Alkali Blues: Blue-Emissive Alkali Metal Pyrrolates

2-Iminopyrroles [H^tBuL, 4-tert-butyl phenyl(pyrrol-2-ylmethylene)amine] are non-fluorescent π systems. However, they display blue fluorescence after deprotonation with alkali metal bases in the solid state and in solution at room temperature. In the solid state, the alkali metal 2-imino pyrrolates, M(^tBuL), aggregate to dimers, [M(^tBuL)(NCR)]₂ (M=Li, R=CH₃, CH(CH₃)CNH₂), or polymers, [M(^tBuL)]_n (M=Na, K). In solution (solv=CH₃CN, DMSO, THF, and toluene), solvated, uncharged monomeric species M(^tBuL)(solv)_m with N,N′-chelated alkali metal ions are present. Due to the electron-rich pyrrolate and the electron-poor arylimino moiety, the M(^tBuL) chromophore possesses a low-energy intraligand charge-transfer (ILCT) excited state. The chelated alkali cations rigidify the chromophore, restricting intramolecular motions (RIM) by the chelation-enhanced fluorescence (CHEF) effect in solution and, consequently, switch-on a blue fluorescence emission.     

Alkali Metal Complexes of a Bis(diphenylphosphino)methane Functionalized Amidinate Ligand: Synthesis and Luminescence

A novel bis(diphenylphosphino)methane (DPPM) functionalized amidine ligand (DPPM−C(N-Dipp)₂H) (Dipp=2,6-diisopropylphenyl) was synthesized. Subsequent deprotonation with suitable alkali metal bases resulted in the corresponding complexes [M{DPPM−C(N-Dipp)₂}(L_n)] (M=Li, Na, K, Rb, Cs; L=thf, Et₂O). The alkali metal complexes form monomeric species in the solid state, exhibiting intramolecular metal-π-interactions. In addition, a caesium derivative [Cs{PPh₂CH₂-C(N-Dipp)₂}]₆ was obtained by cleavage of a diphenylphosphino moiety, forming an unusual six-membered ring structure in the solid state. All complexes were fully characterized by single crystal X-ray diffraction, NMR spectroscopy, IR spectroscopy as well as elemental analysis. Furthermore, the photoluminescent properties of the complexes were thoroughly investigated, revealing differences in emission with regards to the respective alkali metal. Interestingly, the hexanuclear [Cs{PPh₂CH₂-C(N-Dipp)₂}]₆ metallocycle exhibits a blue emission in the solid state, which is significantly red-shifted at low temperatures. The bifunctional design of the ligand, featuring orthogonal donor atoms (N vs. P) and a high steric demand, is highly promising for the construction of advanced metal and main group complexes.     

Planar Hypercoordinate Carbons in Alkali Metal Decorated CE32− and CE22− Dianions

After exploring the potential energy surfaces of M_mCE₂^p (E=S−Te, M=Li−Cs, m=2, 3 and p=m-2) and M_nCE₃^q (E=S−Te, M=Li−Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE₂²⁻ and Y-shaped CE₃²⁻ dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals.     

Structural Diversity in Alkali Metal and Alkali Metal Magnesiate Chemistry of the Bulky 2,6‐Diisopropyl‐N‐(trimethylsilyl)anilino Ligand

Bulky amido ligands are precious in s‐block chemistry, since they can implant complementary strong basic and weak nucleophilic properties within compounds. Recent work has shown the pivotal importance of the base structure with enhancement of basicity and extraordinary regioselectivities possible for cyclic alkali metal magnesiates containing mixed n‐butyl/amido ligand sets. This work advances alkali metal and alkali metal magnesiate chemistry of the bulky arylsilyl amido ligand [N(SiMe₃)(Dipp)]^? (Dipp=2,6‐iPr₂‐C₆H₃). Infinite chain structures of the parent sodium and potassium amides are disclosed, adding to the few known crystallographically characterised unsolvated s‐block metal amides. Solvation by N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDETA) or N,N,N′,N′‐tetramethylethylenediamine (TMEDA) gives molecular variants of the lithium and sodium amides; whereas for potassium, PMDETA gives a molecular structure, TMEDA affords a novel, hemi‐solvated infinite chain. Crystal structures of the first magnesiate examples of this amide in [MMg{N(SiMe₃)(Dipp)}₂(μ‐nBu)]_∞ (M=Na or K) are also revealed, though these breakdown to their homometallic components in donor solvents as revealed through NMR and DOSY studies.     

