Super‐Bulky Penta‐arylcyclopentadienyl Ligands: Isolation of the Full Range of Half‐Sandwich Heavy Alkaline‐Earth Metal Hydrides

Facile, Reversible Hydrogen Activation by Low-Coordinate Magnesium Oxide Complexes

New approaches to achieve facile and reversible dihydrogen activation are of importance for synthesis, catalysis, and hydrogen storage. Here we show that low-coordinate magnesium oxide complexes [{(RDipnacnac)Mg}2(μ-O)] 1, with RDipnacnac = HC(RCNDip)2, Dip = 2,6-iPr2C6H3, R = Me (1a), Et (1b), iPr (1c), readily react with dihydrogen under mild conditions to afford mixed hydride-hydroxide complexes [{(RDipnacnac)Mg}2(μ-H)(μ-OH)] 4. Dehydrogenation of complexes 4 is strongly dependent on remote ligand substitution and can be achieved by simple vacuum-degassing of 4c (R = iPr) to regain 1c. Donor addition to complexes 4 also releases hydrogen and affords donor adducts of magnesium oxide complexes. Computational studies suggest that the hydrogen activation mechanism involves nucleophilic attack of an oxide lone pair at a weakly bound H2···Mg complex in an SN2-like manner that induces a heterolytic dihydrogen cleavage to yield an MgOH and an MgH unit. Alternative synthetic routes into complex 4b from a magnesium hydride complex have been investigated and the ability of complexes 1 or 4 to act as catalysts for the hydrogenation of 1,1-diphenylethene (DPE) has been tested.

Stabilisation of a Strontium Hydride with a Monodentate Carbazolyl Ligand and its Reactivity

The molecular strontium hydride 2 [(dtbpCbz)SrH(L)]2 (L=benzene, toluene) was isolated and stabilized by employing a sterically demanding carbazole ligand (dtbpCbz=1,8-bis(3,5-ditertbutylphenyl)-3,6-ditertbutylcarbazolyl). Compound 2 was synthesized via phenylsilane metathesis with the corresponding amide (dtbpCbz)SrN(SiMe3)2 and characterized by 1H NMR, XRD and vibrational spectroscopy methods. We further investigated the stoichiometric reactivity of 2 towards carbon monoxide, azobenzene and trimethylsilylacetylene, showing three distinct reactivity pathways: addition, reduction and deprotonation. The reaction of 2 with carbon monoxide yields the ethenediolate complex 4 via addition, while with azobenzene reduction of the N-N double bond and release of hydrogen were observed, affording a heteroleptic strontium complex with a radical azobenzenyl ligand (5). The terminal alkyne is deprotonated by the hydride moiety to give the acetylide complex 6.

Carbene-Functionalized Bulky-Cyclopentadiene Rings

A series of significantly bulky mono- and di-substituted cyclic alkyl-amino carbene (cAAC)- functionalized cyclopentadiene ring (Cp) compounds were synthesized. The functionalization of the Cp ring with cAAC ligands makes them significantly bulkier, while retaining their ligation properties. These compounds display interesting fluorescence properties. In these compounds, intra-molecular charge transfer is observed from electron-rich carbene to electron-deficient cyclopentadiene unit. These high yielding compounds have been characterized by X-ray single-crystal diffraction and their emission properties have been studied. Rotational conformers (via C-C bond rotation) play a pivotal role with possible different extent of intramolecular charge-transfer (ICT) from carbene to cyclopentadiene ring. Variable temperature-dependent NMR studies were performed along with NOSY, COSY and different 2D NMR techniques to estimate the energy barriers and 1,5-Hydrogen shift.

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.

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.

