A Stable Calcium Alumanyl

Synthesis, Isolation, and Reactivity Studies of 'Naked' Acyclic Gallyl and Indyl Anions

By exploiting the electronic capabilities of the N-heterocyclic boryloxy (NHBO) ligand, we have synthesized "naked" acyclic gallyl [Ga{OB(NDippCH)2}2]- and indyl [In{OB(NDippCH)2}2]- anions (as their [K(2.2.2-crypt)]+ salts) through K+ abstraction from [KGa{OB(NDippCH)2}2] and [KIn{OB(NDippCH)2}2] using 2.2.2-crypt. These systems represent the first O-ligated gallyl/indyl systems, are ultimately accessed from cyclopentadienyl GaI/InI precursors by substitution chemistry, and display nucleophilic reactivity which is strongly influenced by the presence (or otherwise) of the K+ counterion.

Fresh Impetus in the Chemistry of Calcium Peroxides

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn4CaO5 cluster, in which the Ca2+ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca2+ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O2, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K+ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O2 activation and advances the understanding of Ca/ROS systems.

Application of Bis(amido)alkyl Magnesiates toward the Synthesis of Molecular Rubidium and Cesium Hydrido-magnesiates

Rubidium and cesium are the least studied naturally occurring s-block metals in organometallic chemistry but are in plentiful supply from a sustainability viewpoint as highlighted in the periodic table of natural elements published by the European Chemical Society. This underdevelopment reflects the phenomenal success of organometallic compounds of lithium, sodium, and potassium, but interest in heavier congeners has started to grow. Here, the synthesis and structures of rubidium and cesium bis(amido)alkyl magnesiates [(AM)MgN'2alkyl]∞, where N' is the simple heteroamide -N(SiMe3)(Dipp), and alkyl is nBu or CH2SiMe3, are reported. More stable than their nBu analogues, the reactivities of the CH2SiMe3 magnesiates toward 1,4-cyclohexadiene are revealed. Though both reactions produce target hydrido-magnesiates [(AM)MgN'2H]2 in crystalline form amenable to X-ray diffraction study, the cesium compound could only be formed in a trace quantity. These studies showed that the bulk of the -N(SiMe3)(Dipp) ligand was sufficient to restrict both compounds to dimeric structures. Bearing some resemblance to inverse crown complexes, each structure has [(AM)(N)(Mg)(N)]2 ring cores but differ in having no AM-N bonds, instead Rb and Cs complete the rings by engaging in multihapto interactions with Dipp π-clouds. Moreover, their hydride ions occupy μ3-(AM)2Mg environments, compared to μ2-Mg2 environments in inverse crowns.

Reductions of Arenes using a Magnesium-Dinitrogen Complex

In this contribution, we present "Birch-type", and other reductions of simple arenes by the potassium salt of an anionic magnesium dinitrogen complex, [{K(TCHPNON)Mg}2(μ-N2)] (TCHPNON=4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene), which acts as a masked dimagnesium(I) diradical in these reactions. This reagent is non-hazardous, easy-to-handle, and in some cases provides access to 1,4-cyclohexadiene reduction products under relatively mild reaction conditions. This system works effectively to reduce benzene, naphthalene and anthracene through magnesium-bound "Birch-type" reduction intermediates. Cyclohexadiene products can be subsequently released from the magnesium centres by protonolysis with methanol. In contrast, the reduction of substituted arenes is less selective and involves competing reaction pathways. For toluene and 1,3,5-triphenylbenzene, the structural authentication of "Birch-type" reduction intermediates is conclusive, although the formation of corresponding 1,4-cyclohexadiene derivatives was low yielding. Reduction of anisole did not yield an isolable "Birch-type" intermediate, but instead gave a C-O activation product. Treating triphenylphosphine with [{K(TCHPNON)Mg}2(μ-N2)] resulted in the extrusion of both biphenyl and dinitrogen to afford a magnesium(II) phosphanide [{K(TCHPNON)Mg(μ-PPh2)}2]. Reduction of fluorobenzene proceeded via C-F activation of the arene, and isolation of the magnesium(II) fluoride [{K(TCHPNON)Mg(μ-F)}2]. Finally, the two-electron reduction of 1,3,5,7-cyclooctatetraene (COT) with [{K(TCHPNON)Mg}2(μ-N2)] yielded a complex, [{K(TCHPNON)Mg}2(μ-COT)], incorporating the aromatic dianion (COT2-).

Mercury-Group 13 Metal Covalent Bonds: A Systematic Comparison of Aluminyl, Gallyl and Indyl Metallo-ligands

Bimetallic compounds containing direct metal-group 13 element bonds have been shown to display unprecedented patterns of cooperative reactivity towards small molecules, which can be influenced by the identity of the group 13 element. In this context, we present here a systematic appraisal of group 13 metallo-ligands of the type [(NON)E]- (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) for E=Al, Ga and In, through a comparison of structural and spectroscopic parameters associated with the trans L or X ligands in linear d10 complexes of the types LM{E(NON)} and XM'{E(NON)}. These studies are facilitated by convenient syntheses (from the In(I) precursor, InCp) of the potassium indyl species [{K(NON)In}⋅KCp]n (1) and [(18-crown-6)2K2Cp] [(NON)In] (1'), and lead to the first structural characterisation of Ag-In and Hg-E (E=Al, In) covalent bonds. The resulting structural, spectroscopic and quantum chemical probes of Ag/Hg complexes are consistent with markedly stronger σ-donor capabilities of the aluminyl ligand, [(NON)Al]-, over its gallium and indium counterparts.

