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.

Dialumene-Mediated Production of Phosphines through P4 Reduction.

The formation of phosphorus-rich alanes featuring butterfly-like geometries is achieved. The two-electron reduction products feature a unique P4 2- structure and can act as a source of P3-. The treatment of these phosphorus containing products with electrophiles under mild conditions results in the formation of different phosphines. This approach eliminates the need for high temperatures and/or high pressures, which are commonly required in industrial processes for the preparation of useful phosphines.The activation and further functionalization of white phosphorus (P4) by main group complexes has become an increasingly studied topic in recent times. Herein, we report the controlled formation of phosphorus-rich alanes featuring butterfly-like geometries from the selective reaction of P4 with dialumenes, ([L(IiPr)Al]2) (1: L=Tripp=2,4,6-iPr3C6H2; 2: L=tBu2MeSi; IiPr=[MeCN(iPr)]2C)). The two-electron-reduction product of P4 features a P4 2- structure and is shown to be able to act as a source of P3-. Treatments of different electrophiles (e.g., chlorotrimethylsilane (Me3SiCl), iodotrimethylsilane (Me3SiI), HCl, or acetyl chloride (CH3COCl)) with these alanes under mild conditions gave the corresponding phosphines (e.g., P(SiMe3)3, PH3, or P(COCH3)3).

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.

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.

Structural Snapshots in Reversible Phosphinidene Transfer: Synthetic, Structural, and Reaction Chemistry of a Sn═P Double Bond.

The reaction of amido-substituted stannylenes with phospha-Wittig reagents (Me3PPR) results in release of hexamethyldisilazane and tethering of the resulting -CH2PMe2PR fragment to the tin center to give P-donor stabilized stannylenes featuring four-membered Sn,C,P,P heterocycles. Through systematic increases in steric loading, the structures of these systems in the solid state can be tuned, leading to successive P-P bond lengthening and Sn-P contraction and, in the most encumbered case, to complete P-to-Sn transfer of the phosphinidene fragment. The resulting stannaphosphene features a polar Sn═P double bond as determined by structural and computational studies. The reversibility of phosphinidene transfer can be established by solution phase measurements and reactivity studies.

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 [CnOn]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.

Taming Heavier Group 14 Imine Analogues: Accessing Tin Nitrogen [Sn=N] Double Bonds and their Cycloaddition/Metathesis Chemistry.

A systematic study to access stable stannaimines is reported, by combining different heteroleptic stannylenes with a range of organic azides. The reactions of terphenyl-/hypersilyl-substituted stannylenes yield the putative tin nitrogen double bond, but is directly followed by 1,2-silyl migration to give SnII systems featuring bulky silylamido ligands. By contrast, the transition from a two σ donor ligand set to a mixed σ-donor/π-donor scaffold allows access to three new stannaimines which can be handled at room temperature. The reactivity profile of these Sn=N bonded species is crucially dependent on the substituent at the nitrogen atom. As such, the Sn=NMes (Mes=2,4,6-Me3 C6 H2 ) system is capable of activating a broad range of substrates under ambient conditions via 1,2-addition reactions, [2+2] and [4+2] cycloaddition reactions. Most interestingly, very rare examples of main group multiple bond metathesis reactions are also found to be viable.

A Stable Homoleptic Divinyl Tetrelene Series.

The synthesis of the new bulky vinyllithium reagent (Me IPr=CH)Li, (Me IPr=[(MeCNDipp)2 C]; Dipp=2,6-iPr2 C6 H3 ) is reported. This vinyllithium precursor was found to act as a general source of the anionic 2σ, 2π-electron donor ligand [Me IPr=CH]- . Furthermore, a high-yielding route to the degradation-resistant SiII precursor Me IPr⋅SiBr2 is presented. The efficacy of (Me IPr=CH)Li in synthesis was demonstrated by the generation of a complete inorganic divinyltetrelene series (Me IPrCH)2 E: (E=Si to Pb). (Me IPrCH)2 Si: represents the first two-coordinate acyclic silylene not bound by heteroatom donors, with dual electrophilic and nucleophilic character at the SiII center noted. Cyclic voltammetry shows this electron-rich silylene to be a potent reducing agent, rivalling the reducing power of the 19-electron complex cobaltocene (Cp2 Co).

Generation of a π-Bonded Isomer of [P4 ]4- by Aluminyl Reduction of White Phosphorus and its Ammonolysis to PH3.

