Hypervalent Hydrosilicates Connected to Light Alkali Metal Amides: Synthesis, Structure, and Hydrosilylation Catalysis

Formation and Reactivity of a Hexahydridosilicate [SiH6 ]2- Coordinated by a Macrocycle-Supported Strontium Cation

The cationic benzyl complex [(Me4 TACD)Sr(CH2 Ph)][A] (Me4 TACD=1,4,7,10-tetramethyltetraazacyclododecane; A=B(C6 H3 -3,5-Me2 )4 ) reacted with two equivalents of phenylsilane to give the bridging hexahydridosilicate complex [(Me4 TACD)2 Sr2 (thf)4 (μ-κ3  : κ3 -SiH6 )][A]2 (3 a). Rapid phenyl exchange between phenylsilane molecules is assumed to generate monosilane SiH4 that is trapped by two strontium hydride cations [(Me4 TACD)SrH(thf)x ]+ . Complex 3 a decomposed in THF at room temperature to give the terminal silanide complex [(Me4 TACD)Sr(SiH3 )(thf)2 ][A], with release of H2 . Upon reaction with a weak Brønsted acid, CO2 , and 1,3,5,7-cyclooctatetraene SiH4 was released. The reaction of a 1 : 2 mixture of cationic benzyl and neutral dibenzyl complex with phenylsilane gave the trinuclear silanide complex [(Me4 TACD)3 Sr3 (μ2 -H)3 (μ3 -SiH3 )2 ][A], while n OctSiH3 led to the trinuclear (n-octyl)pentahydridosilicate complex [(Me4 TACD)3 Sr3 (μ2 -H)3 (μ3 -SiH5 n Oct)][A].

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.

Accelerating role of deaggregation agents in lithium-catalysed hydrosilylation of carbonyl compounds

A combined computational and experimental approach demonstrates the accelerating role of deaggregation agents, especially HMPA, in the Li-catalysed hydrosilylation of acetophenone in THF solution under very mild conditions.

Alkali Metal Triphenyl- and Trihydridosilanides Stabilized by a Macrocyclic Polyamine Ligand

Potassium silanide [KSiH₃]_∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me₄TACD (1,4,7,10‐tetramethyl‐1,4,7,10‐tetraaminocyclododecane, L) stabilizes the full range of triphenylsilyl complexes [(L)MSiPh₃]_n (M=Li–Cs), which react with H₂ or PhSiH₃ to form molecular [(L)MSiH₃]_n that can be isolated in soluble form and fully characterized.

Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation

Chemo- and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d- and f-block metal complexes. The cationic hydride of calcium [CaH]+ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti-Markovnikov products, while aryl-substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di- and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy- and siloxy-substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.

Magnesium hydride alkene insertion and catalytic hydrosilylation

The β-diketiminato magnesium hydride, [(BDI)MgH]]₂, reacts with alkenes and catalyses their hydrosilylation with PhSiH₃.

