Substituent-controlled preference of carbonyl group-metal coordination in d8 metal complexes with non-symmetric pentadentate ligands. Structural and stereochemical aspects

Article abstract of DOI:10.1039/c3dt53312c

Chirally switchable Ni(ii) and Pd(ii) complexes were synthesized and fully characterized by X-ray crystallography and additionally by spectroscopic means (NMR and MS). The syntheses and characterization of their ligands are also reported. It was found that control of the stereochemical preference between (S*,S*) and (S*,R*) diastereomers by substituent modification of the ligand sidearms was possible in the solid state with the preferred atomic coordinations of the sidearms consistent with expectations based on the electron-withdrawing properties of the substituent -o-CF ₃ group. The dilemma arising in terms of assigning the absolute configuration descriptors resulting from selecting between strictly following only the covalent bonds of the ligand and disregarding the nature of the bonds altogether and thus bringing the coordinate bonds into consideration to determine the stereochemical priority sequence is hereby resolved by declaring that the latter option is to be preferred. The results obtained here provide a clear indication that sidearm substitution of the Ni(ii) and Pd(ii) complexes need not disturb macromolecular stereochemical arrangements leading to quasidiastereomeric relationships. This permits the design of molecular systems sensitive to external stimuli with predictable macromolecular structure. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53312c

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Substituent-controlled preference of carbonyl
group–metal coordination in d8 metal complexes
with non-symmetric pentadentate ligands.
Structural and stereochemical aspects†
Cite this: Dalton Trans., 2014, 43,
5375
Jianlin Han,a Taizo Ono,b Hidehiro Uekusa,c Karel D. Klikad,e and
Vadim A. Soloshonok*f,g
Chirally switchable Ni(II) and Pd(II) complexes were synthesized and fully characterized by X-ray crystallo-
graphy and additionally by spectroscopic means (NMR and MS). The syntheses and characterization of
their ligands are also reported. It was found that control of the stereochemical preference between (S*,S*)
and (S*,R*) diastereomers by substituent modification of the ligand sidearms was possible in the solid
state with the preferred atomic coordinations of the sidearms consistent with expectations based on the
electron-withdrawing properties of the substituent –o-CF3 group. The dilemma arising in terms of assign-
ing the absolute configuration descriptors resulting from selecting between strictly following only the
covalent bonds of the ligand and disregarding the nature of the bonds altogether and thus bringing the
coordinate bonds into consideration to determine the stereochemical priority sequence is hereby
resolved by declaring that the latter option is to be preferred. The results obtained here provide a clear
indication that sidearm substitution of the Ni(II) and Pd(II) complexes need not disturb macromolecular
stereochemical arrangements leading to quasidiastereomeric relationships. This permits the design of
molecular systems sensitive to external stimuli with predictable macromolecular structure.
Received 24th November 2013,
Accepted 17th January 2014
DOI: 10.1039/c3dt53312c
www.rsc.org/dalton
fine balance between structural predictability and coordination
flexibility, thereby allowing the compounds to be responsive to
Introduction
Coordination compounds play an important role in the design external stimuli.2 In this respect, one successful approach was
and synthesis of molecules that can organize into supramole- based on the application of ligands containing carbonyl
cular assemblies possessing useful chemical and physical pro- groups specifically intended to be weakly coordinating.3
perties.1 One of the major challenges in this field is to attain a Recently, we introduced a new molecular system based on the
coordination of achiral pentadentate ligands with d8 metals
(Scheme 1).4
aSchool of Chemistry and Chemical Engineering, Nanjing University,
Nanjing, 210093, China
bNational Institute of Advanced Industrial Science and Technology (AIST),
Nagoya, Aichi Prefecture, Japan
cDepartment of Chemistry and Materials Science, Tokyo Institute of Technology,
Ookayama 2, Meguro-ku, Tokyo 152-8551, Japan
An innovative feature of this system is that the coordination
of the achiral ligand 1 (X = H) with Ni(II) or Pd(II) results in the
generation of two elements of chirality: the stereogenic center
dMolecular Structure Analysis, German Cancer Research Center (DKFZ),
Im Neuenheimer Feld 280, D-69009 Heidelberg, Germany
eDepartment of Chemistry, University of Turku, Vatselankatu 2, FIN-20014 Turku,
Finland
fDepartment of Organic Chemistry I, Faculty of Chemistry, University of the Basque
Country UPV/EHU, 20018 San Sebastián, Spain. E-mail: vadym.soloshonok@ehu.es;
Fax: +34 943-015270; Tel: +34 943-015177
gIKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
†Electronic supplementary information (ESI) available: NMR spectral data for
compounds 5–14; X-ray data for Ni(II) complexes 12 and 13 and the Pd(II)
complex 14. CCDC 951881 for (12), 951882 for (13) and 951883 (14). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt53312c
Scheme 1 Formation of diastereomerically switchable complexes 2
and 3 from achiral and chiral ligands 1.
