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Research paper
Structural, spectral and theoretical study of the coordination of 3,6-bis(2-pyridyl)tetrazine ligand with zinc(II) and mercury(II)
Lotfali Saghatforoush, Keyvan Moeini, Vali Golsanamlou, Vahid Amani, Akbar
Bakhtiari, Zahra Mardani
PII:
DOI:
Reference:
S0020-1693(17)31892-3
https://doi.org/10.1016/j.ica.2018.08.023
ICA 18423
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
16 December 2017
5 July 2018
19 August 2018
Please cite this article as: L. Saghatforoush, K. Moeini, V. Golsanamlou, V. Amani, A. Bakhtiari, Z. Mardani,
Structural, spectral and theoretical study of the coordination of 3,6-bis(2-pyridyl)tetrazine ligand with zinc(II) and
mercury(II), Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.08.023
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Structural, spectral and theoretical study of the coordination of 3,6-bis(2pyridyl)tetrazine ligand with zinc(II) and mercury(II)
Lotfali Saghatforousha*, Keyvan Moeinia, Vali Golsanamloub, Vahid Amanic,
Akbar Bakhtiaria, Zahra Mardanib
a
Department of Chemistry, Payame Noor University, 19395-4697 Tehran, Iran
b
Inorganic Chemistry Department, Faculty of Chemistry, Urmia University, 57561-51818
Urmia, Iran
c
Department of Chemistry, Farhangian University, Tehran, Iran
Corresponding author (*): Lotfali Saghatforoush
E-mail: saghatforoush@gmail.com, l_saghatf@pnu.ac.ir
Fax: +98 4612332556
Tel: +98 4612349868
ABSTRACT
We report the synthesis and single crystal X-ray structures of three zinc(II) and mercury(II)
coordination polymers (1D-zigzag-{[Zn(μ-BPTZ)Cl2]·DMSO}n (1), 1D-zigzag-{[Zn(μBPTZ)Br2]·CH3CN}n (2) and 2D-herringbone [Hg2(μ-BPTZ)(μ-CN)2(CN)2]n (3)) and a
binuclear 3D-supramolecular coordination compound of mercury(II), [Hg2(μ-BPTZ)(SCN)4]
(4), with 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (BPTZ) ligand. The compounds are
characterized by elemental analysis and by FT-IR, 1H NMR spectroscopies. Stability of syn
and anti conformers of BPTZ and the energy barrier for converting the isomers was
calculated by DFT method. 1 and 2 are isostructural coordination polymers in which the
1
zinc(II) ion has a distorted octahedral coordination environment N 4X2 (X: Cl (1), Br (2)). The
coordination environment of mercury(II) ion in 3 (with N3C2 donor set) and 4 (with N2S2
donor set) is close to square-pyramidal and seesaw geometries, respectively. The binuclear
[Hg2(μ-BPTZ)(SCN)4] units in 4 are linked via Hg….Nthiocyanate interactions to form a 3Dsupramolecular coordination network. In addition, all coordination modes of BPTZ
derivatives, reported in crystal structure database (CSD), were extracted and discussed.
Keywords: Zinc(II); Mercury(II); Coordination polymer; Crystal structure; DFT studies; CSD
studies
1. Introduction
Coordination polymers (CPs) are metal-organic materials in which metal ions or metalcontaining clusters (acting as nodes) and organic ligands (spacers) are linked via coordination
bonds to form one-, two- or three dimensional extended structures [1]. 3,6-Bis(2pyridyl)tetrazine and its derivatives are used as linkers in coordination chemistry. They have
been used as linking group in paramagnetic squares [2], a rigid multitopic spacer ligand to
selectively assemble different metal ions [3], auxiliary in pentagon metallacyclophanes [4, 5]
and a chromophore for sensitisation of the metal luminescence [6]. In addition, these ligands
find use in coordination polymers [7, 8], co-crystal compounds [9] and single molecule
magnets (SMMs) [10].
The polytopic and pi-acidic BPTZ unit is suitable for the design of supramolecular
architectures [11, 12] with main group [13] and transition metals [12]. BPTZ stabilize the
lower oxidation states of metals in the oxo-centered clusters by its low-lying and strong πaccepting π* orbitals [14].
2
The ability of BPTZ derivatives to form one electron reduced anion radicals [15, 16] has been
used to interconvert between different architectures of complexes [17]. The accessible lowest
unoccupied molecular orbital (LUMO) of BPTZ facilitates one-electron reduction of these
compounds to generate stable radical anions [18].
Here, syntheses, crystal structures, spectroscopy and DFT study of four new complexes,
{[Zn(μ-BPTZ)Cl2]·DMSO}n (1), {[Zn(μ-BPTZ)Br2]·CH3CN}n (2), [Hg2(μ-BPTZ)(μCN)2(CN)2]n (3) and [Hg2(μ-BPTZ)(SCN)4] (4), with 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine
ligand (BPTZ, Scheme 1) are described.
