Comparison of the Spectroscopically Measured Catalyst Transformation and Electrochemical Properties of Grubbs' First- and Second-Generation Catalysts.

According to UV-vis spectroscopy (0.10 mM, CH(2)Cl(2) at 25 °C), the catalyst transformation (which could possibly include ligand dissociation with active catalyst formation, dimer formation, and decomposition) rate constants (k (obs)) of Grubbs' first (1) and second (2) generation catalysts are 7.48 × 10(-5) and 1.52 × 10(-4) s(-1), respectively. From (31)P NMR (0.1 M, CD(2)Cl(2), at 25 °C), the catalyst transformation was 5.1% for 1 and 16.5% for 2 after 72 h. However, due to the larger concentrations of the NMR samples compared to the UV-vis samples, the extent of transformation did not correspond. The oxidation potential of the Ru(II)/Ru(III) couple of 2 (E°' = 27.5 mV at v = 200 mV s(-1)) was considerably lower than that of 1 (E°' = 167 mV at v = 200 mV s(-1)). In the case of 1, a second reduction peak appeared at slow scan rates. This may probably be ascribed to an electrochemically active compound that was formed from the intermediate cation 1 (•+) and the subsequent reduction of the latter. The oxidation/reduction of 1 proceeds according to an E(r)C(i) electrochemical mechanism (E(r) = electrochemically reversible step, C(i) = chemically irreversible step), whereas 2 proceeds according to an E(r)C(r) electrochemical mechanism (E(r) = electrochemically reversible step, C(i) = chemically reversible step).