Synthesis, characterization, and biodistribution of a Technetium-99m ‘3+1’ fatty acid derivative. The crystal and molecular structures of a series of oxorhenium model complexes

The ‘3+1’ design of complexes of the {ReVO}3+ core was exploited in the preparation of two series of compounds incorporating fatty acid components. In one case, the fatty acid subunit is attached to the monodentate ligand ‘1’ of the [ReO(tridentate)(monodentate)] complex, with the tridentate component consisting of the sulfido–dithiolate donor, {(SCH2CH2)2S}2−, and represented by the complexes [ReO{η3-(SCH2CH2)2S}{η1-S(CH2)4CO2H}] (1) and [ReO{η3-(SCH2CH2)2S}{η1-S(CH2)10CO2H}] (2). The second series exhibits the fatty acid attachment at the tridentate site, {(SCH2CH2)2NR}2−, in the complexes of the general type [ReO{η3-(SCH2CH2)2NR}{η1-SC6H4X}] (4: R=–(CH2)7CH3, X=–Cl; 5: R=–(CH2)7CH3, X=–Br; 6: R=–(CH2)7CH3, X=–OCH3) and [ReO{η3-(SCH2CH2)2NR}{η1-SCH2C6H4X}] (7: R=–(CH2)7CH3, X=Cl; 8: R=–(CH2)15CH3, X=–H). Compounds 1 and 2 exhibit square-pyramidal geometry, while 4–8 approach more closely the trigonal bipyramidal prototype. The 99mTc analogue [99mTcO{η3-(SCH2CH2)2N(CH2)15CO2H}{η1-SCH2C6H5}] (9) was also prepared. Preliminary biodistribution studies in rats indicate poor heart-to-blood ratios (less than 1.0), suggesting that the complex is not a useful reagent for heart imaging.