Surface binding vs. sequestration; The uptake of benzohydroxamic acid at iron(III) oxide surfaces

Article abstract of DOI:10.1039/b808805e

Benzohydroxamic acid is shown to be an unexpectedly good ligand for iron(iii) oxides, favouring surface attachment to the formation of trisbenzohydroxamato complexes, which are known to have very high thermodynamic stability in solution. The Royal Society

Full text of DOI:10.1039/b808805e

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Surface binding vs. sequestration; the uptake of benzohydroxamic
acid at iron(III) oxide surfacesw
Iria M. Rio-Echevarria,a Fraser J. White,a Euan K. Brechin,a Peter A. Tasker*a
and Steven G. Harris*b
Received (in Cambridge, UK) 27th May 2008, Accepted 10th July 2008
First published as an Advance Article on the web 6th August 2008
DOI: 10.1039/b808805e
Benzohydroxamic acid is shown to be an unexpectedly good
ligand for iron(III) oxides, favouring surface attachment to the
formation of trisbenzohydroxamato complexes, which are
known to have very high thermodynamic stability in solution.
The extraordinary stability of tris(hydroxamato)iron(III) com-
plexes has been well documented1,2 and underpins the mode of
operation of iron transport in vivo by siderophores such as
desferrioxamine B.1 These siderophores are produced by
microorganisms which are very effective in solubilising iron(III)
in the environment.2–5 Consequently, it is surprising that
hydroxamic acids also find application as ‘‘collectors’’ in
extractive metallurgy, binding to the surface of iron-contain-
ing minerals and rendering them sufficiently hydrophobic to be
recovered in froth flotation processes.6–9 It would be assumed
that dissolution processes such as
Fe2O3(s) + 6LH " 2FeL3 + 3H2O
Fig. 1 Uptake of benzohydroxamic acid, L1H, (’) and 3-(4-methyl-
benzoyl)propionic acid, L2H, (m) on high surface area goethite
would be driven by the very high stability of the tris-hydro-
xamate complexes, FeL3, in solution and that formation of
(22.5 m2 gꢀ1) from 95% methanol–water.
surface complexes would be unfavourable.
As part of a wider programme to develop ligands to
of the removal of the free ligand to form iron(III) complexes in
solution. The binding constant derived from the isotherm
‘‘engineer’’ the properties of metal oxide surfaces we have
considered the development of ‘‘co-collectors’’ which would
shows that benzohydroxamic acid (L1H) binds more strongly
to the surface of goethite than Irgacor 419s, 3-(4-methylben-
operate in froth flotation processes for sulfide ores to enhance
zoyl)propionic acid, a waterborne corrosion inhibitor for mild
the recovery of ores that have undergone oxidation and
present surfaces with significant areas of iron(III) oxides.10
steel. The efficacy of the latter has been ascribed to its ability to
show ‘‘multisite attachment’’ to iron(III) oxides, models for
In order to identify candidate co-collectors, we assumed that
which are provided by X-ray crystal structures of polynuclear
they should have ligating groups with a high affinity for
iron(III) oxides, and a screening programme was set up using
complexes such as [Fe11O6(OH)6(L2)15].11
adsorption isotherm measurementsz to monitor binding
strengths to high surface area goethite. The uptake of benzo-
hydroxamic acid from methanol–water (Fig. 1) was followed
by the reduction of intensity of the band at 224 nm in its
electronic spectrum. As no iron could be detected in the
supernatant solutions by ICP-OES analysis, we can be con-
fident that the steep slope of the isotherm is not a consequence
a University of Edinburgh, School of Chemistry, West Mains Road,
Edinburgh, UK. E-mail: peter.tasker@ed.ac.uk;
Fax: (+44) 131 650 6453; Tel: (+44) 131 650 4706
b Infineum UK Ltd, PO Box 1, Milton Hill, Abingdon, Oxon,
There are no comparable structures of polynuclear iron(III)
complexes of hydroxamic acids recorded in the CSD, although
it has been shown that a deprotonated form of the NOH
group can form bridged copper and manganese complexes.12
Our initial attempts to prepare FeIII complexes of
OX13 6BB, UK. E-mail: steve.harris@infineum.com;
Fax: (+44) 1235 46 9371; Tel: (+44) 1235 46 9368
