Low valent silicon
Currently the chemistry of silicon is mainly based on silicon(IV), whereas that of silicon(II) is still in its infancy. A silylene (R2Si:) is a molecule with a divalent neutral silicon atom holding a lone pair of electrons. Until 1994, silylenes were generally considered to be very reactive unstable species that decompose or polymerize rea- dily at temperatures above 77 K. This situation changed when Denk, West et al. reported the first N-heterocyclic silylene that is stable at room temperature[1].
Recently we entered the field of low valent siliconin a close colla- boration with Prof.Dr. Herbert W. Roesky in our Institut.
We elaborated the synthesis of the first base-stabilized dichloro- silylene that is stable at room temperature. It is formed under mild reaction conditions by reductive elimination of HCl from trichloro- silane in the presence of NHC (Fig. 1). The structure features a trigonal pyramidal threefold-coordinated silicon atom with the stereo-chemically active lone pair at the apex. The Cl-Si-Cl angle of only 97.3°, the side-on coordination of the carbene and the shape of the lone-pair suggest that the silicon atom is barely sp2-hybri- dized and that the lone-pair adopts predominantly s-character[2].
Of course several hydrides are known from silicon at oxidation state +4. However, the corresponding stable silicon compound of oxi- dation state +2 was elusive to date. Therefore we were pleased to synthesise a stable Lewis acid-base stabilized silicon(II) mono- hydride [{PhC(NtBu)2}SiH(BH3)] in good yield, stable at room temperature[3]. Charge density investigations from a high-reso- lution, low-temperature diffraction experiment revealed, that there seems to be only one consistent interpretation of the electronic structure. LSiH(BH3) is the first silicon(II) monohydride, stabilized through a covalent shared interaction to a sp3-hybridized boron atom. The positively charged H–Si–BH3 moiety is coordinated by the lone-pairs of the benzamidinate ligand L[4]. From this expe- riment conclusive evidence to the electronic interpretation was gained by integration of all symmetry independent atomic basins. The atomic charges display prominent positive values for silicon (+1.68 e), boron (+1.21 e), and even for C1 (+0.60 e), mainly counterbalanced by N1 (-1.31 e), N2 (-1.21 e), and the hydrogen atoms bound to silicon (-0.53 e) and boron (av.: -0.53 e).
Although the anti-aromatic cyclopentadienyl cation Cp+ is syn- thetically still out of reach the isoelectronic replacement of most of the carbon atoms by elements from the lithosphere such as silicon and phosphorus, makes the heavier congener accessible[5]. Obviously, the third row elements stabilize the anti-aromatic five-membered ring considerably and phosphorus as an excellent π-donor provides diagonal relationship without mimicking a simple carbon-copy (Fig.3). Although this anti-aromatic ring is 24 kcal/mol higher in energy than the virtual aromatic anion the silicon and phosphorus atoms offer sufficient stabilization allowing the isolation of the cation. Most surprisingly the counter anion of the five-membered cationic ring system is a simple chloride anion which does not interact with the ring system at all and therefore does not contribute to the ring’s stability.
The activation of white phosphorus has always attracted con- siderable interest owing to its demand in industrial applications for a more facile production of organo-phosphorus derivatives. Al- though the activation of phosphorus by subvalent Group 14 elements has flowered into an engaging, intriguing, and emerging area of chemistry in a surprisingly short period of time, a neutral acyclic P4 chain stabilized by an N-heterocyclic silylene was still elusive. So, we were pleased to present the synthesis of a neutral acyclic P4 chain stabilized by the recently isolated silicon(II) bis(trimethylsilyl)amide [PhC(NtBu)2SiN(SiMe3)2], (Fig. 4). The mo- lecular structure shows the presence of the Z-diphosphene isomer. It consists of two Si atoms and four P atoms, which together form a neutral acyclic Si2P4(Si=P–P=P–P=Si) chain with 6π-electrons con- tained in adiphosphene and two phosphasilene units[6].
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[1] M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov, H. P. Verne, A. Haaland, M. Wagner, N. Metzler J. Am. Chem. Soc. 1994, 116, 2691-2692.
[2] R. S. Ghadwal, H. W. Roesky, S. Merkel, J. Henn, D. Stalke Angew. Chem. 2009, 121, 5793-5796; Angew. Chem. Int. Ed. 2009, 48, 5683-5686.
[3] A. Jana, D. Leusser, I. Objartel, H. W. Roesky, D. Stalke Dalton Trans. (Hot Article) 2011, 40, 5458-5463 and inside front cover, highlighted by A. Schnepf in Nachr. Chem. 2011, 59, 698.
[4] D. Stalke, M. Wedler, F. T. Edelmann J. Organomet. Chem. 1992, 431, C1-C5.
[5] S. S. Sen, J. Hey, M. Eckhardt, R. Herbst-Irmer, R. A. Mata, H. W. Roesky, M. Scheer, D. Stalke Angew. Chem. 2011, 123, 12718-12721; Angew. Chem. Int. Ed. 2011, 50, 12510-12513.
[6] S. Khan, R. Michel, S. S. Sen, H. W. Roesky, D. Stalke Angew. Chem. 2011, 123, 11990-11993; Angew. Chem. Int. Ed. 2011, 50, 11786-11789.