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

The zwitterionic Rh(I) monocarbonyl complex [Rh(EtSNS)(CO)] (1, EtSNS =EtNC(S)Ph(2)P=NPPh(2)C(S)NEt(-)) was reacted with iodomethane in dichloromethane, yielding the stable acetyl-Rh(III) complex [Rh(EtSNS)(COCH(3))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) x 10(-2) M(-1) s(-1) at 25 degrees C CH(2)Cl(2)] and the activation parameters Delta H(double dagger) [28(3) kJ mol(-1)] and Delta S(double dagger)[-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 (1)H NMR, under second-order conditions ([Rh]/[CH(3)I]=1). Complex 1 can be mono- and biprotonated with HX (X=PF(6), OTf, NO(3)), forming [Rh(HEtSNS)(CO)]X (2 center dot X) and [Rh(H(2)EtSNS)(CO)]X(2) (3 center dot 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 center dot X were also reacted with iodomethane, forming complexes [Rh(HEtSNS)(COCH(3))I]X (5 center dot 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 center dot NO(3)=24.6 (6) x 10(-5) M(-1) s(-1); 2 center dot OTf=12.7 (3) x 10(-5) M(-1) s(-1); 2 center dot PF(6)=2.50 (6) x 10(-5) M(-1) s(-1)]. The activation parameters for 2 center dot PF(6) 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 center dot 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 center dot X series (experimental=5.48 x 10(-2) M(-1) s(-1); calculated=1.29 x 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 center dot X(2) reacted too slowly, yielding complex 4 and unidentified decomposition products, hindering the determination of the rate constants.