Molecular Magnesium Hydrides

Reactivity of Strontium Hydride Supported by the Superbulky Hydrotris(pyrazolyl)borate Ligand

Hydrogenolysis of [(TpAd,iPr)Sr{CH(SiMe3)2}] (1) (TpAd,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) in hexane solution under 20 atm of H2 allowed for the isolation of strontium hydride [(TpAd,iPr)Sr(μ-H)]2 (2) in good yield. Complex 2 exhibits the dimeric nature in solid state, featuring two different bond modes between the Sr center and TpAd,iPr ligand. Treatment of complex 2 with PhC(H)═NtBu or PhCH2Bpin (Bpin = pinacolateborane) afforded the strontium amide complex [(TpAd,iPr)Sr{N(CH2Ph)(tBu)}] (4) and hydroborate complex [(TpAd,iPr)Sr{μ-HBpin(CH2Ph)}] (5), respectively. Reactions of complex 2 with 2-picoline, 2-phenylquinoline, or 2-phenylpyridine led to the formation of strontium 2-pyridylmethylene/2-picoline complex [(TpAd,iPr)Sr(2-CH2-Py)(2-picoline)] (6), reductively coupling diphenyl-biquinolide complex [{(TpAd,iPr)Sr}2(2,2'-Ph2-4,4'-dihydro-4,4'-biquinolide)] (7), and diphenyl-bipyridyl radical complex [(TpAd,iPr)Sr(6,6'-Ph2-2,2'-bipyridyl)] (8), separately. All of the complexes have been well characterized, including NMR spectrum and single-crystal X-ray analysis.

Synthesis and reactivity of a heteroleptic magnesium hydride on a dearomatized picolyl-based NNN-chelator

Heteroleptic magnesium hydrides are important for their stoichiometric and catalytic reduction chemistry. Their primary nucleophilic site is typically the hydride, while the ancillary ligands commonly used are mostly spectators. Chemically non-innocent ligands in comparison are rarely applied on MgH as their reactivity can be complex. Milstein et al. have recently reported Mg-mediated alkyne hydrogenation by using their metal-ligand cooperation (MLC) concept on a dearomatized picolyl-based PNP pincer that is non-innocent with a nucleophilic nature. A '(PNP)MgH' is noted as the active catalyst in hydrogenation but without structural validation. Inspired by the same, we report herein a novel NNN-chelator (MesL) with a dearomatized picolyl moiety and its well-defined MgH. Having two prominent nucleophilic sites, the present MgH shows metal-ligand competition while reacting with certain electrophiles. It also distinguishes nonpolar alkynes and polar carbonyls by cleanly inserting itself into the former but not the latter. The nucleophilicities of the two sites are also probed by DFT methods and compared with Milstein's (PNP)MgH. Although the present system shows no MLC-type H2 activation, the addition of a CS2 molecule in that way is realized.

A series of neutral alkaline earth metal hydride complexes supported by a bulky, unsymmetrical β-diketiminate ligand, [{(Dip/TCHPNacnac)M(μ-H)}2] (M = Mg, Ca, Sr or Ba)

A bulky, unsymmetrical β-diketiminate ligand, [HC{MeCN(Dip)}{MeCN(TCHP)}]- (Dip/TCHPNacnac; Dip = 2,6-diisopropylphenyl, TCHP = 2,4,6-tricyclohexylphenyl), has been utilised in the preparation of a series of magnesium alkyl and calcium, strontium and barium amide complexes. Reaction of these with PhSiH3 afforded the first complete series of β-diketiminato heavier group 2 metal hydride complexes, [{(Dip/TCHPNacnac)M(μ-H)}2] (M = Mg, Ca, Sr or Ba). The unsymmetrical nature of the β-diketiminate ligand seemingly promotes stabilising interactions of ligand Dip groups with the metal centres in the Ca, Sr and Ba hydride complexes.

Heteroleptic Magnesium n-Butyl on a Chemically Non-innocent 2-Anilidomethylpyridine Ligand Leading to Diverse Magnesium Hydrides

