Synthesis, structure and catalytic application of lead(ii) complexes in cyanosilylation reactions

Article abstract of DOI:10.1039/c4dt02316a

Hydrothermal reactions of a lead(ii) salt with 3-aminopyrazine-2-carboxylic acid (HL1) gave rise to a series of lead(ii) coordination compounds (1-6) having zero, one, two and three dimensional structures. X-ray diffraction structural analyses reveal that

Full text of DOI:10.1039/c4dt02316a

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Synthesis, structure and catalytic application of
lead(II) complexes in cyanosilylation reactions†
Cite this: Dalton Trans., 2015, 44,
268
Anirban Karmakar,* Susanta Hazra, M. Fátima C. Guedes da Silva* and
Armando J. L. Pombeiro*
Hydrothermal reactions of a lead(II) salt with 3-aminopyrazine-2-carboxylic acid (HL1) gave rise to a series
of lead(II) coordination compounds (1–6) having zero, one, two and three dimensional structures. X-ray
diffraction structural analyses reveal that complexes [Pb(L1)
possess dinuclear structures, containing a centre of symmetry. Complexes [Pb
and [Pb (L1) ·(H O) ·(2.5DMF) (4) have 1D chain like structures, and [Pb (L1)
HCOO)] ·(DMF) ·(MeOH)
and 3-aminopyrazine-2-carboxylate anions. The hydrothermal reaction of lead(II) nitrate with HL1 in DMF
led to in situ formation of 3,3’-(methylenebis(azanediyl))bis(pyrazine-2-carboxylic acid) [H L2] which pro-
duces the 3D framework [Pb (L2) ·(2DMF) ·(H O)
(6). The L1− and L22− ligands bind the metal cations
by means of a pyrazine N-atom and one, or both, carboxylate O-atoms. The carboxylate group of
L1− presents a diversity of coordination modes, viz., monodentate (1 and 3), bridging µ
(3) and bridging
(4), monodentate bridging µ (1, 2, 4, 5 and 6) and bridging chelate µ (5). The carboxylate moiety of
2
]
2
2
(1) and [Pb(L1)(OCHNH )(η-OCHO)]
2
(2)
2
(L1)
4
(NHMeCHO)
2
]
n
(3)
2
4
]
n
2
n
n
5
7
(η-NO )(µ-HCOO)(η-
3
n
n
n 5 5
(5) shows a 2D sheet like structure constructed by the [Pb O (HCOO)] cluster
2
2
2
]
n
n
2
n
2
µ
3
2
2
2−
Received 31st July 2014,
L2 in 6 binds the metal in a bridging µ fashion. The Pb(II) ions display coordination numbers from 5 to 8
2
Accepted 20th October 2014
with hemi- or holodirected coordination environments. The Pb(II) complexes act as heterogeneous cata-
DOI: 10.1039/c4dt02316a
lysts for the cyanosilylation reaction, at 15 °C, of different aldehydes with trimethylsilyl cyanide (TMSCN)
www.rsc.org/dalton
and can be recycled at least three times without losing activity.
Introduction
Syntheses of discrete and polymeric metal–organic frameworks
are currently attracting considerable attention not only for
their interesting molecular topologies, but also for their poten-
1
2
tial applications in functional materials, ion exchange, cata-
3
4
5
lysis, nonlinear optics, molecular magnetic materials,
6
7
electrical conductivity and gas storage.
Lead is a heavy toxic metal which is commonly used in
numerous industrial applications such as paints and bat-
Scheme 1
8
teries. Lead(II), which accumulates in the liver, kidneys,
9
directed and holodirected coordination geometries (Scheme 1)
with various coordination numbers of 2–10. They form stable
complexes with both soft and hard donor atom ligands.
Remarkable progress has been made in recent years in the
development of metal–organic networks using lead(II) ions and
carboxylic acid ligands as building blocks. A few lead-based
metal organic frameworks with carboxylate linkers, such as
those derived from terephthalic acid,
carboxylic acid, biphenyl dicarboxylic acid, polycarboxylate
bones, and other parts of the body, is difficult to remove.
Therefore the coordination chemistry of lead attracts a great
deal of interest from inorganic chemists, namely for the devel-
opment of suitable ligands as extractants, lead-poisoning treat-
1
1
1
0
ment agents, and sensors.
1
2
Lead(II) ions bearing a stereochemically active electron lone-
pair and with a large ionic radius can display both hemi-
1
3
1,3-benzenedi-
1
4
15
1
6
17
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisbon, Portugal. E-mail: anirbanchem@gmail.com,
fatima.guedes@tecnico.ulisboa.pt, pombeiro@tecnico.ulisboa.pt
linkers, and heterobimetallic coordination polymers have
been reported. These frameworks can show interesting pro-
18
19
perties such as luminescence, nonlinear optics and ion
†
Electronic supplementary information (ESI) available. CCDC 1017108–1017113.
2
0
exchange, but applications in the field of catalysis of lead(II)
complexes and frameworks are still lacking in scope.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4dt02316a
268 | Dalton Trans., 2015, 44, 268–280
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Cyanosilylation is an important classic organic reaction
used, e.g., to synthesize cyanohydrins from aldehydes. Cyano-
hydrins are relevant organic compounds for both synthetic
2
1,22
chemistry and biological processes.
They can be readily
converted into important species such as α-hydroxycarboxylic
2
3
acids or β-amino alcohols. For the preparation of cyano-
hydrins, the use of a catalyst with a Lewis acid or base charac-
ter, capable of activating both the substrate and the cyanide
precursor, is necessary, affording the highly versatile cyanohy-
drin trimethylsilyl ethers. A number of discrete complexes and
2
4
25
coordination polymers containing various d-block, f-block
2
6
and a few p-block metal ions has been utilized as homo-
geneous catalysts for such transformations, but only a few
2
7
studies have been addressed at heterogeneous catalysts. To
our knowledge, no reports are available of Pb(II) complexes or
coordination polymers which have been used for cyanosilyla-
tion of aldehydes. Hence, the development of efficient Pb(II)
catalysts (in particular heterogeneous ones) for cyanosilylation
of carbonyl compounds with trimethylsilyl cyanide (TMSCN) is
a subject that deserves to be explored.
Scheme 2
oxylic acid (HL1) with lead(II) nitrate hexahydrate in the pres-
ence of formamide (for 1), a DMF and formamide mixture (for
2
), N-methylformamide (for 3), only DMF (for 4) and a DMF
and methanol mixture (for 5).
During the hydrothermal processes that led to compounds
In this context, we have chosen 3-aminopyrazine-2-carb-
2
and 5, DMF was hydrolyzed (Scheme 2A) to dimethylamine
oxylic acid (HL1) which can be readily deprotonated to
produce L1− which can act as a multidentate ligand (O- or
and formic acid, the latter being deprotonated to the formate
2
9
anion which ultimately coordinated to the metal cations.
N-donor) with versatile metal-binding and hydrogen-bonding
capabilities. Recently, it has drawn extensive attention for the
construction of metal organic frameworks with attractive archi-
HL1 can be converted in situ to 3,3′-(methylenebis(azane-
diyl))bis(pyrazine-2-carboxylic acid) (H L2, Scheme 2B). We
2
2
8
consider that the formic acid, obtained from DMF hydrolysis,
condensed with the amine groups of HL1 thus producing
tectures. However, most of the reported work has focused on
2
8a,b
28c,d
28e
s-block,
d-block
and f-block metal ions, whereas the
H L2 (see below), at the end forming the 3D polymer
2
corresponding chemistry of the p-block metal ions is still
underexplored. Lead(II) complexes with multicarboxylate
ligands have been widely investigated for their intriguing struc-
[
Pb (L2) ] ·(2DMF) ·(H O) (6).
2 2 n n 2 n
The FT-IR spectra of complexes 1–6 show the expected
−
1
1
2–16
bands, revealing the characteristic asymmetric (1613–1597 cm
)
tural topologies and potential applications,
but to our
and symmetric (1385–1358 cm− ) stretching of carboxylic
groups. A strong band at 1654–1623 cm− is due to δ(N–H) of
the amine group, whereas the asymmetrical and symmetrical
1
knowledge no study on the Pb(II) coordination chemistry with
1
3
-aminopyrazine-2-carboxylic acid (HL1) has been reported.
Thus, the main objectives of the current work are as
−
1
ν(N–H) of the amine groups appear in the 3436–3310 cm
follows: (i) synthesis and characterization of various Pb(II) com-
plexes and polymers using 3-aminopyrazine-2-carboxylic acid
as a linker and study of their thermal decomposition pro-
perties; (ii) investigation of the catalytic activity of the syn-
thesized complexes in the cyanosilylation reaction of
aldehydes.
The catalytic performances of the obtained complexes, as
heterogeneous catalysts, in terms of activity, heterogeneity and
recyclability, were successfully tested towards the cyanosilyla-
tion reaction of aldehydes with TMSCN (trimethylsilyl
cyanide).
region. The bands at 1324–1318 cm− are assigned to ν(C–N)
1
−
1
(
1
aromatic amines), while those in the 1623–1654 cm and
436–1459 cm ranges are attributed to ν(CvC) of aromatic
rings. These complexes are also characterized by X-ray diffr-
action analysis, elemental (see the Experimental section) and
thermogravimetric (see ESI†) analyses.
−
1
3
0
Crystal structure analysis
General outline. The structures of 1 and 2 consist of lead(II)
dimers built up around centrosymmetric Pb
2 2
O cores; those of
3
and 4 are 1D polymers, while 5 and 6 are 2D and 3D net-
works, respectively. The metal cations in these structures are
penta- (in 1 and Pb1 in 4), hexa- (in 2, 3, Pb1 in 5, and 6),
hepta- (Pb2 and Pb4 in 5) or octa-coordinated (Pb2 in 4 and
Pb3 in 5). For all the complexes the Pb(II) centres display hemi-
2 2 2
The syntheses of compounds [Pb(L1) ] (1), [Pb(L1)(OCHNH )- directed environments, except for complex 5 in which the Pb2
Results and discussion
Syntheses and characterization
1
2
(
(
(
η-OCHO)] (2), [Pb (L1) (NHMeCHO) ] (3), [Pb (L1) ] ·(H O) · and Pb4 centres are present in holodirected settings.
