Thermal behavior of volatile palladium(II) complexes with tetradentate Schiff bases containing propylene-diimine bridge

Article abstract of DOI:10.1007/s10973-018-7371-z

Palladium(II) complexes with tetradentate Schiff bases, Pd(acacpda)(I) and Pd(acacdmpda)(II) (acacpda^2?—N,N′-(propylene)-bis(acetylacetoneiminato-), acacdmpda^2?—N,N′-(2,2-dimethylpropylene)-bis(acetylacetoneiminato-)), have been synthesized, purified and characterized by elemental analysis, IR and NMR spectroscopy. Complex I was obtained for the first time, and its solid-state structure was investigated by single-crystal X-ray diffraction. It has a discrete molecular structure with the Pd²⁺ ion being in a distorted square-planar coordination environment formed by N and O atoms of the acacpda^2? ligand. The thermal properties of complexes I and II in condensed phase were investigated by thermogravimetry. Compound II transfers into the gas phase almost completely, while compound I evaporates with a partial decomposition. The temperature dependences of saturated vapor pressure were measured by the flow method in the temperature intervals (423–457) and (422–478) K for I and II, respectively. Both complexes were shown to exhibit an excellent stability upon sublimation. The thermodynamic parameters of sublimation processes were calculated to be: Δ_subH₄₄₀ = (133 ± 4) kJ mol^?1, Δ_subS₄₄₀⁰= (210 ± 9) J (mol K)^?1 for I, Δ_subH₄₅₀ = (135 ± 1) kJ mol^?1, Δ_subS₄₅₀⁰= (218 ± 3) J (mol K)^?1 for II. It has been found that the complex II with a more bulky Schiff base ligand possesses slightly higher vapor pressure values as compared to I, which is probably due to the lower contribution of weak intermolecular interactions occurring in the crystal structure of II.

Full text of DOI:10.1007/s10973-018-7371-z

Journal of Thermal Analysis and Calorimetry
https://doi.org/10.1007/s10973-018-7371-z(0123456789(). -, vo Vl
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Thermal behavior of volatile palladium(II) complexes with tetradentate
Schiff bases containing propylene-diimine bridge
1
Sergey A. Cherkasov Nataliya S. Nikolaeva1 • Anton I. Smolentsev1,2 •
2 •
Evgeniia S. Vikulova
•
1
1
Sergey V. Sysoev
•
Natalya B. Morozova
Received: 28 December 2017 / Accepted: 11 May 2018
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2018
Abstract
Palladium(II) complexes with tetradentate Schiff bases, Pd(acacpda)(I) and Pd(acacdmpda)(II) (acacpda —N,N -
2
-
0
2
propylene)-bis(acetylacetoneiminato-), acacdmpda —N,N -(2,2-dimethylpropylene)-bis(acetylacetoneiminato-)), have
-
0
(
been synthesized, purified and characterized by elemental analysis, IR and NMR spectroscopy. Complex I was obtained for
the first time, and its solid-state structure was investigated by single-crystal X-ray diffraction. It has a discrete molecular
2
structure with the Pd ion being in a distorted square-planar coordination environment formed by N and O atoms of the
?
2
-
acacpda ligand. The thermal properties of complexes I and II in condensed phase were investigated by thermo-
gravimetry. Compound II transfers into the gas phase almost completely, while compound I evaporates with a partial
decomposition. The temperature dependences of saturated vapor pressure were measured by the flow method in the
temperature intervals (423–457) and (422–478) K for I and II, respectively. Both complexes were shown to exhibit an
excellent stability upon sublimation. The thermodynamic parameters of sublimation processes were calculated to be:
-
1
0
-1
-1
0
-
DsubH440 = (133 ± 4) kJ mol , D
=
S
sub 440
= (210 ± 9) J (mol K)
for I, DsubH450 = (135 ± 1) kJ mol , D
sub 450
(218 ± 3) J (mol K) for II. It has been found that the complex II with a more bulky Schiff base ligand possesses
S
-
1
slightly higher vapor pressure values as compared to I, which is probably due to the lower contribution of weak inter-
molecular interactions occurring in the crystal structure of II.
