Influence of C-H/X (X = S, Cl, N, Pt/Pd) Interactions on the Molecular and Crystal Structures of Pt(II) and Pd(II) Complexes with Thiomorpholine-4-carbonitrile: Crystallographic, Thermal, and DFT Study

Article abstract of DOI:10.1021/acs.cgd.9b01661

Pt(II) and Pd(II) complexes (1 and 2, respectively) with thiomorpholine-4-carbonitrile (TM-CN), an N-substituted thiomorpholine derivative, were synthesized from tetrachlorido precursors in water. Structural analysis has shown that 1 represents the first monomeric metal complex with this ligand type with an axial M-S bond with respect to the TM-CN ring chair conformation, while in 2 a typical equatorial M-S bond position with respect to the ring chair conformation was observed. A detailed DFT investigation revealed that axial conformers are more stable for molecular forms of both metals, while intermolecular interactions in the crystals stabilize the axial conformer for Pt(II) and the equatorial conformer for Pd(II). The magnitude of this stabilization in the case of 2 is large enough to change the most stable axial conformer in the molecular form to the equatorial conformer in the crystal. Further investigation of the strength of individual intermolecular interactions revealed significant differences of some interactions between the two structures. The likely cause of the difference in the crystal structures of experimentally obtained complexes is the fact that 1 and 2 exhibit different dominant interactions: C-H/M and C-H/S are more dominant in 1 and C-H/Cl interactions are more dominant in 2. In addition, DFT calculations have shown that while the axial position of the Pt-S bond with respect to the ring chair conformation results in a significantly shorter C-H/Pt interaction distance than that in the hypothetical equatorial conformer, there is very little difference in C-H/Pd interaction distances in conformers with axial and equatorial positions of Pd-S bond with respect to the ring chair conformation.

Full text of DOI:10.1021/acs.cgd.9b01661

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Article
Influence of C−H/X (X = S, Cl, N, Pt/Pd) Interactions on the Molecular
and Crystal Structures of Pt(II) and Pd(II) Complexes with
Thiomorpholine-4-carbonitrile: Crystallographic, Thermal, and DFT
Study
Predrag Ristic,
́
Vladimir Blagojevic,
́
Goran Janjic,
́
Marko Rodic,
́
Predrag Vulic,
́
Morgan Donnard,
and Nenad Filipovic*
Mihaela Gulea, Agnieszka Chylewska, Mariusz Makowski, Tamara Todorovic,
́
́
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sı Supporting Information
ABSTRACT: Pt(II) and Pd(II) complexes (1 and 2, respectively)
with thiomorpholine-4-carbonitrile (TM-CN), an N-substituted
thiomorpholine derivative, were synthesized from tetrachlorido
precursors in water. Structural analysis has shown that 1 represents
the first monomeric metal complex with this ligand type with an
axial M−S bond with respect to the TM-CN ring chair
conformation, while in 2 a typical equatorial M−S bond position
with respect to the ring chair conformation was observed. A
detailed DFT investigation revealed that axial conformers are more
stable for molecular forms of both metals, while intermolecular
interactions in the crystals stabilize the axial conformer for Pt(II)
and the equatorial conformer for Pd(II). The magnitude of this
stabilization in the case of 2 is large enough to change the most stable axial conformer in the molecular form to the equatorial
conformer in the crystal. Further investigation of the strength of individual intermolecular interactions revealed significant differences
of some interactions between the two structures. The likely cause of the difference in the crystal structures of experimentally
obtained complexes is the fact that 1 and 2 exhibit different dominant interactions: C−H/M and C−H/S are more dominant in 1
and C−H/Cl interactions are more dominant in 2. In addition, DFT calculations have shown that while the axial position of the Pt−
S bond with respect to the ring chair conformation results in a significantly shorter C−H/Pt interaction distance than that in the
hypothetical equatorial conformer, there is very little difference in C−H/Pd interaction distances in conformers with axial and
equatorial positions of Pd−S bond with respect to the ring chair conformation.
INTRODUCTION
(TM). There have been 12 crystal structures of TM complexes
deposited in the CSD (Table 1). Three binding modes are
observed for TM: monodentate N (n-N), bidentate bridging N
■
1
Thiane (TH) is a simple monodentate sulfur-containing six-
membered heterocyclic ligand that can coordinate to metals,
forming axial or equatorial M−S bonds with respect to the ring
chair conformation. Due to 1,3-diaxial interactions, which
contribute to the higher energy of the axial conformer relative
to the equatorial form, the formation of an equatorial M−S
bond with respect to the ring chair conformation is preferable.
This is in line with the results of a Cambridge Crystallographic
Database (CSD) search where, out of 13 reported crystal
structures, there is no crystal structure with an exclusively axial
position of the M−S bond with respect to the TH ring chair
(
μ-N), and tridentate bridging S (n-N:μ-S). The last
coordination mode includes both axial and equatorial positions
of the M−S bond (where S is the bridging sulfur atom) with
respect to a ring chair conformation, and it was only observed
in the case of one Cu-based TM complex. TM could act as N,S
bidentate chelator, but this is unfavorable since a boat
conformation would be required for the chelating coordination
2
mode. There have been no reported crystal structures where
TM is coordinated to metal exclusively via the sulfur atom.
1
conformation (Table 1). In 11 structures the M−S bond is in
Received: December 12, 2019
Revised: March 28, 2020
Published: March 30, 2020
equatorial position, while in the case of four complexes the
sulfur atom coordinates as a double donor with formation of
both axial and equatorial M−S bonds with respect to the TH
ring chair conformation.
The introduction of a nitrogen donor in the TH ring results
in another heterocyclic sulfur-based ligandthiomorpholine
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.cgd.9b01661
Cryst. Growth Des. XXXX, XXX, XXX−XXX
A

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Article
8
Table 1. Coordination Modes of TH, TM and N-TM ligands
in Crystal Structures of Complexes Deposited in the CSD
ward Pd(II) and Pt(II). Both chosen ions have a d electronic
configuration and almost the same ionic radii; thus, their
complexes with the same ligand systems are often isostructural.
However, the extended electronic density and higher basicity
of the 5d Pt(II) ion in comparison to the 4d Pd(II) ion result
in stronger M···H−X (X = C, N, O) interactions for Pt(II),
16
which lead to a difference in the structure of the complexes.
Thus, these two systems provide an excellent opportunity to
study the effect of intermolecular interactions on the stability
of molecular and crystal structures, given that four different
types of intermolecular interactions can be formed. We present
two nonisostructural square-planar Pd(II) and Pt(II) com-
plexes with a TM-CN ligand, showing the difference in the M−
S bond position with respect to the ring chair conformation.
