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PubMed2006

Novel cation [N(SO2NMe3)2]+ and its synthesis and crystal structure. Dichloride of imido-bis(sulfuric) acid HN(SO2Cl)2. Part 1. Crystal structures of KN(SO2Cl)2.(1/2)CH3CN, KN(SO2Cl)2.(1/6)CH2Cl2, and [PCl4][N(SO2Cl)2].

Several salts of bis(chlorosulfonyl)imide HN(SO2Cl)2 (1), namely, two solvates of its potassium salt, KN(SO2Cl)2.(1/2)CH3CN (1K1), KN(SO2Cl)2.(1/6)CH2Cl2 (1K2), and its tetrachlorophosphonium salt, [PCl4][N(SO2Cl)2] (2), were prepared and structurally characterized. The reaction of HN(SO2Cl)2 with Me3N gives the [N(SO2Cl)2]- salt of a novel cation, [N(SO2NMe3)2]+. This cation is analogous to the [HC(SO2NMe3)2]+ cation, but in contrast to the latter, it is fairly stable to hydrolysis. The salt [N(SO2NMe3)2]+[N(SO2Cl)2]- (3) can be converted into salts of other anions by being treated with diluted aqueous solutions of the respective acids, and thus NO3-, Cl-.H2O, SeO3(2-), CH3COO-, HSO4-, (COO)2(2-) salts were prepared. Treatment of 3 with concentrated HNO3 gave the [N(SO2NMe3)2]+ [O2NO-H-ONO2]- salt, and the addition of an HCl-acidified FeCl3 aqueous solution yielded the FeCl4- salt. Methanolysis resulted in the formation of MeOSO3- and [MeOSO2NSO2OMe]- salts. All salts have been characterized by chemical analysis, vibrational spectroscopy, and X-ray structure determinations.

Journal Article↗
PubMed2006

Influence of ionic liquids bearing functional groups in dye-sensitized solar cells.

Ionic liquids containing the nitrile and vinyl functional groups attached to imidazolium cations combined with various anions, e.g., iodide, bis[(trifluoromethyl)sulfonyl]imide ([TFSI]-), or dicyanamide ([N(CN)2]-), have been prepared and characterized. These ionic liquids have been successfully used as electrolytes for dye-sensitized solar cells based on nanocrystalline TiO2 with the amphiphilic ruthenium sensitizer [ruthenium (4,4'-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-hexyloxystyryl)-2,2'-bipyridine)][NCS]2 (coded K-19). The iodide salt was used in 3-methoxypropionitrile-based electrolytes, and the performances of both types of devices were evaluated on the basis of their photocurrent density-voltage characteristics and dark current measurements, demonstrating that the functional groups do not exert a detrimental effect on the performance. The solid-state structure of the nitrile-functionalized salt [C1C3CN(im)]I has also been established by single-crystal X-ray diffraction, revealing extensive hydrogen bonding between the cation protons and the iodide.

Journal Article↗
PubMed2006

Observation of triplet intraligand excited states through nanosecond step-scan Fourier transform infrared spectroscopy.

Nanosecond step-scan Fourier transform infrared spectroscopy permits the observation of triplet intraligand ((3)IL) character in the excited states of [Ru(bpy)2(PNI-phen)]2+ and [Ru(PNI-phen)3]2+ where PNI is 4-piperidinyl-1,8-naphthalimide. After pulsed 355-nm laser excitation, the two ground-state imide C=O bands in each compound are bleached and two substantially lower energy vibrations are produced; the lower energy feature appears as two distinct bands split by an overlapping transient bleach. Model studies confirm that the time-resolved vibrational data are consistent with photoinduced sensitization of the 3IL excited state. Density functional theory calculations also support these assignments because localization of triplet electron density on the PNI moiety is expected to lead to red-shifted C=O vibrations of magnitude similar to those measured experimentally. The current results illustrate that triplet electron density can be directly tracked by time-resolved infrared measurements in metal-organic chromophores and that frequency shifts comparable to those observed in charge-transfer systems can be realized.

