Binding and photodissociation of CO in iron(ii) complexes for application in positron emission tomography (PET) radiolabelling

Article abstract of DOI:10.1039/c0dt01614d

(R-DAB)FeI₂ complexes containing bidentate diimide ligands (R-DAB = RNCH-CHNR; R = ⁱPr, c-C₆H₁₁) have been investigated for their ability to react with carbon monoxide to form iron(ii) dicarbonyl complexes, (R-D

Full text of DOI:10.1039/c0dt01614d

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PAPER
Binding and photodissociation of CO in iron(II) complexes for application in
positron emission tomography (PET) radiolabelling†
Chloe R. Child,a,b Steven Kealey,b Harriet Jones,c Philip W. Miller,a Andrew J. P. White,a Anthony D. Geed and
Nicholas J. Long*a
Received 19th November 2010, Accepted 22nd December 2010
DOI: 10.1039/c0dt01614d
i
(R-DAB)FeI2 complexes containing bidentate diimide ligands (R-DAB = RN CH–CH NR; R = Pr,
c-C6H11) have been investigated for their ability to react with carbon monoxide to form iron(II)
dicarbonyl complexes, (R-DAB)FeI2(CO)2. Solution IR spectroscopy revealed two nCO stretches
between 2000 and 2040 cm-1 corresponding to a cis-arrangement of the carbonyl ligands around the
iron. Photochemical decarbonylation was achieved by UV irradiation (365 nm), which occurred within
5 min as evidenced by solution IR spectroscopy. (c-C6H11-DAB)FeI2 has been characterised by X-ray
crystallography. Reactions using 11C-labelled carbon monoxide were investigated and revealed that both
(R-DAB)FeI2 species were not effective as trapping complexes due to the low concentrations of [11C]CO
used in these experiments. A Fe(TPP)(THF)x (TPP = tetraphenylporphyrin) complex was investigated
with unlabelled CO and the monocarbonyl adduct Fe(TPP)(THF)CO was formed in situ as identified
by IR spectroscopy (nCO = 1966 cm-1) yet was stable to CO loss upon UV irradiation. Carbonylation
reactions of in situ-generated Fe(TPP)(THF)x using [11C]CO revealed that 97% of the [11C]CO stream
could be trapped in one pass of the gas at room temperature and at atmospheric pressure.
two precursor moieties to form a labelled molecule containing a
[11C]carbonyl core. A common example is the aminocarbonylation
Introduction
reaction between an amine and an aryl halide with [11C]CO to
form a [11C-carbonyl]amide. Despite its great potential for tracer
synthesis, the widespread use of [11C]CO in PET has been hindered
by its poor solubility, leading to low radiochemical yields under
standard conditions. This has led researchers to examine methods
of increasing its reactivity through the use of high pressure
autoclave reactors,7,8 or by improving solubility at low pressures
through chemical complexation in solution for subsequent release
using heat9 or ligand addition.10
In order to extend this solution chemistry, we decided to
examine the potential of iron complexes to trap CO in solution
for subsequent mild (low temperature, competing ligand-free) CO
release via UV irradiation. Iron(II) complexes, were particularly
appealing due to the high abundance, low cost and biological and
environmental compatibility of iron.11 Here, we report the reactiv-
ity of iron(II) complexes containing bidentate, 1,4-diazabuta-1,3-
diene (R-DAB), and tetradentate, tetraphenylporphyrin (TPP),
N-donor ligands towards [12C]CO and [11C]CO and examine their
potential for rapid decarbonylation via UV photolysis.
Positron emission tomography, PET, is a non-invasive technique
that is used to image biological processes such as metabolic
function and receptor distribution in vivo.1 While most frequently
used for oncology,2 PET is increasingly being used by the
pharmaceutical industry in drug development,3,4 offering the
potential to visualise receptor sites and drug affinity, aiding in
drug dosage calculations and determining pharmaceutical effects
at a molecular level.5
Radiolabelling with 11C is particularly appealing due to the
prevalence of carbon in natural products and pharmaceuticals.