Keywords Palladium(II) complexes Á Schiff bases Á MOCVD precursors Á Crystal structure Á Vapor pressure
Introduction
[9, 10] and in catalysis of a number of chemical reactions
such as oxidation, reduction, hydrogenation, cross-coupling
and others [10–15].
Nowadays, bimetallic films and nanoparticles based on
palladium are widely used in different fields of application,
for example, as materials for magnetooptical recording
devices [1, 2], biosensors for methotrexate detection [3],
water purification and decolorization systems [4, 5],
hydrogen purification membranes [6–8], hydrogen sensors
The perspective techniques to obtain such bimetallic
structures are metal organic chemical vapor deposition
(MOCVD) and related methods such as atomic layer
deposition (ALD), etc. The main advantages of these
methods are complete preside control over the properties
(composition and microstructure/size) of the produced
films and nanoparticles by means of variation a number of
deposition parameters as well as the possibility of forma-
tion of target structures on the complicated shape objects
and the absence of side components (for example, pro-
tecting agents in the case of the colloidal synthesis meth-
ods). The investigation of the formation of Pd-based
bimetallic systems by chemical vapor-phase deposition
techniques has been developing ever more actively since
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10973-018-7371-z) contains supplementary
material, which is available to authorized users.
&
Evgeniia S. Vikulova
lazorevka@mail.ru
1
Nikolaev Institute of Inorganic Chemistry, Siberian Branch
of the Russian Academy of Sciences, Acad. Lavrentiev Ave.
3
, Novosibirsk, Russia 630090
1990s: various structures have been obtained with a num-
2
Novosibirsk State University, Pirogova Street 2, Novosibirsk,
Russia 630090
ber of metals including silver [16, 17], platinum [18–21],
123

E. S. Vikulova et al.
ruthenium [21], gold [22], copper [23] and nickel [24–26].
However, it should be noted that in order to realize all the
advantages of the MOCVD and related methods, the
volatile precursors of both components apart from the
standard requirements for thermal properties (sufficient
volatility, thermal stability and others [27]) should possess
their experimental compatibility. Thus, in order to ensure
further progress in the area discussed, it is necessary to
expand a number of thermochemically characterized
volatile palladium complexes to select the optimal com-
binations of the precursors for effective deposition of each
bimetallic system.
Experimental
Materials and methods
The following reagents were used in the presented work:
palladium chloride (Krastsvetmet, palladium content
59.845%), acetylacetone (Acros Organics, purity [ 99%),
propylenediamine (Merck, purity [ 99%), 2,2-dimethyl-
propylenediamine (Aldrich, purity [ 99%). Acetonitrile
(Kriokhrom, purity [ 99%), hexane (‘‘Reaktiv’’, purity [
98%), toluene (‘‘Reaktiv’’ , purity [ 98%) were used as a
solvents. All of the reactants were used as supplied.
The elemental analysis was carried out on a CHNS-
analyzer vario MICRO cube with the accuracy 0.5%. IR
spectra were recorded on a Scimitar FTS 2000 spectrom-
To date, palladium complexes with the following
ligands have been proposed as volatile MOCVD/ALD
precursors: b-diketonates, b-iminoketonates, b-enamin-
odiones, dialkyldithiocarbamates, dialkyldithiophosphates,
cyclopentadienyls, allyls, alkenols, etc. [28–36]. This work
is devoted to a new class of palladium MOCVD precur-
sors—Pd(II) complexes with tetradentate Schiff bases
-
1
eter in the 400–4000 cm range; the samples were pre-
pared by pressing the pellets with KBr. NMR spectra were
recorded on an Avance 500 spectrometer at 500 MHz for
1
13
H NMR and 125 MHz for C NMR. The primary spectral
(
Fig. 1). Preliminary research has shown that the replace-
data are presented in Supplementary Materials. The melt-
ing points (m.p.) were determined by the visual method
using the Kofler bench.
ment of the ethylene diimine bridge in the ligand with
propylene one increases the volatility of the formed com-
plexes [37]. So, the objects of the present investigation are
palladium complexes with propylene-diimine bridge
containing Schiff bases: Pd(acacpda) I and Pd(acacdmpda)
Synthesis of Schiff bases and palladium
complexes
2
-
0
II (Fig. 1, acacpda —N,N -(propylene)-bis(acetylace-
2
-
0
toneiminato-), acacdmpda —N,N -(2,2-dimethylpropy-
lene)-bis(acetylacetoneiminato-)). The paper presents the
synthesis, detailed characterization and thermochemical
investigation of the complexes including the crystal struc-
ture of compound I obtained for the first time.
