76
4 Technetium Coordinated by Organic Ligands …
Much more complex electrochemical behavior of technetium species was
observed for oxo-Tc(V) species containing Schiff bases (Refosco et al. 1988; Tisato
et al. 1989, 1990). A loss of a ligand in the cis- position to Tc = O bond was
discussed as a result of a one-electron reduction of Tc(V) (in TcOCl(L B ) 2 ) to Tc(IV).
Electrogenerated Tc(IV) complex undergoes isomerization to more stable product
with vacant site trans- to Tc = O linkage. The latter complex is reduced to Tc(III),
which also undergoes isomerization. Based on the results, the authors stated that
8-quinolinol ligands (especially 5-nitrido derivatives) effectively stabilize Tc(IV)
and Tc(III) oxidation states. Tisato et al. (1989) observed two well-developed oneelectron redox couples for quinquedentate Schiff base ligands. Linden et al. (1994)
studied electrochemistry of technetium(V)oxo nitroimidazole complex, which is a
very promising imaging agent that can be applied in the analysis of regional hypoxia.
The CV curve has shown an irreversible process at low potentials.
Bolzati et al. (1997) successfully synthesized nitrido Tc(V) complexes with
ferrocenedithiocarboxylate. Electrochemical studies of this type of complexes reveal
the existence of quasi-reversible couples located very close to Fe(III)-Fe(II) systems
characteristic for free FcCS 2 ligand.
Unusual Tc(VI) complexes with 3,5-di-tert-butylcatechol (DBCat) ligand were
synthesized by deLearie et al. (1989). Tc(DBCat) 3 was synthesized in a simple reaction between pertechnetates and the ligand. The latter played the dual role of reductor
and complexing agent. CV curves recorded for these complexes revealed three oneelectron reversible reactions, including Tc(VI)/Tc(V) and Tc(V)/Tc(IV) couples and
reversible reactions of the ligand. The redox process characteristic for this ligand was
shifted toward more positive values by 0.86–0.87 V as compared with Tc(VI)/Tc(V).
Additional two irreversible oxidation processes took place at much higher potentials
(c.a 1 V and 1.3 V vs. Tc(VI)/Tc(V) couple). A comparison of the electrochemical
properties of the respective Tc and Re complexes revealed that cationic technetium
complex is more stable than its rhenium analogs.
Technetium of a low valency (especially +4, +3) can also exist as μ-oxo technetium complexes. For example, Kastner et al. (1986) or Clarke et al. (1988) examined such types of complexes with halide and pyridine ligands. CV curves of these
compounds revealed two well-separated one-electron redox processes. The dissymmetric ligands (1. type: [X(L) 3 XTc-O-TcX(L)X 3 ]) turned out to be generally less
stable than the asymmetric ones (2. type: [X(L) 4 Tc-O-Tc(L)X 4 ]. The differences
between E 1/2 of Tc
IV -Tc
III /Tc
III -Tc
III and Tc
III -Tc
III /Tc
III -Tc
II redox couples were
equal to 0.97–1.16 V and 1.35–1.63 V for the first and the second type of Tc
complexes, respectively. Later, Linden et al. (1989) synthesized a dimeric Tc
IV -
Tc
IV complex, [(TCTA)Tc(μ-O) 2 Tc(TCTA)]
2− . Voltammetric curves recorded for
these species revealed the existence of three redox systems therein pH independent
Tc(IV-IV)/Tc(IV-III) couple at a potential of 0.167 V versus SCE.
New oxo-bridged complexes of Tc containing bipiridine were described in the
next article by Lu et al. (1993). Three well-separated one-electron redox couples
were observed. Noteworthy is the fact that (μ−O)[X (L) 2 Tc]
2+
2 was relatively stable
4 Technetium Coordinated by Organic Ligands …
Much more complex electrochemical behavior of technetium species was
observed for oxo-Tc(V) species containing Schiff bases (Refosco et al. 1988; Tisato
et al. 1989, 1990). A loss of a ligand in the cis- position to Tc = O bond was
discussed as a result of a one-electron reduction of Tc(V) (in TcOCl(L B ) 2 ) to Tc(IV).
Electrogenerated Tc(IV) complex undergoes isomerization to more stable product
with vacant site trans- to Tc = O linkage. The latter complex is reduced to Tc(III),
which also undergoes isomerization. Based on the results, the authors stated that
8-quinolinol ligands (especially 5-nitrido derivatives) effectively stabilize Tc(IV)
and Tc(III) oxidation states. Tisato et al. (1989) observed two well-developed oneelectron redox couples for quinquedentate Schiff base ligands. Linden et al. (1994)
studied electrochemistry of technetium(V)oxo nitroimidazole complex, which is a
very promising imaging agent that can be applied in the analysis of regional hypoxia.
The CV curve has shown an irreversible process at low potentials.
Bolzati et al. (1997) successfully synthesized nitrido Tc(V) complexes with
ferrocenedithiocarboxylate. Electrochemical studies of this type of complexes reveal
the existence of quasi-reversible couples located very close to Fe(III)-Fe(II) systems
characteristic for free FcCS 2 ligand.
Unusual Tc(VI) complexes with 3,5-di-tert-butylcatechol (DBCat) ligand were
synthesized by deLearie et al. (1989). Tc(DBCat) 3 was synthesized in a simple reaction between pertechnetates and the ligand. The latter played the dual role of reductor
and complexing agent. CV curves recorded for these complexes revealed three oneelectron reversible reactions, including Tc(VI)/Tc(V) and Tc(V)/Tc(IV) couples and
reversible reactions of the ligand. The redox process characteristic for this ligand was
shifted toward more positive values by 0.86–0.87 V as compared with Tc(VI)/Tc(V).
Additional two irreversible oxidation processes took place at much higher potentials
(c.a 1 V and 1.3 V vs. Tc(VI)/Tc(V) couple). A comparison of the electrochemical
properties of the respective Tc and Re complexes revealed that cationic technetium
complex is more stable than its rhenium analogs.
Technetium of a low valency (especially +4, +3) can also exist as μ-oxo technetium complexes. For example, Kastner et al. (1986) or Clarke et al. (1988) examined such types of complexes with halide and pyridine ligands. CV curves of these
compounds revealed two well-separated one-electron redox processes. The dissymmetric ligands (1. type: [X(L) 3 XTc-O-TcX(L)X 3 ]) turned out to be generally less
stable than the asymmetric ones (2. type: [X(L) 4 Tc-O-Tc(L)X 4 ]. The differences
between E 1/2 of Tc
IV -Tc
III /Tc
III -Tc
III and Tc
III -Tc
III /Tc
III -Tc
II redox couples were
equal to 0.97–1.16 V and 1.35–1.63 V for the first and the second type of Tc
complexes, respectively. Later, Linden et al. (1989) synthesized a dimeric Tc
IV -
Tc
IV complex, [(TCTA)Tc(μ-O) 2 Tc(TCTA)]
2− . Voltammetric curves recorded for
these species revealed the existence of three redox systems therein pH independent
Tc(IV-IV)/Tc(IV-III) couple at a potential of 0.167 V versus SCE.
New oxo-bridged complexes of Tc containing bipiridine were described in the
next article by Lu et al. (1993). Three well-separated one-electron redox couples
were observed. Noteworthy is the fact that (μ−O)[X (L) 2 Tc]
2+
2 was relatively stable