Alkali Metal Amides Research Articles

Molecular Structures of THF‐Solvated Alkali‐Metal 2,2,6,6‐Tetramethylpiperidides Finally Revealed: X‐ray Crystallographic, DFT, and NMR (including DOSY) Spectroscopic Studies

The often studied THF solvates of the utility alkali-metal amides lithium and sodium 2,2,6,6-tetramethylpiperidide are shown to exist in the solid state as asymmetric cyclic dimers containing a central M(2)N(2) ring and one molecule of donor per metal to give a distorted trigonal planar metal coordination. DFT studies support these structures and confirm the asymmetry in the ring. In C(6)D(12) solution, the lithium amide displays a concentration-dependent equilibrium between a solvated and unsolvated species which have been shown by diffusion-ordered NMR spectroscopy (DOSY) to be a dimer and larger oligomer, respectively. A third species, a solvated monomer, is also present in very low concentration, as proven by spiking the NMR sample with THF. In contrast, the sodium amide displays a far simpler C(6)D(12) solution chemistry, consistent with the solid-state dimeric arrangement but with labile THF ligands.

Iron(II) Cage Complexes of N-Heterocyclic Amide and Bis(trimethylsilyl)amide Ligands: Synthesis, Structure, and Magnetic Properties

Metallation of hexahydropyrimidopyrimidine (hppH) by [Fe{N(SiMe(3))(2)}(2)] (1) produces the trimetallic iron(II) amide cage complex [{(Me(3)Si)(2)NFe}(2)(hpp)(4)Fe] (2), which contains three iron(II) centers, each of which resides in a distorted tetrahedral environment. An alternative, one-pot route that avoids use of the highly air-sensitive complex 1 is described for the synthesis of the iron(II)-lithium complex [{(Me(3)Si)(2)N}(2)Fe{Li(bta)}](2) (3) (where btaH = benzotriazole), in which both iron(II) centers reside in 3-coordinated pyramidal environments. The structure of 3 is also interpreted in terms of the ring laddering principle developed for alkali metal amides. Magnetic susceptibility measurements reveal that both compounds display very weak antiferromagnetic exchange between the iron(II) centers, and that the iron(II) centers in 2 and 3 possess large negative axial zero-field splittings.

Extremely bulky amido-group 14 element chloride complexes: Potential synthons for low oxidation state main group chemistry

The preparation of a series of extremely bulky secondary amines, Ar*N(H)SiR(3) (Ar* = C(6)H(2){C(H)Ph(2)}(2)Me-2,6,4; R(3) = Me(3), MePh(2) or Ph(3)) is described. Their deprotonation with either LiBu(n), NaH or KH yields alkali metal amide complexes, several monomeric examples of which, [Li(L){N(SiMe(3))(Ar*)}] (L = OEt(2) or THF), [Na(THF)(3){N(SiMe(3))(Ar*)}] and [K(OEt(2)){N(SiPh(3))(Ar*)], have been crystallographically characterised. Reactions of the lithium amides with germanium, tin or lead dichloride have yielded the first structurally characterised two-coordinate, monomeric amido germanium(II) and tin(II) chloride complexes, [{(SiR(3))(Ar*)N}ECl] (E = Ge or Sn; R = Me or Ph), and a chloride bridged amido-lead(II) dimer, [{[(SiMe(3))(Ar*)N]Pb(μ-Cl)}(2)]. DFT calculations on [{(SiMe(3))(Ar*)N}GeCl] show its HOMO to exhibit Ge lone pair character and its LUMO to encompass its Ge based p-orbital. A series of bulky amido silicon(IV) chloride complexes have also been prepared and several examples, [{(SiR(3))(Ar*)N}SiCl(3)] (R(3) = Me(3), MePh(2)) and [{(SiMe(3))(Ar*)N}SiHCl(2)], were crystallographically characterised. The sterically hindered group 14 complexes reported in this study hold significant potential as precursors for kinetically stabilised low oxidation state and/or low coordination number group 14 complexes.

