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Memorial Issue Dedicated to Dr. Howard D. Flack: The Man...
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Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter

A special issue of Chemistry (ISSN 2624-8549). This special issue belongs to the section "Crystallography".

Deadline for manuscript submissions: closed (31 March 2021) | Viewed by 58923

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A printed edition of this Special Issue is available here.

Special Issue Editors

Prof. Dr. Catherine Housecroft

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Guest Editor
Department of Chemistry, University of Basel, Building 1095, Mattenstrasse 22, Postfach, CH-4002 Basel, Switzerland
Interests: light harvesting using inorganic coordination complexes as dyes in dye-sensitized solar cells (DSCs); development of emissive complexes for application in light-emitting electrochemical cells (LECs); water splitting and water oxidation catalysts; functional coordination polymers and networks; chemical education
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Katharina M. Fromm

E-Mail Website
Guest Editor
Department of Chemistry, University of Fribourg, Chemin du Musée 9, 1700 Fribourg, Switzerland
Interests: crystallography; antimicrobial compounds; bioinorganic chemistry of silver; nanomaterials; coordination compounds; batteries
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue of Chemistry is dedicated to Dr. Howard D. Flack (1943–2017), a renowned crystallographer who transformed the way in which, by using single crystal X-ray diffraction, we are able to determine the absolute structure of a crystalline material and thereby determine the absolute configuration of molecular species within material. Any of us who routinely use cifs are familiar with the 'Flack Parameter', and younger generations take it for granted. Not everyone realizes that this is a relatively recent implementation (‘’On Enantiomorph-Polarity Estimation’’, H.D. Flack, Acta Crystallogr. 1983, A39, 876, doi:10.1107/S0108767383001762).

Howard Flack studied at the University of Cambridge, UK and moved to take up the position of Maître-Assistant in the Laboratoire de Cristallographie at the University of Geneva, Switzerland in 1972. He remained in Geneva working as a crystallographer for his entire career and contributed his knowledge, enthusiasm, and encouragement to the establishment of crystallographic facilities at other Swiss Universities. His invaluable computational and scholarly contributions to single-crystal X-ray diffraction have left an extraordinary mark on the field, and this Special Issue provides a timely opportunity to pay tribute to him.

Prof. Dr. Catherine Housecroft
Prof. Dr. Katharina Fromm
Guest Editors

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Editorial

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3 pages, 206 KiB  
Editorial
Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter
by Catherine E. Housecroft and Katharina M. Fromm
Chemistry 2021, 3(3), 818-820; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3030058 - 27 Jul 2021
Viewed by 1766
Abstract
This Special Issue of Chemistry is dedicated to Dr. Howard D. Flack (1943–2017), a renowned crystallographer who transformed the way in which, by using single crystal X-ray diffraction, we are able to determine the absolute structure of a crystalline material, and thereby determine [...] Read more.
This Special Issue of Chemistry is dedicated to Dr. Howard D. Flack (1943–2017), a renowned crystallographer who transformed the way in which, by using single crystal X-ray diffraction, we are able to determine the absolute structure of a crystalline material, and thereby determine the absolute configuration of molecular species within the material [...] Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
7 pages, 259 KiB  
Editorial
The Publications of Howard Flack (1943–2017)
by Edwin Charles Constable
Chemistry 2020, 2(3), 645-651; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2030040 - 02 Jul 2020
Cited by 1 | Viewed by 2538
Abstract
Howard Flack was a driving force in the development of modern crystallography. Today “the Flack parameter” has entered into the common parlance of crystallography but his influence was far wider. This article provides an overview of his scientific output and a full bibliography. [...] Read more.
Howard Flack was a driving force in the development of modern crystallography. Today “the Flack parameter” has entered into the common parlance of crystallography but his influence was far wider. This article provides an overview of his scientific output and a full bibliography. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)

