Modular syntheses of H4octapa and H2dedpa, and yttrium coordination chemistry relevant to 86Y/90Y radiopharmaceuticals

Article abstract of DOI:10.1039/c4dt00239c

The ligands H₂dedpa, H₄octapa, p-SCN-Bn-H ₂dedpa, and p-SCN-Bn-H₄octapa were synthesized using a new protection chemistry approach, with labile tert-butyl esters replacing the previously used methyl esters as pr

Full text of DOI:10.1039/c4dt00239c

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Modular syntheses of H4octapa and H2dedpa, and
yttrium coordination chemistry relevant to 86Y/90Y
radiopharmaceuticals†
Cite this: Dalton Trans., 2014, 43,
7176
Eric W. Price,a,b Jacqueline F. Cawthray,a,b Michael J. Adamb and Chris Orvig*a
The ligands H2dedpa, H4octapa, p-SCN-Bn-H2dedpa, and p-SCN-Bn-H4octapa were synthesized using a
new protection chemistry approach, with labile tert-butyl esters replacing the previously used methyl
esters as protecting groups for picolinic acid moieties. Additionally, the ligands H2dedpa and p-SCN-Bn-
H2dedpa were synthesized using nosyl protection chemistry for the first time. The use of tert-butyl esters
allows for deprotection at room temperature in trifluoroacetic acid (TFA), which compares favorably to
the harsh conditions of refluxing HCl (6 M) or LiOH that were previously required for methyl ester clea-
vage. H4octapa has recently been shown to be a very promising 111In and 177Lu ligand for radiopharma-
ceutical applications; therefore, coordination chemistry studies with Y3+ are described to assess its
potential for use with 86Y/90Y. The solution chemistry of H4octapa with Y3+ is shown to be suitable via
solution NMR studies of the [Y(octapa)]− complex and density functional theory (DFT) calculations of the
predicted structure, suggesting properties similar to those of the analogous In3+ and Lu3+ complexes. The
molecular electrostatic potential (MEP) was mapped onto the molecular surface of the DFT-calculated
coordination structures, suggesting very similar and even charge distributions between both the Lu3+ and
Y3+ complexes of octapa4−, and coordinate structures between 8 (ligand only) and 9 (ligand and one
H2O). Potentiometric titrations determined H4octapa to have a formation constant (log KML) with Y3+ of
18.3 0.1, revealing high thermodynamic stability. This preliminary work suggests that H4octapa may be a
competent ligand for future 86Y/90Y radiopharmaceutical applications.
Received 22nd January 2014,
Accepted 13th March 2014
DOI: 10.1039/c4dt00239c
www.rsc.org/dalton
isotope such as 177Lu or 90Y, to deliver a site-specific thera-
peutic dose.2–4 The acyclic chelating ligand H4octapa has
Introduction
When designing radiometal-based radiopharmaceuticals, an demonstrated its facile radiolabeling kinetics and excellent
attractive feature is the ability to choose a bifunctional chelat- in vitro/in vivo stability with the single photon emission com-
ing ligand (BFC) that can effectively radiolabel with multiple puted tomography (SPECT) radiometal 111In (t1/2 ∼ 2.8 days),
different radiometals.1 The benefit to this modular aspect of and the β− therapeutic radiometal 177Lu (t1/2 ∼ 6.6 days).5–9
BFC-based radiopharmaceuticals is that by changing the radio- Another attractive isotope is the β− emitting 90Y, with a shorter
metal, the same molecular construct can be used for different half-life (t1/2 ∼ 2.7 days) and higher β− emission energy (2288
types of imaging and therapy, with a variety of half-lives and keV vs. 498 keV) than 177Lu, allowing 90Y to treat much larger
emission energies.1 A radiopharmaceutical containing an and more poorly vascularized tumors (maximum in vivo β−
appropriate BFC can be radiolabeled with an imaging isotope range of ∼12 mm for 90Y vs. ∼2 mm 177Lu).2,3,5,10–12 The
such as 111In for pre-therapy imaging, to confirm tumor local- increased range of β− particles emitted by 90Y can provide
ization and calculate dosimetry. Pre-therapy imaging can then therapeutic effects to neighboring tumors up to ∼550 cell
be followed with the same BFC-containing radiopharmaceuti- diameters away via the “crossfire effect”.2,3,5,10–12 The yttrium
cal (e.g. DOTA-trastuzumab), radiolabeled with a therapeutic isotopologue 86Y emits β+ particles for positron emission tomo-
graphy (PET) and is well suited for pre-therapy imaging
and dosimetry. The substitution of 86Y (PET) for 90Y (therapy)
is seamless, with the aqueous chemistry and radiolabeling
aMedicinal Inorganic Chemistry Group, Department of Chemistry, University of
British Columbia, 2036 Main Mall, Vancouver, BC V6 T 1Z1, Canada
bTRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6 T 2A3, Canada.
properties of both isotopes of yttrium being identical; however,
the nuclear properties of 86Y are not ideal.13–18 Although see-
E-mail: orvig@chem.ubc.ca; Fax: +1(604) 822-2847; Tel: +1(604) 822-4449
mingly more attractive than 111In due to its β+ emission for PET
†Electronic supplementary information (ESI) available: 1H/13C NMR spectra and
and its identical chemistry to 90Y for exchange, the serious
FT-ATR-IR spectra of final synthesized compounds. See DOI: 10.1039/c4dt00239c
7176 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
limitations of 86Y include a shorter half-life (14.7 h) than the proficiency with 111In and 177Lu (25 °C, 15 min, >95% RCY),
common therapeutic radiometals 177Lu and 90Y, high energy β+ H4octapa should demonstrate similarly fast radiolabeling kine-
emission (2019, 2335 keV) that decrease image quality, and a tics and robust stability with 86Y/90Y. Towards this end, we
number of high energy γ emissions (1077, 1153, 1854, 1921 have performed potentiometric titrations with H4octapa and
keV) that present radiation dose concerns and require substan- Y3+ to determine the thermodynamic formation constants (log
tial radioactive shielding for handling and transport.5 For KML) and pM value, and have prepared the coordination
these reasons, BFCs such as DOTA and CHX-A″-DTPA, which complex of H4octapa with non-radioactive Y3+ to compare its
can effectively bind multiple radiometals such as 111In, 177Lu, properties to the previously studied complexes with In3+ and
and 86Y/90Y, are valued (Chart 1). 3p-C-NETA is a new NOTA Lu3+.7,8 Future studies with radioactive 86Y/90Y will assess
derivative that has shown excellent properties with 177Lu and radiolabeling performance and in vitro/in vivo stability with
86Y/90Y, radiolabeling in only 5 minutes at room temperature, H4octapa. The current challenges facing yttrium radiochem-
but its radiolabeling properties with 111In are uncertain istry are the scarce availability of 86Y in North America, the
(Chart 1).19–21
non-ideal nuclear properties of 86Y, and the lack of concomi-
Although excellent thermodynamic stability and kinetic tant β+/γ emissions of 90Y that make detection and handling
inertness are crucial properties for radiometal-BFC complexes, more challenging.
fast radiolabeling at ambient temperature is also a very attrac-
Antibody conjugates are large biomolecules (∼150 kDa) that
tive property, because these radiometals are often used with have long biological half-lives (∼2–3 weeks), and are therefore
antibody-based radiopharmaceuticals that are heat sensi- well matched with longer-lived isotopes such 111In, 177Lu, and
tive. H4octapa has been shown to radiolabel with 111In and 90Y.27 Peptide conjugates typically have much faster localiz-
177Lu in under 15 minutes at room temperature, where DOTA ation and clearance times than do antibodies in vivo, but have
requires temperatures of 37–90 °C over
a period of still been successfully used with the above-mentioned radio-
30–60 minutes.1,7,22–26 CHX-A″-DTPA can effectively radiolabel metals – the octreotide-based somatostatin receptor targeting
at ambient temperatures (30–60 min), but to achieve optimal peptide-conjugates DOTA-TOC and DOTA-TATE.11,12,23,28–35 In
radiolabeling yields it typically requires conditions of 37–75 °C order to facilitate the inclusion of H2dedpa and H4octapa into
over a period of 30–60 minutes (Chart 1).1,7,22–26 Although the peptide conjugates, we have developed new protection chem-
optimal radiolabeling temperatures for DOTA and CHX-A″- istry to allow them to conjugate to peptides synthesized on-
DTPA are high (60–100 °C), antibody-conjugates can be effec- resin (e.g. Wang resin), and deprotect under standard con-
tively radiolabeled at 37 °C, albeit with longer reaction times, ditions (e.g. mixture of trifluoroacetic acid–dichloromethane–
lower yields, and typically inconsistent yields.8 Because of its triisopropylsilane). Standard peptide coupling reactions of
Chart 1 H2dedpa, H4octapa, and BFC derivatives p-SCN-Bn-H2dedpa and p-SCN-Bn-H4octapa; the current “gold standard” chelating ligands for
86Y/90Y p-SCN-Bn-DOTA (C-DOTA) and p-SCN-Bn-CHX-A’’-DTPA, and the promising new 86Y/90Y ligand 3p-C-NETA.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7176–7190 | 7177

View Article Online
Paper
Dalton Transactions
these fully deprotected ligands with peptides is not possible,
as the presence of 2 (H2dedpa) or 4 (H4octapa) free carboxylic
acid groups can also form amide linkages. Previously pub-
Results and discussion
Synthesis and characterization
lished synthetic routes for H2dedpa and H4octapa have relied Previously, only methyl-ester protection chemistry was compa-
on methyl ester protection of the picolinic acid moiety, requir- tible with the picolinic acid moiety of the “pa” family of
ing harsh deprotection conditions of LiOH or refluxing HCl ligands (Chart 1), and attempts to utilize tert-butyl esters had
(6–12 M). By utilizing nosyl protection chemistry, we have failed due to incompatibility with reductive amination
incorporated tert-butyl ester protection of the picolinic acid methods.36 The commercially available starting material
moiety, allowing for room temperature deprotection in trifluor- 6-methylpicolinic acid was tert-butyl ester protected following
oacetic acid. This new synthetic route is reported for H2dedpa, a modified literature procedure, which utilized tert-butyl-2,2,2-
H4octapa, p-SCN-Bn-H2dedpa, and p-SCN-Bn-H4octapa. The trichloroacetimidate (Scheme 1).37 The tert-butyl protected
potential of the ligand H4octapa for use with 86Y/90Y is also product (1) was synthesized here for the first time, and then
investigated, through formation and study of the non- transformed to the alkyl-bromide derivative (2) using a modi-
radioactive [Y(octapa)]− complex, DFT calculations of the fied literature procedure with N-bromosuccinimide and
[Y(octapa)]− coordination structure and molecular electrostatic benzoyl peroxide as radical initiator.38 The general synthetic
potential (MEP), and potentiometric titrations of H4octapa scheme relies heavily on the nosyl protection group, and
with Y3+ to determine its solution thermodynamic stability follows the same general pathway as the recently published
parameters (pM, log KML).
nosyl-based synthesis of p-SCN-Bn-H4octapa.8,9 The enhanced
Scheme 1 Syntheses of H2dedpa (6) and H4octapa (8) using tert-butyl ester protection chemistry. (i) CH2Cl2, BF3·etherate (20 μL mmol−1 starting
material), RT, 20 h; (ii) CCl4, Bz2O2, N-bromosuccinimide (NBS, 0.7 equiv.), 70 °C, 4 h; (iii) THF, NaHCO3, RT, 24 h; (iv) DMF, Na2CO3, 80 °C, 48 h; (v)
THF, thiophenol (2.3 equiv.), K2CO3, 50 °C, 48 h; (vi) trifluoroacetic acid (TFA) (1 mL) and CH2Cl2 (1 mL), RT, 16 h, (cumulative yield of ∼26% over 4
steps); (vii) MeCN, Na2CO3, 60 °C, 48 h; (viii) trifluoroacetic acid (TFA) (1 mL) and CH2Cl2 (1 mL), RT, 16 h (cumulative yield of ∼27% over 5 steps).
