Packing polymorphism in 3-amino-2-pyrazinecarboxylate based tin(ii) complexes and their catalytic activity towards cyanosilylation of aldehydes

Article abstract of DOI:10.1039/C8NJ03805H

Reactions of 3-amino-2-pyrazinecarboxylic acid (HL) with SnCl₂ under different hydrothermal conditions and crystallization processes produced two new mononuclear Sn(ii) compounds (1 and 2) of general formula [Sn(κN,O-L)₂] (L = 3-amin

Full text of DOI:10.1039/C8NJ03805H

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Packing polymorphism in 3-amino-2-pyrazinecarboxylate based tin(II)
complexes and their catalytic activity towards cyanosilylation of
aldehydes
Anirban Karmakar*, Susanta Hazra, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva and
Armando J. L. Pombeiro*
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1
049–001, Lisbon, Portugal. E-mail: anirbanchem@gmail.com and pombeiro@tecnico.ulisboa.pt.
Abstract
Reactions of 3-amino-2-pyrazinecarboxylic acid (HL) with SnCl under different hydrothermal conditions
2
and crystallization processes produced two new mononuclear Sn(II) compounds (1 and 2) of general
formula [Sn(κN,O-L) ] (L = 3-amino-2-pyrazinecarboxylate), the single crystal X-ray diffraction analysis
2
indicating that they are packing polymorphs, complex 1 crystallizing in the monoclinic C2/c space group
whereas 2 in the monoclinic P2 /c. The Hirshfeld surface analysis show that the supramolecular features
1
of both forms are guided by Sn···O and Sn···N contacts, as well as by strong N-H···O and N-H···N
hydrogen bonds that differentiate the packing in the two forms. Complex 1 shows one dimensional
hydrogen bonded network whereas 2 shows a 2D hydrogen bonded network. Best of our knowledge no
examples of packing polymorphs based on tin(II) complexes have been reported so far. These
polymorphs are interconvertible by immersing in suitable solvent or mixture of solvents. They also act as
heterogeneous catalysts for the cyanosilylation reaction of aldehydes with trimethylsilyl cyanide
(
TMSCN) and can be recycled several times without losing activity.
Keywords: Packing polymorphism; Mononuclear tin(II) complex; 3-Amino-2-pyrazinecarboxylic acid;
Topological analysis; Hirshfeld surface analysis; heterogeneous catalysis
Introduction
1
Polymorphism concerns the ability of a molecule (or compound) to exhibit different crystalline forms .
Different packing arrangements or conformations can lead to significant changes of the physical and
2
chemical properties of polymorphs, which make them attractive in supramolecular chemistry . The
crystallization process leading to a polymorph is generally sensitive to a variation in the temperature,
3
4
pressure and solvent . Although polymorphism has been extensively investigated in the organic field ,
particularly focused on drug design, a growing attention is being devoted to the polymorphism of
1

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5
6
7
transition metal complexes , coordination polymers , and organometallic compounds . A few types of
polymorphism are known, such as solvo- or solvatomorphism, pseudopolymorphism, packing,
8a-c
conformational and synthon polymorphism . One of the lowest predicted ones is the packing
polymorphism, although it can be important for medicinal chemistry. Packing polymorphs differ in the
8d-g
three-dimensional crystal packing, without noteworthy changes in conformation of the molecules
.
An increased attention has been paid to low valent tin complexes due to their application in the
9
activation of small molecules . The large size of the heavy p-block element combined with the possibility
of stereochemical influence from a lone pair of electrons allows for several possible coordination
geometries, redox and catalytic properties. Tin coordination chemistry, in the majority of cases, deals
10
with Sn(IV) and organotin(IV) species . Complexes of tin(II) have been known since long, but the rich
11
Sn(II) structural chemistry is only slowly being revealed .
