Key to the isolation of the first alkyl strontium complex was the synthesis of a strontium hydride complex that is stable towards ligand exchange reactions. This goal was achieved by using the super bulky β‐diketiminate ligand DIPePBDI (CH[C(Me)N‐DIPeP]2, DIPeP=2,6‐diisopentylphenyl). Reaction of DIPePBDI‐H with Sr[N(SiMe3)2]2 gave (DIPePBDI)SrN(SiMe3)2, which was converted with PhSiH3 into [(DIPePBDI)SrH]2. Dissolved in C6D6, the strontium hydride complex is stable up to 70 °C. At 60 °C, H–D isotope exchange gave full conversion into [(DIPePBDI)SrD]2 and C6D5H. Since H–D exchange with D2 is facile, the strontium hydride complex served as a catalyst for the deuteration of C6H6 by D2. Reaction of [(DIPePBDI)SrH]2 with ethylene gave [(DIPePBDI)SrEt]2. The high reactivity of this alkyl strontium complex is demonstrated by facile ethylene polymerization and nucleophilic aromatic substitution with C6D6, giving alkylated aromatic products and [(DIPePBDI)SrD]2.

The first strontium hydride complex has been obtained by simply treating Sr[N(SiMe ) ] with PhSiH in the presence of PMDTA. The Sr complex Sr H [N(SiMe ) ] ⋅(PMDTA) crystallizes as an "inverse cryptand": an interstitial H is surrounded by a Sr H cage decorated with amide and PMDTA ligands. The analogous Ca complex could also be obtained and both retain their solid-state structures in solution: H NMR spectra in C D show two doublets and one nonet (4:4:1). Up to 90 °C, no coalescence is observed. The Ca cluster was investigated by DFT calculations and shows atypically low charges on Ca (+1.14) and H (-0.59) which signifies an unexpectedly low ionicity. AIM 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.

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).

Reaction of Ba[N(SiMe ) ] with PhSiH in toluene gave simple access to the unique Ba hydride cluster Ba H [N(SiMe ) ] that can be described as a square pyramid spanned by five Ba ions with two flanking BaH[N(SiMe ) ] units. This heptanuclear cluster is well soluble in aromatic solvents, and the hydride H NMR signals and coupling pattern suggests that the structure is stable in solution. At 95 °C, no coalescence of hydride signals is observed but the cluster slowly decomposes to undefined barium hydride species. The complex Ba H [N(SiMe ) ] is a very strong reducing agent that already at room temperature reacts with Me SiCH=CH , norbornadiene, and ethylene. The highly reactive alkyl barium intermediates cannot be observed and deprotonate the (Me Si) N ion, as confirmed by the crystal structure of Ba H [N(SiMe ) ] [(Me Si)(Me SiCH )N] .

Alkaline earth (Ae) metal complexes with the alanate anion AlH 4 − have been prepared by salt metathesis between NaAlH 4 and AeCl 2 in THF and could be isolated as Mg(AlH 4 ) 2 •(THF) 4 , Ca(AlH 4 ) 2 •(THF) 4 , and Sr(AlH 4 ) 2 •(THF) 5 . The previously reported crystal structure of the Mg alanate complex shows bonding of AlH 4 − with one bridging hydride, H 3 Al-(μ-H)-Mg, while the Ca and Sr alanates show a combination of H 3 Al-(μ-H)-Ae and H 2 Al-(μ-H) 2 -Ae bridging. The heteroleptic βdiketiminate complexes ( DIPP BDI)Mg(AlH 4 )•THF and ( DIPP BDI)Ca(AlH 4 )•(THF) 2 have been prepared by reaction of the corresponding Ae hydride complexes with AlH 3 •(THF) 2 [ DIPP BDI = DIPP-NC(Me)C(H)C(Me)N-DIPP, where DIPP = 2,6diisopropylphenyl]. Crystal structures show H 2 Al-(μ-H) 2 -Ae bridging. The Ca complex decomposes at room temperature by reduction of the β-diketiminate anion. Density functional theory calculations (B3PW91/ def2tzvpp) show that the formation of Ae(AlH 4 ) 2 from AeH 2 and AlH 3 is exothermic by ΔH (kilocalories per mole): Be, −68.8; Mg, −66.1; Ca, −95.4; Sr, −100.9; Ba, −112.3. Calculations of NPA charges on LiAlH 4 and the Ae alanate complexes (Ae = Mg, Ca, or Sr) show that these are highly ionic salts in which the charge on AlH 4 − of approximately −0.95 is hardly dependent on the countercation. Compared to LiAlH 4 , the Ae alanates are very efficient catalysts for imine hydrogenation, clearly extending the substrate scope. In addition to aldimines RC(H)NR′ (R/R′ = Ph/tBu, tBu/tBu, nPr/tBu, or Ph/Ph), ketimine PhC(Me)NtBu could be reduced. The salt [Bu 4 N + ][AlH 4− ] is catalytically not active, which shows that the s-block metal is crucial. The highest activities were found for the heterobimetallic Ca and Sr alanates.