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A complex of a metal in its zero oxidation state can be considered a stabilized, but highly reactive, form of a single metal atom. Such complexes are common for the more noble transition metals. Although rare examples are known for electronegative late-main-group p-block metals or semimetals.sup.1-6, it is a challenge to isolate early-main-group s-block metals in their zero oxidation state.sup.7-11. This is directly related to their very low electronegativity and strong tendency to oxidize. Here we present examples of zero-oxidation-state magnesium (that is, magnesium(0)) complexes that are stabilized by superbulky, monoanionic, [beta]-diketiminate ligands. Whereas the reactivity of an organomagnesium compound is typically defined by the nucleophilicity of its organic groups and the electrophilicity of Mg.sup.2+ cations, the Mg.sup.0 complexes reported here feature electron-rich Mg centres that are nucleophilic and strongly reducing. The latter property is exemplified by the ability to reduce Na.sup.+ to Na.sup.0. We also present a complex with a linear Mg.sub.3 core that formally could be described as a Mg.sup.I-Mg.sup.0-Mg.sup.I unit. Such multinuclear mixed-valence Mg.sub.n clusters are discussed as fleeting intermediates during the early stages of Grignard reagent formation. Their remarkably strong reducing power implies a rich reactivity and application as specialized reducing agents. Strongly reducing [beta]-diketiminate complexes containing magnesium in its zero oxidation state are reported, among which is a compound with a linear triatomic Mg-Mg-Mg core., Author(s): B. Rösch [sup.1] , T. X. Gentner [sup.1] , J. Eyselein [sup.1] , J. Langer [sup.1] , H. Elsen [sup.1] , S. Harder [sup.1] Author Affiliations: (1) Department of [...]

Low-valent ( Me BDI)Al and ( Me BDI)Ga and highly Lewis acidic cations in [( tBu BDI)M + ⋅C 6 H 6 ][(B(C 6 F 5 ) 4 - ] (M=Mg or Zn, Me BDI=HC[C(Me)N-DIPP] 2 , tBu BDI=HC[C(tBu)N-DIPP] 2 , DIPP=2,6-diisopropylphenyl) react to heterobimetallic cations [( tBu BDI)Mg-Al( Me BDI) + ], [( tBu BDI)Mg-Ga( Me BDI) + ] and [( tBu BDI)Zn-Ga( Me BDI) + ]. These cations feature long Mg-Al (or Ga) bonds while the Zn-Ga bond is short. The [( tBu BDI)Zn-Al( Me BDI) + ] cation was not formed. Combined AIM and charge calculations suggest that the metal-metal bonds to Zn are considerably more covalent, whereas those to Mg should be described as weak Al I (or Ga I )→Mg 2+ donor bonds. Failure to isolate the Zn-Al combination originates from cleavage of the C-F bond in the solvent fluorobenzene to give ( tBu BDI)ZnPh and ( Me BDI)AlF + which is extremely Lewis acidic and was not observed, but ( Me BDI)Al(F)-(μ-F)-(F)Al( Me BDI) + was verified by X-ray diffraction. DFT calculations show that the remarkably facile C-F bond cleavage follows a dearomatization/rearomatization route., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

First evidence for the coordination of norbornadiene (nbd) and dicyclopentadiene (dcpd) with the main group metal Mg is provided by the crystal structures of adducts with cationic β-diketiminate (BDI) Mg complexes. While the dcpd complex is thermally stable, [(BDI)Mg+·nbd][B(C6F5)4-] shows slow room temperature retro-Diels-Alder decomposition to give a complex with the cation (BDI)Mg(C5H5)Mg(BDI)+.

