Mechanism and Thermodynamics of Reductive Cleavage of Carbon-Halogen Bonds in the Polybrominated Aliphatic Electrophiles.

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Mechanism and Thermodynamics of Reductive Cleavage of CarbonHalogen Bonds in the Polybrominated Aliphatic Electrophiles Sergiy V. Rosokha, Emoke Lukacs, Jeremy Ritzert, and Adam Wasilewski J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11410 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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The Journal of Physical Chemistry

1

Mechanism and Thermodynamics of Reductive Cleavage of Carbon-Halogen Bonds in the Polybrominated Aliphatic Electrophiles Sergiy V. Rosokha,* Emoke Lukacs, Jeremy T. Ritzert,† Adam Wasilewski Department of Biological, Chemical and Physical Sciences Roosevelt University, Chicago, IL, 60605

ACS Paragon Plus Environment


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2 Abstract Quantum-mechanical computations revealed that, despite the presence of electron-withdrawing and/or -acceptor substituents, the lowest unoccupied molecular orbitals (LUMO) of the polybromosubstituted aliphatic molecules R-Br, (R-Br = C3Br2F6, CBr3NO2, CBr3CN, CBr3CONH2, CBr3CO2H, CHBr3, CFBr3, CBr4, CBr3COCBr3), are delocalized mostly over their brominecontaining fragments. The singly-occupied molecular orbitals in the corresponding verticallyexcited anion radicals, (R-Br-)*, are characterized by essentially the same shapes, and show nodes in the middle of the C-Br bonds. An injection of an electron into the antibonding LUMO results in the barrierless dissociation of the anion-radical species and the concerted reductive cleavages of C-Br bonds leading to the formation of the loosely-bonded {R…Br-} associates. The interaction energies between the fragments of these ion-radical pairs vary from about 10 kcal mol-1 to 20 kcal mol-1 in the gas phase and from 1 kcal mol-1 to 3 kcal mol-1 in acetonitrile. In accord with the concerted mechanism of reductive cleavage, all R-Br molecules showed completely irreversible reduction waves in the voltammograms in the whole range of the scan rates employed (from 0.05 V s-1 to 5 V s-1). Also, the transfer coefficients, , established from the width of these waves and dependence of reduction peak potentials, E p, on the scan rates, were significantly lower than 0.5. The standard reduction potentials of the R-Br electrophiles, EoR-Br/R·+X-, and the corresponding R radicals, EoR·/R-, were calculated in acetonitrile using the appropriate thermodynamic cycles. In agreement with these calculations, which indicated that the R radicals resulting from the reductive cleavage of the R-Br molecules are stronger oxidants than their parents, the reduction peaks’ currents in cyclic voltammograms were consistent with the twoelectron transfer processes.


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3 Introduction Reductive cleavage of carbon-halogen bonds in halogenated organic compounds, R-X, represents one of the most important examples of electron transfer (ET) reactions accompanied by formation or dissociation of chemical bonds.1-4 Computational and/or experimental studies4-13 of these processes revealed that reductive cleavages of the majority of aryl halides proceed via stepwise pathway, which involves transient formation of a radical anion (eq 1). R-X + e-

R-X-

R + X-

(1)

The reductive cleavages of the aliphatic halides mostly follow a single-step concerted mechanism: R-X + e-

R + X-

(2)

Indeed, quantum-mechanical computations of many alkyl halides showed that their LUMOs represent, in essence, * antibonding orbitals with respect to the carbon-halogen (C-X) bond.3,12 Injection of an electron into such orbitals is accompanied by the barrierless dissociation of the CX bond and the reductive cleavage of such species follows the concerted mechanism. Yet, earlier studies revealed that mechanisms of the reductive cleavage of some polyhalogenated aliphatic electrophiles may vary with the conditions of the experiments.8,9 Furthermore, if aliphatic halides bear -acceptor substituents or other -fragments, the injected electrons may initially occupy lowenergy *-orbitals and the reduction of these molecules may result in the transient formation of radical anions (i.e., their reductive cleavage occur via the stepwise mechanism).8,14 The ESR identification of radical anions of tetrabromomethane and chloro-, bromo- and iodoperfluoroalkanes resulting from the -irradiation of the corresponding molecules in low-temperature matrices also pointed out the possibility of the stepwise mechanism of their reductive cleavage.3 Reductive cleavage of the carbon-halogen bonds may also be significantly affected by the donor/acceptor interaction in the ET precursor complex. Saveant et al. have shown that if the



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4 interaction is relatively weak, the kinetics of many concerted DET processes can be successfully accounted for by the “sticky” dissociative ET model, which was developed based on the Marcus outer-sphere ET theory and takes into account C-X bond dissociation energies and interactions between ion-radical products.2,4,15-18 Yet, experimental rates of some DET reactions are many orders of magnitude higher than those predicted by the outer-sphere theory, which implies that they proceed via the inner-sphere mechanism involving strongly-bonded intermediates.19 It should be mentioned in this respect that intermolecular attractions between covalently-bound electrophilic halogen atoms (such as those in CBr4 or bromo- and iodoperfluoroalkanes) and electron-rich centers, referred to as halogen bonding, emerged recently as a powerful tool for crystal engineering, anion transport, drug design and other applications.20-24 Moreover, our studies of association between bromosubstituted aliphatic electrophiles (Chart 1) and neutral or anionic nucleophiles

Chart 1. Structures of the R-Br molecules (X= H, F, Br, CN, NO2, Y = CBr3, OH, NH2). and several recent experimental and computational analyses of halogen-bonded complexes revealed that the charge-transfer (weakly-covalent) interactions represent a vital component of halogen bonding.25-32 This implies electronic coupling of the halogen-bonded species, which may considerably attenuate barriers for the ET reactions (similar to that in the -bonded donor / acceptor complexes33,34). In fact, we have demonstrated recently that the involvement of the halogen-bonded precursor complexes resulted in a significant decrease in the barriers for the redox-reactions between the strong electron donors and acceptors (e.g. tetramethyl-p-phenylenediamine or I- anions and CBr3NO2) as compared to the hypothetical outer-sphere pathways. Importantly, the rate constants of the inner-sphere redox-reactions calculated using the spectral


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5 and structural characteristics of the halogen-bonded complexes agreed with the experimental kinetics which supported the suggested mechanism.31,32 Significant lowering of the ET barrier due to halogen bonding has important implications not only for redox-reactions, but also for many chemical transformations of halogenated species which potentially involve ET steps, from electrophilic substitution and addition to initiation of radical polymerization and atmospheric chemistry.19, 33-37 While the analysis of the halogen-bond-assisted ET reactions mentioned above indicated concerted mechanism of the reductive cleavage of CBr3NO2 and CBr4 molecules,31 the electron-withdrawing and π-acceptor fragments (-F, -CN, NO2, -C=O) in the electrophiles in Chart 1 imply that some of them may undergo stepwise reductive cleavage. In order to corroborate the role of halogen bonding in ET reactions, as well as to facilitate the analysis of the redox-transformations of these and similar species in chemical and biochemical systems and in the environment, it is necessary to verify the mechanisms and thermodynamics of the reductive cleavages of carbon-halogen bonds in the R-Br molecules. So far, however, the studies of the dissociative ET reactions of halogenated aliphatic species were focused mainly on the monohalogenated compounds.1-3 While many chemical reagents and environmental pollutants are polyhalogenated molecules, there are only a limited number of works dealing with the reductive cleavage of polychlorinated molecules (e.g., polychloroalkanes and polychloroacetamides).1,4, 7, 38-41 As for the bromo-compounds, only mechanistic analyses of the reductive debromination of polybrominated aromatics, such as brominated diphenyl ethers, that are additive flame retardants, as well as dibromo- and tetrabromomethanes have been published.3, 42,43 Also, a few electrochemical studies and/or theoretical evaluation of the redox potentials of bromosubstituted molecules were reported (e.g., CHBr3, CBr4, CBr3NO2, dihaloadamantanes).31, 44,45 Thus, to fill these voids and to facilitate a mechanistic analysis of the redox-reactions of polybrominated



