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]

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.

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.
^ ^a ^b ^c Buchwald, S. L.; Nielsen, R. B. "Group 4 Metal Complexes of Benzynes, Cycloalkynes, Acyclic Alkynes, and Alkenes" Chem. Rev. 1988, volume 88, 1047-1058
^ ^a ^b 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
^ 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
^ ^a ^b ^c ^d 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.

[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 (7): 2717–2719. doi:10.1021/ja00189a058.

[Buchwald-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

[Buchwald2-3] 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

[Schrock-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

[Bennett-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.

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v t e Organometallic chemistry
Principles	Crystal field theory Ligand field theory 18-electron rule d electron count Polyhedral skeletal electron pair theory Isolobal principle π backbonding Dewar–Chatt–Duncanson model Hapticity spin states Agostic interaction Metal–ligand multiple bond
Reactions	Oxidative addition / reductive elimination Migratory insertion β-hydride elimination Transmetalation Carbometalation (Hydrometalation)
Types of compounds	Gilman reagents Grignard reagents Cyclopentadienyl complexes Transition metal indenyl complexes Transition metal fullerene complexes Metallocenes Metal tetranorbornyls Sandwich compounds Half sandwich compounds Transition metal acyl complexes Transition metal carbene complexes Transition metal carbyne complexes Transition metal alkene complexes Transition metal alkyne complexes Transition-metal allyl complexes Transition metal carbides Arene complexes of univalent gallium, indium, and thallium
Applications	Carbonylation Cativa process Grignard reaction Monsanto process Olefin metathesis Shell higher olefin process Ziegler–Natta process
Related branches of chemistry	Organic chemistry Inorganic chemistry Bioinorganic chemistry
Category

v t e Coordination complexes
H donors:	H⁻ H₂ BH₄^-
B donors:	BR−2 B_mH_n
C donors:	R⁻ RC(O)⁻ HC(O)⁻ CH₂=CHCH−2 (allyl) C(CH₂)₃ CH₂=CH₂ CR=CR₂ RC₂R C₆H₄ CN⁻ CO CO₂ C⁴⁻ C₆R₆ C₆₀ & C₇₀ RNC =CR₂ ≡CR C₅H−5 C₉H−7 =C=CR₂
Si donors:	H_nSiR_4−n R₃Si⁻
N donors:	NH₃ H₂NCH₂CH₂NH₂, etc. N−3 imidazole NO RNO NO−2 py amino acid N³⁻ RN²⁻ RCN bipy phen porphyrin (Me₃Si)₂N⁻ N₂ ⁻NCS
P donors:	PR₃ PR−2 PR₂OH
As donors:	AsR₃
Bi donors:	R_nBi_n RBi
O donors:	H₂O OH⁻ R₂O RO⁻ O²⁻ O₂ CO2−3 & HCO−3 C₂O2−4 RCO−2 RCONR'₂ OC(NR₂)₂ acac R₂CO ONO⁻ NO−3 ClO−4 C₅H₅NO OSR₂ SO2−4 PO3−4 OPR₃
S donors:	R₂NCS−2 RS⁻ HS⁻ and H₂S R₂S R₂C₂S2−2 SO SO₂ SO2−3 S₂O2−3 SR₂O NCS⁻
Halide donors:	F⁻ F₂ Cl⁻ Br⁻ I⁻

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