Unprecedented insertion reaction of a silylene into a B-B bond and generation of a novel borylsilyl anion by boron-metal exchange reaction of the resultant diborylsilane

A kinetically stabilized diarylsilylene, Tbt(Mes)Si: (1, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = mesityl), thermally generated from overcrowded disilene Tbt(Mes)Si=Si(Mes)Tbt (2) or stable silylene-isocyanide complex (3a), was found to insert into a B-B bond of bis(pinacolato)diboron, B2(pin)2 (4), and the boron-lithium exchange reaction of the resulting diborylsilane gave the first borylsilyl anion.     

Tin-chalcogen double-bond compounds, stannanethione and stannaneselone: synthesis, structure, and reactivities

The first isolation of diarylstannanethione (tin-sulfur double-bond compound) and diarylstannaneselone (tin-selenium double-bond compound), Tbt(Ditp)Sn=X (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Ditp = 2,2' '-diisopropyl-m-terphenyl-2'-yl; X = S and Se) was accomplished by dechalcogenation of the corresponding highly hindered tetrachalcogenastannolanes, Tbt(Ditp)SnX(4). The (119)Sn NMR of stannanethione, Tbt(Ditp)Sn=S, and stannaneselone, Tbt(Ditp)Sn=Se, showed only one low-field broad signal at 531 and 440 ppm, respectively, characteristic of a tricoordinated tin, and hence, the stannanethione and stannaneselone display an intrinsic nature of tin-chalcogen double-bond compounds. The X-ray crystallographic analysis of the isolated stannaneselone, Tbt(Ditp)Sn=Se 5a, revealed a completely trigonal geometry around the central tin with a remarkably short Sn-Se bond length, indicative of structural similarity to a ketone.     

The Chemistry of Stable Silabenzenes

Stable silabenzenes ( 1a; R = Tbt, 1b; R = Bbt) were synthesized by taking advantage of extremely bulky and efficient steric protection groups, 2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6‐bis‐[bis(trimethylsilyl)methyl]‐4‐[tris(trimethylsilyl)methyl]phenyl (Bbt). The structure of Tbt‐substituted 1a was determined by X‐ray crystallographic analysis, which demonstrated the complete delocalization of the π‐electrons of the silabenzene ring. It was found that silabenzene 1a reacted with C–C and C–O multiple bond compounds to give the corresponding [4+2]‐cycloadducts via 1,4‐addition, while 1a underwent both 1,2‐ and 1,4‐additions by the reaction with methanol. Silabenzene 1a dimerized very gradually to afford its [4+2]‐dimer, although 1b showed no change under the same conditions. Photochemical reaction of 1a gave the corresponding silabenzvalene isomer instead of the Dewar silabenzene isomer.     

Synthesis and properties of the first stable silylene-isocyanide complexes

The first stable silylene-isocyanide complexes, [Tbt(Mes)SiCNAr] (5 c: Ar=Tip, 5 d: Ar=Tbt, 5 e: Ar=Mes*; Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes=mesityl, Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) were successfully synthesized by the reaction of a kinetically stabilized disilene, [Tbt(Mes)Si=Si(Mes)Tbt] (1), with bulky isocyanides, ArNC (3c-e). The spectroscopic data of 5 c-e and theoretical calculations for a model molecule indicated that 5 c-e are not classical cumulative compounds but the first stable silylene-Lewis base complexes. The reactions of 5 c-e with triethylsilane and 1,3-dienes gave the corresponding silylene adducts, and they underwent isocyanide-exchange reactions in the presence of another isocyanide at room temperature. These results indicate dissociation of complexes 5 c-e to the corresponding silylene 2 and isocyanides 3 c-e under very mild conditions. The reaction of 5 c with methanol gave the MeOH adduct 16, [Tbt(Mes)SiHC(OMe)NTip], which has a hydrogen atom on the silicon atom. This regioselectivity can be explained in terms of the contribution of zwitterionic resonance structures D and E, which have an anion on the silicon atom. This result indicates that 5 c is not a classical cumulene having Si=C double bonds that should react with methanol to give adducts bearing a methoxyl group on the silicon atom. Although the reactions of 5 c-e with electrophilic reagents such as methanol, hydrogen chloride, and methyl iodide gave the formal silylene adducts, the studies on the reaction mechanism by trapping experiments and the observation of the intermediate suggested that the reaction mainly or partially proceeds by initial nucleophilic attack of the silicon atom, as is the case in the formation of 16 in the reaction of 5 c with methanol. It was revealed that 5 c-e show the nucleophilicity of the silicon atom, most likely resulting from the contribution of the zwitterionic resonance structures D and E.     

