Schiff Bases as Inspirational Motif for the Production of Ni(II) and Pd(II) Coordination and Novel Non-Symmetric Ni(II)-POCOP Pincer Complexes

Article abstract of DOI:10.1002/ejic.202100146

The synthesis of Ni(II) and Pd (II) coordination complexes containing Schiff's bases derived from the condensation of 2,4-dihydroxybenzaldehyde with 4-chloroaniline (L1) or 3,5-dichloroaniline (L2) is described. The condensation of ligands L1 and L2 with

Full text of DOI:10.1002/ejic.202100146

European Journal of Inorganic Chemistry
Accepted Article
Title: Schiff Bases as Inspirational Motif for the Production of Ni(II) and
Pd(II) Coordination and Novel Non-Symmetric Ni(II)-POCOP
Pincer Complexes.
Authors: Jaime Alberto Rosas-Ortiz, Jose Roberto Pioquinto-Mendoza,
Lucero González-Sebastián, Simon Hernandez-Ortega,
Marcos Flores-Alamo, and David Morales-Morales
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100146
Link to VoR: https://doi.org/10.1002/ejic.202100146
01/2020

10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Schiff Bases as Inspirational Motif for the Production of Ni(II) and Pd(II)
Coordination and Novel Non-Symmetric Ni(II)-POCOP Pincer Complexes.
M. Sc. Jaime Alberto Rosas-Ortiz,a Dr. J. Roberto Pioquinto-Mendoza,a Dr. Lucero
González-Sebastián,b M.Sc. Simon Hernandez-Ortega,a Dr. Marcos Flores-Alamoc and
Prof. Dr. David Morales-Moralesa,*
aInstituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior,
Ciudad Universitaria, Ciudad de México, C.P. 04510, México. bDepartamento de
Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco No.
186, Ciudad de México, C.P. 09340, México. cFacultad de Química, División de Estudios
de Posgrado, Universidad Nacional Autónoma de México, Circuito Exterior, CU, 04510,
México.
*Corresponding author. E-mail address: damor@unam.mx (D. Morales-Morales)
Phone: +52-5556224514
https://iquimica.unam.mx/david-morales-morales
Twitter: @dmorales_group
Abstract
The synthesis of Ni(II) and Pd (II) coordination complexes containing Schiff´s bases
derived from the condensation of 2,4-dihydroxybenzaldehyde with 4-chloroaniline (L1)
or 3,5-dichloroaniline (L2) is described. The condensation of ligands L1 and L2 with
chlorodiisopropylphosphine in the presence of a base produced the corresponding
non-symmetric pincer ligands POCOP-1 and POCOP-2 with the unexpected C-P bond
formation by a hydrophosphorylation of the imine process. The new C-P bond
formation probably occurring by a Kabachnik-Field mechanism type without the
requirement of catalysts or additional reagents. Further treatment of ligands POCOP-1
or POCOP-2 with NiCl2 afforded the non-symmetric pincer complexes Ni(II)-POCOP-1
and Ni(II)-POCOP-2. The crystal structures of L1, 3, Ni(II)-POCOP-1 and Ni(II)-POCOP-2
were unequivocally determined by single crystal X-ray diffraction studies. In addition,
the metal complexes were used as catalysts in Suzuki-Miyaura couplings under
microwave irradiation, being the Pd(II) derivatives the best catalysts of the series when
compared to their Ni(II) analogues under the same reaction conditions.
Keywords: Non symmetric pincer complexes; Schiff base ligands; Suzuki-Miyaura
Couplings; Catalysis; Nickel and Palladium Complexes.
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
1. Introduction.
In coordination and organometallic chemistry, Schiff´s bases are a class of ligands that
have widespread applications in different fields, e.g. in organometallic synthesis,[1] in
catalytic materials[2] and promoting unusual coordination environments.[3] Schiff base
ligands contain a modular framework that converts strong aryloxide donors into
chelating motifs that are prepared in a facile, high yielding manner. The
straightforward synthesis of these ligands provides an easily accessible and tuneable
platform to exploit the coordination chemistry of various metals.[4]
The π-system in a Schiff base often imposes a geometrical constriction and
affects the electronic structure as well. These types of complexes have been widely
explored in recent years and such studies have been the subject of numerous papers
and reviews;[4-5] Many of these are centred on the catalytic activity of Schiff base
complexes in a large number of homogeneous and heterogeneous reactions.[6]
Among the several classes of Schiff bases those that can be prepared by
condensation of different types of primary amines and salicylaldehyde derivatives to
form salicylaldimine compounds with potential to serve as O, N-donors are very
popular due to their diverse chelating ability and their easy synthesis.[6b, 7]
On the other hand, since the discovery and introduction of pincer metal
complexes in organometallic chemistry by Moulton & Shaw in 1976,[8] these systems
have become a privileged ligand platform to produce many of the most versatile
homogeneous metal catalysts known to date, characterized by their robustness and
high thermal stability catalysing a wide range of organic reactions.[9] Besides, the
evolution of organometallic chemistry has led to the creative modification of the
original PCP pincer ligand backbone, extending their applications and producing
improved catalysts able to activate very strong chemical bonds.[10] In this context, an
attractive characteristic of these species is the tunning of the steric and electronic
properties of their metal derivatives through changes of their substituents in the donor
groups (phosphines, PR2, amines, NR2, thioethers, SR, selenoethers, SeR and N-
heterocyclic carbenes or a pair of donor groups among other functionalities) and/or
the backbone configuration itself.[9c, 11]
2
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Furthermore, among the pincer ligands, those including non-symmetrical
backbones standout and have attracted enormous attention in recent years due to
their chemical properties and catalytic applications (Figure 1).[12]
Figure 1. Potential modifiable sites on a pincer framework to produce non-symmetric
pincer complexes.
As part of our continuous work on non-symmetric POCOP-based pincer
complexes by the metal-substitution on the main pincer backbone and our interest on
the incorporation of biologically active fragments to produce compounds with
potential catalytic and biological applications, herein we present the synthesis and
characterization of a series of Ni(II)-coordination compounds and non-symmetric Ni(II)-
POCOP pincer complexes derived from Schiff´s base ligands, in which a P-C bond is
formed without the presence of a metal catalyst (Figure 2).[13] In this context, Schiff´s
bases containing N, O donor atoms have shown a broad biological activity and are of
special interest due to the variety of ways in which they are bonded to metal ions.[2b,14]
3
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Previous
work
N
S
HN
OH
O
9
H
O
O
O
O
O
O
O
O
Ni
Cl
R2P
PR2
M
R2P
Ni
Cl
PR2
R2P
Pd
Cl
R2P
PR2
PR2
Cl
M=Ni, Pd, Pt
[A]
[B]
[C]
[D]
This
work
O
(iPr)2P
Cln
Cln
N
N
H
OH
O
M
O
O
O
HO
N
(iPr)2P
P(iPr)2
Ni
Cl
Cln
M=Ni, Pd
Figure 2.
