Cationic Magnesium π–Arene Complexes

Synthesis of Superbulky Amide Ligands by Addition of Polar Reagents to Sila-Imine

The chemistry of extremely bulky amide ligands is troubled by difficulties in deprotonation of the parent amine. As an alternative route to superbulky amide reagents, the addition of polar reagents to a sila-imine has been investigated. Attempts to synthesize the superbulky amide anion (tBu3Si)2N- by addition of tBuLi to tBu2Si=N(SitBu3) failed and gave tBu3Si(tBu2HSi)NLi and isobutene. Reaction of the sila-imine with KOtBu successfully led to tBu3Si[tBu2(tBuO)Si]NK which crystallized as a separated ion-pair. Reaction with the slightly bulkier KOAd (Ad=1-adamantyl) led in presence of THF to ether ring-opening. Reaction with tBuOH gave tBu3Si[tBu2(tBuO)Si]NH but this amine cannot be easily deprotonated. Reaction with (BDI*)MgnBu in presence of THF gave (BDI*)Mg+ ⋅ (THF)2 and the non-coordinating anion tBu3Si[tBu2(nBu)Si]N-; BDI*=ß-diketiminate ligand HC[C(tBu)N-DIPP]2, DIPP=2,6-diisopropylphenyl. Reaction of Mg(nBu)2 with tBu2Si=N(SitBu3) led to a Mg complex with one amide ligand: tBu3Si[tBu2(nBu)Si]N-. The other superbulky amide anion isomerized by internal deprotonation of a tBu-substituent to give a primary carbanion that is also coordinated to Mg. Although the amide-to-carbanion isomerization is highly contrathermodynamic, it allows for coordination of both anions to a single Mg center. The new bulky amides are rare cases of halogen-free weakly coordinating anions.

Enforcing Metal-Arene Interactions in Bulky p-Terphenyl Bis(anilide) Complexes of Group 2 Metals (Be-Ba): Potential Precursors for Low-Oxidation-State Alkaline Earth Metal Systems

An extremely bulky p-terphenyl bis(aniline), p-C6H4{C6H4[N(H)TCHP]-2}2 (TCHP = 2,4,6-tricyclohexylphenyl) TCHPTerphH2, has been developed. Deprotonation of a less bulky analogue, DipTerphH2 (Dip = 2,6-diisopropylphenyl), with BePh2 affords the bimetallic system, [(BePh)2(μ-DipTerph)] 1. Treating either TCHPTerphH2 or DipTerphH2 with Mg{CH2(SiMe3)}2 gives the monomeric bis(anilide) complexes [Mg(ArTerph)] (Ar = Dip 2, TCHP 3) which display rare examples of η6-arene coordination to the metal center. Treating 2 with THF leads to partial dissociation of the Mg···arene interaction and formation of [Mg(DipTerph)(THF)] 4. Reactions of the bis(aniline)s with the group 2 metal amides [M{N(SiMe3)2}2] afford dimeric, structurally analogous compounds [{M(ArTerph)}2] (Ar = Dip, M = Ca 5, Sr 6, Ba 7; Ar = TCHP, M = Ca 8, Sr 9, Ba 10) which display intermolecular M···arene interactions in the solid state. Computational studies have shown that the intramolecular M···η6-arene interactions in models of the ether-free metal bis(anilide) compounds are largely electrostatic in nature. Reductions of these compounds with alkali metals led to mixtures of unidentified products.

Ca(II) and Yb(II) complexes featuring M(C≡C)4 structural motif: enforced proximity or genuine η2 -bonding?

