NMR and TRLFS studies of Ln(III) and An(III) C5-BPP complexes

Article abstract of DOI:10.1039/c4sc03103b

C5-BPP is a highly efficient N-donor ligand for the separation of trivalent actinides, An(iii), from trivalent lanthanides, Ln(iii). The molecular origin of the selectivity of C5-BPP and many other N-donor ligands of the BTP-type is still not entirely understood. We present here the first NMR studies on C5-BPP Ln(iii) and An(iii) complexes. C5-BPP is synthesized with 10% ¹⁵N labeling and characterized by NMR and LIFDI-MS methods. ¹⁵N NMR spectroscopy gives a detailed insight into the bonding of C5-BPP with lanthanides and Am(iii) as a representative for trivalent actinide cations, revealing significant differences in ¹⁵N chemical shift for coordinating nitrogen atoms compared to Ln(iii) complexes. The temperature dependence of NMR chemical shifts observed for the Am(iii) complex indicates a weak paramagnetism. This as well as the observed large chemical shift for coordinating nitrogen atoms show that metal-ligand bonding in Am(C5-BPP)₃ has a larger share of covalence than in lanthanide complexes, confirming earlier studies. The Am(C5-BPP)₃ NMR sample is furthermore spiked with Cm(iii) and characterized by time-resolved laser fluorescence spectroscopy (TRLFS), yielding important information on the speciation of trace amounts of minor complex species.

Full text of DOI:10.1039/c4sc03103b

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NMR and TRLFS studies of Ln(III) and An(III) C5-BPP
complexes†
Cite this: Chem. Sci., 2015, 6, 1548
Christian Adam,*ab Bjorn B. Beele, Andreas Geist, Udo Mullich, Peter Kaden
and Petra J. Panak
ab
a
a
a
¨
¨
ab
C5-BPP is a highly efficient N-donor ligand for the separation of trivalent actinides, An(III), from trivalent
lanthanides, Ln(III). The molecular origin of the selectivity of C5-BPP and many other N-donor ligands of
the BTP-type is still not entirely understood. We present here the first NMR studies on C5-BPP Ln(III) and
15
An(III) complexes. C5-BPP is synthesized with 10% N labeling and characterized by NMR and LIFDI-MS
15
methods. N NMR spectroscopy gives a detailed insight into the bonding of C5-BPP with lanthanides
15
and Am(III) as a representative for trivalent actinide cations, revealing significant differences in
N
chemical shift for coordinating nitrogen atoms compared to Ln(III) complexes. The temperature
dependence of NMR chemical shifts observed for the Am(III) complex indicates a weak paramagnetism.
This as well as the observed large chemical shift for coordinating nitrogen atoms show that metal–ligand
Received 9th October 2014
Accepted 9th December 2014
bonding in Am(C5-BPP)
3
has a larger share of covalence than in lanthanide complexes, confirming
NMR sample is furthermore spiked with Cm(III) and characterized by
earlier studies. The Am(C5-BPP)
3
DOI: 10.1039/c4sc03103b
time-resolved laser fluorescence spectroscopy (TRLFS), yielding important information on the speciation
www.rsc.org/chemicalscience
of trace amounts of minor complex species.
It has been shown that extractants containing either so
Introduction
sulfur or so nitrogen donor atoms exhibit the required
6
In 2010 about 13% of the world's electricity is supplied by selectivity. Heterocyclic N-donor ligands derived from the
1
nuclear power plants, producing 10 500 tons of spent nuclear terpyridine motif have shown higher complex strengths towards
2
7
fuel annually. Among the major challenges of used nuclear fuel trivalent actinides than towards trivalent lanthanides. Among
are the long-term radiotoxicity and long-term thermal power these, heteroaromatic nitrogen donor ligands 2,6-bis(1,2,4-
that are dominated by plutonium and the minor actinides (MA triazine-3-yl)pyridines (BTPs) were the rst extractants to ach-
¼
Np, Am, and Cm).
ieve separation factors for Am(III) over Eu(III) higher than 100
Both problems are addressed by the Partitioning and from nitric acid solutions. They show good solubility in a
7,8
3
Transmutation strategy (P&T) that could have a benecial range of organic diluents and form stable and isostructural 1 : 3
3,4
9–16
impact on the design of a safe nal repository. It involves complexes with lanthanides and actinides.
Furthermore,
separating plutonium and the minor actinides from the used they co-extract nitrate anions from the aqueous phase and,
fuel and converting them into shorter-lived ssion products by unlike other similar extracting agents, do not need additional
17–19
neutron-induced nuclear reactions. In this context the separa- lipophilic anion sources such as 2-bromocarboxylic acid.
In
tion of trivalent actinides An(III) from ssion lanthanides Ln(III) order to attain a fundamental understanding of the BTP-type
is the key step, as some lanthanides have high neutron cross ligands' selectivity on a molecular level, the tridentate N-donor
sections, consequently diminishing the efficiency of the trans- ligand C5-BPP was synthesized and tested for its extraction
20
mutation step. Due to the similarity of An(III) and Ln(III) both in behavior. It was found that C5-BPP serves as a useful extract-
chemical properties and ionic radii, highly selective extracting ing agent with separation factors for Am(III) over Eu(III) over 100.
5
agents are needed to achieve a reasonable separation.
However, it does not co-extract nitrate anions from the aqueous
phase and is thus dependent on a lipophilic anion source. The
ability to form stable 1 : 3 complexes and the different extrac-
tion behavior made C5-BPP an interesting target for investiga-
tions on the reason of the observed selectivity, especially in
a
Karlsruhe Institute of Technology (KIT), Institute for Nuclear Waste Disposal (INE),
P.O. Box 3640, 76021 Karlsruhe, Germany. E-mail: christian.adam@kit.edu; Fax:
21
comparison to recent studies with nPrBTP.
+
49 721 608 23927
b
University of Heidelberg, Institute of Physical Chemistry, Im Neuenheimer Feld 253,
The molecular reason for the observed selectivity of some N-
donor ligands is still largely unclear. A larger degree of cova-
lence in the actinide–ligand bond, compared to lanthanide
6
9120 Heidelberg, Germany
†
Electronic supplementary information (ESI) available: LIFDI-MS spectra and
additional NMR spectra. See DOI: 10.1039/c4sc03103b
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complexes, has been assumed to account for the observed electron density can normally only be detected over a few
22–24
extraction behavior.
A more covalent bonding might result covalent bonds. Only in some cases nuclei more than three
from a better overlap of the so nitrogen lone pair with the bonds away from coordinating atoms are inuenced by FCS.
diffuse 5f-orbitals of the actinide ions. In this case, the ratio of Unfortunately, obtaining resonance signals in one-dimensional
1
5
covalent to dative electrostatic bonding in actinide–N-donor direct excitation spectra from N atoms at natural abundance
complexes is expected to be larger than in isostructural is impossible in a time-effective manner. This is due to the fact
1
5
lanthanide compounds. Results from K-edge XAS spectroscopy that N has a low natural abundance of 0.364% and a low
25
on An(III) complexes with ligands containing sulfur, oxygen negative gyromagnetic ratio (g ¼ ꢀ0.28), resulting in low
2
6
13
and chlorine seem to support this explanation.
receptivity of the nucleus (about 1% of the C receptivity at
37
Actinide compounds are a challenge for quantum chemistry natural abundance). Furthermore, a negative gyromagnetic
due to various reasons, like for example the inclusion of rela- ratio means that the Nuclear Overhauser Effect will decrease the
1
5
1
tivistic effects. So far, prediction of bonding modes and NMR signal intensity for N if H broadband decoupling is used.
shis is limited to simple systems and hardly implemented in In paramagnetic coordination compounds the overall expe-
commercial soware packages. As an example for these prob- rienced chemical shi dtot has several contributions:
lems, quantum chemical treatment of Am(III) and Eu(III)
d
tot ¼ ddia + dcon + dpc + danion
(1)
complexes with Cyanex 301 seemed to show a more covalent
bonding in the actinide case, based on consideration of the
27
ddia is the diamagnetic (or orbital) shi of the compound,
con represents the Fermi contact shi, a through-bond effect,
pc is the pseudo contact shi that originates from coupling of
the electron magnetic moment on the metal ion and the ligand
nuclei spins and danion is the inuence of the counter-anion. All
published methods for the separation of these terms have in
common that they rely on an isostructural diamagnetic analog
to the paramagnetic complexes. The purely paramagnetic shi
bond length as a marker for covalence. Yet this produces
misleading results, as calculated lanthanide–ligand bond
lengths are too long in comparison to experimental data and
bond lengths calculated by more sophisticated quantum-
d
d
28
chemical methods.
Recently we were able to obtain the rst NMR spectroscopic
proof of a fundamentally different binding mode in Am(III)
21
complexes with N-donor ligands.
d
para ¼ dcon + dpc is calculated by simply subtracting the chem-
In general, NMR is an excellent spectroscopic method for the
investigation of bonding interactions: the electrons of so
donor ligands can interact with positively charged cations either
by sharing electron density in overlapping frontier orbitals or by
electrostatic interactions. Both mechanisms lead to a rear-
rangement of electron density, which is monitored very
precisely as the chemical shi in NMR spectroscopy. NMR
focusing on the paramagnetism of the compounds allows the
separation of the overall chemical shi into a part that is due to
delocalization of electron spin density through covalent bonds
ical shi values of the diamagnetic reference compound from
the measured chemical shis of the paramagnetic complexes
(
eqn (2)). If reference and paramagnetic complexes have the
same counter-anion, danioncancels out.
dpara ¼ dtot ꢀ ddia
(2)
In the lanthanide series, La(III) and Lu(III) are diamagnetic
ions and their complexes are generally used as diamagnetic
reference compounds. Furthermore, Y(III) oen forms
complexes which are isostructural to the lanthanide
compounds and can also be used as a reference.