2. Results and discussion.
2.1. Synthesis of ligands L1 and L2.
Schiff base ligands were synthesized by a typical condensation reaction of 2,4-
dihydroxybenzaldehyde with 4-chloroaniline and 3,5-dichloroaniline to give L1 and L2,
respectively, which were isolated as yellow (L1) and green (L2) solids in excellent yields
1
(Scheme 1). These compounds were further characterized by H and 13C{1H} NMR
spectroscopy, elemental analysis, FT-IR and mass spectrometry. Compounds L1 and L2
display a key resonance for the imine group (CH=N) on their 1H NMR spectra at δ 8.77
and 8.81 ppm, respectively. The -OH(a) proton resonance appears as a rather broad
singlet at 13.13 (L1) and 12.87 (L2) ppm due to the OH(a)····N protons involved in
intramolecular H-bonding, observed in the solid state in the crystal structure of L1
(vide infra). Besides, the -OH(b) resonance is observed at about δ 9.2 ppm in both
compounds. On the other hand, in the 13C{1H} NMR spectra, typical resonances for the
carbon of the imine group were observed at δ 164.4 (L1) and 166.2 (L2) ppm. Further,
4
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
characterization by FT-IR afforded spectra exhibiting characteristic absorptions for the
stretching vibrations of the CH=N group at 1615-1641 cm-1.[15]
Scheme 1. Synthesis of ligands L1 and L2.
Confirmation of the structure of L1 was achieved by single crystal X-ray
diffraction. The molecular structure of L1 is shown in Figure 3. Crystallographic data
and details of structure determination for this species are given in Table 1. The ligand
L1 crystalized in a R-3 trigonal system. The bond distance in C(7)-N(1) is 1.319(3) Å,
which is similar to that found in salicylaldimine analogous compounds. The planes of
the aryl moieties present an interplanar angle of 6.8o.
Figure 3. Molecular structure for ligand (L1), H atoms are omitted for clarity. Selected
bond distances (Å): C(7)–N(1)= 1.319(3), C(8)–N(1)= 1.422(3), C(6)–C(7)= 1.399(3),
C(1)–O(1)= 1.302(3), C(3)–O(2)= 1.342(3), C(11)–Cl(1)= 1.737(3).
L1 also displays an intramolecular hydrogen bond O1–H···N1 (d(H···N1), 1.822
Å) providing a six-membered ring and an intermolecular hydrogen bond O2–H···O1
(d(H···O1), 1.666 Å) generating a helical arrangement (Figure 4).
5
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Figure 4. Inter- and intra-molecular hydrogen bonds in the crystal structure of L1.
Table 1. Crystallographic data for L1, 3, Ni(II)-POCOP-1 and Ni(II)-POCOP-2.
Compound
L1
3
Ni(II)-POCOP-1
Ni(II)-POCOP-2
Formula
C27H22Cl2N2O5Pd
631.76
C13H10ClNO2
247.67
Trigonal
R-3
C64H106Cl4N2Ni2O7P6S
1492.60
C31H49Cl3NNiO3P3
741.68
Formula weight
Crystal system
Monoclinic
C2/c
Triclinic
Triclinic
Space group
P-1
P-1
a (Å)
21.974(2)
21.974(2)
13.7819(9)
90
29.075(4)
7.9476(10)
11.0668(14)
90
12.0041(8)
13.4414(8)
13.9648(4)
96.566(4)
96.252(4
116.181(6)
1976.5(2)
1
11.8248(4)
13.5987(4)
13.6771(4)
81.0290(9)
85.6237(9)
67.0981(9)
2000.86(11)
2
b (Å)
c (Å)
α (°)
β (°)
90
96.883(2)
90
 (°)
120
V (Å3)
5763.4(13)
18
2538.9(6)
Z
4
 calc (g/cm3)
1.653
1.284
1.254
1.231
Absorption Coefficient
0.983
0.287
0.805
0.833
μ(mm −1
F(000)
T (K)
)
2304
1272
790
780
130(2)
298(2)
130(2)
298(2)
Crystal size
 range (°)
0.350 × 0.090 × 0.080
3.651 a 29.517
0.395 × 0.098 × 0.078
2.658 a 25.330
0.510 × 0.410 × 0.220
3.488 a 25.681
0.382 × 0.251 × 0.220
2.070 a 25.354
Reflections Collected
6929
9862
39265
32702
Indep. Reflections
Completeness to  = 25.242o
Refinement method
3132 [Rint = 0.0451]
98.4
2308 [Rint = 0.1260]
99.4
7507 [Rint = 0.0333]
99.7 %
7303 [Rint = 0.0900]
99.8 %
Full-matrix least-squares
Full-matrix least-squares
Full-matrix least-squares
Full-matrix least-squares
on F2
on F2
on F2
on F2
Data/restraints/parameters
3132 / 1 / 160
2308 / 3 / 176
7507 / 1 / 417
7303 / 145 / 446
Goodness-of-fit on F2
1.114
0.994
1.066
1.055
R1 = 0.0630, wR2 = 0.1615
R1 = 0.1113, wR2 = 0.1825
0.513 −0.468
R1 = 0.0428, wR2 = 0.0894
R1 = 0.0654, wR2 = 0.1024
0.549, −0.563
R1 = 0.0561, wR2 = 0.1702
R1 = 0.0663, wR2 = 0.1808
2.128 −0.610
R1 = 0.0465, wR2 = 0.1287
R1 = 0.0532, wR2 = 0.1348
0.553 −0.542
Final R indices [I>2 (I)]
R indices (all data)
Largestdiff.Peakandhole[e/Å3]
2.2. Synthesis of coordination complexes 1-4.
Coordination compounds 1-4 were synthesized by the reaction of the ligands L1
and L2 with [Ni(AcO)2]·4H2O or Pd(AcO)2 in a 2:1 molar ratio in methanol for 24 h to
6
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
afford the corresponding nickel and palladium complexes 1-4 in moderate yields
(Scheme 2).
Scheme 2. Synthesis of Ni(II) and Pd(II) coordination compounds.
The formation of the palladium complexes was fully confirmed using a
combination of NMR spectroscopy, FT-IR, mass spectrometry and elemental analysis.
1
By comparison of the H NMR spectra of the free ligands L1 and L2 with those of
complexes 3 and 4, it was possible to observe the downfield shift (∆δ ≈ 1 ppm) of the
imine (H7) and aryl protons (H2) (Table 2). In addition, the resonances due to -OH(a)
1
protons are absent in the H NMR spectra of Pd complexes 3 and 4. These results
indicate that the ligands L1 and L2 are coordinated to the palladium in a chelate
fashion through the N and O atoms. Additionally, the 13C{1H} NMR spectra reveal the
different shifts of the C nuclei relative to the free ligands, while the signals for the C7
(CH=N) and C1 (Caryl-O-) expose different field shifts for the Pd(II) metal complexes
confirming the metal coordination. Hence, in the 13C{1H} NMR spectra, the resonance
for carbon of the imine was found at δ 162.9 ppm for L1, 165.0 ppm for L2, and at δ
161.8 ppm for complex 3 and 162.5 ppm for compound 4. Additionally, the C1 (Caryl
-
OH) resonances in the free ligands L1 and L2 are observed at δ 163.2 and 163.2 ppm,
respectively, whereas the resonance due to C1 (Caryl-O-Pd) in the complexes appear at
δ 165.9 ppm for 3 and 165.9 ppm for 4.
Table 2. 1H NMR chemical shifts (δ, in ppm) of the coordination compounds 3 and 4
and their precursor ligands (in DMSO-d6).
