Lecture 21 February 23, 2011 CH4 CH3OH catalysis Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy William A. Goddard, III, firstname.lastname@example.org 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 <email@example.com> Caitlin Scott <firstname.lastname@example.org>
OLEFIN METATHESIS Catalytically make and break double bonds! Mechanism: actual catalyst is a metal-alkylidene
Well-defined metathesis catalysts Schrock 1991 alkoxy imido molybdenum complexa Grubbs 1991 ruthenium benzylidene complexb Grubbs 1999 1,3-dimesityl-imidazole-2-ylidenes P(Cy)3 mixed ligand system”c Bazan, G. C.; Oskam, J. H.; Cho, H. N.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc.1991,113, 6899-6907 Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett.1999,40, 2247-2250. Wagener, K. B.; Boncella, J. M.; Nel, J. G. Macromolecules1991, 24, 2649-2657
Examples 2nd Generation Grubbs Metathesis Catalysts General mechanism of Metathesis
Ru-Methylidene Double Bond z x Cz=Cpp Ruxz Ru dxz-C pzRu-C Pi bond Cs 3B1 CH2 Ru2xx-yy-zz Ru dx2 - C sp2 Ru-C Sigma bond CH2 is triplet state with singly occupied s and p orbitals get spin pairing s bond to Ru dx2 and p bond to Ruxz
Ru-Methylidene Double Bond CH2 is triplet state with singly occupied s and p orbitals get spin pairing s bond to Ru dx2 and p bond to Ruxz z x Ru-C Sigma bond (covalent) Ru dx2 - C sp2 Ru-C Pi bond (covalent) Ru dxz - C pz Bond dist. Theory Experiment Ru-CH2 1.813 1.841 Ru-Carbene2.109 2.069
Carbene sp2-Ru dz2 Don-Accep Bond Ru-Carbene Sigma donor bond (Lewis base-Lewis acid) C sp2 Ru dz2 Planar N with 3 s bonds and 2 e in pp orbital Planar N with 3 s bonds and 2 e in pp orbital Singlet methylene or carbene with 2 s bonds to C and 2 electrons in Cs lone pair but empty pp orbital Singlet Carbene (Casey Carbene or Fisher carbene stablized by donation of N lone pairs, leads to LUMO Bond dist. Theory Experiment Ru-CH2 1.813 1.841 Ru-Carbene2.109 2.069
Carbene sp2-Ru dz2 Don-Accep Bond Ru-Carbene Sigma donor bond (Lewis base-Lewis acid) C sp2 Ru dz2 Carbene p- LUMO) Antibonding to N lone pairs
Ru-dyz - Carbene py Don-Accep Bond Ru dyz Lone Pair (Lewis base-Lewis acid) Ru dyz Carbene py LUMO Ru dyz Lewis Base to Carbene py pi acid stabilizes the RuCH2 in the xy plane This aligns RuCH2 to overlap incoming olefin Carbene p- LUMO) Antibonding to N lone pairs
Ru LP and Ru-CH2 Acceptor Orbitals Because RuCH2 is perpendicular to plane, the empty antibonding orbital overlaps the bonding pi orbital of the incoming olefin Ru dxy Lone Pair Want perpendicular to C-Ru-C plane Avoid overlap with NCN bonds Orients Methylidene Perpendicular to Plane Ru-CH2 * (antibonding) LUMO Acceptor for olefin bond Orients Olefin Perpendicular to plane
Ru Electronic Configuration Ru(CH2)Cl2(phosphine)(carbene) Ru-Cl bonds partially ionic (50% charge transfer), consider as RuII (Cl-)2 RuII: (dxz)1(dx2)1 (dxy)2(dyz)2(dz2)0 Ru (dx2)1 covalent sigma bond to singly-occupied sp2 orbital of CH2 Ru (dxz)1 covalent pi bond to singly-occupied pz orbital of CH2 ( the CH2 is in the triplet or methylidene form) Ru (dxy)2 nonbonding Ru (dyz)2 overlaps empty carbene y orbital stabilizing RuCH2 in xy plane Ru (dz2)0 stabilizes the carbene and phosphine donor orbitals RuCH2p* (antibonding) LUMO overlaps the bonding orbital of incoming olefin stabilizing it in the confirmation required for metallacycle formation.
Generally Accepted Mechanism E or Z olefin products
Metal [2+2] cycloaddition is thermally allowed All-carbon [2+2] cycloaddition is forbidden d orbital has different phase overlaps; other orbitals available HOMO LUMO (more details to follow in upcoming lectures!) Woodward-Hoffman rules still apply, but d-orbitals now participate
Grubbs Catalyst 2nd Generation Product-Substrate exchange is rate determining step B3LYP B3LYP
Catalyst Challenges for the Selective Chemistry needed for Sustainable Development • Challenge: improved catalysts for industrial applications including • Low temperature conversion of methane to fuels and organic feedstocks • High selectivity and activity for converting alkanes to organic feedstocks • Fuel cell cathode catalysts for the oxygen reduction reaction (ORR) with decreased overpotential, much less Pt, and insensitive to deactivation • Fuel cell anode catalysts capable of operating with a variety of fuels but insensitiveto CO and to deactivation • A methane fuel cell (CH4 + H2O CO2 + power [8 (H+ and e-)] • Efficient catalysts for photovoltaic production of energy and H2 • Efficient catalysts for storing and recovering hydrogen • Catalysts for high performance Li ion and F ion batteries Enormous experimental efforts have been invested in solving these problems but better solutions are needed more quickly I claim that Theory and Modeling are poised to provide guidance to achieve these goals much more quickly
Projects in Catalysis: First establish mechanism then use mechanism to design improved catalyst • Propane ammoxidation - structure of new phases in Mixed Metal Oxide (Mitsubishi, BP) catalysts: MoVNbTaTeOx TUESDAY • butane MA over VOPO and ODH over V2O5 • Fuel Cell cathode electrocatalysis: nonPt and CoPt,NiPt alloys • Direct methanol fuel cell: PtRu-RuOHy at anode • CuSix catalysis of MeCl to Si(Me)2Cl2 and role additives • Organometallic Catalysts CH4 to liquids: Pt, Ir, Os, Re, Rh, Ru TODAY • Pd-mediated activation of molecular oxygen • Mechanism of the Wacker reaction in aqueous solution • Single Site Polymerization catalysts for polar monomers
Role of Theory in Developing Catalysts 1. Establish Mechanism of current catalysts: Use QM to predict all plausible reaction paths, Determine transition states (TS) and stable reaction intermediates (RI) Calculate vibrational frequencies (vf) to prove TS(one negative curvature)and RI Use frequencies to calculate entropy, Cp. Use QM and Poisson-Boltzmann to get free solvation energy. Get free energy at reaction temperatures G = Eelec + ZPE + Hvib(T) + Hlib(T) –TSvib – TSlib + Gsolv Use to estimate rates This provides the conceptual framework to interpret experiments 2. Validation: Predict new experiments to test mechanism 3. Lead discovery: Combinatorial Computational Rapid Prototyping In silico search for new lead candidates for Ligands, Metals, Solvents 4. Experiments: optimize best predicted ligands and reaction conditions. Continue theory and simulation in collaboration with experiments Critical to new role of theory: accuracy and reliability for novel systems Must trust the theory well enough to do only 1 to 10% of the systems Focus experiments on these 1% to 10% predicted to be best
Has theory ever contributed to catalysis development? Over last 30 years quantum mechanics (QM) theory has played an increased role in analyzing and interpreting experimental results on catalytic systems But has QM led tonew catalysts before experiment and can we count on the results from theory to focus experiments on only a few systems? Case study: New catalysts for low temperature activation of CH4 and functionalization to form liquids (CH3OH)
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
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)
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+)
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
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
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
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
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.
Evaluating multi-oxidation state cycles for nucleophilic metals 0.5 V OsV OsIV OsIII 1 Volt OsII (trpy)(bpy)Os(OHn) Oxidation states VI→II are present within ~0.5 V window. Aqua ligands stabilize many oxidation states. Odd-electron oxidations are common. Ligands,anions influence the redox properties over a very wide range. (trpy)Os(OHn)3 Pipes and Meyer, Inorg. Chem.1986, 25, 4042. Meyer, et al. Inorg. Chem.1984, 23, 1845.
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
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
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
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
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
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
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
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
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.
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
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.
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
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.
A simple kinetic model can be used to illustrate the dependence of selectivity and product concentration over the course of a batch reaction: Begin with 100% selectivity, no product. A (bpym)PtCl2 reaction in 102% sulfuric acid has a best selectivity of 80%, which is why we need dry sulfuric acid (large KP) and a large ratio of k1:k3. Meeting these requirements will be a challenge for less electrophilic metals.
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
QMRP: computational analogue of combinatorial chemistry Three criteria for CH4 activation: Thermodynamic Criterion: Energy cost for formation of R-CH3 must be less than 10 kcal mol-1. Fast to calculate because need only minimize stable M-CH3 Reaction Intermediate Poisoning Criterion: Species must be resistant to poisoning from water (i.e. water complexation is endothermic)Fast to calculate because minimize only M-H2O intermediate. Kinetic Criterion: Barrier to product formation must be less than 35 kcal mol-1. Test for minimized M-(CH4). Barrier only a few kcal/mol higher. Slower to calculate because of weakly bound anion and CH4, but minimize only intermediate. Do real barriers only when DH3 is less than 35 kcal/mol Quantum Mechanical Rapid Prototyping pilot Many cases of Metal, ligand, solvent 1 2 3 4 exper Small set systems for lab experiment Muller, Philipp, Goddard Topics in Catalysis2003, 23, 81
Tri-site ligands We considered first a class of tri-site ligands analogous to those studied by Brookhart in Fe and Cr based catalysts for olefin polymerization. However we considered alternate ligands in which the 3 coordination sites [(N,N,N) in this case] are be replaced by various other ligands such as C, O, P, S We simplify the ligands to include the parts that affect the chemistry but not the modifications (ligands on the outer N such as mesityl, the embedding the middle N into an aromatic ring) used to protect and stabilize the catalyst under experimental conditions (but which are expected to have only a modest effect on controlling rates). We validated the accuracy of the simplified ligands by doing the Brookhart catalysts both ways. We also consider various metals and oxidation states.
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)
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
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