New hexaphosphane ligands 1,3,5-C6H3{p-C 6H4N(PX2)2}3 [X = Cl, F, C6H3OMe(C3H5)]: Synthesis, derivatization and palladium(ii) and plat

Article abstract of DOI:10.1039/c1dt10459d

A novel dodecachlorohexaphosphane, 1,3,5-C₆H₃[p- C₆H₄N(PCl₂)₂]₃ (1) was synthesized by reacting 1,3,5-tris(4′-anilino)benzene with phosphorus trichloride. Fluorination of 1 with

Full text of DOI:10.1039/c1dt10459d

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COMMUNICATION
New hexaphosphane ligands 1,3,5-C6H3{p-C6H4N(PX2)2}3 [X = Cl, F,
C6H3OMe(C3H5)]: Synthesis, derivatization and palladium(II) and
platinum(II) complexes†
Sowmya Rao,a Chelladurai Ganesamoorthy,a Shaikh M. Mobinb and Maravanji S. Balakrishna*a
Received 18th March 2011, Accepted 11th April 2011
DOI: 10.1039/c1dt10459d
A
novel
dodecachlorohexaphosphane,
1,3,5-C6H3[p-
C6H4N(PCl2)2]3 (1) was synthesized by reacting 1,3,5-tris(4¢-
anilino)benzene with phosphorus trichloride. Fluorination
of 1 with SbF3 produces 1,3,5-C6H3[p-C6H4N(PF2)2]3 (2).
The derivatization of chlorohexaphosphane with an aryloxy
substituent and its palladium(II) and platinum(II) complexes
are also described.
The design and synthesis of polydentate ligands remains as an
active field of research for many years as their metal complexes
show structural diversity and unique catalytic properties.1 The
well-defined ligands of this class can readily form multinuclear
complexes with metals in close proximity to each other which
can enhance the catalytic performance, if there are any, through
cooperativity effects.2 Further the rigid polydentate ligands have
been widely used for constructing macromolecules and metal–
organic frameworks with desired porosity and functionality for
small molecule encapsulation.3 Various pyridyl and carboxylate
containing multidentate aromatic ligands have been designed and
extensively used in these disciplines. Among the pyridyl based
systems the preeminent ones are: 1,3,5-triazene, 4,4¢-bipyridine
and 2,4,6-tris(pyridine-4-yl)-1,3,5-triazine (Chart 1) which have
been exploited as bridging units to generate a series of supramolec-
ular structures.4 However the phosphane analogous are scarce
except for the tri-, tetra-, and hexaphosphanes of the type shown
in Chart 1.5,6 In view of this, recently we have developed novel
tetraphosphanes, {(PX2)2NC6H4N(PX2)2} (X = halides, alhoxy
and aryloxy) which behave like a pair of sideways-fused 4,4¢-
bipyridines and offered unusual reactivity with transition metals
to give multinuclear complexes and 3-D coordination polymers
with large porosity.6
Chart 1 Multidentate pyridyl and phosphane systems.
its aryloxy derivative, 1,3,5-C6H3[p-C6H4N{P(OR)2}2]3 [R = -
C6H3OMe(C3H5)] (3). The compound 1 and its derivatives are
ideal candidates to introduce the concept of metallophilicity in 2-
and 3-D coordination networks by bringing the adjacent metals
in close proximity.
The bis(dihalophosphino)amines of the type C12P–N(R)–PC12
(R = alkyl or aryl) are generally prepared in good yields by
reacting the primary amines with excess PCl3.7 Similar attempts
to synthesize 1 resulted in the formation of a mixture of products
which required tedious purification steps. However, the reaction of
1,3,5-tris(4¢-aminophenyl)benzene with phosphorus trichloride in
presence of three equivalents of pyridine afforded the hexaphos-
phane 1 as a pale yellow crystalline solid in 20% yield. The yield has
been further improved by carrying out the reaction in presence of
a strong base like triethylamine with a catalytic amount of N,N-
dimethyl-4-aminopyridine (Scheme 1). The compound 1 readily
decomposes on exposure to air and moisture. The fluorination of
1 with antimony trifluoride produces the fluoro analogue, 1,3,5-
C6H3[p-C6H4N(PF2)2]3 (2) in good yield (Scheme 2). The 31P NMR
spectrum of 1 consists of a single peak at 153.7 ppm, whereas 2
shows the A portion of an AA¢X2X2¢ multiplet centered at 130.4
ppm with |1JPF|, |3JPF| and |2JPP| couplings of 1246, 124 and
372 Hz, respectively. The 1H NMR spectra of 1 and 2 show singlets
around 7.88 ppm corresponding to the protons of the central
phenyl ring, while the aromatic protons of phenylene groups
In this communication, we report
a convenient one-
pot synthesis of a novel dodecachlorohexaphosphane, 1,3,5-
C6H3[p-C6H4N(PCl2)2]3 (1) and PdII and PtII complexes of
aPhosphorus Laboratory, Department of Chemistry, Indian Institute of
Technology Bombay, Powai, Mumbai, 400 076, India. E-mail: krishna@
chem.iitb.ac.in; Fax: +91 22 2576 7152/2572 3480; Tel: +91 22 2576 7181
bNational Single Crystal X-ray Diffraction Facility, Indian Institute of
Technology Bombay, Powai, Mumbai, 400 076, India
† Electronic supplementary information (ESI) available: Experimental
details. CCDC reference number 818410. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt10459d
1
resonate as two doublets at 7.36 and 7.82 ppm. The H NMR
and analytical data are consistent with the proposed structures of
