Lanthanoid complexes supported by retro-Claisen condensation products of β-triketonates

Article abstract of DOI:10.1039/c8dt03585g

β-Triketonates have been recently used as chelating ligands for lanthanoid ions, presenting unique structures varying from polynuclear assemblies to polymers. In an effort to overcome low solubility of the complexes of tribenzoylmethane, four β-triketones with higher lipophilicity were synthesised. Complexation reactions were performed for each of these molecules using different alkaline bases in alcoholic media. X-ray diffraction studies suggested that the ligands were undergoing decomposition under the reaction conditions. This is proposed to be caused by in situ retro-Claisen condensation reactions, consistent with two examples that have been reported previously. The lability of the lanthanoid cations in the presence of a varying set of potential ligands gave rise to structures where one, two, or three of the molecules involved in the retro-Claisen condensation reaction were linked to the lanthanoid centres. These results, along with measurements of ligand decomposition in the presence of base alone, suggest the solvent used will modulate the impact of the retro-Claisen condensation in these complexes.

Full text of DOI:10.1039/c8dt03585g

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PAPER
Lanthanoid complexes supported by retro-Claisen
condensation products of β-triketonates†
Cite this: Dalton Trans., 2018, 47,
17469
a,b
c
b
Laura Abad Galán,
Alexandre N. Sobolev, Eli Zysman-Colman,
*
a
a
Mark I. Ogden * and Massimiliano Massi
*
β-Triketonates have been recently used as chelating ligands for lanthanoid ions, presenting unique struc-
tures varying from polynuclear assemblies to polymers. In an effort to overcome low solubility of the
complexes of tribenzoylmethane, four β-triketones with higher lipophilicity were synthesised.
Complexation reactions were performed for each of these molecules using different alkaline bases in
alcoholic media. X-ray diffraction studies suggested that the ligands were undergoing decomposition
under the reaction conditions. This is proposed to be caused by in situ retro-Claisen condensa-
tion reactions, consistent with two examples that have been reported previously. The lability of the lantha-
noid cations in the presence of a varying set of potential ligands gave rise to structures where one, two,
or three of the molecules involved in the retro-Claisen condensation reaction were linked to the lantha-
noid centres. These results, along with measurements of ligand decomposition in the presence of base
alone, suggest the solvent used will modulate the impact of the retro-Claisen condensation in these
complexes.
Received 4th September 2018,
Accepted 13th November 2018
DOI: 10.1039/c8dt03585g
rsc.li/dalton
ettbmH (tris(4-ethylbenzoyl)methane), butbmH (tris(4-butyl-
benzoyl)methane) and t-butbmH (tris(4-tert-butylbenzoyl)
Introduction
A variety of strategies for the design of luminescent trivalent methane) (see Fig. 1). Given that the crystallised species are
lanthanoid compounds have been developed in the last few dependent on their specific solubility, the modification of the
decades, due to their potential use as materials for biological substituents may have an impact on products isolated from the
imaging, display applications, night vision devices and reaction mixture. Hence changes in the synthetic conditions
1
–7
telecommunication. β-Triketones are one of the most recent were investigated with the use of different solvents (methanol
ligand systems found to effectively sensitise the lanthanoid and ethanol) and different alkali salts (KOH, CsOH).
cations, with remarkable photophysical properties for the
near-infrared emitters, in particular. Furthermore, lanthanoid two cases of a possible in situ retro-Claisen condensation
In carrying out such work, it is important to note that
8
1
1
β-triketonate-based complexes present characteristic structures reaction were reported for both tbmH (tribenzoylmethane)
+
10
where discrete tetranuclear assemblies Ae Ln (where Ae is an and mtbmH (tris(4-methylbenzoyl)methane) molecules,
2
2
alkali metal cation) can be linked to form polymers {Ae Ln }
suggesting that the stability of the β-triketonates in basic etha-
2
2 n
9
,10
depending on the synthetic conditions.
The low solubility nolic solutions during complexation may be an issue. There is
of these complexes in non-polar solvents drove us to synthesise some precedent for a similar behaviour with the β-diketonates,
more lipophilic examples of β-triketones, and we report here including an example where retro-Claisen condensation gives
the syntheses of dmtbmH (tris(3,5-dimethylbenzoyl)methane), rise to a Ti/Sr complex of the diketonate and associated carbox-
1
2
ylate, but detailed studies of this phenomenon under con-
ditions relevant for metal complexation do not appear to have
a
School of Molecular and Life Sciences and Curtin Institute for Functional Molecules
and Interfaces, Curtin University, Kent Street, Bentley 6102, WA, Australia.
E-mail: m.massi@curtin.edu.au, m.ogden@curtin.edu.au
b
Organic Semiconductor Centre, EaStCHEM School of Chemistry,
University of St Andrews, St Andrews, Fife, KY16 9ST, UK.
E-mail: eli.zysman-colman@st-andrews.ac.uk
c
School of Molecular Sciences and CMCA, M310, University of Western Australia,
Crawley 6009, WA, Australia
†
Electronic supplementary information (ESI) available. CCDC 1846428–1846438.
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/c8dt03585g
1
Fig. 1 Chemical structures of ligands used for this investigation.
Dalton Trans., 2018, 47, 17469–17478 | 17469
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been carried out. In situ oxidation of dibenzoylmethane has Synthesis
been proposed to explain the presence of phenylglyoxylate in a
1
3
lanthanum cluster, but the persistence of the diketonate in
Synthesis of di(3,5-methylbenzoyl)methane (dmdbmH), di(4-
the triketonate systems mentioned above suggests that a retro- ethylbenzoyl)methane (ettbmH), di(4-butylbenzoyl)methane
Claisen condensation is the more plausible decomposition (butbmH) and di(4-tert-butylbenzoyl)methane (t-butbmH). The
pathway.
dmdbmH precursor was synthesised following the same pro-
1
4
We report here the isolation of different families of com- cedure that was previously reported for mdbmH. Ethyl 3,5-di-
plexes resulting from the reaction of these new β-triketones with methylbenzoate (3 mmol, 1 eq.) and 1-(3,5-dimethylphenyl)
lanthanoid cations. The results suggest that the impact of the ethanone (3 mmol, 1 eq.) were combined in 40 mL THF. To
retro-Claisen condensation reaction can be significant under the this solution, a suspension of NaH (60% in mineral oil,
reaction conditions used. Photophysical studies were conducted 9.0 mmol, 3 eq.) in 40 mL THF was added dropwise at 0 °C.
in the solid state to elucidate the effect of the composition and After one hour, the combined mixture was heated to 40 °C for
structure on the emission properties of the complexes.