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6 species, the current work presents the results of computational and experimental studies of the mechanism and thermodynamics of reductive cleavage of the electrophiles R-Br from Chart 1. Experimental section. Materials. Commercially available tetrabromomethane, bromoform, tribromofluoromethane, hexabromoacetone, tribromoacetic acid and 1,2-dibromohexafluoropropane were purified by sublimation or distillation. Tribromonitromethane was synthesized by bromination of nitromethane, tribromoacetamide was prepared by reaction of hexabromoacetone with NH4OH, and tribromoacetonitrile was synthesized by dehydration of tribromoacetamide with P 2O5.27 Cyclic voltammetry studies were performed on a (computer-controlled) BASi Epsilon Electrochemical workstation in a three-electrode cell system using (positive-feedback) IR compensation. Measurements were carried out in dry acetonitrile solutions containing 5 mM of R-Br compound, 5 mM of ferrocene (Fc) and 0.1 M of supporting electrolyte, Bu4NPF6 (which was dried at 120 oC before use) under a dry N2 atmosphere at 20 oC. A glassy carbon (GC) highly polished disk (3 mm diameter) and a platinum wire were used as the working and auxiliary electrodes, respectively. Prior to each experiment, the GC electrode was rinsed with acetone and deionized water, polished on a microfiber cloth with alumina paste, followed by ultrasonic rinsing for 5 min in ethanol and drying.12 Then, a control CV run was carried out using a solution containing only supporting electrolyte and ferrocene. Alternatively, the GC electrode was dipped for a few seconds in 6 M HNO3, rinsed several times with H2O and dried.46 A non-aqueous reference electrode from BASi (a silver wire in a AgNO3/Bu4NPF6 solution in CH3CN) was used as a reference electrode. The apparent redox-potentials obtained in this way were converted to the aqueous saturated calomel electrode (SCE) scale based on the values measured in the same run for the Fc+/Fc couple, which was used as an internal standard (EoFc+/Fc = 0.391V vs SCE).47


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7 Computations were carried out using the Gaussian 09 suite of programs. 48 Since the reductive cleavage of R-Br molecules involves formation of more or less separated ion-radical pairs, the energetics of these processes were examined using DFT calculations with B97X-D49 and M062X functionals50 and 6-311+G(d,p) basis set. Theoretical analyses showed that these functionals provide superior characteristics of long-distance interactions at a reasonable computational cost.51 Also, in our previous study of the halogen-bonded complexes of R-Br electrophiles, they provided the best agreement between experimental and calculated characteristics of the intermolecular associates.27,52,53 Geometries of neutral R-Br molecules were optimized without constraints in the gas phase and in acetonitrile (optimizations in solutions were carried out using polarizable continuum model).54 The optimized R-Br structures were used for the single-point calculations of the vertically-excited (R-Br-•* radical anions. Geometry optimizations (without constraints) of these radical anions resulted in the elongation of the C-Br bond and formation of R/Br- radical / ion pairs as the minimum-energy structures. Energies and atomic coordinates of the optimized RBr molecules and R/Br- radical/ ion pairs are listed in Table S1 the Supporting Information. Atomic charges were calculated Natural Population Analysis (NPA) phase of Natural Bond Orbital analysis.55, 56 Frontier orbital shapes were evaluated at 0.03 isovalue. Geometry optimizations and calculations of energies, charges, and spin densities for open-shell species were carried out using both restricted and unrestricted open-shell methods. Both procedures afforded nearly identical results (see Table S2 in the Supporting Information). To avoid problems resulting from the spin contamination, molecular orbital shapes were derived only from the restricted open-shell computations. To study dissociation of the C-Br bond in the (R-Br-•* radical anions, constrained optimizations of the non-stationary species were carried out. In these computations, one of the C-Br bonds



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8 in the anion-radical was frozen at a certain value (starting from the bond-length in the neutral species, dR-Br, and then increased incrementally by 0.1 Å). It should be mentioned that constrained optimizations of the (R-Br-•* radical anions which one of the C-Br bonds was frozen at values close to dR-Br (typically, lower than dR-Br + 0.4 Å) resulted in dissociation of the other C-Br bond. As such, the optimizations of the non-stationary species, in which the length of the bond undergoing dissociation was fixed in the range between dR-Br and dR-Br + 0.6 Å, were carried out with additional constraints imposed on the values of the other C-Br bond lengths. In the transitional range (e.g., dR-Br + 0.5 Å), the dual computations at the same points along the C-Br dissociation pathways verified that the energies of the species resulting from the optimizations with constrains imposed on all C-Br bonds are close to those resulting from the optimizations with only one C-Br bond frozen. The energies of interaction, E, between radical and anion in the optimized {R…Br}- radical-ion pairs were determined as: complex – [R· + EBr-] + BSSE

(3)

where complex, R• and EBr- are sums of the electronic and zero-point energies of {R…Br}- complexes and isolated R• and Br- counterparts, and BSSE is a basis set superposition error. Zero-point energies and thermal corrections were taken from unscaled vibrational frequencies. The basis set superposition errors were determined via the counterpoise method.57 Reduction potentials of the R-Br molecules in acetonitrile were calculated as:58 Eredo =1/F×(-BDE + TS - Gsolv) + 1.60

(4)

where F is Faraday constant, BDE and S are bond dissociation energy and entropy of R-Br molecule in the gas phase and Gsolv= Gsolv(R•) + Gsolv (Br•) - Gsolv(R-Br) is the difference between solvation energies of the R-Br molecule and products of homolytic bond breaking of its


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9 C-Br bond, and 1.60 is the reduction potential of the Br atom in acetonitrile. Bond dissociation energies for R-Br molecules, were calculated as: 19 BDE = -[H(R-Br) – H(R•) – H(Br•)]

(5)

Following Lin et al.,19 energies and enthalpies of species in the gas phase were calculated using a high-level composite G3(MP2)-RAD(+) method that approximates (UR)CCSD(T) energies with a large triple basis set via additivity corrections at the R(O)MP2 level of theory.59,60 Solvation energies were calculated using the conductor-like polarizable continuum model, CPCM, level for the structures optimized at HF/6-31+G(d) level with UAHF radii.58 The details of the computations are presented in Tables S3 and S4 in the Supporting Information. Reduction potentials of the R• radicals resulting from the reductive cleavage of R-Br molecules were calculated in acetonitrile (in V vs SCE) as: Eored =-G’AN(R•/R-)/F - 4.429

(6)

where G’AN(R•/R-) is a free energy change of the reduction of the radical, and the value of 4.429 V is obtained by adding the SCE potential (0.241 V) to the absolute SHE potential (4.281V) 61 and subtracting the interliquid potential (0.093 V).19 The GAN(R•/R-) values were calculated as: G’AN(R•/R-) = G’gas(R•/R-) +G’solv(R•/R-)

(7)

The free energy changes in the gas-phase were established based on enthalpies and entropies calculated at G3(MP2)-RAD level:19 G’gas(R•/R-)=Hgas(R•/R-) -TSgas(R•/R-)

(8)

where Hgas(R•/R-) = H(R-) – H(R) and S(R•/R-) = S(R-) – S(R•). The differences of the solvation energies of the radical and anions, G’solv(R•/R-), were calculated as: G’solv(R•/R-) = G’solv(R-) – G’solv(R•)

(9)

The details of the calculations are presented in Tables S3 and S4 in the Supporting Information.



