Unusual reactivity of a sterically hindered diphosphazane ligand, EtN{P(OR)2}2, (R = C6H3(Pr i)2-2,6) towards (η3-allyl)palladium precursors

Article abstract of DOI:10.1039/b703308g

The reactivity of (η³-allyl)palladium chloro dimers [(1-R-η³-C₃H₄)PdCl]₂ (R = H or Me) towards a sterically hindered diphosphazane ligand [EtN{P(OR) ₂}₂] (R = C₆H

Full text of DOI:10.1039/b703308g

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Unusual reactivity of a sterically hindered diphosphazane ligand,
EtN{P(OR)2}2, (R = C6H3(Pri)2-2,6) towards (g3-allyl)palladium
precursors†‡
Heera Krishna,a Setharampattu S. Krishnamurthy,*a Munirathinam Nethaji,a Ramaswamy Murugavelb and
Ganesan Prabusankarb
Received 5th March 2007, Accepted 23rd April 2007
First published as an Advance Article on the web 9th May 2007
DOI: 10.1039/b703308g
3
3
The reactivity of (g -allyl)palladium chloro dimers [(1-R-g -C3H4)PdCl]2 (R = H or Me) towards a
sterically hindered diphosphazane ligand [EtN{P(OR)2}2] (R = C6H3(Pri)2-2,6), has been investigated
under different reaction conditions. When the reaction is carried out using NH4PF6
3
as the halide scavenger, the cationic complex [(1-R-g -C3H4)Pd{EtN(P(OR)2)2}]PF6 (R = H or Me) is
formed as the sole product. In the absence of NH4PF6, the initially formed cationic complex,
3
[(g -C3H5)Pd{EtN(P(OR)2)2}]Cl, is transformed into a mixture of chloro bridged complexes over a
3
period of 4 days. The dinuclear complexes, [(g -C3H5)Pd2(l-Cl)2{P(O)(OR)2}{P(OR)2(NHEt)}] and
[Pd(l-Cl){P(O)(OR)2}{P(OR)2(NHEt)}]2 are formed by P–N bond hydrolysis, whereas the
3
3
octa-palladium complex [(g -C3H5)(2-Cl-g -C3H4)Pd4(l-Cl)4(l-EtN{P(OR)2}2)]2, is formed as a result
of nucleophilic substitution by a chloride ligand at the central carbon of an allyl fragment. The reaction
3
of [EtN{P(OR)2}2] with [(g -C3H5)PdCl]2 in the presence of K2CO3 yields a stable dinuclear
3
3
(g -allyl)palladium(I) diphosphazane complex, [(g -C3H5)[l-EtN{P(OR)2}2Pd2Cl] which contains a
coordinatively unsaturated T-shaped palladium center. This complex exhibits high catalytic activity and
high TON’s in the catalytic hydrophenylation of norbornene.
and the diphosphazane ligand which partly destabilizes the syn-
Introduction
isomer.3 If the relative proportion of the anti-isomer is increased,
the reaction with nucleophiles might result in an increased amount
of Z-substitution products in allylic alkylation reactions.4 Hence,
we reasoned that introducing more bulky substituents in the
auxiliary diphosphazane ligand might further enhance the relative
proportion of the anti-isomer. As an initial step towards this goal,
3
The reaction of (g -ally1)palladium complexes with nucleophiles
to afford allylic alkylation products is of wide ranging synthetic
utility.1 One of the most significant aspects of this chemistry
is the prospect of controlling the chemo-, regio-, stereo- and
enantioselectivity of the nucleophilic attack on the allyl moiety
through appropriate choice of the reaction conditions and the
ancillary ligands attached to palladium.2 As a result, there is con-
siderable current interest in investigating the electronic and steric
interactions that govern the selectivity in catalytic transformations
3
we set out to explore the reactivity of (g -allyl)palladium chloro
dimers towards the sterically encumbered diphosphazane ligand
[EtN{P(OR)2}2], (R = C6H3(Pri)2-2,6) viz., (L) under various
experimental conditions. In addition to the expected cationic allyl
palladium complexes, several unusual products have been isolated
and their structures established by single crystal X-ray diffraction
studies.
3
proceeding through (g -allyl)palladium intermediates.
A recent report by Mandal et al. has shown that the reaction
of a sterically bulky diphosphazane ligand [MeN{P(OR)2}2] (R =
3
3
C6H3Me2-2,6), with [(1,3-Me2-g -C3H3)Pd(l-Cl)]2 and [(1-Me-g -
C3H4)Pd(l-Cl)]2, resulted in a mixture of isomers, of which the
normally less favored syn/anti isomer (in the former case) and the
anti-isomer (in the latter case) were present to a greater extent than
that observed for other diphosphazane ligands. This result has
been attributed to the steric interactions between the allyl moiety
Results and discussion
3
The reaction of [Pd(g -C3H5)(l-Cl)]2 with (L) in the presence of
3
NH4PF6 as the chloride scavenger gives the cationic complex [(g -
C3H5)Pd(L)](PF6) (1a) as the sole product. This complex could be
isolated in a pure form and characterized by C, H, N elemental
analysis and NMR spectroscopic data. The same reaction in
aDepartment of Inorganic and Physical Chemistry, Indian Institute of Sci-
ence, Bangalore, 560 012, India. E-mail: sskrish@ipc.iisc.ernet.in; Tel: +91
080 22932401
3
the absence of NH4PF6 initially gives the cationic complex [(g -
bDepartment of Chemistry, Indian Institute of Technology-Bombay, Powai,
Mumbai, 400 076, India
C3H5)Pd(j2-L)]Cl (1b) analogous to complex (1a) as shown by the
1
31P{ H} NMR spectrum of the reaction mixture. The spectrum
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Reactivity of (L)
shows a single resonance at dP 111.0 ppm; this chemical shift is
close to that observed for complex (1a). Efforts to isolate this
complex were unsuccessful. It was found to be highly sensitive to
air and moisture, and over a period of 96 h was slowly transformed
3
towards substituted (g -allyl) palladium chloro dimers, a comparison of
bondlengths and torsion angles in diphosphazane bridged palladium
complexes, structural details of complex (4) and ligand monoxide (Lꢀ).
See DOI: 10.1039/b703308g
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into a mixture of complexes (2), (3), (4) and the ligand monoxide
(Lꢀ) (Scheme 1). In addition, two unidentified products with reso-
nances at dP 65.0 and 130.3 ppm were found in the reaction mixture.
The structures of complexes (2), (3) and (4) have been established
by single crystal X-ray diffraction studies. These products are
formed apparently by hydrolysis, reduction or oxidation. Even
though the reaction was carried out under an atmosphere of
dry nitrogen, traces of moisture present in the glass surface or
ingress of moisture during crystallization led to the formation of
these products. The synthetic procedures were optimized to obtain
complexes (2), (3) and (4) (See Experimental section).
result of hydrolytic cleavage of the P–N bond of the diphosphazane
1
ligand and loss of one or more allyl fragments. The 31P{ H} NMR
spectra of complexes (3) and (4) are of the AX type; the chemical
shift of {P(OR)2(NHEt)} (86.4 or 78.8 ppm) lies downfield
from that of {P(O)(OR)2} (32.1 or 29.1 ppm). An alternate
route to complex (4) involves the reaction of [PdCl2(COD)]
and [EtN{P(OR)2}2] (L) in dichloromethane at 323 K for 16 h
(Scheme 2). The initial product gives rise to a sharp singlet at dP
65.0 ppm, which has been tentatively assigned to the complex
[PdCl2(j2-L)]n, since the dP value lies in the range normally
observed for a chelating diphosphazane ligand.11 Allyl complexes
of phosphoramidites such as complexes (1) and (3) are rare in the
literature. Recently, Pregosin et al. have reported the structures
and solution dynamics of several mono-phosphoramidite and bis-
phosphoramidite 2-methallyl and 1,3-diphenylallyl complexes of
palladium.12
Scheme 2 An alternate synthetic procedure for complex (4).
Scheme 1 Reaction of [EtN{P(OR)2}2], (R = C6H3 (Pri)2-2,6) (L) with
The unique reactivity of the ligand (L) prompted us to
investigate the base-induced reduction of [Pd(g -C3H5)(l-Cl)]2
3
3
[(g -C3H5)Pd(l-Cl)]2.
precursor in the presence of (L). Base-induced reduction of Pd(II)
complexes to give Pd(0) species in the presence of phosphine
ligands is well documented in the literature.13 The significance of
such reactions lies in the involvement of Pd(0) species in various
catalytic transformations. A palladium(0) species is implicated
in numerous catalytic reactions, including carbon–carbon and
carbon–heteroatom cross-coupling and carbonylation reactions.14
3
The most remarkable product obtained in the reaction of [Pd(g -
3
C3H5)(l-Cl)]2 and ligand (L) is the octa-palladium complex, [(g -
3
C3H5)(2-Cl-g -C3H4)Pd4(l-Cl)4(l-EtN{P(OR)2}2)]2, (2) formed as
a result of nucleophilic substitution at the central allyl carbon
of one of the allyl fragments. The 31P{ H} NMR spectrum of
1
(2) displays a sharp singlet at dP 98.0 ppm. The first report on
nucleophilic attack at the central allyl carbon of a transition metal
(g -allyl) complex was by Green and coworkers in 1976.5 Since
3
When the reaction between [Pd(g -C3H5)(l-Cl)]2 and (L) was
3
carried out in the presence of K2CO3 in acetone at 298 K in
open atmosphere, the coordinatively unsaturated palladium(I)
then, there are several reports of similar reactivity in transition
metal (p-allyl) complexes of Mo, W, Pt, Pd, Mn, Zr, Ti, Ir and
Rh.6 The anions that act as nucleophiles in these reactions include
malonate and enolate-type anions, simple alkyl (e.g., RLi, RMgX)
or hydride reagents (e.g., NaBH4).7 Despite the fact that Cl− is one
of the simplest available species for nucleophilic attack in syn-
thetically useful palladium catalysed transformations, examples
3
complex [(g -C3H5)[l-(EtN{P(OR)2}2)Pd2Cl] (6) was obtained in
70% yield (Scheme 3). The orange–red crystalline compound
was found to be remarkably stable over a period of 12 months
when stored in a closed vial. The reaction of diphosphazane
3
ligands [MeN{P(OR)2}2] (R = C6H5 or C6H3Me2-2,6) with (g -
allyl)palladium chloride dimers are known to form ligand-bridged
dinuclear Pd(I) complexes by the reduction of Pd(II) to Pd(I) and
the loss of the allyl fragment.3,15
3
of chloride migration in (g -allyl)palladium complexes are rare.
Szabo has reported the first theoretical study complemented by
experimental verification on ligand assisted chloride migration in
(g -allyl) palladium complexes.8 There is only one other example
3
in the literature of chloride migration from a palladium center
to the allylic carbon terminus.9 To our knowledge, complex (2)
represents the first example of nucleophilic attack by a chloride
3
ligand at the central carbon atom of a (g -allyl) palladium complex.
A clear understanding of the regiochemistry of nucleophilic attack
at the central allyl carbon observed in complex (2) calls for detailed
theoretical investigations.6,10
Scheme 3 Synthesis of the dinuclear three-coordinated Pd(I) complex (6).
