Observation of a thermally accessible triplet state resulting from rotation around a main-group π†bond.

We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main-group π bond, that is, the SiSi double bond of tetrakis(di-tert-butylmethylsilyl)disilene (1). The highly twisted ground-state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350-410 K. DFT-calculated zero-field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half-field transition. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet-triplet energy gap of only 7.3 kcal mol(-1) .

Isolable aryl-substituted silyl radicals: synthesis, characterization, and reactivity.

Isolable aryl-substituted silyl radicals (tBu2 MeSi)2(Ar)Si(·) (Ar = C6H5, 4-tBuC6H4, 4-PhC6H4, 3,5-tBu2C6H3) were synthesized by the reaction of the corresponding iodosilane with an equimolar amount of potassium graphite (KC8 ) in tetrahydrofuran (THF). The crystal structure of 3,5-tBu2C6H3 derivative, which was determined by X-ray crystallography, showed a planar geometry around the Si atom for the radical center. EPR studies of all four radicals revealed the lack of the delocalization of the unpaired electron over the aromatic ring. Reactivity and spectroscopic studies of the less-hindered phenyl-substituted silyl radical showed that it exists as an equilibrium mixture of the radical and its silene-type dimer in solution.

An isolable NHC-stabilized silylene radical cation: synthesis and structural characterization.

The silyl-substituted silylene-NHC complex bis(tri-tert-butylsilyl)silylene-(1,3,4,5-tetramethylimidazol-2-ylidene) [((t)Bu(3)Si)(2)Si:←NHC(Me), 2] was synthesized and isolated as air- and moisture-sensitive orange crystals by reductive debromination of the dibromosilane ((t)Bu(3)Si)(2)SiBr(2) (1) with 2.0 equiv of KC(8) in the presence of NHC(Me). In addition, the silylene-NHC complex 2 cleanly underwent one-electron oxidation with 1.0 equiv of Ph(3)C(+)·Ar(4)B(-) (Ar(4)B(-) = tetrakis[4-(tert-butyldimethylsilyl)-2,3,5,6-tetrafluorophenyl]borate) in benzene to afford the NHC-stabilized silylene radical cation [((t)Bu(3)Si)(2)Si←NHC(Me)](•+)·Ar(4)B(-) (3). The radical cation 3 was isolated as air- and moisture-sensitive yellow crystals and structurally characterized by X-ray crystallography and electron paramagnetic resonance spectroscopy, which showed that 3 has a planar structure with a π-radical nature.

A new disilene with π-accepting groups from the reaction of disilyne RSi≡SiR (R = Si(i)Pr[CH(SiMe3)2]) with isocyanides.

The reaction of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne (1) with tert-butylisocyanide or tert-octylisocyanide produced the corresponding disilyne-isocyanide adducts [RSiSiR(CNR')(2)] (R = Si(i)Pr[CH(SiMe(3))(2)](2), R' = (t)Bu (2a) or CMe(2)CH(2)(t)Bu (2b)), which are stable below -30 °C and were characterized by spectroscopic data and, in the case of 2a, X-ray crystallography. Upon warming to room temperature, 2 underwent thermal decomposition to produce 1,2-dicyanodisilene R(NC)Si═Si(CN)R (3) and 1,2-dicyanodisilane R(NC)HSiSiH(CN)R (4) via C-N bond cleavage and elimination of an alkane and an alkene. The 1,2-dicyanodisilene derivative 3 was characterized by X-ray crystallography.

Access to a stable Si2N2 four-membered ring with non-Kekulé singlet biradical character from a disilyne.

The reactions of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne 1 with an equivalent amount of trans- and cis-3,3',5,5'-tetramethylazobenzenes produced a Si(2)N(2) four-membered ring biradicaloid [RSi(µ-NAr)(2)SiR] 2 (R = Si(i)Pr[CH(SiMe(3))(2)](2), Ar = 3,5-Me(2)C(6)H(3)), which was isolated as air- and moisture-sensitive dark purple crystals. Compound 2 displays no EPR signal, and the molecular structure of 2 was characterized by NMR spectroscopy and X-ray crystallography, revealing that 2 has a planar centrosymmetric Si(2)N(2) four-membered ring. The Si1-Si1' distance is 2.63380(9) Å, and there is no bond interaction between the Si1 and Si1' atoms of 2. The reactions of 2 with methanol and carbon tetrachloride show that 2 has both closed-shell and radical-type reactivity.

Isolable p- and m-[((t)Bu2MeSi)2Si]2C6H4: disilaquinodimethane vs triplet bis(silyl radical).

Isomeric p- and m-disilaquinodimethanes 2 and 4 were synthesized by the reductive dehalogenation of the corresponding p- and m-bis(halosilyl)benzenes 1 and 3, respectively, and were isolated and structurally characterized. The X-ray diffraction and solid-state NMR studies of 2 revealed its singlet quinodimethane structure featuring two exocyclic Si═C double bonds with some singlet biradical contribution. In contrast, the X-ray crystallography and EPR measurements of 4 disclosed its biradical nature, described as a triplet ground state bis(silyl radical).

