Heavy analogues of the 6π-electron anionic ring systems: Cyclopentadienide ion and cyclobutadiene dianion

Aromaticity of Group 14 Organometallics: Experimental Aspects

“…Thus, the NICS(1) for the model compound [ 18 ′ − Li + ] (H 3 Si groups instead of the t Bu 2 MeSi substituents) was negative (−8.4). This value is similar to (albeit smaller than) those of the classical aromatic compounds with six π electrons: C 5 H 5 Li (NICS(1)=−10.3) and C 6 H 6 (NICS(1)=−11.2) 72b. Accordingly, the 7 Li NMR resonance of [ 18 − Li + (thf)] in toluene was observed at very high field ( δ =−5.4 ppm), which is typical for the aromatic lithium cyclopentadienide derivatives 74.…”

“…One of the best methods for making EE′ double bonds is the dehalogenative reduction of the 1,2‐dihalogen precursors of the type E(X)E′(X) (X=halogen) with alkali metals. For this reason, for the preparation of the heavy analogues of cyclobutadiene dianions, initially we designed the appropriate precursors: trans ‐1,2‐dichloro‐ 3 Δ‐1,2,3,4‐disiladigermetene 21 81 and trans , trans , trans ‐1,2,3,4‐tetrabromo‐1,2,3,4‐tetrasiletane 22 72b. The formation of the target 1,2‐disila‐3,4‐digerma‐ and 1,2,3,4‐tetrasilacyclobutadiene dianions [ 23 2− {K + (thf) 2 } 2 ] and [ 24 2− {K + (thf) 2 } 2 ] was smoothly achieved by the reduction of 21 or 22 with potassium graphite KC 8 (4.2 and 6.9 equiv, respectively) in THF (Scheme ) 82.…”

“…The crystal structures of both [ 23 2− {K + (thf) 2 } 2 ] and [ 24 2− {K + (thf) 2 } 2 ]82, 72b (Figure 6) are remarkably different from that of the above‐mentioned carbon analogue [(Me 3 Si) 4 C 4 2− Li + 2 ] featuring D 4 h symmetry 80a. First, both four‐membered {Si 2 Ge 2 } and {Si 4 } rings of [ 23 2− {K + (thf) 2 } 2 ] and [ 24 2− {K + (thf) 2 } 2 ] significantly depart from planarity, being markedly puckered with folding angles of 43 and 34°.…”

“…Interestingly, whereas the solid‐state structures of both [ 23 2− {K + (thf) 2 } 2 ] and [ 24 2− {K + (thf) 2 } 2 ] are virtually identical (Scheme ), their solution structures are appreciably different, which was clearly manifested in their 29 Si NMR spectra 82. 72b Thus, the resonances of the skeletal Si atoms in [ 23 2− K + 2 ] were observed at low field ( δ =113.7 ppm), characteristic of doubly bonded Si atoms,75 thus pointing to the most important contribution of the resonance structure in Scheme with both negative charges located on the more electronegative Ge atoms. Indeed, methylation of [ 23 2− {K + (thf) 2 } 2 ] with Me 2 SO 4 proceeded exclusively at the Ge atoms to form the 1 Δ‐1,2,3,4‐disiladigermetene 26 as the only product (Scheme ) 82.…”

Coordination chemistry of 9-sila- and 9-germa-phenanthrenes — unique coordination modes in their metallene complexes

“…Tilley and co-workers [15][16][17][18][19] have utilized monosilaor monogerma-cyclopentadienides ( 5 -SiC 4 R 5 and 5 -GeC 4 R 5 , respecitvely) to synthesize the corresponding ruthenocene, 15,16 hafnocene, 17,18 zirconocene, 18 and ferrocene 19 analogues. Sekiguchi and co-workers [20][21][22] have reported the syntheses of cyclopentadienides containing three heavy group 14 elements as skeletal atoms ( 5 -Si 2 GeC 2 R 5 8 and 5 -Si 3 C 2 R 5 9 ) and the application to the synthesis of a ferrocene analogue bearing two disilagermacyclopentadienyl ligands 20,21 and rhodium half-sandwich complexes of trisilacyclopentadienyl ligands. 22 Recently, Saito and co-workers 23 have reported bis(stannylene)-bridged dinuclear ruthenium complexes by the reactions of dilithiostannole.…”