Performance of a liquid scintillator containing azirconium β-keto ester complex developed for the
ZICOS experiment
Yoshiyuki Fukuda, Narengerile, Akira Obata*
Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan

Shigetaka Moriyama
Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Hida,
Gifu 506-1205, Japan

Izumi Ogawa
Faculty of Engineering, Fukui University, Fukui, Fukui 910-8507, Japan

Takahiro Gunji, Satoru Tsukada, Ryohei Hayami
Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan

Abstract.
A liquid scintillator containing a zirconium β-keto ester complex has been developed for the
ZIrconium Complex in Organic Scintillator (ZICOS) neutrinoless double beta decay experiment.
We are aiming to develop a detector which has a good energy resolution (4% at 2.5 MeV),a large light yield (60% that of BC505) and a low background rate (0.1 counts/tonne·year)with several tonnes of 96 Zr isotope, so we have investigated the zirconium β-keto ester
complexes tetrakis(isopropyl acetoacetato)zirconium and tetrakis(ethyl acetoacetato)zirconium,
which have high solubility (over 10 wt.%) in anisole. We measured the performance of liquid
scintillators containing these zirconium β-keto ester complexes and obtained 40% of the lightyield of BC505 and energy resolution of 4.1% at 2.5 MeV assuming 40% photo coverage of thephotomultiplier in the ZICOS detector. Thus we almost achieved our initial goal. Preliminary
investigations indicate that tetrakis(diethyl malonato)zirconium will give us no quenching of
the light yield and an energy resolution of 2.9% at 2.5 MeV. This will be a suitable complex for
the ZICOS experiment, if it has a large solubility.

keyword :Neutrino （ニュートリノ）
Neutrinoless Double Beta Decay （ニュートリノを放出しない２重ベータ崩壊）
Majorana Mass (マヨラナ質量）
Zirconium （ジルコニウム）
Liquid Scintillator （液体シンチレータ）

1. Introduction
Recent results from long baseline neutrino experiments [1] and reactor neutrino experiments[2] have confirmed a non-zero θ13 , and reliable neutrino mass matrix elements, except for themass hierarchy and CP phase in the lepton sector, can now be obtained by combining resultsfrom all observations of atmospheric [3], solar [4], and reactor neutrinos [5]. However, neither
the Standard Model nor SUSY extensions of it explain why neutrinos should have a mass.
On the other hand, recent leptogenesis models postulate the existence of heavy right-handed
neutrinos with masses as large as the Planck scale [6], which are also generally present in the
See-Saw model [7], and strongly favor the existence of Majorana neutrinos. The observation ofneutrinoless double beta decay would confirm the Majorana nature of the neutrino and would
also provide more information about the neutrino mass scale and the mass hierarchy.
Neutrinoless double beta decay (0νββ) is expected to occur in special nuclei, such as thoselisted in Table 1. These nuclei cannot undergo single beta decay into the nucleus with an atomic
number one higher because of its larger mass. However, if the nucleus with an atomic number
two higher has a lower mass than the original nucleus, then double beta decay could occur. In
ordinary double beta decay (2νββ) two electrons and two neutrinos are emitted at the sametime. In this case, the total energy of the two electrons should be spread out between 0 and the
Q-value, because the two neutrinos would take out some energy. On the other hand, 0νββ does
not emit neutrinos (emits only two electrons), because the neutrino emitted from one nucleon
should be absorbed by another nucleon in same nucleus due to the Majorana nature of theneutrino. Therefore, the total energy of the two electrons should be unique and be the same as
the Q-value of the initial and final nuclei.
To determine the Majorana neutrino mass from 0νββ, we must measure the half-life. The
half-life of 0νββ is given by
0ν +
2
[T12
(0 → 0+ )]−1 = G0ν M0ν

m2ν
m2e

(1)

where G0ν is the kinematic phase space factor, M0ν is the matrix element of the target nuclei
including Fermi, Gamow-Teller and tensor contributions, me is the electron mass, and mν is theeﬀective Majorana neutrino mass. According to Eq. 1, we must be able to measure a half-life of
the order of 1025 years assuming the neutrino mass is less than 100 meV. On the other hand,
the half-life can also be expressed experimentally as
0ν
T12
∼a

Isotope (decay process)48 Ca →48 Ti
150 Nd →150 Sm
96 Zr →96 Mo
100 Mo →100 Ru
82 Se →82 Kr
116 Cd →116 Sn
130 Te →130 Xe
136 Xe →136 Ba
124 Sn →124 Te
76 Ge →76 Se
110 Pd →110 Cd

