Metal binding characteristics of a laterally nonsymmetric aza cryptand upon functionalization with a π-acceptor group

Article abstract of DOI:10.1021/ic034183c

The laterally nonsymmetric aza cryptand synthesized by condensing tris(2-aminoethyl)amine (tren) with tris{[2-(3(oxomethyl)phenyl)oxy] ethyl}amine readily forms mononuclear inclusion complexes with both transition and main-group metal ions. In these complexes, the metal ion occupies the tren-end of the cavity making bonds with the three secondary amino and the bridgehead N atoms. When a strong π-acceptor group such as 2,4-dinitrobenzene is attached to one of the secondary amines, the binding property of the cryptand changes drastically. When perchlorate or tetrafluoroborate salts of Ni(II), Cu(II), Zn(II), or CD(II) are used, the metal ion enters the cavity which can be monitored by the hypsochromic shift of the intramolecular charge-transfer transition from the donor amino N atom to the acceptor dinitrobenzene. However, in the presence of coordinating ions such as Cl^-, N₃^-, and SCN^-, the metal ion comes out of the cavity and binds the cryptand outside the cavity at a site away from the dinitrobenzene moiety. Four such complexes are characterized by X-ray crystallography. Thus, a metal ion can translocate between inside and outside of the cryptand cavity depending upon the nature of the counter anion.

Full text of DOI:10.1021/ic034183c

Inorg. Chem. 2003, 42, 4955−4960
Metal Binding Characteristics of a Laterally Nonsymmetric Aza Cryptand
upon Functionalization with a π-Acceptor Group
,†
Pritam Mukhopadhyay,† Bijay Sarkar,† Parimal K. Bharadwaj,* Kalle Na1ttinen,‡ and Kari Rissanen‡
Departments of Chemistry, Indian Institute of Technology, Kanpur 208016, India,
and UniVersity of JyVa¨skyla¨, P.O. Box-35, FIN-40351 Finland
Received February 18, 2003
The laterally nonsymmetric aza cryptand synthesized by condensing tris(2-aminoethyl)amine (tren) with tris{[2-(3-
(oxomethyl)phenyl)oxy]ethyl}amine readily forms mononuclear inclusion complexes with both transition and main-
group metal ions. In these complexes, the metal ion occupies the tren-end of the cavity making bonds with the
three secondary amino and the bridgehead N atoms. When a strong π-acceptor group such as 2,4-dinitrobenzene
is attached to one of the secondary amines, the binding property of the cryptand changes drastically. When perchlorate
or tetrafluoroborate salts of Ni(II), Cu(II), Zn(II), or Cd(II) are used, the metal ion enters the cavity which can be
monitored by the hypsochromic shift of the intramolecular charge-transfer transition from the donor amino N atom
to the acceptor dinitrobenzene. However, in the presence of coordinating ions such as Cl-, N -, and SCN-, the
3
metal ion comes out of the cavity and binds the cryptand outside the cavity at a site away from the dinitrobenzene
moiety. Four such complexes are characterized by X-ray crystallography. Thus, a metal ion can translocate between
inside and outside of the cryptand cavity depending upon the nature of the counter anion.
Introduction
inside the cryptand as it forms stable complexes. The extra
stability of these metal inclusion complexes is termed the
cryptate effect.7 However, binding of a cryptand from outside
is reported for Ag(I) ion with an octaazamacrobicyclic
cryptand.8 Translocation of a metal ion inside and outside a
cryptand cavity has not been reported in the literature to date.
Such translocation, if possible, could lead to a new class of
molecular machines9-11 adding a new dimension to the
kinetic, thermodynamic, photophysical, electron-transfer, and
multitude of other properties in these molecules. A pertinent
Macrobicyclic cryptands occupy an important position in
supramolecular chemistry due to the enormous potential1,2
these molecules offer in diverse areas of chemistry, bio-
chemistry, and materials research. Cryptands incorporating
nitrogen atoms placed at strategic positions in the three
bridges can readily complex transition metal ions. We have
been particularly interested in using laterally nonsymmetric
aza cryptands for complexation of transition3 as well as main-
group4 metal ions and molecular recognition of neutral
molecules5 with the goal of using such cryptates for further
applications.6 In all these complexes, the metal ion is included
(6) (a) Chand, D. K.; Bharadwaj, P. K. Inorg. Chem. 1997, 36, 5658. (b)
Ghosh, P.; Bharadwaj, P. K.; Roy, J.; Ghosh, S. J. Am. Chem. Soc.
