CO2 reduction using aluminum hydride: Generation of in-situ frustrated Lewis pairs and small molecule activation therein

CO2 reduction is appealing for the long-term production of high-value fuels and chemicals. Herein, using density functional theory (DFT) based calculations, we study the CO2 reduction pathway to formic acid using aluminum hydride and phosphine derivatives. Our primary focus is on aluminum hydride derivatives, aimed at improving the efficiency of the CO2 reduction process. Substituents with σ-donating properties at the aluminum center are discovered to lower the activation barriers. We demonstrate how di-tert-butylphosphine oxide (LB-O)/di-tert-butylphosphine sulfide (LB-S)/di-tert-butylphosphanimine (LB-N) work together with aluminum hydride to facilitate CO2 reduction process and generate in-situ frustrated Lewis pairs (FLPs), such as FLP-O, FLP-S, and FLP-N. The activation strain model (ASM) analysis reveals the significance of strain energy in determining activation barriers. EDA-NOCV and PIO analyses elucidate the orbital interactions at the corresponding transition states. Furthermore, the study delves into the activation of various small molecules, such as dihydrogen, acetylene, ethylene, carbon dioxide, nitrous oxide, and acetonitrile, using those in-situ generated FLPs. The study highlights the low activation barriers and emphasizes the potential for small molecule activation in this context.

Synthesis and Reactivity of Bis-tris(pyrazolyl)borate Lanthanide/Aluminum Heterobimetallic Trihydride Complexes

Molecular heterobimetallic hydride complexes of lanthanide (Ln) and main-group (MG) metals exhibit chemical properties unique from their monometallic counterparts and are highly reactive species, making their synthesis and isolation challenging. Herein, molecular Ln/Al heterobimetallic trihydrides [Ln(Tp)2(μ-H)2Al(H)(N″)] [2-Ln; Ln = Y, Sm, Dy, Yb; Tp = hydrotris(1-pyrazolyl)borate; N″ = N(SiMe3)2] have been synthesized by facile insertion of aminoalane [Me3N·AlH3] into the Ln-N amide bonds of [Ln(Tp)2(N″)] (1-Ln). Thus, this is a simple synthetic strategy to access a range of Ln/Al hydrides. Reactivity studies demonstrate that 2-Ln is a heterobimetallic hydride, with evidence for the cooperative nature of 2-Ln shown by the catalytic amine-borane dehydrocoupling under ambient conditions in contrast to its monomeric counterparts.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Aluminum-Hydride-Catalyzed Hydroboration of Carbon Dioxide

This study describes the first use of a bis(phosphoranyl)methanido aluminum hydride, [ClC(PPh₂NMes)₂AlH₂] (2, Mes = Me₃C₆H₂), for the catalytic hydroboration of CO₂. Complex 2 was synthesized by the reaction of a lithium carbenoid [Li(Cl)C(PPh₂NMes)₂] with 2 equiv of AlH₃·NEtMe₂ in toluene at -78 °C. 2 (10 mol %) was able to catalyze the reduction of CO₂ with HBpin in C₆D₆ at 110 °C for 2 days to afford a mixture of methoxyborane [MeOBpin] (3a; yield: 78%, TOF: 0.16 h^-1) and bis(boryl)oxide [pinBOBpin] (3b). When more potent [BH₃·SMe₂] was used instead of HBpin, the catalytic reaction was extremely pure, resulting in the formation of trimethyl borate [B(OMe)₃] (3e) [catalytic loading: 1 mol % (10 mol %); reaction time: 60 min (5 min); yield: 97.6% (>99%); TOF: 292.8 h^-1 (356.4 h^-1)] and B₂O₃ (3f). Mechanistic studies show that the Al-H bond in complex 2 activated CO₂ to form [ClC(PPh₂NMes)₂Al(H){OC(O)H}] (4), which was subsequently reacted with BH₃·SMe₂ to form 3e and 3f, along with the regeneration of complex 2. Complex 2 also shows good catalytic activity toward the hydroboration of carbonyl, nitrile, and alkyne derivatives.

Designable Aluminum Molecular Rings: Ring Expansion and Ligand Functionalization

Presented herein are the Al^III molecular ring architectures from 8-ring to 16-ring. Although there are numerous reported cyclic coordination compounds based on transition metals, gallium, or lanthanides, the Al versions are less developed due to the fast hydrolysis nature of Al³⁺ ion. With the assistant of monohydric alcohols, a series of atomic precisely Al molecular rings based on benzoates are synthesized. The ring expansion of these Al-rings from 8-ring to 16-ring is related to the monohydric alcohol structure-directing agents. Moreover, the organic ligands on the Al-rings can be modified by using various benzoate derivatives, which lead to tunable surface properties of the Al-rings from hydrophilicity to ultra-hydrophobicity. Importantly, 4-aminobenzoic acid bridged 16-ring is soluble in organic solvents and exhibits high solution stability revealed by mass spectroscopy. Ligand substitution also can be performed between these Al-rings, which reveal controllable ligand functionalization of these Al-rings.

Reaction of Dialumane Incorporating Bulky Eind Groups with Pyridines

The reaction of the bulky Eind-based dialumane, (Eind)HAl(μ-H)2AlH(Eind) (1) (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), with pyridines is described. When 1 was treated with pyridine (Py) in toluene, the Py adduct of aryldihydroalumane, Py→AlH2(Eind) (2), was initially formed. Then, the hydroalumination of Py took place to yield the Py-bound aryl(1,4-dihydropyrid-1-yl)hydroalumane, Py→AlH(1,4-dihydropyrid-1-yl)(Eind) (3). A similar reaction with a stronger Lewis base, 4-pyrrolidinopyridine (PPy), produced the stable PPy adduct, PPy→AlH2(Eind) (4). The resulting organoaluminum compounds have been fully characterized by NMR spectroscopy as well as X-ray crystallography. The reaction mechanism from 1 to 3 via 2 has been examined by deuterium labeling experiments using (Eind)DAl(μ-D)2AlD(Eind) (1-d4).