w Electronic supplementary information (ESI) available: Ligand
syntheses for L3 and L4. CCDC reference numbers 689505 and
689506. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b808805e
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benzohydroxamic acid resulted in mononuclear complexes
[Fe(L1)3].y Both the mer- and fac- forms were present in
crystals of [Fe(L1)3]ꢂ1.5MeOH. The geometries of their coor-
dination spheres are similar and correspond closely to those of
reported trishydroxamatoiron(III) structures.13 A dinuclear
complex,z [Fe2(m2-L1)2(L1)2Br2],y separated as almost black
crystals from the recrystallization of a purple product arising
from the reaction of FeBr2, L1H and benzoic acid.y In this
centrosymmetric complex (Fig. 2) the bridging benzo-
hydroxamate ligands form bonds of approximately equal
lengths from their oximato oxygen atom, O1, to the two iron
atoms, and formation of these bridges does not greatly change
the geometry of the chelate ring.8
The implication that a dinucleating/bridging mode such as that
in [Fe2(m2-L1)2(L1)2Br2] can be provided by a relatively strain-free
form of a benzohydroxamate ligand when complexed to an oxide
surface was tested by modelling the docking of the Fe2(m2-L1)
motif onto iron(III) oxy/hydroxides. The (110) surface of lepido-
crocite was chosen initially for these studies on the grounds that it
contains Fe2O2 units with similar geometry (Feꢂ ꢂ ꢂFe: 3.060 and
Oꢂ ꢂ ꢂO: 2.740 A, O–Fe–O: 83.5 and Fe–O–Fe: 91.61) to those in a
range of other iron(III) oxide/hydroxides and is one of the
dominant faces of crystals of the mineral.
In Fig. 3 the bridging benzohydroxamate unit in the crystal
structure of [Fe2(m2-L1)2(L1)2Br2] has been used to replace a
terminal and an adjacent m2-bridging hydroxyl group on the
Fig. 3 Two unit cells in the a direction showing the topmost layer of
oxide with two benzohydroxamate molecules per unit cell. In order to
bind, the hydroxamate displaces both a terminal hydroxide and a m2
hydroxide. In order to preserve charge balance, the m3 hydroxides on the
right hand side of the binding site are deprotonated, leaving m3 oxides.
surface. The geometry of the ligand was then optimised using
the UFF14 with partial atomic charges derived by charge
equilibration,15 whilst keeping the iron and the other oxygen
atom positions in the substrate fixed. Bond lengths and angles
associated with the ligand in the energy-minimised structures
of the surface complex compare well with those in [Fe2(m2-
L1)2(L1)2Br2]. The minimized energy of the chelate unit in
the surface structure is higher (16 kJ molꢀ1) than that in the
dinuclear complex, but this might be expected because the
surface structure was not allowed to relax from that of bulk
lepidocrocite to meet the requirements of the hydroxamate
ligand. The surface area required per ligand in the model,
38.25 A2, is significantly smaller than that observed for
saturation in the isotherm, 240(12) A2. A similar difference
(45 vs. 238(15) A2) was observed in modelling the uptake of the
commercial corrosion inhibitor L2H on goethite for which a
different surface binding motif is proposed.16 In both cases this
most probably arises because only ca. 18% of the (110) type
binding sites are accessible in the goethite used in the isotherm
determination. There are no significant intermolecular con-
tacts between benzohydroxamate ligands in adjacent sites in
the model shown in Fig. 3. However, if an N-methyl sub-
stituent is introduced, as in L3H, this is would be expected to
lead to steric interactions between ligands. The isotherm for
this ligand showed both weaker binding and reduced surface
coverage. Replacing the hydroxyl group in L1H with a meth-
oxy group (L4H) will prevent the formation of hydroxamato
Fig. 2 The structure of [Fe2(m2-L1)2(L1)2Br2]. Coordination sphere
bond lengths are: Fe1–O1: 2.087(2); Fe1–O1A: 2.093(1); Fe1–O2:
2.015(1); Fe1–O11: 1.955(1); Fe1–O12: 2.017(1) and Fe1–Br1:
2.4448(4) A. Contact distances and angles in the central Fe2O2 unit
are: Fe1ꢂ ꢂ ꢂFe1A: 3.2978(6) and O1ꢂ ꢂ ꢂO1A: 2.569(3) A, and
Fe1–O1–Fe1A: 104.16(6) and O1–Fe1–O1A: 75.84(6)1.