Journal Article↗
PubMed2006

Dinuclear copper(II) complexes with {Cu2(mu-hydroxo)bis(mu-carboxylato)}+ cores and their reactions with sugar phosphate esters: A substrate binding model of fructose-1,6-bisphosphatase.

Reactions of CuX2.nH2O with the biscarboxylate ligand XDK (H2XDK = m-xylenediamine bis(Kemp's triacid imide)) in the presence of N-donor auxiliary ligands yielded a series of dicopper(II) complexes, [Cu2(mu-OH)(XDK)(L)2]X (L = N,N,N',N'-tetramethylethylenediamine (tetmen), X = NO3 (1a), Cl (1b); L = N,N,N'-trimethylethylenediamine (tmen), X = NO3 (2a), Cl (2b); L =2,2'-bipyridine (bpy), X = NO3 (3); L = 1,10-phenanthroline (phen), X = NO3 (4); L = 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), X = NO3 (5); L = 4-methyl-1,10-phenanthroline (Mephen), X = NO3 (6)). Complexes 1-6 were characterized by X-ray crystallography (Cu...Cu = 3.1624(6)-3.2910(4) A), and the electrochemical and magnetic properties were also examined. Complexes 3 and 4 readily reacted with diphenyl phosphoric acid (HDPP) or bis(4-nitrophenyl) phosphoric acid (HBNPP) to give [Cu2(mu-phosphate)(XDK)(L)2]NO3 (L = bpy, phosphate = DPP (11); L = phen, phosphate = DPP (12), BNPP (13)), where the phsophate diester bridges the two copper ions in a mu-1,3-O,O' bidentate fashion (Cu...Cu = 4.268(3)-4.315(1) A). Complexes 4 and 6 with phen and Mephen have proven to be good precursors to accommodate a series of sugar monophosphate esters (Sugar-P) onto the biscarboxylate-bridged dicopper centers, yielding [Cu2(mu-Sugar-P)(XDK)(L)2] (Sugar-P = alpha-D-Glc-1-P (23a and b), D-Glc-6-P (24a and b), D-Man-6-P (25a), D-Fru-6-P (26a and b); L = phen (a), Mephen (b)) and [Cu2(mu-Gly-n-P)(XDK)(Mephen)2] (Gly-n-P = glycerol n-phosphate; n = 2 (21), 3 (22)), where Glc, Man, and Fru are glucose, mannose, and fructose, respectively. The structure of [Cu2(mu-MNPP)(XDK)(phen)2(CH3OH)] (20) was characterized as a reference compound (H2MNPP = 4-nitrophenyl phosphoric acid). Complexes 4 and 6 also reacted with d-fructose 1,6-bisphosphate (D-Fru-1,6-P2) to afford the tetranuclear copper(II) complexes formulated as [Cu4(mu-D-Fru-1,6-P2)(XDK)2(L)4] (L = phen (27a), Mephen (27b)). The detailed structure of 27a was determined by X-ray crystallography to involve two different tetranuclear complexes with alpha- and beta-anomers of D-Fru-1,6-P2, [Cu4(mu-alpha-D-Fru-1,6-P2)(XDK)2(phen)4] and [Cu4(mu-beta-D-Fru-1,6-P2)(XDK)2(phen)4], in which the D-Fru-1,6-P2 tetravalent anion bridges the two [Cu2(XDK)(phen)2]2+ units through the C1 and C6 phosphate groups in a mu-1,3-O,O' bidentate fashion (Cu...Cu = 4.042(2)-4.100(2) A). Notably, the structure with alpha-D-Fru-1,6-P2 demonstrated the presence of a strong hydrogen bond between the C2 hydroxyl group and the C1 phosphate oxygen atom, which may support the previously proposed catalytic mechanism in the active site of fructose-1,6-bisphosphatase.

Binding Sites↗
PubMed2006

Cu(NO3)2.3H2O-mediated synthesis of 4'-(2-pyridyl)-2,2':6',2' '-terpyridine (L2) from N-(2-pyridylmethyl)pyridine-2-methylketimine (L1). A C-C bond-forming reaction and the structure of {[Cu(L2)(OH)(NO3)][Cu(L2)(NO3)2]}.2H2O.

