Design principles from multiscale simulations to predict nanostructure in self-assembling ionic liquids.

Molecular dynamics simulations (up to the nanoscale) were performed on the 3-methyl-1-pentylimidazolium ionic liquid cation paired with three anions; chloride, nitrate, and thiocyanate as aqueous mixtures, using the effective fragment potential (EFP) method, a computationally inexpensive way of modeling intermolecular interactions. The simulations provided insight (preferred geometries, radial distribution functions and theoretical proton NMR resonances) into the interactions within the ionic domain and are validated against 1H NMR spectroscopy and small- and wide-angle X-ray scattering experiments on 1-decyl-3-methylimidazolium. Ionic liquids containing thiocyanate typically resist gelation and form poorly ordered lamellar structures upon mixing with water. Conversely, chloride, a strongly coordinating anion, normally forms strong physical gels and produces well-ordered nanostructures adopting a variety of structural motifs over a very wide range of water compositions. Nitrate is intermediate in character, whereby upon dispersal in water it displays a range of viscosities and self-assembles into nanostructures with considerable variability in the fidelity of ordering and symmetry, as a function of water content in the binary mixtures. The observed changes in the macro and nanoscale characteristics were directly correlated to ionic domain structures and intermolecular interactions as theoretically predicted by the analysis of MD trajectories and calculated RDFs. Specifically, both chloride and nitrate are positioned in the plane of the cation. Anion to cation proximity is dependent on water content. Thiocyanate is more susceptible to water insertion into the second solvent shell. Experimental 1H NMR chemical shifts monitor the site-specific competition dependence with water content in the binary mixtures. Thiocyanate preferentially sits above and below the aromatic ring plane, a state disallowing interaction with the protons on the imidazolium ring.

Self-Assembly Directed Organization of Nanodiamond During Ionic Liquid Crystalline Polymer Formation.

The UV-initiated free radical polymerization of a lyotropic mesophase prepared by co-assembly of an aqueous mixture of an ionic liquid (IL) monomer, 3-decyl-1-vinylimidazolium chloride, in a dimethyl sulfoxide dispersion of an IL-monomer nanodiamond conjugate yields a well-ordered 2D hexagonally structured network-polymer composite. The IL monomer is covalently bound to carboxylated detonation diamond via ester-linked 3-decyl-1-vinylimidazolium bromide. Successful preparation of the amphiphile-functionalized nanodiamond is determined by ATR/FT-IR, thermogravimetric analysis, and small-angle X-ray scattering (SAXS). Mesophase and composite structure are evaluated by SAXS, revealing a columnar architecture composed of amphiphilic ionic liquid cylinders containing solvent-rich cores. Self-assembly directed site localization of the nanodiamond positions the particles in the alkyl chain continuum upon polymerization. The composite reversibly swells in ethanol allowing structural variation and modulation of the nanoparticle internal packing arrangement. This work demonstrates that through careful molecular design, self-organization and site-directed assembly of nanodiamond into chemically distinct regions of a nanostructured organogel can be achieved.

Synthesis and Characterization of Quinuclidinium Derivatives of the [closo-1-CB11H12](-) Anion as Potential Polar Components of Liquid Crystal Materials.

Antipodal substitution of the [closo-1-CB11H12](-) anion with a 4-pentylquinuclidinium fragment and alkyl groups in positions C(1) and B(12) gave polar zwitterions 1[n] and 2[n]. The molecular structure of 1[5] was established using X-ray diffraction (XRD) methods: P1̅, a = 15.162(2) Å, b = 16.546(3) Å, c = 19.794(3) Å; α = 84.871(2)°, ß = 84.057(2)°, γ = 84.058(3)°; Z = 8. All compounds exhibit high temperature in-plane ordered smectic phases that are stabilized by dipolar interactions. The ordered phases were investigated by powder XRD methods. Thermal and dielectric parameters for two derivatives, 1[0] and 1[6], were evaluated in nematic hosts, ClEster and BPhF. The dielectric data were analyzed with the Maier-Meier formalism augmented with density functional theory methods, and the results were compared to those for similar zwitterions previously reported.

