On the role of noncovalent interactions in electrocatalysis. Two cases of mediated reductive dehalogenation

Electrochimica Acta 110 (2013) 619–627

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On the role of noncovalent interactions in electrocatalysis. Two cases of mediated reductive dehalogenation a,∗ ´ Piotr P. Romanczyk , Klemens Noga b,c , Mariusz Radon´ b,c,∗ , a Grzegorz Rotko , Stefan S. Kurek a,∗ a

Faculty of Chemical Engineering and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Kraków, Poland Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland c Academic Computer Center CYFRONET, ul. Nawojki 11, 30-950 Kraków, Poland b

a r t i c l e

i n f o

Article history: Received 9 December 2012 Received in revised form 27 April 2013 Accepted 5 May 2013 Available online 18 May 2013 Keywords: Electrocatalysis Reductive dehalogenation Hydrogen bond Halogen bond Dispersive interactions

a b s t r a c t Two cases of mediated electron transfer are presented: chloroform reduction catalysed by MoII/I alkoxy scorpionates and debromination of hexabromocyclododecane (HBCD) in the presence of free-base tetraphenylporphyrin (H2 TPP). Although H2 TPP should act as a typical outer-sphere mediator, it is not active towards analogous dehalogenation of 1,2-dibromocyclododecane. The observed phenomena can be rationalised by considering the catalytically relevant transient adducts formed owing to noncovalent interactions (C H hydrogen bonds and dispersive C halogen· · ·␲ interactions or directional halogen bonding), which warrants the close and prolonged contact between the catalyst and its substrate, thus increases the probability of electron transfer, and decisively accelerates the reaction. Crucial for this action is thermodynamic stability of the adducts, which can only be explained if dispersive van der Waals interactions are properly accounted for, e.g., as by dispersion-corrected density functional theory (DFT-D) calculations. The structures involving strong and anisotropic interactions, like the surprisingly short C H· · ·Oalkoxide H-bonding in the MoI –chloroform adduct, may be reasonably well described by standard DFT calculations and the energy needs only be corrected for dispersion without the need for structure re-optimisation at the DFT-D level. The latter is, however, a method of choice for the prediction of supramolecular structures chiefly controlled by weak non-directional van der Waals forces. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Attractive, generally weak, noncovalent forces like C H· · ·A (A = O, N or ␲-acceptor) hydrogen bonds [1–4] and dispersive C halogen· · ·␲ interactions [5], or directional halogen bonding [6], bringing about a certain degree of organisation, may decide on the direction of processes involving much higher energies. This effect has been known in nature since long, very often manifested in gripping and orienting the substrate in enzymes [7], and only much later noticed in purely chemical systems [8]. The existence of these phenomena and that they can help rationalise the course of reactions was intuitively accepted by chemists, but there were no tools to prove their role quantitatively. While hydrogen and halogen bonds – rooted to a large extent in electrostatic and to some degree in orbital interactions – are described qualitatively well by density functional theory (DFT) methods, the same is not true for ubiquitous dispersive interactions. The dispersion

∗ Corresponding authors. Tel.: +48 12 6282770. ´ E-mail addresses: [email protected] (P.P. Romanczyk), ´ [email protected] (S.S. Kurek). [email protected] (M. Radon), 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.006

forces originate from long-range correlation effects which are not properly accounted for by standard density functional theory (DFT), whereas correlated wave function methods are usually not applicable to large models representing catalytic systems for their extremely high computational cost. Only recently, the dispersioncorrected DFT, the so-called DFT-D approach, was developed, in which dispersion interactions are described by adding a damped R−6 interatomic potential to standard density functionals [9–13]. This approach can give reliable results for relatively large systems at no additional cost [14]. The significance of dispersion forces may be surprising; interestingly, they are able even to overcome strong Coulombic repulsion between like-charge ions, as in an adduct formed by cationic species [15,16]. Thus, the need for accurate functionals that describe van der Waals (vdW) interactions is postulated also for calculations of electrochemical processes [17]. Chloroform, like other polyhalogenated alkanes, undergoes reduction coupled with a Cl− anion dissociation in a concerted mechanism with a substantial activation barrier without catalyst [18], since the electron is transferred onto the relatively high-lying C∗ Cl molecular orbital. However, the molecule is special, for it may form non-conventional C H hydrogen bonds, even in solution [19]; the H-bond stabilises also the

