Stereochemistry of Hexacoordinated Zn(II), Cu(II), Ni(II), and Co(II) Complexes with Iminodiacetamide Ligands

Article abstract of DOI:10.1021/acs.inorgchem.9b02200

Metal complexes of iminodiacetamide (imda) ligands and metal ions Zn(II), Cu(II), Ni(II), and Co(II) were prepared using eight imda ligands (L1-L8) substituted with groups of different steric and electronic properties on the central amine N atom (H atom,

Full text of DOI:10.1021/acs.inorgchem.9b02200

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Stereochemistry of Hexacoordinated Zn(II), Cu(II), Ni(II), and Co(II)
Complexes with Iminodiacetamide Ligands
†
†
†
‡
‡
́
́
́
̌
Natalija Pantalon Juraj, Goran I. Miletic, Berislav Peric, Zora Popovic, Neven Smrecki,
Robert Vianello,† and Srecko I. Kirin*
,†
́
†
̌
́
̌
Ruđer Boskovic Institute, Bijenicka c. 54, HR-10000 Zagreb, Croatia
‡Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia
S
* Supporting Information
ABSTRACT: Metal complexes of iminodiacetamide (imda) ligands and metal ions Zn(II), Cu(II), Ni(II), and Co(II) were
prepared using eight imda ligands (L1−L8) substituted with groups of different steric and electronic properties on the central
amine N atom (H atom, methyl, isopropyl, and benzyl) and the para position of the phenyl rings (nitro and dimethylamino).
The effect of these substituents on the stoichiometry (ML and ML2), geometry, and stereochemistry (mer, trans-fac, cis-fac) of
the complexes was studied in the solid state, in solution, and by density functional theory calculations. Single-crystal and powder
X-ray diffraction, thermogravimetry, and IR spectroscopy showed that in the solid state imda ligands preferentially form trans-
fac ML2 complexes, with the exception of the cis-fac complex 7Zn. NMR spectroscopy of diamagnetic Zn(II) and paramagnetic
Co(II) complexes revealed the formation of both ML and ML2 complexes in solution, which was also confirmed by UV−vis
titrations. Variable-temperature NMR was used to study the effect of the substituent on the central amine N atom on the Zn−N
bond strength and nitrogen inversion. The relative stabilities of the isomers were rationalized by computations and the
optimized structures used for geometry analysis.
Imda derivatives can serve as tridentate ligands for transition
metals that form hexacoordinated complexes with ML and
ML2 stoichiometry. Generally, for a coordination number of 6,
two coordination polyhedra are possible: octahedron and
trigonal prism.13,14 For octahedral ML2 complexes, different
geometrical isomers are known: mer, trans-fac, and Δ- or Λ-cis-
fac (Figure S2).15 This potential coordination variety of imda
complexes is in opposition to hexacoordinated EDTA
complexes that can only form a cis-fac isomer because of the
ethylene bridge linking the amine N atoms. By analysis of the
structures of ML2 complexes with imda ligands in the
Cambridge Structural Database, it is apparent that they
preferentially form trans-fac isomers. Out of the 23 known
[M(imda)2]2+ crystal structures, 22 are trans-fac isomers;16,17
the only reported cis-fac structure is with the parent H-imda
ligand.18 So far, metal complexes of imda ligands are
extensively characterized in the solid state, while the
characterization in solution is seldom reported.
INTRODUCTION
■
Aminopolycarboxylic acids, most notably ethylenediaminete-
traacetic acid (EDTA), diethylenetriaminepentaacetic acid, and
nitrilotriacetic acid (NTA), are some of the most widely used
chelating agents. Since the first syntheses of EDTA and NTA
were reported by Munz in the 1930s, aminopolycarboxylic
acids have found applications in contemporary research but
also in everyday life.1,2 For example, in the laboratory,
aminopolycarboxylic acids are used as metal chelators,3−5
while Ni-NTA agarose is used for the purification of proteins
via affinity chromatography (the His-tag method).6 In
addition, aminopolycarboxylic acids are widely used in a
number of industrial processes, including paper pulp bleaching,
and also as additives for household cleaning products like
laundry detergents and bathroom cleansers.1
Iminodiacetamide (imda) is an aminopolycarboxylic acid
derivative that can act as a tridentate O,N,O′-chelator, having a
structural motif similar to that of EDTA, with amide instead of
carboxylic groups. Until now, imda and their derivatives have
been studied as chelators for PET imaging,7,8 chelators for the
separation of lanthanides9 or mercury,10 and ionophores for
optical Zn-ion-selective sensors,11 and some showed antitumor
activity.12
In this work, we report on the synthesis and characterization
of eight imda ligands (L1−L8) and their Zn(II), Cu(II),
Ni(II), and Co(II) metal complexes (1Zn−8Zn). In order to
Received: July 23, 2019
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of Ligands L1−L8 and Complexes 1Zn−8Zn
a
Reaction conditions: (a) DIPEA, KI, DMF, microwave 50 W, 100 °C, 1 h, open vessel; (b) H2/Pd, 20 h; (c) 1/2MA2, M = Zn(II), Cu(II), Co(II),
−
−
Ni(II), A = BF4 , NO3 , methanol.
Figure 1. Crystal structures of trans-fac (1Zn) and cis-fac (7Zn) isomers. (For ORTEP diagrams and crystal packing of ligand L2 and other
complexes, see Figures S3−S15). H atoms are omitted for clarity.
increase the solubility and enable characterization of the metal
complexes in common organic solvents, terminal amides of the
imda ligands were substituted by phenyl groups. In addition,
starting from the unsubstituted ligand, electron-donating or
-withdrawing substituents were added to the imda system in
order to evaluate their influence on the stereochemistry,
stoichiometry, geometry, and energetics of the studied metal
complexes.
substitution with KI as the catalyst. The addition of KI
converts the chloroacetamides to more reactive iodoaceta-
mides, accelerating the reaction.20
The second synthetic step was initially attempted according
to a previously reported procedure,15,16 by nucleophilic
substitution in acetonitrile using potassium carbonate as a
base, resulting in a mixture of mono- and disubstituted amines.
Ligand L1 was obtained in moderate yield (41%) because of
low solubility of the intermediate, a monosubstituted amine in
acetonitrile, which precipitates from the reaction mixture and is
not available for further substitution.
By changing the reaction solvent to dimethylformamide
(DMF) and the base to N,N-diisopropylethylamine (DIPEA)
and carrying out the reaction in a microwave reactor, a higher
yield of L1 (87%) was obtained in a shorter reaction time.
Those conditions were successfully applied to prepare ligands
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of ligands was carried out in
several simple steps (Scheme 1). The first step was
chloroacetylation19 of aniline or its derivatives with electron-
donating [−N(CH3)2] or electron-withdrawing (−NO2)
groups (R) at the para position of the phenyl rings, obtaining
chloroacetamides P1−P3. The second step was nucleophilic
B
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a
Table 1. Selected Bond Lengths and Angles for Metal Complexes 1M−7M
complex
M−O1i (Å)
M−O2i (Å)
M−N2i (Å)
O1i−M−O2i (deg)
N2−M−N2i (deg)
1Zn
1Co
1Ni
2.0951(11)
2.0733(11)
2.0457(13)
2.3954(14)
2.4003(13)
2.022(2)
2.0325(14)
2.079(2)
2.133(3)
2.087(3)
2.122(3)
2.127(3)
2.081(3)
2.0944(11)
2.0749(11)
2.0452(12)
1.9409(13)
1.9726(13)
2.067(2)
2.0919(13)
2.024(2)
2.079(4)
2.112(4)
2.116(4)
2.112(4)
2.098(4)
2.1822(14)
2.1977(14)
2.1419(15)
2.0715(15)
2.0582(16)
2.154(2)
2.0743(16)
2.099(3)
2.164(4)
2.162(4)
2.142(4)
2.136(4)
2.161(4)
96.46(4)
96.63(4)
95.85(5)
96.22(5)
99.87(5)
90.02(9)
87.60(6)
92.72(9)
84.17(15)
83.11(14)
83.73(15)
83.30(14)
83.29(13)
84.97(16)
98.10(17)
99.54(17)
180.00
180.00
180.00
180.00
180.00
180.00
180.00
180.00
173.93(15)
1
1
Cu(1)
Cu(2)
3Ni
4nNi
5nNi
6
6
6
Zn(1)
Zn(2)
Zn(3)
178.92(16)
172.45(14)
103.53(19)
2.147(3)
2.116(4)
2.099(4)
2.098(3)
2.097(4)
2.094(4)
2.141(4)
2.140(5)
2.142(5)
7Zn
a
i = symmetry related atom for 1Zn−5nNi. These are related to a crystallographic center of symmetry located in the metal atom for 6Zn(1)−6Zn(3)
these are related by an approximate center of symmetry located in the metal atom.
L2−L6, having −Me, −iPr, or −Bn as R′ substituents on the
central amine N atom of the ligands and/or −H, −N(CH3)2,
or −NO2 groups at the para position of the two imda phenyl
rings, with yields of 44−79%.
Ligands L7 and L8 were prepared by catalytic hydrogenation
using Pd on C as a catalyst to remove the benzyl groups
attached to the central N atom from ligands L5 and L6.
Ligands L1−L8 were purified by flash chromatography and
characterized by 1H and 13C NMR, UV−vis, and IR
spectroscopy as well as electrospray ionization (ESI) and
matrix-assisted laser desorption/ionization (MALDI) mass
spectrometry (MS). NMR shifts of L1 were assigned using
heteronuclear single quantum coherence (HSQC) and
correlation (COSY) spectroscopy.
literature for other imda ligands.23,24 In the crystal structure of
L2, intermolecular hydrogen bonds contribute to the
formation of two-dimensional layers.
