www.elsevier.nl/locate/jorganchem Journal of Organometallic Chemistry 613 (2000) 208 – 219 Synthesis, structure and spectroscopic properties of branched oligosilanes Silke Chtchian a , Rhett Kempe b , Clemens Krempner a, * a Fachbereich Chemie der Uniersita ¨t Rostock, Buchbinderstrasse 9, D-18051 Rostock, Germany b Institut fu ¨r Organische Katalyseforschung an der Uniersita ¨t Rostock e. V., D-18055 Rostock, Germany Received 3 April 2000; received in revised form 17 July 2000 Abstract A number of various branched heptasilanes [(Me 3 Si) 2 MeSi] 2 MeSiX (2:X =Ph; 3:X =Me; 6:X =Br; 7:X =I; 8:X =Cl; 9: X =F; 10:X =OH) bearing two (Me 3 Si) 2 MeSi groups as well as branched decasilanes [(Me 3 Si) 2 MeSi] 3 SiX(12:X =H; 13: X =Br; 14:X =Cl; 15:X =F; 16:X =OH) bearing three (Me 3 Si) 2 MeSi groups at one silicon centre were synthesized. The structures of the compounds prepared were elucidated on the basis of comprehensive NMR and MS studies. Additionally, the molecular structures of the decasilanes 12 and 14 – 16 were obtained from X-ray diffraction data, which verify a ca. spherical shape with the core silicon atom at the centre of the sphere. The spatial demand of three (Me 3 Si) 2 MeSi groups forces a widening of the SiSiSi angles of the XSiSi 3 tetrahedra toward the group X (X =H, Cl, F, OH) and a remarkable elongation of the central SiX bonds. The analysis of the SiSiSiSi dihedral angles of the tetrasilane subunits in the decasilanes 12 and 14 – 16 indicates the existence of three conformations denoted as anti, ortho and gauche and four different arrangements of pentasilane subunits, denoted as anti – gauche, anti – ortho, ortho – gauche and ortho – ortho. The absorption spectra of the heptasilanes 2 – 10 exhibit broadened absorption maxima, shifted strongly to the red relative to those of the decasilanes 11 – 16. As a result of strongly limited conformational flexibility due to steric overcrowding, the singlet excitation in 11 – 16 is at much higher energy than in linear pentasilanes. A comparison of NMR chemical shifts of 2 – 16 with those of branched silanes of the type (Me 3 Si) 3 SiX and (Me 3 Si) 2 MeSiX reveals that the replacement of methyl groups by additional SiMe 3 groups in the -position leads to a strong low-field shift of the signal of the central nucleus. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Silanes; Branched oligosilanes; Structure elucidation; UV – vis-spectroscopy 1. Introduction Compounds containing branched oligosilyl sub- stituents of type (Me 3 Si) 2 RSi — (R =oligosilyl, trimethylsilyl, aryl, alkyl, hydrogen) have been shown to display unusual chemical behaviour as well as novel electronic properties [1]. For example, Bock et al. demonstrated that radical anions as well as radical cations of tris(trimethylsilyl)silyl derivatives are stabi- lized by delocalizing the negative or positive charge, respectively, into the oligosilyl groups [2]. The ability of branched oligosilyl substituents to act as a -acceptor was discussed by Pitt [3] in the course of studies of the electronic spectra of heptamethyltrisil-2-yl derivatives substituted by electronegative donor groups and by West et al. [4] in the course of ESR studies of the phenyl-tris(trimethylsilyl)silane radical anion. Further- more, the remarkable steric requirements of branched oligosilyl groups, especially of the space-filling (Me 3 Si) 3 Si group, made these substituents versatile lig- ands in main group [5] and in transition metal chem- istry [6]. Of special interest in the search for sterically more demanding ligands for the kinetic stabilization of low- valent intermediates are compounds in which two or three bulky (Me 3 Si) 2 RSi substituents are fixed at the same central atom. Some of those derivatives, mainly of heavier main Group IV elements, have been prepared and for a few of those compounds the X-ray crystal structures were reported [7]. Recently, Apeloig, [8] * Corresponding author. Tel.: +49-381-4981841; fax: +49-381- 4981763. E-mail address: clemens.krempner@chemie.uni.rostock.de (C. Krempner). 0022-328X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0022-328X(00)00518-0