CO reduction by calcium and ytterbium hydride complexes with a bulky monodentate carbazolyl ligand

The bulky monodentate carbazolyl ligand 1,8-bis(3,5-ditertbutylphenyl)-3,6-ditertbutylcarbazole (dtbpCbz) was employed in the synthesis of monomeric heteroleptic amido carbazolyl complexes of Ca and Yb. For both central metal atoms, dimeric hydride complexes [(dtbpCbz)Ca(benzene)H]2, [(dtbpCbz)Ca(THF)H]2, [(dtbpCbz)Yb(benzene)H]2 and [(dtbpCbz)Yb(THF)H]2 were obtained, which show remarkably poor solubility in organic solvents. The characteristic hydride 1H NMR resonance of [(dtbpCbz)Ca(benzene)H]2 was observed at 2.07 ppm, and for the first time, characteristic vibrational modes of the Ca2H2 and Yb2H2 moiety are discussed. Despite their poor solubility, the hydride complexes could be reacted with CO to yield the corresponding ethenediolate complexes.

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.

Multi-electron redox reactivity of a samarium(ii) hydrido complex

Well-defined low-valent molecular rare-earth metal hydrides are rare, and limited to Yb2+ and Eu2+ centers. Here, we report the first example of the divalent samarium(ii) hydrido complex [(CpAr5)SmII(μ-H)(DABCO)]2 (4) (CpAr5 = C5Ar5, Ar = 3,5-iPr2-C6H3; DABCO = 1,4-diazabicyclooctane) supported by a super-bulky penta-arylcyclopentadienyl ligand, resulting from the hydrogenolysis of the samarium(ii) alkyl complex [(CpAr5)SmII{CH(SiMe3)2}(DABCO)] (3). Complex 4 exhibits multi-electron redox reactivity toward a variety of substrates. Exposure of complex 4 to CO2 results in the formation of the trivalent samarium(iii) mixed-bis-formate/carbonate complex [(CpAr5)SmIII(μ-η2:η1-O2CH)(μ-η2:η2-CO3)(μ-η1:η1-O2CH)SmIII(CpAr5)(DABCO)] (8), mediated by hydride insertion and reductive disproportionation reactions. Complex 4 shows four-electron reduction toward four equivalents of CS2 to afford the trivalent samarium(iii) bis-trithiocarbonate complex [(CpAr5)SmIII(μ-η2:η2-CS3)(DABCO)]2 (9). A mechanistic study of the formation of complex 8 was carried out using DFT calculations.

Copper-catalysed perarylation of cyclopentadiene: synthesis of hexaarylcyclopentadienes

While hexaphenylsilacyclopentadiene (hexaphenylsilole) is viewed as an archetypal Aggregation-Induced Emission (AIE) luminogen, its isostructural hydrocarbon surrogate hexaphenylcyclopentadiene has strikingly never been investigated in this context, most probably due to a lack of synthetic availability. Herein, we report a straightforward synthesis of hexaphenylcyclopentadiene, via the direct perarylation of cyclopentadiene upon copper(i) catalysis under microwave activation, with the formation of six new C-C bonds in a single synthetic operation. Using zirconocene dichloride as a convenient source of cyclopentadiene and a variety of aryl iodides as coupling partners, this copper-catalysed cross-coupling reaction gave rise to a series of unprecedented hexaarylcyclopentadienes. The latter are direct precursors of extended π-conjugated polycyclic compounds, and their cyclodehydrogenation under Scholl reaction conditions yielded helicenic 17,17-diarylcyclopenta[l,l']diphenanthrenes. These structurally complex polyannelated fluorene derivatives can now be prepared in only two synthetic steps from cyclopentadiene.

Calcium-Ligand Cooperation Promoted Activation of N2O, Amine, and H2 as well as Catalytic Hydrogenation of Imines, Quinoline, and Alkenes

Bond activation and catalysis using s-block metals are of great significance. Herein, a series of calcium pincer complexes with deprotonated side arms have been prepared using pyridine-based PNP and PNN ligands. The complexes were characterized by NMR and X-ray crystal diffraction. Utilizing the obtained calcium complexes, unprecedented N2O activation by metal-ligand cooperation (MLC) involving dearomatization-aromatization of the pyridine ligand was achieved, generating aromatized calcium diazotate complexes as products. Additionally, the dearomatized calcium complexes were able to activate the N-H bond as well as reversibly activate H2, offering an opportunity for the catalytic hydrogenation of various unsaturated molecules. DFT calculations were applied to analyze the electronic structures of the synthesized complexes and explore possible reaction mechanisms. This study is an important complement to the area of MLC and main-group metal chemistry.

