N-Heterocyclic tridentate aromatic ligands bound to [ln(hexafluoroacetylacetonate)3] units: Thermodynamic, structural, and luminescent properties

Article abstract of DOI:10.1002/chem.201102827

Herein, we discuss how, why, and when cascade complexation reactions produce stable, mononuclear, luminescent ternary complexes, by considering the binding of hexafluoroacetylacetonate anions (hfac^-) and neutral, semi-rigid, tridentate 2,6-bis(benzimidazol-2-yl)pyridine ligands (Lk) to trivalent lanthanide atoms (Ln^III). The solid-state structures of [Ln(Lk)(hfac)₃] (Ln=La, Eu, Lu) showed that [Ln(hfac)₃] behaved as a neutral six-coordinate lanthanide carrier with remarkable properties: 1)the strong cohesion between the trivalent cation and the didentate hfac anions prevented salt dissociation; 2)the electron-withdrawing trifluoromethyl substituents limited charge-neutralization and favored cascade complexation with Lk; 3)-nine-coordination was preserved for [Ln(Lk)(hfac) ₃] for the complete lanthanide series, whilst a counterintuitive trend showed that the complexes formed with the smaller lanthanide elements were destabilized. Thermodynamic and NMR spectroscopic studies in solution confirmed that these characteristics were retained for solvated molecules, but the operation of concerted anion/ligand transfers with the larger cations induced subtle structural variations. Combined with the strong red photoluminescence of [Eu(Lk)(hfac)₃], the ternary system Ln^III/hfac ^-/Lk is a promising candidate for the planned metal-loading of preformed multi-tridentate polymers.

Full text of DOI:10.1002/chem.201102827

FULL PAPER
DOI: 10.1002/chem.201102827
N-Heterocyclic Tridentate Aromatic Ligands Bound to
[Ln(hexafluoroacetylacetonate)3] Units: Thermodynamic, Structural, and
Luminescent Properties
Amir Zaꢀm,[a] Homayoun Nozary,[a] Laure Guꢁnꢁe,[b] Cꢁline Besnard,[b]
Jean-FranÅois Lemonnier,[a] Stꢁphane Petoud,*[c] and Claude Piguet*[a]
Abstract: Herein, we discuss how, why,
and when cascade complexation reac-
tions produce stable, mononuclear, lu-
minescent ternary complexes, by con-
sidering the binding of hexafluoroace-
tylacetonate anions (hfacꢀ) and neu-
tral, semi-rigid, tridentate 2,6-bis(benzi-
midazol-2-yl)pyridine ligands (Lk) to
trivalent lanthanide atoms (LnIII). The
dentate hfac anions prevented salt dis-
sociation; 2) the electron-withdrawing
trifluoromethyl substituents limited
charge-neutralization and favored cas-
cade complexation with Lk; 3) nine-co-
ordination was preserved for [Ln(Lk)-
AHCTUNGTERN(GNUN hfac)3] for the complete lanthanide
series, whilst a counterintuitive trend
showed that the complexes formed
with the smaller lanthanide elements
were destabilized. Thermodynamic and
NMR spectroscopic studies in solution
confirmed that these characteristics
were retained for solvated molecules,
but the operation of concerted anion/
ligand transfers with the larger cations
induced subtle structural variations.
Combined with the strong red photolu-
solid-state structures of [Ln(Lk)
(Ln=La, Eu, Lu) showed that [Ln-
(hfac)3] behaved as a neutral six-coor-
dinate lanthanide carrier with remark-
able properties: 1) the strong cohesion
between the trivalent cation and the di-
(hfac)3]
minescence of [Eu(Lk)ACTHUGNTERNNU(G hfac)3], the ter-
nary system LnIII/hfacꢀ/Lk is a promis-
ing candidate for the planned metal-
loading of preformed multi-tridentate
polymers.
Keywords: lanthanides · ligand ef-
fects · nitrogen heterocycles · pho-
tophysical properties · thermody-
namics
ACHTUNGTRENNUNG
Introduction
Although neutral six-coordinate lanthanide beta-diketonates
building blocks, [Ln(b-diketonate)3], are famous for their
exceptional luminescent properties,[1] some renewed interest
has focused on their specific interactions with additional di-
dentate or tridentate chelating receptors to produce engi-
neered materials for metal–organic chemical-vapor deposi-
tion (MOCVD)[2,3] and for organic light-emitting diodes
(OLEDs).[4] For instance, neutral [Eu(dibenzoylmethanide)3]
units were recently used for chelating to didentate 1,10-phe-
nanthroline binding sites that were incorporated within pho-
toluminescent conducting polymers,[5] whilst [Ln(hexafluor-
Scheme 1. a) Postulated molecular structure of the monomeric unit in
a photoluminescent conducting europium-containing polymer;[5] b) chem-
ical structure of [Ln(hfac)3(diglyme)], which was designed as a precursor
for volatile materials with tunable second-order nonlinear optical proper-
ties.[6]
[a] A. Zaꢀm, Dr. H. Nozary, Dr. J.-F. Lemonnier, Prof. Dr. C. Piguet
Department of Inorganic, Analytical,
oacetylacetonate)3], that is, [LnACHTNUTRGNEUNG(hfac)3], were reacted with
tridentate diglyme for the preparation of transparent films
and Applied Chemistry, University of Geneva
30 quai E. Ansermet, 1211 Geneva 4 (Switzerland)
E-mail: Claude.Piguet@unige.ch
with non-linear optical responses (Scheme 1).[6]
Whilst countless reports have described the solid-state
structures and the metal-centered luminescence of ternary
[Ln(L)(b-diketonate)3] complexes, where L is a didentate N-
donor receptor typically derived from 2,2’-bipyridine or
1,10-phenanthroline,[2,7] much-less attention has been fo-
cused on analogous complexes that incorporate the extend-
ed tridentate 2,2’:6’,2’’-terpyridine (terpy) derivatives.[8]
Beyond 1) the C1-symmetric molecular structure found in
[b] Dr. L. Guꢁnꢁe, Dr. C. Besnard
Laboratory of Crystallography, University of Geneva
24 quai E. Ansermet, 1211 Geneva 4 (Switzerland)
[c] Prof. Dr. S. Petoud
Centre de Biophysique Molꢁculaire, CNRS UPR 4301
Rue Charles-Sadron, 45071 Orlꢁans Cedex 2 (France)
E-mail: Stephane.Petoud@cnrs-orleans.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102827.
the crystals of nine-coordinate [LnACTHNUTRGNEUNG(terpy)(b-diketona-
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te)3][8c,e] and 2) the detection of remarkable luminescence
quantum yields for [Eu(terpy)(b-diketonate)3] in the solid
pentyl (L4)[15] or 3,5-dimethoxybenzyl groups (L5)[16] at the
1-position of the benzimidazole side-arms in ligands L4 and
L5 drastically limits the affinity of these ligands for trivalent
lanthanides. Structural investigations have attributed this
negative effect to the steric bulk of these substituents when
the tridentate ligand adopts the planar cis–cis conformation
required for its coordination to LnIII (see Figure 2).[15,16]
Indeed, the connection of more-compact linear lipophilic
octyl chains (L6) endows sufficient
AHCTUNGTRENNUNG
state,[1,8] little is known about the structures, speciations, and
stabilities of these ternary complexes in solution. This lack
of reliable information is common in lanthanide coordina-
tion chemistry and, during our quest for identifying unsatu-
rated neutral [LnX3] units for cascade complexation with
semi-rigid tridentate ligands L1–L8 (Scheme 2), we were
stability in [Ln(L6)ACHTUNTRGNEUNG(NO3)3] for their
quantitative formation in acetoni-
trile at millimolar concentrations, al-
though difficult isolation, purifica-
tion, and characterization of these
waxy materials hinders detailed
photophysical investigations and fur-
ther exploitations.[17] With this in
mind, we envisioned the use of 3-
methyl-1-butyl residues in L2 and
L3 for optimizing their solubility in
organic solvents whilst minimizing
the structural expansion responsible
for the thermodynamic penalty in
the
associated
complexes
(Scheme 3). Compound L3 was ob-
tained by a standard acidic activa-
tion of the carboxylic groups in the
Scheme 2. Chemical structures of ligands L1–L8 in their trans–trans conformations.