To understand the difference in the crystal structure of
synthesized complexes, DFT calculations of TM-CN coordi-
nation in a molecular structure, Hirshfeld surface and
fingerprint plot analysis, DFT calculations of periodic
structures with comparison of intermolecular interaction
energies, crystallographic and quantum chemical analysis of
observed interactions, and detailed thermal stability inves-
tigations were performed.
a
EXPERIMENTAL SECTION
Complexes with two TH ligands with different coordination modes.
■
b
Polydentate coordination via N of the TM ring and other donor
Materials and Methods. All of the employed reagents and
solvents were of analytical grade and were used without further
purification. K [PdCl ] (≥99.9%) and K [PtCl ] (98%) were
atoms from R groups.
2
4
2
4
obtained from Sigma-Aldrich. Elemental analyses (C, H, N, S) were
performed by standard micro methods using an ELEMENTAR Vario
ELIII C, H, N, SO analyzer. The vibrational spectra were recorded
with a Spectrum Two instrument (PerkinElmer) in the wavenumber
range 4000−450 cm , as well as in the range 600−300 cm using
the universal attenuated total reflection (UATR) technique. The
method used was insensitive to sample thickness or shape due to the
presence of the UATR hard crystal material. The apparatus was
N-substituted TM analogues (N-TMs) have also been used
for the preparation of metal complexes, and there are currently
1
1
8 reported crystal structures deposited in the CSD (Table
1
−1
−1
). The coordination modes of N-TMLs observed in the
crystal structures of the complexes are monodentate
coordination via the S atom, mixed N,S chelation, and
coordination of the N atom together with other donor atoms
from the R substituent. Exclusive coordination via the sulfur
atom was observed in the case of polymeric Cu complexes
where ligands contain electron-withdrawing amide groups
equipped with a diamond material with a measurable surface of about
2
4
mm , allowing good contact with both samples. Molar conductivity
measurements were performed at ambient temperature (298 K) on a
Crison Multimeter MM41 instrument. The NMR spectral measure-
ments were performed on a Bruker Avance III 500 spectrometer or
Agilent 400-MR spectrometer equipped with a broad-band direct
probe. The spectra were recorded at room temperature in DMSO-d6,
CDCl , or CD NO . Chemical shifts are given on the δ scale relative
3
−6
bonded to N-TMLs.
A typical, exclusive axial conformer
was observed only in the case of two Cu(I) coordination
4
,7
polymers. It is worth mentioning that, in the mixed N,S
bidentate chelates, N-TMs coordinate in the higher energy
boat conformation. There have been several reports on the
possible application of metal complexes with N-TMs. A
comparative study of the catalytic activity of Cu(II) binuclear
complexes with a phenol-based ligand with N-(2-aminoethyl)-
thiomorpholine side arms, as well as its morpholine and
piperidine analogues, showed that a TM-based complex is the
3
3
2
1
13
to tetramethylsilane as an internal standard for H and C.
Assignments of signals in NMR spectra are given in the Supporting
Information. A high-resolution electron spray ionization mass
spectrum (HRMS-ESI) of the ligand was recorded on a Agilent
Technologies 6210-1210 TOF-LC-ESI-MS instrument operating in
the positive ion mode. The thermogravimetric analysis was performed
by means of a TG209 Nietzsche thermal equalizer coupled with FT-
IR. The analyzer was equipped with a programmable temperature
controller, which automatically maintains constant temperature
during thermal events. The TG weight-loss measurements were
performed in the 20−950 °C temperature range at a heating rate of 15
°C/min, in an alumina crucible. The IR spectra of gaseous
decomposition products were registered using a Bruker IFS 66
spectrophotometer. All experiments were carried out under an argon
atmosphere and verified at least twice.
8
most active and potent catechol oxidant. Also, two Ru-based
complexes with thioamide-based ligands showed potential to
be used as detectors for NO due to directly observed
9
fluorescence after NO binding to the metal center. Finally,
the ligand 2-(cyclohexylthio)-1-thiomorpholinoethanone
forms three Cu(I) coordination polymers which undergo
crystal to crystal transformation with and without solvent,
while variations of the distance between copper atoms with
temperature are responsible for the observed luminescence
Synthesis of the Ligand Thiomorpholine-4-carbonitrile
3
(TM-CN). The ligand was synthesized according to the literature
thermochromism.
17
procedure: white solid; mp 41−43 °C. Anal. Calcd for C H N S
5
8
2
Taking into account that many properties of chemical
(
MW = 128.20): C, 46.85; H, 6.29; N, 21.85; S, 25.01. Found: C,
6.97; H, 6.43; N, 21.77; S, 24.96. H NMR (CDCl , 500.26 MHz):
δH 2.70 (t, 4H), 3.46 (t, 4H). C NMR (CDCl , 126 MHz): δ 26.1,
50.8, 117.4. HRMS (ESI): m/z calcd for C H N S (M + H)
systems are defined not only by the molecular structure but
1
4
3
10−15
1
3
also by weak intermolecular interactions,
we found it
3
C
+
interesting to study the coordination ability of one unexplored
5 9 2
N-TM ligand, thiomorpholine-4-carbonitrile (TM-CN), to-
129.0481, found 129.0478.
B
https://dx.doi.org/10.1021/acs.cgd.9b01661
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
pubs.acs.org/crystal
Article
−
1
Figure 1. UATR spectra in the 4000−450 cm region of trans-[PtCl (TM-CN) ] (1) (A) and trans-[PdCl (TM-CN) ] (2) (C). UATR spectra in
2
2
2
2
−
1
the 450−300 cm region of 1 (B) and 2 (D).
Synthesis of [PtCl (TM-CN) ] (1). Solid K [PtCl ](0.081 g, 0.2
mmol) was added to a solution of TM-CN (0.050 g, 0.390 mmol) in
bonded to carbon atoms in 1 and 2 were introduced in idealized
positions and refined using riding model structures and validated
2
2
2
4
22
23
H O (10 mL). A pale yellow precipitate formed immediately. The
using PLATON 4and CSD (v. 5.40, updates Aug. 2019) using
2
2
reaction mixture was stirred at 50 °C for 1 h, after which the
precipitate was separated by filtration, washed with small portions of
MERCURY CSD.
The X-ray powder diffraction (XRPD) investigation was conducted
on a Rigaku Smartlab X-ray diffractometer in θ−θ geometry (with the
sample in a horizontal position) in parafocusing Bragg−Brentano
geometry using a D/teX Ultra 250 strip detector in 1D standard mode
with a Cu Kα1,2 radiation source (U = 40 kV and I = 30 mA). The
XRPD patterns were collected in the 5−65° 2θ range, with a step of
0.01° and a data collection speed of 5°/min with a horizontal sample
rotation of 20 rpm. For every sample a small amount of single crystals
was pulverized, and a low-background single-crystal silicon sample
holder was used to minimize the background. The crystal phases
present in the samples were identified with dedicated Rigaku PDXL
2.0 software, comparing them with a user database comprised of
crystallographic information files (CIF) previously obtained by single-
crystal X-ray diffraction (SC XRD) structure determinations.
cold H O and EtOH, and dried in a desiccator. Yield: 0.091 g (90%).
2
Good-quality single crystals were obtained by slow diffusion of EtOH
vapor into a DMSO solution of the product. Anal. Calcd for
C H Cl N PtS (MW = 522.38): C, 22.99; H, 3.09; N, 10.73; S,
1
0
16
2
4
2
−
3
1
2.27. Found: C, 22.78; H, 3.15; N, 10.74; S, 12.48. Λ (1 × 10 M,
M
−1 2 −1
MeCN) = 18.2 Ω cm mol .
Synthesis of [PdCl (TM-CN) ] (2). Solid K [PdCl ](0.050 g, 0.15
2
2
2
4
mmol) was added to a solution of TM-CN (0.010 g, 0.075 mmol) in
H O (10 mL). The reaction mixture was stirred at 50 °C for 1 h. A
2
yellow precipitate was separated under vacuum and dried in the
desiccator. Yield: 0.024 g (72%). Good-quality single crystals were
obtained by slow diffusion of pentan-2-one vapor into a nitromethane
solution of the product. After 2 days yellow single crystals were
filtered off and washed with cold water. Anal. Calcd for
C H Cl N PdS (MW = 433.69): C, 27.69; H, 3.72; N, 12.92; S,
DFT Calculations. Geometry optimization of individual molecules
25
was performed using Gaussian09 software, with the WB97XD
method, 6-31+G** basis set for nonmetals (C, H, N, S, and Cl), the
LANL2DZ basis set for metal ions (Pd and Pt), and the SMD
1
0
16
2
4
2
−
3
1
4.78. Found: C, 27.48; H, 3.59; N, 12.74; S, 14.84. Λ (1 × 10 M,
MeCN) = 10.10 Ω cm mol .
M
−1 2 −1
26
X-ray Crystallography. Single-crystal X-ray diffraction was
performed on an Oxford Diffraction Gemini S kappa geometry
diffractometer, equipped with Mo Kα radiation (λ = 0.71073 Å) from
a sealed-tube source and a Sapphire CCD detector. Data collection
calculations, data reduction, cell refinement ,and absorption
solvation model. A lack of imaginary frequencies in the frequency
calculation was taken as a confirmation of a true minimum. DFT
calculations on a periodic system were conducted on a unit lattice
27
28
using the ABINIT 8.10 software package with PBE0 functional
and norm-conserving ONCVPSP-3.2.3.1 pseudopotential. The energy
18
−7
correction were performed with CRYSALISPRO. Structures were
cutoff was 990 eV, and the SCF tolerance was 5 × 10 eV per atom.
19
solved using SHELXT and refined with anisotropic displacement
To get insight into the strength of particular interactions involving
in crystal packing of the considered complexes (C−H/Cl−M, C−H/
S−M, C−H/NC, and C−H/M interactions), DFT calculations on
20
parameters for all non-hydrogen atoms using SHELXL-2014/6. The
21
program SHELXLE was used as a graphical user interface for
structure solution and refinement procedures. Hydrogen atoms
a model CH /metal complex system were performed. Unlike the
4
C
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Article
Figure 2. TG and DTG curves of complexes 1 (A) and 2 (B).
previous set of calculations, these were conducted only on the
equatorial conformers, because this provided the best model where
these specific interactions could be isolated by eliminating any chance
of potential additional interactions of the TM-CN ligand and CH4.
Therefore, only structures with equatorial M−S bonds with respect to
the ring chair conformation were used in the calculations. During
these calculations the distance of the H atom to the corresponding
acceptor atom (Cl, S, N, Pt, and Pd) changed in the range from 2.5 to
therefore the complexes can be represented by the general
formula [MCl (TM-CN) ] (M = Pt, Pd). They are soluble in
2
2
DMSO, DMF, MeCN, and MeNO and sparingly soluble in
2
MeOH and EtOH. Molar conductivity measurements for both
compounds were performed in MeCN solution. The obtained
values are lower than those characteristic for 1:1 electrolytes,
indicating that molecular complexes were obtained, which is in
agreement with the proposed general formula.
The detailed assignment of the vibrations of 1 and 2 and the
corresponding mode descriptions are reported in Table S1 in
3
.5 Å.
Hirshfeld Surface Analysis and Calculation of Intermolec-
ular Interaction Energies. For visualization of Hirshfeld surfaces,
CIF files were used. Hirshfeld surface visualization, presentation of
results as dnorm, shape index, and curvedness, and calculation of 2D
the Supporting Information. Methylene (CH ) group
2
vibrations of the TM-CN ligand in both complexes are
fingerprint plots with d and d distances were generated using Crystal
e
i
−1
29,30
observed as intense peaks in the range 2900−3000 cm
Explorer v.17.5.
The distance from the surface to the nearest
(
Figure 1), originating from the overlapping of the absorption
nucleus of the atom on the outside of the surface is denoted as de,
while the distance from the surface to the closest nucleus of the atom
peaks of symmetric and asymmetric stretching of the CH
2
3
2,33
on the inside of the surface is denoted as d . Surfaces are mapped over
group in the coordinated ligand.
Additionally, these
i
a standard color scale, and 2D fingerprint plots are calculated using de
stretching frequencies are associated with vibrational modes
and d values in the range 0.4−2.8 Å.
i
such as rocking and twisting as well as wagging modes related
Intermolecular interaction energies were calculated using Crysta-
lExplorer v17.5, with wave functions calculated using Gaussian09
34
to the polarization of vibrations. The CH stretching region
2
25
of the coordinated TM-CN ligand in the complexes is
surprisingly similar to that of coordinated morpholine or
piperazine, with the exception of changes in relative intensities.
This suggests that the lone pair orbitals on sulfur may play a
role similar to that of oxygen or nitrogen in shifting the
software with the B3LYP method and 6-31G(d,p) basis set for
nonmetals (C, H, N, S, and Cl) and the DGDZVP basis set for the
metal center.
Crystallographic Analysis of X−H/Cl−M, X−H/S−M, X−H/M,
and X−H/NC Interactions. The crystallographic analysis of X−
H/Cl−M, X−H/S−M, X−H/M, and X−H/NC interactions is
35
position of certain CH bands. Among the other vibrations of
2
31
based on the crystal structures extracted from the CSD. The study
of X−H/M interactions is based on structures with the M ion from
square-planar complexes. The geometric parameters used for
searching the CSD were the same for all analyzed interactions: the
distance between the H atom from X−H group and the acceptor atom
the CH group of TM-CN ligand in both complexes, the bands
2
−1
at 1280−1113 cm region are related to wagging and twisting.
