rh^I‐metallacyclic silylene exhibits a distorted tetrahedral geometry around the rhodium atom and a considerably shortened Si?Rh bond (2.138 Å) compared to classical Si?Rh single bonds (ca. 2.30–2.35 Å). A theoretical investigation reveals that the geometrical deviation around the rhodium center from the classical square‐planar to a tetrahedral geometry increases the π‐donating and σ‐accepting character of the rhodium atom, thereby efficiently stabilizing the silylene moiety.     

Unusual Fragmentation and Transformation of an N‐Heterocyclic Carbene by a Stable Phosphonium–Borane peri‐Functionalized Naphthalene

A 1‐phosphonium‐8‐borane‐decorated naphthalene molecule 2 has been found to react with N,N′‐dimethylimidazol‐2‐ylidene (IMe), a popular member of the N‐heterocyclic carbene (NHC) family, which converts it into two vinyl‐amine fragments one of which is trapped between the phosphonium and borane unit by the formation of a C?C and a B?N bond. The same reactivity was not observed for larger NHC molecules. Control experiments and mechanistic studies have established the involvement of an ylide–borane molecule and an imidazolium salt in addition to IMe carbene in this new transformation of an NHC.     

Programmed Site‐Selective Palladium‐Catalyzed Arylation of Thieno[3,2‐b]thiophene

Mono‐, di‐, tri‐, and tetraarylated thieno[3,2‐b ]thiophenes were synthesized by direct site‐selective Pd‐catalyzed C−H activation reactions with various aryl bromides in the presence of a phosphine‐free Pd(OAc)₂/KOAc catalyst system in N ,N ‐dimethylacetamide (DMAc). The arylation of 2‐arylthieno[3,2‐b ]thiophene took place at the C3 position if the 2‐aryl substituents possessed electron‐withdrawing groups and at the C5 position if they were bulky and possessed electron‐donating groups.     

Study of the Air‐Tolerant 1,3‐Diphosphacyclobutane‐2,4‐diyl through the Direct Arylation

Installing π‐functional substituents on the skeletal phosphorus atoms of the air‐tolerant 1,3‐diphosphacyclobutane‐2,4‐diyl unit are promising for tuning the open‐shell singlet P‐heterocyclic chromophore. The sterically encumbered 1,3‐diphosphaCycloButen‐4‐yl Anion ( CBA ), generated from the phosphorus‐carbon triple bond, was available for the regioselective arylation via nucleophilic aromatic substitution (S_NAr) reaction, addition to arynes, and single‐electron transfer (SET) process affording the corresponding P‐arylated 1,3‐diphosphacyclobutane‐2,4‐diyls. The photo‐absorption and redox properties correlated with the effects of the aryl substituents on the 1,3‐diphosphacyclobutane‐2,4‐diyl unit. The X‐ray analyses enabled not only to discuss the metric parameters but also to visualize the radicalic electrons via the electron‐density distribution analysis. The electron‐donating character of the P‐heterocyclic chromophores induced the p‐type semiconductor behavior. Detection of hydrogen fluoride via formation of the 1λ⁵,3λ⁵‐diphosphete derivative was also developed.     

A Bis(silylene)‐Substituted ortho‐Carborane as a Superior Ligand in the Nickel‐Catalyzed Amination of Arenes

The synthesis and structure of the first 1,2‐bis(NHSi)‐substituted ortho‐carborane [(LSi:)C]₂B₁₀H₁₀ (termed SiCCSi) is reported (NHSi=N‐heterocyclic silylene; L=PhC(NtBu)₂). Its suitability to serve as a reliable bis(silylene) chelating ligand for transition metals is demonstrated by the formation of [SiCCSi]NiBr₂ and [SiCCSi]Ni(CO)₂ complexes. The CO stretching vibration modes of the latter indicate that the Si^II atoms in the SiCCSi ligand are even stronger σ donors than the P^III atoms in phosphines and C^II atoms in N‐heterocyclic carbene (NHC) ligands. Moreover, the strong donor character of the [SiCCSi] ligand enables [SiCCSi]NiBr₂ to act as an outstanding precatalyst (0.5 mol % loading) in the catalytic aminations of arenes, surpassing the activity of previously known molecular Ni‐based precatalysts (1–10 mol %).     

磷叶立德CH2PH3和类磷叶立德自由基∙CHPH3优势构型和C—P键结构的研究

采用MP4/6-311++G(d,p)和B3LYP/6-311++G(d,p)对磷叶立德CH₂PH₃和类磷叶立德自由基∙CHPH₃进行构型优化，从电子密度拓扑分析的角度对C—P键的键结构进行了探讨。得到如下结论：类磷叶立德自由基和磷叶立德的C—P键性质类似，但磷叶立德中π键由两个电子形成，类磷叶立德自由基中π键由一个电子形成，所以前者的π性明显，而后者的π性不明显。类磷叶立德自由基中的这个单电子在碳原子附近，垂直于对称面的方向上运动，有p_(C→P)配键的特征，所以类磷叶立德自由基∙CHPH₃中的C—P键比相应的产物∙CH₂PH₂中的C—P键要弱一些。     

