pH-Induced formation of metalloligand: Increasing structure dimensionality by tuning number of ligand functional sites

Article abstract of DOI:10.1039/b705603f

Two complexes, 2D [Cu₂(CN)₂(4-Hpcih)]_n and 3D {[Cu₂(CN)_1.5(4-pcih)]·1.25H₂O} _n (4-Hpcih = 4-pyridinecarbaldehyde isonicotinoyl hydrazone), were obtained using a synthetic approach of pH-induced formation of metalloligands, successfully demonstrating a strategy to increase structure dimensionality by tuning the number of ligand functional sites. The Royal Society of Chemistry.

Full text of DOI:10.1039/b705603f

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pH-Induced formation of metalloligand: increasing structure
dimensionality by tuning number of ligand functional sites{
Wen-Xiu Ni, Mian Li, Xiao-Ping Zhou, Zhen Li, Xiao-Chun Huang and Dan Li*
Received (in Cambridge, UK) 12th April 2007, Accepted 2nd July 2007
First published as an Advance Article on the web 24th July 2007
DOI: 10.1039/b705603f
8
Two complexes, 2D [Cu
2
(CN) (4-Hpcih)]
2
n
and 3D
pH-induced formation of the metalloligand, increasing the
number of ligand functional sites, should lead to the generation
of distinct polymeric coordination networks during the self-
assembly process. In other words, such a strategy is based on the
different ligating behaviors between the neutral and anionic forms
of the Schiff-base ligand, which depends on the ligand deprotonat-
{
[Cu (CN) (4-pcih)]?1.25H O} (4-Hpcih = 4-pyridinecarbal-
n
2
1.5
2
dehyde isonicotinoyl hydrazone), were obtained using a
synthetic approach of pH-induced formation of metalloligands,
successfully demonstrating a strategy to increase structure
dimensionality by tuning the number of ligand functional sites.
9
ing or not.
Current increasing interest in coordination polymers is justified not
only by the potential applications of these compounds as
The ligand 4-pyridinecarbaldehyde isonicotinoyl hydrazone
(4-Hpcih) was prepared conveniently in a high yield by the
reaction of 4-pyridinecarboxaldehyde and isonicotinic acid hydra-
zide in ethanol. Solvothermal reactions of CuCN and 4-Hpcih
(molar ratio 2 : 1) in acetonitrile under acidic and neutral
1,2
functional materials but also by the particular beauty and the
3
,4
intriguing structural diversity of the architectures. A general
principle for the construction of coordination polymers is to regard
5
metal ions as connectors and organic ligands as linkers. The
conditions yielded two different complexes: [Cu
(1) and {[Cu (CN)1.5(4-pcih)]?1.25H O} (2) (see the ESI for
synthetic and crystallographic details).{{
2 2 n
(CN) (4-Hpcih)]
design of multifunctional ligands containing appropriate coordina-
tion sites linked by a spacer with specific positional orientation
is especially crucial. Besides the use of organic ligands, the
2
2
n
X-ray crystallography reveals that 1 is a 2D double-interpene-
trated structure containing two undulating sheets (Fig. 1). In each
sheet, each copper(I) atom is bound by two C/N atoms [Cu–C/N
6,7
metalloligand approach, using metal complexes with vacant
donor groups for further coordination, has also been developed.
In our research on constructing coordination polymers with a
Schiff-base compound 4-pyridinecarbaldehyde isonicotinoyl
hydrazone (4-Hpcih) as ligand, we have noticed that 4-Hpcih is
a multifunctional ligand that can either act as a linear double-
coordination-site linker or be successively deprotonated, showing
equilibrium between keto and enol forms. When suitable metal
ions exist, 4-pcih can chelate a metal ion with two carbonyl-O and
two imine-N atoms, generating a metalloligand with square-planar
quadruple coordination sites (Scheme 1). We expected that the
2
˚
1.849(4)–1.958(4) A] from two CN anions and one N atom from
˚
a Schiff-base ligand [Cu–N 2.069(3)–2.128(3) A], adopting a
T-shape three-coordination geometry (Fig. 1a). The adjacent
n
[CuCN] chains are linked by the neutral ligands through pyridyl-
N atoms in a linear exo-bidentate coordination mode. Six
copper(I) atoms, four cyanides and two 4-Hpcih ligands compose
Scheme 1 pH-induced formation of Schiff-base metalloligand.
Department of Chemistry, Shantou University, Guangdong, 515063,
P. R. China. E-mail: dli@stu.edu.cn; Fax: +86 754 290-2767;
Tel: +86 754 290-2741
Fig. 1 Structure of 1: (a) a 2D undulating sheet; (b) two identical
interpenetrating 2D undulating sheets shown in red and green. All
hydrogen atoms are omitted for clarity.
{
Electronic supplementary information (ESI) available: X-Ray crystal-
lographic data, experimental details and supplemental figures. See DOI:
0.1039/b705603f
1
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Chem. Commun., 2007, 3479–3481 | 3479