In addition, rocking modes of the CH group at 980 cm are
also observed. The other bands of the CH group are
−1
2
36
2
observed in the expected region and are presented in Table S1
in the Supporting Information. Moreover, the UATR bands of
ring vibrations are identified in 1283−1113, 980−740, and
(
A= Cl, S, M) is less than 2.9 Å (H···A or d distance), and the X−H···
A angle (α angle) is larger than 110°. Analyzed structures had to
satisfy the following criteria: (a) a crystallographic R factor of less
than 10%, (b) error-free coordinates according to the criteria used in
the CSD system, (c) no crystallographic disorder, (d) no polymeric
structures, and (e) positions of all hydrogen atoms normalized.
−
1
5
80−540 cm regions as very weak and weak peaks for 2 and
medium-strong and strong peaks for 1. The lower frequencies
of these peaks in comparison to those of the free ligand are
probably due to the coordination of the TM-CN ligand to the
RESULTS AND DISCUSSION
37
■
metal centers.
The absorption region between 2260 and 2200 cm− is
assigned to the stretching vibrations of the CN group. In IR
spectra of the complexes, the CN stretching bands are very
intense, although the peak observed for 1 is more intense than
that of 2. The in-plane bending of the N−CN bond is obtained
1
Synthesis and Characterization of the Complexes.
The complexes were obtained by the reaction of TM-CN with
K [MCl ] (M = Pt, Pd) in water as a solvent in the molar ratio
2
4
ligand:metal salt = 2:1. Pale yellow (Pt complex 1) and orange
Pd complex 2) precipitates were obtained after 1 h at 50 °C.
Single crystals suitable for X-ray diffraction (XRD) analysis
vide infra) were obtained by a vapor diffusion method. The
(
−
1
in low-frequency regions at 540 and 543 cm for 1 and 2,
respectively, overlapping with the C−C in-plane bending
vibration. Thiols or thioamides generally exhibit IR absorption
(
results of elemental analysis indicate that the molar ratio of the
ligand and metal ions is preserved in the obtained complexes;
−1
38
at 1750−1400 cm , with ring-size, metal ionic center bound,
D
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Article
Table 2. Thermal Analysis Data for 1 and 2
mass loss (%)
complex
step
onset (°C)
endset (°C)
leaving group
exptl
calcd
exptl (calcd) residue
1
1
2
3
1
2
3
20
250
420
20
330
475
250
410
725
330
475
730
NH3
CO /NO /H S/CO
2.53
38.44
24.93
10.72
20.07
33.11
3.24
38.17
24.61
10.11
20.99
33.56
97.47 (96.76)
59.03 (58.59)
PtO 34.10 (33.98)
89.28 (89.89)
69.21 (68.90)
PdO 36.10 (35.34)
2
2
2
Cl /CO/NO
2
2
CO2
NO /H S/NO
2
2
NH /Cl /CO/NO
3
2
and steric and electronic effects resulting in significant shifts in
Information. Complexes 1 and 2 crystallize in a monoclinic
39
CS absorption frequencies. In 1 (Figure 1A) and 2 (Figure
C) the C−S stretching vibrations are observed at 1652−1401
crystal system, but in different space groups (I2/a and P2 /c,
1
1
respectively). Both complexes are centrosymmetric, with metal
ions residing on a special position with site symmetry −1
(Wyckoff letter a for 1 and d for 2). Thus, asymmetric units of
the obtained trans isomers consist of a metal center, one TM-
CN coordinated via an S atom, and one chloride ion. The
molecular structures of the obtained square-planar complexes 1
and 2 are depicted in Figure 3. Although Pt and Pd atoms lie in
−
1
cm for both compounds. Additionally, the intense peaks
observed at 505 cm for 1 and 504 cm for 2 are assigned to
the stretching vibrations of M−S bonds.
−
1
−1
In the square-planar complexes 1 and 2, ν(M−Cl) bands are
40
sensitive to the ligand’s trans influence, which has been
studied extensively using UATR spectroscopy. Vibrational
spectra of trans isomers of 1 and 2 exhibit several ν(M−Cl)
i
i
the Cl1,S1,Cl1 ,S1 plane (1, i = 1 − x, 1 − y, 1 − z; 2, i = 1 −
x, −y, 1 − z) and trans bond angles are exactly 180°, the
coordination geometry around metal centers in both
complexes is distorted since cis bond angles deviate slightly
from an ideal value of 90° (Table S3 in the Supporting
Information). It is worth noting that, although in both
complexes the thiomorpholine rings are in a chair con-
formation, the positions of M−S bonds with respect to the ring
chair conformation are different. In 1, the Pt−S bond is in an
axial position, while the Pd−S bond in 2 is in an equatorial
position with respect to the ring chair conformation. Overlays
of both structures are depicted in Figure 3C,D. These
conformational differences are also reflected in the values of
M−S−Cg angles, where Cg represents a centroid of the
thiomorpholine ring (Table S3 in the Supporting Informa-
tion), as well as in the values of the corresponding torsion
angles M−S1−C1−C2 and M−S1−C3−C4 (Table S3 in the
Supporting Information). Due to the contribution of a
resonance structure with a positive charge on the thiomorpho-
−
1
bands in the infrared below 400 cm . Additionally, the UATR
spectra of 1 (Figure 1B) and 2 (Figure 1D) appear to confirm
the stereochemistry of these compounds, as well as indicate the
presence of M−Cl bonds inside the coordination spheres of
both compounds (Table S1 in the Supporting Information).
Thermal Behavior. Thermal analyses of 1 and 2 were
carried out by the TG and DTG techniques. The volatile
products of the early stages of decomposition were collected
and identified by IR spectroscopy. The experimental results for
both compounds studied revealed that the degradation
occurred in three main steps (Figure 2). The study of the
thermal decomposition of 1 indicated its increased heat
endurance. It decomposed in the temperature range 170−730
°C. Both complexes are stable up to about 170 °C. Above this
temperature, the elimination of the greatest part of TM-CN
begins. The temperature ranges and the relative observed and
calculated weight losses are given inTable 2. From the weight
loss the stoichiometries of the resultant intermediate species
are assigned. The data corresponding to temperatures between
+
−
line nitrogen atom ( N1C5N2 ), the CN substituent in
both complexes is bonded to the thiomorpholine ring in a
position which is between axial and equatorial with respect to
the ring chair conformation. This is also reflected in the sum of
the corresponding bond angles around N1 (343.9° in 1 and
359.1° in 2). The N−CN fragment is almost linear with a N−
CN angle of 178.3° in 1 and 177.5° in 2.
