Bis(trimethylsilyl)amide complexes of s-block metals with bidentate ether and amine ligands

Potassium Hexamethyldisilazide (KHMDS): Solvent-Dependent Solution Structures

Solution structures of potassium hexamethyldisilazide [KHMDS] and labeled [15N]KHMDS were examined using a number of analytical methods including 29Si NMR spectroscopy and density functional theory computations. A combination of 15N-29Si couplings, 29Si chemical shifts, and the method of continuous variations reveals dimers, monomers, and ion pairs. Weakly coordinating monofunctional ligands such as toluene, N,N-dimethylethylamine, and Et3N afford exclusively dimers. 1,3-Dioxolane, THF, dimethoxyethane, hexamethylphosphoramide, and diglyme provide dimers at low ligand concentrations and monomers at high ligand concentrations. N,N,N',N'-Tetramethylethylenediamine and N,N,N',N'-tetramethylcyclohexanediamine provide exclusively dimers at all ligand concentrations at ambient temperatures and significant monomer at -80 °C. Studies of 12-crown-4 ran into technical problems. Equimolar 15-crown-5 forms a dimer, whereas excess 15-crown-5 affords a putative ion pair. Whereas equimolar 18-crown-6 also affords a dimer, an excess provides a monomer rather than a solvent-separated ion pair. [2.2.2]cryptand affords what is believed to be a contact-ion-paired cryptate. Solvation was probed using largely density functional theory (DFT) computations. Thermally corrected energies are consistent with lower aggregates and higher solvates at low temperatures, but the magnitudes of the computed temperature dependencies were substantially larger than the experimentally derived data.

s-Block metal complexes of superbulky (tBu3Si)2N-: a new weakly coordinating anion?

Sterically hindered amide anions have found widespread application as deprotonation agents or as ligands to stabilize metals in unusual coordination geometries or oxidation states. The use of bulky amides has also been advantageous in catalyst design. Herein we present s-block metal chemistry with one of the bulkiest known amide ligands: (tBu3Si)2N- (abbreviated: tBuN-). The parent amine (tBuNH), introduced earlier by Wiberg, is extremely resistant to deprotonation (even with nBuLi/KOtBu superbases) but can be deprotonated slowly with a blue Cs+/e- electride formed by addition of Cs0 to THF. (tBuN)Cs crystallized as a separated ion-pair, even without cocrystallized solvent. As salt-metathesis reactions with (tBuN)Cs are sluggish and incomplete, it has only limited use as an amide transfer reagent. However, ball-milling with LiI led to quantitative formation of (tBuN)Li and CsI. Structural characterization shows that (tBuN)Li is a monomeric contact ion-pair with a relatively short N-Li bond, an unusual T-shaped coordination geometry around N and extremely short Li⋯Me anagostic interactions. Crystal structures are compared with Li and Cs complexes of less bulky amide ligands (iPr3Si)2N- (iPrN-) and (Me3Si)2N- (MeN-). DFT calculations show trends in the geometries and electron distributions of amide ligands of increasing steric bulk (MeN- < iPrN- < tBuN-) and confirm that tBuN- is a rare example of a halogen-free weakly coordinating anion.

Carbon-Nitrogen Bond Formation Using Sodium Hexamethyldisilazide: Solvent-Dependent Reactivities and Mechanisms

The solvent-dependent reactivity of sodium hexamethyldisilazide (NaHMDS) toward carbon-centered electrophiles reveals reactions that are poorly represented or unrepresented in the literature, including direct aminolysis of aromatic methyl esters to give carboxamides, nitriles, or amidines, depending on the choice of solvent. SNAr substitutions of aryl halides and opening of terminal epoxides are also examined. A combination of 1H and 29Si nuclear magnetic resonance (NMR) spectroscopic studies using [15N]NaHMDS, kinetic studies, and computational studies reveals the complex mechanistic basis of the preferences for simple aryl carboxamides in toluene and dimethylethylamine and arylnitriles or amidines in tetrahydrofuran (THF). A prevalence of dimer- and mixed dimer-based chemistry even starting from the observable NaHMDS monomer in THF solution is notable.

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

Rare examples of heavier alkali metal manganates [{(AM)Mn(CH₂ SiMe₃ )(N^'Ar )₂ }_∞ ] (AM=K, Rb, or Cs) [N^'Ar =N(SiMe₃ )(Dipp), where Dipp=2,6-iPr₂ -C₆ H₃ ] 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 [(Et₂ O)₂ Na(ⁿ Bu)Mn[(N^'Ar )₂ ], a monomeric, dinuclear structure prevented from aggregating by two blocking ether ligands bound to sodium.

