Silicon–silicon π single bond

Abstract

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) Å) 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 29 Si NMR spectroscopy, and theoretical calculations.

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

“…[58][59][60][61][62][63][64] Very recently, veand four-membered cyclic silicon analogues of psingle bonded species were reported. 58,65,66 In 2012, macrocyclic structures were designed to kinetically stabilise carbon-carbon singlet diradicaloids (Scheme 2). 67 In these scaffolds, structural rigidity precludes the s bond formation between the radical centres, a phenomenon termed "stretch effect" (Scheme 2a).…”

A double bond with weak σ- and strong π-interactions is still a double bond

“…The structure of the molecule was taken from the Supplementary Information of the original paper by Kyushin et al 4 and reoptimized at two levels of theory, B3LYP/6-31g(d) as the original paper (Table S1 ) and B3LYP/def2-TZVP (Table S2 ) for more accuracy by Gaussian 16 20 . No symmetry was imposed during optimization.…”

“…In a recent study, Kyushin et al reported synthesis and characterization of 1,2,2,3,4,4-hexa- tert -butylbicyclo[1.1.0]tetrasilane ( 2 ), a four-membered singlet diradical silicon ring with a 1,3 silicon-silicon single π-bond and identified a single π-bond by analyses based on canonical molecular orbitals (CMOs) and natural bond orbitals 5 (NBOs). Although MO-based analyses confirm that the two sp 2 hybridized silicon atoms are connected via a π-MO, the HOMO of the molecule 4 , a deep analysis on the basis of topological approaches indicates a different story. Here, I study this system in more details and show that this bond is not a single π-bond, but rather a unique double bond in which the role of σ- and π bonds is reversed, i.e., a weak σ-bond and a strong π-bond.…”

“…Synthesis of 1,3 singlet diradicals in cyclic organic molecules has been the main route toward the formation of single π bonds, i.e., π-bonded atoms without σ-bonding 1 – 4 by forming a long-range π-bond between unpaired electrons of the radicals that occupy p -atomic orbitals. In a recent study, Kyushin et al reported synthesis and characterization of 1,2,2,3,4,4-hexa- tert -butylbicyclo[1.1.0]tetrasilane ( 2 ), a four-membered singlet diradical silicon ring with a 1,3 silicon-silicon single π-bond and identified a single π-bond by analyses based on canonical molecular orbitals (CMOs) and natural bond orbitals 5 (NBOs).…”

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

“…In our original paper 1 , we reported synthesis of 1,2,2,3,4,4-hexa- tert -butylbicyclo[1.1.0]tetrasilane ( 2 , Fig. 1 ).…”