Zirconium complex boratabenzene

Various bridged salt metathesis, including ethylene-bridged bis(A,A-diisopropylaminoboratabenzene)zirconium complexes 1235, dimethylsilylene-bridged bis(A,A-diisopropylaminoboratabenzene)zirconium dichloride 1236, and dimethylmethylene-bridged (cyclopenta-dienyl)(A,A-diisopropylaminoboratabenzene)zirconium complexes 1237.944 As the crystal structures of the dimethyl... 

Monoanionic boratabenzene ligands are isoelectronic to cyclopentadienide anion and can be nsed to synthesize Zr equivalent metallocenes (eqnation 29). A more congested complex of 9-phenyl-9-borata-anthracene (C 1-synunetry) was obtained by a similar way. Mono and bis boratabenzene complexes can also be prepared by reaction of a nentral boratabenzene-PMes adduct with tetrabenzyl zirconium or hafnium. Boratabenzene ansa metallocenes with CH2-CH2 or Si(CH3)2 bridges as well as Cp-boratabenzene mixed complexes or CG complexes were synthesized. 

The zwitterionic complexes 17 and 18 behave in the expected fashion. In particular, the typical boratabenzene distortions observed with neutral zirconium complexes are retained. The exchange process, which requires generation of the ion separated intermediate II in Scheme 4, occurs more readily for R = NMes than for R = Ph. We attribute the foster rates to better stabilization of the positive charge at zirconium by the... 

The sterically less demanding ethoxyboratabenzene zirconium complex 1228 (Scheme 292), when activated with MAO, produces a-olefins from ethylene, as a result of the much enhanced / -hydrogen transfer rate as compared with the AyV-diisopropylaminoboratabenzene zirconium derivative.939 Zirconium complexes 1229 bearing the phenylborataben-zene ligands are obtained by salt metathesis, where both boratabenzene rings in the structurally characterized 1229... 

Starting from stannacyclohexadienes and BHaL, Ashe and coworkers reported in 1971 a general synthesis of lithium boratabenzenes, which can easily be converted into other metal complexes (75PAC513). The zirconium(IV) complexes with two boratabenzene rings and two halogen... 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

Variations in Zr-B bond distances for complexes of the type (CjHjB-R ZrCfe indicate that the nature of R affects the bonding relationship between zirconium and boratabenzene (Table 1). These data imply that for R = Ph, OEt and Me, the interactions between Zr and B are strong. Boratabenzene with R = NPr2 binds in an r -pentadienyl fashion with little overlap between B and Zr. Unfortunately, these metrical parameters do not give sufficient insight into differences in the electron density at the metal. Furthermore, they are measurements of precatalyst structures and it is unclear how they relate to the putative cationic catalytic species. 

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