2
2
4
2 n
2
4 n
2
n
−
2.5DMF)
n
(4) and [Pb
5
(L1)
7
(η-NO
3
)(µ-HCOO)(η-HCOO)]
n
·
The coordination modes of L1 in 1–6 are depicted in Fig. 1
(5) were carried out under hydrothermal/ and are worth stressing, in particular regarding what concerns
DMF) ·(MeOH)
n
n
solvothermal conditions, by reacting 3-aminopyrazine-2-carb- the carboxylate moieties. Indeed, they occur as monodentate
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−
Fig. 1 Various coordination modes of 3-aminopyrazine-2-carboxylate (L1 ) in compounds 1–6. The carboxylate group is represented as mono-
dentate (a) [observed in complexes 1 and 3], monodentate bridging µ (b) [observed in complexes 1, 2, 4, 5 and 6], bridging µ (c) [observed in
complex 3], bridging chelate µ (d) [observed in complex 5] and bidentate bridging µ (e) [observed in complex 4].
2
2
2
3
Fig. 2 (A) Crystal structure of complex 1 with partial atom labelling scheme. Symmetry operations to generate equivalent atoms: 1 − x, −y, −z.
B) Schematic presentation of complex 1.
(
Fig. 3 (A) Crystal structure of complex 2 with partial atom labelling scheme. Symmetry operations to generate equivalent atoms: −x, −y, −z.
B) Schematic presentation of complex 2.
(
(
in 1 and 2), bridging µ (in 2) and µ (in 4), monodentate brid- H-bond distances, angles and selected bonding parameters are
2 3
ging µ2 (in 1, 3 and 4) and bridging chelate µ (in 5). The given in ESI in Tables S1–S3,† respectively.
2
2
−
Description of the structures. In 1 the L1− species act as
2
ligand L2 binds the metals in a bridging µ fashion in
complex 6.
bidentate chelating and as bridging µ ligands (Fig. 2). They
2
The crystal structures and schematic representation of com- coordinate via carboxylate oxygen atoms, one of them in a
plexes 1–6 are depicted in Fig. 2–7, respectively and represen- bridging fashion between the two metals [Pb–O in the range
tations of their H-bond interactions and/or polymeric 2.271(3)–2.531(3) Å, Table S3†], and via pyrazine nitrogen
networks are shown in Fig. S1 (ESI†). Crystallographic data, atoms [Pb–N, 2.608(3) and 2.585(3) Å, Table S3†]. The Pb1
270 | Dalton Trans., 2015, 44, 268–280
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Fig. 4 (A) Crystal structure of complex 3 with partial atom labelling scheme. Symmetry operations to generate equivalent atoms: (i) −1 + x, 1 + y, z;
ii) 1 + x, −1 + y, z. (B) Schematic presentation of complex 3.
(
centre presents an almost perfect PbO
3 2
N square pyramidal nitrogens and by the N-methylformamide O atom [Pb–O and
geometry (τ = 0.01) with the ligands coordinated to the metal Pb–N in the ranges 2.387(6)–2.880(9) Å and 2.477(6)–2.501(6) Å,
5
in a hemidirected fashion. The distance between the two sym- respectively, Table S3†]. The intra-chain metal⋯metal dis-
metry related cations is of 4.133(2) Å and the shortest inter- tances in 3 assume, alternatively, values of 6.121 and 6.417 Å,
molecular metal⋯metal distance is of 5.902 Å.
and the shortest inter-chain reaches a value of 7.649 Å.
The asymmetric unit of 2 contains a metal cation co-
The crystal structure of the 1D polymer 4 (Fig. 5) is com-
ordinated by a L1− ligand, a formate anion and a formamide posed of two Pb(II) ions, four L1− ligands and a crystallization
−
molecule (Fig. 3). In 2, L1 is chelated to the Pb(II) centre by a water molecule per asymmetric unit. The Pb1 centres exhibit
pyrazine N [Pb1–N1, 2.613(3) Å] and one of the carboxylate O square pyramidal Pb O N geometries (τ = 0.06) filled by the
1
4
1
5
atoms which forms a bridge between two Pb(II); a formate ion O1, O5, O7′, N1 (in the basal plane) and O3 (in the apical posi-
also chelates to each metal cation and the formamide mole- tion) from four L1− ligands. The Pb2 centre shows a PbO
cule is monodentate. On the whole, the metal cations present geometry being coordinated by five carboxylate oxygen atoms
a highly distorted PbO hexacoordinate environment (quad- from five ligands and by three pyrazine N atoms from three
5 3
N
5 1
N
2
ratic elongation of 1.562 and angle variance 622.24° ). The Pb– ligands. The Pb–O bond lengths vary from 2.444(5) to 2.677(5) Å
O bond lengths assume values between 2.335(3) and 2.784(4) Å and the Pb–N bond distances from 2.519(7) to 2.688(6) Å
(
Table S3†) and the Pb–N1 distance is of 2.613(3) Å. The dis- (Table S3†).
