Lecture 21 February 23, 2011 CH4 CH 3 OH catalysis. Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy. William A. Goddard, III, [email protected] 316 Beckman Institute, x3093 - PowerPoint PPT Presentation
•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
Projects in Catalysis: First establish mechanism then use mechanism to
Cl1. 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 ratesThis provides the conceptual framework to interpret experiments
2. Validation: Predict new experiments to test mechanism3. Lead discovery: Combinatorial Computational Rapid PrototypingIn 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 experimentsCritical to new role of theory: accuracy and reliability for novel systemsMust trust the theory well enough to do only 1 to 10% of the systemsFocus experiments on these 1% to 10% predicted to be best
t½ = 15 minRate ok, but decompose far too fast. Why?
(NH3)2PtCl2TOF: 1x10-2 s-1
t½ = 15 minRate ok, but decompose far too fast. Why?
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hoursNot decompose but rate 10 times too slowAlso poisoned by H2O productHow improve rate and eliminate poisoning
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hoursNot decompose but rate 10 times too slowAlso poisoned by H2O productHow improve rate and eliminate poisoning
Experimental discovery: Periana et al., Science, 1998
Catalytica: Many $$$ trying to solve these problems experimentally, failed, cancelled project. Periana came to USC, teamed up with Goddard, Chevron funded. Found success
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
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 atomsUse these forces to determine optimum geometry of solute in solution.Can treat solvent stabilized zwitterionsDifficult 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
Extremely important for these systems (pH from -10 to +30) in very highly polar solvents: accuracy of predicting Solvation effects in QM
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)
The Poisson-Boltzmann Continuum Model in Jaguar/Schrödinger is extremely accurate
Pipes and Meyer, Inorg. Chem. 1986, 25, 4042.Meyer, et al. Inorg. Chem. 1984, 23, 1845.
OsV
OsIV OsIII
OsII
(trpy)Os(OHn)3
1 Volt
0.5 V
Evaluating multi-oxidation state cycles for nucleophilic metals
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.
After first turnover, the catalyst is (bpym) PtCl(OSO3H) not
(bpym)PtCl2
Start here
1st turnover
Catalytic step
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L2PtCl2 – Water Inhibition
Theory: Complexation of water to activated catalyst is 7 kcal/mol exothermic, making barrier 7 kcal/mol higher. Product formation 0Thus inhibition is a ground state effectChallenge: since H2O is a product of the reaction, must make the catalyst less attractive to H2O but still attractive to CH4
Theory: Complexation of water to activated catalyst is 7 kcal/mol exothermic, making barrier 7 kcal/mol higher. Product formation 0Thus inhibition is a ground state effectChallenge: since H2O is a product of the reaction, must make the catalyst less attractive to H2O but still attractive to CH4
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?
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?
Mechanism for the C‑H activation of methane by the Periana-Catalytica catalyst. Free energies (kcal/mol) at 500 K including solvation by H2SO4.
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
• QMRP: computational analogue of combinatorial chemistry• Three criteria for CH4 activation:
1. 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
2. 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.
3. 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.
4. Do real barriers only when 3 is less than 35 kcal/mol
Small set systems for lab experiment
Muller, Philipp, Goddard Topics in Catalysis 2003, 23, 81
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
Fe(II), sd6, S=0 (singlet, low-spin), cationB3LYP/LACVP* (LACVP** for )
FeN
FeN
NN
NN
-24.5
A
+3.8
CB
C
0.0
-3.4
B(1)
+
=
-0.6
N
MN
N
H
N
MN
NH
B(2)
N
MN
NH
N
MN
N
H
N
MN
NH
+9.1
w/o bulkysidegroups
with bulkysidegroups
-26.3
-8.6-13.5
-17.1
+0.4
EWe 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.
We considered first a class of tri-site ligands analogous to those studied by Brookhart in Fe and Cr based catalysts for olefin polymerization.
• The mechanism is a concerted oxidative hydrogen migration with a 14.1 kcal/mol barrier.• The d-orbital used to form the Os-H bond stays at the same energy during the reaction.
• Singly occupied orbital is ‘delta’ with respect to Os-H bond, since a doubly occupied d-orbital is used to bond to the hydrogen.
• 2.42 A Os-C bonds suggest moderate backbonding from Os d-orbitals to benzene.
CH activation by OsIII hydroxides:We’d like to work in an inert medium, like water.The higher oxidation state (e.g. OsIII vs OsII) instantly extends the catalyst’s stability another ~0.5V, but the CH activation barriers are much higher than for OsII-OH and OsIII-Ph:
Through several ligand/anion combinations, no OsIII-OH has yielded a calculated H-CH3 activation barrier <35 kcal/mol.
• OH- is bound more strongly to OsIII (than to OsII) due to the decrease in d-p repulsion, so the coordination step is endothermic.
• The singly-occupied orbital “follows” the remaining hydroxide lone-pair, making the hydroxide less basic. Since the hydroxide lone-pair accepts the hydrogen from methane, the cleavage barrier is also high. (This contrasts the Os-Ph case, where a doubly occupied d-orbital was oriented to stabilize the hydrogen in the TS.)
Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.180°C, 27 bar CH4, TOF 10-3 s-1
Cycle: oxidation → CH activation →
SN2 attack
Accessibility of both AuI and AuIII oxidation states prevents deactivation due to oxidization of catalyst1. CH activation by electrophilic substitution. 2. Functionalization by nucleophilic attack by HSO4
Electronegative Metals Pt, Au, Hg, Pd: ∙ good selectivity, rates, and stability∙ product protection by esterification -but-∙ inhibited by water and methanol∙ require strong oxidantsConsequently we shifted to the nucleophilic paradigm, which can coordinate CH4 under milder acid or concentrated base conditions.
