Structure-dependent regioselectivity of a roll-over cyclopalladation occuring at 2,2′-bipyridine-type ligands

Article abstract of DOI:10.1016/j.jorganchem.2021.121780

In this work, different bipyridine-analogue ligands bearing a dimethylamino group in the meta-position of one of the heterocyclic rings were synthesized and reacted with palladium(II) acetate under identical conditions. Cyclometallated palladium(II) complexes with C,N- or C,N,N’-coordinating chelate ligands are formed which were characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy, and single crystal X-ray diffraction analysis. In the case of the mononuclear, C,N,N’-coordinated complex, which is formed by an attack of the palladium(II) site at of the N-methyl groups, the primarily coordinating acetato ligand is exchanged against a chlorido ligand, which is liberated from the solvent dichloromethane by a nucleophilic substitution reaction. In contrast, cyclometallation occurring at one of the six-membered heterocycles leads to dinuclear acetato-bridged palladium(II) complexes.

Full text of DOI:10.1016/j.jorganchem.2021.121780

Journal of Organometallic Chemistry 940 (2021) 121780
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Structure-dependent regioselectivity of a roll-over cyclopalladation
ꢀ
occuring at 2,2 -bipyridine-type ligands
a
b
a
a
a,∗
Yanik Becker , Florian Schön , Sabine Becker , Yu Sun , Werner R. Thiel
a
Technische Universität Kaiserslautern, Fachbereich Chemie, Erwin-Schrödinger-Straße 54, D-67663 Kaiserslautern, Germany
b
Fakultät für Chemie und Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, different bipyridine-analogue ligands bearing a dimethylamino group in the meta-position
of one of the heterocyclic rings were synthesized and reacted with palladium(II) acetate under identical
conditions. Cyclometallated palladium(II) complexes with C,N- or C,N,N’-coordinating chelate ligands are
formed which were characterized by elemental analysis, 1H and 13 C NMR spectroscopy, and single crystal
X-ray diffraction analysis. In the case of the mononuclear, C,N,N’-coordinated complex, which is formed
by an attack of the palladium(II) site at of the N-methyl groups, the primarily coordinating acetato lig-
and is exchanged against a chlorido ligand, which is liberated from the solvent dichloromethane by a
nucleophilic substitution reaction. In contrast, cyclometallation occurring at one of the six-membered
heterocycles leads to dinuclear acetato-bridged palladium(II) complexes.
Received 12 February 2021
Revised 8 March 2021
Accepted 9 March 2021
Available online 12 March 2021
Keywords:
Roll-over cyclometallation
N-donor
Palladium(II)
NMR spectroscopy
X-ray structure analysis
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
N,N’-chelate complex. However, thermodynamically a cyclometal-
lated species may be favored. To achieve this with our example,
ꢀ
In 1973 Trofimenko defined a cyclometallation reaction as a
chemical transformation leading to a (small) ring system that con-
tains a metal site and a carbon atom directly bound to each other
one of the M-N bonds of 2,2 -bipyridine has to be broken and a
rotation around the C-C bond between the two pyridyl rings has
to occur, allowing the metal to attack at the ortho-C-H unit of
the ligand. This often enables a direct comparison of the proper-
ties of classical chelate complexes with those of their cyclometal-
lated congeners. During the last years, we were able to show that
roll-over cyclometallation may provide profound benefits for ho-
mogeneous catalysis [8]. A common feature of the ligand systems
we used for these studies is the presence of a 2-aminopyrimidin-
4-yl ring, that undergoes a type of roll-over cyclometallation re-
action which is mechanistically closely related to an electrophilic
aromatic substitution reaction. A series of studies that combined
deuterium-labeling experiments with ESI-MS studies in combina-
tion with DFT calculations supported this mechanism [9].
[
1]. However, cyclometallation reactions have been known much
longer. Bähr and Müller for example described a first cyclometal-
lated aluminum compound in 1955 [2]. In this early synthesis, the
M–C bond was formed by activation of a C–X bond (X = halogen).
However, the activation of a C-H bond is also possible and in par-
ticular this strategy has led to a multitude of cyclometallated sys-
tems during the last decades [3–5].
The situation may become complicated in case there is more
than one position in a ligand molecule where a cyclometallation
can occur. While the activation of aromatic units is usually consid-
ered to be easier achievable than the activation of aliphatic groups
[
6], there are electronic as well as steric effects leading to some
In case there are different sites in one ligand where a cyclomet-
allation can occur, it is important to be able to predict the regiose-
lectivity of the attack. This is in particular true for the application
of cyclometallated transition metal complexes in catalysis. We here
report on the regioselectivity of cyclopalladation reactions occur-
ring at a series of structurally closely related N-donor ligands.
modifications of this general rule. Friedrich and Cope for exam-
ple established that five-membered metallacyclones usually exhibit
particularly high stabilities [7].
Some time ago, we started to investigate a special type of cy-
clometallation reactions in more detail, the so-called roll-over cy-
ꢀ
clometallation. Herein, a chelating ligand such as 2,2 -bipyridine
plays the central role, which kinetically will provide access to a
2. Results and discussion
A common feature of the ligands 1–3 (Scheme 1) we applied
in this study is a six-membered nitrogen containing heterocycle
bearing a dimethylamino group next to the nitrogen atom. This
∗
Corresponding author.
E-mail address: thiel@chemie.uni-kl.de (W.R. Thiel).
https://doi.org/10.1016/j.jorganchem.2021.121780
0
022-328X/© 2021 Elsevier B.V. All rights reserved.

