Telescoping the Synthesis of the [18F]CABS13 Alzheimer's Disease Radiopharmaceutical via Flow Microfluidic Rhenium(I) Complexations

Article abstract of DOI:10.1002/ejic.202000433

The syntheses of rhenium(I) complexes were achieved under flow microfluidic conditions. The use of a single microreactor was applied towards complexation of the 6-chloro-2,2'-bipyridine diimine ligand, with ideal complexation conditions around 170 °C. Subsequent radiolabelling with [¹⁸F]fluoride was further achieved by flowing through a second heated microreactor, alongside a stream of dried radiofluorination media. Temperature modulation across both microreactors resulted in 23.6 % and 37.0 % radiochemical yield (RCY) of [¹⁸F]6-fluoro-2,2'-bipyridine and its associated [¹⁸F]tricarbonyl(2-fluoro-2,2'-bipyridine)rhenium(I) chloride complex, respectively. Translation of this set-up to the synthesis of the [¹⁸F]CABS13 Alzheimer's disease positron emission tomography (PET) imaging agent was achieved with the incorporation of a third microreactor to enable thermal control of the complexation, fluorination and decomplexation pathways. Optimal RCYs of 2.7 % and 1.9 % of [¹⁸F]CABS13 and its rhenium(I) complexation were achieved in-flow, respectively. However, discrepancies in the RCYs were found to arise from differences in the grade of anhydrous dimethyl sulfoxide (DMSO) employed in the continuous-flow reactions. Anhydrous DMSO from Sigma-Aldrich (≤ 99.9 %) in former experiments afforded higher yielders in comparison to replicate experiments employing anhydrous DMSO from Merck Millipore (≤ 99.7 %), thus demonstrating that control of the solvent grade is key to optimizing reaction RCYs.

Full text of DOI:10.1002/ejic.202000433

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
Rhenium Radiofluorination
Telescoping the Synthesis of the [18F]CABS13 Alzheimer's
Disease Radiopharmaceutical via Flow Microfluidic Rhenium(I)
Complexations
Mitchell A. Klenner,*[a,b] Benjamin H. Fraser,[a] Vaughan Moon,[a,c] Brendan J. Evans,[a,c]
Massimiliano Massi,[b] and Giancarlo Pascali*[a,d,e]
Abstract: The syntheses of rhenium(I) complexes were (PET) imaging agent was achieved with the incorporation of a
achieved under flow microfluidic conditions. The use of a single third microreactor to enable thermal control of the complexa-
microreactor was applied towards complexation of the 6- tion, fluorination and decomplexation pathways. Optimal RCYs
chloro-2,2′-bipyridine diimine ligand, with ideal complexation of 2.7 % and 1.9 % of [18F]CABS13 and its rhenium(I) complexa-
conditions around 170 °C. Subsequent radiolabelling with tion were achieved in-flow, respectively. However, discrepancies
[18F]fluoride was further achieved by flowing through a second in the RCYs were found to arise from differences in the grade
heated microreactor, alongside a stream of dried radiofluorina- of anhydrous dimethyl sulfoxide (DMSO) employed in the con-
tion media. Temperature modulation across both microreactors tinuous-flow reactions. Anhydrous DMSO from Sigma-Aldrich (≤
resulted in 23.6 % and 37.0 % radiochemical yield (RCY) of 99.9 %) in former experiments afforded higher yielders in com-
[18F]6-fluoro-2,2′-bipyridine and its associated [18F]tricarbonyl- parison to replicate experiments employing anhydrous DMSO
(2-fluoro-2,2′-bipyridine)rhenium(I) chloride complex, respec- from Merck Millipore (≤ 99.7 %), thus demonstrating that con-
tively. Translation of this set-up to the synthesis of the trol of the solvent grade is key to optimizing reaction RCYs.
[18F]CABS13 Alzheimer's disease positron emission tomography
1. Introduction
payments being made for the long-term health care and hos-
pice services of ≥ 65 year-old patients suffering from dementia
in the same year.[3] Many efforts have hence been undertaken
to elucidate the mechanisms underlying AD and to develop
diagnostic agents for early interventional treatment. Two preva-
lent characteristics of AD, exempting genetic factors, are neuro-
fibrillary tangles (NFTs) formed from the hyperphosphorylation
of tau proteins within the neurons which lead to cytoskeletal
collapse alongside malfunctions in neurotransmission,[4] and
amyloid beta (Aꢀ) plaques which form from entangled oligom-
ers and disrupt synaptic networks.[5] Concerning the latter char-
acteristic, Vasdev, et al. developed a PET radiopharmaceutical,
2-[18F]fluoro-8-hydroxyquinoline ([18F]CABS13), for the in vivo
imaging of Aꢀ plaques.[6] The tracer, [18F]CABS13, takes into
account that Zn, Cu and Fe ions are involved in Aꢀ plaque
deposition and stabilization,[7] and that metal chelating agents
can facilitate dissolution of Aꢀ deposits by preventing metal–
Aꢀ interactions,[8] thereby presenting both therapeutic and di-
agnostic strategies for AD and related dementias. The radio-
pharmaceutical was first administered to an amyloid precursor
protein and presenilin-1 (APP/PSEN1) transgenic mouse model
of AD, which exhibited a significant temporal difference in both
brain uptake and retention compared with the wild-type con-
trol model.[6] PET neuroimaging in non-human primate models
of AD, however, demonstrated comparatively low brain uptake
and rapid in vivo metabolism of the tracer.[9] The vast discrep-
ancies in pharmacokinetic behavior between the species have
prolonged investigations concerning the efficacy of the radio-
Alzheimer's disease (AD) is currently the sixth leading cause of
death in the United States, and the cause of 60–70 % of demen-
tia cases.[1] The rate of cases is projected to increase, with a
2016 report from the Alzheimer's Association claiming that a
new person develops the neurodegenerative disease every 66
seconds.[2] These cases have amounted to an estimated 5.8 mil-
lion Americans living with AD, as of 2019, with $290 billion in
[a] Dr. M. A. Klenner, Dr. B. H. Fraser, V. Moon, B. J. Evans, Prof. G. Pascali
National Deuteration Facility (NDF) & Human Health, Australian Nuclear
Science and Technology Organisation (ANSTO),
2234 Lucas Heights, NSW. Australia
E-mail: mitchk@antsto.gov.au
g.pascali@unsw.edu.au
[b] Dr. M. A. Klenner, Prof. M. Massi
School of Molecular and Life Sciences, Curtin University,
6102 Bentley, WA. Australia
[c] V. Moon, B. J. Evans
Department of Molecular Sciences, Macquarie University,
2109 Macquarie Park, NSW. Australia
[d] Prof. G. Pascali
Prince of Wales Hospital,
2031 Randwick, NSW. Australia
[e] Dr. G. Pascali
School of Chemistry, University of New South Wales (UNSW),
2052 Kensington, NSW. Australia
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000433.
Part of the Special Collection for the 44th ICCC Conferences.
Eur. J. Inorg. Chem. 2020, 3554–3564
3554
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
tracer for PET diagnoses of AD in humans, particularly in light 2. Results and Discussion
of nuclear medicines such as carbon-11 labelled Pittsburgh
The precursors and non-radioactive standards were synthesized
Compound B ([11C]PiB) currently being trialed for diagnoses of
human cerebral Aꢀ pathologies.[10] However, alternative
hydroxyquinoline-based treatments for neurodegeneration
such as 5-chloro-7-iodo-8-hydroxyquinoline (clioquinol, CQ),
have demonstrably reversed the in vivo progression of AD and
resulted in rapid restoration of cognitive ability in APP trans-
genic mouse models.[11] Notably, CQ also outcompetes APP de-
rived oligomers for metal ions without affecting the activity of
Zn/Cu-dependent enzymes,[11,12] thus warranting continued
study of hydroxyquinoline radiopharmaceutical analogues. To-
wards furthering the investigation of [18F]CABS13 as an AD im-
aging agent, a few radiosynthetic pathways have been de-
scribed in literature. Each pathway has been characterized by
multiple-step radiosyntheses involving radiofluorination of an
alcohol-protected precursor, using benzyl or benzyloxymethyl
protecting groups, followed by deprotection of the radiola-
belled agent, either by acid-catalyzed hydrolysis or Pd/C-cata-
lyzed hydrogenation.[6,9,13] One recent such method, which we
reported in a 2020 study, utilized a rhenium(I) complexation-
dissociation approach to enable radiofluorination of N-hetero-
cyclic bidentate ligands which were unable to be radiosynthe-
sised hitherto.[14] A mechanism was posited, based on density
functional theory (DFT) modelling analyses, which involved
fluorination of the cis-carbonyl ligand, thus forming an acyl
[18F]fluoride intermediate in proximity to the leaving group and
facilitating nucleophilic aromatic substitution (SNAr). In the case
of hydroxyquinoline ligands, the rhenium(I) centre formed da-
tive bonds with the N and O atoms, serving as both an activator
for SNAr and a protecting group for the alcohol simultaneously.
This experiment incorporated only a single microreactor, how-
ever, which provided limited control over the thermally medi-
ated radiofluorination and decomplexation processes. As a
means of simplifying the radiosynthesis of [18F]CABS13 to a sin-
gle radiosynthetic reaction, we have sought to couple our rhe-
nium(I) complexation-dissociation approach[14,15] with a multi-
ple microreactor in-flow microfluidic set-up hypothesized to en-
able thermal control of the fluorination and dissociation condi-
tions. Only one other two-step telescoping radiotracer produc-
tion method has been reported, which expedited syntheses of
[18F]fluorocholine derivatives.[16] Complexation of the precursor
ligand to a source of rhenium(I) was also trialed in-flow, both
for CABS13 and a chloro-substituted bipyridine compound, to
spare the additional step of synthesis typically required to pre-
pare the rhenium(I) precursor. Such a method accompanies the
few literature examples describing automated multiple step in-
flow syntheses of fluorine-18 labelled PET radiopharmaceuti-
cals.[17] However, by telescoping such radiosyntheses, ensuring
that the first reaction does not negatively interfere with the
second, we present a rarely implemented strategy designed to
reduce the burden on synthetic chemists tasked with preparing
radiolabelling precursors. More significantly, this approach rep-
resents the first demonstration of in-flow rhenium(I) complexa-
tion, particularly as applied under microfluidic conditions,
as per our previously published methodology via the scheme
depicted in Figure 1.[14] 6-Chloro-2,2′-bipyridine (6ClBiPy) was
first complexed to a source of pentacarbonylrhenium(I) chloride
(Re(CO)5Cl) in toluene solution to afford the tricarbonyl(6-
chloro-2,2′-bipyridine)rhenium(I)
chloride
([Re(6ClBiPy)-
(CO)3Cl]) precursor. Starting material 6ClBiPy was also fluorin-
ated via SNAr with potassium fluoride in dimethyl sulfoxide
(DMSO) solution using 18-crown-6 ether as a phase-transfer cat-
alyst (PTC) in azeotropically distilled anhydrous acetonitrile so-
lution, which required four days of refluxing to afford the 6-
fluoro-2,2′-bipyridine (6FBiPy) non-radioactive standard. The
6FBiPy ligand was then complexed with Re(CO)5Cl in toluene
to afford the desired tricarbonyl(6-fluoro-2,2′-bipyridine)-
rhenium(I) chloride ([Re(6FBiPy)(CO)3Cl]) non-radioactive stan-
dard.
Figure 1. Synthetic route used to obtain the 6ClBiPy and
[Re(6ClBiPy)(CO)3Cl] precursors, as well as the 6FBiPy and
[Re(6FBiPy)(CO)3Cl] non-radioactive standards.
In-flow complexation was achieved via the microfluidic set-
up assembled as per Figure 2. Here, the supply and flow rate
of 6ClBiPy in DMSO (0.74 mg mL–1, 3.45 μmol mL–1) solution
was delivered by pump 1 (P1) from the relevant storage loop.
The first 50 μL of the reagent was dispensed to a waste vial via
backpressure created from aspirating and dispensing anhy-
drous DMSO solvent to ensure a uniform concentration of
6ClBiPy in the reaction bolus. Reagent Re(CO)5Cl in DMSO
(1.36 mg mL–1, 4.30 μmol mL–1) was likewise aspirated and dis-
pensed from a storage loop via pump 2 (P2). Volumetric ratios
of 1:1 were applied for the precursor boluses, thus ensuring a
1.25 equiv. excess of Re(CO)5Cl to 6ClBiPy similar to the bulk
synthesis. The reaction boluses (10 μL) of both reagents were
eluted through microreactor 1 (MR1) with anhydrous DMSO at
a 20 μL min–1 flow rate and eluted into the final product vial
for analysis by high-performance liquid chromatography
(HPLC). The internal volume of MR1 was 15.6 μL, which affords
a residence time of approximately 23 s given the combined
flow rate of 40 μL min–1 from both pumps. The temperature
applied to MR1 was modulated in 20 °C increments via a heater
plate to determine the optimal conditions for the complexation
reaction. As the heat supplied to the reaction drive both the
which may have far-reaching implications for the manufactur- forward and reverse processes for complexation (Kc) and disso-
ing of many similar rhenium(I) tricarbonyl tracers. ciation (K-d), respectively, the temperature for maximal produc-
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3555
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
tion of [Re(6FBiPy)(CO)3Cl] was hypothesized to lie between
the two extremes of the 70–190 °C range trialed. Figure 3 de-
picts the 215 nm UV-absorbance channel for the resulting chro-
matograms at each of the tested temperatures. While the exact
quantities of the products and unreacted starting materials are
dependent on the molar absorptivity of each analyte at 215 nm,
comparative assessments of production and consumption can
be determined from the increase and decrease of the relative
integrated peak areas, respectively. Evidently, higher tempera-
tures
resulted
in
increased
production
of
the
[Re(6FBiPy)(CO)3Cl] complex, as quantified in Figure 4 which
depicts the integrated peaks areas for each reagent expressed
as a percentage of the entire chromatographic area at 215 nm
alongside the associated standard deviations. The peak area of
Re(CO)5Cl notably decreases at temperatures greater than 90 °C
and results in a considerable increase in the relative amount of
[Re(6FBiPy)(CO)3Cl] forming at 170 °C. At 190 °C the produc-
tion of [Re(6FBiPy)(CO)3Cl] is observed to decrease, suggesting
that the increased heat supply favours the KC rate constant up
to 170 °C, whereafter the ratio of [Re(6FBiPy)(CO)3Cl] complex-
ation-to-dissociation rate decreases. An additional experiment
was also employed at 190 °C, whereby the molar equivalence
of Re(CO)5Cl was scaled up two-fold (2.5 equiv.) to assess
whether the rate of [Re(6FBiPy)(CO)3Cl] complexation could
be increased. The increased molar equivalence was the result
of doubling the Re(CO)5Cl solution bolus to 20 μL (i.e. by apply-
ing 1:2 volumetric ratio). This reaction is depicted by the dashed
line of Figure 3, clearly indicating the retention time of
Re(CO)5Cl on the column and that excess reagent remained
in the reaction media. Consequently, the [Re(6FBiPy)(CO)3Cl]
complex was afforded in a greater than two-fold increase in
yield, as evidenced by Figure S1 of the SI which plots the inte-
grated peak area with standard deviation, proportional to the
relative mass of each reagent, for each compound at the given
MR1 reaction temperature.
Figure 3. UV absorbance chromatograms showing the in-flow complexation
of 6ClBiPy with Re(CO)5Cl to form [Re(6ClBiPy)(CO)5Cl at varying tempera-
tures. A two-fold equivalence of Re(CO)5Cl was added in an additional 190 °C
experiment to aid in driving the reaction. A gradient HPLC system of water
and acetonitrile (5–95 %) was utilized with a flow rate of 2 mL min–1 and data
collected at 215 nm wavelength.
ing an in-flow rhenium(I) complexation approach. The micro-
fluidic set-up used to automate these reactions is illustrated in
Figure 5. Solutions of 6ClBiPy (1.47 mg mL–1, 7.69 μmol mL–1)
and Re(CO)5Cl (2.76 mg mL–1, 7.69 μmol mL–1) were still sup-
plied by P1 and P2 in 10 μL boluses, respectively, and passed
through heated MR1 to provide [Re(6ClBiPy)(CO)5Cl. In this
setup, a second microreactor (MR2) was placed in series to MR1
and temperature controlled to optimize the radiofluorination
reaction. Cyclotron produced [18F]fluoride in oxygen-18 en-
riched water, formed from the 18O(p,n)18F nuclear reaction, was
eluted though a QMA ion exchange resin with tetraethylammo-
nium (TEA) bicarbonate in a 90 % acetonitrile in water solution
by pump 3 (P3). Adequate elution of [18F]fluoride was con-
firmed by a decline in radioactivity, as measured by a γ-scintilla-
tion detector in close proximity, and a solenoid valve (SV) en-
sured the recycling of [18O]water. The resulting TEA [18F]Fluor-
ide ([18F]TEAF) solution was eluted into an azeotropic distilla-
tion vial, as confirmed by a nearby radioactivity detector,
whereupon the solvent was evaporated under heated condi-
tions with a nitrogen gas flow and vacuum applied through
two SVs. The resulting residue was reconstituted with portions
of anhydrous acetonitrile in order to azeotropically remove tra-
ces of water. The [18F]TEAF complex was then finally reconsti-
tuted in anhydrous DMSO solution and transferred into a stor-
age loop by pump 4 (P4). A fraction of the radioactive solution
was dispensed into a waste vial to ensure homogeneity of the
reaction bolus and 10 2 MBq solutions of [18F]TEAF were
eluted through MR2 alongside the output of MR1 containing
[Re(6ClBiPy)(CO)5Cl]. Using the solution eluted from MR1 we
aimed to telescope the synthesis of [18F]6FBiPy, rather than
its rhenium complexed analogue, by exploiting our previously
described rhenium(I) complexation-dissociation approach.
Thus, the temperature at MR2 was fixed at 190 °C in order to
increase not only the rate of fluorination (Kf) forming
Figure 2. Microfluidic set-up for the automated syntheses of
[Re(6ClBiPy)(CO)3Cl] in-flow, used to determine the optimal temperature for
complexation via heating of MR1.
After confirming that rhenium(I) ligand complexations could [18F][Re(6FBiPy)(CO)3Cl], though also the rate of decomplexa-
be achieved in-flow, we shifted our attention to the two-step tion (Kd) forming [18F]6FBiPy. However, in this optimization, we
radiosynthesis of [18F]6FBiPy and [18F][Re(6FBiPy)(CO)5Cl us- again tested the variation of temperature of MR1 from 50–
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3556
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
Figure 4. Relative percentage of integrated areas with associated standard deviations determined from the 215 nm UV-absorbance chromatograms for 6ClBiPy
and Re(CO)5Cl complexation reactions forming [Re(6ClBiPy)(CO)5Cl at varying MR1 temperatures.
Figure 5. Microfluidic set-up for the complexation of 6ClBiPy with Re(CO)5Cl in MR1 to afford [Re(6ClBiPy)(CO)3Cl], followed by radiofluorination and
decomplexation in MR2 to afford both [18F][Re(6FBiPy)(CO)3Cl] and [18F]6FBiPy radioproducts.
190 °C in 20 °C increments. The flow rates of P1 and P2 were
both set to 10 μL min–1, thus affording a combined flow of
20 μL min–1 through MR1. As the internal volume of MR1 was
15.6 μL, the residence time for the complexation reaction was
thus approximately 47 s. P4 also dispensed the [18F]TEAF solu-
tion at a rate of 10 μL min–1, however MR2 consisted of a larger
internal volume (31.2 μL) and thus a longer residence time of
62 s was allowed for the fluorination and decomplexation path-
ways despite a combined 30 μL min–1 flow rate. Additionally, a
radioactivity detector was also placed in proximity of MR2 to
monitor the elution of the radioactive solution into the final
product vial for radioHPLC analysis.
Table 1. RCYs of [18F]6FBiPy and [18F][Re(6FBiPy)(CO)3Cl] obtained from in-
flow radiosyntheses applying MR1 (non-radioactive complexation) at the
listed temperatures. MR2 (radiofluorination & dissociation) was also applied
in-series at a fixed temperature of 190 °C.
MR1
[
18F]6FBiPy
[
18F][Re(6FBiPy)(CO)3Cl]
Temperature [°C]
(% RCY)
(% RCY)
50
4.7
2.4
70
4.7
4.8
90
3.7
5.3
12.9
23.6
21.7
21.9
23.9
37.0
33.4
5.8
1.2
1.4
110
130
150
170
190
The radiochemical yields (RCY), as determined from the
integrated peak area of the radioproducts divided by the total
radiochromatographic area, for both [18F]6FBiPy and of MR1 and MR2 in Table 1. RCYs of less than 5 % were obtained
[18F][Re(6FBiPy)(CO)3Cl], are listed alongside the temperatures for both radioproducts for an MR1 temperature of no greater
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3557
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
than 70 °C. At 90 °C, however, the radiosynthesis of plexation, a greater quantity of complex was fluorinated to af-
[18F][Re(6FBiPy)(CO)3Cl] was found to increase to 23.9 %, ford [18F][Re(6FBiPy)(CO)3Cl which in turn afforded a greater
RCY of [18F]6FBiPy. These results suggest that with a longer
residence time in MR2 for the 90–130 °C temperature range,
the RCY of [18F]6FBiPy could have been improved, as both reac-
tions are governed by the Kc and Kd rate constants. While rapid
heat transfer in microfluidic laminar flow systems would be ex-
pected, it could be possible that the residual heat of the reac-
tion bolus leaving MR1 affected reactions at MR2 by reducing
the time required for uniform heating in the coil, thus account-
ing for the significant difference in RCYs at higher temperatures.
followed by maximum RCYs of 37.0 % and 33.4 % at
110 and 130 °C, respectively. Thereafter, the RCY of
[18F][Re(6FBiPy)(CO)3Cl] notably decreased as the complex dis-
sociated to liberate [18F]6FBiPy at a maximum of 23.6 % RCY
at 150 °C, followed by consistent RCYs of 21.7 % and 21.9 % at
170 and 190 °C, respectively. As an example, the radiochromato-
gram acquired at 130 °C is depicted in Figure 6, which illustrates
the retention of [18F]6FBiPy at 2.15 min and
[18F][Re(6FBiPy)(CO)3Cl] at 5.68 min, with minor radio by-prod-
ucts eluting around 4.5–5.0 min and unreacted [18F]fluoride
eluting around 30 s into the run. These radioactive by-products
may possibly coincide with the minor by-products observed in
the Figure 3 chromatograms for [Re(6ClBiPy)(CO)3Cl] complex-
ation eluting around 5.2 min. The HPLC elution of [18F]6FBiPy
from this two-reactor experiment slightly differs from the elu-
tion of 6ClBiPy from the non-radioactive single-reactor com-
plexation experiment; however, as the gradient was held at
100 % aqueous phase for the first 2 min to ensure the reten-
tions of [18F]6FBiPy and [18F]fluoride did not overlap. The peak
identities were again verified via retention time comparisons
with the non-radioactive 6FBiPy and [Re(6FBiPy)(CO)3Cl] stan-
dards using the UV channel. The observed RCYs can be rational-
ized through consideration of Figure 7, which depicts the rate
constants for the forward and reverse reactions concerning:
i) complexation (Kc) of 6ClBiPy with Re(CO)5Cl to form
[Re(6ClBiPy)(CO)3Cl] and reverse thermal dissociation (K-d) of
[Re(6ClBiPy)(CO)3Cl] liberating 6ClBiPy, ii) fluorination (Kf) of
[Re(6ClBiPy)(CO)3Cl] forming [18F][Re(6FBiPy)(CO)3Cl] and re-
verse defluorination (K-df) with formerly displaced chloride ions
forming [Re(6ClBiPy)(CO)3Cl], and iii) thermal dissociation (Kd)
of [18F][Re(6FBiPy)(CO)3Cl] to liberate [18F]6FBiPy and reverse
complexation (K-c) with excess Re(CO)5Cl to reform the
[18F][Re(6FBiPy)(CO)3Cl] labelled complex. The fluorination of
6ClBiPy is too slow afford an appreciable rate constant, as evi-
denced by the 96 h synthesis time of the non-radioactive stan-
dard and our former experiments demonstrating no discernible
yield of [18F]6FBiPy from the ligand precursor.[14] Reactions be-
tween [18F]TBAF and Re(CO)5Cl across the trialed temperatures
likewise afforded no radioproducts. Thus, the incorporation of
fluorine-18 is principally restricted to the radioactive species
illustrated in Figure 7. For reactions at temperatures 50 and
70 °C, the Kc rate constant is clearly lower in value thus resulting
in minimal [Re(6ClBiPy)(CO)3Cl] required to form the radio-
products from subsequent fluorination. For temperatures of
90 °C onwards it is curious to note how differences in the MR1
temperature affected radiotracer production, given that the Kf
and Kd rate constants should remain unperturbed given the
stable 190 °C MR2 temperature. Evidently, the amount of
[18F][Re(6FBiPy)(CO)3Cl] and [18F]6FBiPy formed was linked to
the amount of [Re(6ClBiPy)(CO)3Cl] produced in MR1. Between
the 90–130 °C temperature range, a reasonable quantity of
Figure 6. Radiochromatogram acquired from the complexation and radioflu-
orination of [Re(6ClBiPy)(CO)3Cl], with MR1 set to 130 °C and MR2 set to
190 °C, depicting the retention of
[
[
18F]6FBiPy at 2.15 min and
18F][Re(6FBiPy)(CO)3Cl] at 5.68 min. The gradient was held at 100 % water
for the first 2 min to avoid coelution of [18F]6FBiPy with [18F]fluoride. A full
description of the HPLC gradient profile is provided in Table 3 of Section 4 –
Materials and Methods.
Figure 7. Rate constants for the forward and reverse reactions concerning
complexation (Kc), reverse dissociation [K]d), fluorination (Kf), reverse defluori-
nation [K]df), dissociation (Kd) and reverse complexation [K]c) leading to the
formation of [18F]6FBiPy.
Given the promising dual-reactor in-flow production of
[18F]6FBiPy, we were keen to translate the design methodology
to the production of [18F]CABS13, an AD PET diagnostic agent.
The synthetic routes used to obtain the precursors for radiosyn-
thesis and non-radioactive standards for retention time compar-
[Re(6ClBiPy)(CO)3Cl]
was
radiofluorinated
to
form
[18F][Re(6FBiPy)(CO)3Cl] in solution, though not enough to
warrant significant dissociation. Whereas between 150–190 °C,
within the range previously confirmed to afford optimum com-
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3558
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
isons is illustrated in Figure 8. A solution of Re(CO)5Cl was first
refluxed in acetonitrile under anhydrous and deoxygenated yielding, the microfluidic set-up was further modified to incor-
As radiosyntheses of [18F]CABS13 can be particularly low
conditions to afford the [Re(CO)3(NCCH3)2Cl] intermediate, to
which 2Cl8HQ was added and further refluxed. The resulting
[Re(2Cl8HQ)(CO)3(NCCH3)] complex was then found to precipi-
tate from solution. Formation of the non-radioactive fluorinated
standards required additional steps of synthesis. 2-chloro-8-hy-
droxyquinoline (2Cl8HQ) was first dissolved in dimethylform-
amide (DMF) and treated with benzyl chloride in the presence
of potassium carbonate to afford the 2Cl8HQ-OBn benzyl pro-
tected alcohol. Subsequent fluorination using potassium fluor-
ide and an 18-crown-6 PTC in DMSO afforded CABS-13-OBn,
which was then reduced over hydrogen gas in the presence
of palladium and palladium hydroxide on carbon catalysts in
acetonitrile solution to afford the CABS13 standard. The
porate three microreactors in series, as shown in Figure 9. The
temperatures were modified to control the rates of complexa-
tion, fluorination and dissociation at MR1, MR2 and MR3, re-
spectively. An additional radioactivity detector was placed near
the site of MR3 to monitor entry and departure of the radioac-
tive solution into the final product vial for radioHPLC analysis.
