HfC5H52Cl2

Hafnium cyclopentadienyl dichloride, often written Cp2HfCl2 or Hf(C5H5)2Cl2, is an organometallic compound of hafnium. The molecule features a hafnium(IV) center bound to two η5-cyclopentadienyl ligands and two chloride ligands, creating a sandwich-like metallocene with a pseudo-octahedral coordination environment. The compound corresponds to a 16-electron species, with each cyclopentadienyl ligand donating six electrons and each chloride providing two electrons.

Structure and properties

Cp2HfCl2 adopts the typical metallocene geometry of two parallel cyclopentadienyl rings occupying adjacent coordination sites with two chloride ligands completing the coordination sphere. This arrangement yields a relatively rigid, cation-like metal center that is reactive toward activation by cocatalysts in catalytic cycles. The solid is generally handled under inert atmosphere due to sensitivity to air and moisture.

Synthesis

A common preparation method involves a metathesis reaction between hafnium tetrachloride and sodium cyclopentadienide: HfCl4 + 2 NaCp → Cp2HfCl2 + 2 NaCl. Reactions are typically performed in dry solvents under inert gas, such as THF or diethyl ether, to minimize hydrolysis and decomposition.

Reactivity and applications

Cp2HfCl2 serves as a precatalyst in olefin polymerization and copolymerization after activation with cocatalysts, notably methylaluminoxane (MAO) or boron-based activators. The hafnium analogs of other metallocenes (e.g., Cp2ZrCl2) are investigated for catalyst performance in polymerization, enabling control of polymer molecular weight and tacticity under suitable activation conditions.

Safety and handling

The compound is moisture- and air-sensitive and should be handled under inert atmosphere. It can hydrolyze upon exposure to water or air, producing hafnium oxides/hydroxides and releasing hydrogen chloride. Proper lab safety practices, including gloves and eye protection, should be followed, and exposure to dust or fumes should be minimized.