Synthesis of a Carbodicyclopropenylidene: A Carbodicarbene based Solely on Carbon

Metal-Mediated Synthesis of a Mixed Arduengo-Fischer Carbodicarbene Ligand

Carbodicarbenes are strong C-donor ligands, which have found numerous applications in organometallic and main group element chemistry. Herein, we report a structurally distinct carbodicarbene ligand, which is formed by dinitrogenative coupling of a Fischer carbene complex with an N-heterocyclic diazoolefin. The resulting carbonyl complex serves as a stable source for the mixed Arduengo-Fischer carbodicarbene ligand. Facile ligand transfer reactions were demonstrated to occur with gold(I), copper(I), palladium(II), and rhodium(I) complexes.

Bisgermylene-Stabilized Stannylone: Catalytic Reduction of Nitrous Oxide and Nitro Compounds via Element-Ligand Cooperativity

This study describes the synthesis, structural characterization, and catalytic application of a bis(germylene)-stabilized stannylone (2). The reduction of digermylated stannylene (1) with 2.2 equiv of potassium graphite (KC8) leads to the formation of stannylone 2 as a green solid in 78% yield. Computational studies showed that stannylone 2 possesses a formal Sn(0) center and a delocalized 3-c-2-e π-bond in the Ge2Sn core, which arises from back-donation of the p-type lone pair electrons on the Sn atom to the vacant orbitals of the Ge atoms. Stannylone 2 can serve as an efficient precatalyst for the selective reduction of nitrous oxide (N2O) and nitroarenes (ArNO2) with the formation of dinitrogen (N2) and hydrazines (ArNH-NHAr), respectively. Exposure of 2 with N2O (1 atm) resulted in the insertion of two oxygen atoms into the Ge-Ge and Ge-Sn bonds, yielding the germyl(oxyl)stannylene (3). Moreover, the stoichiometric reaction of 2 with 1-chloro-4-nitrobenzene afforded an amido(oxyl)stannylene (4) through the complete scission of the N-O bonds of the nitroarene. Stannylenes 3 and 4 serve as catalytically active species for the catalytic reduction of nitrous oxide and nitroarenes, respectively. Mechanistic studies reveal that the cooperation of the low-valent Ge and Sn centers allows for multiple electron transfers to cleave the N-O bonds of N2O and ArNO2. This approach presents a new strategy for catalyzing the deoxygenation of N2O and ArNO2 using a zerovalent tin compound.

Ligand Exchange at Carbon: Synthetic Entry to Elusive Species and Versatile Reagents

How different is carbon compared to other elements in the periodic table? Can carbon compounds be regarded as coordination complexes with carbon as the central element undergoing a facile exchange of its ligands? Although carbon clearly plays a special role among the elements of the periodic table, recent studies have drawn parallels between the bonding situation and the reactivity of carbon compounds to transition metal complexes. This Perspective summarizes recent reports about ylidic and zwitterionic compounds that were shown to exhibit ambiguous bonding situations that can be interpreted as donor-acceptor interactions similar to the bond between a metal and a neutral ligand. Based on this conception, ligand exchange reactions prototypical of transition metal complexes were realized at carbon atoms, enabling new synthetic strategies for the synthesis of reactive species and building blocks. In particular, the exchange of N2, CO, and phosphine ligands led to the development of a mild method for accessing new compounds and reagents with unusual properties, such as vinylidene ketenes or stable ketenyl anions, that open up a diverse but still poorly explored follow-up chemistry.

