C. Corey Hines

Synthesis and X-ray structure determination of highly active Pd(II), Pd(I), and Pd(0) complexes of di(tert-butyl)neopentylphosphine (DTBNpP) in the arylation of amines and ketones.

The air-stable complex Pd(η(3)-allyl)(DTBNpP)Cl (DTBNpP = di(tert-butyl)neopentylphosphine) serves as a highly efficient precatalyst for the arylation of amines and enolates using aryl bromides and chlorides under mild conditions with yields ranging from 74% to 98%. Amination reactions of aryl bromides were carried out using 1-2 mol % Pd(η(3)-allyl)(DTBNpP)Cl at 23-50 °C without the need to exclude oxygen or moisture. The C-N coupling of the aryl chlorides occurred at relatively lower temperature (80-100 °C) and catalyst loading (1 mol %) using the Pd(η(3)-allyl)(DTBNpP)Cl precatalyst than the catalyst generated in situ from DTBNpP and Pd(2)(dba)(3) (100-140 °C, 2-5 mol % Pd). Other Pd(DTBNpP)(2)-based complexes, (Pd(DTBNpP)(2) and Pd(DTBNpP)(2)Cl(2)) were ineffective precatalysts under identical conditions for the amination reactions. Both Pd(DTBNpP)(2) and Pd(DTBNpP)(2)Cl(2) precatalysts gave nearly quantitative conversions to the product in the α-arylation of propiophenone with p-chlorotoluene and p-bromoanisole at a substrate/catalyst loading of 100/1. At lower substrate/catalyst loading (1000/1), the conversions were lower but comparable to that of Pd(t-Bu(3)P)(2). In many cases, the tri-tert-butylphosphine (TTBP) based Pd(I) dimer, [Pd(μ-Br)(TTBP)](2), stood out to be the most reactive catalyst under identical conditions for the enolate arylation. Interestingly, the air-stable Pd(I) dimer, Pd(2)(DTBNpP)(2)(μ-Cl)(μ-allyl), was less active in comparison to [Pd(μ-Br)(TTBP)](2) and Pd(η(3)-allyl)(DTBNpP)Cl. The X-ray crystal structures of Pd(η(3)-allyl)(DTBNpP)Cl, Pd(DTBNpP)(2)Cl(2), Pd(DTBNpP)(2), and Pd(2)(DTBNpP)(2)(μ-Cl)(μ-allyl) are reported in this paper along with initial studies on the catalyst activation of the Pd(η(3)-allyl)(DTBNpP)Cl precatalyst.

Flexible coordination environments of lanthanide complexes grown from chloride-based ionic liquids

Hydrated lanthanide(III) chlorides, LnCl3·xH2O (Ln = La, Pr, Nd, Sm, Eu, Gd; x = 6–7) readily dissolve in the low melting ionic liquid 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl) in an open vessel at 110 °C, and upon cooling crystallize as the anhydrous [C2mim]3[LnCl6]. The crystal structures exhibit a face-centered packing arrangement of the [LnCl6]3− anions, with the cations located as slip aligned pairs in the void spaces which participate in hydrogen-bonding to chlorides. A second crystalline form of the Gd3+ complex, GdCl3(OH2)4·2([C2mim]Cl), was isolated when the above reaction was conducted in a sealed system. For comparison, a third Gd3+ compound was grown from the ionic liquid 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) using the same unsealed conditions as above, and was found to be [C4mim]3[GdCl6]. This compound exhibits a different packing arrangement to that observed for the [C2mim]+ analogs. Based on these findings, ILs would appear to offer new crystallization process options based on their often high thermal stabilities and low to negligible vapor pressures.