H acacpda was synthesized according to the method [38]
2
through the interaction of 2.15 mL (21.0 mmol) acety-
lacetone and 0.90 mL (10.5 mmol) propylenediamine by
means of stirring under reflux in the absence of a solvent.
The product was purified by recrystallization from hexane,
yield 80%. Anal. Calcd. for C H N O (mass. %): C
1
3 22 2 2
6
5.5, H 9.3, N 11.8. Found C 65.2, H 9.2, N 11.7. IR
-
spectrum (cm ): 726, 751, 1026, 1088, 1138, 1282, 1353,
1
1440, 1514, 1568, 1607, 2875, 2953, 3547.
H acacdmpda was synthesized and purified similarly to
2
X
H acacpda from 1.95 mL (18.8 mmol) of Hacac and
2
1.20 mL (9.41 mmol) of 2,2-dimethylpropylenediamine.
Yield 80%. Anal. Calcd. for C H N O : C 67.6, H 9.8, N
1
5 26 2 2
-
1
N
O
N
1
7
1
0.5. Found: C 67.7, H 9.7, N 10.7. IR spectrum (cm ):
51, 807, 975, 1023, 1104, 1197, 1290, 1359, 1442, 1521,
572, 1611, 2880, 2970, 3388.
Pd
Pd(acacpda)(I) was obtained according to modified
procedure [39]. Firstly, Pd(CH CN) Cl complex was
3
2
2
prepared by dissolving of 0.518 g (2.92 mmol) PdCl in
2
O
7
0 mL of acetonitrile at slight heating and then the 20 mL
of acetonitrile solution of H acacpda (0.696 g, 2.92 mmol)
2
was added to reaction mixture which was further kept for
X = CH CH CH →
2
2
2
Pd(acacpda) (I)
3
5
0 min. Then, the NaOH aqueous solution (0.234 g,
.85 mmol) was added. The product was separated as a
X = CH C(CH ) CH → Pd(acacdmpda) (II)
2
3 2
2
precipitate after 30 min of stirring at room temperature by
adding of 500 mL of water. An additional amount of the
Fig. 1 General structure formula of palladium(II) complexes with
tetradentate Schiff bases studied
1
23

Thermal behavior of volatile palladium(II) complexes with tetradentate Schiff bases containing…
product was obtained by toluene extraction of the filtrate.
The complex was purified by vacuum sublimation (443 K,
Data Centre under the reference number CCDC 1576778
and are available free of charge via www.ccdc.cam.ac.uk/
data_request/cif.
6
.67 Pa). Yield 62% (0.615 g). m. p. 468 K. Anal. Calcd.
for PdC H N O (I): C 45.6, H 5.9, N 8.2. Found: C 45.4,
1
3 20 2 2
-
H 6.0, N 8.1. IR spectrum (cm ): 423, 460, 758, 952,
1
Thermal investigation
1
2
015, 1119, 1252, 1274, 1359, 1405, 1466, 1512, 1577,
1
913, 2938. H NMR (CDCl , 7.26 ppm): 1.9564 (s, 6H,
The thermogravitmetric investigation (TG/DTA) was per-
formed on a Netzsch TG 209 F1 Iris thermal analyzer in an
3
CH C=N), 1.9767 (s, 6H, CH C=O), 2.0532 (m, 2H,
3
3
-
1
(
CH )CH (CH )), 3.0389 (t, 4H, (CH )CH (CH )), 4.8548
2
open Al O crucible in a helium flow (30.0 mL min ).
2 3
-1
The heating rate was 10 K min ; sample mass was
(10 ± 1) mg.