In this particular set-up P1 delivered a solution of 2Cl8HQ
(0.88 mg mL–1, 4.88 μmol mL–1) in DMSO, whereas P2 continued
to supply Re(CO)5Cl (2.12 mg mL–1, 5.86 μmol mL–1) dissolved
in acetonitrile. P3 and P4 reprised the former roles of azeotropi-
cally drying and supplying the [18F]TEAF solution in DMSO. Bo-
luses (20 μL) of 2Cl8HQ and Re(CO)5Cl were pumped through
to MR1 in a 1:1 volumetric ratio, ensuring an excess 1.20 equiv.
of Re(CO)5Cl, and each at a flow rate of 5 μL min–1. Thus, the
summative flow rate of 10 μL min–1 in the 15.6 μL void volume
of MR1 resulted in a residence time of 94 s. P3 supplied the
[18F]TEAF (61 14 MBq) solution at a rate of 10 μL min–1, thus
resulting in a cumulative flow rate of 20 μL min–1 in both MR2
and MR3. Given that the internal volumes (31.2 μL) were the
same for both MR1 and MR2, the residence time of the reaction
boluses was also 94 s in all microreactors. Figure 10 depicts the
rate constants for the forward and reverse reactions intended
to be controlled at each microreactor; i) complexation (Kc) of
2Cl8HQ with Re(CO)5Cl to form [Re(2Cl8HQ)(CO)3(NCCH3)] at
CABS13
[Re(CO)3(NCCH3)2Cl] intermediate, again formed from refluxing
Re(CO)5Cl in acetonitrile, to afford the desired
[Re(CABS13)(CO)3(NCCH3)] fluorinated standard.
ligand
was
then
complexed
with
the
MR1
and
reverse
thermal
dissociation
(K-d)
of
[Re(2Cl8HQ)(CO)3(NCCH3)] liberating 2Cl8HQ, ii) fluorination
(Kf) of [Re(2Cl8HQ)(CO)3(NCCH3)] forming [18F][Re(CABS13)-
(CO)3(NCCH3)] at MR2 and reverse defluorination (K-df) with for-
merly displaced chloride ions forming [Re(2Cl8HQ)(CO)3-
(NCCH3)], and iii) thermal dissociation (Kd) of [18F][Re(CABS13)-
(CO)3(NCCH3)] to liberate [18F]CABS13 at MR3 and reverse
complexation (K-c) with excess Re(CO)5Cl to reform the
[18F][Re(CABS13)(CO)3(NCCH3)] labelled complex. Table 2 lists
Figure 8. Synthetic route used to obtain the 2Cl8HQ and
[Re(2Cl8HQ)(CO)3(NCCH3)] precursors, as well as the CABS13 and
[Re(CABS13)(CO)3(NCCH3)] non-radioactive standards.
Figure 9. Triple reactor microfluidic set-up for the complexation of 6ClBiPy with Re(CO)5Cl in MR1 to afford [Re(6ClBiPy)(CO)3Cl], followed by radiofluorination
to afford [18F][Re(6FBiPy)(CO)3Cl] in MR2 and dissociation in MR3 to afford [18F]6FBiPy.
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3559
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
appeared to favour thermal decomplexation represented by the
K-d pathway, and hence further modulation of the MR1 temper-
ature conditions were discontinued. These former complexation
studies were trialed solely in DMSO media, and the laminar
composition of acetonitrile with DMSO could in part account
for the lesser RCYs. Nonetheless, one final experiment using this
microfluidic set-up was undertaken, whereby the temperature
of MR3 was increased to 210 °C. The temperature represents
the upper end of our instrumental capabilities which we do not
routinely apply for sustainability purposes, hence why all other
reactions did not exceed 190 °C. Applying temperatures of 130,
190 and 210 °C at MR1, MR2 and MR3, respectively, still only
resulted in middle range RCYs of 1.7 % [18F]CABS13 and 1.5 %
[18F][Re(CABS13)(CO)3(NCCH3)Cl].
Figure 10. Rate constants for the forward and reverse reactions concerning
complexation (Kc), reverse dissociation [K]d), fluorination (Kf), reverse defluori-
nation [K]df), dissociation (Kd) and reverse complexation [K]c) leading to the
formation of [18F]CABS13.
Suspecting the lower RCYs of [18F]CABS13 and
[18F][Re(CABS13)(CO)3(NCCH3)] to be principally linked to the
quality of the solvents employed, we repeated the single
reactor
microfluidic
experiment
employing
the
the RCYs for [18F]CABS13 and [18F][Re(CABS13)(CO)3-
(NCCH3)Cl] determined from radioHPLC analyses of each solu-
tion acquired from modulation of the three microreactor tem-
peratures. The incorporation of both DMSO and acetonitrile sol-
vents would have resulted in continuous exchange at the rhe-
nium(I) coordination sphere, given the lability of both ligands.
However, as acetonitrile was used in the HPLC eluent mobile
phase, any coordinated DMSO ligand was likely displaced for
acetonitrile during radioHPLC analysis, thus leading to the ob-
served retention time comparison around 7.5 min with the non-
radioactive standard. The amount of both radioproducts
formed were relatively the same, independent of the tempera-
tures applied to the microreactors, with the RCY of [18F]CABS13
ranging between 1.2–2.7 % and the RCY of [18F][Re(CABS13)-
(CO)3(NCCH3)Cl] ranging between 1.0–1.9 %. The highest yield
of 2.7 % [18F]CABS13 was acquired through temperatures of
130 °C applied to MR1 and 190 °C applied to both MR2 and
MR3. Whereas the highest yield of 1.9 % [18F][Re(CABS13)-
(CO)3(NCCH3)Cl] was obtained through applying temperatures
of 170 °C to MR1 and MR2, followed by 190 °C at MR3. These
comparatively low yields could be rationalized through a partic-
ularly low Kc rate constant which constrained the amount of
[Re(2Cl8HQ)(CO)3(NCCH3)] present to form the desired radio-
[Re(2Cl8HQ)(CO)3(NCCH3)] precursor (0.80 μmol) dissolved in
DMSO. The precursor was treated with [18F]TEAF (29 10 MBq)
over a 7.80 min residence time in a microreactor heated to
190 °C. Such conditions formerly yielded 5 % [18F]CABS13 and
18 % [18F][Re(CABS13)(CO)3(NCCH3)].[14] However, when re-
peating this experiment we found the yields to be approxi-
mately half as those expected, with [18F]CABS13 being pro-
duced in only 2.3 % RCY and [18F][Re(CABS13)(CO)3(NCCH3)]
being produced in only 9.8 % RCY, as shown by the radiochro-
matogram in Figure 11. The only significant change between
this experimental set-up and the previous was the anhydrous
DMSO solvent employed, which was replaced as a cost-saving
initiative within our department. Formerly we had used
≥ 99.9 % anhydrous DMSO purchased from Sigma-Aldrich
(product ID: 276855), whereas for the experiments reported in
this work we used ≥ 99.7 % DMSO purchased from Merck Milli-
pore (product ID: 109678). This was before Merck recently ac-
quired Sigma-Aldrich for $17 billion USD.[18] Evidently, the 0.2 %
of water makes a considerable difference due to the low con-
centrations and sensitivity of radiochemical reactions employ-
ing fluorine-18. Such a requirement for anhydrous radiofluori-
nation conditions has been observed in the production of many
radiopharmaceutical agents.[19] Despite the routine azeotropic
tracers through the subsequent Kf and Kd channels. However, drying of the fluorination media, as all of the reagents were
higher MR1 temperatures in our former complexation studies dissolved in DMSO with a slightly higher water content it would
Table 2. RCYs of [18F]CABS13 and [18F][Re(CABS13)(CO)3(NCCH3)] obtained from in-flow radiosyntheses applying MR1 (non-radioactive complexation), MR2
(radiofluorination) and MR3 (thermal dissociation) microreactors in-series at the specified temperatures.
MR1
MR2
MR3
[
18F]CABS13
[
18F][Re(CABS13)(CO)3
Temperature [°C]
Temperature [°C]
Temperature [°C]
(% RCY)
(NCCH3)] (% RCY)
130
130
130
150
150
150
170
170
190
130
150
170
190
190
150
170
170
190
190
190
190
190
190
190
190
190
190
190
190
210
1.2
1.5
2.7
2.1
2.0
2.1
2.1
1.9
1.8
1.7
1.0
1.2
1.8
1.6
1.7
1.7
1.9
1.7
1.8
1.5
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3560
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
appear that the RCYs of [18F]CABS13, [18F]6FBiPy and their re- noted in our dual microreactor experiments designed to im-
spective rhenium(I) complexes were all comparatively lower prove [18F]6FBiPy and [18F][Re(6FBiPy)(CO)3Cl] syntheses,
than our former experiments.[14] This is despite the implemen- which also afforded lower RCYs when compared to our previ-
tation of multiple reactors which should otherwise have im- ously reported single-reactor experiments with the same rea-
proved the reaction RCYs. Nonetheless, our former experiments gents. However, it cannot be overlooked that the lower RCYs
have fortunately shown that radiofluroinations of rhenium(I) could also be the result of lesser complexation at the first mic-
complexes can tolerate slightly hydrous conditions to afford ra- roreactor and replicate experiments using ≤ 99.9 % anhydrous
dioproducts and, as such, the data presented here still provides DMSO would be needed to verify this hypothesis. Nonetheless,
a useful analysis of which temperature conditions are best util- we have demonstrated that in-flow rhenium(I) complexation re-
ized in multireactor in-flow complexation reactions.
actions are indeed feasible under microfluidic conditions and
that such products can be telescoped for subsequent radiola-
belling with fluorine-18. These results mark significant progress
towards the automated manufacturing of similar reagents, and
we have demonstrated the optimum temperature conditions
required to balance the competing complexation and dissocia-
tion pathways towards heightened RCYs.
4. Experimental Section
Materials and Methods: Instruments and Reagents: NMR analy-
ses were performed using a Bruker Spectrospin 400 MHz UltraShield
NMR spectrometer. Fourier transform infrared (FTIR) analyses were
performed using a Bruker ALPHA-p IR spectrometer. UV/Vis analyses
were performed using an Agilent technologies Cary 100 UV/Vis
spectrometer. Low resolution mass spectrometry (LRMS) analyses
were performed using a Waters micromass ZQ, equipped with an
electrospray ionisation (ESI) source running in positive ionisation
mode with the following parameters applied; voltages: 3 kV capil-
lary voltage, 25 V cone voltage for organic molecules and 70 V
cone voltage for rhenium complexes (required to dissociate Re–Cl
bonds), 5 V extractor voltage and 0.2 V radiofrequency (RF) lens
voltage; Temperatures: 45 °C source temperature and 300 °C desolv-
ation temperature; and gas flows: 350 L h–1 desolvation nitrogen
gas flow and 90 L h–1 cone nitrogen gas flow. Samples for high
resolution mass spectrometry (HRMS) were sent to the Mass Spec-
trometry User Resource and Research Facility at the University of
Wollongong and samples for elemental analysis (EA) were sent to
the Campbell Microanalytical Laboratory at the University of Otago.