16 hours under nitrogen atmosphere. The solvent was then
removed under reduced pressure and water (20 mL) was added
to the residual solid paste. The pH was adjusted approximately
to neutral by addition of HCl solution (1 M). The aqueous
phase was then extracted with ethyl acetate (3 × 15 mL) and
Experimental
General procedures
4
the combined organic phase was dried over MgSO . After fil-
All reagents and solvents were purchased from chemical sup- tration, the crude product was obtained by removal of the
pliers and used as received without further purification. solvent under reduced pressure and the target compound was
3
+
3+
3+
Hydrated LnCl (Ln = Eu , Er , Yb ) was prepared by the isolated, following purification via column chromatography in
3
reaction of Ln O with hydrochloric acid (5 M), followed by a mixture hexane : ethylacetate (90 : 10), as a yellow solid.
2
3
evaporation of the solvent under reduced pressure. Infrared
spectra (IR) were recorded on solid-state samples using an atte- stituting in the appropriate ester and ketone with yields of 60–80%.
A similar procedure was followed for the other precursors, sub-
1
5
nuated total reflectance PerkinElmer Spectrum 100 FT-IR. IR The spectroscopic data match those previously reported.
spectra were recorded from 4000 to 650 cm ; the intensities
−
1
Synthesis of dmtbmH, ettbmH, butbmH and t-butbmH
of the IR bands are reported as strong (s), medium (m), or (R-tbmH). mdtbmH: 3,5-Dimethylbenzoic acid (625 mg,
weak (w), with broad (br) bands also specified. Melting points 4.8 mmol) was added to thionyl chloride (5 mL) and heated at
were determined using a BI Barnsted Electrothermal 9100 reflux for 2 hours. After this time, the solvent was removed
apparatus. Elemental analyses were obtained at Curtin under reduced pressure and the remaining solid 3,5-dimethyl-
University, Australia. Nuclear magnetic resonance (NMR) benzoyl chloride was immediately added to diethyl ether
spectra were recorded using a Bruker Avance 400 spectrometer (20 mL). NaH (60% in mineral oil, 144 mg, 3.6 mmol) and
1
13
(
400.1 MHz for H; 100 MHz for C) at 300 K. The data were dmdbmH (300 mg, 1.2 mmol) were combined in 20 mL
acquired and processed by the Bruker TopSpin 3.1 software. All diethyl ether and the suspension was maintained at 0 °C. To
of the NMR spectra were calibrated to residual solvent signals.
this suspension, the solution of 3,5-dimethylbenzoyl chloride
in diethyl ether was added dropwise. After the addition, the
mixture was stirred under nitrogen atmosphere at 40 °C for
Selected equations
1
6 hours. The formed precipitate was filtered and washed with
R
The values of the radiative lifetime (τ ), and intrinsic quantum
Ln
29
a HCl solution (1 M). The solid was then dried under reduced
pressure for several hours, and afforded the target compound
as a white solid. Yield 88%. M.p. 194–196 °C. Elemental ana-
yield (Φ ), can be calculated with the following equations.
Ln
1
τR
ITot
IMD
ꢀ
1
3
¼
14:65s ꢁ n ꢁ
ð1Þ
lysis calcd (%) for C28
28 3
H O : C, 81.52; H, 6.84; found: C, 81.66;
H, 6.77. IR (ATR): ν = 2918 w, 2863 w, 1735 s, 1661 s, 1640 s,
In eqn (1), the refractive index (n) of the solvent is used
−
1
1
1
1
8
596 m, 1444 m, 1381 m, 1357 w, 1326 m, 1298 s, 1242 m,
197 w, 1175 s, 1161 s, 1125 m, 1100 s, 1082 m, 1067 m,
(
assumed value of 1.5 in the solid state). The value 14.65 s is
7
5
1 0
the spontaneous emission probability of the F ← D tran-
039 m, 1011 w, 995 w, 947 w, 929 m, 895 m, 867 m, 848 m,
sition reported previously. I
is the total integration of the
Tot
1
3
+
12 m, 772 m, 748 m, 713 m. H-NMR (400 MHz, CDCl ) δ 7.54
Eu emission spectrum, and I
is the integration of the
3
MD
7
5
(
1
dt, J = 1.6, 0.8 Hz, 6H), 7.24 (dt, J = 1.6, 0.8 Hz, 3H), 7.08 (s,
H), 2.36 (s, 18H). C-NMR (101 MHz, CDCl ) δ 192.67, 138.61,
3
1 0
F ← D transition.
1
3
τobs
Ln
Ln
Φ
¼
ð2Þ 135.59, 126.46, 66.14, 21.27.
A similar procedure was followed for the ligands, substitut-
The sensitisation efficiency (ηsens) can be determined using ing in the appropriate ester and ketone.
eqn (3) below:
ettbmH: Yield 60%. M.p. 210–212 °C. Elemental analysis
calcd (%) for C28 : C, 81.52; H, 6.84; found: C, 81.59; H,
τR
28 3
H O
L
Φ
Φ
Ln
Ln
Ln
ηsens
¼
ð3Þ 6.84. IR (ATR): ν = 3058 w, 2967 w, 1668 s, 1629 m, 1603 s,
1571 m, 1509 w, 1464 m, 1412 m, 1355 m, 1287 s, 1214 m,
17470 | Dalton Trans., 2018, 47, 17469–17478
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1
8
181 s, 1122 m, 1059 w, 1019 w, 1007 m, 961 w, 928 w, 909 w,
[Eu(dmdbm)
4
(Cs)]
n
(5). M.p. 267–269 °C. Elemental analysis
1
63 s, 822 s, 741 m. H-NMR (400 MHz, CDCl ) δ 7.86 (d, calcd (%) for C H CsEuO : C, 65.10; H, 5.46; found: C, 4.77;
3
76 76
8
J = 8.4 Hz, 6H), 7.27 (d, J = 8.6 Hz, 6H), 7.08 (s, 1H), 2.70 (q, J = H, 5.08. IR (ATR): ν = 2912 w, 1551 s, 1503 s, 1459 s, 1402 s,
1
3
7
.6 Hz, 6H), 1.24 (t, J = 7.6 Hz, 9H). C-NMR (101 MHz, CDCl
δ 191.97, 151.19, 133.71, 129.16, 128.61, 77.16, 66.41, 29.13,
5.20.
3
)
1362 m, 1317 m, 1211 m, 1157 m, 944 w, 857 w, 781 s.
[Yb(dmdbm) (K)] (6). M.p. 243–245 °C. Elemental analysis
4
n
1
calcd (%) for C76
H
8 2
76KO Yb·(H O): C, 67.74; H, 5.83; found: C,
butbmH: Yield 55%. M.p. 260–262 °C. Elemental analysis 67.53; H, 5.73. IR (ATR): ν = 2914 w, 1551 m, 1508 s, 1459 m,
calcd (%) for C H O ·H O: C, 79.34; H, 8.22; found: C, 79.44; 1384 m, 1323 m, 1210 m, 1159 m, 1103 w, 996 w, 856 m, 782 s,
3
4
40
3
2
H, 7.89. IR (ATR): ν = 2929 m, 2957 m, 2859 m, 1930 w, 1688 s, 730 w.
1
1
1
7
7
667 s, 1604 s, 1571 m, 1508 w, 1459 w, 1433 w, 1412 m,
377 w, 1038 m, 1284 s, 1261 m, 1213 m, 1181 s, 1120 w, 1006 s, lysis calcd (%) for C H O Yb·(2.5H O): C, 64.02; H, 6.11;
3 2
[Yb(etdbm) (HOMe)] (7). M.p. 249–251 °C. Elemental ana-
5
8
61
7
2
011 m, 960 w, 931 w, 910 w, 860 s, 832 m, 819 w, 778 w, 730 w, found: C, 63.73; H, 5.84. IR (ATR): ν = 2965 w, 1586 m, 1520 s,
1
43 w. H-NMR (400 MHz, CDCl
3
) δ 7.85 (d, J = 8.3 Hz, 6H), 1489 s, 1422 s, 1379 s, 1309 s, 1225 m, 1182 s, 1112 w, 1063 s,
.08 (s, 1H), 2.73–2.57 (m, 6H), 1.67–1.54 (m, 7H), 1.44–1.29 1016 w, 937 w, 849 w, 792 m, 717 w.
1
3
(
m, 6H), 0.92 (t, J = 7.3 Hz, 9H). C-NMR (101 MHz, CDCl
δ 191.79, 149.78, 133.55, 128.98, 66.25, 35.72, 33.10, 22.31, tal analysis was variable probably due to co-precipitation of
3.87. multiple species. IR (ATR): ν = 2966 w, 1590 s, 1541 m, 1523 s,
t-butbmH: Yield 88%. M.p. 267–269 °C. Elemental analysis 1489 s, 1447 s, 1390 m, 1308 m, 1224 m, 1182 m, 1110 m,
calcd (%) for C34 : C, 82.22; H, 8.12; found: C, 81.91; H, 11 063 m, 1017 m, 941 w, 850 m, 792 m, 715 w.
.15. IR (ATR): ν = 2961 w, 2919 w, 2866 w, 1736 m, 1697 m, [Eu(ettbm) (Cs·HOEt)] (9). M.p. 267–269 °C. The elemental
661 m, 1639 m, 1600 s, 1473 m, 1406 w, 1380 w, 1357 w, 1298 analysis was variable probably due to co-precipitation of multiple
3
)
4 n
[Yb(etdbm) (Cs·OHMe)] (8). M.p. 211–213 °C. The elemen-
1
40 3
H O
8
1
4
2
s, 1242 s, 1188 m, 1176 m, 1161 m, 1125 m, 1102 s, 1067 w, species. IR (ATR): ν = 2964 w, 1637 w, 1604 w, 1577 m, 1540 s,
1
7
1
008 m, 948 w, 929 w, 909 w, 895 w, 861 s, 847 m, 812 m, 1452 w. 1412 m, 1365 s, 1307 m, 1277 m, 1184 w, 1152 m,
1
48 m, 713 m. H-NMR (400 MHz, CDCl
3
) δ 7.88 (d, J = 8.7 Hz, 1116 w, 1080 w, 1020 w, 964 w, 900 s, 837 m, 796 m, 772 w.
1
3
H), 7.47 (d, J = 8.7 Hz, 1H), 7.13 (s, 1H), 1.32 (s, 5H). C-NMR
4 n
[Eu(budbm) Cs] (10). M.p. 214–216 °C. Elemental analysis
(
101 MHz, CDCl ) δ 191.91, 157.92, 133.35, 130.80, 128.92, calcd (%) for C H CsEuO : C, 67.93; H, 6.69; found: C,
3
92 108
8
1
26.10, 125.40, 77.16, 66.25, 35.37, 31.14.
67.63; H, 6.75. IR (ATR): ν = 2925 w, 1590s, 1520 s, 1496 s, 1441
s, 1397 m, 1303 m, 1223 m, 1183 m, 1110 w, 1018 w, 939 w,
General procedure for the synthesis of lanthanoid complexes
850 m, 772 s, 710w.
AeOH (Ae = K, Cs) (4 eq.) was added to a mixture containing
Crystallography
mdtbmH, ettbmH, or butbmH (4 eq.) and hydrated LnCl
ca. 20 mg) in ethanol or methanol (10 mL). The mixture was Crystallographic data for the structures were collected at 100(2) K
3
(
heated at reflux for 30 minutes and filtered over a glass frit on an Oxford Diffraction Gemini or Xcalibur diffractometer fitted
while still hot. The filtered solution was then left undisturbed with Mo-Kα or Cu-Kα radiation. Data were corrected for Lorentz
at ambient temperature and slow evaporation of the solvent and polarisation effects and absorption correction. The structures
over several days afforded crystals suitable for X-ray diffraction. were solved by direct methods and refined by full-matrix least-
2
30
[Eu(dmba)(dmdbm)(dmtbm)HOEt]
2
(1). M.p. 147–149 °C. squares on F using the SHELX-2014 crystallographic package.
