To Invert or Not To Invert Reasons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems

Transcription

1 To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems Bernd Speiser Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, D 7276 Tübingen, Germany bernd.speiser@uni-tuebingen.de To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.1/36

2 Multi-Electron Transfers: Examples basic research: thermodynamics and kinetics mechanisms natural processes: redox enzymes photosynthesis nitrogen fixation (nitrogenase and model systems) technical applications: catalysis fuel cells To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.2/36

3 Two-Electron Transfers and Dis/Comproportionation A A 1+/- E 1 E 2 A 2+/- P P disproportionation/comproportionation: 2 A 1+/- A A + A 2+/- K disp = ][A 2+/ [A ] = exp[ F [A 1+/ ] 2 T E ] 8 E = < : E2 E 1 (E2 E 1 ) for oxidation for reduction To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.3/36

4 Normal and Inverted Potential Ordering expected potential difference between two successive electron transfers: several volts solvation effects several 1 mv 2nd electron transfer more difficult: normal potential ordering equilibrium on side of A 1+/ A 1+/ stable against disproportionation To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.4/36

5 Normal and Inverted Potential Ordering expected potential difference between two successive electron transfers: several volts solvation effects several 1 mv 2nd electron transfer more difficult: normal potential ordering equilibrium on side of A 1+/ A 1+/ stable against disproportionation 2nd electron transfer more easy: inverted potential ordering (Evans) equilibrium on side of A and A 2+/ A 1+/ unstable against disproportionation structural changes during electron transfer? To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.4/36

6 Normal and Inverted Potential Ordering expected potential difference between two successive electron transfers: several volts solvation effects several 1 mv 2nd electron transfer more difficult: normal potential ordering equilibrium on side of A 1+/ A 1+/ stable against disproportionation 2nd electron transfer more easy: inverted potential ordering (Evans) equilibrium on side of A and A 2+/ A 1+/ unstable against disproportionation structural changes during electron transfer? a many-facetted mechanism (Heinze) cyclic voltammetry as analytical technique To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.4/36

7 Multi-Electron Transfer: Examples and Some Systematics examples from organic (sterically hindered anilines, hexaaminobenzenes, hexathiobenzenes, meso-ionic dithiocarboxylates) organometallic (u(arene) complexes, fc-substituted silsesquioxanes) inorganic (boron subhalides) chemistry electrons are transferred to single redox center (= 1 electroactive group) complex redox center 8 < (> 1 electroactive groups) : fully delocalized conjugated with some separation multiple redox centers (> 1 electroactive groups) To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.5/36

8 Multi-Electron Transfer: Examples and Some Systematics examples from organic (sterically hindered anilines, hexaaminobenzenes, hexathiobenzenes, meso-ionic dithiocarboxylates) organometallic (u(arene) complexes, fc-substituted silsesquioxanes) inorganic (boron subhalides) chemistry electrons are transferred to single redox center (= 1 electroactive group) complex redox center 8 < (> 1 electroactive groups) : fully delocalized conjugated with some separation multiple redox centers (> 1 electroactive groups) interaction To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.5/36

9 An Octa-Ferrocenyl Silsesquioxane Structure cage compound cube as core structural element Si O Si NH redox-active centers bound by Si-C linker Si O O O Si O O O Fe O O Si O O O Si Si O Si synthesis: D. uiz Abad and H.A. Mayer To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.6/36

10 An Octa-Ferrocenyl Silsesquioxane Structure cage compound cube as core structural element Si O Si NH redox-active centers bound by Si-C linker Si O O O Si O O O Fe O O Si O O O Si Si O Si synthesis: D. uiz Abad and H.A. Mayer model compound for redox-actively modified silica nanoparticles To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.6/36

11 An Octa-Ferrocenyl Silsesquioxane Cyclic Voltammetry,4 1,,2,5,, -,2 -,5 i / µa -,4 -,6 -,8-1, -1,2-1, E / mv c =.12 mm v =.1 Vs 1 i / µa -1, -1,5-2, -2,5-3, -3, E / mv c =.12 mm v =.2 Vs 1 DMSO/.1 M NBu 4 PF 6 ; Pt electrode To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.7/36

12 An Octa-Ferrocenyl Silsesquioxane Cyclic Voltammetry,4 1,,2,5,, -,2 -,5 i / µa -,4 -,6 -,8-1, -1,2-1, E / mv c =.12 mm v =.1 Vs 1 i / µa -1, -1,5-2, -2,5-3, -3, E / mv c =.12 mm v =.2 Vs 1 two redox signals follow-up reaction at small v DMSO/.1 M NBu 4 PF 6 ; Pt electrode To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.7/36

13 An Octa-Ferrocenyl Silsesquioxane Cyclic Voltammetry, Analysis 1,,5, i / µa -,5-1, -1,5-2, -2,5 I c =.12 mm v =.2 Vs 1-3, -3, E / mv To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.8/36

14 An Octa-Ferrocenyl Silsesquioxane Cyclic Voltammetry, Analysis 1, i / µa,5, -,5-1, -1,5-2, -2,5 I c =.12 mm v =.2 Vs 1 i / µa p 1,2 1,,8,6,4 oxidation I: i p v c =.12 mm c =.71 mm -3,,2 c =.23 mm -3, E / mv, v / mv/s oxidation I: adsorbed species To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.8/36

15 An Octa-Ferrocenyl Silsesquioxane Cyclic Voltammetry, Analysis 1,,5, -,5 i / µa -1, -1,5-2, -2,5 II c =.12 mm v =.2 Vs 1-3, -3, E / mv oxidation I: adsorbed species To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.8/36

16 An Octa-Ferrocenyl Silsesquioxane Cyclic Voltammetry, Analysis 1, 16 i / µa,5, -,5-1, -1,5-2, -2,5-3, -3,5 II c =.12 mm v =.2 Vs 1 i / µa p oxidation II: i p v 1/2 c =.12 mm c =.71 mm c =.23 mm E / mv v 1/2 / (mv/s) 1/2 oxidation I: adsorbed species oxidation II: diffusing species To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.8/36

17 An Octa-Ferrocenyl Silsesquioxane Cyclic Voltammetry, Analysis 1,,5 oxidation II i / µa, -,5-1, -1,5-2, -2,5 II I c =.12 mm v =.2 Vs 1 signal shape and E p : 1 e resulting D is much too large assumption: n = 8, similar E -3, -3, E / mv D cm 2 s 1 scales with molecular weight as compared to Fc oxidation I: adsorbed species oxidation II: diffusing species To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.8/36

18 An Octa-Ferrocenyl Silsesquioxane Multi-Electron Transfer eight electrons transferred independently equal (or at least very similar) formal potential example of separated, non-interacting redox centers To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.9/36

19 uthenium Complexes with Arene Ligands u 2+ e - u + e - u u 42 o Pierce und Geiger, 1992 u 2+ 2 BF 4-2 BF 4 - u 2+ 2 BF 4-2 BF 4 - u 2+ u u u 2+ 2 BF 4-2 BF 4 - Boekelheide et al., To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.1/36

20 Cyclic Voltammetry of Bis([2 2 ]paracyclophane)u(ii) in CH 2 Cl c =.19 mm v =.1 Vs 1 c =.19 mm v = 1. Vs 1-25 c =.19 mm v = 1 Vs c =.39 mm v =.1 Vs c =.39 mm v = 1. Vs 1-5 c =.39 mm v = 1 Vs CH 2 Cl 2 /.1 M NBu 4 PF 6 ; GC electrode; circles: experiment, line: simulation E 1 =.875 V, E 2 = 1.6 V, k s1 =.77 cm/s, k s2 =.41 cm/s To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.11/36

21 Cyclic Voltammetry of Bis([2 2 ]paracyclophane)u(ii) in PC 5 5. c =.71 mm v =.1 Vs 1-5 c =.71 mm v = 1. Vs 1-5 c =.71 mm v = 1 Vs c = 1.6 mm -1 c = 1.6 mm v =.1 Vs c = 1.6 mm v = 1 Vs 1 v = 1. Vs propylene carbonate/.1 M NBu 4 PF 6 ; GC electrode E 1 =.938 V, E 2 = 1.15 V, k s1 =.18 cm/s, k s2 =.12 cm/s To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.12/36

22 Cyclic Voltammetry of Bis([2 2 ]paracyclophane)u(ii) Comparison u 2+ 2 BF 4 - E1 E2 propylene carbonate dichloromethan =.938 V.875 V = 1.15 V 1.6 V k s1 =.18 cm s 1.77 cm s 1 k s2 =.12 cm s 1.41 cm s 1 normal potential ordering E in CH 2 Cl 2 larger than in PC peak splitting 1st electron transfer faster than 2nd electron transfer in CH 2 Cl 2 four times faster than in PC effect of τ L (Marcus theory) solvent effect on kinetics and thermodynamics of electron transfers To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.13/36