N-(2-Pyridylmethyl)pyridine-2-methylketimine (L1) was synthesized from equimolar quantities of (2-pyridyl)methylamine and 2-acetylpyridine. Methanolic solution of L1 reacted readily with Cu(NO3)2.3H2O in air, affording green solid of composition {[Cu(L2)(OH)(NO3)][Cu(L2)(NO3)2]}.2H2O, where L2 is 4'-(2-pyridyl)-2,2':6',2' '-terpyridine. Oxidation of the active methylene group of L1 to an imide and then condensation with 2-acetylpyridine involving a C-C bond-forming reaction, mediated by a Cu2+ ion, are the essential steps involved in the conversion of L1 to L2. L2 is isolated by extrusion of Cu2+ with EDTA(2-). The copper center in [Cu(L2)(OH)(NO3)] has a mer-N3O3 environment, and that in [Cu(L2)(NO3)2] has a distorted trigonal-bipyramidal geometry. Two H2O molecules held by C-H...O interactions are present in the predominantly hydrophobic channels of approximate cavity dimension 7.60 x 6.50 A created by aromatic rings through pi-pi interactions.

Copper↗
PubMed2006

Activation of atmospheric nitrogen and azobenzene N=N bond cleavage by a transient Nb(III) complex.

Atmospheric N2 is activated by two transient Nb(III) "(PNP)NbCl2" (PNP- = N[2-P(CHMe2)2-4-methylphenyl]2) fragments to form the bridging diimido [(PNP)NbCl2]2(mu-N2) (1). Complex 1 can also be independently synthesized from Nb(IV) and Nb(V) precursors via one-electron and transmetalation reactions, respectively. In the presence of azobenzene, the transient Nb(III) intermediate, prepared from Li(PNP) and NbCl3(DME) (DME = dimethoxyethane) under Ar, cleaves the N=N bond via a metal-ligand cooperative four-electron reduction to form niobium imide and phosphoranimine functionalities. Structural studies are presented and discussed for various Nb systems bearing the pincer-type framework PNP as well as the N2 and azobenzene activated products. Theoretical studies addressing the Nb-N2-Nb core in 1 are also presented.

Journal Article↗
PubMed1997

Use of Manganese(II)-Schiff Base Complexes for Carrying Polar Organometallics and Inorganic Ion Pairs.

This report concerns the carrier properties of [Mn(acacen)]-derived compounds toward polar organometallics, inorganic ion pairs, and salts. Such properties are the consequence of Mn(II) behaving as a Lewis acid and the O&arcraise;O bite of the bidentate Schiff base ligand toward alkali cations. The starting compounds, which occur in a dimeric form, [Mn(acac-L-en)](2) [L' = CH(2)CH(2) (1); L" = C(6)H(10) (2); L"' = R,R-C(6)H(10) (3)] have been synthesized either via a metathesis reaction from MnCl(2) or using [Mn(3)Mes(6)]. The reaction of 1-3 with lithium organometallics allowed the isolation of [Mn(acac-L-en)(R)Li(DME)] [R = Me, L = L' (4); R = Ph, L = L' (5); R = Mes, L = L' (6); R = Me, L = L" (7); R = Me, L = L"' (8)] as metalated forms, where the alkyl or aryl group is sigma-bonded to Mn(II), while the lithium cation is anchored to the Schiff base ligand. The metalated forms 4-8 react with PhCHO to give the corresponding lithium alkoxide, which remains bound in its ion-pair form to the [Mn(acacen)] skeleton in [Mn(2)(acac-L'-en)(2)Li(2)(OCH(Ph)Me)(2)](n)() (9). The use of 8, which has a chiral bridge across two nitrogen atoms, did not lead to a significant asymmetric induction in the reaction with PhCHO, because of the long separation between the lithium cation and the stereogenic center. The metalated form 4 was able to transfer the methyl group to the nitrile function to give the corresponding lithium-imide which then remains bonded to [Mn(acacen)] as the ion pair in a dimeric structure, as revealed for [Mn(2)(acac-L'-en)(2)Li(2)(DME){N=C(Ph)Me}(2)](n)() (10). Their reaction with 1 appears to depend on the steric bulkiness of the alkyl group in NaOR, resulting in either monomeric adducts, i.e. in [Mn(acac-L'-en)(2,6-Bu(t)(2)C(6)H(3)O)Na(DME)(2)] (11.2DME), or polymeric structures, like in [Mn(acac-L'-en)Na(DME)(&mgr;-OEt)](n)() (13). All the dimeric units reported in this paper show a slight antiferromagnetic coupling between the two Mn(II) assisted by bridging alkoxo groups.

