Pathway to a molecular calcium methyl

The dimeric β-diketiminato calcium hydride, [(BDI)CaH]2 (BDI = HC{(Me)CN-2,6-iPr2C6H3}2), reacts with ZnMe2 to afford the bimetallic calcium zincate complex, [(BDI)Ca(μ-CH3)2Zn(μ-H)]2, which subsequently undergoes an intramolecular reaction to effect the formation of [(BDI)CaMe]2, a notable omission from the homologous series of β-diketiminato alkylcalcium derivatives.

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.

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.

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.

Organocalcium Hydride-Catalyzed Intramolecular C(sp3)-H Annulation of Functionalized 2,6-Lutidines

This work reports an intramolecular C(sp³)-H annulation of functionalized 2,6-lutidines catalyzed by an organocalcium hydride [{(^DIPPnacnac)CaH(thf)}₂] (^DIPPnacnac = CH{(CMe)(2,6-ⁱPr₂-C₆H₃N)}₂). This reaction constitutes a streamlined approach for producing a new family of tetrahydro-1,5-naphthyridines and hexahydropyrido[3,2-b]azocines derivatives in good to excellent yields with high atom efficiency and broad substrates scope. A calcium alkyl complex was isolated from the stoichiometric reaction between calcium hydride and the substrate through deprotonation, which was structurally characterized and confirmed as the catalytic intermediate.

Calcium Hydride Cation Dimer Catalyzed Hydrogenation of Unactivated 1-Alkenes and H2 Isotope Exchange: Competitive Ca-H-Ca Bridges and Terminal Ca-H Bonds

Recently, it was shown that the double Ca-H-Ca bridged calcium hydride cation dimer complex [LCaH₂ CaL]²⁺ (macrocyclic ligand L=NNNN-tetradentate Me₄ TACD) exhibited remarkable activity in catalyzing the hydrogenation of unactivated 1-alkenes as well as the H₂ isotope exchange under mild conditions, tentatively via the terminal Ca-H bond of cation monomer LCaH⁺ . In this DFT mechanistic work, a novel substrate-dependent catalytic mechanism is disclosed involving cooperative Ca-H-Ca bridges for H₂ isotope exchange, competitive Ca-H-Ca bridges and terminal Ca-H bonds for anti-Markovnikov addition of unactivated 1-alkenes, and terminal Ca-H bonds for Markovnikov addition of conjugation-activated styrene. THF-coordination plays a key role in favoring the anti-Markovnikov addition while strong cation-π interactions direct the Markovnikov addition to terminal Ca-H bonds.

Origin of the Ligand Ring-Size Effect on the Catalytic Activity of Cationic Calcium Hydride Dimers in the Hydrogenation of Unactivated 1-Alkenes

Recently, it was shown that the double Ca-H-Ca-bridged calcium hydride cation dimer [LCaH₂ CaL]²⁺ when stabilized by a larger macrocyclic N,N',N'',N''',N''''-pentadentate ligand showed evidently higher activity than when stabilized by a smaller N,N',N'',N'''-tetradentate ligand in the catalytic hydrogenation of unactivated 1-alkenes. In this DFT-mechanistic work, the origin of the observed ring-size effect is examined in detail using 1-hexene, CH₂ =CH₂ and H₂ as substrates. It is shown that, at room temperature, both the N,N',N'',N''',N''''-stabilized dimer and the monomer are not coordinated by THF in solution, while the corresponding N,N',N'',N'''-stabilized structures are coordinated by one THF molecule mimicking the fifth N-coordination. Catalytic 1-alkene hydrogenation may occur via anti-Markovnikov addition over the terminal Ca-H bonds of transient monomers, followed by faster Ca-C bond hydrogenolysis. The higher catalytic activity of the larger N,N',N'',N''',N''''-stabilized dimer is due to not only easier formation of but also due to the higher reactivity of the catalytic monomeric species. In contrast, despite unfavorable THF-coordination in solution, the smaller N,N',N'',N'''-stabilized dimer shows a 3.2 kcal mol^-1 lower barrier via a dinuclear cooperative Ca-H-Ca bridge for H₂ isotope exchange than the large N,N',N'',N''',N''''-stabilized dimer, mainly due to less steric hindrance. The observed ring-size effect can be understood mainly by a subtle interplay of solvent, steric and cooperative effects that can be resolved in detail by state-of-the-art quantum chemistry calculations.

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.

Synthesis, characterisation and reactivity of group 2 complexes with a thiopyridyl scorpionate ligand