tance [4.187(3) Å] between the two symmetry related Pb(II) Every pair of metal ions within the Pb1⋯Pb2⋯Pb1′ trio is
centres as well as the shortest intermolecular metal⋯metal bridged by five carboxylate entities, four being coordinated via
distance [6.110 Å] are slightly longer than those in 1. bridging modes and another one coordinates to the metal
The asymmetric unit of 3 contains two metal cations, two centre by a µ bridging mode (see above). The intra-chain
µ
3
2
−
N-methylformamide molecules and four L1 ligands. The com- metal⋯metal distances in 4 assume, alternatively, values of
pound features a 1D polymer by means of syn–anti bridging 4.147 and 4.306 Å.
chelate µ
2
carboxylates (Fig. 4). Both Pb centres have a highly
The crystal structure of 5 comprises five Pb(II) cations, seven
distorted PbO N octahedral geometry (for Pb1 and Pb2 the L1− ligands, and two formate and one nitrate moieties per
4
2
quadratic elongations/angle variances are of 1.178/397.58 and formula unit and it can be envisaged as 1D chains intercon-
2
1
.209/412.12° , in this order) and are coordinated by carb- nected by formate anions thus constructing 2D infinite sheets
−
oxylate oxygens of three distinct L1 ligands, by two pyrazine (Fig. 6). It features tetranuclear {Pb (µ-O) } clusters, each
4
5
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Fig. 5 (A) The 1D structure of 4 with partial atom labelling scheme. H atoms and crystallization water molecules are omitted for clarity. Symmetry
operations to generate equivalent atoms: (i) −x, −1/2 + y, 1.5 − z; (ii) x, −1 + y, z; (iii) x, 1 + y, z; (iv) −x, −1.5 + y, 1.5 − z; (v) −x, 1.5 + y, 1.5 − z.
(
B) Schematic representation of the 1D polymer 4.
linked to three {PbO
7
} aggregates via oxygen bridges from two atoms in a one dimensional chain that runs along the crystal-
−
L1 ligands and one formate moiety. Several lead(II) complexes lographic a-axis. These chains are then gathered by the
3
1
32
2−
based on oxo and hydroxo bridges are known but no L2 ligands giving rise to 1D tubular chain motifs connecting
complex with the [Pb
5
O
5
(HCOO)] cluster has been reported.
2D wave-like ribbons (Fig. 7B).
The coordination spheres of the metal are different, i.e., of
Hydrogen bond interactions. Non-covalent interactions are
the PbO N (for Pb1), PbO N (for Pb2), PbO (for Pb3) and present in every compound (Fig. S1 and Table S2†). Concern-
3
3
5
1
8
PbO
7
(for Pb4) types. The Pb1, Pb2 and Pb4 cations are in the ing the intramolecular contacts, in every case the amine
same plane but Pb3 is slightly deviated from it (by 0.94 Å). The groups donate to the O-carboxylate.
shortest Pb–Pb distance found in 5 pertains to the tetranuclear
For compounds 1–3 and 5 the amine moieties simul-
cluster and involves Pb2 and Pb3 (3.887 Å). The Pb–O and taneously donate intermolecularly to pyrazine N atoms of
Pb–N bond distances (Table S3†) are in the 2.210(2)–2.791(10) vicinal entities. In 1 and 2 such interactions give rise to 1D
Å and 2.436(13)–2.791(10) Å ranges, respectively.
chains while in 3 they spread the structure to a second dimen-
The asymmetric unit of 6 contains two L2 ligands and sion. Moreover, by means of interactions between the forma-
two hemidirected Pb(II) cations. The PbO coordination mide molecule (as a donor) and both the carboxylate and
2
−
4 2
N
spheres of each metal (quadratic elongations and angle var- formate moieties (as acceptors), compound 2 forms 2D
2
iances with average values of 1.357 and 914.67° , respectively) frameworks.
make use of atoms from four different ligands. In turn, each
In 5, as a result of H-bond contacts the two dimensional
side of the L2 ligand coordinates to two metal centres in a sheets are interconnected to form a 3D network structure.
κN,O:2κO fashion (Fig. 7). The N–C–N angle of the methane- Such interactions involve some of the amine groups (as
2
−
1
2
−
diamine moiety in L2 (113° and 115°) and the C–N bond dis- donors) and oxygen atoms of carboxylate and formate ligands
tances [1.406(2)–1.506(2) Å] indicate a sp carbon. The Pb–O as well as the non-coordinated pyrazine N as acceptors.
3
and Pb–N bond lengths fall in the ranges of 2.456(6)–2.534(7) Å
and 2.570(9)–2.635(9) Å, respectively. The shortest distance also present in 4 but this structure, being further assembled
between the lead(II) ions is of 3.999(6) Å. by H-bond contacts involving the crystallization water mole-
The aforementioned interactions of the amine groups are
A close inspection of the 3D structural arrangement in 6 cule (as a donor) and the carboxylate O-atoms of a vicinal
indicates that the metals are assembled via bridging oxygen chain, is extended to the third dimension.
