Complexation properties of a heteroditopic cryptand towards CuII and NiII, crystal structures of the cryptand and its nickel(II) cascade complex

Article abstract of DOI:10.1039/a605948a

The crystal structure of the heteroditopic cryptand (L) formed by Schiff-base condensation of tris(2-aminoethyl)amine and 2,2′,2″-nitrilotris(ethyIeneoxy)tris(benzaldehyde) followed by reduction with NaBH₄ has been determined. The molecule has an endo-endo conformation with a pseudo-three-fold symmetry axis passing through the two bridgehead nitrogens. This symmetry is maintained in chloroform solution as indicated by its ¹H NMR spectrum. The cryptand accepts a metal ion (Cu^II, Zn^II or Ni^II) inside the cavity forming mononuclear cryptates of general formula [ML][ClO₄]₂. The metal ion occupies the N₄ end of the cavity. The light blue CuN₄ chromophore thus formed exhibits an axial EPR spectrum with very small A_∥ value (63 × 10^-4 cm^-1) and a low-energy ligand-field band. Nickel(II) initially forms a blue [NiL][ClO₄]₂ complex which on dissolving in moist MeCN solution crystallizes as purple rectangular parallelepipeds with the formulation [NiL(H₂O)(MeCN)][ClO₄]₂·H ₂O·2MeCN. The crystal structure of this complex has also been determined. The Ni^II ion is octahedrally co-ordinated with the equatorial sites occupied by the three secondary amino N atoms of the cryptand and the N atom of the bound MeCN. The axial sites are occupied by the bridgehead N and the O atom of the water molecule which enters the cavity forming a cascade complex.

Full text of DOI:10.1039/a605948a

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Complexation properties of a heteroditopic cryptand towards CuII
and NiII, crystal structures of the cryptand and its nickel(II) cascade
complex †
Pradyut Ghosh, Sayam Sen Gupta and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
The crystal structure of the heteroditopic cryptand (L) formed by Schiff-base condensation of
tris(2-aminoethyl)amine and 2,2Ј,2Љ-nitrilotris(ethyleneoxy)tris(benzaldehyde) followed by reduction with NaBH4
has been determined. The molecule has an endo-endo conformation with a pseudo-three-fold symmetry axis
passing through the two bridgehead nitrogens. This symmetry is maintained in chloroform solution as indicated
by its 1H NMR spectrum. The cryptand accepts a metal ion (CuII, ZnII or NiII) inside the cavity forming
mononuclear cryptates of general formula [ML][ClO4]2. The metal ion occupies the N4 end of the cavity. The light
blue CuN4 chromophore thus formed exhibits an axial EPR spectrum with very small A|| value (63 × 10Ϫ4 cmϪ1
and a low-energy ligand-field band. Nickel() initially forms a blue [NiL][ClO4]2 complex which on dissolving
in moist MeCN solution crystallizes as purple rectangular parallelopipeds with the formulation
)
[NiL(H2O)(MeCN)][ClO4]2ؒH2Oؒ2MeCN. The crystal structure of this complex has also been determined. The
NiII ion is octahedrally co-ordinated with the equatorial sites occupied by the three secondary amino N atoms of
the cryptand and the N atom of the bound MeCN. The axial sites are occupied by the bridgehead N and the O
atom of the water molecule which enters the cavity forming a cascade complex.
The synthesis of macropolycyclic compounds with recognition
site(s) for transition-metal ions is a subject of considerable
N
OH
O
O
CHO
current interest 1–10 due to their use in many research areas. It
CHO
O
CHO
N
(i)
has been observed, that macrobicyclic cryptands incorporating
•HCl
+
CHO
nitrogen donors readily accept transition-metal ion(s) inside
Cl
Cl
Cl
11–13
their cavities forming cryptates. We have been involved
in
synthesizing heteroditopic cryptands through [1 + 1] Schiff-base
condensation of tripodal triamines with tripodal trialdehydes.
Only one end of the cavity of such a cryptand is recognized by
transition-metal ions leaving the other end vacant. Such cryp-
tates are useful in homogeneous catalysis,14 or in stabilizing
metal–ligand bonds which are otherwise unstable.13 Besides,
they are well suited for studying chelation-enhanced fluor-
escence,15 and for making ordered structures like vesicles,16 etc.
It is, therefore, important to probe the metal-binding abilities of
the heteroditopic cryptands. Here we describe an improved syn-
thesis of a heteroditopic cryptand (L) and its complexing prop-
erties towards CuII and NiII. The structures of L and of the
nickel() cryptate, [NiL(H2O)(MeCN)][ClO4]2ؒH2Oؒ2MeCN,
are also described. The latter provides the first example of a
water molecule bonded to the metal ion inside the cavity in a
cascade fashion. A copper() ion bonded to a water molecule
inside the cavity of an oxaaza macrobicycle has been reported
previously.17
N
O
N
O
O
NH
2 NH2
NH2
(ii)
NH NH
HN
N
L
Scheme 1 (i) NaOH, EtOH, reflux; (ii) 5 ЊC, MeOH + thf, NaBH4
in situ
described here. In a typical experiment, the tripodal trialdehyde
(Scheme 1) (3.68 g, 8 mmol) dissolved in a mixture of dry
MeOH (500 cm3) and dry tetrahydrofuran (thf) (50 cm3) was
cooled to 5 ЊC. To this was added a solution of tris(2-
aminoethyl)amine (1.17 g, 8 mmol) in MeOH (300 cm3) over a
period of 12 h with constant stirring while maintaining the
temperature at 5 ЊC. After the addition was complete the light
yellow solution was allowed to warm to room temperature and
kept for another 4 h. The Schiff base formed was not isolated
but immediately treated with an excess of solid NaBH4 (≈1.6 g)
and refluxed for 2 h. After complete removal of the solvent the
white solid left was shaken with water (40 cm3) and the desired
cryptand (L) was extracted with CHCl3 (4 × 50 cm3). The
organic layer after drying over anhydrous Na2SO4 was com-
pletely evaporated to obtain L as a colourless solid in 65% yield,
m.p. 144 ЊC (uncorrected). NMR (CDCl3): 1H (80 MHz), δ 2.5
(m, 12 H), 3.2 (t, 6 H), 3.6 (s, 6 H), 4.1 (t, 6 H) and 7.1 (m, 12 H);
Experimental
Reagent-grade chemicals (Aldrich) were used as provided. All
the solvents (SD Fine Chemicals, India) were purified prior to
use. Spectroscopic and microanalytical data for the complexes
were collected as described 18 earlier.
Preparations
Cryptand L. The synthesis of L by tripod–tripod Schiff-base
condensation in the presence of CsI/RbI as the templating metal
ion has been reported 11 by us. An improved method on the
multigram scale without involving any templating metal ion is
† Non-SI unit employed: µB ≈ 9.27 × 10Ϫ24 J TϪ1
.
J. Chem. Soc., Dalton Trans., 1997, Pages 935–938
935

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13C (20.1 MHz), δ 47.06, 48.60, 54.37, 55.30, 67.55, 111.29,
120.63, 127.16, 128.70, 131.33 and 156.35. FAB mass spectrum:
m/z 560 (L+) (Found: C, 70.25; H, 8.3; N, 12.4. C33H45N5O3
requires C, 70.8; H, 8.1; N, 12.5%). Colourless X-ray-quality
crystals of L were grown by slow evaporation of an acetonitrile
solution at room temperature.
Table 1 Crystallographic and refinement data for L and complex 2a *
L
2a
Formula
C33H45N5O3
(C35H50Cl2N6NiO12)ؒ
H2Oؒ2CH3CN
976.558
Orthorhombic
P212121
M
559.758
Triclinic
¯
P1
Crystal system
Space group
[CuL][ClO4]2 1. A solution of [Cu(H2O)6][ClO4]2 (0.18 g, 0.5
mmol) in MeOH (15 cm3) was added to a stirred solution of L
(0.28 g, 0.5 mmol) dissolved in MeOH (15 cm3) at room tem-
perature. A blue solid precipitated immediately and was filtered
off, washed with MeOH and dried in vacuo. Yield 82% (Found:
C, 48.15; H, 5.45; N, 8.6. C33H45Cl2CuN5O11 requires C, 48.2; H,
5.5; N, 8.5%). FAB mass spectrum: m/z 723 (1 Ϫ ClO4Ϫ) and
623 (1 Ϫ 2ClO4Ϫ).
a/Å
b/Å
c/Å
α/Њ
10.133(2)
11.972(3)
13.433(3)
106.69(2)
90.93(2)
91.38(2)
1560(1)
2
13.937(4)
14.547(3)
22.951(7)
β/Њ
γ/Њ
U/Å3
4653(2)
4
1.394
0.61
Z
Dc/g cmϪ3
1.192
0.08
µ/mmϪ1
[NiL][ClO4]2 2. This complex was prepared as above by treat-
ing [Ni(H2O)6][ClO4]2 with L in warm (40 ЊC) MeOH. It was
isolated as a blue solid in 68% yield upon partial evaporation
of the solvent and dried in vacuo (Found: C, 48.65; H, 5.5; N,
8.4. C33H45Cl2N5NiO11 requires C, 48.5; H, 5.55; N, 8.55%). FAB
mass spectrum: m/z 717 (2 Ϫ ClO4Ϫ) and 617 (2 Ϫ 2ClO4Ϫ).
