Accepted Manuscript

Title: The effect of C−H···O bonding and Cl···� interactionsin electrocatalytic dehalogenation of C2 chlorides containingan acidic hydrogen

Author: Piotr P. Romanczyk Grzegorz Rotko Klemens NogaMariusz Radon Gleb Andryianau Stefan S. Kurek

PII: S0013-4686(14)00955-4DOI: http://dx.doi.org/doi:10.1016/j.electacta.2014.04.175Reference: EA 22681

To appear in: Electrochimica Acta

Received date: 23-1-2014Revised date: 11-4-2014Accepted date: 29-4-2014

Please cite this article as: P.P. Romanczyk, G. Rotko, K. Noga, M. Radon,G. Andryianau, S.S. Kurek, The effect of CminusHcdotcdotcdotO bondingand Clcdotcdotcdotrmpi interactions in electrocatalytic dehalogenation ofC2 chlorides containing an acidic hydrogen, Electrochimica Acta (2014),http://dx.doi.org/10.1016/j.electacta.2014.04.175

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The effect of C−H···O bonding and Cl···π interactions in electrocatalytic

dehalogenation of C2 chlorides containing an acidic hydrogen

Piotr P. Romańczyka,*, Grzegorz Rotkoa, Klemens Nogab, Mariusz Radońb, Gleb

Andryianaua, Stefan S. Kureka,*

a Faculty of Chemical Engineering and Technology, Cracow University of Technology, ul. Warszawska 24,

31-155 Kraków, Poland b Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland;

Academic Computer Center CYFRONET, ul. Nawojki 11, 30-950 Kraków, Poland

Abstract

A tungsten alkoxy scorpionate shows activity in the electrocatalytic reductive dehalogenation of

pentachloroethane and trichloroethylene owing to the formation of hydrogen and halogen bonded

adducts with the substrates, which is further reinforced by dispersive interactions. The ensuing

proximity between the substrate molecule and the metal centre promotes dechlorination in a concerted

process. Two-electron reduction of pentachloroethane yields trichloroethylene that undergoes further,

non-catalysed, reactions that ultimately give acetylenes. Interestingly, pentachloroethane proved to be

a highly efficient proton donor for the transient anions, in extremely exothermic and rapid proton

transfer concerted with chloride anion abstraction, which yields perchloroethylene. The total process

and the mechanism thereof were verified based on DFT and coupled cluster (CC) calculations. The

calculations evaluated feasibility of various pathways in the mechanism. Standard redox potentials for

the environmentally relevant species, participating in the studied reactions, were accurately computed

employing the explicitly correlated CCSD(T)-F12 method that provides an improved C-Cl bond

energy, of essential importance to the dissociative potentials.

Keywords: Electrocatalytic dehalogenation; Pentachloroethane, Trichloroethylene, Noncovalent

interactions; DFT calculations

                                                            * Corresponding authors.

E-mail addresses: piotrom@chemia.pk.edu.pl (P.P. Romańczyk), skurek@chemia.pk.edu.pl (S.S. Kurek).

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1. Introduction

Polychlorinated hydrocarbons, pervasive environmental pollutants, are biodegraded via reductive

dehalogenation, the mechanism of which has been the subject of intensive both experimental [1]– [2–

8] and theoretical [9]– [10–13] investigations. Reduction leading to halide abstraction, particularly,

when electron transfer is concerted with carbon-halide bond cleavage, is associated with a high energy

barrier, hence there is a need for a catalyst. Typically, model complexes of enzymes that are active in

nature, like cobalamin (for chloroethylenes) and cytochrome P450cam (for chloroethanes) were applied.

Interestingly, halogenated substrates bind to the active site of the latter enzyme with free energy

correlated with the number of chlorine atoms (Cl3 to Cl6) indicating the importance of chlorine atoms

interactions with the enzyme. The acceleration of pentachloroethane to trichloroethylene conversion

rate was attributed to the proximity effects warranted by the relatively tight binding of C2HCl5 [Error!

Bookmark not defined.].

Alkyl halides that contain many electron-withdrawing chlorine atoms, like chloroform and

pentachloroethane, have an acidic hydrogen atom, which may be easily abstracted by strong bases like

hydroxide or alkoxide anions. The initially formed carbanion dissociates into dichlorocarbene and

chloride, as in the case of CHCl3 [14], or concerted elimination of proton and Cl− occurs [15].

Chloroform may also serve as H-bonding donor for O, N or aromatic π-system acceptors, even in

solution [16]. Moreover, chlorine atoms might also attractively interact with π-electron density. We

have recently shown [17] how these noncovalent interactions, i.e., the exceptionally short (dH···O equal

1.82 Å) and nearly linear C−H···Oalkoxide bonding along with cooperative Cl···πpyrazolyl dispersive

interactions in {MoI(NO)(TpMe2)(Oalkoxide)}•−···HCCl3 adduct ([TpMe2]− = κ3-hydrotris(3,5-

dimethylpyrazol-1-yl)borate) facilitates concerted dissociative electron transfer triggering a radical

autocatalytic cycle. The formation of the transient adduct (∆Ebind = −52.3 kJ·mol−1) warrants the close

and prolonged contact between the catalyst and its substrate, increasing the probability of ET, and

possibly stabilising the transition state, mimicking effects occurring in enzymatic catalysis.