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Fig. 6 (A) The 2D structure of 5 with partial atom labelling scheme. Symmetry operations to generate equivalent atoms: (i) x, −y, z; (ii) 1/2 − x, −1/2
y, −z; (iii) 1/2 − x, 1/2 − y, −z; (iv) x, 1 + y, z; (v) 1/2 − x, −1/2 − y, −z; (vi) x, −1 + y, z; (vii) x, 1 − y, z; (viii) x, −1 − y, z. (B) Schematic representation of
the 2D polymer 5. (C) Structural fragment showing the two dimensional nature of 5.
+
Catalytic study
is considered to be the conversion of benzaldehyde into the
same product. This high conversion indicates that complex 1
We have tested the catalytic activity of the above lead com- acts as an efficient catalyst for this reaction. With 2, 3, 4, 5 and
plexes (1–6) as solid heterogeneous catalysts in the cyanosilyla- 6, conversions of 61%, 94%, 59%, 49% and 91% were
tion reaction of various aldehydes. In this catalytic reaction, a obtained, respectively (entries 26, 30–32 and 36, Table 1).
mixture of aldehyde, trimethylsilyl cyanide and Pb-catalyst was Upon extending the reaction time to 20 h, no significant
stirred at various temperatures (15 °C, 30 °C and 50 °C) in increase in the yield was detected. The plot of yield versus time
CH Cl (DCM) for the desired time. The crude product mix- for the cyanosilylation reaction of benzaldehyde and TMSCN
2
2
1
tures were analyzed by H NMR (ESI†).
with complex 1 is presented in Fig. 8A.
By using benzaldehyde as a test compound, complex 1
Blank tests were performed without any catalyst, at 15 °C,
shows the highest activity. Therefore, the optimization of the 30 °C and 50 °C, giving conversions of 23%, 36% and 49%
reaction conditions (temperature, reaction time, amount of after a reaction time of 16 h, respectively (entries 37–39,
catalyst, solvent) was carried out with this complex as the cata- Table 1). The use of Pb(NO
3 2 2
) ·6H O or of the ligand HL1
lyst, in a model TMSCN–benzaldehyde system (Scheme 3 and instead of 1 led to the conversion of 31% or 29%, respectively
Table 1).
When solid 1 is used as the catalyst (3 mol%), a conversion
(entries 40 and 41, Table 1).
We have also tested the effect of temperature, catalyst
of 96% (after 16 h reaction) of benzaldehyde into 2-phenyl-2- amount and solvents in the cyanosilylation reaction. The
(trimethylsilyl)oxy)acetonitrile is reached (entry 7, Table 1). increase of the catalyst amount from 1.0 to 3.0 mol% enhances
No other products were detected and the yield of this product the product yield from 21 to 96%, respectively, but a further
(
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Fig. 7 (A) Crystal structure of 6 with partial atom labelling scheme (hydrogen atoms are omitted for clarity). Symmetry operations to generate equi-
valent atoms: (i) −x, 1 − y, 1 − z; (ii) −1 + x, y, z; (iii) 1 − x, 1/2 + y, 1/2 − z; (iv) 1 − x, 1 − y, 1 − z; (v) −1 + x, 1/2 − y, 1/2 + z; (vi) 1 − x, 1 − y, 1 − z;
(
vii) 1 + x, 1.5 − y, −1/2 + z. (B) 3D packing diagram of 6 showing the 1D tubular chain motifs connecting 2D wave-like ribbons. (C) Schematic rep-
resentation of complex 6.
able yields of 66% and 63% in acetonitrile and THF,
respectively. Additionally, upon raising the temperature from
5 to 50 °C, the TOF increased from 6.1 h− to 27 h , consist-
1
−1
1
ent with the increase of the reaction rate with the temperature.
The plots of yield versus time for the cyanosilylation reaction
of benzaldehyde and TMSCN with complex 1 at different temp-
eratures are presented in Fig. 8B.
