New functionalised C,C-pyridylpyrazoles: Synthesis and cation binding properties

Article abstract of DOI:10.3184/030823409X402546

The synthesis of two new C,C-pyridylpyrazole isomers with a functionalised arm is described. The complexation capabilities of these ligands compared to their homologues towards bivalent metals (Hg²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Zn²⁺) and alkali metal ions (K ⁺, Na⁺, Li⁺) were investigated using a liquid-liquid extraction process. The percentage limits of extraction were determined by atomic absorption measurements.

Full text of DOI:10.3184/030823409X402546

72 RESEARCH PAPER
FEBRUARY, 72–74
JOURNAL OF CHEMICAL RESEARCH 2009
New functionalised C,C-pyridylpyrazoles: synthesis and cation binding
properties
Smaail Radia*, Ahmed Attayibata, Abdelkrim Ramdania and Maryse Bacquetb
aChemistry Department, Med I University, 60 000 Oujda, Morocco
bLaboratoire de Chimie Macromoléculaire, Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq, France
The synthesis of two new C,C-pyridylpyrazole isomers with a functionalised arm is described. The complexation
capabilities of these ligands compared to their homologues towards bivalent metals (Hg2+, Cd2+, Pb2+, Cu2+, Zn2+
)
and alkali metal ions (K+, Na+, Li+) were investigated using a liquid–liquid extraction process. The percentage limits
of extraction were determined by atomic absorption measurements.
Keywords: C,C-pyridylpyrazole, liquid–liquid extraction, cations, atomic absorption
Chelating ligands based on the pyrazole ring have been
described extensively in the literature1-6 including several
reviews.7-9 In our recent work, a series of acyclic pyrazole
compounds containing one, two, three or four pyrazole rings
were prepared and demonstrated to extract only transition
metal cations,10-15 whereas macrocyclic pyrazolic compounds
are expected to also form stable complexes with alkali
metals.16-18 This aptitude is mainly due to the presence of sp2
hybrid nitrogen donors with the involvement of geometry and
the nature of the ligands.
Pyrazole-associated pyridine groups showed also the ability
to complex transition metal ions.19-24 However, complexation
studies of pyridylpyrazole compounds are less well-known in
theliterature.Itwasthereforeinterestingtoincreasethediversity
of pyridylpyrazole-based ligands with a view to study their
complexation capabilities compared to ligands with pyrazoles
units only.
Here, we report the synthesis of two new C,C-pyridylpyrazole
isomers 5 and 6 (Fig. 1) with a donor heteroatom in a side chain.
The complexation capabilities of these new functionalised
C,C-pyridylpyrazole isomers 5 and 6 compared to their
homologues such as unfunctionalised C,C-pyridylpyrazole
7,23 C,N-pyridylpyrazole 824 and pyrazolylpyrazole 911 ligands
were investigated using a liquid–liquid extraction process
towards bivalent metal ions (Hg2+, Cd2+, Pb2+, Cu2+, Zn2+) and
alkaline metal ions (Li+, Na+, K+). The relative capabilities of
theses receptors in extracting theses cations were determined
by the measurement of extracted cation percentage by atomic
absorption.
Results and discussion
The preparation of the desired ligands 5 and 6 was carried
out in two ways and the strategy is given below (Scheme 1).
7KHꢀ¿UVWꢀZD\ꢀLQFOXGHVꢀRQO\ꢀRQHꢀVWHSꢀDQGꢀLQYROYHVꢀFRQGHQVDWLRQꢀ
of the diketone 125 with 2-hydroxyethylhydrazine using the
same method as described in the literature for the synthesis
of N-(2-hydroxyethyl)-3,5-dimethylpyrazole.26 Under this
method, the E-isomer 6 and the D-isomer 5 were obtained after
VHSDUDWLRQꢀDQGꢀSXUL¿FDWLRQꢀRQꢀVLOLFDꢀJHOꢀFKURPDWRJUDSK\ꢁꢀLQꢀꢂꢃꢀ
DQGꢀꢄꢅꢀ\LHOGVꢀUHVSHFWLYHO\ꢆꢀ7KHVHVꢀVWUXFWXUHVꢀZHUHꢀLGHQWL¿HGꢀ
on the basis of their spectroscopic data.
