Alpha-gamma hybrid peptides that contain the conformationally constrained gabapentin residue: characterization of mimetics of chain reversals

The crystal structures of four dipeptides that contain the stereochemically constrained gamma-amino acid residue gabapentin (1-(aminomethyl)cyclohexaneacetic acid Gpn) are described. The molecular conformation of Piv-Pro-Gpn-OH (1), reveals a beta-turn mimetic conformation, stabilized by a ten atom C[bond]H...O hydrogen bond between the Piv CO group and the pro S hydrogen of the Gpn CH(2)[bond]CO group. The peptides Boc-Gly-Gpn-OH (2), Boc-Aib-Gpn-OH (3), and Boc-Aib-Gpn-OMe (4) form compact, folded structures, in which a distinct reversal of polypeptide chain direction is observed. In all cases, the Gpn residue adopts a gauche,gauche (g,g) conformation about the C(gamma)[bond]C(beta) (theta(1)) and C(beta)[bond]C(alpha) (theta(2)) bonds. Two distinct Gpn conformational families are observed. In peptides 1 and 3, the average backbone torsion angle values for the Gpn residue are phi=98 degrees, theta(1)=-62 degrees, theta(2)=-73 degrees, and psi=79 degrees, while in peptide 2 and 4 the average values are phi=-103 degrees, theta(1)=-46 degrees, theta(2)=-49 degrees, and psi=-92 degrees. In the case of 1 and 3, an intramolecular nine-membered O[bond]H...O hydrogen bond is formed between the C[double bond]O of the preceding residue and the terminal carboxylic acid OH group. All four alpha-gamma dipeptide sequences yield compact folded backbone conformations; this suggests that the Gpn residue may be employed successfully in the design of novel folded structures.     

A Marcus treatment of rate constants for protonation of ring-substituted alpha-methoxystyrenes: intrinsic reaction barriers and the shape of the reaction coordinate

Rate and equilibrium constants were determined for protonation of ring-substituted -methoxystyrenes by hydronium ion and by carboxylic acids to form the corresponding ring-substituted alpha-methyl alpha-methoxybenzyl carbocations at 25 degrees C and I = 1.0 (KCl). The thermodynamic barrier to carbocation formation increases by 14.5 kcal/mol as the phenyl ring substituent(s) is changed from 4-MeO- to 3,5-di-NO2-, and as the carboxylic acid is changed from dichloroacetic to acetic acid. The Br?nsted coefficient alpha for protonation by carboxylic acids increases from 0.67 to 0.77 over this range of phenyl ring substituents, and the Br?nsted coefficient beta for proton transfer increases from 0.63 to 0.69 as the carboxylic acid is changed from dichloroacetic to acetic acid. The change in these Br?nsted coefficients with changing reaction driving force, (inverted theta)alpha/ (inverted theta) deltaG(av) degrees=(inverted theta)beta/(inverted theta)delta G(av) degrees= 1/8lambda = 0.011, is used to calculate a Marcus intrinsic reaction barrier of lambda= 11 kcal/mol which is close to the barrier of 13 kcal/mol for thermoneutral proton transfer between this series of acids and bases. The value of alpha= 0.66 for thermoneutral proton transfer is greater than alpha= 0.50 required by a reaction that follows the Marcus equation. This elevated value of beta may be due to an asymmetry in the reaction coordinate that arises from the difference in the intrinsic barriers for proton transfer at the oxygen acid reactant and resonance-stabilized carbon acid product.     

Electronic structures and spectroscopic properties of nitrido-osmium(VI) complexes with acetylide ligands [OsN(C[Triple Bond]CR)4]- R=H, CH3, and Ph by density functional theory calculation

