Rhenium complexes with //-ligands

Fig. 9. Scheme for syntheses of rhenium complexes with tetradentate NS, tripod ligands (R-NC R = CH2COOCH3)... 

In the bifunctional radiopharmaceuticals, Tc and Re complexes that are suitable for coupling to antibodies are almost square-pyramidal oxo complexes of tetradentate ligands, as mentioned earlier. Spies et al. [65] recently studied the syntheses and structures of rhenium complexes with a tetradentate NS3 tripod ligand 2, 2,, 2"-nitrilotris(ethanethiol) (l in Fig. 9) as an interesting alternative to the oxo complex. Such ligands could lead to weakly polar, trigonal-bipyramidal complexes in which the metal atom is more strongly shielded than in the square-pyramidal oxo complexes. Schemes for the syntheses... 

The ability of polypyridyl ligands to accept electron density from electron-rich rhenium centers and, thus, to contribute to the stabilization of rhenium complexes with the metal in low oxidation states has already been discussed for rhenium(II) compounds. Only small modifications to the polypyridyl ligand or the metal center can create dramatic differences in the properties of the resulting complexes. Generally, the starting materials which have been introduced as precursors for rhenium(II) polypyridyl complexes in Section 5.3.2.6.2, are also appropriate for the synthesis of related rhenium(l) compounds. 

The three rhenium complexes with anionic ligands, /be-[Re (bpylfCOlgfL)] (12 L=NCS, 8 CN , la Cl ), show similar photophysical and electrochemical characteristics (Table IX). 

Synthesis of Platinum and Rhenium Complexes with a Chelating (Phenylthiomethyl)silane Ligand... 

The 6-substituted cyano bipyridine has been generated in 95% yield from bipyridine A-oxide by reaction with trimethylsilylcyanide and dimethylcarbamylchloride.76 This is an improvement on an earlier method that generated 6-cyano bipyridine from the A-oxide in 62% yield using potassium cyanide and benzoyl chloride.77 This ligand has been used in electrochemical studies of rhenium complexes with sterically hindered bipyridine derivatives, as well as a precursor in the synthesis of 6-carbothioamide-bpy, which showed antitumor activity against P-388 lymphocytic leukemia in mice.77... 

Mull ES, Sattigeri VJ, Rodriguez AL et al (2002) Aryl cyclopentadienyl tricarbonyl rhenium complexes novel ligands for the estrogen receptor with potential use as estrogen radiopharmaceuticals. Bioorg Med Chem 10 1381-1398... 

Vpp couplings have allowed Coulliens et to discriminate easily between the cis and trans PP isomers of oxo and phenylimido rhenium complexes with 2-[bis(ethoxyethyl)phosphino]phenolato ligand and its derivatives Vpp of ca. 5 Hz for the cis and Vpp of ca. 235 Hz for the trans isomers have been observed. 

Low-valent rhenium complexes with Tp and bpy (bpy = bipyridine) ligands that do not include CO in the coordination sphere have been synthesized. Complexes of the type TpRe(bpy)Y (Y = Cl or cyclopentene bpy = 2,2 -bipyridyl) are derived from [TpRe bpy)Cl][OTf with the accompanying one or two electron reduction by Zn/Hg or Na/Hg amalgam, respectively (Scheme 60). " TpRe(bpy)(f/ -cyclopentene) is a potent 7i-base as evidenced by the II/I oxidation potential of —0.66 V versus NHE, 0.89 V more cathodic than TpRe(CO)(PMe3)( -cyclohexene). 

Technetium complexes with dtpa, dmsa, or mdp (methylene diphos-phonate) can be prepared by exchange reactions of the respective rhenium complexes with pertechnetate. Despite the complication that redox as well as substitution is involved here, rates correlate with metal-ligand bond strengths. A detailed kinetic study of these reactions would be welcome. Another type of ligand exchange reaction where kinetic studies are needed is the similar situation encountered in the preparation of technetium(III), (IV), or (V) complexes by reduction of pertechnetate with tin(II) complexes of the respective ligands. 

The only technetium complex has been reported (226) is [Tc(CO)4(R2dtc)]. There appear to be few reports of rhenium or technetium complexes with 1,1-dithiolato ligands. 

The dinuclear rhenium disulfido complex with n,rj -S2 ligands (ReCp )2(/U-S2)2 (Cp =7j -C5Me4Et) (type IIa2 complex in Fig. 2) undergoes an insertion reaction of acetylene and ethylene into the S-S bond according to Scheme 57. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

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