Cyclobutane-1,3-diyl analogues Cyclobutane-1,3-diyl Cyclobutane-1,3-diyl is the planar four-membered carbon ring species with radical character localized at the 1 and 3 positions. 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). 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). However, the all-carbon cyclobutane-1,3-diyl is very short-lived and quickly reacts to form the bicyclobutane isomer.
1,3-diphospha-cyclobutane-2,4-diyl In 1995, Niecke and coworkers reported the first synthesis of a
phosphorus analog of cyclobutane-1,3-diyl, [ClC(μ-PMes*)]2. This species consists of a [P2C2]-four-membered heterocycle with radical character centered on the two carbon atoms. The
heterocycle was synthesized from the reaction of aryl(dichloromethylene)phosphene (aryl = Mes*, supermesityl) with
n-butyllithium in a 2:1 ratio, followed by elimination of LiCl (Figure 5).
X-ray diffraction revealed that the [P2C2] unit exists in the planar four-membered ring form, rather than as the bicyclic isomer.
MCSCF calculations predicted a singlet ground state. In addition, the calculated CI wavefunction has contributions from both the doubly occupied HOMO state and the doubly occupied LUMO state; this corresponded to occupation of the HOMO with 1.6 electrons, indicating considerable diradical character. The diphosphacyclobutane heterocycle is thermally stable, and transannular C-C bond formation is thermally forbidden according to the
Woodward-Hoffmann rules. 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. File:Niecke synthetic route towards diradicaloid.jpg|thumb|Figure 5. Synthetic route towards [ClC(μ-PMes*)]2 developed by Niecke et al. Another synthetic route was developed by Yoshifuji and Ito to access a wider variety of substituents at phosphorus (Figure 7). 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. This intermediate can then be alkylated to attach the second phosphorus substituent. This two-step synthetic pathway allows for the synthesis of unsymmetrically substituted 1,3-diphospha-cyclobutane-2,4-diyls. The substituents on carbon are limited to Mes*, however, due to the limitation of the phosphaalkyne starting material. 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.
1,3-diaza-2,4-dipnicta-cyclobutane-2,4-diyl These diradical species consist of a [Pn1(μ-NR)2Pn2] heterocyclic core (Pn =
pnictogen) where the radical sites are centered on the pnictogen atoms. The presence of a
nitrogen atom in the heterocycle is thought to stabilize the planar form relative to the bicyclic isomer. This is believed to result from the inability of Pn-Pn bond formation in the bicyclobutane form to energetically compensate for the increase in Pn-N-Pn angle strain; consequently, the planar form, which allows for larger Pn-N-Pn angles, is more stable. The lack of electron delocalization found in calculations suggests that
aromaticity from the presence of 6π electrons does not play a significant role in stabilization of the planar isomers. 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 synthetic route begins with the chlorinated P2N2 heterocycle, which is then reduced to the diradicaloid with relatively mild titanium(II) or titanium(III) reducing agents (Figure 8). The bulky terphenyl and hypersilyl groups provide kinetic stabilization, preventing dimerization. The terphenyl-substituted diradicaloid is almost indefinitely stable under
argon atmosphere at ambient temperatures as a solid and in solvent. The crystal structure reveals a planar [P2N2] four-membered ring and a long distance between the two phosphorus atoms (2.6186 Å compared to 2.22 Å, the sum of covalent radii), indicating no significant transannular interactions. Computations also support the diradical character of this species and predict a singlet ground state. The calculated CI wavefunction has contributions from both the doubly occupied HOMO state and the doubly occupied LUMO state; this corresponds to occupation of the HOMO with 1.7 electrons, indicating considerable diradical character. File:Schulz synthetic route towards diradicaloid.jpg|thumb|Figure 8. Synthetic route towards a [P2N2] diradicaloid developed by Schulz et al. Using a similar synthetic route, the
arsenic analogue was also synthesized from the chlorinated precursor; reduction using
magnesium metal generated the arsenic centered diradicaloid. The crystal structure confirmed a long As-As distance, and EPR spectroscopy indicated a singlet ground state. A mixed phosphorus-arsenic diradicaloid was also reported in 2015, the first with different radical centers. The crystal structure revealed a kite-shaped planar four membered ring with a transannular As-P distance of 2.790 Å, which is shorter than the sum of van der Waals radii (3.65 Å) but longer than the sum of covalent radii (2.32 Å). 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. Calculations suggest that the antimony and bismuth-centered diradicaloids have higher diradical character than the lighter pnictogen analogues due to the singlet-triplet energy gap decreasing with heavier, larger pnictogens.
