Pyridine-2,6-dihydroxamic acid, a powerful dihydroxamate ligand for Ni2+ and Cu2+ ions

Article abstract of DOI:10.1039/B207271H

Pyridine-2,6-dihydroxamic acid was found to be the most effective ligand for Ni²⁺ and Cu²⁺ ions among the known dihydroxamates. The stability constants of the title complexes are many orders of magnitude higher than those obtained for the other dihydroxamates. Only equimolar species are formed with Cu²⁺ ions, while NiL and bis-complexes are formed in the case of Ni²⁺. The [NIL₂]^2- complex anion contains compressed octahedral hexanitrogen surroundings of two meridionally coordinated ligands. The equatorial Ni-N(Py) bonds are significantly shorter than the axial Ni-N(hydroxamate) distances although the latter are formed by the deprotonated groups.

Full text of DOI:10.1039/B207271H

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Pyridine-2,6-dihydroxamic acid, a powerful dihydroxamate ligand
for Ni2؉ and Cu2؉ ions
a
b
a
´
Jolanta Swia˛tek-Kozłowska,* Elzbieta Gumienna-Kontecka, Agnieszka Dobosz,
Irina A. Golenyac and Igor O. Fritsky*c
a Department of Basic Medical Sciences, Wroclaw Medical University, 51-601 Wroclaw,
Poland. E-mail: JSK@basmed.am.wroc.pl
b Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland
c Department of Chemistry, Taras Shevchenko University, Kiev 01033, Ukraine
Received 24th July 2002, Accepted 24th October 2002
First published as an Advance Article on the web 18th November 2002
Pyridine-2,6-dihydroxamic acid was found to be the most effective ligand for Ni2ϩ and Cu2ϩ ions among the known
dihydroxamates. The stability constants of the title complexes are many orders of magnitude higher than those
obtained for the other dihydroxamates. Only equimolar species are formed with Cu2ϩ ions, while NiL and bis-
complexes are formed in the case of Ni2ϩ. The [NiL2]2Ϫ complex anion contains compressed octahedral hexanitrogen
surroundings of two meridionally coordinated ligands. The equatorial Ni–N(Py) bonds are significantly shorter than
the axial Ni–N(hydroxamate) distances although the latter are formed by the deprotonated groups.
cooling and filtering off the precipitated NaCl] was added on
cooling with an ice bath, and then the mixture was allowed to
Introduction
Aliphatic dihydroxamic acids HONHOC(CH2)nCONHOH
stand in a closed flask at room temperature for 18 h. The yellow
with n = 3–8 have been studied quite intensively with respect to
precipitate formed was filtered off, washed with methanol (30
their coordination abilities towards metal ions.1–4 The most
typical for these ligands is the {O–OЈ}-chelating mode similar
to that found in monohydroxamic acids. The insertion of the
ml) and air-dried. To the obtained powder a slight excess of
dilute hydrochloric acid (1:2) was added, giving a clear solution
of pH ca. 3. The white precipitate formed was filtered, washed
with water, re-crystallised from boiling water and dried under
adjacent N-bonding group close to the hydroxamic function
facilitates the N-bonding mode of the hydroxamic group.1,5,6
vacuum. Yield 3.08 g (78%). Calc. for C7H7N3O4 (197.15): C,
Also in the case of oxalodihydroxamic acid (OxDHA), in which
42.65; H, 3.58; N, 21.31. Found: C, 42.50; H, 3.38; N, 21.34%.
two hydroxamic functions are next to each other, the main
1
CI–MS: 196.9 [Mϩ] (100%). H NMR (DMSO-d6, 200 MHz):
coordination for Ni2ϩ and Cu2ϩ ions occurs via the {N,NЈ}-
δ (ppm) 8.134 (m, 3H, Py-3, Py-4, Py-5), 9.312 (s, 2H, NH),
11.829 (s, 2H, OH).
chelation.7 The latter dihydroxamic acid was found to be a very
powerful chelator both for Ni2ϩ and Cu2ϩ ions. The X-ray struc-
ture has shown that hydroxamic oxygens are involved in the
[PPh4]2[Ni(PyDHA)2]ؒ7H2O (1). The compound NiCl2ؒ
6H2O (0.024 g, 0.1 mmol) dissolved in water (5 ml) was added
to an aqueous solution (10 ml) of PyDHA (0.039 g, 0.2 mmol)
at 80 ЊC with stirring. To the obtained mixture sodium hydrox-
ide (0.018 g, 0.45 mmol) dissolved in 3 ml of water was added.