7178 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
lability of the tert-butyl ester protecting group resulted in some the methyl ester protected picolinic acid moiety by transform-
decomposition during synthesis, which was partially respon- ing them to a mixture of carboxylic acids and primary alcohols,
sible for decreased yields relative to the analogous methyl but for the more labile tert-butyl ester protection group com-
ester-based synthetic route.6–8 Additionally, the solvent metha- plete ester cleavage and reduction to primary alcohols was
nol was problematic, as during silica column chromatography observed, and no product could be isolated.7 It has not been
some methyl ester product was formed from the tert-butyl until our recent implementation of nosyl protection chemistry
ester, creating byproducts and decreasing yields. Because the for the synthesis of these ligands that implementation of tert-
combination of methanol and dichloromethane was most butyl ester protection chemistry could be realized.8
effective for separating intermediates via column chromato-
An important deviation of the synthetic pathway reported
graphy, it was still used; however, methanol was avoided when- here from the previously published nosyl-based synthesis of
ever possible, such as when rinsing, filtering, rotary p-SCN-Bn-H4octapa is the enhanced modularity, with the tert-
evaporating, and heating. The previously used synthetic proto- butyl ester protected precursors (tBu)2dedpa (5) and p-NO2-Bn-
cols for H2dedpa relied upon a reductive amination reaction, (tBu)2dedpa (13) being arranged as precursors to (tBu)4octapa
which utilized NaBH4 to reduce the Schiff-base (imine) to a (7) and p-NO2-Bn-(tBu)4octapa (15) (Schemes 1 and 2). Using
secondary amine.36,39–43 This method produced byproducts at this layout, common synthetic precursors are used for both
Scheme 2 Synthesis of p-SCN-Bn-H2dedpa (14), and p-SCN-Bn-H4octapa (16) using tert-butyl ester protection chemistry. (i) THF, NaHCO3, 50 °C,
24 h; (ii) DMF, Na2CO3, 80 °C, 48 h; (iii) THF, thiophenol (2.3 equiv.), K2CO3, 60 °C, 48 h; (iv) 5 mL of (1 : 1) glacial acetic acid : 3 M HCl, Pd/C (20 wt%),
H2 (g), RT, 1 h; (v) 3 M HCl, heat to reflux for 1 minute; (vi) thiophosgene in DCM (15 equiv.), 3 M HCl, RT, 24 h (cumulative yield of ∼18% in 6 steps);
(vii) MeCN, K2CO3, 80 °C, 48 h; (viii) 5 mL of (1 : 1) glacial acetic acid : 3 M HCl, Pd/C (30 wt%), H2 (g), RT, 1 h; (ix) 3 M HCl, heat to reflux for 1 minute;
(x) thiophosgene in DCM (15 equiv.), 3 M HCl, RT, 24 h (cumulative yield of ∼7% in 7 steps).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7176–7190 | 7179

View Article Online
Paper
Dalton Transactions
H2dedpa and H4octapa, rendering the reaction schemes nearly access to less harsh room temperature deprotection in trifluoro-
identical for each. The utility of this approach is that the 64Cu acetic acid. Synthesis of p-SCN-Bn-(tBu)2dedpa or p-NH2-Bn-
and 67Ga/68Ga ligand H2dedpa can be made in the same pro- (tBu)2dedpa for direct conjugation to on-resin peptides should
cedure as the 111In, 177Lu, and 86Y/90Y ligand H4octapa, afford easy purification and high yields, which was not pre-
sharing common precursors and resulting in broad radiometal viously possible using the methyl ester protected derivative.
application from one synthetic scheme. Previously, in order to
Yttrium coordination chemistry and density functional theory/
molecular electrostatic potential structure prediction
optimize purification and yields, p-SCN-Bn-H4octapa was syn-
thesized using a different reaction ordering that did not allow
for this overlap.8 Despite this alternate approach, the same
reaction pathways (Schemes 1 and 2) can be performed using
the previously published methyl ester protected picolinic acid
group instead of its tert-butyl protected analogue. The draw-
back to the methyl ester protected picolinic acid moiety lies in
the purification of the (Me)2dedpa intermediate, which is
more challenging than the more lipophilic (tBu)2dedpa deriva-
tive; however, the enhanced stability of the methyl ester protec-
tion group affords higher yields, at the price of harsh
deprotection conditions.7,8,36 H2dedpa and H4octapa have pre-
viously been synthesized as HCl salts, but for this new syn-
thesis they were lyophilized after final high-performance
liquid chromatography (HPLC) purification as their trifluoro-
acetic acid salts. The final trifluoroacetic acid salts (formulae
determined from elemental analysis results) H2dedpa·2tri-
fluoroacetic acid·1.5H2O and H4octapa·2trifluoroacetic acid
were more soluble in water than their HCl counterparts, and
were now soluble in methanol.
The Y3+ complex of H4octapa, [Y(octapa)]− (9), was synthesized
by mixing 8 (H4octapa·2trifluoroacetic acid) with YCl3·6H2O in
deionized water, adjusting the pH to ∼4–5 with NaOH (0.1 M),
and stirring at room temperature for 1 hour. After confirming
formation of the coordination complex by mass spectrometry,
the product was studied by NMR in both D2O and DMSO-d6
(Fig. 1, Fig. S32†). The same [Y(octapa)]− NMR sample was run
During the syntheses of the bifunctional derivatives
p-SCN-Bn-H2dedpa (14) and p-SCN-Bn-H4octapa (16), it was
observed that the hydrogenation reaction conditions used with
the precursors p-NO2-Bn-(tBu)2dedpa (13) and p-NO2-Bn-
(tBu)4octapa (15) surprisingly did not effect full tert-butyl ester
deprotection. The hydrogenation was performed using
1 : 1 glacial acetic acid and hydrochloric acid (3 M) as solvent
at room temperature for 1 hour, and mass spectrometry con-
firmed that p-NH2-Bn-(tBu)2dedpa (not isolated, used without
further purification) was formed without tert-butyl ester clea-
vage. Mass spectrometry suggested that p-NH2-Bn-(tBu)2H2-
octapa was formed after hydrogenation, with two tert-butyl
ester groups being cleaved and two remaining attached;
however, this intermediate was not isolated and was used
without further purification. Full deprotection of these inter-
mediates was performed after isothiocyanate formation.
H2dedpa (6) was synthesized in 4 synthetic steps and
H4octapa (8) in 5 steps, with cumulative yields of ∼26% and
∼27%, respectively. The more synthetically challenging bifunc-
tional derivative p-SCN-Bn-H2dedpa (14) was synthesized in 6
steps and p-SCN-Bn-H4octapa (16) in 7 steps, with cumulative
yields of ∼18% and ∼7%, respectively. The previously reported
nosyl-based synthesis of H4octapa and p-SCN-Bn-H4octapa that
utilized methyl ester protection chemistry of the picolinic acid
moieties were produced in higher cumulative yields of ∼45–50%
and ∼25–30%, respectively.8 H2dedpa (6) and p-SCN-Bn-
H2dedpa (14) have not been previously synthesized using nosyl
protection chemistry, and are reported here for the first time.
Largely due to the enhanced lability of the tert-butyl ester pro-
Fig. 1 1H NMR spectra of H4octapa (400 MHz, 25 °C, top, referenced to
H-O-D at 4.75 ppm), [Lu(octapa)]− in D2O (600 MHz, 25 °C, middle),8 and
[Y(octapa)]− in D2O (400 MHz, 25 °C, bottom) showing the 1H NMR splitting
patterns in the D2O spectra of the Y3+ and Lu3+ complexes to be similar,
suggesting that they may possess similar solution structures and coordi-
tected picolinic acid, a penalty in cumulative yield was paid for nation numbers (e.g. a mixture of static and fluxional isomers, vide infra).
7180 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
repeatedly over a period of 3–4 weeks (e.g. 1H, 13C, VT, 2D
NMR) with no discernable changes observed in the acquired
spectra, suggestion that equilibrium was reached quickly. A
comparison of the 1H NMR spectra of [Lu(octapa)]− (Fig. 1,
middle) and [Y(octapa)]− (Fig. 1, bottom) in D2O reveals simi-
larly sharp signals and coupling patterns, but slightly different
chemical shift values, suggesting similar solution structures.8
Different absolute chemical shift values are expected, as Y3+
and Lu3+ have different electronic properties, but these two
metal ions are known to form coordination structures with
similar geometries (CN = 8–9), as they both have similar ionic
radii (CN = 8, 101.9 pm vs. 97.7 pm, and CN = 9, 107.4 pm vs.
103.2 pm, respectively).44 At first glance, the sharp and well-
resolved peaks and couplings observed for both the Y3+ and
Lu3+ complexes of octapa4− suggest little fluxional behavior in
solution at 25 °C, but the large number of signals and compli-
Fig. 2 Variable temperature (VT) NMR experiments with [Y(octapa)]−
(D2O, 400 MHz), showing a mixture of sharp signals and broad signals at
25 °C, with the broad signals rapidly changing and beginning to coalesce
as the temperature was increased to 85 °C in 20 °C increments (blue
arrows), suggesting these signals arise from fluxional isomers, and the
sharp signals (red arrows) changing very little upon increased tempera-
ture, suggesting these signals arise from a stable static isomer.
1
cated coupling patterns relative to the simple H NMR spectra
of [In(octapa)]− and the unbound ligand H4octapa suggest the
presence of multiple static isomers (Fig. 1).7 In addition to
multiple geometric isomers, it is possible that upon coordi-
nation of a single water molecule to the [Y(octapa)]− complex,
the 9-coordinate [Y(octapa)(H2O)]− geometry is lower symmetry
giving a greater number of non-equivalent protons and a more
complicated spectrum. The NMR spectrum of [Y(octapa)]− in
DMSO-d6 was significantly different than in D2O, potentially as
a result of coordination of DMSO to Y3+ (Fig. S32†). The
increased broadening of NMR signals (Fig. S32†) in DMSO-d6
also suggests an increased rate of fluxional isomerization.
