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.

Publisher Correction: Synthesis, structure and reaction chemistry of a nucleophilic aluminyl anion.

In Fig. 1 of this Letter, the hydrogen (H) atoms attached to each of the two nitrogen (N) atoms in the chemical structure of (NON)H2 were inadvertently missing. The original figure has been corrected online.

Synthesis, structure and reaction chemistry of a nucleophilic aluminyl anion.

The reactivity of aluminium compounds is dominated by their electron deficiency and consequent electrophilicity; these compounds are archetypal Lewis acids (electron-pair acceptors). The main industrial roles of aluminium, and classical methods of synthesizing aluminium-element bonds (for example, hydroalumination and metathesis), draw on the electron deficiency of species of the type AlR3 and AlCl31,2. Whereas aluminates, [AlR4]-, are well known, the idea of reversing polarity and using an aluminium reagent as the nucleophilic partner in bond-forming substitution reactions is unprecedented, owing to the fact that low-valent aluminium anions analogous to nitrogen-, carbon- and boron-centred reagents of the types [NX2]-, [CX3]- and [BX2]- are unknown3-5. Aluminium compounds in the +1 oxidation state are known, but are thermodynamically unstable with respect to disproportionation. Compounds of this type are typically oligomeric6-8, although monomeric systems that possess a metal-centred lone pair, such as Al(Nacnac)Dipp (where (Nacnac)Dipp = (NDippCR)2CH and R = t Bu, Me; Dipp = 2,6- i Pr2C6H3), have also been reported9,10. Coordination of these species, and also of (η5-C5Me5)Al, to a range of Lewis acids has been observed11-13, but their primary mode of reactivity involves facile oxidative addition to generate Al(III) species6-8,14-16. Here we report the synthesis, structure and reaction chemistry of an anionic aluminium(I) nucleophile, the dimethylxanthene-stabilized potassium aluminyl [K{Al(NON)}]2 (NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene). This species displays unprecedented reactivity in the formation of aluminium-element covalent bonds and in the C-H oxidative addition of benzene, suggesting that it could find further use in both metal-carbon and metal-metal bond-forming reactions.

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.

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, Kn [(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 .

Anionic Magnesium and Calcium Hydrides: Transforming CO into Unsaturated Disilyl Ethers.

The synthesis, characterisation and reactivity of two isostructural anionic magnesium and calcium complexes is reported. By X-ray and neutron diffraction techniques, the anionic hydrides are shown to exist as dimers, held together by a range of interactions between the two anions and two bridging potassium cations. Unlike the vast proportion of previously reported dimeric group 2 hydrides, which have hydrides that bridge two group 2 centres, here the hydrides are shown to be "terminal", but stabilised by interactions with the potassium cations. Both anionic hydrides were found to insert and couple CO under mild reaction conditions to give the corresponding group 2 cis-ethenediolate complexes. These cis-ethenediolate complexes were found to undergo salt elimination reactions with silyl chlorides, allowing access to small unsaturated disilyl ethers with a high percentage of their mass originating from the C1 source CO.

Inhibition of Alkali Metal Reduction of 1-Adamantanol by London Dispersion Effects.

A series of alkali metal 1-adamantoxide (OAd1 ) complexes of formula [M(OAd1 )(HOAd1 )2 ], where M=Li, Na or K, were synthesised by reduction of 1-adamantanol with excess of the alkali metal. The syntheses indicated that only one out of every three HOAd1 molecules was reduced. An X-ray diffraction study of the sodium derivative shows that the complex features two unreduced HOAd1 donors as well as the reduced alkoxide (OAd1 ), with the Ad1 fragments clustered together on the same side of the NaO3 plane, contrary to steric considerations. This is the first example of an alkali metal reduction of an alcohol that is inhibited from completion due to the formation of the [M(OAd1 )(HOAd1 )2 ] complexes, stabilized by London dispersion effects. NMR spectroscopic studies revealed similar structures for the lithium and potassium derivatives. Computational analyses indicate that decisive London dispersion effects in the molecular structure are a consequence of the many C-H⋅⋅⋅H-C interactions between the OAd1 groups.

Crystalline Germanium(I) and Tin(I) Centered Radical Anions.

An isostructural series of heavy Group 14 E(I) radical anions (Ge, Sn, Pb), stabilized by a bulky xanthene-based diamido ligand are reported. The radical anions were synthesised by the one-electron reduction of their corresponding E(II) precursor complexes with sodium naphthalenide in THF, yielding the radical anions as charge-separated sodium salts. The series of main group radicals have been comprehensively characterized by EPR spectroscopy, X-ray crystallography and DFT analysis, which reveal that in all cases, the spin density of the unpaired electron almost exclusively resides in a p-orbital of π symmetry located on the Group 14 center.

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.

Reductions of M{N(SiMe3)2}3 (M = V, Cr, Fe): Terminal and Bridging Low-Valent First-Row Transition Metal Hydrido Complexes and "Metallo-Transamination".

The reaction of the vanadium(III) tris(silylamide) V{N(SiMe3)2}3 with LiAlH4 in diethyl ether gives the highly unstable mixed-metal polyhydride [V(µ2-H)6[Al{N(SiMe3)2}2]3][Li(OEt2)3] (1), which was structurally characterized. Alternatively, performing the same reaction in the presence of 12-crown-4 affords a rare example of a structurally verified vanadium terminal hydride complex, [VH{N(SiMe3)2}3][Li(12-crown-4)2] (2). The corresponding deuteride 2D was also prepared using LiAlD4. In contrast, no hydride complexes were isolated by reaction of M{N(SiMe3)2}3 (M = Cr, Fe) with LiAlH4 and 12-crown-4. Instead, these reactions afforded the anionic metal(II) complexes [M{N(SiMe3)2}3][Li(12-crown-4)2] (3, M = Cr; 4, M = Fe). The reaction of the iron(III) tris(silylamide) Fe{N(SiMe3)2}3 with lithium aluminum hydride without a crown ether gives the "hydrido inverse crown" complex [Fe(µ2-H){N(SiMe3)2}2(µ2-Li)]2 (5), while treatment of the same trisamide with alane trimethylamine complex gives the iron(II) polyhydride complex Fe(µ2-H)6[Al{N(SiMe3)2}2]2[Al{N(SiMe3)2}(NMe3)] (6). Complexes 2-6 were characterized by X-ray crystallography, as well as by infrared, electronic, and 1H and 13C (complex 6) NMR spectroscopies. Complexes 1 and 6 are apparently formed by an unusual "metallo-transamination" process.

The Aluminyl Anion: A New Generation of Aluminium Nucleophile.

Trivalent aluminium compounds are well known for their reactivity as Lewis acids/electrophiles, a feature that is exploited in many pharmaceutical, industrial and laboratory-based reactions. Recently, a series of isolable aluminium(I) anions ("aluminyls") have been reported, which offer an alternative to this textbook description: these reagents behave as aluminium nucleophiles. This minireview covers the synthesis, structure and reactivity of aluminyl species reported to date, together with their associated metal complexes. The frontier orbitals of each of these species have been investigated using a common methodology to allow for a like-for-like comparison of their electronic structure and a means of rationalising (sometimes unprecedented) patterns of reactivity.

Approaching a "Naked" Boryl Anion: Amide Metathesis as a Route to Calcium, Strontium, and Potassium Boryl Complexes.

Amide metathesis has been used to generate the first structurally characterized boryl complexes of calcium and strontium, {(Me3 Si)2 N}M{B(NDippCH)2 }(thf)n (M=Ca, n=2; M=Sr, n=3), through the reactions of the corresponding bis(amides), M{N(SiMe3 )2 }2 (thf)2 , with (thf)2 Li- {B(NDippCH)2 }. Most notably, this approach can also be applied to the analogous potassium amide K{N(SiMe3 )2 }, leading to the formation of the solvent-free borylpotassium dimer [K{B(NDippCH)2 }]2 , which is stable in the solid state at room temperature for extended periods (48 h). A dimeric structure has been determined crystallographically in which the K+ cations interact weakly with both the ipso-carbons of the flanking Dipp groups and the boron centres of the diazaborolyl heterocycles, with K⋅⋅⋅B distances of >3.1 Å. These structural features, together with atoms in molecules (QTAIM) calculations imply that the boron-containing fragment closely approaches a limiting description as a "free" boryl anion in the condensed phase.

Probing the Extremes of Covalency in M-Al bonds: Lithium and Zinc Aluminyl Compounds.

Synthetic routes to lithium, magnesium, and zinc aluminyl complexes are reported, allowing for the first structural characterization of an unsupported lithium-aluminium bond. Crystallographic and quantum-chemical studies are consistent with the presence of a highly polar Li-Al interaction, characterized by a low bond order and relatively little charge transfer from Al to Li. Comparison with magnesium and zinc aluminyl systems reveals changes to both the M-Al bond and the (NON)Al fragment (where NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), consistent with a more covalent character, with the latter complex being shown to react with CO2 via a pathway that implies that the zinc centre acts as the nucleophilic partner.

Arene C-H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of SN Ar Chemistry.

The reactivity of the electron-rich anionic AlI aluminyl compound K2 [(NON)Al]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) towards mono- and disubstituted arenes is reported. C-H activation chemistry with n-butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer-type transition state. Selectivity is therefore based on similar electronic factors to classical SN Ar chemistry, which implies the destabilisation of transition states featuring electron-donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C-H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C-H activation at a site which is neither ortho nor para to a methyl substituent.