Hence, we have chosen 3-aminopyrazine-2-carboxylic acid (HL) which can be readily deprotonated to
-
produce L able to act as a N,O-multidentate ligand with versatile metal-binding and hydrogen-bonding
capabilities. Lately, it has drawn extensive attention for the construction of metal organic frameworks
1
2
12a-b
with attractive architectures . However, most of the reported work has focused on s-block , d-
12c-d
12e
block
and f-block metal ions, whereas the corresponding chemistry of the p-block metal ions is still
underexplored. To our knowledge, no study on tin(II) coordination chemistry with 3-aminopyrazine-2-
carboxylic acid (HL) (or its conjugate base) has been reported. Moreover, although a few examples of
12f-g
polymorphism in Sn(IV) complexes are known,
no examples of packing polymorphs based on tin(II)
complexes have been reported so far. Thus, aspects related to the coordination chemistry and
polymorphism of tin(II) complexes deserve to be explored, what constitutes an aim of the current study.
13
Cyanosilylation is an important carbon–carbon bond formation reaction in organic chemistry , which is
used to synthesize, e.g., cyanohydrins which constitute as a very significant class of compounds in
chemistry as well as in biology, and have been widely utilized for the synthesis of a variety of important
14
compounds, such as α-hydroxy acids, α-hydroxy aldehydes, and β-amino alcohols . For the synthesis of
cyanohydrins, various cyanating reagents have been employed, and trimethylsilyl cyanide (TMSCN) is
one of the most useful and safe ones for nucleophilic addition to a carbonyl compound to give
15
cyanohydrin trimethylsilyl ether in the presence of a Lewis acid or base type catalyst . Thus, the
development of effective catalysts for cyanosilylation of carbonyl compounds with TMSCN is an
important subject in modern research. A number of discrete complexes and coordination polymers
1
6
17
18
containing various d-block , f-block and a few p-block metal ions have been utilized as homogeneous
2

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catalysts for such transformations, but only a few studies have been addressed to heterogeneous
19
catalysts . To our knowledge, no reports are available of Sn(II) complex which have been used for
cyanosilylation of aldehydes. Hence, the development of efficient Sn(II) catalysts (in particular,
heterogeneous ones) for cyanosilylation of carbonyl compounds with trimethylsilyl cyanide (TMSCN) is a
subject that deserves to be developed.
In the present work we have synthesized and structurally characterized two polymorphic
interconvertible tin(II) complexes, [Sn(κN,O-L) ] (1 and 2). The compounds have been characterized by
2
elemental analysis, FT-IR, thermogravimetric analysis and single-crystal and powder X-ray diffraction
analysis. Diverse intermolecular contacts generate different supramolecular networks, e.g., a 1D double-
chain in 1 and a 2D zig-zag in 2, indicating that they are packing polymorphs. We have performed
topological and Hirshfeld surface analysis for both polymorphs. We have also tested their
heterogeneous catalytic performance towards the cyanosilylation reaction of aldehydes with TMSCN.
Results and discussion
Syntheses and characterization
The syntheses of complexes 1 and 2 were carried out under solvothermal conditions, by reacting 3-
amino-2-pyrazinecarboxylic acid (HL) with tin(II) chloride in the presence of DMF (for 1) or N-
methylformamide and methanol mixtures (for 2) (Scheme 1). Upon immersing complex 1 into a 1:1
mixture of N-methylformamide and methanol for 4 days, it was converted to complex 2. Similarly, in
case of 2 upon immersing in dimethylformamide it converts into complex 1. This interconversion process
was studied by using power X-ray diffraction analysis. Although the main role of the solvents in the
formation of such packing polymorph is still not clear, their different polarities can play a role.
Scheme 1
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The FT-IR spectra of complexes 1 and 2 show similar pattern, revealing the characteristic asymmetric
-
1
-1
-1
(
1598 cm ) and symmetric (1369 cm ) stretching of carboxylic groups. A strong band at 1632 cm is due
-
1
20
to δ(N–H) of the amine group and the band at 1320–1318 cm to ν(C–N) (aromatic amines) . We have
also performed powder X-ray diffraction analysis for both the complexes (Figure S3, supporting
information). They were also characterized by single crystal X-ray diffraction, elemental and
thermogravimetric analyses.
Crystal structure analyses
Single-crystal X-ray diffraction studies reveal that the complexes have similar mononuclear structures
but crystallize in different space groups (1 in C2/c and 2 in P2 /c).
1
A
B
C
Figure 1 Crystal structures of 1 (A) and 2 (B) with partial atom labelling scheme. (C) Overlap of structures
(in red) and 2 (in blue).
