s-Block metal complexes of superbulky (tBu3Si)2N-: a new weakly coordinating anion?

Sterically hindered amide anions have found widespread application as deprotonation agents or as ligands to stabilize metals in unusual coordination geometries or oxidation states. The use of bulky amides has also been advantageous in catalyst design. Herein we present s-block metal chemistry with one of the bulkiest known amide ligands: (tBu3Si)2N- (abbreviated: tBuN-). The parent amine (tBuNH), introduced earlier by Wiberg, is extremely resistant to deprotonation (even with nBuLi/KOtBu superbases) but can be deprotonated slowly with a blue Cs+/e- electride formed by addition of Cs0 to THF. (tBuN)Cs crystallized as a separated ion-pair, even without cocrystallized solvent. As salt-metathesis reactions with (tBuN)Cs are sluggish and incomplete, it has only limited use as an amide transfer reagent. However, ball-milling with LiI led to quantitative formation of (tBuN)Li and CsI. Structural characterization shows that (tBuN)Li is a monomeric contact ion-pair with a relatively short N-Li bond, an unusual T-shaped coordination geometry around N and extremely short Li⋯Me anagostic interactions. Crystal structures are compared with Li and Cs complexes of less bulky amide ligands (iPr3Si)2N- (iPrN-) and (Me3Si)2N- (MeN-). DFT calculations show trends in the geometries and electron distributions of amide ligands of increasing steric bulk (MeN- < iPrN- < tBuN-) and confirm that tBuN- is a rare example of a halogen-free weakly coordinating anion.

Beryllium-Mediated Halide and Aryl Transfer onto Silicon

The reactivity of hexamethylcyclotrisiloxane (D3 ) towards BeCl2 , BeBr2 , BeI2 and [Be3 Ph6 ]3 was investigated. While BeCl2 only showed unselective reactivity, BeBr2 , BeI2 and [Be3 Ph6 ] cleanly react to the trinuclear complexes [Be3 Br2 (OSiMe2 Br)4 ], [Be3 I2 (OSiMe2 I)4 ] and [Be3 Ph2 (OSiMe2 Ph)4 ]. These unprecedented bromide, iodide and phenyl transfer reactions from a group II metal onto silicon offer a versatile access to previously unknown diorgano bromo and iodo silanolates.

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.

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.

Bis(pertrifluoromethylcatecholato)silane: Extreme Lewis Acidity Broadens the Catalytic Portfolio of Silicon

Given its earth abundance, silicon is ideal for constructing Lewis acids of use in catalysis or materials science. Neutral silanes were limited to moderate Lewis acidity, until halogenated catecholato ligands provoked a significant boost. However, catalytic applications of bis(perhalocatecholato)silanes were suffering from very poor solubility and unknown deactivation pathways. In this work, the novel per(trifluoromethyl)catechol, H₂ cat^CF3 , and adducts of its silicon complex Si(cat^CF3 )₂ (1) are described. According to the computed fluoride ion affinity, 1 ranks among the strongest neutral Lewis acids currently accessible in the condensed phase. The improved robustness and affinity of 1 enable deoxygenations of aldehydes, ketones, amides, or phosphine oxides, and a carbonyl-olefin metathesis. All those transformations have never been catalyzed by a neutral silane. Attempts to obtain donor-free 1 attest to the extreme Lewis acidity by stabilizing adducts with even the weakest donors, such as benzophenone or hexaethyl disiloxane.

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.

Construction of Inorganic Crown Ethers by s-Block-Metal-Templated Si-O Bond Activation

We herein report the synthesis, structures, coordination ability, and mechanism of formation of silicon analogs of crown ethers. An oligomerization of 2 D2 (I) (2 Dn ,=(Me4 Si2 O)n ) was achieved by the reaction with GaI3 and MIx (M=Li, Na, Mg, Ca, Sr). In these reactions the metal cations serve as template and the anions (I- /[GaI4 ]- ) are required as nucleophiles. In case of MIx =LiI, [Li(2 D3 )GaI4 ] (1) is formed. In case of MIx =NaI, MgI2 , CaI2 , and SrI2 the compounds [M(2 D4 )(GaI4 )x ] (M=Mg2+ (3), Ca2+ (4), Sr2+ (5) are obtained. Furthermore the proton complex [H(2 D3 )][Ga2 I7 ] (6) was isolated and structurally characterized. All complexes were characterized by means of multinuclear NMR spectroscopy, DOSY experiments and, except for compound 3, also by single crystal X-ray diffraction. Quantum chemical calculations were carried out to compare the affinity of M+ to 2 Dn and other ligands and to shed light on the formation of larger rings from smaller ones.

