On new solvatomorphs of the metalloligand [Ni(o-van-en)]

Article abstract of DOI:10.1016/j.ica.2020.119874

The Schiff-base bis-(2-hydroxy-3-methoxybenzylidene)ethylenediamine, denoted H₂(o-van-en), is a versatile two-compartment polydentate ligand that can bring different guests, such as a transition metal and a lanthanoid, into close proximity to p

Full text of DOI:10.1016/j.ica.2020.119874

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On new solvatomorphs of the metalloligand [Ni(o-van-en)]
Anna Vráblová, Milagros Tomás, Ján Titiš, Juraj Černák, Larry R. Falvello
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https://doi.org/10.1016/j.ica.2020.119874
ICA 119874
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Please cite this article as: A. Vráblová, M. Tomás, J. Titiš, J. Černák, L.R. Falvello, On new solvatomorphs of the
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On new solvatomorphs of the metalloligand [Ni(o-van-en)]
Anna Vráblová,a,c,d Milagros Tomás b, Ján Titiš,e Juraj Černák a, Larry R. Falvello c*
a Department of Inorganic Chemistry, Faculty of Sciences, P. J. Šafárik University in Košice,
Moyzesova 11, 041 54 Košice, Slovakia
bInstituto de Síntesis Quimica y Catálisis Homogénea (ISQCH), Departamento de Química
Inorgánica, Pedro Cerbuna 12, University of Zaragoza–CSIC, E-50009 Zaragoza, Spain
c Departamento de Química Inorgánica, Instituto de Ciencia de Materiales de Aragón (ICMA),
University of Zaragoza-CSIC, Pedro Cerbuna 12, E-50009 Zaragoza, Spain
d
Department of Chemical Theory of Drugs, Faculty of Pharmacy, Comenius University in
Bratislava, Odbojárov 10, 832 32 Bratislava, Slovakia
e Department of Chemistry, FPV, University of SS Cyril and Methodius, , nám. J. Herdu 2,
917 01 Trnava, Slovakia.
Correspondence e-mail: falvello@unizar.es
Abstract
The Schiff-base bis-(2-hydroxy-3-methoxybenzylidene)ethylenediamine, denoted H2(o-van-
en), is a versatile two-compartment polydentate ligand that can bring different guests, such as
a transition metal and a lanthanoid, into close proximity to produce novel complexes with
potentially singular properties. The base, which is prepared by the condensation of o-vanillin
with ethylenediamine, reacts with nickel(II) carbonate to yield a microcrystalline product which
upon recrystallization from acetone, ethanol and isopropanol, yielded three solvatomorphs of
[Ni(o-van-en)]. These products, the hydrate [Ni(o-van-en)]nH2O (n = 1.16, 1), the ethanol-
water solvate [Ni(o-van-en)]H2OEtOH (2) and the isopropanol-water solvate [Ni(o-van-
en)]H2OiPrOH (3), all contain the molecular complex [Ni(o-van-en)], as characterized by
single-crystal x-ray diffraction. The [Ni(o-van-en)] fragment, which has been developed as a
metalloligand for the preparation of mixed-metal (Tr-Ln) complexes with novel magnetic
properties, here forms solvatomorphs whose crystals display pronounced differences in
morphology and stability. The Ni(II) center in each of the three solvatomorphs is coordinated
by the (o-van-en)2- ligand in a square geometry with a cis-N2O2 donor set. Each solvatomorph
contains water solvate molecules; and in addition, ethanol and isopropanol solvate molecules
are present in 2 and 3, respectively. Full Interaction Maps were used for comparison of the
intermolecular interactions, by way of understanding the factors leading to the different
solvation behavior. A review of the known structures containing o-van-en fragments is used as
a contextual backdrop for a discussion of the new solvatomorph structures. Calculations based
1

on an energy-decomposition model are used to characterize the interactions within dimeric
aggregates observed in the structures of 2 and 3.
Keywords: Ni(II) complex, metalloligand, crystal structure, solvate molecules, Schiff base,
thermal properties
1. Introduction
The long-familiar position of single-crystal x-ray structure analysis as the sine qua non
front-line technique for characterizing coordination compounds (and others) has spawned a
number of derivative endeavors for obtaining molecular crystals, characterizing their physical
properties and even attempting to design crystals with specific properties particular to the
molecular crystalline state and aimed at specific applications. As is well known,
recrystallization from a host of different solvents is one of the common approaches to obtaining
adequate crystals, as solvent molecules can act as coformers, completing a pattern of molecules
and non-covalent interactions that the principal molecule alone may not be capable of forming.
As a result, the study of solvates themselves has been considered valuable in the larger context
of obtaining and characterizing useful crystalline samples of molecules with targeted structural
and/or physical properties [1]. Besides the obvious interest in understanding the formation of
solvated crystals [2], the existence of such solvates can thus have important practical
implications for other fields, including magnetism [3-5], pharmacy [6-9], agrochemicals,
energetic materials and others [10].
Within our broader study of magnetically active complexes, we have prepared the
complex [Ni(o-van-en)]. [H2(o-van-en) is the Schiff base bis-(2-hydroxy-3-methoxybenzyli-
dene)ethylenediamine, Scheme 1],
Scheme 1. Structure of H2(o-van-en)
2

