The zwitterionie RhI monocarbonyl complex [Rh(EtSNS)(CO)] (1, EtSNS = EtNC(S)Ph2P= NPPh2C(S)NEt-) was reacted with iodomethane in dichloromethane, yielding the stable acetyl-Rh III 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) × 102 M -1 s-1 at 25 °C CH2Cl2] and the activation parameters ΔHD̊ [28(3) kJ mol-1] and ΔSD̊ [-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 · X 2), 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 ·×were also reacted with iodomethane, forming complexes [Rh(HEtSNS)(COCH3)I]X (5 · 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 · NO 3 = 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 2 ·×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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry