Synthesis, Structure, and Fluxionality of Strained Hypercoordinate Silicon‐Bridged [1]Ferrocenophanes

The first hypercoordinate sila[1]ferrocenophanes [fcSiMe(2‐C 6 H 4 CH 2 NMe 2 )] ( 5 a ) and [fcSi(CH 2 Cl)(2‐C 6 H 4 CH 2 NMe 2 )] ( 5 b ) (fc=( η 5 ‐C 5 H 4 )Fe( η 5 ‐C 5 H 4 )) were synthesized by low‐temperature (−78 °C) reactions of Li[2‐C 6 H 4 CH 2 NMe 2 ] with the appropriate chlorinated sila[1]ferrocenophanes ([fcSiMeCl] ( 1 a ) and [fcSi(CH 2 Cl)Cl] ( 1 d ), respectively). Single‐crystal Xray diffraction studies revealed pseudo‐trigonal bipyramidal structures for both 5 a and 5 b , with one of the shortest reported Si⋅⋅⋅N distances for an sp 3 ‐hybridized nitrogen atom interacting with a tetraorganosilane detected for 5 a (2.776(2) Å). Elongated SiC ipso bonds trans to the donating NMe 2 arms (1.919(2) and 1.909(2) Å for 5 a and 5 b , respectively) were observed relative to both the non‐ trans bonds ( 5 a : 1.891(2); 5 b : 1.879(2) Å) and the SiC ipso bonds of the non‐hypercoordinate analogues ([fcSiMePh] ( 1 b ): 1.879(4), 1.880(4) Å; [fcSi(CH 2 Cl)Ph] ( 1 e ): 1.881(2), 1.884(2)). Solution‐state fluxionality of 5 a and 5 b , suggestive of reversible coordination of the NMe 2 group to silicon, was demonstrated by means of variable‐temperature NMR studies. The Δ G ≠ of the fluxional processes for 5 a and 5 b in CD 2 Cl 2 were estimated to be 35.0 and 37.6 kJ mol −1 , respectively (35.8 and 38.3 kJ mol −1 in [D 8 ]toluene). The quaternization of 5 a and 5 b by MeOTf, to give [fcSiMe(2‐C 6 H 4 CH 2 NMe 3 )][OTf] ( 7 a‐ OTf) and [fcSi(CH 2 Cl)(2‐C 6 H 4 CH 2 NMe 3 )][OTf] ( 7 b‐ OTf), respectively, supported the reversibility of NMe 2 coordination at the silicon center as the source of fluxionality for 5 a and 5 b . Surprisingly, low room‐temperature stability was detected for 5 b due to its tendency to intramolecularly cyclize and form the spirocyclic [fcSi( cyclo ‐CH 2 NMe 2 CH 2 C 6 H 4 )]Cl ( 9 ‐Cl). This process was observed in both solution and the solid state, and isolation and Xray characterization of 9 ‐Cl was achieved. The model compound, [Fc 2 Si(2‐C 6 H 4 CH 2 NMe 2 ) 2 ] ( 8 ), synthesized through reaction of [Fc 2 SiCl 2 ] with two equivalents of Li[2‐C 6 H 4 CH 2 NMe 2 ] at −78 °C, showed a lack of hypercoordination in both the solid state and in solution (down to −80 °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.