432
P. P. Roma´ nczyk and S. S. Kurek
with a much weaker catalytic effect (see Fig. 1d in [36]). The reduction product
of trichloroethylene is cis-C 2 HCl
•
2 · · ·Cl
− , which is also true for the intramolecular
ET within the W
I adduct. If an effective H
• donor is not present in the system, the
cis-C 2 HCl
•
2 radical is rapidly reduced to the corresponding carbanion.
The DFT calculations performed by us made it possible to elucidate why chloroform can be reduced by the Mo
I , whereas pentachloroethane cannot undergo this reaction in identical conditions, despite the fact that the E° potentials for both chloroalkanes are similar. In the case of {Mo
I
−O alk }
•−
· · ·HC 2 Cl 5 adduct, the distance between
the electron donor and acceptor (O alkoxide · · ·C–Cl acceptor ) is ca. 1 Å higher than in the
analogous CHCl 3 adduct, which results in the need to apply a higher overpotential
(W
II /W
I ). The unique autocatalytic character of the CHCl 3 dehalogenation was also
explained. It results from the substantial differences in the reactivity of the CHCl 3
and C 2 HCl 5 degradation products; the branching in the “organic loop” is observed
only in the case of chloroform (cf. Fig. 12a, b).
Importantly, effects similar to those recognised for the presented catalytic system {M
I
−O alk }
•− (M Mo or W), i.e. the proximity and stabilisation effects, and
competitive inhibition, operate in enzymatic catalysis and originate in the Lewis
acid–Lewis base interactions and London dispersion forces. The obtained results
(described above for chloroform and pentachloroethane dehalogenation in the presence of Mo/W complexes, and also presented in the paper [78] for the porphyrin
adducts) enabled a general conclusion to be drawn concerning the features that a
molecule should exhibit in order to be a good redox electrocatalyst. Amongst them,
the most essential appears to be an anion radical form with an appropriate redox potential, rather than dianion, exhibiting rigid structure with a spacious SOMO, showing
strong dispersive interactions enabling the formation of an adduct enhanced by H–
or halogen bonding, preferably in an environment of a low-dielectric constant.
5 Concluding Remarks
This chapter demonstrates how molecular electrochemistry may benefit from the
application of density functional theory methods combined with implicit solvent
models. From the pioneering work of Baik and Friesner [1], a significant progress
has been made giving rise to chemically accurate predictions of the redox potential
of catalytically relevant transition metal complexes, including also bioinorganic systems. These achievements enabled showing how the thermodynamics of key electron
transfer steps in these processes is controlled by metal–ligand interactions, perturbations in the second coordination sphere, the local environment and a variety of
non-covalent interactions. Most pivotal factor for this has been the development of
novel theoretical models, particularly, efficient solvation models (still remaining a
big challenge), dispersion-corrected functionals, and more accurate correlated methods, not to mention an enormous growth in computational power. Crucial to this and
future success is the calibration of the computational DFT protocol based on either
reliable experimental data or proven benchmark computational methods.
P. P. Roma´ nczyk and S. S. Kurek
with a much weaker catalytic effect (see Fig. 1d in [36]). The reduction product
of trichloroethylene is cis-C 2 HCl
•
2 · · ·Cl
− , which is also true for the intramolecular
ET within the W
I adduct. If an effective H
• donor is not present in the system, the
cis-C 2 HCl
•
2 radical is rapidly reduced to the corresponding carbanion.
The DFT calculations performed by us made it possible to elucidate why chloroform can be reduced by the Mo
I , whereas pentachloroethane cannot undergo this reaction in identical conditions, despite the fact that the E° potentials for both chloroalkanes are similar. In the case of {Mo
I
−O alk }
•−
· · ·HC 2 Cl 5 adduct, the distance between
the electron donor and acceptor (O alkoxide · · ·C–Cl acceptor ) is ca. 1 Å higher than in the
analogous CHCl 3 adduct, which results in the need to apply a higher overpotential
(W
II /W
I ). The unique autocatalytic character of the CHCl 3 dehalogenation was also
explained. It results from the substantial differences in the reactivity of the CHCl 3
and C 2 HCl 5 degradation products; the branching in the “organic loop” is observed
only in the case of chloroform (cf. Fig. 12a, b).
Importantly, effects similar to those recognised for the presented catalytic system {M
I
−O alk }
•− (M Mo or W), i.e. the proximity and stabilisation effects, and
competitive inhibition, operate in enzymatic catalysis and originate in the Lewis
acid–Lewis base interactions and London dispersion forces. The obtained results
(described above for chloroform and pentachloroethane dehalogenation in the presence of Mo/W complexes, and also presented in the paper [78] for the porphyrin
adducts) enabled a general conclusion to be drawn concerning the features that a
molecule should exhibit in order to be a good redox electrocatalyst. Amongst them,
the most essential appears to be an anion radical form with an appropriate redox potential, rather than dianion, exhibiting rigid structure with a spacious SOMO, showing
strong dispersive interactions enabling the formation of an adduct enhanced by H–
or halogen bonding, preferably in an environment of a low-dielectric constant.
5 Concluding Remarks
This chapter demonstrates how molecular electrochemistry may benefit from the
application of density functional theory methods combined with implicit solvent
models. From the pioneering work of Baik and Friesner [1], a significant progress
has been made giving rise to chemically accurate predictions of the redox potential
of catalytically relevant transition metal complexes, including also bioinorganic systems. These achievements enabled showing how the thermodynamics of key electron
transfer steps in these processes is controlled by metal–ligand interactions, perturbations in the second coordination sphere, the local environment and a variety of
non-covalent interactions. Most pivotal factor for this has been the development of
novel theoretical models, particularly, efficient solvation models (still remaining a
big challenge), dispersion-corrected functionals, and more accurate correlated methods, not to mention an enormous growth in computational power. Crucial to this and
future success is the calibration of the computational DFT protocol based on either
reliable experimental data or proven benchmark computational methods.