72
4 Technetium Coordinated by Organic Ligands …
are strong oxidants (E
0 M(II)/M(I) > 1.15 V in DCM vs. Ag, AgCl). One may expect
the same behavior also for Tc(II) complexes.
Linder et al. (1986) synthesized nitrosyl complexes of Tc(I) and reported
results of electrochemical studies of these species. An irreversible reduction of
[Tc(NO)(CNCMe 3 ) 5 ]
+ leads to the formation of a single reduction peak at −
0.72 V (vs. SCE). This complex turned out to be stable even in the presence of
water. A quasireversible oxidation of Tc(I) bonded in [Tc(CNCMe 3 ) 6 ] to Tc(II) was
observed at 0.82 V versus SCE. Synthesis and preliminary electrochemical studies
of [Tc
II (NO)(CF 3 COO) 4 F]
2− complex were recently reported by Balasekaran et al.
(2017). CV curves recorded for this complex in trifluoroacetic acid show a oneelectron reduction wave at −0.744 V versus Ag, AgCl (3M KCl). This value was close
within ±0.1 V to those measured for other inorganic nitrosyl-technetium complexes
and reported earlier by Armstrong and Taube (1976) or Balasekaran et al. (2014).
The separation of anodic and cathodic peak was equal to 0.125 V, which excludes
reversibility of the process.
[TcD 3 ]
+ , which electrochemistry was studied by Ichimura et al. (1984), was found
to be relatively stable in aqueous solutions. The oxidation of [Tc(depe) 3 ]
+ takes place
at a potential lower by 0.164 V than the respective reaction of [Tc(dmpe) 3 ]
+ . This
observation indicates that the dmpe ligand is a less effective σ donor than depe. The
E
0 for Tc(II)/Tc(I) couples in [TcD 3 ]
2+/+ complexes turned out to be 1.57–1.83 V
higher than the respective values for the corresponding [Tc
II D 2 X 2 ]
0/−1 complexes. It
means that stabilization of spin-paired d
6 (Tc(I)) is stronger for bis(phosphinine) than
for two halide ligands. Reduction of [Tc
III D 2 X 2 ]
+ in 0.5M KNO 3 leads to the formation of water insoluble [Tc
II D 2 X 2 ]
0 . The complexes of this type were discussed in
an early work of Kirchhoff et al. (1988). E
0 of more water soluble Tc(II)-dmpe
determined in aqueous solutions was found to be higher by ca. 0.2 V than the
value measured in nonaqueous media (Ichimura et al. 1984). Additional experiments
carried out in various solvents (DMF, propylene carbonate, acetonitrile) have shown
that stabilization of dipositive charged Tc(II) complexes increases when the solvent
becomes more basic. Later work of Ichimura et al. (1985) deals with an analysis of
influence of properties of selected Schiff bases and monodentate tertiary phosphine
ligands on the electrochemistry of technetium(III) complexes in propylene carbonate
solutions. A one-electron reversible reduction of Tc(III) to Tc(II) and a one-electron
reversible oxidation of Tc(III) to Tc(IV) were observed for trans-[Tc
III (PR 2 R’) 2 L]
+
where PR 2 R’ is a monodentate tertiary phosphine with R, R’ indicating ethyl/phenyl
and L denoting a tetradentate Schiff base ligand. The difference in E
0 values between
Tc(III)/Tc(II) and Tc(IV)/Tc(III) couples turned out to be in the range from 1.5 V to
1.75 V indicating high stability of the examined Tc(III) complexes.
Complexes of technetium with sexidentate Schiff-base ligands were investigated
by Hunter and Kilcullen (1989). A one-electron reduction of [Tc
IV L
3 ]
+ to neutral
[Tc
III L
3 ] was observed and the authors suggest accumulation of such formed Tc(III)
species at the electrode surface. It is not clear whether the process is reversible or
not. Later, Refosco et al. (1993) synthesized neutral complexes of Tc(III) with O,Pbidentate phosphino-carboxylate ligands ([Tc(L
n ) 3 ], n = 1, 2, 3, see abbreviation
list for Table 4.2 for details). All the investigated systems revealed the existence
4 Technetium Coordinated by Organic Ligands …
are strong oxidants (E
0 M(II)/M(I) > 1.15 V in DCM vs. Ag, AgCl). One may expect
the same behavior also for Tc(II) complexes.
