Homogeneous Catalysis Without Precious Metals: “Cheap Metals for Noble Tasks”

R. Morris Bullock

Pacific Northwest National Laboratory Richland, Washington, USA

Center for Molecular Electrocatalysis

efrc.pnnl.gov an Energy Frontier Research Center funded by by the

U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Chemical Science Roundtable

Workshop on “The Role of Chemical Science in Finding Alternatives to Critical Resources”

September 29, 2011

published Nov. 2010

(shameless hype)

Cheap Metals to Replace Precious Metals

Abundant, Inexpensive metals (green) to replace precious

(noble) metals (red)

Mostly first row metals (but also includes Mo and W)

Cost Savings: Precious Metals vs. Cheap Metals

Approximate Cost (US $) per Mole of Transition Metals

Sc Ti V Cr Mn Fe Co Ni Cu Zn

13,000 13 180 8 5 3 28 8 2 6

Y Zr Nb Mo Tc Ru Rh Pd Ag Cd

660 51 72 23 --- 5,700 67,000 6,600 240 23

La Hf Ta W Re Os Ir Pt Au Hg

600 820 500 37 5,400 15,000 14,000 30,000 17,000 49

Costs calculated in US dollars from Strem '08-'10 catalog using lowest cost metal powder with purity ≥99%. Mercurcy cost calculated from lowest cost pure elemental form.

Pt / Ni ≈ 4,000 Pd/Cu ≈ 3,000 Ru/Fe ≈ 2,000

Pt / Fe ≈ 10,000

Carbon-Carbon Bond Formation by Cross-Coupling Reactions: Dominated by Pd catalysts

Reactions shown above from Suzuki’s Nobel Prize Lecture http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2010/suzuki-lecture.html#

Nobel Prize in Chemistry (2010) to Heck, Negishi and Suzuki "for palladium-catalyzed cross couplings in organic synthesis"

Pd-Catalyzed “Buchwald-Hartwig” C-N Forming Reactions Are Very Powerful and Widely Used in

Organic Synthesis (Pharmaceutical Products, etc.)

Hartwig, Organic Letters 2008, 10, 4109-4112

Buchwald, Acc. Chem. Res. 1998, 31, 805-818 Hartwig, Acc. Chem. Res. 1998, 31, 852-860

Note Pd catalyst loading as low at 10 ppm!

Pd Not Required: Cheap Metals (Cu) Can Catalyze C-C and C-N Bond Forming Reactions

IH3C

NH2

+CuI

K2CO3

H3C NH2

N

H

CO2H

C-N Formation Catalyzed by CuI: Ma, Organic Letters 2003, 5, 2453-2455

Review: Ma, D., in Catalysis Without Precious Metals;

Bullock, R. M., Ed.; Wiley-VCH: Weinheim, 2010

Montgomery, J. Am. Chem. Soc., 2008, 130, 8132–8133

Review: Montgomery, in Catalysis Without Precious Metals; Bullock, R. M., Ed.; Wiley-VCH: Weinheim, 2010.

Nickel Catalysts for C-C Bond Formation

Pd Not Required: Fe Catalysts for Organic Synthesis

Review of Fe-Catalyzed Reactions in Organic Synthesis Bolm, Chem. Rev. 2004, 104, 6217-6254

Caution:

Trace impurities can lead to errors in identification of the true catalyst.

Observation: higher yields with 98% pure FeCl3 than with

99.99% pure FeCl3

Reaction first thought to be catalyzed by FeCl3 was actually catalyzed by ~10 ppm Cu2O impurity:

Buchwald and Bolm: Angew. Chem. Int. Ed. 2009, 48, 5586-5587

Advantages of Cheap Metals over Precious Metals   MUCH less expensive (> 1000 x)   Environmentally more benign   Can tolerate more losses of metal in an industrial process (vs. recovery/

recycling a key issue for using precious metals)   Less toxic (FDA will allow more residual Fe than Pd in pharmaceuticals)

But, the caveats:   Not as well-studied, though receiving increasing attention   Scope of organic reactivity not as broad (yet)

  Aryl iodides more reactive (but more expensive) than aryl chlorides   Functional group tolerance not always as high   Often Fe, Cu, Ni require higher catalyst loading (10%) than Pd   Since pharmaceuticals (high value) are made on a small scale, there

may be less motivation to develop catalysts based on cheap metals

Hydrogenation of C=O Bonds (Ketones, Aldehydes) to Give Alcohols: Dominated by Ru and Rh catalysts

Ru

N NSO2(C6H4CH3)

PhPh

H

H

H

Ru

N

H

H

C

O

!

!

Noyori (Nobel Prize, 2001)

Review: Noyori, J. Org. Chem. 2001, 66, 7931-7944

Non-classical Mechanism; Coordination of Ketone to Metal NOT required

O

C

RR

M

H

O

C

RR

M

HH

H

NOT needed:

Ketone Binding or Insertion

“Most people are more comfortable with old problems than with new solutions. ~Author Unknown

“The conventional view serves to protect us from the painful job of thinking.”