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located at the benzylamine nitrogen and the stereogenic axis
arising from the restricted rotation of the uncoordinated N-(o-
benzophenone) amide moiety about the N–Ci bond. The
switch between diastereomers (S*,S*)-2 and (S*,R*)-3 occurs
via the carbonyl de-coordination/coordination step resulting in
inversion of the stereogenic center chirality whilst retaining
the same sense of axial chirality. It should be noted that due
to the structural rigidity of complexes 2 and 3, this process
occurs with near complete (>99%) stereoselectivity. Due to the
cis and trans relationship of the benzyl and uncoordinated
N-(o-benzophenone) amide groups, diastereomers (S*,S*)-2
and (S*,R*)-3 differ in stability. However, this difference is
rather minute as both diastereomers can be randomly
obtained in the solid state purely by chance.5a In the current
study, to further advance this molecular system in terms of
some predictability, we considered introducing electron-with-
drawing or -donating substituents to influence the preference
for coordination of the N-(o-benzophenone) amide carbonyls.
During the design of the corresponding ligands, we also rea-
lized that we will be faced with an interesting stereochemical
dilemma as the ligands 1 bearing a substituent X = R thereby
already possess a stereogenic center, also located at the same
nitrogen atom that is rendered chiral upon complexation. The
coordination of such chiral ligands with d8 metals can thus
result in unconventional stereochemical assignments taking
into account the priority dictated by covalent vs. coordination
bonds, an aspect which seemingly remains to be properly clari-
fied. The results described herein will, in addition to having
far reaching implications on the deeper understanding of the
correlation between stereochemistry and the presence of sub-
stituents in the switchable framework of molecules 2 and 3,
potentially lead to a general approach for rational control of
their coordination properties.
Scheme 2 Synthesis of pentadentate ligands 10 and 11.
The ligands 10 and 11 are, in principle at least, chiral, as
the benzylamine nitrogen atom, the stereogenic center, bears
four distinct substituents including the unshared pair of elec-
trons. Furthermore, the capability of the carbonyl oxygen atom
of the benzophenone moiety containing the –o-CF3 group is
expected to be diminished towards coordination with a metal
due to the strong electron-withdrawing effect of this group,
deliberately located by design in an ortho position. To generate
the corresponding Ni(II) complexes, pentadentate ligands 10
and 11 were heated in methanol at 60–70 °C for several hours
together with Ni(NO3)2·6H2O and NaOH (Scheme 3). Upon
completion, the red-colored Ni(II) complexes 12 and 13 were
isolated by filtration from ice water and, interestingly, were
stable enough to be further purified by column chromato-
graphy over silica gel.
For the preparation of the corresponding Pd complex, it
was found that Pd(OAc)2 as the metal source gave the best
result. The reaction was also conducted in methanol but using
triethylamine as the base. The formation of the Pd(II) complex
14 occurred at a much higher rate and the reaction was com-
plete in almost half the time. Filtration using a celite pad was
found to be necessary to remove the excess Pd residue, and fol-
lowing column chromatography through a short column of
silica gel, the target Pd(II) complex 14 was obtained in quanti-
tative yield.
Results and discussion
Building on our modular approach5 for the design of various
tetra- or pentadentate ligands and the corresponding Ni(II)
complexes, we prepared from a set of 2-aminobenzophenones
4a–c a series of compounds 5–7 (Scheme 2) encompassing the
unsubstituted benzophenone moiety (5) as well as compounds
bearing electron-donating para-methoxy, –p-OMe, (6) and elec-
tron-withdrawing ortho-trifluoromethyl, –o-CF3, (7) groups.
