Positively charged GdIII cryptates: slow, associative water exchange

Abstract

A combined variable-temperature and multiple field ¹⁷O NMR, EPR and NMRD study has been performed for the first time on gadolinium(III) complexes of cryptand ligands, L¹ and L², where L¹ contains three 2,2′-bipyridine units ([bpy.bpy.bpy]) and L² is the disubstituted methyl ester derivative of L¹. The experimental data have been analysed in a simultaneous fit in order to determine parameters for water exchange, rotational dynamics and electronic relaxation for both complexes. The cryptates have three water molecules in the inner sphere which exchange with a rate of k_ex²⁹⁸ = 1.8 × 10⁶ s⁻¹ and 0.97 × 10⁶ s⁻¹ for [GdL¹(H₂O)₃]³⁺ and [GdL²(H₂O)₃]³⁺, respectively. The k_ex²⁹⁸ values obtained for these positively charged cryptates are smaller than those of the negatively charged Gd-poly(amino carboxylate) complexes. The water exchange mechanism was assessed for [GdL²(H₂O)₃]³⁺ by variable-pressure ¹⁷O NMR relaxation measurements. Based on the activation volume, ΔV^‡ = −2.5 cm³ mol⁻¹, the water exchange is an associative interchange process. The proton relaxivities, r₁, of the cryptate complexes are 9.79 mM⁻¹ s⁻¹ for [GdL¹(H₂O)₃]³⁺ and 11.18 mM⁻¹ s⁻¹ for [GdL²(H₂O)₃]³⁺ (298 K, 20 MHz), which, due to the presence of three inner sphere water molecules, represent much higher values than those obtained for Gd³⁺ poly(amino carboxylate) complexes of similar molecular weight.