Alessandro Bismuto

Alkyltin trihydride [(Me₃Si)₂CHSnH₃] was synthesized and the reductive elimination of hydrogen from this species was investigated. A methyl-substituted N-heterocyclic carbene reacts with the organotrihydride in dependence on stoichiometry and solvent to give a series of products of the reductive elimination and dehydrogenative tin–tin bond formation. Besides characterization of the carbene adduct of the alkyltin(II) hydride, a Sn₄ chain was also isolated, encompassing two stannyl–stannylene sites, which are stabilized each as NHC-adducts. Complete dehydrogenation resulted to give either a carbene-stabilized distannyne or a metalloid Sn₉-cluster salt. Reductive elimination of hydrogen was also achieved with an excess of diethylmethylamine to give the alkyltin(II) hydride as a Lewis base free tetramer [(RSnH)₄]. The method of cluster formation at low temperatures by hydrogen elimination was also transferred to the mesityl-substituted tin trihydride MesSnH₃. In this case [(MesSn)₁₀], showing a [5]prismane structure, was isolated in good yield and characterized. NMR spectroscopic features of the propellane-type cluster [Trip₆Sn₆] are reported.

A can of gas: Reductive elimination of hydrogen from the alkyltin trihydride [(Me₃Si)₂CH-SnH₃] in reactions with the N-heterocyclic carbene ^MeNHC in various stoichiometries or diethylmethylamine leads to a number of products.

The first example of a well-defined binary, low-oxidation-state aluminum hydride species that is stable at ambient temperature, namely the dianion in [{(^DepNacnac)Mg}₂(μ-H)]₂[H₃Al-AlH₃] (^DepNacnac=[(DepNCMe)₂CH]⁻, Dep=2,6-diethylphenyl), has been prepared via a magnesium(I) reduction of the alanate complex, (^DepNacnac)Mg(μ-H)₃AlH(NEt₃). An X-ray crystallographic analysis has shown the compound to be a contact ion complex, which computational studies have revealed to be the source of the stability of the aluminum(II) dianion.

You can call me HAl: The first example of a well-defined binary, low-oxidation-state aluminum hydride species that is stable at ambient temperature, is prepared (see picture). Computational studies reveal the stability of the aluminum(II) hydride dianion to be derived from its complexation with two bulky dimagnesium hydride cations.

The dimesitylphosphinocyclopentene/HB(C₆F₅)₂-derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane–borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO₂. It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C₆F₅)₂] to generate a FLP-η²-formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.

Poised to attack: The trans-1,2-attachment of the B(C₆F₅)₂ and PMes₂ functional groups at a cyclopentane framework leads to a non-interacting frustrated P/B Lewis pair (see Scheme). The diverse reactivity of this system was studied in detail.

Reaction of the donor-stabilized silylene [iPrNC(NiPr₂)NiPr]₂Si (1) with FeBr₂, CoBr₂, NiBr₂⋅MeOCH₂CH₂OMe, ZnCl₂, and ZnBr₂ afforded the respective transition-metal silylene complexes 4–8, the formation of which can be described in terms of a Lewis acid/base reaction (4, 5, 7, 8) or a nucleophilic substitution reaction (6). However, the reactivity profile of silylene 1 is not only based on its strong Lewis base character; the different coordination modes of the two guanidinato ligands (4–6 vs. 7 and 8) add an additional reactivity facet. The paramagnetic compounds 4 and 5 and the diamagnetic compounds 6⋅THF, 7, and 8⋅0.5 Et₂O were structurally characterized by single-crystal X-ray diffraction. In addition, compound 6⋅THF was studied by ¹⁵N and ²⁹Si solid-state NMR spectroscopy, and 7 and 8 were characterized by NMR spectroscopic studies in the solid state (¹⁵N, ²⁹Si) and in solution (¹H, ¹³C, ²⁹Si). Compounds 4–8 represent very rare examples of Fe^II, Co^II, Ni^II, and Zn^II silylene complexes. Four-coordinate silicon(II) compounds with an SiN₃M skeleton (M=Fe, Co, Ni) and M in the formal oxidation state +2 (4–6) have not yet been reported, and five-coordinate silicon(II) compounds with an SiN₄Zn skeleton (7, 8) are also unprecedented.

