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

Abstract

Homoleptic acetonitrile complexes [Gd(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](3) and [Eu(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](2) have been studied in anhydrous acetonitrile by (14)N- and (1)H 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(CH(3)CN)(9)](3+) and [Eu(CH(3)CN)(9)](2+) are 14.5 ± 1.8 ps and 11.8 ± 1.1 ps, respectively. Solvent exchange rate constants determined are (55 ± 15) × 10(6) s(-1) for the trivalent Gd(3+) and (1530 ± 200) × 10(6) s(-1) for the divalent Eu(2+). Surprisingly, for both solvate complexes CH(3)CN exchange is much slower for the less strongly N-binding acetonitrile than for the more strongly coordinated O-binding H(2)O. It is concluded that this exceptional behavior is due to the extremely fast water exchange, whereas the exchange behavior of CH(3)CN 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 Gd(3+) and Eu(2+) having a nuclear spin.