Structure Determination of Biogenic Crystals Directly from 3D Electron Diffraction Data.

Highly reflective assemblies of purine, pteridine, and flavin crystals are used in the coloration and visual systems of many different animals. However, structure determination of biogenic crystals by single-crystal XRD is challenging due to the submicrometer size and beam sensitivity of the crystals, and powder XRD is inhibited due to the small volumes of powders, crystalline impurity phases, and significant preferred orientation. Consequently, the crystal structures of many biogenic materials remain unknown. Herein, we demonstrate that the 3D electron diffraction (3D ED) technique provides a powerful alternative approach, reporting the successful structure determination of biogenic guanine crystals (from spider integument, fish scales, and scallop eyes) from 3D ED data confirmed by analysis of powder XRD data. The results show that all biogenic guanine crystals studied are the previously known ß-polymorph. This study highlights the considerable potential of 3D ED for elucidating the structures of biogenic molecular crystals in the nanometer-to-micrometer size range. This opens up an important opportunity in the development of organic biomineralization, for which structural knowledge is critical for understanding the optical functions of biogenic materials and their possible applications as sustainable, biocompatible optical materials.

Electron crystallography and dedicated electron-diffraction instrumentation.

Electron diffraction (known also as ED, 3D ED or microED) is gaining momentum in science and industry. The application of electron diffraction in performing nano-crystallography on crystals smaller than 1 µm is a disruptive technology that is opening up fascinating new perspectives for a wide variety of compounds required in the fields of chemical, pharmaceutical and advanced materials research. Electron diffraction enables the characterization of solid compounds complementary to neutron, powder X-ray and single-crystal X-ray diffraction, as it has the unique capability to measure nanometre-sized crystals. The recent introduction of dedicated instrumentation to perform ED experiments is a key aspect of the continued growth and success of this technology. In addition to the ultra-high-speed hybrid-pixel detectors enabling ED data collection in continuous rotation mode, a high-precision goniometer and horizontal layout have been determined as essential features of an electron diffractometer, both of which are embodied in the Eldico ED-1. Four examples of data collected on an Eldico ED-1 are showcased to demonstrate the potential and advantages of a dedicated electron diffractometer, covering selected applications and challenges of electron diffraction: (i) multiple reciprocal lattices, (ii) absolute structure of a chiral compound, and (iii) R-values achieved by kinematic refinement comparable to X-ray data.

Synthesis, detailed geometric analysis and bond-valence method evaluation of the strength of π-arene bonding of two isotypic cationic prehnitene tin(II) complexes: [{1,2,3,4-(CH3)4C6H2}2Sn2Cl2][MCl4]2 (M = Al and Ga).

From solutions of prehnitene and the ternary halides (SnCl)[MCl4] (M = Al, Ga) in chloro-benzene, the new cationic SnII-π-arene complexes catena-poly[[chlorido-aluminate(III)]-tri-µ-chlorido-4':1κ2 Cl,1:2κ4 Cl-[(η6-1,2,3,4-tetra-meth-yl-benzene)-tin(II)]-di-µ-chlorido-2:3κ4 Cl-[(η6-1,2,3,4-tetra-methyl-benzene)-tin(II)]-di-µ-chlorido-3:4κ4 Cl-[chlorido-aluminate(III)]-µ-chlorido-4:1'κ2 Cl], [Al2Sn2Cl10(C10H14)2] n , (1) and catena-poly[[chlorido-gallate(III)]-tri-µ-chlor-ido-4':1κ2 Cl,1:2κ4 Cl-[(η6-1,2,3,4-tetra-methyl-benzene)-tin(II)]-di-µ-chlorido-2:3κ4 Cl-[(η6-1,2,3,4-tetra-methyl-benzene)-tin(II)]-di-µ-chlorido-3:4κ4 Cl-[chlor-ido-gallate(III)]-µ-chlorido-4:1'κ2 Cl], [Ga2Sn2Cl10(C10H14)2] n , (2), were isolated. In these first main-group metal-prehnitene complexes, the distorted η6 arene π-bonding to the tin atoms of the Sn2Cl2 2+ moieties in the centre of [{1,2,3,4-(CH3)4C6H2}2Sn2Cl2][MCl4]2 repeating units (site symmetry ) is characterized by: (i) a significant ring slippage of ca 0.4 Šindicated by the dispersion of Sn-C distances [1: 2.881 (2)-3.216 (2) Å; 2: 2.891 (3)-3.214 (3) Å]; (ii) the non-methyl-substituted arene C atoms positioned closest to the SnII central atom; (iii) a pronounced tilt of the plane of the arene ligand against the plane of the central (Sn2Cl2)2+ four-membered ring species [1: 15.59 (11)°, 2: 15.69 (9)°]; (iv) metal-arene bonding of medium strength as illustrated by application of the bond-valence method in an indirect manner, defining the π-arene bonding inter-action of the SnII central atoms as s(SnII-arene) = 2 - Σs(SnII-Cl), that gives s(SnII-arene) = 0.37 and 0.38 valence units for the aluminate and the gallate, respectively, indicating that comparatively strong main-group metal-arene bonding is present and in line with the expectation that [AlCl4]- is the slightly weaker coordinating anion as compared to [GaCl4]-.