Structural Investigation of the Aqueous Eu²⁺ Ion: Comparison with Sr²⁺ Using the XAFS Technique

Structural parameters of the Sr2+ and, for the first time, of the Eu2+ ions in aqueous solution were determined by the XAFS method. For the Sr2+, the use of an improved theoretical approach led to a first shell coordination number of 8.0 (3), a Sr−O distance of 2.600 (3) Å and a Debye−Waller factor of σ2 = 0.0126 (5) Å2. These results were confirmed by an analysis performed with experimental phase and amplitude, extracted from the solid reference compound [Sr(H2O)8](OH)2. The same theoretical approach was used for the analysis of the Eu2+ XAFS spectra in aqueous solution. This gives a first coordination shell of Eu2+ formed by 7.2 (3) water molecules, an Eu−O distance of 2.584 (5) Å, and a high Debye−Waller factor of σ2 = 0.0138 (5) Å2. Whereas Eu3+ occurs as an equilibrium between the [Eu(H2O)8]3+ and the [Eu(H2O)9]3+ ions, Eu2+ occurs in aqueous solution as an equilibrium between a predominant [Eu(H2O)7]2+ ion and a minor [Eu(H2O)8]2+ species

Solution X-ray Absorption Fine Structure Study of the Eu²⁺ and Sr²⁺ Ions: Unexpected Solvent and Metal Ion Dependencies of the Solvation Numbers

Structural parameters of the Sr2+ and, for the first time, of the Eu2+ ions in nonaqueous solutions were determined by the X-ray absorption fine structure (XAFS) method and compared with those of the aqua ions. For both Eu2+ and Sr2+ ions, a decrease in coordination number (N) and metal-to-solvent distances was found along the increasingly solvating MeCN 2O ≪ DMF N = 8 with [Sr(MeCN)8]2+ (2.665(6) Å) and [Sr(H2O)8]2+ (2.600(3) Å) and N = 7 with [Sr(DMF)7]2+ (2.555(5) Å) and [Sr(DMSO)7]2+ (2.540(7) Å); for europium(II), N = 8 with [Eu(MeCN)8]2+ (2.640(4) Å), N = 7 with [Eu(H2O)7]2+ (2.584(5) Å), and N = 6 with [Eu(DMF)6]2+ (2.541(3) Å) and by extrapolation [Eu(DMSO)6]2+ (2.525 Å). Smaller coordination numbers are observed for the Eu2+ ion in O-coordinating solvents. The ionic radii of both Sr2+ and Eu2+ ions are very similar, but the slightly softer character of the Eu2+ ion leads to shorter M−N and longer M−O bonds

Solution X-ray Absorption Fine Structure Study of the Eu²⁺ and Sr²⁺ Ions: Unexpected Solvent and Metal Ion Dependencies of the Solvation Numbers

Structural parameters of the Sr2+ and, for the first time, of the Eu2+ ions in nonaqueous solutions were determined by the X-ray absorption fine structure (XAFS) method and compared with those of the aqua ions. For both Eu2+ and Sr2+ ions, a decrease in coordination number (N) and metal-to-solvent distances was found along the increasingly solvating MeCN 2O ≪ DMF N = 8 with [Sr(MeCN)8]2+ (2.665(6) Å) and [Sr(H2O)8]2+ (2.600(3) Å) and N = 7 with [Sr(DMF)7]2+ (2.555(5) Å) and [Sr(DMSO)7]2+ (2.540(7) Å); for europium(II), N = 8 with [Eu(MeCN)8]2+ (2.640(4) Å), N = 7 with [Eu(H2O)7]2+ (2.584(5) Å), and N = 6 with [Eu(DMF)6]2+ (2.541(3) Å) and by extrapolation [Eu(DMSO)6]2+ (2.525 Å). Smaller coordination numbers are observed for the Eu2+ ion in O-coordinating solvents. The ionic radii of both Sr2+ and Eu2+ ions are very similar, but the slightly softer character of the Eu2+ ion leads to shorter M−N and longer M−O bonds

Bis-Faced Aminoporphyrin Templates for the Synthesis of Chiral Catalysts and Hemeprotein Analogues

Bis-Faced Aminoporphyrin Templates for the Synthesis of Chiral Catalysts and Hemeprotein Analogue