A Striking Mode of Activation of Carbon Disulfide with a Cooperative Bis(silylene).

The reactivity of the 1,4-substituted bis(silylenyl)terphenylene 1, 1,4-[ortho-(LSi)C6 H4 ]2 C6 H4 , (L=RC(NtBu)2 , R=Ph, Mes) towards CS2 is reported. It results in a dearomatization of the phenylene ring, affording the 1,3-substituted cyclohexadiene derivative 2. According to DFT calculations, a transient silene containing a Si=C bond capable of π(C=C) addition at the aromatic phenylene ring is a key intermediate. In contrast, addition of CS2 to the biphenyl-substituted mono-silylene ortho-(LSi)C6 H4 -C6 H5 3 leaves the aromatic π-system intact and forms, in a [1+2] cycloaddition reaction, the corresponding thiasilirane 4 with a three-membered SiSC ring. Further experimental studies led to the isolation of the novel mesoionic five-membered Si2 S2 C heterocycle 6, which reacts with CS2 under C-C bond formation. All isolated new compounds were fully characterized and their molecular structures determined by single-crystal X-ray diffraction analyses.

An Isolable Bis(Silanone-Borane) Adduct.

The reaction of bis(silylenyl)-substituted ferrocene 1 with two molar equivalents of BPh3 yields the corresponding bis(silylene-borane) Lewis adduct 2. The latter is capable to activate CO2 to furnish the borane-stabilized bis(silanone) 3 through mono-oxygenation of the dative SiII →B silicon centers under release of CO. Removal of BPh3 from 3 with PMe3 affords the corresponding 1,3,2,4-cyclodisiloxane and the Me3 P-BPh3 adduct. All isolated new compounds were characterized and their molecular structures were determined by single-crystal X-ray diffraction analyses.

Silicon-Mediated Coupling of Carbon Monoxide, Ammonia, and Primary Amines to Form Acetamides.

For the first time, a direct transformation of CO, NH3 , and primary amines into acetamides, mediated by a main-group element (silicon), is reported. Starting point is the selective deoxygenative reductive homocoupling of two CO molecules by the Fc-bis(silylene) 1 a (Fc=ferrocendiyl) as a reducing agent, which forms the ferrocendiyl-bridged disila(µ-O)(µ-CCO)ketene intermediate 2 a. Exposing 2 a to NH3 (1 bar, 298 K) and benzylamine yields the Fc-disiloxanediamines [Fc(RHNSi-O-SiNHR)] 5 a (R=H) and 5 b (R=benzyl) under release of the respective acetamides H3 CC(O)NHR, as confirmed by 13 C-isotope-labelling experiments. IR and NMR studies of the reaction reveal a four-step mechanism involving an N-silylated carboxamide that can be isolated and fully characterized. The striking reaction mechanism for this unprecedented transformation involves a facile Si-C bond cleavage and ammonolysis of a Si-O bond, and has been demonstrated experimentally and by quantum-chemical calculations.

Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide.

While the transformation of carbon monoxide to multicarbon compounds (fuels and organic bulk chemicals) via reductive scission of the enormously strong CO bond is dominated by transition metals, splitting and deoxygenative reductive coupling of CO under nonmatrix conditions using silicon, the second most abundant nonmetal of the earth's crust, is extremely scarce and mechanistically not well understood. Herein, we report the selective deoxygenative homocoupling of carbon monoxide by divalent silicon utilizing the (LSi:)2Xant 1a [Xant = 9,9-dimethyl-xanthene-4,5-diyl; PhC(N tBu)2] and (LSi:)2Fc 1b (Fc = 1,1'-ferrocenyl) as four-electron reduction reagents under mild reaction conditions (RT, 1 atm), affording the corresponding disilylketenes, Xant(LSi)2(µ-O)(µ-CCO) 2a and Fc(LSi)2(µ-O)(µ-CCO) 2b, respectively. However, the dibenzofuran analogue of 1b, compound 1c, was unreactive toward CO due to the longer distance between the two SiII atoms, which demonstrated the crucial role of the Si···Si distance on cooperative CO binding and activation. This is confirmed by density functional theory (DFT) calculations, and further theoretical investigations on CO homocoupling with 1a and 1b revealed that the initial step of CO binding and scission involved CO acting as a Lewis acid (four-electron acceptor), in sharp contrast to CO activation mediated by transition metals where CO serves as a Lewis base (two-electron donor). This mechanism was strongly reinforced by the reaction of 1a with isocyanide Xyl-NC (Xyl = 2,6-Me2C6H3), isoelectronic with CO. Treatment of 1a with one or two molecules of Xyl-NC furnished the unique (silyl)(imido)silene 3a and the C═C coupled bis(Xyl-NC) product 5, respectively, via the isolable doubly bridged Xant(LSi)2(µ-XylNC)2 intermediate 4. Moreover, compound 3a reacts with 1 molar equivalent of CO to give the disilylketenimine Xant(LSi)2(µ-O)(µ-CCNR) 6, representing, for the first time, a selective heterocoupling product of CO with isoelectronic isocyanide.

