Selective aromatic C-F and C-H bond activation with silylenes of different coordinate silicon

Synthetic, structural and reactivity studies of a boryl-ethynyl Silylene

The salt metathesis of a boryl-ethynyl lithium salt {[(HCDipN)2]B-CC-Li} with a monochlorosilylene [LSi(:)Cl; L = PhC(NtBu)2] produced an isolable boryl-ethynyl silylene {1; [(HCDipN)2]B-CC-Si(L)}. The Si(II) center in 1 possesses a nonbonding lone pair and forms a covalent bond with the ethynyl group. The characterization of 1 was carried out by multinuclear NMR spectroscopy, single-crystal X-ray structure analysis and DFT calculations. Additionally, a reactivity study of 1 was conducted towards oxygen-containing and aryl C-F substrates.

Photoinduced Dual C-F Bond Activation of Hexafluorobenzene Mediated by Boron Atom

The reaction of laser-ablated boron atoms with hexafluorobenzene (C6 F6 ) was investigated in neon and argon matrices, and the products are identified by matrix isolation infrared spectroscopy and quantum-chemical calculations. The reaction is triggered by a boron atom insertion into one C-F bond of hexafluorobenzene on annealing, forming a fluoropentafluorophenyl boryl radical (A). UV-Vis light irradiation of fluoropentafluorophenyl boryl radical causes generation of a 2-difluoroboryl-tetrafluorophenyl radical (B) via a second C-F bond activation. A perfluoroborepinyl radical (C) is also observed upon deposition and under UV-Vis light irradiation. This finding reveals the new example of a dual C-F bond activation of hexafluorobenzene mediated by a nonmetal and provides a possible route for synthesis of new perfluorinated organo-boron compounds.

Dearomatization of C6 Aromatic Hydrocarbons by Main Group Complexes

The dearomatization reaction is a powerful method for transformation of simple aromatic compounds to unique chemical architectures rapidly in synthetic chemistry. Over the past decades, the chemistry in this field has evolved significantly and various important organic compounds such as crucial bioactive molecules have been synthesized through dearomatization. In general, photochemical conditions or assistance by transition metals are required for dearomatization of rigid arenes. Recently, main-group elements, especially naturally abundant elements in the Earth's crust, have attracted attention as they have low toxicity and are cost-effective compared to the late transition metals. In recent decades, a variety of low-valent main-group molecules, which enable the activation of stable aromatic compounds under mild conditions, have been developed. This minireview highlights the developments in the chemistry of dearomatization of C6 aromatic hydrocarbons by main-group compounds leading to the formation of seven-membered EC6 (E=main-group elements) ring or cycloaddition products.

Oxidative Additions of C-F Bonds to the Silanide Anion [Si(C2 F5 )3 ]. Angew Chem Int Ed Engl 2022;61:e202116468. [PMID: 35107847 PMCID: PMC9310575 DOI: 10.1002/anie.202116468]

Compounds exhibiting main group elements in low oxidation states were found to mimic the reactivity of transition metal complexes. Like the latter, such main group species show a proclivity of changing their oxidation state as well as their coordination number by +2, therefore fulfilling the requirements for oxidative additions. Prominent examples of such main group compounds that undergo oxidative additions with organohalides R-X (R=alkyl, aryl, X=F, Cl, Br, I) are carbenes and their higher congeners. Aluminyl anions, which like carbenes and silylenes oxidatively add to strong σ-bonds in R-X species, have been recently discovered. We present the first anion based upon a Group 14 element, namely the tris(pentafluoroethyl)silanide anion, [Si(C2 F5 )3 ]- , which is capable of oxidative additions towards C-F bonds. This enables the isolation of non-chelated tetraorganofluorosilicate salts, which to the best of our knowledge had only been observed as reactive intermediates before.