Journal Article↗
PubMed1997

(&mgr;-Oxo)bis(&mgr;-carboxylato)bis(2,2'-bipyridyl)bis(X)diiron(III) Complexes, X = NCS(-), NCSe(-), and N(3)(-): Synthetic Models of Pseudohalide Derivatives of Carboxylate-Bridged Diiron Proteins.

The neutral (&mgr;-oxo)bis(&mgr;-carboxylato)diiron(III) complexes [Fe(2)O(&mgr;-XDK)(bpy)(2)(NCS)(2)] (1), [Fe(2)O(&mgr;-XDK)(bpy)(2)((15)NCS)(2)] (1a), [Fe(2)O(&mgr;-XDK)(bpy)(2)(NCSe)(2)] (2), [Fe(2)O(&mgr;-XDK)(bpy)(2)(N(3))(2)] (3), and [Fe(2)O(&mgr;-XDK)(bpy)(2)((15)NNN)(2)] (3a), where XDK is the doubly deprotonated form of m-xylylenediamine bis(Kemp's triacid imide) and bpy is 2,2'-bipyridyl, have been synthesized. The octahedrally coordinated iron centers are triply bridged by an oxo group and the two carboxylate groups of XDK. In the X-ray crystal structures of 1 and 2, the diiron centers have C(2) symmetry, whereas in 3 the coordinated azide ligands are also anti with respect to the Fe-O-Fe plane but in different rotational conformations with respect to the Fe-N(3) bond. The optical spectra of these highly colored compounds are characterized by broad ligand-field bands centered approximately 650 nm and a more intense combination of ligand-field and ligand-to-metal charge-transfer bands between 350 and 500 nm. Compounds 1-3 exhibit intense IR bands ( approximately 2050 cm(-)(1)) which are characteristic of terminally bound isothiocyanate, isoselenocyanate, and azide ligands, respectively. These bands shift to lower energies upon substitution with (15)NCS(-) in 1a (Deltanu = 24 cm(-)(1)) and (15)NNN(-) in 3a (Deltanu = 8 cm(-)(1)), and the failure of the latter to reveal two resolved features indicates a need for caution in interpreting analogous data from isotopically substituted protein adducts. Resonance Raman experiments on 1-3 revealed bands ( approximately 520 cm(-)(1)) attributable to the symmetric FeOFe stretching frequency. The Mössbauer spectra of 1-3 at 77 K display symmetric quadrupole doublets with isomer shifts ( approximately 0.5 mm s(-)(1)) and quadrupole splittings ( approximately 1.7 mm s(-)(1)) typical of (&mgr;-oxo)diiron(III) compounds. Crystal data: 1.2C(6)H(6), monoclinic, P2(1)/n, a = 19.6692(2) Å, b = 14.3069(1) Å, c = 22.6502(3) Å, beta = 96.922(1) degrees, V = 6327.4(1) Å(3), Z = 4, T = -90 degrees C; 2.2C(6)H(6), monoclinic, C2/c, a = 22.6834(3) Å, b = 14.4315(2) Å, c = 19.7013(4) Å, beta = 97.837(1) degrees, V = 6389.1(2) Å(3), Z = 4, T = -85 degrees C; 3.CH(2)Cl(2).C(6)H(3)(CH(3))(3), triclinic, P&onemacr;, a = 12.1645(2) Å, b = 14.9196(3) Å, c = 18.0690(3) Å, alpha = 79.688(1) degrees, beta = 78.611(1) degrees, gamma = 83.453(1) degrees, V = 3152.3(1) Å(3), Z = 2, T = -85 degrees C.

