Isolation and characterization of ZnII and HgII coordination polymers with a designed azo-aromatic ligand: Identification of micrometer- and nanometer-sized particles

Article abstract of DOI:10.1002/ejic.200600878

Controlled layering of the deprotonated methanolic solution of the azo-aromatic ligand [L³]^- (HL³ = 2-[3-(pyridylamino)-phenylazo]pyridine) over the aqueous/methanolic solutions of metal chlorides MCl₂ (M = Zn, Hg) afforded nano- and micrometer-sized particles of the metal-organic polymeric complexes of [HgCl(C₁₆H₁₂N₅)]_∞ (1) and [ZnCl-(C₁₆H₁₈N₅O₃)] _∞ (2). Time-dependent growth of the above particles is followed by the SEM and TEM analyses of the samples at different time intervals. The X-ray structure of the mercury polymer reveals that two infinite 1D, zigzag chains composed of [HgCl(L³)]_∞ units run along the a axis antiparalelly. The polynuclear Zn compound, in contrast, agglomerates fast to form hemispherical microcrystals with a diamondoid surface morphology, and no suitable X-ray quality crystal of this complex could be isolated. Powder XRD analyses of the samples as well as thermogravimetric analysis (TGA) are used for their characterization. Unusually, these metal-organic polymers of the reference d¹⁰ metal ions are green and absorb in the low-energy region of the visible spectrum, 660-675 nm. Semiempirical calculations on a representative complex 1 suggest that the transitions in the complexes involve ligand orbitals. These also show multiple emissions in the blue-green region. The Zn complex, which is microporous, shows reversible adsorption/desorption of N₂ and H₂ gasses. The mercury polymer, on the other hand, shows poor adsorption ability. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200600878

FULL PAPER
DOI: 10.1002/ejic.200600878
Isolation and Characterization of ZnII and HgII Coordination Polymers with a
Designed Azo-Aromatic Ligand: Identification of Micrometer- and Nanometer-
Sized Particles
Priyabrata Banerjee,[a] Soumitra Kar,[b] Asim Bhaumik,[b] Gene-Hsiang Lee,[c]
Shie-Ming Peng,[c] and Sreebrata Goswami*[a]
Keywords: Coordination polymers / Azo-aromatic ligands / d10 metal ions / X-ray structures / Nanostructures / Fluores-
cence
Controlled layering of the deprotonated methanolic solution
of the azo-aromatic ligand [L3]– (HL3 = 2-[3-(pyridylamino)-
phenylazo]pyridine) over the aqueous/methanolic solutions
of metal chlorides MCl2 (M = Zn, Hg) afforded nano- and
micrometer-sized particles of the metal–organic polymeric
crystal of this complex could be isolated. Powder XRD analy-
ses of the samples as well as thermogravimetric analysis
(TGA) are used for their characterization. Unusually, these
metal–organic polymers of the reference d10 metal ions are
green and absorb in the low-energy region of the visible
spectrum, 660–675 nm. Semiempirical calculations on a rep-
resentative complex 1 suggest that the transitions in the com-
plexes involve ligand orbitals. These also show multiple
emissions in the blue-green region. The Zn complex, which
is microporous, shows reversible adsorption/desorption of N2
and H2 gasses. The mercury polymer, on the other hand,
complexes
of
[HgCl(C16H12N5)]ϱ
(1)
and
[ZnCl-
(C16H18N5O3)]ϱ (2). Time-dependent growth of the above
particles is followed by the SEM and TEM analyses of the
samples at different time intervals. The X-ray structure of the
mercury polymer reveals that two infinite 1D, zigzag chains
composed of [HgCl(L3)]ϱ units run along the a axis antipara-
lelly. The polynuclear Zn compound, in contrast, agglomer- shows poor adsorption ability.
ates fast to form hemispherical microcrystals with a dia-
mondoid surface morphology, and no suitable X-ray quality
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
common primarily because of the lack of availability of
suitable ligands.[8] Azo-aromatic compounds, with the ex-
ception of their use in dyestuff industry, have extensively
been studied in recent years owing to their novel optical[9]
and redox properties. Low-lying vacant π*(azo) orbitals in
metal–azo-aromatic compounds are responsible for low-en-
ergy transitions and the colour of the compounds. During
recent years we have been working on the coordination
chemistry of a new class of azo-aromatic ligands [HL].
Three different ligands HL1, HL2 and HL3 have been iso-
lated[10–12] following regioselective ortho fusion of suitable
aromatic amines. The deprotonated ligand [L1]– binds in a
bischelating fashion to produce monometallic complexes,
the other two ligands, [L2]– and [L3]–, each containing an
additional pyridyl donor, serve as bridges between two or
more metal centres. Moreover, the coordination mode of
the deprotonated ligand [L3]– is different from that of its
positional isomer [L2]– and is found to be suitable[13] for the
construction of one-dimensional polymeric material,[14,15]
particularly with coordinately unsaturated metal ions.