Low oxidation state and hydrido group 2 complexes: synthesis and applications in the activation of gaseous substrates

Numerous industrial processes utilise gaseous chemical feedstocks to produce useful chemical products. Atmospheric and other small molecule gases, including anthropogenic waste products (e.g. carbon dioxide), can be viewed as sustainable building blocks to access value-added chemical commodities and materials. While transition metal complexes have been well documented in the reduction and transformation of these substrates, molecular complexes of the terrestrially abundant alkaline earth metals have also demonstrated promise with remarkable reactivity reported towards an array of industrially relevant gases over the past two decades. This review covers low oxidation state and hydrido group 2 complexes and their role in the reduction and transformation of a selection of important gaseous substrates towards value-added chemical products.

Synthesis and Reactivity of Discrete Europium(II) Hydride Complexes

The bulky β-diketiminate ligand frameworks [BDIDCHP]- and [BDIDipp/Ar]- (BDI=[HC{C(Me)2N-Dipp/Ar}2]- (Dipp=2,6-diisopropylphenyl (Dipp); Ar=2,6-dicyclohexylphyenyl (DCHP) or 2,4,6-tricyclohexylphyenyl (TCHP)) have been developed for the kinetic stabilisation of the first europium (II) hydride complexes, [(BDIDCHP)Eu(μ-H)]2, [(BDIDipp/DCHP)Eu(μ-H)]2 and [(BDIDipp/TCHP)Eu(μ-H)]2, respectively. These complexes represent the first step beyond the current lanthanide(II) hydrides that are all based on ytterbium. Tuning the steric profile of β-diketiminate ligands from a symmetrical to unsymmetrical disposition, enhanced solubility and stability in the solution-state. This provides the first opportunity to study the structure and bonding of these novel Eu(II) hydride complexes crystallographically, spectroscopically and computationally, with their preliminary reactivity investigated.

Synthesis, characterisation, and catalytic application of a soluble molecular carrier of sodium hydride activated by a substituted 4-(dimethylamino)pyridine

Recently main group compounds have stepped into the territory of precious transition metal compounds with respect to utility in the homogeneous catalysis of fundamentally important organic transformations. Inspired by the need to promote more sustainability in chemistry because of their greater abundance in nature, this change of direction is surprising since main group metals generally do not possess the same breadth of reactivity as precious transition metals. Here, we introduce the dihydropyridylsodium compound, Na-1,2-tBu-DH(DMAP), and its monomeric variant [Na-1,2-tBu-DH(DMAP)]·Me6TREN, and demonstrate their effectiveness in transfer hydrogenation catalysis of the representative alkene 1,1-diphenylethylene to the alkane 1,1-diphenylethane using 1,4-cyclohexadiene as hydrogen source [DMAP = 4-dimethylaminopyridine; Me6TREN = tris(N,N-dimethyl-2-aminoethyl)amine]. Sodium is appealing because of its high abundance in the earth's crust and oceans, but organosodium compounds have been rarely used in homogeneous catalysis. The success of the dihydropyridylsodium compounds can be attributed to their high solubility and reactivity in organic solvents.

Fluorenyl-tethered N-heterocyclic carbene (NHC): an exclusive C-donor ligand for heteroleptic calcium and strontium chemistry

Exclusive C-donating ligands are rarely used with kinetically labile heavier alkaline earths (Ca, Sr, Ba). We report herein the aptitude of a combination of NHC with fluorenyl connected by a flexible -(CH2)2- linker as a ligand support for heteroleptic Ca- and Sr-N(SiMe3)2 and iodides. The Ca-N(SiMe3)2 complex even catalyzes the intramolecular hydroamination of aminoalkenes to showcase the effectiveness of this ligand framework.

