A fresh look at the structural diversity of dibenzoylmethanide complexes of lanthanides

Article abstract of DOI:10.1039/c9nj02059d

An analysis of known synthetic methods for dibenzoylmethanide (dbm^-) complexes of lanthanides of different nuclearity, viz. [Ln(dbm)₃(H₂O)], [Ln₄(dbm)₁₀(OH)₂], [Ln₅(dbm)₁₀(OH)₅] and [KLn(dbm)₄]_n (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) has been carried out. These methods have been optimized, the range of lanthanides for the compounds extended, and novel complex species, viz. [KLn(Me₂CO)(dbm)₄]₂ (Ln = Eu, Dy, Yb) and [Nd₃(dbm)₈(OH)(H₂O)], have been produced. The nature of the lanthanides in some cases affects the composition of the products formed. The compound [KEu(Me₂CO)(dbm)₄]₂ in solution can transform to [KEu(dbm)₃(OBz)]_n, containing benzoate (OBz^-). For the seven new complexes, the crystal structures have been determined by X-ray diffraction. Photoluminescent properties of the europium complexes have been studied. Their composition and structure influence the emission band shape and quantum yields. Two solid solutions of heterolanthanide complexes [Er_xYb_5-x(dbm)₁₀(OH)₅] (x = 1.2, 2.0) have been prepared, as shown by high-resolution powder XRD analysis.

Full text of DOI:10.1039/c9nj02059d

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Konchenko, New J. Chem., 2019, DOI: 10.1039/C9NJ02059D.
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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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DOI: 10.1039/C9NJ02059D
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A fresh look at the structural diversity of dibenzoylmethanide complexes of
lanthanides
Taisiya S. Sukhikha,b*, Dmitriy S. Kolybalova,b, Ekaterina K. Pylovaa,b, Denis A.
Bashirova,b, Vladislav Y. Komarova,b, Natalia V. Kuratievaa,b, Anton I. Smolentseva,
Andrew N. Fitchc and Sergey N. Konchenkoa,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An analysis of known synthetic methods for dibenzoylmethanide (dbm–) complexes of lanthanides of different nuclearity,
viz. [Ln(dbm)3(H2O)], [Ln4(dbm)10(OH)2], [Ln5(dbm)10(OH)5] and [KLn(dbm)4]n (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) has
been carried out. These methods have been optimized, the range of lanthanides for the compounds extended, and novel
complex species, viz. [KLn(Me2CO)(dbm)4]2 (Ln = Eu, Dy, Yb) and [Nd3(dbm)8(OH)(H2O)], have been produced. The nature of
the lanthanides in some cases affects the composition of the products formed. The compound [KEu(Me2CO)(dbm)4]2 in
solution can transform to [KEu(dbm)3(OBz)]n, containing benzoate (OBz–). For the seven new complexes, the crystal
structures have been determined by X-ray diffraction. Photoluminescent properties of the europium complexes have been
studied. Their composition and structure influence the emission band shape and quantum yields. Two solid solutions of
heterolanthanide complexes [ErxYb5–x(dbm)10(OH)5] (x = 1.2, 2.0) have been prepared, as shown by high-resolution powder
XRD analysis.
particular, it is hard to distinguish the solvent molecules in the
coordination sphere of the lanthanide from those present as
solvates. Therefore, in the analysis of the literature, most
attention should be paid to the results of those studies for
which a single crystal X-ray diffraction (XRD) analysis was
performed.
Introduction
Lanthanide complexes are attracting much attention because of
their actual or prospective applications as luminescent
materials for organic light-emitting diodes (OLEDs) . Among
1
them, Ln complexes with β-diketonate ligands are of special
Several types of structurally characterized Ln(III) complexes
2
interest owing to their efficient Ln(III)-centered emission
.
with dbm– ligand are known: mononuclear tris-complexes
Because of the wide-ranging opportunities for selecting and
functionalizing these ligands, one can precisely tune the
[Ln(dbm)3(H2O)] 14-18, tetranuclear [Ln4(dbm)10(OH)2]
,
19, 20
20-25
pentanuclear [Ln5(dbm)10(OH)5]
and anionic tetrakis-
lanthanide coordination environment and structure of the
complexes [MLn(dbm)4]n (M = K, Cs) 26. The structural library of
reported lanthanide hydroxo clusters including those with dbm–
has been analysed in a review articles 9, 10. However, reaction
conditions of these complexes have not been studied in detail.
To the best of our knowledge, structurally characterized
polymeric tetrakis-complexes [KLn(dbm)4]n containing
potassium were prepared only for the early lanthanides, and
attempts to prepare even the corresponding Sm complex were
unsuccessful (although a similar cesium complex was obtained)
3-5
complexes, which strongly influence the properties
.
Furthermore, β-diketonates can act as chelate-bridging ligands
and form complexes with different nuclearity depending on
reaction conditions 6-8. In particular, these attributes are
possessed by dibenzoylmethanide (dbm–) ligand 9-11
.
Attempts to systematize the effect of reaction conditions on the
composition of lanthanide complexes with dbm– were carried
12-14
out previously
. However, their composition was
determined on the basis of elemental analysis, less often using
IR, NMR spectroscopy and thermogravimetry. Since the
structure of these compounds varies widely, one should be very
cautious about the composition attributed to them. In
26
.
The present work fills some gaps in the chemistry of these
complexes: factors influencing the composition and structure of
already known compounds as well as new ones are discussed.
Photoluminecsent properties of a series of Eu complexes have
been studied.
a.Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of
Sciences, 630090 Novosibirsk, Russia..
b.Department of Natural Sciences, National Research University – Novosibirsk State
University, 630090 Novosibirsk, Russia.
