Linear Dimerization of Terminal Alkynes by Bis(tetramethylphenylcyclopentadienyl)titanium-Magnesium Hydride and Acetylide Complexes

Abstract

New tetramethylphenylcyclopentadienyl trinuclear titanocene hydride-magnesium hydride complexes [{Ti(η5-C5Me4R)2(μ-H)2}2Mg], where R = Ph (1) or 4-fluorophenyl (FPh; 2), the dinuclear [Ti(η5-C5Me4Ph)[η5:η1{Ti:Mg}-C5Me4(o-C6H4)](μ-H)2Mg(THF)2] (3) and [Ti(η5-C5Me4Ph)2(μ-H)2MgC(Me)=CHMe] (4) complexes, and the [{Ti(η5-C5Me4Ph)2(η1-C≡CSiMe3)2}-{MgCl(THF)}+] (5) tweezer complex initiated the dimerization of (trimethylsilyl)ethyne (TMSE) or 1-hexyne (HXYN) to exclusively head-to-tail (HTT) dimers at 60 °C with the turnover number ranging from 300 to 500 mol alkyne per mol of the Ti complex. In contrast, all of them were inactive in the dimerization of tert-butylethyne (TBUE). Monitoring of reactions of the 1-5 complexes with the alkynes by electron spin resonance (ESR) method revealed a decay of the initial complexes 1-5 in the dimerizing systems with TMSE and HXYN or a conversion of complexes 3, 4, and 5 into the Ti(III) acetylide [Ti(η5-C5Me4Ph)2(η1-C≡CCMe3)] (6) in systems with TBUE. The acetylide 6 also initiated the dimerization of TMSE and HXYN to HTT dimers only. This fact together with the absence of ESR signals of tweezer complexes and acetylides in the reacting hydride systems allow us to assume that the rate of conversion of complexes 1-4 to tweezer complexes is slower than the rate of dissociation of the tweezer complexes to give the acetylides which are apparently the ultimate catalytic species. Most of the evidence on the catalytic complexes has been obtained from ESR spectra which are reliably characteristic of each type of the complexes. Crystal structures of 4 and 6 were determined.