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.[2]

Structure of Ru(C₆H₄)(PMe₃)₄.[1]

Examples

The studies of metal-benzyne complexes were initiated with the preparation of zirconocene complex by reaction diphenylzirconocene with trimethylphosphine.[3]

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.[4] Upon heating, this complex eliminates methane, leaving the benzyne complex:

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

The second example of a benzyne complex is Ni(η²-C₆H₄)(dcpe) (dcpe = Cy₂PCH₂CH₂PCy₂). It is produced by dehalogenation of the bromophenyl complex NiCl(C₆H₄Br-2)(dcpe) with sodium amalgam. Its coordination geometry is close to trigonal planar.

Reactivity

Benzyne complexes react with a variety of electrophiles, resulting in insertion into one M-C bond.[5] With trifluoroacetic acid, benzene is lost to give the trifluoroacetate Ni(O₂CF₃)₂(dcpe).[5]

Structural trends

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

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Labeled carbon atoms corresponding to bonds with lengths given in table. Bond lengths (Å) in Ta,[3] Ni[5] and Zr[2] 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

Labeled bond angles that are given in table. Bond angles (degrees) in Ni[5] and Zr[2] 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. John F. Hartwig, Richard A. Andersen, Robert G. Bergman (1989). "Synthesis of a Highly Reactive (Benzyne)ruthenium Complex. Carbon-Carbon, Carbon-Hydrogen, Nitrogen-Hydrogen and Oxygen-Hydrogen Activation Reactions". J. Am. Chem. Soc. 111: 2717–2719. doi:10.1021/ja00189a058.CS1 maint: uses authors parameter (link)
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
3. 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
4. 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
5. Bennett, M. A.; Hambley, T. W.; Roberts, N. K.; Robertson, G. B. (1985). "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. 4: 1992–2000. doi:10.1021/om00130a012.CS1 maint: uses authors parameter (link)