Synthesis, characterisation and crystal structure of a novel nickel(II) phosphine complex, trans-[Ni2Cl4{cis-(Cy 2PCH2)2C6H4}2]

Article abstract of DOI:10.1016/j.ica.2010.12.007

The reaction of 1,3-C₆H₄(CH₂PCy ₂)₂ with anhydrous nickel(II) chloride in dry THF produces the non-cyclometallated dinuclear complex trans-[Ni₂Cl ₄{cis-(Cy₂PCH₂

Full text of DOI:10.1016/j.ica.2010.12.007

Inorganica Chimica Acta 367 (2011) 222–224
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Note
Synthesis, characterisation and crystal structure of a novel nickel(II) phosphine
complex, trans-[Ni2Cl4{cis-(Cy2PCH2)2C6H4}2]
⇑
Magnus T. Johnson, Ola F. Wendt
Organic Chemistry, Department of Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2010
Received in revised form 26 November 2010
Accepted 1 December 2010
Available online 10 December 2010
The reaction of 1,3-C6H4(CH2PCy2)2 with anhydrous nickel(II) chloride in dry THF produces the
non-cyclometallated dinuclear complex trans-[Ni2Cl4{cis-(Cy2PCH2)2C6H4}2]. The compound was charac-
terised by 1H, 31P and 13C NMR spectroscopy and its solid state structure was determined by X-ray
diffraction. An improved synthetic procedure of the aromatic pincer ligand 1,3-C6H4(CH2PCy2)2 is
reported.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Nickel
Phosphine
Cyclometallation
Pincer ligands
1. Introduction
lation. The process of cyclometallation through activation of Csp2–
H bonds is usually easier than Csp3–H bonds as expected with orbi-
tals of higher s-character in the transition state of a concerted
mechanism [9].
More specifically, this was shown for Ni(II) by Zargarian and co-
workers [10]. Under certain conditions, non-cyclometallated com-
plexes were obtained when C–H activation was adverse.
In the present work, we present an improved synthesis of ligand
1 and the molecular structure of a dimeric Ni(II) structure 3. The
preparation of 3 is compared and discussed relative to the syn-
thetic procedure for the cyclometallated complex 2.
Transition metal complexes of the PCP-pincer type show inter-
esting properties in many fields of chemistry including catalysis,
strong bond activation and supramolecular chemistry [1]. Among
the most common in the wide range of synthesised ligands are
the diphosphines, 1,3-(R2PCH2)2C6H4 (R = tBu, Ph), originally syn-
thesised by Moulton and Shaw [2]. A closely related analogue
1,3-(Cy2PCH2)2C6H4 1, based on the dicyclohexylphosphine ligand
was synthesised by Stephan and co-workers [3]. This ligand was la-
ter cyclometallated with NiCl2(H2O)6 to form the nickel chloride
complex [1,3-(Cy2PCH2)2C6H3]NiCl 2, which was converted to the
corresponding hydride [4]. Also the (PCP) nickel bromide was
made similarly [5], as well as [1,3-(Ph2PCH2)2C6H3]NiCl [6]. An iso-
electronic PNP amido complex showed interesting catalytic appli-
cations in Kumada couplings with aliphatic nucleophiles made
possible by its advantageous properties of slow b-hydride elimina-
tion [7]. Furthermore, PCPNi complexes with other phosphines
2. Experimental
2.1. General considerations
All experiments were carried out under an atmosphere of argon
or nitrogen using standard Schlenk or high vacuum line techniques
[11] unless otherwise noted. All solvents were distilled under vac-
uum directly to the reaction vessel from sodium/benzophenone
ketyl radical, except water and ethanol which were used as re-
ceived. NMR experiments were carried out using J. Young NMR
tubes. All chemicals were purchased from either Acros or Sigma–
Aldrich. 1H, 13C and 31P NMR experiments were recorded on a Var-
ian Unity INOVA 500 spectrometer operating at 499.77 MHz (1H)
using C6D6. Chemical shifts are given in ppm downfield from
TMS using residual solvent peaks (1H and 13C NMR) or H3PO4 as
reference. Multiplicities are abbreviated as follows: (s) singlet,
(d) doublet, (t) triplet, (m) multiplet and (v) virtual. Elemental
t
i
(R = Ph, Bu, Pr) showed activity in hydrosilylation reactions with
good to moderate yields in low catalytic loadings [8]. Gaining fur-
ther knowledge on the synthesis of PCP pincer complexes is of fun-
damental interest and
reactivity.
a prerequisite for the study of their
The cyclometallation of these ligands is generally believed to
occur via coordination of the strongly donating phosphine atoms
which places the metal in the immediate vicinity of the C–H bond.
This reduces the entropy of activation, thus facilitating cyclometal-
⇑
Corresponding author. Tel.: +46 46 2228153; fax: +46 46 2228209.
E-mail address: ola.wendt@organic.lu.se (O.F. Wendt).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.12.007

M.T. Johnson, O.F. Wendt / Inorganica Chimica Acta 367 (2011) 222–224
223
analyses were performed by H. Kolbe Microanalytisches Laborato-
rium, Mülheim an der Ruhr, Germany.
1. HPCy2, MeOH
2. NaOMe
2.2. Synthesis of 1,3-C6H4(CH2PCy2)2 (1)
Br
Br
Cy2P
PCy2
In a Straus flask, HPCy2 (1.55 ml, 7.65 mmol) was added to a
1
MeOH (30 ml) solution of
a,a-dibromo-m-xylene (1.01 g,
3.82 mmol). The appearingly immiscible mixture became homoge-
neous after approximately 1 min of intense stirring. After 3 h,
NaOMe (413 mg, 7.65 mmol) was added. A highly viscous oil sep-
arated from the solution which was decanted off. The oil solidified
upon drying in vacuum to yield 1.76 g (92%) of 1. Its spectral data
were in agreement with the literature [3].
Scheme 1. Improved synthesis of 1.
stantially simplified as sodium bromide is easily filtered off instead
of suffering the tedious removal of the diversely soluble triethyl-
ammonium bromide salt.
In an attempt to improve the published synthesis [4] of the PCP-
NiCl complex 2 with a reported yield of 60%, anhydrous nickel
chloride in dry THF was used. Upon heating a mixture of anhydrous
nickel chloride with ligand 1 in THF, the solution turned yellow
after 12 h and after 48 h a purple solid formed which after purifi-
cation was identified by single crystal X-ray diffraction as the di-
meric complex 3 (Scheme 2). The yellow intermediate was not
successfully isolated but is tentatively postulated as the non-cyclo-
metallated complex in Scheme 2. Similar complexes were success-
fully isolated with several different ligands and group 10 metals
[10a] [14–17].
Complex 3 is poorly soluble in most of the common organic sol-
vents and full NMR-characterisation was not possible although a
saturated deuterated benzene solution showed weak resonances.
The virtual triplet in the 1H NMR spectrum at 2.88 ppm indicates
a trans-coordination of the phosphines. The complex is stable to
air. The crystal structure contains one molecule of benzene per
molecule of 3, and the benzene could not be completely removed
even after 12 h in high vacuum and thus is reflected in the results
of elemental analysis.
2.3. Synthesis of trans-[Ni2Cl4{cis-(Cy2PCH2)2C6H4}2] (3)
Anhydrous NiCl2 (472 mg, 3.64 mmol) was added to a Straus
flask containing ligand 1 (1.81 g, 3.64 mmol) in a glove box. After
vacuum-transferring THF (30 ml) into the reaction vessel, an or-
ange-yellow solution formed. After heating to 50 °C for 50 min,
the colour changed to dark brown and after stirring at 30 °C over-
night, a heterogenous purple slurry had formed. The slurry was ex-
posed to air and the solids were filtered and washed with
2 Â 15 ml of THF and Et2O, respectively, to yield 420 mg (9%) of
3. X-ray quality crystals were grown from benzene.
1H NMR: d 1.07–2.12 (m, 84H, Cy), 2.49 (m, 4H, Cy), 2.88 (t,
J = 3.8 Hz, 4H, Ar–CH2), 6.96 (d, J = 7.0 Hz, 2H, Ar), 7.08 (t,
J = 7.0 Hz, 1H, Ar). 13C{1H} NMR: d 26.67 (s, Cy), 27.17 (s, Cy),
28.80 (s, Cy), 32.84 (t, J = 10.4 Hz, Cy–CH–P), 33.62 (m, Ar–CH2–