All synthetic reagents were purchased from Sigma-Aldrich except
for trifluoroacetic acid (TFA) and 18-crown-6 ether which were pur-
chased from Alfa Aesar, and 2-chloro-2,2′-bipyrdine and 2-chloro-8-
hydroxyquinoline which were purchased from Bepharm. Finally, all
solvents were purchased from Merck Millipore, including DMSO
(product ID: 109678), and all water solutions used for syntheses and
analyses were first purified using a Merck Millipore Milli-Q integral
water purification system.
Figure 11. Radiochromatogram showing the formation of [18F]CABS13 in
2.3 % RCY (6.32 min) and [18F][Re(CABS13)(CO)3(NCCH3)] in 9.8 % RCY
(7.58 min). The gradient was held at 100 % water for the first 3 min to avoid
coelution of [18F]CABS13 with [18F]fluoride. A full description of the HPLC
gradient profile is provided in Table 3 of Section 4 – Materials and Methods.
Further considerations for increasing the yield of
[18F]CABS13, which were not implemented herein, could in-
volve the use of an acid primed solid supported reagent to
assist in dissociation of the metal centre or the use of light-
induced photodissociation in flow.[20] Photodissociation meth-
ods, specifically, have been widely applied in the decomplexa-
tion of similar group VII manganese(I) tricarbonyl complexes,
typically in application to carbon monoxide therapies and anti-
bacterial activity.[21] Such methods could potentially improve
the efficiency of rhenium(I) decomplexation strategies and
widen the scope of fluorine-18 labelled radiopharmaceuticals
for which the complexation-dissociation reaction could be ap-
plied to.
3. Conclusions
Non-Radioactive Syntheses of Standards and Precursors
Herein, the radiosynthesis of the [18F]CABS13 AD PET neuro-
Tricarbonylchloro(6-chloro-2,2′-bipyridine)rhenium(I) Chloride
imaging agent has been simplified to a single microfluidic set- ([Re(6ClBiPy)(CO)3Cl]): A mass of 6ClBiPy (277.5 mg, 1.456 mmol,
1.1 equiv.) was dissolved in a volume of anhydrous toluene (5 mL). A
mass of pentacarbonylrhenium(I) chloride (478.7 mg, 1.3234 mmol,
1.0 equiv.) was then added and the solution was stirred at reflux
under an inert nitrogen environment (5 h). The solution was then
cooled to –20 °C to afford a precipitate. The precipitate was isolated
via vacuum filtration and was washed with cold diethyl ether. The
crude material was then purified over a neutral Brockman grade II
alumina stationary phase using a DCM mobile phase. The combined
fractions were evaporated under reduced pressure to afford a pure
yellow powder (564.2 mg, 86 % yield). 1H NMR (400 MHz, [D6]DMSO,
up incorporating three microreactors dedicated to the rhe-
nium(I) complexation, radiofluorination and dissociation of the
ligand. Despite the compact set-up, the RCYs of [18F]CABS13
and the [18F][Re(CABS13)(CO)3(NCCH3)] complex were com-
paratively lower than former single microreactor set-ups. Re-
peat analyses of the single-reactor conditions were found to
also afford lower RCYs which were linked to the different grades
of anhydrous DMSO solvents used in the experiments. Here we
highlight the significant impact of a 0.2 % difference in water
content on radiofluorination reactions. This trend was likewise δ/ppm): δ 9.06 (d, Jd = 4.76 Hz, 1H), δ = –8.76 (d, Jd = 8.12 Hz, 2H),
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3561
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
δ 8.35 (app. td, Jd = 1.96, Jt = 8.04 Hz, 2H), δ 8.08 (dd, Jd = 0.68, Acetonitriletricarbonyl(2-chloro-8-quinolinolate)rhenium(I)
8.04 Hz, 1H), δ 7.78 (app. td, Jd = 1.80, Jt = 4.56 Hz, 1H). 13C{1H} ([Re(2Cl8HQ)(CO)3Cl]): A mass of pentacarbonylrhenium(I) chloride
NMR (100 MHz, [D6]DMSO, δ/ppm): δ = 197.5C≡ O, δ = 197.3C≡ O, δ =
(116.1 mg, 0.321 mmol, 1.0 equiv.) was added to a volume of anhy-
189.5C≡ O, δ = 157.2, δ = 155.6, δ = 153.2, δ = 152.8, δ = 142.7, δ = drous acetonitrile (10 mL) and refluxed in an inert nitrogen environ-
140.3, δ = 128.0, δ = 127.7, δ 125.3, δ = 123.1. FTIR (ATR corr. ν = / ment (4 h) to afford the diacetonitriletricarbonlylrhenium(I) chloride
˜
cm–1): ν 3075 (w, C–H sp2 str.), ν 2018 (s, A′(1) C≡ O str.), ν 1882 (s,
intermediate. A mass of 2Cl8HQ (63.4 mg, 0.353 mmol, 1.1 equiv.)
was dissolved in a volume of anhydrous acetonitrile (2 mL) and
˜
˜
˜
A′(2) & A′′ C≡ O str.). UV/Vis (λ/nm, ε/L mol–1 cm–1): λ 380 (dπ→π*, ε
3375.36), λ 301 (π→π*, ε 14990.57), λ 326 (π→π*, ε 10920.28), λ deprotonated via the addition of sodium hydride (48.0 mg,
233 (π→π*, ε 19259.41). LRMS (ESI+): [M – Cl]+ m/z cald: 460.97, m/z 2.000 mmol, 6.2 equiv.) over a 0 °C ice bath. The resulting green
obvs. 460.95. HRMS (ESI+): [M – Cl]+ m/z cald: 460.9703, m/z obvs.
solution was then filtered through a PTFE membrane (0.2 μm) and
460.9694 (Δ –2.0 ppm). EA (%): calcd. C 31.46, H 1.42, N 5.64. found added to the intermediate solution and left to stir at room tempera-
C 31.59, H 1.17, N 5.70.
ture under an inert nitrogen environment (12 h). A volume of aque-
ous trifluoromethanesulfonic acid (0.1 mol L–1, 10 mL) was then
added. The solution was evaporated under reduced pressure to af-
ford a yellow solid in aqueous suspension. The precipitate was iso-
lated via vacuum filtration and purified over a neutral Brockmann
grade II alumina stationary phase using an acetonitrile mobile
phase. The combined fractions were then evaporated under re-
duced pressure to afford a yellow powder (49.8 mg, 32 % yield). 1H
NMR (400 MHz, CD3CN, δ/ppm): δ 8.32 (d, Jd = 8.68 Hz, 1H), δ 7.58
6-Fluoro-2,2′-bipyridine (6FBiPy): A mass of 6ClBiPy (318.7 mg,
1.672 mmol, 1.0 equiv.) was dissolved in a volume of anhydrous
DMSO (10 mL). Masses of potassium fluoride (1.712 g, 29.466 mmol,
17.6 equiv.) and 18-crown-6 ether (1.567 g, 5.928 mmol, 3.5 equiv.)
were azeotropically dried via dropwise addition of anhydrous aceto-
nitrile at 90 °C under an inert nitrogen gas flow. The ligand solution
was then added to the dried reagents, heated to 90 °C and stirred
under an inert nitrogen environment (96 h). The solution was then
cooled and filtered through a polytetrafluoroethylene (PTFE) mem-
brane (0.2 μm). The filtrate was then purified via reverse phase chro-
matography employing a C18 stationary phase and an acetonitrile/
TFA (0.1 % in water) mobile phase. Lyophilization of the combined
fractions afforded a pink salt which was dissolved in a minimum of
water (20 mL), neutralized with sodium bicarbonate and extracted
into DCM. The combined organic fractions were then dried with
sodium sulfate and evaporated under reduced pressure to afford a
white powder (39.2 mg, 13 % yield). 1H NMR (400 MHz, CDCl3, δ/
ppm): δ 8.74 (d, Jd = 4.20 Hz, 1H), δ 8.50 (d, Jd = 6.48 Hz, 1H), δ 8.45
(d, Jd = 8.04 Hz, 1H), δ 7.96 (q, Jq = 15.84, 8.00 Hz, 2H), δ 7.45 (d,
Jt = 6.52 Hz, 1H), δ 7.00 (dd, Jd = 8.08, 2.80 Hz, 1H). 13C{1H} NMR
(100 MHz, CDCl3, δ/ppm): δ = 163.3 (d, Jd = 237 Hz), δ = 155.1 (d,
Jd = 13 Hz), δ = 154.7, δ = 149.4, δ = 142.0 (d, Jd = 7 Hz), δ = 137.2,
(d, Jd = 8.68 Hz, 1H), δ 7.43 (t, Jt = 7.96 Hz, 1H), δ 7.01 (dd, Jd
=
7.96, 0.96 Hz, 1H), δ = 6.92 (dd, Jd = 7.96, 1.04 Hz, 1H), δ 1.96 (s, 3H).