Molecular magnesium hydrides and hydride-rich clusters are of significant interest for applications ranging from catalysis and small molecule activation to hydrogen storage. Here, we investigate the 2-anilidomethylpyridine framework NNL as an ancillary support for magnesium organometallics with a special emphasis on hydrides. The proligand NNLH (N-[2,6-bis(1-methylethyl)phenyl]-α,6-diphenyl-2-pyridinemethanamine) gives [(NNL)Mg(nBu)(thf)] (1) by nbutane elimination from Mg(nBu)2(thf)n. A stronger donor such as DMAP replaces the THF from 1 to give [(NNL)Mg(nBu)(dmap)] (2). Both are air-sensitive, and 1 is adventitiously oxidized into [(NNL)Mg(μ-OnBu)]2 (32). The homoleptic [(NNL)2Mg] (8) is made from 1 and a second equiv of NNLH. 1's terminal nBu group is selectively protonated by HN(SiMe3)2 to give [(NNL)MgHMDS] (4; HMDS = N(SiMe3)2), whereas Ph3SiOH partially protonates the backbone anilide as well to give a mixture of [(NNL)Mg(OSiPh3)(thf)] (5) and free NNLH. Like HN(SiMe3)2, aprotic MeOTf also reacts by selectively abstracting the nBu group from 1 to give [(NNL)Mg(μ:κ2-O,O'-OTf)(thf)]2 (62). Interestingly, screening the common synthetic routes for magnesium hydrides leads to diverse outcomes upon varying the Mg precursors and hydride sources. 1 and PhSiH3 give the hydride cluster [{(NNL)2Mg2(μ-H)}2(μ-H)4Mg] (7), whereas 2 and PhSiH3 give the molecular complex [(NNLde)Mg(dmap)2] (9) with a dearomatized pyridyl backbone. 1 and HBpin (pinacolborane) give a product mixture, from which a different hydride cluster [(NNL)2Mg2(μ-H)}2(μ:κ2-O,O'-O2C2Me4)] (10) is identified, showing a rare instance of complete deborylation of a HBpin molecule. 1 and HBcat (catecholborane) also give a product mixture, one of which is the borylated ligand [(NNL)Bcat] (11). HBpin with 4 as the Mg precursor takes the ligand borylation route more selectively to give [(NNL)Bpin] (12). Last, 1 reacts with iPrNH2BH3 to give [(NNL)Mg{NH(iPr)BH3}] (13), which shows a slow and fractional conversion into the dinuclear mixed hydrido amidoborane [(NNL)2Mg2(μ-H){(μ-NH(iPr)BH3}] (14) by partial β-hydride elimination. In comparison, [(NNL)Mg(iPrNHBH3)(dmap)] (15) arising from the DMAP-bound 2 and iPrNH2BH3 is stable toward such elimination.

In Situ Construction of Interface with Photothermal and Mutual Catalytic Effect for Efficient Solar-Driven Reversible Hydrogen Storage of MgH2

Hydrogen storage in MgH2 is an ideal solution for realizing the safe storage of hydrogen. High operating temperature, however, is required for hydrogen storage of MgH2 induced by high thermodynamic stability and kinetic barrier. Herein, flower-like microspheres uniformly constructed by N-doped TiO2 nanosheets coated with TiN nanoparticles are fabricated to integrate the light absorber and thermo-chemical catalysts at a nanometer scale for driving hydrogen storage of MgH2 using solar energy. N-doped TiO2 is in situ transformed into TiNxOy and Ti/TiH2 uniformly distributed inside of TiN matrix during cycling, in which TiN and Ti/TiHx pairs serve as light absorbers that exhibit strong localized surface plasmon resonance effect with full-spectrum light absorbance capability. On the other hand, it is theoretically and experimentally demonstrated that the intimate interface between TiH2 and MgH2 can not only thermodynamically and kinetically promote H2 desorption from MgH2 but also simultaneously weaken Ti─H bonds and hence in turn improve H2 desorption from the combination of weakened Ti─H and Ti─H bonds. The uniform integration of photothermal and catalytic effect leads to the direct action of localized heat generated from TiN on initiating the catalytic effect in realizing hydrogen storage of MgH2 with a capacity of 6.1 wt.% under 27 sun.

Different Reaction Modes Operating in ansa-Half-Sandwich Magnesium Catalysts

Magnesium-based catalysts are becoming popular for hydroelementation reactions specially using p-block reagents. Based on the seminal report from Schäfer's group (ChemCatChem 2022, 14, e202201007), our study demonstrates that the reaction mechanisms exhibit a far greater degree of complexity than originally presumed. Magnesium has a variety of coordination modes (and access to different hybridizations) which allows this electron-deficient centre to modulate its catalytic power depending on the σ-donor properties of the reagent. DFT calculations demonstrate several reaction channels closely operating in these versatile catalysts. In addition, variations in limiting energy barriers resulting from catalyst modifications were examined as a function of the Hammett constant, thereby predicting enhanced efficiency in reaction conversions.