Research

Jump to: Editorial, Review

10 pages, 2217 KiB  
Article
Novel Ansa-Chain Conformation of a Semi-Synthetic Rifamycin Prepared Employing the Alder-Ene Reaction: Crystal Structure and Absolute Stereochemistry
by Christopher S. Frampton, James H. Gall and David D. MacNicol
Chemistry 2021, 3(3), 734-743; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3030052 - 11 Jul 2021
Cited by 1 | Viewed by 2389
Abstract
Rifamycins are an extremely important class of antibacterial agents whose action results from the inhibition of DNA-dependent RNA synthesis. A special arrangement of unsubstituted hydroxy groups at C21 and C23, with oxygen atoms at C1 and C8 is essential for activity. Moreover, it [...] Read more.
Rifamycins are an extremely important class of antibacterial agents whose action results from the inhibition of DNA-dependent RNA synthesis. A special arrangement of unsubstituted hydroxy groups at C21 and C23, with oxygen atoms at C1 and C8 is essential for activity. Moreover, it is known that the antibacterial action of rifamycin is lost if either of the two former hydroxy groups undergo substitution and are no longer free to act in enzyme inhibition. In the present work, we describe the successful use of an Alder-Ene reaction between Rifamycin O, 1 and diethyl azodicarboxylate, yielding 2, which was a targeted introduction of a relatively bulky group close to C21 to protect its hydroxy group. Many related azo diesters were found to react analogously, giving one predominant product in each case. To determine unambiguously the stereochemistry of the Alder-Ene addition process, a crystalline zwitterionic derivative 3 of the diethyl azodicarboxylate adduct 2 was prepared by reductive amination at its spirocyclic centre C4. The adduct, as a mono chloroform solvate, crystallized in the non-centrosymmetric Sohnke orthorhombic space group, P212121. The unique conformation and absolute stereochemistry of 3 revealed through X-ray crystal structure analysis is described. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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18 pages, 7673 KiB  
Article
Assessment of Computational Tools for Predicting Supramolecular Synthons
by Bhupinder Sandhu, Ann McLean, Abhijeet S. Sinha, John Desper and Christer B. Aakerӧy
Chemistry 2021, 3(2), 612-629; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3020043 - 03 May 2021
Cited by 6 | Viewed by 3121
Abstract
The ability to predict the most likely supramolecular synthons in a crystalline solid is a valuable starting point for subsequently predicting the full crystal structure of a molecule with multiple competing molecular recognition sites. Energy and informatics-based prediction models based on molecular electrostatic [...] Read more.
The ability to predict the most likely supramolecular synthons in a crystalline solid is a valuable starting point for subsequently predicting the full crystal structure of a molecule with multiple competing molecular recognition sites. Energy and informatics-based prediction models based on molecular electrostatic potentials (MEPs), hydrogen-bond energies (HBE), hydrogen-bond propensity (HBP), and hydrogen-bond coordination (HBC) were applied to the crystal structures of twelve pyrazole-based molecules. HBE, the most successful method, correctly predicted 100% of the experimentally observed primary intermolecular-interactions, followed by HBP (87.5%), and HBC = MEPs (62.5%). A further HBC analysis suggested a risk of synthon crossover and synthon polymorphism in molecules with multiple binding sites. These easy-to-use models (based on just 2-D chemical structure) can offer a valuable risk assessment of potential formulation challenges. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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14 pages, 5722 KiB  
Article
Erdmann’s Anion—An Inexpensive and Useful Species for the Crystallization of Illicit Drugs after Street Confiscations
by Matthew R. Wood, Sandra Mikhael, Ivan Bernal and Roger A. Lalancette
Chemistry 2021, 3(2), 598-611; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3020042 - 30 Apr 2021
Cited by 3 | Viewed by 3171
Abstract
Erdmann’s anion [1,6-diammino tetranitrocobaltate(III)] is useful in the isolation and crystallization of recently confiscated street drugs needing to be identified and catalogued. The protonated form of such drugs forms excellent crystals with that anion; moreover, Erdmann’s salts are considerably less expensive than the [...] Read more.
