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.     

Synthetic,Mechanistic, and Theoretical Studies on the Generation of Iridium Hydride Alkylidene and Iridium Hydride Alkene Isomers

Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(Tp^Me2)Ir(H){?C(CH₂R)ArO }] (Tp^Me2=hydrotris(3,5‐dimethylpyrazolyl)borate; R=H, Me; Ar=substituted C₆H₄ group), and their corresponding hydride olefin isomers, [(Tp^Me2)Ir(H){R(H)C? C(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(Tp^Me2)Ir(C₆H₅)₂] with o‐C₆H₄(OH)CH₂R, or with the substituted anisoles 2,6‐Me₂C₆H₃OMe, 2,4,6‐Me₃C₆H₂OMe, and 4‐Br‐2,6‐Me₂C₆H₂OMe. The reactions with the substituted anisoles require not only multiple C? H bond activation but also cleavage of the Me? OAr bond and the reversible formation of a C? C bond (as revealed by ¹³C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140 °C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen‐elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ir–carbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers.     

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.     

Crystalline Radicals Derived from Classical N‐Heterocyclic Carbenes

One‐electron reduction of C2‐arylated 1,3‐imidazoli(ni)um salts (IPr^Ar)Br (Ar=Ph, 3 a ; 4‐DMP, 3 b ; 4‐DMP=4‐Me₂NC₆H₄) and (SIPr^Ar)I (Ar=Ph, 4 a ; 4‐Tol, 4 b ) derived from classical NHCs (IPr=:C{N(2,6‐iPr₂C₆H₃)}₂CHCH, 1 ; SIPr=:C{N(2,6‐iPr₂C₆H₃)}₂CH₂CH₂, 2 ) gave radicals [(IPr^Ar)]^. (Ar=Ph, 5 a ; 4‐DMP, 5 b ) and [(SIPr^Ar)]^. (Ar=Ph, 6 a ; 4‐Tol, 6 b ). Each of 5 a , b and 6 a , b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid‐state characterization of NHC‐derived carbon‐centered radicals 6 a , b by single‐crystal X‐ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.     

Zur reaktion von metall-koordiniertem kohlenmonoxid mit yliden: XI. Einige neuartige η1-vinyl-komplexe des eisens durch deprotonierung kationischer carbenkomplexe mit trimethyl(methylen)phosphoran

Novel η¹-vinyl complexes of the type Cp(CO)(L)FeC(OMe)C(R)R′ (R = R′ = H, Me; R = H, R′ = Me; L = Me₃P, Ph₃P) are obtainied via methylation of the acyl complexes Cp(CO)(L)FeC(O)R (R = Me, Et, i-Pr) with MeOSO₂F and subsequent deprotonation of the resulting carbene complexes [Cp(CO)(L)FeC(OMe)R]SO₃F with the phosphorus ylide Me₃PCH₂. The same procedure can be applied for the synthesis of the pentamethylcyclopentadienyl derivative C₅Me₅(CO)(Me₃P)FeC(OMe)CH₂, while treatment of the hydroxy or siloxy carbene complexes [Cp(CO)(L)FeC(OR)Me]X (R = H, Me₃Si; X = SO₃CF₃) with Me₃CH₂ results in the transfer of the oxygen bound electrophile to the ylidic carbon. Some remarkable spectroscopic properties of the new complexes are reported.     

Palladium‐Catalyzed Direct C2‐Arylation of an N‐Heterocyclic Carbene: An Atom‐Economic Route to Mesoionic Carbene Ligands

Blocking the C2 position of an imidazole‐derived classical N‐heterocyclic carbene (NHC) with an aryl group is an essential strategy to establish a route to mesoionic carbenes (MICs), which coordinate to the metal via the C4 (or C5) carbon atom. An efficient catalytic route to MIC precursors by direct arylation of an NHC is reported. Treatment of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) with an aryl iodide (RC₆H₄I) in the presence of 0.5 mol % of [Pd₂(dba)₃] (dba=dibenzylideneacetone) precatalyst affords the C2‐arylated imidazolium salts {IPr(C₆H₄R)}I (R=H, 4‐Me, 2‐Me, 4‐OMe, 4‐COOMe) in excellent (up to 92 %) yields. Treatment of {IPr(C₆H₅)}I with CuI and KN(SiMe₃)₂ exclusively affords the MIC–copper complex [(IPrPh)CuI].     

Adduct Formation,B−H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1 ) with unsaturated and saturated NHCs as well as CAAC^Me. Mono‐NHC adducts of the type HBcat?NHC (NHC=nPr₂Im, iPr₂Im, iPr₂Im^Me, and Dipp₂Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC^Me with HBcat yielded the B?H activated product CAAC^Me(H)Bcat via insertion of the carbine‐carbon atom into the B?H bond. The saturated NHC Dipp₂SIm reacted in a 2:2 ratio yielding an NHC ring‐expanded product at room temperature forming a six‐membered ?B?C=N?C=C?N? ring via C?N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene‐carbon atom.     

Experimental and Computational Studies of the Molybdenum‐Flanking Arene Interaction in Quadruply Bonded Dimolybdenum Complexes with Terphenyl Ligands

To clarify the nature of the Mo?C_arene interaction in terphenyl complexes with quadruple Mo?Mo bonds, ether adducts of composition [Mo₂(Ar′)(I)(O₂CR)₂(OEt₂)] have been prepared and characterized (Ar′=Ar^Xyl₂, R=Me; Ar′=ArMes₂, R=Me; Ar′=Ar^Xyl2, R=CF₃) (Mes=mesityl; Xyl=2,6‐Me₂C₆H₃, from now on xylyl) and their reactivity toward different neutral Lewis bases investigated. PMe₃, P(OMe)₃ and PiPr₃ were chosen as P‐donors and the reactivity studies complemented with the use of the C‐donors CNXyl and CN₂C₂Me₄ (1,3,4,5‐tetramethylimidazol‐2‐ylidene). New compounds of general formula [Mo₂(Ar′)(I)(O₂CR)₂( L )] were obtained, except for the imidazol‐2‐ylidene ligand that yielded a salt‐like compound of composition [Mo₂(Ar^Xyl2)(O₂CMe)₂(CN₂C₂Me₄)₂]I. The Mo?C_arene interaction in these complexes has been analyzed with the aid of X‐ray data and computational studies. This interaction compensates the coordinative and electronic unsaturation of one of the Mo atoms in the above complexes, but it seems to be weak in terms of sharing of electron density between the Mo and C_arene atoms and appears to have no appreciable effect in the length of the Mo?Mo, Mo?X, and Mo? L bonds present in these molecules.     

Syntheses,Characterizations, Crystal structures,and Antitumor Activities of Arenetelluronic Triorganotin Esters

Six new arenetelluronic triorganotin esters, namely (R₃Sn)₄[ArTe(μ‐O)(OH)O₂)]₂ (Ar = Ph, R = Me: 1 , R = Ph: 2 ; Ar = 3‐Me‐Ph, R = Me: 3 , R = Ph: 4 , Ar = 3‐Cl‐Ph, R = Me: 5 , R = Ph: 6 ), were prepared by treating arenetelluronic acids with the corresponding R₃SnCl (R = Me, Ph) with potassium hydroxide in methanol. All complexes were characterized by elemental analysis, FT‐IR, NMR (¹H, ¹³C, ¹¹⁹Sn) spectroscopy, and X‐ray crystallography. The structural analyses indicate that these complexes are isostructural as Sn₄Te₂ moiety, in which the Te₂(μ₂‐O)₂ units are situated in the center and each Te atom is coordinated with two OSnR₃ groups on the side. Complexes 1 , 3 , and 5 show one‐dimensional chain and two‐dimensional network supramolecular structures by intermolecular C H···O or C H···Cl interactions. The antitumor activities of these complexes reveal that most arenetelluronic triorganotin esters have powerful antitumor activities with certain regularity.     

N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Depending on the amount of methanol present in solution, CO₂ adducts of N‐heterocyclic carbenes (NHCs) and N‐heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO₂Me] and [EMMIm][OCO₂Me]. The reactivity pattern of representative 1‐ethyl‐3‐methyl‐NHO–CO₂ adduct 4 has been investigated and compared with the corresponding NHC–CO₂ zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO₂H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO₂ zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]₂ cyanomalonate salts formed exemplify the first C?C bond‐forming carboxylation reactions with NHO‐activated CO₂. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO₂Me][X]. The first NHO‐SO₂ adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.     