13C{1H} NMR (100 MHz, CD3CN, δ/ppm): δ = 196.6C≡ O, δ 195.3C≡ O, δ
195.3C≡ O, δ = 170.2, δ = 152.3, δ = 144.3, δ = 142.6, δ = 131.2, δ =
130.1, δ = 123.8, δ = 120.8, δ 117.7, δ = 112.4, δ = 30.3. FTIR (ATR
corr. ν = /cm–1): ν 2925.41 (w, C–H sp2 str.), ν 2303.92 (w, N≡ C str.),
˜
˜
˜
ν 2016.96 (s, A′(1) C≡ O str.), ν 1871.84 (s, A′(2) & A′′ C≡ O str.). UV/
˜
˜
Vis (λ/nm, ε/L mol–1 cm–1): λ 437 (dπ→π*, ε 235.06), λ 281 (π→π*,
ε 2340.84), λ 237 (π→π*, ε 3104.80). EA (%): calcd. C 34.32, H 1.65,
N 5.72. found C 34.89, H 1.54, N 5.60.
2-Chloro-8-benzoxyquinoline (2Cl8HQ-OBn): A mass of 2Cl8HQ
(2.40 g, 13.4 mmol, 1.0 equiv.) was dissolved in a volume of anhy-
drous DMF (2.5 mL) alongside a mass of potassium carbonate
(3.69 g, 26.7 mmol, 2.0 equiv.). A volume of benzyl chloride (3.1 mL,
27.4 mmol, 2.0 equiv.) was added dropwise to the solution. The
reaction was stirred at 60 °C in an inert nitrogen environment (3 h).
The solution was then cooled and extracted with DCM (20 × 10 mL).
The combined organic extracts were washed with brine (3 × 20 mL).
The solution was then dried with sodium sulfate and evaporated
under reduced pressure. Recrystallisation from hot ethanolic solu-
tion afforded a pure pink product (3.0 g, 85 % yield). 1H NMR
(400 MHz, CDCl3, δ/ppm): δ 8.06 (d, Jd = 8.60 Hz, 1H), δ 7.50 (d, Jd =
7.36 Hz, 1H), δ 7.40 (d, Jd = 8.56 Hz, 1H), δ 7.36 (m, 1H), δ 7.30 (d,
Jd = 7.06 Hz, 1H), δ 7.06 (dd, Jd = 6.08, 3.00 Hz, 1H). 13C{1H} NMR
(100 MHz, CDCl3, δ/ppm): δ = 153.7, δ = 150.0, δ = 140.0, δ = 138.7,
δ = 137.0, δ = 128.7, δ = 128.3, δ = 127.9, δ = 127.2, δ = 127.1, δ =
127.1, δ = 123.1, δ 119.7, δ = 111.8, δ 71.0.
δ = 124.3, δ = 121.4, δ = 118.3 (d, Jd = 4 Hz), δ = 109.6 (d, Jd
=
37 Hz). 19F{1H} NMR (376 MHz, CDCl3, δ/ppm): δ = –66.77. LRMS
(ESI+): [M + H]+ m/z cald: 175.07, m/z obvs. 175.14.
Tricarbonylchloro(6-fluoro-2,2′-bipyridine)rhenium(I) Chloride
([Re(6ClBiPy)(CO)3Cl]): A mass of 6FBiPy (65.3 mg, 0.363 mmol,
1.1 equiv.) was dissolved in a volume of anhydrous toluene (8 mL).
A mass of pentacarbonylrhenium(I) chloride (120.0 mg, 0.323 mmol,
1.0 equiv.) was then added and the solution was stirred at reflux
under an inert nitrogen environment (3 h). The solution was then
cooled to –20 °C to afford a precipitate. The precipitate was isolated
via vacuum filtration and was washed with cold diethyl ether. The
crude material was then purified over a neutral Brockman grade II
alumina stationary phase using a DCM mobile phase. The combined
fractions were evaporated under reduced pressure to afford a pure
yellow powder (104.1 mg, 65 % yield). 1H NMR (400 MHz, [D6]DMSO,
2-Fluoro-8-benzoxyquinoline (CABS13-OBn): A mass of 2Cl8HQ-
OBn (1.48 g, 5.5 mmol, 1.0 equiv.) was dissolved in a volume of
anhydrous DMSO (10 mL). Masses of potassium fluoride (3.84 g,
δ/ppm): δ 9.06 (app. tq, Jt = 5.48 Hz, Jq = 0.80 Hz, 1H), δ 8.80 (d, 66.1 mmol, 12.0 equiv.) and 18-crown-6 ether (5.84 g, 22.1 mmol,
Jd = 8.16 Hz, 1H), δ 8.70 (d, Jd = 7.92 Hz, 1H), δ 8.52 (q, Jq = 8.04 Hz,
1H), δ 8.36 (td, Jt = 8.08 Hz, Jd = 1.52 Hz, 1H), δ 7.80 (m, 2H). 13C{1H}
4.0 equiv.) were azeotropically dried via dropwise addition of anhy-
drous acetonitrile at 90 °C under an inert nitrogen gas flow. The
NMR (100 MHz, [D6]DMSO, δ/ppm): δ = 197.2C≡ O, δ = 196.6C≡ O (d, ligand solution was then added to the dried reagents, heated to
Jd = 16.59 Hz), δ = 188.9C≡ O, δ = 164.0, δ = 161.5, δ = 154.4 (d, Jd = 160 °C and stirred under an inert nitrogen environment (12 days).
15.92 Hz), δ = 153.1, δ = 145.9 (d, Jd = 10.15 Hz), δ = 140.4, δ = The solution was then cooled and filtered through a polytetrafluo-
128.2, δ 125.1, δ = 121.4 (d, Jd = 2.94 Hz), δ = 112.7 (d, Jd
=
roethylene (PTFE) membrane (0.2 μm). The filtrate was purified via
reverse phase chromatography employing a C18 stationary phase
and an acetonitrile/TFA (0.1 % in water) mobile phase. Lyophiliza-
30.19 Hz). 19F{1H} NMR (376 MHz, [D6]DMSO, δ/ppm): δ –51.75. FTIR
(ATR corr. ν = /cm–1): ν 3040 (w, C–H sp2 str.), ν 2016 (s, A′(1) C≡ O
˜
˜
˜
str.), ν 1880 (s, A′(2) & A′′ C≡ O str.). UV/Vis (λ/nm, ε/L mol–1 cm–1): λ tion of the combined fractions afforded a pink salt which was dis-
˜
369 (dπ→π*, ε 2768.36), λ 296 (π→π*, ε 12485.88), λ 235 (π→π*, ε
15367.33). LRMS (ESI+): [M – Cl]+ m/z cald: 445.00, m/z obvs. 445.10.
EA (%): calcd. C 32.54, H 1.47, N 5.84. found C 32.59, H 1.16, N 5.88.
solved in a minimum of water (60 mL), neutralized with sodium
bicarbonate and extracted into DCM. The combined organic frac-
tions were then dried with sodium sulfate and evaporated under
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3562
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
reduced pressure to afford a white powder (357 mg, 26 % yield).
polymer. RadioHPLC analyses were carried out using a Shimadzu
MP: 107 °C, 26 % yield. 1H NMR (400 MHz, CDCl3, δ/ppm): δ 8.22 (d, system comprised of a CBM-20 controller, LC-20AD pump, SIL-
Jd = 8.40 Hz, 1H), δ 7.51 (d, Jd = 7.40 Hz, 1H), δ 8.35 (m, 5H), δ 7.10 20AHT autoinjector, SPD-M20A PDA detector and a Lablogic Posi-
(m, 2H), δ 5.42 (s, 1H). 13C{1H} NMR (100 MHz, CDCl3, δ/ppm): δ =
RAM gamma detector. Analyses were performed using gradient
162.0, δ = 159.5, δ = 153.6 (Jd = 2 Hz), δ = 142.1 (Jd = 9 Hz), δ = conditions over 15 minutes, whereupon the aqueous phase was
137.7 (Jd = 15 Hz), δ = 137.0, δ = 128.8, δ = 128.2 (Jd = 2 Hz), δ = held for the first 2 minutes ([18F]6FBiPy and complex) or 3 minutes
128.0, δ = 127.2, δ = 126.3 (Jd = 9 Hz), δ 119.7, δ = 111.8, δ = 110.9, ([18F]CABS13 and complex) followed by linear transposition from
δ = 110.4. 19F{1H} NMR (376 MHz, [D6]DMSO, δ/ppm): δ = –61.07.
95:5 to 5:95 water/acetonitrile over the remaining 12 or 13 minutes
at a 2 mL min–1 flow rate (Table 3). Both water and acetonitrile
mobile phases contained 0.1 % v/v of TFA buffer. A Chromolith RP
column (monolith system, Merck 50 × 4.6 mm), demonstrated to
have < 8 % [18F]fluoride retention in low pH ranges,[22] was em-
ployed as the stationary phase. RadioHPLC derived non-isolated
RCYs were calculated via the integrated peak area ratio between
the radioproduct and other radioactive species present, inclusive of
any unreacted [18F]fluoride, using Laura V4.1.70 SP2 HPLC data anal-
ysis software. Integrations were performed over the appropriate
30 s retention time windows of the radiochromatograms corre-
sponding to each peak; 1:50–2:20 min for [18F]6FBiPy, 5:30–
2-Fluoro-8-hydroxyquinoline (CABS13): A mass of 2F8HQ-OBn
(218 mg, 0.859 mmol, 1.0 equiv.) was dissolved in a volume of anhy-
drous acetonitrile (6 mL). Masses of 10wt.-% palladium on activated
carbon (123 mg) and 20wt.-% palladium hydroxide on activated
carbon (125 mg) were added to the reaction in an inert nitrogen
environment. The reaction flask was purged with hydrogen gas and
left to stir (1 h). The solution was filtered through diatomaceous
earth to afford a yellow filtrate. The filtrate was evaporated under
reduced pressure and recrystallised from hot ethanolic solution to
afford a white powder (106 mg), 75 % yield). MP: 62 °C; 1H NMR
(400 MHz, [D6]DMSO, δ/ppm): δ 9.89 (br. s, 1H), δ 8.51 (t, Jt =
8.60 Hz), δ 7.45 (m, 2H), δ 7.32 (dd, Jd = 8.80, 2.76 Hz, 1H), δ 7.14
(dd, Jd = 7.20, 1.64 Hz, 1H). 13C{1H} NMR (100 MHz, [D6]DMSO, δ/
ppm): δ = 159.6 (d, Jd = 238 Hz), δ = 152.5, δ = 143.0 (d, Jd = 10 Hz),
δ = 135.4 (d, Jd = 15 Hz), δ = 127.9, δ = 126.9 (d, Jd = 2 Hz), δ 117.9,
δ = 113.5, δ = 110.2 (d, Jd = 42 Hz). 19F{1H} NMR (376 MHz,
[D6]DMSO, δ/ppm): δ = –63.53. FTIR (ATR corr. ν = /cm–1): ν 2473
6:00 min for
18F]CABS13 and 7:30–8:00 min for
(NCCH3)].