Derivatives 5–7 were obtained in high yields via a simple reac-
tion of 4a–c with bromoacetyl bromide, respectively. Bromides
5 and 6 were then used in a simple reaction with benzylamine
to produce the secondary amines 8 and 9, respectively, also in
high yield. Finally, secondary amines 8 and 9 were trans-
formed to the pentadentate ligands 10 and 11, respectively, by
alkylation with the –o-CF3 group-containing bromide 7. To
prevent possible quaternization of the benzylamine nitrogen,
the reactions were conducted in acetonitrile in the presence of
Hünig’s base,6 thus facilitating isolation of compounds 10 and
11 in high yields.
Scheme 3 The Ni(II) complexes 12 and 13 and the Pd(II) complex 14.
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Complexes 12–14 were only very poorly soluble in suitable
solvents, thus precluding their comprehensive analysis by
NMR, especially with the consequent unavailability of 13C
acquisition. With appreciable conformational mobility includ-
ing the flexible benzyl moiety, fluxional motion of the hetero-
atom rings formed by coordination to the metal, and other
bond rotations etc., in addition to the chiral switching arising
from the de-coordination/coordination steps of the carbonyl
oxygen atoms to the metal, the NMR spectra were either extre-
mely broad or contained multiple resonances from the various
contributing species. Nevertheless, reasonable 1H and 19F
NMR data were attained at various temperatures and were con-
sistent with the expected structures.
However, the dynamics clearly indicate that the potential
for facile switching between the diastereomers and the solu-
tion-state dynamic NMR behavior remains an area for future
study, particularly in conjunction with molecular modeling
which could enable the identification of the various contribut-
ing species and the likely transitions in effect. It was beyond
the scope of the present work to conduct variable-temperature
measurements for the purposes of extracting thermodynamic
data on the transformations at hand, but clearly chiral switch-
ing must be a principal process in effect based on the magni-
tude of the energies involved. An interesting point is that the
δF of the coalesced signal for the –CF3 group for complexes
12–14 is essentially identical in all three cases (−61 ppm), thus
pertaining to at least some dominance of the species where
the carbonyl oxygen in the ligand arm bearing the –CF3 group
is not coordinated to the metal; otherwise it could be expected
that differences in δFs would be apparent – notwithstanding
some quite improbable coincidences in δF and equilibria
positions.
Nevertheless, from the NMR data, it was clear that the intro-
duction of electron-withdrawing or -donating substituents on
the sidearms in ligands 10 and 11 was insufficient for provid-
ing a predictable, overwhelmingly biased coordination of a
particular carbonyl group in solution. The next step therefore
was to ascertain whether an overwhelmingly predominant
coordination mode was forthcoming in the solid state. For
this, X-ray crystallographic analysis was conducted on crystals
of complex 12 grown from a CH2Cl2–CCl4 solution. The struc-
ture is presented in Fig. 1 and the details of data collection
and refinement are summarized in Table 1.
Fig. 1 X-ray structure of complex 12; only the (S,S)-enantiomer is
shown from the unit cell containing both enantiomers.
–o-CF3 group-containing arm. However, its carbonyl is not co-
ordinated to the Ni and we argue that the first point of differ-
ence for stereochemical differentiation purposes should be
considered from the point of view of the coordinated and non-
coordinated carbonyl oxygen atoms, whereby it is the former
that would then yield priority naming rights. The problem is
that the CIP priority rules7 do not explicitly indicate the
manner in which assignment should be considered as they do
not say anything about the type of bonds involved. It is fully
conceivable that different workers could follow different bond
pathways and thus end up with different stereochemical
assignments, as would be the case here. This deficiency,
however, can be rectified by declaring that the convention that
should be followed is that, rather than strictly following only
the covalent bonds of the ligand, the nature of the bonds
involved should be disregarded altogether. This then brings
into consideration the coordinate bonds as we recommend.