In principle, paramagnetic compounds provide a detailed
insight into the bonding mode via the separation of the
observed paramagnetic chemical shi dpara into FCS and PCS.
For this task, several methods have been proposed in the liter-
(Fermi contact shi, FCS) and a distance- and angle-dependent
part due to interaction of the anisotropic electron magnetic
moment, assumed to be located at the metal ion, and the
nuclear magnetic moment of ligand nuclei (pseudo contact
2
9–35
shi, PCS).
FCS and PCS
Currently, several methods for this separation of
are under investigation regarding their appli-
32,36
cability to actinide complexes.
ature. Methods based on the chemical shi dependence on the
The scope of the work presented in this paper is to generate a
reliable base of NMR spectra of diamagnetic and weakly para-
magnetic C5-BPP lanthanide complexes and of the Am(III)
complex. With these data, we aim to elucidate the bonding
mode and potential bonding differences in lanthanide and
actinide C5-BPP complexes, as this is expected to be the driving
force of the ligand's selectivity for actinide over lanthanide
extraction.
38,39
temperature
have been a matter of controversy and their
40
application has to be evaluated very carefully. Currently, the
standard procedure is the evaluation of the purely paramagnetic
shi throughout the complete lanthanide series vs. tabulated
lanthanide-depending constants, i.e. spin-expectation values
29,31,32,35,41
hS
z
i and Bleaney parameters CLn
.
Lanthanide shi
42–45
reagents
and lanthanide probes for protein structure
31,46–48
determination
have been widely used in NMR spectros-
copy and thus lanthanide magnetic properties are quite well
understood. This is not the case for magnetic properties of
elements of the actinide series, thus Bleaney parameters CLn
Theoretical and NMR background
The chemical shi – and thus the electron distribution – of the
coordinating nitrogen atoms are of particular interest for the
investigation of bonding interactions. The effect of covalent
bonding is especially pronounced here, since transferred
and spin expectation values hS i are unknown for these cations.
z
So far, only a small number of proton spectra and a few
heteronuclear spectra of actinide containing compounds are
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available. These are largely limited to uranium in several
oxidation states and hence there is a paucity of NMR studies on
organic complexes with transuranium elements.
The magnetic properties of the free Am(III) ion are still a
matter of debate in literature, as deviations from the expected
diamagnetism arising from a predicted J ¼ 0 ground state have
been found. Optical spectroscopy and DFT calculations show
that the rst non-diamagnetic excited states are some thousand
wavenumbers higher in energy and thus thermally not popu-
49
lated and mixing of the states is not expected. This was also
conrmed by experimental work on an [Am(H O) ](CF SO )
3 3
2
9
3
crystal in solid state, which exhibited a magnetic susceptibility
50
curve that can be interpreted as non-magnetic behavior. On
the other hand, surprisingly large magnetic susceptibilities and
magnetic moments have been reported for different Am(III)
compounds in the solid state, indicating that Am(III) is not
51–54
purely diamagnetic.
Recently, the magnetic susceptibility of
Am(III) in perchloric acid solution was studied using the Evans
55
NMR method. Results show a signicant deviation from the
expected magnetic behavior for Am(III) and Cf(III). Magnetic
susceptibilities for both ions were found to be higher than Scheme 1 Synthesis and labeling of the pyrazole moiety in C5-BPP
15
56
with 10% N; adapted from the synthesis protocol in ref. 20.
expected.
In a recent publication, the effect of spin–orbit coupling on
the alignment of spins in a magnetic eld and the applicability
57
C5-BPP does not displace all nitrate anions from the inner
coordination sphere of the central metal ions during crystalli-
of the Russel–Saunders coupling scheme was discussed. Spin
expectation values for different congurations of Am(III) were
calculated. The authors show that the expected J ¼ 0 ground
20
1
zation. In our case H NMR spectroscopy on Ln(III) complexes
showed that more than one complex species was formed in
samples in which nitrate anions were present. Diffusion-
state has a spin expectation value hS i ¼ 0 and contains the
z
expected Hund's multiplet to 63%. The energy difference to the
60–62
ordered NMR spectroscopy (DOSY)
proved that several
J ¼ 1 state with hS i ¼ 0.5 is only 0.24 eV, which is signicantly
z
complex species with varying diffusion coefficients were
present. This is due to the fact that nitrate anions are strongly
complexing ligands in pure organic solvents. The formation of
numerous different complex species was overcome by using
lower than the expected value of 3.0 for six unpaired electrons.
The authors conclude that there is a signicant deviation from
the multiplets expected from Hund's rules, but that pure j–j-
coupling cannot describe the electronic states as well.
ꢀ
ꢀ
2
43
triate salts (OTf , CF SO ) for which the counter-anion has
3 3
In the case of the Am(nPrBTP) (NO ) complexes, we were
3
3 3
1
13
15
been shown to be non-coordinating. Indeed, in NMR spectra of
C5-BPP lanthanide triate complexes, only the desired 1 : 3
complex and, occasionally, small traces of a 1 : 2 complex, were
found.
able to acquire one- and two-dimensional H, C and
N
2
1
spectra in good quality. The observed linewidths in the spectra
and the range of chemical shis indicate that Am(III) has only a
weak paramagnetism, with effects even smaller than observed
for Sm(III). These results encouraged us to expand our NMR
studies to the Am(III) complexes of C5-BPP.
In order to perform NMR investigations using complexes
with the same counter-anion, Am(OTf) was prepared from an
3
1
5
Am(NO
3
)
3
stock solution. Subsequently N labeled and unla-
](OTf) . To
beled C5-BPP was used to synthesize [Am(C5-BPP)
3
3
Synthesis
avoid potential magnetic impurities due to radiolysis of the
solvent and impurities from radioactive decay products we used
To compensate the unfavourable NMR spectroscopic properties
2
43
1
5
15
15
the long-lived isotope Am (t1/2 ¼ 7370 a).
of N, we synthesized a N enriched C5-BPP ligand, { N}C5-
1
5
BPP, in accordance to the already published N enriched
nPrBTP ligand. The synthesis pathway is shown in Scheme 1.
Successful labeling was conrmed by NMR spectroscopy and
21
5
8,59
LIFDI-MS
(cf. ESI†).
1
5
Using { N}C5-BPP, 1 : 3 complexes with lanthanides (La(III),
Sm(III), Yb(III), Lu(III)) and Y(III) were prepared. In order to
compare these complexes to a 1 : 3 complex with a trivalent
actinide we also prepared a { N}C5-BPP complex with
Fig. 1).
All complexes were prepared in deuterated methanol. Earlier
studies on crystal structures of the Ln(III) complexes report that BPP)
1
5
243
Am
(
243
Fig. 1 Molecular structure and numbering scheme of the [ Am(C5-
3+
3
]
complex.
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series. The smaller metal ions Lu(III) and Y(III), which both have
closed shells, are better suited as reference compounds for the
heavier lanthanides. The error inferred from the reference
Results and discussion
Diamagnetic Ln(III)–(C5-BPP) complexes
3
As a rst step in the investigation of bonding modes in C5-BPP compound on the determination of the purely paramagnetic
complexes of lanthanide and actinide ions, we focused on chemical shi in strongly paramagnetic systems, where shis of
diamagnetic or nearly diamagnetic compounds. A comparison several hundred ppm can occur, are limited. However, the
of spectra of diamagnetic compounds is straightforward, as inuence on weakly paramagnetic systems should not be
signicant changes between isostructural complexes can be underestimated.
attributed to a change in binding mode.
In our studies with nPrBTP we used the Lu(III) complex as a complexes and in the Y(III) C5-BPP complex, resonance signals
diamagnetic reference, since spectra of [La(nPrBTP) ](NO
for the coordinating nitrogen atoms N and N are generally
showed broadened spectral lines. This is due to a relatively found in a 12 ppm range around 266 ppm. For the non-coor-
weak coordination of nPrBTP to the large La(III) ion which dinating nitrogen atom N ,resonance signals are found in a
It should be noted that in all diamagnetic lanthanide C5-BPP
3
3
)
3
1
8
21
9
decreases the complex symmetry and thus results in broad narrow 2 ppm range around 206 ppm (cf. Table 1 and Fig. 2). In
spectral lines. The bigger bite angle of the pyrazole nitrogen comparison to the free ligand, the coordination of C5-BPP to a
lone pairs in C5-BPP should enable this ligand to form struc- M(III) cation hardly inuences the chemical shi of N . N on
9 8
turally rigid complexes even with slightly larger cations. Indeed, the other hand is shied approximately 30 ppm upeld. This is
we found that C5-BPP forms stable complexes with La(III), due to the rearrangement of electron density upon complex
resulting in well-resolved NMR spectra with sharp lines.
