Steric and electronic effects in stabilizing allyl-palladium complexes of P-N-P ligands, X2PN(Me)PX2 (X = OC6H5 or OC6H3Me2-2,6)

Article abstract of DOI:10.1016/j.jorganchem.2006.02.040

The chemistry of η³-allyl palladium complexes of the diphosphazane ligands, X₂PN(Me)PX₂ [X = OC₆H₅ (1) or OC₆H₃Me₂-2,6 (2)] has been investigated.The reactions of the phenoxy derivative, (PhO)₂PN(Me)P(OPh)₂ with [Pd(η³-1,3-R′,R″-C₃H₃)(μ-Cl)]₂ (R′ = R″ = H or Me; R′ = H, R″ = Me) give exclusively the palladium dimer, [Pd₂{μ-(PhO)₂PN(Me)P(OPh)₂}₂Cl₂] (3); however, the analogous reaction with [Pd(η³-1,3-R′,R″-C₃H₃)(μ-Cl)]₂ (R′ = R″ = Ph) gives the palladium dimer and the allyl palladium complex [Pd(η³-1,3-R′,R″-C₃H₃)(1)](PF₆) (R′ = R″ = Ph) (4). On the other hand, the 2,6-dimethylphenoxy substituted derivative 2 reacts with (allyl) palladium chloro dimers to give stable allyl palladium complexes, [Pd(η³-1,3-R′,R″-C₃H₃)(2)](PF₆) [R′ = R″ = H (5), Me (7) or Ph (8); R′ = H, R″ = Me (6)].Detailed NMR studies reveal that the complexes 6 and 7 exist as a mixture of isomers in solution; the relatively less favourable isomer, anti-[Pd(η³-1-Me-C₃H₄)(2)](PF₆) (6b) and syn/anti-[Pd(η³-1,3-Me₂-C₃H₃)(2)](PF₆) (7b) are present to the extent of 25% and 40%, respectively. This result can be explained on the basis of the steric congestion around the donor phosphorus atoms in 2. The structures of four complexes (4, 5, 7a and 8) have been determined by X-ray crystallography; only one isomer is observed in the solid state in each case.

Full text of DOI:10.1016/j.jorganchem.2006.02.040

Journal of Organometallic Chemistry 691 (2006) 2969–2977
www.elsevier.com/locate/jorganchem
Steric and electronic effects in stabilizing allyl-palladium complexes of
q
‘‘P–N–P’’ ligands, X2PN(Me)PX2 (X = OC6H5 or OC6H3Me2-2,6)
Swadhin K. Mandal, Thengarai S. Venkatakrishnan, Arindam Sarkar,
*
Setharampattu S. Krishnamurthy
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India
Received 7 February 2006; received in revised form 28 February 2006; accepted 28 February 2006
Available online 6 March 2006
Abstract
The chemistry of g3-allyl palladium complexes of the diphosphazane ligands, X2PN(Me)PX2 [X = OC6H5 (1) or OC6H3Me2-2,6 (2)]
has been investigated. The reactions of the phenoxy derivative, (PhO)2PN(Me)P(OPh)2 with [Pd(g3-1,3-R0,R00-C3H3)(l-Cl)]2
(R0 = R00 = H or Me; R0 = H, R00 = Me) give exclusively the palladium dimer, [Pd2{l-(PhO)2PN(Me)P(OPh)2}2Cl2] (3); however, the
analogous reaction with [Pd(g3-1,3-R0,R00-C3H3)(l-Cl)]2 (R0 = R00 = Ph) gives the palladium dimer and the allyl palladium complex
[Pd(g3-1,3-R0,R00-C3H3)(1)](PF6) (R0 = R00 = Ph) (4). On the other hand, the 2,6-dimethylphenoxy substituted derivative 2 reacts with
(allyl) palladium chloro dimers to give stable allyl palladium complexes, [Pd(g3-1,3-R0,R00-C3H3)(2)](PF6) [R0 = R00 = H (5), Me (7)
or Ph (8); R0 = H, R00 = Me (6)]. Detailed NMR studies reveal that the complexes 6 and 7 exist as a mixture of isomers in solution;
the relatively less favourable isomer, anti-[Pd(g3-1-Me-C3H4)(2)](PF6) (6b) and syn/anti-[Pd(g3-1,3-Me2-C3H3)(2)](PF6) (7b) are present
to the extent of 25% and 40%, respectively. This result can be explained on the basis of the steric congestion around the donor phospho-
rus atoms in 2. The structures of four complexes (4, 5, 7a and 8) have been determined by X-ray crystallography; only one isomer is
observed in the solid state in each case.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Allyl complexes; P ligands; Diphosphazanes; NMR spectroscopy; Palladium; X-ray crystallography
1. Introduction
favourable anti-configured allyl palladium complex is gen-
erated by the attack of palladium catalyst on the (Z)-allyl
substrate which should subsequently lead to the formation
of (Z)-allyl product provided that no g3–g1–g3 isomeriza-
tion occurs at all or the rate of the isomerization is slower
than the nucleophilic attack. However, in reality the anti-