Erdmann’s anion [1,6-diammino tetranitrocobaltate(III)] is useful in the isolation and crystallization of recently confiscated street drugs needing to be identified and catalogued. The protonated form of such drugs forms excellent crystals with that anion; moreover, Erdmann’s salts are considerably less expensive than the classically used AuCl4− anion to isolate them, while preparation of high-quality crystals is equally easy in both cases. We describe the preparation and structures of the K+CoH6N6O8− and NH4+CoH6N7O8−, salts of Erdmann’s. In addition, herein are described the preparations of this anion’s salts with cocaine (C17H28CoN7O12), with methamphetamine (C10H22CoN7O8), and with methylone (C22H34CoN8O14), whose preparation and stereochemistry had been characterized by the old AuCl4− salts methodology. For all species in this report, the space groups and cell constants were determined at 296 and 100 K, looking for possible thermally induced polymorphism—none was found. Since the structures were essentially identical at the two temperatures studied, we discuss only the 100 K results. Complete spheres of data accessible to a Bruker ApexII diffractometer with Cu–Kα radiation, λ = 1.54178 Å, were recorded and used in the refinements. Using the refined single crystal structural data for the street drugs, we computed their X-ray powder diffraction patterns, which are beneficial as quick identification standards in law enforcement work. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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13 pages, 3946 KiB  
Article
Solid Phase Nitrosylation of Enantiomeric Cobalt(II) Complexes
by Mads Sondrup Møller, Morten Czochara Liljedahl, Vickie McKee and Christine J. McKenzie
Chemistry 2021, 3(2), 585-597; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3020041 - 28 Apr 2021
Cited by 6 | Viewed by 2697
Abstract
Accompanied by a change in color from red to black, the enantiomorphic phases of the cobalt complexes of a chiral salen ligand (L2−, Co(L)·CS2, and Co(L) (L = LS,S or LR,R)) chemisorb NO (g) at atmospheric [...] Read more.
Accompanied by a change in color from red to black, the enantiomorphic phases of the cobalt complexes of a chiral salen ligand (L2−, Co(L)·CS2, and Co(L) (L = LS,S or LR,R)) chemisorb NO (g) at atmospheric pressure and rt over hours for the CS2 solvated phase, and within seconds for the desolvated phase. NO is installed as an axial nitrosyl ligand. Aligned but unconnected voids in the CS2 desorbed Co(LR,R)·CS2 structure indicate conduits for the directional desorption of CS2 and reversible sorption of NO, which occur without loss of crystallinity. Vibrational circular dichroism (VCD) spectra have been recorded for both hands of LH2, Zn(L), Co(L)·CS2, Co(L), Co(NO)(L), and Co(NO)(L)·CS2, revealing significant differences between the solution-state and solid-state spectra. Chiral induction enables the detection of the νNO band in both condensed states, and surprisingly also the achiral lattice solvent (CS2 (νCS at 1514 cm−1)) in the solid-state VCD. Solution-state spectra of the paramagnetic Co(II) complex shows a nearly 10-fold enhancement and more extensive inversion of polarity of the vibrations of dominant VCD bands compared to the spectra of the diamagnetic compounds. This enhancement is less pronounced when there are fewer polarity inversions in the solid state VCD spectra. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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7 pages, 1146 KiB  
Article
Absolute Configuration of In Situ Crystallized (+)-γ-Decalactone
by Michael Patzer, Nils Nöthling, Richard Goddard and Christian W. Lehmann
Chemistry 2021, 3(2), 578-584; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3020040 - 21 Apr 2021
Cited by 2 | Viewed by 2793
Abstract
Knowledge about the absolute configuration of small bioactive organic molecules is essential in pharmaceutical research because enantiomers can exhibit considerably different effects on living organisms. X-ray crystallography enables chemists to determine the absolute configuration of an enantiopure compound due to anomalous dispersion. Here, [...] Read more.
