Synthesis, crystal structure and VT-NMR study of cis-[PdCl 2(CNC6H3Me2-2,6)(PPh3)]

Article abstract of DOI:10.1016/j.ica.2011.11.038

The complex cis-[PdCl₂(CNC₆H₃Me ₂-2,6)(PPh₃)] has been prepared and fully characterized. Its crystal structure has been determined showing the chloro ligands in cis-disposition. A VT-NMR study shows this complex to be in equilibrium with trans-[PdCl₂(PPh₃)₂] and cis-[PdCl ₂(CNC₆H₃Me₂-2,6)₂]. The equilibrium is fast at room temperature but at -50 °C all three species are observed in solution in 10:1:1 M ratio. The activation parameters for this equilibrium have been calculated.

Full text of DOI:10.1016/j.ica.2011.11.038

Inorganica Chimica Acta 382 (2012) 203–206
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Note
Synthesis, crystal structure and VT-NMR study of cis-[PdCl2(CNC6H3Me2-2,6)(PPh3)]
Antonio Jesús Martínez-Martínez a, , María Teresa Chicote a, Delia Bautista b
⇑
a Grupo de Química Organometálica, Departamento de Química Inorgánica, Universidad de Murcia, Spain
b SAI, Universidad de Murcia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2011
Received in revised form 9 November 2011
Accepted 12 November 2011
Available online 26 November 2011
The complex cis-[PdCl2(CNC6H3Me2-2,6)(PPh3)] has been prepared and fully characterized. Its crystal
structure has been determined showing the chloro ligands in cis-disposition. A VT-NMR study shows this
complex to be in equilibrium with trans-[PdCl2(PPh3)2] and cis-[PdCl2(CNC6H3Me2-2,6)2]. The equilibrium
is fast at room temperature but at ꢀ50 °C all three species are observed in solution in 10:1:1 M ratio. The
activation parameters for this equilibrium have been calculated.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Phosphine/isocyanide mixed ligand
complex
Rearrangement equilibrium
VT-NMR study
Activation parameters
1. Introduction
and its variable temperature (VT) 1H, 13C and 31P NMR spectra
showing it to be in equilibrium with cis-[PdCl2(CNXy)2] (2) and
Compounds of the type [PdCl2(isocyanide)(phosphine)] are
rather scarce and only a few of them have been fully characterized
including their crystal structures [1–5]. As far as we are aware, all
of them are of cis-geometry with the only exception of trans-
trans-[PdCl2(PPh3)2] (3).
As far as we are aware, rearrangement equilibria of the type de-
scribed in this paper, 2ꢁ1 ¡ 2 + 3, is unprecedented for these mixed
ligand complexes.
[PdCl2(CNsecBu){PPh(
a-naphthyl)(C6H4R-4)}] (R = Ph, OMe) [6],
attributable to the mutual repulsion of the sterically demanding
neutral ligands if they were in cis position. They have been pre-
pared by rather inefficient, multistep methods, namely (i) by react-
2. Experimental
2.1. Materials
ing complexes of the types [PdCl2(PR3)2] [7], [PdCl(l-Cl)(PR3)]2 [1]
or [PdCl2(P^N)] (P^N = chelating imine–phosphine [3] or imino-
phosphorane-phosphine [5]) with isocyanide; (ii) by reacting com-
plexes cis-[PdCl2(CNR)2] with phosphine [6,8] (some of these
reactions have been used to resolve racemic tertiary phosphines);
[6] (iii) by reacting [PdCl2(CNR)2] with rhodium or palladium
Acetone, Et2O and CH2Cl2 were distilled before use, from
KMnO4, Na/benzofenone and CaH2, respectively. CHCl3 (Baker),
CDCl3 (SDS) and the reagents PdCl2 (Johnson Matthey), XyNC and
PPh3 (Fluka) were used as purchased.
complexes of the types [RhCl(cod)(P^N)] [9,10], [Rh(g
5-C5Me5)
(P^N0)C12] [11], (cod = 1,5-cyclooctadiene, P^N = PPh2(2-pyOMe-
6)), or [Pd{CH2C(Me)CH2}(P^N0)] (P^N0 = PPh2(2-py)) [4]. The reac-
tivity of these complexes towards amines [12,13] nitrilimines [14],
nitrilylides [15], etc., has been explored leading to the synthesis of
a variety of cyclic carbene complexes.