Accordingly, following our declaration, the assignment of
the absolute stereochemistry of the stereogenic nitrogen atom
for the structure depicted in Fig. 1 is therefore S based on the
sequence:
Ni(II) > coordinated CvO > non-coordinated CvO > benzyl
group. To the best of our knowledge, structure 12 is the first
unequivocal example where the formation of a coordinate
bond can introduce a dilemma regarding the stereochemical
priority sequence for assignment purposes from consideration
of the types of bonds involved (viz. only covalent bonds vs. any
type of bond). By this convention, the –o-CF3 group-containing
complex (S,S)-12 and its unsubstituted analog (S,S)-2
(Scheme 1) have the same allocated stereochemistry and, there-
fore presumably, might possess not only similar physico-
chemical functional properties, but could also share similar
chiroptical properties as well, which would hence be a con-
venient outcome of the naming system.
In the crystallographic unit of complex 12, both enantio-
mers (S,S) and (R,R) are present, though only the former is
depicted in Fig. 1. Most importantly, as was intentionally
designed, the electron-deficient carbonyl oxygen atom of the
–o-CF3 group-containing benzophenone moiety is not co-
ordinated to the metal. This result therefore is in concert with
expectations given the strong electron-withdrawing properties
of the –CF3 group.
According to CIP priority rules,7 and considering only
the covalent bonds involved, the first point of difference
between the substituents of the benzylamine nitrogen atom
for differentiation purposes occurs at the ortho position of the
benzophenone phenyl rings, thereby giving priority to the
Crystals of complex 13 with –o-CF3 and –p-OMe groups on
each arm were grown from CH2Cl2 solution and the resulting
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Table 1 Summary of crystal data for complexes 12–14
12
13
14
Formula
M
C
38H28F3N3NiO4
C39H30F3N3NiO5
736.36
C39H30F3N3PdO5
784.06
706.34
Temperature/K
Wavelength/Å
291(2)
0.71073
291(2)
0.71073
293(2)
1.54186
Crystal size/mm
Crystal system
Space group
0.28 × 0.24 × 0.22
Triclinic
P1
0.28 × 0.24 × 0.22
Triclinic
P1
0.09 × 0.02 × 0.02
Orthorhombic
Pbca
a/Å
b/Å
c/Å
α/°
11.869(2)
11.874(3)
14.3120(18)
74.121(2)
78.329(3)
79.1460(10)
1880.9(7)
2
9.648(4)
10.9428(2)
23.9074(4)
26.1834(5)
—
—
—
6849.9(2)
8
1.521
13.897(6)
16.270(7)
66.870(7)
74.393(7)
87.648(8)
1927.0(14)
2
β/°
γ/°
V/Å3
Z
Dc/Mg m−3
1.311
1.300
F (000)
768
780
3184
θ Range for data collection/°
Index ranges, hkl
1.80–26.00
−12 to 14, −14 to 14,
−8 to 17
10 366/7258
1.039
2.20–26.00
−11 to 11, −9 to 17,
−16 to 20
10 590/7415
0.980
4.75–68.17
−13 to 13, −28 to 28,
−31 to 31
74 634/6252
1.109
Reflections unique/observation
Goodness-of-fit on F2
Final R indices {I > 2σ(I)}, R1, wR2
R Indices (all data), R1, wR2
Largest diff. peak, hole/e Å−3
0.0572, 0.1342
0.0735, 0.1395
0.325, −0.270
0.0552, 0.0954
0.0853, 0.1010
0.559, −0.421
0.0584, 0.1344
0.0997, 0.1883
1.308, −1.081
X-ray crystallographic structure is presented in Fig. 2. The p-OMe group is coordinated to Ni in the solid state. This result
details of data collection and refinement of complex 13 are again is in concert with expectations given the strong electron-
summarized in Table 1. In this case too the crystallographic withdrawing properties of the –CF3 group for the ligand arm
unit is also composed of a pair of enantiomers of absolute containing it. The labeling of the absolute configuration of the
configuration (S,S) and (R,R) (portrayed).
two enantiomers of 13 was likewise based on the priority of
Similarly by intentional design, the more electron-rich the coordinated carbonyl oxygen atom over the non-co-
carbonyl oxygen atom conjugated with the electron-donating – ordinated carbonyl oxygen atom. Hence, for the structure
depicted in Fig. 2, the absolute stereochemistry of the stereo-
genic nitrogen atom is considered to be R.