Cascade synthesis of a gold nanoparticle-network polymer composite.

The multi-step, cascade synthesis of a self-supporting, hierarchically-structured gold nanoparticle hydrogel composite is described. The composite is spontaneously prepared from a non-covalent, lamellar lyotropic mesophase composed of amphiphiles that support the reactive constituents, a mixture of hydroxyl- and acrylate-end-derivatized PEO117-PPO47-PEO117 and [AuCl4](-). The reaction sequence begins with the auto-reduction of aqueous [AuCl4](-) by PEO117-PPO47-PEO117 which leads to both the production of Au NPs and the free radical initiated polymerization and crosslinking of the acrylate end-derivatized PEO117-PPO47-PEO117 to yield a network polymer. Optical spectroscopy and TEM monitored the reduction of [AuCl4](-), formation of large aggregated Au NPs and oxidative etching into a final state of dispersed, spherical Au NPs. ATR/FT-IR spectroscopy and thermal analysis confirms acrylate crosslinking to yield the polymer network. X-ray scattering (SAXS and WAXS) monitored the evolution of the multi-lamellar structured mesophase and revealed the presence of semi-crystalline PEO confined within the water layers. The hydrogel could be reversibly swollen without loss of the well-entrained Au NPs with full recovery of composite structure. Optical spectroscopy shows a notable red shift (Δλ ∼ 45 nm) in the surface plasmon resonance between swollen and contracted states, demonstrating solvent-mediated modulation of the internal NP packing arrangement.

Functionalization of closo-Borates via Iodonium Zwitterions.

A simple method for the functionalization of closo-borates [closo-B10 H10 ](2-) (1), [closo-1-CB9 H10 ](-) (2), [closo-B12 H12 ](2-) (3), [closo-1-CB11 H12 ](-) (4), and [3,3'-Co(1,2-C2 B9 H11 )2 ](-) (5) is described. Treatment of the anions and their derivatives with ArI(OAc)2 gave aryliodonium zwitterions, which were sufficiently stable for chromatographic purification. The reactions of these zwitterions with nucleophiles provided facile access to pyridinium, sulfonium, thiol, carbonitrile, acetoxy, and amino derivatives. The synthetic results are augmented by mechanistic considerations.

Synthesis and characterization of 12-pyridinium derivatives of the [closo-1-CB11H12]- anion.

Diazotization of [closo-1-CB11H10-1-R-12-NH2](-)[NMe4](+) (4[NMe4]) in neat 4-methoxypyridine leads to 12-(4-methoxypyridinium) zwitterions [closo-1-CB11H10-1-R-12-(4-MeOC5H4N)] (2) in ∼50% yield. Demethylation of 2 with LiCl in dimethylformamide provides access to 12-pyridones 5[NMe4], which can be O-alkylated with alkyl triflates giving 12-(4-alkoxypyridinium) zwitterions, such as 1. This three-step process is more efficient than direct diazotization of amine 4[NMe4] in neat higher 4-alkoxypyridine. The new method was demonstrated for the synthesis of [closo-1-CB11H10-1-C5H11-12-(4-C7H15OC5H4N)] (1c), which exhibits a smectic A phase. Molecular and electronic structures of 4-methoxypyridinium zwitterion 2b and its C(1) isomer [closo-1-CB11H11-1-(4-MeOC5H4N)] (3b) were investigated by single-crystal X-ray diffraction and spectroscopic methods, respectively, and the experimental results were compared to those obtained with density functional theory methods. Lastly, the mechanism for formation of zwitterions 2 was investigated computationally revealing low energy for dediazoniation of the [closo-1-CB11H10-1-R-12-N2] (14) intermediate (ΔG298 ≈ 25 kcal/mol) to form boronium ylide 15, with weak dependence on substituent R. Dinitrogen derivative 14c was observed by (11)B NMR spectroscopy.

Functionalization of the [closo-1-CB9H10]- anion for the construction of new classes of liquid crystals.