Y. Becker, F. Schön, S. Becker et al.
Journal of Organometallic Chemistry 940 (2021) 121780
Scheme 2. Synthesis of the palladium complexes 4–6.
Scheme 1. Synthesis routes leading to ligands 1–3. i) and iv) HC(NMe2)(OMe)2, 6 h,
reflux; ii) and v) [(Me2N)C(NH2)2]2(SO4), EtOH, reflux, 24 h; iii) MeMgBr in Et2O,
THF, 18 h, 0 °C; vi) CuCN, DMF, 18 h, 150 °C; vii) DMF, 72 h, 160 °C; viii) NaOMe
then NH4Cl, MeOH, 3 h, reflux then EtOH, 1 h, reflux; ix) NaOMe, toluene, 18 h,
reflux.
from simple starting materials, 2,6-dichloropyridine was converted
into 2-chloro-6-cyanopyridine, which gave 2-dimethylamino-6-
cyanopyridine after treatment with DMF at elevated tempera-
tures [11,12]. 1-(6-(Dimethylamino)pyridin-2-yl)amidinium chlo-
ride is obtained by treating 6-dimethylamino-2-cyanopyridine with
sodium methanolate and ammonium chloride [13]. The final clo-
sure of the pyrimidine entity was done by treating the guani-
dinium intermediate with a mixture of appropriate prop-1-enals
which was generated by reacting 1,1-diethoxyhexane [14] with
phosphoroxytrichloride in dimethylformamide [15]. All ligands and
intermediates were fully characterized by NMR and infrared spec-
troscopy (see the Supporting Information) and by elemental analy-
sis.
fragment is combined with either a second pyrimidine or a pyri-
dine ring. Using a simple primary amino (NH ) instead of a ter-
2
tiary dimethylamino group (NMe ) leads to stable N,N-coordinated
2
systems, which do not undergo roll-over cyclometallation at all.
From our mechanistic studies it is clear, that the dimethylamino
substituent is beneficial for a roll-over cyclometallation in differ-
ent ways: 1) it is sterically more demanding than a primary amino
group, which weakens the N-M bond at the N,N’-coordinated inter-
mediate, 2) it is not able to undergo any intramolecular hydrogen
bonding e.g. to a halogenido ligand coordinated at the metal site
which would strongly stabilize the N,N’-coordinated species, and 3)
it is a substituent with a strong +M effect (stronger than the +M
effect of a NH2 group), which allows the cyclometallation to occur
To get a deeper insight in the regioselectivity of the roll-over
cyclometallation reactions occurring with ligands 1–3, they were
treated with palladium(II) acetate under identical conditions in re-
fluxing dichloromethane. The results are summarized in Scheme 2.
While ligands 1 and 2 are undergoing roll-over cyclometallation
at the dimethylamino functionalized heterocycle, one of the methyl
groups of the dimethyl amino substituent is activated with ligand
3. This leads to a trident C,N,N’-coordination mode in the derived
palladium(II) complex 6 (for a detailed structural discussion see
below) while C,N-coordination is observed in compounds 4 (from
ligand 1) and 5 (from ligand 2) leading to acetato-bridged dinu-
clear palladium(II) complexes. At a first glance, it seems strange
as a S Ar reaction. Scheme 1 summarizes all information about the
N
ligand synthesis.
Ligands 1 and 2 are synthesized following the same strat-
egy: The dimethylaminopyrimidine ring is closed by a cyclisa-
tion reaction between an appropriate 3-aminoprop-2-en-1-one
and dimethylaminoamidinium sulfate [8f,10]. 1-(2-Pyridinyl)- re-
spectively 1-(2-pyrimidinyl)-3-aminoprop-2-en-1-one are obtained
from the corresponding acetyl derivatives by treatment with
dimethylformamide dimethylacetale in high yields [8f,10,8b].
While 2-acetyl pyridine is commercially available, the pyrimi-
dine derivative was synthesized by reacting 2-cyanopyrimidine
with methylmagnesium bromide [8b]. The access to ligand 3
ꢀ
that instead of the expected acetato complex 6 , the chlorido com-
plex 6 is obtained. The chlorido ligand probably origins from the
solvent, which undergoes nucleophilic substitution by acetate. The
trident C,N,N’-coordination mode is proved by 1H and 13C NMR
spectroscopy and elemental analysis as well as an X-ray structure
is different, since it bears
a 2-dimethylaminopyridin-6-yl in-
ꢀ
stead of a 2-dimethylaminopyrimidin-6-yl ring. An appropri-
analysis on compound 6 which is present in the raw product in
ate starting material is not commercially available. We there-
fore decided not to construct the 2-dimethylaminopyridin-6-yl
ring but the pyrimidin-2-yl ring. The primary intention to in-
troduce the butyl group in the 4-position of the pyrimidin-2-
yl ring was to enhance the solubility of the derived transi-
tion metal complexes in organic solvents. To achieve ligand 3
traces and could be isolated in crystalline form (see below).
The most striking difference between the 1H NMR spectra of
ligands 1 and 2 and their palladium(II) complexes 4 and 5 is that
the resonance of H5 (for the numbering see the Supporting In-
formation) at the aminopyrimidinyl ring is no longer present in
the spectra of these complexes. By metalation of this ring, the
2

Y. Becker, F. Schön, S. Becker et al.
Journal of Organometallic Chemistry 940 (2021) 121780
ꢀ
Fig. 1. Molecular structures of compounds 4 (left), 5 (middle) and 6 (right) in the solid state. Hydrogen atoms are omitted for clarity. The ellipsoids are at the 50% level.