7
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Cl
H11
H10
H5 H7
H12
Cl
Cl
H11
H10
H5 H7
Cl
H10
H5 H7
H12
Cl
H10
H5 H7
Cl
H4
N
H4
N
H4
H4
OH
OH
Pd
O
O
O
(b) HO
N
N
Pd
H2
Cl
O
(b) HO
N
H2
Cl
N
(b)HO
OH(a)
(b)HO
OH(a)
H2
H2
Cl
L1
3
L2
4
H2
H4
H5
H7
H10
H11
H12
Ha
6.3
5.4
6.0
7.2
7.8
7.3
7.4
–
6.3
6.4
7.5
8.8
7.5
–
7.4
12.9
10.4
5.5
6.0
7.3
7.9
7.4
–
7.6
–
10.1
6.4
7.4
8.8
7.3
7.5
–
13.3
10.4
–
9.9
Hb
Furthermore, the structure of 3 was fully authenticated by single-crystal X-ray
diffraction analysis (Figure 5). Complex 3 crystallised in a monoclinic system (C2/c). The
asymmetric unit contains half of the molecule along with a methanol molecule. Details
of crystal data and refinements are shown in Table 1 (vide supra). Compound 3 adopts
a square-planar geometry around the palladium center. The centrosymmetric
structure of 3 is a mononuclear species in which the Pd(II) atom is coordinated to two
N, O-bidentate ligands L1. The Pd(1)–O(1) and Pd(1)–N(1) distances are comparable to
those reported for analogous compounds. Also, the planarity of the free ligand is lost
after coordination to the palladium and the aryl group from the 4-Cl-aniline is twisted
out by 55.1(4)° of the plane C(7)–N(1)–C(8)–C(13).
Figure 5. Molecular structure for complex (3), H atoms have been omitted for clarity
Selected bond distances (Å): Pd(1)–O(1)= 1.968(2), Pd(1)–N(1)= 2.024(3). Selected bond
angles (°): O(1)–Pd(1)–O(1)= 180.0, N(1)–Pd(1)–N(1)= 180.0, N(1)–Pd(1)–O(1)=
91.58(11), N(1)–Pd(1)–O(1)= 88.42(11), O(1)–Pd(1)–N(1)= 88.42(11).
8
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
On the other hand, the nickel(II) derivatives 1 and 2 were found to be
paramagnetic and magnetic moment measurements were carried out at 25oC. The
Ni(II) compounds presented magnetic moments of 3.18 μB for 1 and 3.15 μB for 2,
indicative of a high spin six-coordinated octahedral geometry around the nickel center
in both complexes.[16] These results along with the elemental analyses for 1 and 2
suggest that the nickel centre is coordinated to two water molecules and two N,O-
bidentate ligands (Figure 6).[17]
Cln
OH
OH2
N
O
N
Ni
O
OH2
HO
Cln
Figure 6. Structural proposal for complexes 1 and 2.
The nickel complexes were also analysed by FT-IR. A comparison with those of
the free Schiff bases showed a characteristic absorption band in the 1615-1614 cm-1
region, assignable to the azomethine group (C=N). This band shifts to lower
frequencies (1565-1601 cm-1) after complexation indicating the coordination of
azomethine nitrogen to the nickel center.[18] This could be due to the donation of
electron density from the nitrogen atom to the empty d-orbitals of the metal
centre.[19] In addition, the mass spectra of the complexes also supported the proposed
structures, clearly exhibiting the molecular ions [M−H]− at 551 m/z for 1, 619.1 m/z for
2, 599 m/z for 3 and 667 m/z for 4. Elemental analyses results for all complexes (1- 4)
were also in full agreement with the proposed structures.
2.3. Synthesis of non-symmetric Ni(II)-POCOP pincer complexes.
Phosphinite ligands POCOP-1 and POCOP-2 and their corresponding pincer
complexes Ni(II)-POCOP-1 and Ni(II)-POCOP-2 were prepared according to published
methods for related compounds (Scheme 3).[13b, 13c, 20]
9
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Scheme 3. Synthesis of the non-symmetric Ni(II)-POCOP pincer complexes.
POCOP-pincer ligands were obtained through reaction of the ligands L1 and L2
with Et3N in toluene at room temperature for 30 min. This was followed by treatment
with the chlorodiisopropylphosphine in refluxing toluene for 24 h under nitrogen
atmosphere. Unexpectedly, the 31P{1H} NMR analysis of the POCOP-ligands showed
three signals (Pb, Pa and Pc): two singlets between δ 144-155 ppm (Pb and Pa), similar to
those reported for analogous non-symmetric POCOP ligands[13b, c, 21] and a third signal
(Pc) at lower field assigned to a phosphorous nuclei from a phosphine oxide[22]
:
POCOP-1 δ 144.2 (Pa), 154.6 (Pb) and 66.6 (Pc) ppm; POCOP-2: δ 147.9 (Pa), 154.7 (Pb)
and 62.6 (Pc) ppm. Noteworthy, the fact that the phosphorous of the
chlorodiisopropylphosphine oxide (Cl(iPr)2P=O) appears at δ 65.4 ppm in the 31P{1H}
NMR spectrum. A proposal for the formation of these POCOP pincer ligands is shown
on Scheme 4 (vide infra).
The new POCOP ligands POCOP-1 and POCOP-2 exhibited NMR purity and were
further used in the metalation process without previous isolation and purification. The
pincer ligands were reacted with NiCl2 in toluene at reflux to produce the desired
products in moderate yields. The Ni(II)-pincer complexes were isolated as yellow solids
and characterized by NMR spectroscopy, mass spectrometry and elemental analysis.
The 31P{1H} NMR spectra of these new complexes reveal their non-symmetric nature as
shown by typical AB patterns of 31P resonances and diagnostic trans-phosphinite
couplings centred at δ 185.5 (d, Pa)/189.2, (d, Pb) (2JP-P= 326.6 Hz) and δ 185.8 (d,
Pa)/190,0 (d, Pb) (2JP-P= 326.4 Hz) for Ni(II)-POCOP-1 and Ni(II)-POCOP-2, respectively.
As can be noticed, the phosphorus resonances in the nickel complexes are shifted ca.
40 ppm up field with respect to those of the free pincer ligands (vide supra) due to the
coordination to the metal centre. Furthermore, singlets assigned to the Pc phosphorus
atom were observed at δ 57.2-57.5 ppm.
10
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
1
The H NMR spectra of these compounds exhibit characteristic signals for the
isopropyl groups at the expected regions, for instance, signals due to CH(iPr) from the
phosphinite are observed at δ 2.4 ppm, while those due to CH(iPr) of the phosphine
oxide can be observed at δ 1.8 ppm. Additionally, in all cases, the C-H bond activation
by the metalation process was evidenced by the disappearance of the ortho-protons
resonances (R2POCaryl-CHaryl-CarylOPR2) of the complexes.
In the 13C{1H} NMR spectra of these complexes, the characteristic signal due to
the metallated carbon was observed as a doublet of doublets at δ 125.0 ppm for Ni(II)-
POCOP-1 and at δ 125.2 ppm for Ni(II)-POCOP-2 with coupling constants (2JC-P) of ca.