Comparison of the three diamagnetic C5-BPP complexes is observed for N
Y(III), La(III) and Lu(III)) shows that although all three metal ions lanthanide compound spectra, we would expect the resonance
formation. Unfortunately, in the free ligand no resonance signal
. Nevertheless, based on the diamagnetic
1
(
1
13
1 8
are diamagnetic, there are signicant differences in H, C and signal for N in the same shi range as N . The same problems
1
5
13
N NMR chemical shis. These differences are strongest in were encountered when we measured C spectra of the free
close proximity to the metal ion, and only very weak at the ligand. We found that resonance signals for the quaternary
aliphatic side chain. Differences between proton spectra of Y(III) carbon atoms C2/6, C , and C10 are severely broadened and
and Lu(III) complexes are small, with a maximum of 0.01 ppm at sometimes, as in the free N enriched ligand, unobservable in
the H
triplet. The maximal discrepancy between proton signals 1D spectra. So far we do not have a clear explanation for this
of the La(III) and Lu(III) complexes is found for the signals of H3/5 behavior.
7
1
5
4
1
3
with 0.04 ppm. The differences are more pronounced in
C
spectra. Again the spectra of Lu(III) and Y(III) complexes strongly
Paramagnetic Ln(III)–(C5-BPP) complexes
3
resemble each other. Only for C2/6 (Dd ¼ 0.3 ppm) and C10 (Dd ¼
0
.28 ppm) signicant discrepancies are observed. Differences In the following we studied the inuence of a weakly and a
between the La(III) and Lu(III) complexes are stronger in partic- strongly paramagnetic central metal cation on the NMR spectra
of the C5-BPP complexes. We used [Sm(C5-BPP) ](OTf) as a
representative for a weakly paramagnetic ion (Sm : meff ¼ 0.85m
ular for the quaternary carbon atoms C (Dd ¼ 0.73 ppm), C
3
3
7
2/6
3+
(
Dd ¼ 1.70 ppm), and C10 (Dd ¼ 1.20 ppm).
B
)
3
+
As the inuence of the central metal ion seems to be strongly and [Yb(C5-BPP)
3
0,43
3
](OTf) as a strongly paramagnetic ion (Yb :
With the N labeled ligand in hand, our focus
3
15
dependent on the distance to the observed nucleus it should be
meff ¼ 4.54m
B
).
1
5
even more pronounced on the nitrogen atoms. In N spectra we was on the inuence of paramagnetism on the resonance
observe only weak shi differences for the non-coordinating N
signals of the coordinating nitrogen atoms. In the Sm(III)
resonance signal is observed at 205 ppm, i.e.
9
(
Y/Lu (Dd ¼ 0 ppm), La/Lu (Dd ¼ 1.2 ppm)). The coordinating complex, the N
9
nitrogen shis show a stronger dependence on the central without additional shi compared to the diamagnetic
metal ion. The shi differences for N (Y/Lu (Dd ¼ 1.0 ppm), La/ compounds. In contrast to the non-coordinating nitrogen, a
1
1
15
Lu (Dd ¼ 4.0 ppm) from H, N-gHMQC spectra) are smaller
than for N
8
(Y/Lu (Dd ¼ 1.7 ppm), La/Lu (Dd ¼ 7.2 ppm)). These
Table 1 Chemical shifts of the nitrogen atoms in M(C5-BPP)
3 3
(OTf)
results coincide with the differences in ionic radii, which are
quite similar for Y(III) (90.0 pm) and Lu(III) (86.1 pm), whereas
complexes and in the free ligand. All values are given in ppm relative to
15
NH
4
Cl (d( N) ¼ 0 ppm)
63
La(III) is signicantly larger (103.2 pm). Changes in the
complex geometry and subsequently changed interaction
between the metal ion and the ligand can explain the observed
behavior.
These results clearly show that the diamagnetic reference
compound needs to be chosen carefully, as the shi differences
between La(III) and Lu(III) compounds are signicant and several
orders of magnitude larger than the spectral resolution. Based Lu
on our results we assume that La(III) is a good diamagnetic
reference compound for the lighter part of the lanthanide
Metal
N
1
N
8
N
9
a
a
none
Y
La
Sm
Yb
—
287
262
272
224
20
205
205
206
205
194
205
212
a
260a
266a
221
—
a
261
265
ꢀ22
a
Am
1
a
1
15
Labeled values are taken from 2D H, N-gHMQC spectra of the 1 : 3
complexes with unlabeled C5-BPP.
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15
Fig. 2
4 9 8
N direct excitation spectra in MeOD-d of all C5-BPP complexes in this study. N signals are labeled with green circles, N signals with
red circles.
8
larger shi is found for the coordinating nitrogen atoms. paramagnetic ions, while N shows a strong dependency on the
Compared to the La(III) complex, N is shied 45 ppm upeld paramagnetism of the ion.
1
and N is shied 48 ppm upeld. These values are in good
The larger shi in the Yb(III) and the Sm(III) cases can be
8
21
agreement with observed shis for nPrBTP complexes. Yb(III) attributed to a stronger PCS (especially for Yb, which predom-
complexes usually show the expected strong paramagnetic inantly exhibits PCS) but as well a non-negligible FCS. Hetero-
shis, but paramagnetic relaxation enhancement for Yb(III) is nuclei directly bonded to paramagnetic cations have only
still weak enough that spectral lines are not too broad to be scarcely been investigated with respect to the different contri-
observed and most multi-dimensional NMR experiments yield butions to the experienced paramagnetic shi. Most research is
good results. Thus, unambiguous assignment of most signals is limited to protons in close proximity to the metal ion center.
possible by heteronuclear correlation spectroscopy. However, However, although the inuence of FCS decreases rapidly along
1
5
35
due to the enhanced relaxation, the N signals for N and N in covalent bonds, it oen cannot be neglected. A strong impact
8
9
1
5
Yb(C5-BPP) (OTf) are only found aer N-labeling. N shows a of the FCS on directly coordinated nuclei can thus be expected,
3
3
9
notable shi of ꢀ10 ppm compared to the diamagnetic refer- and, as in our case, might even contribute to a larger than
ences, which can be attributed to the stronger PCS. The coor- expected share. Further research into this topic is necessary and
dinating N
A comparison of N direct excitation spectra of all investi-
gated 1 : 3 M(III) C5-BPP complexes is shown in Fig. 2. The N
signals (green circles) for the diamagnetic metal ions (Y(III), NMR-spectroscopy on Am(III)–(C5-BPP)
8
is shied by approximately ꢀ200 ppm to 20 ppm.
currently under way in our group.
1
5
9
3
La(III), Lu(III)) show almost identical chemical shi values (green
The spectra of the Am(III)–C5-BPP complexes with and without
N labeling show that more than one complex species was
formed. Upon addition of further ligand solution two of the
complex species could be assigned to the free ligand and the
dotted line), while for N (red circles) there is a notable differ- 15
8
ence (red dotted line). Furthermore, the chemical shi for the
non-coordinating N remains nearly constant even for the
9
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: 2 complex. Signals from the 1 : 3 complex, which forms the
Chemical Science
1
major species present in the sample, increase in intensity with
increasing ligand-to-metal ratio. However, during titration
another minor complex species that contains only one ligand
molecule and a so far unknown contaminant not visible by
NMR spectroscopy is formed. However, the NMR signals of the
1
: 3 complex as the major species could easily be identied and
unambiguously assigned. To further elucidate the composition
of the complex speciation we studied the samples by further
NMR spectroscopic methods and time-resolved laser uores-
cence spectroscopy (TRLFS, see below).
For a sample containing several different components,
diffusion-ordered NMR spectroscopy (DOSY) is a versatile
1
method. H DOSY spectra show three well separated complex
1
9
species. F direct excitation spectra only show one signal at
1
9
ꢀ
79.97 ppm, which corresponds to the triate anion. F DOSY
spectra yield one diffusion coefficient for this peak which differs
from the coefficients for the complex species calculated from
H DOSY measurements. Thus a coordinated triate anion or
1
exchange between a bound and a free form can be excluded. All
1D spectra are well-resolved, and unambiguous assignment of
the signals of the 1 : 3 Am(III) C5-BPP complex is possible. The
1
13
15
complex is fully characterized by H, C and N direct excita-
tion spectroscopy at different temperatures as well as a range of
Fig. 3 Chemical shift differences between the NMR signals in the
Sm(III) (top) and Am(III) (bottom) complexes compared to the La(III)
complex for all nuclei. All values are given in ppm.
2D heteronuclear correlation spectroscopy methods.
Information about magnetic properties and the bonding
situation can be deduced by comparison of the Am(III) complex
spectra and those of a diamagnetic reference compound.
Unfortunately, the diamagnetic actinides Ac(III) and Lr(III) have nitrogen atoms (N
: ꢀ256 ppm, N
Lr) ¼ 3.6 h) and are Shis of a comparable magnitude have only been found for a
not available in sufficient amounts. As we lack a diamagnetic Yb(III) C5-BPP complex which has a considerably stronger
: ꢀ295 ppm) are noteworthy.
1
8
2
27
262
short half-lifes (t1/2
(
Ac) ¼ 21.8 a, t1/2
(
32
actinide reference compound, we have to compare the Am(III) effective magnetic moment (meff ¼ 4.54m
B
).
complex's chemical shis to those of Ln(III) complexes. This
Furthermore, carbon atoms in both the Sm(II) and the Am(III)
complexes show alternating positive and negative chemicals
and [Sm({ N}C5-BPP) ](OTf) compared to [La({ N}C5-BPP) ](OTf). shi differences along the carbon backbones of the ligands (see
1
5
comparison is displayed in Fig. 3 for [Am({ N}C5-BPP) ](OTf)
3
3
15
15
3
3
3
For most nuclei the effect of the Am(III) cation on the
chemical shi is approximately ten times stronger than that of
the Sm(III) cation. For the weakly paramagnetic Sm(III) a
3
0,63
magnetic moment of meff ¼ 0.85m
B
is known.
