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

The effect of temperature and pressure on the water exchange reaction of [Fe II (NTA)(H 2 O) 2 ] − and [Fe II (BADA)(H 2 O) 2 ] − (NTA = nitrilotriacetate; BADA = β‐alanindiacetate) was studied by 17 O NMR spectroscopy. The [Fe II (NTA)(H 2 O) 2 ] − complex showed a water exchange rate constant, k ex , of (3.1 ± 0.4) × 10 6 s −1 at 298.2 K and ambient pressure. The activation parameters ΔH ≠ , ΔS ≠ and ΔV ≠ 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)(H 2 O) 2 ] − , the water exchange reaction is faster than for the [Fe II (NTA)(H 2 O) 2 ] − complex with k ex = (7.4 ± 0.4) × 10 6 s −1 at 298.2 K and ambient pressure. The activation parameters Δ H ≠ , Δ S ≠ and Δ V ≠ 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 Δ V ≠ suggest in both cases a limiting dissociative (D) mechanism for the water exchange process. Copyright © 2008 John Wiley & Sons, Ltd.