Cheuk-Wai So

Hydrosilylation of a Silicon(II) Hydride: Synthesis and Characterization of a Remarkable Silylsilylene

The synthesis and characterization of novel monomeric silylsilylenes [{PhC(NtBu)(2)}Si--Si{(NtBu)(2)C(H)Ph}R] (R=Cl (2), H (4)) are described. Compound 2 was prepared by the treatment of [{PhC(NtBu)(2)}SiHCl(2)] (1) with two equivalents of potassium graphite, whereas compound 4 was synthesized by the treatment of 1 with four equivalents of potassium graphite. The results suggest that silicon(II) hydride intermediate [{PhC(NtBu)(2)}SiH] was formed in the reduction, which underwent a hydrosilylation with the amidinate ligand of [{PhC(NtBu)(2)}SiR] (R=Cl or H) to form 2 and 4, respectively. The existence of [{PhC(NtBu)(2)}SiH] in solution was demonstrated by the treatment of [{PhC(NtBu)(2)}SiCl] (3) with [K{HB(iBu)(3)}]. Compounds 2 and 4 have been characterized by X-ray crystallography and NMR spectroscopy. The results show that compounds 2 and 4 are stable in solution or the solid state, and do not dimerize to form the corresponding disilene. DFT calculations show that the Si--Si bonds in 2 and 4 do not have multiple-bond character.

Facile B-H bond activation of borane by stable carbenoid species.

Stable nucleophilic carbene compounds have recently been shown to be able to mimic in some instances the reactivity of metal fragments in the reaction of unactivated E-H bonds (E = H, R3Si, NH2, R2P). However, the insertion into a B-H bond of the strongly Lewis acidic BH3 molecule has never been observed at a single C atom or even at a metal fragment. Our results show that designed stable, highly electrophilic carbenoid fragments in compounds 4 and 6 can achieve this reactivity in a controlled manner. Density functional theory calculations corroborated the experimental results on the presently designed systems as well as the lack of reactivity on nucleophilic carbenes.

Reactivity of digermylenes toward potassium graphite : synthesis and characterization of germylidenide anions

The synthesis and characterization of the digermylenes [LGe-GeL] [L = L(1) (3A), L(2) (3B)] supported by the 2,6-diiminophenyl (L(1)) and 2-imino-5,6-methylenedioxylphenyl (L(2)) ligands are described. Their reactivities toward potassium graphite are also reported. The reaction of [LGeCl] [L = L(1) (2A), L(2) (2B)] with KC(8) in tetrahydrofuran (THF) at room temperature afforded the digermylenes [LGe-GeL] [L = L(1) (3A), L(2) (3B)], which are the first examples of diaryldigermylenes stabilized by o-imino donor(s). The treatment of 3A with 2 equiv of KC(8) in Et(2)O, followed by the addition of excess tetramethylethylenediamine (TMEDA), results in cleavage of the Ge(I)-Ge(I) bond to afford the germylidenide anion [L(1)GeK·TMEDA] (4A). Similarly, the reaction of 3B with excess KC(8) in THF afforded the germylidenide anion [L(2)GeK] (4B). The molecular structures of compounds 4A and 4B as determined by single-crystal X-ray diffraction analysis show that the K atoms are η(1)-coordinated with the low-valent Ge atoms. Moreover, the negative charges at the Ge atoms in compounds 4A and 4B are stabilized by electron delocalization in the germanium heterocycles.

Synthesis and characterization of a singlet delocalized 2,4-diimino-1,3-disilacyclobutanediyl and a silylenylsilaimine

The synthesis and characterization of a singlet delocalized 2,4-diimino-1,3-disilacyclobutanediyl, [LSi(μ-CNAr)(2)SiL] (2, L: PhC(NtBu)(2), Ar: 2,6-iPr(2) C(6) H(3)), and a silylenylsilaimine, [LSi(=NAr)-SiL] (3), are described. The reaction of three equivalents of the disilylene [LSi-SiL] (1) with two equivalents of ArN=C=NAr in toluene at room temperature for 12 h afforded [LSi(μ-CNAr)(2)SiL] (2) and [LSi(=NAr)-SiL] (3) in a ratio of 1:2. Compounds 2 and 3 have been characterized by NMR spectroscopy and X-ray crystallography. Compound 2 was also investigated by theoretical studies. The results show that compound 2 possesses singlet biradicaloid character with an extensive electronic delocalization throughout the Si(2)C(2) four-membered ring and exocyclic C=N bonds. Compound 3 is the first example of a silylenylsilaimine, which contains a low-valent silicon center and a silaimine substituent. A mechanism for the formation of 2 and 3 is also proposed.

Reactivity of a Disilylene [{PhC(NBut)2}Si]2 toward Bromine: Synthesis and Characterization of a Stable Monomeric Bromosilylene

The reaction of the disilylene [{PhC(NBu(t))(2)}Si](2) (1) with 1 equiv of bromine in toluene afforded novel monomeric bromosilylene [{PhC(NBu(t))(2)}SiBr] (2). The result shows that the Si(I)-Si(I) bond in 1 was cleaved by bromine. An X-ray structure of compound 2 has been determined.

Synthesis and characterization of a germanium bismethanediide complex

The reaction of [(PPh(2)=S)(2)CH(2)] (1) with MeLi followed by a salt elimination reaction with GeCl(4) in toluene afforded a novel germanium bismethanediide complex, 2. The structure of compound 2 has been determined by X-ray crystallography. The topological analysis of the electron densities of compound 2 was performed.

