Full text of DOI:10.1134/S1070363217110159

ISSN 1070-3632, Russian Journal of General Chemistry, 2017, Vol. 87, No. 11, pp. 2605–2611. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © M.A. Kinzhalov, A.V. Buldakov, A.V. Petrov, K.T. Mahmudov, A.Yu. Ivanov, V.V. Suslonov, 2017, published in Zhurnal Obshchei
Khimii, 2017, Vol. 87, No. 11, pp. 1865–1871.
Low-Temperature Equilibriums in Solutions
of Isocyanide-Phosphine Complexes of Palladium(II) Chloride
M. A. Kinzhalova*, A. V. Buldakova, A. V. Petrova,
K. T. Mahmudovb, A. Yu. Ivanova, and V. V. Suslonova
a St. Petersburg State University, Universitetskaya nab. 7–9, St. Petersburg, 199034 Russia
*e-mail: m.kinzhalov@spbu.ru
b Technical University of Lisbon, Lisbon, Portugal
Received September 21, 2017
Abstract―cis-[PdCl2(CNR)(PPh3)] [R = Cy, t-Bu, C(Me)2CH2C(Me)3] have been synthesized via the interaction
of [(PPh3)ClPd(μ-Cl)2PdCl(PPh3)] with isocyanide in CH2Cl2 at room temperature with 90–98% yield and
characterized by means of mass spectrometry as well as 1H, 13C{1H, 31P}, and 31P{1H} NMR spectroscopy. The
complexes structure in the solid phase has been elucidated by means of X-ray diffraction analysis. Dynamic
processes in the solutions of the complexes in CDCl3 and CD2Cl2 at temperature of –95 to 60°С have been
1
studied by means of H and 31P NMR spectroscopy. It has been found that the studied compounds existed
exclusively in the cis-[PdCl2(CNR)(PPh3)] form in the solutions. In the case of cis-[PdCl2(CNCy)(PPh3)] in
CH2Cl2, the conformational transitions of the equilibrium forms (the transition of the substituent in the
cyclohexyl cycle between the equatorial and axial positions) are slowed down, the equatorial conformer
prevailing in the solution (2 : 1 at –95°С). Quantum-chemical simulation (DFT) has revealed that the standard
Gibbs energy of the conformational transition from the axial form of cis-[PdCl2(CNCy)(PPh3)] into the
equatorial one in the CH2Cl2 solution at 178 K equals –2.5 kJ/mol, being in agreement with the experimental data.
Keywords: palladium complex, isocyanide, phosphine, tautomeric equilibrium
DOI: 10.1134/S1070363217110159
Palladium complexes with N-heterocyclic carbene
ligands [Pd]-(NHC) [1–4] and structurally similar
acyclic diaminocarbenes [Pd]-(ADC) [5–7] have been
found promising new generation of catalysts for
various organic processes. Diaminocarbene complexes
of palladium are generally insensitive to the presence
of air oxygen or moisture and allow performing the С–С
cross-coupling reactions under mild conditions and
with small amount of the catalyst [8–13]. The most
common and promising approach to the preparation of
such palladium catalysts is based on the metal-
promoted coupling (including in situ reactions) of
coordinated isocyanides with N-nucleophiles [14–18].
The use of complexes with mixed ligand surrounding
(containing the isocyanide and other neutral ligands
[5–7], for example, phosphines [12]) as the precursors
is of special interest. Such approach allows fine tuning
of electronic and spatial properties of the catalysts
[5–7]. Indeed, coupling of isocyanide-phosphine
palladium(II) complexes with amines [19, 20], nitril-
imines [21], nitrile ylides [22], and 1,3-diimino-
isoindoline [12] has led to various cyclic [19–22] and
acyclic [12] diaminocarbene complexes, efficient
catalysts of the Suzuki reaction [12]. This has triggered
the interest to structure of isocyanide-phosphine com-
plexes of palladium(II) and their behavior in the solution.
Compounds of the [PdCl2(isocyanide)(phosphine)]
type have been relatively scarcely mentioned in the
literature [19, 20, 23–31]. Only a few of them have
been comprehensively studied, including the crystal
structure [25–31]. The compounds found in the
Cambridge Crystallographic Data Centre (CCDC)
exhibit cis configuration in the solid state [25–31]. At
the same time, using the example of the cis-
[PdCl2(CNXyl)(PPh3)] complex, it has been shown
[25] that disproportionation occurs in the solution, and
the complex exists as the cis-[PdCl2(CNXyl)2] and
trans-[PdCl2(PPh3)2] forms in equilibrium. The data on
the behavior of other isocyanide-phosphine complexes
2605

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KINZHALOV et al.
Scheme 1.
Ph3P
Cl
Cl
Cl
Cl
Cl
CNR
PPh3
CH2Cl2
+ 2 CNR
2
Pd
Pd
Pd
20oC
Cl
PPh3
1
R = Cy (2, 5), t-Bu (3, 6), C(Me)2C(Me)3 (4, 7).
of palladium(II) in the solution are absent, and this
lack of knowledge limits further development in the
synthesis of diaminocarbene complexes. Hence, the
study of the stability of isocyanide-phosphine com-
plexes of palladium(II) in the solution is a critical
issue. Herein, we report on the synthesis of isocyanide-
phosphine complexes of palladium(II) chloride cis-
[PdCl2(CNR)(PPh3)] with aliphatic isocyanide ligands
and the study of their structure in the solution and in
[34–37]. According to spin-spin interaction of the
isocyanide carbon atoms with the 14N quadrupole
nucleus, their signals have a fine structure and consist
of three lines of equal intensity [33]. The 31P{1H}
NMR spectra of complexes 5–7 in CDCl3 or CD2Cl2
contained the only signal with chemical shift of
28 ppm. The signals of the phosphorus atoms of
compounds 5–7 were observed at the range typical of
isocyanide-phosphine complexes of palladium(II) [25]
and were shifted to higher frequency as compared to
bisphosphine complexes of palladium(II) chloride [38].
Hence, the only compound, cis-[PdCl2(CNR)(PPh3)],
existed at room temperature in the solution in CDCl3.
1
the solid state using the methods of H, 13C{1H,31P},
and 31P{1H} NMR spectroscopy, X-ray diffraction, and
quantum-chemical simulations (DFT).
The cis-[PdCl2(CNR)(PPh3)] complexes [R = Cy (5),
t-Bu (6), C(Me)2C(Me)3 (tmbu) (7)] were synthesized
using a procedure adopted from [32] via the interaction
of dimeric chloride-phosphine palladium(II) complex
[(PPh3)ClPd(μ-Cl)2PdCl(PPh3)] (1) with isocyanides
2–4 in CH2Cl2 at room temperature. The com-pounds
were isolated as large pale-yellow crystals in 90–98%
yield. The X-ray diffraction study showed that the
crystals had the same unit cell parameters cor-
responding to cis-[PdCl2(CNR)(PPh3)] (5–7) (Scheme 1).
The X-ray diffraction data showed that the metal
centers of complexes 5–7 exhibited almost undistorted
square-planar geometry with cis-positioned chloride
ligands (Figs. 1–3). The Pd–Cl1 bonds (2.338–2.355 Å)
were somewhat longer than the Pd–Cl2 ones (2.296–
2.310 Å), due to the stronger trans-effect of the
phosphine ligand as compared to the isocyanide one.
The C≡N bonds length (1.135–1.151 Å) was typical of
other isocyanide complexes [39–43].
Compounds 5–7 were characterized by means of
mass spectrometry, IR and NMR (1H, 13C{1H, 31P},
and 31P{1H}) spectroscopy. The structure of the
complexes in the solid state was elucidated by means
of X-ray diffraction analysis.
To investigate the dynamic processes in the
solutions of complexes 5–7 in CDCl3 and CD2Cl2, we
1
recorded the H and 31P NMR spectra over a wide
temperature range (–95 to 25°С for CD2Cl2 and 25 to
60°С for CDCl3, with 15°С step). The heating of
solutions of complexes 5–7 in CDCl3 from 25 to 60°С
did not lead to any structural changes as per 1H and 31P
NMR spectroscopy data.