Elemental analysis calcd (%) for C116H122Eu O ·(1.5H O): C, Unless stated below, anisotropic displacement parameters were
2
16
2
6
1
1
6.25; H, 5.99; found: C, 65.88; H, 5.80. IR (ATR): ν = 2916 w, employed for the non-hydrogen atoms. All hydrogen atoms were
534 s, 1505 s, 1426 m, 1349 s, 1318 s, 1269 m, 1213 m, added at calculated positions and refined by use of a riding
161 w, 1131 w, 1089 w, 1038 w, 959 w, 860 m, 790 s, 765 m.
model with isotropic displacement parameters based on those of
[
Tb(µ-dmba)(dmdbm)(dmtbm)HOEt] (2). M.p. 146–148 °C. the parent atom. Mercury 3.10.1 was used to visualise, analyse
2
16
The elemental analysis was variable probably due to co-precipi- and generate all the graphics included in this investigation.
tation of multiple species. IR (ATR): ν = 2916 w, 1535 s, 1507 s,
X-ray data refinement
1
1
425 m, 1351 s, 1320 s, 1269 m, 1212 m, 1161 w, 1131 m,
021 w, 946 w, 859 m, 789 s, 765 m.
[Eu(µ-dmba)(dmdbm)(dmtbm)HOEt]
2 2 164
(1). C116H122Eu O
[
Yb(µ-dmba)(dmdbm) HOMe]
(3). M.p. 288–290 °C. (C H O), M = 2260.32, pale yellow prism, 0.16 × 0.11 ×
2 6
2
2
3
Elemental analysis calcd (%) for C97
H
106
O
15Yb
2
·(3.5H
2
O): C, 0.08 mm , monoclinic, space group P21/n (no. 14), a =
6
1
7
0.78; H, 5.73; found: C, 60.49; H, 5.38. IR (ATR): ν = 2914 w, 15.0498(1), b = 20.7837(1), c = 19.1261(2) Å, β = 110.583(1)°,
3
−3
−1
505 s, 1372 s, 1326 m, 1270 m, 1216 m, 998 w, 946 w, 859 w, V = 5600.56(7) Å , Z = 2, D = 1.340 g cm , μ = 8.475 mm
.
c
88 s, 750 m.
Yb(µ-dmdbm)(dmdbm)
analysis calcd (%) for C114H114O Yb : C, 67.71; H, 5.68; (Rint = 0.0485). Final GooF = 1.000, R1 = 0.0289, wR2 = 0.0667,
F
000 = 2352, CuKα radiation, λ = 1.54178 Å, T = 100(2) K,
[
2 2
] (4). M.p. 310–312 °C. Elemental 2θmax = 134.6°, 61 717 reflections collected, 10 001 unique
12
2
found: C, 67.29; H, 5.46. IR (ATR): ν = 2914 w, 1552 s, 1508 s, R indices based on 8766 reflections with I > 2σ(I) (refinement
2
−3
1
9
459 s, 1373 s, 1312 m, 1271 m, 1210 m, 1160 m, 1103 w, on F ), |Δρ|max = 0.68(6) e Å , 697 parameters, 14 restraints.
97 w, 946 w, 856 m, 783 s, 730w. CCDC 1846430.†
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[
Tb(µ-dmba)(dmdbm)(dmtbm)HOEt]
2
(2). C116
H
122Tb
2
O
164 unique (Rint = 0.0415). Final GooF = 1.002, R1 = 0.0449, wR2 =
3
(
C H O), M = 2274.25, colorless prism, 0.17 × 0.14 × 0.10 mm , 0.1146, R indices based on 28 533 reflections with I > 2σ(I)
2
6
2
−3
monoclinic, space group P21/n (no. 14), a = 15.0467(1), b = (refinement on F ), |Δρ|max = 3.9(1) e Å , 1206 parameters, 13
0.8306(1), c = 19.1237(2) Å, β = 110.933(1)°, V = 5598.36(7) Å , restraints. CCDC 1846438.†
Z = 2, Dc = 1.349 g cm , μ = 6.671 mm . F000 = 2360,
CuKα radiation, λ = 1.54178 Å, T = 100(2) K, 2θmax = 134.6°, colourless needle, 0.24 × 0.19 × 0.08 mm , monoclinic, space
5 865 reflections collected, 9972 unique (Rint = 0.0311). group C2 (no. 5), a = 30.1708(10), b = 8.3962(1), c = 14.9290(9) Å,
Final GooF = 1.002, R1 = 0.0241, wR2 = 0.0607, R indices β = 118.445(4)°, V = 3325.3(2) Å , Z = 2, D = 1.486 g cm , μ =
based on 9188 reflections with I > 2σ(I) (refinement on F ), 2.006 mm . F000 = 1442, MoKα radiation, λ = 0.71073 Å, T =
Δρ|max = 0.46(5) e Å , 715 parameters, 8 restraints. CCDC 100(2) K, 2θmax = 65.4°, 18 749 reflections collected, 18 749
3
2
−
3
−1
[Yb(etdbm) (Cs·OHMe)] (8). C H CsO Yb, M = 1487.40,
4 n 78 84 10
3
5
3
−3
c
2
−1
−
3
|
1
846434.†
unique (Rint = 0.0000). Final GooF = 1.000, R1 = 0.0368, wR2 =
[
Yb(µ-dmba)(dmdbm)
2
HOMe]
2
97 106 2
(3). C H O15Yb , M =
0.1007, R indices based on 17 754 reflections with I > 2σ(I)
3
2
−3
1
857.90, pale yellow plate, 0.30 × 0.27 × 0.12 mm , monoclinic, (refinement on F ), |Δρ|max = 5.5(1) e Å , 414 parameters, 1
space group P2 /c (no. 14), a = 18.0271(3), b = 14.4367(2), c = restraint. Absolute structure parameter = 0.010(5) The struc-
1
7
1
3
3
1
1
4.8079(3) Å, β = 102.091(1)°, V = 8857.9(2) Å , Z = 4, D
c
=
ture refined as a 2-component twin. Component 2 rotated by
.393 g cm− , μ = 4.306 mm . F000 = 3792, CuKα radiation, λ = −179.9953° around [1.00 0.00 0.01] (reciprocal) or [0.71 0.00 0.71]
3
−1
.54178 Å, T = 100(2) K, 2θmax = 135.4°, 26 738 reflections col- (direct) direction. CCDC 1846429.†
lected, 26 738 unique (Rint = 0.0000). Final GooF = 1.251, R1 =
[Eu(ettbm)
4
(Cs·HOEt)]
2
(9). C228
H
228Cs
2 2
Eu O26, M = 3953.84,
3
0