1
70 and 725 °C suggest a multistep decomposition of both
compounds. Then a series of exothermic heat changes, in both
cases, corresponds to a complete decomposition of the residual
sulfur-based TM-CN ligand and the formation of PtO and
PdO, respectively. A period of slow losses of weight followed
by further losses of chlorides and formation of oxides was
complete by about 725 °C. No metal thiocyanates, sulfides, or
sulfates were detected in the course of the decomposition as
was reported in the case of thermal decomposition of metal
42
Due to the similar ionic radii of the metal ions, the lengths
of the corresponding coordination bonds (M−Cl and M−S)
are similar in both systems (Table S3 in the Supporting
Information) and are typical of values found in related Pt(II)
41
chloride thiomorpholine complexes. On this basis, it can be
concluded that the thermal decompositions of 1 and 2 occur
through breaking of metal−sulfur bonds. The residual masses
of 34.10% (PtO) and 36.10% (PdO) for 1 and 2, respectively,
were caused by simultaneous formation of nitrogen, carbon,
and sulfur species (NO, CO, CO , H S) and Cl (Figures S1
2
3,43
and Pd(II) complexes.
The lengths of bonds in the TM
24,44
ring are also in the usual range.
Thiomorpholine methylene groups form intramolecular C−
H/Cl and C−H/M interactions. It is well-known that metal
complexes can form hydrogen bonds in which the metal ion
2
2
2
44
and S2 in the Supporting Information). Moreover, the
theoretical (calculated) and experimental percentages of mass
losses of both complexes are in very good agreement (Table
serves as an acceptor. As a consequence of conformational
differences, H···Cl and H···M distances in the two structures
are different and are shorter in the system with axial M−S
bonds. In 1, each Cl ligand forms two C−H/Cl interactions
with two methylene groups from one ligand and one
interaction with methylene group from another ligand (1:
2
).
XRD Analysis. A summary of data collection and
refinement parameters for 1 and 2 are given in Table S2 in
the Supporting Information, whereas selected bond lengths
and bond angles are presented in Table S3 in the Supporting
i
Cl1···H3A = 2.809 Å, Cl1···H4A = 2.764 Å, Cl1···H1B = 2.689
Å, i = 1 − x, 1 − y, 1 − z). In 2, one methylene group from one
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Figure 4. Intramolecular C−H/Cl (blue) and C−H/M (magenta)
interactions in molecular structures of 1 (A) and 2 (B). Symmetry
operations: i = 1 − x, 1 − y, 1 − z in 1 and i = 1 − x, − y, 1 − z in 2.
prevent the rotational flexibility of coordinative bonds and
47
conformational flexibility of ligands.
The cell packing diagrams for the two complexes are
depicted in Figure 5. Due to the absence of a classical proton
donor, crystal packings in both complexes are based on weak
hydrogen bonds.
Each molecule of 1 is involved in four C−H/S interactions
with two adjacent molecules with a formation of 1-D chains
parallel to [111] (C3−H3B/S1(1/2 − x, 1/2 − y, 1/2 − z) =
4.137 Å, C−H···S = 165°) (Figure 6A). Due to space group
symmetry the same interactions are responsible for the
formation of 1-D chains parallel to [1−11]. These two types
of chains are interlinked through C−H/Pt, C−H/Cl, and C−
H/π interactions, thus forming 3-D crystal packing. Each
molecule of the Pt(II) complex is involved in four C−H/Pt
interactions (C2−H2B/Pt1(1.5 − x, y, 1 − z) = 4.095 Å, C−
H···Pt = 137.6°) (Figure 6B), C−H/π interactions (C4−H4A/
Cg(1.5 − x, y, 1 − z) = 3.520 Å, C−H···Cg = 127.19°; Cg is
the centroid of the carbonitrile bond) with adjacent complex
molecules (Figure 6C) and three C−H/Cl interactions (C1−
H1A/Cl1(x, 1/2 − y, 1/2 + z) = 3.894 Å, C−H···Cl = 137.6°,
C2−H2A/Cl1(1.5 − x, y, 1 − z) = 3.285 Å, C−H···Cl =
119.39°, C3−H3A/Cl1(1/2 − x, y, 1 − z) = 3.088 Å, C−H···
Cl = 122.47°) (Figure 6D).
On the other hand, each molecule of 2 forms four C−H/S
interactions with two adjacent molecules with the formation of
2-D layers parallel to (100) (C3−H3B/S1(1 − x, − 1/2 + y,
1.5 − z) = 3.136 Å, C−H···S = 155.36°) (Figure 7A).
Additionally, each complex molecule is involved in four C−H/
Pd interactions with two adjacent molecules (C2−H2A/
Pd1(x, 1 + y, z) = 3.785 Å, C−H···Pd = 140.08°) where two
interactions are hydrogen bond donors and the other two are
acceptors (Figure 7B). Also, each molecule of 2 interacts
thorough three C−H/Cl contacts (C1−H1A/Cl1 1 − x, 1 − y,
1 − z) = 2.996 Å, C−H···Cl = 153.78°; C1−H1B/Cl1(x, 1 + y,
−1 + z) = 2.858 Å, C−H···Cl = 144.51°; C2−H2A/Cl1(x, 1.5
−y, − 1/2 − z) = 2.888 Å, C−H···Cl = 132.87°) (Figure 7C)
Figure 3. Perspective view and labeling of molecular structure of 1
(
A) and 2 (B). Thermal ellipsoids are shown at the 50% probability
level. Equivalent atoms are generated by the transformation i = 1 − x,
− y, 1 − z for 1 and i = 1 − x, −y, 1 − z for 2. (C) Overlay (through
1
M and four donor atoms) of the molecular structures of 1 (yellow)
and 2 (red). (D) Overlay (through C1, C2, N1, C3 and C4) of the
molecular structures of 1 (capped sticks) and 2 (ball and sticks).
Hydrogen atoms are omitted for clarity (C, D).
ligand is involved in a bifurcated C−H···Cl interaction, while
one methylene group from other ligand forms one interaction
i
with the Cl ligand (2, Cl1···H1B = 2.938 Å, Cl1···H3B = 2.913
i
Å, Cl1···H3A = 3.088 Å, i = 1 − x, − y, 1 − z). Pd(II) forms
two C−H/Pd interactions with one CH group from each
2
ligand, while Pt(II) forms four C−H/Pt interactions with two
CH groups from each ligand (1, Pt1···H2B = 3.141 Å, Pt1···
2
H4A = 3.273 Å; 2, Pd1···H1A = 3.293 Å). Intramolecular
interactions are depicted in Figure 4. Such a distribution of
intramolecular interactions is a consequence of a difference in
the position of the M−S bond (axial or equatorial) with
respect to the ring chair conformation of TM-CN. However, it
should be stressed that the Pt(II) ion forms somewhat stronger
44−46
hydrogen bonds than the Pd(II) ion.
It is also useful to
point out that weak C−H hydrogen bonding has the ability to
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Figure 5. Cell packing diagrams for 1 (A) and 2 (B).
Figure 6. C−H/S (A), C−H/Pt (B), C−H/π (C), and C−H/Cl (D) interactions in the crystal packing of 1.
and one C−H/π contact (C4−H4A/Cg(1 − x, 1 − y, −z) =
single-crystal structures described above. Figure S4 in the
Supporting Information shows a comparison between exper-
imental powder XRD diffractograms of 1 and 2 with simulated
patterns of their analogues with different TM-CN conforma-
tions of the M−S bond with respect to the ring chair
conformation. These show that there are significant differences
3
.087 Å, C−H···Cg = 132.61°; Cg is the centroid of the
carbonitrile bond) (Figure 7D).
