Hydrides and Dihydrogen as Ligands: Hydrogenation Catalysis

Transcription

1 Hydrides and Dihydrogen as Ligands: Hydrogenation Catalysis

2 Synthesis of Organometallic Complex Hydrides Reaction of MCO with OH -, H -, or CH 2 CHR 2 M(CO) n + OH - = M(CO) n-1 (COOH) - = HM(CO) n CO 2 M(CO) n + H - = M(CO) n-1 (C(=O)H) - = HM(CO) n CO M(CO) n CH 2 CHR 2 = HM(CO) n CH 2 =CR 2 + CO Protonation of MCO anion* M(CO) n - + H + = M(CO) n H Hydrogenation of MCO dimer**: M 2 (CO) 2n + H 2 = M(CO) n H Oxidative addition of H 2 to (typically) d 8 metal M(PR 3 ) 3 X + H 2 = M(CO) n X(H) 2 *Oxidative addition of a proton. If a dianion, the resultant MCO hydride will be anionic and may react as a hydride transfer reagent. **The resultant neutral hydride may have acidic characteristics (i.e., the hydrogen may be removed by a base (reductive deprotonation)

3

4 Properties of the M-H functionality Stereochemically active M-H distance range (3d transition metals): Ǻ M-H stretch: cm -1 M-H hydride resonance: typically upfield, -1 to -20 ppm, but little correlation with electron density M-H Bond Dissociation Energy: kcal/mol (Contrast M-C BDE of ca kcal/mol Homolytic cleavage can initiate radical chain reactions Acid/Base character: Varies. HCo(CO) 4 is strong acid, pk a <1; HFe(CO) 4 is weak; Cp 2 W(H) 2 forms Lewis Base/Acid adduct with AlMe 3. Proton loss is slow as in carbon-based acids.

5 Acidity of MCO Hydrides HM(CO) n + OH - H 2 O + M(CO) n - K a HCo(CO) 4 ~2 HCo(CO) 3 PPh HMn(CO) HRe(CO) 5 very weak H 2 Fe(CO) ; CpCr(CO) 3 H CpMo(CO) 3 H CpW(CO) 3 H

6 The M-H Bond Functionality: Reactivity H M No change in M oxidation state; reverse is β-elimination MH M - + H + Formal reduction of M oxidation state by 2; electron Withdrawing ligands stabilize M + H Oxidation state of M reduced by 1; can yield H 2 or initiate radical rxns M + + H - M oxidation state is unchanged; electron donating ligands stabilize

7 Hydride Transfer Reactivity HM(CO) n - + RX XM(CO) n - + RH Rate = k 2 [HM - ][RX] Organometallics, 1984, 3, 646

8 Suppose one protonates the anionic metal hydride of HW(CO) 5-. Is it possible that the resultant H 2 would remain bound to the metal? Heinekey, JACS, 2005, ; Heinekey, JACS, 2006,

9 The 2 -Dihydrogen as Ligand Story Kubas, JACS, 1984, 10, 451.

10 The η 2 -H 2 Complexes - Typically d 6, Oh structures of Cr 0, Mo 0, W 0, Fe II, Ru II, Ir III. - Bonding: Delicate Balance Required for Stability M H H M H H M H H Morris, U. Toronto - donor * acceptor M 2+ (H - ) 2 Kubas, LANL H R 3 P O C - Examples of 2 -H 2 complexes W 0 C O Kubas H CO P PR 3 P H H Fe II C N H Morris ++ P P H H + Ph 3 P H Ir III N PPh 3 C Crabtree Crabtree, Yale

11 Every Molecule Has a Story: The 2 -H 2 Complexes H H O H H + + H H R 3 P C O C W P P R 3 P H Fe Ir PR 3 N PR C P P 3 O C C N H + + H + - H + H H C O R 3 P W PR 3 C O C O H P Fe P C N H P P + H H R 3 P H Ir N PR 3 C + - stability towards H 2 dissociation - oxidative addition to dihydride Kubas, LANL - strong acid - CNH ligand Morris, U. Toronto - resonance forms - H/D exchange Crabtree, Yale

12 H 2 Ir III

13 Vaska s Complex and Its Oxidative Addition Reactions

14 Catalysis 2009 W.H. Freeman

15 Catalyst Development

16 Catalyst Development

17 Inorganic Chemistry Chapter 1: Figure Schematic representation of physisorption and chemisorption of Hydrogen on a nickel metal surface Schematic representation of Diverse sites exposed on a Metal surface a) different Exposed planes, edges; b) steps And kinks from irregularities 2009 W.H. Freeman

18 Hydrogenation of alkenes on supported metal Involves H 2 dissociation and migration of H-atoms to an adsorbed ethene molecule. (Paul Sabatier, 1890) Mechanism: All isotopomers are seen, therefore highly Reversible prior to loss of the ethane. Volcano diagrams relate stability of products on Surface: Temp. for a set rate of release vs. the Enthalpy. Intermediate values of ΔH f, with the rate being a combination of the rate of adsorption and the rate of desorption gives best catalyst. Just right Surface adducts Very weak Surface adducts Very strong Surface adducts The Goldilocks Effect 2009 W.H. Freeman

19 A Prominent Example of Heterogeneous Hydrogenation Catalysis: Ammonia Synthesis CH 4 + H 2 O 3 H 2 + CO er.htm CO + H 2 O H 2 + CO 2 CO/H 2

20 500 x 106 tons/year; Known as population detonator er.htm

21 er.htm

22 Conditions According to the equation, the equilibrium mixture will contain more ammonia: When the temperature is lower (the reaction is exothermic in the ammonia direction) When pressure is higher (4 moles of reactant gas make 2 moles of product gas) In practice, the equilibrium is run under conditions of moderate temperatures and pressure. Low temperatures affect the equilibrium favorably, but the reaction would be too slow. Very high pressures, though favoring product creation, increase the costs of plant construction, and present a greater risk to plant workers. With the conditions used, a yield of approximately % is achieved from each pass over the catalyst. er.htm

23 Some homogeneous catalytic processes (Adapted from J. Halpern, Inorg. Chim. Acta 1981, 50, 11)

24 Hydrogenation of Alkenes: Wilkinson s catalyst and (one of several versions of) the mechanism

25 Hydrosilation of Terminal Alkenes

26 Mechanism: H 2 activation prior to olefin addition Mechanism: Olefins add first to cationic catalyst

27 Wilkinson s Catalyst: Mechanism for Olefin Hydrogenation Pre-catalyst

28 With the Rh(I) cationic precursor: Olefin adds prior to H 2 oxidative addition.* *This mechanistic route followed by asymmetric Hydrogenation process

29 Halpern, Science, 1982

30 Halpern (Science, 1982, p. 401)

31 Halpern (Science, 1982, p. 401)

32 from Collman, Stanford

33

34 Wacker process: Oxidation of olefins

35

36 Inorganic Chemistry Chapter 1: Figure 26.2 Olefin Isomerization 2009 W.H. Freeman

37 from Collman, Stanford

38 Hydroformylation:

39 Hydroformylation : Union Carbide process