[MCl(ligand)]+ complexes (M = Ni, Pd, Pt) with a P,N,N terdentate ligand - Solid state and solution structures and catalytic activity of the PdII derivative in the Heck reaction

Article abstract of DOI:10.1002/ejic.200500491

Ni^II, Pd^II and Pt^II cationic complexes of the general formula [MCl(PNN)]PF₆, where PNN is the terdentate ligand N-[2-(diphenylphosphanyl)benzylidene][2-(2-pyridyl)ethyl]amine, have been synthesised and fully characterised in solution by NMR spectroscopy. The diamagnetic Ni^II complex 1 shows fluxionality, which can be attributed to a conformational rearrangement of the two six-membered chelate rings. On the contrary, the Pd^II 2 and Pt^II 3 derivatives are stereochemically rigid on the NMR timescale. The crystal structures of 1 and 2 are reported, which show a planar geometry for both complexes. Complex 2 has been investigated as a precatalyst in the Heck coupling between styrene and bromobenzene. Interestingly, the nature of the solvent has a critical role in determining the yield of the reaction product. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500491

FULL PAPER
DOI: 10.1002/ejic.200500491
[MCl(ligand)]+ Complexes (M = Ni, Pd, Pt) with a P,N,N Terdentate Ligand –
Solid State and Solution Structures and Catalytic Activity of the PdII
Derivative in the Heck Reaction
Alessandro Del Zotto,*[a] Ennio Zangrando,[b] Walter Baratta,[a] Alessandro Felluga,[a]
Paolo Martinuzzi,[a] and Pierluigi Rigo[a]
Keywords: Nickel / Palladium / Platinum / Terdentate hybrid ligands / Heck reaction
NiII, PdII and PtII cationic complexes of the general formula
[MCl(PNN)]PF6, where PNN is the terdentate ligand N-[2-
(diphenylphosphanyl)benzylidene][2-(2-pyridyl)ethyl]amine,
have been synthesised and fully characterised in solution by
NMR spectroscopy. The diamagnetic NiII complex 1 shows
fluxionality, which can be attributed to a conformational re-
arrangement of the two six-membered chelate rings. On the
contrary, the PdII 2 and PtII 3 derivatives are stereochemically
rigid on the NMR timescale. The crystal structures of 1 and
2 are reported, which show a planar geometry for both com-
plexes. Complex 2 has been investigated as a precatalyst in
the Heck coupling between styrene and bromobenzene.
Interestingly, the nature of the solvent has a critical role in
determining the yield of the reaction product.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
dyl-donating groups have been reported so far.[3] The pres-
ence of two different sp2 N-donors affects the way the li-
gand binds to the metal, with effects on both reactivity and
catalytic activity of the resulting complexes. Effectively,
some of these complexes have shown valuable efficiency in
different catalysed reactions, such as enantioselective conju-
gate addition of diethylzinc to enones,[3e] olefin hydrogena-
tion,[3f] asymmetric allylic alkylation[3h] and asymmetric
olefin cyclopropanation.[3j]
Vrieze and co-workers have described the potentially
terdentate P,N,N ligand N-[2-(diphenylphosphanyl)benzyl-
idene][2-(2-pyridyl)ethyl]amine[4] (PNN) (Figure 1), which
shows the P-sp3, N-sp2 (imine), N-sp2 (pyridyl) donors se-
quence. Several different transition-metal complexes bear-
ing PNN have been reported, in particular with group 10
metals,[4] the ligand acting as both terdentate[4–6] or biden-
tate with a dangling pyridyl arm.[4–6] Among these species,
the PdII η1-allyl cationic complex has shown good catalytic
activity in allylic alkylation[4b] and 1,3-butadiene telomeris-
ation with methanol.[7]
Introduction
In recent years, transition-metal complexes bearing
multidentate ligands that contain significantly different
types of hard (N- or O-) and soft (P-) donor functions have
been extensively studied because of their potential applica-
tion in homogeneous catalysis.[1] These ligands possess
weakly coordinating groups that can be reversibly released
during the catalytic cycle providing unsaturation at the me-
tal centre. Thus, the less strongly bound moiety of this type
of ligand, called hemilabile, can be seen as an intramolecu-
lar solvent molecule that is capable of temporarily holding a
coordination site on the metal. There are several hemilabile
ligands containing a substitutionally inert phosphorus func-
tion and a labile oxygen or nitrogen donor. Among the vari-
ous kinds of P,N hybrid ligands, those showing pyridyl or
imine arms have been widely used to prepare complexes of
different transition metals,[1b,1c,1e,1f] some of which have
been successfully employed in catalytic reactions. Thus, for
example, NiII complexes have been tested in olefin poly-
merisation, while PdII complexes have been found to be
very active in asymmetric allylic alkylation and cross-coup-
ling reactions. Also, our group has worked in this field with
particular interest in complexes with phosphane–pyridyl bi-
dentate ligands.[2]
By contrast, few terdentate P,N,N-type ligands showing
mixed functionalities such as phosphane-, imine- and pyri-
[a] Dipartimento di Scienze e Tecnologie Chimiche, Universitá di
Udine,
Figure 1. The ligand PNN along with the numbering scheme.