11C-radiolabelling is mostly based around [11C]methylation reac-
tions using [11C]methyl iodide, however, this approach is limited
to labelling the periphery of the molecule (e.g. methylation of an
amino group). In the last 15 years, [11C]carbon monoxide ([11C]CO)
has emerged as an important labelling reagent for transition metal-
mediated carbonylation reactions.6 In this way [11C]CO can couple
aDepartment of Chemistry, Imperial College London, London, UK, SW7
2AZ. E-mail: n.long@imperial.ac.uk
bGlaxoSmithKline Clinical Imaging Centre, Hammersmith Hospital, Lon-
don, UK, W12 0NN
Discussion
cThe Abbey School, Reading, UK, RG1 5DZ
dKing’s College London, Rayne Institute, St Thomas’ Hospital, London, UK,
SE1 7EH
1,4-Diazabuta-1,3-dienes (R-DAB) ligands, form a useful class
of bidentate a-diimine ligands which are readily coordinated
to iron(II) halides to form four coordinate complexes.12,13 The
ligands can be readily prepared in high yield by condensation
† Electronic supplementary information (ESI) available. CCDC reference
number 725949. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0dt01614d
6210 | Dalton Trans., 2011, 40, 6210–6215
This journal is
The Royal Society of Chemistry 2011
©

◦
of glyoxal with two equivalents of the appropriate amine.14 In
1999 Spek et al.15 reported that (iPr-DAB)FeI2 could be reversibly
formed by thermal or photochemical decarbonylation of (iPr-
DAB)FeI2(CO)2 which was itself formed by oxidative addition of
iodine to the stable zero-valent iron complex (iPr-DAB)Fe(CO)3.15
We sought to explore this system for its potential use in [11C]CO
trapping experiments in the radiochemical laboratory.
˚
Table 1 Selected bond lengths (A) and angles ( ) for (c-C6H11-DAB)FeI2
Fe–I
2.5662(7)
2.5662(7)
1.263(9)
Fe–N(1)
2.102(5)
2.102(5)
Fe–I¢
Fe–N(1¢)
C(1)–C(1¢)
I–Fe–I¢
N(1)–Fe–I¢
I¢–Fe–N(1¢)¢
N(1)–C(1)
I–Fe–N(1)
I–Fe–N(1¢)
N(1)–Fe–N(1¢)
1.473(12)
117.05(5)
112.28(13)
115.31(13)
115.31(13)
112.28(13)
78.9(3)
Our initial studies found that the corresponding chloro-
substituted complexes, (R-DAB)FeCl2, were unreactive towards
CO. This difference in reactivity may result from small variations in
electron density. We suggest that the chloro complex has, through
greater electronegativity, insufficient electron density available to
stabilise the Fe–CO bond by back-bonding, whereas iodo ligands
are generally better p-donors thus resulting in a more electron-
rich iron centre. As such, we sought a new synthetic approach
to the iodo-substituted species (R-DAB)FeI2 that did not involve
manipulation of the tricarbonyl species, for potential application
in rapid [11C]CO trapping. The direct reaction of FeI2 with the R-
DAB ligands was found to produce undesired side reactions with
no traces of the desired product. However, it was found that the
iodo complexes could be synthesised in situ by ligand exchange
of the corresponding stable (R-DAB)FeCl2 complex using sodium
iodide (Scheme 1).
Fig. 1 The molecular structure of (c-C6H11-DAB)FeI2 (atoms flagged with
a prime (¢) character are at equivalent position: 3/2 - x, y, 1 - z).
(iPr-DAB)FeI2,15 (c-C6H11-DAB)FeI2 displays crystallographic C2
symmetry about an axis that passes through the iron centre and
bisects the C(1)–C(1¢) bond. Bond distances and angles are shown
in Table 1.
i
Scheme 1 Synthesis of (R-DAB)iron(II) dihalide complexes (R = Pr,
c-C6H11).
Both cyclohexyl and isopropyl R groups were chosen as their
moderate steric bulk was hoped to provide a degree of stabilisation
to the coordinated iron centre whilst not blocking the approach
of the CO molecules. The intermediate (R-DAB)FeCl2 complexes
were isolated as purple solids in moderate yield by 1 : 1 reaction
of the corresponding R-DAB ligand with anhydrous iron(II)
chloride in dichloromethane. As tetrahedral 14e- complexes, their
resultant paramagnetism caused significant line broadening in
Trap and release of [12C]carbon monoxide
Carbonylation and photolysis reactions on solutions of (R-
DAB)FeI2 (generated in dichloromethane solution in situ from (R-
DAB)FeCl2) were performed in a Pyrex immersion well apparatus
using a Philips HPK 125, 400 W high-pressure mercury lamp
irradiating at 365 nm (see Fig. 2). During carbonylation of the
complex, CO was bubbled through the solution for 60 min.