Table 1
Characteristic bond lengths [ A˚ ] and angles [°] of the palladium(II) complexes 4, 5 and 6 .
ꢀ
Bond lengths
4
5
6
Pd1-C3 1.965(2)
Pd1-C6 1.951(3)
Pd1-C14 1.995(2)
Pd1-N3 2.016(2)
Pd1-N1 2.016(2)
Pd1-N3 1.943(2)
Pd1-O2 2.128(2)
Pd1-O1 2.140(2)
Pd1-N2 2.190(2)
Pd1-O1A 2.052(2)
Pd1-O2 2.048(2)
Pd1-O1 2.050(1)
Bond angles
4
5
6
C3-Pd1-N3 81.50(9)
C3-Pd1-O1 174.39(8)
C6-Pd1-N1 81.1(1)
C6-Pd1-O1 171.0(1)
C14-Pd1-N3 82.36(7)
C14-Pd1-N2 160.81(7)
C3-Pd1-O2 95.04(8)
N3-Pd1-O2 176.01(8)
C6-Pd1-O2 95.3(1)
O2-Pd1-N1 173.3(1)
C14-Pd1-O1 90.27(6)
N3-Pd1-O1 171.98(6)
N3-Pd1-O1 93.65(7)
N1-Pd1-O1 94.9(1)
N2-Pd1-O1 108.87(6)
O1-Pd1-O2 89.70(7)
O1-Pd1-O2 89.5(1)
N2-Pd1-N3 78.60(6)
two doublets at about 8.5 (H6) and 7.4 ppm (H5) have disap-
peared and one singlet is observed at about 8.1 ppm (H6). The res-
onances of H8 (in 4) respectively H7 (in 5) are shifted to higher
field due to the carbanionic nature of metallated carbon next to
their position. The resonance of the formerly magnetically equiva-
lent protons next to the nitrogen atoms of the pyrimidin-2-yl ring
group at 2.97 ppm, which demonstrates the withdrawing of elec-
tron density by the palladium(II) center. The same effect is ob-
served in the 13C NMR spectrum of compound 6 (N-CH -Pd: 50.8,
2
N-CH : 36.6 ppm).
3
While the complexes 4 and 5 crystallized without any prob-
lems, crystallization of complex 6 led to microcrystalline powders
in most attempts. Compound 4 was achieved crystalline by layer-
ing a chloroform solution with n-hexane, while single crystals of
compound 5 were obtained by slow diffusion of diethyl ether into
a dichloromethane solution. A few crystals from a crystallization
experiment with compound 6 were of sufficient quality for single
crystal X-ray diffraction. The solution and refinement process how-
ever derived the expected C,N,N’coordination but the forth coordi-
(
9.00 ppm) in palladium complex 5 splits into two doublets of
doublets at 8.74 and 8.29 ppm. At this point it has to be men-
tioned, that according to the X-ray structures of compounds 4 and
5
(see below) two different orientations of the C,N donors are pos-
sible in the dimeric, acetato-bridged arrangements leading either
to a C2 symmetric or a CS symmetric structure. In both cases, no
trace of a second isomer could be detected in the NMR spectra,
which suggests that the observed structures are the energetic min-
ima. There is one case in the literature where a C2 symmetric and
a CS symmetric structure are present in the reaction mixture of a
cyclometallation: Zucca et al. reported this for the unsymmetrical
1
nation site was occupied by a η -coordinating acetato instead of a
ꢀ
chlorido ligand. We believe that the acetato complex 6 is the pri-
mary product of the cyclometallation reaction, whereby one equiv-
alent of acetic acid is liberated. Most of this compound, which is
not accessible in larger amounts, is in the following converted into
the chlorido complex 6, by chloride that is liberated from the sol-
vent dichloromethane via a nucleophilic substitution reaction as
ꢀ
precursor 6-methoxy-2,2 -bipyridine [16].
In contrast, the number of aromatic protons does not change for
complex 6. As in compound 5, the two protons of the pyrimidine
ring are no longer chemically equivalent due to the coordination
of one of the pyrimidine nitrogen atoms to the palladium(II) cen-
ter. The resonances of the pyrimidine protons appear at 8.72 and
ꢀ
discussed above. Nevertheless, the molecular structure found for 6
is fully consistent with the 1H and 13C NMR data of 6. Compound
ꢀ
6
crystallizes with one additional molecule of acetic acid, which
8
.70 ppm (AB spin system), which is close to the value being mea-
sured for these protons in ligand 3. This is different to complex
, where the proton next to the nitrogen atom that coordinates to
undergoes hydrogen bonding to the free oxygen atom of the ac-
ꢀ
etato ligand. The solid-state structures 4, 5 and 6 are presented in
5
Fig. 1. Table 1 summarizes the relevant bond parameters.
