Donor Strength Determination of Pyridinylidene-amide Ligands using Their Palladium-NHC Complexes

Article abstract of DOI:10.1021/acs.inorgchem.0c01585

Pyridinylidene-amides (PYAs) are a relatively new type of N-donor ligands that can exist in three isomeric forms and adopt various resonance structures. This makes them electronically flexible, and in order to evaluate their electronic profile using the Huynh electronic parameter (HEP), seven structurally diverse mixed N-heterocyclic carbenes (NHCs)/PYA palladium complexes of the type trans-[PdBr2(iPr2-bimy)(PYA)] were prepared and fully characterized by various spectroscopic and spectrometric methods. This study shows that PYAs are among the strongest, formally neutral N-donors, but they are still weaker than phosphines and organometallic ligands such as NHCs. Notably, the donating abilities of isomeric PYAs are distinct and can be further fine-tuned by the choice of two substituents making them structurally and electronically versatile. These characteristics and the ease of their preparation hold promise for a wide applicability in coordination chemistry.

Full text of DOI:10.1021/acs.inorgchem.0c01585

pubs.acs.org/IC
Article
Donor Strength Determination of Pyridinylidene-amide Ligands
using Their Palladium−NHC Complexes
Han Vinh Huynh* and Jeroen Thomas Vossen
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Pyridinylidene-amides (PYAs) are a relatively new
type of N-donor ligands that can exist in three isomeric forms and
adopt various resonance structures. This makes them electronically
flexible, and in order to evaluate their electronic profile using the
Huynh electronic parameter (HEP), seven structurally diverse
mixed N-heterocyclic carbenes (NHCs)/PYA palladium com-
plexes of the type trans-[PdBr2(iPr2-bimy)(PYA)] were prepared
and fully characterized by various spectroscopic and spectrometric
methods. This study shows that PYAs are among the strongest,
formally neutral N-donors, but they are still weaker than
phosphines and organometallic ligands such as NHCs. Notably,
the donating abilities of isomeric PYAs are distinct and can be
further fine-tuned by the choice of two substituents making them structurally and electronically versatile. These characteristics and
the ease of their preparation hold promise for a wide applicability in coordination chemistry.
INTRODUCTION
■
Pyridinylidene-amides (PYAs) are a new class of ligands that
have been introduced in 2011 by Wright and co-workers.1
Since then, multiple complexes containing PYA ligands have
been synthesized and tested in catalysis primarily by the groups
of Albrecht. For example, ruthenium- and iridium-based
(de)hydrogenation catalysts,2−5 iridium-based water oxidation
catalysts,6,7 and palladium-based cross-coupling1 or olefin
dimerization and isomerization8 catalysts have been reported.
A preliminary study on the cytotoxicity of a platinum-PYA
complex has also been published.9 Similar to N-heterocyclic
carbenes (NHCs), a unique feature of the formally neutral
PYA ligands is that electronically distinct resonance structures
are available, which could impart electronic flexibility (Figure
1). The latter is believed to be particularly important for
catalysis, as such ligands could stabilize electronically distinct
intermediates in a catalytic cycle. Indeed, a dependence of the
resonance state on the solvent polarity and its implications in
catalysis has been demonstrated recently.2,3
Figure 1. Isomeric PYAs and their limiting resonance structures.
Moreover, typical PYAs can exist in three isomeric forms.
Depending on the positions of the exo- and endocyclic
nitrogen atoms relative to each other, these can be called
ortho-, meta-, and para-PYAs (i.e., 2-PYA, 3-PYAs, and 4-
PYAs), respectively (Figure 1). Particularly, meta-PYAs take a
Received: May 30, 2020
special position, since all their (closed-shell) resonance
structures require charge separation, which makes them truly
mesoionic compounds.7 Indeed, electrochemical studies have
shown that they are the strongest donor among the isomeric
PYAs.4,6 In addition, further stereoelectronic diversity in all
three isomers can be achieved by modifications of the R and R′
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
substituents, respectively. All in all, these properties make PYA
ligands interesting study objects especially in terms of their
electronic properties. Notably, a preliminary donor strength
evaluation using a trans-[RhCl(CO)2(para-PYA)] complex1
gave two carbonyl bands at 2069 and 1987 cm−1 (avg 2028
cm−1),10 which are surprisingly lower than those obtained for
the saturated SIMes complex (avg 2038 cm−1).11 This could
imply that PYAs are stronger donating ligands than are
imidazolidin-2-ylidenes, which is less intuitive given the
presence of an electron-withdrawing amide and the (at least
formally cationic) pyridinium group in PYAs.
Clearly, a more in-depth analysis is required for a better
understanding of the electronic impact imposed by these
relatively new nitrogen donors in comparison to other
common and popular ligands. Since we have a long-standing
interest in ligand design12,13 and the study of their stereo-
electronic properties, we herein report the preparation of
mixed NHC/PYA palladium complexes for a systematic
comparison of isomeric PYAs in terms of their donating
power using the Huynh electronic parameter (HEP).14−16 In
addition, the influence of substituents at the amide- and
pyridine-moieties are evaluated.
alkylations of the pyridine moieties can only be affected using
strong electrophiles such as methyl iodide and benzyl chloride.
Overall, precursors to seven structurally and electronically
distinct PYA ligands were prepared. Similar to the C2−H
proton in NHC precursors, N-alkylation leads to a downfield
shift of the N−H signal indicative of increased acidity due to
cation formation and hydrogen bonding to the halides.17
Interestingly, the meta-pyridinium amides 2-H+I− and 5-H+Br−
exhibit N−H resonances that are generally more upfield (less
acidic) than those of their ortho and para isomers. This may
already point to the stronger donating abilities (more basic) of
3-PYAs compared to their isomers.
