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.

Mercury-Group 13 Metal Covalent Bonds: A Systematic Comparison of Aluminyl, Gallyl and Indyl Metallo-ligands

Bimetallic compounds containing direct metal-group 13 element bonds have been shown to display unprecedented patterns of cooperative reactivity towards small molecules, which can be influenced by the identity of the group 13 element. In this context, we present here a systematic appraisal of group 13 metallo-ligands of the type [(NON)E]- (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) for E=Al, Ga and In, through a comparison of structural and spectroscopic parameters associated with the trans L or X ligands in linear d10 complexes of the types LM{E(NON)} and XM'{E(NON)}. These studies are facilitated by convenient syntheses (from the In(I) precursor, InCp) of the potassium indyl species [{K(NON)In}⋅KCp]n (1) and [(18-crown-6)2K2Cp] [(NON)In] (1'), and lead to the first structural characterisation of Ag-In and Hg-E (E=Al, In) covalent bonds. The resulting structural, spectroscopic and quantum chemical probes of Ag/Hg complexes are consistent with markedly stronger σ-donor capabilities of the aluminyl ligand, [(NON)Al]-, over its gallium and indium counterparts.

Boryl Ancillary Ligands: Influencing Stability and Reactivity of Amidinato-Silanone and Germanone Systems in Ammonia Activation

While the nucleophilic addition of ammonia to ketones is an archetypal reaction in classical organic chemistry, the reactivity of heavier group 14 carbonyl analogues (R2E=O; E=Si, Ge, Sn, or Pb) with NH3 remains sparsely investigated, primarily due to the synthetic difficulties in accessing heavier ketone congeners. Herein, we present a room-temperature stable boryl-substituted amidinato-silanone {(HCDippN)2B}{PhC(tBuN)2}Si=O (Dipp=2,6-iPr2C6H3) (together with its germanone analogue), formed from the corresponding silylene under a N2O atmosphere. This system reacts cleanly with ammonia in 1,2-fashion to give an isolable sila-hemiaminal complex {(HCDippN)2B}{PhC(tBuN)2}Si(OH)(NH2). Quantum chemical calculations reveal that the formation of this sila-hemiaminal is crucially dependent on the nature of the ancillary ligand scaffold. It is facilitated thermodynamically by the hemi-lability of the amidinate ligand (which allows for the formation of an energetically critical intramolecular N⋅⋅⋅HO hydrogen bond within the product) and is enabled mech-anistically by a process in which the silanone initially acts in umpolung fashion as a base (rather than an acid), due to the strongly electron-releasing and sterically bulky nature of the ancillary boryl ligand.

A planar per-borylated digermene

A tetraboryl digermene synthesized by the reaction between a dianionic digermanide nucleophile and a boron halide electrophile is dimeric both in the solid state and in hydrocarbon solution. It features both a planar 'alkene-like' geometry for the Ge2B4 core, and an exceptionally short GeGe double bond. These structural features are consistent with the known electronic properties of the boryl group, and with lowest energy (in silico) fragmentation into two triplet bis(boryl)germylene fragments.

Diberyllocene, a stable compound of Be(I) with a Be-Be bond

The complex diberyllocene, CpBeBeCp (Cp, cyclopentadienyl anion), has been the subject of numerous chemical investigations over the past five decades yet has eluded experimental characterization. We report the preparation and isolation of the compound by the reduction of beryllocene (BeCp₂) with a dimeric magnesium(I) complex and determination of its structure in the solid state by means of x-ray crystallography. Diberyllocene acts as a reductant in reactions that form beryllium-aluminum and beryllium-zinc bonds. Quantum chemical calculations indicate parallels between the electronic structure of diberyllocene and the simple homodiatomic species diberyllium (Be₂).

Synthesis of Homo-Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion

The reduction of the boryl-substituted SnII bromide {(HCDippN)2 B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2 [Sn4 {B(NDippCH)2 }4 ], a homo-metallic heavier analogue of the cyclobutadiene dianion. This system is non-aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=-0.28, NICS(1)=-3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2-localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn4 {B(NDippCH)2 }4 H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4 {B(NDippCH)2 }4 ]2- dianion.

Disproportionation and Ligand Lability in Low Oxidation State Boryl-Tin Chemistry

Boryltin compounds featuring the metal in the+1 or 0 oxidation states can be synthesized from the carbene-stabilized tin(II) bromide (boryl)Sn(NHC)Br (boryl={B(NDippCH)2 }; NHC=C{(Ni PrCMe)2 }) by the use of strong reducing agents. The formation of the mono-carbene stabilized distannyne and donor-free distannide systems (boryl)SnSn(IPrMe)(boryl) (2) and K2 [Sn2 (boryl)2 ] (3), using Mg(I) and K reducing agents mirrors related germanium chemistry. In contrast to their lighter congeners, however, systems of the type [Sn(boryl)]n are unstable with respect to disproportionation. Carbene abstraction from 2 using BPh3 , and two-electron oxidation of 3 both result in the formation of a 2 : 1 mixture of the Sn(II) compound Sn(boryl)2 , and the hexatin cluster, Sn6 (boryl)4 (4). A viable mechanism for this rearrangement is shown by quantum chemical studies to involve a vinylidene intermediate (analogous to the isolable germanium compound, (boryl)2 Ge=Ge), which undergoes facile atom transfer to generate Sn(boryl)2 and trinuclear [Sn3 (boryl)2 ]. The latter then dimerizes to give the observed hexametallic product 4, with independent studies showing that similar trigermanium species aggregate in analogous fashion.

Inducing Nucleophilic Reactivity at Beryllium with an Aluminyl Ligand

The reactions of anionic aluminium or gallium nucleophiles {K[E(NON)]}₂ (E = Al, 1; Ga, 2; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) with beryllocene (BeCp₂) led to the displacement of one cyclopentadienyl ligand at beryllium and the formation of compounds containing Be-Al or Be-Ga bonds (NON)EBeCp (E = Al, 3; Ga, 4). The Be-Al bond in the beryllium-aluminyl complex [2.310(4) Å] is much shorter than that found in the small number of previous examples [2.368(2) to 2.432(6) Å], and quantum chemical calculations suggest the existence of a non-nuclear attractor (NNA) for the Be-Al interaction. This represents the first example of a NNA for a heteroatomic interaction in an isolated molecular complex. As a result of this unusual electronic structure and the similarity in the Pauling electronegativities of beryllium and aluminium, the charge at the beryllium center (+1.39) in 3 is calculated to be less positive than that of the aluminium center (+1.88). This calculated charge distribution suggests the possibility for nucleophilic behavior at beryllium and correlates with the observed reactivity of the beryllium-aluminyl complex with N,N'-diisopropylcarbodiimide─the electrophilic carbon center of the carbodiimide undergoes nucleophilic attack by beryllium, thereby yielding a beryllium-diaminocarbene complex.

Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction

Homologation of carbon monoxide is central to the heterogeneous Fischer–Tropsch process for the production of hydrocarbon fuels. C–C bond formation has been modeled by homogeneous systems, with [C_nO_n]^2– fragments (n = 2–6) formed by two-electron reduction being commonly encountered. Here, we show that four- or six-electron reduction of CO can be accomplished by the use of anionic aluminum(I) (“aluminyl”) compounds to give both topologically linear and branched C₄/C₆ chains. We show that the mechanism for homologation relies on the highly electron-rich nature of the aluminyl reagent and on an unusual mode of interaction of the CO molecule, which behaves primarily as a Z-type ligand in initial adduct formation. The formation of [C₆O₆]^4– from [C₄O₄]^4– shows for the first time a solution-phase CO homologation process that brings about chain branching via complete C–O bond cleavage, while a comparison of the linear [C₄O₄]^4– system with the [C₄O₄]^6– congener formed under more reducing conditions models the net conversion of C–O bonds to C–C bonds in the presence of additional reductants.

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.