Bimetallic Salen-Based Compounds and Their Potential Applications

Article abstract of DOI:10.1021/acs.cgd.9b01610

The new bimetallic coordination compounds [LNiAg(NO3)], [LCuMn(NO3)2], [LCuCu(NO3)2], [LCuZn(NO3)2], and [{LCuBi(NO3)3}(ACN)] have been synthesized from a salen-type ligand L containing two distinct coordination sites to accommodate (different) metal ions M1 and M2. In the solid state, the formation of 1:1 compounds (LCu:M2) is always observed, but interactions between the compounds lead to differences in packing. For example, a head-to-tail arrangement along the crystallographic b-axis is observed in the case of the LCuBi compound, and the LNiAg compound is polymeric, while its Cu homologue LCuAg forms a discrete dimer. Given the inherent potential of the preorganizing, two metal ions using L could be used to synthesize nanoscale copper-bismuth and-manganese mixed metal oxides as a function of the temperature. The combination of different bioactive metal ions within one ligand system was furthermore explored for antimicrobial efficiency.

Full text of DOI:10.1021/acs.cgd.9b01610

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Article
Bimetallic Salen-based compounds and their potential applications
Alba Finelli, Sarah-Luise Abram, Nelly Hérault, Aurelien Crochet, and Katharina M. Fromm
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b01610 • Publication Date (Web): 08 Jul 2020
Downloaded from pubs.acs.org on July 8, 2020
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a) The head-to-tail sandwich type arrangement of the known LCuAg 1 complex (top left, #2(1/2-x, 1/2-y, -
z)) showing Cu1–O5 and Ag(I)∙∙∙Ag(I) interactions (purple dashed bond) and the packing arrangement
(bottom left, #3(1/2-x, -1/2+y, 1/2-z), #4(x, -y, -1/2+z), #5(x, 1-y, -1/2+z), #6(1/2-x, 1/2+y, -1/2-z))
showing Cu1–O7 interactions (green dashed bonds) along the b-axis. b) Crystal structure of the asymmetric
unit of compound LNiAg 2 (top right). Packing of the complexes in the unit cell (bottom right, #2(-1+x,y, z),
#3(-2+x, y, z), #4(5/2-x, 1-y, -1/2+z), #5(3/2-x, 1-y, -1/2+z), #6(1/2-x, 1-y, -1/2+z)) showing the Ag(I)
C6 and Ni(II) O7 interactions (green dashed bonds, bottom) along b-axis and c-axis, respectively. H-atoms
are omitted for clarity.
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Structure of the two distinct units of compound LCuMn 3 (top) and its antiparallel packing (#2(3/2-x, y,
3/2-z), #3(1-x, 1-y, 1-z), #4(1+x, y, z), bottom), showing the π π-interactions (blue dashed bonds) and
Cu2 C5 interactions (green dashed bonds). H-atoms have been omitted for clarity.
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The crystal structure of the asymmetric unit of complex LCuCu 4 (top) and the antiparallel packing
arrangement (bottom, #2(1/2+x, 3/2-y,1/2+z), #3(-1/2+x, 3/2-y, 1/2+z), #4(-1+x, y, z)) showing the π–
π interactions (blue dashed bonds) and Cu1–C4 interactions (green dashed bonds) along a-axis. The minor-
occupancy atoms and the H-atoms have been omitted for clarity.
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Crystal structure of the asymmetric unit of complex LCuZn 5 (#2(1-x,1-y, -z), #3(1-x, -1-y, 1-z), #4(x, y,
1+z), #5(-1+x, y, 1+z), #6(-x, 1-y, 1-z), #7 (-1+x, y, z), #8(-x, 1-y, -z), top) showing the π π-
interactions (blue dashed bonds, bottom) and Cu(II) C4/Cu(II) C14 interactions (green dashed bonds,
bottom). H-atoms are omitted for clarity.
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The crystal structure of complex LCuBi 6 presents an antiparallel head-to-tail configuration (top, #3(1-
x,1/2+y, 3/2-z), #4(1-x, -1/2+y, 3/2-z)) via Cu1–N6 and Cu1–O13 additional bonds and a parallel
arrangement of the complex (bottom, #2(1-x, 1-y, -z)) via π–π packing (blue dashed bonds) along the a-
axis. All H-atoms have been omitted for clarity.
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PXRD analysis of a) LCuMn (3) and b) LCuBi (6) upon thermal decomposition at 300°C, 400°C, 500°C and
600°C.