Scheme 3
rise in the former decreases the reaction yield to 79% (entries
The influence of substituents at the aldehyde aromatic
7
–9, Table 1). We have also examined the effect of different sol- ring was also investigated (Table 2), and a dramatic effect on
vents (entries 10 and 11, Table 1). The reactions give compar- the reaction yield was observed: aryl aldehydes bearing
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Table 1 Optimization of the parameters of the cyanosilylation reaction between benzaldehyde and trimethylsilyl cyanide with Pb-complexes as
a
catalysts
Amount of
catalyst
(mol%)
b
c
Entry
Catalyst
Time (h)
T (°C)
Solvent
Yield (%)
TON
1
2
3
4
5
6
7
1
1
1
1
1
1
1
0.5
1
2
4
6
8
16
3.0
3.0
3.0
3.0
3.0
3.0
3.0
15
15
15
15
15
15
15
DCM
DCM
DCM
DCM
DCM
DCM
DCM
10
19
37
58
68
79
96
3.3
6.3
12.3
19.3
22.6
26.3
32.0
8
9
1
1
16
16
1.0
5.0
15
15
DCM
DCM
21
79
21.0
15.8
1
1
0
1
1
1
16
16
3.0
3.0
15
15
CH
THF
3
CN
66
63
22
21
1
1
1
1
1
1
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
0.5
1
2
4
6
8
16
3.0
3.0
3.0
3.0
3.0
3.0
3.0
30
30
30
30
30
30
30
DCM
DCM
DCM
DCM
DCM
DCM
DCM
15
31
51
67
80
96
97
10.6
12.0
17.0
22.3
26.6
32.0
32.3
1
2
2
2
2
2
2
9
0
1
2
3
4
5
1
1
1
1
1
1
1
0.5
1
2
4
6
8
16
3.0
3.0
3.0
3.0
3.0
3.0
3.0
50
50
50
50
50
50
50
DCM
DCM
DCM
DCM
DCM
DCM
DCM
41
68
85
92
97
98
98
13.6
22.6
28.3
30.6
32.3
32.6
32.6
2
2
2
2
3
3
3
3
3
3
3
6
7
8
9
0
1
2
3
4
5
6
2
3
3
3
3
4
5
6
6
6
6
16
2
4
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
15
15
15
15
15
15
15
15
15
15
15
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
61
31
50
71
94
59
49
32
49
72
91
20.3
10.3
16.6
23.6
31.3
19.6
16.3
10.6
16.3
24.0
30.3
8
16
16
16
2
4
8
16
37
38
39
40
41
Blank
Blank
Blank
Pb(NO
HL1
16
16
16
16
16
—
—
—
3.0
3.0
15
30
50
15
15
DCM
DCM
DCM
DCM
DCM
23
36
49
31
29
—
—
—
10.3
9.6
3 2 2
) ·6H O
a
b
Reaction conditions: 3.0 mol% of catalyst 1, solvent (DCM) (2.0 mL), trimethylsilyl cyanide (1.0 mmol) and benzaldehyde (0.50 mmol).
Calculated by H-NMR as the number of moles of product 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile per mole of aldehyde × 100. Number of
1
c
moles of product 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile per mole of catalyst.
electron-withdrawing groups at the para-position exhibit the In addition, to demonstrate the structure conservation, the
highest reactivities (yields of 99–98% for the nitro- or chloro- catalyst was analyzed by FT-IR and PXRD before and after the
substituted species), which may be related with an increase of catalytic reaction, and no appreciable changes were detected
the electrophilicity of the substrate. The effects of steric hin- (Fig. S4†).
drance are also clearly observed for the hydroxyl benz-
There are some reports on coordination polymers and
aldehydes. The order of the observed product yield (para > complexes which are catalytically active for cyanosilylation
2
4–27
meta > ortho) parallels the inverse order of the steric hindrance reaction.
For example, the 1D and 2D zinc(II) coordina-
tion polymers with 1,2-bis(4-pyridyl)ethene, in the reaction
The catalyst could be recycled and reused at least three of benzaldehyde and TMSCN, led to overall yields of only
(Table 2).
2
4b
times without losing appreciably the activity (ESI, Fig. S3†). 14–18% after 24 h reaction time.
In the case of the
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Fig. 8 Plots of cyanohydrin yield vs. time for the cyanosilylation reaction, catalyzed by 1, of benzaldehyde with TMSCN at 15 °C (A) and at different
temperatures (B) (t = 0–16 h).
Table 2 Cyanosilylation reaction between various aldehydes and tri-
a
methylsilyl cyanide with catalyst 1
b
c
Reactant
Product
Yield (%)
TON
99
33
78
26
9
8
8
3
32.6
27.6
Scheme 4
2
2
6
9
2
7
9.6
7.3
3
3b
catalyst at 50 °C for 16 h,
but our catalysts, particularly 1, 3
and 6, exhibit yields of 98, 94 and 91%, respectively, under
mild conditions. Therefore, our catalysts appear to be much
more efficient than other reported heterogeneous catalysts for
this reaction.
22.5
Despite the lack of experimental basis to support a mechan-
2
6
ism, a catalytic cycle is proposed for the cyanosilylation reac-
tion catalyzed by 1 (Scheme 4). It can involve dual activation of
the carbonyl and TMSCN by the Pb(II) centre and the carb-
a
Reaction conditions: 3.0 mol% of catalyst 1, solvent (CH
2
Cl
2
)
(
2.0 mL), trimethylsilyl cyanide (1.0 mmol) and aldehyde (0.50 mmol) oxylate group, respectively, followed by the formation of the
b
1
c
2
6
for 16 h at 15 °C. Calculated by H NMR. Number of moles of
product per mole of catalyst.
C–C bond leading to the cyanohydrin.
Cu
3 2
(benzenetricarboxylate) MOF, a yield of 57% after 72 h
Conclusions
3
3a
was obtained.
