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

Application of Bis(amido)alkyl Magnesiates toward the Synthesis of Molecular Rubidium and Cesium Hydrido-magnesiates

Rubidium and cesium are the least studied naturally occurring s-block metals in organometallic chemistry but are in plentiful supply from a sustainability viewpoint as highlighted in the periodic table of natural elements published by the European Chemical Society. This underdevelopment reflects the phenomenal success of organometallic compounds of lithium, sodium, and potassium, but interest in heavier congeners has started to grow. Here, the synthesis and structures of rubidium and cesium bis(amido)alkyl magnesiates [(AM)MgN'2alkyl]∞, where N' is the simple heteroamide -N(SiMe3)(Dipp), and alkyl is nBu or CH2SiMe3, are reported. More stable than their nBu analogues, the reactivities of the CH2SiMe3 magnesiates toward 1,4-cyclohexadiene are revealed. Though both reactions produce target hydrido-magnesiates [(AM)MgN'2H]2 in crystalline form amenable to X-ray diffraction study, the cesium compound could only be formed in a trace quantity. These studies showed that the bulk of the -N(SiMe3)(Dipp) ligand was sufficient to restrict both compounds to dimeric structures. Bearing some resemblance to inverse crown complexes, each structure has [(AM)(N)(Mg)(N)]2 ring cores but differ in having no AM-N bonds, instead Rb and Cs complete the rings by engaging in multihapto interactions with Dipp π-clouds. Moreover, their hydride ions occupy μ3-(AM)2Mg environments, compared to μ2-Mg2 environments in inverse crowns.

Reductions of Arenes using a Magnesium-Dinitrogen Complex

In this contribution, we present "Birch-type", and other reductions of simple arenes by the potassium salt of an anionic magnesium dinitrogen complex, [{K(TCHPNON)Mg}2(μ-N2)] (TCHPNON=4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene), which acts as a masked dimagnesium(I) diradical in these reactions. This reagent is non-hazardous, easy-to-handle, and in some cases provides access to 1,4-cyclohexadiene reduction products under relatively mild reaction conditions. This system works effectively to reduce benzene, naphthalene and anthracene through magnesium-bound "Birch-type" reduction intermediates. Cyclohexadiene products can be subsequently released from the magnesium centres by protonolysis with methanol. In contrast, the reduction of substituted arenes is less selective and involves competing reaction pathways. For toluene and 1,3,5-triphenylbenzene, the structural authentication of "Birch-type" reduction intermediates is conclusive, although the formation of corresponding 1,4-cyclohexadiene derivatives was low yielding. Reduction of anisole did not yield an isolable "Birch-type" intermediate, but instead gave a C-O activation product. Treating triphenylphosphine with [{K(TCHPNON)Mg}2(μ-N2)] resulted in the extrusion of both biphenyl and dinitrogen to afford a magnesium(II) phosphanide [{K(TCHPNON)Mg(μ-PPh2)}2]. Reduction of fluorobenzene proceeded via C-F activation of the arene, and isolation of the magnesium(II) fluoride [{K(TCHPNON)Mg(μ-F)}2]. Finally, the two-electron reduction of 1,3,5,7-cyclooctatetraene (COT) with [{K(TCHPNON)Mg}2(μ-N2)] yielded a complex, [{K(TCHPNON)Mg}2(μ-COT)], incorporating the aromatic dianion (COT2-).

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.

Stabilisation of the [SiH6]2- Anion within a Supramolecular Assembly

The hypercoordinate [SiH6]2- anion is not stable in solution. Here, we report the room temperature, solution stable molecular [SiH6]2- complex, [{KCa(NON)(OEt2)}2][SiH6] (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene)), where the [SiH6]2- anion is stabilised within a supramolecular assembly that mimics the solid-state environment of the anion in the lattice of K2SiH6. Solution-state reactivity of the complex towards carbon monoxide, benzaldehyde, azobenzene and acetonitrile is reported, yielding a range of reduction and C-C coupled products.

Steric control of Mg-Mg bond formation vs. N2 activation in the reduction of bulky magnesium diamide complexes

Reduction of the magnesium(II) diamide [Mg(TripNON)] (TripNON = 4,5-bis(2,4,6-triisopropylanilido)-2,7-diethyl-9,9-dimethyl-xanthene) with 5% w/w K/KI leads to a good yield of a dianionic dimagnesium(I) species, as its potassium salt, [{K(TripNON)Mg}2]. An X-ray crystallographic analysis shows the molecule to contain a very long Mg-Mg bond (3.137(2) Å). The formation of [{K(TripNON)Mg}2] contrasts with a previously reported reduction of a magnesium(II) complex incorporating a bulkier diamide ligand, which instead afforded a magnesium-dinitrogen complex. In the current study, [{K(TripNON)Mg}2] has been shown to be a viable reagent for the reductive activation of CO, H2 and N2O.

An anionic beryllium hydride dimer with an exceedingly short Be⋯Be distance

Heteroleptic hydride complexes of the group 2 metals have seen considerable attention as Earth-abundant synthetic tools, yet anionic derivatives are exceedingly rare. We described the facile synthesis and in-depth characterisation of an anionic beryllium hydride dimer, featuring a dynamic [Be2H3] cluster at its core with a short Be⋯Be distance. Despite this, there is no formal Be-Be bond in this complex, with only hydride bridging interactions leading to this remarkable structural attribute.

Intramolecular C-N bond activation by a transient boryl anion

Using a flexible diamido framework, a bulky boron bromide has been prepared as a precusor to a boryl anion with an extremely wide N-B-N angle. Reduction of the compound with lithium metal resulted in intramolecular C-N bond activation and migration of an aryl group onto the boron centre. Reaction of the boron bromide with K[FeCp(CO)2] resulted in nucleophilic reactivity of a carbonyl oxygen and the cooperative activation of CO.

Coordination and Activation of N2 at Low-Valent Magnesium using a Heterobimetallic Approach: Synthesis and Reactivity of a Masked Dimagnesium Diradical

The activation of dinitrogen (N2 ) by transition metals is central to the highly energy intensive, heterogeneous Haber-Bosch process. Considerable progress has been made towards more sustainable homogeneous activations of N2 with d- and f-block metals, though little success has been had with main group metals. Here we report that the reduction of a bulky magnesium(II) amide [(TCHP NON)Mg] (TCHP NON=4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene) with 5 % w/w K/KI yields the magnesium-N2 complex [{K(TCHP NON)Mg}2 (μ-N2 )]. DFT calculations and experimental data show that the dinitrogen unit in the complex has been reduced to the N2 2- dianion, via a transient anionic magnesium(I) radical. The compound readily reductively activates CO, H2 and C2 H4 , in reactions in which it acts as a masked dimagnesium(I) diradical.

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.

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.

Beryllium-centred C-H activation of benzene

Reaction of BeCl₂ with the dilithium diamide, [{SiN^Dipp}Li₂] ({SiN^Dipp} = {CH₂SiMe₂NDipp}₂), provides the dimeric chloroberyllate, [{SiN^DippBeCl}Li]₂, en route to the 2-coordinate beryllium amide, [SiN^DippBe]. Lithium or sodium reduction of [SiN^DippBe] in benzene, provides the relevant organoberyllate products, [{SiN^DippBePh}M] (M = Li or Na), via the presumed intermediacy of transient Be(I) radicals.