Experimental and Theoretical Conformational Analysis of 5‐Benzylimidazolidin‐4‐one Derivatives – a ‘Playground’ for Studying Dispersion Interactions and a ‘Windshield‐Wiper’ Effect in Organocatalysis

The PF₆ salts of 5‐benzyl‐1‐isopropylidene‐ and 5‐benzyl‐1‐cinnamylidene‐3‐methylimidazolidin‐4‐ones 1 (Scheme) with various substituents in the 2‐position have been prepared, and single crystals suitable for X‐ray structure determination have been obtained of 14 such compounds, i.e., 2 – 10 and 12 – 16 (Figs. 2–5). In nine of the structures, the Ph ring of the benzyl group resides above the heterocycle, in contact with the cis‐substituent at C(2) (staggered conformation A ; Figs. 1–3); in three structures, the Ph ring lies above the iminium π‐plane (staggered conformation B ; Figs. 1 and 4); in two structures, the benzylic C? C bond has an eclipsing conformation ( C ; Figs. 1 and 5) which places the Ph ring simultaneously at a maximum distance with its neighbors, the CO group, the N?C‐π‐system, and the cis‐substituent at C(2) of the heterocycle. It is suggested by a qualitative conformational analysis (Fig. 6) that the three staggered conformations of the benzylic C? C bond are all subject to unfavorable steric interactions, so that the eclipsing conformation may be a kind of ‘escape’. State‐of‐the‐art quantum‐chemical methods, with large AO basic sets (near the limit) for the single‐point calculations, were used to compute the structures of seven of the 14 iminium ions, i.e., 3, 4 / 12, 5 – 7, 13 , and 16 (Table) in the two staggered conformations, A and B , with the benzylic Ph group above the ring and above the iminium π‐system, respectively. In all cases, the more stable computed conformer (‘isolated‐molecule’ structure) corresponds to the one present in the crystal (overlay in Fig. 7). The energy differences are small (≤2 kcal/mol) which, together with the result of a potential‐curve calculation for the rotation around the benzylic C? C bond of one of the structures, 16 (Fig. 8), suggests that the benzyl group is more or less freely rotating at ambident temperatures. The importance of intramolecular London dispersion (benzene ring in ‘contact’ with the cis‐substituent in conformation A ) for DFT and other quantum‐chemical computations is demonstrated; the benzyl‐imidazolidinones 1 appear to be ideal systems for detecting dispersion contributions between a benzene ring and alkyl or aryl CH groups. Enylidene ions of the type studied herein are the reactive intermediates of enantioselective organocatalytic conjugate additions, Diels–Alder reactions, and many other transformations involving α,β‐unsaturated carbonyl compounds. Our experimental and theoretical results are discussed in view of the performance of 5‐benzyl‐imidazolidinones as enantioselective catalysts.     

Facially coordinating triamine ligands with a cyclic backbone: some structure-stability correlations

Metal complex formation of the two cyclic triamines 6-methyl-1,4-diazepan-6-amine (MeL(a)) and all-cis-2,4,6-trimethylcyclohexane-1,3,5-triamine (Me(3)tach) was studied. The structure of the free ligands (H(x)MeL(a))(x+) and H(x)Me(3)tach(x+) (0 ≤ x ≤ 3) was investigated by pH-dependent NMR spectroscopy and X-ray diffraction experiments. The crystal structure of (H(2)Me(3)tach)(p-O(3)S-C(6)H(4)-CH(3))(2) showed a chair conformation with axial nitrogen atoms for the doubly protonated species. In contrast to a previous report, Me(3)tach was found to be a stronger base than the parent cis-cyclohexane-1,3,5-triamine (tach); pK(a)-values of H(3)Me(3)tach(3+) (25 °C, 0.1 M KCl): 5.2, 7.4, 11.2. The crystal structures of (H(3)MeL(a))(BiCl(6))·2H(2)O and (H(3)MeL(a))(ClO(4))Cl(2) exhibited two distinct twisted chair conformations of the seven membered diazepane ring. [Co(MeL(a))(2)](3+) (cis: 1(3+), trans: 2(3+)), trans-[Fe(MeL(a))(2)](3+) (3(3+)), [(MeL(a))ClCd(μ(2)-Cl)](2) (4), trans-[Cu(MeL(a))(2)](2+) (5(2+)), and [Cu(HMeL(a))Br(3)] (6) were characterized by single crystal X-ray analysis of 1(ClO(4))(3)·H(2)O, 2Br(3)·H(2)O, 3(ClO(4))(3)·0.8MeCN·0.2MeOH, 4, 5Br(2)·0.5MeOH, and 6·H(2)O. Formation constants and redox potentials of MeL(a) complexes were determined by potentiometric, spectrophotometric, and cyclovoltammetric measurements. The stability of [M(II)(MeL(a))](2+)-complexes is low. In comparison to the parent 1,4-diazepan-6-amine (L(a)), it is only slightly enhanced. In analogy to L(a), MeL(a) exhibited a pronounced tendency for forming protonated species such as [M(II)(HMeL(a))](3+) or [M(II)(MeL(a))(HMeL(a))](3+) (see 6 as an example). In contrast to MeL(a), Me(3)tach forms [M(II)L](2+) complexes (M = Cu, Zn) of very high stability, and the coordination behavior corresponds mainly to an "all-or-nothing" process. Molecular mechanics calculations showed that the low stability of L(a) and MeL(a) complexes is mainly due to a large amount of torsional strain within the pure chair conformation of the diazepane ring, required for tridentate coordination. This behavior is quite contrary to Me(3)tach and tacn (tacn =1,4,7-triazacyclononane), where the main portion of strain is already preformed in the free ligand, and the amount, generated upon complex formation, is comparably low.     

Ordered mesoporous MFe(2)O(4) (M = Co, Cu, Mg, Ni, Zn) thin films with nanocrystalline walls, uniform 16 nm diameter pores and high thermal stability: template-directed synthesis and characterization of redox active trevorite

In this paper, we report on ordered mesoporous NiFe(2)O(4) thin films synthesized via co-assembly of hydrated ferric nitrate and nickel chloride with an amphiphilic diblock copolymer, referred to as KLE. We establish that the NiFe(2)O(4) samples are highly crystalline after calcination at 600 °C, and that the conversion of the amorphous inorganic framework comes at little cost to the ordering of the high quality cubic network of pores averaging 16 nm in diameter. We further show that the synthesis method employed in this work can be readily extended to other ferrites, such as CoFe(2)O(4), CuFe(2)O(4), MgFe(2)O(4), and ZnFe(2)O(4), which could pave the way for innovative device design. While this article focuses on the self-assembly and characterization of these materials using various state-of-the-art techniques, including electron microscopy, grazing incidence small-angle X-ray scattering (GISAXS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), as well as UV-vis and Raman spectroscopy, we also examine the electrochemical properties and show the benefits of combining a continuous mesoporosity with nanocrystalline films. KLE-templated NiFe(2)O(4) electrodes exhibit reasonable levels of lithium ion storage at short charging times which stem from facile pseudocapacitance.     

Systematic and in situ energy dispersive X-ray diffraction investigations on the formation of lanthanide phosphonatobutanesulfonates: Ln(O(3)P-C(4)H(8)-SO(3))(H(2)O) (Ln = La-Gd)

Using the flexible linker H(2)O(3)P-C(4)H(8)-SO(3)H (H(3)L) and rare earth ions Ln(3+) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) we were able to synthesize the new isostructural inorganic organic hybrid compounds Ln(O(3)P-C(4)H(8)-SO(3))(H(2)O). High-throughput experiments were employed to study the influence of the molar ratios Ln:H(3)L and pH on the product formation. The crystal structure of the compounds Sm(O(3)P-C(4)H(8)-SO(3))(H(2)O) (1) and Pr(O(3)P-C(4)H(8)-SO(3))(H(2)O) (2) were determined by single crystal diffraction. The structures are built up from chains of edge-sharing LnO(8)-polyhedra that are connected by the phosphonate and sulfonate groups into layers. These layers are linked by the -(CH(2))(4)- group to form a three-dimensional framework. The synthesis of compound 1 was scaled up in a conventional oven as well as in a microwave reactor system. A modification of a microwave reactor system allowed its integration into the beamline F3 at HASYLAB, DESY, Hamburg. The crystallization was investigated in situ by means of energy dispersive X-ray diffraction using conventional as well as microwave heating methods applying temperatures varying from 110 to 150 °C. The formation of Sm(O(3)P-C(4)H(8)-SO(3))(H(2)O) takes place in two steps. In the first step a crystalline intermediate was observed, which transforms completely into compound 1. The method by Sharp and Hancock was used to determine the rate constants, reaction exponents, and the Arrhenius activation energy for both reaction steps. Comparing both heating methods, microwave heating leads to fully crystallized reaction product after shorter reaction times, but neither the temperature nor the heating method has significant influence on the induction time.     

Site-selective lanthanide doping in a nonanuclear yttrium(III) cluster revealed by crystal structures and luminescence spectra

A series of lanthanide-doped nonanuclear yttrium(III) clusters with general formulas (Y(9-x)Ln(x))(acac)(16)(μ(3)-OH)(8)(μ(4)-O)(μ(4)-OH) (Ln = Pr, Eu, Tb, Dy, and Yb) were synthesized. Characterization by single-crystal X-ray diffraction allowed for analysis of relative populations of yttrium (Z = 39) and dopant trivalent lanthanide (Z = 59-70) at every crystallographic metal position. Nonuniform distribution of ions along the three different sites seems to be correlated to the site volume and the ratio of ionic radii. In support, luminescence spectra of europium(III)-doped nonanuclear clusters were measured over a wide range of dopant concentrations. Emission intensities of peaks characteristic of specific sites correlate well with the site population determined through X-ray diffraction.     