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˚
a twelve-membered ring with the cavity about 15.6 6 9.4 A. Each
ring is fused to six other rings forming the 2D undulating sheet
with a dihedral angle of 157.9u between the up and down planes
along the b-axis. As a whole, two undulating sheets entangle with
each other, forming a two-fold interpenetrating structure (Fig. 1b).
As expected, the structure of 2 is more complicated because of
the formation of the metalloligand and thus the increasing ligand
functional sites. The asymmetric unit of 2 contains three crystal-
lographically independent copper atoms (Fig. 2 and S3{). The
necessity of charge balance indicates that the copper atoms adopt
mixed-valence fashion with one Cu(II) and two Cu(I) ions.
Divalent Cu(1), adopting a square-planar coordination sphere, is
˚
coordinated by two carbonyl-O atoms [Cu–O 1.916(2) A] and
˚
two imine-N atoms [Cu–N 1.962(3) A] from two deprotonated
ligands with a trans-N geometry. The O1–Cu1–O1A and
2 2
O
N3–Cu1–N3A bonds are strictly linear with a bond angle of 180u;
II
therefore the [Cu (4-pcih) ] metalloligand moiety bearing four
2
pendant pyridyl-N atoms resembles a square-planar linker for
further coordination. Both Cu(2) and Cu(3) are monovalent.
Cu(2), which lies on a two-fold axis, coordinates two pyridyl-N
˚
atoms [Cu–N 2.185(3) A] and two C atoms from cyanide groups
˚
[Cu–C 1.895(3) A] with a distorted tetrahedral geometry, while
Cu(3) adopts a trigonal-planar geometry surrounded by one
˚
pyridyl-N atom [Cu–N 2.063(3) A] and two C/N atoms [Cu–C/N
˚
1.891(4)–1.895(3) A] from cyanide groups.
Careful examination reveals that 2 exhibits a 3D two-fold
interpenetrated structure, and interestingly, each 3D identical
framework is composed of 2D self-penetrating nets connected by
the tetrahedral Cu(I) atoms. As shown in Fig. 3a, each 2D self-
4c,10
penetrated
sheet is based on 4-connected metalloligand [Cu(4-
pcih)2] centers forming self-catenated 4-membered circuits.
Further, the adjacent 2D self-penetrated sheets are connected by
Cu(2) atoms to form a unique 3D network (Fig. 3b). Along the a
direction, the alternate linkage of Cu(I) and cyanide ions generate
pseudo-helical [CuCN] chains, and the adjacent chains are linked
n
Fig. 3 Structure of 2: (a) 2D self-penetrating net, showing different
catenated circuits in blue and red; (b) one identical 3D framework. The
additional images in pink highlight the pseudo-helical [CuCN] chains
CN : yellow; Cu: green). Water molecules and all hydrogen atoms are
II
by metalloligands [Cu (4-pcih)
2
] to form the 3D network.
n
Moreover, two independent 3D networks interpenetrate with each
other in a parallel fashion with offset face-to-face distances of
2
(
omitted for clarity.
˚
3
.436 A between pyridyl rings (Fig. 4). The whole structure
contains voids (17.8% of the cell volume) occupied by water
molecules (Fig. S4{). To the best of our knowledge, this is the first
entanglement system of a 3D doubly-interpenetrated network
constructed from 2D self-penetrated sheets.
Fig. 2 View with the labeling scheme of the coordination environment of
copper atoms in 2. Symmetry codes: A: 2x + 1, 2y + 1, 2z + 1. B: 2x +
1/2, 2y, z. C: x, 2y 2 1, z + 1/2. D: x, 2y 2 1, z 2 1/2. E: 2x + 1, y,
Fig. 4 3D two-fold interpenetrating framework of 2, showing different
2z + 3/2.
networks in red and green.
3
480 | Chem. Commun., 2007, 3479–3481
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F(000) = 1616, T = 293(2) K, 19 172 reflections collected, 6977 unique with
˚
Table 1 Bond lengths (A) in the C
free ligand and two complexes
b
LN
b
–N
a
(H)–C
a
LO moiety of the
[
R
int = 0.0263], R1[I . 2s(I)] = 0.0575, final (for all data) R1 = 0.0815,
wR2 0.1392, GOF 1.187; Complex 2, CCDC 646864:
27 4 11
C H23Cu N O4.5, orthorhombic, space group Pcca, M = 827.72, a =
=
=
4-Hpcih
1
2
3
˚ ˚ ˚ ˚
1.9969(15) A, b = 7.3578(5) A, c = 20.3447(14) A, V = 3292.8(4) A , Z = 4,