[
18F][Re(6FBiPy)(CO)3Cl], 6:15–6:45 min for
18F][Re(CABS13)(CO)3-
[
[
Table 3. HPLC gradient profiles for the elution of
[
18F]6FBiPy and
18F]CABS13
[ and
[
[
18F][Re(6FBiPy)(CO)3Cl],
as
well
as
18F][Re(CABS13)(CO)3(NCCH3)]. Both analyses were acquired over 15.2 min,
˜
˜
using a 2 mL min–1 flow rate over an octadecylsilane RP monolithic stationary
phase (Merck Chromolith column). Both acetonitrile and water mobile phase
solvents additionally contained 0.1 % v/v of TFA buffer.
(br. m, O-H str.).
Acetonitriletricarbonyl(2-fluoro-8-quinolinolate)rhenium(I)
([Re(2F8HQ)(CO)3Cl]): A mass of pentacarbonylrhenium(I) chloride
(79.6 mg, 0.220 mmol, 1.0 equiv.) was added to a volume of anhy-
drous acetonitrile (10 mL) and refluxed in an inert nitrogen environ-
ment (4 h) to afford the diacetonitriletricarbonlylrhenium(I) chloride
intermediate. A mass of 2Cl8HQ (63.4 mg, 0.353 mmol, 1.1 equiv.)
was dissolved in a volume of anhydrous acetonitrile (2 mL) and
deprotonated via the addition of sodium hydride (7.2 mg,
0.3000 mmol, 1.4 equiv.) over a 0 °C ice bath. The resulting green
solution was then filtered through a PTFE membrane (0.2 μm) and
added to the intermediate solution and left to stir at room tempera-
ture under an inert nitrogen environment (12 h). A volume of aque-
ous trifluoromethanesulfonic acid (0.1 mol L–1, 10 mL) was then
added. The solution was evaporated under reduced pressure to af-
ford a yellow solid in aqueous suspension. The precipitate was iso-
lated via vacuum filtration and purified over a neutral Brockmann
grade II alumina stationary phase using an acetonitrile mobile
phase. The combined fractions were then evaporated under re-
duced pressure to afford a yellow powder (49.8 mg, 14 % yield). 1H
NMR (400 MHz, CD3CN, δ/ppm): δ 8.51 (dd, Jd = 8.92, 7.04 Hz, 1H),
δ 7.43 (t, Jt = 7.96 Hz, 1H), δ 7.33 (dd, Jd = 8.92, 1.24 Hz, 1H), δ 7.06
(d, Jd = 8.00 Hz, 1H), δ = 6.94 (d, Jd = 7.92 Hz, 1H). 13C{1H} NMR
Radiotracers
Time (min)
Acetonitrile [%] Water [%]
2
0
5
95
95
100
95
5
[
[
18F]6FBiPy and
0.1
13
0.1
18F][Re(6FBiPy)(CO3)Cl]
5
3
0.1
0
5
95
95
100
95
5
[
[
18F]CABS13 and
18F][Re(CABS13)(CO3)(NCCH3)] 12
0.1
5
Acknowledgments
This research was funded by the Australian Institute of Nuclear
Science and Engineering (AINSE) via the provision of a Post-
graduate Research Award (PGRA). The National Deuteration Fa-
cility (NDF) is partly funded by the National Collaborative Re-
search Infrastructure Strategy (NCRIS), an Australian Govern-
ment initiative. The authors thank Dr. Calina Betlazar for review-
ing the manuscript. The authors thank Michael Tran and Dr.
Gary Perkins for cyclotron production and National Imaging Fa-
cility (NIF) for support.
(100 MHz, CD3CN, δ/ppm): δ 195.9C≡ O, δ = 194.9C≡ O, δ = 194.2C≡ O
,
δ = 168.0 (d, Jd = 2 Hz), δ = 161.5 (d, Jd = 256 Hz), δ = 145.0 (d,
Jd = 2 Hz), δ = 139.7, δ = 129.6 (d, Jd = 2 Hz), δ = –128.6, δ = 116.5,
δ = 111.5, δ = 109.8 (d, Jd = 33 Hz). 19F{1H} NMR (376 MHz, CD3CN,
δ/ppm): δ = –56.94. FTIR (ATR corr. ν = /cm–1): ν 2925 (w, C–H sp2
˜
˜
Keywords: Alzheimer's disease · Fluorine · Rhenium ·
Microfluidics · Flow chemistry
str.), ν 2853 (w, C–H sp2 str.), ν 2322 (w, N≡ C str.), ν 2020 (s, A′(1)
˜
˜
˜
C≡ O str.), ν 1875 (s, A′(2) & A′′ C≡ O str.).
˜
Protocol for Fluorine-18 Radiolabeling and Analysis: Aqueous
18F]fluoride was produced on an IBA Cyclone 18 Twin Cyclotron via
[1] a) M. M. Mielke, P. Vemuri, W. A. Rocca, Clin. Epidemiol. 2014, 6, 37–48;
b) D. A. Casey, D. Antimisiaris, J. O′Brien, Pharm. Ther. 2010, 35, 208–211.
[2] Alzheimer′s & Dementia 2016, 12, 459–509.
[
the 18O(p,n)18F reaction. Microfluidic radiosyntheses were per-
formed in Discovery Mode using a NanoTek LF Microfluidic Synthe-
sis System (Advion) connected to a standard laptop using NanoTek
software v1.4.0 GMP Lite. Microreactors were made of fused silica
tubing coiled tightly into a brass ring and held by a thermoresistant
[3] Alzheimer′s & Dementia 2019, 15, 321–387.
[4] a) D. S. Eisenberg, M. R. Sawaya, Nature 2017, 547, 170–171; b) M. Goed-
ert, M. G. Spillantini, R. A. Crowther, Brain Pathol. 1991, 1, 279–286; c) C.
Cicognola, G. Brinkmalm, J. Wahlgren, E. Portelius, J. Gobom, N. C. Cullen,
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3563
© 2020 Wiley-VCH GmbH

Full Paper
doi.org/10.1002/ejic.202000433
EurJIC
European Journal of Inorganic Chemistry
O. Hansson, L. Parnetti, R. Constantinescu, K. Wildsmith, H.-H. Chen, T. G.
288–294; c) B. Zhang, B. H. Fraser, M. A. Klenner, Z. Chen, S. H. Liang, M.
Massi, A. J. Robinson, G. Pascali, Chem. Eur. J. 2019, 25, 7613–7617.
[16] G. Pascali, G. Nannavecchia, S. Pitzianti, P. A. Salvadori, Nucl. Med. Biol.
2011, 38, 637–644.
Beach, T. Lashley, H. Zetterberg, K. Blennow, K. Höglund, Acta Neuropa-
thol. 2019, 137, 279–296; d) A. Metaxas, S. J. Kempf, Neural. Regen. Res.
2016, 11, 1579–1581.
[5] a) I. Benilova, E. Karran, B. D. e. Strooper, Nat. Neurosci. 2012, 15, 349–
357; b) J. J. Palop, L. Mucke, Nat. Neurosci. 2010, 13, 812–818; c) R. M.
Koffie, M. Meyer-Luehmann, T. Hashimoto, K. W. Adams, M. L. Mielke, M.
Garcia-Alloza, K. D. Micheva, S. J. Smith, M. L. Kim, V. M. Lee, B. T. Hyman,
T. L. Spires-Jones, Proc. Natl. Acad. Sci. USA 2009, 106, 4012–4017; d) R. M.
Cohen, K. Rezai-Zadeh, T. M. Weitz, A. Rentsendorj, D. Gate, I. Spivak, Y.
Bholat, V. Vasilevko, C. G. Glabe, J. J. Breunig, P. Rakic, H. Davtyan, M. G.
Agadjanyan, V. Kepe, J. R. Barrio, S. Bannykh, C. A. Szekely, R. N. Pechnick,
T. Town, J. Neurosci. 2013, 33, 6245–6256.
[6] N. Vasdev, P. Cao, E. M. van Oosten, A. A. Wilson, S. Houle, G. Hao, X. Sun,
N. Slavine, M. Alhasan, P. P. Antich, F. J. Bonte, P. Kulkarni, MedChemComm
2012, 3, 1228–1230.
[7] a) M. A. Lovell, J. D. Robertson, W. J. Teesdale, J. L. Campbell, W. R. Mark-
esbery, J. Neurol. Sci. 1998, 158, 47–52; b) J. H. Viles, Coord. Chem. Rev.