allyl palladium complexes undergo a fast g3–g1–g3 isomer-
ization to the more favourable syn-allyl palladium isomers
that leads to (E)-allyl product by subsequent nucleophilic
attack. Thus, the g3–g1–g3 isomerization step actually
determines whether a given (Z)-allyl substrate will lead to
retention of configuration or not in the final product. One
way of controlling the g3–g1–g3 isomerization step is to
introduce a sterically bulky substituent in the auxillary
ligand so that the less favourable anti-allyl palladium iso-
mer would be stabilised [3]. Such a possibility has been rea-
lised with phenyl substituted allyl palladium complexes
Palladium catalysed allylic substitution reactions consti-
tute a powerful methodology for C–C bond formation in
organic synthesis [1]. The configuration of the final product
strongly depends on the configuration of an intermediate of
the type [Pd(g3-allyl)(auxillary ligand)]+ and its rigidity in
solution [2]. The catalytic conversion of (E)-allyl substrate
to (E)-allyl product is a straightforward process that pro-
ceeds via the more favourable syn-configured allyl palla-
dium intermediate (Scheme 1). On the other hand, the less
q
Part 25 of the series ‘‘Organometallic Chemistry of diphosphazanes’’;
for Part 24, see [6c].
*
Corresponding author. Tel.: +91 80 22932401; fax: +91 80 23600683/
23601552.
E-mail address: sskrish@ipc.iisc.ernet.in (S.S. Krishnamurthy).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.02.040

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S.K. Mandal et al. / Journal of Organometallic Chemistry 691 (2006) 2969–2977
Pd catalyst
L
Nu
LG
Nu
Pd
L
L
'E'
'E'
Favoured
1
η3-η -η3
LG
Nu
Nu
Pd catalyst
L
Pd
L
L
'Z'
Less-favoured
'Z'
Scheme 1.
which adopt the less favourable syn/anti or anti/anti con-
figuration characterised both in solid state [4] and in solu-
tion [5a]. However the situation becomes more challenging
when the allyl moiety carries relatively small methyl groups
[3].
Me2-C3H3)(l-Cl)]2 [11] and [Pd(g3-1,3-Ph2-C3H3)(l-Cl)]2
[12] were prepared as previously described. The diphosp-
hazane ligand 1 [13a] and 2 [13b] were prepared by
analogous procedures. The NMR spectra were recorded
using Bruker DRX-500 MHz, Bruker AMX-400 MHz
and Bruker ACF-200 MHz spectrometers. Chemical shifts
downfield from the reference standard were assigned posi-
tive values. Elemental analyses were carried out using a
Perkin–Elmer 2400 CHN analyser.
As a part of our ongoing investigations on the organo-
metallic chemistry of diphosphazane ligands, [6,7] we
reported the synthesis and dynamic behaviour of allyl pal-
ladium complexes of a range of diphosphazane and
diphosphazane monosulfide ligands [7]. Diphosphazanes
constitute a class of versatile short-bite bidentate phospho-
rus-donor ligands based on the ‘‘P–N–P’’ framework that
have engendered a varied and extensive transition metal
organometallic chemistry [8]. The allyl palladium com-
plexes of diphosphonite ligands are rare in literature and
this is probably attributed to the higher p-acidity of these
ligands owing to the presence of two electronegative oxy-
gen atoms attached to the phosphorus centre as compared
2.1. Synthesis of palladium complexes
2.1.1. [Pd(g3-1,3-Ph2-C3H3){j2-(PhO)2PN(Me)-
P(OPh)2}](PF6) (4)
A mixture of [Pd(g3-1,3-Ph2-C3H3)(l-Cl)]2 (0.067 g,
0.99 · 10ꢀ4 mol), NH4PF6 (0.033 g, 2.02 · 10ꢀ4 mol) and
ligand 1 (0.095 g, 2.05 · 10ꢀ4 mol) were dissolved in
20 cm3 of acetone. The solution was stirred for 1 h at
298 K and the white precipitate formed during the reac-
tion was filtered off. The resulting yellow orange filtrate
was concentrated under reduced pressure to 10 cm3 and
the solution was layered by adding 10 cm3 of hexane
(b.p. 40–60 ꢁC) to yield yellow crystals. The compound
was purified by crystallization from acetone–hexane (1:1
v/v). The other allyl palladium complexes 5–8 were syn-
thesised by an analogous procedure by varying the allyl
palladium chloro dimer and the diphosphazane ligand.