Knowledge about the absolute configuration of small bioactive organic molecules is essential in pharmaceutical research because enantiomers can exhibit considerably different effects on living organisms. X-ray crystallography enables chemists to determine the absolute configuration of an enantiopure compound due to anomalous dispersion. Here, we present the determination of the absolute configuration of the flavoring agent (+)-γ-decalactone, which is liquid under ambient conditions. Single crystals were grown from the liquid in a glass capillary by in situ cryo-crystallization. Diffraction data collection was performed using Cu-Kα radiation. The absolute configuration was confirmed. The molecule consists of a linear aliphatic non-polar backbone and a polar lactone head. In the solid state, layers of polar and non-polar sections of the molecule alternating along the c-axis of the unit cell are observed. In favorable cases, this method of absolute configuration determination of pure liquid (bioactive) agents or liquid products from asymmetric catalysis is a convenient alternative to conventional methods of absolute structure determination, such as optical rotatory dispersion, vibrational circular dichroism, ultraviolet-visible spectroscopy, use of chiral shift reagents in proton NMR and Coulomb explosion imaging. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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10 pages, 2395 KiB  
Article
Chloropentaphenyldisiloxane—Model Study on Intermolecular Interactions in the Crystal Structure of a Monofunctionalized Disiloxane
by Jonathan O. Bauer and Tobias Götz
Chemistry 2021, 3(2), 444-453; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3020033 - 29 Mar 2021
Cited by 7 | Viewed by 2665
Abstract
Small functional siloxane units have gained great interest as molecular model systems for mimicking more complex silicate structures both in nature and in materials chemistry. The crystal structure of chloropentaphenyldisiloxane, which was synthesized for the first time, was elucidated by single-crystal X-ray diffraction [...] Read more.
Small functional siloxane units have gained great interest as molecular model systems for mimicking more complex silicate structures both in nature and in materials chemistry. The crystal structure of chloropentaphenyldisiloxane, which was synthesized for the first time, was elucidated by single-crystal X-ray diffraction analysis. The molecular crystal packing was studied in detail using state-of-the-art Hirshfeld surface analysis together with a two-dimensional fingerprint mapping of the intermolecular interactions. It was found that the phenyl C–H bonds act as donors for both weak C–H···π and C–H···Cl hydrogen bond interactions. The influence of intramolecular Si–O–Si bond parameters on the acceptor capability of functional groups in intermolecular hydrogen bond interactions is discussed. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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10 pages, 1759 KiB  
Article
Stereosopecificity in [Co(sep)][Co(edta)]Cl2·2H2O
by Peter Osvath, Allen Oliver and A. Graham Lappin
Chemistry 2021, 3(1), 228-237; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry3010017 - 06 Feb 2021
Cited by 4 | Viewed by 2626
Abstract
The X-ray structure of racemic [Co(sep)][Co(edta)]Cl2·2H2O is reported and reveals hetero-chiral stereospecificity in the interactions of [Co(sep)]3+ with [Co(edta)]−. Hydrogen-bonding along the molecular C2-axes of both complexes accounts for the stereospecificity. The structure of [...] Read more.
The X-ray structure of racemic [Co(sep)][Co(edta)]Cl2·2H2O is reported and reveals hetero-chiral stereospecificity in the interactions of [Co(sep)]3+ with [Co(edta)]−. Hydrogen-bonding along the molecular C2-axes of both complexes accounts for the stereospecificity. The structure of Λ-[Co(en)3]∆-[Co(edta)]2Cl·10H2O has been re-determined. Previous structural data for this compound were collected at room temperature and the model did not sufficiently describe the disorder in the structure. The cryogenic temperature used in the present study allows the disorder to be conformationally locked and modeled more reliably. A clearer inspection of other, structurally interesting, interactions is possible. Again, hydrogen-bonding along the molecular C2-axis of [Co(en)3]3+ and the equatorial carboxylates of [Co(edta)]− is the important interaction. The unique nature of the equatorial carboxylates and molecular C2-axis in [Co(edta)]−, straddled by two pseudo-C3-faces where the arrangement of the carboxylate groups conveys the same helicity, is highlighted. Implications of these structures in understanding stereoselectivity in ion-pairing and electron transfer reactions are discussed. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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13 pages, 2442 KiB  
