Syntheses and X-ray Structures of Anionic Sodocene Sandwiches

While, the lithocene anion (Cp 2 Li - ) can simply be generated by reacting 2 equiv of CpLi with PPh 4 Cl in THF, the attempted synthesis of the sodocene anion under similar reaction conditions results in the crystallization of [Cp - ][PPh 4 + ]. The structure consists of “naked” Cp anions pinched between meta phenyl hydrogens of the phosphonium cations. An ansa -sodocene anion in which the two Cp rings are bridged by a −C(Me) 2 −C(Me) 2 − chain is obtained by reaction of Me 4 C 2 (C 5 H 4 Na) 2 with 1 equiv of PPh 4 Cl in THF. Its crystal structure shows a disordered ansa -sodocene anion containing a sandwiched sodium cation solvated by a THF molecule. Unbridged Cp 2 Na - was obtained by pushing the (CpNa + Cp - ) ⇄ Cp 2 Na - equilibrium to the sodocene anion side. Cell dimensions, space group symmetry, and the molecular structure of [Cp 2 Na - ][PPh 4 + ] are remarkably similar to that of [Cp 2 Li - ][PPh 4 + ]. The anion can be described as a centrosymmetric (η 5,η 5 )Cp 2 Na - sandwich (average C−Na = 2.630(3) Å) with approximate D 5 d symmetry.