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-t-butyl-N-salicylidene-2-aminophenolato (dbusap2–) ligands, Et3NH­[(UO2)3(μ3-OH)­(dbusap)3] (Et3NH­[1]) and (Et3NH)2[(UO2)3(μ3-O)­(dbusap)3] ((Et3NH)2[2]) were synthesized and characterized. Single-crystal X-ray structures of both complexes were determined. The oxygen atom on μ3-hydroxido center in [1]− is sp3 hybridized with an average U–(μ3-O)–U bond angle of 109.7(5)°; the μ3-oxido atom in [2]2– is sp2 hybridized with an average U–(μ3-O)–U bond angle of 118.0(10)°. U–(μ3-O) distances in [1]− are long (average of 2.43(1) Å) compared with those in [2]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 = N-salicylidene-2-aminophenolato) from density functional theory (DFT) calculations resemble those in [1]− and [2]2–, respectively. The U-(μ3-O) bond in [2]2– is significantly shorter than that in [1]−, because of the greater negative charge on the central μ3-oxido. A reversible structural conversion between [2]2– and [1]− was conducted by protonation and deprotonation of the μ3-oxido/hydroxido group. The activation enthalpy and entropy of the proton self-exchange reaction between [1]− and [2]2– determined from the temperature dependence of 1H NMR coalescence are ΔH⧧ = 23 ± 2 kJ mol–1 and ΔS⧧ = −77 ± 5 J K–1 mol–1.