Preparation and Structural Characterization of ansa-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

The metathetical reactions of [(C 5 H 4 )SiMe 2 (N-t-Bu)]MCl 2, M = Ti, Zr, with [Mg(C 4 H 6 )(THF) 2 ] n afford the respective group 4 metal butadiene complexes, [(C 5 H 4 )SiMe 2 (N-t-Bu)]M(C 4 H 6 ). The solution NMR and X-ray crystallographic data for [(C 5 H 4 )SiMe 2 (N-t-Bu)]Ti(C 4 H 6 ) 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 [(C 5 H 4 )SiMe 2 (N-t-Bu)]Zr(C 4 H 6 ) supports a zirconacyclopentene structure in solution, the solid-state structure of this compound is actually tetranuclear. The four [(C 5 H 4 )SiMe 2 (N-t-Bu)]Zr units of {[(C 5 H 4 )SiMe 2 (N-t-Bu)]Zr(C 4 H 6 )} 4 are related by a crystallographically imposed S 4 rotation axis and are linked by four unsymmetrically bridging butadiene groups via an unusual ((1,2,3-η 3 )-Zr−(4-μ 2 )-Zr) bonding interaction. The five-coordinate trigonal-bipyramidal geometry about the doubly bridging carbon is characterized by a nearly linear Zr−(μ 2 -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.