Synthesis and Structural Characterization of Alkali Metal Guanidinates

Reactions of 1,3-diisopropylcarbodiimide with alkali metal amides, MN（SiMe3）2 （M=Li or Na） in hexane or THF produced the alkali metal guanidinates { （i-PrN）2C [N（SiMe3）2]Li }2 （1） and { （i-PrN）2C[N（SiMe3）2]Na（THF） } 2 （2） in nearly quantitative yields. Both complexes 1 and 2 were well characterized by elemental analysis, IR spectra, ^1H and ^13C NMR spectra, and X-ray diffraction. It was found that the guanidinates adopt different coordination modes in these complexes.     

Alkali Metal Compounds of Hydroquinone: Synthesis and Crystal Structure

A number of novel routes to the alkali metal compounds of hydroquinone M₂[p‐C₆H₄O₂] (M = Li, Na, K, Rb, Cs) and M[p‐C₆H₄O(OH)] (M = K, Rb, Cs) have been synthetically explored. The selective synthesis of the alkali 4‐hydroxyphenolates and 1, 4‐phenylenediolates is based on optimized reaction conditions (solvents, temperatures). All compounds were structurally characterized by means of powder X‐ray diffraction using Rietveld profile refinement including C—C and C—O bond distance restraints. The atomic arrangement of M₂[p‐C₆H₄O₂](M = Na, K) (tetragonal, space group: P4₂/ncm) is characterized by infinite pillars of [M₂^[3]O₂^[3]]‐units along the c axis connected by [p‐C₆H₄O₂]^2—‐anions with stacking direction along c. The coordinatively unsaturated alkali metals, surrounded by three oxygen atoms, exhibit symmetrical (K) as well as asymmetrical (Na) interactions with the phenylene rings. M[p‐C₆H₄O(OH] (M = K, Rb) (tetragonal, space group: P4/n) forms hydrogen‐bridged linear chains of [p‐C₆H₄O(OH)]^—‐anions along the c direction. The phenylene planes of neighboring chains have an almost orthogonal arrangement while the interchain planes are parallel. K and Rb are fourfold coordinated by two different oxygen coordination spheres.     

Ternary Alkali Metal Transition Metal Acetylides

The first alkali metal transition metal acetylides of general composition A₂M⁰C₂ (A = Na ? Cs, M⁰ = Pd, Pt) were obtained by solid state reactions of alkali metal acetylides with palladium and platinum. They are characterized by chains, which are separated by alkali metal ions. Analogous chains also separated by alkali metal ions are the characteristic structural feature of acetylides of composition AM^IC₂, which are accessible by reacting AC₂H with M^II in liquid ammonia (A = Li ? Cs, M^I = Cu, Ag, Au). Despite their structural similarities they possess different properties, as acetylides of composition A₂M⁰C₂ are semiconductors with very small indirect band gaps and slightly extended C–C distances compared to a C–C triple bond, whereas acetylides of composition AM^IC₂ show a typical salt‐like behavior with C–C distances close to the expected value for a C–C triple bond of 120 pm. But with the help of simple chemical models these differences can be made plausible. Furthermore, it is shown that only by a combination of different methods (powder diffraction with X‐rays and neutrons, solid state NMR spectroscopy, Raman spectroscopy) it was possible to characterize this new class of compounds structurally and chemically.     

Synthesis and some Reactions of Alkali Metal Carbamotelluroates

Sodium and potassium carbamotelluroates [M(R₂NCOTe), M = Na, K, Rb, Cs] were synthesized in moderate to good yields by reacting carbamoyl chloride with the corresponding alkali metal tellurides. The salts readily reacted with trimethylsilyl chloride to form O‐trimethylsilyl carbamotelluroate (R₂NCTeOSiMe₃), which further reacted with RbF and CsF to lead to the corresponding heavy alkali metal carbamotelluroates. These salts reacted with alkyl iodide and carbamoyl chlorides to give the corresponding Te‐alkyl carbamotelluroates and dicarbamoyl tellurides in moderate yields.     

Disilane Cleavage with Selected Alkali and Alkaline Earth Metal Salts

The industry-scale production of methylchloromonosilanes in the Müller–Rochow Direct Process is accompanied by the formation of a residue, the direct process residue (DPR), comprised of disilanes Me_nSi₂Cl_6-n (n=1–6). Great research efforts have been devoted to the recycling of these disilanes into monosilanes to allow reintroduction into the siloxane production chain. In this work, disilane cleavage by using alkali and alkaline earth metal salts is reported. The reaction with metal hydrides, in particular lithium hydride (LiH), leads to efficient reduction of chlorine containing disilanes but also induces disproportionation into mono- and oligosilanes. Alkali and alkaline earth chlorides, formed in the course of the reduction, specifically induce disproportionation of highly chlorinated disilanes, whereas highly methylated disilanes (n>3) remain unreacted. Nearly quantitative DPR conversion into monosilanes was achieved by using concentrated HCl/ether solutions in the presence of lithium chloride.     