MT
∆E B

Q-value (keV)
4271
3667
3350
3034
2995
2802
2533
2479
2288
2040
2013

(2)

Abundance (%)
0.2
6
3
10
9
7
34
9
6
8
12

Table 1. Possible nuclei isotopes for 0νββ.

where a is the abundance of the target isotope, M is the target mass, T is the measurement
time, ∆E is the energy resolution, and B is the background rate. According to Eq. 2, nextgeneration 0νββ experiments should have tonnes of target isotope, a background rate of 0.1–1
counts/(t·year), and an energy resolution of 4% at 2.5 MeV. (Alternatively we could combine a
relatively low target mass with very high energy resolution.)
2. ZICOS experiment
We are going to search for 0νββ signal using 96 Zr in the liquid scintillator. This experimentis named ZIrconium Complex in Organic liquid Scintillator (ZICOS) neutrinoless double beta
decay experiment. The liquid scintillator will be contained inside a balloon If the radius of thisballoon is 3 m, then total volume is 113 m3 , which corresponds to 113 tonnes. This balloon
is located in a cylindrical tank which has a 5 m radius and is 10 m in height. This cylinder
is filled with pure water in order to exclude external γ rays, neutrons and cosmic ray muons.
The attenuation length of the scintillation light (∼ 450 nm emitted from POPOP) measured foranisole, which is used for the liquid scintillator solvent, is about 10 m. Therefore, the balloon issmall enough to transmit the light to the photomultipliers, which will be mounted on the wallof the pure water tank. The photo coverage will be 40% in order to collect scintillation light.
To eliminate background signals, the detector should be located in an underground laboratory,
such as the Kamioka Observatory.
In order to suppress background signals from the surface of the balloon which used in ZICOS,
we have to minimize the size of the balloon as much as possible. Therefore, it is important thatthe complex has high solubility. Assuming 10% solubility in a liquid scintillator solvent, theamount of zirconium should be of the order of 3 tonnes. However, the natural abundance of
96 Zr is 3%, so we have to enrich 96 Zr up to 10% if we are to achieve a target mass of 300 kg.

2m

6.5 m
10m
Figure 1. The conceptual design of the ZICOS detector in an EGADS tank. The liquid
scintillator is contained inside a balloon, which is surrounded by pure water to reduce external
background signals.
We are planning to use an EGADS (Evaluating Gadolinium Action on Detector Systems)
tank, which is currently used for demonstration of GADZOOKS! (Gadolinium Antineutrino
Detector Zealously Outperforming Old Kamiokande, Super!), located in the Kamioka mine, forthe first physics program (see Fig. 1). EGAD tanks are 6.5 m in radius and 6.6 m in height. If
we estimate the radius of balloon as 2 m, then inner volume is 33.5 tonnes and, therefore, theamount of complex should be 3.4 tonnes, which contains almost 50 kg of 96 Zr.

A nucleus of 96 Zr has a Q-value of 3.35 MeV, which is the third largest value in possible
double beta decay nuclei, and 3% natural abundance, as shown in Table.1. Generally speaking,it is diﬃcult to use metals, such as Mo, Nd, and Zr, in a double beta decay experiment. Some
metal nuclei such as Ge, Cd, and Te could be used for solid-state detectors and scintillation
crystals. These detectors should have very good energy resolution, however, the target mass islimited by the size of the detector. Usually the metals are used as a thin foil target and theemitted electrons are tagged by the magnetic field in the track chamber. The curvature of the
trajectory corresponds to the electron energy. In that case, the energy loss in the foil might not
be negligible and anyway in such detectors it is generally diﬃcult to have a tonne of target.
We know that a liquid scintillator should have the scalability to realize tonnes of the target,
and will be easy shield from external background signals by being located in a pure watertank. Although novel techniques, such as using quantum dots to dissolve metals in the liquid
scintillator, have been developed in recent years, the solubility is only of the order of 0.1 wt.%[8]. We have, therefore, chosen to dissolve a metal in the liquid scintillator using a β-diketone
complex. A classical liquid scintillator consists of an organic solvent such as benzene., toluene
or 1,2,4-trimethylbenzene. We choose an anisole (methoxybenzene) as a solvent, because it candissolve a metal complex of acetylacetone at about 10 wt.%. Our standard cocktail is produced
by dissolving 100 mg PPO and 10 mg POPOP in 20 mL anisole. This standard cocktail has
almost same light yield as BC505, as shown in Fig. 2, and a fast decay time (∼ 20 ns).
Number of events

