Multinuclear NMR Studies on Lewis Acid–Lewis Base Interactions between Bis(pentafluorophenyl)borinic Acid and Uranyl β-Diketonato Complexes in Toluene

Abstract

In order to examine the possibility of Lewis acid-Lewis base (LA-LB) interactions between the boron atom of B(C6F5)2OH and the oxo groups ("yl" oxygen atoms) of uranyl β-diketonato complexes, we have measured the 1H, 11B, 17O, 19F NMR and IR spectra of toluene solutions containing β-diketonato complexes [UO2(acac)2DMSO or UO2(dfh)2DMSO, where acac = 2,4-pentanedionate, dfh = 1,1,1,2,2,6,6,7,7,7-decafluoroheptane-3,5-dionate, and DMSO = dimethyl sulfoxide] and B(C6F5)2OH. 11B and 17O NMR spectra of solutions containing UO2(dfh)2DMSO and B(C6F5)2OH showed no change in their chemical shifts regardless of the [B(C6F5)2OH]/[UO2(dfh)2DMSO] ratio. This indicates that there were no apparent interactions between B(C6F5)2OH and UO2(dfh)2DMSO. On the other hand, in the corresponding NMR spectra of solutions containing UO2(acac)2DMSO and B(C6F5)2OH, new signals were observed at a higher field than signals observed in the solutions containing only B(C6F5)2OH or UO2(acac)2DMSO, and their intensity changed with the [B(C6F5)2OH]/[UO2(acac)2DMSO] ratio. These results reveal that a complex with LA-LB interaction (B···O═U) between the boron atom of B(C6F5)2OH and the "yl" oxygen atom of UO2(acac)2DMSO was formed. IR spectra also supported such complex formation; i.e., the asymmetric O═U═O stretching band of UO2(acac)2DMSO was observed to shift from 897 to 810 cm-1 with the addition of B(C6F5)2OH. Moreover, 19F NMR spectra indicated that 1:1 and 2:1 LA-LB complexes exist in equilibrium, UO{OB(C6F5)2OH}(acac)2DMSO + B(C6F5)2OH = U{OB(C6F5)2OH}2(acac)2DMSO. The thermodynamic parameters for this equilibrium were obtained as K = (2.5 ± 0.6) × 102 M-1 (at 25 °C), ΔH = -42.4 ± 5.2 kJ mol-1, and ΔS = -96.7 ± 19.4 J K-1 mol-1. In 1H NMR spectra, the signal due to -CH groups of UO2(acac)2DMSO disappeared, and three signals due to the corresponding -CH groups newly appeared with an increase in the [B(C6F5)2OH]/[UO2(acac)2DMSO] ratio. From these phenomena, it is proposed that 1:1 and 2:1 LA-LB complexes having interactions between the -CH groups of acac and the -OH group of coordinated B(C6F5)2OH are formed depending on the [B(C6F5)2OH]/[UO2(acac)2DMSO] ratio.