Ni(i) and Ni(ii) ring-expanded N-heterocyclic carbene complexes: C-H activation, indole elimination and catalytic hydrodehalogenation

Article abstract of DOI:10.1039/c0cc01335h

Reaction of the bulky 6-membered N-heterocyclic carbene 6-Mes with Ni(cod)₂ gives a C-H activated 6-Mes nickel(ii) complex, but the novel three-coordinate Ni(i) species Ni(6-Mes)(PPh₃)Br if the reaction is performed in the presence of Ni(PPh₃)₂Br ₂. The Ni(i) complex is a precursor for the catalytic hydrodehalogenation of aryl halides. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0cc01335h

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Ni(I) and Ni(II) ring-expanded N-heterocyclic carbene complexes:
C–H activation, indole elimination and catalytic hydrodehalogenationw
Caroline J. E. Davies, Michael J. Page, Charles E. Ellul, Mary F. Mahon and
Michael K. Whittlesey*
Received 10th May 2010, Accepted 7th June 2010
First published as an Advance Article on the web 18th June 2010
DOI: 10.1039/c0cc01335h
Reaction of the bulky 6-membered N-heterocyclic carbene
6-Mes with Ni(cod)2 gives a C–H activated 6-Mes nickel(II)
complex, but the novel three-coordinate Ni(I) species
Ni(6-Mes)(PPh3)Br if the reaction is performed in the presence
of Ni(PPh3)2Br2. The Ni(I) complex is a precursor for the
catalytic hydrodehalogenation of aryl halides.
Nickel–N-heterocyclic carbene (NHC) complexes have been
used in a wide range of catalytic transformations including
cross-coupling reactions, cycloadditions and reductive
coupling.1 Five-membered ring carbenes with bulky
N-substituents are most commonly employed, as these appear
to aid the stabilisation of low-coordinate Ni(0) species
Fig. 1 Molecular structure of 1. Ellipsoids are shown at 30%
proposed to be active in the catalytic cycle. However, it is
often far from clear even what the nickel–NHC starting
complexes are, let alone any intermediates, as such species
are typically generated in situ. We rationalised that 6- and
7-membered ring NHCs could allow the isolation of new,
well-defined nickel complexes suitable for catalytic studies.
These ring-expanded carbenes offer significantly different
steric and electronic properties to their 5-membered
counterparts,2 but have so far received very little attention.3
We now describe reactions of the 6-membered NHC 6-Mes
(Scheme 1) to give new ring-expanded carbene complexes of
Ni.4 A Ni(II) complex has been isolated following carbene C–H
activation, and found to degrade by a novel indole elimination
pathway. A comproportionation of Ni(II) and Ni(0) has
yielded a rare example of three-coordinate Ni(I), which has
been investigated for the catalytic reduction of aryl halides.
The addition of 1 equiv. of the bulky 6-Mes ligand to a
toluene solution of Ni(cod)2 resulted in colour change over 1 h
from yellow to deep red and formation of the C–H activated
probability with all hydrogen atoms (except those on C11) removed
for clarity. Selected bond lengths (A) and angles (1): Ni(1)–C(1)
1.8996(12), Ni(1)–C(11) 1.9796(14), Ni(1)–C(23) 2.0838(13),
Ni(1)–C(24) 1.9547(14), Ni(1)–C(25) 2.0471(14), C(6)–C(11)–Ni(1)
94.32(8).
6-Mes Ni(II) species 1 (Scheme 1). The compound was isolated
in 40% yield and characterised by NMR spectroscopy and
X-ray crystallography.w Of most note in the 1H NMR
spectrum were two signals at d 2.8 and 2.3 arising from
the diastereotopic protons of the Ni–CH2 moiety. These
correlated by 1H–13C HSQC to a low frequency 13C signal
at d 15.6.
The X-ray structure (Fig. 1)5 confirmed the presence of a
metallated 6-Mes ligand and a cyclooctenyl group, arising
presumably via hydrogen transfer from the transient Ni–H
species formed upon C–H activation. The Ni–CNHC and
Ni–CH2 distances (1.8996(12) and 1.9796(14) A) are compar-
able to those reported by Caddick et al. in an analogous
complex of the 5-membered 1,3-di-tert-butylimidazol-2-
ylidene ligand, which is the only other known example of a
C–H activated nickel carbene complex.6
Attempts to employ 1 as a latent source of 12-electron
[Ni 6-Mes] for catalytic applications were thwarted by the
instability of the complex in solution. Upon thermolysis at
70 1C (16 h), 1 decomposed to afford the N-alkyl indole 2,
which was isolated in 62% yield and characterised by a
combination of NMR and X-ray techniques (Scheme 2).w
Cyclooctene was also observed by NMR spectroscopy,
consistent with a pathway involving hydrogen transfer from