Experimental
General considerations
c. European Synchrotron Radiation Faculty, CS 40220, 38043 Grenoble Cedex 9,
France.
Email: sukhikh@niic.nsc.ru
† Electronic Supplementary Information (ESI) available: IR spectra, crystallographic
data, powder XRD patterns and additional figures. CCDC 1911024–1911030. See
DOI: 10.1039/x0xx00000x
The reagents were used as received from suppliers. Methanol
was not further distilled, therefore it contained traces of water.
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The amount of water within certain limits did not affect the method – the first way (acetone/water), yield – 85%.
View Article Online
products. The composition of the products formed in the C45H35GdO7 (845.0) calcd. C, 63.9; H, 4.2.DFOoIu: n10d.1:0C3,96/C49.4N;J0H2,045.93D.
reactions depends substantially on the ratio of the reagents, [Dy(dbm)3(H2O)]: DyCl3 – 0.100 g (0.372 mmol); Hdbm – 0.250
therefore anhydrous lanthanide chlorides (99.5%) were used. g (1.12 mmol); KOH – 0.065 g (1.16 mmol), crystallization
The corresponding hydrates contain a number of water method – the first way (acetone/water), yield – 80%.
molecules, which is difficult to control, and may contain C45H35DyO7 (850.2) calcd. C, 63.6; H, 4.1. Found: C, 64.2; H, 3.9.
oxochlorides. Therefore with their use, the purity and yields of [Eu(dbm)3(H2O)]: EuCl3 – 0.208 g (0.808 mmol); Hdbm – 0.544 g
the products, as well as the reproducibility of the experiments (2.43 mmol); KOH – 0.136 g (2.42 mmol), crystallization method
turned out to be lower. The mixture of the complexes – the first way (acetone/water), yield – 80%. C45H35EuO7 (850.2)
[Nd3(dbm)8(OH)(H2O)]·3.5C7H8 and [Nd4(dbm)10(OH)2]·C7H8 was calcd. C, 64.4; H, 4.2. Found: C, 64.8; H, 4.5.
isolated during the operations described for the synthesis of [KEu(dbm)4]n: EuCl3 – 0.102 g (0.395 mmol); Hdbm – 0.351 g
known tetranuclear Nd complex using NdCl3·xH2O 19 (x ~ 6 – 9). (1.57 mmol); KOtBu – 0.175 g (1.56 mmol), crystallization
Elemental analysis data were obtained with a Eurovector method – the second way (CH2Cl2/hexane), yield – 5-10%.
EuroEA3000 analyzer. Room temperature excitation and C60H44EuKO8 (1084.0) calcd. C, 66.5; H, 4.1. Found: C, 66.2; H,
emission spectra were recorded with a Horiba Jobin Yvon 4.0.
Fluorolog 3 photoluminescence spectrometer equipped with [KYb(dbm)4]n: YbCl3 – 0.050 g (0.179 mmol); Hdbm – 0.159 g
450W ozone-free Xe-lamp, cooled PC177CE-010 photon (0.709 mmol); KOtBu – 0.080 g (0.713 mmol), crystallization
detection module with a PMT R2658 and double grating method – the second way (CH2Cl2/hexane), yield – 5-10%.
excitation and emission monochromators. Quantum yields C60H44KO8Yb (1105.1) calcd. C, 65.2; H, 4.0. Found: C, 64.8; H,
were determined using Quanta-φ integrating sphere. Excitation 3.9.
and emission spectra were corrected for source intensity (lamp [KDy(dbm)4]n: DyCl3 – 0.053 g (0.197 mmol); Hdbm – 0.179 g
and grating) and emission spectral response (detector and (0.798 mmol); KOtBu – 0.089 g (0.796 mmol), crystallization
grating) by standard correction curves. For measurements method – the second way (CH2Cl2/hexane), yield – 5-10%.
powdered samples were placed between two non-fluorescent C60H44DyKO8 (1094.6) calcd. C, 65.8; H, 4.0. Found: C, 65.8; H,
quartz plates. Energy-dispersive X-ray analysis (EDX) was 3.9.
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performed on a Hitachi TM-3000 electron microscope equipped [K(Me2CO)Eu(dbm)4]2: EuCl3 – 0.096 g (0.371 mmol); Hdbm –
with a Bruker Nano EDS analyzer.
0.330 g – 0.164 g (1.47 mmol),
(1.48 mmol); KOtBu
crystallization method – the third way (acetone), yield – 85%.
C126H100Eu2K2O18 (2284.2) calcd. C, 66.2; H, 4.4. Found: C, 66.2;
General Synthesis
Synthesis of the complexes was carried out according to the H, 4.3.
following procedure: Hdbm and a base (KOtBu or NaOH/KOH) [K(Me2CO)Dy(dbm)4]2: DyCl3 – 0.053 g (0.197 mmol); Hdbm –
were dissolved in MeOH (5-10 ml). Then the solution of LnCl3 in 0.179 g (0.798 mmol); KOtBu – 0.089 g (0.796 mmol),
MeOH (5-10 ml) was added, and a pale yellow precipitate was crystallization method – the third way (acetone), yield – 75%.
formed immediately. The mixture was stirred for 1 day, and C126H100Dy2K2O18 (2305.3) calcd. C, 65.6; H, 4.4. Found: C, 65.9;
then evaporated to dryness. Purification and crystallization of H, 4.2.
