Long Carbon–Carbon Bonding beyond 2 Å in Tris(9-fluorenylidene)methane

Abstract

We report on our investigation of C–C bonding longer than 2.0 Å, which can be realized by perpendicularly facing two fluorenyl rings in the title compound. A small orbital overlap between the distantly positioned carbon atoms is observed as a small concentration of electrons on the X-ray electron density map. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the compound originate from the in-phase and out-of-phase interactions of the overlapping orbitals, respectively, with a gap of 2.39 eV. Solid-state 13C NMR spectroscopy shows a sharp peak at 82.9 ppm for the long-bonded carbons, and a CASSCF­(6,6) calculation indicates small diradical character. The experimental and theoretical analyses reveal sufficient covalent-bonding interaction in the long-bonded carbon pair.

Redox‐Active Hydrocarbons: Isolation and Structural Determination of Cationic States toward Advanced Response Systems

“…The X-ray analysis of anionic species consisting of pure hydrocarbons has been achieved only recently. [36][37][38][39][40][41][42][43][44] Since only a few examples of reversible redox response systems are available, they are not considered in this review. Heteroatom-containing hydrocarbons are also excluded from this review because the effect of the introduction of heteroatoms on the redox properties has been well-investigated and reported in several papers.…”

“…In this review, we focus on examples of succeeding in isolation and structural determination of cationic species based on pure hydrocarbons with aromatic rings and introduce the intrinsic electrochemical and spectroscopic properties of such hydrocarbons. The X‐ray analysis of anionic species consisting of pure hydrocarbons has been achieved only recently [36–44] . Since only a few examples of reversible redox response systems are available, they are not considered in this review.…”

Circularly Recyclable Polymers Featuring Topochemically Weakened Carbon–Carbon Bonds

“…For simplicity, these calculations were performed in the gas phase since the intermolecular van der Waals interactions of the condensed phase are typically 1∼2 orders of magnitude smaller than that of covalent bonds. These protocols were validated by calculating the BDE of a few known molecules such as ethane, ethene, and compound 2, which show excellent agreement with reference values. − The DFT results for all bonds and intermediate species are summarized in Tables S1–4 with the BDE and length of the C–C bond connecting the monomers plotted in Figure b. The three polymer backbones exhibit relatively weak bonds (i.e., ranging from ∼40 to ∼50 kcal/mol) at the sterically crowded position between the monomers (Figure b, red dots), while PE, PS, and PMMA exhibit much stronger bonds (∼80 to ∼90 kcal/mol) (Figure b, green dots).…”

“…For example, a hexaphenylethane derivative (compound 1) developed by Mislow and co-workers exhibits a weak C–C bond in the molecule with a bond length of 1.68 Å . A diamondoid (compound 2), synthesized by Schreiner et al, and a polycyclic hydrocarbon (compound 3), synthesized by Ishigaki et al, possess C–C bond lengths beyond 1.7 Å. , Very recently, an impressive extra-long C–C bond length of 2.04 Å was reported by Kubo et al also using a polycyclic hydrocarbon motif . In general, C–C bond lengths have a quasi-linear relationship with bond dissociation energies (BDEs), as evidenced by many experimental and computational results (Figure b, yellow dots) .…”

“…In general, C–C bond lengths have a quasi-linear relationship with bond dissociation energies (BDEs), as evidenced by many experimental and computational results (Figure b, yellow dots) . Importantly, sufficiently elongated bonds are sometimes stabilized in small molecules through secondary intermolecular and intramolecular interactions, as illustrated in compounds 2 and 3. − …”