2. Experimental
2.1 Materials and Instrumentation
All starting chemicals, including solvents, were of reagent or analytical grade and used as
received. The infrared spectra (4000–400 cm–1) were recorded on a FT-IR ShimadzuIRprestige-21 spectrometer using KBr pellets. The 1H NMR spectra were recorded on Bruker
Avance 400 and 300 instrument; chemical shifts δ are given in parts per million, relative to
TMS as an internal standard. The carbon, hydrogen and nitrogen contents were determined
using a Perkin-Elmer 2400 elemental analyzer. The melting points were determined with an
Elrctrothermal 9100 electrically heated apparatus.
2.1.1
Synthesis of {[Zn(μ-BPTZ)Cl2]·DMSO}n (1)
3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine (0.21 g, 0.85 mmol), dissolved in methanol (20 mL) was
added to a solution of ZnCl2 (0.12 g, 0.85 mmol) in CH3CN (20 mL). The resulting purple
solution was stirred for 30 min at 40 °C to produce a purple precipitate. The precipitate was
dissolved in DMSO to form a purple solution. After 24 h, orange needle-like single crystals
of 1, suitable for X-ray analysis, were obtained by methanol diffusion into the DMSO
3
solution. The crystals were removed by filtration and dried in air. Yield: 0.29 g, 73%; m. p.:
215 °C. Anal. Calcd. for C14H14Cl2N6OSZn (Mw: 450.67): C, 37.31; H, 3.13; N, 18.65.
Found: C, 37.12; H, 3.09; N, 18.52%. IR (KBr, cm−1): 3067 w (ν C–H, aromatic), 2856 w (ν
C–H, DMSO), 1595 m (ν C=N), 1485 w (ν N=N), 1444 w (δas CH3 and/or ν C=Caromatic),
1393 s (δs CH3), 1013 m (ν S=O), 962 w (ρr CH3), 744 w and 635 w (γ py). 1H NMR (400
MHz, DMSO-d6, ppm, Hz): δ= 8.95–8.96 )d, 2 H, CaH, J = 4.4), 8.62–8.64 )d, 2 H, CdH, J =
10.4), 8.15–8.20 )t, 2 H, CcH, J = 10.0 & 10.4), 7.73–7.77 ) t, 2 H, CbH, J = 9.2 & 6.8).
2.1.2
Synthesis of {[Zn(μ-BPTZ)Br2]·CH3CN}n (2)
The synthesis of 2 was similar to that of 1, except that ZnCl2 was replaced by ZnBr2 (0.22 g,
0.85 mmol). A purple solution was obtained upon mixing the reagents in solvent. Orange
needle-like single crystals of 2, suitable for X-ray analysis, were obtained by slow
evaporation of solvent at room temperature over four days. The crystals were filtered off and
dried in air. Yield: 0.33 g, 77%; m. p.: 195 °C. Anal. Calcd. for C14H11Br2N7Zn (Mw:
502.49): C, 33.46; H, 2.21; N, 19.51. Found: C, 33.25; H, 2.18; N, 19.38%. IR (KBr, cm−1):
3047 w (ν C–H, aromatic), 2985 w (ν C–H, CH3CN), 2270 w (ν C≡N, CH3CN), 1597 m (ν
C=N), 1475 w (ν N=N), 1394 s (δs CH3), 741 w and 636 w (γ py). 1H NMR (400 MHz,
DMSO-d6, ppm, Hz): δ= 8.95–8.96 )d, 2 H, CaH, J = 4.8), 8.62–8.64 )d, 2 H, CdH, J = 10.8),
8.15–8.20 )t, 2 H, CcH, J = 10.0 & 10.4), 7.73–7.77 ) t, 2 H, CbH, J = 8 & 8.4), 2.08 ) s, 3 H
of acetonitrile).
2.1.3
Synthesis of [Hg2(μ-BPTZ)(μ-CN)2(CN)2]n (3)
3 was prepared as 2, except that ZnBr2 was replaced by Hg(CN)2 (0.22 g, 0.85 mmol) and a
solvent mixture of acetonitrile/methanol (1:1, v/v) was used. Violet block single crystals of 3,
suitable for X-ray analysis, were obtained by slow solvent evaporation at room temperature
over three days. The crystals were filtered off and dried in air. Yield: 0.24 g, 76%; m. p.: 216
4
°C. Anal. Calcd. for C8H4HgN5 (Mw: 370.75): C, 25.92; H, 1.09; N, 18.89. Found: C, 25.75;
H, 1.07; N, 18.75%. IR (KBr, cm−1): 3061 w (ν C–H), 2180 w and 2204 w (ν C≡N, cyanide),
1585 m (ν C=N, aromatic), 1480 w (ν N=N), 1443 m (ν C=C), 742 m and 623 w (γ py). 1H
NMR (300 MHz, DMSO-d6, ppm, Hz): δ= 8.94–8.95 )d, 2 H, CaH, J = 3.7), 8.61–8.63 )d, 2
H, CdH, J = 7.8), 8.14–8.19 )t, 2 H, CcH, J = 7.2 & 7.5), 7.72–7.76 ) t, 2 H, CbH, J = 6.0 &
6.3).
2.1.4
Synthesis of [Hg2(μ-BPTZ)(SCN)4] (4)
4 was prepared as 3, except that Hg(SCN)2 (0.27 g, 0.85 mmol) was used. Violet prismatic
single crystals of 4, suitable for X-ray analysis, were obtained by slow solvent evaporation at
room temperature over three days. The crystals were filtered off and dried in air. Yield: 0.29
g, 79%; m. p.: 183 °C. Anal. Calcd. for C16H8Hg2N10S4 (Mw: 869.78): C, 22.10; H, 0.93; N,
16.10. Found: C, 21.94; H, 0.91; N, 16.01%. IR (KBr, cm−1): 3058 w (ν C–H), 2110 s (ν
C≡N, thiocyanate), 1581 m (ν C=N, aromatic), 1512 s (ν N=N), 1465 s (ν C=C), 743 s and
657 s (γ py), 697 s (ν C–S, thiocyanate). 1H NMR (300 MHz, DMSO-d6, ppm, Hz): δ= 8.94–
8.95 )d, 2 H, CaH, J = 3.9), 8.61–8.63 )d, 2 H, CdH, J = 7.8), 8.14–8.19 )t, 2 H, CcH, J = 7.8),
7.72–7.76 ) t, 2 H, CbH, J = 6.3 & 6.0).
2.2 Crystal structure determination and refinement
Data collection for the compounds were performed on a Bruker APEX II CCD area detector
diffractometer using graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 298 K.
The structures were solved by SHELX-97 [19] and absorption corrections were done using
the SADABS program [20, 21]. Data collection, cell refinement, and data reduction were
performed by applying APEX II, SAINT, SHELXTL and PLATON program packages [20,
22, 23]. The molecular graphics were prepared using ORTEP [24, 25] and Diamond [26].
Crystallographic data and details of the data collection and structure refinement are listed in
5
Table 1. Selected bond lengths and angles are given in Table 2. Also, hydrogen bond
parameters are provided in Table 3. The unit cell of compound 2 has an accessible solvent
volume of 116.8 Å3, which is occupied by disordered solvent molecule. Therefore, this
molecule was excluded from the structure refinement by means of the SQUEEZE subroutine
of PLATON [27].
2.3 Computational details
Gaussian 09 software [28] was applied for Density Functional Theory (DFT) [29]
calculations at B3LYP/6-31+G level of theory on an isolated BPTZ molecule. Molecular
geometry of the compound, extracted from the single crystal X-ray structure analysis of 1,
was optimized at the same level of theory in free gaseous state before all calculations.
3. Results and Discussion
3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine and a metal halide or pseudo halide salts (ZnX2, X = Cl
(1) and Br(2); HgY2, Y = CN(3), SCN(4)) were reacted at 40 °C to make compounds 1−4.
Their single crystals for X-ray diffraction were obtained by solvent diffusion or by slow
solvent evaporation at room temperature. Single crystal X-ray analysis revealed that 1 and 2
are 1D coordination polymers, while 3 is a 2D coordination polymer. However, 4 is a
binuclear metal complex, which forms a 3D-supramolecular coordination structure in solid
state due to weak interactions. The complexes are air-stable, but dissociate to give free ligand
upon dissolution in DMSO. Therefore, 1H NMR spectra of the complexes measured in
DMSO show signals of to free BPTZ ligand.
3.1 Infrared spectra
Relatively weak absorption bands, observed above 3000 cm–1 in the IR spectra of the free
BPTZ ligand and complexes 1−4 (see Supplementary Materials), are due to the C–H
6
vibrational stretching of the aromatic rings. In the IR spectrum of free BPTZ, the bands at
1450−1600 cm−1 are attributed to the ν (C=N) and ν (N=N) vibrations. Compared to the ν
(N=N) absorption band, ν (C=N) is expected to appear at higher energies [30]. Compared to
those of the free ligand, the respective absorption bands in the IR spectra of 1−4 (see
Supplementary Materials) are observed to shift to higher wavenumbers by 8−24 cm−1 and
5−37 cm−1 for ν (N=N) and ν (C=N), respectively. This is evidence that the ligand is
coordinated to the metal ions through the nitrogen atoms of the aromatic rings. However, it
should be noted that, ν (N=N) in the IR spectrum of 2 shows no shift when compared to the
free ligand. In addition, the ring wagging vibrations of the pyridine groups were observed at
750 and 650 cm−1 [31].
Comparing the IR spectra of 1 and 2 with that of the free ligand reveals the solvent
molecules trapped in the crystal packing of these compounds. The respective ν (S=O) (for
DMSO in 1) and ν (C≡N) (for acetonitrile in 2) for absorption bands are at 1013 and 2270
cm−1, respectively.
FT-IR spectroscopic measurements on compound 3 reveal two v (C≡N) absorption bands.
The bands are shifted by 100 and 124 cm−1 to higher wavenumbers, with respect to that
reported for free cyanide ion [32], confirming the coordination of the cyanide ions to the
mercury(II) ion [32]. The higher energy v (C≡N) band (at 2204 cm−1) could be attributed to
the bridging cyano ligand [33], while the other band is due to terminal cyano ligand vibration.
