Studies of the Structure and Composition of Rhenium−1,1-Hydroxyethylidenediphosphonate (HEDP) Analogues of the Radiotherapeutic Agent 186ReHEDP

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Abstract
Complexes of technetium with diphosphonate ligands are widely used for the imaging and diagnosis of bone disease and most especially metastatic bone cancer. Analogous complexes of radioactive rhenium (186Re) with the ligand H4HEDP, 1,1-hydroxyethylidenediphosphonate, have been shown to be effective palliatives for the treatment of the intense pain associated with metastatic bone cancer. We have synthesized several of these analogs using nonradioactive Re and have structurally characterized them using EXAFS (extended X-ray absorption fine structure) spectroscopy. One complex synthesized via the substitution reaction of HEDP with trans-[(py)4(O)2Re]Cl in absolute ethanol appears to be the 1:1 salt of the tris-HEDP complex anion with the starting Re cation, [(py)4(O)2Re][Re(H2HEDP)3]. Three other materials, all synthesized via reduction of perrhenate by stannous chloride in the presence of excess H4HEDP ligand, are quite different in structure from the material formed by substitution. The principal difference is that each of these contains Re−Re bonds and is formulated as oligomers. The material with a large excess of reductant has Re−Re bonds of ca. 2.4 Å and is best modeled as a linear tetramer of rhenium atoms bridged by HEDP ligands which also bind an equivalent number of tin atoms with additional HEDP ligands. It is formulated as Lix[Re4(OH)2Sn4(HEDP)12]. The material formed with the least amount of reducing agent is best modeled as a triangular cluster of rhenium atoms bicapped by two HEDP ligands and bridged to three tin atoms by HEDP to form a complex Lix[Re3Sn3(HEDP)8]. It also has Re−Re bonds but of a significantly longer distance, ca. 2.8 Å. A material with an intermediate amount of reducing agent, prepared in a manner most closely resembling the medically effective palliative agent, appears to contain a mixture of these, and perhaps other, oligomers.

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