Compton spectra for each liquid scintillator
BC505

Number of events

ADC channel

Standard cocktail(anisole 20 mL + PPO 100 mg + POPOP 10 mg)

ADC channel

Figure 2. The Compton spectra from a 60 Co source for (a) BC505 and (b) the standard cocktail(100 mg PPO and 10 mg POPOP in 20 mL anisole).
As described above. there are several nuclei which will possibly undergo neutrinoless double
beta decay. A β-diketone ligand could be formed as complex with one of these metal nuclei athe coordinate bond. Therefore, the number of ligands bonding with metal nuclei is basicallydetermined by the number of valence electrons which are not used for the covalent bond. For
instance, zirconium has three valence electrons. cadmium has two and neodymium has three.
Molybdenum has some compounds with 2, 3, 4, 5, or 6 valence electrons. Because of thestructure of ligands and the molecular size, it is not easy to understand which is the best metal
for formation of the complex. However, we have found that zirconium acetylacetone has thesame solubility (over 10 wt.%) as indium acetylacetone, as reported in Ref. [9] Unfortunately,
this β-diketone ligand has an absorption peak around 270 nm, which partially overlaps theemission spectrum of anisole, as shown in Fig. 3.Therefore, it causes color quenching as theamount of dissolved complex is increased [10]. To avoid this quenching, we synthesized a β-ketoester complex instead of a β-diketon complex in order to shorten the absorption wavelength.

Arbitrary unit

Absorption (Zr(acac)4)
Emission (anisole)

200

250

300

350

400
450
Wavelength [nm]Figure 3. The absorbance spectrum of zirconium acetylacetone and the emission spectrum foranisole measured in hexane.
3. Synthesis of zirconium β-keto ester complex and its properties
We synthesized tetrakis(isopropyl acetoacetato)zirconium (Zr(ipcac)4 ) and tetrakis(ethyl
acetoacetato)zirconium (Zr(etac)4 ) β-keto ester complexes [10]. The chemical structures of thosecomplexes are shown in Fig. 4 and their chemical formulas are Zr(CH3 CCOCHCOOCH(CH3 )2 )4(MW = 711.92) and Zr(CH3 CCOCHCOOCH2 CH3 )4 (MW = 665.81), respectively.

Figure 4. A tetrakis(isopropyl acetoacetato)zirconium complex (left) and a tetrakis(ethyl
acetoacetato)zirconium complex (right).
We measured the solubility of these complexes in anisole and they were over 10 wt.%. We alsomeasured the absorbance spectra which are shown in Fig. 5. The absorption peaks of Zr(iprac)4and Zr(etac)4 were shifted to a shorter wavelength (∼ 240 nm) than the emission wavelengthof anisole. In other words, there is almost no overlap between the absorption spectrum of the
β-keto ester complex and the emission spectrum of anisole in hexane. Optical properties of acomplex, such as absorbance, are generally determined by the ligand and are not aﬀected by
the nucleus. The absorption peak was almost determined by the energy band of the peripheral
electrons of the ligand, and the electron was provided by the nucleus.
As we reported in Ref. [10], we have observed small bump appearing around 290 nm in theabsorbance spectra. We expected that this bump was caused by some impurities in the complex,
which could be separated by precipitation in a solvent, such as hexane or diethyl ether, because
the absorbance spectra showed no bump just a few weeks after the bump was observed. However,
the impurities could not be precipitated in anisole, because the light yield did not recover evenafter a few weeks. Although we could not directly see the absorbance spectra below 300 nm inanisole due to the huge absorption, we considered those impurities to be dissolved in anisole.
Recently, we have started to keep the complex under nitrogen, with the storage vial filled with

200

250

300

(b)

350

Absorption (Zr(etac)4)
Emission (anisole)

250

300

350

400
450
Wavelength [nm]Measured before air contamination
Measured after air contamination

200

400
450
Wavelength [nm]Arbitrary unit
200

Absorbance for Zr(iprac)4
Arbitrary units

Arbitrary units

Absorption (Zr(iprac)4)
Emission (anisole)

250

300

350

400
450
Wavelength [nm]Absorbance for Zr(etac)4
Arbitrary units(a)Measured before air contamination
Measured after air contamination