.0696, wR2 = 0.1772, R indices based on 25 451 reflections colourless needle, 0.25 × 0.09 × 0.06 mm , triclinic, space group
2
−3
with I > 2σ(I) (refinement on F ), |Δρ|
= 2.0(1) e Å , 1050 P ˉ1 (no. 2), a = 17.1680(5), b = 17.4332(4), c = 18.4379(3) Å, α =
max
3
parameters, 7 restraints. CCDC 1846433.†
88.513(2), β = 80.968(2), γ = 65.713(2)°, V = 4963.0(2) Å , Z = 1,
D
c
= 1.323 g cm− , μ = 7.829 mm . F000 = 2040, CuKα
3
−1
[
Yb(µ-dmdbm)(dmdbm)
2
]
2
(4). C114
H
114
O
12Yb
2
, M = 2022.13,
3
yellow needle, 0.38 × 0.25 × 0.08 mm , monoclinic, space group radiation, λ = 1.54178 Å, T = 100(2) K, 2θmax = 134.6°, 109 457
P2
1
/n (no. 14), a = 17.1570(2), b = 14.8841(1), c = 19.6698(2) Å, reflections collected, 17 676 unique (Rint = 0.0557). Final
3
−3
β = 108.494(1)°, V = 4763.60(8) Å , Z = 2, D
c
= 1.410 g cm , μ = GooF = 1.034, R1 = 0.0707, wR2 = 0.1979, R indices based on
.014 mm− . F000 = 2068, MoKα radiation, λ = 0.71073 Å, T = 14 055 reflections with I > 2σ(I) (refinement on F ), |Δρ|
1
2
=
max
2
1
00(2) K, 2θmax = 75.3°, 161 830 reflections collected, 24 607 1.57(9) e Å− , 1049 parameters, 732 restraints. CCDC 1846436.†
3
unique (Rint = 0.0543). Final GooF = 1.003, R1 = 0.0391, wR2 =
[Eu(budbm)
4
Cs]
n
(10). C92
H
108CsEuO
8
, M = 1626.65, colour-
3
0
.0841, R indices based on 18 359 reflections with I > 2σ(I) less plate, 0.38 × 0.25 × 0.18 mm , monoclinic, space group I2
2
−3
(refinement on F ), |Δρ|max = 5.0(1) e Å , 589 parameters, (no. 5), a = 21.3014(2), b = 8.0066(1), c = 23.7700(3) Å, β =
3
−3
0
restraints. CCDC 1846437.†
99.984(1)°, V = 3992.62(8) Å , Z = 2, D
c
= 1.353 g cm , μ =
Eu(dmdbm) (Cs)] (5). C H CsEuO , M = 1402.23, colour- 1.291 mm− . F000 = 1684, MoKα radiation, λ = 0.71073 Å, T =
1
[
4
n
76 76
8
3
less prism, 0.25 × 0.20 × 0.15 mm , orthorhombic, space group 100(2) K, 2θmax = 64.1°, 42 702 reflections collected, 13 119
Pccn (no. 56), a = 14.7842(3), b = 29.7258(5), c = 7.5735(2) Å, V = unique (Rint = 0.0238). Final GooF = 1.004, R1 = 0.0277, wR2 =
3
−3
−1
3
1
6
328.34(12) Å , Z = 2, D = 1.399 g cm , μ = 1.536 mm . F000
=
=
0.0688, R indices based on 12 624 reflections with I > 2σ(I)
c
2
−3
428, MoKα radiation, λ = 0.71073 Å, T = 100(2) K, 2θmax
4.9°, 65 961 reflections collected, 5842 unique (Rint = 0.0393). 1 restraint. Absolute structure parameter = 0.014(7) CCDC
(refinement on F ), |Δρ|max = 1.89(8) e Å , 465 parameters,
17
Final GooF = 1.002, R1 = 0.0505, wR2 = 0.1589, R indices based 1846428.†
2
on 4609 reflections with I > 2σ(I) (refinement on F ), |Δρ|max
6
=
t-butbmH. C34
H
40
O
3
, M = 496.66, colourless needle, 0.28 ×
0.08 × 0.03 mm , monoclinic, space group P21/n (no. 14), a =
Yb(dmdbm) (K)] (6). C H KO Yb, M = 1329.51, colourless 12.7501(3), b = 10.0565(2), c = 22.7503(5) Å, β = 97.672(2)°, V =
.1(2) e Å− , 222 parameters, 13 restraints. CCDC 1846431.†
3
3
[
4
n
76 76
8
3
3
−3
−1
needle, 0.07 × 0.03 × 0.02 mm , orthorhombic, space group 2890.96(11) Å , Z = 4, D
Pccn (no. 56), a = 14.6365(7), b = 29.8941(14), c = 7.4973(7) Å, 000 = 1072, CuKα radiation, λ = 1.54178 Å, T = 100(2) K, 2θmax
= 134.6°, 25 300 reflections collected, 5149 unique (Rint
000 = 1370, CuKα radiation, λ = 1.54178 Å, T = 100(2) K, 0.0570). Final GooF = 1.002, R1 = 0.0438, wR2 = 0.1048,
max = 134.6°, 22 647 reflections collected, 2928 unique (Rint R indices based on 3870 reflections with I > 2σ(I) (refinement
.1196). Final GooF = 1.009, R1 = 0.0851, wR2 = 0.2296, on F ), |Δρ|max = 0.21(4) e Å , 343 parameters, 0 restraints.
c
= 1.141 g cm , μ = 0.553 mm .
F
3
−3
−1
V = 3280.4(4) Å , Z = 2, D = 1.346 g cm , μ = 3.641 mm
.
=
c
F
2
0
θ
=
2
−3
R indices based on 1781 reflections with I > 2σ(I) (refinement CCDC 1846435.†
2
−3
on F ), |Δρ|max = 3.8(2) e Å , 222 parameters, 13 restraints.
CCDC 1846432.†
[
Yb(etdbm)
3 2 61 7
(HOMe)] (7). C58H O Yb, M = 1043.10, yellow
Results and discussion
Synthesis of the lanthanoid assemblies
3
needle, 0.31 × 0.23 × 0.17 mm , monoclinic, space group P2
no. 14), a = 27.6844(3), b = 12.2823(1), c = 30.1388(3) Å, β =
1
/c
(
3
−3
1
1
1
05.316(1)°, V = 9884.06(17) Å , Z = 8, D
c
= 1.402 g cm , μ = The dmtbmH, ettbmH, butbmH and t-butbmH compounds
.945 mm− . F000 = 4280, MoKα radiation, λ = 0.71073 Å, T = were synthesised following the procedure previously reported
1
8
,10
00(2) K, 2θmax = 64.9°, 204 134 reflections collected, 33 939 for tbm and mtbm.