Measurements
using the Evans method yield a magnetic moment of
56
meff ¼ 1.64m
B
for Am(III). Recently, work on the inuence of
radioactive decay and radiolysis product formation on the
accuracy of the Evans method has been published, suggesting a
64
reduced magnetic moment of approximately meff ¼ 1.42m . We
B
therefore expect the paramagnetic inuence of Am(III) to be
stronger than the inuence of Sm(III), but both should produce
paramagnetic chemical shi effects in the same order of
magnitude.
The large differences in the chemical shis cannot be explained
by the difference in the magnetic moments of the cations, but
point to a fundamental change in the binding mode. Fig. 4 shows a
1
15
15
H, N-gHMQC spectrum of [Am({ N}C5-BPP) ](OTf) . Indicated
3
3
with the red boxes are the chemical shis for the coordinating
nitrogen atoms as expected from the free ligand and diamag-
netic Ln(III) compounds. In the red circles are the measured
values that differ vastly from the expectations. For the Am(III)
1
15
15
Fig.
4
3 3
H, N-gHMQC spectrum of Am[({ N}C5-BPP) ](OTf) in
MeOD-d
4
, optimized for a coupling constant JHN ¼ 5 Hz. The
expected value range is taken from similar experiments with the
diamagnetic lanthanide complexes. The correlation signals in the gray
complex, the immense shi differences of the coordinating circles originate from an additional minor complex species.
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Fig. 3). This phenomenon is indicative of the simultaneous upon temperature change: In the monitored temperature range,
existence of spin polarization and spin delocalization at the N shows a 152 ppm shi, for the non-coordinating N the shi
8
9
6
4-67
ligand (polarized spin density delocalization).
This spin is still ꢀ8 ppm. Furthermore, even in the weakly paramagnetic
delocalization would be due to a Fermi contact interaction Sm(III) complex, N shows a temperature-dependent shi of
8
between metal cation and N-donor ligands and thus to a share ꢀ162 ppm in the observed temperature range. Thus it is clear
of covalence in the bonding. The pattern of the shi differences that the paramagnetism of the Am(III) ion is considerably
suggests that a part of the delocalized spin electron density weaker than at least in the Yb(III) ion and cannot satisfactorily
2
15
resides in p
z
orbitals of the sp hybridized carbon atoms. explain the observed higheld shi of the N
8
signal in [Am({ N}
However, if this is true we would expect the signs of the C5-BPP)
3
](OTf) . The smaller temperature-dependent shi in
3
chemical shi difference of the protons to be inverse to the the Am(III) complex, compared to Sm(III), could be due to a
attached carbons' shi differences (two spins that are coupled different ratio of covalent and dipolar bonding: FCS is trans-
electronically over one bond will have opposite signs). We nd mitted through covalent bonds and has a linear temperature
that for Sm(III) and Am(III) all pyridine proton shi differences dependency. PCS, which can be associated to dipolar interac-
ꢀ
2
4
have the same signs. In both cases H is shied more towards tions, has a T dependency. However, as long as no clear
deeper elds than the H3/5 protons. This behavior suggests that separation of the chemical shi contribution can be performed,
two (or even more) different mechanisms take part in the this has to be seen as indicative of a more covalent bond, but
delocalization of electron spin density, showing that the not yet as a proof.
bonding between Am(III) and the so N-donor ligand is a very
The observed behavior of alternating chemical shi effects
complicated matter. An explanation for the downeld shi of in the carbon backbone, but not on the protons in the ligand,
H could be that unpaired electron spin density is also trans- points towards a combination of direct spin delocalization and
4
ferred through s bonds in the aromatic ring or the conjugated polarized spin density delocalization. Both rely on a Fermi
double bonds, respectively.
contact interaction arising from covalent bonding between the
To gain insight into magnetic and bonding behavior, we trivalent metal cation and the nitrogen atoms of the ligands.
acquired NMR spectra at different temperatures between 185 K From comparison of the observed chemical shi differences in
1
5
3 3 8
and 335 K (cf. ESI†). In [La({ N}C5-BPP) ](OTf) , N shows a the slightly paramagnetic Sm(III) complex and the Am(III)
temperature-dependent shi of 0.3 ppm. The N signal shows complex, which cannot be explained by paramagnetism alone.
9
strong line broadening (FWHM 18.54 ꢁ 0.28 Hz) even at low We interpret this fact as indicative of an higher share of cova-
temperatures. Thus, N is only observable up to 315 K. In the lence in the actinide compound, which is consistent with
9
6
8
monitored 130 K temperature range, the N
9
signal shows a recently reported XAS and EXAFS studies. Another effect that
might explain the shi differences between Am(III) and the
temperature-dependent shi of ꢀ1 ppm.
In the case of the Y(III) C5-BPP complex, N
8
experiences a 0.5 Ln(III) complexes is the existence of spin-orbit coupling effects
(FWHM on the metal ion which inuences the shi of the nitrogen
Spin–orbit coupling is strongly dependent on the
The temperature-dependent chemical shi of the Am(III) atomic number of the nucleus and is thus considerably stronger
complex shows a different behaviour (Fig. 5). The non-coordi- in the actinide series than for lanthanides. Spin polarization
ppm downeld shi between 185 K and 335 K, while N
at 185 K: 19.10 ꢁ 0.18 Hz) shows a ꢀ1.4 ppm upeld shi.
9
69–71
atom.
nating N (FWHM at 285 K: 20.04 ꢁ 0.19 Hz) shows an upeld from spin–orbit coupling resembles spin-spin coupling effects
9
71
shi of approximately 1 ppm at 275 K with increasing broad- in NMR spectroscopy that are mediated by s-type orbitals. This
ening of the resonance signal. This signal is not observable is another mechanism that could explain the substantial shis
above 300 K, while a new doublet appears 0.8 ppm downeld of on the nitrogen atoms and why the shi differences cannot be
the last broad signal. Up to a temperature of 335 K this signal is observed on neighboring atoms. As a consequence, both para-
again shied upeld by 0.5 ppm. In total, the temperature- magnetic effects in the form of FCS and spin–orbit coupling
dependent shi of N is 0.6 ppm. The coordinating N , however, seem to play an important role in the observed chemical shis.
9
8
shows a continuous 11.3 ppm downeld shi. This tempera- Fermi-contact interactions and thus the existence of a certain
ture-dependent shi is approximately ten times the shi covalence compared to the lanthanide compounds could thus
measured for diamagnetic reference compounds, which is explain the observed shis on the nitrogen atoms of the
another distinct piece of evidence showing that Am(III) is not americium complex.
diamagnetic.
The observed chemical shi of the coordinating nitrogen
atoms in the Am(III) complex cannot be explained by the
Cm(III) TRLFS studies to identify the minor complex species
different strength of the ions' magnetic moments alone, as a As shown above minor Am(III) complex species are formed in
comparison to the temperature dependent NMR spectra of the addition to the prevailing 1 : 3 Am(III) complex. Small amounts
1
5
strongly paramagnetic [Yb({ N}C5-BPP) ](OTf) shows. As the of impurities or minor complex species cannot be characterized
3
3
chemical shi of N
8
at room temperature is +20 ppm and thus using NMR spectroscopy. Hence we used a different spectro-
close to the observed shi of the Am(III) complex, one could scopic method to elucidate the composition of the minor
ꢀ
8
assume that the two ion's paramagnetism were in the same complex species. Addition of a trace amount (6.6 ꢂ 10 mol ꢂ
ꢀ
1
243
order of magnitude. However, the complex with the more
3 3
L ) of Cm(III) to the [ Am(C5-BPP) ](OTf) NMR sample
paramagnetic Yb(III) cation shows a larger chemical shi range enabled us to make use of the excellent uorescence properties
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15
15
Fig. 5 Sections of the N direct excitation spectra of [Am({ N}C5-BPP)
3 3 4 9 8
](OTf) in MeOD-d at increasing temperatures (N left side, N right
side). All spectra are referenced to the internal standard TMS by the lock signal.
of Cm(III). Furthermore, the chemical properties of Am(III) and known 1 : 2 and 1 : 3 complex species are assigned to the use of
Cm(III) are highly comparable which is for example reected in a deuterated solvent and the high concentration of triate
very similar M(III)–N bond lengths in 1 : 3 complexes in solu- anions.
9
tion (Am(nPr-BTP)
:
3
: Am(III)–N ¼ 256 pm; Cm(nPr-BTP)
3
-
With increasing ligand-to-metal ratio a decreasing ratio of
the 1 : 2 complex species and an increasing ratio of the 1 : 3
13,14
Cm(III)–N ¼ 257 pm).
Aer addition of Cm(III) to the NMR sample the Cm(III) complex species are observed, showing a stepwise complexation
emission bands are recorded with increasing amounts of C5- of Cm(III). The emission band at lmax ¼ 603.2 nm also gains in
BPP ligand. Upon addition of ligand solution to the Cm(III) intensity upon increasing amount of C5-BPP, which conrms
spiked NMR sample the initial Am(III) and Cm(III) metal that does not result from the 1 : 1 Cm(III)–C5-BPP complex, and
concentrations are diluted, and hence the ligand-to-metal ratio is attributed to a Cm(III) complex species with a minor impurity
stepwise increases. The development of the Cm(III) uorescence from the C5-BPP synthesis. At signicantly higher metal
6
0
8
0
emission resulting from the D7/2 / S7/2 transition as function
of the ligand-to-Am(III) ratio are shown in Fig. 6. The spectra are
normalized to the same peak area for better comparison.