intended as a building block for the preparation of magnetically active bimetallic Ni-Ln
complexes (Ln = lanthanoids) [11]; the rationale for this approach has been described in the
literature [12-14]. Briefly, the transition-metal complex of the deprotonated Schiff base acts as
a metalloligand capable of further incorporating a lanthanoid center in the outer cavity to yield
a heterobimetallic (Tr-Ln) system with intriguing properties. With just the transition metal
present, we find that the outer coordination cavity acquires a different function. During
recrystallization of the crude microcrystalline product obtained by the reaction of nickel(II)
carbonate with the free Schiff base H2(o-van-en), we observed the formation of three distinct
solvatomorphs of the neutral complex [Ni(o-van-en)]; we report here the syntheses,
characterization and low temperature crystal structures including detailed characterization of
the supramolecular structures. Pronounced differences in crystal morphology pertain; these
serve as an experimental demonstration of the influence of the solvate shape, size and location
on the shape and stability of the resulting crystals. The three new solvatomorphs are further
compared with previously reported solvatomorphs with chloroform and dimethylformamide
[15,16], along with the room temperature structure of [Ni(o-van-en)]nH2O, (n = 1) [17].
2. Experimental
2.1. Materials
Nickel(II) carbonate, NiCO3, o-vanillin, ethylenediamine, ethanol, acetone and isopropanol
were purchased from commercial sources and used as received.
2.2 Synthesis of the ligand H2(o-van-en)
For the synthesis of H2(o-van-en) a modification of a previously reported method [18] was used.
Ethylenediamine (0.1 ml, 1.5 mmol) was added to 50 ml of an ethanolic solution of o-vanillin
(0.4542 g, 3 mmol). The mixture was placed in a boiling flask and refluxed for 3 hours. The
final solution was filtered and left for crystallization. After a few hours dark yellow crystals of
the product appeared. The crude product was filtered and washed with 2 ml of diethylether.
Yield: 90 %. Purity and identity of the product were confirmed by CHN analysis, IR, UV-Vis,
1H and 13C NMR spectroscopies and by measuring the melting point.
Anal. [%], calculated for C18H20O4N2: C, 65.83; H, 6.15; N, 8.53; found: C, 66.30; H, 6.33; N,
8.57.
3

IR (cm-1) of H2(o-van-en): 3746w, 2997w, 2931w, 2848w, 1631s, 1463s, 1438m, 1408m,
1325w, 1295w, 1246vs, 1189m, 1170m, 1133m, 1080s, 1054m, 1010m, 987m, 962s, 836s,
791s, 782s, 740s, 729s, 620m, 521m, 441m.
UV-Vis (nm) in EtOH: 219, 264; in CHCl3: 265, 334.
1H-NMR: 3.92 m, 6.84 m, 7.267 s, 8.323 s, 13.5 s, b.
13C-NMR: 56.056, 59.467, 114.084, 118.032, 118.419, 123.149, 148.269, 151.402, 166.643.
Melting point: 162.5 °C (165 °C reported [18]).
2.3 Synthesis of complexes
2.3.1 [Ni(o-van-en)]nH2O (n = 1.17, 1)
Solid H2(o-van-en) (2 g, 6 mmol) and nickel(II) carbonate (0.7 g, 6 mmol) were added to 100
ml of water and heated in an open beaker. After one hour of reaction the resulting brown
microcrystalline product was separated by filtration, washed with 2 ml of ethanol and dried in
air. Yield: 79 %.
Anal. [%], calculated for C18H20O5N2NinH2O with n = 1.5 (full occupancy of atom O6): C,
52.46; H, 5.14; N, 6.80; found: C, 52.14; H, 4.79; N, 6.54; calculated with n = 1.17 from X-ray
single crystal analysis: C, 53.27; H, 5.04; N, 6.90.
IR (cm-1): 3454b, 3055w, 2934w, 2835w, 1621s, 1602s, 1547m, 1472s, 1447s, 1392w, 1314s,
1232s, 1167m, 1109m, 1092m, 1080m, 1001m, 973m, 860m, 777w, 722s, 662w, 619m, 571w,
543w, 480m, 426m, 401m.
Single crystals suitable for X-ray data collection were obtained by dissolving the
microcrystalline product in acetone; when left overnight the resulting clear solution yielded a
garnet red crystalline product 1.
2.3.2 [Ni(o-van-en)]H2OEtOH (2)
Single crystals of complex 2 were obtained by the same procedure as was used for complex 1,
with the difference that the crude product was recrystallized from ethanol. After dissolving the
crude product a clear solution was obtained which overnight yielded red prisms of 2 exhibiting
low stability on standing in air. The crystals were used for single crystal X-ray analysis and for
measuring the IR spectrum immediately after harvesting them from the mother liquor.
IR (cm-1): 3446b, 3055w, 2934w, 2834w, 1621s, 1602s, 1547m, 1471s, 1447s, 1392w, 1313s,
1232s, 1167m, 1109m, 1079s, 1000m, 973m, 859m, 777w, 722s, 667w, 619m, 571w, 542w,
480m, 425m, 401s.
2.3.3 [Ni(o-van-en)]H2OiPrOH (3)
4

