A Simple Route to Calcium and Strontium Hydride Clusters

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].

Metallic Barium: A Versatile and Efficient Hydrogenation Catalyst

Ba metal was activated by evaporation and cocondensation with heptane. This black powder is a highly active hydrogenation catalyst for the reduction of a variety of unactivated (non-conjugated) mono-, di- and tri-substituted alkenes, tetraphenylethylene, benzene, a number of polycyclic aromatic hydrocarbons, aldimines, ketimines and various pyridines. The performance of metallic Ba in hydrogenation catalysis tops that of the hitherto most active molecular group 2 metal catalysts. Depending on the substrate, two different catalytic cycles are proposed. A: a classical metal hydride cycle and B: the Ba metal cycle. The latter is proposed for substrates that are easily reduced by Ba0 , that is, conjugated alkenes, alkynes, annulated rings, imines and pyridines. In addition, a mechanism in which Ba0 and BaH2 are both essential is discussed. DFT calculations on benzene hydrogenation with a simple model system (Ba/BaH2 ) confirm that the presence of metallic Ba has an accelerating effect.

Calcium Hydride Catalysts for Olefin Hydrofunctionalization: Ring-Size Effect of Macrocyclic Ligands on Activity

The fifteen-membered NNNNN macrocycle Me5 PACP (Me5 PACP=1,4,7,10,13-pentamethyl-1,4,7,10,13-pentaazacyclopentadecane) stabilized the [CaH]+ fragment as a dimer with a distorted pentagonal bipyramidal coordination geometry at calcium. The hydride complex was prepared by protonolysis of calcium dibenzyl with the conjugate acid of Me5 PACP followed by hydrogenolysis or treating with n OctSiH3 of the intermediate calcium benzyl cation. The calcium hydride catalyzed the hydrogenation and hydrosilylation of unactivated olefins faster than the analogous calcium complex stabilized by the twelve-membered NNNN macrocycle Me4 TACD (Me4 TACD=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Kinetic investigations indicate that higher catalytic efficiency for the Me5 PACP stabilized calcium hydride is due to easier dissociation of the dimer in solution when compared to the Me4 TACD analogue.

The Valence Orbitals of the Alkaline-Earth Atoms

Quantum chemical calculations of the alkaline-earth oxides, imides and dihydrides of the alkaline-earth atoms (Ae=Be, Mg, Ca, Sr, Ba) and the calcium cluster Ca6 H9 [N(SiMe3 )2 ]3 (pmdta)3 (pmdta=N,N,N',N'',N''-pentamethyldiethylenetriamine) have been carried out by using density functional theory. Analysis of the electronic structures by charge and energy partitioning methods suggests that the valence orbitals of the lighter atoms Be and Mg are the (n)s and (n)p orbitals. In contrast, the valence orbitals of the heavier atoms Ca, Sr and Ba comprise the (n)s and (n-1)d orbitals. The alkaline-earth metals Be and Mg build covalent bonds like typical main-group elements, whereas Ca, Sr and Ba covalently bind like transition metals. The results not only shed new light on the covalent bonds of the heavier alkaline-earth metals, but are also very important for understanding and designing experimental studies.

Borate Hydrides as a New Material Class: Structure, Computational Studies, and Spectroscopic Investigations on Sr5 (BO3 )3 H and Sr5 (11 BO3 )3 D

The unprecedented borate hydride Sr5 (BO3 )3 H and deuteride Sr5 (11 BO3 )3 D crystallizing in an apatite-related structure are reported. Despite the presence of hydride anions, the compound decomposes only slowly in air. Doped with Eu2+ , it shows broad-band orange-red emission under violet excitation owing to the 4f6 5d-4f7 transition of Eu2+ . The observed 1 H NMR chemical shift is in good agreement with previously reported 1 H chemical shifts of ionic metal hydrides as well as with quantum chemical calculations and very different from 1 H chemical shifts usually found for hydroxide ions in similar materials. FTIR and Raman spectroscopy of different samples containing 1 H, 2 H, nat B, and 11 B combined with calculations unambiguously prove the absence of hydroxide ions and the sole incorporation of hydride ions into the borate. The orange-red emission obtained by doping with Eu2+ shows that the new compound class might be a promising host material for optical applications.

Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings

Two series of bulky alkaline earth (Ae) metal amide complexes have been prepared: Ae[N(TRIP)2 ]2 (1-Ae) and Ae[N(TRIP)(DIPP)]2 (2-Ae) (Ae=Mg, Ca, Sr, Ba; TRIP=SiiPr3 , DIPP=2,6-diisopropylphenyl). While monomeric 1-Ca was already known, the new complexes have been structurally characterized. Monomers 1-Ae are highly linear while the monomers 2-Ae are slightly bent. The bulkier amide complexes 1-Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. Catalyst 1-Ba can reduce internal alkenes like cyclohexene or 3-hexene and highly challenging substrates like 1-Me-cyclohexene or tetraphenylethylene. It is also active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step in catalytic hydrogenation is formation of an (amide)AeH species, which can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate size but it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has a noticeable influence on the thermodynamics for formation of the (amide)AeH species. Complex 1-Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts, the substrate scope in hydrogenation catalysis could be extended to challenging multi-substituted unactivated alkenes and even to arenes among which benzene.