Ligand L1 was used as the representative case, having
substituents of moderate steric and electronic influence,
namely, isopropyl at the central amine N atom (R′ = iPr)
and unsubstituted phenyl rings (R = H). With L1, complexes
with Zn(II), Cu(II), Ni(II), and Co(II) were studied.
Complexes 1Zn, 1Co, and 1Ni are slightly distorted isostructural
octahedra. The ligands are bound trans-facially, with an angle
180° between the central amine N atoms and a ≈96° (O−M−
O) fac angle, in good agreement with the calculated value of
−95.2° (Table S10). Complexes 1Zn, 1Co, and 1Ni have two
methanol molecules in the crystal structure; the structure of
the bulk sample was in these cases confirmed by
thermogravimetry. The weight loss from 25 to 100 °C
corresponded to the loss of two methanol molecules from
the crystal structure, and after heating to 1000 °C, the
remaining residue was ZnO, CoF2, and NiF2 for 1Zn, 1Co, and
1Ni, respectively. The similarity of their IR (KBr) spectra
further confirms the isomorphism of 1Zn, 1Co, and 1Ni.
Thermograms, IR spectra, and powder X-ray diffractograms
are shown in the Supporting Information.
Metal complexes were prepared using Zn(II), Cu(II),
Co(II), and Ni(II) salts (Scheme 1). Tetrafluoroborate
−
−
(BF4 ) or nitrate (NO3 ) salts were used because of their
weak coordinating ability, to avoid interference with the
ligands being introduced into the metal coordination
sphere.15,21,22 The complexes were prepared by mixing boiling
methanol solutions of the ligand and metal salt, followed by
slow evaporation at room temperature (1Zn, 1Co, 1Ni, 1Cu, 3Ni,
4Zn, 4nNi, and 5nNi) or vapor diffusion of hexane (7Zn) or
diethyl ether (3Zn, 5Zn, and 6Zn). The complexes were obtained
in yields of 43−82% (Table S4).
The Cu complex 1Cu, however, is not isostructural to other
prepared complexes of L1. Because of Jahn−Teller distortion
characteristic for Cu(II) complexes, 1Cu is a distorted
octahedron, with elongated apical Cu−O2 bonds (2.395 and
2.400 Å). Two crystallographically independent complex
cations are present in the crystal structure with differences in
several characteristic torsional angles (for example, in torsional
angles defining the orientation of the phenyl rings; Table S2).
An interesting occurrence observed in 1Cu, and later in 7Zn, is
Solid-State Characterization. Single-crystal structures of
ligand L2, three Zn(II) complexes (1Zn, 6Zn, and 7Zn), four
Ni(II) complexes (1Ni, 3Ni, 4nNi, and 5nNi), one Co(II)
complex (1Co), and one Cu(II) complex (1Cu) were
determined. In all determined crystal structures, imda acts as
a tridentate ligand, forming complexes of ML2 stoichiometry.
Of the nine determined structures, eight are trans-fac isomers
and 7Zn is a cis-fac isomer. In Figure 1, 1Zn is shown as an
example of a trans-fac complex and 7Zn is the only cis-fac
complex.
For ligand L2, the solubility in common organic solvents is
generally very low; therefore, we did not obtain metal
complexes with L2. However, a single-crystal structure of L2
was determined (Figure S3). In the solid state, ligand L2
features intramolecular hydrogen bonding, forming an eight-
membered ring [graph set notation S(8)], also described in the
2−
−
the presence of the SiF6 anion, instead of the BF4 anion
used in the synthesis. In solution, the tetrafluoroborate anion
undergoes decomposition to a F− ion, and the F− anion then
reacts with SiO2 of the glass reaction vessel, forming
25,26
2−
SiF6
.
Trans-fac-ML2 complexes 3Ni, 4nNi, and 5nNi were
synthesized by adding methanol solutions of Ni(NO3)2,
followed by NaBF4, to the methanol solution of the ligand.
In 3Ni, BF4− was incorporated into the crystal structure, and in
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−
4nNi and 5nNi, NO3 was found in the crystal structure. The
phase purities of the bulk samples 3Ni and 4nNi were confirmed
by powder X-ray diffraction, while for 5nNi, a small amount of
the currently unidentified phase was present.
In the crystal structure of 6Zn, three crystallographically
independent complex cations are present. All three independ-
ent cations in 6Zn have only approximate centers of symmetry
located on the Zn atoms. However, some structural features,
like the orientation of the phenyl rings [from Phe−N−(CH3)2
groups] from opposite sides of molecules, differ significantly
from 0°, the value expected for centrosymmetric structures
(Table S3).
Conformations of five-membered chelate rings M−O1−
C7−C8−N2 or M−O2−C10−C9−N2, denoted as E for
“envelope” and T for “twisted” conformations, are given in
Table S2 for all chelate rings in all complexes. These values are
obtained by calculation of the least-squares plane of each ring
and deviations of the individual atoms.27
1
Figure 2. H NMR (CD3CN) titration of L1 with Zn(BF4)2.
The only isolated cis-fac isomer was 7Zn. The angle N−M−N
of 103.53(19)° is evidence that two N atoms occupy one
equatorial position and one axial position of the coordination
octahedron, unlike trans-fac isomers, where this angle is exactly
180° (for structures with crystallographic centers of symmetry)
or nearly 180° (for structures with approximate centers of
symmetry) (Table 1). The bond angles on the metal atom
show distortion from the ideal octahedral coordination.
Complex 7Zn has approximate C2 molecular symmetry, as
expected for cis-fac-ML2 isomers. Although complexes of C2
symmetry have a definite chirality, in the crystal structure of
An NMR titration of L1 with Zn(BF4)2 was performed to
study the stoichiometry of the formed complexes (Figure 2).
When Zn2+ is added, in addition to the initially present singlet
of the α-CH2 protons, two new doublets (4.02 and 3.65 ppm)
indicative of the formation of a [Zn(L1)2]2+ complex are
present. After further addition of Zn2+, two more doublets
appear at lower chemical shifts (3.88 and 3.49 ppm) that
correspond to the [Zn(L1)]2+ complex. In an excess of Zn2+,
[Zn(L1)]2+ is the dominant species in solution. In the NMR
spectra, the peaks of L1, [Zn(L1)2]2+, and [Zn(L1)]2+ are
separated, indicating the formation of stable complexes and a
slow exchange rate between the species compared to the NMR
time scale.29
7Zn, both enantiomers are present (space group symmetry P1
̅
;
Table S1).
Interactions governing crystal packing of the metal
complexes described in this work are of ionic character
because of the existence of charged cations and anions in all
structures. Additionally, some of the amide H atoms form
The effect of the R′ group on the Zn−N bond strength and
N inversion was studied for metal complexes of ligands with
1
different groups on the central amine N atom. H NMR
spectra were recorded for ML2 complexes of ligands L1 and
L3−L8 (Figure 3). For [Zn(L1)2]2+ and [Zn(L3)2]2+, two
sharp doublets were observed for the α-CH2 protons, while
one sharp singlet for [Zn(L4)2]2+, [Zn(L7)2]2+, and [Zn-
(L8)2]2+ or broad peaks for [Zn(L5)2]2+ and [Zn(L6)2]2+
were observed.
−
hydrogen bonds with F atoms from BF4 /SiF62− anions or with
O atoms from solvent molecules. The basic crystal packing
diagrams for all structures are given in Figures S3−S15. In
particular, in 7Zn, additional hydrogen bonds formed by the
central amine H atoms H21N and H22N and F atoms from
2−
1H NMR spectra were also recorded for different
stoichiometries at room temperature (Figure S21). Sharp
the SiF6 anion as acceptors contribute to the stability of the
cis-fac isomer in the solid state (Figure S16). It is interesting to
mention that, in the only previously described cis-fac-ML2
complex of the imda ligands,18 a similar stabilization
interaction with a perchlorate counterion was found.
Characterization in Solution. The stoichiometry and
stereochemistry of the metal complexes were studied in
1
solution. For diamagnetic Zn(II) complexes, H, 13C, two-
1
dimensional, and variable-temperature (VT) H NMR spectra
were recorded, either of isolated complexes or in situ with
different ratios of ligands L and Zn(II) salts dissolved in
1
deuterated acetonitrile. H NMR spectra were also recorded
for paramagnetic Co(II) complexes. UV−vis spectra were
recorded for all complexes and UV−vis titrations of Cu(II)
with ligands L1, L4, and L7 were performed.
1
In the H NMR (CD3CN) spectrum of the free ligand, the
α-CH2 protons are equivalent and show a singlet (3.37 ppm for
L1; Figure 2). When bound to the metal, the two α-CH2
protons are no longer equivalent, showing two doublets with
large geminal coupling (17 Hz for [Zn(L1)2]2+). Two doublets
indicate a mer or trans-fac isomer in solution, as opposed to the
four doublets (two for the axial and two for the equatorial α-
CH2 groups) that would be expected for a cis-fac isomer.15,28
1
Figure 3. H NMR (CD3CN) spectra of ligands L1 and L3−L8 and
Zn(BF4)2 in a 2:1 ratio at room temperature. The α-CH2 protons are
indicated (*). L2 was omitted because of low solubility.
D
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doublets of α-CH2 protons in ML complexes [Zn(L5)]2+,
[Zn(L6)]2+, and [Zn(L7)]2+ indicate a strong Zn−N bond,
while broad signals of the corresponding ML2 complexes show
a weaker bond and a Zn−N cleavage/coordination kinetics
faster than the NMR time scale.15,30 As the Zn−N bond
weakens, N inversion is enabled and α-CH2 protons are
magnetically equivalent. To further study this assumption, VT
NMR was recorded for ML and ML2 complexes of L1 and
Zn2+.