Finally, to attain a broader sense of generality, crystals of
the Pd(II) complex 14 were also prepared from an ethyl acetate–
hexane solution. The X-ray crystallographic structure of 14 is
presented in Fig. 3. The details of data collection and refine-
ment of complex 14 are summarized in Table 1. However, in
contrast to the Ni(II) complexes 12 and 13, the crystallographic
unit of Pd(II) complex 14 is composed of eight molecules
arranged in four sets of (S,S) and (R,R) enantiomeric pairs.
Importantly though, in line with complexes 12 and 13, the less
electron-rich carbonyl oxygen atom, depleted by the electron-
withdrawing effect of the –o-CF3 group, is not coordinated to
the metal. The absolute configuration of the stereogenic nitro-
gen atom in the molecule depicted in Fig. 3 was denoted, as
previously for complexes 12 and 13, on the basis of giving pri-
ority to the coordinated –p-OMe group-containing arm, and
hence is considered to be R.
Thus, there is a consistent general trend observed within
this study in that the carbonyl oxygen of the more electron rich
arm of the ligand coordinates to the metal in the solid state,
as was anticipated. Furthermore, the cis isomers were not
observed at all and thus only the enantiomers (S,S) and (R,R)
were present, representing the trans isomers. Hence the stereo-
chemistry can be effectively controlled by intentional design.
Fig. 2 X-ray structure of complex 13; only the (R,R)-enantiomer is
shown from the unit cell containing both enantiomers.
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X-ray experimental
The crystal data and details of data collection are given in
Table 1. The data were collected on a Siemens SMART CCD
diffractometer {graphite-monochromated Mo-Kα radiation, ω-
scan technique, λ = 0.71073 (complexes 12 and 13) or 1.54186
(complex 14) Å} at 291 (complexes 12 and 13) or 293 (complex
14) K. The structures were solved by direct methods using
SHELXS-9710 and were refined on F2 using SHELXL-97.11 All
non-hydrogen atoms were refined anisotropically and the
hydrogen atoms were placed in calculated positions and
assigned to an isotropic displacement parameter of 0.08 Å2.
Depictions of the X-ray derived structures in Fig. 1–3 were pro-
duced using the GUI GaussView.12
Synthesis
General procedure for the preparation of N-(2-benzoyl-
phenyl)-2-bromoacetamides 5–7 by the condensation of 2-ami-
nobenzophenone and bromoacetyl bromide. A solution of
bromoacetyl bromide (44.38 g, 225 mmol) in CH3CN (2 mL
per gram of bromoacetyl bromide) was slowly added to a
slurry of the appropriate 2-aminobenzophenone 4 (19.72 g,
100 mmol) and potassium carbonate (69.11 g, 500 mmol) in
CH3CN (250 mL). The reaction was stirred at ambient tempera-
ture on a water bath for 1 hour and upon completion as indi-
cated by TLC, the CH3CN was evaporated under vacuum.
Water (200 mL) was then added and the crude mixture was
extracted with CH2Cl2 (3 × 200 mL). The organic portions were
combined, dried and concentrated under vacuum to afford the
corresponding α-bromoamides 5–7 in 98% yield and greater
than 99% chemical purity.
Fig. 3 X-ray structure of complex 14; only the (R,R)-enantiomer is
shown from the unit cell containing both enantiomers.
These results lead to the preeminent question: what are the
stereochemical relationships between the unsubstituted com-
plexes 2 and 3, the monosubstituted complex 12 and the dis-
ubstituted derivatives 13 and 14? In our opinion, the closest
concept in the literature is the concept of quasienantiomers,
defined as constitutionally different yet closely related chemi-
cal species possessing very similar chemical and physicochem-
ical properties.8 In an entirely analogous manner therefore, we
shall apply the term quasidiastereomers to describe the
relationships between, for example, compounds (S,R)-12 and
(R,R)-14.
N-(2-Benzoylphenyl)-2-bromoacetamide (5). 1H NMR δ 4.03
(2 H, s), 7.16 (1 H, m), 7.47–7.53 (2 H, m), 7.59–7.64 (3 H, m),
7.72–7.75 (2 H, m), 8.60 (1 H, dd, J = 8.9, 1.2 Hz), 11.50 (1 H,
bs). 13C NMR δ 29.5, 121.5, 123.1, 124.1, 128.4, 130.0, 132.7,
133.6, 134.2, 138.3, 139.5, 165.3, 199.3. ESI-MS m/z 340 amu
[M + Na]+. Mp 94–95 °C.