Ionic Liquids Based on Azolate Anions

Compartmentalized molecular level design of new energetic materials based on energetic azolate anions allows for the examination of the effects of both cation and anion on the physiochemical properties of ionic liquids. Thirty one novel salts were synthesized by pairing diverse cations (tetraphenylphosphonium, ethyltriphenylphosphonium, N-phenyl pyridinium, 1-butyl-3-methylimidazolium, tetramethyl-, tetraethyl-, and tetrabutylammonium) with azolate anions (5-nitrobenzimidazolate, 5-nitrobenzotriazolate, 3,5-dinitro-1,2,4-triazolate, 2,4-dinitroimidazolate, 4-nitro-1,2,3-triazolate, 4,5-dinitroimidazolate, 4,5-dicyanoimidazolate, 4-nitroimidazolate, and tetrazolate). These salts have been characterized by DSC, TGA, and single crystal X-ray crystallography. The azolates in general are surprisingly stable in the systems explored. Ionic liquids were obtained with all combinations of the 1-butyl-3-methylimidazolium cation and the heterocyclic azolate anions studied, and with several combinations of tetraethyl- or tetrabutylammonium cations and the azolate anions. Favorable structure-property relationships were most often achieved when changing from 4- and 4,5-disubstituted anions to 3,5- and 2,4-disubstituted anions. The most promising anion for use in energetic ionic liquids of those studied here, was 3,5-dinitro-1,2,4-triazolate, based on its contributions to the entire set of target properties.

Synthesis, limitations, and thermal properties of energetically-substituted, protonated imidazolium picrate and nitrate salts and further comparison with their methylated analogs

The possibility of forming simple energetic ionic liquids via the straightforward protonation of heterocyclic amines with nitric or picric acid was explored with 1-alkylimidazoles, 1-alkyl-2-methylimidazoles, and nitro, dinitro, and dicyano-substituted derivatives. The melting points of most of the prepared salts were lower than expected and of the 30 compounds prepared, more than half were found to melt below 100 °C. Limitations in the approach were found as a result of the use of energetic electron withdrawing substituents, such as nitro or cyano, which results in a reduction in nucleophilicity of the heterocycle and an inability to form salts with the acids studied. Interesting thermal behavior was observed with several of the new salts including supercooling and crystallization on heating. Comparison of the simple protonated imidazolium nitrate and picrate salts with their methylated analogs indicated that the protonated ionic liquids do not differ substantially in their melting points from the methylated analogs. However, the thermal stabilities of protonated imidazolium salts are much lower than their alkylated derivatives. Nitrate salts with alkylated cations tend to be more thermally stable than the corresponding picrate salts, but with protonated cations, the picrate salts tend to be approximately 70–80 °C more stable than the nitrate salts. Moreover, accelerating rate calorimetry (ARC) revealed that alkylated salts decompose much less exothermically (in some cases endothermically) than the protonated analogs, and that among all the analyzed salts, the most energetic materials found were protonated 1-methylimidazolium nitrate and 1,2-dimethylimidazolium picrate.

New hydrogen carbonate precursors for efficient and byproduct-free syntheses of ionic liquids based on 1,2,3-trimethylimidazolium and N,N-dimethylpyrrolidinium cores

Two new hydrogen carbonate IL precursors, 1,2,3-trimethylimidazolium and N,N-dimethylpyrrolidinium hydrogen carbonate salts, were synthesized and their structures confirmed by NMR and single-crystal X-ray diffraction. These salts were also evaluated for application in the syntheses of ILs by reacting them with a variety of acids and [NH4][ClO4], which resulted in the clean and quantitative formation of a family of 1,2,3-trimethylimidazolium- and N,N-dimethylpyrrolidinium-based salts. Synthetic protocols for the formation of the hydrogen carbonate salts involved simple alkylation reactions of the chosen neutral amines with dimethyl carbonate, and later conversion of the formed methyl carbonate anion-based salts to hydrogen carbonate salts. The reactions proceed in one step at temperatures close to room temperature using only water. The new organic salts with the chosen anions are formed with only gaseous byproducts (CO2, H2O, and in the case of [NH4][ClO4], NH3), thus eliminating further purification steps. This generalized synthetic protocol for the formation of hydrogen carbonate IL precursors may be used as a cleaner, contaminant-free (halides and metal ions) route to many classes of ILs.

