Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization.

While the dimerization of heavier group 13 carbene analogues to the corresponding alkene analogues is known and relatively well understood, the dimerization of dicoordinate borylenes (LRB:, L = neutral donor; R = anionic substituent) to the corresponding diborenes (LRB═BRL) has never been directly observed. In this study we present the first example of a formal borylene-to-diborene dimerization through abstraction of a labile phosphine ligand from the tricoordinate hydroborylene precursor (CAAC)(MeP)BH (CAAC = cyclic alkyl(amino)carbene) by bulky Lewis-acidic dihaloboranes (BXY, X = Cl, Br, Y = aryl, boryl), generating the corresponding dihydrodiborene (CAAC)HB═BH(CAAC) and (MeP)BXY as the byproduct. An in-depth experimental and computational mechanistic analysis shows that this seemingly simple process (2 LL'BH + 2 BXY → LHB═BHL + 2 L'BXY) is in fact based on a complex sequence of finely tuned processes, involving the one-electron oxidation of and PMe abstraction from the borylene precursor by BXY, multiple halide transfers between (di)boron intermediates and BXY/[BXY], and multiple one-electron redox processes between diboron intermediates and the borylene precursor, which make the reaction ultimately autocatalytic in [(CAAC)(MeP)BH].

A Stable N-Heterocyclic Silylene with a 1,1'-Ferrocenediyl Backbone.

The N-heterocyclic silylene [{Fe(η -C H -NDipp) }Si] (1DippSi, Dipp=2,6-diisopropylphenyl) shows an excellent combination of pronounced thermal stability and high reactivity towards small molecules. It reacts readily with CO and N O, respectively affording (1DippSiO ) C and (1DippSiO) as follow-up products of the silanone 1DippSiO. Its reactions with H O, NH , and FcPH (Fc=ferrocenyl) furnish the respective oxidative addition products 1DippSi(H)X (X=OH, NH , PHFc).

Synthesis and reduction chemistry of mixed-Lewis-base-stabilised chloroborylenes.

The one-electron reduction of (CAAC)BCl (CAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields the dichloroboryl radical [(CAAC)BCl]˙. Furthermore, the twofold reduction of (CAAC)BCl in the presence of a range of Lewis bases (L = CAAC, N-heterocyclic carbene, phosphine) yields a series of doubly base-supported (CAAC)LBCl chloroborylenes, all of which were structurally characterised. NMR and UV-vis spectroscopic and electrochemical data for (CAAC)LBCl show that the boron centre becomes more electron-rich and the HOMO-LUMO gap widens as L becomes less π-accepting.

Making Use of the Direct Process Residue: Synthesis of Bifunctional Monosilanes.

The industrial production of monosilanes Me SiCl (n=1-3) through the Müller-Rochow Direct Process generates disilanes Me Si Cl (n=2-6) as unwanted byproducts ("Direct Process Residue", DPR) by the thousands of tons annually, large quantities of which are usually disposed of by incineration. Herein we report a surprisingly facile and highly effective protocol for conversion of the DPR: hydrogenation with complex metal hydrides followed by Si-Si bond cleavage with HCl/ether solutions gives (mostly bifunctional) monosilanes in excellent yields. Competing side reactions are efficiently suppressed by the appropriate choice of reaction conditions.

The reductive coupling of dinitrogen.

The coupling of two or more molecules of dinitrogen (N) occurs naturally under the radiative conditions present in the ionosphere and may be achieved synthetically under ultrahigh pressure or plasma conditions. However, the comparatively low N-N single-bond enthalpy generally renders the catenation of the strongly triple-bonded N diatomic unfavorable and the decomposition of nitrogen chains a common reaction motif. Here, we report the surprising organoboron-mediated catenation of two N molecules under near-ambient conditions to form a complex in which a [N] chain bridges two boron centers.

Intercepting the Disilene-Silylsilylene Equilibrium.

The equilibrium between disilenes (R Si=SiR ) and their silylsilylene (R Si-SiR) isomers has previously been inferred but not directly observed, except in the case of the parent system H Si=SiH . Here, we report a new method to prepare base-coordinated disilenes with hydride substituents. By varying the bulk of the coordinating base and other silicon substituents, we have been able to control the rearrangement of disilene adducts to their silylsilylene tautomers.

Reactive Dimerization of an N-Heterocyclic Plumbylene: C-H Activation with Pb.

