1. The Application of Redox-Active Ligands in Homogeneous Catalysis
Ryan J. Trovitch
DEPARTMENT OF CHEMISTRY & BIOCHEMISTRY

2. Outline
Lecture 1 - Redox-Active Ligands: What Are They? How Do They Work? and How Might They Be Improved?
Lecture 2 - The Development of a Highly Active Manganese Hydrosilylation Catalyst
Lecture 3 - Hydrosilylation and Beyond: Expanding the Scope of Redox-Active Ligand Assisted Catalysts

3. Dewar-Chatt Model for Ethylene Coordination
Neutral
Dialkyl
σ-donation
π-backbonding
Although coordinated ethylene may react like a dialkyl ligand, it is not redox-active since its π* orbital is not populated!

4. Conjugated Diene Ligands
Neutral
Enediyl
η4-1,3-Butadiene coordination:
Can be considered a redox-active ligand (singlet dianion shown) if electrons are fully transferred to Ψ3.
This is different than backbonding!

5. (Triphos)Fe(COT) “the COT is fluxional and presumably η4-coordinated”
Felkin, H.; Lednor, P. W.; Normant, J.-M.; Smith, R. A. J. J. Organomet. Chem. 1978, 157, C64-C66.

6. (Triphos)Fe(COT) “the COT is fluxional and presumably η4-coordinated”
Felkin, H.; Lednor, P. W.; Normant, J.-M.; Smith, R. A. J. J. Organomet. Chem. 1978, 157, C64-C66.
Can (Triphos)Fe(COT) be prepared from cheaper starting materials than Fe(COT)2?
COT has been shown to coordinate to transition metals in an η2-, η3-, η4-, η5-, η6-, and η8-fashion. What is the true hapticity of the COT ligand?
What is the correct electronic structure description of (Triphos)Fe(COT)?

7. Dihalide Starting Materials
1H NMR (THF-d8, 23˚C, ppm):
98.38 (CH2), (CH2),
34.24 (CH2), (CH2)
Magnetic Susceptibility
(Gouy Balance):
μeff = 4.8 μB [HS Fe(II)]
1H NMR (THF-d8, 23˚C, ppm):
(CH2), (CH2),
(CH2), (CH2)
Magnetic Susceptibility
(Gouy Balance):
μeff = 4.3 μB [IS Fe(III)]

8. (Triphos)FeBr3 Magnetic Susceptibility (Gouy Balance):
μeff = 5.6 μB [HS Fe(III)]

9. (Triphos)FeBr3 Magnetic Susceptibility (Gouy Balance):
μeff = 5.6 μB [HS Fe(III)]
450 Hz at ½ height
31P NMR

10. Reduction Under N2 Prepared from (Triphos)FeCl3 and
(Triphos)FeBr3 (also with K0)
Addition of Triphos improves yield,
complicates purification
Connectivity/Geometry Confirmed by XRD
LS Fe(0)
31P NMR

11. (Triphos)Fe(COT) 1H NMR (C6D6, 23˚C, ppm): 4.93 (s, COT)
31P NMR (C6D6, 23˚C, ppm):
(t), (d)
Diamagnetic…. Low-spin Fe(0)?

12. (Triphos)Fe(COT) Fe(1)-P(1) 2.1903(4) Fe(1)-P(2) 2.1758(4) Fe(1)-P(3)
2.1913(4)
Fe(1)-C(40)
2.2170(14)
Fe(1)-C(41)
2.0332(14)
Fe(1)-C(42)
2.0302(14)
Fe(1)-C(35)
2.1978(14)
C(40)-C(41)
1.432(2)
C(41)-C(42)
1.402(2)
C(42)-C(35)
P(2)-Fe(1)-C(35)
88.79(4)
P(2)-Fe(1)-C(40)
171.60(4)
P(1)-Fe(1)-P(3)
98.434(15)

13. A Closer Look 1.402(2) 1.432(2) 1.432(2) 1.448(2) 1.446(2) 1.358(2)
1.355(2)
1.425(2)

14. A Closer Look
1.402(2)
1.432(2)
1.432(2)
1.448(2)
1.446(2)
1.358(2)
1.355(2)
1.425(2)
The solid-state structure suggests that this complex features a COT radical monoanion that is antiferromagnetically coupled to a low-spin Fe(I) center.

