Syntheses and Structures of Zirconium Tetranuclear Polyhydrides

Compound Cp*ZrCl(BH4)2, 1, was isolated from a reaction of Cp*ZrCl3 with two equivalents of Cp*Zr(BH4)3. The tetranuclear Zr(IV) polyhydride [(μ2-H)(μ3-H)(Cp*ZrCl)]4, 2, was prepared by the reaction of 1 with excess N(C2H5)3, whereas the related Zr(IV) polyhydrides [(μ2-H)3(μ2-η2-BH4){Cp*Zr(BH4)}2]2, 3, and [(μ2-H)3(μ2-H){Cp*Zr(BH4)}2]2, 4, were isolated from the reaction of Cp*Zr(BH4)3 with excess N(C2H5)3. Compound 4 can be also prepared from the reaction of Cp*Zr(BH4)3 with excess N(CH3)3. Single-crystal X-ray structures of 1, 2, 3, and 4 were determined. In compound 1 two tetrahydroborate ligands coordinate to a zirconium atom in a tridentate bonding mode. Compounds 2, 3, and 4 are tetranuclear polyhydrides. Compound 2 features a butterfly arrangement of zirconium atoms surrounded by eight bridging hydrides. Each triangular face is capped by a μ3-bridging hydride, and the other four hydrides are μ2-bridging to the zirconium atoms in the perimeter. Compounds 3 and 4 are rectangular planar polyhydrides where the adjacent Zr atoms in 3 are linked by a bridging tetrahydroborate ligand and three μ2-bridging hydrides, and the adjacent Zr atoms in 4 are linked by a μ2-bridging hydride and three μ2-bridging hydrides. The proton NMR spectrum and the solid-state structure of 4·(toluene) suggested that the tetramer dissociates to [Cp*Zr(BH4)H(μ-H)]2 in solution.