The activation energy for breaking the Zn−N bond and N
inversion in [Zn(L1)]2+ (ΔG⧧
= 15.3 kcal mol−1) was
higher than that for [Zn(L1)2]2+29(8ΔKG⧧298 K = 14.9 kcal mol−1;
Table S6), further supporting a stronger Zn−N bond in the
[Zn(L1)]2+ complex. This is not surprising because steric
crowding of the ligands in ML2 complexes lowers the strength
of the N−Zn bond relative to the corresponding ML
complexes. However, we should take into consideration that
a larger temperature span would be necessary for a more
accurate determination of the energy for [Zn(L1)]2+.
For [Zn(L3)2]2+, a temperature dependence of the α-CH2
protons similar to that for [Zn(L1)2]2+ is observed. Two
doublets are present at room temperature, and broadening of
the peaks is observed at 70 °C (Figure S32).
In VT NMR spectra of both [Zn(L1)2]2+ and [Zn(L1)]2+
complexes, changes of the α-CH2 peaks were observed, while
all other signals remained sharp, indicating that the exchange
process is limited to the α-CH2 group (Figure 4). For the
Different temperature dependences of the α-CH2 proton
signals were observed for ML2 complexes of ligands L4−L8,
and the results can be grouped according to the R′ substituent
on the amine N atom as follows: For R′ = H ([Zn(L7)2]2+,
[Zn(L8)2]2+), the α-CH2 peak is a singlet at room temperature
(Figures S28 and S29), indicating a weak Zn−N bond. The 1H
NMR spectrum of [Zn(L8)2]2+ at −40 °C shows only a slight
broadening of the α-CH2 singlet, indicating a somewhat
stronger bond, but to observe the separation of signals, spectra
should be recorded at a significantly lower temperature, which
is not possible because of technical reasons [mp(CD3CN) =
−46 °C]. For [Zn(L7)2]2+, it appears that the methylene
singlet broadens at 0 °C and separates into two peaks at −20
°C. The poor solubility of [Zn(L7)2]2+ at low temperature
should also be noted.
Complex [Zn(L4)2]2+ (R′ = Me) shows a sharp singlet for
the α-CH2 protons at room temperature, which slightly
broadens at −20 °C; at −40 °C, two species are present
(Figure S25).
Two complexes with R′ = Bn, [Zn(L5)2]2+ and [Zn-
(L6)2]2+, show similar temperature behaviors. At room
temperature, the methylene signals are two separate but
broad peaks. Cooling to 0 °C, the peaks sharpen into two
doublets, indicating a stronger Zn−N bond, and by further
cooling, the peaks start to broaden again because of the
formation of a second species (Figures S26 and S27). The
activation energy found experimentally for [Zn(L5)2]2+ is
lower compared to [Zn(L1)2]2+ and [Zn(L3)2]2+ (Figure
S37).
From the VT NMR studies, we found that the Zn−N bond
strength is influenced by the R′ substituent on the central
amine N atom. Our results suggest that the bond strength
decreases for ligands with different R′ substituents in the
following order: iPr > Bn > Me ≈ H, which is likely correlated
with the electron-donating ability of these substituents in the
same order (see also Computational Details section).
In the 13C NMR spectra of all ML2 complexes, peaks shifted
compared to the free ligand indicate complexation. Expectedly,
the largest shifts were observed for groups attached to the
amine N atom: 2.7 and 3.4 ppm downfield shifts for CH
groups of complexes of isopropyl derivatives L1 and L3 and
2.0 and 2.3 ppm upfield shifts for CH2 groups of benzyl
derivatives L5 and L6. The groups far from the donor atoms
showed the smallest shifts, less than 0.5 ppm shift of the
dimethylamino C atoms of ligands L3, L6, and L8.
Figure 4. VT NMR spectra of [Zn(L1)2]2+ (bottom, stoichiometry
1:2) and [Zn(L1)]2+ (top, stoichiometry 6:1).
[Zn(L1)2]2+ complex, peak broadening of α-CH2 was
observed at 50 °C and coalescence at 70 °C, while for
[Zn(L1)]2+, peak broadening occurred at 70 °C and
coalescence was expected to occur at significantly higher
temperatures, which could not be recorded because of the
boiling point of CD3CN (80.7 °C). These results agree with
the assumption that the Zn−N bond is stronger in [Zn(L1)]2+
than in [Zn(L1)2]2+.
The exchange rate constants for these processes were
determined by simulating spectra using the Mexico software as
part of the SpinWorks program31 and the energies of activation
calculated using the Eyring method.32,33 The simulated spectra
and activation energy determination are shown in Figures
S30−S37.
Paramagnetic 1H NMR spectra34,35 were recorded for
Co(II) complexes of ligands L1, L3, L4, and L7. For the
complex of L1 and Co(II) at a 2:1 ratio, the spectrum shows
the formation of a ML2 complex, with a small amount of
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signals of the free ligand (Figure 5). At a higher ratio of Co(II),
4:1, a mixture of the ML2 and ML complexes can be observed
phenyl rings, as well as with L10−L12, with 3,5-disubstituted
or pentasubstituted phenyl moieties. In addition, complexes
L13 and L14 with unsubstituted terminal amides were also
considered (Table 2). The bond lengths and angles of the
Table 2. Calculated Relative Stabilities of imda (R′/R)
a
Isomers (ΔGTOTAL/kcal mol−1)
complex
substituents
mer
trans-fac
cis-fac
[Zn(L1)2]2+
[Zn(L2)2]2+
[Zn(L3)2]2+
[Zn(L4)2]2+
[Zn(L5)2]2+
[Zn(L6)2]2+
[Zn(L7)2]2+
[Zn(L8)2]2+
[Zn(L9)2]2+
[Zn(L10)2]2+
[Zn(L11)2]2+
[Zn(L12)2]2+
[Zn(L13)2]2+
[Zn(L14)2]2+
iPr/H
0.5
2.6
0.0
0.0
0.0
0.0
0.8
0.3
2.0
1.4
5.4
7.2
1.2
0.0
0.0
0.0
0.7
5.0
4.6
3.7
1.1
1.6
0.0
0.0
0.0
2.1
0.0
5.2
1.9
0.5
3.7
3.8
2.8
3.3
0.0
0.0
1.3
0.5
1.1
0.0
0.5
2.8
iPr/NO2
iPr/NMe2
Me/H
Bn/H
Bn/NMe2
H/H
H/NMe2
tBu/H
iPr/(NO2)2
iPr/(NMe2)2
iPr/(CN)5
iPr/amide
Me/amide
Figure 5. Paramagnetic 1H NMR (CD3CN) of ligand L1 and
Co(BF4)2 in ratios 2:1 (red) and 1:4 (blue).
and, as in the NMR of Zn(II) complexes, an excess of metal
ions is necessary for formation of the ML complex. The same
was observed for L3; however, Co(II) complexes of L4 and L7
(Figures S39−S42) show only the ML species at a 4:1 ratio,
indicating that the equilibrium is shifted to ML already at
lower ratios of Co(II), possibly because of the weaker N−M
bond in complexes of ligands L4 and L7.
UV−vis titrations of Cu(CF3SO3)2 with L1, L4, and L7
show formation of the ML complex at approximately 1 equiv of
added ligand and the ML2 complex at approximately 2 equiv of
added ligand (Figures S43−S45).
Computational Analysis. The relative stabilities of mer,
trans-fac, and cis-fac isomers of the hexacoordinated [Zn-
(L)2]2+ complex cations with ligands L1−L8 were calculated in
an acetonitrile solution. Moreover, complex cations of several
model ligands L9−L14 were included (Figure 6), providing
additional structural and electronic information about the
studied systems. The aim was to gain insight into various
effects contributing to the stereochemical preference of the
ML2 complex, namely, the effect of (i) the central N
substituent, (ii) the presence of the amide N-phenyl moiety,
and (iii) the substitution pattern within the latter aromatic
fragment.
A total of 14 cations were studied using density functional
theory (DFT) M05-2X methodology, in line with our previous
work on metal complexes of the bis(2-picolyl)amine ligand.15
In particular, [ZnL2]2+ complexes with L1−L9 ligands were
calculated, having either unsubstituted or monosubstituted
a
Boldface numbers indicate the thermodynamically most stable
structures.
calculated complexes [Zn(L1)2]2+, [Zn(L6)2]2+, and [Zn-
(L7)2]2+ are in good agreement with the experimentally
determined 1Zn, 6Zn, and 7Zn (Table S14).
For the parent ligand L7, with an unsubstituted central
amine N atom and an amide N-phenyl fragment, the stability
of all three isomers is spanning a narrow range of only 1.1 kcal
mol−1, being the smallest range among all other systems
studied here. Calculations predict the cis-fac isomer of
[Zn(L7)2]2+ as the preferred structure, being 0.8 and 1.1
kcal mol−1 more stable than the analogous mer and trans-fac
isomers, respectively. The experimentally determined single-
crystal X-ray structure of 7Zn was also a cis-fac isomer although
2−
additionally stabilized by hydrogen bonding to the SiF6
anion.
Interestingly, substitution of the central amine N atom
reverts the stability trend among isomers in favor of some
other stereochemistry. Substitution of the amine H atom with
the methyl group, as in L4, promotes mer as the most stable
isomer, being much more stable than both cis-fac (3.8 kcal
mol−1) and trans-fac (5.0 kcal mol−1). This trend, although
with slightly smaller differences, is maintained in the benzyl
derivative L5 as well. Yet, attaching a stronger electron donor,
the isopropyl group as in L1, overcomes the stability of mer,
Figure 6. Ligands L9−L14 used in the calculations.