15. A Well-Understood Non-Innocent Chelate
Neutral
Radical Monoanion
Dianion

16. A Well-Understood Non-Innocent Chelate
Neutral
Radical Monoanion
Dianion
Will 2,2’-bipyridine accept one electron upon coordinating to (Triphos)Fe?
Will the resulting product possess the same electronic structure as (Triphos)Fe(COT)?

17. (Triphos)Fe(bpy) 31P NMR (C6D6, 23˚C, ppm): 112.93 (t), 91.59 (d)
Diamagnetic…. Low-spin Fe(0)?

18. (Triphos)Fe(bpy) 31P NMR (C6D6, 23˚C, ppm): 112.93 (t), 91.59 (d)
Diamagnetic…. Low-spin Fe(0)?
Fe(1)-P(1)
2.1628(8)
Fe(1)-P(2)
2.1608(8)
Fe(1)-P(3)
2.2045(8)
Fe(1)-N(1)
1.956(2)
Fe(1)-N(2)
1.936(2)
N(1)-C(5)
1.383(3)
N(2)-C(6)
1.399(3)
C(5)-C(6)
1.420(4)
P(1)-Fe(1)-P(3)
116.38(3)
N(2)-Fe(1)-P(1)
104.83(7)
N(1)-Fe(1)-P(1)
95.43(7)
P(2)-Fe(1)-P(1)
85.02(3)

19. (Triphos)Fe(bpy) 1.391(3) Å 1.431(3) Å 1.388(3) Å
Gore-Randall, E.; Irwin, M.; Denning, M. S.; Goicoechea, J. M. Inorg. Chem. 2009, 48,

20. (Triphos)Fe(bpy) XRD 1.391(3) Å 1.431(3) Å 1.388(3) Å
Gore-Randall, E.; Irwin, M.; Denning, M. S.; Goicoechea, J. M. Inorg. Chem. 2009, 48,
XRD
1.399(3) Å
1.420(4) Å
1.383(3) Å

21. (Triphos)Fe(bpy) XRD 1.372(3) Å 1.405(3) Å 1.336(3) Å 1.363(3) Å
1.391(3) Å
1.427(3) Å
1.431(3) Å
1.388(3) Å
1.428(3) Å
1.338(3) Å
1.366(3) Å
1.374(3) Å
1.403(4) Å
Gore-Randall, E.; Irwin, M.; Denning, M. S.; Goicoechea, J. M. Inorg. Chem. 2009, 48,
1.366(4) Å
1.408(4) Å
1.366(4) Å
XRD
1.336(3) Å
1.399(3) Å
1.401(4) Å
1.420(4) Å
1.383(3) Å
1.409(4) Å
1.361(3) Å
1.355(4) Å
1.360(4) Å
1.412(4) Å

22. (Triphos)Fe(bpy) XRD Low-spin Fe(I) antiferromagnetically
1.372(3) Å
1.405(3) Å
1.336(3) Å
1.363(3) Å
1.391(3) Å
1.427(3) Å
1.431(3) Å
1.388(3) Å
1.428(3) Å
1.338(3) Å
1.366(3) Å
1.374(3) Å
1.403(4) Å
Gore-Randall, E.; Irwin, M.; Denning, M. S.; Goicoechea, J. M. Inorg. Chem. 2009, 48,
1.366(4) Å
1.408(4) Å
1.366(4) Å
XRD
1.336(3) Å
1.399(3) Å
1.401(4) Å
Low-spin Fe(I)
antiferromagnetically
coupled to (bpy•-)
1.420(4) Å
1.383(3) Å
1.409(4) Å
1.361(3) Å
1.355(4) Å
1.360(4) Å
1.412(4) Å

