Controlled-potential electrochemical oxidation of cis-[Ru(ROCS 2)2(PPh3)2] (R = Et, iPr) yielded corresponding Ru(III) complexes, and the crystal structures of cis-[Ru(ROCS2)2(PPh3)2] and trans-[Ru(ROCS2)2(PPh3)2](PF 6) were determined. Both pairs of complexes exhibited almost identical coordination structures. The Ru-P distances in trans-[Ru III(ROCS2)2(PPh3) 2](PF6) [2.436(3)-2.443(3) Å] were significantly longer than those in cis-[RuII(ROCS2)2- (PPh3)2] [2.306(1)-2.315(2) Å]: the smaller ionic radius of Ru(III) than that of Ru(II) stabilizes the trans conformation for the Ru(III) complex due to the steric requirement of bulky phosphine ligands while mutual trans influence by the phosphine ligands induces significant elongation of the RuIII-P bonds. Cyclic voltammograms of the cis-[Ru(ROCS 2)2-(PPh3)2] and trans-[Ru(ROCS 2)2(PPh3)2]+ complexes in dichloromethane solution exhibited typical dual redox signals corresponding to the cis-[Ru(ROCS2)2(PPh3)2] +/0 (ca. +0.15 and +0.10 V vs ferrocenium/ferrocene couple for R = Et and iPr, respectively) and to trans-[Ru(ROCS2) 2(PPh3)2]+/0 (-0.05 and -0.15 V vs ferrocenium/ferrocene for R = Et and iPr, respectively) couples. Analyses on the basis of the Nicholson and Shain's method revealed that the thermal disappearance rate of transient trans-[Ru(ROCS2) 2(PPh3)2] was dependent on the concentration of PPh3 in the bulk: the rate constant for the intramolecular isomerization reaction of trans-[Ru(iPrOCS2) 2(PPh3)2] was determined as 0.338 ± 0.004 s-1 at 298.3 K (ΔH* = 41.8 ± 1.5 kJ mol -1 and ΔS* = -114 ± 7 J mol-1 K -1), while the dissociation rate constant of coordinated PPh 3 from the trans-[Ru(iPrOCS2) 2(PPh3)2] species was estimated as 0.113 ± 0.008 s-1 at 298.3 K (ΔH* = 97.6 ± 0.8 kJ mol-1 and ΔS* = 64 ± 3 J mol-1 K -1), by monitoring the EC reaction (electrode reaction followed by chemical processes) at different concentrations of PPh3 in the bulk. It was found that the trans to cis isomerization reaction takes place via the partial dissociation of iPrOCS2- from Ru(II), contrary to the previous claim that it takes place by the twist mechanism.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry