THE SOLVENT EFFECT IN THE ELECTROCHEMICAL REDUCTION THE SOLVENT EFFECT IN THE ELECTROCHEMICAL REDUCTION OF ORGANIC BROMIDES ON SILVEROF ORGANIC BROMIDES ON SILVER

Luigi Falciolaa, Armando Gennarob, Abdirisak Ahmed Isseb, Patrizia Romana Mussinia, and Manuela Rossia

aDepartment of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, 20133, Milano (Italy), patrizia.mussini@unimi.itbDepartment of Chemical Sciences, University of Padova, via Marzolo 1, 35131 Padova (Italy), armando.gennaro@unipd.it

R· + X−

R· + X· + e−GC

R− + X·

R· + X−

R· + X· + e−GC

RX · −

R− + X·

R-X + e−GC

R· + X−

R-X + e−GC

R· + X· + e−GC

RX · −

R− + X ·

R· + X−

R-X + e−GC

R· + X· + e−GC

RX · −

R− + X ·

1.1. A model process in organic electrocatalysis studies: the A model process in organic electrocatalysis studies: the electrocatalyticelectrocatalytic reductive cleavage of the carbonreductive cleavage of the carbon−−halogen bondhalogen bondRecently, the electrocatalytic reductive cleavage of the C−X bond in organic halides has been chosen as the most convenient model process in organic electrocatalysis on account of its fundamental and applicative interestand of the circumstance that both specific adsorption of halide anions onto metal electrodes and non-electrocatalytic reduction of organic halides have been thoroughly investigated by authoritative research groups

Focusing on the dissociative electron transfer (DET) step in non-catalytic conditions following the Marcus−Savéant theory

Disproportionation: R(+H) + R(−H)

Exchange: RH + B−

R· + X−

Combination: R2 dimers

Addition: RAExchange: RH + S·Organometallic compound formation:2R·ads + 2Hg → 2RHg· ads → R2Hg +Hg+ e

+ R·

+ R·+ A acceptor

+ SH+ Hg

Dimerization: R2 +X−

R−

+ RX

+ HB

[RX]·−

RX + e

The generalized, complete electroorganic process

The key parameter for mechanism discrimination:αapp = – 1.857 RT / [F(Ep–Ep/2)] = – 0.047 / (Ep–Ep/2)αapp’ = 1.15 RT/F / (dEp/dlogv) = -0.030 / (dEp/dlogv)

from experiments in a wide range of scan rates, with ohmic drop compensationSTEPWISE MECHANISM

( thermodynamically less favoured, kinetically less hindered)

The two alternative pathways In the outer sphere (nonouter sphere (non−−catalytic) casecatalytic) case:

Second (chemical) step determiningαapp > 0.5 First (electron transfer) step determining

αapp < 0.5

CONCERTED MECHANISM ( thermodynamically more favoured, kinetically more hindered), αapp << 0.5

Representationin terms of

Morse−curve diagrams (energy vs

reaction coordinate)RX + e–

R▪ + X–

RX▪– (stable)Stepwise (I)

Stepwise (II)Concerted,with two centers

GCR•

X−

A.A. Isse, L. Falciola, P.R. Mussini, A. Gennaro, Chem. Comm. (2006), 344-346

A. Gennaro, A.A. Isse, L. Falciola, P.R. MussiniSilver electrocatalysis and dissociative electron transfer mechanisms(This Meeting, Poster S8-P-23)

RX + e– + Ag

(R▪ ⋅⋅⋅Ag) + (X– ⋅⋅⋅Ag)

[RX]▪– (stable) + Ag?

