Lanthanide complexes dipicolinates

There are difficulties associated with the use of ordinary electronic absorption spectra of lanthanide complexes in solution to provide detailed information regarding coordination number and geometry. However, difference spectra versus NdCl3 are reported for Nd3+-ligand (L) solutions for the 4/9/2— -4G5/2, 4G7,2 transitions (L = dipicolinate, oxydiacetate, iminodiacet-ate, malate, methyliminodiacetate and Ar,Ar -ethylenebis Af-(o-hydroxyphenyl)glycinate ). Hypersensitive behaviour was examined and transition dipole strengths were discussed in terms of the nature of the complex species present.431... 

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... 

LeBozec and co-workers have reported nonlinear behavior in a series ofterpyri-dyl and dipicolinic acid complexes, with further studies on these complexes by Maury and co-workers [83, 84]. Their research was on new molecular materials for optoelectronics, with studies based on octupolar nonlinear optical molecules showing that molecular quadratic hyperpolarizability values were strongly influenced by the symmetry of the complexes [85]. Other studies on organic-lanthanide complexes with nonlinear optics have also reported second- and third-harmonic generation behavior with simultaneous multiphoton absorption properties [50]. Such studies have shown the importance of coordination chemistry as a versatile tool in the design of nonlinear materials. 

Wliile investigating the effects of ancillary ligands on the binding of dipicolinate, we developed a binding afBnity by competition (BAG) assay to quantify the binding constant of anal5rte to lanthanide complex (Eq. 3) (102). 

We plan to take advantage of ligand-induced enhancement of dipicolinate binding affinity to improve current bacterial spore detection technologies in two ways. The first involves appending terbium(macrocycle) complexes to solid polymer substrates to improve microscopy-based endospore assays. In a second method, we will bind lanthanide complexes to silica to concentrate dipicolinate from very dilute samples on colmnns. 

D Aleo, A., Plcot, A., Beeby, A., et al. (2008) Efficient sensitization of europium, ytterbium, and neodymium functionalized tris-dipicolinate lanthanide complexes through tunable charge-transfer excited states. Inorganic Chemistry, 47, 10258. 

Lanthanide coordination chemistry is still not completely understood, and many attempts are usually required to design specific Inminescent lanthanide complexes. As an alternative to rational design, a combinatorial approach shows promise for the development of specific luminescent lanthanide materials. Shinoda et al. built a combinatorial library to optimize luminescent lanthanide complexes structurally for the selective detection of amino acids. The lanthanide complex library included 196 combinations of 4 lanthanide centers, 7 pyridine ligands, and 7 amino acid substrates (Figure 16.16). The luminescence responses for amino acids depended on the nature of the ligand used. A series of Tb + complexes typically exhibited interesting luminescence responses. The TV+-picolinic acid complex and Tb -pyrazinecar-boxylic acid complex preferred zwitterionic Ala, Val, Phe, and Gin, whereas the Tb complex with dipicolinic acid favored anionic Gin and Asp. 

It might be considered that the way in which to understand the water complex ion chemistry of the lanthanide ions in aqueous solution would be to start from their hydrates. In fact the hydrates prove to be most intransigent complexes. Their structures are still somewhat uncertain. I shall therefore start from a study of the tris-dipicolinates, i.e. tris 2,2 -carboxy-pyridine complexes, Ln(dipic)3, about which a great deal is known. 

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 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. 

In the case of nitriloacetate complexes, the changes in enthalpy have been explained in terms of the consequences of lanthanide contraction (i) increasingly exothermic complexation with decreasing crystal radius and (ii) decreasing exothermic complexation with decreasing hydration of the cation [22]. In the case of dipicolinates and diglycolates these effects become small as the coordination sphere loses water molecules. Thus AH3 and A S3 vary more regularly than A Hi and A Si. 

Dipicolinates (pyridine-2,6-dicarboxylates) have been investigated in some detail with structures determined for several tris complexes such as Na3[Nd(dipic)3]-15H20 and Na3[Yb(dipic)3]13H20, typical of the early and later lanthanides, respectively. The lanthanides have essentially tricapped trigonal prismatic coordination geometries, isostruc-tural along the lanthanide series they have been the subject of important solution NMR studies. Mono and bis complexes can also be synthesized the mono complex [Nd(dipic)(H20)4] CIO4 has a polymeric structure in which each picolinate is bound to three different (nine-coordinate) neodymium ions. 

Keywords Lanthanide Sensor Sensitized luminescence Dipicolinate Macrocycle Ternary complex Bacterial spore Ancillary ligand Gadolinium break Catecholamine Salicylic acid Salicylurate. 

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