Linder et al. (1986) synthesized nitrosyl complexes of Tc(I) and reported
results of electrochemical studies of these species. An irreversible reduction of
[Tc(NO)(CNCMe 3 ) 5 ]
+ leads to the formation of a single reduction peak at −
0.72 V (vs. SCE). This complex turned out to be stable even in the presence of
water. A quasireversible oxidation of Tc(I) bonded in [Tc(CNCMe 3 ) 6 ] to Tc(II) was
observed at 0.82 V versus SCE. Synthesis and preliminary electrochemical studies
of [Tc
II (NO)(CF 3 COO) 4 F]
2− complex were recently reported by Balasekaran et al.
(2017). CV curves recorded for this complex in trifluoroacetic acid show a oneelectron reduction wave at −0.744 V versus Ag, AgCl (3M KCl). This value was close
within ±0.1 V to those measured for other inorganic nitrosyl-technetium complexes
and reported earlier by Armstrong and Taube (1976) or Balasekaran et al. (2014).
The separation of anodic and cathodic peak was equal to 0.125 V, which excludes
reversibility of the process.
[TcD 3 ]
+ , which electrochemistry was studied by Ichimura et al. (1984), was found
to be relatively stable in aqueous solutions. The oxidation of [Tc(depe) 3 ]
+ takes place
at a potential lower by 0.164 V than the respective reaction of [Tc(dmpe) 3 ]
+ . This
observation indicates that the dmpe ligand is a less effective σ donor than depe. The
E
0 for Tc(II)/Tc(I) couples in [TcD 3 ]
2+/+ complexes turned out to be 1.57–1.83 V
higher than the respective values for the corresponding [Tc
II D 2 X 2 ]
0/−1 complexes. It
means that stabilization of spin-paired d
6 (Tc(I)) is stronger for bis(phosphinine) than
for two halide ligands. Reduction of [Tc
III D 2 X 2 ]
+ in 0.5M KNO 3 leads to the formation of water insoluble [Tc
II D 2 X 2 ]
0 . The complexes of this type were discussed in
an early work of Kirchhoff et al. (1988). E
0 of more water soluble Tc(II)-dmpe
determined in aqueous solutions was found to be higher by ca. 0.2 V than the
value measured in nonaqueous media (Ichimura et al. 1984). Additional experiments
carried out in various solvents (DMF, propylene carbonate, acetonitrile) have shown
that stabilization of dipositive charged Tc(II) complexes increases when the solvent
becomes more basic. Later work of Ichimura et al. (1985) deals with an analysis of
influence of properties of selected Schiff bases and monodentate tertiary phosphine
ligands on the electrochemistry of technetium(III) complexes in propylene carbonate
solutions. A one-electron reversible reduction of Tc(III) to Tc(II) and a one-electron
reversible oxidation of Tc(III) to Tc(IV) were observed for trans-[Tc
III (PR 2 R’) 2 L]
+
where PR 2 R’ is a monodentate tertiary phosphine with R, R’ indicating ethyl/phenyl
and L denoting a tetradentate Schiff base ligand. The difference in E
0 values between
Tc(III)/Tc(II) and Tc(IV)/Tc(III) couples turned out to be in the range from 1.5 V to
1.75 V indicating high stability of the examined Tc(III) complexes.
Complexes of technetium with sexidentate Schiff-base ligands were investigated
by Hunter and Kilcullen (1989). A one-electron reduction of [Tc
IV L
3 ]
+ to neutral
[Tc
III L
3 ] was observed and the authors suggest accumulation of such formed Tc(III)
species at the electrode surface. It is not clear whether the process is reversible or
not. Later, Refosco et al. (1993) synthesized neutral complexes of Tc(III) with O,Pbidentate phosphino-carboxylate ligands ([Tc(L
n ) 3 ], n = 1, 2, 3, see abbreviation
list for Table 4.2 for details). All the investigated systems revealed the existence