Oxidative addition of alkyl halides to rhodium(I) and iridium(I) dicarbonyl diiodides: Key reactions in the catalytic carbonylation of alcohols

Article abstract of DOI:10.1021/om00020a039

Alkyl iodides (RI) react with [Rh(CO)₂I₂]^- to give acyl species [Rh(CO)(COR)I₃]^- (R = Et, ⁿPr, ⁱPr) and with [Ir(CO)₂]₂]^- to give alkyl complexes [RIr(CO)₂I₂]^- (R = Et, ⁿPr, ⁱPr, ⁿBu, n-C₅H₁₁, n-C₆H₁₃). The reactions are analogous to the known reactions of MeI with [Rh(CO)₂I₂]^- and [Ir(CO)₂I₂]^-. The products are characterized spectroscopically and by an X-ray crystal structure determination for Ph₄As[(n-C₆H₁₃)Ir(CO)₂I₃] which showed a fac,cis geometry for the anion. [Crystal structure data: monoclinic, a = 9.408(7) ?, b = 19.470(16) ?, c = 19.529(12) ?, β = 94.99(5)°, Z = 4, space group P2₁/n (a nonstandard setting of P2₁/c C_2h⁵, No. 14); 2446 independent reflections (of 5197 measured) for which |F|/σ(|F|) > 4.0; R = 0.0966 (R_w = 0.0921, 238 parameters)]. Kinetic data for the reactions of [Rh(CO)₂I₂] with EtI, ⁿPrI, and ⁱPrI and for [Ir(CO)₂I₂]^- with MeI, EtI, and ⁿPrI show that oxidative addition of RI to [M(CO)₂I₂]^- is first-order in both reactants. For M = Rh, reactions showed clean second-order kinetics below 80°C, though some decomposition occurred at higher temperatures. For M = Ir, clean second-order kinetics were observed with MeI, but reactions with EtI and ⁿPrI showed a more complex kinetic behavior. A competing radical pathway is suggested, which can be quenched by added duroquinone. Second-order rate constants, k₂, evaluated over the temperature ranges 70-80°C (M = Rh) and 35-50°C (M = Ir) gave the following activation parameters: (M = Rh) ΔH^≠/kJ mol^-1 = 50(±1) (R = Me), 56(±10) (R = Et), 51(±10) (R = ⁿPr), 61(±15) (R = ⁱPr); ΔS^≠/J mol^-1 K^-1 = -165(±4) (R = Me), -195(±25) (R = Et), -215(±25) (R = ⁿPr), -180(±30) (R = ⁱPr); (M = Ir) ΔH^≠/kJ mol^-1 = 54(±1) (R = Me), 66(±5) (R = Et), 66(±3) (R = ⁿPr); ΔS^≠/J mol^-1 K^-1 -113(±4) (R = Me), -123(±15) (R = Et), -132(±11) (R = ⁿPr). Comparisons are made between the reactions of methyl iodide and the higher alkyl iodides with both [Rh(CO)₂I₂]^- (relative rates: Me, 1000, Et, 3; ⁿPr, 1.7) and [Ir(CO)₂I₂]^- (relative rates: Me, 1000; Et, 2.3; ⁿPr, 0.75). The similarity to reactivity trends for organic nucleophiles suggests an S_N2 mechanism, but with a competing radical pathway for iridium. Relative rates for the two nucleophiles, k_Ir/k_Rh ca. 150 (R = Me), 220 (R = Et), and 140 (R = ⁿPr), are estimated. Alkyl isomerization (iso → n) is observed for both [Rh(CO)(COPr)I₃]^- and [PrIr(CO)₂I₃]^- and displacement of propene from [Rh(CO)(COPr)I₃]^- by added ethene gives [Rh(CO)(COEt)I₃]^- reversibly. A mechanism involving hydridoalkene intermediates is proposed. The data are consistent with the carbonylation of the higher alcohols (ROH) proceeding via rate determining oxidative addition of RI to [Rh(CO)₂I₂]^-, rather than by a route involving a rhodium hydride addition to an olefin derived from the ROH.