Single crystals of complex 3 were obtained by the same procedure as was used for complex 1,
with the difference that the crude product was recrystallized from isopropanol. Single crystals
of 3, in the form of tiny brown needles, separated overnight and were filtered, washed quickly
with a small portion of cold isopropanol and dried in air.
IR (cm-1): 3508w, 3478w, 3330w, 3056w, 2961w, 2937w, 2840w, 1621s, 1603s, 1548m, 1473s,
1449s, 1408m, 1335w, 1314s, 1243s, 1232s, 1168m, 1136w, 1108m, 1080s, 1000m, 972m,
958m, 893w, 858m, 820w, 782w, 727s, 660w, 620m, 572w, 542w, 480m, 463w, 425m, 400s.
2.4 Physical measurements
CHN analyses were performed on an Elemental Analyzer vario MICRO instrument. Infrared
spectra were recorded in the range of 4000–400 cm-1 on a Nicolet 6700 FT-IR spectrometer
using the ATR technique and with a Smart Orbit attachment. The temperature-dependent FT-
IR spectra of 1 were measured from a KBr pellet on the same instrument using a Smart Proteus
(Thermo Scientific) heating device. X-ray powder diffraction patterns were recorded on a
RIGAKU D-Max/2500 diffractometer with rotating anode and RINT2000 vertical goniometer,
in the 2θ range 7-50º using Cu Kα1,2 radiation ( = 1.54178 Å). The calculated patterns were
obtained using the program Mercury CSD 3.1.1 Development [19]. The measured powder
diffraction pattern was fitted by the LeBail method [20] using the program JANA2006 [21].
Thermal properties were studied using a 2960 SDT TA instrument under the following
conditions: sample weight = 6.9265 mg (1), heating rate = 10 deg/min, dynamic artificial air
atmosphere, temperature range 25–900 C, aluminum oxide crucible.
2.5 Single crystal X-ray diffraction
Single-crystal X-ray data were collected on an Oxford Diffraction Xcalibur diffractometer
equipped with a Sapphire3 CCD detector and a graphite monochromator utilizing Mo-K
radiation ( = 0.71073 Å). Absorption corrections were based on the multi-scan technique using
ABSPACK [22]. The structures were solved by the program SIR92 [23] and refined against F2
using full-matrix least squares methods with the program SHELXL-2014/7 [24]. Anisotropic
displacement parameters were refined for all non-hydrogen atoms. The hydrogen atoms bonded
to carbon atoms were included at idealized positions and refined as riders with isotropic
displacement parameters assigned as 1.2 times the Ueq values of their corresponding bonding
partners, while the hydrogen atoms of the aqua ligands were located in a difference map and
refined with restrained geometry and constrained isotropic thermal parameters [Uiso(H) was
1.5Ueq of the parent atom]. Some geometric parameters were calculated using the program
PARST [25]. The crystal and experimental data are given in Table 1, and selected geometric
5

parameters are given in Table 2. Possible hydrogen bonds are gathered in Table 3. The
structural figures were drawn using Diamond [26].
Table 1. Crystal data and structure refinement for 1, 2 and 3.
1
2
3
CSD number
Empirical formula
Molecular weight
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
1983891
C18H20.31O5.17N2Ni
405.85
orthorhombic
Pbcn
1983890
C20H26N2NiO6
449.14
triclinic
P-1
1983889
C21H28N2NiO6
463.16
monoclinic
P21/n
19.6607(6)
11.8032(4)
15.0266(4)
90
90
90
3487.05(18)
8
1.546
100(2)
1.146
7.8460(2)
11.4533(3)
12.2907(3)
76.702(2)
79.967(2)
70.233(2)
1005.90(5)
2
14.1325(7)
7.4085(4)
20.1164(8)
90
93.527(4)
90
2102.21(18)
4
1.463
γ (°)
V (Å3)
Z
Dcalc (Mg.m-3)
T (K)
1.483
100(2)
1.004
173(2)
0.963
μ (mm-1)
Crystal dimensions (mm)
Crystal color / form
Index ranges
0.217 x 0.240 x 0.503
red block
−22  h  24
−13  k  14
−18  l  18
2.902 – 25.998
27510
0.121 x 0.232 x 0.355 0.017 x 0.033 x 0.379
red block
−9  h  9
−14  k  14
−15  l  15
3.411 – 26.498
35818
brown needle
−17  h  17
−8  k  9
−24  l  24
3.240 – 25.998
14980
 range (°)
Reflections collected
Rint
0.0489
0.0164
0.0718
Independent reflections
Observed reflections [I > 2I)]
Tmin - Tmax
3433
3004
0.602-1.000
4154
3867
0.818-1.000
4130
2988
0.885-1.000
2
Goodness-of-fit on F
1.036
1.038
1.010
R indices [I > 2 (I)]
R1 = 0.0331
wR2 = 0.0840
R1 = 0.0394
wR2 = 0.0887
0.374; -0.287
R1 = 0.0242
wR2 = 0.0609
R1 = 0.0267
wR2 = 0.0625
0.339; -0.225
R1 = 0.0436
wR2 = 0.0826
R1 = 0.0739
wR2 = 0.0922
0.868*; -0.323
R indices (all data)
Diff. peak and hole (e.Å−3
)
* this maximum was located near (0.998 Å) the Ni(II) central atom; the next maximum exhibits a value of 0.33
e.Å−3
2.6. Energy decomposition calculations
A simple energy decomposition scheme has been used to analyze components of interaction
energy between Ni(II) complexes in the dimeric systems 2 and 3 at B3LYP-D3/TZVP level.
All calculations were performed with ORCA 4.2.1 computational package using the
experimental geometry of the complexes under study [27]. The energy decomposition method
has been used as implemented in the Multiwfn v3.3.8 program suite [28].
3. Results and Discussion
6

