A Simple Route to Calcium and Strontium Hydride Clusters

Maitland, Brant; Wiesinger, Michael; Langer, Jens; Ballmann, Gerd; Pahl, Jürgen; Elsen, Holger; Färber, Christian; Harder, Sjoerd

doi:10.1002/anie.201706786

Cited by 103 publications

(76 citation statements)

References 59 publications

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“…Very recently, we reacted MN″ 2 (M = Ca or Sr) with PhSiH 3 in the presence of MeN(CH 2 CH 2 NMe 2 ) 2 (PMDTA) and isolated the first well-defined larger Ca and Sr hydride clusters with molecular weights up to 1,500: M 6 H 9 N″ 3 • (PMDTA) 3 (2, Fig. 4) 47 . Heating these complexes to 100 °C gave decomposition to larger undefined M x N″ y H z clusters.…”

Section: Nature Catalysismentioning

confidence: 99%

Imine hydrogenation with simple alkaline earth metal catalysts

et al. 2018

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W ith molecular hydrogen being one of the cleanest reducing agents, catalytic hydrogenation using the more noble transition metals is among the most studied of all chemical processes 1 . Increasing social pressure towards a sustainable society, however, dictates replacement of costly, and often harmful, precious metals by more abundant first-row transition metals or even biocompatible redox inactive main group metals [2][3][4][5][6] . The alkaline earth metal calcium does not possess partially filled d orbitals for substrate activation, but has recently shown catalytic activities in the hydrogenation of C= C double bonds with molecular H 2 (ref. 7 ). Although restricted to conjugated C= C bonds, this example strikingly broke the dogma that transition metals are needed for alkene hydrogenation. This was followed by the development of metal-free frustrated Lewis pair (FLP) catalysts [8][9][10][11] and, most recently, cationic calcium hydride catalysts that are also able to hydrogenate unactivated alkenes 12 . Figure 1a shows a working hypothesis for styrene hydrogenation with a dibenzylcalcium catalyst (CaBn 2 ) 7 . The first step is the generation of a calcium hydride species, for which ample precedence exists [13][14][15][16][17] . Further reaction with H 2 may cause precipitation of insoluble (CaH 2 ) n , but catalyst loss is partly prevented by aggregation to soluble but undefined Ca x Bn y H z species. Despite a lack of d orbitals, alkene activation proceeds through a weak electrostatic calciumalkene interaction, recently shown to be of importance in calcium catalysis 18 . The benzylic calcium intermediate formed after insertion may, after successive styrene insertions, form polystyrene 19 , but high H 2 pressure (20-100 bar) can prevent this side reaction by promoting σ-bond metathesis. The latter step in the cycle is, like the initiation reaction, formally a deprotonation of H 2 by a resonance-stabilized benzylic carbanion. Considering the high pK a of H 2 (≈ 49) 20 , this reaction seemed questionable. Stoichiometric conversions of model systems, however, underscored the feasibility of this pathway 7 . Independent theoretical calculations illustrate that the final σ-bond metathesis step is indeed highly endergonic: Gibbs free energy of activation Δ G ‡ (60 °C, 20 bar) = 25.7 kcal mol −1 (ref. 21 ).As the highly atom-efficient catalytic reduction of imines by H 2 received much less attention than alkene or ketone hydrogenation [22][23][24] , it remained an important question whether calcium-catalysed hydrogenation can be extended to imine reduction. Current stateof-the-art imine hydrogenation catalysts can be divided into four categories that vary in terms of substrate activation and nucleophilic power ( Fig. 2a-d). Figure 2a shows organometallic metal hydrides that rely on hydride nucleophilicity. Apart from few early transition metal catalysts (Ti 25 , lanthanides 26 ), these are generally based on late transition metals (Rh, Ir) 22 . The aluminium hydride compound (iso-butyl) 2 AlH is an odd example of a ...

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Section: Nature Catalysismentioning

confidence: 99%

Imine hydrogenation with simple alkaline earth metal catalysts

et al. 2018

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“…[33,41] Kinetic stability administered by suitable ancillary ligands has been indispensable to prepare these compounds.F or molecular magnesium hydrides,t he ligands explored are mainly mono-or dianionic L l X x -type (l = 0-3, x = 1,2), while N-heterocyclic carbene in 6 is the single example of aneutral donor. = 6), Pauling electronegativity c = 1.57] to barium [ionic radius = 1.35 (C.N.…”

Section: Summary and Perspectivementioning

confidence: 99%

Molecular Magnesium Hydrides

2017

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Solid magnesium hydride [MgH ] has been pursued as a potential hydrogen-storage material. Organic chemists were rather interested in soluble magnesium hydride reagents from mid-20th century. It was only in the last two decades that molecular magnesium hydride chemistry received a major boost from organometallic chemists with a series of structurally well-characterized examples that continues to build a whole new class of compounds. More than 40 such species have been isolated, ranging from mononuclear terminal hydrides to large hydride clusters with more than 10 magnesium atoms. They provide not only insights into the structure and bonding of Mg-H motifs, but also serve as models for hydrogen-storage materials. Some of them are also recognized to participate in catalytic transformations, such as hydroelementation. Herein, an overview of these molecular magnesium hydrides is given, focusing on their synthesis and structural characterization.

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“…a) Crystal structure of Sr 6 H 9 N'' 3 ·(PMDTA)3 [36]. Forc larity,the Ha toms and Me groups of the PMDTAand N'' ligands, as well as the disorder,have been omitted.b )The Sr 6 H 9 3+ core of the complex;the Sr atoms (red) describe an octahedron.…”

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A Simple Route to Calcium and Strontium Hydride Clusters

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The first strontium hydride complex has been obtained by simply treating Sr[N(SiMe 3 ) 2 ] 2 with PhSiH 3 in the presence of PMDTA. The Sr complex Sr 6 H 9 [N(SiMe 3 ) 2 ] 3 ·-(PMDTA) 3 crystallizes as an "inverse cryptand": an interstitial H À is surrounded by aSr 6 H 8 4+ cage decorated with amide and PMDTAl igands.T he analogous Ca complex could also be obtained and both retain their solid-state structures in solution: 1 HNMR spectra in C 6 D 6 show two doublets and one nonet (4:4:1). Up to 90 8 8C, no coalescence is observed. The Ca cluster was investigated by DFT calculations and shows atypically low charges on Ca (+ 1.14) and H( À0.59) whichs ignifies an unexpectedly lowi onicity.A IM analysis shows hydride···hydride bond paths with considerable electron densities in the bond critical point. The clusters thermally decompose into larger,undefined, metal hydride aggregates.

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A Simple Route to Calcium and Strontium Hydride Clusters

Cited by 103 publications

References 59 publications

Imine hydrogenation with simple alkaline earth metal catalysts

Imine hydrogenation with simple alkaline earth metal catalysts

Molecular Magnesium Hydrides

A Simple Route to Calcium and Strontium Hydride Clusters

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