Synthesis and structural chemistry of CoII, CuII, CuI and PdII complexes containing a flexible monoselenoether ligand

Article abstract of DOI:10.1039/c8nj03621g

Monoselenoether ligand [PySeCH₂Py] (L) reacts with CoCl₂, CuCl₂, CuI and Pd(OAc)₂ resulting in the formation of molecular or polymeric complexes. The synthesis and structural elucidation of the complexes [CoCl₂(L)] (1), [Cu₂Cl₄(L)₂] (2a), [CuCl₂(L)] (2b), [Cu₂Cl₂(PySe)₂] (2c), [Cu(2-PyCOO)₂] (2d), [Cu₂I₂(L)₂] (3), [Cu₂I₂(L)]_n (4) and [Pd₂(CH₃COO)₂(PySeCHPy)₂]·7H₂O (5) demonstrate the versatility of this class of pyridylselenium ligands. For all compounds, the ligand offers N and/or Se atoms as donors and, in the case of 5, the ligand also builds C-Pd bonds to generate a cyclometallated palladium(ii) complex.

Full text of DOI:10.1039/c8nj03621g

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Synthesis and structural chemistry of CoII, CuII,
CuI and PdII complexes containing a flexible
monoselenoether ligand†
Cite this: New J. Chem., 2018,
42, 17185
Received 20th July 2018,
Accepted 26th September 2018
Roberta Cargnelutti, * Cassia P. Delgado, Rodrigo Cervo, Barbara Tirloni,
´
´
Robert A. Burrow
and Ernesto S. Lang*
DOI: 10.1039/c8nj03621g
rsc.li/njc
Monoselenoether ligand [PySeCH2Py] (L) reacts with CoCl2, CuCl2,
CuI and Pd(OAc)2 resulting in the formation of molecular or poly-
meric complexes. The synthesis and structural elucidation of the
complexes [CoCl2(L)] (1), [Cu2Cl4(L)2] (2a), [CuCl2(L)] (2b), [Cu2Cl2(PySe)2]
(2c), [Cu(2-PyCOO)2] (2d), [Cu2I2(L)2] (3), [Cu2I2(L)]n (4) and
[Pd2(CH3COO)2(PySeCHPy)2]ꢀ7H2O (5) demonstrate the versatility
of this class of pyridylselenium ligands. For all compounds, the
ligand offers N and/or Se atoms as donors and, in the case of 5, the
ligand also builds C–Pd bonds to generate a cyclometallated
palladium(II) complex.
Fig. 1 Examples of 2-pyridylselenium compounds.
On the other hand, the syntheses of compounds containing
a monothioether ligand as represented in structure e (Fig. 1)
are well described.3c,e,7 The monothioether e is considered a
flexible ligand in coordination chemistry.3c,7a,b Prakash and
co-workers3e reported the synthesis and structure of half sandwich
complexes of chalcogenated pyridine based bi-(N, S/Se) and
terdentate (N, S/Se, N) ligands with (Z6-benzene)ruthenium(II)
and the application of these complexes in the catalysis of
transfer hydrogenation of ketones and oxidation of alcohols.
The authors reported that the catalytic efficiency generally
varies with chalcogen ligands in the order of Se 4 S, when
other co-ligands and substituents on the donor atoms are the
same. Another observation reported by the aforementioned
authors is that the bidentate (Se, N) type ligand provides
complexes with catalytic better efficiency. Prakash and co-workers3c
also described efficient catalysis with the same kind of reaction
using rhodium(III) and iridium(III) complexes of half-pincer
chalcogenated pyridines as the catalytic system.
Organoselenium compounds are being studied extensively due
to their numerous applications in organic transformations,1
pharmacological properties,1,2 catalytic activity,3 semiconducting
materials4 and coordination chemistry.5 Pyridylselenium deriva-
tives are a special family of ligands which have soft selenium and
intermediate nitrogen donor atoms and may participate in a
variety of coordination modes.5 Some 2-pyridylselenium ligands
are presented in Fig. 1, and the species a–c have been employed to
build mono-, bi- and polynuclear coordination compounds.1,3b,5
Our group, in 2015, described the use of bis(2-pyridyl)diseleno-
ethers (Fig. 1, structure c) as versatile ligands for copper-catalyzed
C–S bond formation in glycerol.3a
Although the chemistry of 2-pyridylselenium derivatives (Fig. 1,
structures a–c) is well-known,3,5 the synthesis of complexes
containing 2-picolyl selenopyridine (Fig. 1, structure d) as a ligand
remains widely unexplored. To our knowledge, there are only two
articles describing the synthesis of 2-picolyl selenopyridine (d)1,6
and no information on the crystal structures of this class of
compounds was found.