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SEM images (with a zoom at 2µm) of LCuMn (3) (top, left) and the mixture of Cu(NO ) and Mn(NO ) after
3 2
3 2
the thermal decomposition (top, right) as well as SEM images of samples of LCuMn after the thermal
decomposition at 300°C, 400°C, 500°C and 600°C for 4 h (bottom).
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SEM images (with a zoom at 2µm) of the ligand LCuBi (6) (top, left) and the mixture of Cu(NO ) and
3 2
Bi(NO ) after the thermal decomposition (top, right) as well as SEM images of samples of LCuBi after the
3 3
thermal decomposition at 300°C, 400°C, 500°C and 600°C (bottom).
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Antimicrobial activity against E. coli of different mono- and bi-metallic complexes of the ligand H L and the
2
respective controls (nitrate salts and ligand alone) by agar diffusion assay. All compounds were used in form
of pressed pellets containing 0.05-0.06 mmol. The table shows the radii of the inhibition zones (ZOI) in mm
measured from the edge of the pellet to the beginning of the bacterial growth.
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a) Synthetic route of H L and b) its two potential coordination sites.
2
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Bimetallic Salen-based compounds and their
potential applications
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Alba Finelli†, Sarah-Luise Abram†, Nelly Hérault†, Aurélien Crochet‡, Katharina M. Fromm*,†
† Department of Chemistry, University of Fribourg, Ch. du Musée 9, 1700 Fribourg, Switzerland
‡
FriMat, Department of Chemistry, University of Fribourg, Ch. du Musée 9, 1700 Fribourg,
Switzerland.
Keywords: Mixed metal compounds, antimicrobial activity, Schiff base, single-source precursors,
mixed metal oxides.
ABSTRACT: The new bimetallic coordination compounds [LNiAg(NO3)], [LCuMn(NO3)2],
[LCuCu(NO3)2], [LCuZn(NO3)2] and [{LCuBi(NO3)3}(ACN)] have been synthesized from a
salen-type ligand L containing two distinct coordination sites to accommodate (different) metal
ions M1 and M2. In the solid state, the formation of 1:1 compounds (LCu:M2) is always
observed, but interactions between the compounds lead to differences in packing. For example, a
head-to-tail arrangement along the crystallographic b-axis is observed in the case of the LCuBi
compound, and the LNiAg compound is polymeric while its Cu homologue LCuAg forms a
discrete dimer. Given the inherent potential of the pre-organizing two metal ions using L could
be used to synthesize nanoscale copper-bismuth and -manganese mixed metal oxides as a
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function of the temperature. The combination of different bioactive metal ions within one ligand
system was furthermore explored for antimicrobial efficiency.
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INTRODUCTION
Arranging different metal ions in space and number through the compartmentalization in a
ligand is essential in material science and in biology.1–3 The close association of at least two
metal ions can enhance existing properties or even lead to new features4 in compounds such as
coordination polymers,5 multimetallic complexes6,7 or mixed metal oxide materials,8 opening
thus new routes towards multifunctional materials.9 Among these, mixed metal oxides are
nowadays especially sought for their magnetic,10 superconducting,11 catalytic12 and optical13
properties. They are typically synthesized through solid state reactions requiring high
temperatures, or by milder methods such as sol-gel techniques14 or metal organic chemical vapor
deposition ((MO-)CVD),15 for which volatile metal compounds such as alkoxides or diketones
are needed.16 However, oligomerization and the lack of metal ion control, together with
insufficient volatilities and stabilities can be major limitations of these procedures.17 Therefore,
the idea to compartmentalize the metal ions within a single-source precursor (SSP),18 such as
salen-derivatives, is an attractive alternative.19 The use of such multitopic precursors allows an
intimate and organized mixing of the metal ions at the molecular level, facilitating the
crystallization process of the mixed metal oxides under mild conditions and decreasing the
formation of side products.20 This synthetic strategy often leads to monodisperse and phase-pure
nanocrystals with particular morphologies.21
Another application that we are interested in in the context of multimetallic compounds is the
combat against bacterial infections.22 Recently, considerable efforts have been undertaken to
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develop metal-based antimicrobially active compounds that could serve as alternatives to
traditional antibiotics regarding the rise in infections caused by multidrug resistant bacteria.23 It
has for example been shown that the combination of different metal ions lowered the bactericidal
concentrations, thus overcoming toxicity limitations known for metal based drugs.24 Such
synergistic effects have also been reported for associations of metal ions with classical