the product was carried out in three ways: 1) the residue was [K(Me2CO)Yb(dbm)4]2: YbCl3 – 0.048 g (0.172 mmol); Hdbm –
extracted with CH2Cl2 (5-10 ml), and yellow solution was 0.151 g (0.673 mmol); KOtBu – 0.077 g (0.686 mmol),
filtered. n-Hexane (10-15 ml) was then slowly added to obtain a crystallization method – the third way (acetone), yield – 75%.
two-layer liquid. After several days, the crystals of the product C126H100K2O18Yb2 (2326.4) calcd. C, 65.1; H, 4.3. Found: C, 64.9;
formed were separated and dried in vacuo. 2) the residue was H, 3.8.
extracted with acetone (5-10 ml), and yellow solution was [Eu4(dbm)10(OH)2]: EuCl3 – 0.100 g (0.387 mmol); Hdbm – 0.217
filtered and slowly evaporated to dryness to give the crystals of g (0.968 mmol); KOtBu – 0.131 g (1.17 mmol), crystallization
the product. 3) the residue was extracted with acetone (5-10 method – the second way (CH2Cl2/hexane), yield – 80%.
ml), and yellow solution was filtered. Water (5-10 ml) then C150H112Eu4O22 (2874.3) calcd. C, 62.7; H, 3.9. Found: C, 62.1; H,
slowly added to obtain a two-layer liquid. After a week, the 3.7.
crystals of the product formed were separated and dried in [Sm5(dbm)10(OH)5]: SmCl3 – 0.114 g (0.446 mmol); Hdbm –
vacuo. Amounts of the reagents and reaction conditions used 0.200 g (0.892 mmol); KOtBu – 0.151 g (1.34 mmol),
are given below. Note that the yield of [KLn(dbm)4]n is low crystallization method – the second way (CH2Cl2/hexane), yield
because of the low solubility of the product in CH2Cl2 and varies – 70%. C150H115O25Sm5 (3069.3) calcd. C, 58.7; H, 3.8. Found: C,
depending on the amount of the solvent used.
58.3; H, 3.7.
Keeping the crystals of [K(Me2CO)Eu(dbm)4]2 (0.100 g, 0.0438 [Er1.2Yb3.8(dbm)10(OH)5]: ErCl3 – 0.016 g (0.060 mmol); YbCl3 –
mmol) under acetone (0.5 mL) for two months resulted in the 0.053 g (0.190 mmol); Hdbm – 0.112 g (0.500 mmol); KOtBu –
formation of yellow prismatic crystals of [KEu(dbm)3(OBz)]n in 0.086 g (0.770 mmol), crystallization method – the second way
the mixture with the starting complex.
(CH2Cl2/hexane), yield – 60%.
[Gd(dbm)3(H2O)]: GdCl3 – 0.307 g (1.16 mmol); Hdbm – 0.773 g [Er2.0Yb3.0(dbm)10(OH)5]: ErCl3 – 0.027 g (0.100 mmol); YbCl3 –
(3.45 mmol); NaOH – 0.141 g (3.52 mmol), crystallization 0.042 g (0.150 mmol); Hdbm – 0.110 g (0.490 mmol); KOtBu –
2 | J. Name., 2012, 00, 1-3
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0.085 g (0.760 mmol), crystallization method – the second way
(CH2Cl2/hexane), yield – 90%.
X-ray diffraction
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Results and discussion
Analysis of known synthetic methods
View Article Online
DOI: 10.1039/C9NJ02059D
Pentanuclear dibenzoylmethanide complexes are usually
prepared as follows: LnCl3·6H2O (Ln = Y, Eu, Tb, Dy, Ho) Hdbm
and a base (NEt3 or KOtBu) react in 1 : 2 : 3 ratio in methanol or
ethanol to give a precipitate. The latter is separated and
recrystallized from CH2Cl2/hexane mixture or toluene to give
Single-crystal XRD data for the compounds were collected by a
Bruker Apex DUO diffractometer equipped with a 4K CCD area
detector using the graphite-monochromated MoKα radiation (λ
= 0.71073 Å) (Table S1, Supporting Information). The φ- and ω-
scan techniques were employed to measure intensities.
Absorption corrections were applied with the use of the
SADABS program 27. The crystal structures were solved by direct
methods and were refined by full-matrix least squares
techniques by means of the SHELXTL package 28 and Olex2 GUI
29. Atomic thermal displacement parameters for non-hydrogen
atoms were refined anisotropically. The positions of hydrogen
atoms were calculated corresponding to their geometrical
conditions and refined using the riding model. High R1-factor
(0.21) (compare with Rint of 0.045) and high residual electron
density for the structure [Nd3(dbm)8(OH)(H2O)]·3C7H8 can be
explained by the fact that it was obtained from an array of Bragg
reflections. Actual diffraction is more complicated: many Bragg
reflections have a pair of less intense satellites (Fig. S12), which
may be due to incommensurate modulation (modulation vector
± (-0.2 0.4 0.4)). Due to the low intensity of the satellites, it was
not possible to solve the modulated structure. Nevertheless,
the structure [Nd3(dbm)8(OH)(H2O)]·3C7H8 was of sufficient
quality to allow discussion of geometry of the complex and
coordination modes of dbm–, and it was submitted to the
Cambridge Structural Database (CSD) as a CSD Communication
(CCDC Refcode BIYPOR). Plate crystals of [K(Me2CO)Eu(dbm)4]2
tend to intergrowth; as a result, for several crystals tested,
residual electron density (5.6 / –7.6 e·Å–3) and R1-factor (0.14)
were high, while some atoms had large maximum to minimum
ADP ratio (ca. 5). Thus, the structure of [K(Me2CO)Eu(dbm)4]2 is
essentially presented to prove the isostructurality of the series.
