426
P. P. Roma´ nczyk and S. S. Kurek
DFT predicts that an electron exchange along the Mo–O–Mo array could therefore be
triggered by exciting the bridge torsional mode (characteristic of a molecular rheostat), using a high-boiling solvent or by irradiation at ca. 50–200 cm
−1 (note that
low-frequency modes are often underestimated owing to anharmonicity effects),
which is also suggested by the temperature-dependent EPR spectra recorded for
[{Mo(NO)(Tp
Me2 )X} 2 (μ-O)]
– (X Cl or O 2 CPh) complexes [76]. It was demonstrated that the steric hindrance (Tp
Me –Tp
Me repulsion) and the weak hydrogen
bonds, i.e. OH· · ·ON, OH· · ·OH and C−H· · ·O (NO/OH) , had no substantial effect
on the molecular geometry.
4 Electrocatalytic Reductive Dehalogenation Driven
by Non-covalent Interactions—Binding and Activation
in Mo/W-Alkoxide System
The Lewis acid–Lewis base interactions, for example the C−H· · ·A (A O, N
or π acceptor) hydrogen bonds supported by halogen interactions, and the London
dispersion forces, are responsible for the formation of receptor–substrate systems,
which are crucial in molecular recognition and (bio)catalysis. It has been shown [35,
36, 78] that these interactions may also play a significant role in electrocatalytic
reductive dehalogenation processes.
Electroreduction of halogenated hydrocarbons is a slow process due to a high
barrier for the electron transfer. There are only a few classes of transition metal
complexes showing catalytic activity in the reduction of these compounds. The
examples include Co, Ni and Fe tetrapyrrole macrocycles (as active sites in vitamin B 12 [79], Factor F 430 [80] and cytochrome P450 [81]) and 1e reduced forms of
{M
II (NO)(Tp
Me2 )}
2+ (M Mo, W) alkoxide complexes. In our earlier studies (e.g.
[45]) we have shown that the latter compounds, having a more cathodic potential
E 1/2 than −1.77 V, effectively catalyse the CHCl 3 electroreduction. The untypical
character of the cyclic voltammetry curve (the sudden, almost vertical drop-down, a
half-peak width, E p/2 − E p , of just a few mV, see Fig. 8b) points to an activated process, occurring rapidly, according to a new mechanism, which has not been described
in the literature so far. Moreover, it has been indicated that the addition of alcohols
inhibits the catalysis. The presence of the Cl 3 C−H· · ·O alkoxide interactions in the
crystal structures of CHCl 3 solvates of the Mo alkoxide complexes suggested a key
role of a step involving an activation of the C−H bond in CHCl 3 placed inside the
cavity formed by the two pz rings of the Tp ligand. It was therefore important to
answer the following questions: Can such adducts exist in solution? and What is
their fate after the Mo
II reduction?
The aim of the electrochemical and theoretical studies conducted by our group
was to elucidate the mechanism of the chloroform [35] and pentachloroethane [36]
electrocatalytic dehalogenation in the presence of Mo
I /W
I alkoxide complexes, and
to explain the inhibition mechanism (Fig. 8). By using voltammetric methods, it was
P. P. Roma´ nczyk and S. S. Kurek
DFT predicts that an electron exchange along the Mo–O–Mo array could therefore be
triggered by exciting the bridge torsional mode (characteristic of a molecular rheostat), using a high-boiling solvent or by irradiation at ca. 50–200 cm
−1 (note that
low-frequency modes are often underestimated owing to anharmonicity effects),
which is also suggested by the temperature-dependent EPR spectra recorded for
[{Mo(NO)(Tp
Me2 )X} 2 (μ-O)]
– (X Cl or O 2 CPh) complexes [76]. It was demonstrated that the steric hindrance (Tp
Me –Tp
Me repulsion) and the weak hydrogen
bonds, i.e. OH· · ·ON, OH· · ·OH and C−H· · ·O (NO/OH) , had no substantial effect
on the molecular geometry.
4 Electrocatalytic Reductive Dehalogenation Driven
by Non-covalent Interactions—Binding and Activation
in Mo/W-Alkoxide System
The Lewis acid–Lewis base interactions, for example the C−H· · ·A (A O, N
or π acceptor) hydrogen bonds supported by halogen interactions, and the London
dispersion forces, are responsible for the formation of receptor–substrate systems,
which are crucial in molecular recognition and (bio)catalysis. It has been shown [35,
36, 78] that these interactions may also play a significant role in electrocatalytic
reductive dehalogenation processes.
Electroreduction of halogenated hydrocarbons is a slow process due to a high
barrier for the electron transfer. There are only a few classes of transition metal
complexes showing catalytic activity in the reduction of these compounds. The
examples include Co, Ni and Fe tetrapyrrole macrocycles (as active sites in vitamin B 12 [79], Factor F 430 [80] and cytochrome P450 [81]) and 1e reduced forms of
{M
II (NO)(Tp
Me2 )}
2+ (M Mo, W) alkoxide complexes. In our earlier studies (e.g.
[45]) we have shown that the latter compounds, having a more cathodic potential
E 1/2 than −1.77 V, effectively catalyse the CHCl 3 electroreduction. The untypical
character of the cyclic voltammetry curve (the sudden, almost vertical drop-down, a
half-peak width, E p/2 − E p , of just a few mV, see Fig. 8b) points to an activated process, occurring rapidly, according to a new mechanism, which has not been described
in the literature so far. Moreover, it has been indicated that the addition of alcohols
inhibits the catalysis. The presence of the Cl 3 C−H· · ·O alkoxide interactions in the
crystal structures of CHCl 3 solvates of the Mo alkoxide complexes suggested a key
role of a step involving an activation of the C−H bond in CHCl 3 placed inside the
cavity formed by the two pz rings of the Tp ligand. It was therefore important to
answer the following questions: Can such adducts exist in solution? and What is
their fate after the Mo
II reduction?
The aim of the electrochemical and theoretical studies conducted by our group
was to elucidate the mechanism of the chloroform [35] and pentachloroethane [36]
electrocatalytic dehalogenation in the presence of Mo
I /W
I alkoxide complexes, and
to explain the inhibition mechanism (Fig. 8). By using voltammetric methods, it was