2
2
2
2
2
1
s, 2H, CH). C NMR (CDCl , 77.2 ppm): 23.0504
3
(
3
(
CH C=N), 24.5162 (CH C=O), 32.4997 (CH )CH (CH ),
2
3
3
2
2
5
1.2816 (CH), 99.4074 (CH –N), 164.3894 (C=N),
2
The temperature dependencies of saturated vapor pres-
sure of the complexes were measured by flow (transpira-
tion) method in the atmosphere of dry inert gas carrier
(helium) using the equipment presented in Fig. 2. The
principles of the method and the experimental procedure
used are described in [42, 43] and [44], respectively.
During the experiment, the substance in source 1 is trans-
ferred to the gas phase by heating and carried by the carrier
gas flow through the porous plugs 5 to the receiver 2 where
it condenses on the cold walls. The amounts of substance in
source and receiver were determined gravimetrically
1
76.7554 (C=O).
Pd(acacdmpda)(II) was synthesized in a similar way to
I from 0.333 g (1.88 mmol) of PdCl , 0.500 g (1.88 mmol)
2
of H acacdmpda, and 0.180 g (4.50 mmol) of NaOH. The
2
complex was purified by vacuum sublimation ((433–443)
K, 6.67 Pa). Yield 88% (0.543 g). m. p. 487 K. Anal.
Calcd. for PdC H N O (II): C 48.4, H 6.5, N 7.6. Found:
1
5 24 2 2
-
1
C 48.0, H 6.5, N 7.6. IR spectrum (cm ): 449, 469, 756,
9
1
1
52, 1013, 1112, 1232, 1273, 1332, 1365, 1398, 1456,
1
511, 1571, 2916, 2956. H NMR (CDCl , 7.26 ppm):
3
-
4
.1141 (s, 6H, C(CH ) ), 1.9522 (s, 6H, CH C=N), 1.9899
3
(uncertainty is 5 9 10 g). The sublimation is performed
by a calibrated heater 6 with a thermocouple sensor 7. The
temperature was measured by thermal control device 8
with uncertainty 0.5 K, the carrier gas flow was measured
by flow mass (11, 12), and helium flow rate was measured
by device 13 with uncertainty 2%. The total relative stan-
dard uncertainty of this method was not above 5%. The
measurements were taken in the quasi-equilibrium condi-
tions, which were proved by the independence of the vapor
pressure on the helium flow rate. The partial pressure of
saturated vapor over complexes was calculated according
to Eq. 1:
2
3
(
s, 6H, CH C=O), 3.0442 (s, 4H, CH C–N), 4.8332 (s, 2H,
3 2
1
3
CH). C NMR (CDCl , 77.2 ppm): 22.4577 (CH C=N),
3
3
2
4.7277 (CH ) C(CH ) , 25.9650 (CH C=O), 36.5668
3 2 2 2 3
(
CH ) C(CH ) , 61.1032 (CH), 99.6479 (CH –N),
3 2 2 2 2
1
63.3507 (C=N), 176.6955 (C=O).
Single-crystal X-ray diffraction
A yellow needle crystal of I was selected under a micro-
scope and then mounted to the tip of the thin glass fiber
with epoxy resin. X-ray intensity data were collected on a
Bruker-Nonius X8 Apex CCD diffractometer at 296(2) K
n
P ¼ Ptotal
ð1Þ
using graphite monochromatized MoKa radiation
n þ N
˚
(
k = 0.71,073 A). The standard technique was used (com-
Ptotal total pressure in a system; n number of moles of
carried compound; N number of moles of helium. The
calculations were based on the assumption that the sub-
stance vaporizes in the monomolecular form. The experi-
mental results were processed statistically using the
objective function recommended in [45]. Data obtained are
presented in Table 1S (Supplementary Materials).
bined u and x scans of narrow frames). Data reduction and
multi-scan absorption were carried out using the SADABS
[
40]. The structure was solved by direct methods and
2
refined by full-matrix least-squares on F using the
SHELXTL software package [40]. All non-hydrogen atoms
2
were refined anisotropically. Hydrogen atoms of acacpda
-
ligand were placed in geometrically calculated positions
and refined as riding on their parent carbon atoms with
U (H) = 1.5U (C) for methyl hydrogen atoms and
iso
eq
Results and discussion
U (H) = 1.2U (C) for methylene and b-hydrogen atoms.
