4.1 Nonaqueous Solutions
75
Tc
V O-triarylcorroles were of interest to Einrem et al. (2016). They observed
that the Tc, which is the coordination center of the complex, is inactive in redox
reactions. The electrochemical HOMO–LUMO gap was equal to 2.04–2.09 V, which
represented the difference in π and π* energies of the macrocycle.
Tc(III) complexes with tetradentate N 2 O 2 Schiff base are intensively investigated
today as potential multidrug-resistant tumor or myocardial perfusion imaging agents
(Baumeister et al. 2018). Reported [Tc(tmpp) 2 (tmf 2 en)]
+ and [Tc(PEt 3 ) 2 (tmf 2 en)]
+
undergo a one-electron reversible reduction at potentials about 0.2–0.23 V higher
than their rhenium analogs. The oxidation of such types of Tc(III) complexes is
irreversible and occurs at ca. 0.82 V versus Ag, AgCl.
Pyridine complexes of technetium which have +3, +2 and +1 valencies were
analyzed by Barrera et al. (1996). At least two (IV/III and III/II or III/II and II/I)
reversible couples were observed for the examined systems. The E
0 of Tc(III/II)
becomes increasingly more negative in the following order of the amine ligands: tpy
< Me 2 bpy < py < tmeda. It suggests that polypyridyl ligands stabilize more stronger
Tc(II), relative to Tc(III), than pyridyl ligands. A substitution of chlorides with a
pyridine ligand results in a shift of E
0 of Tc(II)/Tc(I) couple by 0.39 V. Nicholson
et al. (1991) examined tris-diazene chelate complexes of Tc(I). Quasi-reversible
processes were observed for [Tc(C 8 H 5 N 2 N = NH 3 ) 3 ]
+ at potentials of −0.24 V and
0.004 V versus Ag, AgCl at room temperature.
A very complex electrochemical behavior of boron-capped technetium-dioxime
complexes was reported by Cyr et al. (1993). The species with the formula of
TcX(dioxime) 3 BR (where X = Cl, Br or OH and R = OH, Me, Et, Bu or Ph) were
examined using CV, d.c. polarography and coulometry. The CV curves revealed the
existence of three characteristic redox features. The authors reported that chloroand bromo-complexes undergo a two-electron irreversible reduction in ACN with
possible decomposition at ca. −1.3 V versus Ag/Ag
+ . The products of this reaction
are also irreversibly oxidized at c.a. −0.7 V. The second two-electron reduction peak
was observed at c.a. −2.7 V. The authors concluded that both reduction and oxidation
of boron-capped Tc-dioxime complexes appeared to be biologically inaccessible.
Patterson
et
al.
(1986)
studied
electrochemistry
of
Tris(βdiketonato)technetium(III) and (IV). The redox properties of Tc(IV)/Tc(III)
and Tc(III)/Tc(II) redox couple strongly depend on the nature of the
ligand. Tris(dipivaloylmethanato)technetium(III) was reversible oxidized
at
0.12
V;
tris(trifluoroacetylacetonato)technetium(III)
was
characterized by irreversible current waves at 1.0 and −1.16 V while
tris(hexafluoroacetylacetonato)technetium(III) was quasi-reversible reduced at
0.14 V versus SCE in ACN.
The experiments carried out by Luo (1995) were focused on synthesis and
analysis of chemical properties of new hexadentate ligand. Cyclic voltammetry curves recorded for [Tc
III (HP 2 O 4 )] where H 4 P 2 O 4 is P,P,P’,P’-tetrakis
(o-hydroxyphenyl)diphosphinoethane at potentials from −2 to 2 V versus Ag, AgCl
and at a scan rate of 0.1 V·s
−1 do not reveal electrochemical signals between −2 and
2 V versus Ag, AgCl indicating a very high stability of this complex.
75
Tc
V O-triarylcorroles were of interest to Einrem et al. (2016). They observed
that the Tc, which is the coordination center of the complex, is inactive in redox
reactions. The electrochemical HOMO–LUMO gap was equal to 2.04–2.09 V, which
represented the difference in π and π* energies of the macrocycle.
Tc(III) complexes with tetradentate N 2 O 2 Schiff base are intensively investigated
today as potential multidrug-resistant tumor or myocardial perfusion imaging agents
(Baumeister et al. 2018). Reported [Tc(tmpp) 2 (tmf 2 en)]
+ and [Tc(PEt 3 ) 2 (tmf 2 en)]
+
undergo a one-electron reversible reduction at potentials about 0.2–0.23 V higher
than their rhenium analogs. The oxidation of such types of Tc(III) complexes is
irreversible and occurs at ca. 0.82 V versus Ag, AgCl.
Pyridine complexes of technetium which have +3, +2 and +1 valencies were
analyzed by Barrera et al. (1996). At least two (IV/III and III/II or III/II and II/I)
reversible couples were observed for the examined systems. The E
0 of Tc(III/II)
becomes increasingly more negative in the following order of the amine ligands: tpy
< Me 2 bpy < py < tmeda. It suggests that polypyridyl ligands stabilize more stronger
Tc(II), relative to Tc(III), than pyridyl ligands. A substitution of chlorides with a
pyridine ligand results in a shift of E
0 of Tc(II)/Tc(I) couple by 0.39 V. Nicholson
et al. (1991) examined tris-diazene chelate complexes of Tc(I). Quasi-reversible
processes were observed for [Tc(C 8 H 5 N 2 N = NH 3 ) 3 ]
+ at potentials of −0.24 V and
0.004 V versus Ag, AgCl at room temperature.
A very complex electrochemical behavior of boron-capped technetium-dioxime
complexes was reported by Cyr et al. (1993). The species with the formula of
TcX(dioxime) 3 BR (where X = Cl, Br or OH and R = OH, Me, Et, Bu or Ph) were
examined using CV, d.c. polarography and coulometry. The CV curves revealed the
existence of three characteristic redox features. The authors reported that chloroand bromo-complexes undergo a two-electron irreversible reduction in ACN with
possible decomposition at ca. −1.3 V versus Ag/Ag
+ . The products of this reaction
are also irreversibly oxidized at c.a. −0.7 V. The second two-electron reduction peak
was observed at c.a. −2.7 V. The authors concluded that both reduction and oxidation
of boron-capped Tc-dioxime complexes appeared to be biologically inaccessible.
Patterson
et
al.
(1986)
studied
electrochemistry
of
Tris(βdiketonato)technetium(III) and (IV). The redox properties of Tc(IV)/Tc(III)
and Tc(III)/Tc(II) redox couple strongly depend on the nature of the
ligand. Tris(dipivaloylmethanato)technetium(III) was reversible oxidized
at
0.12
V;
tris(trifluoroacetylacetonato)technetium(III)
was
characterized by irreversible current waves at 1.0 and −1.16 V while
tris(hexafluoroacetylacetonato)technetium(III) was quasi-reversible reduced at
0.14 V versus SCE in ACN.
The experiments carried out by Luo (1995) were focused on synthesis and
analysis of chemical properties of new hexadentate ligand. Cyclic voltammetry curves recorded for [Tc
III (HP 2 O 4 )] where H 4 P 2 O 4 is P,P,P’,P’-tetrakis
(o-hydroxyphenyl)diphosphinoethane at potentials from −2 to 2 V versus Ag, AgCl
and at a scan rate of 0.1 V·s
−1 do not reveal electrochemical signals between −2 and
2 V versus Ag, AgCl indicating a very high stability of this complex.