Herein we report the reactivity of the proligand tris(2-pyridylthio)methane (HTptm) with various Alkaline Earth (AE) reagents: (1) dialkylmagnesium reagents and (2) AE bis-amides (AE = Mg-Ba). Heteroleptic complexes of general formulae [Mg(Tptm)(R)] (R = Me, ⁿBu; Tptm = {C(S-C₅H₄N)₃}^-) and [AE(Tptm)(N'')] (AE = Mg-Ba; N'' = {N(SiMe₃)₂}^-) were targeted from the reaction of HTptm with R₂Mg or [AE(N'')₂]₂. Reaction of the proligand with dialkylmagnesium reagents led to formation of [{Mg(κ³C,N,N-C{Bu}{S-C₅H₄N}₂)(μ-S-C₅H₄N)}₂] (1) and [{Mg(κ³C,N,N-C{Me}{S-C₅H₄N}₂)(μ-OSiMe₃)}₂] (2) respectively, as a result of a novel transfer of an alkyl group onto the methanide carbon with concomitant C-S bond cleavage. However, reactivity of bis-amide precursors for Mg and Ca did afford the target species [AE(Tptm)(N'')] (3-AE; AE = Mg-Ca), although these proved susceptible to ligand degradation processes. DFT calculations show that alkyl transfer in the putative [Mg(Tptm)(ⁿBu)] (1m') system and amide transfer in 3-Ca is a facile process that induces C-S bond cleavage in the Tptm ligand. 3-Mg and 3-Ca were also tested as catalysts for the hydrophosphination of selected alkenes and alkynes, including the first example of mono-hydrophosphination of 4-ethynylpyridine which was achieved with high conversions and excellent regio- and stereochemical control.

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.

A mononuclear divalent ytterbium hydrido complex supported by a super-bulky scorpionate ligand

The first mononuclear divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (2) bearing a terminal hydrido ligand was obtained by hydrogenolysis of the benzyl precursor in hexane. Complex 2 exhibited two different reaction patterns towards allenes: Yb-H addition with cyclohexylallene and deprotonation of 1,1-dimethylallene.

New horizons in low oxidation state group 2 metal chemistry

Since the seminal report on Mg in the +I oxidation state in 2007, low-valent complexes featuring a Mg^I-Mg^I bond developed from trophy molecules to state-of-the-art reducing agents. Despite increasing interest in low-valency of the other group 2 metals, this area was restricted for a long time to a rare example of a Ca^I(arene)Ca^I inverse sandwich. This feature article focuses on the most recent developments in the field, highlighting recent breakthroughs for Be, Mg and Ca. The more exotic metal Be was the first to be isolated as a zero-valent complex which could be oxidized to a Be^I species. There also has been interest in breaking the Mg^I-Mg^I bond with superbulky β-diketiminate ligands (BDI) that suppress (BDI)Mg-Mg(BDI) bond formation. This led to Mg-Mg bond elongation or Mg-N bond cleavage. Several reports on attempts to isolate (BDI)Mg˙ radicals by combinations of ligand bulk, addition of neutral ligands or UV(vis) irradiation led to reduction of the aromatic solvents, underscoring the high reactivity of these open shell species. Only recently, zero-valent complexes of Mg were introduced. Double reduction of a (BDI)MgI complex with Na gave [(BDI)Mg^-]Na⁺. This Mg⁰ complex crystallized as a dimer in which the Na⁺ cations bridge the two (BDI)Mg^- anions which react as Mg nucleophiles. Thermal decomposition led to spontaneous formation of Na⁰ and a trinuclear (BDI)MgMgMg(BDI) complex. This mixed-valence Mg₃-complex is a prime example of the fleeting multinuclear Mg_n intermediates discussed on the way from Mg metal to Grignard reagent. Attempts to prepare low-valent Ca^I compounds by reduction of (BDI)CaI led to dearomatization of the arene solvents: (BDI)Ca(arene)Ca(BDI). Reduction in alkanes prevented this decomposition pathway but led to N₂ reduction and isolation of (BDI)Ca(N₂)Ca(BDI), representing the first example of molecular nitrogen fixation with an early main group metal. As the N₂^2- anion reacts in most cases as a very strong two-electron reductant, LCa(N₂)CaL could be seen as a synthon for hitherto elusive Ca^I-Ca^I complexes. Theoretical calculations suggest that participation of Ca d-orbitals is relevant for N₂ activation. These most recent developments in low-valent group 2 metal chemistry will revive this area and undoubtly lead to new reactivities and applications.

Mechanism of Potassium tert-Butoxide-Catalyzed Ketones Hydrogenation in the Solution Phase

The mechanism of ketones homogeneous hydrogenation with t-BuOK in tert-butanol is currently portrayed as the one proceeding via a six-membered [2 + 2 + 2] cyclic transition state involving the H₂ molecule, the base, and a ketone. However, the concerted nature of the reaction is inconsistent with a number of experimental observations. Here we reanalyze available experimental data and revise the mechanism of this paradigmatic reaction based on the static and dynamic density functional theory (DFT) calculations in solution phase. In contrast to the gas-phase profile where the overall reaction occurs in two elementary steps, there are three consecutive steps in solution: cleavage of the H-H bond in basic tert-butanol to afford potassium hydride, addition of potassium hydride across the C═O bond of a ketone through the rate-determining transition state, and rapid product formation through K/H exchange. Potassium hydride is therefore an important intermediate of the catalytic process. The free energy profile for the prophetic ester homogeneous hydrogenation with t-BuOK in tert-butanol is also computed herein. The reaction seems to be kinetically possible, but slightly harsher conditions need to be applied, consistent with rate-determining nature of the potassium hydride addition.

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 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.

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.

Large decanuclear calcium and strontium hydride clusters

The largest, most hydride-rich, Ca₁₀H₁₆ cluster is formed by condensation of two smaller Ca₆H₉ octahedrons (the isostructural Sr hydride cluster is also reported).