Powder XRD measurements (Figure S3 in the Supporting
Information) show that both 1 and 2 crystallize as a pure single
phase in the powder form. These phases correspond to the
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Figure 7. C−H/S (A), C−H/Pd (B), C−H/Cl (C), and C−H/π (D) interactions in the crystal packing of 2.
Table 3. Lattice Parameters and Calculated Energies for Crystal and Molecular Structures for Equatorial and Axial Conformers
a
of Pd(II) and Pt(II) Complexes
ΔE (kcal/mol)
complex
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
crystal
molecule
1
I2/a
10.1930
11.4399
11.3992
11.2667
10.1930
7.0711
8.2607
11.2667
16.0148
9.5192
10.0984
18.5256
48.5
48.5
96.2
94.2
40.7
66.8
0.00
21.70
0.00
0.00
3.31
0.00
1
2
2
-eq
P2 /c
90.00
90.00
40.7
90.00
90.00
56.0
1
P2 /c
1
-ax
I2/a
1.96
−3.06
a
Energy differences between corresponding structures are shown as ΔE (lattice parameters for axial conformers represent the primitive lattice).
in the powder XRD diffractograms between the two structures
with the same metal ion, indicating that a possible mixture of
phases would be relatively easily identified in the powder XRD,
in particular through the appearance of the most intense peaks
in the 5−10° 2θ region. Since experimental powder XRDs
powdered samples used for XRD measurements are shown
Figures S5 and S6 in the Supporting Information, respectively.
DFT Calculations on Solid-State Periodic and Molec-
ular Systems. In order to investigate which factors are driving
the stability of the crystal structures of 1 and 2 in the solid
state, DFT calculations were performed on molecular systems
and solid-state periodic systems of Pt and Pd in both axial and
equatorial conformers (Table 3). Optimized structures with
axial positions of M−S bonds with respect to the TM-CN ring
chair conformation are more stable than the optimized
structures with equatorial positions. The energy difference
between axial and equatorial conformers of Pt(II) is slightly
larger (3.31 kcal/mol) than in the case of Pd(II) conformers
(3.06 kcal/mol). DFT calculations on a periodic system were
(
Figure S3 and S4 in the Supporting Information) do not show
any peaks of the second predicted phase, this indicates that 1
and 2 crystallize as a single phasean axial conformer for
Pt(II) and an equatorial conformer for Pd(II).
Since these complexes were obtained as well-formed single
crystals, they do not exhibit any particular observable
morphological features that could be correlated to the
energetic effects in their crystal structure (vide infra). Images
of single crystals from optical microscope and SEM images of
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Figure 8. Fingerprint plot, Hirshfeld surfaces mapped with dnorm (two orientations), shape index, and curvedness for 1.
Figure 9. Fingerprint plot, Hirshfeld surfaces mapped with dnorm (two orientations), shape index, and curvedness for 2.
conducted using the PBE0 functional with norm-conserving
pseudopotential for better accuracy of calculated energies.
Hirshfeld Surface and Fingerprint Plot Analysis.
29,49
48
Hirshfeld surface and fingerprint plot analysis
allow
The results show that the axial conformer is more stable for the
Pt(II) complex, while the equatorial conformer is more stable
for the Pd(II) complex (Table 3), which is consistent with
experimentally obtained structures, in both the powder and
single-crystal experiments. A comparison with the energy
differences from the molecular calculations shows the extent of
the influence of intermolecular interactions in the crystal
structure. For the Pt(II) complex, the preferred conformation
does not change, while the energy difference between axial and
equatorial complexes increases by about 18.4 kcal/mol. On the
other hand, for the Pd(II) complex, the most stable structure
changes from axial to equatorial, with the change in energy
difference between two configurations totaling around 5.1
kcal/mol. This suggests that intermolecular interactions in
Pt(II) and Pd(II) complexes stabilize different conformers
axial for Pt(II) and equatorial for Pd(II). In the case of Pd(II),
that change is large enough to change the most stable
conformation to the equatorial form.
visualization and a quick and easy understanding of intra-
and intermolecular interactions in the crystal structures. Before
calculation of the surfaces, the lengths of C−H bonds are
normalized to standard values determined by neutron
diffraction. Hirshfeld surfaces, pseudosymmetric 2D fingerprint
plots, and existing classical and nonclassical interactions, and
relative contributions to the Hirshfeld surface for the major
intermolecular contacts in the crystal structures of 1 and 2 are
depicted in Figures 8 and 9, respectively. In both complexes
the interactions can be observed in the shape-index plot as red
and blue relief regions, as well as in the curvature plot as a flat
zone in the same position of the surface like in the shape-index
plot. Regions on the surfaces through which the complexes
interact are marked with white circles.
50
In the crystal structures, both complexes have the same four
main types of interactions, which are within defined d + d
e
i
distances (Table 4). The rest of the contacts that appear in the
crystal structures of 1 and 2 are C−H/H and C−H/C
contacts. It is apparent that there are some significant
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Table 4. Overview of the Main Types of Intermolecular
Interactions Obtained from Hirshfeld Analysis
differences in interaction distances on going from 1 to 2, such
as C−H/M interactions and C−H/N interactions. In addition,
there are significant differences in the contributions of
individual interactions, with C−H/Cl interactions providing
a much higher contribution in 2, with a shorter interaction
distance. In order to investigate these differences and their
influence on the structure of individual complexes in more
detail, additional DFT calculations were performed on model
systems for the four dominant types of interactions (vide
infra).
d + d (Å)
contribution (%)
e
i
interaction
1
2
1
2
C−H/Cl
C−H/S
C−H/M
C−H/N
3.10
3.20
3.40
3.10
2.85
3.30
3.80
2.45
15.6
8.0
1.4
20.1
5.4
0.8
30.1
27.6
Figure 10. Distributions of d distances (the length of hydrogen bonds) and α angles (the angle of the hydrogen bonds) for intermolecular C−H/
Cl−M, C−H/S−M, and C−H/M interactions in the crystal structures extracted from CSD.
