NHC aluminum chemistry on the rise

This perspective highlights recent developments of the use of N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (cAACs) in alane and aluminum organyl chemistry. Especially in the last few years this flourishing research field led to some remarkable discoveries including various substitution patterns at the central aluminum atom, different oxidation states, neutral and charged compounds with varying coordination numbers and unique reactivities. Thereby NHCs play a vital role in the stabilization of these otherwise highly reactive compounds, which would not be realizable without the use of this intriguing class of ligands. Nevertheless, main group hydrides and especially NHC ligated alanes also tend to undergo NHC decomposition reactions, which are part of ongoing research and provide important information for NHC research in general.

NHC-ligated indenyl- and fluorenyl-substituted Alanes and Gallanes: synthons towards indenyl- and fluorenyl-bridged (AlC)n-heterocycles (n = 2,3)

Indenyl-(Ind) and fluorenyl-(Fl) substituted NHC-stabilized alanes and gallanes (NHC)·EH2R 1-12 (NHC = IiPrMe, IiPr, IMeMe; E = Al, Ga; R = Ind, Fl) were prepared via reaction of the corresponding NHC-iodoalanes and -gallanes with LiInd and LiFl, respectively. Analogously, the alane adducts with two Ind/Fl substituents (NHC)·AlHR213-18 (NHC = IiPrMe, IiPr, IMeMe; R = Ind, Fl) were obtained by using two equivalents of LiInd/LiFl. Elimination of indene and fluorene was induced thermally affording unusual dimeric and trimeric NHC-alane adducts {(NHC)·AlH2}2-μ-Fl 19-20 and {(NHC)·AlH-μ-R}n21-23 (R = Ind, Fl; n = 2, 3) with bridging indenyl and fluorenyl ligands.

Controlling Oxidative Addition and Reductive Elimination at Tin(I) via Hemi-Lability

We report on the synthesis of a distannyne supported by a pincer ligand bearing pendant amine donors that is capable of reversibly activating E-H bonds at one or both of the tin centres through dissociation of the hemi-labile N-Sn donor/acceptor interactions. This chemistry can be exploited to sequentially (and reversibly) assemble mixed-valence chains of tin atoms of the type ArSn{Sn(Ar)H}_n SnAr (n=1, 2). The experimentally observed (decreasing) propensity towards chain growth with increasing chain length can be rationalized both thermodynamically and kinetically by the electron- withdrawing properties of the -Sn(Ar)H- backbone units generated via oxidative addition.

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.

Ligated aluminum cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2)

A wide range of low oxidation state aluminum-containing cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2), were produced via reactions between aluminum cluster anions and hexamethyldisilazane (HMDS). These clusters were identified by mass spectrometry, with a few of them (n = 4, 6, and 7) further characterized by a synergy of anion photoelectron spectroscopy and density functional theory (DFT) based calculations. As compared to a previously reported method which reacts anionic aluminum hydrides with ligands, the direct reactions between aluminum cluster anions and ligands promise a more general synthetic scheme for preparing low oxidation state, ligated aluminum clusters over a large size range. Computations revealed structures in which a methyl-group of the ligand migrated onto the surface of the metal cluster, thereby resulting in "two metal-atom" insertion between Si-CH3 bond.

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.

Donor-Stabilized Monocarba-Bridged Bis(cyclopentadienyl)alanes

Five monocarba-bridged bis(cyclopentadienyl)aluminum halide NHC and thione complexes and one monocarba-bridged bis(cyclopentadienyl)phosphanylalane NHC complex are reported. The former were synthesized by transmetalation of a C[1]magnesocenophane with the corresponding aluminum(III) chloride and aluminum(III) bromide donor adducts. The phosphanylalane complex was obtained by a subsequent functionalization of the corresponding bromoalane with lithium diphenylphosphide. All complexes were characterized in solution by multinuclear NMR spectroscopy and in the solid state by single crystal X-ray diffraction. Bonding energies of the NHC and thione ligands to the aluminum centres were estimated by DFT calculations.

Shedding Light on the Diverse Reactivity of NacNacAl with N-Heterocycles

The aluminum(I) compound NacNacAl (NacNac=[ArNC(Me)CHC(Me)NAr]^- , Ar=2,6-iPr₂ C₆ H₃ , 1) shows diverse and substrate-controlled reactivity in reactions with N-heterocycles. 4-Dimethylaminopyridine (DMAP), a basic substrate in which the 4-position is blocked, induces rearrangement of NacNacAl by shifting a hydrogen atom from the methyl group of the NacNac backbone to the aluminum center. In contrast, C-H activation of the methyl group of 4-picoline takes place to produce a species with a reactive terminal methylene. Reaction of 1 with 3,5-lutidine results in the first example of an uncatalyzed, room-temperature cleavage of an sp² C-H bond (in the 4-position) by an Al^I species. Another reactivity mode was observed for quinoline, which undergoes 2,2'-coupling. Finally, the reaction of 1 with phthalazine produces the product of N-N bond cleavage.