Oxoborane Formation Turns on Formazanate-Based Photoluminescence

The synthesis of compounds containing multiple bonds to boron has challenged main-group chemists for decades. Despite significant progress, the possibility that the formation of such bonds can turn on photoluminescence has received minimal attention. We report an oxoborane (B=O) complex that is electronically stabilized by a formazanate ligand in the absence of significant steric bulk and, unlike the common BX₂ (X=F, Cl) formazanate adducts, exhibits intense photoluminescence. The latter property was rationalized through density-functional calculations which indicated that the B=O bond enhances photoluminescence by drastically reducing differences between the ligand's geometries in the ground and excited states. The title oxoborane compound was synthesized from an air- and moisture-stable BCl₂ formazanate complex and subsequently converted to a redox-active boroxine. Each of these species may also serve as a precursor to functional materials.     

Theoretical designs for germaacetylene (RC≡GeR'): a new target for synthesis

The effect of substitution on the potential energy surfaces of RC≡GeR (R = F, H, OH, CH(3), SiH(3), Tbt, Ar*, SiMe(SitBu(3))(2) and SiiPrDis(2)) was explored using density functional theories (B3LYP/LANL2DZdp and B3PW91/6-31G(d)). Our theoretical studies indicate that all the triply bonded RC≡GeR species prefer to adopt trans-bent geometry, which is in agreement with the theoretical model (mode (B)). Additionally, we show that the stabilities of the RC≡GeR species bearing smaller substituents (R = F, H, OH, CH(3) and SiH(3)) decrease in the order R(2)C=Ge: > RC≡GeR > :C=GeR(2). On the other hand, the triply bonded RC≡GeR molecules with bulkier substituents (R = Tbt, Ar*, SiMe(SitBu(3))(2), SiiPrDis(2)) were found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. That is to say, both electronic and steric effects of bulky substituents play a crucial role in making triply bonded germaacetylenes (RC≡GeR) an interesting synthetic target.     

Thermolysis,photolysis, and oxidation of an overcrowded 1,3,2-dithiastannetane derivative

Reactions of a sterically crowded 1,3,2-dithiastannetane derivative bearing 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (=Tbt) and 2,4,6-triisopropylphenyl (=Tip) groups on the tin atom are described. Both thermolysis and photolysis of the 1,3,2-dithiastannetane [Tbt(Tip)SnS₂CPh₂] resulted in the formation of products derived from the corresponding stannanethione [Tbt(Tip)Sn=S], while the oxidation reaction by m -chloroperbenzoic acid gave a novel tincontaining heterocyclic system, an 1,2,4,5-oxadithiastannolane derivative.     

Triply bonded stannaacetylene (RC≡SnR): theoretical designs and characterization

The effect of substitution on the potential energy surfaces of RC≡SnR (R = F, H, OH, CH(3), SiH(3), Tbt, Ar*, SiMe(SitBu(3))(2), and SiiPrDis(2)) was explored using density functional theories (B3LYP/LANL2DZdp and B3PW91/Def2-QZVP). Our theoretical investigations indicate that all the triply bonded RC≡SnR molecules prefer to adopt a trans-bent geometry, which is in good agreement with the theoretical model (mode B). In addition, we demonstrate that the stabilities of the RC≡SnR compounds bearing smaller substituents (R = F, H, OH, CH(3), and SiH(3)) decrease in the order R(2)C═Sn: > RC≡SnR > :C═SnR(2). On the other hand, the triply bonded R'C≡SnR' molecules with bulkier substituents (R' = Tbt, Ar*, SiMe(SitBu(3))(2), and SiiPrDis(2)) were found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. Further, we used the B3LYP computations to predict the stability of stannaacetylene bearing the very bulky phosphine ligand. Our theoretical observations strongly suggest that both the electronic and the steric effects of bulky substituents play an important role in making triply bonded stannaacetylene (RC≡SnR) an intriguing synthetic target.     

