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Lecture 22 February 25, 2011 Metal Oxide Catalysis

Lecture 22 February 25, 2011 Metal Oxide Catalysis. Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy. William A. Goddard, III, wag@wag.caltech.edu 316 Beckman Institute, x3093

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Lecture 22 February 25, 2011 Metal Oxide Catalysis

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  1. Lecture 22 February 25, 2011 Metal Oxide Catalysis Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy William A. Goddard, III, wag@wag.caltech.edu 316 Beckman Institute, x3093 Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Teaching Assistants: Wei-Guang Liu <wgliu@wag.caltech.edu> Caitlin Scott <cescott@caltech.edu>

  2. Last time

  3. Extremely important for these systems (pH from -10 to +30) in very highly polar solvents: accuracy of predicting Solvation effects in QM ThePoisson-Boltzmann Continuum Model in Jaguar/Schrödinger is extremely accurate Calculate Solvent Accessible Surface of the solute by rolling a sphere of radius Rsolv over the surface formed by the vdW radii of the atoms. Calculate electrostatic field of the solute based on electron density from the orbitals Calculate the polarization in the solvent due to the electrostatic field of the solute(need dielectric constant ) This leads to Reaction Field that acts back on solute atoms, which in turn changes the orbitals. Iterated until self-consistent. Calculate solvent forces on solute atoms Use these forces to determine optimum geometry of solute in solution. Can treat solvent stabilized zwitterions Difficult to describe weakly bound solvent molecules interacting with solute (low frequency, many local minima) Short cut: Optimize structure in the gas phase and do single point solvation calculation. Some calculations done this way Solvent:  = 99 Rsolv= 2.205 A Implementation in Jaguar (Schrodinger Inc): pK organics to ~0.2 units, solvation to ~1 kcal/mol (pH from -20 to +20)

  4. Comparison of Jaguar pK with experiment 6.9(6.7) -3.89(-52.35) 5.8(5.8)-4.96(-49.64) 6.1(6.0) -3.98(-55.11) 5.3(5.3) -3.90(-57.94) 5.0(4.9) -4.80(-51.84) pKa: Jaguar(experiment) E_sol:zero(H+)

  5. Jaguar predictions of Metal-aquo pKa’s Protonated Complex (diethylenetriamine)Pt(OH2)2+ PtCl3(OH2)1- Pt(NH3)2(OH2)22+ Pt(NH3)2(OH)(OH2)1+ cis-(bpy)2Os(OH)(H2O)1+ Experimental pKa 6.3 7.1 5.5 7.4 11.0 Calculated (B3LYP) pKa(MAD: 1.1) 5.5 4.1 5.2 6.5 11.3 Calculated (M06//B3LYP) pKa (MAD: 1.6) 9.1 8.8 6.2 10.9 13.0 15.2 11.0 13.9 5.6 6.3 10.9 cis-(bpy)2Os(H2O)22+ cis-(bpy)2Os(OH)(H2O)1+ trans-(bpy)2Os(H2O)22+ trans-(bpy)2Os(OH)(H2O)1+ cis-(bpy)2Ru(H2O)22+ cis-(bpy)2Ru(OH)(H2O)1+ trans-(bpy)2Ru(H2O)22+ trans-(bpy)2Ru(OH)(H2O)1+ (tpy)Os(H2O)32+ (tpy)Os(OH)(H2O)21+ (tpy)Os(OH)2(H2O) Experimental pKa 7.9 11.0 8.2 10.2 8.9 >11.0 9.2 >11.5 6.0 8.0 11.0

  6. Use theory to predict optimal pH for each catalyst Predict the relative free energies of possible catalyst resting states as a function of pH. LnOsII(OH2)3+2 LnOsII(OH2)2(OH)+ LnOsII(OH2)(OH)2 LnOsII(OH)3- LnOsII(OH2)2(OH)+ never most stable LnOsII(OH)3- is stable LnOsII(OH2)3+2 is stable LnOsII(OH2)(OH)2 is stable

  7. Use theory to predict optimal pH for each catalyst pH-dependent free energies of formation for transition states are added to determine the effective activation barrier as a function of pH. Insertion transition states Resting states Optimum pH region

  8. Use theory to predict optimal pH for each catalyst 34.6 40.0 34.6 32.6 37.9 we determine the pH at which an elementary step’s rate is maximized. Insertion transition states Resting states Best, 2 kcal/mol better than pH 14

  9. Black  experimental data from Meyer, Red is from QM calculation (no fitting) using M06 functional, no explicit solvent Maximum errors: 200 meV, 2pH units Predicted Pourbaix Diagram for Trans-(bpy)2Ru(OH)2 Experiment: Dobson and Meyer, Inorg. Chem. Vol. 27, No.19, 1988.

  10. Experimental discovery: Periana et al., Science, 1998 (bpim)PtCl2 TOF: 1x10-3 s-1 t½ = >200 hours Not decompose but rate 10 times too slow Also poisoned by H2O product How improve rate and eliminate poisoning (NH3)2PtCl2 TOF: 1x10-2 s-1 t½ = 15 min Rate ok, but decompose far too fast. Why? Two Platinum compounds (out of laaarge number examined) catalyze conversion of methane to methylbisulfate in fuming sulphuric acid (102%) CH4 + H2SO4 + SO3 CH3OSO3H + H2O + SO2 CH3OSO3H + H2O  CH3OH + H2SO4 SO2 + ½O2  SO3 Catalytica: Many $$$ trying to solve these problems experimentally, failed, cancelled project. Periana came to USC, teamed up with Goddard, Chevron funded. Found success

  11. First Step: Nature of (Bpym)PtCl2 catalyst Is H+ on the Catalytica Pt catalyst in fuming H2SO4 (pH~-10)? DH kcal/mol (DG kcal/mol) In acidic media (bpym)PtCl2 has one proton

  12. Mechanisms for CH activation To discuss kinetics of C-H activation for (NH3)2PtCl2 and (bpym)PtCl2Need to consider the mechanism Oxidative addition Form 2 new bonds in TS Sigma metathesis (2s + 2s) Concerted, keep 2 bonds in TS Electrophilic addition Stabilize Occupied Orb. in TS

  13. Use QM to determine mechanism: C-H activation step. Requires high accuracy (big basis, good DFT) H(sol, 0K) kcal/mol Oxidative addition Theory led to new mechanism, formation of ion pair intermediate, proved with D/H exchange -bond metathesis Electrophilic addition 1. Form Ion-Pair intermediate 2. Rate determining step is CH4 ligand association NOT CH activation! CH4 complex (bpym)PtCl2 Start 3. Electrophilic Addition wins CH3 complex

  14. C-H Activation Step for (bpymH+)Pt(Cl)(OSO3H) Solution Phase QM (Jaguar) RDS is CH4 ligand association NOT CH activation! Oxidative addition Electrophilic substitution Differential of 33.1-32.4=0.7 kcal/mol confirmed with detailed H/D exchange experiments CH4 complex Form Ion-Pair intermediate Start CH3 complex

  15. Theory based mechanism: Catalytic Cycle Adding CH4 leads to ion pair with displaced anion After first turnover, the catalyst is (bpym) PtCl(OSO3H) not (bpym)PtCl2 Start here 1st turnover Catalytic step

  16. L2PtCl2 – Water Inhibition Experimental Observation: Reaction strongly inhibited by water, shuts off as solvent goes from 102% to 96% Is this because of interaction of water with reactant, catalysis, transition state or product? Barrier 33.1 kcal/mol Barrier 39.9 kcal/mol Theory: Complexation of water to activated catalyst is 7 kcal/mol exothermic, making barrier 7 kcal/mol higher. Product formation  0 Thus inhibition is a ground state effect Challenge: since H2O is a product of the reaction, must make the catalyst less attractive to H2O but still attractive to CH4