Noteworthy, dispersion forces significantly contribute to the adduct stability [18], and hence it is

necessary to use quantum chemical methods that correctly describe dispersion effects, the dispersion-

corrected DFT (DFT-D) approach becoming the method of choice for these large supramolecular

systems.

The objective of this study is to answer the question whether the above mentioned pattern of

bonding and activation described for reduction of CHCl3 electrocatalysed by the Mo/W alkoxides,

accompanied by autocatalysis, may also occur for other acidic hydrogen-containing polyhalogenated

hydrocarbons, i.e., pentachloroethane and trichloroethylene. Moreover, the mechanism of non-

catalysed dehalogenation steps has been discussed based on the quantum chemical calculations for

various plausible organic reactions and very accurate redox potentials obtained for the species

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generated in the presence of C2HCl5, which may serve as an effective proton donor for anionic

transients.

2. Experimental

2.1. Materials

Dichloromethane (Merck Emsure, ethanol-free, stabilized by 50 ppm amylene) was dried by

distillation from CaH2 under argon prior to use. Pentachloroethane (Sigma-Aldrich, analytical

standard) and trichloroethene (Merck Emsure) were used as received. The Mo and W alkoxides,

[Mo(NO)(TpMe2)(OEt)2] and [W(NO)(TpMe2)O(CH2)4O], were synthesised according to the published

methods [19, 20].

2.2. Electrochemical measurements

Measurements were done using a BAS 100B/W Electrochemical Workstation with a C3 Cell

Stand (Bioanalytical Systems, USA) under argon in dry CH2Cl2 with 0.1 M n-Bu4NPF6 (Sigma-

Aldrich, electrochemical grade, vacuum dried) as base electrolyte. Glassy carbon working electrode

(Mineral, Poland) was used together with platinum wire as the auxiliary and Ag/AgCl (3 M NaCl) as

the reference electrode linked via an electrochemical bridge filled with supporting electrolyte solution.

Typically, scan rates of 0.1 V s−1 were used if not otherwise stated. Ferrocene was applied as an

internal standard and potentials throughout this work are quoted against Fc•+/0. Positive feedback iR

compensation was employed in the measurements.

2.3. Computational Details

Density functional theory (DFT) calculations were carried out with the B3LYP hybrid functional

[21, 22]. Open-shell species were treated within a spin-unrestricted scheme. For W scorpionates

interacting with C2 halides and the transients, geometry optimisations and calculations of harmonic

frequencies were performed in Turbomole [23] at the dispersion-corrected DFT level (the DFT-D3

variant [24]), employing the triple-ζ def2-TZVPP [25]– [26, 27] basis set for all atoms, with respective

effective core potential (ECP) for W. For comparison, we carried out some calculations for Mo

analogues at the same level of theory. The conductor-like screening model (COSMO) [28] was used to

account for the effect of CH2Cl2 solvent (ε = 8.93). The [W(NO)(TpMe2)(OMe)2] complex was used as

a model of [W(NO)(TpMe2)O(CH2)4O]. Bonding energies for the WII/I adducts were corrected for basis-

set superposition error (BSSE) estimated from the standard counterpoise procedure [29].

In the study of organic species and their reactions, the geometries were optimised in Gaussian 09

[30] at the B3LYP/6-31G(d,p) level; the final energies reported were obtained from single-point

B3LYP/6-311++G(2d,2p) calculations with the dispersive correction (DFT-D2 [31]). The polarizable

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continuum model (IEF-PCM) [32] corresponding to the CH2Cl2 solvent was used. Standard

thermodynamic corrections, as implemented in Gaussian, were added to the computed electronic

energies to convert them into the Gibbs free energies.

The standard Gibbs energies are given for 1 mol L−1 at 298.15 K. Absolute reduction potentials

were calculated from the total free energy of an electron attachment in solution and were converted to

potentials vs. the Fc•+/0 couple by subtracting the 4.84 V value [33]. We previously found [Error!

