Formation of Trivalent Zirconocene Complexes from ansa-Zirconocene-Based Olefin-Polymerization Precatalysts: An EPR- and NMR-Spectroscopic Study

Reduction of Zr­(IV) metallocenium cations with sodium amalgam (NaHg) produces EPR signals assignable to Zr­(III) metallocene complexes. The chloro-bridged heterodinuclear ansa-zirconocenium cation [(SBI)­Zr­(μ-Cl)2AlMe2]+ (SBI = rac-dimethylsilylbis­(1-indenyl)), present in toluene solution as its B­(C6F5)4– salt, thus gives rise to an EPR signal assignable to the complex (SBI)­ZrIII(μ-Cl)2AlMe2, while (SBI)­ZrIII-Me and (SBI)­ZrIII(μ-H)2AliBu2 are formed by reduction of [(SBI)­Zr­(μ-Me)2AlMe2]+ B­(C6F5)4– and [(SBI)­Zr­(μ-H)3(AliBu2)2]+ B­(C6F5)4–, respectively. These products can also be accessed, along with (SBI)­ZrIII-iBu and [(SBI)­ZrIII]+ AlR4–, when (SBI)­ZrMe2 is allowed to react with HAliBu2, eliminating isobutane en route to the Zr­(III) complex. Further studies concern interconversion reactions between these and other (SBI)­Zr­(III) complexes and reaction mechanisms involved in their formation.