Dramatic Acceleration of the Hopf Cyclization on Gold(111): From Enediynes to Peri-Fused Diindenochrysene Graphene Nanoribbons

Hopf et al. reported the high-temperature 6π-electrocyclization of cis-hexa-1,3-diene-5-yne to benzene in 1969. Subsequent studies using this cyclization have been limited by its very high reaction barrier. Here, we show that the reaction barrier for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), is decreased by nearly half on a Au(111) surface. We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the Hopf cyclization of enediynes 1a,b on Au(111). Enediyne 1a undergoes two sequential, quantitative Hopf cyclizations, first to naphthalene derivative 2, and finally to chrysene 3. Density functional theory (DFT) calculations reveal that a gold atom from the Au(111) surface is involved in all steps of this reaction and that it is crucial to lowering the reaction barrier. Our findings have important implications for the synthesis of novel graphene nanoribbons. Ullmann-like coupling of enediyne 1b at 20 °C on Au(111), followed by a series of Hopf cyclizations and aromatization reactions at higher temperatures, produces nanoribbons 12 and 13. These results show for the first time that graphene nanoribbons can be synthesized on a Au(111) surface using the Hopf cyclization mechanism.

Carbones - A Classification on the Magnetic Criterion

Carbones (carbodiphosphoranes, bent allenes and chalcogen-stabilized carbones) bear the same resonance contributor X+ -C2- -Y+ (X+ , Y+ =PR3 + , CR2 + , SR2 + , SeR2 + , S+ R2 =NR) and exhibit unique bonding and donating properties at the central carbon atom. A classification is given on basis of both the geometry and the magnetic properties (13 C chemical shift of the central carbon atom and the spatial magnetic properties, through-space NMR shieldings (TS NMRSs), actually the anisotropy effect or the ring current effect of aromatic species). TS NMRS values have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The synergy of geometry (linear or bent, orthogonal or twisted structures) and NMR characteristics (extend of the high field shift of the central carbon atom, anisotropy effect of the allene-like C=C double bonds or the ball-like anisotropy effect of carbone-like central carbon atom) provides a comprehensive picture of the dominating resonance contributor.

Electron-deficient cyclopropenium cations as Lewis acids in FLP chemistry

Cyclopropenium cations incorporating electron deficient substituents are Lewis acidic despite the presence of π-electrons. The chloride and electron affinities are examined computationally and experimentally, respectively. These cations form classic Lewis acid-base adducts with PPh3, while sterically demanding phosphines yield frustrated Lewis pairs (FLPs) which participate in FLP additions. Depending on the basicity of the phosphine used, addition to alkynes or alkyne deprotonation is observed. In either case, new C-C bonds are formed, thus extending the utility of the concept of FLP chemistry to these delocalized π-cations.

σ versus π-radical: Tuning the electronic nature of neutral carbon (I) compounds with three non-bonding electrons

The bonding and reactivity of the hypo-coordinated compounds with one, two, and four non-bonding electrons namely, carbon-centered free radical, carbenes, and carbones were well earlier established. Here, we report stability, bonding and reactivity of compounds RCL, where R is one-electron donor group (R = CH₃ (a), CHO (b), and NO₂ (c)) and L is two-electron donor ligand (L = cAAC (1), CO (2), NHC (3) and PMe₃ (4)), having three non-bonding electrons. The ground states of molecules exist in a doublet with a lone pair of electrons and an unpaired electron at the central carbon atom (C1). The spin hops over from π- to σ-type orbitals is observed as the π-acceptor strength of the donor ligand increases. The replacement of the methyl group by CHO and NO₂ indicate that the cAAC and CHO substituted compounds gives a σ-radical except in compound 2c. These molecules show very high proton affinity and exothermic reaction energy for the hydrogen atom addition indicating dual reactivity namely, radical and lone pair reactivity.

An Isolable Bis(Germylene)-Stabilized Plumbylone

We report herein the synthesis of a stable plumbylone (3) by reduction of a bromodigermylplumbylene (2) with 2.2 equiv of potassium graphite (KC₈ ). The molecular structure of 3 was established by a single-crystal X-ray diffraction study and features a two-coordinated Pb center with an acute Ge-Pb-Ge bond angle. Computational studies showed that this complex (3) possesses a singlet electronic ground state with a Pb⁰ center. Its high thermal stability can be most likely ascribed to the delocalization of π electrons over the Ge-Pb-Ge moiety. A preliminary reactivity study demonstrates that complex 3 can deliver Pb⁰ atoms to an organic azide producing a tetrameric imido complex [(PbNDipp)₄ ] (Dipp=2,6-ⁱ Pr-C₆ H₃ , 4) and perform a metathesis reaction with GeCl₂ ⋅dioxane to produce a bis(germylene)-stabilized germylone (5), highlighting the synthetic utility of 3.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF6 ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC8 results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