The yields, melting points and elemental analyses for
these complexes are given in Table 5. Selected 1H,
31P{1H} and 13C NMR spectral values for complexes
4–8 are given in Table 1.
`
to the diphosphane ligands. A recent study by Calabro
et al. [5a] shows that the optically pure diphosphazane
diphosphonite ligand (C20H12O2)PN(R)P(C20H12O2) (R =
Ph or (S)-sec-butyl) effectively forms the palladium allyl
complex characterized by NMR study in solution although
no solid-state structure of any such complexes were
reported. (Allyl) palladium complexes of a diphosphazane
monoxide viz. Ph2PNHP(O)Ph2 have been reported by
Woollins and coworkers [5b]. In this paper, we report the
reactions of various allyl-palladium dimers, [Pd(g3-1,3-R0,
R00-C3H3)(l-Cl)]2 with symmetrically substituted diphosp-
hazane diphosphonite ligands, X2PN(Me)PX2 [X = OC6H5
(1) or OC6H3Me2-2,6 (2)]. The products have been charac-
terised by NMR spectroscopy and X-ray crystallography.
3. X-ray crystallography
2. Experimental
The crystals were mounted on a glass fibre and the
intensity data for all the complexes were obtained at
room temperature from a Bruker SMART APEX CCD
diffractometer equipped with fine focus 1.75 kW sealed
tube Mo Ka X-ray source with increasing x (width of
0.3ꢁ per frame) at a scan speed of n s/frame (n = 15
for 4, n = 10 for 6a, n = 9 for 7a and n = 15 for 8).
All reactions and manipulations were carried out under
an atmosphere of dry nitrogen using standard Schlenk and
vacuum-line techniques. The solvents were purified by
standard procedures [9] and distilled under nitrogen prior
to use. The chlorobridged palladium allyl dimers, [Pd(g3-
C3H5)(l-Cl)]2 [10], [Pd(g3-Me-C3H4)(l-Cl)]2, [Pd(g3-1,3-

S.K. Mandal et al. / Journal of Organometallic Chemistry 691 (2006) 2969–2977
2971
Table 1
Selected 1H, 31P{1H} and 13C NMR spectral values for complexes 4–8 (only allyl protons and carbon nuclei are listed)
Complex 31P{1H}a NMR 1H NMRb
13C NMRb
Hs
Ha
Hs0
Ha0
5.65 m 6.23 t
4.92 m
3.77 m 4.95 m
Hc
Ct
Ct0
Cc
4
108.1 s
–
–
–
–
–
–
93.1 t (24.1)
–
112.6 t (9.1)
124.7 t (14.7)
123.5 t (12.9)
5
6a
109.1 s
4.32 m 2.69 m
4.01 m 2.41 t
(13.5)d,e
3.74 m 2.94 t
(15.7)d,e
–
75.3 br.d (52.2)f
108.8 d and
110.7 d (13.0)c
107.3 d and
111.7 d (4.4)c
110.9 s
111.6 d and
110.3 d (35.7)c
108.5 s
68.2 dd (40.3)f (13.7)g 101.9 dd (40.8)f (12.6)g
6b
5.47 m
–
–
4.61 m
67.3 dd (42.5)f (11.6)g 98.2 dd (37.0)f (10.3)g
118.4 t (12.7)
124.1 t (13.9)
7a
7b
–
–
–
–
3.52 m 5.13 t (11.2)d
–
–
93.8 t (24.8)f
92.3 dd (40.9)f (12.5)g and 119.3 t (13.8)
90.4 dd (36.9)f (12.3)g
5.19 m 4.08 m 4.78 br.t
8
–
–
5.13 m 6.11 m
Abbreviations: d, doublet; br.d, broad doublet; dd, doublets of doublet; t, triplet; br.t, broad triplet; m, multiplet.