Electronic structures and spectroscopic properties of a series of nitrido-osmium (VI) complex ions with acetylide ligands, [OsN(C[Triple Bond]CR)(4)](-) (R[Double Bond]H, (1), CH(3) (2), and Ph (3)) were investigated theoretically. The structures of the complexes were fully optimized at the B3LYP and CIS level for the ground states and excited states, respectively. The calculated bond lengths of Os[Triple Bond]N (1.639 A in 1, 1.642 A in 2, and 1.643 A in 3) and Os-C (2.040 A in 1, 2.043 A in 2, and 2.042 A in 3) in ground state agree well with the experimental results. The bond length of Os[Triple Bond]N bond is lengthened by ca. 0.13 A in the A (3)B(2) excited state compared to the (1)A(1) ground state, which is consistent with the lower vibration frequency of nu(Os-N) ( approximately 780 cm(-1)) in the excited state than that ( approximately 1175 cm(-1)) in the ground state. Among the calculated dipole-allowed absorptions at lambda>250 nm, the intense absorption at 261 nm for 1, 266 nm for 2, and 300 nm for 3 were attributed to the (1)[pi(C[Triple Bond]C)]-->(1)[pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]C)], (1)[pi(C[Triple Bond]C)]-->(1)[pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]C)], and (1)[pi(C[Triple Bond]CPh)]-->(1)[pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]CPh)], respectively. The lowest energy absorption at lambda(max)=393 nm for 1, 400 nm for 2, and 400 nm for 3 were assigned as (1)[d(xy)(Os)+pi(C[Triple Bond]C)]-->(1)[pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]C)], (1)[d(xy)(Os)+pi(C[Triple Bond]C)]-->(1)[pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]C)], and (1)[d(xy)(Os)+pi(C[Triple Bond]CPh)]-->(1)[pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]CPh)], respectively. The calculated phosphorescence emission at lambda(max)=581 nm for 1, 588 nm for 2, and 609 nm for 3 were originated from (3)[(pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]C))(1)(d(xy)(Os)+pi(C[Triple Bond]C))(1)], (3)[(pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]C))(1)(d(xy)(Os)+pi(C[Triple Bond]C))(1)], and (3)[(pi(*)(N[Triple Bond]Os)+pi(*)(C[Triple Bond]CPh))(1)(d(xy)(Os)+pi(C[Triple Bond]CPh))(1)] excited state, respectively.     

Copper(II) diamino acid complexes: quantum chemical computations regarding diastereomeric effects on the energy of complexation

[reaction: see text] Quantum chemical calculations were used to rationalize the observed enantiodifferentiation in the complexation of alpha-amino acids to chiral Cu(II) complexes. Apart from Cu(II)[bond]pi interactions and steric repulsions between the anchoring cholesteryl-Glu moiety and an aromatic amino acid R group, hydrogen bonding also plays a role. In fact, in the case of tryptophan, C[double bond]O...H[bond]N hydrogen bonding between the glutamate moiety and the tryptophan N[bond]H group compensates for the loss of intramolecular hydrogen-bonding and diminished Cu(II)[bond]pi interactions.     

Aromatic pathways in mono- and bisphosphorous singly Möbius twisted [28] and [30]hexaphyrins

Magnetically induced current densities and strengths of currents passing through selected bonds have been calculated for monophosphorous [28]hexaphyrin ((PO)[28]hp) and for bisphosphorous [30]hexaphyrin ((PO)(2)[30]hp) at the density functional theory level using our gauge-including magnetically induced current (GIMIC) approach. The current-density calculations yield quantitative information about electron-delocalization pathways and aromatic properties of singly M?bius twisted hexaphyrins. The calculations confirm that (PO)[28]hp sustains a strong diatropic ring current (susceptibility) of 15 nA T(-1) and can be considered aromatic, whereas (PO)(2)[30]hp is antiaromatic as it sustains a paratropic ring current of -10 nA T(-1). Numerical integration of the current density passing through selected bonds shows that the current is generally split at the pyrroles into an outer and an inner pathway. For the pyrrole with the NH moiety pointing outwards, the diatropic ring current of (PO)[28]hp takes the outer route across the NH unit, whereas for (PO)(2)[30]hp, the paratropic ring current passes through the inner C(β)=C(β) double bond. The main diatropic ring current of (PO)[28]hp generally prefers the outer routes at the pyrroles, whereas the paratropic ring current of (PO)(2)[30]hp prefers the inner ones. In some cases, the ring current is rather equally split along the two pathways at the pyrroles. The calculated ring-current pathways do not agree with those deduced from measured (1)H NMR chemical shifts.     