The free PYAs can be obtained by deprotonation of the N−
H function of the pyridinium amides. This was explored using
two ways as depicted in Scheme 1. Method A employs an
aqueous, concentrated solution of sodium hydroxide in a
biphasic mixture of dichloromethane and water. Upon
deprotonation, the free PYA is expected to be in the organic
phase, and isolation can occur via simple extraction and
evaporation of the solvent. However, an increased tendency of
the free ligands to remain in the aqueous phase was observed
for methyl-substituted compounds 1−3. In method B, the use
of water is avoided, and the reactions are carried out under an
inert argon atmosphere using powdered sodium hydride as a
base in dry acetonitrile. After removal of the solvent, the PYAs
were redissolved in chloroform and separated from the sodium
salts by filtration. Drying the filtrates in vacuo yielded the
products as solids. Apart from compound 2, which undergoes
amide hydrolysis, all other PYAs could be isolated as
analytically pure, but highly hygroscopic, solids. Generally, 4-
PYAs 1, 4, and 7 were obtained in better yields compared to
their ortho and meta isomers. 3-PYA 5 could only be isolated
using sodium hydride, which hints to a generally stronger
basicity of 3-PYAs. This is also in agreement with the more
upfield N−H resonances observed in their salt precursors (vide
supra). With the exception of 2, all PYAs have been
RESULTS AND DISCUSSION
■
Preparation of Ligand Precursors and Free PYAs. The
syntheses of the various pyridinium amides as precursors to
PYAs consist of only two steps and are outlined in Scheme 1.
Commercially available ortho-, meta-, and para-aminopyridines
were routinely converted into respective amides A−D using
acetic anhydride or benzoyl chloride with gradual warming
from 0 °C to ambient temperature (AT). The subsequent N-
Scheme 1. Two-Step Synthesis of Pyridinium Amides as
Precursors to PYA Ligands
1
characterized by ESI mass spectrometry, H and 13C NMR
spectroscopy. The most obvious change from their precursors
1
is the expected absence of the N−H protons in the H NMR
spectra.
For compounds 1 and 7, single crystals suitable for X-ray
analysis were obtained by layering their solutions in chloro-
form with ether and hexane, and their molecular structures are
depicted in Figure 2 along with selected bond distances.
Comparison of the notably shorter N2−C4 (1) and N1−C8
(7) bond lengths of 1.354(1) and 1.353(2) Å to that of 4-
benzamidopyridine18 with 1.400(2) Å reveals significant
contributions of the neutral imine resonance form in these 4-
PYAs. This is further supported by the bond parameters of the
initially aromatic pyridine rings, which show distinct
alternating double and single bonds. These are in the range
of 1.366(2)−1.427(2) Å (1) and 1.359(3)−1.427(3) Å (7)
compared to the typical aromatic bonds of 1380(2)−1.396(2)
Å found for the 4-benzamidopyridine.18
Preparation of Mixed NHC/PYA Palladium Com-
plexes. To compare the donating strengths of the PYAs
using HEP, palladium complexes of the type trans-[PdBr2(iPr2-
bimy)(PYA)] are required. These can be prepared either by
(A) direct reaction of 2 equiv of free PYA with the dipalladium
complex [PdBr2(iPr2-bimy)]2 (I)19 or (B) by in situ
deprotonation of the pyridinium halides with silver(I) oxide
or sodium hydroxide in the presence of complex I (Scheme 2).
The use of silver(I) oxide provides a greater driving force
B
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
compromise selectivity. For 6-H+Br− precursor to the 2-PYA
in complex 13, the lower yield could be due to the close
proximity of the two competing sites. Moreover, metal
coordination of this 2-PYA is anticipated to be more difficult
due to steric hindrance of the N-benzyl group. Ligand
precursor 4-H+Br− to complex 11, however, is free of such
complications, and an excellent yield was obtained.
The positive LR ESI mass spectra of the complexes show
dominant peaks for the [M + Na]+ or [M + H]+ cations.
Presumably, these ions are formed by capture of a sodium
cation or a proton via interaction with the oxygen atoms of the
PYA ligands. HR ESI mass analyses also confirm the formation
of all complexes. For most palladium complexes, only broad 1H
NMR spectra are obtained, which show two sets of broad
isopropyl resonances due to the unsymmetrical nature of the
PYA ligands. The broadness of the signals indicates a certain
degree of fluxionality triggered by potential rotations within
the PYA ligand and around the Pd−N bond.
Figure 2. Molecular structures of 1 (top) and 7 (bottom) in the solid
state. Hydrogen atoms are omitted for clarity; ellipsoids drawn at 50%
probability. 1: O1−C7 1.239(1), N1−C2 1.352(1), N1−C6 1.355(1),
N2−C4 1.354(1), N2−C7 1.363(1), C2−C3 1.367(2), C3−C4
1.421(2), C4−C5 1.427(2), C5−C6 1.366(2) Å. 7: O1−C7 1.238(2),
N1−C8 1.353(2), N1−C7 1.363(2), N2−C11 1.355(2), N2−C10
1.360(2), C8−C12 1.425(2), C8−C9 1.427(3), C9−C10 1.359(3),
C11−C12 1.359(3) Å.
1
Exceptionally, the H NMR spectrum of trans-[PdBr2(iPr2-
bimy)(2-PYAMe2)] (10) with a small PYA ligand shows two
sets of signals in a ratio of ∼70:30. However, these signals are
well-resolved (see Supporting Information). For 2-PYA
ligands, potentially two stereoisomers, e.g., E-3 and Z-3
(Scheme 3), can be anticipated that are caused by the imine
Scheme 2. One-Pot Synthesis of Mixed NHC/PYA
Complexes of Palladium
Scheme 3. E-3 and Z-3 Ligands and Their Pd Complexes
1
double bond. Unfortunately, the H NMR spectrum does not
allow for the differentiation of trans-Z-10 or trans-E-10
complexes that form upon coordination of E-3 and Z-3,
respectively (note the change of priorities upon coordination).
Notably, such isomers are not observed for mesoionic 3-PYA
ligands where imine formation is formally not possible.
Obviously, they also do not exist for the symmetrical 4-PYA
ligands.
through rapid precipitation of very insoluble silver halides. As
such, the synthetic pathways closely resemble those for the
heterobis(carbene) counterparts.11,20 Notably, the use of
iodide salts in the more convenient route B did not lead to
any halide scrambling as silver(I) preferably precipitates with
the heavier halide. Most complexes were obtained in good
nonoptimized yields except for complexes trans-[PdBr2(iPr2-
bimy)(3-PYAMeBn)] (12) and trans-[PdBr2(iPr2-bimy)(2-
PYAMeBn)] (13). Table 1 summarizes selected data for all
complexes 8−14. Notably, the complexes obtained in lower
yields contain PYAs with N-benzyl substituents, and
competing benzylic deprotonations could explain the poorer
outcomes. For example, 3-PYA precursor 5-H+Br− for complex
12 contains the least acidic N−H proton, which could
The 1H NMR spectrum of trans-[PdBr2(iPr2-bimy)(4-
PYAPhBn)] (14) is particularly broad, which hampers signal
assignments. However, it shows two sets of signals for the
benzylic protons with an approximate ratio of 80:20 that could
be due to rotamers (Figure 3, 298 K). Likewise, the 13C NMR
spectrum shows a doubling of signals for most carbon atoms
except for the quaternary ones, i.e., carbene, imine and
carbonyl carbon atoms. The latter observation hints to a
limited rotation of the bulky 4-PYAPhBn ligand with respect to
the Pd−N vector making this process an unlikely reason for
the two sets of signals. To gain insights into other potential
rotational isomers, additional low-temperature 1H NMR
experiments were conducted at −40, −30, −20, and 0 °C
(Figure 3).
Table 1. Selected Data for Complexes 8−14
a
complex
PYA
yield [%]
color
HEP [ppm]
8
9
4-PYAMe2 (1)
3-PYAMe2 (2)
2-PYAMe2 (3)
4-PYAMeBn (4)
3-PYAMeBn (5)
2-PYAMeBn (6)
4-PYAPhBn (7)
75
78
60
99
14
28/47
69
orange
red-brown
orange
yellow
dark-green
brown
164.60
165.34
162.60
164.62
165.68
162.62
163.02
Lower temperatures are expected to slow dynamic processes,
which should lead to better resolved spectra. Indeed, a first
coalescence is observed between 298 and 273 K, where the
broad signal at ∼1.6 ppm separates into two broad signals.
Between 273 and 253 K a second coalescence occurs, and
these signals become two distinct and sharp doublets at 1.7
and 1.3 ppm. Similarly, the very broad signals for the CH
group at ∼5.1 and ∼6.2 ppm, which are barely visible in the
10
11
12
13
14
yellow
a
Measured in CDCl3 and referenced to the solvent signal at 77.7 ppm.
C
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Solid-State Molecular Structures. Single crystals suitable
for X-ray diffraction could be obtained for complexes 8, 9, 11,
12, and 14. The molecular structures of 8, 9 and 14 are
depicted in Figure 4 as representatives, while those for the
others can be found in the Supporting Information. As
expected, all complexes are essentially square-planar and
contain the HEP reporter NHC and the PYA ligand in a
trans arrangement. The carbene planes and those formed by
the CNC atoms of the PYA ligands are perpendicular to the
1
Figure 3. Low-temperature H NMR spectra of 14 at 233, 243, 253,
273, and 298 K (500 MHz, CDCl3). Signals at >6.6 ppm and between
2.0 and 4.4 ppm are not displayed for clarity. The signal for residual
water is marked with an asterisk.
spectrum at 298 K, become defined multiplets at 4.9 and 6.2
ppm, respectively. The coalescence temperature for these
signals is between 273 and 253 K. The large deviation in
chemical shift difference between these methine signals of the
major species compared to those of the minor species at ∼6.4
and ∼6.5 ppm suggests that the benzamide group is oriented
toward one of the isopropyl groups in the major isomer (vide
infra). The anisotropy of the phenyl ring affects only one
isopropyl group causing an upfield shift of its NMR
resonances. Overall, this creates a great difference in the
chemical environment between the two isopropyl groups. For
the minor species, the phenyl group is expected to be oriented
away from the isopropyl group via rotation of the N−C(O)
bond, thus having very little impact on the chemical shift
difference. Peculiarly, an additional benzylic signal is observed
at 253 K that merges again at lower temperatures. We believe
that this signal arises from an additional rotamer that could
form via Pd−N rotations that changes the orientation of the
PYA ligand with respect to the square-planar coordination
plane. Conclusively, this experiment provides evidence for the
presence of rotational isomers.
Figure 4. Molecular structures of 8·CHCl3, 9·0.5CHCl3, and 14·
CHCl3 in the solid state. Hydrogen atoms are omitted for clarity;
ellipsoids drawn at 50% probability. Selected bond lengths [Å] and
angles [deg]: 8: Pd1−C1 1.962(4), Pd1−N3 2.106(4), O1−C20
1.234(5), N3−C14 1.367(6), N3−C20 1.370(6), N4−C17 1.343(6),
N4−C16 1.344(6), C14−C18 1.407(6), C14−C15 1.409(7), C15−
C16 1.362(7), C17−C18 1.372(7); N1−C1−N2 107.2(4), C14−
N3−C20 120.4(4). 9: Pd1−C1 1.961(7), Pd1−N3 2.102(5), N3−
C20 1.368(9), N3−C15 1.391(9), N4−C18 1.343(11), N4−C14
1.351(9), O1−C20 1.234(8), C14−C15 1.395(10), C15−C16
1.401(11), C16−C17 1.384(11), C17−C18 1.366(12); N1−C1−
N2 108.2(6), C20−N3−C15 120.6(6). 14: Pd1−C1 1.956(2), Pd1−
N3 2.1197(18), O1−C14 1.221(3), N3−C21 1.361(3), N3−C14
1.392(3), N4−C23 1.353(3), N4−C24 1.356(3), C21−C22
1.418(3), C21−C25 1.423(3), C22−C23 1.361(3), C24−C25
1.360(3); N2−C1−N1 107.92(18), C21−N3−C14 119.09(17).
D
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
square-planar [PdBr2CN] coordination plane. The Pd−C and
Pd−N distances in all complexes are essentially identical
averaging to 1.959 and 2.109 Å, respectively, which cannot be
used to differentiate 4-PYA and 3-PYA complexes. Moreover,
all C−O distances are short and with an average of 1.229 Å
typical for CO double bonds ruling out any notable
contribution of the imidato resonance form. However,
comparison of the bond parameters of the 4-PYA ligands in
complexes 8, 11, and 14 with those of the free ligands 1 and 7
(Figure 2) reveal that they coordinate to palladium as primarily
neutral imines with short exocyclic N−CPy bonds (1.349−
1.367 Å) and alternating single and double bonds in the
pyridyl moieties. The orientation of the benzamide group in 14
facing one isopropyl group of the NHC is mostly retained in
solution representative of the major isomer as evidenced by 1H
NMR studies (vide supra). The exocyclic N−CPy bonds in
complexes 9 and 12 (1.381−1.391 Å) are slightly longer, and
their pyridyl rings exhibit a greater π-delocalization with typical
aromatic C−C bonds revealing significant contribution of the
amidato resonance form for the 3-PYA ligands (Figure 1).