Continuous‐Flow N‐Heterocyclic Carbene Generation and Organocatalysis

Two methods were assessed for the generation of common N‐heterocyclic carbenes (NHCs) from stable imidazol(in)ium precursors using convenient and straightforward continuous‐flow setups with either a heterogeneous inorganic base (Cs₂CO₃ or K₃PO₄) or a homogeneous organic base (KN(SiMe₃)₂). In‐line quenching with carbon disulfide revealed that the homogeneous strategy was most efficient for the preparation of a small library of NHCs. The generation of free nucleophilic carbenes was next telescoped with two benchmark NHC‐catalyzed reactions; namely, the transesterification of vinyl acetate with benzyl alcohol and the amidation of N‐Boc‐glycine methyl ester with ethanolamine. Both organocatalytic transformations proceeded with total conversion and excellent yields were achieved after extraction, showcasing the first examples of continuous‐flow organocatalysis with NHCs.     

Thermochemistry of N‐heterocyclic carbenes with 5‐, 4‐, 3‐, and 2‐membered rings

N‐heterocyclic carbenes (NHCs) based on imidazole‐2‐ylidene ( 1 ) or the saturated imidazolidine‐2‐ylidene ( 2 ) scaffolds are long‐lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p‐pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS‐QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four‐membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino‐substituted cyclopropenylidenes have been reported but three‐membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino‐substituted cyclopropenylidenes are comparable in stability to the four‐membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of “two‐membered ring” azapropadienenylidene cations of form :C?N?W with W = an electron‐withdrawing agent. We have studied W = NO₂, CH₂(+), CF₂(+), and (CN)₂C(+). Although these systems display a degree of stabilization and carbene‐like electronic structure, the stability of the NHCs is unsurpassed. © 2014 Wiley Periodicals, Inc.     

Stabilization of a Silaaldehyde by its η2 Coordination to Tungsten

Treatment of pyridine‐stabilized silylene complexes [(η⁵‐C₅Me₄R)(CO)₂(H)W?SiH(py)(Tsi)] (R=Me, Et; py=pyridine; Tsi=C(SiMe₃)₃) with an N‐heterocyclic carbene ^MeIⁱPr (1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene) caused deprotonation to afford anionic silylene complexes [(η⁵‐C₅Me₄R)(CO)₂W?SiH(Tsi)][H^MeIⁱPr] (R=Me ( 1‐Me ); R=Et ( 1‐Et )). Subsequent oxidation of 1‐Me and 1‐Et with pyridine‐N‐oxide (1 equiv) gave anionic η²‐silaaldehydetungsten complexes [(η⁵‐C₅Me₄R)(CO)₂W{η²‐O?SiH(Tsi)}][H^MeIⁱPr] (R=Me ( 2‐Me ); R=Et ( 2‐Et )). The formation of an unprecedented W‐Si‐O three‐membered ring was confirmed by X‐ray crystal structure analysis.     

Nitridorhenium(V) Complexes with 1,3‐Dialkyl‐4,5‐dimethylimidazole‐2‐ylidenes

The nitridorhenium(V) complexes [ReNCl₂(PR₂Ph)₃] (R = Me, Et) react with the N‐heterocyclic carbenes (NHC) 1,3‐diethyl‐4,5‐dimethylimidazole‐5‐ylidene (L^Et) or 1,3,4,5‐tetramethylimidazole‐2‐ylidene (L^Me) in absolutely dry THF under complete replacement of the equatorial coordination sphere. The resulting [ReNCl(L^R)₄]⁺ complexes (L^R = L^Me, L^Et) are moderately stable as solids and in solution, but decompose in hot methanol under formation of [ReO₂(L^R)₄]⁺ complexes. With 1,3‐diisopropyl‐4,5‐dimethylimidazole‐5‐ylidene (L^i‐Pr), the loss of the nitrido ligand and the formation of a dioxo species is more rapid and no nitridorhenium intermediate could be isolated. The Re‐C bond lengths in [ReNCl(L^Et)₄]Cl of approximately 2.195Å are relatively long and indicate mainly σ‐bonding in the electron‐deficient d² system under study. The hydrolysis of the nitrido complexes proceeds via the formation of [ReO₃N]^2? anions as could be verified by the isolation and structural characterization of the intermediates [{ReN(PMe₂Ph)₃}{ReO₃N}]₂ and [{ReN(OH₂)(L^Et)₂}₂O][ReO₃N].     