Effects of chromophore orientation and molecule conformation on surface-enhanced Raman scattering studied with alkanoic acids and colloidal silver nanoparticles

Experimental studies have been carried out to gain a better understanding of the effects of chromophore orientation and molecular conformation on surface-enhanced Raman scattering (SERS) based on metal nanostructures. A series of alkanoic acids that contain a phenyl ring separated by methylene groups from the carboxylic acid, including phenylacetic acid, 3-phenylpropionic acid, 4-phenylbutyric acid, 5-phenylvaleric acid, and 6-phenylhexanoic acid, was investigated as model molecules with colloidal silver nanoparticles as SERS substrates. As the number of methylene groups increases, the molecules display an interesting zigzag intensity pattern of the phenyl ring bending mode around 1000 cm(-1) as well as a trend of appearance and disappearance of either the degenerate ring breathing mode or C[Double Bond]O vibrational mode near 1585 and 1630 cm(-1), respectively. Molecules containing an odd number of methylene units display a higher ring bending intensity and degenerate ring breathing mode and are suggested to have a trans conformation on the particle surface. Molecules with an even number of methylene units show a C[Double Bond]O vibrational mode and weaker ring bending in their SERS spectra and are suggested to have a gauche conformation on the silver nanoparticle surface. The different conformation is attributed to the varying interactions of the carboxylic group or the phenyl ring pi electrons with the silver surface. The SERS intensity was found to change little as the length between the phenyl ring and the carboxylic group was increased by adding CH(2) spacers. This is possibly because the effective distance between the phenyl ring and the silver surface does not change much with increasing number of CH(2) spacers as a result of changes in molecular conformation and variations in the phenyl ring orientation with CH(2) addition. The insight gained from this study is important for understanding SERS of complex molecules for which chromophore orientation and molecular conformation must be taken into careful consideration.     

A comparative computational study of hydrogen and lithium-bonded complexes

A computational study of hydrogen-bonded complexes of F(3)CH and C1H and of lithium-bonded complexes of F(3)CLi and CILi, with small molecules such as N(2) and H(2)O was undertaken at the MP2/6-311++G(d,p) level of theory. Bond extensions and redshifts were obtained for the Cl[Single Bond]H bond in the ClH complexes, while bond contractions and blueshifts were obtained for the C[Single Bond]H bond in the F(3)CH complexes. By contrast, bond extensions and blueshifts were obtained for all of the lithium-bonded species. These results were rationalized using a model derived from perturbation theory.     

Mechanism of glucose oxidation by quinoprotein soluble glucose dehydrogenase: insights from molecular dynamics studies

We have generated 3 ns molecular dynamic (MD) simulations, in aqueous solution, of the bacterial soluble glucose dehydrogenase enzyme.PQQ.glucose complex and intermediates formed in PQQ reduction. In the MD structure of enzyme.PQQ.glucose complex the imidazole of His144 is hydrogen bonded to the hydroxyl hydrogen of H[bond]OC1(H) of glucose. The tightly hydrogen-bonded triad Asp163-His144-glucose (2.70 and 2.91 A) is involved in proton abstraction from glucose concerted with the hydride transfer from the C1[bond]H of glucose to the >C5[double bond]O quinone carbon of PQQ. The reaction is assisted by Arg228 hydrogen bonding to the carbonyl oxygen of >C5[double bond]O. The rearrangement of [bond](H)C5(O-)[bond]C4([double bond]O)[bond] of II to [bond]C5(OH)[double bond]C4(OH)[bond] of PQQH(2) hydroquinone is assisted by general acid protonatation of the >C4[double bond]O oxygen by protonated His144 and hydrogen bonds of Arg228 to the oxyanion O5. The continuous hydrogen bonding of the amide side chain of Asn229 to >C4[double bond]O4 oxygen and that of the O5 oxygen of the cofactor to Wat89 is observed throughout the entire reaction.     

