Analysis of the contribution of the π-acidity of the s-tetrazine ring in the crystal packing of coordination polymers

Article abstract of DOI:10.1039/c3ce00058c

Reaction of manganese(ii) with the electron-deficient ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (pbptz) leads to distinct coordination networks whose topologies are influenced by the nature of the anions used. As anticipated, the linear ditopic ligand pbptz is involved in various types of supramolecular π interactions, i.e. π...π, lone pair...π and C-H...π interactions, which clearly play a role in the formation of the different solid-state architectures obtained, as shown by DFT calculations.

Full text of DOI:10.1039/c3ce00058c

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Analysis of the contribution of the p-acidity of the
s-tetrazine ring in the crystal packing of coordination
polymers3
Cite this: DOI: 10.1039/c3ce00058c
a
b
b
c
de
´
Piya Seth, Antonio Bauza, Antonio Frontera,* Chiara Massera, Patrick Gamez*
and Ashutosh Ghosh*e
Reaction of manganese(II) with the electron-deficient ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (pbptz)
leads to distinct coordination networks whose topologies are influenced by the nature of the anions used.
As anticipated, the linear ditopic ligand pbptz is involved in various types of supramolecular p interactions,
Received 10th January 2013,
Accepted 12th February 2013
DOI: 10.1039/c3ce00058c
…
…
…
i.e. p p, lone pair p and C–H p interactions, which clearly play a role in the formation of the different
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solid-state architectures obtained, as shown by DFT calculations.
…
remarkable supramolecular frameworks with short anion
contacts.12,22,23
p
Introduction
Crystal engineering is based on the rational utilization of
intermolecular forces to construct particular molecular archi-
tectures with tailored physical properties.1,2 Most synthetic
strategies exploit hydrogen or/and coordination bonds.3
However, a number of other supramolecular bonding contacts
In the present study, the ditopic ligand 3,6-bis(4-pyridyl)-
1,2,4,5-tetrazine (Scheme 1; pbptz17) has been used to produce
manganese(II) coordination polymers. The aptitude of the
p-acidic tetrazine ring to achieve p interactions with electron-
rich entities, i.e. p-basic arenes (such as a phenyl group), has
been examined, and the crystal-packing effect of these
supramolecular contacts has been evaluated (with theoretical
calculations as well). For this purpose, reactions of pbptz with
different manganese(II) salts have been carried out. An
inorganic anion, namely thiocyanate SCN2, was first used to
observe the interactions taking place between the tetrazine-
based ligands in the solid-state structure of the consequent
Mn compound. Subsequently, a purely organic anion, i.e.
benzoate PhCOO2, was selected to explore the potential
4
5,6
…
…
involving aromatic moieties, i.e. C–H p,
p
…
p (stacking),
7
8
9
…
…
cation p, but also anion
p
and lone pair p
interactions
may be employed as well to build solid-state networks.10,11
Tetrazine rings are particular aromatic units; indeed, these
are electron-deficient with respect to phenyl rings.12–14 As a
…
result, tetrazines have been used to intentionally generate p
p
interactions with electron-rich aromatic systems (as such
interactions are favored14–16), with the objective to engineer
crystal structures.17,18 Since tetrazines exhibit electron defi-
ciency,19 they have also been employed to interact with anions
…
occurrence of p p stacking interactions between the tetrazine
(anion p interactions20); for instance, the ligand 3,6-bis(2-
…
and the phenyl rings (which are electron-rich aromatic
moieties). Lastly, the effect of the incorporation of a methylene
group between the phenyl ring and the carboxylate function,
namely using the phenylacetate anion (which can also be
pyridyl)-1,2,4,5-tetrazine (bptz21
) has allowed to prepare
aDepartment of Chemistry, University College of Science, University of Calcutta, 92 A.
P. C. Road, Kolkata 700009, India
…
involved in p p interactions) instead of benzoate, on the
bDepartment of Chemistry, Universitat de les Illes Balears, Crta. De Valldemossa km
7.5, 07122 Palma de Mallorca (Baleares), Spain. E-mail: toni.frontera@uib.es
cDipartimento di Chimica, University of Parma, Viale delle Scienze 17/A, 43124
crystal packing, was analysed by comparison of the solid-state
structure of the respective compound. The network topologies
Parma, Italy
d
´
`
´
`
Departament de Quımica Inorganica, Universitat de Barcelona, Martı i Franques
1-11, 08028 Barcelona, Spain. E-mail: patrick.gamez@qi.ub.es
www.bio-inorganic-chemistry-icrea-ub.com
e
´
Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluıs
Companys 23, 08010 Barcelona, Spain. E-mail: ghosh_59@yahoo.com
Electronic supplementary information (ESI) available: Crystallographic data in
3
CIF format; Fig. S1–S5 illustrating specific crystal-packing features for
compounds 1–3 and Fig. S6–S9 showing the IR spectra for the free ligand pbptz
and 1–3, respectively. CCDC 918798–918800. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3ce00058c
Scheme 1 Representation of the ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine
(pbptz).