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10 Results and Discussion Computational analysis of the dissociation of R-Br- radical anions Quantum-mechanical computations of the R-Br bromocarbons from Chart 1 revealed that the LUMOs of the molecules with two to four bromine substituents are delocalized more or less symmetrically over all C-Br fragments and show nodes in the middle of C-Br bonds (Figure 1). A

B

C

D

E

F

G

H

I

Figure 1. The LUMOs (0.03 isovalue) of neutral R-Br electrophiles: C3Br2F6 (A), CHBr3 (B), CFBr3 (C), CBr3CN (D), CBr3NO2 (E), CBr3CONH2 (F), CBr3COOH2 (G), CBr4 (H) and CBr3COCBr3 (I); the dark and light blue colors designate the opposite phases of the wave functions (from B97XD/6-311+G(d,p) computations, note that MP2 computations afforded similar LUMOs, see Figure S1 in the Supporting Information). The LUMO of hexabromoacetone (Figure 1I) is delocalized mostly over two of its C-Br bonds (and shows nodes in the middle of these bonds). On the whole, the molecular orbital shapes in Figure 1 indicate that the LUMOs of the R-Br molecules represent antibonding orbitals with regard to the C-Br bonds. The SOMOs of the vertically excited radical anions (R-Br- produced by injecting an electron into the neutral molecules without relaxation (i.e. the anion-radical species with the


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11 nuclear geometries optimized for their neutral parents) are characterized by essentially the same shapes (Figure S2 in the Supporting Information). Geometry optimization of these species resulted in the significant increase of the length of one of the C-Br bonds and formation of the -

{R…Br - adducts. The energy profiles for the dissociations of the R-Br anion-radical in the gas

phase and in acetonitrile are shown in Figure 2 and in Figure S3 in the Supporting Information. Gas-phase 30 15 0 -15 -30 40

E, kcal mol-1 E, kcal mol-1 E, kcal mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CH3CN

C3Br2F6 -●

C2Br4F6 -●

CBr3CN-●

CBr3CN-●

20 0

-20 15

CBr4-●

CBr4-●

45 30 15 0 -15 45 30 15 0 -15 30 15

0

0

-15

-15 1.5

3 4.5 1.5 dC…Br, Å

3 4.5 dC…Br, Å

Figure 2. Potential energy profiles for the cleavage of the C-Br bonds in the R-Br- radical anions (left – in the gas-phase, right – in acetonitrile). Values of E represent relative energies obtained by subtracting energies of the isolated R radical and Br- anion from the energies of non-stationary species, i.e. vertically excited (R-Br- radical anions (○) and {R…Br - species resulted from anion-radical optimization with one C-Br bond frozen () or from optimization with one C-Br bond frozen and the other C-Br bonds constrained (□), see Experimental section for details.

In the gas-phase, all profiles show distinct minima around 2.5 - 2.8 Å. They suggest significant interaction energies between the fragments of these species (from about 10 kcal mol-1 to 20 kcal mol-1). In acetonitrile, the dissociation energy profiles show only very broad and shallow minima of about -1 kcal mol-1 at much higher C…Br separations of 3.5 – 4 Å.



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12 The results of the unconstrained optimizations of the R-Br- radical anions are consistent with the energy profiles in Figure 2. The fully-optimized structures in acetonitrile are presented in Figure 3. They demonstrate that dissociation of most of the R-Br- radical anions proceeds essentially via the elongation of one of their C-Br bonds, accompanied by the planarization of the carbon-containing fragments. However, unconstrained optimizations of the CHBr3- and CBr3CONH2- radical anions afforded somewhat different dyads showing hydrogen bonds between the hydrogen substituents of the organic fragments and bromines (Figure 3H and 3I).62 A

B

C

D

E

F

H

I

G

Figure 3. Structures of (R…Br -adducts resulted from the unconstrained ROB97XD/6311+G(d,p) optimization of radical anions of C3Br2F6 (A) , CBr3F (B), CBr3NO2(C), CBr3CO2H (D), CBr3CN (E), CBr4 (F), CBr3COCBr3 (G), CBr3H (H) and CBr3CONH2 (I) in acetonitrile. The interatomic separations in these fully-optimized structures, dR…Br, in the gas-phase and in acetonitrile are listed in Table 1. This Table also contains average values of C-Br bond length in the neutral R-Br molecules (which show only minor variations with the computational method and media, see Table S5 in the Supporting Information).


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13 Table 1. Interatomic Separations, dR…Br, and Interaction Energies, E, in {R…Br -Complexes, as well as Average C-Br Bond Length, dR-Br, in the Neutral R-Br Molecules. a

R-Br

{R…Br - Gas-phase

R-Br

dR-Br, Å dR…Brc, Å

{R…Br -

a

CH3CN

a

{R…Br - b

CH3CN 

dR…Brc, dR…Brc,   Å Å kcal mol-1 kcal mol-1

 kcal mol-1

CBr4

1.945

2.591

-8.93

4.022

-0.70

3.560

-1.14

CBr3NO2

1.925

2.612

-18.88

3.583

-1.96

3.320

-3.60

CBr3CN

1.945

2.704

-13.00

3.874

-1.16

3.522

-1.80

CBr3COOH

1.943

2.840

-9.45

3.805

-1.47

3.483

-2.40

CBr3F

1.939

2.559

-11.64

3.634

-0.66

3.369

-1.45

C3Br2F6

1.931

2.687

-19.09

2.992

-2.16

2.746

-4.44

CBr3COCBr3

1.947

2.781

-14.59

4.096

-1.42

3.560

-1.99

CBr3CONH2

1.947

2.357d

-19.13

2.583d

-3.38

2.531d

-3.44

1.934

d

-11.49

d

-1.94

d

-1.81

CHBr3

2.388

2.654

2.639

a) From B97XD/6-311+G(d,p) computations. b) From M062X/6-311+G(d,p) computations c) C…Br distance, if not noted otherwise. d) H…Br distance in hydrogen-bonded complex. 62

The data in Table 1 indicate that gas-phase optimizations of R-Br-radical anionsresult in formation of {R…Br -dyads with C…Br distances in the range from 2.56 Å to 2.84 Å (except for the hydrogen-bonded complexes). Such separations are approximately midpoint between the C-Br covalent bond length (~1.9Å) and the sum of van der Waals radii of these atoms (3.65Å63). The energies of these adducts are from 9 to 19 kcal/mol lower than the sums of the energies of the corresponding isolated radicals and bromide anions.62 The {R…Br}- associates resulting from the unconstrained optimization of R-Br- radical anions in acetonitrile show much larger separations between the fragments. Also, these dyads are less stable than the corresponding associates in the gas phase. In particular, the H…Br separation in the hydrogen-bonded {CBr2H…Br}- and {CBr2CONH2…Br}- dyads in acetonitrile are about 0.25 Å larger than those in the gas-phase. Stabilities of these dyads are about 6 times less in acetonitrile as compared to the gas phase. In the absence of hydrogen bonding, the



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14 {R…Br}- associates resulting from the computations with M062X functional in acetonitrile are characterized by C…Br distances that are only 2-8% shorter than the sum of the van der Waals radii. The C…Br distances in the complexes resulting from the B97XD computation in acetonitrile are close to or somewhat higher than the sum of the van der Waals radii. Only the {C3BrF6 …Br}- associate shows a C…Br separation of about 20% shorter than the sum of the van der Waals radii of carbon and bromine atoms. The interaction energies between the fragments of these dyads in acetonitrile are also rather small (about 1 – 4 kcal/mol). To clarify the nature of the loosely-bound {R…Br -complexes and the mechanism of the cleavage of the C-Br bond, we carried out calculations of the charge and spin distribution and molecular orbital shapes of the species at various interatomic C…Br distances. The frontier orbitals transformations accompanying elongation of the C-Br bond from 1.95 Å in the vertically-excited (CBr3CN-) anion-radical (left) to 2.70 Å and 3.87 Å (which correspond to the minimum-energy structures in the gas-phase and in acetonitrile, respectively) are shown in Figure 4. This Figure also shows related orbitals in the isolated CBr 2CN radical and Br- anion. The SOMO of the (CBr3CN-) anion-radical is essentially the same as the LUMO of the corresponding neutral molecule in Figure 1. It is delocalized mostly over the CBr3 fragment and shows nodes in the middle of each C-Br bond indicating its antibonding *C-Br character. As the C…Br separation increases to 2.70 Å, the SOMO converges toward the elongated C-Br fragment and becomes antibonding with respect to this particular bond. The SOMO of the minimumenergy structure in acetonitrile (dC…Br = 3.87 Å) shows only a minor segment located on the departing Br atom, otherwise it is quite similar to that of the isolated CBr2CN radical.