To the resulting solution tetraphenylphosphonium chloride
formation of the effective and short hydrogen bonds as well as
in the very specific binding of Kϩ ions.
Pyridine-2,6-dihydroxamic acid (PyDHA) is a chelating
agent able to coordinate via a three-nitrogen donor set.
Although a pyridine nitrogen is a very acidic donor the possi-
bility of formation of two 5-membered chelate rings makes
(0.075 g, 0.2 mmol) dissolved in methanol (10 ml) was added.
this ligand a good candidate for a very effective chelating agent
The obtained orange-red solution was filtered and set aside for
crystallisation at room temperature. The dark-green single
for both Ni2ϩ and Cu2ϩ ions.
crystals of the product obtained within 24 h were filtered,
washed with water and air-dried. Yield 0.105 g (84%). Calc. for
NiC62H64N6O15P2 (1253.9): C, 59.39; H, 5.14; N, 6.70; Ni, 4.68.
Experimental
All chemicals were commercial products of reagent grade and
were used without further purification. Elemental analyses were
conducted by the Microanalytical Service of the University of
Wroclaw.
Found: C, 59.52; H, 5.16; N, 6.48; Ni, 4.59%.
Potentiometric studies
Titrations involved an ionic background of 0.1 mol dmϪ3
KNO3, ligand concentrations of 2 × 10Ϫ3 and 3 × 10Ϫ3
mol dmϪ3 and metal-to-ligand ratios of 1:4, 1:5 and 1:6.
Initial solutions of 1.5 cm3 were titrated with sodium hydrox-
ide delivered by a 0.25 cm3 micrometer syringe previously
calibrated by weight titrations and titrations of standard
materials. The pH-metric titrations were performed at 25 ЊC in
the pH range 2.3–10.0 using a MOLSPIN automatic titration
system with a Russell CMAW 711 microcombined electrode
calibrated daily in hydrogen-ion concentration using HNO3.9
Titrations were performed in triplicate and the SUPERQUAD
Synthesis
Pyridine-2,6-dihydroxamic acid (PyDHA). The ligand was
synthesised by modification of the reported method.8 The
dimethyl ester of pyridine-2,6-dicarboxylic acid (Aldrich) (3.90
g, 2 mmol) was dissolved in a mixture of 50 ml absolute meth-
anol and 20 ml chloroform. To this solution, a hydroxylamine
solution in methanol [prepared by mixing NH2OHؒHCl (3.34 g,
4.8 mmol) dissolved in 20 ml methanol and a 30% solution of
sodium methylate in methanol (17.2 g, 4.8 mmol) followed by
DOI: 10.1039/b207271h
J. Chem. Soc., Dalton Trans., 2002, 4639–4643
This journal is © The Royal Society of Chemistry 2002
4639

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Table 1 Spectroscopic (UV-Vis and EPR) and potentiometric data for
PyDHA–Cu2ϩ and PyDHA–Ni2ϩ systems at 25 ЊC and I = 0.1 mol dmϪ3
KNO3
Table 2 Crystal data and refinement parameters for 1
1
logβ
λmax/nm (ε/dm3 molϪ1 cmϪ1
)
A||/G
g||
Formula
C62H64N6NiO15P2
M
1253.84
Triclinic
P1
HL
H2L
8.80(1)
7.64(1)
Crystal system
Space group
¯
a/Å
b/Å
c/Å
α/Њ
14.200(3)
14.412(3)
15.629(3)
108.05(3)
98.06(3)
98.03(3)
2953.5(10)
2
CuL
CuHϪ1L
CuHϪ2L
14.39(3)
8.65(5)
Ϫ0.68(6)
631
642
547
184
184
164
2.24
2.24
2.25
β/Њ
NiL
9.80(1)
18.29(1)
10.35(3)
836 (34)
541 (20)
817 (50)
530 (30)
817 (55)
530 (33)
γ/Њ
U/Å3
NiL2
Z
Dc/g cmϪ3
1.410
0.457
1312
NiHϪ1L2
µ(Mo-Kα)/mmϪ1
F(000)
Measured reflections
Unique reflections (Rint
Observed reflections [I > 2σ(I )]
Final R1, wR2 [I > 2σ(I )]
20399
12968 (0.0287)
10495
0.0372, 0.0937
0.0488, 0.1002
)
computer program was used for calculations of stability
constants (βpqr = [MpHrLq]/[M]p[H]r[L]q).10 Standard deviations
quoted refer to random errors only. They are, however, a good
indication of the importance of a particular species in the
equilibrium.