P), 122.37 (s, Ar), 125.34 (s, Ar), 128.35 (s, Ar). 31P{1H} NMR: d
52.01 (s). Anal. Calc. for C64H104Cl4Ni2P4ÁC6H6: C, 62.99; H, 8.31.
Found: C, 61.13; H, 9.1%.
Possibly due to the poor solubility we were unable to transform
3 to 2 in any of the solvents tested, including THF, petroleum ether,
benzene, dichloromethane and acetone. It can be noted that a sim-
ilar dinuclear platinum complex has been shown to transform into
the corresponding cyclometallated complex in benzene [18].
2.4. X-ray crystallography
The intensity data sets for 3 were collected at RT with a MAR-
CCD system using
u
Àscans and synchrotron radiation at MAXLAB
II, Lund, Sweden (k = 1.0376 Å) [12]. CCD data were extracted and
integrated using XDS [13]. The structure was solved by direct
methods and refined by full-matrix least-squares calculations on
F2 using SHELXTL 5.1. Non-H atoms were refined with anisotropic dis-
placement parameters. All hydrogen atoms were constrained to
parent sites, using a riding model. The use of synchrotron radiation
3.2. Crystal structure
Compound 3 crystallises in the monoclinic space group P21/c.
The asymmetric unit consists of half a molecule in the crystal
with a high wavelength gave a low sin
H/k. Also overload at low
NiCl2(H2O)6
EtOH / H2O
H
-values gave a low completeness. However, the quality of the
crystal structure was sufficient to obtain the molecular structure.
All crystallographic data are available in CIF format (CCDC refer-
ence number 800712).
reflux
2
Cy2P
Ni
Cl
PCy2
Cy2P
PCy2
2.4.1. Crystal data and collection and refinement details
anh. NiCl2
dry THF
60 °C
C
64H104Cl4Ni2P4ÁC6H6, M = 1334.68, monoclinic, a = 10.377(2),
b = 28.090(6), c = 12.017(2) Å, b = 91.88(3)° V = 3501.0(12) Å3,
space group P21/c (No. 14), Z = 2,
= 3.22 mmÀ1, Dcalc = 1.266
g cmÀ3
range 4.29–33.92 deg, 24880 reflections measured,
l
,
h
4056 unique (Rint = 0.0895) which were used in all calculations.
The final wR(F2) was 0.3342 and the S value 2.760 (all data). The
R(F) was 0.0957 (I > 2r(I)).
Cy2P
Cl Ni Cl Cl Ni Cl
Cy2P PCy2
PCy2
3
3. Results and discussion
Cy2P
PCy2
3.1. Synthesis
Ni
Cl
Cl
The synthesis of ligand 1 was modified relative to the literature
procedure such that sodium methoxide was used as a base instead
of triethylamine (Scheme 1). As a result, the purification is sub-
Scheme 2. Synthetic paths to complexes 2 and 3, with a postulated unisolated
intermediate.

224
M.T. Johnson, O.F. Wendt / Inorganica Chimica Acta 367 (2011) 222–224
formed to complex 2 using standard aprotic solvents. An improved
synthetic procedure of ligand 1 was developed.
Acknowledgements
Financial support from the Swedish ResearchCouncil, the Crafo-
ord Foundation, the Knut and Alice Wallenberg Foundation and the
Royal Physiographic Society in Lund is gratefully acknowledged.
We thank MAXLAB, Lund and Dr. Marjolein Thunnissen for
beam-time and help with the synchrotron experiments.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.12.007.
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