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Cl− · · ·HCCl2 • pair formed upon chloroform reduction in solid argon [20]. The occurrence of the C H· · ·Oalkoxide H-bonding was evidenced in the X-ray structure of Mo alkoxide scorpionate [21], which contains CHCl3 molecules interacting with the complex. As we previously demonstrated, mono- [22] and dinuclear [23] alkoxides based on {MoII/I (NO)(TpMe2 )} {[TpMe2 ]− = 3 hydrotris(3,5-dimethylpyrazol-1-yl)borate = scorpionate} (abbrev. {MoII/I Oalkoxide }0/•− ) efficiently catalyse chloroform electroreduction. The process proceeds at the redox potential of MoII/I pair, provided it is more cathodic than −1.82 V vs. Fc•+/0 , which is an extraordinary catalytic effect. We also found that the addition of small amounts of alcohols totally inhibits the reaction, as does, unexpectedly, using DMF as a solvent. Recently, we investigated the mechanism of this electrocatalysis and its inhibition in a joint electrochemical and dispersion-corrected DFT study, finding an autocatalytic cycle being triggered by an electron transfer through the C H· · ·O bonding in {MoI Oalkoxide }•− · · ·HCCl3 adduct [24]. Hexabromocyclododecane (HBCD) is a brominated flame retardant in a wide use since 1960s. It is biologically degraded, especially in anaerobic environments under reducing conditions. Enzymes containing cobalamin or Factor F430 , both based on modified porphyrin ring systems, are active in this process. A number of publications have dealt with environmental problems this product presents, but systematic electrochemical studies are lacking. Likewise HBCD, 1,2-dibromocyclododecane (DBCD) also contains only vicinal pairs of bromine atoms. Such compounds are known to undergo reductive elimination yielding respective cycloalkenes [25], a reaction that occurs at potentials less cathodic by ca. 0.4 V than debromination of monobrominated compounds, as shown for 1,2-dibromocyclohexanes. This reaction may be mediated by aromatic anion radicals including those of zinc, copper and free-base porphyrins [26] at appropriately less cathodic potentials in a purely outer-sphere electron transfer process. In our preliminary studies on reductive HBCD debromination in the presence of cobalt tetraphenylporphyrin we obtained surprising results showing that the rate of reaction is almost 6 times higher than that for DBCD [27], which is twice the expected rate ratio and this prompted us to examine the effect of the porphyrin ring itself. In this paper, we describe two puzzling cases of mediated electron transfer (ET) which could be rationalised by noncovalent interaction effects leading to the formation of transient adducts. First, comparing the structures and energetics of the MoII/I adducts with chloroform (or with inhibitor molecules) calculated at the DFT-D versus ordinary DFT level we show that neglecting the vdW forces makes it impossible to explain the stability of the transient adducts formed, and thus to unravel the catalytic and inhibition effects. Second, markedly higher activity of free-base tetraphenylporphyrin (H2 TPP) in mediating ET to HBCD in comparison with DBCD may also be rooted in van der Waals interactions. 2. Experimental 2.1. Materials Dichloromethane (Merck Emsure, ethanol-free, stabilised by 50 ppm amylene) was freshly distilled off from CaH2 under argon prior to use. As even traces of ethanol, often used as a stabiliser in some grades of CH2 Cl2 or CHCl3 , were inhibiting the onset of the catalytic reaction, it was carefully checked by GLC that the solvents used are ethanol free. Purchased dichloromethane was also GLC checked for the presence of traces of chloroform. Chloroform (Alfa-Aesar, ethanol-free, 99+%) was freshly fractionally distilled. DMF (Merck SeccoSolv) was fractionally vacuum distilled at temperatures below 80◦ and kept over A4 molecular sieves in fridge for maximum a couple of days. The Mo alkoxide [Mo(NO)(TpMe2 )(OEt)2 ] was synthesised according to the

published methods [28] and was checked spectroscopically for its purity. Other reagents (Merck, Sigma–Aldrich) were used as received. HBCD is a commercial product containing ca. 70% of the ␥-isomer. 1,2-Dibromocyclododecane was obtained by addition of Br2 to cyclododecene using a general method [29]. Its purity was checked by GLC and 1 H NMR. The product consisted of two isomers in the ratio 1:1.7 (calculated from integrated 1 H NMR signals at 2.15 and 1.98 ppm, and GLC) that could not be easily separated. Cyclododecene for this synthesis was purchased from Aldrich and used as supplied. Unexpectedly, 1 H NMR analysis indicated that it was a mixture of cis- and trans-isomers [30] in the ratio of 2:1, opposite to the reverse ratio often cited in literature [31]. Bromine addition is stereospecific, and it is expected that cis-cycloalkene would give predominantly RR/SS racemate. It was impossible to decide based only on 1 H NMR spectrum which of two 1,2-dibromocyclododecane diastereoisomers (RR/SS or RS/SR) predominates in the product, but knowing that bromination of 1,5,9-cyclododecatrienes is stereospecific with transdouble bond yielding RS/SR and cis-double bond, RR/SS isomer [32,33], we can assume that the product obtained is a mixture of RS/SR and RR/SS 1,2-dibromocyclododecanes in the ratio 1:1.7, respectively. 2.2. Electrochemical measurements A BAS 100B/W Electrochemical Workstation with a C3 Cell Stand (Bioanalytical Systems, USA) was used. Measurements were done under argon in dry dichloromethane containing 0.1 M n-Bu4 NPF6 (Sigma–Aldrich, electrochemical grade, vacuum dried) or in DMF containing 0.1 M n-Bu4 NBF4 (Fluka) as base electrolyte. The solution was additionally dehydrated with 4 A˚ molecular sieves. Glassy carbon and platinum electrodes (Mineral, Poland) were applied for cyclic voltammetry studies generally recorded at 100 mV s−1 scan rate if not otherwise stated. The Ag/AgCl (3 M NaCl) electrode connected to the solution through an electrochemical key filled with supporting electrolyte solution was used as a reference. Potentials are quoted versus Fc•+/0 that was added at the end of each series of measurements as an internal standard. The IR compensation was employed to yield the peak-to-peak separation (Ep ) for the Fc•+/0 couple 60–65 mV. 2.3. Computational details Quantum chemical calculations were carried out with Turbomole package [34]. Some preliminary optimizations were performed using InSilicoLab portal, which provides a web based access to computational chemistry software via a PL-Grid Infrastructure [35]. Geometry optimisations were performed at standard density functional theory (DFT) or dispersion-corrected DFT (the DFT-D3 variant [9]) levels, in both cases with the B3LYP [36,37] hybrid functional for its overall high accuracy and its good performance for the structural and electronic properties of the {Mo(NO)Tpx } complexes we found previously [24,38]. Openshell species were treated within spin-unrestricted scheme. The conductor-like screening model (COSMO) [39] was used with the dielectric constant of dichloromethane (ε = 8.93) or DMF (ε = 38.25). In the study of Mo scorpionates interacting with CHCl3 and inhibitors we employed the triple-␨ def2-TZVPP [40–42] basis set for all atoms, with respective effective core potential (ECP) for Mo. Bonding energies for the MoII/I adducts were corrected for basis-set superposition error (BSSE) estimated from the standard counterpoise procedure [43]. The starting geometry of the {Mo(NO)(TpMe2 )(OCH2 -)} fragment and chloroform molecule was taken from the crystal structure of anti-[Mo(NO)(TpMe2 ){1,4(OCH2 )2 C6 H4 }]2 ·4CHCl3 [21]. Geometries of the MoII/I species were

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621 •

Fig. 1. Chloroform reduction on GCE before and after the addition of [Mo(NO)(TpMe2 )(OEt)2 ] in CH2 Cl2 /0.1 M TBAPF6 ( = 0.1 V s−1 , cMo = 2 mM). Uncatalysed reduction in green (dotted lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

optimised independently. In the study of porphyrin interactions with DBCD and HBCD, geometries were optimised at DFT-D/631G(d,p) level and the final energies reported were obtained from single-point DFT-D/def2-TZVPPD [44] calculations. It was found that in some cases ordinary DFT calculations were able to predict stable structures, in a qualitative agreement with the DFT-D ones, but yielding clearly too small or even positive bonding energies. Generally, free-base porphine (H2 P) was used as a model of H2 TPP. The H2 P0/1−/2− · · ·DBCD/HBCD adducts, and also selected structures with H2 TPP2− , were considered. Maps of electrostatic potential for DBCD, HBCD, H2 P and H2 P2− were obtained on the 0.001 a.u. isosurface of the total B3LYP/6-31G(d,p) electronic density. 3. Results and discussion 3.1. The case of chloroform reduction catalysed by Mo alkoxy scorpionates Following the MoII/I reduction the electron is transferred to chloroform, which is evidenced by the fact that the MoI re-oxidation wave vanishes (Fig. 1). Please note the apparent differences in potentials of uncatalysed chloroform reduction in CH2 Cl2 in CVs recorded on GCE presented in this work, and in [22] on Pt electrode. In neither case does a peak appear. However, they are clearly seen in voltammograms for CHCl3 reduction in CH2 Cl2 after subtracting CVs of the base electrolyte. The apparent potential on Pt is more cathodic because of a higher barrier, but the potentials of MoII/I reduction are the same on both electrodes. The significant shift in potential, similar to that observed on silver [45], with respect to the reduction in the absence of Mo complex is related to formation of the {MoI (NO)(TpMe2 )(Oalkoxide )}•− · · ·HCCl3 adduct, proposed by us based on joint electrochemical and quantum chemical studies [24]. This adduct is crucial for electrocatalysis since the proximity of MoI and CHCl3 enhances the electron transfer between them within the adduct, which initiates an autocatalytic reaction eventually leading to CHCl3 degradation. To understand this process it is vital to see how the computational methods (DFT, DFT-D) describe the adduct formation in terms of the structure and thermodynamic stability. To this end, Fig. 2 compares the ordinary DFT and dispersion corrected