Donor Strength Determination and Comparison.
Using complexes 8−14, the electronic properties of all
isomeric PYA ligands 1−7 can be estimated by means of the
HEP. Their respective values collected in CDCl3 and
referenced to the solvent signal at 77.7 ppm are summarized
in Table 1. With HEP values in the range of 162−166 ppm, the
donating power of PYA ligands is similar to those of other
typical N-donors. However, they are certainly weaker donors
compared to organometallic ligands such as isocyanides,
arsines, phosphites, phosphines, and various types of NHCs,
which all have HEP values of >168 ppm.14,15 This result is
more intuitive than that based on a trans-[RhCl(CO)2(4-
PYA)] complex previously reported (vide supra).
Figure 6. Donor strength comparison between PYAs 1−7.
same type, i.e., 2/5, 1/4, and 3/6, reveal that benzyl groups
could be slightly more electron-releasing compared to methyl
groups in all three types of PYAs. In 3-PYAs, this effect is
enlarged, which implies an increased stabilization of the
cationic pyridinium moiety by the benzyl substituent. Finally,
comparison between ligands 4 and 7 demonstrates that the
donating ability of PYAs can additionally be tuned by variation
of the amide group, whereby benzamides give poorer donors
compared to acetamides. This observation is in agreement with
the greater positive inductive effect of methyl versus phenyl
groups.
CONCLUSIONS
■
Seven mixed NHC/PYA complexes of the general formula
trans-[PdBr2(iPr2-bimy)(PYA)], 8−14, were prepared and fully
characterized by various spectroscopic and spectrometric
techniques. Using these complexes, the donor strengths of
PYA ligands 1−7 was determined using the HEP method and
systematically compared to each other and to those of
common organometallic and Werner-type ligands. Generally,
PYAs are weaker donors than arsines, phosphites, phosphines,
and organometallic ones including isocyanides and various
types of NHCs. However, they are among the strongest
Werner-type ligands superior to pyridine-N-oxide, acetonitrile,
triethylamine, 2,6-dimethylaniline, various pyridines, most
imidazoles, and pyrazoles and are only rivalled by the
organosuperbases TBU and DBU. In addition, they can exist
in three isomeric forms, which allows for a fine-tuning of
electronic properties over a notable range. 3-PYAs are
generally stronger donors followed by 4-PYAs and then 2-
PYAs. Further modifications are possible by choice of the
pyridinium-N-substituent and the amide group. The electronic
and structural diversity of these relatively new ligands and the
ease of their preparation will certainly increase their impact in
coordination chemistry in the years to come.
Figure 5 compares the donating abilities of PYAs to those of
other typical Werner-type ligands on the 13C NMR scale. They
Figure 5. Donor strength comparison of PYAs with other Werner-
typical ligands.
are superior donors compared to pyridine-N-oxide, acetoni-
trile, triethylamine, 2,6-dimethylaniline, various pyridines, most
imidazoles, and pyrazoles.15 Only the organosuperbases TBD
and DBU are stronger donating compared to most PYAs.
A comparison between the PYA ligands 1−7 is depicted in
Figure 6. Notably, 3-PYA ligands 2 and 5 are superior donors
followed by 4-PYAs 1, 4, and 7, respectively. 2-PYAs 3 and 6
are the weakest in this series. These results agree with those
obtained from electrochemical studies4,6 and reflect the
mesoionic nature of 3-PYAs, which favor the amidato
resonance form as supported by the solid-state molecular
structures of complexes 9 and 12. The increased donicity of 4-
PYAs compared to 2-PYAs is also within expectations.
Although both types are likely to adopt the neutral imine
resonance form, the proximity of the electron-withdrawing
pyridyl-nitrogen to the N-donor would render 2-PYAs
significantly less donating. Comparison between pairs of the
EXPERIMENTAL SECTION
■
General Considerations. Unless specifically mentioned, all
operations were performed without taking precautions to exclude
air and moisture, and all solvents and chemicals were used as received.
The identity and purity of all new compounds was confirmed by a
1
combination of H and 13C{1H} NMR spectroscopy, LRMS, HRMS
(ESI), and elemental analysis. NMR chemical shifts were referenced
internally to residual solvent signals relative to tetramethylsilane
(TMS). X-ray data were collected with a Bruker AXS SMART APEX
diffractometer, using Mo Kα radiation with the SMART suite of
programs,21 and refinement parameters are summarized in the Tables
S1 and S2.
General Complexation Route A. Palladium-dimer I (0.141 g,
0.15 mmol, 0.5 equiv) was dissolved in acetonitrile (15 mL), and a
solution of the free PYA ligand (0.30 mmol, 1.0 equiv) in acetonitrile
(3 mL) was added. After stirring for 3 h, the solvent was removed
under reduced pressure, and the product was dried in vacuo. If a
E
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
3
i
precipitate was observed during the reaction, then it was filtered and
washed with acetonitrile (2 × 2 mL) before removal of the solvent
from the filtrate.
