Nickel(0)-catalyzed isomerization of an aryne complex: Formation of a dinuclear Ni(I) complex via C-H rather than C-F bond activation

Article abstract of DOI:10.1021/ja0572553

The reaction of the aryne complex (PEt₃)₂Ni(η²-C₆H₂-4,5-F₂) with a catalytic amount of Ni(PEt₃)₂ results in a dinuclear Ni(I) complex from the coupling of the isomer (

Full text of DOI:10.1021/ja0572553

Published on Web 01/19/2006
Nickel(0)-Catalyzed Isomerization of an Aryne Complex: Formation of a
Dinuclear Ni(I) Complex via C-H Rather than C-F Bond Activation
Alana L. Keen and Samuel A. Johnson*
Department of Chemistry & Biochemistry, UniVersity of Windsor, Windsor, ON, Canada N9B 3P4
Received October 24, 2005; E-mail: sjohnson@uwindsor.ca
There has been great interest lately in the ability of transition
Scheme 1
metal complexes to activate C-H and C-F bonds, both stoichio-
1
metrically and catalytically. These bonds are typically inert, and
the products of activation provide synthetic avenues inaccessible
by conventional procedures. It has been shown that precursors to
3 2
nickel bisphosphine fragments, such as (COD)Ni(PEt ) , are highly
selective in the activation of C-F over C-H bonds in their reaction
2
,3
with a variety of fluorinated aryl and azaheterocycles. Calcula-
tional studies have demonstrated that the chemoselectivity is due
to the fact that the activation of an aromatic C-F bond by Ni-
(
3 2
PEt ) is thermodynamically favorable, whereas C-H bond
4
activation is not. A few examples of C-H bond activation by Ni
are known, where additional factors render the products thermo-
5
dynamically viable, but never in the presence of C-F bonds. The
mechanistic details of C-F bond activation are uncertain; however,
2
3
η -bound arene intermediates have been isolated and characterized,
4
and calculations suggest a concerted C-F bond cleavage step. The
selectivity of Ni(0) complexes toward C-F bonds engenders
synthetic utility; however, if Ni complexes could be tuned to yield
C-H bond activation products, the resultant methodology would
be economically attractive compared to the current alternatives
(JPP ) 10 Hz) and a complex multiplet at δ 9.0, which can be
modeled as a virtual triplet of doublets of doublets (JPP ) 10 Hz,
JPF ) 6.0 Hz, JPF ) 11.4 Hz). The observation of virtual coupling
indicates that a Ni-Ni bond must be present. Moreover, the
observed coupling constants between the hydrogen and fluorine
substituents on the aromatic ring verify that the connectivity
observed in the solid-state structure is correct, and no other isomers
are observed.
which require more expensive 2nd and 3rd row metals.
2
The reaction of the known aryne complex (PEt
,5-F ) (1) with a catalytic amount of Br Ni(PEt ) over Na/Hg
2 2 3 2
3
)
2
Ni(η -C
H
6 2
-
6
4
was found to cleanly produce the dark brown dinuclear Ni(I)
complex 4, as shown in Scheme 1. Crystals of 4 were obtained in
The conversion of aryne complex 1 to the dinuclear nickel
complex 4 occurs over the course of 24 h with Br Ni(PEt ) and
2
3 2
6
0% isolated yield by crystallization from pentane at -40 °C.
The reduction of Br Ni(PEt) presumably produces transient Ni-
PEt , which appears to catalyze this reaction; no reaction is
observed in the absence of a Ni(PEt precursor, and other sources
of Ni(0), such as Ni(PEt , do not result in the formation of 4. At
Na/Hg, and a long-lived intermediate is observed by NMR
spectroscopy. The intermediate is symmetric, with a single aromatic
hydrogen environment at δ 7.33 (vt), a single fluorine environment
at δ -74.03 (m), and a single phosphorus environment at δ 14.8
2
3
(
3 2
)
3 2
)
in the 1H, F, and P{ H} NMR spectra, respectively. This
19
31
1
3
)
4
first glance, 4 appears to result from the coupling of two aryne
moieties of complex 1; however, due to the ortho disposed hydrogen
intermediate could be isolated by the addition of 1 equiv of Br
2
-
Ni(PEt to a pentane solution of 1 stirred over Na/Hg over the
3 2
)
substituents, it is not the product anticipated from the coupling of
course of 1 h. The solution was cooled to -40 °C, and complex 2
was isolated as a crystalline orange solid. The solid-state structure
of 2 was determined by X-ray crystallography and is shown in
Figure 2.
2
2, but rather of the putative intermediate (PEt
3
)
2
Ni(η -C
6 2
H -5,6-
F ) (3), an isomer of 1 which could be formed by aromatic C-H
2
bond activation followed by the formation of an alternate aromatic
