Molecular Electrochemistry of Coordination Compounds …
433
Calibration of computational methods based on comparison with experimental
data mounts also challenges. Measurements require some specific conditions to be
met, and which may affect the measured values, that are not necessarily given in
data tables. Voltammetry needs the presence of relatively high concentrations of a
base electrolyte that influences the determined E° due to ionic strength (or alternatively variation of dielectric constant) and ion-pairing effects. Both effects are less
important in polar solvents. The potential shift direction always results from the
stabilisation of ions; e.g., if a cation (or a more positive cation) is the product of
oxidation, the potential will be less positive. In non-polar media, like chlorinated
aliphatics (extremely non-polar environments, like aliphatic hydrocarbons are not
used in electrochemistry) the potential shift is generally moderate (53 mV for Fc
+ /Fc
in 0.1 M n-Bu 4 NPF 6 in dichloromethane). Ion-pairing brings about the same effect
for species like ferrocene; however, it may become significant for highly charged
species (500 mV for [Ru(bpy) 3 ]
2+/3+ in CH 2 Cl 2 ). This effect can be successfully
modelled by including explicit counterions.
Care should be taken, when using redox potentials in non-aqueous solutions measured against popular aqueous reference electrodes and not versus internal standards.
Conversion parameters may be found in the literature, but it should be remembered
that the liquid junction potential occurring between the aqueous and non-aqueous
phases that adds a contribution to the measured value, is affected by a couple of
factors, not only the solvent types, but also the types and concentrations of electrolytes on both sides. Thus, it is always better to rely on an internal standard, like
ferrocene. The reliable Fc
+ /Fc absolute potentials in solvents commonly employed
in electrochemistry can be calculated by quantum chemical methods using SMD or
COSMO-RS solvation energies, and the SMD solvation model provided values that
are much more consistent with experiment for low-polarity solvents.
It was shown here that it is possible to obtain outstanding accuracy in the prediction of redox potentials for monometallic and bimetallic Mo/W scorpionates in
non-aqueous solvents, clarify unexpected properties of mixed-valence bimetallic
complexes incorporating dioxoalkylene or oxo bridges, enigmatic dehalogenation
catalysis and demonstrate how H-bonding may affect the redox behaviour of Cu
and Fe systems. The presented examples of entirely valence-trapped Mo
I Mo
II systems, despite showing medium to large electrochemical interactions (the splitting of
redox potentials, E 1/2 ), contradict the common practice of correlating E 1/2 with
the degree of electronic coupling. DFT-D calculations allowed solving of the mechanism of (auto)catalytic chloroform or pentachloroethane dehalogenation triggered by
a dissociative electron transfer through a charge-assisted C−H· · ·O hydrogen bond
and the explanation of the catalysis inhibition. It has been proved that these interactions, crucial for enzymatic activity, may play a decisive role not only in substrate
recognition (e.g. undesirable xenobiotics), but also in electron transfer. The obtained
results could be useful in designing the new electrocatalysts and sensors.
433
Calibration of computational methods based on comparison with experimental
data mounts also challenges. Measurements require some specific conditions to be
met, and which may affect the measured values, that are not necessarily given in
data tables. Voltammetry needs the presence of relatively high concentrations of a
base electrolyte that influences the determined E° due to ionic strength (or alternatively variation of dielectric constant) and ion-pairing effects. Both effects are less
important in polar solvents. The potential shift direction always results from the
stabilisation of ions; e.g., if a cation (or a more positive cation) is the product of
oxidation, the potential will be less positive. In non-polar media, like chlorinated
aliphatics (extremely non-polar environments, like aliphatic hydrocarbons are not
used in electrochemistry) the potential shift is generally moderate (53 mV for Fc
+ /Fc
in 0.1 M n-Bu 4 NPF 6 in dichloromethane). Ion-pairing brings about the same effect
for species like ferrocene; however, it may become significant for highly charged
species (500 mV for [Ru(bpy) 3 ]
2+/3+ in CH 2 Cl 2 ). This effect can be successfully
modelled by including explicit counterions.
Care should be taken, when using redox potentials in non-aqueous solutions measured against popular aqueous reference electrodes and not versus internal standards.
Conversion parameters may be found in the literature, but it should be remembered
that the liquid junction potential occurring between the aqueous and non-aqueous
phases that adds a contribution to the measured value, is affected by a couple of
factors, not only the solvent types, but also the types and concentrations of electrolytes on both sides. Thus, it is always better to rely on an internal standard, like
ferrocene. The reliable Fc
+ /Fc absolute potentials in solvents commonly employed
in electrochemistry can be calculated by quantum chemical methods using SMD or
COSMO-RS solvation energies, and the SMD solvation model provided values that
are much more consistent with experiment for low-polarity solvents.
It was shown here that it is possible to obtain outstanding accuracy in the prediction of redox potentials for monometallic and bimetallic Mo/W scorpionates in
non-aqueous solvents, clarify unexpected properties of mixed-valence bimetallic
complexes incorporating dioxoalkylene or oxo bridges, enigmatic dehalogenation
catalysis and demonstrate how H-bonding may affect the redox behaviour of Cu
and Fe systems. The presented examples of entirely valence-trapped Mo
I Mo
II systems, despite showing medium to large electrochemical interactions (the splitting of
redox potentials, E 1/2 ), contradict the common practice of correlating E 1/2 with
the degree of electronic coupling. DFT-D calculations allowed solving of the mechanism of (auto)catalytic chloroform or pentachloroethane dehalogenation triggered by
a dissociative electron transfer through a charge-assisted C−H· · ·O hydrogen bond
and the explanation of the catalysis inhibition. It has been proved that these interactions, crucial for enzymatic activity, may play a decisive role not only in substrate
recognition (e.g. undesirable xenobiotics), but also in electron transfer. The obtained
results could be useful in designing the new electrocatalysts and sensors.