SYNTHESIS AND STUDY OF CYCLIC PI-SYSTEMS CONTAINING SILICON AND GERMANIUM - THE QUESTION OF AROMATICITY IN CYCLOPENTADIENYL ANALOGS

Synthetic routes to the metallole species C(4)Me(4)E(H)R (9, E = Si, R = Si(SiMe(3))(3); 10, E = Si, R = Mes (mesityl); 11, E = Ge, R = Si(S iMe(3))(3); 12, E = Ge, R = Mes), C(4)R(4)E(SiMe(3))(2) (13, E = Si, R = Me; 14, E = Ge, R = Me; 19, E = Si, R = Et; 20, E = Ge, R = Et), an d C(4)Me(4)E(R)E(R)Me(4)C(4) (15, E = Si, R = SiMe(3); 16, E = Si, R = Me; 17, E = Ge, R = SiMe(3); 18, E = Ge, R = Me) are described. In th e presence of 18-crown-6, dihalides 1 and 2 are reduced by potassium i n tetrahydrofuran to give crystalline samples of the silole dianion [K (18-crown-6)(+)](2)[C(4)MeSi(2-)] (21) and the germole dianion [K-4(18 -crown-6)(3)][C(4)Me(4)Ge](2) (22). Compound 21 adopts an inverse-sand wich geometry, while 22 is a dimer with a bridging [K(18-crown-6)K](2) group and eta(5)-binding modes for all of the potassium atoms. The m etallole dianions in these structures appear to possess delocalized pi -systems, as evidenced by nearly equivalent C-C bond lengths in the fi ve-membered rings. Silolyl and germolyl anions have been obtained by v arious methods involving nucleophilic cleavage of bonds to germanium a nd silicon. Deprotonation of 11 and 12 in the presence of a crown ethe r gave the anions [K(18-crown-6)][C(4)Me(4)GeR] (23, R = Si(SiMe(3))(3 ); 24, R = Mes) and [Li(12-crown-4)(2)][C(4)Me(4)GeR] (25, R = Si(SiMe (3))(3); 26, R = Mes). NMR parameters for these species, and X-ray str uctures for 25 and 26, indicate that the anionic rings possess pyramid al germanium centers and bond localization in the diene portion of the ring. Spectroscopic and X-ray crystallographic data for [Na(15-crown- 5)]-[C(4)Me(4)GeMe] (28), prepared by reductive cleavage of the Ge-Ge bond in 18, reveal a similar structure for the germolyl ring. The latt er compound possesses a Na ... Ge interaction in the solid state. Silo lyl and germolyl anions M[C(4)Me(4)E(SiMe(3))] (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 be en prepared by nucleophilic cleavage of the E-SiMe(3) bond in C(4)Me(4 )E(SiMe(3))(2) with MCH(2)Ph (M = Li, K). By similar methods, the mono anionic species [K(18-crown-6)][C(4)Me(4)E(SiMe(3))C(4)Me(4)E] (36, E = Si; 37, E = Ge) were obtained. A crystal structure determination for 33 revealed a highly pyramidalized Si center (the angle between the C 4Si plane and the Si-Si bond is 99.6 degrees) and pronounced double bo nd localization in the ring. Interaction between the [K(18-crown-6)]() cation and the anion is rather weak, as indicated by the K ... Si di stance (3.604(2) Angstrom) and the atomic position for K. By variable- temperature H-1 NMR spectroscopy, inversion barriers for the compounds [Li(12-crown-4)(2)][C(4)Et(4)ESiMe(3)] (38, E = Si; 40, E = Ge) and K [C(4)Et(4)SiMe(3)] (39, E = Si; 41, E = Ge) were estimated. Barriers f or the germolyl anions 40 and 41 (10.5(1) and 9.4(1) kcal mol(-1), res pectively) are distinctly higher than those for the corresponding silo lyl anions 38 and 39, as might be expected from periodic trends. The s ilolyl anions exhibited coalescence temperatures below the freezing po int of tetrahydrofuran (165 K), but upper limits to the inversion barr iers were estimated from spectra recorded at the lowest temperatures ( less than or equal to 8.3 kcal mol(-1) for 38 and <8.4 kcal mol(-1) fo r 39). The measured inversion barriers for compounds 38-41 provide ene rgy differences between the pyramidal anions and their corresponding p lanar (possibly aromatic) structures, and their low values may be attr ibuted to stability imparted to the transition state by delocalization of rr-electron density in the ring.