The primary concern of this paper is to report our results
on the isolation of one-dimensional HgII and ZnII coordi-
nation polymers with the bridging ligand [L3]–. Unusually,
the polymeric d10 metal complexes are intensely coloured
and absorb in the low-energy region of the visible spectrum.
Introduction
Inorganic as well as organic polymeric substances have
attracted intensive attention during the recent years primar-
ily because of their potential applications as functional ma-
terials.[1] In the past decade, several organic and inorganic
materials[2] have been synthesized and investigated. In con-
trast, examples[3,4] of nano- or micrometer-scale particles of
metal–organic coordination polymers are limited. Thus,
their preparation is challenging owing to their ability to tai-
lor their physical and chemical properties through deliber-
ate selection[5] of metal and multifunctional ligands. Vari-
ous types of bridging ligands[6–8] have been used for the
construction of coordination polymers. However, such ex-
amples containing azo-aromatic bridging ligands are un-
[a] Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
Kolkata 700 032, India
Fax: +91-33-2473-2805
E-mail: icsg@iacs.res.in
[b] Department of Material Science, Indian Association for the
Cultivation of Science,
Kolkata 700 032, India
[c] Department of Chemistry, National Taiwan University,
Taipei, Taiwan, Republic of China
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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P. Banerjee, S. Kar, A. Bhaumik, G.-H. Lee, S.-M. Peng, S. Goswami
The green cobalt complex [Co(L3)2]ClO4 was purified on
FULL PAPER
These polymers form different-sized particles (micrometer
to nanometer) at different time intervals under identical ex- a preparative TLC (silica gel) plate by using a chloroform/
perimental conditions. Their growth processes have been toluene (1:1) solvent mixture as eluent and was charac-
1
followed by FESEM, TEM, HRTEM, XRPD, EDS and terized spectroscopically. Its H NMR spectrum is submit-
single-crystal X-ray data analysis of the frameworks.
ted as Figure S1 Supporting Information. Reduction of the
cationic cobalt complex by dilute N2H4 followed by re-
moval of [Co]2+ as CoS led to the isolation of the ligand
[HL3] in moderate yield. The pure ligand was obtained as
a yellow crystalline solid, whose ESI-MS spectrum shows
an intense peak at m/z = 276 amu assigned to the [H2L3]+
1
ion (Figure S2, Supporting Information). The H and 13C
NMR spectra of the ligand [HL3] (Figures S3 and S4, Sup-
porting Information) corroborate fully with its formulation.
The deprotonated ligand [L3]– (generated in situ by addition
of NEt3) reacts spontaneously with the methanolic/aqueous
solutions of MCl2 (M = Zn, Hg) at room temperature. The
dark green compounds, thus produced, are rapidly precipi-
tated. However, by controlled layering of the deprotonated
ligand solution over the solutions of metal salts, we could
achieve the synthesis of stable particles of the reference
metal–organic polymers. It may be noted here that the salt
CdCl2 failed to produce any stable compound under iden-
tical experimental conditions. Chemical compositions of the
polymers were established on the basis of their elemental
analyses. Both are pentacoordinate and have identical re-
peating units [MCl(L3)]. Notably, the morphology and the
growth process of the mercury polymer are totally different
from those of the zinc analogue. The growth processes of
the two products are detailed separately.
Results and Discussion
The ligand HL3 has been synthesized by regioselective
C–N bond fusion of 3-aminopyridine to cobalt(II)-coordi-
nated 2-(phenylazo)pyridine following the reaction strategy
developed by us[10a] (Scheme 1).
Hg–L3 Polymer
For the evaluation of the crystalline mercury coordina-
tion polymer, the synthetic reactions were carried out in
Scheme 1.
Figure 1. FESEM images of the [HgClL3]ϱ polymer at different time intervals: (a) 5 h, (b) 36 h, (c) 5 d, (d) 10 d.