Article
Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy
by Robert D. Bannister, William Levason and Gillian Reid
Chemistry 2020, 2(4), 947-959; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2040060 - 19 Nov 2020
Cited by 6 | Viewed by 2890
Abstract
Bis(diphenylphosphino)methane dioxide (dppmO2) forms eight-coordinate cations [M(dppmO2)4]Cl3 (M = La, Ce, Pr, Nd, Sm, Eu, Gd) on reaction in a 4:1 molar ratio with the appropriate LnCl3 in ethanol. Similar reaction in a 3:1 ratio [...] Read more.
Bis(diphenylphosphino)methane dioxide (dppmO2) forms eight-coordinate cations [M(dppmO2)4]Cl3 (M = La, Ce, Pr, Nd, Sm, Eu, Gd) on reaction in a 4:1 molar ratio with the appropriate LnCl3 in ethanol. Similar reaction in a 3:1 ratio produced seven-coordinate [M(dppmO2)3Cl]Cl2 (M = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), whilst LuCl3 alone produced six-coordinate [Lu(dppmO2)2Cl2]Cl. The complexes have been characterised by IR, 1H and 31P{1H}-NMR spectroscopy. X-ray structures show that [M(dppmO2)4]Cl3 (M = Ce, Sm, Gd) contain square antiprismatic cations, whilst [M(dppmO2)3Cl]Cl2 (M = Yb, Dy, Lu) have distorted pentagonal bipyramidal structures with apical Cl. The [Lu(dppmO2)2Cl2]Cl has a cis octahedral cation. The structure of [Yb(dppmO2)3(H2O)]Cl3·dppmO2 is also reported. The change in coordination numbers and geometry along the series is driven by the decreasing lanthanide cation radii, but the chloride counter anions also play a role. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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10 pages, 9205 KiB  
Article
Twinning in Zr-Based Metal-Organic Framework Crystals
by Sigurd Øien-Ødegaard and Karl Petter Lillerud
Chemistry 2020, 2(3), 777-786; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2030050 - 16 Sep 2020
Cited by 4 | Viewed by 3444
Abstract
Ab initio structure determination of new metal-organic framework (MOF) compounds is generally done by single crystal X-ray diffraction, but this technique can yield incorrect crystal structures if crystal twinning is overlooked. Herein, the crystal structures of three Zirconium-based MOFs, that are especially prone [...] Read more.
Ab initio structure determination of new metal-organic framework (MOF) compounds is generally done by single crystal X-ray diffraction, but this technique can yield incorrect crystal structures if crystal twinning is overlooked. Herein, the crystal structures of three Zirconium-based MOFs, that are especially prone to twinning, have been determined from twinned crystals. These twin laws (and others) could potentially occur in many MOFs or related network structures, and the methods and tools described herein to detect and treat twinning could be useful to resolve the structures of affected crystals. Our results highlight the prevalence (and sometimes inevitability) of twinning in certain Zr-MOFs. Of special importance are the works of Howard Flack which, in addition to fundamental advances in crystallography, provide accessible tools for inexperienced crystallographers to take twinning into account in structure elucidation. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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9 pages, 2279 KiB  
Article
Structural Elucidation of Enantiopure and Racemic 2-Bromo-3-Methylbutyric Acid
by Rüdiger W. Seidel, Nils Nöthling, Richard Goddard and Christian W. Lehmann
Chemistry 2020, 2(3), 691-699; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2030044 - 30 Jul 2020
Cited by 1 | Viewed by 3250
Abstract
Halogenated carboxylic acids have been important compounds in chemical synthesis and indispensable research tools in biochemical studies for decades. Nevertheless, the number of structurally characterized simple α-brominated monocarboxylic acids is still limited. We herein report the crystallization and structural elucidation of (R [...] Read more.