2-Bromo-N-(2-(4-methoxybenzoyl)phenyl)acetamide
(6). 1H
NMR δ 3.90 (3 H, s), 4.01 (2 H, s), 6.98 (2 H, m), 7.17 (1 H, m),
7.56–7.62 (2 H, m), 7.75–7.78 (2 H, m), 8.54 (1 H, dd, J = 8.6,
1.1 Hz), 11.23 (1 H, bs). 13C NMR δ 29.6, 55.6, 113.7, 121.7,
123.1, 125.0, 130.6, 132.7, 132.8, 133.4, 138.9, 163.5, 164.8,
197.3. ESI-MS m/z 370 amu [M + Na]+. Mp 127–128 °C.
Experimental
NMR experimental
2-Bromo-N-(2-(2-(trifluoromethyl)benzoyl)phenyl)acetamide (7).
NMR spectra were acquired using a Bruker Avance NMR 1H NMR δ 4.07 (2 H, s), 7.07 (1 H, m), 7.31 (1 H, dd, J = 8.0, 1.7
spectrometer equipped with a 5 mm normal configuration Hz), 7.37–7.40 (1 H, m), 7.60–7.68 (3 H, m), 7.77–7.82 (1 H, m),
dual coil probe with z-gradient capability at a field strength of 8.78 (1 H, dd, J = 8.5, 1.0 Hz), 12.09 (1 H, bs). 13C NMR δ 29.6,
9.4 T operating at 400, 100 and 376 MHz for H, 13C and 19F 120.7, 122.2, 123.1, 123.5 (qt, JF,C = 273.9 Hz), 126.8 (qt, JF,C
=
1
nuclei, respectively, at 25 °C (other temperatures as indicated 3.8 Hz), 127.9 (qt, JF,C = 32.3 Hz), 128.0, 130.2, 131.6, 134.9,
for the complexes 12–14) with samples contained in CDCl3 (in 135.9, 138.0 (qt, JF,C = 2.1 Hz), 140.9, 165.6, 199.4. ESI-MS m/z
CD2Cl2 for the complexes 12–14). The chemical shifts of 1H 408 amu [M + Na]+. Mp 87–88 °C.
and 13C nuclei are reported relative to TMS incorporated as an
General procedure for the alkylation of N-(2-benzoylphenyl)-
1
internal standard (δ = 0 ppm for both H and 13C) and exter- 2-bromoacetamides with benzylamine to yield the corres-
nally to CF3CO2H in CDCl3 [2–3% v/v] at 25 °C for 19F (δ = ponding N-(2-benzoylphenyl)-2-(benzylamino)acetamides
8
−78.5 ppm). General NMR experimental details have been pre- and 9. To a flask containing 5 or 6 (50 mmol) and CH3CN
viously described.9
(1 mL per gram of 5 or 6) was added benzylamine (2.5 eq.). The
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reaction mixture was stirred at 60–70 °C until the starting
Syntheses of Ni(II) complexes 12 and 13. To a flask containing
material 5 or 6 had been completely consumed as indicated by ligand 10 or 11 (0.86 mmol), Ni(NO3)2·6H2O (0.75 g,
TLC (hexane–AcOEt, 4 : 1). The solvent was then removed under 2.58 mmol) and 5 mL of MeOH was added NaOH (0.21 g,
reduced pressure and water was added to the residue followed 5.16 mmol). The reaction mixture was stirred at 60–70 °C
by extraction with CH2Cl2 (×3). The organic portions were com- under a N2 atmosphere. After 8 h, the reaction mixture was
bined and dried over anhydrous MgSO4. After filtration and poured into ice water and left until complete precipitation had
removal of the solvent followed by column chromatography over occurred. The red solid was filtered off and washed with water.
silica gel, compound 8 or 9 was obtained in 97–99% yield.
