Basics of Catalysis and Kinetics

Transcription

1 Basics of Catalysis and Kinetics Nobel laureates in catalysis: Haber (1918) Ziegler and Natta (1963) Wilkinson, Fischer (1973) Knowles, Noyori, Sharpless (2001) Grubbs, Schrock, Chauvin (2006) Ertl (2007) Heck, Negishi, Suzuki (2010)

2 Definition The concept of catalyst was first introduced by Berzelius in the 19th century. Subsequently, Ostwald came up with the definition that we still use today: A catalyst is a substance that increases the rate of a chemical reaction, without being consumed or produced Berzelius, J. J. Ann. Chim. Phys. 1836, 61, Ostwald, W. Z. Phys. Chem. 1894, 15,

3 The Catalyst and the Rate of a Reaction ΔG non catalyzed catalyzed A B Reaction coordinate The catalyst changes only the rate of the reaction, and does not change the thermodynamics. However, the temperature can affect equilibrium concentrations: ΔG = ΔH - T ΔS

4 Historical basis for the Analysis of Catalytic Kinetics The Michaelis-Menten Kinetics

5 Thermal, Catalyzed, Inhibited, and Promoted Reactions I II III IV uncatalyzed process catalysis inhibition (no catalysis) stoichimetric promotion (no catalysis) A A A cat A A B cat A inhib B inhib A prom B SM is stabilized less than TS B (+ cat) SM is stabilized more than TS --> the uncatalyzed path is preferred B a further reaction is needed to liberate B B prom

6 Thermal, Catalyzed, Inhibited, and Promoted Reactions NH O Ar O CCl 3 xylenes 140 C Ar HN CCl 3 type I CCl 3 CCl 3 O NH PdCl 2 (PhCN) 2 (5 mol%) O NH THF, rt type II O NH O 1. H B NMe 2 2. H 2, Pd/C N N N N Pt N N Me n trispyrazolylborate-platinum(methyl) NH O N N N Pt N H B N N NMe 2 O type III HN O PdCl 2 (MeCN) [H - ] 2 N Pd Cl O stable N O type IV

7 Thermal versus TM-catalyzed Paths Often the catalytic reaction occurs by a mechanism that is completely different from that of the corresponding uncatalyzed process. In the catalyzed process the reaction typically occurs by more steps, but the activation energy of each step is lower than that of the uncatalyzed process. The resulting global energetic barrier is thus lower that of the uncatalyzed reaction. Alkene hydroboration with (RO) 2 BH is an example. - Oxidative addition - Coordination - Migratory insertion - Reductive elimination - Alkene-borane π-complex - Concerted 4-membered step

8 Efficiency of the Catalytic Cycle (TON and TOF) A catalyst is not eternal and after certain number of turns it decomposes, thereby losing its catalytic activity. It is thus useful to define the turnover number (TON) of a catalyst as the number of moles of substrate that a mole of catalyst can convert before becoming inactivated. Of course, the TON is a function of the chemical yield. The turnover frequency (TOF) is the number of moles of product per mole of catalyst per unit time, that is to say, the turnover number per unit time. As the rate may change with the time, reported TOF s are usually average. TON = moles of product moles of catalyst used TOF = TON h B(OH) 2 + Br 2.0 equiv O 1.0 equiv cat: {[PdCl(allyl)] 2 / L} : 1/2 K 2 CO 3 xylenes, 120 C, 7d ArBr / cat TON = O L = TOF = Chem. Commun., 2011, 47,

9 The Hammond s Postulate The Hammond's postulate is a hypothesis derived from transition state theory, and states that the structure of a transition state resembles that of the species nearest to it in free energy. That is to say, that the transition state of an endothermic reaction resembles the products (late transition state), while that of an exothermic reaction resembles the reactants (early transition state). Hammond, G. S. (1955). J. Am. Chem. Soc. 1955, 77,

10 The Curtin-Hammett Principle The Curtin-Hammett principle states that, for a reaction that has a pair of reactive intermediates or reactants that interconvert rapidly (for example conformers), each going irreversibly to a different product, the product ratio will depend both on the free energy of the transition state going to each product (usually essentially) and the difference in energy between the two conformers (usually to a little extent). Hence, the product distribution will not necessarily reflect the equilibrium distribution of the two intermediates Seeman J. I. (1983 Chemical Reviews 1983, 83,

11 Example of a Generic Tolman Catalytic Cycle precatalyst Product cat I Substrate promoter or co-catalysts may be needed cat IV turnover limiting step resting state cat III cat II B (i.e. dimer) dormant A catalyst deactivation The Tolman cycle is a clockwise catalytic cycle composed only by organometallic species. The concentration of the active catalyst is represented by the sum of the concentrations of all catalytic species within the cycle. Maximum efficiency is achieved when all of the catalyst is active and lies within the catalytic cycle. If the rate of a step is sufficiently high, it may overwhelm an unfavorable preceding equilibrium present in the cycle (kinetic coupling).