iPr−CH), 6.12 (d, 2 H, J = 7.3 Hz, (ii) Pr−CH3), 4.18 (s, 3 H, (i)
NCH3), 4.17 (s, 3 H, (ii) NCH3), 2.85 (s, 3 H, (i) COCH3), 2.81 (s,
3 H, (ii) COCH3), 1.74 (d, 3J = 7.1 Hz, (i) iPr−CH3), 1.72z (d, 3J =
7.3 Hz, (ii) iPr−CH3) ppm. 13C{1H} NMR (126 MHz, CDCl3, AT):
δ = 176.0 ((i) NCOCH3), 175.9 ((ii) NCOCH3), 162.6 ((i, ii) NC),
161.79 ((i) HEP), 161.76 ((ii) HEP) 142.7 ((i) Py−C), 142.6 ((ii)
Py−C), 142.34 ((ii) Py−C), 142.30 ((i) Py−C), 134.1 ((ii) Ar−C),
134.0 ((i) Ar−C), 129.7 ((ii) Py−C), 129.6 ((i) Py−C), 122.8 ((i)
Ar−C), 122.7 ((ii) Ar−C), 119.8 ((i) Py−C), 119.7 ((ii) Py−C),
113.11 ((ii) Ar−C), 113.09 ((i) Ar−C), 55.2 ((ii) iPr−CH), 55.1 ((i)
iPr−CH), 46.1 ((ii) NCH3), 45.8 ((i) NCH3), 29.4 ((ii) COCH3),
General Complexation Route B. A solution of the ligand
precursor (0.30 mmol, 1.0 equiv) in acetonitrile (3 mL) was added to
a solution of palladium-dimer I (0.141 g, 0.15 mmol, 0.5 equiv) in
acetonitrile (12 mL). For precursors with a lower solubility, a few
drops of dimethylsulfoxide were added to the dispersion in
acetonitrile until the precursor was fully dissolved. Subsequently,
silver(I) oxide (0.035 g, 0.15 mmol, 0.5 equiv) was added to the
reaction mixture. After 3 h stirring, the solid formed was filtered and
washed with acetonitrile (4 × 5 mL). The solvent of the filtrate was
removed, and the remaining solid was dried in vacuo for 12 h at 50 °C.
Iodide containing precursors were mixed with silver(I) oxide before
adding the mixture to a solution of palladium-dimer I.
Dibromido(1,3-diisopropylbenzimidazolin-2-ylidene)(N-(1-
methylpyridin-4-ylidene)acetamide)palladium(II) (8). Using
route B, the product was obtained as an orange powder. Orange
single crystals could be obtained from a solution of chloroform
layered with ether and hexane. Yield: 0.140 g (0.23 mmol, 76%). Two
sets of signals (i and ii) are seen in the NMR spectra.
C21H28Br2N4OPd (M = 618.71 g mol−1). 1H NMR (500 MHz,
CDCl3): δ = 8.51 (dm, 2 H, (i) Py−H), 8.47 (dm, 2 H, (ii) Py−H),
i
i
28.8 ((i) COCH3), 21.2 ((i) Pr−CH3), 21.1 ((ii) Pr−CH3), 20.7
i
((ii) Pr−CH3) ppm. HRMS (ESI, positive ions): calcd for [M +
Na]+, C21H28Br2N4NaOPd, m/z = 638.9557. Found, m/z = 638.9562.
Dibromido(1,3-diisopropylbenzimidazolin-2-ylidene)(N-(1-
benzylpyridin-4-ylidene)acetamide)palladium(II) (11). Follow-
ing route B, the product was obtained as a yellow powder. Single
crystals were obtained from a solution of chloroform layered with
diethyl ether. C27H32Br2N4OPd (M = 694.81 g mol−1). Yield: 0.207 g
(0.30 mmol, 99%). 1H NMR (500 MHz, CDCl3, AT): δ = 8.61 (t, 2
H, 3J = 7.5 Hz, Py−H), 7.80 (d, 2 H, 3J = 7.5 Hz, Py−H), 7.57 (dd, 2
3
4
H, J = 6.2 Hz, J = 3.2 Hz, Ar−H), 7.44−7.37 (m, 3 H, Bn−H),
3
4
7.30−7.26 (m, 2 H, Bn−H), 7.20 (dd, 2 H J = 6.2 Hz, J = 3.2 Hz,
3
4
Ar−H), 6.45 (br. s, 1 H, iPr−CH), 6.38 (br. s, 1 H, iPr−CH), 5.22 (s,
7.77−7.71 (m, 2 H, Py−H), 7.57 (dd, 2 H, J = 6.2 Hz, J = 3.2 Hz,
3
3
i
Ar−H), 7.20 (dd, 2 H, J = 6.2 Hz, J = 3.2 Hz, Ar−H), 6.60−6.40
2 H, CH2), 3.12 (s, 3 H, COCH3), 1.81 (d 12 H, Pr−CH3) ppm.
i
i
13C{1H} NMR (126 MHz, CDCl3, AT): δ = 181.8 (NCOCH3), 164.6
(br. m, 2 H, Pr−CH), 6.42−6.25 (br. m, 2 H, (i) Pr−CH), 6.25−
6.12 (br. m, 2 H, (ii) iPr−CH), 3.89 (s, 3 H, (i) NCH3), 3.89 (s, 3 H,
(ii) NCH3), 3.08 (s, 2 H, (i) COCH3), 3.03 (s, 2 H, COCH3), 1.82
(HEP), 164.0 (NC), 140.5 (Py−C), 134.14 (Bn−C), 134.13 (Ar−C),
130.24 (Bn−C), 130.20 (Bn−C), 129.0 (Bn−C), 122.8 (Ar−C),
120.5 (Py−C), 113.3 (Ar−C), 62.2 (CH2), 55.1 (iPr−CH), 31.7
(COCH3), 21.4 (iPr−CH3) ppm. Anal. Calcd for C27H32Br2N4OPd:
C, 46.61, H, 4.78, N, 8.05%. Found: C, 46.05, H, 4.65, N, 8.07%.
Better values could not be obtained despite several recrystallizations.
HRMS (ESI, positive ions): calcd for [M + H]+, C27H33Br2N4OPd,
m/z = 693.0050. Found, m/z = 693.0047.
(d, 12 H, (i) Pr−CH3), 1.79 (d, 12 H, (ii) iPr−CH3) ppm. 13C{1H}
i
NMR (126 MHz, CDCl3): δ = 181.4 (NCOCH3), 164.6 (HEP),
163.9 (NC), 153.4, 145.2, 141.5, 141.3, 134.3, 134.1, 122.9, 122.8,
122.7, 120.7, 120.5, 116.4, 113.3, 55.1 (iPr−CH), 45.9 (NCH3), 31.9
i
((ii) COCH3), 31.5 ((i) COCH3), 21.4 (br., Pr−CH3) ppm. Anal.
Calcd for C21H28Br2N4OPd CHCl3: C, 35.80; H, 3.96; N, 7.59%.
