Bonding electrons happy with single life

Atoms are held together in molecules through a network of bonds, common among which are two-electron σ-bonds (σ)2(σ*)0 like that in H2. The situation gets more complicated when we consider one-electron (σ)1(σ*)0 and three-electron σ-interactions (σ)2(σ*)1, which feature in the archetypal species H2+ and Xe2+, respectively. The propensity of electrons to pair up has made it challenging to incorporate two such odd-electron σ-bonds into a single diradical. Yet, a team led by Xinping Wang, Lili Zhao and Gernot Frenking have now met this challenge by installing two pairs of Se atoms on a rigid naphthalene scaffold to give a redox-active diradical they describe in Nature Communications.

First proposed by Linus Pauling in 1931, odd-electron σ-bonding can feature in reactions of radicals with closed-shell species. One can study this by holding two p- or d-block atoms near each other in a cyclic closed-shell structure and see how oxidations and reductions affect σ-bonding. This strategy affords species like [1,8-(BPh2)2-naphthalenediyl], a radical anion with a one-electron B·B bond. Early work in Wang’s group featured this scaffold in three examples of the radical cation [1,8-(EPh)2-naphthalenediyl]+, in which the chalcogen atoms (E = S, Se) participate in three-electron SS, SeS and SeSe σ-bonding.

Credit: David Schilter/Springer Nature Limited

Having developed robust species with three-electron σ-bonds, the team of Wang, Zhao, Frenking and colleagues now aimed to prepare diradicals with two such bonds. Diradicals usually have spins localized in two spatially separated atomic orbitals or delocalized in a π-system — odd-electron σ-bond diradicals were unknown. “We speculated that two-electron oxidation of a tetrachalcogenide on a naphthalene scaffold might give a diradical containing two odd-electron σ-bonds,” reflected Wang. Gratifyingly, cyclic voltammograms of the tetrachalcogenides [1,4,5,8-(EPh)4-naphthalenetetrayl] (E = S, Se) featured two reversible oxidations. Thus, chemical oxidation with two equivalents of NO+ afforded the two new salts [1,4,5,8-(EPh)4-naphthalenetetrayl]{Al[OC(CF3)3]4}2, in which the {Al[OC(CF3)3]4} anion is important for crystallinity and is weakly coordinating, such that it does not perturb the electronic structure of [1,4,5,8-(EPh)4-naphthalenetetrayl]2+.

The S-containing dication [1,4,5,8-(SPh)4-naphthalenetetrayl]2+ has a closed-shell electronic structure and its symmetry can be rationalized by drawing two resonance structures, each with two formally positive S atoms π-bonded to C atoms in same quinone-type rings. Relative to the neutral compound, each pair of adjacent S atoms in the dication are closer, although the Wiberg bond order of 0.19 reflects only weak S–S interactions. The team observed different behaviour for the Se congener [1,4,5,8-(SePh)4-naphthalenetetrayl]2+ (pictured), which exhibits an electron paramagnetic resonance signal similar to that of the monoradical [1,8-(SePh)2-naphthalenediyl]+. The zero-field parameters and weak half-field signal for the biradical confirms that two equivalent spins are in the same molecule, the singlet state of which is slightly more favourable (−0.29 kcal mol−1) than the triplet. The two Se pairs engage in SeSe σ-bonding, with the Wiberg bond order of 0.43 being close to the 0.5 expected for a (σ)2(σ*)1 system.

“it remains unclear whether the unpaired electrons in [1,4,5,8-(SePh)4-naphthalenetetrayl]2+ interact through bonds or through space”

The different behaviours of the S and Se dications reflect the smaller size of S atoms, which are less likely to ‘reach’ each other and more likely to engage in π-bonding with C atoms. The team’s attempts to prepare [1,4,5,8-(TePh)4-naphthalenetetrayl]2+ were met with rapid decomposition, so they are now seeking another scaffold to support a Te dication. “In addition, it remains unclear whether the unpaired electrons in [1,4,5,8-(SePh)4-naphthalenetetrayl]2+ interact through bonds or through space,” concludes Wang.

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Schilter, D. Bonding electrons happy with single life.
Nat Rev Chem (2020).

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