A boron, nitrogen-containing heterocyclic carbene (BNC) as a redox active ligand: synthesis and characterization of a lithium BNC-aurate complex

Stabilization of low oxidation gold anions as aurate or auride by organic ligands has long been a synthetic challenge, owing to the proneness of low-valent gold centres to cluster. Despite being the most electronegative metal, isolable gold(I) aurate complexes have only been obtained from a few σ-withdrawing organo- and organo-main group ligands. Stabilization of highly-reduced gold complexes by π-modulating redox active ligands has only been achieved by cyclic (amino)(alkyl)carbene (CAAC), which is limited to 1e^--reduction to form neutral gold(0) complexes. This work reports a simple modular synthesis of a boron, nitrogen-containing heterocyclic carbene (^ClBNC) at a gold(I) center through metal-assisted coupling between azadiboriridine and isocyanides. The anionic electrophilic ^ClBNC ligand in the gold(I) complex [(^ClBNC)AuPMe₃] (3a and 3b) allows a 2e^--reduction to form the first η¹-carbene aurate complex [(BNC)AuPMe₃]Li(DME) (5a, DME = dimethoxyethane). Single crystal crystallographic analysis and computational studies of these complexes revealed a highly π-withdrawing character of the neutral 4π B,N-heterocyclic carbene (BNC) moiety and a 6π weakly aromatic character with π-donating properties to the gold(I) fragment in its reduced form, showcasing the first cyclic carbene ligand that allows electronic tunability between π-withdrawing (Fischer-type)- and π-donating (Schrock-type) properties.

A cyclic divalent N(I) species isoelectronic to carbodiphosphoranes

The formation of a rare type of diphosphazenium cation is described. Its synthesis features a unique oxidative dealkylation of an iminophosphorane-phosphole by a silver(I) salt. DFT study of this compound reveals the low valent character of the N(I) center.

Air-Stable Thermoluminescent Carbodicarbene-Borafluorenium Ions

Borenium ions, originally synthesized as fundamentally important laboratory curiosities, have attracted significant attention due to their applications in catalysis and frustrated Lewis pair chemistry. However, investigations of the materials properties of these types of compounds are exceptionally rare. Herein, we report the synthesis, molecular structures, and optical properties of a new class of air-stable borenium ions, stabilized by the strongly donating carbodicarbene (CDC) ligand (2, 3, 6). Notably, CDC-borafluorenium ions exhibit thermoluminescence in solution, a result of a twisted intramolecular charge transfer process. The temperature responsiveness, which is observable by the naked eye, is assessed over a 20 to -60 °C range. Significantly, compound 2 emits white light at lower temperatures. In the solid state, these borocations exhibit increased quantum yields due to aggregation-induced emission. CDC-borafluorenium ions with two different counteranions (Br^-, BPh₄^-) were investigated to evaluate the effect of anion size on the solution and solid-state optical properties. In addition, CDCs containing both symmetrical and unsymmetrical N-heterocycles (bis(1-isopropyl-3-methylbenzimidazol-2-ylidene)methane and bis(1,3-dimethyl-1,3-dihydro-2H-benzo[d]imidazol-2-ylidene)methane) were tested to understand the implications of free rotation about the CDC ligand carbon-carbon bonds. The experimental work is complemented by a comprehensive theoretical analysis of the excited-state dynamics.

