Synthesis, Structure, and Bonding of Stable Complexes of Pentavalent Uranyl

Stable complexes of pentavalent uranyl [UO2(salan-tBu2)(py)K]n (3), [UO2(salan-tBu2)(py)K(18C6)] (4), and [UO2(salophen-tBu2)(thf)]K(thf)2}n (8) have been synthesized from the reaction of the complex {[UO2py5][KI2py2]}n (1) with the bulky amine-phenolate ligand potassium salt K2(salan-tBu2) or the Schiff base ligand potassium salt K2(salophen-tBu2) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV−vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [UVO2(salan-tBu2)(py)][Cp*2Co], 6, were also isolated from the reduction of [UVIO2(salan-tBu2)(py)], 5, with Cp*2Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO2py5]I·py (2). The measured redox potentials E1/2 (UVI/UV) are significantly different for 2 (−0.91 and −0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range −1.65 to −1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give μeff of 2.20 and 2.23 μB at 300 K respectively, which is compared with a μeff of 2.6(1) μB (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (gx = 1.98; gy = 1.25; gz = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal−ligand interaction in these complexes leading to a significant lowering of the πu*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO2(salan-tBu2)(py)K] and [UO2(salophen-tBu2)(py)K] complexes with respect to disproportionation and hydrolysis.