1
The crystal structures of the complexes reveal that they contain one tin(II) ion bound to two 3-amino-2-
pyrazinecarboxylate ligands chelating the metal by means of one O- and one N-atoms, thus giving rise to
SnN O environments. For both the complexes the Sn-O and the Sn-N distances are in the ranges of
2
2
2
.134(2) – 2.155(2) Å and 2.386(3) – 2.443(3) Å, respectively. The metal cations, however, further
4

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establish contacts with two (in 1) or just one (in 2) vicinal molecules generating arrangements with
increase the metal coordination numbers. While in 1 such contacts involve the N-amine and a O-atom
from two distinct neighbours [dSn⋅⋅⋅N 3.549 Å; dSn⋅⋅⋅O 3.282 Å] giving rise to SnN O engagements, in 2 they
4
4
involve solely a O-atom from a symmetry generated molecule [dSn⋅⋅⋅O 3.481 Å] creating SnN O3
2
−
−
associations. As a consequence, the ligand is behaving as a 1κN,O;2κN’,O’-L or as a 1κN,O;2κO’-L
chelator in polymorph 1 or 2, respectively, therefore defining 1D infinite chains running along the
crystallographic c (in 1) or b (in 2) axis. The structure of 2 is further stabilized by Sn⋅⋅⋅π(pyridyl)
interactions (d Sn⋅⋅⋅centroid 3.850 Å) (Figure 3B). The O–Sn–O angles assume values of 88.29(11)º (in 1)
and 97.28(11)º (in 2), while the N–Sn–N angles reach 139.43(11) and 127.37(10) in 1 and 2, respectively.
The O–Sn–N metalacycle angles range from 65.59(8) to 71.23(10)º. The bond angles and distances are in
21a
the ranges observed in previously reported compounds . Selected bond distances and angles of
complexes 1 and 2 are listed in Table S3 (supporting information).
A
B
C
Figure 2 (A) One dimensional H-bond networks of 1. (B) Sn⋅⋅⋅N and Sn⋅⋅⋅O interactions in 1. (C) 3D
hydrogen bonding packing diagram of 1 [Hydrogen bonding interaction drawn in black dotted lines].
The packing diagrams brought about by H-bond interactions clearly indicate infinite 1D double chains
(
Figures 2A) for 1 and infinite 2D zig-zig sheets for 2 (Figure 2B). In 1 the amine group intra- and inter-
ꢁ
ꢁ
molecularly donates to O carboxylate atoms defining graph sets descriptors of types ꢀ ꢂ6ꢃ and ꢄ ꢂ6ꢃ ,
ꢁ
ꢁ
respectively.
5

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A
B
C
Figure 3 (A) Two dimensional H-bond networks of 2. (B) Sn⋅⋅⋅O (in black dots) and Sn⋅⋅⋅π (in green dots)
interactions in 2. (C) 3D zig-zag hydrogen bonding packing diagram of 2 [Hydrogen bonding interaction
drawn in black dotted lines].
The hydrogen bond interactions in polymorph 2 also involve the amine groups which simultaneously
ꢁ
ꢁ
donate to carboxylate groups and pyridyl N-atom, generating graph sets of types ꢀ ꢂ6ꢃ and ꢄ ꢂ9ꢃ in the
ꢁ
ꢁ
ꢁ
former case, and ꢄ ꢂ10ꢃ in the latter. Such contacts can be extended in two dimensions, may involve a
ꢁ
ꢆ
maximum of 6 molecules and generate rings of the type ꢅ ꢂ47ꢃ. Moreover, in both complexes we have
ꢆ
observed intramolecular resonance assisted hydrogen bonding (RAHB) interactions between amine NH
and C=O groups (Figure S8, supporting information) and the distance between the H····Oacceptor and
Ndonor····Oacceptor are in the ranges of 2.115 Å-2.151 Å and 2.744 Å-2.776 Å, respectively. On account of
their intramolecular nature, such interactions possibly do not play a significant role in the packing
polymorphism of the complexes.
