Homogeneous Catalysis Under Ultradilute Conditions: TAML/NaClO Oxidation of Persistent Metaldehyde

TAML activators enable homogeneous oxidation catalysis where the catalyst and substrate (S) are ultradilute (pM-low μM) and the oxidant is very dilute (high nM-low mM). Water contamination by exceptionally persistent micropollutants (MPs), including metaldehyde (Met), provides an ideal space for determining the characteristics and utilitarian limits of this ultradilute catalysis. The low MP concentrations decrease throughout catalysis with S oxidation (k_II) and catalyst inactivation (k_i) competing for the active catalyst. The percentage of substrate converted (%Cvn) can be increased by discovering methods to increase k_II/k_i. Here we show that NaClO extends catalyst lifetime to increase the Met turnover number (TON) 3-fold compared with H₂O₂, highlighting the importance of oxidant choice as a design tool in TAML systems. Met oxidation studies (pH 7, D₂O, 0.01 M phosphate, 25 °C) monitored by ¹H NMR spectroscopy show benign acetic acid as the only significant product. Analysis of TAML/NaClO treated Met solutions employing successive identical catalyst doses revealed that the processes can be modeled by the recently published relationship between the initial and final [S] (S₀ and S_∞, respectively), the initial [catalyst] (Fe_Tot) and k_II/k_i. Consequently, this study establishes that ΔS is proportional to S₀ and that the %Cvn is conserved across all catalyst doses in multicatalyst-dose processes because the rate of the k_II process depends on [S] while that of the k_i process does not. A general tool for determining the Fe_Tot required to effect a desired %Cvn is presented. Examination of the dependence of TON on k_II/k_i and Fe_Tot at a fixed S₀ indicates that for any TAML process employing Fe_Tot < 1 × 10^-6 M, small catalyst doses are not more efficient than one large dose.