An N‐Heterocyclic Silylene‐Stabilized Digermanium(0) Complex

The synthesis of an N-heterocyclic silylene-stabilized digermanium(0) complex is described. The reaction of the amidinate-stabilized silicon(II) amide [LSiN(SiMe3)2] (1; L=PhC(NtBu)2) with GeCl2⋅dioxane in toluene afforded the Si(II)-Ge(II) adduct [L{(Me3Si)2N}Si→GeCl2] (2). Reaction of the adduct with two equivalents of KC8 in toluene at room temperature afforded the N-heterocyclic carbene silylene-stabilized digermanium(0) complex [L{(Me3Si)2N}Si→Ge=Ge←Si{N(SiMe3)2}L] (3). X-ray crystallography and theoretical studies show conclusively that the N-heterocyclic silylenes stabilize the singlet digermanium(0) moiety by a weak synergic donor-acceptor interaction.

Amidinate-Stabilized Group 9 Metal–Silicon(I) Dimer and −Silylene Complexes

The coordination chemistry of the amidinate-stabilized silicon(I) dimer toward group 9 metal complexes is described. The reaction of [LSi:]2 (1, L = PhC(NtBu)2) with [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) in toluene at ambient temperature afforded the base-stabilized silicon(I) dimer-iridium complex [LSi{Ir(cod)-μ-Cl-Ir(cod)}SiL][(cod)IrCl2] (2). In contrast, the reaction of 1 with [Rh(cod)Cl]2 in toluene at ambient temperature afforded a mixture of the amidinate-stabilized silicon(I) dimer-rhodium complex [LSi{Rh(cod)-μ-Cl-Rh(cod)}SiL] (3) and the dimeric amidinate-stabilized rhodosilylene [(LSi)μ-{Rh(μ-Cl)2Rh(cod)}]2 (4). Moreover, the latter reacted with PPh3 to afford a mixture of the Wilkinsons catalyst [(PPh3)3RhCl] and the dimeric rhodosilylene complex [(LSi)μ-{RhCl(PPh3)}]2 (5), which underwent a rearrangement to form the rhodosilylene-phosphidorhodium dimer (LSi)[μ-{RhCl(PPh3)}μ-{RhCl(LSiPh)}](PPh2) (6). Compounds 2-6 were characterized by NMR spectroscopy and X-ray crystallography. In addition, DFT calculations of compound 4 were performed to understand its electronic structure.

Synthesis of a Bent 2-Silaallene with a Perturbed Electronic Structure from a Cyclic Alkyl(amino) Carbene-Diiodosilylene

The cyclic alkyl(amino) carbene (cAAC) 1 reacted with SiI4 in toluene, affording the cAAC-silicon tetraiodide complex [(cAACMe)SiI4] (2, cAACMe = :C(CH2)(CMe2)2NAr, Ar = 2,6-iPr2C6H3). It further reacted with two equivalents of KC8 in toluene at room temperature to afford the first cAAC-diiodosilylene [(cAACMe)SiI2] (3). DFT calculations show that the Ccarbene-Si bond in 3 is formed by the donation of the lone pair of electrons on the Ccarbene atom to the SiI2 moiety, while the π-back-bonding of the lone pair of electrons on the Si atom to the Ccarbene atom is negligible. The presence of the lone pair of electrons on the silicon atom in 3 is also evidenced by its reaction with N3SiMe3 to form the cAAC-silaimine complex [(cAACMe)Si(NSiMe3)I2] (4). Compound 3 reacted with IiPrMe (:C{N(iPr)CMe}2) in n-hexane to form the NHC-cAAC-silyliumylidene iodide [cAACMe(SiI)IiPrMe]I (5), which was then reacted with two equivalents of KC8 in toluene to furnish [cAACMeSi(IiPrMe)] (6). The experimental and theoretical studies suggest that 6 can be described as a bent silaallene with a perturbed electronic structure, which can be attributed to the different donor-acceptor properties of cAACMe and IiPrMe. Compounds 3-6 were elucidated by NMR spectroscopy, X-ray crystallography, and theoretical studies.

Group II Metal Complexes of the Germylidendiide Dianion Radical and Germylidenide Anion

The two-electron reduction of a Group 14-element(I) complex [RË⋅] (E=Ge, R=supporting ligand) to form a novel low-valent dianion radical with the composition [RË:]˙(2-) is reported. The reaction of [LGeCl] (1, L=2,6-(CH=NAr)2C6H3, Ar=2,6-iPr2C6H3) with excess calcium in THF at room temperature afforded the germylidenediide dianion radical complex [LGe]˙(2-)⋅Ca(THF)3(2+) (2). The reaction proceeds through the formation of the germanium(I) radical [LGe⋅], which then undergoes a two-electron reduction with calcium to form 2. EPR spectroscopy, X-ray crystallography, and theoretical studies show that the germanium center in 2 has two lone pairs of electrons and the radical is delocalized over the germanium-containing heterocycle. In contrast, the magnesium derivative of the germylidendiide dianion radical is unstable and undergoes dimerization with concurrent dearomatization to form the germylidenide anion complex [C6H3-2-{C(H)=NAr}Ge-Mg-6-{C(H)-NAr}]2 (3).