When the ligands were made to react
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3
+
with hydrated LnCl
3
(Ln = Eu, Tb, Yb) in the presence of
Finally, when analogous complexation reactions were fol-
+
AeOH (Ae = K, Cs) in ethanol or methanol, species containing lowed for the butbmH and t-butbmH molecules, only one crys-
4 n
products of the retro-Claisen condensation reaction of the tri- talline material was isolated (see Fig. 4), [Eu(budbm) (Cs)] (10).
ketonate molecules were found. The isolated complexes were Less polar solvents were also tested for these ligands.
identified by X-ray diffraction and confirmed by means of IR Unfortunately, when one equivalent of Ln(NO ) ·DMSO
n
3
3
3
+
3+
3+
spectroscopy and elemental analysis when possible as co-pre- (Ln = Eu , Yb ) was reacted with four equivalents of the
cipitation of multiple species was observed in several cases t-butbmH in the presence of four equivalents of potassium
(
see Experimental).
The two retro-Claisen condensation products of the tallised (see ESI†).
β-triketone are the associated β-diketonate and benzoate
tert-butoxide in acetonitrile, only the t-butbmH molecule crys-
1
8
NMR studies
anions. Consequently, three potential ligands can be present
in the complexation mixture: β-triketonates, β-diketonates and To further elucidate the stability of the β-triketonate molecules
1
benzoates.
in basic alcoholic conditions, H-NMR studies were first per-
In the case of dmtbmH, the isolated complexes can be formed in the absence of the lanthanoid cations. The different
classified in three distinct families depending on the number solutions, prepared in d -methanol by addition of the corres-
4
−
2
of different species directly coordinated to the lanthanoid ponding β-triketonate and 1 equivalent of KOH (10 M), were
1
centre (see Fig. 2):
monitored by H-NMR every 24 hours. In every case, mixtures
of β-triketonates and β-diketonates, in both tautomeric forms,
and their corresponding benzoates could be detected in solu-
tion over time, resulting in convoluted spectra.
(
a) Three species: [Ln(µ-dmba)(dmdbm)(dmtbm)(HOEt)] ,
2
3
+
where Ln = Eu (1), Tb (2)
(b) Two species: [Yb(µ-dmba)(dmdbm) (HOMe)] (3)
2 2
(
c)
One
species:
[Yb(µ-dmdbm)(dmdbm)
2
]
2
(4),
The ratio between the integration of the aromatic proton,
for the β-triketonate, and the constant integration of the
3
+
+
[
Ln(dmdbm)
4
(Ae)] where Ln = Eu, Yb, Ae = Cs, K (5, 6).
n
A similar behaviour was found for the ettbmH molecule solvent peak, was plotted against time to quantify the pro-
with the isolated complexes confirming retro-Claisen conden- gression of the retro-Claisen condensation in these systems.
sation reactivity; [Yb(etdbm) (HOMe)] (7), [Yb(etdbm) (Cs)] High levels of decomposition were noted for all triketone
8). Furthermore, one structure was identified to contain only molecules, with almost 80% lost in the case of ettbmH after 5
the unreacted β-triketonate ligand, [Eu(ettbm) (Cs·HOEt)] (9), days (Fig. 5). The most stable β-triketonate is tbmH with 30%
following the formula previously reported for the tetranuclear lost after 5 days, which may explain why retro-Claisen conden-
3
4
n
(
4
2
9
assemblies (see Fig. 3).
sation of this molecule was not observed in earlier work on the
Fig. 2 Summary of the isolated assemblies according to reaction conditions for the dmtbmH molecule.
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Fig. 3 Summary of the isolated assemblies according to reaction conditions for the ettbmH molecule.
ties in crystallising the resulting complexes rather than their
initial formation.
As the least stable of the triketonates examined, ettbmH
was then tested in d -ethanol solution under similar con-
6
ditions. In this case, it was found that the stability is signifi-
cantly enhanced with only 2% of ettbm lost over the same
time and therefore there is negligible appearance of the retro-
Claisen condensation products after 5 days. These results are
in agreement with the experimentally isolated structures from
ethanol, which all included unreacted β-triketonate either
exclusively, or as part of a ligand mixture.
Fig. 4 Summary of the isolated assemblies according to reaction con-
ditions for the butbmH molecule.
Finally, similar studies were performed in the presence of
base and the lanthanoid cations. Retro-Claisen condensation
seemed to be significantly more rapid in the presence of the f
elements, which results in highly convoluted spectra that were
not further analysed (see ESI†).
Crystal structures
Products of the reaction of dmtbmH with lanthanoids, Ln =
3
+
3+
3+
Eu , Tb , Yb . The structures for the [Ln(µ-dmba)(dmdbm)
3
+
(
2
dmtbm)(HOEt)] Ln = Eu (1), Tb (2) are isomorphous and
3
+
can be described as a dimer formed by two Ln , two
β-triketonates, two β-diketonates and two bridging benzoates.
Each metal centre is seven coordinated, with one β-triketonate
and one β-diketonate in a bidentate mode, two monodentate
Fig. 5 Ratio of retro-Claisen condensation reactivity over the time for benzoate ligands bridging the lanthanoid centres and one
the tbm (red trace), mtbm (blue trace), dmtbm (black trace), ettbm
molecule of ethanol completing the coordination sphere
(
purple trace), butbm (green trace), t-butbm (orange trace) molecules
3+
(
Fig. 6). The two Ln cations are equivalent, being related by
all in d-MeOH (squares) and d-EtOH (circles).
an inversion centre and separated by distances of Eu, 4.051 Å,
and Tb, 4.021 Å. Each structure is best described as a capped
octahedron (see ESI†).
9
,19
triketonates.
Finally, these results show that butbmH and
A similar structure was found in the case of [Yb(µ-dmba)
t-butbmH undergo the retro-Claisen condensation to similar (dmdbm) (HOMe)] (3) where the dimer is formed by two
2
2
3
+
extents. Therefore, the fact that examples of complexes based seven coordinated Yb and four β-diketonates, rather than
on these ligands have yet to be isolated may be related difficul- two β-triketonates and two β-diketonates (Fig. 6). Two benzoate
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2 2 2
Fig. 6 Representation of the X-Ray crystal structure of (a) [Eu(µ-dmba)(dmdbm)(dmtbm)(HOEt)] , (b) [Yb(µ-dmba)(dmdbm) (HOMe)] and
(
2 2
c) [Yb(µ-dmdbm)(dmdbm) ] where hydrogens bonds are highlighted in blue. Hydrogens have been omitted for clarity.