~J.K. Galbraith

Successful Replacement of Precious Metals: The New (Fe, Ni, …) Will Not “Look” like the Old (Pt, Rh, …)

The old rules (applicable to precious metals) will often not apply to use of cheap metal catalysts

New ligands will usually be required for successful

design of new catalysts with different metals.

The mechanism of catalysis by the new metals will often be different than those found for precious metals.

H2 = H + + H – Heterolytic cleavage of H2

O

CEtEt 23 °C

O

CEtEt

HBAr'4–Mo

PR3

C

C

O CEt

EtO

O

H2 (4 atm)+

H

Bullock and Voges, J. Am. Chem. Soc. 2000, 122,12594-12595

Review: Bullock, R. M., in Catalysis Without Precious Metals; Bullock, R. M., Ed.; Wiley-VCH: Weinheim, 2010

“Ionic Hydrogenation” of C=O Bonds Using Molybdenum Catalysts. Non-Traditional Mechanism

O

CRR

O

CRR

H

H

H

H

MH

HM H

HydridicAcidic

H H

M

H2

An Iron Catalyst for Hydrogenation of C=O Bonds

Casey and Guan, J. Am. Chem. Soc. 2009, 131, 2499-2507

H2 Delivery: H+ from OH H- from Ru-H Catalysis requires regeneration by heterolytic cleavage of H2.

C

O

RR

+H2 (3 atm)

catalystregeneration

C

O

RR

H

H

Fe

C

CHO

O

O

SiMe3

SiMe3

H

Fe

C

CO

O

O

SiMe3

SiMe3

25 °C

Fe

PiPr2

PiPr2

CON

Br

HMilstein, Angew. Chem. Int. Ed. 2011, 50,

2120-2124.

Recent Iron Catalyst for Hydrogenation of C=O Bonds: Mild Conditions and Low Catalyst Loading

0.05 mole % Fe catalyst 4 atm H2, room temperature

R. H. Morris, Angew. Chem. Int. Ed. 2008, 47, 940-943.

N

N

N

Fe

iPr

iPr

N

N

N

N

IIiPr

iPr

Iron Catalyst for Hydrogenation of C=C Bonds “Modern Alchemy”: Replacing Precious Metals with Iron

Turnover Frequencies Up to 1800 h-1 for hydrogenation of 1-hexene

Chirik, J. Am. Chem. Soc. 2004, 126, 13794-13807

Review: Chirik, in Catalysis Without Precious Metals; Bullock, R. M., Ed.; Wiley-VCH: Weinheim, 2010

Catalysis and Electrocatalysis Are Important For Renewable Energy Storage / Delivery Systems

2 e- + 2 H

+H2

Hydrogen Production

Hydrogen Oxidation

Pt O2 + 4 H+ + 4 e- 2 H2O

N2 + 6 H+ + 6 e- 2 NH3

¢heap Metal$ for Noble tasks

(Ni, Fe, Co)

Multi-proton, multi-electron reactions

Energy is stored in chemical bonds. Interconversion between electricity and fuels will

require catalysts for formation or cleavage of bonds to H.

Second Coordination Sphere Control Proton Transfer

First Coordination Sphere Control Energies of Catalytic Intermediates

Roles of Proton Relays in Catalytic Reactions   Accelerate intra- and intermolecular proton transfers   Stabilize binding of H2 or other ligands to a metal   Lower the barrier for heterolytic cleavage of H2

  Facilitate coupled proton-electron transfer reactions

Pendant Amines as Proton Relays in the Second Coordination Sphere

Dan DuBois

Mary Rakowski DuBois

Ni

P

P

N

N

P

P

N

N

R

R R

R

R'R'

R'R'

H

H

Energy Matching of Proton and Hydride Acceptor Abilities

2H+ + 2e-

ener

gy H2

reaction coordinate

uncatalyzed catalyzed

Turnover freq. ≈ 104 s-1 Overpotential ≈ 0.1 V

Fe

S

Fe

S

N

COCNC

ONC

OC

S

cys

[Fe4S4]

Proposed Structure of [FeFe]-Hydrogenase Active Site

Fe

S

Fe

S

N

COCN

H

CONC

OC

S

cys

[Fe4S4]

H

Fe

S

Fe

S

N

COCN

H

CONC

OC

S

cys

[Fe4S4] HH2

Fontecilla-Camps et al., Chem. Rev. 2007, 107, 4273 Structure-Function Relationships of [FeFe] and [FeNi] Hydrogenases