Ring expansion of alumoles with organic azides: selective formation of six-membered aluminum-nitrogen heterocycles

A cyclopentadienyl-substituted alumole is shown to undergo ring expansions with a series of organic azides, affording 1,2-azaaluminabenzenes either with or without an azo group.

Aside from simple Lewis acid–base chemistry, the reaction chemistry of aluminacyclopentadienes, which are commonly referred to as aluminoles or simply alumoles, remains relatively underdeveloped. To date, few attempts to extend their inherent insertion and cycloaddition reactivity to the construction of stable aluminum-containing heterocycles have been reported. Herein, we demonstrate the selective ring expansion of a cyclopentadienyl-substituted alumole with a series of organic azides to form unsaturated six-membered AlN heterocycles. Depending on the substituent on the azide, the reaction proceeds either with or without loss of dinitrogen, leading to incorporation of only the “NR” unit of the azide or the entire azo substituent into the periphery of the heterocycle. A deeper understanding of these ring expansion reactions is reached through computational studies, illustrating deviations in the mechanism as a function of the organic azide employed.

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.

Sequential Oxidation and C-H Bond Activation at a Gallium(I) Center

In situ oxidation of the Ga^I compound NacNacGa by either N₂ O or pyridine oxide results in the generation of a labile monomeric oxide, NacNacGa(O), which can easily cleave the C-H bonds of aliphatic and aromatic substrates featuring good donor sites. The products of this reaction are gallium organyl hydroxides. DFT calculations show that these reactions start with the formation of NacNac-Ga(O)(L) adducts, the oxo ligand of which can easily abstract protons from nearby C-H bonds, even for sp² -hybridized carbon centers. Aliphatic amines do not enter this reaction for kinetic reasons, presumably because of the unfavorable sterics.

Calcium-Catalyzed Arene C-H Bond Activation by Low-Valent AlI

The low‐valent ß‐diketiminate complex (^DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(^DIPPBDI)CaH]₂ at 20 °C led to (^DIPPBDI)Al(Ph)H (^DIPPBDI=CH[C(CH₃)N‐DIPP]₂, DIPP=2,6‐diisopropylphenyl). Similar Ca‐catalyzed C−H bond activation is demonstrated for toluene or p‐xylene. For toluene a remarkable selectivity for meta‐functionalization has been observed. Reaction of (^DIPPBDI)Al(m‐tolyl)H with I₂ gave m‐tolyl iodide, H₂ and (^DIPPBDI)AlI₂ which was recycled to (^DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (^DIPPBDI)Al(H)M(^DIPPBDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (^DIPPBDI)Al(H)Ca(^DIPPBDI). This is in equilibrium with the hydride bridged complex (^DIPPBDI)Al(μ‐H)Ca(^DIPPBDI) which shows strongly increased electron density at Al. The combination of Ca‐arene bonding and a highly nucleophilic Al center are key to facile C−H bond activation.

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.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

NormalandabnormalNHC coordination in cationic hydride iodide complexes of aluminium

Sterically demanding NHC aluminium hydride iodide complexes react with one equivalent of NHC to cationic mixednormal–abnormalNHC Al^IIIcomplexes.

Evidence for single metal two electron oxidative addition and reductive elimination at uranium

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.

Low oxidation state aluminum-containing cluster anions: LAlH- and LAln- (n = 2-4, L = N[Si(Me)3]2)

Several low oxidation state aluminum-containing cluster anions, LAlH^- and LAl_n^- (n = 2-4, L = N[Si(Me)₃]₂), were produced via reactions between aluminum hydride cluster anions, Al_xH_y^-, and hexamethyldisilazane (HMDS). These clusters were characterized by mass spectrometry, anion photoelectron spectroscopy, and density functional theory (DFT) based calculations. Agreement between the experimental and theoretical vertical detachment energies (VDEs) and adiabatic detachment energies (ADEs) validated the computed geometrical structures. Reactions between aluminum hydride cluster anions and ligands promise to be a new synthetic scheme for low oxidation state, ligated aluminum clusters.

Reaction of sterically encumbered phenols, TEMPO-H, and organocarbonyl insertion reactions with L-AlH2 (L = HC(MeCNDipp)2, Dipp = 2,6-diisopropylphenyl)

A bulky aluminum dihydride reacts with R–OH and organocarbonyls to give a variety of products; including OC insertion into the Al–H bond.