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Analysis of C−H/Cl−M, C−H/S−M, C−H/M, and C−H/
NC Interactions. The study of packing in the crystal
structures of 1 and 2 has shown that the arrangement of
molecules is governed by several noncovalent interactions,
where C−H/Cl−M, C−H/S−M, C−H/M, and C−H/NC
interactions have a significant contribution. In order to
describe the geometries of these interactions, a CSD search
was performed to gather information about noncovalent
5
1
interactions. Intermolecular C−H/Cl−M, C−H/S−M, C−
H/M, and C−H/NC interactions can be characterized as
weak hydrogen bonds, because most of the interactions occur
at distances longer than 2.5 Å (Figure 10). Of these four types
of interactions, the C−H/NC and C−H/Cl−M interactions
occur at the shortest distance range, while the C−H/M
interactions occur at the longest distance range, regardless of
the charge of metal ion (M(I), M(II), or M(III)). Therefore, it
can be assumed that the strength of C−H/S−M interactions
lies between these interactions. The distributions of α angle
Figure 12. Dependence of the hydrogen bond length (d) on the
energy of C−H/Cl−M, C−H/S−M, C−H/M and C−H/NC
interactions (M = Pd, Pt), calculated on CH /[MCl (TM-CN) ]
4
2
2
model systems at the wb97xd/6-31+g** + lanl2dz level of theory.
interaction energy on interatomic distance, where C−H/M
interactions exhibit significantly higher energy values in
comparison to other types of interactions. The strength of
C−H/M interactions, with maximum values of −2.95 kcal/mol
for 1 and −2.84 kcal/mol for 2 (Table S4 in the Supporting
Information), should make them relevant to crystal packing.
The increase in the distance between the C−H group and the
metal ion results in a loss of interaction energy, which can be
compensated by the increased opportunities for additional
(
the angle of the hydrogen bonds) show no tendency toward a
linear orientation of interacting species (160° ≤ α ≤ 180°),
due to the tendency of interacting species (M, XH, M−Cl, M−
S, and CN groups) to form simultaneous interactions in the
crystal structures over long d distances. This is in agreement
with previous crystallographic analysis of C−H/X interactions
(
X = O, N, Cl, F) that showed a tendency of most acceptors
46
for nonspecific interactions with many donors. The
distribution of the α angle (Figure 10) shows that, although
there are differences in the trends across different interactions
and different metal ion charges, they are relatively small.
The calculations for prediction of the strength of C−H/Cl−
M, C−H/S−M, C−H/M, and C−H/NC interactions were
52
interactions.
The interaction strengths of intermolecular interactions from
DFT calculations were then calculated for the corresponding
distances observed in 1 and 2, using the crystal geometries of
axial and equatorial conformers obtained using PBE0
calculations. This suggests that these interactions should play
an important role in determining the most stable crystal
structure of the corresponding complexes (Table 5). The
performed on model systems (Figure 11) containing a CH
4
Table 5. Interaction Strength Obtained from the Crystal
Structure Distances for 1 and 2
distance (Å)
energy (kcal/mol)
interaction
1
2
1
2
C−H/Cl
C−H/S
C−H/M
C−H/N
3.00
3.20
3.35
2.90
2.90
3.15
3.80
2.75
0.63
0.9
2.5
0.6
0.75
1.5
0.55
0.57
results show that there is a significant difference in the energy
of C−H/M interactions, where a shorter Pt−H distance leads
to around a 1 kcal/mol higher energy of interaction. Coupled
with stronger C−H/S interactions in 1, this indicates that the
Pt metal center and its immediate surroundings form
considerably stronger hydrogen bonds in comparison to Pd,
which should be enough to affect the crystal structure of the
complex and result in 1 having a stable crystal structure
different from that of 2. This is in agreement with results of
DFT calculations in a periodic system, where 1 exhibits shorter
C−H/M hydrogen bonds than both 2-ax and 1-eq (Table S5
in the Supporting Information). The last two structures exhibit
M−H hydrogen bond lengths similar to that of the equatorial
conformer of the Pd complex. When it is also considered that
the energy difference between the two structures of Pt
conformers is 1 order of magnitude larger than the difference
between two structures of Pd conformers (Table 5), all of this
suggests that the hydrogen bond interactions with the metal
Figure 11. Model systems for energy calculations of C−H/Cl−M
(
C−H···Cl = 180°, M−Cl···H = 180°), C−H/S−M (C−H···S =
1
1
80°, M−S···H = 109°, Cl−M−S···H = 90°), C−H/M (C−H···M =
80°, S−M···H = 90°, Cl···S−M···H = 90°), and C−H/NC (C−
H···N = 180°, CN···H = 180°) interactions (M = Pd, Pt).
molecule and [MCl (TM-CN) ] complexes (M = Pd, Pt),
2
2
where M acts as the acceptor in the hydrogen bond. These
calculations were performed only on equatorial conformers, in
order to completely isolate individual interactions of the
complexes with CH . The results (Figure 12 and Table S4 in
4
the Supporting Information) show the dependence of the
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Table 6. Calculated Values of Intermolecular Interaction Energies for Different Crystal Structures of Pd (in kJ/mol unless
Otherwise Noted)
symmetry operation
R (Å)
electronic
polarization
dispersion
repulsive
total E
2-ax
−
x, y, z
x, y, z
y + 1/2, −x + 1/2, −z + 1/2
10.67
11.27
12.39
5.15
10.58
11.76
−8.3
6.7
−18.6
−17.7
− 1.0
−15.4
−2.5
−0.9
−4.6
−3.2
−0.5
−3.7
−9.1
−3.5
−11.9
− 42.1
−5.1
4.4
0.1
8.8
22.6
0.2
8
−15.9
3.4
−27.9
−43.8
−5.8
−y + 1/2, −x + 1/2, −z + 1/2
x, y, z
−y + 1/2, −x + 1/2, −z + 1/2
−7.3
−20.4
total energy of intermolecular interactions: −110 kJ/mol (−26.4 kcal/mol)
-eq
2
x, y, z
13.45
13.33
11.44
7.07
5.93
11.63
−19.3
−17.2
7.1
−24.7
−34.3
−6.8
−4.5
−4
−1.6
−9.7
−8.9
−2.1
−21.4
−6
−6.4
−46.1
−47.8
−4.9
19.5
13.4
0.7
46.9
51.7
2.9
−30.4
−18
1.2
−44.4
−52.5
−11.2
−x, y + 1/2, −z + 1/2
x, y, z
x, y, z
−
−
x, y + 1/2, −z + 1/2
x, y + 1/2, −z + 1/2
total energy of intermolecular interactions: −155 kJ/mol (−37.1 kcal/mol)
a
Scheme 1. Possible Conformations of 1 and 2 (A) and Intersonversion between Averaged Conformers of 1 and 2 (B)
a
Abbreviations: S.I. = sulfur inversion, R.R. = ring reversal.
center and its immediate surroundings represent a major
determining factor in the stability of the crystal structures of 1
and 2 and can be attributed in part to the difference in the
crystal structures of these two complexes because stronger
hydrogen bond interactions appear to favor the axial position
of the M−S bond with respect to the ring chair conformation.
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On the other hand, Hirshfeld analysis has shown a higher
contribution of C−H/Cl interactions in 2, and they have been
shown to exhibit longer interaction distances (Table 4). This
suggests that, while 1 is stabilized by hydrogen bonds on and
around the metal center, 2 is stabilized by interactions with
longer distances, which afford more opportunities for
interactions, rather than stronger individual interactions.
