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]
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:
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]
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
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
^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 (7): 2717–2719. doi:10.1021/ja00189a058.
^ abcBuchwald, S. L.; Nielsen, R. B. "Group 4 Metal Complexes of Benzynes, Cycloalkynes, Acyclic Alkynes, and Alkenes" Chem. Rev. 1988, volume 88, 1047-1058
^ abBuchwald, 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
^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
^ abcdBennett, 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.