Fixation and Release of Intact E4 Tetrahedra (E=P,As)

By the reaction of [NacnacCuCH₃CN] with white phosphorus (P₄) and yellow arsenic (As₄), the stabilization and enclosure of the intact E₄ tetrahedra are realized and the disubstituted complexes [(NacnacCu)₂(μ,η^2:2‐E₄)] ( 1 a : E=P, 1 b : E=As) are formed. The mono‐substituted complex [NacnacCu(η²‐P₄)] ( 2 ), was detected by the exchange reaction of 1 a with P₄ and was only isolated using low‐temperature work‐up. All products were comprehensively spectroscopically and crystallographically characterized. The bonding situation in the products as intact E₄ units (E=P, As) was confirmed by theory and was experimentally proven by the pyridine promoted release of the bridging E₄ tetrahedra in 1 .     

An acyclic imino-substituted silylene: synthesis, isolation, and its facile conversion into a zwitterionic silaimine

A new type of Si(II): A novel silylene stabilized by a Cp* and an imidazolin-2-iminato ligand has been prepared using two different methods. The X-ray crystallographic structure shows that the silicon(II) center is coordinated to an η(2) -Cp* ligand and the nitrogen atom of an imidazolin-2-iminato ligand. This silylene easily reacts with B(C(6) F(5) )(3) to give a stable borane adduct having a zwitterionic resonance structure.     

On the electronic structures of the 1,3-diboracyclobutane-1,3-diyls and their valence isomers with a B2E2 skeleton (E=N,P, As)

The concept of through-space versus through-bond interactions on the stabilization of biradical structures with a singlet or triplet ground state is evaluated for the 1,3-diboracyclobutane-1,3-diyls and related congeners. Singlet biradicals are favored when the intermediate units E feature singlet character (PH(2) (+), AsH(2) (+)), while E fragments with triplet character (NH(2) (+)) induce small energy separations between the lowest singlet and triplet states. These considerations are supported by quantum chemical calculations with energy optimization at 1) MCSCF level plus MR-MP2 correction, 2) MR-MP2 level, and 3) two different types of density functional levels for the planar (D(2h)) geometries. The singlet-triplet energy separations in the planar compounds increase with increasing singlet stability of the corresponding E fragments. In addition to this newly developed principal features for singlet stabilization, which primarily occurs in bonded structures with higher main-group elements, the corresponding valence isomers with bicyclobutane, cyclobutene and cis-butadiene structures are investigated.     

Transient Dipnictyl Analogues of Acrylamides,R−E=E′−CONR2, and a Related Diphosphadigalletane from Na[OCP] and (R2N)2ECl (E,E′=P,As, Ga)

The reaction of Na[OCP] with (R₂N)₂ECl (E=P or As; R=alkyl) granted direct access to transient amine-substituted diphospha- and arsaphospha-acrylamide analogues, (R₂N)E=P(CONR₂) 1 . Their facile formation allowed for a comprehensive reactivity study. Dimerization yielded the four-membered rings (R₂N)₂E₂P₂(CONR₂)₂, whereas in the presence of excess Na[OCP], a stepwise [2+2] cycloaddition occured, leading to the sodium salts of carboxotripnictides [(R₂N)EP₂CO(CONR₂)]⁻. These salts served as a reservoir of 1 , either by extrusion of Na[OCP] or by reaction with the appropriate (R₂N)₂ECl, giving the [4+2]-cycloaddition products (R₂N)EP(C₆H₁₀)(CONR₂) in the presence of 2,3-dimethylbutadiene. The formal conjugate addition product K[(tBuO)(R₂N)PP(CONR₂)] was obtained by reaction of Na[(R₂N)PP₂CO(CONR₂)] with tBuOK. In addition, a rare diphosphadigalletane with a ladder-type (R₂N)₂Ga₂P₂(CONR₂)₂ core was isolated from the reaction of Na[OCP] with (R₂N)₂GaCl (R=alkyl). The unprecedented pnictogenyl carboxamide compounds were thoroughly characterized, including single-crystal X-ray structure determinations, and mechanisms for their formation were investigated by DFT calculations.     