The presence of the thiocyanate groups in 4 is confirmed by C≡N stretching band at 2110 cm–
1
[34] and C−S stretching band at 697 cm–1 [30] in the IR spectrum. The bands could be used
to determine the coordination modes of the thiocyanato ligand in its complexes [34, 35].
According to [34], thiocyanate ion can coordinate to metal centers as N-donor, S-donor or
bridging N,S-donor ligand. The C≡N stretching frequencies are generally lower in N-bonded
7
complexes (near and above 2050 cm‒1) than in S-bonded (near 2100 cm‒1) and bridging ones
(above 2100 cm‒1) [34]. Based on these rules, it could be deduced that the thiocyanate ions in
4 are coordinated to the mercury(II) ion as S-donor ligands. In addition, the observed ν (C−S)
band in the IR spectrum is in accord with a S-bonded thiocyanato ligand [34].
3.2 Crystal structures
3.2.1
Structural study of 3,6-Bis(2-pyridyl)tetrazine ligands in CSD
3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine unit (BPTZ) with a tetrazine ring and two side pyridine
groups is a potentially hexa-dentate ligand. Survey of the Cambridge Structural Database
(CSD) [36] revealed that this unit could be protonated to form H2BPTZ [7, 37]. Also, stable
one-electron reduced BPTZ, the BPTZ‾● radical anion, is known [14, 15, 18]. In the CSD,
five coordination modes were found for BPTZ ligands (see Table 4). The most common
coordination mode (69%) is antipy-(NpyNtetrazine)2, in which a BPTZ ligand links two metal
centers by forming two anti-five-membered chelate rings. The second common mode (22%)
is synpy-(NpyNtetrazine)2, while the other coordination modes are rarely observed. In addition,
according to CSD analysis, it acts as a N2- or N4-donor ligand and there is no example for
other donations.
The average values, in CSD, for the angles between mean planes through each pyridine ring
and the tetrazine core are calculated for each coordination mode (see ANG in Table 4). In the
coordination mode synpy-(NpyNtetrazine)2, with an average observed ANG value of 13.54°, two
pyridine rings are not coplanar with the tetrazine group. Moreover, it is observed that the
pyridine rings, commonly bend in the opposite directions. In comparison, the pyridine rings
in the coordination mode antipy-(NpyNtetrazine)2 (average measured ANG value of 6.95°) are
almost coplanar with the tetrazine ring.
8
While two anti and syn conformations are possible for free BPTZ, it prefers to crystallize in
anti form [38]. Also, CSD shows that BPTZ mostly coordinates to the metal ions in anti
conformation (see Table 4). These facts may reflect the thermodynamic stability of the anti
conformer with respect to the syn one. This issue is studied by DFT calculations in the
theoretical section of this paper.
CSD shows that the 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine moiety can form mono- [39], bi[40], tri- [14], tetralinear- [8], tetracyclic- [2], pentacyclic-nuclear [4] complexes and coordination
polymers [41]. It seems that the mono- or poly nuclearity of the BPTZ metal complexes is
mostly controlled by the nature of the ligands in the metal ion coordination sphere, rather
than the nature of the metal ion. For example, in the same experimental conditions,
cadmium(II) halides react with BPTZ to form linear tetra-nuclear ([Cd4(BPTZ)2(H2O)2Cl8]),
coordination polymer ({[Cd(BPTZ)Br2].DMSO}n) and mono-nuclear ([Cd(BPTZ)2I2]) [8,
42].
3.2.2
Crystal structure of {[Zn(μ-BPTZ)Cl2]·DMSO}n (1)
Single crystal X-ray diffraction analysis of 1 (Fig. 1) revealed a 1D-zigzag [43] coordination
polymer extending along the crystallographic b axis. The zinc(II) ion is coordinated by four
nitrogen atoms from two BPTZ ligands and two terminal chloro ligands fill the coordination
sphere of the metal ion to form a distorted octahedral coordination geometry (Fig. 2). A CSD
survey shows that 1 is isomorphous with the known {[Cd(μ-BPTZ)Br2].DMSO}n [42]. The
Zn–Npy bond lengths (2.158(10) and 2.159(10) Å) are shorter than those for Zn–Ntetrazine
(2.498(11) and 2.308(11) Å). In 1, BPTZ acts as tetradentate ligand, which forms two fivemembered chelate rings with two zinc(II) ions in opposite directions. Therefore, the
coordination mode of BPTZ in 1 is antipy-(NpyNtetrazine)2 (see Table 4). Two BPTZ ligands,
which are coordinated to the same metal center in 1, are cis with respect to each other. The
9
angle between two mean planes through these ligands is 85.91°. The angle between two
chloride ligands is 101.8(2)°. CSD shows that this coordination is common for the
compounds in which the metal ion has the same coordination environment i.e. two cischloride ions on Zn(II) ion. Among eleven examples found in CSD [44-54], only in trans[ZnL2Cl2] (L: N-octylethylenediamine) [54], where the coordinated nitrogen atoms have sp3
hybridization, trans arrangement of the chloride ligands in the complex is observed.
