Dianionic amidinates at silicon and germanium centers: Four-, six- and eight-membered rings

At variance to an earlier finding, the reaction of Me2SiCl2 with Li[(Me)N–C(Ph)–NH] (1a), in the presence of a base, gives a six-membered ring molecule μ-[(Ph)(MeN)C–N][–SiMe2–N–C(Ph)–N(Ph)–SiMe2–] (s3a), whereas with Li[iPrN–C(Ph)–NH] (1b), a four-membered ring molecule μ-[iPrN(Ph)C–N]2(SiMe2)2 (s4b) was formed. In contrast, with Li[tBuN–C(Ph)–NH] (1c), no such reaction occurred. Obviously, a delicate influence of steric effects has to be taken into account. In fact, the latter amidinate reacts with GeCl2 to form an eight-membered ring molecule [tBuN–C(Ph)–N–Ge]4 (5c) without adding an additional base. The compounds are fully characterized and their structures determined by X-ray diffraction. DFT calculations confirm the dependence on steric influences. The relative energies of ground and transition states give a rationalization the ease of transformations of the various rings via pathways with penta- and hexacoordinate silicon centers, which in turn relates to the experimental results on penta- and hexacoordinate silicon amidinates and their fluctional behavior in solution.