Other hetero-cyclobutane-1,3-diyls In 2002, Bertrand and coworkers synthesized the first
1,3-diphospha-2,4-dibora-cyclobutane-2,4-diyl, in which the diradical character is localized on the
boron atoms. In 2009, Schnöckel and coworkers reported the synthesis of a heavier
aluminum-centered diradical analog. A
silicon-centered diradical (1,3-diaza-2,4-disilacyclobutane-2,4-diyl) is also known, synthesized by Sekiguchi and coworkers in 2011. An analog in which the nitrogen atoms are replaced with carbon, as well as an all-silicon cyclobutane-1,3-diyl, have been synthesized. In 2004, Power and coworkers reported the synthesis of a
germanium-centered diradical, the heavier analog of Sekiguchi's silicon diradical. The corresponding
tin-centered diradicals have also been synthesized by Lappert and coworkers in 2004. 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. 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.
Cyclopentane-1,3-diyl analogues Cyclopentane-1,3-diyl Cyclopentane-1,3-diyl is the planar five-membered carbon ring species with radical character localized at the 1 and 3 positions. The triplet diradical was detected by EPR spectroscopy; the diradical species was generated via irradiation of the precursor diazo compound at 5.5 K in a solid matrix (Figure 11). 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. While the triplet state is predicted to be an energy minimum, the singlet state is predicted to be the transition state for housane inversion.
Hetero-cyclopentane-1,3-diyls 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). 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. Isonitriles can also insert into the same diphosphadiazanediyls to form the corresponding heterocyclic 5-membered diradicaloids (Figure 13a). The insertion reaction is sensitive to the steric bulk of the substituent on the isonitrile; for example, the terphenyl-substituted isonitrile was unable to undergo the insertion reaction, while the smaller 2,6-dimethylphenyl isonitrile was able to insert into the P-N bond. File:Isonitrile insertion into 4-membered ring diradicaloid.jpg|thumb|Figure 13. RNC (isonitrile) insertion into (a) [P2N2] and (b) [PAsN2] diradicaloids, reported by Schulz et al. Isonitrile insertion was also explored with mixed phosphorus-nitrogen and phosphorus-arsenic centered 4-membered ring diradicaloids. With the latter compound, the isonitrile selectively inserts into the arsenic-nitrogen bond over the phosphorus-nitrogen bond (Figure 13b). The resulting five-membered ring species was characterized via X-ray structural analysis, confirming the above connectivity (Figure 14). Calculations revealed a substantial diradical character (\beta=0.24), which agrees with the experimentally observed activation of triple bonds. File:Crystal structure of hetero-cyclopentanediyl diradicaloid.gif|thumb|Figure 14. Crystal structure of hetero-cyclopentanediyl diradicaloid resulting from isonitrile insertion into [PAsN2] diradicaloid.
Other main group diradicaloids Diradicaloid 6-membered heterocycles have been reported. In 2020, a
cyclic alkylaminocarbene-stabilized 9,10-diboraanthracene was synthesized. EPR spectroscopy and quantum calculations indicated a singlet diradical ground state, and the incorporation of boron atoms was demonstrated to lower the
HOMO-LUMO energy gap. In 2021, a cyclic germanium-centered diradicaloid with a [C4Ge2] framework was isolated. Calculations indicated a singlet diradical ground state, and the ability of the germanium species to split dihydrogen at room temperature further supported its diradical character. A 1,2-diborete diradicaloid containing a highly strained [B2C2] framework was reported by Braunschweig and coworkers in 2022. In 2024, the first di
borepin diradicals, in which the boron radical sites are disjointed, were synthesized by Gilliard and coworkers. == Reactivity ==