3
+
ligands bridge the two Yb centres and the coordination
sphere is completed by a coordinating molecule of methanol.
3
+
In this case, the coordination sphere of the two Yb cations is
slightly different with differing degrees of distortion from a
capped octahedron geometry (see ESI†).
A different dinuclear complex was found for the [Yb(µ-
dmdbm)(dmdbm)
2 2
] (4) where only the β-diketonate ligand is
present in the structure (Fig. 6). This arrangement is analo-
3
+
gous to the previously published structure formed by Gd and
2
0
3+
2
,2,6,6-tetramethylheptane-3,5-dione. In this case, each Yb
is seven-coordinated, where the main ligand bridges the two
lanthanoid centres with no molecules of solvent directly co-
3
+
3+
ordinated. The Yb ⋯Yb distance is slightly shorter in this Fig. 7 Representation
of
the
X-ray
crystal
structure
of
[
Yb(etdbm) (HOMe)] . Hydrogen atoms have been omitted for clarity.
3 2
structure, 3.691 Å, in comparison to the mixed ligand dimers.
The structure is best described as a distorted capped trigonal
prism.
Because lanthanoid-lanthanoid distances in complexes 1–4
are quite short, lanthanoid-lanthanoid cross relaxation
quenching should be considered, when studying photo-
2
4
diketonate polymers (5, 6)
and reported triketonate-
1
9
supported tetranuclear assemblies, respectively (see ESI†).
Products of the reaction of butbmH with lanthanoids, Ln =
Eu . The structure of the [Eu(budbm) (Cs)] (10) presents a
similar linear polymeric structure to that already described for
2
1–23
physical properties.
Finally, complexes 5, [Eu(dmdbm) (Cs)] and 6, [Yb
3
+
4
n
4
n
4 n
(dmdbm) (K)] were identified as linear coordination poly-
the dmtbmH and ettbmH complexes (see ESI†).
mers, following a similar formula to the previously reported
2
4
structures with dibenzoylmethane (see ESI†).
Products of the reaction of ettbmH with lanthanoids, Ln =
Photophysical investigation
3
+
3+
Eu , Yb . Complex 7 crystallises as a hydrogen bonded dimer The energy of the triplet state of the β-triketonates and their
involving only the β-diketonate ligand and solvent molecules, corresponding β-diketonates were measured at the 0-phonon
3
+
formulated as [Yb(etdbm)
3
(HOMe)], where each Yb is seven- transition of the phosphorescent emission at 77 K of their
3
+
coordinated with three β-diketonates binding in a bidentate corresponding Gd complexes. These energies were found to
−
1
mode and an oxygen atom from a molecule of solvent metha- lie close in energy in the region 20 202–20 844 cm , indepen-
nol (Fig. 7). The dimer is situated about an inversion centre dently of the specific substituent (see ESI†), as expected.
and is linked by hydrogen bonds between the methanol O Therefore, these ligands would be expected to effectively sensi-
3
+
5
−1
3+
2
−1
atom and keto-O (O22). The resulting pair of lanthanoid ions tise Eu ( D = ∼17 200 cm ) and Yb ( F = ∼10 200 cm
)
0
5/2
are relatively close in distance, 5.704 Å, which could cause excited states.
quenching by energy transfer between lanthanoid excited
The emission properties for complexes where the bulk of
states. The coordination geometry is best described as a dis- the sample was confirmed to be pure by elemental analysis
torted capped trigonal prism.
Complexes 8 ([Yb(etdbm)
(1, 3, 4, 5, 6, 7 and 10) were recorded in the solid state. In every
case, the emission was seen as a consequence of the antenna
4
n
(Cs·OHMe)] ) and 9 ([Eu(ettbm)
4
(
Cs·OHEt)] ) are analogous to the previously described linear effect, evidenced by the broad excitation spectra similar to the
2
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absorption spectra of the ligands (see ESI†). The photophysical β-diketonates complexes, suggesting efficient non-radiative
data, including excited state lifetime decay (τobs), calculated decay pathways. The main quenching process might be energy
R
radiative decay (τ ), intrinsic photoluminescence quantum transfer between the two lanthanoid centres as they are separ-
yield (Φ ) and overall quantum yield (Φ LL n) are reported in ated by a short distance of ∼4 Å. From the Eu spectrum, the
Ln
3+
Ln
Tables 1 and 2.
value of radiative lifetime can be calculated to be 0.78 ms,
Visible light emitters. The emission spectrum of complex 1, giving an intrinsic quantum yield value of 6%. The overall
3
+
7
5
shows the characteristic peaks of Eu , attributed to the F
J
← D
0
quantum yield was measured using an integrating sphere and
2
5
(
J = 0–4) in the region 550–750 nm (Fig. 8). The presence of was found to be 2%, which implies a poor sensitisation
F ← D peak at 580 nm (full width at half efficiency of 33%. In fact, some direct excitation can be
0 0
maximum ∼40 cm ) is an indication of a unique low sym- observed in the excitation plot for emission recorded at
metry Eu centre. This is in agreement with the crystal struc- 612 nm (see ESI†).
ture where the two Eu cations present in the dimer are
related by an inversion centre. The splitting of the magnetic the region 580–750 nm (Fig. 8). The F
dipole ( F ← D ) and the hypersensitive transition ( F ← D ) absent in both cases, suggesting that the Eu ion occupies a
7
5
the narrow
−
1
3
+
3
+
3+
Complexes 5 and 10 show characteristic Eu emission in
7
5
0
← D
0
transition is
7
5
7
5
3+
1
0
2
0
into two and three bands, respectively, is suggestive of trigonal site with higher symmetry than complex 1. The splitting of the
7
5
fields. This result is supported by shape analysis where the magnetic dipole transition, F
1
← D
0
, and the hypersensitive
7
5
geometry is best described as capped octahedron (trigonal, band, F ← D , is slightly different which indicates different
2
0
C3v) (see ESI†). The excited state lifetime decay was satisfac- degrees of distortion between the two coordination spheres.
torily fitted with a monoexponential function and was These trends are confirmed by the results found in the shape
measured to be 47 µs (see ESI†), shorter than analogous analysis (see ESI†). The excited state lifetimes decay values
(τobs) were measured to be much longer than complex 1 with
values of 815 and 485 µs, respectively. These data suggest that
lanthanoid cross relaxation quenching is not effective in the
linear polymers (5, 10) where the Eu⋯Eu distance is longer
than 8 Å.