antibiotics25 or organic acids.26 As a result, the combination of different drugs has turned out to
be an efficient tool to broaden the therapeutic window of a given antibiotic compound.27
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For such biological applications, but also to obtain mixed metal oxide materials, the design and
development of specific ligand systems are thus crucial for the formation of precise and clean
multimetallic compounds.28 Salen-type ligands can efficiently bind to transition-metal ions in a
tetradentate mode through their N2O2 site to form very stable complexes. They are versatile
chelating ligands in coordination chemistry thanks to their tunable design29,30 and multiple
functional compartments can be created by simple substitution of the ligand, generating
macrocycle-type compounds for the coordination of different metal ions.31,32
Here, we report the study of bimetallic compounds based on a salen-derivative, H2L, possessing
two distinct coordination sites. We have previously studied the structural trends of LCu and LNi
compounds in combination with alkali and alkaline earth metal ions and found that the Cu-
compounds formed mostly chains with alternatingly arranged LCu-moieties, while the Ni-
analogues had the LNi entities arranged on the same side of the polymeric chain.33 Now more
interested in the optical, conductive and catalytic properties of copper-manganese and copper-
bismuth oxides34,35 as well as the efficient antimicrobial properties of silver, copper, zinc,
bismuth ions, encouraged us to combine LCu(II) with Ag(I), Mn(II), Cu(II), Zn(II), Bi(III), and,
for comparison, LNi(II) with Ag(I) as bimetallic compounds. A structural study of the different
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compounds was carried out by single crystal X-ray diffraction, and their potential as single-
source precursors for the generation of nanoscale, mixed metal oxides has been investigated
together with their potential antimicrobial properties.
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EXPERIMENTAL SECTION
Material and Methods. All experiments were performed in air and at RT. Ligand H2L was
prepared according to the procedure reported by Avecilla et al. previously.36 Monometallic
complex ligands LCu, LNi and LZn were prepared as described in the literature.37,38 All
chemicals were commercial products of reagent grade and were used without further
purification. For the numbering of the obtained compounds, we chose to start with the lowest
charged M2, hence Ag(I) compounds, then the M(II) ions as they come in the periodic table and
finally the example with a M(III) ion as M2. Elemental analyses for C, H and N were carried out
using a FLASH 2000 Organic Elemental Analyzer. Some of the results indicate residual solvent,
e.g. water, after drying. In most cases, the values found for hydrogen content are lower than
expected despite trying many times, and this phenomenon may be due to the possible
interferences with the diverse metal ions during the mineralization. Indeed, the elemental
analysis can be affected by the incomplete conversion of the compounds or by the formation of
compounds leading to the corrosion of analytical instruments, both possibly leading to
underestimated results. However, mass spectra (ESI-TOF, positive mode) for all compounds
could be recorded with a Bruker esquire HCT spectrometer with a DMF/ACN mixture as
solvent. The crystallographic data of single crystals were collected with Mo-Kα radiation (λ =
0.71073 Å). All measurements were performed at 250K or 200K, with Stoe IPDS-II or IPDS-II T
diffractometers equipped with Oxford Cryosystem open-flow cryostats.39 Single crystals were
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picked under the microscope, and placed in inert oil. All crystals were mounted on loops and all
geometric and intensity data were taken from one single crystal. The absorption corrections were
partially integrated in the data reduction procedure.40 The structures were solved and refined
using full-matrix least-squares on F2 with the SHELX-2014 package.41 All atoms (except
hydrogen atoms) were refined anisotropically. Hydrogen atoms were refined when possible, and
otherwise added using the riding model position parameters. Crystallographic data can be found
in the Supporting Information (see Table S1). CIF files can be obtained from the Cambridge
Crystallographic Data Centre, CCDC-1860979 (1), CCDC-1861009 (2), CCDC-1860982 (3),
CCDC-1860981 (4), CCDC-1860983 (5), and CCDC-1860980 (6). Additional crystal structures
with similar metal ion combinations but different counter ions or solvent (CCDC-1861005
(“LCuMn Cl2”), CCDC-1861004 (“LCuMn ClO4”), and CCDC-1861006 (“LCuMn OAc”)
can be found in the supporting information (Fig, S3-S5, Table S4). Copies of these data can be
obtained free of charge from the Cambridge Crystallographic Data Centre through
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www.ccdc.cam.ac.uk or e-mail, deposit@ccdc.cam.ac.uk. Powder X-ray diffractograms were
collected on a Stoe STADIP using Cu-Kα1 radiation (1.5406 Å) using a Mythen detector (Fig.