1997, 119, 11903. (c) Mukhopadhyay, P.; Bharadwaj, P. K.; Savitha,
G.; Krishnan, A.; Das, P. K. Chem. Commun. 2000, 1815. (d)
Mukhopadhyay, P.; Bharadwaj, P. K.; Krishnan, A.; Das, P. K. J.
Mater. Chem. 2002, 12, 2786. (e) Ghosh, P.; Sengupta, S.; Bharadwaj,
P. K. Langmuir 1998, 14, 5712. (f) Das, G.; Bharadwaj, P. K.; Singh,
U.; Singh, R. A.; Butcher, R. J. Langmuir 2000, 16, 1910.
(7) Lehn, J.-M. Pure Appl. Chem. 1977, 47, 857.
* To whom correspondence should be addressed. E-mail: pkb@iitk.ac.in.
† Indian Institute of Technology.
‡ University of Jyva¨skyla¨.
(1) (a) ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies,
J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J.-M., Eds.; Pergamon:
New York, 1996; Vol. 1. (b) Izatt, R. M.; Pawlak, K.; Bradshaw, J.
S.; Bruening, R. L. Chem. ReV. 1995, 95, 2529.
(8) McKee, V.; Nelson, J.; Speed, D. J.; Town, R. M. J. Chem. Soc.,
Dalton Trans. 2001, 3641.
(2) Bharadwaj, P. K. Prog. Inorg. Chem. 2002, 51, 251.
(3) (a) Chand, D. K.; Bharadwaj, P. K. Inorg. Chem. 1996, 35, 3380. (b)
Chand, D. K.; Bharadwaj, P. K. Inorg. Chem. 1998, 37, 5050. (c)
Ghosh, P.; Sengupta, S.; Bharadwaj, P. K. J. Chem. Soc., Dalton Trans.
1997, 935. (d) Ghosh, P.; Bharadwaj, P. K. J. Chem. Soc., Dalton
Trans. 1997, 2673.
(4) Bandyapadhyay, P.; Bharadwaj, P. K. To be published.
(5) Chand, D. K.; Ragunathan, K. G.; Mak, T. C. W.; Bharadwaj, P. K.
J. Org. Chem. 1996, 61, 1169.
(9) (a) Amendola, V.; Fabbrizzi, L.; Mangano, C.; Pallavicini, P.; Perotti,
A.; Taglietti, A. J. Chem. Soc., Dalton Trans. 2000, 185. (b) Amendola,
V.; Fabbrizzi, L.; Mangano, C.; Pallavicini, P. Acc. Chem. Res. 2001,
34, 488. (c) Amendola, V.; Fabbrizzi, L.; Mangano, C.; Pallavicini,
P.; Perotti, A.; Taglietti, A. Angew. Chem., Int. Ed. 2002, 41, 2553.
(10) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem.,
Int. Ed. 2000, 39, 3348.
(11) Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994,
369, 133.
10.1021/ic034183c CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/02/2003
Inorganic Chemistry, Vol. 42, No. 16, 2003 4955

Mukhopadhyay et al.
(dd, J ) 9.4 Hz, J ) 2 Hz, 1H), 8.54 (d, J ) 2 Hz, 1H). FAB-MS
(m/z) 726 (100%). Anal. Calcd for C39H47N7O7: C, 64.54; H, 6.53;
N, 13.51. Found: C, 64.62; H, 6.61; N, 13.42.
[Cu(L1)Cl(ClO4)]‚H2O, 1. To L1 (0.073 g, 0.1 mmol) dissolved
in 10 mL of MeOH was added Cu(H2O)6‚2ClO4 (0.037 g, 0.1 mmol)
dissolved in 5 mL of MeOH with gentle heating at 50 °C. On
addition of NaCl (0.1 mmol), a dark green solid separated in ∼30
min which was filtered off, washed with MeOH, and dried in vacuo.
X-ray quality crystals were grown from MeOH solution upon slow
evaporation at RT in 85% overall yield. Anal. Calcd for C39H49N7O12-
Cl2Cu: C, 49.71; H, 5.24; N, 10.41. Found: C, 49.84; H, 5.30; N,
10.32.
[Cu(L1)Cl](BF4), 2. To L1 (0.073 g, 0.1 mmol) dissolved in 10
mL of MeOH was added Cu(BF4)2‚xH2O (0.024 g, 0.1 mmol)
dissolved in 5 mL of MeOH, and the solution was heated gently
for 10 min at 50 °C. A dark green solid separated overnight on
addition of NaCl (0.1 mmol) which was collected by filtration,
washed with MeOH, and air-dried. X-ray quality crystals were
grown from MeCN solution upon slow evaporation at RT in 90%
overall yield. Anal. Calcd for C39H47N7O7BF4ClCu: C, 51.38; H,
5.19; N, 10.75. Found: C, 51.48; H, 5.25; N, 10.62.