23. (Triphos)Fe(bpy) Electrochemistry XRD Low-spin Fe(I) 1.420(4) Å
1.399(3) Å
Low-spin Fe(I)
antiferromagnetically
coupled to (bpy•-)
1.420(4) Å
1.383(3) Å

24. (Triphos)Fe(bpy) Electrochemistry DFT XRD Low-spin Fe(I)
1.399(3) Å
Low-spin Fe(I)
antiferromagnetically
coupled to (bpy•-)
1.420(4) Å
1.383(3) Å

25. Mössbauer Comparison δ = 0.013 mm/s ΔEQ = 2.19 mm/s δ = 0.106 mm/s

26. Mössbauer Comparison Low-spin Fe(I) antiferromagnetically
coupled to (COT•-)
δ = mm/s
ΔEQ = 1.08 mm/s
δ = mm/s
ΔEQ = 2.19 mm/s
δ = mm/s
ΔEQ = 1.16 mm/s

27. EPR Confirmation Triplet eigenstates: Ψ1, Ψ2, Ψ3 Singlet eigenstate:Ψ4
Low-spin Fe(I)
antiferromagnetically
coupled to (COT•-)
σ = 2.3%
Consistent with a strong dipolar interaction between two unpaired spins that are within close proximity of one another.

28. COT as a Redox-Active Ligand
~ 40 η4-COT complexes characterized by XRD.
Several feature a disordered COT ligand (unreliable C-C distances)
Do any possess a COT radical anion?

29. COT as a Redox-Active Ligand
~ 40 η4-COT complexes characterized by XRD.
Several feature a disordered COT ligand (unreliable C-C distances)
Do any possess a COT radical anion? ?
1.415(5)
1.382(6)
1.429(6)
μeff = 2.87 μB
Lavallo, V.; El-Batta, A.; Bertrand, G.; Grubbs, R. H. Angew. Chem. Int. Ed. 2011, 50,

30. COT as a Redox-Active Ligand
~ 40 η4-COT complexes characterized by XRD.
Several feature a disordered COT ligand (unreliable C-C distances)
Do any possess a COT radical anion? ?
1.437(4)
1.398(4)
1.429(4)
Brennessel, W. W.; Young, Jr., V. G.; Ellis, J. E. Angew. Chem. Int. Ed. 2002, 41,

31. Outline
Lecture 1 - Redox-Active Ligands: What Are They? How Do They Work? and How Might They Be Improved?
Lecture 2 - The Development of a Highly Active Manganese Hydrosilylation Catalyst
Lecture 3 - Hydrosilylation and Beyond: Expanding the Scope of Redox-Active Ligand Assisted Catalysts

32. Traditional Coordination Compounds
Precious Metals
Two Electron Reaction Pathways Observed

33. Traditional Coordination Compounds
Precious Metals
Two Electron Reaction Pathways Observed
First Row Metals
One Electron Reaction Pathways Observed

34. Overcoming Radical Pathways
Precatalyst Preparation:
Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126,
Hydrogenation:

35. Overcoming Radical Pathways
Precatalyst Preparation:
Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126,
Hydrogenation:
[2π+2π] Cyclization:
Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128,

36. PDI Ligand Redox Activity
Neutral
Radical Monoanion
Dianion
Lowest energy PDI π* orbital is often close in energy to the metal d-obritals.

37. How Does Redox-Activity Help?
[2π+2π] Cyclization:
Reductive Elimination
Ligand Substitution
Reductive C-C
Bond Formation
Iron Center Remains Divalent!
Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794.