Facile Access to Silicon‐Functionalized Bis‐Silylene Titanium(II) Complexes

A series of unprecedented bis‐silylene titanium(II) complexes of the type [(η⁵‐C₅H₅)₂Ti(LSiX)₂] (L=PhC(NtBu)₂; X=Cl, CH_3, H) has been prepared using a phosphane elimination strategy. Treatment of the [(η⁵‐C₅H₅)₂Ti(PMe₃)₂] precursor ( 1 ) with two molar equivalents of the N‐heterocyclic chlorosilylene LSiCl ( 2 ), results in [(η⁵‐C₅H₅)₂Ti(LSiCl)₂] ( 3 ) with concomitant PMe₃ elimination. The presence of a Si? Cl bond in 3 enabled further functionalization at the silicon(II) center. Accordingly, a salt metathesis reaction of 3 with two equivalents of MeLi results in [(η⁵‐C₅H₅)₂Ti(LSiMe)₂] ( 4 ). Similarly, the reaction of 3 with two equivalents of LiBHEt₃ results in [(η⁵‐C₅H₅)₂Ti(LSiH)₂] ( 5 ), which represents the first example of a bis‐(hydridosilylene) metal complex. All complexes were fully characterized and the structures of 3 and 4 elucidated by single‐crystal X‐ray diffraction analysis. DFT calculations of complexes 3 – 5 were also carried out to assess the nature of the titanium–silicon bonds. Two σ and one π‐type molecular orbital, delocalized over the Si‐Ti‐Si framework, are observed.     

New heteroorganic betaines containing the (+)E15—C—E14—X(–) and (+)E15—C—E14(–) structural fragments (E15 = P,As; E14 = Si,Ge, Sn; X = C,S, O,NR)

The review surveys the data on the reactions of phosphorus and arsenic ylides with compounds containing E=X bonds (E = C, Si, Ge, or Sn; X = C or S), cyclic oligomers (R₂ES)_n (n = 2 or 3), and heavier analogs of carbenes. These reactions give rise to two new classes of heteroorganic betaines containing the ⁽⁺⁾E¹⁵—C—E¹⁴—X^(–) (I) and ⁽⁺⁾E¹⁵—C—E^14(–) (II) (E¹⁵ = P or As; E¹⁴ = Si, Ge, or Sn; X = C or S) structural fragments. Procedures for the synthesis of these compounds, their reactivities, the X-ray diffraction structures, and the electronic structures established by high-level quantum-chemical calculations are considered in detail. The carbon analogs of betaines of type I, viz., compounds bearing the ⁽⁺⁾P—C—C—X^(–) fragment (III), are also discussed. The latter were long considered as possible intermediates in the reactions of compounds containing the polar C=X bond (X = C, O, S, NR, etc.) with phosphorus ylides (classical Wittig and Corey—Chaykovsky reactions and related processes).     

Comparison of side-on and end-on coordination of E2 ligands in complexes [W(CO)5E2] (E=N,P, As,Sb, Bi,Si-, Ge-, Sn-, Pb-)

Complexes of W(CO)(5) with neutral diatomic pnictogen ligands N(2), P(2), As(2), Sb(2), and Bi(2) and anionic Group 14 ligands Si(2) (2-), Ge(2) (2-), Sn(2) (2-), and Pb(2) (2-) coordinated in both side-on and end-on fashion have been optimized by using density functional theory at the BP86 level with valence sets of TZP quality. The calculated bond energies have been used to compare the preferential binding modes of each respective ligand. The results were interpreted by analyzing the nature of the interaction between the ligands and the metal fragment using an energy partitioning method. This yields quantitative information regarding the strength of covalent and electrostatic interactions between the metal and ligand, as well as the contributions by orbitals of different symmetry to the covalent bonding. Results show that all the ligands studied bind preferentially in a side-on coordination mode, with the exception of N(2), which prefers to coordinate in an end-on mode. The preference of the heavier homologues P(2)-Bi(2) for binding in a side-on mode over the end-on mode in the neutral complexes [(CO)(5)WE(2)] comes mainly from the much stronger electrostatic attraction in the former species. The energy difference between the side-on and end-on isomers of the negatively charged complexes with the ligands Si(2) (2-), Ge(2) (2-), Sn(2) (2-), and Pb(2) (2-) is much less and it cannot be ascribed to a particular bonding component.     

The Influence of N‐Heterocyclic Carbenes (NHC) on the Reactivity of [Ru(NHC)4H]+ With H2, N2, CO and O2