Solution IR spectra were recorded at regular intervals using
aliquots of reaction solution which were removed from the
apparatus by syringe.
1
their H NMR spectra. Mass spectrometry was used to confirm
the presence of the parent complexes and elemental analysis
provided an excellent match to the calculated values for both (iPr-
DAB)FeCl2 and (c-C6H11-DAB)FeCl2. The corresponding iodo-
substituted complexes (iPr-DAB)FeI2 and (c-C6H11-DAB)FeI2 were
formed by addition of two equivalents of sodium iodide to the
chloro-complexes and stirring overnight, in each case the purple
solution becoming dark green. Filtration followed by removal
of the solvent gave the iodo-species as dark green solids. It
was not possible to obtain an analytically pure sample of (iPr-
DAB)FeI2, although this compound has been isolated previously
and structurally characterised.15
Crystals of the cyclohexyl-substituted species, (c-C6H11-
DAB)FeI2, were grown by slow diffusion of hexane into a
dichloromethane solution and analysed by X-ray crystallography
(Fig. 1). The structure reveals that the iron centre is coordinated
tetrahedrally to two iodo ligands and the two N-donors of the
DAB ligand. As was seen for the previously reported structure of
During the carbonylation of the (R-DAB)FeI2 complexes (R =
i
c-C6H11, Pr), in each case the solution changed from dark green
to dark pink in colour (Fig. 2, RHS) and IR spectroscopy
(Fig. 3) indicated the coordination of CO to the complex via
the appearance of carbonyl peaks at around 2000 and 2040
cm-1. These two peaks suggest that two carbonyl groups are
bound to each iron centre in a cis-arrangement, thus forming
hexacoordinate complexes of the formula (R-DAB)FeI2(CO)2.
Previously reported15 data for (iPr-DAB)FeI2(CO)2 shows two
nCO absorptions at 2400 and 2000 cm-1, in agreement with our
findings.
Following the formation of both (R-DAB)FeI2(CO)2 complexes,
photodissociation reactions were investigated by irradiating each
solution with UV light (365 nm) for 5 min while a continuous
stream of nitrogen gas was bubbled through the solution. This
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6210–6215 | 6211
©

spectra performed on an aliquot of solution removed from the
vessel showed complete removal of the carbonyl stretches for
both species. In order to examine the potential reversibility of
this process, the irradiation was stopped and CO was once
more bubbled through the solutions for 30 min. As before, the
conversion from dark green solution to dark pink solution was
observed along with the appearance of strong bands in the IR
spectra at 2040 and 2000 cm-1 (Fig. 3). This indicates that the
process is indeed reversible as shown in Scheme 2. In addition to
the CO-trap/UV-release reactions, a pair of control reactions were
performed to observe the effect of bubbling N2 gas through the
newly formed (R-DAB)FeI2(CO)2 solutions in the absence of UV
light. For both species, it was found that after 12 h of N2 bubbling
there was no significant reduction in the intensity of the carbonyl
bands in the IR spectra.
Fig. 2 Schematic and photograph of the UV apparatus used in the
carbonylation of (R-DAB)FeI2.
Scheme 2 Carbonylation and photolysis reaction of (R-DAB)FeI2.
In addition to the study of the CO binding to (R-DAB)FeI2
complexes, a synthetic heme analogue consisting of tetraphenyl-
porphyrin (TPP) coordinated to iron(II) was similarly investigated.
Since the Nobel prize-winning determination of the structures of
hemoglobin and myoglobin in the 1960s by Perutz and Kendrew,
considerable work has been directed towards the synthesis of
caused the dark pink solutions to revert back to their original
dark green colour, indicating reformation of the carbonyl-free
species. UV-vis spectra were not collected as the IR data was
more informative, but similar observations were noted to the
analogous compounds reported in reference 15. Solution IR
Fig. 3 IR spectra showing A) the trap and release of [12C]CO to (iPr-DAB)FeI2 and B) the trapping of [12C]CO by (c-C6H11-DAB)FeI2.