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of all three coordination polymers have been probed and
compared.
Single-crystal X-ray crystallography
Suitable single crystals of compounds 1–3 were mounted on a
Bruker SMART diffractometer equipped with a graphite
monochromator and Mo Ka (l = 0.71073 Å) radiation. The
crystallographic data were collected at room temperature. The
crystals were positioned at 60 mm from the CCD 360 frames
were measured with a counting time of 10 s. The structures
were solved by Patterson methods using SHELXS-97.
Subsequent difference Fourier synthesis and least-square
refinement revealed the positions of the remaining non-
hydrogen atoms. Non-hydrogen atoms were refined with
independent anisotropic displacement parameters. Hydrogen
atoms were placed in idealized positions and their displace-
ment parameters were fixed to be 1.2 times larger than those
of the attached non-hydrogen atom. Successful convergence
was indicated by the maximum shift/error of 0.001 for the last
cycle of the least squares refinement. Absorption corrections
were carried out using the SADABS program.26 All calculations
were carried out using SHELXS-97,27 SHELXL-97,28 PLATON
99,29 ORTEP-32,30 and WinGX systemVer-1.64.30 Data collec-
tion, structure refinement parameters and crystallographic
data for the three complexes are given in Table 1.
Crystallographic data for the structures reported in this paper
have been deposited in the Cambridge Crystallographic Data
Centre with CCDC reference numbers 918798–918800 for
complexes 1–3, respectively.
Experimental
Materials and general methods
All
reagents
except
Mn(C6H5COO)2?4H2O
and
Mn(C6H5CH2COO)2?H2O, which were prepared applying a
published procedure,24 were commercially available and used
as received. Elemental analyses (carbon, hydrogen and
nitrogen) were performed using a Perkin-Elmer 240C elemen-
tal analyser. FT-IR spectra (in KBr) were recorded using a
Perkin-Elmer RXI FT-IR spectrophotometer (4000–500 cm21).
Caution! Perchlorate salts of metal coordination compounds
with organic ligands are potentially explosive. Only small
amounts of material should be prepared and handled with
great care.
Preparation of pbptz
The ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (pbptz) was
synthesized as a purple solid, following a literature method
using 4-cyanopyridine and hydrazine monohydrate as starting
reagents.25
Synthesis of [Mn(NCS)2(pbptz)2]n (1).
A solution of
Mn(ClO4)2?6H2O (0.722 g, 2 mmol) in MeOH (5 mL) was
added to a solution of pbptz (0.944 g, 4 mmol) in DCM (10
mL). NH4SCN (0.152 g, 4 mmol) in 10 mL of H2O–MeOH (1 : 1)
was added and the resulting reaction mixture was stirred for
about half an hour and filtered, subsequently. The filtrate was
kept unperturbed at room temperature for the slow evapora-
tion of the solvent. Deep-red single crystals suitable for X-ray
diffraction studies were obtained after a few days. Yield = 78%
(0.501 g). Elemental analysis for C26H16MnN14S2, calcd. (%): C,
48.52; H, 2.51; N, 30.47. Found (%): C, 48.43; H, 2.62; N, 30.31.
IR data (KBr pellet, cm21): 1391 n(NLN), 2076 n(NCS).
Syntheses of [Mn(C6H5CH2COO)2(pbptz)(H2O)2]n (2) and
[Mn3(C6H5COO)6(pbptz)2]n (3). A methanolic solution (10
mL) of Mn(C6H5CH2COO)2?H2O (0.686 g, 2 mmol) was added
to a solution of pbptz (0.944 g, 4 mmol) in 10 mL of DCM. The
resulting reaction mixture was stirred for about half an hour.
Next, the solution was filtered, and the subsequent filtrate was
kept unperturbed for the slow evaporation of the solvent. Red-
coloured single crystals of 2 were collected after a few days.
Compound 3 was obtained applying the same procedure,
using Mn(C6H5COO)2?4H2O (0.738 g, 2 mmol) instead of
Mn(C6H5CH2COO)2?H2O.
Theoretical methods
The energies of compounds 1 and 2 described in the present
study were computed at the BP86-D3/def2-TZVPD level of
theory using the crystallographic coordinates within the
program TURBOMOLE version 5.10.31 The interaction energies
were calculated with correction for the basis set superposition
error (BSSE) by using the Boys–Bernardi counterpoise techni-
que.32 The calculation of the wavefunction to perform the
‘‘atoms-in-molecules’’ (AIM)33 analysis was computed at the
same level of theory. The calculation of AIM properties was
performed with the AIM2000 program.34
Results and discussion
Structure description
Reaction of manganese(II) perchlorate with pbptz in the
presence of ammonium thiocyanate yields red single crystals
of [Mn(NCS)2(pbptz)2]n (1). Compound 1 crystallizes in the
centrosymmetric space group Cmca. Crystallographic data for
1 are given in Table 1. The coordination environment of 1 is
shown in Fig. 1 together with its atomic-numbering scheme.