Designing New Magnesium Pincer Complexes for Catalytic Hydrogenation of Imines and N-Heteroarenes: H2 and N-H Activation by Metal-Ligand Cooperation as Key Steps

Utilization of main-group metals as alternatives to transition metals in homogeneous catalysis has become a hot research area in recent years. However, their application in catalytic hydrogenation is less common due to the difficulty in heterolytic cleavage of the H-H bond. Employing aromatization/de-aromatization metal-ligand cooperation (MLC) highly enhances the H₂ activation process, offering an efficient approach for the hydrogenation of unsaturated molecules catalyzed by main-group metals. Herein, we report a series of new magnesium pincer complexes prepared using PNNH-type pincer ligands. The complexes were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H₂ and N-H bonds by MLC employing these pincer complexes was developed. Using the new magnesium complexes, homogeneously catalyzed hydrogenation of aldimines and ketimines was achieved, affording secondary amines in excellent yields. Control experiments and DFT studies reveal that a pathway involving MLC is favorable for the hydrogenation reactions. Moreover, the efficient catalysis was extended to the selective hydrogenation of quinolines and other N-heteroarenes, presenting the first example of hydrogenation of N-heteroarenes homogeneously catalyzed by early main-group metal complexes. This study provides a new strategy for hydrogenation of C═N bonds catalyzed by magnesium compounds and enriches the research of main-group metal catalysis.

Magnesium Pincer Complexes and Their Applications in Catalytic Semihydrogenation of Alkynes and Hydrogenation of Alkenes: Evidence for Metal-Ligand Cooperation

The development of catalysts for environmentally benign organic transformations is a very active area of research. Most of the catalysts reported so far are based on transition-metal complexes. In recent years, examples of catalysis by main-group metal compounds have been reported. Herein, we report a series of magnesium pincer complexes, which were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H₂ via aromatization/dearomatization metal-ligand cooperation was studied. Utilizing the obtained complexes, the unprecedented homogeneous main-group metal catalyzed semihydrogenation of alkynes and hydrogenation of alkenes were demonstrated under base-free conditions, affording Z-alkenes and alkanes as products, respectively, with excellent yields and selectivities. Control experiments and DFT studies reveal the involvement of metal-ligand cooperation in the hydrogenation reactions. This study not only provides a new approach for the semihydrogenation of alkynes and hydrogenation of alkenes catalyzed by magnesium but also offers opportunities for the hydrogenation of other compounds catalyzed by main-group metal complexes.

Teaming up main group metals with metallic iron to boost hydrogenation catalysis

Hydrogenation of unsaturated bonds is a key step in both the fine and petrochemical industries. Homogeneous and heterogeneous catalysts are historically based on noble group 9 and 10 metals. Increasing awareness of sustainability drives the replacement of costly, and often harmful, precious metals by abundant 3d-metals or even main group metals. Although not as efficient as noble transition metals, metallic barium was recently found to be a versatile hydrogenation catalyst. Here we show that addition of finely divided Fe⁰, which itself is a poor hydrogenation catalyst, boosts activities of Ba⁰ by several orders of magnitude, enabling rapid hydrogenation of alkynes, imines, challenging multi-substituted alkenes and non-activated arenes. Metallic Fe⁰ also boosts the activity of soluble early main group metal hydride catalysts, or precursors thereto. This synergy originates from cooperativity between a homogeneous, highly reactive, polar main group metal hydride complex and a heterogeneous Fe⁰ surface that is responsible for substrate activation.

Elemental iron turns alkaline-earth metal complexes into highly active catalysts for the hydrogenation of alkenes, alkynes, imines and arenes. The proposed mechanism combines homogeneous catalysis by a soluble main group metal hydride complex with heterogeneous catalysis at the iron surface.

Bulking up CpBIG: A Penta-Terphenyl Cyclopentadienyl Ligand

The modification of cyclopentadienyl ligands with carefully selected substituents is a widely used strategy for tuning their steric and electronic properties. We describe the synthesis of an extremely bulky penta-terphenyl cyclopentadienyl ligand (Cp^T5) by arylation of cyclopentadiene. Deprotonation reactions with various group 1 metals and bases afforded a complete series of alkali metal salts MCp^T5 (M = Li–Cs). The compounds were isolated as solvate-free salts, which were characterized by multinuclear nuclear magnetic resonance spectroscopy, ultraviolet–visible spectroscopy, and elemental analysis. Single-crystal X-ray diffraction studies of LiCp^T5, NaCp^T5 (crystallized as a solvate with one tetrahydrofuran molecule per formula unit), and KCp^T5 revealed the formation of metallocene-like sandwich structures in the solid state.