33100 Udine, Italy
E-mail: alessandro.delzotto@dstc.uniud.it
[b] Dipartimento di Scienze Chimiche, Università di Trieste,
34127 Trieste, Italy
For our part, we decided to investigate the coordination
chemistry of PNN towards group 10 metal halides, as the
Eur. J. Inorg. Chem. 2005, 4707–4714
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A. Del Zotto, E. Zangrando, W. Baratta, A. Felluga, P. Martinuzzi, P. Rigo
FULL PAPER
first step of a more detailed study on the catalytic properties
of PNN-containing late transition-metal complexes. In this
paper we present the preparation and characterisation of
the cationic complexes [MCl(PNN)]PF6 [M = Ni (1), Pd
(2), Pt (3)], also describing preliminary results on the cata-
lytic activity of 2 in the Heck C–C coupling[8] using aryl
bromides as starting material (Scheme 1). The choice for
this reaction was stimulated by the fact that PNN is able to
stabilise Pd0 species without the need of additional stabilis-
ing ligands[4b] and that the couple Pd0/PdII is thought to be
involved in the Pd-catalysed Heck reaction.[8]
Results and Discussion
The compound PNN was prepared as described in the
literature[4b] and obtained as a cream-coloured powder after
double recrystallisation from n-hexane. In the original pub-
lication,[4b] besides 31P and 15N NMR spectroscopic data,
1
only selected H and 13C values were reported for PNN.
1
Also two later papers dealt with selected H NMR values
only.[5,6] Therefore, we decided by means of 1H{31P}, 1H–1H
1
COSY, H–13C HETCOR, 13C DEPT and 13C PENDANT
1
experiments to assess all H (confirmed by computer simu-
lation) and 13C parameters. The 1H and 13C NMR spectro-
scopic data have been collected in Table 1 and Table 2,
respectively, to allow a direct comparison with the data of
complexes 1–3. The numbering scheme of carbon and hy-
drogen atoms of PNN is reported in Figure 1.
Solution Structure of Complexes 1–3
Complexes [PdCl(PNN)]PF6 (2) and [PtCl(PNN)]PF6 (3)
have been prepared by the room-temperature reaction be-
tween the appropriate [MCl2(RCN)2] precursor (M = Pd, R
Scheme 1.
Complexes 1–3 have been fully characterised by spectro- = CH3; M = Pt, R = C6H5) and the ligand PNN in a 1:1
scopic and analytical methods. In addition, as the NMR molar ratio in CH2Cl2 solution, followed by addition of an
investigation has evidenced that, unlike complexes 2 and 3, excess of NH4PF6 in propan-2-ol and vacuum elimination
the NiII derivative 1 is fluxional in the whole temperature of CH2Cl2. The complexes were obtained pure as pale yel-
range 293–183 K, the crystal structure determination of 1 low crystals by recrystallisation from CH2Cl2/propan-2-ol.
and 2 has been performed to establish whether the different For both 2 and 3, the best fit of the elemental analysis data
behaviour in solution of the two species could be related to is obtained for the formula [MCl(PNN)]PF6·0.5CH2Cl2.