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21b-c
Intermolecular tetrel interactions
are also observed in both complexes, particularly Sn···O contacts of
3
.281 (in 1) and 3.481 Å (in 2). However, compound 1 presents Sn···H(N) contacts of 3.122 Å, with
21d
N−H···Sn angle of 113.78° (Figure S9A) which, according to Braga et al., can be classified as a Type 2
non-covalent interaction, eventually indicating the existence of a true hydrogen bond contact. In
contrast, the relevant tetrel interaction in 2 (Figure S9B) is the medium-intense Sn⋅⋅⋅π contact involving
the pyrazine rings (Sn⋅⋅⋅centroid 3.849 Å). Such different interactions can possibly account for the
observed packing polymorphism.
22
We have also carried out topological analysis of the hydrogen bonded networks of 1 and 2 . The
hydrogen bonded network of 1 can be represented as a 2-connected uninodal net (Figure 4A) with
topological type 2C1 whereas compound 2 displays a 4-connected uninodal net (Figure 4B) with
topological type sql/Shubnikov tetragonal plane net.
A
B
Figure 4 Node-and-linker-type descriptions of the 1D hydrogen bonded frameworks in 1 and 2D
hydrogen bonded network of 2
Hirshfeld surface analysis
23
Hirshfeld surface analysis has proven to be an effective tool to analyze the differences in crystal
packing between structurally related compounds, especially effective in studying molecular
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24
polymorphism and multicomponent crystals with variable conformers, solvents or counterions . It is a
useful tool for visualizing 3-dimensional electron density, especially that is related to intermolecular
23a
interactions between the molecules in 3D lattice space. We used the program Crystal Explorer for this
task.
Hirshfeld surface plots and 2D fingerprints of intermolecular interactions in polymorphs 1 and 2 are
presented in Figure 5. The relative contributions of different interactions of compounds 1 and 2 are
presented in Figure 5E. For both polymorphs, the most significant contributions come from H···O (24.2-
2
7.8%) and H···H (18.4-19.5%) contacts, which correspond to the N−H···O/C−H···O contacts and van der
Waals interactions, respectively. Besides that, other significant interactions like H···N (13.7-15.3%) and
H···C (11.0-13.0%) interactions are observed. Sn···O (2.2-2.7%) and Sn···N (0.3-2.1%) interactions are of a
lower significance. Hirshfeld surfaces of polymorphs 1 and 2 exhibit significant differences between
intermolecular interactions. As shown in figure 5E, polymorph 2 has 3.6%, 1.1% and 2.0% larger H···O,
H···H and H···C contributions than those for 1. Moreover, polymorph 1 has 1.6% larger H···N
contributions than 2. Significant differences in the Sn···N (0.3% for 1 and 2.1% for 2) and Sn···C (0.2% for
1
and 3.8% for 2) interactions are also observed. Finally, polymorph 1 (3.5%) displays a higher
contribution of C···C interactions than polymorph 2 (2.3%). These differences in the contributions of all
types of interactions found in the crystal structures of compounds 1 and 2 evidence that they are
structurally different.
A
B
8

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C
D
E
Figure 5 Calculated Hirshfeld surfaces of polymorphs 1 (A) and 2 (B) [the red spots on the Hirshfeld maps
correspond to close intermolecular interactions, whereas blue regions are the domains without close
intermolecular contacts]. 2D fingerprint plots of 1 (C) and 2 (D) exhibiting different intermolecular
interactions. (E) The relative contribution of the intermolecular contacts to the Hirshfeld surface area.
Thermogravimetric analyses
Thermogravimetric analyses were carried out under dinitrogen in the range from room temperature to
-
1
ca. 800 ˚C at a heating rate of 5 ˚C min . Features of the thermal stability of complexes 1 and 2 are
illustrated in Figure S7 (supporting information). Both complexes follow a common type of thermal
decomposition. Complex 1 shows a weight loss of 35.4 % between 153 and 378˚C, corresponding to the
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loss of one 3-amino-2-pyrazinecarboxylate ligand (calcd: 34.9 %). Similarly, complex 2 exhibits a weight
loss of 35.2 % in the 134-365˚C temperature range which accounts for such a ligand loss (calcd: 34.9 %).
Above this temperature further decomposition occurs towards the final SnO species.