However, when Zn(II) is coordinated also by four sp2 hybridized nitrogen atoms, the
coordination bonds arrange in the same way as in 1. Finally, it is notable that the observed
angle between the mean planes through pyridine and tetrazine rings in 1 is 4.57° which is
lower than the average value in CSD (Table 4).
3.2.3
Crystal structure of {[Zn(μ-BPTZ)Br2]·CH3CN}n (2)
2, in solid state, forms 1D-zigzag polymeric chains (see Fig. 3) which are extended along the
b axis. Zn(II) ions in 2 are hexacoordinated and have distorted octahedral coordination
environment (see Fig. 4). Four coordination sites are occupied by the nitrogen atoms from
two BPTZ ligands, which show a coordination mode of antipy-(NpyNtetrazine)2. The 5th and 6th
cis-positions are occupied by two bromide ions. Similar to that observed for 1, the Zn–Npy
bond lengths (2.128(12) and 2.141(12) Å) are shorter than Zn–Ntetrazine (2.342(12) and
2.394(11) Å). Also, the measured ANG (4.55°) is lower than the average value in CSD
(Table 4). The angle between mean planes through the BPTZ ligands which are coordinated
to the same metal ion (79.92°) is in accord with the cis arrangement of the ligands. This
value is lower than that measured for 1, which could be attributed to the larger size of
bromide ion. Accordingly, the Br–Zn–Br angle in 2 (103.01(9)°) is larger than the Cl–Zn–Cl
angle in 1. The CSD was searched for motifs similar to that shown in scheme 2, except that
chloride ligands were replaced by bromide. Only one structure (cis-[ZnL2Br2]; L:
10
pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile) [55] was found to be similar to 2,
where Zn(II) is coordinated by four sp2 hybridized N atoms and two cis-bromide ligands.
3.2.4
Crystal structure of [Hg2(μ-BPTZ)(μ-CN)2(CN)2]n (3)
3, in solid state, forms a 2D-herringbone type [43] coordination polymer (see Fig. 5), in
which mercury(II) ions are bridged alternatively by BPTZ (coordination to Hg(II) ions in
antipy-(NpyNtetrazine)2 mode) and cyanide ions linkers (Fig. 6). BPTZ ligands bridge metal ions
to form 1D polymeric chains, while the cyano bridges connect the chains to expand the
polymer in two dimensions. Two bridging and one terminal cyano ligands are coordinated to
each Hg(II) ion. Five crystal structures with similar Hg(II)–CN–Hg(II) bridges have been
reported in[56-60]. The average values of the structural parameters for this motif are given in
Scheme 3. The results are in accord with those observed for 3, indicating that this bridge
commonly is not linear (as in 3) and the average Hg−N bond length is longer than that for
Hg−C bond. Different binding modes of the cyanide ion with the mercury(II) ion were also
investigated in CSD survey. It was revealed that, cyanide commonly coordinates to the
mercury(II) ion as a terminal ligand (61%) and only via the carbon atom. Also there are no
CSD examples for the terminal N-bonded terminal coordination mode. Moreover, when
acting as a bridging ligand, cyanide forms stronger and shorter Hg(II)–C bonds that Hg(II)–N
bonds.
In the crystal structure of 3, the mercury(II) ion is five coordinated by two nitrogen atoms
from a BPTZ ligand, one nitrogen and one carbon atom from two bridging cyanide ions and
one carbon atom from a terminal cyano ligand. CSD shows that 3 is the first example in
which a tetrazine ring in BPTZ ligand is coordinated to the mercury ion. A penta-coordinated
metal complex may adopt either a square pyramidal or a trigonal bipyramidal structure,
which could be identified by applying the formula of Addison et al. [61, 62]. For 3, the
11
angular structural parameter τ (τ = (β – α)/60, where α and β are the two largest angles around
the mercury(II) ion and β ≥ α) was calculated to be 0.48. The calculated τ parameter reveals
an intermediate coordination environment, close to square-pyramidal geometry around the
mercury(II) ion. In the crystal structure of 3, the observed Hg–Npy bond length (2.676(7) Å)
is shorter than that for Hg–Ntetrazine (2.766(7) Å) and the calculated ANG value (4.41°) is
lower than the average CSD value.
3.2.5
Crystal structure of [Hg2(μ-BPTZ)(SCN)4] (4)
In the binuclear complex 4, Hg(II) ions are coordinated by terminal S-donor thiocyanate
ligands and a bridging antipy-(NpyNtetrazine)2 (see Table 4) coordinated BPTZ ligand. The
mercury(II) ions are tetracoordinated by N2S2 donor set in a seesaw geometry (see Fig. 7).
CSD was searched for compounds with HgN2SSCN2 environment in which the S−Hg−S angle
is in the range of 160−180°; however, none were found. In the crystal structure of 4, the axial
Hg−S bond lengths (average: 2.400 Å) are shorter than the equatorial Hg−N ones (average:
2.583 Å). A similar result was reported for mercury(II) ions in seesaw geometry with
different coordination environments [63]. Similar to 1–3, the Hg–Npy bond length is shorter
than Hg–Ntetrazineone, while the ANG (16.17°) is higher than the CSD average (see Table 4).
