Transition metal benzyne complex

Transition metal benzyne complexes are organometallic complexes that contain benzyne ligands (C₆H₄). Unlike benzyne itself, these complexes are less reactive although they undergo a number of insertion reactions.^[1]

Examples

The studies of metal-benzyne complexes were initiated with the preparation of zirconocene complex by reaction diphenylzirconocene with trimethylphosphine.^[2]

Cp₂ZrPh₂ + PMe₃ → Cp₂Zr(C₆H₄)(PMe₃) + PhH

The preparation of Ta(η⁵-C₅Me₅)(C₆H₄)Me₂ proceeds similarly, requiring the phenyl complex Ta(η⁵-C₅Me₅)(C₆H₅)Me₃. This complex is prepared by treatment of Ta(η⁵-C₅Me₅)Me₃Cl with phenyllithium.^[3] Upon heating a solution of this complex, methane was lost, leaving the benzyne complex:

Ta(η⁵-C₅Me₅)(C₆H₅)Me₃ → Ta(η⁵-C₅Me₅)(C₆H₄)Me₂ + CH₄

The nickel derivative Ni(η²-C₆H₄)(Cy₂PCH₂CH₂PCy₂) was the second type metal-benzyne complex to be reported and structurally characterized. Synthesis involves dehalogenation of the chlorophenyl complex NiCl(C₆H₄Br-2)(Cy₂PCH₂CH₂PCy₂) with sodium amalgam, which furnished the yellow nickel(0) benzyne complex. Its coordination geometry is close to trigonal blanar and the benzyne ligand was determined to be planar. However, both the nickel and phosphorus atoms were found to be slightly displaced to one side of the benzyne plane. This complex reacts with a variety of electrophiles.^[4] With trifluoroacetic acid, benzene is lost to give the trifluoroacetate:^[4]

Ni(η²-C₆H₄)(Cy₂PCH₂CH₂PCy₂) + 2 CF₃CO₂H → Ni(O₂CCF₃)₂(Cy₂PCH₂CH₂PCy₂ + C₆H₆

Structural trends

Several benzyne complexes have been examined by X-ray crystallography.

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Bond lengths (Å) in Ta,[2] Ni[4] and Zr[1] benzyne complexes Bond Ta Ni Zr M-C1 2.059 1.868 2.267 M-C2 2.091 1.870 2.228 C1-C2 1.364 1.332 1.364 C2-C3 1.410 1.389 1.389 C3-C4 1.362 1.383 1.383 C4-C5 1.403 0.93 1.380 C5-C6 1.375 1.383 1.377 C6-C1 1.408 1.386 1.406

Bond angles (degrees) in Ni[4] and Zr[1] benzyne complexes Angle Ni Zr 1 41.9 35.3 2 69.2 70.8 3 69.1 73.9 4 122.9 120.2 5 122.3 122.1 6 116.1 (n/a) 7 121.5 (n/a) 8 121.0 (n/a) 9 116.2 (n/a)

References

1. ↑ 1.0 1.1 1.2 Buchwald, S. L.; Nielsen, R. B. "Group 4 Metal Complexes of Benzynes, Cycloalkynes, Acyclic Alkynes, and Alkenes" Chem. Rev. 1988, volume 88, 1047-1058
2. ↑ 2.0 2.1 Buchwald, S. L.; Watson, B. T. The Trimethylphosphine Adduct of the Zirconocene-Benzyne Complex: Synthesis, Reactions, and X-ray Crystal Structure. J. Am. Chem. Soc. 1986, 108, 7411-7413
3. ↑ McLain, S. J.; Schrock, R. R.; Sharp, P. R.; Churchill, M. R.; Youngs, W. J. Synthesis of Monomeric Niobium- and Tantalum-Benzyne Complexes and Molecular Structure of Ta(η⁵-C₅Me₅)(C₆H₄)Me₂" J. Am. Chem. Soc. 1979, volume 101, 263-265. doi:10.1021/ja00495a067
4. ↑ 4.0 4.1 4.2 4.3 Bennett, M. A.; Hambley, T. W.; Roberts, N. K.; Robertson, G. B. "Synthesis and Single-Crystal X-ray Study of the Mononuclear η2-Benzyne (Dehydrobenzene) Nickel(0) Complex Ni(η2-C6H4)((C6H11)2PCH2CH2P(C6H11)2). Insertion Reactions with Simple Molecules and X-ray Crystal Structure of the Nickelaidan Complex Ni(CH2CH2C6H4-o)((C6H11)2PCH2CH2P(C6H11)2)" Organometallics 1985, 4, 1992-2000.doi:10.1021/om00130a012