the palladium site is shifted strongly to lower field compared to
the other one (see above). The absence of this effect indicates one
rather weak Pd–N bond in compound 6. This is due to the steric
consequences of the strong Pd–C bond in the trans-position, which
is formed with one of the methyl groups of the former dimethy-
lamino moiety. In the 1H NMR spectrum the new situation at this
site of the ligand is clearly reflected: The resonance of the methy-
lene group is observed at 4.46 and the resonance of the methyl
The most striking difference between the molecular structures
of complexes 4 and 5 is the relative orientation of the C,N-
donor ligands. While complex 4 occupies a C2 symmetrical ge-
ometry, complex 5 is CS symmetric. At a first glance, C2 sym-
metry should be preferred with respect to some steric repulsion
of the dimethylamino groups. Additionally, the C2 symmetric ar-
rangement should be preferred since it allows for π-stacking of
more and less electron-rich π-systems. It therefore might be that
3

Y. Becker, F. Schön, S. Becker et al.
Journal of Organometallic Chemistry 940 (2021) 121780
the electronic difference between the pyridine rings and the cy-
clometallated aminopyrimidine rings in compound 5 is small and
that attractive interactions of the dimethylamino methyl groups
are stabilizing the CS symmetric arrangement. Aside these differ-
ences, the bond parameters of complexes 4 and 5 are rather similar
and found in the range of related structures, which have been re-
ported in the literature [17]. Labinger et al. have for example stud-
ied in detail the electronic structures of acetato-bridged dimeric
palladium(II) complexes bearing cyclometallated 2-phenylpyridine
ligands in combination with acetate and trifluoro acetate [18]. As
expected, the largely covalent Pd–C bonds are shorter than the
Pd–N bonds. The Pd–O bonds in trans-position to the Pd–C bonds
requires elevated temperatures and the presence of a base [8b,9d].
The Lewis-acidic metal cation requires the deliverance of a cer-
tain amount of electron density from the chelating ligand. In case
of the weaker donating pyrimidin-2-yl ring in ligand 2 (poorer
compared to the pyridine-2-yl ring in ligand 1) the M-N bond to
the 2-dimethylaminopyrimidin-4-yl ring therefore has to become
stronger, which hinders the cyclometallation of this ring. The un-
derstanding of these subtle effects allows to explain the reactiv-
ity of ligand 3: Here the pyrimidin-2-yl ring is a weaker donor,
which strengthens the Pd–N bond to the 6-dimethyaminopyridin-
2-yl ring in a way that the cyclometallation of the pyridine ring
is impossible. However, there are basic acetato ligands not only
at the palladium(II) site, but also in close proximity to the N-
methyl groups, which are already activated for deprotonation by
the electronegative nitrogen atom. The role of the acetato groups
is crucial! We never observed any cyclopalladation with palladium
dichloride. In the case of compound 6, we therefore believe that
the cyclometallation occurs first followed by a slower substitution
reaction at the solvent dichloromethane that delivers the chlorido
˚
are by about 0.1 A shorter than those in the trans-position to the
Pd–N bonds indicating a stronger trans-influence of the carbanion
compared to the nitrogen donor site. Due to the tridentate C,N,N’-
coordination mode of the chelating ligand, the bond parameters
ꢀ
of complex 6 are different. The strong Pd–C bond in combina-
tion with the steric strain of the two five-membered rings that
are annulated to one pyridine ring leads to a pronounced elonga-
tion of the Pd–N bond in the trans-position to the Pd–C bond (see
ꢀ
ligand for the formation of 6 from the acetato intermediate 6 .
˚
–
NMR discussion), which is by almost 0.2 A longer than the Pd N
bonds in complexes 4 and 5. The Pd–N bond in trans-position to
the acetato ligand is in contrast shorter than the Pd–N bonds in
complexes 4 and 5, which can be explained by the weak trans-
influence of the acetato ligand and again with the steric strain
3. Conclusion
Three different bipyridine-analogue ligands bearing dimethy-
lamino units and their cyclometallated palladium(II) complexes
were synthesized and structurally as well as spectroscopically char-
acterized. Despite of closely related ligand structures, the cyclopal-
ladation reaction occurs either at the heteroaromatic fragment or
at the dimethylamino moiety. The regioselectivity of the product
formation depends on subtle effects, in particular on the Pd–N
bond strengths of the intermediately formed N,N’-adducts. A pro-
found knowledge of these effects in the future will allow for a pre-
diction of cyclometallation reactivity, which is of importance, e.g.
for the design of a series of catalysts.
ꢀ
of the two five-membered rings. The Pd–C bond in complex 6 is
slightly longer than in complexes 4 and 5, which can be explained
by the different hybridizations of the involved carbanionic carbon
atoms. In the literature, there is a series of structurally character-
ized C,N,N’ coordinated palladium(II) complexes with bond param-
ꢀ
eters similar to those of compound 6 [19].
At the end arises the question on the stereoelectronic reasons
that are responsible for the different regioselectivities of the roll-
over cyclometallation. A few years ago, Cinellu et al. published the
ꢀ
ꢀ
roll-over cyclometallation of 6,6 -dimethoxy-2,2 -bipyridine with
palladium(II) acetate and found, depending on the reaction con-
ditions, cyclometallation at one of the heterocycles (like in com-
pounds 4 and 5) as well as at the methoxy group, which is com-
4. Experimental section
ꢀ
4.1. General remarks
parable to the process leading to 6 [16]. The corresponding plat-
inum(II) complexes were also obtained. Here the regioselectivity of
the roll-over cyclometallation depends on the reaction conditions
and on the applied palladium(II) precursors.