IR spectra of complexes 5–7 contained strong ab-
sorption band of the ν(C≡N) stretching with maximum
at about 2230 cm–1. The increase in the frequency of
the ν(CN) band upon isocyanide coordination {for
example, ν(CN) band of free CNCy is observed at
2140 cm–1 [33]} pointed at the increase in the
electrophilic character of the isocyanide carbon atom
and, hence, indirectly suggested their enhanced
reactivity with respect to nucleophiles [15]. In the 13C
NMR spectra, coordination of isocyanide at the
palladium atom was accompanied by sharp change in
the chemical shift of the terminal carbon atom to lower
frequency {δC 123.43 (5), 154.5–156.0 ppm (2–4)
[33]}, similarly to other isocyanide complexes
Cooling of the solutions of complexes 6 and 7 in
CD2Cl2 to –95°С did not result in any structural
1
changes as detected by H and 31P NMR spectroscopy
31
as well; at the same time, the 1H and P NMR spectra
of complex 5 revealed temperature-induced changes.
1
In the H NMR spectra of complex 5 in CD2Cl2, the
signal of the methine hydrogen atom of the cyclohexyl
ring observed at room temperature as multiplet (3.38–
3.49 ppm) was strongly broadened at –50°С and
further split (below –65°C) into a broadened triplet
(3.31 ppm, J = 11.8 Hz) and a singlet (3.71 ppm), their
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LOW-TEMPERATURE EQUILIBRIUMS IN SOLUTIONS
2607
Fig. 1. General view of a molecule of complex 5 in the
crystal.
Fig. 2. General view of a molecule of complex 6 in the
crystal.
intensities ratio being 2 : 1 at –95°С (Fig. 4). Similar
pronounced broadening at –50°С was observed also
for the methylene hydrogen atoms of the cyclohexyl
ring, whereas the signals of protons of the triphenyl-
phosphine ligand were not noticeably broadened. In
the 31P NMR spectra of complex 5 in CD2Cl2, the
signal observed at 27.8 ppm at room temperature was
strongly broadened at –50°С and split into two signals
upon further cooling (δ 28.6 and 29.0 ppm, the
intensities ratio 2 : 1 at –95°С) (Fig. 5). The observed
changes could be explained by the decelerated
transition of the cyclohexyl ring substituent between
the equatorial and axial positions, resulting in the
predominance of the equatorial conformer, cis-
[PdCl2(CNR)(PPh3)] complex, in line with the data on
low-temperature conformational equilibrium for other
palladium(II) complex with aromatic isocyanide ligand
cis-[PdCl2(CNXyl)(PPh3)] which exists in equilibrium
with the disproportionation products cis-[PdCl2
(CNXyl)2] and trans-[PdCl2(PPh3)2] [25], the com-
plexes with aliphatic ligands obtained in this study
existed exclusively in the cis-[PdCl2(CNR)(PPh3)] form
over the –95 to 60°С temperature range in the solutions.
EXPERIMENTAL
Commercially available chemicals were used as
received, except for CH2Cl2 which was dried over
P2O5. The [Pd2Cl4(PPh3)2] complex was prepared as
described elsewhere [45].
Mass spectrometry analysis was performed using a
Bruker micrOTOF (Bruker Daltonics) spectrometer
(electrospray ionization, methanol as solvent). The m/z
values were given for the isotopologs with the highest
1
substituted cyclohexanes [44]. Hence, the H and 31P
NMR data revealed that the solutions of complexes
5–7 in CDCl3 and CD2Cl2 contained the only form of
the complex over the –95 to 60°С range, cis-
[PdCl2(CNR)(PPh3)].
The DFT quantum-chemical simulations allowed
estimation of standard Gibbs energy of the tautomeric
transformation between the equatorial and axial forms
of compound 5 in CH2Cl2 at –95°C (178 K). The
obtained value of –2.5 kJ/mol agreed with the experi-
mental data.
In summary, we synthesized a series of isocyanide-
phosphine complexes of palladium(II) with aliphatic
isocyanide ligands, cis-[PdCl2(CNR)(PPh3)] [R = Cy,
t-Bu, C(Me)2CH2С(Me)3], and investigated the equilib-
riums in their solutions in chloroform and dichloro-
1
methane by means of H and 31P spectroscopy. In
Fig. 3. General view of a molecule of complex 5 in the
crystal.
contrast to the earlier described isocyanide-phosphine
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 11 2017

2608
KINZHALOV et al.
δ, ppm
δ, ppm
Fig. 4. 1H NMR spectra of a solution of cis-[PdCl2(CNCy)·
(PPh3)] complex (5) in CD2Cl2 at different temperatures:
(1) –95, (2) –65, (3) –50, (4) –20, and (5) 10°С.