Hydrogen desorption mechanism of a Li–N–H hydrogen storage system

Alkali metal amides may exist in solution, the solid phase, and even the gas phase. Based on a theoretical model of a Li 3N system which adsorbs and desorbs two hydrogen molecules, we examine the possible pathways of the Li 3 N + 2 H 2 ↔ LiNH 2 + 2 LiH reversible reaction. The dehydrogenation process can be separated into two-step reactions, Li 2 NH + LiH → Li 3 N + H 2 (−9.5 kcal/mol exothermic) and LiNH 2 + LiH → Li 2 NH + H 2 (+0.7 kcal/mol endothermic). Along the reaction pathway, two intermediates and a transition state for each reaction were found in our ab initio molecular orbital calculations at the MP2 and CCSD(T) levels of theory. A total of two H 2 molecules can be stored and released at normal temperature and pressure if there are means to substantially raise the energy of the two stable intermediates. Reaction energy profiles from our calculations support the much higher temperature of the first step reaction in experiment.

Rare‐Earth Metal Phenyl(trimethylsilyl)amide Complexes

AbstractThe reactions of common rare‐earth metal chloride, borohydride, and triflate precursors – LnCl3(thf)x, Ln(BH4)3(thf)3, and La(OTf)3 – with alkali metal amide complexes M[N(SiMe3)(C6H3iPr2‐2,6)] (M[NRR′]; M = Li, K) were investigated in thf or hexane as a solvent. A diverse range of products was obtained comprising alkali metal‐free mono(rare‐earth metal) complexes Y[NRR′]Cl2(thf)3 and Ln[NRR′]2(BH4)(thf) (Ln = Nd, La), as well as the mononeodymium ate complex {Nd[NRR′]2(BH4)2}{Li(thf)4} and polymeric ate complexes {La[NRR′]2(thf)(μ‐Cl)K(thf)2(μ‐Cl)}n and {Nd[NRR′]2(μ‐BH4)Li(thf)2(μ‐BH4)}n. All compounds were characterized by NMR (1H and 13C) and FTIR spectroscopy, elemental and X‐ray structure analyses. The lithium salts Li[NRR′](thf)3 and Li4(OTf)4(thf)6 were isolated as side products and structurally characterized. The homoleptic complex La[NRR′]3 was prepared following two synthesis protocols in hexane, either by reacting Ln[NRR′]2(BH4)(thf) with 1 equiv. of K[NRR′] or treatment of LaCl3(thf)1.25 with 3 equiv. of K[NRR′].

Thermochemical transformations in 2 MNH 2–3MgH 2 systems ( M = Li or Na)

Thermochemical reactions between alkali metal amides and magnesium hydride taken in 2:3 molar ratios have been investigated using pressure-composition-temperature, X-ray powder diffraction and residual gas analysis measurements. The thermally induced reactions in both title systems are stoichiometric and proceed as a following solid state transformation: 2 MNH 2 + 3MgH 2 → Mg 3N 2 + 2 MH + 4H 2↑. A total of 6.45 wt.% of hydrogen is released by the 2LiNH 2–3MgH 2 system beginning at 186 °C, and a total of 5.1 wt.% H 2 is released by the 2NaNH 2–3MgH 2 system starting at 130 °C. Combined structure/property investigations revealed that the transformation in the lithium containing system proceeds in two steps. In the first step, lithium amide reacts with MgH 2 to form Li 2Mg(NH) 2 and hydrogen. In the second step, reaction between Li 2Mg(NH) and MgH 2 leads to the formation of the Mg 3N 2 nitride, lithium hydride and additional gaseous hydrogen. The transformation in the sodium containing system appears to proceed through a series of competing solid state processes with formation of Mg(NH 2) 2 and NaMgH 3 intermediates. Partial rehydrogenation in 190 bar hydrogen pressure leading to formation of the MgNH imide was observed in the dehydrogenated 2NaNH 2–3MgH 2 system at 395 °C.

Cis-2,6-Dimethylpiperidide: a structural mimic for TMP (2,2,6,6-tetramethylpiperidide) or DA (diisopropylamide)?

Four novel heterobimetallic ate complexes containing cis-2,6-dimethylpiperidide (cis-DMP) have been prepared and characterised. Two contain one cis-DMP ligand, namely the bisalkyl-amido lithium, and sodium zincates [(TMEDA) x MZn(cis-DMP)(tBu)2] (M = Li for 1, Na for 2). Both 1 and 2 are synthesised by co-complexation of the respective alkali metal amide with di-tert-butylzinc in the presence of a molar equivalent of N,N,N',N'-tetramethylethylenediamine (TMEDA) in a hydrocarbon medium. The third complex, containing two cis-DMP ligands, is the alkyl-bisamido sodium zincate [(TMEDA) x NaZn(cis-DMP)2(tBu)] 3. Complex 3 is prepared from 2 via a disproportionation reaction where the by-product is [(TMEDA) x NaZn(tBu)3]. Another alkyl-diamido sodium zincate, [(TMEDA) x NaZn(DIBA)2(tBu)] 4 is synthesised by utilising diisobutylamine [DIBA(H)]. This reaction emphasises the generality of this disproportionation process. Complex 5 contains three cis-DMP ligands and is a tris-amido sodium magnesiate [(TMEDA) x NaMg(cis-DMP)3]. It is prepared by treating an equimolar mixture of butylsodium and dibutylmagnesium with three and one molar equivalents of cis-DMP(H) and TMEDA respectively, in hydrocarbon solution. By comparison of 1-5 with appropriate complexes from the literature, it has been possible to experimentally determine that the steric bulk of cis-DMP closely resembles that of DA but is considerably less bulky than 2,2,6,6-tetramethylpiperidide (TMP).

