Functionalized Carborane Complexes of the [M(CO)2(NO)]2+ Core (M=99mTc, Re): A New Class of Organometallic Probes for Correlated in Vitro and in Vivo Imaging

Functionalized carborane complexes of the [M(CO)(2)(NO)](2+) (M = Re, Tc-99) core have potential utility as matched pairs of probes for correlated optical (M = Re) and scintigraphic (M = Tc-99m) imaging. A synthetic route to closo-rhenacarboranes of the type [Re(CO)(2)(NO)(RR'C2B9H9)] in a manner suitable for the preparation of the radioactive technetium-99m analogues has been developed, resulting in the isolation of four novel metallacarboranes [Re(CO)(2)(NO)(RR'C2B9H9)], which exhibit both 3,1,2 (6a: R H, R' = Bn) and 2,1,8 (7a: R = R' = H; 8a: R = H, R' = Ph; 9a: R = R' = Bn) cage configurations. These complexes absorb strongly at 300-348 nm; 6a-9a fluoresce weakly at ambient temperature following, excitation at 300 nm (7a) or similar to 340 nm (6a, 8a/9a), with emission maxima ranging between 313 and 411 nm. A microwave-assisted approach was used to prepare radioactive [Tc-99m(CO)(3)](+)-carborane complexes (1b-4b) at the tracer level. When an analogous aqueous nitrosation methodology was applied to generate dicarbonylnitroso-closo-technetacarboranes, the novel complexes 6b, 8b, and 9b were isolated in good radiochemical yields (62-79%). In aqueous solutions, slow decomposition of 6b and 8b, but not 9b was observed. To better understand this process, the radioactive Re-186/188 analogue of 8a was also prepared.