Stabilization of a Two‐Coordinate Mononuclear Cobalt(0) Compound

Compound (Me₂‐cAAC:)₂Co⁰ ( 2 ; Me₂‐cAAC:=cyclic (alkyl) amino carbene; :C(CH₂)(CMe₂)₂N‐2,6‐iPr₂C₆H₃) was synthesized by the reduction of the precursor (Me₂‐cAAC:)₂Co^ICl ( 1 ) with KC₈ in THF. The cyclic voltammogram of 1 exhibited one‐electron reduction, which suggests that synthesis of a bent 2‐metallaallene ( 2 ) from 1 should be possible. Compound 2 contains one cobalt atom in the formal oxidation state zero, which is stabilized by two Me₂‐cAAC: ligands. Bond lengths from X‐ray diffraction are 1.871(2) and 1.877(2) Å with a C‐Co‐C bond angle of 170.12(8)°. The EPR spectrum of 2 exhibited a broad resonance attributed to the unique quasi‐linear structure, which favors near degeneracy and gives rise to very rapid relaxation conditions. The cAAC?Co bond in 2 can be considered as a typical Dewar–Chatt–Duncanson type of bonding, which in turn retains 2.5 electron pairs on the Co atom as nonbonding electrons.     

Treatment of Silylene–Phosphinidene with Chalcogens Resulted Exclusively in the Formation of Silicon-Bonded Chalcogens

Chalcogen-bonded silicon phosphinidenes LSi(E)−P−^MecAAC (E=S ( 1 ); Se ( 2 ); Te ( 3 ); L=PhC(NtBu)₂; ^MecAAC=C(CH₂)(CMe₂)₂N-2,6-iPr₂C₆H₃)) were synthesized from the reactions of silylene–phosphinidene LSi−P−^MecAAC ( A ) with elemental chalcogens. All the compounds reported herein have been characterized by multinuclear NMR, elemental analyses, LIFDI-MS, and single-crystal X-ray diffraction techniques. Furthermore, the regeneration of silylene–phosphinidene ( A ) was achieved from the reactions of 2 – 3 with L′Al (L′=HC{(CMe)(2,6-iPr₂C₆H₃N)}₂). Theoretical studies on chalcogen-bonded silicon phosphinidenes indicate that the Si−E (E=S, Se, Te) bond can be best represented as charge-separated electron-sharing σ-bonding interaction between [LSi−P−^MecAAC]⁺ and E⁻. The partial double-bond character of Si−E is attributed to significant hyperconjugative donation from the lone pair on E⁻ to the Si−N and Si−P σ*-molecular orbitals.     

Reversible Oxidative Addition at Carbon

The reactivity of N‐heterocyclic carbenes (NHCs) and cyclic alkyl amino carbenes (cAACs) with arylboronate esters is reported. The reaction with NHCs leads to the reversible formation of thermally stable Lewis acid/base adducts Ar‐B(OR)₂⋅NHC ( Add1 – Add6 ). Addition of cAAC^Me to the catecholboronate esters 4‐R‐C₆H₄‐Bcat (R=Me, OMe) also afforded the adducts 4‐R‐C₆H₄Bcat⋅cAAC^Me ( Add7 , R=Me and Add8 , R=OMe), which react further at room temperature to give the cAAC^Me ring‐expanded products RER1 and RER2 . The boronate esters Ar‐B(OR)₂ of pinacol, neopentylglycol, and ethyleneglycol react with cAAC at RT via reversible B−C oxidative addition to the carbene carbon atom to afford cAAC^Me(B{OR}₂)(Ar) ( BCA1 – BCA6 ). NMR studies of cAAC^Me(Bneop)(4‐Me‐C₆H₄) ( BCA4 ) demonstrate the reversible nature of this oxidative addition process.     

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent‐Silyliumylidene Cation

The trapping of a silicon(I) radical with N‐heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6‐i Pr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent‐silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1‐F‐2‐I_Me‐C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X‐ray crystallography.     

N-Heterocyclic Carbene Derived 3-Azabutadiene as a π-Base in Classic and Frustrated Lewis Pair Chemistry

N-Heterocyclic carbene (NHC) derived 3-azabutadienes 1 and 2 have been prepared by a single-step reaction of the corresponding NHC with cyclohexyl isocyanide. Compound 1 features π-basic, delocalized nucleophilic sites over the 3-azabutadiene moiety, therefore allowing for coordinating with small Lewis acids, such as AlCl₃, GaCl₃, and Me₂SAuCl, to form diverse classic Lewis adducts 3 – 5 . Combination of 1 with B(C₆F₅)₃ or [Ph₃C][B(C₆F₅)₄] resulted in single-electron transfer and the obtained radical cation was detected by EPR. In addition, a frustrated Lewis pair comprised of the π-basic 1 and BPh₃ effects the splitting of the O−H bond of phenol and the N−H bond of imidazole to give 7 and 8 , respectively. An intrinsic bond orbital (IBO) analysis of the pathway leading to 8 showcases the transformation of the delocalized π-electrons of 1 to a newly formed C−H localized σ-bond.     

A Triatomic Silicon(0) Cluster Stabilized by a Cyclic Alkyl(amino) Carbene

Reduction of the neutral carbene tetrachlorosilane adduct (cAAC)SiCl₄ (cAAC=cyclic alkyl(amino) carbene :C(CMe₂)₂(CH₂)N(2,6‐iPr₂C₆H₃) with potassium graphite produces stable (cAAC)₃Si₃, a carbene‐stabilized triatomic silicon(0) molecule. The Si?Si bond lengths in (cAAC)₃Si₃ are 2.399(8), 2.369(8) and 2.398(8) Å, which are in the range of Si?Si single bonds. Each trigonal pyramidal silicon atom of the triangular molecule (cAAC)₃Si₃ possesses a lone pair of electrons. Its bonding, stability, and electron density distributions were studied by quantum chemical calculations.     

Low-Temperature Isolation of a Labile Silylated Hydrazinium-yl Radical Cation, [(Me3Si)2N−N(H)SiMe3].+

The oxidation of silylated hydrazine, (Me₃Si)₂N−N(H)SiMe₃, with silver salts led to the formation of a highly labile hydrazinium-yl radical cation, [(Me₃Si)₂N−N(H)SiMe₃]^.+, at very low temperatures (decomposition > −40 °C). EPR, NMR, DFT and Raman studies revealed the formation of a nitrogen-centered radical cation along the N−N unit of the hydrazine. In the presence of the weakly coordinating anion [Al{OCH(CF₃)₂}₄]⁻, crystallization and structural characterization in the solid state were achieved. The hydrazinium-yl radical cation has a significantly shortened N−N bond and a nearly planar N₂Si₃ framework, in contrast to the starting material. According to DFT calculations, the shortened N−N bond has a total bond order of 1.5 with a π-bond order of 0.5. The π bond can be regarded as a three-π-electron, two-center bond.     

An Electrophilic Carbene‐Anchored Silylene–Phosphinidene

The cyclic alkyl(amino) carbene‐anchored silylene–phosphinidene was isolated as L−Si−P(:cAAC−Me) (L=benzamidinate) at room temperature, synthesized from the reduction of L−Si(Cl₂)−P(:cAAC−Me) ( 1 ) using two equivalents of KC₈. Compound 1 was prepared by the oxidative addition of a chlorophosphinidene to the benzamidinate substituted silylene center. This is the first molecular example of a silylene–phosphinidene characterized by single‐crystal X‐ray structural analysis. Moreover, ¹H, ³¹P, and also ²⁹Si NMR spectroscopic data supported the formulation of the products. The theoretical calculations of compound 2 are in good agreement with the experimental results.     