836 Eur. J. Inorg. Chem. 2007, 835–845
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ZnII and HgII Coordination Polymers with a Designed Azo-Aromatic Ligand
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five sealed glass tubes (A–E) under the same experimental presence of large quantities of spherical particles embedded
conditions but with varying reaction times. A deprotonated within the structure. Figure 2b shows the magnified TEM
methanolic solution of the ligand was layered over an aque- image of a ribbon, which indicates the presence of a large
ous solution of mercuric chloride and left undisturbed for number of dark, spherical nanoparticles. The TEM image
diffusion (see Experimental Section). Figure 1a–d shows the in Figure 2d reveals the presence of a large number of
SEM images of the as-synthesized products. It reveals the spherical nanoparticles of the polynuclear mercury sample
following features: (i) The product obtained from the tube 1 with diameters that vary between 10 and 30 nm. Some
A (after 5 h) is amorphous (Figure 1a); (ii) The SEM image grain agglomeration is also observed in this image and is
(Figure 1b) of the second sample (tube B, after 36 h) reveals indicated by arrows. It is also interesting to note, from the
fine wirelike morphology; (iii) The SEM shown in Figure 1c TEM images, the layered nature of the agglomerated par-
is obtained from the third sample (tube C, after 5 d) and ticles, which can perhaps explain the reason for the rapid
indicates the formation of nano-ribbonlike morphology. increase in the dimension of the products. Thus, on the ba-
The width of these ribbons varies from a few nm to 500 nm, sis of the SEM and TEM observations we can conclude
and their lengths lie within a few tens of micrometers. The that the crystalline metal–organic framework (MOF) is pro-
facets of these ribbons are not well defined, which is an duced in four steps. The first step is the formation of
indication of their poor crystallinity; (iv) Interestingly, the amorphous nucleation centres, followed by formation of the
fourth sample (tube D, after 10 d) is composed of well-de- one-dimensional amorphous framework; the amorphous
fined microribbons (Figure 1d); and (v) X-ray-quality single nature is also evident from its featureless powder XRD
crystals are obtained from tube E after 3 weeks and their pattern (Figure S5, samples of Figure 1a and b). The nano-
X-ray structures are analyzed (vide infra).
crystalline particles act as the nucleation sites (thus formed
For understanding the growth mechanism, we also inves- within the amorphous network), absorb more reactants
tigated these samples by transmission electron microscopy from the solution and agglomerate with each other to grow
(TEM). The first product is a featureless amorphous spheri- in one-dimension to form fine wire- or ribbonlike products.
cal compound, and the second is wirelike. The fourth sam- In the final step these products crystallize to yield large
ple (tube D) is quite thick for TEM measurement, and the whiskerlike single crystals. This is as expected since the
electron beam failed to penetrate through it. Fairly interest- amorphous products are high-energy systems[16] and have a
ing micrographs are obtained from the third sample (tube natural tendency to minimize their energy by crystalli-
C). Figure 2a shows the TEM image of a single ribbonlike zation. Larger single-crystalline products are achieved if the
morphology. Closer observation of the image reveals the reaction is allowed to take place for 3 weeks or more.
Figure 2. (a) TEM image of the single ribbonlike products of [HgClL3]ϱ. (b) Zoomed TEM image showing spherical nanoparticles of Hg
containing MOF. (c) Representative zoomed HRTEM image of a single metal–ligand nanoparticle. (d) Arrows indicating the agglomera-
tion of the particles.
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P. Banerjee, S. Kar, A. Bhaumik, G.-H. Lee, S.-M. Peng, S. Goswami
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We examined the XRPD pattern for all the samples ob- crometer-sized particles are identified. The SEM image of
tained from the aforementioned five tubes (A–E). The sam- the products obtained after 1 d reaction time shows the for-
ple from tube A exhibits a featureless XRPD pattern, which mation of large numbers of nanorods bundled together
implies that it is amorphous. However, experimental XRPD (Figure 4a). With time, these particles grow in one direc-
patterns of the samples from the three tubes (B–D) are sim- tion, and the basal region crystals penetrate through each
ilar and consistent with the simulated pattern derived from other and agglomerate to form large microcrystals. As these
the single-crystal (tube E). Thus, we conclude that the bulk particles grow on the tube walls they cannot form spheres,
sample has the same structure as that of the single crystal. instead hemispherical particles are isolated (Figure 4b). The
The experimental powder diffraction pattern of a represen- SEM image of a hemispherical single unit (after 6 d) is
tative sample (tube C), along with the simulated pattern, shown in Figure 4c, and a further magnified image of it is
are shown in Figure 3 for comparison. Furthermore, micro- displayed in Figure 4d. This reveals that the individual
elemental analyses of the above mercury samples reveal that basic units with pyramidal tips interpenetrate[16] to form the
they have an identical composition, [HgCl(L3)], which was hemispherical particles. Figure 4e shows the broken basal
further supported by energy dispersive spectroscopy (EDS) region of a hemispherical crystal that indicates agglomera-
(Figures S6 and S7).
tion[17] of closely packed individual units. No suitable single
crystal of this sample could be isolated in this case, possibly
because of fast agglomeration, although the powder diffrac-
tion pattern indicates a crystalline nature (Figure 5). The
Zn samples are too thick for TEM measurements; however,
high resolution transmission electron microscopy
Figure 3. Experimental and simulated XRPD patterns of the as-
synthesized mercury polymer [HgClL3]ϱ.
