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.

On the mechanism of sp2 C-H borylation using ortho-N-substituted pyridinium cations.

ortho-N-Substituted pyridinium cations with the weakly coordinating anion [B(C6F5)4]- have been studied and crucial structural features in the sp2 C-H borylation catalysis of 3-methylthiophene have been identified. The electron-deficiency of the aromatic core of the cation is essential for activity together with accessible protons. The spectroscopic yield of the borylation of 3-methylthiophene with catecholborane (CatBH) was optimized up to 86% and the method was further applied to other substrates such as N-alkylbenzenes. A mechanistic DFT study revealed the rate-limiting step in the catalysis to be the liberation of molecular H2 (ΔG‡ = 27.5 kcal mol-1), whereas the overall reaction was found to be exergonic by 5.1 kcal mol-1.

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.

The structures and reactivity of NHC-supported copper(i) triphenylgermyls.

Deprotonation of triphenyl germane with NHC-supported copper alkoxides afforded four novel (NHC)CuGePh3 complexes. Of these, (IPr)CuGePh3 (IPr = :C{N(2,6-iPr2C6H3)CH}2) was selected for further investigation. Analysis by EDA-NOCV indicates it to be a germyl nucleophile and its σ-bond metathesis reaction with a range of p-block halides confirmed it to be a convenient source of [Ph3Ge]-. The Cu-Ge bond of (IPr)CuGePh3 underwent π-bond insertions with tBuNCS, CS2, and PhNCO to furnish a series of germyl substituted carboxylate derivatives, (IPr)CuXC(Y)GePh3 (X = S, NPh; Y = S, NtBu, O), which were structurally characterised. (IPr)CuGePh3 inserted phenyl acetylene, providing both the Markovnikov and anti-Markovnikov products. The (NHC)CuGePh3 compounds were validated as catalytic intermediates; addition of 10 mol% of NHC-copper(i) alkoxide to a mixture of triphenyl germane and a tin(iv) alkoxide resulted in a tin/germanium cross coupling with concomitant formation of alcohol. Moreover, a catalytic hydrogermylation of Michael acceptors was developed with Ph3GeH adding to 7 activated alkenes in good conversions and yields in the presence of 10 mol% of NHC-copper(i) alkoxide. In all cases, this reaction provided the ß-germylated substrate implicating nucleophilicity at germanium.

An acyclic aluminyl anion.

The potassium aluminyl complex K2[Al{N(Dipp)SiMe3}2]2 was synthesised via reduction of [AlI{N(Dipp)SiMe3}2] (Dipp = 2,6-i-Pr2C6H3). This represents the first example of an aluminyl anion supported by an acyclic ligand framework. Attempts to yield the same structure with a larger ligand framework, {N(Dipp)Si(i-Pr)3}, led to C-H cleavage. K2[Al{N(Dipp)SiMe3}2]2 behaves as a nucleophilic source of aluminium; reaction with an electrophilic ß-diketiminate supported magnesium(II) iodide forms a monomeric, acyclic magnesium aluminyl complex.

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.

Boronic ester functionalised 1,8-diboryl-naphthalene scaffolds: fluoride versus oxide chelation.

1,8-Bis(boronic ester) derivatives of naphthalene, 1,8-C10H6{B(OR)2}2, present an attractive target as receptors for the fluoride ion via B-F-B chelation, but are synthetically challenging to access due to the competing formation of a very stable anhydride containing a B-O-B motif. By contrast, unsymmetrical systems of the type 1,8-C10H6{B(OR)2}(BR'2) can be synthesized for (OR)2 = 1,2-O2C6H4 (i.e. Cat) and R' = Mes. This system is shown to be competent for the uptake of F-, making use of a chelating mode of action and the formation of a bridging B-F-B motif between the two boron centres. However, both experimental and quantum chemical studies indicate that the µ2-F adduct is the kinetic product of fluoride uptake, with an alternative structural motif featuring a terminal B-F bond and a B-O-B bridge using one of the catechol oxygens being (marginally) more favourable thermodynamically.

Oxidative addition or Werner coordination complex? Reactivity of ß-diketiminate supported main group and first-row transition metal complexes towards ammonia.

A series of neutral LM (L = [HC{(H3C)C(Dipp)N}2], Dipp = 2,6-iPr2C6H3, M = group 13: B-In, TM: Fe, Co, Ni, Cu) and L'M (L' = [HC{(CCH2)(CCH3)(DippN)2}], M = group 14: C-Pb) compounds including a main group 13/14 and first-row transition metal complexes were studied computationally by density functional theory (DFT). The optimised complexes were assessed in terms of structural parameters and electronic structures to find trends and characteristics that could be used to predict their reactivity towards ammonia. In addition, the differences in oxidative addition and Werner coordination complex formation depending on the identity of the central element were investigated and the Werner complexes were evaluated by QTAIM and EDA-NOCV approaches. The computational results complement the earlier experimental studies and shed light on the feasibility of isolating novel main group Werner complexes or transition metal oxidative addition products.

Terpene dispersion energy donor ligands in borane complexes.

Structural characterization of the complex [B(ß-pinane)3] (1) reveals non-covalent H⋯H contacts that are consistent with the generation of London dispersion energies involving the ß-pinane ligand frameworks. The homolytic fragmentations of 1, and camphane and sabinane analogues ([B(camphane)3] (2) and [B(sabinane)3] (3)) were studied computationally. Isodesmic exchange results showed that London dispersion interactions are highly dependent on the terpene's stereochemistry, with the ß-pinane framework providing the greatest dispersion free energy (ΔG = -7.9 kcal mol-1) with Grimme's dispersion correction (D3BJ) employed. PMe3 was used to coordinate to [B(ß-pinane)3], giving the complex [Me3P-B(ß-pinane)3] (4), which displayed a dynamic coordination equilibrium in solution. The association process was found to be slightly endergonic at 302 K (ΔG = +0.29 kcal mol-1).

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.

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.

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.

A crystalline radical cation derived from Thiele's hydrocarbon with redox range beyond 1 V.

Thiele's hydrocarbon occupies a central role as an open-shell platform for new organic materials, however little is known about its redox behaviour. While recent synthetic approaches involving symmetrical carbene substitution of the CPh2 termini yield isolable neutral/dicationic analogues, the intervening radical cations are much more difficult to isolate, due to narrow compatible redox ranges (typically < 0.25 V). Here we show that a hybrid BN/carbene approach allows access to an unsymmetrical analogue of Thiele's hydrocarbon 1, and that this strategy confers markedly enhanced stability on the radical cation. 1•+ is stable across an exceptionally wide redox range (> 1 V), permitting its isolation in crystalline form. Further single-electron oxidation affords borenium dication 12+, thereby establishing an organoboron redox system fully characterized in all three redox states. We perceive that this strategy can be extended to other transient organic radicals to widen their redox stability window and facilitate their isolation.

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.

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.

Cooperative N-H bond activation by amido-Ge(ii) cations.

N-heterocyclic carbene (NHC) and tertiary phosphine-stabilized germylium-ylidene cations, [R(L)Ge:]+, featuring tethered amido substituents at R have been synthesized via halide abstraction. Characterization in the solid state by X-ray crystallography shows these systems to be monomeric, featuring a two-coordinate C,N- or P,N-ligated germanium atom. The presence of the strongly Lewis acidic cationic germanium centre and proximal amide function allows for facile cleavage of N-H bonds in 1,2-fashion: the products resulting from reactions with carbazole feature a tethered secondary amine donor bound to a three-coordinate carbazolyl-GeII centre. In each case, addition of the components of the N-H bond occurs to the same face of the germanium amide function, consistent with a coordination/proton migration mechanism. Such as sequence is compatible with the idea that substrate coordination via the pπ orbital at germanium reduces the extent of N-to-Ge π donation from the amide, thereby enhancing the basicity of the proximal N-group.

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.