21 Hz. Also, the reduction of the imine group was clearly evidenced by comparison of
the 13C{1H} NMR spectra of L1 (CH=N, 164.4 ppm) and L2 (CH=N, 166.2 ppm) free
ligands with their corresponding Ni(II)-POCOP complexes, which exhibit a doublet at δ
46.8 ppm, JC-Pc= 65.6 Hz assigned to the carbon of the R2P(O)CH-N- fragment. The FT-IR
spectra of both nickel pincer complexes, Ni(II)-POCOP-1 and Ni(II)-POCOP-2, are
characterized by the appearance of bands at 3235-3250 cm-1 (N-H), 1585 cm-1 (C=C),
~1029.5 cm-1 (P–O) and 1152.5 cm−1 due to (P=O).[23]
Crystals suitable for X-ray diffraction studies were obtained for both Ni(II)-
POCOP-1 and Ni(II)-POCOP-2 compounds by slow evaporation from concentrated
DMSO solutions of the corresponding complexes at room temperature. The molecular
structures are shown in Figure 7. Crystallographic data and details of the structure
̅
determinations are given in Table 1 (vide supra). Both complexes crystallized in P1
triclinic systems. The asymmetric unit of Ni(II)-POCOP-1 contains two molecules of this
compound along with a DMSO molecule whereas in Ni(II)-POCOP-2 there is one
molecule of the complex by asymmetric unit. The coordination sphere in the two
complexes is constituted by one chloride and the respective pincer ligand coordinated
in a tridentate fashion through the two phosphorus atoms (Pa, Pb) and one carbon
atom from the aryl backbone forming two five-membered metallacycles. Both
complexes exhibited distorted square-planar geometries as revealed by their bond
lengths and angles. The Pb-Ni-Pa and C-Ni-Cl angles in both complexes are 163.81o and
175.5o, respectively. The Ni-C (1.876 Å) and Ni-Cl (2.1903 Å) bond lengths are
comparable to those reported for related Ni(II)-POCOP pincer complexes, whereas the
M-P distances exhibited values in the range of 2.1432-2.1633(10) Å. The distances and
11
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
angles of Ni(II)-POCOP-1 and Ni(II)-POCOP-2 are comparable with the interatomic
distances and angles in closely related structures.[13a, 21] Attempts to synthesize the
corresponding Pd-POCOP pincer complexes under different reaction conditions were
unsuccessful.
Ni(II)-POCOP-1
Ni(II)-POCOP-2
Figure 7. Molecular structures for complexes Ni(II)-POCOP-1 and Ni(II)-POCOP-2, H
atoms have been omitted for clarity. Selected bond distances (Å): Ni(II)-POCOP-1
Ni(1)–P(1)= 2.1432(10), Ni(1)–P(2)= 2.1633(10), Ni(1)–Cl(1)= 2.1903(10), Ni(1)–C(2)=
1.876(3); Ni(II)-POCOP-2 Ni(1)–P(1)= 2.1532(7), Ni(1)–P(2)= 2.1577(7), Ni(1)–Cl(1)=
2.1903(8), Ni(1)–C(2)= 1.876(2). Selected bond angles (°): Ni(II)-POCOP-1 P(1)–Ni(1)–
P(2)= 163.81(3), C(2)–Ni(1)–Cl(1)= 175.50(8), C(2)–Ni(1)–P(1)= 82.19(7), C(2)–Ni(1)–
P(2)= 82.22(7), P(1)–Ni(1)–Cl(1)= 98.35(3), P(2)–Ni(1)–Cl(1)= 97.53(3); Ni(II)-POCOP-2
P(1)–Ni(1)–P(2)= 163.81(3), C(2)–Ni(1)–Cl(1)= 175.50(8), ), C(2)–Ni(1)–P(1)= 82.22(7),
C(2)–Ni(1)–P(2)= 82.19(7), P(1)–Ni(1)–Cl(1)= 97.53(3), P(2)–Ni(1)–Cl(1)= 98.35(3).
Considering the above results and the structural confirmation by X-ray
diffraction of the pincer complexes, we proposed two possible ways (a and b) for the
formation of the POCOP-ligands (Scheme 4), on the basis of the following
considerations: 1) water traces present in the reaction mixture, 2) in the polarized
azomethine group the carbon atom is electrophilic and susceptible to a nucleophilic
attack,[22, 24] 3) the chlorodialkylphosphines can suffer hydrolysis in the presence of
water to give dialkylphosphine oxide (or dialkylphosphinous acid) and hydrochloric
acid. Also, the dialkylphosphine oxide is present as an equilibrium mixture of the tetra-
P(V) and tri-coordinated P(III) tautomeric forms,[25] as shown in the Scheme 5. The
mechanism of the reaction is therefore proposed to involve direct attack of the
12
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
nucleophilic phosphorus on the electrophilic substrate thus producing the new C–P
bond through a related Kabachnik-Field mechanism.[26]
Intermolecular proposal
R
N
Cl-
N
O
H
R
R
Cl
O
R2P
O
N
O
R
PR2
N
H
R2P
O
O
O
O
PR2
PR2
R2P
PR2
PR2
IIb
POCOP-Ligand
PATH c
B:
Intramolecular proposal
Cl-
N
R
H
R
H
R
PATH b
O
H
R
O
N
O
N
O
R2P
R2P
O
Cl-
R
R
Et3N-HCl
B:
H2O
R2PCl
BH+Cl-
B:
N
N
N
H
PR2
PR2
PR2
H
O
O
O
O
R2PCl
HO
OH
O
OH
O
O
H
H
R2P
PR2
PR2
PR2
R2P
R2P
B:
I
IVb
IIb
IIIb
PATH a
POCOP-Ligand
B:
R2PCl
B:
O
OH
P
R
R
H
O
Cl-
R2P
O
R2P
O
Cl-
R
N
R
R
R
R
N
H
N
H
N
N
H
HCl
R2P(OH)
H
O
O
O
O
O
O
O
O
BH+Cl-
PR2
PR2
PR2
PR2
PR2
PR2
PR2
PR2
PR2
PR2
I
POCOP-Ligand
IIa
IIIa
IVa
Scheme 4. Mechanistic Proposal for the C-P bond formation.
Scheme 5. Equilibrium of pentavalent and trivalent forms of phosphine oxides.
In both pathways a and b, the formation of I occurs in the first step by
deprotonation of -OH groups from the ligands (L1 and L2) with Et3N and then, their
nucleophilic addition to the P from the R2P-Cl. In path a, I undergoes a N-protonation
(IIa) followed by the nucleophilic attack of the R2POH over the CH=N to afford IVa,
then the deprotonation of the resulting hydroxide ion and the concomitant formation
of the P=O bond yield the final POCOP pincer ligand. Alternatively, in path b, N-
protonation (CH=N) can occur to yield IIb followed by nucleophilic attack by the
phosphorus atom of the phosphinite group to the carbon atom of the azomethine
moiety, producing IIIb. Cleavage of the O-P bond follows after the nucleophilic attack
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
over phosphorus by water, affording IVb, successive deprotonations can then occur to
form a P=O bond. Nucleophilic addition to chlorophosphine regenerates the
phophinite moiety and the POCOP pincer ligand. Alternatively, the process may occur
in a concerted intermolecular pathway invoking the C-O cleavage on the resorcinol
moiety (Path c) (Scheme 4).
