Organoelement Compounds with Al?Al, Ga?Ga, and In?In Bonds

Reductive elimination of [AlH2]+ from a cationic Ga-Al dihydride

Oxidative addition of TMEDA-supported [AlH₂]⁺ to [{BDI}Ga] (BDI = {HC(C(CH₃)N(2,6-iPr₂-C₆H₃))₂}) provides [{BDI}Ga(H)-Al(H)(tmeda)][B(C₆H₃-3,5-Me₂)₄] (TMEDA = N,N,N'N'-tetramethylethylenediamine) with a covalent metal-metal bond. The reaction is readily reversed by substituting TMEDA for an N-heterocyclic carbene or dissolving in THF.

Reactivity of mono- and divalent aluminium compounds towards group 15 nanoparticles

Herein, we present a novel approach towards organometallic group 13/15-compounds, i.e. the reaction of nanoparticular arsenic and antimony with low-valent aluminium species. The reaction of the two-electron reducing agent [AlCp*]₄ (Cp* = C₅Me₅) with arsenic nanoparticles gave rise to a mixture of two unprecedented deca- and dodecanuclear Al-As clusters. In contrast, the analogous transformation with nanoparticular antimony yielded the already known Al-Sb compound [(AlCp*)₃Sb₂]. Additionally, two different dialanes [AlCp*X]₂ (X = Br, I) were employed as one-electron reducing agents, forming calix like coordination compounds upon reaction with nano arsenic. The isolated species significantly enlarge the accessible structural variety of molecular group 13/15 compounds, highlighting the exceptional utility and reactivity of nanoscale group 15 precursors.

Reversible Reductive Elimination in Aluminum(II) Dihydrides

Oxidative addition and reductive elimination are defining reactions of transition‐metal organometallic chemistry. In main‐group chemistry, oxidative addition is now well‐established but reductive elimination reactions are not yet general in the same way. Herein, we report dihydrodialanes supported by amidophosphine ligands. The ligand serves as a stereochemical reporter for reversible reductive elimination/oxidative addition chemistry involving Al^I and Al^III intermediates.

Recent advances in low oxidation state aluminium chemistry

The synthesis and isolation of novel low oxidation state aluminium (Al) compounds has seen relatively slow progress over the 30 years since such species were first isolated. This is largely due to the significant challenges in isolating these thermodynamically unstable compounds. Despite challenges with isolation, their reactivity has been widely explored and they have been utilized in a wide range of processes including the activation of strong chemicals bonds, as ligands to transition metals and in the formation of heterobimetallic M-M compounds. As such, attempts to isolate novel low oxidation state Al compounds have continued in earnest and in the last few years huge advances have been made. In this review we highlight the remarkable recent developments in the low oxidation state chemistry of aluminium and discuss the variety of new reactions these compounds have made possible.

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.

Di-tert-butyldiphosphatetrahedrane: Catalytic Synthesis of the Elusive Phosphaalkyne Dimer

While tetrahedranes as a family are scarce, neutral heteroatomic species are all but unknown, with the only reported example being AsP₃. Herein, we describe the isolation of a neutral heteroatomic X₂Y₂ molecular tetrahedron (X, Y=p‐block elements), which also is the long‐sought‐after free phosphaalkyne dimer. Di‐tert‐butyldiphosphatetrahedrane, (tBuCP)₂, is formed from the monomer tBuCP in a nickel‐catalyzed dimerization reaction using [(NHC)Ni(CO)₃] (NHC=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene (IMes) and 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene (IPr)). Single‐crystal X‐ray structure determination of a silver(I) complex confirms the structure of (tBuCP)₂. The influence of the N‐heterocyclic carbene ligand on the catalytic reaction was investigated, and a mechanism was elucidated using a combination of synthetic and kinetic studies and quantum chemical calculations.

