Mechanism of alkyne insertion into the Ru-C bonds of orthoruthenated compounds featuring similarity of the Ru(II) and Pd(II) reactions

The pseudo-first-order rate constants k(obs) for the reaction between the ring-substituted orthoruthenated N,N-dimethylbenzylamines [Ru-II(eta(G)-C6H6)(o-C(6)H(3)RCH(2)NMe(2))Cl] (R = 4,5-(MeO)(2), 5-Me, H, 5-F), and alkynes R'C drop CR'' (R'/R'' = Ph/Ph, Ph/C6H4CF3-3, Ph/C6H4NO2-4, Et/Et, CO(2)Me/CO(2)Me) to afford the isoquinolinium cations coordinated to the [Ru-0(eta(6)-C6H6)] moiety are a linear function of [R'C drop CR''] in methanol at 20.0-40.0 degrees C in accord with the rate-determining insertion of alkyne into the Ru-C bond. The latter was confirmed by the observation that the electron-donating groups R at the dimethylbenzylamine ruthenocycle favor the insertion and the slope of the corresponding Hammett plot equals -1.6 against sigma(+). The electron-poor alkynes react slower than the electron-rich ones. An X-ray structural investigation of the product of the MeO(2)CC drop CCO(2)Me reaction with the related ruthenocycle [(3-MeC(5)H(3)N-2-C6H4)Ru(mu-I)(CO)(2)](2) demonstrates that the alkyne inserts into the Ru-C bond. Addition of LiCl retards the insertion markedly indicative of the importance of the ionization of the coordinated chloride. The major reaction pathway involves the solvento species [Ru-II(eta(6)-C6H6)(o-C(6)H(3)RCH(2)NMe(2))(MeOH)](+), the existence of which was confirmed by the spectrophotometric study of the starting compound in the presence of LiCl. All the observations reported show that the d(6) system under study is very mechanistically similar to the insertion, of alkynes into the dinuclear d(8) Pd-II complexes [Pd(o-C(6)H(3)RCH(2)NMe(2))mu-Cl)](2), the key difference being the ways of creation of a coordinative site readily accessible by alkyne. These are the ionization of the Ru-CI bond and the cleavage of the [Pd(mu-Cl)(2)Pd] bridge in the ruthenium and palladium cases, respectively.