Bismuth Redox Catalysis: An Emerging Main-Group Platform for Organic Synthesis

Bismuth has recently been shown to be able to maneuver between different oxidation states, enabling access to unique redox cycles that can be harnessed in the context of organic synthesis. Indeed, various catalytic Bi redox platforms have been discovered and revealed emerging opportunities in the field of main group redox catalysis. The goal of this perspective is to provide an overview of the synthetic methodologies that have been developed to date, which capitalize on the Bi redox cycling. Recent catalytic methods via low-valent Bi(II)/Bi(III), Bi(I)/Bi(III), and high-valent Bi(III)/Bi(V) redox couples are covered as well as their underlying mechanisms and key intermediates. In addition, we illustrate different design strategies stabilizing low-valent and high-valent bismuth species, and highlight the characteristic reactivity of bismuth complexes, compared to the lighter p-block and d-block elements. Although it is not redox catalysis in nature, we also discuss a recent example of non-Lewis acid, redox-neutral Bi(III) catalysis proceeding through catalytic organometallic steps. We close by discussing opportunities and future directions in this emerging field of catalysis. We hope that this Perspective will provide synthetic chemists with guiding principles for the future development of catalytic transformations employing bismuth.

Advances in silylation and borylation of fluoroarenes and gem-difluoroalkenes via C-F bond cleavage

Organoboron and organosilane compounds are widely used in organic synthesis and pharmaceuticals. In addition, the C-F bond functionalization is a useful tool for the construction of carbon-carbon and carbon-heteroatom bonds. In particular, the late-stage functionalization of bioactive molecules through defluoroborylation and defluorosilylation reactions will provide good opportunities for the development and diversification of new medicinal compounds. Thus, this feature article summarized the methods for the defluorosilylation and defluoroborylation of unreactive monofluoroarenes and gem-difluoroalkenes from 2000 to 2021, which might create some new ideas and will be helpful for further research in this field. These defluoroborylation and defluorosilylation strategies can be applied to synthesize silylated arenes, borylated arenes, silylated fluoroalkenes, and borylated fluoroalkenes, thus providing impressive advantages over traditional methods for the synthesis of organoboron and organosilane compounds in terms of divergent structures.

C-F bond activation under transition-metal-free conditions

The unique properties of fluorine-containing organic compounds make fluorine substitution attractive for the development of pharmaceuticals and various specialty materials, which have inspired the evolution of diverse C-F bond activation techniques. Although many advances have been made in functionalizations of activated C-F bonds utilizing transition metal complexes, there are fewer approaches available for nonactivated C-F bonds due to the difficulty in oxidative addition of transition metals to the inert C-F bonds. In this regard, using Lewis acid to abstract the fluoride and light/radical initiator to generate the radical intermediate have emerged as powerful tools for activating those inert C-F bonds. Meanwhile, these transition-metal-free processes are greener, economical, and for the pharmaceutical industry, without heavy metal residues. This review provides an overview of recent C-F bond activations and functionalizations under transition-metal-free conditions. The key mechanisms involved are demonstrated and discussed in detail. Finally, a brief discussion on the existing limitations of this field and our perspective are presented.

Catalytic Hydrodefluorination via Oxidative Addition, Ligand Metathesis, and Reductive Elimination at Bi(I)/Bi(III) Centers

Herein, we report a hydrodefluorination reaction of polyfluoroarenes catalyzed by bismuthinidenes, Phebox-Bi(I) and OMe-Phebox-Bi(I). Mechanistic studies on the elementary steps support a Bi(I)/Bi(III) redox cycle that comprises C(sp²)-F oxidative addition, F/H ligand metathesis, and C(sp²)-H reductive elimination. Isolation and characterization of a cationic Phebox-Bi(III)(4-tetrafluoropyridyl) triflate manifests the feasible oxidative addition of Phebox-Bi(I) into the C(sp²)-F bond. Spectroscopic evidence was provided for the formation of a transient Phebox-Bi(III)(4-tetrafluoropyridyl) hydride during catalysis, which decomposes at low temperature to afford the corresponding C(sp²)-H bond while regenerating the propagating Phebox-Bi(I). This protocol represents a distinct catalytic example where a main-group center performs three elementary organometallic steps in a low-valent redox manifold.

The diverse reactivity of NOBF4 towards silylene, disilene, germylene and stannylene

The reactivity of NOBF₄ towards silylene, disilene, germylene, stannylenes has been described. Smooth syntheses of compounds of composition [PhC(NtBu)₂E(= O → BF₃)N(SiMe₃)₂, E = Si (3) and Ge (4)] were accomplished from the corresponding tetrylenes. An unusual heterocycle (10) featuring B, Sn, N, P, and O atoms was obtained from the reaction with a stannylene, while a 1,2-vicinal anti addition of fluoride was observed with a disilene (12).

