Rapid reduction of nitric oxide to dinitrogen by zirconium(II): Kinetic studies on a reaction controlled by gas-liquid transport

Nitric oxide reacts with Cp₂Zr(PMe₃)₂ (1; Cp = η⁵-C₅H₅) in THF or toluene to yield dinitrogen and an oligomeric oxozirconocene species, [Cp(x)ZrO(y)](n). The reduction of NO occurs in two distinct steps: 1 reduces 2 equiv of NO to 1 equiv of N₂O, and then 1 reduces N₂O to N₂. In each step, 1 is converted to a monomeric oxozirconocene species [Cp₂ZrO], which may be trapped by the addition of Me₃SiCl or Cp₂ZrMe₂, to form Cp₂Zr(OSiMe₃)(C1) and [Cp₂ZrMe]₂O, respectively. Kinetics for the reduction of NO by 1 were followed at low temperature (160 < T < 195 K) by monitoring the change in pressure of the system. Complete reaction was observed in less than 15 s, even at the lowest temperatures. The typical kinetic trace displayed two decay regimes, zero- and first-order, which were interpreted as the result of competing mass-transfer and chemical reaction processes. The kinetic results support a rate-limiting bimolecular reaction between NO and 1. The subsequent intermediates and mechanistic possibilities are discussed.