Moreover, the heterometallic coordination
polymer [{Co0.6Ni1.4(H O) (Bpe) }(V O )]·4H O·Bpe (Bpe = 1,2- We successfully isolated six lead(II) complexes which exhibit
2
2
2
4
12
2
di(4-pyridyl)ethylene) gave a yield of 77% by using 10% of the dinuclear (1 and 2), 1D (3 and 4), 2D (5) and 3D (6) structures
276 | Dalton Trans., 2015, 44, 268–280
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with a diversity of coordination modes of the 3-aminopyrazine- Synthesis of complex 2
2
-carboxylate ligand and, in particular, of the carboxylate
Pb(NO
3
)
3
·6H
2
O (10 mg, 0.025 mmol), HL1 (7 mg, 0.050 mmol)
−
2−
groups. Both L1 and L2 ligands coordinate to the Pb(II)
centres via O-carboxylate and N-pyrazine atoms in a chelating
fashion, but the remaining N-pyrazine atom and the amine
group participate in hydrogen bonding interactions and con-
struct 2D and 3D supramolecular assemblies. Complexes 2
and 4,4′-bipyridine (10 mg, 0.060 mmol) were dissolved in
.0 mL of DMF–formamide (1 : 1), sealed in a capped glass
2
vessel and heated to 70 °C for 48 h. Subsequent gradual
cooling to room temperature (0.2 °C min ) afforded colorless
−1
crystals in ca. 72% (7.8 mg) yield (based on Pb). FT-IR (KBr,
and
5
bear formate ligands conceivably formed upon
−1
cm ): 3391 (bs), 3261 (bs), 1692 (s), 1629 (s), 1601 (s), 1520
decomposition of DMF. In the case of complex 6, the ligand
HL1 condensed in situ with formic acid and converted into
another ligand (H L2) forming a 3D polymer.
(
(
(
m), 1443 (s), 1358 (s), 1319 (s), 1239 (s), 1186 (s), 1139 (s), 917
s), 827 (s), 683 (m), 591 (m). Anal. Calcd for C H N O Pb
2
M = 870.73): C, 19.27; H, 1.62; N, 9.63. Found: C, 19.39; H,
.82; N, 10.01.
1
4
16
8
10
2
Complexes 1, 3 and 6 effectively catalyze the cyanosilylation
reaction of various aldehydes with TMSCN producing the
corresponding cyanohydrins in high yields, which depends on
various factors such as the amount of catalyst, the electrophili-
city of the substrates, temperature and the reaction conditions.
Among the six complexes of this study, compound 1 is the
most promising towards the cyanosilylation reaction. The
stability and recyclability of the catalyst were also tested suc-
cessfully. Hence, lead(II) complexes with pyrazine carboxylate
type ligands can be utilized as effective heterogeneous catalysts
in the above reaction, and further studies on their catalytic
applications are in progress.
1
Synthesis of complex 3
Pb(NO ·6H O (20 mg, 0.050 mmol) and HL1 (7 mg,
.050 mmol) were dissolved in 2.0 mL of N-methylformamide.
3
)
3
2
0
Then, the resulting mixture was sealed in a glass vessel and
heated at 75 °C for 48 h. It was subsequently cooled to room
temperature (0.2 °C min− ), affording plate-like colorless crys-
1
−
1
tals. Yield: 68% (10.4 mg) based on Pb. FT-IR (KBr, cm ):
393 (bs), 3297 (bs), 1623 (s), 1546 (s), 1436 (s), 1367 (s), 1319
s), 1231 (s), 1194 (s), 1145 (s), 915 (s), 860 (s), 821 (s), 678 (m),
3
(
565 (w), 541 (w). Anal. Calcd for C H N O Pb (M =
24 26 14 10 2
1084.97): C, 26.62; H, 2.23; N, 18.11. Found: C, 26.12; H, 2.00;
N, 17.61.
Experimental section
Synthesis of complex 4
All the chemicals were obtained from commercial sources
Pb(NO
3 3 2
) ·6H O (20 mg, 0.050 mmol), HL1 (7 mg, 0.050 mmol)
and used as received. The infrared spectra (4000–400 cm−
1
)
and 4,4′-bipyridine (5 mg, 0.030 mmol) were dissolved in
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, hydro-
gen, and nitrogen elemental analyses were carried out by the
Microanalytical Service of the Instituto Superior Técnico.
Thermal properties were analyzed with a Perkin-Elmer Instru-
ment system (STA6000) at a heating rate of 10 °C min− under
a dinitrogen atmosphere. Powder X-ray diffraction (PXRD)
was conducted in a D8 Advance Bruker AXS (Bragg–Brentano
geometry) theta–2theta diffractometer, with copper radiation
2
7
.0 mL of DMF, sealed in a capped glass vessel and heated to
0 °C for 48 h. Subsequent gradual cooling to room tempera-
−1
ture (0.2 °C min ) afforded colorless crystals in ca. 65%
−1
(
18.9 mg) yield (based on Pb). FT-IR (KBr, cm ): 3417 (bs),
3
310 (bs), 1646 (s), 1597 (s), 1545 (s), 1456 (m), 1384 (s), 1324
(
m), 1227 (s), 1176 (s), 919 (s), 823 (s), 686 (w), 568 (w), 451 (w).
1
Anal. Calcd for C27.5H35.5N14.5O11.5Pb (M = 1167.6): C, 28.29;
H, 3.06; N, 17.39. Found: C, 27.79; H, 3.10; N, 16.79.
2
Synthesis of complex 5
(
Cu Kα, λ = 1.5406 Å) and a secondary monochromator,
Pb(NO
3 3 2
) ·6H O (20 mg, 0.050 mmol) and HL1 (7 mg,
operated at 40 kV and 40 mA. Flat plate configuration was
used and the typical data collection range was between 5°
and 40°.
