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Reaction of [(DIPePBDI)SrH]2 with C6H5X (X=Cl, Br, I) led to hydride‐halogenide exchange (DIPePBDI=HC[(Me)CN‐2,6‐(3‐pentyl)phenyl]2). Conversion rates increase with increasing halogen size (F

The chiral building block (R)-(+)-2,2′-diamino-1,1′-binaphthyl, (R)-BINAM, which is often used as backbone in privileged enantioselective catalysts, was converted to a series of N-substituted proligands R1-H2 (R = CH2tBu, C(H)Ph2, PPh2, dibenzosuberane, 8-quinoline). After double deprotonation with strong Mg or Ca bases, a series of alkaline earth (Ae) metal catalysts R1-Ae·(THF)n was obtained. Crystal structures of these C2-symmetric catalysts have been analyzed by quadrant models which show that the ligands with C(H)Ph2, dibenzosuberane and 8-quinoline substituents should give the best steric discrimination for the enantioselective intramolecular alkene hydroamination (IAH) of the aminoalkenes H2C＝CHCH2CR′2CH2NH2 (CR′2 = CPh2, CCy or CMe2). The dianionic R12− ligand in R1–Ae·(THF)n functions as reagent that deprotonates the aminoalkene substrate, while the monoanionic (R1-H)− ligand formed in this reaction functions as a chiral spectator ligand that controls the enantioselectivity of the ring closure reaction. Depending on the substituent R in the BINAM ligand, full cyclization of aminoalkenes to chiral pyrrolidine products as fast as 5 minutes was observed. Product analysis furnished enantioselectivities up to 57% ee, which marks the highest enantioselectivity reported for Ca catalyzed IAH. Higher selectivities are impeded by double protonation of the R12− ligand leading to complete loss of chiral information in the catalytically active species. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Double deprotonation of the amidine DIPP‐N=C(tBu)‐NH2 or β‐diketimine DIPP‐N=C(Me)‐C(H)=C(Me)‐NH2 (DIPP = 2,6‐diisopropylphenyl) with strong benzylcalcium or strontium bases gave metal imido complexes with the anions DIPP‐N=C(tBu)‐N2– (Am2–) and DIPP‐N=C(Me)‐C(H)=C(Me)‐N2– (BDI2–) which crystallized as tetrameric complexes with typical Ca4N4 or Sr4N4 cubane frameworks. Crystal structures of [(Am)Ca·THF]4, [(BDI)Ca·THF]4 and the first Sr imido complex [(Am)Sr·THF]4 are presented. Calculated geometries of [(Am)Ca·THF]4 and [(BDI)Ca·THF]4 (B3PW91/6‐311++G**) are in good agreement with the crystal structures. Also complexes with monoanionic ligands (Am‐H)– and (BDI‐H)– are reported. Charge delocalization in the ligand backbone is discussed. Ligand–metal bonds are calculated to be circa 90 % ionic; the NPA charges on Ca is circa +1.8. The negative charge on the ligand is delocalized over the ligand backbone but there is still considerable electron density on the terminal N (–1.4). Despite this high negative charge, the reactivity of these complexes is generally low due to the strongly bound cubane core. Reaction of [(Am)Ca·THF]4 with phenyl cyanide gave the [(Am)Ca·PhCN]4, clearly demonstrating its low reactivity. Attempts to break the tetrameric cluster in smaller aggregates by addition of 18‐crown‐6 led to protonation and isolation of (Am‐H)2Ca·(18‐crown‐6). Reaction of [(Am)Ca·THF]4 with iPrN=C=NiPr gave after addition a complex with a unique amido‐guanidinate ligand. [ABSTRACT FROM AUTHOR]