The five‐coordinate ruthenium N‐heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr₂Me₂)₄H][BAr^F₄] ( 1 ; IiPr₂Me₂=1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene; Ar^F=3,5‐(CF₃)₂C₆H₃), [Ru(IEt₂Me₂)₄H][BAr^F₄] ( 2 ; IEt₂Me₂=1,3‐diethyl‐4,5‐dimethylimidazol‐2‐ylidene) and [Ru(IMe₄)₄H][BAr^F₄] ( 3 ; IMe₄=1,3,4,5‐tetramethylimidazol‐2‐ylidene) have been synthesised following reaction of [Ru(PPh₃)₃HCl] with 4–8 equivalents of the free carbenes at ambient temperature. Complexes 1 – 3 have been structurally characterised and show square pyramidal geometries with apical hydride ligands. In both dichloromethane or pyridine solution, 1 and 2 display very low frequency hydride signals at about δ ?41. The tetramethyl carbene complex 3 exhibits a similar chemical shift in toluene, but shows a higher frequency signal in acetonitrile arising from the solvent adduct [Ru(IMe₄)₄(MeCN)H][BAr^F₄], 4 . The reactivity of 1 – 3 towards H₂ and N₂ depends on the size of the N‐substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H₂ and N₂ only at low temperature and incompletely, while 3 affords [Ru(IMe₄)₄(η²‐H₂)H][BAr^F₄] ( 7 ) and [Ru(IMe₄)₄(N₂)H][BAr^F₄] ( 8 ) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1 – 3 to give [Ru(NHC)₄(CO)H][BAr^F₄] ( 9 – 11 ). Addition of O₂ to solutions of 2 and 3 leads to rapid oxidation, from which the Ru^III species [Ru(NHC)₄(OH)₂][BAr^F₄] and the Ru^IV oxo chlorido complex [Ru(IEt₂Me₂)₄(O)Cl][BAr^F₄] were isolated. DFT calculations reproduce the greater ability of 3 to bind small molecules and show relative binding strengths that follow the trend CO ? O₂ > N₂ > H₂.     

Density Functional Theory Investigation of the Alkyl–Alkyl Negishi Cross‐Coupling Reaction Catalyzed by N‐Heterocyclic Carbene (NHC)–Pd Complexes

Why bigger is better : A “steric wall” created by the N‐(2,6‐diisopropylphenyl) substituent on the bulky NHC ligand IPr (1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) guides the reactants to and from the Pd center through weak, fleeting (IPr)H–Pd interactions that help the oxidative addition intermediate escape “the anti‐trap”. The alternative “side” approach leads to transmetalation (the rate‐limiting step) for which a novel Pd–Zn interaction was identified.

     

Iridium(I) Hydroxides: Powerful Synthons for Bond Activation

A family of iridium(I) hydroxides of the form [Ir(cod)(NHC)(OH)] (cod=1,5‐cyclooctadiene, NHC=N‐heterocyclic carbene) is reported. Single‐crystal X‐ray analyses and computational methods were used to explore the structural characteristics and steric properties of these new complexes. The model complex [Ir(cod)(IiPr)(OH)] (IiPr=1,3‐(diisopropyl)imidazol‐2‐ylidene) undergoes reaction with a wide variety of substrates including boronic acids and silicon compounds. In addition, O? H, N? H and C? H bond activation was achieved with alcohols, carboxylic acids, amines and various sp‐, sp²‐ and sp³‐hybridised carbon centres, giving access to a wide range of new Ir^I complexes. These studies have allowed us to explore the exciting reactivity of this motif, revealing a versatile and useful synthon capable of activating important chemical bonds under mild (typically room temperature) conditions. No additives were required and, in the case of X? H bond activation, water was the only waste product, rendering this an atom efficient procedure for bond activation. This system has great potential for the construction of new catalytic cycles for organic synthesis and small‐molecule activation.     

From the N‐Heterocyclic Carbene‐Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates

Among various N‐heterocyclic carbenes (NHCs) tested, only 1,3‐bis(tert‐butyl)imidazol‐2‐ylidene (NHC^tBu) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC‐catalyzed 1,4‐addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well‐defined α‐alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]₀/[ROH]₀ molar ratio, were ultimately obtained in N,N‐dimethylformamide at 25 °C. A hydroxyl‐terminated poly(ethylene oxide) (PEO‐OH) macro‐initiator was also employed to directly access PEO‐b‐PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC^tBu.     

The mechanism of C--X (X=F, Cl, Br, and I) bond activation in CX4 by a stabilized dialkylsilylene

The potential-energy surfaces for the abstraction and insertion reactions of dialkylsilylene with carbon tetrahalides (CX4) have been characterized in detail using density functional theory (B3LYP), including zero-point corrections. Four CX4 species, CF4, CCl(4), CBr4, and CI(4), were chosen as model reactants. The theoretical investigations described herein suggest that of the three possible reaction paths, the one-halogen-atom abstraction (X abstraction), the one-CX3-group abstraction (CX3 abstraction), and the insertion reaction, the X-abstraction reaction is the most favorable, with a very low activation energy. However, the insertion reaction can lead to the thermodynamically stable products. Moreover, for a given stable dialkylsilylene, the chemical reactivity has been found to increase in the order CF4相似文献   

The Role of Free N‐Heterocyclic Carbene (NHC) in the Catalytic Dehydrogenation of Ammonia–Borane in the Nickel NHC System

Enders' N‐heterocyclic carbene (NHC) dehydrogenates ammonia–borane with a relatively low barrier, producing NH₂BH₂ and NHC–(H)₂. The nickel NHC catalyst present in the reaction media can activate the NHC–(H)₂ produced to regenerate the free NHC and release H₂. The release of free NHC enables further dehydrogenation of ammonia–borane.