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15 dR…Br, A

1.95

2.70

3.87

Infinity

-1.199

-2.966

-3.504

-3.616

-9.121

-8.741

-8.367

-8.436

-9.509

-8.760

-8.400

-8.436

-9.511

-8.930

SOMO

E (eV) HOMO

E (eV)

HOMO-1

E (eV)

HOMO-2a

E, eV

-8.401

-8.436

Figure 4. Molecular orbitals shapes (at 0.03 isovalue) and energies for {CBr2CN…Br}- species calculated (ROB97XD/6-311+G(d,p)) at various C….Br separations in acetonitrile. The (non-bonding) HOMO of the vertically-excited (CBr3CN- radical anion shows three equivalent fragments located on its three halogen atoms. The HOMO-1 and HOMO-2 are also located mostly on the bromine substituents, but they show some bonding with the segments located on the C-CN fragment. In the adduct with dC…Br = 2.70Å, the HOMO and HOMO-1 become essentially non-bonding lone pairs of the departing Br atom (directed perpendicularly to the C-Br bond undergoing dissociation). At this separation, only very minor segments of these orbitals are located on the organic fragment. At dC…Br = 3.87 Å, the HOMO and HOMO-1 fragments are localized only on the bromine fragment. In comparison, the HOMO-2 of the



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16 adduct with dC…Br = 2.70Å shows more distinctively its (C-Br) bonding character with regard to the elongated C-Br bond. However, at dC…Br = 3.87 Å, it also looks like a lone pair of the bromine (directed along C-Br bond) with the very minor segment remaining on the organic fragment. Noticeably, as the dC…Br distance increases, the energies of these three HOMOs become closer to each other. As a result, in the minimum-energy structure in acetonitrile, the shapes and energies of the SOMO and three HOMOs are similar to those of the isolated CBr2CN radical and (degenerate) HOMOs of the isolated halide anions, respectively. Frontier orbitals changes accompanying dissociation of the C-Br bonds in the other R-Br- radical anions from Chart 1 are analogous to those in the CBr3CNradical anionThe charges and spin distributions in the vertically-excited (CBr3CN- and the other (R-Br- radical anions, as well as in the {R…Br}- dyads resulting from their dissociations agree with the MO pictures in Figure 4. Specifically, NBO analysis indicated that negative charges in the (R-Br- radical anions are distributed over carbon-containing cores and (more or less equally) their bromine substituents (Table 1). The spin densities are also delocalized over whole (R-Br- moieties. Table 2. NBO Charges (qBr) and Spin (SBr) Residing on the Departing Bromine in the Vertically Excited Radical Anions (R-Br- and in the Fully-Optimized {R…Br}- Complexes.a (R-Br-) qBr SBr CBr4 CBr3NO2 CBr3CN CBr3COOH CBr3F C3Br2F6 CBr3COCBr3 CBr3CONH2 CHBr3

-0.072 -0.108 -0.092 -0.151 -0.163 -0.269 0.006 -0.235 -0.170

0.207 0.279 0.254 0.276 0.247 0.403 0.147 0.375 0.260

{R…Br}- (Gas-phase) qBr SBr -0.525 -0.492 -0.578 -0.667 -0.550 -0.512 -0.572 -0.938b -0.934 b

0.570 0.524 0.597 0.660 0.579 0.565 0.556 0.892 b 0.908 b

{R…Br}- (CH3CN) qBr SBr -0.997 -0.988 -0.996 -0.993 -0.987 -0.867 -0.990 -0.977 b -0.979 b

a) From ROB97XD calculations. b) Hydrogen-bonded complex.


0.002 0.008 0.002 0.004 0.009 0.009 0.001 0.0001b 0.001b

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17 Bond dissociations in all R-Br molecules are accompanied by the shift of the negative charges toward departing bromine atoms. As a result, in most of the fully-optimized gas-phase {R…Br}- complexes (dR…Br of 2.60 ± 0.25 Å), the departing bromine atoms bear about half of the negative charge. In the hydrogen-bonded {CBr2CONH2…Br}- and { CBr2H…Br}- pairs, the shift of the negative charges toward the departing bromine atoms are even larger (Table 2). In the minimum-energy structures resulting from the optimizations in acetonitrile, negative charges reside almost completely on the departing bromine atoms. The changes of spin densities on the departing Br atoms with the increase of the dR…Br distances are not monotonous. In the gas-phase {R…Br}- complexes, atomic spin densities on the departing bromine atoms are higher than those in the vertically excited (R-Br- radical anions (Table 2). This is probably related to the fact that the SOMOs of the {R…Br}-species optimized in the gas-phase converge toward the dissociating C-Br bond as compared to their nearly symmetric distribution over all bromine-containing fragments in the vertically excited (RBr- anion-radical. However, as the distances between R and Br fragments increase, spin densities shift toward the organic part and the bromine atoms show only residual spin densities of 0.0001 to 0.01 in the minimum-energy structures in acetonitrile. On the whole, the quantum-mechanical computations indicated that addition of an electron to the R-Br bromocarbons from Chart 1 results in the reductive cleavage of their C-Br bonds. This cleavage proceeds via the concerted mechanism and it leads to the formation of the loosely bound (“sticky”) anion-radical pairs. Interestingly, previous studies of the fragmentations of radical anions of alkyl halides produced weak complexes between the alkyl radicals and the halide anions only in the gas phase, and stable complexes were not found in solutions.10,14 The identification of the stable {R…Br}- associates in acetonitrile in this work is probably related to



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18 the use of the B97XD and M062X functionals (which were specifically designed for the accurate treatment of the long-distance interactions) and/or to stronger electron-acceptor properties of the R fragments resulting from the dissociation of the R-Br electrophiles under study. Most importantly, such an attraction between the fragments facilitates reductive cleavage of the C-Br bonds.4,10 Cyclic voltammetry of R-Br electrophiles. Experimentally, the concerted and stepwise ET mechanisms of the reductive cleavage of the carbon-halogen bonds are commonly distinguished using electrochemical measurements.1,2 The reversible or partially reversible reduction peaks in the voltammogramms of chemical species indicate the presence of the radical anions with the lifetime comparable with the timescale of the CV measurements, i.e. a stepwise mechanism. The molecules leading to an anionradical with the shorter life-times and those undergoing concerted reductive cleavage (in which reduced species dissociate faster than the time of one vibration)64 show completely irreversible reduction peaks. For these molecules, stepwise and concerted mechanisms can be discriminated based on the values of the transfer coefficients, , established from the shape of the irreversible reduction wave or its dependence on the scan rate. Specifically, the values of  can be determined from the variation of the potential of reduction peaks, E p, with the scan rate, :65  = -(1.15RT/F) /(Ep/log)

(10)

Alternatively, it could be established from the half-peak width, p/2Ep/2 – Ep of the waves:  = (1.857RT/F)/p/2)

(11)

The values of  close to 0.5 point to the stepwise mechanism, and the lower values (e.g.,  ~ 0.3) are consistent with the concerted mechanism of the reductive cleavage.12,46,65


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19 Cyclic voltammetry of the R-Br molecules from Chart 1 in acetonitrile (see Experimental for the details) afforded irreversible reduction peaks in the whole range of the scan rates employed, i.e. from 0.05 V s-1 to 5 V s-1 (Figure 5 and Figure S4 in the Supporting Information).

