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Oxidative Addition of Iodomethane to Charge-Tuned Rhodium(I) Complexes

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posted on 13.04.2009, 00:00 by Massimiliano Delferro, Matteo Tegoni, Vincenzo Verdolino, Daniele Cauzzi, Claudia Graiff, Antonio Tiripicchio
The zwitterionic RhI monocarbonyl complex [Rh(EtSNS)(CO)] (1, EtSNS = EtNC(S)Ph2PNPPh2C(S)NEt) was reacted with iodomethane in dichloromethane, yielding the stable acetyl-RhIII complex [Rh(EtSNS)(COCH3)I] (4). Complex 4 was characterized in solution and in the solid state by X-ray diffraction analysis. The rate constant of the reaction [5.48 (7) × 10−2 M−1 s−1 at 25 °C CH2Cl2] and the activation parameters ΔH [28(3) kJ mol−1] and ΔS [−173(10) J mol−1 K−1] were determined, confirming a nucleophilic addition mechanism. The rate constant was obtained by monitoring the acetylic product by 1H NMR, under second-order conditions ([Rh]/[CH3I] = 1). Complex 1 can be mono- and biprotonated with HX (X = PF6, OTf, NO3), forming [Rh(HEtSNS)(CO)]X (2·X) and [Rh(H2EtSNS)(CO)]X2 (3·X2), respectively. A decrease of the calculated DFT Mulliken atomic population on the Rh atom is observed along the series 1 > 2 > 3 in accordance with the variation of the coordinated CO stretching frequency. Compounds 2·X were also reacted with iodomethane, forming complexes [Rh(HEtSNS)(COCH3)I]X (X), stable in solution for a short time, that transform by deprotonation into 4 and into unidentified decomposition products. The rate constants were determined under pseudo-first-order conditions due to the lower reactivity [2·NO3 = 24.6 (6) × 10−5 M−1 s−1; 2·OTf = 12.7 (3) × 10−5 M−1 s−1; 2·PF6 = 2.50 (6) × 10−5 M−1 s−1]. The activation parameters for 2·PF6 were also determined. The influence of the counterion could be explained assuming that the different non-metal-coordinated anions form hydrogen bonding with the NH group of 2·X, which in turn causes a variation of the electron density on the Rh center. A good correlation between the CO stretching frequencies and the rate constants was observed. The experimental rate constant for complex 1 is 1 order of magnitude higher than the one calculated using the linear regression function obtained for the X series (experimental = 5.48 × 10−2 M−1 s−1; calculated = 1.29 × 10−3 M−1 s−1), pointing out that the monoprotonated complexes react more slowly than expected. Both steric and electronic effects were examined and held responsible for this reduced reactivity. Complexes 3·X2 reacted too slowly, yielding complex 4 and unidentified decomposition products, hindering the determination of the rate constants.

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