DFT (abbrev. DFT-D) optimised structures of {MoII/I Oalkoxide }0/ − adducts with CHCl3 (a and b) and HCCl2 • radical (c), which is the product of the ET concerted with Cl− dissociation. The structures were calculated in the actual solvent used (CH2 Cl2 ), as described by the continuum solvation model COSMO. The chloroform molecule interacts with Mo alkoxy scorpionate via a non-conventional C H· · ·Oalkoxide H-bonding, which becomes exceptionally short (dH · · ·O even equal 1.82 A˚ at DFT-D) and nearly linear ( C H···O = 172.2◦ ) in the MoI adduct. In addition to the H-bonding, whose structural effects are already reasonably modelled by ordinary DFT calculations, a further important factor controlling the geometry of these adducts are the Cl· · ·␲pyrazolyl van der Waals (vdW) interactions, correctly described only when DFT is supplemented with a suitable interatomic potential in the DFT-D approach. These interactions manifest themselves by a significant attraction of the Cl atom towards the ␲ system in the cavity formed by scorpionate ligand, which is correctly reproduced only at DFT-D and not at DFT level. In the DFT-D structure, the Cl· · ·␲pyrazolyl distance is shortened even by 0.27 A˚ upon MoII reduction. The difference between DFT-D and DFT structures is particularly pronounced for the {MoII Oalkoxide }· · ·HCCl2 • adduct (Fig. 2c); an entirely different orientation of the CHCl2 • radical, being mainly controlled by vdW interactions, cannot be properly determined by ordinary DFT calculations. Some discrepancies between DFT and DFT-D structures are noticeable also for the {MoII Oalkoxide }· · ·HCCl3 adduct (Fig. 2a). The H-bonding in the DFT-D structure deviates from the linearity because of the weak competitive C H· · ·Npyrazolyl interaction, and Cl atom is weakly attracted towards the ␲pyrazolyl system, which is again missing in ordinary DFT. We point out that the DFT-D geometry of {MoII Oalkoxide }· · ·HCCl3 is in good agreement with the X-ray crystal structure of a closely related (bimetallic) MoII -alkoxy scorpionate with bound chloroform [21]. No experimental structure is available for the transient MoI –chloroform adduct, neither for the MoII -adduct with CHCl2 • radical. As mentioned in Introduction, the addition of an alcohol inhibits the electrocatalysis [22,24]. The calculations rationalise this observation by showing that alcohol may compete with CHCl3 in binding to the {MoI Oalkoxide }•− site, giving an adduct. The formation of a strong, classical O H· · ·Oalkoxide hydrogen bonding in MoI –methanol adduct is reflected in the geometric parameters ˚  C H···O = 173.4◦ and 171.4◦ , cf. Fig. 3a). (dH· · ·O = 1.66 and 1.68 A, Stability of this adduct is further increased when dispersion is included (see below), but the geometry is not much affected. An inhibitory effect is also brought about by an excess of alkene or using DMF as a solvent. However, these species (propene was used as a model) are only weakly held together with {MoI Oalkoxide }•− by weak Csp2 H· · ·Oalkoxide H-bonding and dispersive interactions, resulting in the DFT and DFT-D geometries markedly different. Table 1 compares bonding energies of CHCl3 , CHCl2 • , and the three inhibitors, showing that the long-range dispersion interactions considerably contribute to the stability of these transient adducts. The bonding energies obtained from ordinary DFT calculations are compared with the DFT-D ones performed in two ways: single-point calculation on top of the DFT-optimised structure and with full geometry optimisation at DFT-D level. The latter approach is, clearly, more consistent and presumably more accurate as well. However, already a simple correction for dispersion (without reoptimisation of the structure), yields reasonably close results, here with discrepancies up to 13 kJ mol−1 . For the MoI adducts with CH3 OH and CHCl3 , the bonding energies are already negative at the DFT level (and become even more negative at the DFT-D level). In contrast, the bonding energies of alkene and DMF are predicted positive by ordinary DFT calculations and they become negative only after correcting for dispersion. For