3
calcd = 1.670 g cm , F(000) = 1656, T = 293(2) K, 20 812 reflections
2
r
2
C
C
N
a
LO/C
LN /C
–N
a
–O
1.2117(18)
1.355(2)
1.3765(17)
1.265(2)
1.223(5)/1.229(5)
1.331(5)/1.331(5)
1.383(4)/1.378(4)
1.279(5)/1.280(5)
1.281(4)
1.305(4)
1.390(4)
1.269(4)
a
a
a
–N
a
collected, 4066 unique with [Rint = 0.0333], R1[I . 2s(I)] = 0.0475, final
(for all data) R1 = 0.0673, wR2 = 0.1353, GOF = 1.074. Data collections of
˚
crystals were performed with Mo Ka radiation (l = 0.71073 A) on a Bruker
Smart Apex CCD diffractometer at T = 293(2) K using SMART. See
Fig. S1–S3{ for XP drawings of the ligand and the complexes, respectively.
For crystallographic data in CIF or other electronic format, see DOI:
b
a
b
C
b
LN
The enol form of 4-pcih in the metalloligand can be verified by
10.1039/b705603f
crystallographic and spectroscopic data. The corresponding C–N,
N–N and C–O bond distances in the CLN–N(H)–CLO moiety in
free 4-Hpcih and the two complexes show significant differences
1
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2
in the free ligand and 1, whereas the conjugated CLN–NLC–O
9
moiety is present in 2. The negative charge, arising upon the loss
of the proton originally bound to the amido-N atom, is delocalized
to the carbonyl-O atom. The electron delocalization enhances the
coordination ability of the oxygen atom, hence the oxygen atom
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+
binds to Cu in 2 but stays vacant in 1. The amide N–H (ca. 3448
21
2
1
and 3443 cm , respectively) and CLO (ca. 1688 and 1650 cm
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the free ligand and 1, but are not observed in that of 2. An
21
absorption band at 1614 cm due to the enol resonance in the
2
conjugated CLN–NLC–O moiety, which is absent in the spectra
of the free ligand and 1, exists in the spectrum of 2.
9a,11
It is noted that the formation of the metalloligand in 2 was
accompanied by the oxidation of CuCN to Cu(II), likely due to the
dissolved atmospheric oxygen in the solution. Because the hard
2+
+
base oxygen donors tend to coordinate Cu rather than Cu , the
deprotonation of the ligand seems to facilitate the oxidation of
Cu(I) to form the metalloligand.
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In summary, we have developed a synthetic approach toward
higher dimensionality entangled frameworks by pH-induced
formation of the metalloligand. This work successfully demon-
strates that increasing the coordination sites of the ligand favors
assembling a more complicated and higher-dimensional structure.
The resulting 3D two-fold interpenetrating network constructed by
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We gratefully acknowledge financial support from the National
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4
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9
Notes and references
{
10 4
Crystal data for Ligand, CCDC 643873: C12H N O, monoclinic, space
˚ ˚
1
/n, M = 226.4, a = 7.7099(11) A, b = 14.665(2) A, c =
3
group P2
˚
˚
1
0.2423(15) A, b = 109.166(3)u, V = 1093.8(3) A , Z = 4, rcalcd
=
23
1.374 g cm , F(000) = 472, T = 293(2) K, 7486 reflections collected, 2684
10 (a) H.-L. Sun, S. Gao, B.-Q. Ma and S. R. Batten, CrystEngComm,
2004, 6, 579; (b) A. M. Chippindale, S. M. Cheyne and S. J. Hibble,
Angew. Chem., Int. Ed., 2005, 44, 7942; (c) X.-L. Wang, C. Qin,
E.-B. Wang, Z.-M. Su, L. Xua and S. R. Batten, Chem. Commun., 2005,
4789.
unique with (Rint = 0.0276), R1[I . 2s(I)] = 0.0512, final (for all data)
R1 = 0.0769, wR2 = 0.1419, GOF = 1.028; Complex 1, CCDC 646863:
C
28
H
20Cu
4 12 2 1
N O , monoclinic, space group P2 /n, M = 810.72,
˚
˚
˚
a = 9.4337(6) A, b = 32.490(2) A, c = 10.2061(7) A, a = c = 90u,
b = 105.3930(10)u, V = 3015.9(3) A , Z = 4, rcalcd = 1.785 g cm
3
23
,
˚
11 L. El-Sayed and M. F. Iskander, J. Inorg. Nucl. Chem., 1971, 33, 435.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 3479–3481 | 3481