Journal Article↗
PubMed1998

Monomeric Alanes: Synthesis, Structure, and Thermolysis of MesAl(H)N(SiMe(3))(2) and a One-Pot Synthetic Route to Mes(2)AlH (Mes = -C(6)H(2)-2,4,6-t-Bu(3)).

The reaction of (MesAlH(2))(2) (Mes = -C(6)H(2)-2,4,6-t-Bu(3)) with HN(SiMe(3))(2) affords the monomeric amidoarylalane MesAl(H)N(SiMe(3))(2), 1. This product can also be synthesized by the reaction of [MesAlH(2)](2) with LiN(SiMe(3))(2), which, in addition, yields the byproducts LiAlH(2){N(SiMe(3))(2)}(2), 3, and MesH. Thermolysis of 1 at 175-180 degrees C affords three different the related and the imide [MesAlN(SiMe(3))](n)(), 5. In addition, the previously reported monomeric alane Mes(2)AlH was synthesized in ca. 70% yield by a one-pot reaction between LiMes (generated in situ) and AlH(3).NMe(3). All products were spectroscopically characterized, and the structure of 1 was determined by X-ray crystallography. The Al-N distance (1.819(2) Å) in 1 is relatively long. However, it has a substantial, 18.5 kcal mol(-)(1), Al-N rotation barrier which is attributed to steric congestion rather than Al-N pi bonding.

Journal Article↗
PubMed1998

Mixed-Valence Cu(I)-Cu(II) and Heterodimetallic Cu(I)-M(II) Bis(carboxylate-bridged) Complexes: Structural, Electrochemical, and Spectroscopic Investigations.

The synthesis, spectroscopic, electrochemical, and structural properties of a series of Cu(I)Cu(II) bis(carboxylate-bridged) complexes are described, together with related investigations of Cu(I)M(II) (M = Fe, Zn) analogues. Treatment of previously reported [Cu(2)(XDK)(MeCN)] (1) or [Cu(2)(PXDK)(MeCN)] (2), where H(2)XDK = m-xylylenediamine bis(Kemp's triacid imide) and H(2)PXDK = the propyl derivative of H(2)XDK, with 1 equiv of silver(I) triflate, trifluoroacetate, or tetrafluoroborate in THF afforded mixed-valence complexes [Cu(2)L(&mgr;-X)(THF)(2)], where X = triflate and L = XDK (4), PXDK (5); X = trifluoroacetate and L = XDK (6) and [Cu(2)L(THF)(4)]X, where X = tetrafluoroborate, L = XDK (7), PXDK (8). Compound 8 was also prepared from an equimolar mixture of (Et(4)N)[Cu(PXDK)] (3) and copper(II) triflate. Solid-state structural investigations of 4, 6, and 8 revealed symmetric, square pyramidal coordination environments about each copper atom and short Cu-Cu distances ranging from 2.3988(8) to 2.4246(12) Å. These features imply significant metal-metal bonding character, the nature of which was further interrogated. Comparative structural and ligand exchange studies with mixed-metal analogues [CuZn(PXDK)(OTf)(THF)(2)(H(2)O)] (9), [CuFe(PXDK)(OTf)(NB)(MeCN)](2) (10, NB = norbornene), and [CuZn(PXDK)(OTf)(NB)(H(2)O)] (11) revealed longer metal-metal distances ranging from 3.294(2) to 3.732(2) Å and monodentate, terminal triflate ligation. Variable-temperature and variable-field EPR studies showed that complexes 4-8 have fully delocalized electronic structures in the solid state and solution down to liquid helium temperatures. Molecular orbital calculations on simplified models of 4-8 revealed a Cu-Cu bonding interaction in the SHOMO and SOMO, comprising mainly sigma-type overlap between the d(x)()()2(-)(y)()()2 orbitals. In addition, cyclic voltammetric studies of compound 4 revealed a chemically reversible, electrochemically quasireversible one-electron reduction at a positive potential for a Cu(I)Cu(II) complex having a dianionic, oxygen-rich donor set. The relevance of these properties to the electronically similar Cu-Cu bonded system of the biological Cu(A) center is discussed.

