Chemistry: Electron Transfer

THE UNIVERSITY OF TEXAS AT EL PASO CHEM 3151 APRIL 23RD , 2014 The Layout Introduction to Electron Transfer

M M Mixed-valent Compounds +2 +3 −

Electrochemistry

2 Redox What happens in a reduction reaction?

A) Electrons are removed

B) Electrons are added

C) Protons are added

D) Protons are removed

3 X + e X Redox − − What happens in a reduction reaction? →

A) Electrons are removed

B) Electrons are added

C) Protons are added

D) Protons are removed

4 Electron Transfer In an electron transfer reaction, there is an electron exchange between a donor ( ) and an acceptor ( ).

𝐷𝐷 + 𝐴𝐴 + + − 𝐷𝐷 𝐴𝐴 𝐷𝐷 𝐴𝐴 The exchange results in an oxidized donor ( ) and a reduced acceptor ( ). The donor has lost an electron+ and the acceptor has gained an electron.− 𝐷𝐷 𝐴𝐴

5 Introduction to Electron Transfer ET reactions are commonplace in biological systems such as:

◦ Photosystem II

◦ Nitrogen fixation

◦ Cellular respiration

6 Water Splitting + 𝟐𝟐𝐇𝐇𝟐𝟐𝐎𝐎 → 𝐎𝐎𝟐𝟐 𝟐𝟐𝟐𝟐𝟐𝟐

According to the diagram on the right, the reaction above is:

A) exothermic, spontaneous B) endothermic, spontaneous C) exothermic, non-spontaneous D) endothermic, non-spontaneous

7 Water Splitting + 𝟐𝟐𝐇𝐇𝟐𝟐𝐎𝐎 → 𝐎𝐎𝟐𝟐 𝟐𝟐𝟐𝟐𝟐𝟐

According to the diagram on the right, the reaction above is:

A) exothermic, spontaneous > > B) endothermic, spontaneous ΔG > 0 C) exothermic, non-spontaneous ∆𝑮𝑮 𝟎𝟎ΔH > 0 D) endothermic, non-spontaneous ∆𝑯𝑯 𝟎𝟎

8 Introduction to Electron Transfer: Photosystem II

Photosystem II utilizes ET to split water:

2 +

- + 2H2O O2 + 24e + 4 H 2 (H2O oxidation half-reaction) 𝐻𝐻 𝑂𝑂 +→ 𝑂𝑂- 2𝐻𝐻 + 4H→+ 4e 2H2 (H reduction half-reaction) →

9 Inner versus Outer Inner-sphere electron transfer: e- D A

Outer-sphere electron transfer: e- D A

10 Marcus Theory R.A. Marcus

In 1992 Rudolph Marcus received the Nobel Prize in Chemistry for his contribution to the understanding of the thermodynamics and the kinetics of electron transfer.

11 Mixed-valent compounds Mixed-valent (MV) are capable of inner-sphere electron transfer. D A

The electron donor and electron acceptor are the same element, a metal (not always the case)… M M …but they have different oxidation states. − M M +2 +3

− 12 Mixed-valent Compounds H. Taube One of the first and most thoroughly studied MV compounds was the Creutz-Taube Ion:

H3N NH3 H3N NH3

H3N Ru N N Ru NH3

H3N NH3 H3N NH3

In a MV compound, two metal centers are connected by a linker. M-L-M

13 My Research Dr. Villagrán

N N

N N N Mo N O Mo N O H O N N = N N Mo O N Mo N N N N

N N

N N N N N Mo O N Mo N

N Mo N O Mo N N N N N

14 Equilibrium Equation K M L M + M L M 2 [M L M ] 𝑐𝑐 3+ 3+ 2+ 2+ 3+ 2+ ↔ Assuming the reaction above is at equilibrium, what should the equilibrium constant equation be?

[ ] [ ] A) K = B) K = 3+ [ 2+ 2 ] 3+ [ 3+ 2+] 2+ M L M M L M M L M 3+ 3+ 2+ 2+ 3+ 2+ 2 c M L M M L M c M L M

[ ] [ ] C) K = D) K = 3+ [ 3+ 2+ ] 2+ 3+ [ 2+ ] M L M M L M 2 M L M 3+ 2+ 3+ 3+ 2+ 2+ c 2 M L M c M L M M L M

15 Equilibrium Equation K M L M + M L M 2 [M L M ] 𝑐𝑐 3+ 3+ 2+ 2+ 3+ 2+ ↔ Assuming the reaction above is at equilibrium, what should the equilibrium constant equation be?

[ ] [ ] A) = B) K = 𝟑𝟑+ [ 𝟐𝟐+ 𝟐𝟐 ] 3+ [ 3+ 2+] 2+ 𝐌𝐌 𝐋𝐋 𝐌𝐌 M L M M L M 𝟑𝟑+ 𝟑𝟑+ 𝟐𝟐+ 𝟐𝟐+ 3+ 2+ 2 𝐊𝐊𝐜𝐜 𝐌𝐌 𝐋𝐋 𝐌𝐌 𝐌𝐌 𝐋𝐋 𝐌𝐌 c M L M

[ ] [ ] C) K = D) K = 3+ [ 3+ 2+ ] 2+ 3+ [ 2+ ] M L M M L M 2 M L M 3+ 2+ 3+ 3+ 2+ 2+ c 2 M L M c M L M M L M

16 Equilibrium Equation K aA + bB cC

[C↔] K = [A] [Bc ] a b

17 Equilibrium Constant and MV We can use the equilibrium constant to determine if electron transfer will proceed thermodynamically.

