Table 1 Relative selectivity of ligands 5 or L for Zn2+, Co2+, Cu2+ or Ni2+
In the case of [5·Fe2+] the logK (5) was 3.7 units higher than logK
(6) for [6·Fe2+].
against Fe2+
Although 5 showed similar overall coordination geometry for
the Fe2+ cation as in typical tris(2,2¢-bipyridine)iron(II) complexes,
the trimeric platform offers rigidity and an all-syn conformation of
the coordinating bipyridine moieties leading to a highly preorga-
nized modulated coordination site for octahedral transition metal
cations and thus demonstrating exceptional selectivity properties.
The stability order: Zn2+< Co2+< Ni2+ ª Fe2+ < Cu2+, obtained
for [5·M2+] complexes, differs significantly from the stability
order of tris(5,5-dimethyl-2,2¢-bipyridine) complexes. Of all of
the above metal ions, 5 showed exceptional selectivity towards
Cu2+, probably due the more flexible coordination behaviour of
the Cu(II) cation itself. On the other side, the coordinatively very
rigid Co2+ cation shows a remarkably lower stability with 5. Only in
the case of the coordinatively versatile Zn2+ cation, the stabilities
with cyclophane 5 did not deviate from the reference complex,
tris(5,5¢-dimethyl-2,2¢-bipyridine)Zn(II).
[L3·Fe2+
A
]
[5·Fe2+
A
]
A
a
A
a
Plot [L/5]:[Fe]:[X]
/
A
/
A
—
0.854a
0.799
0.548
0.401
0.251
1.00
0.94
0.64
0.47
0.29
0.882a
0.845
0.821
0.229
0.491
1.00
0.96
0.93
0.26
0.56
Zn2+
Co2+
Cu2+
Ni2+
a The absorption without the competing metal ion [X].
Acknowledgements
The financial support from the Academy of Finland (KRi,
no. 212588 and 218325) and the National Graduate School of
Organic Chemistry and Chemical Biology (KRa) is gratefully
acknowledged. We thank Spec. Lab. Technician Reijo Kauppinen
for his help for recording the NMR spectra.
Notes and references
Chart 1 Relative selectivity [%] of ligand 5 or L for Zn2+, Co2+, Cu2+ and
Ni2+ against Fe2+
.
1 B. P. Hay and R. D. Hancock, Coord. Chem. Rev., 2001, 212, 61.
2 J-M. Lehn, Supramolecular chemistry - scope and perspectives, Wiley-
VCH, 1995.
3 D. J. Cram and J. M. Lehn, J. Am. Chem. Soc., 1985, 107, 3657.
4 A. E. Martell, R. D. Hancock and R. J. Motekaitis, Coord. Chem. Rev.,
1994, 133, 39.
5 D. J. Cram, Science, 1988, 240, 760.
6 Y-J. Li, I. Murase, J. Reibenspies and A. E. Martell, Inorg. Chim. Acta,
1996, 246, 89.
7 E. T. Clarke and A. E. Martell, Inorg. Chim. Acta, 1991, 190, 37.
8 D. K. Cabbiness and D. W. Margerum, J. Am. Chem. Soc., 1969, 91,
6540.
9 See the references in: H. K. Dam, D. N. Reinhoudt and W. Verboom,
Chem. Soc. Rev., 2007, 36, 367.
are; 13.4, 16.1, 17.0, 17.5 and 20.1 for Zn2+, Co2+, Cu2+, Fe2+
and Ni2+ ions, respectively.23 Thus, the stability constants are
in accordance with the order obtained from this competition
experiment. However, the tris(2,2¢-bipy) stability constant values
vary considerably depending on the method and experimental
conditions. For example, a logK as high as 17.85 has been
obtained for a tris(2,2¢-bipyridine)copper(II) complex.24 For our
preorganized cyclophane receptor 5 the stability/selectivity order:
Zn2+ (4%) < Co2+ (7%) < Ni2+ (44%) < Fe2+ (56%) < Cu2+ (74%),
differs significantly from the values of the reference tris-complexes.
The experiment showed exceptionally high selectivity for Cu2+ over
the other metal ions (see the red and blue plots in Chart 1). On
the contrary Co2+ and Ni2+ cations were clearly less favoured when
referred to Fe2+.
10 K. Raatikainen, J. Huuskonen, E. Kolehmainen and K. Rissanen,,
Chem.–Eur. J., 2010, 16, 14554.
11 K. Raatikainen, J. Huuskonen, E. Kolehmainen and K. Rissanen,
Chem.–Eur. J., 2008, 14, 3297.
12 K. Raatikainen and K. Rissanen, Cryst. Growth Des., 2010, 10, 3638.
13 SPARTAN 08, Wavefunction Inc., 18401 Von Karman Ave, Suite 370,
Irvine, California 92612 USA. a) MMFF b) DFT, B3LYP, HF 6-31G*.
14 C. Y. Huang, Methods Enzymol., 1982, 87, 509.
15 P. Job, Ann. Chim, 1928, 9, 113.
Conclusion
16 P. Krumholz, Nature, 1949, 163, 724.
17 D. Buist, N. J. Williams, J. H. Reibenspies and R. D. Hancock, Inorg.
Chem., 2010, 49, 5033.
18 N. J. Williams, N. E. Dean, D. G. Van Derveer, R. C. Luckay and R.
D. Hancock, Inorg. Chem., 2009, 48, 7853.
19 I. Cukrowski, E. Cukrowska, R. D. Hancock and G. Anderegg, Anal.
Chim. Acta, 1995, 312, 307.
20 R. D. Hancock, P. W. Wade, M. P. Ngwenya, A. S. De Sousa and K. V.
Damu, Inorg. Chem., 1990, 29, 1968.
21 R. D. Hancock, Acc. Chem. Res., 1990, 23, 253.
22 R. D. Hancock, S. M. Dobson, A. Evers, P. W. Wade, M. P. Ngwenya,
J. C. A. Boeyens and K. P. Wainwright, J. Am. Chem. Soc., 1988, 110,
2788.
23 H. Irving and D. H. Mellor, J. Chem. Soc., 1962, 5222.
24 L. Onstott, J. Am. Chem. Soc., 1950, 72, 4724.
The simple and high yield synthetic procedures and X-ray struc-
tures of two tripodal and one tetrapodal macrocyclic transition
metal ion receptors are reported. The tripodal receptors 5 and 7
are based on the 27-membered trimeric piperazine cyclophane
platform bearing three 2,2¢-bipyridine derivatives as pendant
moieties in all-syn conformation. On the basis of the detailed
studies of the X-ray structures I, V and VII, we concluded that the
trimeric platform is rigid and highly preorganized. The subsequent
complexation studies in solution confirmed that the preorganized
(5 and 7) vs. non-preorganized system (6) greatly affected the
complex stabilities of octahedral divalent transition metal cations.
5710 | Dalton Trans., 2011, 40, 5706–5710
This journal is
The Royal Society of Chemistry 2011
©