Bookmark not defined.] that B3LYP/LACV3P+ [34] calculations best reproduced the experimental

redox potential of the {MoII/I−Oalk}0/•− couple (a basis set containing a diffuse d function on Mo is

essential) and this level was used to calculate the redox potentials for the {WII/I−Oalk}0/•− and

{WIII/II−Oalk}•+/0 pairs. The E° values for organic species were obtained at the PCM-B3LYP-

D2/6-311++G(2d,2p)//PCM-B3LYP/6-31G(d,p) level (see above). To provide more accurate standard

potentials, particularly for the dissociative reductions, we carried out coupled cluster (CC) calculations

at the RCCSD(T)-F12b/aug-cc-pVTZ level, as implemented in Molpro [35, 36]. Note that the main

advantage of the explicitly correlated (F12) methodology, as compared with conventional coupled-

cluster calculations, is a much faster convergence of the correlation energy with respect to the orbital

basis set, making thus possible to obtain an accurate energetics already for the (augmented) triple-zeta

basis set [37]. In order to provide the CC potentials reported in Table 2, the single-point B3LYP and

CC calculations were carried out in the gas phase (on top of the DFT structures optimised in solution)

and the difference between the both energies (i.e., ΔECC,gas – ΔEDFT,gas) was used to correct the standard

potentials in solution computed at the DFT level.

3. Results and Discussion

3.1. Electrochemical behaviour

We started this investigation with [Mo(NO)(TpMe2)(OEt)2] complex (previously characterised as

electrocatalyst for CHCl3 reduction [Error! Bookmark not defined.,Error! Bookmark not

defined.]), which surprisingly proved to be inactive in pentachloroethane (PCA) dehalogenation,

despite having the standard redox potential more cathodic than that of PCE. That is why we applied a

tungsten analogue, exhibiting redox potential by ca. 0.6 V more cathodic then the Mo complex.

The chelato tungsten dialkoxy scorpionate presently used in this work, [W(NO)(TpMe2)O(CH2)4O]

({WII−Oalk}), is reduced at −2.26 V and oxidised at +0.66 V vs. Fc•+/0 in quasi-reversible processes

[Error! Bookmark not defined.]. The reduction is an evidently slower process with ΔEp = 122 mV,

an effect of a relatively high inner reorganisation energy, characteristic of this type Mo and W alkoxy

scorpionates [Error! Bookmark not defined.].

Figure 1a reports the electrochemical behaviour of PCA without and with addition of the W

complex. PCA is non-catalytically reduced in an entirely irreversible process at a potential more

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cathodic than that of the W complex. Although the onset of the non-catalytic PCA reduction wave is

close to the W complex reduction potential, the process for PCA is very slow. The addition of

{WII−Oalk} significantly accelerates the process, shifting it anodically by nearly 0.2 V and increasing

the current twice (see Fig. 1a). Successive additions of PCA result in a further increase in the current

and the disappearance of the anodic re-oxidation wave for the W complex (Fig. 1b), which is typical of

electrocatalysis. The dependence of the peak current as a function of PCA concentration becomes

linear at higher values of cPCA (Fig. 1c); at lower values its rise is slower. A lack of decrease in the

current of the anodic process at +0.66 V, corresponding to {WIII/II−OMe}•+/0, has been used to check,

whether the complex undergoes degradation during the catalytic process. To test it we were holding

the potential at −2.5 V for 30 s, then jumped the potential to +0.40 V and scanned it at 0.5 V s−1 to

+0.90 V, next, back to +0.40 V. No reduction in the intensity of the wave for this oxidation process

compared with the regular voltammogram starting at 0.0 V was observed, which indicates that the

tungsten complex was fully recovered after the catalytic step.

Trichloroethylene (TCE) also undergoes a slow irreversible reduction at potentials beyond the

WII/I redox pair (Fig. 1d). In this case, however, the voltammogram looks different. The W complex

reduction wave clearly overlaps electrocatalytic reduction of TCE. It can be calculated that ca. 85% of

{WII−Oalk} added gives unaltered voltammogram, which means that only 15% participates in the

catalytic process, evidently slower than in the case of PCA.

Fig. 1. Reduction of (a) 4 mM pentachloroethane and (d) 4 mM trichloroethylene on GCE before and after the addition of [W(NO)(TpMe2)O(CH2)4O] in CH2Cl2/0.1 M TBAPF6 (ν = 0.1 V s−1, cW = 2 mM). Uncatalysed reduction in green. Panel (b) shows the effect of consecutive additions of C2HCl5 with numbers denoting PCA/{WII−Oalk} concentration ratios and (c) the peak current as a function of concentration ratios.

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The studied W-alkoxides revealed no activity towards C2Cl4 despite its reduction potential being

similar to that of C2HCl3; only the reversible reduction of the W complex and non-catalytic C2Cl4

reduction can be observed in the relevant CV.