Formation, Structure and Reactivity of a Beryllium(0) Complex with Mgδ+-Beδ- Bond Polarization

Attempts to create a novel Mg-Be bond by reaction of [(DIPePBDI*)MgNa]2 with Be[N(SiMe3)2]2 failed; DIPePBDI*=HC[(tBu)C=N(DIPeP)]2, DIPeP=2,6-Et2C-phenyl. Even at elevated temperatures, no conversion was observed. This is likely caused by strong steric shielding of the Be center. A similar reaction with the more open Cp*BeCl gave in quantitative yield (DIPePBDI*)MgBeCp* (1). The crystal structure shows a Mg-Be bond of 2.469(4) Å. Homolytic cleavage of the Mg-Be bond requires ΔH=69.6 kcal mol-1 (cf. CpBe-BeCp 69.0 kcal mol-1 and (DIPPBDI)Mg-Mg(DIPPBDI) 55.8 kcal mol-1). Natural-Population-Analysis (NPA) shows fragment charges: (DIPePBDI*)Mg +0.27/BeCp* -0.27. The very low NPA charge on Be (+0.62) compared to Mg (+1.21) and the strongly upfield 9Be NMR signal at -23.7 ppm are in line with considerable electron density on Be and the formal oxidation state assignment of MgII-Be0. Despite this Mgδ+-Beδ- polarity, 1 is extremely thermally stable and unreactive towards H2, CO, N2, cyclohexene and carbodiimide. It reacted with benzophenone, azobenzene, phenyl acetylene, CO2 and CS2. Reaction with 1-adamantyl azide led to reductive coupling and formation of an N6-chain. The azide reagent also inserted in the Cp*-Be bond. The inertness of 1 is likely due to bulky ligands protecting the Mg-Be unit.

Bis(Aluminyl)Magnesium: A Source of Nucleophilic or Radical Aluminium-Centred Reactivity

The homoleptic magnesium bis(aluminyl) compound Mg[Al(NON)]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) can be accessed from K2[Al(NON)]2 and MgI2 and shown to possess a non-linear geometry (∠Al-Mg-Al=164.8(1)°) primarily due to the influence of dispersion interactions. This compound acts a four-electron reservoir in the reductive de-fluorination of SF6, and reacts thermally with polar substrates such as MeI via nucleophilic attack through aluminium, consistent with the QT-AIM charges calculated for the metal centres, and a formal description as a Al(I)-Mg(II)-Al(I) trimetallic. On the other hand, under photolytic activation, the reaction with 1,5-cyclooctadiene leads to the stereo-selective generation of transannular cycloaddition products consistent with radical based chemistry, emphasizing the covalent nature of the Mg-Al bonds and a description as a Al(II)-Mg(0)-Al(II) synthon. Consistently, photolysis of Mg[Al(NON)]2 in hexane in the absence of COD generates [Al(NON)]2 together with magnesium metal.

Reversible [4 + 1] Cycloaddition of Arenes by a "Naked" Acyclic Aluminyl Compound

The large steric profile of the N-heterocyclic boryloxy ligand, -OB(NDippCH)2, and its ability to stabilize the metal-centered HOMO, are exploited in the synthesis of the first example of a "naked" acyclic aluminyl complex, [K(2.2.2-crypt)][Al{OB(NDippCH)2}2]. This system, which is formed by substitution at AlI (rather than reduction of AlIII), represents the first O-ligated aluminyl compound and is shown to be capable of hitherto unprecedented reversible single-site [4 + 1] cycloaddition of benzene. This chemistry and the unusual regioselectivity of the related cycloaddition of anthracene are shown to be highly dependent on the availability (or otherwise) of the K+ countercation.

Fluorenyl-tethered N-heterocyclic carbene (NHC): an exclusive C-donor ligand for heteroleptic calcium and strontium chemistry

Exclusive C-donating ligands are rarely used with kinetically labile heavier alkaline earths (Ca, Sr, Ba). We report herein the aptitude of a combination of NHC with fluorenyl connected by a flexible -(CH2)2- linker as a ligand support for heteroleptic Ca- and Sr-N(SiMe3)2 and iodides. The Ca-N(SiMe3)2 complex even catalyzes the intramolecular hydroamination of aminoalkenes to showcase the effectiveness of this ligand framework.

[{SiNDipp}MgNa]2: A Potent Molecular Reducing Agent

The bimetallic species, [{SiNDipp}MgNa]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2; (Dipp = 2,6-i-Pr2C6H3)], is shown to be a potent reducing agent, able to effect one- or two-electron reduction of either dioxygen, TEMPO, anthracene, benzophenone, or diphenylacetylene. In most cases, the bimetallic reaction products imply that the dissimilar alkaline metal centers react with a level of cooperativity. EPR analysis of the benzophenone-derived reaction and the concurrent isolation of [{SiNDipp}Mg(OCPh2)2], however, illustrate that treatment with such reducible, but O-basic, species can also result in reactivity in which the metals provide independent reaction products. The notable E-stereochemistry of the diphenylacetylene reduction product prompted a computational investigation of the PhC≡CPh addition. This analysis invokes a series of elementary steps that necessitate ring-opening via Mg+ → Na+ amido group migration of the SiNDipp ligand, providing insight into the previously observed lability of the bidentate dianion and its consequent proclivity toward macrocyclization.

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Access to a peri-Annulated Aluminium Compound via C-H Bond Activation by a Cyclic Bis-Aluminylene

Carbocyclic aluminium halides [(ADC)AlX2]2 (2-X) (X=F, Cl, and I) based on an anionic dicarbene (ADC=PhC{N(Dipp)C}2, Dipp = 2,6-iPr2C6H3) framework are prepared as crystalline solids by dehydrohalogenations of the alane [(ADC)AlH2]2 (1). KC8 reduction of 2-I affords the peri-annulated Al(III) compound [(ADCH)AlH]2 (4) (ADCH=PhC{N(Dipp)C2(DippH)N}, DippH=2-iPr,6-(Me2C)C6H3)) as a colorless crystalline solid in 76 % yield. The formation of 4 suggests intramolecular insertion of the putative bis-aluminylene species [(ADC)Al]2 (3) into the methine C-H bond of HCMe2 group. Calculations predict singlet ground state for 3, while the conversion of 3 into 4 is thermodynamically favored by 61 kcal/mol. Compounds 2-F, 2-Cl, 2-I, and 4 have been characterized by NMR spectroscopy and their solid-state molecular structures have been established by single crystal X-ray diffraction.

Bimetallic Synergy Enables Silole Insertion into THF and the Synthesis of Erbium Single-Molecule Magnets

The potassium silole K2 [SiC4 -2,5-(SiMe3 )2 -3,4-Ph2 ] reacts with [M(η8 -COT)(THF)4 ][BPh4 ] (M=Er, Y; COT=cyclo-octatetraenyl) in THF to give products that feature unprecedented insertion of the nucleophilic silicon centre into a carbon-oxygen bond of THF. The structure of the major product, [(μ-η8  : η8 -COT)M(μ-L1 )K]∞ (1M ), consists of polymeric chains of sandwich complexes, where the spiro-bicyclic silapyran ligand [C4 H8 OSiC4 (SiMe3 )2 Ph2 ]2- (L1 ) coordinates to potassium via the oxygen. The minor product [(μ-η8  : η8 -COT)M(μ-L1 )K(THF)]2 (2M ) features coordination of the silapyran to the rare-earth metal. In forming 1M and 2M , silole insertion into THF only occurs in the presence of potassium and the rare-earth metal, highlighting the importance of bimetallic synergy. The lower nucleophilicity of germanium(II) leads to contrasting reactivity of the potassium germole K2 [GeC4 -2,5-(SiMe3 )2 -3,4-Me2 ] towards [M(η8 -COT)(THF)4 ][BPh4 ], with intact transfer of the germole occurring to give the coordination polymers [{η5 -GeC4 (SiMe3 )2 Me2 }M(η8 -COT)K]∞ (3M ). Despite the differences in reactivity induced by the group 14 heteroatom, the single-molecule magnet properties of 1Er , 2Er and 3Er are similar, with thermally activated relaxation occurring via the first-excited Kramers doublet, subject to effective energy barriers of 122, 80 and 91 cm-1 , respectively. Compound 1Er is also analysed by high-frequency dynamic magnetic susceptibility measurements up to 106  Hz.

Alumanyl-Samarium(II): Synthesis, Characterization, and Reactivity Studies

Metal-metal bonded species involving lanthanides are intriguing but rare. The recently reported salt metathesis reaction of an Al anion and SmI2(thf)2 yields novel heterometallic compound possessing two distinctive Al-Sm bonds. Although the Al-Sm bonds were considerably long [3.518(1) and 3.543(1) Å], DFT calculations indicated polar character of the Alδ--Smδ+ bonds. This is the first example of lanthanide species containing X-type Al ligands. Reactivity studies have demonstrated that the introduction of Sm(II) produces unique reactivity. The reaction with carbodiimide led to an insertion of carbodiimide into the Al-Sm bonds and reductive coupling of carbodiimide to create an oxalamidinate moiety, facilitated by Sm(II). Exposure of the Al-Sm-Al complex toward ethylene furnished a Sm(II) salt of anionic aluminacyclopropane that was spontaneously isomerized to a 1,4-dialuminacyclohexane derivative. The important role of Sm(II) to facilitate the ring expansion through an alkyl-relay mechanism was elucidated by DFT calculations.

Synthesis and Modular Reactivity of Low Valent Al/Zn Heterobimetallics Supported by Common Monodentate Amides

Recent advances on low valent main group metal chemistry have shown the excellent potential of heterobimetallic complexes derived from Al(I) to promote cooperative small molecule activation processes. A signature feature of these complexes is the use of bulky chelating ligands which act as spectators providing kinetic stabilization to their highly reactive Al-M bonds. Here we report the synthesis of novel Al/Zn bimetallics prepared by the selective formal insertion of AlCp* into the Zn-N bond of the utility zinc amides ZnR2 (R=HMDS, hexamethyldisilazide; or TMP, 2,2,6,6-tetramethylpiperidide). By systematically assessing the reactivity of the new [(R)(Cp*)AlZn(R)] bimetallics towards carbodiimides, structural and mechanistic insights have been gained on their ability to undergo insertion in their Zn-Al bond. Disclosing a ligand effect, when R=TMP, an isomerization process can be induced giving [(TMP)2AlZn(Cp*)] which displays a special reactivity towards carbodiimides and carbon dioxide involving both its Al-N bonds, leaving its Al-Zn bond untouched.

NHC-Stabilized Mixed Group 13/14/15 Element Hydrides

The syntheses of novel N-heterocyclic carbene (NHC) adducts of group 13, 14 and 15 element hydrides are reported. Salt metathesis reactions between NaPH2 and IDipp ⋅ GeH2 BH2 OTf (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) led to mixtures of the two isomers IDipp ⋅ GeH2 BH2 PH2 (2 a) and IDipp ⋅ BH2 GeH2 PH2 (2 b); by altering the reaction conditions an almost exclusive formation of 2 b was achieved. Attempts to purify mixtures of 2 a and 2 b by re-crystallization from THF afforded a salt [IDipp ⋅ GeH2 BH2  ⋅ IDipp][PHGeH2 BH2 PH2 BH2 GeH2 ] (4) that contains the novel anionic cyclohexyl-like inorganic heterocycle [PHGeH2 BH2 PH2 BH2 GeH2 ]- . In addition, the borane adducts IDipp ⋅ GeH2 BH2 PH2 BH3 (3 a) and IDipp ⋅ BH2 GeH2 PH2 BH3 (3 b) as even longer chain compounds were obtained from reactions of 2 a/2 b with H3 B ⋅ SMe2 and were studied by NMR spectroscopy. Accompanying DFT computations give insight into the mechanism and energetics associated with 2 a/2 b isomerization as well as their decomposition pathways.