iso
eq
The molecular graphics were performed using DIAMOND
program [41]. Crystallographic data and selected refine-
ment details are given in Table 1. The main bond lengths
and angles are presented in Table 2. The atomic positional
and thermal parameters, full lists of bond lengths and
angles were deposited at the Cambridge Crystallographic
Synthesis and characterization
Palladium complexes I and II were synthesized by the
interaction of Pd(CH CN) Cl prepared in situ by dis-
3
2
2
solving of PdCl2 in CH CN under reflux with
3
123

E. S. Vikulova et al.
Table 1 Crystallographic data,
data collection and refinement
parameters for I
Empirical formula
13 20 2 2
C H N O Pd
Formula weight
Temperature/K
342.71
296(2)
3
Crystal size/mm
0.37 9 0.05 9 0.05
Crystal system
Space group
Z
Orthorhombic
1 1 1
P2 2 2
4
˚
a/A
6.39800(10)
12.9005(4)
16.9119(6)
1395.86(7)
1.631
˚
b/A
˚
c/A
3
˚
V/A
-3
Dcalcd./g cm
-
1
l (Mo Ka)/mm
F(000)
1.326
696
h-range for data collection/°
Ranges of h, k, l
1.99–27.74
– 7 B h B 5; – 10 B k B 16; – 22 B l B 22
Reflections collected
7235
Independent reflections
Observed reflections [I [ 2r(I)]
Parameters refined
3164 (Rint = 0.0211)
3015
168
2
2
R [F [ 2r(F )]
R
R
1
1
= 0.0425; wR
2
= 0.0981
2
R(F ) (all data)
= 0.0444, wR
2
= 0.0999
2
Goodness of fit on F
1.055
Table 2 Selected bond lengths
Bond lengths
˚
(
A), bond angles (°) and
dihedral angles (°) for I
Pd(1)–O(1)
Pd(1)–O(2)
Pd(1)–N(1)
Pd(1)–N(2)
1.994(3)
1.996(3)
2.023(3)
2.023(3)
Bond angles
O(1)–Pd(1)–O(2)
O(1)–Pd(1)–N(1)
O(1)–Pd(1)–N(2)
O(2)–Pd(1)–N(1)
O(2)–Pd(1)–N(2)
N(1)–Pd(1)–N(2)
79.73(13)
91.98(14)
171.59(14)
171.52(14)
92.13(13)
96.08(14)
Atoms in plane
Dihedral angles
Angle
O(1)Pd(1)N(1)
O(2)Pd(1)N(2)
N(1)Pd(1)N(2)
N(1)C(6)C(8)N(2)
O(1)C(1)C(3)C(4)N(1)
O(2)C(12)C(11)C(9)N(2)
N(1)C(6)C(8)N(2)
C(6)C(7)C(8)
17.12(12)
20.14(13)
17.91(16)
63.0(3)
corresponding Schiff base in acetonitrile and an aqueous
solution of alkali (Eq. 2). The products were precipitated
by adding a large amount of water. The complexes I and II
are yellow crystalline powders stable on storage in air at
reduced temperature for at least 6 months.
1
23

Thermal behavior of volatile palladium(II) complexes with tetradentate Schiff bases containing…
2
1
5
6
C7
C6
N1
C8
N2
1
3
C5
C4
C3
C10
C9
C11
12
4
3
7
8
Pd1
11
10
C1
C12
O1
O2
9
C2
C13
Fig. 2 Experimental setup for tensimetric measurements by flow
method: 1—source, 2—receiver, 3—analyzed substance in source,
Fig. 3 Thermal ellipsoid representation of I with the atomic num-
bering scheme (ellipsoids are drawn at the 50% probability level)
4
—condensed substance in receiver, 5—porous plugs, 6—heater, 7—
thermocouple sensors, 8—thermal control device, 9—carrier gas
cylinder, 10—gas drying unit, 11, 12—gas consumption regulator
of corresponding Schiff bases [45] shows the absence of
acidic proton signal at about 11 ppm, indicating the
complexation.