At an initial C5-BPP-to-Am(III) + Cm(III) ratio of 3 : 1 (corre-
5
sponding to a ligand-to-Cm(III) ratio of 4.5 ꢂ 10 : 1) an emission
band with a maximum at l
¼ 606.0 nm and two weak
max
shoulders at lmax ¼ 603.2 nm and lmax ¼ 610.3 nm are observed.
With increasing ligand-to-metal ratio the intensity of the
emission band at lmax ¼ 606.0 nm decreases signicantly while
both shoulders gain in intensity. At a nal ligand-to-Am(III) + Cm(III)
ratio of 6.8 : 1 two distinct emission bands with maxima at
l
¼ 603.2 nm and lmax ¼ 610.6 nm are observed with an
max
intensity ratio of approximately 1 : 1.
In earlier studies the emission bands of the Cm(III)–C5-BPP
1
: 1, 1 : 2 and 1 : 3 complex species in methanol were observed
at lmax ¼ 603.7 nm, lmax ¼ 607.7 nm and lmax ¼ 611.6 nm,
20
respectively.
Hence, the observed emission bands at
l
max ¼ 606.0 nm and lmax ¼ 610.6 nm are attributed to a Cm(III)–
C5-BPP 1 : 2 and a Cm(III)–C5-BPP 1 : 3 complex species. The Fig. 6 Normalized fluorescence spectra of Cm(III) in MeOD-d
hypsochromic shi of 1.0 nm in comparison to the literature
4
with
increasing amount of C5-BPP. [Cm(III)]ini ¼ 6.6 ꢂ 10ꢀ mol L
8
ꢀ1
,
ꢀ6
ꢀ1
[
Am(III)]ini ¼ 6.0 ꢂ 10 mol L
.
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concentrations used for NMR studies these minor complex
Further temperature dependent NMR experiments with
species do not play an important role and all signals of the 1 : 3 paramagnetic cations of the entire lanthanide series and further
complex species can be assigned unambiguously (see above).
transuranium element cations are in progress. Moreover, we
endeavor to investigate the contributions to the chemical shi
using quantum chemical calculations. The obtained experi-
mental chemical shi values for all nuclei in the complexes are
important benchmarks for those calculations.
Conclusions
We present the rst NMR study on a series of 1 : 3 complexes of
Ln(III) and Am(III) with the tridentate N-donor ligand C5-BPP. A
key step in our investigations was the synthesis of a C5-BPP
Experimental section
General
1
5
molecule with N enrichment in the pyrazole moieties.
1
5
Using { N}C5-BPP we prepared 1 : 3 complexes with triva-
lent lanthanide ions (La, Sm, Yb, Lu and Y) and Am(III) as a
representative of the trivalent actinides. In diamagnetic
All NMR spectra were recorded at T ¼ 300 K on a Bruker Avance
1
III 400 spectrometer operating at 400.18 MHz for H, 100.63
13
15
MHz for C and 40.56 MHz for N. The spectrometer was
equipped with a z-gradient broadband observe probe optimized
for x-magnetization detection. Chemical shis are referenced
complexes, signals of the non-coordinating N are observed in a
9
small chemical shi range between 195 ppm and 206 ppm. At
room temperature, the coordinating N
8
signals are found in a
15
internally to TMS (d(TMS) ¼ 0 ppm). N chemical shis are
chemical shi range between 224 ppm and 275 ppm.
15
referenced to NH Cl with d(NH Cl) ¼ 0 ppm. For all direct
4
4
Comparing the three diamagnetic complexes Y(III), La(III), and
excitation and correlation spectra, standard Bruker pulse
sequences were used. DOSY spectra were acquired using one-
1
13
15
Lu(III), we found signicant differences in H, C and
N
spectra. This shows that diamagnetic reference compounds for
the extraction of purely paramagnetic shis dpara have to be
chosen with care. We conclude that La(III) serves as good
diamagnetic reference for the lighter part of the lanthanide
series and Lu(III) and Y(III) are better suited for the heavier
lanthanides.
61
shot sequences. All 1D spectra for diamagnetic complexes and
Am(III) were recorded with 32k data points and are zero lled to
15
6
4k points. N spectra were recorded at lower spectral resolu-
tion if necessary, allowing fast pulsing and high repetition rates
to compensate the paramagnetic relaxation enhancement. The
reported chemical shis are taken from 1D spectra unless
1
5
We furthermore prepared the [Am({ N}C5-BPP)
3
](OTf)
3
15
stated otherwise. N data at natural abundances are obtained
complex and showed that NMR resonance signals for this
complex have a stronger temperature dependence than signals
of complexes with diamagnetic Ln(III), but weaker than for
paramagnetic Yb(III) and Sm(III). This indicates a weak para-
magnetism of the Am(III) complex, similar to earlier ndings for
BTP complexes.
1
15
from H, N-HMQC spectra. Deuterated solvents were
purchased from Euriso-Top GmbH. Chemicals for synthesis
15
were purchased from VWR International and used as-is. N-
labeled hydrazine hydrate (98% N) was purchased from
15
Sigma-Aldrich and used as-is. Mass spectra using LIFDI and EI
ionization methods were recorded using a JEOL JMS-700
magnetic sector instrument. Mass spectra using ESI ionization
methods were recorded using a Bruker ApexQe FT-ICR instru-
ment. All mass spectra were recorded at the mass spec facility of
the Institute of Organic Chemistry at the University of Heidel-
In comparison to the diamagnetic lanthanide complexes, the
coordinating N
8
experiences a signicant upeld shi to ꢀ20
ppm, which is in excellent agreement with data from earlier
studies with the Am(III)–nPrBTP complex. As comparison to the
Sm(III) and Yb(III) complexes shows, this extraordinary upeld
shi cannot be explained as paramagnetic effects known from
studies of similar lanthanide complexes, since shis of the
15
berg. All mass spectra of N-labeled compounds were acquired
58,59
using LIFDI-MS technology.
Melting points were measured
using a Stuart SMP30 melting point apparatus.
8
coordinating N in the same order of magnitude have only been
found for the Yb(III) complex, which has a bigger magnetic
moment. Explanations for this behavior are transfer of electron
TRLFS setup
spin density to the nitrogen atoms by several possible mecha- All compounds for TRLFS experiments were used as received.
nisms and spin–orbit coupling effects from Am(III). All transfer Methanol (absolute) was purchased from Merck and stored over
mechanisms rely on the existence of a Fermi contact interac- molecular sieves. The concentration of Cm(III) was set to
ꢀ8
ꢀ1
tion, which is mediated by covalent bonding through s-orbital 6.6 ꢂ 10 mol ꢂ L by adding an aliquot of a stock solution
ꢀ
6
ꢀ1
ꢀ2
ꢀ1
containing binding orbitals.
Our results are an important contribution within current the [ Am({ N}C5-BPP)
research efforts to identify the origin of selectivity of N-donor mass distribution of the Cm(III) solution was 89.7% Cm, 9.4%
[Cm(III)] ¼ 6.7 ꢂ 10 mol L in HClO
4
(1.0 ꢂ 10 mol L ) to
2
43
15
3
](OTf) NMR sample. The isotopic
3
2
48
2
46
243
244
245
247
ligands in actinide–lanthanide separation. They show that NMR
Cm, <0.5% Cm, Cm, Cm, and Cm, determined by
spectroscopy is a versatile and sensitive tool in the elucidation alpha spectroscopy and ICP-MS. TRLFS measurements were
of fundamental bonding mechanisms especially for actinide performed using a Nd:YAG-pumped dye laser system [Surelite II
compounds. Important insights into the metal–ligand bonding laser (Continuum), NARROWscan D-R dye laser (Radiant Dyes
were obtained, which reveal valuable information for an opti- Laser Accessories)]. For Cm(III) excitation a wavelength of 396.6
mized design of future extractants for the separation of acti- nm was used. The emission spectra were recorded at an angle of
ꢃ
nides from lanthanides.
90 to the exciting laser beam. A Shamrock 303i spectrograph
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(ANDOR), equipped with a 300, 900 and 1200 lines per mm added to 3.3 mL (22.9 mmol) 4,4-dimethylpentane-2-one and
grating turret was used for spectral decomposition. Fluores- stirred in an argon atmosphere for 30 min at room temperature.
cence spectra were recorded in the 575–635 nm range using the Subsequently, a solution of 2.1 g (10.7 mmol) dimethyl 2,6-
1200 lines per mm grating of the spectrograph. The uores- pyridinedicarboxylate in 20 mL diethyl ether (abs.) was added
cence emission was detected by an ICCD camera [iStar Gen III, dropwise, and the reaction mixture was reuxed for 5 h.
A-DH 720 18F-63 (ANDOR)]. Rayleigh scattering and shortlived Subsequently, the reaction mixture was cooled to room

uorescence of organic ligands was discriminated by a delay temperature and neutralized with glacial acetic acid. The
time of 1.0 ms before the uorescence light is recorded. The organic phase was washed three times with 30 mL portions of
ꢃ
quartz cuvette was temperature controlled at T ¼ 25 C.
cold water, dried with sodium sulfate and concentrated in
vacuo. The product was obtained in 79% yield as a yellowish
crystalline solid. mp: 109.6 C.
