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.Buchwald, S. L.; Nielsen, R. B. "Group 4 Metal Complexes of Benzynes, Cycloalkynes, Acyclic Alkynes, and Alkenes" Chem. Rev. 1988, volume 88, 1047-1058
Examples The studies of metal-benzyne complexes were initiated with the preparation of zirconocene complex by reaction diphenylzirconocene with trimethylphosphine.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
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.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 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. With trifluoroacetic acid, benzene is lost to give the trifluoroacetate Ni(O2CF3)2(dcpe).
Structural trends Several benzyne complexes have been examined by X-ray crystallography.
+ Bond lengths (Å) in Ta, Ni and Zr 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 and Zr 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)
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