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 {[UO₂py ₅][KI₂py₂]}_n (1) with the bulky amine-phenolate ligand potassium salt K₂(salan-^tBu ₂) 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 [U^VO₂(salan- ^tBu₂)(py)][Cp*₂Co], 6, were also isolated from the reduction of [U^VIO₂(salan-^tBu ₂)(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 E_1/2 (U ^VI/U^V) 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 μ_eff of 2.20 and 2.23 μ_B at 300 K respectively, which is compared with a μ_eff of 2.6(1) μ_B (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g _x = 1.98; g_y = 1.25; g_z = 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 π_u*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO ₂(salan-^tBu₂)(py)K] and [UO ₂(salophen-^tBu₂)( py)K] complexes with respect to disproportionation and hydrolysis.