DMF exchange

The origin of the change in sign of ASt is revealed when the [Ln(DMF)8]3+ exchange process is studied in inert d3-nitromethane diluent (11,93). For Ln3 + = Tb3+, Er3+, Tm3+, the general form of the DMF exchange rate equation is as in Eq. (27). When Ln3+ = Tb3+, the second term on the right-hand side dominates. Both terms are im-... 

Rate Constants and Activation Parameters for DMF Exchange on [Ln(DMF)8]3+ complexes... 

The behaviour of lanthanum in dimethyl formamide (DMF) is quite different from that in methanol and acetonitrile. While perchlorate forms inner sphere complexes with lanthanides in acetonitrile [31], no such complexes are formed in DMF [32]. The coordination properties in DMF solutions were studied by NMR and UV-Vis spectroscopy techniques [33,34], The rate of DMF exchange in the system ytterbium perchlorate-DMF-CD2CI2 was slow enough that 1H NMR resonances permitted the determination of the mean coordination number to be 7.8 0.2. Similar determination in the case of thulium(III) gave a mean coordination number of 7.7 0.2. Thus it was concluded that the predominant species in heavy lanthanides is Ln(DMF)g+ in DMF solutions. In the case of lighter lanthanides, the following equilibrium exists... 

In order to explore the possible change in the mechanism, kinetics of DMF exchange were carried out in nitromethane as a diluent. The rate constants for the DMF exchange with Ln(DMF)g+ in nitromethane are plotted as a function of [DMF] in the case of Tm,... 

Fig. 7.21. Variable pressure data for DMF exchange on Ln(DMF)g+ ions, in neat DMF [35].

Fe " than at Ru, but it should be borne in mind that the bulk of the DMF molecule may affect this comparison (cf. AV values for water " and DMF exchange in Table 8.1). Certainly for solvent exchange at Mn " the mechanism is... 

The differences in solvent exchange in HjO and DMF are also reflected in the exchange mechanisms. The activation volumes for water exchange are negative and, within experimental error, essentially constant for the different lanthanides this suggests a concerted associative (I ) mechanism. By contrast, the data for DMF exchange are more consistent with a gradual transition from an to a D mechanism between Tb " and Yb ". ... 

In square planar platinum(II) chemistry, however, the behaviour of the complexes, particularly in response to anion attack, differs greatly in the two solvents. In the reactions of m .s-[PtCl2(py)2] with radioactive chloride ion in DMSO, the exchange is only through the anion independent path (ks) while in DMF, exchange involves the anion dependent path (A y).Nifi2.N2i8... 

The ligand substitution reactions of the bivalent first-row transition metal ions are the most studied of those of the labile metal ions, probably because the visible d-d spectra of the transition metal ions make them particularly amenable to spectrophotometric study, and also because their reaction timescale is usually well within those of the SF and NMR techniques. Thus it has been shown that the mechanism of dimethylformamide (dmf) exchange on [M(dmf)6] (M = Mn—Ni) varies systematically from L to D, in contrast to the analogous [M(solvent)6] in water, methanol, and acetonitrile where the mechanism varies from L h the number of d electrons increases. This has occasioned a spectrophotometric SF study of the closely related substitution of the bidentate ligands trans-pyndine-2-azo(p-dimethylaniline) (Pada) and diethyldithiocarbamate (Et2DTC) on [M(dmf)6] shown in Eq. (13) (where L-L represents a bidentate ligand) which... 

M(dmf)6- (L-L)] " is an outer-sphere complex in which L-L resides in the second coordination sphere. Thus the data in Table 9.5 pertain to the first bond formation. The trends in lability closely follow those observed earlier for dmf exchange on [M(dmf)6], and the AV values are consistent with a dissociative mode of activation for the ligand substitution. In contrast the formation of ternary complexes of [Ni(nta)(H20)2] with amino acids is thought to involve a fast first bond formation with the chelating step being slow. " ... 

Variable-pressure and variable-temperature NMR studies have been reported of solvent exchange on various lanthanide ions in DMF, while the exchange of 1,1,3,3-tetramethylurea (tmu) on [Lu(tmu)6] in CD3CN solution is apparently dissociatively activated. The kinetics of DMF exchange and anation by SCN and NJ on [U02(DMF)5] have been measured by the NMR and stopped-flow techniques, respectively. 

Proton NMR studies show that DMF exchange on five-coordinate [Cu(Me6tren)(DMF)] has a rate constant (298 K) = 555 s, AH = 43.3 kJ mor, AS = -47.0 J K mol", and A(365 K) = 6.1 cm mol", suggesting a dissociative mode of activation. Anation of this complex by NCS", N3, and Br" is characterized by kinetic data consistent with an Id mechanism. The observance of comparatively slow solvent exchange on the zinc analog also emphasizes the importance of steric hindrance in these compounds. 

Figure 52 Schematic representation of reversible SC-SC DEF/DMF exchange at the metal center (H atoms are removed for clarity). (Reprinted with permission from Ref 65. Copyright (2011) American Chemical Society.)...

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