Synthesis of two novel dinuclear palladium(I) complexes and studies of their catalytic activity in amination reactions

Article abstract of DOI:10.1039/b402283a

Two novel palladium(I) dinuclear complexes have been prepared and their structural characterization has revealed an unprecedented μ²- η³:η³ coordination between a phenyl ring from P^tBu₂Bph (Bph = Biphenyl) and the Pd^I-Pd ^I unit present in the complexes.

Full text of DOI:10.1039/b402283a

Synthesis of two novel dinuclear palladium( ) complexes and studies of
I
their catalytic activity in amination reactions†
Ute Christmann, Ramón Vilar,* Andrew J. P. White and David J. Williams
Department of Chemistry, Imperial College London, London, UK SW7 2AZ. E-mail: r.vilar@imperial.ac.uk;
Fax: +44 20-7594-5804; Tel: +44 20-7594-5755
Received (in Cambridge, UK) 16th February 2004, Accepted 13th April 2004
First published as an Advance Article on the web 5th May 2004
Two novel palladium(
I) dinuclear complexes have been pre-
pared and their structural characterization has revealed an
2
3
3
unprecedented m -h :h coordination between a phenyl ring
t
I
I
from P Bu
2
Bph (Bph = Biphenyl) and the Pd –Pd unit present
in the complexes.
I
I
Dinuclear Pd –Pd complexes tend to react readily with small
molecules, being potential catalysts for a wide range of transforma-
1
tions. Consequently, there is considerable interest in developing
novel synthetic routes to prepare this type of dinuclear compound.
We have previously reported the synthesis of the dinuclear species
t
2,3
[
Pd
strained Pd
reactivity towards a wide range of small molecules (namely CO,
2
(m-X)
2
(P Bu
2
3
)
2
2
] (X = Br, I). These compounds have a
(m-X)
core which is probably the reason for their high
Fig. 1 The molecular structure of 2. Selected bond lengths (Å); Pd(1)–Pd(2)
2.5627(8), Pd(1)–Br(1) 2.4615(9), Pd(1)–P 2.2841(17), Pd(1)–C(1)
2.140(6), Pd(1)–C(2) 2.340(7), Pd(1)–C(6) 2.637(7), Pd(2)–Br(1)
2.5633(10), Pd(2)–Br(2) 2.4931(10), Pd(2)–C(3) 2.443(8), Pd(2)–C(4)
4
5
CNR, O
demonstrated that [Pd
amination of various aryl halides. Since the reactivity of Pd –Pd
2 2 2
and C R ). More recently Hartwig and Prashad have
t
2
2 3 2
(m-Br) (P Bu ) ] is a precatalyst for the
I
I
2
.113(8), Pd(2)–C(5) 2.545(9).
complexes is highly dependent on the nature of the phosphine it is
of interest to synthesise dinuclear complexes with a wide range of
phosphines.
respectively] coupled with a contraction of the P–C(12)–C(7) angle
[115.2(5) and 115.2(7)°], allowing a symmetric approach of C(1)
and C(4) to Pd(1) and Pd(2) respectively. It is interesting to note
that in both these complexes the plane of the phenyl ring is not
As part of our systematic efforts to prepare novel PdI–PdI
compounds we have recently turned our attention to some of the
6
sterically demanding phosphines reported by Buchwald. Herein
2
orthogonal to the Pd X plane. Thus the distances from the metal
we report the formation of the new mono-phosphine palladium
t
centers to each of the ortho and meta-carbon atoms are asymmetric,
although all of them short enough to be considered bonding (see
Fig. 1 and ESI†). These differences in the Pd–C distances are
accentuated by an additional slight skewing of the C(1)…(4) vector
with respect to the Pd(1)–Pd(2) bond direction. Complexes where
complex [Pd(dba)(P Bu
upon addition of [Pd(COD)X
novel dinuclear palladium( ) complexes [Pd
2
Bph)] (1) (where Bph = biphenyl) which
2
] (X = Br, Cl) reacts to form the
t
I
2
X(m-X)(m-P Bu
2
Bph)]
(
2, X = Br; 3, X = Cl).
Using the same conproportionation approach previously reported
I
I
7–11
t
3
an arene is p-coordinated to a Pd –Pd unit are rare.
to the previously reported examples, to the best of our knowledge,
and 3 are the first structurally characterised compounds where an
In contrast
to prepare [Pd
2
(m-Br)
Bph were mixed and reacted for 48 hours. Upon
addition of one equivalent of [Pd(COD)Br ] to the reaction
2 3 2 2 3
(P Bu ) ], one equivalent of [Pd (dba) ] and
t
four of P Bu
2
2
2
I
I
2
3
3
12
arene is coordinated to a Pd –Pd unit in a m -h :h fashion.
mixture, an immediate color change from orange to brown was
observed followed by the precipitation of a dark brown solid which
was isolated and, on the basis of structural, spectroscopic and
analytical techniques, characterized as the novel palladium com-
plex 2 (see Fig. 1 and structural discussion below).‡ As expected,
The solid state structures of 2 and 3 are consistent with the
spectroscopic and analytical characterization of the bulk materials.
The 31P{ H} NMR spectra of both dinuclear compounds show
singlets (at 67.7 and 62.0 ppm for 2 and 3 respectively) while the
FAB(+) mass spectra indicate the presence of peaks for [2 2 Br]
1
I
I
2
is a Pd –Pd dinuclear complex with a bridging bromide, but with
1
13
and [3 2 Cl] respectively. The H and C NMR data are consistent
only one phosphine retained in the dinuclear core—in spite of
2
3
3
with a m -h :h coordinated phenyl group (the resonances of which
are shifted to higher field). Elemental analyses of the bulk materials
were also consistent with the formulations found in the solid
state.
having used the appropriate stoichiometry to form a complex with
t
formulation [Pd
2
(m-Br)
2
(P Bu
2
Bph)
2
]. Carrying out the reaction in
a 2 : 1 palladium : phosphine ratio led to the formation of 2 in higher
yields. When a similar conproportionation reaction was carried out
using [Pd(COD)Cl ] as the palladium(II) source, the dinuclear
2
0
These results indicate that the reaction between a Pd complex—
t
generated from the reaction of [Pd
Pd(COD)X
. Consequently it was of interest to determine the exact nature of
the palladium(0) compound formed in situ prior to the con-
proportionation reactions. One equivalent of [Pd (dba) ] and two of
2
(dba)
3
] with P Bu
2
Bph—and
species 3 was obtained (see Scheme 1).§ Crystals of 2 suitable for
X-ray analysis were obtained by layering a THF solution of 2 with
hexane. The single crystal analysis revealed the molecular structure
shown in Fig. 1.¶
The structure of the related chloride complex 3 was also
determined† and shown to be isostructural with 2. The two rings of
the biphenyl group are rotated substantially with respect to each
other (by 73 and 79° respectively in 2 and 3) thereby facilitating p-
bonding between the terminal ring and the metal centers. In both 2
and 3 there is a significant pyramidalization at C(1), [0.14, 0.15 Å
[
3
2
] leads to the formation of the dinuclear species 2 and
2
3
t
31
1
2
P Bu Bph were reacted for 48 hours. After this time P{ H} NMR
†
Electronic supplementary information (ESI) available: experimental
details and X-ray data for compounds 1–3. See http://www.rsc.org/
suppdata/cc/b4/b402283a/
Scheme 1
1
294
C h e m . C o m m u n . , 2 0 0 4 , 1 2 9 4 – 1 2 9 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