structural modifications due to the nature of the metal ion. The presence of dichloromethane within the crystals has
1
Table 1. H NMR values of the ligand PNN (in CDCl3) and complexes 1–3 (in CD2Cl2) at 293 K.[a,b]
H2
H3
H4
H5
H7
H8
H9
H11
H12
H13
H14
HPh[c]
PNN[d] 7.93 m 7.37 m 7.24 m 6.87 m[e]
8.85 dt
3.88 dt
2.98 t
7.03 m
7.48 m
7.85 dt
7.93 m
8.00 m
7.02 m 8.48 m
7.15–7.45 m
1
7.12 br. d 8.04 br. s 3.61 br. t 4.03 br. m
7.16 m
7.21 m
8.72 br. d 7.30–7.85 m
8.83 m
8.81 m
2[f]
3[g]
7.81 m
7.89 m
8.51 dt
8.75 dt
3.93 m
4.07 m
3.47 m
3.50 m
7.36–7.74 m
7.42–7.72 m
[a] The numbering of the H atoms is shown in Figure 1. [b] The resonances not reported overlap with those of aromatic protons. [c] HPh
= Ho + Hm + Hp. [d] Coupling constants in Hz: J(H2–H3) = 7.7; J(H2–H4) = 1.6; J(H3–H4) = 7.5; J(H3–H5) = 1.3; J(H4–H5) = 7.6;
J(H5–H7) = 4.7; J(H7–H8) = 1.3; J(H11–H12) = 7.7; J(H11–H13) = 1.5; J(H11–H14) = 0.8; J(H12–H13) = 7.5; J(H12–H14) = 1.9; J(H13–H14)
1
= 4.8; J(H2–31P) = 3.9; J(H3–31P) = 0.5; J(H5–31P) = 4.6; J(H7–31P) = 4.7; no splitting due to H–31P coupling is observed on the signal
of H4, as previously reported.[4b] [e] An uncorrected value (7.03) has been previously reported.[4b] [f] Coupling constants in Hz: J(H2–31P)
= 2.1; J(H5–31P) = 10.7; J(H7–31P) = 1.7; J(H14–31P) = 3.3. [g] Coupling constants in Hz: J(H2–31P) = 2.2; J(H5–31P) = 11.0; J(H7–31P)
= 0.5; J(H14–31P) = 3.4; J(H7–195Pt) = 115; J(H8–195Pt) = 26; J(H14–195Pt) = 25.
Table 2. 13C{1H} NMR values of the ligand PNN (in CDCl3) and complexes 1–3 (in CD2Cl2).[a,b]
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
Ci
Co
Cm
Cp
PNN[c] 139.5 127.6 128.8 130.1 133.3 137.3 160.3 60.8 39.4 159.8 123.4 136.0 121.1 149.2 136.2 133.9 128.5 128.7
[17.6] [4.4]
119.4 135.9 134.0 132.6 133.6 134.7 171.0 52.4 35.0 155.8 123.6 140.0 123.5 150.4 122.7 133.4 128.7 132.3
[44.0] [8.2] [7.0] [13.5] [7.6] [56.9] [11.6] [11.7]
120.0 138.1 135.3 134.1 134.6 136.2 168.0 57.1 37.6 158.2 125.1 141.0 124.2 151.7 125.2 134.3 129.6 133.1
[50.5] [8.8] [8.2] [2.9] [2.9] [15.3] [8.5] [3.5] [2.9] [1.2] [62.8] [11.2] [12.3] [2.9]
120.4 137.7 135.2 133.9 134.0 136.2 166.6 57.5 37.8 158.3 125.1 141.5 124.6 151.5 125.2 134.3 129.3 132.8
[59.9] [8.8] [8.2] [2.3] [3.5] [13.5] [7.6] [3.5] [3.5] [1.2] [70.4] [11.1] [12.3] [2.9]
[1.1]
[19.7] [21.1]
[10.0] [20.0] [7.3]
1[d]
2[c]
3[c]
[a] The numbering of the C atoms is shown in Figure 1. [b] J(13C–31P) in Hz are reported in brackets. [c] At 293 K. [d] At 183 K.
4708 Eur. J. Inorg. Chem. 2005, 4707–4714
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[MCl(ligand)]+ Complexes (M = Ni, Pd, Pt) with a P,N,N Terdentate Ligand
1
FULL PAPER
been confirmed by H NMR analysis and crystal structure centration of the solution. The 31P{1H} NMR spectrum of
determination of complex 2 (see below).
1 recorded at 293 K in CD2Cl2 solution shows a broad sing-
1
The H NMR spectra of 2 and 3, which are very similar, let at δ = 22.4 (Δν1/2 Ϸ 340 Hz) along with the regular sharp
indicate that the structures of the cations are static on the septet at δ = –143.9 [1J(19F–31P) = 711 Hz], which is typical
–
NMR timescale. Meaningful shifts of some resonances can of the PF6 ion. On lowering the temperature, the width of
be observed upon coordination of the ligand to the metal the broad signal progressively sharpens and at 183 K, the
centre (Table 1). For example, the signal of the H9 proton, lowest available temperature, Δν1/2 is reduced to 4 Hz (δ =
which is at δ = 2.98 in the free ligand, is shifted to lower 24.7). The process is completely reversible and also the
1
fields in complexes 2 (δ = 3.47) and 3 (δ = 3.50), while room-temperature H and 13C{1H} spectra of complex 1
that of the H8 proton does not show an appreciable shift. exhibit broad signals and no 31P–13C coupling constant is
Furthermore, both methylene signals of the –CH2CH2– shown.