Halogenated carboxylic acids have been important compounds in chemical synthesis and indispensable research tools in biochemical studies for decades. Nevertheless, the number of structurally characterized simple α-brominated monocarboxylic acids is still limited. We herein report the crystallization and structural elucidation of (R)- and rac-2-bromo-3-methylbutyric acid (2-bromo-3-methylbutanoic acid, 1) to shed light on intermolecular interactions, in particular hydrogen bonding motifs, packing modes and preferred conformations in the solid-state. The crystal structures of (R)- and rac-1 are revealed by X-ray crystallography. Both compounds crystallize in the triclinic crystal system with Z = 2; (R)-1 exhibits two crystallographically distinct molecules. In the crystal, (R)-1 forms homochiral O–H···O hydrogen-bonded carboxylic acid dimers with approximate non-crystallographic C2 symmetry. In contrast, rac-1 features centrosymmetric heterochiral dimers with the same carboxy syn···syn homosynthon. The crystal packing of centrosymmetric rac-1 is denser than that of its enantiopure counterpart (R)-1. The molecules in both crystal structures adopt a virtually identical staggered conformation, despite different crystal environments, which indicates a preferred molecular structure of 1. Intermolecular interactions apart from classical O–H···O hydrogen bonds do not appear to have a crucial bearing on the solid-state structures of (R)- and rac-1. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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9 pages, 1690 KiB  
Article
Synthesis and Crystallographic Characterization of X-Substituted 2,4-Dinitrophenyl-4′-phenylbenzenesulfonates
by Brock A. Stenfors, Richard J. Staples, Shannon M. Biros and Felix N. Ngassa
Chemistry 2020, 2(2), 591-599; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2020036 - 15 Jun 2020
Cited by 5 | Viewed by 3345
Abstract
Treatment of 2,4-dinitrophenol with sulfonyl chlorides in the presence of pyridine results in the formation of undesired pyridinium salts. In non-aqueous environments, the formation of the insoluble pyridinium salt greatly affects the formation of the desired product. A facile method of producing the [...] Read more.
Treatment of 2,4-dinitrophenol with sulfonyl chlorides in the presence of pyridine results in the formation of undesired pyridinium salts. In non-aqueous environments, the formation of the insoluble pyridinium salt greatly affects the formation of the desired product. A facile method of producing the desired sulfonate involves the use of an aqueous base with a water-miscible solvent. Herein, we present the optimization of methods for the formation of sulfonates and its application in the production of desired x-substituted 2,4-dinitrophenyl-4′-phenylbenzenesulfonates. This strategy is environmentally benign and supports a wide range of starting materials. Additionally, the intermolecular interactions of these sulfonate compounds were investigated using single-crystal x-ray diffraction data. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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22 pages, 5528 KiB  
Article
The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes
by Neel Deorukhkar, Timothée Lathion, Laure Guénée, Céline Besnard and Claude Piguet
Chemistry 2020, 2(2), 231-252; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2020015 - 02 Apr 2020
Cited by 2 | Viewed by 3934
Abstract
The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) [...] Read more.
The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (VI) shows the selective formation of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This similitude is reinforced by comparable ligand-field strengths (∆oct) and nephelauxetic effects (as measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+ within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of [Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand arm wrestling match operating in bound L5, which prevents the contraction of the coordination sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in [Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic compensation. The decrease of the stability constants by five orders of magnitude observed in going from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this effect, which had been not foreseen by the authors. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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Review