Selected bond distances and angles are listed in Table 2. The
Mn(II) ion lies on a crystallographic inversion centre, in a
slightly distorted octahedral environment (the angles varying
from 86.04(8) to 93.96(8)u). The axial positions are occupied by
two thiocyanate nitrogen atoms with Mn–NSCN distances of
2.158(3) Å. The equatorial plane of the octahedron consists of
four nitrogen atoms (N2, N2a, N2b and N2c) belonging to four
different bridging pbptz ligands, with Mn–Npbptz bond lengths
of 2.309(2) Å, conferring an axially-compressed octahedral
Compound 2: yield = 74% (0.441 g). Elemental analysis for
C28H26N6O6Mn, calcd. (%): C, 56.29; H, 4.39; N, 14.07. Found
(%): C, 56.44; H, 4.47; N, 13.95. IR data (KBr pellet, cm21): 1364
n(NLN), 1577 n(COO), 3432 n(H2O).
Compound 3: yield = 70% (0.318 g). Elemental analysis for
C66H46N12O12Mn3, calcd. (%): C, 58.12; H, 3.40; N, 12.32.
Found (%): C, 58.04; H, 3.32; N, 12.40. IR data (KBr pellet,
cm21): 1400 n(NLN), 1558 ns(COO), 1604 nas(COO).
The IR spectra of compounds 1–3 are shown in the ESI.
3
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Table 1 Crystal data and structure refinement for 1–3
1
2
3
Empirical formula
Fw (g mol21
Crystal system
C26H16MnN14S2
643.61
Orthorhombic
Cmca
C28H26MnN6O6
597.49
Monoclinic
P21/c
C66H46Mn3N12O12
1363.97
Triclinic
¯
P1
)
Space group
Temperature (K)
293(2)
0.03 6 0.05 6 0.07
21.1177(16)
14.9308(11)
8.8929(7)
90
90
90
293(2)
0.05 6 0.06 6 0.07
15.637(5)
5.627(5)
16.468(5)
90
116.561(5)
90
293(2)
0.05 6 0.06 6 0.07
11.336(5)
12.249(5)
13.784(5)
85.864(5)
67.811(5)
74.415(5)
1706.2(12)
1
Crystal size (mm3)
a (Å)
b (Å)
c (Å)
a (u)
b (u)
c (u)
V (Å3)
Z
2804.0(4)
4
1296.1(13)
2
rcalcd
m (mm21
F(000)
h for data collection (u)
Reflections collected/unique
Completeness to theta
1.525
0.666
1308
1.9–24.77
14 626/1246
100
1.531
0.566
618
1.46–28.88
14 727/3407
99.6
1.327
0.614
697
1.60–22.67
6255/4141
90.5
)
Data/restraints/parameters
1246/0/104
1.037
3407/0/203
1.048
4141/0/421
0.903
Goodness-of-fit on F2
Final R indices [I . 2s(I)]
R indices (all data)
R1 = 0.0379, wR2 = 0.0920
R1 = 0.0595, wR2 = 0.1029
0.297 and 20.419
R1 = 0.0329, wR2 = 0.0824
R1 = 0.0429, wR2 = 0.0875
0.377 and 20.215
R1 = 0.0581, wR2 = 0.1360
R1 = 0.1196, wR2 = 0.1639
0.359 and 20.459
Largest diff. peak and hole (e Å3)
…
geometry about the metal ion. The four bridging tetrazine
ligands connect the manganese ion to four adjacent metals,
coordinated pyridine units (S1 Cg1 = 3.730(2) Å; Table 3 and
Fig. S2B, ESI ), which are more p-acidic than free pyridines.35
3
generating
a 2D (4,4) square-grid framework, with net
Compound 2, i.e. [Mn(C6H5CH2COO)2(pbptz)(H2O)2]n, is
obtained by reaction of manganese(II) phenylacetate with
pbptz. X-ray diffraction studies on a red single crystal of 2
dimensions of 15.694 6 15.694 Å2 (Fig. 2).
These large voids inevitably result in interpenetration, (see
Fig. S1, ESI ), as will be illustrated in more details in the
3
topological section (see below). This intricate structural
…
architecture is stabilized by p p interactions involving both
Table 2 Selected bond distances (Å) and angles (u) for 1–3a
aromatic groups of the ligand pbptz, namely the pyridine and
tetrazine rings, with centroid-to-centroid contact distances of
…
1
3.881(2) Å (Cg1 Cg2; Table 3 and Fig. S2A, ESI ), and lone
3
…
Mn1–N1
Mn1–N2
2.158(3)
2.309(2)
N2–Mn1–N2a
N2a–Mn1–N2b
N1–Mn1–N1a
86.04(8)
93.96(8)
180
pair p interactions between thiocyanate sulfur atoms and
b
…
Mn1 Mn1inter
8.689(3)
2
Mn1–O1
Mn1–O3
Mn1–N1
2.159(2)
O1–Mn1–O3
O1–Mn1–O3b
N1–Mn1–N1b
89.92(4)
90.08(4)
180
2.191(2)
2.302(3)
5.627(3)
b
…
Mn1 Mn1inter
3
Mn1–O1
Mn1–O4
Mn1–O5
Mn1–O3
Mn1–N1
Mn1–N2
2.070(5)
2.246(5)
2.092(5)
2.274(5)
2.234(6)
2.412(6)
O1–Mn1–O3
O3–Mn1–O4
O4–Mn1–N1
N1–Mn1–O1
N2–Mn1–O5
108.5(2)
58.5(2)
94.9(2)
95.1(2)
172.5(3)
Mn2–O2
Mn2–O3
Mn2–O6
2.211(5)
2.242(5)
2.159(6)
O2–Mn2–O6
O2c–Mn2–O6
O3–Mn2–O3c
89.02(19)
90.98(19)
180
c
…
…
Mn1 Mn2
3.593(2)
Mn1 Mn1inter
6.067(6)
a
Fig. 1 Representation of the single-crystal X-ray structure of 1. Hydrogen atoms
have been omitted for clarity. Symmetry operations: a = x, 2y, 1 2 z; b = 1 2 x,
2y, 1 2 z; c = 1 2 x, 2y, 2 2 z.
