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Rhenium(I)- and technetium(I) tricarbonyl complexes anchored by bifunctional pyrazole-diamine and pyrazole-dithioether chelators
Authors:Rute F. Vitor
Abstract:The novel pyrazolyl containing ligands 4-(HOOC)pz(CH2)2NH(CH2)2NH2 (L1) and 4-(HOOCCH2)-3,5-Me2pz(CH2)2NH(CH2)2NH2 (L2), and 3,5-Me2pz(CH2)2S(CH2)2SCH2CH3 (L3), 3,5-Me2pz(CH2)2S(CH2)2SCH2COOEt (L4) and 3,5-Me2pz(CH2)2S(CH2)2SCH2COOH (L5) were synthesized, and their ability to stabilise complexes with the fac-[M(CO)3]+ (M = Re,99mTc) moiety was evaluated. Reactions of L1-L5 with the Re(I) tricarbonyl starting materials (NEt4)2[Re(CO)3Br3] and/or [Re(CO)5Br] afforded complexes fac-[Re(CO)33-L)] (L = L1-L5 (1-5)), which contain the pyrazolyl ancillary ligands coordinated in a tridentate fashion. Complexes 1-5 were characterized by the common analytical techniques, which included single crystal X-ray diffraction analysis in the case of 4. The structural analysis of 4 confirmed the tridentate coordination mode of the pyrazole-dithioether ligand, which is facially coordinated to the Re(I) centre through the nitrogen from the pyrazole ring and the two thioether sulphur atoms, without involvement of the terminal ester functional group. The distorted octahedral coordination environment around the metal is completed by the three facial carbonyl ligands. The radioactive congeners of complexes 1, 3 and 4, fac-[99mTc(CO)33-L)]+ (L = L1 (1a), L3 (3a), L4 (4a)), have been prepared by reacting the precursor fac-[99mTc(CO)3(H2O)3]+ with the corresponding ligands, and their identity confirmed by HPLC comparison with the rhenium surrogates. Complexes 1a and 3a have been challenged in the presence of a large excess of histidine or cysteine, in order to evaluate their in vitro stability. Only a negligible displacement was observed, indicating that pyrazole-diamine and pyrazole-dithioether chelators provide a high kinetic inertness and/or stability to organometallic complexes with the fac-[99mTc(CO)3]+ moiety.
Keywords:Rhenium   Technetium   Carbonyl   Pyrazole   Bifunctional chelators   Radiopharmaceuticals
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