ꢃ
TRLFS sample preparation
1
H-NMR (400.18 MHz, MeOD-d , T ¼ 328 K) keto form:
4
2
43
3 3 4
The [ Am(C5-BPP) ](OTf) NMR sample in 600 mL MeOD-d
was transferred from a J. Young-type NMR tube into a quartz
d(ppm) ¼ 8.19 (d, 2H, H3/5), 8.08 (dd, 1H, H
4 8
), 6.87 (s, 4H, H ),
2
.37 (s, 4H, H10), 1.08 (s, 18H, H12); enol form: (12% according
cuvette. 6 mL of an aqueous Cm(ClO ) stock solution (1.0 ꢂ
1
4
3
to H-NMR) d(ppm) ¼ 8.27 (dd, 2H, H3/5)*, 6.80 (s, 4H, H
s, OH), 2.59 (s, 4H, H10), 1.03 (s, 18H, H12). * Value for H
not be assigned unambiguously.
8
), 4.56
could
ꢀ2
ꢀ1
ꢀ6
ꢀ1
10
mol L HClO
4
, [Cm(III)] ¼ 6.7 ꢂ 10 mol L ) was added
(
4
and carefully shaken. The change in volume was limited to 1.0%
vol). Titrations were performed by stepwise addition of a C5-
(
13
C-NMR (100.63 MHz, MeOD-d , T ¼ 328 K) keto form:
4
ꢀ2
ꢀ1
BPP solution (3.0 ꢂ 10 mol L ) in MeOD. Aer each addition
of the ligand solution the sample was carefully shaken and a
Cm(III) uorescence spectrum was recorded.
d(ppm) ¼ 195.6 (C , C ), 183.7 (C , C ), 153.5 (C , C ), 139.7 (C ,
q
9
q
7
q
2/6
t
C ), 125.3 (C , C ), 99.6 (C , C ), 53.1 (C , C ), 32.7 (C , C ),
4
s
3/5
s
8
s
10
q
11
3
1
0.4 (C
53.4 (C
p
, C12); enol form: d(ppm) ¼ 195.5 (C
q
, C
9
), 183.2 (C
q
, C
7
),
q
, C2/6), 139.9 (C
t
, C
4
), 126.5 (C
s
, C3/5), 99.8 (C , C ), 56.6
s 8
Dimethyl 2,6-pyridinedicarboxylate
s 10 q 11 p 12
(C , C ), 31.7 (C , C ), 30.0 (C , C ).
+
HR-MS (EI) calculated for C21
H
29NO
4
[M] 359.2097; found:
+
3
3
3
59.2114; calculated for C20 26NO
H
4
[M ꢀ CH
3
] 344.1862, found:
+
44.1841; calculated for C H NO [M ꢀ C H ] 303.1471, found:
17
21
4
4 8
+
03.1492; calculated for C H NO [M ꢀ C H ] 288.1236,
16
18
4
5 11
+
found: 288.1275; calculated for C H NO [M ꢀ C H O]
1
6
21
3
5
8
2
75.1521, found: 275.1534; calculated for
15
C H18NO
3
+
[
C
M ꢀ C
6
H11O] 260.1287, found: 260.1283; calculated for
+
The preparation of dimethyl 2,6-pyridinedicarboxylate was
14
H19NO
2
[M ꢀ C
7
H
10
O
2
] 233.1416, found: 233.1416; calcu-
204.1025, found: 204.0657; calculated for
12NO 190.0868, found: 190.0488; calculated for C
9.0810, found: 99.0791.
carried out by a modication of a previously published
lated for C12H14NO
3
C
9
72
method. 2,6-Dipicolinic acid (10.0 g, 59.8 mmol) and 2.0 mL
sulphuric acid (conc.) were reuxed in 40 mL methanol for 3 h.
Aer the solution was cooled to room temperature the solution
11
H
6 11
H O
2 3
was neutralized with 1.5 g (14.2 mmol) Na CO . The resulting
white precipitate was separated by ltration and washed three
times with 20 mL portions of cold water. The solid was dried for
ꢃ
2,6-Bis(5-(2,2-dimethylpropyl)1H-pyrazol)-3-yl-pyridine (C5-
2
h at 60 C in high vacuum yielding the desired product (10.65
BPP)
g, 54.6 mmol, 91%) as a white solid.
1
H-NMR (400.18 MHz, CDCl ): d(ppm) ¼ 8.26 (d, 2H, H3/5,
3
3
3
J ¼ 7.8 Hz), 7.99 (t, 1H, H
4
, J ¼ 7.8 Hz), 3.98 (s, 6H, H
8
).
, C
).
1
3
C-NMR (100.63 MHz, CDCl
3
): d(ppm) ¼ 164.9 (C
q
7
),
1
1
48.1 (C
q
, C2/6), 138.3 (C
t
, C
4
), 127.9 (C
t
, C3/5), 53.1 (C
p
, C
8
0
,1 -(Pyridine-2,6-diyl)bis(5,5-dimethylhexane-1,3-dione)
8
.0 mL (129 mmol) N
2
H
4
$H
2
O (80% in H
2
O) were added to a
0
solution of 540 mg (1.5 mmol) 1,1 -(pyridine-2,6-diyl)bis(4,4-
dimethylhexane-1,3-dione) in 40 mL methanol (abs.) and
reuxed for 3 h. Aer the solution was cooled to room
temperature the resulting white precipitate was collected and
0
1
,1 -(Pyridine-2,6-diyl)bis(5,5-dimethylhexane-1,3-dione)
was washed three times each with 30 mL water and 30 mL diethyl
prepared from dimethyl 2,6-pyridinedicarboxylate and 4,4- ether. The desired product was obtained by drying in high
dimethylpentan-2-one in an adapted literature procedure. 5.0 vacuum (0.415 g, 1.18 mmol, 79%) as a white, crystalline solid.
ꢃ
mL sodium methanolate (30% in methanol, 28.6 mmol) were mp: 266.5 C.
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1
H-NMR (400.18 MHz, MeOD-d
4
): d(ppm) ¼ 7.84 (t, 1H, H
4
), centrifuged again (6000 rpm, 2 min) and the supernatant was
7
.69 (s, 2H, H3/5), 6.72 (s, 2H, H ), 2.60 (s, 4H, H ), 1.00 (s, 18H, removed. Following this procedure, the precipitate was
11 12
H ).
washed three times with 1.0 mL portions of NaOH (0.01 mol
1
4
1
3
ꢀ1
C-NMR (100.63 MHz, MeOD-d ): d(ppm) ¼ 153.1 (C , C2/6),
L
) and once with 1.0 mL water. The americium hydroxide
and 10 mL tri-
uoromethanesulfonic acid, forming Am(OTf) For
4
q
1
49.3 (C
04.9 (C
LIFDI-MS (CH
found: 352.21; calculated for C21
51.22.
q
, C
, C11), 43.3 (C
OH) calculated for C21
7
), 144.2 (C
q
, C10), 139.0 (C
t
, C
4
), 119.7 (C
t
, C3/5), was dissolved in 1.0 mL
2
H O
1
t
s
, C12), 32.1 (C
q
, C13), 29.8 (C
p
, C14).
+
3
.
15
3
H
30
N
5
[M + H] : 352.25, complexation of Am(OTf)
3
with C5-BPP or { N}C5-BPP,
solution were heated to dryness
+
H
29
N
5
[M] : 351.24, found: respectively, 420 mL Am(OTf)
3
ꢃ
3
at about 100 C on a heating plate. The obtained pale-pink
solid was subsequently washed with 250 mL D O and heated to
2
1
5
N labeled 2,6-bis(5-(2,2-dimethylpropyl)1H-pyrazol)-3-yl-
dryness. The ligand solution (18 mmol in 600 mL MeOD-d ) was
4
1
5
pyridine ({ N}C5-BPP)
added to the Am(OTf) , carefully mixed and transferred into a
3
1
5
J. Young-type NMR tube.
1
00 mg (1.9 mmol) N H $H O and 1.06 mL (17.3 mmol)
2
4
2
1
4
0
N H $H O were added to a mixture of 690 mg (1.9 mmol) 1,1 -
pyridine-2,6-diyl)bis(4,4-dimethylhexane-1,3-dione) and 10 mL
2
4
2
1
5
[
Y({ N}C5-BPP) ](OTf)
3
(
3
1
methanol (abs.) and reuxed for 3 h. Aer the solution was
cooled to room temperature the resulting white precipitate was
collected and washed three times each with 20 mL water and 20
mL diethyl ether. The desired product was obtained by drying in
high vacuum (0.618 g, 1.76 mmol, 92%) as a white, crystalline
H-NMR (400.18 MHz, MeOD-d ): d(ppm) ¼ 8.15 (t, 1H, H ,
4
4
3
3
J ¼ 7.9 Hz), 7.93 (d, 2H, H , J ¼ 7.9 Hz), 6.77 (s, 2H, H ), 2.45
3
/5
11
2
2
(
1
d, 2H, H12, J ¼ 14.0 Hz), 2.39 (d, 2H, H12, J ¼ 14.0 Hz), 0.70 (s,
8H, H14).
1
3
C-NMR (100.63 MHz, MeOD-d
4
): d(ppm) ¼ 153.6 (C
q
, C
, C
7
),
ꢃ
solid. mp: 266.5 C.