3.2. The {WII/I−Oalk}0/•− adducts and intramolecular ET

Having proved the participation of {MoI(NO)(TpMe2)(Oalkoxide)}•−···HCCl3 adducts in the

dechlorination of CHCl3 electrocatalysed by molybdenum alkoxy scorpionates [Error! Bookmark

not defined.,Error! Bookmark not defined.], we expected the similar mechanism for

pentachloroethane dehalogenation. To check if analogous adducts may take part in this reaction, we

computationally optimised the structures of {WII/I−OMe}0/•− adducts with C2HCl5 and the following

transient radical C2HCl4• (Fig. 2). The DFT-D calculations showed that the pentachloroethane

molecule interacts with WI alkoxy scorpionate in a similar way as chloroform [Error! Bookmark not

defined.,Error! Bookmark not defined.], i.e., with a very short and not far from linear

C−H···Oalkoxide H-bonding (dH···O = 1.87 Å and θC−H···O = 159.9°). The cavity formed by the two

pyrazolyl rings clearly accommodates a molecule larger than chloroform, so now two chlorine atoms

of C2HCl5 are attracted toward the πpyrazolyl system, which stiffens the bound molecule. Formation of

the H-bond is reflected by a large ΔνC−H red-shift of ca. 300 cm−1 (i.e., by 80 cm−1 less than found for

an analogous MoI CHCl3 adduct in [Error! Bookmark not defined.]).

Fig. 2. DFT-D optimised geometries (distances in Å) for (a) [WI(NO)(TpMe2)(OMe)2]•−···HC2Cl5 adduct in CH2Cl2, and (b) {WII−Oalk}···HC2Cl4

•, the product of intramolecular dissociative ET. Dotted lines show C−H···Oalkoxide bonding and C−Cl···πpyrazolyl dispersive interactions. The C2HCl4

• radical is only weakly bound to WII and will be rapidly substituted by CH2Cl2 (solvent) or C2HCl5, thus the next catalytic cycle may start.

Table 1 presents the calculated bonding energies (∆Ebind) and Gibbs energies (∆Gbind) for the WII/I

adducts with the relevant chlorinated molecules and radicals (all adducts in CH2Cl2 solvent), compared

with their Mo analogues. Pentachloroethane is bound to the WI site almost as strongly as CHCl3 to the

MoI site; however, the latter site binds C2HCl5 a bit weaker. The dichloromethane solvent, being in a

large excess, despite less favourable bonding energies (6.7–13.8 kJ·mol−1) competes in the formation

of WI adducts with C2HCl5 and the intermediates resulting from its reduction. Because of that, a small

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fraction of the tungsten complex will form the {WI−Oalk}•−···HC2Cl5 adduct under experimental

conditions.

Table 1 Bonding energiesa (in kJ·mol−1) for various adducts with WII/I(NO)(TpMe2)(OMe)2

0/•− in CH2Cl2 solvent, and the Mo analogues for comparison.

Adduct ∆Ebind ∆Gbind {WI−Oalk}•−···HC2Cl5 −54.8 −5.0 {WII−Oalk}···HC2Cl4

• −46.4 1.7 {WI−Oalk}•−···HC2Cl4

• −56.1 −5.4 {WI−Oalk}•−···HC2Cl3 −44.4 0.4 {WI−Oalk}•−···H2CCl2 −41.0 1.7 {MoI−Oalk}•−···HC2Cl5 −53.6 −2.9 {MoI−Oalk}•−···HC2Cl3 −43.1 1.3 {MoI−Oalk}•−···HCCl3

b −52.3 −6.7 {MoI−Oalk}•−···H2CCl2

b −37.7 3.8 a Full DFT-D3 geometry optimisation; energies corrected for basis set superposition error. b Data from [Error! Bookmark not defined.].

The frontier orbitals for the {WI−OMe}•−…HC2Cl5 are shown in Fig. 3. The SOMO-LUMO gap (Δε)

for {WI−Oalk}•−···HC2Cl5 adduct equals 1.9 eV. This is, interestingly, much smaller than for an

analogous C2HCl5 adduct with the MoI scorpionate (Δε = 3.1 eV) that was found incapable of reducing

pentachloroethane (see Section 3.1). Naturally, the energy of the σ*C−Cl-based LUMO considerably

drops down with the C−Cl bond elongation, facilitating the ET onto the PCA molecule. Indeed, if the

C−Cl bond extending outside the cavity formed by the two pyrazolyl rings is stretched just 0.3 Å from

its equilibrium bond length (1.80 Å), the calculations lead to a spontaneous intramolecular ET (from

WI to C2HCl5), resulting in a complete dissociation of Cl− and formation of the C2HCl4• radical (Fig.

2b). Moreover, the advantageous noncovalent interactions tend to partly compensate for an increase in

energy due to cleavage of the C−Cl bond coupled with the ET, since C2HCl5 becomes even more

firmly bound during the C−Cl bond stretching. This leads possibly to the transition state stabilisation;

the bulky scorpionate ligand screens the ET centre, presumably reducing the outer-sphere

reorganisation energy. The spatial proximity of the WI 5dxy (SOMO, electron donor) and C2HCl5

σ*C−Cl orbitals (LUMO, acceptor) warranted by the noncovalent interactions is expected to favour

intramolecular electron transfer (i.e., increasing the probability of electron tunnelling in accordance

with its exponential distance dependence) in comparison with heterogeneous ET directly from an inert

electrode. However, the electron donor and acceptor (Oalkoxide···C-Clacceptor) are separated by ca. 4 Å

that is by ca. 1 Å more than in the case of the analogous MoI-HCCl3 adduct. This could rationalise the

lack of catalytic effect for MoI complex in the case of C2HCl5 despite its standard redox potential

being more favourable to that of CHCl3. It means that overpotential might be affected by the donor-

acceptor distance in the adduct.

