Reaction of the Ruthenium(II) Indenyl Complex [Ru(η5-C9H7){κ3(P,C,C)-PPh2(CH2CHCH2)}(PPh3)][PF6] with Terminal Alkynes. Mechanisms of 1-Alkyne to η1-Vinylidene Transformation and Kinetic Detection of Hemilability of the Allylphosphine Ligand

Abstract

The reaction of the complex [Ru(η5-C9H7){κ3(P,C,C)-PPh2(CH2CHCH2)}(PPh3)][PF6] (1) with p-XC6H4C⋮CH (X = H, Cl) yields the transient and observable vinylidene species [Ru(η5-C9H7){κ1(P)-PPh2(CH2CHCH2)}(PPh3)(CCH(p-XC6H4))]+, which react further by an intramolecular [2 + 2] cycloaddition process, forming bicyclic alkylidene compounds. The formation of the vinylidene intermediates, associated with a change of the binding mode of the allylphosphine ligand from κ3(P,C,C) to monodentate κ1(P) coordination, has been investigated by kinetic measurements carried out in chloroform-d at 38 °C. Plots of first-order kobs values vs concentration of arylalkyne are linear with a positive intercept on the y axis. The reaction proceeds via two parallel pathways, one which is first order in complex 1 and first order in arylalkyne, second order overall, and one which is zero order in arylalkyne, overall first order. The second-order pathway implies rate-determining nucleophilic attack of the arylalkyne on complex 1, k2 = [5.5(±0.4)] × 10-4 (X = H) and [2.8(±0.3)] × 10-4 M-1 s-1 (X = Cl) being the corresponding rate constants, associated with displacement of the allylic double bond or a haptotropic shift of the indenyl ring. The first-order pathway involves rate-determining formation of a transient 16-electron intermediate arising from complex 1 by reversible decoordination of the allylic double bond and rapid reaction with the incoming arylalkyne. The rate of this route is independent of the concentration and nature of the alkyne (k1 = [9.5(±2.5)] × 10-5 s-1 for X = H; [6.9(±1.5)] × 10-5 s-1 for X = Cl) and corresponds to the rate of formation of the intermediate species [Ru(η5-C9H7){κ1(P)-PPh2(CH2CHCH2)}(PPh3)]+. The vinylidene complexes [Ru(η5-C9H7)(CCRR‘){κ1(P)-PPh2(CH2CHCH2)}(PPh3)][BF4] (R‘ = H, R = Ph (3a), p-MeC6H4 (3b), p-ClC6H4 (3c); R‘ = Me, R = Ph (4a), p-MeC6H4 (4b)) have been independently synthesized by electrophilic attack of HBF4 or MeSO3CF3 on the alkynyl derivatives [Ru(η5-C9H7)(C⋮CR){κ1(P)-PPh2(CH2CHCH2)}(PPh3)] (R = Ph (2a), p-MeC6H4 (2b), p-ClC6H4 (2c)), obtained in turn from [Ru(η5-C9H7)Cl{κ1(P)-PPh2(CH2CHCH2)}(PPh3)].