In order to direct the reaction towards the dominating
formation of the desired D-isomer 5, we tried another way
of synthesis. According to this method, the D-isomer 5 was
prepared in three steps from the diketone 1.25 Action of
hydrazine monohydrate on diketone 1 leads to the formation
of pyridylpyrazole 2 in a 70% yield.23 Alkylation of compound
2 with ethylbromoacetate in THF was carried out under
solid-liquid phase transfer catalysis to favour the D-isomer.27
Thus, one isolated major product 3 as the D-isomer was
formed in 20% yield. The second positional isomer 4 was also
isolated but with a very low yield (1%).
The poor yield of this reaction (20%) can be possibly due
to the more conjugated nature of 3-(2-pyridyl)pyrazole which
delocalises the negative charge when it is deprotonated, making
it a poorer nucleophile. This poor yield was not surprising as
bis[3-(2-pyridyl)pyrazol-1-yl]methane28 was already reported
under the same procedure in only a 14% yield. Compound 3
was then converted in the presence of LiAlH4 to give a 70%
yield of the corresponding alcohol 5.
Me
Me
N
N
N
N
N
N
OH
6
5
OH
Me
Me
Me
Me
N
N
N
N
Me
Me
Me
Me
N
N
N
N
N
N
9
7
8
Fig. 1 Structures of synthesised C,C-pyridylpyrazole isomers 5 and 6 and literature ligands 7,23 824 and 9.11
* Correspondent. E-mail: radi_smaail@yahoo.fr

JOURNAL OF CHEMICAL RESEARCH 2009 73
OH
Me
Me
Me
H2NHN
+
N
EtOH
N
N
O
O
N
N
N
N
OH
H
1
5
6
LiAlH4/THF
NH2NH2, H2O/MeOH
HO
Me
Me
Me
t-BuOK/THF
BrCH2CO2Et
+
NH
N
N
N
CO2Et
N
N
N
N
N
2
4
3
CO2Et
Scheme 1
Liquid–liquid extraction of individual cations
as convergent chelating bidentate donors. The term convergent
refers to the nitrogen donor atoms coordinating to the same
PHWDOꢀFHQWUHꢀOHDGLQJꢀWRꢀDꢀ¿YHꢇPHPEHUHGꢀULQJꢀZKLFKꢀLVꢀWKXVꢀ
part of several such rings when the whole ligand is considered.
It is well known32ꢀWKDWꢀ¿YHꢇPHPEHUHGꢀULQJꢀFKHODWHVꢀDUHꢀPRUHꢀ
stable than six-membered and four-membered ones.
We used this method in order to determine, and thereafter
to compare, the relative complexing capabilities of these
pyridylpyrazole derivatives 5–9 towards bivalent and alkali
metal ions (Hg2+, Cd2+, Pb2+, Cu2+, Zn2+, Li+, Na+ and K+).
Metal nitrates were extracted into the organic phase by
complex formation with the pyridylpyrazole receptors. The
percentage limits of extraction were determined by atomic
absorption and the results are given in Table 1.
We notice that the structure with
a C-N junction,
pyridylpyrazole 8, rather than a C–C junction, pyridylpyrazole
7, has stronger binding properties especially for mercury
EHFDXVHꢀRIꢀJUHDWHUꢀÀH[LELOLW\ꢁꢀDOORZLQJꢀWKHꢀGRQRUꢀVLWHꢀWRꢀDGRSWꢀ
the best binding position.
0RUHRYHUꢁꢀ ZHꢀ QRWLFHGꢀ Dꢀ GRXEOHꢀ DI¿QLW\ꢀ IRUꢀ &ꢁ&ꢇ
pyrazolylpyrazole ligand 9 (type sp2 pyrazole) compared to its
homologue pyridylpyrazole 8 (type sp2 pyridine), we can thus
emphasise the novel complexation properties of the ligands
containing linked pyrazole groups compared to the pyridinic
JURXSVꢆꢀ&RQVHTXHQWO\ꢁꢀWKHꢀDI¿QLW\ꢀWRZDUGꢀPHUFXU\ꢀIROORZVꢀWKHꢀ
following order: C,C-bipyrazole (65%) > C,N-pyridylpyrazole
(32%) > C,C-pyridylpyrazole (4%).