F
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Inorganic Chemistry
Article
making the trans-fac isomer 0.5 kcal mol−1 more stable. This
trend is further confirmed in the tert-butyl derivative (L9),
where trans-fac is as much as 2.0 kcal mol−1 more stable than
mer (and 1.3 kcal mol−1 than cis-fac), indicating that stronger
electron-donating N-alkyl groups promote stability of the trans-
fac stereoisomer. The latter is supported by the corresponding
Hammett substituent σp constants, being −0.09, −0.15, and
−0.20 for the Bn, iPr, and tBu groups, respectively.36
In addition, a lower stability of the cis-fac isomers in N-
substituted L1, L4, and L5 can also be looked at through steric
crowding around the amine N atom, where the introduced
substituents start to interfere with coordination around the Zn
cation and work toward favoring arrangements other than cis-
fac. For example, in the cis-fac isomer of [Zn(L7)2]2+, both
amino H atoms are 4.189 Å apart from each other and come
only within 3.35−3.40 Å from the nearest methylene
(−CH2−) H atoms from the neighboring L7 ligand. On the
other hand, substitution of these amino H atoms with the N-
methyl group moves methyl H atoms to the closest distance of
3.75 Å from each other and within 2.59 Å from the matching
methylene H atoms, which clearly destabilizes the cis-fac
isomer of [Zn(L4)2]2+. Such a steric hindrance is relieved in
the mer isomer, making it 3.8 kcal mol−1 more stable than the
analogous cis-fac isomer.
show that the complexes are somewhat distorted octahedra.
For both [Zn(L1)2]2+ and [Zn(L7)2]2+, the calculated
structures are more distorted than the experimental structures,
as a consequence of different conformations of the five-
membered chelate rings.
CONCLUSIONS
■
Eight imda ligands (L1−L8) were synthesized by microwave-
assisted nucleophilic substitution of primary amines with
chloroacetamide precursors. The imda ligands were used in
the preparation of metal complexes with divalent cations of Zn,
Cu, Ni, and Co. Single-crystal structures were determined for
one ligand (L2) and nine metal complexes. The solid-state
structures of metal complexes of ML2 stoichiometry show
trans-fac stereochemistry, with the exception of a cis-fac isomer
found for 7Zn (R, R′ = H).
NMR spectroscopy in acetonitrile was used to study selected
Zn(II) and Co(II) complexes with various stoichiometries.
Upon a comparison of the ML and ML2 complexes, a stronger
Zn−N bond was observed in the ML complex. On the basis of
the α-CH2 peak shape, the strength of the neighboring Zn−N
bond was found to decrease for ligands with different
substituents on the central N atom, in the order iPr > Bn >
Me ≈ H.
Although distant from the coordinating amide carbonyl
fragments, substitution within the amide N-phenyl moiety
exerts a notable influence on the stereochemical preference,
with a consistent observation that electron-donating groups
work toward the stability of the mer isomer, while electron-
withdrawing substituents promote the stability of the trans-fac
isomer. This is evident in, for example, the fact that electron-
withdrawing p-nitro groups increase the stability of trans-fac
over mer, from 0.5 kcal mol−1 in L1 to 2.6 kcal mol−1 in L2. On
the other hand, electron-donating p-dimethylamino groups
overcome the lower stability of mer in L1, making it 0.7 kcal
mol−1 more stable than trans-fac in L3. Even in L8, the
attached p-NMe2 group increases the stability of its mer
analogue, yet not enough to make it more stable than cis-fac.
Still, the relative difference between mer and trans-fac is
reduced from 0.8 kcal mol−1 in L7 to 0.3 kcal mol−1 in L8.
Unfortunately, the trend revealed is not maintained in di- or
pentasubstituted ligands L10−L12, where steric crowding
starts to predominate over the electronic influence and where
consistent structure−stereochemistry relationships become less
evident. For example, regardless of their electronic character,
both the dinitro- and bis(dimethylamino) substituents in L10
and L11 promote the stability of the corresponding trans-fac
isomers. Interestingly, the pentacyano substitution in L12
strongly promotes the stability of cis-fac, being 7.2 and 2.1 kcal
mol−1 more stable than its mer and trans-fac analogues.
Last, by analyzing the results for the unsubstituted amide
ligands L13 and L14, one concludes that the amide N-phenyl
group has only a modest effect on the preferred stereo-
chemistry. Removal of the aromatic fragments in L1 and L4
leaves the stereochemical preference intact, while their
presence allows the option to fine-tune the stereochemistry
through its substitution.
DFT calculations for three isomers (mer, trans-fac, and cis-
fac) of 14 Zn(II) complex cations with imda ligands employing
the implicit SMD acetonitrile solvation were applied to support
the experimental results. Analysis of both the experimental and
calculated structures using three different methods showed
distorted octahedral geometries. The results show that the
parent unsubstituted ligand L7 forms a cis-fac isomer, and
upon substitution on the central N atom, other orientations
prevail, mer with Me and Bn and trans-fac with the stronger
electron-donating iPr and tBu. In addition, computations
revealed that the presence of the N-phenyl group does not
have a significant impact on the stereochemistry, while
allowing options to fine-tune the desired stereochemistry
through the substitution, with an important conclusion that
electron-donating groups favor mer, while electron-withdraw-
ing substituents promote trans-fac orientations.
In this publication, we show that the stereochemical
preferences of [M(imda)2]2+ complexes can be greatly
influenced by substituent effects. Lessons learned herein can
serve as useful guidelines for the future design of organic
ligands if a preferred stereochemistry of their metal complexes
is desired. Recently, we have developed chiral catalysts
containing specific hydrogen-bonded secondary structures
originally found in ferrocene peptides (Figure 7, top
row).41,42 In those systems, the chirality can be transmitted
by “backdoor induction” from distant amino acids (Figure 7b)
and a metal that is only prochiral can selectively catalyze
asymmetric reactions.41 Similar secondary structures were
utilized to obtain specific supramolecular assemblies.43,44
Herein, complex 7Zn is reported as an example of a cis-fac-
ML2 isomer stabilized by supramolecular interactions (Figure
7c). Because 7Zn has no additional source of chirality, a
racemic mixture of both Δ and Λ cis-fac isomers is formed.
However, 7Zn could be substituted with chiral amino acids, and
if a linker of optimal length and flexibility is chosen, the
formation of chiral hydrogen-bonded structures would be
possible (Figure 7d). Such structures would allow a number of
interesting applications. Research along these lines is in
progress in our laboratories.
The structures of complexes [Zn(L1)2]2+ and [Zn(L7)2]2+
calculated by DFT and obtained by X-ray diffraction were used
to compare the geometry of different isomers (mer, trans-fac,
and cis-fac). The geometry was analyzed by three classification
methods37−40 (see the Experimental Section), and the results
are shown in Tables S7 and S8. All three classification methods
G
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Article
μL, 4.7 mmol), and KI (195.7 mg, 1.2 mmol); automated flash
chromatography, 0% → 5% methanol in dichloromethane, Rf = 0.36,
5% methanol in dichloromethane. Yield: 335.2 mg (1.03 mmol, 87%),
1
white powder. H NMR (300 MHz, CD3CN): δ 9.53 (s, 2H, HN),
7.65 (d, 4H, Ho, J = 7.8 Hz), 7.33 (t, 4H, Hm, J = 7.9 Hz), 7.09 (t, 2H,
Hp, J = 7.4 Hz), 3.37 (s, 4H, Hα), 3.00 (m, 1H, H1), 1.08 (d, 6H, H2, J
= 6.6 Hz). 13C NMR (150 MHz, CD3CN): δ 171.9 (Cβ), 139.8 (Ci),
129.8 (Cm), 124.6 (Cp), 120.4 (Co), 56.8 (Cα), 54.1 (C1), 18.9 (C2).
ESI-MS: m/z 348.1 (M + Na+, 14%), 326.1 (M + H+, 97%). MALDI-
HRMS. Calcd: m/z 326.1863 (C19H23N3O2 + H+). Found: m/z
326.1863. IR (KBr, cm−1): ν
̃
3442, 3199, 3132, 2965, 1686, 1654,
1600, 1552, 1499, 1313, 1248, 941, 752, 691, 503.
(p-O2N-Ph-imda)-iPr, L2. Conditions: isopropylamine (79.4 μL,
0.9 mmol), p-O2N-PhNH-COCH2Cl, P2 (500.0 mg, 2.3 mmol),
DIPEA (644.9 μL, 3.7 mmol), and KI (154.7 mg, 0.9 mmol);
automated flash chromatography, 0% → 5% methanol in dichloro-
methane, Rf = 0.26, 3% methanol in dichloromethane. Yield: 240.3 mg
(0.58 mmol, 62%), yellow powder. 1H NMR (300 MHz, CD3CN): δ
10.03 (s, 2H, HN), 8.24−8.17 (m, 4H, Hm), 7.94−7.85 (m, 4H, Ho),
Figure 7. Top view of a ferrocene amino acid (a), a rhodium
triphenylphosphine catalyst (b), metal complex 7Zn presented herein
(c), and a metal complex with a potential transmission of chirality (d).
The arrows indicate the sign of the helical chirality.
3.46 (s, 4H, Hα), 3.02 (m, 1H, H1), 1.08 (d, 6H, H2, J = 6.6 Hz). 13
C
EXPERIMENTAL SECTION
■
General Remarks. Reactions were carried out in ordinary
glassware, and chemicals were used as purchased from commercial
suppliers without further purification. Reactions were carried out in a
microwave reactor (CEM Discover). Reactions were monitored by
thin-layer chromatography on Silica Gel 60 F254 plates and detected
with a UV lamp (254 nm); ligands were purified using automated
flash chromatography (Teledyne Isco CombiFlash Rf) equipped with
a UV detector (254 nm) and prepacked silica columns. MS spectra
were recorded on a high-performance liquid chromatography
(HPLC)−MS system (Agilent Technologies 1200) coupled with a
6410 Triple-Quadrupole mass spectrometer, operating in a positive
ESI mode. NMR spectra were obtained on a Bruker Avance 300 or
600 spectrometer, operating at 300 or 600 MHz for 1H and 75 or 150
MHz for 13C. If not mentioned otherwise, the spectra were recorded
at room temperature. Chemical shifts, δ (ppm), indicate a downfield
NMR (75 MHz, CD3CN): δ 173.1 (Cβ), 145.6 (Ci), 144.1 (Cp),
125.9 (Cm), 119.8 (Co), 56.9 (Cα), 54.4 (C1), 18.9 (C2). ESI-MS: m/
z 831.2 (2M + H+, 5%), 416.1 (M + H+, 100%). MALDI-HRMS.