Reaction of lithium hexamethyldisilazide (LiHMDS) with water at ultracold conditions

Alkali metal amides are highly reactive reagents that are broadly applied as strong bases in organic synthesis. Here, we use a combined helium nanodroplet IR spectroscopic and theoretical (DFT calculation) study to show that the reaction of the model compound lithium hexamethyldisilazide (LiHMDS) with water is close to barrierless even at ultra-cold conditions. Upon complex formation of dimeric (LiHMDS)₂ with water in helium nanodroplets as ultra-cold nano-reactors (0.37 K) we observed the reaction product (LiOH)₂(HMDS)₂. This can be rationalized as aggregation induced reation upon stepwise addition of water. With increasing water partial pressure, only the product (LiOH)₂(HMDS)₂ is observed experimentally. This implies that the large interaction energy (69 kJ mol^-1) of (LiHMDS)₂ with water is sufficient to overcome the follow-up reaction barriers, in spite of the rapid cooling rates in He nanodroplets.

Sodium β-Diketonate Glyme Adducts as Precursors for Fluoride Phases: Synthesis, Characterization and Functional Validation

Very few sodium complexes are available as precursors for the syntheses of sodium-based nanostructured materials. Herein, the diglyme, triglyme, and tetraglyme (CH₃O(CH₂CH₂O)_nCH₃, n = 2–4) adducts of sodium hexafluoroacetylacetonate were synthesized in a single-step reaction and characterized by IR spectroscopy, ¹H, and ¹³C NMR. Single-crystal X-ray diffraction studies provide evidence of the formation of the ionic oligomeric structure [Na₄(hfa)₆]²⁻•2[Na(diglyme₂]⁺ when the diglyme is coordinated, while a mononuclear seven-coordinated complex Na(hfa)•tetraglyme is formed with the tetraglyme. Reaction with the monoglyme (CH₃OCH₂CH₂OCH₃) does not occur, and the unadducted polymeric structure [Na(hfa)]_n forms, while the triglyme gives rise to a liquid adduct, Na(hfa)•triglyme•H₂O. Thermal analysis data reveal great potentialities for their applications as precursors in metalorganic chemical vapor deposition (MOCVD) and sol-gel processes. As a proof-of-concept, the Na(hfa)•tetraglyme adduct was successfully applied to both the low-pressure MOCVD and the sol-gel/spin-coating synthesis of NaF films.

Silicon-nitrogen bond formation via dealkynative coupling of amines with bis(trimethylsilyl)acetylene mediated by KHMDS

The catalytic synthesis of silylamines mediated by s- and p-block catalysts is largely underdeveloped. Herein, commercially available potassium bis(trimethylsilyl)amide serves as an efficient alternative to transition metal complexes. N-H/Si-C dealkynative coupling was achieved by means of user-friendly main-group catalysis with ample substrate scope and high chemoselectivity.

Ketone Enolization with Sodium Hexamethyldisilazide: Solvent- and Substrate-Dependent E-Z Selectivity and Affiliated Mechanisms

Ketone enolization by sodium hexamethyldisilazide (NaHMDS) shows a marked solvent and substrate dependence. Enolization of 2-methyl-3-pentanone reveals E-Z selectivities in Et₃N/toluene (20:1), methyl-t-butyl ether (MTBE, 10:1), N,N,N',N″,N″-pentamethyldiethylenetriamine (PMDTA)/toluene (8:1), TMEDA/toluene (4:1), diglyme (1:1), DME (1:22), and tetrahydrofuran (THF) (1:90). Control experiments show slow or nonexistent stereochemical equilibration in all solvents except THF. Enolate trapping with Me₃SiCl/Et₃N requires warming to -40 °C whereas Me₃SiOTf reacts within seconds. In situ enolate trapping at -78 °C using preformed NaHMDS/Me₃SiCl mixtures is effective in Et₃N/toluene yet fails in THF by forming (Me₃Si)₃N. Rate studies show enolization via mono- and disolvated dimers in Et₃N/toluene, disolvated dimers in TMEDA, trisolvated monomers in THF/toluene, and free ions with PMDTA. Density functional theory computations explore the selectivities via the E- and Z-based transition structures. Failures of theory-experiment correlations of ionic fragments were considerable even when isodesmic comparisons could have canceled electron correlation errors. Swapping 2-methyl-3-pentanone with a close isostere, 2-methylcyclohexanone, causes a fundamental change in the mechanism to a trisolvated-monomer-based enolization in THF.

