Transition metal benzyne complex explained

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, 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).^[4]

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)

Notes and References

1. 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. 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
4. 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. 10.1021/om00130a012.