Determination of Reaction Paths for Pentacoordinate Metal Complexes with the Structure Correlation Method

Crystal structures potentially deliver far more information than is present in the average structural communication—if sufficient structural data on closely related molecules or molecular fragments are available, it may be possible to infer details of geometric changes occurring along certain reaction pathways for the species of interest. This geometric information is extrapolated from an analysis of the similarities between the structures of the fragment in the various crystalline environments, by a method that is now known as structure correlation analysis. Since it was first proposed twenty years ago, the method has been applied to a large variety of chemical systems, but none have received as much attention as the class of five-coordinate compounds. Comparative analyses of the structures of pentacoordinate complexes have yielded information about the intimate mechanisms of substitution and addition/elimination reactions at tetrahedral and square-planar complexes, and about intramolecular isomerizations of five-coordinate compounds. Since its inception, the structure correlation method has gradually adapted techniques from other branches of science, in particular group-theoretical and multivariate statistical techniques, which have been shown to be enormously powerful tools for probing geometrically complex systems. This review traces the development of the method of structure correlation and the evolution of these co-opted techniques, with a specific emphasis on studies of five-coordinate metal complexes.     

Astellifadiene: Structure Determination by NMR Spectroscopy and Crystalline Sponge Method,and Elucidation of its Biosynthesis

Genome mining of a terpene synthase gene from Emericella variecolor NBRC 32302 and its functional expression in Aspergillus oryzae led to the production of the new sesterterpene hydrocarbon, astellifadiene ( 1 ), having a 6‐8‐6‐5‐fused ring system. The structure of 1 was initially investigated by extensive NMR analyses, and was further confirmed by the crystalline sponge method, which established the absolute structure of 1 and demonstrated the usefulness of the method in the structure determination of complex hydrocarbon natural products. Furthermore, the biosynthesis of 1 was proposed on the basis of isotope‐incorporation experiments performed both in vivo and in vitro. The cyclization of GFPP involves a protonation‐initiated second cyclization sequence, 1,2‐alkyl migration, and 1,5‐hydride shift to generate the novel scaffold of 1 .     

In Situ Gel‐to‐Crystal Transition and Synthesis of Metal Nanoparticles Obtained by Fluorination of a Cyclic β‐Aminoalcohol Gelator

A new fluorinated version of a cyclic β‐aminoalcohol gelator derived from 1,2,3,4‐tetrahydroisoquinoline is presented. The gelator is able to gel various nonprotic solvents through OH???N hydrogen bonds and additional CH???F interactions due to the introduction of fluorine. A bimolecular lamellar structure is formed in the gel phase, which partly preserves the pattern of molecular organization in the single crystal. The racemate of the chiral gelator shows lower gelation ability than its enantiomer because of a higher tendency to form microcrystals, as shown by X‐ray diffraction analysis. The influence of fluorination on the self‐assembly of the gelator and the properties of the gel was investigated in comparison to the original fluorine‐free gel system. The introduction of fluorine brings two new features. The first is good recognition of o‐xylene by the gelator, which induces an in situ transition from gels of o‐xylene and of an o‐xylene/toluene mixture to identical single crystals with unique tubular architecture. The second is the enhanced stability of the toluene gel towards ions, including quaternary ammonium salts, which enables the preparation of a stable toluene gel in the presence of chloroaurate or chloroplatinate. The gel system can be used as a template for the synthesis of spherical gold nanoparticles with a diameter of 5 to 9 nm and wormlike platinum nanostructures with a diameter of 2 to 3 nm and a length of 5 to 12 nm. This is the first example of a synthesis of platinum nanoparticles in an organogel medium. Therefore, the appropriate introduction of a fluorine atom and corresponding nonbonding interactions into a known gelator to tune the properties and functions of a gel is a simple and effective tactic for design of a gel system with specific targets.     

The Formosalides: Structure Determination by Total Synthesis

Total synthesis allowed the constitution of the cytotoxic marine macrolides of the formosalide family to be confirmed and their previously unknown stereostructure to be assigned with confidence. The underlying blueprint was inherently modular to ensure that each conceivable isomer could be reached. This flexibility derived from the use of strictly catalyst controlled transformations to set the stereocenters, except for the anomeric position, which is under thermodynamic control; as an extra safety measure, all stereogenic centers were set prior to ring closure to preclude any interference of the conformation adopted by the macrolactone rings of the different diastereomers. Late‐stage macrocyclization by ring‐closing alkyne metathesis was followed by a platinum‐catalyzed transannular 6‐exo‐dig hydroalkoxylation/ketalization to craft the polycyclic frame. The side chain featuring a very labile unsaturation pattern was finally attached to the core by Stille coupling.     