In light of these previous results and our interest in the
unexplored coordination chemistry of the ligand 2-picolyl
selenopyridine (d), we synthesized and characterized six new
transition metal complexes (1, 2a–b, 3–5) and two complexes
obtained as ligand decomposition products (2c–d) by IR, elemental
analysis, electrospray ionization mass spectrometry and single-
crystal X-ray crystallography.
Department of Chemistry, Universidade Federal de Santa Maria,
The procedure used for the synthesis of ligand L is different
from the one reported by Bhasin and co-workers6 and afforded
the product in good yield when compared with the literature.6
For the synthesis of the ligand [PySeCH2Py] L, the proce-
dure described by Peglow and co-workers1 was adapted using
´
ˆ
LMI – Laboratorio de Materiais Inorganicos, 97105-900, Santa Maria, RS, Brazil.
E-mail: roberta.cargnelutti@ufsm.br, eslang@ufsm.br
† Electronic supplementary information (ESI) available. CCDC 1844932–1844939
(1–5). For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8nj03621g
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Scheme 1 Synthetic route for the synthesis of ligand L.
bis(2-pyridyl)diselenide (f), 2-(chloromethyl)pyridine (g), PEG-400
as the solvent and NaBH4 as the reducing agent under an argon
atmosphere for 3.5 h. The synthesis of L has been summarized in
Scheme 1. The NMR spectra, elemental analysis and ESI+MS data
for ligand L are consistent with its structure which can also be
seen and confirmed by the single-crystal X-ray diffraction studies
of complexes 1–5.
Fig. 3 Molecular structure of the binuclear copper(II) complex
[Cu2Cl4(L)2] (2a) with ellipsoids at the 30% probability level.
2-Picolyl selenopyridine
L was used for complexation
reactions with metal ions of different hardness (CoII, CuII, CuI
and PdII). Scheme 2 summarizes the reactions and their
products. The complexes 1–5 obtained were characterized by
single-crystal X-ray diffraction. Their molecular structures were
designed with Diamond8 and are shown in Fig. 2–9. The
experimental part for the synthesis of the ligand (L) and
complexes 1–5 and also crystallographic details are reported
in the ESI.†
Fig. 4 Molecular structure of [CuCl2(L)] (2b) with ellipsoids at the 50%
probability level.
Complex [CoCl2(L)] (1) was obtained when a solution of L
(0.2 mmol) in 3 mL of dichloromethane was overlayered with
a solution of CoCl2 (0.2 mmol) in 3 mL of methanol. Slow
diffusion of these solutions produced blue crystals of compound 1.
After three days, the crystals were collected and dried under a
Fig. 5 Molecular structure of [Cu2Cl2(PySe)2] (2c) with ellipsoids at the
50% probability level.
Scheme 2 Synthetic routes used to obtain 1–5.
Fig. 6 Molecular structure of [Cu(2-PyCOO)2] (2d) with ellipsoids at the
50% probability level.
vacuum system. The ESI+ mass spectra of the compound show
intense peaks at m/z = 308.9329 for the molecular ion corres-
ponding to [C11H10CoN2Se]+ (calc. 308.9341). In this complex,
the 2-picolyl selenopyridine L acts as a bidentate ligand and is
Fig. 2 Molecular structure of [CoCl2(L)] (1). Thermal ellipsoids represent
30% probability.
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Letter
the mother liquor. An important difference was observed for
the crystal yield of these compounds: 2a 80.2% and 2b 8.0%.
