Cobalt(II) and nickel(II) complexes with N-benzyl- and N-(p-nitrobenzyl)iminodiacetic acids. Structural and vibrational spectroscopic characterization and DFT study

Article abstract of DOI:10.1016/j.ica.2016.07.056

The reactions of N-benzyliminodiacetic acid (BnidaH₂) and N-(para-nitrobenzyl)iminodiacetic acid (p-NO₂BnidaH₂) with cobalt(II) and nickel(II) acetate in aqueous solutions were performed. Three new complexes [Co(Bnida)(H

Full text of DOI:10.1016/j.ica.2016.07.056

Accepted Manuscript
Cobalt(II) and nickel(II) complexes with N-benzyl- and N-(p-nitrobenzyl)imi-
nodiacetic acids. Structural and vibrational spectroscopic characterization and
DFT study
Neven Smrečki, Ozren Jović, Vladimir Stilinović, Boris-Marko Kukovec,
Marijana Đaković, Zora Popović
PII:
S0020-1693(16)30438-8
http://dx.doi.org/10.1016/j.ica.2016.07.056
ICA 17191
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
12 January 2016
13 June 2016
29 July 2016
Please cite this article as: N. Smrečki, O. Jović, V. Stilinović, B-M. Kukovec, M. Đaković, Z. Popović, Cobalt(II)
and nickel(II) complexes with N-benzyl- and N-(p-nitrobenzyl)iminodiacetic acids. Structural and vibrational
spectroscopic characterization and DFT study, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.
2016.07.056
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Cobalt(II) and nickel(II) complexes with N-benzyl- and N-(p-nitrobenzyl)iminodiacetic
acids. Structural and vibrational spectroscopic characterization and DFT study
Neven Smrečkia,*, Ozren Jovićb, Vladimir Stilinovića, Boris-Marko Kukoveca, Marijana
Đakovića and Zora Popovića
a
Laboratory of General and Inorganic Chemistry, Department of Chemistry, Faculty of Science,
University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
b
Laboratory of Physical Chemistry, Department of Chemistry, Faculty of Science, University of
Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
*Corresponding author:
Dr. N. Smrečki
E-mail: nsmrecki@chem.pmf.hr
Fax: +385 1 4606341
Phone: +385 1 4606305
E-mail addresses:
nsmrecki@chem.pmf.hr , neven.smrecki@gmail.com (N. Smrečki)
ozren.jovic@chem.pmf.hr (O. Jović)
vstilinovic@chem.pmf.hr (V. Stilinović)
bkukovec@chem.pmf.hr (B.-M. Kukovec)
mdjakovic@chem.pmf.hr (M. Đaković)
zpopovic@chem.pmf.hr (Z. Popović)
1

Abstract
The
reactions
of
N-benzyliminodiacetic
acid
(BnidaH2)
and
N-(para-
nitrobenzyl)iminodiacetic acid (p-NO2BnidaH2) with cobalt(II) and nickel(II) acetate in
aqueous solutions were performed. Three new complexes [Co(Bnida)(H2O)3] · H2O (1a ·
H2O), [Co(p-NO2Bnida)(H2O)3] · ½H2O (2a · ½H2O) and [Ni(p-NO2Bnida)(H2O)3] · ½H2O
(2b · ½H2O) were prepared and characterized by infrared spectroscopy and thermal analysis
(TG/DTA). The molecular and crystal structures of 1a · H2O and 2b · ½H2O were determined
by X-ray crystallography. The octahedral coordination environments around the Co(II) ion in
1a and Ni(II) ion in 2b consist of an O,N,O'-tridentate N-arylalkyliminodiacetate ion and three
water molecules, resulting in the formation of fac-isomers in both cases. The molecular
geometry and infrared spectra of these three complexes, together with previously synthesized
and characterized complex [Ni(Bnida)(H2O)3] · H2O (1b · H2O) are studied by DFT
calculations using BP86/6-311G(d,p) and mPW1PW91/6-311G(d,p) computational model.
Keywords: cobalt(II); nickel(II); iminodiacetate; IR spectroscopy; X-ray structure
determination; DFT geometry and vibrational calculation
2

1. Introduction
Iminodiacetic acids (RidaH2) are well known complexing agents which are able to
form stable complexes with many metal ions [1-5]. These ligands usually react with Co(II)
and Ni(II) in an identical manner to give analogous complexes, but those of Ni(II) show
greater stability than their Co(II) analogues [1]. Several copper(II) complexes with N-
benzyliminodiacetic acid (BnidaH2) were described in the literature and most of them contain
heterocyclic nitrogen compounds (imidazole, nucleobases and structurally similar derivatives)
[6-10] as additional ligands. The studies on cobalt(II), nickel(II) and copper(II) complexes
with other para-substituted N-benzyliminodiacetic acids (para-methyl, methoxy, fluoro and
chloro derivatives) revealed that Cu(II) complexes exhibit a distorted square-pyramidal
coordination, while those of Co(II) and Ni(II) which showed to be mutually isostructural,
adopt typical octahedral coordination with fac-configuration of the iminodiacetate chelators
[11-13].
The question is whether the electron density within the aromatic ring can be adjusted
to enhance or decrease the stability of these complexes. To check this assumption, we chose
to study Co(II) and Ni(II) complexes with BnidaH2 and p-NO2BnidaH2. The nitro group is a
very strong electron-withdrawing group and because of this, p-nitrobenzylamine is a weaker
base (and a weaker nucleophile) than benzylamine [14,15]. According to this, p-NO2BnidaH2
should be weaker ligand than BnidaH2 and this could give an observable or measurable
difference between the properties of Co(II) and Ni(II) complexes with p-NO2BnidaH2.
However, complicated hydrogen bond networks are observed in the crystal structures of all
complexes with this kind of ligands [4-13,16]. It is, therefore, questionable whether the
expected electron-withdrawing effect will have a pronounced influence on the structure of the
complex in the solid state, or if it will be compensated by the supramolecular environment.
We are thus interested in determining to what extent the influence of the ring substituent will
reflect on thermal properties of the resulting complexes and their infrared spectra. In order to
get more insight into the role of non-coordinating substituents in these systems, we have
prepared three new metal complexes with N-benzyliminodiacetic acid and N-(p-
nitrobenzyl)iminodiacetic acid: [Co(Bnida)(H2O)3]
·
H2O (1a
·
H2O), [Co(p-
NO2Bnida)(H2O)3] · ½H2O (2a · ½H2O) and [Ni(p-NO2Bnida)(H2O)3] · ½H2O (2b · ½H2O)
(Scheme 1). These complexes were, together with the previously synthesized and
characterized [Ni(Bnida)(H2O)3] · H2O (1b · H2O) [16], also studied with the aid of density
3