Journal Article↗
PubMed1998

Reaction of the Primary Alane (2,4,6-t-Bu(3)H(2)C(6)AlH(2))(2) with Nitriles, Isonitriles, and Primary Amines.

The reactions of the sterically encumbered primary alane (MesAlH(2))(2) (Mes = C(6)H(2)-2,4,6-t-Bu(3)) with the nitriles t-BuCN, MesCN (Mes = C(6)H(2)-2,4,6-Me(3)) or MeCN lead eventually to dimeric amido alane products in which one of the ortho t-Bu groups of the Mes ligand is metalated and the nitrile is reduced to the amide ligand N(H)CH(2)R (R = t-Bu, Mes, or Me). The compounds (R = t-Bu, 2 (cis), 3 (trans); Mes, 4 (cis), 5 (trans); Me, 6 (cis)) have been isolated and characterized spectroscopically and also by X-ray crystallography in the cases of 4 and 5. The intermediate, dimeric iminato complex [MesAl(H){&mgr;(2)-NC(H)t-Bu}](2) (1), can also be isolated under carefully controlled, mild conditions. Reaction of (MesAlH(2))(2) with the isonitrile t-BuNC affords the cyclic species {MesAlN(t-Bu)CH(2)}(2) (7) featuring a six-membered (AlNC)(2) ring which, when heated, affords the cyclometalated species (8). Recognition that the products 2-6 and 8 were derivatives of primary amides led to an investigation of synthetic approaches to these product types via the direct reaction of (MesAlH(2))(2) with some primary amines. Treatment of (MesAlH(2))(2) with H(2)NCH(2)Mes affords the dimeric amido alane [Mes(H)Al{&mgr;(2)-N(H)CH(2)Mes}](2) as a mixture of trans (9) and cis (10) isomers. Further heating of 9 and 10 affords the ortho-metalated compounds 4 and 5. The reaction of (MesAlH(2))(2) with H(2)NSiPh(3) furnishes the bis amido aluminum compound MesAl{N(H)SiPh(3)}(2) (11) and [Mes(H)Al{N(H)SiPh(3)}](2). The latter yields the dimeric imide {MesAlNSiPh(3)}(2) (12) at elevated temperature.

Journal Article↗
PubMed2001

Facile synthesis of cyclopropene analogues of aluminum and an aluminum pinacolate, and the reactivity of LAl[eta(2)-C(2)(SiMe(3))(2)] toward unsaturated molecules (L=HC[(CMe)(NAr)](2), Ar=2,6-i-Pr(2)C(6)H(3)).

