Synthesis,Structure,and Fluxionality of Sturained Hypercoordinate SiloconBridged [1] Ferrocenophanes

摘要

The first hypercoordinate sila[1]ferrocenophanes [fcSiMe(2-C_6H_4-CH_2NMe_2)] (5a) (fc=(eta~5-C_5H_4)Fe-(eta~5-C_5h_4)) were synthesized by low temperature (-78 deg C) reactions of Li[2-C_6H_4CH_2NMe_2] with the appropriate chlorinated sila[1] ferrocenophanes ([fcSiMeCl] (1a) and [faSi(CH_2Cl)Cl] (1d) respedctively)Single-crystal X-ray diffection studies reveales pseudotdrigonal bipyramida structures for both 5a and 5b with one of hte shortest reported Si...N didtances for an sp_3hybriddized nitrogen atom interaction with a tetraorgansilane deteected for 5a (2.776(2)A) Elongated Si-C_ipso bonds trans to tghe donatinf NMe_2 arms (1.919)(2) and 1.909(2)A for 5a and 5b respectively) were observed realtive to both te non-trans bonds (5a:1.891(2) 5b:1.879(2) A) and the Si-C_ipso bonds of the non-hypercoordinat analobues ([fcSi(CH_2Cl)ph] (1e) 1.881(2) 1.884(2))Solution-state fluxionality fo 5a and 5b suggestive of reversible coordination of the NMe_2 group to wilcon was demonstrated by means of variable-temperature NMR studies The triangle open G not= of the fluxional processes for 5a and 5b in CD_2Cl_2 were estimated to be 35.0 and 37.6kJmol~-1 respetively The quaternization of 5a and 5b by MeOTf to give [fcSImE(2-C_6H_4CH_2NMe_3)][OTf] (7B-OTf).respectively supported the reversibility of NMe_2 NMe_2 coordination the silicon center as the source fluxionality for 5a and 5b Suprising ly low room-temperature stability was detected for 5b due ot its tendency to intramolecularly cyclized form the spirocyclic [fcSi(cyclo-CH_2NMe_2CH_2NMe_2)_2] (8) synthesized through reaction of [Fc_2SiCl_2] with two equivalents of Li[2-c_6h_4ch_2nmE_2] at -78deg C showed a lack of hypercoordination in both the solid state and in solution (down to -80deg C) This suggests that either the reduced steric hindrance around Si or the unique electronics of the strained sila[1]ferrocenophanes is necessary for hypercoordination to occur.