Formation and Reactivity of a Hexahydridosilicate [SiH6 ]2- Coordinated by a Macrocycle-Supported Strontium Cation

The cationic benzyl complex [(Me4 TACD)Sr(CH2 Ph)][A] (Me4 TACD=1,4,7,10-tetramethyltetraazacyclododecane; A=B(C6 H3 -3,5-Me2 )4 ) reacted with two equivalents of phenylsilane to give the bridging hexahydridosilicate complex [(Me4 TACD)2 Sr2 (thf)4 (μ-κ3  : κ3 -SiH6 )][A]2 (3 a). Rapid phenyl exchange between phenylsilane molecules is assumed to generate monosilane SiH4 that is trapped by two strontium hydride cations [(Me4 TACD)SrH(thf)x ]+ . Complex 3 a decomposed in THF at room temperature to give the terminal silanide complex [(Me4 TACD)Sr(SiH3 )(thf)2 ][A], with release of H2 . Upon reaction with a weak Brønsted acid, CO2 , and 1,3,5,7-cyclooctatetraene SiH4 was released. The reaction of a 1 : 2 mixture of cationic benzyl and neutral dibenzyl complex with phenylsilane gave the trinuclear silanide complex [(Me4 TACD)3 Sr3 (μ2 -H)3 (μ3 -SiH3 )2 ][A], while n OctSiH3 led to the trinuclear (n-octyl)pentahydridosilicate complex [(Me4 TACD)3 Sr3 (μ2 -H)3 (μ3 -SiH5 n Oct)][A].

Strontium Hydride Cations Supported by a Large NNNNN Type Macrocycle: Synthesis, Structure, and Hydrofunctionalization Catalysis

The use of the 15-membered NNNNN macrocyclic ligand Me₅PACP (Me₅PACP = 1,4,7,10,13-pentamethyl-1,4,7,10,13-pentaazacyclopentadecane) allowed the isolation of two cationic strontium hydride complexes by hydrogenolysis of benzyl precursors. Treatment of sparingly soluble [(Me₅PACP)Sr(CH₂Ph)₂] with dihydrogen gave free Me₅PACP, toluene, and oligomeric strontium hydride [SrH₂]_n, while hydrogenolysis in the presence of 1 equiv of the benzyl cation [(Me₅PACP)Sr(CH₂Ph)][B(C₆H₃-3,5-Me₂)₄] enabled isolation of the thermally unstable trihydride cation [(Me₅PACP)₂Sr₂(μ-H)₃][B(C₆H₃-3,5-Me₂)₄]. When the benzyl cation [(Me₅PACP)Sr(CH₂Ph)][BAr₄]₂ (Ar = C₆H₃-3,5-Me₂ or C₆H₄-4-ⁿBu) was reacted with dihydrogen or n-octylsilane, dihydride complexes [(Me₅PACP)₂Sr₂(μ-H)₂][BAr₄]₂ containing a dinuclear core bridged by two hydride ligands were obtained. The soluble dihydride complex [(Me₅PACP)₂Sr₂(μ-H)₂][B(C₆H₄-4-ⁿBu)₄]₂ was tested in olefin hydrogenation and hydrosilylation catalysis. Kinetic analyses for [(Me₅PACP)₂Sr₂(μ-H)₂]²⁺ showed lower catalytic activity as compared to that of the isostructural calcium homologue [(Me₅PACP)₂Ca₂(μ-H)₂]²⁺. This is explained by a shift in the monomer-dimer equilibrium which precedes the catalytic cycle.

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.

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.

Scandium and lanthanum hydride complexes stabilized by super-bulky penta-arylcyclopentadienyl ligands

Hydrogenolysis of the half-sandwich penta-arylcycopentadienyl-supported rare-earth metal dibenzyl complexes [(Cp^Ar5)Ln(p-CH₂-C₆H₄-Me)₂(THF)] (Cp^Ar5 = C₅Ar₅, Ar = 3,5-ⁱPr₂-C₆H₃; Ln = Sc, La) afforded a bimetallic scandium complex [(Cp^Ar5)Sc(H)(μ-OC₄H₉)]₂ (2) with two terminal hydrido ligands, and a double-sandwich bimetallic lanthanum hydride complex [(Cp^Ar5)La(μ-H)]₂ (4) bearing the reduced Cp^Ar5 ligand. DFT calculations were conducted to elucidate the reaction profiles.