Hybrid catalysts consisting of alkaline earth iodides (AeI2) and the Schwesinger base tBuP4 catalyse the intramolecular alkene hydroamination of H2C=CHCH2CR2CH2NH2 [CR2=CPh2, C(CH2)5, CMe2]. Activities decrease along the row: Ca > Sr >> Mg > Ba. Hybrid catalysts consisting of tBuP4 and ZnI2, AlI3, FeCI3 or NaI were found to be fully inactive. Also, the hybrid catalyst tBuP3/CaI2 was not active which means that the base strength of the non‐nucleophilic organic base must be higher than that of tBuP3 (pKa BH+ = 38.6). Combinations of tBuP4 with CaX2 (X = Cl, Br, OiPr, OTf, NTf2) were found to be fully inactive which may in part be explained by poor solubility. The hybrid catalysis method is therefore limited to the combination tBuP4/CaI2 but the iodide ligands may be partially or fully replaced by chiral ligands. Chiral modifications of the hybrid catalysts gave in intramolecular alkene hydroamination ee values up to 33 %. [ABSTRACT FROM AUTHOR]

Double deprotonation of the diamine 1,1′‐(tBuCH2NH)‐ferrocene (1‐H2) by alkaline‐earth (Ae) or EuII metal reagents gave the complexes 1‐Ae (Ae=Mg, Ca, Sr, Ba) and 1‐Eu. 1‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1‐Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1‐Ca, 1‐Sr, and 1‐Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1‐Ae complexes shows that the heavier species 1‐Ca, 1‐Sr, and 1‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed. [ABSTRACT FROM AUTHOR]

Two series of bulky alkaline earth (Ae) metal amide complexes have been prepared: Ae[N(TRIP)2]2 (1‐Ae) and Ae[N(TRIP)(DIPP)]2 (2‐Ae) (Ae=Mg, Ca, Sr, Ba; TRIP=SiiPr3, DIPP=2,6‐diisopropylphenyl). While monomeric 1‐Ca was already known, the new complexes have been structurally characterized. Monomers 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. It is also 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, which can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate size but it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has a noticeable influence on the thermodynamics for formation of the (amide)AeH species. Complex 1‐Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts, the substrate scope in hydrogenation catalysis could be extended to challenging multi‐substituted unactivated alkenes and even to arenes among which benzene. [ABSTRACT FROM AUTHOR]

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

The reaction of Mg[N(SiMe3)2]2 with PhSiH3 in benzene at room temperature gave a white precipitate of approximate constitution MgH1.5N′′0.5. This undefined hydride rich Mg salt can be dissolved by addition of well‐known neutral donors, such as PMDTA, DABCO or the N‐heterocyclic carbene IMeiPr (N,N′‐diisopropyl‐2,3‐dimethylimidazol‐2‐ylidine). This allowed for crystallization and structural characterization of five new magnesium hydride complexes: Mg2(µ‐H)2N′′2·(PMDTA) (1), Mg4(µ‐H)6N′′2·(PMDTA)2 (2), Mg5(µ‐H)7N′′3·(PMDTA)2 (3), Mg2(µ‐H)2N′′2·(DABCO)2 (4) and Mg2(µ‐H)2N′′2·(IMeiPr)2 (5). The PMDTA complexes 1–3 were only isolated as mixtures that could not be separated but complexes 4 and 5 have been fully characterized. The structures of the Mg hydride complexes 1, 4 and 5 are reproduced well by DFT calculations (B3PW91/6‐311++G**). Bonding between Mg and hydride is largely ionic (circa 80 %) and there is very little charge transfer from the neutral donor to Mg2+. The calculated solvation enthalpies (ΔHsolv) per ligand are –12.4 kcal/mol (PMDTA), –15.9 kcal/mol (DABCO) and –20.8 kcal/mol (IMeiPr). In complexes 4 and 5 Bond‐Critical‐Points are found between the two negatively charged hydride ligands. The very high electron densities of 0.15–0.16 e/Å3 in these unusual BCP's are related to the short H–···H– distances of circa 2.55 Å. [ABSTRACT FROM AUTHOR]