Cesium Reduction of a Lithium Diamidochloroberyllate

Room temperature reaction of elemental cesium with the dimeric lithium chloroberyllate [{SiNDipp}BeClLi]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2, where Dipp = 2,6-di-isopropylphenyl, in C6D6 results in activation of the arene solvent. Although, in contrast to earlier observations of lithium and sodium metal reduction, the generation of a mooted cesium phenylberyllate could not be confirmed, this process corroborates a previous hypothesis that such beryllium-centered solvent activation also necessitates the formation of hydridoberyllium species. These observations are further borne out by the study of an analogous reaction performed in toluene, in which case the proposed generation of formally low oxidation state beryllium radical anion intermediates induces activation of a toluene sp3 C-H bond and the isolation of the polymeric cesium benzylberyllate, [Cs({SiNDipp}BeCH2C6H5)]∞.

Mechanochemical Synthesis of Non-Solvated Dialkylalumanyl Anion and XPS Characterization of Al(I) and Al(II) Species

A non-solvated alkyl-substituted Al(I) anion dimer was synthesized by a reduction of haloalumane precursor using a mechanochemical method. The crystallographic and theoretical analysis revealed its structure and electronic properties. Experimental XPS analysis of the Al(I) anions with reference compounds revealed the lower Al 2p binding energy corresponds to the lower oxidation state of Al species. It should be emphasized that the experimentally obtained XPS binding energies were reproduced by delta SCF calculations and were linearly correlated with NPA charges and 2p orbital energies.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Seven-Membered Cyclic Diamidoalumanyls of Heavier Alkali Metals: Structures and C-H Activation of Arenes

Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiNDipp}AlI] ({SiNDipp} = {CH2SiMe2NDipp}2) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiNDipp}AlM]2. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiNDipp}Na2] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C-H oxidative addition of arene C-H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C-H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M+ cation, which becomes increasingly favorable as group 1 is descended.

Aluminum-Boron Bond Formation by Boron Ester Oxidative Addition at an Alumanyl Anion

The potassium diamidoalumanyl, [K{Al(SiNDipp)}]2 (SiNDipp = {CH2SiMe2NDipp}2), reacts with the terminal B-O bonds of pinacolato boron esters, ROBpin (R = Me, i-Pr), and B(OMe)3 to provide potsassium (alkoxy)borylaluminate derivatives, [K{Al(SiNDipp)(OR)(Bpin)}]n (R = Me, n = 2; R = i-Pr, n = ∞) and [K{Al(SiNDipp)(OMe)(B(OMe)2)}]∞, comprising Al-B σ bonds. An initial assay of the reactivity of these species with the heteroallene molecules, N,N'-diisopropylcarbodiimide and CO2, highlights the kinetic inaccessibility of their Al-B bonds; only decomposition at high temperature is observed with the carbodiimide, whereas CO2 preferentially inserts into the Al-O bond of [K{Al(SiNDipp)(OMe)(Bpin)}]2 to provide a dimeric methyl carbonate species. Treatment of the acyclic dimethoxyboryl species, however, successfully liberates a terminal alumaboronic ester featuring trigonal N2Al-BO2 coordination environments at both boron and aluminum.

Evidence of AlII Radical Addition to Benzene

Electrophilic Al^III species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low-valent Al^I aluminyl anions have showcased oxidative additions towards arenes C-C and/or C-H bonds. Herein, we communicate compelling evidence of an Al^II radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC₈ in benzene furnishes a double addition to the benzene ring instead of a C-H bond activation, producing the corresponding cyclohexa-1,3(orl,4)-dienes as Birch-type reduction product. X-ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable Al^II radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.

Inducing Nucleophilic Reactivity at Beryllium with an Aluminyl Ligand

The reactions of anionic aluminium or gallium nucleophiles {K[E(NON)]}₂ (E = Al, 1; Ga, 2; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) with beryllocene (BeCp₂) led to the displacement of one cyclopentadienyl ligand at beryllium and the formation of compounds containing Be-Al or Be-Ga bonds (NON)EBeCp (E = Al, 3; Ga, 4). The Be-Al bond in the beryllium-aluminyl complex [2.310(4) Å] is much shorter than that found in the small number of previous examples [2.368(2) to 2.432(6) Å], and quantum chemical calculations suggest the existence of a non-nuclear attractor (NNA) for the Be-Al interaction. This represents the first example of a NNA for a heteroatomic interaction in an isolated molecular complex. As a result of this unusual electronic structure and the similarity in the Pauling electronegativities of beryllium and aluminium, the charge at the beryllium center (+1.39) in 3 is calculated to be less positive than that of the aluminium center (+1.88). This calculated charge distribution suggests the possibility for nucleophilic behavior at beryllium and correlates with the observed reactivity of the beryllium-aluminyl complex with N,N'-diisopropylcarbodiimide─the electrophilic carbon center of the carbodiimide undergoes nucleophilic attack by beryllium, thereby yielding a beryllium-diaminocarbene complex.

Probing a General Strategy to Break the C-C Bond of Benzene by a Cyclic (Alkyl)(Amino)Aluminyl Anion

The oxidative addition of C-C bonds in aromatic hydrocarbons by low valent main group species has attracted considerable attention from both theoretical and experimental chemists due to the big challenge in breaking their aromaticity. Herein, a general strategy to break the C-C bonds in benzene by cyclic (alkyl)(amino)aluminyl anion is demonstrated via density functional theory (DFT) calculations. The results suggest that the activation of the C-C bond of benzene by this anion is both kinetically and thermodynamically unfavorable whereas introducing electron-withdrawing groups makes such C-C bond activation becomes favorable both kinetically and thermodynamically. Such a sharp change on the kinetics and thermodynamics could be rationalized by the frontier molecular orbital theory by decreasing the lowest unoccupied molecular orbitals of the mono- and disubstituted benzenes. Aromaticity is found to stabilize the transition state for the ring open step. All these findings can help develop the chemistry of small-molecule activation.

The emerging chemistry of the aluminyl anion

The chemistry of low valent p-block metal complexes continues to elicit interest in the research community, demonstrating reactivity that replicates and in some cases exceeds that of their more widely studied d-block metal counterparts. The introduction of the first aluminyl anion, a complex containing a formally anionic Al(I) centre charge balanced by an alkali metal (AM) cation, has established a platform for a new area of chemical research. The chemistry displayed by aluminyl compounds is expanding rapidly, with examples of reactivity towards a diverse range of small molecules and functional groups now reported in the literature. Herein we present an account of the structure and reactivity of the growing family of aluminyl compounds. In this context we examine the structural relationships between the aluminyl anion and the AM cations, which now include examples of AM = Li, Na, K, Rb and Cs. We report on the ability of these compounds to engage in bond-breaking and bond-forming reactions, which is leading towards their application as useful reagents in chemical synthesis. Furthermore we discuss the chemistry of bimetallic complexes containing direct Al-M bonds (M = Li, Na, K, Mg, Ca, Cu, Ag, Au, Zn) and compounds with Al-E multiple bonds (E = NR, CR₂, O, S, Se, Te), where both classes of compound are derived directly from aluminyl anions.

An Aluminium Imide as a Transfer Agent for the [NR]2- Function via Metathesis Chemistry

The reactions of a terminal aluminium imide with a range of oxygen-containing substrates have been probed with a view to developing its use as a novel main group transfer agent for the [NR]^2- fragment. We demonstrate transfer of the imide moiety to [N₂ ], [CO] and [Ph(H)C] units driven thermodynamically by Al-O bond formation. N₂ O reacts rapidly to generate the organoazide DippN₃ (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ), while CO₂ (under dilute reaction conditions) yields the corresponding isocyanate, DippNCO. Mechanistic studies, using both experimental and quantum chemical techniques, identify a carbamate complex K₂ [(NON)Al-{κ² -(N,O)-N(Dipp)CO₂ }]₂ (formed via [2+2] cycloaddition) as an intermediate in the formation of DippNCO, and also in an alternative reaction leading to the generation of the amino-dicarboxylate complex K₂ [(NON)Al{κ² -(O,O')-(O₂ C)₂ N-(Dipp)}] (via the take-up of a second equivalent of CO₂ ). In the case of benzaldehyde, a similar [2+2] cycloaddition process generates the metallacyclic hemi-aminal complex, K_n [(NON)Al{κ² -(N,O)-(N(Dipp)C(Ph)(H)O}]_n . Extrusion of the imine, PhC(H)NDipp, via cyclo-reversion is disfavoured thermally, due to the high energy of the putative aluminium oxide co-product, K₂ [(NON)Al(O)]₂ . However, addition of CO₂ allows the imine to be released, driven by the formation of the thermodynamically more stable aluminium carbonate co-product, K₂ [(NON)Al(κ² -(O,O')-CO₃ )]₂ .

Al-Sc Bonded Complexes: Synthesis, Structure, and Reaction with Benzene in the Presence of Alkyl Halide

An alumanyl anion possessing N,N'-bis(2,6-diisopropylphenyl)-1,3-propanediamine ligand was synthesized and characterized. Transmetalation of this Al anion with diaminoscandium chloride precursors afforded the corresponding Al-Sc complexes possessing an unprecedented Al-Sc bond. The Al-Sc[N(SiMe₃)₂] complex underwent intramolecular C-H cleavage to form a bridged dinuclear complex with μ-hydrido and μ-methylene ligands. The Al-Sc(NⁱPr₂)₂ complex reacted with benzene in the presence of alkyl bromide to furnish a 1,4-dialuminated cyclohexadiene product with a concomitant formation of the alkyl-alkyl coupled product. Although the latter product seems to form through the radical mechanism, DFT calculations revealed an ionic mechanism involving bimetallic reaction pathways to react with alkyl bromide and benzene, which provides new insight into the chemistry of metal-metal bonded compounds.

Synthesis, Characterization, and Structural Analysis of [Al(NONDipp)(H)(SiH2Ph)] ( = Li, Na, K, Rb, Cs) Compounds, Made Via Oxidative Addition of Phenylsilane to Alkali Metal Aluminyls

We report the oxidative addition of phenylsilane to the complete series of alkali metal (AM) aluminyls [AM{Al(NON^Dipp)}]₂ (AM = Li, Na, K, Rb, and Cs). Crystalline products (1-AM) have been isolated as ether or THF adducts, [AM(L)_n][Al(NON^Dipp)(H)(SiH₂Ph)] (AM = Li, Na, K, Rb, L = Et₂O, n = 1; AM = Cs, L = THF, n = 2). Further to this series, the novel rubidium rubidiate, [{Rb(THF)₄}₂(Rb{Al(NON^Dipp)(H)(SiH₂Ph)}₂)]⁺ [Rb{Al(NON^Dipp)(H)(SiH₂Ph)}₂]^-, was isolated during an attempted recrystallization of Rb[Al(NON^Dipp)(H)(SiH₂Ph)] from a hexane/THF mixture. Structural and spectroscopic characterizations of the series 1-AM confirm the presence of μ-hydrides that bridge the aluminum and alkali metals (AM), with multiple stabilizing AM···π(arene) interactions to either the Dipp- or Ph-substituents. These products form a complete series of soluble, alkali metal (hydrido) aluminates that present a platform for further reactivity studies.