The powder XRD analysis of the compounds was performed
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[Ln5(dbm)10(OH)5] usually in low yield (30 – 60%) 20-25. In this
33, 34
regard, we previously proposed a modified procedure
,
according to which both the mother liquor and the precipitate
were used. This procedure allows a 80 – 95% yield of the
complexes [Ln5(dbm)10(OH)5] (Ln = Dy, Er, Yb). Noteworthy, the
use of NEt3 as a base is less preferable than KOtBu, because the
product can be contaminated with the salt NEt3HCl formed.
Tetranuclear complexes [Ln4(dbm)10(OH)2] are known for Ln =
Pr, Nd, Sm, Eu 19, 20. They are usually prepared similarly to
[Ln5(dbm)10(OH)5] with the exception of using another
Ln : Hdbm : base ratio (1 : 2.5 : 3). It should be noted that the
synthetic method for Eu complex was not optimized, and it was
isolated with a low yield of 15% 20. The different nuclearity of
the complexes formed was explained by different ionic radii of
the central atoms. The late lanthanides were suggested to form
the pentanuclear complexes, whereas the early lanthanides
19,
form tetranuclear compounds
23. In the case of the
intermediate Eu, both tetra- and pentanuclear complexes were
obtained. However, different reaction conditions were used for
the late and early lanthanides, viz. the ratio of reagents. To date,
only one work reports reactions under identical conditions with
Ln : Hdbm ratio of 1 : 3 (solvent is EtOH/H2O mixture, base is
NH3) giving different products [Sm4(dbm)10(OH)2] and
[Tb5(dbm)10(OH)5] 25. Thus, one cannot exclude the possibility of
obtaining compounds of the same nuclearity for all lanthanides.
In fact, the pentanuclear complex is known for the late
neodymium, but it has been prepared in a different way using
[Nd(dbm)3(H2O)] as starting compound 35
.
using
a
Shimadzu XRD-7000 diffractometer at room
Similar assumptions were made for polymer complexes
[KLn(dbm)4]n: only the corresponding Nd (late lanthanide)
compound was obtained (with the reagents ratio of 1 : 4 : 4,
KOtBu as a base), and attempts to obtain Sm (intermediate
lanthanide) complex were unsuccessful 26. However, a similar
terakis complex “KEu(dbm)4” was mentioned by L. R. Melby et
al (yield 60%) 14; its composition was assigned on the basis of
elemental analysis. A number of examples of complexes
temperature (CuKα radiation, Ni filter). Powder samples were
slightly ground with heptane in an agate mortar and deposited
on the polished side of a quartz-glass holder. Powder diffraction
patterns were collected in 3–30° 2θ range.
High-resolution powder XRD measurements were carried out at
room temperature at the ID22 beamline of European
Synchrotron Radiation Facility (ESRF, Grenoble, France)
30
equipped with the 9-channel Si(111) multi-analyzer stage
.
+
Cat[Ln(dbm)4] with other conterions (for example, Cat = NHEt3
Calibration of the instrument and refinement of the X-ray
wavelength λ = 0.354216(6) Å were performed via NIST silicon
standard SRM640c. The powders were loaded into 1.5-mm-
diameter borosilicate thin-walled glass capillaries, which were
rotated during measurements to improve the powder
orientation statistics. The raw diffraction data reduction was
performed in 0.5–22.0° 2θ range with 0.002° step 31. The unit-
cell parameters were refined using a Pawley fit via Topas
36), is known, but we will not discuss them in detail.
The complexes [Ln(dbm)3(H2O)] are prepared by refluxing
LnCl3·6H2O (Ln = Pr, Nd, Sm, Eu, Ho, Er), Hdbm and KOH
(1 : 3 : 3) in acetone; the product is crystallized from mother
liquor (yields 60–95%) 14-16. These complexes can be used as
precursors for the preparation of [Ln5(dbm)10(OH)5] and
[KLn(dbm)4]n 37, 38. Synthesis of tris-complexes with a different
number of coordinated water molecules is reported, viz.
Academic v.6 program 32
.
39
“Ln(dbm)3(H2O)2”
and “Ln(dbm)3” 13, 40. Their composition
was assigned on the basis of elemental analysis and
thermogravimetry. Nevertheless, only [Ln(dbm)3(H2O)] has
been characterized by single-crystal XRD.
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Thus, there are a number of gaps in the chemistry these more polar acetone, the novel product formed, viz.
complexes: the factors influencing the composition and [K(Me2CO)Ln(dbm)4]2, which had unchanged metal-ligand
structure of these compounds should be determined. To fill stoichiometry, but contained additional acetone molecules.
these gaps, we studied a number of reactions of LnCl3, Hdbm P.C. Junk, P.W. Roesky et al. reported an interesting reaction of
and KOtBu or KOH in various ratios in methanol. In particular, Hdbm with hydrated LaCl3 and excess of NEt3 to form the
the existence of the complexes [KLn(dbm)4] was extended to complex [La12(OH)12(H2O)4(dbm)18(Phgly)2(CO3)2] containing
View Article Online
DOI: 10.1039/C9NJ02059D
the late lanthanides.
phenylglyoxylate ligand (Phgly–) 41. They proposed catalytic
oxidation of Hdbm by oxygen of air to Phgly– and benzoate
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Synthesis of the complexes
(Scheme 2). The latter was identified by ESI mass spectrometry
41
General scheme of syntheses carried out in this work is shown
.
We
support
this
hypothesis:
the
complex
in Scheme 1. A new complex [Sm5(dbm)10(OH)5] was obtained [K(Me2CO)Eu(dbm)4]2 in acetone solution gradually transforms
using the procedure developed for the pentanuclear complexes to a novel complex [KEu(dbm)3(OBz)]n containing benzoate
(Ln : Hdbm : KOtBu ratio is 1 : 2 : 3, in methanol) 33. Note that (OBz–), which was characterized by single crystal XRD. In this
19
for Sm only a tetranuclear complex was previously reported
.
case, the oxidation process is not very fast. According to powder
Therefore, in this case the size of the lanthanide ion does not XRD data, after several weeks, transformation to benzoate
limit the possibility of complex formation. However, the complex is incomplete (Fig. S5). Direct synthesis of
corresponding pentanuclear Nd complex failed to form under [KEu(dbm)3(OBz)]n is a subject of future research.
these conditions, giving instead the known [Nd4(dbm)10(OH)2], The complex [Nd4(dbm)10(OH)2] can be obtained by the
which implies an influence of the lanthanide’s nature. interaction of NdCl3·6H2O with Hdbm (2.5 equivalents) in the
Nevertheless, [Nd5(dbm)10(OH)5] can be achieved using presence of an excess of NEt3 in 60% yield, as described in ref.
[Nd(dbm)3(H2O)] as starting compound 35. It should be noted 19. Our attempts to reproduce this synthesis gave a mixture of
that in several synthetic procedures for penta-, tetranuclear and the complexes of different nuclearities was formed. A possible
polymer complexes with dbm–, slow addition of LnCl3 solution reason for the formation of a number of phases in our case is
19-21,
to dbm– solution another was mentioned
23. Our the presence of impurities in the starting NdCl3·xH2O (x ~ 6 – 9)
experiments showed that the rate of combining the solutions or its undefined composition which break the required reagents
does not affect the reaction pathway. In addition, we tested the ratio. Two products obtained were characterized by single
order of combining the reagents: 1) the solution of KOtBu was crystal XRD. The first phase is [Nd4(dbm)10(OH)2]·C7H8 isotypical
19
added to the solution of LnCl3 and Hdbm; 2) the solution of LnCl3 to the known neodymium complex [Nd4(dbm)10(OH)2] (see
was added to the solution of Hdbm and KOtBu; both cases gave Crystal Structures Section). The second phase is the novel
the same results.
trinuclear complex [Nd3(dbm)8(OH)(H2O)]·3.5C7H8. The
With the example of ytterbium, it was shown that under similar synthetic method for the latter has not yet been developed.
conditions, but with YbCl3 : Hdbm ratio of 1 : 4, a polymer Using a procedure analogous to the pentanuclear complexes,
[KYb(dbm)4]n is formed. Therefore, the late lanthanides are also but with
able to form this type of compound. The complex [KEu(dbm)4]n tetranuclear complex [Eu4(dbm)10(OH)2] was prepared in a
a a
ratio EuCl3 : Hdbm : KOtBu of 1 : 2.5 : 3,
was prepared in
a similar way. Since the compounds moderate yield (80%) considerably exceeding that in the
[KLn(dbm)4]n have much lower solubility in nonpolar solvents previously described procedure 20. However, along with the
compared to the pentanuclear complexes, it was unreasonable target product, the polymer complex [KEu(dbm)4]n was formed
to recrystallize them in sufficient scale (more than 0.05 g) from as
a minor phase. Note that [Ln5(dbm)10(OH)5] and
a mixture of CH2Cl2/hexane. When the solvent was replaced by [Ln4(dbm)10(OH)2] are readily soluble in CH2Cl2 and poorly
soluble in acetone, whereas the tetrakis-complexes
[KEu(dbm)4]n and [K(Me2CO)Ln(dbm)4]2 – vice versa. Thus, due
to different solubility, these complexes can be separated from
their mixture. Thus, the complex [Eu4(dbm)10(OH)2] can be
purified by washing the mixture of the phases with acetone. It
should be noted that [Yb4(dbm)10(OH)2] failed to form under the
conditions mentioned for Eu, giving instead the mixture of
[Yb5(dbm)10(OH)5] and [KYb(dbm)4]n; this implies an influence of
lanthanide’s nature.
Scheme 1. Synthesis of dibenzoylmethanide complexes of different nuclearity. The
lanthanides in previously reported complexes are marked gray; the lanthanides in new
Scheme 2. Proposed catalytic oxidation of dibenzoylmethanide to phenylglyoxylate and
complexes are marked black. At the first stage, the reactions were carried out in
benzoate.
methanol; at the second stage, the products were recrystallized from the corresponding
solvent.
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We have shown that a known procedure for [Ln(dbm)3(H2O)] 15
modified by using methanol as a solvent at room temperature
gave the target product. This technique was demonstrated for
Ln = Sm, Gd, Dy.
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DOI: 10.1039/C9NJ02059D
The complexes obtained in this work were characterized by
single crystal and powder XRD and elemental analysis. Using IR
spectroscopy, one cannot clearly identify all the products
because of the similarity of the spectra of different complexes
(Fig. S1). However, analysis of ν(C–O) bands allows
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differentiation
between
pairs
of
the
complexes
Figure 1. The structure of [KLn(dbm)4]n (a) exemplified by Ln
= Eu and
[Ln4(dbm)10(OH)2] and [Ln5(dbm)10(OH)5] (ca. 1400 cm–1) from
pairs [KLn(dbm)4]n and [K(Me2CO)Ln(dbm)4]2 (ca. 1420 cm–1)
and from [Ln(dbm)3(H2O)] (ca. 1375 cm–1). It should be noted
that single crystal structures of some compounds revealed the
presence of solvate molecules; however, the bulk samples were
characterized by powder XRD and elemental analysis as solvate-
free due to the quick loss of the solvent.