R1, wR2 (all unique reflections)
Spectroscopic studies
UV-visible absorption spectra were recorded on a Perkin Elmer
Lambda Bio20 spectrophotometer. The metal-ion concen-
trations were 1.5 × 10Ϫ3 mol dmϪ3 and metal-to-ligand ratios
were 1:7 for Cu2ϩ and 1:5 for Ni2ϩ. Electron paramagnetic reson-
ance (EPR) spectra were performed in an ethylene glycol–water
(1:2 v/v) solution at 77 K on a Bruker ESP 300E spectrometer at
the X-band frequency (ca. 9.45 GHz) and equipped with a
Bruker NMR Gauss-meter ER 035M and a Hewlett-Packard
microwave frequency counter HP 5350B. The metal concen-
tration was adjusted to 1 × 10Ϫ3 mol dmϪ3 and the metal-to-
ligand molar ratio was 1:7.
The results of potentiometric, UV-Vis and EPR spectro-
scopy studies are given in Table 1.
X-Ray crystallography
Crystallographic data were collected on a KUMA KM4CCD
κ-axis diffractometer11 with graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å) at 100 K. The data collection was
made using Oxford diffraction programs.12,13 The structure
was solved by direct methods using SHELXS-9714 and refined
2
by full-matrix, least-squares on all Fo using SHELXL-97.15
The non-hydrogen atoms were refined anisotropically. All
aromatic C–H hydrogen atoms were fixed in calculated
positions, the O–H hydrogen atoms of the hydroxamic groups
and water molecules were located from the difference Fourier
map, their positions were included into refinement, and the
thermal parameters were allowed to ride on the parent
atom. Crystallographic data for compound 1 are given in
Table 2.
CCDC reference number 190498.
See http://www.rsc.org/suppdata/dt/b2/b207271h/ for crystal-
lographic data in CIF or other electronic format.
Fig. 1 (a) Species distribution for Ni2ϩ–PyDHA (solid line) and Ni2ϩ
–
.
OxDHA (dashed line) complexes (1:2), CL = 2 × 10Ϫ3mol dmϪ3
(b) Hypothetical competition plot for the Ni2ϩ–PyDHA–OxDHA
Results and discussion
system assuming a 1:2:2 molar ratio and the stability constants
obtained for binary species; CL = 2 × 10Ϫ3 mol dmϪ3
.
Solution studies
Pyridine-2,6-dihydroxamic acid (PyDHA) has two protonation
constants (Table 1) corresponding to protonation of two
hydroxamic groups. The pyridine nitrogen is a very acidic donor
with a pK below 2, which could not be evaluated precisely.
Potentiometric data calculations show the formation of three
very stable Ni2ϩ complexes (Table 1). The metal binding at low
pH is much more effective for the pyridine derivative than that
for oxalodihydroxamic acid, OxDHA7 [Fig. 1(a)]. The stability
constant of the NiL species of PyDHA is almost three orders
of magnitude higher than that of OxDHA (Table 3). This
clearly indicates a tridentate coordination of the PyDHA
ligand with use of the {N,NPy,NЈ} donor set. The NiL and NiL2
species are, according to absorption spectra, of pseudo-
octahedral geometry. The competition plot [Fig. 1(b)] shows
considerably higher efficacy of PyDHA as a chelator for Ni2ϩ
ions when compared with OxDHA.