Reduction of LAlI(2) (1) (L = HC[(CMe)(NAr)](2), Ar = 2,6-i-Pr(2)C(6)H(3)) with potassium in the presence of alkynes C(2)(SiMe(3))(2), C(2)Ph(2), and C(2)Ph(SiMe(3)) yielded the first neutral cyclopropene analogues of aluminum LAl[eta(2)-C(2)(SiMe(3))(2)] (3), LAl(eta(2)-C(2)Ph(2)) (4), and LAl[eta(2)-C(2)Ph(SiMe(3))] (5), respectively, whereas reduction of 1 in the presence of Ph(2)CO gave an aluminum pinacolate LAl[O(2)(CPh(2))(2)] (6), irrespective of the amount of Ph(2)CO employed. The unsaturated molecules CO(2), Ph(2)CO, and PhCN inserted into one of the Al-C bonds of 3 leading to ring enlargement to give novel aluminum five-membered heterocyclic systems LAl[OC(O)C(2)(SiMe(3))(2)] (7), LAl[OC(Ph)(2)C(2)(SiMe(3))(2)] (8), and LAl[NC(Ph)C(2)(SiMe(3))(2)] (9) in high yields. In contrast, 3 reacted with t-BuCN, 2,6-Trip(2)C(6)H(3)N(3) (Trip = 2,4,6-i-Pr(3)C(6)H(2)), and Ph(3)SiN(3) resulting in the displacement of the alkyne moiety to afford LAl[N(2)(Ct-Bu)(2)] (10) with an unprecedented aluminum-containing imidazole ring, and the first monomeric aluminum imides LAlNC(6)H(3)-2,6-Trip(2) (11) and LAlNSiPh(3) (12). All compounds have been characterized spectroscopically. The variable-temperature (1)H NMR studies of 3 and ESR measurements of 3 and 4 suggest that the Al-C-C three-membered-ring systems can be best described as metallacyclopropenes. The (27)Al NMR resonances of 2 and 3 are reported and compared. Molecular structures of compounds 3, 4, 6.OEt(2), 8.OEt(2), and 9 were determined by single-crystal X-ray structural analysis.

Journal Article↗
PubMed2001

Direct measurement indicates a slow cis/trans isomerization at the secondary amide peptide bond of glycylglycine.

Spectral differences between the cis and the trans isomer of a secondary amide peptide bond were used to follow the time course of the cis/trans isomerization of Gly-Gly, Gly-Ala, Ala-Gly, and Ala-Ala dipeptides in the UV/vis region at 220 nm. Isomerization rates and Eyring activation energies were calculated from pH- and LiCl-mediated solvent jump experiments. Rate constants were found to be in a narrow range of 0.29 to 0.64 s(-)(1) for the zwitterionic dipeptides at 25 degrees C. The isomerization rate is about 2-fold higher for the monoionic forms of Gly-Gly. The zwitterionic Gly-Gly has an activation enthalpy DeltaH() of 71.6 +/- 4.9 kJ mol(-)(1) that is in the range of the rotational barriers of aromatic side chain dipeptides that have been measured by (1)H NMR magnetization transfer experiments. Late stages of protein backbone rearrangements often involve crossing the energy barrier for rotational isomerization of imidic peptide bonds. Our findings are consistent with the idea that a wide range of secondary amide peptide bonds are also able to induce slow rate-limiting steps in protein restructuring.

Dipeptides↗
PubMed2001

Rubbing-induced molecular reorientation on an alignment surface of an aromatic polyimide containing cyanobiphenyl side chains.

Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second harmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformation changes at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendent cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis[omega-[4-(4-cyanophenyl)phenoxy]hexyl] 4,4'-diamino-2,2'-biphenyldicarboxylate (nCBBP, n = 6), abbreviated as 6FDA--6CBBP. Uniform alignment layers, possessing high pretilt angles ranging from 39 degrees to 43 degrees, have been achieved after mechanical rubbing of the polyimide thin film surface at room temperature and subsequent annealing. This is the first time that high pretilt angles have been detected to possess a negative angle (-theta(c)) with respect to the rubbing direction (i.e., opposite to the rubbing direction), considerably different from the conventional pretilt angle (theta(c)) observed along the rubbing direction. This observation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomic force microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on PE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are aligned parallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermost molecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both the pendent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at the polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene structure, but also significant conformational rearrangements of the pendent side chains at the surfaces. The molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a uniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layer surface deduced from SHG phase measurements. This conformational rearrangement of the side chains results in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls having the -theta(c) pretilt angle with respect to the rubbing direction.

Journal Article↗
PubMed2002

Micropatterning of polymer thin films with pH-sensitive and cross-linkable hydrogen-bonded polyelectrolyte multilayers.