1
50.2 (C , C , C ), 149.2 (C , C10), 143.0 (C
q
2
6
q
t
, C ), 123.3 (C
4
t
3
,
1
H-NMR (400.18 MHz, MeOD-d ): d(ppm) ¼ 7.82 (t, 1H, H ,
4
4
5 t 11 d 12 q 13 p 14
C ), 106.2 (C , C ), 39.5 (C , C ), 32.2 (C , C ), 29.7 (C , C ).
3
3
1
5
†
J ¼ 7.8 Hz), 7.69 (d, 2H, H , J ¼ 7.7 Hz), 6.71 (s, 2H, H ), 2.58
3
/5
11
N-NMR (40.56 MHz, MeOD-d ): d(ppm) ¼ 264 (N ) , 263 (t,
4
1
1
(
s, 4H, H12), 0.98 (s, 18H, H14).
N , J ¼ 9.4 Hz), 205 (s, N ).
8
9
1
3
C-NMR (100.63 MHz, MeOD-d ): d(ppm) ¼ 153 (C
)*, 152 (C , C2/6)*, 143 (C , br. s., C10)*, 138.9 (C , C
, C3/5), 105.0 (C , C11), 41.7 (C , br. s., C12), 32.1 (C , C13), 29.8
, C14). * Value taken from a H, C-HMBC spectrum.
4
q
, br. s.,
ESI-MS (CH OH) calculated for C H F N O S Y
3
65 87 6 15 6 2
+
C
7
q
q
t
4
), 119.8
[
3 2
Y(C5-BPP) (OTf) ] : 1440.5367, found: 1440.5516; calculated for
+
(
(
C
C
t
t
s
1
q
C
1
65
H
88
F
6
N
15
O
6
S
2
NaK [3C5-BPP + HOTf + OTf + Na + K] :
414.5922, found: 1414.5880; calculated for C64 SY
[Y(C5-BPP) (OTf) ꢀ H] : 1290.5770, found: 1290.5872; calcu-
13
p
86 3 15 3
H F N O
1
5
+
N-NMR (40.56 MHz, MeOD-d ): d(ppm) ¼ 287 (N )*, 206
4
8
3
1
1
15
+
(
N )*. * Value taken from an H, N-HMQC spectrum.
9
64 88 3 15 3
lated for C H F N O SK [3C5-BPP + HOTf + K] : 1242.6504,
1
5
+
+
LIFDI-MS (CH OH) calculated for C H N N [M + H] :
54.24, found: 354.25; calculated for C21H N N [M] : 353.25,
29 3 2
3
21 30
3
5
2
found: 1242.6491; calculated for C H N Y [Y(C5-BPP) ꢀ 2H] :
6
3
85 15
3
1
+
3
1
140.6171, found: 1140.6252; calculated for C H F N O S Y
44 58 6 10 6 2
+
found: 353.28.
[Y(C5-BPP) (OTf) ] : 1089.2945, found: 1089.3011; calculated for
2
2
+
C
43
H
57
F
3
N
10
O
3
SY [Y(C5-BPP)
2
(OTf)
1
ꢀ H] : 939.3346, found:
+
Syntheses of lanthanide complexes
939.3423; calculated for C42
H
58
N
10Na [2C5-BPP + Na] : 725.4744,
H N [C5-BPP + H] : 352.2501,
30 5
+
found: 725.4744; calculated for C21
found: 352.2500.
6
mmol of Ln(OTf)
3
were weighted in a screw-cap glass. 18 mmol
C5-BPP or { N}C5-BPP, respectively, were dissolved in 600 mL
4
with traces of TMS. The C5-BPP or { N}C5-BPP ligand
solution was added to the metal salt. Aer mixing the complex [La({ N}C5-BPP)
1
5
1
5
MeOD-d
1
5
3
](OTf)
3
solution was transferred into an NMR tube. The sample was
degassed by three freeze–pump–thaw cycles and subsequently
1
H-NMR (400.18 MHz, MeOD-d
J ¼ 7.9 Hz), 7.96 (d, 2H, H3/5, J ¼ 7.9 Hz), 6.77 (s, 2H, H11), 2.41
4
): d(ppm) ¼ 8.16 (t, 1H, H
4
,
3
3

ame-sealed. Complexes with the labeled and unlabeled ligand
(s, 4H, H12), 0.70 (s, 18H, H14).
were prepared the same way. The chemical shi values for the
unlabeled complex are equal to those of the labeled complexes
13
C-NMR (100.63 MHz, MeOD-d
4
): d(ppm) ¼ 154.3 (C
, C ), 123.7 (C
C ), 106.4 (C , C ), 39.5 (C , C ), 32.2 (C , C ), 29.7 (C , C ).
q
, C
7
),
1
51.6 (C
q
, C
2
, C
6
), 148.3 (C
q
, C10), 142.9 (C
t
4
t
, C
3
,
1
and are not stated here for brevity. However, N chemical shis
5
t
11
d
12
q
13
p
14
1
15
could only be determined from H, N-gHSQC spectra and are
15
N-NMR (40.56 MHz, MeOD-d ): d(ppm) ¼ 272 (d, N
4
8
labeled accordingly (†).
1
J ¼ 9.9 Hz), 206 (m, N
ESI-MS (CH OH) calculated for
(OTf)
for
9
).
3
C
67
H
92
F
10
N
15
O
10
S
3
La
2
43
Synthesis of [ Am(C5-BPP)
3
](OTf)
3
+
[La(C5-BPP)
3
3 3
+ CH OH + HF] : 1692.5296, found: 1692.5454;
ꢀ
1 243
+
1
(
2
.0 mL of a solution containing 4 mg mL
Am in HNO
3
calculated
C
65
H
87
F
6
N
15
O
6 2
S La
[La(C5-BPP)
3
(OTf)
SLa
ꢀ H] : 1340.5774, found: 1340.5868; calcu-
2
] :
ꢀ1
86 3 15 3
0.5 mol L ) were transferred into a screw-cap glass. A total of 1490.5373, found: 1490.5228; calculated for C64H F N O
80 mL NaOH (2.0 mol L ) was added in portions, resulting in [La(C5-BPP) (OTf)
3 1
ꢀ
1
+
+
precipitation of americium hydroxide. Aer 20 mL of addi- lated for C H N La [La(C5-BPP) ] : 1192.6332, found:
6
3
87 15
3
ꢀ
1
+
tional NaOH (2.0 mol L ) were added, the solution was 1192.6338; calculated for C H N La [La(C5-BPP) ꢀ 2H] :
6
3
85 15
3
centrifuged at 6000 rpm for 3 min. Additional 10 mL NaOH 1190.6176, found: 1190.6127; calculated for C44
H
58
F
6
N
10
O
6
S
2
La
ꢀ
1
+
solution (2.0 mol L ) were added, the solution was [La(C5-BPP)
2
(OTf)
2
] : 1139.2950, found: 1139.3026; calculated
1558 | Chem. Sci., 2015, 6, 1548–1561
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+
15
for C42
58
3 3
H N10Na [2C5-BPP + Na] : 725.4744, found: 725.4803; [Yb({ N}C5-BPP) ](OTf)
2
+
calculated for C H F N O SLa [La(C5-BPP) (OTf) ] :
1
6
4
87
3
15
3
3
1
H-NMR (400.18 MHz, MeOD-d ): d(ppm) ¼ 7.20 (br. s., 1H, H ),
4
4
6
70.7926, found: 670.7924; calculated for C H N La [La(C5-
63 86 15
6
.61 (br. s., 2H, H3/5), 5.17 (s, 2H, H11), 2.83 (s, 4H, H12), 0.41 (s,
2
+
BPP)3 ꢀ H] : 595.8127, found: 595.8125; calculated for
1
8-H, H14).
3
+
C
63
H
87
N
15La [La(C5-BPP)
calculated for C21
52.2511.
3
] : 397.5444, found: 397.5463;
13
C-NMR (100.63 MHz, MeOD-d
, C , C ), 144.1 (C , C10), 141.9 (C
), 101.4 (C , C11), 38.9 (C , C12), 32.1 (C , C13), 28.7 (C
p
4
): d(ppm) ¼ 149.1 (C
, C ), 118.4 (C
, C14).
q
, C
7
),
+
30 5
H N [C5-BPP + H] : 352.2501, found:
1
47.8 (C
q
2
6
q
t
4
t
, C
3
,
3
C
5
t
d
q
1
5
N-NMR (40.56 MHz, MeOD-d ): d(ppm) ¼ 20 (s, N ), 194 (s,
4
8
1
5
N9).
[
Lu({ N}C5-BPP) ](OTf)
3 3
ESI-MS
Yb(C5-BPP)
(CH OH)
calculated
for
C H N NaYb
3
63 87 15
1
H-NMR (400.18 MHz, MeOD-d
4
): d(ppm) ¼ 8.15 (t, 1H, H
4
,
+
[
3
+ Na] : 1250.6555, found: 1250.6565; calculated
3
3
J ¼ 7.9 Hz), 7.93 (d, 2H, H , J ¼ 7.9 Hz), 6.77 (s, 2H, H ), 2.44
+
3
/5
11
88 3 15 3
for C64H F N O SK [3C5-BPP + HOTf + K] : 1242.6504, found:
242.6489; calculated for C63
228.6736, found: 1228.6736; calculated for
3C5-BPP + K] : 1092.6906, found: 1092.6861; calculated for
2
2
(
d, 2H, H , J ¼ 14.0 Hz), 2.40 (d, 2H, H , J ¼ 14.0 Hz), 0.71 (s,
+
1
2
12
1
88 3
H N15Yb [Yb(C5-BPP) + H] :
1
8H, H14).
