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Chemistry, Volume 2, Issue 3 (September 2020) – 14 articles

Cover Story (view full-size image): The crystal and molecular structures of the chiral α-halogenated carboxylic acid (R)-2-bromo-3-methylbutyric acid and its racemate are determined. The enantiopure acid features homochiral dimers in the crystal with approximate local C2 symmetry, whereas the racemate exhibits centrosymmetric heterochiral dimers with the same carboxy syn···syn homosynthon. The molecules in both crystal structures adopt an almost identical staggered conformation, despite different crystal environments, which indicates that the preferred molecular conformation is maintained. The crystal packing of the centrosymmetric racemate is denser than that of its enantiopure counterpart. View this paper.
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9 pages, 2574 KiB  
Perspective
Effect of the Enantiomeric Ratio of Eutectics on the Results and Products of the Reactions Proceeding with the Participation of Enantiomers and Enantiomeric Mixtures
by Emese Pálovics, Dorottya Fruzsina Bánhegyi and Elemér Fogassy
Chemistry 2020, 2(3), 787-795; https://doi.org/10.3390/chemistry2030051 - 21 Sep 2020
Cited by 2 | Viewed by 2704
Abstract
This perspective is focused on the main parameters determining the results of crystallization of enantiomers or enantiomeric mixtures. It was shown that the ratio of supramolecular and helical associations depends on the eutectic composition of the corresponding enantiomeric mixture. The M and P [...] Read more.
This perspective is focused on the main parameters determining the results of crystallization of enantiomers or enantiomeric mixtures. It was shown that the ratio of supramolecular and helical associations depends on the eutectic composition of the corresponding enantiomeric mixture. The M and P ratios together with the self-disproportionation (SDE) of enantiomers define the reaction of the racemic compound with the resolving agent. Eventually, each chiral molecule reacts with at least two conformers with different degrees of M and P helicity. The combined effect of the configuration, charge distribution, constituent atoms, bonds, flexibility, and asymmetry of the molecules influencing their behavior was also summarized. Full article
(This article belongs to the Special Issue Organic Chemistry Research in Hungary)
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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://doi.org/10.3390/chemistry2030050 - 16 Sep 2020
Cited by 4 | Viewed by 3425
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
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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://doi.org/10.3390/chemistry2030049 - 12 Sep 2020
Cited by 7 | Viewed by 3381
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
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17 pages, 5548 KiB  
Review
Membrane-Supported Recovery of Homogeneous Organocatalysts: A Review
by Péter Kisszékelyi, Sándor Nagy, Zsuzsanna Fehér, Péter Huszthy and József Kupai
Chemistry 2020, 2(3), 742-758; https://doi.org/10.3390/chemistry2030048 - 27 Aug 2020
Cited by 8 | Viewed by 3756
Abstract
As catalysis plays a significant role in the development of economical and sustainable chemical processes, increased attention is paid to the recovery and reuse of high-value catalysts. Although homogeneous catalysts are usually more active and selective than the heterogeneous ones, both catalyst recycling [...] Read more.
As catalysis plays a significant role in the development of economical and sustainable chemical processes, increased attention is paid to the recovery and reuse of high-value catalysts. Although homogeneous catalysts are usually more active and selective than the heterogeneous ones, both catalyst recycling and product separation pose a challenge for developing industrially feasible methods. In this respect, membrane-supported recovery of organocatalysts represents a particularly useful tool and a valid option for organocatalytic asymmetric synthesis. However, catalyst leaching/degradation and a subsequent decrease in selectivity/conversion are significant drawbacks. As the effectivity of the membrane separation depends mainly on the size of the catalyst in contrast to the other solutes, molecular weight enlargement of small organocatalysts is usually necessary. In the last few years, several synthetic methodologies have been developed to facilitate their recovery by nanofiltration. With the aim of extending the possibilities for the membrane-supported recovery of organocatalysts further, this contribution presents a review of the existing synthetic approaches for the molecular weight enlargement of organocatalysts. Full article
(This article belongs to the Special Issue Organic Chemistry Research in Hungary)
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15 pages, 5017 KiB  
Article
Preparation and Chiral HPLC Separation of the Enantiomeric Forms of Natural Prostaglandins
by Márton Enesei, László Takács, Enikő Tormási, Adrienn Tóthné Rácz, Zita Róka, Mihály Kádár and Zsuzsanna Kardos
Chemistry 2020, 2(3), 727-741; https://doi.org/10.3390/chemistry2030047 - 18 Aug 2020
Viewed by 3907
Abstract
Four enantiomeric forms of natural prostaglandins, ent-PGF ((−)-1), ent-PGE2 ((+)-2) ent-PGF ((−)-3), and ent-PGE1 ((+)-4) have been synthetized in gram scale by Corey synthesis used in the [...] Read more.