Synthesis and Structural Investigation of a Complete Series of Brightly Colored Alkali Metal 1,3-Dimethylviolurates

A complete series of alkali metal 1,3-dimethylviolurates M(Me₂Vio) was synthesized and fully characterized. The title compounds M(Me₂Vio)(H₂O) [M = Li ( 3 ), Na ( 4 )], K(Me₂Vio)(H₂O)_0.5 ( 5 ) and M(Me₂Vio) [M = Rb ( 6 ), Cs ( 7 )] were prepared by neutralizing 1,3-dimethylvioluric acid (= HMe₂Vio; 2 ) with 1 equiv. of the corresponding metal hydroxides MOH. The resulting salts exhibit striking colors ranging from orange-red ( 3 ) through purple ( 4 , 5 ) to bright blue ( 6 , 7 ). In contrast to the monohydrate 4 , the classical synthesis of sodium 1,3-dimethylviolurate from 1,3-dimethylbarbituric acid and NaNO₂ afforded the purple trihydrate Na(Me₂Vio)(H₂O)₃ ( 4a ). All new compounds have been fully characterized by their IR and NMR (¹H, ¹³C) spectra as well as elemental analyses. X-ray crystal structure determination revealed that the title compounds exist as one- (Li, Na), two- (K, Cs), or three-dimensional (Rb) coordination polymers in the solid state.     

Can Cyclen Bind Alkali Metal Azides? A DFT Study as a Precursor to Synthesis

Can cyclen (1,4,7,10‐tetraazacyclododecane) bind alkali metal azides? This question is addressed by studying the geometric and electronic structures of the alkali metal azide‐cyclen [M(cyclen)N₃] complexes using density functional theory (DFT). The effects of adding a second cyclen ring to form the sandwich alkali metal azide‐cyclen [M(cyclen)₂N₃] complexes are also investigated. N₃^? is found to bind to a M⁺(cyclen) template to give both end‐on and side‐on structures. In the end‐on structures, the terminal nitrogen atom of the azide group (N1) bonds to the metal as well as to a hydrogen atom of the cyclen ring through a hydrogen bond in an end‐on configuration to the cyclen ring. In the side‐on structures, the N₃ unit is bonded (in a side‐on configuration to the cyclen ring) to the metal through the terminal nitrogen atom of the azide group (N1), and through the other terminal nitrogen atom (N3) of the azide group by a hydrogen bond to a hydrogen atom of the cyclen ring. For all the alkali metals, the N₃‐side‐on structure is lowest in energy. Addition of a second cyclen unit to [M(cyclen)N₃] to form the sandwich compounds [M(cyclen)₂N₃] causes the bond strength between the metal and the N₃ unit to decrease. It is hoped that this computational study will be a precursor to the synthesis and experimental study of these new macrocyclic compounds; structural parameters and infrared spectra were computed, which will assist future experimental work.     

Structures of Solvent-Free,Monomeric LiCCH,NaCCH, and KCCH

The simplest alkali metal acetylides MCCH were made in the gas phase for the first time (see drawing), and their bond lengths were determined by millimeter/submillimeter spectroscopy of their isotopomers. The M−C bond lengths r_CM are the shortest known for organoalkali metal compounds. In the case of LiCCH, the experimentally determined Li−C distance of 1.888 Å has an estimated accuracy of ±0.0005 Å, which should allow a rigorous test of theoretical methods.     

Heavy Alkali Metal Manganate Complexes: Synthesis,Structures and Solvent-Induced Dissociation Effects

