Homogeneous Catalysis Without Precious Metals: Cheap Metals for Noble Tasks

Transcription

1 Homogeneous Catalysis Without recious Metals: Cheap Metals for oble Tasks. Morris Bullock acific orthwest ational Laboratory ichland, Washington, USA Center for Molecular Electrocatalysis efrc.pnnl.gov an Energy Frontier esearch Center funded by by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Chemical Science oundtable Workshop on The ole of Chemical Science in Finding Alternatives to Critical esources September 29, 2011

2 Cheap Metals to eplace recious Metals (shameless hype) Abundant, Inexpensive metals (green) to replace precious (noble) metals (red) Mostly first row metals (but also includes Mo and W) published ov. 2010

3 Cost Savings: recious Metals vs. Cheap Metals Approximate Cost (US $) per Mole of Transition Metals Sc Ti V Cr Mn Fe Co i Cu Zn 13, Y Zr b Mo Tc u h d Ag Cd ,700 67,000 6, La Hf Ta W e Os Ir t Au Hg ,400 15,000 14,000 30,000 17, 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. t / i 4,000 d/cu 3,000 u/fe 2,000 t / Fe 10,000

4 Carbon-Carbon Bond Formation by Cross-Coupling eactions: Dominated by d catalysts obel rize in Chemistry (2010) to Heck, egishi and Suzuki "for palladium-catalyzed cross couplings in organic synthesis" eactions shown above from Suzuki s obel rize Lecture

5 d-catalyzed Buchwald-Hartwig C- Forming eactions Are Very owerful and Widely Used in Organic Synthesis (harmaceutical roducts, etc.) Buchwald, Acc. Chem. es. 1998, 31, Hartwig, Acc. Chem. es. 1998, 31, Hartwig, Organic Letters 2008, 10, ote d catalyst loading as low at 10 ppm!

6 d ot equired: Cheap Metals (Cu) Can Catalyze C-C and C- Bond Forming eactions C- Formation Catalyzed by CuI: Ma, Organic Letters 2003, 5, eview: Ma, D., in Catalysis Without recious Metals; Bullock,. M., Ed.; Wiley-VCH: Weinheim, 2010

7 ickel Catalysts for C-C Bond Formation Montgomery, J. Am. Chem. Soc., 2008, 130, eview: Montgomery, in Catalysis Without recious Metals; Bullock,. M., Ed.; Wiley-VCH: Weinheim, 2010.

8 d ot equired: Fe Catalysts for Organic Synthesis eview of Fe-Catalyzed eactions in Organic Synthesis Bolm, Chem. ev. 2004, 104, Caution: Trace impurities can lead to errors in identification of the true catalyst. Observation: higher yields with 98% pure FeCl 3 than with 99.99% pure FeCl 3 eaction first thought to be catalyzed by FeCl 3 was actually catalyzed by ~10 ppm Cu 2 O impurity: Buchwald and Bolm: Angew. Chem. Int. Ed. 2009, 48,

9 Advantages of Cheap Metals over recious 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 d in pharmaceuticals) But, the caveats: ot 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, i require higher catalyst loading (10%) than d Since pharmaceuticals (high value) are made on a small scale, there may be less motivation to develop catalysts based on cheap metals

10 Hydrogenation of C=O Bonds (Ketones, Aldehydes) to Give Alcohols: Dominated by u and h catalysts oyori (obel rize, 2001) eview: oyori, J. Org. Chem. 2001, 66, on-classical Mechanism; Coordination of Ketone to Metal OT required H M H O C M H O C H OT needed: Ketone Binding or Insertion

11 Successful eplacement of recious Metals: The ew (Fe, i, ) Will ot Look like the Old (t, h, ) 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 The old rules (applicable to precious metals) will often not apply to use of cheap metal catalysts ew 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.

12 Ionic Hydrogenation of C=O Bonds Using Molybdenum Catalysts. on-traditional Mechanism Bullock and Voges, J. Am. Chem. Soc. 2000, 122, eview: Bullock,. M., in Catalysis Without recious Metals; Bullock,. M., Ed.; Wiley-VCH: Weinheim, 2010 H 2 = H + + H Heterolytic cleavage of H 2 H H O O C C H H

13 An Iron Catalyst for Hydrogenation of C=O Bonds H 2 Delivery: H + from OH H - from u-h Catalysis requires regeneration by heterolytic cleavage of H 2. Casey and Guan, J. Am. Chem. Soc. 2009, 131,

14 ecent Iron Catalyst for Hydrogenation of C=O Bonds: Mild Conditions and Low Catalyst Loading 0.05 mole % Fe catalyst 4 atm H 2, room temperature Milstein, Angew. Chem. Int. Ed. 2011, 50, H. Morris, Angew. Chem. Int. Ed. 2008, 47,

