Electrochemical Methods

Transcription

1 Electrochemical Methods John F. Berry June 19th, 2014

2 Why perform electrochemistry? Me Me N N N 3 Fe III O N N Me O [(Me 3 cyclam-acetate)fen 3 ]PF 6

3 Current Why perform electrochemistry? Me Me N N N 3 Fe III O N N Me O [(Me 3 cyclam-acetate)fen 3 ]PF 6 Potential (V vs Fc/Fc+) Berry, J. F.; Bill, E.; Bothe, E.; Weyhermüller, T.; Wieghardt, K. J. Am. Chem. Soc. 2005, 127,

4 Why perform electrochemistry? Ren, T. Coord. Chem. Rev. 1998, 175,

5 Why perform electrochemistry? Ren, T. Coord. Chem. Rev. 1998, 175,

6 The Cyclic Voltammetry Experiment Auxiliary Working Reference M analyte (= MW/100 mg in 10 ml) 0.1 M NBu 4 PF 6 (= 387 mg in 10 ml) static solution Ag wire Ag + NO 3 - (0.01M) e - e - e e - - M n+ (n+1)+ n+ M n+ n+ M (n+1)+ M M (n+1)+ (n+1)+ M (n+1)+ n+ Analyte in solution

7 Solvents for Electrochemistry ε r,dielectric Constant 80.1 (water, 20 o C) 37.5 (acetonitrile, 20 o C) 36.7 (DMF, 25 o C) 26.0 (benzonitrile) 7.6 (THF, 25 o C) 64.9 (PC, 20 o C) 8.9 (CH 2 Cl 2, 25 o C) 16.5 (SO 2, 10 o C) 19.8 (NH 3, 10 o C)

8 The Electrodes Working Reference Auxiliary Glassy carbon Ag/Ag(NO 3 ) Pt wire

9 Cyclic Voltammetry Auxiliary Working Reference Ag wire Ag + NO 3 - (0.01M) e - M n+ M n+ M n+ e - M n+ M n+ Analyte in solution Factors Affecting Electrode Reaction Rate and Current 1. Mass Transfer 2. Electron Transfer 3. Chemical Reactions (preceding or following electron transfer) 4. Surface Reactions (e.g., adsorption)

10 Cyclic Voltammetry A. Potential is not high enough for the M to start oxidizing. B. M is oxidized to M +, causing a current to flow. C. The concentration of M at the electrode is depleted, causing the current to drop. D. M + is re-reduced to M. The shape of the return wave looks similar to that of the forward wave for similar reasons.

11 Cyclic Voltammetry Criteria for reversible (Nernstian) behavior 1. i pa /i pc = 1 regardless of scan rate 2. E pa E pc = 2.3RT/nF = (59 mv)/n at 25 o C 3. i pa = C(v) 1/2

12 CE Reaction Irreversible Behavior EC Reaction EC Reaction

13 The CE Reaction Case 1: Fast scan rate Diffusion-controlled Nernestian behavior Case 4: Slow scan rate Steady-state current controlled by the rate of A B

14 The EC Reaction Fast Scan Rates: Reversible behavior; Slow Scan Rates: Irreversible behavior

15 EC The Catalytic Reaction k Case 1: v >> k Case 4: k >> v A limiting current, i, is achieved when k ET = k.

16 Reversible or Not? 30 ma E / V vs. Fc + /Fc 0 Oxidation peak at E > 1 V vs Fc + /Fc DE p 1V in CH 3 CN 0.1 M TBAPF 6 glassy carbon working electrode 200 mv s -1

17 Spectroelectrochemistry The simultaneous application of electrochemical and optical spectroscopic techniques to investigate a phenomenon. Spectro UV-Vis, Near IR, Fluorescence Information about electronic structure Electro Cyclic Voltammetry Redox potentials Coulometry Electrochemical oxidation or reduction

18 Electrochemistry Controlled Potential Coulometry Auxiliary Dip-probe Work Reference 0.1 M NBu 4 PF 6 Mixing is important measure current (I) vs time (t) I x t = charge estimate conversion F = C/mol

19 Abs Spectroelectrochemistry in Action Rh 2 (esp)

20 Abs Spectroelectrochemistry in Action Nitrogen for sparging The Dip Probe Pt wire: Auxiliry electrode (separated by a frit) Ag/Ag(NO 3 ) reference electrode Rh 2 (esp) 2 Pt wire connected to Pt gauze: Working electrode Glassy Carbon: Working electrode (for CV)