Synthese eines funktionalen Aluminiumalkinids,Me3C‐C≡C‐AlBr2, und dessen Reaktionen mit der sperrigen Lithiumverbindung LiCH(SiMe3)2

Synthesis of a Functional Aluminium Alkynide, Me₃C‐C≡C‐AlBr₂, and its Reactions with the Bulky Lithium Compound LiCH(SiMe₃)₂ Treatment of aluminium tribromide with the lithium alkynide (Li)C≡C‐CMe₃ afforded the aluminium alkynide Me₃C‐C≡C‐AlBr₂ ( 1 ) in an almost quantitative yield. 1 crystallizes with trimeric formula units possessing Al₃C₃ heterocycles and the anionic carbon atoms of the alkynido groups in the bridging positions. A dynamic equilibrium was determined in solution which probably comprises trimeric and dimeric formula units. Reaction of 1 with one equivalent of LiCH(SiMe₃)₂ yielded the compound [Me₃C‐C≡C‐Al(Br)‐CH(SiMe₃)₂]₂ ( 2 ), which is a dimer via Al‐C‐Al bridges. Two equivalents of the lithium compound gave a mixture of four main‐products, which could be identified as 2 , Li[Me₃C‐C≡C‐Al{CH(SiMe₃)₂}₃] ( 3 ), Me₃C‐C≡C‐Al[CH(SiMe₃)₂]₂ ( 4 ), and Al[CH(SiMe₃)₂]₃. The lithium atom of 3 is coordinated by the C≡C triple bond and an inner carbon atom of one bis(trimethylsilyl)methyl group. Further interactions were observed to C‐H bonds of methyl groups.     

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent-Silyliumylidene Cation

The trapping of a silicon(I) radical with N-heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6-iPr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent-silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1-F-2-I_Me-C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X-ray crystallography.     

Die Hydroaluminierung von 1,1,4,4‐Tetramethyl‐2,3‐diazabutadien mit Dialkylaluminiumhydriden – Synthese von Dialkylaluminiumhydrazoniden

The Hydroalumination of 1,1,4,4‐Tetramethyl‐2,3‐diazabutadiene by Dialkylaluminium Hydrides – Synthesis of Dialkylaluminium Hydrazonides 1,1,4,4‐Tetramethyl‐2,3‐diazabutadiene reacted with dimethylaluminium hydride by hydroalumination of only one C=N double bond. The hydrazone derivative [Me₂Al–N(CHMe₂)–N=CMe₂]₂ ( 1 ) was formed which gave a dimer possessing a six‐membered Al₂N₄ heterocycle. The hydroalumination of both C=N double bonds was not observed. Also an excess of di(tert‐butyl)‐ or bis(trimethylsilylmethyl)aluminium hydride afforded only the product of a single hydroalumination step, a second dialkylaluminium hydride molecule was attached via a coordinative interaction between its central aluminium atom and the nitrogen atom of the C=N double bond and in addition via a 3 c‐2 e Al–H–Al bond. Compounds [(Me₃C)₂Al][(Me₃C)₂AlH]N(CHMe₂)NCMe₂ ( 2 ) and [(Me₃SiCH₂)₂Al][(Me₃SiCH₂)₂AlH]N(CHMe₂)NCMe₂ ( 3 ) were formed which have five‐membered Al₂N₂H heterocycles. Thermolysis of 2 gave by C–H activation compound [(Me₃C)₂Al]₂[CH₂C(Me)=N–]₂ ( 4 ) in trace amounts which possesses two anellated AlN₂C₂ rings with a common N–N bond. In contrast, the thermal decomposition of 3 yielded by the cleavage of the N–N bond a dimeric dialkylaluminium methylideneamide ( 5 ) which has two intact C=N double bonds. Up to now our attempts to insert a C=N double bond into an Al–C bond remained unsuccessful, and only the formation of an adduct [(Me₃C)₃Al(–N=CMe₂)₂] ( 6 ) was observed upon treatment of tri(tert‐butyl)aluminium with the diazabutadiene derivative.     

