Molecular Electrochemistry of Coordination Compounds …
421
3 Electrochemical Communication Across Saturated
Dioxoalkylene and Oxo Bridges in Dimolybdenum
Scorpionates
Electron transfer between donor–acceptor pairs (D-A) is a process of great importance in chemical, physical and biological systems, particularly metalloproteins,
where covalent σ- and π-bonds, H-bonds [50, 51] and through-space tunnelling
often assist ET [52–55]. The investigation of transition metal complexes in mixedvalence states, of which the Creutz–Taube ion (Ru
II Ru
III ) is the best-known example
[56], reveals the correlation between the magnitude, range and character of D· · ·A
electronic interactions and the structure of the bridging ligand, such as its length,
substitution pattern, geometric/electronic structure [57, 58], and the surrounding
medium (solvent, base electrolyte [59]). Moreover, the relationship between the
localisation/delocalisation of an unpaired electron and the relative orientation of the
local D and A coordinate systems may confer the properties of the molecular switches
and rheostats on complexes [60–62]. According to the Robin and Day seminal paper
[63], mixed-valence systems are divided into three main classes corresponding to
the degree of electronic coupling between D and A along the bridge. In compounds
belonging to class I, there is no coupling between diabatic redox states and the odd
electron is fully localised. Conversely, the charge in class III molecules is totally
delocalised over both redox centres (a single ground-state minimum on the freeenergy surface without ET barrier), and systems in-between fit into class II (a double
single ground-state minimum). This has been discussed in detail in review papers
[52, 58] in the light of Marcus theory [64], with implications for the properties of
mixed-valence complexes.
Metal–metal electronic interactions in a mixed-valence state are manifested in
electrochemical studies. For instance, in the voltammogram, the measure of electrochemical interactions is differences in reduction (or oxidation) potentials, red/ox E 1/2 ,
between both metal centres. The red/ox E 1/2 difference is observed when the change
in electron density on one metal centre (caused by the initial reduction or oxidation) affects the second centre by a combination of through-space (Coulombic) and
through-bond (inductive or resonance contribution) interactions. Thermodynamic
stability of mixed-valence ions is quantified by the comproportionation constant K c ,
for the equilibrium between isovalent and mixed-valence forms:
{M-Q-M}
n + {M-Q-M}
n−2 2{M-Q-M}
n−1
(4)
The value of K c is related to the splitting of redox potentials (E°) by: K c
e
FE°/(RT) (it can range from the statistical value of 4 to more than 10
24 (E 1/2
1440 mV) as reported for the Creutz–Taube analogue [{Mo(NO)(Tp
Me2 )Br} 2 (μpz)]
− [65]). A practical consequence of this equilibrium is that only for sufficiently
large electrochemical interactions (E 1/2 > 200 mV) it is possible to selectively
reduce or oxidise (electrochemically or chemically) just one redox centre.
421
3 Electrochemical Communication Across Saturated
Dioxoalkylene and Oxo Bridges in Dimolybdenum
Scorpionates
Electron transfer between donor–acceptor pairs (D-A) is a process of great importance in chemical, physical and biological systems, particularly metalloproteins,
where covalent σ- and π-bonds, H-bonds [50, 51] and through-space tunnelling
often assist ET [52–55]. The investigation of transition metal complexes in mixedvalence states, of which the Creutz–Taube ion (Ru
II Ru
III ) is the best-known example
[56], reveals the correlation between the magnitude, range and character of D· · ·A
electronic interactions and the structure of the bridging ligand, such as its length,
substitution pattern, geometric/electronic structure [57, 58], and the surrounding
medium (solvent, base electrolyte [59]). Moreover, the relationship between the
localisation/delocalisation of an unpaired electron and the relative orientation of the
local D and A coordinate systems may confer the properties of the molecular switches
and rheostats on complexes [60–62]. According to the Robin and Day seminal paper
[63], mixed-valence systems are divided into three main classes corresponding to
the degree of electronic coupling between D and A along the bridge. In compounds
belonging to class I, there is no coupling between diabatic redox states and the odd
electron is fully localised. Conversely, the charge in class III molecules is totally
delocalised over both redox centres (a single ground-state minimum on the freeenergy surface without ET barrier), and systems in-between fit into class II (a double
single ground-state minimum). This has been discussed in detail in review papers
[52, 58] in the light of Marcus theory [64], with implications for the properties of
mixed-valence complexes.
Metal–metal electronic interactions in a mixed-valence state are manifested in
electrochemical studies. For instance, in the voltammogram, the measure of electrochemical interactions is differences in reduction (or oxidation) potentials, red/ox E 1/2 ,
between both metal centres. The red/ox E 1/2 difference is observed when the change
in electron density on one metal centre (caused by the initial reduction or oxidation) affects the second centre by a combination of through-space (Coulombic) and
through-bond (inductive or resonance contribution) interactions. Thermodynamic
stability of mixed-valence ions is quantified by the comproportionation constant K c ,
for the equilibrium between isovalent and mixed-valence forms:
{M-Q-M}
n + {M-Q-M}
n−2 2{M-Q-M}
n−1
(4)
The value of K c is related to the splitting of redox potentials (E°) by: K c
e
FE°/(RT) (it can range from the statistical value of 4 to more than 10
24 (E 1/2
1440 mV) as reported for the Creutz–Taube analogue [{Mo(NO)(Tp
Me2 )Br} 2 (μpz)]
− [65]). A practical consequence of this equilibrium is that only for sufficiently
large electrochemical interactions (E 1/2 > 200 mV) it is possible to selectively
reduce or oxidise (electrochemically or chemically) just one redox centre.