Bis(carbazolide) complexes M[3,6-tBu2 -1,8-(RC≡C)2 Carb]2 (THF)n (R=SiMe3 , n=0, M=Ca, Yb; R=Ph, n=1, M=Ca, Yb; n=0, M=Yb) were synthesized through transamination reaction of M[N(SiMe3 )2 ]2 (THF)2 with two molar equivalents of carbazoles. The complexes feature M(η2 -C≡C)4 structural motif composed of M(II) ions encapsulated by four acetylene fragments due to atypical for alkaline- and rare-earth metals η2 -interactions with triple C≡C bond. This interaction is evidenced experimentally by X-ray diffraction, Raman spectroscopy in the solid state and by NMR-spectroscopy in the solution. According to QTAIM analysis there are 4 bond critical points (3;-1) between the metal atom and each of the triple bonds, which are connected by a strongly curved, almost T-shaped bond pathway.

Grignard reagents as deprotonation agents for oxazoline-amido-phenolate ligands: structural and catalytic implications with the role of halogen ions

In this study, various halogen-substituted Grignard reagents were assessed as deprotonating agents for the oxazoline-amido-phenolate ligand, leading to the formation of magnesium complexes. The newly synthesized complexes with halogen substituents displayed three distinct coordinative modes, all extensively characterized through crystallographic methods. The introduction of halogen substituents induced changes in the Lewis acid properties of the complexes, thereby impacting their structural attributes and catalytic behavior during the initiation and propagation of ring polymerization of cyclic esters.

Opportunities with calcium Grignard reagents and other heavy alkaline-earth organometallics

More than a century old, magnesium Grignard reagents remain essential to the toolbox of organic chemists. Although similar reagents with the neighbouring group 2 metal Ca have been explored, the considerably higher polarity and reactivity of the Ca-C bond result in undesired decomposition pathways. Ca Grignard reagents have found academic interest but have never fully developed into an established synthetic tool. Recent research activities, however, provide facile access to these highly reactive organocalcium species, including in situ preparation and ball milling approaches to tackle the challenge of controlling their extreme sensitivity. Heavier Grignard reagents are not just more reactive but profit from unique chemical transformations. Insight into the transition metal-like properties of Ca, Sr and Ba is only just emerging. Considering the rapidly developing field of alkaline-earth metal-mediated catalysis, heavy Grignard reagents will probably have a bright future.

The borderless world of chemical bonding across the van der Waals crust and the valence region

The definition of the van der Waals crust as the spherical section between the atomic radius and the van der Waals radius of an element is discussed and a survey of the application of the penetration index between two interacting atoms in a wide variety of covalent, polar, coordinative or noncovalent bonding situations is presented. It is shown that this newly defined parameter permits the comparison of bonding between pairs of atoms in structural and computational studies independently of the atom sizes.

Atom-economic access to cationic magnesium complexes

Cationic alkaline-earth complexes attract interest for their enhanced Lewis acidity and reactivity compared with their neutral counterparts. Synthetic protocols to these complexes generally utilize expensive specialized reagents in reactions generating multiple by-products. We have studied a simple ligand transfer approach to these complexes using (NacNac)MgR and ER3 (NacNac = β-diketiminate anion; E = group 13 element; R = aryl/amido anion) which demonstrates high atom economy, opening up the ability to target these species in a more sustainable manner. The success of this methodology is dependent on the identity of the group 13 element with the heavier elements facilitating faster ligand exchange. Furthermore, while this reaction is successful with aromatic ligands such as phenyl and pyrrolyl, the secondary amide piperidide (pip) fails to transfer, which we attribute to the stronger 3-centre-4-electron dimerization interaction of Al2(pip)6.