Isolation of a Uranium(III)-Carbon Multiple Bond Complex

Low-valent uranium-element multiple bond complexes remain scarce, though there is burgeoning interest regarding to their bonding and reactivity. Herein, isolation of a uranium(III)-carbon double bond complex [(Cp*)2 U(CDP)](BPh4 ) (1) comprising a tridentate carbodiphosphorane (CDP) was reported for the first time. Oxidation of 1 afforded the corresponding U(IV) complex [(Cp*)2 U(CDP)](BPh4 )2 (2). The distance between U and C in 2 is 2.481 Å, indicating the existence of a typical U=C double bond, which is further confirmed by quantum chemical calculations. Bonding analysis suggested that the CDP also serves as both σ- and π-donor in complex 1, though a longer U-C bond (2.666(3) Å) is observed. It implies that 1 is the first isolable mononuclear uranium(III) carbene complex. Moreover, these results suggest that CDPs are promising ligands to establish other low-valent f-block metal-carbon multiple bond complexes.

Bis(silylene)-Stabilized Monovalent Nitrogen Complexes

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1, [1,2-(LSi)2 C2 B10 H10 ; L=PhC(Nt Bu)2 ], reaction with adamantyl azide (AdN3 ) affords the terminal N-μ2 -bridged zwitterionic carborane-1,2-bis(silylium) AdN3 adduct 2 with an open-cage dianionic nido-C2 B10 cluster core. Remarkably, upon one-electron reduction of 2 with C8 K and liberation of N2 and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized NI complex as an anion of 3 with the nido-C2 B10 cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) NI complex 4 with the closo-C2 B10 cluster core. Moreover, the corresponding neutral NI radical complex 5 results from single-electron transfer from 3 to 4.

Carbones and Carbon Atom as Ligands in Transition Metal Complexes

This review summarizes experimental and theoretical studies of transition metal complexes with two types of novel metal-carbon bonds. One type features complexes with carbones CL₂ as ligands, where the carbon(0) atom has two electron lone pairs which engage in double (σ and π) donation to the metal atom [M]⇇CL₂. The second part of this review reports complexes which have a neutral carbon atom C as ligand. Carbido complexes with naked carbon atoms may be considered as endpoint of the series [M]-CR₃ → [M]-CR₂ → [M]-CR → [M]-C. This review includes some work on uranium and cerium complexes, but it does not present a complete coverage of actinide and lanthanide complexes with carbone or carbide ligands.

Divalent NI Compounds: Identifying new Carbocyclic Carbenes to Design Nitreones using Quantum Chemical Methods

Nitreones are compounds with oxidation state 1 at the nitrogen, these compounds carry formal positive charge as well as two lone pairs of electrons at nitrogen center. These compounds are also known as divalent NI compounds and can be represented with the general formula L → N+ ← L, where L is an electron donating ligand. In the recent past, several divalent NI compounds have been reported with L = N-heterocyclic carbene (NHC), remote N-heterocyclic carbene (rNHC), carbocyclic carbene (CCC) and diaminocarbene. Recently, our group reported that a novel six-membered CCC (cyclohexa-2,5-diene-4-[diaminomethynyl]-1-ylidene) can stabilize N+ center in nitreones. As an independent carbene, this species is very unstable. In this work, modulation of this CCC using (a) annulation, (b) heterocyclic ring modification, (c) substitutions adjacent to the carbenic carbon, (d) exocyclic double bond insertion and (e) ring contraction, has been reported. These modulations and quantum chemical analyses helped in the identification of five new six-membered CCCs which carry improved donation and stability properties. Further, these CCCs were employed in the design of new divalent NI compounds (nitreones) which carry coordination bonds between ligands and N+ center. The molecular and electronic structure properties, and the donor→acceptor coordination interactions present in the resultant low oxidation state divalent NI compounds have been explored.

Bent Allenes or Di-1,3-betaines-An Answer Given on the Magnetic Criterion

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of bent allene 1, the corresponding C-extended 1,3-butadiene derivative 2, and a number of related compounds 3-20 have been calculated using the gauge-independent atomic orbital perturbation method, employing the nucleus-independent chemical shift concept and visualized as isochemical shielding surfaces of various sizes and directions. Prior to that, both structures and ¹³C chemical shifts were calculated and compared with available experimental bond lengths and δ(¹³C)/ppm values (also, as a quality criterion for the computed structures). Bond lengths, the δ(¹³C)/ppm, and the TSNMRS values are employed to qualify and quantify the electronic structure of the studied compounds in terms of dative or classical electron-sharing bonds.

Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Chromium and Cobalt Complexes

Carbodicarbenes (CDCs) possess two lone pairs of electrons on their central carbone C atom (C_carbone). Coordination to a transition metal via a σ bond leaves one pair of electrons with appropriate symmetry for π donation to the metal. However, the high energy of the latter also renders the CDC ligand potentially redox-active. Herein, we explore these alternatives in the redox series [Cr(L)₂]ⁿ⁺ and [Co(L)₂]ⁿ⁺ (n = 2-5), where L is a tridentate ligand comprised of a central CDC and two flanking pyridine donors. To this end, all members of both redox series were synthesized and their electronic structures were investigated by using a combination of ¹H NMR, Evans' NMR, IR, UV-vis, and EPR spectroscopies, SQUID magnetometry, X-ray crystallography, and density functional theory studies. Whereas [Co^II(L)₂]²⁺ is a straightforward low-spin (S = ¹/₂) cobalt(II) complex, the corresponding chromium complex was found to feature an electronic structure that is intermediate between the two limiting resonance forms [Cr^III(L^•-)(L)]²⁺ and [Cr^II(L)₂]²⁺. In the case of the tri-, tetra-, and pentacationic complexes, the qualitatively identical electronic structures [M^III(L)₂]³⁺, [M^III(L^•+)(L)]⁴⁺, and [M^III(L^•+)₂]⁵⁺ were observed for both metals. Thus, the metal ions retain a 3+ oxidation state throughout, and the higher redox states contain oxidized ligands. The majority of the unpaired spin on the cation radical ligands was calculated to be localized in π-symmetry orbitals on the coordinated C_carbone atoms. Analogous behavior was previously reported for the corresponding iron redox series and, as such, redox noninnocence in oxidized CDC and, more broadly, carbone complexes is likely widely accessible.

Cerium-carbon dative interactions supported by carbodiphosphorane

A set of complexes containing dative interactions between a rare-earth metal and carbon are reported. Complex 2, Br₃Ce(CDP)(THF), with a Ce←C bond was synthesized by the reaction of CeBr₃ with a carbon(0) ligand, carbodiphosphorane (CDP). More significantly, a trivalent cerium complex 3, [BrCe(CDP)₂](BPh₄)₂, with two σ dative interactions C→Ce←C was also isolated, which represents an unusual example of two dative interactions formed with the same atom in a molecule. Furthermore, π donation by the second lone-pair electrons of the CDP ligand is rather weak. Single-crystal X-ray diffraction shows that the Ce-C bond lengths in these complexes are comparable with those in cerium(iii)-carbene species. Density functional theory calculations support the dative interaction formation in these complexes and the strength of σ-donation in 3 is stronger than that in 2.

Design of non-ionic carbon superbases: second generation carbodiphosphoranes

A new generation of carbodiphosphoranes (CDPs), incorporating pyrrolidine, tetramethylguanidine, or tris(dimethylamino)phosphazene as substituents is introduced as the most powerful class of non-ionic carbon superbases on the basicity scale to date. The synthetic approach as well as NMR spectroscopic and structural characteristics in the free and protonated form are described. Investigation of basicity in solution and in the gas phase by experimental and theoretical means provides the to our knowledge first reported pK BH + values for CDPs in the literature and suggest them as upper tier superbases.

Gold(i)-catalyzed stereoselective cyclization of 1,3-enyne aldehydes by a 1,3-acyloxy migration/Nazarov cyclization/aldol addition cascade

Stereoselective synthesis of bicyclo[3.3.0]octenones from chiral 1,3-enyne aldehydes bearing propargylic acetates is described. The method is based on a Au(i)-catalyzed domino sequence with concomitant transfer of chirality involving 1,3-acyloxy migration followed by Nazarov cyclization and an unprecedented aldol addition. The method furnishes densely functionalized bicyclic structures in high yields, with up to 97% ee and good diastereoselectivity.