By employing the highly reducing aluminyl complex [K{(NON)Al}]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), we demonstrate the controlled formation of P4 2- and P4 4- complexes from white phosphorus, and chemically reversible inter-conversion between them. The tetra-anion features a unique planar π-bonded structure, with the incorporation of the K+ cations implicit in the use of the anionic nucleophile offering additional stabilization of the unsaturated isomer of the P4 4- fragment. This complex is extremely reactive, acting as a source of P3- : exposure to ammonia leads to the release of phosphine (PH3 ) under mild conditions (room temperature and pressure), which contrast with those necessitated for the direct combination of P4 and NH3 (>5 kbar and >250 °C).

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.

Linking Low-Coordinate Ge(II) Centers via Bridging Anionic N-Heterocyclic Olefin Ligands.

We introduce a large-scale synthesis of a sterically encumbered N-heterocyclic olefin (NHO) and illustrate the ability of its deprotonated form to act as an anionic four-electron bridging ligand. The resulting multicenter donating ability has been used to link two low oxidation state Ge(II) centers in close proximity, leading to bridging Ge-Cl-Ge and Ge-H-Ge bonding environments supported by Ge2C2 heterocyclic manifolds. Reduction of a dimeric [RGeCl]2 species (R = anionic NHO, [(MeCNDipp)2C═CH]-; Dipp = 2,6-iPr2C6H3) did not give the expected acyclic RGeGeR analogue of an alkyne, but rather ligand migration/disproportionation transpired to yield the known diorganogermylene R2Ge and Ge metal. This process was examined computationally, and the ability of the reported anionic NHO to undergo atom migration chemistry contrasts with what is typically found with bulky monoanionic ligands (such as terphenyl ligands).

Pushing Chemical Boundaries with N-Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry.

N-Heterocyclic olefins (NHOs) have gone from the topic of a few scattered (but important) reports in the early 1990s to very recently being a ligand/reagent of choice in the far-reaching research fields of organocatalysis, olefin and heterocycle polymerization, and low oxidation state main group element chemistry. NHOs are formally derived by appending an alkylidene (CR2) unit onto an N-heterocyclic carbene (NHC), and their pronounced ylidic character leads to high nucleophilicity and soft Lewis basic character at the ligating carbon atom. These olefinic donors can also be structurally derived from imidazole, triazole, and thiazole-based heterocyclic carbenes and, as a result, have highly tunable electronic and steric properties. In this Account, we will focus on various synthetic routes to imidazole-2-ylidene derived NHOs (sometimes referred to as deoxy-Breslow intermediates) followed by a discussion of the electron-donor ability of this structurally tunable ligand group. It should be mentioned that NHOs have a close structural analogy with Breslow-type intermediates, N-heterocyclic ketene aminals, and ß-azolium ylides; while these latter species play important roles in advancing synthetic organic chemistry, discussion in this Account will be confined mostly to imidazole-2-ylidene derived NHOs. In addition, we will cover selected examples from the literature where NHOs and their anionic counterparts, N-heterocyclic vinylenes, are used to access reactive main group species not attainable using traditional ligands. Added motivation for these studies comes from the emerging number of low coordinate main group element based compounds that display reactivity once reserved for precious metal complexes (such as H-H and C-H bond activation). Moreover, NHOs are versatile precursors to new mixed element (P/C and N/C), and potentially bidentate, ligand constructs of great potential in catalysis, where various metal oxidation states and coordination environments need to be stabilized during a catalytic cycle. The most active area of recent growth for NHOs is their use as nucleophiles to promote efficient organocatalytic transformations, including transesterification, carbonyl reduction, and the conversion of CO2 into value added products. Polyesters have also been generated through the NHO-promoted ring-opening polymerization of lactones, and the highly tunable nature of NHO organocatalysts allows for the rapid screening and enhancement of catalytic performance. Therefore, the growing utility of NHOs in the realm of organic and polymer chemistry can be viewed as evidence of the widespread impact of N-heterocyclic olefins on the chemical community. It is hoped that through this Account others will join this flourishing research domain and that the rapid recent growth of NHO chemistry is sustained for the foreseeable future.

Neutral, Cationic and Hydride-substituted Siloxygermylenes.

The introduction of the labile trimethylsiloxy group to GeII centers in the presence of an N-heterocyclic carbene donor is reported. The new complex IPr⋅GeCl(OSiMe3 ) (IPr=[(HCNDipp)2 C:]; Dipp=2,6-iPr2 C6 H3 ) was readily converted into the structurally unique GeII siloxy(hydrido)germylene IPr⋅GeH(OSiMe3 )⋅BH3 by treatment with lithium borohydride. Additionally, the reactive siloxygermylene cation [IPr⋅Ge(OSiMe3 )]+ was synthesized and clean oxidative addition of CH2 Cl2 was demonstrated. The two-coordinate [IPr⋅Ge(OSiMe3 )]+ cation also promoted the catalytic hydroborylation of sterically hindered ketones under mild conditions, with enhanced reactivity stemming from an open coordination site at Ge.