Diverse reactivity of carbenes and silylenes towards fluoropyridines

The reaction of IDipp with C5F5N led to functionalization of all three carbon atoms of the imidazole ring with HF2- as the counter-anion (1). Reactivity with 2,3,5,6-tetrafluoropyridine gives only C-F bond activation leaving C-H bonds intact (5b). The reaction of SIDipp with C5F5N in the presence of BF3 afforded the ring cleavage product (3). Analogous reactions with silylene led to oxidative addition at the Si(ii) center.

Cationic Phosphorus Compounds Based on a Bis(1-piperidinyl)-Substituted Carbodiphosphorane: Syntheses, Structures, and Csp3–H Activation

The use of the bis(1-piperidinyl)-substituted carbodiphosphorane (Ph₂(Pip)P)₂C (1) as an NCN ligand for the stabilization of phosphorus cations was studied. A simple ligand for halide exchange allowed the synthesis and isolation of a series of phosphorus monocations of the type [1-PR₂]⁺ (with R = Cl, Br, I, CyCl, Ph). These cations exhibit characteristic NMR and structural properties which nicely correlate with the charge at the central phosphorus atom and the interaction between the ligand and the PR₂ moiety. Halide abstraction from the monocations does not result in isolable dicationic compounds but in an unexpected intramolecular C_sp³–H activation in the piperidinyl group. DFT studies show that the selective activation of the CH₂ group next to the nitrogen atom instead of a CH group at the phenyl substituents proceeds via an iminium intermediate formed by hydride transfer from the carbon atom to the cationic phosphorus center. This observation clearly demonstrates the pronounced π acidity of the dicationic phosphorus species in comparison to compounds with a further π-donor substituent.

Activation of small molecules by cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes): a theoretical study

Quantum chemical calculations have been carried out on a series of skeletally modified cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes) to understand their ligand properties and reactivity towards the activation of a variety of small molecules. The installation of boron or silicon atoms into the ring framework of these silylenes/germylenes led to a dramatic increase in their σ-basicity while the incorporation of ylidic moieties resulted in a sharp reduction of their π-acidity although it did help in increasing the electron donation ability. The calculated values of energy barriers for the activation of H-H, N-H, C-H and Si-H bonds by many of the cyclic silylenes considered here are found to be comparable to those for experimentally evaluated systems, indicating the potential of these computationally designed molecules in small molecule activation and calling for synthetic efforts towards their isolation. Furthermore, activations employing CAAGes are found to be more demanding than those with CAASis which may be attributed to the significantly lower Lewis basicity of the former than the latter.

A Mono-Substituted Silicon(II) Cation: A Crystalline "Supersilylene"

Mono-coordinated silicon(II) cations are predicted to be reactive ambiphiles, combining the typically high Lewis acidity of silicon cations with nucleophilicity due to the presence of an electron pair at the same atomic centre. Here, a carbazole-derived scaffold was used to isolate salts with a mono-coordinated silicon(II) cation, [RSi]+ (R=bulky carbazolyl substituent), by halide abstraction from a base-free halosilylene, RSiI, with Ag[Al(Ot BuF )4 ]. Despite the bulk of the carbazolyl moiety, the silylenylium cation [RSi]+ retains high reactivity. It was shown to react with an amine to form three bonds at the silicon atom in one reaction which conforms with the notion of a "supersilylene". The resulting silylium cation [RSi(H)NR'2 ]+ (in the formal oxidation state SiIV ) obtained by oxidative addition of an NH bond at [RSi]+ is even more acidic than the silylenylium cation (SiII ) due to the absence of a lone pair of electrons the silicon atom.

Palladium-catalysed C-F alumination of fluorobenzenes: mechanistic diversity and origin of selectivity

A palladium pre-catalyst, [Pd(PCy₃)₂] is reported for the efficient and selective C–F alumination of fluorobenzenes with the aluminium(i) reagent [{(ArNCMe)₂CH}Al] (1, Ar = 2,6-di-iso-propylphenyl). The catalytic protocol results in the transformation of sp² C–F bonds to sp² C–Al bonds and provides a route to reactive organoaluminium complexes (2a–h) from fluorocarbons. The catalyst is highly active. Reactions proceed within 5 minutes at 25 °C (and at appreciable rates at even −50 °C) and the scope includes low-fluorine-content substrates such as fluorobenzene, difluorobenzenes and trifluorobenzenes. The reaction proceeds with complete chemoselectivity (C–F vs. C–H) and high regioselectivities (>90% for C–F bonds adjacent to the most acidic C–H sites). The heterometallic complex [Pd(PCy₃)(1)₂] was shown to be catalytically competent. Catalytic C–F alumination proceeds with a KIE of 1.1–1.3. DFT calculations have been used to model potential mechanisms for C–F bond activation. These calculations suggest that two competing mechanisms may be in operation. Pathway 1 involves a ligand-assisted oxidative addition to [Pd(1)₂] and leads directly to the product. Pathway 2 involves a stepwise C–H → C–F functionalisation mechanism in which the C–H bond is broken and reformed along the reaction coordinate, guiding the catalyst to an adjacent C–F site. This second mechanism explains the experimentally observed regioselectivity. Experimental support for this C–H activation playing a key role in C–F alumination was obtained by employing [{(MesNCMe)₂CH}AlH₂] (3, Mes = 2,4,6-tri-methylphenyl) as a reagent in place of 1. In this instance, the kinetic C–H alumination intermediate could be isolated. Under catalytic conditions this intermediate converts to the thermodynamic C–F alumination product.

A palladium pre-catalyst, [Pd(PCy₃)₂] is reported for the efficient and selective C–F alumination of fluorobenzenes with the aluminium(i) reagent [{(ArNCMe)₂CH}Al] (Ar = 2,6-di-iso-propylphenyl).

Carbon-fluorine bond cleavage mediated by metalloenzymes

Fluorochemicals are a widely distributed class of compounds and have been utilized across a wide range of industries for decades. Given the environmental toxicity and adverse health threats of some fluorochemicals, the development of new methods for their decomposition is significant to public health. However, the carbon-fluorine (C-F) bond is among the most chemically robust bonds; consequently, the degradation of fluorinated hydrocarbons is exceptionally difficult. Here, metalloenzymes that catalyze the cleavage of this chemically challenging bond are reviewed. These enzymes include histidine-ligated heme-dependent dehaloperoxidase and tyrosine hydroxylase, thiolate-ligated heme-dependent cytochrome P450, and four nonheme oxygenases, namely, tetrahydrobiopterin-dependent aromatic amino acid hydroxylase, 2-oxoglutarate-dependent hydroxylase, Rieske dioxygenase, and thiol dioxygenase. While much of the literature regarding the aforementioned enzymes highlights their ability to catalyze C-H bond activation and functionalization, in many cases, the C-F bond cleavage has been shown to occur on fluorinated substrates. A copper-dependent laccase-mediated system representing an unnatural radical defluorination approach is also described. Detailed discussions on the structure-function relationships and catalytic mechanisms provide insights into biocatalytic defluorination, which may inspire drug design considerations and environmental remediation of halogenated contaminants.

Activation of Aromatic C-F Bonds by a N-Heterocyclic Olefin (NHO)

A N-heterocyclic olefin (NHO), a terminal alkene selectively activates aromatic C-F bonds without the need of any additional catalyst. As a result, a straightforward methodology was developed for the formation of different fluoroaryl-substituted alkenes in which the central carbon-carbon double bond is in a twisted geometry.

Activation of C–F, Si–F, and S–F Bonds by N-Heterocyclic Carbenes and Their Isoelectronic Analogues

Reactions involving C–F, Si–F, and S–F bond cleavage with N-heterocyclic carbenes and isoelectronic species are reviewed. Most examples involve activation of aromatic C–F bond via an SNAr pathway and nucleophilic substitution of fluorine in electron-deficient olefins. The mechanism of the C–F bond activation depends on the reaction partners and the reaction can proceed via addition–elimination, oxidative addition (concerted or stepwise) or metathesis. The adducts formed upon substitution find applications in organic synthesis, as ligands and as stable radical precursors, but in most cases, their full potential remains unexplored.

