Solid-to-solid oxidation of a vanadium(IV) to a vanadium(V) compound: Chemisty of a sulfur-containing siderophore

Article abstract of DOI:10.1021/ic301026b

Visible light facilitates a solid-to-solid photochemical aerobic oxidation of a hunter-green microcrystalline oxidovanadium(IV) compound (1) to form a black powder of cis-dioxidovanadium(V) (2) at ambient temperature. The siderophore ligand pyridine-2,6-bis(thiocarboxylic acid), H₂L, is secreted by a microorganism from the Pseudomonas genus. This irreversible transformation of a metal monooxo to a metal dioxo complex in the solid state in the absence of solvent is unprecedented. It serves as a proof-of-concept reaction for green chemistry occurring in solid matrixes.

Full text of DOI:10.1021/ic301026b

Communication
pubs.acs.org/IC
Solid-to-Solid Oxidation of a Vanadium(IV) to a Vanadium(V)
Compound: Chemisty of a Sulfur-Containing Siderophore
Pabitra B. Chatterjee and Debbie C. Crans*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
S
* Supporting Information
ABSTRACT: Visible light facilitates a solid-to-solid
photochemical aerobic oxidation of a hunter-green micro-
crystalline oxidovanadium(IV) compound (1) to form a
black powder of cis-dioxidovanadium(V) (2) at ambient
temperature. The siderophore ligand pyridine-2,6-bis-
(thiocarboxylic acid), H2L, is secreted by a microorganism
from the Pseudomonas genus. This irreversible trans-
formation of a metal monooxo to a metal dioxo complex
in the solid state in the absence of solvent is
unprecedented. It serves as a proof-of-concept reaction
for green chemistry occurring in solid matrixes.
Figure 1. Structures and colors (hunter green and black) of
compounds 1 (left) and 2 (right) in the solid state.
olid-to-solid transformations have been reported for a
Srange of systems, but the oxidation reactions in solid
matrixes have significantly expanded the versatility of these
reactions.1−5 Most of these solid state reactions occurring
within organic molecules are reversible in nature and are driven
by either light1,6,7 or heat.8,9 Solid-state transformations are
accompanied by numerous changes in chemical and physical
properties such as host−guest behavior,10−13 modification of
the matrix dimensionality,14−16 magnetic communication,17,18
and photochemical rearrangements.7,19−21 In coordination
compounds, structural changes are generally more dramatic
involving many bond-breaking and -forming processes, so that
retention of the product crystallinity becomes a formidable
task.15,19,22,23 Oxidation of a metal oxo species to a metal dioxo
in the solid state by gaseous oxygen is an unprecedented
process approaching a green reaction requiring no solvent or
reagents other than light and air.
Herein we report a photochemical oxidation reaction
occurring in the solid state. Specifically, an oxidovanadium(IV)
compound, [VIVO(L)] (1), containing a sulfur-based side-
rophore, pyridine-2,6-bis(thiocarboxylic acid) (H2L), oxidizes
via solid-to-solid transformation to form a cis-dioxidovanadium-
(V) complex, [VVO2(HL)] (2), in 100% yield (Figure 1). This
reaction is not observed in solution or in the absence of light or
oxygen. We have investigated this oxidation reaction by IR,
electron paramagnetic resonance (EPR), and multinuclear
NMR spectroscopies to document the changes in the metal
oxidation state.
late- and hydroxamate-based, sulfur-containing siderophores are
rare.25 Interest in this natural metabolite increased when it was
shown to be involved in the degradation of CCl4 by P. stutzeri
and hydrolysis of CCl4 to CO2 and HCl by its copper
complex.26,27
The coordination chemistry of H2L was first reported with
iron(2+/3+) showing a tridentate coordination mode with 1:2
metal-to-ligand stoichiometry.28,29 Subsequent studies with
other transition metal ions have shown that mostly divalent
and some trivalent metal ions form complexes with H2L.24,30−33
The seminal contribution by Kruger and Holm demonstrated
̈
that this ligand can stabilize trivalent nickel and can serve as an
excellent model for the coordination chemistry that takes place
in the [NiFe]hydrogenase enzymes.33 Few reports on
vanadium−sulfur chemistry have explored their novel coordi-
nation chemistry;34−44 however, vanadium complexes with a
sulfur-based siderophore, such as H2L, have not yet been
documented.
In the past, H2L has been synthesized following the
procedure of Hildebrand et al.28,29 The use of toxic and
flammable reagents, pyridine and H2S, has limited access to this
ligand.28 We have synthesized the ligand H2L using the more
convenient approach shown in Scheme 1. Upon treatment of
2,6-pyridinedicarboxylic acid (H2dipic) with excess thionyl
chloride, a light-yellow dicarbonyl dichloride, pdcdc, formed.
The pdcdc is then stirred with a saturated aqueous sodium
bisulfide until all of the solid pdcdc dissolved. The ligand H2L is
isolated, in 65−70% yield, by adjusting the pH to 1.8.
Compound 1 (Figure 1) is obtained as a hunter-green
microcrystalline solid by reacting the ligand H2L with
The ligand H2L is a natural product which is produced and
released by several strains of the genus Pseudomonas when
grown in an iron deficient environment.24 At 198 D, it is the
smallest naturally produced extracellular microbial metal
chelator, commonly known as a siderophore.25 While micro-
organisms commonly produce siderophores, mainly catecho-
Received: May 16, 2012
Published: August 10, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Communication
a
reaction, shown in Figure 1, is indicative of the molecular