0.0003

Epc (V vs Fc)

-1.85

-1.95 -2.05 -1.2

i, mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.2 log v

0.8

0.0000

Fc+/Fc

CHBr3

-0.0003 0.20 -0.20 -0.60 -1.00 -1.40 -1.80 -2.20 E, V vs Ag/Ag+

Figure 5. Cyclic voltammetry of CBr3H in acetonitrile with Fc+/Fc internal standard. Insert: Dependence of the peak potential on the scan rate.

The increase of the scan rate was accompanied by the shift of the peak potentials, Ep, toward more negative potentials, but their widths, p/2, remained essentially constant. The values of peak potentials, Ep, and  (determined via eq 8 and 9), are listed in Table 3.



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20 Table 3. Reduction Peak Potentials, Transfer Coefficients, Peak Current Ratios and Number of Transferred Electrons in Electrochemical Reductions of R-Br Electrophiles.a

CBr4 CBr3NO2 CBr3CN CBr3COOH CBr3F C3Br2F6 CBr3COCBr3

Ep,b V vs SCE

c

ip(R-Br)/ip(Fc) d

n

-0.86 -0.52

0.30 0.28

1.22 1.67

1.98 2.88

-0.67 -0.82 -1.18 -1.33 -0.36

0.26 0.33 0.32 0.33 0.31

1.47 0.20 1.31 1.18 1.18

2.57 0.32 2.06 1.83 2.05

0.34 0.75 -1.09 1.17 CHBr3 0.35 1.35 -1.51 1.96 -1 a) Accuracy: Ep:  0.02V, :  0.05, n:  0.2. b) At scan rate of 0.2 V s ; c) Average values from Ep vs log  and p/2. d) Average ratios of the reduction peak current of R-Br molecules to that of the ferrocene internal standard with the same concentration. CBr3CONH2

Notably, the values of  resulting from the electrochemical measurements are lower than 0.5 for all molecules, which support the concerted mechanism of their reductive cleavage. It should be mentioned, however, that besides the dissociation of the C-Br bonds, the reduction waves of R-Br molecules are affected by the follow-up reactions. For example, similar to many other halogenated molecules,19 the radicals resulting from the reductive cleavage of the R-Br species from Chart 1 are stronger electron acceptors than their parents (vide infra). As such, reductive cleavage of the R-Br molecules resulting in formation of the radicals, Ris followed by the fast reduction of the latter into carbanion, R-. Comparison of the reduction peak currents of the R-Br molecules from Chart 1with those of the internal standard, ferrocene, confirmed the multi-electron character of the reduction waves of (poly-)brominated R-Br electrophiles. According to Saveant et al.,14,65 the irreversible peak current, ip(R-Br), of the halogenated electrophile R-Br is related to the overall number of electrons, n, exchanged in the irreversible reduction as: ip(R-Br) = n×0.496×FSCR-Bro (DR-BrF/RT)0.5


(12a)

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21 where DR-Br and CR-Bro are diffusion coefficient and concentration of R-Br, S is electrode surface area, and  is a scan rate. The (reversible) 1e-peak current of the internal standard, ip(Fc) is: ip(Fc) = 0.446×FSCFCo(DFCF/RT)0.5

(12b)

Taking into account that, according to the Stokes-Einstein equation, diffusion coefficients of species are inversely proportional to their radii, the number of electrons, n, can be determined from the experimental ratio of currents, as n = (0.446/0.496) {ip(R-Br)/ip(Fc)}×(rR-Br/rFc)0.5/α0.5

(13)

where rR-Br and rFc are radii of the R-Br molecule and ferrocene, respectively (the values of rR-Br and rFc, calculated from their molecular volumes, are listed in Table S7 in the Supporting Information). The values of n, calculated from the ratios of the peak currents, are listed in the last column in Table 3. For most of the R-Br molecules, these values are close (within the accuracy limits) to two. However, the reduction peak currents for the CBr3CONH2 and CBr3COOH electrophiles were significantly lower than that expected for the 2e-processes (see also Figure S4 in the Supporting Information). This observation is apparently related to the R-/R-Br parent-son protontransfer reactions taking place in these systems. Indeed, since CBr3COOH molecules bear acidic hydrogens, the proton transfer from the neutral molecules to the electro-generated R- anions decreases the concentration of the starting substrate leading to a decrease in the measured current. (Although such father-son reactions could be suppressed by the addition of a strong proton donor,12 we refrained from such an addition for consistent comparison of all R-Br molecules). On the other hand, for the tribromonitromethane and tribromoacetonitrile, the values of n are higher than two. This suggests that the follow-up reactions of the R-• radical anions



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Page 22 of 36

22 bearing the strongest electron-withdrawing (-CN and -NO2) groups produce electroactive species which contribute to the overall reduction process. It should be stressed, however, that while the detailed analysis of electrochemical processes with each particular R-Br molecule from Chart 1 deserve separate consideration, most important for the current work is the fact that the electrochemical data are consistent with the results of quantum-mechanical computations, which indicated the concerted mechanism of reductive cleavage of these species. Computations of reduction potentials of R-Br molecules in acetonitrile. Due to irreversibility of the reduction of R-Br molecules from Chart 1, the redox potentials, EoR-Br/R·+X-which are required for the analysis of the ET reaction of these electrophiles, were determined via theoretical calculations (see the Experimental section). As described earlier by Isse et al.,58 the Gibbs free energy of the reductive cleavage of the R-Br molecules R-Brs + e  Rs + Brs

(14)

(and therefore EoR-Br/R·+X-can be calculated from the sum of the free energies of the C-Br bond dissociation (eq 15) and reduction of bromine atom (eq 16) R-Brs  Rs + Brs

(15)

Brs + e-  Br-s

(16)

where the subscript s indicates the solvated species. The bond dissociation enthalpies and entropies, BDE and S, for the R-Br molecules were calculated as the difference of enthalpies and entropies of the relevant species established via high-level composite G3(MP2)-RAD(+) computations in the gas phase (eq 5); and the differences between the solvation energies of reactants and products, Gsolv, were calculated using the combination of CPCM with R(O)HF/ 6-31+G(d) computations.19 The values of BDE, TS and Gsolv are listed in Table 4. Together


Page 23 of 36

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23 with the reduction potential of the bromine atom in acetonitrile of 1.60 V,58 they led (via eq 4) to the standard reduction potentials of brominated electrophiles, EoR-Br/R·+X-, that are listed in Table 4. This Table also includes the values of the reduction potentials of R radicals. They were calculated via eq 6 based on the free energy changes, GAN’, of the reduction of radicals R in acetonitrile (eq 17, see the Experimental section and the Supporting Information for details).19 R s + e-  R-s.