Single-Electron Transfer Reactions in Frustrated and Conventional Silylium Ion/Phosphane Lewis Pairs.

Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylsilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylsilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu3 P, which does not react with either B(C6 F5 )3 or silylium ions.

Stereoselective Transfer Semi-Hydrogenation of Alkynes to E-Olefins with N-Heterocyclic Silylene-Manganese Catalysts.

The synthesis and structures of the first SiII -donor supported manganese(II) complexes [L1]MnCl2 , [L2]MnCl2 , and [L3]2 MnCl2 are reported, bearing a pincer-type bis(NHSi)-pyridine ligand L1, bidentate bis(NHSi)-ferrocene ligand L2, and two monodentate NHSi ligands L3 (NHSi = N-heterocyclic silylene), respectively. They act as unprecedented very active and stereoselective Mn-based precatalysts (1 mol % loading) in transfer semi-hydrogenations of alkynes to give the corresponding E-olefins using ammonia-borane as a convenient hydrogen source under mild reaction conditions. Complex [L1]MnCl2 shows the best catalytic performance with quantitative conversion rates and excellent E-stereoselectivities (up to 98 %) for different alkyne substrates. Different types of functional groups can be tolerated, except CN, NH2 , NO2 , and OH groups at the phenyl group of 1-phenyl substituted alkynes.

Bis(silylenyl)-substituted ferrocene-stabilized η6-arene iron(0) complexes: synthesis, structure and catalytic application.

Reaction of FeX2(thf)n (X = Cl n = 1.5, Br n = 2) with the chelating 1,1'-bis(silylenyl)-substituted ferrocene ligand SiFcSiA (Fc = ferrocendiyl, Si = PhC(NtBu)2Si:) furnishes the corresponding dihalido Fe(ii) complexes [(SiFcSi)FeX2] (X = Cl, 1 and X = Br, 2) in high yields. Reduction of the latter with an excess of KC8 in the presence of benzene and toluene leads to the unprecedented bis(silylene) stabilized Fe0 complexes [(SiFcSi)Fe-η6(C6H6)] 3 and [(SiFcSi)Fe-η6(C7H8)] 4, respectively. The 57Fe Mössbauer spectrum of 3 at 13 K exhibits parameters (σ = 0.3676 mm s-1; ΔEQ = 1.334 mm s-1) which are consistent with the presence of a pentacoordinated Fe0 atom in a pseudo trigonal-bipyramidal coordination environment, with two dative Si→Fe bonds and three coordination sites occupied by the η6-coordinated arene ligand. Results from DFT calculations, 57Fe Mössbauer parameters and the diamagnetic NMR spectra confirm the redox-innocent nature of these ligands and the zero oxidation state of the iron center. The catalytic ability of 3 was investigated with respect to ketone hydrogenation. In all cases, good to excellent yields to the corresponding alcohols were obtained at 50 °C and 50 bar H2 pressure. Electron-donating as well as -withdrawing substituents were tolerated with excellent to good yields. Conversions of bulkier ketones and unactivated aliphatic ketones lead merely to moderate yields. This represents the first example of a silylene-iron metal complex which has been utilized as a highly active precatalyst in the hydrogenation of ketones. The results underline the powerful ability of chelating bis(N-heterocyclic silylene) ligands acting as strong σ-donor ligands in stabilizing a new generation of low-valent, electron-rich transition metal complexes for catalytic transformations.