To further probe the coordination structure and isomeriza-
tion of the [Y(octapa)]− complex, variable temperature-NMR
(VT-NMR) and 2D-COSY/HSQC experiments were performed.
Upon increasing the NMR sample (D2O) temperature from
25 °C to 85 °C, the multiple sharp peaks observed in the 25 °C
spectrum did not change drastically, which suggests the pres-
ence of a stable static isomer (red arrows, Fig. 2). For the broad
signals observed at 25 °C (blue arrows, Fig. 2), substantial
change in the 1H NMR spectrum was observed after a small
change in temperature between 25 °C and 45 °C, with some
signals broadening to such an extent that they nearly dis-
appeared from the spectrum. This trend of broadening and
coalescing of signals continued up to 85 °C; however, even at
85 °C some signals remained sharp. It can be seen in Fig. 2
that some sharp signals remain the same between 25 °C and
45 °C (red arrows), where other more broad signals that
overlap these sharp signals are observed to rapidly broaden
and coalesce (blue arrows). This observation suggests that a
single major static isomer is present in solution (red arrows,
sharp peaks that remain as temperature is increased), with at
least one fluxional isomer existing at the same time (blue
arrows). The signals highlighted by red arrows (Fig. 2)
resemble the more simple spectra obtained previously for [In-
(octapa)]−, where only a single static isomer was present, with
several doublets and simple multiplets arising from neighbor-
ing protons on the H4octapa scaffold becoming diastereotopic
upon In3+ coordination, and coupling to each other.7 Because
an aqua ligand could be involved in the fluxional isomeriza-
tion of [Y(octapa)]−. These VT-NMR experiments suggest that
one static isomer and at least one set of fluxional isomers are
present under aqueous conditions at ambient temperature,
with the fluxional species rapidly broadening upon heating,
and the static isomer changing very little upon heating (Fig. 2).
The 2D-COSY experiment was used to probe 1H–1H corre-
lations, to see if distinct static isomers could be observed at
ambient temperature and confirm VT NMR results (Fig. 3 and
4). An expansion of the aromatic region of the 2D-COSY spec-
trum revealed strongly correlated cross-peaks between the
sharp signals, which were attributed to a single major static
isomer in solution (Fig. 3). Additionally, weak cross-peaks
Fig. 3 1H–1H COSY NMR (600 MHz, D2O, 25 °C) expansion of aromatic
signals in the spectrum of [Y(octapa)]−, showing no correlations
between broad signals arising from a fluxional species (blue arrows), and
sharp signals arising from a single static isomer (see Fig. S32† for full
yttrium can form 9-coordinate complexes, the coordination of spectrum).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7176–7190 | 7181

View Article Online
Paper
Dalton Transactions
Fig. 4 1H–1H COSY NMR (600 MHz, D2O, 25 °C) expansion of alkyl
signals in the spectrum of [Y(octapa)]−, showing no correlations
between broad signals arising from a fluxional species (blue arrows,
identified by VT-NMR, Fig. 2), and sharp signals arising from a single
static isomer (see Fig. S32† for full spectrum).
Fig. 5 1H–13C HSQC NMR (400/100 MHz, D2O, 25 °C) expansion of
aromatic signals in the spectrum of [Y(octapa)]−, showing correlations to
∼5 unique 13C signals, suggesting the presence of a single static isomer
(red arrows), along with a fluxional species (blue arrows) (13C NMR
spectra externally referenced to MeOH in D2O) (see Fig. S33† for full
spectrum).
between the broad signals that were assigned as a fluxional
isomer in VT NMR experiments (blue arrows, Fig. 3) were
observed, and showed no correlation to the sharp signals. The
same conclusion can be drawn from the COSY alkyl-region
expansion (blue arrows, Fig. 4), where the broad signals which
were observed to rapidly disappear after the temperature was
increased during VT-NMR experiments (Fig. 2), can now be
seen to weakly correlate which each other, and further to show
no correlation with the sharp signals assigned as the major
static isomer. These 2D-NMR experiments confirm the results
of the VT-NMR experiments, suggesting the presence of one
major static isomer in solution, along with at least one fluxio-
nal species. Fluxional species typically produce very weak NMR
data, demonstrated by weak cross peaks correlating to a fluxio-
nal species in the 2D COSY experiments discussed above
(Fig. 3 and 4), and strong cross peaks correlating to the major
static isomer.
the free H4octapa ligand (Fig. 1, top). Expansion of the alkyl
region of the HSQC NMR spectrum revealed strong corre-
lations to ∼4 unique carbon atoms (red arrows, Fig. 6), with
many weaker signals likely corresponding to the fluxional
species (blue arrows, Fig. 6), again suggesting the presence of
a single major static isomer of [Y(octapa)]−, along with a fluxio-
nal species. Because yttrium typically forms 8–9 coordinate
complexes, there is likely a mixture of [Y(octapa)]− and
[Y(octapa)(H2O)]−, with the single aqua ligand potentially
exchanging in solution and producing the observed fluxional
species. It is unclear if these two isomers are a mixture
Because 2D-HSQC experiments utilize 1H–13C heteronuclear
correlations, the signals obtained for fluxional species are even
weaker than those for the COSY spectra, as they rely on 13C
1
1
NMR data collection as well as H NMR. The 2D-HSQC H–13C
single-bond heteronuclear correlation experiment revealed a
strong set of cross-peaks arising from a single static isomer
(red arrows, Fig. 5), with only weak signals and cross-peaks
arising from the observed fluxional species (blue arrows,
Fig. 5). For the unbound and symmetric ligand H4octapa, one
would expect to see 3 distinct aromatic 13C signals arising
from the three distinct C–H pyridines (∼C2v symmetry), and
for a single static isomer of [Y(octapa)]− there would be 3–6
signals (depending on symmetry). It was observed that the
aromatic region of the [Y(octapa)]− HSQC NMR spectrum
displayed ∼5 unique and strong 13C correlation signals,
supporting the hypothesis of a single major static isomer
(Fig. 5). There are 6 unique alkyl-carbon atoms present in
H4octapa/[Y(octapa)]−, which manifest as 3 sharp signals for
Fig. 6 1H–13C HSQC NMR (400/100 MHz, D2O, 25 °C) expansion of
alkyl-region signals in the spectrum of [Y(octapa)]−, showing corre-
lations to ∼4 unique and strong 13C signals (red arrows), suggesting the
presence of 1 static isomer, along with several weak correlations arising
from a fluxional species (blue arrows) (13C NMR spectra externally refer-
enced to MeOH in D2O) (see Fig. S33† for full spectrum).
7182 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
between [Y(octapa)]− and [Y(octapa)(H2O)]−, or different geo- [In(octapa)]− have been calculated and studied previously.7,8
metric isomers. RP-HPLC purification (no buffer, deionized The molecular electrostatic potential (MEP) surfaces shown in
water and acetonitrile) of [Y(octapa)]− revealed a single broad Fig. 7 (bottom) show very similar charge distributions between
peak, as is typically observed for all H2dedpa/H4octapa free the 8- or 9-coordinate complexes of octapa4− with Y3+, and are
ligands and metal complexes to date, suggesting that the two very similar to those calculated for the Lu3+ and In3+ com-
isomers have similar or identical physical properties (e.g. plexes.7,8 These observations suggest that the solution behav-
polarity) (Fig. S34†). These VT-NMR and 2D-NMR studies have ior of the coordination complexes of octapa4− with Lu3+ and
not been performed on the [Lu(octapa)]− complex, but the Y3+ may be very similar, with both metal complexes having the
1
similarities between their basic H-NMR spectra (Fig. 1) would same overall 1− charge and nearly identical charge distri-
suggest that [Lu(octapa)]− also exists in solution as a mixture butions as predicted by DFT/MEP calculations, foreshadowing
of a single static isomer and a fluxional species.
both metal complexes to share in vivo properties.
It is difficult to draw concrete conclusions, but these NMR
studies suggest that [Y(octapa)]− is present as a mixture of a
single major isomer and a fluxional species. The NMR sample
of [Y(octapa)]− (D2O) was run repeatedly over a period of 3–4
weeks with no changes, suggesting that equilibrium was
reached rapidly. These isomers could not be separated by
RP-HPLC (Fig. S34†). It is important to note that these NMR
solution studies cannot be used to predict in vivo kinetic stabi-
lity, and radiochemical experiments (e.g. in vitro serum stabi-
lity and in vivo biodistribution studies with 86Y/90Y) must be
performed.
Coordination geometries were calculated by density func-
tional theory (DFT) calculations for both the binary 8-coordi-
nate [Y(octapa)]− (Fig. 7a) complex and the 9-coordinate
monohydrate [Y(octapa)(H2O)]− (Fig. 7b), as yttrium can form
either 8- or 9-coordinate complexes in solution. The DFT struc-
tures of the complexes [Lu(octapa)]−, [Lu(octapa)(H2O)]−, and
Thermodynamic stability
The ligands H2dedpa and H4octapa have previously been eval-
uated with a number of metal ions, including Cu2+, Ga3+, In3+,
and Lu3+.7,8,36,39,40 Recently, H4octapa has been found to have
exceptional radiolabeling properties with 111In and 177Lu, and
has shown very promising in vivo behavior when compared to
the industry “gold standards” DTPA and DOTA.7,8 Thermo-
dynamic stability constants for the ligands H2dedpa,
H4octapa, DTPA, and DOTA, with the metal ions In3+, Lu3+,
and Y3+ are shown in Table 1. The formation constant (log
KML) and pM value for H4octapa with Y3+ has been experi-
mentally determined for the first time using EDTA compe-
tition potentiometric titrations. The pM (−log[M]) value is a
condition-dependent thermodynamic parameter ([M] = 1 µM,
[L] = 10 µM, pH 7.4, 25 °C) that is considered more biologically
relevant than log KML values, as it additionally takes into
account conditions such as ligand basicity, metal ion hydro-
lysis, physiological pH, and ligand : metal ratio.45–48 As pre-
viously determined, H4octapa has been shown to possess
exceptionally high log KML and pM values with In3+ and Lu3+,
with pM values exceeding those of the ligands DTPA and
DOTA (Table 1). The log KML value for H4octapa with Y3+ has
been experimentally determined here to be 18.3 0.1 (pM =
18.1) (Table 1). A comparison of the thermodynamic stability
constant of H4octapa to other ligands with Y3+ shows the log
Table 1 Formation constants (log KML) and pMa values for In3+, Lu3+
and Y3+ complexes of relevant chelating ligand
,
Ligand
M3+
log KML
pMa
Ref.
dedpa2−
octapa4−
In3+
In3+
Lu3+
Y3+
26.60(4)
26.8(1)
20.08(9)
18.3(1)
29.0
22.6
21.2, 21.9, 22.5
23.9(1)
21.6(1), 23.6, 25, 29.2
24.3, 24.4, 24.9
18.3
25.9
26.5
19.8
18.1
7
7
8
DTPA4−
In3+
Lu3+
Y3+
25.7
51, 53
8, 51
49, 50
45, 51
8, 54–56
49, 50, 57
58
59
Fig. 7 In silico DFT structure predictions: (a) 8-coordinate structure of
19.1
[Y(octapa)]− (top, left); (b) 9-coordinate structure of [Y(octapa)(H2O)]−
17.6–18.3b
18.8
(top, right), as well as the MEP polar-surface area maps (bottom) pre- DOTA4−
In3+
Lu3+
Y3+
17.1
dicting the charge distribution over the solvent-exposed surface of the
19.3–19.8b
18.7
metal complexes (red
= negative, blue = positive, representing a
Transferrin
In3+
Lu3+
maximum potential of 0.200 au and a minimum of −0.200 au, mapped
onto electron density isosurfaces of 0.002 Å−3), demonstrating very little
difference between the 8- and 9-coordinate solution structures in
terms of both geometry and charge distribution, suggesting little
change in physical properties between the 8- and 9-coordinate
geometries.
