Synthesis, Structures, and Proton Self-Exchange Reaction of μ 3 -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, Et3 NH[(UO2 )3 (μ3 -OH)(dbusap)3 ] (Et3 NH[1]) and (Et3 NH)2 [(UO2 )3 (μ3 -O)(dbusap)3 ] ((Et3 NH)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 of1 H NMR coalescence are ΔH⧧ = 23 ± 2 kJ mol-1 and ΔS⧧ = -77 ± 5 J K-1 mol-1 .