presence of o-phenylenediamine to
often faced with drastic limitations, owing to unexpected so-
give compound 3, which was then deprotonated and alkylat-
ed.[18] Because the latter procedure mixed the 5- and 6-posi-
tions within each benzimidazole ring,[18] the stereospecific
connection of two bromine atoms at the 5,5’-positions of the
benzimidazole rings in L2 relied on an alternative two-step
reductive Phillips-modified coupling strategy (Scheme 3).[19]
Because of the average planar C2v-symmetrical arrange-
ment adopted by the free ligands in solution, we only detect-
ed five signals for the aromatic protons in the 1H NMR
spectrum of compound L2 (six signals for L3, see Figure 3a,
Figure 4a; also see the Supporting Information, Tables S1
and S2), together with pairs of enantiotopic methyl groups
for H9 and H10 (atom numbering is given in Scheme 3;
herein H9 and H10 refer to the H atoms bonded to C9 and
C10, respectively).[16] The lack of a nuclear Overhauser en-
hancement effect (NOE) between alkyl protons H6 and pyr-
idine protons H1 or H2 suggested that the three coordinated
nitrogen atoms adopted the standard trans–trans geometry,
which optimized the intramolecular electric dipolar interac-
tions (Scheme 3).[20] The crystal structure of ligand L3 con-
firmed this suggestion and showed two slightly different
molecules in the asymmetric unit, both of which adopted
the expected transoid conformation (N47 was trans to N44
whilst N80 was trans to N44, Figure 1). Typical bond lengths
and angles were observed (see the Supporting Information,
Table S3)[21] but the benzimidazole-pyridine-benzimidazole
aromatic units were not strictly coplanar (interplanar angles
9.6–38.98; see the Supporting Information, Table S4 and Fig-
ure S2) because of the residual helical twists imposed by the
ꢀ
lution behaviors.[9–11] For instance, when X=NO3 or
ꢀ
CF3CO2 , the desired mononuclear nine-coordinate com-
plexes [Ln(L1)(X)3] that were observed in the solid state
systematically dimerized in aprotic polar solvents (see the
Supporting Information, Figure S1).[9,10] For X =SCNꢀ, the
situation was even worse, with the formation of intricate
mixtures of charged complexes, [Ln(L1)
[Ln(L1)2A
(SCN)2]+, which prevented the isolation of the neu-
tral targets, [Ln(L1)
(SCN)3].[11,12] To identify and further ex-
(SCN)4]ꢀ and
ACHTUNGTRENNUNG
CHTUNGTRENNUNG
ACHTUNGTRENNUNG
ploit a neutral [LnX3] lanthanide carrier, we turned our at-
tention toward X=b-diketonate, and, more precisely,
toward the highly soluble [LnACHTNUGTRENUNG(hfac)3] compounds (hfac=
hexafluoroacetylacetonate), which are known to be rather
robust toward dissociation, dimerization, and hydrolysis.[1,13]
Being aware of the reports of some faint thermodynamic af-
finities of [Ln(b-diketonate)3] toward didentate 1,10-phe-
nanthroline in polar solvents,[14] we first embarked on the
quantitative exploration of the intermolecular cascade reac-
tion of these units with the related tridentate N-heterocyclic
ligands L2 and L3.
Results and Discussion
Synthesis, characterization, and molecular structures of li-
gands L2 and L3 and of complexes [Ln(Lk)ACTHNURTGNENG(U hfac)3] (k=2,
3; Ln=La, Eu, Gd, Lu, Y): Although attractive for solubili-
ty and chirality reasons, the substitution of branched neo-
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[Ln(hexafluoroacetylacetonate)3] with N-Heterocyclic Ligands
FULL PAPER
[Ln(Lk)ACTHNURTGENNG(U hfac)3] (yield: 40–60%; see the Supporting Infor-
mation, Table S5). Upon slow evaporation, prisms suitable
for X-ray analysis were obtained for Ln=La, Eu, and Lu
(see the Supporting Information, Table S6). All of these
compounds crystallized in the monoclinic crystal system
(P21/n space group for the large and mid-range La and Eu
cations, C2/c for the small Lu cation) and showed the forma-
tion of mononuclear complex [Ln(Lk)
spection of the crystal packing revealed that either weak in-
termolecular Br–p interactions in [Ln(L2)(hfac)3] (Ln=La,
Eu; see the Supporting Information, Figure S3) or faint aro-
matic p–p stacking in [Ln(L3)(hfac)3] (Ln=La, Eu; see the
ACHTUNGTERN(NNUG hfac)3]. Careful in-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Supporting Information, Figure S4) contributed to the cohe-
sion of the crystal structures for the larger lanthanides,
whilst no remarkable intermolecular interactions were ob-
ꢀ
served for Ln=Lu, except for some short F benzimidazole
distances along the b axis (see the Supporting Information,
Figure S5). We concluded that the geometries of the molec-
ular complexes were weakly affected by packing forces,
which justified the analysis of the coordination bond lengths
in term of their chemical affinities.[24]
The six molecular structures for [Ln(Lk)ACTHNURTGNENG(U hfac)3] were
very similar (Figure 2; also see the Supporting Information,
Figure S6) and their rigid cores were globally superimposa-
ble across the lanthanide series (see the Supporting Infor-
mation, Figures S7–S9). Each metal atom in [Ln(Lk)ACHTUNGTRENNUNG(hfac)3]
(k=2, 3; Ln=La, Eu, Lu) was nine-coordinated by the
three nitrogen atoms of the bound aromatic ligand (cis–cis
conformation) and the six oxygen atoms of three didentate
hexafluoroacetylacetonate anions (Figure 2). One didentate
hfacꢀ ion was almost located within the coordinating plane
defined by the metal and the three bound nitrogen atoms,
whilst the two remaining hfacꢀ ions were arranged on both
sides of this plane, thereby leading to a highly distorted co-
ordination geometry around the metal centers. To minimize
the steric constraints produced by the alkyl chains that were
located close to the hydrogen atoms of the central pyridine
rings,[16,17] the polyaromatic tridentate aromatic ligands devi-
ated from planarity (interplanar pyridine-benzimidazole
Scheme 3. Syntheses of ligands L2 and L3 with atom numbering.
angles of 9.8–31.48 (average: 18(10)8) for [Ln(L2)
and 14.6–27.08 (average: 21(5)8) for [Ln(L3)(hfac)3]; see the
Supporting Information, Tables S7–S9 and S10–S12, respec-
tively). In contrast with the reported intramolecular interli-
gand interactions within the analogous [Ln(iPr-pybox)-
ACHTUNGTRENNUNG(hfac)3]
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTERG(NNNU hfac)3] complexes (iPr-pybox=2,6-bis(5-isopropyloxazolin-
2-yl)pyridine),[23] we did not detect any unusual short con-
tacts between ligand L2 (or L3) and hfacꢀ ions that were
bound to the same metal in [Ln(Lk)ACHTNUTRGNEUNG(hfac)3]. However, thor-
ough analysis of the bond lengths showed a systematic and
intriguing contraction of both the Ln–O and Ln–N distances
Figure 1. ORTEP of the molecular structures of two slightly different li-
gands (A and B) in the asymmetric unit of ligand L3. Thermal ellipsoids
were set at 50% probability.
for a given metal on going from [Ln(L2)
(hfac)3], whilst the bond angles displayed no special trends
(see the Supporting Information, Table S7–S12). Thus, we
resorted to the calculation of bond valences (nLn,N and nLn,O
with Equation (1) for an easy comparison of the strength of
the ligand–metal interactions in the various complexes (see
the Supporting Information, Tables S13–S18):[11,24,25]
ACHTUNGTERN(NNUG hfac)3] to [Ln(L3)-
AHCTUNGTRENNUNG
alkyl residues. This observation agreed with the interplanar
angles of 23.4–27.28 reported for isomeric ligand L4.[15a]
The reactions of stoichiometric amounts of compound L2
)
or L3 with [Ln
ACHTUNGTRENNUNG(hfac)3ACHTUNGTNER(NUGN diglyme)] (Ln=La, Eu, Gd, Lu,
Y)[3,22] in CH2Cl2/MeCN gave anhydrous ternary complexes
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S. Petoud, C. Piguet et al.
noted that: 1) nLn,O-hfac(L2)<
nLn,O-hfac(L3) and nLn,N-ligand(L2)
<nLn,N-ligand(L3) for
a
given
metal, and 2) nLa,j ꢂ nEu,j > nLu,j
for each ligand along the lan-
thanide series (Table 1). These
results suggested that: 1) the
connection of bulky bromine
atoms onto the aromatic ligand
backbone expanded the coordi-
nation sphere and reduced in-
teractions between the donor
atoms and the central cation in
[Ln(L2)ACTHNUGRTENUNG(hfac)3], and 2) the af-
finity of the ligands for the cen-
tral cation decreased along the
lanthanide series; this trend was
opposite to the classically ob-
served electrostatic trend.[26,27]
As expected,[17] the replacement
of the 2,6-bis(benzimidazol-2-
yl)pyridine
[Eu(L3)(hfac)3] with the analo-
gous terpyridine ligand in
[Eu(4-phenyl-terpy)(hfac)3]
scaffolds
in
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(Table 1, entry 7) or with 2,6-bi-
s(oxazolinyl)pyridine ligand in
[Eu(iPr-pybox)
ACHTUNGTRENNUNG
entry 8) only had
a
impact on the geometry of the
coordination sphere.[23] On the
contrary, the replacement of
electron-withdrawing fluoride
atoms in the hfacꢀ ions with
bulky electron-donating methyl
groups in dipivaloymethanate
anions (dpmꢀ) significantly dis-
tanced the nitrogen atoms from
the metal in [EuACTHNUGTRENNUG(terpy)ACHTUNGTRENNUNG(dpm)3]
(Table 1, entry 9). Finally, the
replacement of the six-mem-
bered chelate rings, which were
produced by the didentate
hfacꢀ ions in [Ln(Lk)
ACHTUNGTRENNUNG(hfac)3],
with the four-membered chelat-
ed rings that were produced by
didentate nitrate anions in
Figure 2. The molecular structures of complexes [Ln(L2)(hfac)3] and [Ln(L3)(hfac)3] (Ln=La, Eu, Lu) in the
solid state. Colors: C gray, N dark blue, O red, F light blue, La yellow, Eu magenta, Lu green. H atoms are
omitted for clarity. For atom numbering and thermal ellipsoids, see the Supporting Information, Figure S6.
[Ln(Lk)
tries 10–13)
global anion affinities (nLn,O-hfac >nLn,O-NO ). In terms of bond-
ACHTUNGTRENNUNG
nLn;j ¼ exp½ðRLn;j ꢀ dLn;jÞ=bꢁ
ð1Þ
3
valence, [LnACTHUNRGTNEUNG(hfac)3] is a promising candidate as a neutral
where dLn,j is the bond length, RLn,j corresponds to the bond-
valence parameters, and b=0.37 ꢃ is a universal scaling
constant. From the average bond valences for [Ln(Lk)-
ACHTUNGTRENNUNG(hfac)3] (Table 1, entries 1–6), we found that nLn,O-hfac >nLn,N-
ligand, which was in line with the preference of trivalent lan-
thanides for negatively charged oxygen donors. We also
lanthanide carrier in cascade complexation because:
1) hfacꢀ ions strongly coordinated to the LnIII centers, whilst
2) the electron-withdrawing CF3 group limited charge-deloc-
alization onto the cation to such an extent that the subse-
quent coordination of an additional neutral tridentate poly-
aromatic binding unit was still efficient.