2.4. Preliminary catalytic studies.
Palladium and nickel complexes are among the most efficient catalysts for
cross-coupling reactions leading to the formation of new C-C bonds.[21, 27] In this
context, the Suzuki-Miyaura reaction is amongst the best-studied metal-catalysed
coupling process due to its relevance for the synthesis of active pharmaceutical
ingredients, materials and others industrial relevant intermediates.[27a, 28] Likewise, the
applications of these metal systems and the use of alternative energy sources, such as
microwave irradiation (MW) to generate more eco-friendly, economical and
sustainable protocols are themes of great relevance nowadays.[29] The use of
microwave irradiation has undeniable advantages compared to conventional heating
affording shorter reaction times, faster reaction kinetics and cleaner products. Thus,
we evaluated the catalytic activity of the series of newly synthesized coordination and
organometallic complexes in Suzuki-Miyaura couplings using microwave irradiation in
a model reaction between bromobenzene and phenylboronic acid under optimized
reaction conditions previously established in our laboratory[21] for analogous catalytic
systems (Na2CO3, DMF/H20, 4:1 (5 mL), 150 W, 20 min) as summarized in Table 3
(Entries 1-6). Among all screened complexes, palladium complex 4 was the most
efficient catalyst affording the coupled product in 58.6 % yield (Entry 4). In general,
palladium compounds 3 and 4 showed by far better results than their analogous nickel
compounds 1 and 2, and the Ni(II)-POCOP pincer complexes. On the other hand,
reactions with the Ni(II)-POCOP pincer complexes exhibited better conversions than
the nickel coordination compounds (avg. 36 vs 50 %). Cross-coupling reactions are
strongly dependant on the base used, thus, several bases were tested under the
optimized reaction conditions using the best-found catalyst, 4 (entries 4, 8-11). The
results obtained revealed that Cs2CO3 affords the best yield (62%), but other bases
such as K2CO3 (60%) and Na2CO3 (58.6) also afforded good results. Finally, the use of
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
lower amounts of complex 4 (0.1 and 0.01 mol %) lead to lower conversions (Entries 12
and 13).
Although no apparent decomposition of the catalyst was observed (i.e.
palladium black), many processes considered homogeneous generally proceed via the
decomposition of the catalysts to produce heterogeneous nanoparticulated catalysts.
In order to confirm the homogeneous nature of our system under the optimized
conditions, we investigated the probable involvement of nanoparticles during the
catalysed Suzuki-Miyaura coupling reaction. Hence, the standard catalytic reaction was
carried out adding drops of elemental mercury (Entry 7) wherein the coupling product
was obtained in 58.9% yield, showing that the presence of mercury did not suppress
the catalytic activity of the complex to produce the C-C coupling suggesting that the
palladium compound is responsible of the observed catalytic activity.
Table 3. Suzuki-Miyaura couplings of bromobenzene and phenylboric acid catalysed by
complexes 1, 2, 3, 4, Ni(II)-POCOP-1 and Ni(II)-POCOP-2a
[Cat], Base
Br
(HO)2B
+
DMF/H2O, 90 oC
150 W, 20 min.
Entry
Compound
[Cat] Mol %
Base
Conversion (%)b
1
2
3
4
5
1
2
3
4
1
1
1
1
1
1
1
1
1
1
1
0.1
0.01
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
CsCO3
Rb2CO3
Li2CO3
Cs2CO3
Cs2CO3
36.5
36.2
57.9
58.6
50.5
50.8
58.9
60.0
62,0
39.5
47.3
45.0
13.2
Ni(II)-POCOP-1
Ni(II)-POCOP-2
6
7c
8
4
4
4
4
4
4
4
9
10
11
12
13
aReaction conditions: [Cat] (0.01 mmol), bromobenzene (1 mmol), PhB(OH)2 (1.2
mmol), Base (2 mmol), DMF/H20 4:1 (5 mL), 150 W, 20 min, 90oC. Conversions were
b
obtained by GC–MS and are based on residual aryl bromide and are the average of two
runs. cElemental mercury was added.
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
3. Conclusions
We have shown that Schiff bases can serve as an attractive template for the
synthesis of coordination and organometallic compounds. Thus, a series of Ni(II) and
Pd(II) coordination complexes 1-4 have been synthetized from the corresponding
Schiff base ligands L1 and L2 attained by the reaction of 2,4-dihydroxybenzaldehyde
and chloroaniline derivatives and then with the corresponding metal. In addition, a
couple of non-symmetric Ni(II)-POCOP pincer complexes, meta-substituted on the
aromatic ring of the main framework were also synthesized. Both non-symmetric
pincer ligands POCOP-1 and POCOP-2 were obtained by treatment of ligands L1 and L2
with chlorodiisopropylphosphine. During this reaction a hydrophosphorylation process
of the imine group to form a C-P bond occurred. Further reactions of POCOP-1 and
POCOP-2 with NiCl2 afforded two novel non-symmetric Ni(II)-POCOP complexes Ni(II)-
POCOP-1 and Ni(II)-POCOP-2 in moderate yields (66-67 %). All ligands and metal
complexes were fully characterized based on different analytical techniques. In
addition, the molecular structures of ligand L1 and complexes 3, Ni-POCOP-1 and Ni-
POCOP-2 were authenticated by single-crystal X-ray diffraction techniques. Finally, all
complexes were examined as catalysts in Suzuki-Miyaura couplings under microwave
irradiation, being Pd(II) complex 4 the best catalysts of the series. Current studies
towards the study of potential coordination of the meta positioned fragment as well as
studies of the reactivities of these kind of ligands with other transition metals and
potential use of these complexes in biological and other catalytic applications are
currently under development in our laboratories and will be disclosed in due time.
4. Experimental section.
4.1. General.
All reactions were carried out under nitrogen with standard Schlenk techniques
unless otherwise stated. All chemical compounds were commercially obtained from
Aldrich Chemical Co. and used as received without further purification. Melting points
were recorded on a MelTemp II apparatus and are reported without correction. The
1H, 13C{1H}, 31P{1H} NMR spectra were recorded on a JEOL GX300 spectrometer.
Elemental analyses were performed in a Thermo Scientific Flash 2000 elemental
analyzer, using a Mettler Toledo XP6 AutomatedS Microbalance and sulfanilamide as
16
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
standard (Thermo Scientific BN 217826, attained values N = 16.40%, C = 41.91%, H =
4.65%, y S = 18.63%; certified values N = 16.26%, C = 41.81%, H = 4.71%, y S = 18.62%).
Positive-ion IE mass spectra were recorded on a JEOL JEM-AX505HA mass
spectrometer. Magnetic susceptibility measurements of the complexes in the solid
state were determined by Gouy balance using CuSO4 as the calibrant. GC–MS analyses
were performed with an Agilent 6890 N GC with a 30.0 m DB-1MS capillary column
coupled to an Agilent 5973 Inert Mass Selective detector, using dodecane as internal
standard. Percent of conversion (% Conversion) was determined from the residual aryl
halide, the product obtained and the corresponding standard solutions. Microwave
driven reactions were carried out using a CEM Discover reactor coupled to a CEM
Explorer robotic system.
4.2 General procedure for the synthesis of ligands L1 and L2
Ligands L1 and L2 were synthesized by the reaction of 2,4-
dihydroxybenzaldehyde (1.0 mmol, 0.138 g) and sodium sulphate in 30 mL ethanol
with 4-chloroaniline (1.0 mmol, 0.127 g) for L1 and 3,5-dichloroaniline (1.0 mmol,
0.162 g) for L2. The mixtures were set to reflux for 24 h. After which time, the resulting
reaction mixture was allowed to cool to room temperature and then concentrated
under vacuum. The residues were dissolved in the minimum volume of CH2Cl2 and
then precipitated with hexane. The resulting solids were filtered under vacuum.