Donor→acceptor coordination interactions in 1,3-bis(NHC)triazenyl Cations: An electronic structure analysis

Donor→acceptor coordination interactions (L → N) between ligands and nitrogen center as in L → N^⊕  ← L were reported in the recent past. This article describes the possibility of L → N coordination interactions in triazenyl cation species L → N₃ ^⊕  ← L. A few 1,3-bis(NHC)triazenyl cation species were experimentally known, the electronic structure analysis reported in this work reveals the presence of L → N (donor→acceptor) interactions in these species. Molecular orbital analysis, NBO charge analysis, energy decomposition analysis, and so forth, confirm the possibility of L → N coordination bond character. Ten molecules with the general formula L → N₃ ^⊕  ← L have been designed carrying L → N₃ ^⊕  ← L interactions. © 2019 Wiley Periodicals, Inc.

1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate New Prospects of the Coordination Chemistry of Carbon

Reacting (NHC)(cyclopropenyl)gold(I) complexes with cationic gold complexes [(IPr)AuX] afforded extremely reactive allylium-1,1-diido-bridged digold intermediates. We prove the existence and constitution of this structure with FT-ICR-MS/MS, NMR, and UV-vis-NIR experiments and isolated the nucleophilic addition product [(Me)(Ph)(CCHC){Au(IPr)}₂(SOMe₂)]NTf₂ with DMSO. Our computational investigation unveiled that the bonding situation of this μ-allylium-1,1-diido digold domain was best described as a three-center-four-electron bond with a π-backbond. The valence orbitals showed extreme delocalization and strong π-interactions between the three centers Au¹-C¹-Au². The bridging carbon atom C¹ was best described as trigonal planar sp-hybridized carbon in this structure. Excitation succeeded in UV-vis-NIR measurements with energies as low as near-IR radiation.

Double dative bond between divalent carbon(0) and uranium

Dative bonds between p- and d-block atoms are common but species containing a double dative bond, which donate two-electron pairs to the same acceptor, are far less common. The synthesis of complexes between UCl₄ and carbodiphosphoranes (CDP), which formally possess double dative bonds Cl₄U⇇CDP, is reported in this paper. Single-crystal X-ray diffraction shows that the uranium-carbon distances are in the range of bond lengths for uranium-carbon double bonds. A bonding analysis suggests that the molecules are uranium-carbone complexes featuring divalent carbon(0) ligands rather than uranium-carbene species. The complexes represent rare examples with a double dative bond in f-block chemistry. Our study not only introduces the concept of double dative bonds between carbones and f-block elements but also opens an avenue for the construction of other complexes with double dative bonds, thus providing new opportunities for the applications of f-block compounds.

Observation of Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Iron Complexes

To probe the possibility that carbodicarbenes (CDCs) are redox active ligands, all four members of the redox series [Fe(1)₂ ]ⁿ⁺ (n=2-5) were synthesized, where 1 is a neutral tridentate CDC. Through a combination of spectroscopy and DFT calculations, the electronic structure of the pentacation is shown to be [Fe^III (1^.+ )₂ ]⁵⁺ (S= 1 / 2 ). That of [Fe(1)₂ ]⁴⁺ is more ambiguous, but it has significant contributions from the open-shell singlet [Fe^III (1)(1^.+ )]⁴⁺ (S=0). The observed spin states derive from antiferromagnetic coupling of their constituent low-spin iron(III) centres and cation radical ligands. This marks the first time redox activity has been observed for carbones and expands the diverse chemical behaviour known for these ligands.