A calculation of the energies of intermolecular interactions
in the crystal structure for the axial and equatorial conformers
of Pd shows that the intermolecular interactions stabilize the
equatorial conformer of the Pd complex more than its axial
counterparts (Table 6). The difference in the total energy of
intermolecular interactions between two Pd conformers is
about 10.7 kcal/mol, while a comparison of molecular and
crystal structures in DFT calculations produced a difference of
entity where one of the coordinated TM-CN ligands in 1 is
replaced by the solvent molecule. In that case, according to the
1
ratio of intensities of the signals in the H NMR spectrum, in
DMSO-d solution the ratio of amounts of free TM-CN and
6
the complex species is ca. 8.8:1.2. The existence of the free
ligand and the complex species in the solution is also
evidenced by 2D NMR spectra (COSY, NOESY, and
1
13
H− C HSQC; Figure S10−S12, respectively, in the
1
13
Supporting Information). In H and C NMR spectra of 1
in CD NO there are also two set of signals (Figure S13 in the
3
2
Supporting Information). However, in this noncoordinating
solvent the ligand dissociation does not occur. The two sets of
signals thus can be attributed to different conformers of 1, the
1
axial conformer being the major one. In H NMR spectra of 1
in both solvents all signals are broad, indicating the onset of
the conformation shift (Scheme 1B) at a measurable rate on
the NMR time scale. Yet, interconversion at room temperature
is faster in DMSO probably due to the hydrogen bond
formation ability of this solvent.
5
.1 kcal/mol in the relative stability of the equatorial
conformer in molecular and crystal systems. This confirms
that the interactions between the individual molecules in the
crystal lattice are the deciding factors in the stability of
individual crystal structures, rather than the stability of
individual molecules that make up the crystal lattice.
1
H NMR spectra of 2 in both solvents comprise two very
broad signals (Figures S14 and S15 in the Supporting
Information). This indicates that at ambient temperature the
interconversion process is fast and thus the chemical shifts for
Solution Behavior. NMR spectroscopy was used to study
the solution behavior and stability of the complexes. Thus, the
NMR spectra of the ligand and the complexes were recorded in
two different solventsDMSO as a solvent with a
coordinating ability and capability for hydrogen bond
1
3
all α and all β methylene protons are averaged. The C NMR
spectra of 2 (Figures S14 and S15 in the Supporting
Information) consist of three absorptions due to the α, β,
and carbonitrile carbon atoms of the coordinated ligand.
formation and MeNO as a noncoordinating solvent with a
2
low tendency for hydrogen bond formation. Assignments of
1
13
signals in H and C NMR spectra of the ligand and the
complexes are given in Tables S6 and S7 in the Supporting
Information.
CONCLUSION
Complexes 1 and 2, with the general formula [MCl (TM-
■
2
CN) ], were obtained by the reaction of TM-CN with
2
1
In the H NMR spectra of TM-CN in both solvents two
K [MCl ], where M = Pt, Pd. Both complexes crystallize in the
2
4
signals are observed (Figure S7 in the Supporting Informa-
monoclinic crystal system, but in different space groups. In
order to determine the factors leading to the different crystal
structures with different metal ions, an in-depth theoretical
investigation was conducted, examining the structures of the
two complexes and comparing them with hypothetical
structures with the same metal center and different positions
of M−S bonds with respect to the ligand chair conformation. It
was determined that the most stable molecular structure
implies the coordination of TM-CN via a sulfur atom to both
Pt(II) and Pd(II) with an axial position of the M−S bond with
respect to the ligand chair conformation, although the energy
differences between two structures were around 3 kcal/mol.
This small energy difference proved significant, as the
calculations in the crystal system revealed that the most stable
conformers were axial for 1 and equatorial for 2, corresponding
to the experimentally obtained structures. A comparison of
calculations in the molecular and crystal systems revealed that
it is likely that intermolecular interactions stabilize the axial
conformer for Pt(II) and equatorial conformer for Pd(II),
leading to a change in the most stable conformation from axial
to equatorial in the case of Pd(II). This was confirmed using
calculations of intermolecular interaction energy in the crystal,
where equatorial Pd(II) systems exhibited higher values of
intermolecular interaction energy in comparison to its axial
counterpart. The magnitude of differences in intermolecular
energy between the corresponding axial and equatorial
conformers was in general agreement with the results of
DFT calculations in molecular and crystal systems. The
strength of individual interactions in 1 and 2 was calculated
using a series of model systems in order to determine the
particular type of interaction responsible for the differences
tion). A multiplet at a higher field corresponds to the
equivalent CH groups which are closer to the sulfur atom
2
(
α- and α′-CH ), while a lower field signal corresponds to the
2
other two equivalent CH groups (β- and β′-CH ). The shape
2
2
of both signals can be described as a poorly resolved doublet of
doublets originating from geminal and trans-diaxial couplings
13
of adjacent CH groups. The C NMR spectra of TM-CN in
2
both solvents (Figure S8 in the Supporting Information)
consist of three absorptions due to the α, β, and carbonitrile
carbon atoms of the ligand ring.
Analysis and interpretation of NMR spectra of the
complexes requires consideration of possible conformations
of the complexes. Conformational changes in the case of 1 and
53
2
can be derived by analogy with a similar system. They are
caused by chair to chair reversal of TM-CN ring (R.R.) and
pyramidal inversion at the coordinated sulfur atom (S.I.)
5
3
(
Scheme 1A). The total number of conformers of each of the
complexes is greater than those depicted in Scheme 1A, where
the conformation of one ligand ring is considered independent
of the conformation of the other. Since conformers i and iii,
where the M−S bond is equatorial to the TM-CN ring, or
conformers ii and iv, where the bond is axial (Scheme 1A),
1
53
cannot be distinguished by H NMR spectroscopy,
interconversion between averaged conformational isomers of
the complexes can be represented as in Scheme 1B.
1
13
In H and CNMR spectra of 1 in DMSO-d there are two
6
sets of signals, indicating the presence of two species in
solutionthe free ligand and one complex species (Figure S9
in the Supporting Information). It is reasonable to assume that
the complex species in the solution represents the coordination
M
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Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Article
between two metal complexes. The difference in the crystal
structure of two complexes can be attributed in part to the
apparent inability of the Pd(II) system to form shorter C−H/
M interaction distances in the axial conformation, where
interaction distances remain relatively the same as in the
equatorial conformation, combined with generally weaker C−
H/S interactions. The equatorial conformer is then stabilized
by increased opportunities for longer range interactions, such
as an increased contribution of C−H/Cl interactions, effecting
a change in the most stable conformer on going from a
molecular to a periodic crystal system.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
Ministry of Education, Science and Technological Develop-
ment of the Republic of Serbia (Project No. OI 172057).
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Marko Rodic − Department of Chemistry, Faculty of Sciences,
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