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J. Chem. Soc., Dalton Trans., 2002, 4639–4643

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Table 3 Protonation constants (logK) and overall stability constants (logβ) for dihydroxamate ligands
logK
logβ
Ligand
Hϩ
Cu2ϩ
Ni2ϩ
Ref.
a
C4DHA
C6DHA
C7DHA
C8DHA
C4DHA
9.38
8.81
CuHL
CuL
17.06
13.11
NiHL
NiL
14.43
7.63
a
a
a
b
9.61
8.92
CuHL
CuL
17.53
13.40
—
—
—
—
9.65
9.02
CuHL
CuL
—
13.60
NiHL
NiL
14.89
8.92
9.52
8.82
CuHL
CuL
—
13.54
NiHL
NiL
—
8.78
9.67
9.02
CuHL
CuL
17.24
—
NiHL
NiL
14.60
—
Cu2L2
28.19
Ni2L2
17.85
b
c
C6DEDHA
Dha1
9.83
8.77
CuHL
CuL
Cu2L2
17.76
—
28.22
NiHL
NiL
Ni2L2
14.54
—
17.92
9.25
8.48
Precipitation
—
NiHL
NiL
13.89
7.44
Ni2L3
20.94
c
Dha2
9.64
8.86
CuHL
CuL
17.28
12.83
NiHL
NiL
14.71
8.33
Ni2L3
22.66
d
L1
9.66
8.72
6.05
CuHL
CuL
19.75
—
NiHL
NiL
15.50
8.82
d
L2
9.88
9.16
7.97
CuHL
CuL
21.44
—
NiHL
NiL
17.20
—
e
PIPDMAHA
9.53
8.45
6.67
2.44
CuH2L
CuHL
CuL
23.66
18.55
12.50
—
—
f
DACHDMAHA
DOCYDMAHA
10.26
8.48
6.88
2.93
CuH2L
CuHL
CuL
25.01
22.43
16.85
—
—
—
—
g
9.46
8.39
6.67
4.02
CuH2L
CuHL
CuL
CuHϪ1L
CuHϪ2L
22.99
18.95
14.48
6.76
Ϫ2.30
h
OxDHA
8.68
6.74
CuL2
18.22
10.22
0.37
26.17
22.75
NiL
NiL2
NiHϪ1L2
7.07
12.82
4.04
CuHϪ1L2
CuHϪ2L2
Cu2L2
Cu2HϪ1L2
a See ref. 3. b B. Kurzak, L. Nakonieczna, G. Rusek, H. Kozlowski and E. Farkas, J. Coord. Chem., 1993, 28, 17. c See ref. 2. d M. A. Santos,
R. Grazina, M. Pinto and E. Farkas, Inorg. Chim. Acta, 2001, 321, 42. e See ref. 4. f M. A. Santos, M. A. Esteves, M. C. T. Vaz and M. L. Simoes
Goncalves, Inorg. Chim. Acta, 1993, 214, 47. g M. A. Santos, M. Gaspar and M. T. Amorim, Inorg. Chim. Acta, 1999, 284, 20. h See ref. 7.