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Summarising, the first step of electrocatalytic reduction of PCE, related to its interaction with the

WI scorpionate and the intramolecular ET, is analogous as we previously found for the Mo scorpionate

interacting with CHCl3 [Error! Bookmark not defined.,Error! Bookmark not defined.]. However,

as will be shown below, there are remarkable differences in the subsequent steps, explaining the

different electrochemical behaviour observed in these electrocatalytic processes.

Fig. 3. SOMO and LUMO orbitals for {WI−Oalk}•−···HC2Cl5 adduct in equilibrium geometry (contour plots for isovalue of 0.05 bohr−3/2). The SOMO is mainly W 5d-based (with participation of Oalk and NO), while LUMO is predominantly C2HCl5 σ*C−Cl orbital. Note that at C-Cl distance of 2.05 Å, just before ET, Δε = 0.70 eV (see text). For analogous MoI adduct in equilibrium geometry the orbital energies are: −3.12 (SOMO) and −0.02 eV (LUMO).

3.3. Reduction of C2HCl4• radical

Upon the dissociative ET onto pentachloroethane, the generated C2HCl4• radical is only weakly

bound to the WII site (cf. Fig. 2b and Table 1). The radical may be thus easily reduced at the electrode

(vide infra) or directly at the W centre, if the latter is reduced again to the WI state before the radical

detaches. As can be expected, the radical is more strongly bound to the WI than to the WII centre (cf.

Table 1), which is reflected in the geometry of the C-H···O moiety within the {WI−Oalk}•−···HC2Cl4•

adduct (Fig. 4a). The parameters of the H-bonding in the latter adduct, dH···O = 1.85 Å and θC−H···O =

163.8°, resemble these in the {WI−Oalk}•−···HC2Cl5 adduct, whereas the bonding energy is even larger

for the radical. This is in contrast to the situation of the previously studied CHCl3 and CHCl2• adducts

with the MoI scorpionate, where the radical binds weaker than the chloroform [Error! Bookmark not

defined.]. This suggests that in the present case an additional stabilising interaction should contribute

to the bonding between the C2HCl4• and WI scorpionate. We calculated the electrostatic potential map

for C2HCl4• as shown in Fig. 4a, which revealed the presence of an electrophilic region (σ-hole) on one

of the chlorine atoms. The analogous σ-hole was also found for C2HCl5, but the one for the radical is

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deeper and spatially larger than for C2HCl5, making the radical prone to form a more favourable

halogen bonding with a pyrazolyl ring; the greater spatial size of the σ-hole makes the energy of the

halogen bonding less sensitive to the halogen bond angle.

Fig. 4. DFT-D optimised geometries (distances in Å) for (a) {WI−Oalk}···HC2Cl4•, and (b)

{WI−Oalk}•−···HC2Cl3 formed upon intramolecular dissociative ET into C2HCl4•. Dotted lines show

C−H···Oalkoxide bonding and C−Cl···πpyrazolyl interactions, which in (a) exhibit a weak halogen bonding character. Electrostatic potential for C2HCl4

• on the 0.001 a.u. isosurface of the total B3LYP/6-311++G(2d,2p) electronic density is shown; the potential energies are presented in the −25 (or less) to +84 kJ·mol−1 (or more) range.

The subsequent ET onto the C2HCl4• radical is concerted with the Cl− dissociation (our

calculations indicated that the hypothetical C2HCl4− carbanion is unstable), yielding trichloroethylene,

C2HCl3, which in turn might accept the next electron from the WI centre (Fig. 4b) or from the

electrode, in any case undergoing a dissociative ET (for the reduction potential see Table 2 in Section

3.4 below). The DFT results show that the ET coupled with the dissociation of chloride interacting

with the pyrazolyl ring is favoured (by ca. 10.5 kJ·mol−1) over the cleavage of its geminal neighbour.

However, the stability of {WI−Oalk}•−···HC2Cl3 is much lower than that of its analogue with C2HCl5,

which is a plausible reason of lowering the catalytic effect (see Fig. 1d). Hence, further dechlorination

requires a potential more cathodic than that of the WII/I pair.

3.4. Calculated redox potentials

Before analysing the mechanism of further dehalogenation steps we would like to show the

calculated redox potentials (Table 2) for all plausible compounds and transients that may occur in the

mechanism. The significance of the potentials will be discussed in the next section. The redox

potential was also calculated for [W(NO)(TpMe2)(OMe)2] that was used as a model of

[W(NO)(TpMe2)O(CH2)4O] (the experimental E1/2 values of these two complexes differ by 0.16 V, the

potential for the dimethoxy complex being more cathodic). Note that the experimental redox potential

of the {WII/I−OMe}0/•− pair (and also of {WIII/II−OMe}•+/0) is reproduced very accurately by the present

DFT calculations. In passing we note that the difference between the SOMO energies for Mo and W

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dimethoxy scorpionates (0.58 eV) correlates nicely with the difference between their E1/2 values

(0.56 V).