(
flow mass), 13—carrier gas flow rate measuring device
According to single-crystal X-ray diffraction analysis,
the complex I has a molecular structure consisting of dis-
crete Pd(acacpda) molecules (Fig. 3), which can be best
compared with the structure of the related complex
Pd(acacdmpda), II [37]. All atoms of the complex mole-
cule are crystallographycally independent and occupy
CH3CN
HL; NaOH
PdCl À! PdðCH CNÞ Cl À! PdL;
2
3
2
2
T
CH3
ð2Þ
HL-Hacacpda; Hacacdmpda
The comparison of IR spectra of the complexes with IR
spectra of corresponded Schiff bases is presented in
Table 3. The presence of stretching vibrational bands
m(Pd–N), m(Pd–O) in the IR spectra of the complexes as
well as the absence of bands m(N–H), m(O–H) indicates the
complexation. Shifts of absorption bands m(C=O), m(C=N)
toward lower frequencies also confirm the formation of the
complexes.
2
?
general positions. Within the molecule, the Pd ion has a
distorted square-planar coordination environment formed
by imino N and carbonyl O atoms of the tetradentate
2
-
acacpda ligand. The bond lengths and angles around the
palladium center are, as expected, close to the corre-
sponding values found in II (Table 2). In particular, the
average values for Pd–O and Pd–N bond lengths are
1
13
˚
1.995(1) and 2.023(1) A, respectively, while the chelate N–
The expected signal positions in the H NMR and
C
1
NMR spectra, as well as the integrated intensities of the H
NMR spectra signals of the complexes, also confirm their
structure in solution (Fig. 1). The proton signals of the
CH C=N, CH C=O groups are located closely, with a
Pd–O angles are 91.98(14) and 92.13(13)°. The N–Pd–N
and O–Pd–O angles are 96.08(14) and 79.73(13)°,
respectively. Due to the tetradentate coordination of the
2
-
acacpda ligand, the Pd(acacpda) molecule contains two
conjugated six-membered metallocycles (PdOCCCN) and
one six-membered metallocycle (PdNCCCN). Both
(PdOCCCN) metallocycles are rather nonplanar, with the
angles of folding along the O(1)N(1) and O(2)N(2) lines
3
3
larger shift of CH C=O proton group toward the stronger
3
field. The signals of the protons of other aliphatic frag-
ments are easily recognizable and typical. Comparison of
1
the H NMR spectra of complexes I and II with the spectra
-1
Table 3 The assignment of absorption bands (cm ) in IR spectra of the complexes I and II and corresponded Schiff bases
Compound
m(Pd–N), m(Pd–O)
d(CCC)
m(C=C)
m(C=O), m(C=N)
m(C–H)
m(N–H), m(O–H)
H
2
acacpda
Pd(acacpda)(I)
acacdmpda
Pd(acacdmpda)(II)
–
726, 751, 1026
758, 952, 1015
751, 807, 975, 1023
756, 962, 1013
1282, 1353, 1440
1252, 1274, 1405
1290, 1359, 1442
1232, 1273, 1398
1514, 1568, 1607
1466, 1512, 1577
1521, 1572, 1611
1456, 1511, 1571
2875, 2953
2913, 2938
2880, 2970
2916, 2956
3547
–
423, 460
–
H
2
3493
–
449, 469
123

E. S. Vikulova et al.
Fig. 4 Packing diagram of
I viewed along the a axis. H
atoms are omitted for clarity.
Hydrogen bonds C–HÁÁÁO are
shown as dashed lines
Pd
O
b
N
C
c
a
being 17.1 and 20.1°, respectively. The (PdNCCCN) met-
allocycle contains no conjugated bonds and deviates con-
siderably from planarity; its geometry is characterized by
the angles of folding along the C(6)C(8) and N(1)N(2)
lines, which are 63.0° and 17.9°, respectively. Therefore,
the (PdNCCCN) metallocycle can be best described as
having a distorted boat conformation with the palladium
atom at the front and the opposing carbon atom at the back.
