Molecular Calcium Hydride: Dicalcium Trihydride Cation Stabilized by a Neutral NNNN‐Type Macrocyclic Ligand

Leich, Valeri; Spaniol, Thomas P.; Maron, Laurent; Okuda, Jun

doi:10.1002/anie.201600552

Cited by 80 publications

(113 citation statements)

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“…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.…”

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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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Imine hydrogenation with simple alkaline earth metal catalysts

et al. 2018

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“…In situ generation of LAeH is aprerequisite for its successful isolation. [11,12] Using carefully controlled reaction conditions,gave the first amidinate calcium hydride complex (4). Synthetic routes to heavier alkaline-earth metal hydride complexes.…”

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A Simple Route to Calcium and Strontium Hydride Clusters

et al. 2017

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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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“…[9] Dieser Komplex kann als Addukt aus einem neutralen [L 4 CaH 2 ]-und einem kationischen [L 4 CaH] + -Fragment, stabilisiert durch ein stark basisches Tr iphenylsilyl-Anion, angesehen werden. [9] Der Komplex katalysierte die Hydrierung von 1,1-Diphenylethen (1,, wobei die katalytisch aktive Spezies in Lçsung ungewiss blieb.D ie Hydrierung von Alkenen durch Übergangsmetall-Homogenkatalysatoren [10] ist weit verbreitet, die Aktivitätvon Hydrierkatalysatoren auf Basis von "frustrierten" Lewis-Paaren (FLP) [11] und Hauptgruppenmetallen [12] beschränkt sich jedoch auf aktivierte (konjugierte) C-C-Doppelbindungen. [9] Der Komplex katalysierte die Hydrierung von 1,1-Diphenylethen (1,, wobei die katalytisch aktive Spezies in Lçsung ungewiss blieb.D ie Hydrierung von Alkenen durch Übergangsmetall-Homogenkatalysatoren [10] ist weit verbreitet, die Aktivitätvon Hydrierkatalysatoren auf Basis von "frustrierten" Lewis-Paaren (FLP) [11] und Hauptgruppenmetallen [12] beschränkt sich jedoch auf aktivierte (konjugierte) C-C-Doppelbindungen.…”

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“…Im 1 H-NMR-Spektrum des Filtrats wurden freier Ligand und andere Zersetzungsprodukte beobachtet. [9] Die chemischen Verschiebungen des Me 4 TACD-Liganden und des Carbanions bestätigen die vermutete Struktur. [4][5][6] Dibenzyl-Komplex 1 reagierte mit HSiPh 3 in THF zu dem durch zwei silylierte Benzylanionen [PhCHSiPh 3 ] À stabilisierten, dikationischen Dihydrid 2 (Schema 1).…”

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“…[9] Die Tr ihydridkomplexe 6 sind unlçslich in aliphatischen und aromatischen Kohlenwasserstoffen aber lçslich in THF.D ie Stabilitäti n THF ist vergleichbar mit der des Trihydridkomplexes 3 (t 1/2 % 14 d). Die Elementaranalysen und NMR-Spektren von 6a und 6b sind konsistent mit der vorgeschlagenen Struktur.Die Hydridsignale (6a: d = 4.73 ppm; 6b: d = 4.71 ppm) werden tieffeldverschoben zu denen in Dihydrid 5 beobachtet und sind vergleichbar mit der chemischen Verschiebung der Hydride in [Ca 2 H 3 (Me 4 TACD) 2 ]-(SiPh 3 )( d = 4.72 ppm), was auf einen analogen kationischen [Ca 2 H 3 (Me 4 TACD) 2 ] + -Kern hindeutet.…”

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