Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013. 

Heteroatom-Based Diradical(oid)s.

Localised singlet diradicals are key intermediates in bond homolysis processes. Generally, these highly reactive species undergo radical–radical coupling reaction immediately after their generation. Therefore, their short-lived character hampers experimental investigations of their nature. In this study, we implemented the new concept of “stretch effect” to access a kinetically stabilised singlet diradicaloid. To this end, a macrocyclic structure was computationally designed to enable the experimental examination of a singlet diradicaloid with π-single bonding character. The kinetically stabilised diradicaloid exhibited a low carbon–carbon coupling reaction rate of 6.4 × 103 s−1 (155.9 μs), approximately 11 and 1000 times slower than those of the first generation of macrocyclic system (7.0 × 104 s−1, 14.2 μs) and the parent system lacking the macrocycle (5 × 106 s−1, 200 ns) at 293 K in benzene, respectively. In addition, a significant dynamic solvent effect was observed for the first time in intramolecular radical–radical coupling reactions in viscous solvents such as glycerin triacetate. This theoretical and experimental study demonstrates that the stretch effect and solvent viscosity play important roles in retarding the σ-bond formation process, thus enabling a thorough examination of the nature of the singlet diradicaloid and paving the way toward a deeper understanding of reactive intermediates.

/pdf/impact-of-the-macrocyclic-structure-and-dynamic-solvent-2v4s36cwr8.pdf

Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character

Transition-metal complexes of cyclopropenes occur as fleeting intermediates of numerous metal-catalyzed organic transformations. A heavier analogue has now been obtained from the reaction of an NHC-stabilized silagermenylidene, bis(1,5-cyclooctadiene)nickel(0), and 1 equiv of N-heterocyclic carbene (NHC). The residual chloro functionality at the germanium end of the coordinated Ge═Si moiety of the thus formed 1H-disilagermirene is easily exchanged by treatment with anionic nucleophiles, which provides access to a series of differently substituted Si2Ge-cyclopropenes as nickel complexes in excellent yields. NMR spectroscopic data, X-ray crystallographic analysis, and DFT calculations indicate a coordination mode different from the metallacyclopropane and π-complex extremes of the Dewar-Chatt-Duncanson model: the σ-component of the Ge═Si double bond acts as donor and acceptor, leaving behind a nearly unsupported Si-Ge π-bond.

Transition-Metal Complexes of Heavier Cyclopropenes: Non-Dewar-Chatt-Duncanson Coordination and Facile Si═Ge Functionalization.

Heavy element analogues of bicyclo[1.1.0]butane have attracted much attention as they often exhibit skeletal isomerisations that are not observed for all-carbon bicyclo[1.1.0]butanes. The reaction of a bicyclo[1.1.0]tetrasil-1(3)-ene with CCl4 at low temperature provided orange crystals of a 1,3-dichlorobicyclo[1.1.0]tetrasilane containing a planar Si4 ring with a π-type bridgehead Si–Si bond. The equilibrium between the bicyclotetrasilane and an isomer, 1-chloro-2-(chlorosilyl)cyclotrisilene in solution, which is an unprecedented interconversion mode of Si4R6 isomers, was identified by NMR spectroscopy and also the formation of its 4-(dimethylamino)pyridine (DMAP) complex.

Interconversion between a planar 1,3-dichlorobicyclo[1.1.0]tetrasilane and a (chloro)(chlorosilyl)cyclotrisilene

This paper presents an ab initio investigation, performed in the framework of density functional theory, on the properties of functionalized few-layer silicene nanosheets, denoted as Si2X2 bilayers and Si4X2 trilayers with X = B, N, Al, and P. Searching for stable phases, we computed the structural, energetic, thermodynamic, dynamic, elastic, and electronic properties of those systems in several stacking configurations, labeled as AA', AB, AA'A'', and ABC. The results revealed that AA'-Si2N2, AB-Si2N2, AA'-Si2P2, and AB-Si2P2 bilayers, as well as ABC-Si4B2, ABC-Si4Al2, AA'A''-Si4P2, and ABC-Si4P2 trilayers are all dynamically stable, based on their respective phonon dispersion spectra. Particularly, there is scarce literature regarding functionalized trilayer silicene systems and, in this work, we found four new nanosheet systems with interesting physical properties and promising applications. Additionally, according to their standard enthalpies of formation and by exploring their electronic properties, we established that those structures could be experimentally accessed, and we discovered that those silicene nanosheets are indirect band gap semiconductors when functionalized with N or P atoms and metallic with B or Al ones. Finally, we envision potential applications for those nanosheets in alkali-metal ion batteries, van der Waals heterostructures, UV-light devices, and thermoelectric materials. 

/pdf/functionalized-few-layer-silicene-nanosheets-stability-cow7wpsc.pdf

Functionalized few-layer silicene nanosheets: stability, elastic, structural, and electronic properties

A carbon–carbon double bond consists of a σ bond and a π bond. Recently, the concept of a π single bond, where a π bond is not accompanied by a σ bond, has been proposed in diradicals containing carbon and heteroatom radical centers. Here we report a closed-shell compound having a silicon–silicon π single bond. 1,2,2,3,4,4-Hexa-tert-butylbicyclo[1.1.0]tetrasilane has a silicon−silicon π single bond between the bridgehead silicon atoms. The X-ray crystallographic analysis shows that the silicon−silicon π single bond (2.853(1) A) is far longer than the longest silicon−silicon bond so far reported. In spite of this unusually long bond length, the electrons of the 3p orbitals are paired, which is confirmed by measurement of electron paramagnetic resonance, and magnetic susceptibility and natural bond orbital analysis. The properties of the silicon−silicon π single bond are studied by UV/Vis and 29Si NMR spectroscopy, and theoretical calculations. Examples of π single bonds in diradicals are rare, as they are typically unstable. Here, the authors describe a room temperature-stable closed-shell bicyclic tetrasilane that features a Si-Si π single bond between two of the non-adjacent Si centers.

/pdf/silicon-silicon-p-single-bond-1tm17og5st.pdf

Silicon–silicon π single bond

/pdf/reply-to-a-double-bond-with-weak-s-and-strong-p-interactions-1b7r3vuv8c.pdf

Reply to: "A double bond with weak σ- and strong π-interactions is still a double bond".

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Silicon–silicon π single bond

Reply to: "A double bond with weak σ- and strong π-interactions is still a double bond".