[KLn(Me2CO)(dbm)4]2 exemplified by Ln = Dy (b). Short K–O contacts are dashed; K and
Ln atoms are connected by a transparent line. Chelate dbm– ligands are coloured orange.
H atoms are omitted, phenyl groups are simplified.
The angles between the Ln···Ln and K···K lines in these
molecules are 90°. The nearest coordination environment of
each K atom consists of four O atoms of the dbm–and one O
atom of acetone. The experimental powder XRD patterns of the
complexes are identical, but they do not agree with the
simulated ones due to the loss of acetone (Fig. S4).
The complex [KEu(dbm)3(OBz)]n reveals {Eu(dbm)3(OBz)} units
connected by K atoms via K–O contacts (five for each K atom),
forming polymer chains (Fig. 3a). In fact, the structure
resembles that of [KLn(dbm)4]n, with the exception of one dbm–
ligand being replaced by OBz–. Due to smaller O···O distance in
OBz– comparing to dbm–, the {KEu} chain is not straight, but has
a zigzag shape showing K–Eu–K angle of 133.8°.
The molecule of the complex [Nd3(dbm)8(OH)(H2O)]·3.5C7H8
contains eight dbm– ligands, five of them are chelate, two
chelate-bridging ligands and one is bis-chelate-bridging that
connect Nd atoms forming a triangle (Fig. 3b). The OH– ligand is
μ3-bridging, and the water molecule is coordinated as a
monodentate ligand to one of the Nd atoms. Thus, the
coordination number of all the central atoms is eight.
Thus, using methanol as a solvent, varying the base and the
ratio of reagents, one can selectively prepare five types of
lanthanide complexes of different nuclearities.
Crystal structures of the complexes
All complexes [Ln(dbm)3(H2O)] (Ln = Pr, Nd, Sm, Eu, Ho, Er 14-18
,
Gd and Dy) are isostructural. The molecule of these complexes
contains three chelate dbm– ligands and one water molecule;
hence, the coordination number of the central atom is seven
(Fig. 1). Average Ln–O bond lengths are listed in Table S2. The
compounds do not contain solvate molecules, and their
experimental powder diffraction patterns are consistent with
the simulated ones (Fig. S2).
In the case of isostructural polymer complexes
[KLn(dbm)4]n·2nCH2Cl2 (Ln = Nd 26, Eu, Yb), the Ln atoms
coordinate four chelate dbm– ligands. The K atoms connect the
{Ln(dbm)4} units via short K–O contacts (four for each K atom;
the distances listed in Table S2) with the neighboring ligands,
forming polymer chains (Fig. 2a). The K-Ln-K angles are 180°
since these atoms are located on the crystallographic twofold
rotation axis. The experimental powder XRD patterns of the
complexes are identical, but they do not agree with the
simulated ones due to the loss of the solvate molecules (Fig. S3).
In the binuclear complexes [KLn(Me2CO)(dbm)4]2 (Ln = Eu, Dy),
two crystallographically independent molecules (halves) are
observed. They also contain {Ln(dbm)4} units, but the K atoms
are located not on the same line with the Ln atoms (Fig. 2b). In
the first independent molecule, K atoms lie on the twofold axis,
while in the second molecule, Ln atoms lie on the twofold axis.
Figure 3. The structure of [KEu(dbm)3(OBz)]n (a) and [Nd3(dbm)8(OH)(H2O)]·3.5C7H8 (b).
Short K–O contacts are dashed; K and Ln atoms are connected by a transparent line. OH–
and H2O ligands are coloured pink, chelate dbm– are coloured orange, chelate-bridging
dbm– are coloured violet, bis-chelate-bridging dbm– ligands are coloured gray. H atoms
are omitted; phenyl groups of dbm– are simplified.
Figure 2. The structure of the complexes [Ln(dbm)3(H2O)] exemplified by Ln = Dy. H2O
ligand is coloured pink, chelate dbm– are coloured orange. H atoms are omitted, phenyl
groups are simplified.
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Molecule of the compounds [Ln4(dbm)10(OH)2]·nsolv (n = 0 – 3; chelate (six ligands) and chelate-bridging (four ligands). The
View Article Online
solv = toluene, C7H8; Ln = Pr, Nd, Sm, Eu) 19, 20 contains {Ln4(OH)2} complexes crystallize in the group P4/n 20, 22, 25, 33, 37
DOI: 10.1039a/Cn9dN/Jo0r2n05o9nD-
core. Within this core, four Ln atoms define a parallelogram and isostructural P21/c 24, 33, 37, 42. Despite the fact that compounds
are bridged by two μ3-OH– ligands, which are located on both of these structural types contain solvate molecules, global
sides of the metal plane (Fig. 4a). In this case, three coordination structural rearrangements do not occur with solvate loss, as
modes of dbm–are observed: chelate (six ligands), chelate- indicated by powder XRD patterns for desolvated samples (Figs.
bridging (two ligands) and bis-chelate-bridging (two ligands). S8, S9).
The coordination number of each atom Ln is eight. All Powder XRD data obtained with a laboratory diffractometer
compounds reveal similar crystal packing of the molecules of allow identification of all five types of complexes in a single
the complexes (Fig. S6), but are not isostructural due to the phase; however, even a semi-quantitative identification of
different number of solvate molecules. Apparently, even a slight several phases in the mixture is difficult due to low-intensity
change in the conditions for the crystallization of complexes broadened diffraction peaks (Fig. S10).
