Repulsive interaction can be a key design element of molecular rotary motors.

Low-barrier molecular rotary motors having rotaxane architecture can be constructed using a cucurbituril host and a polyyne guest serving as stator and rotator, respectively. The repulsive interaction between these components is supported by molecular mechanics calculations with model systems and experimentally verified by X-ray crystallography with several synthetic host-guest complexes, all suggesting that the diyne rod floats at the center of the macrocyclic host with no apparent van der Waals contacts between them. Further support for these interactions is suggested by microcalorimetry measurements.

The role of template in the synthesis of meso-hexamethyl-meso-hexaphenyl-calix[6]pyrrole: trihalogenated compounds as templates for the assembly of a host with a trigonal cavity.

Trihalogenated compounds act as effective and selective templates in the template-assisted synthesis of meso-hexaphenyl-calix[6]pyrrole.

Self-assembled calix[6]pyrrole capsules: solid-state encapsulation of different guests in preorganized calix[6]pyrrole capsules.

meso-hexamethyl-meso-hexaphenyl calix[6]pyrrole assembles into well-defined dimeric capsules in the crystalline state; the preorganized capsule serves as an efficient host for different organic guests as well as for solvent molecules.

Photochemistry of Diaryl Vicinal Tetraketones and Chemistry of Intermediate (Aroyloxy)arylketenes(1).

Photolysis of diaryl vic-tetraketones results in formation of tricyclic-gamma-lactones 3 and 4 with low quantum but high chemical yield. (Aroyloxy)arylketenes and carbon monoxide are the initial photochemical products of these photolyses. Subsequent reaction of ketene with ground-state tetraketone results in formation of the observed photoproducts via intermediate beta-lactones. The latter are formed with a high degree of stereoselectivity. The failure of a cyclic tetraketone to react is attributed to its inability to undergo the cyclic mechanism proposed for formation of ketenes from tetraketones. Dimesityl and di-tert-butyl tetraketones react by a competing intramolecular hydrogen atom transfer mechanism. Dimerization of (benzoyloxy)phenylketene and its reactions with a number of tetraketones are described.

Polymerization of propylene promoted by zirconium benzamidinates.

New bis(N,N-trimethylsilylarylamidinate) zirconium dichloride complexes with various carbon substituents were prepared, and their solid as well as solution state structures were studied. In the polymerization of propylene, after activation by MAO, these catalysts provided two fractions. Ether soluble polymers were obtained at a low activity as sticky polymers with lower molecular weights, except with the o-OMe substituted complex. The solid fractions were composed of a highly isotactic polymer and a moderately syndiotactic polymer. An interesting linear correlation was found between the rates of the 2,1 and 3,1 insertions for the ether soluble fractions.

Anisotropic lattice distortions in biogenic calcite induced by intra-crystalline organic molecules.

We have performed precise structural measurements on five different calcitic seashells by high-resolution X-ray powder diffraction on a synchrotron beam line and by laboratory single crystal X-ray diffraction. The unit cell parameters a and c of biogenic calcite were found to be systematically larger than those measured in the non-biogenic calcite. The maximum lattice distortion (about 2.10(-3)) was detected along the c-axis. Under heat treatment above 200 degrees C, a pronounced lattice relaxation was observed, which allowed us to conclude that anisotropic lattice swelling in biogenic calcite is induced by organic macromolecules incorporated within the single crystal calcitic prisms during biomineralization. This conclusion is supported by the results of crystallization experiments in the presence of specific protein extracted from one of the shells.

Sensitive and selective PET-based diimidazole luminophore for Zn(II) ions: a structure-activity correlation.

3-(3-Ethoxymethyl-1H-imidazol-2-yl)-3-(1-ethoxymethyl-1H-imidazol-2-yl)-3H-benzo[de]isochromen-1-one, 4, is a novel photoinduced electron transfer (PET) chemosensor that becomes fluorescent upon binding metal ions and shows a strong preference toward Zn(II) ions. The new bisimidazol PET sensor and its zinc complex were prepared and characterized in terms of their crystal structures, absorption and emission spectra, and orbital energy diagrams. Free 4 is a weakly luminescent species. On the basis of detailed DFT calculations, we suggest that the poor luminescence yield of free 4 originates from its orbital structure in which two pi-orbitals of the two imidazole rings, HOMO and HOMO-1, are situated between two pi-orbitals of the isochromene-one system, HOMO-2 and LUMO. The absorption and emission processes occur between the two pi-orbitals of the isochromene-one system, HOMO-2 and LUMO, and the two pi-imidazole orbitals serve as quenchers for the excited state of the molecule through nonradiative processes. Upon binding Zn(II) ions, 4 becomes a highly luminescent species having a luminescence maximum peaking at 375 nm (lambda(ex) = 329 nm). The significant 900-fold enhancement in luminescence upon binding of the Zn(II) ions is attributed to the stabilization of the pi-orbitals of the imidazole rings upon their engagement in new bonds with the zinc ion. The affinity of 4 to zinc ions in acetonitrile is found to be very high, Ka > 3 x 10(6) M(-1), while with other metals ions, the association constants are considerably weaker.

Reversible and persistent electrical bistability in single crystals of a self-assembled pi-conjugated tetraaryl system: a submicrometer scale electrical characterization.

A new pi-conjugated electroactive 4,4'-bipyridinium that bears two pyridazone self-complementary units was prepared and characterized. The new system readily assembles and forms at least two stable charge-transfer crystalline structures having a thermal population of unpaired electrons. The crystals of one phase exhibit electrical bistability, displaying either high (ON) and low (OFF) conductivity, at the same applied bias, depending on the electrical history of the system. The relation between the supramolecular structure of the crystals and their electrical properties is also discussed.

2-Chloro-4,6-dimethoxy-1,3,5-triazine.

The molecules of 2-chloro-4,6-dimethoxy-1,3,5-triazine, C(5)H(6)ClN(3)O(2), lie on a crystallographic mirror plane. There is a close contact of 3.180 (3) A between one of the methyl C atoms and the N atom of a neighboring molecule. Differential scanning calorimetry measurements show that methyl rearrangement does not take place in the solid state, despite the close proximity of the methyl group to the N atom.

Derivatives of 2,4-dimethoxy-1,3,5-triazine.

The crystal structure of three derivatives of 2,4-dimethoxy-1,3,5-triazine are described. In 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)morpholine, C(9)H(14)N(4)O(3), the morpholine moiety adopts a chair conformation, and the dimethoxytriazine molecule adopts a butterfly conformation with respect to the two methoxy groups. In 3-(4,6-dimethoxy-1,3,5-triazin-2-yloxy)-2-methylphenol, C(12)H(13)N(3)O(4), the dimethoxytriazine moiety adopts a propeller conformation with respect to the methoxy groups, and the molecules form dimers held together by O-H.N hydrogen bonds. 4-(4,6-Dimethoxy-1,3,5-triazin-2-yloxy)phenyl phenyl ketone, C(18)H(15)N(3)O(4), crystallizes with two crystallographically independent molecules in the asymmetric unit. The two molecules adopt different conformations with respect to the geometric relations between the phenyl ketone and triazine moieties.

Two hydrates of 6-methoxypurine.

6-Methoxypurine crystallizes from N,N-methylformamide as the hemihydrate, C(6)H(6)N(4)O.0.5H(2)O, and from water as the trihydrate, C(6)H(6)N(4)O.3H(2)O. Both forms crystallize in the triclinic crystal system. Upon heating the trihydrate, molecules of water are liberated successively; the hemihydrate is formed at 383 K. In the hemihydrate, the H atom on the imidazole N atom is disordered between the two N atoms. The water molecule in the hemihydrate and the H atoms of a water molecule in the trihydrate are also disordered. In the hemihydrate, the organic moieties are connected by N-H...N hydrogen bonds, while they are connected via water molecules in the trihydrate.

Two dimorphs of 5-methylsulfanyl-1H-tetrazole.

5-Methylsulfanyl-1H-tetrazole, C(2)H(4)N(4)S, crystallizes in dimorphic forms; the alpha-form crystallizes at room temperature in the monoclinic crystal system, space group P2(1)/m, and the beta-form crystallizes by sublimation at 423 K in the orthorhombic crystal system, space group Pbcm. In both forms, the molecules occupy crystallographic mirror planes and are connected to one another via N-H...N hydrogen bonds, the amino H atoms being disordered. The two forms differ from one another in their packing; there are polar layers in the alpha-form and non-polar layers in the beta-form.

Formation of inclusion organoactinide complexes with boron-containing macrocycles.

Reaction of the organoactinide complexes (C5Me5)2AnMe2 (An = Th, U) with catecholborane yields an inclusion complex where the actinide is encapsulated inside a 15-membered, hexaoxo, trianionic macrocycle built from alternating catechol and catecholborate fragments. In the presence of LiOH, a dimer of two encapsulated actinide macrocycles is formed. The X-ray molecular structure for all the complexes is presented.

TpPt(IV)Me(H)(2) forms a sigma-CH(4) complex that is kinetically resistant to methane liberation.

Complex 1 undergoes H/D scrambling in methanol without concomitant liberation of either methane or dihydrogen (k(H)/k(D) = 0.76, 55 degrees C). The measured isotope effect was proposed to directly relate to the initial reductive coupling step in reductive elimination reactions.

Some errors from the crystallographic literature, some amplifications and a questionable result.

