Stereoelectronic influence of the type of bifunctional ansa-monocyclopentadienyidimethylsilylamido 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 cyclopentaclienyl-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 Zr-2(mu-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) Angstrom in 1 and 2.542(1) and 2.745(1) Angstrom 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 (70-80degrees) 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.