At a supramolecular level, both complexes I and II
exhibit C–HÁÁÁO hydrogen bonds, leading to a chain-like
arrangement of molecules in the crystals (Fig. 4). Despite
the weakness of these interactions, their relative strength
can be compared based on the CÁÁÁO distances. In I, the
central (a) hydrogen atom of the b-diketone unit partici-
pates in the formation of hydrogen bond, showing the CÁÁÁO
atoms make weaker hydrogen bonds with the CÁÁÁO dis-
˚
tances of 3.48 and 3.51 A. Moreover, the hydrogen-bonded
chains of Pd(acacpda) and Pd(acacdmpda) molecules are
organized in different ways and, as one can expect, their
packing motifs and densities are rather different. In par-
ticular, the chains of Pd(acacpda) molecules in I have a
zigzag shape and are characterized by the following PdÁÁÁPd
˚
distances: 7.89 A between the adjacent Pd(acacpda)
˚
molecules within the chain and 6.40 A between the Pd(a-
cacpda) molecules from the adjacent chains. In II, the
chains of Pd(acacdmpda) molecules have a more linear
shape with the corresponding PdÁÁÁPd distances of 6.18 and
˚
8.05 A [37]. Most likely, the reason for the differences in
hydrogen-bonding patterns in I and II lies in the higher
2
bulkiness of acacdmpda ligand as compared to that of
-
2
acacpda ligand. The presence of two methyl groups in
-
˚
distance of 3.32 A, while in II, the methylene hydrogen
1
23

Thermal behavior of volatile palladium(II) complexes with tetradentate Schiff bases containing…
DTA curves at 467 K and 485 K (onset temperatures) for
I and II, respectively, correspond to the melting process,
that have been confirmed visually on the Kofler’s table.
The mass loss curves are presented in Fig. 5. Under the
conditions of the TG experiments, the complexes evaporate
in the temperature range (498–598) K. Compound II
transfers into gas phase almost quantitatively (mass loss
1
00
8
6
4
2
0
0
0
0
0
98.0%), while compound I evaporates with a partial
decomposition (mass loss 87.3%). Temperatures of 50%
mass loss (T50%) are close (576 and 573 K for I and II,
respectively); consequently, the complexes appear to be
comparable in volatility.
I
II
In quasi-equilibrium conditions of tensimetric experi-
ments, in the temperature intervals (423–457) K and
3
00 350 400 450 500 550 600 650 700 750
Temperature/K
(422–478) K for I and II, respectively, the difference in
Fig. 5 Mass loss curves of complexes Pd(acacpda)(I) and
Pd(acacdmpda)(II)
volatility of the complexes was more accurately revealed.
The mass of substance in receiver was equal to the mass of
evaporated one in the all investigated temperature range,
indicating the stability of the complexes during sublimation
processes. The results obtained are presented in Fig. 6 and
Table 1S (Supplementary Materials). The thermodynamic
parameters of sublimation and logarithmic temperature
dependences of the saturated vapor pressure are listed in
Table 4. It should be noted that the differences in experi-
mental pressure values from the calculated ones (Table 1S)
are not exceed standard uncertainties in temperature and
pressure measurements, while the random character of
these deviations indicates the absence of a systematic error
in the measurements.
–7.7
–8.4
–9.1
–9.8
II
–10.5
–11.2
–11.9
–12.6
I
The data obtained allow us to conclude that the insertion
of methyl substituents in the diimine bridge slightly
increases the volatility of the complexes: vapor pressure
values of complexes I and II at 433 K are 0.82 and
2
.10
2.15
2.20
2.25
2.30
2.35
3
–1 –1
1
0 T /K
Fig. 6 Temperature dependences of the saturated vapor pressure of
complexes Pd(acacpda)(I) and Pd(acacdmpda)(II)
1.35 Pa, respectively. It should be noted that the same
regularities were observed for analogous copper(II) com-
plexes by means of TG method [46]. The slight difference
in volatility observed for the complexes under study could
be associated with the weak intermolecular interactions
revealed in their crystal packing. In fact, the structure of
less volatile complex I is characterized by stronger C–
HÁÁÁO hydrogen bonds described above. Moreover, the
packing of complex I is more dense as it could be also
concluded from the higher calculated density of this com-
2
-
propylene-diimine bridge of acacdmpda ligand results in
a less dense packing of the Pd(acacdmpda) molecules in
the crystal of II.
