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.

The low‐valent ß‐diketiminate complex (DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(DIPPBDI)CaH]2 at 20 °C led to (DIPPBDI)Al(Ph)H (DIPPBDI=CH[C(CH3)N‐DIPP]2, DIPP=2,6‐diisopropylphenyl). Similar Ca‐catalyzed C−H bond activation is demonstrated for toluene or p‐xylene. For toluene a remarkable selectivity for meta‐functionalization has been observed. Reaction of (DIPPBDI)Al(m‐tolyl)H with I2 gave m‐tolyl iodide, H2 and (DIPPBDI)AlI2 which was recycled to (DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (DIPPBDI)Al(H)M(DIPPBDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (DIPPBDI)Al(H)Ca(DIPPBDI). This is in equilibrium with the hydride bridged complex (DIPPBDI)Al(μ‐H)Ca(DIPPBDI) which shows strongly increased electron density at Al. The combination of Ca‐arene bonding and a highly nucleophilic Al center are key to facile C−H bond activation.

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).

A new dipyrromethene (DPM) ligand with bulky DIPP-substituents is introduced (DIPP = 2,6diisopropylphenyl). The ligand, abbreviated as DIPP DPM, was deprotonated with ZnEt 2 to give ( DIPP DPM)ZnEt, which reacted with I 2 to form ( DIPP DPM)ZnI. Reaction of the latter with K[N( i Pr)HBH 3 ] afforded a labile Zn amidoborane complex which, after β-hydride elimination, formed ( DIPP DPM)ZnH. Crystal structures of ( DIPP DPM)ZnX (X = Et, I, H) revealed their monomeric nature. The Zn−N bond distances are somewhat longer than those in the corresponding monomeric β-diketiminate complexes ( DIPP BDI)ZnX ( DIPP BDI = CH[C(Me)N-DIPP] 2 ). This is in agreement with calculated NPA charges, which are lower on the N atoms of DPM compared to those on BDI. Reaction of ( DIPP DPM)ZnH with CO 2 gave ( DIPP DPM)Zn(O 2 CH), which crystallized as a monomer with a symmetrically bound η 2 -formate ligand. In contrast, the β-diketiminate complex crystallizes as a dimer [( DIPP BDI)Zn(O 2 CH)] 2 with bridging formate ligands. Reaction of ( DIPP DPM)Zn(O 2 CH) with various silanes regenerated the hydride complex ( DIPP DPM)ZnH. Catalytic CO 2 hydrosilylation with (EtO) 3 SiH using ( DIPP DPM)ZnH as a catalyst gave full reduction to [Si]−OMe species, whereas the catalyst ( DIPP BDI)ZnH only partially reduced CO 2 to [Si]−OC(O)H. The advantage of the DIPP DPM ligand is the arrangement of the DIPP-substituents, which form a pocket around the Zn−X unit, preventing dimerization and influencing its reactivity. In addition, in contrast to the negatively charged central backbone carbon in the DIPP BDI ligand, that in DIPP DPM is neutral. This makes it less nucleophilic and Brønsted basic, as expected for a true spectator ligand.

While magnesium hydride complexes are generally stabilized by hard, bulky N-donor ligands, softer ligands with a broad variety of coordination modes are shown to efficiently adapt themselves to the large variety of Mg centers in a growing magnesium hydride cluster. A P,N-chelating ligand is introduced that displays coordination modes between that of enamide, aza-allyl, and phosphinomethanide. Slight changes in the ligand bite angle have dramatic consequences for the structure type. The hitherto largest neutral magnesium hydride clusters are isolated either in a nonanuclear sheet-structure (brucite-type) or a dodecanuclear ring structure.