Transition Metal-like Oxidative Addition of C–F and C–O Bonds to an Aluminum(I) Center

A Stable Calcium Alumanyl

A seven‐membered N,N′‐heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg−Al and Ca−Al bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

The unique β-diketiminate ligand in aluminum(i) and gallium(i) chemistry

Herein we present an overview of the last 10 years for aluminum(i) and gallium(i) stabilized by β-diketiminate ligands that undergo a series of oxidative addition reactions with molecules containing single and multiple bonds.

Salt metathesis as an alternative approach to access aluminium(i) and gallium(i) β-diketiminates

Aluminium(i) and gallium(i) β-diketiminates are accessed by a new route that provides better overall yields. In the case of aluminium it is also much faster, but some molecules turn into a dead end and merge into a dinuclear aluminium(iii) hydride.

C–F bond activation by pentamethylcyclopentadienyl-aluminium(i): a combined experimental/computational exercise

Although (AlCp*)₄ is one of the longest known stable aluminium(i) compounds, it still has surprising reactivities in store as illustrated by the ability to break strong C–F bonds.

Facile oxidative addition of O2 and S8 by an indium bis(carbene) analogue

The oxidative addition of oxygen towards an indium bis(carbenoid) yields an almost planar In₄O₂ ring, an isovalence-electronic analogue of dioxane, in which the indium is in the formal oxidation state +ii. The weaker oxidant sulfur, however, yields an eight-membered ring in a deck-chair conformation containing four indium(iii) centres and four sulfides in an alternating fashion. According to DFT calculations, this reactivity difference originates from kinetic rather than thermodynamic reasons. When the reaction with S₈ is performed with the in situ generated bis(carbenoid), a β-diketimine (NacNac) incorporating an S₄-chain bridging the two γ-carbon atoms of each NacNac unit is derived.

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 .

Dinuclear Aluminum Halide Complexes Based on Bis(β-diketiminate) Ligands: Synthesis, Structures, and Electrochemical Characterization

Twenty-three dinuclear aluminum halide complexes based on bis(β-diketiminate) ligands with various flexible and rigid bridging groups have been synthesized and fully characterized by NMR and IR spectroscopy, cyclic voltammetry, and elemental analysis. In addition, nine complexes were structurally characterized by means of single-crystal X-ray diffraction. Attempts to reduce the dinuclear species using C₈K remained unsuccessful, but the reduction potentials investigated by cyclic voltammetry indicate the principle feasibility.

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.

Can the solvent enhance the rate of chemical reactions through C-H/π interactions? insights from theory

The current computational study with density functional theory (DFT) shows that the rate of chemical reactions can be influenced through non-covalent C-H/π interactions between substrates and the solvent. It is shown that intramolecular carbon-carbon interaction and CO2 activation by a low valent silicon complex are both favourably affected by the explicit presence of the solvent toluene, due to C-H/π interactions between toluene and the silicon complex. Furthermore, ab initio molecular dynamics (AIMD) simulations demonstrate that even if the C-H/π interacting solvent molecule is displaced from the complex, another would quickly take its place, thus maintaining the interaction. Hence, the current work shows how non-covalent interactions between solvent and substrate can enhance the rate of the reaction and expands our understanding of the role and influence of the solvent in effecting important chemical transformations.

Reversible alkene binding and allylic C-H activation with an aluminium(i) complex

The monomeric molecular aluminium(i) complex 1 [{(ArNCMe)₂CH}Al] (Ar = 2,6-di-iso-propylphenyl) reacts with a series of terminal and strained alkenes including ethylene, propylene, allylbenzene and norbornene to form alkene bound products.

The monomeric molecular aluminium(i) complex 1 [{(ArNCMe)₂CH}Al] (Ar = 2,6-di-iso-propylphenyl) reacts with a series of terminal and strained alkenes including ethylene, propylene, allylbenzene and norbornene to form alkene bound products. Remarkably all these reactions are reversible under mild conditions (298–353 K) with alkene binding being disfavoured at higher temperatures due to the positive reaction entropy. Van't Hoff analyses have allowed quantification of the binding events with . Calculations and single crystal X-ray diffraction studies are consistent with the alkene bound species being metallocyclopropane complexes. Alkene binding involves a reversible redox process with changes from the +1 to +3 aluminium oxidation state. Under more forcing conditions the metallocyclopropane complexes undergo non-reversible allylic C–H bond activation to generate aluminium(iii) allyl hydride complexes. This represents a rare example of redox-based main group reactivity in which reversible substrate binding is followed by a further productive bond breaking event. Analysis of the mechanism reveals a reaction network in which alkene dissociation and reformation of 1 is required for allylic C–H activation, a realisation that has important implications for the long-term goal of developing redox-based catalytic cycles with main group compounds.