Catalytic activity
We have tested the activity of both tin complexes as heterogeneous catalysts for the cyanosilylation
reaction of different aldehydes under mild reaction condition (Scheme 2). Initially, we have optimized
the reaction conditions (temperature, solvent and catalyst amount) by using benzaldehyde as a model
substrate with trimethylsilyl cyanide (TMSCN) and tin complex 1 or 2 as catalyst. An increase of the
o
o
reaction temperature from 25 C to 50 C enhanced the product yield from 71 to 92% when we have
used 1 as catalyst (entries 9 - 11, Table 1), whereas for 2 the yield increases from 79 to 99% (entries 12-
1
4, Table 1); further raise in temperature did not improve the reaction yield.
Scheme 2
We have carried out experiments in the absence (entries 18 and 22, Table 1) and in the presence of
solvent (DCM, CH CN, THF or MeOH; entries 1-8, 15-17 and 18-21, Table 1). The results indicate that
3
dichloromethane (DCM) is the best solvent (92% for 1 and 99% for 2), whereas the worst is CH CN. For
3
other solvents and in solvent free condition, the yields are in the range of 75-89% for 1 and 73-91% for
2
. Concerning the effect of catalyst amount, an increase from 1.0 to 2.0 mol% enhances the product
yield from 88 to 92% for 1 (entries 23 and 4, Table 1) and 92 to 99% for 2 (entries 25 and 8, Table 1), but
a further increase (up to 5%) results only in an insignificant yield improvement (entries 24 and 26, Table
1
), with a marked decrease of the TON value. Thus, our optimized reaction conditions for the
o
cyanosilylation of aldehyde with trimethylsilyl cyanide (TMSCN) was 2 mol% of Sn-catalyst at 50 C in
DCM medium.
We have also checked the reactivity of SnCl towards the cyanosilylation reaction and the obtained yield
2
is only 19% (entry 28, Table 1). A blank test was carried out with benzaldehyde in the absence of any
catalyst and yielded only 12% of the product after a reaction time of 4 h (entry 27, Table 1). An identical
yield (13%, entry 29) was obtained when using the pro-ligand (HL) instead of the metal catalyst, showing
that free HL has no catalytic activity.
1
0

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Table 1 Optimization of the parameters of the cyanosilylation reaction of benzaldehyde with
a
trimethylsilyl cyanide (TMSCN) with complexes 1 and 2 as catalysts
b
c
Entry Catalyst Time (h)
Amount of
Temperature
Solvent
Yield
(%)
36
TON
o
Catalyst (mol%)
( C)
1
2
3
4
5
6
7
8
1
1
1
1
2
2
2
2
0.5
1
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
50
50
50
50
50
50
50
50
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
18
25
35
46
17
24
72
49
51
2
71
4
92
0.5
1
35
48
2
73
4
99
Different Temperatures
9
1
1
1
2
2
2
4
4
4
4
4
4
2.0
25
DCM
DCM
DCM
DCM
DCM
DCM
71
86
92
79
89
99
35
43
46
39
45
49
1
0
1
2
3
4
2.0
2.0
2.0
2.0
2.0
40
1
1
1
1
50
25
40
50
Different Solvents
1
1
1
1
1
2
2
2
5
6
7
8
9
0
1
2
1
1
1
1
2
2
2
2
4
4
4
4
4
4
4
4
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
50
CH CN
67
82
89
75
63
78
91
73
33
41
44
37
31
39
45
36
3
50
THF
50
MeOH
50
Solvent free
50
CH CN
3
50
THF
50
MeOH
50
Solvent free
Catalyst Amount
2
2
2
2
3
4
5
6
1
1
2
2
4
4
4
4
1.0
5.0
1.0
5.0
50
50
50
50
DCM
DCM
DCM
DCM
88
93
88
18
92
20
92
100
2
2
2
7
8
9
Blank
SnCl2
HL
4
4
4
-
50
50
50
DCM
DCM
DCM
12
19
13
-
5
-
2.0
2.0
a
Reaction conditions (unless stated otherwise): benzaldehyde (1 mmol), trimethylsilyl cyanide (500 µl, 4
b
1
mmol), 2.0 mol% (relative to the aldehyde) of catalyst, at the desired temperature. Calculated by H-
1
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NMR as the number of moles of product 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile per mole of
c
aldehyde X 100. Number of moles of product 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile per mole of
catalyst.
The plot of yield vs. time (until 4 h) for the cyanosilylation reaction of benzaldehyde with TMSCN in the
presence of 1 or 2 is presented in Fig. 6A. Moreover, no other products were detected and the yield of
this product corresponds to the conversion of benzaldehyde.