1 Introduction

1.1 The C–F Bond

1.2 C–F Bond Activation: A Short Summary

1.3 C–F Bond Activation: A Special Case of SNAr

1.4 N-Heterocyclic Carbenes (NHCs)

1.5 The Purpose of this Article

2 C–F bond Activation in Acyl Fluorides

3 Activation of Vinylic C–F Bonds

4 Activation of Aromatic C–F Bonds

5 X–F Bond Activation (X = S or Si)

6 C–F Bond Activation by Main Group Compounds Isoelectronic with NHCs

7 Conclusions and Outlook

Reactivity of boraguanidinato germylenes toward carbonyl compounds and isocyanides: C-O, C-F and C-N bond activation

The reactions of two equivalents of germylene [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge (1) with carbonyl compounds RC(O)R' resulted in carbonyl functionality activation and the formation of 4-(R,R')-1,2-digerma-3-oxa-cyclobutanes (R/R' = Ph/CF₃ (2) or C₆F₅/H (3)). Surprisingly, the analogous reaction of 1 with C₆F₅C(O)Me led to the insertion of the germanium atom into the C-F bond of the perfluorophenyl group, thus producing a spiro compound (4) with a germanium atom sharing 1,2-digerma-3,5-diaza-4-bora-cyclopentane and 1-germa-2,4-diaza-3-boracyclobutane rings. Furthermore, the reaction of 1 with 2e^- donors was investigated. In the case of 4-dimethylaminopyridine (DMAP), an expected complex [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge(DMAP) (5) was isolated, but using t-BuNC resulted in the formation of germanium(iv) cyanide [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge(CN)(t-Bu) (6) as a result of C-N bond activation in the starting isocyanide. In contrast, mixing other isocyanides RNC (R = Cy or Ad) with 1 in solution led only to an equilibrium between the starting compounds and most probably the corresponding complexes [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge(CNR) (R = Cy (7a) or Ad (8a)) based on NMR studies. From these equilibrium mixtures, fortuitously, single crystals of digerma-spiro-complexes (7 and 8) containing two germanium atoms (one of them coordinated to a particular isocyanide) were obtained and structurally authenticated by the X-ray diffraction technique.

A 1,5-Oligosilanylene Dianion as Building Block for Oligosiloxane Containing Cages, Ferrocenophanes, and Cyclic Germylenes and Stannylenes

Starting out from dipotassium 1,5-oligosiloxanylene diide 2, a 3,7,10-trioxa-octasilabicyclo[3.3.3]undecane was prepared, which represents the third known example of this cage structure type. Reaction of 1,3-dichlorotetramethyldisiloxane with 1,1'-bis[bis(trimethylsilyl)potassiosilyl]ferrocene gave a ferrocenophane with a disiloxane containg bridge. The compound can be further derivatized by conversion into a 1,5-oligosilanyl diide. Reacting 1,5-oligosiloxanylene diide 2 with SnCl2 or GeCl2·dioxane in the presence of PMe3 gave cyclic disilylated tetrylene PMe3 adducts. Release of the base-free stannylene led to a dimerization process which gave a bicyclic distannene as the final product. Abstraction of the PMe3 from the cyclic disilylated germylene PMe3 adduct with B(C6F5)3 caused oxidative addition of the germylene into a para-C-F bond of Me3P·B(C6F5)3.

Where silylene–silicon centres matter in the activation of small molecules

Small molecules such as H₂, N₂, CO, NH₃, O₂ are ubiquitous stable species and their activation and role in the formation of value-added products are of fundamental importance in nature and industry.

Saturated N-Heterocyclic Carbene Based Thiele's Hydrocarbon with a Tetrafluorophenylene Linker

The synthesis of a SIPr [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] derived Kekulé diradicaloid with a tetrafluorophenylene spacer (3) has been described. Two synthetic routes have been reported to access 3. The cleavage of C-F bond of C₆ F₆ by SIPr in the presence of BF₃ led to double C-F activated compound with two tetrafluoro borate counter anions (2), which upon reduction by lithium metal afforded 3. Alternatively, 3 can be directly accessed in one step by reacting SIPr with C₆ F₆ in presence of Mg metal. Compounds 2 and 3 were well characterized spectroscopically and by single-crystal X-ray diffraction studies. Experimental and computational studies support the cumulenic closed-shell singlet state of 3 with a singlet-triplet energy gap (ΔE_S-T ) of 23.7 kcal mol^-1 .