Carbon Monoxide Activation by a Molecular Aluminium Imide: C-O Bond Cleavage and C-C Bond Formation.

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K2 [(NON)Al(NDipp)]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene; Dipp=2,6-diisopropylphenyl) structural characterization by X-ray crystallography reveals a short Al-N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al-N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C-C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al-O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium-bound isocyanate fragment.

Reversible, Room-Temperature C-C Bond Activation of Benzene by an Isolable Metal Complex.

The activation of C-C bonds is of fundamental interest in the construction of complex molecules from petrochemical feedstocks. In the case of the archetypal aromatic hydrocarbon benzene, C-C cleavage is thermodynamically disfavored, and is brought about only by transient highly reactive species generated in situ. Here we show that the oxidative addition of the C-C bond in benzene by an isolated metal complex is not only possible, but occurs at room temperature and reversibly at a single aluminium center in [(NON)Al]- (where NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene). Selectivity over C-H bond activation is achieved kinetically and allows for the generation of functionalized acyclic products from benzene.

Borylated N-Heterocyclic Carbenes: Rearrangement and Chemical Trapping.

This study details attempts to access N-heterocyclic carbenes (NHCs) featuring the diazaborolyl group, {(HCNDipp)2 B}, as one or both of the N-bound substituents, to generate strongly electron-donating and sterically imposing new carbene ligands. Attempts to isolate N-heterocyclic carbenes based around imidazolylidene or related heterocycles, are characterized by facile N-to-C migration of the boryl substituent. In the cases of imidazolium precursors bearing one N-bound diazaborolyl group and one methyl substituent, deprotonation leads to the generation of the target carbenes, which can be characterized in situ by NMR measurements, and trapped by reactions with metal fragments and elemental selenium. The half-lives of the free carbenes at room temperature range from 4-50 h (depending on the pattern of ancillary substituents) with N-to-C2 migration of the boryl function being shown to be the predominant rearrangement pathway. Kinetic studies show this to be a first-order process that occurs with an entropy of activation close to zero. DFT calculations imply that an intramolecular 1,2-shift is mechanistically feasible, with calculated activation energies of the order of 90-100 kJ mol-1 , reflecting the retention of significant aromatic character in the imidazole ring in the transition state. Trapping of the carbene allows for evaluation of steric and electronic properties through systems of the type LAuCl, LRh(CO)2 Cl, and LSe. A highly unsymmetrical (but nonetheless bulky) steric profile and moderately enhanced σ-donor capabilities (compared with IMes) are revealed.

Trapping and Reactivity of a Molecular Aluminium Oxide Ion.

Aluminium oxides constitute an important class of inorganic compound that are widely exploited in the chemical industry as catalysts and catalyst supports. Due to the tendency for such systems to aggregate via Al-O-Al bridges, the synthesis of well-defined, soluble, molecular models for these materials is challenging. Here we show that reactions of the potassium aluminyl complex K2 [(NON)Al]2 (NON=4,5-bis(2,6-diiso-propylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) with CO2 , PhNCO and N2 O all proceed via a common aluminium oxide intermediate. This highly reactive species can be trapped by coordination of a THF molecule as the anionic oxide complex [(NON)AlO(THF)]- , which features discrete Al-O bonds and dimerizes in the solid state via weak O⋅⋅⋅K interactions. This species reacts with a range of small molecules including N2 O (to give a hyponitrite ([N2 O2 ]2- ) complex) and H2 , the latter offering an unequivocal example of heterolytic E-H bond cleavage across a main group M-O bond.

Reduction of Carbon Oxides by an Acyclic Silylene: Reductive Coupling of CO.

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO2 to [C2 O2 ]2- .

Reversible C-H Activation, Facile C-B/B-H Metathesis and Apparent Hydroboration Catalysis by a Dimethylxanthene-Based Frustrated Lewis Pair.

A dimethylxanthene-based phosphine/borane frustrated Lewis pair (FLP) is shown to effect reversible C-H activation, cleaving phenylacetylene, PhCCH, to give an equilibrium mixture of the free FLP and phosphonium acetylide in CD2 Cl2 solution at room temperature. This system also reacts with B-H bonds although in a different fashion: reactions with HBpin and HBcat proceed via C-B/B-H metathesis, leading to replacement of the -B(C6 F5 )2 Lewis acid component by -Bpin/-Bcat, and transfer of HB(C6 F5 )2 to the phosphine Lewis base. This transformation underpins the ability of the FLP to catalyze the hydroboration of alkynes by HBpin: the active species is derived from the HB(C6 F5 )2 fragment generated in this exchange process.