Dipicolinate complexes

Different coordination modes have been reported in transition metal-dipicolinate complexes. These are shown in Figure 2 ... 

The structure of the deep red diaquoperoxotitanium(IV) dipicolinate complex, [Ti02(C7H304N)(H20)2].2H20 was reported. The complex (see... 

A further application of relaxation rate measurements is that similar 1/71 ratios in a series of lanthanide complexes may be taken to indicate an isostructural series. However, this approach has the limitation that if only part of the complex is studied, perhaps an organic ligand, its 71 ratios would be independent of changes, for example changes in the extent of hydration in the remainder of the complex, provided that the conformation of the ligand relative to the lanthanide ion were preserved. An excellent example of the use of 71 data in a quite different way is its use to determine hydration numbers of lanthanide dipicolinate complexes.562... 

The enthalpy and entropy of complex formation between Zn11 and picolinate and dipicolinate anions in aqueous solution have been determined by calorimetry and from formation constant data. The greater stability of the dipicolinate complex compared to the picolinate complex reflects an entropy effect, and Ais actually less favourable. These anions are well known to have a low basicity to H+ compared to their complexing ability to metals. In the present case, this probably reflects the coplanarity of the carboxylate anions and the pyridine ring, so that the oxygen atoms are in a favourable position to coordinate.800... 

Table 2. Isotropic shifts of Ln(III) tris-dipicolinate complexes obtained from solution containing 1 2.5 and 1 4 mole ratio of metal DPA measured at 270 MHz 300 °K...

Further use of relaxation data, now studying the water and the ligand protons34 36, leads to an estimate of the outer sphere hydration of the lanthanides. We know there are no water molecules in the first coordination sphere of course. These outer sphere relaxation data for the different cations are proportional to susceptibilities and electron relaxation times and become very useful in the study of the inner sphere hydration of other complexes M(dipic) (H20)x and M(dipic)2(H20)y, see below. Note that there is no evidence of further association of the Ln(III) tris-dipicolinate complexes with small cations such as sodium ions. Later we shall show that these anions can bind to biological cationic surfaces and act as shift or relaxation probes. 

We pass next to the Ln(III) bis-dipicolinate complexes, shown in Fig. 6. There is no crystal structure for the complexes. Flowever detailed examination of the proton NMR spectra at room temperature shows that the shift ratios are again constant throughout the series and that absolute shifts follow Bleaney s predicted values, Table 3. The complexes must be isostructural and must have axial symmetry. Again the use of relaxation data gives an independent assessment of the relative distances of meta and para protons. We can put all the data together and give a structure for the complex ion as in Fig. 6 leaving three water molecules in the inner sphere. To prove that this is so we must analyse the proportions of the water, both bound and outer sphere. 

We can turn finally to the mono dipicolinate complexes. The same analysis as above shows that in solution the M(dipic) complexes are isostructural. The exact structure has been determined using shift and relaxation data as above, see Refs. 34—36. Knowledge of the relaxation data for both ligand and water protons and the known relaxation of the contribution to water relaxation from the outer sphere then permits calculation of the number of water molecules in M(dipic)(H20)n. We have shown that n = 6 for all the lanthanides. 

Table 4. Values of the relaxation enhancements of solvent water, MR1E, in presence of different Ln(III)-dipicolinate complexes measured at 20 MHz, 24 1 °C ...

FIGURE 38 Top variation of of the tris(dipicolinate) complex versus the metal ion concentration. Bottom variation of Qlfjj versus pH (redrawn from Chauvin et al., 2004). 

Bacterial spore walls are like cell walls, but surrounded by a calcium dipicolinate complex, exterior to which is a protein shell, rich in disulfide bonds (Gould and Hitchins, 1963). 

Turning to the case of R(III) complexes, it should be first noted that the theoretical radiative lifetime of a given R(III) complex may be (very) different from that of the solvated ion (Werts et al., 2002). Therefore, the lifetime values of R(III) complexes in various solvents should be interpreted with caution. However, some experimental facts are in contradiction with eq. (13) (i) the deactivation efficiency of a ligand can be even lower than that of D2O in one case, an Eu(III) complex in D2O has a lifetime (tobs = 4.66 ms) larger than that of Eu " aq in D2O (Elhabiri et al., 1999) (ii) the lifetime of a Tb complex in D2O is decreased by further complexation with various anions such as OH , NOs", CH3C02, HCOs", citrate or C03 (Dickins et al., 1998). In fact, the radiative lifetime value of a given complex is rather difficult to derive and very few calculations (or data) are available so that comparison between/ rad and obs values is only possible for Eu-dipicolinate complexes (Werts et al., 2002) (see table 5). Nevertheless, it can be seen that the ligand has an important impact onto the lifetime, even in D2O (see table 5). In particular, it was shown in a veiy detailed study on Eu, Gd, Tb and Yb (Beeby et al., 1999), that the -OH, -NH and -CH bonds have an effect A similar study has... 

Figure 5.9 From left to right structure of water soluble tris-dipicolinate complexes (Ln = Tb or Eu), three photon excited (lex = S00nm) luminescence, variation of the luminescence intensity with the incident laser power for a two (right, top) or three (right, bottom) photon excitation (Zex = 532 or 800 nm respectively) In linear or log scale, respectively (between parenthesis is indicated the experimental determination of the multi-photon absorption)...

In 2007, our group reported the hrst microscopic observation of two-photon excited Tb(III) emission, in bulk crystals of tris-dipicolinate complex or in protein crystals (e.g., HEWL, hen eggs white lysozyme [102]) doped with the same complex supress crystals [60]. In spite of the very low 2PA efficiency of this complex, the microscopy experiment was successful... 

Saravanan, K. and Govindarajan, S. (2002) Dipicolinate complexes of main group metals with hydrazinium cation. Proceedings of the Indian Academy of Sciences — Chemical Sciences, 112, 25-36. 

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