Synthesis,Characterization, and Theoretical Investigation of Two‐Coordinate Palladium(0) and Platinum(0) Complexes Utilizing π‐Accepting Carbenes

An elegant general synthesis route for the preparation of two coordinate palladium(0) and platinum(0) complexes was developed by reacting commercially available tetrakis(triphenylphosphine)palladium/platinum with π‐accepting cyclic alkyl(amino) carbenes (cAACs). The complexes are characterized by NMR spectroscopy, mass spectrometry, and single‐crystal X‐ray diffraction. The palladium complexes exhibit sharp color changes (crystallochromism) from dark maroon to bright green if the C‐Pd‐C bond angle is sharpened by approximately 6°, which is chemically feasible by elimination of one lattice THF solvent molecule. The analogous dark orange‐colored platinum complexes are more rigid and thus do not show this phenomenon. Additionally, [(cAAC)₂Pd/Pt] complexes can be quasi‐reversibly oxidized to their corresponding [(cAAC)₂Pd/Pt]⁺ cations, as evidenced by cyclic voltammetry measurements. The bonding and stability are studied by theoretical calculations.     

Oxidative Cleavage of C=S and P=S Bonds at an AlI Center: Preparation of Terminally Bound Aluminum Sulfides

The treatment of cyclic thioureas with the aluminum(I) compound NacNacAl ( 1 ; NacNac=[ArNC(Me)CHC(Me)NAr]^?, Ar=2,6‐Prⁱ₂C₆H₃) resulted in oxidative cleavage of the C=S bond and the formation of 3 and 5 , the first monomeric aluminum complexes with an Al=S double bond stabilized by N‐heterocyclic carbenes. Compound 1 also reacted with triphenylphosphine sulfide in a similar manner, which resulted in cleavage of the P=S bond and production of the adduct [NacNacAl=S(S=PPh₃)] ( 8 ). The Al=S double bond in 3 can react with phenyl isothiocyanate to furnish the cycloaddition product 9 and zwitterion 10 as a result of coupling between the liberated carbene and PhN=C=S. All novel complexes were characterized by multinuclear NMR spectroscopy, and the structures of 5 , 9 , and 10 were confirmed by X‐ray diffraction analysis. The nature of the Al=S bond in 5 was also probed by DFT calculations.     

Wechselwirkungen in Kristallen, 176. Mitteilung,Redox‐Reaktionen von Hexahydropyren: Kristallstrukturen seiner Radikalanion‐Salze sowie von Trihydropyrenylium‐tetrachloroaluminat und Dichtefunktionaltheorie‐Berechnungen

Redox Reactions of Hexahydropyrene: Crystal Structures of Its Radical‐Anion Salts as well as of Trihydropyrenylium Tetrachloroaluminate and Density‐Functional‐Theory Calculations Hexahydropyrene 1 , a doubly propane‐1,3‐diyl‐bridged peri‐naphthalene derivative wtih 10 π‐electrons allows both oxidation to its cation as well as reduction to its radical‐anion salts, which could be crystallized and structurally characterized – a rather rare case for small unsaturated hydrocarbons. The unexpected formally threefold dehydrogenation by the oxidizing system AlCl₃/H₂CCl₂ (Bock's reagent) generated the hitherto unknown 1,2,3‐trihydropyrene cation in two polymorphic crystals, which contain 12 π‐electrons delocalized over three anellated six‐membered rings comprising 13 π‐centers. Structural comparison of the altogether four crystallized redox products [K⁺_solv][M^.−] 2a , [K⁺_solv][M^.−]_∞ 2b , [Na⁺_solv][M^.−] 2c , and [(M−3 H)⁺][AlCl⁻₄] 3 with the neutral hydrocarbon 1 reveals only small differences in bond lengths and angles, but establishes solvation contacts, π(η⁶)⋅⋅⋅K⁺ coordination in the polymer 2b , the flattening of one molecular half in the trihydropyren cation of 3 and ten H‐bonds CH⋅⋅⋅Cl to the AlCl₄⁻ counter anion of 3 . DFT/NBO Charge distributions, calculated based on the experimental structural parameters, show charge accumulation in the propanediyl bridges as well as in the peripheral naphthalene C−C bonds of the radical anions. The largest changes result expectedly for the formally triply dehydrogenated 1 , i.e. the trihydropyren cation of 3 , with two slightly positive and partly considerably less negative π‐centers.     