Ligand L1 was isolated as a yellow solid in 92 % yield. Melting point: 127–128
1
°C. H NMR (400 MHz, Acetone-d6, δ ppm): 13.13 (br s, 1H, Ha), 9.27 (br s, 1H, Hb),
3
8.77 (s, 1H, H7), 7.45 (d, JH11,H10 = 8.9 Hz, 2H, H11), 7.42 (d, 3JH5,H4 = 8.8 Hz, 1H, H5),
3
4
7.38 (d, 3JH10,H11 = 8.9 Hz, 2H, H10), 6.49 (dd, JH4,H5 = 8.4 Hz, JH4,H2 = 2.4 Hz, 1H, H4),
4
6.40 (d, JH2,H4 = 2.4 Hz, 1H, H2). 13C{1H} NMR (75.5 MHz, Acetone-d6, δ ppm): 164.6
(C1), 164.4 (C7), 163.5 (C3), 148.6 (C9), 135.7 (C5) 132.0 (C12), 130.5 (C11), 123.7
(C10), 113.5 (C6), 108.8 (C4), 103.5 (C2). MS-DART+: (m/z):248.0 [M]+. FT-IR (KBr, cm-1):
FT-IR (KBr, cm-1): 2435 (O-H···N), 1615 (C=N), 1582, 1491, 1243, 1198 (C–O), 1112,
823, 795 (C–Cl), 509, 483, 404. Anal. Calcd. (%) for C13H10ClNO2: C, 63.04; H, 5.66; N,
4.07. Found: C, 62.98; H, 5.62; N, 4.04.
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Ligand L2 was isolated as an olive green solid in 94 % yield. Melting point: 133–
1
134°C. H NMR (400 MHz, Acetone-d6, δ ppm): 12.87 (br s, 1H, Ha), 8.81 (s, 1H, H7),
3
4
4
7.45 (d, JH5,H4 = 8.5 Hz, 1H, H5), 7.37 (d, JH10,H12 = 1.9 Hz, 2H, H10), 7.33 (d, JH12,H10
=
3
4
1.9 Hz, 1H, H12), 6.51 (dd, JH4,H5 = 8.5 Hz, 4JH4,H2 = 2.3 Hz, 1H, H4), 6.41 (d, JH2,H4 = 2.3
Hz, 1H, H2). 13C{1H} NMR (75.5 MHz, Acetone-d6, δ ppm): 166.2 (C7), 164.7 (C1), 164.2
(C3), 152.5 (C9), 136.3 (C5) 126.3 (C12), 136.2 (C11), 121.1 (C10), 113.4 (C6), 109.1
(C4), 103.6 (C2). MS-DART+ (m/z): 282.0 [M]+. FT-IR (KBr, cm-1): 3513 (O-H), 1614 (C=N),
1562, 1510, 1337, 1250, 1198 (C–O), 1107, 839, 801 (C–Cl). Anal. Calcd. (%) for
C13H9Cl2NO2: C, 55.34; H, 3.22; N, 4.96. Found: C, 53.79; H, 3.36; N, 4.88.
4.3. General procedure for the synthesis of coordination complexes 1-4.
A solution of either [Pd(AcO)2] (0.12 mmol, 28 mg) or [Ni(AcO)2]·4H2O (0.12
mmol, 31 mg) in absolute methanol (15 mL) was added to a solution of the
corresponding ligand in methanol (15 mL): L1 (0.25 mmol, 62 mg) or L2 (0.25 mmol, 70
mg). The resulting reaction mixtures were magnetically stirred at room temperature
for 24 h. After this time, the mixtures were evaporated to dryness. The solids obtained
were washed with ethyl ether and dried in vacuo producing either green (1) or yellow
(2) products.
Complex 1 was isolated as green solid in 62 % yield. Decomposition point: 258–
259 °C. MS-ESI- (m/z): 550.9 [M]−. FT-IR (KBr, cm-1): 3288 (O-H), 1565 (C=N), 1541,
1432, 1195 (C–O), 1125, 948, 800, 501. Anal. Calcd. (%) for C26H22Cl2N2NiO6: C, 53.10;
H, 3.77; N, 4.76. Found: C, 53.01; H, 3.78; N, 4.72.
Complex 2 was isolated as green solid in 62 % yield. Decomposition point: 270–
271 °C. MS-ESI- (m/z): 619.1 [M]−. FT-IR (KBr, cm-1): 3120 (O-H), 1601 (C=N), 1574,
1542, 1437, 1193 (C–O), 1128, 799, 670. Anal. Calcd. (%) for C26H20Cl4N2NiO6: C, 47.54;
H, 3.07; N, 4.26. Found: C, 47.48; H, 3.04; N, 4.23.
Complex 3 was isolated as bright yellow solid in 65 % yield. Melting point: 127–
128 °C. 1H NMR ((400 MHz, DMSO-d6, δ ppm): 9.99 (br s, 2H, Hb), 8.71 (s, 2H, H7), 7.44
3
3
3
(d, JH11,H10 = 8.6 Hz, 4H, H11), 7.30 (d, JH10,H11 = 8.6 Hz, 4H, H10), 7.23 (d, JH5,H4 = 8.8
18
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
3
4
4
Hz, 2H, H5), 6.04 (dd, JH4,H5 = 8.8 Hz, JH4,H2 = 2.3 Hz, 2H, H4), 5.41 (d, JH2,H4 = 2.3 Hz,
2H, H2) ppm. 13C{1H} NMR (400 MHz, DMSO-d6): 165.9 (C1), 161.8 (C7), 148.1 (C9),
137.0 (C5) 130.0 (C12), 127.7 (C11), 126.9 (C10), 113.5 (C6), 103.4 (C4), 99.5 (C2) ppm.
MS-ESI- (m/z): 599.0[M]−. FT-IR (KBr, cm-1): 3629 (O-H), 1617(C=N), 1541, 1483, 1439,
1252, 1193 (C–O), 1170, 1129, 786, 698, 441, 422. Anal. Calcd. (%) for C26H18Cl2N2O4Pd:
C, 52.07; H, 3.03; N, 4.67. Found: C, 52.17; H, 3.08; N, 4.68.
Complex 4 was isolated as bright yellow solid in 72 % yield. Melting point:
1
300°C. H NMR ((400 MHz, DMSO-d6, δ ppm): 10.10 (br s, 2H, Hb), 7.90 (s, 2H, H7),
4
4
3
7.59 (t, JH12,H10 = 1.9 Hz, 2H, H12), 7.42 (d, JH10,H12 = 1.9 Hz, 4H, H10), 7.26 (d, JH5,H4
=
3
4
4
8.8 Hz, 2H, H5), 6.08 (dd, JH4,H5 = 8.8 Hz, JH4,H2 = 2.2 Hz, 2H, H4), 5.45 (d, JH2,H4 = 2.2
Hz, 2H, H2). 13C{1H} NMR (400 MHz, DMSO-d6, δ ppm): 165.9 (C1), 164.7 (C3), 162.5
(C7), 151.2 (C9), 137.3 (C5) 125.3 (C12), 133.0 (C11), 124.4 (C10), 113.6 (C6), 107.0
(C4), 103.2 (C2). MS-ESI- (m/z): 666.9 [M]−. FT-IR (KBr, cm-1): 3545 (O-H), 1616 (C=N),
1573, 1537, 1440, 1256, 1201, 1178 (C–O), 1135, 843, 801, 508, 431. Anal. Calcd. (%)
for C26H16Cl4N2O4Pd: C, 46.70; H, 2.41; N, 4.19. Found: C, 46.65; H, 2.38; N, 4.17.
