Stable Radical Cation and Dication of an N‐Heterocyclic Carbene Stabilized Digallene: Synthesis,Characterization and Reactivity

One‐ and two‐electron oxidation of a digallene stabilized by an N‐heterocyclic carbene afforded the first stable gallium‐based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu₃SnH and cyclo‐S₈ was studied; a digallium–hydride cation salt containing a Ga?Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder‐like Ga₄S₄ core structure were obtained, respectively.     

Cyclic(Alkyl)(Amino)Carbene (CAAC)-Supported Zn Alkyls: Synthesis,Structure and Reactivity in Hydrosilylation Catalysis

The reactivity of Zn^II dialkyl species ZnMe₂ with a cyclic(alkyl)(amino)carbene, 1-[2,6-bis(1-methylethyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene (CAAC, 1 ), was studied and extended to the preparation of robust CAAC-supported Zn^II Lewis acidic organocations. CAAC adduct of ZnMe₂ ( 2 ), formed from a 1:1 mixture of 1 and ZnMe₂, is unstable at room temperature and readily undergoes a CAAC carbene insertion into the Zn−Me bond to produce the ZnX₂-type species (CAAC-Me)ZnMe ( 3 ), a reactivity further supported by DFT calculations. Despite its limited stability, adduct 2 was cleanly ionized to robust two-coordinate (CAAC)ZnMe⁺ cation ( 5⁺ ) and derived into (CAAC)ZnC₆F₅⁺ ( 7⁺ ), both isolated as B(C₆F₅)₄⁻ salts, showing the ability of CAAC for the stabilization of reactive [ZnMe]⁺ and [ZnC₆F₅]⁺ moieties. Due to the lability of the CAAC−ZnMe₂ bond, the formation of bis(CAAC) adduct (CAAC)₂ZnMe⁺ cation ( 6⁺ ) was also observed and the corresponding salt [ 6 ][B(C₆F₅)₄] was structurally characterized. As estimated from experimental and calculations data, cations 5⁺ and 7⁺ are highly Lewis acidic species and the stronger Lewis acid 7⁺ effectively mediates alkene, alkyne and CO₂ hydrosilylation catalysis. All supporting data hints at Lewis acid type activation–functionalization processes. Despite a lower energy LUMO in 5⁺ and 7⁺ , their observed reactivity is comparable to those of N-heterocyclic carbene (NHC) analogues, in line with charge-controlled reactions for carbene-stabilized Zn^II organocations.     

L3C3P3: Tricarbontriphosphide Tricyclic Radicals and Cations Stabilized by Cyclic (alkyl)(amino)carbenes

Alkynes usually oligomerize to give rings with a conjugated π‐electron system. In contrast, phosphaalkynes, R?C≡P, frequently give compounds with polycyclic structures, which are thermodynamically more stable than the corresponding π‐conjugated isomers. The syntheses of the first C₃P₃ tricyclic compounds are reported with either radical or cationic ground states stabilized by cyclic (alkyl)(amino)carbenes (CAACs). These compounds may be considered as examples of tricarbontriphosphide coordinated by carbenes and are likely formed via trimerization of the corresponding mono‐radicals CAAC‐CP^.. The mechanism for the formation of these tricarbontriphosphide radicals has been rationalized by a combination of experiments and DFT calculations.     

P=C bonds as building blocks for three- and four-membered heterocyclic cations: synthesis, structures and mechanistic studies

The activation of the P=C bond of phosphaalkenes with electrophiles is investigated as a means to prepare and characterize unusual organophosphorus compounds. Treatment of RP=CHtBu (1a: R=tBu; 1b: R=1-adamantyl) with HOTf (0.5 equiv) affords diphosphiranium salts [RP-CHtBu-PR (CH(2)tBu)]OTf ([2a]OTf and [2b]OTf), each containing a three-membered P(2)C ring. In contrast, the addition of MeOTf (0.5 equiv) to either 1a or 1b affords diphosphetanium salts [RP-CHtBu-P(Me)R-CHtBu]OTf ([3a]OTf and [3b]OTf) containing four-membered P(2)C(2) heterocycles. The phosphenium triflate [tBuP(CH(2)tBu)]OTf ([5a]OTf) and methylenephosphonium triflate [tBu(Me)P=CHtBu]OTf ([7a]OTf) are identified spectroscopically as intermediates in the formation of [2a](+) and [3a](+), respectively. The phosphenium triflate intermediate can be trapped with 2-butyne to afford phosphirenium salt [MeC=CMe-tBuPCH(2)tBu]OTf ([6a]OTf). Treatment of diphosphetanium [3a]OTf with an excess MeOTf affords [Me(2)P-CHtBu-PMetBu-CHtBu](OTf)(2) ([4a](OTf)(2)), a compound containing a diphosphetanium dication. The molecular structures are reported for [2a]OTf, [2b][H(OTf)(2)], [3a]I, [3b]I, [4a](OTf)(2), and [6a]OTf.     

α-Radical Phosphines: Synthesis,Structure, and Reactivity

A series of phosphines featuring a persistent radical were synthesized in two steps by condensation of dialkyl-/diarylchlorophosphines with stable cyclic (alkyl)(amino)carbenes (cAACs) followed by one-electron reduction of the corresponding cationic intermediates. Structural, spectroscopic, and computational data indicate that the spin density in these phosphines is mainly localized on the original carbene carbon from the cAAC fragment; thus, it remains in the α-position with respect to the central phosphorus atom. The potential of these α-radical phosphines to serve as spin-labeled ligands is demonstrated through the preparation of several Au^I derivatives, which were also structurally characterized by single-crystal X-ray diffraction.     