S6-S10). TGA were recorded on a Mettler Toledo TGA/SDTA851. Being paramagnetic, 1H-
NMR measurements could not be performed on the compounds 1-6. The ligand H2L and the
complex LCu obtained by chelating the ligand to the copper ion were synthesized as described in
the literature.36,37 Specific surface areas of the samples were determined using the Brunauer–
Emmett–Teller (BET) method by performing nitrogen sorption measurements using a
Micromeritics ASAP 2010 instrument. The adsorption and desorption experiments were carried
out at 77K after initial pre-treatment of the samples by degassing at 250°C for 3 hours.
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Synthesis of LCuAg (1). A solution of AgNO3 (41.61 mg, 0.245 mmol) in acetonitrile/ethanol
(7:3, 30 ml) was added to a solution of LCu (100 mg, 0.245 mmol), previously dissolved in a
mixture of acetonitrile/ethanol (7:3, 30 ml). After stirring overnight at room temperature, the
precipitate was collected and dried under vacuum to afford the compound 1 (119.32 mg, 0.212
mmol, 87%). After few days, red, block-like single crystals suitable for X-ray crystallographic
analysis were obtained by slow evaporation of the mother liquor. ESI-MS (m/z): 558.95 [M+H]+,
where M corresponds to “LCuAg”. Anal. Calcd for C18H18AgCuN3O7, corresponding to
“LCuAg” (5): C, 38.62; H, 3.24; N, 7.51%. Found: C, 38.65; H, 1.80; N, 7.42 %.
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Synthesis of [LNiAg(NO3)] as “LNiAg” (2), [LCuMn(NO3)2] as “LCuMn” (3),
[LCuCu(NO3)2] “LCuCu” (4), [LCuZn(NO3)2] a “LCuZn” (5) or [{LCuBi(NO3)3}(ACN)]
“LCuBi” (6). These compounds were prepared by the same method as described above, by
reacting LCu in an acetonitrile/ethanol (7:3) solution with one equivalent of the appropriate
metal salt. The metal salts used were as follows: AgNO3 (41.61 mg, 0.245 mmol) for 1 and 2,
Mn(NO3)2∙4H2O (61.49 mg, 0.245 mmol) for 3, Cu(NO3)2 (45.95 mg, 0.245 mmol) for 4,
Zn(NO3)2 (46.40 mg, 0.245 mmol) for 5 and Bi(NO3)3∙5H2O (118.84 mg, 0.245 mmol) for 6.
Data for LNiAg (2). Orange needle-like single crystals suitable for X-ray crystallographic
analysis were obtained by slow evaporation of the mother liquor. Yield: 82% (0.201 mmol,
111.48 mg). ESI-MS (m/z): 553.95 [M+H]+, where M corresponds to “LNiAg”. Anal. Calcd for
C18H18AgCuN3O7 (5): C, 38.62; H, 3.24; N, 7.51%. Found: C, 38.65; H, 1.80; N, 7.42 %.
Data for LCuMn (3). Red plate-like single crystals suitable for X-ray crystallographic
analysis were obtained by vapor phase diffusion of diethyl ether into the mother liquor of the
reaction in a long tube. Yield: 90% (0.225 mmol, 125.43 mg). ESI-MS (m/z): 506.95 [M-NO3]+,
where M corresponds to “LCuMn”. Anal. Calcd for C18H20CuMnN4O11, corresponding to
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“LCuMn” (3) with one molecule of water: C, 36.8; H, 3.44; N, 9.36%. Found: C, 36.8; H, 1.7; N,
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6
9.49%.
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Data for LCuCu (4). Purple plate-like single crystals suitable for X-ray crystallographic
analysis were obtained by vapor phase diffusion of diethyl ether into the mother liquor of the
reaction. Yield: 82% (0.201 mmol, 116 mg). ESI-MS (m/z): 513.95 [M-NO3]+, where M
corresponds to “LCuCu”. Anal. Calcd for C18H18Cu2N4O10, corresponding to “LCuCu” (3): C,
37.44; H, 3.14; N, 9.70%. Found: C, 37.09; H, 2.48; N, 9.45%.
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Data for LCuZn (5). Purple plate-like single crystals suitable for X-ray crystallographic
analysis were obtained by slow evaporation of the mother liquor reaction. Yield: 91% (0.223
mmol, 129.15 mg). ESI-MS (m/z): 577.96 [M+H]+, where M corresponds to the asymmetric unit
of “LCuZn”. Anal. Calcd for C18H18CuN4O10Zn, corresponding to the asymmetric unit of
“LCuZn” (4): C, 37.32; H, 3.13; N, 9.67%. Found: C, 37.44; H, 2.27; N, 9.25%.