Recent progress in beryllium organometallic chemistry

Beryllium possesses a unique amalgamation of characteristics, its electronegativity included, that not only make it a vital component in a wide range of technical sectors and consumer industries, but also make it an interesting candidate for forming covalently bonded compounds. However, the extremely toxic nature of beryllium, which can cause chronic beryllium disease, has limited the exploration of its chemistry, making beryllium one of the least studied (non-radioactive) elements. The development of selective chelating ligands, sterically encumbered substituents and, moreover, the boom of N-heterocyclic carbenes in organometallic chemistry and main group chemistry has revived the interest in beryllium chemistry. Therefore, some quite remarkable progress in the coordination and organometallic chemistry of beryllium has been made in the last two decades. For example, low oxidation state beryllium compounds, antiaromatic/aromatic beryllium compounds, where beryllium is involved in π-electron delocalization, and the isolation of beryllium-beryllium bonded species have all been achieved. This article provides an oversight over the recent developments in the organometallic chemistry of beryllium.

Lanthanide Mimicking by Magnesium for Oxazolidinone Synthesis

In the last decade, magnesium complexes have emerged as a viable alternative to transition-metal catalysts for the hydrofunctionalization of unsaturated bonds. However, their potential for advanced catalytic reactions has not been thoroughly investigated. To address this gap, we have developed a novel magnesium amide compound (3) using a PNP framework that is both bulky and flexible. Our research demonstrates that compound 3 can effectively catalyze the synthesis of biologically significant oxazolidinone derivatives. This synthesis involves a tandem reaction of hydroalkoxylation and cyclohydroamination of isocyanate using propargyl alcohol. Furthermore, we conducted comprehensive theoretical calculations to gain insights into the reaction mechanism. It is important to note that these types of transformations have not been reported for magnesium and would significantly enhance the catalytic portfolio of the 7th most abundant element.

A Convenient One-Pot Synthesis of a Sterically Demanding Aniline from Aryllithium Using Trimethylsilyl Azide, Conversion to β-Diketimines and Synthesis of a β-Diketiminate Magnesium Hydride Complex

This work reports the one-pot synthesis of sterically demanding aniline derivatives from aryllithium species utilising trimethylsilyl azide to introduce amine functionalities and conversions to new examples of a common N,N'-chelating ligand system. The reaction of TripLi (Trip = 2,4,6-iPr3-C6H2) with trimethylsilyl azide afforded the silyltriazene TripN2N(SiMe3)2 in situ, which readily reacts with methanol under dinitrogen elimination to the aniline TripNH2 in good yield. The reaction pathways and by-products of the system have been studied. The extension of this reaction to a much more sterically demanding terphenyl system suggested that TerLi (Ter = 2,6-Trip2-C6H3) slowly reacted with trimethylsilyl azide to form a silyl(terphenyl)triazenide lithium complex in situ, predominantly underwent nitrogen loss to TerN(SiMe3)Li in parallel, which afforded TerN(SiMe3)H after workup, and can be deprotected under acidic conditions to form the aniline TerNH2. TripNH2 was furthermore converted to the sterically demanding β-diketimines RTripnacnacH (=HC{RCN(Trip)}2H), with R = Me, Et and iPr, in one-pot procedures from the corresponding 1,3-diketones. The bulkiest proligand was employed to synthesise the magnesium hydride complex [{(iPrTripnacnac)MgH}2], which shows a distorted dimeric structure caused by the substituents of the sterically demanding ligand moieties.

Three Oxidative Addition Routes of Alkali Metal Aluminyls to Dihydridoaluminates and Reactivity with CO2

Three distinct routes are reported to the soluble, dihydridoaluminate compounds, AM[Al(NONDipp )(H)2 ] (AM=Li, Na, K, Rb, Cs; [NONDipp ]2- =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ) starting from the alkali metal aluminyls, AM[Al(NONDipp )]. Direct H2 hydrogenation of the heavier analogues (AM=Rb, Cs) produced the first examples of structurally characterized rubidium and caesium dihydridoaluminates, although harsh conditions were required for complete conversion. Using 1,4-cyclohexadiene (1,4-CHD) as an alternative hydrogen source in transfer hydrogenation reactions provided a lower energy pathway to the full series of products for AM=Li-Cs. A further moderation in conditions was noted for the thermal decomposition of the (silyl)(hydrido)aluminates, AM[Al(NONDipp )(H)(SiH2 Ph)]. Probing the reaction of Cs[Al(NONDipp )] with 1,4-CHD provided access to a novel inverse sandwich complex, [{Cs(Et2 O)}2 {Al(NONDipp )(H)}2 (C6 H6 )], containing the 1,4-dialuminated [C6 H6 ]2- dianion and representing the first time that an intermediate in the commonly utilized oxidation process of 1,4-CHD to benzene has been trapped. The synthetic utility of the newly installed Al-H bonds has been demonstrated by their ability to reduce CO2 under mild conditions to form the bis-formate AM[Al(NONDipp )(O2 CH)2 ] compounds, which exhibit a diverse series of eyecatching bimetallacyclic structures.