A
B
Figure 6 (A) Plot of 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile yield vs. time for the cyanosilylation
reaction, catalyzed by 1 (red curve) and 2 (black curve), of benzaldehyde with TMSCN at room
temperature. (B) Effect of the catalyst 1 (blue pillars) and 2 (red pillars) recycling on the yield of 2-
phenyl-2-((trimethylsilyl)oxy)acetonitrile.
We have also explored the activity of catalysts 1 and 2 towards a variety of aromatic aldehydes and
ketones. The corresponding cyanohydrin derivatives were obtained with yields ranging from 42% to
1
00% (Table 2). Aryl aldehydes bearing strong electron-withdrawing substituents (nitro and chloro)
exhibit the highest yields (entries 1-3, Table 2) whereas the aldehydes containing electron-donating
groups (methyl or methoxy) show lower reaction yields (entries 5 and 6, Table 2). Moreover, the
aromatic 4-chloroacetophenone shows a much lower reactivity (entry 8, Table 2) than the
1
9a,19i
corresponding aromatic aldehyde, as obtained in other cases
.
Table 2: Cyanosilylation reaction of various aldehydes and ketones with trimethylsilyl cyanide
a
(
TMSCN) with catalysts 1 and 2
b
b
Entry Substrate
Yield (%) with 1
Yield (%) with 2
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1
2
3
4
5
6
7
8
3-Nitrobenzaldehyde
4-Nitrobenzaldehyde
4-Chlorobenzaldehyde
4-Hydroxybenzaldehyde
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
Acetophenone
97
98
96
85
63
51
42
53
99
100
98
89
71
55
50
61
4-Chloroacetophenone
a
Reaction conditions: 2.0 mol% (relative to the aldehyde) of catalyst 1 or 2, trimethylsilyl cyanide (500
o
b
1
µl, 4 mmol) and aldehyde (1 mmol) for 4 h at 50 C. Calculated by H NMR.
Catalyst 2 is more active than 1 conceivably on account of the less hindered (more accessible) metal
centre in the former case, displaying an SnN O association instead of SnN O in 1. A comparison of the
2
3
4
4
activity of our Sn(II)-catalyst with other reported compounds for the cyanosilylation of benzaldehyde
with TMSCN is presented below. For example, a dinuclear Pb(II) complex of 3-amino-2-
o
18a
pyrazinecarboxylate led to an overall yield of 97% after 8h at 50 C . The heterometallic coordination
2
5a
polymer [{Co Ni (H O) (Bpe) }(V O )]·4H O·Bpe gave a yield of 77% at 50 °C for 16 h . In the case of
0.6 1.4
2
2
2
4
12
2
o
25b
the 3D MOF [Cu (benzenetricarboxylate) ] , a yield of 50% after 48 h at 40 C was obtained . Moreover,
3
2 n
that reaction catalysed by a Pr(III)-MOF of 3,5-disulfobenzoic acid and 1,10-phenathroline showed an
o
25c
overall yield of 78% after 3h reaction time at 50 C under solvent free conditions . Besides that, a
25d
lanthanide-supported molybdovanadate MOF gave a yield of 90% after 5 h of reaction . Several
mononuclear complexes can also catalyze the cyanosilylation reactions of aldehydes. For example, a
mononuclear copper(II) complex of 2-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)benzenesulfonate and
25e
ethylenediamine led to an overall yield of 85% after 16 h . A mononuclear Zn(II) complex with an
amide-based ligand containing thiazole gave an overall yield of 90% after 6 h by using 5 mol% of
25f
25g
catalyst . Using a Ce(III) complex of D-proline as catalyst ensued a yield of 90% after 40 h . Such
results indicate that our catalysts are quite effective for the cyanosilylation of benzaldehyde with
TMSCN in terms of reaction yields.