Synthesis,Structure, and Reactivity of Lewis Base Stabilized Plumbacyclopentadienylidenes

Plumbacyclopentadienylidenes, in which the lead atoms have divalent states and are coordinated by THF, pyridine and N‐heterocyclic carbene, were synthesized and characterized. The THF‐ and pyridine‐stabilized compounds can be regarded as rare examples of hypervalent 10‐X‐4 species. The equilibrium between the THF adduct and the free plumbacyclopentadienylidene was evidenced by spectroscopic analysis and theoretical calculations. The THF adduct in benzene converted into a plumbylene dimer, where one of the lead centers is coordinated by THF and the other lead atom is coordinated by a divalent lead atom, the dimer gradually decomposing into spiroplumbole. The THF adduct unexpectedly reacted with trifluoroborane and trichlorogallane to afford fluoroborole and chlorogallole, which are the first examples of non‐annulated fluoroborole and gallole, respectively.     

Intermolecular and Regioselective Access to Polysubstituted Benzo‐ and Dihydrobenzo[c]azepine Derivatives: Modulating the Reactivity of Group 6 Non‐Heteroatom‐Stabilized Alkynyl Carbene Complexes

We highlight the versatility of non‐heteroatom‐stabilized tungsten–carbene complexes 3 synthesized in situ, which have been used in a modular approach to access 2‐benzazepinium isolable intermediates 5 . By employing very mild conditions, benzazepinium derivatives 5 have been obtained in high yield from simple compounds, such as acetylides 2 , Fischer‐type alkoxycarbenes 1 , and phenylimines 4 . The process, involving a formal [4+3] heterocycloaddition, occurs in a totally regioselective manner, which differs from the approach previously observed in similar procedures for other carbene analogues. This work, which involves three components, reveals a control of the reactivity of non‐heteroatom‐stabilized carbene complexes 3 ([4+3] vs. [2+2]‐heterocycloaddition reactions) depending on the acetylide substitution pattern. The influence of the substitution pattern in the behavior of the complexes has been computationally analyzed and rationalized. Finally, elaboration of the 2‐benzazepinium intermediates allows access to 3H‐benzo[c]azepines 6 and 3H‐1,2‐dihydrobenzo[c]azepines 7 – 9 with high control of the substitution of the nine positions of the heterocycle.     

Transition-Metal-Like Reversible Cycloadditions of [tBuSP-W(CO)5] with Alkenes and Alkynes

tert-Butylthiophosphinidene complex [tBuSP-W(CO)₅] was generated by dissociation of 1-(tert-butylthio)phosphirane–W(CO)₅ complex under mild conditions. The formation of transient [tBuSP-W(CO)₅] was indicated by trapping reactions with 2,3-dimethyl-1,3-butadiene, alkynes, phenanthrene-9,10-dione, and methanol. The LUMO of [MeSP-W(CO)₅] is significantly lower in energy than those of [Me₂NP-W(CO)₅], [MeOP-W(CO)₅], and [Me₂PP-W(CO)₅]. The HOMO of [MeSP-W(CO)₅] contains a significant contribution from the in-plane lone pair of P and the LUMO shows a typical π* characteristic. Since stabilized by sulfur lone pair and coordinated by W(CO)₅, [tBuSP-W(CO)₅] undergoes facile and reversible cycloadditions with alkenes and alkynes.     

Coordination Chemistry of Cyclopropenylidene‐Stabilized Phosphenium Cations: Synthesis and Reactivity of Pd and Pt Complexes

A straightforward synthesis of cyclopropenylidene‐stabilized phosphenium cations 1 a – g through the reaction of [(iPr₂N)₂C₃⁺Cl]BF₄ with secondary phosphines is described. Their donor ability was evaluated by analysis of the CO stretching frequency in Rh complexes [RhCl(CO)L₂](BF₄)₂ and electrochemical methods. The cyclopropenium ring induces a phosphite‐type behavior that can be tuned by the other two substituents attached to the phosphorus atom. Despite of the positive charge that they bear, phosphenium cations 1 a – g still act as two‐electron donor ligands, forming adducts with Pd^II and Pt^II precursors. Conversely, in the presence of Pd⁰ species, an oxidative insertion of the Pd atom into the C_carbene–phosphorus bond takes place, providing dimeric structures in which each Pd atom is bonded to a cyclopropenyl carbene while two dialkyl/diaryl phosphide ligands serve as bridges between the two Pd centers. The catalytic performance of the resulting library of Pt^II complexes was tested; all of the cationic phosphines accelerated the prototype 6‐endo‐dig cyclization of 2‐ethynyl‐1,1′‐biphenyl to afford pentahelicene. The best ligand 1 g was used in the synthesis of two natural products, chrysotoxene and epimedoicarisoside A.     

A Cationic Zinc Hydride Cluster Stabilized by an N‐Heterocyclic Carbene: Synthesis,Reactivity, and Hydrosilylation Catalysis

The trinuclear cationic zinc hydride cluster [(IMes)₃Zn₃H₄(THF)](BPh₄)₂ ( 1 ) was obtained either by protonation of the neutral zinc dihydride [(IMes)ZnH₂]₂ with a Brønsted acid or by addition of the putative zinc dication [(IMes)Zn(THF)]²⁺. A triply bridged thiophenolato complex 2 was formed upon oxidation of 1 with PhS? SPh. Protonolysis of 1 by methanol or water gave the corresponding trinuclear dicationic derivatives. At ambient temperature, 1 catalyzed the hydrosilylation of aldehydes, ketones, and nitriles. Carbon dioxide was also hydrosilylated under forcing conditions when using (EtO)₃SiH, giving silylformate as the main product.