11.08
—
a Calculated for 10 μM total ligand and 1 μM total [M3+] at pH 7.4 and
25 °C. b pM values for DTPA and DOTA with Y3+ were calculated for
49
this work, based upon previously determined log KML
,
ligand
pKa,50,51 and metal ion pKa values.52
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7176–7190 | 7183

View Article Online
Paper
Dalton Transactions
KML value to be similar to DTPA, but lower than the current (Siliaplate™, 60 Å pore size, 250 μM plate thickness, Silicycle,
“gold standard” DOTA (Table 1). Values of pM were not avail- Quebec, QC). Flash column silica gel was purchased from Sili-
able for DTPA and DOTA from the literature, and so values cycle (Siliaflash® Irregular Silica Gels F60, 60 Å pore size,
49
were calculated based on previously determined log KML
,
40–63 mm particle size, Silicycle, Quebec, QC). Automated
ligand pKa,50,51 and metal pKa values.52 From a range of log column chromatography was performed using a Teledyne Isco
KML values that have been published for DTPA (21.2, 21.9) and (Lincoln, NE) CombiFlash® Rf automated system with solid
DOTA (24.4, 24.9), pM values were calculated to be 17.6–18.3 load cartridges packed with flash column silica gel and
(DTPA) and 19.3–19.8 (DOTA) (Table 1). Although thermo- RediSep Rf Gold® reusable normal-phase silica columns
1
dynamic parameters are very valuable, the in vivo stability and neutral alumina columns (Teledyne Isco, Lincoln, NE). H
(kinetic inertness/stability at high dilution in vivo) is the most and 13C NMR spectra were recorded on Bruker AV300, AV400,
important factor for determining the usefulness of radiometal or AV600 instruments; all spectra were internally referenced to
complexes, as thermodynamic stability may not correlate to residual solvent peaks except for 13C NMR spectra in D2O,
in vivo stability. To this end, serum stability assays and studies which were externally referenced to a sample of CH3OH–D2O.
in mice must be performed in the future with the 86Y/90Y com- Low-resolution mass spectrometry was performed using a
plexes of H4octapa to properly assess the potential utility for Waters liquid chromatography-mass spectrometer (LC-MS)
radiopharmaceutical applications.
consisting of a Waters ZQ quadrupole spectrometer equipped
with an ESCI electrospray/chemical ionization ion source and
a Waters 2695 HPLC system (Waters, Milford, MA). High-
resolution electrospray-ionization mass spectrometry (EI-MS)
was performed on a Waters Micromass LCT time of flight
Conclusions
The ligands H2dedpa, H4octapa, p-SCN-Bn-H2dedpa, and instrument. Microanalyses for C, H, N were performed on a
p-SCN-Bn-H4octapa were synthesized using a new tert-butyl Carlo Erba EA 1108 elemental analyzer. The HPLC system used
ester protection group scheme, allowing for deprotection at for purification of compounds consisted of a semi-preparative
room temperature in TFA, which compares favorably to the reverse phase C18 Phenomenex Synergi hydro-RP (80 Å pore
harsh conditions of refluxing HCl (6–12 M) or LiOH that were size, 250 × 21.2 mm, Phenomenex, Torrance, CA) column con-
previously required for methyl ester cleavage. Additionally, the nected to a Waters 600 controller, a Waters 2487 dual wave-
ligands H2dedpa and p-SCN-Bn-H2dedpa were synthesized length absorbance detector, and a Waters delta 600 pump.
using nosyl protection chemistry for the first time. The less
tert-Butyl 6-(methyl)picolinate (1). Commercially available
harsh deprotection conditions afforded by the tert-butyl ester 6-methylpicolinic acid (15.00 g, 109.38 mmol) was suspended
groups may allow these ligands to be incorporated into sensi- in CH2Cl2 (500 mL). To the reaction mixture was added tert-
tive molecular scaffolds in the future, such as peptides syn- butyl-2,2,2-trichloroacetimidate (47.80 g, 218.8 mmol), fol-
thesized on-resin (e.g. Wang resin), accommodating the lowed by BF3·etherate (20 μL mmol−1 of starting material,
standard deprotection conditions of a mixture of trifluoro- ∼2.19 mL), and the mixture was stirred overnight at ambient
acetic acid, dichloromethane, and triisopropylsilane. H4octapa temperature. The reaction mixture volume was reduced
has been recently shown to be a very promising 111In and 177Lu in vacuo to ∼100 mL; the resulting white solid was filtered
ligand for radiopharmaceutical applications, and density func- through a fritted glass filter and discarded, and the filtrate
tional theory (DFT) calculations of the predicted structures of reduced to dryness in vacuo. The crude product was then resus-
H4octapa with Y3+, In3+ and Lu3+ look similar. Potentiometric pended in hexanes (∼50 mL) and filtered again to remove
titrations have determined H4octapa to have a formation con- additional white precipitate (unreacted tert-butyl-2,2,2-trichloro-
stant (log KML) with Y3+ of 18.3 0.1 (pM = 18.1), revealing acetimidate, visualized by ninhydrin staining of TLC plates).
high thermodynamic stability. These results suggest that along The crude product was purified by silica chromatography
with 111In and 177Lu, H4octapa may be a competent ligand for (CombiFlash Rf automated column system; 80 g HP silica; A:
86Y/90Y radiopharmaceutical applications.
CH2Cl2, B: MeOH, 100% A to 95% A gradient) to yield product
1 as a clear colourless oil, which later solidified into a off-
white wax (73%, ∼15.4 g) (Rf: 0.6, TLC in 95% CH2Cl2: 5%
MeOH). 1H NMR (400 MHz, CDCl3, 25 °C) δ: 7.75 (d, J =
7.8 Hz, 1H), 7.60 (t, J = 7.7 Hz, 1H), 7.21 (d, J = 7.9 Hz, 1H),
2.56 (s, 3H), 1.55 (s, 9H). 13C NMR (100 MHz, CDCl3, 25 °C)
Experimental
General remarks
All solvents and reagents were purchased from commercial δ: 163.9, 158.7, 148.5, 136.7, 126.0, 121.6, 81.8, 27.8, 24.3.
suppliers (Sigma Aldrich, St. Louis, MO; TCI America, Port- HR-ESI-MS calcd for [C11H15NO2 + H]+: 194.1181; found:
land, OR; Fisher Scientific, Waltham, MA) and were used as 194.1183, [M + H]+, PPM = 1.0.
received unless otherwise indicated. Methyl-6-bromomethyl-
tert-Butyl 6-(bromomethyl)picolinate (2). Compound
1
picolinate was synthesized according to a literature protocol.7 (1.82 g, 9.45 mmol) was dissolved in CCl4 (15 mL), followed by
Water used was ultra-pure (18.2 MΩ cm−1 at 25 °C, Milli-Q, addition of N-bromosuccinimide (NBS, 1.18 g, 6.62 mmol) and
Millipore, Billerica, MA). The analytical thin-layer chromato- benzoyl peroxide (Bz2O2, 0.2 g, ∼10 wt%). The reaction mixture
graphy (TLC) plates were aluminum-backed ultrapure silica gel was brought to reflux for 4 h, removed from heat and allowed
7184 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
to cool to room temperature, filtered through a fritted glass 123.9, 123.7, 82.0, 53.3, 46.5, 28.0. HR-ESI-MS calcd for
filter, reduced to dryness in vacuo, resuspended in CH2Cl2, [C36H40N6O12S2 + Na]+: 835.2043; found: 835.2042, [M + Na]+,
again filtered through a fritted glass filter and reduced to PPM = −0.1.
dryness. The crude product was purified by silica chromato-
N,N′-[6-(tert-Butoxycarbonyl)pyridin-2-yl]methyl-1,2-diamino-
graphy (CombiFlash Rf automated column system; 80 g HP ethane (5). To a solution of 4 (0.234 g, 0.288 mmol) in tetra-
silica; A: hexanes, B: ethyl acetate, 100% A to 75% A gradient) hydrofuran (5 mL) was added thiophenol (68 mL, 0.662 mmol)
to yield the product 2 as a waxy faint-yellow solid (40%, and potassium carbonate (excess, ∼0.5 g). The reaction
∼1.04 g) (Rf: 0.4, TLC in 75% hexanes : 25% ethyl acetate). In mixture was stirred at 50 °C for 48 h, during which time a
addition, ∼1.15 g of starting material was recovered (Rf: 0.3, colour change from colourless to dark yellow occurred. In
TLC in 75% hexanes : 25% ethyl acetate). The major byproduct Chapter 3, the potassium carbonate had become sticky
observed was the dibrominated product (Rf: 0.5, TLC in 75% during the course of the reaction, and would clog fritted glass
1
hexanes : 25% ethyl acetate). H NMR (300 MHz, CDCl3, 25 °C) filters, and so it was removed via centrifugation in 20 mL cen-
δ: 7.86 (d, J = 7.1 Hz, 1H), 7.75 (t, J = 7.6 Hz, 1H), 7.57 (d, J = trifuge tubes.8,9 Here the crude reaction mixture was less sticky
7.1 Hz, 1H), 4.56 (s, 2H), 1.54 (s, 9H). 13C NMR (75 MHz, than previously found, and was filtered with a large fritted
CDCl3, 25 °C) δ: 163.2, 157.0, 148.5, 137.8, 126.3, 123.7, 82.1, glass filter, rinsed liberally with THF and CH3CN, and then
33.1, 27.8. HR-ESI-MS calcd for [C11H14NO2Br + H]+: 272.0286; concentrated to dryness in vacuo. The resulting crude yellow
found: 272.0289, [M + H]+, PPM = 1.1.