2012, 256, 2271–2284; c) S. Bolognin, L. Messori, D. Drago, C. Gabbiani,
L. Cendron, P. Zatta, Int. J. Biochem. Cell Biol. 2011, 43, 877–885; d) A. R.
White, X. Huang, M. F. Jobling, C. J. Barrow, K. Beyreuther, C. L. Masters,
A. I. Bush, R. Cappai, J. Neurochem. 2001, 76, 1509–1520.
[8] a) R. A. Cherny, J. T. Legg, C. A. McLean, D. P. Fairlie, X. Huang, C. S.
Atwood, K. Beyreuther, R. E. Tanzi, C. L. Masters, A. I. Bush, J. Biol. Chem.
1999, 274, 23223–23228; b) J.-Y. Lee, J. E. Friedman, I. Angel, A. Kozak,
J.-Y. Koh, Neurobiol. Aging 2004, 25, 1315–1321; c) J.-S. Choi, J. J. Braymer,
R. P. R. Nanga, A. Ramamoorthy, M. H. Lim, Proc. Natl. Acad. Sci. USA
2010, 107, 21990–21995; d) B. P. Cary, A. F. Brooks, M. V. Fawaz, X. Shao,
T. J. Desmond, G. M. Carpenter, P. Sherman, C. A. Quesada, R. L. Albin,
P. J. H. Scott, ACS Med. Chem. Lett. 2015, 6, 112–116.
[17] a) G. Tang, M. Wang, X. Tang, M. Gan, L. Luo, Nucl. Med. Biol. 2005, 32,
553–558; b) D. Kobus, Y. Giesen, R. Ullrich, H. Backes, B. Neumaier, Appl.
Radiat. Isot. 2009, 67, 1977–1984; c) V. Paolillo, S. Riese, J. G. Gelovani,
M. M. Alauddin, J. Labelled Compd. Radiopharm. 2009, 52, 553–558; d) T.
Bourdier, I. Greguric, P. Roselt, T. Jackson, J. Faragalla, A. Katsifis, Nucl.
Med. Biol. 2011, 38, 645–651; e) B. G. Hockley, M. N. Stewart, P. Sherman,
C. Quesada, M. R. Kilbourn, R. L. Albin, P. J. H. Scott, J. Labelled Compd.
Radiopharm. 2013, 56, 595–599; f) A. Haslop, A. Gee, C. Plisson, N. Long,
J. Labelled Compd. Radiopharm. 2013, 56, 313–316; g) Y. Liu, L. Zhu, K.
Plössl, S. R. Choi, H. Qiao, X. Sun, S. Li, Z. Zha, H. F. Kung, Nucl. Med. Biol.
2010, 37, 917–925; h) J. Henrottin, C. Lemaire, D. Egrise, A. Zervosen, B.
Van den Eynde, A. Plenevaux, X. Franci, S. Goldman, A. Luxen, Nucl. Med.
Biol. 2016, 43, 379–389; i) M. Patt, A. Schildan, B. Habermann, S. Fischer,
A. Hiller, W. Deuther-Conrad, S. Wilke, R. Smits, A. Hoepping, G. Wagen-
knecht, J. Steinbach, P. Brust, O. Sabri, Appl. Radiat. Isot. 2013, 80, 7–11;
j) S. Guillouet, D. Patin, O. Tirel, J. Delamare, F. Gourand, J. B. Deloye, M.
Leporrier, L. Barré, Mol. Imaging Biol. 2014, 16, 28–35; k) B. W. Schoultz,
B. J. Reed, J. Marton, F. Willoch, G. Henriksen, Molecules 2013, 18, 7271–
7278; l) S. K. Nandy, M. G. R. Rajan, Appl. Radiat. Isot. 2010, 68, 1944–
1949; m) J. Liu, V. Kepe, A. Žabjek, A. Petrič, H. C. Padgett, N. Satyamurthy,
J. R. Barrio, Mol. Imaging Biol. 2007, 9, 6–16; n) D. Thonon, D. Goblet, E.
Goukens, G. Kaisin, J. Paris, J. Aerts, S. Lignon, X. Franci, R. Hustinx, A.
Luxen, Mol. Imaging Biol. 2011, 13, 1088–1095.
[18] A. Philippidis. Genet. Eng. Biotechn. N. 2018, 38, 6.
[19] a) C.-C. Lee, G. Sui, A. Elizarov, C. J. Shu, Y.-S. Shin, A. N. Dooley, J. Huang,
A. Daridon, P. Wyatt, D. Stout, H. C. Kolb, O. N. Witte, N. Satyamurthy,
J. R. Heath, M. E. Phelps, S. R. Quake, H.-R. Tseng, Science 2005, 310, 1793;
b) M.-W. Wang, Y.-P. Zhang, Y.-J. Zhang, C. Shen, J. Radioanal. Nucl. Chem.
2011, 289, 191; c) W. Sun, J. Wilson, P. Kumar, E. Knaus, L. Wiebe, Curr.
Radiopharm. 2009, 2, 75–82; d) O. Jacobson, D. O. Kiesewetter, X. Chen,
Bioconjugate Chem. 2015, 26, 1–18.
[20] a) M. Pahl, M. Mayer, M. Schneider, D. Belder, K. R. Asmis, Anal. Chem.
2019, 91, 3199–3203; b) P. G. Graham, D. C. Weatherburn, Aust. J. Chem.
1981, 34, 291–300; c) B. Fecher, H. Elias, Inorg. Chim. Acta 1990, 168,
179–187; d) P. Ketan, Y. Bernard, R. G. John, I. M. Veronique, L. Wen-
Hsuan, A. P. John, B. B. Aaron, F. J. Klavs, J. Flow Chem. 2011, 1, 53–55.
[21] a) R. Sakla, A. Singh, R. Kaushik, P. Kumar, D. A. Jose, Inorg. Chem. 2019,
58, 10761–10768; b) S. Pai, M. Hafftlang, G. Atongo, C. Nagel, J. Niesel, S.
Botov, H.-G. Schmalz, B. Yard, U. Schatzschneider, Dalton Trans. 2014, 43,
8664–8678; c) M. J. Stout, A. Stefan, B. W. Skelton, A. N. Sobolev, M.
Massi, A. Hochkoeppler, S. Stagni, P. V. Simpson, Eur. J. Inorg. Chem. 2020,
2020, 292–298; d) N. Rana, H. E. Jesse, M. Tinajero-Trejo, J. A. Butler, J. D.
Tarlit, M. L. von Und Zur Muhlen, C. Nagel, U. Schatzschneider, R. K. Poole,
Microbiology 2017, 163, 1477–1489; e) K. Ling, F. Men, W.-C. Wang, Y.-Q.
Zhou, H.-W. Zhang, D.-W. Ye, J. Med. Chem. 2018, 61, 2611–2635; f) H.-
M. Berends, P. Kurz, Inorg. Chim. Acta 2012, 380, 141–147.
[9] S. H. Liang, J. P. Holland, N. A. Stephenson, A. Kassenbrock, B. H. Rotstein,
C. P. Daignault, R. Lewis, L. Collier, J. M. Hooker, N. Vasdev, ACS Chem.
Neurosci. 2015, 6, 535–541.
[10] a) J. Fripp, P. Bourgeat, O. Acosta, P. Raniga, M. Modat, K. E. Pike, G. Jones,
G. O′Keefe, C. L. Masters, D. Ames, K. A. Ellis, P. Maruff, J. Currie, V. L.
Villemagne, C. C. Rowe, O. Salvado, S. Ourselin, NeuroImage 2008, 43,
430–439; b) A. B. Rodell, G. O'Keefe, C. C. Rowe, V. L. Villemagne, A.
Gjedde, Front. Aging Neurosci. 2017, 8; c) G. D. Rabinovici, A. J. Furst, J. P.
O′Neil, C. A. Racine, E. C. Mormino, S. L. Baker, S. Chetty, P. Patel, T. A.
Pagliaro, W. E. Klunk, C. A. Mathis, H. J. Rosen, B. L. Miller, W. J. Jagust,
Neurology 2007, 68, 1205–1212.
[11] R. A. Cherny, C. S. Atwood, M. E. Xilinas, D. N. Gray, W. D. Jones, C. A.
McLean, K. J. Barnham, I. Volitakis, F. W. Fraser, Y.-S. Kim, X. Huang, L. E.
Goldstein, R. D. Moir, J. T. Lim, K. Beyreuther, H. Zheng, R. E. Tanzi, C. L.
Masters, A. I. Bush, Neuron 2001, 30, 665–676.
[12] S. R. Bareggi, U. Cornelli, CNS Neurosci. Ther. 2012, 18, 41–46.
[13] S. H. Liang, T. L. Collier, B. H. Rotstein, R. Lewis, M. Steck, N. Vasdev, Chem.
Commun. 2013, 49, 8755–8757.
[14] M. A. Klenner, B. Zhang, G. Ciancaleoni, J. K. Howard, H. E. Maynard-
Casely, J. K. Clegg, M. Massi, B. H. Fraser, G. Pascali, RSC Adv. 2020, 10,
8853–8865.
[22] D. Ory, J. Van den Brande, T. de Groot, K. Serdons, M. Bex, L. Declercq, F.
Cleeren, M. Ooms, K. Van Laere, A. Verbruggen, G. Bormans, J. Pharm.
Biomed. Anal. 2015, 111, 209–214.
[15] a) M. A. Klenner, G. Pascali, B. Zhang, T. R. Sia, L. K. Spare, A. M. Krause-
Heuer, J. R. Aldrich-Wright, I. Greguric, A. J. Guastella, M. Massi, B. H.
Fraser, Chem. Eur. J. 2017, 23, 6499–6503; b) M. A. Klenner, G. Pascali, B.
Zhang, G. Ciancaleoni, M. Massi, B. H. Fraser, Aust. J. Chem. 2019, 72,
Received: May 3, 2020
Eur. J. Inorg. Chem. 2020, 3554–3564
www.eurjic.org
3564
© 2020 Wiley-VCH GmbH