Four enantiomeric forms of natural prostaglandins, ent-PGF ((−)-1), ent-PGE2 ((+)-2) ent-PGF ((−)-3), and ent-PGE1 ((+)-4) have been synthetized in gram scale by Corey synthesis used in the prostaglandin plants of CHINOIN, Budapest. Chiral HPLC methods have been developed to separate the enantiomeric pairs. Enantiomers of natural prostaglandins can be used as analytical standards to verify the enantiopurity of synthetic prostaglandins, or as biomarkers to study oxidation processes in vivo. Full article
(This article belongs to the Special Issue Organic Chemistry Research in Hungary)
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13 pages, 2700 KiB  
Review
Results in Chemistry of Natural Organic Compounds. Synthesis of New Anticancer Vinca Alkaloids and Flavone Alkaloids
by Szabolcs Mayer, András Keglevich, Csilla Sepsey Für, Hedvig Bölcskei, Viktor Ilkei, Péter Keglevich and László Hazai
Chemistry 2020, 2(3), 714-726; https://doi.org/10.3390/chemistry2030046 - 10 Aug 2020
Cited by 5 | Viewed by 4336
Abstract
The antitumor indole–indoline alkaloids of the evergreen Catharanthus roseus—namely vinblastine and vincristine—are widely used in chemotherapy of cancer. Many efforts were made to synthesize more efficient derivatives with less side-effect. The 14,15-cyclopropane derivative of vinblastine was synthesized successfully by a five-step procedure [...] Read more.
The antitumor indole–indoline alkaloids of the evergreen Catharanthus roseus—namely vinblastine and vincristine—are widely used in chemotherapy of cancer. Many efforts were made to synthesize more efficient derivatives with less side-effect. The 14,15-cyclopropane derivative of vinblastine was synthesized successfully by a five-step procedure starting from vindoline. Vincristine, vinorelbine and several derivatives condensed with a cyclopropane ring were synthesized. Various hybrid molecules were prepared by the coupling reaction of vindoline and methyl ester of tryptophan, which were conjugated by carrier peptides of octaarginine. Studying the halogenation reactions of vindoline and catharanthine some fluorine derivatives were obtained which showed promising antitumor activity on various tumor types. The synthesis of the Aspidospermane alkaloid bannucine and 5′-epibannucine were carried out using N-acyliminium intermediates. The same intermediate was also applied in the first synthesis of sessiline. The research group have synthesized of flavonoid alkaloids: dracocephins A and B. Further three flavonoid alkaloids, namely 8-(2”-pyrrolidinon-5′′-yl)quercetin, 6-(2′′-pyrrolidinon-5′′-yl)-(−)- and 8-(2′′-pyrrolidinon-5′′-yl)-(−)-epicatechin were prepared by acid-catalyzed regioselective Mannich reaction starting from the corresponding flavonoid precursor. Vindoline was also coupled to synthetic pharmacophores, such as triphenylphosphine and various N-heterocycles. Some of these hybrid molecules showed significant antitumor activity. Furthermore, 7-OH and 7-NH modified flavonoid derivatives were synthesized by a regioselective alkylation followed by Smiles rearrangement and hydrolysis. Full article
(This article belongs to the Special Issue Organic Chemistry Research in Hungary)
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14 pages, 2447 KiB  
Article
Hydrogen and Halogen Bond Mediated Coordination Polymers of Chloro-Substituted Pyrazin-2-Amine Copper(I) Bromide Complexes
by Aaron Mailman, Rakesh Puttreddy, Manu Lahtinen, Noora Svahn and Kari Rissanen
Chemistry 2020, 2(3), 700-713; https://doi.org/10.3390/chemistry2030045 - 05 Aug 2020
Cited by 2 | Viewed by 4070
Abstract
A new class of six mono- (1; 3-Cl-, 2; 5-Cl-, 3; 6-Cl-) and di-(4; 3,6-Cl, 5; 5,6-Cl-, 6; 3,5-Cl-) chloro-substituted pyrazin-2-amine ligands (16) form complexes with copper (I) bromide, to give [...] Read more.