All reactions were carried out under an atmosphere of dini-
trogen. The solvents were dried and degassed before use accord-
ing to standard techniques. Other reagents were obtained from
commercial suppliers and used as received. 1H and 13C NMR
spectra were recorded on BRUKER Spectrospin Avance 400 and
At this point it is helpful to remember the mechanism that ex-
plains the roll-over cyclometallation of an aromatic ring system
as it occurs with complexes 4 and 5. This reaction can be con-
sidered as an electrophilic aromatic substitution with the metal
cation acting as the electrophile and one of its ligands acting as
a base that takes the proton that is substituted during the cy-
clometallation. In the course of the roll-over cyclometallation it is
known that the chelating ligand coordinates first with its two ni-
trogen atoms. To allow the metal site to attack the carbon atom,
one of the M–N bonds has to be broken. Therefore, the strength
of this bond is critical. The σ-donor ability of the nitrogen donor
is lowered by σ-accepting fragments such as additional nitrogen
atoms that are located in the heterocycle or directly bound to it.
In addition, the dimethylamino group next to the donating nitro-
gen atom weakens the M-N bond by steric repulsion with the lig-
and in the cis-position at the metal site. This explains why the
6
00 spectrometers at room temperature (unless otherwise de-
noted). The chemical shifts are referenced to internal solvent reso-
nances and the assignment of the resonances refers to the num-
bering schemes provided Scheme 3 and in the Supporting In-
formation to this manuscript. Infrared spectra were recorded on
a Perkin Elmer FT-ATR-IR spectrometer Spectrum 100 equipped
with a diamond coated ZnSe window. Elemental analyses (C,H,N)
were carried out with a vario MICRO cube elemental analyzer
at the Analytical Laboratory of the Fachbereich Chemie. All com-
mercially available starting materials were purchased from Sigma
Aldrich and used without any further purification. Toluene and
dichloromethane were dried in a MB-SPS solvent dryer. Tetrahydro-
furane was dried over potassium/benzophenone, acetonitrile was
dried over CaH . 2-(2-Dimethylaminopyrimidin-4-yl)pyridine (1)
2
-dimethyaminopyrimidin-4-yl ring is undergoing cyclometallation
2
and 2-(2-dimethylaminopyrimidin-4-yl)pyrimidine (2) were syn-
so easily. However, the dimethyamino group is also a strong π-
donor, which is favorable in the sense of an electrophilic aromatic
substitution, since it stabilizes the intermediate where the metal
site is σ-bound to the ring. Astonishingly, the second ring, that
does not undergo cyclometallation, also has an influence on the cy-
clometallation of the first ring. Ligand 1 e.g. reacts spontaneously
thesized according to published procedures [8b,f].
4.2. 6-Chloro-2-picolylnitrile
50.0 g of 2,6-dichloropyridine (338 mmol) and 15.3 g of cop-
per(I) cyanide (169 mmol) were added to 100 mL of dry DMF. The
∗
with [Cp IrCl ] to give the cyclometallated product, while ligand 2
2
2
4

Y. Becker, F. Schön, S. Becker et al.
Journal of Organometallic Chemistry 940 (2021) 121780
mixture was heated to 160 °C for 18 h. After cooling to room tem-
perature, the resulting suspension was poured onto 400 mL of a
saturated aqueous solution of Na CO and extracted three times
with 4 × 30 mL of dichloromethane and 4 × 30 mL of diethyl
ether. The combined organic phases were dried over MgSO and
4
the solvent was removed under vacuum. According to the 1H NMR
spectrum a 2:3 mixture of 2-butyl-1-ethoxyprop-1-enal and 2-
butyl-1-dimethylamino-prop-1-enal was obtained, which was nor
further purified. The two components were not separated because
they react in the same way in the next step. Yield: 95% of a brown
2
3
with 300 mL of ethyl acetate. The combined organic phases were
dried over MgSO . After filtration of the drying agent, the solvent
4
was removed under reduced pressure and the raw product was pu-
rified by column chromatography (ethyl acetate/n-hexane 1:4 v/v).
Yield: 9.84 g (42%) of a colorless solid. Elemental analysis calcd.
for C H ClN (138.55): C 52.01, H 2.18, N 20.22; found: C 52.01, H
oil. 1H NMR (CDCl , 600 MHz) of the minor ethoxy derivative: δ
3
9.18 (s, 1H, H1), 6.92 (s, 1H, H3), 4.14 (q, 3JHH = 7.1 Hz, 2H, H8),
6
3
2
2
.25, N 20.30%. H NMR (CDCl , 400 MHz): δ 7.83 (dd, 3JHH = 8.6,
1
2.20 (t, 3JHH = 7.7 Hz, 3H, H9), 1.34 (m, 6H, H4, H5, H6), 0.88 (t,
3
3JHH = 7.1 Hz, H3, 1H), 7.65 (dd, 3JHH = 7.6 Hz, 4JHH = 0.9 Hz, H4,
3JHH = 7.2 Hz, 3H, H7). H NMR (CDCl , 600 MHz) of the major
1
3
4
13
1
1
H), 7.58 (dd, 3
J
= 8.2 Hz,
J
= 0.9 Hz, H2, 1H). C{ H} NMR
dimethylamino derivative: δ 8.72 (s, 1H, H1), 6.46 (s, 1H, H3), 3.09
HH
HH
(
CDCl , 101 MHz): δ 152.9 (C5), 139.8 (C3), 133.6 (C1), 128.6 (C4),
(s, 6H, H8), 2.35–2.29 (m, 2H, H4), 1.35–1.17 (m, 4H, H5, H6), 0.88–
3
27.3 (C2), 116.0 (CN). IR (ATR, cm 1): ν3081 w, 3059 w, 2240 w
−
˜
0.80 (m, 3H, H7). For the 13C{ H} NMR spectrum of the product
1
1
(
ν ), 1570 m, 1553 m, 1430 m, 1143 m, 985 m, 805 s, 678 m.
