Reactivity of rhodium(I) complexes bearing nitrogen-containing ligands toward CH3I: synthesis and full characterization of neutral cis-[RhX(CO)2(L)] and acetyl [RhI(µ-I)(COMe)(CO)(L)]2 complexes.

The neutral rhodium(I) square-planar complexes [RhX(CO)(2)(L)] [X = Cl (3), I (4)] bearing a nitrogen-containing ligand L [diethylamine (a), triethylamine (b), imidazole (c), 1-methylimidazole (d), pyrazole (e), 1-methylpyrazole (f), 3,5-dimethylpyrazole (g)] are straightforwardly obtained from L and [Rh(µ-X)(CO)(2)](2) [X = Cl (1), I (2)] precursors. The synthesis is extended to the diethylsulfide ligand h for 3h and 4h. According to the CO stretching frequency of 3 and 4, the ranking of the electronic density on the rhodium center follows the order b > a ≈ d > c > g > f ≈ h > e. The X-ray molecular structures of 3a, 3d-3f, 4a, and 4d-4f were determined. Results from variable-temperature (1)H and (13)C{(1)H} NMR experiments suggest a fluxional associative ligand exchange for 4c-4h and a supplementary hydrogen-exchange process in 4e and 4g. The oxidative addition reaction of CH(3)I to complexes 4c-4g affords the neutral dimeric iodo-bridged acetylrhodium(III) complexes [RhI(µ-I)(COCH(3))(CO)(L)](2) (6c-6g) in very good isolated yields, whereas 4a gives a mixture of neutral 6a and dianionic [RhI(2)(µ-I)(COCH(3))(CO)][NHMeEt(2)](2) and 4h exclusively provides the analogue dianionic complex with [SMeEt(2)](+) as the counterion. X-ray molecular structures for 6d(2) and 6e reveal that the two apical CO ligands are in mutual cis positions, as are the two apical d and e ligands, whereas isomer 6d(1) is centrosymmetric. Further reactions of 6d and 6e with CO or ligand e gave quantitatively the monomeric complexes [RhI(2)(COCH(3))(CO)(2)(d)] (7d) and [RhI(2)(COCH(3))(CO)(e)(2)] (8e), respectively, as confirmed by their X-ray structures. The initial rate of CH(3)I oxidative addition to 4 as determined by IR monitoring is dependent on the nature of the nitrogen-containing ligand. For 4a and 4h, reaction rates similar to those of the well-known rhodium anionic [RhI(2)(CO)(2)](-) species are observed and are consistent with the formation of this intermediate species through methylation of the a and h ligands. The reaction rates are reduced significantly when using imidazole and pyrazole ligands and involve the direct oxidative addition of CH(3)I to the neutral complexes 4c-4g. Complexes 4c and 4d react around 5-10 times faster than 4e-4g mainly because of electronic effects. The lowest reactivity of 4f toward CH(3)I is attributed to the steric effect of the coordinated ligand, as supported by the X-ray structure.

Direct involvement of the acetato ligand in the reductive elimination step of rhodium-catalyzed methanol carbonylation.

For the last step of rhodium-catalyzed methanol carbonylation, high-pressure NMR, and kinetic and experimental data supported by density functional theory calculations are consistent with substitution of I(-) by an AcO(-) ligand on the [RhI(3)(COCH(3))(CO)(2)](-) species followed by reductive elimination of acetic anhydride, which immediately reacts with water to afford acetic acid.

Isolation and structural characterization of anionic and neutral compounds resulting from the oxidative addition of HI or CH3I to [IrI2(CO)2](-).

The active iridium species in the methanol carbonylation reaction has been crystallized as the [PPN][IrI(2)(CO)(2)] complex and the X-ray structure solved, showing a cis-geometry and a square planar environment. Hydriodic acid reacts very quickly with this compound to provide [PPN][IrHI(3)(CO)(2)], the X-ray crystal structure of which has been determined. The two CO ligands remain in mutual cis-position in a pseudooctahedral environment. The same cis-arrangement has been observed from the X-ray structure for [PPN][IrI(3)(CH(3))(CO)(2)] resulting from the slower oxidative addition of CH(3)I to [PPN][IrI(2)(CO)(2)]. By iodide abstraction with InI(3), the anionic methyl complex gave rise to the dimeric neutral complex [Ir(2)(mu-I)(2)I(2)(CH(3))(2)(CO)(4)]. An X-ray structure showed that the methyl ligands are in the equatorial positions of the two octahedrons sharing an edge, formed by the two bridging iodide ligands. All these four complexes have been fully characterized by mass spectrometry, (1)H and (13)C NMR, and infrared both in solution and in the solid state. When necessary, the (13)CO- or (13)CH(3)-enriched complexes have been prepared and analyzed.

A mechanistic investigation of oxidative addition of methyl iodide to [Tp*Rh(CO)(L)].

Reaction of methyl iodide with square planar [kappa(2)-Tp*Rh(CO)(PMe(3))] 1a (Tp* = HB(3,5-Me(2)pz)(3)) at room temperature affords [kappa(3)-Tp*Rh(CO)(PMe(3))(Me)]I 2a, which was fully characterized by spectroscopy and X-ray crystallography. The pseudooctahedral geometry of cationic 2a, which contains a kappa(3)-coordinated Tp* ligand, indicates a reaction mechanism in which nucleophilic attack by Rh on MeI is accompanied by coordination of the pendant pyrazolyl group. In solution 2a transforms slowly into a neutral (acetyl)(iodo) rhodium complex [kappa(3)-Tp*Rh(PMe(3))(COMe)I] 3a, for which an X-ray crystal structure is also reported. Kinetic studies on the reactions of [kappa(2)-Tp*Rh(CO)(L)] (L = PMe(3), PMe(2)Ph, PMePh(2), PPh(3), CO)] with MeI show second-order behavior with large negative activation entropies, consistent with an S(N)2 mechanism. The second-order rate constants correlate well with phosphine basicity. For L = CO, reaction with MeI gives an acetyl complex, [kappa(3)-Tp*Rh(CO)(COMe)I]. The bis(pyrazolyl)borate complexes [kappa(2)-Bp*Rh(CO)(L)] (L = PPh(3), CO) are much less reactive toward MeI than the Tp* analogues, indicating the importance of the third pyrazolyl group and the accessibility of a kappa(3) coordination mode. The results strengthen the evidence in favor of an S(N)2 mechanism for oxidative addition of MeI to square planar d(8) transition metal complexes.