Gadolinium porphyrin complexes

Another approach has been tackled with by Sessler et al. [37, 38] who described gadolinium(III) complexes with texaphyrins as potential MRI contrast agents. When the expanded porphyrin ring is subtituted by water solubilizing groups (Fig. 5), the chelate is sufficiently water soluble to be injected. It is... 

Porphyrin complexes of yttrium, lanthanum, cerium, praesodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and thorium can be synthesized using this method. 

The solvent effects on the absorption spectrum of Gd(TPP)(acac) have been studied by Radzki and Giannotti (1993). As shown in fig. 7, both the Soret and Q(1,0) bands are substantially shifted in different solvents, in particular in solvents with donor atoms. The magnitude is about two times larger than that for metal-free porphyrin H2(TPP) which can be attributed to complex formation between the gadolinium porphyrin and the Lewis base solvents. 

Weiss et al. have employed the latter method to synthesize the gadolinium analogues [144]. Treatment of Li2Pc with 2equiv. of Gd(acac)3- H20 produces Gd(Pc)(acac), which further reacts with 1 equiv. of H2(TPP) giving GdPc(TPP) in 65% yield and a small amount of Gd2(Pc)(TPP)2. However, by using 4 equiv. of the metal salt and 2.5 of the metal-free porphyrin, the binuclear complex Gd2(Pc)(TPP)2 has been isolated in 78% yield. 

As shown in Fig. 7.6, texaphyrins have a larger cavity than porphyrins so they can form complexes with lanthanide metals such as gadolinium (XCYTRIN ), that enhances the efficacy of treatment for certain brain tumours, and lutetium (LUTRIN ), used as a sensitizer for photodynamic therapy of recurrent breast cancer [17], Crucial to their success is the increased number of donor atoms available as the more lanthanide binding sites that a ligand can satisfy, the more stable the complex. 

As noted above, the porphyrins fail to form stable complexes with gadolinium(III) [13,74,98]. Since Gd(III) is inherently more paramagnetic than Mn(III), it is intrinsically more attractive for use in MRI contrast applications [6]. In view of this, the authors proposed that a larger porphyrin-like macrocycle, capable of coordinating gadolinium(III) in a stable, nonlabile manner, could prove useful as an... 

Figure 17 The CD spectra of gadolinium(III) porphyrinate (31) in benzene on incorporation of L- or D-phenylglycines (L-32 solid line, d-32 dashed line) that were extracted from their aqueous solution. (Reprinted from Inorg. Chim. Acta, 300-302, H. Tami-aki, N. Matsumoto, S. Unno, et al.. Induced circular dichroism active complexes of synthetic gadolinium(III) porphyrinates with chiral amino acids, 243, Copyright (2000), with permission from Elsevier.)...

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