38. Outline
Lecture 1 - Redox-Active Ligands: What Are They? How Do They Work? and How Might They Be Improved?
Lecture 2 - The Development of a Highly Active Manganese Hydrosilylation Catalyst
Lecture 3 - Hydrosilylation and Beyond: Expanding the Scope of Redox-Active Ligand Assisted Catalysts

39. Catalyst Design Inspiration
Donor Substituted Redox-Active Ligands Might:
- Obviate the Need for Steric Bulk in Catalyst Design
- Improve Activity by Stabilizing High Energy Catalytic Intermediates
- Participate in Bifunctional Catalysis

40. Chelate Preparation
Hagit Ben-Daat

41. Chelate Preparation
- Easy to Prepare
- Highly Modular
Hagit Ben-Daat

42. Rh  Diamagnetic, Well-Investigated
1H and 13C NMR indicate top to bottom ligand equivalence
Hagit Ben-Daat

43. Rh  Backbonding Over Redox-Activity
Rh(1)-N(1)
2.030(3)
Rh(1)-N(2)
1.884(3)
Rh(1)-N(3)
Rh(1)-Cl(1)
2.3476(9)
N(1)-C(2)
1.316(4)
N(3)-C(8)
1.303(4)
C(2)-C(3)
1.452(5)
C(7)-C(8)
1.467(4)
N(1)-Rh(1)-N(2)
79.76(11)
N(1)-Rh(1)-N(3)
159.19(11)
N(2)-Rh(1)-N(3)
79.44(11)
N(2)-Rh(1)-Cl(1)
178.68(8)
Rh(1)-N(1)
2.030(3)
Rh(1)-N(2)
1.889(3)
Rh(1)-N(3)
2.032(3)
Rh(1)-Cl(1)
2.3621(9)
N(1)-C(2)
1.316(5)
N(3)-C(8)
1.303(5)
C(2)-C(3)
1.462(5)
C(7)-C(8)
1.475(5)
N(1)-Rh(1)-N(2)
79.27(13)
N(1)-Rh(1)-N(3)
159.13(12)
N(2)-Rh(1)-N(3)
79.86(7)
N(2)-Rh(1)-Cl(1)
176.92(9)
Hagit Ben-Daat

44. Rh  Backbonding Over Redox-Activity
Rh(1)-N(1)
2.030(3)
Rh(1)-N(2)
1.884(3)
Rh(1)-N(3)
Rh(1)-Cl(1)
2.3476(9)
N(1)-C(2)
1.316(4)
N(3)-C(8)
1.303(4)
C(2)-C(3)
1.452(5)
C(7)-C(8)
1.467(4)
N(1)-Rh(1)-N(2)
79.76(11)
N(1)-Rh(1)-N(3)
159.19(11)
N(2)-Rh(1)-N(3)
79.44(11)
N(2)-Rh(1)-Cl(1)
178.68(8)
Rh(1)-N(1)
2.030(3)
Rh(1)-N(2)
1.889(3)
Rh(1)-N(3)
2.032(3)
Rh(1)-Cl(1)
2.3621(9)
N(1)-C(2)
1.316(5)
N(3)-C(8)
1.303(5)
C(2)-C(3)
1.462(5)
C(7)-C(8)
1.475(5)
N(1)-Rh(1)-N(2)
79.27(13)
N(1)-Rh(1)-N(3)
159.13(12)
N(2)-Rh(1)-N(3)
79.86(7)
N(2)-Rh(1)-Cl(1)
176.92(9)
Due to radial expansion (vs. first row metals), Rh d-orbitals backbond efficiently into the π* orbital of PDI, destabilizing it such that it remains unoccupied.
Hagit Ben-Daat

45. Enabling Amine Coordination
Variable temperature NMR spectroscopy revealed slow arm exchange at ambient temperature.
Hagit Ben-Daat

46. Enabling Amine Coordination
Variable temperature NMR spectroscopy revealed that chelate arm exchange is fast at ambient temperature.
Hagit Ben-Daat

47. Crystallographic Evidence
Rh(1)-N(1)
2.016(2)
Rh(1)-N(2)
1.902(2)
Rh(1)-N(3)
2.084(2)
Rh(1)-N(4)
2.143(2)
N(1)-C(2)
1.309(3)
N(3)-C(8)
1.305(3)
C(2)-C(3)
1.460(4)
C(7)-C(8)
1.475(4)
N(1)-Rh(1)-N(2)
79.52(9)
N(1)-Rh(1)-N(3)
157.74(9)
N(2)-Rh(1)-N(3)
78.49(9)
N(2)-Rh(1)-N(4)
173.11(9)
κ4-N,N,N,N-PDI coordination observed
Hagit Ben-Daat

