First cyclotrisiloxanes with three different R2Si-units of the general formula R21Si(OSiR22)(OSiR23)O (R21 = Me2Si(NSiMe3)2, R2 = Cl, Ot-Bu, R3 = Me)

The reaction of compound Me2Si(NSiMe3)2Si(OH)Cl with Me2SiCl2 leads to the disiloxane Me2Si(NSiMe3)2Si(Cl)OSi(Me2)Cl (1). Hydrolysis of 1 in the presence of pyridine results in Me2Si(NSiMe3)2Si(OH)OSi(Me2)OH (2), which is allowed to react with SiCl4 to give cyclotrisiloxane [Me2Si(NSiMe3)2Si](OSiMe2)(OSiCl2)O (3). The treatment of 1 with (t-BuO)2Si(OH)2 forms cyclotrisiloxane [Me2Si(NSiMe3)2Si](OSiMe2)[OSi(Ot-Bu)2]O (4). Compound 3 is obtained as a crystalline solid while 4 is an oily liquid. The ring size of these new types of cyclotrisiloxanes with three different R2Si-units is confirmed by cryoscopy in benzene, 29Si NMR chemical shifts and in case of 3, additionally by a single X-ray diffraction study. The different electronegativities of the substituents in the R2Si-units lead to different bond lengths and bond angles within the Si3O3 cycle, which are discussed in detail in the molecular structure of 3.