Michael Cowley

Bonding with Al: Treatment of a dialumene-benzene adduct (1) with naphthalene and anthracene afforded the corresponding dialumene-arene adducts at room temperature. A 1,2-dialuminacyclobutene derivative was also obtained by the exchange of the C₆H₆ moiety of 1 with bis(trimethylsilyl)acetylene. These findings exhibit the potential of 1 as a synthetic equivalent of a dialumene.

The tetrasilacyclobutadiene [LSi(μ-SiL′)₂SiL] (L=PhC(NtBu)₂, L′=2,6-iPr₂C₆H₃NSiMe₃) consists of an aromatic silicon-containing four-membered ring in which two π, two σ, and two lone-pair electrons are cyclically delocalized. The electron delocalization was illustrated by theoretical studies and reactivity with elemental sulfur to form the allylic zwitterionic cyclic compound [(LSi)₂(μ-SiL′)(μ-Si(S)L′)] with 2π-electron delocalization along the Si₃ skeleton.

Give me a B: A stepwise chloride abstraction of [Cp*BCl₂(IMes)] resulted in the linear di-substituted boron dication [Cp*B(IMes)]²⁺. The hypercoordinated boron dication, stabilized in the pentagonal pyramidal [C₅B]²⁺ cluster, reacts with superhydride to yield the N-heterocyclic carbene-stabilized borabenzene, [C₅Me₅B(IMes)] (see scheme).

Metallacyclopentadienes have attracted much attention as building blocks for synthetic chemistry as well as key intermediates in many metal-mediated or metal-catalyzed reactions. However, metallacyclopentadienes of the alkaline-earth metals have not been reported, to say nothing of their structures, reaction chemistry, and synthetic applications. In this work, the first series of magnesiacyclopentadienes, spiro-dilithio magnesiacyclopentadienes, and dimagnesiabutadiene were synthesized from 1,4-dilithio 1,3-butadienes. Single-crystal X-ray structural analysis of these magnesiacycles revealed unique structural characteristics and bonding modes. Their reaction chemistry and synthetic application were preliminarily studied and efficient access to amino cyclopentadienes was established through their reaction with thioformamides. Experimental and DFT calculations demonstrated that these magnesiacyclopentadienes could be regarded as bis(Grignard) reagents wherein the two MgC(sp²) bonds have a synergetic effect when reacting with substrates.

Mg in the middle: The first series of spiro-dilithio magnesiacyclopentadienes, magnesiacyclopentadienes, and dimagnesiabutadienes has been synthesized and structurally characterized. Unique structural characteristics and bonding were revealed, and the reaction chemistry and synthetic applications were studied. An efficient method for synthesizing amino cyclopentadienes was established by using thioformamides.

Thermolysis of 9-azido-9-borafluorene in heptane solution produces the tetramer of a BN-phenanthryne. The isolation of the self-trapping product provides evidence for the involvement of the BN-aryne in the thermolysis reaction. Its formation may be rationalized by denitrogenation of the azide and ring enlargement.

Making a first appearance: A BN-aryne analogous to 9,10-phenanthryne can be formed by thermolysis of the 9-azido-9-borafluorene and can undergo cyclotetramerization.

The synthesis of dimeric magnesium(I) compounds of the general type RMgMgR (R=monoanionic substituent) is still a challenging synthetic task and limited to few examples with sterically demanding ligands with delocalized CN-frameworks that all have been accessed by Na or K metal reduction of magnesium(II) halide precursors. Here we report on the synthesis of a novel diiminophosphinato magnesium(I) compound that has been synthesized by a facile redox reaction using a known magnesium(I) complex. The synthetic strategy may be applicable to other ligand systems and can help expand the class of low oxidation state magnesium complexes even if reductions with Na or K are unsuccessful. The new dimeric magnesium(I) complex has been structurally characterized and undergoes a CC coupling reaction with tert-butylisocyanate.

The new switcheroo: A dimeric magnesium(I) complex has been prepared by a simple redox reaction using a known magnesium(I) complex. The new complex is stabilized by a high oxidation state phosphorane-based ligand and undergoes a CC coupling reaction with tert-butylisocyanate.

Cyclopropylidene is a transient intermediate of the allene–propyne–cyclopropene isomerization. The incorporation of heavier Group 14 elements into the cyclopropylidene scaffold has to date been restricted to the formal replacement of the carbenic carbon atom by a base-coordinated silicon(II) center. Herein we report the synthesis and characterization of NHC-coordinated heavier cyclopropylidenes (Si₂GeR₃X, and Si₃R₃Br; X=Cl, Mes; R=Tip=2,4,6-iPr₃C₆H₂) in which the three-membered ring is exclusively formed by silicon and germanium. In case of the chloro-substituted Si₂Ge-cyclopropylidene, a stable heavier cycloprop-1-yl-2-ylidene cation is obtained by NHC-induced chloride dissociation.

He is heavy and he′s my brother: NHC-coordinated cyclopropenylidene analogues with molecular scaffolds exclusively formed by heavier Group 14 elements are accessible from the corresponding vinylidene isomers by exchange of the NHC ligand for a smaller NHC. The residual chloride in one of these heavier cyclic carbenes can be expelled by a second equivalent of NHC to generate cationic derivatives of the imidazolium type.

