Synthesis and X-ray crystal structure analysis of the first nickel bisoxazoline pincer complex

Article abstract of DOI:10.1016/j.jorganchem.2004.06.055

The reaction in toluene between 2-iodo-1,3-bis(4′,4′-dimethyl-2′-oxazolinyl)benzene and Ni(COD)₂ gave [2,6-bis(4′,4′ -dimethyl-2′-oxazolinyl)phenyl-N,C¹, N′]iodonickle(II) isolated in 69% yield. The structure of this novel nickel bisoxazoline pincer complex was confirmed by a X-ray crystal structure analysis.

Full text of DOI:10.1016/j.jorganchem.2004.06.055

Journal of Organometallic Chemistry 689 (2004) 3056–3059
www.elsevier.com/locate/jorganchem
Synthesis and X-ray crystal structure analysis of the first
nickel bisoxazoline pincer complex
*
John S. Fossey, Christopher J. Richards
Department of Chemistry, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
Received 29 June 2004; accepted 29 June 2004
Available online 3 August 2004
Abstract
The reaction in toluene between 2-iodo-1,3-bis(40,40-dimethyl-20-oxazolinyl)benzene and Ni(COD)2 gave [2,6-bis(40,40-dimethyl-
20-oxazolinyl)phenyl-N,C1,N0]iodonickle(II) isolated in 69% yield. The structure of this novel nickel bisoxazoline pincer complex
was confirmed by a X-ray crystal structure analysis.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Bisoxazoline; Crystal structure; Nickel; Pincer complexes
1. Introduction
synthesis, properties and X-ray crystal structure analysis
of the first nickel bisoxazoline complex.
Complexes containing an anionic terdentate ligand
ECE, where E is a neutral donor atom such as phospho-
rus, nitrogen or sulfur, are currently the subject of con-
siderable attention, particularly in respect to their
application in homogeneous catalysis [1]. A major sub-
category of these so-called pincer complexes, and the
subject of much of the work in this area, are NCN com-
plexes 1 based upon, amongst others, the group 10 met-
als nickel, palladium and platinum [2]. Our own work in
this area has focused on the synthesis of bisimine and
bisoxazoline palladium and platinum complexes of gen-
eral structures 2 [3] and 3 [4,5], and following halide
abstraction with silver salts, application of the resulting
cationic species as Lewis acid catalysts. Bisoxazoline
complexes 3 are of particular interest for application
in asymmetric catalysis, but to the best of our knowl-
edge no corresponding nickel complex of structure 3
has been reported. In this note, we report on the
X
Cl
Pt
R1 R
X
R
R1
O
R
R
M
M
Me2N
NMe2
N
N
N
N
O
1a M = Ni
1b M = Pd
1c M = Pt
2
3a M = Pd
3b M = Pt
X = halogen
2. Results and discussion
Nickel complex 1a (X = Br) was first synthesised by
the addition of Ni(COD)2 to 2-bromo-1,3-(N,N-
dimethylaminomethyl)benzene [2b]. This type of oxida-
tive addition protocol has also been applied to the
synthesis of palladium complexes 3a, generated in good
yield on combination of Pd2(dba)3 and 2-bromo-
1,3-bis(20-oxazolinyl)benzenes [5a]. This type of ap-
proach thus appeared appropriate for the generation
*
Corresponding author. Tel.: +44-(0)20-7882-3271; fax: +44-(0)20-
7882-7427.
E-mail address: c.j.richards@qmul.ac.uk (C.J. Richards).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.06.055