A new class of six mono- (1; 3-Cl-, 2; 5-Cl-, 3; 6-Cl-) and di-(4; 3,6-Cl, 5; 5,6-Cl-, 6; 3,5-Cl-) chloro-substituted pyrazin-2-amine ligands (16) form complexes with copper (I) bromide, to give 1D and 2D coordination polymers through a combination of halogen and hydrogen bonding that were characterized by X-ray diffraction analysis. These Cu(I) complexes were prepared indirectly from the ligands and CuBr2 via an in situ redox process in moderate to high yields. Four of the pyrazine ligands, 1, 46 were found to favor a monodentate mode of coordination to one CuI ion. The absence of a C6-chloro substituent in ligands 1, 2 and 6 supported N1–Cu coordination over the alternative N4–Cu coordination mode evidenced for ligands 4 and 5. These monodentate systems afforded predominantly hydrogen bond (HB) networks containing a catenated (μ3-bromo)-CuI ‘staircase’ motif, with a network of ‘cooperative’ halogen bonds (XB), leading to infinite polymeric structures. Alternatively, ligands 2 and 3 preferred a μ2-N,N’ bridging mode leading to three different polymeric structures. These adopt the (μ3-bromo)-CuI ‘staircase’ motif observed in the monodentate ligands, a unique single (μ2-bromo)-CuI chain, or a discrete Cu2Br2 rhomboid (μ2-bromo)-CuI dimer. Two main HB patterns afforded by self-complimentary dimerization of the amino pyrazines described by the graph set notation R22(8) and non-cyclic intermolecular N–H∙∙∙N’ or N–H∙∙∙Br–Cu leading to infinite polymeric structures are discussed. The cooperative halogen bonding between C–Cl∙∙∙Cl–C and the C–Cl∙∙∙Br–Cu XB contacts are less than the sum of the van der Waals radii of participating atoms, with the latter ranging from 3.4178(14) to 3.582(15) Å. In all cases, the mode of coordination and pyrazine ring substituents affect the pattern of HBs and XBs in these supramolecular structures. Full article
(This article belongs to the Special Issue Supramolecular Chemistry in the 3rd Millennium)
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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://doi.org/10.3390/chemistry2030044 - 30 Jul 2020
Cited by 1 | Viewed by 3234
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
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17 pages, 5940 KiB  
Review
Synthesis and Chemistry of 1,2,3-Benzothiadiazine 1,1-Dioxide Derivatives: A Comprehensive Overview
by Imre Gyűjtő, Gyula Simig, Márta Porcs-Makkay and Balázs Volk
Chemistry 2020, 2(3), 674-690; https://doi.org/10.3390/chemistry2030043 - 21 Jul 2020
Cited by 4 | Viewed by 5182
Abstract
1,2,4-Benzothiadiazine 1,1-dioxide derivatives (e.g., chlorothiazide, hydrochlorothiazide) have been long used in the human therapy as diuretic and antihypertensive agents. Marketed drugs containing the structurally related phthalazinone scaffold are applied for the treatment of various diseases ranging from ovarian cancer to diabetes and allergy. [...] Read more.