Early Main Group Metal Catalysts for Imine Hydrosilylation

The efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K (THF)]∞ , (DMAT)2 Ca⋅(THF)2 and (DMAT)2 Sr⋅(THF)2 (DMAT=2-dimethylamino-α-trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph2 C=NPh could be successfully reduced by PhSiH3 at temperatures between 25-60 °C with catalyst loadings down to 2.5 mol %. Also, simple amides like KN(SiMe3 )2 or Ae[N(SiMe3 )2 ]2 (Ae=Ca, Sr, Ba) catalyze this reaction. Activities increase with metal size. For most substrates the activity increases along the row K Collapse

Key Words

• alkaline earth metals
• density functional calculations
• hydrosilylation
• imines
• potassium

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Affiliation(s)

Holger Elsen Inorganic and Organometallic ChemistryFriedrich-Alexander-Universität Erlangen-NürnbergEgerlandstraße 191058ErlangenGermany
Christian Fischer Inorganic and Organometallic ChemistryFriedrich-Alexander-Universität Erlangen-NürnbergEgerlandstraße 191058ErlangenGermany
Christian Knüpfer Inorganic and Organometallic ChemistryFriedrich-Alexander-Universität Erlangen-NürnbergEgerlandstraße 191058ErlangenGermany
Ana Escalona Inorganic and Organometallic ChemistryFriedrich-Alexander-Universität Erlangen-NürnbergEgerlandstraße 191058ErlangenGermany
Sjoerd Harder Inorganic and Organometallic ChemistryFriedrich-Alexander-Universität Erlangen-NürnbergEgerlandstraße 191058ErlangenGermany

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Nucleophilic Aromatic Substitution at Benzene with Powerful Strontium Hydride and Alkyl Complexes

Key to the isolation of the first alkyl strontium complex was the synthesis of a strontium hydride complex that is stable towards ligand exchange reactions. This goal was achieved by using the super bulky β-diketiminate ligand ^DIPeP BDI (CH[C(Me)N-DIPeP]₂ , DIPeP=2,6-diisopentylphenyl). Reaction of ^DIPeP BDI-H with Sr[N(SiMe₃ )₂ ]₂ gave (^DIPeP BDI)SrN(SiMe₃ )₂ , which was converted with PhSiH₃ into [(^DIPeP BDI)SrH]₂ . Dissolved in C₆ D₆ , the strontium hydride complex is stable up to 70 °C. At 60 °C, H-D isotope exchange gave full conversion into [(^DIPeP BDI)SrD]₂ and C₆ D₅ H. Since H-D exchange with D₂ is facile, the strontium hydride complex served as a catalyst for the deuteration of C₆ H₆ by D₂ . Reaction of [(^DIPeP BDI)SrH]₂ with ethylene gave [(^DIPeP BDI)SrEt]₂ . The high reactivity of this alkyl strontium complex is demonstrated by facile ethylene polymerization and nucleophilic aromatic substitution with C₆ D₆ , giving alkylated aromatic products and [(^DIPeP BDI)SrD]₂ .

Direct Synthesis of Heavy Grignard Reagents: Challenges, Limitations, and Derivatization

The direct synthesis of organocalcium compounds (heavy Grignard reagents) by the reduction of organyl halides with activated calcium powder succeeded in a straightforward manner for organic bromides and iodides that are bound at sp² -hybridized carbon atoms. Extension of this strategy to alkyl halides was very limited, and only the reduction of trialkylsilylmethyl bromides and iodides with activated calcium allowed the isolation of the corresponding heavy Grignard reagents. Substitution of only one hydrogen atom of the methylene moiety by a phenyl or methyl group directed this reduction toward the Wurtz-type coupling and the formation of calcium halide and the corresponding C-C coupling product. The stability of the methylcalcium and benzylcalcium derivatives in ethereal solvents suggests an unexpected reaction behavior of the intermediate organyl halide radical anions. Quantum chemical calculations verify a dependency between the ease of preparative access to organocalcium complexes and the C-I bond lengths of the organyl iodides. The bulkiness of the trialkylsilyl group is of minor importance. Chloromethyltrimethylsilane did not react with activated calcium; however, halogen-exchange reactions allowed the isolation of [Ca(CH₂ SiMe₃ )(thf)₃ (μ-Cl)]₂ . Furthermore, the metathetical approach of reacting [Ca(CH₂ SiMe₃ )I(thf)₄ ] with KN(SiMe₃ )₂ and the addition of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdeta) allowed the isolation of heteroleptic [CaCH₂ SiMe₃ {N(SiMe₃ )₂ }(pmdeta)]. In the reaction of this derivative with phenylsilane, the trimethylsilylmethyl group proved to be more reactive than the bis(trimethylsilyl)amido substituent.