Calcd: m/z 416.1564 (C19H21N5O6 + H+). Found: m/z 416.1573. IR
(KBr, cm−1): ν
̃
3349, 3180, 3137, 2969, 1688, 1598, 1545, 1511,
1362, 1174, 1080, 841, 752, 665, 495, 403. Ligand L2 (41.5 mg) was
heated in 5 mL of methanol and left to cool at room temperature.
After 1 h, light-green prismatic crystals suitable for single-crystal X-ray
analysis were obtained.
(p-Me2N-Ph-imda)-iPr, L3. Conditions: isopropylamine (80.8 μL,
0.9 mmol), p-Me2N-PhNH-COCH2Cl, P3 (500.0 mg, 2.4 mmol),
DIPEA (650.9 μL, 3.8 mmol), and KI (156.1 mg, 0.9 mmol);
automated flash chromatography, 0% → 1% methanol in dichloro-
methane, Rf = 0.35, 5% methanol in dichloromethane. Yield: 171.4 mg
1
(0.42 mmol, 44%), white powder. H NMR (300 MHz, CD3CN): δ
1
shift from the residual solvent signal (1.94 ppm of CD3CN for H
9.25 (s, 2H, HN), 7.46−7.38 (m, 4H, Ho), 6.76−6.68 (m, 4H, Hm),
NMR; 118.26 ppm of CD3CN or 49.00 ppm of CD3OD for 13C
NMR). Coupling constants, J, are given in hertz. The paramagnetic
1H NMR spectra of Co(II) complexes were acquired with a spectral
window of 51020 Hz, a pulse width of 30°, and an acquisition time of
0.16 s with no relaxation delay.45,46 An exponential line broadening of
2 Hz was applied prior to Fourier transformation. UV−vis spectra
were recorded on a Cary 100 spectrophotometer, using HPLC-grade
acetonitrile. The UV−vis titrations were performed by adding the
ligand in the solution of Cu(II) to a solution of Cu(II) to avoid
dilution. Cu(CF3SO3)2 was chosen instead of Cu(BF4)2 because of
the hygroscopic nature of Cu(BF4)2. IR spectra were recorded using
KBr pellets with a Bruker Alpha FT-IR spectrometer, in the 4000−
350 cm−1 region. The powder diffractograms were measured on a
PANalytical Aeris instrument. Conditions: Bragg−Brentano geometry
(θ − 2θ), source Cu Kα (λ = 1.5418 Å), measurement from 5° to 70°
(2θ), with 5.2° min−1 (0.0216° step and 0.25 s step−1). The high-
resolution MS (HRMS) spectra were obtained with a MALDI time-
of-flight (TOF)/TOF instrument with α-cyano-4-hydroxycinnamic
acid as the matrix. Complexes 1Cu, 3Zn, 3Ni, 4Zn, 5Zn, 5nNi, 6Zn, 7Zn,
and 8Zn were desalted using Thermo Scientific Aspire RP30 desalting
tips. For complexes 1Cu and 7Zn, the molecular ion could not be
3.29 (s, 4H, Hα), 2.99 (m, 1H, H1), 2.87 (s, 12H, HN(CH ) ), 1.07 (d,
3
2
6H, H2, J = 6.6 Hz). 13C NMR (75 MHz, CD3CN): δ 171.0 (Cβ),
148.8 (Cp), 129.5 (Ci), 122.2 (Co), 113.8 (Cm), 56.5 (Cα), 54.0 (C1),
41.1 (CN(CH ) ), 18.8 (C2). ESI-MS: m/z 823.4 (2M + H+, 6%), 412.2
3
2
(M + H+, 100%). MALDI-HRMS. Calcd: m/z 412.2707
(C23H33N5O2 + H+). Found: m/z 412.2694. IR (KBr, cm−1): ν
3439, 3273, 3050, 2887, 2806, 1664, 1543, 1520, 1351, 1257, 818,
677, 522.
̃
(Ph-imda)-Me, L4. Conditions: methylamine hydrochloride (63.7
mg, 0.9 mmol), PhNH-COCH2Cl, P1 (400.0 mg, 2.4 mmol), DIPEA
(652.9 μL, 3.8 mmol), and KI (156.6 mg, 0.9 mmol); automated flash
chromatography, 0% → 10% methanol in dichloromethane, Rf = 0.32,
5% methanol in dichloromethane. Yield: 221.5 mg (0.74 mmol, 79%),
1
white powder. H NMR (300 MHz, CD3CN): δ 9.16 (s, 2H, HN),
7.64 (d, 4H, Ho, J = 7.9 Hz), 7.34 (t, 4H, Hm, J = 7.9 Hz), 7.10 (t, 2H,
Hp, J = 7.4 Hz), 3.32 (s, 4H, Hα), 2.48 (s, 3H, H1). 13C NMR (150
MHz, CD3CN): δ 170.0 (Cβ), 139.5 (Ci), 129.8 (Cm), 124.8 (Cp),
120.7 (Co), 62.4 (Cα), 43.8 (C1). ESI-MS: m/z 320.1 (M + Na+,
21%), 298.1 (M + H+, 59%). MALDI-HRMS. Calcd: m/z 298.1550
(C17H19N3O2 + H+). Found: m/z 298.1560. IR (KBr, cm−1): ν
̃
3449,
2−
3300, 3266, 2947, 1689, 1657, 1604, 1541, 1443, 1320, 1044, 854,
758, 692, 539, 507.
observed because of the difficulty of working with SiF6 salts.
The syntheses of the chloroacetamide precursors P1−P3 are
described in the Supporting Information.
(Ph-imda)-Bn, L5. Conditions: benzylamine (128.8 μL, 1.2
mmol), PhNH-COCH2Cl, P1 (500.0 mg, 2.9 mmol), DIPEA
(815.9 μL, 4.7 mmol), and KI (195.7 mg, 1.2 mmol); automated
flash chromatography, EtOAc/hexane gradient, Rf = 0.44, EtOAc:hex-
ane = 8:2, Rf = 0.33, 3% methanol in dichloromethane. Yield: 284.6
Synthesis of Ligands L1−L8, General Procedure. A mixture of
the amine (1 equiv), chloroacetamide (2.5 equiv), DIPEA (4 equiv),
and KI (1 equiv) in DMF (4 mL) was heated in a microwave reactor
for 1 h (50 W, 100 °C). The product was extracted into ethyl acetate
and washed three times with saturated NaHCO3 and brine, and the
organic layer was dried over anhydrous sodium sulfate, filtered, and
evaporated in a vacuum. The crude ligand was purified by automated
flash chromatography on a prepacked silica gel column (12 g).
(Ph-imda)-iPr, L1. Conditions: isopropylamine (100.4 μL, 1.2
mmol), PhNH-COCH2Cl, P1 (500.0 mg, 2.9 mmol), DIPEA (815.9
mg (0.76 mmol, 65%), colorless oil. 1H NMR (600 MHz, CD3CN): δ
9.29 (s, 2H, HN), 7.60 (d, 4H, Ho, J = 8.2 Hz), 7.43 (d, 2H, Ho1, J =
7.8 Hz), 7.36−7.30 (m, 6H, Hm, Hm1), 7.25 (t, 1H, Hp1, J = 7.3 Hz),
7.09 (t, 2H, Hp, J = 7.4 Hz), 3.89 (s, 2H, H1), 3.44 (s, 4H, Hα). 13C
NMR (75 MHz, CD3CN): δ 170.7, 139.5, 138.9, 130.3, 129.7, 129.4,
128.4, 124.7, 120.5, 60.3, 59.6. ESI-MS: m/z 396.1 (M + Na+, 12%),
H
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374.1 (M + H+, 100%). MALDI-HRMS. Calcd: m/z 374.1863
ν
̃
3640, 3542, 3336, 3104, 2981, 1644, 1598, 1567, 1500, 1452, 1327,
(C23H23N3O2 + H+). Found: m/z 374.1887. IR (KBr, cm−1): ν
̃
3450,
1082, 758, 692, 498, 448. UV−vis [CH3CN; λmax, nm (ε, M−1 cm−1)]:
201 (87464), 242 (54817).
3060, 3029, 1664, 1600, 1543, 1444, 1249, 1195, 754, 692, 504.
(Me2N-Ph-imda)-Bn, L6. Conditions: benzylamine (109.2 μL, 0.9
mmol), p-Me2N-PhNH-COCH2Cl, P3 (500.0 mg, 2.4 mmol),
DIPEA (650.9 μL, 3.8 mmol), and KI (156.1 mg, 0.9 mmol);
automated flash chromatography, 0% → 10% methanol in dichloro-
methane, Rf = 0.37, 5% methanol in dichloromethane. Yield: 262.4 mg
[Co(L1)2](BF4)2·2CH3OH, 1Co. Conditions: ligand L1 (64.7 mg,
0.2 mmol) and Co(BF4)2·6H2O (33.7 mg, 0.1 mmol). The vial was
partly covered and left in the fume hood for slow evaporation at room
temperature for 1 day. Yield: 57.9 mg (0.06 mmol, 61%), pink
crystals, suitable for single-crystal X-ray analysis. MALDI-HRMS.
(0.57 mmol, 61%), white powder. H NMR (300 MHz, CD3CN): δ
Calcd for C38H45N6O4Co+ (M − 2BF4 − H+): m/z 708.2823.