In addition to these four dative interactions in the crystal structure of 4, each mercury(II) ion
weakly interacts in equatorial positions with two thiocyanate nitrogen atoms from two
adjacent complexes (average atomic distances: 2.900 Å) to complete a pseudo-octahedral
geometry [63]. The observed Hg···N distances are longer than the coordination bond
distances in CSD, while are shorter than the sum of the respective atomic van der Waals radii.
Therefore, 4 could be best described as a 3D-supramolecular coordination compound.
3.2.6 Crystal network interactions
12
In the crystal network of the complexes (Figures 1, 3, 8, 9) are found intermolecular C–H···O
(1), C–H···N (2 and 4) and weak C–H···X (X: Cl (1), Br (2)) hydrogen bonding interactions.
Therefore, the nitrogen, oxygen and halide ions act as proton acceptors while the C−H groups
act as donors in hydrogen bonding. In addition to the hydrogen bonds, the crystal network of
2 is further stabilized by π–π stacking interactions between two pyridine rings of the adjacent
ligands (centroid–centroid distance: 3.761 Å; the angle between the planes: 0°; the
perpendicular distance between the planes: 3.495 Å). The rings are not exactly on top of each
other (slippage value is 1.389 Å) and two nitrogen atoms on two pyridine rings are anti to the
each other. Solvent molecules are trapped by intermolecular interactions in the crystal
structure of 1 and 2 (DMSO in 1 and acetonitrile in 2; see Figs. 1 and 3, respectively). Two
1D polymeric chains form a cage around two solvent molecules in the crystal packing of the
compounds. Similar cages were observed previously (around the solvent [42] and anion [2,
4]) for this type of ligand.
Total intermolecular interactions energy1 for one complex 4 were calculated by
Mercury software using CSD-materials tool [64, 65]. Sum of the intermolecular interactions
energy in a molecular packing shell containing 100 molecules around one complex was
calculated to be −460.18 kJ/mol (Fig. 10). In 4, according to the calculations, 50 and 93% of
total intermolecular energy corresponds to the interactions with four and fourteen
surrounding molecules, respectively (Fig. 10).
3.3 Theoretical studies
A CSD survey revealed that the dihedral angle between tetrazine and pyridine rings is
variable. For a theoretical study, geometrical coordinates of BPTZ were extracted from the
cif file of 1. The energy level of the free gaseous ligand was calculated by DFT method (180
1
This parameter can be calculated only for non-polymeric structures
13
scans) when the dihedral angles of Ny/Cf/Ce/Cd (see Scheme 1) vary between −179.66° to
0.34° (see Fig. 11). The study revealed that BPTZ has the lowest energy when the dihedral
angle between pyridine and tetrazine rings is 9.85° and the two pyridine rings are anti to each
other. With increasing the dihedral angles, the energy increases and the molecule has
maximum energy when the dihedral angle is 89.31º. The energy barrier for rotation of the
pyridine rings about the Cpy−Ctetrazine bond and for converting two conformers is 4.35
kcal/mol (calculated for the isolated molecule in free gaseous state). It is found that the anti
form of the ligand is −0.57 kcal/mol more stable than the syn form. The results are in accord
with those observed in CSD (see section 3.2.1).
4. Conclusion
In this work, four complexes of 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine ligand (BPTZ),
including 1D-zigzag-{[Zn(μ-BPTZ)Cl2]·DMSO}n (1), 1D-zigzag-{[Zn(μBPTZ)Br2]·CH3CN}n (2), 2D- herringbone [Hg2(μ-BPTZ)(μ-CN)2(CN)2]n (3) and a 3Dsupramolecular coordination structure, [Hg2(μ-BPTZ)(SCN)4] (4), were synthesized and their
spectral (IR, 1H NMR) and structural properties were investigated. CSD studies confirm that
the compounds are the first examples in which a tetrazine ligand (BPTZ) is coordinated to
zinc and mercury ions. A CSD search revealed that among the five coordination modes of
BPTZ, the antipy-(NpyNtetrazine)2 mode is the most abundant one (69%). Moreover, DFT
calculations confirmed that the the antipy conformer is more stable than the synpy one.
According to the calculations, the energy barrier for rotation of the pyridine rings about the
Cpy−Ctetrazine bond and for converting two conformers is 4.35 kcal/mol in isolated molecule. In
the crystal structure of 1, the zinc(II) ion has a N4Cl2 coordination environment in distorted
octahedral geometry in which the nitrogen atoms come from two bridging BPTZ ligands.
Replacing the chloride ions with bromide give 2, in which Zn(II) ion has a geometry similar
14
to that in 1. The mercury(II) ion in 3 (with C.N. = 5 and N3C2 environment) is observed to
have an intermediate coordination geometry, close to be square pyramidal. The mercury(II)
ion in 4 has a rare seesaw coordination geometry and N2S2 environment.
Appendix A. Supplementary data
CCDC 1586604, 1586601, 1586602 and 1586603, respectively, contain the supplementary
crystallographic data for 1–4 in this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgement
We thank the research Office of Payame Noor University, Urmia University and Farhangian
University for supporting this work.
15
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19
Table 1. Crystal data and structure refinement for 1–4.