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•

Fig. 2. Comparison of DFT-D (sharp, Cl atoms yellow) and DFT (diffused, Cl atoms green) optimised geometries for {MoII/I Oalkoxide }0/ HCCl2 • radical (c) in CH2 Cl2 . Dotted lines show C H· · ·Oalkoxide bonding and C Cl· · ·␲pyrazolyl dispersive interactions; distances in Å. Table 1 • Bonding energiesa (in kJ mol−1 ) for adducts with MoII/I (NO)(TpMe2 )(OMe)2 0/ − in CH2 Cl2 obtained from DFT and DFT-D calculations. Adduct

DFT

DFT-D//DFTb

DFT-D optc

{Mo } HCCl3 {MoI }• − HCCl3 {MoII } HCCl2 • {MoI }• − HOCH3 {MoI }• − HC( O)N(CH3 )2 d {MoI }• − H2 C CHCH3

4.2 −4.2 2.1 −22.2 10.5 12.6

−32.2 −42.7 −23.8 −52.3 −22.6 −11.3

−44.4 −52.3 −37.2 −56.1 −30.1 −22.6

II

a b c d

Corrected for basis set superposition error. DFT-D3 energy on top of the DFT optimised geometry. Full DFT-D3 geometry optimisation, data from [24]. In DMF.

both situations, however, the vdW corrections are very significant, 33–46 kJ mol−1 . Ordinary DFT calculations thus seriously underestimate the stability of these adducts, thus cannot rationalise their existence at room temperature in solution. Here one should bear in mind that in order to get a thermodynamically relevant G of binding, the energies given in Table 1 need to be supplemented with zero-point and thermal energy as well as entropic corrections. This will produce a positive contribution to Gbind (of about 38–42 kJ mol−1 ), mainly due to the loss of translational and rotational entropy when binding a small molecule to the Mo scorpionate species. Therefore, the formation of considered adducts can be rationalised only by taking into account the dispersioncorrected DFT energies, which are negative enough to counteract the loss of entropy. We notice there is an approximate compensation between the dispersion interactions (ca. −42 kJ mol−1 ) and entropy effects (ca. +42 kJ mol−1 ), which may be quite general [46]. Considering the relatively large negative bonding energy of {MoI Oalkoxide }•− · · ·HCCl3 , which would give slightly negative Gbind , a fraction of CHCl3 molecules will be bound to the MoI complex under experimental conditions (which is crucial for the electrocatalysis). Even more stable should be the adduct with

−

adducts with CHCl3 (a and b) and

methanol, but those with alkene and DMF are predicted much less stable. The inhibitory effect of the latter two species chiefly results from trapping of transient CHCl2 • radical by alkene or:CCl2 by DMF [24].

3.2. Tetraphenylporphyrin–free base – a redox mediator Reductive debromination of vicinal dibromo compounds goes through the antiperiplanar conformer and, if such cannot be formed due to steric reasons, it requires much more cathodic potentials typical of monobrominated species [25]. In the case of brominated cyclododecanes it would mean that RR/SS diastereoisomers should be more reactive than the RS/SR ones. And indeed, higher activity in isomerisation of HBCD, with similar transients taking part, was observed for RR/SS configurations [33]. Commercial HBCD contains ␣, ␤ and ␥ isomers with two RS/SR and one RR/SS structures, and other isomers in much minor amounts. An interesting observation from the studies on biodegradation [47], where it was proven that HBCD indeed undergoes sequential reductive elimination, is that the second step of reductive elimination occurs very rapidly and does not control the overall reaction rate. This means that in the case of a 12-membered ring the structure is more flexible and there are no critical differences in rates of dehalogenation between RS/SR and RR/SS configurations. Concluding, we might expect similar activity of Br atom pairs in both HBCD and DBCD used by us, which means statistically grounded three times higher reaction rate for HBCD. Fig. 4 presents the cyclic voltammetry of DBCD and HBCD reduction in the absence and in the presence of free-base tetraphenylporphyrin. HBCD and DBCD are reduced on GCE at about the same potentials of −2.5 V vs. Fc•+/0 couple. The current intensities are, as expected, about three times higher for HBCD. Monobromocyclododecane requires potentials more cathodic by 0.4 V to undergo the same process (not shown in Fig. 4). It agrees with the observation [26] that vicinal dibromo compounds are

•

Fig. 3. Comparison of DFT-D (sharp) and DFT (diffused) optimised geometries for {MoII/I Oalkoxide }0/ − adducts with CH3 OH (a), DMF (b) and propene (c) in CH2 Cl2 , except DMF adduct (in DMF used also as solvent). Dotted lines show C H· · ·Oalkoxide bonding and C H· · ·␲pyrazolyl interactions; distances in Å.

