Tw oseries of bulkyalkaline earth (Ae) metal amide complexes have been prepared:A e[N(TRIP) 2 ] 2 (1-Ae) and Ae[N(TRIP)(DIPP)] 2 (2-Ae) (Ae = Mg, Ca, Sr,B a; TRIP = SiiPr 3 ,D IPP = 2,6-diisopropylphenyl). While monomeric 1-Ca was already known, the new complexes have been structurally characterized.M onomers 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.I ti sa lso 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,w hich can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate sizeb ut it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has an oticeable influence on the thermodynamics for formation of the (amide)AeH species.C omplex 1-Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts,t he substrate scope in hydrogenation catalysis could be extended to challenging multi-substituted unactivated alkenes and even to arenes among which benzene. Scheme 4. Energy profiles (DH in kcal mol À1 )for a) the hydrogenation of ethylene by catalysts 1-Ca(orange), 1-Ba (black) and CaN'' 2 (red), and b) benzene hydrogenation by 1-Ba;B3PW91/def2tzvpp including correction for dispersion (GD3BJ) and solvent (PCM = benzene).

The efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K (THF)]∞, (DMAT)2Ca⋅(THF)2 and (DMAT)2Sr⋅(THF)2 (DMAT=2‐dimethylamino‐α‐trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph2C=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

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.

Hydrogenation of unsaturated bonds is a key step in both the fine and petrochemical industries. Homogeneous and heterogeneous catalysts are historically based on noble group 9 and 10 metals. Increasing awareness of sustainability drives the replacement of costly, and often harmful, precious metals by abundant 3d-metals or even main group metals. Although not as efficient as noble transition metals, metallic barium was recently found to be a versatile hydrogenation catalyst. Here we show that addition of finely divided Fe0, which itself is a poor hydrogenation catalyst, boosts activities of Ba0 by several orders of magnitude, enabling rapid hydrogenation of alkynes, imines, challenging multi-substituted alkenes and non-activated arenes. Metallic Fe0 also boosts the activity of soluble early main group metal hydride catalysts, or precursors thereto. This synergy originates from cooperativity between a homogeneous, highly reactive, polar main group metal hydride complex and a heterogeneous Fe0 surface that is responsible for substrate activation.

Reaction of a MgN" 2 /BaN" 2 mixture (N" = N(SiMe 3 ) 2 ) with PhSiH 3 gave three unique heterometallic Mg/Ba hydride clusters: Mg 5 Ba 4 H 11 N" 7 • (benzene) 2 (1), Mg 4 Ba 7 H 13 N" 9 • (toluene) 2 (2) and Mg 7 Ba 12 H 26 N" 12 (3). Product formation is controlled by the Mg/ Ba ratio and temperature. Crystal structures are described. While 3 is fully insoluble, clusters 1 and 2 retain their structures in aromatic solvents. DFT calculations and AIM analyses indicate highly ionic bonding with MgÀ H and BaÀ H bond paths. Also unusual H À •••H À bond paths are observed. Catalytic hydrogenation with MgN" 2 , BaN" 2 and the mixture MgN" 2 /BaN" 2 has been studied. Whereas MgN" 2 is only active in imine hydrogenation, alkene and alkyne hydrogenation needs the presence of Ba. The catalytic activity of the MgN" 2 /BaN" 2 mixture lies in general between that of its individual components and strong cooperative effects are not evident.