Reaching for two new stable ambiphilic quinoline‐derived N‐heterocyclic carbenes at DFT level

A detailed investigation on the thermodynamic and kinetic stability of four carbenic tautomers of quinoline 1 , including quinoline‐2‐ylidene 2 , quinoline‐3‐ylidene 3 , quinoline‐4‐ylidene 4 , and 3,4‐dihydroquinoline‐4‐ylidene 5 , reveals that singlet planar six‐membered ring N‐heterocyclic carbenes (NHCs) 2 and 4 have less stability than Arduengo type NHC but seems to have enough conceivably for reaching at B3LYP/aug‐cc‐pVTZ//B3LYP/6–31+G* and B3LYP/6–311++G**//B3LYP/6–31+G* levels. All these six‐membered NHCs are extremely ambiphilic with the more nucleophilic and electrophilic characters compared to the Arduengo type one. The aromaticity of singlet 2 and 4 is a significant contributor to their stability which is confirmed through their Nucleus‐independent chemical shift(1)_zz values. Finally, among 2–5 , the normal NHC 2 is thermodynamically preferred but the remote NHC 4 is kinetically proffered over the other isomeric carbenes. The effects of different N‐ or C‐substituted NHCs of 2 are studied using appropriate isodesmic reactions. The trimethylsilyl substituent exhibits slightly larger carbene stabilization in quinoline‐derived NHCs than the pyridine analogue. © 2014 Wiley Periodicals, Inc.     

Synthesis and Reactivity of Cationic Triruthenium Clusters Derived from 2‐Methyl‐ and 4‐Methylpyrimidines: From Conventional Cyclometalated Ligands to Novel Types of N‐Heterocyclic Carbenes

The methylation of the uncoordinated nitrogen atom of the cyclometalated triruthenium cluster complexes [Ru₃(μ‐H)(μ‐κ²N¹,C⁶‐2‐Mepyr)(CO)₁₀] ( 1 ; 2‐MepyrH=2‐methylpyrimidine) and [Ru₃(μ‐H)(μ‐κ²N¹,C⁶‐4‐Mepyr)(CO)₁₀] ( 9 ; 4‐MepyrH=4‐methylpyrimidine) gives two similar cationic complexes, [Ru₃(μ‐H)(μ‐κ²N¹,C⁶‐2,3‐Me₂pyr)(CO)₁₀]⁺( 2 ⁺) and [Ru₃(μ‐H)(μ‐κ²N¹,C⁶‐3,4‐Me₂pyr)(CO)₁₀]⁺ ( 9 ⁺), respectively, whose heterocyclic ligands belong to a novel type of N‐heterocyclic carbenes (NHCs) that have the C_carbene atom in 6‐position of a pyrimidine framework. The position of the C‐methyl group in the ligands of complexes 2 ⁺ (on C²) and 9 ⁺ (on C⁴) is of key importance for the outcome of their reactions with K[N(SiMe₃)₂], K‐selectride, and cobaltocene. Although these reagents react with 2 ⁺ to give [Ru₃(μ‐H)(μ‐κ²N¹,C⁶‐2‐CH₂‐3‐Mepyr)(CO)₁₀] ( 3 ; deprotonation of the C²‐Me group), [Ru₃(μ‐H)(μ₃‐κ³N¹,C⁵,C⁶‐4‐H‐2,3‐Me₂pyr)(CO)₉] ( 4 ; hydride addition at C⁴), and [Ru₆(μ‐H)₂{μ₆‐κ⁶N¹,N^1′,C⁵,C^5′,C⁶,C^6′‐4,4′‐bis(2,3‐Me₂pyr)}(CO)₁₈] ( 5 ; reductive dimerization at C⁴), respectively, similar reactions with 9 ⁺ have only allowed the isolation of [Ru₃(μ‐H)(μ₃‐κ²N¹,C⁶‐2‐H‐3,4‐Me₂pyr)(CO)₉] ( 11 ; hydride addition at C²). Compounds 3 and 11 also contain novel six‐membered ring NHC ligands. Theoretical studies have established that the deprotonation of 2 ⁺ and 9 ⁺ (that have ligand‐based LUMOs) are charge‐controlled processes and that both the composition of the LUMOs of these cationic complexes and the steric protection of their ligand ring atoms govern the regioselectivity of their nucleophilic addition and reduction reactions.     