Synthesis and characterization of functionalized ferrocenylsilanes bearing a bulky substituent

Several types of overcrowded ferrocenylsilanes, Tbt(Fc)SiX₂(Tbt=2, 4, 6-tris[bis(trimethylsilyl)methyl]phenyl, Fc=ferrocenyl, X = H, OH, and Br) were synthesized and characterized. In addition, DFT calculations for ferrocenylsilanediol systems indicated the existence of the intramolecular hydrogen bonding between the Fe atom and H–O moiety in Tbt(Fc)Si(OH)₂. Dedicated to Professor Mitsuo Kira on the occasion of his 2005 Wacker Awardy     

Germabenzenylpotassium: A Germanium Analogue of a Phenyl Anion

Reduction of the stable germabenzene, 1‐Tbt‐2‐tert ‐butyl‐germabenzene (Tbt=2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl), with KC₈ resulted in the formation of 2‐tert ‐butylgermabenzenylpotassium, that is, the germanium analogue of phenylpotassium, under concomitant elimination of the aryl group from the Ge atom. Under an inert atmosphere, this germabenzenylpotassium could be isolated in the form of stable yellow crystals. In the crystalline state, as well as in solution, the germabenzenyl moiety adopts a monomeric form, even though the X‐ray diffraction analysis suggests the presence of highly reactive Ge=C double bonds. The spectroscopic and X‐ray crystallographic analyses, in combination with theoretical calculations indicate an ambident character for this germabenzenyl anion, with contributions from aromatic and germylene resonance structures.     

Synthesis and characterization of a coordinated oxoborane: Lewis acid stabilization of a boron-oxygen double bond

The first example of a stable oxoborane monomer (LB=O) stabilized by complexation to AlCl3 has been prepared by the reaction of LAlCl2 with BCl3 followed by treatment with H2O in CH2Cl2 (L = [HC(CMe)2(NC6F5)2]). DFT calculations reveal that considerable boron-oxygen double bond character is retained in this complex.     

Synthesis and characterization of an extremely hindered tetraaryl-substituted digermene and its unique properties in the solid state and in solution

An extremely hindered digermene (E)-Tbt(Mes)GeGe(Mes)Tbt (1; Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Mes=mesityl) was synthesized. X-ray crystallographic analysis of the hexane solvated single crystal [1·0.5hexane] revealed that 1 has an extremely long germanium–germanium double bond [2.416(2) Å] as that of a carbon-substituted digermene. The temperature-dependent change of UV–Vis absorption of digermene 1 in solution indicated the quantitative interconversion between 1 and the corresponding germylene Tbt(Mes)Ge: (3). The thermodynamic parameters (ΔH=14.7±0.2 kcal mol⁻¹ and ΔS=42.4±0.8 cal mol⁻¹ deg⁻¹) for the dissociation of digermene 1 to germylene 3 were obtained from temperature dependence of the absorption of 1. Since the reactivity of germylene 3 is much higher than that of digermene 1, almost all the intermolecular reactions of 1 in solution proceeded via dissociated 3. For instance, the reaction of 1 with oxygen in solution resulted in a non-stereospecific formation of the cis- and trans-1,3,2,4-dioxadigermetanes 11 and 7 via the initial formation of germanone 12 derived from oxygenation of the dissociated germylene 3. In case of the oxidation in the solid state, however, digermene 1 reacted with O₂ without dissociation to give the corresponding trans-substituted 1,3,2,4-dioxadigermetane stereospecifically. The reaction of digermene 1 with W(CO)₅(THF) was also examined to give the corresponding terminal tungsten complex of the dissociated germylene 3, i.e. Tbt(Mes)GeW(CO)₅ (23), as a marginally stable orange yellow paste.     

Unusual carbon-sulfur bond cleavage in the reaction of a new type of bulky hexathioether with a zerovalent palladium complex

The reaction of a bulky hexathioether, TbtS(o-Phen)S(o-Phen)SS(o-Phen)S(o-Phen)STbt (o-Phen = o-phenylene, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) (1), with 3 molar amounts of Pd(PPh3)4 afforded trinuclear palladium complex bridged by two benzenedithiolato ligands via a three-step palladium insertion reaction into one sulfur-sulfur and two carbon-sulfur bonds of 1.     

Chalcogenation reactions of a stable ferrocenyldiphosphene: Formation of thia‐, selena‐, and telluradiphosphiranes

The reactions of TbtPPTbt (Tbt = 2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl) with elemental sulfur and selenium in the presence of triethylamine resulted in the formation of the corresponding thia‐ and selenadiphosphiranes in 71% and 65%, respectively, whereas the tellurization reaction was unsuccessful probably due to the steric reason. On the other hand, treatment of TbtPPFc (Fc = ferrocenyl) with elemental sulfur, selenium, and tributylphosphine telluride afforded the corresponding thia‐, selena‐, and telluradiphosphiranes in 84%, 88%, and 66% yields, respectively. The molecular structures of these three‐membered heterocyclic compounds were confirmed by spectroscopic analysis. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:443–449, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20449     

Synthesis of an All‐Ferric Cuboidal Iron–Sulfur Cluster [FeIII4S4(SAr)4]

An unprecedented, super oxidized all‐ferric iron–sulfur cubanoid cluster with all terminal thiolates, Fe₄S₄(STbt)₄ ( 3 ) [Tbt=2,4,6‐tris{bis(trimethylsilyl)methyl}phenyl], has been isolated from the reaction of the bis‐thiolate complex Fe(STbt)₂ ( 2 ) with elemental sulfur. This cluster 3 has been characterized by X‐ray crystallography, zero‐field ⁵⁷Fe Mössbauer spectroscopy, cyclic voltammetry, and other relevant physico‐chemical methods. Based on all the data, the electronic ground state of the cluster has been assigned to be S_tot=0.     

