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

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

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!

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!

(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.

(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)?

Dihalide Starting Materials 1H NMR (THF-d8, 23˚C, ppm): 98.38 (CH2), 43.10 (CH2), 34.24 (CH2), 26.20 (CH2) Magnetic Susceptibility (Gouy Balance): μeff = 4.8 μB [HS Fe(II)] 1H NMR (THF-d8, 23˚C, ppm): -19.34 (CH2), -24.32 (CH2), -46.02 (CH2), -49.62 (CH2) Magnetic Susceptibility (Gouy Balance): μeff = 4.3 μB [IS Fe(III)]

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

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

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

(Triphos)Fe(COT) 1H NMR (C6D6, 23˚C, ppm): 4.93 (s, COT) 31P NMR (C6D6, 23˚C, ppm): 116.33 (t), 95.71 (d) Diamagnetic…. Low-spin Fe(0)?

(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)

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)

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.

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

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)?

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

(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)

(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, 8304-8316.

(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, 8304-8316. XRD 1.399(3) Å 1.420(4) Å 1.383(3) Å

(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, 8304-8316. 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

(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, 8304-8316. 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

(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

(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

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

Mössbauer Comparison Low-spin Fe(I) antiferromagnetically coupled to (COT•-) δ = 0.086 mm/s ΔEQ = 1.08 mm/s δ = 0.013 mm/s ΔEQ = 2.19 mm/s δ = 0.106 mm/s ΔEQ = 1.16 mm/s

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.

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?

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, 268-271.

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, 1211-1215.

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

Traditional Coordination Compounds Precious Metals Two Electron Reaction Pathways Observed

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

Overcoming Radical Pathways Precatalyst Preparation: Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 13794. Hydrogenation:

Overcoming Radical Pathways Precatalyst Preparation: Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 13794. Hydrogenation: [2π+2π] Cyclization: Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13340.

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

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.

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

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

Chelate Preparation Hagit Ben-Daat

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

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

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

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

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

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

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

Stronger Field Chelate Arms 31P NMR (DMSO-d6, ppm): 43.64 (d, JRhP = 135 Hz) Hagit Ben-Daat

Stronger Field Chelate Arms 31P NMR (DMSO-d6, ppm): 43.64 (d, JRhP = 135 Hz) 31P NMR (DMSO-d6, ppm): 42.42 (d, JRhP = 135 Hz) Hagit Ben-Daat

Stronger Field Chelate Arms 31P NMR (DMSO-d6, ppm): 32.88 ppm (d, JRhP = 138 Hz) Hagit Ben-Daat

Stronger Field Chelate Arms 31P NMR (DMSO-d6, ppm): 32.88 ppm (d, JRhP = 138 Hz) 1H NMR (DMSO-d6, ppm): 4.33 (m, COD), 2.37 (m, COD) 31P NMR (DMSO-d6, ppm): 31.63 (d, JRhP = 138 Hz) Hagit Ben-Daat

Stronger Field Chelate Arms 31P NMR (DMSO-d6, ppm): 32.88 ppm (d, JRhP = 138 Hz) κ5-PDI coordination observed for Et- and Pr- bridged PDI chelates 1H NMR (DMSO-d6, ppm): 4.33 (m, COD), 2.37 (m, COD) 31P NMR (DMSO-d6, ppm): 31.63 (d, JRhP = 138 Hz) Hagit Ben-Daat

[(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

[(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

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

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

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