C≡N
mixture see the Supporting Information.
4
.3. 6-(Dimethylamino)-2-picolylnitrile
4.6. 5-Butyl-2-(6-dimethylaminopyridin-2-yl)pyrimidine (3)
5
.00 g (36.1 mmol) of 6-chloro-2-picolylnitrile were dissolved in
316 mg (13.8 mmol) of sodium were dissolved in 25 mL
of methanol. The solvent was removed under reduced pressure
and the remaining solid was suspended in 30 mL of toluene.
2.50 g (12.5 mmol) of 6-(dimethylamino)pyridin-2-yl)guanidinium
chloride were added and the mixture was stirred for 30 min
at room temperature. Then 1.95 g (ca. 12.5 mmol) of the mix-
ture of 2-butyl-1-ethoxyprop-1-enal and 2-butyl-1-dimethylamino-
prop-1-enal described above were added and the reaction mixture
was heated to reflux for 18 h. After cooling to room temperature,
20 mL of saturated brine and 10 mL of water were added. The or-
ganic phase was separated and the aqueous phase was extracted
with 3 × 30 mL of dichloromethane. The combined organic solu-
tions were dried over Na SO and the solvents were removed un-
2
0 mL of DMF and heated to 160 °C for 48 h. After cooling to room
temperature, the solvent was removed under vacuum and the re-
sulting solid was in a first step purified by column chromatography
(
ethyl acetate/n-hexane 1:5 v/v). Final purification was carried out
by subliming off residuals of 6-chloro-2-picolylnitrile. Yield: 2.94 g
(
(
55 %) of a colorless solid. Elemental analysis calcd. for C H N
8 9 3
147.18): C 65.29, H 6.16, N 28.55; found: C 65.07, H 6.19, N 28.51%.
1
H NMR (CDCl , 400 MHz): δ 7.45 (dd, 3JHH = 8.8, 3JHH = 7.2 Hz,
HH
3
H3, 1H), 6.86 (d, 3JHH = 7.2 Hz, H4, 1H), 6.66 (d,
3
J
= 8.8 Hz, H2,
13
1
1
H), 3.06 (s, 6H, NMe ). C{ H} NMR (CDCl , 101 MHz): δ 159.0
2 3
(
(
C5), 137.3 (C3), 131.5 (C1), 118.3 (CN), 116.2 (C4), 109.8 (C2), 37.8
−1
NMe ). IR (ATR, cm ): ν˜ = 2915 w, 2230 m, 1739 m, 1599 s,
2
2
4
1
550 m, 1380 s, 1186 s, 784 s.
.4. 6-(Dimethylamino)pyridin-2-yl)amidinium chloride
9.2 mg (3.9 mmol) of sodium were dissolved in 15 mL of
der reduced pressure. The raw product obtained this way was pu-
rified by column chromatography (ethyl acetate / n-hexane, 1:5).
Yield: 897 g (28%) of a pale yellow solid. Elemental analysis calcd.
4
for C15H20N4 × (C H O )
(256.35): C 69.03, H 7.97, N 20.13;
4
8
2 0.25
found: C 69.16, H 8.02, N 20.15%. 1H NMR (400.1 MHz, CDCl ): C
8
3
69.03, H 7.97, N 20.13; found: C 69.16, H 8.02, N 20.15%. 1H NMR
methanol. 2.94 g (19.4 mmol) of 6-(dimethylamino)-2-picolylnitrile
were added and the mixture was stirred for 18 h at room temper-
ature. Then 1.25 (23.3 mmol) of ammonium chloride were added
and the resulting solution was stirred under reflux for 3 h. After
cooling to room temperature the solvent was stripped off under
reduced pressure, the remaining solid was treated with 20 mL of
ethanol and heated to reflux for another hour. After filtration of
the hot solution, the solvent was removed under vacuum and the
resulting rw product was recrystallized from a 1:1 mixture of n-
hexane and isopropanol. Yield: 2.61 g (67%) of a colorless solid. El-
emental analysis calcd. for C H13ClN4•(NH Cl) (200.67): C 43.27,
(400.1 MHz, CDCl ): δ 8.69 (s, 2H, H7), 7.66 (d, 3JHH = 7.2 Hz,
3
1H, H2), 7.62–7.56 (m, 1H, H3), 6.63 (d, 3JHH = 8.3 Hz, 1H, H4),
3.20 (s, 6H, N(CH ) ), 2.64 (t, 3JHH = 7.6 Hz, 2H, H9), 1.63 (p,
HH
3
2
3JHH = 7.7 Hz, 2H, H10), 1.38 (m, 2H, H11), 0.94 (t,
3
J
= 7.3 Hz,
3H, H12). 13C{ H} NMR (CDCl , 101 MHz): δ 162.9 (C6), 159.61
1
3
(C5), 157.4 (C7), 153.3 (C1), 138.0 (C3), 133.8 (C8), 111.7 (C2), 107.23
(C4), 38.2 (N(CH ) ), 33.0 (C9), 30.1 (C10), 22.2 (C11), 13.9 (C12) IR
3
2
(ATR, cm 1): IR (ATR, cm ): ν2957 w, 2929 w, 2858 w, 1594 m,
−
−1
˜
1414 s, 983 m, 783 s.