Data for LCuBi (6). Green needle-like single crystals suitable for X-ray crystallographic
analysis were obtained by vapor phase diffusion of diethyl ether into the mother liquor of the
reaction. Yield: 88% (0.215 mmol, 169.22 mg). ESI-MS (m/z): 722.00 [M-NO3]+, where M
corresponds to “LCuBi” without ACN molecule. Anal. Calcd for C18H20BiCuN5O14,
corresponding to “LCuBi” with one molecule of water (2): C, 26.93; H, 2.51; N, 8.72%. Found:
C, 26.33; H, 1.24; N, 8.37%.
Thermal processes. Samples of the two compounds 3 and 6 were heated under air to four
different temperatures: 300°C, 400°C, 500°C and 600°C during 4h. The morphology and the
composition of the obtained materials were explored by scanning electron microscopy (SEM)
and powder X-ray diffraction (PXRD). Average crystallite sizes were estimated using the
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Scherrer method. Using BET, the surface area (SBET) and pore size distribution (VBJH) were
calculated. In order to follow the decomposition of 3 and 6 upon heating, the thermal behavior of
the mixed metal compounds 3 and 6 was studied by TGA over a range of 30° to 600°C in air.
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As both compounds 3 and 6 were prepared from nitrate salts, for comparison, a mixture of
stoichiometric amounts (1:1) of Cu(NO3)2 and Mn(NO3)2 or Bi(NO3)3 was ball-milled and heated
at 600°C in order to compare the results with the mixed metal oxides resulting from the
combustion of our single-source precursors.
Agar diffusion test. An overnight culture of Escherichia coli 25922 (E. coli) in Müller-Hinton
(MH) broth was diluted to the concentration of 1*106 cfu/mL using the McFarland standard. 100
μL were spread on 8.5 cm diameter MH agar dishes. After 5 min drying, pellets of the
compounds were placed on the agar and gently pressed with sterile tweezers. The agar plates
were incubated upside down at 37°C overnight. The diameter of the inhibition zone (ZOI) was
measured using ImageJ on photographs of the agar plates with the diameter of the plates as
reference scale. The initial diameter of the pressed pellets is subtracted before to divide by two
for the ZOI radius.
Broth microdilution test. An overnight culture of E. coli in Müller Hinton (MH) broth was
diluted to 1*107 cfu/mL using the McFarland standard. The powders of the compounds or the
respective nitrate salts were suspended by sonication in MH broth and diluted to create a
concentration series that was then inoculated with the overnight E. coli culture to reach a final
inoculum of 1.106 cfu/mL. Duplicates of 0.5 mL for each concentration were incubated in suitable
well plates for 24 h at 37°C while shaking at 160 rpm. 100 µL of each well were spread directly
on MH agar dishes for MBC determination and a second 100 µL aliquot was diluted 1:1000 before
spreading on agar for MIC determination as any optical detection of bacterial growth was impeded
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by the turbidity of the compound suspensions. The agar dishes were incubated overnight and
checked for bacterial colonies. The MIC corresponds to the lowest tested concentration where the
bacterial growth did not exceed the starting inoculum of 1.106 cfu/mL. The MBC is defined as the
lowest concentration without visible bacterial growth on the agar dishes. All compounds were
tested against at least three independent E. coli cultures.
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RESULTS AND DISCUSSION
Synthesis. The salen-based ligand H2L has been synthetized following the literature
procedure36 by a straightforward synthetic route involving 2-hydroxy-3-methoxybenzaldehyde
(o-vanillin) and ethane-1,2-diamine (Scheme 1).
Scheme 1: a) Synthetic route of H2L and b) its two potential coordination sites.
The ligand system (H2L) is composed of an imine-based recognition site, N2O2, typically able to
coordinate transition metal ions after deprotonation. This coordination pre-organizes the ligand
in a Ω-shape and allows the formation of a neutral compound “LM1”, providing an additional
coordination site, O2O2. With two phenol and two methoxy groups, the newly generated
compartment allows the binding of further metal ions. Throughout this study, Cu(II) and Ni(II)
metal ions have been used to fit into the N2O2 cavity of H2L, yielding the previously reported
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LCu and LNi complexes37, which were then employed as metallo-ligand for the coordination of
additional metal ions. New mixed metal compounds have then been synthesized from LCu in
combination with Mn(II), Cu(II), Zn(II) or Bi(III) ions. The previously known bimetallic copper-
silver compound42 has also been taken into account during this study as well as its new nickel
analogue for comparison.