200

250

300

350

400
450
Wavelength [nm]Figure 5.
Left hand panel shows the absorbance spectra for (a) tetrakis(isopropyl
acetoacetato)zirconium, and (b) tetrakis (ethyl acetoacetato) zirconium. The absorption peaks
are around 240 nm. Right hand panel shows the same absorbance spectra before and after air
contamination. This shows that the small bump appearing around 290 nm was produced by air,particularly oxygen.
nitrogen instead of air. We found that the newly delivered complex did not exhibit such abump in the absorbance spectra as shown in right hand side of Fig. 5. On one occasion air was
accidentally introduced into the storage vial. Then, the newly delivered complex showed thebump again. This indicates that the impurities might be produced by air, particularly oxygen.
4. Performance of liquid scintillators containing zirconium β-keto ester complexes
The performance of a liquid scintillator form the point of view of neutrinoless double beta decay
should be evaluated by its energy resolution. To distinguish between 2νββ and 0νββ, and avoid
γ rays from 208 Tl in U/Th (total energy ∼ 2.7 MeV), our initial goal was that (a) the light yieldshould be larger than 60% that of BC505, and (b) the energy resolution should be 4% at 2.5
MeV for a 10 wt.% concentration of zirconium β-keto ester complex.
To measure the light yield and energy resolution, we must use not only the Compton edge
but also the single energy peak obtained by using the backscattering method. The left figure of
Fig. 6 shows the configuration of the 60 Co radioactive source, the liquid scintillator containing
zirconium β-keto ester complex, and a NaI scintillator. To select the scattering angle of 150
degrees, we collimated the γ rays using lead blocks, as shown in the left panel of Fig. 6. The
right panel of Fig. 6 shows typical energy distributions of the liquid scintillator and the NaI
scintillator. We could see the single peak in both detectors. The calculated energy of NaI
obtained from the scattering angle was 224 keV and the fitted value was 221 keV; therefore theexperimental measurements should be correct.
To perform measurements at 10 wt.% concentration, it was necessary to have about 2 gof Zr(iprac)4 and Zr(etac)4 ; however, we had only about 1 g of each complex. Therefore, we
could not measure the light yield and energy resolution for 10 wt.% concentration. However,
the absorbance spectra of the complex and the ligand should be basically the same because
the optical properties of the complex should be determined by the energy band of the orbital
electrons in the ligand. Fig. 7 shows that the absorbance spectra of Zr(iprac)4 and isopropyl
acetoacetate are actually quite similar. Therefore, at this time, we measured the performancePulse height distribution of LS using BS method

Number of events

12
10
8
6
4
2
0

NaI
scintillator

Number of eventsCo Source

Liquidscintillator

2000

3000

4000

5000
6000
ADC channel

Energy distribution of BS gammas with NaI

50

60

1000

40
30

Expected energy : 225keV

20
10
0

100

200

300

400
500
Observed energy [keV]

Figure 6. Setup for measurement of the single peak for the liquid scintillator using thebackscattering method.
using isopropyl acetoacetate instead of Zr(iprac)4 for large concentrations.
Absorbance spectra of Zr(ipcrc)4 and ipcrc in hexane
Arbitrary units

Zr(iprac)4 (complex)

200

iprac (ligand)

250

300

350

400
450
Wavelength [nm]Figure 7. Absorbance spectra for tetrakis(isopropyl acetoacetato)zirconium, and isopropyl
acetoacetate. The shapes of these spectra are quite similar.
The left panel of Fig. 8 shows the measured light yield fraction for the standard cocktail asa function of the concentration of Zr(iprac)4 , isopropyl acetoacetate, and Zr(acac)4 . At highconcentrations we used isopropyl acetoacetate to measure the light yield, because of a lack ofZr(iprac)4 complex, as described above. To evaluate at same concentration of ligand, we used afourfold larger molar number for isopropyl acetoacetate. For comparison, the light yield fractionfor Zr(acac)4 is also shown in same figure. The light yield of Zr(iprac)4 obtained by fitting
the data is almost 30% to 40% of that of the standard cocktail at a 10 wt.% concentration. It
appears that there is a diﬀerence between isopropyl acetoacetate and Zr(iprac)4 ; namely, thelight yield of the complex may be about half that of the ligand. We thought that this might bedue to the diﬀerence in absorbance shape, as the complex has a small bump appearing around
290 nm. If we can use the controlled complex that should have the same absorbance shape as
the ligand, then we will get the same light yield of about 30% that of BC505. This number isquite a bit smaller than our initial goal which was 60% of the light yield of BC505. However, thisdoes not restrict the performance of the liquid scintillator, because the most important property
is the energy resolution.
The right panel of Fig. 8 shows the measured energy resolution as a function of theconcentration of Zr(iprac)4 equivalent. Again these data were obtained using isopropyl