21 Abs Spectroelectrochemistry in Action Apply potential here Rh 2 (esp)

22 Abs Spectroelectrochemistry in Action Apply potential here Rh 2 (esp)

23 Abs Abs Spectroelectrochemistry in Action e Rh 2 (esp) 2 Rh 2 (esp) Kornecki, K. P.; Berry, J. F. Chem. Eur. J. 2011, 17,

24 Isosbestic Points Wieghardt et al., Inorganic Chemistry 1999, 38, 5131.

25 Isosbestic Points Wieghardt et al., Inorganic Chemistry 1999, 38, 5131.

26 Isosbestic Points Kornecki, K. P.; Berry, J. F. Chem. Eur. J. 2011, 17,

27 Reversible or Not? 30 ma E / V vs. Fc + /Fc 0 Oxidation peak at E > 1 V vs Fc + /Fc DE p 1V in CH 3 CN 0.1 M TBAPF 6 glassy carbon working electrode 200 mv s -1

28 Spectroelectrochemistry of Irreversible Fe 2 Compounds 10 5 x % conversion x 5 2 Products stable at ca 25ºC Reversible at low T ( -25 ºC) 0 / 10 3 M -1 cm x 10 3 Compound E 0, V vs Fc + /Fc x [([9]aneN 3 ) 2 Fe III 2 (m-o)(m-aco) 2 ] [(Me 3 [9]aneN 3 ) 2 Fe III 2 (m-o)(m-aco) 2 ] [(tpb) 2 Fe III 2 (m-o)(m-ch 3 CH 2 ) 2 ] [(tpb)fe III (m-o)(m-aco) 2 Fe III (Me 3 [9]aneN 3 )] [([9]aneN 3 )Fe III (m-o)(m-aco) 2 Cr III (Me 3 [9]aneN 3 )] / nm Slep, L.D.; Mijovilovich, A.; Meyer-Klaucke, W.; Weyhermüller, T.; Bill, E.; Bothe, E.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125,

29 A Potentiometric titration methodology to determine E [(tpb) 2 Fe III 2 (m-o)(m-ch 3 CH 2 ) 2 ] 0 red -1e - ox Nernst equation 1.2 Plot of measured absorbances vs open circuit potential 450 nm E 0 RT [ ox] E log nf [ red ] [ ox] [ red] c A [ red] [ ox] red 0 ox nm 340 nm E / V Slep, L.D.; Mijovilovich, A.; Meyer-Klaucke, W.; Weyhermüller, T.; Bill, E.; Bothe, E.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125,

30 Normalized Intensity Further Characterization of Coulometrically Oxidized Compounds [(Me 3 [9]aneN 3 ) 2 Fe 2 III,IV (m-o)(m-aco) 2 ] 3+ g - factors T S = 3/2 Fe(III)(S = 5/2) Fe(IV)(S = 1) T ox E, ev T E, ev B / mt g iso = 1.97 E/D = v / mm s - 1 Fe IV d (mm s -1 ) DE Q (mm s -1 ) h A eff /g N b N (T) 5.3

31 %T Thin-Layer Electrolysis Cells for IR 105 Spectroelectrochemistry Fe(II) Fe(III) Fe(IV) Me Me N N N 3 Fe III O O N N Me 1677 C=O 75 N=N=N Wavenumber (cm -1 )

32 Electrosynthesis: Controlled- Current Electrolysis CPE, -0.2 V -e - CH 3 CN [Rh 2 (CH 3 CN) 10 ] 4+

33 Electrosynthesis: Controlled- Current Electrolysis CPE, -0.2 V -e - precipitate CH 3 CN [Rh 2 (CH 3 CN) 10 ] 4+

34 Electrosynthesis: Controlled- Current Electrolysis CCE, 2.0 μa 3 weeks -e - Single crystals, grown on the electrode! CH 3 CN [Rh 2 (CH 3 CN) 10 ] 4+

35 A Solvated Rh Wire Finniss, G. M.; Canadell, E.; Campana, C.; Dunbar, K. R. Angew. Chem. Int. Ed. 1996, 35,

36 Further Reading Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2 nd ed. John Wiley & Sons, Inc.: Hoboken, Questions???