The strong hydrogen bond, in which a hydrogen atom is centered between two electronegative atoms in a single minimum potential has been widely discussed but rarely definitively observed. The compound 4-cyano-2,2,6,6-tetramethyl-3,5-heptanedione has the shortest symmetrical O-H‚‚‚O hydrogen bond yet reported, at 239.3 pm. Neutron crystallography reveals the hydrogen-bonded proton to be nearly centered between the two oxygens, with a highly elongated thermal ellipsoid. Inelastic neutron scattering measurements of the normal and isotope labeled molecule reveal a hydrogen-bond vibrational frequency at 46.0 meV (371 cm -1 ). This frequency is too low to be compatible with a single-minimum potential and indicates a low-barrier double minimum. The temperature dependence of the NMR properties confirms the existence of a thermally accessible vibrationally excited state for the bond. Because other short hydrogen bonds show similar NMR behavior, it is likely that no "strong" hydrogen bond of this sort has yet been discovered.

Two monomeric, five-coordinate lanthanide complexes, [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]samarium[2,6-bis(tert-butyl)-4-methylphenoxide] and [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]erbium[2,6-bis(tert-butyl)-4-methylphenoxide], were isolated from the reactions of 2,6-bis(tert-butyl)-4-methylphenol with [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]lanthanide[bis(trimethylsilyl)amido] (lanthanide = Er(3+) and Sm(3+)). The purified phenoxides were recovered in excellent yields and analytical purity, and the reactions proceeded cleanly without Schiff-base degradation or cluster formation. Analogously, [bis-3,3'-(1,3-propanediyldiimino)-1-phenyl-2-butene-1-onato]erbium[bis(trimethylsilyl)amido] was also directly converted to [bis-3,3'-(1,3-propanediyldiimino)-1-phenyl-2-butene-1-onato]erbium[2,6-bis(tert-butyl)-4-methylphenoxide]; however, a less sterically demanding alcohol (i.e., ethanol) yielded a neutral trinuclear oxo alkoxide species with each dianionic Schiff base asymmetrically bridging through micro-oxo interactions. In this polynuclear cluster, each symmetry-related, seven-coordinate erbium(III) ion exhibits monocapped trigonal prismatic geometry, which assembles by sharing triangular capped faces. Single-crystal X-ray diffraction revealed square-pyramidal metal coordination in each five-coordinate lanthanide ion with varied S(4) ruffling of the "square base" donor atoms and the six-membered propylene diamine chelate ring adopting the boat conformation. To contrast the effect of subtle ligand changes, we also report the synthesis and characterization of [bis-5,5'-(2,2-dimethyl-1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]samarium[bis(trimethylsilyl)amido], having gem-dimethyl substituents appended to the propylene bridge central carbon. The six-membered diamine chelate ring in this compound adopts the chair conformation without metal-hydrocarbon interaction. Also presented are qualitative activity observations and polymerization data for the polymerization of rac-lactide and epsilon-caprolactone using the five-coordinate lanthanide amidos and phenoxides.

In a systematic study of molecular recognition of amino acid derivatives in solid-state ␤-cyclodextrin (␤-CD) complexes, we have determined crystal structures for complexes of ␤-cyclodextrin͞N-acetyl-L-phenylalanine at 298 and 20 K and for N-acetyl-D-phenylalanine at 298 K. The crystal structures for the N-acetyl-L-phenylalanine complex present disordered inclusion complexes for which the distribution of guest molecules at room temperature is not resolvable; however, they can be located with considerable confidence at low temperature. In contrast, the complex with N-acetyl-D-phenylalanine is well ordered at room temperature. The latter complex presents an example of a complex in this series in which a water molecule is included deeply in the hydrophobic torus of the extended dimer host. In an effort to understand the mechanisms of molecular recognition giving rise to the dramatic differences in crystallographic order in these crystal structures, we have examined the intermolecular interactions in detail and have examined insertion of the enantiomer of the D-complex into the chiral ␤-CD complex crystal lattice.C hiral cyclodextrin (CD) hosts have been used extensively as models for investigating chiral and molecular recognition. Solution studies of CD inclusion complexes (1-3) and determination of binding constants (2), have provided thermodynamic data useful for chromatographic applications (4-8). In the solid phase, x-ray diffraction studies of inclusion complexes with chiral guest molecules can provide direct information regarding the mechanism of chiral recognition. In the relatively few studies with native cyclodextrins, mixed results have been reported (9-15). The most complete studies are those in which structures are determined for complexes formed with both enantiomers separately and for the racemate (9-14). For example, significant discrimination was observed for the ␤-CD inclusion complex with (R,S)-fenoprofen (11) in the solid state, whereas there was no apparent discrimination for (R,S)-flurbiprofen (9-14). These results and others suggest that features such as guest fit to the cavity, solvent interactions, hydrogen bonding potential of the guest molecules, and host crystal packing arrangement combine to play a significant role in chiral discrimination.We have initiated a study in which the crystal lattice of ␤-cyclodextrin complexes that crystallize in the intermediate (Im) packing motif (16-18) has been characterized as a binding pocket useful for the crystallographic study of structural aspects of molecular recognition at high resolution (19). Briefly, the crystal lattice consists of close packed hydrogen bonded dimers of the host ␤-cyclodextrin molecules stacked, with an intervening layer of water molecules, along the a-axis. Typically two amino acid derivative guest molecules are included in the extended torus of the host dimer. To differing extents, the more hydrophilic backbones of the guest molecules interact with water molecules between the sheets and, in most cases, via bridging water molec...

Cyclodextrins (CDs) are cyclic oligosaccharides that encapsulate various small organic molecules, forming inclusion complexes. Because CD complexes are held together purely by noncovalent interactions, they function as excellent models for the study of chiral and molecular recognition mechanisms. Recently, room-temperature crystallographic studies of both the 2:2 N-acetyl-L-phenylalanine methyl ester/beta-CD and 2:2 N-acetyl-L-phenylalanine amide/beta-CD complexes were reported. The effect of changes in carboxyl backbone functional group on molecular recognition by the host CD molecule was examined for the nearly isomorphous supramolecular complexes. A new perturbation of the system is now examined, specifically perturbation of the aromatic side chain. We report a room-temperature crystal structure determination for the 2:2 N-acetyl-p-methoxy-L-phenylalanine methyl ester/beta-CD inclusion complex. The complex crystallizes isomorphously with the two previously reported examples in space group P1; the asymmetric unit consists of a hydrated head-to-head host dimer with two included guest molecules. The crystal packing provides both a nonconstraining extended hydrophobic pocket and an adjacent hydrophilic region, where hydrogen-bonding interactions can potentially occur with primary hydroxyl groups of neighboring CD molecules and waters of hydration. The rigid host molecules show no sign of conformational disorder, and water of hydration molecules exhibit the same type of disorder observed for the other two complexes, with a few significant differences in locations of water molecules in the hydrophilic region near guest molecules. There is evidence for modest disorder in the guest region of an electron density map. In comparing this system with the two previously reported complexes of phenylalanine derivatives, it is found that the packing of the guest molecules inside the torus of the CD changes upon substitution of a methoxy group at the para position of the aromatic phenyl ring. Backbone hydrogen-bonding interactions for the guest molecules with the CD primary hydroxyls and waters also change. This structure determination is a new and revealing addition to a small but growing database of amino acid and peptidomimetic interactions with carbohydrates.