Using a Chloride-Free Magnesium Battery Electrolyte to Form a Robust Anode-Electrolyte Nanointerface

Magnesium bis(hexamethyldisilazide) (Mg(HMDS)₂)-based electrolytes are compelling candidates for rechargeable magnesium batteries due to their high compatibility with magnesium metal anode. However, the usual combination of Mg(HMDS)₂ with chloride salts limits their practical application due to severe corrosion of cell components and low anodic stability. Herein, we report for the first time, a chloride-free Mg(HMDS)₂-based electrolyte in 1,2-dimethoxyethane. By chemically controlling the moisture content using tetrabutylammonium borohydride as a moisture scavenger, the electrolyte demonstrates outstanding electrochemical performance in magnesium plating/stripping, with an average Coulombic efficiency of 98.3% over 150 cycles, and is noncorrosive to cell components. Surface analysis and depth profiling of the magnesium metal anode reveals the formation of a robust solid electrolyte interphase at the anode-electrolyte nanointerface, which allows magnesium plating/stripping to occur reversibly. The electrolyte also demonstrates good compatibility with a copper sulfide nanomaterial cathode, which exhibits a high initial discharge capacity of 261.5 mAh g^-1.

Aggregation and Solvation of Sodium Hexamethyldisilazide: Across the Solvent Spectrum

We report solution structures of sodium hexamethyldisilazide (NaHMDS) solvated by >30 standard solvents (ligands). These include: toluene, benzene, and styrene; triethylamine and related trialkylamines; pyrrolidine as a representative dialkylamine; dialkylethers including THF, tert-butylmethyl ether, and diethyl ether; dipolar ligands such as DMF, HMPA, DMSO, and DMPU; a bifunctional dipolar ligand nonamethylimidodiphosphoramide (NIPA); polyamines N,N,N',N'-tetramethylenediamine (TMEDA), N,N,N',N″,N″-pentamethyldiethylenetriamine (PMDTA), N,N,N',N'-tetramethylcyclohexanediamine (TMCDA), and 2,2'-bipyridine; polyethers 12-crown-4, 15-crown-5, 18-crown-6, and diglyme; 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane ([2.2.2] cryptand); and tris[2-(2-methoxyethoxy)ethyl]amine (TDA-1). Combinations of 1H, 13C, 15N, and 29Si NMR spectroscopies, the method of continuous variations, X-ray crystallography, and density functional theory (DFT) computations reveal ligand-modulated aggregation to give mixtures of dimers, monomers, triple ions, and ion pairs. 15N-29Si coupling constants distinguish dimers and monomers. Solvation numbers are determined by a combination of solvent titrations, observed free and bound solvent in the slow exchange limit, and DFT computations. The relative abilities of solvents to compete in binary mixtures often match that predicted by conventional wisdom but with some exceptions and evidence of both competitive and cooperative (mixed) solvation. Crystal structures of a NaHMDS cryptate ion pair and a 15-crown-5-solvated monomer are included. Results are compared with those for lithium hexamethyldisilazide, lithium diisopropylamide, and sodium diisopropylamide.

Crystal structures of [Li7(i-PrO)3(C4H10NO)3]2O and [Na(i-PrOH)2(C8H18NO2)]2

The title compounds, hexa-kis-[μ3-2-(di-methyl-amino)-ethano-lato]hexa-μ2-iso-propano-lato-μ4-oxido-tetra-deca-lithium(I), [Li7(i-PrO)3(C4H10NO)3]2O (1), and {3-[(2-meth-oxy-eth-yl)(meth-yl)amino]-1,1-dimethylpropano-lato}diiso-prop-an-o-lsodium(I), [Na(i-PrOH)2(C8H18NO2)] (2), were crystallized in the presence of 2-propanol (i-PrOH, C3H7OH). The structure 1 has monoclinic symmetry (C2/c) and the asymmetric unit contains half of the compound. Title compound 2 has triclinic symmetry (P ) and the asymmetric unit is half of an inversion-symmetric aggregate. Both compounds consist of an alkali metal, an amino-alkoxide and a 2-propanol compound. Furthermore, the dimeric sodium aggregate 2 is build up by hydrogen bonding through the 2-propanol and the alkoxides. Compound 1 does not exhibit hydrogen bonding, due to the fact that the 2-propanol is deprotonated. In compound 1, benzene appeared as co-crystallate, but was suppressed by solvent masking because of strong disorder. The formula mass and density do not take account of the solvent.

Sodium Hexamethyldisilazide: Using 15N-29Si Scalar Coupling to Determine Aggregation and Solvation States

²⁹Si NMR spectroscopy, the method of continuous variations, and density functional theory computations show that sodium hexamethyldisilazide (NaHMDS) is a disolvated dimer in toluene, a mixture of disolvated dimer and tetrasolvated monomer in THF/toluene, and exclusively monomer in neat THF. The dioxane-solvated NaHMDS only partially deaggregates to monomer even in neat dioxane. ¹⁵N-²⁹Si coupling constants and ²⁹Si chemical shifts show a high and dependable correlation with the aggregation state. Monitoring either chemical shift or coupling constant versus THF concentration even in the high-temperature, rapid-exchange limit affords the solvation numbers consistent with DFT computations. The preparation of ¹⁵N-labeled NaHMDS has been improved.