Stepwise (II)

Stepwise (I)

R• ⋅⋅⋅

X−⋅⋅⋅Ag

Concerted,with three centers

2. Our extension to the inner sphere (catalytic) DET case on Ag 2. Our extension to the inner sphere (catalytic) DET case on Ag cathodescathodes

• We are taking into account the modification of the Morse curves associated with the presence of a “third body” (the catalytic Ag surface)

• The presence of catalytic effects always appear associated with a concerted reaction mechanism; intrinsically stepwise mechanisms become (or tend to) concerted, intrinsically concerted mechanisms remain concerted

• The catalytic effects in the process are modulated by many parameters…

…each of them requiring a specific, systematic investigation

molecular structure of RXmolecular structure of RX→→ intrinsic DET mechanismintrinsic DET mechanismnature of the halide leaving group X-

adsorption auxiliary groups

surfacesurface

morphology

state ((re)activation procedures, cfr. SERS)

nature (Ag vs other cathode materials)

mediummediumsolvent

supportingelectrolyte

substituents with inductive effects

π-network delocalization of the radical anion charge

This presentation

Solvation modifies the process kinetics, also modifying the

energy of the reaction intermediates. It also affects,

through its viscosity, the rate of rate of mass transfermass transfer to the electrode

and hence the peak currents will show a strong solvent

dependency.

The solventmodifies the

thermodynamicsthermodynamicsof the process,

modifying the freeenergies of both

reagents and products through

their solvation

XFXF⋅⋅−−→→ XX⋅⋅ ++ FF−−

2A1σ (σ)

2A’ (σ)

2A2 (π)

A. B. Pierini, D. M. Vera, JOC 68 (2003) 9191

Gas phase

ACN phase

Solvation modifies the free energy profile along the whole reaction coordinate. Such a modification may be huge and, above all, asymmetric. For instance, a polar solvent such as ACN will result in preferential stabilization of the species having the highest charge density, i.e., in the current case, the reaction products including halide anions that bear a net negative charge

3.3. Focusing on the solvent effectFocusing on the solvent effect L. Falciola, A. Gennaro, A.A. Isse, P.R. Mussini, M. Rossi, J. Electroanal. Chem., 593 (1-2), ( 2006) 47-56

Last, dealing with an heterogeneous catalytic process, the extent and mode of solvent (co)adsorptionsolvent (co)adsorption on the working electrode can strongly influence the inner-sphere electron transfer to/from the reacting molecule

(b) Determination of the threshold potential for the (b) Determination of the threshold potential for the specific adsorption of Xspecific adsorption of X-- on Ag in each solvent studied)on Ag in each solvent studied)

Studying the solvent effects required two preliminary investigatStudying the solvent effects required two preliminary investigations ions ::

(a) Identification of a reliable standard redox couple(a) Identification of a reliable standard redox couplefor Intersolvental normalization of the CV characteristicsfor Intersolvental normalization of the CV characteristics

6. The solvent effect on the reduction of two model bromides in6. The solvent effect on the reduction of two model bromides in six model organic solvents six model organic solvents on catalytic Ag electrode and on the nonon catalytic Ag electrode and on the non−−catalytic glassy carbon onecatalytic glassy carbon one

•The catalytic effects of silver (peak potential anticipation) are huge in all cases considered

•In all cases the DET mechanism is concerted (both on GC and Ag), as predicted by Savéant for most aliphatic halides (which can hardly give a stable radical anion)

• When, as a consequence of the catalytic effect, the reduction takes place within the potential range of bromide adsorption (shadowed areas), the CV patterns become more complex in multiplicity and shape (peaks of mixed diffusive and adsorptive character)

•Neatly diffusive peak currents satisfactorily comply with Stokes’ law(i.e. the diffusion coefficients of the reacting molecule, obtained from limiting currents Il after peak convolution, linearly decrease with increasing viscosity), pointing to a viscous motion of the reactant molecule with negligible solvent interactions

����

The solvent modulation on the peak potentials on the non-catalytic and

the catalytic electrode The sequences of both the normalized reduction peak potentials and the catalytic effects in the six solvents point to a key role of the solvation of the Br- ion produced in the process, resulting in flat characteristics on the non-catalytic GC, and on steeply linear, parallel characteristics on highly catalytic Ag.