When the light blue solid was dissolved in moist MeCN the
solution gradually changed to purple overnight. X-Ray-quality
crystals (2a) were grown upon slow evaporation of this purple
solution at 5 ЊC.
Crystal size/mm
Transmission (maximum,
minimum)
hkl Ranges
0.20 × 0.30 × 0.30 0.20 × 0.20 × 0.15
0.9771, 0.9658 0.8745, 0.8360
0–15, Ϫ14 to р14, 0–16, 0–17, 0–27
Ϫ30 to 30
Total reflections measured
No. unique reflections
No. reflections used
[I > 2σ(I)]
5808
5467
2987
4589
4563
2935
No. variables
370
560
R
RЈ
0.060
0.054
2.968
0.060
0.052
2.625
[CuL(N3)][ClO4] 3. Complex 1 (0.21 g, 0.25 mmol) was dis-
solved in MeCN (20 cm3) and stirred with a solution of NaN3
(0.16 g, 0.25 mmol) in water (2 cm3). The solution immediately
changed to dark green which on evaporation afforded a green
solid in 75% yield (Found: C, 50.95; H, 5.7; N, 14.45. C33H45-
ClCuN8O7 requires C, 51.85; H, 5.95; N, 14.65%). FAB mass
spectrum: m/z 723 (3 Ϫ N3Ϫ) and 623 (3 Ϫ N3Ϫ Ϫ ClO4Ϫ).
Goodness of fit
Largest and mean shift/e.s.d. 0.001, 0.0006
Minimum, maximum peak Ϫ0.42, +0.45
in Fourier map/e ÅϪ3
0.003, 0.0006
Ϫ0.46, +0.53
* Details in common: θ–2θ scans, range 0.80 + 0.35 tan θ; 2θ range 2–
¹
2

2
50Њ; R = Σ |Fo| Ϫ |Fc | /Σ|Fo|; RЈ = [Σw(|Fo| Ϫ |Fc|) /Σw(Fo) ]², w = 1/σ(F).
Table 2 Selected bond lengths (Å), angles (Њ) and non-bonded
distances (Å)
[NiL(N3)][ClO4] 4. This complex was prepared by treating 2
with NaN3 in 1:1 molar ratio as in the case of 3. The solution
immediately changed to purple and a purple solid precipitated
gradually and was filtered off and dried in vacuo. Yield 65%
(Found: C, 52.3; H, 5.75; N, 14.4. C33H45ClN8NiO7 requires C,
52.15; H, 5.95; N, 14.75%). FAB mass spectrum: m/z 717
(4 Ϫ N3Ϫ) and 617 (4 Ϫ N3Ϫ Ϫ ClO4Ϫ).
Complex 2a
Ni᎐O(1w)
Ni᎐N(1)
Ni᎐N(2)
2.042(6)
2.063(7)
2.167(6)
Ni᎐N(4)
Ni᎐N(5)
Ni᎐N(61)
2.147(5)
2.180(5)
2.152(4)
N(1)᎐Ni᎐O(1w)
N(1)᎐Ni᎐N(2)
N(1)᎐Ni᎐N(4)
N(1)᎐Ni᎐N(5)
N(1)᎐Ni᎐N(61)
N(2)᎐Ni᎐O(1w)
N(2)᎐Ni᎐N(4)
N(2)᎐Ni᎐N(5)
175.6(2)
84.87(2)
84.04(4)
84.85(3)
95.37(2)
92.26(3)
90.89(2)
162.68(5)
N(2)᎐Ni᎐N(61)
N(4)᎐Ni᎐N(5)
N(4)᎐Ni᎐N(61)
N(4)᎐Ni᎐O(1w)
N(5)᎐Ni᎐O(1w)
N(5)᎐Ni᎐N(61)
N(61)᎐Ni᎐O(1w)
86.69(4)
101.86(5)
177.55(3)
93.74(2)
98.48(3)
80.44(4)
86.73(4)
CAUTION: Care should be taken while treating organic
compounds with metal perchlorates as potentially explosive
mixtures may be formed.