Results in Table 1 show that in analogy to our previous
work10-18 in which acyclic pyrazoles extract only the bivalent
metal cations when the macrocyclic pyrazolic compounds are
expected to form stable complexes both with bivalent and
DONDOLꢀPHWDOVꢁꢀZHꢀGHPRQVWUDWHꢀDOVRꢀKHUHꢀDQꢀDI¿QLW\ꢀRIꢀWKHVHꢀ
new acyclic ligands (type sp2 nitrogen only) with the bivalent
metal cations, with no complexation being observed toward
alkali cations.
The strong ability of nitrogen to complex mercury was
demonstrated for our previous ligands.10-18 However, the present
ligands 5–7ꢀKDYHꢀDꢀYHU\ꢀZHDNꢀDI¿QLW\ꢀWRZDUGVꢀWKLVꢀPHWDOꢆ
:HꢀQRWLFHꢀIRUꢀWKHVHꢀW\SHVꢀRIꢀOLJDQGVꢀDꢀPRGHVWꢀDI¿QLW\ꢀRQO\ꢀ
WRZDUGVꢀFRSSHUꢀDQGꢀOHDGꢀZLWKꢀDꢀYHU\ꢀZHDNꢀRUꢀDꢀ]HURꢀDI¿QLW\ꢀ
towards others metals. We can thus suggest an important
selectivity for these ligands towards copper and lead.
Contrary to the literature, in which a donor atom in a side
chain of lariat ethers increases the binding ability of the
macrocycle,29-31 the comparison between 5 or 6 with a donor
atom in a side chain and 7 without a donor atom shows that
there is little change in the percentage of complexation.
Indeed, we can conclude here that the complexation was due
to the ligand nitrogens without contribution of a side arm.
Moreover compound 3 acts in a similar way to compounds 5
and 7ꢆꢀ7KLVꢀFRQ¿UPVꢀWKHꢀDERYHꢇPHQWLRQHGꢀDVVXPSWLRQꢆ
The spacing of this contribution makes the chelating effect
responsible for the complexation ability. Indeed, when the
geometry supports this effect, a stable complex is formed. In
most cases, the bipyrazole groups and similar derivatives act
Conclusion
In conclusion, new acyclic C,C-pyridylpyrazole-based
ligands were prepared and only form complexes with bivalent
metal cations. They do not complex alkali metal cations at
DOOꢆꢀ$ꢀPRGHVWꢀDI¿QLW\ꢀZDVꢀREVHUYHGꢀWRZDUGVꢀFRSSHUꢀDQGꢀOHDGꢆꢀ
However, ligands with pyrazole-type sp2 nitrogen are the
most effective.
Experimental
All solvents and other chemicals, obtained from usual commercial
VRXUFHVꢁꢀZHUHꢀRIꢀDQDO\WLFDOꢀJUDGHꢀDQGꢀXVHGꢀZLWKRXWꢀIXUWKHUꢀSXUL¿FDWLRQꢆꢀ
The NMR spectra were obtained with a Bruker AC 300 spectrometer.
Elemental analyses were performed by Microanalysis Central Service
(CNRS). Molecular weights were determined on a JEOL JMS DX-
300 Mass Spectrometer. Atomic absorption measurements were
performed by Spectra Varian A.A. 400 Spectrophotometer.
Synthesis of 3 and 4: A mixture of 2 (3 g; 18.87 u 10-3 mol) and
potassium tert-butoxide (2.22 g; 19.82 u 10-3 mol) in anhydrous THF
Table 1 Yields of extraction of various studying metals by ligands 5–9
Hg2+
Cd2+
Pb2+
Cu2+
Zn2+
Li+
Na+
K+
5
9
4
0
3
3
1
9
7
15
13
20
9
0
0
0
0
6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
6
12
10
13
27
7
4
8
32
65
911
17

74 JOURNAL OF CHEMICAL RESEARCH 2009
ꢈꢉꢊꢀP/ꢋꢀZDVꢀUHÀX[HGꢀIRUꢀꢄꢀKꢆꢀ$IWHUꢀFRROLQJꢀWRꢀꢊ°C, a solution of
ethylbromoacetate (3.85 g; 23.05 u 10-3 mol) in THF (20 mL) was
slowly added. The reaction mixture was stirred for one night at room
WHPSHUDWXUHꢀWKHQꢀ¿OWHUHGꢀDQGꢀWKHꢀVROYHQWꢀZDVꢀHYDSRUDWHGꢀWRꢀGU\QHVVꢆꢀ
7KHꢀREWDLQHGꢀUHVLGXHꢀZDVꢀSXUL¿HGꢀRQꢀVLOLFDꢀXVLQJꢀWKHꢀPL[WXUHꢀꢈꢌꢊꢅꢀ
ethylacetate/90% hexane) as eluant to give a 20% yield of 3 as a
white solid and 1% yield of 4 (yellow oil).
complexation was followed by measuring the concentration of
cations in the aqueous phase by atomic absorption. The temperature
remained constant during all the experiments at 25°C and at pH 7
measured by a pH-meter.