The dimeric β-diketiminato magnesium hydride, [(BDI)MgH]₂, reacts at 80 °C with the terminal alkenes, 1-hexene, 1-octene, 3-phenyl-1-propene and 3,3-dimethyl-butene to provide the respective n-hexyl, n-octyl, 3-phenylpropyl and 3,3-dimethyl-butyl magnesium organometallics. The facility for and the regiodiscrimination of these reactions are profoundly affected by the steric demands of the alkene reagent. Reactions with the phenyl-substituted alkenes, styrene and 1,1-diphenylethene, require a more elevated temperature of 100 °C with styrene providing a mixture of the 2-phenylethyl and 1-phenylethyl products over 7 days. Although the reaction with 1,1-diphenylethene yields the magnesium 1,1-diphenylethyl derivative as the sole reaction product, only 64% conversion was achieved over a 21 day timeframe. Reactions with the α,ω-dienes, 1,5-hexadiene and 1,7-octadiene, provided divergent results. The initial 5-alkenyl magnesium reaction product of the shorter chain diene undergoes 5-exo-trig cyclisation via intramolecular carbomagnesiation to provide a cyclopentylmethyl derivative, which was shown by X-ray diffraction analysis to exist as a three-coordinate monomer. In contrast, 1,7-octadiene provided a mixture of two compounds, a magnesium oct-7-en-1-yl derivative and a dimagnesium-octane-1,4-diide, as a result of single or two-fold activation of the terminal C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C double bonds. The magnesium hydride was unreactive towards internal alkenes apart from the strained bicycle, norbornene, allowing the characterisation of the resultant three-coordinate magnesium norbornyl derivative by X-ray diffraction analysis. Computational analysis of the reaction between [(BDI)MgH]₂ and 1-hexene using density functional theory (DFT) indicated that the initial Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion process is rate determining and takes place at the intact magnesium hydride dimer. This exothermic reaction (ΔH = –14.1 kcal mol^–1) traverses a barrier of 18.9 kcal mol^–1 and results in the rupture of the dinuclear structure into magnesium alkyl and hydride species. Although the latter three-coordinate hydride derivative may be prone to redimerisation, it can also provide a further pathway to magnesium alkyl species through its direct reaction with a further equivalent of 1-hexene, which occurs via a lower barrier of 15.1 kcal mol^–1. This Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion reactivity provides the basis for the catalytic hydrosilylation of terminal alkenes with PhSiH₃, which proceeds with a preference for the formation of the anti-Markovnikov organosilane product. Further DFT calculations reveal that the catalytic reaction is predicated on a sequence of Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion and classical Si–H/Mg–C σ-bond metathesis reactions, the latter of which, with a barrier height of 24.9 kcal mol^–1, is found to be rate determining.

Weak Interactions Get Strong: Synergy between Tetrel and Alkaline-Earth Bonds

Weak and strong noncovalent interactions such as tetrel bonds and alkaline-earth bonds, respectively, cooperate and get reinforced when acting together in ternary complexes of general formula RN··· SiH₃F···MY, where MY is a Be or Mg derivative and RN is a N-containing Lewis base with different hybridization patterns. Cooperativity has been studied in the optimized MP2/aug'-cc-pVTZ ternary complexes by looking at changes on geometries, binding energies, ²⁹Si NMR chemical shifts, and topological features according to the atoms in molecules theoretical framework. Our study shows that cooperativity in terms of energy is in general significant: more than 40 kJ/mol, and up to 83.6 kJ/mol in the most favorable case. The weakest the isolated interaction, the strongest the reinforcement in the ternary complex; in this sense, the tetrel bond is shortened enormously, between 0.3 and 0.6 Å. This dramatic reinforcement of the tetrel bond is also nicely reflected in the positive variations of the ²⁹Si chemical shifts in all the ternary complexes. At the same time the ternary complexes are characterized by the presence of totally planar silyl group, due to the pentacoordination of the Si atom. Both the hybridization of the N base and the geometry imposed by the alkaline-earth ligands have a strong influence on the binding energies, as they modify the donor ability of N and the Lewis acid character of the alkaline-earth metal.

Titanium Carbene Complexes Stabilized by Alkali Metal Amides

Facile α-H elimination from tetrakis(trimethylsilylmethyl)titanium precursors to give adducts of (alkylidene)bis(alkyl)titanium complexes is induced by light alkali metal amides of the NNNN-type macrocyclic anionic ligand Me₃ TACD [(Me₃ TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane]. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH₂ group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by the reaction with benzophenone and terminal acetylenes.

Recent progress in the application of group 1, 2 & 13 metal complexes as catalysts for the ring opening polymerization of cyclic esters

This review focuses on recent developments concerned with the use of well-defined main group complexes as (pre-)catalysts for the ROP of cyclic esters to give aliphatic polyesters; factors influencing catalytic activity, selectivity and polymer properties are all discussed.