Synthesis, characterization and applications of novel iminophosphinites

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Phosphorus, Sulfur, and Silicon
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Synthesis, Characterization
and Applications of Novel
Iminophosphinites
a
a
a
Jake Yorke , Sarah Beaton , Aibing Xia & Hilary
b
Jenkins
a
Department of Chemistry , Mount Saint Vincent
University , Halifax, Nova Scotia, Canada
b
Department of Chemistry , Saint Mary's University ,
Halifax, Nova Scotia, Canada
Published online: 21 Jun 2008.
To cite this article: Jake Yorke , Sarah Beaton , Aibing Xia & Hilary Jenkins
(
2008) Synthesis, Characterization and Applications of Novel Iminophosphinites,
Phosphorus, Sulfur, and Silicon and the Related Elements, 183:2-3, 652-656, DOI:
0.1080/10426500701796017
1
To link to this article: http://dx.doi.org/10.1080/10426500701796017
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Phosphorus, Sulfur, and Silicon, 183:652–656, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701796017
Synthesis, Characterization and Applications of Novel
Iminophosphinites
1
1
1
Jake Yorke, Sarah Beaton, Aibing Xia,
2
and Hilary Jenkins
Department of Chemistry, Mount Saint Vincent University, Halifax,
Nova Scotia, Canada
1
2
Department of Chemistry, Saint Mary’s University, Halifax,
Nova Scotia, Canada
Keywords Iminophosphinites; palladium complexes; Suzuki reaction; unsymmetrical
ligands
INTRODUCTION
Ligand design plays an important role in organometallic synthesis and
catalysis. The combination of a soft donor and a hard donor induces
different interactions between metal centers and the unsymmetrical
1
,2
donors, which may lead to hemilabile properties. Recently unsym-
metrical ligands with two or more different donors such as phosphorus
and nitrogen (P,N type) donors have attracted increasing attention due
3
to their bonding versatility and catalytic applications. Here, we report
the synthesis, characterization and catalytic applications of two novel
bidentate and potentially hemilabile iminophosphinites, and a palla-
dium complex bearing one of the iminophosphinite ligands.
The reactions between Schiff bases, 2-(2,6- diisopropylphenylim-
ino)methyl)phenol from salicylaldehyde and 2,6-di-isopropylaniline4
or 2-(diphenylmethyleneamino)phenol from 2-aminophenol and
5
benzophenone, and chlorodiphenylphosphine in the presence of
4
-dimethylaminopyridine (DMAP) in Et2O afforded iminophosphinite
6,7
compounds 1 and 2 (Schemes 1 and 2).
Both compounds can be
We thank National Sciences and Engineering Research Council of Canada and Canada
Foundation for Innovation for financial support. We also thank the Atlantic Region Mag-
netic Resonance Centre for help with NMR data collection.
Address correspondence to Aibing Xia, Department of Chemistry, Mount Saint Vin-
cent University, Halifax, Nova Scotia, B3M 2J6, Canada. E-mail: aibing.xia@msvu
652

Novel Iminophosphinites
653
SCHEME 1
SCHEME 2
recrystallized from ether/hexane. The crystals are fairly air stable
and remain unchanged in the air after several days. Reaction of
iminophosphinite 2 and bis(benzonitrile)palladium chloride in CH2Cl2
8
afforded the expected palladium complex 3 (Scheme 2). Single crystals
of 3 were grown from CH2Cl2/hexane through slow evaporation. In
the solid state of 3, palladium ion adopts a slightly distorted square
planar geometry, with the sum of the bond angles surrounding the Pd
◦
ion at 360.16(3) . The IR spectra show characteristic C N stretches
−1
at 1627 and 1638 cm for 1 and 2 respectively. Upon coordination
of 2 to palladium, the C N stretch decreased to 1596 cm
−
1
in 3,
indicating back-bonding from the electron-rich palladium ion. The
3
1
P signal increased from 111.93 ppm in 2 to 131.55 ppm in 3 after
coordination.
Reactions of dichlorobis(benzonitrile)palladium with iminophosphi-
3
1
nite 1 at either 1:1 or 1:2 ratio afforded mixtures, as suggested by the
P
NMR spectra which gave more than one phosphorus signals. This may
be indicative of the lability of this type of iminophosphinte as it can only
form seven-membered palladacycles if both P and N coordinate to Pd,
which is not as stable as six-membered ones formed by iminophosphi-
nite 2. Similarly, recent studies have shown that longer bisphosphines
may act as monodentate donors instead of bidentate ones, leading to
lower stability in the metal complexes.9
The catalytic activities of compounds 1 and 2, and complex 3 were
investigated in Suzuki coupling reaction,10 an important carbon-carbon
bond formation reaction. We employed a deactivated aryl bromide, 4-
bromoanisole, and phenylboronic acid in the studies [Equation (1)].
In comparative studies, we found that with 2% Pd catalyst and
ligand loading, systems based on iminophosphinite 1 gave better
yields (up to 99%) than those based on iminophosphinite 2 and its
palladium complex 3. These systems are generally better than those

6
54
J. Yorke et al.
SCHEME 3
based on related but monodentate ligands such as triphenylphos-
phine and ethyl diphenylphosphinite (Table I). The higher activ-
ity of systems base on iminophosphinite 1 may be attributed to
the hemilabile property of the iminophosphinite ligand, which may
facilitate aryl bromide oxidative addition. On the other hand, the
stability of palladium complex bearing iminophosphinite 2 hinders
palladium interaction with organic substrates which makes it less
active.
In conclusion, we synthesized and characterized two novel
iminophosphinite ligands and investigated their coordination to palla-
dium metal center. In Suzuki coupling studies, the potentially hemi-
labile iminophosphinite 1 showed better catalytic activity than the
iminophosphinite 2 under similar conditions. We are attempting to
t
syntheisize iminophosphinites with bulky alkyl (e.g., Bu) substituents
on the phosphorus atom, which should lead to enhanced σ-donating
ability and thus improved catalytic activity. Furthermore, we will
attempt to make the imine moiety more labile by introducing electron-
1
1
withdrawing substituents such as nitro group on the phenyl ring.
SCHEME 4

Novel Iminophosphinites
655
TABLE I Pd(II) Catalyzed Suzuki Reactions
Entry
Ligand or Complex, 2% loading
Solvent
Yield (%)a
1
2
3
4
5
6
7
8
1
2
3
1
2
3
Toluene
Toluene
Toluene
Dioxane
Dioxane
Dioxane
Toluene
Toluene
99
59
74
54
33
62
43
26
PPh3
EtOPPh2
aYields are calculated using single point external standard with authentic samples
through GC/MS analysis.
SUPPLEMENTARY INFORMATION
CCDC 642499 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre: via www.ccdc.cam.ac.uk
/
data request/cif
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[
[
◦
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6] Characterization of 1: m.p. 109–111 C. IR (KBr, cm ): 2957, 1627, 1596, 1457,
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1
7
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1
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6
56
J. Yorke et al.
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1
(
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◦
−1
1
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1
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1
8
3
1
1
P H NMR (CD2Cl2): δ 131.55.
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3