Carsten Präsang

An Ylide-like Phosphasilene and Striking Formation of a 4π-Electron, Resonance-Stabilized 2,4-Disila-1,3-diphosphacyclobutadiene

The first N-donor-stabilized phosphasilene LSi(SiMe(3))═PSiMe(3) (L = PhC(NtBu)(2)) has been synthesized in 87% yield through 1,2-silyl migration of the (Me(3)Si)(2)P-substituted, N-heterocyclic silylene [LSi-P(SiMe(3))(2)]. Remarkably, the latter reacts with dichlorotriphenylphosphorane Ph(3)PCl(2) to give the unprecedented 4π-electron Si(2)P(2)-cycloheterobutadiene [(LSi)(2)P(2)] with two-coordinate phosphorus atoms. The striking molecular structures as well as the (29)Si and (31)P NMR spectroscopic features of both products indicate the presence of zwitterionic Si═P bonds which is also in accordance with results by DFT calculations.

The Striking Stabilization of Ni0(η6-Arene) Complexes by an Ylide-Like Silylene Ligand†

The right mix does the trick: Elusive {Ni(0)(eta(6)-arene)} moieties can be dramatically stabilized by the N-heterocyclic silylene ligand 1, which has a zwitterionic mesomeric structure. The sigma, pi-acid-base synergism between nickel and 1 explains the unexpectedly high stability of the new silylene complexes 2, which enables arene exchange studies at a Ni(0) center. Addition of B(C(6)F(5))(3) to 2 affords the zwitterionic silylene complex 3 (see scheme, R=2,6-iPr(2)C(6)H(3)).

Hydrosilylation of alkynes by Ni(CO)3-stabilized silicon(II) hydride.

Not copy and paste: Although β-diketiminato ligands have been employed for the stabilization of Ge(II) and Sn(II) hydrides, the corresponding Si(II) hydride is not accessible. However, coordination of silicon(II) to a {Ni(CO)(3)} fragment allowed the isolation of the first Si(II) hydride metal complex 1. This complex was used for the first silicon(II)-based and Ni(0)-mediated, stereoselective hydrosilylation of alkynes. R = phenyl, tolyl.

Metal-Free Activation of EH3 (E=P, As) by an Ylide-like Silylene and Formation of a Donor-Stabilized Arsasilene with a HSiAsH Subunit†

that have inspired and encouraged many research groups to gain access to novel types of silicon-containing functional groups. This work has recently enabled the discovery of new reactivity patterns, such as the metal-free activation of N H bonds of ammonia or amines with multiply bonded silicon. Apart from one example published by Kira et al., all known isolable silylenes are stabilized by adjacent nitrogen donor(s) and can be classified as N-heterocyclic silylenes (NHSi molecules) or donor-stabilized silicon(II) compounds. The reactivity of NHSis is comparable to that of well-known N-heterocyclic carbenes (NHCs), with the silicon center and the Si N bonds being the predominant reactive sites. Because of the yilde-like (zwitterionic) electronic structure of silylene 2 (Scheme 1), its chemistry is distinctively different from that of other known NHSi molecules. Compound 2 exhibits an electron-rich butadiene moiety in the backbone that can be utilized for the metal-free activation of E H bonds or the addition of Lewis acids. NHSi 2 thus features three reactive sites instead of two: The basic silicon lone pair, a formally empty (acidic) 3p orbital at silicon, and a basic butadiene moiety. The straightforward formation of silathioformamide 3 from 2 and H2S [10] is the only example to date where all three of these sites have been involved in a single reaction. However, it is possible to selectively trigger one or two of the donor–acceptor functional groups if the right choice of reagent is made. For example, strong Lewis and Brønsted acids add to the exocyclic electron-rich methylene group (compounds 4 a,b, Scheme 1), whereas the silicon(II) lone pair can be utilized as a strong s donor for the preparation of silylene–metal complexes, as shown for complex 5 in which the zwitterionic character of the ligand is retained. The 1,1-addition to the silicon center, which is also well-documented for other NHSis, and the 1,4-addition across the C3N2Si heterocycle are the most common reactivity features of 2. Both pathways have even been observed simultaneously during the reaction of water with two equivalents of NHSi 2 to yield donor-stabilized siloxysilylene 6 (Scheme 1). It has been shown that 2 and even the {Ni(CO)3} complex 5 are capable of activating the N H bonds of ammonia and hydrazines, C H bonds of alkynes and arenes, and C F bonds of fluoroarenes to eventually yield the corresponding thermodynamic 1,1-addition products. As part of our ongoing investigations on ylide-like silylene 2, we were curious about its reactivity towards the more Brønsted acidic heavier Group 15 hydrides EH3 (E = P, As). This reactivity could possibly lead to unprecedented donor-stabilized Si=E multiple bonds akin to the formation of silathioformamide 3 formed from 2 and H2S. Herein we present the strikingly different reactivity of 2 towards PH3 and AsH3. Remarkably, whilst PH3 leads merely to the 1,1-addition product, the activation of AsH3 furnishes a donor-stabilized arsasilene (silylidenearsane, Si=As) with a HSi = AsH subunit in a twostep process, which could be isolated in the form of deep blue crystals. The reaction of 2 with a twentyfold excess of PH3 furnishes merely the 1,1-addition product, that is, silylphosphane 7a, even at low temperatures (Scheme 2). The progress of the PH3 addition can easily be monitored by the slow disappearance of the characteristic yellow color of 2 in Scheme 1. Isolable N-heterocyclic silylenes 1 and 2 and adducts 3–6 derived from silylene 2. LA =H, (C6F5)3B ; Ar= 2,6-iPr2C6H3.