1,2,4-Benzothiadiazine 1,1-dioxide derivatives (e.g., chlorothiazide, hydrochlorothiazide) have been long used in the human therapy as diuretic and antihypertensive agents. Marketed drugs containing the structurally related phthalazinone scaffold are applied for the treatment of various diseases ranging from ovarian cancer to diabetes and allergy. 1,2,3-Benzothiadiazine 1,1-dioxides combine the structural features of these two compound families, which led to their more intensive research since the 1960s. In the present review, we summarize the literature of this period of more than half a century, including all scientific papers and patent applications dealing with the synthesis and reactions of this compound family, briefly hinting at their potential therapeutic application as well. Full article
(This article belongs to the Special Issue Organic Chemistry Research in Hungary)
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12 pages, 4826 KiB  
Communication
Peptide Binding Sites of Connexin Proteins
by Ágnes Simon, Csaba Magyar, László Héja and Julianna Kardos
Chemistry 2020, 2(3), 662-673; https://doi.org/10.3390/chemistry2030042 - 14 Jul 2020
Cited by 7 | Viewed by 2940
Abstract
Intercellular gap junction (GJ) contacts formed by the coupling of connexin (Cx) hemichannels (HCs) embedded into the plasma membranes of neighboring cells play significant role in the development, signaling and malfunctions of mammalian tissues. Understanding and targeting GJ functions, however, calls for finding [...] Read more.
Intercellular gap junction (GJ) contacts formed by the coupling of connexin (Cx) hemichannels (HCs) embedded into the plasma membranes of neighboring cells play significant role in the development, signaling and malfunctions of mammalian tissues. Understanding and targeting GJ functions, however, calls for finding valid Cx subtype-specific inhibitors. We conjecture the lack of information about binding interactions between the GJ interface forming extracellular EL1 and EL2 loops and peptide mimetics designed to specifically inhibit Cx43HC coupling to Cx43GJ. Here, we explore active spots at the GJ interface using known peptide inhibitors that mimic various segments of EL1 and EL2. Binding interactions of these peptide inhibitors and the non-peptide inhibitor quinine has been modelled in combination with the use of blind docking molecular mechanics (MM). The neuron-specific Cx36HC and astrocyte-specific Cx43HC subtypes were modelled with a template derived from the high-resolution structure of Cx26GJ. GJ-coupled and free Cx36HC and Cx43HC models were obtained by dissection of GJs (GJ-coupled) followed by 50 ns molecular dynamics (free). Molecular mechanics (MM) calculations were performed by the docking of inhibitors, explicitly the designed Cx43 EL1 or EL2 loop sequence mimetics (GAP26, P5 or P180–195, GAP27, Peptide5, respectively) and the Cx36 subtype-specific quinine into the model structures. In order to explore specific binding interactions between inhibitors and CxHC subtypes, MM/Generalized Born Surface Area (MM/GBSA) ΔGbind values for representative conformers of peptide mimetics and quinine were evaluated by mapping the binding surface of Cx36HC and Cx43HC for all inhibitors. Quinine specifically contacts Cx36 EL1 residues V54-C55-N56-T57-L58, P60 and N63. Blocking the vestibule by the side of Cx36HC entry, quinine explicitly interacts with the non-conserved V54, L58, N63 residues of Cx36 EL1. In addition, our work challenges the predicted specificity of peptide mimetics, showing that the docking site of peptides is unrelated to the location of the sequence they mimic. Binding features, such as unaffected EL2 residues and the lack of Cx43 subtype-specificity of peptide mimetics, suggest critical roles for peptide stringency and dimension, possibly pertaining to the Cx subtype-specificity of peptide inhibitors. Full article
(This article belongs to the Special Issue Organic Chemistry Research in Hungary)
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10 pages, 2404 KiB  
Communication
Unexpected Ethyltellurenylation of Epoxides with Elemental Tellurium under Lithium Triethylborohydride Conditions
by Damiano Tanini and Antonella Capperucci
Chemistry 2020, 2(3), 652-661; https://doi.org/10.3390/chemistry2030041 - 06 Jul 2020
Cited by 3 | Viewed by 3124
Abstract
The one-pot multistep ethyltellurenylation reaction of epoxides with elemental tellurium and lithium triethylborohydride is described. The reaction mechanism was experimentally investigated. Dilithium ditelluride and triethyl borane, formed from elemental tellurium and lithium triethylborohydride, were shown to be the key species involved in the [...] Read more.