functional theory (DFT) calculations. Experimental infrared spectra of the complexes were
fully assigned and compared with their calculated spectra.
Many research papers which include combined experimental and theoretical
investigations of metal complexes have been published [17-21], but DFT structural and
vibrational studies for metal complexes with iminodiacetate ligands are very scarce in the
literature, as only few examples on such topic were found [13, 22-27]. Similar studies based
on theoretical comparison of cobalt(II) and nickel(II) complexes with iminodiacetate ligands
were not reported in the literature until now.
Scheme 1. Structural formulae of N-benzyliminodiacetic acid (BnidaH2) and N-(p-
nitrobenzyl)iminodiacetic acid (p-NO2BnidaH2).
2. Experimental
2.1. Reagents and physical measurements
All chemicals for the syntheses were purchased from commercial sources (Aldrich,
Acros or Alfa Aesar) and used as received without further purification. The CHN analyses
were performed on a Perkin-Elmer 2400 Series II CHNS analyzer in the Analytical Services
Laboratories of the Ruđer Bošković Institute, Zagreb, Croatia. The FTIR spectra were
obtained from KBr pellets in the range 4000–400 cm−1 on a Perkin-Elmer Spectrum TwoTM
FTIR-spectrometer. The TG/DTA measurements were performed at a heating rate of 10 °C
min−1 in the temperature range 25–600 °C, under nitrogen or oxygen flow of 150 mL min−1 on
an instrument Mettler-Toledo TG/SDTA 851e. Approximately 10 mg of sample was placed in
a standard aluminium crucible (40 µL). The DSC measurements were performed at heating
rate of 10 °C min−1 in the temperature range 25–500 °C, under nitrogen flow of 180 mL min−1
on an instrument Mettler-Toledo DSC 823e. Approximately 2–5 mg of sample was placed in
the standard aluminium crucible (40 µL). The NMR spectra of the ligands were recorded in
the NMR Centre of the Ruđer Bošković Institute, Zagreb, Croatia, on a Bruker AV 600
4

1
13
spectrometer, operating at 600 MHz for the H nucleus and at 150 MHz for the C nucleus.
The samples were measured in DMSO-d6 solutions at 298 K, using 5 mm NMR tubes. The
chemical shifts in ppm were referenced to TMS. The powder X-ray diffraction (PXRD)
experiments on the samples were performed on a PHILIPS PW 1840 X-ray diffractometer
with CuKα1 (1.54056 Å) radiation at 40 mA and 40 kV. The scattered intensities were
measured with a scintillation counter. The angular range (2θ) was from 3 to 50° with steps of
0.02°, and the measuring time was 0.5 s per step. The data collection and analysis were
performed using the program package Philips X'Pert [28-30].
The diffraction data were collected at 296 K for both compounds. Diffraction
measurements were performed on an Oxford Diffraction Xcalibur Kappa CCD X-ray
diffractometer with graphite-monochromated MoKα (λ = 0.71073Å) radiation [31]. The data
sets were collected using the ω scan mode over the 2θ range up to 54°. The structures were
solved by direct methods and refined using the SHELXS and SHELXL programs [32, 33].
The structural refinement was performed on F2 using all data. Hydrogen atoms not involved
in hydrogen bonding were placed in calculated positions and treated as riding on their parent
atoms [Car-H = 0.93 Å and Uiso(H) = 1.2 Ueq(C); Cal-H = 0.97 Å and Uiso(H) = 1.2 Ueq(C)]. All
calculations were performed using the WINGX crystallographic suite of programs [34]. The
crystal parameters, data collection and refinement details are listed in Supplementary Table
S1.
2.2. Preparation of the ligands
N-benzyliminodiacetic acid (BnidaH2) was prepared as previously reported [16]. N-(p-
nitrobenzyl)iminodiacetic acid hemihydrate (p-NO2BnidaH2 · ½H2O) was prepared by a
procedure different from the reported [10].
A mixture of p-nitrobenzylamine hydrochloride (10.0 g; 0.05 mol) [Note 1] and
chloroacetic acid (11.7 g; 0.12 mol) was dissolved in water (120 mL), after which sodium
carbonate (10.6 g; 0.10 mol) was carefully added in few portions (CAUTION – extensive
foaming!). When the foaming subsided, the reaction mixture was refluxed for 1 hour. The
resulting yellow solution, containing a small amount of red-brown resinous material was
boiled for 5 minutes with 0.25 g of decolourising carbon and filtered while still hot. The
filtrate was mixed first with ethanol (100 mL) and acidified to pH ≈2 by the addition of
diluted hydrochloric acid (20 mL; 1:1 v/v). The crude yellow product (8.3 g) was filtered off,
washed with ice water (30 mL) and recrystallized from boiling water (100 mL) with the
5