Concentration dependence of light yield■ Zr(acac)4

0.8● Zr(iprac)4 sample1○ Zr(iprac)4 sample2▼ iprac (ligand)□ iprac (ligand) BS

Concentration dependence of energy resolution
14.0
12.0
Energy resolution [%]Light yield ratio with respect to 0% concentration

1.0

10.0

0.6

0.4

8.0
●
6.0
4.0

0.2
2.0

Standard cocktail : anisole 20 mL
with PPO (100 mg) and POPOP (10 mg)
0.0
0.0
2.0
4.0
6.0
8.0
10.0
Concentration [wt.%]

0.0Standard cocktail : anisole 20 mL
with PPO (100 mg) and POPOP (10 mg)
2.0

4.0

6.0
8.0
10.0
Concentration [w.t.%]

Figure 8. Light yield fraction as a function of the concentration of Zr(iprac)4 and isopropyl
acetoacetate (left panel). Measured energy resolution as a function of the concentration ofZr(iprac)4 and isopropyl acetoacetate (right panel).
acetoacetate, not Zr(iprac)4 . It appears that the energy repulsion obeys the usual expectation
σ = √ σ0 , where E, E0 , and σ0 correspond to the electron energy, the reference energy, andE/E0

the energy resolution for the reference energy, respectively. The energy should be proportional
to the light yield. The obtained energy resolution around 10 wt.% concentration was 14% at 1
MeV, which was estimated by measuring the energy of NaI. In this case, the observed energy
in the liquid scintillator was 1 MeV, because we used 60 Co as the γ source (1.33 MeV and 1.17
MeV) and the expected energy deposited in the liquid scintillator would be calculated as 1.025
MeV (the 0.225 MeV value observed in the NaI scintillator should be subtracted). In this setup,the scintillation photons were collected by two photomultipliers, as shown in the right panel ofFig. 6. The photo coverage of this setup was estimated to be about 8.5% using Monte Carlosimulation. On the other hand, the ZICOS detector will have 40% coverage of photomultipliers,
so that the energy resolution for the ZICOS detector should be 6.5% at 1.0 MeV. Therefore,
the actual energy resolution at 2.5 MeV was estimated to be 4.1%. This value is quite similar
to our initial goal. In conclusion, our liquid scintillator system with 10 wt.% concentration ofZr(iprac)4 in anisole has almost achieved our initial goals.
5. Tetrakis(diethyl malonato)zirconium
As described in previous section, it is possible, using Zr(iprac)4 , to obtain a liquid scintillator
system for the ZICOS detector, which should have the necessary energy resolution for theneutrinoless double beta decay search. However, the light yield is still lower than our goal,so the energy resolution of 4.1% at 2.5 MeV may not be enough, because this is almost the same
performance as achieved by the present KamLAND experiment. KamLAND needs better energy
resolution to distinguish the background and 136 Xe signals. In our case, in order to achieve a
better energy resolution, we need to have a much better complex than Zr(iprac)4 .
The β-keto ester complex shortened the absorption wavelength by introducing an ester groupin the place of a methyl group. Therefore, if we exchange another methyl group with anester group, the absorption peak should be moved to an even shorter wavelength than 240
nm. Tetrakis(diethyl malonato)zirconium (Zr(deml)4 ) which is shown in the left panel of Fig. 9

is one possible complex which has double ester ligand instead of a β-keto ester ligand.
The absorbance spectrum of diethyl malonate was measured and the result is shown in theright panel of Fig. 9.

Arbitrary units

Absorption (deml)
Emission (anisole)

