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.