Unravelling the Potential of Ylides in Stabilizing Low-Valent Group 13 Compounds: Theoretical Predictions of Stable, Five-Membered Group 13 (Aluminum and Gallium) Carbenoids Capable of Small-Molecule Activation

Computational investigations provide evidence toward the remarkable ability of strongly electron-donating ylidic functionalities in stabilizing singlet group 13 carbenoids with promising ligand properties. All of the proposed carbenoids are found to be considerably nucleophilic and possess significant singlet-triplet energy separation values. The calculated activation barriers and reaction free energies obtained for the cleavage of different enthalpically strong bonds by these carbenoids are found to be either comparable to or lower than those of the experimentally evaluated aluminum and gallium carbenoids, thereby indicating their potential in small-molecule activation.

Isolating elusive 'Al(μ-O)M' intermediates in CO2 reduction by bimetallic Al-M complexes (M = Zn, Mg)

The reaction of compounds containing Al-Mg and Al-Zn bonds with N₂O enabled isolation of the corresponding Al(μ-O)M complexes. Electronic structure analysis identified largely ionic Al-O and O-M bonds, featuring an anionic μ-oxo centre. Reaction with CO₂ confirmed that these species correspond to the proposed intermediates in the formation of μ-carbonate compounds.

Synthesis of a low-valent Al4+ cluster cation salt

Low-valent aluminium compounds are very reactive main-group species and have therefore been widely investigated. Since the isolation of a stable molecular Al(I) compound in 1991, [(AlCp*)₄] (Cp* = [C₅Me₅]^-), a variety of highly reactive neutral or anionic low-valent aluminium complexes have been developed. By contrast, their cationic counterparts have remained difficult to access. Here, we report the synthesis of [Al(AlCp*)₃]⁺[Al(OR^F)₄]^- (R^F = C(CF₃)₃) through a simple metathesis reaction between [(AlCp*)₄] and Li[Al(OR^F)₄]. Unexpectedly, the [Al(AlCp*)₃]⁺ salt forms a dimer in the solid state and concentrated solutions. Addition of Lewis bases results in monomerization and coordination to the unique formal Al⁺ atom, giving [(L)_xAl(AlCp*)₃]⁺ salts where L is hexaphenylcarbodiphosphorane (x = 1), tetramethylethylenediamine (x = 1) or 4-dimethylaminopyridine (x = 3). The Al⁺-Al_Cp* bonds in the resulting [(L)_xAl(AlCp*)₃]⁺ cluster cations can be finely tuned between very strong (with no ligand L) to very weak and approaching isolated [Al(L)₃]⁺ ions (when L is dimethylaminopyridine).

Alkali metal influences in aluminyl complexes

The previously reported potassium aluminyl complex [(BDI-H)Al^-K⁺]₂ was converted in Li⁺ or Na⁺ salts by a salt metathesis reaction with Li(BPh₄) or Na(BPh₄), respectively; BDI-H = dianionic [(DIPP)N-C(Me)C(H)-C(CH₂)-N(DIPP)^2-] and DIPP = 2,6-diisopropylphenyl. The Rb and Cs aluminyl complexes were obtained by reaction of (BDI)Al with RbC₈ or CsC₈; BDI = HC[C(Me)N(DIPP)]₂. Crystal structures of two monomers, (BDI-H)Al^-Li⁺·(Et₂O)₂ and (BDI-H)Al^-Na⁺·(Et₂O)(TMEDA), and four dimers [(BDI-H)Al^-M⁺]₂ (M = Li, Na, Rb, Cs) are discussed. Lewis base-free dimers [(BDI-H)Al^-M⁺]₂ crystallize either as slipped dimers (Li⁺, Na⁺) in which each Al center features only one Al-M contact or as a symmetric dimer (K⁺, Rb⁺, Cs⁺) in which the cation bridges both Al centers. The dimer with the largest cation (Cs⁺) shows Cs⋯CH₂C interactions between dimers, resulting in a coordination polymer. AIM and charge analysis reveal highly ionic Al-M bonds with strong polarization of the Al lone-pair towards the smaller cation Li⁺ and Na⁺. The Al-M bonds become weaker from Li to Cs. Calculated dimerization energies suggest that in apolar solvents only complexes with the heavier metals Rb and Cs may be in a monomer-dimer equilibrium. This is confirmed by DOSY measurements in benzene. Dimeric aluminyl complexes with heavier alkali metals (K-Cs) react with benzene to give a double C-H activation in para-positions.

Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction

Homologation of carbon monoxide is central to the heterogeneous Fischer–Tropsch process for the production of hydrocarbon fuels. C–C bond formation has been modeled by homogeneous systems, with [C_nO_n]^2– fragments (n = 2–6) formed by two-electron reduction being commonly encountered. Here, we show that four- or six-electron reduction of CO can be accomplished by the use of anionic aluminum(I) (“aluminyl”) compounds to give both topologically linear and branched C₄/C₆ chains. We show that the mechanism for homologation relies on the highly electron-rich nature of the aluminyl reagent and on an unusual mode of interaction of the CO molecule, which behaves primarily as a Z-type ligand in initial adduct formation. The formation of [C₆O₆]^4– from [C₄O₄]^4– shows for the first time a solution-phase CO homologation process that brings about chain branching via complete C–O bond cleavage, while a comparison of the linear [C₄O₄]^4– system with the [C₄O₄]^6– congener formed under more reducing conditions models the net conversion of C–O bonds to C–C bonds in the presence of additional reductants.

