Search for: All records

Several new bimetallic and trimetallic complexes containing the [Y(COT)2]- unit in different coordination environments are synthesized and fully characterized using X-ray crystallography and NMR spectroscopy. These complexes include [K([2.2.2]cryptand)][Y(COT)2] (1), [K(18-crown-6)(THF)][Y(COT)2] (2), [Y2K2(COT)4(THF)4] (3), and [YKCa(COT)3(THF)3] (4). In the presence of secondary ligands, bimetallic ionic pairs that comprise a [Y(COT)2]- sandwich and a solvent-wrapped K+ ion are formed in 1 and 2. In contrast, 3 represents a bimetallic tetradecker oligomer with two [Y(COT)2]- units bridged by a K+ ion and a terminal [K(THF)4]+ moiety. Complex 4 is a heterotrimetallic triple-decker with an axial arrangement of three metal centers to form [Y(COT)2K(COT)Ca(THF)3]. Structural analysis shows the Y–COTcentroid distances in the [Y(COT)2]- sandwich are equidistant (1.884(6) Å) in 1 but become asymmetric in 2–4 (1.857(3)–1.952(3) Å), reflecting additional external coordination. The two COT rings are parallel in 1 and become increasingly tilted from 0.8° to 27.5° in 2–4, respectively. Multinuclear NMR spectroscopy measurements reveal solution behavior of the sandwich and heterometallic multi-decker COT-based products. The observed NMR spectroscopic trends corroborate with weak interactions between the [Y(COT)2]- unit and cationic moieties persisting in solutions of 2–4. 

An overview of structural responses of helicenes with increasing dimensions and complexity to stepwise electron addition reveals charge- and topology-dependent outcomes ranging from reversible to irreversible core transformations and site-specific reactivity. 

Abstract The two‐fold reduction of tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT, or tetraphenylene,1) with K, Rb, and Cs metals reveals a distinctive core transformation pathway: a newly formed C−C bond converts the central eight‐membered ring into a twisted core with two fused five‐membered rings. This C−C bond of 1.589(3)–1.606(6) Å falls into a single σ‐bond range and generates two perpendicular π‐surfaces with dihedral angles of 110.3(9)°–117.4(1)° in the1_TR²⁻dianions. As a result, the highly contorted1_TR²⁻ligand exhibits a “butterfly” shape and could provide different coordination sites for metal‐ion binding. The K‐induced reduction of1in THF affords a polymeric product with low solubility, namely [{K⁺(THF)}₂(1_TR²⁻)] (K₂‐1_TR²⁻). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb⁺(18‐crown‐6)]₂[1_TR²⁻] (Rb₂‐1_TR²⁻) and [Cs⁺(18‐crown‐6)]₂[1_TR²⁻] (Cs₂‐1_TR²⁻). Both internal and external coordination are observed inK₂‐1_TR²⁻, while the bulky 18‐crown‐6 ligand only allows external metal binding inRb₂‐1_TR²⁻andCs₂‐1_TR²⁻. The reversibility of the two‐fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. Computational analysis shows that the heavier alkali metals enable effective charge transfer from the1_TR²⁻TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected. 

A unique trimeric radical cation with unequal charge distribution is obtained by chemical oxidation of triphenylene with GaCl₃. XRD determined structure is combined with computational modeling showing stabilizing pancake bonding in the π-stacks. 

Abstract Cyclooctatetraene (COT) and COT²⁻dianion are well‐known as archetypical non‐aromatic and aromatic systems, respectively. However, despite a wealth of studies the effect of one electron addition to the eight‐membered ring remains equivocal. Herein, we report the first stepwise electron addition to tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT or tetraphenylene), accompanied by isolation and structural characterization of the mono‐ and doubly‐reduced anions. The X‐ray crystallographic study reveals only a small asymmetric distortion of the saddle‐shaped core upon one electron uptake. In contrast, the doubly‐reduced product exhibits a severely twisted conformation, with a new C−C bond separating the COT ring into two fused 5‐membered rings. The reversibility of the two‐fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. In agreement with experimental results, computational analysis confirms that the reduction‐induced core rearrangement requires the addition of the second electron.