addition of ethanol (200 mL). The pure product (6.7 g; 48%) was obtained in the form of pale
yellow crystals, melting at 187.8 °C after the initial dehydration.
Anal. Calc. for C11H13N2O6.5 (277.21): C, 47.66; H, 4.73; N, 10.10. Found: C, 47.35;
H, 4.57; N, 10.26%. The assigned IR data (KBr), ṽ / cm−1: 3480(m, br, ν(OH)), 1737(vs,
1
ν(C=O)), 1606(s, νa(COO−)), 1520(vs, νa(NO2)), 1401(s, νs(COO−)), 1347(vs, νs(NO2)). H
NMR (600 MHz, DMSO-d6), δ / ppm: 3.45 (s, 4H), 4.00 (s, 2H), 7.64 (d, 2H, J = 8.7 Hz),
8.14 (d, 2H, J = 8.7 Hz). 13C NMR (150 MHz, DMSO-d6), δ / ppm: 54.51 (CH2), 57.05
(CH2), 123.10 (CH), 129.45 (CH), 147.01 (C), 147.24 (C), 171.81 (COOH).
Note 1. p-nitrobenzylamine hydrochloride was obtained via Reichert-Dornis cleavage
of N-(p-nitrobenzyl)hexamethylenetetraminium chloride [35], which was prepared from p-
nitrobenzyl chloride and hexamethylenetetramine by the Delépine reaction [36].
2.3. Preparation of the complexes
The complexes were prepared by pouring the aqueous solution of cobalt(II) acetate
tetrahydrate or nickel(II) acetate tetrahydrate (0.25 g; 1.0 mmol in 10 mL) into boiling
aqueous solution of BnidaH2 or p-NO2Bnida (1.0 mmol in 40 mL). The resulting complexes
were filtered off, washed with water and dried in air. Complexes 1a · H2O and 2b · ½H2O
gave crystals suitable for X-ray structural analysis directly from the above described reaction
mixtures. Ni(II) complexes 1b · H2O and 2b · ½H2O are stable substances, but the analogous
cobalt(II) complexes 1a · H2O and 2a · ½H2O should be isolated and analysed quickly after
they appear in the reaction mixture because they decompose upon standing.
[Co(Bnida)(H2O)3] · H2O (1a · H2O): from 0.23 g of N-benzyliminodiacetic acid;
rose-red crystals after about 12 hours, yield: 0.27 g (77%). Anal. Calc. for C11H19NO8Co
(352.20): C, 37.51; H, 5.44; N, 3.98. Found: C, 37.23; H, 5.17; N, 4.09%. IR (KBr), ṽ / cm−1:
3464(s, br), 2925(m), 1620(vs), 1580(m), 1495(w), 1458(w), 1410(m), 1384(w), 1363(w),
1327(w), 1265(w), 1225(w), 1205(w), 1109(w), 1094(w), 1076(w), 1027(w), 972(w),
942(w), 921(w), 896(w), 828(w), 762(w), 706(w), 629(w), 515(w).
[Co(p-NO2Bnida)(H2O)3] · ½H2O (2a · ½H2O): from 0.28 g of N-(p-nitrobenzyl)
iminodiacetic acid hemihydrate; pink powder after 10-15 minutes, yield: 0.28 g (72%). Anal.
Calc. for C11H17N2O9.5Co (388.18): C, 34.03; H, 4.41; N, 7.21. Found: C, 33.85; H, 4.27; N,
7.39%. IR (KBr), ṽ / cm−1: 3435(s, br), 2925(w), 2855(w), 1607(vs), 1516(m), 1442(w),
1415(m), 1349(s), 1318(w), 1235(w), 1092(w), 985(w), 926(w), 873(w), 854(w), 808(w),
760(w), 735(w), 704(w), 630(w).
6

[Ni(p-NO2Bnida)(H2O)3] · ½H2O (2b · ½H2O): from 0.28 g of N-(p-nitrobenzyl)
iminodiacetic acid hemihydrate; blue-green crystals after few minutes, yield: 0.36 g (95%).
Anal. Calc. for C11H17N2O9.5Ni (387.97): C, 34.05; H, 4.42; N, 7.22. Found: C, 34.32; H,
4.16; N, 7.47%. IR (KBr), ṽ / cm−1: 3456(s, br), 2929(w), 2852(w, sh), 1619(vs), 1518(m),
1429(w), 1464(w), 1410(m), 1352(s), 1314(w), 1224(w), 1086(w), 994(w), 905(w), 872(w),
855(w), 806(w), 765(w), 740(w), 703(w), 624(w).
[Ni(Bnida)(H2O)3] · H2O (1b · H2O) was prepared and characterized earlier [16].
2.4. Computational details
The geometry optimizations were carried out using the ORCA 3.02 version [37]
employing the density functional theory functionals BP86 [38], B97-D [39], B3LYP [40] and
mPW1PW91 [41], using several Pople style basis sets [42] (see the list in Supplementary
table S2). The computations were conducted on 2b, 1b, 2a and 1a. The optimal basis set
chosen gave the minimum root mean square deviation (RMSD) between experimentally and
theoretically determined bond lengths of 2b (disregarding bond lengths that include hydrogen
atoms) [43-45] using BP86 functional. The optimal basis set was shown up to be 6-311G(d,p)
and it was further used for the selection of optimal functional with lowest RMSD between
experimentally and DFT determined bond lengths, which was determined to be mPW1PW91,
also used in similar systems [46]. Calculations were conducted both in vacuum and in water
using COSMO solvation model (εr = 80, n = 1.33) as dielectric polarizable continuum with
ORCA defaults [47]. Additional models forming hydrogen bonds were considered. For Co(II)
complexes we calculated both high and low spin for 1a and 2a. Initial input coordinates for
geometry optimizations were obtained from experimentally determined X-ray data. Grid4 was
used for all these calculations [48]. 6-311++G(2df,2pd) was used for energy evaluation [49].
The frequency calculations were performed for the optimal functional as well as for
BP86 and B97-D. RI-approximation shortened some calculations by approximately factor of
eight with negligible difference in obtained frequencies (≈ 0.1-0.2 cm−1).
The binding energy of one tridentate iminodiacetate ligand to metal centre (Co2+ or
Ni2+) was calculated as the single point electronic energy difference between the energy of
geometry optimized complex and sum of the corresponding energies of geometry optimized
tridentate ligand and metal centre (Ebind = E(ML) – [E(M2+)+E(L2–)]). Geometry optimization
for each ligand was performed with atom position inputs extracted from DFT optimized Ni(II)
complexes. Calculations were carried out using BP86-D3 and B97-D, taking into account zero
point energy (ZPE) and basis set superposition error correction. The energies calculated in
7