7-Metalla-1,4-diphosphanorbornadienes: cycloaddition of monovalent group 13 NacNac complexes to a stable 1,4-diphosphinine

Monovalent group 13 NacNac complexes reacted as dienophiles with a tricyclic 1,4-diphosphinine to furnish first examples of 7-metalla-1,4-diphosphanorbornadienes via reversible [4 + 1]-cycloaddition reactions.

Four- and five-coordinate aluminum complexes supported by N,O-bidentate β-pyrazylenolate ligands: synthesis, structure and application in ROP of ε-caprolactone and lactide

Two series of alkyl aluminum complexes LAlMe2 and L2AlMe supported by N,O-bidentate β-pyrazylenolate ligands have been synthesized and applied to the ROP of ε-CL and rac-LA.

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.

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.

Reactions of Fluoroalkanes with Mg-Mg Bonds: Scope, sp3 C-F/sp2 C-F Coupling and Mechanism

sp³C−F Bonds of fluoroalkanes (7 examples; 1°, 2° and 3°) undergo addition to a low‐valent Mg−Mg species generating reactive organomagnesium reagents. Further reactions with a series of electrophiles results in a net C−F to C−B, C−Si, C−Sn or C−C bond transformation (11 examples, diversity). The new reactivity has been exploited in an unprecedented one‐pot magnesium‐mediated coupling of sp³C−F and sp²C−F bonds. Calculations suggest that the sp³C−F bond activation step occurs by frontside nucleophilic attack of the Mg−Mg reagent on the fluoroalkane.

En Route to Bis-Carbene Analogues of the Heavier Group 13 Elements: Consideration of Bridging Group and Metal(I) Source

With the aim of exploring the effect of steric constraints imposed on the metal-metal interaction of bis-carbene analogues of thallium by the linking scaffold, seven dinuclear thallium diyls with a series of rigid, semirigid, and flexible bridging scaffolds were synthesized. The solid-state molecular structures were determined for four of these compounds by single-crystal XRD and compared with the results of DFT calculations, which were performed for all substances. These compounds serve as models to investigate the metal-metal distance in the absence of co-coordinated molecules (additional ligands, solvent molecules). In addition, the effect of the metal(I) precursor, and more specifically the counterion, on the synthetic access to bis-carbene analogues of indium and thallium was investigated. For indium, only InI yields the desired dinuclear indium diyl. With InBr no reaction was observed, and using InCl gave rise to a mononuclear indium(III) compound. For thallium, both TlI and TlBr allow access to the related bis-carbene analogue, although the yield with the latter is significantly lower. In contrast, no reactions were observed with TlCl and TlBF₄ .

Activation of diverse carbon-heteroatom and carbon-carbon bonds via palladium(II)-catalysed β-X elimination

Chemists' ability to synthesize structurally complex, high-value organic molecules from simple starting materials is limited by methods to selectively activate and functionalize strong alkyl C(sp³) covalent bonds. Recent activity has focused on the activation of abundant C-O, C-N and C-C bonds via a mechanistic paradigm of oxidative addition of a low-valent, electron-rich transition metal. This approach typically employs nickel(0), rhodium(I), ruthenium(0) and iron catalysts under conditions finely tuned for specific, electronically activated substrates, sometimes assisted by chelating functional groups or ring strain. By adopting a redox-neutral strategy involving palladium(II)-catalysed C-H activation followed by β-heteroatom/carbon elimination, we describe here a catalytic method to activate alkyl C(sp³)-oxygen, nitrogen, carbon, fluorine and sulfur bonds with high regioselectivity. Directed hydrofunctionalization of the resultant palladium(II)-bound alkene leads to formal functional group metathesis. The method is applied to amino acid upgrading with complete regioselectivity and moderate to high retention of enantiomeric excess. Low-strain heterocycles undergo strong-bond activation and substitution, giving ring-opened products.