200

250

300

350

400
450
Wavelength [nm]Figure 9. Chemical structure of Zr(deml)4 and absorbance spectrum of diethyl malonate.
The absorption peak has shifted to around 210 nm, a much shorter wavelength than that ofethyl acetylacetone. This indicates that the light yield and energy resolution of tetrakis(diethyl
malonato)zirconium could be improved by the non-overlapping of the absorption spectrum of
the complex and the emission spectrum of anisole. The Compton edge distributions of γ raysfrom the 60 Co radioactive source for each ligand (isopropyl acetoacetate, methyl acetoacetate,
and diethyl malonate) at almost same concentration (6 –7 wt.%) are shown in Fig. 10, whichshows that the Compton edge of a liquid scintillator containing diethyl malonate is same asthat of the standard cocktail. This means that there is no quenching for a liquid scintillator
containing diethyl malonate. According to these results, we expect that tetrakis(diethyl
malonato)zirconium (Zr(deml)4 ) will give us the same light yield as BC505, and the energy
resolution will be about 2.9% at 2.5 MeV for the ZICOS detector. This value is better than our
initial goal, and might be enough to distinguish background and signals. In other words, it is
crucial for discrimination in a non-tracking detector such as a liquid scintillator.
6. Conclusion
Tetrakis(isopropyl acetoacetato)zirconium and tetrakis(ethyl acetoacetato)zirconium have aabsorption peak around 240 nm, which enables us to obtain a liquid scintillator containing
those β-keto ester complexes having a light yield of 30% to 40% relative to that of BC505
and an expected energy resolution of 4.1% at 2.5 MeV for the ZICOS experiment. Also, theattenuation length for 450 nm light, which is emission peak of POPOP, in anisole is almost 6
m. Therefore, we are almost ready to realize this experiment. We are now developing anothercomplex, tetrakis(diethyl malonato)zirconium, to shorten the absorption wavelength to around
210 nm. We expect that a liquid scintillator containing tetrakis(diethyl malonato)zirconium will
have the same light yield as BC505 and an energy resolution of 2.9% at 2.5 MeV. This will be
the final complex for the ZICOS experiment, if it has a suﬃciently large solubility.
Synthesis
Tetrakis(isopropyl acetoacetato)zirconium
ZrCl4 (3.10 g, 13.3 mmol) in benzene (40 mL) was added to tetrahydrofuran (2 mL), andstirred overnight. The slight cloudy solution was added to isopropyl acetoacetate (8.63 g, 59.9mmol), and stirred for 1 h at ambient temperature. The pale yellow suspension was added

Scintillation light yield for several ligands

150

Anisole 20 mL +PPO (100 mg) + POPOP (10 mg)

100
50
0

0

1000

2000

3000

4000

5000

6000

ADC channel

150

Zr(acac)4 1053mg (1 wt.%)

100
50
0

0

1000

2000

3000

4000

150

5000

6000

ADC channel

iprac 1250mg (6.7 wt.% equiv.)

100
50
00

1000

2000

3000

4000

5000

6000

ADC channel

150

etac 1124mg (6.2 wt.% equiv.)

100
50
00

1000

2000

3000

4000

5000

6000

ADC channel

150

deml 1383mg (7.3 wt.% equiv.)

100
50
0

0

1000

2000

3000

4000

5000

6000

ADC channel

Figure 10. Scintillation yield using the Compton edge for (a) the standard cocktail, (b)Zr(acac)4 , (c) isopropyl acetoacetate, (d) ethyl acetoacetate, and (e) diethyl malonate.
dropwise to triethylamine (5.96 g, 58.9 mmol) over 30 min and then refluxed for 2 h. Aftercooling, the mixture was filtered to remove amine salt and volatile compounds were removed
using a rotary evaporator. The orange residue was recrystallized from isopropyl alcohol. The
crystal was collected by filtration and dried under reduced pressure at room temperature toprovide Zr(iprac)4 as white crystals (4.63 g, 52.4%).
tetrakis(ethyl acetoacetato)zirconium
ZrCl4 (3.94 g, 16.9 mmol) in benzene (50 mL) was added to tetrahydrofuran (4 mL), andstirred overnight. The slight cloudy solution was added to ethyl acetoacetate (8.86 g, 68.1 mmol),and stirred for 1 h at ambient temperature. The pale yellow suspension was added dropwise
to triethylamine (7.03 g, 69.4 mmol) over 30 min and then refluxed for 2 h. After cooling, themixture was filtered to remove amine salt and volatile compounds were removed using a vacuum
pump. A yellow viscous liquid was obtained, which gave pale yellow solid after sitting for several
weeks at room temperature. The solid was collected by filtration, dissolved in a small amount of
EtOH, and then cooled to -30◦ C. After 1 month, Zr(etac)4 was obtained as a white solid (8.10g, 79%).
Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas (No.
24104501 and No. 26105502) from the Ministry of Education, Culture, Sports, Science andTechnology (MEXT), and a Grant-in-Aid for Scientific Research (C) (No.24540295) from theJapanese Society for the Promotion of Science (JSPS). We would like to thank Professor Takashi
Muramatsu for help in the measurement of the absorbance of the complex.
References

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