A new dipyrromethene ligand with 9‐anthracenyl substituents for steric shielding of the metal has been introduced. The ligand, AnthDPM‐H (3), was directly deprotonated with MN′′2 {M = Mg, Ca, Sr, Ba, Zn; N′′ = N(SiMe3)2} to give (AnthDPM)MN′′ (4–8) in the form of highly fluorescent substances that are intensively colored from deep‐red to purple. Crystal structures reveal in all cases monomeric complexes with tricoordinate metals. Also the crystal structure of (AnthDPM)K, prepared by reaction of 3 with KN′′, is presented. The metal's coordination sphere in complexes 4–8 is further saturated by anagostic M···MeSi interactions which are most important for the smaller metals Mg and Zn. For the larger metals, Ca–Ba, the anthracenyl‐substituents assist in metal coordination. These secondary metal π‐interactions result in very high thermal stabilities. The least stable complex (AnthDPM)BaN′′ decomposes at circa 100 °C. The dipyrromethene complexes are clearly more stable than the corresponding β‐diketiminate complexes (DIPPBDI)MN′′ {DIPPBDI = CH[C(Me)N‐DIPP]2, DIPP =2,6‐diisopropylphenyl} which partially decompose at room temperature. These heteroleptic complexes of similar constitution, with metals ranging from Mg to Ba, form a new series that is potentially useful for investigations towards metal effects in early main group metal catalysis. [ABSTRACT FROM AUTHOR]

Alkaline earth metal (Ae) chemistry with the anion [N(C6F5)2]− has been explored. Deprotonation of the amine (C6F5)2NH, abbreviated in here as NFH, with 0.5 equivalent of AeN′′2 (N′′ = N(SiMe3)2) is fast and gave, dependent on the solvent, the complexes AeNF2, AeNF2·(THF)2 and AeNF2·(Et2O)2 (Ae = Mg, Ca, Sr). Using a 1/1 ratio, mixed amide complexes were obtained: NFAeN′′ (Ae = Mg, Ca, Sr). Crystal structures of the monomers AeNF2·(THF)2 (Ae = Mg, Ca, Sr) and AeNF2·(Et2O)2 (Ae = Mg, Ca) are presented and compared with those of AeN′′2·(THF)2. In addition, crystal structures of the homoleptic dimer (MgNF2)2 and the heteroleptic dimers (NFAeN′′)2 (Ae = Mg, Ca, Sr) are discussed. All structures are strongly influenced by very short Ae⋯F contacts down to circa 2.11 Å (Mg), 2.50 Å (Ca) and 2.73 Å (Sr). AIM analysis illustrates that, although Ae⋯F contacts are short, there is no bond-critical-point along this axis, indicating an essentially electrostatic interaction. The monomeric complexes feature strong C6F5⋯C6F5 π-stacking, resulting in unusually acute NF–Ae–NF angles as small as 95°. Heteroleptic (NFAeN′′)2 complexes retain their dimeric structure in C6D6 solution and there is no indication of ligand scrambling by the Schlenk equilibrium, suggesting that an electron withdrawing ligand may stabilize heteroleptic complexes. According to DFT calculations, the heteroleptic arrangement is 70 kJ mol−1 more stable than the homoleptic dimers. The Lewis acidity of MgNF2 has been quantified with the Gutmann–Beckett method and by calculation of the Fluoride-Ion-Affinity. The latter calculations show that the Lewis acidity of MgNF2 and CaNF2 is comparable to that of B(C6F5)3. Dimeric (MgNF2)2 fully abstracts Et3PO from Et3PO·B(C6F5)3 and may have potential in Lewis acid catalysis. [ABSTRACT FROM AUTHOR]