N,N′-(2-Nitrobenzenesulfonamide)-1,2-diaminoethane
oil was purified by silica chromatography (CombiFlash Rf auto-
(3). mated column system; 24 g neutral alumina; A: dichloro-
Compound 3 was prepared according to a literature pro- methane, B: methanol, 100% A to 75% A gradient) to yield 5 as
cedure:8 ethylenediamine (548 μL, 8.2 mmol) was dissolved in clear colourless oil (80%, ∼0.102 g). Compound 5 was purified
THF (10 mL) and the reaction vessel placed in an ice bath. using column chromatography on neutral alumina; silica
Sodium bicarbonate (∼2 g) was then added, followed by slow should be avoided as 5 has a high affinity for silica and
addition of 2-nitrobenzenesulfonyl chloride (4.00 g, requires the use of ammonium hydroxide and >20% methanol
18.1 mmol). The reaction mixture was allowed to warm to for elution, giving partial tert-butyl ester deprotection and dis-
ambient temperature and stirred overnight. The yellow mixture solution of some silica. 1H NMR (400 MHz, CDCl3, 25 °C)
was filtered to remove sodium bicarbonate, reduced to dryness δ: 7.87 (d, J = 7.6 Hz, 2H), 7.74 (t, J = 7.6 Hz, 2H), 7.52 (d, J =
in vacuo to yield a red oil, and then dissolved in a minimum 7.6 Hz, 2H), 4.00 (s, 4H), 2.81 (s, 4H), 2.26 (s, –NH–, 2H), 1.61
volume of dichloromethane and placed in the freezer over- (s, 18H). 13C NMR (100 MHz, CDCl3, 25 °C) δ: 164.1, 160.5,
night. The precipitated product was filtered and then washed 148.8, 137.1, 125.0, 122.9, 99.9, 81.9, 55.0, 49.0, 28.0.
with cold dichloromethane (3 × 10 mL). This process was HR-ESI-MS calcd for [C24H34N4O4 + K]+: 481.2217; found:
repeated with the filtrate once more to recover more material. 481.2213, [M + K]+, PPM = −0.8.
The faint yellow powder (3) was dried in vacuo for a yield of
H2dedpa,
N,N′-[(6-carboxylato)pyridin-2-yl)methyl]-1,2-di-
87% (∼3.07 g). 1H NMR (300 MHz, DMSO-d6, 25 °C) δ: 8.16 aminoethane (6). Compound 5 (49.3 mg, 0.114 mmol) was dis-
(br s, 2H), 8.00–7.94 (m, 4H), 7.90–7.83 (m, 4H), 3.00 (s, 4H). solved in a mixture of trifluoroacetic acid (TFA) (1 mL) and
13C NMR (75 MHz, DMSO-d6, 25 °C) δ: 147.6, 134.2, 132.8, CH2Cl2 (1 mL) and stirred overnight at room temperature. The
132.4, 129.4, 124.5, 42.4. HR-ESI-MS calcd for [C14H14N4O8S2 + reaction mixture was concentrated in vacuo and then purified
Na]+: 453.0151; found: 453.0154 [M + Na]+, PPM = 0.7.
via semi-preparative reverse-phase HPLC (10 mL min−1, gradi-
N,N′-(2-Nitrobenzenesulfonamide)-N,N′-[6-(tert-butoxycarb- ent: A: 0.1% TFA in deionized water, B: 0.1% TFA in CH3CN.
onyl)pyridin-2-yl]methyl]-1,2-diaminoethane (4). To a solution 5% to 50% B linear gradient 30 min. tR = 14.8 min, broad).
of 3 (0.206 g, 0.479 mmol) in dimethylformamide (10 mL, Product fractions were pooled and lyophilized to a white
dried over molecular sieves, 4 Å) was added 2 (0.391 g, powder overnight. The trifluoroacetic acid salt H2dedpa·2
1.44 mmol) and sodium carbonate (∼1 g). The faint yellow trifluoroacetic acid·1.5H2O (6) was obtained as a white solid
reaction mixture was stirred at 80 °C for 48 h, filtered to (∼45 mg, 67% yield, using the molecular weight of the trifluoro-
remove sodium carbonate, and concentrated in vacuo. The sep- acetic acid salt as determined by elemental analysis), with a
aration of the mono- and di-alkylated products was very cumulative yield of 26% over 4 steps. 1H NMR (300 MHz,
difficult, and so the reaction was left until as complete as poss- MeOD, 25 °C) δ: 8.19 (d, J = 7.7 Hz, 2H, pyr–H), 8.11 (t, J =
ible. The crude product was purified by silica chromatography 7.7 Hz, 2H, pyr–H), 7.76 (d, J = 7.8 Hz, 2H, pyr–H), 4.78 (s, 4H,
(CombiFlash Rf automated column system; 40 g HP silica; A: Pyr–CH2–N), 3.72 (s, 4H, ethylene–H). 13C NMR
hexanes, B: ethyl acetate, 100% A to 50% A slow gradient) to (75 MHz, MeOD, 25 °C) δ: 167.8, 153.3, 148.6, 140.8, 127.7,
yield the product 4 as light yellow fluffy solid (55%, ∼0.213 g) 126.5, 50.9, 45.6. IR (neat, ATR-IR): ν = 1719 cm−1 (CvO),
(Rf: 0.9, TLC in 95% dichloromethane : 5% MeOH). 1H NMR 1679/1591 cm−1 (CvC py). HR-ESI-MS calcd for [C16H18N4O4 +
(400 MHz, CDCl3, 25 °C) δ: 8.18–8.16 (m, 2H), 7.86 (d, J = H]+: 331.1406; found [M
+ = 0.9.
H]+: 331.1409, PPM
7.6 Hz, 2H), 7.73 (t, J = 7.6 Hz, 2H), 7.69–7.62 (m, 4H), Elemental analysis: calcd% for H2dedpa·2CF3COOH·1.5H2O
7.60–7.56 (m, 2H), 7.50 (d, J = 7.6 Hz, 2H), 4.73 (s, 4H), 3.56 (s, (C16H18N4O4·2CF3COOH·1.5H2O = 585.408): C 41.03, H 3.96,
4H), 1.58 (s, 18H). 13C NMR (100 MHz, CDCl3, 25 °C) δ: 163.4, N 9.57; found: C 40.92 (Δ = 0.11), H 3.97 (Δ = 0.01), N 9.32
156.1, 148.9, 147.9, 137.7, 133.5, 132.6, 132.1, 131.4, 125.3, (Δ = 0.25).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7176–7190 | 7185

View Article Online
Paper
Dalton Transactions
N,N′-[(tert-Butoxycarbonyl)methyl-N,N′-[6-(tert-butoxycarbonyl)- DMSO-d6, 25 °C) δ: 176.2, 168.8, 168.5, 168.2, 158.7, 157.5,
pyridin-2-yl]methyl]-1,2-diaminoethane (7). To a solution of 5 156.3, 155.7, 152.9, 152.8, 152.3, 141.5, 140.8, 140.4, 131.2,
(22.8 mg, 0.0515 mmol) in acetonitrile (5 mL) was added tert- 130.5, 129.2, 124.7, 124.3, 124.1, 123.7, 122.9, 122.7, 122.6,
butylbromoacetate (18.3 μL, 0.124 mmol) and sodium carbon- 122.6, 122.4, 82.4, 62.9, 62.5, 62.1, 58.2, 56.9, 55.0, 53.7, 47.9,
ate (∼300 mg). The reaction mixture was stirred at 60 °C for 40.0. HR-ESI-MS calcd for [C20H1889YN4O8 + 2Na]+: 576.9979;
48 h. Sodium carbonate was removed by filtration and the found: 576.9966, [M + 2Na]+, PPM = −2.2.
crude reaction mixture was concentrated in vacuo. The crude
1-(p-Nitrobenzyl)ethylenediamine (10). Compound 10 was
oil was purified by column chromatography (CombiFlash prepared according to a literature procedure,60 and was puri-
Rf automated column system; 40 g HP silica; A: dichloro- fied with a modified procedure using column chromatography
methane, B: methanol, 100% A to 80% A gradient) to afford (CombiFlash Rf automated column system; 40 g HP silica, A:
the product 7 as light yellow oil (96%, ∼33.2 mg) (Rf: 0.65, TLC 95% dichloromethane 5% ammonium hydroxide, B: 95%
in 80% dichloromethane: 20% MeOH). 1H NMR (400 MHz, methanol 5% ammonium hydroxide, 100% A to 30% B gradi-
CDCl3, 25 °C) δ: 7.90–7.89 (m, 4H), 7.55–7.53 (m, 2H), 3.86 (s, ent) to afford a 10 as brown/amber oil in a cumulative yield of
1
4H), 3.05 (s, 4H), 2.68 (s, 4H), 1.50 (s, 18H), 1.33 (s, 18H). 13C 40% over 3 steps. H NMR (300 MHz, CDCl3, 25 °C) δ: 7.91 (d,
NMR (100 MHz, CDCl3, 25 °C) δ: 171.6, 163.8, 158.1, 148.1, J = 8.9 Hz, 2H), 7.18 (d, J = 8.5 Hz, 2H), 2.80–2.57 (m, 3H),
138.7, 126.7, 123.5, 82.9, 82.0, 60.5, 56.1, 53.3, 27.8. 2.45–2.31 (m, 2H). 13C NMR (75 MHz, CDCl3, 25 °C) δ: 147.1,
HR-ESI-MS calcd for [C36H54N4O8 + H]+: 671.4020; found: 147.7, 129.4, 122.8, 54.2, 47.5, 41.4. HR-ESI-MS calcd for
671.4019, [M + H]+, PPM = −0.1.
[C9H13N3O2 + H]+: 196.1086; found: 196.1084 [M + H], PPM =
H4octapa, N,N′-[(6-carboxylato)pyridin-2-yl)methyl]-N,N′-di- −1.0.