0.050 mmol) were dissolved in 2.0 mL of DMF–MeOH (1 : 3),
sealed in a capped glass vessel and heated to 75 °C for 48 h.
Subsequent gradual cooling to room temperature (0.2 °C
−
1
min ) afforded block shaped colorless crystals in ca. 77%
(
17.4 mg) yield (based on Pb). FT-IR (KBr, cm− ): 3393 (bs),
1
Synthesis of complex 1
3
1
301(bs), 1631 (s), 1602 (s), 1545 (s), 1438 (s), 1365 (s), 1299 (s),
195 (s), 1142 (m), 1041 (m), 1041 (m), 915 (m), 860 (w), 822
Pb(NO ) ·6H O (20 mg, 0.050 mmol) and HL1 (7 mg,
3
3
2
0
.050 mmol) were dissolved in 2.0 mL of formamide, sealed in
(
m), 682 (m). Anal. Calcd for C H N O Pb (M = 2333.01):
44 48 24 24 5
a capped glass vessel and heated to 75 °C for 48 h. Subsequent
−
1
C, 22.65; H, 2.07; N, 14.41. Found: C, 22.02; H, 1.91; N, 14.06.
gradual cooling to room temperature (0.2 °C min ) afforded
block shaped colorless crystals in ca. 60% (14.5 mg) yield
−1
Synthesis of complex 6
(
based on Pb). FT-IR (KBr, cm ): 3393 (bs), 3297 (bs), 1633 (s),
1
1
4
598 (s), 1546 (s), 1438 (s), 1366 (s), 1318 (s), 1229 (s), The mixture of Pb(NO ) ·6H O (20 mg, 0.050 mmol) and HL1
3 3 2
194 (m), 1147 (s), 915 (s), 821 (s), 682 (m), 591 (w), 541 (w), (7 mg, 0.050 mmol) was dissolved in 1.0 mL of DMF.
50 (w). Anal. Calcd for C20 Pb (M = 966.83): C, 24.85; The resulting mixture was sealed in a glass vessel and heated
H
16
N
12
O
8
2
H, 1.67; N, 17.38. Found: C, 24.33; H, 1.47; N, 16.75.
at 75 °C for 48 h. Subsequent gradual cooling to room
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temperature (0.2 °C min− ) afforded block shaped colorless scattering, for 4, 5 and 6, in this order. The electron counts
1
crystals. Yield: 37% (10.6 mg) based on Pb. FT-IR (KBr, cm− ): suggest, per asymmetric unit, the presence of ca. 2.5 DMF
1
3
(
8
C
436 (bs), 1654 (s), 1613 (s), 1507 (s), 1459 (m), 1385 (s), 1320 (in 4), 2 DMF and 1 MeOH (in 5) or 2 DMF and 1 H
m), 1226 (m), 1175 (s), 1140 (m), 1098 (w), 1053 (w), 885 (w), These disordered solvents were removed from the models but
26 (m), 806 (m), 739 (w), 661 (w), 562 (w). Anal. Calcd for included in the empirical formulae. The strategy improved the
(M = 1155.05): C, 29.12; H, 2.79; N, 16.98. factor by 29.12 (4), 8.20 (5) or 13.84% (6). Thermo-
2
O (in 6).
28
H
32
N
14
O
11Pb
2
R
1
Found: C, 29.59; H, 2.10; N, 16.81.
gravimetric and elemental analysis data support these results.
Crystallographic data are summarized in Table S1 (ESI†), and
selected bond distances and angles are presented in Table S2.†
Cyanosilylation of aldehydes catalyzed by Pb complexes
A dichloromethane (2.0 mL) suspension of an aromatic alde- CCDC 1017108–1017113 for compounds 1–6, contain the sup-
hyde (0.50 mmol), trimethylsilyl cyanide (1.0 mmol) and plementary crystallographic data for this paper.
3
1
mol% catalyst (14.5 mg for 1, 13.1 mg of 2, 16.2 mg of 3,
7.5 mg of 4, 33.8 mg of 5 and 17.2 mg of 6) was stirred at 15 °C
for 16 h and the progress of the reaction was monitored by TLC.
After the desired time, the catalyst was removed by filtration,
Acknowledgements
washed with dichloromethane and recovered for reuse. After This work was supported by the Foundation for Science and
evaporation of the solvent from the filtrate in a rotary evapor- Technology (FCT), Portugal (project PEst-OE/QUI/UI0100/
ator, the crude solid was obtained. The crude product was dis- 2013). Authors A. Karmakar and S. Hazra express their grati-
1
solved in CDCl
3
and analyzed by H NMR (ESI, Fig. S5†).
tude to the FCT for post-doctoral fellowships (Ref. no. SFRH/
In order to perform the recycling experiment, the catalyst BPD/76192/2011 and SFRH/BPD/78264/2011). The authors
isolated by filtration was washed with dichloromethane and acknowledge the Portuguese NMR Network (IST-UL Centre) for
dried at 25 °C for 1 h. The following steps were performed as access to the NMR facility.
described above.
Crystal structure determinations
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