Oxidatively stable, aqueous europium(II) complexes through steric and electronic manipulation of cryptand coordination chemistry.

The magnetic and optical properties of the divalent state of europium make this ion extremely attractive for use in materials, catalysis, luminescence, magnetic, and diagnostic-medical applications. A major hindrance to the use of Eu in many of these applications is the extreme propensity of the ion to oxidize to Eu, especially in aqueous solution. Research efforts aimed at increasing the stability of aqueous Eu have yielded little success: even the most stable aqueous Eu complex reported (4,7,13,16,21,24hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane europium(II), 1-Eu) is not stable enough in aqueous solution for practical use. Our research group has generated Eu complexes in aqueous solution, and here we report the most oxidatively-stable aqueous Eu complexes known. Our strategy for favoring Eu over Eu in aqueous solution involves the synthesis and use of ligands that would preferentially coordinate to large, soft, electron-rich metals like Eu. The template for our ligand design was cryptand 1 because 1-Eu is the most oxidatively-stable, aqueous Eu complex previously reported. The stability of 1-Eu is partially due to the better size match of the cavity of cryptand 1 (1.4 ) to the Eu ion (1.25 ) relative to the Eu ion (1.07 ). We hypothesized that further oxidative stabilization could be achieved by modifying the structure of cryptand 1 using four principles of coordination chemistry to stabilize electron-rich metals. Specifically, our goals were 1) to increase the steric bulk surrounding cryptand 1 to minimize interactions between Eu and its environment; 2) to reduce the Lewis basicity of cryptand 1 to favor the electronrich Eu over Eu ; 3) to change the cavity size of the cryptand to match the size of the Eu ion preferentially; and 4) to modify the hard–soft, acid–base (HSAB) properties of cryptand 1 to coordinate Eu over Eu. To implement these strategies, we studied cryptands 1–6 (Scheme 1).