On the existence of low-valent magnesium-calcium complexes

DFT-Calculations predict that a low-valent complex (BDI)Mg-Ca(BDI) with bulky β-diketiminate (BDI) ligands is thermodynamically stable. It was attempted to isolate such a complex by salt-metathesis between [(^DIPePBDI*)Mg^-Na⁺]₂ and [(^DIPePBDI)CaI]₂ (^DIPePBDI = HC[C(Me)N-DIPeP]₂; ^DIPePBDI* = HC[C(tBu)N-DIPeP]₂; DIPeP = 2,6-CH(Et)₂-phenyl). Whereas in alkane solvents no reaction was observed, salt-metathesis in C₆H₆ led to immediate C-H activation of benzene to give (^DIPePBDI*)MgPh and (^DIPePBDI)CaH, the latter crystallizing as a THF-solvated dimer [(^DIPePBDI)CaH·THF]₂. Calculations suggest reduction and insertion of benzene in the Mg-Ca bond. The activation enthalpy for the subsequent decomposition of C₆H₆^2- into Ph^- and H^- is only 14.4 kcal mol^-1. Repeating this reaction in the presence of naphthalene or anthracene led to heterobimetallic complexes in which naphthalene^2- or anthracene^2- anions are sandwiched between (^DIPePBDI*)Mg⁺ and (^DIPePBDI)Ca⁺ cations. These complexes slowly decompose to their homometallic counterparts and further decomposition products. Complexes in which naphthalene^2- or anthracene^2- anions are sandwiched between two (^DIPePBDI)Ca⁺ cations were isolated. The low-valent complex (^DIPePBDI*)Mg-Ca(^DIPePBDI) could not be isolated due to its high reactivity. There is, however, strong evidence that this heterobimetallic compound is a fleeting intermediate.

s-Block chemistry in weakly coordinating solvents

Alkaline earth metal catalysis has been a growing field in recent years. To enhance reactivity and to understand the metal-substrate interactions in more detail, reactions are increasingly carried out in weakly coordinating solvents. This article gives an overview over the two main approaches to facilitate this, which are either through the employment of highly dipolar haloaryls as solvents, or by increasing the solubility of the ligand systems. The resulting coordination modes and reactivities are presented together with the synthetic strategies. Additionally, the latest results of group 1 complex chemistry in aliphatic solvents are illustrated and future challenges are highlighted.

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.

Retro-Diels-Alder decomposition of norbornadiene mediated by a cationic magnesium complex

First evidence for the coordination of norbornadiene (nbd) and dicyclopentadiene (dcpd) with the main group metal Mg is provided by the crystal structures of adducts with cationic β-diketiminate (BDI) Mg complexes. While the dcpd complex is thermally stable, [(BDI)Mg+·nbd][B(C6F5)4-] shows slow room temperature retro-Diels-Alder decomposition to give a complex with the cation (BDI)Mg(C5H5)Mg(BDI)+.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

Strongly Lewis acidic cationic aluminium complexes, stabilized by β–diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C₆F₅)₄⁻ salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the ^R1,R2BDI ligand (^R1,R2BDI=HC[C(R1)N(R2)]₂) influence sterics and Lewis acidity. Ligand bulk increases along the row ^Me,DIPPBDI<^Me,DIPePBDI≈^tBu,DIPPBDI<^tBu,DIPePBDI; DIPP=2,6‐C(H)Me₂‐phenyl, DIPeP=2,6‐C(H)Et₂‐phenyl. The Gutmann‐Beckett test showed acceptor numbers of: (^tBu,DIPPBDI)AlMe⁺ 85.6, (^tBu,DIPePBDI)AlMe⁺ 85.9, (^Me,DIPPBDI)AlMe⁺ 89.7, (^Me,DIPePBDI)AlMe⁺ 90.8, (^Me,DIPPBDI)AlH⁺ 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation (^tBu,DIPPBDI)AlMe⁺ shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured.

Reversible Addition of Carbon Dioxide to Main Group Metal Complexes at Temperatures about 0 °C

The reaction of dialane [LAl-AlL] (1; L=dianion of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene, dpp-bian) with carbon dioxide results in two different products depending on solvent. In toluene at temperatures of about 0 °C, the reaction gives cycloadduct [L(CO₂ )Al-Al(O₂ C)L] (2), whereas in diethyl ether, the reaction affords oxo-bridged carbamato derivative [L(CO₂ )(Et₂ O)Al(μ-O)AlL] (3). The DFT and QTAIM calculations provide reasonable explanations for the reversible formation of complex 2 in the course of two subsequent (2+4) cycloaddition reactions. Consecutive transition states with low activation barriers were revealed. Also, the DFT study demonstrated a crucial effect of diethyl ether coordination to aluminium on the reaction of dialane 1 with CO₂ . The optimized structures of key intermediates were obtained for the reactions in the presence of Et₂ O; calculated thermodynamic parameters unambiguously testify the irreversible formation of the product 3.

Unsupported Mg-Alkene Bonding

The first intermolecular early main group metal-alkene complexes were isolated. This was enabled by using highly Lewis acidic Mg centers in the Lewis base-free cations (Me BDI)Mg+ and (tBu BDI)Mg+ with B(C6 F5 )4 - counterions (Me BDI=CH[C(CH3 )N(DIPP)]2 , tBu BDI=CH[C(tBu)N(DIPP)]2 , DIPP=2,6-diisopropylphenyl). Coordination complexes with various mono- and bis-alkene ligands, typically used in transition metal chemistry, were structurally characterized for 1,3-divinyltetramethyldisiloxane, 1,5-cyclooctadiene, cyclooctene, 1,3,5-cycloheptatriene, 2,3-dimethylbuta-1,3-diene, and 2-ethyl-1-butene. In all cases, asymmetric Mg-alkene bonding with a short and a long Mg-C bond is observed. This asymmetry is most extreme for Mg-(H2 C=CEt2 ) bonding. In bromobenzene solution, the Mg-alkene complexes are either dissociated or in a dissociation equilibrium. A DFT study and AIM analysis showed that the C=C bonds hardly change on coordination and there is very little alkene→Mg electron transfer. The Mg-alkene bonds are mainly electrostatic and should be described as Mg2+ ion-induced dipole interactions.

C-H and C-F coordination of arenes in neutral alkaline earth metal complexes

A series of neutral magnesium and calcium complexes bearing an extremely bulky diamido ligand have been synthesised and crystallographically characterised. A number of these complexes feature rare group 2 metalaromatic interactions, such as the η6-coordination of benzene and 'agostic-like' C-H coordination, the latter previously unseen in neutral Mg and Ca complexes.

Magnesium-halobenzene bonding: mapping the halogen sigma-hole with a Lewis-acidic complex

Complexes of the Lewis base-free cations (^MeBDI)Mg⁺ and ( ^tBuBDI)Mg⁺ with Ph-X ligands (X = F, Cl, Br, I) have been studied (^MeBDI = HC[C(Me)N-DIPP]₂ and ^tBuBDI = HC[C(tBu)N-DIPP]₂; DIPP = 2,6-diisopropylphenyl). For the smaller β-diketiminate ligand (^MeBDI) only complexes with PhF could be isolated. Heavier Ph-X ligands could not compete with bonding of Mg to the weakly coordinating anion B(C₆F₅)₄ ^-. For the cations with the bulkier ^tBuBDI ligand, the full series of halobenzene complexes was structurally characterized. Crystal structures show that the Mg⋯X-Ph angle strongly decreases with the size of X: F 139.1°, Cl 101.4°, Br 97.7°, I 95.1°. This trend, which is supported by DFT calculations, can be explained with the σ-hole which increases from F to I. Charge calculation and Atoms-In-Molecules analyses show that Mg⋯F-Ph bonding originates from electrostatic attraction between Mg²⁺ and the very polar C ^δ+-F ^δ- bond. For the heavier halobenzenes, polarization of the halogen atom becomes increasingly important (Cl < Br < I). Complexation with Mg leads in all cases to significant Ph-X bond activation and elongation. This unusual coordination of halogenated species to early main group metals is therefore relevant to C-X bond breaking.