spectroscopy indicated that most of the free phosphine had been
consumed and only one new phosphine-containing species [d =
properties of 2 and 3. Four amination reactions were studied and the
preliminary results are summarized in Table 1.
5
9.5 ppm (s)] had formed. The solvent was evaporated under
The expected coupling products were obtained in good yields
indicating that 2 and 3 catalyse the amination of aryl halides. It is
worth noting that the coupling reaction that yields di-p-tolylamine
reduced pressure and the remaining oily material was recrystallized
t
from a diethyl ether–hexane mixture to yield [Pd(dba)(P Bu
1). The crystal structure of 1 shows it to be very similar to the
closely related PCy (phenanthrene) and PCy (dimethoxybiphenyl)
analogues previously reported by Buchwald.
2
Bph)]
16
(
has been previously reported by Buchwald using [Pd
2
(dba)
3
] and
t
2
2
1
P Bu Bph (reported yield of 90% at 80 °C). As can be seen in Table
2
3,14
The palladium is
1, 2 and 3 catalyse this reaction at room temperature, in a
comparable timescale and in reasonably good yields. We are
currently studying a wider range of substrates and experimental
conditions to optimise the cross-coupling reactions and elucidate
the mechanism of the catalytic process. In summary, two novel
p-bonded to one of the olefinic units of the dba and s-bonded to the
phosphorus atom, the third coordination site on the metal being
occupied by a “linkage” to one of the ortho carbon atoms of the
terminal phenyl ring of the biphenyl unit (Fig. 2).§ The coordina-
tion geometry in 1 is analogous to that observed in the recently
dinuclear palladium(
strated to have an unusual structure with a m -h :h coordinated
phenyl ring bound to the Pd –Pd unit of the complexes.
Furthermore, these compounds have been shown to catalyse the
amination of aryl halides in good yields at room temperature.
We thank EPSRC for financial support (U.C.) and Johnson
I
) compounds have been prepared and demon-
reported14 Pd(dba){PCy
bond” between the palladium centre and the aromatic ring
(dimethoxybiphenyl)} complex where the
2
3
3
2
I
I
“
involves only a single carbon atom. As was observed in 2 and 3 the
two rings of the biphenyl group in 1 are rotated substantially with
respect to each other (66°) thereby facilitating p-bonding between
the terminal ring and the metal centre. The solid state structure of
2
Matthey for a loan of PdCl .
1
is consistent with the spectroscopic and analytical character-
3
1
1
ization of the bulk material. The P{ H} spectrum of the complex
shows a singlet at 59.5 ppm while the molecular peak (m/z = 638
amu) in the FAB(+) mass spectrum corresponds to the proposed
formulation.
Notes and references
‡
3
Selected data for 2: Yield: 45%. Anal. Calcd for C20
H
27Br
2
PPd
2
: C,
5.80, H, 4.06. Found: C, 35.89, H, 4.01. 31P{ H} NMR (THF-d
1
8
) d 67.7
(
§
s); FAB(+) MS: m/z 591 (M 2 Br)
Buchwald has established that different palladium sources in
combination with biaryl-based phosphines catalyse cross coupling
reactions.15 It was thus of interest to evaluate the catalytic
