Jean M Pearson

The University of Sheffield, Sheffield, ENG, United Kingdom

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Publications (5)14.26 Total impact

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    ABSTRACT: Kinetic measurements are reported for the oxidative addition reactions of methyl chloride and acetyl chloride with [Ir(CO)2Cl2]−. At 40°C the second-order rate constant for MeCOCl addition is estimated to be nearly 40 000 times larger than that for MeCl addition. The Ir(III) products, [Ir(CO)2Cl3R]− (R = Me, COMe) have been isolated and characterised by spectroscopy and x-ray crystallography. In the absence of excess organic chloride, both Ir(III) complexes undergo reductive elimination of RCl. Kinetic measurements show these reactions to be first order in the Ir(III) complex with elimination of MeCOCl estimated to be ca 7000 times faster than MeCl elimination at 40°C. Combination of activation parameters for the forward and reverse reactions allows calculation of thermodynamic parameters for oxidative addition. Both MeCl and MeCOCl additions are exothermic (by 44 and 68 kJ mol−1, respectively) but disfavoured entropically. The trends are predicted satisfactorily by ab initio and DFT computational methods. The results for MeCl addition to [Ir(CO)2Cl2]− are compared with data for MeI addition to [Ir(CO)2I2]−. Kinetic data are also reported for carbonylation of [Ir(CO)2Cl3Me]− into [Ir(CO)2Cl3(COMe)]− under mild conditions in PhCl–MeOH. It is concluded that the low activity of iridium–chloride carbonylation catalysts is due primarily to the relatively slow reaction of [Ir(CO)2Cl2]− with MeCl. Copyright © 2004 John Wiley & Sons, Ltd.
    Journal of Physical Organic Chemistry 08/2004; 17(11):1007 - 1016. · 1.58 Impact Factor
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    ABSTRACT: The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu(4)NI) are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H(2)O levels > or = 5% w/w) is fac,cis-[Ir(CO)(2)I(3)Me](-), 2. The stoichiometric carbonylation of 2 into [Ir(CO)(2)I(3)(COMe)](-), 6, is accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)(3)I(2)](2), [Ru(CO)(2)I(2)](n), InI(3), GaI(3), and ZnI(2)). The rate increase is approximately proportional to promoter concentration for promoter:Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO)(3)I(3)](-), [Ru(CO)(2)I(4)](2)(-)) do not promote carbonylation of 2 and Bu(4)NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give [Ir(CO)(3)I(2)Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700 times faster for 4 than for 2 (85 degrees C, PhCl), representing a lowering of Delta G(++) by 20 kJ mol(-1). Ab initio calculations support a more facile methyl migration in 4, the principal factor being decreased pi-back-donation to the carbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)(2)I(3)(COMe)](-), 6a (as its Ph(4)As(+) salt), was characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)(m)I(n)] (M = Ru, In; m = 3, 0; n = 2, 3) binds I(-) to form [M(CO)(m)I(n+1)](-)H(3)O(+) and catalyzes the reaction HI(aq) + MeOAc --> MeI + HOAc. This moderates the concentration of HI(aq) and so facilitates catalytic turnover via neutral 4.
    Journal of the American Chemical Society 03/2004; 126(9):2847-61. · 10.68 Impact Factor
  • Anthony Haynes, James McNish, Jean M Pearson
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    ABSTRACT: Infrared spectroscopic studies show that the cis–trans isomerisation of [Rh(CO)2I4]− (1) reaches an equilibrium in solution favouring the trans isomer (Keq≈10 in CH2Cl2 at 25°C). The approach to equilibrium obeys a first order rate law with a half life of ca. 40 min at 25°C. Activation parameters for the isomerisation are ΔH‡ 103 (±3) kJ mol−1, ΔS‡ +32 (±9) J mol−1 K−1 in CH2Cl2 and ΔH‡ 99 (±2) kJ mol−1, ΔS‡+25 (±4) J mol−1 K−1 in THF. In polar coordinating solvents, formation of monocarbonyl complexes [Rh(CO)I4(sol)]− occurs in competition with cis–trans isomerisation, and in the presence of added iodide salt, [Rh(CO)I5]2− is formed. Isotopic labelling experiments show that added 12CO is incorporated into trans-1 on isomerisation of cis-[Rh(13CO)2I4]−. Addition of CO to the monocarbonyls, [Rh(CO)I4(sol)]− and [{Rh(CO)I4}2]2− gives predominantly trans-[Rh(CO)2I4]−. A mechanism is proposed for cis–trans isomerisation involving dissociation of a CO ligand. Elemental and infrared spectroscopic analysis of crystals obtained from the residues of carbonylation experiments on [Ir(CO)2I3(Me)]− are consistent with a new trans isomer of [Ir(CO)2I4]−. The same species is formed in the reaction of [{Ir(CO)I4}2]2− with CO. Isomerisation to cis-[Ir(CO)2I4]− only occurs in the presence of added CO. The ν(CO) frequencies for isotopomers of cis- and trans-[M(CO)2I4]− (M=Rh, Ir) are analysed using C–O factored force-fields. The force constants obtained indicate stronger metal–CO back-donation for the cis relative to the trans isomers and also for iridium relative to rhodium.