Empirical formula
Formula weight, g mol–1
Crystal size, mm3
Temperature, K
Crystal system
Space group
Unit cell dimensions (Å, °)
a
b
c
α
β
γ
Volume, Å3
Z
Calculated density, g cm–3
Absorption coefficient, mm–1
F(000), e
θ range for data collection, deg
h, k, l ranges
Reflections collected / independent /
Rint
Data / restraints / parameters
Goodness-of-fit on F2
R1 / wR2 (I  2 σ(I))
R1 / wR2 (all data)
Largest diff. peak / hole, e Å–3
1
C14H14Cl2N6OSZn
450.67
0.11 × 0.06 × 0.05
298(2)
Triclinic
Pī
2
C14H11Br2N7Zn
502.49
0.13 × 0.07 × 0.05
298(2)
Triclinic
Pī
3
C8H4HgN5
370.75
0.30 × 0.22 × 0.21
298(2)
Monoclinic
P21/c
4
C16H8Hg2N10S4
869.78
0.50 × 0.30 × 0.20
298(2)
Monoclinic
P21/n
8.819(2)
10.136(3)
10.822(3)
98.27(2)
102.49(2)
94.97(2)
927.7(5)
2
1.613
1.74
456
2.0 to 25.0
−10 ≤ h ≤ 10
−12 ≤ k ≤ 12
−12 ≤ l ≤ 11
6848 / 3254 /
0.190
3254 / 0 / 229
0.90
0.0811 / 0.1821
0.2254 / 0.2327
0.89 / −1.34
8.9868(11)
9.1485(11)
11.9149(14)
73.443(9)
87.789(10)
85.822(10)
936.32(19)
2
1.782
5.59
488
2.3–27.0
–11 ≤ h ≤ 11
–10 ≤ k ≤ 11
0 ≤ l ≤ 15
4074 / 4074 /
0.000
4074 / 0 / 208
0.91
0.1002 / 0.1769
0.2118 / 0.2097
0.89 / –1.00
7.0725(14)
8.0470(16)
17.186(3)
90.00
93.67(3)
90.00
976.1(3)
4
2.523
15.73
668
2.4–27.0
–9 ≤ h ≤ 8
–10 ≤ k ≤ 10
–21 ≤ l ≤ 21
5911 / 2116 /
0.059
2116 / 0 / 128
0.95
0.0328 / 0.0760
0.0606 / 0.1019
1.39 / – 1.88
9.413(2)
10.7422(19)
11.684(2)
90.00
108.934(14)
90.00
1117.4(4)
2
2.585
14.12
796
2.4–27.0
–11 ≤ h ≤ 12
–13 ≤ k ≤ 13
–14 ≤ l ≤ 13
6474 / 2422 /
0.065
2422 / 0 / 145
0.93
0.0301 / 0.0596
0.0474 / 0.0637
0.70 / – 1.40
Table 2. Selected bond lengths (Å) and angles (°) for 1–4 with estimated standard deviations in parentheses.
20
Angles
Distances
1
2
3
4
Zn1−N1
2.16(1)
Zn1−N1
2.14(1)
Hg1−N1
2.676(7)
Hg1−N1
2.516(5)
Zn1−N2
2.50(1)
Zn1−N2
2.34(1)
Hg1−N2
2.766(7)
Hg1−N2
2.650(4)
Zn1−N4
2.16(1)
Zn1−N4
2.13(1)
Hg1−N4
2.77(1)
Hg1−S1
2.383(2)
Zn1−N5
2.31(1)
Zn1−N5
2.40(1)
Hg1−C7
2.03(1)
Hg1−S2
2.417(2)
Zn1−Cl1
2.305(4)
Zn1−Br1
2.476(2)
Hg1−C8
2.04(1)
Zn1−Cl2
2.299(5)
Zn1−Br2
2.445(3)
N4−Zn1−N1
143.7(4)
N4−Zn1−N1
147.8(5)
N1−Hg1−N2
60.5(2)
N1−Hg1−N2
63.4(1)
N4−Zn1−N2
77.2(4)
N4−Zn1−N2
83.8(5)
N1−Hg1−N4
82.1(3)
N1−Hg1−S1
99.2(1)
N4−Zn1−N5
73.7(4)
N4−Zn1−N5
71.4(4)
N1−Hg1−C8
97.1(3)
N1−Hg1−S2
92.9(1)
N4−Zn1−Cl1
98.3(3)
N4−Zn1−Br1
101.3(3)
N1−Hg1−C7
92.4(3)
S1−Hg1−N2
100.2(1)
N4−Zn1−Cl2
105.3(3)
N4−Zn1−Br2
98.7(3)
N4−Hg1−C8
93.6(4)
S1−Hg1−S2
163.31(6)
Cl1−Zn1−Cl2
101.8(2)
Br1−Zn1−Br2
103.01(9)
C8−Hg1−C7
170.6(4)
S2−Hg1−N2
95.5(1)
21
Table 3. Hydrogen bonds parameters (Å and °) in 1–4.