23 Bis(η 6 -triphenylene)u(ii) u 2+ 2 BF 4 - To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.14/36

24 Cyclic Voltammetry of Bis(η 6 -triphenylene)u(ii) in PC c =.75 mm v =.1 Vs 1-5 c =.75 mm v =.2 Vs 1-25 c =.75 mm v = 5.12 Vs c = 1.75 mm v =.1 Vs 1-1 c = 1.75 mm v =.2 Vs 1-5 c = 1.75 mm v = 5.12Vs propylene carbonate/.1 M NBu 4 PF 6 ; GC electrode To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.15/36

25 eduction Mechanism of Bis(η 6 -triphenylene)u(ii) in PC u (II) E irrev E rev Disp 2 + utp 2 C 2 u (I) E qrev Disp u () k s (II/I) =.19 cm s 1 α(ii/i) =.5 k s (I/) =.2 cm s 1 α(i/) =.5 E (II/I) =.775 V E (I/) =.754 V K disp = k f,disp = M 1 s 1 k C = 84 M 1 s 1 potential inversion by 2 mv 2nd electron transfer very slow To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.16/36

26 Two-Electron eduction of u-complexes in Comparison differences: inverted potential ordering for triphenylene complex 2nd electron transfer slower for triphenylene complex by factor 1 u BF 4 u 2+ 2 BF 4 - To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.17/36

27 Two-Electron eduction of u-complexes in Comparison differences: inverted potential ordering for triphenylene complex 2nd electron transfer slower for triphenylene complex by factor 1 u BF 4 u 2+ 2 BF 4 - change of hapticity η 6 η 4 To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.17/36

28 Two-Electron eduction of u-complexes in Comparison differences: inverted potential ordering for triphenylene complex 2nd electron transfer slower for triphenylene complex by factor 1 u BF 4 u 2+ 2 BF 4 - structural distortion of aromatic rings in triphenylene complex more difficult To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.17/36

29 Imidazolium-2-dithiocarboxylates: Synthesis N N S S = CH 3 = C 2 H 5 = i-c 3 H 7 synthesis from carbene and CS 2 : N. Kuhn et al., Z. Naturf. 49b, (1994) To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.18/36

30 Imidazolium-2-dithiocarboxylates: Chemical eduction N N S S + 2 e - 2 e - - N N S S reducing agent potassium: N. Kuhn et al., J. Chem. Soc. Chem. Commun. 1997, To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.19/36

31 Imidazolium-2-dithiocarboxylates: Structures neutral molecule: N. Kuhn et al., J. Chem. Soc. Chem. Commun. 1997, orthogonal dianion: N. Kuhn et al., Z. Naturf. 49b, (1994) planar To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.2/36

32 Imidazolium-2-dithiocarboxylates: Structures neutral molecule: N. Kuhn et al., J. Chem. Soc. Chem. Commun. 1997, dianion: N. Kuhn et al., Z. Naturf. 49b, (1994) electrochemical reduction? To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.2/36

33 Imidazolium-2-dithiocarboxylates: Structures neutral molecule: N. Kuhn et al., J. Chem. Soc. Chem. Commun. 1997, dianion: N. Kuhn et al., Z. Naturf. 49b, (1994) cyclic voltammetry as analytical technique To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.2/36

34 Cyclic Voltammetry of 1,3-Dimethyldithiocarboxylate 5-5 c =.36 mm v =.5 Vs c =.36 mm v =.5 Vs c =.36 mm v = 2 Vs c =.67 mm v =.5 Vs c =.67 mm v =.5 Vs c =.67 mm v = 2 Vs THF/.2 M NBu 4 PF 6 ; GC electrode To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.21/36

35 Cyclic Voltammetry of 1,3-Diethyldithiocarboxylate -5 c =.23 mm v =.5 Vs c =.23 mm v =.5 Vs 1-25 c =.23 mm v = 2 Vs c =.49 mm v =.5 Vs c =.49 mm v =.5 Vs c =.49 mm v = 2 Vs THF/.2 M NBu 4 PF 6 ; GC electrode To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.22/36