6212 | Dalton Trans., 2011, 40, 6210–6215 This journal is The Royal Society of Chemistry 2011
©

synthetic heme analogues as models to study dioxygen and carbon
monoxide binding to heme proteins.16 The well-established binding
of CO to the iron centre in heme units provided an ideal system
for application in the rapid binding of [11C]CO for potential
radiolabelling experiments. Furthermore, photodissociation of
CO from iron(II) porphyrins is well known, with flash photolysis
having being used to measure the kinetics of O2 and CO binding.16
The tetraphenylporphyrin iron(II) complex, (TPP)Fe(THF)x
was synthesised in situ by the reduction of tetraphenylporphyrin
iron(III) chloride in THF giving the characteristic deep red
solution. CO was bubbled through this solution following the
same procedure as for the (R-DAB)FeI2, complexes and solution
IR spectra were recorded periodically. Formation of the carbonyl
adduct was confirmed by solution IR spectroscopy which showed
a CO stretch appearing at 1966 cm-1 within 1 min of CO bubbling,
while prolonged bubbling did not lead to a significant increase in
the intensity of this band (Fig. 4). This CO stretch falls within
the typical region for a six coordinate, low-spin Fe(II)(porphyrin)
complex,17 and is in reasonable agreement with that previously
reported for (TPP)FeCO (1973 cm-1)18 and excellent agreement
with the low temperature IR spectrum of (TPP)Fe(THF)CO
reported by Kurtikyan et al.19 Formation of the dicarbonyl species
would be expected to lead to an increase in nCO to around
2040 cm-1 as a result of competitive p-back bonding between the
trans CO groups and no such absorption was observed during
our experiments. These observations led us to conclude that the
monocarbonyl adduct (TPP)FeCO(THF) has been formed in our
experiments.
As for the (R-DAB)FeI2 complexes, photodissociation experi-
ments were attempted by irradiating the solution with UV light
at 365 nm. This did not lead to any noticeable reduction in the
intensity of the CO stretch in the solution IR spectrum despite
prolonged (1 h) irradiation. It is thought that this phenomenon
might be caused by rapid recombination of CO to the complex
despite the presence of the purging nitrogen gas stream.17 Alterna-
tively, it may indicate the need for an irradiating light source of a
different wavelength and this will be the subject of future studies.
[11C]CO trapping experiments
Having observed rapid CO uptake by both the bidentate and
tetradentate iron(II) complexes, using unlabelled CO, the uptake
of 11C-labelled carbon monoxide, [11C]CO was investigated in
the radiochemical laboratory. The conditions for radiolabelling
experiments vary significantly from those used with unlabelled
CO due to the small amounts of 11C generated by the cyclotron.
In a typical experiment it is estimated that nanomolar quantities
of [11C]CO are produced in the radiochemical laboratory, with
the gas being delivered to the reaction solution diluted in a
helium carrier gas stream. In our experiments the gas stream
was delivered via a needle to a glass sample vial containing the
iron(II) complex in THF (1 mL of a 11 mM solution). For each
experiment the [11C]CO/He gas stream was bubbled through the
solution at a flow rate of 20 mL min-1 and the waste gases, having
passed through the vial, were collected in a gas bag situated in a
dose calibrator providing real-time radioactivity measurements
of the untrapped gases. Following delivery of the [11C]CO to
the vial, the radioactivity of the sample vial was measured.
From this the trapping efficiency was calculated as the fraction
of the radioactivity of the sample vial over the total amount
of [11C]CO delivered to the system (vial radioactivity + waste
gas radioactivity). The trapping efficiencies of (iPr-DAB)FeI2, (c-
C6H11-DAB)FeI2 and (TPP)Fe(THF)x from these experiments are
shown in Table 2.
As can be seen, the trapping efficiencies of the (R-DAB)FeI2
complexes were very low compared with (TPP)Fe(THF)x. The (R-
DAB)FeI2 trapping efficiencies are comparable to that found in
the solvent alone, indicating that the iron(II) complexes do not
contribute significantly to CO binding. This is in contrast to the
rapid CO binding observed in the corresponding reactions using
unlabelled CO, and is attributed to the reversal in stoichiometries
that occurs when switching to the radiochemical laboratory. In
the latter, there is an excess of iron(II) complex relative to the
[11C]CO meaning that formation of the dicarbonyl species (R-
DAB)FeI2 is statistically highly unlikely. Since the monocarbonyl
species has not been isolated or observed in our experiments, it is
assumed that this species is energetically unstable and hence does
Table 2 [11C]CO Trapping efficiencies of Fe(II) solutions
Trapping efficiency/%a
(iPr-DAB)FeI2
(c-C6H11-DAB)FeI2
(TPP)Fe(THF)x
6.1 0.2
12.5 0.7
96.9 0.1
a Trapping efficiency calculated as a fraction of [11C]CO trapped in vial
versus total amount of [11C]CO delivered to the vial. Average of two runs.