Symmetry operations: a = x, 2y, 1 2 z; b = 1 2 x, 2y, 1 2 z; c = 1
b
c
…
2 x, 2y, 2 2 z. Closest inter-trinuclear Mn Mn distance. Closest
inter-mononuclear Mn Mn distance.
…
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Fig. 3 Representation of the single-crystal X-ray structure of 2. Only the water
hydrogen atoms are shown for clarity. Symmetry operations: b = 1 2 x, 2y, 1 2
z.
coordinated oxygen atoms (O2) from phenylacetato ligands of
adjacent 1D chains (Fig. S3A, ESI ).
3
…
These hydrogen bonds (O3–H41 O1; Table 3) thus connect
the coordination polymers to each other along the crystal-
lographic b axis, producing a 2D supramolecular sheet in the
Fig. 2 2D (4,4) square grid in the structure of 1 with net dimensions. Hydrogen
atoms have been omitted for clarity.
ab plane (Fig. S3B, ESI ). As anticipated, the electron-rich
3
phenyl units (Cg3) of the organic anions are stacking with the
electron-deficient tetrazine rings (Cg2) of neighbouring 2D
sheets, in the crystallographic c direction, generating a 3D
reveal that it crystallizes in the centrosymmetric space group
P21/c. Crystallographic data are given in Table 1 and selected
bond distances and angles are listed in Table 2. The
coordination environment of 2 exhibits one crystallographi-
cally unique manganese(II) atom lying on an inversion centre,
one pbptz ligand, two phenylacetate anions and two water
molecules (see Fig. 3, in which two pbptz ligands are shown to
complete the coordination around the metal). Hence, each
metal centre is six-coordinated to two oxygen atoms (O1 and
O1b) belonging to two phenylacetato ligands, two water
molecules (O3 and O3b) and two nitrogen atoms from distinct
pbptz ligands (N1 and N1b). The resulting regular octahedron
(the equatorial angles being very close to the ideal value of 90u;
Table 2) is characterized by equatorial bond distances of
2.159(2) Å (Mn–O1; Table 2) and 2.191(2) Å (Mn–O3; Table 2),
and by slightly elongated axial Mn–N1 bond lengths of 2.302(3)
Å (Table 2). The bridging, ditopic pbptz ligands connect
manganese(II) ions to generate a 1D coordination polymer
along the crystallographic a axis (Fig. 4), which exhibits strong
intramolecular hydrogen-bonding interactions between coor-
dinated phenylacetato ligands and water molecules (O3–
framework (Fig. S4, ESI ). These strong parallel-displaced p
3
stacking interactions36 are characterized by a centroid-to-
…
…
centroid distance Cg2 Cg3 of 3.581(3) Å, with short C1 C9
…
and C11 N3 contact distances of respectively 3.366(4) and
3.249(4) Å (see Table 3 and Fig. S4, ESI ), which are in the
3
range of the corresponding sum of the van der Waals radii of C
and C (3.40 Å), and C and N (3.25 Å).
Comparison of the solid-state structures of 1 and 2 discloses
some important features. First, most likely as the result of the
steric hindrance of the phenylacetate anions (in contrast to the
linear thiocyanate ions), only two pbptz ligands coordinate to
the manganese(II) ion in 2 while four of them are bound to the
metal centre in 1. Consequently, the coordination sphere of 2
contains two water molecules as small ligands, impeding the
formation of a 2D square-grid network. However, these water
molecules are involved in hydrogen-bonding interactions,
giving rise to a 2D supramolecular framework (while it is a
2D coordination framework for 1). Second, as expected, the
phenyl group of the organic anion strongly interacts with the
tetrazine rings; these supramolecular bonding interactions
actually drive the formation of a non-interpenetrated (in
contrast to 1) 3D architecture (thus resulting from the
p-stacking induced self-assembly of the hydrogen-bonded 2D
sheets).
