Low-spin to low-spin valence tautomeric transition in cobalt bis-dioxolenes.

Cobalt dioxolenes are a well-known class of switchable coordination compounds showing intramolecular electron transfer, which is always accompanied by a spin state change at the cobalt center. Here, we present the first example of thermally switchable cobalt bis-dioxolenes where intramolecular electron transfer seems to take place, but the spin state change is suppressed. This leads to the detection of thermal transition between a common ls-CoIII(SQ˙-)(Cat2-) and an extremely rare ls-CoII(SQ˙-)2 electronic state (hs - high-spin, ls - low-spin, SQ˙- - benzosemiquinonate(1-) radical and Cat2- - catecholate(2-)). Parallel to the present work, a similar work but on cobalt mono-dioxolenes has just appeared (Chem. Eur. J., 2023, 29, e202300091), suggesting thermal transition between ls-CoIII(Cat2-) and ls-CoII(SQ˙-) electronic states.

Experimental and Theoretical Structure Elucidation of the [2 : 1] Complex Ion of Carbo[n]helicene with n=6, 7 and 8 and Ag.

The front cover artwork illustrates the competition of [6]-, [7]- and [8]helicene for attaining a silver(I) cation. This struggle takes place in the electrospray process during solvent evaporation, leading to the well-known tweezer-like surrounding of Ag+ by the helicene in the [1:1] complex. In this competition, the larger helicenes outperform the smaller ones. The main topic of our investigation, however, is the resulting [2:1] complex in which a second helicene attaches via π-π stacking to the [1:1] tweezer complex. Read the full text of the Research Article at 10.1002/cphc.202300496.

Experimental and Theoretical Structure Elucidation of the [2 : 1] Complex Ion of Carbo[n]helicene with n=6, 7 and 8 and Ag.

Gas-phase complexes of [n]helicenes with n=6, 7 and 8 and the silver(I) cation are generated utilizing electrospray ionization mass spectrometry (ESI-MS). Besides the well-established [1 : 1] helicene/Ag+ -complex in which the helicene provides a tweezer-like surrounding for the Ag+ , there is also a [2 : 1] complex formed. Density functional theory (DFT) calculations in conjunction with energy-resolved collision-induced dissociation (ER-CID) experiments reveal that the second helicene attaches via π-π stacking to the first helicene, which is part of the pre-formed [1 : 1] tweezer complex with Ag+ . For polycyclic aromatic hydrocarbons (PAHs) of planar structure, the [2 : 1] complex with silver(I) is typically structured as an Ag+ -bound dimer in which the Ag+ would bind to both PAHs as the central metal ion (PAH-Ag+ -PAH). For helicenes, the Ag+ -bound dimer is of similar thermochemical stability as the π-π stacked dimer, however, it is kinetically inaccessible. Coronene (Cor) is investigated in comparison to the helicenes as an essentially planar PAH. In analogy to the π-π stacked dimer of the helicenes, the Cor-Ag+ -Cor-Cor complex is also observed. Competition experiments using [n]helicene mixtures reveal that the tweezer complexes of Ag+ are preferably formed with the larger helicenes, with n=6 being entirely ignored as the host for Ag+ in the presence of n=7 or 8.

Silver cation tagged on 5,7,12,14-tetraphenyl-6,13-diazapentacene and its dihydro-form.

The attachment of silver(I) cations to 5,7,12,14-tetraphenyl-6,13-diazapentacene and its reduced dihydro-form has been studied by electrospray ionization mass spectrometry (ESI-MS). The structure elucidation of the Ag+ complexes has been accomplished in gas-phase collision experiments in conjunction with density functional theory (DFT) calculations. The oxidized form provides a favourable cavity for the Ag+ ion, leading to the [1 : 1] complex with the highest resilience towards dissociation and severely hindering the attainment of a second molecular ligand. When the nitrogen is hydrogenated in the reduced dihydro-form, the cavity is partly blocked. This leads to a less strongly bound [1 : 1] complex ion but facilitates the attachment of a second molecular ligand to the Ag+. The resulting complex is the most stable among the [2 : 1] complexes. DFT calculations provide valuable insight into the geometries of the complex ions. Adding silver(I) to the reduced dihydro-form for cationization also induces its oxidation in solution. The oxidative dehydrogenation reaction, for which a mechanism is proposed, proceeds by first order kinetics and is markedly accelerated by day light.

Molecular Valence Tautomeric Metal Complexes for Chemosensing.

Switchable valence tautomeric metal complexes have been long suggested for applications as chemosensors. However, no such molecular sensors have been yet reported. Here, we present a concept for sensing and the first prototype molecular sensor based on valence tautomeric cobalt-dioxolenes. A valence tautomeric cobalt-dioxolene complex [ls-CoIII(SQ•)(Cat)(stypy)2] ⇄ [hs-CoII(SQ•)2(stypy)2] 1 (ls = low spin, hs = high spin, Cat = 3,5-di-tert-butylcatecholate(2-), SQ = one-electron oxidized, benzosemiquinone(1-) form of Cat, stypy = trans-4-styrylpyridine) has been used as a molecular sensor. The lability of axial stypy ligands of 1 in solution allows us to exchange stypy ligands by dimethyl sulfoxide and simple pyridine analytes in a controllable way, which triggers colorimetric and magnetic responses.

Aromatic Nanosandwich Obtained by σ-Dimerization of a Nanographenoid π-Radical.

A 139-π-electron nanographenoid radical was obtained by expanding the periphery of a naphthalimide-azacoronene hybrid with a methine bridge. The radical was isolated in the form of its σ-dimer, which was shown to possess a conformationally restricted two-layer structure both in the solid state and in solution. The dimer is cleaved into its parent radicals when exposed to ultraviolet or visible radiation in toluene solutions but is resistant to thermally induced dissociation. Under inert conditions, the radicals recombine quantitatively into the σ-dimer with observable kinetics, but they are oxidized into a ketone derivative in the presence of atmospheric oxygen. Combined structural, spectroscopic, and theoretical evidence shows that the σ-dimer contains a weak C(sp3)-C(sp3) bond, but is stabilized against thermal dissociation by a very strong dispersive interaction between the overlapping π surfaces.