3.1 Preparation and identification
The direct reaction of the Schiff base H2(o-van-en) with nickel(II) carbonate under reflux leads
to a microcrystalline product of 1 [Ni(o-van-en)]nH2O (n = 1.17; 1). Diffraction data for 1 were
measured at T = 100 K (LT) and at T = 293 K (RT). We report here the results derived from
the LT data set. A summary of crystal data and refinement results for 1 at RT is given in Table
S1; the RT results have also been deposited in the CSD. The identity and purity of 1 were
corroborated by powder X-ray diffraction (Fig. 1), using the RT single-crystal analysis as a
point of departure for the Le Bail refinement. Complex 1 is easily soluble in different organic
solvents. We recrystallized this product form acetone, ethanol and isopropanol. Single crystals
of various solvatomorphs were obtained in the form of the hydrate (recrystallization from
acetone) 1, the monohydrate-monoethanol solvate (from ethanol) 2, and the monohydrate-
monoisopropanol solvate (recrystallization from isopropanol) 3. While crystals of 1 present
more regular proportions, crystals of 3 are small, thin needles; compound 2 crystallized with a
shape intermediate between those of 1 and 3 (Fig. S1 in the Electronic Supplementary
Information). A literature survey has shown that the hydrate 1 in the form of the monohydrate
(n = 1) prepared by crystallization from chloroform was already structurally elucidated based
on room temperature data with a higher final residual value of R = 7.1 % [17]. Additional
systems comprising [Ni(o-van-en)] complex molecules are also recorded in the CSD [29]: the
chloroform solvate, [Ni(o-van-en)]CHCl3 (Ref-code TEJGIZ) which was prepared by the solid
state reaction of the Schiff base H2(o-van-en) with nickel(II) acetate tetrahydrate followed by
recrystallization from chloroform [15], and the dimethylformamide solvate, [Ni(o-van-
en)]Me2N-CHO (ULAMIE) prepared by recrystallization from dimethylformamide [16]. In
addition, several co-crystals of [Ni(o-van-en)] with various tin complexes were reported, e.g.,
[Ni(o-van-en)](H2O)SnMe2Cl2 (WAVYOH) [17]. We note also the existence of a report on
the synthesis and crystal structure of [Ni(o-van-en)(H2O)] (YUDFIN) with a pentacoordinated
Ni(II) atom [30].
Commented [JČ1]: Formatting only
7

Figure 1 Results of Le Bail refinement of the powder diffraction pattern of 1, with the unit-cell
parameters taken from the known crystal structure of [Ni(o-van-en)]nH2O (n = 1.16, see Table
1). The refined cell parameters are a = 19.6242 (3) Å, b = 11.873 (2) Å, c = 15.224 (2) Å. R(p)
= 0.0172, R(wp) = 0.0258, goodness of fit = 8.34.
8

All three solvatomorphs 1-3 display content-rich IR spectra. The positions of the main
absorption bands are gathered in the experimental section and the respective spectra are
depicted in Fig. 2 (complex 1), Fig. S2 (Schiff-base), Fig. S3 (2) and Fig. S4 (3) (Figures S2-
S4 are in the Supplementary File). Characteristic signals include pairs of strong absorption
bands positioned at 1621 and 1602 cm−1 in 1, 1621 and 1602 cm−1 in 2, and 1622 and 1603
cm−1 in 3, respectively, which, according to the literature, can be assigned to ν(C=N) vibrations
[31,32]. In the IR spectrum of the uncoordinated Schiff base H2(o-van-en) this band appeared
at 1631 cm−1 and this value is very close to that of 1633 cm-1 reported previously [31]. Weak to
very weak but easily identifiable are the absorption bands positioned slightly above 3000 cm−1
[3055 cm−1 (1), 3055 cm−1 (2) and 3056 cm−1 (3)] due to ν(Car–H) stretching vibrations; the
corresponding band in the Schiff base was observed at 3087 cm–1. Medium-broad absorption
bands observed around 3500 cm−1 (3454 cm−1 in 1, 3446 cm−1 in 2 and 3330 cm-1 in 3) were,
in accord with the literature [33], assigned to ν(O–H) vibrations; we note the presence of two
sharper absorption bands positioned at 3508 and 3478 cm-1 in the IR spectrum of 3 which can
be assigned to the ν(O–H) vibrations from the isopropanol solvate molecule.
Commented [JČ2]: The figure was displaced
Figure 2 IR spectrum of 1 at RT (red) and at 140 °C (blue).
Thermodynamic data were measured for the hydrate solvatomorph 1 with the aim of
elucidating its thermal stability and to corroborate the presence of water solvate molecules (Fig.
3). As can be seen from Fig. 3 complex 1 begins to lose weight smoothly, already at room
temperature and this process is endothermic with DTAmax at 95 C. The experimental weight
loss (5.0 %) corresponds well to the calculated value for full dehydration (5.1 % calculated for
n = 1.16). Upon further heating a plateau on the TG curve was observed which indicates
9

formation of the anhydrous complex [Ni(o-van-en)], stable within the temperature range 100-
200 C. The IR spectrum of 1 measured at 140 C (blue line in Fig. 3, Fig. S5) does not contain
absorption bands of the (OH) and (H2O) types (weak shoulder at 1670 cm-1) corroborating
the full dehydration of 1. Further heating in the temperature range of
320-420 C causes the total decomposition of the dehydrated sample. The solid residue
represents 17.0 % of the initial weight; the calculated values for NiO and metallic nickel are
18.4 and 14.5 %, respectively.
Figure 3. Thermal curves for 1.
3.2 Crystal structures
The Schiff base type ligand (o-van-en)2- exhibits two compartments, a smaller one with cis-
N2O2 donor set and a larger one with O4 donor set; the O atoms formed by deprotonation of the
hydroxido groups are common donor atoms for both compartments, usually forming O-
monoatomic bridges between the M1 and M2 central atoms placed in the smaller and larger
cavities, resp.
Because of its stereochemical properties and its ability to act as a chaperone for two
entities that would not otherwise come into proximity, this ligand is often used for syntheses of
new complexes. A search in the CSD [29] yielded 349 hits for complexes formed by (o-van-
en)2- with any metallic element M1 present in the smaller compartment. As expected, the most
numerous complexes are those with M1 = Cu(II) with 106 hits in CSD, among these 72 hits for
tetracoordinated (square) and 32 hits for pentacoordinated (square pyramidal) Cu(II) and 2 hits
for hexacoordinated Cu(II). It seems the smaller compartment, due to its size, contributes to the
10