Although cis-[PdCl2(CNXy)(PPh3)] (1, Xy = Xylyl = C6H3Me2-2,6)
had been previously prepared from cis-[PdCl2(CNXy)2] and PPh3
[16], it was only characterized by its melting point and IR spectrum
[17]. We report here the one-pot, 100% atom-efficient synthesis of
1 from equimolar amounts of PdCl2, XyNC and PPh3 (Scheme 1)
2.2. Physical measurements
Elemental analyses of carbon, hydrogen and nitrogen were car-
ried out with a Carlo Erba 1106 microanalyzer. The melting point
was determined on a Reichert apparatus and is uncorrected. The
IR spectrum was recorded on a Perkin-Elmer 16F PC FT-IR spec-
trometer in the range of 4000–200 cmꢀ1 region, with Nujol mulls
between polyethylene sheets. The NMR spectra were recorded in
a Bruker Avance 400 NMR spectrometer, every 10 °C in the range
of 25 to ꢀ50 °C using CDCl3 solutions. Chemical shifts are refer-
enced to TMS (1H and 13C{1H}) and H3PO4 31P{1H}). The NMR
(
⇑
assignments were performed with the help of APT, HMQC and
HMBC experiments.
Corresponding author. Tel.: +34 868 88 74 83; fax: +34 868 88 77 85.
E-mail address: ajmtinez@um.es (A.J. Martínez-Martínez).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.11.038

204
A.J. Martínez-Martínez et al. / Inorganica Chimica Acta 382 (2012) 203–206
Cl
Details of the crystal data and structure refinement of the complex
are listed in Table 1.
PdCl2 + PPh3 + XyNC
Cl Pd CNXy
PPh3
1
3. Results and discussion
Scheme 1. Synthesis of complex 1 from PdCl2, PPh3 and XyNC.
3.1. Synthetic aspects and general properties
2.3. Synthesis of cis-[PdCl2(CNXy)(PPh3)] (1) and VT 1H and 31P NMR
study of the equilibrium. 2ꢁcis-[PdCl2(CNXy)(PPh3)] ¡ cis-
[PdCl2(CNXy)2] + trans-[PdCl2(PPh3)2]
The method here described for the synthesis of complex 1, from
equimolar amounts of PdCl2, XyNC and PPh3 (Scheme 1) is more
straightforward and atom-efficient than any of those previously
described for complexes of this type, some of them being rather
cumbersome. In fact, with respect to the more efficient ones,
[PdCl2(CNR)2] + phosphine or [PdCl2(phosphine)2] + RNC, our
method not only avoids one step but also saves one equivalent of
isocyanide or phosphine, respectively.
To a suspension of PdCl2 (162.5 mg, 1.39 mmol) in acetone
(20 mL) were successively added XyNC (182 mg, 1.39 mmol) and
PPh3 (364 mg, 1.39 mmol). After 1 h of stirring, the resulting green-
ish-yellow suspension was filtered. The solid collected was dis-
solved in CHCl3 (10 mL), filtered through a short pad of Celite,
concentrated under vacuum (5 mL), and Et2O (25 mL) was added.
The suspension was filtered and the solid collected was dried, first
by suction and then in an oven at 70 °C for 7 h, to give cis-
[PdCl2(PPh3)(CNXy)] (1) as a pale yellow solid. Yield: 542 mg,
When we did the reaction in acetone, 1 precipitated from the
reaction mixture and was isolated in 70% yield after recrystalliza-
tion from CHCl3/Et2O, which was necessary in order to separate a
small amount of PdCl2. In spite of the fact that the variable
temperature NMR study described below proves that an equilib-
rium exists in CDCl3 solution between the mixed ligand complex
1, cis-[PdCl2(CNXy)2] (2) and trans-[PdCl2(PPh3)2] (3), the proce-
dure described in Section 2 leads to the precipitation of pure 1,
as shown by the coincidence of its melting point and IR spectrum
with those previously reported [18], and also with those obtained
from a crop of single crystals that we dried in an oven at 70 °C for
0.95 mmol, 70%. Mp: 232 °C (dec). IR (Nujol, cmꢀ1):
m(C„N) 2206
(strong),
m
(Pd–Cl) 347 (medium), 298 (strong). 1H NMR
(400 MHz, CDCl3, 25 °C, TMS): d 1.95 (s, 6H, Me), 6.98 (d, 2H,
3
3JHH = 7.6 Hz, meta-Xy), 7.18 (t, 1H, JHH = 7.6 Hz, para-Xy), 7.35–
7.50 (various m, 9 H, PPh3), 7.72 (‘‘d’’, 6H, ortho-PPh3, 3JHH = 7.2 Hz).
