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

The first hypercoordinate sila[1]ferrocenophanes [fcSiMe(2‐C6H4CH2NMe2)] (5 a) and [fcSi(CH2Cl)(2‐C6H4CH2NMe2)] (5 b) (fc=(η5‐C5H4)Fe(η5‐C5H4)) were synthesized by low‐temperature (−78 °C) reactions of Li[2‐C6H4CH2NMe2] with the appropriate chlorinated sila[1]ferrocenophanes ([fcSiMeCl] (1 a) and [fcSi(CH2Cl)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 sp3‐hybridized nitrogen atom interacting with a tetraorganosilane detected for 5 a (2.776(2) Å). Elongated SiCipso bonds trans to the donating NMe2 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 SiCipso bonds of the non‐hypercoordinate analogues ([fcSiMePh] (1 b): 1.879(4), 1.880(4) Å; [fcSi(CH2Cl)Ph] (1 e): 1.881(2), 1.884(2)). Solution‐state fluxionality of 5 a and 5 b, suggestive of reversible coordination of the NMe2 group to silicon, was demonstrated by means of variable‐temperature NMR studies. The ΔG≠ of the fluxional processes for 5 a and 5 b in CD2Cl2 were estimated to be 35.0 and 37.6 kJ mol−1, respectively (35.8 and 38.3 kJ mol−1 in [D8]toluene). The quaternization of 5 a and 5 b by MeOTf, to give [fcSiMe(2‐C6H4CH2NMe3)][OTf] (7 a‐OTf) and [fcSi(CH2Cl)(2‐C6H4CH2NMe3)][OTf] (7 b‐OTf), respectively, supported the reversibility of NMe2 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‐CH2NMe2CH2C6H4)]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, [Fc2Si(2‐C6H4CH2NMe2)2] (8), synthesized through reaction of [Fc2SiCl2] with two equivalents of Li[2‐C6H4CH2NMe2] 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. The first strained hypercoordinate sila[1]ferrocenophanes 1 have been synthesized and their fluxionality in solution has been demonstrated. The reversible dissociation of an intramolecular dimethylamino group allows quaternization at