Self-assembly of linear and cyclic siloxane-containing mesogens: investigation of layered structures in bulk and thin films

Silicon-containing materials which possess the ability to form mesophases are promising systems for applications in the fields of electro-optical devices, nonlinear optics, and information storage media. In this work, the formation of supramolecular assemblies of a series of low molecular weight siloxane-containing mesogens is presented. Besides a novel synthesis route via Ru(II) -catalyzed hydrosilylation of phenyl acetylene derivatives, mesophase characterization by modern analysis techniques is performed. As linker groups, leading to bi- and tetramesogens, linear disiloxane and cyclic tetrasiloxane are utilized. In the resulting class of materials, high thermal stability, induced by the formation of layered smectic-type structures, is predominant. The smectic-type phases were found to be monotropic. Layer distances in the assemblies, as well as the phase transition temperatures, can be controlled by the substitution motif on the mesogens (number and length of alkyl chains). In spin-cast thin films, the layered domains are visualized by atomic force microscopy; furthermore, domain dimensions and electron densities are determined by grazing-incidence small-angle X-ray scattering.     

Automatic Structure Determination of Organic Molecules： Principle and Implementation of the LSD Program

The LSD (Logic for Structure Determination) program gener-ates organic molecular structures from 1D and 2D NMR data without resorting to chemical shift databases. Its use in the res-olution of natural product structure determination problems has been already reported in the literature. This paper describes how data and structures are internally represented and pro-cessed by LSD to build solution structures.     

Structure Determination of [Au18(SR)14]

Unravelling the atomic structures of small gold clusters is the key to understanding the origin of metallic bonds and the nucleation of clusters from organometallic precursors. Herein we report the X‐ray crystal structure of a charge‐neutral [Au₁₈(SC₆H₁₁)₁₄] cluster. This structure exhibits an unprecedented bi‐octahedral (or hexagonal close packing) Au₉ kernel protected by staple‐like motifs including one tetramer, one dimer, and three monomers. Until the present, the [Au₁₈(SC₆H₁₁)₁₄] cluster is the smallest crystallographically characterized gold cluster protected by thiolates and provides important insight into the structural evolution with size. Theoretical calculations indicate charge transfer from surface to kernel for the HOMO–LUMO transition.     

Synthesis,Structure, and Photoluminescence Properties of Novel KBaSc2(PO4)3:Ce3+/Eu2+/Tb3+ Phosphors for White‐Light‐Emitting Diodes

A series of novel KBaSc₂(PO₄)₃:Ce³⁺/Eu²⁺/Tb³⁺phosphors are prepared using a solid‐state reaction. X‐ray diffraction analysis and Rietveld structure refinement are used to check the phase purity and crystal structure of the prepared samples. Ce³⁺‐ and Eu²⁺‐doped phosphors both have broad excitation and emission bands, owing to the spin‐ and orbital‐allowed electron transition between the 4f and 5d energy levels. By co‐doping the KBaSc₂(PO₄)₃:Eu²⁺ and KBaSc₂(PO₄)₃:Ce³⁺ phosphors with Tb³⁺ ions, tunable colors from blue to green can be obtained. The critical distance between the Eu²⁺ and Tb³⁺ ions is calculated by a concentration quenching method and the energy‐transfer mechanism for Eu²⁺→Tb³⁺ is studied by utilizing the Inokuti–Hirayama model. In addition, the quantum efficiencies of the prepared samples are measured. The results indicate that KBaSc₂(PO₄)₃:Eu²⁺,Tb³⁺ and KBaSc₂(PO₄)₃:Ce³⁺,Tb³⁺ phosphors might have potential applications in UV‐excited white‐light‐emitting diodes.     

Desolvation of a Novel Microporous Hydrogen-Bonded Framework: Characterization by In Situ Single-Crystal and Powder X-ray Diffraction

Despite significant structural rearrangement upon desolvation of a three-dimensional molecular framework of hexaaquacobalt cations and redox-active functionalized tetrathiafulvalene anions (see the picture; the area filled with water molecules is shown in gray), monocrystallinity and microporosity are retained. X-ray analyses show that a unique combination of hydrogen bonds and π⋅⋅⋅π interactions within the framework gives this material a structural flexibility not seen in zeolites or their analogues.