The heterogeneity of 1 and 2 has also been tested for the cyanosilylation reaction, according to the
26
following method . We have removed the catalyst by centrifugation after 2 h (yield 71% or 73%) and
kept the catalyst-free reaction solution under the same conditions stirred for additional 2 h. After
1
3

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removal of the solid catalyst, the product yield did not increase significantly. This demonstrates the
heterogeneous nature of the catalyst. Moreover, we have also determined the amount of tin in the
reaction mixture after removing the catalyst upon completion of the reaction. Only an insignificant
amount (0.015-0.017% of the quantity used in the reaction) of tin was present in the solution, thus
ruling out any significant leaching of the catalyst. We have also checked the structural integrity of 1 and
2
after catalysis by performing powder X-ray diffraction analyses of the catalyst before and after the
catalysis reaction. The structure of the catalysts remained intact after the catalytic reaction (Figure S4).
We have also tested the recyclability of the catalyst 1 and 2. For this purpose, upon completion of a
reaction cycle, we have separated the catalyst by centrifugation, washed with dichloromethane and
dried at room temperature before further use. We have recycled five times catalysts 1 and 2 and the
catalytic system maintained almost the full activity as shown in figure 6B.
A possible catalytic cycle for the cyanosilylation reaction catalyzed by a Sn(II) complex is proposed in
Scheme 3. It can involve activation of the substrate carbonyl group and TMSCN by the Sn(II) centre and a
ligated carboxylate group, respectively (represented in Scheme 3 as [O-Sn]), promoting the nucleophilic
attack of the cyano group to the carbonyl carbon, leading to the formation of C-C bond and thus to the
1
9a,19i
cyanohydrin
.
Scheme 3
Conclusion
1
4

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Depending on the method of synthesis and of crystallization, two interconvertible tin(II) packing
polymorphs (1 and 2) with 3-amino-2-pyrazinecarboxylate are obtained. X-ray diffraction analysis
−
−
revealed that the organic ligand can be considered as a 1κN,O;2κN’,O’-L or as a 1κN,O;2κO’-L chelator,
respectively, therefore defining 1D infinite chains running along the crystallographic c (in 1) or b (in 2)
axis. Hydrogen bond interactions in 1 lead to a one dimensional network, while in 2 infinite two
dimensional zig-zag sheets are formed, demonstrating that they are packing polymorphs. From the
Hirshfeld surface analysis we observed that both polymorphs have different contributions in all types of
interactions and different hydrogen bonding networks.
Complexes 1 and 2 effectively catalyze heterogeneously the cyanosilylation reaction of various
aldehydes with TMSCN producing the corresponding cyanohydrins in yields as high as 99% with 2 mol%
of catalyst 2 at 50 ºC for 2 h. These are the first examples of Sn(II) complexes with pyrazine carboxylate
type ligands which are utilized as effective heterogeneous catalysts for cyanosilylation reactions of
aldehydes, and further studies on their catalytic applications are in progress.
Experimental
Materials and physical methods
The synthetic work was performed in air. All the chemicals were obtained from commercial sources and
-1
used as received. The infrared spectra (4000–400 cm ) were recorded on a Bruker Vertex 70 instrument
in KBr pellets; abbreviations: s = strong, m = medium, w = weak, bs = broad and strong, mb = medium
and broad. Carbon, hydrogen and nitrogen elemental analyses were carried out by the Microanalytical
Service of the Instituto Superior Técnico. Thermal properties were analyzed with a Perkin-Elmer
-
1
Instrument system (STA6000) at a heating rate of 5˚C min under a dinitrogen atmosphere. Powder X-
ray diffraction (PXRD) was conducted in a D8 Advance Bruker AXS (Bragg Brentano geometry) theta-2-
theta diffractometer, with copper radiation (Cu Kα, λ = 1.5406 Å) and a secondary monochromator,
operated at 40 kV and 40 mA. Flat plate configuration was used and the typical data collection range
was between 5˚ and 40˚.
Synthesis of 1
A mixture of SnCl (18.9 mg, 0.1 mmol) and 3-amino-2-pyrazinecarboxylic acid (27.8 mg, 0.2 mmol) was
2
dissolved in 2 mL of dimethylformamide. The resulting colorless solution was sealed in a 4 mL glass
-
1
vessel and heated at 75˚C for 48 h. Subsequent gradual cooling to room temperature (0.1 ˚C min )
afforded plate-like yellow crystals. The compound was not soluble in the common organic solvents.
