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

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.

Enabling nucleophilic reactivity in molecular calcium fluoride complexes

Calcium fluoride is the ultimate source of all fluorochemicals. Current synthetic approaches rely on the use of HF, generated from naturally occurring fluorspar and sulfuric acid. Methods for constructing E-F bonds directly from CaF2 have long been frustrated by its high lattice energy, low solubility and impaired fluoride ion nucleophilicity. Little fundamental understanding of the reactivity of Ca-F moieties is available to guide methodology development; well-defined molecular species containing Ca-F bonds are extremely rare, and existing examples are strongly aggregated and evidence no nucleophilic fluoride delivery. Here, by contrast, we show that by targeting anionic systems of the type [Ln(X)2CaF]-, monomeric calcium fluoride complexes containing single Ca-F bonds can be synthesized, including via routes involving fluoride abstraction from existing C-F bonds. Comparative structural and spectroscopic studies of mono- and dinuclear systems allow us to define structure-activity relationships for E-F bond formation from molecular calcium fluorides.

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex

Addition of CO to a tetrametallic magnesium hydride cluster results in both carbon-carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]- which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]- fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3- fragment, which undergoes subsequent deoxygenation.

Main group metal-mediated strategies for C-H and C-F bond activation and functionalisation of fluoroarenes

With fluoroaromatic compounds increasingly employed as scaffolds in agrochemicals and active pharmaceutical ingredients, the development of methods which facilitate regioselective functionalisation of their C-H and C-F bonds is a frontier of modern synthesis. Along with classical lithiation and nucleophilic aromatic substitution protocols, the vast majority of research efforts have focused on transition metal-mediated transformations enabled by the redox versatilities of these systems. Breaking new ground in this area, recent advances in main group metal chemistry have delineated unique ways in which s-block, Al, Ga and Zn metal complexes can activate this important type of fluorinated molecule. Underpinned by chemical cooperativity, these advances include either the use of heterobimetallic complexes where the combined effect of two metals within a single ligand set enables regioselective low polarity C-H metalation; or the use of novel low valent main group metal complexes supported by special stabilising ligands to induce C-F bond activations. Merging these two different approaches, this Perspective provides an overview of the emerging concept of main-group metal mediated C-H/C-F functionalisation of fluoroarenes. Showcasing the untapped potential that these systems can offer in these processes; focus is placed on how special chemical cooperation is established and how the trapping of key reaction intermediates can inform mechanistic understanding.

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.

Calcium-stabilised transition metal bis(formyl) complexes: structure and bonding

Reaction of a molecular calcium hydride with a series of group 9 dicarbonyl complexes [M(η5-C5Me5)(CO)2] (M = Co, Rh, Ir) led to the formation of both mono(formyl) and bis(formyl) complexes. The bis(formyl) complexes are unique. They have been characterised by multinuclear NMR spectroscopy and examples have been crystallographically characterised for the first time.

Carbon-Carbon Bond Formation from Carbon Monoxide and Hydride: The Role of Metal Formyl Intermediates

Current examples of carbon chain production from metal formyl intermediates with homogeneous metal complexes are described in this Minireview. Mechanistic aspects of these reactions as well as the challenges and opportunities in using this understanding to develop new reactions of CO and H2 are also discussed.

Anionic Magnesium and Calcium Hydrides: Transforming CO into Unsaturated Disilyl Ethers

The synthesis, characterisation and reactivity of two isostructural anionic magnesium and calcium complexes is reported. By X-ray and neutron diffraction techniques, the anionic hydrides are shown to exist as dimers, held together by a range of interactions between the two anions and two bridging potassium cations. Unlike the vast proportion of previously reported dimeric group 2 hydrides, which have hydrides that bridge two group 2 centres, here the hydrides are shown to be "terminal", but stabilised by interactions with the potassium cations. Both anionic hydrides were found to insert and couple CO under mild reaction conditions to give the corresponding group 2 cis-ethenediolate complexes. These cis-ethenediolate complexes were found to undergo salt elimination reactions with silyl chlorides, allowing access to small unsaturated disilyl ethers with a high percentage of their mass originating from the C₁ source CO.

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.