A General Route to Metal-Substituted Dipnictenes of the Type [L(X)M]2 E2

Two equivalents of LGa (L=HC[C(Me)N(2,6-iPr₂ C₆ H₃ )]₂ ) reacted with PX₃ (X=Cl, Br) with insertion into two P-X bonds and formation of [L(X)Ga]₂ PX (X=Cl 1, Br 2), whereas the analogous reaction with AsCl₃ occurred with twofold insertion and subsequent elimination of LGaCl₂ and formation of the Ga-substituted diarsene [L(Cl)Ga]₂ As₂ (3). Analogous findings were observed in the reactions with Me₂ NAsCl₂ , yielding the unsymmetrically-substituted diarsene [L(Cl)Ga]As=As[Ga(NMe₂ )L] (4). The reaction of As(NMe₂ )₃ with LGa gave [L(Me₂ N)Ga]₂ As₂ (5) after heating at 165 °C for five days, whereas the reaction with LAl gave [L(Me₂ N)Al]₂ As₂ (6) after heating at only 80 °C for one day. Finally, two equivalents of LGa reacted with Bi(NEt₂ )₃ to give [L(Et₂ N)Ga]₂ Bi₂ (7). Complexes 1-7 were characterized by NMR spectroscopy (¹ H, ¹³ C, ³¹ P), elemental analysis, and single-crystal X-ray diffraction (except for 1 and 5). The bonding situations in 4, 6, and 7 were analyzed by quantum chemical calculations.

A combined experimental and computational study on the reaction of fluoroarenes with Mg-Mg, Mg-Zn, Mg-Al and Al-Zn bonds

Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to Mg–Mg bonds has been determined as a concerted S_NAr-like pathway in which one Mg centre acts as a nucleophile and the other an electrophile.

Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to Mg–Mg bonds has been determined as a concerted S_NAr-like pathway in which one Mg centre acts as a nucleophile and the other an electrophile. The experimentally determined Gibbs activation energy for the addition of C₆F₆ to a Mg–Mg bond of a molecular complex, ΔG‡298 K(experiment) = 21.3 kcal mol^–1 is modelled by DFT with the ωB97X functional, ΔG‡298 K(DFT) = 25.7 kcal mol^–1. The transition state for C–F activation involves a polarisation of the Mg–Mg bond and significant negative charge localisation on the fluoroarene moiety. This transition state is augmented by stabilising closed-shell Mg···F_ortho interactions that, in combination with the known trends in C–F and C–M bond strengths in fluoroarenes, provide an explanation for the experimentally determined preference for C–F bond activation to occur at sites flanked by ortho-fluorine atoms. The effect of modification of both the ligand coordination sphere and the nature and polarity of the M–M bond (M = Mg, Zn, Al) on C–F activation has been investigated. A series of highly novel β-diketiminate stabilised complexes containing Zn–Mg, Zn–Zn–Zn, Zn–Al and Mg–Al bonds has been prepared, including the first crystallographic characterisation of a Mg–Al bond. Reactions of these new M–M containing complexes with perfluoroarenes were conducted and modelled by DFT. C–F bond activation is dictated by the steric accessibility, and not the polarity, of the M–M bond. The more open coordination complexes lead to enhanced Mg···F_ortho interactions which in turn lower the energy of the transition states for C–F bond activation.

Imino-stabilised phosphinidene (or azaphosphole?) and some of its derivatives

Diiminophenyl (dimph) proved to be an excellent ligand platform to stabilise a low-valent phosphorus centre.

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.

The Unusual Role of Aromatic Solvent in Single-Site AluminumI Chemistry: Insights from Theory

The single-site activation of strong σ bonds (such as that of H-H, P-H, and N-H) remains a significant challenge in main-group chemistry, with only a few cases reported to date. In this regard, recent exciting experiments performed with aluminum^I complexes hold significance because they have been seen to activate a variety of strong σ bonds. Such chemistry is generally seen to occur in aromatic solvents. Current computational studies with DFT reveal the interesting reason for this: an explicit aromatic solvent molecule acts as a catalyst by converting the aluminum^I complex into aluminum^III during the process. Different cases of σ-bond activation by aluminum^I complexes have been investigated and the efficiency of H-X (X=H, NHtBu, PPh₂ ) bond activation in the presence of an explicit benzene solvent molecule is orders of magnitude higher than that in its absence. The current work therefore reveals the chemistry of aluminum^I complexes to be richer and more complex than that previously realized, and shows it to be dependent on metal-solvent cooperativity; the first known example of its kind in main-group chemistry.

Aluminum(i) β-diketiminato complexes activate C(sp2)-F and C(sp3)-F bonds by different oxidative addition mechanisms: a DFT study

DFT computations reveal different reaction mechanisms for the oxidative addition of C(sp²)-F and C(sp³)-F bonds to the Al(i) complexes: a concerted mechanism for C(sp²)-F and a stepwise mechanism for C(sp³)-F involving fluoride transfer and the formation and recombination of an ion pair.

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.