Mn11 complexes

In the case of Mn11 complexes as potential MRI contrast agents, the presence of a water molecule in the first coordination sphere is crucial to obtain reasonable relaxivity. Given the lower spin of Mn11, the outer sphere contribution to the overall relaxivity is lower than for Gdm complexes. Thus, for complexes that lack inner sphere water the relaxivity is less than 50% as compared to complexes with coordinated water (233-235). 

Thermodynamic Stability (log of Selected Mn11 Complexes and Their Comparison with Other Biogenic Metal Ions at 298 K... 

A series of 4- and 5-coordinated Mn11 complexes of salen Schiff bases (Figure 5) [20] have either N202 or N202C1 chromophores and dismutation rates similar to those of the previously reported desferrioxamine [20,32a,45,46] complexes. 

Several dinuclear Mn11 complexes based on the dinucleating L9 ligand [L9 = m-xylenediamine bis(Kemp s triacid imide)] have a (p.-carboxylato)2 core [lOOp] (Figure 18). 

A Mn11 complex has been formed with the tripodal heptadentate ligand (126), which turns out to be the only ligand coordinated to the metal ion and therefore achieves seven coordination.203... 

When the chelating ligands are able to occupy the equatorial plane of a Mn11 complex at the same time, the monodentate ligands... 

Fig. 26) (230). This led to a Mn11 complex having a square prismatic geometry. The second complex is composed of a cryptate ligand that yields a complex with a cubic symmetry (Fig. 27) (231). The third structure is built up from two tetradentate tripodal amine ligands about the Mn I ions (Fig. 26) (232). 

A Mn11 complex with an all-nitrogen first-coordination sphere was reported in 1996 to be a competent MnSOD mimic (436). In this case the complex is composed of two meridonally binding 2,6-bis(benzimi-dazol-2-yl)-pyridine ligands. When tested against the NBT system, this complex was reported to exhibit an LCB0 value of 0.72 pmol dm 3. This level of activity is comparable to that observed for the other systems tested by the nitroblue tetrazolium system. 

Most manganese complexes that have been reported to react with superoxide, however, do not react catalytically. Several have been reported by various groups in attempts to generate catalytic systems, but these only show stoichiometric interactions with superoxide. Examples of this group of complexes includes the recently reported seven-coordinate Mn11 complex, with the same ligand and a structure similar to the complex shown in Fig. 25 (229). 

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