Intermolecular arene C-H activation by nickel(II)

Article abstract of DOI:10.1021/ja065505p

Intermolecular arene C-H activation mediated by a divalent nickel species under extremely mild conditions is described. The reactions of either Ni(COD)₂ with H[N(o-C₆H₄PR₂)₂] (H[R-PNP]; R = ⁱPr, Cy) in benzene at room temperature or [R-PNP]NiCl with LiBHEt₃ in THF at -35 °C produced the corresponding [R-PNP]NiH in high yield. Addition of 1 equiv of B(C₆F₅)₃ to a benzene solution of [R-PNP]NiH at room temperature led to the formation of a mixture that contains [R-PNP]NiPh and [R-PNP]Ni(C₆F₅), both of which are proposed to evolve from zwitterionic [R-PNP]Ni(μ-H)B(C₆F₅)₃. In contrast, the reaction of [R-PNP]NiH with AlMe₃ in benzene at room temperature afforded exclusively the corresponding [R-PNP]NiPh. Similar results were also observed for intermolecular toluene and m-xylene C-H activation by [R-PNP]NiH. A parallel study involving [R-PNP]NiMe (R = Ph, ⁱPr, Cy) on the reactivity of intermolecular arene activation reveals the significance of π basicity of Ni(II) in these molecules. The remarkable reactivity of inexpensive Ni(II) species established in this study is attractive, particularly from an economic viewpoint, as compared to the current alternatives of 4d and 5d metals. Copyright