Coupling constants in Hz are given in parenthesis. Hs and Ha are the syn and anti allylic protons at the unsubstituted allyl terminus, respectively. H0s and
H0 are the syn and anti allylic protons at the substituted allyl terminus, respectively. Hc is the central allyl proton. Ct is unsubstituted terminal allyl carbon.
C0ta is substituted terminal allyl carbon. Cc is central allyl carbon.
a
Acetone was used as solvent.
CDCl3 was used as solvent.
J(P,P).
J(H,H).
b
c
d
e
J(P,H).
f
2J(P,C)trans
.
g
2J(P,C)cis
.
Details of crystallographic data collection are summa-
rised in Table 2. The SMART [14a] software was used
for cell-refinement and data acquisition and the SAINT
[14b] software was used for data reduction. Lorentzian
and polarization corrections were made on the intensity
data. An absorption correction was made on the inten-
sity data using the SADABS [14c] program. All the struc-
tures were solved using SHELXTL [14d] and the WINGX
graphical user interface [15]. Least-square refinements
were performed by the full-matrix method with SHELXL-
97 [16]. All non-hydrogen atoms were refined anisotrop-
ically and hydrogen atoms were refined isotropically.
Table 2
Details of X-ray crystallographic data collection for the complexes 4 and 7a
Complex
4
7a
C38H48NO4F6P3Pd
896.08
Light yellow
Monoclinic, P21/c
9.818(7)
21.389(15)
19.731(14)
90.000
94.969(12)
90.000
Empirical formula
Formula weight
Colour
C
908.01
Yellow
40H36NO4F6P3Pd
Crystal system, space group
Monoclinic, P21/n
10.243(1)
15.601(2)
24.886(3)
90.000
95.721(2)
90.000
3956.7(9)
4
˚
a (A)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
4128(5)
4
1.442
0.631
Z
Density (calcd) (Mg/mm3)
Absorption coefficient (mmꢀ1
F(000)
1.524
0.660
1840
)
1840
Crystal size (mm)
h Range (ꢁ)
0.275 · 0.159 · 0.102
1.54–27.5
0.75 · 0.44 · 0.18
1.90–27.7
Index range
Reflections collected
ꢀ13 6 h 6 13, ꢀ20 6 k 6 20, ꢀ32 6 l 6 28
ꢀ12 6 h 6 11, ꢀ28 6 k 6 28, ꢀ26 6 l 6 25
34226
35871
Independent reflections [Rint
Refinement method
Data/restraints/parameters
Goodness of fit on F2
Final R indices (I > 2rI)
R indices (all data)
]
9338 [0.0458]
9708 [0.0414]
Full-matrix least squares on F2
9338/0/496
Full-matrix least squares on F2
9708/0/478
0.799
1.030
R1 = 0.0424, wR2 = 0.1172
R1 = 0.0655, wR2 = 0.1305
0.609 and ꢀ0.505
R1 = 0.0514, wR2 = 0.1433
R1 = 0.0703, wR2 = 0.1542
1.186 and ꢀ0.557
ꢀ3
˚
Largest differential peak and hole (e A
)

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S.K. Mandal et al. / Journal of Organometallic Chemistry 691 (2006) 2969–2977
4. Results and discussion
thesis consists of treatment of (PhO)2PN(Me)P(OPh)2 with
a palladium(II) complex, [PdCl2(COD)] [6d]. The absence
of any palladium allyl complex in the reaction depicted in
Scheme 2 is probably related to the nature of the phosphorus
ligand, (PhO)2PN(Me)P(OPh)2. The two phosphonite phos-
phorus atoms in this ligand have a high p-acceptor capability
which would destabilize palladium-allyl p-bonding. This
inference is borne out by the reaction of the same ligand,
(PhO)2PN(Me)P(OPh)2 with 1,3-diphenyl-allyl-dimer in the
presence of NH4PF6, in which the major product is the pal-
ladium-allyl-diphosphazane complex 4 (Scheme 3). The
chloro palladium dimer 3 is also formed in this reaction as
shown by the 31P NMR spectrum of the reaction mixture.