Heavy Alkali Metal Amides: Role of Secondary Interactions in Metal Stabilization

The coordination chemistry of the bis(diphenylmethylsilyl)amine ligand, HN(SiMePh(2))(2), with the heavy alkali metals potassium and rubidium has been investigated to study its effect on the structure of the resulting compounds. The compounds exhibit extensive intra- and intermolecular M-pi interactions, creating 1-D coordination polymers, [K{N(SiMePh(2))(2)}](infinity) (1) and [Rb{N(SiMePh(2))(2)}](infinity) (3). This motif is maintained in the presence of tetrahydrofuran (THF), as seen in [K{N(SiMePh(2))(2)}thf](infinity) (2). In contrast, use of the tridentate PMDTA allows isolation of a monomeric species, K(pmdta)N(SiMePh(2))(2) (4), which exhibits both agostic and M-pi interactions. The incorporation of macrocyclic 18-crown-6, results in the formation of a rare non-metal bound amido species, {[K(18-crown-6)N(SiMePh(2))(2)]}(infinity) (5), stabilized by M-pi interactions from adjacent [N(SiMePh(2))(2)](-) ligands propagating 1-D coordination polymers.

Structural Elucidation of tmeda‐Solvated Alkali Metal Diphenylamide Complexes

AbstractLithium, sodium and potassium salts of diphenylamine have been prepared by using a deprotonative route and characterised in both, solid state (by X‐ray crystallography) and solution (by NMR spectroscopic studies). In each case the metal atom's coordination sphere is completed by coordination to the synthetically important co‐ligand N,N,N′,N′‐tetramethylethylenediamine (tmeda). Complexes 1 and 2 [{(tmeda)M(NPh2)}2] (M = Li for 1, Na for 2) can be prepared by treating 1 mol.‐equiv. of the parent amine with an equimolar quantity of nBuM and tmeda in hexane solution. In the solid state, 1 and 2 are essentially isostructural, being dimeric with a four‐atom M–N–M–N framework. The coordination sphere of each M atom is completed by a bidentate tmeda molecule. Complex 3 [{(tmeda)3/2K(NPh2)}2] has been prepared in a similar way to 1 and 2 except that benzylpotassium has been utilised as the metallating agent. In addition, 4 mol‐equiv. of tmeda is required to fully solubilise the heavy alkali metal amide mixture. In the solid state, 3 exists as a polymeric array of dinuclear K–N–K–N rings. Akin to 1 and 2, the K atom is coordinated to a bidentate tmeda molecule; however, each K atom is also bound to another tmeda molecule that acts as a monodentate bridge, thus producing the coordination polymer. Crystalline 1 and 2 are soluble in C6D6; hence, NMR spectroscopic studies could be performed. These show that the solid‐state structure appears to stay intact in arene solution. On the other hand, 3 is not soluble in C6D6; so solution studies have been performed in [D8]thf. These studies reveal that 3 readily looses tmeda during in‐vacuo isolation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Recyclable hydrogen storage system composed of ammonia and alkali metal hydride

Ammonia (NH 3) reacts with alkali metal hydrides MH ( M = Li, Na, and K) in an exothermic reaction to release hydrogen (H 2) at room temperature, resulting that alkali metal amides ( MNH 2) which are formed as by-products. In this work, hydrogen desorption properties of these systems and the condition for the recycle from MNH 2 back to MH were investigated systematically. For the hydrogen desorption reaction, the reactivities of MH with NH 3 were better following the atomic number of M on the periodic table, Li < Na < K. It was confirmed that the hydrogen absorption reaction of all the systems proceeded under 0.5 MPa of H 2 flow condition below 300 °C.

