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 ...
A range of symmetric amidinate ligands RAmAr (R is backbone substituent, Ar is N substituent) have been investigated for their ability to stabilize calcium hydride complexes of the type RAmArCaH. It was found that the precursors of the type RAmArCaN(SiMe3)2 are only stable toward ligand exchange for Ar = DIPP (2,6-diisopropylphenyl). The size of the backbone substituent R determines aggregation and solvation. The following complexes could be obtained: [RAmDIPPCaN(SiMe3)2]2 (R = Me, p-Tol), RAmDIPPCaN(SiMe3)2·Et2O (R = Np, tBu), AdAmDIPPCaN(SiMe3)2·THF, and AdAmDIPPCaN(SiMe3)2. Reaction of these heteroleptic calcium amide complexes with PhSiH3 gave only for larger backbone substituents (R = tBu, Ad) access to the dimeric calcium hydride complexes (RAmArCaH)2. (N,aryl)-coordination of the amidinate ligand seems crucial for the stability of these complexes, and the aryl···Ca interaction is found to be strong (17 kcal/mol). Addition of polar solvents led to a new type of trimeric calcium hydride complex exemplified by the crystal structures of (tBuAmDIPPCaH)3·2Et2O and (AdAmDIPPCaH)3·2THF. The overall conclusion of this work is that minor changes in sterics (tBu vs Ad) or coordinated solvent (THF vs Et2O) can have large consequences for product formation and stability.
A series of (DIPPnacnac)CaN(SiMe)·S complexes (DIPPnacnac = HC[C(Me)N(2,6-iPr-CH)]; S = solvent) could be obtained by the addition of S = THF, DME or N-Me-morpholine (Morph) to (DIPPnacnac)CaN(SiMe)·OEt or (DIPPnacnac)CaN(SiMe). Crystal structures for complexes with S = DME and Morph are compared to literature-known structures with S = none, THF or EtO. Bulkier and weaker Lewis bases like the tertiary amines EtN, TMEDA and DABCO did not interact with (DIPPnacnac)CaN(SiMe). The reaction of (DIPPnacnac)CaN(SiMe) with PhSiH gave conversion to a calcium hydride complex that dismutated in (DIPPnacnac)Ca and CaH. The reaction of (DIPPnacnac)CaN(SiMe)·S with PhSiH gave [(DIPPnacnac)CaH·S] for S = THF, EtO or N-Me-morpholine (Morph). For S = DME, high reaction temperatures were needed and dismutation into (DIPPnacnac)Ca and CaH was observed. Extensive NMR investigations (VT-NMR and PGSE) confirm the dimeric nature of [(DIPPnacnac)CaH·THF] in aromatic solvents or in THF. Thermal decomposition of [(DIPPnacnac)CaH·THF] (release of H at 200 °C) is compared to that of Mg and Zn analogues. Weakly coordinating EtO in [(DIPPnacnac)CaH·OEt] could be replaced by THF, Morph or DABCO but not with EtN. The addition of TMEDA led to the formation of CaH and unidentified products. The addition of DME led to the decomposition of EtO and complex [(DIPPnacnac)CaOEt] was obtained. Crystal structures of the following compounds are presented: (DIPPnacnac)CaN(SiMe)·S (S = Morph, DME), [(DIPPnacnac)CaH·S] (S = EtO, Morph and DABCO) and [(DIPPnacnac)CaOEt]. Although bulky ligands have long been thought to be the key to the stabilization of calcium hydride complexes, the presence of a polar, strongly coordinating, co-solvent is also crucial.
An anionic N-heterocyclic olefin ligand was serendipitously obtained by reaction of an amidinate calcium hydride complex with 1,3-dimethyl-2-methyleneimidazole (NHO). Instead of anticipated addition to the polarized C=CH bond to form an unstabilized alkylcalcium complex, deprotonation of the NHO ligand in the backbone was observed. Preference for deprotonation versus addition is explained by loss of aromaticity in the latter conversion. Theoretical calculations demonstrate the substantially increased ylidic character of this anionic NHO ligand which, like N-heterocyclic dicarbenes, shows strong bifunctional coordination.
Addition of a calcium hydride complex to diphenylacetylene gave a complex in which the stilbene dianion symmetrically bridges two Ca ions. DFT calculations discuss the effect of the metal stilbene coordination. The stilbene complex reacts as a base (with H) or an electron donor (with I) and catalyzes the reduction of diphenylacetylene.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.