  17. Summary less positive Pt leads to easier CH4 oxidation addition activation more positive Pt makes electrophilic substitution easier. Lower oxidation state, easier oxidation step Lower oxidation state, less water inhibition A weak Pt-Cl bond facilitates electrophilic substitution A strong Pt-L bond prevents precipitation

  18. A catalyst that can activate CH4 should even more easily activate CH3OH. CH bond CH4 is 105 kcal/mol CH bond of CH3OH is 94 kcal/mol How can the Periana Catalyst work? Product Protection, the Key to Developing High Performance Methane Selective Oxidation Catalysts, M. Ahlquist, RJ Neilsen, RA Periana, and wag JACS, just published Marten Ahlquist

  19. Recall mechanism (1 mM of CH4 in solution) Assuming a 1 mM of CH4 in solution, reaction barrier for methane coordination 27.5 kcal/mol, Followed by insertion of Pt into CH bond and Reductive deprotonation to give the platinum(II) methyl intermediate Pt-CH Add CH4 deprotonation Mechanism for the C‑H activation of methane by the Periana-Catalytica catalyst. Free energies (kcal/mol) at 500 K including solvation by H2SO4.

  20. Next step: Oxidation of the PtII‑Me intermediate by sulfuric acid CH3-O-SO3H Get CH3OSO3H + SO2 products Free energies (kcal/mol) at 500 K including solvation by H2SO4. SO2

  21. reaction path for C‑H activation of methyl bisulfate by the Periana-Catalytica catalyst. 41.5 kcal/mol Barrier react with CH3-O-SO3H 27.5 kcal/mol Barrier react with CH4 27.2 kcal/mol Barrier react with CH3OH Get product protection Free energies (kcal/mol) at 500 K including solvation by H2SO4.

  22. Proposed pathway for oxidation ofactivated CH3-O-SO3H The rate limiting step in the oxidation of methyl bisulfate is C‑H cleavage (41.5) rather than oxidation (35.3) For methane the activation barrier is (27.5) while the oxidation barrier is 32.4

  23. Activation of CH3OH by the Periana Catalyst include the energy for formation of free methanol from methyl bisulfate, Assuming free methanol, Free energies (kcal/mol) at 500 K including solvation by H2SO4.

  24. Quantum Mechanics Rapid Prototyping (QM-RP) With an understanding of basic mechanistic steps, use QM to quickly test other ligands and metals computationally Other metals (Ir, Rh, Pd?) Other activating Ligands X Other stabilizing ligands L Identify leads for further theory For best cases do experiment synthesis, characterization Other solvents

  25. Switch from IrIII NCN to IrIII NNC Eliminate trans-effect by switching ligand central C to N Get some water inhibition, but low ligand lability Continue 20.6 -OH- 8.0 -H2O 0.0 Solvated (H2O)

  26. Further examine IrIII NNC CH4 activation by Sigma bond metathesis - Neutral species - Kinetically accessible with total barrier of 28.9 kcal/mol 28.9 8.0 -H2O 0.0 -9.0 Solvated (H2O) Passes Test

  27. Oxidize with N2O prior to Functionalization 24.5 +N2O -N2 -7.4 -OH- -9.0 Solvated (H2O) -19.8 IrIII - NNC Passes Test Oxidation by N2O Kinetically accessible

  28. Re-examine Functionalization for IrIII NNC Passes Test 8.3 -2.1 -11.2 -19.8 Thus reductive elimination from IrV Is kinetically accessible Solvated (H2O) -65.9

  29. CH activation CH4 CH3OH A solution IrIII – NNC 28.9 +CH4 8.0 -H2O 0.0 -9.0 24.5 +N2O -N2 -7.4 -OH- -9.0 -19.8 To avoid H2O poisoning, work in strong base instead of strong acid. Use lower oxidation states, e.g. IrIII and IrI QM optimum ligands (Goddard) 2003 Tested experimentally (Periana) 2009 It works Oxidation Functionalization 8.3 Experimental ligand -2.1 -11.2 -19.8 Predicted: Muller, Philipp, Goddard Topics in Catalysis2003, 23, 81 -65.9

