NMR and Electron Paramagnetic Resonance Studies of [Gd(CH3CN)(9)](3+) and [Eu(CH3CN)(9)](2+): Solvation and Solvent Exchange Dynamics in Anhydrous Acetonitrile

Homoleptic acetonitrile complexes [Gd(CH3CN)(9)]Al(OC(CF3)(3))(4) and [Eu(CH3CN)(9)]Al(OC(CF3)(3))(4) have been studied in anhydrous acetonitrile by N-14- and H-1 NMR relaxation as well as by X- and Q-band EPR. For each compound a combined analysis of all experimental data allowed to get microscopic information on the dynamics in solution. The second order rotational correlation times for Gd(CH3CN)(9) and Eu(CH3CN)(9) are 14.5 +/- 1.8 ps and 11.8 +/- 1.1 ps, respectively. Solvent exchange rate constants determined are (55 +/- 15) x 10(6) s(-1) for the trivalent Gd3+ and (1530 +/- 200) x 10(6) s(-1) for the divalent Eu2+. Surprisingly, for both solvate complexes CH3CN exchange is much slower for the less strongly N-binding acetonitrile than for the more strongly coordinated O-binding H2O. It is concluded that this exceptional behavior is due to the extremely fast water exchange, whereas the exchange behavior of CH3CN is more regular. Electron spin relaxation on the isoelectronic ions is much slower than on the O-binding water analogues. This allowed a precise determination of the hyperfine coupling constants for each of the two stable isotopes of Gd3+ and Eu2+ having a nuclear spin.