William A. Goddard, III, wag@kaist.ac.kr Lecture 22, November 24, 2009 Nature of the Chemical Bond with applications to catalysis, materials science,

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 Room E11-101 Hours: Tuesday and Thursday William A. Goddard, III, 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: Manager of Center for Materials Simulation and Design (CMSD) Teaching Assistant: Ms. Ga In Lee: Special assistant: Tod EEWS 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 wag lecturing Seattle and Pasadena; no lectures, Dec. 11, Friday, 2pm, L27, additional lecture, room 101 EEWS Goddard-L15

Last time EEWS Goddard-L15

EEWS 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 to new 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 + ½O  SO3 Background: structure of the two most efficient catalysts for methane to methanol conversion. 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 The Poisson-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= 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
pKa: Jaguar (experiment) E_sol: zero (H+) 6.9 (6.7) (-52.35) 5.8 (5.8) (-49.64) 6.1 (6.0) (-55.11) 5.0 (4.9) (-51.84) 5.3 (5.3) (-57.94)

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)2Pt Cl2 and (bpym)PtCl2 Need 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

Electrophilic addition 3. Electrophilic Addition wins
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

Electrophilic substitution
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 =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 The catalytic activity dies down with time, as the concentration of product builds up. During product formation water is formed, and calculations show that water binds very stronly to the metal. At low concentrations of water, the strong acicid media ensures that all free water is protonated, and would not bind. As the concentration increases, free non-protonated water builds up. 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 Many cases of Metal, ligand, solvent 1 2 3 4 exper pilot Small set systems for lab experiment Muller, Philipp, Goddard Topics in Catalysis 2003, 23, 81

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

More exotic 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)+ Sample of screened ligands (all in all, over 100 systems were screened). These two systems fail the first QMRP test, and are thus discarded. DE(A-C) too high for both complexes

QMRP: OsII NCN and PtIV NNN ligands, reject
(NCN)Pt(IV) (NCN)Os(II) First system fails the QMRP, second passes (thermodynamics <10 kcal/mol) 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) passes first two tests, but fails the third (severe water inhibition) (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) system passes all three tests, thus warranting more detailed studies. (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. However, Ir(I) not compatible with acidic media, quickly converted to Ir(IIII). Hypothesis then – move to Ir(III) Oxidation state of IrI too low move to IrIII 4.6

Very slight water inhibition, low ligand lability,
QMRP: IrIII NCN Even though (NCN)IrICl failed QC-RP tests, could (NCN)IrIII(OH)2 be viable? 18.6 -OH- Initial concern – the (NCN)IrCl2 failed the QCRP tests. However, some unrelated calculations suggested that hydroxides could be viable ligands. Calculations show that the (NCN)Ir(OH)2 complex suffers from very little water inhibition, and also very low ligand lability. -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 Exploratory calculations of C-H activation pathways showed that oxidative addition is energetically inaccessible. 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 Could this effect be eliminated by moving the carbon to the side, so the trans influence would only affect the (inert) ligand? Calculations show similar low ligand lability, but somewhat higher water inhibition. -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 However, the sigma-bond metathesis transition state is accessbile, even from the (NNC)Ir(OH)2(OH2) starting point. 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 With C-H activation shown to be feasible, can this system functionalize the methyl group? Follow-up calculations show that reductive elimination from the Ir(III) complex is kinetically inaccessible, most likely due to the low oxidation state of the metal. Both the M-C and M-O bonds are very strong, while at higher oxidation states they would be weakened. -7.0 Solvated (H2O) -1.3 -9.0 Maybe problem is that IrIII -> IrI unfavorable Need to Oxidize to IrV prior to functionalization?

Oxidize with N2O prior to Functionalization
-9.0 24.5 -7.4 -19.8 -OH- +N2O -N2 Solvated (H2O) IrIII - NNC Passes Test Oxidation using N2O as a model oxidant is favourable, with an overall barrier of 33.5 kcal/mol. It should be noted that N2O is not a very reactive oxidant, even though it is powerful, so more practical oxidants such as H2SeO4 are expected to be more rapid. Current work is evaluating these more realistic oxidants. Oxidation by N2O Kinetically accessible

Re-examine Functionalization for IrIII NNC
Passes Test 8.3 -2.1 -11.2 -19.8 Reductive elimination from the oxidized Ir(V) is accessible, however. The overall barrier is only 28 kcal/mol, and leads directly to methanol. Thus reductive elimination from IrV Is kinetically accessible Solvated (H2O) -65.9

Predicted: Muller, Philipp, Goddard Topics in Catalysis 2003, 23, 81
CH activation 0.0 28.9 8.0 -H2O -9.0 +CH4 CH4  CH3OH A solution IrIII – NNC 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 -9.0 24.5 -7.4 -19.8 -OH- +N2O -N2 Oxidation Functionalization 8.3 However, the sigma-bond metathesis transition state is accessbile, even from the (NNC)Ir(OH)2(OH2) starting point. Experimental ligand -2.1 -11.2 -19.8 Predicted: Muller, Philipp, Goddard Topics in Catalysis 2003, 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): (2009)

Thermal ellipsoid plot of 1-TFA with 50% probability.
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): (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):

Final step: QM for Experimental Ligand
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 = Å). Chem. Comm., (22): (2009)

Catalytic cycle: Au in H2SO4/H2SeO4
Product. AuI to III Cycle: oxidation → CH activation → SN2 attack Act. CH4 Act. CH4 Au uses electrophilic substitution 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

Pt Au Ir Hg Os Re W Pd Ag Rh Cd Ru Tc Mo Tl Ta
Early successes in methane functionalization used the electrophilic paradigm: 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. Introduction / review (NH3)2PtCl2 TOF: 1x10-2 s-1 t½ = 15 min (bpim)PtCl2 TOF: 1x10-3 s-1 t½ = >200 hours Pt: Periana et al., Science, 1998 Au: Periana, wag; Angew. Chem. 2004 Hg: Periana et al., Science, 1993

Progress towards CH4 + ½O2→ CH3OH
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) Known catalysts and their shortcomings Progress, but major problems Need new strategy

Ru, Re, Os, Ir are good nucleophilic metals for base or weak acid
Pt Au Ir Hg Os Re W Pd Ag Rh Cd Ru Tc Mo Tl Ta Nucleophilic Electrophilic pH = 14 Solvent pH pH < 0 K+/Na+ OH- 1M OH H2O M H H2SO4 Oxidant We have learned many relationships between METALS / OXIDATION STATES / OXIDANTS / pH / ANIONS that determine compatible combinations of these (H2O) DMSO H2SeO3 H2SO H2SeO4 Product protection CH3O CH3OH CH3OH2+ Ru, Re, Os, Ir are good nucleophilic metals for base or weak acid

We have identified 3 Mechanistic pathways
LnM-X CH3X CH4 Insertion New mechanisms for nucleophilic metals Electrophilic Nucleophilic Base-assisted Substitution New mechanisms must be found which are consistent with electron-rich metals and negatively polarized –methyl groups. Functionalization CH Activation LnM-CH3 We are discovering new and manipulating old mechanistic steps that will be active for less electrophilic metals operating in aqueous solution.

Functionalization by nucleophilic attack (SN2)
(bpy)IrIII(pyr)(OH)2(CH3) Separate Au/Pt and Ir/Os (trpy)OsIV(OH)2(CH3) SN2 barriers (reductive functionalization) very high for earlier (electron-rich) metals.

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] Electrophilic backside attack is feasible. Electrophilic “3+2” is on the verge of feasible in this system. Barriers should come down with more electron-rich Os(II)-CH3 compared to these Os(IV)-CH3 3+2 G298K, pH = 14 Barriers are pH dependent. This oxidant, [cis-(acac)2OsVI(O)2], is privileged. Backside attack

Functionalization of (acac)2OsIV(CH3)(OH)
Reactant M-CH3 bond [Oxidant] Now we are working on incorporating these into catalytic cycles. MO’s show electrons being drained from M-CH3 sigma bond into the oxidant’s pi-antibonding orbital. In the past we have also demonstrated Baeyer-Villiger pathway. In 31.9, triplet is lower than quintet (spin crossing to two OsIV triplets probably happens after TS). No spin density in C-O bond (2-electron oxidation) Triplet 21t is 8.8 up 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).