Alkaline earth metal amides (AeN′′2: Ae=Ca, Sr, Ba, N′′=N(SiMe3)2) catalyze alkene hydrogenation (80–120 °C, 1–6 bar H2, 1–10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p‐MeO‐styrene, α‐Me‐styrene, Ph2C=CH2, trans‐stilbene, cyclohexadiene, 1‐Ph‐cyclohexene), semi‐activated C=C bonds (Me3SiCH=CH2, norbornadiene), or non‐activated (isolated) C=C bonds (norbornene, 4‐vinylcyclohexene, 1‐hexene) could be reduced. The results show that neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For activated alkenes (e.g. styrene), polymerization is fully suppressed due to fast protonation of the highly reactive benzyl intermediate by N′′H (formed in the catalyst initiation). Using cyclohexadiene as the H source, the first Ae metal catalyzed H‐transfer hydrogenation is reported. DFT calculations on styrene hydrogenation using CaN′′2 show that styrene oligomerization competes with styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate with N′′H is a low‐energy escape route, thus avoiding oligomerization. Keep it simple: The widely used, easily accessible, alkaline earth metal amides M(NR2)2 (M=Ca, Sr, Ba, R=SiMe3) are highly active alkene hydrogenation catalysts. Alkene oligomerization is fully suppressed by trapping the reactive intermediate with R2NH. A first example of Group 2 metal hydrogen transfer catalysis is given. [ABSTRACT FROM AUTHOR]

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One‐pot reaction of 2,6‐iPr2‐aniline (DIPP‐NH2) with (Me2SiO)3 and Sr[N(SiMe3)2]2 (SrN′′2) gave a tetranuclear cluster consisting of four dianions [OSiMe2N‐DIPP]2– and four Sr2+ ions solvated each by one THF ligand. The general applicability of this method was investigated by variation of amine and metal. Anilines with smaller substituents led to insoluble uncharacterized coordination polymers, whereas bulkier anilines gave soluble product mixtures that could not be purified. Primary alkylamines neither led to isolable products. Introduction of a tBu group in para‐position of DIPP‐NH2, however, gave an isostructural cluster with increased solubility. Similar clusters could be obtained with barium. Both, the Sr and Ba clusters, were found to be active catalysts for a wide range of transformations: intramolecular alkene hydroamination, alkene hydrophoshination, pyridine hydroboration, pyridine hydrosilylation, and alkene hydrosilylation. The Ba catalysts were generally more active than the Sr catalysts. The tBu‐substituent in para‐position also had an accelerating effect, which is likely due to improved solubility. [ABSTRACT FROM AUTHOR]

Reaction of Ba[N(SiMe3)2]2 with PhSiH3 in toluene gave simple access to the unique Ba hydride cluster Ba7H7[N(SiMe3)2]7 that can be described as a square pyramid spanned by five Ba2+ ions with two flanking BaH[N(SiMe3)2] units. This heptanuclear cluster is well soluble in aromatic solvents, and the hydride 1H 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 Ba7H7[N(SiMe3)2]7 is a very strong reducing agent that already at room temperature reacts with Me3SiCH=CH2, norbornadiene, and ethylene. The highly reactive alkyl barium intermediates cannot be observed and deprotonate the (Me3Si)2N− ion, as confirmed by the crystal structure of Ba14H12[N(SiMe3)2]12[(Me3Si)(Me2SiCH2)N]4. [ABSTRACT FROM AUTHOR]

Anilido imine-magnesium bromide complexes, (LHMgBr)2 and (LMeMgBr)2, have been prepared [LH = DIPP-N(C6H4)C(H)N-DIPP and LMe = DIPP-N(C6H4)C(Me)N-DIPP (DIPP = 2,6- iPr2C6H3)]. Their dimeric crystal structures are compared to the dimer (DIPP-nacnacMgBr)2 [DIPP-nacnac = DIPP-NC(Me)C(H)C(Me)N-DIPP]. The anilido imine ligands show some degree of charge delocalization but are clearly more asymmetric than the DIPP-nacnac complex: one of the N atoms has considerable anilido character, whereas the other one is close to an imine. Calculated natural population analysis (NPA) charges (B3PW91/6-311++G**) and an atoms-in-molecules (AIM) study on model systems quantify the asymmetry in charge distribution. Quantification of the Lewis acidity of LHMgBr and LMeMgBr by the Gutmann-Beckett test gave acceptor numbers of 58.9 and 58.3, respectively. The complexes themselves are inert to a number of electrophiles; however, in combination with PPh3 the substrate 1-butene oxide was ring-opened. The resulting betaine-type alkoxide complex is unstable and decomposes at room temperature through a Wittig-type reaction. At higher temperatures also a reverse Wittig-type decomposition is observed. [ABSTRACT FROM AUTHOR]

