Highly Lewis acidic cationic alkaline earth metal complexes

Stable Mg2+ Dication Weakly Stabilized/Coordinated in Solution: Synthesis, Structure, Reactivity, and Use in Catalysis

The first soluble and stable Mg2+ dication stabilized only by weakly coordinating and chemically robust carborate anions [HexCB11Cl11]- is described. Mg[HexCB11Cl11]2 (1), prepared by reaction of Mg(nBu)2 with 2 equiv of [Ph3C][HexCB11Cl11], consists, in the solid state, of a central Mg2+ surrounded by two [HexCB11Cl11]- anions. In solution, experimental and classical molecular dynamics simulations (cMD) agree with cation/anion association being retained, reflecting the high electrophilicity of the Mg center. Yet, reflecting only weak anion/cation interactions, species 1 polymerizes 1-hexene and coordinates alkynes. However, 1 displays no reaction with HSiEt3 at room temperature, consistent with a low hydridicity of the hard (HSAB) Mg2+ center. Contrasting with 1 (FIA = 264 kJ mol-1; FIA: Fluoride Ion Affinity), salt Mg[(nBu)3NB12H4Cl7]2 (2), incorporating the more basic ammoniododecaborate [(nBu)3NB12H4Cl7]- anion, is significantly less Lewis acidic (FIA = 214.7 kJ mol-1) and unreactive with alkenes and alkynes. Salt 1 effectively catalyzes alkene/alkyne hydrosilylation via an initial alkene/alkyne coordination/initiation, as suggested by experimental and computational data. It also efficiently catalyzes (with a catalyst loading down to 0.1 mol%) the hydrosilylation of CO2 to CH4 in the presence of HSiEt3. Salt 1 smoothly promotes the catalytic transfer hydrogenation of 1,1-diphenylethylene, and it is also an active imine hydrogenation catalyst in the presence of H2.

Low-coordinate potassium alkoxide - an efficient trap for arenes: the role of ηn non-covalent bonding in substrate activation for C-H bond metalation

Metalation of bulky tris(2-(piperidin-1-yl-methyl)phenyl)methanol [(C5H10N)CH2C6H4-o]3COH with (Me3Si)2NK in Et2O results in a dimeric potassium alkoxide {[(C5H10N)CH2C6H4-o]3C(μ2-O)K(Et2O)}2 (1). The Et2O molecule can be removed from the K+ coordination sphere affording coordinatively unsaturated alkoxide species which readily traps π-donor molecules. In the presence of excess arene, the reactions result in ηn-π-complexes, retaining in the crystal state a dimeric core {[(C5H10N)CH2C6H4-o]3C(μ2-O)K(ηn-arene)}2 (arene = C6H6 (2), CH3C6H5 (3), C10H8 (4)). With C6H5OMe and C6H5NMe2 molecules containing competing n- and π-donating sites, the reactions proceed differently: the former coordinates to K+ through an oxygen lone pair resulting in {[(C5H10N)CH2C6H4-o]3C(μ2-O)K(κ1-O(Me)C6H5)}2 (5) while for the latter, π-arene interaction turns out to be preferable, yielding {[(C5H10N)CH2C6H4-o]3C(μ2-O)K(η2-C6H5NMe2)}2 (6). The reactions with equimolar amounts of benzene or thiophene afford coordination polymers [{[(C5H10N)CH2C6H4-o]3C(μ2-O)K}2(μ-C6H6)]n (7) and [{[(C5H10N)CH2C6H4-o]3C(μ2-O)K}2(μ-C4H4S)]n (8), in which benzene and thiophene molecules are μ-bridging two K+ ions. The treatment of {[(C5H10N)CH2C6H4-o]3C(μ2-O)K(η2-CH3C6H5))}2 with Me3SiCH2Li or n-BuLi (1.2 eq.) in hexane at 20 °C results in the facile metalation of the Me group of toluene, forming [PhCH2K]n and lithium alkoxide. This model reaction provides a deeper insight into the probable mechanism of metalation of CH bonds under Lochmann-Schlosser superbasic conditions, and the role and the nature of the synergistic effect of two metals. The calculations and QTAIM analysis were performed for 1-8 and model molecules as well.