J. Chem. Soc., Dalton Trans., 2002, 4639–4643
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Table 4 Bond lengths (Å) and angles (Њ) for complex 1
Ni(1)–N(5)
Ni(1)–N(2)
Ni(1)–N(3)
1.999(2)
2.008(2)
2.087(2)
Ni(1)–N(1)
Ni(1)–N(6)
Ni(1)–N(4)
2.099(2)
2.104(2)
2.109(2)
N(5)–Ni(1)–N(2)
N(5)–Ni(1)–N(3)
N(2)–Ni(1)–N(3)
N(5)–Ni(1)–N(1)
N(2)–Ni(1)–N(1)
N(3)–Ni(1)–N(1)
N(5)–Ni(1)–N(6)
N(2)–Ni(1)–N(6)
166.52(6)
98.23(6)
76.37(6)
109.76(6)
75.93(6)
151.96(6)
76.18(6)
91.35(6)
N(3)–Ni(1)–N(6)
N(1)–Ni(1)–N(6)
N(5)–Ni(1)–N(4)
N(2)–Ni(1)–N(4)
N(3)–Ni(1)–N(4)
N(1)–Ni(1)–N(4)
N(6)–Ni(1)–N(4)
90.06(6)
94.85(6)
76.50(6)
115.98(6)
95.13(6)
93.07(6)
152.64(6)
O(1)–N(1)
O(4)–N(3)
O(5)–N(4)
O(8)–N(6)
1.418(2)
1.410(2)
1.416(2)
1.413(2)
O(2)–C(1)
O(3)–C(2)
O(6)–C(8)
O(7)–C(9)
1.269(2)
1.270(2)
1.276(2)
1.281(2)
N(1)–C(1)
N(3)–C(2)
1.315(2)
1.316(2)
N(4)–C(8)
N(6)–C(9)
1.315(2)
1.314(2)
With Cu2ϩ PyDHA forms the set of four equimolar species
(Table 1), while bidenate OxDHA forms only the bis-complexes
(Table 3). This result strongly suggests the tridentade binding
mode of the pyridine ligand. The EPR and absorption spectra
clearly support the three-nitrogen coordination in all species
formed (Table 1). The d–d transition band around 630 nm and
A|| around 180 G correspond well to the {N,NPy,NЈ} binding
mode in CuL and CuHϪ1L complexes. The deprotonation of
the CuL complex with logK = 5.74 occurs at one of the
hydroxamic functions. The logK value for the CuHϪ1L
CuHϪ2L deprotonation reaction of 9.33 may correspond to the
deprotonation of an equatorially bound water molecule16 or of
a second hydroxamic function. The formation of the CuHϪ2L
species results in a distinct shift of the d–d band from 642
to 547 nm. This energy variation strongly suggests that the
deprotonation of the hydroxamic group dominates this process.
The deprotonation of the hydroxamic oxygen increases the
electron density on the coordinated nitrogen distinctly, which
results in a strong change of the d–d transition energy.1,17
The comparison of the coordination abilities of all, to our
knowledge, dihydroxamates collected in Table 3 indicates that
the most efficient ligands for Cu2ϩ ions are OxDHA and
DACHDMAHA. The comparison of PyDHA with both lig-
ands indicates that also in the case of Cu2ϩ ions the tridentate
PyDHA is a much more powerful ligand than OxDHA or
DACHDMAHA and the most powerful chelating agent among
known dihydroxamic acids [Figs. 2(a) and 2(b); see also ref. 7).
Fig. 2 (a) Species distribution for Cu2ϩ–PyDHA (solid line) and
Cu2ϩ–DACHDMAHA (dashed line) complexes (1:2), CL = 2 × 10Ϫ3mol
dmϪ3
.
(b) Hypothetical competition plot for Cu2ϩ–PyDHA–
DACHDMAHA system a assuming 1:2:2 molar ratio and the stability
constants obtained for binary species; CL = 2 × 10Ϫ3 mol dmϪ3
.
Crystal and molecular structure of [PPh4]2[Ni(PyDHA)2]ؒ
7H2O (1)
The molecular structure and numbering scheme for compound
1 are presented in Fig. 3, and selected bond parameters are given
in Table 4. The structure of compound 1 is ionic and consists of
tetraphenylphosphonium cations, the double-charged com-
plex anions [Ni(PyDHA)2]Ϫ and solvate water molecules. The
elements of the structure are united between each other by an
extensive system of H-bonds and also by the offset stacking
interactions between the aromatic Py and phenyl rings present
both in the cations and anions.