Found: C, 35.82; H, 3.83; N, 7.79%. HRMS (ESI): calcd for [M +
Na]+, C21H28Br2N4NaOPd, m/z = 638.9557. Found, m/z = 638.9539.
Dibromido(1,3-diisopropylbenzimidazolin-2-ylidene)(N-(1-
methylpyridin-3-ylidene)acetamide)palladium(II) (9). After a
reaction according to route B, the product was obtained as a brown
powder, which was crystallized from a solution in chloroform layered
with diethyl ether and hexane to give red-brown single crystals.
C21H28Br2N4OPd (M = 618.71 g mol−1). Yield: 0.146 g (0.24 mmol,
Dibromido(1,3-diisopropylbenzimidazolin-3-ylidene)(N-(1-
benzylpyridin-4-ylidene)acetamide)palladium(II) (12). Precur-
sors not showing a successful conversion with silver(I) oxide were
treated with an excess of sodium hydroxide (0.15 mL, 8.7 M, 4.4
equiv) as a base. The formation of a precipitate upon base addition
could be observed for this reaction. The remaining solid after
removing the solvent was dispersed in chloroform (20 mL), and the
organic phase was washed with water (3 × 10 mL) to remove traces
of salts. After drying the organic phase over sodium sulfate for 1 h, the
solvent of the filtrate was removed, and the residue was dried in vacuo
for several hours at 50 °C. The complex was obtained as dark green
crystals after crystallization from a solution of chloroform layered with
diethyl ether. C27H32Br2N4OPd (M = 694.81 g mol−1). Yield: 0.030 g
(0.043 mmol, 14%). 1H NMR (500 MHz, CDCl3, AT): δ = 9.91 (s, 1
H, Py−H), 9.37 (dm, 1 H, Py−H), 7.69 (d, 1 H, Py−H), 7.61−7.54
(m, 3 H, Py−H, Bn−H), 7.44−7.40 (m, 3 H, Bn−H), 7.35 (dd, 2 H,
1
3
78%). H NMR (500 MHz, CDCl3): δ = 9.52 (d, 1 H J = 6.3 Hz,
Py−H), 9.26 (d, 1 H, 3J = 8.7 Hz, Py−H), 9.21 (d, 1 H, 3J = 8.7 Hz,
Py−H), 7.77 (dm, 1 H, 3J = 5.1 Hz, Py−H), 7.61 (t, 1 H, 3J = 6.3 Hz,
3
4
Py−H), 7.56 (dd, 2 H, J = 6.1 Hz, J = 3.2 Hz, Ar−H), 7.18 (dd, 2
3
4
i
H, J = 6.1 Hz, J = 3.2 Hz, Ar−H), 6.56−6.13 (m, 2 H, Pr−CH),
4.16 (s, 3 H, NCH3), 2.95 (s, 3 H, COCH3), 2.90 (s, 3 H, COCH3),
i
i
1.79 (d, 12 H, Pr−CH3), 1.76 (d, 12 H, Pr−CH3) ppm. 13C{1H}
NMR (126 MHz, CDCl3): δ = 178.3 (NCOCH3), 165.3 (HEP),
152.9 (NC), 141.3 (Py−C), 140.1 (Py−C), 139.8 (Py−C), 134.2
(Ar−C), 134.1 (Ar−C), 133.0 (Py−C), 126.8 (Py−C), 126.7 (Py−
C), 122.8 (Ar−C), 122.7 (Ar−C), 113.2 (Ar−C), 55.12 (iPr−CH),
55.05 (iPr−CH), 49.2 (NCH3), 30.8 (COCH3), 30.3 (COCH3), 21.4
(iPr−CH3), 21.2 (iPr−CH3), 20.8 (iPr−CH3) ppm. Anal. Calcd for
C21H28Br2N4OPd·0.5CHCl3: C, 38.07, H, 4.23, N, 8.26%. Found: C,
38.39, H, 4.12, N, 7.93%. HRMS (ESI, positive ions): calcd for [M +
Na]+, C21H28Br2N4NaOPd, m/z = 638.9557. Found, m/z = 638.9564.
Dibromido(1,3-diisopropylbenzimidazolin-2-ylidene)(N-(1-
methylpyridin-2-ylidene)acetamide)palladium(II) (10). Follow-
ing route B, the complex was obtained as an orange powder. A
mixture of trans-E- and trans-Z-complex was obtained, and NMR
signals will be listed accordingly as (i) and (ii) with (i) as the
predominant complex. C21H28Br2N4OPd (M = 618.71 g mol−1).
Yield: 0.112 g (0.18 mmol, 60%). 1H NMR (500 MHz, CDCl3, AT):
δ = 8.52 (s, 1 H, 3J = 8.7 Hz, (ii) Py−H), 8.46 (s, 1 H, 3J = 8.7 Hz, (i)
Py−H), 8.20−8.10 (m, 2 H, (ii) Py−H), 8.08−7.96 (m, 1 H, (i) Py−
H), 7.64−7.55 (m, 2 H, (ii) Ar−H), 7.52 (dd, 2 H, 3J = 6.2 Hz, 4J =
3.2 Hz, (i) Ar−H), 7.26−7.20 (dm, 2 H, (ii) Ar−H), 7.16 (dd, 2 H, 3J
3J = 6.4 Hz, 4J = 3.0 Hz, Ar−H), 7.19 (dd, 2 H, J = 6.4 Hz, J = 3.0
3
4
Hz, Ar−H), 6.40 (br. s, 2 H, iPr−CH), 5.41 (s, 2 H, CH2), 2.99 (s, 3
H, COCH3), 1.79 (d, 12 H, Pr−CH3) ppm. 13C{1H} NMR (126
i
MHz, CDCl3, AT): δ = 178.6 (NCOCH3), 165.7 (HEP), 153.3
(NC), 141.1 (Py−C), 140.3 (Py−C), 134.2 (Ar−C), 132.8 (Py−C),
131.4 (Bn−C), 130.8 (Bn−C), 130.5 (Bn−C), 129.2 (Bn−C), 126.9
(Py−C), 122.7 (Ar−C), 113.2 (Ar−C), 65.9 (CH2), 55.1 (iPr−CH),
30.4 (COCH3), 21.4 (iPr−CH3) ppm. Anal. Calcd for
C27H32Br2N4OPd·CHCl3: C, 41.31, H, 4.09, N, 6.88%. Found: C,
41.73, H, 4.11, N, 6.80%. HRMS (ESI, positive ions): calcd for [M +
H]+, C27H33Br2N4OPd, m/z = 693.0050. Found, m/z = 693.0048.
