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 t Bu)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 CC 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.
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 tBu P, which does not react with either B(C F ) or silylium ions.
The synthesis and structures of the first Si -donor supported manganese(II) complexes [L1]MnCl , [L2]MnCl , and [L3] MnCl 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]MnCl 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, NH , NO , and OH groups at the phenyl group of 1-phenyl substituted alkynes.
Silyliumionen unterliegen einer Einelektronenreduktion mit Phosphanen unter Bildung von kurzlebigen Silylradikalen und den entsprechenden stabilen Phosphanradikalkationen. Experimentelle Untersuchungen, unterstützt durch DFT‐Rechnungen, zeigen, dass Phosphane mit elektronenreichen 2,6‐disubstituierten Arylgruppen ausreichend starke Reduktionsmittel sind, um diese Einelektronenübertragung zu ermöglichen. Frustration, wie sie in kinetisch stabilisierten Triarylsilyliumion/Phosphan‐Lewis‐Paaren auftritt, ist nicht zwingend erforderlich. So gehen Silylphosphoniumionen, die durch konventionelle Lewis‐Adduktbildung aus lösungsmittelstabilisierten Trialkylsilyliumionen und Phosphanen gebildet werden, ebenfalls diesen radikalischen Mechanismus ein. Das Tritylkation, eine Lewis‐Säure mit höherer Elektronenaffinität, oxidiert sogar Trialkylphosphane wie tBu3P, das weder mit B(C6F5)3 noch mit Silyliumionen reagiert.
Reaction of FeX(thf) (X = Cl n = 1.5, Br n = 2) with the chelating 1,1'-bis(silylenyl)-substituted ferrocene ligand SiFcSiA (Fc = ferrocendiyl, Si = PhC(NBu)Si:) furnishes the corresponding dihalido Fe(ii) complexes [(SiFcSi)FeX] (X = Cl, 1 and X = Br, 2) in high yields. Reduction of the latter with an excess of KC in the presence of benzene and toluene leads to the unprecedented bis(silylene) stabilized Fe complexes [(SiFcSi)Fe-η(CH)] 3 and [(SiFcSi)Fe-η(CH)] 4, respectively. The Fe Mössbauer spectrum of 3 at 13 K exhibits parameters (σ = 0.3676 mm s; ΔE = 1.334 mm s) which are consistent with the presence of a pentacoordinated Fe atom in a pseudo trigonal-bipyramidal coordination environment, with two dative Si→Fe bonds and three coordination sites occupied by the η-coordinated arene ligand. Results from DFT calculations, Fe 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 H 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.
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