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of the P–N–P skeletons with dihedral angle of 90.68(1)◦ (P2–
N–P1 vs. C7–C12). The observed P–N bond lengths in 1 vary
˚
from 1.689(3) to 1.696(3) A and are comparable with those
found in the analogous tetraphosphane, p-C6H4[N(PCl2)2].6a The
˚
P–Cl bond distances vary from 2.025(2) to 2.066(2) A. In the
molecular structure of 1, the three independent P–N–P bond
angles [111.3(2)◦, 112.2(2)◦ and 111.4(2)◦] are virtually the same
and are smaller than those found in free ligands of the type
PhN{P(OR)2}2 (R = alkyl or aryl groups).7
Scheme 1 Synthesis of compound 1.
4-Allyl-2-methoxyphenol is an ideal candidate to functionalize
the periphery of the hexaphosphane 1 with the dangling olefin
functionality, and the presence of o-methoxy groups can fine-
tune the electronic properties of the transition metals during
catalysis. The hexaphosphane 1 reacts smoothly with the ap-
propriate amount of 4-allyl-2-methoxyphenol in the presence
of triethylamine to afford the hexaphosphonite, 1,3,5-C6H3[p-
C6H4N{P(OR)2}2]3 [R = -C6H3OMe(C3H5)] (3) in quantita-
tive yield. Treatment of 1 with the sodium salt of 4-allyl-2-
methoxyphenol in THF also produces 3 in quantitative yield as
shown in Scheme 2. The 31P NMR spectrum of 3 shows a single
resonance at 128.2 ppm. The 1H NMR spectrum of 3 exhibits two
singlets at 3.65 and 7.82 ppm corresponding to the o-methoxy and
central phenyl ring protons, respectively. The allyl groups show
two multiplets centered at 5.03 and 5.92 ppm whereas the -CH2-
Scheme 2 Syntheses of compounds 2 and 3.
1 and 2 and the structure of compound 1 was confirmed via an
X-ray structure determination.
3
protons exhibit a doublet at 3.32 ppm with a JHH coupling of
6.4 Hz. The 1H NMR and analytical data are consistent with the
proposed structure of compound 3.
A perspective view of the molecular structure of 1 with the atom
numbering scheme is shown in Fig. 1. Crystal data and the details
of the structure determination along with selected bond lengths
and bond angles are given in Supporting Information (Tables S1
and S2).
Compounds 1–3 are potential hexadentate ligands and it would
be interesting to explore their coordination behavior with various
transition metal salts. In a preliminary study, the air- and moisture-
stable ligand 3 has been used for the preparation of platinum
group metal complexes. Treatment of 3 with three equivalents of
[M(COD)Cl2] (M = Pd or Pt) in dichloromethane afforded the
chelate complexes 1,3,5-C6H3[p-C6H4N{P(OR)2}2(MCl2)]3 [R =
-C6H3OMe(C3H5)] (4, M = Pd and 5, M = Pt) in good yields
(Scheme 3). The 31P NMR spectra of complexes 4 and 5 show
single resonances at 63.2 and 57.9 ppm, respectively, which are
considerably shielded compared to the free ligand. The platinum
1
complex exhibits a large JPt–P coupling of 5913 Hz, which is
consistent with the proposed cis geometry around the platinum
center. The spectral and analytical data support the proposed
formulations of complexes 4 and 5.
Fig. 1 Molecular structure of 1. All hydrogen atoms have been omitted
for clarity. Displacement ellipsoids are drawn at the 50% probability level.
Crystals of 1 suitable for X-ray diffraction analysis were grown
from saturated PCl3 solution. The compound 1 crystallizes in
the monoclinic crystal system with space group P21/c. In 1,
the Cl atoms are disposed approximately equally above and
below the respective P–N–P planes while there is a distorted
pyramidal geometry about the phosphorus centers and a planar
environment around the nitrogen centers with the sum of the
angles around nitrogen almost 360◦ in all cases. Further the
bridging phenylene rings are almost perpendicular to the plane
Scheme 3 Reactions of 3 with [M(COD)Cl2] (M = Pd and Pt).
In summary, we report for the first time an efficient one-pot
synthesis of a dodecachlorohexaphosphane 1 which would serve as
a molecular synthon for producing large number of multidentate
phosphanes in good yield. The easy access of hexaphosphane 1
5842 | Dalton Trans., 2011, 40, 5841–5843
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circumvents the general problems associated with the synthesis
of these systems, particularly the multi-steps and low yield.
In addition, the hexaphosphanes (1–3) are ideally suited to
design multinuclear complexes especially with Group 11 metals
as they are known to show metallophilic interactions. Further
organometallic chemistry and catalytic reactions with this system
are under active investigation in our laboratory.
We thank the Department of Science and Technology (DST),
New Delhi, for financial support of this work through grant
SR/S1/IC-17/2010. We also thank the Department of Chemistry
Instrumentation Facilities and National Single Crystal X-ray
Diffraction Facility, IIT Bombay, for spectral, analytical data and
X-ray structure determination.
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©