[Zn(L1)N3)2], 3. To L1 (0.073 g, 0.1 mmol) dissolved in 10 mL
of MeOH was added [Zn(H2O)6][ClO4]2 (0.037 g, 0.1 mmol)
dissolved in 5 mL of MeOH with gentle heating at 50 °C. Upon
addition of 2 equiv of NaN3, the solution became yellowish-orange
in color. After the solution stood overnight, orange crystalline solids
were isolated which were collected by filtration and air-dried.
Crystals suitable for X-ray diffraction were grown by slow
evaporation of MeCN solution. Yield 88%. Anal. Calcd for
C39H47N13O7Zn: C, 53.51; H, 5.41; N, 20.80. Found: C, 53.61;
H, 5.47; N, 20.72.
Figure 1. Laterally nonsymmetric cryptand and its monoderivative.
question in this regard is the following: When can such a
cryptand be forced to bind a metal ion from outside? The
easiest way this can be achieved would be to alter the donor
characteristics of an amino nitrogen by attaching an electron
withdrawing group to it. This way, less electron density will
be available for binding the metal ion inside the cryptand
and coaxing it to bind from outside. The ultimate aim will
be having cryptands where donor properties can be tuned to
effect translocation of a metal ion inside and outside the
cavity depending on various factors such as counteranions,
dielectric constant of the solvent, temperature, and so on.
In our initial attempt in this direction, we present here a
laterally nonsymmetric aza cryptand (Figure 1),12 which is
monosubstituted with the π-accepting 2,4-dinitrobenzene
group. This transformation causes the lone-pair of the amino
N atom to be in conjugation with the π-acceptor reducing
its availability inside the cryptand. The coordination proper-
ties of this π-A functionalized cryptand L1 toward metal ions
such as Cu(II), Zn(II) and Cd(II) are described.
[Cd(L1)(NCS)2]‚1/2MeOH‚1/2MeCN‚2H2O, 4. To L1 (0.073
g, 0.1 mmol) dissolved in 10 mL of MeOH was added [Cd(NO3)2]
(0.031 g, 0.1 mmol) dissolved in 5 mL of MeOH with gentle heating
at 50 °C. Upon addition of 2 equiv of KSCN, the solution became
bright yellow which afforded a yellow solid within 1 h. The solid
was washed with methanol and then dissolved in 100 mL of MeCN.
Crystals suitable for X-ray diffraction were grown by slow
evaporation of this MeCN solution. Yield 72%. Anal. Calcd for
Experimental Section
Materials. Reagent grade tris(2-aminoethyl)amine (Aldrich),
salicylaldehyde, triethanolamine, and 2,4-dinitrochlorobenzene (SD
Fine Chemicals) were used as received. All the solvents (SD Fine
Chemicals) were purified prior to use following standard procedures.
For chromatographic separation, 100-200 mesh silica gel (Acme
Synthetic Chemicals) was used.
C42.5H51.5N9.5O9.5S2Cd: C, 51.46; H, 5.23; N, 13.42. Found: C,
12
Synthesis. Cryptand L1. To a suspension of the cryptand Lo
51.51; H, 5.37; N, 13.62.
(0.56 g; 1 mmol) in dry EtOH (40 mL) was added anhydrous K2-
CO3 (0.44 g; 2 mmol), and the reaction mixture was stirred for 10
min. Subsequently, a solution of 2,4-dinitrochlorobenzene (0.65 g;
1.8 mmol) in dry EtOH (10 mL) was added dropwise over a period
of 15 min, and the reaction mixture was allowed to stir at RT for
2 h when the solution became yellow-orange in color. For
completion of the reaction, the mixture was allowed to reflux for
4 h. After cooling to RT, the solvent was removed under vacuo,
and the yellow solid product obtained was repeatedly washed with
water (5 × 100 mL). This product contains a mixture of all the
three derivatives. The tris and the bis derivatives were removed by
column chromatography in silica gel with chloroform/methanol
mixed solvent with 99.5:0.5 v/v and 98.5:1.5 v/v, respectively, as
the eluent. The desired mono derivative (L1) was eluted out finally
using a chloroform/methanol ratio of 95.5:4.5 (v/v). This product
was isolated as a bright yellow crystalline solid upon evaporation
All attempts to grow single crystals of L1 with perchlorate,
triflate, or picrate salts of the aforementioned metal ions remained
unsuccessful.
Caution! Care must be taken while performing complexation
of organic compounds with metal perchlorates as potentially
explosive mixtures may be formed.