1H and 13C NMR studies of lanthanide complex with 5-sulfosalicylic acid

In this paper, paramagnetic shifts have been measured for all ¹H and ¹³C nuclei of 5-sulfosalicylic acid (SSA) in the presence of the lanthanide ions in the second half of the series. The ligand forms isostructural complexes with these ions in aqueous solution. The separation of LISs was carried out by the use of the Reilley method and the calculated dipolar shifts were used to simulate the coordination structure of the complex. The result reveals that SSA is coordinated to lanthanide ion via two oxygens, one from the carboxylic group and the other from the phenolic group with Ln–O bond lengths equal to 2.47 Å. The lanthanide ion lies on the benzene plane and the carboxylic group is twisted 20° from the benzene ring. Of all the nuclei examined, those in the six-membered chelate ring experience significant dipolar interactions and contact interactions. Small |G/A| ratios were obtained for two protons five bonds away from the central lanthanide ion, which shows that the number of bonds alone cannot be used as a criterion for neglecting contact shifts in aromatic ligand.     

C[bond]H- - -O hydrogen bonds in beta-sheetlike networks: combined X-ray crystallography and high-pressure infrared study

Close interactions of the C(alpha)[bond]H- - -O type have been analyzed via X-ray crystallography and high-pressure infrared spectroscopy. The results demonstrate that the C(alpha)[bond]H- - -O interactions can offer an additional stability to the beta-sheet formation. X-ray structural data suggest that while 1-acetamido-3-(2-pyrimidinyl)-imidazolium bromide exhibits a bilayer stacking, the PF(6)(-) salt reveals a beta-sheetlike pattern. The appearance of the free-NH infrared absorption indicates that the conventional N[bond]H- - -O or N[bond]H- - -N hydrogen bonds do not fully dominate the packing for the PF(6)(-) salt. The high-pressure infrared study suggests that the C(alpha)[bond]H- - -O hydrogen bonds are the important determinants for the stability of the PF(6)(-) salt. This study also verifies that the imidazolium C[bond]H stretching frequency shifts to a longer wavelength upon the formation of the C[bond]H- - -O hydrogen bonds.     

Molecular structures of benzoic acid and 2-hydroxybenzoic acid, obtained by gas-phase electron diffraction and theoretical calculations

The structures of benzoic acid (C6H5COOH) and 2-hydroxybenzoic acid (C6H4OHCOOH) have been determined in the gas phase by electron diffraction using results from quantum chemical calculations to inform restraints used on the structural parameters. Theoretical methods (HF and MP2/6-311+G(d,p)) predict two conformers for benzoic acid, one which is 25.0 kJ mol(-1) (MP2) lower in energy than the other. In the low-energy form, the carboxyl group is coplanar with the phenyl ring and the O-H group eclipses the C=O bond. Theoretical calculations (HF and MP2/6-311+G(d,p)) carried out for 2-hydroxybenzoic acid gave evidence for seven stable conformers but one low-energy form (11.7 kJ mol(-1) lower in energy (MP2)) which again has the carboxyl group coplanar with the phenyl ring, the O-H of the carboxyl group eclipsing the C=O bond and the C=O of the carboxyl group oriented toward the O-H group of the phenyl ring. The effects of internal hydrogen bonding in 2-hydroxybenzoic acid can be clearly observed by comparison of pertinent structural parameters between the two compounds. These differences for 2-hydroxybenzoic acid include a shorter exocyclic C-C bond, a lengthening of the ring C-C bond between the substituents, and a shortening of the carboxylic single C-O bond.     

Electron ionization mass spectral fragmentation of C19 isoprenoid aldehydes and carboxylic acid methyl and trimethylsilyl esters

The electron ionization (EI) mass spectra of saturated and alpha,beta-unsaturated C(19) isoprenoid aldehydes and carboxylic acid methyl and trimethylsilyl esters are reported. Different pathways are proposed in order to explain the main fragmentations observed. The conjugated double bond migrates more or less readily before gamma-hydrogen rearrangement according to the structure of the considered compound. Configurations of the double bond of alpha,beta-unsaturated C(19) isoprenoid aldehydes and fatty acid methyl and trimethylsilyl esters can be easily determined thanks to the peaks at m/z 97, 127 and 185, respectively, which are much more abundant in the mass spectra of the Z isomers owing to the formation of a cyclic ion. In the case of trimethylsilyl esters, subsequent fragmentation of the cyclic ion at m/z 185 affords two other diagnostic ions at m/z 95 and 169.     