chain appear as unsymmetric multiplets whose lines have
We have examined the possibility that the broadening of
about the same intensity. This spectral feature is diagnostic the signals may arise from the presence of a paramagnetic
of the presence of diastereotopic pairs of protons within NiII species. In this regard, it should be noted that com-
each methylene group, arising from the rigidity of the che- plexes [NiX2(P-N)], where P-N is a phosphane-pyridyl[10] or
late N,N ring. The H7 resonance, which is at δ = 8.85 in the a phosphane-imine[10d,11] ligand, are tetrahedral, with mag-
free ligand, is shifted to higher fields in complexes 2 (δ = netic moments in the range 1.82–2.62 μB. Furthermore, for
8.51) and 3 (δ = 8.75). On the contrary, the H14 resonance, a NiII derivative with a PNNX donor set the possibility for
which is at δ = 8.48 in the free ligand, is markedly shifted a temperature-dependent equilibrium between diamagnetic
to lower fields (1, δ = 8.83; 2, δ = 8.81 ppm).
square-planar and high-spin tetrahedral structures should
In the free ligand, H2, H3, H5 and H7 protons are cou- be considered.[12] However, determinations of the magnetic
pled to the 31P nucleus. In complexes 2 and 3 the H–31P moment in dichloromethane (183–293 K) and 1,2-dichloro-
1
coupling constants are not lost but considerably modified. ethane (293–343 K) solutions show that complex 1 is dia-
4
In particular, the J(1H–31P) value on H7, which is 4.7 Hz magnetic in agreement with a square-planar structure. This
in free PNN, is reduced to 1.7 Hz in 2 and 0.5 Hz in 3. This finding fits well with the observation that there is no colour
observation contrasts with the lack of coupling between H7 change of the solutions in the whole temperature range
and 31P nuclei found in previously reported PdII and PtII 183–343 K. Another NiII complex with a X(P–N–N) donor
complexes bearing PNN.[4b] It is worth noting that also H14 set has been found to be diamagnetic.[13]
shows a coupling with phosphorus both in 2 [4J(1H–31P)
In the 13C{1H} spectrum of 1 recorded at 183 K almost
= 3.3 Hz] and 3 [4J(1H–31P) = 2.9 Hz]. This feature again all signals are rather sharp and 31P–13C couplings are
confirms that the pyridyne nitrogen of the ligand is coordi- shown by C7 and several carbon atoms of both phenyl and
nated to the metal centre.
phenylene rings. Unfortunately, the slow-exchange limit
Interestingly, the H7, H8 and H14 resonances in the spec- spectrum is not reached and the resonances of C8 and C9
trum of 3 show the presence of satellites due to H–195Pt appear as still broad signals. However, the comparison be-
1
coupling. While H8 and H14 show similar low J values (28 tween the 13C{1H} spectra of 1 (at 183 K) and 2 (at 293 K)
3
and 26 Hz, respectively), the coupling constant on H7 is allows us to conclude that, as chemical shifts and coupling
115 Hz. As can be seen in Figure 2, these hydrogen atoms constants do not exhibit remarkable differences (see
are the closest to the metal centre, all others being separated Table 2), the “frozen” structure of 1 is analogous to that of
from it by four or more bonds. The 1H–195Pt coupling value 2. Most probably, the dynamic process observed for com-
found for H7 is much higher than that reported by Pudde- plex 1 in solution is due to a conformational flexibility of
phatt and co-workers for the imine proton in other PtII the chelate ring, which allows both chair and boat confor-
four-coordinate complexes.[9]
mations. In this regard, several different transition-metal
complexes bearing bidentate ligands that form a six-mem-
bered chelate ring,[14] including square-planar species of the
type [Ni(P-P)X2],[14b] exhibit a fluxional behaviour due to
δ-twist-boat−λ-twist-boat rearrangement through a higher-
energy chair conformation. It is likely that the dynamic pro-
cess shown by complex 1 involves a rearrangement of both
rings; that formed by the nitrogen donors being more flexi-
ble, as suggested by the observation that the most signifi-
cant modifications of the spectral pattern mainly involve
the resonances of the –CH2CH2– bridge of the PNN ligand.
It should be noted that the other chelate ring exhibits a
chain of four sp2-hybridised atoms (–C–C–C–N–), which
Figure 2. Structure of the cationic complexes (the two phenyl rings
bound to phosphorus have been omitted for clarity).
The orange cationic diamagnetic complex [NiCl(PNN)]- reduces the conformational flexibility of the cycle. At very
PF6 (1) was prepared by reacting equimolar amounts of low temperatures, the rate of the rearrangement processes
NiCl2·6H2O and PNN in methanol, followed by addition is strongly slowed down and the two rings most probably
of an excess of NH4PF6 dissolved in propan-2-ol and con- adopt the conformation characteristic of the solid-state
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A. Del Zotto, E. Zangrando, W. Baratta, A. Felluga, P. Martinuzzi, P. Rigo
FULL PAPER
structure of the complex (see below). Another interpret- in 1 because of the smaller radius of the NiII ion. The bond
ation of the fluxional behaviour of 1 invoking the rupture lengths and angles are reported in Table 3, and data of the
of the metal–nitrogen bond trans to the P atom is less pre- Pd derivative have values similar to those measured in sim-
cedented.
ilar complexes.[4] In both complexes, the ring formed by the
N,N donor set shows a nearly ideal boat conformation,
while the other one can be seen as a slightly twisted half-
chair (Figure 3). It should be noted that the former is con-
sidered to be an unconventional form.[14c] These conforma-
tions cause a twist of the ligand around the metal centre.
