Room-Temperature Synthesis
of Re(I) and Tc(I) Metallocarboranes
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Abstract
A series of carborane derivatives bearing guanidine substituents
were prepared and characterized, and their reactivity toward Re(I)
and Tc(I) in aqueous media was evaluated. Guanidinylation was achieved
by treating 1-aminomethyl-1,2-<i>closo</i>-dodecaborane
with <i>N</i><sup>1</sup>,<i>N</i><sup>2</sup>-di-Boc-1<i>H</i>-pyrazole-1-carboxamidine, and the associated <i>N</i>-ethyl derivative, which produced the desired products
in good (circa 50%) yield. These were deprotected and converted to
the corresponding <i>nido</i>-carboranes, which, when combined
with [M(CO)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>]<sup>+</sup> (M
= Re and <sup>99m</sup>Tc) at room temperature for 3 h or 35 °C
for 1 h, afforded the corresponding η<sup>5</sup>-metallocarborane
complexes. Corresponding reactions involving carboranes without basic
substituents generally require microwave heating at temperatures greater
than 150 °C. The rate, yields, and the temperature of the reaction
appear to be dependent on the basicity of the guanidines tested. The
biodistribution of two of the <sup>99m</sup>Tc complexes, which are
stable indefinitely in solution, were evaluated in vivo in CD1 mice
and showed that the <sup>99m</sup>Tc–carboranyl guanidine complexes
clear key nontarget organs and tissues within one half-life (6 h)
and have properties that are desirable for developing targeted molecular
imaging probes