Fig. 5. 31P{1H} NMR spectra of a solution of cis-
[PdCl2(CNCy)(PPh3)] complex (5) in CD2Cl2 at different
temperatures. (1) –95, (2) –65, (3) –50, (4) –20, and (5) 25°С.
content. IR spectra were recorded using a Shimadzu
IRAffinity-1 spectrometer (4000–400 cm–1, KBr
Crystallographic Data Centre database (CCDC 1574362,
1574720, and 1574721 for complexes 5–7, respectively).
1
pellets). H, 13С{1H,31P}, and 31P{1H} NMR spectra
Quantum-chemical simulations were performed
using the density functional theory (DFT) method with
the HCTH exchange-correlation functional [50, 51] in
DMol3 software package [52, 53]. The geometry was
optimized using the DNP basis set, the solvation
effects were accounted for using the COSMO solvent
model [54, 55]. CH2Cl2 was chosen as the solvent.
were recorded using a Bruker Avance II+ spectrometer
[500.03 (1Н), 125.73 (13C), and 202.42 MHz (31P)],
T 25±1°C if not stated otherwise, СDCl3 and CD2Cl2
as solvents.
X-ray diffraction analysis was performed using
SuperNova and Xcalibur (Agilent Technologies) diffracto-
meters (100 K, monochromatic MoKα radiation). Unit
cell parameters were refimed by means of least squares
method basing on 9555, 3889, and 5339 reflections
over 2θ 6°–55°. The structures were solved via the
Charge Flipping method implemented in Superflip
software [46] and refined using SHELXL software
[47] integrated in the OLEX2 graphical shell [48]. The
absorption was accounted for using CrysAlisPro
software package [49] using the multi-scan method
implemented in the SCALE3 ABSPACK algorithm.
Hydrogen atoms were refined in the calculated
positions. The structures were deposited at the Cambridge
Synthesis of cis-[PdCl2(CNR)(PPh3)] complexes.
A solution of the corresponding isocyanide 2–4
(0.10 mmol) in CH2Cl2 (2 mL) was added dropwise to
a suspension of [Pd2Cl4(PPh3)2] 1 (46 mg, 0.05 mmol)
in CH2Cl2 (2 mL). The product was precipitated via
vapor diffusion of Et2O into the reaction mixture at
room temperature. The formed yellow crystals were
separated off and dried in air at room temperature.
cis-[PdCl2(CNCy)(PPh3)] (5). Yield 52 mg (95%).
IR spectrum, ν, cm–1: 3067, 2930, 2860 (C–H); 2240
(C≡N). 1H NMR spectrum (CDCl3), δ, ppm: 1.14–1.62
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 11 2017

LOW-TEMPERATURE EQUILIBRIUMS IN SOLUTIONS
2609
m (10H, CH2, Cy), 3.38–3.49 m (1H, CH, Cy), 7.41–
7.49 m (6H, m-CH), 7.50–7.55 m (3H, p-CH), 7.68–
7.76 m (6H, o-CH). 13C{1H,31P} NMR spectrum
(CD2Cl2), δС, ppm: 22.30 (CH2), 24.35 (CH2), 31.15
(CH2), 55.10 (CH, Cy), 123.43 (C≡N), 128.64 (o-CH),
129.29 (ipso-C), 131.69 (p-CH), 134.36 (m-CH). 31P
{1H} NMR spectrum (CDCl3): δР 27.82 ppm. Mass
spectrum (ESI), m/z: 570.0130 [M + Na]+ (calculated
for C25H26NCl2NaPPd+: 570.0107). X-ray diffraction
analysis parameters: C25H26Cl2NPPd, M 548.74, T
100(2) K, monoclinic, space group P21/n, a 13.1335(5),
b 10.7142(4), c 17.2936(6) Å, β 108.165(4), V
2312.17(15) Å3, Z 4, dcalc 1.576 g/cm3, µ 1.116 mm‒1,
crystal size 0.2×0.2×0.2 mm, 21120 total reflections,
5296 independent reflections with I > 2σ(I) (Rint
0.0523), R1(|F0| ≥ 4σF)/R1 (overall) = 0.0295/0.0386,
wR2 (|F0| ≥ 4σF)/wR2 (overall) = 0.0650/0.0722,
ρmax/ρmin = 0.61/–0.82 e/Å3.
