The preparation and characterization of functionalized carboranes and Re/Tc-metallocarboranes as platforms for developing molecular imaging probes: Structural and cage isomerism studies

A study of the structure and reactivity of a series of carborane ligands [(RC2B9H11)−] towards Re(I) and 99mTc(I) was undertaken in which the arising products [(M(CO)3RC2B9H10)−, M = Re, 99mTc] can be used as platforms to develop organometallic molecular imaging agents. Synthetic methods are reported that produce good yields of the target ligands and metal complexes where the organometallic complexes were prepared in aqueous solvents at both the macroscopic scale and at the tracer level. NMR and X-ray crystallography studies confirmed the structures of the closo and nido-ligands and demonstrated that the rhenacarborane complexes exist as 3,1,2 and/or 2,1,8 isomers where the product distribution depends upon on the steric bulk and electronic influence of the substituent linked to the carbon atom of the carborane cage. Experiments at the tracer level with 99mTc(I) demonstrated for one ligand a difference in the ratio of isomers produced and the reactivity of the ligands compared to that for Re(I). Data is also presented to show for the first time that a 3,1,2-technetium carborane forms initially during radiolabeling and then upon heating isomerizes to the more stable 2,1,8 isomer. The comparable process is not observed for rhenium. The log D values for the technetium complexes were also measured and are within the range needed to cross the blood–brain-barrier making the reported compounds viable candidates for imaging key targets in the central nervous system.

Graphical abstractThe prepared rhenacarborane complexes exist as isomers where the product distribution depends upon on the identity of the substituent linked to the carbon atom of the carborane cage. The 3,1,2-technetium carborane forms initially during radiolabeling and then upon heating isomerizes to the more stable 2,1,8 isomer, which is not observed for rhenium.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Targeted carborane and metallocarborane complexes of Re(I) and Tc(I) were prepared and characterized. ► X-ray crystal structures show the formation of different isomers depending on the nature of the substituent. ► Unlike for Re, a 3,1,2-Tc-carborane isomer can be isolated and then converted to the 2,1,8 configuration by heating. ► The lipophilicities of 99mTc complexes were measured and are within the ideal range needed to cross the BBB.