Stronger Br- solvation results in more positive reduction peaks and higher catalytic effects (the protic and highly Br−coordinating MeOH resulting in huge catalytic effects). This might be accounted for by the solvent assistance in the turnover of the catalytic sytes.

Both the peak potentials on the catalytic electrode and the catalytic effects exhibit a linear dependence on the primary medium effects on

the Br- ion (decreasing with increasingly stronger halide solvation)

7. The solvent modulation on the catalytic effects of the Ag ele7. The solvent modulation on the catalytic effects of the Ag electrodectrodeThe solvent modulation on the catalytic effects,

accounted for by the reduction potential differences between the catalytic electrode

and the non-catalytic one, (Ep, Ag −Ep, GC)

Me10Fc+|Me10Fc emerges as the best intersolvental redox reference because:

•it shows the best electrochemical and chemical reversibility features;• it shows no anomalies with respect to water, the aqueous redox potential

being aligned with the organic solvent ones (unlike the Fc+|Fc case); • its diffusion coefficients give the best agreement with Stokes’ law (purely viscous

motion with no specific interactions with the sorrounding solvent);• it shows constant potential differences with its analogous permethylated cobaltocene

in the tested solvents, notwithstanding their remarkable distance.These evidences point to the interactions between this molecule and the investigated solvents being very little if any.An additional bonus is that the Me10Fc redox potentials of this couple are very near the SCE ones; therefore the potentials normalized vs this couple fall in a more familiar range than those normalized vs Fc

4.4. A reliable standard for intersolvental normalization of the CV cA reliable standard for intersolvental normalization of the CV characteristics:haracteristics:the decamethylferricinum|decamethylferrocene couplethe decamethylferricinum|decamethylferrocene couple

Fe

0/+

In this context, we have carried out along a careful protocol an exhaustive voltammetric reactivity study on ferrocene, either as such or functionalized on the cyclopentadienyl rings, together with other reversible redox couples, on both stationary and rotating electrodes, in a series of model organic solvents with constant supporting electrolyte.

Voltammetric studies in nonaqueous solvents are currently referred to the ferricinium|ferrocene redox couple, following the 1986 IUPAC document [1], assuming the above redox process to take place at an invariant potential in all solvents. However, the above assumption of intersolvental invariancy of the Fc+|Fc redox potential has been recently questioned [2, 3], suggesting that only permethylated ferrocene derivatives (“electron-reservoir sandwich complexes”) could be considered as unaffected by solvent coordination and therefore to be reliable intersolvental references.

[1] G.Gritzner, J. Kuta, Pure Appl. Chem. 56 (1984) 461. [2] J. Ruiz, D. Astruc, Comptes Rendus de l'Academie des Sciences Series IIB Mechanics Physics Chemistry Astronomy 1(1) (1998) 21.

[3] I. Noviandri, K. N. Brown. D. S. Fleming, P.T. Gulyas, P.A. Lay, A.F. Masters, L. Phillips, J. Phys. Chem. B 103 (1999) 6713

E. Castello, L. Falciola, P.R. Mussini, T. Mussini, M. Rossi, A comparative investigation of reference redox couples for intersolvental comparison of electrode potential scales ( this Meeting, Poster S8-P-23)

Accordingly, in the present study all potentials have been compaAccordingly, in the present study all potentials have been compared after red after normalization vs the decamethylferrocene ones in the same solvenormalization vs the decamethylferrocene ones in the same solventsnts

Me10Fc+|Me10Fc Br- I-

Before (-0.7 V(SCE), triangles); at (-1.1, -1.2 V(SCE), circles); and after (-1.5, -1.7 V(SCE), squares) the potential of the adsorption/desorption process

ACN, polycrystalline Ag

Rsol = electrolyte resistanceC = double layer capacitanceRads, Cads = adsorption resistance and capacitance (slow adsorption process)Wads = Warburg diffusion parameter (slow diffusion process)

Best fitting equivalent circuit at Ethresh

Br- I-I-

Before (-0.7 V(SCE), triangles); at (-1.1, -1.2 V(SCE), circles); and after (-1.5, -1.7 V(SCE), squares) the potential of the adsorption/desorption process