(17)

Table 4. Calculated Standard Reduction Potentialsa of R-Br Molecules and R Radicals together with the Main Components Employed in their Computations. o -d BDE, b -TS, b -Gsolv,c E R-Br/R·+X , -G’gas,e -G’solv,f EoR·R-, g kcal mol-1 cal mol-1K-1 kcal mol-1 V vs SCE kcal mol-1 kcal mol-1 V vs SCE

R-Br CBr3H CBr3NO2

h

CBr3CN

64.91

9.51

0.02

-0.80

41.65

50.72

-0.42

56.77

8.83

2.07

-0.39

67.66

50.95

0.76

52.70

9.64

1.05

-0.22

57.25

45.16

0.01

h

58.69

9.81

0.40

-0.50

56.92

46.98

0.08

CBr3F

64.46

9.71

0.57

-0.75

56.28

49.65

0.16

CBr3COOH

55.29

9.14

0.90

-0.36

54.89

47.99

0.03

CBr3CONH2

56.37

9.06

1.32

-0.40

49.08

48.38

-0.20

C3Br2F6

71.91

11.59

1.49

-0.95

79.79

46.42

1.04

CBr4

i

CBr3COCBr3 57.2 10.5 0.90 -0.38 66 • a) In CH3CN, at 298.15K. b) From G3(MP2)-RAD(+) computations; c) Gsolv= Gsolv(R ) + Gsolv (Br•) - Gsolv(R-Br), where solvation energies of species were taken from the CPCM computations for the structures optimized at HF/6-31+G(d) level with UAHF radii; d) From eq 4. e) From eq. ; f) From eq  g) From eq. 6. h) From Ref. 31; i) Extrapolated from the BDE values for CBr3COCH3 and CBr3COCBrH2 (see Supporting Information). Consideration of the data in Table 4 allowed us to draw several conclusions. First, the changes of the reduction potentials of R-Br molecules are mostly related to the changes of the CBr bond strength. In fact, the variations of the other terms in eq 4 are relatively small and the correlation between the values of EoR-Br/R·+X- and the BDE are characterized by R2 of 0.97



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Page 24 of 36

24 (Figure S5 in the Supporting Information). 66,67 Noticeably, the presence of the carbonyl-, cyanoor nitrogroups leads to significant weakening of the carbon-bromine bonds, and the R-Br electrophiles bearing these groups are characterized by less negative reduction potentials than the polyhalogenated alkane molecules. It should be mentioned in this respect that earlier studies demonstrated that the strength of the carbon-halogen bond is a critical factor determining the mechanism of dissociative electron transfer. Specifically, it was found that the weaker the bond, the greater the tendency for the concerted mechanism to prevail over the stepwise one. 9 Thus, weakening of the C-Br bonds results not only in the easy reduction of the R-Br species bearing -acceptor substituents (with reduction potentials in the range from -0.2 to -0.4 V vs SCE), but it also underlies the concerted mechanism of their reductive cleavage. Finally, as with the other alkyl halides, the radicals resulting from the reductive cleavage of all R-Br electrophiles under study are stronger oxidants than their parent molecules. The differences in the reduction potentials of R-Br molecules and R radicals vary significantly – from as little as 0.23 V for tribromoacetic acid to more than 2 V for 1,2-dibromohexafluoropropane. This suggests that the electrochemical reduction of the R-Br molecules is followed by the fast reduction of the generated Rradical, in accord with the 2e-reduction waves observed for most of the electrophiles under study (vide supra). Conclusions Results of the computational analysis and electrochemical measurements of the series of brominated aliphatic compounds clarified the mechanism and thermodynamics of their reductive cleavage critical for understanding of the chemical transformation of these and similar species in chemical systems and in the environment. Specifically, we found that despite the presence of strong electron-withdrawing and/or -acceptor substituents, LUMOs of polybrominated aliphatic


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25 R-Br electrophiles (and SOMOs of the corresponding vertically-excited radical anions) show, in essence, *C-Br -antibonding character. The addition of an electron to the R-Br molecules results in their barrierless dissociations and formation of the loosely-bound {R…Br-} associates in the gas-phase and in acetonitrile. The concerted mechanism of the reductive cleavage of carbonbromine bonds in the R-Br molecules, suggested by the quantum-mechanical computations, is supported by the results of their cyclic voltammetry measurements. Thus, while the -acceptor substituents in the R-Br molecules imply the presence of low-laying *-orbitals and the possibility for the transient formation of radical anions, the significant weakening of the C-Br bonds related to the same substituents and interactions between (strong electron acceptor) R radical and Br- anion in the loosely-bound product {R…Br-} of reductive cleavage are apparently dominant factors, which determine the concerted mechanism of dissociative electron transfer in these systems. ASSOCIATED CONTENT Supporting Information. Details of the quantum-mechanical computations (energies, Cartesian coordinates, MO shapes, charges and spin distributions) and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Telephone: +1-847-330-4519 Present Address † Feinberg School of Medicine, Northwestern University, Chicago, IL, 60611 Author Contributions



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26 The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Funding Sources National Science Foundation grant CHE-1112126 ACKNOWLEDGMENT We thank the National Science Foundation (grant CHE-1112126) for financial support of this work.

REFERENCES 1. Houmam, A. Electron transfer initiated reactions: bond formation and bond dissociation, Chem. Rev. 2008, 108, 2180-2237. 2. Saveant, J.-M. Acc. Chem. Res. Electron transfer, bond breaking, and bond formation. 1993, 26, 455-461. 3. Eberson, L. Problems and prospects of the concerted dissociative electron transfer mechanism. Acta Chem. Scand. 1999, 53, 751 – 764. 4. Costentin, C.; Robert, M.; Saveant, J.-M. Electron transfer and bond breaking: recent advances. Chem. Phys. 2006, 324, 40-56. 5. Saveant J.-M. A simple model for the kinetics of dissociative electron transfer in polar solvents. Application to the homogeneous and heterogeneous reduction of alkyl halides. J. Am. Chem. Soc. 1987, 109, 6788-6795. 6. Saveant, J.-M. Dissociative electron transfer. New tests of the theory in the electrochemical and homogeneous reduction of alkyl halides. J. Am. Chem. Soc. 1992, 114, 10595-10602.


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27 7. Pause, L.; Robert, M.; Saveant, J. M. Reductive cleavage of carbon tetrachloride in a polar solvent. An example of a dissociative electron transfer with significant attractive interaction between the caged product fragments. J. Am. Chem. Soc. 2000, 122, 9829-9835. 8. Andrieux, C. P.; Le Gorande, A.; Saveant, J. M. Electron transfer and bond breaking. Examples of passage from a sequential to a concerted mechanism in the electrochemical reductive cleavage of arylmethyl halides. J. Am. Chem. Soc. 1992, 114, 6892-6904. 9. Andrieux, C. P.; Robert, M.; Saeva, F.D.; Saveant, J. M. Passage from concerted to stepwise dissociative electron transfer as a function of the molecular structure and of the energy of the incoming electron. Electrochemical reduction of aryldialkyl sulfonium cations. J. Am. Chem. Soc. 1994, 116, 7864-7871. 10. Cardinale, A.; Isse, A. A.; Gennaro, A.; Robert, M.; Saveant, J.-M. Dissociative electron transfer to haloacetonitriles. An example of the dependency of in-cage ion-radical interactions upon the leaving group. J. Am. Chem. Soc. 2004, 126, 16051-16057. 11. Costentin, C.; Robert, M.; Saveant, J.-M. Fragmentation of aryl halide  anion radicals. Bending of the cleaving bond and activation vs driving force relationships. J. Am. Chem. Soc. 2002, 124, 13533-13539. 12. Isse, A. A.; Gennaro, A.; Lin, C. Y.; Hodgson, J. L.; Coote, M. L.; Guliashvili, T. Mechanism of carbon-halogen bond reductive cleavage in activated alkyl halide initiators relevant to living radical polymerization: theoretical and experimental study. J. Am. Chem. Soc. 2011, 133, 6254 – 6264.



