Room-Temperature Synthesis of Re(I) and Tc(I) Metallocarboranes

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-closo-dodecaborane with N¹,N²-di-Boc-1H-pyrazole-1-carboxamidine, and the associated N-ethyl derivative, which produced the desired products in good (circa 50%) yield. These were deprotected and converted to the corresponding nido-carboranes, which, when combined with [M­(CO)₃(H₂O)₃]⁺ (M = Re and ^99mTc) at room temperature for 3 h or 35 °C for 1 h, afforded the corresponding η⁵-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 ^99mTc complexes, which are stable indefinitely in solution, were evaluated in vivo in CD1 mice and showed that the ^99mTc–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.