1H NMR (400 MHz, CDCl3, ꢀ50 °C, TMS): cis-[PdCl2(CNXy)(PPh3)]
3
7 h (margin of error:
DMp, 3 °C; Dm
, 3 cmꢀ1). The purity of the iso-
(1, 71.4%) d: 1.97 (s, 6H, Me), 7.03 (d, 2H, JHH = 7.6 Hz, meta-Xy),
7.23 (t, 1H, 3JHH = 7.6 Hz, para-Xy, overlapped with para-Xy of com-
plex 2), 7.46–7.55 (various m, 9H, meta- + para-PPh3, overlapped
with the homologous resonances in 3), 7.78 (dd, 6H, ortho-PPh3,
lated complex 1 could be attributed to the fact that 1 is the major
species in the reaction mixture (1:2:3 = 10:1:1 in CDCl3 at ꢀ50 °C)
and/or to differences in solubility. In fact, 1 was the only species
that crystallized when Et2O was layered over a solution of the
reaction mixture in CH2Cl2.
4
3JHH = 7.8 Hz, JHH = 4.8 Hz); cis-[PdCl2(CNXy)2] (2, 14.3%): 1.65 (s,
3
3H, Me), 6.95 (d, 2H, meta-Xy, JHH = 7.6 Hz), 7.20 (t, 1H, para-Xy,
3JHH = 7.6 Hz, overlapped with para-Xy of complex 1); trans-
[PdCl2(PPh3)2] (3, 14.3%): 7.46–7.55 (various m, 9H, meta- + para-
PPh3, overlapped with the homologous resonances in 1), 7.69 (m,
6H, ortho-PPh3). 13C{1H} APT NMR (100.8 MHz, CDCl3, 25 °C,
TMS): d 18.2 (Me), 127.9 (meta-Xy), 128.8 (br s, meta-Ph, PPh3),
130.3 (para-Xy), 131.8 (br s, para-Ph, PPh3), 134.4 (br s, ortho-Ph,
PPh3), 135.6 (ortho-Xy). Pd–CNXy and ipso-Ph(PPh3) not observed.
13C{1H} APT NMR (100.8 MHz, CDCl3, ꢀ50 °C, TMS): cis-
[PdCl2(CNXy)(PPh3)] (1) d: 18.1 (Me), 124.5 (Pd–CN), 127.7
(meta-Xy, coincident with meta-Xy for complex 2), 128.0 (d, ipso-
PPh3, JCP = 58.5 Hz), 128.6 (d, meta-PPh3, JCP = 12 Hz), 130.3 (para-
Xy), 131.9 (d, para-PPh3, JCP = 2 Hz), 134.0 (d, ortho-PPh3,
JCP = 11.0 Hz), 135.4 (ortho-Xy); cis-[PdCl2(CNXy)2] (2): d 17.8
(Me), 123.3 (Pd–CNXy), 127.7 (meta-Xy, coincident with meta-Xy
for complex 1), 130.9 (para-Xy), 134.3 (ortho-Xy); trans-
[PdCl2(PPh3)2] (3): d 126.8 (‘‘t’’, ipso-PPh3, N = 54.0 Hz), 129.0 (‘‘t’’,
meta-PPh3, N = 11 Hz), 132.1 (s, para-PPh3), 133.7 (‘‘t’’, ortho-PPh3,
N = 12 Hz). 31P{1H} NMR (162.3 MHz, CDCl3, 25 °C): d 26.2 (v br
3.2. Structure description
The crystal structure of 1 (Fig. 1 and Table 1) shows the
palladium atom in a slightly distorted square planar environment,
coordinated to a PPh3, a XyNC and two chloro ligands in mutually
cis-disposition. The bond distances and angles are similar to those
Table 1
Crystal data and structure refinement of complex 1.
Formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
C27H24Cl2NPPd
570.74
100(2)
triclinic
ꢀ
P1
9.8101(6)
11.8776(7)
12.0372(7)
87.239(2)
67.363(2)
78.693(2)
1268.82(13)
2
1.494
1.020
576
0.14 ꢂ 0.09 ꢂ 0.02
1.83–28.18
14774
5704 (0.0316)
0.9799–0.8555
0/291
b (Å)
c (Å)
s).
31P{1H}
NMR
(162.3 MHz,
CDCl3,
ꢀ50 °C):
cis-
a
(°)
b (°)
[PdCl2(CNXy)(PPh3)] (1): d 29.3; trans-[PdCl2(PPh3)2] (3): d 22.8.
