Barbara Gehrhus

Chemistry of thermally stable bis(amino)silylenes

Abstract This paper provides a comprehensive review of the chemistry of the thermally stable bis(amino)silylenes Si[(NCH 2 Bu t ) 2 C 6 H 4 -1,2] [abbreviated as Si(NN)] 1 and Si[N(Bu t )CH] 2 [abbreviated as Si(N′N′)] 2 and the somewhat less stable Si[N(Bu t )CH 2 ] 2 [abbreviated as Si(N″N″)] 3 . In a brief introduction, comments are made on the importance of transient silylenes in organosilicon chemistry and on thermally stable mononuclear silicon(II) compounds having a coordination number greater than two for silicon. The chemistry of 1 – 3 is discussed under the headings: synthesis, molecular and electronic structures, and reactions. The latter are classified into eight categories: (i) nucleophilic additions to an unsaturated organic substrate; (ii) insertions (oxidative addition reactions to the silylene) into a compound containing a OH, CCl, CBr, CI or BC bond; (iii) insertions into a main group element compound having a MN (M=Li, Na, K), LiC, LiSi, or M′(II)X (M′=Ge, Sn or Pb; X=C, N, O or Cl) bond; (iv) insertions into a transition metalX bond; (v) further (see ii) oxidative addition reactions of a chalcogen or Bu t NC; (vi) reduction reactions; (vii) silylenes as ligands in transition or 4f block metal chemistry, and (viii) silylenes as electrophiles.

Synthesis, structures and reactions of new thermally stable silylenes

Two representatives 1a and 1b of a new series of stable but reactive bis(amino)silylenes, derived from the N,N′-dineopentyl-1,2-phenylenediamido ligand RC6H3[(CH2But)]2, have been prepared by reductive elimination from RC6H3[[graphic omitted]SiCl2 and characterised by NMR spectroscopy and for 1a X-ray crystallography; the silyienes [graphic omitted](CH2But)]2C6H3-1,2-R (R = H 1a or 4-Me 1b) readily undergo oxidative addition with EtOH or Mel.

Synthesis and characterisation of bis(amino)silylene–nickel(0), –palladium(II), –platinum(0), –platinum(II) and copper(I) complexes

Abstract The bis(amino)silylene Si[(NCH2tBu)2C6H4-1,2] ( SiNN) behaved as a ligand in displacing COD or PPh3 from [Ni(COD)2], [Pt(PPh3)4] or [CuI(PPh3)3], yielding the crystalline d10 complexes [Ni(SiNN)4] (crystallised from Et2O as the 1:1-adduct 2), [Pt(PPh3)(SiNN)3] (5) or [CuI(PPh3)2(SiNN)] (6). Under the same conditions (in C6H6 at ambient temperature), SiNN and [MCl2(PPh3)2] yielded the crystalline (i) d8 complex trans-[M{SiNN(Cl)}2(SiNN)2] [M=Pt (3), Pd (4)] or (ii) the d10 complexes [Ni(PPh3)4−n(SiNN)n] (n=4 (2) or 3 (1), depending on stoichiometry; with Cl2SiNN as co-product). The X-ray structures of 2, 4, 5 and 6 are presented; those of 1 and 3 appeared in a preliminary communication. Compound 3 was shown to undergo three fluxional processes in CDCl3 solution, as shown by detailed variable temperature NMR spectroscopic experiments. These involved in order of increasing energy (a) two conformers of 3, (b) one of these and the cis-isomer, and (c) Si→Si 1,3-shifts of Cl−. These results are placed into the context of related published silylenemetal chemistry.

Reactions of the stable bis(amino)silylene with multiply bonded compounds

Abstract Treatment of the title silylene [abbreviated as Si(NN)] under mild conditions with various unsaturated organic compounds has yielded the following series of tetravalent silicon compounds: [XY = OCRR′ (R,R′ = Ph, Me or Ad′; Ad′ = Ad minus H, Ad = adamantyl), NCBu t or C(SiMe 3 )CPh], [R, R′ = SiMe 3 ,Ph or Bu t ,(H)Si(NN)], (R = CH 2 Bu t ), Si(NN)(Bu t )X [X  CN or Si(NN)CN],

Synthesis and reactivity of functional 1-methyl-silacyclopentenes

Abstract The silylenes MeSiCl, MeSiOMe and MeSiNMe 2 can be thermally generated from disilanes and trapped by butadiene, isoprene and 2,3-dimethylbutadiene to give functionally-substituted silacyclopentenes. Some examples of cycloadditions to heterodienes are included to demonstrate the scope and mechanism of the reaction. The reactivity of 1-chloro-1-methyl-silacyclopent- 3-ene has been studied.