Measurements. Spectroscopic data were collected as follows:
IR (KBr disk, 400-4000 cm-1) Perkin-Elmer model 1320, elec-
tronic absorption spectra (at 295 K, freshly distilled MeCN as
solvent) JASCO V-570 UV-vis-NIR spectrophotometer, 1H NMR
(400 MHz, CDCl3, TMS, 298 K) JEOL JNM-LA400 FT instrument,
FAB mass spectrometry (CHCl3 solvent, argon carrier gas, 298 K)
JEOL SX 102/DA-6000, EPR (X-band, 298 and 77 K, solid and
solution in MeCN) Varian E-109 with DPPH as the external
standard. Melting points were determined with an electrical melting
point apparatus by PERFIT, India, and were uncorrected. Mi-
croanalyses were obtained either from IIT Kanpur or from CDRI,
Lucknow.
1
of the solvent. Yield 32%. Mp 145 °C. H NMR: δ 2.42-2.51
(m, 4H), 2.64-2.75 (m, 4H), 2.89-2.94 (m, 2H), 3.23-3.26 (m,
4H), 3.35-3.41 (m, 4H), 3.62 (d, J ) 13 Hz, 2H), 3.91 (d, J ) 13
Hz, 2H), 4.23-4.26 (m, 6H), 4.48 (s, 2H), 5.33 (br, 2H), 6.82-
6.99 (m, 8H), 7.1 (d, J ) 9.4 Hz, 1H), 7.22-7.28 (m, 4H), 8.07
X-ray Structural Studies. Single-crystal X-ray data on 1-3
were collected at room temperature on a P4 Bruker X-ray
diffractometer using graphite monochromated Mo KR radiation (λ
) 0.71073 Å). The data for 4 were collected on a Nonius Kappa
(12) Ragunathan, K. G.; Bharadwaj, P. K. Tetrahedron Lett. 1992, 33, 7581.
4956 Inorganic Chemistry, Vol. 42, No. 16, 2003

Metal Binding of a Nonsymmetric Aza Cryptand
Table 1. Crystal Data and Structure Refinement for 1, 2, 3, and 4
empirical formula
fw
C39H49N7O12Cl2Cu
942.29
C39H47N7O7BF4ClCu
911.64
C39H47N13O7Zn
875.27
C42.5H51.5N9.5O9.5CdS2
1023.95
T
293(2) K
0.71073 Å
monoclinic
P21/c
8.649 (11)
16.927 (2)
29.394 (4)
90
87.304 (2)
90
4299 (10)
4
293(2) K
0.71073 Å
monoclinic
P21/c
17.290 (5)
13.170 (5)
18.210 (5)
90
97.00 (5)
90
4116 (2)
4
293(2) K
0.71073 Å
triclinic
173(2) K
0.71073 Å
triclinic
wavelength
cryst syst
space group
a, Å
b, Å
c, Å
R (deg)
â (deg)
γ (deg)
V, Å3
P1h
P1h
10.907(10)
12.089(10)
17.099(2)
108.160(10)
100.460(10)
95.770(10)
2076.7(4)
2
12.430(3)
13.771(3)
16.613(5)
66.672(10)
77.723(10)
70.084(10)
2411.5(11)
2
Z
F
calcd Mg/m3
1.440
0.703
1964
1.471
0.672
1892
1.400
0.657
916
1.410
0.602
1058
µ, mm-1
F(000)
reflns collected
ndep reflns
refinement method
7372
4085
full-matrix
least-squares
on F2
8740
7223
full-matrix
least-squares
on F2
8328
7099
full-matrix
least-squares
on F2
15799
9038
full-matrix
least-squares
on F2
data/params
GOF
7372/554
1.024
7223/540
0.943
7099/534
0.848
9038/581
1.027
final R indices
[I > 2σ(I)]
R indices
(all data)
R1 ) 0.0753
wR2 ) 0.1644
R1 ) 0.1442
wR2 ) 0.2024
R1 ) 0.0833
wR2 ) 0.2272
R1 ) 0.1299
wR2 ) 0.2915
R1 ) 0.0723
wR2) 0.1945
R1 ) 0.1026
wR2 ) 0.2295
R1 ) 0.0554
wR2) 0.1311
R1 ) 0.0919
wR2 ) 0.1480
CCD diffractometer with graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å) at 173 K. The cell parameters in each case were
determined by least-squares refinement of the diffractometer setting
angles from 25 centered reflections that were in the range 20° e
2θ e 25°. Three standard reflections were measured every hour to
monitor instrument and crystal stability. The linear absorption
coefficients, scattering factors for the atoms, and the anomalous
dispersion corrections were taken from the International Tables for
X-ray Crystallography.13 The structures were solved by the direct
method using SIR9214 and were refined on F2 by full-matrix least-
squares technique using the SHELXL-9715 program package. In
each case, some of the H atoms could be located in the difference
maps while the rest were calculated assuming ideal geometries of
the atoms concerned. The H atom positions or thermal parameters
were not refined but included in the structure factor calculations.