acetic acid-1,2-diaminoethane (8). Compound 7 (33.2 mg,
N,N′-(2-Nitrobenzenesulfonamide)-1-(p-nitrobenzyl)-1,2-di-
0.0495 mmol) was dissolved in a mixture of trifluoroacetic acid aminoethane (11). Compound 11 was prepared according to a
(TFA) (1 mL) and CH2Cl2 (1 mL) and stirred overnight at room literature procedure.8 Briefly, 10 (578 mg, 2.96 mmol) was dis-
temperature. The reaction mixture was concentrated in vacuo solved in THF (10 mL) in a round bottom flask and placed in
and then purified via semi-preparative reverse-phase HPLC an ice bath, then sodium bicarbonate (∼2 g) was added, fol-
(10 mL min−1, gradient: A: 0.1% TFA in deionized water, B: lowed by slow addition of 2-nitrobenzenesulfonyl chloride
0.1% TFA in CH3CN. 5% to 50% B linear gradient 30 min. tR = (1.58 g, 7.1 mmol). The reaction mixture was heated to 50 °C
14.8 min, broad). Product fractions were pooled and lyophi- and stirred overnight. The yellow/orange mixture was filtered
lized to a white powder overnight. The trifluoroacetic acid salt to remove sodium bicarbonate, rotary evaporated to an orange
H4octapa·2trifluoroacetic acid (8) was obtained as white solid oil, dissolved in a minimum volume of dichloromethane and
(∼45 mg, 74% yield, using the molecular weight of the trifluoro- then placed in the freezer. The precipitated product was
acetic acid salt as determined by elemental analysis), with a filtered and then washed with cold dichloromethane
cumulative yield of ∼27% over 5 steps. 1H NMR (300 MHz, (3 × 10 mL); this process was repeated with the filtrate twice
MeOD, 25 °C) δ: 8.04 (d, J = 7.6, 2H, pyr–H), 7.95 (t, J = 7.7, 2H, more to recover more product. The faint yellow powder (11)
pyr–H), 7.63 (d, J = 7.5, 2H, pyr–H), 4.59 (s, 4H, Pyr–CH2–N), was dried in vacuo for a yield of 74% (∼1.24 g) (Rf: 0.90, TLC in
4.07 (s, 4H, HOOC–CH2–N), 3.63 (s, 4H, ethylene–H). 13C NMR 10% methanol in dichloromethane). 1H NMR (300 MHz,
(75 MHz, MeOD, 25 °C) δ: 171.8, 167.4, 156.7, 148.9, 140.0, acetone-d6, 25 °C) δ: 8.18–8.15 (m, 1H), 7.99–7.94 (m, 2H),
128.6, 125.8, 58.9, 56.0, 52.3. IR (neat, ATR-IR): ν = 1687/ 7.79–7.57 (m, 5H), 7.33 (d, J = 8.5 Hz, 2H), 7.04–6.98 (m, 2H),
1672 cm−1 (CvO), 1618/1594 cm−1 (CvC py). HR-ESI-MS 4.01 (br s, 1H), 3.41–3.38 (m, 2H), 3.26 (dd, J = 3.4, 13.7 Hz,
calcd for [C20H22N4O8 + H]+: 447.1516; found [M + H]+: 1H), 2.98 (m, 1H). 13C NMR (75 MHz, acetone-d6, 25 °C) δ:
447.1515, PPM = −0.2. Elemental analysis: calcd% for H4octapa· 206.3, 149.2, 148.1, 147.6, 146.7, 135.2, 134.8, 134.2, 133.9,
2CF3COOH (C20H22N4O8·2CF3COOH
= 674.457): C 42.74, 133.8, 133.6, 131.7, 131.4, 130.9, 126.0, 125.7, 123.9, 57.7, 49.3,
H 3.59, N 8.31; found: C 42.51 (Δ = 0.23), H 3.69 (Δ = 0.10), 38.2. HR-ESI-MS calcd for [C21H19N5O10S2 + Na]+: 588.0471;
N 8.38 (Δ = 0.07).
found: 588.0465 [M + Na]+, PPM = −1.0.
[Na][Y(octapa)] (9). H4octapa (8) (10 mg, 0.015 mmol) was
N,N′-(2-Nitrobenzenesulfonamide)-N,N″-[6-(methoxycarbonyl)-
suspended in 0.1 M HCl (1.0 mL) and YCl3·6H2O (5.5 mg, pyridin-2-yl]methyl]-1-(p-nitrobenzyl)-1,2-diaminoethane (12).
0.018 mmol) was added. The pH was adjusted to ∼4.0–4.5 To a solution of 11 (188 mg, 0.332 mmol) in dimethylform-
using 0.1 M NaOH and then the solution was stirred at room amide (5 mL, dried over molecular sieves, 4 Å) was added 2
temperature. After 1 hour the product was confirmed via mass (225.8 mg, 0.830 mmol) and sodium carbonate (∼0.5 g). The
spectrometry and the solvent was removed in vacuo to yield yellow reaction mixture was stirred at 80 °C for 48 h, over
[Na][Y(octapa)] (9). 1H NMR (400 MHz, D2O, 25 °C) which time the colour slowly changed to red. The reaction
δ: 8.11–7.99 (m, 4H), 7.75–7.61 (m, 2H), 4.34–4.30 (m, 1H), mixture was filtered to remove sodium carbonate and concen-
4.10–4.06 (m, 1H), 3.94 (m, 1H), 3.80–3.77 (m, 1H) 3.62 (m, trated in vacuo. As with the analogous compound 4, chromato-
1H), 3.49–3.26 (m, 2H), 3.16 (m, 2H), 2.95–2.94 (m, 1H), graphic separation of the mono- and di-alkylated product was
2.48–2.45 (m, 1H), 2.14–2.13 (m, 1H). 1H NMR (300 MHz, very difficult, with the Rf difference being ∼0.1 in
DMSO-d6, 25 °C) δ: 8.12–7.48 (m, 4H), 7.60–7.28 (m, 2H), 50 : 50 hexanes–ethyl acetate. The crude product was purified
4.50–3.80 (m, 6H), 3.25–2.50 (m, 6H). 13C NMR (150 MHz, by silica chromatography (CombiFlash Rf automated column
7186 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
system; 40 g HP silica; A: hexanes, B: ethyl acetate, 100% A to ionized water, and injected directly onto a semi-preparative
50% A gradient) to yield 12 as faint yellow solid (81%, HPLC column for purification (A: 0.1% TFA in deionized
1
∼255 mg) (Rf = 0.65 in 50 : 50 EtOAc–Hex). H NMR (300 MHz, water, B: 0.1% TFA in CH3CN, 95% A to 60% B gradient over
CDCl3, 25 °C) δ: 8.00–7.83 (m, 4H), 7.78–7.60 (m, 7H), 40 min). p-SCN-Bn-H2dedpa·2trifluoroacetic acid·H2O (14) was
7.59–7.7.42 (m, 4H), 7.38–7.21 (m, 1H), 7.01 (d, J = 9 Hz, 2H), found in the largest peak at Rt = 29 min, lyophilized overnight,
4.97–4.65 (m, 4H), 4.36–4.29 (m, 1 H), 3.40–3.24 (m, 2H), and was isolated as a white solid (∼23 mg, 43% over 3 steps
3.20–3.11 (m, 1H), 3.00–2.93 (m, 1H), 1.58 (s, 9H), 1.56 (s, 9H). from 13, using the molecular weight of the trifluoroacetic acid
1
13C NMR (75 MHz, CDCl3, 25 °C) δ: 163.5, 163.2, 157.5, 155.9, salt as determined by elemental analysis). H NMR (400 MHz,
148.7, 148.6, 147.7, 146.9, 146.1, 144.7, 137.9, 137.7, 133.7, MeOD, 25 °C) δ: 8.18–8.08 (m, 4H), 7.77–7.72 (m, 2H), 7.36 (d,
133.3, 132.5, 132.3, 132.1, 132.0, 131.4, 129.6, 126.5, 125.3, J = 8.5 Hz, 2H), 7.23 (m, J = 8.5 Hz, 2H), 4.95–4.84 (m, 3H),
124.0, 123.9, 123.8, 123.8, 122.9, 82.2, 82.1, 57.8, 53.0, 51.4, 4.68 (d, J = 16.7 Hz, 1H), 4.03–3.97 (m, 1H), 3.71–3.65 (m, 1H),
49.8, 34.4, 27.9. HR-ESI-MS calcd for [C43H45N7O14S2 + Na]+: 3.52–3.47 (m, 2H), 3.01–2.95 (m, 1H). 13C NMR (100 MHz,
970.2364; found: 970.2355, [M + Na]+, PPM = −0.9.
MeOD, 25 °C) δ: 167.7, 167.6, 153.9, 153.5, 153.3, 148.4, 140.9,
N,N′-[[6-(Methoxycarbonyl)pyridin-2-yl]methyl]-1-(p-nitro- 140.3, 137.7, 135.5, 132.3, 132.2, 132.0, 130.6, 127.7, 127.4,
benzyl)-1,2-diaminoethane (13). To a solution of 12 (125 mg, 127.3, 126.6, 126.5, 59.8, 59.5, 50.7, 35.8. IR (neat, ATR-IR): ν =
0.132 mmol) in tetrahydrofuran (5 mL) was added thiophenol 2097 cm−1 (SvCvN–), 1665 cm−1 (CvO), 1594 cm−1 (CvC
(31 μL, 0.304 mmol) and potassium carbonate (excess, py). HR-ESI-MS calcd for [C24H23N5O4S + H]+: 478.1549; found
∼300 mg). The reaction mixture was stirred at 60 °C for 48 h, [M + H]+: 478.1548, PPM = −0.2. Elemental analysis: calcd%
over which time the colour slowly changed from colourless to for p-SCN-Bn-H2dedpa·2CF3COOH·1H2O (C24H23N5O4S·2CF3-
dark yellow. The crude reaction mixture was filtered with a COOH·1H2O = 723.597): C 46.48, H 3.76, N 9.68; found: C
fritted glass filter, rinsed ad libitum with THF and CH3CN, and 46.68 (Δ = 0.20), H 3.73 (Δ = 0.03), N 9.60 (Δ = 0.08).
then concentrated to dryness in vacuo. The resulting crude
N,N′-[(tert-Butoxycarbonyl)methyl]-N,N′-[[(6-tert-butoxycarb-
yellow oil was purified by neutral alumina column chromato- onyl)pyridin-2-yl]methyl]-1-(p-nitrobenzyl)-1,2-diaminoethane
graphy (CombiFlash Rf automated column system; 24 g neutral (15). To a solution of 13 (78.1 mg, 0.135 mmol) in acetonitrile
alumina; A: dichloromethane, B: methanol, 100% A to 75% A (15 mL) was added tert-butylbromoacetate (∼48 μL,
gradient) to yield 13 as light yellow oil (70%, ∼76 mg). 1H NMR 0.324 mmol) and potassium carbonate (∼500 mg). The reac-
(400 MHz, CDCl3, 25 °C) δ: 8.09 (d, J = 8.9 Hz, 2H), 7.86 (d, J = tion mixture was stirred at 80 °C for 48 h. Potassium carbonate
7.9 Hz, 2H), 7.74–7.69 (m, 2H), 7.46 (t, J = 8.6 Hz, 2H), 7.34 (d, was removed by filtration and the crude reaction mixture was
J = 8.5 Hz, 2H), 4.06–3.88 (m, 4H), 3.01–2.96 (m, 2H), 2.88–2.82 concentrated in vacuo. The crude oil was purified by column
(m, 1H), 2.69–2.66 (m, 1H), 2.56–2.52 (m, 1H), 1.60 (s, 18H). chromatography (CombiFlash Rf automated column system;
13C NMR (100 MHz, CDCl3, 25 °C) δ: 163.9, 163.9, 160.4, 148.7, 24 g HP silica; A: dichloromethane, B: methanol, 100% A to
148.6, 147.3, 146.4, 137.2, 137.1, 130.1, 125.1, 125.0, 123.4, 80% A gradient) to afford the product 15 as light yellow oil
122.9, 81.9, 58.3, 54.9, 52.4, 51.7, 39.2. HR-ESI-MS calcd for (63%, ∼69 mg) (Rf = 0.61 in 80 : 20 CH2Cl2–MeOH). 1H NMR
[C31H39N5O6 + H]+: 578.2979; found: 578.2984, [M + H]+, PPM (400 MHz, CDCl3, 25 °C) δ: 8.02–7.97 (m, 3H), 7.89–7.73 (m,
= 0.9.