Strongly Lewis acidic cationic aluminium complexes, stabilized by β-diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C 6 F 5 ) 4 - salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the R1,R2 BDI ligand ( R1,R2 BDI=HC[C(R1)N(R2)] 2 ) influence sterics and Lewis acidity. Ligand bulk increases along the row Me,DIPP BDI< Me,DIPeP BDI≈ tBu,DIPP BDI< tBu,DIPeP BDI; DIPP=2,6-C(H)Me 2 -phenyl, DIPeP=2,6-C(H)Et 2 -phenyl. The Gutmann-Beckett test showed acceptor numbers of: ( tBu,DIPP BDI)AlMe + 85.6, ( tBu,DIPeP BDI)AlMe + 85.9, ( Me,DIPP BDI)AlMe + 89.7, ( Me,DIPeP BDI)AlMe + 90.8, ( Me,DIPP BDI)AlH + 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation ( tBu,DIPP BDI)AlMe + shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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[double bond, length as m-dash]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.

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 Ba 0 , that is, conjugated alkenes, alkynes, annulated rings, imines and pyridines. In addition, a mechanism in which Ba 0 and BaH 2 are both essential is discussed. DFT calculations on benzene hydrogenation with a simple model system (Ba/BaH 2 ) confirm that the presence of metallic Ba has an accelerating effect., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Complexes of the Lewis base-free cations ( Me BDI)Mg + and ( t Bu BDI)Mg + with Ph-X ligands (X = F, Cl, Br, I) have been studied ( Me BDI = HC[C(Me)N-DIPP] 2 and t Bu BDI = HC[C( t Bu)N-DIPP] 2 ; DIPP = 2,6-diisopropylphenyl). For the smaller β-diketiminate ligand ( Me BDI) only complexes with PhF could be isolated. Heavier Ph-X ligands could not compete with bonding of Mg to the weakly coordinating anion B(C 6 F 5 ) 4 - . For the cations with the bulkier t Bu BDI ligand, the full series of halobenzene complexes was structurally characterized. Crystal structures show that the Mg⋯X-Ph angle strongly decreases with the size of X: F 139.1°, Cl 101.4°, Br 97.7°, I 95.1°. This trend, which is supported by DFT calculations, can be explained with the σ-hole which increases from F to I. Charge calculation and Atoms-In-Molecules analyses show that Mg⋯F-Ph bonding originates from electrostatic attraction between Mg 2+ and the very polar C δ + -F δ - bond. For the heavier halobenzenes, polarization of the halogen atom becomes increasingly important (Cl < Br < I). Complexation with Mg leads in all cases to significant Ph-X bond activation and elongation. This unusual coordination of halogenated species to early main group metals is therefore relevant to C-X bond breaking., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

The bulk of a recently reported superbulky β-diketiminate ligand was further increased by introducing tBu substituents in the ligand backbone. Attempts to isolate free Mg radicals with this extremely bulky ligand failed. Instead, a dinuclear Mg(i) complex with one chelating and one monodentate β-diketiminate ligand was isolated. Asymmetry in metal coordination results in a polarized Mg-Mg bond.

The reagent RK [R=CH(SiMe 3 ) 2 or N(SiMe 3 ) 2 ] was expected to react with the low-valent ( DIPP BDI)Al ( DIPP BDI=HC[C(Me)N(DIPP)] 2 , DIPP=2,6-iPr-phenyl) to give [( DIPP BDI)AlR] - K + . However, deprotonation of the Me group in the ligand backbone was observed and [H 2 C=C(N-DIPP)-C(H)=C(Me)-N-DIPP]Al - K + (1) crystallized as a bright-yellow product (73 %). Like most anionic Al I 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 C 6 H 6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C 6 H 6 , 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 ( DIPP BDI)Al. Calculations show that both Al I and K + work in concert and determines the reactivity of 1., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Two large decanuclear hydride clusters of the general formula Ae10H16[N(R)R']4·(PMDTA)2 (Ae = Ca, Sr) were isolated. Both derivatives feature a [Ae10H16]4+ core, consisting of two edge shared metal octahedra with interstitial hydrides, while the other hydrides cap triangular faces.

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=SiiPr 3 , 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., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)