Crystals of 1 suitable for an X-ray diffraction study grew at 4 °C
after layering Et2O over a CH2Cl2 solution containing an equimolar
mixture of PdCl2, PPh3 and XyNC. After drying the crystals in an
oven at 70 °C for 7 h, the melting point and IR spectrum coincided
with those of the crude material.
c
(°)
V (Å3)
Z
qcalcd (Mg mꢀ3
)
l
(Mo K
a
) (mmꢀ1
)
F(000)
Crystal size (mm)
h (°)
Number of reflections collected
Number of independent reflections (Rint
Transmission
2.4. X-ray crystallography
)
Complex 1 was measured on a Bruker Smart APEX machine.
Restraints/parameters
Goodness-of-fit (GOF) on F2
1.057
0.0373
0.0820
0.802/ꢀ0.526
Data were collected using monochromated Mo K
a radiation. The
R1 (I > 2
wR2 (all reflections)
Largest difference in peak and hole (eÅꢀ3
r(I))
structure was solved by direct methods. It was refined anisotropi-
cally on F2. The methyl groups were refined using rigid groups, and
the other hydrogen atoms were refined using a riding model.
)

A.J. Martínez-Martínez et al. / Inorganica Chimica Acta 382 (2012) 203–206
205
Fig. 1. Crystal structure of complex 1. Thermal ellipsoid representation plot (50%
probability) of complex 1. Selected bond lengths (Å) and angles (°): Pd(1)–C(1)
1.916(3), Pd(1)–P(1) 2.2607(7), Pd(1)–Cl(1) 2.3079(7), Pd(1)–Cl(2) 2.3463(7), C(1)–
N(1) 1.152(4). C(1)–Pd(1)–P(1) 93.58(8), P(1)–Pd(1)–Cl(1) 88.86(3), C(1)–Pd(1)–
Cl(2) 84.08(9), Cl(1)–Pd(1)–Cl(2) 93.29(3), N(1)–C(1)–Pd(1) 173.7(3).
found in the few other palladium complexes of this type character-
ized by their crystal structures [1–5]. The different Pd–Cl bond dis-
tances, Pd(1)–Cl(1) 2.3079(7), Pd(1)–Cl(2) 2.3463(7) found in 1,
and also in its homologous complexes, account for the greater trans
influence of phosphines with respect to isocyanides.
Fig. 2. Double zig-zag chain along the b axis in 1 resulting from C–HꢁꢁꢁCl hydrogen
bonds. Bond distances (Å) and angles (°): H(25)ꢁꢁꢁCl(2)#1, 2.77; C(25)ꢁꢁꢁCl(2)#1,
3.603; H(35)ꢁꢁꢁCl(1)#2, 2.73; C(35)ꢁꢁꢁCl(1)#2, 3.519; C(25)–H(25)–Cl(2)#1, 146.8;
C(35)–H(35)–Cl(1)#2, 141.2.
Non-classical hydrogen bonds of the type C–HꢁꢁꢁCl form in
which two C–H groups, each of them on one phenyl ring of the
same molecule, connect the chloro ligands of two other molecules,
giving rise to a double zig-zag chain along the b axis (Fig. 2).
3.3. VT-NMR studies
3.3.1. The 2ꢁ1 ¡ 2 + 3 equilibrium
At room temperature, the 1H NMR spectrum of 1 showed the
expected resonances. However, a very broad signal in the 31P
NMR spectrum, along with the fact that 13C APT spectrum did
not show the ipso-C resonance for the PPh3 ligand and the remain-
ing phosphine nuclei gave broad resonances instead of the ex-
pected doublets, suggested to us that a dynamic process could be
occurring in solution that we decided to investigate by means of
a variable temperature NMR study.
We show below VT 1H and 31P NMR spectra measured, every
10 °C, in the ranges of 25 to ꢀ50 °C (1H, Fig. 3) or 25 to ꢀ20 °C
(
31P, Fig. 4), respectively. Fig. 3 shows, for instance, that the 1H
NMR resonances appearing at room temperature at 1.95 (Me)
and 6.98 (meta-Xy) ppm, coalesce at ꢀ5 and ꢀ15 °C, respectively,
and each of them clearly splits, at ꢀ50 °C into two resonances at
1.97 and 1.65 (Me) or at 6.95 and 7.03 (meta-Xy) ppm, with 1:5
intensities ratio in both cases. We assigned the resonances at
1.65 and 6.95 to complex 2, based on the spectrum of a pure sam-
ple of this complex measured at ꢀ50 °C, which means that the
more intense resonances at 1.97 and 7.03 ppm correspond to com-
plex 1 and that the 1:2 M ratio is 10. Similarly, the doublet at
7.72 ppm in the room temperature NMR spectrum, assigned to
the ortho-PPh3 protons, coalesces at around ꢀ10 °C and splits at
ꢀ50 °C into a doublet of doublets and an apparent quartet centred
at 7.78 and 7.69 ppm, respectively, with 5:1 intensities ratio. In
turn, the very broad resonance appearing in the room temperature
31P NMR spectrum at around 26 ppm, splits below 15 °C into two
Fig. 3. VT 1H NMR spectra (400 MHz, CDCl3) showing the resonances of complexes
cis-[PdCl2(CNXy)(PPh3)] (1, circle), cis-[PdCl2(CNXy)2] (2, square) and trans-
[PdCl2(PPh3)2] (3, triangle).
