Metal complexes of a biconcave porphyrin with D4-structure--versatile chiral shift agents.

Representative metal complexes of a biconcave D4-symmetric porphyrin were synthesised by metalion insertion into the porphyrin ligand 1. The NMR spectra suggested D4-symmetry for the ZnII and dioxo-RuVI complexes of 1 and C4-symmetry for the unsymmetrically ligated RuII and RhIII complexes. Metal complexes of 1 proved to be versatile chiral 1H NMR shift agents for a broad spectrum of organic amines, alcohols, carboxylic acids, esters, nitriles and nonpolar fullerene derivatives. A practical analysis of chiral substrates with 1 covers enantiomeric excesses beyond 99%. An X-ray structure of (1:1)-cocrystals of an achiral, biconcave CoII porphyrinate and C60 provided the first detailed insights into the structure of such a biconcave metallo-porphyrinate. It also showed remarkable packing of the carbon sphere against the main concave units of the porphyrin and gave clues about the relevant interactions between biconcave porphyrins and fullerenes.

Structural complexity of a polar perhydrotriphenylene inclusion compound brought to light by synchrotron radiation.

The complex diffraction pattern of the heavily disordered co-crystals of perhydrotriphenylene and 1-(4-nitrophenyl)piperazine (5C(18)H(30) x C(10)H(13)N(3)O(2)) has been investigated with synchrotron radiation and an area detector. Five (almost) complete, three-dimensional data sets have been obtained from the tips and the centre of a needle-like crystal at room temperature and 120 K. They revealed a rich variety of features including one,- two- and three-dimensional diffuse scattering, as well as incommensurate satellites. At the centre and one tip of the crystal the symmetry appears to be orthorhombic, whereas at the other tip the symmetry of the satellites and of some of the diffuse scattering is clearly monoclinic, indicating that the crystal is not homogeneous. Most of the scattering could be assigned to R/S occupational disorder of the chiral host molecules, to positional disorder of the guest molecules or to local distortions of the average structure. Assignments are based on the disorder deduced from the average structure and the molecular form factors of host and guest molecules which show characteristic patterns in reciprocal space. Two smaller, orthorhombic twin fragments and an additional phase with hexagonal symmetry have also been found.

Motion and disorder in crystal structure analysis: measuring and distinguishing them.

Dynamic processes in crystalline solids are reflected in the atomic displacement amplitudes determined, together with the atomic coordinates, by crystal structure analysis. The interpretation of such amplitudes poses two severe problems: (a) The relative phases of the atomic displacements are lost; and (b) the amplitudes may reflect disorder in the structure and systematic error in the diffraction experiment in addition to motion, but the three contributions cannot be separated on the basis of measurements at a single temperature. Several approximate ways to solve these problems, e.g. rigid-body and segmented-rigid-body analysis, are reviewed together with their limitations. A more recent approach that represents a significant advance with respect to both difficulties is also described: Crystal structures are determined over a range of temperatures; the mean square amplitude quantities are interpreted by taking explicit account of their temperature dependence, i.e. by exploiting the difference in behavior of a microscopic oscillator in the low-temperature, quantum regime and in the high-temperature, classical regime. A distinction between low-frequency and high-frequency motion, disorder, and systematic error becomes possible with this model; this is illustrated with the help of case studies.

ZrIV-tetraphenylporphyrinates as nuclease mimics: structural, kinetic and mechanistic studies on phosphate diester transesterification.

The Zr(IV)-tetraphenylpor-phyrinates Zr(TPP)(X,X'), (X,X' = -OAc, -OMe, Cl ) 4-6, 8 were prepared and their complexing properties as well as catalytic properties towards solvolysis of the phosphate diesters hpp (2), dmp (3) and pmp (16) characterised. The diesters 2 and 16, representing model phosphates for RNA and DNA, were substrates for the catalyst Zr(TPP)Cl2 (4), and rate accelerations over background by 6-9 orders of magnitude were measured. These accelerations are comparable to those of dinuclear transition metal catalysts and lanthanide ions. Catalytic turnover was observed. Kinetic studies revealed that the catalytically active species of 4 in the solvolysis of 2 and 16 in methanol-containing solvents are dinuclear complexes containing either one or two phosphate esters depending upon the phosphate concentration. Besides the usual solvolysis pathway of the RNA model hpp (2), which proceeds via the cyclophosphate 20, a second, unusual pathway via direct substitution of the hydroxypropyl substituent was found. X-ray analysis of the Zr(TPP)(dmp) complex 19 revealed a dinuclear structure with two bridging dmp ligands and one monomethyl phosphate unit. In 19 one of the two dmp residues occurs in a very unusual high energy ac,ap conformation. Based on this structure and on the kinetic data, mechanistic models for the two solvolysis reaction pathways were developed. From an extensive CSD search on phosphodiester structures no correlation between P-O ester bond lengths and diester conformations could be found. However, P-O ester bonds decrease in length with increasing formal charge of the complexing metal ions. This underlines the higher importance of electrostatic activation relative to stereoelectronic effects in phosphodiester hydrolysis.

Crystal structure and optical spectroscopy of Er2[Pt(CN)4]3.21H2O and Er2[Pt(CN)4]2.SO4.11.5H2O.

We have synthesized two forms of erbium tetracyanoplatinates, Er2[Pt(CN)4]3.21H2O (red form) and Er2[Pt(CN)4]2.SO4.11.5H2O (yellow form), and determined their crystal structures by X-ray diffraction. While the red form crystallizes in the orthorhombic space group Pbcn, with a = 15.4848(3) A, b = 13.8186(2) A, c = 19.07820(10) A, alpha = beta = gamma = 90 degrees, and Z = 4, the yellow form precipitates in the tetragonal space group I4cm, with a = b = 14.321(2) A, c = 13.338(3) A, alpha = beta = gamma = 90 degrees, and Z = 4. Both forms show [Pt(CN)4]2- chains but differ markedly in color and morphology. This is due to the incorporation of sulfate ions in the latter modification, leading to an increased Pt-Pt distance. The observed optical absorption and emission behavior of the title compounds is correlated with the Pt-Pt distances.

PEANUT: computer graphics program to represent atomic displacement parameters.

PEANUT is an easy to use computer graphics program for the visualization and real-time manipulation of the atomic displacement parameters of small molecules. A flexible, dynamic data structure allows the user to compute complicated, nonspherical atomic surfaces and to handle the point group symmetry of the molecules automatically. Pictures with hidden-line removal may be plotted in publication quality on appropriate output devices.