Fig. 4 IR spectra of (TPP)Fe(THF)x in THF solution upon reaction with
CO.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6210–6215 | 6213
©

not form in these experiments. (TPP)Fe(THF)x, however, is able
to trap the [11C]CO almost quantitatively (97%). This compares
favourably with the existing solution-based trapping media used in
PET radiolabelling experiments. Cu(Tp*) complexes of the same
concentration (11 mM) have shown a 96% trapping efficiency,10
while BH3·THF been reported to trap >90% of the [11C]CO.9
crystals leading to a brown solution with two layers. The top
layer was separated and left to cool forming tan coloured crystals
(2.15 g, 15 mmol, 62%). Mpt.: 53–56 ◦C. IR (nujol): n(C N)
1
1632 cm-1. H NMR (CDCl3) d = 7.94 (s, 2H, –CH N), 3.53
3
[septet, 2H, –CH(Me)2, JHH 6.3 Hz], 1.23 [d, 12H, –CH(CH3)2,
3JHH = 6.3 Hz]. 13C NMR (300 K, 400 MHz, CDCl3): d = 159.7 [s,
–CH N], 61.2 [s, –CH(CH3)2], 23.7 [s, –CH(CH3)2]. UV (THF):
lmax = 222, 277 nm. MS (FAB): m/z = 99 (60%, [C5H9N2]+) 140
(27%, [C8H16N2]+).
Conclusions
(iPr-DAB)FeI2 and (c-C6H11-DAB)FeI2 complexes have been
found to rapidly combine with CO to form dicarbonyl (R-
DAB)FeI2(CO)2 species, with UV triggered decarbonylation be-
ing observed within 5 min as observed by IR spectroscopy.
(TPP)Fe(THF)x also showed rapid CO binding yet decarbony-
lation could not be triggered under our experimental conditions.
Despite their potential for rapid trapping and release of unlabelled
CO, the (R-DAB)FeI2 complexes were not successful at trapping
[11C]CO due to the small quantities of this gas being produced in
PET experiments. (TPP)Fe(THF)x however, provided an excellent
system for rapid [11C]CO capture and we are currently examining
methods of releasing this gas for use in radiolabelling experiments
for the synthesis of PET tracer molecules and biologically-relevant
compounds.
Synthesis of c-C6H11-DAB [compound previously reported by
Kliegman and Barnes14]
Cyclohexylamine (0.92 ml, 8 mmol) in methanol (10 ml) was
cooled to 0 ◦C. Glyoxal (40 wt% in water) (0.27 ml, 4 mmol)
was added to the solution and stirred for 15 min. The solution was
warmed to room temperature and left to stir for 2 h forming a white
crystalline solid which was collected by filtration and recrystallised
from methanol–water (0.27 g, 1.2 mmol, 31%). Mpt.: 151–157 ◦C.
1
IR (nujol): n(C N) 1622 cm-1. H NMR (CDCl3): d = 1–2 [m,
20H, cyclohexyl-H], 3.19 [m, 2H, HC–N], 7.96 [s, 2H, N CH].
13C NMR: d = 24.6 [–CH2–], 25.5 [–CH2], 34.0 [–CH2CH], 69.5
[–CH–N], 160.1 [N CH]. UV (diethyl ether): 232, 280 nm. MS
(FAB): m/z = 55 (100%, [C2H2N2]+), 82 (95%, [C6H10]+), 177 (80%,
[C11H17N2]+) 220 (7%, [C14H24N2]+).
Experimental
Synthesis of (iPr-DAB)FeCl2
All reagents were used as received from commercial suppliers.
HPLC quality solvents were dried with alumina beads and sparged
with nitrogen. All glassware was pre-dried in an oven at 120 C
iPr-DAB (0.70 g, 5 mmol) and iron(II) dichloride (0.63 g, 5 mmol)
were dissolved in dry diethyl ether (50 ml) and stirred for 30 min.