Addition of [(η5-C5Me5)IrH4] to a zwitterionic silylene: stepwise formation of iridium(V)-silyl and iridium(III)-silylene complexes

The first zwitterionic silyl-iridium(v) complex is generated by insertion of silylene into an iridium-hydrogen bond of the iridium(v) hydride , [(eta(5)-C(5)Me(5))IrH(4)]. Complex undergoes proton migration from an IrH moiety to the terminal CH(2) group of the silyl ligand to furnish the N-donor stabilised Ir(iii)-silylene complex .

N-Heterocyclic Silylene (NHSi) Rhodium and Iridium Complexes: Synthesis, Structure, Reactivity, and Catalytic Ability

Reaction of the zwitterionic N-heterocyclic silylene (NHSi) 1 L′Si: (L′u2009=u2009[HC(CMeNAr)(C(CH2)NAr)], Aru2009=u20092,6-iPr2C6H3) with HCl at low temperatures affords the kinetically stable 1,4-addition product of 1, LSiCl (Lu2009=u2009[HC(CMeNAr)2], Aru2009=u20092,6-iPr2C6H3) (9a), which upon reaction with [Rh(Cl)cod]2 and [Ir(Cl)cod]2 (codu2009=u20091,5-cyclooctadiene) selectively affords the NHSi complexes [L(Cl)Si:→Rh(Cl)cod] (10a) and [L(Cl)Si:→Ir(Cl)cod] (10b), respectively. The latter were employed as pre-catalysts in the catalytic reduction of amides in the presence of silanes. Remarkably, they show strikingly different activities and selectivities. While complex 10a yields selectively the C–O cleavage product, 10b affords both cleavage products (C–O and C–N). Moreover, the total conversion of the catalytic amide reduction with 10b is significantly higher than the conversion with a benchmark system [Ir(Cl)cod]2 highlighting the enhanced catalytic activity afforded by the coordination of the NHSi ligand. Introducing the hydride source Li[HBEt3] into the catalytic reactions retards the catalyst performance due to a competitive decomposition pathway. This appears to occur via a H-shift onto the cod ligand with concomitant liberation of cyclooctene, which is also presented. The different reactivity of 10a and 10b towards nucleophiles such as MeLi is also discussed. The reaction of 10a with MeLi affords an intractable array of products, while the reaction of 10b with one equivalent of MeLi selectively affords [L(Cl)Si:→Ir(CH3)cod] (14) with selective methylation at the Ir centre. The analogous reaction with two equivalents of 10b affords the double methylated product [L(CH3)Si:→Ir(CH3)cod] (15).