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623

Fig. 4. DBCD (a) and HBCD (b) reduction in the presence of 1 mM free-base tetraphenylporphyrin in DMF/0.1 M TBABF4 ( = 0.1 V s−1 ) on GCE. Inserts show uncatalysed DBCD and HBCD (c = 0.5 mM) reduction.

reduced at less cathodic potentials, because the first electron transfer is followed by immediate second electron injection leading to the elimination of two bromides with alkenes as products. HBCD used by us, contains primarily ␥-isomer, with lesser amounts of ␣- and ␤-, all three of them exhibiting two RS/SR and only one RR/SS CHBr CHBr configuration. 1,2-DBCD applied in this study, however, is a mixture with prevailing RR/SS isomer content (cf. Section 2). Reactions involving Br atoms in such structures generally proceed via an antiperiplanar transient that may be readily formed from RR or SS structure, but with a higher energy barrier from RS/SR diastereoisomers, which was experimentally proven for isomerisation of HBCD [33] and is believed to be true also for debromination. In our experiments the current ratio for HBCD and DBCD reduction on GCE was equal 3 for a wide range of concentrations indicating that the type of CHBr CHBr configuration may not be critical for this reaction, in contrast to 1,2-dibromocyclohexanes [25], possibly because of higher flexibility of cyclododecane rings. A slightly less cathodic potential seen for uncatalysed HBCD reduction may be attributed to entropy effects caused by the presence of three times more Br atom pairs (an effect of the order of RT/F · ln(3) = 28 mV). However, the behaviour of DBCD and HBCD is dramatically different in the presence of free-base tetraphenylporphyrin (H2 TPP). Unexpectedly, the porphyrin dianion is capable of transferring the electron only to HBCD, but not to DBCD. The latter compound is reduced only at potentials characteristic of the uncatalysed process, although it still somehow interacts with reduced porphyrin, increasing the third reduction wave. The vanishing oxidation peak corresponding to the porphyrin second reduction wave in the case of HBCD indicates that the electrons were transferred. A characteristic bromide oxidation wave seen for both HBCD and DBCD confirms that debromination indeed occurs. The experimental results thus suggest that H2 TPP2− is a more effective electron donor for HBCD than for DBCD, which does not conform to the common view of free-base porphyrins as being non-specific redox electron transfer mediators [26]. It is therefore interesting how the porphyrin ring interacts with the two compounds and, in particular, whether the transient adducts may form. Since the calculations for H2 TPP0/1−/2− · · ·DBCD/HBCD structures turned out to be immensely time-consuming, they were performed only for selected structures, while for the entire set

of adducts we used unsubstituted porphine (H2 P) as a tractable model of H2 TPP, interacting with either DBCD (1S,2S) or ␥-HBCD (1S,2S,5S,6R,9R,10S). Three charge states were considered for the porphine and the respective adducts: the neutral (H2 P), the 1e reduced (H2 P− ), and the 2e reduced (H2 P2− ). We recall that the CV curves suggest that DBCD/HBCD can be reduced only following the second reduction of porphyrin, i.e., the active form of the electrocatalyst is H2 TPP2− . Some comment must be then made on the spin state of the dianion. Although one may naively expect the triplet ground state, since the two extra electrons are accommodated in the doubly degenerate eg LUMO, the two eg -type orbitals are in fact not degenerated in free-base porphine due to non-equivalent protonation of the porphine’s nitrogen atoms: the one having significant contribution from the 2pz on the protonated nitrogens (eg ) is lower in energy than the other one (eg ). The present DFT calculations (with the larger basis set) indicated the singlet (eg )2 ground state for H2 P2− to be below the triplet and open-shell singlet corresponding to the (eg )1 (eg )1 occupations. Indeed, the singlet ground state is experimentally recognised for 2e reduced porphyrins even in the case of formally more symmetric metalloporphyrins and -porphyrazines [48,49], where it is stabilised by a pseudo Jahn–Teller distortion. Therefore, the bonding energies were calculated with respect to the closed-shell singlet state of H2 P2− , and the same spin state was also assumed for the H2 P2− fragment in the adducts with DBCD/HBCD. In the course of DFT-D optimizations in DMF (COSMO model), we identified several local energy minima for the H2 P0/1−/2− · · ·DBCD and H2 P0/1−/2− · · ·HBCD adducts, of which the structures corresponding to the lowest energy are shown in Fig. 5. Closer inspection of the structures obtained reveals that the H2 P0/2− · · ·DBCD geometries are determined by an interplay of weak C H hydrogen bonds and C Br· · ·␲ interactions, while the structures of HBCD adducts seem to be mainly governed by Br· · ·␲ dispersion forces (Fig. 5c–e). In the case of the H2 P0/1−/2− · · ·HBCD adducts, two conformations with very close energies were identified, illustrated for the dianion in Fig. 5(d) and (e). The conformation (e), lying less than 4 kJ mol−1 above the lowest energy one (d), reveals somewhat closer packing of the two fragments and a significant saddle distortion of the porphine ring. Analogous structures were also observed for the neutral and 1e reduced porphine (not shown in Fig. 5). More important, however, the H2 P0/2− · · ·HBCD