The central atom in the complex anion is nested in a distorted
axially compressed octahedral hexanitrogen surrounding. The
two doubly deprotonated ligands are coordinated in a tri-
dentate mode via the nitrogen atom of the Py rings and the
nitrogen atoms of N-deprotonated hydroxamic functions, thus
forming two fused five-membered chelate rings with a common
Ni–N(Py) bond. Two ligands are disposed in a meridional
fashion being situated in perpendicular planes in an idealized
octahedron. However, the mutual disposition of the ligands
deviates noticeably from perpendicularity which is indicative by
Fig. 3 Molecular structure and numbering scheme for complex 1.
decreased values of the trans-angle between the Py nitrogens
[N(5)–Ni(1)–N(2) = 166.52(6)Њ] and by a significant difference
in the values of the interligand cis-angles associated with the
same Ni–N(Py) bond [91.35(6) and 115.98(6)Њ for the angles
involving the Ni(1)–N(2) bond, and 98.23(6) and 109.76(6) for
the Ni(1)–N(5) bond]. The dihedral angle between the mean
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J. Chem. Soc., Dalton Trans., 2002, 4639–4643

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planes defined by the nickel ion and the atoms forming two
condensed chelate rings is 82.34(4)Њ. The conformation of the
coordinated ligands deviates from planarity: the hydroxamic
groups form dihedral angles of 4.3(1)–8.8(1)Њ with the plane of
the Py rings, and 8.1(2) and 11.6(1)Њ between each other within
the same ligand.
Acknowledgements
This work was supported by the Polish State Committee
for Scientific Research and COST D21 Action. I. O. F. thanks
to the Royal Society of Chemistry for a grant within the
Programme Journals Grants for International Authors.
The Ni–N(Py) bonds [1.999(2) and 2.008(2) Å] occupying the
equatorial positions are significantly shorter than the equatorial
Ni–N(hydroxamate) [2.087(2)–2.109(2) Å] contacts although
the later are formed by the deprotonated groups. This difference
is dictated by the bis(chelating) mode of the ligands in which
the Ni–N(Py) bonds appear to be shared by two condensed
five-membered chelate rings. A similar effect was recently
observed in a Ni2ϩ anionic complex with the tridentate ligand
2,6-bis[N-(phenyl)carbamoyl]pyridine having the same binding
mode as H2PyDHA and very close geometrical parameters of
the co-ordination sphere.18 It is noteworthy that the Ni–N(Py)
bond lengths are significantly shorter than the typical values
observed in the complexes where this bond is not constrained
(ca. 2.10–2.20 Å).19 In contrast, the Ni–N(hydroxamate)
bonds are markedly longer (by more than 0.2 Å) than those
observed in the reported square-planar Ni2ϩ complexes with an
N-coordinated hydroxamic function.6,7,20 Note, that 1 repre-
sents the first example of an octahedral Ni2ϩ complex with
N-coordinated hydroxamate. The values of the bite angles are
significantly decreased to 75.93(6)–76.50(6)Њ, while the trans-
angles between the Ni–N(hydroxamate) bonds are decreased to
151.96(6) and 152.64(6)Њ. All four five-membered chelate rings
indicate a distorted envelope conformation with quite different
values of deviation of the nickel ion from the mean planes
defined by the four other atoms forming the chelate rings [by
0.054(3)–0.392(3) Å].
It is interesting to note that the complex anion contains four
free (O,O)-bidentate pockets available for extra metal ion chel-
ation formed by non-coordinated hydroxamic oxygen atoms.
This fact makes it possible to consider the regarded complex
as a suitable building block for polynuclear assemblies and
possibly also as an efficient chelating agent for metal ions
preferring an oxygen environment and having a high affinity
for hydroxamic groups (e.g. Fe3ϩ, VO2ϩ). The protonated
hydroxamic oxygens form intramolecular contacts with the
adjacent carbonyl oxygen atoms which can be interpreted as
weak H-bonds [the O ؒ ؒ ؒ O separations are 2.586(2)–2.678(2)
Å, the angles near H atoms are 118(2)–130(2)Њ]. In the coordin-
ated hydroxamic groups the C–O, N–O and C–N distances are
indicative of the hydroxamic rather than the oximic form.6,7
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