stability of manganese(III); there are 68 hits for such complexes and only 2 hits for Mn(II)
complexes. Also numerous complexes with M1 = Ni(II) in square coordination (54 hits) and
those with Zn(II) being almost exclusively pentacoordinated (53 hits, only 1 structure contains
a hexacoordinated Zn(II) center). Among the other 3d metals maybe surprisingly we find only
eight crystal structures with M1 = Co(III) and an additional two with Co(II). Much less
numerous are complexes with vanadium(IV, V), with 12 hits (coordination numbers 5 or 6),
iron (10 hits); and for the 3d metals titanium(IV) and chromium(III) there were two and one
entries, respectively.
Surprisingly, there are additional crystal structures reported with larger central M1
atoms, vis.: Cd (1 hit), Ru (6 hits), U (3), Pt (5, square), Pd(II) (2, square), Tb(III) (1 hit), Dy(III)
(1), Ho(III) (1), Bi(III) (1), Th(IV) (1). It is interesting that in the case of Dy(III) complex
PIDXIM and Ho(III) complex PIDXOS a sandwich-like arrangement of two (o-van-en)2-
ligands around the Ln(III) atom is formed [34 ].
As mentioned above, there are 55 hits in CSD for compounds containing the structural unit
{Ni(o-van-en)}; in 54 cases the Ni(II) atom is in square coordination while in one hit (YUDFIN)
unusual pentacoordinated Ni(II) atom is reported [30]. In all these complexes the smaller
compartment is occupied by a square coordinated Ni(II) atom while the larger compartment is
available for further use (Table S2). Three different uses (functionalization of the O4 donor set)
for the larger compartment exist, as recorded in the CSD:
1. The second compartment is occupied by an another metallic atom, usually with a larger
ionic radius (4d-, 5d-, p- or 4f-metal) with one exception for the 3d-metal Mn(II). In this
case all donor atoms O4 form coordination bonds to the metallic element which
completes its coordination sphere with additional co-ligands. If these co-ligands are
terminal or chelating, dinuclear complexes are formed. If at least one co-ligand exhibits
bridging character, trinuclear (some hexacyanidocomplexes) or even complexes with
higher nuclearities can be formed with 10 being the highest. Sometimes dimerization of
the dinuclear unit enhances the nuclearity.
2. The second compartment is occupied by an alkali metal or alkali-earth cation. In this
case various structures were reported. In some cases the metal atom enters the larger
cavity and is placed within the plane of metalloligand {Ni(o-van-en)}, while in other
cases the metal is placed inside the space between a pair of {Ni(o-van-en)} structural
units and this arrangement often leads to a chain-like arrangement of parallel-stacked
{Ni(o-van-en)} structural units with M placed between them.
3. The second compartment lacks any metal atom. In this case the structural unit {Ni(o-
van-en)} can be viewed as the host part of a host-guest compound and is used for
accommodation of an additional guest molecule, like water (also as aqua ligand),
ethanol, ammonium cation, etc. Usually, the guest molecule is not situated in the plane
of the {Ni(o-van-en)} as they are pushed outside the plane by formation of hydrogen
bonding interactions with the inner O atoms. If only solvent molecules are present in
the structure, formation of solvatomorphs is observed. On the other hand, the presence
of additional molecules (not solvent) results in the formation of co-crystals.
In conclusion, for the {Ni(o-van-en)} structural unit (in the solid state) a high tendency to
use/functionalize the four inner oxygen atoms is typical, presumably in order to enhance
11

the lattice energy Ug and this contributes to the stability of the crystal structure with at least
a solvent molecule.
3.2.1 Molecular structure of the complex molecule
All three solvatomorphs 1, 2 and 3 contain the same complex molecule [Ni(o-van-en)] with the
Ni(II) central atom residing within the square O2N2 donor set of the (o-van-en)2- anion (NiN2O2
chromophore) formed by two nitrogen atoms from the ethane-1,2-diamine fragment and two
phenoxido oxygen atoms (Figs. 4-6). The room temperature crystal structure of 1, characterized
as [Ni(o-van-en)]nH2O with n = 1, has been reported twice; in both cases, WAVZAU [17]
and WAVZAU01 [35], the final R1 were 0.071. We have collected low temperature data which
yielded a better final R1 = 0.0333 (Table 1), and, in addition, the results of structure refinement
suggests the presence of an additional, partially occupied site for a water solvate molecule
resulting in n = 1.16 (see below). The lower temperature of our data collection (100 K)
manifests itself in a smaller unit cell volume of 3487.05(18) Å3 as compared to the value of
3574.26 Å3 (RT) reported for WAVZAU [17].
The same complex molecule [Ni(o-van-en)] is also present in two further solvatomorphs
with chloroform [Ni(o-van-en)]CHCl3 (TEJGIZ, 4) [15] and dimethylformamide (C3H7NO,
DMFA) [Ni(o-van-en)]DMFA (ULAMIE, 5) [16]. It should be noted that all solvatomorphs 1-
5 crystallize in centrosymmetric space groups so both possible conformations of the ethane-
1,2-diamine fragment in (o-van-en)2- are present in the unit cell in equimolar ratio.
Figure 4 Molecular structure of 1 showing the atom labeling scheme. The thermal ellipsoids
are drawn at 30 % probability level. The position for O6 water molecule is partially occupied.
Dashed lines represent possible hydrogen bonds. Symmetry code: 1i: 1-x, y, 0.5-z
12

Figure 5 Molecular structure of 2 showing the atom labeling scheme. The thermal ellipsoids
are drawn at 30 % probability level. Only the more populated position (s.o.f. = 0.60(2)), of O6
atom as well as the corresponding hydrogen atoms of the ethanol solvate molecule, are
depicted. Dashed lines represent possible hydrogen bonds.
13