However, the overall configuration of 1 and 2 is closely
comparable and there are no significant geometrical differ-
ences. The slight differences in the orientation of pyridine
and of the imine moiety C(7)–N(2)–C(8)–C(9) with respect
to the coordination plane likely originate in the crystal
packing. On the other hand, a detailed examination of the
coordination environment shows a slight tetrahedral distor-
Crystal Structure Determination of Complexes 1 and 2
Structural analysis of complexes 1 and 2 in the solid state
was undertaken in order to understand the different dy-
namic behaviour of the two species observed in solution.
Orange-red needles of 1, as well as pale yellow needles of
2, suitable for X-ray analysis were obtained by slow dif-
fusion of propan-2-ol into a dichloromethane solution of
the complex. The molecular structures of 1 and 2 are pre-
sented in Figure 3 and Figure 4, respectively. The metal in
both the complexes exhibits the expected square-planar ge-
ometry with coordination distances that are slightly shorter
Figure 4. X-ray structure of the cation of complex 2 (ORTEP draw-
ing, thermal ellipsoids at 40% probability level).
Table 3. Coordination bond lengths [Å] and angles [°] and selected
geometrical data for 1 and 2.
1
2
Δ
M–N(1)
M–N(2)
M–P(1)
M–Cl(1)
N(1)–M–N(2)
N(1)–M–P(1)
N(1)–M–Cl(1)
N(2)–M–P(1)
N(2)–M–Cl(1)
P(1)–M–Cl(1)
1.944(3)
1.904(3)
2.155(1)
2.143(1)
89.38(12)
172.97(9)
89.87(9)
88.41(9)
173.05(10)
93.12(4)
2.125(4)
2.037(4)
2.226(1)
2.282(1)
87.12(16)
170.28(11)
91.73(12)
85.05(12)
178.09(11)
96.25(5)
0.181
0.133
0.071
0.139
N(2)–C(8)
N(2)–C(7)
C(8)–N(2)–C(7)
C(8)–N(2)–Ni
C(7)–N(2)–Ni
C(5)–C(6)–C(7)–N(2)
1.283(5)
1.499(5)
115.9(3)
128.5(3)
115.5(2)
60.1(4)
1.269(7)
1.490(6)
117.7(4)
125.8(3)
116.4(3)
59.0(6)
Py/coord. plane
56.9(1)
50.8(1)
47.7(2)
0.066(2)
Imino group/ coord plane 44.7(3)
Figure 3. X-ray structure of the cation of complex 1 (ORTEP draw-
ing, thermal ellipsoids at 40% probability level) and a view of the
conformations of the two chelate rings.
Dev [Å][a]
0.112(1)
[a] Dev = mean deviation of donors in the square-planar geometry.
4710
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[MCl(ligand)]+ Complexes (M = Ni, Pd, Pt) with a P,N,N Terdentate Ligand
FULL PAPER
tion in the square-planar geometry of the Ni derivative reaction rate is too low to obtain an acceptable yield (entry
[ 0.112(1) Å] in contrast to the more coplanar donors, 9). Surprisingly, the use of C6H5I instead of C6H5Br does
0.066(2) Å in the case of Pd. The packing in compound 1 not correspond to the expected neat increase of the reaction
–
shows a PF6 anion located close to the nickel ion with a rate[8g] (entry 10). The mechanism of the reaction is cur-
Ni–F distance of 3.335 Å. This is a feature often encoun- rently under investigation, but some preliminary results can
tered in square-planar complexes containing this anion.[15]
be briefly discussed. 31P NMR measurements indicate that
complex 2 is stable at 120 °C in EGME alone and even in
the presence of both C6H5Br and styrene, but immediately
after the introduction of Cs2CO3 the pale yellow colour of
the solution turns dark orange. The 31P{1H} NMR spec-
trum of this solution shows that a singlet at δ = 33.0 has
Catalytic Properties of Complexes 1 and 2 in the Heck
Reaction
To evaluate the catalytic activity of complex 2 in the completely replaced the signal at δ = 31.1 because of com-
Heck reaction, the process of formation of trans-stilbene plex 2, while no other resonance is detectable. Thus, the
from bromobenzene and styrene has been chosen as a starting complex is no longer present in solution under
model (Scheme 2). The results of the investigation are col- catalytic conditions but is transformed into a new species,
lected in Table 4. For all trials, samples of the reaction mix- most probably the effective catalyst. Attempts to isolate this
ture have been extracted and then analysed by GC after 4, derivative were unsuccessful. A prolonged warming of the
8 and 20 h. First, we have examined how the nature of the dark orange solution at 120 °C results in the formation of
solvent influences yield and selectivity. Thus, the reaction increasing amounts of palladium black.
