Technetium radiopharmaceutical development. 1. Synthesis, characterization, and structure of dichloro[hydrotris(l-pyrazolyl)borato]oxotechnetium(V)

The title complex, Tc(HBPz3)Cl2O, has been prepared by reduction of /sup 99/TcO4- in aqueous 3 M HCl in the presence of the HBPz3- ion, and subsequently characterized by infrared, Raman, and 1H NMR spectroscopy, as well as by a single-crystal x-ray structure determination. This molecular complex crystallizes in the triclinic space group P anti l, Z = 2, with a = 7.786 (2) A, b = 9.052 (2) A, c = 11.328 (2) A, a = 103.20 (2)0, β = 92.07 (3)0, and γ = 113.83 (3)0. Refinement based on 1941 independent reflections led to residual indices R1 - 0.025 and R2 = 0.024. The technetium center is in a distorted octahedral coordination environment, the six ligating atoms being the oxo oxygen atom, two chlorine atoms, and the three nitrogen atoms of the HBPz3- ligand which occupy one face of the coordination octahedron. The oxo ligand induces a significant structural trans effect, the pyrazolyl ring trans to the oxo oxygen atom being ca. 0.2 A further from the technetium center than the two rings which are trans to the chlorine atoms. Implications of the synthesis and structure of lipophilic Tc(HBPz3)Cl2O, and of the properties of the spin-paired Tc/sup V/ = O center, for the development and understanding of /sup 99m/Tc radiopharmaceuticals are discussed. 1 figure, 7 tables