Figure 6 Molecular structure of 3 showing the atom labeling scheme. The thermal ellipsoids
are drawn at 30 % probability level. Dashed lines represent possible hydrogen bonds.
Selected geometric parameters of the solvatomorphs 1-3 along with those reported for
the known compounds 4 and 5 are gathered in Table 2. As can be seen from this table the
Ni-N and Ni-O bond distances fall within the narrow ranges 1.843(2) (in 4) - 1.8552(12) Å (in
2) and 1.8392(17) (in TEJGIZ) - 1.8651(18) Å (in 3), respectively, in line with the rigidity of
the square coordination. In nickel(II) complexes with the analogous tetradentate Schiff base
obtained by condensation of 1,3-propanediamine with 2-hydroxybenzophenone in a 1:2 molar
ratio, the corresponding Ni-N and Ni-O bond distances are in the ranges 1.877(2) - 1.883(2) Å
and 1.824(2) - 1.852(2) Å, respectively, demonstrating somewhat longer Ni-N distances [32]
presumably as a consequence of the larger metallocycle formed by 1,3-diamine fragment with
respect to the 1,2-diamine in our case. The small variation of coordination bond distances also
manifests itself in near uniformity of the values of the cis- N-Ni-N and O-Ni-O angles in 1-5;
these lie in the range 84.18(7) (O-Ni-O angle in 4) – 86.60(9) (N-Ni-N angle in 4). The
corresponding geometric parameters in the RT hydrate (1, n = 1) are very close to these ranges
[17].
Table 2 Comparison of selected geometric parameters [Å, ].
14

1
2
3
4 TEJGIZ
[15]
5 ULAMIE
[16]
(n = 1.16)
This work
T = 100 K
1.8509(16)
1.8547(18)
1.8469(13)
1.8607(14)
85.67(7)
85.68(6)
10.9(3)
This work
T = 100 K
This work
T = 173 K
T = 296 K
T = 110 K
Ni-N
Ni-O
1.8461(12)
1.8552(12)
1.8577(10)
1.8599(10)
86.03(5)
85.17(4)
2.1(2)
1.849(2)
1.852(2)
1.8650(18)
1.8651(18)
85.78(10)
85.96(8)
12.9(4)
1.846(2)
1.843(2)
1.8392(17)
1.8458(17)
86.60(9)
84.18(7)
-6.93
1.8502(17)
1.8503(17)
1.8609(13)
1.8594(13)
85.71(8)
85.14(6)
-6.90
cis (N-Ni-N)
cis (O-Ni-O)
C1-O1-C2-C3
C18-O4-C16-C15
5.9(3)
-9.2(2)
-6.3(4)
16.17
3.20
Angle between the 3.76(5)
7.54(5)
4.27(8)
3.41
3.87
aromatic rings
Angles between the
aromatic rings and
eq. plane {N2NiO2}
5.78(4)
7.28(4)
3.72(8)
5.38
8.47
3.448
7.65
9.50(5)
1.03(4)
5.73(8)
6.89
Shortest NiNi
distance and
6.2080(4)
3.3379(3)
3.3712(6)
3.534; 3.671
1-x, 1-y, 1-z 1-x, 1-y, 1-z
dimer dimer
type of supramo-
lecular structure
0.5-x, 0.5+y, z
monomer
(dimer)
(chain)
The complex molecules [Ni(o-van-en)] in the respective solvatomorphs deviate
somewhat from planarity, presumably due to the action of packing forces (Figs. S6-S8). The
deviation from planarity can be demonstrated by the non-co-planarity of the two aromatic rings
(atoms C2-C7 and atoms C12-C17) of the vanillin fragments as well as by the non-zero angles
between the aromatic rings of the vanillin fragments and the equatorial mean plane formed by
the {NiN2O2} atoms. The range for the dihedral angle between the aromatic rings is
3.41 - 7.54 (5) with the greatest value for 2 (water and ethanol solvate molecules); and the
range for the second angle is 1.03 (4) - 9.50 (5) with the largest value found in 1. Different
packing forces manifest themselves also in the variation of the positions of the methoxy groups
which are sometimes significantly pushed out from the plane of the complex molecule with the
highest values of 12.9(4) and 16.17 observed in 3 and 4, respectively, as indicated by the
values of the corresponding C-O-C-C torsion angles (Table 2). The asymmetry in the positions
of the two methoxy groups within the same complex molecule is also noteworthy.
15

The small but significant conformational differences among the complex molecules
[Ni(o-van-en)] in the respective solvatomorphs 1, 2 and 3, can be observed on a pairwise basis
in Figure 7; comparisons of the solvatomorph 1 with those of 4 and 5 are depicted on Fig. S9a
and Fig. S9b. A comparison of the molecular structure of 1 at 100 K (n = 1.16) with the RT
structure (n = 1; [17] (Fig. S9c) indicates only minor conformational differences which can be
ascribed to temperature differences.
(a)
(b)
(c)
Figure 7. Comparison of the molecular structures of the complex molecules [Ni(o-van-en)] in
the pairs of solvatomorphs: a) 1 (red) and 2 (green); b) 1 (red) and 2 (green); c) 1 (red) and 2
(green). The structures are superimposed in such a way that the positions of the atoms in the
{NiN2O2} fragment are overlapped as closely as possible.
3.2.2 Intramolecular interactions between the complex molecules and solvate molecules
As a result of its shape the [Ni(o-van-en)] complex molecule possesses a compartment located
between the two oxygen atoms from deprotonated hydroxyl groups and two methoxy groups.
16