36 Cyclic Voltammetry of 1,3-Diisopropyldithiocarboxylate 5-5 c =.24 mm v =.5 Vs c =.24 mm v =.5 Vs 1 c =.24 mm v = 2 Vs c =.46 mm v =.5 Vs 1 c =.46 mm v =.5 Vs c =.46 mm v = 2 Vs THF/.2 M NBu 4 PF 6 ; GC electrode To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.23/36

37 Imidazolium-2-dithiocarboxylates: edox Potentials CH 3 E E 2 + in mv E vs Fc/Fc C 2 H i-c 3 H To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.24/36

38 Imidazolium-2-dithiocarboxylates: edox Potentials CH 3 E E 2 + in mv E vs Fc/Fc C 2 H i-c 3 H potential inversion for isopropyl derivative To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.24/36

39 Imidazolium-2-dithiocarboxylates: Kinetic Constants k s1 /cm s 1 k s2 /cm s 1 k comp /M 1 s 1 D/ cm 2 s 1 k f / s 1 CH C 2 H i-c 3 H To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.25/36

40 Imidazolium-2-dithiocarboxylates: Kinetic Constants k s1 /cm s 1 k s2 /cm s 1 k comp /M 1 s 1 D/ cm 2 s 1 k f / s 1 CH C 2 H i-c 3 H nd electron transfer slower To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.25/36

41 Imidazolium-2-dithiocarboxylates: Kinetic Constants k s1 /cm s 1 k s2 /cm s 1 k comp /M 1 s 1 D/ cm 2 s 1 k f / s 1 CH C 2 H i-c 3 H based on Marcus theory: structural change in 2nd step To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.25/36

42 Imidazolium-2-dithiocarboxylates: Structural eorganisation To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.26/36

43 Imidazolium-2-dithiocarboxylates: Structural eorganisation substituents hinder planarization To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.26/36

44 Hexakis(dimethylamino)benzene and its Dication: Structures (CH 3 ) 2 N N(CH 3 ) 2 N(CH 3 ) 2 (CH 3 ) 2 N N(CH 3 ) 2 N(CH 3 ) 2 Hückel-aromatic compound; planar two polymethine-units; twist To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.27/36

45 Hexakis(dimethylamino)benzene: Cyclic Voltammetry 25 (CH 3 ) 2 N (CH 3 ) 2 N N(CH 3 ) 2 N(CH 3 ) 2 N(CH 3 ) potential, E /V N(CH 3 ) potential, E /V CH 3 CN/CH 2 Cl 2 1:1,.1 M NBu 4 PF 6 ; Pt electrode; v = 1. Vs 1, c =.24 mm To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.28/36

46 Hexakis(dimethylamino)benzene: Cyclic Voltammetry 25 (CH 3 ) 2 N (CH 3 ) 2 N N(CH 3 ) 2 N(CH 3 ) 2 N(CH 3 ) potential, E /V N(CH 3 ) 2 extreme potential inversion for two-electron oxidation potential, E /V CH 3 CN/CH 2 Cl 2 1:1,.1 M NBu 4 PF 6 ; Pt electrode; v = 1. Vs 1, c =.24 mm To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.28/36

47 Hexakis(dimethylamino)benzene: Cyclic Voltammetry 25 (CH 3 ) 2 N (CH 3 ) 2 N N(CH 3 ) 2 N(CH 3 ) 2 N(CH 3 ) potential, E /V N(CH 3 ) 2 change from aromatic to bis(polymethine) structure potential, E /V CH 3 CN/CH 2 Cl 2 1:1,.1 M NBu 4 PF 6 ; Pt electrode; v = 1. Vs 1, c =.24 mm To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.28/36

48 * * * * * * * * * * * * * * * * * * * * * * * * * Boron Subhalides: Cluster Structures * * * * * * * * * * *... B 1 X 1 B 9 X 9 B 8 X 8... B 6 X 6... B 4 X 4 X = Cl, Br, I dianions: closo structure according to Wade s rules neutral molecules: hypercloso cluster with electron deficit To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.29/36

49 Boron Subhalides: Cyclic Voltammetry of B 8 X 8 and B 9 X 9 5 B 9 Cl 9 25 B 8 Cl v =.1 V/s c =.29 mm v =.2 V/s c 2 mm CH 2 Cl 2 /.1 M NBu 4 PF 6 ; Pt electrode To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.3/36