Zn–L3 Polymer
The results obtained in this case were notably different
Figure 5. XRPD pattern of the polycrystalline polymer
from that of its mercury analogue. Here, two types of mi- [ZnClL3·3H2O]ϱ.
Figure 4. FESEM images of the [ZnClL3·3H2O]ϱ polymer at different time intervals: (a) 1 d, (b) 6 d. (c) Zoomed SEM image of one of
the representative hemispheres. (d) Magnified surface image of (c). (e) Broken basal region of a hemisphere.
838
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ZnII and HgII Coordination Polymers with a Designed Azo-Aromatic Ligand
FULL PAPER
(HRTEM) on the crushed sample reveals that these par- ligand and its coordination at the Hg centre result in zigzag
ticles are crystalline with a highly ordered arrangement of chains that adopt a saw-tooth-like structure (Figure 8a).
the micropores (Figure 6). Microelemental analyses on the The two independent polymeric chains are closely packed/
Zn samples indicate that their compositions are similar to locked through metal–pi interactions[21a] along with face-
those of the mercury analogues, with the exception that the to-face π–π ring interactions.[21b] Notably, the metal–pi in-
Zn polymer contains three H2O molecules of crystallization teractions predominate over the π–π interactions. For exam-
per formula weight ([ZnCl(L3)]·3H2O) of the repetitive ple, the perpendicular distance between the centroids of the
building unit (TG analysis, vide infra). Energy dispersive two interacting polymeric chains lies between 3.44 and
spectra of these samples (Figures S8a and S8b) also corro- 3.45 Å, whereas the distance between the Hg metal and
borate the above formulation.
centroid (Cg) lies in the range 3.37–3.41 Å. Selected bond
lengths and angles, and the crystallographic data for the
crystal [HgCl(L3)]ϱ are collected in Table 1 and Table 2,
respectively. The secondary interactions in this structure
(Figure 8b) work together to bring the two consecutive
chains close together (Table 3 and Table 4). Notably, the
closest distances between the two mercury atoms (both in-
ter- and intramolecular) are quite long: intramolecular
Hg(1)···Hg(1a) and Hg(2)···Hg(2a) distances are 6.243 Å
and 6.307 Å, respectively, which are appreciably longer than
the sum of the van der Waals radii[12c,22] between two con-
secutive mercury atoms (3.50 Å). The distance between two
mercury atoms to complete one pitch [irrespective of Hg(1)
or Hg(2)] is 10.145 Å. The Hg–π and π–π interactions are
depicted in Figure 8b. For comparison, we wish to note that
the ligand HL2 reacts with HgCl2 to produce the soluble
dinuclear mercury(II) blue complex, in which the two [L2]–
ligands bridge the two mercury centres.[12c]
Spectral Properties
Infrared spectra of the two polymeric complexes
[MClL3] (M = Hg, Zn) are almost identical, and νC=N and
˜
ϱ
ν
appear[12] in the range 1585–1600 cm–1 and 1305–
˜
N=N
1325 cm–1, respectively. These complexes also show charac-
teristic ν
bands that appear[12] near 275 cm–1. Both
˜
M–Cl
complexes are insoluble in common organic solvents, and
their UV/Vis spectra in the solid state were studied. The
Figure 6. HRTEM images of the as-synthesized [ZnClL3·3H2O]ϱ lowest-energy transition in free [L3]– (460 nm) is redshifted
polymer: (a) low magnification, (b) high magnification.
considerably by ca. 200 nm in the corresponding metal
complexes. Whereas the mercury complex shows a broad
transition at 660 nm, its zinc analogue absorbs at 675 nm.
Notably, examples of d10 metal ion complexes that absorb
Crystal Structure of [HgCl(L3)]ϱ (1)
in such a low-energy part of the visible region are uncom-
The three-dimensional structure of the mercury complex mon.[12] In order to gain some insight into the nature of
(sample obtained from tube E) was determined by single- the orbitals involved in electronic transitions of the above
crystal X-ray diffraction. In this structure, two 1D infinite complexes, semiempirical EHMO calculations on the repre-
polymeric chains made of [HgCl(L3)]ϱ units run antipara- sentative mercury complex [HgClL3]ϱ was performed with
lelly along the a axis.[18] As shown in Figure 7, each mer- the program CACAO[23] created by Mealli and Proserpio.
cury atom in this molecule is in a pentacoordinated N4Cl The calculations were made on the basis of atomic coordi-
environment, and the geometry around each HgII ion may nates obtained from the single-crystal X-ray data analysis
be described as distorted square pyramidal[19] with a τ value of compound 1. The results reveal that HOMO–1, HOMO,
of 0.21. In this context, we note that higher-coordination- LUMO and LUMO+1 all are essentially ligand orbitals
number (Ͼ4) complexes of divalent mercury[20] are scarce with (Ͼ90% ligand contribution, see Figure S9). Thus the
in the literature. In the molecule 1, Hg(1) sits above the lowest-energy transitions in the present polymeric com-
plane formed by the three coordinating nitrogen atoms, plexes are ascribed to electronic transitions within the li-
N(1), N(3) and N(4), by 0.0464(1) Å; this pattern is re- gand orbitals (π–π*). Modification of the properties of the
peated throughout the polymeric chain. The bending of the ligand orbitals in the present examples is due to increased
Eur. J. Inorg. Chem. 2007, 835–845
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P. Banerjee, S. Kar, A. Bhaumik, G.-H. Lee, S.-M. Peng, S. Goswami
FULL PAPER
Figure 7. ORTEP and atom-numbering scheme for [HgClL3]ϱ. Hydrogen atoms are omitted for clarity.
Table 1. Selected bond lengths [Å] and bond angles [°] for
[HgClL3]ϱ (1).
Bond lengths
Hg(1)–N(1)
Hg(1)–N(3)
Hg(1)–N(4)
Hg(1)–N(5)
Hg(1)–Cl(1)
N(2)–N(3)
N(3)–C(6)
C(6)–C(11)
C(11)–N(4)
N(4)–C(12)
2.53(2)
2.41(1)
2.27(1)
2.43(1)
2.40(1)
1.24(1)
1.41(2)
1.40(2)
1.31(2)
1.42(2)
Hg(2)–N(6)
Hg(2)–N(8)
Hg(2)–N(9)
Hg(2)–N(10)
Hg(2)–Cl(2)
N(7)–N(8)
N(8)–C(22)
C(22)–C(27)
C(27)–N(9)
N(9)–C(28)
2.50(1)
2.34(1)
2.26(1)
2.40(1)
2.40(1)
1.26(1)
1.34(2)
1.46(2)
1.36(2)
1.39(2)
Bond angles
N(3)–Hg(1)–N(4) 71.7(5)
N(9)–Hg(2)–N(8) 70.5(4)
N(1)–Hg(1)–N(3) 63.0(5)
N(8)–Hg(2)–N(6) 66.2(4)
planarity of the ligand and also to extensive charge delocal-
ization along the ligand backbone. A similar type of finding
was reported recently on the ZnII and CdII complexes[24,25]
of heterocyclic ligands. In such systems, heteroatoms de-
crease the π and π* orbital energies considerably, and as a
result, the HOMO and LUMO lack contribution from the
metal atoms. In addition, multiple electronic transitions are
Figure 8. (a) Space-filling model for [HgClL3]ϱ, showing the chain
that adopts a saw-tooth-like backbone. (b) Different weak interac-
tions constructed by π–π and (Hg) metal–π interactions between
the adjacent polymer units running along the a axis.
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ZnII and HgII Coordination Polymers with a Designed Azo-Aromatic Ligand
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Table 2. Crystallographic data for complex 1.
Complex
[C16H12ClHgN5]ϱ
Empirical formula
Molecular mass
T [K]
C16H12ClHgN5
510.35
295(2)
Crystal system
Space group
Wavelengths [Å]
a [Å]
b [Å]
c [Å]
orthorhombic
Pca21
0.71073
10.1452(5)
18.1918(9)
17.3237(9)
90.00
α [°]
β [°]
90.00
γ [°]
90.00
3197.3(3)
8
V [Å3]
Z
Dcalcd. [gcm–3
]
2.120
Crystal dimension [mm]
θ range for data collection [°]
GOF on F2
0.20ϫ0.10ϫ0.02
1.12–27.50
1.042
Reflections collected
29113
Unique reflections
7341
Final R indices [IϾ2σ (I)]
R1 = 0.0595, wR2 = 0.0947
0.974, –1.466
Largest diff. between peak and hole [eÅ–3
]
Figure 9. An overlay of UV/Vis spectra for the two MOFs in the
solid state: (i) [HgClL3]ϱ (----), (ii) [ZnClL3·3H2O]ϱ (Ϫ).
Table 3. π–π interactions (face-to-face) in complex 1.
emission spectra of its Zn analogue have been submitted as
Supporting Information (Figure S10). It should be noted
that the ligand [HL3] shows very weak luminescence in the
solid state upon excitation at 255 and 335 nm. The photo-
physical properties of complexes 1 and 2 are unique in the
sense that no other reported compound of this and similar
azo-aromatic ligands exhibit detectable luminescence. The
luminescence properties in these examples thus may be at-
tributed to the[27–28] polymeric nature of the complexes.