4.1. Reactions of X2PN(Me)PX2 [X = OC6H5 (1)] with
[Pd(g3-1,3-R0,R00-C3H3)(l-Cl)]2
The reactions of (PhO)2PN(Me)P(OPh)2 with the [Pd(g3-
1,3-R0,R00-C3H3)(l-Cl)]2 (R0 = R00 = H or Me; R0 = H,
R00 = Me) in the presence of NH4PF6 gives exclusively the
known palladium chloro-dimer [17], [Pd2{l-P,P-(PhO)2-
PN(Me)P(OPh)2}2Cl2] (3) in which the palladium center is
reduced to the mono-valent oxidation state as shown in
Scheme 2. The formation of the palladium dimer 3 is con-
firmed by its characteristic 31P{1H} NMR spectrum (singlet
1
1
at 113.4 ppm) as well as by its H and IR spectra. The di-
The allyl complex 4 is characterized by H, 31P{1H} and
palladium complex 3 has been synthesized previously by
two other methods. The first method of synthesis involves
a disproportionation reaction between a palladium(0) and
a palladium(II) precursor [17]. The second method of syn-
13C NMR spectra (see Table 1) and single crystal X-ray crys-
tallography. The formation of the allyl complex 4 only in the
case of 1,3-diphenyl-allyl precursor suggests that the pres-
ence of the two electron-releasing phenyl groups enhances
Me
N
(Ph O)2P
Cl Pd
P(OPh)2
Pd Cl
Me
NH 4PF6
N
[Pd(η3-1,3-R’,R”-C3H3)(μ-Cl)]2
+
(Ph O)2P
P(O Ph)2
R' = R" = H or M e;
R' = H , R" = M e
P(OPh)2
(Ph O)2P
1
N
Me
3
Scheme 2.
Me
N
+
Me
N
PF
NH4PF
PhO)2P
P(OPh)2
6
6
+
3-1,3-Ph2-C3H3 -Cl)]2
3
+
(PhO)2P
P(OPh)2
Pd
H
H
a
a
1
Ph
Ph
4
Scheme 3.
Me
Me
+
X
N
NH PF
X
P
X
N
X
+
X
4
6
PF
X
X
[Pd(η3-1,3-R’,R”-C3H3)(μ-Cl)]2
P
6
P
P
X
Pd
H
H
X = OC6H3Me -2,6 (2)
2
R'
R"
5; R'= R"= H
6; R'= H, R" = Me
7; R'= R"= Me
8; R'= R"= Ph
Scheme 4.

S.K. Mandal et al. / Journal of Organometallic Chemistry 691 (2006) 2969–2977
2973
the p-donor character of the allyl moiety and in turn enables
effective p-back donation from the metal to the diphosphaz-
ane ligand. Previously we had reported a similar electronic
effect in the allyl palladium complexes of the diphosphazane
ligand, Ph2PN(CHMe2)PPh(N2C3HMe2-3,5) which exhibits
ambident coordination behaviour (P,P- or P,N-mode) only
in the case of 1,3-diphenyl-allyl complex [7b].
The reactions of X2PN(Me)PX2 (X = OC6H3Me2-2,6) with
[Pd(g3-1,3-R0,R00-C3H3)(l-Cl)]2 (R0 = R00 = H, Me or Ph;
R0 = H or R00 = Me) in the presence of NH4PF6 yield the
allyl complexes 5–8 as shown in Scheme 4. The 31P{1H}
4.2. Reactions of X2PN(Me)PX2, X = OC6H3Me2-2,6 (2)
with [Pd(g3-1,3-R0,R00-C3H3)(l-Cl)]2
The incorporation of two methyl groups at the 2- and 6-
positions of the phenyl rings in the ligand (PhO)2PN(Me)-
P(OPh)2 can have two effects. The first one is that it can
enhance the electron donating capacity of the two phos-
phorus atoms owing to the inductive effect of the methyl
groups on the phenyl rings. Secondly, the methyl groups
at the 2- and 6-positions of the phenyl rings would provide
steric crowding around the donating phosphorus centres.
Fig. 1. The 31P{1H} NMR (161.9 MHz, CDCl3) spectrum of [Pd(g3-Me-
C3H4){j2-X2PN(Me)PX2}](PF6) [X = OC6H3Me2-2,6 (6)] revealing two
isomers in solution.
+
Me
N
PF6
O
O
P
O
P
O
Pd
Ha
Ha
Hs
Hs
Hc
5
+
+
Me
Me
PF6
PF6
N
N
O
O
O
O
P
P
P
P
O
O
O
O
Pd
Pd
Ha
Ha
Ha'
Hs
Hs
Hs'
Hc
Hc
Syn-, 6a
Anti-, 6b
+
+
Me
N
Me
N
PF6
PF6
O
O
O
O
O
O
P
P
P
P
O
O
Pd
Pd
Ha'
Ha'
Ha'
R'
R"
R"
R'
Hs'
Hc
Hc
Syn/anti-, 7b; R' = R" = Me
Syn/syn-, 7a; R' = R" = Me
Syn/syn-, 8; R' = R" = Ph
Fig. 2. The allyl complexes, [Pd(g3-1,3-R0-R00-C3H3){j2-P,P-X2PN(Me)PX2}](PF6) (5–8) of the diphosphazane ligand X2PN(Me)PX2 (X = OC6H3Me2-
2,6).