Synthesis and Crystal Structures of Sodium and Calcium ­Complexes with the Ligand N‐(2,6‐Dimethylphenyl)diphenylphosphinic Amide

The sodium complex [{Ph₂P(O)NH(2,6‐Me₂C₆H₃)}Na{Ph₂P(O)N(2,6‐Me₂C₆H₃)}]₂ ( 2 ) with the ligand N‐(2,6‐dimethylphenyl)diphenylphosphinic amide was synthesized involving the reaction of the neutral ligand [Ph₂P(O)NH(2,6‐Me₂C₆H₃)] ( 1 ) and sodium bis(trimethylsilyl)amide in toluene at 60 °C. The calcium complex [{Ph₂P(O)NH(2,6‐Me₂C₆H₃)CaI(THF)₃}I] ( 3 ) was obtained by the reaction between the neutral ligand 1 and anhydrous calcium diiodide in THF at ambient temperature. The solid‐state structures of the complexes were established by single‐crystal X‐ray diffraction analysis. In the solid‐state structure of 2 , the sodium ion is coordinated through the chelation of oxygen atom attached to the phosphorus atom. Two different P–N and P–O bond lengths are observed, which indicates that one ligand moiety is anionic, whereas the second one is neutral. In the solid‐state structure of 3 , the calcium atom adopts distorted octahedral arrangement through the ligation of two phosphinic amide ligands, three THF molecules, and one iodide ion.     

Sydnone Methides: Intermediates between Mesoionic Compounds and Mesoionic N-Heterocyclic Olefins

Sydnone methides represent an almost unknown class of mesoionic compounds which possess exocyclic carbon substituents instead of oxygen (sydnones) or nitrogen (sydnone imines) in the 5-position of a 1,2,3-oxadiazolium ring. Unsubstituted 4-positions give rise to the generation of anionic N-heterocyclic carbenes by deprotonation. Preparations of new sydnone methides are described here. They can be represented by mesomeric structures with either exocyclic carbanionic groups like −C(CN)₂⁻, −C(CN)(COOMe)⁻, −C(CN)(CONH₂)⁻, and −C(CN)(SO₂Me)⁻, or with the corresponding exocyclic C=C double bonds as a common feature with mesoionic N-heterocyclic olefins. An X-ray single structure analysis revealed a length of 140.7(3) pm of the exocyclic bond in the solid state. From the coalescence temperature (55 °C) determined by a series of ¹³C NMR experiments (150 MHz) at various temperatures, an energy of rotation of 18.5 Kcal/mol was calculated for this bond. The ¹³C NMR signals of the anionic N-heterocyclic carbenes, from which the 2-mesityl-substituted anionic NHC proved to be stable up to 10 °C, are highly shifted upfield (δ_carbene=157.9 ppm−160.5 ppm). The carbenes can be reacted in situ with elemental selenium and chlorophosphanes to yield sydnone methide selenoethers after methylation and 4-phosphanylsydnone methides in good to excellent yields, respectively.     