     

Bonding of multiple noble-gas atoms to CUO in solid neon: CUO(Ng)n (Ng=Ar, Kr, Xe; n=1, 2, 3, 4) complexes and the singlet-triplet crossover point

Laser-ablated U atoms co-deposited with CO in excess neon produce the novel CUO molecule, which forms distinct Ng complexes (Ng=Ar, Kr, Xe) with the heavier noble gases. The CUO(Ng) complexes are identified through CO isotopic and Ng reagent substitution and comparison to results of DFT frequency calculations. The U[bond]C and U[bond]O stretching frequencies of CUO(Ng) complexes are slightly red-shifted from neon matrix (1)Sigma(+) CUO values, which indicates a (1)A' ground state for the CUO(Ng) complexes. The CUO(Ng)(2) complexes in excess neon are likewise singlet molecules. However, the CUO(Ng)(3) and CUO(Ng)(4) complexes exhibit very different stretching frequencies and isotopic behaviors that are similar to those of CUO(Ar)(n) in a pure argon matrix, which has a (3)A" ground state based on DFT vibrational frequency calculations. This work suggests a coordination sphere model in which CUO in solid neon is initially solvated by four or more Ne atoms. Up to four heavier Ng atoms successively displace the Ne atoms leading ultimately to CUO(Ng)(4) complexes. The major changes in the CUO stretching frequencies from CUO(Ng)(2) to CUO(Ng)(3) provides evidence for the crossover from a singlet ground state to a triplet ground state.     

Reaction of N‐Heterocyclic Silylenes with Thioketone: Formation of Silicon–Sulfur Three (Si‐C‐S)‐ and Five (Si‐C‐C‐C‐S)‐Membered Ring Systems

Three‐ and five‐membered rings that bear the (Si‐C‐S ) and (Si‐C‐C‐C‐S ) unit have been synthesized by the reactions of L SiCl ( 1 ; L =PhC(NtBu)₂) and L′ Si ( 2 ; L′ =CH{(C?CH₂)(CMe)(2,6‐iPr₂C₆H₃N)₂}) with the thioketone 4,4′‐bis(dimethylamino)thiobenzophenone. Treatment of 4,4′‐bis(dimethylamino)thiobenzophenone with L SiCl at room temperature furnished the [1+2]‐cycloaddition product silathiacyclopropane 3 . However, reaction of 4,4′‐bis(dimethylamino)thiobenzophenone with L′ Si at low temperature afforded a [1+4]‐cycloaddition to yield the five‐membered ring product 4 . Compounds 3 and 4 were characterized by NMR spectroscopy, EIMS, and elemental analysis. The molecular structures of 3 and 4 were unambiguously established by single‐crystal X‐ray structural analysis. The room‐temperature reaction of 4,4′‐bis(dimethylamino)thiobenzophenone with L′ Si resulted in products 4 and 5 , in which 4 is the dearomatized product and 5 is formed under the 1,3‐migration of a hydrogen atom from the aromatic phenyl ring to the carbon atom of the C? S unit. Furthermore, the optimized structures of probable products were investigated by using DFT calculations.     

EH3 (E=N,P, As) and H2 Activation with N‐Heterocyclic Silylene and Germylene Homologues

DFT calculations have been used to probe the mechanism of the addition reaction of group 15 hydrides EH₃ (E=N, P, As) and H₂ to a N‐heterocyclic silylene and its germylene homologue. Nitrogen lone pair donation into the vacant p‐orbital of Si and Ge is the first step of ammonia activation, whereas silylene and germylene behave as nucleophiles toward dihydrogen, phosphane, and arsane. Formation of 1,4‐addition products is kinetically favoured in all cases. In excess ammonia, the assistance of a second molecule drastically lowers the 1,1‐addition energy barriers, enabling formation of 1,1‐addition products. The participation of a second molecule in the P? H bond activation of phosphane also lowers the 1,1‐addition energy barriers, but not enough to cause inversion.     

Reversible Dimerization of Phosphine‐Stabilized Silylenes by Silylene Insertion into SiII–H and SiII–Cl σ‐Bonds at Room Temperature

Contrary to the classical silylene dimerization leading to a disilene structure, phosphine stabilized hydro‐ and chloro‐silylenes ( 2 a , b ) undergo an unique dimerization via silylene insertion into Si? X σ‐bonds (X=H, Cl), which is reversible at room temperature. DFT calculations indicate that the insertion reaction proceeds in one step in a concerted manner.