The molecular structure of 2a (Fig. 3) shows a binuclear
copper(II) complex, and the copper metal atom adopting a
distorted square planar geometry. The ESI+ mass spectrum of
the compound shows an intense peak at m/z = 312.9319, which
can be assigned to an ion of the composition [C11H10CuN2Se]+
(calc. 312.9305). Fig. 4 shows the molecular structure of
complex 2b. In this complex, the copper atom adopts a dis-
torted tetrahedral geometry formed by two N atoms of the same
ligand and two chloride ligands.
Trace amounts of the complexes [Cu2Cl2(PySe)2] (2c) and
[Cu(2-PyCOO)2] (2d) were obtained as the by-products in the
mother liquor of complexes 2a–b. The orange (2c) and blue (2d)
crystals were analyzed by single-crystal X-ray crystallography
and the molecular structures are shown in Fig. 5 and 6,
respectively.
Fig. 7 Molecular structure of [Cu2I2(L)2] (3). Thermal ellipsoids represent
30% probability. Symmetry designator: (0) = 1 ꢁ x, 1 ꢁ y, ꢁz.
Kienitz and collegues,9 in 2000, described the synthesis of a
[Cu(m-Br)-(N,N0-PySeSePy)] complex that is structurally similar
to complex 2c. The molecular structure of 2c shows a core with
two copper(I) atoms being connected by two m2-chlorine atoms,
in a tetrahedral geometry. This structure is different from
the one obtained in the direct reaction between copper(II)
chloride and bis(2-pyridil)diselenide (PySeSePy) described by
our group5a in 2015. In the formation of these by-product
complexes, the copper and selenium reduction (2c) and the
oxidation of the alkyl carbon of the ligand L formed the
carboxylate group (2d). The structure of the planar mononuclear
copper(II) complex 2d is well established in the literature.10
In 2012, Wang and colleagues synthesized a picolinate copper(II)
complex by promoting a reaction between Cu(CH3COO)2ꢀH2O and
picolinic acid in a mixture of methanol and water as solvents.
These authors also reported that this copper(II) complex shows
typical intercalation with DNA, due to its nearly planar structure.
Complex [Cu2I2(L)2] (3) was obtained when a solution of L
(0.2 mmol) in 3 mL of dichloromethane was overlayered with a
solution of CuI (0.2 mmol) in 3 mL of methanol. Yellow crystals
of 3 were obtained in the mother liquor reaction and a single-
crystal was characterized by X-ray analysis. The structure of 3
(Fig. 7) shows a core with two copper atoms being connected by
two m2-iodine atoms. Surrounding the core, four nitrogen atoms
from two ligands L complete the tetrahedral coordination
environment of the core. This coordination mode is similar
to the one observed for compound 2c.
Fig. 8 Polymeric structure of [Cu2I2(L)]n (4). Thermal ellipsoids represent
30% probability. Symmetry designator: (0) = 1 ꢁ x, ꢁ0.5 + y, 1 ꢁ z;
(00) = 1 ꢁ x, 0.5 + y, 1 ꢁ z.
Fig. 9 Molecular structure of [Pd2(CH3COO)2(PySeCHPy)2]ꢀ7H2O (5).
Thermal ellipsoids represent 50% probability. Water solvate molecules
have been omitted for clarity.
The same starting material stoichiometry used to synthesize
compound 3 was also used to obtain the polymer [Cu2I2(L)]n (4),
but, in this reaction, the ligand L and copper(I) were solubilized
coordinated with a cobalt metal atom through nitrogen atoms. in anhydrous methanol and stirred for 2 hours under reflux
In the crystal structure of [CoCl2(L)] (1), the CoII adopts a typical and an argon atmosphere. The ESI+ mass spectrum of the
tetrahedral geometry formed by two nitrogen atoms from the compound shows an intense peak at m/z = 502.7662, which
2-picolyl selenopyridine ligand and two chlorine ligands, as can be assigned to an ion of the composition [C11H10Cu2IN2Se]+
shown in Fig. 2.
(calc. 502.7646). The yellow crystals observed in the mother
Compounds [Cu2Cl4(L)2] (2a) and [CuCl2(L)] (2b) were liquor were analyzed by X-ray crystallography and the structure
prepared in a way similar to that used for the synthesis of 1, obtained is represented in Fig. 8.
but using copper(II) chloride instead of cobalt(II) chloride. After
The one-dimensional polymeric chains grow along the
some days, green (2a) and yellow (2b) crystals were observed in crystallographic b axis. Each copper(I) atom adopted a distorted
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structures are formed by agostic interactions between Pd and the
benzilic C–H bond. The metallacycle formation (complex 5)
results from the C–H bond cleavage with the successive exclusion
of HOAc and the formation of a C–Pd bond.