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22 pages, 1683 KiB  
Review
The Crystal Chemistry of Inorganic Hydroborates
by Radovan Černý, Matteo Brighi and Fabrizio Murgia
Chemistry 2020, 2(4), 805-826; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2040053 - 29 Sep 2020
Cited by 31 | Viewed by 6342
Abstract
The crystal structures of inorganic hydroborates (salts and coordination compounds with anions containing hydrogen bonded to boron) except for the simplest anion, borohydride BH4−, are analyzed regarding their structural prototypes found in the inorganic databases such as Pearson’s Crystal Data [...] Read more.
The crystal structures of inorganic hydroborates (salts and coordination compounds with anions containing hydrogen bonded to boron) except for the simplest anion, borohydride BH4−, are analyzed regarding their structural prototypes found in the inorganic databases such as Pearson’s Crystal Data [Villars and Cenzual (2015), Pearson’s Crystal Data. Crystal Structure Database for Inorganic Compounds, Release 2019/2020, ASM International, Materials Park, Ohio, USA]. Only the compounds with hydroborate as the only type of anion are reviewed, although including compounds gathering more than one different hydroborate (mixed anion). Carbaborane anions and partly halogenated hydroborates are included. Hydroborates containing anions other than hydroborate or neutral molecules such as NH3 are not discussed. The coordination polyhedra around the cations, including complex cations, and the hydroborate anions are determined and constitute the basis of the structural systematics underlying hydroborates chemistry in various variants of anionic packing. The latter is determined from anion–anion coordination with the help of topology analysis using the program TOPOS [Blatov (2006), IUCr CompComm. Newsl. 7, 4–38]. The Pauling rules for ionic crystals apply only to smaller cations with the observed coordination number within 2–4. For bigger cations, the predictive power of the first Pauling rule is very poor. All non-molecular hydroborate crystal structures can be derived by simple deformation of the close-packed anionic lattices, i.e., cubic close packing (ccp) and hexagonal close packing (hcp), or body-centered cubic (bcc), by filling tetrahedral or octahedral sites. This review on the crystal chemistry of hydroborates is a contribution that should serve as a roadmap for materials engineers to design new materials, synthetic chemists in their search for promising compounds to be prepared, and materials scientists in understanding the properties of novel materials. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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9 pages, 700 KiB  
Review
Howard Flack and the Flack Parameter
by David John Watkin and Richard Ian Cooper
Chemistry 2020, 2(4), 796-804; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2040052 - 23 Sep 2020
Cited by 14 | Viewed by 5764
Abstract
The Flack Parameter is now almost universally reported for all chiral materials characterized by X-ray crystallography. Its elegant simplicity was an inspired development by Howard Flack, and although the original algorithm for its computation has been strengthened by other workers, it remains an [...] Read more.
The Flack Parameter is now almost universally reported for all chiral materials characterized by X-ray crystallography. Its elegant simplicity was an inspired development by Howard Flack, and although the original algorithm for its computation has been strengthened by other workers, it remains an essential outcome for any crystallographic structure determination. As with any one-parameter metric, it needs to be interpreted in the context of its standard uncertainty. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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18 pages, 2343 KiB  
Review
When Stereochemistry Raised Its Ugly Head in Coordination Chemistry—An Appreciation of Howard Flack
by Edwin C. Constable and Catherine E. Housecroft
Chemistry 2020, 2(3), 759-776; https://0-doi-org.brum.beds.ac.uk/10.3390/chemistry2030049 - 12 Sep 2020
Cited by 7 | Viewed by 3408
Abstract
Chiral compounds have played an important role in the development of coordination chemistry. Unlike organic chemistry, where mechanistic rules allowed the establishment of absolute configurations for numerous compounds once a single absolute determination had been made, coordination compounds are more complex. This article [...] Read more.
Chiral compounds have played an important role in the development of coordination chemistry. Unlike organic chemistry, where mechanistic rules allowed the establishment of absolute configurations for numerous compounds once a single absolute determination had been made, coordination compounds are more complex. This article discusses the development of crystallographic methods and the interplay with coordination chemistry. Most importantly, the development of the Flack parameter is identified as providing a routine method for determining the absolute configuration of coordination compounds. Full article
(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)
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