Stack by Stack: From the Free Cyclopentadienylgermanium Cation Via Heterobimetallic Main-Group Sandwiches to Main-Group Sandwich Coordination Polymers

Heterobimetallic cationic sandwich complexes [M(μ-Cp)M'Cp]+ of group 13 (M=Ga, In) and group 14 (M'=Ge, Sn) elements have been prepared as [WCA]- salts (WCA=Al(ORF )4 ; ORF =OC(CF3 )3 ). Their molecular structures include free apical gallium or indium atoms. The sandwich complexes were formed in the reactions of [M(HMB)]+ [WCA]- (HMB=C6 Me6 ) with the free metallocenes [M'Cp2 ]. Their structures are related to known stannocene and stannocenium salts; the unprecedented germanium analogues, namely the free germanocenium cation [GeCp]+ and the corresponding triple-decker complex cation [CpGe(μ-Cp)GeCp]+ , are described herein. By variation of the reaction conditions, these sandwich complexes can be transformed into the group 13/14 mixed cationic coordination polymer [{In(HMB)(μ-SnCp2 )}n ][WCA]n . This polymeric chain motif was also successfully replicated by the synthesis of complexes [{Ga/In(HMB)(μ-FeCp2 )}n ][WCA]n containing FeCp2 as a bridging ligand.

Low-Symmetry β-Diketimine Aryloxide Rare-Earth Complexes: Flexible, Reactive, and Selective

We report the synthesis, characterization, and reactivity of a new low-symmetry β-diketimine featuring a pendant amino(methyl)phenol donor and its corresponding heteroleptic rare-earth (RE) complexes. This includes the first structurally characterized examples of alcoholysis and insertion from an isolated RE^III amide in a β-diketimine framework. The flexible methylene linkage leads to RE^III complexes with tunable dynamic solution behavior that defines their stoichiometric and catalytic reactivity. The addition of a strong neutral donor ligand, tricyclohexylphosphine oxide, suppresses a prevalent catalyst degradation pathway (base-promoted elimination) and dramatically enhances the catalyst performance in the stereospecific ring-opening polymerization of rac-β-butyrolactone. Our results further demonstrate the importance of ligand reorganization in the stoichiometric and catalytic activity of RE^III ions.

A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

The potential of a dicationic strontium ansa‐arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base‐free strontium‐perfluoroalkoxyaluminate Sr[Al(OR^F)₄]₂ (OR^F=OC(CF₃)₃). Addition of an ansa‐arene yielded the highly Lewis acidic, dicationic strontium ansa‐arene complex. In preliminary experiments, the complex was successfully applied as a catalyst in CO₂‐reduction to CH₄ and a surprisingly controlled isobutylene polymerization reaction.

Boosting Low-Valent Aluminum(I) Reactivity with a Potassium Reagent

The reagent RK [R=CH(SiMe3 )2 or N(SiMe3 )2 ] was expected to react with the low-valent (DIPP BDI)Al (DIPP BDI=HC[C(Me)N(DIPP)]2 , DIPP=2,6-iPr-phenyl) to give [(DIPP BDI)AlR]- K+ . However, deprotonation of the Me group in the ligand backbone was observed and [H2 C=C(N-DIPP)-C(H)=C(Me)-N-DIPP]Al- K+ (1) crystallized as a bright-yellow product (73 %). Like most anionic AlI complexes, 1 forms a dimer in which formally negatively charged Al centers are bridged by K+ ions, showing strong K+ ⋅⋅⋅DIPP interactions. The rather short Al-K bonds [3.499(1)-3.588(1) Å] indicate tight bonding of the dimer. According to DOSY NMR analysis, 1 is dimeric in C6 H6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C6 H6 , two C-H bond activations are observed and a product with a para-phenylene moiety was exclusively isolated. DFT calculations confirm that the Al center in 1 is more reactive than that in (DIPP BDI)Al. Calculations show that both AlI and K+ work in concert and determines the reactivity of 1.