Dibromido(1,3-diisopropylbenzimidazolin-2-ylidene)(N-(1-
benzylpyridin-2-ylidene)acetamide)palladium(II) (13). Both
routes have been applied to synthesize this complex giving similar
yields. The raw product was obtained as yellow powder.
C27H32Br2N4OPd (M = 694.81 g mol−1). Yield: 0.098 g (0.141
mmol, 47%). 1H NMR (400 MHz, CDCl3, AT): δ = 8.46 (dd, 1 H 3J
= 7.5 Hz, Py−H), 7.98 (tm, 1 H, 3J = 7.5 Hz, Py−H), 7.71 (dm, 1 H,
3J = 6.6 Hz, Py−H), 7.54 (dd, 2 H, J = 6.2, J = 3.2 Hz, Ar−H),
7.45−7.39 (m, 4 H, Bn−H), 7.37−7.29 (m, 1 H, Bn−H), 7.18 (dd, 2
3
4
3
3
= 6.2 Hz, J = 3.2 Hz, (i) Ar−H), 6.25 (sept, 2 H, J = 7.1 Hz, (i)
F
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
H, 3J = 6.2 Hz, 4J = 3.2 Hz, Ar−H), 7.11 (t, 1 H, 3J = 6.6 Hz, Py−H),
6.31 (sept, 2 H, 3J = 7.1 Hz, iPr−CH), 5.75 (s, 2 H, CH2), 2.90 (s, 3
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
3
i
H, COCH3), 1.76 (d, 12 H, J = 7.1 Hz, Pr−CH3) ppm. 13C{1H}
NMR (126 MHz, CDCl3, AT): δ = 176.9 (NCOCH3), 162.6 (HEP),
162.0 (NC), 142.2 (Py−C), 139.8 (Py−C), 134.1 (Py−C), 133.9
(Ar−C), 131.5 (Bn−C), 130.14 (Bn−C), 130.09 (Bn−C), 129.4
(Bn−C), 122.8 (Ar−C), 119.3 (Py−C), 113.1 (Ar−C), 58.4 (iPr−
CH), 55.2 (CH2), 29.0 (NCOCH3), 21.3 (iPr−CH3) ppm. HRMS
(ESI, positive ions): calcd for [M + H]+, C27H33Br2N4OPd, m/z =
693.0050. Found, m/z = 693.0035.
Funding
Singapore Ministry of Education (WBS R-143−000−669−
112)
Notes
The authors declare no competing financial interest.
Dibromido(1,3-diisopropylbenzimidazolin-2-ylidene)(N-(1-
benzylpyridin-4-ylidene)benzamide)palladium(II) (14). Follow-
ing route A, the complex was obtained as a yellow powder. Yellow
single crystals were obtained from a solution of chloroform layered
with diethyl ether and hexane. Two sets of signals in a ratio of 80:20
are observed in the NMR spectra, which will be listed accordingly as
(i) and (ii). C32H34Br2N4OPd (M = 756.88 g mol−1). Yield: 0.158 g
(0.21 mmol, 69%). 1H NMR (500 MHz, CDCl3, AT): δ = 8.84 (d, 2
ACKNOWLEDGMENTS
■
This work was supported by the National University of
Singapore. Technical support from staff at the Chemical,
Molecular and Materials Analysis Centre (CMMAC) of our
department is appreciated.
3
3
H, J = 7.4 Hz, (ii) Py−H), 8.57 (d, 2 H, J = 7.4 Hz, (ii) Py−H),
8.42 (d, 2 H, 3J = 7.5 Hz, (i) Py−H), 8.36 (dd, 2 H, (ii) Ph−H), 8.28
(dd, 2 H, (i) Ph−H), 7.83 (d, 2 H, 3J = 7.5 Hz, (i) Py−H), 7.62−7.56
(m, 3 H, (ii) Ph−H), 7.56−7.49 (m, 3 H, (i) Ph−H), 7.45 (dd, 2 H,
(ii) Ar−H), 7.43−7.38 (m, 5 H, (i, ii) Bn−H), 7.32 (dd, 2 H, (i) Ar−
H), 7.18−7.12 (m, 2 H, (ii) Ar−H), 7.11 (dd, 2 H, (i) Ar−H), 6.54
(sept, 1 H, 3J = 6.7 Hz, (ii) iPr−CH), 6.42 (sept, 1 H, 3J = 6.2 Hz, (ii)
REFERENCES
■
(1) Boyd, P. D. W.; Wright, L. J.; Zafar, M. N. Extending the Range
of Neutral N-Donor Ligands Available for Metal Catalysts: N-[1-
Alkylpyridin-4(1H)-ylidene]amides in Palladium-Catalyzed Cross-
Coupling Reactions. Inorg. Chem. 2011, 50, 10522−10524.
(2) Leigh, V.; Carleton, D. J.; Olguin, J.; Mueller-Bunz, H.; Wright,
L. J.; Albrecht, M. Solvent-Dependent Switch of Ligand Donor Ability
and Catalytic Activity of Ruthenium(II) Complexes Containing
Pyridinylidene Amide (PYA) N-Heterocyclic Carbene Hybrid
Ligands. Inorg. Chem. 2014, 53, 8054−8060.
i
iPr−CH3), 6.4−5.9 (br. m, 1 H, (i) Pr−CH3), 5.77 (s, 2 H, (ii)
CH2), 5.28 (s, 2 H, (i) CH2), 5.4−4.9 (br. m, 1 H, (i) iPr−CH), 1.81
3
i
3
(d, 6 H, J = 6.7 Hz, (ii) Pr−CH3), 1.63 (d, 6 H, J = 6.2 Hz, (ii)
iPr−CH3), 2.0−1.1 (br. m, 12 H, (i) iPr−CH3) ppm. 13C NMR (126
MHz, CDCl3, AT): δ = 180.6 (NCOPh), 164.9 (NC), 163.0 (HEP),
154.2 (Ph−C), 144.5, 144.4, 141.8, 141.7, 140.5, 131.4, 130.2, 130.2,
(3) Donnelly, K. F.; Segarra, C.; Shao, L. X.; Suen, R.; Muller-Bunz,
̈
H.; Albrecht, M. Adaptive N-Mesoionic Ligands Anchored to a
Triazolylidene for Ruthenium-Mediated (De)Hydrogenation Catal-
ysis. Organometallics 2015, 34, 4076−4084.
