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

The reaction of the complex [Ru(η 5 -C 9 H 7 ){κ 3 ( P,C,C )-PPh 2 (CH 2 CH CH 2 )}(PPh 3 )][PF 6 ] ( 1 ) with p -XC 6 H 4 C⋮CH (X = H, Cl) yields the transient and observable vinylidene species [Ru(η 5 -C 9 H 7 ){κ 1 (P)-PPh 2 (CH 2 CH CH 2 )}(PPh 3 )( C CH( p -XC 6 H 4 ))] +, 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 κ 3 (P, C, C) to monodentate κ 1 (P) coordination, has been investigated by kinetic measurements carried out in chloroform- d at 38 °C. Plots of first-order k obs 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, k 2 = [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 ( k 1 = [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(η 5 -C 9 H 7 ){κ 1 ( P )-PPh 2 (CH 2 CH CH 2 )}(PPh 3 )] + . The vinylidene complexes [Ru(η 5 -C 9 H 7 )(C C RR‘){κ 1 ( P )-PPh 2 (CH 2 CH CH 2 )}(PPh 3 )][BF 4 ] (R‘ = H, R = Ph ( 3a ), p -MeC 6 H 4 ( 3b ), p -ClC 6 H 4 ( 3c ); R‘ = Me, R = Ph ( 4a ), p -MeC 6 H 4 ( 4b )) have been independently synthesized by electrophilic attack of HBF 4 or MeSO 3 CF 3 on the alkynyl derivatives [Ru(η 5 -C 9 H 7 )(C⋮CR){κ 1 ( P )-PPh 2 (CH 2 CH CH 2 )}(PPh 3 )] (R = Ph ( 2a ), p -MeC 6 H 4 ( 2b ), p -ClC 6 H 4 ( 2c )), obtained in turn from [Ru(η 5 -C 9 H 7 )Cl{κ 1 ( P )-PPh 2 (CH 2 CH CH 2 )}(PPh 3 )].