The space groups of [[Mo(2)(O(2)CCH(3))(4)('linker')](n)] are corrected from P1 to C2/m for 'linker' = pyrazine and 1,4-diazabicyclo[2.2.2]octane (dabco) and from P1 to C2/c for 'linker' = 4,4'-bipyridine. Also, [[tris-(2-pyridylmethyl)amine]BrV(mu-O)VBr[tris-(2-pyridylmethyl)amine]]Br.H(2)O is corrected from P1 to C2/c. These Laue class changes allow more reliable crystallochemical comparisons to be made among families of related structures. Space groups are corrected for 4-methyl-2,6-bis(4-methylbenzylidene)cyclohexanone, 2,6-bis(4-dimethylaminobenzylidene)cyclohexanone, K[Cr(tetramethylenediamine-N,N,N',N'-tetraacetate)].H(2)O and [bis(11,-11-dimethyl-3,4:8,9-dibenzobicyclo[4.4.1]undeca-3,8-diene)-(tetracyanoethylene)]. The conflicting reports for Cu(H(2)O)-(phenanthroline)(2)(X)(2), where X = ClO(4), NO(3) and BF(4), are resolved. Three related examples of open framework host-guest structures with space groups 'Cc or C2/c' are discussed. Adding centers to 2,2'-bi-1H-imidazolium dipicrate and [tris(2,2'-bi-1H-imidazole)bis[2-(2-1H-imidazolyl)-1H-imidazolium]bis(iodide)] corrects discrepancies of up to 0.38 A between chemically similar hydrogen-bond distances. Wrongly identified atoms are corrected in theta-[bis(ethylenedithio)tetrathiafulvalene](2) (CsCd)(SCN)(4) and (purported) diaquadihydroxotetrakis(m-nitrobenzenesulfonate)discandium(III). The reported difference between the crystal structure of (CH(3)NH(3))(4)YbCl(7) and those of the other members of this family of (CH(3)NH(3))(4)MX(7) (M = In, Fe, V; X = Cl, Br) structures is pointed out in the context of possibly different N-H...Cl hydrogen bonding in the Yb structure.

Inclusion compounds of 2,5-diphenylhydroquinone.

The crystal structures of three 1:2 inclusion compounds that consist of host molecule 2,5-diphenylhydroquinone (C(18)H(14)O(2)) and the guest molecules 2-pyridone (C(5)H(5)NO), 1,3-diphenyl-2-propen-1-one (chalcone, C(15)H(12)O) and 1-(4-methoxyphenyl)-3-phenyl-2-propen-1-one (4'-methoxychalcone, C(16)H(14)O(2)) were determined in order to study the ability of guest molecules in inclusion compounds to undergo photoreaction. All of the crystals were found to be photoresistant. The three inclusion compounds crystallize in triclinic space group P1. In each case, the host/guest ratio is 1:2, with the host molecules occupying crystallographic centers of symmetry and the guest molecules occupying general positions. The guest molecules in each of the inclusion compounds are linked to the host molecules by hydrogen bonds. In the inclusion compound where the guest molecule is pyridone, the host molecule is disordered so that the hydroxy groups are distributed between two different sites, with occupancies of 0.738 (3) and 0.262 (3). The pyridone molecules form dimers via N-H...O hydrogen bonds.

Different packing in three polymorphs of 2,4,6-trimethoxy-1,3,5-triazine.

2,4,6-trimethoxy-1,3,5-triazine was found to exhibit three different polymorphs. The alpha-polymorph undergoes reversible phase transformation to the beta-polymorph at 340 K with an enthalpy of 3.9 kJ mol(-1). The heat of fusion of the beta-polymorph at 404 K is 18.1 kJ mol(-1). The low-temperature phase (alpha-polymorph) crystallizes in the orthorhombic space group Pnma. The high-temperature phase (beta-polymorph) can be obtained from the melt and crystallizes in the monoclinic space group P2(1). The gamma-polymorph is obtained by crystallization from a 1:1 mixture of methanol and methylenechloride from hydrolyzed 2,4-dimethoxy-6-oxybenzophenone-1,3,5-triazine. The gamma-polymorph melts at 409 K with an enthalpy of 11.4 kJ mol(-1). The gamma-polymorph crystallizes in the trigonal space group R3c. The molecules occupy a crystallographic threefold axis. Molecules of 2,4,6-trimethoxy-1,3,5-triazine are planar in all three polymorphs. The major difference between the three polymorphs is the mode of packing of the molecules in the crystal.

Catalytic coupling of terminal alkynes with isonitriles promoted by organoactinide complexes.

The coupling reaction of terminal alkynes and tert-butylisonitrile to yield substituted alpha,beta-acetylenic aldimines is catalyzed by the organoactinide neutral complexes Cp*2AnMe2 (Cp* = C5Me5, An = Th, U) and the cationic complex [(Et2N)3U][BPh4]. The reaction proceeds by a 1,1-insertion of the isonitrile into the metal-acetylide bond. Additional insertion products can be obtained by altering the catalyst and the reactant ratios. A plausible mechanism for the catalytic reaction is presented, in addition to the crystal structure of Cp*2UMe2

Some 60 new space-group corrections.

Some 60 examples of crystal structures are presented which can be better described in space groups of higher symmetry than used in the original publications. These are divided into three categories: (A) incorrect Laue group (33 examples), (B) omission of a center of symmetry (22 examples), (C) omission of a center of symmetry coupled with a failure to recognize systematic absences (nine examples). Category A errors do not lead to significant errors in molecular geometry, but these do accompany the two other types of error. There are 19 of the current set of examples which have publication dates of 1996 or later. Critical scrutiny on the part of authors, editors and referees is needed to eliminate such errors in order not to impair the role of crystal structure analysis as the chemical court of last resort.