Complexes of platinum(II) containing ferrocenylethynyl ligands: synthesis, characterization and spectroscopic and electrochemical properties

A series of homoleptic and heteroleptic platinum(ii) complexes [Pt(C[triple bond, length as m-dash]CFc)(2)(L-L)] (L-L = COD , 1,1'-bis(diphenylphosphino)ferrocene (dppf) ), Q(2)[cis/trans-Pt(C(6)F(5))(2)(C[triple bond, length as m-dash]CFc)(2)] (cis, Q = PMePh(3), ; trans, Q = NBu(4), ), (NBu(4))[Pt(bzq)(C[triple bond, length as m-dash]CFc)(2)] (Hbzq = 7,8-benzoquinoline) and (NBu(4))(2)[Pt(C[triple bond, length as m-dash]CFc)(4)] has been synthesized and characterized spectroscopically and the structures of .2CHCl(3), and .2H(2)O.2CH(2)Cl(2) confirmed by single-crystal X-ray studies. The anion of complex , shows strong O-Hpi(C[triple bond, length as m-dash]C) interactions and weaker C-Clpi(C[triple bond, length as m-dash]C) contacts between the protons of two water and two CH(2)Cl(2) molecules and the C(alpha)[triple bond, length as m-dash]C(beta) of mutually cis alkynyl groups. In this complex the presence of additional O-HH-C(Cp) and C-ClH-C(Cp) contacts gives rise to an extended bidimensional network. The optical and electrochemical properties of all derivatives have been examined. It is remarkable that for complexes and a facile oxidatively induced coupling, giving rise to 1,4-diferrocenylbutadiyne, is observed, this also having been proven by chemical oxidation.     

O‐Arylation of Carboxylic Acids using (Phenyl)[2‐(trimethylsilyl)phenyl]iodonium Triflate as a Precursor of Arynes

Using (phenyl)[2‐(trimethylsilyl)phenyl]iodonium triflate as a precursor of arynes, Larock's method for O‐arylation of carboxylic acids and arynes was developed. A variety of acids including simple aliphatic carboxylic acids, aromatic carboxylic acids, allenoic acids, and p‐toluenesulfonic acid under mild reaction conditions could generate the aryl esters.     

Coupling of functional hydrogen bonds in pyridoxal-5'-phosphate-enzyme model systems observed by solid-state NMR spectroscopy

We present a novel series of hydrogen-bonded, polycrystalline 1:1 complexes of Schiff base models of the cofactor pyridoxal-5'-phosphate (PLP) with carboxylic acids that mimic the cofactor in a variety of enzyme active sites. These systems contain an intramolecular OHN hydrogen bond characterized by a fast proton tautomerism as well as a strong intermolecular OHN hydrogen bond between the pyridine ring of the cofactor and the carboxylic acid. In particular, the aldenamine and aldimine Schiff bases N-(pyridoxylidene)tolylamine and N-(pyridoxylidene)methylamine, as well as their adducts, were synthesized and studied using 15N CP and 1H NMR techniques under static and/or MAS conditions. The geometries of the hydrogen bonds were obtained from X-ray structures, 1H and 15N chemical shift correlations, secondary H/D isotope effects on the 15N chemical shifts, or directly by measuring the dipolar 2H-15N couplings of static samples of the deuterated compounds. An interesting coupling of the two "functional" OHN hydrogen bonds was observed. When the Schiff base nitrogen atoms of the adducts carry an aliphatic substituent such as in the internal and external aldimines of PLP in the enzymatic environment, protonation of the ring nitrogen shifts the proton in the intramolecular OHN hydrogen bond from the oxygen to the Schiff base nitrogen. This effect, which increases the positive charge on the nitrogen atom, has been discussed as a prerequisite for cofactor activity. This coupled proton transfer does not occur if the Schiff base nitrogen atom carries an aromatic substituent.     

