A Genuine Trivalent Bis-Acylphosphide (BAP) Complex of Uranium

The genuine trivalent uranium complex, K[UIII(mesBAP)4], employing four bidentate bis(2,4,6-trimethylbenzoyl)phosphide (mesBAP) chelating ligands, is reported and obtained by the reduction of the literature-known tetravalent analog [UIV(mesBAP)4]. Hence, the bis(acyl)phosphide mesBAP ligand allowed to establish a fully reversible redox couple with uranium in the oxidation states +III and +IV. In both complexes [U(mesBAP)4]0/-, the uranium ions are coordinated to four mesBAP ligands in a square antiprismatic geometry. All new compounds have been characterized by single-crystal XRD analysis, 1H and 31P NMR, and UV/Vis/NIR electronic absorption spectroscopy, as well as SQUID magnetization and electrochemical measurements, and CW X-band EPR in the case of the trivalent complex. A trivalent uranium complex, K[UIII(mesBAP)4], has been synthesized and spectroscopically, electrochemically, and magnetochemically characterized. An X-ray diffraction analysis on single-crystals of [K(2.2.2-crypt)][UIII(mesBAP)4] reveals the molecular structure, consisting of four bis(acyl)phosphide chelates coordinated to the electron-rich uranium ion in a distorted tetragonal antiprismatic geometry. [UIII(mesBAP)4]- complements the known [UIV(mesBAP)4], thus providing an isostructural U(III/IV) redox pair with axial cavities for small molecule activation.image