J.S. Fossey, C.J. Richards / Journal of Organometallic Chemistry 689 (2004) 3056–3059
3057
I
Ni
N
N
N
I
N
N
N
i) THF, LDA,
TMEDA
O
O
O
O
O
O
Ni(COD)2
PhCH3
ii) I2
4
5 42%
Scheme 1.
6 70%
of a corresponding nickel bisoxazoline complex. To gen-
erate a precursor 2-halo-1,3-bisoxazoline we utilised the
known regioselective lithiation of 1,3-bis(40,40-dimethyl-
20-oxazolinyl)benzene [6] and followed this by the
addition of iodine to give 2-iodo-1,3-bis(40,40-dimethyl-
20-oxazolinyl)benzene 5 in 42% yield (Scheme 1). Disso-
lution of 5 in dry toluene, addition of Ni-(COD)2 and
stirring the resulting mixture at room temperature re-
sulted in a slow change in the colour of the mixture from
yellow to orange. After nineteen hours, work-up of the
reaction mixture gave novel complex 6, the identity of
which was confirmed by an X-ray structure analysis of
crystals obtained by the slow evaporation of a diethyl
ether solution in air (Fig. 1). The resulting bright orange
crystals have been stored in air at room temperature for
several months without decomposition.
the two independent crystal structures of
6
are
˚
1.895(4), 1.972(3) and 1.977(3) A, respectively. As palla-
dium–ligand bonds in complexes 3a are longer than the
corresponding platinum based bonds in 3b [4d], metal–
terdentate ligand bond lengths in group 10 bisoxazoline
complexes follow the expected pattern Ni < Pt < Pd.
In a recent study, a large number of metal chlorides
were examined as Lewis acid catalysts for the reaction
of a silyl enol ether with a mixture of benzaldehyde
and N-benzylideneaniline [7]. This revealed both NiCl2
and PdCl2 to be inactive whereas PtCl2 showed weak
activity for the activation of the imine. This result mir-
rors our recent findings that cationic Pt complexes de-
rived from 3b are more active than the corresponding
palladium complexes derived from 3a in the Michael
reaction between ethyl a-cyanoacetate and methyl vinyl
ketone [4d]. Nickel complexes 1a have previously been
applied as catalysts for the Kharasch addition which
proceeds via a radical mechanism and includes the for-
mation of a nickel(III) complex [Ni(NCN)X2] as a per-
sistent radical [8]. We were thus curious to examine
the applicability of cationic Ni–bisoxazoline complexes
as Lewis acid catalysts for the Michael reaction of acti-
vated nitriles. To generate the required complex we com-
bined 6 with one equivalent of silver triflate in reagent
grade acetone protected from light (Scheme 2). Analysis
of the reaction mixture by TLC revealed consumption of
the starting material, but attempted isolation of the ex-
pected cationic Ni(II) complex 7 only resulted in a green
amorphous solid that could not be further purified.
Some evidence for the formation of 7 was obtained from
the mass spectrum (FAB) in which a signal arising from
the molecular ion was observed (m/z 496, 53%), together
with a signal arising from the loss of the triflate counter
The structure of 6 reveals the shorter metal–nitrogen
and metal–carbon bond distances expected of a first row
transition metal complex compared to its second and
third row congeners. For example, complex 3b
(R = iPr, R1 = H, X = Cl) has a Pt–C bond length of
˚
1.928(10) A and Pt–N bond lengths of 2.032(10) and
˚
2.035(10) A [5c]. The corresponding values for one of
1
ion (m/z 347, 51%). The H NMR of the reaction prod-
uct in acetone-d6 was broad and inconclusive and the
13C NMR spectrum contained more peaks than ex-
pected. It is of note that a cationic analogue of 7 derived
from 1 is sensitive to oxidation to Ni(III), and even that
samples with no particular evidence of oxidation can
still give broadened signals in a NMR spectrum [2b].
We thus attempted catalysis of the Michael reaction be-
tween ethyl cyanoacetate and methyl vinyl ketone on the
crude product arising from the addition of silver triflate
to 6. Addition of this (5 mol% based on 6) and 10% Hu-
nigs base to a CH2Cl2 solution of ethyl cyanoacetate and
methyl vinyl ketone maintained at 4 ꢁC, resulted in a
Fig. 1. Representation of one of the two independent crystal structures
of 6 (corresponding values for the unshown second structure are given
˚
in parenthesis). Selected bond distances (A) and angles (ꢁ):
Ni(1)–C(11) = 1.859(4) [1.857(4)], Ni(1)–N(1) = 1.972(3) [1.968(3)]
Ni(1)–N(2) = 1.977(3) [1.973(3)], Ni(1)–I(1) = 2.5804(5) [2.5861(5)]
N(1)– C(5) = 1.298(5) [1.296(5)] N(2)–C(12) = 1.294(5) [1.292(5)],
C(11)–Ni(1)–N(1) = 80.53(15) [80.38(15)] C(11)–Ni(1)–N(2) = 80.73
(15) [80.82(15)] N(1)–Ni(1)–N(2) = 161.04(13) [159.94(13)] C(11)–
Ni(1)–I(1) = 172.61(11) [171.36(11)].