Synthesis and characterisation of bulky guanidines and phosphaguanidines: precursors for low oxidation state metallacycles

Reactions of alkali metal amides or phosphides with the bulky carbodiimide, ArNCNAr (Ar = C6H3Pri2-2,6), followed by aqueous work-ups, have yielded several guanidines, ArNC(NR2)N(H)Ar (R = cyclohexyl (GisoH) or Pri (PrisoH); NR2 = cis-NC5H8Me2-2,6 (PipisoH)), a bifunctional guanidine, {ArNCN(H)Ar}2{μ-N(C2H4)2N} (Pip(GisoH)2), and two phosphaguanidines, ArNC(PR2)N(H)Ar (R = cyclohexyl (CyP-GisoH) or Ph (PhP-GisoH)). A very bulky guanidine, ArNC{N(Ar)SiMe3}N(H)Ar (ArSi-Giso), and an aryl coupled bifunctional guanidine, {ArN(H)C(NPri2)NC6H2Pri2-2,6-}2 (PrisoH)2, have been prepared by other routes. All compounds have been crystallographically characterised and shown to exist in a number of isomeric forms in the solid state. These appear to be largely retained in solution. The deprotonation of GisoH with BunLi in either hexane or THF led to crystallographically characterised dimeric and monomeric complexes respectively, viz. [Li{Li(κ2-N,N′-Giso)2}] and [Li(THF)(η1-N,η3-Ar-Giso)]. Deprotonation of PrisoH and Pip(GisoH)2 with K[N(SiMe3)2] gave the unsolvated polymer, [{K(η1-N,η6-Ar-Priso)}∞], and the solvated complex, [{K(THF)2}{Pip(Giso)2}{K(THF)3}], respectively.

Crystal Chemistry of Hydrogen Storage Materials

AbstractWe review here work on two classes of compounds that have been promoted as potential hydrogen storage materials; alkali metal amides and borohydrides, highlighting how their crystal structure and chemical properties may be used to influence the key hydrogen absorption and desorption parameters in these materials.

Synthesis and Structural Characterization of Alkali Metal Guanidinates

AbstractReactions 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.

Remarkable Cocatalytic Effect of Alkali Metal Amides and Alkoxides in the Rhodium-Catalyzed Polymerization of Phenylacetylene

Polymerizations of phenylacetylene (1) with [(nbd)RhCl]2 (nbd: 2,5-norbornadiene) in the presence of alkali metal amides (2−5) in toluene at 30 °C reached completion almost instantaneously. These polymerizations are much faster than the polymerization catalyzed by the well-known conventional catalyst system, [(nbd)RhCl]2/triethylamine (Et3N) (polymer yield is 75% after 1 h under the same conditions). The molecular weights of the formed polymers (2, Mn = 270 000; 3, Mn = 297 000; 4, Mn = 310 000; 5, Mn = 396 000; after 1 h) were clearly higher than that with the [(nbd)RhCl]2/Et3N system (Mn = 118 000). KO-tBu (6) also served as cocatalyst but it was less effective than were 2−5. In the polymerization of 1 with [(nbd)RhCl]2/4−6, toluene was the optimal solvent in terms of polymer yield and molecular weight. The stoichiometric amount of cocatalyst to the Rh metal was found to be the most suitable with respect to polymer yield, beyond which there was no discernible change in the polymerization behavior. The ...

Recent developments in the field of organic heterobimetallic compounds of the alkaline-earth metals

Heterobimetallic compounds of the alkaline-earth metals show a wide structural variety with strongly differing reactivity patterns. The combination of magnesium and alkali metal amides yields cyclic molecules with an extreme high reactivity which often are considered as "inverse crowns" with the metal atoms as coordination sites for Lewis bases. In other metallates of the alkaline-earth metals an activation of alkyl groups succeeds. In alkaline-earth metal zincates an inverse coordination of the type M(2)[(mu-R)(2)ZnR](2) is observed and the alkyl groups are in bridging positions between zinc and the s-block metals thus forming a very reactive M-C-Zn three-center-two-electron bond. Furthermore, the metals of the carbon group form alkaline-earth metal-silicon, -germanium and -tin bonds or, in the presence of very strong Lewis bases, even solvent-separated ion pairs. For electronegative substituents at tin an inverse coordination mode such as M[(mu-R)(2)SnR](2) is observed.

Ketone deprotonation mediated by mono- and heterobimetallic alkali and alkaline earth metal amide bases: structural characterization of potassium, calcium, and mixed potassium-calcium enolates.

Potassium, calcium, and mixed potassium-calcium amide combinations have been shown to be efficient reagents in enolization reactions, and a set of representative intermediate mono- and heterobimetallic enolates have been successfully isolated and crystallographically characterized.

Structural motifs in secondary ammonium halides: ring-stacking and ring-laddering in the organic solid state.

The ring-stacking and ring-laddering concepts of structural inorganic chemistry may be applied to rationalize motifs observed for secondary ammonium halides R(2)NH(2)X (X = Cl, Br) in the organic solid state. General examination of the directional preferences of N(+)...X(-) contacts in 166 crystal structures confirms that the shortest contacts (3.0-3.2 and 3.2-3.4 A, X = Cl, Br) are N(+)-H...X(-) hydrogen bonds lying approximately along the directions of the N(+)-H bond vectors. The next shortest N(+)...X(-) contacts display two preferred directions of approach: i) contacts in the distance range 3.2-3.5 (X = Cl) and 3.2-3.9 A (X = Br) lie close to the H-N(+)-H plane, along the direction of the bisector of the H-N(+)-H angle; ii) contacts in the distance range 4.0-4.2 (X = Cl) and 4.0-4.4 A (X = Br) lie close to the H-N(+)-H plane, along the direction of an axis extending to the rear of one of the N(+)-H bonds. Both directions of approach lead frequently to association of R(2)NH(2) (+)X(-) ion pairs into laddered motifs. Stacking association is also observed, giving rise in one case to discrete cubanes and in several other cases to extended stacked-cube arrangements. In each case, the distribution of N(+)...X(-) contacts reflects a balance between the directional properties of the N(+)-H...X(-) hydrogen bonds and (primarily steric) interactions between the R groups of the organic moieties. The ladder and stack motifs of the organic ammonium halides are in many cases directly comparable to those in alkali metal amides, [R(2)NM](n), and information derived from the extensive organic sample provides insight into the motifs adopted by the inorganic complexes.

Reaction of the silylene Si[(NCH2But)2C6H4-1,2] with the alkali metal silylamides M[N(SiMe3)R](M = Li, Na or K; R = SiMe3 or SiMe2Ph).

The thermally stable silylene Si[(NCH(2)Bu(t))(2)C(6)H(4)-1,2] 1 undergoes oxidative addition reactions with the alkali metal silylamides MN(SiMe(3))(2)(M = Li, Na or K) to afford the new alkali metal amides MN(SiMe(3))[(1)SiMe(3)][M = Li (2), Na (3) or K (4)]. Reaction of two equivalents of 1 with LiN(R)(SiMe(3)) leads in a two-step process to the compound LiN[(1)R][(1)SiMe(3)][R = SiMe(2)Ph (5) or SiMe(3) (6)]. Alternatively, 1 reacts with 3 to afford NaN[(1)SiMe(3)](2) (7). The structures of 2-5 and are presented and the formation of 2-7 is discussed.

A homologous series of crown-ether-complexed alkali metal amides as discrete ion-pair species: synthesis and structures of [M(12-crown-4) 2][PyNPh · PyN(H)Ph] (M=Li, Na and K)

Reaction between one equivalent of n-butyllithium and 2-phenylaminopyridine, in the presence of 12-crown-4 ether (12C4) in toluene, affords the discrete ion-pair complex [{Li(12C4) 2}{N(Py)(Ph) · HN(Py)(Ph)}] ( 1), in which the secondary amine proton is hydrogen-bonded to the secondary amide anion. Compound 1 is the sole isolable product of this reaction, irrespective of the amount of lithium alkyl employed, metallation never exceeding 50%. Metathesis of 1 with sodium- or potassium- tert-butoxide affords the homologous complexes [{Na(12C4) 2}{N(Py)(Ph) · HN(Py)(Ph)}] ( 2) and [{K(12C4) 2}{N(Py)(Ph) · HN(Py)(Ph)}] ( 3), respectively. Compounds 1, 2 and 3 have all been characterised by X-ray crystallography, NMR spectroscopy and elemental analysis.

Intercalation of alkali metal cations into layered organotitanium oxides.

Deprotonation of the complex [{Ti(η5-C5Me5)(μ-O)}3(μ3-CH)] with different alkali metal alkyl and amide reagents provides a method for the intercalation of alkali-metal ions into layered organometallic titanium oxides (see picture). The process and the structures were analyzed by DFT calculations.