Mechanistic Features in Al(I)-Mediated Oxidative Addition of Aryl C-F Bonds: Insights From Density Functional Theory Calculations

The oxidative addition of a range of robust aryl C–F bonds to a single Al(I) center supported by a (NacNac)⁻ bidentate ligand ((NacNac)⁻ = [ArNC(Me)CHC(Me)NAr]⁻ and Ar = 2,6–Pr2iC₆H₃) have been explored by density functional theory calculations. Our calculations demonstrate that the Al(I) center-mediated C–F insertion generally proceeds via the concerted mechanism that involve both the donation (nAl→σC-F*) and back-donation (σF(p)→πAl(p)*) interactions. In addition, the predicted free energy barriers for the C–F bond activation show good agreement with the experimental information available. Finally, the comparative studies show that B(I) is the most active among group III metals (B, Al, Ga), thus supplying a testable prediction for experiments.

Silylene induced cooperative B–H bond activation and unprecedented aldehyde C–H bond splitting with amidinate ring expansion

Silylene mediated B–H and aldehyde C–H bond splitting were realized under ambient conditions.

Magnesium-mediated arylation of amines via C–F bond activation of fluoroarenes

Exploiting the high nucleophilic power of structurally defined β-diketiminate stabilised magnesium amides, a new method for amine arylation via rapid C–F bond activation of fluoroarenes is introduced.

Cyclometallation of a germylene ligand by concerted metalation-deprotonation of a methyl group

The reaction of [CH{(CMe)(2,6-iPr2C6H3N)}2]GeCl with LiN(SiMe3)2 was previously reported, which led to the formation of a hetero-fulvene type germylene, [CH{(CMe)(C[double bond, length as m-dash]CH2)(2,6-iPr2C6H3N)}2]Ge through the deprotonation of the C-H bond from the methyl substituents. In this paper, we attempted the analogous reaction with (Dipp)NCMeCHCOMeGeCl using LiN(SiMe3)2 which gave rise to a metathesis product, (Dipp)NCMeCHCOMeGeN(SiMe3)2 (2). However, the reactions of 2 with [M2Cl2(μ-Cl)2(η5-Cp*)2] (M = Rh and Ir) resulted in cyclometallated Rh and Ir complexes through the activation of the C-H bond from the germylene ligand. The complexes were characterized by single crystal X-ray analysis, which authenticated the presence of Ge-Rh and Ge-Ir bonds. DFT studies have been performed to understand the mechanism.

Cycloaddition versus Cleavage of the C=S Bond of Isothiocyanates Promoted by Digallane Compounds with Noninnocent α-Diimine Ligands

Whereas the chemistry of single-bond activation by compounds of the main group elements has undergone some development in recent years, the cleavage of multiple bonds remains underexplored. Herein, the reactions of two digallanes bearing α-diimine ligands, namely, [L¹ Ga-GaL¹ ] (1, L¹ =dpp-dad=[(2,6-iPr₂ C₆ H₃ )NC(CH₃ )]₂ ) and [L² Ga-GaL² ] (2, L² =dpp-bian=1,2-[(2,6-iPr₂ C₆ H₃ )NC]₂ C₁₀ H₆ ), with isothiocyanates are reported. Reactions of 1 or 2 with isothiocyanates in 1:2 molar ratio proceeded with [2+4] cycloaddition of the C=S bond across the C₂ N₂ Ga metallacycle with formation of C-C and S-Ga single bonds to afford [L¹ (RN=C-S)Ga-Ga(S-C=NR)L¹ ] (3, R=Me; 4, R=Ph) and [L² (RN=C-S)Ga-Ga(S-C=NR)L² ] (8, R=allyl; 9, R=Ph). In the cases of 8 and 9, this cycloaddition is reversible. The digallanes reacted with 2 equiv of PhNCS in the presence of Na metal or at high temperatures through a unique reductive cleavage of the C=S bond to yield the disulfide-bridged digallium species [Na(THF)₃ ]₂ [L¹ Ga(μ-S)₂ GaL¹ ] (5), [L² Ga(μ-S)₂ GaL² ] (10), and [Na(DME)₃ ][L² Ga(μ-S)₂ GaL² ] (11). Moreover, products 4 and 5 can further react with an excess of isothiocyanate, through cleavage of the C=S bond or cycloaddition, to give the bis- or mono-S-bridged complexes [Na(THF)₂ ]₂ [L¹ (PhN=C-S)Ga(μ-S)₂ Ga(S-C=NPh)L¹ ] (6) and [L¹ (PhN=C-S)Ga(μ-S)Ga(S-C=NPh)L¹ ] (7). All the newly prepared compounds were characterized by elemental analysis, single-crystal X-ray diffraction, IR spectroscopy, NMR (3-9) or ESR spectroscopy (11), and DFT calculations.