3058
J.S. Fossey, C.J. Richards / Journal of Organometallic Chemistry 689 (2004) 3056–3059
_OTf
+
I
OH2
Ni
Ni
N
N
N
N
O
O
O
O
CH3COCH3/H2O
AgOTf
Detected but
not purified.
6
7
Scheme 2.
55% conversion to the Michael product 5-cyano-5-eth-
oxycarbonyl-2,8-nonadione after 1 h. However, subse-
quent analyses (2, 3 and 15 h) revealed no significant
progression of the reaction revealing deactivation of
the catalyst under the conditions employed.
cooled to ꢀ78 ꢁC, containing a solution of 1,3-
bis(40,40-dimethyl-20-oxazolinyl)benzene
(0.210 g,
4
0.77 mmol) and TMEDA (0.330 g, 2.84 mmol) in
THF (7 mL). After the addition the resulting deep red
solution was stirred at room temperature for 5 h prior
to the addition of iodine (0.400 g, 1.58 mmol). The sol-
vent was removed in vacuo and the crude product dis-
solved in CH2Cl2 (50 mL). After washing with
aqueous sodium thiosulfate solution (5 · 50 mL), this
was dried (MgSO4), filtered and the solvent removed
in vacuo. Column chromatography of the residue
(EtOAc) gave 5 as a pale brown crystalline solid
(0.130 g, 42%). M.p. 103–110 ꢁC; IR (mmax; nujol) 1652
3. Conclusions
The first example of a nickel bisoxazoline complex
was synthesised in good yield by the addition to Ni-
(COD)2 of 2-iodo-1,3-bis(40,40-dimethyl-20-oxazolinyl)-
benzene. The identity of the resulting complex 6 was
confirmed by a X-ray crystal structure analysis. At-
tempted halide abstraction from 6 gave some evidence
for the formation of the desired cationic complex 7,
but this could not be purified and appeared sensitive
to oxidation. In contrast, the palladium and platinum
congeners of 7 are stable easily isolated complexes,
making these significantly better suited as Lewis
acid catalysts for the transformation of organic
substrates.
(C‚N) cmꢀ1 1H NMR (d; CDCl3) 1.40 (12H, s,
;
CH3), 4.14 (4H, s, OCH2), 7.37 (1H, t, J 7.7, 5-H),
7.50 (2H, d, J 7.4, 4- and 6-H); 13C{1H} NMR (d;
CDCl3) 28.6 (CH3), 68.6 (C(CH3)2), 80.0 (OCH2), 96.2
(2-C), 128.2 (5-C), 132.0 (4- and 6-C), 137.0 (1- and 3-
C), 163.7 (C‚N); m/z (EI) 398 (M+, 56), 271 (4), 128
(18), 55 (85), 41 (100): (Found: M+, m/z (EI) 398.0498.
C16H19IN2O2 requires 398.0491).
4.3. Synthesis of [2,6-bis(4040-dimethyl-20-oxazolinyl)-
phenyl-N,C1,N0]iodonickel (II) 6
4. Experimental
A mixture of 5 (0.23 g, 0.6 mmol) and Ni(COD)2
(0.20 g, 0.7 mmol) were stirred in freshly distilled de-
gassed toluene (20 mL) for 19 h at room temperature,
by which time the reaction was deemed complete by
TLC (CH2Cl2). The reaction mixture was filtered
through silica eluting with ethyl acetate. The orange
band was collected and the solvent removed in vacuo
(for an extended period) to give [2,6-bis(40,40-dimethyl-
20-oxazolinyl)phenyl-N,C1,N0]iodonickle(II) as an or-
ange/brown solid (0.184 g, 70% yield based on 5. A sam-
ple suitable for X-ray analysis was prepared by slow
evaporation of a diethyl ether solution in air to give
bright orange plates. M.p. 214 ꢁC (decomp.); Anal.
Found: C, 42.24; H, 4.22; N, 5.96. Calcd for C16H19IN2-
NiO2: C, 42.06; H, 4.19; N, 6.13; IR (mmax; CH2Cl2) 1616
4.1. General methods
Reactions were performed under an atmosphere of
nitrogen employing standard Schlenk techniques. THF
was distilled under nitrogen from sodium benzophenone
ketal and toluene was distilled under nitrogen from mol-
ten sodium metal, other solvents were not specifically
dried. Column chromatography was performed on
SiO2(40–63 lm). Compound 4 was prepared as previ-
ously reported [6].
4.2. Synthesis of 2-iodo-1,3-bis(40,40-dimethyl-20-oxazol-
inyl)benzene 5
1
To a solution of diisopropylamine (0.234 g, 2.31
mmol) in THF (1 mL), cooled to ꢀ78 ꢁC, was added
BuLi in hexanes (2.54 mmol) and the resulting mixture
stirred at ꢀ78 ꢁC for 20 min, followed by a further 30
min at room temperature. After re-cooling to ꢀ78 ꢁC,
this was added via cannula to a separate flask, also
(C‚N) cmꢀ1; H NMR (d; CDCl3) 1.63 (12H, s, CH3),
4.30 (4H, s, CH2), 7.07 (1H, app dd, J 5.9 and 8.6, Ar 4-
H), 7.13 (2H, d, J 6.2, Ar 3- and 5-H); 13C{1H} NMR (d;
CDCl3) 29.0 (CH3), 66.2 (C(CH3)), 84.4 (OCH2), 128.9
(4-C), 124.3 (3- and 5-C), 124.91 (2- and 3-C), 130.0
(1-C), 172.4 (C‚N); MS (m/z; FAB) 329 (M+ ꢀ I,

