| The primary objective of this thesis is the resolution of an octanuclear iron-pyrazolate racemate, which has been proposed as a possible Magnetic Resonance Imaging (MRI) contrast agent, into its P and M enantiomers.;The resolution of the octanuclear iron-pyrazolate complexes was attempted through four chiral separation methods: (1) Enantioselective synthesis, (2) Crystallization, (3) Chromatography and (4) Diastereomer formation. Three chiral pyrazoles were successfully synthesized, although the enantioselective synthesis of their Fe8 complexes was not achieved. Neither crystallization nor chromatography were able to resolve efficiently the Fe8 racemic mixture. Diastereomer formation through substitution of terminal chlorides by chiral phenols, Ar*OH, on the peripheral position of the [Fe8(μ 4-O)4(4-Cl-pz)12Cl4] complex, showed to be the most promising method for the resolution.;The substitution of chlorides by (S)-(+)- and ( R)-(-)-4-sec-butylphenol on the [Fe8(μ 4-O)4(μ-4-R-pz)12Cl4] (where R = H (Fe8Cl4) or R = Cl (Fe8(4-Cl-pz)12 Cl4)) was confirmed by UV-vis spectroscopy, Elemental Analysis, and X-ray crystallography. The Fe8 complex with simple pyrazole did not show preference between the racemic and enantiopure 4-sec-butylphenols, because it crystallized in an achiral C2/c space group. However, the complex with R = Cl crystallized in chiral P213 space group with the racemic as well as with the enantiopure 4-sec-butylphenols, which also permitted to identify the R and S enantiomers of the ligand. Manual separation of two different products of the complex allow the identification of the diastereomer pairs M(R)/P(R) and M(S)/P( S) by X-ray crystallography and Circular Dichroism. The M(R) isomer was found in the first crop of crystals, while the P(R) was found in the recrystallized residue. Similarly, the M( S) crystallized on the bottom of the vial, while the P(S) on the wall. The separation the two diastereomeric pairs was confirmed by the opposite signs of absorption bands in their CD spectra.;This investigation resulted in the first ever resolution of a chiral multinuclear complex of such complexity—a molecule containing four chiral axes—confirming the original hypothesis. The availability of enantiopure Fe8 complexes opens up new areas of research, such as the use of these complexes as chiral catalysts and the study of their interactions with biological substrates. |
