A Fourfold Gold(I)−Aryl Macrocycle with Hyperbolic Geometry and its Reductive Elimination to a Carbon Nanoring Host

An ethylene glycol‐decorated [6]cyclo‐meta‐phenylene (CMP) macrocycle was synthesized and utilized as a subunit to construct a fourfold AuI2−aryl metallacycle with an overall square arrangement. The corners consist of rigid dinuclear gold(I) complexes previously known to form only triangular metallacycles. The interplay between the conformational flexibility of the [6]CMP macrocycle and the rigid dinuclear gold(I) moieties enable the square geometry, as revealed by single‐crystal X‐ray diffraction. The formation of the gold complex shows size‐selectivity compared to an alternative route using platinum(II) corner motifs. Upon reductive elimination, an all‐organic ether‐decorated carbon nanoring was obtained. Investigation as a host for the complexation of large guest molecules with a suitable convex π‐surfaces was accomplished using isothermal NMR binding titrations. Association constants for [6]cycloparaphenylene ([6]CPP), [7]CPP, C60, and C70 were determined.Fonds der Chemischen Industrie http://dx.doi.org/10.13039/100018992Peer Reviewe

A Planar Five‐Membered Aromatic Ring Stabilized by Only Two π‐Electrons

Many chemicals known today are partially or fully aromatic, since a ring framework experiences additional stabilization through the delocalization of π‐electrons. While aromatic rings with equal numbers of π‐electrons and ring atoms such as benzene are particularly stable, those with the minimally required two π‐electrons are very rare and yet remain limited to three‐ and four‐membered rings if not stabilized in the coordination sphere of heavy metals. Here we report the facile synthesis of a dipotassium cyclopentagallene, a unique example of a five‐membered aromatic ring stabilized by only two π‐electrons. Single‐crystal X‐ray diffraction revealed a planar Ga5 ring with almost equal gallium–gallium bond lengths, which together with computational and spectroscopic data confirm its aromatic character. Our results prove that aromatic stabilization goes far beyond what has previously been assumed as minimum π‐electron count in a five‐atom ring fragment.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Verband der Chemischen Industrie http://dx.doi.org/10.13039/100007215Peer Reviewe

An All-Organic Photochemical Magnetic Switch with Bistable Spin States

Controlling the electronic spin state in single-molecules through an external stimulus is of interest in developing devices for information technology, such as data storage and quantum computing. We report the synthesis and operation mode of two all-organic molecular spin state switches that can be photochemically switched from a diamagnetic (EPR silent) to a paramagnetic (EPR active) form at cryogenic temperatures due to a reversible electrocyclic reaction of its carbon skeleton. Facile synthetic substitution of a configurationally stable 1,14-dimethyl-[5]helicene with radical stabilizing groups at the 4,11-positions afforded two spin state switches as 4,11-dioxo or 4,11-bis(dicyanomethylidenyl) derivatives in a closed diamagnetic form. After irradiation with an LED light source at cryogenic temperatures a stable paramagnetic state is readily obtained, rendering this system a bistable magnetic switch that can reversibly react back to its diamagnetic form through a thermal stimulus. The switching can be monitored with UV/Vis spectroscopy, EPR spectroscopy, or induced by electrochemical reduction and reoxidation. Variable-temperature EPR spectroscopy of the paramagnetic species revealed an open-shell triplet ground state with an experimentally determined triplet–singlet energy gap of < 0.1 kcal/mol. The inherent chirality and the ability to separate the enantiomers turns this helical motif into a potential chiroptical spin state switch. The herein developed 4,11-substitution pattern on the dimethyl[5]helicene introduces a platform for designing future generations of organic molecular photomagnetic switches that might find applications in spintronics and related fields

Halogen Bonding of (Iodoethynyl)benzene Derivatives in Solution

Halogen bonding (XB) between (iodoethynyl)benzene donors and quinuclidine in benzene affords binding free enthalpies (ΔG, 298 K) between −1.1 and −2.4 kcal mol–1, with a strong LFER with the Hammett parameter σpara. The enthalpic driving force is compensated by an unfavorable entropic term. The binding affinity of XB acceptors increases in the order pyridine < C═O < S═O < P═O < quinuclidine. Diverse XB packing motifs are observed in the solid state.ISSN:1523-7060ISSN:1523-705