…
H40 O2; Fig. 4 and Table 3). In addition, the water molecules
(O3) are involved in a second hydrogen-bonding network with
Table 3 Supramolecular bond distances (Å) and angles (u) for 1–3
1
…
…
Cg1 Cg2
3.881(2)
Cg1 S1
3.730(2)
2
…
…
O3–H40 O2
O3–H41 O1
Cg2 Cg3
C1 C9
2.658(3)
2.772(3)
3.581(3)
3.366(4)
2.658(3)
O3–H40–O2
O3–H41–O1
162(3)
167(3)
…
…
…
C11 N3
3.249(4)
162(3)
…
Fig. 4 Formation of a 1D coordination polymer along the crystallographic a axis.
The green dotted lines symbolize the intramolecular strong hydrogen bonds
involving the coordinated phenylacetate ligands (O2) and water molecules (O3).
O3–H40 O2
O3–H40–O2
3
…
…
…
Cg9 Cg13
3.669(5)
C16 C66
3.377(12)
O3–H40 O2 = 2.658(3) Å and /O3–H40–O2 = 162(3)u.
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Fig. 5 Representation of the single-crystal X-ray structure of 3. Hydrogen atoms
have been omitted for clarity. Symmetry operations: c = 1 2 x, 2y, 2 2 z; d = 2x,
1 2 y, 1 2 z.
Compound 3, i.e. [Mn3(C6H5COO)6(pbptz)2]n, is prepared by
reaction of manganese(II) benzoate with the ligand pbptz.
X-ray diffraction studies on a red single crystal of 3 reveal that
the coordination compound crystallizes in the triclinic space
Fig. 6 2D rhombohedral grid in the structure of 3 with net dimensions.
Hydrogen atoms have been omitted for clarity.
¯
group P1. Crystallographic data for 3 are given in Table 1 and
…
selected bond lengths and angles are listed in Table 2. A
representation of the coordination environment of 3 is
depicted in Fig. 5. 3 exhibits two types of manganese(II) ions,
namely Mn1 and Mn2.
Table 3), as evidenced by the C16(tetrazine) C66(phenyl)
contact distance of 3.377(12) Å, producing an interdigitated
packing structure (Fig. S5C, ESI ).
3
In a first instance, the difference between the phenylacetate
and benzoate anions appears insignificant. However, the solid-
state structures of 2 and 3 present several differences. The
binding mode of the two anionic ligands is not the same; while
phenylacetate acts as a monodentate ligand, benzoate func-
tions as a bridging bidentate or tridentate ligand. In fact, the
benzoate anion has a clear tendency to behave as a bridging
ligand. Indeed, a search at the Cambridge Structural Database
(CSD v 5.33 including three updates (May 2012) using
ConQuest v 1.14) reveals that out of the 1,490 solid-state
structures containing the benzoato ligand, 945 include
bridging benzoato ligands. Actually, C6H5COO2 is often used
for its bridging aptitudes to prepare metal clusters.37,38 For the
phenylacetate, only 72 structures are found in the CSD, of
which 40 have this anion as bridging ligand. The formation of
a linear trinuclear cluster in 3 allows the bonding of two pbptz
ligands to the external manganese(II) ions, which permits the
formation of a 2D coordination framework that resembles that
of 1. Similarly to 1, compound 3 does not exhibit coordinated
water molecules (unlike 2); hence, no hydrogen-bonding
network is observed. However, since 3 contains an organic
Mn1 is characterized by a strongly distorted octahedral
coordination geometry, which is apparently due to the very
small bite angle, i.e. O3–Mn1–O4 = 58.5(2)u (Table 3), of the
m-O,O9,O9 benzoato ligand [O3, O4]. The equatorial plane is
constituted of two oxygen atoms belonging to the m-O,O9,O9
benzoato ligand (O3 and O4), one oxygen atom from a m-O,O9
benzoato ligand (O1), and one nitrogen atom from a pbptz
ligand (N1). The distorted octahedron is completed by the
nitrogen atom of a second pbptz ligand (N2) and one oxygen
atom belonging to a third benzoato ligand (O5), at the axial
positions. The Mn–O and Mn–N bond distances can be
considered as normal for this type of MnN2O4 coordination
environment (Table 2).
Mn2 lies on an inversion centre and its coordination
environment is an almost perfect octahedron, as illustrated
by the angles of 89.02(19) and 90.98(19)u (close to the ideal
value of 90u). At the equatorial plane, Mn2 is coordinated by
four oxygen atoms (O2, O3, O2c and O3c; Fig. 5) from four
different benzoato ligands. The axial positions are occupied by
the two oxygen atoms O6 and O6c belonging to two m-O,O9
benzoato ligands. Hence, Mn2 is bridged to two symmetry-
related Mn1 ions via four m-O,O9 benzoato ligands and two
m-O,O9,O9 benzoato ligands, generating a linear trinuclear
[Mn1, Mn2, Mn1d] cluster (see Fig. 5).
…
anion with an aryl group, p p interactions take place with the
electron-deficient tetrazine ring that connect the 2D rhombo-
hedral grids to produce a 3D architecture, as in compound 2.