1,3-Diisopropoxybenzene can be selectively metalated by nBuLi or by the superbase mixtures nBuLi/NaOC(Me)2Et or nBuLi/KOC(Me)2Et to give 2,6-diisopropoxyphenyllithium (71 %), 2,6-diisopropoxyphenylsodium (61 %), or 2,6-diisopropoxyphenylpotassium (59 %) as isolated products. The attempted synthesis of bis(2,6-diisopropoxyphenyl)calcium by the salt metathesis route using 2,6-diisopropoxyphenylsodium and CaI2 gave an undefined mixture of products, whereas the attempted synthesis of bis(2,6-diisopropoxyphenyl)calcium by the same approach using 2,6-diisopropoxyphenylpotassium and CaI2 gave the calcate complex [{( iPrO)2C6H3}3Ca]-K+, which is the first arylcalcate complex to have been structurally characterized. The trend in the 13C NMR chemical shifts of the aryl C ipso atoms shows that the calcate complex displays an aryl-metal bond polarity (nucleophilicity/basicity) similar to that in arylsodium compounds. [ABSTRACT FROM AUTHOR]

Reaclion of the diprotic ligand [Xanthdim]H2 (a ligand system where two adjacent β-dialdimine units are linked by a xanthyt backbone) with 2 equiv of potassium hydride or benzylcesium gave access to bimetallic alkali metal complexes. These complexes were structurally characterized by X-ray diffraction, which showed lhat the β-diminato units are orientated in a W-conformation. Treatment with 2 equiv of Mg[N(SiMe3)2]2(THF)2 led to the formation of the heteroleptic complex [Xanthdim][MgN(SiMe3)2(THF)]2, that crystallized as a highly strained monomer. The heteroteptic Mg complex is remarkably stable against ligand exchange but is not active in Co2/cyclohexene oxide copolymerization. Reaction with Ca[N(SiMe3)2]2(THF)2 gave the homoteptic complex [Xanthdim][Ca[THF)]. Both alkaline-earth metal complexes display considerable distortions in their solid state structure. [ABSTRACT FROM AUTHOR]

The role of the alkaline-earth metal on the catalytic hydrogenation of imines is elucidated by means of conceptual quantum chemical techniques, revealing the main factors behind the striking reactivity differences between Mg and the heavier Ae metals. [Display omitted] Alkaline-earth metal (Ae) catalysts have been recently developed for challenging imine and alkene hydrogenation at moderate reaction conditions, providing a sustainable alternative to transition metal catalysis. The understanding of catalytic hydrogenations mediated by group 2 metals is underdeveloped and mechanistic studies are scarce from experimental and computational sides. Herein, we examine the role of the metal on the catalytic hydrogenation of imines by Ae[N(SiMe 3) 2 ] 2 (Ae = Mg, Ca, Sr, Ba) using state-of-the-art computational techniques. Trends in energy barriers and turnover frequencies agree remarkably well with the experimentally observed increase in catalytic activity upon descending group 2 (Mg ≪ Ca < Sr < Ba). Structural and chemical bonding differences in the key intermediates were found to be the main driving force behind the enhanced reactivity of heavier Ae catalysts. More specifically, the N - A e - H ^ bond angle is drastically reduced in the Ca, Sr, and Ba catalytic species driven by the participation of the d-orbitals in the chemical bonding. The activation strain model reveals that these catalytic reactions are strain controlled and the higher activation barriers for the Mg catalyst originates from unfavourable bond angles in the Mg hydride species featuring linear structures and a more covalent metal-hydride bond. Further decomposition of the interaction energy reveals that stronger repulsive interactions destabilize the Mg species, indicating that the steric congestion due to the small Mg centre impedes reaction kinetics. Overall, the different aspects to be considered in the Ae catalyst design for imine hydrogenations are the strength and flexibility of the Ae-H bond, the bond ionicity, the N - A e - H ^ angle and the strength of the noncovalent interactions in the TOF-determining intermediate. [ABSTRACT FROM AUTHOR]