The N-heterocyclic plumbylene [Fe{(η -C H )NSiMe } Pb:] is in equilibrium with an unprecedented dimer in solution, whose formation involves the cleavage of a strong C-H bond and concomitant formation of a Pb-C and an N-H bond. According to a mechanistic DFT assessment, dimer formation does not involve direct Pb insertion into a cyclopentadienyl C-H bond, but is best described as an electrophilic substitution. The bulkier plumbylene [Fe{(η -C H )NSitBuMe } Pb:] shows no dimerization, but compensates its electrophilicity by the formation of an intramolecular Fe-Pb bond.

Hydrosilane Synthesis by Catalytic Hydrogenolysis of Chlorosilanes and Silyl Triflates.

Hydrogenolysis of the chlorosilanes and silyl triflates (triflate = trifluoromethanesulfonate, OTf) MeSiX (X = Cl, OTf; n = 0, 1) to hydrosilanes at mild conditions (4 bar of H, room temperature) is reported using low loadings (1 mol %) of the bifunctional catalyst [Ru(H)CO( HPNP )] ( HPNP = HN(CHCHP( iPr))). Endergonic chlorosilane hydrogenolysis can be driven by chloride removal, e.g.

Lewis Base Catalyzed Selective Chlorination of Monosilanes.

A preparatively facile, highly selective synthesis of bifunctional monosilanes R SiHCl, RSiHCl and RSiH Cl is reported. By chlorination of R SiH and RSiH with concentrated HCl/ether solutions, the stepwise introduction of Si-Cl bonds is readily controlled by temperature and reaction time for a broad range of substrates. In a combined experimental and computational study, we establish a new mode of Si-H bond activation assisted by Lewis bases such as ethers, amines, phosphines, and chloride ions.

Thermal Synthesis of Perchlorinated Oligosilanes: A Fresh Look at an Old Reaction.

A combined experimental and theoretical study of the high-temperature reaction of SiCl and elemental silicon is presented. The nature and reactivity of the product formed upon rapid cooling of the gaseous reaction mixture is investigated by comparison with the defined model compounds cyclo-Si Cl , n-Si Cl and n-Si Cl . A DFT assessment provides mechanistic insight into the oligosilane formation.

Unraveling the Amine-Induced Disproportionation Reaction of Perchlorinated Silanes-A DFT Study.

A detailed quantum-chemical study on the amine-induced disproportionation reaction of perchlorinated silanes to neo-Si5 Cl12 is reported. The key intermediate in the resulting mechanistic scenario is a dichlorosilylene amine adduct, which is in tune with recent experimental findings. Yet, at variance with the generally accepted notion of silicon-chain growth by concerted silylene insertion into Si-Cl bonds of lower silanes, the formation of neo-Si5 Cl12 follows more complex pathways.

A Disilene Base Adduct with a Dative Si-Si Single Bond.

An experimental and theoretical study of the base-stabilized disilene 1 is reported, which forms at low temperatures in the disproportionation reaction of Si2 Cl6 or neo-Si5 Cl12 with equimolar amounts of NMe2 Et. Single-crystal X-ray diffraction and quantum-chemical bonding analysis disclose an unprecedented structure in silicon chemistry featuring a dative Si→Si single bond between two silylene moieties, Me2 EtN→SiCl2 →Si(SiCl3 )2 . The central ambiphilic SiCl2 group is linked by dative bonds to the amine donor and the bis(trichlorosilyl)silylene acceptor, which leads to push-pull stabilization.

Chloride-induced aufbau of perchlorinated cyclohexasilanes from Si2Cl6: a mechanistic scenario.

A surprisingly simple preparative procedure, addition of Si2Cl6 to a solution of [nBu4N]Cl in CH2Cl2, leads to the formation of the chloride-complexed cyclic dianions [Si6Cl12⋅2Cl](2-), [(SiCl3)Si6Cl11⋅2Cl](2-), or [1,y-(SiCl3)2Si6Cl10⋅2Cl](2-) (y = 1, 3, 4), depending on the stoichiometric ratio of the reactants and the reaction temperature (25-85 °C). Below -40 °C the open-chain oligosilane chloride adducts [Si3Cl9](-), [Si3Cl10](2-), [Si4Cl11](-), and [Si6Cl15](-) are formed, again depending on the reaction conditions chosen. All species were characterized by X-ray crystallography.