Sapphyrins metallation

Sandwich complexes nickel. 5, 35 Sapphyrins, 2, 888 demetallation, 2, 891 metallation, 2, 891 reactions, 2, 891 synthesis, 2, 889 Sarcoplasmic reticulum calcium/magnesium ATPase, 6, 566 skeletal muscle... 

In view of the above, it is not surprising that significant efforts have been dedicated to the problem of generating synthetic chloride receptors. Indeed, a number of such systems are known. Unfortunately, many of them are hydrophilic polycations and, accordingly, suffer from low solubility in organic media. " " Others are metal- or metalloid-derived systems. The clinical utility of these is clouded by metal toxicity. ° The sapphyrins, being both wholly organic and... 

Sapphyrins (39) are metallated with divalent metal aoetates in the presence of sodium acetate (Scheme 109).274 For the Zn11 and Co11 complexes, structure (44) was proposed with the dianionic macrocycle coordinating to the metal ion as a tetradentate ligand. These metallosapphyrins experience ring contraction to porphyrin complexes under mass spectral conditions. 

Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe2+ and Mg2+ (e.g. haemoglobin and chlorophylls, Sections 2.3-2.5) however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5A in diameter, in which (particularly when protonated) the Nff... 

To date very little is known about the coordination chemistry of the sapphyrin macrocycle, and no complexes have been reported for the dioxosapphyrin or the thiosapphyrin. In the free-base form, sapphyrin is a potential trianionic ligand and thus, on paper at least, seems perfectly suited for complexing the normally trivalent cations of the lanthanide series. Presumably, lanthanide(III) complexes of sapphyrin, which would be neutral (and potentially useful for magnetic resonance imaging applications see Sect. 12.2), would be expected to form easily under typical porphyrin metalation conditions. However, in spite of the apparent correspondence in the sapphyrin core size and the ionic radii of the lanthanides, to date no lanthanide cation has been inserted into the core of the sapphyrin macrocycle using a variety of standard metal insertion techniques [156]. Nor, have any other pentaligated complexes of any other metal cations been reported to... 

Because of their basic resemblance to porphyrins, it was initially expected that the sapphyrins would mimic, at least on some level, the rich coordination chemistry displayed by the porphyrins. However, the larger core size ca. 5.5 A inner N-N diameter vs. ca. 4.0 A for porphyrins), the greater number of potentially chelating heteroatom centers, and the fact that pentaazasapphyrins when fully deprotonated are potentially trianionic ligands made sapphyrin a likely candidate for large metal chelation, particularly as a potential ligand for the trivalent lanthanides and actinides. Unfortunately, in spite of extensive effort, this hope remains largely unrealized. Nonetheless, some metal complexes of sapphyrins and heterosapphyrins have been successfully prepared and characterized. Their preparation and properties are reviewed in this section. 

Quite early on, investigations into the metal-chelating chemistry of the sapphyrins were carried out by Woodward and his group. However, only a few poorly characterized metal complexes resulted from these efforts, and these primarily involved first-row transition elements. For instance, these workers found that, when decamethyl sapphyrin was treated with the acetate salts of NP", Fe, ... 

Unfortunately, crystals of X-ray diffraction quality for the above cobalt and zinc derivatives could not be obtained. Thus, tentative assignments of structure were made on the basis of mass spectrometric analysis. Such studies were consistent with the apparent complexation of only one metal atom by the sapphyrin core. They also revealed a parent ion for each of these complexes that was considered most in accord with the symmetrical structures represented by 5,83 and 5.84 (Figure 5.5.1). In other words, based on these analyses. Woodward and coworkers proposed cobalt(II) and zinc(II) sapphyrin complexes wherein only four of the five possible nitrogen centers interacted with the metal center, and only two of the three possible NH protons became lost upon complexation. ... 

Several years after the initial report of Woodward, et al., Sessler and coworkers found that sapphyrin 5.21 would in fact form a stable complex with uranyl cation. It was determined, however, that 1102 " complexation was accompanied by reduction of the sapphyrin macrocycle. Specifically, it was found that metal insertion occurred concurrent with addition of methanol to one of the meso-Xike carbon centers. The net result was an overall neutral complex of a modified, nonaromatic sapphyrin-like system (Scheme 5.5.2). 

In spite of the obvious structural differences, macrocycles 6.26 and 6.27 exhibit physical and spectral properties that are similar to those of the parent sapphyrins. For example, organic solutions of these macrocycles are green, while as solids they are metallic blue. The bishydrochloride salt of the alkyne-containing macrocycle (6.26b) in CH2CI2 displays a rather intense Soret-like absorbance band at 476 nm... 

IR spectra, 469 soils, 962 structure, 471 Salt hydrates, 296 Samarium(III) complexes salicylic acid crystal structure, 481 Sapphyrins, 888 demetallation, 891 metallation, 891 reactions, 891 synthesis, 889 Scandium reactions... 

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