between C6H5Br and styrene was performed under argon
at 120 °C, using Cs2CO3 as the base, in the following sol-
vents: N-methylpyrrolidinone (NMP), dimethyl sulfoxide
(DMSO), dimethylformamide (DMF), ethylene glycol mon-
omethyl ether (EGME) and 2,2Ј-thiodiethanol (TDE).
NMP, DMF and DMSO are commonly used solvents for
the Heck reaction, while the same is not true for the protic
solvents EGME and TDE, which have been chosen mainly
to ensure a complete dissolution of both the cationic com-
plex and the base. The high final yield (89%) of trans-stil-
bene obtained using EGME (entry 5) with respect to the
other solvents (entries 1–4) is rather surprising. Conversely,
both regio- and stereoselectivity are rather low, as 9% of
a mixture of 1,1-diphenylethene (main byproduct) and cis-
stilbene is formed. The use of DMF resulted in the immedi-
Scheme 2.
ate formation of an insoluble brown material and only very
small amounts of products were detected. The almost negli-
gible activity of complex 2 in DMSO is most probably due
to the strong coordinating ability of this solvent, which does
Finally, the NiII derivative 1 does not show any catalytic
not allow substrate activation on the metal centre. Notably, property in the reaction between C6H5Br and styrene, or in
using EGME the yield is very high within a short time the presence of an excess of zinc dust (entry 11). It is well
(83% after 4 h), despite the formation of palladium black known that NiII compounds are less active than PdII com-
after only a few minutes. We have found that the catalytic pounds, but the beneficial role of a reductant such as zinc
activity of complex 2 drastically decreases after the addition has been demonstrated in several cases.[17] The complete
of a second batch of reagents. Thus, the trans-stilbene yield lack of catalytic activity of 1 is most probably due to the
drops from 83 to 49% after 4 h in the second run. Such a more difficult β-elimination and HBr removal steps for NiII
finding agrees with the observed increasing formation of with respect to PdII.[18]
palladium black in the reaction vessel. In this regard, se-
It should be noted that α,β-unsaturated esters, used as
veral authors have reported that palladium black is not an starting material in combination with bromobenzene in
active Heck catalyst.[16] Apparently, the effective catalyst is EGME, undergo a transesterification process, promoted by
longer lived, but slightly less active, in TDE, the final yield Cs2CO3, giving the corresponding 2-methoxyethyl ester,
being good nevertheless (85%). The best base to use in which then undergoes the Heck reaction. Thus, for example,
combination with EGME is Cs2CO3, as with dimethylcyclo- a mixture of cis- and trans-2-methoxyethyl-2-methyl-3-
hexylamine, K2CO3 and CsOH·H2O lower yields have been phenylpropenoate (major product) has been obtained from
obtained (entries 6–8). In the case of cesium hydroxide the ethyl methacrylate.
conversion of C6H5Br into products is almost complete, but
In conclusion, complex 2 shows a good catalytic per-
a dramatic lowering of the selectivity is observed. To reduce formance in the Heck reaction starting from aryl bromides,
palladium black formation, the reaction has been per- and the use of EGME as solvent is crucial to achieve good
formed at lower temperature (80 °C), but in this case the yields.
Eur. J. Inorg. Chem. 2005, 4707–4714
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4711

A. Del Zotto, E. Zangrando, W. Baratta, A. Felluga, P. Martinuzzi, P. Rigo
Table 4. Catalytic activity of complexes 1 and 2 (1 mol-%) in the Heck reaction.[a]
FULL PAPER
Entry
Complex
Aryl halide
Alkene
Solvent[b]
Base[c]
Temp.
