The impact of tripodal chelates on water exchange kinetics and mechanisms: A variable temperature and pressure O-17 NMR study to clarify the structure-reactivity relationship in [Fe-II(L)(H2O)(2)](-) complexes

摘要

The effect of temperature and pressure on the water exchange reaction of [Fe-II(NTA)(H2O)(2)](-) and [Fe-II(BADA)(H2O)(2)](-) (NTA = nitrilotriacetate; BADA = beta-alanindiacetate) was studied by O-17 NMR spectroscopy. The [Fe-II(NTA)(H2O)(2)](-) complex showed a water exchange rate constant, k(ex), of (3.1 +/- 0.4) x 10(6) s(-1) at 298.2 K and ambient pressure. The activation parameters Delta H-not equal, AS(not equal) and Delta V not equal for the observed reaction are 43.4 +/- 2.6 kJ mol(-1), +25 +/- 9 J K-1 mol(-1) and +13.2 +/- 0.6 cm(3) mol(-1), respectively. For [Fe-II(BADA)(H2O)(2)](-), the water exchange reaction is faster than for the [Fe-II(NTA)(H2O)(2)](-) complex with kex = (7.4 +/- 0.4) x 10(6) s(-1) at 298.2 K and ambient pressure. The activation parameters Delta H-not equal, Delta S-not equal and Delta V-not equal for the water exchange reaction are 40.3 +/- 2.5 kJ mol(-1), +22 +/- 9 J K-1 mol(-1) and +13.3 +/- 0.8 cm(3) mol(-1), respectively. The effect of pressure on the exchange rate constant is large and very similar for both systems, and the numerical values for Delta V-not equal suggest in both cases a limiting dissociative (D) mechanism for the water exchange process.