  30. New material

  31. Catalytic cycle: Au in H2SO4/H2SeO4 Product. AuI to III Cycle: oxidation → CH activation → SN2 attack Act. CH4 Act. CH4 I Problem: Inhibited by water AuI to III Accessibility of both AuI and AuIII oxidation states prevents deactivation due to oxidization of catalyst 1. CH activation by electrophilic substitution. 2. Functionalization by nucleophilic attack by HSO4-. Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626. 180°C, 27 bar CH4, TOF 10-3 s-1

  32. Consider AuIII in H2SO4/H2SeO4: CH activation by AuIII Add CH4 to AuIII complex H extracted by bound HSO4- Assisted by solvent H2SO4 Form Au-CH3 bond to AuIII complex Equilibrium Complex with Au-CH3 Protonated AuIII complex Start with AuIII CH activation relies on solvent, H2SO4, or conjugate base. Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.

  33. AuIII in H2SO4/H2SeO4: Functionalization CH3OSO3H product Separate by adding H2O HSO4- solvent SN2 attack on Au-CH3 bond Functionalization relies on solvent, H2SO4, or conjugate base. Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.

  34. General strategy to developing new catalysts CH4 LnM-X CH3OH Identify and elucidate elementary mechanistic steps for activation, functionalization/oxidation and reoxidation that connect to provide a complete, electronically consistent cycle. Y ½ O2 functionalization YO reoxidation CH Activation LnM-CH3 + HX

  35. Mo Tc Ru Rh Pd Ag Cd Early successes in methane functionalization used the electrophilic paradigm: Ta W Re Os Ir Pt Au Hg Tl Electronegative Metals Pt, Au, Hg, Pd: ∙ good selectivity, rates, and stability ∙ product protection by esterification -but- ∙ inhibited by water and methanol ∙ require strong oxidants Consequently we shifted to the nucleophilic paradigm, which can coordinate CH4 under milder acid or concentrated base conditions. (bpim)PtCl2 TOF: 1x10-3 s-1 t½ = >200 hours (NH3)2PtCl2 TOF: 1x10-2 s-1 t½ = 15 min Pt: Periana et al., Science, 1998 Au: Periana, wag; Angew. Chem. 2004 Hg: Periana et al., Science, 1993

  36. PtCl4= (Shilov) (not commercial, requires strong oxidant) Au,Hg/H2SO4 (not commercial, inhibited by water, Au requires strong oxidant) (bpym)PtCl2/H2SO4 (impressive, but not commercial, inhibited by water) 70% one pass yield 95% selectivity for CH3OSO3H TOF ~ 10-3 s-1, TON > 1000 PdII/H2SO4 (modest selectivity for CH3COOH) (NNC)IrIII(OH)2 (requires strong oxidant) Progress towards CH4 + ½O2→ CH3OH Progress, but major problems Need new strategy

  37. Mo Tc Ru Rh Pd Ag Cd Ta W Re Os Ir Pt Au Hg Tl Nucleophilic Electrophilic pH = 14 Solvent pH pH < 0 K+/Na+ OH- 1M OH- H2O 1M H+ H2SO4 Oxidant (H2O) DMSO H2SeO3 H2SO4 H2SeO4 Product protection CH3O- CH3OHCH3OH2+ Ru, Re, Os, Ir are good nucleophilic metals for base or weak acid

  38. We have identified 3 Mechanistic pathways LnM-X CH4 CH3X Insertion New mechanisms for nucleophilic metals Electrophilic Nucleophilic Base-assisted Substitution CH Activation Functionalization LnM-CH3 We are discovering new and manipulating old mechanistic steps that will be active for less electrophilic metals operating in aqueous solution.

  39. Functionalization by nucleophilic attack (SN2) (bpy)IrIII(pyr)(OH)2(CH3) (trpy)OsIV(OH)2(CH3) SN2 barriers (reductive functionalization) very high for earlier (electron-rich) metals.

  40. Switch to less electronegative metals, e.g. Os Functionalize (acac)2OsIV(CH3)(OH) using (acac)2OsVI(=O)(=O) 3+2 VI IV Migratory Insertion [Oxidant] 3+2 G298K, pH = 14 Barriers are pH dependent. This oxidant, [cis-(acac)2OsVI(O)2], is privileged. Backside attack

  41. Functionalization of (acac)2OsIV(CH3)(OH) Reactant M-CH3 bond [Oxidant] Oxidant LUMO accepting 2 electrons and CH3 in TS Electrophilic attack on methyl by the more stable [trans-(acac)2OsVI(O)2] is exciting. Oxidation is consistently 2-electron in the backside attack mechanism, regardless of Mn-CH3 oxidation state (n = II, III, IV).

  42. Functionalization using transfer of CH3 to Se SN2 process

  43. Full cycle Re(CO)5-OH Re(CO)5-CH3 Catalytic Oxy-Functionalization of a Low Valent Metal Carbon Bond with Se(IV) William J. Tenn, III, Brian L. Conley, Mårten Ahlquist, Robert J. Nielsen, ‡ Jonas Oxgaard, William A. Goddard, III and Roy A. Periana

  44. Homogeneous CH4 functionalization: how to best choose new metals Our QM mechanistic studies for a variety of complexes from AuIII to ReI show the continuum of charge transfer to methane Charge transfer Electron-rich methyl groups HX + Electron-poor methyl groups

  45. CH activation and functionalization by nucleophilic d6 metals M-CH3 polarization based on C1s chemical shift The carbon 1s orbital energy is an excellent measure of the electron density on the methyl carbon. This illustrates the extremes of the polarity scale, which require very different functionalization mechanisms.

  46. Ongoing Work in Homogeneous CH4 Functionalization We modeled bipyridine complexes of RuII, OsII and ReI to determine the dependence of ground states (protonation), H3C-H activation barriers (substitution and insertion) and functionalization barriers on metal and p-donating ligand substituents. Insertion Substitution Going forward, we are considering the kinetics of these steps using d5 and d4 metals and new coordinated bases (i.e. –NH2).

  47. Ongoing Work in Homogeneous CH4 Functionalization (bpy)2Ru(OH)2 complexes do not participate in insertion mechanisms (i.e., the products are not a minimum on the potential energy surface), only in the substitution path. (bpy)2Os(OH)2 complexes allow both pathways (each are identifiable saddle points). However, the insertion pathway is preferred. substitution insertion Electron-donating substituents labilize hydroxide, creating vacancies. Insertion barriers decrease with the electron-donating ability of the substituent. The catalyst’s susceptibility to oxidation also increases with the C-H activation rate. After the resting state switches to Ru(OH)(OH2), the substituents weakly effect substitution barriers. Insertion barriers can be tuned over an extreme range by varying the ligand and metal. Substitution barriers cannot be similarly tuned.

  48. CH4 functionalization with homogeneous catalysts Going forward: Determine what combinations of Group 9 and 10 metals, ligands and nucleophiles will allow SN2 functionalization with thermally accessible barriers. Barrier Periodic table Reductive functionalization mechanisms (red. elim., SN2) well known for late metals (M-CH3d+). With Periana we have sought complimentary mechanisms appropriate for electron rich metals: Baeyer-Villiger Nucleophilic attack Electrophilic attack HX + Reductive elimination Transalkylation

  49. Going forward in homogeneous CH4 functionalization We explore functionalization mechanisms in which the oxidant is a higher oxidation state of the hypothetical CH activation catalyst: OsVI + OsIV L = (acac)2 OsVI + OsIII L = terpyridine

  50. Catalytic cycle: Au in H2SO4/H2SeO4 Product. AuI to III Cycle: oxidation → CH activation → SN2 attack Act. CH4 Act. CH4 I Problem: Inhibited by water AuI to III Accessibility of both AuI and AuIII oxidation states prevents deactivation due to oxidization of catalyst 1. CH activation by electrophilic substitution. 2. Functionalization by nucleophilic attack by HSO4-. Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626. 180°C, 27 bar CH4, TOF 10-3 s-1

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