Hungry for charge – how a beryllium scorpionate complex “eats” a weakly coordinating anion

We present the reaction of a tris(pyrazolyl) beryllium scorpionate (TpBe) complex with a weakly coordinating anion (WCA), which yields the heteroleptic complex TpBeOC(CF3)3 1 (TpBeOR F). The product 1 has been characterized by multinuclear NMR spectroscopy (1H, 9Be, 13C) and single-crystal X-ray diffraction (scXRD). Quantum chemical calculations (DFT, NPA, LOL) were performed to study the bonding nature in 1.

Soft interactions with hard Lewis acids: generation of mono- and dicationic alkaline-earth metal arene-complexes by direct oxidation

The synthesis of the first unsupported dicationic arene complexes of calcium and strontium [(η⁶-HMB)AE(oDFB)₄]²⁺ is reported (HMB = hexamethylbenzene; AE = alkaline earth metal; oDFB = ortho-difluorobenzene). They were prepared by direct oxidation of the elemental metals employing the ligand-forming radical cation salt [HMB][WCA] as an oxidant (WCA = [Al(OR^F)₄] or [μF-{Al(OR^F)₃}₂]; R^F = C(CF₃)₃). In addition, monocationic η⁶-HMB complexes of calcium, strontium and barium supported by coordination of the monodentate anion [F-Al(OR^F)₃]^- are reported. In all examples, almost undistorted η⁶-HMB coordination is observed with rather short M-arene_centroid distances approaching those observed with the isoelectronic but negatively charged pentamethylcyclopentadienyl ligand. The structure and bonding, thermodynamic stability and Lewis acidity (fluoride/hydride ion affinities, FIA/HIA) of the generated complexes were assessed by DFT methods. It followed that the gaseous dications [(η⁶-HMB)AE(oDFB)₄]²⁺ are extremely hard Lewis acids that retain FIAs close to superacidity in solution.

Hydrocarbon-soluble, hexaanionic fulleride complexes of magnesium

Fullerene C₆₀ reacts with dimagnesium(i) compounds LMgMgL, where L is a monoanionic β-diketiminate ligand, to contact ion complexes [(LMg)_nC₆₀], where n is predominantly 2, 4 or 6.

The reaction of the magnesium(i) complexes [{(^Arnacnac)Mg}₂], (^Arnacnac = HC(MeCNAr)₂, Ar = Dip (2,6-iPr₂C₆H₃), Dep (2,6-Et₂C₆H₃), Mes (2,4,6-Me₃C₆H₂), Xyl (2,6-Me₂C₆H₃)) with fullerene C₆₀ afforded a series of hydrocarbon-soluble fulleride complexes [{(^Arnacnac)Mg}_nC₆₀], predominantly with n = 6, 4 and 2. ¹³C{¹H} NMR spectroscopic studies show both similarities (n = 6) and differences (n = 4, 2) to previously characterised examples of fulleride complexes and materials with electropositive metal ions. The molecular structures of [{(^Arnacnac)Mg}_nC₆₀] with n = 6, 4 and 2 can be described as inverse coordination complexes of n [(^Arnacnac)Mg]⁺ ions with C₆₀^n– anions showing predominantly ionic metal–ligand interactions, and include the first well-defined and soluble complexes of the C₆₀^6– ion. Experimental studies show the flexible ionic nature of the {(^Arnacnac)Mg}⁺···C₆₀^6– coordination bonds. DFT calculations on the model complex [{(^Menacnac)Mg}₆C₆₀] (^Menacnac = HC(MeCNMe)₂) support the formulation as an ionic complex with a central C₆₀^6– anion and comparable frontier orbitals to C₆₀^6– with a small HOMO–LUMO gap. The reduction of C₆₀ to its hexaanion gives an indication about the reducing strength of dimagnesium(i) complexes.