Glycosylidene Carbenes,Part 29 , Insertion into B−C and Al−C Bonds: Glycosylborinates, ‐boranes,and ‐alanes

Insertion of the glycosylidene carbenes derived from the diazirines 1 , 14 , and 15 into the B−alkyl bond of the B‐alkyl‐9‐oxa‐10‐borabicyclo[3.3.2]decanes 5 , 6 , and 7 yielded the stable glycosylborinates 8 / 9 (55%, 55 : 45), 10 / 11 (31%, 65 : 35), 12 / 13 (47%, 60 : 40), 16 / 17 (55%, 55 : 45), 18 / 19 (47%, 45 : 55), and 20 / 21 (31%, 30 : 70). Crystal‐structure analysis of 17 and NOEs of 9 and 19 show that 17 , 9 , and 19 adopt similar conformations. The glycosylborinates are stable under acidic, basic and thermal conditions. The unprotected glycosylborinate 25 was obtained in 80% by hydrogenolysis of 12 . Insertion of the glycosylidene carbene derived from the diazirine 1 into a B−C bond of BEt₃, BBu₃, and BPh₃ led to unstable glycosylboranes that were oxidised to yield the hemiacetals 29 (55%), 31 (45%), and 33 (48%), respectively, besides the glucals 30 (13%), 32 (20%), and 34 (20%), respectively. Insertion of the glycosylidene carbenes derived from 14 and 15 into a B−C bond of BEt₃ led exclusively to hemiacetals; only 15 yielding traces of the glucal 40 besides the hemiacetal 39 . The glycosylidene carbene derived from 1 reacted with Al(ⁱBu)₃ and AlMe₃ to generate reactive glycosylalanes that were hydrolysed, yielding the C‐glycosides 46 (21%) and 49 (30%), respectively, besides the glucals 48 (26%) and 51 (30%); deuteriolysis instead of protonolysis led to the monodeuterio analogues of 46 and 49 , respectively, which possess an equatorial ²H‐atom at the anomeric center.     

A Stable Dimer of SiS2 Arranged between Two Carbene Molecules

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.     

Building up Strain in One Step: Synthesis of an Edge-Fused Double Silacyclobutene from an Extensively Trichlorosilylated Butadiene Dianion

The exhaustive trichlorosilylation of hexachloro-1,3-butadiene was achieved in one step by using a mixture of Si₂Cl₆ and [nBu₄N]Cl (7:2 equiv) as the silylation reagent. The corresponding butadiene dianion salt [nBu₄N]₂[ 1 ] was isolated in 36 % yield after recrystallization. The negative charges of [ 1 ]²⁻ are mainly delocalized across its two carbanionic (Cl₃Si)₂C termini (α-effect of silicon) such that the central bond possesses largely C=C double-bond character. Upon treatment with 4 equiv of HCl, [ 1 ]²⁻ is converted into neutral 1,2,3,4-tetrakis(trichlorosilyl)but-2-ene, 3 . The Cl⁻ acceptor AlCl₃, induces a twofold ring-closure reaction of [ 1 ]²⁻ to form a six-membered bicycle 4 in which two silacyclobutene rings are fused along a shared C=C double bond (84 %). Compound 4 , which was structurally characterized by X-ray crystallography, undergoes partial ring opening to a monocyclic silacyclobutene 2 in the presence of HCl, but is thermally stable up to at least 180 °C.     

The microstructure determination of ethylene‐styrene‐propylene terpolymers at triad level by high‐temperature two‐dimensional NMR spectra

To further extend temperature range of application and low temperature performance of the ethylene‐styrene copolymers, a series of poly(ethylene‐styrene‐propylene) samples with varying monomer compositions and relatively low glass‐transition temperatures (T_g = −28 – 22 °C) were synthesized by Me₂Si(Me₄Cp)(N‐t‐Bu)TiCl2/MMAO system. Since the ¹³C NMR spectra of the terpolymers were complex and some new resonances were present, 2D‐¹H/¹³C heteronuclear single quantum coherence and heteronuclear multiple bond correlation experiments were conducted. A complete ¹³C NMR characterization of these terpolymers was performed qualitatively and quantitatively, including chemical shifts, triad sequence distributions, and monomer average sequence lengths. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 340–350