Confined Spaces in [n]Cyclo-2,7-pyrenylenes

A series of [n]cyclo-2,7-pyrenylenes was synthesized. K-region functionalization of the pyrene moieties led to a confined space within those macrocycles. This confined space is especially pronounced for n=5 and resulted in a host-in-host binding with shape complementary alkaline crown ethers. A multifaceted approach combining computation, spectroscopy, and single-crystal X-ray analysis elucidated the binding. A set of strained aromatic macrocycles based on [n]cyclo-2,7-(4,5,9,10-tetrahydro)pyrenylenes is presented with size-dependent photophysical properties. The K-region of pyrene was functionalized with ethylene glycol groups to decorate the outer rim and thereby confine the space inside the macrocycle. This confined space is especially pronounced for n=5, which leads to an internal binding of up to 8.0×104 m−1 between the ether-decorated [5]cyclo-2,7-pyrenylene and shape-complementary crown ether–cation complexes. Both the ether-decorated [n]cyclo-pyrenylenes as well as one of their host–guest complexes have been structurally characterized by single-crystal X-ray analysis. In combination with computational methods the structural and thermodynamic reasons for the exceptionally strong binding have been elucidated. The presented rim confinement strategy makes cycloparaphenylenes an attractive supramolecular host family with a favorable, size-independent read-out signature and binding capabilities extending beyond fullerene guestsPeer Reviewe

Halogen Bonding of (Iodoethynyl)benzene Derivatives in Solution

Halogen bonding (XB) between (iodoethynyl)benzene donors and quinuclidine in benzene affords binding free enthalpies (ΔG, 298 K) between −1.1 and −2.4 kcal mol–1, with a strong LFER with the Hammett parameter σpara. The enthalpic driving force is compensated by an unfavorable entropic term. The binding affinity of XB acceptors increases in the order pyridine < C═O < S═O < P═O < quinuclidine. Diverse XB packing motifs are observed in the solid state.ISSN:1523-7060ISSN:1523-705

Supramolekulare Bindungstaschen in [n]Cyclo‐2,7‐pyrenylenen

Eine Reihe gespannter aromatischer Makrozyklen basierend auf [n]Cyclo-2,7-(4,5,9,10-tetrahydro)pyrenylenen wurde an der K-Region der Pyreneinheit mit Ethylenglykol-Gruppen funktionalisiert, um im Innenraum des Makrozyklus eine definierte Bindungstasche zu formen. Diese ist für den fünfgliedrigen Makrozyklus besonders ausgeprägt, was zu einer Bindungskonstante von 8.0×104 m−1 zwischen dem Ether-dekorierten [5]Cyclo-2,7-pyrenylen und einem komplementären Kronenether-Kation-Komplex führt. Die Ether-dekorierten [n]Cyclopyrenylene sowie einer der Wirt-Gast-Komplexe wurden durch Kristallstrukturanalyse charakterisiert. In Kombination mit computergestützten Berechnungsmethoden wurden die strukturellen und thermodynamischen Hintergründe für die besonders starken nicht-kovalenten Bindungseigenschaften erklärt. Die hier vorgestellte Strategie zur Formung von Bindungstaschen erschließt die Stoffklasse der Cycloparaphenylene als eine attraktive supramolekulare Wirt-Familie, die größenunabhängige Ausleseparameter und Bindungseigenschaften über Fulleren-Gäste hinaus zeigt.Verband der Chemischen Industrie http://dx.doi.org/10.13039/100007215Peer Reviewe

Photoswitching neutral homoaromatic hydrocarbons

The identification and characterization of homoaromatic compounds bearing an interrupted -system are fundamental challenges for the understanding of electronic interactions in organic molecules. This is further hampered by the fact that no stable neutral homoaromatic hydrocarbon is known. We present the preparation of a new class of neutral homoaromatic compounds, which are supported by experimental evidence (ring current observed by NMR spectroscopy, equalization of bond lengths through X-ray structure analysis) as well as computational analysis (NICS and ACID). Furthermore, we show that one homoaromatic hydrocarbon is a photoswitch through a reversible photochemical [1,11] sigmatropic rearrangement. We provide evidence that the local 6 homoaromatic state vanishes upon photoswitching to produce a purely global 10 homoaromatic state. These results of stable and accessible homoaromatic neutral hydrocarbons and their photoswitching behaviour provide new insights for the understanding and study of homoconjugative interactions in organic molecules, and for the design of new responsive molecular materials

Halogen Bonding of (Iodoethynyl)benzene Derivatives in Solution

Halogen bonding (XB) between (iodoethynyl)­benzene donors and quinuclidine in benzene affords binding free enthalpies (Δ<i>G</i>, 298 K) between −1.1 and −2.4 kcal mol^–1, with a strong LFER with the Hammett parameter σ_para. The enthalpic driving force is compensated by an unfavorable entropic term. The binding affinity of XB acceptors increases in the order pyridine < CO < SO < PO < quinuclidine. Diverse XB packing motifs are observed in the solid state