The chloro bridged dinuclear Pd(II) diphosphazane com-
3
plexes [(g -C3H5)Pd2(l-Cl)2{P(O)(OR)2}{P(OR)2(NHEt)}], (3)
Allyl palladium complexes of diphosphazane ligands bearing
phenoxy substituents are relatively rare in the literature because
and [Pd(l-Cl){P(O)(OR)2}{P(OR)2(NHEt)}]2, (4) are formed as a
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Table 1 Selected bond distances and angles for complex (2)
of their high p-acidity owing to the presence of two electroneg-
ative oxygen atoms attached to each phosphorus center.3 The
high p-acceptor nature of (L) combined with its large steric
3
3
[(g -C3H5)(2-Cl-g -C3H4)Pd4(l-Cl)4(l-EtN{P(OR)2}2)]2, (2)
3
bulk destabilizes the cationic complex [(g -C3H5)Pd(j2-L)]Cl (1b),
˚
Bond lengths/A
3
making it susceptible for the loss of the (g -allyl) fragment which
Pd(1) · · · Pd(2)
Pd(2)–Pd(3)
Pd(3) · · · Pd(4)
Pd(1) · · · Pd(1ꢀ)a
Pd(1)–Cl(1ꢀ)
Pd(1)–Cl(2)
3.356
2.5309(9)
3.413
Pd(2)–Cl(3)
Pd(3)–Cl(4)
Pd(3)–Cl(5)
Pd(4)–Cl(5)
Pd–P<>
C(52)–Cl(1)
(C–C)allyl <>
Pd–Callyl<>
2.4270(19)
2.420(2)
2.443(2)
2.376(2)
2.1644[17]
1.53(2)
triggers the subsequent reactions. Apparently, the chloride ion
released during the initial bridge-splitting reactions to form the
cationic complex (1b) acts as the chloride source for nucleophilic
substitution to yield complex (2). The steric strain imposed by
the bulky diphosphazane ligand L in complex (1b) is released
during P–N bond rupture, and this acts as the driving force for
the formation of complexes (3) and (4). Similar solvolytic cleavage
of diphosphazane ligands bound to transition metals by water or
protic solvents has been reported.16,17
3.554
2.476(2)
2.410(2)
2.425(2)
2.442(2)
Pd(1)–Cl(3)
Pd(2)–Cl(2)
1.35[3]
2.13[12]
Bond angles/Torsion angles/◦
P(1)–N(1)–P(2)
Cl(3)–Pd(2)–
Pd(3)–Cl(4)
P(1)–Pd(2)–
Pd(3)–P(2)
111.9(3)
67.11(8)
Single crystal X-ray diffraction studies
46.26(6)
3
3
The solid-state structures of [(g -C3H5)(2-Cl-g -C3H4)Pd4(l-Cl)4-
3
(l-EtN{P(OR)2}2)]2
(2),
[(g -C3H5)Pd2(l-Cl)2{P(O)(OR)2}-
a Pd(1ꢀ) indicates that this atom is at (1 − x, 1 − y, −z).
{P(OR)2(NHEt)}] (3), [Pd(l-Cl){P(O)(OR)2}{P(OR)2(NHEt)}]2
3
(4), [(g -C3H5)[l-(EtN{P(OR)2}2)Pd2Cl] (6) and the ligand
monoxide (Lꢀ) have been determined by X-ray crystallography.
The salient structural features of complexes (2), (3) and (6) only
are discussed below. The crystal and molecular structures of
complex (4) and the diphosphazane ligand monoxide (Lꢀ) are
discussed in the ESI.‡
A remarkable feature of the molecule is that the two tetra-
palladium units are bridged by two chloro allyl moieties. Nucle-
ophilic substitution occurs at C(52) of one allyl fragment (Fig. 1),
and this chloride coordinates to an adjacent palladium center
giving rise to a six-membered ring structure; the Pd(1) · · · Pd(1)ꢀ
˚
separation is 3.554 A. The two ‘Pd(l-Cl)2Pd’ mean planes flanking
Crystal and molecular structure of [(g3-C3H5)(2-Cl-g3-
C3H4)Pd4(l-Cl)4(l-EtN{P(OR)2}2)]2 (2). The ORTEP plot of
complex (2) is shown in Fig. 1. Selected bond distances and angles
are listed in Table 1. The molecular framework consists of two
tetra-palladium units with an inversion center at the midpoint
of a six-membered ring, viz., Pd(1)–C(52)–Cl(1)–Pd(1ꢀ)–C(52ꢀ)–
Cl(1ꢀ). The asymmetric unit consists of a tetra-palladium chain in
which the adjacent metal centers are bridged by either two chloride
ligands or the bidentate diphosphazane ligand in an alternating
fashion. The terminal metal atoms of the asymmetric unit (Pd1
the ‘Pd(l-L)Pd’ unit are nearly orthogonal (dihedral angle: 87◦)
as a result of the huge steric bulk of the diphosphazane ligand
(L). Consequently, the terminal (g -C3H5)Pd(l-Cl)2 mean plane
3
◦
3
subtends an angle of 63 with the (2-Cl-g -C3H5)Pd(l-Cl)2 mean
plane of the same asymmetric unit. This steric factor is also
reflected in the P(1)–Pd(2)–Pd(3)–P(2) torsion angle of 46.26(6)◦
spanned by (L) across the Pd(2)–Pd(3) bond. The bridging ‘P–N–
P’ ligand holds the two metal centers together with a short M–
˚
M bond distance of 2.5309(9) A. This distance is shorter than
the M–M bond distances observed for dimeric Pd(I) complexes
containing bridging diphosphazanes.11,18,19 The twist along the
Pd–Pd axis is attributed to the anti-bonding interactions of filled
metal d-orbitals that are minimized when the torsion angle equals
45◦.20 The five-coordinate palladium adopts a distorted square
pyramidal geometry with a chloride ligand occupying the axial
coordination site. The crystal lattice also contains one molecule
of water and one molecule of acetone. The water oxygen atom
O(6) shows a weak interaction with the Pd(4) atom (Pd(4)–O(6)
3
and Pd4) are capped by two allyl fragments bound in an g -mode.
The dihedral angle between the square plane containing the allyl
carbon atoms, and the plane containing the ‘Pd(l-Cl)2’ unit bound
to it, is ∼120◦.
˚
distance 2.51(3) A).
Crystal and molecular structure of [(g3-C3H5)Pd2(l-Cl)2-
{P(O)(OR)2}{P(OR)2(NHEt)}] (3). The molecular structure of
complex (3) is shown in Fig. 2. It consists of a neutral, chloro
3
bridged dinuclear species capped by an g -allyl fragment at one
terminus (Fig. 2). The ligand [EtN{P(OR)2}2], (R = C6H3(Pri)2-
2,6), (L) has apparently undergone P–N bond rupture followed by
hydrolysis to form P(O)(OR)2 and P(NHEt)(OR)2 moieties which
are bound to palladium. Selected bond distances and angles are
Fig. 1 The ORTEP view of the basic framework in complex (2). The
atoms are represented by their vibrational ellipsoids of 30% probability.
All carbon atoms except those of the allyl fragment, all hydrogen atoms,
the water molecule and acetone present in the lattice have been omitted
for clarity. The prime in Pd(1ꢀ) indicates that this atom is at (1 − x,
1 − y, −z).
˚
listed in Table 2. The P(1)–N(1) bond distance (1.615(3) A) is less
than that (1.674(5) A) observed for the ligand (L).21 The geometry
˚
around the metal centers is essentially planar. The dihedral angle
between the square plane containing the allyl carbon atoms, and
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Fig. 2 The ORTEP plot of complex (3). The atoms are represented by
their vibrational ellipsoids of 30% probability. The CH3 carbon atoms of
the isopropyl groups and all hydrogen atoms (except the NH proton) have
been omitted for clarity.
Fig. 3 The ORTEP plot of complex (6). The atoms are represented by
their vibrational ellipsoids of 30% probability. The hydrogen atoms, CH3
carbon atoms of isopropyl groups and acetone present in the lattice have
been omitted for clarity.
Table 2 Selected bond distances and angles for complex (3)
3
[(g -C3H5)Pd2(l-Cl)2{P(O)(OR)2}{P(OR)2(NHEt)}], (3)
˚
Bond lengths/A
Table 3 Selected bond distances and angles for complex (6)
Pd(1) · · · Pd(2)
Pd–Cl <>
Pd–Callyl<>
Pd(1)–P(1)
Pd(1)–P(2)
N(1) · · · O(5)
3.358
2.4003[11]
2.107[5]
2.2385(10) P(2)–O(3)
2.2516(10) P(2)–O(4)
2.708
N(1)–P(1)
P(1)–O(1)
P(1)–O(2)
1.615(3)
1.601(3)
1.611(3)
1.609(3)
1.610(3)
1.482(3)
3
(g -C3H5)Pd2Cl(l-EtN{P(OR)2}2), (6)
˚
Bond lengths/A
P(2)–O(5)
Pd(1)–Cl(1)
Pd(1)–Pd(2)
P(1)–Pd(1)
P(2)–Pd(2)
P(1)–O(1)
P(1)–O(2)
P(2)–O(3)
2.3158(13) P(2)–O(4)
2.5755(7) P(1)–N(1)
2.1376(10) P(2)–N(1)
2.2588(11) N(1)–C(25)
1.625(2)
1.635(3)
1.620(3)
1.615(3)
1.698(3)
1.685(3)
1.485(5)
2.180(6)
2.112(6)
2.169(5)
Bond angles/◦
C(51)–Pd(2)
C(52)–Pd(2)
C(53)–Pd(2)
N(1)–H(1) · · · O(5)
P(1)–Pd(1)–P(2)
P(1)–Pd(1)–Cl(1)
158
93.77(4)
87.59(4)
Cl(1)–Pd(2)–Cl(2)
Cl(2)–Pd(1)–P(2)
Cl(1)–Pd(1)–Cl(2)
87.17(4)
93.55(4)
85.75(4)
Bond angles/Torsion angles/◦
the plane containing the ‘Pd(l-Cl)2’ unit bound to it, is ∼102◦.
The N–H proton is hydrogen bonded to O(5), the O(5) · · · N(1)
P(1)–N(1)–P(2)
112.89(17)
◦
O(1)–P(1)–O(2)
98.41(13)
100.66(15)
40.63(4)
˚
distance being 2.708 A (∠O(5)–H(1)–N(1) = 158 ). The average
O(3)–P(2)–O(4)
˚
P–O (aryl) single bond distance (1.608(1) A) is slightly reduced
P(1)–Pd(1)–Pd(2)–P(2)
˚
when compared to that (1.650(5) A) in the free ligand (L). The
˚
P(2)–O(5) bond distance is 1.482(3) A as would be expected for a
ring comprising the two palladium atoms and the PNP skeleton
adopts a ‘half-chair’ conformation with one of the palladium
atoms slightly projecting out of the mean plane. The coordination
geometry around each metal is approximately planar, but the two
metal coordination planes are twisted so that the P(1)–Pd(1)–
Pd(2)–P(2) torsion angle is 40.63(4)◦, thereby minimizing the anti-
bonding interactions of filled metal d-orbitals.20,23 The twisted
conformation also allows the ligands to span a shorter metal–
metal bond. The remarkable stability of the three-coordinate
complex can be attributed to the steric crowding imposed by the
bulky ligand L at the unsaturated palladium center. A discussion
of Pd–Pd bond distances and torsion angles observed in complexes
(2), (6) and other dinuclear Pd(I) complexes of diphosphazanes is
given in the ESI.‡
=
P O bond.