Complexation and Release of N-Heterocyclic Carbene-Aminoborylene Ligands from Group VI and VIII Metals

The coordination chemistry and stability of aminoborylene ligands bearing different N-heterocyclic carbene (NHC) stabilizing groups has been investigated with Group VI and VIII metals. NHC-aminoborylene complexes have been accessed via reduction of NHC-dihaloaminoborane adducts with Na₂[M(CO) _x] species (M = Fe, Ru, Cr, W). Imidazol-2-ylidene-stabilized aminoborylene ligands were found to afford thermally robust metal-borylene complexes, which are inert to oxidation, hydrolysis, and insertion of unsaturated substrates. Such ligands have additionally been demonstrated to be significantly more electron releasing than NHCs and other carbon-based ligands by infrared spectroscopy, and can be regarded as unique examples of highly nucleophilic borylene ligands isolobal to classical NHCs. In contrast, cyclic alkylaminocarbene (CAAC)-bound dihaloaminoboranes were found to be reduced by one or two electrons upon reaction with Na₂[M(CO) _x] species to form either a stable borane-centered radical, or the free CAAC-aminoborylene complex, which further reacts to form a carbonyl-stabilized aminoborylene. Borylene-to-CO migration was also observed upon reaction of a ruthenium imidazol-2-ylidene aminoborylene complex with B(C₆F₅)₃, where the product borylene remains trapped by the Ru center.

NL2+ Systems as New-Generation Phase-Transfer Catalysts

Phase-transfer catalysts (PTCs), currently, are one of the most important tools of chemists for performing organic reactions. PTCs accelerate several types of reactions in biphasic systems, giving excellent yields of the desired product. Most of the PTCs belong to the general formula NR₄⁺X^-. In the recent past, several compounds possessing a novel scaffold with the general formula NL₂⁺X^- have been reported as PTCs. In the NL₂⁺ species, a nitrogen atom with a formal positive charge accepts electron density from electron-donating ligands. Electronic structure studies reported in the literature confirmed the possibility of L → N coordination (donor-acceptor) interactions in these species, and thus, this class of compounds are known as divalent N^I compounds. These species are reported to exhibit better catalytic potential in comparison to the traditional NR₄⁺ systems. Some of the NL₂⁺ systems are found to be useful in asymmetric phase-transfer catalysis. Thus, these systems offer extensive opportunities for exploring the catalytic properties and novel mechanistic aspects associated with their unique electronic structure. In this paper, the synthesis, electronic, and structural properties and the applications in catalysis of the NL₂⁺-based PTCs are reviewed with their bright future scope in catalytic organic chemistry.

Molecular Manipulations of a Utility Nitrogen-Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes

Expanding the scope and applications of anionic N-heterocyclic carbenes (NHCs), a novel series of magnesium NHC complexes is reported using a mixed sodium-magnesium approach. Sequential reactivity of classical imidazol- 2-ylidene carbene IPr with NaR and MgR₂ (R=CH₂ SiMe₃ ) affords [(THF)₃ Na(μ-IPr^- )MgR₂ (THF)] (2) [IPr^- =:C{[N(2,6-iPr₂ C₆ H₃ )]₂ CHC] containing an anionic NHC ligand, whereas surprisingly sodium magnesiate [NaMgR₃ ] fails to deprotonate IPr affording instead the redistribution coordination adduct [IPr₂ Na₂ MgR₄ ] (1). Compound 2 undergoes selective C2-methylation when treated with MeOTf furnishing novel abnormal NHC complex [{aIPr^Me MgR₂ }₂ ] (3). Dissolving 3 in THF led to the dissociation of this complex into MgR₂ and aIPr^Me with the latter isomerizing to the olefinic NHC IPr=CH₂ . The ability of 2 and 3 to transfer their anionic and abnormal NHC ligands, respectively to Au^I metal fragments has been investigated allowing the isolation and structural characterization of [RAu(μ-IPr^- )MgR(THF)₂ ] (4) and [aIPr^Me AuR] (5) respectively. In both cases transfer of an alkyl R group is observed. However while 3 can also transfer its abnormal NHC ligand to give 5, in 4 the anionic NHC still remains coordinated to Mg via its C4 position, whereas the {AuR} fragment occupies the C2 position previously filled by a donor-solvated {Na(THF)₃ }⁺ cation.