The First Rotational Isomers of Stable Selenobenzaldehydes and their η1-Tungsten Complexes

The first rotational isomers of stable selenoaldehydes are synthesized by deselenation of cyclic polyselenides having an efficient steric protection group, 2, 4, 6-tris[bis(trimetylsilyl)methyl]phenyl (Tbt). Reactions of these selenoaldehydes with W(CO)₅.THF gives the corresponding η¹-selenoaldehyde tungsten complexes, the structures of which are established by X-ray crystallography.     

Nucleophilic attack toward group 4 metal complexes bearing reactive 1-Aza-1,3-butadienyl and imido moieties

Unique (1-aza-2-butenyl)titanium complexes bearing a phosphonium ylide moiety [Ti=NTbt{C(Me)(PR3)CH=C(Me)N(Mes)}Cl] (3-5, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = 2,4,6-trimethylphenyl) were formed by the nucleophilic attack of PMe3, P(n-Bu)3, and 1,2-bis(dimethylphosphino)ethane (dmpe) toward the corresponding (1-aza-1,3-butadienyl)titanium complex, [Ti=NTbt{C(Me)CHC(Me)N(Mes)}(mu-Cl)2Li(tmeda)] (2a). The reaction of a lithium beta-diketiminate, [Li{N(Tbt)C(Me)CHC(Me)N(Mes)}] (1) with [TiIICl2(dmpe)2] also resulted in the formation of the same complex 5. Density functional theory calculation indicated that the negative charge of the model molecule of 3 was slightly delocalized to the C3N plane. In addition, the calculation of the model molecule of 2a suggested the electrophilicity of 2a at the carbon atom connecting to the titanium atom. Interestingly, the reaction of zirconium and hafnium analogues (2b and 2c) with PMe3 and dmpe did not proceed. In contrast to the cases of phosphine reagents, pyridine which was found to undergo the nucleophilic attack toward the titanium center of 2a gave the pyridine-coordinated titanium-imide [Ti=NTbt{C(Me)CHC(Me)N(Mes)}Cl(py)] (7).     

Systematic Studies on Homo- and Heteronuclear Doubly Bonded Compounds of Heavier Group 15 Elements

The first stable phosphabismuthene (R 1 --P=Bi--R 2 ) and stibabismuthene (R 1 --Sb=Bi--R 2 ) were successfully synthesized by taking advantage of efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), 2,6-bis[bis(trimethylsilyl)-methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt), and 2,4,6-tri- t -butylphenyl (Mes*). Their spectroscopic properties and structural parameters were systematically compared with those of previously reported stable, homonuclear, doubly bonded systems, such as diphosphene, diarsene, distibene, and dibismuthene.     

Synthesis and properties of stable 2-metallanaphthalenes of heavier group 14 elements

The first stable neutral stannaaromatic compound, 2-stannanaphthalene , was synthesized by taking advantage of an extremely bulky and efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). The molecular structure and aromaticity of were discussed on the basis of X-ray crystallographic analysis, NMR, UV-vis, and Raman spectroscopy, cyclic voltammetry, and theoretical calculations. 2-Germanaphthalene , which has a framework similar to that of , was synthesized for comparison, and systematic elucidation was made for the properties of 2-metallanaphthalene systems containing a heavier group 14 element (Si, Ge, or Sn).     

Stable 2H-azasilirene and 2H-phosphasilirene: Addition reaction of an overcrowded silylene to a nitrile and a phosphaalkyne

In this paper, we present the reaction of an overcrowded silylene bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group with pivalonitrile and t-butylphosphaalkyne to give the corresponding [1+2] cycloadducts, 2H-azasilirene and 2H-phosphasilirene derivatives. This is the first exampleof the isolation of a stable 2H-azasilirene derivative, and the X-ray crystallographic analysis unambiguously revealed its three-membered ring structure in the solid state. In addition, DFT calculations supported three-membered ring character in the structures of the 2H-azasilirene and 2H-phosphasilirene.