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 {Re^VO}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}{η ¹-S(CH₂)₄CO₂H}] (1) and [ReO{η³-(SCH₂CH₂)₂S}{η ¹-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}{η ¹-SC₆H₄X}] (4: R=-(CH₂)₇CH₃, X=-Cl; 5: R=-(CH₂)₇CH₃, X=-Br; 6: R=-(CH₂)₇CH₃, X=-OCH₃) and [ReO{η³-(SCH₂CH₂)₂NR}{η ¹-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}{η ¹-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.