The laser-induced fluorescence spectrum, Renner-Teller effect, and molecular quantum beats in the A (2)Pi(i)-X (2)Pi(i) transition of the jet-cooled HCCSe free radical

The selenoketyl (HCCSe) radical has been positively identified for the first time as a product of an electric discharge through selenophene vapor. Laser-induced fluorescence, wavelength resolved emission, and fluorescence decay studies of jet-cooled HCCSe and DCCSe have given a detailed picture of the ground and excited state. The 418-400 nm band system of the HCCSe radical is assigned as A (2)Pi(i)-X (2)Pi(i) and the available evidence suggests that the radical is linear in the ground state and quasilinear in the excited state. The fluorescence decays of some upper state rotational levels show field-free molecular quantum beats, ascribed to an internal conversion interaction with high vibrational levels of the ground state. A comparison of the molecular structures and bonding in the HCCX (X=O,S,Se) free radicals shows that nonlinear ground state HCCO is best described as the ketenyl radical (H[Single Bond]C[Double Bond]C[Double Bond]O) with the unpaired electron on the terminal carbon atom, whereas HCCS and HCCSe have linear ground state acetylenic (H[Single Bond]C[Triple Bond]C[Single Bond]X) structures with the unpaired electron on the heteroatom. On electronic excitation, B (2)Pi HCCO reverts to the linear acetylenic structure, and A (2)Pi HCCS and HCCSe become quasilinear with the allenic structure.     

Aspartic acid side chain effect—Experimental and theoretical insight

Gas-phase H/D exchange and density functional theory study of the Asp and Glu side-chain carboxylic group intrinsic reactivity is reported. H/D exchange site specific treatment and some additional theoretical calculations showed that a side-chain carboxylic group may initiate proton transfer along with bond formation to one of its oxygens, i.e., possibility to initiate selective of cleavage peptide bond ("aspartic acid effect"). That finding is used to select aspartic acid cleavage mechanisms (side-chain proton transfer either to backbone carbonyl or to amide nitrogen) for further computational study. B3LYP/6-31G(d) and G3(MP2)//B3LYP potential energy profiles of both mechanisms on a model system CH3CO-Asp-NHCH3 were constructed. Although energy employed in low-energy collision induced dissociation suffices for both mechanisms thresholds, energy transferred to specific modes suggests a complex one-step mechanism of proton transfer (from the side-chain carboxylic group to the backbone amide group), bond formation (between deprotonated carboxylic group and carbon atom of the backbone carbonyl), and peptide bond cleavage as favorable.     

Stereochemistry and mechanism of the Brønsted acid catalyzed intramolecular hydrofunctionalization of an unactivated cyclic alkene

Through employment of deuterium-labeled substrates, the triflic acid catalyzed intramolecular exo addition of the X-H(D) (X=N, O) bond of a sulfonamide, alcohol, or carboxylic acid across the C=C bond of a pendant cyclohexene moiety was found to occur, in each case, with exclusive formation (≥90%) of the anti-addition product without loss or scrambling of deuterium as determined by (1)H and (2)H NMR spectroscopy and mass spectrometry analysis. Kinetic analysis of the triflic-acid-catalyzed intramolecular hydroamination of N-(2-cyclohex-2'-enyl-2,2-diphenylethyl)-p-toluenesulfonamide (1a) established the second-order rate law: rate=k(2)[HOTf][1a] and the activation parameters ΔH(++)=(9.7±0.5) kcal mol(-1) and ΔS(++)=(-35±5) cal K(-1) mol(-1). An inverse α-secondary kinetic isotope effect of k(D)/k(H) =(1.15±0.03) was observed upon deuteration of the C2' position of 1a, consistent with partial C-N bond formation in the highest energy transition state of catalytic hydroamination. The results of these studies were consistent with a mechanism for the intramolecular hydroamination of 1a involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C3' position of 1a coupled with intramolecular anti addition of the pendant sulfonamide nitrogen atom to the alkenyl C2' position.     

