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Iminoether Complexes of the Type, fac-[Re(CO)3L{HNC(CH3)OCH3}]BF4 (L = Bipyridine or a Substituted Bipyridine): Synthesis and Properties



The methyl acetimidate (an iminoether) complexes, fac-[Re(CO)3L{HNC(CH3)OCH3}]BF4 [where L = 2,2′-bipyridine (bipy), 4,4′-Me2bipy, 5,5′-Me2bipy, or 6,6′-Me2bipy], were formed when fac-[Re(CO)3(CH3CN)3]BF4 in acetonitrile/methanol was treated with 2,2′-bipyridine (bipy) or the dimethyl-2,2′-bipyridines (4,4′-Me2bipy, 5,5′-Me2bipy, or 6,6′-Me2bipy). Structural analysis of the four fac-[Re(CO)3L{HNC(CH3)OCH3}]BF4 complexes revealed that all of the complexes crystallize with the iminoether ligand in the Z configuration and that distortions in L that involved the two pyridyl rings are minor except for 6,6′-Me2bipy, which is highly distorted. This distortion of the 6,6′-Me2bipy ligand is reflected in the NMR-spectroscopic data. Upon complex formation, the methyl group signal in the 13C NMR spectrum shifted downfield significantly for 6,6′-Me2bipy, but this did not occur in the 13C NMR spectrum of the 5,5′-Me2bipy complex. The reaction times for forming the respective fac-[Re(CO)3L{HNC(CH3)OCH3}]BF4 complexes for these two ligands were comparable, which indicated that the differences in the distortion of L and the methyl substituent position have little influence on the ease of iminoether formation. However, a higher steric bulk of the alcohol (methanol, ethanol, and2-propanol) decreases the ease of fac-[Re(CO)3(5,5′-Me2bipy){HNC(CH3)OR}]BF4 formation. Isopropanol did not form the iminoether complex after two days of heating the reaction mixture at reflux. The low reactivity of the alcohols tested versus the amines that were tested previously suggests that selective bioconjugation to the fac-{99mTc(CO)3}+ core through an amidine linkage is feasible for biomolecules that have both amine and hydroxy groups. Finally, a comparison of the C≡N bond lengths that were obtained for the fac-[Re(CO)3(CH3CN)3]BF4/PF6 complexes with those values obtained for other rhenium acetonitrile complexes revealed that there is no significant difference between the C≡N bond lengths and that there is no correlation to the acetonitrile reactivity. It is possible that the fac-{Re(CO)3}+ core stabilizes the transition state for amine or alcohol addition.

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