Structures and dynamic features of dinuclear tartratozirconocene complexes

Abstract

The reaction of dimethylzirconocene with 2 R,3 R-diethyltartrate proceeds with evolution of two equivalents of methane and yields the chiral diethyltartratozirconocene dimer 2. The analogous reaction between Cp 2Zr(CH 3) 2 and 2 S,3 S-diethyltartrate furnishes ent- 2 in almost quantitative yield. Complex 2 contains a dimetallatricyclic framework (tetraoxadizirconatricyclo[5.3.0.0 2,6]decane) in solution as well as in the solid state. Complex 2 was characterized by X-ray diffraction. It crystallizes in space group P2 1 with cell parameters a=9.494(1), b=20.122(2), c=9.915(1) Å, β=108.36(1)°, Z=2, R=0.039 and R w=0.046. In solution the tartratozirconocene dimer 2 undergoes an intramolecular automerization reaction (Δ G ‡ intra (265 K)≈ 13±1 kcal mol −1) which probably proceeds via a ten-membered metallacyclic intermediate or transition state. A cross-over experiment between the enantiomeric complexes 2 and ent- 2 showed the activation barrier for intermolecular exchange of mononuclear dioxazirconacyclopentane units Cp 2 ZrOCH(E)CH(E)O (ECO 2C 2H 5) leading to equilibration with the isomeric dinuclear tartratozirconocene complex meso- 2 to be at Δ G ‡ inter (290 K)≈ 22±1 kcal mol −1. Complex meso-2 shows NMR spectra compatible with a static ten-membered dimetallamonocyclic ring structure. The reaction of dimethylzirconocene with meso-dimethyltartrate yields the ( meso-dimethyltartrato)zirconocene dimer 4 (two isomers, 4a and 4b in a 60:40 ratio, both of which possess dynamic dimetallatricyclic structures in solution (Δ G‡ intra (224 K)≈11±1 kcal mol −1 ( 4a); Δ G‡ intra (285 K)≈14±1 kcal mol −1 ( 4b); Δ G‡ inter (300 K)=24.7±1.0 kcal mol −1 ( 4a→-4b). Crystallization from methylene chloride gives single crystals of the isomeric ( meso-dimethyltartrato)zirconocene dimer 5a which contains a monocyclic ten-membered ring structure. Complex 5a crystallizes in space group P2 1/ c with cell parameters a=11.261(2), b=14.511(2), c=11.562(2) Å, β=117.81(1)°; Z=2, R=0.047 and R w=0.063.