Kinetics of ferrocenoylacetonato substitution from (1,5-cyclooctadiene)(ferrocenoylacetonato)rhodium(I) with 2,2′-dipyridyl and derivatives of 1,10-phenanthroline

Treatment of MeOH solutions of [Rh(cod)(fca)] (cod = 1,5-cyclooctadiene, fca = ferrocenoylacetonato) with seven derivatives of 1,10-phenanthroline (N,N), as well as with the (N,N) ligand 2,2'-dipyridyl, gave [Rh(cod)(N,N)](+). The kinetics of these reactions follow the rate law: Rate = k[Rh(cod)(fca)][N,N]. The temperature dependence of all the studied substitutions resulted in activation entropies, DeltaS(not equal), more negative than -100 J K-1 mol(-1) which is indicative of associative mechanisms. The pK(a)'s of the incoming phenanthroline derivatives were between 3.03 and 6.31 but did not influence the reaction rate to any significant extent. This implies that the rate determining step during the substitution involves Rh-O bond breaking and not Rh-N bond formation. Substitution of fca with 2,2'-dipyridyl was slightly faster (k = 118 dm(3) mol(-1) s(-1)) than with the 1,10-phenanthroline derivatives (k(average) = 14.2 dm(3) mol(-1) s(-1)) and may be attributed to the free rotation capability of the two pyridyl rings about the 1-1' carbon-carbon axis in 2,2'-dipyridyl. 1,10-Phenanthroline cannot rotate about the corresponding carbon axis.