Crystal and molecular structure of [(g3-C3H5)[l-(EtN{P-
(OR)2}2)Pd2Cl] (6). The molecular structure of complex (6)
(shown in Fig. 3) consists of a bimetallic Pd(I) unit bridged by
the diphosphazane ligand, (L). The allyl fragment is bonded to
3
one palladium center in an g -fashion thereby coordinatively sat-
urating it, whereas the other tri-coordinate Pd(I) center is bonded
to a chloride. The dihedral angle between the plane containing
the allyl carbon atoms and the plane containing the ‘Pd–P–N–
P’ unit bound to it, is ∼167◦. The geometry around the low-
coordinate palladium is essentially T-shaped with a P(1)–Pd(1)–
Cl(1) angle measuring 174.71(4)◦ which is more linear compared
to the P(1)–Pd–N angle in the 14e T-shaped Pd(II) complexes of
the type [(4-MeO–C6H4)Pd(PBut2R)(N((3,5-CF3)2C6H3)2)] (R =
But, ferrocenyl, or 1ꢀ,2ꢀ,3ꢀ,4ꢀ,5ꢀ-pentaphenyl-1-ferrocenyl), reported
by Yamashita and Hartwig.22 There are no agostic interactions;
the hydrogen atom H(7) (attached to C(7)), which is nearest to
Catalytic studies using the coordinatively unsaturated complex
(6). Coordinatively unsaturated organopalladium(II) complexes
are often proposed as intermediates in metal-mediated or-
ganic transformations such as C–C bond forming reactions,24
hydroamination,25 etherification,26 and Stille coupling reactions,27
and hence have attracted considerable scientific interest. Sterically
encumbered phosphine ligands that can facilitate the formation of
˚
Pd(1), is at a distance of 2.932 A from the metal center. The
average P–N bond distance (1.691(8) A) is slightly longer than that
(1.674(5) A) observed in the free ligand (L). The Pd(1)–P(1) bond is
shorter than the Pd(2)–P(2) bond (see Table 3). The five-membered
˚
˚
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3
catalytically active low coordinate species are well known in the
literature.28 To the best of our knowledge, there are no studies so
far using T-shaped complexes as catalysts in homogenous hydro-
phenylation reactions. Complex (6) exhibits extremely high turn-
over numbers (TON’s) in catalytic homogeneous hydroarylation
reaction (hydrophenylation of norbornene with iodobenzene)29
even when aerobic conditions are employed. Carbon–carbon
coupling in the presence of NEt3/formic acid as hydride source
and catalytic amounts of (6) gives exo-phenylnorbornane in
quantitative yield (isolable yield: 98%) (Scheme 4). TON’s of
2.07 × 106 are obtained even when the catalyst loading is as low
as 4.8 × 10−5 mol%. Catalytic hydrophenylation could not be
was added to a solution of [(g -C3H5)Pd(l-Cl)]2 (0.050 g, 1.37 ×
10−4 mol) and NH4PF6 (0.045 g, 2.74 × 10 −4 mol) in dry acetone
(20 mL, distilled twice from K2CO3) at 25 ◦C. The reaction mixture
was stirred at room temperature for 2 h and was filtered to remove
the precipitated ammonium chloride. Solvent was evaporated from
the reaction mixture and acetone (5 mL) added. The solution was
layered with petrol. Colorless micro-crystals of 1a were formed
over a period of 24 h. Yield: 0.120 g, 79% Anal. Calcd. for
Pd1F6P3O4 N1C53H78: C, 57.6; H, 7.1; N, 1.3. Found: C, 57.5; H,
7.2; N, 1.2. 1H NMR (CDCl3, 400 MHz): d 1.0–1.33 (m, 48H, CH3
of 2,6-Pri), 1.87 (t, 3H, CH2CH3, 3JHH = 7 Hz), 2.59 (br, 2H, Ha of
allyl), 2.9 (br, 2H, Hs of allyl), 3.10–3.17 (m, 8H, CH of 2,6-Pri),
4.14–4.3 (m, 2H, CH2CH3), 5.31 (m, 1H, Hc of allyl), 6.85–7.28(m,
3
achieved using a 1 : 1 molar ratio of [(g -C3H5)Pd(l-Cl)]2 and the
1
ligand [EtN{P(OR)2}2] (L) as catalyst instead of complex (6).
12H, aryl protons). 31P{ H} NMR (CDCl3, 161.9 MHz): d 111.0,
(s). 13C NMR (allyl carbons,* CDCl3, 293 K): 73.9 (dd, {12.5,
2J(P,C)cis, 54.1, J(P,C)trans,} Ct), 124.2 (t, Cc). IR (Nujol, cm−1):
2
3131(s), 2968 (w), 2873(s), 1587(s), 1464(w), 1437(m), 1255(s),
1100(w), 931, 889, 863, 769, 667, 558, 476. *(Ct: terminal allyl
carbon atoms; Cc: central allyl carbon).
Preparation of [(g3-C3H5)(2-Cl-g3-C3H4)Pd4(l-Cl)4(l-EtN-
{P(OR)2}2)]2, (2). The ligand L (0.059 g, 0.73 × 10−4 mol)
Scheme 4 Hydrophenylation of norbornene catalyzed by complex (6).