Using ionic liquids to trap unique coordination environments: polymorphic solvates of ErCl3(OH2)4·2([C2mim]Cl)

Two polymorphs of ErCl(3)(OH(2))(4).2([C(2)mim]Cl) solvates were isolated from the same solution of 1-ethyl-3-methylimidazolium chloride when HCl(aq) was added, while [C(2)mim](3)[ErCl(6)] was isolated without HCl addition, illustrating how ionic liquids can be used to trap unusual coordination environments in the solid state.

Azolium azolates from reactions of neutral azoles with 1,3-dimethyl-imidazolium-2-carboxylate, 1,2,3-trimethyl-imidazolium hydrogen carbonate, and N,N-dimethyl-pyrrolidinium hydrogen carbonate

Utilizing previously reported synthetic protocols for the halide- and metal-free synthesis of organic salts, we have prepared a new group of imidazolium and pyrrolidinium azolate anion-based salts demonstrating the general applicability of the methodology and expanding our investigation into non ion exchange routes to potentially energetic ionic liquids. Eighteen salts, out of which six exhibit melting points below 100 °C, were prepared by a simple decarboxylation reaction, which resulted in clean formation of the new compounds without the need for extensive purification. The low stability of the H2CO3 by-product, and its decomposition to CO2 and H2O in aqueous media, allows for purification of the salts by evaporation only.

Lanthanide polyether complexation chemistry: the interaction of hydrated lanthanide(III) nitrate salts with an acyclic 18-crown-6 analog, pentaethylene glycol

The complexation reactions of 1 : 1 molar ratios of M(NO3)3·nH2O (M = Y, La–Pr, Sm–Lu) and pentaethylene glycol (EO5) in 3 : 1 CH3CN : CH3OH were investigated. Crystalline complexes were isolated for all metals investigated and X-ray structural analyses performed. The M(NO3)3–EO5 complexes structurally characterized are remarkably similar to the corresponding 18-crown-6 complexes. Six structurally unique types of anhydrous complexes with the early- to mid-lanthanides (M = La–Nd, Sm–Dy) were found. For the largest metals studied, the twelve coordinate species [M(NO3)3(EO5)] (M = La, Ce) were isolated. A second form of the type [Ce(NO3)2(EO5)]7[Ce(NO3)6][NO3]4 was isolated for M = Ce from the same reaction mixture as the above complex. Praseodymium through dysprosium form ten coordinate species which all have the same basic formula, [M(NO3)2(EO5)][NO3], however, four structurally unique forms have been characterized with the major differences between each arising from the way in which the uncoordinated nitrate anion is hydrogen bonded to the glycol ligand, and the glycol conformation. The smallest lanthanides investigated, M = Ho–Lu, Y, form outer-sphere complexes [M(OH2)3(NO3)3]·EO5. Structural results suggest that the flexibility of the acyclic EO5 ligand allows for the formation of inner sphere complexes across most of the series, whereas the cyclic 18-crown-6 is too constrained to permit such complexation to the mid- to late-lanthanides.

Ionic Liquid-Based Routes to Conversion or Reuse of Recycled Ammonium Perchlorate

New, potentially green, and efficient synthetic routes for the remediation and/or re-use of perchlorate-based energetic materials have been developed. Four simple organic imidazolium- and phosphonium-based perchlorate salts/ionic liquids have been synthesized by simple, inexpensive, and nonhazardous methods, using ammonium perchlorate as the perchlorate source. By appropriate choice of the cation, perchlorate can be incorporated into an ionic liquid which serves as its own electrolyte for the electrochemical reduction of the perchlorate anion, allowing for the regeneration of the chloride-based parent ionic liquid. The electrochemical degradation of the hazardous perchlorate ion and its conversion to harmless chloride during electrolysis was studied using IR and (35)Cl NMR spectroscopies.

CCDC 902238: Experimental Crystal Structure Determination

Related Article: Marcin Smiglak, C. Corey Hines, W. Matthew Reichert, Julia L. Shamshina, Preston A. Beasley, Parker D. McCrary, Steven P. Kelley and Robin D. Rogers|2013|New J.Chem.|37|1461|doi:10.1039/C3NJ00147D