Synthesis and crystal structure of trimeric sodium 2,2,6,6- tetramethylpiperidide (NaTMP)

Abstract The title compound has been synthesised in powder form by two distinct methods: namely by sodiation of the parent amine 2,2,6,6-tetramethylpiperidine with n-butylsodium in a hydrocarbon solvent or by transmetallation from lithium 2,2,6,6-tetramethylpiperidide through the action of sodium t-butoxide in hexane solution. An alternative crystalline specimen suitable for X-ray crystallographic study was grown from a solution which additionally contained n,s-dibutylmagnesium. This study reveals a trimeric molecule of C3h symmetry, its salient feature being a strictly planar (NaN)3 ring. The sterically encumbered amide anions are separated by large NNaN bond angles of 143.76(6)°, the bonding in which is slightly asymmetrical (NaN bond lengths: 2.307(2) and 2.362(2) A). Discussion focuses on the contrast between the tetrameric ring arrangement of the lithium congener, which was reported several years ago.

Electronic structure of stable benzodiazasilylenes: photoelectron spectra and quantum-chemical investigations

Photoelectron spectra of 2,3-dihydro-1,3-di(neopentyl)-1H-1,3,2-benzodiazasilol-2-ylidene were investigated and compared to those of 2,3-dihydro-1,3-di(tert-butyl)-1H-1,3,2-diazasilol-2-ylidene and the related 2,3-dihydro-1,3-di(neopentyl)-1H-1,3,2-benzodiazasilole. The experimental spectra were assigned with the help of quantum-chemical calculations using the MP2/6-31G* level of theory. Trends in the orbital energies correspond well with the observed ionization energies. A 0.41 eV stabilization of the highest occupied molecular orbital level from the silane to the related silylene derivative indicates the incorporation of the silicon empty out-of-plane orbital into the π system. A good agreement was found between the calculated and experimental geometries. Delocalization stabilization of the investigated molecules is also supported by appropriate isodesmic reactions. The reaction energy indicates significant aromatic stabilization of the silylene derivatives. The stabilization is, however, reduced by a considerable ring strain.

The stable silylene Si[(NCH2tBu)2C6H4-1,2]: insertion into LiC or LiSi bonds of lithium alkyls LiR or [Li{Si(SiMe3)3}(THF)3] [R=Me, tBu or CH(SiMe3)2]

Abstract The new crystalline amino-functionalised lithium silyls Li[Si{(NCH2tBu)2C6H4-1,2}R](L) [R=Si(SiMe3)3 (sisyl), L=(THF)2 (2); R=Me, L=(OEt2)2 (3); R=tBu, L=(THF)3 (4a) or THF(OEt2) (4b); R=CH(SiMe3)2, L=(THF)2 (5)] have been obtained in high yield from the silylene Si[(NCH2tBu)2C6H4-1,2] and LiR under mild conditions. A by-product in the reaction leading to 4 was the crystalline disilane [1,2-C6H4(NCH2tBu)2(tBu)Si]2 (6). Each of 2–6 was fully characterised by microanalysis, multinuclear NMR spectra and X-ray structures. Each of the compounds 3–6 was thermally stable in solution, whereas the sisyl derivative 2 retained its integrity only at low temperatures, being completely dissociated into its factors at ambient temperature.

A crystalline carbene–silylene adduct 1,2-C6H4[N(R)]2C-Si[N(R)]2C6H4-1,2 (R = CH2But); synthesis, structure and bonding in model compounds†

The red–brown, crystalline carbene–silylene adduct, 1,2-C6H4[N(R)]2C-Si[N(R)]2C6H4-1,2 (R = CH2But) 4, was obtained from its factors, the carbene 3 and silylene 1, or from Ni{C[N(R)]2C6H4-1,2}2 and 1; the X-ray structure of 4 shows a long C–Si bond [2.162(5) A] and NMR spectral data indicate significant C+–Si– bond polarity, features consistent with DFT calculations at the B3LYP/6-311+G** level on [(CH)2(NH)2]C–Si[(NH)2(CH)2], (H2N)2C–Si(NH2)2 or even [(CH)2(NH)2]C–SiH2 and (H2N)2C–SiH2, but not H2CSi(NH2)2 or H2CSi[(NH)2(CH)2].

Reaction of the Silylene Si[(NCH2But)2C6H4-1,2] with the Alkali Metal Silylamides M[N(SiMe3)R] (M = Li, Na or K; R = SiMe3 or SiMe2Ph)

The thermally stable silylene Si[(NCH(2)Bu(t))(2)C(6)H(4)-1,2] 1 undergoes oxidative addition reactions with the alkali metal silylamides MN(SiMe(3))(2)(M = Li, Na or K) to afford the new alkali metal amides MN(SiMe(3))[(1)SiMe(3)][M = Li (2), Na (3) or K (4)]. Reaction of two equivalents of 1 with LiN(R)(SiMe(3)) leads in a two-step process to the compound LiN[(1)R][(1)SiMe(3)][R = SiMe(2)Ph (5) or SiMe(3) (6)]. Alternatively, 1 reacts with 3 to afford NaN[(1)SiMe(3)](2) (7). The structures of 2-5 and are presented and the formation of 2-7 is discussed.