D–H···A
d(D–H)
d(H···A)
<(DHA)
d(D···A)
Symmetry code on A atom
C2−H2∙∙∙O1
C3−H3∙∙∙Cl1
C7−H7∙∙∙Cl1
C10−H10∙∙∙Cl2
C13−H13A∙∙∙Cl1
0.93
0.93
0.93
0.93
0.96
2.71
2.717
2.853
2.948
2.877
139.7
152
129.5
130.8
165
3.48(2)
3.56(2)
3.52(1)
3.62(1)
3.82(2)
−1 + x, −1 + y, z
−1 + x, y, z
1 − x, 1 − y, 1 − z
−1 + x, y, z
1 − x, 1 − y, 2 − z
C2−H2∙∙∙Br1
C3−H3∙∙∙Br1
C7−H7∙∙∙Br1
C8−H8∙∙∙N7
C9−H9∙∙∙Br2
C13−H13B∙∙∙Br1
C13−H13C∙∙∙Br2
0.93
0.93
0.93
0.93
0.93
0.96
0.96
2.865
3.021
2.933
2.74
2.949
3.012
2.996
168
134
128
134
151
132
131
3.78(2)
3.73(2)
3.58(2)
3.45(2)
3.79(2)
3.72(2)
3.70(2)
x, y, −1 + z
1 − x, − y, − z
− x, 1 − y, − z
−1 + x, y, z
−1 + x, y, z
x, 1 + y, z
x, y, z
C3−H3∙∙∙N5
0.930
2.685
149.7
3.52(1)
3 − x, − y, 1 – z
1
2
4
22
Table 4. Coordination mode of the BPTZ ligand from the CSD analysis. In the coordination mode column, the
parentheses refer to chelate ring. syn and anti refers to the position of the nitrogen atoms of two pyridine rings
with respect to each other. ANG is the average angle between mean planes through each pyridine ring and the
mean plane of tetrazine ring.
Coordination
Modes
Structure
Hits
(%)
ANG* (°)
synpy(NpyNtetrazine)2
12
21.8
13.54
antipy(NpyNtetrazine)2
38
68.8
6.95
synpy(NpyNtetrazine)
1
1.8
10.81
antipy(NpyNtetrazine)
2
3.8
4.50
antipy-Npy2
2
3.8
18.72
23
Figure Captions
Scheme 1. 3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (BPTZ) ligand.
Scheme 2. The motif searched in CSD to find structures similar to 1.
Scheme 3. The average values found for structural parameters of Hg−CN−Hg bridges in
CSD.
Figure 1. Crystal packing of 1, showing the hydrogen bonds and the 1D-polymeric chain in
bc plane. Each ZnN4Cl2 unit is shown as octahedron. Only the hydrogen atoms
involved in hydrogen bonding are shown.
Figure 2. The ORTEP diagram of the molecular structure of 1. The ellipsoids are drawn at the
30% probability level.
Figure 3. Crystal packing of 2, showing the hydrogen bonds and the 1D-polymeric chain in
bc plane. Each ZnN4Br2 unit is shown as octahedron. Only the hydrogen atoms
involved in hydrogen bonding are shown. The orange molecules are trapped
acetonitrile molecules.
Figure 4. The ORTEP diagram of the molecular structure of 2. The ellipsoids are drawn at the
30% probability level.
Figure 5. 2D-herringbone type coordination polymer 3. The hydrogen and non-bridging
cyanide ligands are removed for clarity.
Figure 6. The ORTEP diagram of the molecular structure of 3. The ellipsoids are drawn at the
30% probability level.
24
Figure 7. The ORTEP diagram of the molecular structure of 4. The ellipsoids are drawn at the
30% probability level.
Figure 8. Crystal packing of 3, showing the hydrogen bonds. Each HgN4C unit is shown as
square plane. Only the hydrogen atoms involved in hydrogen bonding are shown.
Figure 9. Crystal packing of 4, showing the hydrogen bonds and seesaw geometry around the
mercury ion. Only the hydrogen atoms involved in hydrogen bonding are shown.
Figure 10. Total intermolecular interactions energy (E) diagram for complex 4 varying with
increasing the number of surrounding molecules.
Figure 11. DFT calculated energy changes for free gaseous BPTZ with changes in the
dihedral angle between pyridine and tetrazine rings.
25
Scheme 1.
26
Scheme 2.
27
Scheme 3.
28
Figure 1.
29
Figure 2.
30
Figure 3.
31
Figure 4.
32
Figure 5.
33
Figure 6.
34
Figure 7.
35
Figure 8.
36
Figure 9.
37
Figure 10.
38
Figure 11.
39
Graphical abstract
Four new complexes of 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (BPTZ) including, 1Dzigzag-{[Zn(μ-BPTZ)Cl2]·DMSO}n (1), 1D-zigzag-{[Zn(μ-BPTZ)Br2]·CH3CN}n (2) and 2Dherringbone-[Hg2(μ-BPTZ)(μ-CN)(CN)]n (3) and a 3D-supramolecular coordination
compound, [Hg2(μ-BPTZ)(SCN)4] (4), were prepared and their spectral and structural
properties were investigated. Experimental data are compared with CSD database and
theoretical results.
40
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