The [closo-1-CB(9)H(10)](-) anion is a member of an extensive family of σ-aromatic closo-boranes that possess impressive stability and functionalization characteristics. In contrast to its bigger, more extensively studied brother, the [closo-1-CB(11)H(12)](-) anion, convenient access to the [closo-1-CB(9)H(10)](-) anion has only been recently established, and researchers have only begun to develop and understand its fundamental chemistry. The geometrical and electronic properties of the [closo-1-CB(9)H(10)](-) anion make it an attractive structural element of novel classes of either zwitterionic or ionic liquid crystals suitable for electro-optical and ion transport applications, respectively. Such materials require a 1,10-difunctionalized [closo-1-CB(9)H(10)](-) anion that permits for the formation of molecules of elongated shape. The covalent attachment of an onium fragment or the use of a counterion compensates for the negative charge. This Account highlights the progress made in the advancement and understanding of the fundamental chemistry of the [closo-1-CB(9)H(10)](-) anion. We also describe the development of 1,10-difunctionalized derivatives as key intermediates in the preparation of new classes of liquid crystalline materials. We obtained the first isomerically pure 1,10-difunctionalized derivative of the [closo-1-CB(9)H(10)](-) anion, iodo acid [closo-1-CB(9)H(8)-1-COOH-10-I](-), from decaborane through the Brellochs reaction. Functional group transformation of the C(1)-carboxyl group led to a 1-amino derivative and, subsequently, to a synthetically valuable 1-dinitrogen derivative. The latter exhibits reactivity typical for PhN(2)(+) and undergoes diazocoupling and Gomberg-Bachmann arylation reactions. The B(10)-iodine participated in Negishi alkylation and Buchwald-Hartwig amination reactions, leading to 10-hexyl and 10-amino carboxylic acids, respectively. We converted the 10-amino carboxylic acid to a 10-dinitrogen acid [closo-1-CB(9)H(8)-1-COOH-10-N(2)], which proved to be synthetically valuable in the preparation of 10-pyridinium and 10-sulfonium zwitterionic acids and their liquid crystalline esters. We investigated several intermediates using structural, spectroscopic, and kinetic methods.

Electrochemical activity of glucose oxidase on a poly(ionic liquid)-Au nanoparticle composite.

Glucose oxidase (GOx) adsorbed on an ionic liquid-derived polymer containing internally organized columns of Au nanoparticles exhibits direct electron transfer and bioelectrocatalytic properties towards the oxidation of glucose. The cationic poly(ionic liquid) provides an ideal substrate for the electrostatic immobilization of GOx. The encapsulated Au nanoparticles serve to both promote the direct electron transfer with the recessed enzyme redox centers and impart electronic conduction to the composite, allowing it to function as an electrode for electrochemical detection.

Transmission of electronic effects through the {closo-1-CB9} and {closo-1-CB11} cages: apparent dissociation constants for series of [closo-1-CB9H8-1-COOH-10-X] and [closo-1-CB11H10-1-COOH-12-X] acids.

The apparent ionization constants pK(a)' for series of carboxylic acids [closo-1-CB(9)H(8)-1-COOH-10-X](-) (1) and [closo-1-CB(11)H(10)-1-COOH-12-X](-) (2), where X = H, I, n-C(6)H(13), (+)NMe(3), (+)N(2), (+)SMe(2), OC(5)H(11), were measured in EtOH/H(2)O (1/1, v/v) at 24 °C. Correlation analysis of the pK(a)' values using Hammett substituent constants σ(p)(X) gave the reaction constant ρ = 0.87 ± 0.04 for series 1 and ρ = 1.00 ± 0.09 for series 2. These values are higher than for derivatives of PhCH═CHCOOH (ρ = 0.70 ± 0.09 in 55% EtOH) and correspond to 56% and 65% efficiencies in transmission of electronic effects by [closo-1-CB(9)H(10)](-) (E) and [closo-1-CB(11)H(12)](-) (F), respectively, as compared to benzene (A). Experimental results were supported with DFT calculations of relative acidity for series of acids derived from A, E, and F in aqueous medium.

The preparation of 3-substituted-1,5-dibromopentanes as precursors to heteracyclohexanes.

The methodology to prepare 3-substituted 1,5-dibromopentanes I and their immediate precursors, which include 3-substituted 1,5-pentanediols VII or 4-substituted tetrahydropyrans VIII, is surveyed. Such dibromides I are important intermediates in the preparation of liquid crystalline derivatives containing 6-membered heterocyclic rings. Four dibromides 1a-1d containing simple alkyl and more complex fragments at the 3-position were prepared. 3-Propyl- and 3-pentyl-pentane-1,5-diol (2a,b) were prepared starting from either glutaconate or malonate diesters, while tetrahydropyrans 3c and 3d were obtained from tetrahydro-4H-pyran-4-one. The advantages and disadvantages of each route are discussed. Dibromides 1c and 1d were used to prepare sulfonium zwitterions 11c and 11d.

Diazotization of the amino acid [closo-1-CB9H8-1-COOH-6-NH3] and reactivity of the [closo-1-CB9H8-1-COO-6-N2]- anion.

A comparative study of the reactivity of dinitrogen acids [closo-1-CB(9)H(8)-1-COOH-10-N(2)] (3[10]) and [closo-1-CB(9)H(8)-1-COOH-6-N(2)] (3[6]) was conducted by diazotization of a mixture of amino acids [closo-1-CB(9)H(8)-1-COOH-6-NH(3)] (1[6]) and [closo-1-CB(9)H(8)-1-COOH-10-NH(3)] (1[10]) with NO(+)BF(4)(-) in the presence of a heterocyclic base (pyridine, 4-methoxypyridine, 2-picoline, or quinoline). The 10-amino acid 1[10] formed an isolable stable 10-dinitrogen acid 3[10], while the 6-dinitrogen carboxylate 3[6](-) reacted in situ, giving products of N-substitution at the B6 position with the heterocyclic solvent (4[6]). The molecular and crystal structures for pyridinium acid 4[6]a were determined by X-ray crystallography. The electronic structures and reactivity of the 6-dinitrogen derivatives of the {1-CB(9)} cluster were assessed computationally at the B3LYP/6-31G(d,p) and MP2/6-31G(d,p) levels of theory and compared to those of the 10-dinitrogen, 2-dinitrogen, and 1-dinitrogen analogues.

Anionic amino acid [closo-1-CB9H8-1-COO-10-NH3]- and dinitrogen acid [closo-1-CB9H8-1-COOH-10-N2] as key precursors to advanced materials: synthesis and reactivity.

Amino acid [closo-1-CB(9)H(8)-1-COO-10-NH(3)](-) (4) was prepared by amination of iodo acid [closo-1-CB(9)H(8)-1-COOH-10-I](-) (1) with LiHMDS in a practical and reproducible manner. The apparent dissociation constants, pK(2) = 5.6 and pK(1) > 11, were measured for 4[NMe(4)] in 50% aq. EtOH. Diazotization of 4 with NO(+)PF(6)(-) under mildly basic conditions afforded stable dinitrogen acid [closo-1-CB(9)H(8)-1-COOH-10-N(2)] (5). Activation parameters (DeltaH(++) = 33.9 +/- 1.4 kcal mol(-1) and DeltaS(++) = 10 +/- 3.5 cal mol(-1) K(-1)) for thermolysis of its methyl ester [closo-1-CB(9)H(8)-1-COOMe-10-N(2)] (11) in PhCN were established, and the heterolysis of the B-N bond is believed to be the rate-determining step. Electrochemical analysis showed a partially reversible reduction process for 11 (E(1/2)(red) = -1.03 V) and 5(-) (E(1/2)(red) = -1.21 V), which are more cathodic than reduction of [closo-1-CB(9)H(9)-1-N(2)] (17). The dinitrogen acid 5 was reacted with pyridine and N,N-dimethylthioformamide, to form pyridine acid 6 and protected mercapto acid 7, respectively, through a boronium ylide intermediate 18. Compound 7 was converted to sulfonium acid 8. The molecular and crystal structures for 5 [C(2)H(9)B(9)N(2)O(2) monoclinic, P2(1)/n, a = 7.022(2) A, b = 11.389(4) A, c = 12.815(4) A, beta = 96.212(5) degrees ; V = 1018.8(6) A(3), Z = 4,], 6 [C(7)H(14)B(9)NO(2), monoclinic, P2(1)/n, a = 14.275(4) A, b = 12.184(3) A, c = 30.538(8) A, beta = 95.377(4) degrees ; V = 5288(3) A(3), Z = 16], and 8 [C(7)H(19)B(9)O(2)S, monoclinic, P2(1)/c, a = 15.988(5) A, b = 19.377(6) A, c = 9.655(3) A, beta = 98.348(5) degrees; V = 2959.4(16) A(3), Z = 8] were determined by X-ray crystallography and compared with results of density functional theory (DFT) and MP2 calculations. Electronic structures of 5, 6, and related species were elucidated with electronic spectroscopy and assessed computationally at the B3LYP/6-31G(d,p), MP2/6-31G(d,p), and ZINDO//MP2 levels of theory.