Can Remote N-Heterocyclic Carbenes Coordinate with Main Group Elements? Synthesis, Structure, and Quantum Chemical Analysis of N+ -Centered Complexes

Remote N-heterocyclic carbenes (rNHCs), such as N-methyl-4-pyridylidene, are known to form coordination complexes with TMs. Herein, it is established that rNHCs can also coordinate to the N⁺ centre. Synthesis of some novel divalent N^I complexes with the general formula (rNHC)→N⁺ ←(NHC) and (rNHC)→N⁺ ←(rNHC) was achieved, and X-ray diffraction studies supported the coordination bond character between the rNHCs and the N⁺ centre. Quantum chemical analysis established the presence of divalent N^I character at the central nitrogen in these systems.

Phosphine/Sulfoxide-Supported Carbon(0) Complex

A new carbon(0) complex 2 with two different L ligands, a phosphine and a sulfoxide, was synthesized and fully characterized. This new type of carbone exhibits excellent coordination ability, in contrast to the related phosphine/sulfide-supported carbon(0) complexes. Several organometallic complexes were isolated and, of special interest, the ν_av (CO) value of Rh^I -dicarbonyl complex indicates that 2 has a donor capability superior to classical NHCs.

Metalated Ylides: A New Class of Strong Donor Ligands with Unique Electronic Properties

The development and design of new ligand systems with special donor properties has been essential for crucial advances made in main-group-element and transition-metal chemistry over the years. This Forum Article focuses on metalated ylides as novel ligand systems. These anionic congeners of bisylides possess likewise two lone pairs of electrons at the central carbon atom and can thus function as X,L-type ligands with strong donor abilities. This article highlights recent efforts in the isolation and application of metalated ylides with a focus on work from this laboratory. We summarize structural and electronic properties and their use in organic synthesis as well as main-group-element and transition-metal chemistry.

Synthesis, Electronic Structure, and Reactivities of Two-Sulfur-Stabilized Carbones Exhibiting Four-Electron Donor Ability

Bis(sulfane)carbon(0) (BSC; Ph₂ S→C←SPh₂ (1)) is successfully synthesized by deprotonation of the corresponding protonated salt 1⋅HTfO. The diprotonated salt 1⋅(HTfO)₂ as the starting material can be also easily accessed by the deimination of iminosulfane(sulfane)carbon(0) (iSSC)⋅HBF₄ . Density functional theory calculations revealed the peculiar electronic structure of 1, which has two lone pairs of electrons at the central carbon atom. The largest proton affinities (PA(1): 297.5 kcal mol^-1 ; PA(2): 183.7 kcal mol^-1 ) and the highest energy levels of the HOMOs (HOMO: -4.89 eV; HOMO-1: -5.02 eV) for 1 among the two-sulfur-stabilized carbones clearly indicate the strong donor ability of carbon center stabilized by two S^II ligands. The donating ability of these lone pairs of electrons is demonstrated by the C-diaurated and C-proton-aurated complexes, which provide the first experimental evidence for two-sulfurstabilized carbones behaving as four-electron donors. Furthermore, the syntheses and application of Ag^I carbone complexes as carbone transfer agents are also reported.

Donor-Stabilized Silylene/Phosphine-Supported Carbon(0) Center with High Electron Density

An isolable donor-stabilized silavinylidene phosphorane was synthesized. This molecule, which can also be regarded as a new carbon(0) complex featuring a phosphine and a donor-stabilized silylene ligand, presents a central carbon atom with a remarkably high electron density (-1.82). Furthermore, the experimental electron-density study of this compound demonstrates the delocalization of the σ-lone pair at the central carbon atom toward the silicon center, a feature which is remarkably different from electronic situation of other bent-allene-type molecules. This result clearly demonstrates the powerful electron-donating ability of donor-stabilized silylene ligands, as well as their excellent electron-acceptor properties.