Heavier alkaline earth and heterobimetallic s-block "ate" complexes of a di(amido)siloxane ligand: solid-state structure and dynamic solution-phase behaviour

The diverse solid-state structures and solution-phase dynamics of both neutral and heterometallic s-block "ate" complexes of the heavier alkaline earth metals (Ae; Ca-Ba) supported by a chelating and flexible di(amido)siloxane ligand ([NON-DippL]2- = [O(SiMe2NDipp)2]2-) are described, enabling comparison with those of closely related di(amido) ligands based on either flexible aliphatic or rigid xanthene-based backbones. Three dimeric alkaline earth complexes [(NON-DippL)Ae]2 (Ae = Ca (2), Sr (3) and Ba (4)) which feature a κ3-N,O,N'-κ1-N'-tridentate coordination mode were prepared from protonolysis reactions between NON-DippLH2 with (Ae = Ca, Sr and Ba); N'' = [N(SiMe3)2]-. In tetrahydrofuran, these complexes were readily converted into the monomeric adducts [(NON-DippL)Ae(thf)n] (n = 2, Ae = Ca (5); n = 3, Ae = Sr (6) and Ba (7)). Heterometallic Ae/K amide "ate" complexes were afforded through two routes: reaction of previously reported [(NON-DippL)Mg]2 (1) with two equivalents of KN'' at elevated temperatures resulted in [(NNO-DippL)Mg(μ-N'')K]n (8; NNO-DippL = [OSiMe2NDippSiMe2NDipp]2-), whereas the equimolar reaction of NON-DippLH2 with led to [(NON-DippL)Ae(μ-N'')K]n (Ae = Ca (9), Sr (10) and Ba (11)). Complexes 8-11 exist as one-dimensional coordination polymers propagated by K+-aryl π-facial interactions in the solid-state. The mixed amide/siloxide "NNO" ligand in 8 results from a 1,3-silyl retro-Brook rearrangement of the original di(amido)siloxane ligand, while the larger Ae2+ congeners readily accommodate the coordination of KN'' with the di(amido)siloxane ligand retaining a κ3-N,O,N'-tridentate motif in 9-11. Finally, the solution-phase behaviour of 8-11 in both toluene and thf were investigated indicating the reversible dissociation of KN'' from 9-11 and the thermodynamic parameters of this process were elucidated.

Promoting π-Facial Interactions in Phenyl-Substituted 1,8-Bis(silylamido)naphthalene Alkaline Earth Complexes

Bimetallic 1,8-bis(silylamido)naphthalene alkaline earth complexes [(R3 L)Ae]2 ([R3 L]2- = [1,8-{(R3Si)N}2C10H6)]2-, where R3 = Ph2Me, Ae = Ca (1), Sr (2), and Ba (3); R3 = Ph3, Ae = Ca (4), Sr (5), and Ba (6) were prepared via protonolysis reactions of the phenyl-substituted proligands Ph3 LH2 and Ph2MeLH2 with [AeN″2]2 (N″ = [N(SiMe3)2]-) in benzene. X-ray crystallographic analysis showed that 1, 2, and 4 crystallize as nitrogen-bridged dimers. Conversely, 5 and 6 display a naphthalene-bridged motif, while the structure of 3 is intermediate between the two distinct classes. NMR spectroscopic analysis of isolated samples of 1-6 in thf-d 8 confirmed their conversion into the monomeric thf-d 8 adducts [(R3 L)Ae(thf-d 8) n ]; crystallographic verification of the structural motif was provided by the X-ray crystal structure of [(Ph3 L)Sr(thf)3] (7). The structural range of dimers 1-6 was influenced by the electron-withdrawing nature of the phenyl substituents of the ligand and the ability to form "soft" multihaptic π-facial interactions with the metal ions, which was preferential for the larger Sr2+ and Ba2+ cations as well as the relative strength of the metal-N bonds. This has been rationalized through complementary computational studies. This work provides insight into the structure and bonding preferences of heavy alkaline earth complexes with rigid bis(amido) ligands.

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.

