Hydrides and Dihydrogen as Ligands: Lessons from Organometallic Chemistry. Lecture 9

Transcription

1 ydrides and Dihydrogen as Ligands: Lessons from Organometallic Chemistry Lecture 9

2 Inorganic Chemistry Chapter 1: Figure W.. Freeman

3 Synthesis of Organometallic Complex ydrides Reaction of MCO with O -, -, or C 2 CR 2 M(CO) n + O - = M(CO) n-1 (COO) - = M(CO) n CO 2 M(CO) n + - = M(CO) n-1 (C(=O)) - = M(CO) n CO M(CO) n C 2 CR 2 = M(CO) n C 2 =CR 2 + CO Protonation of MCO anion* M(CO) n = M(CO) n ydrogenation of MCO dimer**: M 2 (CO) 2n + 2 = M(CO) n Oxidative addition of 2 to (typically) d 8 metal M(PR 3 ) 3 X + 2 = M(CO) n X() 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)

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

6 Acidity of MCO ydrides M(CO) n + O - 2 O + M(CO) n - K a Co(CO) 4 ~2 Co(CO) 3 PPh Mn(CO) Re(CO) 5 very weak 2 Fe(CO) ; CpCr(CO) CpMo(CO) CpW(CO)

7 Isolobal Analogies: R. offmann

8 Isolobal species:, Methyl, M(CO) s

9 Metal Carbonyl Anions: Nucleophilicity Anion {M}CO + RX RM(CO) + X CN Anion Rate = k [MCO][RX] Product CN Relative nucleophilicity CpFe(CO) 2 5 CpFe(CO) 2 R x 10 6 CpRu(CO) 2 5 CpRu(CO) 2 R x 10 6 CpNi(CO)ˉ 4 CpNi(CO)R x 10 6 Ru(CO) 5 5 Ru(CO) 5 R x 10 4 CpW(CO) 3 6 CpW(CO) 3 R 7 ~500 Mn(CO) 5 5 Mn(CO) 5 R 6 77 CpMo(CO) 3 6 CpMo(CO) 3 R 7 67 CpCr(CO) 3 6 CpCr(CO) 3 R 7 4 Co(CO) 4 4 RCo(CO) V(CO) 6-6 RV(CO) 6 7 << 1 Fe(CO) RFe(CO) 4-5 >> 7.0 x 10 6

10 Another electron rich metal center: Also reacts with Organic Electrophiles: Vaska s Complex

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

12 Nucleophilicity/ydricity of Anionic MCO ydrides

13 ydride Transfer Reactivity M(CO) n - + RX XM(CO) n - + R Rate = k 2 [M - ][RX] Organometallics, 1984, 3, 646

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

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

16 The η 2-2 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 M M Morris, U. Toronto σ - donor σ* acceptor M 2+ ( - ) 2 Kubas, LANL R 3 P O C - Examples of η 2-2 complexes W 0 C O Kubas CO P PR 3 P Fe II C N Morris ++ P P + Ph 3 P Ir III N PPh 3 C Crabtree Crabtree, Yale

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

18 omogeneous Catalysis: ydrogenation of Alkenes: Wilkinson s catalyst and (one of several versions of) the mechanism

19 Inorganic Chemistry Chapter 1: Figure Schematic representation of physisorption and chemisorption of ydrogen 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.. Freeman

20 ydrogenation of alkenes on supported metal Involves 2 dissociation and migration of -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 Δ 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 2009 W.. Freeman