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.

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.

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.

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.

Coordination polymers of alkali metal cyclosiloxazanides with one- and two-dimensional structures

The reaction of O(Si₂Me₄Cl)₂ with ammonia yielded the cyclic siloxazane O(Si₂Me₄)₂NH (1), which was used as a precursor for the synthesis of siloxazanide-type alkali metal salts. The metalation of 1 with the strong bases BzA (A = Na, K, Rb, Cs and Bz = benzyl) results in different dimensional structures depending on the alkali metal ion used. These results give new insights into framework design with inorganic building blocks and the coordination ability of siloxanes.

Heavier Alkaline-Earth Catalyzed Dehydrocoupling of Silanes and Alcohols for the Synthesis of Metallo-Polysilylethers

The dehydrocoupling of silanes and alcohols mediated by heavier alkaline-earth catalysts, [Ae{N(SiMe₃ )₂ }₂ ⋅(THF)₂ ] (I-III) and [Ae{CH(SiMe₃ )₂ }₂ ⋅(THF)₂ ], (IV-VI) (Ae=Ca, Sr, Ba) is described. Primary, secondary, and tertiary alcohols were coupled to phenylsilane or diphenylsilane, whereas tertiary silanes are less tolerant towards bulky substrates. Some control over reaction selectivity towards mono-, di-, or tri-substituted silylether products was achieved through alteration of reaction stoichiometry, conditions, and catalyst. The ferrocenyl silylether, FeCp(C₅ H₄ SiPh(OBn)₂ ) (2), was prepared and fully characterized from the ferrocenylsilane, FeCp(C₅ H₄ SiPhH₂ ) (1), and benzyl alcohol using barium catalysis. Stoichiometric experiments suggested a reaction manifold involving the formation of Ae-alkoxide and hydride species, and a series of dimeric Ae-alkoxides [(Ph₃ CO)Ae(μ₂ -OCPh₃ )Ae(THF)] (3 a-c, Ae=Ca, Sr, Ba) were isolated and fully characterized. Mechanistic experiments suggested a complex reaction mechanism involving dimeric or polynuclear active species, whose kinetics are highly dependent on variables such as the identity and concentration of the precatalyst, silane, and alcohol. Turnover frequencies increase on descending Group 2 of the periodic table, with the barium precatalyst III displaying an apparent first-order dependence in both silane and alcohol, and an optimum catalyst loading of 3 mol % Ba, above which activity decreases. With precatalyst III in THF, ferrocene-containing poly- and oligosilylethers with ferrocene pendent to- (P1-P4) or as a constituent (P5, P6) of the main polymer chain were prepared from 1 or Fe(C₅ H₄ SiPhH₂ )₂ (4) with diols 1,4-(HOCH₂ )₂ -(C₆ H₄ ) and 1,4-(CH(CH₃ )OH)₂ -(C₆ H₄ ), respectively. The resultant materials were characterized by NMR spectroscopy, gel permeation chromatography (GPC) and DOSY NMR spectroscopy, with estimated molecular weights in excess of 20,000 Da for P1 and P4. The iron centers display reversible redox behavior and thermal analysis showed P1 and P5 to be promising precursors to magnetic ceramic materials.

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.

Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation

Chemo- and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d- and f-block metal complexes. The cationic hydride of calcium [CaH]+ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti-Markovnikov products, while aryl-substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di- and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy- and siloxy-substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.