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments

New developments in the field of Lewis acidity are highlighted, with the focus of novel Lewis acids and Lewis superacids of group 2, 13, 14, and 15 elements. Several important basics, illustrated by modern examples (classification of Donor-Acceptor (DA) complexes, amphoteric nature of any compound in terms of DA interactions, reorganization energies of main group Lewis acids and the role of the energies of frontier orbitals) are presented and discussed. It is emphasized that the Lewis acidity phenomena are general and play vital role in different areas of chemistry: from weak "atomophilic" interactions to the complexes of Lewis superacids.

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.

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.

Molecular Influences on the Quantification of Lewis Acidity with Phosphine Oxide Probes

Gutmann-Beckett-type measurements with phosphine oxide probes can be used to estimate effective Lewis acidity with ³¹P nuclear magnetic resonance spectroscopy, but the influence of the molecular structure of a given probe on the quantification of Lewis acidity remains poorly documented in experimental work. Here, a quantitative comparison of triethyl (E), trioctyl (O), and triphenyl (P) phosphine oxides as molecular probes of Lewis acidity has been carried out via titration studies in MeCN with a test set of six mono- and divalent metal triflate salts. In comparison to E, the bulkier O displays a similar range of chemical shift values and binding affinities for the various test metal ions. Spectral linewidths and speciation properties vary for individual cation-to-probe ratios, however, confirming probe-specific properties that can impact the data quality. Importantly, P displays a consistently narrower dynamic range than both E and O, illustrating how electronic changes at phosphorus can influence the NMR response. Comparative parametrizations of the effective Lewis acidities of a broader range of metal ions, including the trivalent rare earth ions Y³⁺, Lu³⁺, and Sc³⁺ as well as the uranyl ion (UO₂²⁺), can be understood in light of these results, providing insight into the fundamental chemical processes underlying the useful approach of single-point measurements for quantification of effective Lewis acidity. Together with a study of counteranion effects reported here, these data clarify the diverse ensemble of factors that can influence the measurement of Lewis acid/base interactions.

Alkaline-Earth Metal Mediated Benzene-to-Biphenyl Coupling

Complex [(DIPeP BDI)Ca]2 (C6 H6 ), with a C6 H6 2- dianion bridging two Ca2+ ions, reacts with benzene to yield [(DIPeP BDI)Ca]2 (biphenyl) with a bridging biphenyl2- dianion (DIPeP BDI=HC[C(Me)N-DIPeP]2 ; DIPeP=2,6-CH(Et)2 -phenyl). The biphenyl complex was also prepared by reacting [(DIPeP BDI)Ca]2 (C6 H6 ) with biphenyl or by reduction of [(DIPeP BDI)CaI]2 with KC8 in presence of biphenyl. Benzene-benzene coupling was also observed when the deep purple product of ball-milling [(DIPP BDI)CaI(THF)]2 with K/KI was extracted with benzene (DIPP=2,6-CH(Me)2 -phenyl) giving crystalline [(DIPP BDI)Ca(THF)]2 (biphenyl) (52 % yield). Reduction of [(DIPeP BDI)SrI]2 with KC8 gave highly labile [(DIPeP BDI)Sr]2 (C6 H6 ) as a black powder (61 % yield) which reacts rapidly and selectively with benzene to [(DIPeP BDI)Sr]2 (biphenyl). DFT calculations show that the most likely route for biphenyl formation is a pathway in which the C6 H6 2- dianion attacks neutral benzene. This is facilitated by metal-benzene coordination.

Synthesis of Molecular Phenylcalcium Derivatives: Application to the Formation of Biaryls

Hydrocarbon-soluble β-diketiminato phenylcalcium derivatives, which display various modes of Ca-μ2 -Ph-Ca bridging, are accessible from reactions of Ph2 Hg and [(BDI)CaH]2 . Although the resultant compounds are inert toward the C-H bonds of benzene, they yield selective and uncatalyzed biaryl formation when reacted with readily available aryl bromides.

Ion-pairs as a gateway to transmetalation: aryl transfer from boron to nickel and magnesium

Gas-phase ion-ion reactions between tris-1,10-phenantholine metal dications, [(phen)₃M]²⁺ (where M = Ni and Mg), and the tetraphenylborate anion yield the ion-pairs {[(phen)₃M]²⁺[BPh₄]^-}⁺. The ion-pairs undergo transmetalation upon loss of a phen ligand to give the organometallic complexes [(phen)₂M(Ph)]⁺. DFT calculations, used to determine the energy barriers for the transmetalation reactions and the hydrolysis reactions, are entirely consistent with the experimental results.

Geometrically Constrained Cationic Low-Coordinate Tetrylenes: Highly Lewis Acidic σ-Donor Ligands in Catalytic Systems

A novel non-innocent ligand class, namely cationic single-centre ambiphiles, is reported in the phosphine-functionalised cationic tetrylene Ni0 complexes, [PhR DippENi(PPh3 )3 ]+ (4 a/b (Ge) and 5 (Sn); PhR Dipp={[Ph2 PCH2 SiR2 ](Dipp)N}- ; R=Ph, i Pr; Dipp=2,6-i Pr2 C6 H3 ). The inherent electronic nature of low-coordinate tetryliumylidenes, combined with the geometrically constrained [N-E-Ni] bending angle enforced by the chelating phosphine arm in these complexes, leads to strongly electrophilic EII centres which readily bind nucleophiles, reversibly in the case of NH3 . Further, the GeII centre in 4 a/b readily abstracts the fluoride ion from [SbF6 ]- to form the fluoro-germylene complex PhR DippGe(F)Ni(PPh3 )3 9, despite this GeII centre simultaneously being a σ-donating ligand towards Ni0 . Alongside the observed catalytic ability of 4 and 5 in the hydrosilylation of alkynes and alkenes, this forms an exciting introduction to a multi-talented ligand class in cationic single-centre ambiphiles.

Phosphine oxides as NMR and IR spectroscopic probes for geometry and energy of PO···H–A hydrogen bonds

In this work we evaluate the possibility to use the NMR and IR spectral properties of P=O group to estimate the geometry and strength of hydrogen bonds which it forms...

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.

A Thermally Stable Magnesium Phosphaethynolate Grignard Complex

The 2-phosphaethynolate (OCP) anion has found versatile applications across the periodic table but remains underexplored in group 2 chemistry due to challenges in isolating thermally stable complexes. By rationally modifying their coordination environments using 1,3-dialkyl-substituted N-heterocyclic carbenes (NHCs), we have now isolated and characterized thermally stable, structurally diverse, and hydrocarbon soluble magnesium phosphaethynolate complexes (2, 4^Me, and 8-10), including the novel phosphaethynolate Grignard reagent (2^iPr). The methylmagnesium phosphaethynolate and magnesium diphosphaethynolate complexes readily activate dioxane with subsequent H-atom abstraction to form [(NHC)MgX(μ-OEt)]₂ [X = Me (3) or OCP (8 and 9)] complexes. Their reactivities increased with the Lewis acidity of the Mg²⁺ cation and may be attenuated by Lewis base saturation or a slight increase in carbene sterics. Solvent effects were also investigated and led to the surreptitious isolation of an ether-free sodium phosphaethynolate (NHC)₃Na(OCP) (6), which is soluble in aromatic hydrocarbons and can be independently prepared by the reaction of NHC and [Na(dioxane)₂][OCP] in toluene. Under forcing conditions (105 °C, 3 days), the magnesium diphosphaethynolate complex (NHC)₃Mg(OCP)₂ (10) decomposes to a mixture of organophosphorus complexes, among which a thermal decarbonylation product [(NHC)₂P^I][OCP] (11) was isolated.

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.

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.

Cu-Catalyzed Arylation of Bromo-Difluoro-Acetamides by Aryl Boronic Acids, Aryl Trialkoxysilanes and Dimethyl-Aryl-Sulfonium Salts: New Entries to Aromatic Amides

We describe a mechanism-guided discovery of a synthetic methodology that enables the preparation of aromatic amides from 2-bromo-2,2-difluoroacetamides utilizing a copper-catalyzed direct arylation. Readily available and structurally simple aryl precursors such as aryl boronic acids, aryl trialkoxysilanes and dimethyl-aryl-sulfonium salts were used as the source for the aryl substituents. The scope of the reactions was tested, and the reactions were insensitive to the electronic nature of the aryl groups, as both electron-rich and electron-deficient aryls were successfully introduced. A wide range of 2-bromo-2,2-difluoroacetamides as either aliphatic or aromatic secondary or tertiary amides were also reactive under the developed conditions. The described synthetic protocols displayed excellent efficiency and were successfully utilized for the expeditious preparation of diverse aromatic amides in good-to-excellent yields. The reactions were scaled up to gram quantities.