Received 5 September 2008; accepted 13 December 2008
Paper 08/0155 doi:10.3184/030823409X402546
Published online: 24 February 2009
3: White solid. M.p. = 58–60°C (ether); 1H NMR (300 MHz,
CDCl3, 25°C): G = 1.26 (t, JH,H = 7.17 Hz, 3 H, CH2–CH3), 2.29 (s,
3 H, –CH3), 4.22 (q, JH,H = 7.17 Hz, 2 H, –CH2–CH3), 4.90 (s, 2 H,
N–CH2–), 6.71 (s, 1 H, Pz-H), 7.17 (m, 1 H, Py-HE), 7.69 (m, 1 H,
Py-HJ), 7.85 (m, 1 H, Py-HG), 8.60 (m, 1 H, Py-HD) ppm; 13C NMR
(300 MHz, CDCl3, 25°C): G = 11.20 (CH3), 14.14 (–CH3), 51.06 (N–
CH2–), 61.89 (O–CH2–), 104.81 (Pz C–H), 120.16 (Py C–HG), 122.41
(Py C–HE), 136.76 (Py C–HJ), 141.01 (Pz CCH3), 149.12 (Py C–HD),
150.40 (Py C), 167.68 (C=O) ppm; Anal. Calcd for C13H15N3O2: C
63.66, H 6.16, N 17.13. Found: C 63.60, H 6.15, N 17.22%; m/z 245
(M+.). IR (KBr): Q(C=O) = 1740 cm-1.
References
1
C. Dowling, V.J. Murphy and G. Parkin, Inorg. Chem., 1996, 35, 2415.
W.J. Haanstra, W.L. Driessen, M. Van Roon, A.L.E. Stoffels and
J. Reedijk, J. Chem. Soc., Dalton Trans., 1992, 481.
W.L. Driessen, W.G.R. Wiesmeijer, M. Schipper-Zablotskaja, R.A.G.
De Graaff and J. Reedijk, Inorg. Chim. Acta, 1989, 162, 233.
Y.C.M. Pennings, W.L. Driessen and J. Reedijk, Polyhedron, 1988, 7,
2583.
2
3
4
5
6
7
4: Yellow oil. 1H NMR (300 MHz, CDCl3, 25°C): G = 1.17 (t, JH,H
= 7.17 Hz, 3 H, CH2–CH3), 2.23 (s, 3 H, –CH3), 4.13 (q, JH,H = 7 Hz,
2 H, –CH2–CH3); 5.4 (s, 2 H, N–CH2–), 6.47 (s, 1 H, Pz-H), 7.20 (m,
1 H, Py-HE), 7.69 (m, 2 H, Py-HJ,G), 8.65 (d, 1 H, Py-HD) ppm; Anal.
Calcd for C13H15N3O2: C 63.66, H 6.16, N 17.13. Found: C 63.57,
H 6.12, N 17.25%; m/z 245 (M+.). IR (KBr): Q(C=O) = 1760 cm-1.
(5): To a solution of LiAlH4 (450 mg; 11.84 u 10-3 mol) in
anhydrous diethyl ether (20 mL), cooled at 0°C, was slowly added
pyridylpyrazole 3 (1.15 g; 4.69 u 10-3 mol) in anhydrous diethyl ether
ꢈꢄꢊꢀP/ꢋꢆꢀ7KHꢀPL[WXUHꢀZDVꢀVWLUUHGꢀXQGHUꢀUHÀX[ꢀIRUꢀꢄꢀKꢆꢀ$IWHUꢀFRROLQJꢁꢀ
water (0.45 mL), aqueous sodium hydroxide (15%, 0.45 mL) and
then water (1.35 mL) were added successively to the mixture at 0°C.