In summary, while the crystal structures of 1 and 2 are
drastically distinct, the 2D framework of 3 is related to that of
The external manganese(II) ions Mn1 and Mn1d exhibit two
ditopic pbptz ligands that are perpendicularly coordinated (the
N1–Mn1–N2 angle amounts to 88.0(2)u). Accordingly, each
trimanganese unit is connected to four adjacent trinuclear
clusters, producing a 2D rhombohedral grid structure, as
depicted in Fig. 6. This coordination framework is character-
ized by net dimensions of 17.767 6 20.084 Å2. In contrast to 1,
interpenetration is not observed; actually, the 2D layers
…
1, and its 3D architecture is driven by pphenyl ptetrazine
interactions, like 2.
Network topologies
The topology of the three coordination compounds has been
examined in more details using the TOPOS software.39,40 As
mentioned earlier, the structure of 1 consists of a 2D (4,4)
square-grid framework, in which the metal is the node of the
net while the pbptz ligand behaves as 2-coordinated con-
nector. Applying to this structure the simplification process
assemble in an alternated fashion (see Fig. S5A and S5B,
…
ESI ) by means of strong p p interactions between phenyl
3
…
(Cg13) and tetrazine (Cg9) rings (Cg9 Cg13 = 3.669(5) Å;
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Fig. 8 View of the parallel 2D sheets describing the structures of 2 and 3. The
blue spheres are the Mn atoms acting as nodes of the net.
In the case of compound 2, a meaningful topological
analysis should include the noncovalent interactions present
in the crystal structure. In fact, as previously shown in Fig. S3,
ESI, the 1D coordination polymers are organized into non-
3
interpenetrating, parallel layers via hydrogen bonds between
coordinated water molecules and phenylacetato ligands. If
these H-bonds are formally treated as valence bonds, and a
simplification is carried out on the structure to remove 0-, 1- or
2-connected units, each of these layers can be again described
as a sql/Shubnikov tetragonal plane net (Fig. 8).
Finally, the topology of compound 3 can be best described
using the cluster representation,44 in which the pbptz ligands
(2-c linkers) connect [Mn3(C6H5COO)6] units, considered as
cluster nodes. Following the simplification of the organic
ligand, the structure results again in a series of parallel, non-
interpenetrating sheets (Fig. 6 and 8) of the topological type
sql, as for compound 2 (even if, in contrast to 2, the 2D
framework is based on coordinative interactions and not on
hydrogen bonding).
Fig. 7 A) View of the 2D+2D inclined interpenetration network. The two
different sets of parallel planes are shown in blue and red. B) View of the 4-4-
Hopf link. C) View of the 4-4 non-Hopf link. D) Combined view of the possible
links. The blue 4-cycle on the left forms a Hopf link with the red one, while the
one on the right just penetrates the window of the red cycle with one angle.
Computational studies
The noncovalent interactions attributable to the different
anions (namely thiocyanate and phenylacetate) used to
prepare compounds 1 and 2 have been analysed in more
detail by theoretical calculations. More conventional interac-
tions such as hydrogen bonding and coordination bonds are
very important as elements of cohesion within the networks
studied herein. However, we have focused our attention to the
which removes the 2-c units, the resulting framework can be
described as a sql/Shubnikov tetragonal plane net {44.62} with
vertex symbol [4.4.4.4.*.*].41
…
role played by lp p, C–H/p and p–p interactions, which are
A further analysis shows that the 2D sheets are related to
each other through polycatenation, giving rise to a parallel/
parallel inclined interpenetration, which occurs when each
grid of a sheet contains rods passing through them from other
comparatively less studied noncovalent forces that are also
fundamental for the final architecture of the networks.
A
remarkable supramolecular feature observed in the
sheets (Fig. 7A and S1, ESI ).42,43 More specifically, the
tridimensional architecture of
1 is represented by the
3
45
…
(bifurcated) lone-pair p interactions that occur between
resulting 3D network is formed by two sets of parallel planes,
namely (0, 2, 1) and (0, 2, 21) with a dihedral angle of 80u; the
planes cross through the line [1, 0, 0]. From a more detailed
analysis it is also possible to classify the types of 4-cycles
involved in the 3D net and to show how they are related to
each other. Two cases are present: i) pairs of 4-cycles belonging
to two different sets in which only one type of crossing exists.
Each cycle crosses another cycle one time forming one link
(knot) of the Hopf type, and on the whole each cycle is crossed
by four other cycles (Fig. 7B, the cycles belonging to different
sets are coloured in blue and red). ii) Pairs of 4-cycles involved
into two crossings which are not links (the cycles are not
entangled); each cycle is crossed by two other cycles (Fig. 7C).
the sulfur atom of the SCN2 ligand and the pyridine ring of
the pbptz ligand (Fig.
9 and S2B, ESI ). In fact, the
3
coordination of the pyridine moiety to the manganese(II) ion
increases the p-acidity of the aromatic ring, thus favouring the
…
lp
p bonding contact. This issue has been examined
theoretically. In the theoretical model used to evaluate this
interaction, the coordination sphere of the metal centre has
been saturated with ammonia molecules to simulate the pbptz
ligands (Fig. 9). We have used this small molecule as ligand
model instead of other possible larger models like pyridine in
order to keep the size of the systems computationally feasible.