4.4. General procedure for the synthesis of POCOP pincer complexes.
A solution of L1 (0.5 mmol, 124 mg) or L2 (0.5 mmol, 141 mg) and Et3N (1,1
mmol) in toluene was stirred at room temperature for 30min. and then
chlorodiisopropylphosphine (1.5 mmol) was added. The reaction mixture was refluxed
for 24 h. After reaction time, the solution was cooled to room temperature and filtered
through cannula. To this solution, NiCl2 0.5 mmol (65 mg) was added in one portion.
Then, the reaction mixture was refluxed for 24 h. After this time, the solution was
cooled to room temperature and all volatiles were removed under high vacuum. The
crude products were purified by chromatographic column using a 4:1 hexane/ethyl
acetate mixture as eluent.
Ligand POCOP-1. 31P{1H} NMR (121.6 MHz, CDCl3, δ ppm): 154.6 (s, Pb), 144.2
(s, Pa), 66.6 (s, Pc).
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Ligand POCOP-2. 31P{1H} NMR (121.6 MHz, CDCl3, δ ppm): 154.7 (s, Pb), 147.9
(s, Pa), 62.6 (s, Pc).
Complex Ni(II)-POCOP-1 was isolated as yellow solid in 67 % yield. Melting
point: 224–226°C. 31P{1H} NMR (121.6 MHz, CDCl3, δ ppm): 189.2 (d, 2JPb,Pa = 326.6 Hz,
Pb), 185.5 (d, 2JPa,Pb = 326.6 Hz, Pa), 57.2 (s, Pc). 1H NMR (400 MHz, CDCl3, δ ppm): 7.18
(d, 3JH5,H4 = 8.3 Hz, 1H, H5), 7.02 (d, 3JH11,H10 = 8.2 Hz, 2H, H11), 6.52 (d, 3JH10,H11 = 8.2 Hz,
2H, H10), 6.4 (dd, 3JH4,H5 = 8.3 Hz, 1H, H4), 5.21 (br s, 1H, H8), 4.87 (m, 1H, H7), 2.4 (m,
H13a,b), 1.8 (m, H13c), 1.3 (m, H14). 13C{1H} NMR (75.6 MHz, CDCl3, δ ppm): 168.6 (d,
2
3
2JC3,Pb = 9.7 Hz, C3), 165.2 (d, JC1,Pa = 8.9 Hz, C1), 144.7 (d, JC9,Pc = 10.4 Hz, C9), 129.1
2
2
(C5), 129.1 (C10), 134.9 (tv, JC2,Pa = 21.8 Hz, JC2,Pb = 21.1 Hz, C2), 122.4 (C12), 114.4
(C11), 114.3 (d, 2JC6,Pc = 8.7 Hz, C6), 107.0 (d, 3JC4,Pb = 11.0 Hz, C4), 46.8 (d, 1JC7,Pc = 65.6
Hz, C7), 27 (C13), 16 (C14). MS-FAB+ (m/z): 708 [M]+. FT-IR (KBr, cm-1): 3235, 2961,
1585 (N–H), 1450, 1153 (P–O), 1028, 986, 836, 677, 652, 497 (Ni–C). Anal. Calcd. (%)
for C31H50Cl2NNiO3P3: C, 52.65; H, 7.13; N, 1.98. Found: C, 52.52; H, 7.07; N, 1.99.
Complex Ni(II)-POCOP-2 was isolated as yellow solid in 66 % yield. Melting
point: 214–216 °C. 31P{1H} NMR (121.6 MHz, CDCl3, δ ppm): 190.0 (d, 2JPb,Pa = 326.4 Hz,
Pb), 185.8 (d, 2JPa,Pb = 326.6 Hz, Pa), 57.5 (s, Pc). 1H NMR (400 MHz, CDCl3, δ ppm): 7.26
3
4
4
(d, JH5,H4 = Hz, 2H, H5), 7.59 (t, JH12,H10 = 1.9 Hz, 2H, H12), 7.42 (d, JH10,H12 = 1.9 Hz,
4H, H10), , 6.08 (dd, 3JH4,H5 = 8.8 Hz, 4JH4,H2 = 2.2 Hz, 2H, H4), 5.45 (d, 4JH2,H4 = 2.2 Hz, 2H,
2
H2). 13C{1H} NMR (75.6 MHz, CDCl3, δ ppm): 168.6 (d, JC3,Pb = 11.7 Hz, C3), 165.2 (d,
3
3
2JC1,Pa = 13.2 Hz, C1), 148.1 (d, JC9,Pc = 9.9 Hz, C9), 129.1 (d, JC5,Pc = 2.8 Hz, C5), 134.9
2
2
2
(tv, JC2,Pa = 21.4 Hz, JC2,Pb = 20.6 Hz, C2), 117.5 (C12), 113.9 (d, JC6,Pc = 10.1 Hz, C6),
111.4 (C10), 114.4 (C11), 107.0 (d, 3JC4,Pb = 11.2 Hz, C4), 46.4 (d, 1JC7,Pc = 64.5 Hz, C7), 27
(C13), 16 (C14). MS-FAB+ (m/z): 743 [M]+. FT-IR (KBr, cm-1): 3250 (N–H), 2962, 2605,
1584 δ(N–H), 1448, 1152 (P–O), 1108, 1029, 985, 836, 673, 653, 499 (Ni–C) cm−1. Anal.
Calcd. (%) for C31H49Cl3NNiO3P3: C, 50.20; H, 6.66; N, 1.89. Found: C, 50.12; H, 6.61; N,
1.87
20
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4.5. General procedure for Suzuki-Miyaura couplings.
Suzuki-Miyaura couplings were carried out in 10 mL reaction tubes suitable for
using in a microwave apparatus equipped with an inner magnetic stirring bar. A
reaction mixture of 5 mL of a DMF/H2O 4:1 containing [Cat] (0.01 mmol), aryl bromide
(1 mmol), PhB(OH)2 (1.2 mmol) and a base (2 mmol) was stirred and heated in a
o
microwave reactor at 90 C and 150 W for 20 min. After the prescribed reaction time,
the mixture was cooled to room temperature, extracted with CH2Cl2 (3 X 3 mL) and
dried with anhydrous Na2SO4, filtered through celite and analysed by GC-MS.
4.6. Mercury drop experiments[30]
Following the above described procedures; additionally, adding two drops of
elemental Hg(0) to the reaction mixture. After the prescribed reaction times, the
solution was filtered and analysed by GC–MS: no significant difference in conversion
between these experiments and those in the absence of mercury was observed,
indicating that heterogeneous Pd(0) is not involved.
4.7. Data collection and refinement for L1, 3, Ni(II)-POCOP-1 and Ni(II)-POCOP-2.
4.7.1. Complexes 3 and Ni(II)-POCOP-2.
Yellow prisms of compounds 3 (CCDC 2062937) and Ni(II)-POCOP-2 (CCDC
2062938) were grown independently from CH2Cl2/diethyl ether and mounted on glass
fibers, then placed on a Bruker Smart Apex II diffractometer with a Mo-target micro-
focus X-ray source (λ = 0.71073Å). The detector was placed at a distance of 5.0 cm
from the crystals frames were collected with a scan width of 0.5 in ω and an exposure
time of 10 s/frame, which were integrated with the Bruker SAINT software package
using a narrow-frame integration algorithm. Non-systematic absences and intensity
statistics were used for monoclinic C2/c space group for compound 3 and triclinic P-1
space group for Ni(II)-POCOP-2. The structures were solved using Patterson methods
using SHELXS-2018 program.[31] The remaining atoms were located via a few cycles of
least squares refinements and difference Fourier maps. Hydrogen atoms were input at
calculated positions and allowed to ride on the atoms to which they are attached.