Electrophilic fluorination: the aminopyridine dilemma

An unusually high yielding fluorination of aminopyralid (3) using F-TEDA (SELECTFLUOR™) in warm water, followed by kinetic resolution (via iterative esterification/saponification) of the crude fluorination product with dry HCl in methanol produced pure ring-fluorinated pyridine 2 in an overall yield of 31% for the two steps.     

On the hydration of the phosphocholine headgroup in aqueous solution

The hydration of the phosphocholine headgroup in 1,2-dipropionyl-sn-glycero-3-phosphocholine (C(3)-PC) in solution has been determined by using neutron diffraction enhanced with isotopic substitution in combination with computer simulation techniques. The atomic scale hydration structure around this head group shows that both the -N(CH(3))(3) and -CH(2) portions of the choline headgroup are strongly associated with water, through a unique hydrogen bonding regime, where specifically a hydrogen bond from the C-H group to water and a strong association between the water oxygen and N(+) atom in solution have both been observed. In addition, both PO(4) oxygens (P=O) and C=O oxygens are oversaturated when compared to bulk water in that the average number of hydrogen bonds from water to both X=O oxygens is about 2.5 for each group. That water binds strongly to the glycerol groups and is suggestive that water may bind to these groups when phosophotidylcholine is embedded in a membrane bilayer.     

Submillimeter-wave and far-infrared spectroscopy of high-J transitions of the ground and ν2 = 1 states of ammonia

Complete and reliable knowledge of the ammonia spectrum is needed to enable the analysis and interpretation of astrophysical and planetary observations. Ammonia has been observed in the interstellar medium up to J=18 and more highly excited transitions are expected to appear in hot exoplanets and brown dwarfs. As a result, there is considerable interest in observing and assigning the high J (rovibrational) spectrum. In this work, numerous spectroscopic techniques were employed to study its high J transitions in the ground and ν(2)=1 states. Measurements were carried out using a frequency multiplied submillimeter spectrometer at Jet Propulsion Laboratory (JPL), a tunable far-infrared spectrometer at University of Toyama, and a high-resolution Bruker IFS 125 Fourier transform spectrometer (FTS) at Synchrotron SOLEIL. Highly excited ammonia was created with a radiofrequency discharge and a dc discharge, which allowed assignments of transitions with J up to 35. One hundred and seventy seven ground state and ν(2)=1 inversion transitions were observed with microwave accuracy in the 0.3-4.7 THz region. Of these, 125 were observed for the first time, including 26 ΔK=3 transitions. Over 2000 far-infrared transitions were assigned to the ground state and ν(2)=1 inversion bands as well as the ν(2) fundamental band. Of these, 1912 were assigned using the FTS data for the first time, including 222 ΔK=3 transitions. The accuracy of these measurements has been estimated to be 0.0003-0.0006?cm(-1). A reduced root mean square error of 0.9 was obtained for a global fit of the ground and ν(2)=1 states, which includes the lines assigned in this work and all previously available microwave, terahertz, far-infrared, and mid-infrared data. The new measurements and predictions reported here will support the analyses of astronomical observations by high-resolution spectroscopy telescopes such as Herschel, SOFIA, and ALMA. The comprehensive experimental rovibrational energy levels reported here will permit further refinement of the potential energy surface to improve ammonia ab initio calculations and facilitate assignment of new high-resolution spectra of hot ammonia.     

Optical response of small closed-shell sodium clusters

Absorption spectra of closed-shell Na(2), Na(3) (+), Na(4), Na(5) (+), Na(6), Na(7) (+), and Na(8) clusters are calculated using a complex Bethe-Salpeter equation derived using a conserving linear response method. In the framework of a quasiparticle approach, we determine electron-hole correlations in the presence of an external field. The calculated results are in excellent agreement with experimental spectra, and some possible cluster geometries that occur in experiments are analyzed. The position and the broadening of the resonances in the spectra arise from a consistent treatment of the scattering and dephasing contributions in the linear response calculation. Comparison between the experimental and the theoretical results yields information about the cluster geometry, which is not accessible experimentally.     

Femtosecond midinfrared study of aggregation behavior in aqueous solutions of amphiphilic molecules

We study the spectral and orientational dynamics of HDO molecules in aqueous solutions of different concentrations of tertiary butyl alcohol (TBA) and trimethylamine-N-oxide (TMAO). The spectral dynamics is investigated with femtosecond two-dimensional infrared spectroscopy of the O-H stretch vibration of HDO:D(2)O, and the orientational dynamics is studied with femtosecond polarization-resolved pump-probe spectroscopy of the O-D stretch vibration of HDO:H(2)O. Both the spectral and orientational dynamics are observed to show bimodal behavior: part of the water molecules shows spectral and orientational dynamics similar to bulk liquid water and part of the water molecules displays a much slower dynamics. For low solute concentrations, the latter fraction of slow water increases linearly as a function of solute molality, indicating that the slow water is contained in the solvation shells of TBA and TMAO. At higher concentrations, the fraction of slow water saturates. The saturation behavior is much stronger for TBA solutions than for TMAO solutions, indicating the aggregation of the TBA molecules.