Full text of DOI:10.1021/ja065505p

Published on Web 11/14/2006
Intermolecular Arene C-H Activation by Nickel(II)
Lan-Chang Liang,* Pin-Shu Chien, and Yu-Lun Huang
Department of Chemistry and Center for Nanoscience & Nanotechnology, National Sun Yat-sen UniVersity,
Kaohsiung 80424, Taiwan
Received July 31, 2006; E-mail: lcliang@mail.nsysu.edu.tw
Selective hydrocarbon activation and functionalization has long
been identified as a major challenge to synthetic chemists.1 The
search for practical methods in this regard has thus far led to
extensive investigation in systems predominantly focused on
second- and third-row transition metals.2,3 The preference for the
heavier metals instead of their corresponding first-row congeners
is due likely to comparatively stronger M-H and M-C bond
strengths for the former. Notably, while detailed examinations on
Pt(II)- and Pd(II)-mediated C-H activation processes emerge
significantly,2,4 parallel studies involving Ni(II) remain relatively
unexplored.1c The pursuit of feasible methods for hydrocarbon
activation by Ni(II) is attractive, particularly from an economic
viewpoint, as compared to the current alternatives of more expensive
4d and 5d metals. We note that, although examples of well-defined
Ni(0)-mediated and chelate-assisted hydrocarbon activation pro-
cesses are known,5 intermolecular arene C-H activation mediated
by a divalent nickel complex is virtually unprecedented.
The reaction was periodically monitored by 31P{1H} NMR spec-
troscopy. After 40 h, the 31P{1H} NMR spectrum of the homoge-
neous reaction aliquot indicates the concurrent presence of the
starting material [iPr-PNP]NiH (29%), an intermediate (30%) that
we tentatively formulate as zwitterionic [iPr-PNP]Ni(µ-H)B(C6F5)3
(δp 51 ppm), the desired benzene C-H activation product [iPr-
PNP]NiPh (12%, vide infra), and [iPr-PNP]Ni(C6F5) (29%, vide
infra) (Scheme 1). Unsatisfactorily, the relative ratio of the desired
[iPr-PNP]NiPh is consistently low (up to 20%) over a period of
110 h as compared to the other components present in the reaction
mixture. Selective isolation of the presumed [iPr-PNP]Ni(µ-H)B-
(C6F5)3 seems unlikely as it is always accompanied by a significant
amount of [iPr-PNP]NiH and/or [iPr-PNP]Ni(C6F5) throughout
the reaction period as indicated by the 31P{1H} NMR spectroscopy.
The concomitant formation of the major [iPr-PNP]Ni(C6F5) and
the minor [iPr-PNP]NiPh is indicative of competitive reactions
between C6F5 transfer and intermolecular benzene C-H activation,
respectively. Both phenyl and pentafluorophenyl complexes are
presumably evolved from the putative [iPr-PNP]Ni(µ-H)B(C6F5)3
intermediate. The transfer of C6F5 from B(C6F5)3 to a transition
metal has also been reported in some cases.12 As anticipated for
the inherent thermal stability of pentafluorophenyl complexes of
late transition metals, [iPr-PNP]Ni(C6F5) can be efficiently isolated
by heating the reaction mixture at 110 °C for 1 day. Similar
phenomena were also observed for reactions involving [Cy-PNP]-
NiH, although at a much slower rate (>7 days at room temperature),
consistent with the larger steric size of cyclohexyl than isopropyl
and thus the corresponding intermolecular steric repulsion of
[R-PNP]NiH with B(C6F5)3. The putative [Cy-PNP]Ni(µ-H)B-
(C6F5)3 intermediate was observed at 43 ppm in the 31P{1H} NMR
spectroscopy.
Inspired by Fryzuk’s pioneering work in -SiMe2CH2- derived
amido phosphine complexes,6 we became interested in exploratory
chemistry of diarylamido phosphine compounds.7 It has been
demonstrated that Pt(II) complexes of [N(o-C6H4PR2)2]- ([R-
PNP]-; R ) Ph) are active for intermolecular benzene C-H
i
activation.8 Ni(II) complexes of [R-PNP]- (R ) Ph, Pr, Cy)
effectively promote the C-X (X ) Cl, Br, I) bond cleavage of a
variety of halogenated hydrocarbons.9a Encouraged by these results,
we envision that intermolecular arene C-H activation may also
be accessed by [R-PNP]- complexes of inexpensive Ni(II). In this
contribution, we describe the reactivity studies of [R-PNP]NiR′
i
(R ) Pr, Cy; R′ ) H, Me) in this aspect, which represents a rare
example of Ni(II)-mediated intermolecular arene C-H activation
under extremely mild conditions.
Complexes [iPr-PNP]NiH and [Cy-PNP]NiH were readily
prepared in high yield by oxidative addition reactions of Ni(COD)2
with H[R-PNP]9a in benzene at room temperature. Alternatively,
metathetical reactions of [R-PNP]NiCl9a with LiBHEt3 in THF at
-35 °C also afforded the corresponding [R-PNP]NiH. A similar
nickel hydride complex supported by a tolyl-derived amido diphos-
phine ligand was also reported very recently.10 Surprisingly,
attempts to isolate [Ph-PNP]NiH by identical synthetic routes were
not successful. Reminiscent of the previously reported [R-PNP]-
NiX (X ) halide or alkyl),9 complexes [iPr-PNP]NiH and [Cy-
PNP]NiH display solution C2V symmetry on the NMR time scale,
indicating a square-planar geometry for these molecules. The nickel-
bound hydride ligand appears as a characteristic triplet resonance
at -18.5 ppm with JHP of 63 Hz in the H NMR spectroscopy.
Strong Lewis acidic boron and aluminum compounds are often
effective to abstract an alkyl or hydride ligand from transition metal
complexes.11 Addition of 1 equiv of B(C6F5)3 to a benzene solution
of [iPr-PNP]NiH at room temperature led immediately to a change
in color from yellow to red. No precipitate was observed from the
reaction mixture even after a prolonged period of time (>5 days).
Red crystals of [Cy-PNP]Ni(C6F5) suitable for X-ray diffraction
analysis were grown from a concentrated diethyl ether solution at
-35 °C. Figure S1 (see Supporting Information) illustrates the X-ray
structure of this molecule in which the C6F5 group is trans to the
amido nitrogen donor in a square-planar geometry. The Ni-N
distance in [Cy-PNP]Ni(C6F5) (1.912(6) Å) is slightly shorter than
that in [Cy-PNP]NiMe (1.947(5) Å),9a suggesting that the trans
influence of the pentafluorophenyl ligand is somewhat lower than
that of methyl. The C6F5 ring in [Cy-PNP]Ni(C6F5) lies ap-
proximately perpendicular to the mean coordination plane with a
dihedral angle of 79.5°.
Interestingly, addition of 1 equiv of AlMe3 to a benzene solution
of [R-PNP]NiH at room temperature led exclusively to the
formation of the corresponding benzene C-H activation product
[R-PNP]NiPh (R ) iPr, Cy; Scheme 2). The 1H and 31P{1H} NMR
spectroscopy indicates that these reactions are complete in 4 h, a
rate that is notably much faster than that employing B(C6F5)3. No
intermediate was observed as indicated by 31P{1H} NMR spec-
troscopy; only the disappearance of [R-PNP]NiH and the formation
of [R-PNP]NiPh could be detected. The identity of byproducts
2
1
9
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J. AM. CHEM. SOC. 2006, 128, 15562-15563
10.1021/ja065505p CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Scheme 2
[Cy-PNP]NiMe likely underscores the significance of π basicity
of Ni(II) in the latter complexes for rather sufficient back π bonding
from formally electron-deficient Ni(II) to the σ* orbital of the
benzene C-H bond. The possibility of ineffective methyl abstrac-
tion from [Ph-PNP]NiMe by AlMe3, however, cannot be ruled
out.
In summary, we have demonstrated an efficient intermolecular
arene C-H activation process mediated by Ni(II) complexes of
[R-PNP]- under extremely mild conditions. Of particular note is
the remarkable reactivity of inexpensive Ni(II) as compared to the
current alternatives of 4d and 5d metals. Studies directed to delineate
the reaction mechanism and reactivity with applicable hydrocarbons
are currently underway.
formed from these reactions is currently under investigation.
Analysis of the H NMR spectra of reactions performed in a J.
1
Young NMR tube in C6D6 led us to propose the production of
methane and dimethylalane. No hydrogen gas, however, was
detected. Remarkably, reactions employing a substoichiometric
amount (e.g., 0.2 equiv) of AlMe3 also afforded quantitatively the
corresponding [R-PNP]NiPh. The identity of these phenyl com-
plexes was further confirmed by independent preparation of these
molecules from the reactions of [R-PNP]NiCl (R ) iPr, Cy)9a with
PhMgCl in ethereal solutions at -35 °C. It is worth noting that the
reactivity of [R-PNP]NiH toward intermolecular benzene C-H
activation is somewhat better than that of [Ph-PNP]- complexes
of Pt(II), which cleave benzene C-H bonds in 31 h at room
temperature in the presence of a Lewis acid.8
Acknowledgment. We thank the National Science Council of
Taiwan for financial support (NSC 94-2113-M-110-004), and Mr.
Ting-Shen Kuo (NTNU) for crystallographic assistance.
Supporting Information Available: Experimental details, spec-
troscopic data, and X-ray crystallographic data in CIF format for [Cy-
PNP]Ni(C6F5) and [iPr-PNP]Ni(p-tolyl). This material is available free
of charge via the Internet at http://pubs.acs.org.
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2
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1
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