Bonding in Barium Boryloxides, Siloxides, Phenoxides and Silazides: A Comparison with the Lighter Alkaline Earths

Barium complexes ligated by bulky boryloxides [OBR₂ ]^- (where R=CH(SiMe₃ )₂ , 2,4,6-ⁱ Pr₃ -C₆ H₂ or 2,4,6-(CF₃ )₃ -C₆ H₂ ), siloxide [OSi(SiMe₃ )₃ ]^- , and/or phenoxide [O-2,6-Ph₂ -C₆ H₃ ]^- , have been prepared. A diversity of coordination patterns is observed in the solid state for both homoleptic and heteroleptic complexes, with coordination numbers ranging between 2 and 4. The identity of the bridging ligand in heteroleptic dimers [Ba(μ₂ -X₁ )(X₂ )]₂ depends largely on the given pair of ligands X₁ and X₂ . Experimentally, the propensity to fill the bridging position increases according to [OB{CH(SiMe₃ )₂ }₂ )]^- <[N(SiMe₃ )₂ ]^- <[OSi(SiMe₃ )₃ ]^- <[O(2,6-Ph₂ -C₆ H₃ )]^- <[OB(2,4,6-ⁱ Pr₃ -C₆ H₂ )₂ ]^- . This trend is the overall expression of 3 properties: steric constraints, electronic density and σ- and π-donating capability of the negatively charged atom, and ability to generate Ba ⋅ ⋅ ⋅ F, Ba ⋅ ⋅ ⋅ C(π) or Ba ⋅ ⋅ ⋅ H-C secondary interactions. The comparison of the structural motifs in the complexes [Ae{μ₂ -N(SiMe₃ )₂ }(OB{CH(SiMe₃ )₂ }₂ )]₂ (Ae = Mg, Ca, Sr and Ba) suggest that these observations may be extended to all alkaline earths. DFT calculations highlight the largely prevailing ionic character of ligand-Ae bonding in all compounds. The ionic character of the Ae-ligand bond encourages bridging coordination, whereas the number of bridging ligands is controlled by steric factors. DFT computations also indicate that in [Ba(μ₂ -X₁ )(X₂ )]₂ heteroleptic dimers, ligand predilection for bridging vs. terminal positions is dictated by the ability to establish secondary interactions between the metals and the ligands.

Cationic strontium hydride complexes supported by an NNNN-type macrocycle

A trinuclear strontium hydride [(Me4TACD)3Sr3(μ2-H)4(thf)][B(C6H3-3,5-Me2)4]2 (Me4TACD = 1,4,7,10-tetramethyltetraazacyclododecane) and a mixed calcium strontium hydride [(Me4TACD)2CaSr(μ-H)2(thf)][B(C6H3-3,5-Me2)4]2 were isolated by hydrogenolysis of cationic benzyl precursors. A solution of [(Me4TACD)2CaSr(μ-H)2(thf)][B(C6H3-3,5-Me2)4]2 shows hydride ligand exchange between calcium and strontium centers and higher affinity of the hydride ligand toward calcium.

Calcium-mediated C(sp3)-H Activation and Alkylation of Alkylpyridines

Main-group metal calcium-mediated alkylpyridine benzylic C(sp³)-H activation and functionalization have been achieved. The reaction of a calcium hydride complex [{(^DIPPnacnac)CaH(thf)}₂] (^DIPPnacnac = CH{(CMe)(2,6-ⁱPr₂-C₆H₃N)}₂) with two equivalents of 2,6-lutidine rapidly yields a monomeric calcium alkyl complex with the release of dihydrogen. A hydride/carbon-bridged binuclear calcium complex [{(^DIPPnacnac)Ca}₂(μ-H){2-Me-6-(μ-CH₂)-Py}(thf)] is obtained from an equimolar treatment of calcium hydride and 2,6-lutidine that is readily converted into mono- or binuclear calcium alkyl complexes upon subsequent addition of 2,6-lutidine. DFT calculations and kinetic studies are conducted to determine their reaction profiles. More significantly, this calcium hydride complex catalyzes regioselective benzylic C-H bond addition of alkylpyridines to a variety of alkenes, affording linear or branched alkylated pyridine derivatives.