Stibinyl and bismuthinyl radicals are recognized as representative intermediates of antimony and bismuth compounds, but still elusive in the condensed phase. We successfully synthesized persistent stibinyl and bismuthinyl radicals in solution by facile dissociation of the corresponding dimers with bulky substituents. We characterized the radicals by NMR and UV/Vis spectroscopy and estimated the thermodynamic parameters for the dissociation equilibria. The radicals show np (HOMOSOMO) transition bands at 497 nm (stibinyl) and 543 nm (bismuthinyl) in 3-methylpentane and react with a stable nitroxyl radical to give the cross-radical coupling products in good yields.

Persistent: The facile dissociation of distibines and dibismuthines with bulky bidentate alkyl groups leads to the corresponding persistent radicals in solution. The stibinyl and bismuthinyl radicals were spectroscopically characterized, and the thermodynamic parameters for their dissociation equilibria were estimated. The radicals reacted with a stable nitroxyl radical to give the cross-radical coupling products in good yields.

Common wisdom has it that organoboranes are readily oxidized. Described herein is that also their reduction can result in remarkable chemistry. Treatment of dimeric 9H-9-borafluorene with Li metal in toluene yields two strikingly different classes of compounds. One part of the sample reacts in a way similar to B₂H₆, thus affording an aryl(hydro)borane cluster reminiscent of the [B₃H₈]⁻ anion. The other part furnishes a dianionic boron-doped graphene flake devoid of hydrogen substituents at the boron centers and featuring a central BB bond. A change in the solvent to THF allows an isolation of this dibenzo[g,p]chrysene analogue in good yields.

What a difference an electron can make: Extensive rearrangements of the 9H-9-borafluorene scaffold occur upon injection of electrons into the system using Li metal. Two major reduction products are a [B₃H₈]⁻ cluster analogue and a graphene flake with a central BB bond (see picture; C black, H white, B green).

We present isolable examples of formal zinc hydride cations supported by N-heterocyclic carbene (NHC) donors, and investigate the dual electrophilic and nucleophilic (hydridic) character of the encapsulated [ZnH]⁺ units by computational methods and preliminary hydrosilylation catalysis.

A positive approach: Isolable examples of formal zinc hydride cations supported by N-heterocyclic carbene donors are prepared. The dual electrophilic and nucleophilic (hydridic) character of the encapsulated [ZnH]⁺ units was investigated by computational methods and hydrosilylation catalysis.

A robust catalyst for the selective dehydrogenation of formic acid to liberate hydrogen gas has been designed computationally, and also successfully demonstrated experimentally. This is the first such catalyst not based on transition metals, and it exhibits very encouraging performance. It represents an important step towards the use of renewable formic acid as a hydrogen-storage and transport vector in fuel and energy applications.

From models to reality: A robust catalyst for the selective dehydrogenation of formic acid to liberate hydrogen gas has been designed computationally, and also demonstrated experimentally. This catalyst represents an important step towards the use of renewable formic acid as a hydrogen-storage and transport vector in fuel and energy applications.

The simple synthetic development of acyclic pincer bis(pyridine)carbodicarbene is depicted herein. Presented is the first isolated structural pincer carbodicarbene with a C-C-C angle of 143°, larger than the monodentate framework. More importantly, theoretical analysis showed that this carbodicarbene embodies a more allene-like character. Palladium complexes supported by this pincer ligand are active catalysts for Heck–Mizoroki and Suzuki–Miyaura coupling reactions.

The acyclic pincer ligand bis(pyridine)carbodicarbene was synthesized, isolated, and characterized. It features a C-C-C angle of 143°, which is larger than that in the monodentate framework. Palladium complexes supported by this ligand are active catalysts in Heck–Mizoroki and Suzuki–Miyaura coupling reactions.

N-methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4-(N,N-dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6-lutidine which activates H₂, even in the presence of H₂O. Anion effects dominate reactivity, with both solubility and rate of H₂ cleavage showing marked anion dependency. With the optimal anion, a N-methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine–boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over-reduction to the alkanes.

Softly does it: N-methylacridinium salts are shown to be versatile Lewis acids within frustrated Lewis pairs for stoichiometric and catalytic transformations by through H₂ activation.

Monovalent RAl (R=HC[C(Me)N(2,6-iPr₂C₆H₃)]₂) reacts with E₂Et₄ (E=Sb, Bi) with insertion into the weak EE bond and subsequent formation of RAl(EEt₂)₂ (E=Sb 1; Bi 2). The analogous reactions of RGa with E₂Et₄ yield a temperature-dependent equilibrium between RGa(EEt₂)₂ (E=Sb 3; Bi 4) and the starting reagents. RIn does not interact with Sb₂Et₄ under various reaction conditions, but formation of RIn(BiEt₂)₂ (5) was observed in the reaction with Bi₂Et₄ at low temperature.

Doing the shuffle: Reactions of monovalent RAl with E₂Et₄ (E=Sb, Bi; Ar=2,6-iPr₂C₆H₃) proceed with EE bond cleavage and formation of RAl(EEt₂)₂, whereas RGa forms a reversible chemical equilibrium with E₂Et₄ and RGa(EEt₂)₂. RIn does not react with Sb₂Et₄, but also forms a reversible equilibrium with Bi₂Et₄ at low temperatures.