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Fig. 5. Selected DFT-D optimised geometries for DBCD and HBCD adducts with porphine0/2− in DMF. Dotted lines show weak C H hydrogen bonds or C Br· · ·␲ dispersive interactions; distances in Å. The energy for structure (e) is higher by only 3.3 kJ mol−1 in comparison with (d).

adducts presented in Fig. 5(c) and (d) show a very short ˚ with nearly linear C Br· · ·Nporphine Br· · ·Nporphine contact (3.22 A) arrangement, which might point to the presence of halogen bonding. In order to check whether the characteristic ␴-hole [6,50] would appear in the system, we calculated electrostatic potential maps for DBCD, HBCD and porphine0/2− , as presented in Fig. 6. Indeed, for HBCD (but not for DBCD) there is a positive potential region at the top of bromine atoms (the ␴-hole), exactly along the extension of the C Br bond, as expected for halogen bonding donors. In the adduct, the Br atom shown in Fig. 6(b) is directed towards the unprotonated N atom of porphine (cf. Fig. 5(c) and (d)), which is the centre of the negative electrostatic potential (cf. Fig. 6(c) and (d)), hence a perfect halogen bond acceptor. It cannot be excluded that the resulting linear arrangement, and thus a greater overlap of the HBCD C∗ Br with the porphyrin ␲ orbital,

contributes to an increase in the electronic coupling parameter Hab and thus to acceleration of ET in comparison with DBCD. But we believe that the primary effect responsible for this electrocatalysis is, like for the Mo scorpionates considered above, the proximity of electron donor (porphyrin) and acceptor (DBCD/HBCD) fragments held together by noncovalent interactions. Even if the considered interactions may seem at first glance weak and ephemeral, their overall energetic effect is quite significant as shown in Table 2. The binding energy at DFT-D level is as large as 42 kJ mol−1 for the adduct with neutral H2 P. Although it becomes somewhat lower for the adducts with H2 P− , H2 P2− (presumably due to unfavourable electrostatic interactions between the bromine lone electron pairs and the negative charge on the anions as well as stronger solvation of these anions by DMF), these adducts are still notably stabilised and may be considered plausible

Fig. 6. The maps of electrostatic potential (kJ mol−1 ) for DBCD (a), HBCD (b), H2 P0 (c), and H2 P2− (d) on the 0.001 a.u. isosurface of electronic density.

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625

Fig. 7. DFT-D optimised geometries for DBCD (a) and HBCD (b) adducts with H2 TPP2− in DMF. Dotted lines show weak C H hydrogen bonds or C–Br· · ·␲ interactions. Distances (Å) are as follows: d1 = 3.31, d2 = 2.59, d3 = 2.34 (a), and d1 = 3.39, d2 = 3.22, d3 = 2.35, d4 = 2.44 (b).

