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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

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posted on 20.08.2012, 00:00 by Romain J. Adcock, Duc Hanh Nguyen, Sonia Ladeira, Carole Le Berre, Philippe Serp, Philippe Kalck
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 > ad > c > g > fh > e. The X-ray molecular structures of 3a, 3d3f, 4a, and 4d4f were determined. Results from variable-temperature 1H and 13C­{1H} NMR experiments suggest a fluxional associative ligand exchange for 4c4h and a supplementary hydrogen-exchange process in 4e and 4g. The oxidative addition reaction of CH3I to complexes 4c4g affords the neutral dimeric iodo-bridged acetylrhodium­(III) complexes [RhI­(μ-I)­(COCH3)­(CO)­(L)]2 (6c6g) in very good isolated yields, whereas 4a gives a mixture of neutral 6a and dianionic [RhI2(μ-I)­(COCH3)­(CO)]­[NHMeEt2]2 and 4h exclusively provides the analogue dianionic complex with [SMeEt2]+ as the counterion. X-ray molecular structures for 6d2 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 6d1 is centrosymmetric. Further reactions of 6d and 6e with CO or ligand e gave quantitatively the monomeric complexes [RhI2(COCH3)­(CO)2(d)] (7d) and [RhI2(COCH3)­(CO)­(e)2] (8e), respectively, as confirmed by their X-ray structures. The initial rate of CH3I 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 [RhI2(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 CH3I to the neutral complexes 4c4g. Complexes 4c and 4d react around 5–10 times faster than 4e4g mainly because of electronic effects. The lowest reactivity of 4f toward CH3I is attributed to the steric effect of the coordinated ligand, as supported by the X-ray structure.

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