Metallic Barium: A Versatile and Efficient Hydrogenation Catalyst

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.

Calcium Hydride Catalysts for Olefin Hydrofunctionalization: Ring-Size Effect of Macrocyclic Ligands on Activity

The fifteen-membered NNNNN macrocycle Me5 PACP (Me5 PACP=1,4,7,10,13-pentamethyl-1,4,7,10,13-pentaazacyclopentadecane) stabilized the [CaH]+ fragment as a dimer with a distorted pentagonal bipyramidal coordination geometry at calcium. The hydride complex was prepared by protonolysis of calcium dibenzyl with the conjugate acid of Me5 PACP followed by hydrogenolysis or treating with n OctSiH3 of the intermediate calcium benzyl cation. The calcium hydride catalyzed the hydrogenation and hydrosilylation of unactivated olefins faster than the analogous calcium complex stabilized by the twelve-membered NNNN macrocycle Me4 TACD (Me4 TACD=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Kinetic investigations indicate that higher catalytic efficiency for the Me5 PACP stabilized calcium hydride is due to easier dissociation of the dimer in solution when compared to the Me4 TACD analogue.

Reactivity of a Molecular Calcium Hydride Cation ([CaH]+) Supported by an NNNN Macrocycle

The hydride ligand in the cationic calcium hydride supported by a NNNN-type macrocycle, [(Me₄TACD)₂Ca₂(μ-H)₂(THF)][BAr₄]₂ (1; Me₄TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane; THF = tetrahydrofuran; BAr₄ = B(C₆H₃-3,5-Me₂)₄), shows, in addition to its Brönsted basicity toward weak acids, a pronounced nucleophilicity resulting in nucleophilic substitution or insertion (addition) at a silicon or sp² carbon center. Terminal acetylenes RC≡CH (R = SiMe₃, cyclopropyl) as well as 1,4-diphenylbutadiene were deprotonated by 1 to give dinuclear complexes [(Me₄TACD)₂Ca₂(μ-C≡CR)₂][BAr₄]₂ (2a, R = SiMe₃; 2b, R = cyclopropyl) and [(Me₄TACD)₂Ca₂(μ₂-η⁴-1,4-Ph₂C₄H₂)][BAr₄]₂ (3) with H₂ evolution. The addition reaction with BH₃(THF) gave a tetrahydridoborate complex, [(Me₄TACD)Ca(BH₄)(THF)₂][BAr₄] (4), with κ₂-H₂BH₂ coordination in the solid state, suggesting a pronounced Lewis acidic calcium center. The behavior resulting from both Lewis acidity and hydricity becomes apparent in the nucleophilic substitution of fluorobenzene by 1 to give benzene and the dimeric fluoride complex [(Me₄TACD)₂Ca₂(μ-F)₂(THF)][BAr₄]₂·2.5THF (5). Analogous nucleophilic substitution reaction is observed for heterofunctionalized organosilanes XSiR₃ [X = I, N(SiHMe₂)₂, N₃; R = Me₃ or HMe₂], which resulted in the formation of calcium complexes [(Me₄TACD)Ca(X)(THF)_n][BAr₄] (6-8) containing an X ligand along with hydrosilane HSiR₃. An insertion reaction by 1 was observed with CO₂ and CO to give dinuclear formato complex [(Me₄TACD)₂Ca₂(μ-OCHO)₂][BAr₄]₂ (9) and cis-enediolato complex [(Me₄TACD)₂Ca₂(μ-OCH═CHO)][BAr₄]₂·3.5THF (10), respectively. The latter is believed to have been formed as a result of the dimerization of an initially generated formyl or oxymethylene complex, [(Me₄TACD)Ca(OCH)]⁺.

Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings

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.