Group 4 dimethylmetallocenes: improved synthesis and reactivity studies.

Group 4 dimethylmetallocenes are catalyst precursors for the methylmetallocenium/borate catalyst systems for olefin polymerization, and they are usually prepared by methylation (with MeMgCl or MeLi) of the parent metallocene dichlorides. We describe here a simpler preparation of a series of bisindenyldimethylmetallocenes carried out by reacting the pi-ligand with a 2-fold excess of MeLi, and then MtCl(4) (Mt = Ti, Zr, Hf). This simple, one-pot method produces the dimethylated complexes in higher overall yield, and saves on reaction time and solvents. Ind(2)MtMe(2) (1, Mt = Ti; 2a, Mt = Zr; 3, Mt = Hf), (4,7-Me(2)Ind)(2)ZrMe(2) (4), rac/meso-[C(2)H(4)(Ind)(2)]ZrMe(2) (5), meso-[C(2)H(4)(4,7-Me(2)Ind)(2)]ZrMe(2) (m-6a), and meso-[C(2)H(4)(4,7-Me(2)Ind)(2)]HfMe(2) (m-7a) have been prepared in 40-80% isolated yields. 2a reacts with 1-4 equiv of t-BuOH to give the mono-tert-butoxy derivative 2b, Ind(2)ZrMe(O-t-Bu), while reaction with 2 equiv of C(6)F(5)OH cleanly affords Ind(2)Zr(OC(6)F(5))(2) (2c). Analogously, in the presence of 2 equiv of t-BuOH, m-6a gives meso-[C(2)H(4)(4,7-Me(2)Ind)(2)]ZrMe(O-t-Bu) (m-6b) with replacement of the outward methyl group only, as established by NMR analysis; meso-[C(2)H(4)(4,7-Me(2)Ind)(2)]Zr(OC(6)F(5))(2) (m-6c) is obtained by reaction with 2 equiv of C(6)F(5)OH. The molecular structures of m-6a and m-6c are also described.

[Re]

A chair conformation comparable to that observed for six-membered rings composed of tetrahedral carbon atoms is found for the cluster anion [Re(6)(µ-H)(5)(CO)(24)](-) (see picture; black spheres: Re, white spheres: µ-H; CO ligands omitted for clarity) in spite of the octahedral coordination at the Re centers. This is the first example of a carbonyl cluster exhibiting a cyclohexane-like geometry of the metallic framework.