Selected data for 3: Yield: 71%. Anal. Calcd for C20
H
27Cl
2 2
PPd : C,
2
41.26, H, 4.60. Found: C 40.99, H 4.42. 31P{
1
H} NMR (DCM–d ): d 62.0
(s); FAB(+) MS: m/z 549 (M–Cl)
¶
6 1
Crystal data for 1: C37H41OPPd·0.5C H14, M = 682.15, P2 /c (no. 14), a
=
18.999(3), b = 9.9530(18), c = 19.111(6) Å, b = 102.658(16)°, V =
3
23
21
3
=
0
4
C
526.0(13) Å , Z = 4, D
203 K; 6208 independent measured reflections, R
.119, 3922 independent observed absorption corrected reflections [|F
s(|F |), 2qmax 50°], 386 parameters. CCDC 224107. For 2:
27Br PPd , M = 671.01, I2/a (no. 15), a = 16.195(3), b = 8.037(2),
c = 34.199(9) Å, b = 98.901(11)°, V = 4397.8(18) Å , Z = 8, D
c
= 1.285 g cm , m(Mo–Ka) = 0.600 mm , T
1
= 0.063, wR
2
=
o
| >
o
=
20
H
2
2
3
c
= 2.027
g cm , m(Mo–Ka) = 5.344 mm , T = 203 K; 6418 independent
measured reflections, R 0.054, wR 0.091, 3989 independent
observed absorption corrected reflections [|F | > 4s(|F |), 2qmax = 60°],
26 parameters. CCDC 224108. See http://www.rsc.org/suppdata/cc/b4/
b402283a/ for crystallographic data in .cif format.
23
21
1
=
2
=
o
o
2
1
2
T. Murahashi and H. Kurosawa, Coord. Chem. Rev., 2002, 231, 207.
R. Vilar, D. M. P. Mingos and C. J. Cardin, J. Chem. Soc., Dalton
Trans., 1996, 4313.
3
4
5
V. Dura-Vila, D. M. P. Mingos, R. Vilar, A. J. P. White and D. J.
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Fig. 2 Molecular structure of 1. Selected bond lengths (Å); Pd–P
2
002, 41, 4746.
M. Prashad, X. Y. Mak, Y. Liu and O. Repic, J. Org. Chem., 2003, 68,
163.
2.3269(18), Pd–C(2) 2.401(6), Pd–C(22) 2.165(6), Pd–C(23) 2.124(6).
1
Table 1 Amination of aryl halidesa
6 A. Aranyos, D. W. Old, A. Kiyomori, J. P. Wolfe, J. P. Sadighi and S.
L. Buchwald, J. Am. Chem. Soc., 1999, 121, 4369.
7
8
9
G. Allegra, G. Tettamanti Casagrande, A. Immirzi, L. Porri and G.
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J. Dupont, M. Pfeffer, M. A. Rotteveel, A. De Cian and J. Fischer,
Organometallics, 1989, 8, 1116.
Yield
(%)
b
Cat. Aryl halide Amine
Product
Time/h
19
2
3
Ph
2
NH
NH
86
76
S. Kannan, A. J. James and P. R. Sharp, J. Am. Chem. Soc., 1998, 120,
2
15.
Ph
2
19
10 M. Sommovigo, M. Pasquali, P. Leoni, D. Braga and P. Sabatino, Chem.
Ber., 1991, 124, 97.
1
1
1
1
1 M. Retboll, A. J. Edwards, A. D. Rae, A. C. Willis, M. A. Bennett and
E. Wenger, J. Am. Chem. Soc., 2002, 124, 8348.
2 For a compilation of Pd(I)–Pd(I) distances see: W. Lin, S. R. Wilson and
G. S. Girolami, Inorg. Chem., 1994, 33, 2265.
3 J. Yin, M. P. Rainka, X.-X. Zhang and S. L. Buchwald, J. Am. Chem.
Soc., 2002, 124, 1162.
4 S. D. Walker, T. E. Barder, J. R. Martinelli and S. L. Buchwald, Angew.
Chem., Int. Ed., 2004, 43, 1871.
2
3
3
78
81
3
a
The reactions were carried out at RT, in THF (1 mL) using either 1 mol%
of either 2 or 3 and on a 1 mmol scale (see ESI for details). b Isolated
yields.
15 A. R. Muci and S. L. Buchwald, Top. Curr. Chem., 2002, 219, 131.
1
6 J. P. Wolfe, H. Tomori, J. P. Sadighi, J. Yin and S. L. Buchwald, J. Org.
Chem., 2000, 65, 1158.
C h e m . C o m m u n . , 2 0 0 4 , 1 2 9 4 – 1 2 9 5
1295