Functionalization using transfer of CH3 to Se
SN2 process

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

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)+ pKa’s for this complex are known, maximum deviation from experiment is 2 pH units. LnOsII(OH2)(OH)2 LnOsII(OH)3- LnOsII(OH2)2(OH)+ never most stable LnOsII(OH2)3+2 is stable LnOsII(OH2)(OH)2 is stable LnOsII(OH)3- is stable

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 We can iteratively optimize pH and the identity of the anion for given metal/ligand combinations Resting states Optimum pH region

we determine the pH at which an elementary step’s rate is maximized. 32.6 34.6 40.0 37.9 Insertion transition states Resting states Best, 2 kcal/mol better than pH 14

Plan for bringing to pilot new CH4 to liquids catalysts
Pt Au Ir Hg Os Re W Pd Ag Rh Cd Ru Tc Mo Tl Ta Middle Transition Metals Now couple our new functionalization mechanisms with our proven CH activation mechanisms using either nucleophilic substitution or insertion mechanisms with product protection by acid or base. Plan Use theory to identify and study scope of new functionalization mechanisms, and to study the effect of high pH on CH activation of CH4 and OCH3-. Late Transition Metals Mechanistic steps sufficient to get through a complete cycle, with mechanisms for protection, are proven and understood. Plan: Use theory to address the likely performance-limiting aspect of each metal, then design the ligand, pH, and oxidant around the rate-limiting step.

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

Get product protection
Proposed 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 Get product protection Free energies (kcal/mol) at 500 K including solvation by H2SO4.

Proposed pathway for oxidation of activated 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.

Simple kinetic model to determine overall selectivity
Kinetic model relating product protection and selectivity for the Periana-Catalytica catalyst

Commercial success if get here
Effect of product protection on selectivity and product concentration for the Periana catalyst. KP=0  no protection; KP=10∞  maximum protection. protection drops significantly already at 99%. Commercial success if get here Selectivity and product conc. Catalytica reaction starting at 102% H2SO4

Method Comparison in the Prediction of Stable
Isomers of Ru Olefin Metathesis Catalysts in Solution Geometry B3LYP M06-L Experiment SP Energy M06 Structure Relative Energy (kcal mol−1) Relative Abundance 1H-NMR 5a 0.0 9.8 15.9 95.9 10 5b 0.36 0.44 2.21 5.4 7.6 2.3 4 5c 0.29 0.78 2.82 6.0 4.3 0.8 2 5d 1.35 1.64 2.70 1.0 1 5e 0.25 0.02 4.88 6.5 15.4 N.O. 5f 1.67 1.98 5.61 0.6 5g 1.70 2.57 7.76 0.2 M06 leads to slightly better relative free energies (DG298) (by 2 to 3 kcal/mol) and relative abundances of isomers of 5 in CH2Cl2 at 298K than B3LYP Stewart, Benitez, O'Leary, Tkatchouk, Day, Goddard, Grubbs, J. Am. Chem. Soc., 2009, 131, 1931–1938.

Method Comparison in the Prediction of Stable
Isomers of Ru Olefin Metathesis Catalysts in Solution Geometry B3LYP M06-L Experiment SP Energy M06 Structure Relative Energy (kcal mol−1) Relative Abundance 1H-NMR 3a 0.13 0.0 2.9 1.2 7.0 6.7 (syn) 3c 0.37 0.45 3b 0.75 0.66 1.15 1 1 (anti) 3d 0.40 0.04 0.95 M06 leads to slightly better (0.5 kcal/mol) relative free energies (DG298) and relative abundances of isomers in CH2Cl2 at 298K than B3LYP Benitez, Tkatchouk, Goddard Organometallics 2009, 28, 2643–2645.

OLEFIN METATHESIS Catalytically make and break double bonds!
Mechanism: actual catalyst is a metal-alkylidene

Applications of olefin metathesis
Ring closing metathesis (RCM) Ring opening metathesis polymerization (ROMP) Acyclic diene metathesis (ADMET)

Well-defined metathesis catalysts
Schrock 1991 alkoxy imido molybdenum complexa Grubbs 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, Wagener, K. B.; Boncella, J. M.; Nel, J. G. Macromolecules 1991, 24, Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40,

History of Olefin Metathesis Catalysts

Examples 2nd Generation Grubbs Metathesis Catalysts
General mechanism of Metathesis

Structure Grubbs Carbene Catalyst
Ru-Carbene 2.109 CH2-Ru-Carb º CH2 Cl(1)-Ru-Cl(2) º Ru-CH P(iPr)3