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28 13. Isse, A. A.; Mussini, P. R.; Gennaro, A. New insights into electrocatalysis and dissociative electron transfer mechanisms: The case of aromatic bromides. J. Phys. Chem. C, 2009, 113, 14983 - 14992. 14. Costentin, C.; Robert, M.; Saveant, J.-M. Successive removal of chloride ions from organic polychloride pollutants. Mechanism of reductive electrochemical elimination in aliphatic gem-polychlorides, ,-polychloroalkenes and ,-polychloroalkanes in mildly protic medium. J. Am. Chem. Soc. 2003, 125, 10729-10739. 15. Marcus, R. A. The theory of oxidation-reduction reactions involving electron transfer. J. Chem. Phys. 1956, 24, 966-78. 16. Marcus, R.A.; Sutin, N. Electron transfers in chemistry and biology. Biochim. Biophys. Acta, 1985, 811, 265-322. 17. Marcus, R.A. Electron-transfer reactions in chemistry: theory and experiment (Nobel lecture). Angew. Chem. Int. Ed.. 1993, 32, 1111-21. 18. Brunschwig, B.S.; Sutin, N. Energy surfaces, reorganization energies and coupling elements in electron transfer. Coord. Chem. Revs., 1999, 187, 233 - 54. 19. Lin, C. Y.; Coote, M. L.; Gennaro, A.; Matyjaszewski, K. Ab initio evaluation of the thermodynamic and electrochemical properties of alkyl halides and radicals and their mechanistic implications for atom transfer radical polymerization. J. Am. Chem. Soc. 2008, 130, 12762-74. 20. Metrangolo, P.; Resnati, G. Halogen bonding: a paradigm in supramolecular chemistry. Chem. Eur. J., 2001, 7, 2511.


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29 21. Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Halogen bonding based recognition processes: a world parallel to hydrogen bonding. Acc. Chem. Res., 2005, 38, 386-95. 22. Lu Y.X.; Wang Y.; Zhu W.L. Nonbonding interactions of organic halogens in biological systems: implications for drug discovery and biomolecular design. Phys. Chem. Chem. Phys. 2010, 12, 4543-4551 23. Pennington, W. T.; Resnati, G.; Taylor, M. S. Halogen bonding: from self-assembly to materials and biomolecules. CrystEngComm 2013, 15, 3057. 24. Gilday, L. C.; Robinson, S. W.; Barendt, T. A.; Langton, M. J.; Mullaney, B. R.; Beer, P. D. Halogen bonding in supramolecular chemistry. Chem. Rev., 2015, 115, 7118–7195 25. Rosokha, S.V.; Neretin, I. S.; Rosokha, T.Y.; Hecht J., Kochi J.K. Charge-transfer character of halogen bonding: molecular structures and electronic spectroscopy of carbon tetrabromide and bromoform complexes with organic - and -donors. Heteroat. Chem., 2006, 17, 449-59. 26. Rosokha, S. V.; Kochi, J. K. X-ray structures and electronic spectra of the -halogen complexes between halogen donors and acceptors with  -receptors, in: Halogen Bonding: Fundamentals and Applications. In Structure and Bonding, 2008, 126, 137-60. 27. Rosokha, S.V.; Stern, C.L.; Ritzert, J.T. Experimental and computational probes of the nature of halogen bonding: complexes of bromine-containing molecules with bromide anions, Chem. Eur. J. 2013, 19, 8774-8788. 28. Rogachev, A. Yu.; Hoffmann, R. Iodione (I2) as a Janus-faced ligand in organometallics. J. Am. Chem. Soc., 2013, 135, 3262. 29. Wang, C.; Danovich, D.; Mo, Y.; ; Shaik, S. On the nature of the halogen bond. J. Chem. Theor. Comput., 2014, 10, 3726-3737.



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30 30. Robinson, S. W.; Mustoe, C. L.; White, N. G.; Brown, A.; Thompson, A. L.; Kennepohl, P.; Beer, P. D. Evidence for halogen bond covalency in acyclic and interlocked halogen-bonding receptor anion recognition. J. Am. Chem. Soc., 2015, 137, 499-507 31. Rosokha, S.V; Vinakos, M.K. Halogen bond-assisted electron transfer reactions of aliphatic bromosubstituted electrophiles. Phys. Chem. Chem. Phys., 2014, 16, 1809 - 1813. 32. Rosokha, S.V; Traversa, A. From charge transfer to electron transfer in halogen-bonded complexes of electrophilic bromocarbons with halide anions. Phys.Chem.Chem.Phys., 2015, 17, 4989-4999. 33. Rosokha, S. V.; Kochi, J. K. The preorganization step in organic reaction mechanisms. Charge-transfer complexes as precursors to electrophilic aromatic substitutions. J. Org. Chem., 2002, 67, 1727-37. 34. Rosokha, S. V.; Kochi, J. K. Fresh look at electron-transfer mechanisms via the donor/ acceptor bindings in the critical encounter complex. Acc. Chem. Res., 2008, 41, 641-53. 35. Schwarzenbach, R.P.; Gshwend, P.M.; Imboden, D.W. Environmental Organic Chemistry, Wiley: New York, 1993. 36. Hill, V. L.; Manley, S. L. Release of reactive bromine and iodine from diatoms and its possible role in halogen transfer in polar and tropical oceans. Limnology and Oceanography, 2009, 54, 812-822. 37. Bruckmann, A.; Pena, M.A.; Bolm, C. Organocatalysis through halogen-bond activation. Synlett, 2008, 900-2.


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31 38. Arulmozhiraja, S.; Morita, M. Electron affinities and reductive dechlorination of toxic polychlorinated dibenzofurans: A density functional theory study. J. Phys. Chem. A, 2004. 108, 3499-3508. 39. Bylaska, E. J.; Dupuis, M.; Tratnyek, P. G. One-electron-transfer reactions of polychlorinated ethylenes: Concerted and stepwise cleavages. J. Phys. Chem. A, 2008, 112, 3712-3721. 40. Costentin, C.; Louault, C.; Robert, M.; Teillout, A. L. Sticky dissociative electron transfer to polychloroacetamides. In-cage ion-dipole interaction control through the dipole moment and intramolecular hydrogen bond. J. Phys. Chem. A, 2005. 109, 2984-2990. 41. Isse, A. A.; Sandonà, G.; Durante, C.; Gennaro, A. Voltammetric investigation of the dissociative electron transfer to polychloromethanes at catalytic and non-catalytic electrodes. Electrochim. Acta, 2009. 54, 3235-3243. 42. Rotko, G.; Romanczyk, P. P.; Andryianau, G., Kurek, S.S. Stepwise and concerted dissociative electron transfer onto a *-type orbital in polybrominated aromatics. Electrochem. Commun. 2014, 43, 117 – 120. 43. Luo, J.; Hu, J.; Zhuang, Y.; Wei, X.; Huang, X. Electron-induced reductive debromination of 2,3,4-tribromodiphenyl ether: a computational study. J. Mol. Model. 2013, 19, 3333-3338. 44. Adcock, W.; Clark, C. I.; Houmam, A.; Krstic, A. R.; Pinson, J.; Savéant, J.-M.; Taylor, D. K.; Taylor, J. F. Dissociative electron transfer to dihaloalkanes. Electrochemical reduction of 1,3 -dihaloadamantes,1,4-dihalobicyclo[2.2.2]octanes, and 1,3-dihalobicyclo[1.1.1]pentanes. J. Am. Chem. Soc., 1994, 116, 4653-4659.



