Synthesis, Structures, and Proton Self-Exchange Reaction of μ₃‑Oxido/Hydroxido Bridged Trinuclear Uranyl(VI) Complexes with Tridentate Schiff-Base Ligands

New μ₃-hydroxido/oxido bridged trinuclear uranyl­(VI) complexes with 3,5-di-<i>t</i>-butyl-<i>N</i>-salicylidene-2-aminophenolato (dbusap^2–) ligands, Et₃NH­[(UO₂)₃(μ₃-OH)­(dbusap)₃] (Et₃NH­[<b>1</b>]) and (Et₃NH)₂[(UO₂)₃(μ₃-O)­(dbusap)₃] ((Et₃NH)₂[<b>2</b>]) were synthesized and characterized. Single-crystal X-ray structures of both complexes were determined. The oxygen atom on μ₃-hydroxido center in [<b>1</b>]⁻ is sp³ hybridized with an average U–(μ₃-O)–U bond angle of 109.7(5)°; the μ₃-oxido atom in [<b>2</b>]^2– is sp² hybridized with an average U–(μ₃-O)–U bond angle of 118.0(10)°. U–(μ₃-O) distances in [<b>1</b>]⁻ are long (average of 2.43(1) Å) compared with those in [<b>2</b>]^2– (average of 2.23(2) Å). The optimized geometries of the [(UO₂)₃(μ₃-OH)]⁵⁺ core in [(UO₂)₃(μ₃-OH)­(sap)₃]⁻ and the [(UO₂)₃(μ₃-O)]⁴⁺ core in [(UO₂)₃(μ₃-O)­(sap)₃]^2– (where sap = <i>N</i>-salicylidene-2-aminophenolato) from density functional theory (DFT) calculations resemble those in [<b>1</b>]⁻ and [<b>2</b>]^2–, respectively. The U-(μ₃-O) bond in [<b>2</b>]^2– is significantly shorter than that in [<b>1</b>]⁻, because of the greater negative charge on the central μ₃-oxido. A reversible structural conversion between [<b>2</b>]^2– and [<b>1</b>]⁻ was conducted by protonation and deprotonation of the μ₃-oxido/hydroxido group. The activation enthalpy and entropy of the proton self-exchange reaction between [<b>1</b>]⁻ and [<b>2</b>]^2– determined from the temperature dependence of ¹H NMR coalescence are Δ<i>H</i>^⧧ = 23 ± 2 kJ mol^–1 and Δ<i>S</i>^⧧ = −77 ± 5 J K^–1 mol<sup>–1