Thermal properties
Thermal properties of the complexes in condensed phase
were investigated by TG/DTA. The endoeffects on the
-
1
pound (1.631 g cm ) in comparison with that of complex
Table 4 The thermodynamic
-1
0
-1
Compound
DT/K
N
ln(p/p ) = A - B/T
0
D
sub
H
T*/kJ mol
sub
D ST*/J (mol K )
parameters of sublimation and
temperature dependences of
saturated vapor pressure of
complexes I and II
A
B
Pd(acacpda)(I)
423–457 11 25.34
15,994
16,238
133 ± 4
135 ± 1
210 ± 9
218 ± 3
Pd(acacdmpda)(II) 422–478 13 26.23
DT temperature interval; T* middle temperature of the interval investigated; N number of experimental
points; p = 1 atm = 760 Torr = 101,325 Pa
0
123

E. S. Vikulova et al.
-
1
II (1.526 g cm ), whereas the molecular weight of com-
plex I is lower, and the number of molecules in the unit cell
is the same. The loosening of the crystalline package due to
methyl substituents introduced into the diimine bridge of
the Schiff base ligand could also be one of the factors that
lead to an increase in compound volatility.
It has been shown that the introducing methyl groups to
the central atom of diimine bridge in Schiff base ligand
(complex II vs. complex I) does not lead to significant
changes in the palladium coordination polyhedron being a
distorted square-planar formed by imino N and carbonyl O
atoms. However, the modification of the ligand under
consideration results in noticeable changes in the crystal
packing, which is reflected in the thermal properties of the
complexes. In particular, complex II is characterized by
slightly higher volatility than I (the difference in vapor
pressure values is about 0.2 order of magnitude). This
observation could be explained in terms of extenuation of
weak hydrogen intermolecular interactions in the crystal
structure of II with a more bulky Schiff base ligand and its
lower packing density.
At the same time, palladium chelates I and II are
characterized by the lower volatility in comparison with
palladium complexes with bidentate Schiff bases (b-imi-
noketonates). For example, the saturated vapor pressure of
the simplest representative of palladium b-iminoketonates
with alkyl substituent at the nitrogen atom, Pd(Mei-acac)
-
Mei-acac —4-methylamino-3-penten-2-onato-), is almost
2
(
3
0 times higher (34.85 Pa at 433 K) [32]. The same reg-
ularity seems to be typical for complexes of 3d-transition
metals with coordination polyhedrons close to the square-
planar namely Cu(II), Co(II), Ni(II) [46–51]. However,
prior to the present work, the quantitative tensimetric data
were available only for nickel complexes (saturated vapor
Although the evaporation of complex I is accompanied
by a partial decomposition, the sublimation processes for
both complexes have been shown to proceed purely.
Consequently, the thermal stability of the complexes I and
II as well as the vapor pressure values are sufficient for the
use in MOCVD processes.
pressure of Ni(acacen) at 433 K is 0.83 Pa, Ni(i-acac) —
2
2
2.1 Pa (acacen —N,N -(ethylene)-bis(acetylacetoneimi-
-
0
3
-
nato-), i-acac —4-amino-3-penten-2-onato-) [50, 51],
whereas for the other complexes it was shown only qual-
itatively (by TG experiments).
Acknowledgements The authors thank Federal Agency for Scientific
Organizations for funding.
Nevertheless, it is worth noting that the vapor pressure
values and stability of complexes I and II during subli-
mation make it possible to use these precursors in the
typical MOCVD reactors. In this aspect, the tensimetric
data obtained are useful to select the temperature param-
eters of the evaporator (precursor vapor source) in
MOCVD experiments to control the partial pressure of
palladium precursor during deposition of bimetallic
nanostructures.
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