i
129.0, 128.5, 122.6, 122.5, 120.3, 117.9, 113.1, 64.1 ((ii) Pr−CH),
i
i
62.2 ((i) Pr−CH), 57.7 ((ii) CH2), 54.7 ((i) CH2), 21.2 ((i) Pr−
i
(4) Melle, P.; Manoharan, Y.; Albrecht, M. Modular Pincer-Type
Pyridylidene Amide Ruthenium(II) Complexes for Efficient Transfer
Hydrogenation Catalysis. Inorg. Chem. 2018, 57, 11761−11774.
(5) Melle, P.; Albrecht, M. Ruthenium Complexes with PYA Pincer
Ligands for Catalytic Transfer Hydrogenation of Challenging
Substrates. Chimia 2019, 73, 299−303.
CH3), 21.1 ((ii) Pr−CH3) ppm. Anal. Calcd for C32H34Br2N4OPd·
CHCl3: C, 45.23, H, 4.03, N, 6.39%. Found: C, 45.36, H, 3.97, N,
6.09%. HRMS (ESI, positive ions): calcd for [M + Na]+,
C27H34Br2N4NaOPd, m/z = 777.0027. Found, m/z = 777.0025.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01585.
■
(6) Navarro, M.; Smith, C. A.; Li, M.; Bernhard, S.; Albrecht, M.
Optimization of Synthetically Versatile Pyridylidene Amide Ligands
for Efficient Iridium-Catalyzed Water Oxidation. Chem. - Eur. J. 2018,
24, 6386−6398.
(7) Navarro, M.; Li, M.; Bernhard, S.; Albrecht, M. A Mesoionic
Nitrogen-Donor Ligand: Structure, Iridium Coordination, and
Catalytic Effects. Dalt. Trans. 2018, 47, 659−662.
(8) Navarro, M.; Rosar, V.; Montini, T.; Milani, B.; Albrecht, M.
Olefin Dimerization and Isomerization Catalyzed by Pyridylidene
Amide Palladium Complexes. Organometallics 2018, 37, 3619−3630.
(9) Muller, C. M. A.; Babak, M. V.; Kubanik, M.; Hanif, M.;
Jamieson, S. M. F.; Hartinger, C. G.; Wright, L. J. Reprint of: Pt(II)
Pyridinium Amidate (PYA) Complexes: Preparation and in Vitro
Anticancer Activity Studies. Inorg. Chim. Acta 2017, 454, 247−253.
(10) The mode or media of the IR data collection was not indicated.
(11) Huynh, H. V. Electronic Properties of N-Heterocyclic Carbenes
and Their Experimental Determination. Chem. Rev. 2018, 118, 9457−
9492.
sı
Experimental details for PYAs and their precursors,
molecular structures, selected crystallographic data
tables, NMR spectra and ESI mass spectra (PDF)
Accession Codes
CCDC 2006195−2006201 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
(12) Jothibasu, R.; Huynh, H. V. Versatile Coordination Chemistry
of Indazole-Derived Carbenes. Chem. Commun. 2010, 46, 2986.
(13) Han, Y.; Huynh, H. V. Pyrazolin-4-Ylidenes: A New Class of
Intriguing Ligands. Dalton Trans. 2011, 40, 2141−2147.
(14) Teng, Q.; Huynh, H. V. A Unified Ligand Electronic Parameter
Based on 13C NMR Spectroscopy of N-Heterocyclic Carbene
Complexes. Dalton Trans. 2017, 46, 614−627.
Han Vinh Huynh − Department of Chemistry, National
University of Singapore3, 117543, Singapore; orcid.org/
0000-0003-4460-6066; Phone: (65) 65162670;
Email: chmhhv@nus.edu.sg; Fax: (65) 67791691
(15) Teng, Q.; Ng, P. S.; Leung, J. N.; Huynh, H. V. Donor
Strengths Determination of Pnictogen and Chalcogen Ligands by the
Huynh Electronic Parameter and Its Correlation to Sigma Hammett
Constants. Chem. - Eur. J. 2019, 25, 13956−13963.
(16) Yan, X.; Feng, R.; Yan, C.; Lei, P.; Guo, S.; Huynh, H. V. A
Palladacyclic N-Heterocyclic Carbene System Used to Probe the
Author
Jeroen Thomas Vossen − Department of Chemistry, National
University of Singapore3, 117543, Singapore
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01585
G
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Donating Abilities of Monoanionic Chelators. Dalton Trans. 2018, 47,
7830−7838.
(17) Huynh, H. V.; Lam, T. T.; Luong, H. T. T. Anion Influences on
Reactivity and NMR Spectroscopic Features of NHC Precursors. RSC
Adv. 2018, 8, 34960−34966.
(18) Noveron, J. C.; Lah, M. S.; Del Sesto, R. E.; Arif, A. M.; Miller,
J. S.; Stang, P. J. Engineering the Structure and Magnetic Properties of
Crystalline Solids via the Metal-Directed Self-Assembly of a Versatile
Molecular Building Unit. J. Am. Chem. Soc. 2002, 124, 6613−6625.
(19) Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Palladium(II)
Complexes of a Sterically Bulky, Benzannulated N-Heterocyclic
Carbene with Unusual Intramolecular C-H···Pd and CCarbene...Br
Interactions and Their Catalytic Activities. Organometallics 2006, 25,
3267−3274.
(20) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. 13C NMR
Spectroscopic Determination of Ligand Donor Strengths Using N-
Heterocyclic Carbene Complexes of Palladium(II). Organometallics
2009, 28, 5395−5404.
(21) APEX 2, version 2008.4.; Bruker AXS Inc.: Madison, WI, 2008.
H
https://dx.doi.org/10.1021/acs.inorgchem.0c01585
Inorg. Chem. XXXX, XXX, XXX−XXX