Diverse reactivity of an Al(I)-centred anion towards ketones

The reactivity of a seven-membered cyclic potassium diamidoalumanyl toward a variety of ketone small molecules has been assessed. Whilst acetophenone generates an aluminium pinacolate derivative, reductive C-C coupling is induced between the ketyl and ortho-carbon centres of two equivalents of benzophenone. In contrast, whereas oxidative addition of an enolisable proton is observed with 2,4-dimethyl-3-pentanone, 2,2,4,4-tetramethyl-3-pentanone undergoes an unprecedented hydroalumination process, where the reducing hydride may be traced to intramolecular oxidative addition of a (sp³)C-H bond.

Reactivity of Unsupported Transition Metal-Aluminyl Complexes: A Nucleophilic TM-Al Bond

Despite the long history of research in transition metal (TM) complexes, the study of TM-aluminyl complexes is still in its early stage of development. It is expected that the presence of an electropositive Al donor atom would open up new possibilities in TM complex reactivity, and indeed TM-aluminyl has shown an early sign of success in small-molecule activation. On the other hand, the existing reports on TM-aluminyl reactivity are often explained to readers with different understanding on individual cases, and a general picture of TM-aluminyl reactivity is still not available. In this work, we have attempted to provide a systematic picture to explain some early explorations in this field, specifically a series of recently reported heteroallene insertion reactions involving unsupported TM-aluminyl complexes. Through density functional theory calculations of a number of TM-aluminyl complexes, covering both Au and Cu centers, we found that their reactivity against heteroallenes (including CO₂ and carbodiimides) is mostly based on the strong nucleophilicity of the TM-Al σ-bond.

Sterically shielded primary anilides of the alkaline-earth metals of the type (thf)nAe(NH-Ar*)2 (Ae = Mg, Ca, Sr, and Ba; Ar* = bulky aryl)

Metalation of 2,4,6-triphenylphenylamine (H₂N-C₆H₂-2,4,6-Ph₃, 1a) and 4-methyl-2,6-bis(diphenylmethyl)aniline (2,6-bis(diphenylmethyl)-p-toluidine, H₂N-C₆H₂-4-Me-2,6-(CHPh₂)₂, 2a) with dibutylmagnesium and Ae[N(SiMe₃)₂]₂ with a stoichiometric 1 : 2 ratio in THF at room temperature yields the corresponding primary anilides [(thf)_nAe{N(H)-C₆H₂-2,4,6-Ph₃}₂] (Ae/n = Mg/2 (1b), Ca/2 (1c), Sr/3 (1d), and Ba/3 (1e)) and [(thf)_nAe{N(H)-C₆H₂-4-Me-2,6-(CHPh₂)₂}₂] (Ae/n = Mg/2 (2b), Ca/3 (2c) and Sr/2 (2d)), respectively. The 1 : 1 reaction of Mg(n/sBu)₂ and MgPh₂ with 2a leads to the formation of heteroleptic [(thf)₂Mg(R){N(H)-C₆H₂-4-Me-2,6-(CHPh₂)₂}] (R = n/sBu (2bBu), Ph (2bPh)). At 50 °C, the strontium complex 2d liberates one equivalent of 2avia intramolecular deprotonation of the triarylmethyl functionality yielding dinuclear [(thf)₂Sr{N(H)-C₆H₂-4-Me-2-(CPh₂)-6-(CHPh₂)₂}]₂ (2d'). The barium compound is significantly more reactive and regardless of applied stoichiometry the isotypic barium congener [(thf)₂Ba{N(H)-C₆H₂-4-Me-2-(CPh₂)-6-(CHPh₂)₂}]₂ (2e') forms. The molecular structures of 1c, 2d, 2d', and 2e' are stabilized by metal-phenyl π-interactions.

Potassium Aluminyl Promoted Carbonylation of Ethene

The potassium aluminyl [K{Al(NON^Dipp)}]₂ ([NON^Dipp]²⁻=[O{SiMe₂NDipp}₂]²⁻, Dipp=2,6‐iPr₂C₆H₃) activates ethene towards carbonylation with CO under mild conditions. An isolated bis‐aluminacyclopropane compound reacted with CO via carbonylation of an Al−C bond, followed by an intramolecular hydrogen shift to form K₂[Al(NON^Dipp)(μ‐CH₂CH=CO‐1κ²C^1,3‐2κO)Al(NON^Dipp)Et]. Restricting the chemistry to a mono‐aluminium system allowed isolation of [Al(NON^Dipp)(CH₂CH₂CO‐κ²C^1,3)]⁻, which undergoes thermal isomerisation to form the [Al(NON^Dipp)(CH₂CH=CHO‐κ²C,O)]⁻ anion. DFT calculations highlight the stabilising influence of incorporated benzene at multiple steps in the reaction pathways.

Sigma/pi Bonding Preferences of Solvated Alkali-Metal Cations to Ditopic Arylmethyl Anions

Arylmethyl anions allow alkali‐metals to bind in a σ‐fashion to the lateral carbanionic centre or a π‐fashion to the aryl ring or in between these extremities, with the trend towards π bonding increasing on descending group 1. Here we review known alkali metal structures of diphenylmethane, fluorene, 2‐benzylpyridine and 4‐benzylpyridine. Next, we synthesise Li, Na, K monomers of these diarylmethyls using polydentate donors PMDETA or Me₆TREN to remove competing oligomerizing interactions, studying the effect that two aromatic rings has on negative charge (de)localisation via NMR, X‐ray crystallographic and DFT studies. Diphenylmethyl and fluorenyl anions maintain C(H)−M interactions regardless of alkali‐metal, although the adjacent arene carbons engage in interactions with larger alkali‐metals. Introducing a nitrogen atom into the ring (at the 2‐ or 4‐position) encourages relocalisation of negative charge away from the deprotonated carbon and onto nitrogen. Phenyl(2‐pyridyl)methyl moves from an enamide formation at one extremity (lithium) to an aza‐allyl formation at the other extremity (potassium), while C‐ or N‐coordination modes become energetically viable for Na and K phenyl(4‐pyridyl)methyl complexes.