water were obtained after outlying charge correction. Mayer bond orders in water were
calculated for geometry optimized complexes using BP86/6-311G** level of theory.
3. Results and discussion
3.1. Preparation and general properties of the complexes
The complexes were prepared by the reaction of metal acetates with corresponding
iminodiacetic acids in aqueous solutions. Under the described experimental conditions Co(II)
and Ni(II) gave complexes of analogous chemical composition and very similar infrared
spectra. 1a · H2O and 2b · ½H2O gave suitable crystals for x-ray structural analysis directly
from the original reaction mixture. Although of analogous composition and similar spectral
features, Co(II) and Ni(II) complexes show significant difference in their stability. Nickel(II)
complexes are stable substances whose colour, infrared spectra and water content remain
unchanged even after few weeks. On the contrary, the analogous cobalt(II) complexes lose
their water molecules upon standing and consequently show different physical appearance,
infrared spectra and thermograms than the original samples. Both 1a · H2O and 2a · ½H2O
change their colour after a day or two (1a · H2O from rose-red to purple, 2a · ½H2O from pink
to purple).
3.2. Molecular and crystal structures of the complexes
The coordination environment around Co(II) and Ni(II) in the molecular structures of
1a and 2b consist of a O,N,O’-tridentate N-arylalkyliminodiacetate ion and three water
molecules. Both compounds exhibit the usual non-planar fac-binding mode of the
iminodiacetate ion. ORTEP-3 drawings of the molecular structures of 1a and 2b are depicted
in Fig. 1 and the selected molecular geometry parameters are listed in Table 1.
Nickel(II) and cobalt(II) ions are octahedrally coordinated by O1, O2 and O3 atoms
from three water molecules and by O,N,O’-tridentate Bnida2- (in 1a) or p-NO2Bnida2- ligands
(in 2b) via the imine nitrogen (N1) and two carboxylate oxygen O4 and O5 atoms, forming
two five-membered chelate rings (defined by M1/O4/C1/C2/N1 and M1/O5/C3/C4/N1 atoms;
M = Co, Ni; Fig. 1) with approximately perpendicular mean ring planes (dihedral angle of
79.6° in 1a and angles of 81.4° and 84.9° in 2b). Complex 1a is isostructural with the
previously reported 1b [16]. The geometry of the coordination sphere in both complex
molecules in the asymmetric unit of 2b is essentially the same as those in 1a and 1b, with the
analogous bond distances and angles in 1a, 2a and 2b comparable to each other (Table 1) and
8

also in accordance with the corresponding values reported in the literature for the analogous
cobalt(II) and nickel(II) complexes with N-arylalkyl derivatives of iminodiacetic acid [4, 5,
11-13]. On the basis of the similarity of infrared spectra of 2a · ½H2O with that of its
nickel(II) analogue 2b · ½H2O and previously determined crystal structures of Co(II) and
Ni(II) complexes with similar ligands [11-13], it is likely that 2a has the analogous molecular
structure as 2b. This argument is additionally strengthened by the similarity of the powder
diffraction patterns (Supplementary material) of the isolated solids, 2a · ½H2O and 2b ·
½H2O, which indicates that they crystallize isostructurally.
Fig. 1. ORTEP plots of asymmetric units in the crystal structures of a) [Co(Bnida)(H2O)3] ·
H2O (1a · H2O), b) [Ni(Bnida)(H2O)3] · H2O (1b · H2O) [16], and c) [Ni(p-
NO2Bnida)(H2O)3]2 · H2O (2b · ½H2O) with the atomic numbering scheme. Thermal
ellipsoids are drawn at 30% probability and hydrogen atoms are shown as spheres of arbitrary
radius.
9