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bridges between FeIII atoms and, as expected, no uptake of
L4H onto goethite was detected.
group C2/c (no. 15), Z = 4, 43 236 reflections measured, 4680 unique
(Rint = 0.0422) which were used in all calculations. The final wR(F2)
was 0.0338 (3511 data). Crystal data for [Fe(L1)3]ꢂ1.5MeOH.
C45H48N6O15Fe2, M = 1024.59, monoclinic, a = 20.2477(8), b =
11.3483(4), c = 22.4035(9) A, b = 115.874(2)1, V = 4631.8(3) A3,
T = 150 K, space group P21/c (no. 14), Z = 4, 62 262 reflections
measured, 10 127 unique (Rint = 0.0442) which were used in all
calculations. The final wR(F2) was 0.0371 (8129 data).
8 The bite angle, O1–Fe–O2; 76.74(5)1 and the carbonylato oxygen-
to-iron bond length, O2–Fe; 2.0151(13) A, are similar to those in the
non-bridging chelate ring and to the mean values in the mononuclear
complexes [Fe(L1)3], 79.69(5)1 and 2.0167(14) A, and 78.671 and
2.037 A, respectively.
Despite the well defined propensity to form very stable
mononuclear iron(III) complexes, it is clear that simple hydro-
xamic acids can also bind sufficiently strongly to the surfaces
of iron(III) oxides that dissolution of iron to tris-hydroximato
complexes is not the favoured reaction. This observation has
relevance to the modes of action of hydroxamate-containing
siderophores in their role of scavenging for iron from solid
materials and also for the stabilisation of colloidal iron(III)
oxyhydroxides. It is also possible that the efficacy of benzo-
hydroxamic acid as a collector in flotation processes could
depend on the formation of very stable complexes with
oxidized pyrite on the surface of sulfide minerals.
1 C. J. Marmion, D. Griffith and K. B. Nolan, Eur. J. Inorg. Chem.,
2004, 3003–3016.
2 R. J. Bergeron, J. Wiegand, J. S. McManis, W. R. Weimar and G.
Huang, in Adv. Exp. Med. Biol., CRC, Boca Raton, 2002,
pp. 167–184.
3 K. J. Wallace, M. Gray, Z. Zhong, V. M. Lynch and E. V. Anslyn,
Dalton Trans., 2005, 2436–2441.
4 W. R. Harris, C. J. Carrano, S. R. Cooper, S. R. Sofen, A. E.
Avdeef, J. V. McArdle and K. N. Raymond, J. Am. Chem. Soc.,
1979, 101, 6097–6104.
We thank the EPSRC and Rio Tinto and Infineum
for CASE studentships, EaStCHEM for funding and
Dr D. Nagaraj (Cytec Industries) and Drs Chris Cross and
Lucy Esdaile (Rio Tinto) for very helpful discussions.
5 M. Gaspar, R. Grazina, A. Bodor, E. Farkas and M. A. Santos,
J. Chem. Soc., Dalton Trans., 1999, 799–806.
Notes and references
6 D. W. Fuerstenau, Proceedings of the 19th International Mineral
Processing Congress, San Francisco, 1995, vol. 3, pp. 3–17.
