A Remarkable Isostructural Homologous Series of Mixed Lithium−Heavier Alkali Metal tert-Butoxides [(t-BuO)8Li4M4] (M = Na, K, Rb or Cs)

THF-solvated Heavy Alkali Metal Benzyl Compounds (Na, Rb, Cs): Defined Deprotonation Reagents for Alkali Metal Mediation Chemistry

The incorporation of heavy alkali metals into substrates is both challenging and essential for many reactions. Here, we report the formation of THF‐solvated alkali metal benzyl compounds [PhCH₂M ⋅ (thf)_n] (M=Na, Rb, Cs). The synthesis was carried out by deprotonation of toluene with the bimetallic mixture n‐butyllithium/alkali metal tert‐butoxide and selective crystallization from THF of the defined benzyl compounds. Insights into the molecular structure in the solid as well as in solution state are gained by single crystal X‐ray experiments and NMR spectroscopic studies. The compounds could be successfully used as alkali metal mediating reagents. The example of caesium showed the convenient use by deprotonating acidic C−H as well as N−H compounds to gain insight into the aminometalation using these reagents.

Structural Motifs of Alkali Metal Superbases in Non-coordinating Solvents

Lochmann-Schlosser superbases (LSB) are a standard reagent in synthetic chemistry to achieve an exchange of a proton on an organic framework with an alkali metal cation, which in turn can be replaced by a wide range of electrophilic groups. In standard examples, the deprotonating reagent consists of an equimolar mixture of n-butyllithium and potassium t-butoxide. However, the nature of the reactive species could not be pinned down either for this composition or for similar mixtures with comparable high reactivity. Despite the poor solubility and the fierce reactivity, some insights into this mixture were achieved by some indirect results, comparison with chemically related systems, or skillful deductions. Recent results, mainly based on new soluble compounds, delivered structural evidence. These new insights lead to advanced and more detailed conclusions about the interplay of the involved components.

Towards the Next Generation of Lochmann-Schlosser Superbases: A Potassium Neopentyl/Alkoxy Aggregate used in the Tetra-Functionalization of Ferrocene

Lochmann-Schlosser superbases are formed by mixing alkyllithium with potassium alkoxides. These reagents could prove their synthetic usefulness and reliability in many reactions over five decades. However, despite many efforts, the real source of the exceptional reactivity remained a secret. The seemingly manageable system of four components (lithium, potassium atoms, alkyl groups, and alkoxy groups) and their interaction is obscured by poor solubility and fierce reactivity. Recent progress was achieved by using neopentyllithium, leading to alkane-soluble aggregates with varying lithium/potassium content and a flexible alkyl/alkoxy ratio. Herein, we isolated two new alkane-soluble alkyl/alkoxy mixed aggregates, [Li₄ KNp₂ (OtBu)₃ ] and [K₄ Np(OtAm)₃ ]. The latter compound is a thermally stable three-component potassium alkyl/alkoxy base with well-defined stoichiometry, in contrast to lithium-containing Lochmann-Schlosser bases with variable metal and alkyl/alkoxy content. In a simple protocol, this potassium-base gave tetrametalated ferrocene, which was converted into 1,1',3,3'-ferrocenetetracarboxylic acid by reaction with CO₂ . A subsequent conversion into the methyl ester allowed its separation from accompanying di- and tri-substituted ferrocenes.

The hetero-cubane structures of the heavy alkali metal tert-amyloxides [MOCMe2Et]4 (M = K, Rb, Cs)

Hetero-cubane structures were revealed for the tert-amyloxide compounds of potassium, rubidium, and caesium.

Combining Neopentyllithium with Potassium tert-Butoxide: Formation of an Alkane-Soluble Lochmann-Schlosser Superbase

Mixtures of alkyllithium and heavier alkali-metal alkoxides are often used to form alkyl compounds of heavier alkali metals, but these mixtures are also known for their high reactivity in deprotonative metalation reactions. These organometallic mixtures are often called LiC-KOR superbases, but despite many efforts their constitution remains unknown. Herein we present mixed alkali-metal alkyl/alkoxy compounds produced by reaction of neopentyllithium with potassium tert-butoxide. The key to success was the good solubility and temperature-stability of neopentyl alkali-metal compounds, leading to hexane-soluble mixtures, which allowed handling at ambient temperatures and isolation by crystallization. The compounds in solid state and in solution were identified by X-ray crystallography and NMR spectroscopy as mixtures of lithium/potassium neopentyl/tert-butoxy aggregates of varying compositions Lix Ky Npz (OtBu)x+y-z .