Table 2 Bonding energies (in kJ mol−1 ) for the DBCD and HBCD adducts with porphyrins0/1−/2− in DMF. Adduct

DFT-D Ebind

H2 P· · ·DBCD H2 P· · ·HBCD H2 P• − · · ·DBCD H2 P• − · · ·HBCD H2 P2− · · ·DBCD H2 P2− · · ·HBCD H2 TPP2− · · ·DBCD H2 TPP2− · · ·HBCD

−43.1 −42.3 −37.7 −38.9 −21.8 −24.3 −39.7 −46.0

a

the probability of electron tunnelling (even if the intrinsic barrier for electron transfer remains unchanged). This proximity effect may be considered analogous to that occurring in enzymatic catalysis.

a

Full DFT-D geometry optimisation.

transient species, even in the solution and at room temperature. Moreover, there is an apparent tendency for HBCD to bind stronger than DBCD, upon consecutive steps of the porphyrin reduction (cf. Table 2). As it was mentioned above, we carried out the calculations also for adducts with H2 TPP2− besides for its simplified model (H2 P2− ) containing no meso-phenyl groups. The obtained geometries are shown in Fig. 7 with key distances indicated, and bonding energies given in Table 2. The presence of four phenyl groups does not diminish the accessibility of porphine ␲-electrons density to DBCD/HBCD; on the contrary, it clearly gives rise to bond strengthening with the binding energy nearly doubled. This is due not only to an enhancement in dispersive interactions but also to screening of the porphyrin dianion by phenyl groups from an ‘excessive’ solvation in the absence of DBCD/HBCD (an effect which reduces the binding energy by stabilising one of the fragments). Most importantly, in the extended model with H2 TPP2− , the energetic discrimination between HBCD and DBCD is larger and more distinct (6.3 kJ mol−1 , in favour of HBCD) than in the case of H2 P2− , confirming that this effect is responsible for their different voltammetric behaviour. Nevertheless, one should be aware of intrinsic limitations of the presently chosen methodology. Statistical sampling or molecular dynamics simulation may be necessary to systematically probe the conformational diversity and properly take care of the connected contribution to entropy. Indeed, the entropy may further discriminate between the HBCD and DBCD adducts, presumably in favour of the HBCD ones where more closelying conformations are expected.

• The generally weak C H· · ·O hydrogen bond may unexpectedly become strong, especially if it is charge assisted and involves chlorocarbons as the H-donor. This is the case of {MoI Oalkoxide }•− · · ·HCCl3 , where the adduct stability is additionally enhanced by vdW interactions. • The proximity effect in the case of HBCD-porphyrin adduct, owing to dispersive C Br· · ·␲ interactions and directional halogen bonding, seems to be responsible for the observed enhancement of electrocatalysis. • The formation of adducts stabilised by noncovalent interactions relevant to electrocatalysis (redox not chemical catalysis) has not yet been postulated in the literature, according to our best knowledge. In particular, free-base porphyrin has so far been considered as a non-specific redox mediator. However, in the present study it would be hardly possible to explain the experimental CV results without assuming the formation of transient adducts with porphyrin, stabilised by noncovalent interactions, whose existence is supported by dispersion-corrected DFT calculations. • Including noncovalent interactions by means of dispersioncorrected DFT is necessary to explain the thermodynamic stability of these electrocatalytically relevant adducts. The dispersion correction to bonding energies, close to 42 kJ mol−1 for the systems studied here, in part counteracts the loss of translational and rotational entropy when binding two fragments into a single adduct. Structures of the adducts with strong, anisotropic interactions (like H-bonding in the MoI –chloroform adduct) may still be described by standard DFT calculations reasonably well, but those chiefly controlled by weak non-directional vdW forces (like the porphyrin adducts with DBCD/HBCD) have to be optimised at the DFT-D level. • Our results suggest a general idea what features a molecule should exhibit to be a good redox electrocatalyst. Among them the most essential seem to be: an anion radical form with appropriate redox potential, rather than dianion, exhibiting rigid structure with spacious SOMO, with a great deal of dispersion enabling the formation of an adduct enhanced by H- or halogen bonding, preferably in an environment of a low-dielectric constant.

4. Conclusions Acknowledgments In this paper we have shown two examples, in which electrocatalytic effect can be enhanced by noncovalent interactions that stabilise the transient adducts and thus place the electron donor and acceptor close to each other for a prolonged time, increasing

The authors gratefully acknowledge funding from the Polish Ministry of Science and Higher Education: Projects IP2011 045871 (P.R.), and N N305 363139 (S.S.K.). This scholarly work was made

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thanks to POWIEW project, which is co-funded by the ERDF as a part of the Innovative Economy program, PL-Grid Infrastructure, and computational grants from the Academic Computer Centre in Krakow (CYFRONET) and Wroclaw Networking and Supercomputing Centre (WCSS, Grant No. 181). The publication was made possible through financial support provided by the Foundation for Polish Science (START scholarship for M.R.) and the European Union through the ESF within Cracow University of Technology Development Program, Contract No. UDA-POKL.04.01.01-00-029/10-00 (scholarship for P.R.).

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