Here we report that attempted preparation of low-valent CaI complexes in the form of LCa-CaL (where L is a bulky β-diketiminate ligand) under dinitrogen (N2) atmosphere led to isolation of LCa(N2)CaL, which was characterized crystallographically. The N22ˉ anion in this complex reacted in most cases as a very potent two-electron donor. Therefore, LCa(N2)CaL acts as a synthon for the low-valent CaI complex LCa-CaL, which was the target of our studies. The N22ˉ anion could also be protonated to diazene (N2H2) that disproportionated to hydrazine and N2. The role of Ca d orbitals for N2 activation is discussed.

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 ...

The reagent RK [R=CH(SiMe3)2 or N(SiMe3)2] was expected to react with the low‐valent (DIPPBDI)Al (DIPPBDI=HC[C(Me)N(DIPP)]2, DIPP=2,6‐iPr‐phenyl) to give [(DIPPBDI)AlR]−K+. However, deprotonation of the Me group in the ligand backbone was observed and [H2C=C(N‐DIPP)−C(H)=C(Me)−N−DIPP]Al−K+ (1) crystallized as a bright‐yellow product (73 %). Like most anionic AlI complexes, 1 forms a dimer in which formally negatively charged Al centers are bridged by K+ ions, showing strong K+⋅⋅⋅DIPP interactions. The rather short Al–K bonds [3.499(1)–3.588(1) Å] indicate tight bonding of the dimer. According to DOSY NMR analysis, 1 is dimeric in C6H6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C6H6, two C−H bond activations are observed and a product with a para‐phenylene moiety was exclusively isolated. DFT calculations confirm that the Al center in 1 is more reactive than that in (DIPPBDI)Al. Calculations show that both AlI and K+ work in concert and determines the reactivity of 1.

N-Heterocyclic carbenes (NHCs) are extremely valuable as nucleophilic organocatalysts. They are widely applied as ligands in transition-metal catalysed reactions, where they are known as particularly potent s-donors. They are commonly viewed as workhorses exhibiting reliable, but undramatic, chemical behaviour. The N / C carbene p-donation stabilises NHCs at the expense of low reactivity towards nucleophiles. In contrast to NHCs, stable (alkyl)(amino)carbenes exhibit spectacular reactivity, allowing, for example, the splitting of hydrogen and ammonia and the fixation of carbon monoxide. NHCs have been judged to be electronically not suitable for showing similar reactivity. Here, we demonstrate that a ferrocene-based NHC is able to add ammonia, methyl acrylate, tert-butyl isocyanide, and carbon monoxide-reactions typical of (alkyl)(amino)carbenes, but unprecedented for diaminocarbenes. We also show that even the simplest stable diaminocarbene, C(NiPr 2 ) 2 , adds CO. This reaction affords a b-lactam by a subsequent intramolecular process involving a C-H activation. Our results shed new light on the chemistry of diaminocarbenes and offer great potential for synthetic chemistry and catalysis.

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.