X-Ray crystallography
Crystallographic data for compounds L and 2a are listed in
Table 1. In the case of L a crystal of suitable size was mounted
on a glass fibre with epoxy cement. For 2a a crystal was sealed
inside a Lindemann capillary containing a small amount of the
mother-liquor. Cell parameters and reflection intensities were
measured at 298 K on an Enraf-Nonius CAD4-mach diffrac-
tometer with graphite-monochromated Mo-Kα radiation
(λ = 0.71073 Å). The structures were solved by direct methods
and completed by Fourier-difference syntheses. All non-
hydrogen atoms were anisotropically refined by full-matrix
least-squares calculations based on F. In each case a few H
atoms could be located in the subsequent Fourier-difference
maps while others were added at calculated positions. For 2a
the H atoms of the water molecule inside the cavity could be
located in the Fourier-difference map. Hydrogen atoms were
given a fixed Ueq (0.2 for L, 0.1 Å2 for 2a). Their positions or
thermal parameters were not refined. Computations were made
with the XTAL 3.2 program package19 on a 486-DX personal
computer (IBM compatible, PCL, India). Selected bond lengths
and angles for both structures are collected in Table 2.
L
2a
N(1) ؒ ؒ ؒ N(3)
N(2) ؒ ؒ ؒ N(4)
N(2) ؒ ؒ ؒ N(5)
N(4) ؒ ؒ ؒ N(5)
O(1) ؒ ؒ ؒ O(2)
O(1) ؒ ؒ ؒ O(3)
O(2) ؒ ؒ ؒ O(3)
H(1)O(w) ؒ ؒ ؒ N(3)
H(1)O(w) ؒ ؒ ؒ O(1)
H(2)O(w) ؒ ؒ ؒ O(1)
H(2)O(w) ؒ ؒ ؒ O(3)
H(2)O(w) ؒ ؒ ؒ N(3)
6.249(5)
4.036(6)
4.336(5)
3.536(5)
4.862(7)
5.365(5)
3.992(3)
6.410(6)
3.082(5)
4.295(7)
3.354(4)
4.603(6)
5.450(6)
3.602(4)
3.118(4)
3.282(5)
2.988(4)
2.535(5)
2.013(5)
graphic Data Centre (CCDC). See Instructions for Authors,
J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the
CCDC for this material should quote the full literature citation
and the reference number 186/384.
Results and Discussion
The cryptand readily forms mononuclear complexes with CuII,
Atomic coordinates, thermal parameters, and bond lengths
and angles have been deposited at the Cambridge Crystallo-
936
J. Chem. Soc., Dalton Trans., 1997, Pages 935–938

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Table 3 Ligand-field bands, magnetic and EPR spectral data for the
cryptates
Cryptate
λ(d–d)a/nm µeff/µB
g||
g⊥
104A||/cmϪ1
b
1
990 (sh)
810 (335)
680 (260)
415 (430)
940 (20)
565 (15)
360 (55)
960 (25)
870 (20)
560 (30)
805 (175)
670 (250)
1.89
2.07 (gav)c
2.12d
2.07
2.06
63
95
2.18e
2
2.98
2a
3
2.04
3.07
2.09 (gav)c
2.03d
2.13
2.26
65
64
2.01e
Fig. 1 An ORTEP 22 drawing of L showing the thermal ellipsoids and
4
950 (15)
570 (15)
the atomic numbering scheme. Hydrogen atoms are omitted for clarity
a
Electronic spectra of 1–4 at 298 K in MeOH or in MeCN; molar
absorption coefficients (ε/dm3 molϪ1 cmϪ1) are in parentheses. b At
298 K. c Solid sample at 298 K. d In MeCN at 298 K. e In MeCN glass
at 77 K.
(H2O)25 bond distances are within normal ranges. The non-
bonding distances between the secondary amino N atoms are
only slightly shorter than those in L (Table 2). Hence, there is a
high degree of preorganization of the cryptand prior to metal
binding.
For complex 1 in MeOH the ligand-field band positions and
their intensities (Table 3) are similar to those obtained for a
copper() cryptate where the co-ordination geometry is26 dis-
torted trigonal pyramidal. The corresponding bands of 3 are
consistent with a trigonal-bipyramidal geometry.27,28
The nickel cryptate 2 in MeOH shows three weak ligand-field
bands. The geometry around the NiII is probably distorted tri-
gonal pyramidal (when H2O and the MeCN are not bonded as
in 2a). While pseudo-tetrahedral NiN4 complexes are rare,29
no nickel() complex with a distorted trigonal-pyramidal
geometry is known. The bands shift gradually when exposed to
moist air and correspond 30 to that obtained for a typical octa-
hedral nickel() complex (Table 3). In case of 4 the electronic
spectral data compare well31 with those for high-spin
trigonal-bipyramidal nickel() complexes.
