Synthesis and Characterization of Chiral Group 4 Metallocene Alkyne Complexes: (η⁵-menthyl-C₅H₄)₂M(η²-Me₃SiC₂SiMe₃), M = Ti, Zr

Abstract

show a similar complexation behaVior and reactiVity to their achiral metallocene congeners. For example, the reaction with ethylene was studied; whereas the zirconium complex 2-Zr yields the zirconacyclopentane 3 in a well-defined reaction, the titanium analogue 2-Ti produces only an as yet unseparable mixture of different compounds. 2-Ti, 2-Zr, and 3 were characterized by X-ray crystal structure analysis.For stoichiometric and catalytic reactions of organometallic compounds it is necessary to have suitable complex fragments that are coordinatively and electronically unsaturated. In group 4 chemistry such complex fragments are titanocene "Cp′ 2 Ti", zirconocene "Cp′ 2 Zr", and hafnocene "Cp′ 2 Hf" (Cp′ ) substituted or unsubstituted η 5 -cyclopentadienyl), which are considered as generally unstable 14-electron compounds having a d 2 configuration. 1 The aim is to find an adequate precursor that liberates the very reactive core complex under mild conditions. There are several systems known that are able to generate metallocene fragments. 2 We used acetylene ligands to stabilize the metallocene fragments. The complexation of bis(trimethylsilyl)acetylene by metallocenes leads to three-membered metallacyclopropenes Cp′ 2 M(L)(η 2 -Me 3 SiC 2 SiMe 3 ), for example, Cp 2 Zr(py)(η 2 -Me 3 SiC 2 SiMe 3 ). 1 The acetylene is easily exchangeable by substrate molecules. The presence of the additional donor "L" depends on either the metal or the nature of the cyclopentadienyl. Such titanium and zirconium compounds show a very broad chemistry: by using different metals, Cp ligands, and additional ligands a fine-tuning of the complexes is possible. A chiral modification was tried by using (S)-(-)-nicotine as additional ligand in the complex rac-(ebthi)Zr(L)(η 2 -Me 3 Si-C 2 SiMe 3 ), with ebthi ) 1,2-ethylene-1,1′-bis(η 5 -tetrahydroindenyl). 3 Recently, Gansäuer et al. published a convenient method to synthesize chiral metallocene dichlorides Cp′ 2 MCl 2 (Cp′ ) η 5 -menthyl-C 5 H 4 ) (1-M, M ) Ti, Zr), which have menthylsubstituted Cp ligands and were first synthesized by Kagan. 4a,b,d Very recently, we used a modified procedure to prepare the metallocene dichlorides and the corresponding difluorides Cp′ 2 MF 2 (M ) Ti, Zr, Hf). 4c These complexes do not undergo any racemization. This offers the possibility for enantioselective reactions with enantiomeric pure complexes.

Reaction of Zirconocene–Carboryne with Alkenes: Synthesis and Structure of Zirconacyclopentanes with a Carborane Auxiliary

“…18 The Zr−C(1) distances (2.389(3)−2.502(4) Å) are very close to the corresponding values observed in zirconocene− carboranyl complexes. 19 The Zr−C(4) distances fall in the range 2.268(8)−2.401(2) Å, which are comparable to the corresponding values of 2.289(4)/2.307(4) Å in rac-[1,2ethylene-1,1′-bis(η 5 -tetrahydroindenyl)]Zr(C 4 H 8 ), 15 2.302(2)/ 2.307(2) Å in (η 5 -menthyl-C 5 H 4 ) 2 Zr(C 4 H 8 ), 20 and 2 21 On the other hand, the sum of five interior angles on the five-membered zirconacyclopentane ring falls in the range 527.6−529.4°( Table 2), suggestive of a nonplanar geometry, which is different from those of zirconacyclopentenes incorporating a carboranyl unit. 9…”

Syntheses and Molecular Structures of [(thf)4Li] [{(thf)Li}M(C4H8)3] (M = Zr, Hf) and Their Solution Behavior

“…It is noteworthy that in the 13 C{ 1 H} NMR spectra the α‐carbon atoms were found at lower field than the β‐carbon atoms ( 1‐Zr in [D 8 ]THF: α‐C 49.9 ppm, β‐C 36.2 ppm; in [D 8 ]toluene: α‐C 47.9 ppm, β‐C 33.9 ppm; 1‐Hf in [D 8 ]THF: α‐C 56.1 ppm, β‐C 36.2 ppm; in [D 8 ]toluene: α‐C 57.2 ppm, β‐C 34.9 ppm). Such shifts of the α‐carbon atoms to lower field have also been observed earlier for zircona‐ and hafna‐ cyclo ‐pentane congeners and was explained with the heavy atom effect induced by these metal atoms. The 1 H NMR resonances of the butane‐1,4‐diide ligands lie in the expected ranges ( 1‐Zr in [D 8 ]THF: α‐CH 2 0.08 ppm, β‐CH 2 1.78 ppm; in [D 8 ]toluene: α‐CH 2 0.83 ppm, β‐CH 2 2.76 ppm; 1‐Hf in [D 8 ]THF: α‐CH 2 0.05 ppm, β‐CH 2 1.96 ppm; in [D 8 ]toluene: α‐CH 2 0.86 ppm, β‐CH 2 2.89 ppm).…”

Group 4 Transition Metal Sandwich Complexes: Still Fresh after Almost 60 Years

“…The hindered acetylene Me 3 SiCCSiMe 3 has been shown to be particularly effective for this purpose, and numerous applications to organic synthesis have been described . This chemistry and related cumulene compounds, including a wealth of titanium chemistry, has been the subject of many publications, including reviews. − Because of this extensive coverage, specific details of this important work will not be presented here. Attempts to extend this approach to the corresponding hafnium compounds have produced different outcomes, owing to the strength of the metal−alkyne interaction, which inhibits dissociation of the acetylene .…”