Nickel Catalysts for Oxidation of H2

Overpotential = 0.7 V Turnover frequency < 0.5 s-1

ΔG°(H2) = -6.0 kcal/mol Overpotential < 0.1 V Turnover frequency < 0.5 s-1

No proton relay

Two Flexible proton relays

Ni

PP

PP

N

Bz

NBz

N

Bz

NBz

2+

Cy

Cy

Cy

Cy

Two Positioned proton relays

ΔG°(H2) = -3.1 kcal/mol Turnover frequency 10 s-1

2 e- + 2 H+H2

NiPP

PPNMe

NMe

Et2

Et2Et2

Et2

2+

NiPP

PP

Et2

Et2Et2

Et2

2+

Ni

PP

PP

N

N

N

N

Ni

PP

PP

N

N

N

N

2+

H

H

NiP

P

N

N

+H

+

H2

- e-

2+

Base

HBase+

- e-

Base

HBase+

P

P

N

N

NiP

P

N

N

+H

P

P

N

N

NiP

P

N

N

H

P

P

N

N

2+

NiP

P

N

NP

P

N

N

NiPP

PP

N

N

N

N

2+H

H

Proposed Mechanism of Catalytic Oxidation of H2

NH, not NiH avoids NiIII intermediate

H2 Oxidation Catalyzed by [Ni(PCy2NtBu

2)2]2+

Jenny Yang

Estimated ΔGH2 = −6 kcal/mol for Ni(PCy2NtBu

2)22+

Not inhibited by CO

icat/ip ~ 22 TOF ~ 50 s-1 (1 atm H2)

2 e- + 2 H+H2

Hydrogen Oxidation

Chem. Comm. 2010, 8618

Dependence of Catalytic Rate of H2 Production on pKa of Pendant Base

Acid = [(H)DMF]+OTf-

k = 740 s-1, overpotential = 280 mV for X = Br

pKa(CH3CN) = 6.1

Uriah Kilgore

2 e- + 2 H+ H2

Hydrogen Production

John Roberts J. Am. Chem. Soc. 2011, 133, 5867-5872

NiPP

PP

N

N

N

N

2+H

HPh

Ph

Ph

X

X

X

X

Ph

A P2N1 Ligand: One Pendant Amine

Prof. Monte Helm (sabbatical visitor at PNNL from Fort Lewis College, Colorado)

P

P

P

N

Ph

Ph

2 + Ni

P

P

P

Ph

PhN

Ph

PhN

2+

(BF4-)2

Ph

Ph

Ph

[Ni(CH3CN)6]2+

E1/2 = -1.13 V (overlapping II/I and I/0 couples)

Catalyst + H+ + H2O [Ni] = 1.0 mM

[DMF(H)]+ = 0.42 M [H2O] = 1.2 M υ = 10 V s-1

Very Fast Catalysis For H2 Production Prevent formation of catalytically inactive isomers: Ni

P

PN

N

H

R

R

R'

R'

P

Ni

P

P

P

Ph

PhN

Ph

PhN

2+

Ph

Ph

TOF = 106,000 s-1

Overpotential = 0.62 V

Faster than the [FeFe] Hydrogenase Enzyme (9,000 s-1 at 30 °C; Frey, ChemBioChem 2002, 3, 153)

Science 2011, 333, 861

X

Electrochemical Oxidation of Formate Using [Ni(P2N2)2]2+ Catalysts

Brandon Galan

HCOO- H+ + CO2 + 2e-

First example of a molecular (homogeneous) catalyst for oxidation of formate. First example of a catalyst NOT based on a precious metal. DuBois (PNNL), Kubiak, (UC-SD) et al., J. Am. Chem. Soc. 2011, 133, 12767-12779.

NiP

P

N

N

2+

P

P

N

N

Ph

Ph Ph

Ph

R

R

R

R

“It's not that I'm so smart, it's just that I stay with problems longer.” (Albert Einstein)

Scientists engaged in basic research sometimes have more patience than funding agencies, but substantial

progress over the last ~15 years shows that systematic, focused studies can lead to catalysts of cheap metals that

have high activity.

Substantial Progress on Replacing Precious Metals With Abundant, Inexpensive Metals Will Require

Years of Research (…and Funding! )

Conclusions – Homogeneous Catalysis Without Precious Metals

Cost of abundant metals can be >1000 x less than precious metals In addition to cost savings, cheap metals are often more environmentally benign.

“Cheap Metals for Noble Tasks” research (academic / basic research) gaining more interest and recognition in recent years.

Catalysts using cheap metals will often require different ligands than precious metals, and the mechanism of reaction will be different.

Fundamental research (and funding) needed to sustain/accelerate discovery and development of catalysts with abundant metals.

Notable successes found for replacing Pd with Cu, Ni or Fe for organic synthesis

Pt for fuel cells and energy applications: fundamental research shows that Ni or Fe can catalyze the same reactions

Thanks to: U.S. Department of Energy (Energy Frontier Research Center); Office of

Science, Office of Basic Energy Sciences

U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences

Dan DuBois

Mary Rakowski DuBois

Jenny Yang

John Roberts

Uriah Kilgore Leo Liu