Compare QM and (Xray) Bond Lengths (Å)
Ru-CH (1.841) Ru-P (2.419) Ru-Carbene (2.069) Ru-Cl(2) (2.383) Ru-Cl(1) (2.393) C(1)-N(1) (1.366) Carb-N(2) (1.354) C(2)-C(3) (1.296) Bond Angles (deg) CH2-Ru-Carb (99.2) CH2-Ru-Cl(2) 90.0 (87.1) Carb-Ru-Cl(2) (86.9) CH2-Ru-Cl(1) 94.3 (104.3) Cl(1)-Ru-Cl(2) (168.6) CH2-Ru-P 93.9 (97.1) Carb-Ru-P (163.2) Cl(1)-Ru-P 89.4 (89.9) Carb-N(1)-C(2) (112.1) N(1)-C(1)-N(2) (101.0) Important Torsion Angles (deg) Cl(1)-Ru-CH2-H N(1)-Carb-Ru-Cl Carb-Ru-CH2-H 88.6 N(1)-Carb-Ru-CH

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

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

Carbene sp2-Ru dz2 Don-Accep Bond
Ru-Carbene Sigma donor bond (Lewis base-Lewis acid) C sp Ru dz2 Carbene p- LUMO) Antibonding to N  lone pairs

Carbene sp2-Ru dz2 Don-Accep Bond
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 Ru-Carbene Sigma donor bond (Lewis base-Lewis acid) C sp Ru dz2 Singlet Carbene (Casey Carbene or Fisher carbene stablized by donation of N  lone pairs, leads to LUMO

Ru-dyz - Carbene py Don-Accep Bond
Ru dyz Lewis Base to Carbene py pi acid stabilizes the RuCH2 in the xy plane This aligns RuCH2 to overlap incoming olefin Ru dyz Lone Pair (Lewis base-Lewis acid) Ru dyz Carbene py LUMO Carbene p- LUMO) Antibonding to N  lone pairs

Ru LP and Ru-CH2 Acceptor Orbitals
the empty RuCH2 antibonding orbital overlaps the bonding pi orbital of the incoming olefin IF it is perpendicular to plane Ru dxy Lone Pair No special role Ru-CH2 * (antibonding) LUMO Acceptor for olefin  bond

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 pz 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 RuCH2 p* (antibonding) LUMO overlaps the  bonding orbital of incoming olefin stabilizing it in the confirmation required for metallacycle formation.

2 plausible intermediates for Ruthenium Metathesis
Trans Cis Trans is direct product of initiation. All previous mechanistic studies have assumed Trans. Either could explain propagation Trans Cis

But cis initiates more rapidly than trans
Previous mechanisms have assumed that the Ru-Cl bonds remain trans throughout the reaction  “trans” products To probe the mechanism Grubbs designed a ligand that could go into either cis or trans Cl structure For this constrained ligand, cis is more stable than trans by 0.8 kcal/mol But cis initiates more rapidly than trans

Use DFT QM to determine Structures and Energetics for Isomerization between cis and trans

Validation of DFT calculations
DG (kcal/mol) CH2Cl2: ε=9.1, R0=2.4A Experiment: K=3.5  ΔG = kcal/mol Theory: ΔG = kcal/mol Experiment: benzene solvent only observe trans  ΔG > 2 kcal/mol Theory: ΔG = 2.2 kcal/mol (ε=2.3, R0=2.6A) Theory: polar solvent (ε>20) leads to 100% cis Thus can tune stereochemistry of product by solvent polarity Not tested experimentally

Analysis of results Trans Cis
The strong dependence on solvent polarity results from the enormous difference in the dipole moment from the wavefunctions of the complexes (in methylene chloride) 1.5 Debye for trans and 12.4 Debye for cis This difference arises from the polarity in the Ru-Cl bonds, which cancel in the trans geometry. This marked difference in polarity translates to very different solvation energies calculated 14.8 kcal for trans and 22.7 kcal for cis, which dramatically increases the relative stability of the cis chloride structure.

Analysis of cis-trans Cl isomerization Rates of metathesis initiation
initiates much slower than experimentally kcal/mol Trans 11.7 barrier Cis 18.4 barrier Thus expect cis initiation should be much slower than trans: agrees with experiment

William A. Goddard, III, wag@kaist.ac.kr Lecture 22, November 24, 2009 Nature of the Chemical Bond with applications to catalysis, materials science,

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William A. Goddard, III, wag@kaist.ac.kr Lecture 22, November 24, 2009 Nature of the Chemical Bond with applications to catalysis, materials science,

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Presentation on theme: "William A. Goddard, III, wag@kaist.ac.kr Lecture 22, November 24, 2009 Nature of the Chemical Bond with applications to catalysis, materials science,"— Presentation transcript:

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