3
and [Pd(g -C3H5)(l-Cl)]2 (0.038 g, 1.04 × 10−4 mol) were
introduced into a 100 mL round bottomed flask equipped
with a magnetic stirring bar and an inlet for nitrogen. Acetone
(20 mL) was added and the reaction mixture was stirred for
96 h. The resultant red–orange solution was filtered in an open
atmosphere using a Whatman No. 42 filter paper and the filtrate
was evaporated to dryness. The residue was redissolved in a
minimum amount of acetone and the solution left aside. Deep
red rhombus-shaped crystals of (2) were formed over a period of
24 h along with colorless crystals of complexes (3), (4) and the
ligand monoxide (Lꢀ). The crystals of complex (2) were separated
by handpicking them from the mother liquor, washed with cold
acetone and air-dried. Complex (2) was found to be air-stable for
a period of 5–6 weeks. Yield: (0.060 g), 20%. Anal. Calcd. for
C112H164N2O8P4Pd8Cl10.CH3COCH3·H2O (found): C, 45.2 (45.7);
H, 5.6 (5.2); N, 0.9 (1.2). IR (Nujol, cm−1): 3049, 2964, 2870,
1583, 1565, 1456, 1439, 1383, 1327, 1256, 1157, 1092, 913, 874,
Conclusions
In conclusion, we have shown for the first time that nucleophilic
attack by a chloride ligand can take place at the central carbon
atom of a (p-allyl)palladium complex. We also present here
the first example of a dinuclear Pd(I) complex containing a
sterically encumbered diphosphazane, which displays a true three-
coordinate T-shaped geometry at one Pd center, and demonstrates
its potential as an efficient hydroarylation catalyst under aerobic
conditions. Because of its high stability and ready accessibility, the
T-shaped complex offers a wide scope for studying small molecule
insertion reactions across the Pd(I)–Pd(I) bond.30 The reactivity of
3
(g -allyl)palladium precursors with other “P–N–P” type ligands
under various reaction conditions warrants further study.
Experimental
1
762, 734, 662, 438, 405, 368, 305. 31P{ H} NMR: d 98.0, (s). 13C
General details
ꢀ
ꢀ
(allyl carbons): 52.5 (s, Ct), 62.2 (s, Ct ), 110.4 (s, Cc), 206 (s, Cc ).
The NMR spectra were recorded in CDCl3 using a Bruker Avance-
400 spectrometer with Me4Si as an internal standard for 1H
and 13C NMR measurements and 85% H3PO4 as an external
standard for 31P NMR measurements. Chemical shifts downfield
from the standard were assigned positive values. Infrared spectra
were recorded using a Perkin-Elmer Model 457 spectrometer.
Elemental analyses were carried out using a Perkin-Elmer 2400
CHN analyzer. Triethylamine and petrol (bp 60–80 ◦C) were
purified by conventional procedures and freshly distilled prior to
use. DMSO was distilled over CaH2, and stored over molecular
sieves. Unless noted otherwise, acetone was distilled over KMnO4
and used without further drying. The precursor complexes [Pd(1-
Preparation of [(g3-C3H5)Pd2(l-Cl)2{P(O)(OR)2}{P(OR)2-
−4
(NHEt)}] (3). The ligand (L) (0.111 g, 1.37 × 10
mol) was
3
added to a solution of [(g -C3H5)Pd(l-Cl)]2 (0.050 g, 1.37 ×
◦
10 −4 mol) in acetone (20 mL) at 25 C and the reaction mixture
stirred for 12 h. Solvent was evaporated to give a foamy yellow
solid. Acetone (5 mL) was added and the solution “layered”
with petroleum ether. Colorless crystals of 3 were formed over
a period of 24 h. Yield: 0.053 g, 40% Anal. Calcd for Pd2Cl2P2O5
N1C53H79: C, 55.1; H, 6.9; N, 1.2. Found: C, 55.0; H, 7.1; N, 1.3.
3
1H NMR (CDCl3, 400 MHz): d 0.48 (t, 3H, CH2CH3, JHH
=
7 Hz), 0.99–1.28 (m, 48H, CH3 of 2,6-Pri), 2.59 (m, 2H, Ha of
allyl), 2.9 (br, 2H, Hs of allyl), 3.66–3.82 (m, 8H, CH of 2,6-Pri),
3.9–4.16 (m, 2H, CH2CH3), 5.4 (m, 1H, Hc of allyl), 6.95–7.10
3
3
Me-g -C3H4)(l-Cl)]2 and [Pd(g -C3H5)(l-Cl)]2,31 and the ligand
(L), [EtN{P(OR)2}2]21 were prepared according to published
procedures.
(m, 12H, aryl protons), 8.05(br, NH). 31P{ H} NMR (CDCl3,
1
161.9 MHz): d 86.4 d, (dPA), 32.1 d, (dPX), (2JP–P = 99 Hz).
13C NMR (allyl carbons, CDCl3, 293 K): 69.8 (br, Ct), 116.2
(br, Cc). IR (Nujol, cm−1): 3417(m), 3057(s), 2959(w), 2866(s),
Preparation of [(g3-C3H5)Pd{EtN(P(OC6H3(Pri)2-2,6)2)2}]PF6
(1a). The ligand [EtN{P(OR)2}2] (L) (0.223 g, 2.74 × 10 −4 mol)
2912 | Dalton Trans., 2007, 2908–2914
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1922(m), 1713(s), 1586(s), 1463(w), 1154(w), 1091(w), 759(s),
654(w), 578(m), 514(s), 480(w), 451(s).