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Description of the Crystal Structures. In order to explore the coordination mode of the
metallo-ligand LCu with the different metal ions (and LNi with Ag(I)), single crystal X-ray
analysis was performed on all compounds. The crystal structures are described below following
the charge of the M2 as well as the ionic radii of the cation, starting thus from Ag(I) and ending
by Bi(III).[{LCuAg (NO3)}2] is thereafter abbreviated as “LCuAg” (1) and [LNiAg(NO3)] as
“LNiAg” (2), [LCuMn(NO3)2] as “LCuMn” (3), [LCuCu(NO3)2] as “LCuCu” (4),
[LCuZn(NO3)2] as “LCuZn” (5), [{LCuBi(NO3)3}(ACN)] as “LCuBi” (6), (Table 1) (Bond
lengths are gathered in the supporting information, Table S3).
Single crystals of the six compounds were obtained from the reaction between LCu/LNi with
the corresponding metal ion nitrate salt. Each crystal structure reveals a 1:1:1 stoichiometry of
the LM1M2 compound. In each compound, Cu(II)/Ni(II) as M1 is coordinated in a square planar
fashion by the phenolate and imine groups of the N2O2 chelating site of the ligand, with angle
sums close to 360°.
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Table 1 : Crystallographic data of 1-6.
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“LCuAg” (1)
“LNiAg” (2)
C18H18AgN3NiO7
536.79
“LCuMn” (3)
C18H18CuMnN4O10
568.84
“LCuCu” (4)
C18H18Cu2N4O10
577.44
“LCuZn” (5)
C36H36Cu2N8O20Zn2
1158.55
“LCuBi” (6)
C20H21BiCuN6O13
825.95
Formula
Mw
C18H18AgCuN3O7
559.76
9
T [K]
250
200
200
250
250
200
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Lattice
Space group
a [Å]
Monoclinic
C2/c
orthorhombic
P212121
6.7481(2)
16.3551(5)
17.5677(5)
90
monoclinic
P2/n
monoclinic
P21/n
triclinic
orthorhombic
P212121
8.6303(9)
15.3489(16)
19.784(2)
90
P-1
23.049(4)
14.9081(16)
11.611(2)
90
13.1448(6)
10.9131(4)
14.2298(9)
90
8.8762(6)
22.1395(16)
10.5889(7)
90
7.7915(8)
10.5248(10)
13.3319(13)
83.988(8)
78.881(8)
72.117(7)
1019.70(18)
2
b [Å]
c [Å]
α [°]
β [°]
104.674(13)
90
90
99.361(4)
90
95.903(6)
90
90
γ [°]
V [Å3]
90
90
3859.6(10)
8
1928.87(10)
4
2014.09(18)
4
2069.8(2)
4
2620.7(5)
4
Z
dcalc
1.927
1.839
1.876
1.853
1.887
2.093
[g cm-3]
R1/ wR2
[I>2sigma(I)]
0.0354/0.0782
0.0440/0.0938
0.1053/ 0.2530
0.0419/ 0.1019
0.0228/ 0.0521
0.0498/ 0.1169
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In the already known LCuAg bimetallic compound (1), Ag(I) is coordinated by all four oxygen
atoms of the ligand.42 It is found ca. 0.3 Å out of the Cu-N2O2 mean plane of the metalloligand
and oriented towards the silver ion of an antiparallel neighbor moiety. The silver ion completes
its coordination sphere by two O-atoms O5 and O6 belonging to one nitrate anion. O5 further
coordinates to the copper ion of a neighboring entity, the copper ion thus reaching a bond
valence sum of 1.96.42 The coordination of Ag(I) with LCu leads to dimers by symmetry
(inversion center in the middle of the Ag(I)-Ag(I) contact) with a head-to-tail sandwich-type
arrangement through Cu1–O5 bonds and Ag1···Ag1 interactions (2.516(4) Å and 3.053(7) Å,
respectively) (Fig. 1A).43
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Interestingly, our new compound LNiAg (2) does not present the same structural
characteristics than its Cu-analogue. Compound 2 is composed by a Ni(II) cation coordinated by
two O-atoms, O2 and O3, and two N-atoms, N1 and N2, in a perfect square planar fashion and
with a Ag(I) cation occupying the O2O2 cavity. The silver cation completes its coordination
sphere with two O-atoms, O5-O6, from the nitrate counterion. In contrast to 1, Ni(II) reaches a
bond valence sum of 1.99 solely by the O2N2 donors and hence does not need to complete its
coordination sphere through additional coordination, as already observed in previous studies with
alkali metal ions.33 This leads to an offset parallel arrangement of the complex moieties into a 1D
coordination polymer along the crystallographic b-axis, governed by Ag1–C6 interactions of
2.690(1) Å. This interaction further leads to a non-planarity of the ligand, with the two aromatic
rings forming an angle of 17.04°. The packing of these 1D strands in the unit cell is defined by a
very weak coordination of the third O-atom of the anion, O7, to the Ni(II) with a distance of
3.020(9) Å along the crystallographic c-axis (Fig. 1B).