The crystal data for the four structures are collected in Table 1.
avoid the hassles of multistep protection and deprotection
of amines and isolate the compounds in good yields.
All the metal complexes are isolated in high yields. The
crystals, once formed, become sparingly soluble in common
organic solvents such as alcohol, acetonitrile, acetone,
chloroform, and THF but are soluble in DMF and DMSO.
Spectral and Magnetic Studies. The unsubstituted cryptand
(Lo) does not absorb to any significant extent at energies
lower than 250 nm. The mono substituted cryptand L1
exhibits an intense absorption band centered at 380 nm (ꢀmax
) 17180 M-1 cm-1) attributable17 to the intramolecular
charge transfer (ICT) transition from the donor N atom to
the acceptor dinitrobenzene moiety which can accept electron
density into its available π-symmetry orbital. The metal
inclusion complexes of the unsubstituted cryptand do not
absorb in the range 300-450 nm, to any noticeable extent.
Therefore, the ICT transition can be utilized to monitor the
inclusion of a metal ion inside the cryptand as the bound
metal ion will modulate the donor ability of the N atom
attached to the dinitrobenzene moiety. The alkali or alkaline-
earth metal ions do not exhibit any changes in the absorption
spectra since these ions occupy the upper deck of the
cryptand and, hence, are not able to communicate with the
D-π-A chromophore unit. Addition of a metal ion such as
Ni(II), Cu(II), Zn(II), or Cd(II) as the perchlorate or
tetrafluoroborate salt induces a blue-shift17 with concomitant
lowering of intensity of the ICT band (Figure 2) as these
metal ions enter and occupy the tren-end of the cavity and
become directly involved with the D-π-A chromophore
Results and Discussion
The unsubstituted cryptand (Lo) has three derivatizable
secondary amino groups in the three bridges. With the pKa
values16 for these being different, selective functionalization
of the cryptand can be made. It has been empirically found
that upon reacting 1.8 equiv of 2,4-dinitrochlorobenzene with
the unsubstituted cryptand (Lo), mono, bis, and tris deriva-
tives are formed in almost equal amounts. The mono π-A
functionalized cryptand (L1) can be separated by column
chromatography from the tris and the bis derivatives. Thus,
by utilizing this simple and straightforward strategy we can
(13) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974; Vol. IV.
(14) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Gualardi, A. J. Appl.
Crystallogr. 1993, 26, 343.
(15) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(16) Bazzicalupi, C.; Bencini, C.; Bianchi, A.; Bharadwaj, P. K.; Bandyo-
padhyay, P.; Giorgi, C.; Valtancoli, B.; Biswas, D.; Butcher, R. J.
Eur. J. Inorg. Chem. 2000, 2111.
(17) Cryptands and aza crown ethers incorporating D-π-A chromophoric
units have been utilized to probe metal ion binding in their cavities
by studying the changes in the absorption maximum (λmax) and the
intensities of the ICT band. Please see the following:. (a) Forgues, S.
F.; Le Bris, M.-T.; Guette, J.-P.; Valeur, B. J. Phys. Chem. 1988, 92,
6233. (b) Houbrechts, S.; Kubo, Y.; Tozawa, T.; Tokita, S.; Wada,
T.; Sasabe, H. Angew. Chem., Int. Ed. 2000, 39, 3859.
Inorganic Chemistry, Vol. 42, No. 16, 2003 4957

Mukhopadhyay et al.
Figure 2. Effect of addition of metal perchlorates on the ICT band of L1:
(s) metal free L1; (- -) upon addition of Cu(II); (- ‚ ‚ -) upon addition
of Cd(II) added; (- ‚ -) upon addition of Ni (II); (- ‚ ‚ ‚ -) upon addition
of Zn(II). Concentration of L1 was 5 × 10-4 M and that of the metal
perchlorates was 5 × 10-3 M.