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[Ln(hexafluoroacetylacetonate)3] with N-Heterocyclic Ligands
FULL PAPER
[a]
Table 1. Average bond valences (nLn,j) and bond-valence sums (VLn,j
)
in
The downfield shift of H1 and the concomitant upfield
shift of H2 in diamagnetic complexes [Ln(Lk)ACTHNURGTNEUNG(hfac)3] (k=
the crystal structures of [Ln(Lk)(hfac)3], [Ln(Lk)(NO3)3], and related
complexes.
2,3; Ln=La, Y, Lu) were diagnostic for the complexation of
the central pyridine ring to the cationic metal,[30] whilst the
NOE effect between H2 and H6 attested to the cis–cis con-
formation that was adopted by the ligand upon coordination
of the benzimidazole side-arms (Figure 2).[27a] However, con-
trary to the crystal structures, in which only a twofold axis
could be considered, we observed an average pseudo-trigo-
nal symmetry on the NMR timescale for complexes
Complex
nLn,N-ligand nLn,O-hfac nLn,O-NO VLn Reference
3
[La(L2)(hfac)3]
[Eu(L2)(hfac)3]
[Lu(L2)(hfac)3]
[La(L3)(hfac)3]
[Eu(L3)(hfac)3]
[Lu(L3)(hfac)3]
0.32(4)
0.32(3)
0.31(3)
0.36(3)
0.35(2)
0.31(5)
0.38(3)
0.35(4)
0.34(7)
0.41(3)
0.39(4)
0.35(6)
0.37(2)
0.35(3)
0.39(5)
3.22 this work
3.05 this work
2.96 this work
3.53 this work
3.37 this work
3.00 this work
[Eu(4-Ph-terpy)(hfac)3][b] 0.33(2)
3.17
2.99
[8d]
[23]
[8b]
[9a]
[17]
[17]
[15a]
[Eu(iPr-pybox)(hfac)3][c]
[Eu(terpy)(dpm)3][d]
[Lu(L1)(NO3)3]
0.30(3)
0.26(3)
0.37(4)
0.38(7)
[Ln(Lk)ACHTGNUTRNE(NUG hfac)3], with a single signal for the protons
3.14
(Figure 3; also see the Supporting Information, Table S1)
and for the fluorine atoms (see the Supporting Information,
Figure S12 and Table S19) of the three didentate hexafluor-
oacetylacetonate anions. Such dynamic behavior is common
for lanthanide complexes and a straightforward explanation
involved fast exchange of the axial and equatorial didentate
hfacꢀ ions, which made them equivalent on the NMR time-
scale, thereby leading to a dynamically averaged C2v symme-
try for the remaining coordinated tridentate aromatic ligand
0.31(2) 2.96
0.32(2) 3.06
0.28(5) 3.04
0.27(3) 2.99
[Lu(L6)(NO3)3]
[Eu(L8)(NO3)3(CH3OH)] 0.36(6)
[Eu(L4)(NO3)3(CH3CN)] 0.38(9)
P
[a] VLn
¼
VLnj . [b] 4-Ph-terpy=4’-phenyl-2,2’:6’,2’’-terpyridine. [c] iPr-
j
pybox=2,6-bis(5-isopropyl-oxazolin-2-yl)pyridine.
terpyridine, dpm=dipivaloylmethanate.
[d] terpy=2,2’:6’,2’’-
Speciation and structures of complexes [Ln(Lk)
ACHTUNGTERNN(UNG hfac)3] (k=
2, 3; Ln=La, Eu, Lu, Y) in solution: Monitoring the titra-
in [Ln(Lk)
tions with [Ln(1,10-phenanthroline)ACTHUNGTRENNUNG
ACHTUNGTRENNUNG
1
tion of Lk (k=2, 3) with [Ln
G
(diglyme)] by H NMR
spectroscopy in CDCl3 showed the stepwise disappearance
of the signals for the free ligand and the appearance of ten
new peaks for ligand L2 (eleven peaks for L3), which were
characteristic of the formation of the single C2v-symmetrical
trifluoro-1-2-thienyl)-1,3-butanedione), Destri and co-work-
ers used paramagnetic NMR spectroscopy and pseudo-con-
tact shift analysis for proposing an alternative trigonal struc-
ture in solution, in which the neutral heterocyclic ligand lay
on one side of the C3 axis of a distorted facial “static” trigo-
complex [Ln(Lk)ACHTUNGTRENNUNG(hfac)3] (Figure 3; also see the Supporting
nal prism that was produced by the [LnACTHNUTRGNEUNG
(TTA)3] moiety.[31]
Fast rotation of the didentate phenanthroline ligand around
the threefold axis on the NMR timescale was also required
for producing local C2 symmetry for the aromatic ligand.
Extending this reasoning for [Ln(Lk)ACTHNUTRGEN(UNG hfac)3] was difficult
because the 13C NMR (see the Supporting Information, Fig-
ure S11) and 19F NMR patterns (see the Supporting Infor-
mation, Figure S12) pointed to six equivalent CF3 groups for
the three coordinated hfacꢀ anions (global C3h, D3, or D3h
point group) on the NMR timescale, which was incompati-
ble with the formation of a “static” trigonal prism produced
by the [LnACTHNURGTNEUNG(hfac)3] moiety with Lk coordinated on one side
of the threefold axis (C3 or C3v point group). As expected,
the replacement of diamagnetic metals with paramagnetic
EuIII in [Eu(Lk)
ACTHUNTRGNE(UGN hfac)3] showed considerable lanthanide-in-
duced shifts, with a maximum effect for H5 because of its lo-
cation close to the metallic center (Figure 2, also see the
Supporting Information, Table S1).[32] Repeating these titra-
tions in more-polar CD3CN provided similar results for
Ln=Eu, Y, Lu with the exclusive formation of [Ln(Lk)-
Figure 3. 1H NMR spectra of a) ligand L2 and its diamagnetic complexes
b) [Lu(L2)(hfac)3], c) [Y(L2)(hfac)3], and d) [La(L2)(hfac)3] in CDCl3
(total ligand concentration: 5 mm, 293 K, atom numbering is given in
Scheme 3).
AHCTUNGTERG(NNUN hfac)3] (k=2, 3), according to Equilibrium (2) (Figure 4;
Information, Table S1 and Figure S10). At a total ligand
also see the Supporting Information, Figure S13 and
concentration of 5 mm and [Ln
G
Table S2). For the smaller Lu cation, we noted a significant
LnðhfacÞ3,Lk
for the free ligand disappeared, which agreed with Equilibri-
reduction in b
in acetonitrile, and we detected non-
(hfac)3] in
slow exchange on the NMR timescale (total ligand concen-
tration: 5 mm, [Lu(hfac)3]/Lk=1.0; Figure 4b, also see the
Supporting Information, Surprisingly,
1,1
LnðhfacÞ3,Lk
um (2), for which b
ꢂ5ꢄ105 (in CHCl3).[28] This
negligible amounts of free ligand and free [LuAHCTUNGTRENNUNG
1,1
value was in line with association constants of 107 that have
been reported for the formation of [Ln(1,10-phenantroline)-
AHCTUNGTRENNUNG
(hfac)3] in dichloromethane.[29]
Figure S13b).
1H NMR titration of Lk with larger lanthanum cations
showed the formation of two C2v-symmetric complexes at
LnðhfacÞ3þLk Ð ½LnðLkÞðhfacÞ3ꢁ
Chem. Eur. J. 2012, 18, 7155 – 7168
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7159

S. Petoud, C. Piguet et al.
MS (ESI) spectra of solutions of [La(Lk)
nitrile confirmed the proposed bi-exchange process, with the
detection of prominent signals for [La(Lk)2A
(hfac)2]+ (m/z
1771 for k=2 and 1456 for k=3, positive mode) and for
[La
(hfac)4]ꢀ (m/z 967.3, negative mode), whilst 19F NMR
spectroscopy showed three singlets, which were assigned to
[La(Lk) 2A
(hfac)3] (d=ꢀ77.52 ppm), [La(Lk)
(hfac)2]+ (d=
ꢀ77.08 ppm), and [La
(hfac)4]ꢀ (d=ꢀ77.46 ppm; see the
Supporting Information, Table S19 and Figure S14). We con-
cluded that, in acetonitrile, [La(Lk)(hfac)3] co-existed with
2A
(hfac)4]ꢀ, accord-
ACHTUNGTERN(NGNU hfac)3] in aceto-
CTHUNGTRENNUNG
AHCTUNGTRENNUNG
A
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(hfac)2]+ and [La
its ionized form, [La(Lk) CHTUNGRETNNUG AHCTUNGTRENNUNG
ing to Equilibrium (3).[35] Because of the slow exchange on
the NMR timescale, the integrated intensities of the same
proton in Lk, [La(Lk)
the titration of Lk with [LaACTHNUGRTNE(NUG hfac)3] could be exploited for
(hfac)2]+ along
ACHUTGTNERN(NGU hfac)3], and [La(Lk)2AHCTUNGTRNENGUN
La,Lk
the estimation of thermodynamic constants K
([Equilib-
Figure 4. 1H NMR spectra of a) ligand L2 and its diamagnetic complexes
b) [Lu(L2)(hfac)3] (*=free ligand, #=free Lu(hfac)3), c) [Y(L2)(hfac)3],
and d) [La(L2)(hfac)3] in CD3CN (total ligand concentration: 5 mm,
293 K, atom numbering is given in Scheme 3).
exch
LaðhfacÞ3,Lk
rium (3)]) and b
([Equilibrium (2)]; also see the
Supporting Information, Appendix 2). At room tempera-
1,1
La,L2
exch
La,L3
exch
ture, we obtained K
=0.07(4) and K
=0.04(3) (see
the Supporting Information, Table S21), which translated
qffiffiffiffiffiffiffiffiffiffiffiffi
jLaðLkÞ2ðhfacÞ2j
La,Lk
into
=
K
=0.26(7) and 0.20(8) for ligands
jLaðLkÞ2ðhfacÞ3j
exch
[La
(hfac)3]/Lk=1.0: [La(Lk)
(hfac)3]-A and [La(Lk)
G
L2 and L3, respectively; that is, a ligand speciation of about
B (Figure 4d; also see the Supporting Information, Fig-
ure S13d and Table S2).
Comparison of the H NMR chemical shifts of lanthanum
70% in favor of the target complexes [La(Lk)
able-temperature NMR spectra for the most-soluble com-
ACHTUNGTERN(NUNG hfac)3]. Vari-
1
plex, [La(L2)ACHTNGUTERNNU(G hfac)3], showed a significant increase in the
jLaðLkÞ2ðhfacÞ2j
complexes with those of their analogous diamagnetic com-
ratio at low temperatures (see the Supporting In-
jLaðLkÞ2ðhfacÞ3j
plexes [Y(Lk)
demonstrated that [La(Lk)
the expected neutral mononuclear nine-coordinate complex
[La(Lk)(hfac)3]. The striking upfield shift observed for the
aromatic protons H4, H5, and H11 in the second complex
[La(Lk)(hfac)3]-B suggested some local diamagnetic aniso-
tropy produced by their specific location in the shielding
cone of neighboring aromatic rings, as found in diamagnetic
complexes [Ln(L8)2]3+[33] and [Ln(L8)3]3+,[27a] which con-
tained two or three polyaromatic tridentate units. Accord-
ingly, diffusion-ordered spectroscopy (DOSY NMR) dis-
played smaller translational self-diffusion coefficients for
G
E
formation, Table S21 and Figure S15), from which a van’t
R
Hoff plot gave DHꢃLa,L2 =ꢀ23(1) kJmolꢀ1, DSꢃLa,L2
=
exch
exch
ꢀ98(4) Jmolꢀ1 Kꢀ1, and DGꢃLa,L2 =6(1) kJmolꢀ1 (see the
exch
N
Supporting Information, Figure S16). The considerable en-
tropic penalty of Equilibrium (3) followed the charge-neu-
tralization principle, which entropically strongly disfavors
anion/cation dissociation in polar solvents.[36] Thus, the de-
ACHTUNGTRENNUNG
tection of non-negligible amounts of [La(Lk)2ACTHNUTRGNEUNG
(hfac)2]+ was
driven by the enthalpic gain that accompanied the Lk/hfac
bi-exchange process. Beyond the minor cooperative/anti-co-
operative intramolecular interACTHNUGRTNENUGliCAHUTGTNRENgUNG and interactions that con-
tributed to the latter equation (see below), we suspected
that solvation processes played a crucial role in stabilizing
[La(Lk)ACHTUNGTRENNUNG(hfac)3]-B, which was in agreement with the exis-
tence of larger molecular aggregates, including additional li-
gands (see the Supporting Information, Table S20). Quanti-
tative analysis of the self-diffusion coefficients with the help
of the Stokes–Einstein equation showed that the molecular
weights increased by DMMBHꢀA ¼ MMLHaꢀB ꢀ MMLHaꢀA =578-
the ionic products, [La(Lk)
substantiate this hypothesis, we noted that the H NMR ti-
tration of ligand L2 with [La(hfac)3] in CD3NO2, a solvent
with a dielectric constant very similar to that of CD3CN,
also showed the concomitant formation of [La(L2)(hfac)3],
[La(L2)2A
(hfac)2]+, and [La(hfac)4]ꢀ (see the Supporting In-
(hfac)2]+ and [La(hfac)4]ꢀ. To
2ACHUTNGTRENNUG ACHUTNGTRENNUGN
1
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
A
C
ACHTUNGTRENNUNG
on going from [La(Lk)
E
E
formation, Figure S17); this phenomenon stepwise disap-
Supporting Information, Table S20, Appendix 1).[34] These
values matched reasonably well with the computed changes
in molecular weight for the replacement of one hfacꢀ anion
with a tridentate neutral ligand to give the ten-coordinate
pears when non-polar CDCl3 was added to CD3CN (see the
La,L2
exch
Supporting Information, Figure S18). Moreover, K
was
highly sensitive to the ionic strength of the solution, and
La,L2
exch
K
=0.005 when 1.4m LiClO4 was added in CD3CN. With
cations [La(L2)
this result in mind, we considered the values
log(b1L,a1ðhfacÞ ,L2)=3.3(9) and log(b1L,a1ðhfacÞ ,L3)=3.6(9), which
were estimated for Equilibrium (2) by using NMR data, to
only be mere estimations for the thermodynamic stability
constants extrapolated at infinite dilution. Finally, the reluc-
tance of smaller lanthanides for reaching ten-coordination
in [Ln(L2)2(hfac)2]+ resulted in such a rapid decrease of
3
3
[La(L3)2A
CHTUNGTRENNUNG
Equilibrium (3).
La,Lk
2 ½LaðLkÞðhfacÞ3ꢁ Ð ½LaðLkÞ2ðhfacÞ2ꢁþþ½LaðhfacÞ4ꢁꢀ
K
exch
ð3Þ
7160
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Chem. Eur. J. 2012, 18, 7155 – 7168

[Ln(hexafluoroacetylacetonate)3] with N-Heterocyclic Ligands
FULL PAPER
LnðhfacÞ3
La,Lk
Table 2. Thermodynamic-formation constants (logb
ated microscopic affinities (log(f
,Lk) and associ-
K
along the lanthanide series that the latter complexes
1,1
exch
) and DGLLnkðhfacÞ3) obtained by spec-
trophotometric titrations of ligands L2, L3, and L8 with
LnðhfacÞ3
Lk
were only detected for Ln=La, Ce, and Pr.
[Ln(hfac)3(diglyme)] in MeCN (298 K).[a]
Thermodynamic behavior of complexes [Ln(Lk)(hfac)3]
(k=2, 3; Ln=La, Nd, Sm, Eu, Gd, Tb, Tm, Lu, Y) in aceto-
nitrile: Thermodynamic stability constants b
Ln
CN¼9
LnðhfacÞ3,Lk
LnðhfacÞ3
Lk
LnðhfacÞ3
DG
Lk
Ligand
LnIII
R
[ꢃ][b]
logb
log(f
)
1,1
[kJmolꢀ1
]
LnðhfacÞ3,Lk
for
1,1
L2
L2
L2
L2
L2
L2
L2
L2
L2
L3
L3
L3
L3
L3
L3
L3
L3
L3
L8
L8
L8
La
Nd
Sm
Eu
Gd
Tb
Y
Tm
Lu
La
Nd
Sm
Eu
Gd
Tb
Y
1.216
1.163
1.132
1.120
1.107
1.095
1.075
1.052
1.032
1.216
1.163
1.132
1.120
1.107
1.095
1.075
1.052
1.032
1.216
1.120
1.032
5.06(7)
5.89(13)
6.18(16)
6.41(12)
6.06(15)
5.48(8)
5.45(8)
4.23(32)
4.28(30)
5.68(11)
6.22(9)
6.31(10)
5.94(9)
6.14(11)
5.74(7)
5.15(5)
4.18(30)
4.26(24)
5.57(8)
5.87(6)
4.29(31)
4.60(7)
5.41(13)
5.70(16)
5.93(12)
5.58(15)
5.00(8)
4.97(8)
3.75(32)
3.80(30)
5.20(11)
5.74(9)
5.83(10)
5.46(9)
5.66(11)
5.26(7)
4.67(5)
3.70(30)
3.78(24)
5.09(8)
5.39(6)
3.91(31)
ꢀ26.3(6)
ꢀ30.9(6)
ꢀ33(1)
Equilibrium (2), but extrapolated at zero ionic strength,
were obtained by spectrophotometric titrations of ligands
L2 and L3 at low concentration (10ꢀ4 m in CH3CN and
10ꢀ4 m diglyme) with [Ln(hfac)3(diglyme)] (Ln=La, Nd, Sm,
Eu, Gd, Tb, Tm, Lu, Y; jLnjtot/jLkjtot =0.1!2.6; see the
Supporting Information, Figure S19). The trans–trans!cis–
cis conformational change of the tridentate ligand, which ac-
companied the complexation process, induced some signifi-
cant changes in the electronic structure,[37] which were easily
monitored in the UV part of the absorption spectra (see the
Supporting Information, Figure S19a).[16,18b,38] Correcting the
spectrophotometric data for the absorption of free
[Ln(hfac)3] (see the Supporting Information, Appendix 3)
showed the classical splitting of the ligand-centered p!p*
transition, which resulted from the coordination of ligand
Lk to the metal center in [Ln(Lk)(hfac)3] (33110 and
ꢀ33.8(6)
ꢀ32.0(6)
ꢀ28.7(6)
ꢀ28.7(6)
ꢀ21(1)
ꢀ21(1)
ꢀ29.8(6)
ꢀ32.7(6)
ꢀ33.2(6)
ꢀ30.9(6)
ꢀ32.0(6)
ꢀ29.8(6)
ꢀ27.0(6)
ꢀ21(2)
Tm
Lu
La
Eu
Lu
ꢀ21(1)
ꢀ29.2(6)
ꢀ30.9(6)
ꢀ21.8(2)
28170 cmꢀ1
;
see the Supporting Information, Fig-
ure S19b),[37–39] whilst the existence of isosbestic points was
diagnostic for the existence of only two absorbing species in
the solution (excluding [Ln(hfac)3]; see the Supporting In-
formation, Figure S19b). The single end-point for jLnjtot/j
Lkjtot =1.0 (see the Supporting Information, Figure S19c)
confirmed the operation of Equilibrium (2), and the spectro-
photometric data were fitted with non-linear least-square
[a] MeCN contains a fixed total concentration (10ꢀ4 m) of diglyme for sta-
bilizing [Ln(hfac)3]. [b] Effective ionic radii for nine-coordinate LnIII
.
