NMR studies on the dynamic behavior of zirconaaziridinium ion pairs in solution

The fluxional behavior of zirconaaziridinium ion pairs [Cp₂Zr(η²-CH₂NMePh][X] (X– = MeB(C₆F₅)₃– (1a), B(C₆F₅)₄– (1b)) in solution has been investigated by means of 2D EXSY and line shape analysis NMR methods at different temperatures. It is found that two dynamic processes are operative: (i) the inversion of the absolute configuration of the coordinated nitrogen atom (PI process) and (ii) the dynamic symmetrization of the ion pair occurring through a 180° rotation around Zr–CH₂ bond and anion relocation from one side of the cation to the other (BS process). Analysis of the kinetic data indicates that both processes necessitate nitrogen decoordination from the metal that, by the way, is not the rate-determining step; the nature of the borate anion has a negligible effect on the rates of both PI and BS processes.