Synthesis and Reactivity of Alkali Metal Hydrido-Magnesiate Complexes which Exhibit Group 1 Metal Counter-Cation Specific Stability

Reactions of the series of alkali metal amides M(HMDS) (M = Li-Cs; HMDS = [N(SiMe3)2]-) with the neutral magnesium(II) hydride compound [Mg(BDIDipp)(μ-H)]2 (BDIDipp = [CH{C(Me)NDipp}2], Dipp = 2,6-iPr2-C6H3) have been carried out. When M = Li or Na, the reactions yielded Mg(BDIDipp)(HMDS) and MH as the primary products. In the sodium amide reaction, [Na2(HMDS)][{Mg(BDIDipp)}2(H)3] was obtained as a low-yield by-product. When M = K-Cs, the reactions gave the group 1 metal hydrido-magnesiates, M2[Mg(BDIDipp)(HMDS)(H)]2·(benzene)n (n = 0 or 1), the thermal stability of which increases with the increasing molecular weight of the alkali metal involved. Reactions of Cs2[Mg(BDIDipp)(HMDS)(H)]2·(benzene) with 18-crown-6 and CO gave the first monomeric alkali metal hydrido-magnesiate [Cs(18-crown-6)][Mg(BDIDipp)(HMDS)(H)] and the ethenediolate complex Cs2[{Mg(BDIDipp)(HMDS)}2(μ-C2H2O2)], respectively. The new synthetic route to alkali metal hydrido-magnesiates described herein may facilitate further reactivity studies of this rare compound class.

Reactivity of a quasi-four-coordinate butylmagnesium cation

We present the reactivity of the Mg-C and the β-CH bonds in the trigonal pyramidal [(pmdta)Mg(nBu)]⁺ exhibiting a weak Mg⋯F interaction with counter anion, [B(C₆F₅)₄]^-. Instantaneous β-hydride reactivity with benzophenone, reductive alkylation of phenyl benzoate, and straightforward synthesis of [(pmdta)MgH]⁺via metathesis with pinacolborane/phenylsilane are discussed.

The Continuum Between Hexagonal Planar and Trigonal Planar Geometries

New heterometallic hydride complexes that involve the addition of {Mg-H} and {Zn-H} bonds to group 10 transition metals (Pd, Pt) are reported. The side-on coordination of a single {Mg-H} to Pd forms a well-defined σ-complex. In contrast, addition of three {Mg-H} or {Zn-H} bonds to Pd or Pt results in the formation of planar complexes with subtly different geometries. We compare their structures through experiment (X-ray diffraction, neutron diffraction, multinuclear NMR), computational methods (DFT, QTAIM, NCIPlot), and theoretical analysis (MO diagram, Walsh diagram). These species can be described as snapshots along a continuum of bonding between ideal trigonal planar and hexagonal planar geometries.

A Planar Nickelaspiropentane Complex with Magnesium-Based Metalloligands: Synthesis, Structure, and Synergistic Dihydrogen Activation

The nature of transition-metal-olefin bonding has been explained by the Dewar-Chatt-Duncanson model within a continuum of two extremes, namely, a π-complex and a metallacyclopropane. The textbook rule suggests that a low-spin late-transition-metal-ethylene complex more likely forms a π-complex rather than a metallacyclopropane. Herein, we report a low-spin late-transition-metal-bis-ethylene complex forming an unprecedented planar metalla-bis-cyclopropane structure with magnesium-based metalloligands. Treatment of LMgEt (L = [(DippNCMe)₂CH]^-, Dipp = 2,6-ⁱPr₂C₆H₃) with Ni(cod)₂ (cod = 1,5-cyclooctadiene) formed the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂, which features a linear Mg-Ni-Mg linkage and a planar coordination geometry at the nickel center. Both structural features and computational studies strongly supported the Ni(C₂H₄)₂ moiety as a nickelaspiropentane. The exposure of (LMg)₂Ni(C₂H₄)₂ to 1 bar H₂ at room temperature produced a four-hydride-bridged complex (LMg)₂Ni(μ-H)₄. The profile of H₂ activation was elucidated by density functional theory calculations, which indicated a novel Mg/Ni cooperative activation mechanism with no oxidation occurring at the metal center, differing from the prevailing mono-metal-based redox mechanism. Moreover, the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂ catalyzed the hydrogenation of a wide range of unsaturated substrates under mild conditions.