ACN, polycrystalline Ag

Rsol = electrolyte resistanceC = double layer capacitanceRads, Cads = adsorption resistance and capacitance (slow adsorption process)Wads = Warburg diffusion parameter (slow diffusion process)

Best fitting equivalent circuit at Ethresh

5. Determining the threshold potential for the specific adsorpt5. Determining the threshold potential for the specific adsorption of Xion of X−− on Ag in the working on Ag in the working organic solventsorganic solvents L. Falciola, P.R. Mussini, S. Trasatti, L. M. Doubova, J. Electroanal. Chem., 593 (1-2), (2006), 185-193;

L. Falciola, Oral presentation, This Symposium, Tuesday 29th, 17.50A parallel research project had to be devoted to the investigation of the potential range for specific halide anion adsorption on

silver in each tested solvent. Since the task was a much more critical one with respect to the thoroughly investigated aqueous case, we employed three different, “synergic” techniques, namely

(b) Differential capacity (c) Impedance

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

3.5E-05

-1.90 -1.80 -1.70 -1.60 -1.50 -1.40 -1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60 -0.50

E /V(10MeFc+| 10MeFc)

C/F

Fondo ACN+TBA P 0.1M[TEABr]=0.0005M[TEABr]=0.002M[TEABr]=0.004M[TEABr]=0.006M[TEABr]=0.008M[TEABr]=0.01M[TEABr]=0.014M[TEABr]=0.018M[TEABr]=0.026M

[TEABr]=0.05M

ACNA

B

0.0E+ 00

5.0E- 06

1.0E- 05

1.5E- 05

2.0E- 05

2.5E- 05

3.0E- 05

3.5E- 05

-1.9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

E/V (10MeFc+| 10MeFc)

C/F

Fondo ACN+T BA P 0 .1 M

[TE AI]=0.0005 M

[TE AI]=0.002 M

[TE AI]=0.004 M

[TE AI]=0.006 M

[TE AI]=0.0079 M

[TE AI]=0.01 M

[TE AI]=0.014 M

[TE AI]=0.018 M

ACN

A

B

Br-

I-

Adsorption thresholds from CV method

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

3.5E-05

-1.90 -1.80 -1.70 -1.60 -1.50 -1.40 -1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60 -0.50

E /V(10MeFc+| 10MeFc)

C/F

Fondo ACN+TBA P 0.1M[TEABr]=0.0005M[TEABr]=0.002M[TEABr]=0.004M[TEABr]=0.006M[TEABr]=0.008M[TEABr]=0.01M[TEABr]=0.014M[TEABr]=0.018M[TEABr]=0.026M

[TEABr]=0.05M

ACNA

B

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

3.5E-05

-1.90 -1.80 -1.70 -1.60 -1.50 -1.40 -1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60 -0.50

E /V(10MeFc+| 10MeFc)

C/F

Fondo ACN+TBA P 0.1M[TEABr]=0.0005M[TEABr]=0.002M[TEABr]=0.004M[TEABr]=0.006M[TEABr]=0.008M[TEABr]=0.01M[TEABr]=0.014M[TEABr]=0.018M[TEABr]=0.026M

[TEABr]=0.05M

ACNA

B

0.0E+ 00

5.0E- 06

1.0E- 05

1.5E- 05

2.0E- 05

2.5E- 05

3.0E- 05

3.5E- 05

-1.9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

E/V (10MeFc+| 10MeFc)

C/F

Fondo ACN+T BA P 0 .1 M

[TE AI]=0.0005 M

[TE AI]=0.002 M

[TE AI]=0.004 M

[TE AI]=0.006 M

[TE AI]=0.0079 M

[TE AI]=0.01 M

[TE AI]=0.014 M

[TE AI]=0.018 M

ACN

A

B

0.0E+ 00

5.0E- 06

1.0E- 05

1.5E- 05

2.0E- 05

2.5E- 05

3.0E- 05

3.5E- 05

-1.9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

E/V (10MeFc+| 10MeFc)

