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Mechanistic studies on activation of dioxygen using iridium and platinum triphenylphosphine complexes : protonation of coordinated peroxide, and oxygenation of coordinated hydride

University of British Columbia

Kinetic and mechanistic studies on the activation of dioxygen using iridium and platinum triphenylphosphine complexes via oxygenation-protonation (i.e. protonation of coordinated peroxide) and protonation-oxygenation (i.e. oxygenation of coordinated hydride) processes are described. Dioxygen is activated in the form of a dioxygen (peroxide) metal complex, which upon protonation produces H₂O₂ stoichiometricaUy. Oxidation of the oxidizable ligands or substrates sometimes occurs when free H₂O₂ is present in the reaction systems. The reversible oxygenation of trans-IrCl(CO)(PPh₃)₂ (eq. 1) has been re-examined at 25 °C to obtain data in benzene (k₁ = (3.4 ± 0.3) x 10⁻² M⁻¹s⁻¹, k₋₁= ~1.5 x 10⁻⁵ s⁻¹) and in dichloromethane (k₁ = (6.6 ± 0.3) x 10⁻⁷s⁻¹mm Hg⁻¹, k₋₁= (1.6 ± 0.1) x 10⁻⁵ s⁻¹).[See Thesis for Equations] Complex II reacts stoichiometrically with trifluoroacetic acid to produce H₂O₂ which further oxidizes the CO and PPI13 ligands, but with HCl the oxidation of CO or PPH₃ ligand by H₂O₂ does not take place; II reacts with free PPh₃ to give I and OPPH₃. The reaction of IrHCl₂(CO)(PPh₃)₂, III, with O₂ follows the sequence of steps outlined in eqs. 2, 1, 3 and 4. [See Thesis for Equations] This reaction sequence is proposed based on kinetic studies and product analysis. At 25 °C in dichloromethane, the rate constant (k₂) of the reductive-elimination reaction is (3.1 ± 0.1) x 10⁻⁵ s⁻¹ and the rate constant (k₋₂) of the oxidative-addition reaction is (4.4 ± 0.2) x 10² M⁻¹s⁻¹. Within the reversible reactions (2) and (1), HCl and O₂ compete with the intermediate I. Reaction (1) is eventually favoured because of the rapid protonation reaction (3) that scavenges the free HCl and prevents the reverse of reaction (2). Reaction (3) is therefore the driving force of the overall reaction between III and O₂. However, the forward step of reaction (2) is rate-determining in the complex process. Removal of one chloride ligand from III leads to the elimination of HCl from III and the formation of a cationic iridium(I) species, which reacts with O₂ slowly at room temperature. Reaction between IrH(CO)(PPh₃)₃ and O₂ gives IrH(O₂)(CO)(PPh₃)₂ that decomposes to form the final products, outlined in eqs. 5 to 7. [See Thesis for Equations] The complex Pt(PPh₃)₄ dissociates in benzene and to luene as described in eq. 8. [See Thesis for Equations] with ³¹P{¹H} NMR data giving K = (2.0 ± 0.2) x 10⁻¹ M at 22 °C in toluene (ΔH° = 2.7 ± 0.3 kcal mole⁻¹; ΔS° = 5.9 ± 0.6 e.u.). The Pt(PPh₃)₃ species does not measurably dissociate further to produce Pt(PPh₃)₂ in contrast to some literature reports. The exchange of PPh₃ between Pt(PPh₃)₄ and Pt(PPh₃)₃ studied via ³¹P{¹H} and ¹⁹⁵Pt{¹H} NMR spectroscopy, is frozen out at -90 °C. The reactions of O₂ with Pt(PPh₃)[sub n] (n = 3, 4) are irreversible and take place through the reaction steps shown in eqs. 8, 9, and 10. [See Thesis for Equations] One stage involves the formation of Pt(O₂)(PPh₃)₂ and PPh₃ and a later stage, the net oxygen transfer from Pt(O₂)(PPh₃)₂ to free PPh₃, presumably via liberated peroxide, as described previously in the literature. Kinetic studies indicate a first order dependence on both Pt(O₂)(PPh₃)₂ and PPh₃ for reaction 10, the rate constants being (0.9 ± 0.2) M⁻¹s⁻¹ for raction 9 and (1.38 ± 0.01) x 10⁻¹M⁻¹s⁻¹ for reaction 10 at 25 °C in benzene. The final products of the reaction of Pt(PPh₃)[sub n] with O₂ are Pt(O₂)(PPh₃)₂ and OPPh₃. The Pt(O₂)(PPh₃)₂ complex reacts with HCl in solution to produce stoichiometrically H₂O₂ and cis-PtCl₂(PPh₃)₂, the H₂O₂ being unable to oxidize the PPh₃ in cis-PtCl₂(PPh₃)₂. In contrast to a literature report, there is no facile reaction between trans-PtHCl(PPh₃)₂, or [PtH(S)(PPh₃)₂]PF₆ (S = solvent) and O₂, at ambient temperatures in dichloromethane, acetone, or tetrahydrofuran. However, over several days, oxidation reactions result from a slow loss of HCl from PtHCl(PPh₃)₂, as outlined in eqs. 11 to 14. [See Thesis for Equations]

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2010-10-18

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http://hdl.handle.net/2429/29301

Chemistry

University of British Columbia

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