(2JP–P = 188 Hz). 13C NMR (allyl carbons, CDCl3, 293 K): 71.6 (d,
2
ꢀ
*Ct, J(P,C) trans = 38 Hz), 117.1 (br, s *Ct ), 124.2 (br, *Cc). IR
(KBr, cm−1): 2962(s), 2868(s), 1713(s), 1460(m), 1324(m), 1253(s),
1153(m), 1088(w), 865(w), 762(s), 671(w). IR (Nujol, cm−1): 588,
Preparation of [Pd(l-Cl){P(O)(OR)2}{P(OR)2(NHEt)}]2 (4).
The ligand (L) (0.223 g, 2.74 × 10 −4 mol) was added to a solution of
ꢀ
515, 406, 387, 297, 209. *(Ct: allyl carbon trans to Pd–P bond; Ct :
3
[(g -C3H5)Pd(l-Cl)]2 (0.050 g, 1.37 × 10 −4 mol) in acetone (20 mL)
allyl carbon trans to Pd–Pd bond; Cc: central allyl carbon).
at 25 ◦C and the reaction mixture stirred for 24 h. The solution was
concentrated to ∼5 mL and layered with petrol. Colorless crystals
of 4 were formed over a period of 24 h. Yield: 0.130 g, 49% Anal.
Calcd for Pd2Cl2P4O10N2C100H148·2CH3COCH3: C, 62.8; H, 8.0; N,
1.4. Found: C, 62.6; H, 8.1; N, 1.2. 1H NMR (CDCl3, 400 MHz):
d 0.32–0.6 (br, 2 × 3H, CH2CH3), 0.99–2.07 (m, 2 × 48H, CH3 of
2,6-Pri), 3.02–3.96 (m, 2 × 8H, CH of 2,6-Pri), 4.96 (m, 2 × 2H,
CH2CH3), 6.39–7.37 (m, 2 × 12H, aryl protons), 8.33(br, 2 × 1H
Procedure for the catalytic hydrophenylation of norbornene using
iodobenzene
To 5 mL of dry DMSO were added iodobenzene (1 mmol),
norbornene (3 mmol), Et3N (4 mmol), and formic acid (3 mmol).
An appropriate amount of catalyst (6·CH3COCH3), dissolved in
0.5 mL acetone was introduced into the stirred solution. The
reaction mixture was maintained at 120 ◦C for 12 h and allowed
to attain room temperature. Water (5 mL) was added and the
reaction mixture was extracted with EtOAc (10 mL × 3). The
combined organic phase was dried over MgSO4, filtered, and
concentrated under reduced pressure. The product was purified
by column chromatography using benzene–petrol (1 : 4 v/v) as
1
NH). 31P{ H} NMR (CDCl3, 161.9 MHz): d 78.8 d, (dPA), 29.1
d, (dPX), (2JP–P = 94 Hz). IR (Nujol, cm−1): 3419(w), 2964(br),
1713(m), 1443(m), 1368(m), 1253(s), 1155(m), 1092(br), 905(br),
758(s), 656(br), 577(m), 514(s), 477(br).
Preparation of [(g3-C3H5)[l-(EtN{P(OR)2}2Pd2Cl] (6). The
ligand (L) (0.111 g, 1.37 × 10−4 mol) was added to a suspension
1
the eluant. Phenylnorbornane: H NMR (CDCl3, 400 MHz) d
3
−4
of [(g -C3H5)Pd(l-Cl)]2 (0.050 g, 1.37 × 10
mol) and K2CO3
1.74–1.27 (m, 8H, CH and CH2), 2.36 (s, 2H, CH2), 2.74 (m, 1H,
CH), 7.26–7.15 (m, 5H, Ar).
◦
(0.009 g, 6.85 × 10−5 mol) in acetone (20 mL) at 25 C and the
reaction mixture stirred for 6 h during which time the solution
turned deep yellow. Solvent was evaporated from the reaction
mixture and freshly distilled acetone (10 mL) was added. The
solution was passed through a short plug of Celite (7 cm × 2 cm)
and concentrated to ∼5 mL. Orange crystals of 6 were formed over
a period of 24 h. Yield: 0.110 g, 69%. Anal. Calcd for Pd2Cl1P2O4
N1C53H78·CH3COCH3: C, 57.9; H, 7.3; N, 1.2. Found: C, 58.0; H,
7.1; N, 1.3. 1H NMR (CDCl3, 293 K): d 0.92–1.3 (m, 48H, CH3 of
2,6-Pri), 1.85 (t, 3H, CH2CH3, 3JHH = 7 Hz), 2.18 (br, 1H, CH2 of
allyl), 3.0 (br, 1H, CH2 of allyl) 3.25–3.75 (m, 8H, CH of 2,6-Pri,
1H, CH2 of allyl), 4.42 (m, 2H, CH2CH3), 4.6 (m, 1H, CH2 of
allyl), 5.2 (m, 1H, CH of allyl), 7.09–7.19 (m, 12H, aryl protons).