7KHꢀVROLGꢀPDWHULDOꢀZDVꢀ¿OWHUHGꢀDQGꢀWKHꢀUHVLGXHꢀZDVꢀZDVKHGꢀZLWKꢀKRWꢀ
7+)ꢆꢀ7KHꢀ¿OWUDWHꢀDQGꢀ7+)ꢀZDVKLQJVꢀZHUHꢀFRQFHQWUDWHGꢀXQGHUꢀUHGXFHGꢀ
pressure to give a 70% yield of 5 as a white solid. M.p. = 50–52°C
(CCl4); 1H NMR (300 MHz, CDCl3, 25°C): G = 2.33 (s, 3 H, CH3),
4.09 (t, JH,H = 4.32 Hz and 4.71, 2 H, N–CH2), 4.19 (t, JH,H = 4.32
Hz and 4.71, 2 H, –CH2OH), 6.68 (s, 1 H, pz-H), 7.21 (t, 1 H, HE),
7.73 (m, 1 H, HJ), 7.88 (d, 1 H, HG), 8.60 (d, 1 H, HD) ppm; 13C NMR
(300 MHz, CDCl3, 25°C): G = 11.24 (CH3), 50.45 (N–CH2), 61.59
(–CH2–OH), 104.12 (PzC-H), 120.05 (PyC-HG), 122.44 (PyC-HE),
136.83 (PyC-HJ), 140.64 (CCH3), 149.10 (PyC-HD), 149.10 (PzC),
150.37 (PyC) ppm; Anal. Calcd for C11H13N3O: C 65.02, H 6.40, N
20.69. Found: C 64.90, H 6.39, N 20.71%; m/z 203 (M+.). IR (KBr):
Q(OH) = 3200 cm-1.
J.B. Veldhuis, W.L. Driessen and J. Reedijk, J. Chem. Soc., Dalton Trans.,
1986, 537.
H.L. Blonk, W.L. Driessen and J. Reedijk, J. Chem. Soc., Dalton Trans.,
1985, 1699.
R. Mukherjee, Coord. Chem. Rev., 2000, 203, 151.
ꢀ ꢍꢀ 6ꢆꢀ7UR¿PHQNRꢁꢀChem. Rev., 1993, 93, 943.
ꢀ ꢎꢀ 6ꢆꢀ7UR¿PHQNRꢁꢀProg. Inorg. Chem., 1986, 34, 115.
10 A. Attayibat, S. Radi, A. Ramdani, Y. Lekchiri, B. Hacht, M. Bacquet,
S. Willai and M. Morcellet, Bull. Kor. Chem. Soc., 2006, 27, 1648.
11 A. Attayibat, S. Radi, Y. Lekchiri, A. Ramdani, B. Hacht, M. Bacquet,
S. Willai and M. Morcellet. J. Chem. Res., 2006, 10, 655.
12 F. Malek, A. Ramdani, I. Zidane, A. Yahyi and S. Radi, Tetrahedron,
2005, 61, 2995.
13 F. Malek, A. Ramdani and S. Radi, J. Chem. Res., 2004, 9, 640.
14 S. Radi, A. Ramdani, Y. Lekchiri, M. Morcellet, G. Crini, L Janus and
M. Bacquet, New J. Chem., 2003, 27, 1224.
15 S. Radi, A. Ramdani, Y. Lekchiri, M. Morcellet, G. Crini, J. Morcellet and
L. Janus, Eur. Polym. J., 2000, 36, 1885.
16 S. Radi, A. Ramdani, Y. Lekchiri, M. Morcellet, G. Crini, J. Morcellet and
L. Janus, J. Chem. Res., 2003, 11, 712.
17 S. Radi, A. Ramdani, Y. Lekchiri, M. Morcellet, G. Crini and L. Janus,
Tetrahedron, 2004, 60, 939.
18 S. Radi, A. Yahyi, A. Ramdani, I. Zidane and B. Hacht, Tetrahedron,
2006, 62, 9153.
19 J.A. Campo, M. Cano, J.V. Heras, M.C. Lagunas, J. Perles, E. Pinilla and
M.R. Torres, Helv. Chim. Acta, 2002, 85, 1079.