Transition-metal silylene complexes: The donor-stabilized silylene [iPrNC(NiPr₂)NiPr]₂Si reacts with FeBr₂, CoBr₂, NiBr₂⋅MeOCH₂CH₂OMe, ZnCl₂, and ZnBr₂ to afford compounds 1–5, a series of very rare examples of Fe^II, Co^II, Ni^II, and Zn^II silylene complexes. The reactivity profile of the silylene is not only based on its strong Lewis base character; the different coordination modes of the two guanidinato ligands (bridging vs. nonbridging) add an additional reactivity facet.

The catalytic activity of the NHI-substituted aluminum hydrides {L^MesNAlH₂}₂ (1a), {L^DipNAlH₂}₂ (1b), {L^MesNAl(H)OTf}₂ (2a), and {L^DipNAl(H)OTf}₂ (2b) in the hydroboration of terminal alkynes (for 1), as well as carbonyl compounds (for 2) with pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) is investigated (L^MesN = 1,3-dimesityl-imidazolin-2-imino, Mes = 2,4,6-trimethylphenyl, L^DipN = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-imino, Tf = triflyl). With the implementation of 1 as a catalyst, the hydroboration of selected terminal arylalkynes requires elevated temperature (80 °C) to proceed. The less sterically congested 1a produces faster conversions than the bulkier 1b. With the use of 2 as a catalyst, the hydroboration of carbonyl compounds occurs at room temperature. An increased steric hindrance of the catalyst (2a vs. 2b) does not mitigate the rate of conversion to a relevant degree.

Silylated germylene–PMe₃ adducts exchange their phosphane moiety smoothly for an N-heterocyclic carbene or isocyanide species to form their respective base adducts. Reaction of the silylated germylene–PMe₃ adducts with monosubstituted alkynes produce germylene adducts with the alkyne inserted into a Ge−Si bond. A computational study of this process provides evidence for the initial formation of a germirene, which rearranges to a vinylgermylene species. The thermodynamic driving force for this reaction is provided by subsequent adduct formation with PMe₃. Reaction of the PMe₃ adduct of bis[(trimethylsilyl)silyl]germylene with disubstituted alkynes leads to the formation of stable germirenes, which can be isomerized further to silagermetes.

Insert here: Monosubstituted alkynes can insert into one of the Ge−Si bonds of disilylated germylene–PMe₃ adducts (see figure). The driving force for this reaction is PMe₃–vinylgermylene adduct formation.

Treatment of the donor-stabilized silylene [iPrNC(Ph)NiPr]₂Si with Ph–C≡C–Ph resulted in a [2+1] cycloaddition reaction to afford a silicon(IV) compound, the first silacyclopropene with a six-coordinate silicon atom. This compound was structurally characterized by single-crystal X-ray diffraction and multinuclear NMR spectroscopic studies in the solid state and in solution.

The donor-stabilized bis(amidinato)silylene [iPrNC(Ph)NiPr]₂Si reacts with Ph–C≡C–Ph in a [2+1] cycloaddition reaction to afford a silicon(IV) compound, the first silacyclopropene with a six-coordinate silicon atom.

The activation of perfluoroalkyl iodides by the frustrated Lewis pair tris(pentafluorophenyl)borane and tri-tert-butylphosphine is described. By abstraction of both a fluorine and an iodine atom, an iodophosphonium fluoroborate salt is formed. In the presence of alkenes the corresponding iodoperfluoroalkylation products are generated regioselectively. First mechanistic investigations support a radical mechanism.

Frustrated Lewis pairs mediate the regioselective addition of perfluoroalkyl iodides to non-activated olefins. In the absence of an alkene, an iodophosphonium fluoroborate salt is formed by a fluoride elimination process. For the activation of the C−I bond, dual action of both the phosphine and the borane is required (see scheme).