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.