Table 1. Selected bond distances (Å) and angles (˚) for 1a and 2b.
2bb
1a
2b
Bond distances
M1–O1a (Ni2–O10)b
2.124(2)
2.050(2)
2.111(1)
2.067(2)
2.039(1)
2.194(2)
2.117(1)
2.030(1)
2.039(1)
2.046(1)
2.004(1)
2.137(1)
2.056(1)
2.033(1)
2.049(1)
2.030(1)
2.013(1)
2.151(1)
M1–O2 (Ni2–O11)
M1–O3 (Ni2–O12)
M1–O4 (Ni2–O13)
M1–O5 (Ni2–O14)
M1–N1 (Ni2–N3)
Bond angles
O5–M1–O2 (O14–Ni2–O11)
O5–M1–O4 (O14–Ni2–O13)
O2–M1–O4 (O11–Ni2–O13)
O5–M1–O1 (O14–Ni2–O10)
O2–M1–O1 (O11–Ni2–O10)
O4–M1–O1 (O13–Ni2–O10)
O5–M1–O3 (O14–Ni2–O12)
O2–M1–O3 (O11–Ni2–O12)
O4–M1–O3 (O13–Ni2–O12)
O1–M1–O3 (O10–Ni2–O12)
O5–M1–N1 (O14–Ni2–N3)
O2–M1–N1 (O11–Ni2–N3)
O4–M1–N1 (O13–Ni2–N3)
O1–M1–N1 (O10–Ni2–N3)
O3–M1–N1 (O12–Ni2–N3)
174.27(9)
93.54(9)
92.01(9)
88.98(8)
85.42(9)
176.50(8)
89.75(6)
91.61(7)
90.46(8)
91.99(8)
80.78(6)
98.78(7)
80.36(6)
97.66(7)
166.32(8)
172.34(6)
93.68(5)
92.76(6)
90.08(5)
83.60(6)
175.92(5)
93.19(6)
91.22(6)
87.58(5)
90.65(6)
83.62(5)
93.25(5)
81.52(5)
100.49(5)
168.39(6)
176.37(5)
88.97(5)
93.60(5)
90.02(6)
87.41(7)
178.98(6)
93.22(6)
89.39(6)
88.98(5)
91.19(6)
83.49(5)
94.37(6)
81.03(5)
98.74(5)
169.52(6)
a
b
M = Co in 1a · H2O and Ni in 2b · ½H2O; equivalent parameters in the second molecule of [Ni(p-
NO2Bnida)(H2O)3] unrelated by symmetry in 2b.
The crystal packing of 1a · H2O is governed by an extensive network of intermolecular
O–H⋅⋅⋅O hydrogen bonds which interconnect the molecules of 1a and water into bilayers
perpendicular to the crystallographic b axis (Fig. 2). Single layers can be described as a 2D
3
2
network of 1a molecules comprising two types of ring motives, R3 (16) and R3 (12), which
3
alternate along the c axis. The larger rings (R3 (16)) are additionally bridged by water
molecules which act as hydrogen bond donors to two coordinated carbonyl oxygen atoms of
one ring, and as an acceptor of a hydrogen bond with a coordinated water from a complex
10

molecule belonging to the second layer. This leads in a head-to-head interconnection of two
layers into a bilayer with all groups capable of hydrogen bonding in the centre and the
hydrophobic phenyl groups on the surfaces. Further interconnection of the bilayers is
accomplished only through weak C–H⋅⋅⋅π hydrogen bonds.
Fig. 2. a) Layers of molecules perpendicular to the crystallographic b axis in the crystal
structure of 1a · H2O, including the bridging water molecules; b) Bilayers formed by head-to-
head interconnection of layers in the crystal structure of 1a · H2O viewed along the
crystallographic c axis.
Similarly to 1a · H2O, the crystal structure of 2b · ½H2O also comprises hydrogen bond
bilayers, although with a substantially different topology than in 1a · H2O (Fig. 3). The single
2
4
layers here comprise three distinct ring motives, acentric R3 (8), R4 (20), and
2
centrosymmetric R2 (12). Uncoordinated water molecules here also bridge molecules of the
4
larger rings (R4 (20)). However, here they act both as hydrogen bond donors (to coordinated
carbonyl oxygen) and acceptors (from coordinated water molecules), leaving the second
hydrogen atom free to form a hydrogen bond with a coordinated carbonyl oxygen atom of a
molecule from the second layer. This, together with direct hydrogen bonding between 2b
molecules of the two layers (absent in 1a · H2O) forms head-to-head interconnected bilayers
with nitro-phenyl groups on the surface, similarly as in the crystal structure of 1a · H2O.
11

Fig. 3. a) Layers of molecules laying in the bc plane in the crystal structure of 2b · ½H2O,
including the bridging water molecules, viewed along the crystallographic a axis. b) Bilayers
formed by head-to-head interconnection of layers in the crystal structure of 2b · ½H2O viewed
along the crystallographic b axis.
3.3. Thermal properties of the complexes
Co(II) complexes start to lose their water molecules at lower temperatures than those
of Ni(II), especially 2a · ½H2O which starts to decompose immediately after being exposed to
either nitrogen or oxygen flow. The initial dehydration (co-crystallized water molecules) of all
four complexes is overlapped with further loss of the coordinated water molecules. The
corresponding endothermic peaks are observed in DTA curves at 96.8 °C (1a · H2O), 149.2
°C (1b · H2O), 112.5 °C (2a · ½H2O) and 144.9 °C (2b · ½H2O) in nitrogen atmosphere and at
87.3 °C (1a · H2O), 153.2 °C (1b · H2O), 117.1 °C (2a · ½H2O), 143.8 °C (2b · ½H2O) in
oxygen atmosphere, indicating the greater thermal stability of Ni(II) complexes. The
following complete degradation of the complexes starts at ca. 300 °C in nitrogen atmosphere
and 270 °C in oxygen atmosphere for all complexes except 2a · ½H2O, for which was
observed at lower temperatures (270 °C under nitrogen and 220 °C under oxygen). 2a · ½H2O
is therefore the least stable complex of this series while 1b · H2O and 2b · ½H2O showed to be
mutually similar concerning their thermal stability. TG/DTA curves for all four complexes are
given in the Supplementary material.
12