8
4
0.4
H 6.63, N 27.75; found C 43.11, H 6.75, N 27.90%. 1H NMR (CDCl3,
4.7. General method for the synthesis of the palladium complexes 4–6
4
1
1
00 MHz): δ 9.31 (br. s, NH , 4H), 7.75 (dd, 3JH,H = 8.7 Hz, 7.3 Hz,
H,H
2
H, H3), 7.51 (d, 3JH,H = 7.3 Hz, H2, 1H), 7.00 (d, 3J
= 8.5 Hz, H4,
Palladium(II) acetate (1.0 equiv.) and the appropriate ligand
(1.05 equiv.) were filled into a crimp-cap vial. The vial was closed
and the atmosphere was exchanged against nitrogen. Then 5.0 mL
of dichloromethane were added and the vial was heated to 60 °C
for 18 h. After this time, the solvent was removed under reduced
pressure. Single crystals for x-ray were obtained either by ether
diffusion into a dichloromethane solution (5) or by layering a chlo-
roform solution with n-hexane (4 and 6).
13
1
H), 3.10 (s, 6H, NMe ). C{ H} NMR (CDCl , 101 MHz): δ 162.5
2
3
(
(
C6), 158.2 (C5), 141.4 (C1), 138.5 (C3), 111.4 (C4), 110.4 (C2), 37.7
−1
˜
C7). IR (ATR, cm ): ν3360 m, 3053 s, 1686 m, 1599 s, 1513 s,
383 m, 1190 m, 799 m, 740 m.
1
4
1
.5. 2-Butyl-1-ethoxyprop-1-enal and 2-Butyl-1-dimethylamino-prop-
-enal [15]
2
12.9 mL (21.3 g, 139.0 mmol) of phosphoroxytrichloride were
4.8. Bis[μ -acetato(2-(2-dimethylaminopyrimidin-4-
cooled to 0 °C. 14.2 mL (13.4 g, 183.0 mmol) of dimethylfor-
yl)pyridine)palladium(II)] (4)
mamide were added dropwise over a period of 30 min. 11.0 g
(
63.3 mmol) of 1,1-diethoxyhexane were added drop-wise over a
period of 30 min at 0 °C. The mixture was stirred for another
0 min at 0 °C. It was heated to 75 °C for 2 h and then poured
onto 150 g of crushed ice. By addition of K CO the mixture was
124 mg (617 μmol) of 1, 132 mg (588 μmol) of palladium(II) ac-
etate, yield: 156 mg (73%) of a red solid. Elemental analysis calcd.
for C26H28N O Pd (729.39): C 42.81, H 3.87, N 15.36; found: C
2
8
4
2
42.84, H 3.60, N 15.62%. 1H NMR (400.1 MHz, CDCl ): δ 8.00 (d,
2
3
3
brought to pH = 9. The aqueous phase was extracted four times
3JHH = 5.3 Hz, 1H, H5), 7.88 (s, 1H, H8), 7.57 (m, 2H, H3, H4), 6.86
5

Y. Becker, F. Schön, S. Becker et al.
Journal of Organometallic Chemistry 940 (2021) 121780
Table 2
Crystallographic data, data collection and refinement.
ꢀ
6
4
5
Empirical formula
Formula weight
Crystal size [mm]
T [K]
C26H28N8O4Pd2
729.36
C12H13N5O2Pd
365.67
C17H22N4O2Pd(C2H4O2)
480.84
0.04 × 0.14 × 0.16
150
0.20 × 0.33 × 0.44
150
0.08 × 0.21 × 0.36
100
λ [ A˚ ]
1.54184
1.54184
0.71073
Crystal system
monoclinic
P21/n
monoclinic
C2/c
triclinic
P1¯
Space group
a [ A˚ ]
b [ A˚ ]
c [ A˚ ]
10.7994(2)
16.8431(2)
14.9851(2)
90
16.8803(3)
12.8934(2)
11.9995(2)
90
8.0745(4)
8.6118(4)
15.0409(8)
81.143(2)
78.060(2)
88.612(2)
1011.02(9)
2
α [°]
β [°]
γ [°]
98.579(2)
90
92.034(1)
90
V [ A˚ 3
]
2695.22(7)
4
2609.98(8)
8
Z
calcd. [g cm 3]
−
1.798
1.861
1.579
ρ
μ [mm 1]
−
11.184
11.574
0.950
θ-range [o]
3.974–62.775
19,003
4.316–62.720
8074
2.394–30.506
63,494
Refl. coll.
Indep. refl.
4318
2097
6177
Data/restr./param.
Final R indices [I>2σ(I)] a
R indices (all data)
GooF b
4318/0/367
0.0196, 0.0488
0.0220, 0.0496
1.012
2097/0/184
0.0284, 0.0726
0.0288, 0.0730
1.115
6177/130/289
0.0298, 0.0636
0.0386, 0.0668
1.068
max/min (e• A˚ −3)
ꢀρ
0.590/-0.378
0.516/-1.126
1.298/-0.941
a
R1 = ꢁ||Fo| - |Fc||/ꢁ|Fo|, ωR2 = [ꢁω(Fo - Fc2)2/ꢁωFo2]1/2. b GooF = [ꢁω(Fo - Fc2)2/(n-p)]1/2
2
2
.
1
3
1
−1
˜
(
(
s, 1H, H2), 3.08 (s, 6H, NMe ), 2.21 (s, 3H, O CCH ). C{ H} NMR
(C12). IR (ATR, cm ): ν2955 m, 2928 m, 2871 m, 1586 s, 1560 s,
1361s, 1013 m, 766 s.