change. Both light and oxygen are required for the oxidation
reaction to occur, as documented from control experiments
carried out in the dark and/or under an argon atmosphere for
similar and longer time periods. The black solid no longer
exhibits a strong band at 964 cm−1 in the IR spectrum; instead,
a strong twin-band pattern appears at 990 and 927 cm−1,
characteristic of the cis-VO2 moiety.46 The UV−vis spectrum
shows no band corresponding to the d−d transition. These data
are consistent with the oxidation of oxidovanadium(IV) to a
dioxidovanadium(V) compound in the solid state. Moreover,
the appearance of a weak band at 2506 cm−1 attests to the
presence of an SH group in compound 2.
Scheme 1. Synthetic Route to Siderophore H2L
a
Conditions: (a) SOCl2, stir; (b) NaHS, H2O, stir; (c) HCl.
VO(OR)3, where R is either Me, Et, or iPr. Elemental analysis,
thermogravimetric analysis (TGA), and electrospray ionization
mass spectrometry (ESI-MS) support the composition [VIVO-
(L)(H2O)]. A strong band at 964 cm−1 in the IR spectrum of
this compound is assigned to the VO stretching mode of
oxidovanadium(IV).23 The UV−vis spectrum shows a d−d
band as a shoulder at 465 nm. The ESI-MS (positive-ion mode)
of compound 1 in CH3OH shows a peak at m/z 264 ([1-H2O
+ H]+). TGA of 1 shows a weight loss of 6.3% (calcd 6.4%),
which is equivalent to one water per formula unit and thus
supports the molecular composition of 1 being [VIVO(L)-
(H2O)]. The EPR spectrum of compound 1 recorded in a
CH3OH solution at room temperature (Figure S1) displays
eight well resolved lines (51V, I = 7/2) with ⟨g⟩ = 1.997 and ⟨A⟩
= 106 × 10−4 cm−1, which attests to the existence and stability
of the 4+ oxidation state of vanadium.34,45
To further characterize the oxidation state of vanadium in 2,
1
1H NMR spectroscopy was used. The H NMR spectrum of 2
is shown in Figure 3b and documents that this material no
Reacting 2,2′-bipyridyl with compound 1 resulted in a light-
brown compound (3) in moderate yield (ca. 47%). The IR
spectrum of this compound also contains a strong band at 964
cm−1 corresponding to the VO stretching mode of
oxidovanadium(IV).23 The structure of compound 3 was
determined by single-crystal X-ray analysis; see Figure 2. The
Figure 3. 1H NMR spectra of the ligand H2L (bottom, a) and
compound 2 (top, b) acquired in CD2Cl2 and CD3OD, respectively.
The inset c shows the 51V NMR spectrum of 2 in CD3OD. Spectra
were recorded at 25 °C.
longer contains vanadium(IV). The pyridine ring protons
appear at 8.31 and 8.63 ppm, with the couplings, integration,
and chemical shifts of a metal complex of H2L (Figure 3b). The
singlet at 7.90 ppm in Figure 3b is assigned to the SH proton
from the thiocarboxylic acid group. Coordination of the SH
group to the vanadium(V) center shifts this singlet SH proton
downfield from the free ligand H2L signal at 5.72 ppm (Figure
3a). The 51V NMR spectrum (Figure 3c) shows only one signal
at −548 ppm and confirms the presence of vanadium(V) in
compound 2.
Compound 1 is the first metal complex that undergoes solid-
to-solid oxidation in the presence of light and is also unique
with regard to its coordination chemistry exhibited in the solid
state. In complex 2, the coordination number remains the same;
however, the oxidation of vanadium results in conversion of an
aqua group to an oxo group. Two systems have been reported
to undergo solid-to-solid oxidations.2,4,5 However, in these
systems, oxidation takes place after the crystal was ground to a
powder and suspended in a solvent such as DMSO or H2O
containing an oxidant such as I2 or O2.2,4,5 These reactions
provided the impetus for the development of a solid-to-solid
oxidation reaction that takes place even in the absence of
Figure 2. Perspective view and atom-numbering scheme of compound
3.
vanadium is in a distorted octahedral geometry in which the
three donor atoms (S1, S2, and N3) from the ligand L2− and
the N1 atom from the bipyridyl ligand define the basal plane.
The V1−O3 [1.605(3) Å]41,42 and average V−S (2.4313 Å)
distances are in the expected ranges for oxidovanadium(IV)
thio compounds.41,42 The V1−N2 bond length is large
[2.280(4) Å] because of the trans effect of the terminal oxo
group O3.43 Because 3 is obtained by derivatization of 1, we
infer that 1 contains oxidovanadium(IV).
Microcrystals of 1, placed in a petri dish, were allowed to
stand in open air at ambient temperature for about 3 months.
During this period, the initial hunter-green compound
underwent a solid-to-solid oxidation reaction to yield a black
product 2, which is amorphous. The color change during the
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Inorganic Chemistry
Communication
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ASSOCIATED CONTENT
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* Supporting Information
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Synthesis and characterization details, tables of relevant
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the EPR spectrum of compound 1 in CH3OH. This material is
available free of charge via the Internet at http://pubs.acs.org.
CCDC 867913 contains the supplementary crystallographic
data for this paper (compound 3). These data can be obtained
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: debbie.crans@colostate.edu.
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was supported by the National Science Foundation
(Grants CHE-0628260 and CHE-0750079).
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