(°C)
trans-Stilbene yield (%)[d]
4 h
42
8 h
45
20 h
1
2
2
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5I
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
NMP
TDE
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMCA
K2CO3
120
120
120
120
120
120
120
120
80
46 (53)
2
58
–
71
1
83 (89)
2 (5)
3
2
DMSO
DMF
4
2
1
2
3 (7)
5
2
EGME
EGME
EGME
EGME
EGME
EGME
EGME
83
13
64
69
11
87
–
86
28
67
74
14
88
–
89 (98)
41 (45)
68 (74)
75 (98)
18 (24)
90 (100)
–
6
2
7
2
8
2
CsOH·H2O
Cs2CO3
Cs2CO3
Cs2CO3
9
2
10
11
2
120
1[e]
C6H5Br
120
[a] Experimental conditions: aryl halide (1.0 equiv.), alkene (1.2 equiv.), base (1.2 equiv.). [b] NMP = 1-methyl-2-pyrrolidinone, DMSO =
dimethyl sulfoxide, TDE = 2,2Ј-thiodiethanol, EGME = ethylene glycol monomethyl ether. [c] DMCA = dimethylcyclohexylamine. [d] GC
yield. Aryl halide conversion after 20 h is reported in parenthesis. [e] With 10% Zn (dust) added.
tion of NiCl2·6H2O (90 mg, 0.38 mmol). The red solution was
stirred for 10 min and then NH4PF6 (196 mg, 1.2 mmol) dissolved
Experimental Section
General Remarks: All reagents were purchased from Aldrich and
used without further purification. Commercial solvents were dried
according to standard methods[19] and freshly distilled under argon
before use. All syntheses and manipulations were carried out under
argon using standard Schlenk techniques. The ligand N-[2-(di-
in propan-2-ol (20 mL) was added. An orange powder immediately
formed, which was filtered off, washed with methanol and dried.
The product was then recrystallised from dichloromethane/propan-
2-ol. Needle-shaped red crystals were obtained by slow diffusion of
propan-2-ol into a dichloromethane solution of the complex. Yield:
195 mg, 81%. C26H23ClF6N2NiP2 (633.57): calcd. C 49.29, H 3.66,
N 4.42; found C 48.93, H 3.62, N 4.37. 31P NMR (CD2Cl2, 183 K):
δ = 24.7 ppm. 1H and 13C NMR spectroscopic data are reported
in Table 1 and Table 2, respectively.
phenylphosphanyl)benzylidene][2-(2-pyridyl)ethyl]amine
(PNN)
was prepared as described in the literature[4b] and was obtained
very pure as a cream-coloured powder after double recrystallisation
from n-hexane.
1
Instrumentation and Analyses: The H, 13C and 31P NMR spectra
Synthesis of [PdCl(PNN)]PF6 (2): A mixture of [PdCl2(CH3CN)2]
(156 mg, 0.60 mmol) and PNN (245 mg, 0.62 mmol) in dichloro-
methane (20 mL) was stirred for 30 min. NH4PF6 (294 mg,
1.8 mmol) dissolved in propan-2-ol (25 mL) was then added to the
yellow solution and the dichloromethane was pumped off. The pale
yellow precipitate, which was filtered off, washed with propan-2-ol
and vacuum dried, was then recrystallised from dichloromethane/
propan-2-ol. Needle-shaped pale yellow crystals were obtained by
slow diffusion of propan-2-ol into a dichloromethane solution of
the complex. The crystals contain half mol of CH2Cl2 per mol of
complex. Yield: 379 mg, 87%. C26H23ClF6N2P2Pd·0.5CH2Cl2
(723.76): calcd. C 43.98, H 3.34, N 3.87; found C 43.72, H 3.37, N
(at 200.13, 50.32 and 81.02 MHz, respectively) were recorded with
a Bruker AC 200 F QNP spectrometer equipped with a variable-
temperature probe. The 1H and 13C chemical shifts were referenced
to SiMe4, while positive 31P chemical shifts were downfield from
85% H3PO4 as external standard. The temperature in the range
273–183 K was calibrated with methanol. The simulation of the
293 K 1H spectrum of PNN was performed with an Aspect 2000
computer using the programme PANIC (Bruker Spectrospin AG).
The GC-MS analyses, run to control the identity of the compounds
obtained in the catalytic trials, were carried out with a Fisons
TRIO 2000 gas chromatograph-mass spectrometer working in the
positive ion 70 eV electron impact mode. Injector temperature was
kept at 250 °C and the column (Supelco® SE-54, 30 m long,
0.25 mm i.d., coated with a 0.5 μm phenyl methyl silicone film)
temperature was programmed from 50 °C to 310 °C with a gradient
of 10 °C/min. The GC analyses were run on a Fisons GC 8000
Series gas chromatograph equipped with a Supelco® PTA-5 column
[30 m long, 0.53 mm i.d., coated with a 3.0 μm poly(5% diphenyl/
95% dimethylsiloxane) film]. Injector and column temperatures
were as indicated above. The elemental analyses (C, H, N) were
carried out in the Microanalytical Laboratory of our department.
1
3.81. 31P NMR (CD2Cl2, 293 K): δ = 31.0 ppm. H and 13C NMR
spectroscopic data are reported in Table 1 and Table 2, respectively.