New supramolecular assemblies in heterobimetallic chemistry: synthesis of a homologous series of unsolvated alkali-metal zincates

Using an interlocking co-complexation approach, a homologous series of unsolvated alkali-metal zincates [MZn(CH2SiMe3)3] (M = Li 1, Na 2, K 3) was prepared by reacting equimolar amounts of Zn(CH2SiMe3)2 with the relevant alkali-metal alkyl M(CH2SiMe3) employing non-coordinating hexane as a solvent. X-ray crystallographic studies reveal that these heterobimetallic compounds exhibit unprecedented supramolecular assemblies made up exclusively of a three-fold combination of M-CH2, Zn-CH2 and M···Me interactions. Revealing an important alkali-metal effect, 1 displays a linear chain structure; whereas 2 and 3 form much more intricate 3D and 2D coordination networks respectively. Shedding new light into the formation of these solvent-free zincates, DFT calculations indicate that the infinite degree of aggregation observed in 1-3 plays a major role in thermodynamically driving the co-complexation reactions of their homometallic precursors. NMR spectroscopic studies suggest that in C6D6 solution 1-3 exist as discrete contacted ion-pair species, where the alkali-metal is partially solvated by molecules of deuterated solvent. The supramolecular assemblies of 1-3 can be easily deaggregated by adding the polydentate N-donors PMDETA (N,N,N',N'',N''-pentamethyldiethylenetriamine) or TMEDA (N,N,N',N'-tetramethylethylenediamine), affording monomeric [(PMDETA)LiZn(CH2SiMe3)3] (4) and [(TMEDA)2NaZn(CH2SiMe3)3] (5).

Novel CuII–MII–CuII (M = Cu or Ni) trinuclear and [NaI2CuII6] hexanuclear complexes assembled by bi-compartmental ligands: syntheses, structures, magnetic and catalytic studies

Complexes 5–7 synthesized from two in situ generated ligands, HL¹ and HL² exhibited excellent TON for epoxidation of olefins.

50 years of superbases made from organolithium compounds and heavier alkali metal alkoxides

A review of reactions of organolithium compounds (RLi) with alkali metal alkoxides is presented. On the one hand, simple lithium alkoxides form adducts with RLi the reactivity of which differs only slightly from that of RLi. On the other hand, after mixing heavier alkali metal alkoxides (R’OM, M = Na, K, Rb, Cs) with RLi, a new system is formed, which has reactivity that dramatically exceeds that of the parent RLi. A metal interchange, according to the equation RLi + R’OM = RM + R’OLi, occurs in this system, giving rise to a superbase. This reaction is frequently used for the preparation of heavier alkali metal organometallic compounds. Similar metal interchange takes place between R’OM and compounds such as lithium amides and lithium enolates of ketones or esters, thus demonstrating the general nature of this procedure. Superbases react easily with many types of organic compounds (substrates), resulting in the formation of a heavier alkali metal derivative of the substrate (metalation). The metalated substrate can react in situ with an electrophile to yield the substituted substrate, a procedure that is frequently used in synthetic and polymer chemistry. An improved mechanism of metal interchange and reaction of superbases with substrates is proposed.

Developing a hetero-alkali-metal chemistry of 2,2,6,6-tetramethylpiperidide (TMP): stoichiometric and structural diversity within a series of lithium/sodium, lithium/potassium and sodium/potassium TMP compounds

Studied extensively in solution and in the solid state, Li(TMP) (TMP=2,2,6,6-tetramethylpiperidide) is an important utility reagent popular as a strongly basic, weakly nucleophilic tool for C-H metallation. Recently, there has been a surge in interest in mixed metal derivatives containing the bulky TMP anion. Herein, we start to develop hetero (alkali metal) TMP chemistry by reporting the N,N,N',N'-tetramethylethylenediamine (TMEDA)-hemisolvated sodium-lithium cycloheterodimer [(tmeda)Na(μ-tmp)(2) Li], and its TMEDA-free variant [{Na(μ-tmp)Li(μ-tmp)}(∞)], which provides a rare example of a crystallographically authenticated polymeric alkali metal amide. Experimental observations suggest that the former is a kinetic intermediate en route to the latter thermodynamic product. Furthermore, a third modification, the mixed potassium-lithium-rich cycloheterotrimer [(tmeda)K(μ-tmp)Li(μ-tmp)Li(μ-tmp)], has also been synthesised and crystallographically characterised. On moving to the bulkier tridentate donor N,N,N',N'',N''-pentamethyldiethylenediamine (PMDETA), the additional ligation forces the sodium-lithium and potassium-dilithium ring species to open giving the acyclic arc-shaped complexes [(pmdeta)Na(μ-tmp)Li(tmp)] and [(pmdeta)K(μ-tmp)Li(μ-tmp)Li(tmp)], respectively. Completing the series, the potassium-lithium and potassium-sodium derivatives [(pmdeta)K(μ-tmp)(2) M] (M=Li, Na) have also been isolated as closed structures with a distinctly asymmetric central MN(2) K ring. Collectively, these seven new bimetallic compounds display five distinct structural motifs, four of which have never hitherto been witnessed in TMP chemistry and three of which are unprecedented in the vast structural library of alkali metal amide chemistry.

