Preparation and Structural Characterization of <i>ansa</i>-Mono(cyclopentadienyl)dimethylsilylamido Group 4 Metal Butadiene Complexes. Observation of a Tetranuclear Structure for [(C₅H₄)SiMe₂(N-t-Bu)]Zr(C₄H₆) Featuring Bridging Butadiene Ligands

Abstract

The metathetical reactions of [(C5H4)SiMe2(N-t-Bu)]MCl2, M = Ti, Zr, with [Mg(C4H6)(THF)2]n afford the respective group 4 metal butadiene complexes, [(C5H4)SiMe2(N-t-Bu)]M(C4H6). The solution NMR and X-ray crystallographic data for [(C5H4)SiMe2(N-t-Bu)]Ti(C4H6) indicate that the butadiene moiety adopts a symmetrical prone structure consistent with a Ti(II) π-diene bonding representation. Alternatively, the solution and solid-state structures for its Zr analogue are markedly different. Whereas the observed chemical shift dispersion for the anti and meso butadiene protons of [(C5H4)SiMe2(N-t-Bu)]Zr(C4H6) supports a zirconacyclopentene structure in solution, the solid-state structure of this compound is actually tetranuclear. The four [(C5H4)SiMe2(N-t-Bu)]Zr units of {[(C5H4)SiMe2(N-t-Bu)]Zr(C4H6)}4 are related by a crystallographically imposed S4 rotation axis and are linked by four unsymmetrically bridging butadiene groups via an unusual ((1,2,3-η3)-Zr−(4-μ2)-Zr) bonding interaction. The five-coordinate trigonal-bipyramidal geometry about the doubly bridging carbon is characterized by a nearly linear Zr−(μ2-C)−Zr bond angle of 173.2(2)° and two rather long Zr−C bond distances of 2.636(5) and 2.530(5) Å.