N-(2-Benzoylphenyl)-2-(benzylamino)acetamide (8). 1H NMR δ
3.47 (2 H, s), 3.88 (2 H, s), 7.09–7.14 (1 H, m), 7.24–7.31 (3 H,
After column chromatography through a short column of silica
gel, the desired Ni(II) complexes 12 or 13 were obtained.
Ni(II) complex 12. 1H NMR (−80 °C; major species) δ 3.32
m), 7.42–7.45 (2 H, m), 7.42–7.64 (5 H, m), 7.77–7.80 (1 H, m), (1 H, d, J = 15.6 Hz), 3.67 (1 H, d, J = 16.1 Hz), 3.99 (1 H, d,
8.65 (1 H, dd, J = 8.3, 0.6 Hz), 11.70 (1 H, bs). 13C NMR δ 52.8, J = 15.7 Hz), ∼4.09 (2 H, vbs), 4.27 (1 H, d, J = 16.3 Hz),
54.0, 121.7, 122.4, 124.8, 127.3, 128.4, 128.5, 130.2, 132.7, 6.92–7.86 (19 H, m), 8.06 (1 H, bd, J = 8.5 Hz), 8.20 (2 H, bd, J =
132.9, 133.7, 138.5, 139.2, 139.3, 171.4, 198.4. ESI-MS m/z 367 7.4 Hz). 19F NMR (35 °C) δ −61.08.
Ni(II) complex 13. 1H NMR (35 °C) δ ∼3.30 (2 H, vb), ∼3.74
amu [M + Na]+. Mp 80–81 °C.
N-(2-(4-Methoxybenzoyl)phenyl)-2-(benzylamino)acetamide (9). (3 H, vb), 3.79 (3 H, s), ∼4.10 (1 H, vb), 6.75–7.06 (4 H, m),
1H NMR δ 3.44 (2 H, s), 3.84 (2 H, s), 3.90 (3 H, s), 6.96–6.99 7.16–7.80 (15 H, m), 7.87–7.98 (2 H, m). 19F NMR (35 °C)
(2 H, m), 7.10–7.15 (1 H, m), 7.23–7.33 (3 H, m), 7.40–7.43 δ −60.89.
Synthesis of Pd(II) complex 14. To a flask containing ligand
11 (0.10 g, 0.17 mmol), Pd(OAc)2 (0.077 g, 0.34 mmol) and
(2 H, m), 7.51–7.58 (2 H, m), 7.80–7.83 (2 H, m), 8.59 (1 H, dd,
J = 8.3, 0.4 Hz), 11.44 (1 H, bs). 13C NMR δ 52.8, 54.0, 55.6,
113.6, 121.8, 122.4, 125.9, 127.3, 128.5, 128.6, 130.8, 132.0,
132.8, 133.0, 138.7, 139.2, 163.5, 171.2, 196.7. ESI-MS m/z 375
amu [M + H]+.
General procedure for the syntheses of ligands 10 and 11. A
solution of compound 8 or 9 (10 mmol) in CH3CN (10 mL) was
added to compound 7 (1.1 eq.) and N-ethyl-N-isopropylpropan-
2-amine (1.5 eq.). The reaction mixture was stirred at 60–70 °C
until compound 8 or 9 had been completely consumed as indi-
cated by TLC (hexane–AcOEt, 3 : 1). After evaporation of the
solvent, aqueous NH4Cl was added to the residue and the
organic layer was extracted with CH2Cl2 (3×). The organic por-
tions were combined and dried over anhydrous MgSO4. After
removal of the solvent followed by column chromatography
over silica gel, the desired ligand 10 or 11 was obtained in
93–95% yield.
5
mL of MeOH was added triethylamine (0.059 mL,
0.43 mmol). The reaction mixture was stirred at 60–70 °C for
4.5 h whilst being monitored by TLC (acetone–CHCl3, 1 : 4).
The reaction mixture was then filtered through Celite with the
aid of acetone for the removal of Pd residue. After removal of
the solvent followed by column chromatography through a
short column of silica gel, the desired Pd(II) complex 14 was
1
obtained in quantitative yield. H NMR (35 °C) δ ∼3.61 (1 H,
vbd), 3.62 (1 H, bd, J = 15.2 Hz), 3.77–3.97 (2 H, vb), 3.82 (3 H,
s), 3.93 (1 H, bd, J = 15.4 Hz), ∼4.05 (1 H, vbd, J = ∼10.5 Hz),
6.87–7.03 (4 H, m), 7.15–7.39 (6 H, m), 7.48–7.88 (11 H, m).