3.4. Geometry Optimization, binding energy calculations and population analysis
The experimentally and DFT determined bond lengths and bond angles for 2b at
mPW1PW91/6-311G(d,p) level of theory are displayed in Table 2. As it can be seen, high
discrepancies between experimentally determined and calculated bond lengths (highest
difference ≈ 0.1 Å) and bond angles (highest difference ≈ 19°) are obtained for optimization
in vacuum. Using the water as dielectric continuum, with addition of five H2O molecules
forming hydrogen bonds calculated and experimentally determined geometries come in much
better agreement [46, 50]. Even by neglecting hydrogen bonds DFT optimized geometry for
2b in water solution is still highly acceptable when compared to DFT optimized geometry in
vacuum (Table S3). Very good agreement between the experiment and DFT is also attained
for 1b in water (for models with and without hydrogen bonds, Tables 2, S4, S7).
Table 2. DFT optimized geometry for [Ni(p-NO2Bnida)(H2O)3] (2b) (Fig. 1.c) and for
[M(Bnida)(H2O)3]: 1b (M = Ni) (Fig. 1.b) and 1a (M = Co) (Fig. 1.a) with mPW1PW91/6-
311G(d,p)a. The optimizations were performed in vacuum (vac.) and in water solution (H2O).
[Ni(p-NO2Bnida)(H2O)3]
[Ni(Bnida)(H2O)3]
[Co(Bnida)(H2O)3]
expb
DFT
expb
DFT
expb
DFT
(vac.)
DFT
(H2O)c
2.126
2.082
2.064
2.084
2.010
2.010
95.7
(H2O)c
(H2O)c
−N1)/Å
d(M
d(M−O1)/Å
2.137
2.117
2.030
2.039
2.046
2.004
100.5
93.3
2.127
2.069
2.04
2.068
2.041
2.015
97.3
2.121
2.066
2.083
2.087
2.011
2.011
95.4
2.194
2.124
2.050
2.111
2.067
2.039
97.7
2.185
2.094
2.104
2.149
2.042
2.088
97.6
2.078
2.144
2.127
2.121
1.975
1.984
105.4
101.4
157.3
82.3
−O2)/Å
d(M
d(M−O3)/Å
−O4)/Å
d(M
d(M−O5)/Å
α(N1-M-O1)/°
α(N1-M-O2)/°
α(N1-M-O3)/°
α(N1-M-O4)/°
α(N1-M-O5)/°
α(O1-M-O5)/°
α(O3-M-O5)/°
α(O4-M-O5)/°
95.0
96.5
96.0
98.8
166.3
80.4
103.2
165.9
80.5
168.4
81.5
172.4
83.2
168.0
82.6
172.2
83.9
83.6
90.1
93.2
93.7
84.5
75.0
86.1
112.9
83.8
82.9
90.5
89.2
90.7
83.2
88.0
91.3
90.6
80.8
89.0
89.8
93.5
76.5
89.5
90.3
98.4
86.8
92.2
90.4
RMSD(b.l.)/Å d
0.0023 0.0467 0.0187
0.15 5.65 2.08
0.0033
0.18
0.0176
1.89
0.0028
0.17
0.0252
2.78
RMSD(b.a.)/°d
RMSD(b.l.*)/Åe 0.0010 0.0787 0.0313
0.0020
0.11
0.0222
2.50
0.0017
0.11
0.0373
3.36
RMSD(b.a.*)/° e
0.07 8.07 2.94
a Basis set used for all atoms except for Ni and Co where used basis was {611111111|51111|311|1}.
b Experimental results are for 2b · ½H2O, for 1b · H2O and for 1a · H2O and include many hydrogen bonds.
c For 2b and 1b models with additional 5 H2O and for 1a model with additional 7 H2O molecules attained lower
RMSD for bond lengths than models without hydrogen bonds in water solution (Tables S3-S8).
d Root-mean squared error for bond lengths (b.l./Å) and bond angles (b.a./°) with hydrogen atoms excluded.
e Root-mean squared error for bond lengths (b.l.*/Å) and bond angles (b.a.*/°) including metal atom only.
13

The selected DFT optimized geometry parameters for 1a (high-spin Co(II) complex)
are also shown in Tables 2, S5 and S8. RMSD for bond lengths corresponding to a low-spin
complex at the same level of theory is 0.067 Å. Single point energy evaluation revealed that
−5.6 kcal/mol for BP86, −19.1 kcal/mol for mPW1PW91)
high-spin state is more stable (
than
−3.5 kcal/mol
low-spin state for 1a as well as for 2a (
for BP86 and −19.0 kcal/mol for
mPW1PW91), which is in agreement with former studies [51]. In Table 2 are shown data for
1a · 7H2O although it differs almost negligibly in RMSD between the data for 1a (without
hydrogen bonded H2O molecules). Full geometry data for 2b, 1b and 1a with and without
additional water molecules are all given in supplementary tables S3-S8. When DFT optimized
geometries in water solution with and without hydrogen bonds are compared, the important
difference is that hydrogen bonds increase calculated M-O4 and M-O5 bond lengths, and
decrease M-O1, M-O2 and M-O3 bond lengths. Fig. 4 displays the comparison between
experimentally obtained and DFT optimized geometries for the complexes 1a, 1b and 2b.
Fig. 4. Comparison between experimentally obtained (red) and DFT optimized geometries
(green) for the complexes 1a, 1b and 2b.
Computed Ni-N1 bond orders are higher than Co-N1 bond orders in the corresponding
complexes (supplementary table S9). Ni-N1 and Co-N1 bond lengths are slightly longer for p-
NO2Bnida2− ligand and, consequently, of slightly lower bond order than in analogous
14