Backbone Reactivity of Lithium β-Diketiminate (NacNac) Complexes with CO2 , tBuNCO and iPrNCO

Though alkali metal NacNac (β-diketiminate) complexes have been utilised in synthesis as NacNac-transfer agents, studies of them in their own right with small molecules are exceptionally rare. Here, the lithium compound of the common 2,6-diisopropylphenyl-β-methyldiketiminate [NacNac(Dipp, Me)] ligand is investigated with carbon dioxide and isocyanates. In all four cases reaction occurs at the backbone γ-C atom of the NacNac ligand, which redistributes electronically into a diimine. Insertion of CO₂ gives an eight-atom carboxylate (Li₂ O₄ C₂ ) ring at the γ-C site in a dimer. Insertion of tBuNCO gives a secondary amide at the γ-C site in a monomer with TMEDA chelating lithium. Double insertion of tBuNCO and (adventitious) oxygen gives a dimer with a (LiO)₂ central core involving the latter source. Insertion of less bulky (iPrNCO) gives a dimer with dimerisation through the C=O bonds of the emergent secondary amide function.

N-Heterocyclic Carbene-Supported Aryl- and Alk- oxides of Beryllium and Magnesium

Recently, we have witnessed significant progress with regard to the synthesis of molecular alkaline earth metal reagents and catalysts. To provide new precursors for light alkaline earth metal chemistry, molecular aryloxide and alkoxide complexes of beryllium and magnesium are reported. The reaction of beryllium chloride dietherate with two equivalents of 1,3-diisopropyl-4,5-dimethylimidizol-2-ylidine (sIPr) results in the formation of a bis(N-heterocyclic carbene) (NHC) beryllium dichloride complex, (sIPr)2BeCl2 (1). Compound 1 reacts with lithium diisopropylphenoxide (LiODipp) or sodium ethoxide (NaOEt) to form the terminal aryloxide (sIPr)Be(ODipp)2 (2) and alkoxide dimer [(sIPr)Be(OEt)Cl]2 (3), respectively. Compounds 2 and 3 represent the first beryllium alkoxide and aryloxide species supported by NHCs. Structurally related dimers of magnesium, [(sIPr)Mg(OEt)Brl]2 (4) and [(sIPr)Mg(OEt)Me]2 (5), were also prepared. Compounds 1-5 were characterized by single crystal X-ray diffraction studies, 1H, 13C, and 9Be NMR spectroscopy where applicable.

Stepwise Reduction at Magnesium and Beryllium: Cooperative Effects of Carbenes with Redox Non-Innocent α-Diimines

In the past two decades, the organometallic chemistry of the alkaline earth elements has experienced a renaissance due in part to developments in ligand stabilization strategies. In order to expand the scope of redox chemistry known for magnesium and beryllium, we have synthesized a set of reduced magnesium and beryllium complexes and compared their resulting structural and electronic properties. The carbene-coordinated alkaline earth-halides, (^Et2CAAC)MgBr₂ (1), (SIPr)MgBr₂ (2), (^Et2CAAC)BeCl₂ (3), and (SIPr)BeCl₂ (4) [^Et2CAAC = diethyl cyclic(alkyl)(amino) carbene; SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene] were combined with an α-diimine [2,2-bipyridine (bpy) or bis(2,6-diisopropylphenyl)-1,4-diazabutadiene (^DippDAB)] and the appropriate stoichiometric amount of potassium graphite to form singly- and doubly-reduced compounds (^Et2CAAC)MgBr(^DippDAB) (5), (^Et2CAAC)MgBr(bpy) (6), (^Et2CAAC)Mg(^DippDAB) (7), (^Et2CAAC)Be(bpy) (8), and (SIPr)Be(bpy) (9). The doubly-reduced compounds 7-9 exhibit substantial π-bonding interactions across the diimine core, metal center, and π-acidic carbene. Each complex was fully characterized by UV-vis, FT-IR, X-ray crystallography, ¹H, ¹³C, and ⁹Be NMR, or EPR where applicable. We use these compounds to highlight the differences in the organometallic chemistry of the lightest alkaline earth metals, magnesium and beryllium, in an otherwise identical chemical environment.