In situ Grignard Metalation Method, Part II: Scope of the One-Pot Synthesis of Organocalcium Compounds

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot strategy to synthesize alkaline-earth metal amides in multi-gram scale with high yields via addition of bromoethane to an ethereal suspension of a primary or secondary amine and magnesium (Part I) or calcium (Part II). This method is highly advantageous because no activation of calcium is required prior to the reaction. Contrary to the magnesium-based iGMM, there are some limitations, the most conspicuous one is the large influence of steric factors. The preparation of Ca(hmds)₂ succeeds smoothly within a few hours with excellent yields opening the opportunity to prepare large amounts of this reagent. Side reactions and transfer of the iGMM to substituted anilines and N-heterocycles as well as other H-acidic substrates such as cyclopentadienes are studied. Bulky amidines cannot be converted directly to calcium amidinates via the iGMM but stoichiometric calciation with Ca(hmds)₂ enables their preparation.

CO, CO2 and CS2 activation by divalent ytterbium hydrido complexes

Treatment of a divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (1) with CO, CO₂ and CS₂ resulted in the formation of a divalent ytterbium ethenediolate complex [(Tp^Ad,iPr)Yb]₂(cis-OCHCHO) (2), a formate complex [(Tp^Ad,iPr)Yb(κ²-O₂CH)(THF)] (3), and a trivalent ytterbium ethenetetrathiolate complex [(Tp^Ad,iPr)Yb^III]₂(C₂S₄) (4), respectively. DFT calculations were carried out to elucidate the reaction profiles of complexes 3 and 4.

Reduction of CO2 with Aluminum Hydrides Supported with Ar-BIAN Radical-Anions (Ar-BIAN = 1,2-Bis(arylimino)acenaphthene)

The reactions of H₂AlCl with [(dpp-Bian)Na(Et₂O)_n] and [(Ar^BIG-Bian)Na(THF)] produce respective aluminum hydrides supported by radical-anionic 1,2-bis(arylimino)acenaphthene ligands, [(dpp-Bian)AlH₂] (1) and [(Ar^BIG-Bian)AlH₂(THF)] (2) (dpp-Bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene); Ar^BIG-Bian = 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene). The reaction of 1 with CO₂ proceeds with reduction of both C═O bonds and results in diolate [{(dpp-Bian)Al(μ-O₂CH₂)}₂] (3). Complex 2 reacts with CO₂ to carbonate [{(Ar^BIG-Bian)Al(μ-OCH₂OCO₂)}₂] (4) that is a result of the insertion of CO₂ into the Al-O bond in diolate species formed initially. Aluminum monohydrides [(dpp-Bian)AlH(X)] (X = Cl, 5; Me, 6) react with CO₂ to form respective alumoxanes [{(dpp-Bian)AlX}₂(μ-O)] (X = Cl, 7 and X = Me, 8). Compounds 1-4, 7, and 8 have been characterized by ESR and IR spectroscopy, and their molecular structures have been determined by single-crystal X-ray analysis.

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

The cationic benzyl complex [(Me₄TACD)Sr(CH₂Ph)][A] (Me₄TACD=1,4,7,10‐tetramethyltetraazacyclododecane; A=B(C₆H₃‐3,5‐Me₂)₄) reacted with two equivalents of phenylsilane to give the bridging hexahydridosilicate complex [(Me₄TACD)₂Sr₂(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 [(Me₄TACD)SrH(thf)_x]⁺. Complex 3 a decomposed in THF at room temperature to give the terminal silanide complex [(Me₄TACD)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 [(Me₄TACD)₃Sr₃(μ₂‐H)₃(μ₃‐SiH₃)₂][A], while ⁿOctSiH₃ led to the trinuclear (n‐octyl)pentahydridosilicate complex [(Me₄TACD)₃Sr₃(μ₂‐H)₃(μ₃‐SiH₅ⁿOct)][A].

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.

Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C-F Bond Cleavage

Low-valent (Me BDI)Al and (Me BDI)Ga and highly Lewis acidic cations in [(tBu BDI)M+ ⋅C6 H6 ][(B(C6 F5 )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 AlI (or GaI )→Mg2+ 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.

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.