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32 45. Fedurco, M.; Sartoretti, C. J.; Augustynski, J. Medium effects on the reductive cleavage of the carbon-halogen bond in methyl and methylene halides. J. Phys. Chem. B 2001, 105, 2003 – 2009. 46. Klinger, R. J.; Kochi, J. K. Heterogonous rates of electron transfer. Application of cyclic voltammeric techniques to irreversible electrochemical processes. J. Am. Chem. Soc. 1980, 102, 4790-4798. 47. Pavlishchuk, V. V.; Addison, A. W. Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25 C. Inorg. Chim. Acta, 2000 298, 97-102. 48. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford CT, 2009. 49. Chai, J.-D. ; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom–atom dispersion corrections. Phys. Chem. Chem. Phys., 2008, 10, 6615-20. 50. Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215-241. 51. Bauza, A.; Alkorta, I.; Frontera, A.; Elguero, J. On the reliability of pure and hybrid DFT methods for the evaluation of halogen, chalcogen, and pnicogen bonds involving anionic and neutral electron donors. J. Chem. Theory Comput. 2013, 9, 5201-5210.


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33 52. Rosokha, S.V.; Stern, C.L.; Swartz, A.; Stewart, R. Halogen bonding of electrophilic bromocarbons with pseudohalide anions. Phys. Chem. Chem. Phys., 2014, 16, 12968 - 12979. 53. Rosokha, S.V; Loboda, E.A. Interplay of halogen and - charge-transfer bondings in intermolecular associates of bromo- oriododinitrobenzene with tetramethyl-p-phenylenediamine. J. Phys. Chem. A, 2015, 119, 3833–3842. 54. Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models. Chem. Rev. 2005, 105, 2999−3093. 55. Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1. 56. Weinhold, F.; Landis, C. R. Discovering Chemistry with Natural Bond Orbitals, Wiley, Hoboken, New Jersey, 2012. 57. Boys, S. F.; Bernardi, F. The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors. Mol. Phys. 1970, 19, 553−566. 58. Isse, A. A.; Lin, C. Y.; Coote, M. L.; Gennaro, A. Estimation of standard reduction potentials of halogen atoms and alkyl halides. J. Phys. Chem. B, 2011, 115, 678-684. 59. For the thermodynamics and mechanism of reductive cleavage of N-Br bond, see: O’Reilly, R.J.; Karton, A.; Radom, L. Effect of substituents on the preferred model of one-electron reductive cleavage of N-Cl and N-Br bonds. J. Phys. Chem. A, 2013, 117, 460-472. 60. See also: Pattison, D.I.; O’Reilly R.J.; Skaff, O.; Radom, L.; Anderson, R.F.; Davies, M.J. One-electron reduction of N-chlorinated and N-brominated species is a source of radical and bromine atom formation. Chem. Res. Toxicol., 2011, 24, 371-382.



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34 61. Isse, A. A.; Gennaro, A. Absolute potentials of the standard hydrogen electrode and the problem of interconversion of potentials in different solvents. J. Phys. Chem. B, 2010, 114, 7894-7899. 62. The DFT computations of R-Br-• radical anions with double-hybrid B2PLYPD functional produced adducts analogous to those in Figure 3 (including hydrogen-bonded complexes for CBr3CONH2-• and CHBr3-• radical anions). The interatomic separations in the corresponding structures resulting from the computations with B2PLYPD and B97XD functionals were similar (R2= 0.91, see Table S6 and Figure S6 in the Supporting Information). Furthermore, the variations of the interaction energies in the complexes resulting from the computations with these functionals also showed similar trends (R2 = 0.90, see Figure S6), although interaction in the adducts resulting from the B2PLYPD computations were somewhat weaker. 63. Bondi, A. van der Waals volumes and radii. J. Phys. Chem. 1964, 68, 441-451. 64. Note, however, that the demonstration of the passage from the stepwise to concerted mechanism with variation of conditions of experiments8,9 pointed out that concerted pathway of DET might be viable (and more favorable) even for the systems in which the radical anions exist. 65. Andrieux, C. P.; Saveant, J. M. In: Investigation of Rates and Mechanisms of Reactions, Techniques in Chemistry; Bernasconi, C., Ed. ; Wiley: New York, 1986; Vol. 6, 4E, Part 2, pp 305 -390. 66. Note, that the calculated BDE values of 58.7 kcal mol-1 and 64.9 kcal mol-1 for the CBr4 and CBr3H molecules, respectively, in Table 4 are within accuracy limits of the available in literature experimental values for these molecules (57.9  2.0 kcal mol-1 and 65.7  3.1 kcal


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35 mol-1, for CBr4 and CBr3H, respectively).67 In comparison, the BDE values calculated using enthalpies from the composite G4(MP2) calculations were 49.2 kcal mol-1 and 60.1 kcal mol-1 for CBr4 and CBr3H, respectively. 67. CRC Handbook of Chemistry and Physics. 96th ed. Cleveland, Ohio: CRC Press, 2014. 68. Note that under the kinetic control of the ET, experimental reduction peak potentials of the RBr molecules, Ep , are related to the corresponding calculated redox potentials, EoR-Br/R·+X as: Ep = Eo – 0.78(RT/αF) + (RT/αF) ln (k×(RT/αFDv) where k is a standard rate constant which depend on the bond dissociation energy. 8,12, 65



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36 ToC graphics

CBr3CN/CBr2CN•+Br-

Fc+/Fc

0.0

E, V

-1.0

R-Br + e- → R + Br-

(CBr3CN-•) 2.0

dC…Br, Å

{CBr2CN•…Br-} 4.0


Difluorocarbene-Derived Trifluoromethylthiolation and [(18)F]Trifluoromethylthiolation of Aliphatic Electrophiles.

Reductive cross-coupling reactions between two electrophiles.

Nickel-catalyzed reductive and borylative cleavage of aromatic carbon-nitrogen bonds in N-aryl amides and carbamates.

Cleavage at aspartyl-prolyl bonds.

Bidentate cycloimidate palladium complexes with aliphatic and aromatic anagostic bonds.

Linkage analysis using reductive cleavage method.

Unusually short chalcogen bonds involving organoselenium: insights into the Se-N bond cleavage mechanism of the antioxidant ebselen and analogues.

Reductive Insertion of Elemental Chalcogens into Boron-Boron Multiple Bonds.

The mechanism of peptide bonds cleavage and volatile compounds generated from pentapeptide to heptapeptide via Maillard reaction.

The seeming lack of CF···HO intramolecular hydrogen bonds in linear aliphatic fluoroalcohols in solution.

Reductive Cleavage of Carbon Monoxide by a Disilenide.

Reductive cleavage of hydroperoxides by cytochrome P-450.

Reductive debromination of the commercial polybrominated biphenyl mixture firemaster BP6 by anaerobic microorganisms from sediments.

Nickel-catalyzed asymmetric reductive cross-coupling between vinyl and benzyl electrophiles.

Kinetic mechanism of the aliphatic amidase from Pseudomonas aeruginosa.

Cleavage of unactivated amide bonds by ammonium salt-accelerated hydrazinolysis.

Analysis of Macrocystis pyrifera and Pseudomonas aeruginosa alginic acids by the reductive-cleavage method.

Nickel-catalyzed direct arylation of C(sp3)-H bonds in aliphatic amides via bidentate-chelation assistance.

Ni-catalyzed direct reductive amidation via C-O bond cleavage.

The presence and cleavage of interpeptide disulfide bonds in viral glycoproteins.

Transition metal-free intramolecular regioselective couplings of aliphatic and aromatic C-H bonds.

Structural Basis of Stereospecificity in the Bacterial Enzymatic Cleavage of β-Aryl Ether Bonds in Lignin.

Studies of model compounds for the analysis of ester-containing polysaccharides by the reductive-cleavage method.

Chemoselective hydroxylation of aliphatic sp3 C-H bonds using a ketone catalyst and aqueous H2O2.