The two new disilene compounds, 1,2-bis(thiophen-2-yl)disilene (1) and 1,2-bis(2,2′-bithiophen-5-yl)disilene (2), supported by the fused-ring bulky Eind groups (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) have been obtained as orange and purple crystals, respectively, by the reduction of the corresponding dibromosilanes. Their X-ray structures and spectroscopic properties demonstrate the effective π-conjugation between a Si═Si double bond and thiophene units. Notably, the π-extended 2 exhibits a distinct emission both in solution and in the solid state at room temperature based on the essentially coplanar 1,2-bis(bithienyl)disilene skeleton. The structural features and electronic properties of 1 and 2 have been thoroughly characterized both experimentally and computationally.

AbstractThe reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHCP(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol−1 for 2 b.

AbstractThe reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHCP(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol−1 for 2 b.

(Z)-1,2-Di(1-pyrenyl)disilene containing bulky 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) groups has been obtained as purple crystals by the reductive coupling reaction of the corresponding dibromosilane with lithium naphthalenide. An X-ray crystallographic analysis revealed an Eind- and pyrenyl-meshed molecular gear around the disilene core adopting the Z configuration, in which the two pyrenyl groups intramolecularly interact through the π–π stacking with a distance of 3.635 Å between the centers of the two pyrene rings. The disilene π-system exhibits a π(Si–Si) → π*(pyrene) intramolecular charge-transfer (ICT) fluorescence at room temperature, whose wavelengths depend on the solvent polarity. The photophysical properties are theoretically supported by computational studies including excited-state calculations.

The bulky aryl alcohols, (Rind)OH (1) [Rind = EMind (a) and Eind (b)], based on the rigid fused-ring 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacene skeleton were prepared by the reaction of (Rind)Li with nitrobenzene followed by protonation. The treatment of 1 with nBuLi affords the lithium aryloxide dimers [(Rind)OLi(THF)]2 (2) or trimers [(Rind)OLi]3 (3), depending on the employed solvents (THF = tetrahydrofuran). The salt metathesis reaction of [(EMind)OLi(THF)]2 (2a) with TiCl4(THF)2 leads to the formation of the mononuclear diamagnetic mono- and bis(aryloxide) Ti(IV) complexes, [(EMind)O]TiCl3(THF) (4a) and [(EMind)O]2TiCl2 (5a). We also isolated a trace amount of the tris(aryloxide) Ti(IV) complex, [(EMind)O]3TiCl (6a). The reaction between 2a and TiCl3(THF)3 resulted in the isolation of the mononuclear paramagnetic mono- and bis(aryloxide) Ti(III) complexes, [(EMind)O]TiCl2(THF)2 (7a) and [(EMind)O]2TiCl(THF)2 (8a). The discrete monomeric structures of the titanium complexes 4a, 5a, 6a, 7a, and 8a were determined by X-ray crystallography.

The bulky Eind-based aryllithium, (Eind)Li (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), reacted with BF3·OEt2 in Et2O to form the corresponding aryldifluoroborane (Eind)BF2 (1), along with a trace amount of the sterically congested diarylborane, (Eind)2BH (2). The reaction of 1 with LiAlH4 in THF led to the isolation of the corresponding lithium trihydroborate [Li(thf)]2[(Eind)BH3]2 (3), which can be transformed into the diborane(6), (Eind)HB(μ-H)2BH(Eind) (4), with treatment with Me3SiCl. The Eind-based lithium trihydroaluminate [Li(OEt2)]2[(Eind)AlH3]2 (5) has been synthesized by the reaction of (Eind)Li with LiAlH4 in Et2O. The subsequent addition of Me3SiCl to a solution of 5 in toluene produced the dialumane(6), (Eind)HAl(μ-H)2AlH(Eind) (6), the heavier congener of 4. The dialumane(6) 6 reacted with lithium metal in a mixed solvent of Et2O and toluene to give the diarylalumane, (Eind)2AlH (7), via a disproportionation reaction along with the cleavage and recombination of the Al–C bond. The discrete monomeric structures of 1, 2, and 7 and dimeric structures of 3, 4, 5, and 6 have been determined by X-ray crystallography.

A series of oligo(p-phenylenedisilenylene)s (Si-OPVs 1–4), silicon analogues of oligo(p-phenylenevinylene)s, up to the tetramer have been synthesized and isolated by the introduction of a newly developed protecting group [(HexO)MEind] for improving their solubility. The experimental and theoretical studies of the Si-OPVs 1–4 demonstrate the fully extended π-conjugation of the Si-OPV main chains. Single crystal X-ray analyses of the monomer 1 and the dimer 2 revealed the highly coplanar Si-OPV backbones facilitating the effective extension of the π-conjugation, which has further been validated by the significant increases in the absorption maxima from 465 nm for the monomer 1 to 610 nm for the tetramer 4. The absorption maxima exhibit an excellent fit to Meier’s equation, leading to the estimation of an effective conjugation length (ECL) of 9 repeat units (nECL = 9) and the absorption maximum of 635 nm for the infinite chain (λ∞ = 635 nm). In sharp contrast to other nonemissive disilenes, the Si-OPVs 2–4 show an intense fluorescence from 613 to 668 nm at room temperature with the quantum yields up to 0.48. All the data presented here provide the first evidence for the efficient extended π-conjugation between the Si═Si double bonds and the carbon π-electron systems over the entire Si-OPV skeleton. This study reveals the possibility for developing the conjugated disilene π-systems, in which the Si═Si double bonds would be promising building blocks, significantly optimizing the intrinsic photophysical and electrochemical properties of the carbon-based π-conjugated materials.