3H), 7.64–7.60 (m, 1H), 7.56–7.46 (m, 3H), 4.03–3.93 (m, 4H),
p-SCN-Bn-H2dedpa, N,N′-[[(6-carboxylato)pyridin-2-yl]methyl]- 3.43–3.21 (m, 4H), 3.03–2.95 (m, 2H), 2.86–2.81 (m, 2H),
1-(p-benzylisothiocyanato)-1,2-diaminoethane (14). Compound 2.50–2.45 (m, 1H), 1.62 (m, 18H), 1.46 (m, 18H). 13C NMR
13 (43.1 mg, 0.0746 mmol) was dissolved in glacial acetic acid (100 MHz, CDCl3, 25 °C) δ: 170.6, 170.5, 163.9, 160.2, 160.1,
(2.5 mL) with hydrochloric acid (2.5 mL, 3 M), palladium on 148.7, 148.6, 148.2, 146.2, 136.9, 136.9, 130.2, 126.1, 125.6,
carbon (20 wt%), and hydrogen gas (balloon). The reaction 123.4, 123.2, 123.1, 82.0, 81.9, 81.1, 80.9, 61.4, 60.8, 56.9, 56.6,
mixture was stirred vigorously at rt for 1 hour, then filtered to 54.1, 52.4, 35.7, 29.6, 28.1, 28.1, 28.0. HR-ESI-MS calcd for
remove Pd/C and washed ad libitum with acetonitrile and [C43H59N5O10 + H]+: 806.4340; found: 806.4339, [M + H]+,
hydrochloric acid (3 M). The product was confirmed by low- PPM = −0.1.
resolution mass spectrometry (LRMS), and revealed that the
p-SCN-Bn-H4octapa, N,N′-[(Carboxylato)methyl]-N,N′-[[(6-car-
tert-butyl ester protecting groups remained intact. The crude boxylato)pyridin-2-yl]methyl]-1-(p-benzylisothiocyanato)-1,2-di-
product was dissolved in HCl (2–3 mL, 3 M), and heated with a aminoethane (16). Compound 15 (59.6 mg, 0.0739 mmol) was
heat gun for 1 minute. Following this heating step, quantitat- dissolved in glacial acetic acid (2.5 mL) with hydrochloric acid
ive removal of the tert-butyl ester protecting groups was con- (2.5 mL, 3 M), palladium on carbon (30 wt%), and hydrogen
firmed by LRMS. Without purification, the crude reaction gas (balloon). The reaction mixture was stirred vigorously at rt
mixture (in 2–3 mL of 3 M HCl) was reacted with thiophosgene for 1 hour, filtered to remove Pd/C and washed ad libitum with
(suspension in chloroform) in ∼0.2 mL of additional chloro- acetonitrile and hydrochloric acid (3 M). The product was con-
form (∼86 μL, 1.12 mmol) overnight at ambient temperature firmed by low-resolution mass spectrometry (LRMS), and
with vigorous stirring. The reaction mixture was washed with revealed that two of the tert-butyl ester protecting groups
chloroform (5 × 1 mL) by vigorous biphasic stirring, followed remained intact and two had been removed. The crude
by decanting of the organic phase with a pipette to remove product was dissolved in HCl (2–3 mL, 3 M), and heated with a
excess thiophosgene, diluted to a volume of 4.5 mL with de- heat gun for 1 minute to effect full ester deprotection. Follow-
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7176–7190 | 7187

View Article Online
Paper
Dalton Transactions
ing heating, quantitative removal of the tert-butyl ester protect- bration data were analyzed by standard computer treatment
ing groups was confirmed by LRMS. Without purification, the provided within the program MacCalib62 to obtain the cali-
crude reaction mixture (in 2–3 mL of 3 M HCl) was reacted bration parameters E0. Ligand solutions were prepared
with thiophosgene (suspension in chloroform) in ∼0.2 mL of 24 hours in advance of titrations to allow for equilibration.
additional chloroform (∼85 μL, 1.11 mmol) overnight at Electrode equilibration times for titrations were up to 10 min
ambient temperature with vigorous stirring. The reaction for pKa titrations and up to 3 h for metal complex titrations
mixture was washed with chloroform (5 × 1 mL) by vigorous (<0.2 mV min−1 drift allowed). Ligand and metal concen-
biphasic stirring followed by decanting of the organic phase trations were 0.75–1.0 mM for potentiometric titrations. The
with a pipette to remove excess thiophosgene, diluted to a data were treated with Hyperquad2008.63 The proton dis-
volume of 4.5 mL with deionized water, and injected directly sociation constants corresponding to hydrolysis of Y3+
ion
(aq)
onto a semi-preparative HPLC column for purification (A: included in the calculations were taken from Baes and
0.1% TFA in deionized water, B: 0.1% TFA in CH3CN, 100% A Mesmer.52 The KML value for the yttrium-EDTA complex was
to 40% A gradient over 40 min). p-SCN-Bn-H4octapa·2trifluoro- taken from Martell.51 It was necessary to include a ML(OH)
acetic acid·0.5H2O·0.5CH3CN (16) was found in the largest species in the equilibrium model used for fitting where log
peak at Rt = 34.5 min, lyophilized overnight, and was isolated KML(OH) = 10.6(1). This is consistent with a 9-coordinate struc-
as a white solid (∼11 mg, 25% over 3 steps from 15, using the ture and only becomes relevant above pH 9.5. Values of pM
molecular weight of the trifluoroacetic acid salt as determined were calculated at physiologically relevant conditions of pH
by elemental analysis). 1H NMR (400 MHz, MeOD, 25 °C) δ: 7.4, 10 μM ligand, and 1 μM metal. All values and errors rep-
8.03–7.90 (m, 4H), 7.61–7.56 (m, 2H), 7.21–7.11 (m, 4H), 5.07 resent the average of at least three independent experiments.
(m, 2H), 4.70 (m, 2H), 4.52–4.48 (m, 2H), 4.16–4.13 (m, 1H),
4.00–3.94 (m, 2H), 3.68–3.57 (m, 2H), 3.25–3.20 (m, 1H),
Molecular modeling
2.68–2.62 (m, 1H). 13C NMR (150 MHz, MeOD, 25 °C) δ: 174.5,
Calculations were performed using the Gaussian 0964 and
169.3, 167.8, 167.4, 167.2, 160.8, 152.7, 149.1, 148.6, 140.0,
GaussView packages. Molecular geometries and electron den-
139.9, 138.9, 131.8, 131.7, 131.1, 129.2, 128.0, 127.0, 126.1,
sities were obtained from density functional theory calcu-
125.3, 125.1, 61.1, 55.5, 52.1, 50.7, 38.1, 34.7. IR (neat, ATR-IR):
lations, with the B3LYP functional employing the 6-31+G(d,p)
ν = 2096 cm−1 (SvCvN–), 1703/1660 cm−1 (CvO), 1593 cm−1
basis set for 1st and 2nd row elements and the Stuttgart–
(CvC py). HR-ESI-MS calcd for [C28H27N5O8S + H]+: 594.1659;
Dresden effective core potential, SDD for yttrium.65–67 Solvent
found [M + H]+: 594.1650, PPM = −1.5. Elemental analysis:
(water) effects were described through a continuum approach
calcd% for p-SCN-Bn-H4octapa·2CF3COOH·0.5H2O·0.5CH3CN
by means of the IEF PCM as implemented in G09. The electro-
(C28H27N5O8S ·2CF3COOH·0.5H2O·0.5CH3CN = 851.188): C
static potential was mapped onto the calculated electron
46.57, H 3.73, N 9.05; found: C 46.92 (Δ = 0.35), H 4.00 (Δ =
density surface. The corresponding harmonic vibration fre-
quencies were computed at the same level to characterize the
0.27), N 8.67 (Δ = 0.38).
geometry as a minima.
Solution thermodynamics
The experimental procedures and details of the apparatus
closely followed our reported studies of H2dedpa/Ga3+, and
H4octapa with In3+ and Lu3+.7,8,36 As a result of the strength of
the binding of the Y3+ complex [Y(octapa)]−, the complex for-
mation constant with this ligand could not be determined
directly and ligand–ligand competition using Na2H2EDTA was
Acknowledgements
We acknowledge NSERC for CGS-M/CGS-D fellowships (E.W.P.)
and many years of research support (C.O.), as well as the Uni-
versity of British Columbia for a 4YF fellowship (E.W.P.). C.O.
acknowledges the Canada Council for the Arts for a Killam
Research Fellowship (2011–2013) and the Alexander von Hum-
boldt Foundation for a Research Award, as well as Prof. Dr
Peter Comba and his research group in Heidelberg for hospi-
tality and interesting discussions.
used. Potentiometric titrations were performed using
a
Metrohm Titrando 809 equipped with a Ross combination pH
electrode and a Metrohm Dosino 800. Data were collected in
triplicate using PC Control (Version 6.0.91, Metrohm). The
titration apparatus consisted of a water-jacketed glass vessel
maintained at 25.0 ( 0.1 °C, Julabo water bath). Prior to and
during the course of the titration, a blanket of nitrogen,
passed through 10% NaOH to exclude any CO2, was main-
tained over the sample solution. Yttrium ion solutions were
prepared by dilution of the appropriate atomic absorption
standard (AAS) solution. The exact amount of acid present in
the yttrium standard was determined by titration of an equi-
molar solution of Y3+ and Na2H2EDTA. The amount of acid
present was determined by Gran’s method.61 Calibration of
the electrode was performed prior to each measurement by
titrating a known amount of HCl with 0.1 M NaOH. Cali-
Notes and references
1 E. W. Price and C. Orvig, Chem. Soc. Rev., 2014, 43, 260–
290.
2 T. J. Wadas, E. H. Wong, G. R. Weisman and
C. J. Anderson, Chem. Rev., 2010, 110, 2858–2902.
3 C. F. Ramogida and C. Orvig, Chem. Commun., 2013, 49,
4720–4739.
7188 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
4 B. M. Zeglis and J. S. Lewis, Dalton Trans., 2011, 40, 6168– 25 M. W. Brechbiel, O. A. Gansow, R. W. Atcher, J. Schlom,
6195.
J. Esteban, D. Simpson and D. Colcher, Inorg. Chem., 1986,
25, 2772–2781.
5 J. P. Holland, M. J. Williamson and J. S. Lewis, Mol.
Imaging, 2010, 9, 1–20.
6 G. A. Bailey, E. W. Price, B. M. Zeglis, C. L. Ferreira,
E. Boros, M. J. Lacasse, B. O. Patrick, J. S. Lewis,
M. J. Adam and C. Orvig, Inorg. Chem., 2012, 51, 12575–
12589.
26 A. Harrison, C. A. Walker, D. Parker, K. J. Jankowski,
J. P. L. Cox, A. S. Craig, J. M. Sansom, N. R. A. Beeley,
R. A. Boyce, L. Chaplin, M. A. W. Eaton,
A. P. H. Farnsworth, K. Millar, A. T. Millican, A. M. Randall,
S. K. Rhind, D. S. Secher and A. Turner, Int. J. Radat. Appl.
Instrum., Part B, 1991, 18, 469–476.