6: To a solution of 1-pyridin-2-yl-butane-1,3-dione 1 (1.5 g;
9.2 u 10-3 mol) in absolute ethanol (50 mL) at 0°C, was slowly
added a solution of 2-hydroxyethylhydrazine (0.7 g; 9.2 u 10-3 mol)
in absolute ethanol (10 mL). The mixture was stirred at room
temperature for 2 h. Then, the solvent was removed under reduced
SUHVVXUHꢀ DQGꢀ WKHꢀ REWDLQHGꢀ UHVLGXHꢀ ZDVꢀ SXUL¿HGꢀ RQꢀ VLOLFDꢀ JHOꢀ XVLQJꢀ
(20% ethanol/80% ether) to give 54% yield of 6 as a brown solid and
2% yield of 5 as a white solid. The solid 6 was recrystallised using
methanol to give colourless crystals of 6. M.p. = 90–93°C (ether);
1H NMR (300 MHz, CDCl3, 25°C): G = 2.34 (s, 3 H, CH3), 4.12
(t, JH,H = 4.86 Hz and 5.40 Hz, 2 H, –CH2OH), 4.59 (t, JH,H = 4.86
Hz and 5.40 Hz, 2 H, N–CH2–), 6.37 (s, 1 H, pz-H), 7.32 (m, 1 H,
HE), 7.60 (d, 1 H, HG), 7.82 (t, 1 H, HJ), 8.62 (d, 1 H, HD) ppm; 13C
NMR (300 MHz, CDCl3, 25°C): G = 13.48 (–CH3), 52.29 (N–CH2–),
62.91 (–CH2–OH), 106.37 (Pz C–H), 122.94 (Py C–HE), 123.62 (Py
C–HG), 137.73 (Py C–HJ), 142.22 (Pz C), 148.05 (Pz CCH3), 148.31
(Py C–HD), 148.95 (Py C) ppm. Anal. Calcd for C11H13N3O: C 65.02,
H 6.40, N 20.69. Found: C 65.00, H 6.39, N 20.74%; m/z 203 (M+.).
IR (KBr): Q(OH) = 3260 cm-1.
20 M.A. Halcrow, E.J.L. Mc Innes, F.E. Mabbs, I.J. Scowen, M. Mc Partlin,
H.R. Powell and J.E. Davies, J. Chem. Soc., Dalton Trans., 1997, 4025.
21 M.J. Mayoral, M.C. Torralba, M. Cano, J.A. Campo and J.V. Heras, Inorg.
Chem. Commun., 2003, 6, 626.
22 P.J. Steel, F. Lahousse, D. Lerner and C. Marzin, Inorg. Chem., 1983, 22,
1488.
23 A. Satake and T. Nakata, J. Am. Chem. Soc., 1998, 120, 10391.
24 N. Saha and S.K. Kar, J. Inorg. Nucl. Chem., 1977, 39, 1236.
25 K.P. Strotmeyer, I.O. Fritsky, R. Ott, H. Pritzkow and R. Krämer,
Supramol. Chem., 2003, 15, 529.
26 W.G. Haanstra, W.L. Driessen, J. Reedijk, U. Turpeinen and
R. Hämäläinenn, J. Chem. Soc., Dalton Trans, 1989, 2309.
27 J. Fifani, A. Ramdani and G. Tarrago, New J. Chem., 1977, 1, 521.
28 K.L.V. Mann, J.C. Jeffery, J.A. McCleverty, P. Thornton and M.D. Ward,
J. Chem. Soc., Dalton Trans., 1998, 89.
29 R.A. Schultz, D.M. Dishong and G.W. Gokel, Tetrahedron Lett., 1981,
22, 2623.
30 R.B. Davidson, R.M. Izatt, J.J. Christensen, R.A. Schultz, D.M. Dishong
and G.W. Gokel, J. Org. Chem., 1984, 49, 5080.
31 A. Kaifer, D.A. Gustowski, L.A. Echegoyen, V.J. Gatto, R.A. Schultz,
T.P. Cleary, C.R. Morgan, D.M. Goli, A.M. Rios and G.W. Gokel, J. Am.
Chem. Soc., 1985, 107, 1958.
Extraction experiments: A solution of 7 u 10-5 M of every
pyridylpyrazole in CH2Cl2 (25 mL) was stirred for 2 h with an
aqueous solution (25 mL) of metal nitrates 7
u
10-5 M; the
32 J.M. Lehn and J.P. Sauvage, J. Am. Chem. Soc., 1975, 97, 6700.