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Figure 1 : a) The head-to-tail sandwich type arrangement of the known LCuAg 1 (top left, #2(1/2-x,
1/2-y, -z)) showing Cu1–O5 and Ag(I)∙∙∙Ag(I) interactions (purple dashed bond) and the packing
arrangement (bottom left, #3(1/2-x, -1/2+y, 1/2-z), #4(x, -y, -1/2+z), #5(x, 1-y, -1/2+z), #6(1/2-x, 1/2+y, -
1/2-z)) showing Cu1–O7 interactions (green dashed bonds) along the b-axis. b) Crystal structure of the
asymmetric unit of compound LNiAg 2 (top right). Packing of the complex moieties in the unit cell
(bottom right, #2(-1+x,y, z), #3(-2+x, y, z), #4(5/2-x, 1-y, -1/2+z), #5(3/2-x, 1-y, -1/2+z), #6(1/2-x, 1-y, -
1/2+z)) showing the Ag(I) C6 and Ni(II) O7 interactions (green dashed bonds, bottom) along b-axis and
c-axis, respectively. H-atoms are omitted for clarity.
In compound LCuMn (3), the asymmetric unit is composed of two halves of two distinct
complexes, named complex A for the one containing Cu1, and B for the one containing Cu2. The
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entire units are obtained through a C2 rotation axis running through the Cu(II) and Mn(II) ion, in
which the Cu(II) ions are coordinated by the N-and O- atoms of the imine and the phenoxy
groups, while the Mn(II) atoms fit perfectly into the O2O2 cavity provided by the phenoxy and
the methoxy groups. Complex A differs from the B-complex in the coordination of Mn(II). Thus,
Mn1 in A is slightly more out of the aromatic plane with a distance of ca. 0.19 Å (compared to
ca. 0.04 Å for Mn2 in the B-complex), and has a longer Mn1–O1 distance to the methoxy group
(2.481(7) Å) than the equivalent bond in the Cu2-complex (Mn2–O3 2.475(6)). Contrarily, the
distance Mn1–O2 distance to the phenoxy group is shorter in A than in B (see SI). The
coordination sphere of the Mn(II) ions is further completed by four O-atoms of two symmetry
equivalent nitrate ions (O5 and O6 and their symmetry equivalents for Mn1, and O8 and O9 and
their symmetry equivalents for Mn2). The Mn–nitrate distances in complex B are longer and
more asymmetric than the equivalent distances in complex A. In both LCuMn complexes, the
bond valence sum for Cu(II) is around 1.95, while it is ca. 2 for Mn(II) in A and 1.96 in B. The
packing of these moieties is controlled by Cu1–C14, Cu2–C5 and π–π interactions (3.502(8) Å,
3.317(7) Å and 3.497(2) Å, respectively) along the crystallographic c-axis, leading to alternating
antiparallel strands (Fig. 2).
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Changing the counter ions to chloride, perchlorate and acetate, water-containing compounds
were obtained as described in Table S4 and Figures S3-S5.
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Figure 2 : Structure of the two distinct units A (top left) and B (top right) of compound LCuMn 3 (top)
and its antiparallel packing (#2(3/2-x, y, 3/2-z), #3(1-x, 1-y, 1-z), #4(1+x, y, z), bottom), showing the π
π-interactions (blue dashed bonds) and Cu2 C5 interactions (green dashed bonds). H-atoms have been
omitted for clarity.
In the compound LCuCu (4), Cu1 has the known square planar environment, while the second
copper cation Cu2 is bound by all four O-atoms of the O2O2 compartment of the metalloligand
LCu. Its coordination sphere is completed by three further O-atoms belonging to two different
nitrate anions (O5, O6 and O8) found in the axial positions of a distorted octahedron. One nitrate
moiety is on a roughly 50:50 split position (refined at 51.5(9) and 48.5(9)%), and Cu2 is then
connected to O8 with a distance of 1.933(9) Å or to O8A at 1.942(8) Å. Compound 4 is not
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completely planar, as the angle between the two aromatic rings of the ligand of 3.8° indicates.