The effective magnetic moments (µeff/µB) for the copper()
cryptates after diamagnetic corrections range between 1.89 and
2.04 at 301 K consistent with 32 discrete, mononuclear formu-
lations for these complexes. For the nickel() cryptates 2 and 4,
µeff is 2.98 and 3.07 µB respectively consistent with four-33 and
five31-co-ordination. Both 1 and 3 display broad EPR signals
(Table 3) in the solid state at 298 K which become sharper at 77
K without revealing any fine structure. Complex 1 in MeCN
(ca. 1 × 10Ϫ3 mol dmϪ3) at 298 K exhibits an axial spectrum.
In acetonitrile glass (77 K) the signal is still axial although
with significant changes in the g|| and A|| values. This type of
behaviour has been observed previously26 and is attributable to
a change in the ligand superstructure or solvent co-ordination
at low temperature. In case of 3 the EPR parameters in MeCN
at 298 and 77 K are consistent 27 with a distorted trigonal-
bipyramidal geometry.
Fig. 2 An ORTEP 22 drawing of complex 2a. Details as in Fig. 1
1, and NiII, 2, as their perchlorate salts. When 1 and 2 are
allowed to react with NaN3 in 1:1 molar ratio, five-co-ordinate
cascade complexes [CuL(N3)][ClO4] 3 and [NiL(N3)][ClO4] 4
are formed respectively with the azide group directly bonded to
the metal ion. For 3 and 4, νasym shows20 blue shifts of about 25
cmϪ1 with respect to the free azide group and compares well
with the literature data.21 Complexes 1–4 exhibit a strong and
broad peak at 1100 cmϪ1 attributable to ionic perchlorates. The
molar conductivity values in MeCN (ca. 1 × 10Ϫ3 mol dmϪ3) are
consistent with the formulations. The complexes are stable in
air and soluble in common organic solvents.
In the solid state L has an endo-endo conformation with a
distance of 6.249(5) Å between the bridgehead nitrogens
through which a pseudo-three-fold symmetry axis passes (Fig.
1). The bond distances and angles in the molecule are normal
within statistical errors. Both 1H and 13C NMR data also
indicate a three-fold symmetry in the molecule in chloroform
solution.
The structure of complex 2a consists of the cation [NiL-
(H2O)(MeCN)]2+ (Fig. 2) and two perchlorate anions. In add-
ition, two molecules of MeCN and one of H2O are present in
the crystal lattice. The NiII recognizes the N4-end of the cavity.
The water molecule occupies the empty space inside the cavity
making hydrogen bonds with the bridgehead N and ether O
atoms of the cryptand while its O atom binds the metal ion
(Table 2) forming a cascade complex. Similar cascade com-
plexes are known for sodium 23 and copper().17 The two
bridgehead N atoms in 2a are in an endo-endo conformation
with a separation of 6.410(6) Å which compares well with that
found in L. The NiII achieves six-co-ordination by binding a
MeCN molecule from outside the cavity. The Ni᎐N 24 and Ni᎐O
Conclusion
The cryptand L recognizes CuII and NiII at the N4 end of the
cavity forming four-co-ordinate mononuclear cryptates with
the other end vacant. The nickel() cryptate accepts a water
molecule inside the cavity which binds, the H atoms of which
are bonded to two of the ether O and bridgehead N atoms. The
strong tendency of NiII to six-co-ordination is manifested in the
crystal structure of the nickel() cryptate where the metal ion
J. Chem. Soc., Dalton Trans., 1997, Pages 935–938
937

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13 K. G. Ragunathan and P. K. Bharadwaj, Proc. Indian Acad. Sci.
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co-ordination.
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14 D. K. Chand and P. K. Bharadwaj, unpublished work.
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20 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
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Acknowledgements
Financial support for this work from the Department of Science
and Technology (DST), New Delhi, India (No. SP/S1/F-13/91)
is gratefully acknowledged. The structural studies were made
at the National Facility on Single Crystal X-Ray Diffraction at
the Department of Chemistry, Indian Institute of Technology,
Kanpur. P. K. B. thanks the DST for funding the facility (under
the IRPHA Programme; No. 6P/S1/F-47/89).
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Received 28th August 1996; Paper 6/05948A
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