    Journal of Organometallic Chemistry 01/1998; 551(s 1–2):339–347. · 2.00 Impact Factor
  • Anthony Haynes, James McNish, Jean M Pearson
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    ABSTRACT: Infrared spectroscopic studies show that the cis–trans isomerisation of [Rh(CO)2I4]− (1) reaches an equilibrium in solution favouring the trans isomer (Keq≈10 in CH2Cl2 at 25°C). The approach to equilibrium obeys a first order rate law with a half life of ca. 40 min at 25°C. Activation parameters for the isomerisation are ΔH‡ 103 (±3) kJ mol−1, ΔS‡ +32 (±9) J mol−1 K−1 in CH2Cl2 and ΔH‡ 99 (±2) kJ mol−1, ΔS‡+25 (±4) J mol−1 K−1 in THF. In polar coordinating solvents, formation of monocarbonyl complexes [Rh(CO)I4(sol)]− occurs in competition with cis–trans isomerisation, and in the presence of added iodide salt, [Rh(CO)I5]2− is formed. Isotopic labelling experiments show that added 12CO is incorporated into trans-1 on isomerisation of cis-[Rh(13CO)2I4]−. Addition of CO to the monocarbonyls, [Rh(CO)I4(sol)]− and [{Rh(CO)I4}2]2− gives predominantly trans-[Rh(CO)2I4]−. A mechanism is proposed for cis–trans isomerisation involving dissociation of a CO ligand. Elemental and infrared spectroscopic analysis of crystals obtained from the residues of carbonylation experiments on [Ir(CO)2I3(Me)]− are consistent with a new trans isomer of [Ir(CO)2I4]−. The same species is formed in the reaction of [{Ir(CO)I4}2]2− with CO. Isomerisation to cis-[Ir(CO)2I4]− only occurs in the presence of added CO. The ν(CO) frequencies for isotopomers of cis- and trans-[M(CO)2I4]− (M=Rh, Ir) are analysed using C–O factored force-fields. The force constants obtained indicate stronger metal–CO back-donation for the cis relative to the trans isomers and also for iridium relative to rhodium.
    Journal of Organometallic Chemistry - J ORGANOMET CHEM. 01/1998; 551(1):339-347.
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    ABSTRACT: Carbonylation of the anionic iridium(III) methyl complex, [MeIr(CO)2I3]− (1) is an important step in the new iridium-based process for acetic acid manufacture. A model study of the migratory insertion reactions of 1 with P-donor ligands is reported. Complex 1 reacts with phosphites to give neutral acetyl complexes, [Ir(COMe)(CO)I2L2] (L = P(OPh)3 (2), P(OMe)3 (3)). Complex 2 has been isolated and fully characterised from the reaction of Ph4As[MeIr(CO)2I3] with AgBF4 and P(OPh)3; comparison of spectroscopic properties suggests an analogous formulation for 3. IR and 31P NMR spectroscopy indicate initial formation of unstable isomers of 2 which isomerise to the thermodynamic product with trans phosphite ligands. Kinetic measurements for the reactions of 1 with phosphites in CH2Cl2 show first order dependence on [1], only when the reactions are carried out in the presence of excess iodide. The rates exhibit a saturation dependence on [L] and are inhibited by iodide. The reactions are accelerated by addition of alcohols (e.g. 18× enhancement for L = P (OMe)3 in 1:3 MeOH-CH2Cl2). A reaction mechanism is proposed which involves substitution of an iodide ligand by phosphite, prior to migratory CO insertion. The observed rate constants fit well to a rate law derived from this mechanism. Analysis of the kinetic data shows that k1, the rate constant for iodide dissociation, is independent of L, but is increased by a factor of ∼18 on adding 25% MeOH to CH2Cl2. Activation parameters for the k1 step are ΔH≠ = 71 (±3) kJ mol−, ΔS≠ = −81 (±9) J mol−1 K−1 in CH2Cl2 and ΔH≠ = 60(±4) kJ mol−1, ΔS≠ = −93(± 12) J mol−1 K−1 in 1:3 MeOH-CH2Cl2. Solvent assistance of the iodide dissociation step gives the observed rate enhancement in protic solvents. The mechanism is similar to that proposed for the carbonylation of 1.
    Inorganica Chimica Acta - INORG CHIM ACTA. 01/1998; 270(1):382-391.