Sigma-Bond Metathesis Reactions of Zirconocene Alkyl Cations with Phenylsilane

The zirconocene methyl cations [(C5R5)2ZrMe(ClC6D5)][B(C6F5)4] (C5R5 = C5H5 (1a),
C5H4Me (1b)) react with PhSiH3 in the dark to yield [{(C5R5)2Zr(μ-H)}2][B(C6F5)4]2 (5a,b)
and a mixture of PhxMeyHzSi products. The reaction proceeds by initial Zr−C/Si−H σ-bond
metathesis via a four-center transition state in which Si is β to Zr. In the presence of light,
significant amounts of [{(C5R5)2Zr(μ-Cl)}2][B(C6F5)4]2 (4a,b) are formed by photochemical
reaction of (C5R5)2ZrH+ species with the chlorobenzene solvent. The azazirconacycle [rac-(EBI)Zr{η2(C,N)-CH2CHMe(6-phenyl-2-pyridyl)}][B(C6F5)4] (2, EBI = 1,2-ethylene-bis-indenyl) does not react with PhSiH3 at 23 °C. However at 85 °C, 2 deinserts propene to afford
the η2-pyridyl complex [rac-(EBI)Zr{η2(C,N)-(6-phenyl-2-pyridyl)}][B(C6F5)4] (6), which is
catalytically isomerized to [rac-(EBI)Zr{η2(C,N)-2-(2-pyridyl)phenyl}][B(C6F5)4] (7) by PhSiH3.
The key step in this process is Zr−C/Si−H σ-bond metathesis of 6 with PhSiH3 via a transition
state in which Si is α to Zr. The less crowded azazirconacycle [Cp2Zr{η2(C,N)-CH2CHMe(6-methyl-2-pyridyl)}][B(C6F5)4] (3) reacts with PhSiH3 directly to afford [{Cp2Zr(SiPhH2)}2][B(C6F5)4]2 (8) via a transition state in which Si is α to Zr. Steric factors may play a role in
determining the selectivity of these reactions.