Monomeric Cp3tAl(i): synthesis, reactivity, and the concept of valence isomerism

With the isolation of Cp3tAl (1), the first monomeric Cp-based Al(i) species could be realized in a pure form via a three-step reaction sequence (salt elimination/adduct formation/adduct cleavage) starting from readily available AlBr3. Due to its monomeric structure, reactions involving 1 were found to proceed more selectively, faster, and under milder conditions than for tetrameric (Cp*Al)4. Thus, 1 readily formed simple Lewis acid-base adducts with tBuAlCl2 (6) and AlBr3 (7), reactions that before have always been interfered with by the presence of aluminum halide bonds. In addition, the 2 : 1 reaction of 1 with AlBr3 enabled the realization of the very rare trialuminum adduct species 8. 1 also reacted rapidly with N2O and PhN3 at room temperature to afford Al3O3 and Al2N2 heterocycles 9 and 10, respectively. With the structural characterization of products 4 and 5, the reaction of monovalent 1 with Cp3tAlBr2 (2) provided the first experimental evidence for the concept of valence isomerism between dialanes and their Al(i)/Al(iii) Lewis adducts.

Dialumination of unsaturated species with a reactive bis(cyclopentadienyl) dialane

A new cyclopentadienyl-containing dialane is prepared and found to undergo dialumination reactions with unsaturated species.

Stability and donor-acceptor bond in dinuclear organometallics CpM1-M2Cl3 (M1, M2 = B, Al, Ga, In; Cp = η 5-C5H5)

The geometries and stabilities of the dinuclear organometallics CpM₁-M₂Cl₃ (M₁, M₂ = B, Al, Ga, In; Cp = η ⁵-C₅H₅) have been investigated by density functional theory (DFT) at M06 L/6-311G(d, p) levels. The nature of the donor-acceptor M₁ → M₂ bond was also studied based on the atoms in molecules (AIM) theory, energy decomposition analysis (EDA) and natural bond orbital (NBO) analysis. The results show that the electronegativity of the M atom determines the stability and covalent character of the dinuclear organometallics CpM₁-M₂Cl₃. The compounds in which the M with larger electronegativity acts as the donor are more stable than in those in which it acts as the acceptor in the donor-acceptor bond, and the donor-acceptor bond has more covalent characteristics. The strength and polarity of the M₁ → M₂ donor-acceptor bond is determined by the periodicity of the M atom. When the period number of the M₁ atom is smaller than that of M₂, the strength of the M₁ → M₂ bond is larger than that of the M₂ → M₁ bond. For homonuclear dinuclear organometallics, the polarity of the M-M bond increases with increasing atomic number of the M atom. For heteronuclear complexes, the polarity of the M₁-M₂ bond for a given M₁ also increases in the sequence of M₂ = B, Al, Ga, and In. Graphic abstract Molecular graph and electron location function isosurfaces map of CpM 1 -M 2 Cl 3(small red spheres represent bond critical points).

A Stable Neutral Compound with an Aluminum-Aluminum Double Bond

Homodinuclear multiple-bonded neutral Al compounds, aluminum analogues of alkenes, have been a notoriously difficult synthetic target over the past several decades. Herein, we report the isolation of a stable neutral compound featuring an Al═Al double bond stabilized by N-heterocyclic carbenes. X-ray crystallographic and spectroscopic analyses demonstrate that the dialuminum entity possesses trans-planar geometry and an Al-Al bond length of 2.3943(16) Å, which is the shortest distance reported for a molecular dialuminum species. This new species reacts with ethylene and phenyl acetylene to give the [2+2] cycloaddition products. The structure and bonding were also investigated by detailed density functional theory calculations. These results clearly demonstrate the presence of an Al═Al double bond in this molecule.

An open route to asymmetric substituted Al–Al bonds using Al(i)- and Al(iii)-precursors

An applicable synthetic pathway for asymmetric Al(ii)–Al(ii) compounds was developed using Al(iii) and Al(i) precursors.

Influences of the substituents on the M–M bonding in Cp4Al4 and Cp2M2X2 (M = B, Al, Ga; Cp = C5H5, X = halogen)

The different stabilities of Cp*4Al₄ and Cp₄Al₄ (Cp = C₅H₅) have been explained by theoretical methods.

Reversal of Stability on Metalation of Pentagonal-Bipyramidal (1-MB6H72-,1-M-2-CB5H71-,and 1-M-2,4-C2B4H7) and Icosahedral (1-MB11H122-,1-M-2-CB10H121-,and 1-M-2,4-C2B9H12) Boranes (M = Al, Ga, In, and Tl): Energetics of Condensation and Relationship to Binuclear Metallocenes

The usual assumption of the extra stability of icosahedral boranes (2) over pentagonal-bipyramidal boranes (1) is reversed by substitution of a vertex by a group 13 metal. This preference is a result of the geometrical requirements for optimum overlap between the five-membered face of the ligand and the metal fragment. Isodesmic equations calculated at the B3LYP/LANL2DZ level indicate that the extra stability of 1-M-2,4-C(2)B(4)H(7) varies from 14.44 kcal/mol (M = Al) to 15.30 kcal/mol (M = Tl). Similarly, M(2,4-C(2)B(4)H(6))(2)(1-) is more stable than M(2,4-C(2)B(9)H(11))(2)(1-) by 9.26 kcal/mol (M = Al) and by 6.75 kcal/mol (M = Tl). The preference for (MC(2)B(4)H(6))(2) over (MC(2)B(9)H(11))(2) at the same level is 30.54 kcal/mol (M = Al), 33.16 kcal/ mol (M = Ga) and 37.77 kcal/mol (M = In). The metal-metal bonding here is comparable to those in CpZn-ZnCp and H(2)M-MH(2) (M= Al, Ga, and In).

Metalloid Al- and Ga-clusters: a novel dimension in organometallic chemistry linking the molecular and the solid-state areas?

Formation and fragmentation of metal-metal bonds on the way between stable metal compounds in which the metal atoms are oxidised (e.g. isolated species in solution or metal salts in bulk) and the bulk metal are the fundamental steps to understand this process in which formation and chemical behaviour of metalloid Al and Ga clusters as intermediates are essential. Many examples of metalloid Al and Ga clusters show that their formation reflects a high degree of complexity like that of the simple seeming formation of the bulk metal itself: starting from metastable Al(i) and Ga(i) solutions containing small molecular entities, metalloid clusters grow during many self-organization steps including aggregation as well as irreversible redox cascades. This novel class of clusters seems to open a new dimension in chemistry between the molecular and the solid-state area, because, for the first time, it is shown that under well selected conditions definite molecular species, i.e. metalloid clusters, grow via the formation of additional metal-metal bonds and that the solid metal represents the final step.

From group 13–group 13 donor–acceptor bonds to triple-decker cations

Donor-acceptor bonding between group 13 elements seems counter-intuitive because one normally thinks of e.g. boron and aluminium compounds as classical Lewis acids. Indeed, many such compounds have achieved industrial prominence in this regard. Recently, however, it has become possible to stabilize these and other group 13 elements in the +1 oxidation state as opposed to the archetypical +3 oxidation state. Moreover, it turns out that in the +1 oxidation state these species are excellent donors--hence the formation of these unprecedented donor-acceptor bonds. The discovery of such bonds has led, albeit indirectly, to the development of triple-decker main group cations. This aspect is also covered in the review.

Structure and bonding in the aluminum radical species Al x NH(3), HAlNH(2), HAlNH(2) x NH(3), and Al(NH(2))(2) studied by means of matrix IR spectroscopy and quantum chemical calculations

Experimental matrix IR spectra in alliance with extensive quantum chemical calculations provide a framework for the detailed evaluation of the structures and electronic properties of the doublet species Al x NH(3), Al(NH(3))(2), HAlNH(2), HAlNH(2) x NH(3), and Al(NH(2))(2). These species were the products of the reaction of Al atoms with NH(3) in an Ar matrix. While the two species Al x NH(3) and HAlNH(2) were already sighted in previous experiments, the results described herein lead to the first identification and characterization of HAlNH(2) x NH(3) and Al(NH(2))(2), the products of the reaction of Al atoms with two NH(3) molecules. The results allow a detailed reaction scheme leading to all the product species to be established. The unpaired electron in each of the species Al x NH(3), Al(NH(3))(2), HAlNH(2), HAlNH(2) x NH(3), and Al(NH(2))(2) is located near the Al atom, but there is a significant degree of delocalization, especially in Al(NH(2))(2), due to pi bonding interactions. The consequences for the barrier to pyramidalization at the N-atom are discussed.