Synthesis and reactivity of heterometallic complexes containing Mg- or Zn-metalloligands

Heteronuclear metal complexes comprising main group metals and transition metals have attracted widespread attention from researchers due to their applications in stoichiometric and catalytic activation of small molecules with possible cooperative effects. Herein, the advances of heterometallic complexes containing Mg- or Zn-metalloligands over the past ten years are reviewed. They consist of two parts: (i) synthetic approaches to heterometallic complexes. Only a brief discussion is made on the different Mg/Zn precursors since they have been summarized before. (ii) Stoichiometric and catalytic reactivities of heterometallic complexes containing Mg/Zn metalloligands. The exploration of the cooperative catalytic reaction of heterometallic complexes is still in its infancy, promising but challenging; thus, further investigations are required in the future.

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

The addition of a B-H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon-carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.

Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

The electron-rich cyclopentadienyl and the analogous indenyl and fluorenyl ligands (collectively denoted here as Cp') have been impactful in stabilizing electron-deficient metal centers including the highly electropositive alkaline earths. Being in the s-block, the group 2 metals follow a major periodic variation in their atomic and ionic properties which is reflected in those Cp' compounds. This article presents an overview of this class of compounds for all the five metals from beryllium to barium (radium is excluded for its radioactivity), highlighting their systematic variation.

Theoretical investigation of the structures, stabilities, and vibrational and rotational spectroscopic parameters of linear HOMgNC and HMgNCO molecules by density functional theory and coupled-cluster method

Linear HOMgNC and HMgNCO molecules: two appropriate candidates for interstellar observation and experimental preparation.

Lithium and magnesium complexes from the employment of pyridyl-pendanted unsymmetrical β-diketiminates: syntheses and utilization as catalysts for the hydroboration of carbonyl compounds

The push for environmentally benign and sustainable chemical processes has reinforced the demand to displace transition metals with cheap, nontoxic and naturally abundant metals. To fulfil this requirement, we endeavored...

Generation of organo-alkaline earth metal complexes from non-polar unsaturated molecules and their synthetic applications

Organomagnesium compounds, represented by the Grignard reagents, are one of the most classical yet versatile carbanion species which have widely been utilized in synthetic chemistry. These reagents are typically prepared via oxidative addition of organic halides to magnesium metals, via halogen-magnesium exchange between halo(hetero)arenes and organomagnesium reagents or via deprotonative magnesiation of prefunctionalized (hetero)arenes. On the other hand, recent studies have demonstrated that the organo-alkaline earth metal complexes including those based on heavier alkaline earth metals such as calcium, strontium and barium could be generated from readily available non-polar unsaturated molecules such as alkenes, alkynes, 1,3-enynes and arenes through unique metallation processes. Nonetheless, the resulting organo-alkaline earth metal complexes could be further functionalized with a variety of electrophiles in various reaction modes. In particular, organocalcium, strontium and barium species have shown unprecedented reactivity in the downstream functionalization, which could not be observed in the reactivity of organomagnesium complexes. This perspective will focus on the newly emerging protocols for the generation of organo-alkaline earth metal complexes from non-polar unsaturated molecules and their applications in chemical synthesis and catalysis.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Synthesis, Structure, and Reactivity of a Terminal Cadmium Hydride Compound, [κ3-TismPriBenz]CdH

The terminal cadmium hydride compound, [κ³-Tism^PriBenz]CdH, which features the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, may be obtained via the reactions of either [κ³-Tism^PriBenz]CdN(SiMe₃)₂ or [Tism^PriBenz]CdOSiPh₃ with PhSiH₃. The Cd-H bond of [κ³-Tism^PriBenz]CdH undergoes (a) metathesis reactions with MeI, Me₃SiX (X = Cl, Br, I, NCO), and Me₃SnX (X = Cl, Br, I) to afford the corresponding [Tism^PriBenz]CdX derivative, (b) insertion with CO₂ and CS₂ to afford respectively [Tism^PriBenz]Cd(κ¹-O₂CH) and [Tism^PriBenz]Cd(κ¹-S₂CH), and (c) hydride abstraction with B(C₆F₅)₃ to afford {[Tism^PriBenz]Cd}[HB(C₆F₅)₃] that possesses a rare trigonal monopyramidal geometry for cadmium.

Synthesis of α-Alkynylnitrones via Hydromagnesiation of 1,3-Enynes with Magnesium Hydride

A protocol for the synthesis of α-alkynylnitrones from 1,3-enynes has been developed. The process is triggered by hydromagnesiation of 1,3-enynes with magnesium hydride (MgH₂), which is prepared in situ through solvothermal treatment of magnesium iodide (MgI₂) with sodium hydride (NaH) in tetrahydrofuran. Downstream functionalization of the resulting propargylmagnesium intermediates with organo nitro compounds affords α-alkynylnitrones, which could be used as versatile precursors for the construction of various nitrogen-containing compounds.