[41]
with simple methyl groups (L8) had a negligible impact on
the thermodynamic constants (Figure 5), the switch from the
standard electrostatic trend, which characterized the connec-
tion of Lk to [Ln(NO3)3] (Lk=L7 or L8, [Equilibri-
um (4)]),[11] to the reverse behavior, for the addition of Lk
to [Ln(hfac)3] (Lk=L2, L3, or L8, [Equilibrium (2)]), was
assigned to the choice of counteranions:
techniques[40] to give the associated formation constants
LnðhfacÞ3,Lk
b
(Table 2 and Figure 5).
1,1
LnðNO3Þ3,Lk
LnðNO3Þ3þLk Ð ½LnðLkÞðNO3Þ3ꢁ
b
ð4Þ
1,1
Altogether, 1) the stability constants collected for
[Ln(Lk)(hfac)3] along the major part of the lanthanide
series (i.e. log(b1L,n1ðhfacÞ ,Lk) > log(b1L,n1ðNO Þ ,Lk)) combined with
2) the large solubility brought by the branched alkyl resi-
dues in L2 and L3, and 3) the remarkable bowl-shaped ther-
modynamic selectivity[27a] make [Ln(hfac)3] very attractive
for the planned loading of multi-tridentate polymeric ligands
3
3 3
that contain 2,6-bis(benzimidazol-2-yl)pyridine binding
LnðhfacÞ3,L2
units. However, we noted that the trend
b
<
1,1
LnðhfacÞ3
Figure 5. Variations of log(b
,Lk) for ligands L2 (red squares), L3
LnðhfacÞ3,L3
1,1
b
suggested by the crystal-structure analysis was not
pertinent in solution, probably as a result of compensating
solvation effects.
A deeper insight into the thermodynamic complexation
process benefitted from the site-binding model,[42] which de-
ciphered the various energetic contributions to the forma-
tion of [Ln(Lk)(hfac)3]. In Equation (5), the microscopic in-
termolecular affinity of ligand Lk for the metal unit
1,1
(black diamonds), and L8 (green triangles) as a function of the inverse of
the nine-coordinate ionic radii for the lanthanide series.[41] The dashed
LnðNO3Þ3,L7
lines are only guides for the eyes. The related variation of log(b
is taken from reference [11] (yellow disks).
)
1,1
As inferred from the bond-valence analysis in the solid
LnðhfacÞ3,Lk
state,
b
decreases across the lanthanide series
1,1
(Figure 5), which demonstrates the operation of a counterin-
tuitive trend for this system.[26,27] Because the replacement
of the bulky 3-methy-1-butyl groups (ligands L2 and L3)
[Ln(hfac)3] was estimated by the connection parameter
LnðhfacÞ3
f
(Table 2, column 5), which was a microscopic de-
Lk
scriber that included desolvation processes, whilst the purely
Chem. Eur. J. 2012, 18, 7155 – 7168
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7161

S. Petoud, C. Piguet et al.
entropic statistical factor wc1h,1iral·w1L,n1ðhfacÞ ,Lk =3 (associated
½LaðCH3CNÞ9ꢁ3þþ2 Lkþ2 hfacꢀ
Ð
3
ð9Þ
½LaðLkÞ2ðhfacÞ2ꢁþþ9 CH3CN
b
with Equilibrium (2)) was obtained by the method of the
La,Lk,hfac
1,2,2
symmetry numbers (Figure 6a).[43]
La,Lk,hfac
1,2,2
La
Lk
2
La
hfac
2
4
b
¼ 72ðf Þ ðf Þ ðuLk,hfacÞ ðuhfac,hfacÞðuLk,Lk
Þ
ð10Þ
LaðhfacÞ3,Lk
chiral
1,1
LnðhfacÞ3,Lk
LaðhfacÞ3,Lk
LaðhfacÞ3
Lk
b
¼ w
ꢄ w
ꢄ f
¼ 3f
ð5Þ
1,1
1,1
1,1
LaðhfacÞ3,Lk
Once the experimental values for b
([Equilibri-
1,1
La,Lk
La,Lk,hfac
um (2)], Table 2), K
[Equilibrium (3)], b
(see the
exch
1,0,3
After fixing the standard concentration of the reference
state to 1m,[44] the van’t Hoff isotherm transformed fLnðhfacÞ3
Supporting Information, [Equilibrium (S28)] and Table S22),
Lk
La,Lk,hfac
LnðhfacÞ3
and b
(see the Supporting Information, [Equilibri-
1,0,4
into their free energy counterparts ꢀ34ꢅDG
ꢅ
con,Lk
ꢀ21 kJmolꢀ1 across the lanthanide series for ligands L2, L3,
um (S30)] and Table S22) were in hand, the missing stability
La,Lk,hfac
1,1,3
La,Lk,hfac
1,2,2
constants b
[Equilibrium (7)] and b
[Equilibri-
and L8 (Table 2, column 6). These values compared well
LnðNO3Þ3
um (9)] were deduced by using Equations (11) and (2) (see
the Supporting Information, Table S22).
with those reported for DG
, but were significantly
L7
LnðCF3SO3Þ3
less-negative than those found for DG
and
L7
LnðClO4Þ3
DG
in pure acetonitrile.[11] More detailed information
L7
La,Lk,hfac
La,Lk,hfac
LaðhfacÞ3,Lk
b
¼ b
ꢄ b
ð11Þ
ð12Þ
could not be obtained from the determination of a single
stability constant. However, the operation of Equilibri-
um (3) and the formation of the two ternary complexes
[La(Lk)(hfac)3] and [La(Lk)2(hfac)2]+ opens up new per-
spectives when one considers Equilibrium (2) to be the
result of a cascade reaction of the solvated metal with li-
gands Lk and hfacꢀ ([Equilibrium (6)]).
1,1,3
1,0,3
1,1
La,Lk,hfac
2
La,Lk
ðb
Þ ꢄ K
1,1,3
exch
La,Lk,hfac
b
¼
1,2,2
La,Lk,hfac
b
1,0,4
Multi-linear least-square fits of the nine Equations
(Eq. (11) and (12) as well as Eq. (S18)–(S31) in the Support-
ing Information) converged for the five microscopic thermo-
dynamic describers (Table 3), which satisfyingly reproduced
the experimental formation constants (see the Supporting
Information, Table S22 and Figure S21).[45]
La,Lk,hfac
1,1,3
La3þþLkþ3 hfacꢀ Ð ½LaðLkÞðhfacÞ3ꢁ
b
ð6Þ
Since the statistical factors only marginally contributed to
the total free-energy change,[42] rough values were deduced
by fixing an arbitrarily common coordination number of
CN=9 around each lanthanide atom, except for
[La(Lk)2(hfac)2]+ (CN=10) and [La(hfac)4]ꢀ (CN=8; see
the Supporting Information, Figure S20). Moreover, the
donor atoms in the first coordination sphere were assumed
to occupy the position of an idealized tricapped trigonal
prism, whilst solvent molecules filled the vacant positions.
With these assumptions in mind, Equilibrium (6) was trans-
Table 3. Fitted microscopic thermodynamic parameters for LaIII/Lk/hfacꢀ
(k=2, 3; MeCN; 298 K).[a]
Entry
Parameters
L2
L3
La
Lk
La
con,Lk
La
hfac
La
1
2
3
4
5
6
7
8
log(f
DG
log(f
DG
)
5.5(4)
ꢀ31(2)
6.2(3)
ꢀ35(2)
ꢀ0.4(4)
2(3)
4.3(5)
ꢀ25(3)
6.2(4)
ꢀ35(2)
0.8(6)
ꢀ5(3)
0.1(2)
0(1)
[kJmolꢀ1
]
)
[kJmolꢀ1
]
con,hfac
La
log(u
)
Lk,Lk
La
DE
[kJmolꢀ1
]
Lk,Lk
La
Lk,hfac
log(u
)
ꢀ0.5(2)
formed into Equilibrium (7), whose stability constant
La
DE
[kJmolꢀ1
)
]
3(1)
Ln
Lk
(bL1,a1,,L3k,hfac) could be modeled by Equation (8) (Figure 6b, f
Lk,hfac
La
hfac,hfac
9
10
log(u
ꢀ0.4(2)
2(1)
ꢀ0.4(3)
Ln
and f
are the intermolecular microscopic affinities of
L;L
La
DE
[kJmolꢀ1
]
2(2)
hfac
hfac,hfac
DE =RT
each ligand for Ln3+ and uL;L ¼ eꢀ
are the Boltz-
ligand
ð
Þ
[a] MeCN contains a fixed total concentration (10ꢀ4 m) of diglyme for sta-
bilizing [Ln(hfac)3].
mann factors that account for the intramolecular interACHUTNERGNNUG AHCTUNGRENNUG
interactions that occurred within the coordination
sphere).[42]
In line with the well-known oxophilicity of trivalent lan-
thanides and the preference for charge-neutralization in
polar solvents, the intermolecular connection of the hfacꢀ
½LaðCH3CNÞ9ꢁ3þþLkþ3 hfacꢀ
Ð
ð7Þ
ð8Þ
La,Lk,hfac
½LaðLkÞðhfacÞ3ꢁþ9 CH3CN
b
1,1,3
À
Á
La
La
anion to La3+ (DG
¼ ꢀRT ln f
=ꢀ35(2) kJmolꢀ1,
con;hfac
hfac
Table 3, entry 4) made a prominent contribution to the sta-
La,Lk,hfac
1,1,3
La La
Lk hfac
3
3
3
b
¼ 48f ðf Þ ðuLk,hfacÞ ðuhfac,hfacÞ
bility of [La(Lk)(hfac)3]. The related interactions with the
neutral tridentate N-donor ligands Lk with La3+ were slight-
La
Lk
The same strategy was followed for modeling Equilibri-
ly
less
favorable
(ꢀ31ꢅDGLcoan,Lk =ꢀRTln(f )ꢅ
um (9) with the stability constant given in Equation (10).