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leads to the formation of different solvates. For example, the
known complex [Nd4(dbm)10(OH)2] does not contain solvate
Heterolanthanide complexes
19
molecules, while [Nd4(dbm)10(OH)2]·C7H8, obtained in this work, Of interest is the preparation of heterolanthanide complexes,
contains and is isostructural to the corresponding Pr compound. containing two or more different lanthanides. This strategy can
It is possible that the phases of tetranuclear complexes are a allow achieving effective interplay between optical properties
mixture of different solvates, but they cannot be identified by of near-infrared (NIR) luminescent functional materials 43. To
powder XRD and elemental analysis because of the rapid loss of the
best
of
our
knowledge,
heterobimetallic
solvate molecules. In this case, phase transformations take dibenzoylmethanide compounds have been described in the
place, therefore the experimental powder XRD patterns do not only work, which were characterized by energy-dispersive X-ray
44
agree with the simulated ones (Fig. S7). However, the analysis (EDX) and unit-cell determination for single crystals
.
experimental diffraction patterns of tetranuclear complexes of In the present work, we synthesized two heterolanthanide
different lanthanides (Nd and Eu) are identical. complexes [ErxYb5–x(dbm)10(OH)5]. Their synthesis was carried
Molecules of the complexes [Ln5(dbm)10(OH)5]·nsolv (n = 0 – 3; out similarly to the Er and Yb pentanuclear complexes with the
20-25, 37, 42
solv = C7H8, CH2Cl2; Ln = Y, Eu, Gd, Tb, Dy, Ho, Er, Yb
,
exception of using a ErCl3/YbCl3 mixture of reagents. Single
Sm) contain five Ln atoms, which are located at the vertexes of crystals of the products likely contained CH2Cl2 solvate
a square pyramid. Four μ3-OH– ligands are located above the molecules; however, the crystals out of the mother liquor
triangular faces, and one μ4-OH– ligand is located above the cracked due to the quick loss of solvate preserving the general
base of the pyramid (Fig. 4b). dbm– is coordinated by two ways: structure, as well as those of parent Er and Yb compounds. Thus,
the solvate-free products were studied. The Er content in the
complexes was determined using EDX to be x = 1.2 and 2.0. This
value was similar for several samples tested and close to the
initial ratio of the reagents ErCl3/YbCl3 (Table 1).
Heterolanthanide complexes prepared, as well as their Er and
Yb congeners, were characterized by high-resolution powder
XRD analysis using synchrotron radiation (Fig. 5).
Heterolanthanide compounds are solid solutions of tetragonal
(P4/n) modification of [Ln5(dbm)10(OH)5]. The positions of the
reflections are very close – for the reflections at small angles
(200 and 002, Fig. S11), the difference in positions is comparable
to the resolution in the determination of the angles (ca. 0.002°).
However, the use of the Pawley method of simulated diffraction
profile refinement in the range of 0.5 – 10.0° (d = 40 – 2 Å)
revealed noticeable differences in the unit cell parameters. The
parameters (Table 2) increase with the increase of the content
Table 1. Er/Yb molar content (x/5–x) in the complexes [ErxYb5–x(dbm)10(OH)5] determined
using EDX analysis for different samples.
initial ErCl3/YbCl3 ratio Er/Yb content in the samples
1.2/3.8
1.2/3.8
1.3/3.7
1.3/3.7
2.1/2.9
1.9/3.1
1.8/3.2
Figure 4. The structure of [Ln4(dbm)10(OH)2] (a) exemplified by Ln
= Nd and
[Ln5(dbm)10(OH)5] exemplified by Ln = Er (b). {Ln5} and {Ln4} units are marked gray. OH–
ligands are coloured pink, chelate dbm– are coloured orange, chelate-bridging dbm– are
coloured violet, bis-chelate-bridging dbm– ligands are coloured gray. H atoms are
omitted, phenyl groups are simplified.
2.0/3.0
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Figure 5. High-resolution powder XRD patterns of the complexes [Er5(dbm)10(OH)5]
(Er5), [Yb5(dbm)10(OH)5] (Yb5), [Er1.2Yb3.8(dbm)10(OH)5] (Er1.2Yb3.8 and
and simulated pattern on the example of
)
[Er2.0Yb3.0(dbm)10(OH)5] (Er2.0Yb3.0
)
[Yb5(dbm)10(OH)5]
Table 2. a and c unit cell parameters for the complexes [ErxYb5–x(dbm)10(OH)5], x = 0 – 5,
defined using high-resolution powder XRD patterns
Volume, Å3
x
a, Å
c, Å
6930.8(8)
6944.03(13)
6957.5(2)
0
19.3464(1) 18.5175(1)
1.2 19.3659(1) 18.5154(2)
2.0 19.3769(2) 18.5304(3)
Figure 6. a) Normalized photoluminescence solid state spectra of the
complexes [Eu(dbm)3(H2O)] ([Eu]), [KEu(dbm)4]n ([KEu]n),
[K(Me2CO)Eu(dbm)4]2 ([KEu]2) and [Eu4(dbm)10(OH)2] ([Eu4]) exemplified by
λexc = 415 nm. b) Normalized excitation spectra at λem = 612 nm
5
19.3974(2) 18.5458(3) 6978.07(17)
of Er in the samples, which is obviously due to a smaller size of
Yb(III) ion compared to Er(III). To reveal clear dependence
further studies are needed.
identical, only minor differences in the relative intensities of the
bands are revealed. Narrow bands at 464 nm, observed in all
recorded excitation spectra, appear due to the 5D0←7F2
transition 45. Triboluminescence was not detected in the
complexes obtained.