1
−
−1
Found: m/z 708.2846. IR (KBr, cm ): ν
̃
3534, 3362, 3068, 2972,
9.04 (s, 2H, HN), 7.47−7.23 (m, 9H, HAr) 7.78−6.69 (m, 4H, HAr),
3.85 (s, 2H, H1), 3.36 (s, 4H, Hα), 2.87 (s, 12H, HN(CH ) ). 13C NMR
1637, 1598, 1568, 1452, 1327, 1053, 759, 692, 499, 454. UV−vis
[CH3CN; λmax, nm (ε, M−1 cm−1)]: 201 (88853), 242 (54656), 442
(9.20), 478 (10.79), 510 (12.38), 530 sh (11.19), 607 sh (4.78).
[Ni(L1)2](BF4)2·2CH3OH, 1Ni. Conditions: ligand L1 (64.7 mg, 0.2
mmol), Ni(NO3)2·6H2O (30.1 mg, 0.1 mmol), and NaBF4 (22.3 mg,
0.2 mmol). The vial was partly covered and left in the fume hood for
slow evaporation at room temperature for 1 h. Yield: 77.4 mg (0.08
3
2
(75 MHz, CD3CN): δ 169.9, 148.9, 139.0, 130.2, 129.3, 129.2, 128.4,
122.3, 113.7, 60.3, 59.6, 41.0. ESI-MS: m/z 460.2 (M + H+, 100%).
MALDI-HRMS. Calcd: m/z 459.2634 (C27H33N5O2). Found: m/z
459.2633. IR (KBr, cm−1): ν
̃
3445, 3261, 3061, 2883, 2800, 1657,
1600, 1521, 1320, 1256, 982, 817, 744, 700, 519.
Catalytic Hydrogenation, General Procedure. To a solution
of L5 or L6 in methanol was added palladium on carbon (10%) as a
catalyst, and the reaction in a hydrogen atmosphere was performed
overnight. The product was filtered and evaporated in a vacuum. The
crude ligand was purified by automated flash chromatography on a
prepacked silica gel column (12 g), yielding products in the form of
white powder.
mmol, 82%), blue-green crystals, suitable for single-crystal X-ray
−
analysis. MALDI-HRMS. Calcd for C38H45N6O4Ni+ (M − 2BF4
−
H+): m/z 707.2845. Found: m/z 707.2867. IR (KBr, cm−1): ν
̃
3530,
3362, 3066, 2976, 1636, 1598, 1567, 1453, 1327, 1062, 692, 500, 458.
UV−vis [CH3CN; λmax, nm (ε, M−1 cm−1)]: 201 (101561), 242
(63564), 629 br (5.92).
[Cu(L1)2]2(BF4)2(SiF6)·2H2O, 1Cu. Conditions: ligand L1 (21.5
mg, 0.07 mmol) and Cu(BF4)2·H2O (7.84 mg, 0.03 mmol). The vial
was partly covered and left in the fume hood for slow evaporation at
room temperature for 4 days. Yield: 12.9 mg (0.01 mmol, 45%), pale
(Ph-imda)-H, L7. Conditions: L5 (459.6 mg, 1.23 mmol);
automated flash chromatography, 0% → 10% methanol in dichloro-
methane, Rf = 0.25, 5% methanol in dichloromethane. Yield: 271.0 mg
1
(0.96 mmol, 78%), white powder. H NMR (300 MHz, CD3CN): δ
green crystals, suitable for single-crystal X-ray analysis. IR (KBr,
9.05 (s, 2H, HN), 7.62 (d, 4H, Ho, J = 8.0 Hz), 7.33 (t, 4H, Hm, J =
7.9 Hz), 7.09 (t, 2H, Hp, J = 7.4 Hz), 3.42 (s, 4H, Hα). 13C NMR
(150 MHz, CD3CN): δ 171.0 (Cβ), 139.5 (Ci), 129.7 (Cm), 124.7
(Cp), 120.5 (Co), 53.9 (Cα). ESI-MS: m/z 306.1 (M + Na+, 28%),
284.1 (M + H+, 100%). MALDI-HRMS. Calcd: m/z 306.1213
−1
cm ): ν 3444, 3288, 3211, 3099, 2978, 1668, 1621, 1596, 1566,
1384, 1345, 1125, 1083, 754, 693, 520. UV−vis [CH3CN; λmax, nm
(ε, M−1 cm−1)]: 201 (81034), 242 (48218), 707 (39).
̃
[Zn(L3)2](BF4)2, 3Zn. Conditions: ligand L3 (18.3 mg, 0.04 mmol),
Zn(BF4)2·H2O (5.3 mg, 0.02 mmol). The vial was partly covered, left
in the fume hood for slow evaporation for 1 week, and then placed in
a tank with diethyl ether (10 mL) for diffusion for 2 weeks. Yield: 14.2
mg (0.01 mmol, 61%), white solid. 1H NMR (600 MHz, CD3CN): δ
9.26 (s, 4H), 7.33 (d, 8H, J = 8.6 Hz), 6.78 (d, 8H, J = 8.5 Hz), 3.96
(d, 4H, J = 16.5 Hz), 3.58 (d, 4H, J = 16.5 Hz), 3.42−3.27 (m, 2H),
2.92 (s, 24H), 1.29 (d, 12H, J = 6.2 Hz). 13C NMR (151 MHz,
CD3CN): δ 171.7, 149.2, 127.0, 123.2, 114.0, 57.4, 56.5, 41.2, 18.3.
MALDI-HRMS. Calcd for C46H65N10O4Zn+ (M − 2BF4− − H+): m/z
(C16H17N3O2 + Na+). Found: m/z 306.1220. IR (KBr, cm−1): ν
̃
3373,
3281, 3056, 2877, 1656, 1599, 1530, 1442, 1300, 1151, 754, 692, 560,
485.
(p-Me2N-Ph-imda)-H, L8. Conditions: L6 (260.0 mg, 0.57
mmol); automated flash chromatography, 0% → 5% methanol in
dichloromethane, Rf = 0.16, 5% methanol in dichloromethane. Yield:
77.3 mg (0.21 mmol, 37%), white powder. 1H NMR (600 MHz,
CD3CN): δ 8.75 (s, 2H, HN), 7.43−7.38 (m, 4H, Ho), 6.75−6.70 (m,
4H, Hm), 3.36 (s, 4H, Hα), 2.88 (s, 12H, HN(CH ) ). 13C NMR (75
885.4471. Found: m/z 885.4482. IR (KBr, cm−1): ν
̃
3318, 3082, 2980,
3
2
MHz, CD3CN): δ 170.2 (Cβ), 148.9 (Cp), 129.2 (Ci), 122.4 (Co),
113.8 (Cm), 53.9 (Cα), 41.0 (CN(CH ) ). ESI-MS: m/z 739.3 (2M +
2889, 2807, 1633, 1522, 1325, 1083, 819, 520. UV−vis [CH3CN;
λmax, nm (ε, M−1 cm−1)]: 198 (110255), 276 (76340).
3
2
H+, 26%), 392.1 (M + Na+, 13%), 370.1 (M + H+, 100%). MALDI-
[Ni(L3)2](BF4)2, 3Ni. Conditions: ligand L3 (17.4 mg, 0.04 mmol),
Ni(NO3)2·6H2O (6.1 mg, 0.02 mmol), and NaBF4 (4.6 mg, 0.04
mmol). The vial was partly covered and left in the fume hood for slow
evaporation for 1 day. Yield: 15.1 mg (0.02 mmol, 82%), light-blue
platelike crystals suitable for single-crystal X-ray analysis. MALDI-
HRMS. Calcd: m/z 370.2237 (C20H27N5O2 + H+). Found: m/z
370.2260. IR (KBr, cm−1): ν
̃
3442, 3272, 2922, 2800, 1656, 1639,
1535, 1522, 1382, 1353, 948, 817, 602, 521.
Synthesis of Metal Complexes (ML2), General Procedure.
Saturated methanol solutions of the ligand (2 equiv) and metal salt (1
equiv) were heated and briefly boiled in separate beakers until
completely dissolved. The metal salt solution was added to the ligand
solution, and the mixture was cooled to room temperature and left
partially covered for slow evaporation until crystals appeared (1 h to 1
month). The solvent was decanted, and the crystals were washed with
diethyl ether (2 × 2 mL) and air-dried. Complexes that did not
crystallize by the method of slow evaporation were placed in a tank
with hexane or diethyl ether for slow diffusion.
[Zn(L1)2](BF4)2·2CH3OH, 1Zn. Conditions: ligand L1 (64.7 mg,
0.2 mmol) and Zn(BF4)2·H2O (23.8 mg, 0.1 mmol). The vial was
partly covered and left in the fume hood for slow evaporation at room
temperature for 1 week. Yield: 49.0 mg (0.05 mmol, 51%), colorless
crystals, suitable for single-crystal X-ray analysis. 1H NMR (300 MHz,
CD3CN): δ 9.34 (s, 4H, HN), 7.48 (d, 8H, Ho, J = 8.0 Hz), 7.40 (t,
8H, Hm, J = 7.8 Hz), 7.25 (t, 4H, Hp, J = 7.3 Hz), 4.02 (d, 4H, Hα, J =
17.0 Hz), 3.65 (d, 4H, Hα, J = 16.8 Hz), 3.47−3.35 (m, 2H, H1), 1.31
(d, 12H, H2, J = 6.5 Hz). 13C NMR (151 MHz, CD3CN): δ 173.0
(Cβ), 136.8 (Ci), 130.2 (Cm), 127.3 (Cp), 122.1 (Co), 57.8 (Cα), 56.8
(C1), 18.4 (C2). MALDI-HRMS. Calcd for C38H45N6O4Zn+ (M −
2BF4− − H+): m/z 713.2783. Found: m/z 713.2889. IR (KBr, cm−1):
HRMS. Calcd for C46H65N10NiO4 (M − 2BF4 − H+): m/z
+
−
−1
879.4533. Found: m/z 879.4570. IR (KBr, cm ): ν 3441, 3060, 2975,
̃
1626, 1521, 1384, 1083, 946, 818, 532, 523. UV−vis [CH3CN; λmax
,
nm (ε, M−1 cm−1)]: 200 (101245), 283 (67665), 587 (12.24), 634
(11.68).