The sample was concentrated to dryness under vacuum and the
resultant solid washed with hexane (2 ¥ 20 ml). Dichloromethane
(50 ml) was added and the solution was filtered and the filtrate
concentrated to dryness under vacuum to give a dark purple solid
◦
overnight prior to use. All manipulations were performed under a
nitrogen atmosphere using standard Schlenk line techniques. All
analytical data were recorded in the Department of Chemistry,
1
Imperial College London, unless otherwise stated. H and 13C
NMR spectra were obtained at room temperature using Bruker
400 MHz spectrometers. Chemical shifts were recorded relative to
internal solvent standards. IR spectra were obtained using a Perkin
Elmer FT-IR Spectrometer Paragon 1000. Photolyses were carried
out in a Pyrex immersion well apparatus using a Philips HPK 125
high-pressure mercury lamp. Mass spectra were recorded using
EI, FAB or electrospray methods. Diffraction data were collected
on an Oxford Diffraction Xcalibur 3 diffractometer using Mo-Ka
radiation. Microanalyses were carried out by Mr. Stephen Boyer
at the London Metropolitan University. [11C]Carbon dioxide
was produced using a Siemens Eclipse HP cyclotron by proton
bombardment of a target containing nitrogen and 1% oxygen.
[11C]CO was produced using an Eckert and Zeigler modular lab
system using a molybdenum reductant (850 ◦C) and delivered to
the trapping solution in a helium stream at a flow rate of 20 mL
min-1. The R-DAB ligands were synthesised via modification of
a procedure reported by Kleigman and Barnes.14 Complexation
of R-DAB ligands with FeCl2 was performed according to the
procedure by Dieck and Dietrichs.20
1
(0.43 g, 1.9 mmol, 32%). H NMR (CDCl3): d = 7.29 [s(br), –
HC N], 3.78 [s(br), N–CH(CH3)2], 1.89 [s(br), N–CH(CH3)2].
MS (FAB): m/z = 231 (10%, [C8H16N2FeCl]+). Anal. (%) calc.
(found) for C8H16Cl2FeN2: C, 36.0 (36.1); H, 6.0 (6.0); N, 10.5
(10.4).
Synthesis of (c-C6H11-DAB)FeCl2
c-C6H11-DAB (1.10 g, 5 mmol) and iron(II) dichloride (0.64 g,
5 mmol) were dissolved in dry dichloromethane (50 ml) and
stirred overnight. Dichloromethane was removed under vacuum
and the precipitate washed with hexane (2 ¥ 20 ml). The
precipitate was dissolved in dichloromethane, the solution was
filtered and the solvent evaporated to dryness to leave a purple
solid (0.53 g, 1.5 mmol, 30%). 1H NMR (CDCl3): d = 5.36 [s(br),
–HC N), 3.80 (s(br), cyclohexyl-CH2], 2.45 [s(br), –N–CH], 1.90
[s(br), cyclohexyl-CH2CH2]. Mass spec (FAB): m/z = 311 (65%,
[C14H24N2FeCl]+). Anal. (%) calc. (found) for C14H24Cl2FeN2: C,
48.4 (48.4); H, 7.0 (6.8); N, 8.1 (8.0).
Synthesis of iPr-DAB [compound previously reported by Kleigman
and Barnes14]
Synthesis of (iPr-DAB)FeI2 [compound previously reported by
Breuer et al.15]
Isopropylamine (4.29 ml, 50 mmol) was cooled to 0 ◦C and glyoxal
(40% wt in water) (2.86 ml, 25 mmol) added dropwise to produce
an off-white solution. This was stirred for 24 h at room temperature
producing white crystals. The solid was heated to dissolve the
(iPr-DAB)FeCl2 (0.10 g, 0.37 mmol) was dissolved in
dichloromethane (50 ml) and sodium iodide (0.11 g, 0.74 mmol)
added. The dark purple solution was stirred overnight turning
dark green and on removal of the solvent it formed a sticky black
6214 | Dalton Trans., 2011, 40, 6210–6215
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The Royal Society of Chemistry 2011
©

1
solid. H NMR (CDCl3): 7.73 [s, N CH], 4.26 [s, –CH(CH3)2],
Synthesis of (TPP)Fe[11C]CO(THF)
1.35 [s, –CH(CH3)2].