Imidazolin‐2‐ylidenaminophosphines as Highly Electron‐Rich Ligands for Transition‐Metal Catalysts

A variety of chemical transformations benefit from the use of strong electron‐donating ancillary ligands, such as alkylphosphines or N‐heterocyclic carbenes when electron‐rich metal centers are required. Herein, we describe a facile and highly modular access to monodentate and bidentate imidazolin‐2‐ylidenamino‐substituted phosphines. Evaluation of the phosphine’s electronic properties substantiate that the formal replacement of alkyl or aryl groups by imidazolin‐2‐ylidenamino groups dramatically enhance their donor ability beyond that of alkylphosphines and even N‐heterocyclic carbenes. The new phosphines have been coordinated onto palladium(II) centers, and the beneficial effect of the novel substitution patterns has been explored by using the corresponding complexes in the palladium‐catalyzed Suzuki–Miyaura cross‐coupling reaction of non‐activated aryl chloride substrates.     

Theoretical strategies toward stabilization of singlet remote N‐heterocyclic carbenes

Theoretical investigations predict that the singlet states of ylide‐substituted remote carbenes are significantly stable and comparable to those of experimentally known NHCs. They are also found to be strongly σ‐donating in nature as evident from an evaluation of the carbonyl stretching frequencies (ν_CO) of their complexes with the [Rh(CO)₂Cl] fragment. NICS and QTAIM based bond magnetizability calculations indicate the presence of cyclic electron delocalization in majority of the molecules. © 2016 Wiley Periodicals, Inc.     

Substituent effects in trans‐p,p′‐disubstituted azobenzenes: X‐ray structures at 100 K and DFT‐calculated structures

The crystal and molecular structures of two para‐substituted azobenzenes with π‐electron‐donating –NEt₂ and π‐electron‐withdrawing –COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the –NEt₂ group in N,N,N′,N′‐tetraethyl‐4,4′‐(diazenediyl)dianiline, C₂₀H₂₈N₄, (I), may be caused by steric hindrance and the π‐electron‐donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp²‐hybridization and the other is slightly shifted from the plane of the surrounding atoms. The molecule of the second azobenzene, diethyl 4,4′‐(diazenediyl)dibenzoate, C₁₈H₁₈N₂O₄, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecules. The most significant changes are observed in the values of the N=N—C—C torsion angles, which for the isolated molecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II).     

New Dual Donor–Acceptor (2D‐π‐2A) Porphyrin Sensitizers for Stable and Cost‐Effective Dye‐Sensitized Solar Cells

A series of porphyrin sensitizers that featured two electron‐donating groups and dual anchoring groups that were connected through a porphine π‐bridging unit have been synthesized and successfully applied in dye‐sensitized solar cells (DSSCs). The presence of electron‐donating groups had a significant influence on their spectroscopic, electrochemical, and photovoltaic properties. Overall, the dual anchoring groups gave tunable electronic properties and stronger attachment to TiO₂. These new dyes were readily synthesized in a minimum number of steps in gram‐scale quantities. Optical and electrochemical data confirmed the advantages of these dyes for use as sensitizers in DSSCs. Porphyrins with electron‐donating amino moieties provided improved charge separation and better charge‐injection efficiencies for the studied dual‐push–pull dyes. Attenuated total reflectance–Fourier‐transform infrared (ATR‐FTIR) and X‐ray photoelectron spectroscopy of the porphyrin dyes on TiO₂ suggest that both p‐carboxyphenyl groups are attached onto TiO₂, thereby resulting in strong attachment. Among these dyes, cis-Zn2BC2A , with two electron‐donating 3,6‐ditertbutyl‐phenyl‐carbazole groups and dual‐anchoring p‐carboxyphenyl groups, showed the highest efficiency of 4.07 %, with J_SC=9.81 mA cm^?2, V_OC=0.63 V, and FF=66 %. Our results also indicated a better photostability of the studied dual‐anchored sensitizers compared to their mono‐anchored analogues under identical conditions. These results provide insight into the developments of a new generation of high‐efficiency and thermally stable porphyrin sensitizers.     

Effects of Phenyl‐ and Methyl‐Substituents on p‐Phenylenediamine,an Electrochemical and Spectral Study

Two series of substituted p‐phenylenediamines have been studied for their electronic effects on redox potential and spectral properties. p‐Phenylenediamines and N,N,N′,N”‐tetramethyl‐p‐phenylenediamine substituted with different numbers of phenyl groups have been synthesized and their cyclic voltammograms have been obtained. The correlation between the substituent number and the redox potential appears linear. The slope reflects the additive effect of electron‐donating methyl and electron‐withdrawing phenyl groups. The absorption spectra of the cation radicals indicate that phenyl‐substituted ones have broad intervalence‐charge transfer bands. The p‐phenylenediamines exhibit different properties from triphenylamines in that the oxidized forms are more stable in CH₃CN then those in CH₂Cl₂. Some of the cation radicals or dications could undergo follow‐up chemical reactions and form products that are more easily oxidized.