50 Boron Subhalides: Cyclic Voltammetry of B 8 X 8 and B 9 X 9 5 B 9 Cl 9 25 B 8 Cl v =.1 V/s c =.29 mm v =.2 V/s c 2 mm CH 2 Cl 2 /.1 M NBu 4 PF 6 ; Pt electrode normal potential ordering; no dramatic structural changes To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.3/36

51 Boron Subhalides: Cyclic Voltammetry of B 8 X 8 and B 9 X 9 5 B 9 Cl 9 25 B 8 Cl v =.1 V/s c =.29 mm v =.2 V/s c 2 mm CH 2 Cl 2 /.1 M NBu 4 PF 6 ; Pt electrode π-back bonding compensates electron deficit in hypercloso clusters To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.3/36

52 Sterically Hindered Biphenylamines: Structure and Cyclic Voltammograms = H c = 1.3 mm v =.5 Vs 1 = OCH 3 c = 1.5 mm v =.5 Vs 1 = N(CH 3 ) 2 c = 1.5 mm v =.5 Vs 1 B. Speiser, A. ieker, and S. Pons, J. Electroanal. Chem. 147, (1983). CH 3 CN/.1 M NEt 4 ClO 4 ; Pt electrode; potentials vs. Ag/Ag + (.1 M in CH 3 CN) To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.31/36

53 Sterically Hindered Biphenylamines: Two-Electron Oxidation Mechanism NH NH 2 NH 2 e e E 1 E 2 e NH 2 NH 2 + Hammett correlation with σ + substituent constants stabilization of radical cation by planarization normal potential ordering To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.32/36

54 Conclusions: Multi-Electron Transfers mechanistic variations side/follow-up reactions each case to be analyzed carefully ensemble of voltammograms simulation, parameter fitting thermodynamics and kinetics various examples for normal and inverted potential ordering structural change solvation, ion pairing To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.33/36

55 Conclusions: Multi-Electron Transfers mechanistic variations side/follow-up reactions each case to be analyzed carefully ensemble of voltammograms simulation, parameter fitting thermodynamics and kinetics various examples for normal and inverted potential ordering structural change solvation, ion pairing however: not fully systematic To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.33/36

56 Conclusions: Multi-Electron Transfers mechanistic variations side/follow-up reactions each case to be analyzed carefully ensemble of voltammograms simulation, parameter fitting thermodynamics and kinetics various examples for normal and inverted potential ordering structural change solvation, ion pairing two reaction steps, three species To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.33/36

57 Conclusion: Normal vs. Inverted Potential Ordering G norm,rel 1. A 2+/.5 A 1+/ G = nfe. A equal normal inverted formal potentials normalized relative energies of species in general two-electron transfer system To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.34/36

58 Conclusion: Normal vs. Inverted Potential Ordering G norm,rel 1..5 G = nfe. =Me =Et =ipr normalized relative energies of species in dithiocarboxylate system To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.34/36

59 Conclusion: Normal vs. Inverted Potential Ordering G norm,rel 1. A 2+/.5 A 1+/ G = nfe. A equal normal inverted formal potentials relative energies of three species are decisive normalized relative energies of species in general two-electron transfer system To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.34/36

60 Conclusion: Normal vs. Inverted Potential Ordering G norm,rel 1. A 2+/.5 A 1+/ G = nfe A. equal normal inverted formal potentials stabilization of A 2+/ or destabilization of A 1+/ leads to potential inversion normalized relative energies of species in general two-electron transfer system To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.34/36

61 Conclusion: Normal vs. Inverted Potential Ordering G norm,rel 1. A 2+/.5 A 1+/ G = nfe. A equal normal inverted formal potentials structural rearrangement can be stabilizing normalized relative energies of species in general two-electron transfer system To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.34/36

62 Conclusion: The Message The Message: two is not enough... all redox states must be considered! To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.35/36

63 Acknowledgements: Co-workers and Cooperation Partners Fc-silsesquioxanes: Thomas eißig, David uiz Abad, Hermann Mayer u(arene) complexes: Bernhard Gollas, Stefan Dümmling Dithiocarboxylates: Stefan Dümmling, Gerd Weyers, N. Kuhn Hexaminobenzenes: Marc Würde, Cäcilie Maichle-Mössmer, Jens J. Wolff Boron Subhalides: Carsten Tittel, Tina Wizemann, Wolfgang Einholz Biphenylamines: Peter Hertl, Anton ieker To Invert or Not To Invert easons for the Occurrence of Normal and Inverted Formal Potentials in Molecular Multi-Electron Transfer Systems p.36/36