Synthesis of [PtCl(PNN)]PF6 (3): A mixture of [PtCl2(C6H5CN)2]
(118 mg, 0.25 mmol) and PNN (103 mg, 0.26 mmol) in dichloro-
methane (20 mL) was stirred for 30 min. After the addition of
NH4PF6 (130 mg, 0.8 mmol) dissolved in propan-2-ol (20 mL), the
dichloromethane was eliminated in vacuo affording a pale yellow
product, which was filtered off, washed with propan-2-ol and vac-
uum dried. The crude product was recrystallised from dichloro-
methane/propan-2-ol. Needle-shaped pale yellow crystals were ob-
tained by slow diffusion of propan-2-ol into a dichloromethane
solution of the complex. The crystals contain half mol of CH2Cl2
Synthesis of [NiCl(PNN)]PF6 (1): A methanol solution (10 mL) of
PNN (158 mg, 0.40 mmol) was added to 15 mL of a methanol solu-
4712
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2005, 4707–4714

[MCl(ligand)]+ Complexes (M = Ni, Pd, Pt) with a P,N,N Terdentate Ligand
FULL PAPER
per mol of complex. Yield: 186 mg, 91%. C26H23ClF6N2P2Pt· located at geometrically calculated positions. All the calculations
0.5CH2Cl2 (812.42): calcd. C 39.18, H 2.98, N 3.45; found C 38.94,
were performed using the WinGX System, version 1.70.00.[23] Crys-
tallographic data and details of structure refinements are reported
in Table 5.
1
H 3.03, N 3.40. 31P NMR (CD2Cl2, 293 K): δ = 1.4; J(31P–195Pt)
= 3459 Hz. 1H and 13C NMR spectroscopic data are reported in
Table 1 and Table 2, respectively.
CCDC-282001 (for 2) and -282002 (for 1) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Magnetic Moment Measurements on Complex 1 in Solution: Mea-
surements were run on CH2Cl2 and ClCH2–CH2Cl solutions of the
complex, at temperatures below and above 293 K, respectively. The
method described by Evans[20] was adopted using a 5-mm NMR
tube with a 2-mm coaxial capillary inside containing the pure sol-
vent. The solutions (5×10–2 m) were prepared by dissolving
15.8 mg (25 μmol) of 1 into 0.5 mL of the appropriate solvent.
Spectra were recorded at 183, 193, 213, 233, 253, 273, 293, 313,
333 and 343 K.
Acknowledgments
This research has been financially supported by the Italian Minis-
tero dell’Istruzione, dell’Università e della Ricerca (MIUR), Rome.
We wish to thank Mr. P. Polese for carrying out elemental analyses.
General Procedure for the Heck Reaction: Under argon, a 10-mL
Schlenk flask containing a magnetic stir bar was charged with the
catalyst (2.0 mg for each single experiment, 1 mol-%) and appropri-
ate amounts of aryl halide (1.0 equiv.), alkene (1.2 equiv.) and base
(1.2 equiv.). After addition of the solvent (2 mL), the flask was
capped with a silicon stopper and heated at 120 °C in an oil bath.
The reaction mixture was extracted from the flask by syringe after
4, 8 and 20 h. The samples were treated with water, extracted with
ethyl ether, dried with Na2SO4 and then analysed by GC.
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for 1 and 2.
1
2·0.5CH2Cl2
Formula
Mr
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C26H23ClF6N2NiP2 C26.5H24Cl2F6N2P2Pd
633.56
723.72
triclinic
triclinic
¯
¯
P1
P1
10.125(3)
12.214(3)
13.101(4)
103.51(2)
111.03(3)
106.81(2)
1340.5(7)
2
8.544(2)
13.097(4)
13.643(4)
102.97(2)
103.63(2)
97.57(2)
1418.1(7)
2
α [°]
β [°]
γ [°]
Volume [Å3]
Z
Dcalcd. [g cm–3]
μ (Mo-Kα) [mm–1]
F(000)
1.570
1.695
3.647
644
8.627
722
θmax [°]
64.56
64.70
20902
Reflections collected 15767
Unique reflections
Rint
3954
0.0751
3404
343
4352
0.0438
4024
355
1.073
0.0564
0.1521
0.885, –0.984
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Observed I Ͼ 2σ(I)
Parameters
Goodness of fit (F2) 1.078
R1 [I Ͼ 2σ(I)][a]
0.0631
0.1808
0.364, –0.429
[b]
wR2
Δρ [e/Å3]
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4713

A. Del Zotto, E. Zangrando, W. Baratta, A. Felluga, P. Martinuzzi, P. Rigo
FULL PAPER
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Received: June 1, 2005
Published Online: October 25, 2005
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Eur. J. Inorg. Chem. 2005, 4707–4714