1
[
63 87 15
C H N K
1
3
C-NMR (100.63 MHz, MeOD-d
49.9 (C , C , C ), 149.5 (C
), 106.2 (C , C11), 39.6 (C
N-NMR (40.56 MHz, MeOD-d
4
): d(ppm) ¼ 153.6 (C
q
, C
, C
, C14).
7
),
+
1
q
2
6
q
, C10), 142.9 (C
t
, C
4
), 123.2 (C
t
3
,
+
C H F N O SNa [2C5-BPP + OTf + Na] : 874.4264, found:
4
3
58 3 10 3
C
5
t
d
, C12), 32.2 (C
q
, C13), 29.7 (C
p
+
8
74.4351; calculated for C H N Na [2C5-BPP + Na] : 725.4744,
found: 725.4754; calculated for C H N K [3C5-BPP + H + K] :
46.8492, found: 546.8461; calculated for
Yb(C5-BPP)
42 58 10
1
5
†
4
): d(ppm) ¼ 267 (N
1
) , 265 (d,
2+
6
3 88 15
1
N
8
, J ¼ 9.4 Hz), 205 (m, N
9
).
5
[
63 84
C H N15Yb
ESI-MS (CH OH) calculated for C H F N O S Lu
Lu(C5-BPP) (OTf) ] : 1526.5717, found: 1526.5793; calculated for
3+
3
65 87 6 15 6 2
3
ꢀ 3H] : 408.2141, found: 408.2091; calculated for
+
[
+
3
2
C
21 29 5
H N Na [C5-BPP + Na] : 374.2320, found: 374.2321.
+
64 86
C H F
3
N
15
O
3
SLu [Lu(C5-BPP)
3
(OTf)
1
ꢀ H] : 1376.6119, found:
+
1376.6199; calculated for C63
H
85
N
15Lu [Lu(C5-BPP)
3
ꢀ 2H] :
[
1
Am(C5-BPP) ](OTf)
3
3
1
226.6520, found: 1226.6662; calculated for C44
H
58
F
6
N
10
O
6
S
2
Lu
+
H-NMR (400.18 MHz, MeOD-d ): d(ppm) ¼ 7.64 (d, 2H, H3/5,
4
[Lu(C5-BPP)
2
(OTf)
2
] : 1175.3294, found: 1175.3356; calculated
3
3
+
J ¼ 7.9 Hz), 7.47 (t, 1H, H , J ¼ 7.9 Hz), 6.29 (s, 2H, H ), 2.91
4
11
for C43
H
57
F
3
N
10
O
3
SLu [Lu(C5-BPP)
2
(OTf)
1
ꢀ H] : 1025.3696,
SK [2C5-BPP +
+ K] : 891.4081, found: 891.4008; calculated for
2
2
(
d, 2H, H , J ¼ 13.9 Hz), 2.55 (d, 2H, H , J ¼ 13.9 Hz), 0.64 (s,
1
2
12
59 3 10 3
found: 1025.3748; calculated for C43H F N O
+
18H, H ).
14
HOTf
1
3
2
+
C-NMR (100.63 MHz, MeOD-d ): d(ppm) ¼ 164.8 (C , C ),
4
q
7
C H F N O SLu [Lu(C5-BPP) (OTf) ] : 688.8098, found:
6
4
87
3
15
3
3
1
+
164.2 (C , C , C ), 147.9 (C , C ), 147.7 (C , C ), 116.3 (C , C ,
q
2
6
t
4
q
10
t
3
688.8129; calculated for C21
H
30
N
5
[C5-BPP + H] : 352.2501,;
C
5
), 101.7 (C
t
, C11), 38.7 (C
d
, C12), 33.9 (C
q
, C13), 29.6 (C
)*, 1 (N
8
22 (N )*. * Value taken from an H, N-HMQC spectrum.
p
, C14).
found: 352.2498.
1
5
†
N-NMR (40.56 MHz, MeOD-d
4
1
): d(ppm) ¼ 216 (N
9
1
) ,
15
ꢀ
19
1
5
[
Sm({ N}C5-BPP)
3
](OTf)
3
F-NMR (376.54 MHz, MeOD-d ): d(ppm) ¼ ꢀ80.00 (s,
4
ꢀ
1
CF SO ).
3
3
H-NMR (400.18 MHz, MeOD-d
4
): d(ppm) ¼ 8.30 (t, 1H, H
4
,
3
3
J ¼ 7.9 Hz), 8.12 (d, 2H, H3/5, J ¼ 7.9 Hz), 6.90 (s, 2H, H11), 2.41
1
5
[
3 3
Am({ N}C5-BPP) ](OTf)
(
s, 4H, H ), 0.81 (s, 18H, H ).
12 14
1
3
1
C-NMR (100.63 MHz, MeOD-d ): d(ppm) ¼ 156.0 (C , C ),
H-NMR (400.18 MHz, MeOD-d ): d(ppm) ¼ 7.64 (d, 2H, H3/5,
4
q
7
4
3
3
1
53.4 (C , C , C ), 148.4 (C , C ), 143.4 (C , C ), 122.9 (C , C ,
J ¼ 7.9 Hz), 7.47 (tr, 1H, H , J ¼ 7.9 Hz), 6.28 (s, 2H, H ), 2.91
q
2
6
q
10
t
4
t
3
4
11
2
2
C
5
), 106.2 (C
t
, C11), 39.5 (C
d
, C12), 32.3 (C
q
, C13), 29.8 (C
p
, C14). (d, 2H, H , J ¼ 13.9 Hz), 2.55 (d, 2H, H , J ¼ 13.9 Hz), 0.64 (s,
†
1
2
12
1
5
N-NMR (40.56 MHz, MeOD-d
), 205 (s, N ).
ESI-MS (CH OH) calculated for
3C5-BPP + HOTf + OTf + Na + K] : 1414.5922, found: 1414.5916; C ), 101.6 (C , C ), 38.7 (C , C ), 33.9 (C , C ), 29.6 (C , C ).
4
): d(ppm) ¼ 221 (s, N
1
14
) , 224 18H, H ).
1
3
(s, N
8
9
C-NMR (100.63 MHz, MeOD-d ): d(ppm) ¼ 164.8 (C , C ),
4
q
7
3
C
65
H
88
F
6
N
15
O
6
S
2
NaK 164.2 (C , C , C ), 147.9 (C , C ), 147.7 (C , C ), 116.2 (C , C ,
q
2
6
t
4
q
10
t
3
+
[
5
t
11
d
12
q
13
p
14
+
15
calculated for C H F N O SSm [Sm(C5-BPP) (OTf) + 2H] :
N-NMR (40.56 MHz, MeOD-d ): d(ppm) ¼ 217 (s, N ),
64
89
3
15
3
3
1
4
9
† 1
1
[
356.6143, found: 1356.6115; calculated for C H N KSm 1 (N ) , ꢀ23 (d, N , J ¼ 9.6 Hz). * Value taken from an
6
3
87 15
1
8
+
1
15
Sm(C5-BPP) + K] : 1244.6103, found: 1244.6114; calculated for
H, N-HMQC spectrum.
3
+
19
C
64
H
88
F
3
N
15
O
3
SK [3C5-BPP + HOTf + K] : 1242.6504, found:
F-NMR (376.54 MHz, MeOD-d ): d(ppm) ¼ ꢀ80.00 (s,
4
+
ꢀ
1242.6508; calculated for C63
H
88
N
15Sm [Sm(C5-BPP)
3
+ H] : CF SO ).
3
3
1
206.6545, found: 1206.6468; calculated for C63
87
H N15Sm
+
[Sm(C5-BPP)
3
] : 1205.6466, found: 1205.6438; calculated for
3
SK [2C5-BPP + HOTf + K] : 891.4081, found:
Acknowledgements
+
C
43
H
59
F
3
N
10
O
+
8
7
91.4008; calculated for C H N Na [2C5-BPP
+
Na] : This work was supported by the German Federal Ministry of
4
2
58 10
25.4744, found: 725.4783; calculated for C H N Sm Education and Research (BMBF) under contract numbers
6
3
87 15
2
+
[Sm(C5-BPP) ] : 602.8233, found: 602.8167; calculated for 02NUK020A and 02NUK020D. The authors wish to thank Lisa
3
+
C
21
H
29
N
5
Na [C5-BPP + Na] : 374.2320, found: 374.2323; calcu- B ¨o ringer and Julia Sch ¨a fer (KIT-INE) for support with the lab-
+
lated for C21
H
30
N
5
[C5-BPP + H] : 352.2501, found: 352.2514.
work. We are grateful to Prof. Dr Frank Breher (KIT-AC,
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Karlsruhe) for cooperation during these funded projects
involving the NMR spectrometer and associated equipment. We
M. A. Denecke and A. Geist, Inorg. Chem., 2012, 51, 5199–
5207.
thank Dr J u¨ rgen H. Gross, Doris Lang and Norbert Nieth 21 C. Adam, P. Kaden, B. B. Beele, U. M u¨ llich, S. Trumm,
(
Institute of Organic Chemistry, University of Heidelberg) for
A. Geist, P. J. Panak and M. A. Denecke, Dalton Trans.,
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the LIFDI and ESI MS measurements.
2
2
2 S. Trumm, P. J. Panak, A. Geist and T. Fangh ¨a nel, Eur. J.
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3 S. Trumm, G. Lieser, M. R. S. J. Foreman, P. J. Panak, A. Geist
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