15 Iron Catalyst for Hydrogenation of C=C Bonds Modern Alchemy : eplacing recious Metals with Iron Turnover Frequencies Up to 1800 h -1 for hydrogenation of 1-hexene Chirik, J. Am. Chem. Soc. 2004, 126, eview: Chirik, in Catalysis Without recious Metals; Bullock,. M., Ed.; Wiley-VCH: Weinheim, 2010

16 Catalysis and Electrocatalysis Are Important For enewable Energy Storage / Delivery Systems Hydrogen Oxidation Energy is stored in chemical bonds. Interconversion between electricity and fuels will require catalysts for formation or cleavage of bonds to H. H 2 2 e H + Hydrogen roduction heap Metal$ for oble tasks (i, Fe, Co) t Multi-proton, multi-electron reactions

17 endant Amines as roton elays in the Second Coordination Sphere Second Coordination Sphere Control roton Transfer Dan DuBois First Coordination Sphere Control Energies of Catalytic Intermediates oles of roton elays in Catalytic eactions Accelerate intra- and intermolecular proton transfers Stabilize binding of H 2 or other ligands to a metal Mary akowski DuBois Lower the barrier for heterolytic cleavage of H 2 Facilitate coupled proton-electron transfer reactions

18 energy Energy Matching of roton and Hydride Acceptor Abilities Turnover freq s -1 Overpotential 0.1 V Fontecilla-Camps et al., Chem. ev. 2007, 107, 4273 Structure-Function elationships of [FeFe] and [Fei] Hydrogenases H 2 uncatalyzed catalyzed 2H + + 2e - reaction coordinate

19 ickel Catalysts for Oxidation of H 2 H 2 2 e H + o proton relay Overpotential = 0.7 V Turnover frequency < 0.5 s -1 Two Flexible proton relays G (H 2 ) = -6.0 kcal/mol Overpotential < 0.1 V Turnover frequency < 0.5 s -1 Two ositioned proton relays G (H 2 ) = -3.1 kcal/mol Turnover frequency 10 s -1

20 roposed Mechanism of Catalytic Oxidation of H 2 H, not ih avoids i III intermediate

21 H 2 Oxidation Catalyzed by [i( Cy 2 tbu 2) 2 ] 2+ Hydrogen Oxidation H 2 2 e H + Jenny Yang i cat /i p ~ 22 TOF ~ 50 s -1 (1 atm H 2 ) Estimated ΔG H2 = 6 kcal/mol for i( Cy 2 tbu 2) 2 2+ ot inhibited by CO Chem. Comm. 2010, 8618

22 Current (µa) Hydrogen Oxidation and roduction by a Single Complex, [i( h 2 CH 2 CH 2 OMe 2) 2 ] 2+ Current (µa) Estimated ΔG H2 = +1 kcal/mol Stuart Smith Catalytic H 2 Oxidation and roduction Catalytic H + eduction Overpotential = 24 mv Cp 2 Fe +/0 II/I I/0 otential vs Cp 2 Fe +/0 (V) otential vs Cp 2 Fe +/0 (V) 360 mv positive shift from protonation before reduction

23 Dependence of Catalytic ate of H 2 roduction on pk a of endant Base Hydrogen roduction 2 e H + H 2 X X h h H H i h h 2+ X X k = 740 s -1, overpotential = 280 mv for X = Br Acid = [(H)DMF] + OTf - pk a (CH 3 C) = 6.1 J. Am. Chem. Soc. 2011, 133, John oberts Uriah Kilgore

24 A 2 1 Ligand: One endant Amine E 1/2 = V (overlapping II/I and I/0 couples) rof. Monte Helm (sabbatical visitor at L from Fort Lewis College, Colorado)

25 Very Fast Catalysis For H 2 roduction revent formation of catalytically inactive isomers: TOF = 106,000 s -1 Overpotential = 0.62 V i H Catalyst + H + + H 2 O [i] = 1.0 mm [DMF(H)] + = 0.42 M [H 2 O] = 1.2 M = 10 V s -1 Faster than the [FeFe] Hydrogenase Enzyme (9,000 s -1 at 30 C; Frey, ChemBioChem 2002, 3, 153) Science 2011, 333, 861

26 Electron-withdrawing C 6 F 5 on Cp - to Increase Acidity of Fe(H 2 ) + Bz C 6 F 5 t Bu C 6 F 5 Bz t Bu Fe H. A. Deck, Coord. Chem. ev. 2006, 250, aul Deck et al., Organometallics 1996, 15, Leo Liu -Fe-: Fe-H: 1.49 Å Fe-: 2.15 Å 9

27 Catalytic H 2 Oxidation by [ C6F5 CpFe( tbu 2 Bz 2)(H)] t k 2 s -1 Leo Liu Catalytic chemical oxidation of D 2 by Cp 2 Fe + F 6 - was confirmed by detection of i r 2 Et-D + by 2 H M, as well as [Cp 2 Fe] by UV.