The following data on the photoluminescence properties of
complexes are described in the literature. The emission
spectrum of [Eu5(dbm)10(OH)5] was reported in the works 23, 24
Luminescent properties of the Eu complexes
The
complexes
[Eu(dbm)3(H2O)],
[Eu4(dbm)10(OH)2],
[KEu(dbm)4]n and [KEu(Me2CO)(dbm)4]2 in the solid state at
room temperature exhibit a characteristic Eu(III)-centered
;
5
luminescence (Fig. 6a), revealing the transitions D0→7F0 (578
the data confirm the low symmetry of the Eu centers. The
photoluminescence spectra of compounds “MEu(dbm)4” (M =
Li, Na, K) were recorded at 77K 38; apparently, the complexes
5
5
5
nm), D0→7F1 (591 nm), D0→7F2 (612 nm), D0→7F3 (650 nm)
and D0→7F4 (700 nm). Due to the diverse ligand environment
5
of Eu atom in the complexes, the fine structure of the bands and
their relative intensity differ 45. The most intense band is at 612
nm; in all compounds discussed, it is split with the exception of
[KEu(dbm)4]n, characterized by the most symmetrical ligand
environment of Eu. For the complexes with a lower symmetry
of the Eu environment, the band has two maxima (in the case
of [Eu(dbm)3(H2O)] and [KEu(Me2CO)(dbm)4]2) or multiple
splitting (in the case of [Eu4(dbm)10(OH)2]). The relative intensity
Table 3. Relative intensities of the bands and absolute quantum yields (Q) of the
luminescence of the Eu complexes. The excitation wavelengths, nm, are given in
parentheses
compound
[Eu(dbm)3(H2O)]
[Eu4(dbm)10(OH)2]
[KEu(dbm)4]n
I(5D0→7F2)/
I(5D0→7F0)
50 (290)
I(5D0→7F2)/
I(5D0→7F0)
50 (435)
Q, %
Q, %
<0.5
(290)
~1
(300)
14
<0.5
(435)
36 (300)
30 (425)
3 (425)
5
45
of the forbidden transition D0→7F0 also correlates with the
symmetry of the central atom: the highest for
[Eu4(dbm)10(OH)2] and the lowest for [KEu(dbm)4]n (Table 3). In
the excitation spectra of the complexes (Fig. 6b) at λem = 612
nm, two broadened bands are observed at ca. 300 and 420 nm.
The emission spectra recorded at these wavelengths are almost
–
1 (300)
–
1 (415)
37
(415)
42
(300)
16
[K(Me2CO)Eu(dbm)4]2
146 (300)
160 (415)
(300)
(415)
1 – The intensity of the band is at the noise level
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have the structure [KEu(dbm)4]n. The shape of the spectra
differs somewhat from that presented in our work by the
splitting of the main band, probably because of the lower
temperature. The spectrum of the complex ”MEu(dbm)4”
recorded at 300K in another work is almost identical to our
spectrum. In the same work, the spectrum of “Eu(dbm)3(H2O)”
Acknowledgements
View Article Online
DOI: 10.1039/C9NJ02059D
Dr. A.P. Zubareva and Dr. B.M. Kuchumov are gratefully
acknowledged for elemental analysis. Dr. A.A. Ryadun is
thanked for recording of the photoluminescence spectra. The
support provided by the students I.V. Lyakisheva and I.A.
Trofimov. We acknowledge the European Synchrotron
Radiation Facility for provision of synchrotron radiation
facilities.
14
is shown; it reveals the main band not split, probably because
5
of low resolution. In a later work, a split D0→7F2 band was
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demonstrated for [Eu(dbm)3(H2O)] 46. We determined the
absolute quantum yields (Q) for the Eu complexes obtained. The
values
for
the
complexes
[KEu(dbm)4]n
and
Notes and references
[KEu(Me2CO)(dbm)4]2 are significantly higher compared to
[Eu4(dbm)10(OH)2] and [Eu(dbm)3(H2O)] due to quenching of the
luminescence of the latter through O–H oscillations, which is
common for lanthanide complexes. For all compounds
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Schepers, S. Bräse and N. P. Kuzmina, J. Lumin., 2018, 202, 38-46.
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Eur. J. Inorg. Chem., 2014, 2014, 2849-2854.
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and P. W. Roesky, JACS, 2013, 135, 7454-7457.
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discussed, the higher
Q is observed using excitation
wavelengths of 415–435 nm than of 290 – 300 nm. This
phenomenon appears to be due to a higher probability of
luminescence quenching during energy transfer to Eu(III) from
higher energy levels of ligands compared to lower energy levels.
Conclusions
This work reports an overview of known procedures for the
synthesis of dibenzoylmethanide complexes of lanthanides,
which may contain water molecules or hydroxy ligands. Based
on the literature data and the experiments carried out in this
work, we can arrive at the following conclusions: using
methanol as a solvent, varying the base and the ratio of LnCl3
and Hdbm, it is possible selectively to obtain five types of
complexes:
[Ln(dbm)3(H2O)],
[Ln4(dbm)10(OH)2],
[Ln5(dbm)10(OH)5], [KLn(dbm)4]n and [KLn(Me2CO)(dbm)4]2. The
number of possible products is apparently incomplete, and
small changes in the reaction conditions and in-depth analysis
of the products can lead to novel species, e.g.
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Conflicts of interest
There are no conflicts to declare.
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New Journal of Chemistry
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Synthetic methods for dibenzoylmethanide lanthanide complexes as well as heterolanthanide ones
View Article Online
were systematized revealing several species.
DOI: 10.1039/C9NJ02059D
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