7 E. W. Price, J. F. Cawthray, G. A. Bailey, C. L. Ferreira,
E. Boros, M. J. Adam and C. Orvig, J. Am. Chem. Soc., 2012, 27 C. A. Boswell and M. W. Brechbiel, Nucl. Med. Biol., 2007,
134, 8670–8683. 34, 757–778.
8 E. W. Price, B. M. Zeglis, J. F. Cawthray, C. F. Ramogida, 28 W. A. P. Breeman, M. de Jong, T. J. Visser, J. L. Erion and
N. Ramos, J. S. Lewis, M. J. Adam and C. Orvig, J. Am.
Chem. Soc., 2013, 135, 12707–12721.
E. P. Krenning, Eur. J. Nucl. Med. Mol. Imaging, 2003, 30,
917–920.
9 E. W. Price, B. M. Zeglis, J. S. Lewis, M. J. Adam and 29 T. Clifford, C. A. Boswell, G. B. Biddlecombe, J. S. Lewis
C. Orvig, Dalton Trans., 2014, 43, 119–131.
and M. W. Brechbiel, J. Med. Chem., 2006, 49, 4297–
10 R. Weiner and M. Thakur, BioDrugs, 2005, 19, 145–163.
4304.
11 F. Forrer, C. Waldherr, H. R. Maecke and J. Mueller-Brand, 30 M. Cremonesi, M. Ferrari, S. Zoboli, M. Chinol,
Anticancer Res., 2006, 26, 703–707.
12 F. Forrer, H. Uusijärvi, D. Storch, H. R. Maecke and
J. Mueller-Brand, J. Nucl. Med., 2005, 46, 1310–1316.
M. G. Stabin, F. Orsi, H. R. Maecke, E. Jermann,
C. Robertson, M. Fiorenza, G. Tosi and G. Paganelli,
Eur. J. Nucl. Med. Mol. Imaging, 1999, 26, 877–886.
13 T. K. Nayak, K. Garmestani, D. E. Milenic, K. E. Baidoo and 31 M. Gabriel, C. Decristoforo, D. Kendler, G. Dobrozemsky,
M. W. Brechbiel, PLoS One, 2011, 6, e18198.
14 T. K. Nayak, K. Garmestani, K. E. Baidoo, D. E. Milenic and
M. W. Brechbiel, Int. J. Cancer, 2010, 128, 920–926.
D. Heute, C. Uprimny, P. Kovacs, E. Von Guggenberg,
R. Bale and I. J. Virgolini, J. Nucl. Med., 2007, 48, 508–
518.
15 S. Palm, R. M. Enmon, C. Matei, K. S. Kolbert, S. Xu, 32 M. Lin, M. Welch and S. Lapi, Mol. Imaging Biol., 2013, 15,
P. B. Zanzonico, R. L. Finn, J. A. Koutcher, S. M. Larson
and G. Sgouros, J. Nucl. Med., 2003, 44, 1148–1155.
16 L. I. Gordon, T. E. Witzig, G. A. Wiseman, I. W. Flinn,
S. S. Spies, D. H. Silverman, C. Emmanuolides, L. Cripe,
1–8.
33 S. Pauwels, R. Barone, S. Walrand, F. Borson-Chazot,
R. Valkema, L. K. Kvols, E. P. Krenning and F. Jamar,
J. Nucl. Med., 2005, 46, 92S–98S.
M. Saleh, M. S. Czuczman, T. Olejnik, C. A. White and 34 A. Romer, D. Seiler, N. Marincek, P. Brunner, M. T. Koller,
A. J. Grillo-López, Semin. Oncol., 2002, 29, 87–92.
17 M. F. Giblin, B. Veerendra and C. J. Smith, In Vivo, 2005,
19, 9–29.
Q. K. T. Ng, H. R. Maecke, J. Müller-Brand, C. Rochlitz,
M. Briel, C. Schindler and M. A. Walter, Eur. J. Nucl. Med.
Mol. Imaging, 2014, 41, 214–222.
18 H. Herzog, F. Rösch, G. Stöcklin, C. Lueders, S. M. Qaim 35 R. Kashyap, P. Jackson, M. S. Hofman, P. Eu,
and L. E. Feinendegen, J. Nucl. Med., 1993, 34, 2222–
2226.
J.-M. Beauregard, D. Zannino and R. J. Hicks, Eur. J. Nucl.
Med. Mol. Imaging, 2013, 40, 1853–1860.
19 H.-S. Chong, K. Garmestani, D. Ma, D. E. Milenic, 36 E. Boros, C. L. Ferreira, J. F. Cawthray, E. W. Price,
T. Overstreet and M. W. Brechbiel, J. Med. Chem., 2002, 45,
3458–3464.
B. O. Patrick, D. W. Wester, M. J. Adam and C. Orvig, J. Am.
Chem. Soc., 2010, 132, 15726–15733.
20 H.-S. Chong, H. A. Song, X. Ma, D. E. Milenic, E. D. Brady, 37 A. Armstrong, I. Brackenridge, R. F. W. Jackson and
S. Lim, H. Lee, K. Baidoo, D. Cheng and M. W. Brechbiel,
Bioconjugate Chem., 2008, 19, 1439–1447.
21 C. S. Kang, X. Sun, F. Jia, H. A. Song, Y. Chen, M. Lewis and
H.-S. Chong, Bioconjugate Chem., 2012, 23, 1775–1782.
22 L. Camera, S. Kinuya, K. Garmestani, C. Wu,
M. W. Brechbiel, L. H. Pai, T. J. McMurry, O. A. Gansow,
J. M. Kirk, Tetrahedron Lett., 1988, 29, 2483–2486.
38 Y. Gultneh, B. Ahvazi, Y. T. Tesema, T. B. Yisgedu and
R. J. Butcher, J. Coord. Chem., 2006, 59, 1835–1846.
39 E. Boros, C. L. Ferreira, D. T. T. Yapp, R. K. Gill, E. W. Price,
M. J. Adam and C. Orvig, Nucl. Med. Biol., 2012, 39,
785–794.
I. Pastan, C. H. Paik and J. A. Carrasquillo, J. Nucl. Med., 40 E. Boros, J. F. Cawthray, C. L. Ferreira, B. O. Patrick,
1994, 35, 882–889.
23 G. J. Förster, M. J. Engelbach, J. J. Brockmann, H. J. Reber,
M. J. Adam and C. Orvig, Inorg. Chem., 2012, 51,
6279–6284.
H. G. Buchholz, H. R. Mäcke, F. R. Rösch, H. R. Herzog 41 R. Ferreiros-Martinez, D. Esteban-Gomez, C. Platas-Iglesias,
and P. R. Bartenstein, Eur. J. Nucl. Med., 2001, 28, 1743–
1750.
A. d. Blas and T. Rodriguez-Blas, Dalton Trans., 2008,
5754–5765.
24 C. Wu, H. Kobayashi, B. Sun, T. M. Yoo, C. H. Paik, 42 E. Balogh, M. Mato-Iglesias, C. Platas-Iglesias, E. Toth,
O. A. Gansow, J. A. Carrasquillo, I. Pastan and
K. Djanashvili, J. A. Peters, A. de Blas and T. Rodriguez-
M. W. Brechbiel, Bioorg. Med. Chem., 1997, 5, 1925–1934.
Blas, Inorg. Chem., 2006, 45, 8719–8728.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7176–7190 | 7189

View Article Online
Paper
Dalton Transactions
43 C. Platas-Iglesias, M. Mato-Iglesias, K. Djanashvili, 58 W. R. Harris, Y. Chen and K. Wein, Inorg. Chem., 1994, 33,
R. N. Muller, L. V. Elst, J. A. Peters, A. de Blas and
T. Rodríguez-Blas, Chem. – Eur. J., 2004, 10, 3579–3590.
44 R. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
Theor. Gen. Cryst., 1976, 32, 751–767.
45 E. T. Clarke and A. E. Martell, Inorg. Chim. Acta, 1991, 190,
37–46.
4991–4998.
59 W. R. Harris, B. Yang, S. Abdollahi and Y. Hamada, J. Inorg.
Biochem., 1999, 76, 231–242.
60 A. Kumar Mishra, P. Panwar, M. Chopra, R. Kumar
Sharma and J.-F. Chatal, New J. Chem., 2003, 27, 1054–
1058.
46 W. R. Harris and V. L. Pecoraro, Biochemistry, 1983, 22, 61 G. Gran, Analyst, 1952, 77, 661–671.
292–299. 62 P. Duckworth, Private communication.
47 K. N. Raymond, G. Müller and B. Matzanke, in Topics in 63 P. Gans, A. Sabatini and A. Vacca, Talanta, 1996, 43, 1739–
Current Chemistry, ed. F. L. Boschke, Springer-Verlag,
Berlin, Heidelberg, 1984, vol. 123, pp. 49–102.
48 M. J. Kappel and K. N. Raymond, Inorg. Chem., 1982, 21,
3437–3442.
49 M. Koudelková, H. Vinšová and V. Jedináková-Křížová,
J. Chromatogr. A, 2003, 990, 311–316.
50 K. Kumar, C. A. Chang, L. C. Francesconi, D. D. Dischino,
M. F. Malley, J. Z. Gougoutas and M. F. Tweedle, Inorg.
Chem., 1994, 33, 3567–3575.
51 A. E. Martell and R. M. Smith, Critical Stability Constants,
Plenum Press, New York, 1974–1989, vol. 1–6.
52 C. F. Baes Jr. and R. E. Mesmer, The Hydrolysis of Cations,
Wiley-Interscience, New York, 1976.
53 C. F. G. C. Geraldes, R. Delgado, A. M. Urbano, J. Costa,
F. Jasanada and F. Nepveu, J. Chem. Soc., Dalton Trans.,
1995, 327–335.
54 Ė. Tóth and E. Brücher, Inorg. Chim. Acta, 1994, 221, 165–167.
55 W. P. Cacheris, S. K. Nickle and A. D. Sherry, Inorg. Chem.,
1987, 26, 958–960.
56 M. F. Loncin, J. F. Desreux and E. Merciny, Inorg. Chem.,
1986, 25, 2646–2648.
57 C. J. Broan, J. P. L. Cox, A. S. Craig, R. Kataky, D. Parker,
1753.
64 G. W. T. M. J. Frisch, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
GAUSSIAN 09 (Revision C.01), Gaussian, Inc., Wallingford,
CT, 2010.
65 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
Matter, 1988, 37, 785–789.
A. Harrison, A. M. Randall and G. Ferguson, J. Chem. Soc., 66 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
Perkin Trans. 2, 1991, 87–99.
67 X. Cao and M. Dolg, J. Chem. Phys., 2001, 115, 7348–7355.
7190 | Dalton Trans., 2014, 43, 7176–7190
This journal is © The Royal Society of Chemistry 2014