Indeed, due to the large number of unpaired electrons, SCF
convergence problems are commonly encountered with
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Fig. 10 Models used to compute the interaction energies of the binary (DE4 and
DE4BEN) and ternary (DE5) complexes observed in the solid-state structure of
compound 2. Values in italics correspond to energies computed using the MP2
method.
a bifurcated hydrogen bond involving aromatic protons.47 This
is due to an enhancement of the acidity of the hydrogen atoms
of the pyridine ring upon coordination to the manganese(II)
ions.
…
Next, the p p stacking interactions noticed between the
Fig. 9 A) Model of a fragment of the structure of 1 used to evaluate
…
theoretically the bifurcated lp p interactions; B) theoretical model used to
central tetrazine ring of the bidentate ditopic ligand and the
phenyl group of the organic anion (Fig. S4, ESI ) in the crystal
packing of 2 have been examined. Two aspects have been
taken into consideration. Possible cooperativity effects have
been first evaluated since each tetrazine ring is establishing
…
assess the role played by the Mn ions in the lp p interactions; C) model of a
3
…
fragment of the structure of 1 used to evaluate theoretically the S
H
interactions.
…
two concurrent p p stacking interactions with two phenyl
units (Fig. 10 and S4, ESI ). Subsequently, the influence of the
calculations carried out on such complexes, which are
minimized by reducing the size of the system. However, in
the simpler systems studied and discussed below we have
examined the influence of the sp2/sp3 nature of the model
ligand on the interaction energy and we have not found
significant differences.
3
coordination of the manganese ion on the interaction energy
has been investigated (Fig. 11). These calculations have been
carried out using the BP86 functional with the latest available
correction for dispersion (D3) in order to properly assess the
p-stacking interactions. Moreover, we have computed the
interaction energies DE4 and DE5 using the ab initio MP2
method and the same basis set. We have included the MP2
energies in italics in Fig. 10. It can be observed that the
interaction energies are comparable, giving reliability to the
BP86-D3/def2-TZVPD level of theory.
The two theoretical models used to analyse the stacking
interactions between the pbptz ligand and phenyl group(s) in
the absence of manganese are depicted in Fig. 10. In both the
binary and ternary complexes, the phenyl–tetrazine interac-
tion(s) involve(s) two different types of supramolecular
The interaction energy of this system where two bifurcated
lp–p interactions are established is DE1 = 220.2 kcal mol21
…
(Fig. 9A). Therefore, each lp
p contact contributes to
approximately 25 kcal mol21, suggesting a more favourable
bonding interaction than that calculated for lp p interactions
…
involving hexafluorobenzene (ca. 23 kcal mol21).46 This
energy increase is probably due to an alteration of the
electronic nature of the pyridine ring upon coordination to
the manganese(II) ion that gives rise to an enhancement of the
interaction. To verify this hypothesis, the interaction energy of
a model for which one Mn ion has been removed and one
pbptz ligand has been maintained at the position found in the
crystal structure (see Fig. 9B) has been computed. Using this
system, the interaction energy is lessened to 22.7 kcal mol21
(instead of 25 kcal mol21; see above). This result clearly
demonstrates that the presence of the coordinated metal
…
interactions, namely p p stacking contact(s) (with a cen-
troid-to-centroid distance of 3.58 Å and a centroid to mean
plane distance of 3.35 Å), and C–H/p interaction(s) between the
methylene unit of the organic anion and the ligand pyridine
ring (with a hydrogen atom-to-centroid distance of 2.99 Å). The
aromatic ring of the phenylacetate is located above the
s-tetrazine ring and the methylene group over the pyridine
ring (Fig. 10). The interaction energy for the binary complex is
29.7 kcal mol21 (DE4), which is stronger than that expected for
weak noncovalent p p and C–H/p interactions. This surpris-
ingly high value is most likely due to particularly favoured p
interactions ascribed to the electronic nature of the interacting
rings (i.e. donor–acceptor interactions are taking place
…
centre significantly augments the lp p interaction. In addi-
…
tion, this also explains why the lp p interaction is established
with the pyridine ring instead of the more p-acidic s-tetrazine
ring, which would be the situation expected without coordi-
nated metal ions. Finally, we have also evaluated the relevant
…
…
…
p
intermolecular C–H S hydrogen bonds formed between the
sulfur atom of the SCN2 ligand and the pyridine ring of the
pbptz ligand (Fig. 9C). The interaction energy of this system
where two bifurcated hydrogen bonds are established is DE3 =
213.4 kcal mol21. This energy is higher than that expected for
between the interacting partners). To estimate the contribu-
…
tion of each interaction, the interaction energy of the p
p
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Fig. 12 Distribution of the critical points in crystal fragment of A) compound 1
and B) compound 2. Bond-, ring- and cage-critical points are represented by red,
yellow and blue spheres, respectively. The bond paths are also indicated.