−
complexes with Bnida2 (Table S9). Binding energies follow the same trend regardless of the
level of theory used (Tables 3 and S10). The calculated stability order of these complexes (1b
> 2b > 1a > 2a) is mostly consistent with experimentally obtained for their thermal stability
(1b · H2O ≈ 2b · ½H2O > 1a · H2O > 2a · ½H2O) (section 3.3.). The explanation why 2b ·
½H2O showed similar thermal stability as 1b · H2O might be due to additional stabilization of
the second coordination sphere by extensive hydrogen bonding, which was neglected in our
calculations. This effect was obviously not sufficient to stabilize cobalt(II) complex 2a
because of decreased stability in its first coordination sphere. Since Co(II) complex 1a is
already less stable than its Ni(II) analogue 1b, this destabilizing influence of nitro group
might have greater impact on 2a than 2b.
Table 3. Binding energy between the metal center (M) (Ni2+ or Co2+) and one ligand (L)
−
−
(NO2Bnida2 or Bnida2 ) forming ML, using 6-311++G(2df,2pd)a basis set.
Ebind(ML) / kcal mol−1
Functional
2b
1b
2a
1a
BP86-D3
B97-D
−276.47 −284.13 −250.58 −258.26
−266.94 −269.56 −242.61 −245.08
a Basis set used for all atoms except for Ni and Co, where used basis was {6111111111|5111111|3111|11|1}.
3.5. Assignment and comparison of the experimental and calculated infrared spectra
The features observed in the experimental infrared spectra of these complexes are
consistent with their molecular structures and the calculated IR spectra. Infrared spectra of 1a
· H2O and 1b · H2O [16] are mutually very similar, as well as those of 2a · ½H2O and 2b ·
½H2O (Figs. 5 and 6). The positions of the bands in the spectra, however, differ between the
Co(II) and Ni(II) complex with the same ligand. The spectra of all four complexes are
characterized by the very strong band of antisymmetric stretching of the carboxylate ion,
νas(COO−) [52, 53] (the strongest band in the spectra), occurring mostly at ≈1620 cm−1. The
band is, however, shifted to remarkably lower value in the least stable complex 2a · ½H2O
(1607 cm−1). The bands of medium intensity, assigned to the symmetric stretching of the
carboxylate ion, νs(COO−), are observed at ≈1410 cm−1. The spectra of 2a · ½H2O and 2b ·
½H2O are also characterized by the presence of antisymmetric (νas(NO2) ≈ 1515 cm−1) and
symmetric stretching of the nitro group (νs(NO2) ≈ 1350 cm−1). The positions of these two
bands are much less dependent on the metal ion (Co/Ni) since nitro group does not participate
in metal coordination or hydrogen bonding and is also located far away from the coordination
15

sphere in the molecule of 2b (and presumably 2a). The peak positions of broad and strong
bands of v(O−H) are found at almost the same positions for both nickel(II) complexes 1b ·
H2O (3453 cm−1) and 2b · ½H2O (3456 cm−1), in contrary to their cobalt(II) analogues. The
band was found at 3464 cm−1 for 1a · H2O and 3435 cm−1 for 2a · ½H2O. The region below
500 cm−1 accounts for ring deformations, torsions and metal-ligand donor atoms vibrational
modes (e.g. stretching of Ni−N, Ni−O, Co−N and Co−O). The assignment of the most
important vibrational bands for all four compounds are given for both BP86 and mPW1PW91
in Supplementary Tables S11 and S12. The results for 1a and 1b with mPW1PW91 functional
by also taking into account hydrogen-bonded water molecules are shown in Table 4.
16

Fig. 5. The compared experimental IR spectra of 1a · H2O, 1b · H2O, 2a · ½H2O and 2b ·
½H2O.
Fig. 6. The compared calculated IR spectra with BP86/6-311G** of 1a, 1b, 2a and 2b
(without hydrogen bonds considered).
17

Table 4. Experimental and calculated harmonic frequencies for selected modes of 1a · 5 H2O
and 1b · 5 H2O (forming hydrogen bonds, Tables S7-S8 and Cartesian coordinates below)
1a · 5 H2O
mPW1PW91 a exp.
3600-3100 b 3496-3149 3600-3100 c 3525-3151
1b · 5 H2O
exp.
mPW1PW91 a mode
v(OH in H2O (H bonds))
vas(CH in CH2)
2925
2998-2981
2950-2932
1620, 1611
1588-1532
1470
2930
3013-2991
2963-2940
1632, 1610
1593-1535
1473
vs(CH in CH2)
1620
1580
1495
1458
1410
1384
1623
1581
1496
1458
1411
1389
vas(COO), dsci(H2O)
dsci(H2O)
v(ring), dinp(CH in ring)
dsci(CH2)
1426
1437
1405
1410
vs(COO), dsci(CH2), v(C–N)
vs(COO), doutp(CH2), dsci(CH2), v(C–C),
1405-1349
(max. at 1378)
1328
1410-1357
(max. at 1382) v(C−N),
1363
1327
1362
1330
1328
dwag(CH2)
1325-1300
1319-1306
dinp(CH in ring), v(C–N), dwag(CH2),
dtwi(CH2)
1265
1225
1205
1109
1094
1076
1027
972
1275-1227
1198
1273
1222
1206
1105
1092
1074
1028
974
1265-1257
1191
v(C−N), dtwi(CH2), dinp(CH in ring)
dtwi(CH2), v(C–C)
1183
1187
dinp(CH in ring), v(C–C)
d(CH in ring), doutp(CH2), v(ring)
v(C–N), doutp(CH2, ring, COOCH2)
v(C–N), dtwi(CH2), dinp(CH in ring)
v(ring), dinp(CH in ring)
1097
1088
1085
1083
1057
1060
1013
1016
980-961
932-921
978-959
951-926
doutp(ring, CH2, H2O), drock(CH2)
drock(CH2), doutp(CH in ring, COOCH2),
v(C–N)
942
945
921
828
762
706
629
515
908-901
809
922
821
765
707
632
522
914-902
808
doutp(ring, CH2, COOCH2), v(C–C)
doutp(ring), d(CCN)
755-745
730-707
629
751-749
721-708
640
doutp(ring, H2O), d(CCN)
doutp(ring, H2O), dsci(COO)
doutp(H2O),drock(H2O)
524-494
516-497
doutp(ring, H2O), drock(H2O), d(CCN)
a For mPW1PW91 functional, scaling factor of 0.957 was used for all obtained frequencies [54].
b Peak at 3464 cm−1.
c Peak at 3453 cm−1.
v − stretching, d – bending, dinp – in plane bending, doutp – out of plane bending, vs − symmetric stretching, vas −
antisymmetric stretching, dsci – scissoring, dtwi – twisting dwag – wagging, drock − rocking
18