X-Ray crystallography
Crystal data for all the complexes were collected on a BRUKER
SMART APEX CCD diffractometer (graphite-monochromated
32
˚
Mo-Ka radiation, k = 0.71073 A) using SMART software. The
data was integrated with SAINT33 and an empirical absorption
correction was applied using SADABS.34 The structures were
solved by the Patterson method using SHELXS-97 and refined by
full-matrix least squares methods against F2 (SHELXL-97).35 All
non-hydrogen atoms were refined with anisotropic displacement
parameters. The hydrogen atoms except the NH proton of complex
(3) were placed in their calculated positions; the NH proton of
31P{ H} NMR (CDCl3, 293 K): d 147.6 d, (dPA), 125.8 d, (dPB),
1
Table 4 Summary of X-ray diffraction data for the ligand monoxide (Lꢀ) and complexes (2), (3), (4) and (6)
Compound
Ligand monoxide (Lꢀ) Complex (2)
Complex (3)
Complex (4)
Complex (6)
Formula
M
C50H73N1O5P2
830.03
Triclinic
P-1
13.465(7)
14.582(8)
15.016(8)
68.274(9)
70.498(8)
74.301(9)
2545(2)
2
0.128
29347
11659
0.0271
0.0905
0.0637
C115H172Cl10N2 O10P4Pd8 C53H79Cl2N1O5P2Pd2 C106 H158Cl2N2 O12P4Pd2 C56H84Cl1N1O5P2Pd2
3072.13
Triclinic
P-1
1155.81
Monoclinic
P21/n
2059.92
Triclinic
P-1
1161.43
Monoclinic
P21/n
Crystal system
Space group
˚
a/A
13.714(3)
15.294(3)
16.424(3)
78.172(4)
81.154(4)
80.768(4)
3301.8(12)
1
10.6926(13)
22.839(3)
24.066(3)
90.00
98.332(2)
90.00
5814.9(12)
4
0.807
12.134(2)
17.393(3)
27.747(5)
85.016(4)
79.956(4)
73.461(4)
5523.4(18)
2
10.616(2)
40.964(9)
13.602(3)
90.00
96.722(4)
90.00
5874(2)
4
0.756
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
U/A
Z
3
˚
M(Mo-Ka)/mm−1
Total reflections
Unique reflections
Rint
1.366
0.487
38813
15311
51983
13990
43509
14545
47088
11990
0.0328
0.101
0.0737
0.0546
0.0989
0.0656
0.1114
0.1491
0.0890
0.0615
0.0837
0.0529
R1 (all data)
R1 (Io > 2r(Io))
wR2
(Io > 2r(Io))
0.1522
28.1
0.221
28.02
0.1167
27.98
0.1796
22.56
0.1193
26.37
2h max/◦
wR2
(all data)
0.1662
0.2411
0.1265
0.2050
0.1305
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The Royal Society of Chemistry 2007
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©

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complex (3) could be located. Hydrogen atoms attached to the
allyl carbon atoms in complex (2) were not fixed owing to disorder
(manifested in the large anisotropic temperature factors of these
atoms). The oxygen atom O(6) of the water molecule coordinated
to Pd(4) and the acetone molecule present in the lattice of (2) have
been assigned a partial occupancy factor of 0.5. The oxygen atom
1890; C. Amatore, A. Jutand and M. J. Medeiros, New J. Chem., 1996,
20, 1143.
14 S. S. Stahl, J. L. Thorman, N. d. Silva, I. A. Guzei and R. W. Clark,
J. Am. Chem. Soc., 2003, 125, 12.
15 The reduction of Pd(II) to Pd(I) is also observed in the reaction of
[PdCl2(COD)] with the strong p-acceptor ligand [MeN{P(OR)2}2],
(R = CH2CF3) to form a dinuclear palladium(I) complex; see: M.
Ganesan, S. S. Krishnamurthy and M. Nethaji, J. Organomet. Chem.,
2005, 690, 1080.
16 R. B. King, Acc. Chem. Res., 1980, 13, 243; S. C. Browning and D. H.
Farrar, J. Chem. Soc., Dalton Trans., 1995, 2005; R. B. King and
J. Gimeno, Inorg. Chem., 1978, 17, 2396; J. T. Mague and Z. Lin,
Organometallics, 1994, 13, 3027.
17 A. Badia, L. R. Falvello, R. Navarro and E. P. Urriolabeitia,
J. Organomet. Chem., 1997, 547, 121.
18 R. Uson, J. Fornies, R. Navarro, M. Tomas, C. Fortuno, J. I. Cebollada
and A. J. Welch, Polyhedron, 1989, 8, 1045; S. C. Browning, D. H.
Farrar, D. C. Frankel and J. J. Vittal, Inorg. Chim. Acta, 1997, 254, 329;
Also see ref. 11 and 15.
˚
O(6) is at a distance of 1.89 A from the acetone carbon atom C(57).
A summary of the crystallographic data for complexes (2), (3), (4),
(6) and (Lꢀ) is given in Table 4.
CCDC reference numbers 637406 (2), 637407 (3), 637408 (4),
618420 (6) and 637409 (Lꢀ).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b703308g
Acknowledgements
19 For dinuclear palladium(I) complexes, the Pd–Pd bond ranges from
2+
˚
2.488(1) A in [Pd2(MeCN)4(PPh3)2] which does not contain any
We thank the Department of Science and Technology, New Delhi,
India, for financial support and the CCD X-ray facility (IISc) for
data collection. We also thank Prof. T. N. Guru Row, SSCU, IISc,
for helpful discussions on crystallography. S. S. K. thanks Indian
National Science Academy, New Delhi for the position of a Senior
Scientist.
bridging ligand to 3.185(1) A in [Pd2(l-C4H6)2]2+, which contains
˚
bridging 1,3-butadiene molecules. See: K. Tani, S. Nakamura, T.
Yamagata and Y. Kataoka, Inorg. Chem., 1993, 32, 5398; T. Murahashi,
T. Otani, E. Mochizuki, Y. Kai and H. Kurosawa, J. Am. Chem. Soc.,
1998, 120, 4536.
20 G. Besenyei, L. Parkanyi, E. Gacs-Baitz and B. R. James, Inorg. Chim.
Acta, 2002, 327, 179.
21 G. Prabusankar, N. Palanisamy, R. Murugavel and R. J. Butcher,
Dalton Trans., 2006, 2140.
22 M. Yamashita and J. F. Hartwig, J. Am. Chem. Soc., 2004, 126,
5344.
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