Diradicaloid

[5] Diradicals have historically been characterized as transient species describing the transition state of a bond breaking and/or making process, but recently, the introduction of steric strain to prevent bond formation and substitution of carbon atoms with main-group elements have been found to significantly stabilize diradical species, leading to their isolation and structural characterization.

[7] Diradicaloids have found applications in small molecule activation, molecular switching, nonlinear optics, and spintronics.

Both the open-shell singlet and triplet states must be considered to fully describe the electronic structure of diradical(oid) species.

The covalent component represents the electron configuration in which both localized orbitals are singly occupied; this corresponds to diradical character.

[19] Lastly, if the calculated A-B distance (where A and B are the two radical centers) is elongated compared to the sum of the covalent radii (the typical A-B distance of a closed-shell molecule) but is shorter than the sum of the van der Waals radii, this may also suggest the presence of a diradicaloid.

The singlet cyclobutane-1,3-diyl is predicted to be the transition state for the ring inversion of bicyclobutane, proceeding via homolytic cleavage of the transannular carbon-carbon bond (Figure 3).

[20] A 1,3-dimethyl substituted derivative in the triplet state was detected by electron paramagnetic resonance spectroscopy; the diradical species was generated via irradiation of the precursor diazo compound below 25 K in a solid matrix (Figure 4).

X-ray diffraction revealed that the [P2C2] unit exists in the planar four-membered ring form, rather than as the bicyclic isomer.

Heating at 100 °C in toluene led to the cleavage of the P-C bond, likely generating a ring-opened carbene intermediate that subsequently performed intramolecular C-H activation.

Another synthetic route was developed by Yoshifuji and Ito to access a wider variety of substituents at phosphorus (Figure 7).

[24] 2 equivalents of Mes*-substituted phosphaalkyne can be reacted with the lithiated compound of the first substituent on phosphorus, forming the anionic [P2C2] four-membered ring.

Most diradicaloids of this type can be handled in air and display high kinetic stability due to the steric protection provided by the Mes* substituents on the carbon radical centers.

The presence of a nitrogen atom in the heterocycle is thought to stabilize the planar form relative to the bicyclic isomer.

[25] In 2011, Schulz and coworkers synthesized the first example of a [P2N2] four-membered ring diradicaloid (here, Pn = phosphorus) with meta-terphenyl and hypersilyl substituents on the nitrogen atoms.

The terphenyl-substituted diradicaloid is almost indefinitely stable under argon atmosphere at ambient temperatures as a solid and in solvent.

The crystal structure confirmed a long As-As distance, and EPR spectroscopy indicated a singlet ground state.

[28] Heavier derivatives (where Pn = antimony and bismuth) were observed in situ but could not be isolated due to rapid decomposition to the allyl analogues in the presence of magnesium; however, the corresponding diradicaloids could be trapped through [2+2] cycloadditions with alkynes, thereby providing evidence for their existence.

[36] In 2017, N-heterocyclic carbene-stabilized phosphorus-centered diradicals were reported; like the Niecke-type diradicaloid, the core heterocycle is a [P2C2] four-membered ring, but the radical centers are located on phosphorus rather than carbon.

[37] Lastly, one of the first hetero-cyclobutanediyl derivates synthesized is N2S2, disulfur dinitride, but its diradical character has been widely discussed in the literature and is still disputed today.

[39][40] Due to its very short lifetime, all-carbon cyclopentane-1,3-diyl cannot be isolated, but heating cyclopentane-1,3-diyl leads to the formation of a transannular C-C bond, producing the housane isomer.

[41] Five-membered diradicals with radical character localized on pnictogen atoms can be synthesized via the insertion of carbon monoxide and isonitriles into the corresponding pnictogen-centered cyclobutane-1,3-diyls.

In 2015, Schulz and coworkers reported the first stable cyclopentane-1,3-diyl species generated from the ring expansion of terphenyl-substituted diphosphadiazanediyl using carbon monoxide (Figure 12).

[42] The computed structural data support an almost planar five-membered ring, and the HOMO/LUMO contributions to the CI wavefunction indicate an occupation of the HOMO with 1.44 electrons, suggesting diradical character.

Experimentally, additions of phosphaalkyne and elemental sulfur across the phosphorus atom are consistent with diradicaloid reactivity.

[44] The resulting five-membered ring species was characterized via X-ray structural analysis, confirming the above connectivity (Figure 14).

[45] EPR spectroscopy and quantum calculations indicated a singlet diradical ground state, and the incorporation of boron atoms was demonstrated to lower the HOMO-LUMO band gap.

[48] Diradicaloids, depending on the reaction conditions and extent of diradical character, can display both closed-shell and open-shell reactivity.

[51] For example, the phosphorus-centered diradicaloid [P(μ-NTer)2]2 can undergo stepwise radical addition reactions with alkyl bromides (Figure 18).

[11] Upon exposure to red light, the planar five-membered ring diradical isomerizes to the bicyclic housane species.

After irradiation, the thermally induced reverse reaction occurs, breaking the transannular bond to regenerate the planar diradicaloid species.

Figure 1. The occupation of 2 orbitals with 2 electrons results in 6 different electronic configurations. These configurations need to be considered to describe the singlet and triplet states of a diradical species.
Figure 2. Change of basis from delocalized molecular orbital basis to localized atomic orbital basis. Reprinted with permission from Chem. Rev. 2023, 123, 16, 10468–10526. Copyright 2023 American Chemical Society.
Figure 3. The ring inversion of bicyclobutane is proposed to involve a biradical transition state in which the transannular C-C bond is broken.
Figure 4. Generation of a longer-lived cyclobutane-1,3-diyl species through N2 extrusion.
Figure 5. Synthetic route towards [ClC(μ-PMes*)]2 developed by Niecke et al.
Figure 7. Alternative synthetic route towards 1,3-diphospha-cyclobutane-2,4-diyl compounds developed by Yoshifuji and Ito et al.
Figure 8. Synthetic route towards a [P2N2] diradicaloid developed by Schulz et al.
Figure 11. Generation of cyclopentane-1,3-diyl through N2 extrusion.
Figure 12. CO insertion generates hetero-cyclopentane-1,3-diyl, reported by Schulz et al.
Figure 13. RNC (isonitrile) insertion into (a) [P2N2] and (b) [PAsN2] diradicaloids, reported by Schulz et al.
Figure 14. Crystal structure of hetero-cyclopentanediyl diradicaloid resulting from isonitrile insertion into [PAsN2] diradicaloid.
Figure 15. Examples of closed-shell reactivity of [P(μ-NTer)2]2.
Figure 17. Reactivity of the zwitterionic form of [P(μ-NTer)2]2 with Lewis bases and Lewis acids.
Figure 18. Example of open-shell reactivity of [P(μ-NTer)2]2.
Figure 19. A phosphorus-centered cyclopentane-1,3-diyl diradicaloid demonstrates molecular switching behavior. Exposure to red light generates the bicyclic closed-shell species, turning “off” isonitrile insertion reactivity.