SYNTHESIS AND INSERTION CHEMISTRY OF CATIONIC ZIRCONIUM(IV) PYRIDYL COMPLEXES - PRODUCTIVE SIGMA-BOND METATHESIS

The reaction of Cp2Zr(CH3)(THF)+ (1) with pyridine produces CH4 and a mixture of Cp2Zr(η2-N,C-pyridyl)(THF)+ (2) and Cp2Zr(η2-N,C-pyridyl)(pyridine)+ (3) via ortho-C–H bond activation. Complex 2 is converted to 3 by reaction with excess pyridine. Complex 1 reacts similarly with 2-methylpyridine (α-picoline) to yield the picolyl complex Cp2Zr{η2-N,C-NC5H3(6-Me)}(THF)+ (6). An intermediate picoline adduct Cp2Zr(CH3)(picoline)+ (7a) is observed in this reaction. Low-temperature NMR studies of 7a and the (C5H4Me)2Zr analogue 7b reveal a high-field1H shift and a reduced JC-H for the ortho C–H of the coordinated picoline suggestive of an agostic interaction. The three-membered Zr–N–C ring structures of 2, 3, and 6 are assigned on the basis of1H and13C NMR spectral data and confirmed by X-ray crystallographic analysis of Cp2Zr{η2-N,C-NC5H3(6-Me)}(PMe3)+ (8), which is formed by reaction of 6 with PMe3. Complex 8 crystallizes in space group Cc with a = 9.571 (4) Å, b = 17.524 (11) Å, c = 21.861 (7) Å, β = 92.55 (3)°, V = 3662.9 (5.1) Å3, and Z = 4. The picolyl ligand of 8 lies in the plane between the two Cp ligands, and the Zr–C (2.29 (2) Å) and Zr–N (2.21 (1) Å) bond lengths are similar. Complex 6 reacts readily with ethylene, propylene, and 2-butyne to yield insertion products 9-11, which have five-membered chelate ring structures. These reactions proceed via initial THF dissociation from 6 followed by coordination of substrate and insertion into the Zr–C bond. Complex 9 crystallizes in space group P1 with a = 10.387 (2) Å, b = 12.129 (3) Å, c = 13.844 (4) Å, α = 87.78 (2)°, β = 76.37 (2)°, γ = 76.63 (2)°, V = 1648.8 (9) Å3, and Z = 2. The chelate ring of 9 is relatively unstrained. Complexes 9 and 10 react with CH3CN to yield 2-ethenyl-6-methylpyridine and 2-(1-methylethenyl)-6-methylpyridine (14), respectively, and Cp2Zr(N═CHMe)(CH3CN)+ (13). These reactions proceed by ligand-induced ring opening, β-H elimination, and trapping of the Zr–H product by CH3CN insertion. Complex 10 reacts similarly with PMe3 to yield 14 and Cp2Zr(H)(PMe3)2+ (17) via ring opening, β-H elimination, and trapping of the Zr–H product by PMe3 coordination. These Zr-mediated olefin/picoline coupling reactions illustrate how productive reaction schemes may be constructed by combining σ-bond metathesis, insertion, and β-H elimination reactions. © 1990, American Chemical Society. All rights reserved.