1
5

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Yield: 76% (based on Sn). Anal. Calcd. for C H N O Sn (M = 394.92): C, 30.41; H, 2.04; N, 21.28; Found:
1
0
8
6
4
-
1
C, 30.50; H, 2.16; N, 21.33. FT-IR (KBr, cm ): 3394 (bs), 1632 (s), 1598 (s), 1545 (s), 1435 (s), 1369 (s),
320 (s), 1220 (s), 1184 (m), 1137 (s), 920 (s), 825 (s), 689 (m), 594 (w).
1
Synthesis of 2
A mixture of SnCl (18.9 mg, 0.1 mmol) and 3-amino-2-pyrazinecarboxylic acid (27.8 mg, 0.2 mmol) was
2
dissolved in 2 mL of N-methylformamide: methanol (1:1 v/v) mixture. The resulting colorless solution
was sealed in a 4 mL glass vessel and heated at 80˚C for 24 h. Subsequent gradual cooling to room
-
1
temperature (0.1 ˚C min ) afforded block-like yellowish crystals. The compound was not soluble in the
common organic solvents. Yield: 81% (based on Sn). Anal. Calcd. for C H N O Sn (M = 394.92): C, 30.41;
10
8
6
4
-
1
H, 2.04; N, 21.28; Found: C, 30.50; H, 2.16; N, 21.33. FT-IR (KBr, cm ): 3397 (bs), 1632 (s), 1598 (s), 1544
s), 1436 (s), 1370 (s), 1322 (s), 1221 (s), 1184 (m), 1135 (s), 920 (s), 824 (s), 689 (m), 596 (w).
(
Interconversion studies
0 mg of complex 1 or 2 were taken into a 50 mL beaker and 10 mL of N-methylformamide: methanol
1:1 v/v) mixture or of dimethylformamide, respectively, were added into it. The mixture was heated for
0 min and kept at room temperature for 4 days. The complex was then isolated by filtration, dried in air
5
(
3
and the conversion was confirmed by using powder X-ray diffraction analysis.
Crystal structure determinations
X-ray quality single crystals of the compounds were immersed in cryo-oil, mounted in a nylon loop and
measured at room temperature (1-2). Intensity data were collected using a Bruker APEX-II PHOTON 100
diffractometer with graphite monochromated Mo-Kα (λ 0.71069) radiation. Data were collected using
phi and omega scans of 0.5° per frame and a full sphere of data was obtained. Cell parameters were
2
7
27a
retrieved using Bruker SMART software and refined using Bruker SAINT on all the observed
27a
reflections. Absorption corrections were applied using SADABS . Structures were solved by direct
2
7b
27b
methods by using the SHELXS-97 package and refined with SHELXL-97 . Calculations were performed
27c
using the WinGX System–Version 1.80.03 . The hydrogen atoms attached to carbon atoms and to the
nitrogen atoms were inserted at geometrically calculated positions and included in the refinement using
the riding-model approximation; Uiso(H) were defined as 1.2Ueq of the parent nitrogen atoms or the
carbon atoms for phenyl. Least square refinements with anisotropic thermal motion parameters for all
the non-hydrogen atoms and isotropic ones for the remaining atoms were employed. Crystallographic
data are summarized in Table S1 (Supplementary Information file) and selected bond distances and
1
6

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angles are presented in Table S2. CCDC 1859233-1859234 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work has been supported by the Foundation for Science and Technology (FCT) (project
UID/QUI/00100/2013), Portugal. Authors A. Karmakar and S. Hazra express their gratitude to the FCT for
post-doctoral fellowships (Ref. No. SFRH/BPD/76192/2011 and SFRH/BPD/78264/2011).
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New Journal of Chemistry
Page 22 of 22
View Article Online
DOI: 10.1039/C8NJ03805H
Packing polymorphism in 3-amino-2-pyrazinecarboxylate based
tin(II) complexes and their catalytic activity towards cyanosilylation
of aldehydes
Anirban Karmakar*, Susanta Hazra, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva and
Armando J. L. Pombeiro*
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1
049–001, Lisbon, Portugal. E-mail: anirbanchem@gmail.com and pombeiro@tecnico.ulisboa.pt.
3
-amino-2-pyrazinecarboxylic acid is used to synthesize two new mononuclear interconvertible
packing polymorphs of tin(II) which act as heterogeneous catalysts for the cyanosilylation of
aldehydes with trimethylsilyl cyanide.