This slight distortion can be explained by an antiparallel arrangement of the moieties, allowing
Cu1 to interact with neighboring entities by Cu1–C4 interactions (3.249(2) Å), while the
opposite aromatic rings stack as well with π–π interactions of 3.622(2) Å (ring-center to ring-
center) along the crystallographic a-axis (Fig. 3).
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Figure 3 : The crystal structure of the asymmetric unit of LCuCu 4 (top) and the antiparallel packing
arrangement (bottom, #2(1/2+x, 3/2-y,1/2+z), #3(-1/2+x, 3/2-y, 1/2+z), #4(-1+x, y, z)) showing the π–π
interactions (blue dashed bonds) and Cu1–C4 interactions (green dashed bonds) along a-axis. The minor-
occupancy atoms and the H-atoms have been omitted for clarity.
Compound LCuZn (5) adopts a similar structure to 4, with Zn(II) being perfectly lodged in the
O2O2 cavity. The main difference with 4 is that the zinc ion completes its coordination sphere
with three O-atoms from two nitrate ions, both adopting two distinct orientations with respective
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occupancy of 2/3 and 1/3 (refined to 67.8(9)% for the form N3 and N4 and of 32.2(9)% for the
form N3A and N4A). The zinc cation is thus bound by O5, O6 and O8 or by O5A, O8A and
O9A depending on the occupancy. The two aromatic rings of the complex entity form an angle
of 7.81°, and LCuZn is thus not completely planar. This effect is probably again induced by an
antiparallel arrangement of the moieties, which leads to alternating strands through Cu1–C14 and
Cu1–C4 interactions as well as π–π interactions (3.397(3) Å, 3.463(2) Å and 3.500(2) Å,
respectively) along the crystallographic c-axis (Fig. 4).
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Figure 4 : Crystal structure of the asymmetric unit of LCuZn 5 (#2(1-x,1-y, -z), #3(1-x, -1-y, 1-z),
#4(x, y, 1+z), #5(-1+x, y, 1+z), #6(-x, 1-y, 1-z), #7 (-1+x, y, z), #8(-x, 1-y, -z), top) showing the π π-
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interactions (blue dashed bonds, bottom) and Cu(II) C4/Cu(II) C14 interactions (green dashed bonds,
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bottom). H-atoms are omitted for clarity.
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In compound LCuBi (6), the bismuth ion is coordinated by all four O-atoms of the O2O2
chelating site of L and completes its coordination sphere with six oxygen atoms from three
different nitrate ions, as it is charged 3+. Being furthermore much larger than e.g. Mn(II) (ionic
radii 67 pm for Mn(II), low spin, and ca. 100 pm for Bi(III)45), Bi(III) is ca. 0.438 Å out of the
Cu-N2O2 mean plane, thus disturbing the planarity of the metallo-ligand as evidenced by the
angle of 5.16° between the two aromatic rings of the ligand. This in turn leads to a less good
coordination of the N2O2 cavity with the Cu(II), as shown by a lower bond valence sum than in
the other compounds, reaching only 1.90. As a consequence, a close interaction between Cu(II)
and N6 of a solvent molecule of acetonitrile (2.810(2) Å) acting as a terminal ligand, and with
O13 of a nitrate anion (2.940(2) Å), which acts as bridging ligand between copper and bismuth
ions of neighboring entities, is observed. Cu(II) has thus a distorted octahedral geometry, as
expected from the Jahn-Teller effect. Moreover, the elongated bond generated by the nitrate
bridging ligand produces then an alternating head-to-tail arrangement of the units along the b-
axis. This compound features a parallel arrangement based on π–π interactions between neighbor
complex entities along the a-axis (Fig.5).
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Figure 5 : The crystal structure of LCuBi 6 presents an antiparallel head-to-tail arrangement (top, #3(1-
x,1/2+y, 3/2-z), #4(1-x, -1/2+y, 3/2-z)) by Cu1–N6 and Cu1–O13 additional bonds and a parallel
arrangement of the complexes (bottom, #2(1-x, 1-y, -z)) through π–π packing (blue dashed bonds) along
the a-axis. All H-atoms have been omitted for clarity.
The formation of 1:1:1 complexes (L:M1:M2) is always observed in the solid state. As already
observed previously,33 LCuM2 with M2 being a 3rd row element, can lead to alternating
arrangements of the LCu entities in order to allow interactions of the Cu(II) with anions, but also
parallel arrangements with Cu(II)-π interactions are observed. For the heavier cations, packing
with a head-to-tail arrangement into coordination polymers along the crystallographic b-axis are
found in the case of LCuBi, respectively discrete antiparallel dimer formation in the case of
LCuAg occurs. These results can be explained by the fact that larger cations are slightly pushed
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