This compartment, as mentioned above, can serve for accommodation of an additional metal
atom such as a lanthanide [11,36] or alkali metal [37,38]. In the absence of such a metal atom
the resulting space is (possibly partially) occupied by a hydrogen-bonded solvate molecule,
sometimes slightly displaced from the plane of the complex. In the structures of 1-3 the closest
solvate molecule is water (Table 3), and its presence is complemented by an additional partially
occupied water molecule (O6) in 1, and by ethanol and isopropanol molecules in 2 and 3,
respectively.
Figure 8 FIM showing the acceptor (oxygen atoms, red) and donor (water oxygens, blue)
regions around the [Ni(o-van-en)] complex molecule in 1. Dots, wireframe and nods represent
frequencies of _2, _4 and _6 times those expected for a random distribution of contacts. The
position of the O5 water solvate molecule is shown.
The calculated Full Interaction Map (FIM) [39] for 1 considering only the [Ni(o-van-
en)] complex molecule indicates a blue donor region located in or nearby the larger
compartment of the complex molecule. The O5 water solvate molecule is located within this
region (Figure 8). The same observation pertains to the positions of the water solvate molecules
in 2 and 3; the corresponding figures are in the Supplementary File (Fig. S10). In line with this
the O5 water solvate molecules within the respective solvatomorphs 1-3 form O-H···O
hydrogen bonds (HBs) with the oxygen atoms inside the complex, with O···O distances in the
range 2.581(4) Å (in 1) - 3.057(3) Å (in 3) (Table 3, Figs. 4-6). In solvatomorph 1 the O6 water
17

molecule is located on the twofold axis and is only partially occupied, but it forms hydrogen
bonds with O5 and its symmetry related congener O51i (1i: 1-x, y, 0.5-z) yielding a trimeric
arrangement (Fig. 4).
In the solvatomorphs 2 and 3 molecules of the additional crystallization solvent are
present, namely ethanol in 2 and isopropanol in 3. In both cases these alcohol solvate molecules
are bonded to the water solvate molecule by O-HO hydrogen bonds (Table 3, Fig. 5, 6). We
note that in 2 the ethanol solvate molecule suffers positional disorder of the O6 oxygen atom.
Table 3. Possible hydrogen bonds in 1-5 [Å, ].
a) Complex 1
D-H···A
D-H
H···A
D···A
DHA
130(2)
161(3)
150(3)
174(2)
154
O5-H1w···O3
O5-H1w···O4
O5-H2w···O2
O6-H3W···O5
C8-H8···O51i
C11-H11···O11ii
C13-H13···O61iii
0.836(10)
0.836(10)
0.832(10)
0.839(10)
0.95
2.41(2)
2.217(12)
2.262(17)
1.746(10)
2.60
3.018(2)
3.021(2)
3.014(2)
2.581(4)
3.477(3)
3.492(3)
3.209(6)
0.95
2.57
164
0.95
2.29
162
Symmetry codes: 1i: x, 1-y, 1/2+z; 1ii: 1.5-x, y-1/2, z ; 1iii : x-1/2, y-1/2, 1/2-z.
b) Complex 2 (1063)
D-H···A
D-H
H···A
D···A
DHA
O5w-H5w1···O1
O5w-H5w1···O2
O5w-H5w2···O3
O5w-H5w2···O4
O6A-H6···O5w
O6B-H6···O5w
C8-H8···O5w2i
0.830(9)
0.830(9)
0.828(9)
0.828(9)
0.840(10)
0.841(10)
0.952(17)
2.204(15)
2.202(15)
2.346(17)
2.082(12)
1.901(10)
1.901(10)
2.554(17)
2.9026(16) 142(2)
2.9252(14) 146(2)
2.9718(15) 133.0(17)
2.8468(16) 154(2)
2.706(5)
2.702(9)
160(3)
159(3)
3.2264(18) 127.8(13)
Symmetry codes: 2i: -x, 1-y, 1-z; 2ii: x, 1+y, z;
c) Complex 3
D-H···A
D-H
H···A
D···A
DHA
O5w-H5w1···O1
O5w-H5w1···O2
O5w-H5w2···O3
0.835(10)
0.835(10)
0.840(10)
2.078(19)
2.34(2)
2.37(2)
2.781(3)
3.057(3)
3.057(3)
141(3)
144(2)
139(3)
18

O5w-H5w2···O4
O6-H6···O5W
C15-H15···O5W3i
0.840(10)
0.836(10)
0.95
2.041(16)
1.985(12)
2.56
2.781(3)
2.813(3)
3.472(4)
147(3)
171(3)
161
Symmetry codes: 3i: 0.5-x, y-0.5, 0.5-z
3.2.3 Intermolecular interactions in 1-3
From the shape of the [Ni(o-van-en)] complex molecule it follows that it can participate only
in weak hydrogen bonding interactions. The full interaction map (FIM) for 1 (Fig. 8) indicates
that there are two acceptor regions (red) around the complex molecule, namely those
corresponding to the C8-H8 and C11-H11 imine groups; both these imine groups are involved
in weak (N=)C-HO hydrogen bonds in 1 (Table 3a). These weak hydrogen bonds along with
a weaker hydrogen bonding interaction involving the C13-H13 group leads to the formation of
cavities within the supramolecular structure in which are enclosed the water solvate molecules
(Fig. 9, Fig. S11). The packing forces also include -stacking interactions between the C2-C7
aromatic ring and its congener, related by an inversion center, with distance Cg1···Cg11iv being
3.6221(12) Å (1iv: 1-x, 1-y, 1-z). The same aromatic ring, on its opposite face, acts as an
acceptor in a much weaker C151v-H151v···Cg1 interaction (1v: 0.5-x, 1.5-y, 0.5+z) with
H151v···Cg1 = 2.84 Å, C151v···Cg1 = 3.614(2) Å and C151v-H151v···Cg1 = 140 (Fig. S12).
We note that due to the packing arrangement the Ni(II) central atoms are well separated with a
shortest NiNi distance of 6.2080(4) Å.
19