19F NMR (35 °C) δ −60.96.
Conclusions
Ligand 10. 1H NMR δ 3.47 (2 H, s), 3.48 (2 H, s), 3.95 (2 H,
s), 6.84–6.89 (2 H, m), 7.02 (1 H, m), 7.11 (1 H, m), 7.21–7.67
(17 H, m), 7.78–7.80 (1 H, m), 8.15 (1 H, d, J = 8.2 Hz), 8.76
(1 H, dd, J = 8.3 Hz), 10.75 (1 H, bs), 12.11 (1 H, bs). 13C NMR δ
58.6, 59.1, 59.8, 121.0, 122.2, 122.5, 122.8, 123.4, 123.6 (qt,
JF,C = 274.4 Hz), 126.8 (qt, JF,C = 4.1 Hz), 127.4, 127.8, 127.8 (qt,
JF,C = 32.5 Hz), 128.2, 128.3, 128.6, 129.7, 130.0, 130.1, 131.6,
132.0, 132.6, 132.9, 134.8, 135.9, 136.6, 137.9, 138.1, 138.2 (qt,
JF,C = 2.0 Hz), 141.2, 169.4, 170.1, 197.6, 199.2. ESI-MS m/z 672
amu [M + Na]+. Mp 56–57 °C.
The dilemma arising, as demonstrated in this work, in terms
of assigning the absolute configuration descriptors resulting
from selecting between strictly following only the covalent
bonds of the ligand and disregarding the nature of the bonds
altogether and thus bringing the coordinate bonds into con-
sideration, to determine the stereochemical priority sequence
is hereby resolved by declaring that the latter option is to be
preferred. The results obtained here provide a clear indication
that substituents need not disturb macromolecular stereo-
chemical arrangements leading to quasidiastereomeric
relationships and allow for the structural design of systems
sensitive to external stimuli, a subject we are rigorously pursu-
ing at present. Finally, the results markedly demonstrate that
coordination, and thus the stereochemistry, can be controlled
in the solid state by the electronic properties of the substitu-
ents introduced into the coordinating sidearms.
Ligand 11. 1H NMR δ 3.46 (2 H, s), 3.48 (2 H, s), 3.84 (3 H,
s), 3.93 (2 H, s), 6.84–6.89 (2 H, m), 7.02 (1 H, ddd, J = 8.2, 6.0,
1.2 Hz), 7.12 (1 H, dt, J = 7.6, 1.1 Hz), 7.21–7.49 (9 H, m),
7.55–7.70 (5 H, m), 7.77–7.80 (1 H, m), 8.10 (1 H, d, J = 7.6 Hz),
8.75 (1 H, dd, J = 8.4, 0.8 Hz), 10.53 (1 H, bs), 12.10 (1 H, bs).
13C NMR δ 55.5, 58.5, 59.0, 59.7, 113.6, 121.0, 122.3, 122.5,
123.1, 123.4, 123.6 (qt, JF,C = 273.9 Hz), 126.7 (qt, JF,C = 3.8 Hz),
127.8, 127.9 (qt, JF,C = 32.3 Hz), 128.2, 128.3, 128.6, 129.7,
130.0, 130.5, 131.3, 131.5, 132.2, 132.6, 134.8, 135.9, 136.7,
137.3, 138.2 (qt, JF,C = 2.0 Hz), 141.1, 163.4, 169.3, 170.1, 196.0,
199.2. ESI-MS m/z 702 amu [M + Na]+. Mp 58–60 °C.
The potential for facile switching between the diastereo-
mers in the solution-state as vehemently indicated by the
dynamic NMR behavior remains an area for future study. The
atomic coordinates on hand from the X-ray crystal structure
5380 | Dalton Trans., 2014, 43, 5375–5381
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Acknowledgements
Guangqian Zhang is thanked for assistance with synthetic
preparations and NMR acquisitions. Financial support from
the Center for International Mobility (CIMO) and Turun Ylio-
pistosäätiö is gratefully acknowledged.
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