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S.K. Mandal et al. / Journal of Organometallic Chemistry 691 (2006) 2969–2977
NMR spectrum of the reaction mixture shows the exclusive
formation of allyl complexes in each of these reactions and
a palladium dimer of the type 3 is not formed. The com-
plexes 5–8 are characterized by NMR spectroscopy and
elemental analyses. The 31P{1H} NMR spectra of 6
(Fig. 1) and 7 show the presence of two isomers (see Table
1). The 31P chemical shifts for the allyl complexes 4–8 lie
upfield compared to those of the ligands 1 and 2 (dp
135.5 and 134.9, respectively). This trend is reverse of that
observed for the (g3-allyl) palladium complexes of the ‘‘P–
N–P’’ ligands, Ph2PN(R)PPh2 [7c].
1
The H and 13C NMR spectral data for 5–8 are pre-
1
sented in Table 1. The H–1H COSY and NOESY spectra
suggest that the major isomer 6a contains a syn allyl-methyl
group with respect to the central allyl proton whereas the
minor isomer 6b possesses an anti arrangement of the
allyl-methyl group (see Fig. 2). The 31P{1H} NMR spectrum
of the 1,3-dimethyl-allyl complex, [Pd(g3-1,3-Me2-C3H3){j2-
P,P-X2PN(Me)PX2}](PF6) (X = OC6H3Me2-2,6) (7) reveals
that complex 7 exists as a mixture of two isomers; the spec-
trum displays a singlet and two doublets (AB pattern) in
the ratio 1.4:1.0. The observed AB pattern in the 31P{1H}
spectrum for the minor isomer indicates that the two palla-
dium bound allyl termini are no longer equivalent to each
other. On the basis of a detailed NMR investigation
1
1
(1H–1H NOESY, H–1H ROESY and H–1H COSY) the
major isomer is assigned the syn/syn-configuration while
the minor isomer is assigned the syn/anti-configuration as
shown in Fig. 2. The H–1H NOESY spectrum (Fig. 3)
1
clearly points to the syn/anti-configuration for the minor
isomer 7b. The anti-allyl-methyl protons centred at 0.50 ppm
shows a strong NOE cross-peak to the anti-allyl proton
ðH0aÞ at 4.08 ppm but does not show any NOE cross-peak
to the central allyl proton Hc at 4.78 ppm. Both 6 and 7
do not show any dynamic behaviour at 298 K as revealed
by their phase sensitive 1H–1H NOESY and ROESY spec-
tra; presumably the inter-conversion is slow on the NMR
time scale at this temperature.
The less favourable anti-isomer 6b and the syn/anti-iso-
mer 7b are observed in solution to the extent of 25% and
40%, respectively, as revealed by their 31P{1H} and 1H
NMR spectra. These are the highest relative abundances
of less favourable isomers observed in solution for any pal-
ladium methyl-substituted-allyl complexes bearing the
diphosphazane ligands [5,7]. This result may be attributed
to the increasing steric demand of the 2- and 6-methyl
groups on the phenyl rings which would interact with the
syn allyl-methyl group forcing the syn-isomer to adopt an
anti-configuration to a relatively larger extent. It is reported
that the allyl-palladium complexes tend to adopt a syn-
configuration and the less stable anti-isomer is generally
present in less than 10% relative abundance except in some
special cases [18]. Exceptions arise when the steric interac-
tions either between the allyl moiety and the ligand or
between the substituents on the allyl moiety destabilize
the syn-isomer [3,18,19]. The formation of the anti-isomer
is also significant from the point of view of obtaining a
Fig. 3. The 1H–1H NOESY (400 MHz, CDCl3) spectrum of [Pd(g3-1,3-
Me2-C3H3){j2-P,P-X2PN(Me)PX2}](PF6) [X = OC6H3Me2-2,6 (7)] reveal-
ing the minor isomer (7b) as a syn/anti-isomer. Empty box indicates the
absence of any NOE cross-peak between anti-allyl-methyl protons and Hc
in the minor isomer, 7b.
Z-configured product after reaction with a nucleophile
irrespective of the configuration of the starting material
provided no p–r–p isomerization occurs (see Scheme 1)
[20]. Generally, in the palladium-promoted catalytic reac-
tions, the E-configured products are obtained and selective
conversion of (Z)-allyl substrate with retention of the olefin
geometry remains as one of the most difficult problems to
solve [21]. The reaction of 2 with [Pd(g3-1,3-Ph2-C3H3)-
(l-Cl)]2 gives an orange colour product as the major
component, which is insoluble in common organic solvents
and could not be characterized, and a syn/syn-1,3-diphenyl-
allyl complex [Pd(g3-1,3-Ph2-C3H3){j2-P,P-X2PN(Me)PX2}]-
(PF6) (8, X = OC6H3Me2-2,6) in less than 10% yield. The
structure of 8 has been determined by X-ray crystallography
(see below).
5. Solid state structures of 4, 5, 7a and 8
The molecular structure of 4, 5, 7a and 8 has been deter-
mined by X-ray crystallography. The complexes are iso-
structural to each other and show similar trends in their

S.K. Mandal et al. / Journal of Organometallic Chemistry 691 (2006) 2969–2977
2975
Table 3
Selected bond distances and angles for 4 and 7a
Complex 4
Complex 7a
˚
Bond distances (A)
Pd(1)–C(1)
Pd(1)–C(2)
Pd(1)–C(3)
Pd(1)–P(1)
Pd(1)–P(2)
P(1)–N(1)
P(2)–N(1)
2.230(3)
2.205(3)
2.220(3)
2.301(1)
2.287(1)
1.663(3)
1.676(3)
2.211(6)
2.169(4)
2.172(6)
2.296(1)
2.315(2)
1.666(3)
1.675(3)
Bond angles (ꢁ)
P(1)–Pd(1)–P(2)
C(1)–Pd(1)–C(3)
C(1)–Pd(1)–P(1)
C(1)–Pd(1)–P(2)
C(3)–Pd(1)–P(1)
C(3)–Pd(1)–P(2)
P(1)–N(1)–P(2)
O(1)–P(1)–O(2)
O(3)–P(2)–O(4)
69.42(3)
66.28(11)
112.22(9)
171.13(8)
175.59(9)
111.39(8)
102.96(14)
98.34(12)
98.48(14)
70.00(4)
65.7(2)
112.32(16)
176.5(3)
Fig. 4. The molecular structure of [Pd(g3-1,3-Ph2-C3H3){j2-(PhO)2PN-
(Me)P(OPh)2}] (4) in the solid state; hexafluorophosphate and hydrogen
atoms are omitted for clarity. Thermal ellipsoids are drawn at 30%
probability level.
169.9(4)
112.55(18)
104.68(15)
99.87(12)
100.22(12)
Selected structural parameters of complexes 4 and 7a are
listed in Table 3.
The solid state structures of complexes 4, 5, 7a and 8
reveal a distorted square planar geometry around the pal-
ladium. The distances and dihedral angles between the allyl
moiety and mean plane formed by P(1), P(2) and Pd(1) are
listed in Table 4. The plane of the allyl ligand is tilted from
an axis perpendicular to the P–Pd–P coordination plane
from its idealised position [ideally the dihedral angle
between the allyl moiety and the coordination plane
formed by P(1), Pd(1) and P(2) should be 90ꢁ]. Interest-
ingly, the maximum deviation (48.2ꢁ) from the ideal value
is observed for 7a in which case the maximum population
(40%) of the relatively less favourable syn/anti-isomer
(7b) is observed in solution. The conversion of the syn/
syn-isomer 7a observed in the solid state into a mixture
of 7a and 7b upon dissolution in chloroform can be
explained by considering the well-known g3–g1–g3 isomer-
ization process in solution observed for other related palla-
dium allyl complexes bearing diphosphazane ligands [7].
The bonding parameters fall in the expected ranges
[7,22,23]. In complex 8 (see Supplementary material) the
two Pd–P and Pd–C (terminal) bond lengths [Pd(1)–
Fig. 5. The molecular structure of [Pd(g3-1,3-Me2-C3H3){j2-
X2PN(Me)PX2}](PF6) [X = OC6H3Me2-2,6 (7)] revealing only one isomer
7a in the solid state; hexafluorophosphate and hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at 30% probability level.
structural parameters. For a comparison of the structural
features of these complexes, selected data of 4 and 7a only
are included here. The data for the complexes 5 and 8 are
given in the Supplementary material. The molecular struc-
tures of 4 and 7a are shown in Figs. 4 and 5, respectively.
˚
˚
P(1) = 2.381(1) A, Pd(1)–P(2) = 2.267(1) A, Pd(1)–C(1) =
Table 4
˚
The distances (A) and dihedral angles (ꢁ) between the allyl moiety and mean plane formed by P(1), P(2) and Pd(1)
Distancesa (A) of terminal allyl
Distancea (A) of central
Dihedral angleb
˚
˚
Compound
carbon atoms
allyl carbon atom
C(1)
C(3)
4
ꢀ0.34
0.07
0.10
ꢀ0.24
ꢀ0.17
0.03
0.355
0.624
0.144
0.32
61.6 (28.4)
67.9 (22.1)
41.8 (48.2)
63.4 (26.6)
5
7a
8
ꢀ0.36
ꢀ0.35
a
The distances are from the mean plane formed by P(1), P(2) and Pd(1). Negative sign indicates the atom is located below the mean plane and the
positive sign indicates the atom is located above the plane.
b
Angle between the allyl plane and the mean plane formed by P(1), P(2) and Pd(1) atoms and the deviation from idealized values are in the parentheses.

2976
S.K. Mandal et al. / Journal of Organometallic Chemistry 691 (2006) 2969–2977
Table 5
Yield, melting point and elemental analysis data for g3-allyl palladium complexes 4–8
Compound
Yield (%)
m.p. (ꢁC)
Elemental analysisa
%C
%H
%N
4
5
6
7
8
a
50
76
79
72
50
152–155 (dec)
169–172 (dec)
163–165 (dec)
150–153 (dec)
200–202 (dec)
52.92 (52.78)
49.58 (49.82)
50.34 (50.39)
50.84 (50.94)
56.50 (56.69)
4.01 (3.96)
5.16 (5.07)
5.08 (5.22)
5.37 (5.36)
5.14 (4.98)
1.67 (1.54)
1.46 (1.61)
1.69 (1.58)
1.60 (1.56)
1.37 (1.27)
Calculated values are in parentheses.
˚
˚
2.183(4) A (trans to P(1)), Pd(1)–C(3) = 2.337(4) A (trans
to P(2))] differ significantly. Probably the crowding
between 1,3-diphenyl allyl moiety and the methyl groups
of ligand 2 in complex 8 forces the allyl moiety to bind
unsymmetrically with the Pd centre. A similar type of bond
distortion originating purely from steric constraints was
observed previously in the case of 1,3-diphenyl-allyl com-
plex of a symmetrically substituted P-stereogenic ligand,
1,10-bis(naphthylphenylphosphino)ferrocene [22a].
(fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the Department of Science and Technology
(DST), New Delhi, India for financial support and CCD
X-ray facility at IISc Bangalore set up under IRHPA pro-
gramme. We thank NMR research center (formerly
Sophisticated Instruments Facility), IISc, Bangalore for
NMR measurements. SSK thanks Indian National Science
Academy, New Delhi for the position of a Senior Scientist.
6. Conclusion
The interplay of steric and electronic effects is evident in
the reactions of ‘P–N–P’ ligands of the type X2PN(Me)PX2
with (allyl) chloro palladium dimers. The formation of the
chloro palladium(I) dimer [ClPd(l-X2PN(Me)PX2)]2 and
the absence of a (allyl) palladium complex in the reaction
of [Pd(g3-1,3-R0, R00-C3H3)(l-Cl)]2 (R0 = R00 = H or Me;
R0 = H, R00 = Me) with X2PN(Me)PX2 (X = OPh) ligand
indicates that this ligand bearing two phosphonite phospho-
rus atoms behaves as a strong p-acceptor and destabilizes
metal to allyl p-interactions. A moderately stable allyl com-
plex could be obtained when electron releasing phenyl
groups are introduced in the allyl moiety. On the other
hand, when the r-donor ability of the auxiliary ligand is
increased by incorporating eight methyl groups at the
2- and 6-positions of the aryl rings of the ligand X2PN(Me)PX2
(X = OC6H3Me2-2,6), (allyl) palladium complexes are the
only products. At the same time, the methyl groups at 2-
and 6-positions of the aryl rings of the ligand X2PN(Me)PX2
(X = OC6H3Me2-2,6) exert a significant steric effect in deter-
mining the relative abundance of two isomers of (allyl) pal-
ladium complexes 6 and 7. The isomers with less favourable
allylic arrangements anti (6b) and syn/anti (7b) are formed in
exceptionally high amounts as compared to the other allyl
complexes of diphosphazane ligands.
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