Fluoroarene Separations in Metal-Organic Frameworks with Two Proximal Mg2+ Coordination Sites

Fluoroarenes are widely used in medicinal, agricultural, and materials chemistry, and yet their production remains a critical challenge in organic synthesis. Indeed, the nearly identical physical properties of these vital building blocks hinders their purification by traditional methods, such as flash chromatography or distillation. As a result, the Balz-Schiemann reaction is currently employed to prepare fluoroarenes instead of more atom-economical C-H fluorination reactions, which produce inseparable mixtures of regioisomers. Herein, we propose an alternative solution to this problem: the purification of mixtures of fluoroarenes using metal-organic frameworks (MOFs). Specifically, we demonstrate that controlling the interaction of fluoroarenes with adjacent coordinatively unsaturated Mg²⁺ centers within a MOF enables the separation of fluoroarene mixtures with unparalleled selectivities. Liquid-phase multicomponent equilibrium adsorption data and breakthrough measurements coupled with van der Waals-corrected density functional theory calculations reveal that the materials Mg₂(dobdc) (dobdc^4- = 2,5-dioxidobenzene-1,4-dicarboxylate) and Mg₂(m-dobdc) (m-dobdc^4- = 2,4-dioxidobenzene-1,5-dicarboxylate) are capable of separating the difluorobenzene isomers from one another. Additionally, these frameworks facilitate the separations of fluoroanisoles, fluorotoluenes, and fluorochlorobenzenes. In addition to enabling currently unfeasible separations for the production of fluoroarenes, our results suggest that carefully controlling the interaction of isomers with not one but two strong binding sites within a MOF provides a general strategy for achieving challenging liquid-phase separations.

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.

On the Use of Aqueous Metal-Aqua pKa Values as a Descriptor of Lewis Acidity

The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in nonaqueous conditions, commonly used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a ³¹P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. Plots of the pK_a values of the corresponding metal-aqua species, [M(H₂O)_m]ⁿ⁺, measured in H₂O vs the ³¹P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d₃-MeCN) and deuterated dichloromethane (CD₂Cl₂), display tightly colinear relationships, suggesting similar behavior for these ions in water, d₃-MeCN, and CD₂Cl₂. This colinearity reinforces the utility of the common approach of using the aqueous pK_a values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d₃-MeCN suggest a 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Together, these data suggest that both aqueous pK_a values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.

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.

Alkaline-earth complexes with macrocyclic-functionalised bis(phenolate)s and bis(fluoroalkoxide)s

The synthesis and structural features of several families of unsolvated molecular complexes of the heavy alkaline earths (Ae = calcium, strontium and barium) supported by bis(phenolate)s or bis(fluoroalkoxide)s are described. These dianionic, multidentate ligands are built around diaza-macrocycles that contain either five or six N- and O-heteroatoms. Several of these complexes have been characterised by X-ray diffraction crystallography. A list of comparative features was drawn upon close examination of the molecular structures of these complexes. It highlights the subtle influences of the identity of the central Ae metal, denticity and nature -fluoroalkoxide vs. phenolate- of the anionic tethers in the ligands. All complexes are seven- or eight-coordinate. It is observed in particular that a decrease of the number of heteroatoms in the macrocyclic backbone of the ligand will be compensated by the establishment of intramolecular AeF interactions (accounting for ca. 3.8-6.4% of the pertaining coordination spheres according to bond valence sum analysis), dimerisation of the complex, or, in one case, solvent (thf) retention. Attempts to gauge the Lewis acidity in these series of complexes were carried out by three independent methods (Childs, Gutmann-Beckett and global electrophilicity index). However, conflicting results were obtained and no clear trend can be delineated, even if on the whole, these measurements concur to suggest relatively low Lewis acidity.

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.