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Table 2 Redox potentials in CH2Cl2 (V vs. Fc•+/0) for the W complex and organic species and transients occurring in C2HCl5 dehalogenation obtained from DFT (E0

DFT) and high-level coupled cluster CCSD(T)-F12b calculations (E0

CC). Species E0

calcd(DFT) E0cacld(CC)

a E0exp

{WII−OMe}/{WI−OMe}•− −2.45 b −2.40 c {WIII−OMe}•+/{WII−OMe} +0.68 b +0.63 c C2HCl5/C2HCl4

• + Cl− −0.66 −1.34 (−1.08)

−0.98 d

C2HCl4•/C2HCl3 + Cl− +0.93 +0.72

C2HCl3/cis-C2HCl2• + Cl− −1.66 −2.15

(−1.89)

−1.86 d,e cis-C2HCl2

•/cis-C2HCl2− −0.42 −0.47

cis-C2H2Cl2/cis-C2H2Cl• + Cl− −1.97 −2.40 (−2.14)

−2.09 d

cis-C2H2Cl•/C2H2 + Cl− +0.99 +0.90 (+1.16)

C2Cl4/C2Cl4•− −2.32

(−2.14) −2.65

(−2.47)

−2.11 d,e C2Cl4/C2Cl3

• + Cl− −1.60 −2.12 (−1.86)

−1.73 d,e

C2Cl3•/C2Cl3

− −0.24 −0.23 C2HCl/C2H•···Cl− −2.12 −2.54

(−2.31)

C2HCl/C2H• + Cl− −2.72 −2.96 (−2.69)

C2H•/C2H− +0.68 +0.56 C2Cl2/C2Cl•···Cl− −1.99 −2.36

(−2.14)

C2Cl2/C2Cl• + Cl− −2.44 −2.91 (−2.63)

C2Cl•/C2Cl− +0.71 +0.88 a Values calculated for comparison in DMF solution are given in parentheses. b CC calculations are presently too expensive for these large W complexes. c This work. d In DMF from [Error! Bookmark not defined.]. We have noticed by comparing the same data in [Error! Bookmark not defined.] and [Error! Bookmark not defined.] that the calomel electrode used by the authors had the potential equal to +0.268 V vs. NHE and we used this value along with Fc•+/0 = 0.400 V vs. SHE to convert potentials taken from [Error! Bookmark not defined.]. e In DMF from [Error! Bookmark not defined.], potentials converted to the ferrocene scale assuming Fc•+/0 = 0.400 V vs. SHE.

However, we are aware that the B3LYP functional (used in the DFT calculations), despite its

overall high accuracy for organic and inorganic reactions, underestimate the energy of the C−Cl bond

by as much as 41.8 kJ·mol−1 [Error! Bookmark not defined., 38]. This DFT error (due to limitation

of the approximate exchange-correlation functional in accurate description of the correlation energy)

considerably affects the dissociative potentials calculated for the chlorinated compounds and is,

obviously, not remedied by including the dispersive correction. Therefore, to provide correct values

for dissociative reduction potentials we also performed more accurate coupled cluster (CC)

calculations, whose methodology is described in the Computational Details. Here we only notice that

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these calculations are based on the explicitly-correlated CCSD(T)-F12 method which is expected to

provide the results nearly converged with respect to the basis set limit for already a moderate basis set

(see Computational Details); the results are thus expected to be more accurate than the results from

conventional CCSD(T) calculations with the similar basis set. As can be seen from Table 2, the

difference between the DFT and CC-corrected potentials is only significant for the case of dissociative

reductions (concerted with the C-Cl cleavage, the energy of which is inaccurately described by

DFT:B3LYP), where the E°cacld(CC) fall in a good agreement with the experimental data in contrast to

E0calcd(DFT). An exception is the (non-dissociative) reduction potential of C2Cl4, where the DFT and CC

results do also differ considerably, the former providing a better agreement with the experimental

potential. For comparison with literature data some of the dissociation potentials and of C2Cl4/C2Cl4•−

were calculated also in DMF solvent; note that for the former the difference between potential values

in DMF and CH2Cl2 is 0.26-0.28 V, which is mainly due to the different solvation energy of the Cl−

anion in both solvents (0.26 eV).

3.5. C2HCl3 dechlorination in the presence of C2HCl5

Non-catalysed processes starting with C2HCl3 together with the discussed catalytic cycle are

shown in Fig. 5.