28 Electrochemical Oxidation of Formate Using [i( 2 2 ) 2 ] 2+ Catalysts HCOO - H + + CO 2 + 2e - Brandon Galan First example of a molecular (homogeneous) catalyst for oxidation of formate. First example of a catalyst OT based on a precious metal. DuBois (L), Kubiak, (UC-SD) et al., J. Am. Chem. Soc. 2011, 133,

29 Substantial rogress on eplacing recious Metals With Abundant, Inexpensive Metals Will equire Years of esearch ( and Funding! ) 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.

30 Conclusions Homogeneous Catalysis Without recious 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 oble 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. otable successes found for replacing d with Cu, i or Fe for organic synthesis t for fuel cells and energy applications: fundamental research shows that i or Fe can catalyze the same reactions

31 Thanks to: U.S. Department of Energy (Energy Frontier esearch 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 Jenny Yang Uriah Kilgore Leo Liu Mary akowski DuBois John oberts Stuart Smith

32 H 2 Addition to i( 2 2 ) 2 2+ : Two-Electron eduction of i(ii) to i(0). rotonation of Two endant Amines; o i-h Two -H Bonds Formed o i-h

33 roposed Mechanism of Catalytic Oxidation of H 2 inched -H- wrong isomers can slow down catalytic activity

34 H 2 Oxidation or H 2 roduction: Tuning the Free Energy of Hydrogen Addition Tuning DG o H2 i 2+ [i( 2 2 ) 2 ] 2+ + H - [Hi( 2 2 ) 2 ] + -DG o H- DG o H2 more negative H 2 oxidation catalyst [Hi( 2 2 ) 2 ] + + H + [Hi( 2H 2 )] pk a H 2 H + + H - 76 kcal/mol [i( 2 2 ) 2 ] 2+ + H 2 [Hi( 2H 2 )] 2+ DG o H2 Hydride acceptor ability H 2 2 e H + DG o H2 more positive H 2 production catalyst roton acceptor ability Increase hydride acceptor ability; increase size of substituent on Increase proton acceptor ability; increase pk a of -H + t Bu > Cy > h t Bu > Me > Bz > h

35 Catalysis of H 2 Oxidation or H 2 roduction: Tuning the Free Energy of Hydrogen Addition H 2 oxidation [i( h 2 Me 2) 2 ] kcal/mol [i( h 2 Bz 2) 2 ] kcal/mol [i( h 2 h 2) 2 ] 2+ (9 kcal/mol) Hydride acceptor ability [i( Cy 2 tbu 2) 2 ] 2+ ( 6 kcal/mol) [i( Cy 2 Bz 2) 2 ] kcal/mol [i( Cy 2 h 2) 2 ] 2+ (1 kcal/mol) H + reduction

36 Mechanism of Formate Oxidation solv i 2+ -solv +HCO 2 - O C H O i 1+ Aaron Appel -1e - +solv rds -CO 2 Brandon Galan i 1+ -1e - -H + H i 1+ John Linehan Galan, B..; Schoffel, J.; Linehan, J. C.; Seu, C.; Appel, A. M.; oberts, J. A.; Helm. M. L.; Kilgore, U. J.; Yang, J. Y.; DuBois, D. L.; Kubiak, C.. J. Am. Chem. Soc. 2011, 133,

37 Electrocatalytic Oxidation of Formate HCO 2 - CO 2 + H + + 2e - Kinetic Studies First order in catalyst Initially first order in [HCO 2- ] TOF (s -1 ) endant amine is required for catalysis and influences the catalytic rate.

38 Et 2 Et 2 i Et 2 Et 2 H 2 Et 3 2+ Me Et 2 Et 2 i Et 2 Et 2 2+ Me i H 2 H 2 2+ h 2+ h h i h h h - H 2 2 H + i HEt 3 H Me Et 2 H Et 2 i Et 2 Et 2 2+ Me H 2+ i H h h H h i h H h h h h 2+ TOF < 0.2 s -1 otential = 0.0 V (vs. Cp 2 Fe +/0 ) TOF < 0.2 s -1 otential = V TOF = 10 s -1 otential = V TOF = 106,000 s -1 otential = -1.1 V