Abstract

We have synthesized the [Cp*MIII(CO)3Br]+ complexes (M = Re, 99Tc) and studied their basic chemistry in water and in organic solvents in order to understand if these complexes could be synthons for the preparation of new Re- and 99Tc-based cyclopentadienyl cores for (radio)pharmaceutical applications. The [Cp*MIII(CO)3Br]Br [M = Re (1), 99Tc (1a)] complexes were obtained in nearly quantitative yield from the reaction of the corresponding [Cp*MI(CO)3] with Br2 in cold toluene. Compounds 1 and 1a are photo- and thermally unstable and undergo rapid, bromide concentration-dependent redox reactions at room temperature generating the stable [Cp*MIII(CO)3Br][(CO)3MI(-Br)3MI(CO)3] [M = Re (2), 99Tc (2a)] species as main products. Reaction of 1 with AgSbF6 gives rise to the redox-stable complex [Cp*ReIII(CO)3Br]SbF6 (3). In water, 1 and 1a produces a mixture of cis/trans-[Cp*MIIIBr2(CO)2] isomers [M = Re (cis/trans-4), 99Tc (cis/trans-4a)] via CO release. In methanol, 3 reacts with the solvent to generate the methoxycarbonyl complex trans-[Cp*ReIII(CO)2Br(COOCH3)] (5). Compound 5 is stable under basic conditions. In acidic media it is converted into [Cp*ReI(CO)3] as the major product. Kinetic studies with 13C labelled formic acid indicate that formic acid, generated from rapid hydrolysis of methyl formate released from 5, is the reducing agent and the source of CO. Reaction of 1 with 3-fluorobenzyl alcohol (3-FBA), chosen as a simple model of fluorouracil, gives the corresponding alkoxycarbonyl complex [Cp*ReIII(CO)2Br(COOCH2-C6H4F)] (7). Under acidic conditions 7 rapidly releases 3-FBA to give [Cp*ReI(CO)3]. Compounds 2, 2a, cis-4, trans-4a and 5 were structurally characterized.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)