Synthesis and characterization of aluminum(III) and tin(II) complexes supported by diiminophosphinate ligands and their application in ring‐opening polymerization catalysis of ε‐caprolactone

A series of Al(III) and Sn(II) diiminophosphinate complexes have been synthesized. Reaction of Ph(ArCH₂)P(?NBu^t)NHBu^t (Ar = Ph, 3 ; Ar = 8‐quinolyl, 4 ) with AlR₃ (R = Me, Et) gave aluminum complexes [R₂Al{(NBu^t)₂P(Ph)(CH₂Ar)}] (R = Me, Ar = Ph, 5 ; R = Me, Ar = 8‐quinolyl, 6 ; R = Et, Ar = Ph, 7 ; R = Et, Ar = quinolyl, 8 ). Lithiated 3 and 4 were treated with SnCl₂ to afford tin(II) complexes [ClSn{(NBu^t)₂P(Ph)(CH₂Ar)}] (Ar = Ph, 9 ; Ar = 8‐quinolyl, 10 ). Complex 9 was converted to [(Me₃Si)₂NSn{(NBu^t)₂P(Ph)(CH₂Ph)}] ( 11 ) by treatment with LiN(SiMe₃)₂. Complex 11 was also obtained by reaction of 3 with [Sn{N(SiMe₃)₂}₂]. Complex 9 reacted with [LiOC₆H₄Bu^t‐4] to yield [4‐Bu^tC₆H₄OSn{(NBu^t)₂P(Ph)(CH₂Ph)}] ( 12 ). Compounds 3–12 were characterized by NMR spectroscopy and elemental analysis. The structures of complexes 6 , 10 , and 11 were further characterized by single crystal X‐ray diffraction techniques. The catalytic activity of complexes 5–8 , 11 , and 12 toward the ring‐opening polymerization of ε‐caprolactone (CL) was studied. In the presence of BzOH, the complexes catalyzed the ring‐opening polymerization of ε‐CL in the activity order of 5 > 7 ≈ 8 > 6 ? 11 > 12 , giving polymers with narrow molecular weight distributions. The kinetic studies showed a first‐order dependency on the monomer concentration in each case. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4621–4631, 2006     

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.     

Generating and Trapping Metalla‐N‐Heterocyclic Carbenes

By means of a combined experimental and theoretical approach, the electronic features and chemical behavior of metalla‐N‐heterocyclic carbenes (MNHCs, N‐heterocyclic carbenes containing a metal atom within the heterocyclic skeleton) have been established and compared with those of classical NHCs. MNHCs are strongly basic (proton affinity and pK_a values around 290 kcal mol^?1 and 36, respectively) with a narrow singlet–triplet gap (around 23 kcal mol^?1). MNHCs can be generated from the corresponding metalla‐imidazolium salts and trapped by addition of transition‐metal complexes affording the corresponding heterodimetallic dicarbene derivatives, which can serve as carbene transfer agents.     

N‐Heterocyclic Carbene Stabilized Dicarbondiphosphides: Strong Neutral Four‐Membered Heterocyclic 6π‐Electron Donors

In contrast to cyclic π‐conjugated hydrocarbons, the coordination chemistry of inorganic heterocycles is less developed. Dicarbondiphosphides stabilized by N‐heterocyclic carbenes (NHCs) NHC→C₂P₂←NHC ( 1 a , b ) (NHC=IPr or SIPr) contain a four‐membered C₂P₂ ring with an aromatic 6π‐electron configuration. These heterocycles coordinate to a variety of complex fragments with metals from groups 6, 9, and 10, namely [M⁰(CO)₃] (M=Cr, Mo), [Co^I(CO)₂]⁺, or [Ni^IIBr₂], through an η⁴‐coordination mode, leading to complexes 2 a , b , 3 a , b , 5 a , b , and 6 a , b , respectively. These complexes were characterized by X‐ray diffraction methods using single crystals, IR spectroscopy, and DFT calculations. In combination these methods indicate that 1 a , b behave as exceptionally strong 6π‐electron donors.