Lithium-Aluminate-Catalyzed Hydrophosphination Applications

Synthesized, isolated, and characterized by X‐ray crystallography and NMR spectroscopic studies, lithium phosphidoaluminate iBu₃AlPPh₂Li(THF)₃ has been tested as a catalyst for hydrophosphination of alkynes, alkenes, and carbodiimides. Based on the collective evidence of stoichiometric reactions, NMR monitoring studies, kinetic analysis, and DFT calculations, a mechanism involving deprotonation, alkyne insertion, and protonolysis is proposed for the [iBu₃AlHLi]₂ aluminate catalyzed hydrophosphination of alkynes with diphenylphosphine. This study enhances further the development of transition‐metal‐free, atom‐economical homogeneous catalysis using common sustainable main‐group metals.

NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis

The catalytic dehydrocoupling of amine–boranes has recently received a great deal of attention due to its potential in hydrogen storage applications. The use of aluminum catalysts for this transformation would provide an additional cost-effective and sustainable approach towards the hydrogen economy. Herein, we report the use of both N-heterocyclic imine (NHI)- and carbene (NHC)-supported Al(III) hydrides and their role in the catalytic dehydrocoupling of Me2NHBH3. Differences in the σ-donating ability of the ligand class resulted in a more stable catalyst for NHI-Al(III) hydrides, whereas a deactivation pathway was found in the case of NHC-Al(III) hydrides.

Dehydrogenation of Amine-Boranes Using p-Block Compounds

Amine-boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition- and main-group metal catalysts for the dehydrogenation of amine-boranes. During the past decade, new systems started to emerge solely based on p-block elements that promote the dehydrogenation of amine-boranes through hydrogen-transfer reactions, polymerization initiation, and main-group catalysis. In this review, we highlight the development of these p-block based systems for stoichiometric and catalytic amine-borane dehydrogenation and discuss the underlying mechanisms.

Cationic Complexes of Boron and Aluminum: An Early 21st Century Viewpoint

Boron and aluminum are lighter Group 13 elements, found in daily life commodities, and considered environmentally benign. Nevertheless, they markedly differ in their elemental properties (e.g., metal character, atomic radius). The use of Lewis acidic complexes of boron and aluminum for methods of bond activation and catalysis (e.g., hydrogenation of unsaturated substrates, polymerization of olefins and epoxides) is quickly expanding. The introduction of cationic charge may boost the metalloid-centered Lewis acidity and allow for its fine-tuning particularly with regard to preference for "hard" or "soft" Lewis bases (i.e., substrates). Especially the isolation of low-coordinate cations (number of ligand atoms smaller than four) demands elaborate techniques of thermodynamic and kinetic stabilization (i.e., electronic saturation and steric shielding) by a ligand system. Furthermore, the properties of the solvent and the counteranion must be considered with care. Here, selected examples of boron and aluminum cations are described.

Comparing Neutral (Monometallic) and Anionic (Bimetallic) Aluminum Complexes in Hydroboration Catalysis: Influences of Lithium Cooperation and Ligand Set

Bimetallic lithium aluminates and neutral aluminum counterparts are compared as catalysts in hydroboration reactions with aldehydes, ketones, imines and alkynes. Possessing Li–Al cooperativity, ate catalysts are found to be generally superior. Catalytic activity is also influenced by the ligand set, alkyl and/or amido. Devoid of an Al−H bond, iBu₂Al(TMP) operates as a masked hydride reducing benzophenone through a β‐Η transfer process. This catalyst library therefore provides an entry point into the future design of Al catalysts targeting substrate specific transformations.

Dehydrocoupling routes to element–element bonds catalysed by main group compounds

This Tutorial Review focuses on recent applications of main group compounds in the catalytic synthesis of heteronuclear element–element bonds within the p-block.

Use of crown ethers to isolate intermediates in ammonia-borane dehydrocoupling reactions

The presence of 18-crown-6 in the Lewis acid-promoted dehydrocoupling reaction of ammonia borane permits isolation of [(THF)BH₂NH₃]⁺ cation.

Cationic aluminum hydride complexes: reactions of carbene–alane adducts with trityl-borate

Reaction of (Idipp)AlH₃ with [Ph₃C][B(C₆F₅)₄] in toluene affords the dimeric aluminum dication [((Idipp)AlH(μ-H))₂][B(C₆F₅)₄]₂ with the corresponding reaction of (IBn)AlH₃ gives [(IBn)₂AlH][B(C₆F₅)₄]₂.

Stoichiometric and catalytic Si–N bond formation using the p-block base Al(NMe2)3

The aluminium reagent Al(NMe₂)₃ acts as a stoichiometric or catalytic reagent in dehydrogenic Si–N bond formation using amines and silanes. The observed catalytic rate law suggests a mechanism involving the silane component in the deprotonation of the amine.

Multiple deprotonation of primary aromatic diamines by LiAlH4

Reaction of LiAlH₄with 1,2-phenylenediamine (1H₄) in THF gives [{Al(1H₂)}₂{Al(1H)₂}₂][Li(THF)₂]₄, containing the largest aluminate of its type so far reported.

TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases: lithium-mediated alumination or lithiation–alkylaluminium-trapping reagents?

Surprisingly lithium TMP-aluminate reagents are not capable of directly aluminating anisole as previously thought but operateviasequential lithiation–alkylaluminium trapping.