This process was systematically repeated for the related
equilibria, thereby leading to the formation of [La(Lk)n]3+
(n=1–3; see the Supporting Information, S18–S23) and
[La(hfac)n](3ꢀn)+ (n=1–4; see the Supporting Information,
S24–S31, Appendix 4, and Figure S20).
ꢀ25 kJmolꢀ1, Table 3, entry 2), but they closely matched
those found for the connection of Lk onto [La(hfac)3]
LaðhfacÞ
(ꢀ30ꢅDG
3 =ꢀRTln(fLLkaðhfacÞ3)ꢅꢀ26 kJmolꢀ1, Table 2),
con,Lk
which was in agreement with a negligible influence of the
charge neutralization brought by the complexation of hfacꢀ
ions on the cascade reaction with ligands L2 and L3. As
7162
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Chem. Eur. J. 2012, 18, 7155 – 7168

[Ln(hexafluoroacetylacetonate)3] with N-Heterocyclic Ligands
FULL PAPER
became more repulsive than
their hetero
ACHUTGTNRENNlGU iACTHNGURTENNUgG and counterpart
La
(DEhLfaac,hfac +DELLka,Lk >2DE
).
Lk,hfac
This trend was assigned to an
Ln
Lk,Lk
increase in DE
for the heav-
ier lanthanide cations.
Photophysical properties of
complexes [Ln(Lk)(hfac)3] (k=
2, 3; Ln=La, Eu, Gd): Taking
log(Db1L,n1ðhfacÞ ,Lk)ꢆ5.5 for Equi-
3
librium (2) with Ln=Eu, Gd,
Tb, we predicted that partial
ligand-decomplexation amount-
ed to 6% at millimolar concen-
trations, but reached 16%
(10ꢀ4 m) and 43% (10ꢀ5 m) at
the concentrations typically
used for recording unbiased
photophysical data. Therefore,
we limited the investigation of
the luminescent properties to
solid-state samples, for which
quantitative complexation had
been firmly established by X-
ray diffraction, whilst the ab-
sorption electronic spectra (re-
corded in 10ꢀ4 m MeCN solu-
tion) were systematically cor-
Figure 6. Application of the site-binding model,[42] which shows the determination of symmetry numbers (sext
s
,
int, schiral),[43] for a) Equilibrium (2) and b) Equilibrium (7). The symmetry point groups are those expected for
idealized arrangements of the donor groups of the ligands in the first coordination sphere of the lanthanide.
a corollary, the intramolecular hetero
G
G
rected for partial dissociation (see the Supporting Informa-
tion, Appendix 5). According to the standard procedure,[4,46]
the ligand-centered photophysical properties were deduced
for the Gd complexes [Gd(Lk)(hfac)3] because paramagnet-
ic GdIII induced a mixing of the ligand and metal wavefunc-
tions that was very similar to that expected for the complex-
ation of luminescent EuIII (heavy-atom effect and paramag-
netic coupling),[47] without possessing accessible low-lying
metal-centered excited states.[48] However, the situation for
the ternary complex [Gd(Lk)(hfac)3] was delicate because
both types of ligands (i.e. Lk and hfacꢀ) possessed delocal-
ized p-aromatic chromophores, which may contribute to the
light-harvesting and sensitization processes.
La
DELLka,hfac =ꢀRTln(u
Lk, hfac
this trend was mirrored by the homo
li
E
La
Table 3, entry 6) and hfac–hfac (DE
hfac,hfac
interactions within experimental errors. Finally, the intro-
duction of Equations (8), (10), and (S31) from the Support-
ing Information into Equation (12) gave:
ðuLhfaac,hfacÞ ꢄ ðuLLka,LkÞ
3
4
La,Lk
exch
K
¼
La
2
ðu
Þ
Lk,hfac
which was transformed by using the van’t Hoff isotherm
into the standard mixing rule [Eq. (14)]:[45]
The electronic absorption spectrum of coordinated hfacꢀ
ions in [Gd(hfac)3(diglyme)] showed a broad band envelope
ꢀ
ꢁ
ꢀ2DE
3
4
mix
Lk,hfac
La,Lk
1
for the spin-allowed 1n,1p! p* transitions, centered at
DE
¼ DG
þRTln
þ DE
exch
33200 cmꢀ1 with a shoulder at lower energy (30000 cmꢀ1,
La,Lk
hfac,hfac
La
La
Lk,hfac
Figure 7a). Excitation into the hfac(1pp*) level at n˜exc
=
¼ DE
Lk,Lk
33330 cmꢀ1 produced short-lived fluorescence (0-0 phonon
transition at E0–0(1pp*)=27000 cmꢀ1, Figure 7b) and long-
lived phosphorescence (E0–0(3pp*)=21550 cmꢀ1, t(3pp*)=
1.17(2) ms at 77 K, Figure 7c; also see the Supporting Infor-
mation, Table S23). To decipher the photophysical conse-
quences of the subsequent cascade reaction of
[Gd(hfac)3(diglyme)] with Lk, we first noted that the inten-
Under statistical conditions, the sum of the homoACTHNUTRGENNlUG iACHUTNGTERNNUGgand
La
Lk,Lk
interactions (DELhfaac,hfac +DE
)
exactly overcame the
La
scaled hetero
li
gand interactions (2DE
Lk,hfac
0.[45] This result was close to that found for [La(L2)(hfac)3]
(DELm2ix,hfac =ꢀ1(3) kJmolꢀ1)
and
for
(DELm3ix,hfac =ꢀ2(4) kJmolꢀ1). On the contrary, the decrease in
Ln,Lk
exch
1
1
K
for the smaller lanthanide cations corresponded to
sity of the p! p* transitions, centered on the non-coordi-
nated tridentate ligand Lk, was similar to that observed for
[Gd(hfac)3(diglyme)], but red-shifted by about 2000 cmꢀ1
a considerable anti-cooperative process (DEmLkix,hfac @0) pro-
duced by the sum of the homoACTHNURGTENNlUG iACHUTGTNRENNUGgand interactions, which
Chem. Eur. J. 2012, 18, 7155 – 7168
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7163

S. Petoud, C. Piguet et al.
(Ln=Gd, Eu; Figures 7a, also see the Supporting Informa-
tion,
Figure S23a).[50]
Therefore,
irradiation
of
[Gd(L2)(hfac)3] and [Gd(L3)(hfac)3] at n˜exc =27780 cmꢀ1
produced similar emission spectra, which reflected the elec-
tronic structure of the coordinated tridentate ligands with-
out significant contributions from the hfac ligands (0–0
phonon transition for fluorescence at E0–0(1pp*)
ꢆ25400 cmꢀ1 (Figures 7b, see the Supporting Information,
Figure S23b), and long-lived phosphorescence at E0-0(3pp*)
ꢆ21000 cmꢀ1, t(3pp*)=0.4–1.0 ms at 77 K (Figures 7c; also
see the Supporting Information, Figure S23c and Table S23).
When GdIII was replaced with emissive EuIII in the com-
plexes [Eu(Lk)(hfac)3], irradiation in the Lk-centered excit-
ed states at n˜exc =27780 cmꢀ1 produces faint residual ligand-
1
centered fluorescence (1pp*! pp) together with an intense
red signal that arose from Lk!Eu energy-transfer followed
by Eu(5D1) and Eu(5D0)-centered luminescence (Figure 8).
Figure 8. Solid-state luminescence emission spectra of [Eu(Lk)(hfac)3]
(k=2, 3; 77 K; n˜exc =27780 cmꢀ1).
The intensities of the Eu(5D1! FJ) transitions were ex-
7
tremely small compared to the luminescence arising from
Figure 7. a) Absorption (10ꢀ4 m in CH3CN, 293 K, corrected for partial
dissociation, see the Supporting Information), b) fluorescence (solid-
state, 77 K) and c) phosphorescence spectra (solid-state, 77 K, delay time
after excitation flash: 0.05 ms) recorded for L2 (c, n˜exc =31250 cmꢀ1),
[Gd(hfac)3(diglyme)] (a, n˜exc =33330 cmꢀ1), and [Gd(L2)(hfac)3] (d,
n˜exc =27780 cmꢀ1). All emission spectra were arbitrarily normalized to 1.
7
the Eu(5D0! FJ) transitions, and the emission spectra were
dominated by the hypersensitive forced electric dipolar
7
Eu(5D0! F2) transition, centered at 16240 cmꢀ1. These two
spectral characteristics have been well-documented for low-
symmetry tris-b-diketonate EuIII complexes (Figure 8).[1,2,51]
The experimental absolute quantum yields FLEu (determined
upon excitation of the ligand excited states and monitoring
of the Eu3+ emission) reached 29(2)% for [Eu(L2)(hfac)3]
and 30(2)% for [Eu(L3)(hfac)3] (solid-state, 293 K) and tes-
tified to the efficiency of the sensitization process brought
by tridentate ligands L2 or L3. The latter quantum yields
were marginally smaller than the 40%ꢅFLEu ꢅ60% recently
reported for optimized [Eu(L)(b-diketonate)3] complexes,
where L were chelating N-donor or O-donor ligands and b-
diketonate was the unsymmetrical 2-thienoyltrifluoroaceto-
(Table S23, Figure 7a); this pattern was retained in their
emission spectra (Figure 7b; also see the Supporting Infor-
mation, Table S23).[49] Upon complexation of Lk to
[Gd(hfac)3], the characteristics of each of the contributing
chromophores were easily recognized in the absorption
spectra of [Gd(Lk)(hfac)3] (Figures 7a; also see the Sup-
porting Information, Figure S23a).