Polyelectrolyte multilayers of poly(acrylic acid) (PAA) and polyacrylamide (PAAm) were prepared via hydrogen-bonding interactions. These multilayers as assembled were stable at low pH but dissolved quickly in neutral pH water. We developed methods for stabilizing these multilayers to high pH through cross-linking by heating or UV-irradiation. Thermal treatment of the multilayers, which resulted in a partial imidization between carboxylic acid and amide groups, gave the multilayer good stability at high pH. In addition, we introduced photoreactive groups in the multilayer, which rendered the film insoluble after UV irradiation. Using these selective stabilization approaches, we have succeeded in micropatterning these films by ink-jet printing and photolithography to create subtractive patterns.

Acrylic Resins↗
PubMed2003

Modeling the reaction mechanisms of the amide hydrolysis in an N-(o-carboxybenzoyl)-L-amino acid.

Reaction mechanisms of the amide hydrolysis from the protonated, neutral, and deprotonated forms of N-(o-carboxybenzoyl)-l-amino acid have been investigated by use of the B3LYP density functional method. Our calculations reveal that in the amide hydrolysis the reaction barrier is significantly lower in solution than that in the gas phase, in contrast with the mechanism for imide formation in which the solvent has little influence on the reaction barrier. In the model reactions, the water molecules function both as a catalyst and as a reactant. The reaction mechanism starting from the neutral form of N-(o-carboxybenzoyl)-l-amino acid, which corresponds to pH 0-3, is concluded to be the most favored, and a concerted mechanism is more favorable than a stepwise mechanism. This conclusion is in agreement with experimental observations that the optimal pH range for amide hydrolysis of N-(o-carboxybenzoyl)-l-leucine is pH 0-3 where N-(o-carboxybenzoyl)-l-leucine is predominantly in its neutral form. We suggest that besides the acid-catalyzed mechanism the addition-elimination mechanism is likely to be an alternative choice for cleaving an amide bond. For the reaction mechanism initiated by protonation at the amidic oxygen (hydrogen ion concentration H(0) < -1), the reaction of the model compound with two water molecules lowers the transition barrier significantly compared with that involving a single water molecule.

Amides↗
PubMed2002

C2-amidoglycosylation. Scope and mechanism of nitrogen transfer.

A one-pot C2-amidoglycosylation reaction for the synthesis of 2-N-acyl-2-deoxy-beta-pyranosides from glycals is described. Glycal donors activated by the reagent combination of thianthrene-5-oxide (11) and Tf2O, followed by treatment with an amide nucleophile and a glycosyl acceptor, lead to the formation of various C2-amidoglycoconjugates. Both the C2-nitrogen transfer and the glycosidic bond formation proceed stereoselectively, allowing for the introduction of both natural and nonnatural amide functionalities at C2 with concomitant anomeric bond formation in a one-pot procedure. Tracking of the reaction by low-temperature NMR spectroscopy employing 15N- and 18O-isotope labels suggests a mechanism involving the formation of the C2-sulfonium glycosyl imidate 39 as well as oxazoline 37 as key intermediates in this novel oxidative glycosylation process.

Aza Compounds↗
PubMed2002

A general, highly enantioselective method for the synthesis of D and L alpha-amino acids and allylic amines.

Catalytic and enantioselective synthesis of amino acids is a subject of intense interest in the field of asymmetric catalysis. Traditionally, researchers have concentrated their efforts largely on the design and discovery of enantiopure catalysts for the Strecker reaction, alkylation of tert-butyl gylcinate-benzophenone, electrophilic amination of carbonyl compounds, and hydrogenation of N-acyl-aminoacrylic acid; however, the scope of these reactions is limited. In this paper, we report on a different approach to amino acids based on an expeditious route to enantiopure allylic amines. A highly enantioselective and catalytic vinylation of aldehydes leads to allylic alcohols that are then transformed to the allylic amines via Overman's [3,3]-sigmatropic rearrangement of imidates. Oxidative cleavage of the allylic amines furnishes the amino acids in good yields and excellent ee's. The scope and utility of this method are demonstrated by the synthesis of challenging allylic amines and their subsequent transformation to valuable nonproteinogenic amino acids, including both D and L configured (1-adamantyl)glycine.

Alcohols↗