unit. On gradual addition of the perchlorate salts, the
absorption spectra show6d a well-defined isosbestic point
suggesting a 1:1 stoichiometry with L1. Compound L1
exhibits a maximum blue-shift in the presence of cupric
perchlorate (21 nm with regard to free L1). However, when
the chloride salts of these metal ions are used, no shift or
change of intensity of the ICT band is observed. Also, when
Figure 3. Crystal structure of 1.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of the
Complexes
[Cu(L1)(Cl)(ClO4)]‚H2O, 1
Cu-N1
Cu-N1
Cu-O11
2.055(5)
2.055(5)
2.391(5)
Cu-N1b
Cu-Cl
2.045(5)
2.280(19)
-
Cl-, SCN-, or N3 ion is added to the metal perchlorate
N1a-Cu-N1b
N1-Cu-N1b
N1a-Cu-O11
N1b-Cu-Cl
Cl-Cu-O11
168.1(2)
83.7(2)
89.7(2)
95.9 (1)
96.1(1)
N1-Cu-N1a
N1-Cu-Cl
84.4(2)
174.6(2)
89.2(2)
88.9(2)
and L1 mixture, the ICT band position and intensity slowly
change (∼24 h) to the values obtained on free L1. These
results lead us to the suggestion that, in the presence of a
coordinating anion like chloride, azide, etc., the metal ion
has a strong preference to bind the cryptand from outside.
N1-Cu-O11
N1b-Cu-O11
[Cu(L1)(Cl)](BF4), 2
Cu-N1a
Cu-O1a
Cu-Cl
2.012(7)
2.48 (6)
2.264(5)
Cu-N1
2.073(6)
2.010(6)
The effective magnetic moment values (µeff/µB) for the
Cu(II) complexes after diamagnetic corrections lie in the
range 1.94-2.01 at 295 K, consistent18 with the discrete,
mononuclear formulation of the complexes. The EPR spectral
studies of the Cu(II) complexes were carried out in the solid
state and in dry DMF as the solvent. In the solid state, a
broad signal with gav ≈ 2.05 is observed in each case at
ambient temperature; the signal sharpens somewhat at 77
K. These pure solids are not magnetically dilute enough to
yield ESR spectra indicative of the solid-state stereochem-
istry. On the other hand, in DMF solution at 298 K, a typical
four-line signal (gav ) 2.14 for 1 and 2.11 for 2) is observed
in each case. At 77 K, the signal changes to an axial one
with g| ) 2.22 and g ) 2.02 for 1 and g| ) 2.30 and g )
2.04 for 2. The A| values for 1 and 2 are 170 × 10-4 and
175 × 10-4 cm-1, respectively. It follows, therefore, from
the ESR results that the stereochemistry in solution is also
most likely to be square pyramidal as in the solid-state X-ray
structure.
Cu-N1b
N1a-Cu-N1b
N1-Cu-Cl
N1a-Cu-O1a
N1b-Cu-N1
N1b-Cu-O1a
165.5(3)
175.9(2)
84.24(2)
85.5(2)
N1b-Cu-O1a
N1-Cu-O1a
N1a-Cu-N1
N1a-Cu-Cl
94.1(2)
98.26(2)
84.2(3)
95.4(2)
107.37(2)
[Zn(L1)(N3)2], 3
2.089(4)
1.943(7)
Zn-N1a
Zn-N1z
Zn-N1b
Zn-N4z
2.050(4)
1.989(8)
N1a-Zn-N1b
N1a-Zn-N4z
N1b-Zn-N4z
110.7(2)
105.5(3)
111.2(3)
N1a-Zn-N1z
N1b-Zn-N1z
N1z-Zn-N4z
108.8(3)
106.7(2)
113.9(3)
[Cd(L1)(NCS)2]‚1/2MeOH‚1/2MeCN‚2H2O, 4
Cd-N1
Cd-N1a
Cd-N1b
2.416(3)
2.408(4)
2.362(4)
Cd-N11
Cd-N12
Cd-N12′
2.228(4)
2.444(4)
2.499(4)
N11-Cd-N1b
N1b-Cd-N1a
N1b-Cd-N1
N11-Cd-N12
N1a-Cd-N12
N11-Cd-N12
N1a-Cd-N12
N12-Cd-N12
93.54(16)
107.27(14)
77.50(13)
96.89(16)
162.81(13)
99.53(15)
85.50(14)
78.54(17)
N11-Cd-N1a
N11-Cd-N1
N1a-Cd-N1
N1b-Cd-N12
N1-Cd-N12
N1b-Cd-N12
N1-Cd-N12
91.98(16)
162.25(15)
76.45(13)
86.91(14)
97.88(12)
161.44(13)
93.02(12)
Description of the Structures. [Cu(L1)(Cl)(ClO4)]‚H2O,
1. The structure of 1 consists of discrete, neutral [Cu(L1)-
(Cl)(ClO4)] units and water molecules. A perspective view
of the molecule showing the atom numbering scheme is given
in Figure 3 while the bond distances and angles are collected
in Table 2. The Cu(II) ion is bonded to the two secondary
amino N atoms and the bridgehead N atom at the tren-end,
in an unprecedented fashion outside the cavity. The other
two coordination sites are occupied by one chloride ion and
one oxygen of a perchlorate ion. The dinitrobenzene moiety
remains furthest from the metal ion. As a result of this
binding mode, the cryptand has undergone a significant
conformational change. The two bridgehead N atoms have
(18) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
Wiley: New York, 1988; p 768.