2
2
3
100.6 MHz, CDCl ): δ 182.4 (O CCH ), 167.7 (C9), 162.1, 160.2 (C5,
3
2
3
C8), 159.3, 150.1 (C1, C6), 138.1 (C7), 125.2, 124.3, 120.9 (C2, C3,
C4), 37.3 (N(CH ) ), 24.4 (O CCH ). IR (ATR, cm 1): IR (ATR, cm ):
˜
−
−1
3
2
2
3
4
.11. X-ray structure analyses
ν2926 w, 2853 w, 1547 s, 1517 s, 1393 s, 981 m, 773 s.
Crystal data and refinement parameters are collected in Table 2.
2
4
.9. Bis[μ -acetato(2-(2-dimethylaminopyrimidin-4-
All structures were solved using direct method, completed by sub-
sequent difference Fourier syntheses, and refined by full-matrix
least-squares procedures based on F² [20]. Analytical numeric
absorption correction was carried out to compounds 4 and 5,
while semi-empirical absorption correction (multi-scan) was ap-
yl)pyrimidine)palladium(II)] (5)
9
4.0 mg (467 μmol) of 2, 100 mg (445 μmol) of palladium(II)
acetate, yield: 120 mg (74%) of a reddish-brown solid. Elemen-
tal analysis calcd. for C24H26N10O Pd (731.38): C 39.41, H 3.58,
ꢀ
4
2
plied to complex 6 [21]. All non-hydrogen atoms were refined
N 19.15; found: C 39.26, H 3.88, N 18.85%. 1H NMR (400.1 MHz,
with anisotropic displacement parameters. All hydrogen atoms ex-
CDCl ): δ 8.74 (dd, 3JHH = 4.8, 4JHH = 2.2 Hz, 1H, H4), 8.29 (dd,
ꢀ
3
cept hydrogen atom H3 (complex 6 ) were placed in calculated po-
3JHH = 5.6, 4JHH = 2.2 Hz, 1H, H2), 8.06 (s, 1H, H10), 6.89 (dd,
3JHH = 5.5, 3JHH = 4.9 Hz, 1H, H3), 3.15 (s, 6H, N(CH ) ), 2.22 (s,
sitions and refined by using a riding model. The position of H3
was located in the difference Fourier synthesis, and was refined
semi-freely with the help of a distance restraint, while constrain-
ing its U-value to 1.5 times the U(eq) value of the bonded oxy-
gen atom O3. CCDC 2061925–2061927 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data centre via
www.ccdc.cam.ac.uk.
3
2
13
1
3
H, O CCH ). C{ H} NMR (100.6 MHz, CDCl ): δ 182.8 (O CCH ),
2 3 3 2 3
1
69.1 (C8), 164.1 (C1), 160.9 (C5), 159.8, 158.7, 156.6 (C2, C4, C7),
24.8 (C6), 119.8 (C3), 37.5 (NMe ), 24.4 (O CCH ). IR (ATR, cm 1):
−
1
2 2 3
˜
ν3039 w, 2857 w, 1584 w, 1547 vs, 1462s, 1401 vs, 1386 vs, 1336
s, 1267 m, 1225 m, 1195 m, 1163s, 1070 m, 1020 m, 983 m, 951 m,
8
40 w, 816 m, 780 s, 734 s, 686 m.
4
.10. Chlorido(N-(6-(2-(5-butyl-pyrimidin-2-yl)pyridyl)))(N-
Declaration of Competing Interest
methyl)amino)methylpalladium(II) (6)
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
6
8.1 mg (166 μmol) of 2, 56.8 mg (253 μmol) of palladium(II)
acetate, yield: 81 mg (76%) of a red solid. Elemental analysis
calcd. for C15H19 ClN Pd × (C H10O)0.3 × (H O) (433.85): C 44.85,
4
4
2
0.8
H 5.48, N 12.91; found C 44.80, H 5.21, N 12.66%. 1H NMR
(
400.1 MHz, CDCl ): δ 8.72, 8.70 (2 × d, 4JHH = 2.8 Hz, 2H, H7,
Acknowledgement
3
H7’), 7.58 (dd, 3
J
= 8.8, 7.3 Hz, 1H, H3), 7.38 (dd,
HH
3
J
= 7.3,
HH
HH
4JHH = 0.6 Hz, 1H, H2), 6.41 (d,
3
J
= 8.5 Hz, 1H, H4), 4.49 (s, 2H,
We gratefully thank the DFG-funded transregional collabora-
tive research center SFB/TRR 88 “Cooperative effects in homo-
and heterometallic complexes (3MET)” for the financial sup-
port. We also thank S. Schindler and J. Becker, Justus-Liebig-
University, Gießen, for providing assistance with the crystal
measurement.
H14), 3.03 (s, 3H, H13), 2.71–2.65 (m, 2H, H9), 1.67–1.60 (m, 2H,
H10), 1.38 (sextett, 3 HH
J
= 7.5 Hz, 2H, H11), 0.94 (t, 3JHH = 7.4 Hz,
3
H, H12). 13C NMR (CDCl , 101 MHz): δ 161.9, 160.1 (C5, C6), 157.9,
3
1
55.4 (C7, C7’), 149.9 (C1), 137.9, 137.1 (C2, C3), 110.2, 109.5 (C4,
C8), 50.8 (C14), 36.6 (C13), 32.8 (C9), 30.3 (C10), 22.4 (C11), 13.8
6

Y. Becker, F. Schön, S. Becker et al.
Journal of Organometallic Chemistry 940 (2021) 121780
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