Accessing Low-Valent Inorganic Cations by Using an Extremely Bulky N-Heterocyclic Carbene.

The extremely bulky N-heterocyclic carbene (NHC), ITr (ITr=[(HCNCPh3 )2 C:]) featuring sterically shielding umbrella-shaped trityl (CPh3 ) substituents was prepared. This NHC features the highest percent buried volume (%Vbur ) to date, and was used to form a thermally stable quasi one-coordinate thallium(I) cation [ITr-Tl]+ . This TlI adduct and the corresponding lithium complex [ITr⋅Li(OEt2 )]+ are versatile "all-in-one" transmetalation/ligation reagents for preparing low-coordinate inorganic species inaccessible by pre-existing routes.

Investigation of N-Heterocyclic Carbene-Supported Group 12 Triflates as Pre-catalysts for Hydrosilylation/Borylation.

N-Heterocyclic carbene (NHC) complexes of Cd and Hg triflates (OTf) were prepared and their attempted conversion into rare cadmium and mercury hydrides was explored. In contrast to zinc, which forms stable [ZnH]+ complexes with NHCs, the heavier Cd and Hg congeners could not be formed; the increased instability of Cd-H and Hg-H units was rationalized with the aid of computations. It was also discovered that the dimeric adduct [IPr⋅Cd(µ-OTf)2 ]2 (IPr=[(HCNDipp)2 C:]; Dipp=2,6-iPr2 C6 H3 ) is an active precatalyst for the hydrosilylation and hydroborylation of hindered aldehydes and ketones. The related zinc congener was inactive as a catalyst highlighting a distinct advantage of using heavy Group 12 metals to promote catalytic hydrosilylation/borylation.

Reaction of a bis(pentafulvene)titanium complex with an N-heterocyclic olefin: C-H-activation leads to resonance between a titanium vinyl and titanium alkylidene complex.

The N-heterocyclic olefin (NHO) ImMe4CH2 (2) (ImMe4CH2 = (MeCNMe)2CCH2) was employed for the synthesis of the titanium complex 3 derived from an NHO ligand precursor. By reacting 2 with the bis(π-η5:σ-η1-pentafulvene)titanium complex 1a, the terminal ylidic methylene unit of 2 is deprotonated by the quaternary exocyclic carbon atom of one pentafulvene ligand of 1a yielding the titanium complex 3 which bears an anionic NHO ligand (ImMe4CH-). 3 was characterized by NMR spectroscopy, single crystal X-ray diffraction and quantum chemical calculations. The latter highlight that 3 is best described as a titanium vinyl complex with significant contribution of the titanium alkylidene resonance structure.

A vinyl silylsilylene and its activation of strong homo- and heteroatomic bonds.

The facile synthesis of a rare two-coordinate acyclic silylene (R2Si:) that is stabilized using a bulky vinylic N-heterocyclic olefin ligand and the strongly σ-donating hypersilyl group [Si(SiMe3)3]- is reported. This vinyl-substituted silylene exhibits an excellent combination of prolonged thermal stability along with high reactivity towards small molecules. Despite being stable for months in solution, the reactivity of this new silylene is manifest in its ambient temperature activation of strong B-H, Si-Cl, C-O, P-P and C-H bonds.

Approaching monocoordination at a silver(i) cation.

The recently reported bulky N-heterocyclic carbene ITr (ITr = [(HCNCPh3)2C:]) was found to stabilize low-coordinate Ag(i) environments. These electrophilic species were crystallographically identified as the weak solvates [(ITr)Ag(sol)]+ (sol = PhF, MesH or CH2Cl2) and as a solvent-free dimer [(ITr)Ag]22+. The highly electrophilic nature of the [(ITr)Ag]+ cation was further demonstrated by the calculation of a very high methyl ion affinity (MIA) and the synthesis of [(ITr)Ag(PCO)] which features substantial side-on electron donation from a P-C π bond to Ag.

Desorption of hydrogen from light metal hydrides: concerted electronic rearrangement and role of H···H interactions.

A theoretical study of the desorption of hydrogen from rhombic Group 1 metal hydride dimers reveals a concerted reorganisation of the electron density for the M-H and H-H moieties as the reaction coordinate is traversed and a closed-shell H···H interaction evolves into a covalent H2 bond. The central role played by homopolar dihydrogen bonding in this process is revealed and analysed.