48. Stronger Field Chelate Arms
31P NMR (DMSO-d6, ppm): (d, JRhP = 135 Hz)
Hagit Ben-Daat

49. Stronger Field Chelate Arms
31P NMR (DMSO-d6, ppm): (d, JRhP = 135 Hz)
31P NMR (DMSO-d6, ppm): (d, JRhP = 135 Hz)
Hagit Ben-Daat

50. Stronger Field Chelate Arms
31P NMR (DMSO-d6, ppm): ppm (d, JRhP = 138 Hz)
Hagit Ben-Daat

51. Stronger Field Chelate Arms
31P NMR (DMSO-d6, ppm): ppm (d, JRhP = 138 Hz)
1H NMR (DMSO-d6, ppm): (m, COD), 2.37 (m, COD)
31P NMR (DMSO-d6, ppm): (d, JRhP = 138 Hz)
Hagit Ben-Daat

52. Stronger Field Chelate Arms
31P NMR (DMSO-d6, ppm): ppm (d, JRhP = 138 Hz)
κ5-PDI coordination observed for Et- and Pr- bridged PDI chelates
1H NMR (DMSO-d6, ppm): (m, COD), 2.37 (m, COD)
31P NMR (DMSO-d6, ppm): (d, JRhP = 138 Hz)
Hagit Ben-Daat

53. [(Ph2PPrPDI)Rh][(COD)RhCl2]
Rh(1)-N(1)
2.029(3)
Rh(1)-N(2)
1.926(3)
Rh(1)-N(3)
2.046(3)
Rh(1)-P(1)
2.2926(9)
Rh(1)-P(2)
2.3101(10)
N(1)-C(2)
1.335(5)
N(3)-C(8)
1.322(5)
C(2)-C(3)
1.427(5)
C(7)-C(8)
1.435(5)
N(1)-Rh(1)-N(2)
78.95(12)
N(1)-Rh(1)-N(3)
157.10(12)
N(2)-Rh(1)-P(1)
119.79(8)
N(2)-Rh(1)-P(2)
141.15(8)
P(1)-Rh(1)-P(2)
99.06(3)
Hagit Ben-Daat

54. [(Ph2PPrPDI)Rh][(COD)RhCl2]
Rh(1)-N(1)
2.029(3)
Rh(1)-N(2)
1.926(3)
Rh(1)-N(3)
2.046(3)
Rh(1)-P(1)
2.2926(9)
Rh(1)-P(2)
2.3101(10)
N(1)-C(2)
1.335(5)
N(3)-C(8)
1.322(5)
C(2)-C(3)
1.427(5)
C(7)-C(8)
1.435(5)
N(1)-Rh(1)-N(2)
78.95(12)
N(1)-Rh(1)-N(3)
157.10(12)
N(2)-Rh(1)-P(1)
119.79(8)
N(2)-Rh(1)-P(2)
141.15(8)
P(1)-Rh(1)-P(2)
99.06(3)
Increased covalency, ligand remains redox-innocent!
Hagit Ben-Daat

55. Summary
Lecture 1 - Redox-Active Ligands: What Are They? How Do They Work? and How Might They Be Improved?
COT, Bpy, and PDI all behave as redox-active ligands

56. Summary
Lecture 1 - Redox-Active Ligands: What Are They? How Do They Work? and How Might They Be Improved?
COT, Bpy, and PDI all behave as redox-active ligands
PDI ligands can aid catalysis by reversibly storing electrons

57. Summary
Lecture 1 - Redox-Active Ligands: What Are They? How Do They Work? and How Might They Be Improved?
COT, Bpy, and PDI all behave as redox-active ligands
PDI ligands can aid catalysis by reversibly storing electrons
Electron transfer not observed for Rh due to radial expansion