Alkali metal influences in aluminyl complexes

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.

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.

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.

Three Oxidative Addition Routes of Alkali Metal Aluminyls to Dihydridoaluminates and Reactivity with CO2

Three distinct routes are reported to the soluble, dihydridoaluminate compounds, AM[Al(NONDipp )(H)2 ] (AM=Li, Na, K, Rb, Cs; [NONDipp ]2- =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ) starting from the alkali metal aluminyls, AM[Al(NONDipp )]. Direct H2 hydrogenation of the heavier analogues (AM=Rb, Cs) produced the first examples of structurally characterized rubidium and caesium dihydridoaluminates, although harsh conditions were required for complete conversion. Using 1,4-cyclohexadiene (1,4-CHD) as an alternative hydrogen source in transfer hydrogenation reactions provided a lower energy pathway to the full series of products for AM=Li-Cs. A further moderation in conditions was noted for the thermal decomposition of the (silyl)(hydrido)aluminates, AM[Al(NONDipp )(H)(SiH2 Ph)]. Probing the reaction of Cs[Al(NONDipp )] with 1,4-CHD provided access to a novel inverse sandwich complex, [{Cs(Et2 O)}2 {Al(NONDipp )(H)}2 (C6 H6 )], containing the 1,4-dialuminated [C6 H6 ]2- dianion and representing the first time that an intermediate in the commonly utilized oxidation process of 1,4-CHD to benzene has been trapped. The synthetic utility of the newly installed Al-H bonds has been demonstrated by their ability to reduce CO2 under mild conditions to form the bis-formate AM[Al(NONDipp )(O2 CH)2 ] compounds, which exhibit a diverse series of eyecatching bimetallacyclic structures.

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.

Controlled reductive C-C coupling of isocyanides promoted by an aluminyl anion

We report the reaction of the potassium aluminyl, K[Al(NON)] ([NON]^2- = [O(SiMe₂NDipp)₂]^2-, Dipp = 2,6-iPr₂C₆H₃) with a series of isocyanide substrates (R-NC). In the case of tBu-NC, degradation of the isocyanide was observed generating an isomeric mixture of the corresponding aluminium cyanido-κC and -κN compounds, K[Al(NON)(H)(CN)]/K[Al(NON)(H)(NC)]. The reaction with 2,6-dimethylphenyl isocyanide (Dmp-NC), gave a C₃-homologation product, which in addition to C-C bond formation showed dearomatisation of one of the aromatic substituents. In contrast, using adamantyl isocyanide Ad-NC allowed both the C₂- and C₃-homologation products to be isolated, allowing a degree of control to be exercised over the chain growth process. These data also show that the reaction proceeds through a stepwise addition, supported in this study by the synthesis of the mixed [(Ad-NC)₂(Dmp-NC)]^2- product. Computational analysis of the bonding within the homologised products confirm a high degree of multiple bond character in the exocyclic ketenimine units of the C₂- and C₃-products. In addition, the mechanism of chain growth was investigated, identifying different possible pathways leading to the observed products, and highlighting the importance of the potassium cation in formation of the initial C₂-chain.

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.

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.

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.