Michela Cattabriga

Organometallic radiopharmaceuticals: rhenium(I) carbonyl complexes of natural bile acids and derivatives

Abstract The precursor [Re(CO)3(PPh3)2Cl] reacts with natural bile acids to give new carboxylato–carbonyl(phosphine) rhenium(I) complexes [Re(CO)2(PPh3)2(L)] (HL=natural bile acids). Coupling reactions between the carboxylic group of bile acids with the NH2 moiety of 2-aminothiazole, 2-(2-aminoethyl)pyridine and hydrazine form derivatives which coordinate the metal ion through the deprotonated amidic group. For a comparison, the carboxylato–rhenium(I) complex [NEt4][Re(CO)3Cl(L1)] (HL1=3α,7α,12α-trihydroxy-5β-cholic acid) was synthesized starting from the [Re(CO)3Cl3]2− precursor. The complex [Tc(CO)3(PPh3)2Cl] reacts with HL1 to form the corresponding carboxylato–carbonyl compound [Tc(CO)2(PPh3)2(L1)]. No difference in reactivity is observed with respect to the reactions carried out with the Re(I) starting materials. The complexes were characterized by elemental analyses, IR and NMR spectra, and a few also by mass spectrometry. An octahedral geometry may be assigned to these complexes. The two phosphine ligands are located in the axial position, and the two cis-CO groups having in trans the chelating ligand define the equatorial plane.

[PtOTf(triphos)]OTf and [PtMe2(triphos-P,P′)] as versatile synthons of platinum(II)-triphos species

The new complex [PtOTf(triphos)]OTf (triphos=bis(2-diphenylphosphinoethyl)phenyl-phosphine, OTf=CF3SO3) (1) can be most efficiently prepared by adding triflic acid to the known complex [PtMe2(triphos-P,P′)] (5) where triphos acts as a bidentate ligand. The fluxional behaviours of 1 and 5 in solution and their reactivity have been investigated by NMR: [PtOTf(triphos)]OTf is a very electrophilic complex and its reactivity is dominated by the tendency of the labile ligand OTf to be replaced by a variety of nucleophiles, while the chemistry of [PtMe2(triphos-P,P′)] is controlled by the proclivity of the third phosphorus to coordinate to platinum, as soon as a vacancy is created via Pt–Me protonolysis.

Platinum assisted cyclization of S-methyl 3-acyl-2-methyldithiocarbazates under mild conditions. Crystal structure of [Pt2(µ-SMe)(terpy)2][ClO4]3†

The reaction of the newly synthesized aqua complex [Pt(terpy)(OH2)][BF4]2 with S-methyl 3-acyl-2-methyldithiocarbazates under a variety of experimental conditions has been studied. Using a 2∶1 metal to ligand ratio in methanol, a platinum assisted cyclization was observed. The reaction products were Δ2-1,3,4-oxadiazoline-5-thione derivatives and the binuclear tricationic complex [Pt2(µ-SMe)(terpy)2]3+ whose molecular structure has been determined by X-ray crystallography. This platinum assisted transformation is proposed as a new synthetic route to Δ2-1,3,4-oxadiazoline-5-thiones under mild conditions. In the presence of an excess of a non-co-ordinating acid (HClO4, CH3SO3H or CF3SO3H) the cyclization is completely quenched and complexes with co-ordinated S-methyl 3-acyl-2-methyldithiocarbazates have been isolated. A general mechanism which accounts for the observed transformations is proposed on the basis of 1H NMR and UV/Vis evidence.