The kinetics of the exchange reactions of N-deuterated ethylenediamine with paramagnetic Ln(en-d4)43+(Ln3+= Pr3+, Nd3+, Eu3+, Er3+, Yb3+) complexes in deuterated acetonitrile were investigated over the temperature range 233-343 K using‘H NMR. The data were analyzed by line shape analysis using the equation for a two-site exchange. The mean ligand residence times, τm, were observed to increase across the lanthanide series. The erbium and ytterbium systems demonstrated both the slow-and fast-exchange limits over this temperature range on both the 300-and 100-MHz time scales; however, exchange involving complexes of the larger metal ions revealed coalescence of the coordinated and free ligand peaks even at the lowest temperature studied (233 K). A linear dependency of 1 /τmon the concentration of free ligand was observed for complexes derived from the larger ions (Pr3+, Nd3+, Eu3+), corresponding to a rate law that is first order in ethylenediamine concentration. The mechanism proposed for these reactions is discussed in terms of a nine-coordinate intermediate. At 298 K: kʺ= (1.9 ± 0.2) X 10377(Pr), (2.0 ʺ 0.2) X 106(Nd), (1.5 ± 0.15) X 106M-1s-1(Eu); ΔH≠ = 9.7 ± 1 (Pr), 16 ± 1 (Nd), 11 ± 1 kJ.mol-1(Eu); ΔS≠ = -92 ± 9 (Pr), -71 ± 5 (Nd), -91 ± 8 J-K-'-mol (Eu). For complexes of the smaller ions (Ln3+= Er3+, Yb3+), 1 /τmwas nearly independent of the ethylenediamine concentration at higher temperatures and revealed a nonlinear dependency on ethylenediamine concentration at lower temperatures. Parallel Id and D exchange mechanisms are proposed to be operating in these systems. At 298 K: kD= (3.6 ± 0.4) X 104(Er), (1.3 ± 0.1) X 104s-1(Yb); ΔH≠ = 53 ± 5 (Er), 53 ± 5 kJ-mol-1(Yb); ΔS≠ = 17 ± 5 (Er), 12 ± 5 J.K-1-mol (Yb).
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry