William A. Goddard, III, wag@kaist.ac.kr

Lecture 22, November 24, 2009 Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy Course number: KAIST EEWS 80.502 Room E11-101 Hours: 0900-1030 Tuesday and Thursday William A. Goddard, III, wag@kaist.ac.kr WCU Professor at EEWS-KAIST and Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Senior Assistant: Dr. Hyungjun Kim: linus16@kaist.ac.kr Manager of Center for Materials Simulation and Design (CMSD) Teaching Assistant: Ms. Ga In Lee: leeandgain@kaist.ac.kr Special assistant: Tod Pascal:tpascal@wag.caltech.edu EEWS-90.502-Goddard-L15

Schedule changes Nov. 24, Tuesday, 9am, L22, as scheduled Nov. 26, Thursday, 9am, L23, as scheduled Dec. 1, Tuesday, 9am, L24, as scheduled Dec. 2, Wednesday, 3pm, L25, additional lecture, room 101 Dec. 3, Thursday, 9am, L26, as scheduled Dec. 7-10 wag lecturing Seattle and Pasadena; no lectures, Dec. 11, Friday, 2pm, L27, additional lecture, room 101 EEWS-90.502-Goddard-L15

Last time EEWS-90.502-Goddard-L15

EEWS-90.502-Goddard-L17

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+)

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 Start here Adding CH4 leads to ion pair with displaced anion After first turnover, the catalyst is (bpym) PtCl(OSO3H) not (bpym)PtCl2 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

New material

Quantum Mechanical Rapid Prototyping 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 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

A few of the prototype ligand/metal sets evaluated by QMRP

More exotic ligand/metal sets evaluated by QMRP. Since calculations are fast, a couple of hours, can try wild guesses

QMRP: PtII NCN and NNN ligands, reject (NCN)Pt(II) (NNN)Pt(II)+ DE(A-C) too high for both complexes

QMRP: OsII NCN and PtIV NNN ligands, reject (NCN)Pt(IV) (NCN)Os(II) DE(A-C) too high for (NCN)Os(II), but acceptable for (NCN)Pt(IV)

QMRP: IrIII NCN, passes 1,3, fails 2,reject (NCN)Ir(III) system passes QM-RP tests 1 and 3, but is not resistant to water (test 2)

QMRP: IrI NNN, passes 1,2,3 examine further (NNN)Ir(I) (NNN)Ir(I) picked as focal point for more detailed studies (NNN)Ir(I) system passes all three tests!

QMRP: further examination of IrI NNN.Not stable in acid media,reject +H3O+ -H2O + H2O 0.0 -28.7 -32.7 + H2O Ir(I) not compatible with acidic media – protonation to Ir(III) predicted to be rapid and irreversible. Oxidation state of IrI too lowmove to IrIII 4.6

QMRP: IrIII NCN Even though (NCN)IrICl failed QC-RP tests, could (NCN)IrIII(OH)2 be viable? 18.6 -OH- -H2O Very slight water inhibition, low ligand lability, Both good 2.0 0.0 Solvated in (H2O)

QMRP: Further examine IrIII - NCN ? ? 46.1 +CH4 20.1 Unfavorable to have covalent Ir-CH3 bond trans to Ir-Ph bond Oxidative addition Thermodynamically Inaccessible. Thus reject 10.6 Solvated (H2O)

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

Examine Functionalization for IrIII - NNC Reductive Elimination to form CH3OH Kinetically inaccessible 44.3 -7.0 -1.3 Solvated (H2O) -9.0 Maybe problem is that IrIII -> IrI unfavorable Need to Oxidize to IrV prior to functionalization?

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

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

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

Experimental Synthesis of IrIII NNC system Experimental realization of catalytic CH4 hydroxylation predicted for an iridium NNC pincer complex, demonstrating thermal, protic, and oxidant stability; Young, KJH; Oxgaard, J; Ess, DH; Meier SK, Stewart T, Goddard WA, Periana RA;Chem. Comm., (22): 3270-3272 (2009)

Xray of IrIII NNC Experimental realization of catalytic CH4 hydroxylation predicted for an iridium NNC pincer complex, demonstrating thermal, protic, and oxidant stability; Young, KJH; Oxgaard, J; Ess, DH; Meier SK, Stewart T, Goddard WA, Periana RA;Chem. Comm., (22): 3270-3272 (2009) bond lengths (Å): Ir(1)-N(2) 2.017(6), Ir(1)-C(16) 2.078(8), Ir(1)-C(27) 2.174(9), Ir(1)-N(1) 2.164(6), Ir(1)-C(29) 2.081(11), Ir(1)-O(1) 2.207(6). bond angles (deg): N(2)-Ir(1)-C(16) 78.7(3), N(2)-Ir(1)-C(27) 161.0(3), N(2)-Ir(1)-N(1) 76.8(2), C(16)-Ir(1)-N(1) 155.4(3), C(27)-Ir(1)-N(1) 84.2(3), C(29)-Ir(1)-O(1) 171.1(5). Thermal ellipsoid plot of 1-TFA with 50% probability. Hydrogens, and benzene co-solvent removed for clarity. bond lengths (Å): bond angles (deg):

Message: it took 2 years of experiments to synthesize the desired ligand and incorporate the Ir in the correct ox. state. Periana persisted only because he was confident it would work. Not practical to do this for the 1000’s of cases examined in QMRP Final step: QM for Experimental Ligand enthalpy solvent corrections in kcal mol-1 (453K) for HTFA ( = 8.42 radius = 2.479 Å). Chem. Comm., (22): 3270-3272 (2009)

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

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.

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.

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

Early successes in methane functionalization used the electrophilic paradigm: Mo Tc Ru Rh Pd Ag Cd 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

William A. Goddard, III, wag@kaist.ac.kr