Fig. 5. Suggested mechanism of C2HCl5 reduction electrocatalysed by {W(NO)(TpMe2)}2+ alkoxides, based on calculated ΔE‡, ΔE and E°. After C2HCl4

• has been detached (1) it may be reduced at the electrode or in {WI−Oalk}•−···HC2Cl4

• adduct (2); the product of this reaction, C2HCl3, undergoes outer-sphere reduction rather

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than by ET from WI complex. Species in red may be reduced by WI. Dotted lines denote a feasible pathway that may occur at potentials more cathodic than the WII/I redox potential; C2HCl, C2H2Cl2 and C2Cl2 undergo further dehalogenation to acetylene as a final product.

The plausible dehalogenation steps that can follow are based on the calculated Gibbs free energies

(ΔG0) and activation free energies (Δ‡G0) of various possible organic reactions, and the redox

potentials (E0, see Table 2 above) of the chemically and electrochemically generated transient species,

describing their ability to be further reduced under the experimental conditions.

The first steps of the organic reaction pathway, i.e., generation of the C2HCl4• radical and its

reduction by WI leading to TCE, were already described above. We notice that the C2HCl4• radical

might alternatively react with C2HCl5 via a H• atom transfer (eq 1)

C2HCl4• + C2HCl5 = C2Cl5

• + C2H2Cl4 (1)

but this reaction is neither favoured thermodynamically, nor kinetically, i.e., ΔG0 = 5.9 kJ·mol−1 and

Δ‡G0 = 67.4 kJ·mol−1.

Concerning TCE, there is an experimental [Error! Bookmark not defined.] and theoretical [Error!

Bookmark not defined.] evidence that for strong reducing agents the reduction follows a stepwise

mechanism, i.e., π* anion radical C2HCl3•− is an intermediate, which then breaks into C2HCl2

• + Cl−

(the same mechanism has been found plausible also for C2Cl4 and C2H2Cl2 reduction). It was found,

however, that for some reducing agents a concerted cleavage may occur yielding lower activation

barriers. Interestingly, calculations indicate that dissociative ET to C2HCl3 leads selectively to the cis-

C2HCl2•···Cl− pair. This selectivity was attributed to the tendency to form a cis ion-dipole complex

over its trans isomer [Error! Bookmark not defined.]. Our calculations confirmed it, and moreover,

indicated that this is also true in the case of intramolecular ET within the adduct {WI−Oalk}•−···HC2Cl3

(vide supra). It is known that the cis-1,2-dichloroethen-1-yl radical may abstract H• from an alcohol

molecule [Error! Bookmark not defined.]) or be interconverted to the trans isomer (that is why

small amounts of trans-DCE are present in the products of C2HCl3 reduction by outer-sphere electron-

transfer agents [Error! Bookmark not defined.]). Since no effective H• donor is present in our

system and the potentials are more cathodic than −2.45 V, the cis-C2HCl2• radical should be rapidly

reduced to the corresponding carbanion.

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Fig. 6. Geometries for (a) encounter complex and (b) transition state for the concerted proton transfer and Cl− abstraction from C2HCl5 to cis-C2HCl2

−, eventually giving cis-C2H2Cl2 and C2Cl4; distances in Å.

The cleavage of cis-dichlorovinyl carbanion (cis-C2HCl2−) to chloroacetylene and Cl− is a very likely

path, since it is a thermodynamically favoured and almost a barrierless process (ΔG0 = −101.3

kJ·mol−1 and Δ‡G0 = 2.5 kJ·mol−1). However, the calculations indicate that cis-C2HCl2− may be

alternatively protonated by C2HCl5, if sufficient amount of this proton donor is available.

cis-C2HCl2− + C2HCl5 → cis-C2H2Cl2 + C2Cl4 + Cl−  (2)

This very exothermic reaction (ΔG0 = −214.2 kJ·mol−1) proceeds rapidly from the well bound

encounter complex (Fig. 6a) with moderate C−H···O hydrogen bonding (which compensates the

negative entropy of activation) through an early transition state (Δ‡G0 = 2.9 kJ·mol−1). The proton

migration and the Cl− dissociation occur simultaneously (Fig. 6b), since the stable C2Cl5− carbanion

does not exist [Error! Bookmark not defined.], as also confirmed by our DFT calculations. The

formation of both cis-C2H2Cl2 and chloroacetylene were experimentally evidenced in the reduction of

C2HCl3 [Error! Bookmark not defined.].

The outer-sphere dissociative reduction of the generated C2Cl4 (eqn 1) yields C2Cl3•,† which in turn is

rapidly reduced to the C2Cl3− carbanion. The latter may meet a similar fate as cis-C2HCl2

−, namely the

decomposition or protonation by C2HCl5. The first process, giving dichloroacethylene and Cl−, is also

a favourable and fast reaction (ΔG0 = −76.1 kJ·mol−1 and Δ‡G0 = 15.1 kJ·mol−1). The second one, even

more thermodynamically and kinetically favourable (ΔG0 = −201.3 kJ·mol−1 and Δ‡G0 = 6.3 kJ·mol−1),

will regenerate C2HCl3 and C2Cl4 (eqn 3).