J.S. Fossey, C.J. Richards / Journal of Organometallic Chemistry 689 (2004) 3056–3059
3059
100%), 273 (M+ ꢀ I ꢀ Ni, 51%), (m/z; ES) 273 (M+ ꢀ
Acknowledgements
I ꢀ Ni, 100%).
We thank EPSRC for a studentship (J.S.F.) and M.E.
Light and M.B. Hursthouse of the EPSRC National
Crystallographic Service and for the X-ray crystal struc-
ture determination. In addition we thank Mark Stark
for carrying out preliminary experiments in this area
and Huy V. Nguyen for assistance with the completion
of the manuscript.
4.4. Attempted synthesis of [2,6-bis(40,40-dimethyl-20-
oxazolinyl)phenyl-N,C1,N0]nickel (II) triflate 7
[2,6-Bis(40,40-dimethyl-20-oxazolinyl)phenyl-N,C1,N0]-
iodonickel(II) 6 (0.044 g, 0.1 mmol) and silver triflate
(0.026 g, 0.1 mmol) were combined in acetone (10 mL)
and stirred protected from light. After stirring at room
temperature for 27 h the reaction mixture was filtered
through celite, eluting with acetone, to remove a grey/
white precipitate, consistent with the formation of silver
iodide. Removal of the solvent in vacuo gave a green
References
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been deposited with the Cambridge Crystallographic
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information may be obtained free of charge from The
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