4958 Inorganic Chemistry, Vol. 42, No. 16, 2003

Metal Binding of a Nonsymmetric Aza Cryptand
Figure 5. Crystal structure of 3.
The difference in the binding mode is due to the less
-
-
coordinating nature of BF4 compared to that of the ClO4
Figure 4. Crystal structure of 2.
anion. The coordination geometry (τ ) 0.17) is also more
distorted from an ideal square pyramid19 compared to 1, and
the metal ion is situated 0.221 Å above the least-squares
plane described by the four atoms in the basal plane and
toward the axial O atom. All the Cu-N and the Cu-Cl bond
distances are, however, within normal ranges.20 The axial
Cu-O(ethereal) bond is significantly long, as otherwise the
cryptand moiety would have undergone severe distortion.
The two bridgehead N atoms are closer at 4.952 Å compared
to that in 1 in the overall endo-exo conformation. The bond
distances and angles within the cryptand moiety are normal.
The Cl- ion shows positional disorder in the structure.
[Zn(L1)(N3)2], 3. The X-ray structure of 3 reveals a slightly
different binding mode to L1. Here, the Zn(II) ion is bonded
to the cryptand through two secondary amino N atoms (the
bridgehead N atom remains uncoordinated) while the other
two coordination sites are occupied by two azide anions
forming an almost ideal tetrahedral coordination geometry
(Figure 5). One of the azide groups is disordered. All the
Zn(II)-N bond distances are normal.22 The metal ion has
pulled the two amino N atoms away from the cryptand
causing severe conformational distortion, and as a result, the
two bridgehead N atoms are much closer compared to 1 and
2 at 4.652 Å. All the bond distances and angles within the
cryptand are again normal. The packing diagram of this
complex shows that the metal-bound N atom of one of the
azide groups makes a C-H‚‚‚N intermolecular hydrogen
bonding interaction (C‚‚‚N, 3.308 Å) with an aromatic proton
of the neighboring cryptand forming an infinite supramo-
lecular linear array in the crystal lattice (Figure 6).
come much closer at 5.281 Å from the distance of 6.249 Å
in Lo.3c The metal ion has pulled the three N donors of the
cryptand moiety toward itself causing these three atoms to
be inverted with their lone-pairs directed at the metal ion.
Thus, the cryptand has changed to an endo-exo conforma-
tion upon metal binding. All the bond distances and angles
in the cryptand moiety are, however, within normal statistical
errors.
The coordination geometry around the metal ion is square
pyramidal where the three N and the Cl- occupy the basal
plane with the O atom at the axial position. The coordination
geometry has a very small trigonal bipyramidal component19
with τ ) 0.1. For a perfect square pyramidal geometry, this
value should be 0. The Cu(II) ion is only 0.045 Å above the
least-squares plane described by the four basal atoms and
toward the axial O atom. All Cu-N bond distances are within
the normal range,20 and the Cu-Cl distance is also within
-
the normal range found in other compounds. The ClO4
anion is tightly held to the metal ion as the Cu-O bond is
shorter21 compared to other perchlorate bound Cu complexes.
[Cu(L1)Cl](BF4), 2. The X-ray structure of 2 consists of
-
discrete [Cu(L1)Cl]+ cations and BF4 anions. As in 1, the
metal ion is bound to the cryptand from outside the cavity
(Figure 4) although there is a significant difference in the
mode of binding in the two cases. In 2, the metal ion is again
square pyramidal with two amino and one bridgehead N
atoms and the Cl- ion forming the basal plane, and the axial
site is occupied by an ethereal O atom of the cryptand moiety.
(19) (a) Deeth, R. J.; Gerloch, M. Inorg. Chem. 1984, 23, 3853. (b) Addison,
A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem.
Soc., Dalton Trans. 1984, 1349. (c) Carugo, O.; Castellani, C. B. J.
Chem. Soc., Dalton Trans. 1990, 2895.
(20) (a)Anderson, O. P. Inorg. Chem. 1975, 14, 730. (b) Motekaitis, R. J.;
Rudolf, P. R.; Martell, A. E.; Clearfield, A. Inorg. Chem. 1989, 28,
112. (c) Bazzicalupi, C.; Bencini, A.; Bianchi, A.; Fusi, V.; Giorgi,
C.; Paoletti, P.; Valtancoli, B. J. Chem. Soc., Dalton Trans. 1994,
3581. (d) Costa, R.; Lo´pez, C.; Molins, E.; Espinosa, E.; Pe´rez, J. J.