Roles of Ti-Based Catalysts on Magnesium Hydride and Its Hydrogen Storage Properties

Magnesium-based hydrides are considered as promising candidates for solid-state hydrogen storage and thermal energy storage, due to their high hydrogen capacity, reversibility, and elemental abundance of Mg. To improve the sluggish kinetics of MgH2, catalytic doping using Ti-based catalysts is regarded as an effective approach to enhance Mg-based materials. In the past decades, Ti-based additives, as one of the important groups of catalysts, have received intensive endeavors towards the understanding of the fundamental principle of catalysis for the Mg-H2 reaction. In this review, we start with the introduction of fundamental features of magnesium hydride and then summarize the recent advances of Ti-based additive doped MgH2 materials. The roles of Ti-based catalysts in various categories of elemental metals, hydrides, oxides, halides, and intermetallic compounds were overviewed. Particularly, the kinetic mechanisms are discussed in detail. Moreover, the remaining challenges and future perspectives of Mg-based hydrides are discussed.

C-N and C-H Activation of an N-Heterocyclic Carbene by Magnesium(II) Hydride and Magnesium(I) Complexes

Reactions of the hindered N-heterocyclic carbene, :C{(MesNCH)₂} (IMes; Mes = mesityl), with a series of β-diketiminatomagnesium(II) hydride and dimagnesium(I) complexes were carried out at 80 °C. The reactions involving the magnesium hydrides, [{(^ArNacnac)Mg(μ-H)}₂] [^ArNacnac = [(ArNCMe)₂CH]^-, where Ar = 2,6-diethylphenyl (Dep) or 2,6-diisopropylphenyl (Dip)], proceeded via activation of an exocyclic C-N bond of IMes, giving magnesium imidazolyl compounds [(^ArNacnac)Mg(μ-H)(μ-Imid)Mg(^ArNacnac)] (Imid = [NC₂H₂N(Mes)C]^-) and mesitylene. A low-yield IMes methyl C-H activation product, [(^DepNacnac)Mg(IMes^-H)], was also obtained, via H₂ elimination, from the reaction between IMes and [{(^DepNacnac)Mg(μ-H)}₂]. Reactions between IMes and dimagnesium(I) compounds [{(^ArNacnac)Mg}₂] [Ar = 2,6-dimethylphenyl (Xyl) or Mes] afforded isostructural C-H activation products [(^ArNacnac)Mg(IMes^-H)] but in higher yields. Density functional theory calculations suggest that the reactions do not progress via stable adduct complex intermediates, which are sterically inaccessible.

Enantiopure dimagnesium(i) and magnesium(ii) hydride complexes incorporating chiral amidinate or β-diketiminate ligands

A known chiral β-diketiminate, [HC{MeCN-(S)-(-)-CHMePh}(MeCNDip)]^- (Dip = 2,6-diisopropylphenyl) L¹, and a new chiral amidinate, [Bu^tC(NAr*){N-(S)-(-)-CHMePh}]^- L² (Ar* = C₆H₂{C(H)Ph₂}₂Me-2,6,4) have been utilised in the synthesis of the first examples of enantiopure, dinuclear magnesium(i), [(L¹)Mg-Mg(L¹)], and magnesium(ii) hydride complexes, [(L²)Mg(μ-H)₂Mg(L²)] and [(L²)(THF)Mg(μ-H)₂Mg(THF)(L²)]. A related enantiopure β-diketiminato magnesium hydride cluster compound, [(L¹)₄Mg₅H₆], has also been synthesised, presumably forming via a partial redistribution of in situ generated [(L¹)Mg(μ-H)₂Mg(L¹)].

Nickel(0)-Induced β-H Elimination of Magnesium Alkyls: Formation and Reactivity of Heterometallic Hydrides

We report the synthesis and reactivity of heterometallic Mg-Ni complexes with bridging hydrides. Treatment of magnesium monoalkyl complexes, which are supported by a tridentate β-diketiminato ligand bearing a pendent phosphine group, with nickel(0) reagent Ni(COD)₂ (COD: 1,5-cyclooctadiene) at a molar ratio of 2:1 resulted in the formation of a heterotrimetallic hydride-bridged [Mg-Ni-Mg] complex via facile elimination of the corresponding alkenes. A heterobimetallic hydride-bridged [Mg-Ni] complex served as an intermediate species for the formation of the [Mg-Ni-Mg] complex. Computational studies revealed that the reaction was initiated by coordination of nickel to magnesium followed by an alkyl group transfer. β-H elimination at the nickel center subsequently occurred to give the heterometallic hydride-bridged complex. Density functional theory analysis also highlighted a three-center two-electron interaction for the Mg-H-Ni unit. The hydride-bridged [Mg-Ni-Mg] complex showed diverse reactivity toward unsaturated small molecules. For instance, reactions with isocyanides provided heterometallic species by coordination of isocyanides to the nickel center, with no subsequent reduction detected. Isocyanides could also be dissociated at 80 °C. In contrast, hydromagnesiation occurred upon treatment of the heterotrimetallic hydride with carbodiimides, affording C₃-symmetric complexes with three heteroleptic magnesium mixed β-diketiminate/amidinate moieties. The hydride-bridged heterotrimetallic complex underwent dehydrogenation reaction with phenyl acetylene to produce an acetylide-bridged [Mg-Ni-Mg] complex.