3
+
Table 1 Photophysical data for the Eu complexes in the solid state
Ln
Ln
L
Complex
τobs (µs)
τ
R
(ms)
Φ
(%)
Φ
(%)
Ln
Ln
The intrinsic quantum yields (Φ ) were found to be 68%
Ln
1
5
47
815
485
0.79
1.19
0.96
6
68
50
2
45
43
and 50%, respectively, based on the radiative lifetimes calcu-
lated from the emission spectra. Overall quantum yields were
measured to be 45 and 43%, respectively. The photophysical
data for the two complexes are therefore comparable, showing
high sensitisation efficiency in both cases.
10
3
+
Table 2 Photophysical data for the Yb complexes in the solid state
3
+
NIR light emitters. Characteristic Yb NIR emission for
complexes 3, 4, 6 and 7 was observed in the 900–1100 nm
a
R
Ln
Ln
Complex
τ
obs (µs)
τ
(ms)
Φ
(%)
2
2
region corresponding to the F7/2 ← F5/2 transition (Fig. 9).
Different splitting of this transition is found in the case of the
3
4
6
7
10.4
12.7
14.0
8.0
1.2
1.2
1.2
1.2
0.86
1.1
1.2
0.7
3
+
7-coordinated Yb dimers (3, 4 and 7) suggestive of different
coordination environments of the lanthanoid centres, which is
in agreement with the shape analysis studies (see ESI†). In
a
3+ 26
R
Literature τ for Yb .
Fig. 8 Normalised emission plots for 1 (black trace), 5 (red trace) and
Fig. 9 Normalised emission plots for 3 (black trace), 4 (red trace), 6
(blue trace) and 7 (green trace) Yb complexes, with excitation wave-
length at 350 nm.
3
+
3+
1
0 (blue trace) Eu complexes, with excitation wavelength at 350 nm.
7
5
Inset: highlight of the peaks corresponding to the F
1
← D
0
transition.
17476 | Dalton Trans., 2018, 47, 17469–17478
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contrast, this transition is split into four main bands in the Grant. EZ-C thanks EPSRC (EP/M02105X/1) for support. L. A. G
3
+
case of the 8-coordinated Yb polymer (6). In every case, thanks Curtin University for the postgraduate scholarship.
emission from hot bands is present as a shoulder in the
2
6
9
30–960 nm region.
The observed lifetime decays (τobs) for all the Yb com-
3
+
Notes and references
plexes were satisfactorily fitted to monoexponential functions.
The lifetimes decay values for the Yb dimers 3, 4 and 7, are
3
+
1 A. de Bettencourt-Dias, Dalton Trans., 2007, 22, 2229–2241.
2 S. V. Eliseeva and J.-C. G. Bünzli, New J. Chem., 2011, 35,
1165.
1
0.4, 12.7, 8.0 µs, respectively. These values are comparable to
2
7,28
similar reported β-diketonate complexes.
This suggests
that the presence of a molecule of methanol coordinated to
the lanthanoid cations in complexes 3 and 7 is not dramati-
cally quenching the NIR emission which might be explained
by the presence of intramolecular hydrogen bonds with
short O⋯O distances involving the methanol OH groups in
both structures (3: O10⋯O22, 2.70(1); O20⋯O12, 2.74(1).
3 J.-C. G. Bünzli, Eur. J. Inorg. Chem., 2017, 2017, 5058–5063.
4 S. V. Eliseeva and J.-C. G. Bünzli, Chem. Soc. Rev., 2010, 39,
189–227.
5 J. Kido and Y. Okamoto, Chem. Rev., 2002, 102, 2357–2368.
6 J.-C. G. Bünzli, Luminescence Bioimaging with Lanthanide
Complexes, 2014, vol. 1.
7
: O1⋯O22, 2.576(3); O2⋯O51, 2.578(3) Å). Nevertheless, the
7 J.-C. G. Bünzli and S. V. Eliseeva, J. Rare Earths, 2010, 28,
824–842.
8 B. L. Reid, S. Stagni, J. M. Malicka, M. Cocchi, G. S. Hanan,
M. I. Ogden and M. Massi, Chem. Commun., 2014, 50,
11580–11582.
3+
lifetime decay for the Yb polymer (6), which has no coordinated
MeOH molecules, was found to be slightly longer with a value
of 14.0 µs. However, other quenching effects such as self-
3
+
quenching could also be occurring, since the Yb cations sit
relatively close in distance (∼4 Å) for complexes 3, 4 and 7.
9 B. L. Reid, S. Stagni, J. M. Malicka, M. Cocchi,
A. N. Sobolev, B. W. Skelton, E. G. Moore, G. S. Hanan,
M. I. Ogden and M. Massi, Chem. – Eur. J., 2015, 21, 18354–
18363.
Conclusions
1
0 L. Abad Galán, B. L. Reid, S. Stagni, A. N. Sobolev,
B. W. Skelton, M. Cocchi, J. M. Malicka, E. Zysman-colman,
E. G. Moore, M. I. Ogden and M. Massi, Inorg. Chem., 2017,
56, 8975–8985.
In this report four new β-Triketones were synthesised and fully
characterised, with the aim of using these as ligands to form
lipophilic lanthanoid complexes. It was found that the lantha-
noid complexation reaction under basic conditions in polar 11 L. Abad Galán, A. N. Sobolev, B. W. Skelton, E. Zysman-
solvents, methanol or ethanol, yielded structures where their
retro-Claisen condensation products were incorporated. The
colman, I. Mark and M. Massi, Dalton Trans., 2018, 47,
12345–12352.
high lability of the lanthanoid cations in the presence of a 12 C. Pettinari, F. Marchetti, R. Pettinari, V. Vertlib,
dynamically changing ligand set gave rise to complexes where
one, two or even the three retro-Claisen condensation reactants
A. Drozdov, I. Timokhin, S. Troyanov, Y. S. Min and
D. Kim, Inorg. Chim. Acta, 2003, 355, 157–167.
and products were linked to the lanthanoid centres. These 13 P. C. Andrews, T. Beck, C. M. Forsyth, B. H. Fraser,
results show that ligand stability is a significant issue under
the complexation reaction conditions that were used. Indeed,
P. C. Junk, M. Massi and P. W. Roesky, Dalton Trans., 2007,
9226, 5653–5736.
when the stability of the β-triketones in methanol was studied 14 J. Ishida, H. Ohtsu, Y. Tachibana, Y. Nakanishi,
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Conflicts of interest
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