OXIDATIVE ADDITION OF ALKYL-HALIDES TO RHODIUM(I) AND IRIDIUM(I) DICARBONYL DIIODIDES - KEY REACTIONS IN THE CATALYTIC CARBONYLATION OF ALCOHOLS

Alkyl iodides (RI) react with [Rh(CO)2I2]- to give acyl species [Rh(CO)(COR)I3]- (R = Et, (n)Pr, (i)Pr) and with [Ir(CO)2]2]- to give alkyl complexes [RIr(CO)2]2]- (R = Et, (n)Pr, (i)Pr, (n)Bu, 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) angstrom, b = 19.470(16) angstrom, c = 19.529(12) angstrom, beta = 94.99(5)-degrees, Z = 4, space group P2(1)/n (a nonstandard setting of P2(1)/c C2h5, No. 14); 2446 independent reflections (of 5197 measured) for which \F\/sigma(\F\) > 4.0; R = 0.0966 (R(w) = 0.0921, 238 parameters)]. Kinetic data for the reactions of [Rh(CO)2I2]-with EtI. (n)PrI, and (i)PrI and for [Ir(CO)2I2]- with MeI, EtI, and (n)PrI 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-degrees-C, though some decomposition occurred at higher temperatures. For M = Ir, clean second-order kinetics were observed with MeI, but reactions with EtI and (n)PrI 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-degrees-C (M = Rh) and 35-50-degrees-C (M = Ir) gave the following activation parameters: (M = Rh) DELTAH(double dagger)/kJ mol-1 = 50(+/-1) (R = Me), 56(+/-10) (R = Et), 51(+/-10) (R = (n)Pr), 61(+/-15) (R = (i)Pr); DELTAS(double dagger)/J mol-1 K-1 = -165(+/-4) (R = Me), -195(+/-25) (R = Et), -215(+/-25) (R = (n)Pr), -180(+/-30) (R = (i)Pr); (M = Ir) DELTAH(double dagger)/kJ mol-1 = 54(+/-1) (R = Me), 66(+/-5) (R = Et), 66(+/-3) (R = (n)Pr); DELTAS(double dagger)/J mol-1 K-1 -113(+/-4) (R = Me), -123(+/-15) (R = Et), -132(+/-11) (R = (n)Pr). 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; (n)Pr, 1.7) and [Ir(CO)2I2]- (relative rates: Me, 1000; Et, 2.3; (n)Pr, 0.75). The similarity to reactivity trends for organic nucleophiles suggests an SN2 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 = (n)Pr), 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)Is]-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.