As can be seen, the agreement between experimental and calculated frequencies is
quite satisfactory by taking into account hydrogen bonds and harmonic approximation. If all
hydrogen bonds were disregarded for 1a and 1b (Table S11), there would be a high
discrepancy between experimental and calculated O–H stretching vibration, and also a
slightly higher frequency (≈20-30 cm–1) of vas(COO) and dsci(H2O) modes would be observed.
For vs(COO) calculated frequency would be ≈20 cm–1 lower than in Table 4. But generally
even if hydrogen bonds are completely disregarded, the agreement between experimental and
calculated frequencies is still mostly satisfactory (see also Figures 5 and 6). The assignment of
the vibrational bands for 2a and 2b (Table S12) also shows a satisfactory agreement between
most experimental and calculated frequencies, especially for νs(NO2) at ≈ 1350 cm−1 and ν(C–
NO2) at ≈ 1090 cm−1. An additional frequency analysis on 2b · 5 H2O was also carried out
and (in comparison to 2b) shows similar improvements in agreement of v(O–H), vas(COO),
vs(COO) and dsci(H2O) modes between experiment and DFT as in former cases. The effect of
hydrogen bonds was also studied with 2b · H2O and 1b · H2O (Supplementary figure S1). The
results show that frequency for v(O–H) of the hydrogen bond is at 3481 cm–1 for 2b and 3476
cm–1 for 1b, which agrees with experimental frequencies (Fig. 5). Water molecule forming
hydrogen bond with COO– group slightly lowers the frequency of vas(COO) and dsci(H2O)
modes (on average by 10 cm–1), which is consistent with our former analyses.
4. Conclusions
Cobalt(II) and nickel(II) complexes with N-benzyliminodiacetic acid and N-(p-
nitrobenzyl)iminodiacetic acid were prepared and characterized by X-ray diffractometry,
infrared spectroscopy and thermoanalytical methods (TG/DTA), with the aid of DFT
calculations. All these complexes have analogous molecular structures with fac-configuration
of the chelators. Nickel(II) complexes with these ligands show greater thermal stability over
their cobalt(II) analogues. DFT calculated binding energies are also greater for nickel(II)
complexes.
The structural and vibrational spectroscopic study, enhanced by DFT calculations
allowed a successful assignment of the main vibrational modes, leading to a good agreement
between the theory and experiment. The calculated spectra and molecular geometry agree
very well with the experimental results despite the extensive hydrogen bonding frameworks
which are present in the crystal structures of these compounds. When hydrogen bonds are
taken into account for DFT calculations, the agreement between calculated and
experimentally determined frequencies is excellent.
19

Acknowledgements
This research was supported by the Ministry of Science, Education and Sports of the
Republic of Croatia (Grant No. 119-1193079-1332). The authors are grateful to Dr. Marijana
Vinković for the recorded NMR spectra.
Conflict of interest
The authors declare that they are not in a conflict of interest.
Appendix A. Supplementary material
DSC curves for the ligands (recorded in the nitrogen)
TG/DTA curves for the ligands (recorded in the nitrogen)
TG/DTA curves for the complexes (recorded in the nitrogen and oxygen)
Powder X-ray diffraction patterns (PXRD) for the complexes
Supplementary Tables S1-S12.
Supplementary Figure S1.
Cartesian coordinates for DFT optimized complexes 1a, 1a · 5H2O, 1a · 7H2O, 1b, 1b · H2O,
1b · 5H2O, 1b · 7H2O, 2a, 2b, 2b · H2O, 2b · 5H2O mostly in water at mPW1PW91/6-311G**
but some also in vacuum and/or with BP86/6-311G**; Cartesian coordinates are also given
for DFT optimized NO2Bnida2− and Bnida2− in water for both BP86-D3/6-311G** and B97-
D/6-311G**.
CCDC 1443097 and 1443098 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (or from
the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44(0)1223-336033; email: deposit@ccdc.cam.ac.uk).
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Graphical Abstract – Synopsis
Cobalt(II) and nickel(II) complexes with N-benzyliminodiacetic acid (BnidaH2) and N-(para-
nitrobenzyl)iminodiacetic acid (p-NO2BnidaH2), namely [Co(Bnida)(H2O)3] · H2O (1a · H2O),
[Co(p-NO2Bnida)(H2O)3] · ½H2O (2a · ½H2O) and [Ni(p-NO2Bnida)(H2O)3] · ½H2O (2b ·
½H2O) were characterized by IR spectroscopy and studied by DFT. Crystal structures were
determined by X-ray structural analysis.
25

Highlights
o Cobalt(II) and nickel(II) complexes were studied.
o The complexes were characterized by X-ray crystallography.
o O,N,O'-tridentate iminodiacetate ligands are bound in fac-configuration.
o The complexes were studied by DFT structural and vibrational calculations.
o Calculated geometry and IR spectra agree very well with experimental results.
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