Rare examples of heavier alkali metal manganates [{(AM)Mn(CH₂SiMe₃)(N^‘Ar)₂}_∞] (AM=K, Rb, or Cs) [N^‘Ar=N(SiMe₃)(Dipp), where Dipp=2,6-iPr₂-C₆H₃] have been synthesised with the Rb and Cs examples crystallographically characterised. These heaviest manganates crystallise as polymeric zig-zag chains propagated by AM⋅⋅⋅π-arene interactions. Key to their preparation is to avoid Lewis base donor solvents. In contrast, using multidentate nitrogen donors encourages ligand scrambling leading to redistribution of these bimetallic manganate compounds into their corresponding homometallic species as witnessed for the complete Li - Cs series. Adding to the few known crystallographically characterised unsolvated and solvated rubidium and caesium s-block metal amides, six new derivatives ([{AM(N^‘Ar)}_∞], [{AM(N^‘Ar)⋅TMEDA}_∞], and [{AM(N^‘Ar)⋅PMDETA}_∞] where AM=Rb or Cs) have been structurally authenticated. Utilising monodentate diethyl ether as a donor, it was also possible to isolate and crystallographically characterise sodium manganate [(Et₂O)₂Na(ⁿBu)Mn[(N^‘Ar)₂], a monomeric, dinuclear structure prevented from aggregating by two blocking ether ligands bound to sodium.     

The Cadmium Oxide Route to Alkali Metal Oxometallates with Uncommon Structural Features

The aim of this research report is to give an introduction to a specific solid state reaction applied in the field of alkali metal rich transition metal compounds. The approach is an explorative investigation of the oxidation of transition metals with CdO in the presence of alkali metal oxides and / or compounds with complex anions such as Na₂SO₄. Thereby, a range of unusual compounds have been obtained and structurally characterized. In particular, low valences and uncommon coordination numbers (C.N.) of the late 3d transition metals, as well as interesting varieties in the anionic part of the structures are accessible. The presented examples were selected mainly in order of their inherent aspects concerning the reactivity, structural features or electronic structures.     

Synthetic, structural, and theoretical investigations of alkali metal germanium hydrides--contact molecules and separated ions

The preparation of a series of crown ether ligated alkali metal (M=K, Rb, Cs) germyl derivatives M(crown ether)_nGeH₃ through the hydrolysis of the respective tris(trimethylsilyl)germanides is reported. Depending on the alkali metal and the crown ether diameter, the hydrides display either contact molecules or separated ions in the solid state, providing a unique structural insight into the geometry of the obscure GeH₃^? ion. Germyl derivatives displaying M? Ge bonds in the solid state are of the general formula [M([18]crown‐6)(thf)GeH₃] with M=K ( 1 ) and M=Rb ( 4 ). The compounds display an unexpected geometry with two of the GeH₃ hydrogen atoms closely approaching the metal center, resulting in a partially inverted structure. Interestingly, the lone pair at germanium is not pointed towards the alkali metal, rather two of the three hydrides are approaching the alkali metal center to display M? H interactions. Separated ions display alkali metal cations bound to two crown ethers in a sandwich‐type arrangement and non‐coordinated GeH₃^? ions to afford complexes of the type [M(crown ether)₂][GeH₃] with M=K, crown ether=[15]crown‐5 ( 2 ); M=K, crown ether=[12]crown‐4 ( 3 ); and M=Cs, crown ether=[18]crown‐6 ( 5 ). The highly reactive germyl derivatives were characterized by using X‐ray crystallography, ¹H and ¹³C NMR, and IR spectroscopy. Density functional theory (DFT) and second‐order Møller–Plesset perturbation theory (MP2) calculations were performed to analyze the geometry of the GeH₃^? ion in the contact molecules 1 and 4 .     

Alkali Metal Species in the Reversible Activation of H2

MP(tBu)₂ (M=Li, Na, K), KH and KN(SiMe₃)₂ are shown to activate HD reversibly. In the case of MP(tBu)₂ this leads to isotopic scrambling and the formation of H₂, D₂, H(D)P(tBu)₂ and MH(D) in C₆D₆. In toluene, KP(tBu)₂ reacts with H₂ but also leads to isotopic scrambling into the methyl groups of the solvent toluene. DFT calculations reveal that these systems effect H₂ activation via cooperative interactions with the Lewis acidic alkali metal and the basic phosphorus, carbanion, or hydride centres, mimicking frustrated Lewis pair (FLP) behaviour.     

Titanium Carbene Complexes Stabilized by Alkali Metal Amides

Facile α‐H elimination from tetrakis(trimethylsilylmethyl)titanium precursors to give adducts of (alkylidene)bis(alkyl)titanium complexes is induced by light alkali metal amides of the NNNN‐type macrocyclic anionic ligand Me₃TACD [(Me₃TACD)H=1,4,7‐trimethyl‐1,4,7,10‐tetraazacyclododecane]. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH₂ group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by the reaction with benzophenone and terminal acetylenes.     

