Cyclische diazastannylene: XXIII. Zur bildungsweise eines zwitterions mit einem phosphonium-kation und einem triorganylstannat-anion

Abstract

On heating the acid-base adduct of 1,3-di-t-butyl-2,2-dimethyl-1,3,2,4λ 2-diazasilastannetidine with triphenylmethylenphosphorane (II) to 120°C in toluene in a glass tube, a quantitative decomposition takes place yielding bis(t-butylamino)dimethylsilane and the novel pentacyclic compound Sn 2(CH) 2P 2(C 6H 4) 2(C 6H 5) 4 (III). The X-ray structure determination reveals that the centrosymmetric III consists of a central (CH) 2Sn 2 square (SnC 229.3(5) pm), two edges of which are members of irregular SnCPC 2-pentagons, one edge of these belonging to C 6-hexagons (monoclinic, P2 1/ c with a 885.3(4), b 1127.8(5), c 1779.6(6) pm, β 110.0(2)° and Z = 2). In III the tin atoms are bonded to three carbon atoms thus forming formally stannate(II) anions, while the phosphorus atoms adopt fourfold coordination and may be considered as cations. It can be shown by NMR-kinetics, that the decomposition of II follows a first order rate law. Triphenyldideuteromethylenphosphorane is used to establish that: (i) the hydrogen (deuterium) transfer is responsible for the rate; (ii) that only one of the hydrogen (deuterium) atoms on the methylene group reacts, while the second hydrogen atom, which has to migrate, must come from a phenyl group on the phosphorous atom. By thorough investigation of the reaction conditions we succeeded in crystallizing “an intermediate”. This intermediate VI has the identical composition of II but a different structure (monoclinic, P2 1/ c, a 1884.2(5), b 1204.8(3), c 1517.9(4) pm, β 116.8(3)° and Z = 4). The prominent feature in this adduct of a methylenphosphorane bonded to a stannylene (SnC 244.2(5) pm) is an intramolecular hydrogen bridge between an o-carbon atom of a phenyl group and a nitrogen atom of the four-membered SiN 2Sn cycle. This hydrogen bridge, which follows from distances as well as from the angles around the nitrogen atom, must be considered to be responsible for a hydrogen transfer. In a series of structure pictures a possible mechanism for the decomposition reaction of II is illustrated.