An isolable β-diketiminato chlorosilylene

The first β-diketiminate ligated chlorosilylene has been synthesised and isolated from the corresponding β-diketiminato dichlorohydrosilane through dehydrochlorination with an N-heterocyclic carbene.

Silicon-fluorine chemistry: from the preparation of SiF2to C–F bond activation using silylenes and its heavier congeners

The feisty nature of silicon(ii) fluorides has been harnessed by two cyclic alkyl amino carbene (cAAC) ligands and (cAAC)₂SiF₂has been isolated at room temperature and structurally characterized.

Evidence for single metal two electron oxidative addition and reductive elimination at uranium

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.

The reactivity of f-block complexes is primarily defined by single-electron oxidations and σ-bond metathesis. Here, Liddle and co-workers provide evidence that a uranium complex can undergo reversible oxidative addition and reductive elimination, demonstrating transition metal-like reactivity within f-block chemistry.

Facile Conversion of Bis-Silylene to Cyclic Silylene Isomers: Unexpected C-N and C-H Bond Cleavage

Reaction of thiolate 1 with carbene-stabilized diiodo-bis-silylene (2) (in a 2:1 ratio) in THF unexpectedly gives both the first five-membered, sulfur-containing, zwitterionic silylene ring (3) via insertion of the "Si^I₂" unit of 2 into the olefinic C-H bond of the imidazole ring of 1 and four-membered cyclic silylene (4) via insertion of a silicon(I) atom of 2 into the C_phenyl-N bond of the carbene ligand. The parallel reaction in toluene only gives 3 as the major product. The nature of the bonding in isomeric 3 and 4 was probed by experimental and theoretical methods.

DFT Study on C-F Bond Activation by Group 14 Dialkylamino Metalylenes: A Competition between Oxidative Additions versus Substitution Reactions

The C-F bond activation of pentafluoropyridine (PFP) by group 14 dialkylamino metalylenes has been studied employing DFT calculations. Emphasis is placed on the group 14 central atom (M = Si^II, Ge^II, and Sn^II) and substituents (-NMe₂, -NⁱPr₂, -Cl, -NH₂, and -PH₂) dependent switching of oxidative addition to the metathesis/substitution reaction route, using state-of-the-art theoretical methods (M062X/def2-QZVP(SMD)//M062X/def2-TZVP) to provide a systematic classification of the individual mode of reactions. Moreover, an energy decomposition analysis (EDA) is implemented to get a brief insight into the physical factors that control the activation barriers originating via the different mode of reactions, viz., oxidative addition and metathesis routes. The key finding is that the distortion of PFP is the principal guiding factor in the oxidative addition reaction, while distortions imposed on both the PFP and metalylenes are inevitable toward the origin of the metathesis reaction barrier. The preferable oxidative addition reaction over metathesis of substituted silylenes can be explained on the basis of electron concentration and the HOMO-LUMO gap between the reacting substrates. However, the dramatic switch between oxidative addition and metathesis reaction in substituted germylenes depends on both the electronic and steric nature of the substituents. Similar observations are also noted for the reactivity of substituted stannylenes.

Unusual Reactions of NacNacAl with Urea and Phosphine Oxides

The reaction of cyclic urea 1,3-dimethyl-2-imidazolidinone with the aluminum(I) compound NacNacAl (1) gives an unexpected adduct of urea with the isomerized aluminum(III) hydride NacNac'AlH(O═SIMe) (3). A related reaction of 1 with phosphine oxides results in cleavage of the P═O bond and formation of hydroxyl derivatives NacNac'Al(OH)(O═PR₃) [R = Ph (5) and Et (6)]. Density functional theory calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic aluminum(I) center or the transient Al═O species deprotonate the methyl group of the NacNac ligand.