7 D. R. Nagaraj, Trans. Indian Inst. Met., 1997, 50, 355–363.
8 Mining Chemicals Handbook, Cytec Industries Inc., New Jersey,
2002.
z Different concentrations of each benzohydroxamic acid in metha-
nol–water (10 ml, 95 : 5 v/v) were added to accurately weighed samples
(ca. 0.40 g) of high surface area (22.5 m2 ꢀ1) goethite in polycarbo-
g
nate centrifuge tubes. The suspensions were stirred for 2 h at 25 1C
and then centrifuged. Aliquots of the supernatant solutions were
filtered through glass microfibre filter paper, which was then washed
(3 ꢃ 0.5 ml) with methanol–water (95 : 5 v/v) and the combined filtrate
and washings made up to volume. For L1 the absorbance at 224 nm,
and for L3 and L4 the sulfur-content measured by ICP-OES, were used
to determine the concentration of ligand remaining in solution. For
L1 the adsorption constant and surface coverage are 16(3) ꢃ 103 and
15.5(8) ꢃ 10ꢀ6 mol gꢀ1. A double Langmuir equation was needed to fit
isotherm data for L3.
9 K. K. Das and Pradip, Process Technol. Proc., 1988, 7,
305–316.
10 I. M. Rio Echevarria, PhD Thesis, University of Edinburgh,
2007.
11 M. Frey, S. G. Harris, J. M. Holmes, D. A. Nation, S. Parsons,
P. A. Tasker, S. J. Teat and R. E. P. Winpenny, Angew. Chem.,
Int. Ed., 1998, 37, 3245–3248.
12 CSD Copper Refcodes: LOPBAT; MEMDUD; MEMFAL;
MIHMOF; QOFJOK; QOFJUQ; REXTIY; SUGMIQ; VIJRUB;
WUTGIX; WUTCOD; YELTAK; YELTEO; YELTIS;
YELTOY. Manganese Refcodes: FILFOW; FILGAJ; FIPTAA;
HUHPIJ; IDUYOV; IDUYUB; SEDBOS; UMIQUC; YOBYUJ;
YOBZEU; YOBZEW.
13 CSD Refcodes: BURDIB; FEBOAH; FEBOAH01; FEBOAH02;
SISSAO; SISSES; SUXREI.
14 A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard-III and
W. M. Skiff, J. Am. Chem. Soc., 1992, 114, 10024–10035.
15 A. K. Rappe and W. A. Goddard-III, J. Phys. Chem., 1991, 95,
3358–3363.
16 M. Frey, S. G. Harris, J. M. Holmes, D. A. Nation, S. Parsons,
P. A. Tasker and R. E. Winpenny, Chem.–Eur. J., 2000, 6,
1407–1415.
y The complex [Fe2(m2-L1)2(L1)2Br2] was synthesised by dissolving in
methanol equimolar amounts of FeBr2 (0.300 g, 1.39 mmol),
NaOOCPh (0.200 g, 1.39 mmol) and benzohydroxamic acid (0.191 g,
1.39 mmol). The resulting purple solution was stirred overnight at
room temperature, after which a purple precipitate was obtained and
removed by filtration. Layering of the filtrate with diethyl ether gave
black crystals of [Fe2(m2-L1)2(L1)2Br2] after 4 weeks. Yield 5%.
(Found: C, 41.26; H, 3.04; N, 6.78. Calc. for C28H24Br2Fe2N4O8: C,
41.21; H, 2.96; N, 6.87). nmax/cmꢀ1 1596 (CQO), 555 (Fe–O).
z Crystal data for [Fe2(l2-L1)2(L1)2Br2]. C28H24Br2N4O8Fe8, M =
816.01, monoclinic,
a = 14.5538(9), b = 18.2543(10), c =
12.9684(8) A, b = 91.814(4)1, V = 3066.3(3) A3, T = 150 K, space
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4572 | Chem. Commun., 2008, 4570–4572