This imparts rigidity in the ligand framework and thus re-
duces energy loss through a nonradiative relaxation path-
way. In this context, it should be noted that blue-green lu-
minescent coordination complexes have been studied inten-
sively because of their possible applications in material sci-
ence.[29]
ring(i) Ǟ ring(j)
Dihedral angle Angle between first Ќ distance
between ring ring normal and the of centroid(i)
planes (i,j) [°] line joining ring Cgs from ring(j)
(i,j) [°]
[Å]
R(1) Ǟ R(2)[a]
R(3) Ǟ R(4)[b]
4.16
1.70
39.02
40.29
3.441
3.451
[a] Symmetry code: = 1+x, y, z. [b]Symmetry code: = x, y, z.
Table 4. (Hg) metal–π ring interactions in complex 1.[a]
R(i)–Hg(j)
Metal-to-Cg distance [Å] Angle between joining
line and ring normal [°]
R(3)–Hg(2)
R(4)–Hg(1)
3.373
3.412
24.08
26.63
[a] Where R(1) represents N1–C1–C2–C3–C4–C5; R(2) represents
N6–C17–C18–C19–C20–C21; R(3) represents C6–C7–C8–C9–
C10–C11; R(4) represents C22–C23–C24–C25–C26–C27.
Sorption Properties
also noted in the high-energy region. Spectral data of 1 and
2 are presented in the Experimental Section, and the spectra
are shown in Figure 9.
The presence of peaks at low 2θ range in the XRPD
pattern of the above polymers suggest that the possible or-
The emission properties of the complexes were studied in dering of the building units of these functional materials[30]
the solid state at room temperature. The emission spectra is similar to that of microporous[31] systems. Whereas the
are found to be dependent on the excitation wavelength. Zn complex indeed shows the moderate ability of adsorp-
For example, the mercury complex 1 shows a relatively tion of small molecules like N2 and H2, its HgII analogue
weak emission band at 690 nm upon excitation at 640 nm. shows poor adsorption ability.
Interestingly, multiple blue-green emission bands with max-
The N2 sorption experiment on the Zn polymer 2 at 77 K
ima at 420, 450 and 485 nm are observed upon excitation suggests a type I isotherm at low P/P0 ratio of N2 that cor-
at 335 nm, whereas emission bands with maxima at 425 nm responds to the presence of micropores with a BET surface
and 460 nm are observed upon excitation with the very area of 214 m2 g–1. Pore size distribution of the sample, ob-
short wavelength of 235 nm. Multiple emission bands were tained by using Horvath–Kawazoe method,[32] is shown in
reported[26] previously in other d10 metal complexes. A sim- Figure 11. Two broad maxima can be seen at 9.2 and 14.1 Å
ilar behaviour in the emission spectrum of the correspond- in the micropore region.The average pore diameter of 9.2 Å
ing zinc complex is also observed. The emission spectra of can be explained from the low-range 2θ peaks in the powder
the mercury MOF are displayed in Figure 10, whereas the XRD pattern (Figure 5). The pore diameter of this sample
Eur. J. Inorg. Chem. 2007, 835–845
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841

P. Banerjee, S. Kar, A. Bhaumik, G.-H. Lee, S.-M. Peng, S. Goswami
FULL PAPER
complete reversible adsorption and desorption for H2, and
a steady increment in the uptake of H2 with increasing pres-
sure.
Figure 11. Pore size distribution curve for [ZnClL3·3H2O]ϱ.
Figure 12. Isotherms for the adsorption and desorption of H2 gas
for [ZnClL3·3H2O]ϱ.
The observed BET surface area of the mercury polymer
is small, 13.5 m2g–1, and is quite low relative to that of
the zinc sample. This indicates that nitrogen gas adsorption
occurs primarily at the external surface of the mercury
polymer and that the adsorbate nitrogen molecule cannot
freely diffuse into the internal channels of this solid, as ob-
served for other microporous solids.[33]
Thermogravimetric (TGA) analyses of samples 1 and 2
were performed to estimate the stability of the complexes
at elevated temperatures. Figure S11 in the Supporting In-
formation shows the TGA curves for samples 1 and 2. The
mercury sample 1 is stable only up to 97 °C. A continuous
weight loss (Ͼ35%) followed by another massive weight
Figure 10. Multiple emission spectra for [HgClL3]ϱ at different ex-
citation wavelengths.
estimated from the TEM analysis (vide supra) [Figure 6] loss of (Ͼ 50%) are noted in the temperature range 100–
agrees well with the N2 sorption data. The presence of the 225 °C and 225–375 °C, respectively. The weight loss in the
peak at a pore diameter of 14.1 Å may be attributed to the temperature range 100–225 °C may be attributed to the loss
large micropores generated at the interparticle cages. We of the ligand, 2-(phenylazo)pyridine. In comparison, the Zn
have also carried out H2 sorption on the reference Zn poly- polymer 2 shows a slow weight loss of 7.2% in the range
mer. The H2 adsorption/desorption isotherm at 77 K is 25–275 °C, which is attributed to the loss of adsorbed water.
