C. J. Marmion et al.
FULL PAPER
hydroxamate systems.[14] Precipitation was observed at pH
Ͼ 5.97. The 1:1 complexes give spectra with λmax ≈ 510 nm,
the 1:2 complexes spectra with λmax ≈ 480 nm and the 1:3
complexes spectra with λmax ≈ 425 nm. The UV/Vis spectra
of the FeIIIL2 system are shown in Figure 13 as a represen-
tative example. The spectrophotometric results complement
our potentiometry data and further confirm the formation
of FeL, FeL2 and FeL3 complexes.
at the Department of Chemistry, Trinity College Dublin, Ireland.
The residual undeuterated DMSO signal was used as an internal
reference at δ = 2.505 ppm.
Preparation of the Ligands: These were prepared as reported pre-
viously.[6]
3-Pyridinehydroxamic Acid (3-pyhaH, L1): Hydroxylamine hydro-
chloride (5.07 g, 72 mmol) was added to sodium hydroxide (5.83 g,
146 mmol) in deionised water (37 mL) and the resulting solution
was added dropwise to a solution of methyl nicotinate (5.00 g,
36 mmol) in methanol (55 mL). The solution was stirred at room
temperature for 72 h, after which it was acidified to pH = 5.5 with
5% HCl. The solvent was removed in vacuo to yield a yellow solid.
Methanol (60 mL) was added and sodium chloride was filtered off.
The solvent was removed in vacuo to yield a light pink solid, which
was recrystallised from water. Yield: 3.00 g (22 mmol, 66%).
C6H6N2O2 (138.12): calcd. C 52.17, H 4.38, N 20.28; found C
51.98, H 4.24, N 20.14. 1H NMR [400 MHz, (CD3)2SO, 25 °C]: δ =
11.41 (s, 1 H, OH), 9.24 (br. s, 1 H, NH), 8.90 (d, 4JH,H = 2.00 Hz, 1
3
4
H, H2), 8.70 (dd, JH,H = 5.04, JH,H = 1.52 Hz, 1 H, H4), 8.10
(dt, 3JH,H = 8.00, 4JH,H = 2.00 Hz, 1 H, H6) 7.51 (dd, 3JH,H = 8.04,
4JH,H = 5.04 Hz, 1 H, H5) ppm. IR (KBr disc): ν
= 3196 vs
˜
max
(hydroxamate N–H), 2802 s (hydroxamate O–H), 1660 vs and
1643 vs (hydroxamate C=O), 1594 s, 1557 s (hydroxamate C–N)
cm–1.
Figure 13. UV/Vis spectra of aqueous solutions of FeIII (0.1 m)
and L2 (0.3 m) in KNO3 (0.1 m) at pH = 2.75, 4.11 and 6.07.
4-Pyridinehydroxamic Acid (4-pyhaH, L2): This compound was pre-
pared according to the same method as for 3-pyhaH except ethyl
isonicotinate was used instead of methyl nicotinate. Yield: 55%.
C6H6N2O2 (138.12): calcd. C 52.17, H 4.38, N 20.28; found C
Conclusions
The affinity of hard NiII, ZnII and FeIII centres for hard
oxygen donors facilitates the formation of a series of novel
metal pyridinehydroxamates where the coordination mode
is exclusively hydroxamato-O,OЈ with the ligand chelating
1
51.94, H 4.29, N 20.12. H NMR [400 MHz, (CD3)2SO, 25 °C]: δ
= 11.41 (s, 1 H, OH), 9.37 (br. s, 1 H, NH), 8.70 (d, 3JH,H = 5.6 Hz,
3
2 H, H2/6), 7.68 (d, JH,H = 6.0 Hz, 2 H, H3/5) ppm. IR (KBr
disc): νmax = 3186 vs. (hydroxamate N–H), 2850b (hydroxamate O–
˜
to the metal centre as a monoanionic species. The crystal H), 1642 vs. and 1608 s (hydroxamate C=O), 1551 s, 1531 s (hydrox-
{amate C–N) cm–1.
and molecular structures of two FeIII tris(pyridinehydroxa-
mate) building blocks have been determined and found to
have different packing systems despite the similar nature of
the two complexes. A comparative analysis of the crystal
packing systems allows us to suggest that the different
packing motifs found in 1a and 2a might, at least partly, be
explained by the participation of the pyridine N-atom in
hydrogen bonding to the pyridinehydroxamate ring in 2a,
while in 1a hydrogen bonds are formed only between the
pyridinehydroxamate rings and pyridine N-atoms are in-
volved in hydrogen bonding with water molecules only.
Preparation of Metal Pyridinehydroxamates
[FeIII(3-pyha)3]·6.5H2O (1): 3-pyhaH (150 mg, 1.086 mmol) was
dissolved in tepid deionised water (5 mL) and, after allowing it to
return to room temperature, an aqueous solution (5 mL) of
FeCl3·6H2O (97.8 mg, 0.362 mmol) was added. The pH was ad-
justed to 5.5 with 5% NaOH, which resulted in the precipitation
of a red solid. This was filtered, washed with water (3 mL) and
dried over P2O5. Yield: 141 mg (0.253 mmol, 70%).
C18H28FeN6O12.5 (584.29): calcd. C 37.00, H 4.83, N 14.38, Fe
9.56; found C 37.00, H 2.87, N 14.00, Fe 9.22. IR (KBr disc): ν
˜
max
These novel pyridinehydroxamato complexes of FeIII
,
= 3186 vs (hydroxamato N–H), 1600 vs (hydroxamato C=O),
1549 s, 1518 s (hydroxamato C–N) cm–1. UV/Vis (H2O): λmax
=
NiII and ZnII lend themselves to further investigation as
potential platforms for the design of novel heterodimetallic
supramolecular assemblies.
430 nm (ε = 2200 –1 cm–1, LMCT). Single crystals of [FeIII(3-
pyha)3]·5.125H2O (1a) suitable for an X-ray diffraction study, were
obtained from the filtrate after standing at room temperature for
2 d.
[FeIII(4-pyha)3]·1.5H2O (2): 4-pyhaH (150 mg, 1.086 mmol) was
dissolved in tepid deionised water (5 mL) and, after allowing it to
return to room temperature, an aqueous solution (5 mL) of
FeCl3·6H2O (97.8 mg, 0.362 mmol) was added. The pH was ad-
justed to 5.5 with 5% NaOH, which resulting in the precipitation
of a red solid. This was filtered, washed with water (3 mL) and
dried over P2O5. Yield: 120 mg (0.243 mmol, 67%).
C18H18FeN6O7.5 (494.22): calcd. C 43.74, H 3.67, Fe 11.30, N
17.00; found C 43.98, H 3.38, Fe 11.40, N 16.93. IR (KBr disc):
Experimental Section
Materials and Methods: All reagents, deuteriated solvents and
metal salts were purchased from Sigma Aldrich and used without
further purification. IR spectra were recorded as KBr discs with a
Mattson Genesis II CSI FTIR spectrometer in the 4000–400 cm–1
region. UV/Vis spectra were recorded with a Helios Alpha Thermo
Spectronic Spectrophotometer in a quartz cell. C, H, N, Fe, Ni
and Zn elemental analyses were performed at the Microanalytical
Laboratories, University College Dublin, Ireland. 1H NMR spectra
were recorded with a Bruker Advance DPX 400 FT spectrometer
ν
= 3186 vs (hydroxamato N–H), 1595 vs (hydroxamato C=O),
˜
max
1555 s (hydroxamato C–N) cm–1. UV/Vis (H2O): λmax = 430 nm (ε
1378
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Eur. J. Inorg. Chem. 2007, 1373–1380