The one-pot multistep ethyltellurenylation reaction of epoxides with elemental tellurium and lithium triethylborohydride is described. The reaction mechanism was experimentally investigated. Dilithium ditelluride and triethyl borane, formed from elemental tellurium and lithium triethylborohydride, were shown to be the key species involved in the reaction mechanism. Epoxides undergo ring-opening reaction with dilithium ditelluride to afford β-hydroxy ditellurides, which are sequentially converted into the corresponding β-hydroxy-alkyl ethyl tellurides by transmetalation with triethyl borane, reasonably proceeding through the SH2 mechanism. Full article
(This article belongs to the Special Issue 2020 Profile Papers by Chemistry' Editorial Board Members)
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7 pages, 259 KiB  
Editorial
The Publications of Howard Flack (1943–2017)
by Edwin Charles Constable
Chemistry 2020, 2(3), 645-651; https://doi.org/10.3390/chemistry2030040 - 02 Jul 2020
Cited by 1 | Viewed by 2527
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
19 pages, 7082 KiB  
Article
Supramolecular Assemblies of Trinuclear Copper(II)-Pyrazolato Units: A Structural, Magnetic and EPR Study
by Kaige Shi, Logesh Mathivathanan, Radovan Herchel, Athanassios K. Boudalis and Raphael G. Raptis
Chemistry 2020, 2(3), 626-644; https://doi.org/10.3390/chemistry2030039 - 01 Jul 2020
Cited by 4 | Viewed by 3591
Abstract
Two anionic complexes, {[Cu33-OH)(µ-4-Ph-pz)3Cl3]2[Cu(4-Ph-pzH)4](µ-Cl)2}2− (1) and [Cu33-OH)(µ-pz)31,1-N3)2(N3)] (2), crystallize [...] Read more.
Two anionic complexes, {[Cu33-OH)(µ-4-Ph-pz)3Cl3]2[Cu(4-Ph-pzH)4](µ-Cl)2}2− (1) and [Cu33-OH)(µ-pz)31,1-N3)2(N3)] (2), crystallize as one-dimensional polymers, held together by weak Cu-(µ-Cl) and Cu-(µ-N3) interactions, respectively. Variable temperature magnetic susceptibility analyses determined the dominant antiferromagnetic intra-Cu3 exchange parameters in the solid state for both complexes, whereas the weak ferromagnetic inter-Cu3 interactions manifested also in the solid state EPR spectra, are absent in the corresponding frozen solution spectra. DFT calculations were employed to support the results of the magnetic susceptibility analyses. Full article
(This article belongs to the Special Issue Supramolecular Chemistry in the 3rd Millennium)
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13 pages, 1704 KiB  
Article
Towards a Generalized Synthetic Strategy for Variable Sized Enantiopure M4L4 Helicates
by Stephanie A. Boer, Winnie Cao, Bianca K. Glascott and David R. Turner
Chemistry 2020, 2(3), 613-625; https://doi.org/10.3390/chemistry2030038 - 28 Jun 2020
Cited by 7 | Viewed by 3329
Abstract
The reliable and predictable synthesis of enantiopure coordination cages is an important step towards the realization of discrete cages capable of enantioselective discrimination. We have built upon our initial report of a lantern-type helical cage in attempts to expand the synthesis into a [...] Read more.
The reliable and predictable synthesis of enantiopure coordination cages is an important step towards the realization of discrete cages capable of enantioselective discrimination. We have built upon our initial report of a lantern-type helical cage in attempts to expand the synthesis into a general approach. The use of a longer, flexible diacid ligand results in the anticipated cage [Cu4(L1)4(solvent)4] with a similar helical pitch to that previously observed and a cavity approximately 30% larger. Using a shorter, more rigid ligand gave rise to a strained, conjoined cage-type complex when using DABCO as an internal bridging ligand, [{Co4(L2)4(DABCO)(OH2)x}2 (DABCO)]. The expected paddlewheel motif only forms for one of the Co2 units within each cage, with the other end adopting a “partial paddlewheel” with aqua ligands completing the coordination sphere of the externally facing metal ion. The generic approach of using chiral diacids to construct lantern-type cages is partially borne out, with it being apparent that flexibility in the core group is an essential structural feature. Full article
(This article belongs to the Special Issue Supramolecular Chemistry in the 3rd Millennium)
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