74
4 Technetium Coordinated by Organic Ligands …
more difficult (by 23 mV) in comparison to cis-[Tc(SC 6 H 5 ) 2 (dmpe) 2 ] due to the
rigidity of (diars) ligand and its lower ability to function as a π-acid.
Similar technetium complexes were examined also by Dilworth et al. (1992).
Tc(III or V) were coordinated by 2-(diphenylphosphino)benzenethiol as the ligand.
Neutral [Tc
III (dppbt) 3 ] exhibited reversible redox processes, i.e., two oxidation reactions and one reduction process. This is in contrast to [Tc
V OCl(dppbt) 2 ], which does
not undergo oxidation processes even at potentials as high as 1 V. Moreover, reduction
of the latter complex becomes reversible only at extremely low temperatures.
Crown thioether (trithiacyclononane) as a ligand in Tc-complexes was described
by White et al. (1992) and Mullen et al. (2000). They observed reversible or quasireversible redox Tc(III)Tc(II) couples. Pasqualini and Duatti (1992) reported a high
stability of [TcN(noet) 2 ] complex where (noet) is N-ethyl-N-ethoxydithiocarbamate
ligand. These species do not reveal any oxidation or reduction signals in ACN solutions in a wide potential window from −1.75 V to c.a. 1.225 V versus SCE. For the
latter potential, an irreversible two-electron oxidation process was observed.
Technetium complexes with mixed phosphine and dithiocarbamate ligands were
of interest to Okamoto et al. (1993a). All the examined systems containing dithiocarbamate ligands were characterized by reversible Tc(III)/Tc(II) and Tc(II)/Tc(I)
couples observed at ca. 0.3 and −0.5 V versus Ag, AgCl (3M NaCl), respectively. The
trans-geometry is preferred for [Tc
III (SR) 2 (dmpe) 2 ]
+ (R-alkyl or benzyl) complexes
with saturated carbon bounded to sulfur while for phenyl or phenyl derivatives acting
as R the cis-geometry is usually observed. The redox potentials of technetium coordinated by zwitter-ionic ligand (SCP) and phosphine are shifted toward more negative potentials as compared with other discussed systems. Apart from reversible
Tc(III)/Tc(II) couple also irreversible Tc(II)/Tc(I) system was observed (Okamoto
et al. 1993b).
Technetium complexes with tridentate dithiolato fragments were of interest
to Pietzsch et al. (2001, 2003). According to these authors, these types of
nonpolar, liphophilic and sterically shielded oxo-free Tc(III) complexes can be
used for the synthesis of new radiotracers. The redox stability window of synthesized “3 + 2” complexes turned out to be equal to ca. 0.7–0.8 V being
ca. 0.1–0.2 V larger than for Tc(III) “3 + 1 + 1” species. “Sulfur rich”
bis(perthiobenzoato)(dithiobenzoato)technetium(III) was of interest to Mévellec
et al. (2002). Two one-electron reversible Tc(IV)/Tc(III) and Tc(III)/Tc(II) couples
were observed for this complex. These redox couples were separated by 1.45 V. It
turned out that the examined Tc complex is more difficult to oxidize and easier to
reduce than its Re analog.
Technetium complexes with chelating thiourea ligands, including arylthioureas
and N-(N”, N”-dialkylaminothiocarbonyl)-N’-substituted benzamidines, were of
interest to Huy (2009). This author noted that Tc(III) with tridentate benzamidine ligands, [Tc
III (PPh 3 )(L
1b )(morphbtu)] can be oxidized in two well-separated
(0.86 V) one-electron steps to [Tc
V (PPh 3 )(L
1b )(morphbtu)]
2+ . The latter form turned
out to be stable in the presence of Ar. Oxo-Tc(V) complex was described as a more
thermodynamically stable in the presence of air than the Tc(III) complex.
4 Technetium Coordinated by Organic Ligands …
more difficult (by 23 mV) in comparison to cis-[Tc(SC 6 H 5 ) 2 (dmpe) 2 ] due to the
rigidity of (diars) ligand and its lower ability to function as a π-acid.
Similar technetium complexes were examined also by Dilworth et al. (1992).
Tc(III or V) were coordinated by 2-(diphenylphosphino)benzenethiol as the ligand.
Neutral [Tc
III (dppbt) 3 ] exhibited reversible redox processes, i.e., two oxidation reactions and one reduction process. This is in contrast to [Tc
V OCl(dppbt) 2 ], which does
not undergo oxidation processes even at potentials as high as 1 V. Moreover, reduction
of the latter complex becomes reversible only at extremely low temperatures.
Crown thioether (trithiacyclononane) as a ligand in Tc-complexes was described
by White et al. (1992) and Mullen et al. (2000). They observed reversible or quasireversible redox Tc(III)Tc(II) couples. Pasqualini and Duatti (1992) reported a high
stability of [TcN(noet) 2 ] complex where (noet) is N-ethyl-N-ethoxydithiocarbamate
ligand. These species do not reveal any oxidation or reduction signals in ACN solutions in a wide potential window from −1.75 V to c.a. 1.225 V versus SCE. For the
latter potential, an irreversible two-electron oxidation process was observed.
Technetium complexes with mixed phosphine and dithiocarbamate ligands were
of interest to Okamoto et al. (1993a). All the examined systems containing dithiocarbamate ligands were characterized by reversible Tc(III)/Tc(II) and Tc(II)/Tc(I)
couples observed at ca. 0.3 and −0.5 V versus Ag, AgCl (3M NaCl), respectively. The
trans-geometry is preferred for [Tc
III (SR) 2 (dmpe) 2 ]
+ (R-alkyl or benzyl) complexes
with saturated carbon bounded to sulfur while for phenyl or phenyl derivatives acting
as R the cis-geometry is usually observed. The redox potentials of technetium coordinated by zwitter-ionic ligand (SCP) and phosphine are shifted toward more negative potentials as compared with other discussed systems. Apart from reversible
Tc(III)/Tc(II) couple also irreversible Tc(II)/Tc(I) system was observed (Okamoto
et al. 1993b).
Technetium complexes with tridentate dithiolato fragments were of interest
to Pietzsch et al. (2001, 2003). According to these authors, these types of
nonpolar, liphophilic and sterically shielded oxo-free Tc(III) complexes can be
used for the synthesis of new radiotracers. The redox stability window of synthesized “3 + 2” complexes turned out to be equal to ca. 0.7–0.8 V being
ca. 0.1–0.2 V larger than for Tc(III) “3 + 1 + 1” species. “Sulfur rich”
bis(perthiobenzoato)(dithiobenzoato)technetium(III) was of interest to Mévellec
et al. (2002). Two one-electron reversible Tc(IV)/Tc(III) and Tc(III)/Tc(II) couples
were observed for this complex. These redox couples were separated by 1.45 V. It
turned out that the examined Tc complex is more difficult to oxidize and easier to
reduce than its Re analog.
Technetium complexes with chelating thiourea ligands, including arylthioureas
and N-(N”, N”-dialkylaminothiocarbonyl)-N’-substituted benzamidines, were of
interest to Huy (2009). This author noted that Tc(III) with tridentate benzamidine ligands, [Tc
III (PPh 3 )(L
1b )(morphbtu)] can be oxidized in two well-separated
(0.86 V) one-electron steps to [Tc
V (PPh 3 )(L
1b )(morphbtu)]
2+ . The latter form turned
out to be stable in the presence of Ar. Oxo-Tc(V) complex was described as a more
thermodynamically stable in the presence of air than the Tc(III) complex.