Amidomagnesium cations

We report the synthesis, structure and reactivity of molecular amidomagnesium cations bearing tris{2-(dimethylamino)-ethyl}amine (Me₆TREN). Me₆TREN binds to the cationic magnesium centre exhibiting κ⁴ and κ³ coordination modes in [Me₆TREN-Mg-N(SiHMe₂)₂]⁺ and [Me₆TREN-Mg-N(SiMe₃)₂]⁺ respectively. [Me₆TREN-Mg-N(SiHMe₂)₂]⁺ reacts with benzophenone resulting in the insertion of the carbonyl group across β-SiH bond. The reaction between [Me₆TREN-Mg-N(SiMe₃)₂]⁺ and CO₂ leads to [Me₆TREN-Mg-OSiMe₃]⁺, while the reaction with H₂O results in [Me₆TREN-Mg-OH]₂²⁺. Attempts to prepare hydridomagnesium cations from [Me₆TREN-Mg-N(SiMe₃)₂]⁺ using KH resulted in the precipitation of MgH₂ and the isolation of [(Me₆TREN)K(THF)₃]⁺.

Bulky cationic β-diketiminate magnesium complexes

Cationic β-diketiminate Mg complexes with the bulky tBuBDI ligand and the weakly coordinating anion B(C6F5)4- have been prepared by the reaction of (tBuBDI)MgnBu with [Ph3C]+[B(C6F5)4]-; tBuBDI = CH[C(tBu)N-Dipp]2 and Dipp = 2,6-diisopropylphenyl. Their structures are compared to the previously reported cationic (MeBDI)Mg+ complexes; MeBDI = CH[C(Me)N-Dipp]2. Crystallization of [(tBuBDI)Mg]+[B(C6F5)4]- from chlorobenzene gave a unique (tBuBDI)Mg+·ClC6H5 cation with a rather short MgCl and consequently long C-Cl bond. Crystallization from chlorobenzene/arene solvent mixtures gave (tBuBDI)Mg+·arene complexes (arene = benzene, toluene, m-xylene) but in the presence of mesitylene the chlorobenzene complex was formed. Due to the greater shielding of the metal, none of these complexes display Mg(F5C6)4B- interactions. Crystal structures of the arene complexes show in all cases η2-coordination of the arene ligands. Ring slippage from a more favorable η2-coordination can be explained by the steric bulk of the tBuBDI ligand. The smaller arenes, benzene and toluene, also bind to (tBuBDI)Mg+ in bromobenzene solution. The Lewis acidity of these cationic Mg complexes was determined by the Gutmann-Beckett test. The acceptor number for (tBuBDI)Mg+ (AN = 76.0) is substantially higher than that estimated for (MeBDI)Mg+ (AN = 70.3). Calculation of the atomic NPA charges by DFT shows that the Mg2+ ion in (tBuBDI)Mg+ is slightly more positively charged than the metal in (MeBDI)Mg+, confirming its higher Lewis acidity. The lower benzene complexation energy calculated for (tBuBDI)Mg+versus (MeBDI)Mg+ is due to steric congestion of the metal in the (tBuBDI)Mg+ cation which allows only for Mg(η1)C6H6 instead of Mg(η6)C6H6 bonding. This ring slippage, however, results in a significant polarization of the electron density in the benzene ring, making it susceptible for nucleophilic attack.

Calcium stannyl formation by organostannane dehydrogenation

Reaction of the dimeric calcium hydride, [(BDI)CaH]₂ (1), with Ph₃SnH ensues with elimination of H₂ to provide [(BDI)Ca-μ₂-H-(SnPh₃)Ca(BDI)] (3) and [(BDI)Ca(SnPh₃)]₂ (4) alongside dismutation to Ph₄Sn, H₂ and Sn(0).

Lewis acidic alkaline earth metal complexes with a perfluorinated diphenylamide ligand

Alkaline earth metal chemistry with the anion [N(C₆F₅)₂]⁻ has been explored. Complexes are stabilized by metal⋯F interactions. Mg[N(C₆F₅)₂]₂ is a stronger Lewis acid than B(C₆F₅)₃.