Metal-Ligand Cooperation Facilitates Bond Activation and Catalytic Hydrogenation with Zinc Pincer Complexes

A series of PNP zinc pincer complexes capable of bond activation via aromatization/dearomatization metal–ligand cooperation (MLC) were prepared and characterized. Reversible heterolytic N–H and H–H bond activation by MLC is shown, in which hemilability of the phosphorus linkers plays a key role. Utilizing this zinc pincer system, base-free catalytic hydrogenation of imines and ketones is demonstrated. A detailed mechanistic study supported by computation implicates the key role of MLC in facilitating effective catalysis. This approach offers a new strategy for (de)hydrogenation and other catalytic transformations mediated by zinc and other main group metals.

Magnesium hydrides bearing sterically demanding amidinate ligands: synthesis, reactivity and catalytic application

The synthesis and characterisation of a series of magnesium complexes bearing sterically demanding amidinate ligands is reported; this includes magneisum amides (1a and 1b), hydrides (3a and 3b) and alkyl complexes (2b). The solid and solution state behaviour of the complexes has been investigated using single crystal X-ray diffraction and NMR spectroscopy, revealing the magnesium hydrides to exist as dimers in the solid state, dispite the sterically demanding ligand systems and showing a degree of monomeric character in solution. The stoichiometric and catalytic activity of the amidinate complexes were investigated, with the complexes found to efficiently mediate both the hydroamination of N,N'-diisopropylcarbodiimide and the Tishchenko reaction. The metal hydrides are highly reactive towards coordinating substrates, showing a significant increase in catalytic rate compared with more ubiquitous β-diketiminate magnesium hydrides.

Stereo-controlled anti-hydromagnesiation of aryl alkynes by magnesium hydrides

A concise protocol for anti-hydromagnesiation of aryl alkynes was established using 1 : 1 molar combination of sodium hydride (NaH) and magnesium iodide (MgI₂) without the aid of any transition metal catalysts. The resulting alkenylmagnesium intermediates could be trapped with a series of electrophiles, thus providing facile accesses to stereochemically well-defined functionalized alkenes. Mechanistic studies by experimental and theoretical approaches imply that polar hydride addition from magnesium hydride (MgH₂) is responsible for the process.

Anti-hydromagnesiation of aryl alkynes was facilitated solely by magnesium hydride. The resulting alkenylmagnesium intermediates were functionalized with various electrophiles to afford stereochemically defined tri-substituted alkenes.

N-Methylation and Trideuteromethylation of Amines via Magnesium-Catalyzed Reduction of Cyclic and Linear Carbamates

A new reduction of carbamates to N-methyl amines is presented. The magnesium-catalyzed reduction reaction allows the conversion of cyclic and linear carbamates, including N-Boc protected amines, into the corresponding N-methyl amines and amino alcohols which are of significant interest due to their presence in many biologically active molecules. Furthermore, the reduction can be extended to the formation of N-trideuteromethyl labeled amines.

A Hexagonal Planar Metal Complex

A six-coordinate [ML₃ Z₃ ]-type transition-metal complex with a hexagonal planar geometry has been isolated and characterized, extending the scope of six-coordinate metal coordination compounds to those with a geometry beyond octahedral and trigonal prismatic.

A Stable Calcium Alumanyl

A seven-membered N,N'-heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg-Al and Ca-Al bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

A Stable Calcium Alumanyl

A seven‐membered N,N′‐heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg−Al and Ca−Al bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

A versatile nitrogen ligand for alkaline-earth chemistry

A readily available and versatile bis(imino)carbazolate ligand is shown to allow for the synthesis of a broad range of solution stable heteroleptic complexes of the alkaline-earth metals Mg, Ca, Sr and Ba.

Mononuclear calcium complex as effective catalyst for alkenes hydrogenation

Mononuclear calcium unsubstituted alkyl complex [(Tp^Ad,iPr)Ca{(CH₂)₄Ph}(THP)], proposed as the catalytic alkene hydrogenation intermediate, was isolated for the first time.