Chem. Soc., Dalton Trans. 2001, 2833.
(21) (a) Lee, T.-J.; Lee T.-Y.; Hong, C.-Y.; Wu, D.-T.; Chung, C.-S. Acta
Crystallogr. 1986, C42, 999. (b) Cheon, M.; Suh, M. P.; Shin, W.
Bull. Korean Chem. Soc. 1992, 13, 363. (c) Elmali, A.; Elerman, Y.;
Svoboda, I. Z. Naturforsch., B: Chem. Sci. 2002, 57b, 1129.
[Cd(L1)(NCS)2]‚1/2MeOH‚1/2MeCN‚2H2O, 4. In the
asymmetric unit, a Cd(II) ion is bonded to the cryptand
through its two secondary amines and two thiocyanate anions.
The molecular structure (Figure 7) reveals a dimeric unit,
where two Cd(II) ions are bonded to two bridging thiocyanate
anions, and in addition, each metal ion is bonded to one L1
and one terminal thiocyanate anion. The crystal lattice also
(22) (a) Adams, H.; Bradshaw, D.; Fenton, D. E. 2002, 925. (b) Ghosh,
P.; Bharadwaj, P. K. J. Chem. Soc., Dalton Trans. 1997, 1467.
Inorganic Chemistry, Vol. 42, No. 16, 2003 4959

Mukhopadhyay et al.
compared to other complexes in the series. Thus, the cryptand
moiety in 4 has undergone maximum distortion from its
pseudo-3-fold symmetry. Although thiocyanate anions are
known23 to bind Cd(II) through both S and N atoms, none
of the S atoms here are bonded to the metal ions. The Cd-
(II)-N bond distances involving the bridging thiocyanates
are significantly longer compared to the known structures24
due to the steric hindrance provided by the dinitrobenzene
moiety.
Conclusion
The present study shows that it is possible to bind a metal
ion outside the cavity of a cryptand if it is modified suitably.
More interestingly, a metal ion bound inside the cavity can
be forced outside by changing the counteranions. The
cryptand is not very rigid and can distort to a different extent
for optimum binding of a metal ion outside the cavity. It
will be of interest to probe if a cryptand which is much more
rigid compared to the present one can translocate a metal
ion in a similar fashion. This result is significant because
this offers a possibility of controlling any property of the
metal cryptate in a reversible manner simply by changing
the counteranion. It is thus possible, in principle, to attach
fluorophoric groups to such cryptand derivatives for signaling
the presence of transition metal ions inside the cavity. As
the metal ion can be forced out by coordinating anions, these
fluorescent signaling systems can be made reversible.
Besides, the metal ions bound to the cryptand from outside
can be utilized to have metal assembled giant structures with
cryptands. We are presently working along these lines.
Figure 6. Packing of 3 in the crystal lattice showing an infinite
supramolecular array.
Acknowledgment. Financial support for this work from
the Council of Scientific and Industrial Research (CSIR),
New Delhi, India (to P.K.B.), is gratefully acknowledged.
Figure 7. Crystal structure of 4.
Supporting Information Available: Additional figures, table,
and crystallographic information. This material is available free of
charge via the Internet at http://pubs.acs.org.
contains disordered solvent molecules such as methanol,
acetonitrile, and water. Each Cd(II) ion is bonded to six N
atoms in a distorted octahedral environment (Table 2): two
secondary amino N atoms, the bridgehead N at the tren-end
of the cryptand moiety, a terminal thiocyanate, and two
bridging thiocyanates.
All the Cd(II)-N bond distances are within the normal
range found in other complexes23 of Cd(II). The two
bridgehead N atoms have come much closer (4.523 Å)
IC034183C
(23) (a) Yang, G.; Zhu, H.-G.; Liang, B.-H.; Chen, X.-M. J. Chem. Soc.,
Dalton Trans. 2001, 580. (b) Taniguchi, M.; Ouchi, A. Bull. Chem.
Soc. Jpn. 1987, 60, 1192. (c) Mondal, N.; Saha, M. N.; Mitra, S.;
Gramlich, V. J. Chem. Soc., Dalton Trans. 2000, 3218.
(24) (a) Hou, H.; Meng, X.; Song, Y.; Fan, Y.; Zhu, Y.; Lu, H.; Du, C.;
Shao, W. Inorg. Chem. 2002, 41, 4068. (b) Mautner, F. A.; Abu-
Youssef, M. A. M.; Goher, M. A. S. Polyhedron 1997, 16, 235.
4960 Inorganic Chemistry, Vol. 42, No. 16, 2003