the activated methylene arm to the cyclooctenyl ligand
and elimination of 2 from the nickel centre. A dark grey
precipitate, presumed to be metallic Ni, was also deposited.
Although there are a number of decomposition pathways for
Scheme 1
Department of Chemistry, University of Bath, Claverton Down, Bath
BA2 7AY, UK. E-mail: chsmkw@bath.ac.uk;
Fax: +44 (0)1225 386231; Tel: +44 (0)1225 383748
w Electronic supplementary information (ESI) available: Experimental
data for 1–3. CCDC 776076–776078 (1–3). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc01335h
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Table 1 Hydrodehalogenation catalysed by 3w
Substrate
Product
% Conversiona (time/h)
Scheme 2
p-BrC6H4F
p-ClC6H4F
C6H5F
C6H5F
C6H5F
C6H6
>99 (0.5)b
68 (4); 73 (48)b
18 (22)c
metal–NHC complexes,7 we are unaware of any previous
reports that describe products comparable to 2.
a
Yields based on integrations of 19F NMR spectra relative to an
It seemed plausible to us that a short-lived Ni(0) 6-Mes
complex must exist prior to the formation of 1. We thus
adopted the approach employed by Sigman et al. of reacting
Ni(0) with carbene in the presence of Ni(II) in an effort to
generate a Ni(I) 6-Mes complex.8 To our delight, the reaction
of Ni(cod)2 with 6-Mes in the presence of Ni(PPh3)2Br2 did
indeed yield Ni(I) in the form of the 3-coordinate species
Ni(6-Mes)(PPh3)Br (3, Scheme 1).w The X-ray structure
(Fig. 2)9 revealed a distorted trigonal planar geometry
(sum of the bond angles 360.001), in contrast to the T-shaped
structure reported recently for the bis-carbene Ni(I) product
Ni(IPr)2Cl (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene).10 The solution behaviour of 3 also differs from that
of Ni(IPr)2Cl, with no evidence of a monomer–dimer equili-
brium involving either 6-Mes or PPh3 dissociation. The 1H
NMR spectrum of 3 exhibited a series of broad, paramagne-
tically shifted resonances in the range +30 to À17 ppm, while
we were unable to observe any signal at all for the PPh3 ligand
in the 31P{1H} NMR spectrum. A solution magnetic moment
(meff) of 1.13 mB was measured in C6D6 (Evans method).
Well-characterised examples of Ni(I) species are rare11 and,
as a result, very little is known about their role in catalysis.10,12
We decided to target the hydrodehalogenation of aryl halides,
which has been the subject of a number of reports using
Ni/NHC combinations.13 Our initial findings on the activity
of 3 for the hydrodehalogenation of disubstituted aryl halides
are shown in Table 1. Thus, 1-bromo-4-fluorobenzene is
completely reduced to fluorobenzene at room temperature
within 30 min using NaOiPr as the hydrogen source
b
internal standard of a,a,a-trifluorotoluene. Reactions performed in
thf at 25 1C with 4.5 mol% 3 and 3 equiv. of NaOiPr. Reaction
c
performed at 70 1C.
(NaOMe proved to be totally ineffective) in the presence of
4.5 mol% 3. The Ni(I) complex also proved active for the
reduction of 1-chloro-4-fluorobenzene to fluorobenzene at
room temperature, although conversion was incomplete even
after 48 h. When the reaction was repeated in the presence of
10 equivalents PPh3, complete reduction was now achieved in
the same period, suggesting that the phosphine helps to
maintain the integrity of the catalyst. However, it also appears
to hinder activity as a somewhat slower initial reaction
rate (5% conversion after 1.5 h) was observed in comparison
to the value found at the same time in the absence
of PPh3 (36%).
No hydrodefluorination was observed for either p-BrC6H4F
or p-ClC6H4F. Moreover, we observed no reduction of fluoro-
benzene itself at room temperature. Under more forcing
conditions (70 1C), we did achieve 18% conversion to benzene
after 22 h, implying that 3 can bring about reduction of even
the most unreactive of the carbon–halogen bonds.14
In summary, new Ni(II) and Ni(I) complexes incorporating
ring-expanded N-heterocyclic carbene ligands have been
prepared. The catalytic potential of the Ni(II) product,
which contains a C–H activated NHC, is limited by a new
decomposition pathway involving indole elimination. In
contrast, the rare, three-coordinate Ni(I) species 3 proved to
be a useful precursor for catalytic hydrodehalogenation.
Further studies to investigate the potential of 3 in catalytic
and stoichiometric transformations are ongoing.
We thank the EPSRC for support (MJP).
Notes and references
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