Thermal parameters were refined for hydrogen atoms on the phenyl groups using a
Ueq = 1.2 Å and a Ueq = 1.5 Å for methyl groups to precedent atom in all cases. For all
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
complexes, the final cycle of refinement was carried out on all non-zero data using
SHELXL-2014/7.[32] Absorption correction was applied using SADABS program.[33]
Compound 3 crystallized with CH3OH as crystallization solvent which fall in
special position, the structure was solved as a half of molecule and symmetry code (-
x+1/2, -y+1/2, -z+1) is needed for complete the molecule. In the crystal packing
intermolecular H-bond O3-H3----O2 are observed on the plane bc and along a axis. In
the refinement 5 disagreeable reflection were omitted.
In the Ni(II)-POCOP-2 structure, the coordination center is almost planar with
2,4-dichloroaniline moiety rotated by 66.46(12)°. Two isopropyl groups are disordered
and were refined in two major contributors using a site occupational factor (SOF). The
ratio for SOF was 0.76/0.24 and 0.78/0.22 for the isopropyl groups. 145 restraints were
applied as follow: 126 for SIMU and 12 for DELU, and 6 for SAME/SADI. 7 disagreeable
reflection were omitted.
4.7.2. Ligand L1 and complex Ni(II)-POCOP-1.
Suitable single crystals of compounds L1 (CCDC 2062896)and Ni-POCOP-1
(CCDC 2062897) were mounted on glass fibers; crystallographic data were collected
with an Oxford Diffraction Gemini "A" diffractometer with a CCD area detector at 130
K, with λ = 0.71073. Unit cell parameters were determined with a set of three runs of
15 frames (1° in ω). The double pass method of scanning was used to exclude any
noise [34]. The collected frames were integrated by using an orientation matrix
determined from the narrow frame scans. Final cell constants were determined by a
global refinement; collected data were corrected for absorbance by using analytical
numeric absorption correction using a multifaceted crystal model based on
expressions upon the Laue symmetry with equivalent reflections [35]. Structures
solutions and refinement were carried out with the SHELXS-2014 [36] and SHELXL-
2014 [32] packages. WinGX v2020.1 [37] software was used to prepare material for
publication. Full-matrix least-squares refinement was carried out by minimizing (Fo2 –
Fc2)2. All non-hydrogen atoms were refined anisotropically. The H atoms of the
hydroxy and amine groups were located in a difference map and refined isotropically
with Uiso(H) = 1.5 for H–O and Uiso(H) = 1.2 for H–N. H atoms attached to C atoms were
placed in geometrically idealized positions and refined as riding on their parent atoms,
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
with C–H = 0.95 – 1.00 Å and with Uiso(H) = 1.2 Ueq(C) for aromatic and methine groups
and Uiso(H) = 1.5 Ueq(C) for methyl groups. For L1 attempts made to model the solvent
molecule were not successful; the SQUEEZE option in PLATON [38] indicated there was
a large solvent cavity of 299 Å3. In the final cycles of refinement, this contribution of 54
electrons to the electron density was removed from the observed data. For the
electron density, F(000) value in the molecular weight and the formula are given
without taking into account the results obtained with SQUEEZE.
5. Supplementary information
Supplementary data for compounds L1, 3, Ni(II)-POCOP-1 and Ni(II)-POCOP-2
was deposited at the Cambridge Crystallographic Data Centre. Copies of this
information are available free of charge on request from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033; e-mail deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk) quoting the deposition numbers 2062937-
2062938 and 2062896-2062897.
6. Declaration of Competing Interest
The authors declare that they have no known competing financial interests or
personal relationship that could have appeared to influence the work reported in this
paper.
7. Acknowledgments
We would like to thank Dr. Francisco Javier Pérez Flores, Q. Eréndira García
Ríos, M.Sc. Lucia de Carmen Márquez Alonso, M.Sc. Lucero Ríos Ruiz, M.Sc. Alejandra
Núñez Pineda (CCIQS), Q. María de la Paz Orta Pérez, Q. Rocio Patiño-Maya and Ph.D.
Nuria Esturau Escofet for technical assistance. The financial support of this research by
PAPIIT-DGAPA-UNAM (PAPIIT IN210520) and CONACYT A1-S-33933 is gratefully
acknowledged.
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European Journal of Inorganic Chemistry
Graphical Abstract
Schiff Bases as Inspirational Motif for the Production of Ni(II) and Pd(II)
Coordination and Novel Non-Symmetric Ni(II)-POCOP Pincer Complexes.
M. Sc. Jaime Alberto Rosas-Ortiz,a Dr. J. Roberto Pioquinto-Mendoza,a Dr. Lucero
González-Sebastián,b M.Sc. Simon Hernandez-Ortega,a Dr. Marcos Flores-Alamoc and
Prof. Dr. David Morales-Moralesa,*
aInstituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior,
Ciudad Universitaria, Ciudad de México, C.P. 04510, México. bDepartamento de
Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco No.
186, Ciudad de México, C.P. 09340, México. cFacultad de Química, División de Estudios
de Posgrado, Universidad Nacional Autónoma de México, Circuito Exterior, CU, 04510,
México.
*Corresponding author. E-mail address: damor@unam.mx (D. Morales-Morales)
Phone: +52-5556224514
https://iquimica.unam.mx/david-morales-morales
Twitter: @dmorales_group
26
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10.1002/ejic.202100146
European Journal of Inorganic Chemistry
Table of Contents Text
Schiff Bases as Inspirational Motif for the Production of Ni(II) and Pd(II)
Coordination and Novel Non-Symmetric Ni(II)-POCOP Pincer Complexes.
M. Sc. Jaime Alberto Rosas-Ortiz,a Dr. J. Roberto Pioquinto-Mendoza,a Dr. Lucero
González-Sebastián,b M.Sc. Simon Hernandez-Ortega,a Dr. Marcos Flores-Alamoc and
Prof. Dr. David Morales-Moralesa,*
aInstituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior,
Ciudad Universitaria, Ciudad de México, C.P. 04510, México. bDepartamento de
Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco No.
186, Ciudad de México, C.P. 09340, México. cFacultad de Química, División de Estudios
de Posgrado, Universidad Nacional Autónoma de México, Circuito Exterior, CU, 04510,
México.
*Corresponding author. E-mail address: damor@unam.mx (D. Morales-Morales)
Phone: +52-5556224514
https://iquimica.unam.mx/david-morales-morales
Twitter: @dmorales_group
N-O chelating Schiff´s bases from 2,4-dihydroxybenzaldehyde were synthesized, and
further functionalized with (iPr)2PCl to produce non-symmetric POCOP pincer ligands
with an unexpected C-P bond formation. Reactions of the ligands with Ni(II) or Pd(II)
lead to the corresponding coordination or organometallic POCOP pincer compounds.
All complexes were used as catalysts in Suzuki-Miyaura couplings.
27
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