Asymmetric ketone hydroboration catalyzed by alkali metal complexes derived from BINOL ligands

The ability of alkali metal complexes featuring functionalized BINOL-derived ligands to catalyze ketone hydroboration reactions was explored. The reduced products were formed in excellent yields and with variable enantioselectivities dependent upon the nature of the ligand and the alkali metal cation.

Combined KH/alkaline-earth metal amide catalysts for hydrogenation of alkenes

Breaking saline KH: The combined KH/alkaline-earth metal amide catalysts display much better catalytic activity than their components in the hydrogenation of alkenes, suggesting the degradation and activation of saline KH with the metal amides.

Barium tetraalkylaluminate complexes supported by the super-bulky hydrotris(pyrazolyl)borate ligand

A heteroleptic barium aminobenzyl complex [(Tp^Ad,iPr)Ba(o-CH₂C₆H₄-NMe₂)] (1) was obtained in excellent yield from a simple one-pot reaction. Treatment of [(Tp^Ad,iPr)Ba(o-CH₂C₆H₄-NMe₂)] (1) with two equivalents of AlR₃ (R = Me, Et) led to the formation of barium tetraalkylaluminate complexes [(Tp^Ad,iPr)Ba(AlR₄)]_n (R = Me, n = 2, 2; R = Et, n = 1, 3) as dimers or monomers in the solid state. The Tp^Ad,iPr ligand-free peralkylated barium complex [Ba(AlEt₄)₂]_n was isolated by the addition of ten equivalents of AlEt₃ under the same conditions. The donor-induced aluminate cleavage is not applicable when donor solvents are added to complexes 2 and 3. In the solution of complexes 2 and 3, the alkylaluminate moieties and Tp^Ad,iPr ligands show a rapid fluxional behavior in [D₈]toluene solution over the temperature range of -70 to 25 °C, without any significant decoalescence of the corresponding proton signals.

Magnesium hydride alkene insertion and catalytic hydrosilylation

The β-diketiminato magnesium hydride, [(BDI)MgH]]₂, reacts with alkenes and catalyses their hydrosilylation with PhSiH₃.

The dimeric β-diketiminato magnesium hydride, [(BDI)MgH]₂, reacts at 80 °C with the terminal alkenes, 1-hexene, 1-octene, 3-phenyl-1-propene and 3,3-dimethyl-butene to provide the respective n-hexyl, n-octyl, 3-phenylpropyl and 3,3-dimethyl-butyl magnesium organometallics. The facility for and the regiodiscrimination of these reactions are profoundly affected by the steric demands of the alkene reagent. Reactions with the phenyl-substituted alkenes, styrene and 1,1-diphenylethene, require a more elevated temperature of 100 °C with styrene providing a mixture of the 2-phenylethyl and 1-phenylethyl products over 7 days. Although the reaction with 1,1-diphenylethene yields the magnesium 1,1-diphenylethyl derivative as the sole reaction product, only 64% conversion was achieved over a 21 day timeframe. Reactions with the α,ω-dienes, 1,5-hexadiene and 1,7-octadiene, provided divergent results. The initial 5-alkenyl magnesium reaction product of the shorter chain diene undergoes 5-exo-trig cyclisation via intramolecular carbomagnesiation to provide a cyclopentylmethyl derivative, which was shown by X-ray diffraction analysis to exist as a three-coordinate monomer. In contrast, 1,7-octadiene provided a mixture of two compounds, a magnesium oct-7-en-1-yl derivative and a dimagnesium-octane-1,4-diide, as a result of single or two-fold activation of the terminal C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C double bonds. The magnesium hydride was unreactive towards internal alkenes apart from the strained bicycle, norbornene, allowing the characterisation of the resultant three-coordinate magnesium norbornyl derivative by X-ray diffraction analysis. Computational analysis of the reaction between [(BDI)MgH]₂ and 1-hexene using density functional theory (DFT) indicated that the initial Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion process is rate determining and takes place at the intact magnesium hydride dimer. This exothermic reaction (ΔH = –14.1 kcal mol^–1) traverses a barrier of 18.9 kcal mol^–1 and results in the rupture of the dinuclear structure into magnesium alkyl and hydride species. Although the latter three-coordinate hydride derivative may be prone to redimerisation, it can also provide a further pathway to magnesium alkyl species through its direct reaction with a further equivalent of 1-hexene, which occurs via a lower barrier of 15.1 kcal mol^–1. This Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion reactivity provides the basis for the catalytic hydrosilylation of terminal alkenes with PhSiH₃, which proceeds with a preference for the formation of the anti-Markovnikov organosilane product. Further DFT calculations reveal that the catalytic reaction is predicated on a sequence of Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion and classical Si–H/Mg–C σ-bond metathesis reactions, the latter of which, with a barrier height of 24.9 kcal mol^–1, is found to be rate determining.