shown in Figure 12. The nature of the isotherms suggests Between 275 °C and 325 °C, there are multiple stages of
842
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Eur. J. Inorg. Chem. 2007, 835–845

ZnII and HgII Coordination Polymers with a Designed Azo-Aromatic Ligand
FULL PAPER
Synthesis of Compounds
weight loss, which amount to 14.4%, presumably due to the
loss of water present from the large micropore region. A
weight loss of 11.1% in the range 325–370 °C and of 14.0%
in the range 370–500 °C, as a result of the collapse of the
metal–ligand framework, is observed. We wish to note here
that the Zn–L3 framework remains intact up to 325 °C, as
evidenced by their near identical XRPD pattern; above this
temperature, the network collapses.
HL3 Synthesis: Two steps were involved: (i) ortho-amination on
[Co(pap)3]2+ followed by (ii) ligand isolation from the cobalt com-
plex.
Isolation of [Co(L3)2]ClO4 from [Co(pap)3](ClO4)2: A mixture of
[Co(pap)3](ClO4)2 (0.2 g, 0.25 mmol) and 3-aminopyridine (0.1 g,
1.06 mmol) was heated on a steam bath for 7 h. The initial brown
colour gradually became intense green. The crude product, which
contained mainly the compound [Co(L3)2]ClO4, was further puri-
fied on a preparative TLC plate (silica gel) with a acetonitrile/chlo-
roform mixture (1:4) as eluent. A yellowish band of unreacted 3-
aminopyridine moved first, followed by a major green band of the
cobalt compound [Co(L3)2]ClO4. The major green band was col-
lected and was thoroughly washed with diethyl ether. Crystalline
pure green [Co(L3)2]ClO4 was obtained from a dichloromethane/
hexane mixture. Yield: 0.125 g, 70%. ESI-MS: m/z = 607.15 [M –
Conclusions
In this work we have described the isolation and charac-
terization of a polymeric network of ZnII and HgII with a
designed azo-aromatic ligand. The gradual growth of the
particles of the polymeric materials has been followed by
successive product isolation at different time intervals. Un-
usually, the polymers are blue-green, have five-coordinate
metal centres and display emission in the blue-green region
of the visible spectrum. The Zn polymer is microporous and
its sorption isotherm is reversible. Our work in the area
of designing multifunctional azo-aromatic bridging ligands
continues.
–
ClO4 ]. C32H24ClCoN10O4 (706.93): calcd. C 54.36, H 3.42, N
19.81; found C 54.40, H 3.40, N, 19.75. IR (KBr): ν = 1600 (s)
˜
[C=N], 1325 (vs) [N=N], 1085 (s), 620 (s) [ClO4 ] cm–1. UV/Vis (in
–
dichloromethane, λmax): 315, 400, 720, 790, 880 nm.
Isolation of [3-(2-Pyridylamino)phenyl]azopyridine (HL3) from
[Co(L3)2]ClO4: The cobalt complex [Co(L3)2]ClO4 (0.15 g,
0.21 mmol) was dissolved in ethanol (30 mL), and hydrazine hy-
drate (5 mL) and yellow ammonium sulfide (5 mL) were added.
The mixture was then stirred for 30 min at room temperature. The
resulting orange-yellow solution was evaporated to dryness, and
the product was extracted with dichloromethane and loaded on a
preparative TLC plate (silica gel) for purification. An orange-yel-
low band was eluted with a toluene/chloroform mixture (2:1),
which on evaporation yielded orange crystals of HL3. Yield:
0.075 g, 65%. M.p. 98 °C. ESI-MS: m/z = 276.10. pKa = 8.7Ϯ0.1.
C16H13N5 (275.28): calcd. C 69.81, H 4.75, N 25.44; found. C
Experimental Section
Materials: The starting complex [Co(pap)3](ClO4)2 was prepared
by a reported procedure.[34] The salts ZnCl2 and HgCl2 were ob-
tained from Merck India Limited and Qualigens, respectively. 3-
Aminopyridine was obtained from S.D. Fine-chemical Limited.
Solvents and chemicals used for synthesis were of analytical grade.
69.75, H 4.80, N 25.40. IR (KBr): ν = 1585 (s) [C=N], 1315 (vs)
˜
[N=N) cm–1. UV/Vis (solid, λmax): 255, 335, 460 nm.