Unsymmetrically substituted disilyne Dsi(2)(i)PrSi-Si≡Si-SiNpDsi(2) (Np = CH(2)(t)Bu): synthesis and characterization.

The unsymmetrically substituted disilyne, Dsi(2)(i)PrSi-Si≡Si-SiNpDsi(2) (Np = CH(2)(t)Bu) 2, was synthesized and characterized by X-ray crystallography to show a trans-bent structure with a silicon-silicon triple bond length of 2.0569(12) Å. The (29)Si chemical shifts of the triply bonded silicon atoms of 2 are quite different, being observed at 62.6 ppm for the Dsi(2)(i)PrSi side and 106.3 ppm for the Dsi(2)NpSi side, indicating different hybridizations on the triply bonded silicon atoms at each site.

Addition of amines and hydroborane to the disilyne RSi[triple bond]SiR (R = Si(i)Pr[CH(SiMe3)2]2) giving amino- and boryl-substituted disilenes.

The reaction of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne 1 with diethylamine or diphenylamine produced the corresponding amino-substituted disilenes R(R'(2)N)Si=SiHR 2a, b (R = Si(i)Pr[CH(SiMe(3))(2)](2), R' = Et (2a), Ph (2b)). The reaction of 1 with 9-borabicyclo[3.3.1]nonane afforded the boryl-substituted disilene R(R''(2)B)Si=SiHR 3 (R''(2)B = 9-borabicyclo[3.3.1]nonan-9-yl). Spectroscopic and X-ray crystallographic analyses of 2a, b, and 3 showed that 2a and 3 have a coplanar arrangement of the Si=Si double bond and amino or boryl groups, giving pi-conjugation between the Si=Si double bond and the lone pair on the nitrogen atom or vacant 2p orbital on the boron atom, whereas 2b exhibits no such conjugation.

Reactivity of the disilyne RSi[triple bond]SiR (R = Si(i)Pr[CH(SiMe3)2]2) toward silylcyanide: two pathways to form the bis-adduct [RSiSiR(CNSiMe3)2] with some silaketenimine character and a 1,4-diaza-2,3-disilabenzene analogue.

The reaction of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne 1 with trimethylsilylcyanide produced the bis-adduct [RSiSiR(CNSiMe(3))(2)] (R = Si(i)Pr[CH(SiMe(3))(2)](2)) 2 and a 1,4-diaza-2,3-disilabenzene analogue 3, which were characterized by spectroscopic data as well as X-ray crystallography (for 2). These two products were produced by different isomers of trimethylsilylcyanide reacting with the disilyne; bis-adduct 2 was formed from the isocyanide form, and compound 3 was produced from the cyanide form followed by cyclization. The bis-adduct 2 is shown on the basis of X-ray crystallography structural data and theoretical studies to exhibit some bis(silaketenimine) character with zwitterionic contributions. The dl-form of dicyano compound 4 was preferentially produced by the reaction of 2 with methanol.

The isolable cation radical of disilene: synthesis, characterization, and a reversible one-electron redox system.

The highly twisted tetrakis(di-tert-butylmethylsilyl)disilene 1 was treated with Ph3C+.BAr4- (BAr4-: TPFPB = tetrakis(pentafluorophenyl)borate) in toluene, producing disilene cation radical 3 upon one-electron oxidation. Cation radical 3 was isolated in the form of its borate salt as extremely air- and moisture-sensitive red-brown crystals. The molecular structure of 3 was established by X-ray crystallography, which showed a highly twisted structure (twisting angle of 64.9 degrees) along the central Si-Si bond with a bond length of 2.307(2) A, which is 2.1% elongated relative to that of 1. The cation radical is stabilized by sigma-pi hyperconjugation by the four tBu2MeSi groups attached to the two central sp2-Si atoms. An electron paramagnetic resonance (EPR) study of the hyperfine coupling constants (hfcc) of the 29Si nuclei indicates delocalization of the spin over the central two Si atoms. A reversible one-electron redox system between disilene, cation radical, and anion radical is also reported.

Solid-state (29)Si NMR study of RSiSiR: a tool for analyzing the nature of the Si-Si bond.

The first solid state 29Si NMR of a disilyne, that is, RSiSiR, R = Si(CH(SiMe3)2)2(i-Pr) (1) was measured: delta11 = 364 +/- 20; delta22 = 221 +/- 16 and delta33 = -350 +/- 13; CSA = -643 ppm. These measured values as well as calculations for model disilynes strongly support the description of the Si-Si bond in bent disilynes as a triple bond, although with weakened pi-bonds and a reduced bond order of 2.6.