Interestingly, the additional splitting of the Lk-centered
1p! p* transitions[37] produced a low-energy component at
nate.[1,2,31,52] Interestingly, the Eu(5D0) excited lifetime of
1
Eu
obs
27740 cmꢀ1,[18b] which was exploited for the selective excita-
t
=0.97(1) ms observed for [Eu(L2)(hfac)3] and [Eu-
tion of the coordinated tridentate ligands in [Ln(Lk)(hfac)3]
(L3)(hfac)3] (solid-state, 293 K; see the Supporting Informa-
7164
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 7155 – 7168

[Ln(hexafluoroacetylacetonate)3] with N-Heterocyclic Ligands
FULL PAPER
tion, Table S23) was close to the radiative Eu(5D0) lifetime
(tEr u =1.13 ms) recently estimated for [Eu(hfac)3(H2O)2]
under the same experimental conditions.[54c] The subsequent
rough estimation of the intrinsic Eu-centered quantum
yields (FEEuu =toEbus=tEr u =0.97/1.13=0.86) in [Eu(L2)(hfac)3]
and [Eu(L3)(hfac)3] confirmed the standard statement that
the global quantum yield (FLEu) in ternary complexes [Eu(-
L)(hfac)3] was limited by the sensitization process, which
combined the efficiencies of the successive inter-system
4) The exclusive detection of monomeric, strongly lumines-
cent [Eu(Lk)(hfac)3] units in solution, even though their
analogous compounds [Eu(Lk)(NO3)3] and [Eu(-
Lk)(CF3CO2)3] only existed as complicate mixtures of
monomers and dimers, represented a decisive argument
for the exploitation of [Ln(hfac)3] as neutral lanthanide
carriers for the planned metallic loading of linear multi-
site polymers that incorporate 2,6-bis(benzimidazole-2-
yl)pyridine binding units.
1
3
crossing p*! p* and L!Eu (5Dj) energy-transfer processes
(see the Supporting Information, Figure S24).[52]
Experimental Section
Conclusion
Chemicals were purchased from Strem, Acros, Fluka AG, and Aldrich,
and used without further purification unless otherwise stated. The hexa-
fluoroacetylacetonate salts, [Ln(hfac)3C8H14O3], were prepared from
their corresponding oxide (Aldrich, 99.99%).[22] MeCN and CH2Cl2 were
distilled over calcium hydride. Silica gel plates (Merck 60 F254) were used
for thin layer chromatography (TLC) and Fluka silica gel 60 (0.04–
0.063 mm) or Acros neutral activated alumina (0.050–0.200 mm) was
used for preparative column chromatography.
ꢀ
The replacement of didentate nitrate (X=NO3 ) or carbox-
ꢀ
ylate groups (X=CF3CO2 , both four-membered chelating
anions) with didentate hexafluoroacetylacetonate (X=
hfacꢀ, a six-membered chelating anion) around trivalent lan-
thanide atoms of the formula [LnX3] offers remarkable ad-
vantages for cascade reactions with tridentate N-heterocyclic
ligands:
Preparation of N-3-methylbutyl-(4-bromo-2-nitrophenyl)amine (1): 2,5-
Dibromonitrobenzene (6, 19.97 g, 71.09 mmol) and 3-methylbutylamine
(70% in water) were heated in an autoclave at 1108C for 24 h. The dark
mixture was evaporated to dryness, extracted with CH2Cl2 (100 mL), and
successively washed with half-saturated aqueous NH4Cl solution (3ꢄ
50 mL) and water (50 mL). The organic layer was dried (Na2SO4), the
solvent was evaporated, and the resulting oil was crystallized from n-
hexane to give 20.02 g (69.72 mmol, yield 98%) of compound 1 as orange
crystals. 1H NMR (CDCl3, 400 MHz): d=1.00 (d, 6H, 3J=6.6 Hz), 1.65
(q, 2H, 3J=7.2 Hz), 1.78 (n, 1H, 3J=6.7 Hz), 3.32 (q, 2H, 3J=7.2 Hz),
6.79 (d, 1H, 3J=9.2 Hz), 7.51 (dd, 1H, 3J=9.2 Hz, 4J=2.3 Hz), 8.34 ppm
1) The thermodynamic affinities of the tridentate Lk li-
gands for [Ln(hfac)3] with large and mid-range trivalent
lanthanides (Ln=La–Ho; MeCN; 293 K; Figure 5) were
1–2 orders of magnitude larger than those found upon
reaction with [Ln(CF3CO2)3][10] and [Ln(NO3)3].[11]
2) The bowl-shaped best-fit curve displayed by
log(bL1,n1ðhfacÞ ,Lk) along the lanthanide series contrasted
3
with the standard monotonous electrostatic behavior dis-
4
(s, 1H, J=2.3 Hz). MS (ESI, CH2Cl2): m/z: 288.1 [M+H]+.
played by log(b1L,n1ðNO Þ ,Lk). The crossing of the two curves
at around Ln=Ho reversed the selectivity for the heavi-
er cations (Ln=Tm–Lu), which preferred complexation
3
3
Preparation of pyridine-2,6-dicarboxylic acid bis[(4-bromo-2-nitrophen-
yl)-(3-methylbutyl)amide] (2): Pyridine-2,6-dicarboxylic acid (2.99 g,
17.89 mmol) and DMF (30 mL) were heated to reflux in freshly distilled
thionyl chloride (15 mL) for 1 h. Excess thionyl chloride was distilled
from the reaction mixture, which was then co-evaporated with dry
CH2Cl2 (3ꢄ20 mL) and dried under vacuum. The solid was re-dissolved
in freshly distilled CH2Cl2 (20 mL) and a solution of N-3-methylbutyl-(4-
bromo-2-nitrophenyl)-amine (1, 5.01 g, 17.45 mmol) in CH2Cl2 (20 mL)
was slowly added under an inert atmosphere. The resulting mixture was
heated to reflux for 24 h and the pH value was kept close to pH 9 by
adding small amounts of N,N-diisopropylethylamine. The mixture was
partitioned between CH2Cl2 (60 mL) and half-saturated aqueous NH4Cl
(60 mL). The organic layer was separated and the aqueous phase was fur-
ther extracted with fresh CH2Cl2 (2ꢄ60 mL). The combined organic
phases were dried with anhydrous sodium sulfate, evaporated to dryness,
and the crude product was purified by column chromatography on silica
gel (CH2Cl2/MeOH, 100:0!99.5:0.5) to give 4.4 g (6.24 mmol, yield
75%) of compound 2 as a yellow powder. MS (ESI, CH2Cl2): m/z: 704.8
[M+H]+.
ꢀ
with [Ln(NO3)3] (Figure 5). The anomalously long Lu N
bond-lengths for the smallest LuIII cation in the crystal
structure of [Lu(Lk)(hfac)3] confirmed the operation of
this rare anti-electrostatic trend in the solid state. How-
ever, nine-coordination was retained for the complete
series of [Lu(Lk)(hfac)3]; this trend was in contrast to
the change in coordination numbers (CN=10!9!8)
found for [Ln(Lk)(NO3)3].[9,11]
3) The negligible values observed for the microscopic inter-
ACHTUNGTRENNUNGliACHTUNGTRENNUNGgand interactions that operated in the coordination
sphere of the largest LaIII cation in [La(Lk)(hfac)3] were
responsible for the concomitant formation of
[La(Lk)2(hfac)2]+ (10-coordinate) and [La(hfac)4]ꢀ in
MeCN at room temperature, through a solvent-depen-
dent enthalpy-driven Lk/hfac bi-exchange process. Be-
cause of the increasing Lk–Lk repulsive interactions that
accompanied the contraction of the lanthanide ionic
radius in [Ln(Lk)2(hfac)2]+, the latter side-reaction was
strictly limited to the largest cations Ln=La, Ce, and Pr
in MeCN. Replacement of MeCN with CHCl3 restored
the exclusive formation of [Ln(Lk)(hfac)3] in solution for
the complete lanthanide series.
Preparation of 2,6-bis[1-(3-methylbutyl)-5-bromobenzimidazol-2-yl]pyri-
dine (L2): Pyridine-2,6-dicarboxylic acid bis[(4-bromo-2-nitrophenyl)(3-
methylbutyl)amide] (2, 0.5037 g, 0.714 mmol) was dissolved in DMF
(3 mL) and a solution of Na2S2O4 (85%, 1.35 g, 6.56 mmol) in EtOH
(3 mL) was added. The slurry solution was heated to 858C, then deion-
ized water (3 mL) was added and the resulting mixture was heated to
reflux for 20 h under an inert atmosphere. The reaction was allowed to
cool to RT and KOH (4m, 5 mL) was added to the solution. The solvents
were evaporated under vacuum, the crude product was dissolved in
CH2Cl2 (30 mL), washed with water (3ꢄ20 mL), dried with anhydrous
sodium sulfate, and evaporated to dryness. Purification by column chro-
matography on silica gel (CH2Cl2/MeOH, 100:0!99:1) gave 199.7 mg
Chem. Eur. J. 2012, 18, 7155 – 7168
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7165

S. Petoud, C. Piguet et al.
(0.327 mmol, yield 46%) of ligand L2 as a white powder. 1H NMR
(CDCl3, 400 MHz): d=0.69 (d, 12H, 3J=6.6 Hz), 1.38 (n, 2H, 3J=
6.6 Hz), 1.60 (q, 4H, 3J=7.4 Hz), 4.67 (t, 4H, 3J=7.7 Hz), 7.33 (d, 2H,
3J=8.6 Hz), 7.47 (d, 2H, 3J=8.6 Hz), 8.01 (s, 2H), 8.09 (t, 1H, 3J=
7.9 Hz), 8.30 ppm (d, 2H, 3J=7.9 Hz); 13C NMR (CDCl3, 100 MHz): d=
22.10 (primary C); 38.73, 43.64 (secondary C); 25.73, 111.48, 123.12,
125.82, 126.60, 138.42, 149.58, 150.87 (tertiary C); 115.71, 135.06,
144.00 ppm (quaternary C); MS (ESI, CH2Cl2): m/z: 608.5 [M+H]+; ele-
mental analysis calcd (%) for C29H31N5Br2: C 57.16, H 5.13, N 11.49;
found: C 56.95, H 5.15, N 11.29.