CH3, tmbu), 1.22 s (6H, CH3, tmbu), 1.39 s (2H, CH2,
tmbu), 7.50–7.57 m (6H, m-CH), 7.58–7.64 m (3H,
p-CH), 7.72–7.79 m (6H, o-CH). 13C{1H,31P} NMR
spectrum (CDCl3), δС, ppm: 30.01 (CMe3), 30.80
[C(Me)2CH2], 31.62 (CH2CMe3), 52.94 (CH2), 128.78
(o-CH), 129.58 (ipso-C), 131.78 (p-CH), 134.55
(m-CH); the signals of isocyanide carbon atom and the
carbon atom adjacent to the isocyanide group were
note detected. 31P{1H} NMR spectrum (CDCl3): δР
27.80 ppm. 31P{1H} NMR spectrum (CD2Cl2): δР
27.66 ppm. Mass spectrum (ESI), m/z: 600.0574 [M +
Na]+ (calculated for C27H32NCl2NaPPd+: 600.0576). X-
ray diffraction analysis parameters: C27H32Cl2NPPd,
M 578.80, T 100(2) K, rhombic, space group Pbca,
a 17.7369(5), b 13.7382(4), c 21.8928(6) Å, V
5334.7(3) Å3, Z 8, dcalc 1.441 g/cm3, µ(MoKα) 0.971
mm–1, crystal size 0.1×0.1×0.1 mm, 16534 total
reflections, 6122 independent reflections with I ≥ 2σ(I)
6122 (Rint 0.0351), R1(|F0| ≥ 4σF)/R1 (overall) =
0.0643/0.0744, wR2(|F0| ≥ 4σF)/wR2 (overall) =
0.1533/0.1592, ρmax/ρmin = 5.03/–1.90 e/Å3.
cis-[PdCl2(CNt-Bu)(PPh3)] (6). Yield 47 mg
(90%). IR spectrum, ν, cm–1: 3072, 2983, 2831 (C–H);
2227 (C≡N). 1H NMR spectrum (CDCl3), δ, ppm: 1.17
s (9H, CH3, t-Bu), 7.45–7.52 m (6H, m-CH), 7.53–7.58
1
ACKNOWLEDGMENTS
m (3H, p-CH), 7.71–7.78 m (6H, o-CH). H NMR
spectrum (CD2Cl2), δ, ppm: 1.19 s (9H, CH3, t-Bu),
7.49–7.57 m (6H, m-CH), 7.57–7.65 m (3H, p-CH),
7.72–7.80 m (6H, o-CH). 13C{1H,31P} NMR spectrum
(CDCl3), δС, ppm: 29.38 (Me), 128.76 (o-CH), 129.58
(ipso-C), 131.78 (p-CH), 134.52 (m-CH); the signals
of isocyanide carbon atom and the carbon atom
adjacent to the isocyanide group were note detected.
31P{1H} NMR spectrum (CDCl3, 25°C): δР 27.74 ppm.
31P{1H} NMR spectrum (CD2Cl2): δР 27.56 ppm. Mass
spectrum (ESI), m/z: 543.9948 [M + Na]+ (calculated
for C23H24NCl2NaPPd+: 543.9950). X-ray diffraction
analysis parameters: C23H24Cl2NPPd, M 522.70, T 100
(2) K, monoclinic, space group P21/n, a 9.4580(5), b
13.2701(8), c 17.7557(12) Å, β 95.861(5), V 2216.9(2)
Å3, Z 4, dcalc 1.566 g/cm3, µ(MoKα) 1.159 mm‒1,
crystal size 0.2×0.2×0.2 mm, 10121 total reflections,
5086 independent reflections with I ≥ 2σ(I) (Rint
0.0450), R1(|F0| ≥ 4σF)/R1 (overall) = 0.0367/0.0436,
This study was financially supported by the Russian
Science Foundation (project no. 14-43-00017-P).
Measurements were performed at the Center for
Magnetic Resonance, Center for X-ray Diffraction
Studies, Center for Chemical Analysis and Materials
Research and Chemistry Educational Centre (all in
Saint Petersburg State University).
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