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