Synthesis and reactivity of [closo-1-CB9H9-1-N2]: functional group interconversion at the carbon vertex of the {closo-1-CB9} cluster.

The dinitrogen derivative [closo-1-CB(9)H(9)-1-N(2)] (1) was prepared from amine [closo-1-CB(9)H(9)-1-NH(3)] (2) and reacted with three types of nucleophiles: activated arenes (phenolate and aniline), divalent sulfur compounds (Me(2)S and Me(2)NCHS), and pyridine, giving products of substitution at C(cage). The reaction of 1 with pyridine gave all four isomers 11a-11d, indicating the Gomberg-Bachmann mechanism, which involves radical anion [closo-1-CB(9)H(9)](-*) (26). The radical and also closed-shell electrophilic aromatic substitution mechanisms were probed with the aid of DFT and MP2 computational methods and compared to those of phenylation of pyridine. Overall, experimental results supported by computational analysis suggest two mechanisms for the substitution of the N(2) group in 1: (i) thermal heterolytic cleavage of the C(cage)-N bond and the formation of electrophilic carbonium ylide [closo-1-CB(9)H(9)] (19) and (ii) electron-transfer-induced homolytic cleavage of the C(cage)-N bond and the formation of 26. Decomposition of 1 in MeCN is believed to proceed by the nonradical mechanism involving formation of the ylide 19 as the rate-determining step with experimental activation parameters DeltaH(double dagger) = 38.4 +/- 0.8 kcal mol(-1) and DeltaS(double dagger) = 44.5 +/- 2.5 cal mol(-1) K(-1). The electron-transfer-induced formation of 26 is consistent with the relatively high reduction potential of 1 (E(pc) = -0.54 V), which is more cathodic than that of PhN(2)(+) by 0.38 V. Transformations of the phenol 8a and the Me(2)NCHS adduct 10 were demonstrated by O-methylation of the former and hydrolysis of 10 followed by S-alkylative cyclization. Direct products and their derivatives were investigated by UV-vis spectroscopy and analyzed with the ZINDO computational method.

A practical synthesis of isomerically pure 1,10-difunctionalized derivatives of the [closo-1-CB9H10] anion.

The isomer-free [closo-1-CB9H(8)-1-COOH-10-I]- anion was prepared in four steps and 10% overall yield from B10H14. The key step is the skeletal isomerization of the [closo-2-CB9H8-2-COOH-7-I]- anion to a mixture of the 10- and 6-iodo derivatives of [closo-1-CB9H(9)-1-COOH]- formed in up to a 3:1 ratio. The carboxylic acid 4 was converted to the amine [closo-1-CB9H(8)-1-NH(2)-10-I]- using the Curtius reaction. The relative thermodynamic stability of each product was calculated at the DFT and MP2 levels of theory. The regioselectivity of electrophilic substitution in [closo-CB9H10]- derivatives was briefly investigated using the NBO population analysis of the MP2 wave function.