Pop-the-cork strategy in synthetic utilization of imines: stabilization by complexation and activation via liberation of the ligated species

Treatment of trans-[PtCl(4)(RCN)(2)] (R = Me, Et) with ethanol allowed the isolation of trans-[PtCl(4)[E-NH[double bond]C(R)OEt](2)]. The latter were reduced selectively, by the ylide Ph(3)P[double bond]CHCO(2)Me, to trans-[PtCl(2)[E-NH[double bond]C(R)OEt](2)]. The complexed imino esters NH[double bond]C(R)OEt were liberated from the platinum(II) complexes by reaction with 2 equiv of 1,2-bis(diphenylphosphino)ethane (dppe) in chloroform; the cationic complex [Pt(dppe)(2)]Cl(2) precipitates almost quantitatively from the reaction mixture and can be easily separated by filtration to give a solution of NH[double bond]C(R)OEt with a known concentration of the imino ester. The imino esters efficiently couple with the coordinated nitriles in trans-[PtCl(4)(EtCN)(2)] to give, as the dominant product, [PtCl(4)[NH[double bond]C(Et)N[double bond]C(R)OEt](2)] containing a previously unknown linkage, i.e., ligated N-(1-imino-propyl)-alkylimidic acid ethyl esters. In addition to [PtCl(4)[NH[double bond]C(Et)N[double bond]C(Et)OEt](2)], another compound was generated as the minor product, i.e., [PtCl(4)(EtCN)[NH[double bond]C(Et)N[double bond]C(Et)OEt]], which was reduced to [PtCl(2)(EtCN)[NH[double bond]C(Et)N[double bond]C(Et)OEt]], and this complex was characterized by X-ray single-crystal diffraction. The platinum(IV) complexes [PtCl(4)[NH[double bond]C(Et)N[double bond]C(R)OEt](2)] are unstable toward hydrolysis and give EtOH and the acylamidine complexes trans-[PtCl(4)[Z-NH[double bond]C(Et)NHC(R)[double bond]O](2)], where the coordination to the Pt center results in the predominant stabilization of the imino tautomer NH[double bond]C(Et)NHC(R)[double bond]O over the other form, i.e., NH(2)C(Et)[double bond]NC(R)[double bond]O, which is the major one for free acylamidines. The structures of trans-[PtCl(4)[Z-NH[double bond]C(Et)NHC(R)[double bond]O](2)] (R = Me, Et) were determined by X-ray studies. The complexes [PtCl(4)[NH[double bond]C(Et)N[double bond]C(R)OEt](2)] were reduced to the appropriate platinum(II) compounds [PtCl(2)[NH[double bond]C(Et)N[double bond]C(R)OEt](2)], which, similarly to the appropriate Pt(IV) compounds, rapidly hydrolyze to yield the acylamidine complexes [PtCl(2)[NH[double bond]C(Et)NHC(R)[double bond]O](2)] and EtOH. The latter acylamidine compounds were also prepared by an alternative route upon reduction of the corresponding platinum(IV) complexes. Besides the first observation of the platinum(IV)-mediated nitrile-imine ester integration, this work demonstrates that the application of metal complexes gives new opportunities for the generation of a great variety of imines (sometimes unreachable in pure organic chemistry) in metal-mediated conversions of organonitriles, the "storage" of imino species in the complexed form, and their synthetic utilization after liberation.     

Crystal chemistry of melilite [CaLa]2[Ga]2[Ga2O7]2: a five dimensional solid electrolyte

Melilite-type [A(2)](2)[B(I)](2)[B(II)(2)O(7)](2) gallates are promising ion conducting electrolytes for deployment in solid oxide fuel cells. Single crystals of [CaLa](2)[Ga](2)[Ga(2)O(7)](2), grown in an optical floating zone furnace, were investigated using a combination of transmission electron microscopy and single crystal X-ray diffraction. Strong anisotropic displacements of oxygen arise from the structural misfit between the interlayer Ca/La cations and the [Ga]-[Ga(2)O(7)] tetrahedral layers. A model employing two-dimensional modulation achieves bond lengths and bond angles that preserve satisfactory bond valence sums throughout the structure. The melilite belongs to the tetragonal superspace group P42(1)m(α, α, 0)00s(α, α, 0)000, α = 0.2160(5), with a subcell metric of a = 7.9383(2) ?, c = 5.2641(3) ?, onto which modulation vectors are superimposed: q(1) = α (a* + b*), q(2) = α (-a* + b*). Both displacive (cation and anion) and occupational (cation) modulations contribute to incommensuration. The analysis of structural adjustments that accompany changes in temperature and composition provides assurance that the crystal chemical model is correct. By better understanding the flexibility of this modulated structure a rational approach toward crystallochemical optimization of electrolyte performance by enhancing oxygen mobility becomes feasible.