Unprecedented pairs of uranium (iv/v) hydroxido and (iv/v/vi) oxido complexes supported by a seven-coordinate cyclen-anchored tris-aryloxide ligand

Abstract

We present the synthesis and reactivity of a newly developed, cyclen-based tris-aryloxide ligand precursor, namely cyclen(Me)(^t-Bu,t-BuArOH)₃, and its coordination chemistry to uranium. The corresponding uranium(III) complex [U^III((OAr^t-Bu,t-Bu)₃(Me)cyclen)] (1) was characterized by ¹H NMR analysis, CHN elemental analysis and UV/vis/NIR electronic absorption spectroscopy. Since no single-crystals suitable for X-ray diffraction analysis could be obtained from this precursor, 1 was oxidized with methylene chloride or silver fluoride to yield [(cyclen(Me)(^t-Bu,t-BuArO)₃)U^IV(X)] (X = Cl (2), F (3)), which were unambiguously characterized and successfully crystallized to gain insight into the molecular structure by single-crystal X-ray diffraction analysis (SC-XRD). Further, the activation of H₂O and N₂O by 1 is presented, resulting in the U(IV) complex [(cyclen(Me)(^t-Bu,t-BuArO)₃)U^IV(OH)] (4) and the U(V) complex [(cyclen(Me)(^t-Bu,t-BuArO)₃)U^V(O)] (6). Complexes 2, 3, 4, and 6 were characterized by ¹H NMR analysis, CHN elemental analysis, UV/vis/NIR electronic absorption spectroscopy, IR vibrational spectroscopy, and SQUID magnetization measurements as well as cyclic voltammetry. Furthermore, chemical oxidation of 3, 4, and 6 with AgF or AgSbF₆ was achieved leading to complexes [(cyclen(Me)(^t-Bu,t-BuArO)₃)U^V(F)₂] (5), [(cyclen(Me)(^t-Bu,t-BuArO)₃)U^V(OH)][SbF₆] (7), and [(cyclen(Me)(^t-Bu,t-BuArO)₃)U^VI(O)][SbF₆] (8). Finally, reduction of 7 with KC₈ yielded a U(IV) complex, spectroscopically and magnetochemically identified as K[(cyclen(Me)(^t-Bu,t-BuArO)₃)U^IV(O)].