C-H bond.
The solid-state molecular structure of 4 was determined by X-ray
crystallography, and a simplified ORTEP depiction is shown in
Figure 1. The molecule has approximate C
crystallographic symmetry. The two Ni(PEt
asymmetrically bridged by a (C biarylyl fragment, with a
Ni-Ni bond distance of 2.3710(5) Å. The Ni(1)-C(1) and Ni-
2)-C(8) distances of 1.965(3) and 1.952(3) Å are shorter than
2
symmetry, but no
3 2
)
fragments are
6 2 2 2
F H )
(
the Ni(1)-C(8) and Ni(2)-C(1) contacts of 2.317(3) and 2.308(3)
Å.
Solution NMR spectroscopy demonstrates that the approximate
2
C symmetry observed for the solid-state structure of 4 is maintained
in solution at low temperature. In the 213 K P{ H} NMR
spectrum, two environments are observed, a virtual triplet at δ -5.2
Figure 1. Solid-state molecular structure of 4 as determined by X-ray
crystallography. Ethyl substituents on the phosphine ligands and hydrogen
atoms are omitted for clarity.
31
1
1806
9
J. AM. CHEM. SOC. 2006, 128, 1806-1807
10.1021/ja0572553 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
4
and Na/Hg in pentane produced only the deuterated product 4-d .
Further investigations are underway to distinguish this mechanism
from another plausible mechanism, where C-C bond formation
precedes C-H bond rearrangement.11
The rearrangement of aryne 1 to its isomer 3 is reminiscent of
the chain walking of alkenes in their reaction with metal hydride
complexes. The driving force for the reaction appears to be the
formation of a more stable isomer of 1; hydrocarbyl complexes
bearing proximal fluoro substituents have stronger M-C bonds,
2
and DFT calculations on the model complexes (PMe
4
3 2
)
Ni(η -C
6 2
H -
2
2 3 2 6 2 2
,5-F ) and (PMe ) Ni(η -C H -3,4-F ) demonstrate that the latter
is more stable by 9 kcal/mol. Although aromatic C-H bond
activations by Ni bisphosphine complexes are typically thermody-
namically unfavorable, clearly these reactions are kinetically
accessible and, under suitable conditions, can provide products
resulting from C-H bond activation, even in the presence of C-F
bonds. The development of methods to trap the transitory C-H
bond activation products could render Ni(0) bisphosphine complexes
as useful reagents for synthetic methods which utilize C-H bond
activation.
Figure 2. Solid-state molecular structure of 2 as determined by X-ray
crystallography. The methyl group of the ethyl substituents on the phosphine
ligands and hydrogen atoms are omitted for clarity.
Scheme 2
Acknowledgment. Acknowledgment is made to the NSERC
of Canada and the Ontario Research and Development Challenge
Fund (ORDCF) for their financial support.
Supporting Information Available: Full experimental details;
crystal data collection and refinement parameters; coordinates and
energies from DFT calculations; CIF files for 2 and 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
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(
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2
7
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1
8
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2
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(
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(
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(
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(
3 2
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1
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3 2
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of solutions of 1 to 4 in the absence of Na/Hg. No other fluorine-
19
(9) A Meissenheimer complex is also a viable intermediate to hydrogen
migration.
containing products are observed by F NMR after 24 h. The
mechanism shown in Scheme 2 may be operative, where C-H bond
activation occurs in preference to C-F bond activation due to the
(
10) A complex related to the possible transition state is known: Buchwald,
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398.
9
proximity of the C-H bond. This is followed by a transfer of the
(
11) A related product resulting from biarylyl ligand rearrangement has been
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loss of Ni(PEt
3 2
) forms 3, which can dimerize to form 4. The
2
reaction of (PEt ) Ni(η -C D -4,5-F ), (1-d ) with 9% Br Ni(PEt )
3 2 6 2 2 2 2 3 2
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J. AM. CHEM. SOC.
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