Solvent Exchange and Jahn-Teller Inversion on Cu2+ in Water and N,N'-Dimethylformamide: A High-Pressure 17O NMR Study

Abstract

We have measured the longitudinal and transverse l70 NMR relaxation rates and chemical shifts in solutions of Cu2+ in DMF as a function of temperature, pressure, and magnetic field. The temperature and magnetic field dependence of the transverse relaxation rates and chemical shifts yields the following kinetic parameters for DMF exchange on [Cu(DMF)(,]2+: k«298 = (9.1 ± 2.4) x 108 s-1 [lifetime rm298 = (1.1 ± 0.4) x 10-9 s] with activation parameters AH* = 24.3 ± 1.2 kJ mol-1 and AS* = 8.1 ± 4.6 J K.-1 mol-1. The pressure dependence of the transverse relaxation rates yielded the activation volume, AV* = +8.4 ± 0.4 cm3 mol-1, which indicates a dissociative activation mode for solvent exchange. The unusually large magnetic field dispersion of the longitudinal relaxation rates at high temperatures is similar to that observed in aqueous Cu2+ solutions and is consistent with a model of relaxation due to a scalar interaction modulated by a dynamic Jahn-Teller distortion. We estimate the characteristic lifetime for inversion of the distortion axis to be rr98 = (6.0 ± 2.5) x 10-12 s, i.e., the process is very much faster than either solvent exchange or rotation of the solvated ion. We have also measured the pressure dependence of the transverse l70 NMR relaxation rates in aqueous Cu2+ solutions. This study complements our earlier variable-temperature and magnetic field study of the same system and yields the activation volume, AV* = +2.0 ± 1.5 cm3 mol-1, consistent with a dissociative activation mode for water exchange. We compare all these results with kinetic data for solvent exchange on other divalent first-row transition metal ions. We conclude that solvent exchange on hexasolvento Cu2+ occurs via the dissociatively activated loss of a solvent molecule from an axial position in the tetragonally elongated complex: the high lability of this relatively distant axial site is conferred to all the bound solvent molecules by the rapid inversion of the distortion axis.

Structure, stability, and ligand exchange of copper(II) complexes with oxidized glutathione

“…At the same time the substitution reactions of poorly nucleophilic ligands including solvent molecules on copper(II) complexes were found to proceed by a dissociative interchange (I d ) mechanism [39,40,[52][53][54][55][56]. This is evidenced, in particular, by positive values of the activation volume (DV 6 ¼ ) for solvent exchange reactions of methanol (+8.3 cm 3 mol À1 ) [53], water (+2.0 cm 3 mol À1 ) [54], and DMF (+8.4 cm 3 mol À1 ) [54] on hexasolvento Cu 2+ .…”

Dynamic Study of Metal-Ion Incorporation into Porphyrins Based on the Dynamic Characterization of Metal Ions and on Sitting-Atop Complex Formation

“…37 In Fig. 5 is visualized the trend in variation of the rate constant, kf, of the TPP-SAT complex formation in AN for a series of M 2+ ions and in the rate constant, kex, of the solvent exchange in H2O, [81][82][83][84][85] methanol, [86][87][88] DMF, [88][89][90][91] and AN. 88,[92][93][94][95] Although the values of kex for the solvent exchange on the Cu 2+ and Zn 2+ ions in AN are not available, the variation in kex for a series of M 2+ ions is similar for various solvents, as expected from Fig.…”

Application of High Pressure in the Elucidation of Inorganic and Bioinorganic Reaction Mechanisms

“…It was determined to be þ 2.1 cm 3 /mol in a later report signifying the likelihood of an I d mechanism. 188 It was also shown that DMF was exchanged on [Cu(DMF) 6 ] 2 þ by an I d mechanism (DV " ¼ þ 8.4 cm 3 /mol), a reflection of the greater bulk of the DMF ligand. Indeed, DMF exchange on [Mn(DMF) 6 ] 2 þ is characterized by a positive value of DV " , whereas the hexaaqua Mn(II) ion has a small negative DV " and proceeds to exchange water with an I a mechanism (see above).…”

“…Indeed, DMF exchange on [Mn(DMF) 6 ] 2 þ is characterized by a positive value of DV " , whereas the hexaaqua Mn(II) ion has a small negative DV " and proceeds to exchange water with an I a mechanism (see above). 188 The mechanism of solvent exchange has not been in dispute, but relatively recently based on neutron diffraction experiments and supported by molecular dynamics simulations, it was proposed that the Cu(II) aqua ion exhibits five-fold coordination. 189 It was argued that the earlier experimental data could be rationalized on the basis of a five-coordinate metal center, and the k EX value for water was modified to k EX ¼ 5.7 Â 10 9 s À1 .…”