Crystal structure of a heavier alkali metal diisopropylamide complex: a discrete (KN)2 ring dimer with TMEDA chelation and short intramolecular K⋯H(C) contacts

Crystal structure of potassium tri-ethyl-hydridoborate ('superhydride')

In the title compound, formally K+·C6H16B-, the contact sphere of potassium consists of eleven hydrogen atoms from three different anions, assuming an arbitrary cut-off of 3 Å. The shortest inter-action, 2.53 (2) Å, involves the hydridic hydrogen H01, which fulfils a bridging function in the formation of chains of KHBEt3 units parallel to the a axis [K1-H01i 2.71 (2) Å, K1-H01-K1ii 126.7 (9)°, operators x∓1/2, -y + , -z + 1].

Alkali-Metal Mediation: Diversity of Applications in Main-Group Organometallic Chemistry

Organolithium compounds have been at the forefront of synthetic chemistry for over a century, as they mediate the synthesis of myriads of compounds that are utilised worldwide in academic and industrial settings. For that reason, lithium has always been the most important alkali metal in organometallic chemistry. Today, that importance is being seriously challenged by sodium and potassium, as the alkali-metal mediation of organic reactions in general has started branching off in several new directions. Recent examples covering main-group homogeneous catalysis, stoichiometric organic synthesis, low-valent main-group metal chemistry, polymerization, and green chemistry are showcased in this Review. Since alkali-metal compounds are often not the end products of these applications, their roles are rarely given top billing. Thus, this Review has been written to alert the community to this rising unifying phenomenon of "alkali-metal mediation".

Preparation of Functionalized Aryl, Heteroaryl, and Benzylic Potassium Organometallics Using Potassium Diisopropylamide in Continuous Flow

We report the preparation of lithium-salt-free KDA (potassium diisopropylamide; 0.6 m in hexane) complexed with TMEDA (N,N,N',N'-tetramethylethylenediamine) and its use for the flow-metalation of (hetero)arenes between -78 °C and 25 °C with reaction times between 0.2 s and 24 s and a combined flow rate of 10 mL min-1 using a commercial flow setup. The resulting potassium organometallics react instantaneously with various electrophiles, such as ketones, aldehydes, alkyl and allylic halides, disulfides, Weinreb amides, and Me3 SiCl, affording functionalized (hetero)arenes in high yields. This flow procedure is successfully extended to the lateral metalation of methyl-substituted arenes and heteroaromatics, resulting in the formation of various benzylic potassium organometallics. A metalation scale-up was possible without further optimization.

Exploring the solid state and solution structural chemistry of the utility amide potassium hexamethyldisilazide (KHMDS)

The coordination chemistry of the important potassium amide KHMDS has been explored both in the solid state and in solution.

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 -C6 H3 ). 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.

Synthetically important alkali-metal utility amides: lithium, sodium, and potassium hexamethyldisilazides, diisopropylamides, and tetramethylpiperidides

Most synthetic chemists will have at some point utilized a sterically demanding secondary amide (R2 N(-) ). The three most important examples, lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS), lithium diisopropylamide (LiDA), and lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-the "utility amides"-have long been indispensible particularly for lithiation (Li-H exchange) reactions. Like organolithium compounds, they exhibit aggregation phenomena and strong Lewis acidity, and thus appear in distinct forms depending on the solvents employed. The structural chemistry of these compounds as well as their sodium and potassium congeners are described in the absence or in the presence of the most synthetically significant donor solvents tetrahydrofuran (THF) and N,N,N',N'-tetramethylethylenediamine (TMEDA) or closely related solvents. Examples of hetero-alkali-metal amides, an increasingly important composition because of the recent escalation of interest in mixed-metal synergic effects, are also included.

A hetero-alkali-metal version of the utility amide LDA: lithium-potassium diisopropylamide

Designed to extend the synthetically important alkali-metal diisopropylamide [N(i)Pr(2); DA] class of compounds, the first example of a hetero-alkali-metallic complex of DA has been prepared as a partial TMEDA solvate. Revealed by an X-ray crystallographic study, its structure exists as a discrete lithium-rich trinuclear Li(2)KN(3) heterocycle, with TMEDA only solvating the largest of the alkali-metals, with the two-coordinate lithium atoms being close to linearity [161.9(2)°]. A variety of NMR spectroscopic studies, including variable temperature and DOSY NMR experiments, suggests that this new form of LDA maintains its integrity in non-polar hydrocarbon solution. This complex thus represents a rare example of a KDA molecule which is soluble in non-polar medium without the need for excessive amounts of solubilizing Lewis donor being added.

Sodium congener of the classical lithium methylchromate dimer: synthetic, X-ray crystallographic, and magnetic studies of Me8Cr2[Na(OEt2)]4

One of the milestone structures in the development of transition-metal complexes with metal-metal bonds of multiple bond order was the lithium methylchromate dimer Me(8)Cr(2)[Li(donor)](4) (donor = THF or Et(2)O). Using a simple salt metathesis reaction mixing this compound with sodium tert-butoxide, the sodium congener Me(8)Cr(2)[Na(OEt(2))](4) has been synthesized as a green crystalline compound and isolated in 51% yield. Its solid-state structure was determined by single-crystal X-ray diffraction. Exhibiting exact crystallographic C(4h) symmetry, this heavier alkali-metal chromate structure is also dimeric, formally comprising a (Me(8)Cr(4))(4-) tetranionic core with four peripheral Na(+) cations carrying supporting ether ligands. Its salient feature is the long Cr···Cr distance of 3.263(2) Å, which is remarkably elongated compared to that in the lithium THF-solvated congener [1.968(2) Å]. With respect to the methyl C atoms, the Cr coordination is distorted-square-planar. Each Na(+) interacts with four methyl C atoms, and there are also some short Na···H(C) contacts. Unlike for lithium chromate, no NMR spectroscopic data could be obtained for sodium chromate. The paramagnetic character of sodium chromate was confirmed by variable-temperature magnetization measurements, which indicated antiferromagnetic behavior.

Syntheses and Structures of the First Heavy-Alkali-Metal Tris(trimethylsilyl)germanides

The first heavy-alkali-metal tris(trimethylsilyl)germanides were obtained in high yield and purity by a simple one-pot reaction involving the treatment of tetrakis(trimethylsilyl)germane, Ge(SiMe3)4, with various alkali metal tert-butoxides. The addition of different sizes of crown ethers or the bidentate TMEDA (TMEDA=N,N,N',N'-tetramethylethylenediamine) provided either contact or separated species in the solid state, whereas in aromatic solvents the germanides dissociate into separated ions, as shown by 29Si NMR spectroscopic studies. Here we report on two series of germanides, one displaying M-Ge bonds in the solid state with the general formula [M(donor)n Ge(SiMe3)3] (M=K, donor=[18]crown-6, n=1, 1; Rb, donor=[18]crown-6, n=1, 4; and M=K, donor=TMEDA, n=2, 6). The silicon analogue of 6, [K(tmeda)2Si(SiMe3)3] (7) is also included to provide a point of reference. The second group of compounds consists of separated ions with the general formula [M(donor)2][Ge(SiMe3)3] (M=K, donor=[15]crown-5, 2; M=K, donor=[12]crown-4, 3; and M=Cs, donor=[18]crown-6, 5). While all target compounds are highly sensitive towards hydrolysis, use of the tridentate nitrogen donor PMDTA (PMDTA=N,N,N',N'',N''-pentamethyldiethylenetriamine) afforded even more reactive species of the composition [K(pmdta)2Ge(SiMe3)3] (8). We also include the silanide analogue [K(pmdta)2Si(SiMe3)3] (9) for sake of comparison. The compounds were typically characterized by X-ray crystallography, and 1H, 13C, and 29Si NMR and IR spectroscopy, unless extremely high reactivity, as observed for the PMDTA adducts 8 and 9, prevented a more detailed characterization.