[Zn(L4)2](BF4)2, 4Zn. Conditions: ligand L4 (22.3 mg, 0.07 mmol)
and Zn(BF4)2·H2O (9.0 mg, 0.04 mmol). The vial was partly covered
and left in the fume hood for slow evaporation at room temperature
for 3 days. Yield: 25.1 mg (0.03 mmol, 80%), colorless platelike
crystals. 1H NMR (300 MHz, CD3CN): δ 9.47 (s, 2H), 7.56 (d, 4H, J
= 8.0 Hz), 7.39 (t, 4H, J = 7.9 Hz), 7.24 (t, 2H, J = 7.4 Hz), 3.94 (s,
4H), 2.64 (s, 3H). 13C NMR (75 MHz, CD3CN): δ 171.8, 137.1,
130.2, 127.2, 121.9, 61.0, 45.0. MALDI-HRMS. Calcd for
−
C34H37N6O4Zn+ (M − 2BF4 − H+): m/z 657.2157. Found: m/z
657.2151. IR (KBr, cm−1): ν
̃
3335, 3158, 3103, 1635, 1598, 1566,
1499,1453, 1321, 1083, 757, 691, 504, 478. UV−vis [CH3CN; λmax
,
nm (ε, M−1 cm−1)]: 200 (120598), 242 (67553).
[Ni(L4)2](NO3)2, 4nNi. Conditions: ligand L4 (59.5 mg, 0.2 mmol)
and Ni(NO3)2·6H2O (30.1 mg, 0.1 mmol). The vial was partly
covered and left in the fume hood for slow evaporation at room
temperature for 1 day. Yield: 59.4 mg (0.07 mmol, 71%), blue
I
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
crystals, suitable for single-crystal X-ray analysis. MALDI-HRMS.
dissolved in 1 mL of the L1 solution [c(Zn2+) = 83.7 mM] to avoid
dilution.47 The ligand solution (600 μL) was placed in an NMR tube,
and the spectrum of the free ligand was acquired. For each following
measurement, an aliquot of Zn2+ in the L1 solution was added until
the ratio of Zn2+/L1 was 3:1.
Paramagnetic Co(II) NMR. NMR spectroscopy is mainly used for
the characterization of diamagnetic compounds. However, by
adjustment of the acquisition parameters, the spectra of paramagnetic
species can be recorded, giving valuable information about the
structure of the sample. The observed shift in the paramagnetic
complexes is a sum of the hypothetical shift of the isostructural
diamagnetic complex and the paramagnetic contribution, which is
further divided into contact (scalar) and dipolar (pseudocontact)
shifts (eq 1).48,49
Calcd for C34H37N6O4Ni+ (M − 2BF4− − H+): m/z 651.2219. Found:
m/z 651.2229. IR (KBr, cm−1): ν
̃
3432, 3216, 3063, 2927, 1634,
1597, 1566, 1499, 1452, 1384, 1325, 1042, 964, 900, 767, 751, 690,
585, 499, 455, 429. UV−vis [CH3CN; λmax, nm (ε, M−1 cm−1)]: 200
(97404), 243 (59902), 605 (7.86).
[Zn(L5)2](BF4)2, 5Zn. Conditions: ligand L5 (19.0 mg, 0.05 mmol)
and Zn(BF4)2·H2O (6.1 mg, 0.02 mmol). The vial was partly covered,
left in the fume hood for slow evaporation for 1 week, and then placed
in a tank with diethyl ether (10 mL) for diffusion for 1 week. Yield:
15.1 mg (0.02 mmol, 60%), colorless platelike crystals. 1H NMR (300
MHz, CD3CN): δ 9.59 (s, 4H), 7.67−7.15 (m, 30H), 4.40−3.48 (m,
12H). 13C NMR (75 MHz, CD3CN): δ 171.8, 137.1, 133.2, 130.2,
129.8, 127.2, 122.1, 58.3, 57.1. MALDI-HRMS. Calcd for
−
C46H45N6O4Zn+ (M − 2BF4 − H+): m/z 809.2783. Found: m/z
809.2825. IR (KBr, cm−1): ν
̃
3442, 3068, 1632, 1566, 1082, 751, 706,
δTexp = δTpara + δdia = δTcon + δTdip + δdia
(1)
486. UV−vis [CH3CN; λmax, nm (ε, M−1 cm−1)]: 201 (98950), 243
(54132).
The contact shift, δcTon, describes the influence of the unpaired
electron spin on the nuclear chemical shifts due to through-bond
hyperfine coupling. As the number of bonds between the examined
nucleus and paramagnetic metal ion increases, the value of the contact
shift decreases and becomes negligible if the number of bonds
between the paramagnetic metal ion and nucleus is greater than four
and there are no π bonds. The dipolar shift, δdTip, describes the
through-space interaction of the magnetic moments of the unpaired
electron and nucleus and can be observed at distances of up to 60 Å
from the paramagnetic ion.
[Ni(L5)2](NO3)2, 5nNi. Conditions: ligand L5 (16.2 mg, 0.04
mmol) and Ni(NO3)2·6H2O (6.32 mg, 0.02 mmol). The vial was
partly covered and left in the fume hood for slow evaporation at room
temperature for 10 days. Yield: 8.4 mg (0.01 mmol, 43%), light-blue
platelike crystals, suitable for single-crystal X-ray analysis. MALDI-
+
−
HRMS. Calcd for C46H45N6NiO4 (M − 2BF4 − H+): m/z
803.2845. Found: m/z 803.2845. IR (KBr, cm−1): ν
̃
3441, 3219, 3052,
1628, 1384, 1327, 761, 702. UV−vis [CH3CN; λmax, nm (ε, M−1
cm−1)]: 199 (131925), 243 (67545), 595 (11.35).
[Zn(L6)2](BF4)2, 6Zn. Conditions: ligand L6 (26.5 mg, 0.06 mmol)
and Zn(BF4)2·H2O (6.9 mg, 0.03 mmol). The vial was partly covered,
left in the fume hood for slow evaporation for 1 week, and then placed
in a tank with diethyl ether (10 mL) for diffusion for 1 week. Yield:
20.6 mg (0.02 mmol, 61%), colorless platelike crystals, suitable for
single-crystal X-ray analysis. 1H NMR (300 MHz, CD3CN): δ 9.37 (s,
4H), 7.46 (s, 10H), 7.39 (d, 8H, J = 9.1 Hz), 6.72 (d, 8H, J = 9.1 Hz),
4.01 (s, 4H), 3.97−3.43 (m, 8H), 2.89 (s, 24H). 13C NMR (151
MHz, CD3CN): δ 170.4, 150.0, 133.2, 130.1, 129.7, 126.1, 123.3,
113.2, 58.0, 56.7, 40.6. MALDI-HRMS. Calcd for C54H65N10O4Zn+
X-ray Crystallography. The X-ray intensity data were collected
on an Oxford Diffraction Xcalibur CCD diffractometer using
monochromatic Cu Kα (λ = 1.54184 Å) radiation. For temperature
conditions, see Table S1. The data were processed with the
CrysalisPro program50 (unit cell determination and data reduction).
Because of the absence of several symmetry-independent reflections at
higher angles, the data up to θmax = 65° were used in calculations for
the crystals of 1Zn, 1Co, 1Ni, and 5nNi. The crystal of 7Zn diffracted up
to θmax = 62°. The structures were solved by direct methods with the
SIR2011 program51 and refined against F2 on all data by a full-matrix
least-squares procedure with the SHELXL-97 program.52 The
exception was the structure of 6Zn, where each least-squares cycle
was separated into five subcycles (BLOC instruction) because of the
large number of parameters in the structure (three symmetrically
nonequivalent [Zn(L6)2]2+ complex cations and six symmetrically
−
(M − 2BF4 − H+): m/z 981.4471. Found: m/z 981.4503. IR (KBr,
cm−1): ν
̃
3431, 2888, 1627, 1521, 1082, 816, 747, 703, 520. UV−vis
[CH3CN; λmax, nm (ε, M−1 cm−1)]: 199 (118088), 277 (80949).
[Zn(L7)2](SiF6)·4CH3OH, 7Zn. Conditions: ligand L7 (22.2 mg,
0.08 mmol) and Zn(BF4)2·H2O (9.4 mg, 0.04 mmol). The vial was
placed in a tank with hexane (10 mL) for diffusion for 1 month. Yield:
21.5 mg (0.03 mmol, 71%), colorless crystals, suitable for single-
crystal X-ray analysis. 1H NMR (300 MHz, CD3CN): δ 9.21 (s, 4H),
7.56 (d, 8H, J = 7.9 Hz), 7.37 (t, 8H, J = 7.8 Hz), 7.20 (t, 4H, J = 7.4
Hz), 3.80 (s, 8H). 13C NMR (75 MHz, CD3CN): δ 172.9, 137.7,
−
nonequivalent BF4 anions). In first subcycle, only the position and
displacement parameters for residue 1 were refined (first [Zn(L6)2]2+
complex); in the second (third) subcycles, similar parameters for
residue 2 (3) were refined (second and third [Zn(L6)2]2+ complexes,
respectively); in fourth subcycle, the position and displacement
parameters for BF4− anions were refined (residues 4−9); in last (fifth)
subcycle, only the position parameters for all atoms were refined.
These five subcycles were repeated until an overall convergence of all
parameters in the structure was reached. All non-H atoms in all
structures were refined anisotropically, and rigid group restraints
(DELU) were applied only for atoms in the structure of 6Zn because
130.1, 126.5, 121.5, 52.8. IR (KBr, cm−1): ν
̃
3443, 3344, 1641, 1564,
1084, 756, 691. UV−vis [CH3CN; λmax, nm (ε, M−1 cm−1)]: 201
(90105), 242 (57136).