Biosystems API 150EX LC/MS System equipped with a Turbo Ionspray
source. Elemental analyses were performed by K. L. Buchwalder from
the Microchemical Laboratory of the University of Geneva. Electronic
absorption spectra in the UV/Vis were recorded at 208C from solutions
in CH2Cl2 with a Perkin–Elmer Lambda 900 spectrometer using quartz
cells of 10 or 1 mm path length. Excitation and emission spectra as well
as lifetime measurements were recorded on a Perkin–Elmer LS-50B
spectrometer equipped for low-temperature measurements. Lumines-
cence spectra in the visible were measured using a Jobin Yvon-Horiba
Fluorolog-322 spectrofluorimeter equipped with a Hamamatsu R928.
Spectra were corrected for both excitation and emission responses (exci-
tation lamp, detector and both excitation and emission monochromator
responses). Quartz tube sample holders were employed. Quantum yield
measurements of the solid state samples were measured on quartz tubes
with the help an integration sphere developed by Frꢁdꢁric Gumy and
Jean-Claude G. Bꢅnzli (Laboratory of Lanthanide Supramolecular
Chemistry, ꢆcole Polytechnique Fꢁderale de Lausanne (EPFL), BCH
1402, CH- 1015 Lausanne, Switzerland) commercialized by GMP S.A.
(Renens, Switzerland).
Preparation of 2,6-bis(benzimidazol-2-yl)pyridine (3):[18] Pyridine 2,6-di-
carboxylic acid (10.43 g, 62 mmol) was stirred with o-phenylenediamine
(15 g, 13.8 mmol) in syrupy polyphosphoric acid (120 mL) at 2208C for
5 h. The colored melt was poured onto 3.5 L of vigorously stirring cold
water. When cooled, the bulky blue–green precipitate was collected by
filtration and slurried in a hot aqueous sodium carbonate solution (10%,
1.5 L). The resulting solid was filtered and recrystallized from MeOH to
give colorless prisms (12.6 g, 40.3 mmol, 65% yield). 1H NMR
([D6]DMSO, 400 MHz): d=7.23 (t, 2H, 3J=7.3 Hz), 7.31 (t, 2H, 3J=
7.3 Hz), 7.69 (d, 2H, 3J=7.3 Hz), 7.73 (d, 2H, 3J=7.3 Hz), 8.13 (t, 1H,
X-ray crystallography: For a summary of the crystal data, intensity meas-
urements, and structure refinements for ligand L3, [Ln(L2)(hfac)3], and
[Ln(L3)(hfac)3] (Ln=La, Eu, Lu), see the Supporting Information,
Table S6. All crystals were mounted on quartz fibers with protection oil.
Cell dimensions and intensities were measured between 120–200 K on
a Stoe IPDS diffractometer with graphite-monochromated MoKa radia-
tion (l=0.71073 ꢃ). Data were corrected for Lorentz and polarization
effects and for absorption. The structures were solved by direct methods
(SIR92[54] or SIR97)[55] or by charge-flipping methods (superflip).[56] All
other calculation were performed with ShelX97[57] or Crystals[58] systems
and ORTEP[59] programs. CCDC-843152 (L3), CCDC-843153 ([La(L2)-
(hfac)3]), CCDC-843154 ([Eu(L2)(hfac)3]), CCDC-843155 ([Lu(L2)-
(hfac)3]), CCDC-843156 ([La(L3)(hfac)3]), CCDC-843157 ([Eu-
(L2)(hfac)3]), contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3
3J=7.8 Hz), 7.23 ppm (d, 2H, J=7.8 Hz).
Preparation of 2,6-bis[1-(3-methylbutyl)benzimidazol-2-yl]pyridine (L3):
2,6-Bis(benzimidazol)pyridine (3, 2 g, 6.42 mmol) was dissolved in dry
DMF (100 mL) under a N2 atmosphere. The solution was cooled to 08C
and a suspension of sodium hydride (640 mg, 16.06 mmol) in DMF
(5 mL) was added. After stirring for 2 h at room temperature, 1-bromo-2-
methylbutane (2.91 g, 19.27 mmol) was added and the solution was
stirred for a further 24 h at room temperature under an inert atmosphere.
Water (100 mL) was added and the aqueous phase was extracted with
CH2Cl2 (5ꢄ50 mL). The combined organic phases were washed with
water (5ꢄ50 mL), dried (Na2SO4), and evaporated to dryness, and the
crude product was purified by column chromatography on silica gel
(CH2Cl2/MeOH, 99.9:0.1!99:1) followed by crystallization in hot n-
hexane to give 1.615 g of 2,6-bis[1-(3-methylbutyl)benzimidazol-2-yl]pyri-
dine (L3, 3.57 mmol, yield 56%) as transparent crystals. 1H NMR
(CDCl3, 400 MHz): d=0.72 (d, 12H, 3J=6.6 Hz), 1.42 (n, 2H, 3J=
6.6 Hz), 1.65 (dd, 4H, 3J=7.1, 3J=8.2 Hz), 4.74 (t, 4H, 3J=7.8 Hz), 7.39
(m, 4H), 7.49 (d, 2H, 3J=7.1 Hz), 7.90 (d, 2H, 3J=7.1 Hz), 8.09 (t, 1H,
3J=7.9 Hz), 8.34 ppm (d, 2H, 3J=7.9 Hz); 13C NMR (CDCl3, 100 MHz):
d=22.16 (primary C); 38.79, 43.47(secondary C); 25.78, 110.27, 122.80,
123.57, 125.58, 138.26, 149.95, 150.09 (tertiary C); 120.33, 136.14,
142.76 ppm (quaternary C); MS (ESI, CH2Cl2): m/z: 452.4 [M+H]+; ele-
mental analysis calcd (%) for C29H33N5: C 77.13, H 7.37, N 15.51; found:
C 77.09, H 7.39, N 15.52.
The Supporting Information contains details for the calculation of hydro-
dynamic molecular weights (Appendix 1), for the determination of stabil-
ity constants (Appendix 2), for the correction of electronic absorption
spectra (Appendices 3 and 5), and for thermodynamic modeling (Appen-
dix 4). Tables of 1H NMR spectroscopic shifts, elemental analysis, crystal
data, geometric parameters and bond valences, self-diffusion coefficients,
and photophysical data are also provided. Figures showing molecular
structures with atom numbering, molecular superimpositions, crystal
1
packing, symmetry numbers, H, 13C, and 19F NMR spectra, and electron-
ic absorption and emission spectra are also given.
Preparation of complexes [Ln(Lk)(hfac)3] (k=2, 3; Ln=La, Eu, Gd, Lu,
Y): Stoichiometric amounts of Lk and [Ln(hfac)3(diglyme)] were reacted
in MeCN/CH2Cl2 (1:1) at RT. Slow evaporation of CH2Cl2 provided
single-crystals of anhydrous [Ln(Lk)(hfac)3] suitable for X-ray diffraction
that gave satisfactory elemental analysis data (see the Supporting Infor-
mation, Table S5).
Acknowledgements
Spectroscopic measurements: 1H, 19F and 13C NMR spectra were record-
ed at 293 K on Bruker Avance 400 MHz and Bruker DRX-300 MHz
spectrometers. Chemical shifts are given in ppm with respect to TMS.
DOSY- NMR data used the pulse sequence implemented in the Bruker
program ledbpgp2s[53] which employed stimulated echo, bipolar gradients
and longitudinal eddy current delay as the z filter. The four 2 ms gradient
pulses had sine-bell shapes and amplitudes ranging linearly from 2.5 to
50 Gcmꢀ1 in 32 steps. The diffusion delay was in the range 60–140 ms de-
pending on the analyte diffusion coefficient, and the no. of scans was 32.
The processing was done using a line broadening of 5 Hz and the diffu-
sion coefficients were calculated with the Bruker processing package.
VT-1H NMR measurements of samples were measured on a Bruker
Avance 400 spectrometer equipped with a variable temperature unit. The
integrated intensities of the relevant peaks were obtained by deconvolut-
ing using Matlab or Excel (one Lorentz function per peak) after Fourier
transform and phasing of the spectrum using mnova. Fitting of van’t Hoff
plots was done using Excel. Pneumatically-assisted electrospray (ESI-
MS) mass spectra were recorded from 10ꢀ4 m solutions on an Applied
Financial support from the Swiss National Science Foundation is grateful-
ly acknowledged. S.P. acknowledges supports from la Ligue contre le
Cancer and from the Institut National de la Santꢁ et de la Recherche
Mꢁdicale (INSERM). The work in France was carried out within the
COST Action D38.
[1] H. F. Brito, O. M. L. Malta, M. C. F. C. Felinto, E. E. S. Teotonio,
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[Ln(hexafluoroacetylacetonate)3] with N-Heterocyclic Ligands
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1,1
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b
(Figure 5) is acceptable.
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3
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Received: September 9, 2011
Published online: April 30, 2012
7168
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