Unusual reactivity of acetylacetone with imidazole/histamine complexes and fac-M (OH₂)₃(CO)₃⁺ (M = Re, ^99mTc)

The fac-M(CO)3+ (M = Re, ^99mTc) moiety was investigated with a known bidentate ligand, acetylacetone (acac) and imidazole based ligands in a 2 + 1 and tridentate approach. In the 2 + 1 approach, the fac-Re(CO) ₃(acac)(OH₂) was reacted with imidazole or 1-methylimidazole to yield a unique anionic μ-bridging imidazol-1-ide dimer fac-[(imidazol-1-ide)-bis-(fac-acetylacetonatotricarbonylrhenium (I)], 1, or the monomer version fac-M(CO)₃(acac)(1-methylimidazole), 2, respectively. Based on the Schiff base condensation of a ketone and a primary amine, a tridentate ligand approach for fac-M(OH2)3(CO)3+ utilizing the reactivity of acac and histamine was explored in a didactic manner: (1) an in situ ligand synthesis approach reacting fac-M(CO)₃(acac)(OH ₂) with histamine (2 + 2 = 3) that led to the formation of an unexpected dimer μ-acachistimine, compound, 3, or (2) direct complexation of fac-M(OH2)3(CO)3+ with the prepared ligand acachistimine, 4, to yield the monomer product fac-M(CO)₃(acachistimine), 5. The results observed with rhenium complexes characterized by standard chemical analysis and X-ray analysis correlated with the radioactive experiments conducted with 99mTc(CO)3+.