C2Cl3− + C2HCl5 → C2HCl3 + C2Cl4 + Cl−  (3)

The chloroacetylenes, C2Cl2 and C2HCl, formed by a breakdown of C2Cl3− and cis-C2HCl2

−,

respectively, may undergo further concerted reductive cleavage (see Table 2). Interestingly, our

calculations indicate that the corresponding R•···Cl− pairs (where R• is one of the alkyne radicals) are

tightly bound (dR•···Cl− is as small as 2.38 Å) and energetically stabilised, owing to which interaction

the reduction potential for chloroacetylenes is greatly lowered as compared with a purely dissociative

process (see Table 2). The last radical in the path of chloroacetylenes dehalogenation, C2H•, is

immediately reduced to a carbanion that subsequently abstracts a proton giving acetylene.

cis-Dichloroethene (the product of cis-C2HCl2− protonation) also has a very cathodic reduction

potential. This explains why cis-C2H2Cl2 often accumulates in anoxic environments, in which PCE or

                                                            † Even if the C2Cl4

•− anion radical would form, its protonation by C2HCl5, i.e., C2Cl4− + C2HCl5 → C2HCl4

• + C2Cl4 + Cl−, is predicted by our calculations to be rather slow (Δ‡G0 = 20.9 kJ·mol−1) compared with the C-Cl bond cleavage. That is why the C2Cl3

• radical, and not C2Cl4•−, was taken into account in our mechanism.

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TCE undergo reductive dechlorination [Error! Bookmark not defined.]. The product of concerted

DET onto cis-dichloroethene, i.e., cis-C2H2Cl• radical, is readily reduced (note a very high driving

force because of the positive E0 value) to yield acetylene coupled with a chloride anion expulsion.

In sum, the “organic loop” in Fig. 5 remains open for the applied range of potentials. This is because

the chemical and electrochemical processes described above finally lead to species whose potentials

are too cathodic for their efficient reduction close to the WII/I redox potential. Thus, although the W

scorpionate is capable to mediate the dissociative reduction of C2HCl5 and the resulting C2HCl4•

radical, the autocatalysis comparable to the one described previously for CHCl3 reduction (mediated

by Mo scorpionates) will not be triggered in the case of PCA, in agreement with the presented CVs.

4. Conclusions

The identified mechanism of binding and activation for CHCl3 reductive catalytic dehalogenation

driven by the Mo/W-alkoxides may operate in the case of other acidic H-containing polyhalogenated

hydrocarbons, like pentachloroethane and trichloroethylene, however, no activity toward C2Cl4 was

observed.

The calculations clearly showed that C2HCl5 binds to the WI alkoxy scorpionate, almost as

strongly as CHCl3 to the analogous MoI, due to the formation of a short, charge assisted C−H···Oalkoxide

H-bond and dispersive interactions of the chlorine atoms with π-electron density. The close

Oalkoxide···C-Clacceptor separation (ca. 4 Å) and stiffening the bound molecule are essential for the

intramolecular concerted electron transfer. The C2HCl4• radical thus produced may also interact with

the WI site, even stronger, because of the better halogen bonding donor ability. However, the binding

energy of the WI adduct with trichloroethylene is much lower than for C2HCl5, hence the catalytic

effect is weak, which could also rationalise the effect observed for P450. Autocatalytic dehalogenation

of C2HCl5 in the studied system does not occur due to the lack of reactive species formed after C2HCl4•

has been released, which may undergo immediate reduction at mild potentials, as in the case of CHCl3.

The mechanism of non-catalysed total dehalogenation of C2HCl5 to C2H2 has been verified based

on the DFT and coupled cluster (CC) calculations – consonant with all the experimentally detected

intermediate products, described in the literature. The present calculations evaluated feasibility of

various reaction pathways in the studied dechlorination mechanism. In this regard several potentially

interesting aspects were pointed out in the considered mechanism, for instance that pentachloroethane

may be easily deprotonated in an extremely exothermic and rapid reaction with anionic transients in a

concerted proton transfer and Cl− abstraction. Another observation is that the reduction potential of

chloroacetylenes are greatly lowered owing to the formation of the strongly interacting R•···Cl− pairs.

Last but not least, redox potential for the environmentally relevant species (participating in the studied

reactions) were accurately computed employing the explicitly correlated CC methodology.

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Acknowledgments

The authors gratefully acknowledge funding from the Polish Ministry of Science and Higher

Education within the project no. IP2011 045871 (P.R.) and IP2011 044471 (M.R.). This work was

made possible thanks to PL-Grid Infrastructure, computational grants from the Academic Computer

Centre in Krakow (CYFRONET) and through financial support provided by the European Union

through the ESF within the Cracow University of Technology Development Program, contract no.

UDA-POKL.04.01.01-00-029/10-00 (scholarship for G.R.).

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Graphical Abstract (for review)