Synthesis and Study of Cyclic π-Systems Containing Silicon and Germanium. The Question of Aromaticity in Cyclopentadienyl Analogues

Synthetic routes to the metallole species C4Me4E(H)R (9, E = Si, R = Si(SiMe3)3; 10, E = Si, R = Mes (mesityl); 11, E = Ge, R = Si(SiMe3)3; 12, E = Ge, R = Mes), C4R4E(SiMe3)2 (13, E = Si, R = Me; 14, E = Ge, R = Me; 19, E = Si, R = Et; 20, E = Ge, R = Et), and C4Me4E(R)E(R)Me4C4 (15, E = Si, R = SiMe3; 16, E = Si, R = Me; 17, E = Ge, R = SiMe3; 18, E = Ge, R = Me) are described. In the presence of 18-crown-6, dihalides 1 and 2 are reduced by potassium in tetrahydrofuran to give crystalline samples of the silole dianion [K(18-crown-6)+]2[C4Me4Si2-] (21) and the germole dianion [K4(18-crown-6)3][C4Me4Ge]2 (22). Compound 21 adopts an inverse-sandwich geometry, while 22 is a dimer with a bridging [K(18-crown-6)K]2+ group and η5-binding modes for all of the potassium atoms. The metallole dianions in these structures appear to possess delocalized π-systems, as evidenced by nearly equivalent C−C bond lengths in the five-membered rings. Silolyl and germolyl anions have been obtained by various methods involving nucleophilic cleavage of bonds to germanium and silicon. Deprotonation of 11 and 12 in the presence of a crown ether gave the anions [K(18-crown-6)][C4Me4GeR] (23, R = Si(SiMe3)3; 24, R = Mes) and [Li(12-crown-4)2][C4Me4GeR] (25, R = Si(SiMe3)3; 26, R = Mes). NMR parameters for these species, and X-ray structures for 25 and 26, indicate that the anionic rings possess pyramidal germanium centers and bond localization in the diene portion of the ring. Spectroscopic and X-ray crystallographic data for [Na(15-crown-5)][C4Me4GeMe] (28), prepared by reductive cleavage of the Ge−Ge bond in 18, reveal a similar structure for the germolyl ring. The latter compound possesses a Na···Ge interaction in the solid state. Silolyl and germolyl anions M[C4Me4E(SiMe3)] (30, E = Si, M = Li; 31, E = Si, M = K; 32, E = Si, M = Li(12-crown-4)2; 33, E = Si, M = K(18-crown-6); 34, E = Ge, M = K; 35, E = Ge, M = K(18-crown-6)) have been prepared by nucleophilic cleavage of the E−SiMe3 bond in C4Me4E(SiMe3)2 with MCH2Ph (M = Li, K). By similar methods, the monoanionic species [K(18-crown-6)][C4Me4E(SiMe3)C4Me4E] (36, E = Si; 37, E = Ge) were obtained. A crystal structure determination for 33 revealed a highly pyramidalized Si center (the angle between the C4Si plane and the Si−Si bond is 99.6°) and pronounced double bond localization in the ring. Interaction between the [K(18-crown-6)]+ cation and the anion is rather weak, as indicated by the K···Si distance (3.604(2) Å) and the atomic position for K. By variable-temperature 1H NMR spectroscopy, inversion barriers for the compounds [Li(12-crown-4)2][C4Et4ESiMe3] (38, E = Si; 40, E = Ge) and K[C4Et4ESiMe3] (39, E = Si; 41, E = Ge) were estimated. Barriers for the germolyl anions 40 and 41 (10.5(1) and 9.4(1) kcal mol-1, respectively) are distinctly higher than those for the corresponding silolyl anions 38 and 39, as might be expected from periodic trends. The silolyl anions exhibited coalescence temperatures below the freezing point of tetrahydrofuran (165 K), but upper limits to the inversion barriers were estimated from spectra recorded at the lowest temperatures (≤8.4 kcal mol-1 for 38 and <8.4 kcal mol-1 for 39). The measured inversion barriers for compounds 38−41 provide energy differences between the pyramidal anions and their corresponding planar (possibly aromatic) structures, and their low values may be attributed to stability imparted to the transition state by delocalization of π-electron density in the ring.