[Zn(L8)2](BF4)2, 8Zn. Conditions: ligand L8 (12.85 mg, 0.04
mmol) and Zn(BF4)2·H2O (4.2 mg, 0.02 mmol). The vial was partly
covered, left in the fume hood for slow evaporation for 1 week, and
then placed in a tank with diethyl ether (10 mL) for diffusion for 1
week. No crystallization occurred, so the product was evaporated to
dryness. Yield: 10.9 mg (0.01 mmol, 64%), light-green solid. 1H NMR
(600 MHz, CD3OD): δ 7.40 (d, 8H, J = 9.0 Hz), 6.76 (d, 8H, J = 9.0
Hz), 3.58 (s, 8H), 2.90 (s, 24H). 13C NMR (151 MHz, CD3OD): δ
171.6, 150.2, 127.8, 123.2, 114.0, 52.7, 41.0. MALDI-HRMS. Calcd
for C40H53N10O4Zn+ (M − 2BF4− − H+): m/z 801.3532. Found: m/z
−
of the large number of parameters. Also, in the structure of 6Zn, BF4
anions were treated as rigid bodies of ideal tetrahedral symmetry with
a B−F bond length of 1.36 Å. Additionally, in the structure of 6Zn, the
two solvent-accessible voids of volume 600 Å3 (forming infinite
cylindrical shapes) contained unresolved solvent contributions. The
electron densities from these voids were treated by the SQUEEZE
option in the PLATON program,53,54 and the contributions of 202
electrons per void (as found by the SQUEEZE calculation) were
removed from the observed intensities. The final refinement of the
801.3640. IR (KBr, cm−1): ν
̃
3432, 2922, 1631, 1522, 1323, 1083,
818, 731, 522, 477. UV−vis [CH3CN; λmax, nm (ε, M−1 cm−1)]: 199
(101071), 278 (77958).
−
rest of the structure (three [Zn(L6)2]2+ complexes and six BF4
In Situ NMR Measurements. Complexes were prepared by
dissolving the ligand and metal salt in different ratios in approximately
0.6 mL of deuterated solvent in an NMR tube. For the preparation of
ML2 complexes, the ratio was 2:1 L/M2+, and for the preparation of
ML complexes, the ratio was 1:4−6 L/M2+.
NMR Titrations. Ligand L1 (5.4 mg) was dissolved in 2 mL of
CD3CN [c(L1) = 8.3 mM], and Zn(BF4)2·H2O (20 mg) was
anions) was performed without contributions from these voids.
During the refinement of structure 7Zn, orientational disorder of
one phenyl ring (with an attached N−H group) from one of the
ligands coordinated to the Zn atom was observed. The electron
density was modeled as two identical parts, and the occupation
parameter for each part was refined, with the constraint of the sum of
the occupations is 1. The phenyl rings in these parts were treated as
J
DOI: 10.1021/acs.inorgchem.9b02200
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
perfect hexagons with C−C bond lengths of 1.39 Å. C−N bond
lengths, which included disordered N atoms, were restrained to be
equal to other chemically identical bond lengths in the molecule
(SADI restraint), while the displacement parameters were restrained
to behave isotropically (ISOR restraint). Amide H atoms on all
compounds were refined freely and isotropically; the exceptions were
the structures of 6Zn and 7Zn, which were calculated from the position
parameters of atoms on which they are bonded, assuming ideal sp2
hybridization, a N−H bond length of 0.86 Å, and U(H) = 1.2Ueq(N)
(HFIX 43). Secondary amine H atoms bonded to coordinated N
atoms in the structure 7Zn were also calculated, assuming ideal sp3
hybridization, a N−H bond length of 0.91 Å, and U(H) = 1.2Ueq(N)
(HFIX 13).
the s/h ratio in the case of a regular octahedron is 1.22. The trigonal
prism has a compression ratio (s/h) uniformly close to 1.00 (range
0.96−1.04).37
The second method B38 defines three geometrical parameters
(Scheme 3): the θ angle between the triangle plane (basis) and the
Scheme 3. Method B for the Classification of
Hexacoordinated Polyhedra with Three Parameters: θ, ρ,
and ω38
All H atoms bonded to C atoms were included in the structural
model at geometrically calculated positions. For methyl H atoms, the
torsional angles around the C−C (N−C) bonds were refined (HFIX
137), except in the structure 6Zn, where the staggered conformation
with respect to the second N−C(Phe) bond was used (HFIX 33).
Hydroxyl H atoms on several methanol solvent molecules were
constrained to an ideal distance of 0.96 Å and a C−O−H angle of
109°, while their torsions with respect to the C−O bond were refined
(HFIX 147). Additional distance restraints with the nearest possible
acceptor atoms were used. During refinement, it was observed that
−
anisotropic displacement parameters in BF4 anions were unusually
large, especially in the structure 6Zn. Such disorder is very often in
structures containing these anions.55 Their occupancies were not
refined (they were constrained to 1) because this would violate the
charge neutrality of the structures. All details for X-ray diffraction
studies in this publication are collected in Table S1.
Computational Details. All molecular geometries were opti-
mized using a very efficient DFT M05-2X/6-31+G(d)/LanL2DZ +
ECP model, known to be successful in reproducing geometries, dipole
moments, and hemolytic bond energies in various Zn complexes.56,57
To account for the effect of the acetonitrile solution, during geometry
optimization, we included the implicit SMD solvation model (ε =
35.688), being in line with our earlier results.15 Thermal corrections
were extracted from the corresponding frequency calculations, and all
of the presented results corresponded to differences in the Gibbs free
energies. This was followed by calculation of the energies of the
excited states responsible for the experimental UV−vis spectra of
several Zn complexes through time-dependent DFT computations at
the same level of theory, considering the 32 lowest singlet electronic
excitations. The choice of this setup was prompted by its recent
success in modeling UV−vis spectra of organic heterocycles and their
metal complexes in various solvents.58,59 All calculations were
performed using the Gaussian 16 software.60 Cartesian coordinates
for all computed molecules are collected in a single text file readable
by the Mercury program (version 3.3 or later).61
plane defined by one atom in the triangle plane, metal ion, and atom
in the lower triangle plane regarded as “analogous” to the atom in the
above triangle plane, the ρ angle between two “bonds” of “trans”
atoms in two triangle planes to the metal ion, and the third parameter
ω dependent on whether the polyhedron in question is regarded as an
octahedron or trigonal prism. If the polyhedron is regarded as an
octahedron, then ω is the angle between two opposite planes. If it is
regarded as a trigonal prism, then ω is an angle between neighboring
planes. Therefore, in this scheme, it is necessary to assume the type of
polyhedron before calculating the ω parameter, making the scheme
somewhat arbitrary. In the part of the calculations according to the
first two methods, the SymPy library was employed,62 and the
parameters shown in Tables S7 and S8 (with the exception of
parameter h in method A) are average values of the calculated
parameters.
The third method C39 presents a generalization with respect to the
former two schemes37,38 because it treats different coordination
numbers (from 2 to 9) and the set of corresponding polyhedra. On
the contrary, methods A and B take into account only two possible
hexacoordinated polyhedra: octahedron and trigonal prism. In
addition, in methods A and B, the orientation of the polyhedron
(choice of the triangle bases) is not unique, providing additional
imprecision in the subsequent analysis. Method C is implemented in
the program FindGeo,40 which is available as a standalone or web
application. The FindGeo program compares the polyhedra around
the metal ion with templates from a library and determines the best
match, e.g., the lowest root-mean-square deviation for different atom−
atom pairings between the analyzed polyhedron and each template
from the library.39 It should be noted that the algorithm even takes
into account the possibility of ligand atom vacancy around the
analyzed polyhedron.
Analysis of the Coordination Geometry. The first method A37
employs two structural parameters (Scheme 2): the twist angle φ
between two triangles and the s/h ratio (s = length of the triangle side
and h = distance between two triangles). Ideal values for φ are 60°
(regular octahedron) and 0° (trigonal prism), and an ideal value of
Scheme 2. Method A for the Classification of
Hexacoordinated Polyhedra with Two Parameters: φ and
the s/h Ratio13,37
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02200.
■
S
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DOI: 10.1021/acs.inorgchem.9b02200
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: Srecko.Kirin@irb.hr.
10396.
̌
̌
(15) Skalamera, Đ.; Sanders, E.; Vianello, R.; Marsavelski, A.; Pevec,
A.; Turel, I.; Kirin, S. I. Synthesis and Characterization of ML and
ML2 Metal Complexes with Amino Acid Substituted Bis(2-Picolyl)-
Amine Ligands. Dalton Trans. 2016, 45 (7), 2845−2858.
(16) Smrecki, N.; Kukovec, B. M.; Đakovic, M.; Popovic, Z. A
Diversity of Hydrogen Bond Motifs in the Crystal Structures of
Nickel(II) and Copper(II) Complexes with N-Arylalkyliminodiaceta-
mides. Polyhedron 2015, 93, 106−117.
ORCID
́
Berislav Peric: 0000-0002-2397-2836
Robert Vianello: 0000-0003-1779-4524
̌
́
́
́
Srecko I. Kirin: 0000-0002-0500-2422
Notes
The authors declare no competing financial interest.
̌
́
̌
(17) Smrecki, N.; Jovic, O.; Molcanov, K.; Kukovec, B.-M.; Kekez, I.;
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Matkovic-Calogovic, D.; Popovic, Z. Influence of the Non-
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ACKNOWLEDGMENTS
■
These materials are based on work financed by the Croatian
Science Foundation (Projects IP-2014-09-1461 and IP-2014-
(18) Sekizaki, M. Bis(Iminodiacetamide)Nickel(II) Perchlorate.
Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1976, B32,
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09-3386). The authors thank Ernest Sanders (Ruđer Boskovic
Institute) for technical assistance.
(19) Mu
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ller, E.; Bayer, O.; Meerwein, H.; Ziegler, K. Band XI/2
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