On the reactivity of Al-group 11 (Cu, Ag, Au) bonds

Reactions of the seven-membered heterocyclic potassium diamidoalumanyl, [K{Al(SiN^Dipp)}]₂ (SiN^Dipp = {CH₂SiMe₂NDipp}₂; Dipp = 2,6-di-isopropylphenyl), with a variety of Cu(I), Ag(I) and Au(I) chloride N-heterocyclic carbene (NHC) adducts are described. The resultant group 11-Al bonded derivatives have been characterised in solution by NMR spectroscopy and, in the case of [{SiN^Dipp}Al-Au(NHC^iPr)] (NHC^iPr = N,N'-di-isopropyl-4,5-dimethyl-2-ylidene), by single crystal X-ray diffraction. Although similar reactions of LAgCl and LAuCl, where L is a more basic cyclic alkyl amino carbene (CAAC), generally resulted in reduction of the group 11 cations to the base metals, X-ray analysis of [(^CyCAAC)AgAl(SiN^Dipp)] (^CyCAAC = 2-[2,6-bis(1-methylethyl)phenyl]-3,3-dimethyl-2-azaspiro[4.5]dec-1-ylidene) provides the first solid-state authentication of an Ag-Al σ bond. The reactivity of the NHC-supported Cu, Ag and Au alumanyl derivatives was assayed with the isoelectronic unsaturated small molecules, N,N'-di-isopropylcarbodiimide and CO₂. While these reactions generally provided products consistent with nucleophilic attack of the group 11 atom at the electrophilic heteroallene carbon centre, treatment of the NHC-supported copper and silver alumanyls with N,N'-di-isopropylcarbodiimide yielded less symmetric Cu-C and Ag-C-bonded isomers. In contrast to the previously described copper and silver alumanyl derivatives, [(NON)Al(O₂C)M(Pt-Bu₃)] (M = Cu or Ag; NON = 4,5-bis(2,6-di-isopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), which were prone to facile CO extrusion and formation of carbonate derivatives, the NHC-supported dioxocarbene species, [(NHC^iPr)M(CO₂)Al(SiN^Dipp)] (M = Cu, Ag, Au), are all stable at room and moderately elevated temperatures. The stabilising role of the NHC co-ligand was, thus, assessed by preparation of the t-Bu₃P adducted copper-alumanyl, [(t-Bu₃P)CuAl(SiN^Dipp)]. Treatment of this latter compound, which was also structurally characterised by X-ray analysis, with both N,N'-di-isopropylcarbodiimide and CO₂ again provided smooth heteroallene insertion and formation of the relevant Cu-C-bonded products. Although both compounds were quite stable at room temperature, heating of [(t-Bu₃P)Cu(CO₂)Al(SiN^Dipp)] at 60 °C induced elimination of CO and formation of the analogous carbonate, [(t-Bu₃P)Cu(OCO₂)Al(SiN^Dipp)], which was identified by ¹³C and ³¹P NMR spectroscopy. Reflective of the more reliable nucleophilic behaviour of the gold centres in these group 11 alumanyls, computational (QTAIM and NBO) analysis highlighted a lower level of covalency of the Al-Au linkage in comparison to the analogous Al-Cu and Al-Ag interactions. Although substitution of the co-ligand significantly perturbs the charge distribution across the Cu-Al bond of [LCuAl(SiN^Dipp)] (L = NHC^iPr or t-Bu₃P), only a negligible difference is observed between the phosphine-coordinated copper systems derived from either the [SiN^Dipp]- or (NON)-based alumanyl ligands. Computational mapping of the reaction profiles arising from treatment of the various group 11 alumanyls with N,N'-di-isopropylcarbodiimide indicates that the observed formation of the Cu-N and Ag-N bound isomers do not provide the thermodynamic reaction outcome. In contrast, examination of the CO₂-derived reactions, and their potential toward CO extrusion and subsequent carbonate formation, implies that the identity of the co-ligand exerts a greater influence on this aspect of reactivity than the architecture of the diamidoalumanyl anion.

A Tetraorganyl-Alumaborane with An Al-B σ-Bond and Two Adjacent Lewis-Acidic Centers

A tetraorganyl-alumaborane (3) that contains an Al-B bond and twisted Al and B planes was synthesized and structurally characterized. UV-vis absorption spectroscopy, electrochemical measurement, and DFT calculations were employed to reveal the electronic properties of 3. The reactivity of 3 toward DMSO and CO was studied to demonstrate its deoxygenating abilities. On the basis of the results of the DFT calculations, a detailed reaction mechanism was developed, which highlighted the important role of the distinct Lewis acidity of the group-13 elements Al and B in 3.

Rubidium and caesium aluminyls: synthesis, structures and reactivity in C-H bond activation of benzene

Expanding knowledge of low valent aluminium chemistry, rubidium and caesium aluminyls are reported to complete the group 1 (Li-Cs) set of metal aluminyls. Both compounds crystallize as a contacted dimeric pair supported by M⋯π(arene) interactions with a pronounced twist between aluminyl units. Density functional theory calculations show symmetrical bonding between the M and Al atoms, with an Al centred lone-pair donating into vacant Rb and Cs orbitals. Interestingly, despite their structural similarity the Cs aluminyl enables C-H bond activation of benzene, but not the Rb aluminyl reflecting the importance of the alkali metal in these heterobimetallic systems.