Exploiting σ/π coordination isomerism to prepare homologous organoalkali metal (Li, Na, K) monomers with identical ligand sets

Tetraamine Me(6)TREN has been used as a scaffold support to provide coordinative saturation in the complexes PhCH(2)M⋅Me(6)TREN (M=Li, Na, K). The Li derivative displays a Li−−C σ interaction with a pyramidalized CH(2) both in the solid state and in solution, and represents the first example of η(4) coordination of Me(6)TREN to lithium. In the sodium derivative, the metal cation slips slightly towards the delocalized π electrons whilst maintaining a partial σ interaction with the CH(2) group. For the potassium case, coordinative saturation successfully yields the first monomeric benzylpotassium complex, in which the anion binds to the metal cation exclusively through its delocalized π system resulting in a planar CH(2) group.

Analogy of the Coordination Chemistry of Alkaline Earth Metal and Lanthanide Ln2+ Ions: The Isostructural Zoo of Mixed Metal Cages [IM(OtBu)4{Li(thf)}4(OH)] (M=Ca, Sr, Ba, Eu), [MM′6(OPh)8(thf)6] (M=Ca, Sr, Ba, Sm, Eu, M′=Li, Na), and their Derivatives with 1,2-Dimethoxyethane

As previously shown, alkali and alkaline earth metal iodides in nonaqueous, aprotic solvents behave like transition metal halides, forming cis- and trans-dihalides with various neutral O-donor ligands. These compounds can be used as precursors for the synthesis of new mixed alkali/alkaline earth metal aggregates. We show here that Ln2+ ions form isostructural cluster compounds. Thus, with LiOtBu, 50% of the initial iodide can be replaced in MI2, M=Ca, Sr, Ba, Eu, to generate the mixed-metal alkoxide aggregates [IM(OtBu)4{Li(thf)}4(OH)], for which the M--OH contacts were investigated by theoretical methods. With M'OPh (M'=Li, Na), a new mixed-metal aryloxide cluster type [MM'6(OPh)8(thf)6] is obtained for M=Ca, Sr, Ba, Sm, Eu. Their stability versus DME (DME=1,2-dimethoxyethane) as bidentate ligand is studied.

Organometallic Polymers Assembled from Cation–π Interactions: Use of Ferrocene as a Ditopic Linker Within the Homologous Series [{(Me3Si)2NM}2⋅(Cp2Fe)]∞ (M=Na, K, Rb, Cs; Cp=cyclopentadienyl)

Addition of ferrocene to solutions of alkali metal hexamethyldisilazides M(HMDS) in arenes (in which M=Na, K, Rb, Cs) allows the subsequent crystallization of the homologous series of compounds [{(Me(3)Si)(2)NM}(2) (Cp(2)Fe)](infinity) (1-4). Similar reactions using LiHMDS led to the recrystallization of the starting materials. The crystal structures of 1-4 reveal the formation of one-dimensional chains composed of dimeric [{M(HMDS)}(2)] aggregates, which are bridged through neutral ferrocene molecules by eta(5)-cation-pi interactions. In addition, compounds 3 and 4 also contain interchain agostic M--C interactions, producing two-dimensional 4(4)-nets. Whereas 1 and 2 were prepared from toluene, the syntheses of 3 and 4 required the use of tert-butylbenzene as the reaction media. The attempted crystallization of 3 and 4 from toluene resulted in formation of the mixed toluene/ferrocene solvated complexes [{(Me(3)Si)(2)NM)(2)}(2) (Cp(2)Fe)(x)(Tol)(y)](infinity) (in which M=Rb, x=0.6, y=0.8, 5; M=Cs, x=0.5, y=1, 6). The extended solid-state structures of 5 and 6 are closely related to the 4(4)-sheets 3 and 4, but are now assembled from a combination of cation-pi, agostic, and pi-pi interactions. The charge-separated complex [K{(C(6)H(6))(2)Cr}(1.5)(Mes)][Mg(HMDS)(3)] (15) was also structurally characterized and found to adopt an anionic two-dimensional 6(3)-network through doubly eta(3)-coordinated bis(benzene)chromium molecules. DFT calculations at the B3 LYP/6-31G* level of theory indicate that the binding energies of both ferrocene and toluene to the M(HMDS) dimers increases in the sequence Li<Na<K. This pattern is a consequence of the larger metals allowing more open coordination spheres to support cation-pi contacts. By comparison, binding of the isolated metal cations to the aromatic groups follow the reverse order K<Na<Li. A combined analysis of theoretical and experimental data suggest that ferrocene is a stronger cation-pi donor than toluene for the lighter metals, but that this difference is eliminated on descending the group.

Synthesis and Characterization of a Series of Rubidium Alkoxides and Rubidium−Titanium Double Alkoxides

This report investigates the structural aspects of the products isolated from the reactions of a series of titanium alkoxides [[Ti(OR)4]n n = 2, OR = OCH2C(CH3)3 (ONep) (1); n = 1, OC6H3(CH3)2-2,6 (DMP) (2)] with rubidium alkoxides [[Rb(OR)]infinity where OR = (ONep) (3), (DMP) (4), and OC6H3(CH(CH3)2)2-2,6 (DIP) (5)]. The resultant double alkoxides were determined by single crystal X-ray diffraction to be [Rb(mu-ONep)4(py)Ti(ONep)]2 (6), [Rb(mu-DMP)Ti(DMP)4]infinity (7), and [Rb(mu-DMP)2(mu-ONep)2Ti(ONep)]infinity (8). Compound 1 is the previously reported dinculear species with trigonal bipyramidal Ti metal centers whereas compound 2 is a monomer with a tetrahedral Ti center. Suitable X-ray quality crystals of 3 were not isolated. Compounds 4 and 5 demonstrate extended polymeric networks with Rb coordination ranging from two to five utilizing terminal mu- and mu3-OR ligands and pi-interactions of neighboring OAr ligands. The double alkoxide 6 revealed a simple tetranuclear structure with mu-ONep acting as the bridge, terminal ONep ligands on the Ti, and one terminal py on the Rb. For 7 and 8, the pi-interaction facilitated the formation of extended polymeric systems. All complexes were further characterized by FT-IR and multinuclear NMR spectroscopy.

Lithiation of (tBuNH)3PNSiMe3 and Formation of Tetraimidophosphate Complexes Containing M3O3 Rings (M = Li, K): X-ray Structure of the Stable Radical {(Me3SiN)P(μ3-NtBu)3[μ3-Li(THF)]3(OtBu)}

The reaction of ((t)BuNH)(3)PNSiMe(3) (1) with 1 equiv of (n)BuLi results in the formation of Li[P(NH(t)Bu)(2)(N(t)Bu)(NSiMe(3))] (2); treatment of 2 with a second equivalent of (n)BuLi produces the dilithium salt Li(2)[P(NH(t)Bu)(N(t)Bu)(2)(NSiMe(3))] (3). Similarly, the reaction of 1 and (n)BuLi in a 1:3 stoichiometry produces the trilithiated species Li(3)[P(N(t)Bu)(3)(NSiMe(3))] (4). These three complexes represent imido analogues of dihydrogen phosphate [H(2)PO(4)](-), hydrogen phosphate [HPO(4)](2)(-), and orthophosphate [PO(4)](3)(-), respectively. Reaction of 4 with alkali metal alkoxides MOR (M = Li, R = SiMe(3); M = K, R = (t)Bu) generates the imido-alkoxy complexes [Li(3)[P(N(t)Bu)(3)(NSiMe(3))](MOR)(3)] (8, M = Li; 9, M = K). These compounds were characterized by multinuclear ((1)H, (7)Li, (13)C, and (31)P) NMR spectroscopy and, in the cases of 2, 8, and 9.3THF, by X-ray crystallography. In the solid state, 2 exists as a dimer with Li-N contacts serving to link the two Li[P(NH(t)Bu)(2)(N(t)Bu)(NSiMe(3))] units. The monomeric compounds 8 and 9.3THF consist of a rare M(3)O(3) ring coordinated to the (LiN)(3) unit of 4. The unexpected formation of the stable radical [(Me(3)SiN)P(mu(3)-N(t)Bu)(3)[mu(3)-Li(THF)](3)(O(t)Bu)] (10) is also reported. X-ray crystallography indicated that 10 has a distorted cubic structure consisting of the radical dianion [P(N(t)Bu)(3)(NSiMe(3))](.2)(-), two lithium cations, and a molecule of LiO(t)Bu in the solid state. In dilute THF solution, the cube is disrupted to give the radical monoanion [(Me(3)SiN)((t)BuN)P(mu-N(t)Bu)(2)Li(THF)(2)](.-), which was identified by EPR spectroscopy.

Synthesis and structure of [Na11(OtBu)10(OH)]:1H NMR shift of a hydroxide ion encapsulated in a 21-vertex alcoholate cage

[Na11(OtBu)10(OH)], a hydroxide enclosing 21-vertex cage compound, was found to crystallize from mixtures of sodium tert.butanolate with sodium hydroxide. Its structure can be derived from the known (NaOtBu)6-hexaprismane by replacing one butanolate unit with OH- and capping the latter with five additional units of NaOtBu. The hydroxide shows a signal at -3.21 ppm in the 1H NMR spectrum.