12 Kinetics of the Catalytic Cycle In a catalytic cycle the net rates of step are identical Although the rate constants of the different steps in a catalytic cycle may be different, once the system has reached the steady state, the net rates 1 of each step are identical. Indeed, the rate of each step is proportional to the rate constant and to the concentration of the reagents and species involved in the catalytic cycle. As a consequence, the concentrations of the species that lie in the cycle vary to make the rate of each step of the catalytic cycle identical! The rate with the smallest pseudo-first order 2 rate constant (k n [reagent n ]) is called turnover-limiting step (TLS). This step controls the efficiency of the catalytic process as the rate of a catalytic reaction cannot exceed k TLS. [cat n ][reagent n ]. Most of the catalyst consists of the species preceding the TLS, 3 which is normally the resting state, the species present in the highest amount. 1) For a step cat n + reagent n cat n+1 the net rate is k n [cat n ][reagent n ] menus the reverse rate 2) Within the cycle the concentration of the catalyst remains constant. 3) It is erroneous to call the TLS the slow step, as all the steps proceed with the same net rate.

13 The Energetic Span E The energetic span ( E) is related to the turnover frequency TOF of the catalytic cycle. The smallest it is, the fastest the catalysis. Thus, a good catalytic cycle must consist of low-lying transition states with high-lying intermediates! Amatore, C.; Jutand, A. J. Organomet. Chem. 1999, 576, 254. Kozuch, S.; Amatore, C.; Jutand, A. Shaik, S. Organometallics, 2005, 24, Kozuch, S.; Shaik, S. Acc. Chem. Res. 2011, 44, Kozuch, S.; Martin, J. M. L. ChemPhysChem 2011, 12, 1413.

14 Recall: Solvent Effect in SN2 Nu - + RCH 2 -X > Nu-CH 2 R + X - Nu - = F -, I -

15 The Energetic Span E E E a1 E a cat c cat a cat 1 cat b E E cat a1 E1 cat b1 cat c1 cat n1 E1 cat n Reaction coordinate cat n1 Reaction coordinate Trying to improve the rate of the slowest step of a catalytic cycle by increasing the energy of its reactant, so as to decreases its E a, has (in contrast to a stoichiometric reaction) no chance of success, if the catalytic sequence is preserved otherwise. Indeed, the result is opposite, as the gain in lowering Ea is more than lost due to the increase of ΔE! As a result, the net effect is that the energetic span E increases and the global rate decreases.

16 About the Definitions of Rate Determing Step 1. The step with the smallest rate constant 2. The step with the highest energy TS 3. The step with the rate constant that exerts the strongest effect (IUPAC) smallest k: OK highest energy TS: OK smallest k: NON highest energy TS: OK smallest k: OK highest energy TS: NON E T 1 TLS E T 2 TLS k 1 < k 2! E T 1 TLS k -1 k 1 k 2 I 1 I 2 T 2 I 3 I 1 k 1 T 1 k k -1 2 I 2 I 3 k 1 I 1 I 2 k 2 T 2 I 3 k -1 < k 2 steady state Rxn coordinate k 1 I 1 I 2 k -1 I 3 k 2 k -1 > k 2 pre-equilibrium Rxn coordinate k 1 k I 1 I 1 2 k -1 < k 2 I 1 I 2 k -1 k both irreversible 2 k 2 I 3 Rxn coordinate I 3 Result: The RDS is a bad model. The States [TOF determining Intermediate (TDI) and TOF determining TS (TDTS)] exert the strongest effect on the overall rate.

17 The Energetic Span Product I 1 Substrate lowest k and highest TS but not RD step! I 3 I 2 Substrate I 1 k 1 T 1 k 3 T 3 TDTS T 1' Product T 3' TDTS TDI I 2 k 2 T 2 I 3 energetic span G r I 1' T 2' I 2' I 3' I 4' TDI energetic span T 3 I 2 > T 1 I 1

18 The Energetic Span T 1 T 2 rate limiting TS (TDTS) Product I 1 Substrate E I 3 I 2 I 1 G r resting state (TDI) I 2 E T 3 I 1' G r T 1' T 2' I 3 T 3' maximal energetic span I 2' I3' Cycle 1 Cycle 2 Rxn flow T 2 I 2 > T 1 I 3 > T 1 I 1

19 PhBr + PhB(OH) 3 - PhPh (L = SPhos) HO Br HO OH B cat - B(OH) 3, -Br cat MeO MeO Cy Cy P Pd P Cy Cy OMe OMe Kozuch, S.; Martin, J. M. L. Chem. Commun., 2011, 47, 4935

20 PhBr + PhB(OH) 3 - PhPh (L = InPhos) HO Br HO OH B cat - B(OH) 3, -Br cat MeO MeO *Cy Cy* P Pd P *Cy Cy* OMe OMe cy* = Me