On the molecular architectures of siloxane coordination compounds: (re-)investigating the coordination of the cyclodimethylsiloxanes Dn(n= 5–8) towards alkali metal ions

Cyclodimethylsiloxanes have been used as complex ligands and silyl-ether bonding towards various cations is explored stepwise. Compounds with unprecedented coordination modes as well as novel molecular architectures are obtained and characterized.

Alkaline Earth Metal Template (Cross-)Coupling Reactions with Hybrid Disila-Crown Ether Analogues

Alkaline earth metal iodides were used as templates for the synthesis of novel silicon‐based ligands. Siloxane moieties were (cross‐)coupled and ion‐specific, silicon‐rich crown ether analogues were obtained. The reaction of 1,2,7,8‐tetrasila[12]crown‐4 (I) and 1,2‐disila[9]crown‐3 (II) with MgI₂ yielded exclusively [Mg(1,2,7,8‐tetrasila[12]crown‐4)I₂] (1). The larger Ca²⁺ ion was then employed for cross‐coupling of I and II and yielded the complex [Ca(1,2,7,8‐tetrasila[15]crown‐5)I₂] (2). Cross‐coupling of I and 1,2,4,5‐tetrasila[9]crown‐3 (III) with SrI₂ enables the synthesis of the silicon‐dominant 1,2,4,5,10,11‐hexasila[15]crown‐5 ether complex of SrI₂ (3). Further, the compounds [Sr(1,2,10,11‐tetrasila[18]crown‐6)I₂] (4), [Sr(1,2,13,14‐tetrasila[24]crown‐8)I₂] (5), and [Sr(1,2,13,14‐tetrasila‐dibenzo[24]crown‐8)I₂] (6) were obtained by coupling I, 1,2‐disila[12]crown‐4 (IV) or 1,2‐disila‐benzo[12]crown‐4 (V), respectively. Using various anions, the (cross‐)coupled ligands were also observed in an X‐ray structure within the mentioned complexes. These template‐assisted (cross‐)couplings of various ligands are the first of their kind and a novel method to obtain macrocycles and/or their metal complexes to be established. Further, the Si−O bond activations presented herein might be of importance for silane or even organic functionalization.

Not Non-Coordinating at All: Coordination Compounds of the Cyclodimethylsiloxanes Dn (D = Me2SiO; n = 6, 7) and Group 2 Metal Cations

We present the coordination chemistry of the cyclodimethylsiloxanes D₆ and D₇ toward alkaline earth metal salts. The coordination chemistry of these macrocycles toward alkaline earth metals has been unprecedented to date, and we could show that these ligands coordinate better than previously thought. Direct reaction of alkaline earth metal salts with these ligands yields stable complexes even with a relatively strongly coordinating iodide anion. A handful of counterintuitive coordination compounds could be characterized by single-crystal X-ray diffraction analysis. Quantum chemical calculations of suited Born-Haber cycles showed that these complexes are indeed stable, for Mg²⁺ and Ca²⁺ even with iodide employed as the anion and for Sr²⁺ and Ba²⁺ in the presence of GaI₃.

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.

Chemistry of a 1,5-Oligosilanylene Dianion Containing a Disiloxane Unit

Synthesis of a number of disiloxane containing cyclo- and bicyclooligosilanes is described starting from the dipotassium 1,5-oligosiloxanylene diide derived from 1,3-bis[tris(trimethylsilyl)silyl]tetramethyldisiloxane. In addition, the use of this particular fragment as ligand for zinc and group 4 metallocene complexes was studied. Both types of compounds exhibit marked structural differences compared to related compounds containing Si-Si-Si units instead of the Si-O-Si fragment.

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.

Coordination of arenes and phosphines by charge separated alkaline earth cations

Generation of ‘naked’ β-diketiminato magnesium and calcium cations allows the isolation of benzene, toluene, 1,4-difluorobenzene and terminal phosphine adducts.

The coordination behaviour and reactivity of partially silicon based crown ethers towards beryllium chloride

The reaction of partially silicon based crown ethers with BeCl₂ yields eight-membered Be–O-heterocycles, which are annulated by two six-membered Be–O-cycles.