Early successes in methane functionalization used the electrophilic
paradigm:N
N
Pt
Cl
Cl
N
NH3N
H3N
Pt
Cl
Cl
(NH3)2PtCl2TOF: 1x10-2 s-1
t½ = 15 min
(NH3)2PtCl2TOF: 1x10-2 s-1
t½ = 15 min
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hours
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hours
Pt: Periana et al., Science, 1998Au: Periana, wag; Angew. Chem. 2004Hg: Periana et al., Science, 1993
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).
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
CH Activation Functionalization
LMn-OH
LMn-CH3
CH4
H2O Y
YO
+ H2O
+ CH3OH
Net Reaction: CH4 + 1/2 O2 CH3OH
Oxidation
1/2 O2
Se
O
OHH3C
Se
O
OHHORe(CO)5-CH3
Re(CO)5-OH
Full cycle
76
Homogeneous CH4 functionalization: how to best choose new metals
HX + Electron-poor
methyl groups
Electron-rich methyl groups
Charge transfer
Our QM mechanistic studies for a variety of complexes from AuIII to ReI show the continuum of charge transfer to methane
77
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.
Pt
H
N
ClN
CH3
N
HN
ReO CH3
O
O
-46.4 kcal/mol
CH4
31.0
25.2
22.721.3
21.3
17.416.5
0.0
M OH2NN
CH3N
N
H2N
H2N
H2N
H2N
q+
2+
M OH2NN
CH3N
N
q+M = Re Ru Os
Re = MRuOs
CH activation and functionalization by nucleophilic d6 metalsM-CH3 polarization based on C1s chemical shift
78
Ongoing Work in Homogeneous CH4 Functionalization
M OHNN
OHN
N+ CH4
MOH
NN
HN
N
+1
CH3
-OH-X
X
X
X
X = NO2, H, NH2, O-
M = RuII, OsII, ReI
MOH
NN
H
N
N
+1
H3C
X
X
X
X
X
X
X
X
Insertion
Substitution
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 -donating ligand substituents.
MOH
NN
CH3N
N
X
X
X
X
MOH
NN
CH3N
N
X
X
X
X
O
Os(acac)2
O
Going forward, we are considering the kinetics of these steps using d5 and d4 metals and new coordinated bases (i.e. –NH2).
79
metal substituent
RuII X=H -1.1 0.0 5.2 31.1 12.7 n.a. n.a.
=NH2 -7.5 0.0 -1.7 23.6 9.4 n.a. n.a.
OsII X=H 7.8 0.0 11.1 41.8 21.7 31.7 30.7
=NH2 1.5 0.0 3.1 34.5 17.1 20.5 17.0
ReI X=H 4.8 0.0 4.0 41.9 26.5 26.9 18.5
=NH2 0.1 0.0 -0.6 36.6 23.6 19.4 7.1
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.
OH2
L4M OH
OH
L4M OH L4M OH
H3C
L4M OH
HCH3
L4M OH2
CH3
L4M OH
H CH3
L4M OH
H
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.
insertionsubstitution
Insertion barriers can be tuned over an extreme range by varying the ligand and metal. Substitution barriers cannot be similarly tuned.
80
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.
Nucleophilic attack
MCH3
SeOO
Se OHHO
O
H2O H3C Se OH
O
HX +
CH3
M
XCH3X
M CH3
O
Y
OCH3M
Y
H2O2
IO4-
PhIO
M CH3 O
M'
CH3O
M'
O
M'
Transalkylation
Baeyer-Villiger
Reductive elimination
Electrophilic attack
CH3M X- CH3X
Periodic tableBarr
ier
Reductive functionalization mechanisms (red. elim., SN2) well known for late metals (M-CH3
+).With Periana we have sought complimentary mechanisms appropriate for electron rich metals:
81
We explore functionalization mechanisms in which the oxidant is a higher oxidation state of the hypothetical CH activation catalyst:
CH3
G 27.6 kcal/mol
OsIIIHO
OH
OsVI
O
O
HO(trpy)OsIII(OH)2(CH3)
(trpy)OsVI(O)2(OH)+
+OsIV
O
O
O
OCH3
OHOsVI
O
O
O
O
O
O
Os
O
O
O
O
O
O Os(acac)2
CH3
G 23.0 kcal/mol
G 37.0 kcal/mol
+
OsIV O
O
O
O
CH3OH
OsVIO
O
O
O
O O
OsVI + OsIV
L = (acac)2 OsVI + OsIII
L = terpyridine
Going forward in homogeneous CH4 functionalization
82
Catalytic cycle: Au in H2SO4/H2SeO4
Y=O
HX
CH4
Y
M CH3
MIII
X
XMI X
HX
CH4
MIII
X
CH3Y=O Y
X-
CH3X
X
X
Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.180°C, 27 bar CH4, TOF 10-3 s-1
Cycle: oxidation → CH activation →
SN2 attack
Accessibility of both AuI and AuIII oxidation states prevents deactivation due to oxidization of catalyst1. CH activation by electrophilic substitution. 2. Functionalization by nucleophilic attack by HSO4
Late Transition MetalsMechanistic 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.
Middle Transition MetalsNow 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
-.
Plan for bringing to pilot new CH4 to liquids catalysts