In summary, the syntheses of complexes 1–5 demonstrate
the versatility of the ligand 2-picolyl selenopyridine (L) toward
the formation of molecular, polymeric or cyclometallated
complexes.
Compounds 1, 2c, 3, 4 and 5 show that this class of ligand
can stabilize metal centers at different oxidation levels, such as
CoII, CuI and PdII. It is well established that these transition
metals in these oxidation states can be used in different organic
reactions as a catalyst.12 In metal catalysis systems it is impor-
tant that the use of a ligand may increase the solubility of the
metal salts in the solvents.3a,b Furthermore, it is reported3e that
bidentate (Se, N) type ligands provide complexes with better
catalytic efficiency.
Scheme 3 Mechanistic pathways proposed for the formation of
[Pd2(CH3COO)2(PySeCHPy)2] (5) through agostic C–H bond activation.
Complexes 1, 2c, 3, 4 and 5 allow us to predict its applica-
tions as a catalyst in coupling reactions. Some examples of
reactions that use these transition metals as a catalyst are cross-
tetrahedral geometry, with the atoms of the ligands coordinating dehydrogenative coupling12d (CoII), cross-coupling reactions
differently to the metal center. For example, the Cu20 atom is of arylboronic esters and aryl halides12g (CoII), cross-coupling
coordinated by two m3-iodine, one m2-iodine and one N atom of of iodobenzoates with bromozincdifluorophosphonate12b (CuI),
the ligand L (Fig. 8 – light violet tetrahedron) and the Cu1 atom Heck-type cross-coupling12h (PdII) and regioselective arylation of
is coordinated by one m3-iodine, one m2-iodine, one N and one imidazoles12i (PdII).
selenium atom of the same ligand L (Fig. 8 – light green
tetrahedron). The observed copper-copper distances Cu1ꢀ ꢀ ꢀCu20
Conflicts of interest
and Cu2ꢀ ꢀ ꢀCu100 of 2.6232 (7) Å can be interpreted as weak
interactions and are shorter than the copper-copper distance
reported for complex [Cu4I4((2-PySe)2CH2)2] 2.8481 (3).3b Among
the complexes 1–5, complex 4 was the only one in which the
There are no conflicts to declare.
selenium soft atom coordinated with the copper(I) soft atom, Acknowledgements
and this complex was only formed when the reaction was
performed under reflux conditions.
This study was supported by funds from the Brazilian agencies
FAPERGS (2017/1 ARD 17/2551-0000902-9), CNPq, CAPES and
UFSM (FIT-BIT, FIPE Jr and FIPE ARD).
The cyclometallated palladium complex [Pd2(CH3COO)2-
(PySeCHPy)2]ꢀ7H2O (5) was obtained under reflux conditions
by the reaction of Pd(OAc)2 with the ligand L in methanol. The
ESI+ mass spectrum of the compound shows a peak at m/z =
769.8181, which can be assigned to an ion of the composition
[C24H21N4O2Pd2Se2 + H]+ (calc. 769.8137). Fig. 9 shows the
molecular structure of 5 determined by single-crystal X-ray
analysis. This complex is dimeric, with the palladium(II) atoms
acquiring a distorted square planar geometry. The coordination
around each palladium atom is defined by one O atom of the
acetate ion, two N and one C atom of the metallated mono-
selenoethers. In this complex, one of the pyridyl rings on both
the monoselenoether ligands are parallel to each other and are
separated by 3.4690(2) Å, which may be attributed to p-stacking,
according to the literature.11a Complex 5 has a palladium–
carbon bond and the proposed mechanistic pathways for the
formation of this compound, based on the literature,11 is
presented in Scheme 3. We believe that the first step involves
the coordination of each palladium atom by two N atoms of
the ligands (Scheme 3, structure A). In the remaining steps,
considering the reported theoretical studies,11 a five-membered
intermediate (structures B and C) can be proposed. Both B and C
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