C/F

Fondo ACN+T BA P 0 .1 M

[TE AI]=0.0005 M

[TE AI]=0.002 M

[TE AI]=0.004 M

[TE AI]=0.006 M

[TE AI]=0.0079 M

[TE AI]=0.01 M

[TE AI]=0.014 M

[TE AI]=0.018 M

ACN

A

B

Br-

I-

Adsorption thresholds from CV method

Peak B: breaking of the compact saturated bromide layer; Peak A: reorientation of the

solvent molecule dipoles

ACN, polycrystalline Ag, Me10Fc reference

-1.9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5E / V (Me10Fc+|Me10Fc)

MeOHACNPCDMF

-1.25

-1.21

-1.00

-1.50

-1.21

-1.43

-1.40

iodide adsorption on Ag

-1.39

bromide adsorption

on Ag

•The three methods converge on the same results →

•The threshold potentials for iodide anion adsorption are always significantly more negative than the bromide ones, in agreement with Kads, I >> Kads, Br

•For a given anion, the threshold adsorption potentials are increasingly more negative with decreasing primary medium effect on the ion considered (i.e. ion coordination ability of the solvent): adsorption should be regarded as a competition between ion coordination by the surface and ion coordination by the solvent

P.R. Mussini, S. Ardizzone, G. Cappelletti,M. Longhi, S. Rondinini, L. Doubova, J. Electroanal. Chem. 552 (2003) 213-22

(a) CV (a recently proposed: indirect method [1] based on the monitoring of the negative shift of the reduction potential of an organic halide “probe” in the presence of increasing amounts of halide anions)

RTEFcK

RTr

Xads +≈ ,0lnϑLinearization by a Frumkin-like isotherm

-0.00040

-0.00035

-0.00030

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

E / V (SCE)

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEAI

O

Br

Ac OOAc

AcO

Ac OH2 C

TeaI Additions

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEABr

O

Br

AcOOAc

AcO

Ac OH2C

TeaBr Additions

-0.00040

-0.00035

-0.00030

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

E / V (SCE)

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEAI

O

Br

Ac OOAc

AcO

Ac OH2C

TeaI Additions

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEABr

O

Br

AcOOAc

AcO

Ac OH2C

TeaBr Additions

Solvents: ACNACN PCPC DMFDMF

∆Ethresh (the threshold negative potential shift) exhibits excellent linear correlations:

•for more halides in a given solvent, with pKs, AgX

•for two halides in a solvent series, with the halide adsorption valency ratio

RTEFcK

RTr

Xads +≈ ,0lnϑLinearization by a Frumkin-like isotherm

RTEFcK

RTr

Xads +≈ ,0lnϑLinearization by a Frumkin-like isotherm

-0.00040

-0.00035

-0.00030

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

E / V (SCE)

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEAI

O

Br

Ac OOAc

AcO

Ac OH2C

TeaI Additions

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEABr

O

Br

AcOOAc

AcO

Ac OH2C

TeaBr Additions

-0.00040

-0.00035

-0.00030

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

E / V (SCE)

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEAI

O

Br

Ac OOAc

AcO

Ac OH2C

TeaI Additions

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

I/A

ACN + TEATFB 0.1 M

concentrazione crescente di TEABr

O

Br

AcOOAc

AcO

Ac OH2C

TeaBr Additions

Solvents: ACNACN PCPC DMFDMFSolvents: ACNACN PCPC DMFDMF

∆Ethresh (the threshold negative potential shift) exhibits excellent linear correlations:

•for more halides in a given solvent, with pKs, AgX

•for two halides in a solvent series, with the halide adsorption valency ratio

∆Ethresh (the threshold negative potential shift) exhibits excellent linear correlations:

•For more halides in a given solvent, with pKS, AgX

•For two halides in a solvent series, with the halide adsorption valency ratio

Linearization by a Frumlin-like isotherm