Figure 9 View of the packing in solvatomorph 1, showing the positions of the water solvate
molecules (space filling fashion, pink spheres) in the formed cavities. The Ni(II) atoms are
shown as green balls, the remaining atoms are displayed in wire mode.
In contrast, a typical feature of the packing for solvatomorphs 2 and 3 is the formation
of centrosymmetric [Ni(o-van-en)]2 dimers with short NiNi distances, namely 3.3379(3) and
3.3712(6) Å for 2 and 3, respectively (Table 2, Fig. 10 and Fig. S13). Similar pairing of
complex molecules, or even the formation of a chain-like arrangement, were reported for
solvatomorphs 4 and 5 (Table 2) [15,16].
Previously it was suggested, that the observed short NiNi distances may correspond
to metal-metal interactions [40,41]. In order to verify this conjecture, theoretical calculations
were done. In the model that was used to analyze the dimeric aggregates in 2 and 3, the total
energy variation on forming a complex molecular system is decomposed as
Etot  Eadduct

E fragment  Eels  EEx  Eorb  Edisp
i



i
where Eels is an electrostatic interaction term, normally negative if the two fragments are
neutral; EEx is the exchange repulsion term, which comes from the Pauli repulsion effect and is
invariably positive. For convenience, it is customary to combine these two terms as steric term
(Esteric). Eorb in the above formula is the orbital interaction term, which arises from the mixing
of occupied MOs and virtual MOs. For large molecules, it is necessary to include dispersion
20

forces (van der Waals interaction) in the total interaction energy This was done using
Grimme’s DFT-D3 method to evaluate dispersion correction to interaction energy [42].
According to the previous calculations, we found for 2 and 3:
Etot
Eorb
Esteric
Edisp
2
28.52
204.17
84.78
147.90
3
16.40
197.63


*Energies in kJ/mol.
namely the total interaction energy between the Ni(II) complexes is ~ 200 kJ/mol. The orbital
interaction energy slightly stabilized the adducts, while the steric energy, which is the sum of
electrostatic and exchange repulsion energy, destabilized the adducts by ~16 and 29 kJ/mol,
respectively. However, the largest contribution to the stabilization energy comes from the
dispersion interactions of the ligand systems (~65% of the total interaction energy). Initial DFT
calculations also provided Mayer bond order values, which for intermolecular atom-atom
contacts are < 0.1. We note that the calculations have shown that the estimated contribution
from NiNi interaction represent only about 11 % of the whole attractive forces and that the
main contribution comes from dispersion interactions between the ligands.
The presence of only weak dispersion interactions between the [Ni(o-van-en)]2 dimers
in 2 and 3, permits the accommodation of ethanol and isopropanol solvate molecules in the
respective crystal structures (Table 3b and 3c, Figs. S14 and S15). In 2 the ethanol molecules
are accommodated within parallel layers of the dimeric complex aggregates while in 3 the
solvent isopropanol molecules are trapped in channels (Figs. 11 and 12). Thus, the resulting
supramolecular structures in 2 and 3 enable diffusion across the quasi layers and channels and
this may be the cause of the low stability of these crystals. A comparison of the densities (Table
1) shows that the crystal structure of the hydrate 1 is more compact (1.546 g/cm3) than are the
other two solvatomorphs 2 (1.483 g/cm3) and 3 (1.463 g/cm3).
21

Figure 10 Formation of weakly bound dimer with Ni···Ni interaction (green dashed line) in 2.
Symmetry code: 2iv: 1-x, 1-y, 1-z.
Figure 11 Packing of the complex molecules (wire model, nickel atoms as green balls) in 2
with solvent water and EtOH molecules (O atoms are large pink balls, the carbon atoms in
EtOH are large grey balls) located in the channels. Hydrogen atoms are omitted for clarity.
22

Figure 12 Packing of the complex molecules (wire model, nickel atoms as green balls) in 3
with solvent water and i-PrOH molecules (space filling) located in the channels (O atoms are
large pink balls, the carbon atoms in i-PrOH are large grey balls). Hydrogen atoms are omitted
for clarity.
4. Conclusions
Three different solvatomorphs were prepared by recrystallization of the crude nickel complex
from various solvents, and were structurally characterized. Each solvatomorph is composed of
[Ni(o-van-en)] complex molecules, a water solvate molecule placed near the larger
compartment of the complex, and additional water (1), ethanol (2) or isopropanol (3) molecules
which are linked by hydrogen bonds to the proximal water molecules. The water-containing
solvatomorph (1) crystallizes with discrete, isolated complex molecules with water molecules
trapped in the cavities, leading to the most compact structure. In contrast, in the solvatomorphs
containing additional ethanol and isopropanol molecules, dimers of the complex molecules are
formed, with short NiNi separations, leading to less compact structures. Theoretical
calculations have shown that the formation of dimers is mainly supported by attractive
dispersion interactions between the ligands. Thermal analysis and temperature-dependent IR
spectroscopy confirmed that dehydration is the first step in the decomposition of 1.
Acknowledgements
This work was supported by the Slovak grants APVV-18-0016 and VEGA (grant No.
1/0063/17). Funding from the Ministry of Science and Innovation (Spain) under grant
23

PGC2018–093451-B-I00, from the European regional development fund (FEDER), and from
the Diputación General de Aragón (Project M4, E11_20R) is gratefully acknowledged.
Supplementary data
CCDC 1983889 (3), 1983890 (2) and 1983891 (1) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
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