K M L M + M L M 2 [M L M ] c 3+ 3+ 2+ 2+ 3+ 2+ ↔

[M L M ] K = M L M3+ [M 2+ L2 M ] c 3+ 3+ 2+ 2+

18 What should the axis labels be for the graph below?

A) X = current, Y = potential B) X = potential, Y = current

C) X = time, Y = potential D) X = time, Y = current

19 Electrochemistry What should the axis labels be for the graph below?

A) X = current, Y = potential B) X = potential, Y = current

C) X = time, Y = potential D) X = time, Y = current

20 Cyclic voltammetry

In the graph above, A refers to what event?

A) Oxidation B) Reduction

C) Both A and B D) None of the above

21 Cyclic voltammetry

In the graph above, A refers to what event?

A) Oxidation B) Reduction

C) Both A and B D) None of the above

22 Electrochemistry: Cyclo voltammetry

-

+

23 Half-wave Potential

+ = / 2 𝐸𝐸𝑝𝑝𝑝𝑝 𝐸𝐸𝑝𝑝𝑝𝑝 𝐸𝐸1 2

24 Half-wave potential: Electron Transfer

I. II. 𝐸𝐸𝑝𝑝𝑝𝑝� M-L-M 𝐸𝐸𝑝𝑝𝑝𝑝� II. I. E / = E / E /

𝐸𝐸𝑝𝑝𝑝𝑝� II I 𝐸𝐸𝑝𝑝𝑝𝑝� ∆ 1 2 1 2 − 1 2

25 Half-wave potential: Electron Transfer

In the cyclovoltammogram below, would the / be relatively large or small? ∆𝐸𝐸1 2

A) Small B) Large C) I dunno D) None

26 Half-wave potential: Electron Transfer

In the cyclovoltammogram below, would the / be relatively large or small?

∆𝐸𝐸1 2

A) Small B) Large C) I dunno D) None

27 Free Energy and Equilibrium Constant Free Energy of reaction:

= ° +

∆𝐺𝐺 ∆𝐺𝐺 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 Free Energy and potential relation:

= ° = °

∆𝐺𝐺 −𝑛𝑛𝑛𝑛𝑛𝑛 ∆𝐺𝐺 −𝑛𝑛𝑛𝑛𝐸𝐸

28 Equilibrium Constant and Potential Potential in free energy equation:

= ° +

−𝑛𝑛𝑛𝑛𝑛𝑛 −𝑛𝑛𝑛𝑛𝐸𝐸 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = ° 𝑅𝑅𝑅𝑅 Nernst Equation: 𝐸𝐸 𝐸𝐸 − 𝑙𝑙𝑙𝑙𝑙𝑙 𝑛𝑛𝑛𝑛

° = exp( ) 𝐹𝐹𝐸𝐸 𝐾𝐾 𝑅𝑅𝑅𝑅

29 K and E / Half-wave potential and Nernst equation c 1 2 ° ∆ / = ∆𝐸𝐸1 2 𝐸𝐸 =

𝐾𝐾𝑐𝑐 𝐾𝐾

= exp( / ) 𝐹𝐹∆𝐸𝐸1 2 𝐾𝐾𝑐𝑐 𝑅𝑅𝑅𝑅

30 Equilibrium

Kept at constant temperature, what does a large / mean?

∆𝐸𝐸1 2 A) Equilibrium constant will be large, formation of products favored.

B) Equilibrium constant will be small, formation of reactants favored.

C) Equilibrium constant will be 1, both sides of reaction favored.

D) None of the above.

31 Equilibrium

Kept at constant temperature, what does a large / mean?

∆𝐸𝐸1 2 A) Equilibrium constant will be large, formation of products favored.

A) Equilibrium constant will be small, formation of reactants favored.

B) Equilibrium constant will be 1, both sides of reaction favored.

C) None of the above.

32 Robin-Day Classification The Robin-Day classification scheme divides MV molecules into three different classes (Class I, Class II, Class III) according to the ’s equilibrium constant.

Class I. < 10 (low) 2 𝐾𝐾𝑐𝑐

Class II. 10 106 (intermediate) 2 ≤ 𝐾𝐾𝑐𝑐 ≤

Class III. > 106 (high)

𝐾𝐾𝑐𝑐

33 Class III

Toma, H. E.; Journal of the Brazilian Chemical Society, 2003, 14, 6.

34 Class II

Toma, H. E.; Journal of the Brazilian Chemical Society, 2003, 14, 6.

35 Class I

Toma, H. E.; Journal of the Brazilian Chemical Society, 2003, 14, 6.

36 Electrochemistry In this experiment we will be doing cyclic voltammetry experiments on three different ferrocene compounds. The first compound is just ferrocene. Ferrocene is composed of a single ion that is sandwiched between two cyclopentadienyl ions.

Fe

37 The Ferrocene Dimers you will use

Fe

Fe Fe Fe

Z E

38 The Experiment The Ferrocene compounds have already been synthesized.

You will prepare their own solution with samples for electrochemical analysis.

You will need to prepare one sample for each ferrocene compound.

You will be required to use potentiostats to run cyclic voltammetry experiments on the compounds.

39 The Report You will have two weeks to submit a lab report on today’s experiment.

The format of the lab report should conform to the requirements listed on your class syllabus.

You should focus on the conclusion and the discussion of your results.

Be able to explain your results using your knowledge of electrochemistry and electron transfer.

40 This material is based upon work supported by the National Science Foundation under Grant Number DUE-1140469. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.