Tetraphenylporphyrin iron(III) chloride (38.3 mg, 54.5 mmol)
was dissolved in THF (5 ml) and sodium borohydride
(3.8 mg, 0.1 mmol) was added to form the reduced complex
(TPP)Fe(THF)x in solution. A 5 ml glass vial was flushed with
nitrogen and (TPP)Fe(THF)x (1 ml, 11 mmol of Fe, [Fe] = 11 mM)
added. A [11C]CO/He gas stream was delivered to the vial, with
the waste gases being collected in a bag situated inside a dose
calibrator. Once the radioactivity of the vial had reached a
maximum as judged by a radioactivity sensor placed adjacent
to the vial (pin diode detector), the vial was sealed and its
radioactivity measured in a dose calibrator.
Synthesis of (c-C6H11-DAB)FeI2
(c-C6H11-DAB)FeCl2 (0.10 g, 0.3 mmol) was dissolved in
dichloromethane (50 ml) and sodium iodide (0.09 g, 0.6 mmol)
added. The solution was stirred overnight, filtered and the solvent
removed under vacuum to give a dark green precipitate. Single
crystals suitable for X-ray analysis were grown by slow diffusion
of hexane into a dichloromethane solution. Anal. (%) calc. (found)
for C14H24FeI2N2: C, 31.7 (31.9); H, 4.6 (4.5); N, 5.3 (5.2). Mass
spec (FAB): m/z = 403 (60%, [C14H24N2FeI]+).
Crystal data for (c-C6H11-DAB)FeI2: C14H24FeI2N2, M = 530.00,
monoclinic, I2/a (no. 15), a = 13.0697(4), b = 7.0217(4), c =
20.4319(8) A, b = 98.020(4) , V = 1856.73(11) A , Z = 4 (C2
symmetry), Dc = 1.896 g cm-3, m(Cu-Ka) = 32.535 mm-1, T =
173 K, brown blocks, Oxford Diffraction Xcalibur PX Ultra
Trap and release of carbon monoxide from (R-DAB)FeI2 [based on
method reported by Breuer et al.15]
◦
3
˚
˚
i
(R-DAB)FeCl2 (0.10 g, [R = Pr 0.38 mmol; c-C6H11: 0.29 mmol])
and sodium iodide (0.09 g, 0.60 mmol) was dissolved in
dichloromethane (50 mL) and stirred overnight forming a dark
green solution. The solution was filtered and transferred into the
UV apparatus and dichloromethane (100 mL) added. An excess
of CO was then bubbled through the solution for 60 min causing
the solution to become dark pink in colour. Solution IR spectra
were recorded at regular time intervals using an aliquot of solution
removed from the vessel. (iPr-DAB)FeI2(CO)2: IR (DCM): nCO =
1998, 2039 cm-1. (c-C6H11-DAB)FeI2(CO)2: IR (DCM): nCO =
1997, 2038 cm-1. Following the carbonylation reactions, the CO
stream was stopped and replaced by a nitrogen stream and the
solution irradiated by UV light at 365 nm for 5 min. Solution
IR spectra were recorded for each complex using an aliquot of
solution removed from the vessel and in each case the absence of
nCO indicated that the CO had dissociated.
diffractometer; 1789 independent measured reflections (Rint
=
0.0480), F2 refinement, R1(obs) = 0.062, wR2(all) = 0.158, 1604
independent observed absorption-corrected reflections [|Fo| >
4s(|Fo|), 2qmax = 143◦], 88 parameters. CCDC 725949.
Acknowledgements
We acknowledge the Leverhulme Trust for the award of a Research
Fellowship to NJL, and the Nuffield Foundation for a summer
student bursary to HJ. PWM is grateful to EPSRC for a LSI
Fellowship award (EP/E039278/1).
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i
(R-DAB)FeI2 [R = Pr 2.94 mg; c-C6H11 3.81 mg] (11 mmol, [Fe] =
11 mM) was placed in a 5 ml glass vial and flushed with nitrogen.
Dry THF (1 ml) and sodium iodide was added to form the iodide
complex in solution. A [11C]CO/He gas stream was delivered to
the vial, with the waste gases being collected in a bag situated
inside a dose calibrator. Once the radioactivity of the vial had
reached a maximum as judged by a radioactivity sensor placed
adjacent to the vial (pin diode detector), the vial was sealed and
its radioactivity measured in a dose calibrator.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6210–6215 | 6215
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