Anionic Copolymerization of Carbonyl Sulfide with Epoxides via Alkali Metal Alkoxides

Carbonyl sulfide (COS), an analogue of carbon dioxide (CO₂), can be converted to CO₂ via the carbonic anhydride enzymes widely existing in nature. COS is an ideal monomer for making poly(monothiocarbonate)s, which are difficult to synthesize by traditional methods. Herein, for the first time, we describe an anionic copolymerization of COS with epoxides using alkali metal alkoxides as the catalysts (initiators), affording poly(monothiocarbonate)s with 100% alternating degree, >99% tail‐to‐head (T‐H) content, high number‐average molecular weights (M_ns, up to 90.3 kg/mol) with narrow molecular weight distributions (Đ=M_w/M_n, 1.05—1.31 for COS/propylene oxide copolymers) under solvent‐free and mild conditions. Oxygen‐sulfur exchange reaction (O/S ER), which can result in the production of contaminated dithiocarbonate and carbonate units in the main chain, was nearly completely depressed at 0 ^oC. In addition, in contrast to previously reported salen chromium (iron) complexes that required multiple synthetic steps, this work provides simple, low‐cost, and effective catalysts for making colorless sulfur‐containing polymers.     

Antiferromagnetic Alkali Metal Oxohydroxoferrates(III) with Correlated Hydrogen Bonding Systems

The oxohydroxoferrates(III) A₂[Fe₂O₃(OH)₂] (A=K, Rb, Cs) were synthesized under hydroflux conditions. Approximately equimolar mixtures of the alkali metal hydroxides and water were reacted with Fe(NO₃)₃ ⋅ 9H₂O at about 200 °C. The product formation depends on the hydroxide concentration, therefore also other reaction products, such as KFeO₂, K_2−x[Fe₄O_7−x(OH)_x] or α-Fe₂O₃, are obtained. The crystal structures of the oxohydroxoferrates(III) A₂[Fe₂O₃(OH)₂] follow the same structural principle, yet differ in their layer stacking or/and their hydrogen bonding systems depending on A and temperature. In the resulting four different orthorhombic structure types, [FeO₃OH]⁴⁻ tetrahedra share their oxide corners to create folded Fe₂O₃(OH)₂]²⁻ layers. The terminal hydroxide ligands form hydrogen bonds between and/or within the layers. The positions of the hydrogen atoms in these networks are correlated. The A⁺ cations are located between the folded anionic layers as well as in their trenches. Under reaction conditions, the potassium compound crystallizes in the space group Cmce (Pearson symbol oC88), showing a bimodal disorder of the hydrogen atoms in hydrogen bridges. In a virtually hysteresis-less first-order transition at 340(2) K, the structure slightly distorts into the room-temperature modification with the subgroup Pbca (oP88), and the hydrogen atoms order. The rubidium and caesium compounds are isostructural to each other but not to the potassium compound, and are always obtained as mixtures of two modifications with space groups Cmce (oC88′) and Immb (oI88). Upon heating, the oxohydroxoferrates decompose into their anhydrides AFeO₂ and water. The type of hydrogen bonding network influences the decomposition temperature, the structure and the morphology of the crystals. Despite the presence of iron(III), which was confirmed by ⁵⁷Fe-Mössbauer spectroscopy, K₂[Fe₂O₃(OH)₂] is diamagnetic in the investigated temperature range between 1.8 and 300 K. Neutron diffraction revealed strong antiferromagnetic coupling of the magnetic moments, which are inverted in neighboring tetrahedra.     

Solid-State Isolation of Cyclic Alkyl(Amino) Carbene (cAAC)-Supported Structurally Diverse Alkali Metal-Phosphinidenides

Cyclic alkyl(amino) carbene (cAAC)-supported, structurally diverse alkali metal-phosphinidenides 2 – 5 of general formula ((cAAC)P-M)_n(THF)_x [ 2 : M=K, n=2, x=4; 3 : M=K, n=6, x=2; 4 : M=K, n=4, x=4; 5 : M=Na, n=3, x=1] have been synthesized by the reduction of cAAC-stabilized chloro-phosphinidene cAAC=P-Cl ( 1 ) utilizing metallic K or KC₈ and Na-naphthalenide as reducing agents. Complexes 2 – 5 have been structurally characterized in solid state by NMR studies and single crystal X-ray diffraction. The proposed mechanism for the electron transfer process has been well-supported by cyclic voltammetry (CV) studies and Density Functional Theory (DFT) calculations. The solid state oligomerization process has been observed to be largely dependent on the ionic radii of alkali metal ions, steric bulk of cAAC ligands and solvation/de-solvation/recombination of the dimeric unit [(cAAC)P-M(THF)_x]₂.