Stereoelectronic Influence of the Type of Bifunctional ansa-Monocyclopentadienyldimethylsilylamido Ligand on the Molecular Structures Displayed by Zirconium Dichloride and 1,4-Diphenylbutadiene Complexes

A series of organozirconium dichloride and 1,4-diphenylbutadiene complexes featuring a dianionic bifunctional ligand with a cyclopentadienyl-type functionality and an appended amido N donor have been prepared and structurally characterized. [(C₅H₄)SiMe₂(N-t-Bu)]ZrCl₂, 1, [(C₉H₆)SiMe₂(N-t-Bu)]ZrCl₂, 2, and [(C₅Me₄)SiMe₂(N-i-Pr)]ZrCl₂, 3, were prepared in two steps, with ligand chelation accomplished by an amine elimination reaction followed by treatment of the diamido Zr intermediate with an excess of SiMe₃Cl. X-ray structural analyses reveal that in the solid state 2 is monomeric, whereas 1 and 3 are centrosymmetric dimers linked by a pair of bridging chlorides. The level of asymmetry displayed by the central Zr₂(μ-Cl)₂ moiety is indicated by the variation in the pair of independent bridging Zr−Cl bond distances, which are 2.618(1) and 2.657(1) Å in 1 and 2.542(1) and 2.745(1) Å in 3, respectively. The metathetical reactions of [Mg(C₄H₄Ph₂)(THF)₃]_n with 1, 2, 3, and [(C₅Me₄)SiMe₂(N-t-Bu)]ZrCl₂ proceed to afford the corresponding 1,4-diphenylbutadiene derivatives 4, 5, 6, and 7, respectively. Solution NMR data show that 6 is obtained exclusively as the supine isomer, whereas compounds 4, 5, and 7 exist as >20:1, 6:1, and 2:1 mixtures of the supine and prone isomers at ambient temperature. The molecular structures of the supine forms of 4, 5, 6, and 7 are appreciably folded (70−80°) along the line of intersection between the plane containing the Zr and the two terminal butadiene carbons and the plane of the cis-butadiene fragment. An increase in the folding is accompanied by a decrease in the difference between the average Zr−C(terminal) and Zr−C(internal) bond distances and leads to a more pronounced long−short−long C−C bond sequence within the coordinated butadiene.