The impact of tripodal chelates on water exchange kinetics and mechanisms: a variable temperature and pressure 17O NMR study to clarify the structure-reactivity relationship in [Fe(II)(L)(H2O2](-) complexes

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 17O 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 DeltaH( not equal), DeltaS( not equal) and DeltaV( 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 k(ex) = (7.4 +/- 0.4) x 10(6) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH( not equal), DeltaS( not equal) and DeltaV( 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 DeltaV( not equal) suggest in both cases a limiting dissociative (D) mechanism for the water exchange process.