The Uptake and Assembly of Alkanes within a Porous Nanocapsule in Water: New Information about Hydrophobic Confinement

In Nature, enzymes provide hydrophobic cavities and channels for sequestering small alkanes or long‐chain alkyl groups from water. Similarly, the porous metal oxide capsule [{Mo^VI₆O₂₁(H₂O)₆}₁₂{(Mo^V₂O₄)₃₀(L)₂₉(H₂O)₂}]^41? (L=propionate ligand) features distinct domains for sequestering differently sized alkanes (as in Nature) as well as internal dimensions suitable for multi‐alkane clustering. The ethyl tails of the 29 endohedrally coordinated ligands, L, form a spherical, hydrophobic “shell”, while their methyl end groups generate a hydrophobic cavity with a diameter of 11 Å at the center of the capsule. As such, C₇ to C₃ straight‐chain alkanes are tightly intercalated between the ethyl tails, giving assemblies containing 90 to 110 methyl and methylene units, whereas two or three ethane molecules reside in the central cavity of the capsule, where they are free to rotate rapidly, a phenomenon never before observed for the uptake of alkanes from water by molecular cages or containers.     

Coverage dependent structures of oligopyridine adlayers on (111) oriented Ag films

The coverage dependent formation of ordered structures in vapor deposited 2,4'-bis(terpyridine)derivatives (2,4'-BTP) on (111) oriented Ag films was investigated by STM. Following a two-dimensional (2D) disordered gas phase at lowest coverages, a sequence of at least three ordered structures is observed with increasing coverage. These include a 'parallel chain structure' (PCS), a 'quasi-quadratic network' (QQN) structure, and a 'packed windmill structure' (PWS), with ideal coverages of 0.37, 0.4, and 0.44 molecules nm(-2). At intermediate coverages between 0.37 and 0.44 molecules nm(-2), these structures coexist in larger islands. The energetics of the different phases, whose structures are mainly determined by attractive C-H[dot dot dot]N bridges, are discussed in a picture including C-H[dot dot dot]N and C-H[dot dot dot]H-C interactions and lateral variations in the substrate-adsorbate interactions.     

Transesterification and amide cis-trans isomerization in Zn and Cd complexes of the chelating amino acid ligand Boc-Asp(Dpa)-OBzl

The amino acid derivative Boc-Asp-OBzl (Boc=N-butyloxycarbonyl; Asp=aspartic acid; Bzl=benzyl) was functionalized by coupling its carboxylate side chain to dipicolylamine. This yielded the tridentate nitrogen donor ligand Boc-Asp(Dpa)-OBzl (-OBzl). The compound -OBzl contains three different carbonyl groups: a tertiary amide linkage between Asp and Dpa, a C-terminal benzyl ester function, and an N-terminal urethane protecting group. NMR spectra were used to compare the reactivity of these moieties. The Boc protecting group gives rise to two isomers, (E, 9%) and (Z, 91%). Coordination of Cd(NO3)2 and Zn(NO3)2 yielded the complexes and. These compounds have significantly reduced barriers to rotation about the tertiary amide C-N bond compared with the free ligand (-OBzl:18.5 kcal mol-1 in CDBr3;: 12.9 kcal mol-1 in (CD3)2CO;: 13.8 kcal mol-1 in (CD3)2CO). Both complexes readily undergo transesterification in methanol or CD3OD. Experimental pseudo-first order rate constants were determined in CD3OD and (CD3)2CO:CD3OD (3:1;). It was found that the zinc complex (k=(2.28+/-0.02)x10(-4) s-1) is significantly more reactive than the cadmium complex (k=(1.41+/-0.03)x10(-6) s-1). In order to study their tertiary amide cis-trans isomerization, the cadmium complex [(-OCH3)Cd(NO3)2] was synthesized, and the zinc complex [(-OCD3)Zn(NO3)2] was generated in situ in (CD3)2CO:CD3OD (3:1). The barriers to rotation were determined (:14.1 kcal mol-1 in CD3OD;: 13.4 kcal mol-1 in (CD3)2CO:CD3OD (3:1)). Our results show that the stronger Lewis-acid zinc(II) is significantly more active than cadmium(II) in the acceleration of the transesterification. This is in marked contrast to the tertiary amide bond rotation which is comparably fast with both metal ions.     

Selectivity of guest-host interactions in self-assembled hydrogen-bonded nanostructures observed by NMR

We studied the incorporation of various small guest molecules into calix[4]hydroquinone nanotubes and nanoclusters using solid-state proton NMR spectroscopy in combination with quantum chemical calculations. While the molecules exhibit different types of hydrogen bonding and van der Waals interactions, they show different affinities to the nanotube host structures. As the guest molecules are located inside the complexes, they experience a shift in the NMR resonance line caused by screening effects from the aromatic electrons of the host superstructure. The abilities to fill the otherwise empty space within the tubes can hence be measured indirectly by the displacement of the NMR lines relative to the free molecules. In this way, we can probe which guest molecules are recognized by the calix[4]hydroquinones as suitable for filling their nanoporous superstructures. Selective guest-host interactions have been successfully achieved for the three component mixture of water and acetone with either 2-methyl-2-propanol or 2-propanol. In both cases, the alcohols were superior to acetone in filling the CHQ tubes.     

Mimicking biological cation-transport based on sphere-surface supramolecular chemistry: simultaneous interaction of porous capsules with molecular plugs and passing cations

Spherical capsules of the type [{(Mo)Mo(5)}(12){Mo(2)(ligand)}(30)](n-) exhibiting 20 {Mo(9)O(9)} pores with crown ether functions allowed us to perform a sophisticated study of the title phenomenon based on synthetic work as well as NMR spectroscopy. The pores of the host system interact in solution specifically with guests that can be noncovalently bonded, such as formamidinium and acetamidinium cations, while having different affinities to the pores. The exchange between the guest species present in solution and in the pores was investigated, including, besides the extreme scenarios of complete pore closing and complete opening, that of stepwise pore plugging. Because of this option it was possible to model for the first time passive transmembrane cation transport based on gated pores/channels. These have the appropriate dimensions and can even adopt different structure flexibilities in response to different cations. The present investigation is based on related syntheses as well as on numerous detailed (7)Li NMR studies of Li(+) transport/exchange equilibria in dependence of the pore environment/guest situations. One compound containing capsules with sulfate ligands (2) could be obtained in which all the pores are plugged with formamidinium cations and another corresponding one was obtained with additionally encapsulated Ca(2+) ions (3); these were taken up after temporary release of some of the formamidinium plugs/guests upon short heating of the related solution.     

Rates of ligand exchange between >FeIII-OH2 functional groups on a nanometer-sized aqueous cluster and bulk solution

Variable-temperature 17O NMR experiments were conducted on the nanometer-sized Keplerate Mo72Fe30 cluster, with the stoichiometry [Mo72Fe30O252(CH3COO)12[Mo2O7(H2O)]2[H2Mo2O8(H2O)](H2O)91]. approximately 150H2O. This molecule contains on its surface 30 Fe(H2O) groups forming a well-defined icosidodecahedron, and we estimated the rates of exchange of the isolated >FeIII-OH2 waters with bulk aqueous solution. Both longitudinal and transverse 17O-relaxation times were measured, as well as chemical shifts, and these parameters were then fit to the Swift-Connick equations in order to obtain the rate parameters. Correspondingly, we estimate: k(ex)298 = 6.7(+/-0.8) x 106 s-1, which is about a factor of approximately 4 x 104 times larger than the corresponding rate coefficient for the Fe(OH2)63+ ion of k(ex)298 = 1.6 x 102 s-1 (Grant and Jordan, 1981; Inorg. Chem. 20, 55-60) and DeltaH and DeltaS are 26.3 +/- 0.6 kJ mol-1 and -26 +/- 0.9 J mol-1 K-1, respectively. High-pressure 17O NMR experiments were also conducted, but the cluster decomposed slightly under pressure, which precluded confident quantitative estimation of the DeltaV. However, the increase in the reduced transverse-relaxation time with pressure suggests a dissociative character, such as a D or Id mechanism. The enhanced reactivity of waters on the Mo72Fe30 cluster is associated with an increase in the FeIII-OH2 bond length in the solid state of approximately 0.1 A relative to the Fe(OH2)63+ ion, suggesting that a correlation exists between the FeIII-OH2 bond length and k(ex)298. Although there are only few high-spin Fe(III) complexes where both exchange rates and structural data are available, these few seem to support a general correlation.     

Quadrature Formulas for the Wiener Measure

We present a new method for the approximation of Wiener integrals and provide an explicit error bound for a class of smooth integrands. The purely deterministic algorithm is a sequence of quadrature formulas for the Wiener measure, where the knots are piecewise linear functions. It uses ideas of Smolyak, as well as the multiscale decomposition of the Wiener measure due to Lévy and Ciesielski. For the class we obtain n()max(1, 2⁻⁴), where n() is the number of integrand evaluations needed to guarantee an error at most for f .