Fig. 11 Models used to compute the interaction energies of the binary (DE6)
and ternary (DE7) complexes observed in the solid state of compound 2,
including two coordinated manganese(II) ions. Two different model ligands
have been used to evaluate the interactions.
Finally, the Bader’s theory of ‘‘atoms in molecules’’ (AIM),33
which provides an unambiguous definition of chemical
bonding, has been used to further describe the noncovalent
interactions described above for compounds 1 and 2. The AIM
theory has been successfully applied to characterize and better
understand a great variety of interactions including those
investigated herein. The representation of critical points and
bond paths computed for the present assemblies are depicted
in Fig. 12. For compound 1, each bifurcated lp–p interaction is
characterized by the presence of two bond critical points that
stacking complex between the pbptz ligand and benzene
(instead of phenylacetate) located exactly at the same position
(denoted as DE4BEN) has been computed (see Fig. 10, bottom).
BEN
DE4
amounts to 27.9 kcal mol21; hence, the energy
difference with DE4 can be used to appraise the contribution
of C–H/p contacts, which is 21.8 kcal mol21. This estimation
agrees very well with values reported for a number of C–H/p
connect the sulfur atom of the SCN2 ligand to two carbon
…
atoms of two pyridine rings (Fig. 12A). For 2, each p
p
systems, i.e. in the range 1.5–2.5 kcal mol21 48
Comparison of
.
interaction involving the central tetrazine ring of the pbptz
ligand and the phenylacetate is characterized by the presence
of two bond critical points connecting one carbon and one
nitrogen atoms of the tetrazine ring to two carbon atoms of the
phenyl ring. This interaction is further described by the
presence of two ring- and one cage-critical points. In addition,
each C–H/p interaction is characterized by the presence of one
bond-critical point that connects the hydrogen atom of the
methylene group to one carbon atom of the pyridine ring
(Fig. 12B).
The presence of the aforementioned critical points confirm
the presence of the noncovalent interactions in the solid-state
structures of compounds 1 and 2, which have been energeti-
cally evaluated by DFT calculations; these supramolecular
bonding interactions certainly influence the solid-state archi-
tecture of the two coordination polymers.
the interaction energies DE4 and DE5 clearly indicates lack of
cooperativity in the ternary complex. In fact, the interaction
energy is additive since the value for the ternary complex is
twice that of the binary complex (219.5 versus 29.7 kcal
mol21).
A theoretical analysis has been also performed with the
pbptz ligand coordinated to two manganese(II) ions. The other
ligands bound to the metal centres (found in the crystal
structure) have been replaced by smaller molecules to keep the
size of the model system feasible computationally. Hence, the
phenylacetate ligands have been changed to formates and the
two external pbptz ligands have been replaced by either
ammonia or pyridine molecules (Fig. 11). Pyridine was
employed in these particular systems to validate the use of
ammonia; indeed, the real structure includes a p-ligand that
may have some electronic influence on the metal. Actually, the
interaction energies are comparable (see bottom of Fig. 11) for
both ligands, i.e. the difference is ,0.6 kcal mol21; thus
validating the utilization of the smaller molecule as a
theoretical model. It is interesting to compare the interaction
energies gathered in Fig. 10 (DE4 and DE5; without
manganese(II) ions) with those shown in Fig. 11 where exactly
the same interactions are studied in the presence of the metal
centre. The interaction (DE6 = 213.0 kcal mol21) of the binary
complex is clearly more favourable when the ligand is
coordinated to two metal ions. In fact, the coordination of
the manganese(II) ions increases the p-acidity of the s-tetrazine
Conclusions
In the present study, a s-tetrazine-based linear and ditopic
ligand has been used to generate manganese-containing
coordination networks. Hence, different anions, one inorganic
(i.e. thiocyanate) and two organic (i.e. phenylacetate and
benzoate), have been used to investigate their potential
noncovalent interaction with the ligand 3,6-bis(4-pyridyl)-
1,2,4,5-tetrazine (pbptz) in the respective solid-state structures
obtained. As anticipated pbptz, which includes the electron-
deficient tetrazine ring and two pyridine groups that become
electron-poor upon coordination, is involved in a number of p
interactions. For compound 1 with the NCS2 ion, the organic
…
ring, therefore strengthening the p p stacking interactions. A
significant intensification of the interaction is also observed
for the ternary complex, since the DE7 interaction energy value
of 226.4 kcal mol21 is considerably more negative than the
DE5 one (Fig. 10), computed for the ternary complex in the
absence of manganese(II) ions.
…
tetrazine unit undergoes p p interactions and the coordinated
…
pyridine rings are implicated in lone pair p bonding contacts
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Acknowledgements
PS is thankful to UGC for Junior Research Fellowship (UGC/20/
Jr. Fellow (Sc) dated 10.01.2011). PG acknowledges the
´
Institucio Catalana de Recerca I Estudis Avançats (ICREA).
´
PG and AF acknowledge the Spanish Ministerio de Economıa y
Competitividad (MINECO) for funding (Projects CTQ2011-
27929-C02-01 and CTQ2011-27512/BQU, and CONSOLIDER
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