Polynuclear Mercury(II) Complex [HgClL3]ϱ (1): In a long neck
crystal tube, an aqueous solution (5 mL) containing HgCl2
(100 mg, 0.365 mmol), water (2 mL) and a methanolic solution of
a mixture of HL3 (100 mg, 0.365 mmol) and of NEt3 (2 drops) were
layered gradually in the above order. In between the water and
methanol solvent layer, a green ring formed almost immediately.
The tube was stoppered and left undisturbed at room temperature.
Slow diffusion between the two solutions afforded dark green crys-
Physical Measurements: A JASCO V-570 Spectrophotometer was
used to record electronic spectra. The IR spectra were recorded
with a Perkin–Elmer 783 spectrophotometer. 1H NMR spectra
were measured in CDCl3 with a Bruker Avance DPX 300 spectrom-
eter, and SiMe4 (TMS) was used as the internal standard. A Per-
kin–Elmer 240C elemental analyzer was used to collect microana-
lytical data (C, H and N). The compositional analyses were evalu-
ated by energy dispersive spectroscopy (EDS). Microstructures of
the nano forms were studied by scanning electron microscopy
(SEM, Hitachi S-3200), transmission electron microscopy (TEM)
and high resolution transmission electron microscopy (HRTEM,
JEOL JEM 2010 TEM at an accelerating voltage of 200 kV). Solid
state emission spectra were recorded at room temperature at dif-
ferent excitation wavelengths with a Perkin–Elmer LS 55 Lumines-
cence Spectrometer. ESI mass spectra were recorded on a micro-
mass Q-TOF mass spectrometer (serial no. YA-263). Powder XRD
data were recorded on a Seifert 3000P diffractometer, on which
small- and wide-angle goniometers were mounted, at 40 kV and
20 mA by using Cu-Kα radiation (λ = 1.5406 Å) with a scan speed
of 2°min–1. Sorption isotherm studies were performed by using a
Quantachrome AUTOSORB 1C-TCD sorption instrument at
77 K. Prior to gas adsorption measurements, samples were de-
gassed for 2 h at 100 °C for 1 and at 120 °C for 2 to remove all
guest molecules. Thermogravimetric (TGA) analyses for the repre-
sentative samples were conducted at a scan rate of 5.00 °Cmin–1
by using a Perkin–Elmer DIAMOND TG/DTA thermogravimeter.
Melting points were determined with the help of a capillary fitting
Mel. Temp. II (Laboratory Devices Inc., USA) apparatus.
tals of complex
1 after 3 weeks. Yield: 0.130 g, 70%.
C16H12ClHgN5 (510.32): calcd. C 37.66, H 2.37, N 13.72; found C
37.65, H 2.39, N 13.67. IR (KBr): ν = 1590 (s) [C=N], 1300 (vs)
˜
[N=N] cm–1. UV/Vis (solid, λmax): 245, 345, 500, 660 nm.
The growth process of the mercury particles was followed by carry-
ing out the synthetic reaction in five sealed tubes (A–E) by using
an identical reaction mixture as stated above but by varying the
reaction time. The samples were collected after 5 h, 36 h, 5 d, 10 d
and 21 d.
Polynuclear Zinc(II) Complex [ZnClL3·3H2O]ϱ (2): In a long neck
crystal tube, the deprotonated methanolic ligand solution of [L3]–
(100 mg, 0.365 mmol) was layered over a methanolic solution of
ZnCl2 (50 mg, 0.365 mmol). The tube was stoppered and left undis-
turbed at room temperature. A dark polycrystalline complex of the
zinc polymer was isolated from the tube after 6 d. Yield: 0.125 g,
80%. C16H18ClN5O3Zn (429.16): calcd. C 44.77, H 4.22, N 16.31;
found C 44.70, H 4.22, N 16.26. IR (KBr): ν = 1595 (s) [C=N],
˜
1310 (vs) [N=N], 3440 (s) [H2O] cm–1. UV/Vis (solid, λmax): 255,
335, 380, 675 nm.
Eur. J. Inorg. Chem. 2007, 835–845
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843

P. Banerjee, S. Kar, A. Bhaumik, G.-H. Lee, S.-M. Peng, S. Goswami
FULL PAPER
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X-ray Crystallography: Crystallographic data for compound 1 is
collected in Table 2. Specific details are given below.
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Suitable
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[7]
Acknowledgments
Financial support received from the Council of Scientific and In-
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CSIR for his fellowship. We are also thankful to Dr. Golam
Mostafa of Jadavpur University for his help.
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844
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ZnII and HgII Coordination Polymers with a Designed Azo-Aromatic Ligand
FULL PAPER
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Received: September 19, 2006
Published Online: January 10, 2007
Eur. J. Inorg. Chem. 2007, 835–845
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