Rare Earth Metal Complexes of Bidentate Nitroxide Ligands: Synthesis and Electrochemistry

Abstract

We report rare earth metal complexes with tri- and bidentate ligands including strongly electron-donating nitroxide groups. The tridentate ligand 1,3,5-tris(2'-tert-butylhydroxylaminoaryl)benzene (H3arene-triNOx) was complexed to cerium(IV) in a 2:1 ligand-to-metal stoichiometry as Ce(Harene-triNOx)2 (1). Cyclic voltammetry of this compound showed stabilization of the tetravalent cerium cation with a Ce(IV/III) couple at E1/2 = -1.82 V versus Fc/Fc(+). On the basis of the uninvolvement of the third nitroxide group in the coordination chemistry with the cerium(IV) cation, the ligand system was redesigned toward a simpler bidentate mode, and a series of rare earth metal-arene-diNOx complexes were prepared with La(III), Ce(IV), Pr(III), Tb(III), and Y(III), [RE(arene-diNOx)2](-) ([2-RE](-), RE = La, Pr, Y, Tb) and Ce(IV)(arene-diNOx)2, where H2arene-diNOx = 1,3-bis(2'-tert-butylhydroxylaminoaryl)benzene. The core structures were isostructural throughout the series, with three nitroxide groups in η(2) binding modes and one κ(1) nitroxide group coordinated to the metal center in the solid state. In all cases except Ce(IV)(arene-diNOx)2, electrochemical analysis described two subsequent, ligand-based, quasi-reversible redox waves, indicating that a stable [N-O•] group was generated on the electrochemical time scale. Chemical oxidation of the terbium complex was performed, and isolation of the resulting complex, Tb(arene-diNOx)2·CH2Cl2 (3·CH2Cl2), confirmed the assignment of the cyclic voltammograms. Magnetic data showed no evidence of mixing between the Tb(III) states and the states of the open-shell ligand.

Neodymium(III) Complexes Capable of Multi‐Electron Redox Chemistry

“…While redox‐active ligands have been proven to impart interesting magnetic or electrochemical properties to the rare earth elements due to their ability to stabilize ligand radicals, fewer examples exist that demonstrate productive electron movement for new bond formation. Mazzanti et al.…”

Neodymium(III) Complexes Capable of Multi‐Electron Redox Chemistry

“…While redox-active ligands have been proven to impart interesting magnetic [8,9] or electrochemical [10][11][12] properties to the rare earth elements due to their ability to stabilize ligand radicals, [13,14] fewer examples exist that demonstrate productive electron movement for new bond formation. Mazzanti et al reported tetradentate Schiff base complexes of Nd, Eu, Tb,Ybthat, upon reduction, store two electrons in anew CÀC s bond.…”

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

“…Ce(IV) alkoxides ( Table 7 ) have been applied as protonolysis reagents for the synthesis of Ce(IV) complexes, as demonstrated by Kim et al with the synthesis of 8-coordinate complex [Ce(Harene-TriNOx) 2 ] ( 213 , HareneTriNOx = {[(2- t BuNO)C 6 H 4 ] 2 [(2- t BuNOH)C 6 H 4 ]C 6 H 3 } 2– ) ( Scheme 63 ). 560 Gulino et al attempted to use Ce(O i Pr) 4 in salt elimination reactions with Tl(Cp) or Mg(Cp) 2 without success, observing reduction to Ce(III) instead. 561 Nonetheless, when the tin reagent R 3 SnCp (R = Me, n Bu) was employed, they were able to isolate Ce(Cp) 3 (O i Pr) ( 214 ), though structural authentication was not obtained ( Scheme 63 ).…”