resonances (22.8 and 29.3 ppm at ꢀ20 °C) with 1:5 intensities ratio
(Fig. 4). We attribute them to complexes 3 and 1, respectively,
based on the 31P NMR spectrum of a pure sample of 3 measured
at the same temperatures. The same behaviour can be deduced

206
A.J. Martínez-Martínez et al. / Inorganica Chimica Acta 382 (2012) 203–206
limiting step in the overall process, even though the formation of
a chloro-bridged dinuclear intermediate can not be ruled out.
Since, in the absence on an added ligand, dissociation of 1 is neces-
sary in order to provide the PPh3 and XyNC ligands responsible for
the nucleophilic attacks leading to 2 and 3, the robustness of the
Pd–PPh3 and Pd–CNXy bonds could account for the
found.
D
S– value
4. Conclusion
We describe a highly atom-efficient, one-pot synthesis of com-
plex cis-[PdCl2(CNXy)(PPh3)] and the equilibrium in solution be-
tween this complex, cis-[PdCl2(CNXy)2] and trans-[PdCl2(PPh3)2]
which is unprecedented for this type of complexes.
Fig. 4. VT 31P NMR spectra (162.3 MHz, CDCl3) showing the resonances of cis-
[PdCl2(CNXy)(PPh3)] (1, circle) and trans-[PdCl2(PPh3)2] (3, triangle).
Acknowledgements
Cl
Cl
Cl
We thank Ministerio de Educación y Ciencia (Spain), FEDER
(CTQ2007-60808/BQU) and Fundación Séneca (04539/GERM/06
and 03059/PI/05) for financial support and for a grant to A.J.M.M.
+
2
Cl Pd CNXy
Cl Pd CNXy
Ph3P Pd PPh3
PPh3
CNXy
Cl
1
2
3
Appendix A. Supplementary material
Scheme 2. Equilibrium between 1, 2 and 3.
CCDC 827505 contains the supplementary crystallographic data
for complex 1. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. The Supplementary material includes also the
checkcif for complex 1, the 13C{1H} APT NMR spectra showing
the resonances in the equilibrium 2ꢁ1 ¡ 2 + 3 at 25 and at ꢀ50 °C
and a VT NMR study allowing the determination of the rate con-
stants and activation parameters for this equilibrium. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.ica.2011.11.038.
from the 13C APT NMR spectra measured at 25 and ꢀ50 °C (see
Fig. S1, Supporting information). These data prove that an equilib-
rium exists in CDCl3 solution between complexes 1, 2 and 3
(Scheme 2). Its composition depends on the temperature and, at
ꢀ50 °C, is 10:1:1. The equilibrium is fast at room temperature
and the NMR spectra show the average resonances which, as ex-
pected, are closer to those of the most abundant species 1.
In support of the above conclusions is the fact that a CDCl3 solu-
tion containing equimolar amounts of independently prepared cis-
[PdCl2(CNXy)2] (2) and trans-[PdCl2(PPh3)2] (3) gave, at each tem-
perature, identical 1H, 13C and 31P NMR spectra as those described
above.
References
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3.3.2. Activation parameters for the 2ꢁ1 ¡ 2 + 3 equilibrium
Line shape analyses of both the Me (1H, 223–298 K) and the
PPh3 (31P, 223–253 K) resonances allowed us to determine a series
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enthalpy,
9 J Kꢀ1 molꢀ1) equations. From these data, the activation free en-
ergy can be calculated for given temperature (at 298 K,
G–298K = 48 1 kJ molꢀ1, for details see Supplementary material).
D
H– = 107 4 kJ molꢀ1
;
and entropy, D
S– = 199
a
D
We have not found in the literature any data to which our values
could be compared. Although it is likely that the process we are
studying differ in many respects from common substitution reac-
tions, the high positive value found for the activation entropy is
still surprising and suggests that a dissociative process is the rate
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