Long but Strong C-C Single Bonds: Challenges for Theory

Theoretical challenges in describing molecules with anomalously long single C-C bonds are analyzed in terms of the relative contributions of stabilizing and destabilizing intramolecular interactions. Diamondoid dimers that are stable despite the presence of C-C bonds up to 1.7 Å long, as well as other bulky molecules stabilized due to intramolecular noncovalent interactions (London dispersions) are discussed. The unexpected stability of highly crowded molecules, such as diamondoid dimers and tert-butyl-substituted hexaphenylethanes, calls for reconsideration of the "steric effect" traditionally thought to destabilize the molecule. Alternatively, "steric attraction" helps to understand bonding in sterically overloaded molecules, whose structural and energetic analysis requires a proper theoretical description of noncovalent interactions.

Carbon-based Biradicals: Structural and Magnetic Switching

Sterically hindered C═C double bonds often deform into a bent or twisted geometry. Thus, many overcrowded ethylenes or anthraquinodimethanes can adopt multiple conformations, such as a folded form or a twisted form, which are interconvertible under the application of external stimuli. A perpendicular form with biradical character can also be adopted when designed to incorporate a stable carbon-based radical unit, which is involved in stimuli-responsive magnetic switching accompanied by a structural change. This review focuses on recent advances in the development of such strained π-electron systems and reveals the factors that affect the mutual interconversion and switching behavior. The energy barrier for the interconversion of conformational isomers is affected by the tricyclic skeleton or bulky substituents on the C═C double bonds, whereas the relative stability of the perpendicular biradical form increases with the additional insertion of 9,10-anthrylene units into the C═C double bonds.

Five shades of green: substituent influence on the (spectro-) electrochemical properties of diferrocenyl(phenyl)methylium dyes

Five new, intensely green diferrocenylphenylmethylium complexes 1+-5+ with electron donating (EDG: 4-MeO, 4-Me, 4-Br) or withdrawing (EWG: 3,5-CF₃, 4-ⁿC₆F₁₃) substituents were synthesized and fully characterized. The substituent influence on their electrochemical and spectroscopic properties was investigated by cyclic voltammetry, UV/Vis/NIR and T-dependent EPR spectroscopy of the cationic as well as the oxidized (12+-52+) and reduced (1˙-5˙) species. The reduced forms equilibrate with their corresponding dimers (65-83%) with a clear substituent influence as expressed by their Hammett parameters in an ordering 4+ > 5+ > 3+ > 2+ > 1+. The structures of all five precursor carbinols 1-OH-5-OH and those of three of the diferrocenylphenylmethylium cations (1+, 4+-5+) were established by X-ray crystallography.

How Long Can A C-C σ-Single Bond Be?

In a specifically designed molecular structure, two sp²-hybridized carbon atoms align their unhybridized 2p_z orbitals in the same orientation to form an extremely long C_2pz-C_2pz σ-single bond that goes beyond 3 Å in length. This new type of C-C σ-bond (coined as a fringe bond) can be made more than 1 Å longer than the current experimental world record (∼1.8 Å) and 300 kJ/mol weaker than the ordinary C-C σ-bond (∼330 kJ/mol). If such molecular monomers are polymerized into infinite chains, the extended long C-C saturated σ-bonding framework manifests band gaps similar to those of some conventional iconic organic-conducting polymers based on conjugated networks of unsaturated bonds.

Controlling Through-Space and Through-Bond Exchange Pathways in Bis-Cobaltocenes for Molecular Spintronics

Pinching molecules via chemical strain suggests intuitive consequences, such as compression at the pinched site and clothespin‐like opening of other parts of the structure. If this opening affects two spin centers, it should result in reduced communication between them. We show that for naphthalene‐bridged biscobaltocenes with competing through‐space and through‐bond pathways, the consequences of pinching are far less intuitive: despite the known dominance of through‐space interactions, the bridge plays a much larger role for exchange spin coupling than previously assumed. Based on a combination of chemical synthesis, structural, magnetic, and redox characterization, and a newly developed theoretical pathway analysis, we can suggest a comprehensive explanation for this non‐intuitive behavior. These results are of interest for molecular spintronics, as naphthalene‐linked cobaltocenes can form wires on surfaces for potential spin‐only information transfer.

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl-Substituted Tritylium Dyes

Four monoferrocenyl tritylium derivatives with donor-substituted (OMe, NMe₂ ) aryl rings are reported, along with their spectroscopic and electrochemical properties. All the complexes show a one-electron reduction and a quasi-reversible ferrocene oxidation at a very positive potential. Small quadrupole splittings, ΔE_Q , in Mößbauer spectra agree with highly electron-deficient ferrocenes. Comparison of the experimental half-wave potentials for ferrocene oxidation, E_1/2 (Fc/Fc⁺ ), with those estimated from established correlations of E_1/2 (Fc/Fc⁺ ) with ΔE_Q indicates that the E_1/2 values of the anisyl-substituted congeners FcOMe⁺ and FcMeOMe⁺ are affected by Coulombic repulsion between the positive charges at the Fe ion and the neighboring methylium site. Electronic spectra are recorded and interpreted with the aid of quantum chemical calculations. UV/Vis spectroelectrochemical measurements as well as chemical reduction provide insight into the redox-induced color changes upon ferrocene oxidation or upon reduction to the neutral trityl radicals. The neutral radicals reversibly form EPR-silent dimers. This process is studied by temperature-dependent EPR spectroscopy, and thermodynamic data for their dimerization are determined. Experimental and quantum chemical data suggest that the dimers assume classical hexaarylethane structures as opposed to normal or "offset" Jacobson-Nauta-type structures.

Expandability of a long C–O bond by a scissor effect in acenaphthofuran

The angle strain induced by ring annulation on a spiro-type naphthofuran weakens its C(sp³)–O bond at the opposite peri-position and endows expandability, so that quite different bond lengths [1.493(3)–1.526(6) Å] are determined for the elongated C(sp³)–O bond of the corresponding spiro-acenaphthofuran derivative upon X-ray analyses of its pseudopolymorphs.

9,10-Dihydrophenanthrene with Two Spiro(dibenzocycloheptatriene) Units: A Highly Strained Caged Hydrocarbon Exhibiting Reversible Electrochromic Behavior

The title dispiro hydrocarbon 1 was designed as a new electrochromic material. This multiply clamped hexaphenylethane-type electron donor was prepared from 2,2′-diiodobiphenyl via biphenyl-2,2′-diylbis(dibenzotropylium) 2²⁺ salt. X-ray analysis of 1 revealed a highly strained structure as reflected by an elongated “ethane” bond [bond length: 1.6665(17) Å] and nearly eclipsed conformation. The weakened bond was cleaved upon two-electron oxidation to regenerate the deeply colored dication 2²⁺. The reversible interconversion between 1 and 2²⁺ is accompanied not only by a drastic color change but also by C–C bond formation/cleavage. Thus, the voltammogram showed a pair of well-separated redox waves, which is characteristic of “dynamic redox (dyrex)” behavior. The tetrahydro derivative of 1 with two units of spiro(dibenzocycloheptadiene), which suffers from more severe steric congestion, was also prepared. The crystallographically determined bond length for the central C–C bond [1.705(4) Å] is greatest among the values reported for 9,9,10,10-tetraaryl-9,10-dihydrophenanthrene derivatives.